Field of the Invention

[0001] The present invention relates to water oxidation reaction catalysts, and methods of making them.

Background [0002] Renewable energy systems such as photovoltaic devices [1 -3] require that electrical energy be efficiently stored and released. A promising method to store electrical energy on an industrial scale utilizes the electrolysis of water [4-6] to produce hydrogen and oxygen. The resulting hydrogen can be stored and converted back into electricity with fuel cells such as proton exchange membrane fuel cells, PEMFC [7-9] . The water oxidation reaction (WOR) at the anode of an electrolyser requires an electrocatalyst [10- 14] . While the standard potential for the WOR (E°) is 1 .23 VRHE, the kinetics of the WOR are slow, resulting in significant anode overpotentials in water electrolyzers [ 1 1 ] .

[0003] Many single and multicomponent metallic catalysts have been investigated to accelerate the WOR in acid [ 10- 13, 15- 18] . Catalysts containing iridium (Ir), regarded as derivatives of Ir(IV)-oxo species, have a favourable combination of activity and stability in acid. Catalysts containing ruthenium (Ru) are also active, but Ru is not stable to prolonged operation at WOR potentials in acid [19, 20] . As a result, Ir oxide nanoparticles are presently the benchmark for the WOR in acid [21 ] , and much of the research in this area is directed towards optimizing utilization of Ir [ 10, 12, 21 -25] . For example, Berlinguette et al. recently published a photochemical decomposition of Ir(acetylacetonate) ₃ deposits to form amorphous IrOx films that were active towards the WOR in acid [22] . Among the more active acid WOR catalysts are those reported by Strasser et al. [10- 13] . With a series of dealloyed Ir oxide core- shell catalysts, they obtained mass activities in acid as high as ~ 40 A g ^"1 at 0.25 V

overpotential [ 10] . The most active IrNi@IrO _x catalyst in this series was three times more active than pure Ir. More recently, a mass activity of ~ 90 A g ^"1 Ir, with Eworking = 1 .51 VRHB at 1 80° C, using a IrNiO _x catalyst on antimony-doped tin oxide has been reported [1 1 ] .

[0004] However, there remains a need in the art for alternative methods of preparing stable and active catalysts for WORs.

Summary Of The Invention

[0005] Aspects and embodiments of the invention may be summarized as follows. [0006] In one aspect, the invention may comprise a method of preparing mixed metal oxide nanoparticles, comprising the steps of preparing a solution of an iridium salt and a nickel salt, and adding a hydroxide in the presence of oxygen. In one embodiment, the iridium salt is iridium chloride, the nickel salt is nickel chloride, and the hydroxide is OH.

[0007] Preferably, the molar ratio of iridium to nickel is between about 4: 1 and about 16: 1 , or any ratio therebetween, and more preferably the ratio is about 8 : 1. In another embodiment, the molar ratio of iridium to nickel may be between about 2 to about 20.

[0008] In another aspect, the invention may comprise an electrode comprising or coated with nanoparticles comprising mixed iridium oxide and nickel oxide. [0009] In another aspect, the invention may comprise a method of electrolyzing water, using an electrode comprising or coated with nanoparticles comprising mixed iridium oxide and nickel oxide.

Brief Description of the Drawings

[0010] The drawings attached to or embedded in the description form part of the specification and are included to further demonstrate certain embodiments or various aspects of the invention. In some instances, embodiments of the invention can be best understood by referring to the accompanying drawings in combination with the detailed description presented herein. The description and accompanying drawings may highlight a certain specific example, or a certain aspect of the invention. However, one skilled in the art will understand that portions of the example or aspect may be used in combination with other examples or aspects of the invention.

[0011] Fig. 1A. SEM (on GC) and Fig. IB TEM (on a Cu grid), and voltammetric profiles in 0.1 M H ₂ S0 ₄ at 50 mV s ^"1 (Fig. 1 C, E = 0 - 1.25 VRHE) and at 10 mV s ^"1 (Fig. I D; 1.0-1.55 VRHE) of NP deposits on GC containing 3.4 μg Ir and 1 wt. % Nafion™ on GC. The size distribution (B, inset) was determined by measuring ~ 100 particles.

[0012] Fig. 2. A: Mass normalized activities (A g ^_1 Ir) of the IrO _x and IrNiO _x NP (3.4 μ ) at 1.48 VRHE in 0.1 M H ₂ S0 ₄ at 10 mV s ^"1 . B : Specific activities (A mmor'ir) obtained by number of moles of Ir from the Ir electrochemistry (see Fig. 3) in 0.1 M H ₂ S0 ₄ .

