AND ELECTROCATALYST INCLUDING THE SAME

Cross-reference to related application

The present application claims priority to Korean patent application No. 10-2016-0177502 filed on December 23, 2016, the whole content of this application being incorporated herein by reference. Should the disclosure of any patents, patent applications, and publications which are incorporated herein by reference conflict with the description of the present application to the extent that it may render a term unclear, the present description shall take precedence.

TECHNICAL FIELD

[0001] The present disclosure relates to a hollow composite, a method of preparing the hollow composite, an electrocatalyst including the hollow composite, an electrode including the hollow composite, a cell including the electrode, an electronic ink including the hollow composite, and an electronic paper and display device including the electronic ink.

BACKGROUND

[0002] Increasing energy demands and foreseen reduced fossil fuels supplies have paved the need to an efficient development of clean and sustainable energy conversion, and storage systems at reduced cost. In particular, low-emission fuel cells and metal-air batteries with high energy density and reusability have been highlighted as promising energy sources in recent years. The sluggish oxygen reduction reaction (ORR) has been hinted to limit the performance of these technologies. Pt supported on carbon (Pt/C) has been italicized as the catalyst of choice in ORR in both acidic and alkaline conditions, despite its high cost and poor durability. On the contrary, precious transition metal oxides, chalcogenides, and their composites with carbon materials tend to exhibit poor electronic conductivity and durability in strong acidic and basic environment. The exploration of low-cost and efficient ORR catalysts made from earth-abundant elements has thus become of paramount importance, with a plethora of exploratory works focusing on promising alternatives with enhanced applicability. In this context, Ti0 ₂ has been focus of interest due to its abundance, low-cost, non-toxicity and high stability in a wide pH range. Recent reports have shown that Pt/Ti0 ₂ exhibited comparable ORR activity and superior stability than commercial Pt/carbon black. However, the application of Ti0 ₂ as a sole ORR electrocatalyst has remained scarce in the literature with only low activity values being reported. The relatively low intrinsic conductivity and poor reactivity of Ti0 ₂ are believed to hinder an efficient use of this material in fuel cells and electrocatalytic applications that require fast electron transport.

[0003] To tentatively improve the electrochemical properties of Ti0 ₂ , a number of approaches have emerged in the literature (e.g., elemental doping and coupling with carbon materials). Literature reports focusing on self-doping of Ti ³⁺ accompanied by the formation of oxygen vacancies into the crystal lattice of Ti0 ₂ have gathered significant attention. Self-doping of Ti ³⁺ has been shown to substantially narrow the bandgap of the Ti0 ₂ , increasing the donor density and electric conductivity of the Ti0 ₂ . Facet- and defect-engineered Ti ³⁺ doped Ti0 ₂ nanocrystals were recently reported to exhibit competitive ORR activity, excellent durability and superior tolerance to methanol. Nevertheless, the stability of Ti ³⁺ -doped Ti0 ₂ systems has remained a challenge due to the easy oxidation of Ti ³⁺ species.

[0004] In an attempt to improve the electric conductivity of Ti0 ₂ , hybridization with high conductive carbonaceous materials has been reported. Owing to its superior electronic conductivity, high mechanical strength, structural flexibility, and high surface area, carbonaceous materials have found effective application in photovoltaics, photocatalysis, and electrocatalysis. In particular, rGO/Ti0 ₂ hybrid materials have shown higher conductivity and improved thermal and chemical stability. Despite the progress achieved in photocatalysis, water splitting and solar cells[J. Du, X. Lai, N. Yang, J. Zhai, D. Kisailus, F. Su, D. Wang, L. Jiang, ACS Nano 2011, 5, 590. ; G. Xie, K. Zhang, B. Guo, Q. Liu, L. Fang, J. . Gong, Adv. Mater. 2013, 25, 3820. ; J. T.-W. Wang, J. M. Ball, E. M. Barea, A. Abate, J. A. Alexander-Webber, J. Huang, M. Saliba, I. Mora-Sero, J. Bisquert, H. J. Snaith, R. J. Nicholas, Nano Lett. 2014, 14, 724.], to the best of our knowledge, however, the application of rGO/Ti0 ₂ catalytic systems remains largely unexplored in electrocatalytic applications.

[0005] The attempts to simultaneously consider the study of rGO/Ti0 ₂ hybrid systems and the evaluation of Ti ³⁺ self-doped Ti0 ₂ have remained unexplored in the art.

[0006] In this regard, U.S. Patent Laid-open Publication No. 2013-0330659 discloses a method for producing a fuel cell electrode catalyst. SUMMARY

[0007] In view of the foregoing, the present disclosure provides a hollow composite, including a shell which includes a hybrid containing a metal oxide and a carbonaceous structure, wherein the hybrid is self-doped by a metal element included in the metal oxide, and wherein an inner surface of the shell in the hollow composite includes carbon.

[0008] The present disclosure provides a method of preparing the hollow composite, an electrocatalyst including the hollow composite, an electrode including the hollow composite, a cell including the electrode, an electronic ink including the hollow composite, and an electronic paper and a display device including the electronic ink.

[0009] However, problems to be solved by the present disclosure are not limited to the above-described problems. Although not described herein, other problems to be solved by the present disclosure can be clearly understood by those skilled in the art from the following descriptions.

[0010] In accordance with a first aspect of the present disclosure, there is provided a hollow composite, including a shell which includes a hybrid containing a metal oxide and a carbonaceous structure, wherein the hybrid is self-doped by a metal element included in the metal oxide, and an inner surface of the shell in the hollow composite includes carbon.

[0011] In accordance with a second aspect of the present disclosure, there is provided a method for preparing a hollow composite, including coating a polymer core particle with a mixture solution including a precursor for a metal oxide and a precursor for a carbonaceous structure to form a core-shell composite in which the precursors are coated on the polymer core particle, and a first calcination of the core-shell composite to obtain a hollow composite, wherein a shell of the hollow composite includes a hybrid containing the metal oxide and the carbonaceous structure, and wherein the hybrid is self-doped by a metal element included in the metal oxide, and the first calcination includes heating the core-shell composite at a first temperature to carbonize and remove the polymer core particle so as to obtain the hollow composite.

[0012] In accordance with a third aspect of the present disclosure, there is provided an electrocatalyst, including the hollow composite according to the first aspect of the present disclosure.

[0013] In accordance with a fourth aspect of the present disclosure, there is provided an electrode, including the hollow composite according to the first aspect of the present disclosure.

[0014] In accordance with a fifth aspect of the present disclosure, there is provided a cell, including the electrode according to the fourth aspect of the present disclosure.

[0015] In accordance with a sixth aspect of the present disclosure, there is provided an electronic ink, including the hollow composite according to the first aspect of the present disclosure.

[0016] In accordance with a seventh aspect of the present disclosure, there is provided an electronic paper or a display device, including the electronic ink according to the sixth aspect of the present disclosure. EFFECTS OF INVENTION

[0017] In accordance with an embodiment of the present disclosure, the hollow composite comprises a shell which includes a hybrid containing a metal oxide and a carbonaceous structure, wherein the hybrid is self-doped by a metal element included in the metal oxide and an inner surface of the shell in the hollow composite includes carbon, so that electronic conductivity of the hollow composite is improved, and the hollow composite exhibits electrocatalytic activity for oxygen reduction reaction, which is similar to the activity of Pt/C.

[0018] In accordance with an embodiment of the present disclosure, the hollow composite includes hybridization of the metal oxide and the carbonaceous structure, self-doping by the metal element, and carbon, so that its stability in an alkaline media in the oxygen reduction reaction is improved and particularly, its methanol tolerance is increased.

[0019] In accordance with an embodiment of the present disclosure, the hollow composite may be used for an electrocatalyst, in particular, an electrocatalyst for oxygen reduction reaction, an electrode, in particular, an electrode for oxygen reduction reaction, a cell including the electrode, an electronic ink, and an electronic paper and a display device including the electronic ink.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 provides SEM images of (a) PS nanospheres, (b) PS@GO/Ti0 ₂ , (c) Ti0 _2, and (d) rGO(10%)/TiO ₂ hollow composite after carbonization in an example of the present disclosure; FIG. 2 provides FESEM images of (a) PS@Ti0 ₂ , (b) rGO(5%)/Ti0 ₂ , and (c) rGO(20%)/TiO ₂ hollow composite and a cross-sectional image of (d) rGO(10%)/TiO ₂ hollow composite (scale bar is 1 μηι in all figures) in an example of the present disclosure;

FIG. 3 provides TEM images of (a) Ti0 ₂ (air), (b) Ti0 ₂ , (c) rGO(20%)/TiO ₂ showing the hollow nature, and H TEM image of (d) rGO(20%)/TiO ₂ in an example of the present disclosure; FIG. 4 provides (a) TEM image of Ti0 ₂ hollow composite, (b) corresponding HRTEM image evidencing the presence of a thin carbon layer onto the Ti0 ₂ nanoparticles, and (c) TEM and (d) HRTEM images of rGO(10%)/TiO ₂ hollow composite in an example of the present disclosure;

FIG. 5A shows the synchrotron X-ray diffraction (SXRD) patterns of crystalline phase of Ti0 ₂ and rGO/Ti0 ₂ hollow composites in an example of the present disclosure;

FIG. 5B shows Raman spectra of the hybrid structures collected to investigate the bonding nature of rGO and Ti0 ₂ in an example of the present disclosure;

FIG. 6A shows the normalized XANES spectra of a pristine Ti0 ₂ reference calcined in air (Ti0 ₂ air), and Ti0 ₂ and rGO(10%)/TiO ₂ hollow nanospheres in an example of the present disclosure;

FIG. 6B shows the magnified image of XANES spectra showing the pre-edge position of a pristine Ti0 ₂ reference calcined in air (Ti0 ₂ air), and Ti0 ₂ and rGO(10%)/TiO ₂ hollow nanospheres in an example of the present disclosure;

FIG. 7A shows Nyquist plots of all evaluated samples in an example of the present disclosure; FIG. 7B shows the result of Mott-Schottky measurements of the corresponding samples in an example of the present disclosure;

FIG. 8A shows CV curves of rGO/Ti0 ₂ and Pt/C in an example of the present disclosure;

FIG. 8B shows LSV curves of different samples in an example of the present disclosure;

FIG. 8C shows LSV curves of rGO(10%)/TiO ₂ samples at different rotation rates in an example of the present disclosure;

FIG. 8D shows Tafel plots of rGO/Ti0 ₂ composites derived from the corresponding RDE data in an example of the present disclosure;

FIG. 9 shows CV curves of various samples and LSV curves of rGO(5%)/Ti0 ₂ rGO(20%)/TiO ₂ and Pt/C, respectively, at different rotation rates in an example of the present disclosure; FIG. 10A to FIG. 10D shows K-L plots for (a) rGO(5%)/Ti0 ₂ , (b) rGO(10%)/TiO ₂ , (c) rGO(20%)/TiO ₂ derived from corresponding O plots at different potentials and electron transfer number of (d) rGO(5%)/Ti0 ₂ , rGO(10%)/TiO ₂ , rGO(20%)/TiO ₂ and Pt/C in an example of the present disclosure; and

FIG. 11 shows chronoamperometric responses and methanol tolerances of respective samples in an example of the present disclosure.

DETAILED DESCRIPTION

[0020] Hereinafter, examples of the present disclosure will be described in detail with reference to the accompanying drawings so that the present disclosure may be readily implemented by those skilled in the art. However, it is to be noted that the present disclosure is not limited to the examples but can be embodied in various other ways. In drawings, parts irrelevant to the description are omitted for simplicity of explanation, and like reference numerals denote like parts through the whole document.

[0021] Through the whole document, the term "connected to" or "coupled to" that is used to designate a connection or coupling of one element to another element includes both a case that an element is "directly connected or coupled to" another element and a case that an element is "electronically or electrostatically connected or coupled to" another element via still another element.

[0022] Through the whole document, the term "on" that is used to designate a position of one element with respect to another element includes both a case that the one element is adjacent to the another element and a case that any other element exists between these two elements.

[0023] Further, through the whole document, the term "comprises or includes" and/or "comprising or including" used in the document means that one or more other components, steps, operation and/or existence or addition of elements are not excluded in addition to the described components, steps, operation and/or elements unless context dictates otherwise. Through the whole document, the term "about or approximately" or "substantially" are intended to have meanings close to numerical values or ranges specified with an allowable error and intended to prevent accurate or absolute numerical values disclosed for understanding of the present disclosure from being illegally or unfairly used by any unconscionable third party. Through the whole document, the term "step of" does not mean "step for".

[0024] Through the whole document, the term "combination of" included in Markush type description means mixture or combination of one or more components, steps, operations and/or elements selected from a group consisting of components, steps, operation and/or elements described in Markush type and thereby means that the disclosure includes one or more components, steps, operations and/or elements selected from the Markush group.

[0025] In a first aspect of the present disclosure, there is provided a hollow composite, including a shell which includes a hybrid containing a metal oxide and a carbonaceous structure, wherein the hybrid is self-doped by a metal element included in the metal oxide, and wherein an inner surface of the shell in the hollow composite includes carbon.

[0026] In an embodiment of the present disclosure, the metal oxide may include a semiconductor oxide, and the semiconductor oxide may include, for example, a member selected from the group consisting of Ti0 ₂ , Sn0 ₂ , ZnO, V0 ₂ , ln ₂ 0 ₃ , NiO, Mo0 ₃ , SrTi0 ₃ , Fe-doped SrTi0 ₃ , Fe ₂ 0 ₃ , W0 ₃ , CuO, BiV0 ₄ , and combinations thereof.

[0027] In an embodiment of the present disclosure, the carbonaceous structure may include a member selected from the group consisting of reduced graphene oxide (rGO), graphene, graphite, single-walled carbon nanotube, multi-walled carbon nanotube, carbon nanohorns, vapor- grown carbon fibers and carbon nanofibers, synthetic carbon sources including acetylene black, carbon black coming from the combustion of hydrocarbon or polymer (e.g., Ketjenblack), and combinations thereof.

[0028] In an embodiment of the present disclosure, the metal element may be in a more reduced state than a metal cation included in the metal oxide. The reduced metal cation is unstable and thus easily oxidized. However, the reduced metal cation may be stabilized by carbon formed on the inner surface of the shell and the carbonaceous structure so as to self-dop the hybrid. For example, if the metal oxide in the shell in the hollow composite includes Ti0 ₂ , Ti ⁴⁺ contained in the Ti0 ₂ may be partially reduced to Ti ³⁺ which is stabilized in the hollow composite so that the hybrid can be self-doped by the stabilized Ti ³⁺ .

