METAL NITRIDE CATALYSTS FOR PROMOTING A HYDROGEN EVOLUTION REACTION

Abstract

The present invention provides metal nitrides and methods of making and using same, including using such metal nitrides as catalysts for hydrogen evolution reactions.

Description

This application claims benefit of U.S. Provisional Application No. 61/862,369, filed Aug. 5 2013, which application is incorporated herein by reference in its entirety.

STATEMENT OF GOVERNMENT RIGHTS

This invention was made with Government support under contract number DE-AC02-98CH 10886, awarded by the U.S. Department of Energy. The Government has certain rights in the invention.

FIELD OF THE INVENTION

The present disclosure generally relates to the field of hydrogen evolution reaction catalysts. More particularly, to metal nitride catalysts.

BACKGROUND

Hydrogen production through the splitting of water has attracted great scientific interest due to its relevance to renewable energy storage and its potential for creating an energy carrier free of carbon dioxide emissions. Electrocatalytic systems for H₂ generation typically incorporate noble metals such as platinum (Pt) in the catalysts because of their low overpotential and fast kinetics for driving the hydrogen evolution reaction (HER). However, the high cost and limited world-wide supply of these noble metals makes their use an obstacle to a viable commercial process. Several non-noble metal materials, such as transition metal chalcogenides, carbides and complexes, as well as metal alloys have been widely investigated recently, and characterized as catalysts and supports for application in hydrogen evolution.

Early transition metal nitrides have been demonstrated to have excellent catalytic activities in a variety of reactions. One of the primary interests in the applications of nitrides in these reactions was to use them in conjunction with cheaper alternative metals to replace group VIII noble metals. For example, the function of molybdenum nitride as a catalyst for hydrocarbon hydrogenolysis resembles that of platinum. The catalytic and electronic properties of transition metal nitrides are governed by their bulk and surface structure and stoichiometry.

For example, Ni and NiMo are known electrocatalysts for hydrogen production in alkaline electrolytes, and in the bulk form have exhibited exchange current densities between 10⁻⁶ and 10⁻⁴ A cm⁻², compared to 10⁻³ A cm⁻² for Pt (Huot, et al., (1991) J. Electrochem. Soc. 138:1316-1321). Jak{hacek over (s)}ić et al. ((1998) Int. J. Hydrogen Energy 23:667-681) and Jak{hacek over (s)}ić, M. M. ((2001) J. Hydrogen Energy 26:559-578) postulated the hypo-hyper-d-electronic interactive effect between Ni and Mo yields the synergism for the HER. Owing to their poor corrosion stability, few studies in acidic media have been reported.

Owing to these and other disadvantages in the current state of the art, a more affordable and efficient method for hydrogen production is needed.

SUMMARY

Embodiments of the invention provide for catalysts which may be used in affordable and efficient methods for hydrogen production. Embodiments of catalysts include catalysts made from metal nitrides.

In an embodiment, a catalyst for promoting a hydrogen evolution reaction is presented. The catalyst comprises a metal nitride having the formula:

M′_xM″_yN_z, where:

M′ is selected from the group consisting of Ag, Al, Ca, Co, Cr, Cu, Fe, Ga, In, Li, Mg, Mn, Na, Ni, Sc, Ti, V, Y, Zn, and mixtures thereof,

M″ is selected from the group consisting of Hf, Mo, Nb, Re, Ru, Ta, W, Zr, and mixtures thereof,

x is a number from 0 to 1,

y is a number from 1 to 2, and

z is a number greater than 1.8 and less than 2.2.

The metal nitride has a hexagonal lattice with a four-layered stacking sequence that comprises two formula units of mixed close packed structure with alternating layers of M″ metals in trigonal prismatic coordination; and M′, or M′ and M″ metals in octahedral coordination.

In another embodiment, a method of producing hydrogen is presented. The method includes: (a) providing a one chamber electrochemical cell comprising an anode, supporting electrolytes and a reagent, a cathode including a hydrogen evolution reaction catalyst, and (b) applying current to the electrochemical cell, whereby hydrogen is produced at the cathode. The hydrogen evolution reaction catalyst includes a metal nitride having a formula:

M′_xM″_yN_z where:

M′ is selected from the group consisting of Ag, Al, Ca, Co, Cr, Cu, Fe, Ga, In, Li, Mg, Mn, Na, Ni, Sc, Ti, V, Y, Zn, and mixtures thereof,

M″ is selected from the group consisting of Hf, Mo, Nb, Re, Ru, Ta, W, Zr, and mixtures thereof,

x is a number from 0 to 1,

y is a number from 1 to 2, and

z is a number greater than 1.8 and less than 2.2.

The metal nitride has a hexagonal lattice with a four-layered stacking sequence that comprises two formula units of mixed close packed structure with alternating layers of M″ metals in trigonal prismatic coordination; and M′, or M′ and M″ metals in octahedral coordination.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1.a) X-ray powder diffraction patterns of Co₃Mo₃N, Co_0.6Mo_1.4N₂, and δ₁-MoN. The impurity peak of cobalt metal is indicated by the asterisk. b) Four-layered crystal structure of Co_0.6Mo_1.4N₂. c) Rietveld refinements of X-ray diffraction for Co_0.6Mo_1.4N₂. Inset: neutron and X-ray diffraction data in the d-spacing range between 1.0 and 3.0 Å.