[0013] Fig. 3. Voltammograms for IrO and IrNi _y O _x nanoparticle in 0.1 M H ₂ S0 ₄ (Potential sweep rate 50 mV s ^"1 ). No hydrogen signals are observed at low potential limits, which indicates all metals exist in oxide forms. The peaks over the high potential limit (i.e. 0.8 VRHE and higher) are ascribed to Ir (3+/4+) and further to 5+ (the poorly defined shoulder following the redox peaks on both directions over potentials higher than 1.1 VRHE) conversions.

[0014] Fig. 4A: Voltammograms (10 mV s ^"1 ) for the GC, GC/IrO _x , and GC/IrNio.mOx electrodes in 0.1 M H2SO4. Inset: WOR (0.087 mglr cm ^"2 ) at 1 mA cm ^"2 _ge ometric. Figure 4B: TEM for IrNio.i2sO _x .

Detailed Description

[0015] In general terms, the present invention comprises a method of preparing non-precious metal oxide nanoparticles by stirring metal chloride salt precursors with a hydroxide in the presence of oxygen. [0016] In one embodiment, the method may comprise a method of preparing colloidal nanoparticle suspensions of catalysts comprising iridium and nickel. In the prior art, colloidal suspensions of IrO _x nanoparticles (NPs) may be prepared but contain organic stabilizers that protect the surface of the oxide particles [26-30]. Embodiments of the present invention result in colloidal IrNi _y O _x NPs in relatively high concentrations as stable suspensions in water, without a stabilizer such as an organic stabilizer. This colloidal suspension may be formed by reacting an iridium salt, a nickel salt and hydroxide solutions in the presence of oxygen. The resulting NPs may be active towards the WOR in acid and in base, depending at least in part on the molar ratios between Ir and Ni. Advantageously, synthesis of the non-precious metal NP catalysts with the methods described herein may be completed in less than about 24 hours, compared to the multi-day procedures in the prior art for Ir-containing catalysts. [0017] As used herein, a nanoparticle is a particle that has at least one dimension which is less than about 100 nm, preferably less than about 50 nm, and more preferably less than about 10 nm. In one embodiment, the nanoparticles described herein have an average diameter of between about 1 to 2 nm.

[0018] Colloidal suspensions of iridium oxide NPs may be prepared by dissolving iridium chloride (IrCl3.3H ₂ 0) in water. Then, a hydroxide such as KOH or NaOH may be added slowly. Upon adding the base, the color of the solution changes from dark brown to green, and then to blue after about two days. The color does not change afterwards. The resulting IrOx NP colloidal suspension has been found to stable for about two months at room temperature. [0019] IrNiyOx mixed NPs may be prepared by adding a base, such as KOH, to solutions of IrCl ₃ and NiCl ₂ . The molar ratio (y) of Ni ²⁺ /Ir ³⁺ may be in the range of y = 0.01 to about 1.0. In a preferred embodiment, y may be about 0.05 to about 0.20. and more preferably y may be about 0.125. Where y > 0.25, precipitates may form from the resulting colloidal suspensions.

[0020] Experiments in which IrNiyO* nanoparticles were also made in pure 0 ₂ atmosphere, and pure N ₂ atmosphere were compared to those made under air. The NPs made in pure oxygen showed almost the same activity as the case made under air, while the NPs made in pure nitrogen took a longer time (about a week overall) to give appearance and color of the final product (dark bluish purple), and although still active, demonstrated inferior activity to those made under air and 0 ₂ . [0021] Experiments with the same procedure were tested to make Ir-Co, Ir-Fe, Ir-Pd, and Ir- Ru. All gave inferior performances to the Ir-Ni and to pure Ir. Without restriction to a theory, the enhanced activity of Ir-Ni is not believed to be the result of the physical presence of a second metal, but rather the specific combination of iridium and nickel.

[0022] In an alternative embodiment, colloidal suspensions of IrNiyO* nanoparticles with low Ir content may be made with the same procedure. The resulting NPs may be used in electrodes used in WOR in basic solution. Better activities than pure Ni may be achieved where the molar ratio of Ir/Ni is in the range of between about 0.05 to about 0.5 (2<y <20), and preferably between about 0.1 and 0.2 (4<y <8).

[0023] In either case, the resulting colloidal suspension may be used to coat or form electrodes in a conventional manner. Working electrodes may be prepared by mixing the colloidal suspension with a binder such as an ionomer such as Nafion™, and coating an electrode with the mixture. In one embodiment, a IrNi _y O _x NP suspension may be diluted with a solution of Nafion™, to give a final 1% wt. Nafion™ per total mass of Ir + Ni. This suspension may then be sonicated or otherwise well agitated to ensure homogeneity, and applied to an electrode using standard techniques.

