DOPED NANOCARBON CATALYSTS

Related Applications

[0001] This application claims benefit of priority under PCT Chapter I, Article 8, and 35 U.S.C. § 1 19(e) of U.S. Provisional Patent Application No. 61/981 ,485, entitled "DOPED NANOCARBON CATALYSTS," filed on April 18, 2014, and U.S. Provisional Patent Application No. 61/981 ,759, entitled "DOPED NANOCARBON CATALYSTS," filed on April 19, 2014, both of which are incorporated herein by reference.

Statement Regarding Federally Sponsored Research or Development [0002] This invention was made with government support under Grant No. DE-SC000101 1 awarded by the Department of Energy. The U.S. Government has certain rights in the invention.

Technical Field

[0003] This invention relates to catalysts, namely, electrocatalysts containing doped carbon nanomaterials optionally with nitrogen-containing polymer as a co- catalyst. Also disclosed are methods for reducing carbon dioxide to formate.

Background Art

[0004] Accumulation of carbon dioxide in the atmosphere has been considered as a major contributor to climate change through global warming. Once captured, C0 ₂ is a potentially useful feedstock if converted into formate/formic acid, CO, or more highly reduced hydrocarbon products. Electrochemical and

photoelectrochemical C0 ₂ reduction could become an integral part of an energy storage strategy with solar or wind-generated electricity used to store energy in the chemical bonds of carbon-based fuels. Electrochemical reduction of C0 ₂ has yet to be achieved on appropriately large scales due, in part, to the lack of efficient, robust catalysts operating at low overpotentials with high selectivities and current densities. Summary of Invention

[0005] We report here results of an ammonia plasma study on nitrogen and oxygen doping of carbon nanotubes. In some embodiments, nitrogen doping initiates electrocatalytic reduction of C0 ₂ to formate in aqueous solutions. We also report on a co-catalytic effect by an overlayer film of polyethylenimine (PEI), a polymer with amine functional groups which can be used as a C0 ₂ absorbent. The combination of N doping of carbon nanotubes and PEI overlayer in certain embodiments leads to a significant reduction in overpotential and enhanced Faradaic efficiencies and current densities for C0 ₂ reduction to formate in water. Broadly, carbon nanomaterials such as, for example, carbon nanotubes, carbon black, mesoporous carbon, graphite, graphene, or a combination of two or more thereof can be formed on or attached to an electrically-conductive surface such as, for example, glassy carbon, carbon papers, carbon cloth, or a gas diffusion electrode; optionally, the carbon

nanomaterial is graphenated before or after attaching to the electrically-conductive surface, or after forming on that surface; the carbon nanomaterial is doped with a catalytic activity enhancing dopant such as, for example, oxygen, nitrogen, boron, phosphorus, sulfur, or a combination thereof; and the carbon nanomaterial can be placed in catalytically-enhancing contact with a nitrogen-containing polymer such as, for example, polyethylenimine, poly(diallyldimethylammonium chloride), or poly(allylamine hydrochloride), or combinations of two or more thereof; to make an electrocatalytic electrode, according to certain embodiments of the present invention. Such an electrocatalytic electrode can be used to reduce carbon dioxide to formate, in further embodiments. Formate, or its protonated form formic acid, is used as a preservative and antibacterial agent in livestock feed, a coagulant in the production of rubber, a hydrogen storage material, and as the anode fuel in direct formic acid fuel cells.

[0006] Accordingly, some embodiments of the present invention relate to electrocatalytic electrodes, such an electrode comprising:

an electrically-conductive surface in electrical communication with at least one carbon nanomaterial, and a nitrogen-containing polymer in catalytically-enhancing contact with the at least one carbon nanomaterial. [0007] Other embodiments of the present invention relate to methods for reducing carbon dioxide to formate, comprising: providing an electrocatalytic electrode in a suitable electrocatalytic cell, wherein the electrocatalytic electrode comprises: an electrically-conductive surface in electrical communication with at least one carbon nanomaterial, and a nitrogen-containing polymer in catalytically- enhancing contact with the at least one carbon nanomaterial;

exposing the electrocatalytic electrode to a concentration of carbon dioxide in a fluid composition;

applying a reducing potential to the electrocatalytic electrode and allowing at least some of the carbon dioxide to react; thereby reducing the carbon dioxide to formate.

[0008] Still other embodiments relate to methods for making an

electrocatalytic electrode, comprising:

depositing on an electrically-conductive surface at least one carbon nanomaterial; exposing the at least one carbon nanomaterial to a plasma environment for a time sufficient to dope the at least one carbon nanomaterial with a catalytic activity enhancing amount of a catalytic activity enhancing dopant, thereby forming doped carbon nanomaterial; and optionally bringing at least one nitrogen-containing polymer into catalytically-enhancing contact with the doped carbon nanomaterial; thereby making the electrocatalytic electrode.

[0009] Additional embodiments relate to methods for reducing carbon dioxide to formate, comprising:

providing an electrocatalytic electrode in a suitable electrocatalytic cell, wherein the electrocatalytic electrode comprises: an electrically-conductive surface in electrical communication with a plurality of carbon nanotubes, and a nitrogen-containing polymer in catalytically-enhancing contact with the plurality of carbon nanotubes; exposing the electrocatalytic electrode to a concentration of carbon dioxide in a fluid composition;

applying a reducing potential to the electrocatalytic electrode and allowing at least some of the carbon dioxide to react; thereby reducing the carbon dioxide to formate.

[0010] Further additional embodiments involve electrocatalytic electrodes, comprising: an electrically-conductive surface in electrical communication with a plurality of carbon nanotubes, and a nitrogen-containing polymer in catalytically- enhancing contact with the plurality of carbon nanotubes. [001 1] Yet other embodiments involve methods for making an electrocatalytic electrode, comprising:

depositing on an electrically-conductive surface a plurality of carbon nanotubes; and exposing the carbon nanotubes to a plasma environment for a time sufficient to dope the carbon nanotubes with a catalytic activity enhancing amount of a catalytic activity enhancing dopant, thereby forming doped carbon nanotubes; thereby making the electrocatalytic electrode.

[0012] Throughout this application, the following abbreviations will be used:

CNT carbon nanotubes

CNT/GC carbon nanotubes on glassy carbon electrode

NCNT N-doped carbon nanotubes

NCNT/GC N-doped carbon nanotubes on glassy carbon electrode

OCNT O-doped carbon nanotubes

OCNT/GC O-doped carbon nanotubes on glassy carbon electrode

PEI polyethylenimine

PEI-NCNT polyethylenimine on N-doped carbon nanotubes

PEI-NCNT/GC polyethylenimine on N-doped carbon nanotubes on glassy carbon electrode

GCNT graphenated carbon nanotubes

NGCNT N-doped graphenated carbon nanotubes

NGCNT/GC N-doped graphenated carbon nanotubes on

glassy carbon electrode

PEI-NGCNT polyethylenimine on N-doped graphenated carbon nanotubes

PEI-NGCNT/GC polyethylenimine on N-doped graphenated carbon nanotubes on glassy carbon electrode

Sometimes, it should be noted, that 7GC" may be implied from context, such as, for example, when the word "electrodes" follows the naming of electrochemically-tested embodiments.

[0013] While the disclosure provides certain specific embodiments, the invention is not limited to those embodiments. A person of ordinary skill will appreciate from the description herein that modifications can be made to the described embodiments and therefore that the specification is broader in scope than the described embodiments. All examples are therefore non-limiting.

Brief Description of the Drawings [0014] Figure 1 depicts, for one embodiment of the present invention, a scheme showing fabrication of nitrogen-doped carbon nanotubes on glassy carbon electrodes (NCNT/GC) with an overlayer of polyethylenimine (PEI-NCNT/GC).

[0015] Figure 2 depicts, for one embodiment, (a) XPS spectra, (b) Raman spectra, and (c) formate partial current density Tafel plots at NCNT and PEI-NCNT. Tafel plot data were obtained in 0.1 M KHC0 ₃ /C0 ₂ (saturated) water.