FIG. 2. Thermogravimetric response of Co_0.6Mo_1.4N₂ heated under oxygen from room temperature to 600° C.

FIG. 3. Neutron PDF fit for Co_0.6Mo_1.4N₂ from r=1.7 Å to 20 Å with a P6₃/mmc model.

FIG. 4. TEM image of single crystallite of Co_0.6Mo_1.4N₂.

FIG. 5. SEM image of Co_0.6Mo_1.4N₂.

FIG. 6. SEM image of δ₁-MoN synthesized at 600° C.

FIG. 7. TEM image of catalyst ink comprised of Co_0.6Mo_1.4N₂ dispersed on carbon black.

FIG. 8. Left: Mo³⁺ electron configuration in octahedral site. Right: Mo⁴⁺ electron configuration in trigonal prismatic site.

FIG. 9. XPS spectra of Co_0.6Mo_1.4N₂: Co 2p, Mo 3d and N 1s.

FIG. 10. HER activities of nitrides: Polarization curves for Co₃Mo₃N and Co_0.6Mo_1.4N₂ on GC electrode in Ar saturated 0.1 M HClO₄.

FIG. 11. HER activities of nitrides: Polarization curves for MoS₂, δ₁-MoN, Co_0.6Mo_1.4N₂ and Pt (0.46 μg cm⁻² for low loading and 3.44 μg cm⁻² for high loading) on GC electrode in Ar saturated 0.1 M HClO₄. Tafel plots of corresponding samples are presented in inset.

FIG. 12. Polarization curves for Co_0.6Mo_1.4N₂ on carbon paper in H₂ saturated 0.1 M HClO₄ with and without iR corrected data. Inset shows Tafel plot of Co_0.6Mo_1.4N₂ with iR corrected data.

DETAILED DESCRIPTION

Embodiments of the invention provide for catalysts which may be used in affordable and efficient methods for hydrogen production. Embodiments of catalysts include catalysts made from metal nitrides.

Metal Nitrides

In one aspect, the present invention provides metal nitrides. In one embodiment, the metal nitride has the formula (I):

M′_xM″_yN_z   (I)

M′ is a metal and may be Ag, Al, Ca, Co, Cr, Cu, Fe, Ga, In, Li, Mg, Mn, Na, Ni, Sc, Ti, V, Y, Zn, or mixtures thereof. In certain embodiments, M′ is selected from the group consisting Ag, Ca, Co, Cr, Fe, Mg, Mn, Ni, Zn, and mixtures thereof. In certain embodiments M′ is Co.

M″ is a metal and may be Hf, Mo, Nb, Re, Ru, Ta, W, Zr, or mixtures thereof. In certain embodiments M″ is Mo.

x is any number from 0 to 1. All individual values and subranges are included herein and disclosed herein; for example, x may be from a lower limit of about 0, 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 0.55, 0.6, 0.65, 0.7, 0.75, 0.8, 0.85, or 0.9 to an upper limit of about 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 0.55, 0.6, 0.65, 0.7, 0.75, 0.8, 0.85, 0.9, 0.95, or 1. In certain embodiments, x is a number greater than 0.4 and less than 0.8 or a number greater than 0.5 and less than 0.7. In certain embodiments, x is about 0.6.

y is any number from 1 to 2. All individual values and subranges are included herein and disclosed herein; for example, y may be from a lower limit of about 1, 1.05, 1.1, 1.15, 1.2, 1.25, 1.3, 1.35, 1.4, 1.45, 1.5, 1.55, 1.6, 1.65, 1.7, 1.75, 1.8, 1.85, or 1.9 to an upper limit of about 1.2, 1.25, 1.3, 1.35, 1.4, 1.45, 1.5, 1.55, 1.6, 1.65, 1.7, 1.75, 1.8, 1.85, 1.9, 1.95, or 2. In certain embodiments, y is a number greater than 1.2 and less than 1.6 or a number greater than 1.3 and less than 1.5. In certain embodiments, y is about 1.4.

z is a number greater than 1.8 and less than 2.2. All individual values and subranges are included herein and disclosed herein; for example, y may be from a lower limit of about 1.8, 1.85, 1.9, 1.95, 2, 2.05, or 2.1 to an upper limit of about 1.9, 1.95, 2, 2.05, 2.1, 2.15, or 2.2. In certain embodiments, z is a number greater than 1.9 and less than 2.1. In certain embodiments, z is about 2.

In an embodiment, M′ is Co, M″ is Mo, x is about 0.6, y is about 1.4, and z is about 2.