[0029] In an embodiment of the present disclosure, a size of the hollow composite may range from about 50 nm to about 5,000 nm. The size of the hollow composite may have a different meaning depending on the shape of the hollow composite. For example, if the hollow composite has a sphere shape, the size may mean the diameter of the sphere, and if the hollow composite has an oval shape, the size may mean the diameter of its longer axis or the diameter of its shorter axis. The size of the hollow composite may range, for example, from about 50 nm to about 5,000 nm, from about 100 nm to about 5,000 nm, from about 300 nm to about 5,000 nm, from about 500 nm to about 5,000 nm, from about 700 nm to about 5,000 nm, from about 1,000 nm to about 5,000 nm, from about 1,300 nm to about 5,000 nm, from about 1,500 nm to about 5,000 nm, from about 1,700 nm to about 5,000 nm, from about 2,000 nm to about 5,000 nm, from about 2,500 nm to about 5,000 nm, from about 3,000 nm to about 5,000 nm, from about 3,500 nm to about 5,000 nm, from about 4,000 nm to about 5,000 nm, from about 4,500 nm to about 5,000 nm, from about 50 nm to about 4,500 nm, from about 50 nm to about 4,000 nm, from about 50 nm to about 3,500 nm, from about 50 nm to about 3,000 nm, from about 50 nm to about 2,500 nm, from about 50 nm to about 2,000 nm, from about 50 nm to about 1,700 nm, from about 50 nm to about 1,500 nm, from about 50 nm to about 1,300 nm, from about 50 nm to about 1,000 nm, from about 50 nm to about 700 nm, from about 50 nm to about 500 nm, from about 50 nm to about 300 nm, from about 50 nm to about 100 nm, from about 100 nm to about 4,000 nm, from about 300 nm to about 4,000 nm, from about 500 nm to about 4,000 nm, from about 700 nm to about 4,000 nm, from about 1,000 nm to about 4,000 nm, from about 1,300 nm to about 4,000 nm, from about 1,500 nm to about 4,000 nm, from about 1,700 nm to about 4,000 nm, from about 2,000 nm to about 4,000 nm, from about 2,500 nm to about 4,000 nm, from about 3,000 nm to about 4,000 nm, from about 3,500 nm to about 4,000 nm, from about 100 nm to about 3,500 nm, from about 100 nm to about 3,000 nm, from about 100 nm to about 2,500 nm, from about 100 nm to about 2,000 nm, from about 100 nm to about 1,700 nm, from about 100 nm to about 1,500 nm, from about 100 nm to about 1,300 nm, from about 100 nm to about 1,000 nm, from about 100 nm to about 700 nm, from about 100 nm to about 500 nm, from about 100 nm to about 300 nm, from about 200 nm to about 2,000 nm, from about 300 nm to about 2,000 nm, from about 400 nm to about 2,000 nm, from about 500 nm to about 2,000 nm, from about 600 nm to about 2,000 nm, from about 700 nm to about 2,000 nm, from about 800 nm to about 2,000 nm, from about 900 nm to about 2,000 nm, from about 1,000 nm to about 2,000 nm, from about 1,100 nm to about 2,000 nm, from about 1,200 nm to about 2,000 nm, from about 1,300 nm to about 2,000 nm, from about 1,400 nm to about 2,000 nm, from about 1,500 nm to about 2,000 nm, from about 1,600 nm to about 2,000 nm, from about 1,700 nm to about 2,000 nm, from about 1,800 nm to about 2,000 nm, from about 1,900 nm to about 2,000 nm, from about 200 nm to about 1,900 nm, from about 200 nm to about 1,800 nm, from about 200 nm to about 1,700 nm, from about 200 nm to about 1,600 nm, from about 200 nm to about 1,500 nm, from about 200 nm to about 1,400 nm, from about 200 nm to about 1,300 nm, from about 200 nm to about 1,200 nm, from about 200 nm to about 1,100 nm, from about 200 nm to about 1,000 nm, from about 200 nm to about 900 nm, from about 200 nm to about 800 nm, from about 200 nm to about 700 nm, from about 200 nm to about 600 nm, from about 200 nm to about 500 nm, from about 200 nm to about 400 nm, or from about 200 nm to about 300 nm, from about 400 nm to about 1,800 nm, from about 500 nm to about 1,800 nm, from about 600 nm to about 1,800 nm, from about 700 nm to about 1,800 nm, from about 800 nm to about 1,800 nm, from about 900 nm to about 1,800 nm, from about 1,000 nm to about 1,800 nm, from about 1,100 nm to about 1,800 nm, from about 1,200 nm to about 1,800 nm, from about 1,300 nm to about 1,800 nm, from about 1,400 nm to about 1,800 nm, from about 1,500 nm to about 1,800 nm, from about 1,600 nm to about 1,800 nm, from about 1,700 nm to about 1,800 nm, from about 400 nm to about 1,700 nm, from about 400 nm to about 1,600 nm, from about 400 nm to about 1,500 nm, from about 400 nm to about 1,400 nm, from about 400 nm to about 1,300 nm, from about 400 nm to about 1,200 nm, from about 400 nm to about 1,100 nm, from about 400 nm to about 1,000 nm, from about 400 nm to about 900 nm, from about 400 nm to about 800 nm, from about 400 nm to about 700 nm, from about 400 nm to about 600 nm, from about 400 nm to about 500 nm, from about 600 nm to about 1,600 nm, from about 700 nm to about 1,600 nm, from about 800 nm to about 1,600 nm, from about 900 nm to about 1,600 nm, from about 1,000 nm to about 1,600 nm, from about 1,100 nm to about 1,600 nm, from about 1,200 nm to about 1,600 nm, from about 1,300 nm to about 1,600 nm, from about 1,400 nm to about 1,600 nm, from about 1,500 nm to about 1,600 nm, from about 600 nm to about 1,500 nm, from about 600 nm to about 1,400 nm, from about 600 nm to about 1,300 nm, from about 600 nm to about 1,200 nm, from about 600 nm to about 1,100 nm, from about 600 nm to about 1,000 nm, from about 600 nm to about 900 nm, from about 600 nm to about 800 nm, from about 600 nm to about 700 nm, from about 800 nm to about 1,400 nm, from about 900 nm to about 1,400 nm, from about 1,000 nm to about 1,400 nm, from about 1,100 nm to about 1,400 nm, from about 1,200 nm to about 1,400 nm, from about 1,300 nm to about 1,400 nm, from about 800 nm to about 1,300 nm, from about 800 nm to about 1,200 nm, from about 800 nm to about 1,100 nm, from about 800 nm to about 1,000 nm, or from about 800 nm to about 900 nm.

[0030] In an embodiment of the present disclosure, a content of the carbonaceous structure may be from about 1 part by weight to about 20 parts by weight with respect to 100 parts by weight of the hollow composite. For example, the content of the carbonaceous structure with respect to 100 parts by weight of the hollow composite may be from about 1 part by weight to about 20 parts by weight, from about 2 parts by weight to about 20 parts by weight, from about 3 parts by weight to about 20 parts by weight, from about 4 parts by weight to about 20 parts by weight, from about 5 parts by weight to about 20 parts by weight, from about 6 parts by weight to about 20 parts by weight, from about 7 parts by weight to about 20 parts by weight, from about 8 parts by weight to about 20 parts by weight, from about 9 parts by weight to about 20 parts by weight, from about 10 parts by weight to about 20 parts by weight, from about 11 parts by weight to about 20 parts by weight, from about 12 parts by weight to about 20 parts by weight, from about 13 parts by weight to about 20 parts by weight, from about 14 parts by weight to about 20 parts by weight, from about 15 parts by weight to about 20 parts by weight, from about 16 parts by weight to about 20 parts by weight, from about 17 parts by weight to about 20 parts by weight, from about 18 parts by weight to about 20 parts by weight, from about 19 parts by weight to about 20 parts by weight, from about 1 part by weight to about 19 parts by weight, from about 1 part by weight to about 18 parts by weight, from about 1 part by weight to about 17 parts by weight, from about 1 part by weight to about 16 parts by weight, from about 1 part by weight to about 15 parts by weight, from about 1 part by weight to about 14 parts by weight, from about 1 part by weight to about 13 parts by weight, from about 1 part by weight to about 12 parts by weight, from about 1 part by weight to about 11 parts by weight, from about 1 part by weight to about 10 parts by weight, from about 1 part by weight to about 9 parts by weight, from about 1 part by weight to about 8 parts by weight, from about 1 part by weight to about 7 parts by weight, from about 1 part by weight to about 6 parts by weight, from about 1 part by weight to about 5 parts by weight, from about 1 part by weight to about 4 parts by weight, from about 1 part by weight to about 3 parts by weight, or from about 1 part by weight to about 2 parts by weight. If the content of the carbonaceous structure is less than 1 part by weight with respect to 100 parts by weight of the hollow composite, electric conductivity of the hollow composite may be decreased due to a small content of the carbonaceous structure that enables easier electron transport. If the content of the carbonaceous structure is more than 20 parts by weight with respect to 100 parts by weight of the hollow composite, a shape of the composite may not be induced to a hollow composite and the shell may be increased in thickness, so that electric conductivity may be decreased.

[0031] In an embodiment of the present disclosure, a charge transfer resistance of the hollow composite may range from about 10 Ω to about 200 kQ, but may not be limited thereto. For example, the charge transfer resistance of the hollow composite may range from about 10 Ω to about 200 kQ, from about 100 Ω to about 200 kQ, from about 300 Ω to about 200 kQ, from about 500 Ω to about 200 kQ, from about 700 Ω to about 200 kΩ, from about 1 kΩ to about 200 kΩ, from about 10 kΩ to about 200 kΩ, from about 20 kΩ to about 200 kΩ, from about 30 kΩ to about 200 kΩ, from about 40 kΩ to about 200 kΩ, from about 50 kΩ to about 200 kΩ, from about 60 kΩ to about 200 kΩ, from about 70 kΩ to about 200 kΩ, from about 80 kΩ to about 200 kΩ, from about 90 kΩ to about 200 kΩ, from about 100 kΩ to about 200 kΩ, from about 120 kΩ to about 200 kΩ, from about 140 kΩ to about 200 kΩ, from about 160 kΩ to about 200 kΩ, from about 180 kΩ to about 200 kΩ, from about 10 Ω to about 180 kΩ, from about 10 Ω to about 160 kΩ, from about 10 Ω to about 140 kΩ, from about 10 Ω to about 120 kΩ, from about 10 Ω to about 100 kΩ, from about 10 Ω to about 90 kΩ, from about 10 Ω to about 80 kΩ, from about 10 Ω to about 70 kΩ, from about 10 Ω to about 60 kΩ, from about 10 Ω to about 50 kΩ, from about 10 Ω to about 40 kΩ, from about 10 Ω to about 30 kΩ, from about 10 Ω to about 20 kΩ, from about 10 Ω to about 10 kΩ, from about 10 Ω to about 1 kΩ, from about 10 Ω to about 700 Ω, from about 10 Ω to about 500 Ω, from about 10 Ω to about 300 Ω, or from about 10 Ω to about 100 Ω, but may not be limited thereto. The charge transfer resistance is closely correlated to electric conductivity of the hollow composite, and as the charge transfer resistance is decreased, the electric conductivity of the hollow composite is increased. A high electric conductivity (low charge transfer resistance) of the hollow composite may be due to the carbonaceous structure included in the hollow composite.

[0032] In an embodiment of the present disclosure, the hollow composite may use as an electrocatalyst for oxygen reduction reaction, and the hollow composite may facilitate oxygen diffusion, increase the number of active sites, and facilitate electrolyte diffusion for the oxygen reduction reaction.

[0033] In an embodiment of the present disclosure, the hollow composite may further enhance the inherently poor electric conductivity of the metal oxide by including the carbonaceous structure.

[0034] In a second aspect of the present disclosure, there is provided a method for preparing a hollow composite, including coating a polymer core particle with a mixture solution including a precursor for a metal oxide and a precursor for a carbonaceous structure to form a core- shell composite in which the precursors are coated on the polymer core particle, and a first calcination of the core-shell composite to obtain a hollow composite, wherein a shell of the hollow composite includes a hybrid containing the metal oxide and the carbonaceous structure, and the hybrid is self-doped by a metal element included in the metal oxide, and wherein the first calcination includes heating the core-shell composite at a first temperature to carbonize and remove the polymer core particle so as to obtain the hollow composite.

[0035] Detailed descriptions of the repeated parts as described in the first aspect of the present disclosure will be omitted. Although omitted in the second aspect of the present disclosure, the description of the first aspect of the present disclosure may also be applied in the same manner to the second aspect.

[0036] In an embodiment of the present disclosure, in S100, by coating a polymer core particle with a mixture solution including a precursor for a metal oxide and a precursor for a carbonaceous structure, a core-shell composite in which the precursors are coated on the polymer core particle is formed.

[0037] In an embodiment of the present disclosure, coating of the precursor for the metal oxide and the precursor for the carbonaceous structure on the polymer core particle may be attributed to electrostatic interaction between functional groups included in the polymer core particle and the precursor for the metal oxide and the precursor for the carbonaceous structure. For example, the coating may be attributed to electrostatic interaction between carbonyl group of the polymer core particle and hydroxyl group included in the precursor for the metal oxide and the precursor for the carbonaceous structure.

[0038] In an embodiment of the present disclosure, coating of the precursor for the metal oxide and the precursor for the carbonaceous structure on the polymer core particle may be carried out to modify the polymer core particle.

[0039] In an embodiment of the present disclosure, a content of the precursor for the carbonaceous structure may be from about 1 part by weight to about 30 parts by weight with respect to 100 parts by weight of the mixture solution including the precursor for the metal oxide and the precursor for the carbonaceous structure, but may not be limited thereto. For example, the content of the precursor for the carbonaceous structure may be from about 1 part by weight to about 30 parts by weight, from about 2 parts by weight to about 30 parts by weight, from about 3 parts by weight to about 30 parts by weight, from about 4 parts by weight to about 30 parts by weight, from about 5 parts by weight to about 30 parts by weight, from about 6 parts by weight to about 30 parts by weight, from about 7 parts by weight to about 30 parts by weight, from about 8 parts by weight to about 30 parts by weight, from about 9 parts by weight to about 30 parts by weight, from about 10 parts by weight to about 30 parts by weight, from about 11 parts by weight to about 30 parts by weight, from about 12 parts by weight to about 30 parts by weight, from about 13 parts by weight to about 30 parts by weight, from about 14 parts by weight to about 30 parts by weight, from about 15 parts by weight to about 30 parts by weight, from about 16 parts by weight to about 30 parts by weight, from about 17 parts by weight to about 30 parts by weight, from about 18 parts by weight to about 30 parts by weight, from about 19 parts by weight to about 30 parts by weight, from about 20 parts by weight to about 30 parts by weight, from about 21 parts by weight to about 30 parts by weight, from about 22 parts by weight to about 30 parts by weight, from about 23 parts by weight to about 30 parts by weight, from about 24 parts by weight to about 30 parts by weight, from about 25 parts by weight to about 30 parts by weight, from about 26 parts by weight to about 30 parts by weight, from about 27 parts by weight to about 30 parts by weight, from about 28 parts by weight to about 30 parts by weight, from about 29 parts by weight to about 30 parts by weight, from about 1 part by weight to about 29 parts by weight, from about 1 part by weight to about 28 parts by weight, from about 1 part by weight to about 27 parts by weight, from about 1 part by weight to about 26 parts by weight, from about 1 part by weight to about 25 parts by weight, from about 1 part by weight to about 24 parts by weight, from about 1 part by weight to about 23 parts by weight, from about 1 part by weight to about 22 parts by weight, from about 1 part by weight to about 21 parts by weight, from about 1 part by weight to about 20 parts by weight, from about 1 part by weight to about 19 parts by weight, from about 1 part by weight to about 18 parts by weight, from about 1 part by weight to about 17 parts by weight, from about 1 part by weight to about 16 parts by weight, from about 1 part by weight to about 15 parts by weight, from about 1 part by weight to about 14 parts by weight, from about 1 part by weight to about 13 parts by weight, from about 1 part by weight to about 12 parts by weight, from about 1 part by weight to about 11 parts by weight, from about 1 part by weight to about 10 parts by weight, from about 1 part by weight to about 9 parts by weight, from about 1 part by weight to about 8 parts by weight, from about 1 part by weight to about 7 parts by weight, from about 1 part by weight to about 6 parts by weight, from about 1 part by weight to about 5 parts by weight, from about 1 part by weight to about 4 parts by weight, from about 1 part by weight to about 3 parts by weight, or from about 1 part by weight to about 2 parts by weight, with respect to 100 parts by weight of the mixture solution, but may not be limited thereto.