[0016] Figure 3 depicts XPS spectra of carbon nanotubes (CNT) (a) before and (b) after ammonia plasma treatment.

[0017] Figure 4 depicts, for another embodiment, (a) XPS spectrum and (b)

Raman spectra of OCNT.

[0018] Figure 5 depicts XPS spectra of purified carbon nanotubes.

[0019] Figure 6 depicts, for another embodiment, cyclic voltammetry curves of various electrodes in in 5 mM K ₃ Fe(CN) ₆ /0.1 M KCI solution. Scan rate: 5 mV s-1.

[0020] Figure 7 depicts the structure of branched polyethylenimine (PEI).

[0021] Figure 8 (a) cathodic linear sweep voltammetry (LSV) scans at 50 mV s ^"1 in a C0 ₂ saturated aqueous 0.1 M KHC0 ₃ solution; (b) plot of Faradaic efficiencies for formate production vs. applied potential on CNT/GC, NCNT/GC, and

PEI-NCNT/GC electrodes.

[0022] Figure 9 depicts (a, b) side view SEM images of GCNT, and (c) controlled potential electrolyses at -1 .8 V vs. SCE at various electrodes in 0.1 M KHC0 ₃ /C0 ₂ (saturated) water, y ^' totai is the geometric current density.

[0023] Figure 10 depicts, for another embodiment, in situ electrochemical Raman spectra of PEI-NCNT at controlled potential electrolyses in 0.1 M KHC0 ₃ with C0 ₂ flow.

[0024] Figure 1 1 depicts formate partial current density Tafel plots at (a) PEI- NCNT and (b) NCNT electrodes. [0025] Figure 12 depicts different resolution SEM images of graphenated carbon nanotubes (a and b for cross section; c and d for top view) on silicon wafer and commercial CNT (e and f) on gold coated silicon wafer.

[0026] Figure 13 depicts XPS spectra of (a) as prepared GCNT, (b) NGCNT prepared by ammonia plasma, and (c) PEI-NGCNT.

[0027] Figure 14 depicts Raman spectra of some embodiments of GCNT, NGCNT, and PEI-NGCNT.

[0028] Figure 15 depicts Cyclic voltammetry (CV) curves of NGCNT/GC and PEI-NGCNT/GC electrodes in 5 mM K ₃ Fe(CN) ₆ /0.1 M KCI solution. Scan rate: 5 mV s

[0029] Figure 16 depicts a proposed mechanism for C0 ₂ reduction at PEI functionalized, nitrogen-doped carbon nanomaterials, in some embodiments of the present invention.

[0030] Figure 17 depicts deconvoluted N1 s spectra for NCNT.

Description of Embodiments

[0031] As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various forms. The figures are not necessarily to scale, and some features may be exaggerated to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention.

[0032] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as is commonly understood by one of ordinary skill in the art to which this disclosure belongs. In the event that there is a plurality of definitions for a term herein, those in this section prevail unless stated otherwise.

[0033] Where ever the phrase "for example," "such as," "including" and the like are used herein, the phrase "and without limitation" is understood to follow unless explicitly stated otherwise. Similarly "an example," "exemplary" and the like are understood to be non-limiting.

[0034] The term "substantially" allows for deviations from the descriptor that don't negatively impact the intended purpose. Descriptive terms are understood to be modified by the term "substantially" even if the word "substantially" is not explicitly recited.

[0035] The term "about" when used in connection with a numerical value refers to the actual given value, and to the approximation to such given value that would reasonably be inferred by one of ordinary skill in the art, including

approximations due to the experimental and or measurement conditions for such given value.

[0036] The terms "comprising" and "including" and "having" and "involving" (and similarly "comprises", "includes," "has," and "involves") and the like are used interchangeably and have the same meaning. Specifically, each of the terms is defined consistent with the common United States patent law definition of

"comprising" and is therefore interpreted to be an open term meaning "at least the following," and is also interpreted not to exclude additional features, limitations, aspects, etc. Thus, for example, "a device having components a, b, and c" means that the device includes at least components a, b and c. Similarly, the phrase: "a method involving steps a, b, and c" means that the method includes at least steps a, b, and c.

[0037] As stated above, certain embodiments of the present invention relate to electrocatalytic electrodes, comprising: an electrically-conductive surface in electrical communication with at least one carbon nanomaterial, and a nitrogen-containing polymer in catalytically-enhancing contact with the at least one carbon nanomaterial. Catalytically-enhancing contact can be detected when an electrode is tested in any suitable manner with and without the nitrogen-containing polymer, and the electrode with the polymer performs better. Any suitable components can be used on these electrocatalytic electrodes. In some cases, the electrically-conductive surface comprises glassy carbon, carbon paper, carbon cloth, or a combination thereof. In other cases, the electrically-conductive surface comprises a gas diffusion electrode. The electrocatalytic electrode is substantially free of metal catalysts, in further embodiments.

[0038] Any suitable carbon nanomaterial can be used. Certain instances provide at least one carbon nanomaterial chosen from carbon nanotubes, carbon black, mesoporous carbon, graphite, graphene, and combinations of two or more thereof. For example, a carbon nanomaterial such as carbon nanotubes can be graphenated. See Figures 9a and 9b. In other cases, mixtures of two or more carbon nanomaterials can be used. As used herein, a carbon nanomaterial has at least one dimension less than about ten microns. For example, as seen in Figures 9a and 9b, certain graphenated carbon nanotubes useful in some embodiments of the present invention have a diameter of about 2.5 microns or less. A carbon nanomaterial in some cases has no dimension greater than about 1 micron. "Carbon nanotubes" indicates single-walled carbon nanotubes, multi-walled carbon nanotubes, as-made or pristine carbon nanotubes, and graphenated carbon nanotubes. As demonstrated below, some embodiments comprise carbon

nanomaterial that is substantially free of iron impurity. In some cases, carbon nanotubes appear that are substantially free of iron impurity. Any suitable carbon nanotubes can be used on the electrocatalytic electrodes of the present invention. In some cases, the carbon nanotubes comprise graphenated carbon nanotubes.

[0039] The carbon nanomaterial further comprises a catalytic activity enhancing amount of a catalytic activity enhancing dopant, in some embodiments of the present invention. A catalytic activity enhancing amount of dopant is at least the minimum amount that can be observed to increase the catalytic activity of the carbon nanomaterial, according to any suitable test. In some cases, the catalytic activity enhancing amount ranges from about one to about twenty atomic percent of the catalytic activity enhancing dopant relative to the carbon nanomaterial. For example, a carbon nanotube has a catalytic activity enhancing amount of nitrogen atoms when it comprises at least about one atomic percent nitrogen. For another example, a carbon nanotube has a catalytic activity enhancing amount of nitrogen atoms when it comprises no more than about twenty atomic percent nitrogen. In other cases, the catalytic activity enhancing amount ranges from about 5 to about 10 atomic percent of the catalytic activity enhancing dopant relative to the carbon nanomaterial. In still other cases, the catalytic activity enhancing amount is about 7.6 atomic percent of the catalytic activity enhancing dopant relative to the carbon nanomaterial.

[0040] Any suitable catalytic activity enhancing dopant can be used. For example, oxygen, nitrogen, boron, phosphorus, sulfur, or a combination thereof, can be used to dope the carbon nanomaterial. Nitrogen appears as a catalytic activity enhancing dopant in certain instances. In some cases, the dopant comprises atoms of the dopant material. In other cases, clusters of atoms can be detected. Any suitable doping procedure can be used to dope the catalytic activity enhancing dopant into the carbon nanomaterial. For example, as illustrated below, suitable plasmas effectively introduced dopants into carbon nanotubes.

[0041] Any suitable nitrogen-containing polymer can be used. Certain instances provide a nitrogen-containing polymer comprising secondary amine groups, tertiary amine groups, or a combination thereof. Certain other instances provide the nitrogen-containing polymer comprising polyethylenimine,

poly(diallyldimethylammonium chloride), poly(allylamine hydrochloride), or a combination thereof. Any suitable amount of nitrogen-containing polymer can be used. In some cases, the nitrogen-containing polymer is present in an amount ranging from about 5% to about 20% by mass, relative to the weight of the doped carbon nanomaterials. In other cases, the nitrogen-containing polymer is present in an amount ranging from about 10% to about 15% by mass, relative to the weight of the doped carbon nanomaterials. Still other cases provide the nitrogen-containing polymer being present in an amount of about 12.5% by mass, relative to the weight of the doped carbon nanomaterial.