[0024] In one embodiment, no heat treatment is necessary, and the IrNiyO* nanoparticles may be used without a separation or refining step. Also, no potential cycling or activation is necessary before use of the electrode. In one embodiment, heat treating the electrode, for example overnight (~12 hours) at 60° C may improve its properties. [0025] In one embodiment, the as-prepared IrNi _y O _x NPs can be used directly to prepare inks for WOR catalysts on an electrode, such as a glassy carbon electrode. In testing, the WOR activity in acid of the deposited IrO _x colloid was excellent, 90 A g ^"1 Ir at 1.48 VRHE, and is considerably increased by adding Ni. In one preferred embodiment, the atomic ratio of Ir to Ni is 8/1 (GC/IrNio.i ₂₅ Ox), giving 140 A g ¹ Ir at 1.48 VRHE. The highest activity for an Ir/Ni catalyst reported in the literature is ~ 40 A g ^"! Ir at 1.48 VRHE (0.25 V overpotential). Catalysts with higher relative loadings of Ni are less active, and those with Ir/Ni less than 16/1 had activities similar to IrO _x . The moderate-term stability of IrO _x and IrNi ₀ .i ₂ 5O _x towards the galvanostatic WOR at 1 mA cm ^"2 were similar, with the IrNio.mOx operating at ~ 50 mV lower than that of IrO _x , and ~ 100 mV lower than those of the most active IrNi systems in the literature [10].

[0026] EASA (Electrochemical active surface area) measurements showed that the improvement in activity upon addition of Ni was not a surface area effect, and the low amount of Ni in the optimum catalyst (Ir:Ni = 8: 1) suggests that Ni is not acting in a third-body effect, either [33]. [0027] 24-hour constant- current (1 mA cm ^"2 ) water oxidation tests on the Ir-Ni and pure Ir in acid were performed for comparison of longevity. Both showed very good durability, while the Ir-Ni was even more stable than pure Ir. This is somewhat surprising as Ni is readily soluble in acid over high potentials. Without restriction to a theory, it may be that Ni is trapped inside the nanoparticle structure. HRTEM analysis has confirmed that the lattice spacing in the mixed Ir-Ni oxide nanoparticles (0.22 nm) matches the (200) plane d-spacing of the IrO reported in the literature [34]. Also, elemental mapping of a single nanoparticle has shown that the Ni is distributed throughout the nanoparticle. Therefore, it is believed that Ir forms an octahedron by connecting with 6 surrounding oxygen atoms, and in some octahedra, some Ir is replaced by Ni. The structure is a continuous network connected by O.

[0028] Examples - The following examples are provided solely to illustrate specific embodiment of the present invention, and are not intended to limit the claimed invention. [0029] Sulfuric acid (Alfa Aesar; 99.9999%), potassium hydroxide (Aldrich; semiconductor grade, 99.99%), iridium chloride trihydrate (A.B. Mackay Chemicals), nickel chloride (alfa Aesar; anhydrous, 98%), and Nafion™ (5 wt%, ElectroChem, Inc.) were used as received. Triply distilled water was used throughout the experiments.

[0030] Preparation of nanoparticle solutions [0031] Colloidal suspensions of the mixed metal oxide NPs were prepared by modifying the procedure described for iridium oxides [17]. To prepare a pure Ir-oxide NP suspension (IrO _x NP), 0.0704 g IrCl ₃ .3H ₂ 0 was dissolved in 20 mL water (i. e. 0.01 M solution of Ir ³⁺ ) to form a dark brown solution. Then, 2 mL of 1 M KOH diluted to 2.5 mL was added dropwise over 5 min. Upon adding the KOH, the dark brown color changed first to green, and then to blue after two days. The color did not change afterwards. The IrO NP solution was then stored at room temperature. Control experiments (voltammetry in 0.1 M H ₂ S0 ) confirmed that the IrOx NP suspension was stable for about two months at room temperature. IrNi _y O _x (y:

Ni ²⁺ /Ir ³⁺ molar ratio) mixed NP were prepared by adding 1 M KOH to 20 mL solutions of IrCl ₃ and NiCl ₂ in the weights and volumes shown in Table 1. The molar ratios of the precursor mixtures were IrNio.o6, IrNio.125 IrNio.25, IrNio.s, and IrNii.o. There was no visual evidence of a precipitate after several weeks for the suspensions with y = 0-0.125. The nanoparticles had an average diameter of about 1.3 nm. Precipitates formed from suspensions with y > 0.25.