[0040] In an embodiment of the present disclosure, a size of the core-shell composite in which the precursor for the metal oxide and the precursor for the carbonaceous structure are coated on the polymer core particle may range from about 50 nm to about 5,000 nm, but may not be limited thereto. The size of the core-shell composite may have a different meaning depending on the shape of the core-shell composite. For example, if the core-shell composite has a sphere shape, the size may mean the diameter of the sphere, and if the core-shell composite has an oval shape, the size may mean the diameter of its longer axis or the diameter of its shorter axis. The size of the core-shell composite may range, for example, from about 50 nm to about 5,000 nm, from about 100 nm to about 5,000 nm, from about 300 nm to about 5,000 nm, from about 500 nm to about 5,000 nm, from about 700 nm to about 5,000 nm, from about 1,000 nm to about 5,000 nm, from about 1,300 nm to about 5,000 nm, from about 1,500 nm to about 5,000 nm, from about 1,700 nm to about 5,000 nm, from about 2,000 nm to about 5,000 nm, from about 2,500 nm to about 5,000 nm, from about 3,000 nm to about 5,000 nm, from about 3,500 nm to about 5,000 nm, from about 4,000 nm to about 5,000 nm, from about 4,500 nm to about 5,000 nm, from about 50 nm to about 4,500 nm, from about 50 nm to about 4,000 nm, from about 50 nm to about 3,500 nm, from about 50 nm to about 3,000 nm, from about 50 nm to about 2,500 nm, from about 50 nm to about 2,000 nm, from about 50 nm to about 1,700 nm, from about 50 nm to about 1,500 nm, from about 50 nm to about 1,300 nm, from about 50 nm to about 1,000 nm, from about 50 nm to about 700 nm, from about 50 nm to about 500 nm, from about 50 nm to about 300 nm, from about 50 nm to about 100 nm, from about 100 nm to about 4,000 nm, from about 300 nm to about 4,000 nm, from about 500 nm to about 4,000 nm, from about 700 nm to about 4,000 nm, from about 1,000 nm to about 4,000 nm, from about 1,300 nm to about 4,000 nm, from about 1,500 nm to about 4,000 nm, from about 1,700 nm to about 4,000 nm, from about 2,000 nm to about 4,000 nm, from about 2,500 nm to about 4,000 nm, from about 3,000 nm to about 4,000 nm, from about 3,500 nm to about 4,000 nm, from about 100 nm to about 3,500 nm, from about 100 nm to about 3,000 nm, from about 100 nm to about 2,500 nm, from about 100 nm to about 2,000 nm, from about 100 nm to about 1,700 nm, from about 100 nm to about 1,500 nm, from about 100 nm to about 1,300 nm, from about 100 nm to about 1,000 nm, from about 100 nm to about 700 nm, from about 100 nm to about 500 nm, from about 100 nm to about 300 nm, from about 200 nm to about 2,000 nm, from about 300 nm to about 2,000 nm, from about 400 nm to about 2,000 nm, from about 500 nm to about 2,000 nm, from about 600 nm to about 2,000 nm, from about 700 nm to about 2,000 nm, from about 800 nm to about 2,000 nm, from about 900 nm to about 2,000 nm, from about 1,000 nm to about 2,000 nm, from about 1,100 nm to about 2,000 nm, from about 1,200 nm to about 2,000 nm, from about 1,300 nm to about 2,000 nm, from about 1,400 nm to about 2,000 nm, from about 1,500 nm to about 2,000 nm, from about 1,600 nm to about 2,000 nm, from about 1,700 nm to about 2,000 nm, from about 1,800 nm to about 2,000 nm, from about 1,900 nm to about 2,000 nm, from about 200 nm to about 1,900 nm, from about 200 nm to about 1,800 nm, from about 200 nm to about 1,700 nm, from about 200 nm to about 1,600 nm, from about 200 nm to about 1,500 nm, from about 200 nm to about 1,400 nm, from about 200 nm to about 1,300 nm, from about 200 nm to about 1,200 nm, from about 200 nm to about 1,100 nm, from about 200 nm to about 1,000 nm, from about 200 nm to about 900 nm, from about 200 nm to about 800 nm, from about 200 nm to about 700 nm, from about 200 nm to about 600 nm, from about 200 nm to about 500 nm, from about 200 nm to about 400 nm, or from about 200 nm to about 300 nm, from about 400 nm to about 1,800 nm, from about 500 nm to about 1,800 nm, from about 600 nm to about 1,800 nm, from about 700 nm to about 1,800 nm, from about 800 nm to about 1,800 nm, from about 900 nm to about 1,800 nm, from about 1,000 nm to about 1,800 nm, from about 1,100 nm to about 1,800 nm, from about 1,200 nm to about 1,800 nm, from about 1,300 nm to about 1,800 nm, from about 1,400 nm to about 1,800 nm, from about 1,500 nm to about 1,800 nm, from about 1,600 nm to about 1,800 nm, from about 1,700 nm to about 1,800 nm, from about 400 nm to about 1,700 nm, from about 400 nm to about 1,600 nm, from about 400 nm to about 1,500 nm, from about 400 nm to about 1,400 nm, from about 400 nm to about 1,300 nm, from about 400 nm to about 1,200 nm, from about 400 nm to about 1,100 nm, from about 400 nm to about 1,000 nm, from about 400 nm to about 900 nm, from about 400 nm to about 800 nm, from about 400 nm to about 700 nm, from about 400 nm to about 600 nm, from about 400 nm to about 500 nm, from about 600 nm to about 1,600 nm, from about 700 nm to about 1,600 nm, from about 800 nm to about 1,600 nm, from about 900 nm to about 1,600 nm, from about 1,000 nm to about 1,600 nm, from about 1,100 nm to about 1,600 nm, from about 1,200 nm to about 1,600 nm, from about 1,300 nm to about 1,600 nm, from about 1,400 nm to about 1,600 nm, from about 1,500 nm to about 1,600 nm, from about 600 nm to about 1,500 nm, from about 600 nm to about 1,400 nm, from about 600 nm to about 1,300 nm, from about 600 nm to about 1,200 nm, from about 600 nm to about 1,100 nm, from about 600 nm to about 1,000 nm, from about 600 nm to about 900 nm, from about 600 nm to about 800 nm, from about 600 nm to about 700 nm, from about 800 nm to about 1,400 nm, from about 900 nm to about 1,400 nm, from about 1,000 nm to about 1,400 nm, from about 1,100 nm to about 1,400 nm, from about 1,200 nm to about 1,400 nm, from about 1,300 nm to about 1,400 nm, from about 800 nm to about 1,300 nm, from about 800 nm to about 1,200 nm, from about 800 nm to about 1,100 nm, from about 800 nm to about 1,000 nm, or from about 800 nm to about 900 nm.

[0041] Then, in S200, by the first calcination, the polymer core particle of the core-shell composite may be carbonized and removed, so that an inner surface of the shell in the hollow composite may include carbon which is formed by carbonizing surface functional groups of the polymer core particle contacting the inner surface of the shell in the core-shell composite, and at least a part of metal cations in the metal oxide may be reduced to form the metal element for self- doping the hybrid. [0042] In an embodiment of the present disclosure, the first calcination may be performed at a temperature ranging from about 400 ^° C to about 800 ^° C, but may not be limited thereto. For example, the first calcination may be performed at a temperature ranging from about 400 ^° C to about 800 ^° C, from about 450 ^° C to about 800 ^° C, from about 500 ^° C to about 800 ^° C, from about 550 ^° C to about 800 ^° C, from about 600 ^° C to about 800 ^° C, from about 650 ^° C to about 800 ^° C, from about 700 ^° C to about 800 ^° C, from about 750 ^° C to about 800 ^° C, from about 400 ^° C to about 750 ^° C, from about 400 ^° C to about 700 ^° C, from about 400 ^° C to about 650 ^° C, from about 400 ^° C to about 600 ^° C, from about 400 ^° C to about 550 ^° C, from about 400 ^° C to about 500 ^° C, or from about 400 ^° C to about 450 ^° C, but may not be limited thereto. If the first calcination is performed at a temperature less than 400 ^° C, the polymer core particle may not be completely carbonized and removed. If the first calcination is performed at a temperature higher than 800 ^° C, the metal oxide may be transformed to a rutile crystalline phase and thus catalytic activity may be decreased, and the hollow composite may be unstable while the polymer core particle is carbonized and removed.

[0043] In an embodiment of the present disclosure, while the polymer core particle is carbonized and removed by the first calcination, a content of the carbonaceous structure may be decreased.

[0044] In an embodiment of the present disclosure, carbon formed on the inner surface of the shell in the hollow composite may form a layer having a thickness of from about 1 nm to about 5 nm, but may not be limited thereto. For example, the carbon formed on the inner surface of the shell in the hollow composite may form a layer having a thickness of from about 1 nm to about 5 nm, from about 2 nm to about 5 nm, from about 3 nm to about 5 nm, from about 4 nm to about 5 nm, from about 1 nm to about 4 nm, from about 1 nm to about 3 nm, or from about 1 nm to about 2 nm, but may not be limited thereto.

[0045] Then, the method of preparing a hollow composite may further include a second calcination (S300) which includes heating the hollow composite obtained by the first calcination at a second temperature higher than the first temperature to crystallize the metal oxide included in the hybrid.

[0046] In an embodiment of the present disclosure, the second temperature at which the second calcination is performed may be higher than the first temperature at which the first calcination is performed, and may range from about 500 ^° C to about 850 ^° C, but may not be limited thereto. For example, the second calcination may be performed at a temperature ranging from about 500°C to about 850°C, from about 550°C to about 850°C, from about 600°C to about 850°C, from about 650°C to about 850°C, from about 700°C to about 850°C, from about 750°C to about 850°C, from about 800°C to about 850°C, from about 500°C to about 800°C, from about 500°C to about 750°C, from about 500°C to about 700°C, from about 500°C to about 650°C, from about 500°C to about 600°C, or from about 500°C to about 550°C, but may not be limited thereto. If the second calcination is performed at a temperature less than 500 ^° C, the metal oxide may not be crystallized. If the second calcination is performed at a temperature higher than 850 ^° C, the metal oxide may be transformed to a rutile crystalline phase and thus catalytic activity may be decreased, and the hollow composite may be unstable while the metal oxide is crystallized.