[0042] Certain other embodiments relate to methods for reducing carbon dioxide to formate, comprising: providing an electrocatalytic electrode in a suitable electrocatalytic cell, wherein the electrocatalytic electrode comprises: an electrically- conductive surface in electrical communication with at least one carbon

nanomaterial, and a nitrogen-containing polymer in catalytically-enhancing contact with the at least one carbon nanomaterial; exposing the electrocatalytic electrode to a concentration of carbon dioxide in a fluid composition; applying a reducing potential to the electrocatalytic electrode and allowing at least some of the carbon dioxide to react; thereby reducing the carbon dioxide to formate. [0043] A suitable electrocatalytic cell relates to any arrangement sufficient to cause electrocatalysis at an electrocatalytic electrode. Any suitable electrocatalytic electrode can be used, such as those described herein. Two-electrode cells, containing the electrocatalytic electrode in ionic communication through an electrolyte with a counter electrode such as platinum or other suitable material, can be mentioned. Three-electrode cells, further including a suitable reference electrode, also can be mentioned. The electrodes of the cell should be connected through a suitable external circuit so that a reducing potential can be applied to the electrocatalytic electrode.

[0044] The cell should provide a way to expose a concentration of carbon dioxide in a fluid composition to the electrocatalytic electrode. Any suitable fluid composition can be used, such as, for example, liquid, semi-liquid such as a slurry, gel, aerosol, vapor, or gas. The fluid can be stationary, agitated such as by stirring or bubbling C0 ₂ -containing gas, or arranged in a flowing manner to bring fresh C0 ₂ to the electrocatalytic electrode and to remove formate. In some cases, the fluid composition comprises water. In further cases, the fluid composition comprises carbonate salts, bicarbonate salts, phosphate salts, biphosphate salts, or a combination thereof. Certain cases provide the fluid composition comprising potassium bicarbonate, sodium bicarbonate, or a combination thereof. The fluid composition can comprise carbon dioxide, and can be saturated with carbon dioxide, such as, for example, by bubbling C0 ₂ or a C0 ₂ -containing gas such as C0 ₂ /N ₂ or air through the fluid composition.

[0045] Any suitable reducing potential can be applied to the electrocatalytic electrode. In some cases, the reducing potential can be at least -1 V versus SCE, or at least -1 .2 V versus SCE. In other cases the reducing potential is no more reducing than about -1 .8 V versus SCE, or no more reducing than about -2.0 V versus SCE.

[0046] Exposing the electrocatalytic electrode to a concentration of carbon dioxide can occur before, after, or at the same time as applying a reducing potential to the electrocatalytic electrode. The reduction of carbon dioxide to formate can proceed in a batch manner, a continuous manner, or a combination thereof.

[0047] Still other embodiments relate to methods of making the

electrocatalytic electrodes described herein. Any suitable methods can be used to make the electrodes and the components thereof. Certain embodiments relates to method for making an electrocatalytic electrode, comprising: optionally graphenating at least one carbon nanomaterial; depositing on an electrically-conductive surface the at least one carbon nanomaterial; and exposing the at least one carbon nanomaterial to a plasma environment for a time sufficient to dope the at least one carbon nanomaterial with a catalytic activity enhancing amount of a catalytic activity enhancing dopant, thereby forming doped carbon nanomaterial; and optionally bringing at least one nitrogen-containing polymer into catalytically-enhancing contact with the doped carbon nanomaterial; thereby making the electrocatalytic electrode. Graphenating a carbon nanomaterial can occur according to any suitable method, such as, for example, the microwave plasma-enhanced chemical vapor deposition method described below. Depositing the carbon nanomaterial can occur according to any suitable method. Dip coating, spin coating, painting, spraying, drop casting, and combinations thereof can be mentioned. Exposing the carbon nanomaterial to a plasma environment can occur by any suitable method. A time sufficient to dope the at least one carbon nanomaterial with a catalytic activity enhancing amount of a catalytic activity enhancing dopant includes any suitable time. In some cases, the time sufficient to dope the at least one carbon nanomaterial is at least one minute. In other cases, the time sufficient to dope the at least one carbon nanomaterial is no more than about one hour. In still other cases, the time sufficient to dope the at least one carbon nanomaterial is at least about 40 minutes. Bringing at least one nitrogen-containing polymer into catalytically-enhancing contact with the doped carbon nanomaterial, or contacting the electrocatalytic electrode with a nitrogen- containing polymer, can occur by any suitable method. Dip coating, spin coating, painting, spraying, drop casting, in situ polymerization, and combinations thereof can be mentioned.

Preparation and C0 ₂ reduction reactivity of nitrogen-doped carbon nanotubes.

[0048] As described in Figure 1 , multi-walled carbon nanotubes (CNT) were first dispersed in dimethylformamide (DMF) by sonication to yield a homogeneous CNT suspension which was drop cast onto a pre-polished glassy carbon (GC) electrode. Nitrogen doped carbon nanotubes (NCNT) were synthesized by exposing the CNT/GC electrodes to an ammonia plasma. The plasma treatment is a facile, room temperature doping method with the dopant content varied by changing plasma power intensities, chamber pressures, and exposure time. In the present study, the extent of nitrogen doping was controlled by varying exposure time with the results, monitored by XPS analysis (Figures 2a and 3), shown in Table 1 .

Table 1 The relationship between Faradaic efficiencies for formate, nitrogen contents, and ammonia plasma exposure time. The doped nitrogen content in CNT first increased with exposure time and then leveled off after 40 min of treatment. The maximal Faradaic efficiencies for formate (59 %) were achieved using NCNT with nitrogen content over 7 %, further N-doping did not show any observable improvement in electrolyses.

Ammonia plasma Nitrogen content Faradaic efficiency

Samples

exposure time / min % for formate / %

As received 0 0 5

\# 10 3.6 23

2# 15 5.2 42

3# 20 7.6 59

4# 40 9.2 54

5# 60 9.3 56

[0049] From these results, the nitrogen content in the doped CNT first increased with exposure time, leveling off after 40 min of exposure, to give the NCNT coated glassy carbon electrodes (NCNT/GC) used in subsequent electrochemical experiments.

[0050] Oxygen doped carbon nanotubes on glassy carbon (OCNT/GC) were prepared by using a similar procedure but with use of an oxygen plasma. Physical characterization, Figures 3 and 4, confirmed the doping of nitrogen and oxygen into the CNT. The plasma treatment provided a reliable method for obtaining consistent dopant concentrations in the electrochemical experiments. As seen in Figure 3a, no obvious N1 s peak was found at as received carbon nanotubes ("CNT"). By contrast, after ammonia plasma treatment, N1 s peak was clearly observed, indicating the successful nitrogen doping into CNT, producing "NCNT." See Figure 3b. The relationship between nitrogen content and exposure time is summarized in Table 1 : the doped nitrogen content in CNT first increased with exposure time and then leveled off after 40 min of treatment.

[0051] After oxygen plasma, oxygen content increased from 4.6 % to 14.8 % in carbon nanotubes. Looking at the Raman spectra of as-received CNT and oxygen-doped CNT ("OCNT") in Figure 4b, the G band is related to the in-plane bond-stretching motion of pairs of sp ² -C atoms. The D band ("disordered" band) is the breathing mode of the sp ² -rings of the graphene layer that is related to a series of defects: bond-angle disorder, bond-length disorder, and hybridization, which are caused by heteroatom (nitrogen/oxygen) doping and structure defects by plasma treatment. So the ratio of D band over G band increased from 1.120 to 1.206 after oxygen plasma treatment, which can be attributed to the oxygen doping into carbon nanotubes. Figure 4a shows the XPS spectrum of OCNT.