Table 1. Amounts used for the synthesis of the IrO _x and IrNi _y O _x NPs.

Ir/Ni ratio K¾ / g ^NiCh 8 Added KOH (1 M) / mL

Ir 0.0597 - 2

IrNii 0.0597 0.0259 4

IrNio.5 0.0597 0.0130 3

IrNio.25 0.0597 0.0065 2.5

IrNio. i25 0.0597 0.0032 2.25

IrNio.06 0.0597 0.0016 2.15

[0032] Preparation of working electrodes

[0033] The IrO _x or IrNi _y O _x NP suspensions were diluted by a factor of five using water that contained the appropriate amounts of Nation™ to give a final 1% wt. Nafion™ per total mass of Ir + Ni, calculated from the amounts in Table 1. Using a micropipette and graded microtips, 10 μΐ, of the NP/Nafion™ suspension (sonicated for 2 minutes) was drop coated onto a bare glassy carbon (GC; 0.196 cm ² ) electrode. The ink was dried over 20 minutes at 60° C then left at room temperature for 20 minutes. To check the accuracy of the metal weighing, aliquots of the NP suspensions (for IrO _x and IrNiiOx) that nominally contained 3.4 μg Ir were analyzed by ICP-MS. The accuracy of the nominal mass was ~ 95% that of the actual ICP-MS value. [0034] The electrochemical experiments were performed with a Solartron SI 1287

Electrochemical Interface controlled by CorrWare for Windows Version 2-3 d software. The reference electrode was a saturated calomel electrode (SCE; Fisher Scientific); however, all potentials in this paper are reported versus reversible hydrogen electrode (RHE). A graphite rod formed the counter electrode. Uncompensated resistance was estimated by impedance (20 Ω) and corrected for. SEM was performed with a Hitachi S-4800 instrument. A JEOL 201 1 transmission electron microscope (The Microscopy and Microanalysis Facility, University of New Brunswick) was used for TEM analysis. Samples for the inductively coupled plasma- mass spectrometry (ICP-MS) analysis were dissolved in 0.3 M HNO3 and measured with Perkin Elmer Elan 6000. [0035] Fig. 1 shows the SEM (A, on GC with Nafion™) and TEM (B, on Cu grid) of the IrO _x NPs. The average particle size, ~ 1 nm, and the disparity were consistent with those reported previously [17]. Fig. 1 (C) shows the voltammetric profile (0 - 1.25 V, 50 mV s ^"1 ) of the IrO _x NP/Nafion™ deposit on GC in 0.1 M H2SO4. The profile contains Ir(III)-Ir(IV) redox peaks at 1.07 (anodic) and 0.93 VRHE (cathodic) [31], along with other oxidation and reduction processes that are evident from the ill-defined shoulders on both directions. There are no hydrogen signals, indicating that metallic Ir is absent from the deposit. Fig ID shows the WOR activity of the IrO _x NP deposit (with 3.4 μg Ir) in the extended anodic range in 0.1 M H2SO4. The oxidation onset is ~ 1.42 VRHE, corresponding to ~ 0.19 V overpotential. The mass-corrected WOR activity at 1.48 VRHE (i.e. 0.25 V overpotential) is ~ 93 A g ^"1 Ir. [0036] A series of IrNi _y O _x NPs were prepared using the same procedure with adding N1CI2.

Fig. 2A plots the mass normalized (relative to Ir) activities of the GC/Ir, GC/IrNi ₀ , ₀₆ , GC/IrNio.ns, GC/IrNi ₀ .2s, GC/IrNio.s, and GC/IrNij oxide deposits in 0.1 M H ₂ S0 ₄ . The presence of Ni increased the activity up to the atomic ratio of 8/1 Ir/Ni (GC/IrNi ₀ ,i ₂₅ ), beyond which the activity decreased. The mass activities of GC/IrNi ₀ . ₅ and GC/IrNii were ~ 76 and 61 A g ^"1 Ir at 1.48 VRHE, respectively, significantly lower than that of GC/IrO _x (93 A g ^"1 Ir). All of these electrochemical experiments were performed at least three times to ensure reproducibility. The electrochemical active surface areas (EASAs) of the GC/IrO _x and

GC/IrNiyOx electrodes were estimated by integrating voltammetric currents over the range 0.8 - 1.2 VRHE, and assuming one electron transfer for the Ir ³⁺ /Ir ⁴⁺ redox couple [10]. Figure 3 presents typical CVs of the GC/IrO _x and GC/IrNi _y O _x electrodes in 0.1 M H ₂ S0 ₄ . Figure 2B shows the specific activities (A mmor'lr ³ ^ ^4"1" ), showing nearly the same trend as the mass activity, with the GC/IrNio. i2 ₅ O _x deposit being the most active.