[0047] In an embodiment of the present disclosure, the hollow composite that is obtained by the steps (S100 to S300) according to the second aspect of the present disclosure may have a size ranging from about 50 nm to about 5,000 nm. The size of the hollow composite may have a different meaning depending on the shape of the hollow composite. For example, if the hollow composite has a sphere shape, the size may mean the diameter of the sphere, and if the hollow composite has an oval shape, the size may mean the diameter of its longer axis or the diameter of its shorter axis. The size of the hollow composite may range, for example, from about 50 nm to about 5,000 nm, from about 100 nm to about 5,000 nm, from about 300 nm to about 5,000 nm, from about 500 nm to about 5,000 nm, from about 700 nm to about 5,000 nm, from about 1,000 nm to about 5,000 nm, from about 1,300 nm to about 5,000 nm, from about 1,500 nm to about 5,000 nm, from about 1,700 nm to about 5,000 nm, from about 2,000 nm to about 5,000 nm, from about 2,500 nm to about 5,000 nm, from about 3,000 nm to about 5,000 nm, from about 3,500 nm to about 5,000 nm, from about 4,000 nm to about 5,000 nm, from about 4,500 nm to about 5,000 nm, from about 50 nm to about 4,500 nm, from about 50 nm to about 4,000 nm, from about 50 nm to about 3,500 nm, from about 50 nm to about 3,000 nm, from about 50 nm to about 2,500 nm, from about 50 nm to about 2,000 nm, from about 50 nm to about 1,700 nm, from about 50 nm to about 1,500 nm, from about 50 nm to about 1,300 nm, from about 50 nm to about 1,000 nm, from about 50 nm to about 700 nm, from about 50 nm to about 500 nm, from about 50 nm to about 300 nm, from about 50 nm to about 100 nm, from about 100 nm to about 4,000 nm, from about 300 nm to about 4,000 nm, from about 500 nm to about 4,000 nm, from about 700 nm to about 4,000 nm, from about 1,000 nm to about 4,000 nm, from about 1,300 nm to about 4,000 nm, from about 1,500 nm to about 4,000 nm, from about 1,700 nm to about 4,000 nm, from about 2,000 nm to about 4,000 nm, from about 2,500 nm to about 4,000 nm, from about 3,000 nm to about 4,000 nm, from about 3,500 nm to about 4,000 nm, from about 100 nm to about 3,500 nm, from about 100 nm to about 3,000 nm, from about 100 nm to about 2,500 nm, from about 100 nm to about 2,000 nm, from about 100 nm to about 1,700 nm, from about 100 nm to about 1,500 nm, from about 100 nm to about 1,300 nm, from about 100 nm to about 1,000 nm, from about 100 nm to about 700 nm, from about 100 nm to about 500 nm, from about 100 nm to about 300 nm, from about 200 nm to about 2,000 nm, from about 300 nm to about 2,000 nm, from about 400 nm to about 2,000 nm, from about 500 nm to about 2,000 nm, from about 600 nm to about 2,000 nm, from about 700 nm to about 2,000 nm, from about 800 nm to about 2,000 nm, from about 900 nm to about 2,000 nm, from about 1,000 nm to about 2,000 nm, from about 1,100 nm to about 2,000 nm, from about 1,200 nm to about 2,000 nm, from about 1,300 nm to about 2,000 nm, from about 1,400 nm to about 2,000 nm, from about 1,500 nm to about 2,000 nm, from about 1,600 nm to about 2,000 nm, from about 1,700 nm to about 2,000 nm, from about 1,800 nm to about 2,000 nm, from about 1,900 nm to about 2,000 nm, from about 200 nm to about 1,900 nm, from about 200 nm to about 1,800 nm, from about 200 nm to about 1,700 nm, from about 200 nm to about 1,600 nm, from about 200 nm to about 1,500 nm, from about 200 nm to about 1,400 nm, from about 200 nm to about 1,300 nm, from about 200 nm to about 1,200 nm, from about 200 nm to about 1,100 nm, from about 200 nm to about 1,000 nm, from about 200 nm to about 900 nm, from about 200 nm to about 800 nm, from about 200 nm to about 700 nm, from about 200 nm to about 600 nm, from about 200 nm to about 500 nm, from about 200 nm to about 400 nm, or from about 200 nm to about 300 nm, from about 400 nm to about 1,800 nm, from about 500 nm to about 1,800 nm, from about 600 nm to about 1,800 nm, from about 700 nm to about 1,800 nm, from about 800 nm to about 1,800 nm, from about 900 nm to about 1,800 nm, from about 1,000 nm to about 1,800 nm, from about 1,100 nm to about 1,800 nm, from about 1,200 nm to about 1,800 nm, from about 1,300 nm to about 1,800 nm, from about 1,400 nm to about 1,800 nm, from about 1,500 nm to about 1,800 nm, from about 1,600 nm to about 1,800 nm, from about 1,700 nm to about 1,800 nm, from about 400 nm to about 1,700 nm, from about 400 nm to about 1,600 nm, from about 400 nm to about 1,500 nm, from about 400 nm to about 1,400 nm, from about 400 nm to about 1,300 nm, from about 400 nm to about 1,200 nm, from about 400 nm to about 1,100 nm, from about 400 nm to about 1,000 nm, from about 400 nm to about 900 nm, from about 400 nm to about 800 nm, from about 400 nm to about 700 nm, from about 400 nm to about 600 nm, from about 400 nm to about 500 nm, from about 600 nm to about 1,600 nm, from about 700 nm to about 1,600 nm, from about 800 nm to about 1,600 nm, from about 900 nm to about 1,600 nm, from about 1,000 nm to about 1,600 nm, from about 1,100 nm to about 1,600 nm, from about 1,200 nm to about 1,600 nm, from about 1,300 nm to about 1,600 nm, from about 1,400 nm to about 1,600 nm, from about 1,500 nm to about 1,600 nm, from about 600 nm to about 1,500 nm, from about 600 nm to about 1,400 nm, from about 600 nm to about 1,300 nm, from about 600 nm to about 1,200 nm, from about 600 nm to about 1,100 nm, from about 600 nm to about 1,000 nm, from about 600 nm to about 900 nm, from about 600 nm to about 800 nm, from about 600 nm to about 700 nm, from about 800 nm to about 1,400 nm, from about 900 nm to about 1,400 nm, from about 1,000 nm to about 1,400 nm, from about 1,100 nm to about 1,400 nm, from about 1,200 nm to about 1,400 nm, from about 1,300 nm to about 1,400 nm, from about 800 nm to about 1,300 nm, from about 800 nm to about 1,200 nm, from about 800 nm to about 1,100 nm, from about 800 nm to about 1,000 nm, or from about 800 nm to about 900 nm.

[0048] In an embodiment of the present disclosure, with respect to the size of the core- shell composite prior to the consecutive first calcination and second calcination, the size of the hollow composite after the first calcination and second calcination may be shrunk by from about 5% to about 40%, but may not be limited thereto. For example, by the first calcination and second calcination, the size of the hollow composite may be shrunk by from about 5% to about 40%, from about 10% to about 40%, from about 15% to about 40%, from about 20% to about 40%, from about 25% to about 40%, from about 30% to about 40%, from about 35% to about 40%, from about 5% to about 35%, from about 5% to about 30%, from about 5% to about 25%, from about 5% to about 20%, from about 5% to about 15%, or from about 5% to about 10%, with respect to the size of the core- shell composite, but may not be limited thereto. The size shrinkage of the hollow composite may be ascribed to the condensation/polymerization of a template associated with the sintering contraction of the metal oxide during the first calcination and second calcination.

[0049] In an embodiment of the present disclosure, the metal oxide may include a semiconductor oxide, and the semiconductor oxide may include, for example, a member selected from the group consisting of Ti0 ₂ , Sn0 ₂ , ZnO, V0 ₂ , ln ₂ 0 ₃ , NiO, Mo0 ₃ , SrTi0 ₃ , Fe-doped SrTi0 ₃ , Fe ₂ 0 ₃ , W0 ₃ , CuO, BiV0 ₄ , and combinations thereof.

[0050] In an embodiment of the present disclosure, the carbonaceous structure may include a member selected from the group consisting of reduced graphene oxide (rGO), graphene, graphite, single-walled carbon nanotube, multi-walled carbon nanotube, carbon nanohorns, vapor- grown carbon fibers and carbon nanofibers, synthetic carbon sources including acetylene black, carbon black coming from the combustion of hydrocarbon or polymer (e.g., Ketjenblack), and combinations thereof.

[0051] In an embodiment of the present disclosure, the metal element may be in a more reduced state than a metal cation included in the metal oxide. The reduced metal cation is unstable and thus easily oxidized. However, the reduced metal cation may be stabilized by carbon formed on the inner surface of the shell and the carbonaceous structure so as to self-dope the hybrid. For example, if the metal oxide in the shell in the hollow composite includes Ti0 ₂ , Ti ⁴⁺ contained in the Ti0 ₂ may be partially reduced to Ti ³⁺ which is stabilized in the hollow composite so that the hybrid can be self-doped by the stabilized Ti ³⁺ .

[0052] In an embodiment of the present disclosure, a content of the carbonaceous structure may be from about 1 part by weight to about 20 parts by weight with respect to 100 parts by weight of the hollow composite. For example, the content of the carbonaceous structure with respect to 100 parts by weight of the hollow composite may be from about 1 part by weight to about 20 parts by weight, from about 2 parts by weight to about 20 parts by weight, from about 3 parts by weight to about 20 parts by weight, from about 4 parts by weight to about 20 parts by weight, from about 5 parts by weight to about 20 parts by weight, from about 6 parts by weight to about 20 parts by weight, from about 7 parts by weight to about 20 parts by weight, from about 8 parts by weight to about 20 parts by weight, from about 9 parts by weight to about 20 parts by weight, from about 10 parts by weight to about 20 parts by weight, from about 11 parts by weight to about 20 parts by weight, from about 12 parts by weight to about 20 parts by weight, from about 13 parts by weight to about 20 parts by weight, from about 14 parts by weight to about 20 parts by weight, from about 15 parts by weight to about 20 parts by weight, from about 16 parts by weight to about 20 parts by weight, from about 17 parts by weight to about 20 parts by weight, from about 18 parts by weight to about 20 parts by weight, from about 19 parts by weight to about 20 parts by weight, from about 1 part by weight to about 19 parts by weight, from about 1 part by weight to about 18 parts by weight, from about 1 part by weight to about 17 parts by weight, from about 1 part by weight to about 16 parts by weight, from about 1 part by weight to about 15 parts by weight, from about 1 part by weight to about 14 parts by weight, from about 1 part by weight to about 13 parts by weight, from about 1 part by weight to about 12 parts by weight, from about 1 part by weight to about 11 parts by weight, from about 1 part by weight to about 10 parts by weight, from about 1 part by weight to about 9 parts by weight, from about 1 part by weight to about 8 parts by weight, from about 1 part by weight to about 7 parts by weight, from about 1 part by weight to about 6 parts by weight, from about 1 part by weight to about 5 parts by weight, from about 1 part by weight to about 4 parts by weight, from about 1 part by weight to about 3 parts by weight, or from about 1 part by weight to about 2 parts by weight. If the content of the carbonaceous structure is less than 1 part by weight with respect to 100 parts by weight of the hollow composite, electric conductivity of the hollow composite may be decreased due to a small content of the carbonaceous structure that enables easier electron transport. If the content of the carbonaceous structure is more than 20 parts by weight with respect to 100 parts by weight of the hollow composite, a shape of the composite may not be induced to a hollow composite and the shell may be increased in thickness, so that electric conductivity may be decreased.

[0053] In an embodiment of the present disclosure, the hollow composite may use as an electrocatalyst for oxygen reduction reaction, and the hollow composite may facilitate oxygen diffusion, increase the number of active sites, and facilitate electrolyte diffusion for oxygen reduction reaction.

[0054] In an embodiment of the present disclosure, the hollow composite may further enhance the inherently poor electric conductivity of the metal oxide by including the carbonaceous structure. [0055] In a third aspect of the present disclosure, there is provided an electrocatalyst, including the hollow composite according to the first aspect of the present disclosure. The hollow composite includes a shell which includes a hybrid containing a metal oxide and a carbonaceous structure, and the hybrid is self-doped by a metal element included in the metal oxide, and an inner surface of the shell in the hollow composite includes carbon.

[0056] In an embodiment of the present disclosure, the electrocatalyst may be for the oxygen reduction reaction.

[0057] In a fourth aspect of the present disclosure, there is provided an electrode, including the hollow composite according to the first aspect of the present disclosure.

[0058] In an embodiment of the present disclosure, the electrode may be for the oxygen reduction reaction.

[0059] In a fifth aspect of the present disclosure, there is provided a cell, including the electrode according to the fourth aspect of the present disclosure.

[0060] Detailed descriptions of the repeated parts as described in the first aspect and the second aspect of the present disclosure will be omitted. Although omitted in the third aspect, the fourth aspect, and the fifth aspect of the present disclosure, the description of the first aspect and the second aspect of the present disclosure may also be applied in the same manner to the third aspect, the fourth aspect, and the fifth aspect.

[0061] Hereinafter, the third aspect, the fourth aspect, and the fifth aspect of the present disclosure will be described in detail.

[0062] In an embodiment of the present disclosure, the cell may be a fuel cell or a secondary cell, but may not be limited thereto. For example the secondary cell may be a metal-air battery, but may not be limited thereto. [0063] In an embodiment of the present disclosure, if the cell is a fuel cell, the fuel cell is an electrochemical device that converts chemical energy of fuels directly into electric energy to provide a clean and highly efficient source of power for electric vehicles potentially. The fuel cell system may be a solid oxide fuel cell, but may not be limited thereto. The fuel cell system may include an electrolyte membrane, a first electrode, and a second electrode, but may not be limited thereto.

[0064] In an embodiment of the present disclosure, the first electrode and/or second electrode may be a glassy carbon electrode or a rotating disk electrode, and a material of the electrodes may include a member selected from the group consisting of carbon, a metal, a metal oxide, a conductive polymer, and combinations thereof, but may not be limited thereto.

[0065] In an embodiment of the present disclosure, an electrochemical reaction of the fuel cell system may be carried out in a reverse direction with respect to a water splitting system, and cations may be produced by a hydrogen oxidation reaction in the first electrode and water may be produced by an oxygen reduction reaction in the second electrode, but the present disclosure may not be limited thereto. Since electrons are generated from the first electrode and electrons are consumed by the second electrode, electricity may flow by connecting the first electrode and the second electrode, but the present disclosure may not be limited thereto.

[0066] In an embodiment of the present disclosure, each of the first electrode and the second electrode may include a semiconductor or conductive material, and the second electrode may be an electrode for oxygen reduction reaction including the hollow composite according to the first aspect of the present disclosure.

[0067] In an embodiment of the present disclosure, the electrolyte membrane may include a proton conductive polymer film, and may separate the first electrode and the second electrode and also enable the flow of protons between the electrodes. The conductive polymer film may include, for example, nafion, but may not be limited thereto. [0068] In an embodiment of the present disclosure, in order to carry out an oxidation reaction and a reduction reaction at high reaction rate in the fuel cell system and also carry out the reactions even at a reduced potential, the electrode including the hollow composite according to the first aspect of the present disclosure may be used as the second electrode. In the electrode including the hollow composite, the hollow composite may act as an electrocatalyst for oxygen reduction reaction, which may provide improvement in electrocatalytic activity caused by an increase in number of active sites in an oxygen reduction reaction, improvement in durability in alkaline media, and improvement in methanol tolerance.

[0069] In an embodiment of the present disclosure, a hollow composite includes a shell which includes a hybrid containing a metal oxide and a carbonaceous structure, wherein the hybrid is self-doped by a metal element included in the metal oxide, and wherein an inner surface of the shell in the hollow composite includes carbon.

[0070] In an embodiment of the present disclosure, the metal oxide may include a semiconductor oxide, and the semiconductor oxide may include, for example, a member selected from the group consisting of Ti0 ₂ , Sn0 ₂ , ZnO, V0 ₂ , ln ₂ 0 ₃ , NiO, Mo0 ₃ , SrTi0 ₃ , Fe-doped SrTi0 ₃ , Fe ₂ 0 ₃ , W0 ₃ , CuO, BiV0 ₄ , and combinations thereof.

[0071] In an embodiment of the present disclosure, the carbonaceous structure may include a member selected from the group consisting of reduced graphene oxide (rGO), graphene, graphite, single-walled carbon nanotube, multi-walled carbon nanotube, carbon nanohorns, vapor- grown carbon fibers and carbon nanofibers, synthetic carbon sources including acetylene black, carbon black coming from the combustion of hydrocarbon or polymer (e.g., Ketjenblack), and combinations thereof.

[0072] In an embodiment of the present disclosure, the metal element may be in a more reduced state than a metal cation included in the metal oxide. The reduced metal cation is unstable and thus easily oxidized. However, the reduced metal cation may be stabilized by carbon formed on the inner surface of the shell and the carbonaceous structure so as to self-dope the hybrid. For example, if the metal oxide in the shell in the hollow composite includes Ti0 ₂ , Ti ⁴⁺ contained in the Ti0 ₂ may be partially reduced to Ti ³⁺ which is stabilized in the hollow composite so that the hybrid can be self-doped by the stabilized Ti ³⁺ .