Prior to electrochemical measurements, the doped electrodes were subjected to an additional electrochemical purification step (see Examples) to remove residual Fe particles within the carbon nanotubes. After purification, there was no residual peak for elemental Fe by high resolution XPS analysis, note Figures 5a and 5b. The electroactive surface areas of the electrodes were evaluated by cyclic voltammetry (CV) with the ferri-/ferrocyanide couple

([Fe(CN) ₆ ] ^3" ' ^4" ) used as the reference probe (see Figure 6). Specifically, the electro- active surface areas of as-made electrodes were evaluated by cyclic voltammetry (CV) with [Fe(CN) ₆ ] ^3" ' ^4" as a probe in 0.1 M KCI solution at a scan rate of 5 mV s ^' From the CV scans in Figure 6, it can be calculated according to the Randles-Sevcik equation at room temperature: / _p =2.69* 10 ⁵ *n ³ *A*D ^l *v ^l *C where A is electrochemical active surface area (cm ² ), / _p is peak current (A), and n=1 , D=4.34*10 ^"6 cm ² s ^"1 , v is scan rate (V s ^"1 ), C is the concentration of potassium ferricyanide (5*10 ^"6 mol cm ^"3 ). Electrochemical active surface area were calculated to be 0.228 cm ² for PEI-NCNT, 0.161 cm ² for NCNT, 0.185 cm ² for O-CNT, 0.146 for CNT, and 0.098 cm ² for GC electrode. This showed electroactive surface areas have increased after ammonia and oxygen plasma treatment.

The increase can be attributed to the fact that the surface layer of carbon nanotubes was partially etched away by the plasma during the nitrogen or oxygen doping allowing accessibility of inner carbon layers to the external solution.

[0052] Electrocatalytic C0 ₂ reduction by CNT/GC, NCNT/GC and OCNT/GC electrodes was evaluated by controlled potential electrolyses in 0.1 M KHCO ₃ solution saturated with C0 ₂ (Tables 1 , 2, and 3). The products of C0 ₂ reduction were analyzed by both ¹ H-NMR for the liquid phase and gas chromatography for the headspace. Formate, hydrogen, and trace amounts of carbon monoxide were produced during the electrolyses. Both NCNT and OCNT electrodes exhibited twofold higher current densities (over 3 mA/cm ² ) relative to CNT (1 .3 mA/cm ² ), but the reduction products differ significantly. Optimized NCNT electrodes demonstrated significantly higher Faradaic efficiencies for formate (59 %) than either OCNT (7 %) or CNT (5 %), with 39 % H ₂ and 2 % of CO also formed. OCNT and CNT electrodes mainly gave H ₂ as the product (>90 %) with a small amount of CO (<1 %). Maximum Faradaic efficiencies for formate were achieved by using NCNT electrodes with a nitrogen content of > 7 At. % with further N-doping providing no further improvement. These results suggest that nitrogen doping plays a non-trivial role in C0 ₂ reduction to formate.

Polyethylenimine co-catalysis and enhanced C0 ₂ reduction activity.

[0053] PEI, a polymer with multiple amine groups (see Figure 7 for structure), has a high adsorption capacity and selectivity toward C0 ₂ adsorption. In aqueous solutions it is positively charged because of partial protonation at the amine nitrogens with pK _a of 7-9 under neutral pH conditions in the external solution. It can be attached to the surfaces of carbon nanotubes through non-covalent, dispersion interactions based on van der Waals forces, driven largely by elimination of the hydrophobic interface between the carbon nanotubes and water. [0054] In some embodiments, a PEI overlayer was applied to NCNT/GC electrodes by dip-coating followed by rinsing with excess water to obtain PEI- NCNT/GC electrodes. Based on the XPS spectra in Figure 2a, the nitrogen content was increased from 7.6 At. % in NCNT to 1 1 .3 At. % at the PEI-NCNT interface demonstrating the presence of the polyethylenimine at the surfaces of the carbon nanotubes in -17 mass %. Raman spectra, Figure 2b, show that PEI

functionalization of the nanotube surfaces results in a slight increase in the intensity ratio of the D band over the G band, consistent with a slightly more disordered nanotube structure. A decrease in peak position of ~4 cm ^"1 was also observed suggesting that charge transfer from the electron-donating PEI to carbon nanotubes induces slightly more disorder in the underlying carbon nanotube structure. This observation is also consistent with reports of PEI used as an electron donor to modify CNT.

[0055] PEI functionalization reduced catalytic overpotential and enhanced both Faradaic efficiency and current densities for formate production. As shown in Figure 8a, PEI-NCNT exhibited a more positive onset potential than the other two electrodes, NCNT and CNT, subjected to Cathodic linear sweep voltammetry (LSV) scans at 50 mV s ^"1 in a C0 ₂ -saturated aqueous 0.1 M KHC0 ₃ solution. Figure 9c shows the result of a controlled potential electrolysis of a PEI-NCNT/GC electrode at -1 .8 V vs. SCE in 0.1 M KHC0 ₃ /C0 ₂ (saturated) water for 24 h which resulted in a steady state catalytic current density of 7.2 mA/cm ² . Electrodes having PEI-NGCNT, NCNT, and CNT were also tested, and geometric current densities, y ^' totai, are shown in Figure 9c. The appearance of sustained currents shows that the PEI overlayer is stable on the surface of NCNT.

[0056] Formate was shown to be the dominant electrolysis product. It was formed in 85 % yield at PEI-NCNT/GC electrodes and detected directly by in situ electrochemical Raman monitoring (see Figure 10 for details). Figure 10 depicts in situ electrochemical Raman spectra of PEI-NCNT at controlled potential electrolyses in 0.1 M KHC0 ₃ with C0 ₂ flow. The weak peak at around 1069 cm ^"1 can be assigned to the C-H bending of HCOO ^" produced during C0 ₂ reduction. As shown in Figure 8b, the Faradaic efficiency for formate at PEI-NCNG/GC electrodes reaches a maximum at -1 .8 V and decreases at more negative potentials due to an increase in competing background hydrogen evolution. Figure 8b plots Faradaic efficiencies for formate production vs. applied potential on CNT/GC, NCNT/GC, and PEI-NCNT/GC electrodes.

Table 2 Current densities (J) and Faradaic efficiencies for formate (F _f0 rmate) during controlled potential electrolyses at -1 .8 V versus SCE in 0.1 M KHCO3/CO2 aqueous solution at various electrodes.

Samples CNT OCNT NCNT ^ ^EI^ j geometric ^3 g g _{3 Q} 3 3 _{J 2} 9 5

/(mA/cm )

Velectroactive

0.6 1 .4 1 .3 1 .3 2.2 3.8

/(mA/cm ² )

formate /% 5 7 59 8 85 87

Table 3 Current densities and Faradaic efficiencies during Controlled potential electrolyses at -1.8 versus SCE in 0.1 M KHCO3/CO2 aqueous solution at various electrodes.

PEI- PEI- PEI-

Samples CNT OCNT NCNT NGCNT

CNT NCNT NGCNT

Current

density

/ mA cm ^"2 1.3 3.6 3.0 3.9 3.8 7.2 9.5 geometric

Current

density

/ mAcm ^"2 0.6 1.4 1.3 2.0 1.3 2.2 3.8 electroactive

Faradaic

efficiency

for

5 7 59 59 8 85 87 formate

/ %

Partial

current

density for

0.03 0.09 0.78 1.16 0.10 1.88 3.28 formate/

mA cm ^"2

electroactive [0057] Current densities normalized for electroactive surface area in Table 2 were 2.2 mA/cm ² for PEI-NCNT and 1 .3 mA/cm ² for NCNT. Reduction at -1 .2 V (an overpotential of 0.54 V for C0 ₂ reduction with £° = -0.66 V vs. SCE for the

C0 ₂ /HCOO ^" couple at pH = 6.8) produced formate in 4 % yield at PEI-NCNT. By contrast, the onset potentials for C0 ₂ reduction to formate were ca. -1 .8 V for CNT and ca. -1 .4 V for NCNT. These results suggest that PEI functions as a co-catalyst in promoting C0 ₂ reduction at NCNT.