[0037] Fig 4A shows the extended anodic range voltammograms (main panel) in 0.1 M H ₂ S0 ₄ for the GC/IrO _x , GC/IrNi ₀ .i ₂₅ O _x , and GC (blank) electrodes to better illustrate the differences in WOR activity. The GC/IrNio.i ₂₅ O _x electrode is substantially superior to the GC/IrOx electrode over the range, whereas the onset potentials appear similar. Galvanostatic WOR at 1 mA cm ^"2 _ge ometric was conducted to indicate the relative stability of these catalysts [10, 1 1]. The GC/IrO _x and GC/IrNio. i2 ₅ O _x electrodes produced similar durability patterns, and showed no significant (over)potential increase over the measurement timescale. The potential of the GC/IrNio. i2 ₅ Ox catalyst was offset at ~ 1.51 VRHE, about 50 mV smaller than that of the GC/IrOx catalyst. TEM of the IrNio. i2 ₅ O _x in Fig. 4B shows similar particles sizes and shapes to the IrOx (Fig. IB). Consistently, the EASA of the GC/IrO _x and GC/IrNio, i2 ₅ O _x electrodes are 0.578 and 0.570 cm ² (0.396 mC cm ^"2 conversion coefficient [32]), respectively. Definitions and Interpretation

[0038] All terms and phrases used in this specification have their ordinary meanings as one of skill in the art would understand, except where specifically defined. Such ordinary meanings may be obtained by reference to technical dictionaries, such as Hawley 's Condensed

Chemical Dictionary 14 ^th Edition, by R.J, Lewis, John Wiley & Sons, New York, N.Y., 2001. [0039] To the extent that the following description is of a specific embodiment or a particular use of the invention, it is intended to be illustrative only, and not limiting of the claimed invention. The following description is intended to cover all alternatives, modifications and equivalents that are included in the spirit and scope of the invention, as defined in the appended claims. References in the specification to "one embodiment", "an embodiment", etc., indicate that the embodiment described may include a particular aspect, feature, structure, or characteristic, but not every embodiment necessarily includes that aspect, feature, structure, or characteristic. Moreover, such phrases may, but do not necessarily, refer to the same embodiment referred to in other portions of the specification. Further, when a particular aspect, feature, structure, or characteristic is described in connection with an embodiment, it is within the knowledge of one skilled in the art to affect or connect such aspect, feature, structure, or characteristic with other embodiments, whether or not explicitly described.

[0040] The corresponding structures, materials, acts, and equivalents of all means or steps plus function elements in the claims appended to this specification are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. [0041] It is further noted that the claims may be drafted to exclude any optional element. As such, this statement is intended to serve as antecedent basis for the use of exclusive terminology, such as "solely," "only," and the like, in connection with the recitation of claim elements or use of a "negative" limitation. The terms "preferably," "preferred," "prefer," "optionally," "may," and similar terms are used to indicate that an item, condition or step being referred to is an optional (not required) feature of the invention.

[0042] The singular forms "a," "an," and "the" include the plural reference unless the context clearly dictates otherwise. The term "and/or" means any one of the items, any combination of the items, or all of the items with which this term is associated.

[0043] As will be understood by one skilled in the art, for any and all purposes, particularly in terms of providing a written description, all ranges recited herein also encompass any and all possible sub-ranges and combinations of sub-ranges thereof, as well as the individual values making up the range, particularly integer values. A recited range (e.g., weight percents or carbon groups) includes each specific value, integer, decimal, or identity within the range. Any listed range can be easily recognized as sufficiently describing and enabling the same range being broken down into at least equal halves, thirds, quarters, fifths, or tenths. As a non-limiting example, each range discussed herein can be readily broken down into a lower third, middle third and upper third, etc. As will also be understood by one skilled in the art, all language such as "up to", "at least", "greater than", "less than", "more than", "or more", and the like, include the number recited and such terms refer to ranges that can be

subsequently broken down into sub-ranges as discussed above. In the same manner, all ratios recited herein also include all sub-ratios falling within the broader ratio. [0044] The term "about" can refer to a variation of ± 5%, ± 10%, ± 20%, or ± 25% of the value specified. For example, "about 50" percent can in some embodiments carry a variation from 45 to 55 percent. For integer ranges, the term "about" can include one or two integers greater than and/or less than a recited integer at each end of the range. Unless indicated otherwise herein, the term "about" is intended to include values and ranges proximate to the recited range that are equivalent in terms of the functionality of the composition, or the embodiment.

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