[0073] In an embodiment of the present disclosure, a size of the hollow composite may range from about 50 nm to about 5,000 nm. The size of the hollow composite may have a different meaning depending on the shape of the hollow composite. For example, if the hollow composite has a sphere shape, the size may mean the diameter of the sphere, and if the hollow composite has an oval shape, the size may mean the diameter of its longer axis or the diameter of its shorter axis. The size of the hollow composite may range, for example, from about 50 nm to about 5,000 nm, from about 100 nm to about 5,000 nm, from about 300 nm to about 5,000 nm, from about 500 nm to about 5,000 nm, from about 700 nm to about 5,000 nm, from about 1,000 nm to about 5,000 nm, from about 1,300 nm to about 5,000 nm, from about 1,500 nm to about 5,000 nm, from about 1,700 nm to about 5,000 nm, from about 2,000 nm to about 5,000 nm, from about 2,500 nm to about 5,000 nm, from about 3,000 nm to about 5,000 nm, from about 3,500 nm to about 5,000 nm, from about 4,000 nm to about 5,000 nm, from about 4,500 nm to about 5,000 nm, from about 50 nm to about 4,500 nm, from about 50 nm to about 4,000 nm, from about 50 nm to about 3,500 nm, from about 50 nm to about 3,000 nm, from about 50 nm to about 2,500 nm, from about 50 nm to about 2,000 nm, from about 50 nm to about 1,700 nm, from about 50 nm to about 1,500 nm, from about 50 nm to about 1,300 nm, from about 50 nm to about 1,000 nm, from about 50 nm to about 700 nm, from about 50 nm to about 500 nm, from about 50 nm to about 300 nm, from about 50 nm to about 100 nm, from about 100 nm to about 4,000 nm, from about 300 nm to about 4,000 nm, from about 500 nm to about 4,000 nm, from about 700 nm to about 4,000 nm, from about 1,000 nm to about 4,000 nm, from about 1,300 nm to about 4,000 nm, from about 1,500 nm to about 4,000 nm, from about 1,700 nm to about 4,000 nm, from about 2,000 nm to about 4,000 nm, from about 2,500 nm to about 4,000 nm, from about 3,000 nm to about 4,000 nm, from about 3,500 nm to about 4,000 nm, from about 100 nm to about 3,500 nm, from about 100 nm to about 3,000 nm, from about 100 nm to about 2,500 nm, from about 100 nm to about 2,000 nm, from about 100 nm to about 1,700 nm, from about 100 nm to about 1,500 nm, from about 100 nm to about 1,300 nm, from about 100 nm to about 1,000 nm, from about 100 nm to about 700 nm, from about 100 nm to about 500 nm, from about 100 nm to about 300 nm, from about 200 nm to about 2,000 nm, from about 300 nm to about 2,000 nm, from about 400 nm to about 2,000 nm, from about 500 nm to about 2,000 nm, from about 600 nm to about 2,000 nm, from about 700 nm to about 2,000 nm, from about 800 nm to about 2,000 nm, from about 900 nm to about 2,000 nm, from about 1,000 nm to about 2,000 nm, from about 1,100 nm to about 2,000 nm, from about 1,200 nm to about 2,000 nm, from about 1,300 nm to about 2,000 nm, from about 1,400 nm to about 2,000 nm, from about 1,500 nm to about 2,000 nm, from about 1,600 nm to about 2,000 nm, from about 1,700 nm to about 2,000 nm, from about 1,800 nm to about 2,000 nm, from about 1,900 nm to about 2,000 nm, from about 200 nm to about 1,900 nm, from about 200 nm to about 1,800 nm, from about 200 nm to about 1,700 nm, from about 200 nm to about 1,600 nm, from about 200 nm to about 1,500 nm, from about 200 nm to about 1,400 nm, from about 200 nm to about 1,300 nm, from about 200 nm to about 1,200 nm, from about 200 nm to about 1,100 nm, from about 200 nm to about 1,000 nm, from about 200 nm to about 900 nm, from about 200 nm to about 800 nm, from about 200 nm to about 700 nm, from about 200 nm to about 600 nm, from about 200 nm to about 500 nm, from about 200 nm to about 400 nm, or from about 200 nm to about 300 nm, from about 400 nm to about 1,800 nm, from about 500 nm to about 1,800 nm, from about 600 nm to about 1,800 nm, from about 700 nm to about 1,800 nm, from about 800 nm to about 1,800 nm, from about 900 nm to about 1,800 nm, from about 1,000 nm to about 1,800 nm, from about 1,100 nm to about 1,800 nm, from about 1,200 nm to about 1,800 nm, from about 1,300 nm to about 1,800 nm, from about 1,400 nm to about 1,800 nm, from about 1,500 nm to about 1,800 nm, from about 1,600 nm to about 1,800 nm, from about 1,700 nm to about 1,800 nm, from about 400 nm to about 1,700 nm, from about 400 nm to about 1,600 nm, from about 400 nm to about 1,500 nm, from about 400 nm to about 1,400 nm, from about 400 nm to about 1,300 nm, from about 400 nm to about 1,200 nm, from about 400 nm to about 1,100 nm, from about 400 nm to about 1,000 nm, from about 400 nm to about 900 nm, from about 400 nm to about 800 nm, from about 400 nm to about 700 nm, from about 400 nm to about 600 nm, from about 400 nm to about 500 nm, from about 600 nm to about 1,600 nm, from about 700 nm to about 1,600 nm, from about 800 nm to about 1,600 nm, from about 900 nm to about 1,600 nm, from about 1,000 nm to about 1,600 nm, from about 1,100 nm to about 1,600 nm, from about 1,200 nm to about 1,600 nm, from about 1,300 nm to about 1,600 nm, from about 1,400 nm to about 1,600 nm, from about 1,500 nm to about 1,600 nm, from about 600 nm to about 1,500 nm, from about 600 nm to about 1,400 nm, from about 600 nm to about 1,300 nm, from about 600 nm to about 1,200 nm, from about 600 nm to about 1,100 nm, from about 600 nm to about 1,000 nm, from about 600 nm to about 900 nm, from about 600 nm to about 800 nm, from about 600 nm to about 700 nm, from about 800 nm to about 1,400 nm, from about 900 nm to about 1,400 nm, from about 1,000 nm to about 1,400 nm, from about 1,100 nm to about 1,400 nm, from about 1,200 nm to about 1,400 nm, from about 1,300 nm to about 1,400 nm, from about 800 nm to about 1,300 nm, from about 800 nm to about 1,200 nm, from about 800 nm to about 1,100 nm, from about 800 nm to about 1,000 nm, or from about 800 nm to about 900 nm.

[0074] In an embodiment of the present disclosure, a content of the carbonaceous structure may be from about 1 part by weight to about 20 parts by weight with respect to 100 parts by weight of the hollow composite. For example, the content of the carbonaceous structure with respect to 100 parts by weight of the hollow composite may be from about 1 part by weight to about 20 parts by weight, from about 2 parts by weight to about 20 parts by weight, from about 3 parts by weight to about 20 parts by weight, from about 4 parts by weight to about 20 parts by weight, from about 5 parts by weight to about 20 parts by weight, from about 6 parts by weight to about 20 parts by weight, from about 7 parts by weight to about 20 parts by weight, from about 8 parts by weight to about 20 parts by weight, from about 9 parts by weight to about 20 parts by weight, from about 10 parts by weight to about 20 parts by weight, from about 11 parts by weight to about 20 parts by weight, from about 12 parts by weight to about 20 parts by weight, from about 13 parts by weight to about 20 parts by weight, from about 14 parts by weight to about 20 parts by weight, from about 15 parts by weight to about 20 parts by weight, from about 16 parts by weight to about 20 parts by weight, from about 17 parts by weight to about 20 parts by weight, from about 18 parts by weight to about 20 parts by weight, from about 19 parts by weight to about 20 parts by weight, from about 1 part by weight to about 19 parts by weight, from about 1 part by weight to about 18 parts by weight, from about 1 part by weight to about 17 parts by weight, from about 1 part by weight to about 16 parts by weight, from about 1 part by weight to about 15 parts by weight, from about 1 part by weight to about 14 parts by weight, from about 1 part by weight to about 13 parts by weight, from about 1 part by weight to about 12 parts by weight, from about 1 part by weight to about 11 parts by weight, from about 1 part by weight to about 10 parts by weight, from about 1 part by weight to about 9 parts by weight, from about 1 part by weight to about 8 parts by weight, from about 1 part by weight to about 7 parts by weight, from about 1 part by weight to about 6 parts by weight, from about 1 part by weight to about 5 parts by weight, from about 1 part by weight to about 4 parts by weight, from about 1 part by weight to about 3 parts by weight, or from about 1 part by weight to about 2 parts by weight. If the content of the carbonaceous structure is less than 1 part by weight with respect to 100 parts by weight of the hollow composite, electric conductivity of the hollow composite may be decreased due to a small content of the carbonaceous structure that enables easier electron transport. If the content of the carbonaceous structure is more than 20 parts by weight with respect to 100 parts by weight of the hollow composite, a shape of the composite may not be induced to a hollow composite and the shell may be increased in thickness, so that electric conductivity may be decreased. [0075] In an embodiment of the present disclosure, a charge transfer resistance of the hollow composite may range from about 10 Ω to about 200 kQ, but may not be limited thereto. For example, the charge transfer resistance of the hollow composite may range from about 10 Ω to about 200 kQ, from about 100 Ω to about 200 kQ, from about 300 Ω to about 200 kQ, from about 500 Ω to about 200 kQ, from about 700 Ω to about 200 kΩ, from about 1 kΩ to about 200 kΩ, from about 10 kΩ to about 200 kΩ, from about 20 kΩ to about 200 kΩ, from about 30 kΩ to about 200 kΩ, from about 40 kΩ to about 200 kΩ, from about 50 kΩ to about 200 kΩ, from about 60 kΩ to about 200 kΩ, from about 70 kΩ to about 200 kΩ, from about 80 kΩ to about 200 kΩ, from about 90 kΩ to about 200 kΩ, from about 100 kΩ to about 200 kΩ, from about 120 kΩ to about 200 kΩ, from about 140 kΩ to about 200 kΩ, from about 160 kΩ to about 200 kΩ, from about 180 kΩ to about 200 kΩ, from about 10 Ω to about 180 kΩ, from about 10 Ω to about 160 kΩ, from about 10 Ω to about 140 kΩ, from about 10 Ω to about 120 kΩ, from about 10 Ω to about 100 kΩ, from about 10 Ω to about 90 kΩ, from about 10 Ω to about 80 kΩ, from about 10 Ω to about 70 kΩ, from about 10 Ω to about 60 kΩ, from about 10 Ω to about 50 kΩ, from about 10 Ω to about 40 kΩ, from about 10 Ω to about 30 kΩ, from about 10 Ω to about 20 kΩ, from about 10 Ω to about 10 kΩ, from about 10 Ω to about 1 kΩ, from about 10 Ω to about 700 Ω, from about 10 Ω to about 500 Ω, from about 10 Ω to about 300 Ω, or from about 10 Ω to about 100 Ω, but may not be limited thereto. The charge transfer resistance is closely correlated to electric conductivity of the hollow composite, and as the charge transfer resistance is decreased, the electric conductivity of the hollow composite is increased. A high electric conductivity (low charge transfer resistance) of the hollow composite may be due to the carbonaceous structure included in the hollow composite.

[0076] In an embodiment of the present disclosure, the hollow composite may use as an electrocatalyst for oxygen reduction reaction, and the hollow composite may facilitate oxygen diffusion, increase the number of active sites, and facilitate electrolyte diffusion for the oxygen reduction reaction. [0077] In an embodiment of the present disclosure, the hollow composite may further enhance the inherently poor electric conductivity of the metal oxide by including the carbonaceous structure. [0078] In a sixth aspect of the present disclosure, there is provided an electronic ink, including the hollow composite according to the first aspect of the present disclosure. In a seventh aspect of the present disclosure, there is provide an electronic paper or a display device, including the electronic ink according to the sixth aspect of the present disclosure.

[0079] Detailed descriptions of the repeated parts as described in the first aspect to the fifth aspect of the present disclosure will be omitted. Although omitted in the sixth aspect and the seventh aspect of the present disclosure, the description of the first aspect to the fifth aspect of the present disclosure may also be applied in the same manner to the sixth aspect and the seventh aspect.

[0080] Hereinafter, the sixth aspect and the seventh aspect of the present disclosure will be described in detail.

[0081] An electronic paper is a display device configured to display characters or images using flexible substrates, and can be repeatedly used millions of times.

[0082] A display device of the electronic paper displays data by distributing conductive particles between the flexible substrates and then changing an alignment direction of the particles (or charged particles) by changing polarity of an electric field. The electronic paper is of reflective type without the use of a separate light source and thus requires lower production cost than a conventional flat display panel and does not need a backlight and continuous recharging unlike a liquid crystal display device. Therefore, the electronic paper consumes less power. Further, the electronic paper provides super high definition clarity and a wide viewing angle. Furthermore, the electronic paper has a memory function by which characters do not disappear even when power is turned off since a previous state is maintained by an internal balance between positively or negatively charged particles.

[0083] An electronic paper can be implemented on various substrates, for example, a substrate or flexible substrate selected from the group consisting of plastic, metal, paper, and combinations thereof, and can be implemented in a large area like a conventional paper. Therefore, the electronic paper can be mass-produced by a roll-to-roll process using flexible substrates.

[0084] An electronic paper may include a lower substrate and an upper substrate provided to face each other with a predetermined space therebetween, and a display unit between the lower substrate and the upper substrate, but may not be limited thereto.

[0085] The display unit may include walls configured to define a unit pixel for determining resolution, lower electrodes separated from each other in the respective walls, electronic ink layers formed on the respective lower electrodes, and upper electrodes formed on the respective electronic ink layers, but may not be limited thereto.

[0086] The electronic ink layer includes a film, and ink capsules are dispersed within the film. Each of the ink capsules may include an inorganic or organic fluid which is transparent or tinted with a certain color, a negatively charged particle that transmits a light dispersed in the fluid, and a positively charged particle that does not transmit the light. For example, if the ink capsule includes a negatively charged black particle and a positively charged white particle, when a positive voltage is applied to the upper electrode and a negative voltage is applied to the lower electrode, the negatively charged black particle is aligned on the upper electrode and the positively charged white particle is aligned on the lower electrode, so that the electronic paper may display black on the screen. In an embodiment of the present disclosure, the negatively charged particle and/or the positively charged particle may include a hollow composite according to the first aspect of the present disclosure. [0087] In an embodiment of the present disclosure, the negatively charged particle and/or the positively charged particle may include a hollow composite according to the first aspect of the present disclosure.

[0088] In an embodiment of the present disclosure, the hollow composite used as the negatively charged particle and/or the positively charged particle, comprises a shell which includes a hybrid containing a metal oxide and a carbonaceous structure, wherein the hybrid is self-doped by a metal element included in the metal oxide, and wherein an inner surface of the shell in the hollow composite includes carbon.

[0089] In an embodiment of the present disclosure, the metal oxide may include a semiconductor oxide, and the semiconductor oxide may include, for example, a member selected from the group consisting of Ti0 ₂ , Sn0 ₂ , ZnO, V0 ₂ , ln ₂ 0 ₃ , NiO, Mo0 ₃ , SrTi0 ₃ , Fe-doped SrTi0 ₃ , Fe ₂ 0 ₃ , W0 ₃ , CuO, BiV0 ₄ , and combinations thereof.