Tafel plots for C0 ₂ reduction are shown in Figure 2c. From these data, Tafel slopes were 142 mV/dec and 134 mV/dec for NCNT/GC and PEI-NCNT/GC electrodes, respectively (Figures 1 1 a and 1 1 b). Specifically, Tafel plot data were collected in 0.1 M KHCO ₃ /CO ₂ (saturated) aqueous solution. The Tafel relationship for formate production can be derived as:

η =E - E ₀ = b · log/o - b · log _formate ( 1 ) a=2.3RT/bF (2) where E is the applied potential, E ₀ is the equilibrium potential ( - 0.67 V vs. SCE for the CO ₂ /HCOO ^" couple) in pH6.8 aqueous solution, η is the overpotential for C0 ₂ /HCOO ^~ couple, b is the Tafel slope, a is transfer coefficient, k is the exchange current density, and /formate is the partial current density for formate production during C0 ₂ reduction.

[0058] Both Tafel slope values for NCNT/GC and PEI-NCNT/GC electrodes are close to 1 18 mV/dec which is expected for rate limiting single electron transfer at the electrode. Transfer coefficients (a) were 0.42 and 0.44 at NCNT/GC and PEI- NCNT/GC. Exchange current densities, / ₀ , were ca. 1 .4*10 ^"7 and 4.6x 10 ^"7 mA/cm ² for NCNT and PEI-NCNT, respectively. The / ₀ value for PEI-NCNT is about three fold higher than for NCNT. It is also larger than the / ₀ value at Hg (1 .5x10 ^"9 mA/cm ² ) and approaches /Ό for Sn (1 .2x10 ^"6 mA/cm ² ) in both cases for C0 ₂ reduction to formate. The magnitude of /Ό reflects the free energy barrier to C0 ₂ reduction at the reversible potential and is a measure of the intrinsic catalytic activities of electrode materials and interfaces. [0059] From the comparisons presented here, the intrinsic catalytic activity of PEI-NCNT is comparable to that for the best metal electrodes for C0 ₂ reduction to formate.

[0060] Graphenated carbon nanotubes (GCNT) were utilized to further increase catalytic current density. With its unique three dimensional (3D)

nanostructure, it has shown promise as an electrocatalytic substrate. GCNT has the advantage of high electrical conductivity and offers both the high surface area framework of CNTs combined with the high edge density and reactivity of graphene nanosheets. In our studies, GCNT was prepared by chemical vapor deposition, resulting in graphene foliates along the length of aligned carbon nanotubes (see ref. 19 for details of the synthesis), and then transferred onto GC electrodes (GCNT/GC). N-doped NGCNT/GC and PEI-NGCNT/GC electrodes were obtained by using the procedure in Figure 1 .

[0061] Figures 9a and 9b depict SEM cross sectional images of a typical film (with ~ 20 μιη thick films shown in Figure 12) of aligned carbon nanotubes with graphene foliates along their length, at different resolutions. For the embodiments imaged in Figures 12a-12f, it is apparent that three dimensional GCNT have porous structure with much larger pore size than CNT, which may facilitate the mass transport of C0 ₂ through them. The diameter of GCNT is about 200 nm, much larger than that of commercial CNT. XPS spectra in Figure 13 and Raman spectra in Figure 14 were used to demonstrate adsorption of PEI on the surface of NGCNT. Specifically, XPS spectra of as-prepared GCNT, NGCNT prepared by ammonia plasma, and PEI-NGCNT, appear in Figures 13a, 13b, and 13c, respectively. No obvious N1 s peak was observed at as prepared GCNT, while nitrogen contents were evaluated to be about 7.8 mass% for NGCNT and 12.6 % for PEI-NGCNT. Raman spectra of GCNT, NGCNT, and PEI-NGCNT appear in Figure 14. The ratios of D band over G band increased and their peak positions negatively shifted in the following sequence: GCNT, NGCNT, and PEI-NGCNT. This indicates the successful nitrogen doping and PEI overlay coating. Electrolyses at PEI-NGCNT/GC electrodes (Figure 9c) were conducted at -1 .8 V for 24 h in C0 ₂ saturated 0.1 M KHC0 ₃ aqueous solution. The Faradaic efficiency for formate production at PEI-NGCNT/GC was 87 %, comparable to that found for PEI-NCNT. The geometric current density was increased to 9.5 mA/cm ² and the catalytic current density, normalized for electroactive surface area (Figure 15), of 3.8 mA/cm ² was also higher than for PEI- NCNT. The cyclic voltammetry (CV) curves of NGCNT/GC and PEI-NGCNT/GC electrodes in 5 mM K ₃ Fe(CN) ₆ /0.1 M KCI solution were measured at a scan rate of 5 mV s ^"1 , and are shown in Figure 15. The electroactive surface areas were calculated to be about 0.136 cm ² for NGCNT/GC and 0.175 cm ² for PEI-NGCNT/GC, which are smaller than those for NCNT/GC and PEI-NCNT/GC electrodes. This can be attributed to the much larger diameter of GCNT than CNT, as shown in Figure 12. The rate enhancement can be attributed to higher edge densities and C0 ₂ transport rates for the three dimensional graphenated carbon nanotubes compared to conventional carbon nanotubes. The results summarized in Table 2 and Table 3 also confirm that PEI functionalization significantly improves performance toward electrocatalytic C0 ₂ reduction at the nitrogen-doped, graphenated carbon

nanotubes.

[0062] The co-catalytic role of PEI is a notable finding. It significantly improves performance toward C0 ₂ reduction with nitrogen doped carbon

nanomaterials. PEI functionalized, but undoped, PEI-CNT electrodes did not exhibit the same magnitude of improvement over CNT electrodes (Table 2). This points to a concerted interaction between PEI and the NCNT interface in a rate limiting step or steps in the catalytic reduction of C0 ₂ that is absent with a PEI overlayer on pristine CNT. It is expected that a similar effect will be observed with nitrogen-containing polymers interacting with carbon nanomaterials doped with other dopants, such as oxygen, boron, phosphorus, and sulfur. See Table 2.

[0063] The results of the Tafel analysis of the data in Figure 2c point to rate- determining electron transfer to C0 ₂ to give the C0 ₂ ^" anion radical. The

thermodynamic potential for C0 ₂ reduction to C0 ₂ ^" is -1 .90 V vs. SHE compared to - 0.43 V for 2e ^" reduction to HCOO ^" at pH 7. With nitrogen doping, the single-electron reduction onset shifts anodically, to -1 .4 V at NCNT (Figure 8), a net 400 mV decrease in overpotential compared with that for CNT.

[0064] From the high resolution N1 s XPS spectra of NCNT in Figure 17, pyridinic-N (62.5%) and pyrrolic-N (23.7%) are the dominant nitrogen states. In both, the N atoms are polarized negatively due to electron withdrawing effects in the graphene π system with the adjacent carbon atoms polarized positively. Specifically, Figure 17 elucidates the existence of four main nitrogen species, that is, for pyridinic N (B.E. ~ 398.9 eV), pyrrolic N (B.E. ~ 400.1 eV), quaternary N (B.E. ~ 401 .5 eV), and nitrogen oxide (B.E. ~ 402.2 eV). The quantitative analyses demonstrate that the fraction of the various nitrogen species is approximately, 62.5% for pyrrolic, 23.7 % for pyridinic, 8.2 % for quaternary, and 5.6 % for nitrogen oxide. These doped nitrogen would increase the density of electrons of graphene at its Fermi level and open the band gap of carbon nanotubes. The pyridinic-N and pyrrolic-N were the dominant nitrogen states. In the proposed mechanism in Figure 16, without wishing to be bound by theory, C0 ₂ is presumably first adsorbed to the basic nitrogen binding sites in NCNT where it is reduced to C0 ₂ ^'" . The PEI overlayer may stabilize C0 ₂ ^" by a hydrogen bond interaction,

NCNT-N-C(0)0 ^" H-N-PEI, thus lowering the onset potential for reducing C0 ₂ to

C0 ₂ ^" by creating a stabilizing environment. This hypothesis is supported by the fact that the overpotential for C0 ₂ reduction to formate is further reduced by 200 mV after adding the PEI overlayer to NCNT. In addition, given its known ability to adsorb C0 ₂ , the adsorbed PEI overlayer may concentrate C0 ₂ on the electrode surface from the bulk solution increasing its effective local concentration.