[0090] In an embodiment of the present disclosure, the carbonaceous structure may include a member selected from the group consisting of reduced graphene oxide (rGO), graphene, graphite, single-walled carbon nanotube, multi-walled carbon nanotube, carbon nanohorns, vapor- grown carbon fibers and carbon nanofibers, synthetic carbon sources including acetylene black, carbon black coming from the combustion of hydrocarbon or polymer (e.g., Ketjenblack), and combinations thereof.

[0091] In an embodiment of the present disclosure, the metal element may be in a more reduced state than a metal cation included in the metal oxide. The reduced metal cation is unstable and thus easily oxidized. However, the reduced metal cation may be stabilized by carbon formed on the inner surface of the shell and the carbonaceous structure so as to self-dope the hybrid. For example, if the metal oxide in the shell in the hollow composite includes Ti0 ₂ , Ti ⁴⁺ contained in the Ti0 ₂ may be partially reduced to Ti ³⁺ which is stabilized in the hollow composite so that the hybrid can be self-doped by the stabilized Ti ³⁺ . [0092] In an embodiment of the present disclosure, a size of the hollow composite may range from about 50 nm to about 5,000 nm. The size of the hollow composite may have a different meaning depending on the shape of the hollow composite. For example, if the hollow composite has a sphere shape, the size may mean the diameter of the sphere, and if the hollow composite has an oval shape, the size may mean the diameter of its longer axis or the diameter of its shorter axis. The size of the hollow composite may range, for example, from about 50 nm to about 5,000 nm, from about 100 nm to about 5,000 nm, from about 300 nm to about 5,000 nm, from about 500 nm to about 5,000 nm, from about 700 nm to about 5,000 nm, from about 1,000 nm to about 5,000 nm, from about 1,300 nm to about 5,000 nm, from about 1,500 nm to about 5,000 nm, from about 1,700 nm to about 5,000 nm, from about 2,000 nm to about 5,000 nm, from about 2,500 nm to about 5,000 nm, from about 3,000 nm to about 5,000 nm, from about 3,500 nm to about 5,000 nm, from about 4,000 nm to about 5,000 nm, from about 4,500 nm to about 5,000 nm, from about 50 nm to about 4,500 nm, from about 50 nm to about 4,000 nm, from about 50 nm to about 3,500 nm, from about 50 nm to about 3,000 nm, from about 50 nm to about 2,500 nm, from about 50 nm to about 2,000 nm, from about 50 nm to about 1,700 nm, from about 50 nm to about 1,500 nm, from about 50 nm to about 1,300 nm, from about 50 nm to about 1,000 nm, from about 50 nm to about 700 nm, from about 50 nm to about 500 nm, from about 50 nm to about 300 nm, from about 50 nm to about 100 nm, from about 100 nm to about 4,000 nm, from about 300 nm to about 4,000 nm, from about 500 nm to about 4,000 nm, from about 700 nm to about 4,000 nm, from about 1,000 nm to about 4,000 nm, from about 1,300 nm to about 4,000 nm, from about 1,500 nm to about 4,000 nm, from about 1,700 nm to about 4,000 nm, from about 2,000 nm to about 4,000 nm, from about 2,500 nm to about 4,000 nm, from about 3,000 nm to about 4,000 nm, from about 3,500 nm to about 4,000 nm, from about 100 nm to about 3,500 nm, from about 100 nm to about 3,000 nm, from about 100 nm to about 2,500 nm, from about 100 nm to about 2,000 nm, from about 100 nm to about 1,700 nm, from about 100 nm to about 1,500 nm, from about 100 nm to about 1,300 nm, from about 100 nm to about 1,000 nm, from about 100 nm to about 700 nm, from about 100 nm to about 500 nm, from about 100 nm to about 300 nm, from about 200 nm to about 2,000 nm, from about 300 nm to about 2,000 nm, from about 400 nm to about 2,000 nm, from about 500 nm to about 2,000 nm, from about 600 nm to about 2,000 nm, from about 700 nm to about 2,000 nm, from about 800 nm to about 2,000 nm, from about 900 nm to about 2,000 nm, from about 1,000 nm to about 2,000 nm, from about 1,100 nm to about 2,000 nm, from about 1,200 nm to about 2,000 nm, from about 1,300 nm to about 2,000 nm, from about 1,400 nm to about 2,000 nm, from about 1,500 nm to about 2,000 nm, from about 1,600 nm to about 2,000 nm, from about 1,700 nm to about 2,000 nm, from about 1,800 nm to about 2,000 nm, from about 1,900 nm to about 2,000 nm, from about 200 nm to about 1,900 nm, from about 200 nm to about 1,800 nm, from about 200 nm to about 1,700 nm, from about 200 nm to about 1,600 nm, from about 200 nm to about 1,500 nm, from about 200 nm to about 1,400 nm, from about 200 nm to about 1,300 nm, from about 200 nm to about 1,200 nm, from about 200 nm to about 1,100 nm, from about 200 nm to about 1,000 nm, from about 200 nm to about 900 nm, from about 200 nm to about 800 nm, from about 200 nm to about 700 nm, from about 200 nm to about 600 nm, from about 200 nm to about 500 nm, from about 200 nm to about 400 nm, or from about 200 nm to about 300 nm, from about 400 nm to about 1,800 nm, from about 500 nm to about 1,800 nm, from about 600 nm to about 1,800 nm, from about 700 nm to about 1,800 nm, from about 800 nm to about 1,800 nm, from about 900 nm to about 1,800 nm, from about 1,000 nm to about 1,800 nm, from about 1,100 nm to about 1,800 nm, from about 1,200 nm to about 1,800 nm, from about 1,300 nm to about 1,800 nm, from about 1,400 nm to about 1,800 nm, from about 1,500 nm to about 1,800 nm, from about 1,600 nm to about 1,800 nm, from about 1,700 nm to about 1,800 nm, from about 400 nm to about 1,700 nm, from about 400 nm to about 1,600 nm, from about 400 nm to about 1,500 nm, from about 400 nm to about 1,400 nm, from about 400 nm to about 1,300 nm, from about 400 nm to about 1,200 nm, from about 400 nm to about 1,100 nm, from about 400 nm to about 1,000 nm, from about 400 nm to about 900 nm, from about 400 nm to about 800 nm, from about 400 nm to about 700 nm, from about 400 nm to about 600 nm, from about 400 nm to about 500 nm, from about 600 nm to about 1,600 nm, from about 700 nm to about 1,600 nm, from about 800 nm to about 1,600 nm, from about 900 nm to about 1,600 nm, from about 1,000 nm to about 1,600 nm, from about 1,100 nm to about 1,600 nm, from about 1,200 nm to about 1,600 nm, from about 1,300 nm to about 1,600 nm, from about 1,400 nm to about 1,600 nm, from about 1,500 nm to about 1,600 nm, from about 600 nm to about 1,500 nm, from about 600 nm to about 1,400 nm, from about 600 nm to about 1,300 nm, from about 600 nm to about 1,200 nm, from about 600 nm to about 1,100 nm, from about 600 nm to about 1,000 nm, from about 600 nm to about 900 nm, from about 600 nm to about 800 nm, from about 600 nm to about 700 nm, from about 800 nm to about 1,400 nm, from about 900 nm to about 1,400 nm, from about 1,000 nm to about 1,400 nm, from about 1,100 nm to about 1,400 nm, from about 1,200 nm to about 1,400 nm, from about 1,300 nm to about 1,400 nm, from about 800 nm to about 1,300 nm, from about 800 nm to about 1,200 nm, from about 800 nm to about 1,100 nm, from about 800 nm to about 1,000 nm, or from about 800 nm to about 900 nm.

[0093] In an embodiment of the present disclosure, a content of the carbonaceous structure may be from about 1 part by weight to about 20 parts by weight with respect to 100 parts by weight of the hollow composite. For example, the content of the carbonaceous structure with respect to 100 parts by weight of the hollow composite may be from about 1 part by weight to about 20 parts by weight, from about 2 parts by weight to about 20 parts by weight, from about 3 parts by weight to about 20 parts by weight, from about 4 parts by weight to about 20 parts by weight, from about 5 parts by weight to about 20 parts by weight, from about 6 parts by weight to about 20 parts by weight, from about 7 parts by weight to about 20 parts by weight, from about 8 parts by weight to about 20 parts by weight, from about 9 parts by weight to about 20 parts by weight, from about 10 parts by weight to about 20 parts by weight, from about 11 parts by weight to about 20 parts by weight, from about 12 parts by weight to about 20 parts by weight, from about 13 parts by weight to about 20 parts by weight, from about 14 parts by weight to about 20 parts by weight, from about 15 parts by weight to about 20 parts by weight, from about 16 parts by weight to about 20 parts by weight, from about 17 parts by weight to about 20 parts by weight, from about 18 parts by weight to about 20 parts by weight, from about 19 parts by weight to about 20 parts by weight, from about 1 part by weight to about 19 parts by weight, from about 1 part by weight to about 18 parts by weight, from about 1 part by weight to about 17 parts by weight, from about 1 part by weight to about 16 parts by weight, from about 1 part by weight to about 15 parts by weight, from about 1 part by weight to about 14 parts by weight, from about 1 part by weight to about 13 parts by weight, from about 1 part by weight to about 12 parts by weight, from about 1 part by weight to about 11 parts by weight, from about 1 part by weight to about 10 parts by weight, from about 1 part by weight to about 9 parts by weight, from about 1 part by weight to about 8 parts by weight, from about 1 part by weight to about 7 parts by weight, from about 1 part by weight to about 6 parts by weight, from about 1 part by weight to about 5 parts by weight, from about 1 part by weight to about 4 parts by weight, from about 1 part by weight to about 3 parts by weight, or from about 1 part by weight to about 2 parts by weight. If the content of the carbonaceous structure is less than 1 part by weight with respect to 100 parts by weight of the hollow composite, electric conductivity of the hollow composite may be decreased due to a small content of the carbonaceous structure that enables easier electron transport. If the content of the carbonaceous structure is more than 20 parts by weight with respect to 100 parts by weight of the hollow composite, a shape of the composite may not be induced to a hollow composite and the shell may be increased in thickness, so that electric conductivity may be decreased.

[0094] In an embodiment of the present disclosure, a charge transfer resistance of the hollow composite may range from about 10 Ω to about 200 kQ, but may not be limited thereto. For example, the charge transfer resistance of the hollow composite may range from about 10 Ω to about 200 kQ, from about 100 Ω to about 200 kQ, from about 300 Ω to about 200 kQ, from about 500 Ω to about 200 kQ, from about 700 Ω to about 200 kQ, from about 1 kQ to about 200 kQ, from about 10 kQ to about 200 kQ, from about 20 kQ to about 200 kQ, from about 30 kQ to about 200 kQ, from about 40 kQ to about 200 kQ, from about 50 kQ to about 200 kQ, from about 60 kQ to about 200 kQ, from about 70 kQ to about 200 kQ, from about 80 kQ to about 200 kQ, from about 90 kQ to about 200 kQ, from about 100 kQ to about 200 kQ, from about 120 kQ to about 200 kQ, from about 140 kQ to about 200 kQ, from about 160 kQ to about 200 kQ, from about 180 kQ to about 200 kQ, from about 10 Ω to about 180 kQ, from about 10 Ω to about 160 kQ, from about 10 Ω to about 140 kQ, from about 10 Ω to about 120 kΩ, from about 10 Ω to about 100 kΩ, from about 10 Ω to about 90 kΩ, from about 10 Ω to about 80 kΩ, from about 10 Ω to about 70 kΩ, from about 10 Ω to about 60 kΩ, from about 10 Ω to about 50 kΩ, from about 10 Ω to about 40 kΩ, from about 10 Ω to about 30 kΩ, from about 10 Ω to about 20 kΩ, from about 10 Ω to about 10 kΩ, from about 10 Ω to about 1 kΩ, from about 10 Ω to about 700 Ω, from about 10 Ω to about 500 Ω, from about 10 Ω to about 300 Ω, or from about 10 Ω to about 100 Ω, but may not be limited thereto. The charge transfer resistance is closely correlated to electric conductivity of the hollow composite, and as the charge transfer resistance is decreased, the electric conductivity of the hollow composite is increased. A high electric conductivity (low charge transfer resistance) of the hollow composite may be due to the carbonaceous structure included in the hollow composite.

[0095] In an embodiment of the present disclosure, the hollow composite may further enhance the inherently poor electric conductivity of the metal oxide by including the carbonaceous structure. Accordingly, such improved electric conductivity of the hollow composite enhances performance of the electronic ink including the hollow composite, and the electronic paper and the display device including the electronic ink. [0096] Hereinafter, the present disclosure will be explained in more detail with reference to Examples. However, the following Examples are illustrative only for better understanding of the present disclosure but do not limit the present disclosure.

[0097] [Examples]

[0098] 1. Preparation of rGO/Ti0 ₂ hollow nanospheres

[0099] (1) Synthesis of PS microspheres

[00100] Monodispersed polyvinylpyrrolidone (PVP)-modified polystyrene (PS) microspheres were synthesized by dispersion polymerization method using styrene as monomer, 2,2'- azobisisobutyronitrile (AIBN) as an initiator, and PVP as stabilizer. Ideally PVP (4 g) was dissolved in ethanol (55 mL) and purged under Ar flow for 30 min, before adding the styrene monomer (6.4 mL) and AIBN (1 wt%) pre-dissolved in ethanol (5 mL). After polymerization at 70°C for 24 h, monodispersed PS microspheres with an average diameter of 1 μηι were obtained. The obtained PS microspheres were consecutively washed with ethanol and centrifuged thrice, before being re- dispersed in ethanol (15 wt.% solution).

[00101] (2) Synthesis of graphene oxide (GO)

[00102]GO was synthesized by the modified Hummers method. In brief, graphite powder (1 g) was added to 70 ml H ₂ S0 ₄ (98%). KMn0 ₄ (3 g) and NaN0 ₃ (0.5 g) were then added gradually in an ice bath. The mixture was stirred for 4 h, after which deionized water (100 mL) was added to the mixture. The resulting preparation was maintained at that temperature for 30 min. A H ₂ 0 ₂ solution (30%) was then gradually added into the solution while stirring until the suspension turned to brilliant brown indicating fully oxidization of graphite. The mixture was washed repeatedly with 5% HCI and Dl-water and collected by centrifugation. The obtained graphite oxide powder was added to ethanol and exfoliated by sonication for 6 h. The resulting suspension was centrifuged for 30 min at 3000 rpm to remove precipitates and used to obtain a stable GO-ethanol suspension. The GO- ethanol suspension was further centrifuged at high rpm to separate exfoliated GO nanosheets.

[00103] (3) Fabrication of rGO/Ti0 ₂ hollow nanospheres

[00104] A rGO/Ti0 ₂ precursor solution was prepared according to the following procedure.

GO suspended in 7.6 mL of ethanol containing 0.2 mL of HCI (37%) was sonicated for 2 h. Upon the addition of 3.4 mL of titanium butoxide (TBOT), the solution was stirred at room temperature for 24 h. The amount of GO was adjusted to obtain wt% of GO of 5%, 10% and 20% in the resulting GO/Ti0 ₂ precursor solution. Ti0 ₂ is first nucleated and believed to react with the available functional groups of GO, to form 3D interconnected GO/Ti0 ₂ networks.

[00105] In the preparation of GO/Ti0 ₂ -coated PS nanospheres (PS@Ti0 ₂ ), the as-prepared precursor solution (350 μί) mixed with colloidal PS (1 mL) was sonicated for 30 min to enhance the interaction between GO/Ti0 ₂ and the PS nanospheres. Initial experiments revealed an impact of the ratio of GO/Ti0 ₂ precursor solution to colloidal PS nanospheres suspensions in order to obtain well- defined and reproducible PS@GO/Ti0 ₂ composites. The ratio herein applied was optimized from those initial experiments. Each prepared suspension (200 μί) was then drop-cast onto a quartz substrate (2.5 x 8 cm ² ), and dried at room temperature. During the drying process, the Ti0 ₂ precursor exposed to the moisture in air and hydrolysed into metal oxide sols, which subsequently formed a homogeneous, dense, thin coating around each polystyrene bead. A two-step calcination method was considered in the synthesis of all rGO/Ti0 ₂ hybrids. Samples were firstly carbonized at 500°C under Ar flow to efficiently remove the PS core while simultaneously ensuring the stability of the formed hollow composite. The temperature was then increased to 800°C to realize full crystallization of Ti0 ₂ in rGO/Ti0 ₂ hybrid structures. According to the selected GO wt% used in the precursor solution, corresponding synthesized materials are hereafter denoted as rGO(5%)/Ti0 ₂ , rGO(10%)/TiO ₂ and rGO(20%)/TiO ₂ . Prepared rGO/Ti0 ₂ were evaluated together with synthesized Ti0 ₂ hollow nanospheres as a reference.

[00106] 2. Analyses of characteristics

[00107] (1) Characterization techniques

[00108]The surface morphology was characterized using scanning electron microscopy (SEM; JEOL JSM6700-F) and transmission electron microscopy (TEM; JEOL JSM-2100F operated at 200 kV). Synchrotron XRD were obtained by synchrotron X-ray diffraction (SXRD) at beamline 17-BM at Advanced Photon Source (APS) at Argonne National Laboratory (ANL). Samples were attached to Kapton ^® tapes and measured in transmission mode. A PerkinElmer ^® amorphous silicon flat panel detector was used to collect two-dimensional XRD data. Integration of the 2D data to conventional plots of intensity versus 2-theta was performed with Program GSAS(II). The wavelength used was 0.72768 A. Raman spectra were recorded on a T64000 (HORIABA Jobin Yvon, France). X-ray photoelectron spectroscopy (XPS) spectra were measured on a Thermo Scientific K-Alpha XPS, using a dual beam source and ultra-low energy electron beam for charge compensation. X-ray absorption near edge spectroscopy (XANES) was carried out in transmission mode at beamline 9-BM of APS, ANL. Data reduction and data analysis were performed with the Athena software packages. The pre- edge was linearly fitted and subtracted. The post-edge background was determined by using a cubic- spline-fit procedure and then subtracted. Normalization was performed by comparing the data to the height of the absorption edge at 50 eV. The monochromator was detuned to 80% of the maximum intensity at those Ti K edges to minimize the presence of higher harmonics. The X-ray beam was calibrated using the Ti metal foil K edge at 4966 eV.

[00109] (2) Calibration of SCE and conversion to RHE [00110]The calibration of SCE reference electrode was performed by the modified method described in literature [Y. Li, W. Zhou, H. Wang, L. Xie, Y. Liang, F. Wei, J.-C. Idrobo, S. J. Pennycook, H. Dai, Nat Nano 2012, 7, 394.]. A standard three-electrode system was utilized with Pt/C deposited glassy carbon electrode and polished Pt wires as the working and counter electrodes respectively, and the SCE as the reference electrode. Electrolytes were pre-purged and saturated with high purity H ₂ . Linear scanning voltammetry (LSV) was then run at a scan rate of 0.1 mV/s, and the potential at which the current crossed zero was taken to be the thermodynamic potential (vs. SCE) for the hydrogen electrode reactions. The zero current point was observed at -0.998 V, so

[00111] (3) Electrochemical measurements

[00112] For electrochemical measurements, 5 mg of the catalyst mixed with 50 μί of nafion were ultrasonically dropped in isopropanol (500 μί) for one hour. Then, a 3 μί ink was dispersed onto a 3 mm rotating glassy carbon electrode and dried in air. Electrochemical linear sweep voltammetry (LSV) and cyclic voltammetry (CV) measurements were performed using a computer- controlled potentiostat with a three-electrode glass electrochemical cell. The catalyst loaded glassy carbon electrode was used as the working electrode, a Pt wire as the counter electrode and a saturated calomel electrode (SCE; calibrated and converted to H E) was used as the reference electrode. Prior to each measurement, KOH electrolyte (0.1 M) was bubbled with either N ₂ or 0 ₂ for 20 min and continued 0 ₂ flow was maintained during the measurement to ensure continuous 0 ₂ saturation. The potential range was cathodically scanned between 0.05 V and 1.05 V versus RH E at a scan rate of 5 mV/s. The samples were compared with a commercial 20 wt% Pt/carbon black (Pt/C, Alpha Aeser) prepared as aforementioned. [00113]3. Results and analyses

[00114] (1) Morphology

[00115] Functionalized PS nanospheres were strategically coated with GO/Ti0 ₂ in presence of a GO/Ti0 ₂ precursor solution. The resulting core/shell composite was drop-casted on a quartz substrate and calcined at 500°C under Ar, then up to 800°C under air flow to obtain hollow rGO/Ti0 ₂ nanospheres.

[00116] epresentative SEM images were collected to evaluate the size and morphology of the materials synthesized. Prepared hybrid materials synthesized with 10 wt% of GO were present example compared with Ti0 ₂ -based hollow nanospheres as a comparative example. SEM images were firstly collected to verify the synthesis of monodispersed PS nanospheres with smooth surface and uniform particle size of ca. 1 μηι (FIG. 1A). SEM photographs of core/shell structured composite PS@GO/Ti0 ₂ (FIG. IB) and PS@Ti0 ₂ (FIG. 2A) nanospheres suggested a uniform coating of Ti0 ₂ and GO/Ti0 ₂ over the PS template, respectively. The effective synthesis of these core/shell composite structures was attributed to the strong electrostatic interaction between the carbonyl-groups located on the surface of PS and the hydroxyl-groups of Ti0 ₂ and GO. Upon deposition of Ti0 ₂ and GO/Ti0 ₂ , the average diameter of the resulting materials could be increased up to ca. 1.1 μηι and 1.24 μηι with PS@Ti0 ₂ and PS@GO(10%)/TiO ₂ , respectively. Interestingly, an increase of the average diameter of PS@GO/Ti0 ₂ was suggested with increasing GO concentration in the precursor solution. FIG. 2 shows the FESEM images of (a) PS@Ti0 ₂ , (b) rGO(5%)/Ti0 ₂ , and (c) rGO(20%)/TiO ₂ hollow composite and a cross-sectional image of (d) rGO(10%)/TiO ₂ hollow composite (scale bar is 1 μηι in all figures). The observation was tentatively attributed to the presence of abundant functional groups of GO, which were believed to enhance the interaction with the carbonyl groups of PS. SEM images of uniform Ti0 ₂ and rGO(10%)/TiO ₂ hollow shells corroborated the effective removal of PS in the following carbonization (FIG. 1C and FIG. ID). Although the spherical morphology could be retained in both cases, size shrinkage of from ca. 10% to 15% was evidenced upon comparison with the uncalcined counterparts. This observation was ascribed to the condensation/polymerization of template associated with the sintering contraction of Ti0 ₂ frameworks during the high temperature crystallization reaction. FIG. 3 provides TEM images of (a) Ti0 ₂ (air), (b) Ti0 ₂ , (c) rGO(20%)/TiO ₂ showing the hollow nature, and H TEM image of (d) rGO(20%)/TiO ₂ . A close inspection of the collected SEM images and a HRTEM image revealed the presence of Ti0 ₂ nanoparticles aggregated in the Ti0 ₂ hollow nanospheres (FIG. 1C).

[00117] epresentative TEM images of both Ti0 ₂ and rGO(10%)/TiO ₂ hollow spheres were collected from the following the 2-step carbonization procedure. FIG. 4A and FIG. 4C show the TEM image of well-defined Ti0 ₂ and rGO(10%)/TiO ₂ composite hollow spheres respectively, with a diameter of ca. 800 nm. Whereas the Ti0 ₂ hollow spheres were composed of randomly distributed Ti0 ₂ nanoparticles with the size ranging from 10 nm to 50 nm, the thick and continuous shell of rGO(10%)/TiO ₂ revealed uniform coating of averaged 10 nm Ti0 ₂ nanoparticles on a rGO sheet. In the present example, the abundant functional groups of rGO were assumed to reduce the particle size and agglomeration of the Ti0 ₂ nanoparticles with the rGO(10%)/TiO ₂ sample of the present disclosure. Interestingly, high-resolution TEM photographs revealed the presence of a 2-3 nm-thick carbon coating-layer over individual Ti0 ₂ nanoparticles with the both evaluated samples (FIG. 4B and FIG. 4D). The formed layer was assumed to be carbonaceous in nature and the result of the carbonization of the carbonyl groups in the modified PS nanospheres. The assumption followed a recent literature report documenting carbon coating of Ti0 ₂ nanoparticles. The porous texture of all the samples was evaluated by N ₂ sorption measurement. The strong uptake of N ₂ at high relative pressures corroborated the presence of large macropores in the hollow structures. Most importantly, with all rGO/Ti0 ₂ hybrid materials, N ₂ assessment revealed type-IV isotherms with a clear H2-type hysteresis loop, characteristic of mesoporous materials. The observation was present example ascribed to the presence of intraparticle voids resulting from the aggregation of crystalline nanoparticles with an average size of 10 nm. [00118] (2) Structural and chemical characterization

[00119]The crystalline phase of Ti0 ₂ and rGO/Ti0 ₂ hollow nanospheres were assessed by synchrotron X-ray diffraction (SXRD) (FIG. 5A). The SXRD patterns identified in all structures were found to be consistent with both Ti0 ₂ anatase and rutile-crystalline phases (JCPDS No.21-1272 and No.21-1276, respectively). Changes in the anatase-to-rutile ratio were found to be negligible upon mixing Ti0 ₂ with varying rGO amount. No characteristic rGO diffraction peaks were detected in the rGO/Ti0 ₂ hybrid materials as a result of a presumed weak diffraction intensity of the latter. Raman spectra were collected to investigate the bonding nature of rGO and Ti0 ₂ in the hybrid structures (FIG. 5B). Bands located at 155 (E _g ) and 398 cm ^"1 (B _lg ) were confirmed to be in agreement with Ti0 ₂ anatase-crystal phase, whereas scattering peaks at 250 (A _lg ), 440 (E _g ) and 612 cm ^"1 (A _lg ) were assigned to Ti0 ₂ rutile. These results further reflected the presence of mixed Ti0 ₂ anatase and rutile crystalline phases evidenced by XRD data. Mixed Ti0 ₂ crystalline phases had been shown to exhibit superior catalytic activity, as the difference in their conduction bands was believed to suppress the recombination of charge carriers. Interestingly, the most intense E _g Raman mode of anatase at 155 cm ^"1 in rGO/Ti0 ₂ was shifted by 8 cm ^"1 upon comparison with a pristine Ti0 ₂ sample. A similar result was observed with the E _g peak at 440 cm ^"1 . The observed blue-shifts were believed to be the result of oxygen vacancies or disorder induced Ti ³⁺ centres. Bands at 1350 cm ^"1 and 1604 cm ^"1 were assigned to the D and G peaks of disordered sp ² carbon and the ordered graphitic structure of graphene, respectively (FIG. 5B). The both bands were further observed at a minor extent with Ti0 ₂ hollow nanospheres, corroborating the nature of the carbon-coating layer covering the surface of Ti0 ₂ nanoparticles upon carbonization under Ar at 500°C (FIG. 4B). For comparison, Ti0 ₂ -based hollow nanospheres were directly calcined in atmospheric air at 800°C (Ti0 ₂ (air) in FIG. 4B). Upon characterization, results confirmed the absence of both D and G bands in the sample, suggesting a complete removal of C species during the calcination treatment. [00120]To investigate the chemical state and effective integration of rGO and Ti0 ₂ , high- resolution X-ray photoelectron spectroscopy (XPS) measurements were performed. Peaks in the ranges of from 282 eV to 292 eV, from 526 eV to 536 eV, and from 455 eV to 468 eV were assigned to C Is, O Is, and Ti 2p, respectively. After subtracting the spectrum background by Shirley's method, the collected spectra were conveniently deconvoluted into the expected component peaks. rGO(10%)/TiO ₂ hollow nanospheres unveiled Ti2p signals at 459.1 eV and 464.8 eV, corresponding to Ti 2p ^{3 2} and Ti 2p ^{1 2} spin-orbital splitting photoelectrons in Ti ⁴⁺ state, respectively. Upon comparison with a pristine Ti0 ₂ sample calcined under air up to 800°C, it was clear that both signals were slightly shifted from corresponding 458.1 eV and 464.1 eV peaks to higher energy regions. The shift of binding energy towards the higher energy region was attributed to the transfer of electron density from Ti to rGO in the prepared rGO/Ti0 ₂ composite (M. S. A. Sher Shah, A. . Park, K. Zhang, J. H. Park, P. J. Yoo, ACS Appl. Mater. Interfaces 2012, 4, 3893). rGO/Ti0 ₂ exhibited two small shoulderlike peaks at 457.4 eV and 463.2 eV, which were assigned to Ti ³⁺ defects. Ti ³⁺ species on the surface or in bulk Ti0 ₂ are known to be unstable and therefore easily oxidized upon calcination, representing a clear challenge to obtain stable Ti ³⁺ -doped Ti0 ₂ . In the present example, however, it is believed that these species can be stabilized upon Ti0 ₂ hybridization with rGO. The presence of Ti ³⁺ species was observed at a lower extent in the case of Ti0 ₂ -based hollow nanospheres calcined under Ar. The observation was ascribed to the presence of the formed carbon layer over the Ti0 ₂ nanocrystals observed in collected TEM images (FIG. 4B). The presence of this carbon layer was assumed to stabilize the doped Ti ³⁺ species and surface oxygen vacancies into the crystal lattice, even at high temperature levels. The hypothesis was corroborated upon comparison with the pristine Ti0 ₂ reference in which no characteristic Ti ³⁺ peaks were observed. In the absence of a carbon layer at 800°C, the Ti ³⁺ are expected to be easily and completely oxidized. This result suggested that the generation and stabilization of 0-Ti ³⁺ species in Ti0 ₂ and rGO/Ti0 ₂ is thus closely correlated with both the carbon coating and rGO layers. [00121]Ols XPS spectra of Ti0 ₂ and rGO(10%)/TiO ₂ hollow nanospheres exhibited peaks at 529.4 eV and 531.5 eV corresponding to the lattice oxygen (Ti-O) in Ti0 ₂ , and to surface hydroxyl groups (Ti-OH) or 0-C=0, respectively. The shoulder-peak at 532.3 eV evidenced HO-C or carboxyl species in the presence of carbon and rGO. The peak at 530.3 eV was assigned with the both samples to 0-Ti ³⁺ , previously evidenced with collected Ti XPS data, with a noticeable Ti ³⁺ species content being observed in rGO/Ti0 ₂ . Cls XPS spectra revealed a high intense deconvoluted peak centred at 284.4 eV assigned to the C-C, C=C, and C-H bonds. The peak at 284.4 eV was assigned to the sp ² C=C of rGO with the evaluated rGO/Ti0 ₂ hollow nanospheres, and attributed to the formed C=C in the graphitic-phase carbon layer in the Ti0 ₂ nanoparticles following previous evidence. Oxygen-containing carbonaceous bonds were deconvoluted at higher binding energy levels, i.e., 285.3 eV, 286.7 eV, and 287.7 eV corresponding to C-OH, C=0 and -COOH groups, respectively, in good agreement with collected Ols XPS data.