[0065] In the mechanism in Figure 16, once formed, the stabilized C0 ₂ ^°" radical is protonated and further reduced with the probable proton source HCO ₃ ^" given its pKa (10.33) compared to H ₂ 0 (15.7). Protonation is followed by, or in concert with, a rapid, second electron transfer reduction to give formate as the product.

Industrial Applicability

[0066] In summary, ammonia plasma treatment with N doping followed by adsorption of PEI, for example, and oxygen dopants for another example, have been used here to create a facile and efficient local environment for selective reduction of C0 ₂ to formate on carbon surfaces. The plasma treatment results in the doping of high surface area carbon nanomaterials with nitrogen with notable enhancements in performance toward electrocatalytic C0 ₂ reduction to formate. The combination of N- doping with a PEI overlayer results in a synergistic effect by creating a local environment in which reduction of C0 ₂ to C0 ₂ ^" occurs at a greatly reduced overpotential. The decrease in overpotential is accompanied by corresponding enhancements in Faradaic efficiency and current density for formate production. [0067] In C0 ₂ saturated aqueous KHC0 ₃ solutions, maximum Faradaic efficiencies for formate production of 87 % have been reached with current densities of 9.5 mA/cm ² on three dimensional graphenated carbon nanotubes. The resulting catalytic electrodes are highly stable in extended controlled potential electrolyses. The overall performance of the optimized metal-free electrodes is on par with the most efficacious metal electrodes and their fabrication is straightforward.

[0068] The results of this study may open a new avenue for highly efficient C0 ₂ reduction catalysis based on inexpensive, easily prepared carbon-based materials, and inspire application of related co-catalysis strategies to other reactions of interest.

[0069] Some embodiments of the present invention can be used to reduce carbon dioxide to formate. Formate, or its protonated form formic acid, is used as a preservative and antibacterial agent in livestock feed, a coagulant in the production of rubber, a hydrogen storage material, and as the anode fuel in direct formic acid fuel cells.

[0070] Other industrially-useful applications appear mentioned throughout this application.

EXAMPLES

1 Materials and preparation of electrodes

[0071] All chemicals were purchased from commercial sources if not mentioned otherwise. Polyethylenimine (average Mw -25,000) and potassium bicarbonate (99.99% purity) were purchased from Sigma Aldrich. Deionized water was further purified by using a Milli-Q Synthesis A10 Water Purification system. C0 ₂ (National Welders, research grade) was of 99.999% purity with less than 3ppm H ₂ 0 and used as received.

[0072] Multi-walled carbon nanotubes (CNT) of 13-18 nm in diameter (Cheap Tubes Inc.) were first dispersed in dimethylformamide (DMF) by sonication for 20 min to yield a homogeneous CNT suspension, and then this solution was drop- casted onto a pre-polished glassy carbon (3 mm in diameter) electrode. Nitrogen doped carbon nanotubes (NCNT) were synthesized by exposing the CNT/GC electrode to an ammonia plasma using plasma enhanced chemical vapor deposition system (PECVD, Advanced Vacuum Vision 310). The plasma treatment is a facile, mild doping method at room temperature with the dopant contents tunable by changing plasma power intensities, chamber pressures, and the exposure time. In the present study, the amount of doped nitrogen was controlled by changing the exposure time and quantified using XPS spectra. In brief, the electrode was placed in the plasma chamber, which was backfilled with an ammonia atmosphere at a pressure of 200 mTorr. Plasma power was 100 W, and exposure time was 0, 10, 15, 20, 40, and 60 min. The relationship between nitrogen content and exposure time was summarized in Table 1 : the doped nitrogen content in CNT first increased with exposure time and then leveled off after 40 min of exposure. After ammonia plasma treatment, the as-prepared nitrogen doped carbon nanotubes coated glassy carbon (NCNT/GC) electrodes were used for subsequent electrochemical experiments. The above NCNT/GC electrode was immersed into 5 mass % polyethylenimine (PEI) aqueous solution for 30 min and rinsed with plenty of Dl water. The adsorbed amount of PEI on CNT was evaluated to be about 17 mass %. Oxygen doped carbon nanotubes coated glassy carbon (OCNT/GC) electrode was prepared using the same procedure except under oxygen plasma instead of ammonia plasma.

[0073] The synthesis of graphenated carbon nanotubes (GCNT) follows. In brief, they were grown using a 915 MHz microwave plasma enhanced chemical vapor deposition (MPECVD) system. To prepare the substrates, 5-nm iron films were deposited on Silicon(100) wafers using a CHA electron beam evaporation system. Prior to growth, the coated substrates were heated to 1050 °C in 150 seem of NH ₃ , followed by striking and stabilizing a plasma at 21 Torr and 2.1 kW of magnetron input power. The pressure of 21 Torr was maintained throughout the pretreatment and growth steps. Substrates were then pretreated for 180 s in the plasma.

Following pretreatment, growth of the g-CNTs was accomplished by changing the gas flow to 150 seem CH ₄ and 50 seem NH ₃ for 360 s. So silicon supported GCNT film was obtained. Then it was immersed into an aqueous solution of nitric acid (1 M) to peel off the GCNT film followed by rinsing with Dl water. The free standing GCNT film was then transferred onto the surface of a GC electrode, followed by fixing with 5 μΙ of Nafion solution (0.05 mass% in isopropanol). N-doped NGCNT/GC and PEI- NGCNT/GC electrodes were obtained using the same procedure as the case of CNT. See Figures 9a and 9b for side view SEM images of GCNT. 2 Electrochemical measurements

[0074] Iron catalyst for the growth of carbon nanotubes may still remain inside the carbon nanotube even after chemical purification and impact the electrocatalytic performance of carbon materials. To remove the residual Fe catalyst inside carbon nanotubes, all the above electrodes were subjected to an electrochemical purification process prior to subsequent electrochemical measurements. In brief, the electrodes were purified by electrochemical oxidation in a phosphate buffered solution (pH 6.8) at a potential of 1 .7 V (vs. Ag/AgCI) for 300 s, followed by potential sweeping from 0.0 V to 1 .4 V in 0.5 M H ₂ S0 ₄ until a stable voltammogram was achieved. After purification, no peak of element Fe was detected by high resolution XPS shown in Figure 3.

[0075] The electrochemical measurements were carried out in a gas-tight two compartment electrochemical cell system controlled with a CHI601 D station (CH Instruments, Inc., USA) with Pt coil as counter electrode and Saturated Calomel Reference Electrode (SCE) as reference electrode. The working electrodes were prepared by loading sample suspension onto the pre-polished glass carbon electrodes. The well-prepared electrodes were dried at room temperature overnight before the electrochemical tests. Linear sweep voltammetric (LSV) scans were recorded in C0 ₂ saturated 0.1 M KHCO ₃ , while controlled potential electrolysis was performed in 0.1 M KHCO ₃ electrolyte with C0 ₂ flow.

3 Physical characterization

[0076] X-ray photoelectron spectra (XPS) were obtained at the Chapel Hill Analytical and Nanofabrication Lab (CHANL) at UNC. A Kratos Analytical Axis UltraDLD spectrometer with monochromatized X-ray Al Ka radiation (1486.6 eV) with an analysis area of 1 mm2 was used. A survey scan was first performed with a step size of 1 eV, a pass energy of 80 eV, and a dwell time of 200 ms. High resolution scans were then taken for each element present with a step size of 0.1 eV and a pass energy of 20 eV. The binding energy for all peaks was referenced to the C 1 s peak at 284.6 eV.

[0077] NMR analysis was used to quantify the yield of formate during controlled potential electrolysis. NMR spectra were recorded on Bruker NMR spectrometers (AVANCE-400). 1 H NMR spectra were referenced to residual solvent signals. At the end of electrolysis periods, gaseous samples (0.8 ml) were drawn from the headspace by a gas-tight syringe (Vici) and injected into the GC (Varian 450-GC, pulsed discharge helium ionization detector, PDHID). Calibration curves for H ₂ and CO were determined separately.

[0078] Raman spectra were collected by Raman measurements (Renishaw) with a 514 nm laser, which was coupled with an electrochemical station for the in situ electrochemical Raman study. Scanning electron microscopy (SEM) and energy dispersive X-ray spectrometer (EDS) were obtained on a FEI Helios 600 Nanolab Dual Beam System focused ion beam (FIB) equipped with an Oxford Instruments, INCA PentaFET-x3 X-ray detector with the electron beam set to 20 keV and a beam current of 0.69 nA.

REFERENCES

(1 ) Karl, T. R.; Trenberth, K. E. Science 2003, 302, 1719.

(2) (a) Rosen, B. A.; Salehi-Khojin, A.; Thorson, M. R.; Zhu, W.; Whipple, D. T.;

Kenis, P. J. A.; Masel, R. I. Science 2011 , 334, 643. (b) Lu, Q., Rosen, J., Zhou, Y., Hutchings, G.S., Kimmel Y.C., Chen, J.G., Jiao, F. Nat. Commun. 2014, 5, 3242. (c) Kang, P.; Cheng, C; Chen, Z.; Schauer, C. K.; Meyer, T. J.; Brookhart, M. J. Am. Chem. Soc. 2012, 134, 5500. (d) Kang, P.; Meyer, T. J.; Brookhart, M. Chem. Sci. 2013, 4, 3497.

(3) (a) Kuhl, K. P.; Cave, E. R.; Abram, D. N.; Jaramillo, T. F. Energy Environ. Sci. 2012, 5, 7050. (b) Zhang, S.; Kang, P.; Meyer, T. J. J. Am. Chem. Soc. 2014, 736, 1734. (c) Wu, J. J.; Risalvato, F. G.; Ke, F. S.; Pellechia, P. J.; Zhou, X. D. J.

Electrochem. Soc. 2012, 759, F353. (d) DiMeglio, J. L; Rosenthal, J. J. Am. Chem. Soc. 2013, 735, 8798. (e) Hori, Y.; Wakebe, H.; Tsukamoto, T.; Koga, O.

Electrochim. Acta 1994, 39, 1833. (f) Costentin, C; Drouet, S.; Robert, M.; Saveant, J.-M. Science 2012, 338, 90.

(4) (a) Chen, Y.; Li, C. W.; Kanan, M. W. J. Am. Chem. Soc. 2012, 734, 19969. (b) Li, C. W.; Kanan, M. W. J. Am. Chem. Soc. 2012, 734, 7231 . (c) Le, M.; Ren, M.; Zhang, Z.; Sprunger, P. T.; Kurtz, R. L.; Flake, J. C. J. Electrochem. Soc. 2011 , 758, E45. (5) (a) Kumar, B.; Asadi, M.; Pisasale, D.; Sinha-Ray, S.; Rosen, B. A.; Haasch, R.; Abiade, J.; Yarin, A. L; Salehi-Khojin, A. Nat. Commun. 2013, 4, 2819. (b) Nakata, K.; Ozaki, T.; Terashima, C; Fujishima, A.; Einaga, Y. Angew. Chem. Int. Ed. 2014, 53, 871 .

(6) Aydin, R.; Koleli, F. J. Electroanal. Chem. 2002, 535, 107.

(7) (a) Barton Cole, E.; Lakkaraju, P. S.; Rampulla, D. M.; Morris, A. J.; Abelev, E.; Bocarsly, A. B. J. Am. Chem. Soc. 2010, 732, 1 1539. (b) Riduan, S. N.; Zhang, Y.; Ying, J. Y. Angew. Chem. Int. Ed. 2009, 48, 3322. (c) Seshad , G.; Lin, C; Bocarsly, A. B. J. Electroanal. Chem. 1994, 372, 145. (d) Yan, Y.; Zeitler, E. L.; Gu, J.; Hu, Y.; Bocarsly, A. B. J. Am. Chem. Soc. 2013, 735, 14020. (e) Morris, A. J.; McGibbon, R. T.; Bocarsly, A. B. ChemSusChem 2011 , 4, 191 .

(8) (a) Tornow, C. E.; Thorson, M. R.; Ma, S.; Gewirth, A. A.; Kenis, P. J. A. J. Am. Chem. Soc. 2012, 734, 19520. (b) Zheng, Y.; Jiao, Y.; Chen, J.; Liu, J.; Liang, J.; Du, A.; Zhang, W.; Zhu, Z.; Smith, S. C; Jaroniec, M.; Lu, G. Q.; Qiao, S. Z. J. Am.

Chem. Soc. 2011 , 733, 201 16. (c) Liu, R. L; Wu, D. Q.; Feng, X. L; Mullen, K.

Angew. Chem. Int. Ed. 2010, 49, 2565. (d) Li, Y. G.; Zhou, W.; Wang, H. L; Xie, L. M.; Liang, Y. Y.; Wei, F.; Idrobo, J. C; Pennycook, S. J.; Dai, H. J. Nat. Nanotechol. 2012, 7, 394.

(9) (a) Gong, K. P.; Du, F.; Xia, Z. H.; Durstock, M.; Dai, L. M. Science 2009, 323, 760. (b) Wang, S.; Yu, D.; Dai, L. J. Am. Chem. Soc. 2011 , 733, 5182.

(10) Zhao, Y.; Nakamura, R.; Kamiya, K.; Nakanishi, S.; Hashimoto, K. Nat.

Commun. 2013, 4, 2390.

(1 1 ) Sanz, R.; Calleja, G.; Arencibia, A.; Sanz-Perez, E. S. J. Mater. Chem. A 2013, 7, 1956.

(12) Zhang, S.; Shao, Y.; Yin, G.; Lin, Y. Angew. Chem. Int. Ed. 2010, 49, 221 1 .

(13) Shao, Y. Y.; Zhang, S.; Engelhard, M. H.; Li, G. S.; Shao, G. C; Wang, Y.; Liu, J.; Aksay, I. A.; Lin, Y. H. J. Mater. Chem. 2010, 20, 7491 .

(14) Jiao, L.; Zhang, L.; Ding, L.; Liu, J.; Dai, H. Nano Res. 2010, 3, 387.

(15) Kozlovskaya, V.; Kharlampieva, E.; Drachuk, I.; Cheng, D.; Tsukruk, V. V. Soft Matter 2010, 6, 3596.

(16) Tasis, D.; Tagmatarchis, N.; Bianco, A.; Prato, M. Chem. Rev. 2006, 706, 1 105.

(17) (a) Ryu, J.; Andersen, T. N.; Eyring, H. J. Phys. Chem 1972, 76, 3278. (b) Prakash, G. K. S.; Viva, F. A.; Olah, G. A. J. Power Sources 2013, 223, 68. (18) Gileadi, E. Electrode Kinetics for Chemists, Chemical Engineers and Materials Scientists; Wiley, 1993.

(19) Parker, C. B.; Raut, A. S.; Brown, B.; Stoner, B. R.; Glass, J. T. J. Mater. Sci. 2012, 27, 1046.

(20) Kim, H.; Lee, K.; Woo, S. I.; Jung, Y. Phys. Chem. Chem. Phys. 2011 , 73, 17505.

(21 ) Shao, Y. Y.; Zhang, S.; Engelhard, M. H.; Li, G. S.; Shao, G. C; Wang, Y.; Liu, J.; Aksay, I. A.; Lin, Y. H. J. Mater. Chem. 2010, 20, 7491 .

(22) Parker, C. B.; Raut, A. S.; Brown, B.; Stoner, B. R.; Glass, J. T. J. Mater. Sci. 2012, 27, 1046.

(23) Cui, H.; Zhou, O.; Stoner, B. R. J. Appl. Phys. 2000, 88, 6072.

(24) Wang, L.; Ambrosi, A.; Pumera, M. Angew. Chem. Int. Ed. 2013, 52, 13818.