[00122]Ti and the Ti valence of the synthesized sample were further assessed by X-ray absorption near-edge structure (XANES) spectroscopic analyses (FIG. 6). The pre-edge features give rise to three peaks usually denoted Al, A2, and A3, which yield to a specific ion's immediate environment. In particular, the relative intensity of the A2 peak, sensitive to Ti site geometry, reflects the local atomic arrangement around titanium ions. In the present example, the overall spectral features of the synthesized samples were found to be rather similar to that of anatase-type Ti0 ₂ , corroborating previous observations. Most interestingly, the relative intensity of A2 was slightly increased compared with the pristine Ti0 ₂ reference (FIG. 6B). This change was more notable with the rGO(10%)/TiO ₂ hollow nanospheres of the present example, indicating an increasing modification of the Ti coordination sphere as a consequence of carbon and rGO addition. The results suggest that the Ti species in the prepared hollow nanospheres exist in a lower average oxide state, corroborating the aforementioned XPS data. [00123] (3) Conductivity

[00124] Electrochemical impedance spectroscopy (EIS) was carried out at open circuit potential in a 10 ^"2 to 10 ^s Hz frequency range to investigate the electronic conductivity of the prepared samples. The Nyquist plots of all the evaluated samples were depicted in FIG. 7A. Whereas a diameter of the Nyquist plot in the high frequency region was related to the charge transfer resistance ( _ct ) of the evaluated electrode, a smaller radius implied a more efficient charge transfer. Calculated R _ct values were observed to follow the order Ti0 ₂ (114.2 kQ) > rGO(5%)/Ti0 ₂ (48.5 kQ) > rGO(20%)/TiO ₂ (29.9 kQ) > rGO(10%)/TiO ₂ (16.8 kQ). Results reflected the impact of the introduced rGO layers in enabling easier electron transport, which was assumed to have a crucial impact in the resulting ORR activity of these catalysts. A slightly higher charge transfer resistance attained with rGO(20%)/TiO ₂ could be attributed to a thicker nature of the shell, evidenced in corresponding SEM images.

[00125]To shed light on the impact of the formation of the thin carbon layer evidenced in TEM on the conductivity of the evaluated samples, results were compared as described above with a pristine Ti0 ₂ reference calcined under air flow at 800 ^° C. The reference sample disclosed a significantly higher resistance value (182.4 kQ), showcasing the poor intrinsic electronic conductivity of individual Ti0 ₂ nanoparticles. This result additionally reflected the scarcity in the application of Ti0 ₂ as a sole ORR electrocatalyst, and therefore the novelty of the present disclosure. Most importantly, the cooperative effect of both rGO and the formed carbon layer was concluded to result in a remarkable 11-fold improvement of the conductivity of Ti0 ₂ with the best sample (rGO(10%)/TiO ₂ ) of the present example.

[00126] Mott-Schottky measurements were additionally carried out with all rGO/Ti0 ₂ and Ti0 ₂ -based hollow nanospheres at a frequency of 5 kHz, to further investigate the impact of self- doping and rGO incorporation on Ti0 ₂ (FIG. 7B). In agreement with the previous results, a Ti0 ₂ sample calcined under atmospheric air was further evaluated to investigate the impact of the formed carbon layer upon carbonization. All the samples revealed a positive slope as expected with n-type semiconductor materials (FIG. 7B). Values were calculated from the slopes of Mott-Schottky plots according to <Equation 1>:

[00127]<Equation 1>

[00129]with e ₀ , the electron charge, ε, the dielectric constant of Ti0 ₂ determined based on the ratio of Ti0 ₂ crystalline-phase composition (XRD), ε ₀ , the permittivity of vacuum, N _d , the donor density, and V, the applied bias at the electrode. Calculated electron densities followed the order rGO(10%)/TiO ₂ (3.5xl0 ²¹ ) > rGO(5%)/Ti0 ₂ (2.7xl0 ²¹ ) > rGO(20%)/TiO ₂ (9.3xl0 ²⁰ ) > Ti0 ₂ (2.1xl0 ²⁰ ) > Ti0 ₂ (air) (6.2xl0 ¹⁹ ). Notably, the presence of a thin carbon layer in the Ti0 ₂ nanoparticles appeared herein to play a crucial role in remarkable boost in donor density. The enhanced donor density followed corroborating XPS data evidencing Ti ³⁺ doping and increasing oxygen vacancies known to be electron donors with Ti0 ₂ . The increased charge-carrier density gave a rise to the aforementioned improved electronic conductivity and a better electron charge separation within electrode.

[00130] (4) Electrocatalytic performances

[00131] Electrocatalytic measurements were carried out as described in the 2. Analysis of characteristics above. Prepared samples were conveniently compared with a Pt/C conventional catalyst as reference (comparative example). The ORR electrocatalytic activity of all the samples was initially evaluated through loading of the active materials on glassy carbon electrode for cyclic voltammetry (CV) in a potential window from 0 V to 1.2 V at a constant scan rate of 20 mV/s in 0 ₂ saturated 0.1M KOH aqueous solution (FIG. 8A). The CV of Ti0 ₂ and rGO/Ti0 ₂ hollow nanospheres exhibited distinctive cathodic ORR peaks around 0.7 V vs. RHE, suggesting the pronounced electrocatalytic activities of these new materials for the ORR. No distinctive CV curves within the evaluated range were witnessed in N ₂ -saturated solution (FIG. 9). [00132]The ORR activity of all the samples was further investigated in rotating disk electrode (RDE) measurements. Linear sweep voltammograms (LSV) recorded in a rotation speed range of from 400 rpm to 2500 rpm evidenced an enhancement of the current density with increasing rotation, which was attributed to a higher availability of diffused oxygen at the electrodes surface (FIG. 8C). Accordingly, at lower values, the ORR curve displayed mass transport limitation by the diffusion of the dissolved oxygen in the KOH aqueous solution.

[00133] LSV curves for all prepared samples were detailed at a fixed rotation rate of 1,600 rpm and compared with a commercial Pt/C (FIG. 8B). In line with the impact of the presence of a thin carbon layer on the enhanced conductivity of the Ti0 ₂ sample, results were compared with a pristine Ti0 ₂ -reference calcined in atmospheric air. Catalytic parameters were conveniently summarized in Table 1.

[00134] <Table 1>

[00135]

[00136] rGO(10%)/TiO ₂ and rGO(20%)/TiO ₂ exhibited oxygen reduction with similar onset potential value (0.82 V), whereas Pt/C displayed a 1.01 V value. As expected, the rGO/Ti0 ₂ samples showed superior onset potential compared to the value (0.70 V) attained with the pristine Ti0 ₂ - reference calcined in atmospheric air. Similarly, the ORR half-wave potential of the aforementioned rGO/Ti0 ₂ samples, defined as the potential at which the magnitude of the current is half of the limiting current (herein, 0.70 V), revealed a remarkable improvement compared with the Ti0 ₂ reference (0.54 V). In each case, results highlighted a superior ORR electrocatalytic activity as suggested upon comparison with the Pt/C sample (0.82 V). The current density at 0.1 V vs. RH E under the aforementioned operating conditions could be increased up to stable -4.5, -5.0 and -5.5 mA/cm ² values with increasing the rGO content of 5%, 10%, and 20%, respectively, attaining a comparable value with Pt/C (-5.4 mA/cm ² ) (Table 1). The result italicized the impact of the hybridization of Ti0 ₂ with rGO to increase the ORR performance of pristine Ti0 ₂ materials.

[00137]The enhanced ORR activities could be attributed to a facilitated charge transfer across the interface depending on the Fermi level difference. The presence of Ti ³⁺ sites was further believed to increase the electron density and to shift the Fermi level of Ti0 ₂ toward the conduction band. The upward shift of the Fermi level facilitated charge separation at the electrode/electrolyte interface. The formation of a thin carbon layer over the formed Ti0 ₂ nanoparticles was further evidenced to play a crucial role in the methodology (Table 1). The resulting carbon-coating layer marked an improvement of the electrocatalytic properties of Ti0 ₂ , followed higher conductivity levels, enabled the presence of oxygen vacancies and reduced Ti ³⁺ and reduced the contact resistance between active material particles.

[00138]The kinetic analysis was carried out using the following Koutecky-Levich (K-L) equation <Equation 2>, to determine the corresponding electron transfer number (n) and kinetic current density (i _k ):

[00139] <Equation 2>

[00140]- = - +— ^—j-

^{1 1} 1 i _k B x ω ¹ / ²

[00141]with i, the measured current, i _k , the kinetic current and ω, the electrode rotation rate. The K-L plots were obtained from the currents at different potentials (FIG. 10) and varying rotating speeds. The number of transferred electrons in ORR, n, was calculated from the slopes of straight lines at different potentials according to <Equation 3>:

[00142] <Equation 3> [00143]β = 0.62 x n x F XC _Q2 X D^X V ^{" 1} / ²

[00144]with n, the number of electrons transferred per oxygen molecule, F, the Faradic constant (96,485 C mol ^"1 ), C ₀₂ , the oxygen concentration saturated in a 0.1M KOH aqueous solution (1.2 x 10 ^"6 mol cm ^"3 ), D ₀₂ , the oxygen diffusion coefficient (1.73 x 10 ^s cm ² s ^"1 ), and v, the kinematic viscosity of the solution (0.01 cm ² s ^"1 ). The number of transferred electrons (n) calculated from the slope of K-L plots in the 0 to 0.5 V vs H E range implied a two-electron reaction pathway from 0 ₂ to hydrogen peroxide.

[00145] For a better comparison of the rGO/Ti0 ₂ hybrid electrocatalysts, the ORR activity was further investigated according to the Tafel slopes at low overpotentials in 0 ₂ -saturated 0.1 M KOH aqueous solution (FIG. 8D). The E versus log(-/ ^' ) curves in the range of from 0.80 V to 0.85 V indicated rather similar change in the reaction mechanisms with the potential. rGO(10%)/TiO ₂ exhibited smaller Tafel slope (66 mV/dec) compared to both rGO(5%)/Ti0 ₂ (114 mV/dec) and rGO(20%)/TiO ₂ (73 mV/dec) under the investigated operating conditions. Based on CV and RDE measurements, rGO/Ti0 ₂ samples exhibited superior ORR catalytic activity with more positive ORR onset potential and a more positive half-wave potential than Ti0 ₂ , indicating that rGO hybridization further enhanced ORR catalytic activity of Ti0 ₂ . The evaluation of the impact of concentration of rGO in the prepared samples suggested a higher ORR electrocatalytic activity of rGO(10%)/TiO ₂ among all samples.

[00146]To evaluate the impact of the electrochemically accessible surface area enhancement, rGO(10%)/TiO ₂ hybrids were further prepared under the same synthesis conditions without the use of the PS template. The resulting rGO(10%)/TiO ₂ structures exhibited remarkably poor electrocatalytic activity compared with corresponding hollow nanospheres-counterparts, underlining the impact of the morphology of these materials in the ORR activity. Therefore, aside from the formed carbon-coating layer, Ti ³⁺ -doping and rGO hybridization, a higher contact area between the electrode and the electrolyte (KOH) and higher number active sites were believed to contribute to higher ORR activity.

[00147]To evaluate the applicability of the samples described in the present disclosure, corresponding durability and methanol tolerance ability were further evaluated. The durability of the rGO/Ti0 ₂ hollow nanospheres was examined at a 0.5 V vs RHE and 1,600 rpm in KOH (0.1 M) saturated in 0 ₂ , for 20,000 s. Higher relative current values were found with the new rGO/Ti0 ₂ hybrid samples of the present disclosure, emphasizing a superior stability compared to conventional Pt/C samples (FIG. 11A). The unique design of the novel material of the present disclosure as a hollow nanosphere was believed to play a crucial role in increasing the durability of rGO/Ti0 ₂ , providing higher contact area between electrode and electrolyte and larger active sites for ORR. rGO/Ti0 ₂ hollow nanospheres were further tested for methanol crossover via chronoamperometric responses under the abovementioned operating conditions (FIG. 11B). Upon the addition of methanol (3 wt%) into the electrolyte solution, the Pt/C catalyst disclosed a rapid drop in its performance, highlighting one of the major drawbacks in long cyclic performance of Pt/C as a conventional ORR catalyst. On the contrary, both rGO(5%) and rGO(10%)/TiO ₂ hollow nanospheres revealed stable amperometric responses. The excellent methanol tolerance herein underlined further emphasized the promising applicability of these hollow nanospheres and, in particular, of the best rGO(5%)/Ti0 ₂ sample in the present disclosure. As suggested with the result attained with rGO(20%)/TiO ₂ however, a continuous increase in the rGO content seemed to result in lower methanol tolerance.

[00148]The above description of the present disclosure is provided for the purpose of illustration, and it would be understood by those skilled in the art that various changes and modifications may be made without changing technical conception and essential features of the present disclosure. Thus, it is clear that the above-described examples are illustrative in all aspects and do not limit the present disclosure. For example, each component described to be of a single type can be implemented in a distributed manner. Likewise, components described to be distributed can be implemented in a combined manner.

[00149]The scope of the present disclosure is defined by the following claims rather than by the detailed description of the embodiment. It shall be understood that all modifications and embodiments conceived from the meaning and scope of the claims and their equivalents are included in the scope of the present disclosure.