(25) Gong, K. P.; Du, F.; Xia, Z. H.; Durstock, M.; Dai, L. M. Science 2009, 323, 760.

(26) Zhang, S.; Kang, P.; Meyer, T. J. J. Am. Chem. Soc. 2014, 736, 1734.

(27) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications; Wiley, 2000.

(28) Tasis, D.; Tagmatarchis, N.; Bianco, A.; Prato, M. Chem. Rev. 2006, 706, 1 105.

(29) lchinohe, Y.; Wadayama, T.; Hatta, A. J. Raman Spectrosc. 1995, 26, 335.

(30) Gileadi, E. Electrode Kinetics for Chemists, Chemical Engineers and Materials Scientists; Wiley, 1993.

(31 ) Wang, Y.; Shao, Y.; Matson, D. W.; Li, J.; Lin, Y. Acs Nano 2010, 4, 1790.

EMBODIMENTS

[0079] Embodiment 1 . An electrocatalytic electrode, comprising:

an electrically-conductive surface in electrical communication with

at least one carbon nanomaterial, and

at least one nitrogen-containing polymer in catalytically-enhancing contact with the at least one carbon nanomaterial. [0080] Embodiment 2. The electrocatalytic electrode of embodiment 1 , wherein the at least one carbon nanomaterial comprises carbon nanotubes, graphenated carbon nanotubes, carbon black, mesoporous carbon, graphite, graphene, or a combination of two or more thereof. [0081] Embodiment 3. The electrocatalytic electrode of any one of

embodiments 1 -2, wherein the electrically-conductive surface comprises glassy carbon, carbon paper, carbon cloth, or a combination thereof.

[0082] Embodiment 4. The electrocatalytic electrode of any one of

embodiments 1 -3, wherein the electrically-conductive surface comprises a gas diffusion electrode. [0083] Embodiment 5. The electrocatalytic electrode of any one of

embodiments 1 -4, wherein the at least one carbon nanomaterial is substantially free of iron impurity.

[0084] Embodiment 6. The electrocatalytic electrode of any one of

embodiments 1 -5, wherein the electrocatalytic electrode is substantially free of metal catalysts.

[0085] Embodiment 7. The electrocatalytic electrode of any one of

embodiments 1 -6, wherein the at least one carbon nanomaterial comprises carbon nanotubes.

[0086] Embodiment 8. The electrocatalytic electrode of any one of

embodiments 1 -7, wherein the at least one carbon nanomaterial comprises graphenated carbon nanotubes.

[0087] Embodiment 9. The electrocatalytic electrode of any one of

embodiments 1 -8, wherein the at least one carbon nanomaterial further comprises a catalytic activity enhancing amount of a catalytic activity enhancing dopant. [0088] Embodiment 10. The electrocatalytic electrode of embodiment 9, wherein the catalytic activity enhancing dopant comprises oxygen, nitrogen, boron, phosphorous, sulfur, or a combination thereof. [0089] Embodiment 1 1 . The electrocatalytic electrode of any one of embodiments 9-10, wherein the catalytic activity enhancing dopant comprises oxygen atoms, nitrogen atoms, or a combination thereof. [0090] Embodiment 12. The electrocatalytic electrode of any one of embodiments 9-1 1 , wherein the catalytic activity enhancing dopant comprises nitrogen atoms.

[0091] Embodiment 13. The electrocatalytic electrode of any one of embodiments 9-12, wherein the catalytic activity enhancing amount ranges from about 1 to about 20 atomic percent of the catalytic activity enhancing dopant relative to the carbon nanotubes.

[0092] Embodiment 14. The electrocatalytic electrode of any one of embodiments 9-13, wherein the catalytic activity enhancing amount ranges from about 5 to about 10 atomic percent of the catalytic activity enhancing dopant relative to the carbon nanotubes.

[0093] Embodiment 15. The electrocatalytic electrode of any one of embodiments 9-14, wherein the catalytic activity enhancing amount is about 7.6 atomic percent of the catalytic activity enhancing dopant relative to the carbon nanotubes.

[0094] Embodiment 16. The electrocatalytic electrode of any one of embodiments 1 -15, wherein the at least one nitrogen-containing polymer comprises secondary amine groups, tertiary amine groups, or a combination thereof.

[0095] Embodiment 17. The electrocatalytic electrode of any one of embodiments 1 -16, wherein the at least one nitrogen-containing polymer comprises polyethylenimine, poly(diallyldimethylammonium chloride), or poly(allylamine hydrochloride), or a combination thereof. [0096] Embodiment 18. The electrocatalytic electrode of any one of embodiments 1 -17, wherein the at least one nitrogen-containing polymer comprises polyethylenimine. [0097] Embodiment 19. The electrocatalytic electrode of any one of embodiments 1 -18, wherein the at least one nitrogen-containing polymer is present in an amount ranging from about 5% to about 20% by mass, relative to the weight of the at least one carbon nanomaterial. [0098] Embodiment 20. The electrocatalytic electrode of any one of embodiments 1 -19, wherein the at least one nitrogen-containing polymer is present in an amount ranging from about 10% to about 15% by mass, relative to the weight of the at least one carbon nanomaterial. [0099] Embodiment 21 . The electrocatalytic electrode of any one of embodiments 1 -20, wherein the at least one nitrogen-containing polymer is present in an amount of about 12.5% by mass, relative to the weight of the at least one carbon nanomaterial. [00100] Embodiment 22. A method for reducing carbon dioxide to formate, comprising:

providing the electrocatalytic electrode of any one of embodiments 1 -21 in a suitable electrocatalytic cell;

exposing the electrocatalytic electrode to a concentration of carbon dioxide in a fluid composition;

applying a reducing potential to the electrocatalytic electrode and allowing at least some of the carbon dioxide to react;

thereby reducing the carbon dioxide to formate. [00101] Embodiment 23. The method of embodiment 22, wherein the fluid composition comprises water. [00102] Embodiment 24. The method of any one of embodiments 21 -23, wherein the fluid composition comprises carbonate salts, bicarbonate salts, phosphate salts, biphosphate salts, or a combination thereof. [00103] Embodiment 25. The method of any one of embodiments 21 -24, wherein the fluid composition comprises potassium bicarbonate, sodium

bicarbonate, or a combination thereof.

[00104] Embodiment 26. The method of any one of embodiments 21 -25, wherein the fluid composition comprises carbon dioxide.

[00105] Embodiment 27. The method of any one of embodiments 21 -26, wherein the fluid composition is saturated with carbon dioxide. [00106] Embodiment 28. The method of any one of embodiments 21 -27, wherein the reducing potential is at least -1 V versus SCE.

[00107] Embodiment 29. The method of any one of embodiments 21 -28, wherein the reducing potential is at least -1 .2 V versus SCE.

[00108] Embodiment 30. The method of any one of embodiments 21 -29, wherein the reducing potential is no more reducing than about -1 .8 V versus SCE.

[00109] Embodiment 31 . The method of any one of embodiments 21 -29, wherein the reducing potential is no more reducing than about -2.0 V versus SCE.

[001 10] Embodiment 32. A method for making the electrocatalytic electrode of any one of embodiments 1 -31 , comprising:

optionally graphenating at least one carbon nanomaterial;

depositing on an electrically-conductive surface the at least one carbon

nanomaterial;

exposing the at least one carbon nanomaterial to a plasma environment for a time sufficient to dope the at least one carbon nanomaterial with a catalytic activity enhancing amount of a catalytic activity enhancing dopant, thereby forming doped carbon nanomaterial; and

optionally bringing at least one nitrogen-containing polymer into catalytically- enhancing contact with the doped carbon nanomaterial;

thereby making the electrocatalytic electrode.

[001 11] As previously stated, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various forms. It will be appreciated that many modifications and other variations stand within the intended scope of this invention as claimed below. Furthermore, the foregoing description of various embodiments does not necessarily imply exclusion. For example, "some" embodiments may include all or part of "other" and "further" embodiments within the scope of this invention. In addition, "a" does not mean "one and only one;" "a" can mean "one and more than one."