The metal nitride may consist of a hexagonal lattice with a four-layered stacking sequence that comprises two formula units of mixed close packed structure with alternating layers of M″ metals in trigonal prismatic coordination; and M′, or M′ and M″ metals in octahedral coordination. Embodiments also encompass the metal nitride occupying a P6₃/mmc space group. Embodiments further encompass the metal nitride having lattice parameters of approximately 2.8×2.8×11 Å. In certain embodiments, the lattice parameters are approximately 2.85×2.85×11.0 Å. Furthermore, the metal nitride may consist of nanoparticles having diameters of about 50 nm to about 500 nm. In certain embodiments the nanoparticles have diameters of less than about 100 nm.

The nanoparticles may be at least 95% free, at least 99% free, or virtually completely free of amorphous materials and/or impurities. Examples of amorphous materials include organic surfactants. Examples of impurities include an element different from the recited elements of the nanoparticles and a vacancy.

The nanoparticles can be in isolated form or can be in a plurality. Examples of pluralities of nanoparticles include stacked nanosheets and aggregates of nanoparticles. Stacked nanosheets include from about two to hundreds of stacked nanosheets. Typically, the diameter of an aggregate of nanoparticles is about 0.1 μm to about 10 μm, more typically, about 0.8μm to about 1.2 μm, and about 0.5 μm to about 1.5 μm.

In one embodiment, the metal nitride catalysts are physically supported by a carbon-based support material, i.e., a carbon support. That is, metal nitride particles are on the surface of a carbon support. A carbon support is any material that contains carbon. Examples of carbon supports include activated carbon, carbon black, carbon nanotubes, carbon nanohorns, graphene or reduced graphene oxides.

Catalysts and Methods of Using Same

In one aspect of the present invention, catalysts are provided. The catalysts comprise the metal nitride embodiments of the present invention, typically including the carbon support.

The catalysts are resistant to acid corrosion. Thus, the present invention provides methods by which reactions can be carried out in acidic media, thereby exploiting a low overpotential.

In one embodiment, the catalysts are used for hydrogen evolution reactions (HER), i.e., for methods of producing hydrogen. The HER takes place on the cathode in an electrochemical cell. For instance, a one-chamber electrochemical cell can comprise a cathode, an anode, a reference electrode, in an electrolyte. In such cell, the cathode comprises a catalyst of the present invention. For instance, a thin film of the catalyst can be prepared on a carbon electrode. The solution in the chamber can be, for example, at a pH of about 0 to 3, 0 to 2, or 1 to 2. Low overpotential of the HER on the catalysts can thus be exploited. Potential is applied to the electrochemical cell, whereby hydrogen is produced at the cathode.

Typically, at a pH of about 0 to 3, the catalysts of the present invention promotes the hydrogen evolution reaction with a low onset potential between about −1 mV and about −200 mV. In another embodiment, the onset potential is between about 10 mV and about −200 mV. In certain embodiments, the onset potential is between about −75 mV and about −150 mV. In one embodiment, the onset potential is about −100 mV.

Methods of Making the Catalysts

In another aspect, methods of making the metal nitride catalysts of the present invention are provided. In an embodiment, an oxide precursor, M′M″O₄ may be prepared by dropwise addition of an aqueous solution of M′Cl₂ into an aqueous solution of Na₂M″O₄. In certain embodiments, M′Cl₂ is CoCl₂ and Na₂M″O₄ is Na₂MoO₄. The resulting suspension may be stirred for about an hour, filtered, washed, and dried at about 120° C. M′₃M″₃N may be synthesized by annealing M′M″O₄ at about 750° C. under flowing ammonia inside a suitable vessel, such as a fused quartz tube. M′_xM″_yN_z may be obtained by treating M′₃M″₃N in NH₃ for about an hour at 400° C.

In another aspect, methods of making metal nitride catalyst inks are provided. For instance, the metal nitride inks can be prepared by combining metal nitrides with Vulcan XC-72R carbon black and water to obtain a metal nitride catalyst ink loading of about 30 wt % total M″ metal. The ink may then be deposited onto an electrode such as a glassy carbon electrode or a carbon paper electrode and dried at room temperature or elevated temperature.

In the specification, numerous specific details are set forth in order to provide a thorough understanding of the present embodiments. It will be apparent, however, to one having ordinary skill in the art that the specific detail need not be employed to practice the present embodiments. In other instances, well-known materials or methods have not been described in detail in order to avoid obscuring the present embodiments.

In this specification, groups of various parameters containing multiple members are described. Within a group of parameters, each member may be combined with any one or more of the other members to make additional sub-groups. For example, if the members of a group are a, b, c, d, and e, additional sub-groups specifically contemplated include any two, three, or four of the members, e.g., a and c; a, d, and e; b, c, d, and e; etc.

In some cases, the members of a first group of parameters, e.g., a, b, c, d, and e, may be combined with the members of a second group of parameters, e.g., A, B, C, D, and E. Any member of the first group or of a sub-group thereof may be combined with any member of the second group or of a sub-group thereof to form additional groups, i.e., b with C; a and c with B, D, and E, etc.

The present disclosure may be better understood with reference to the examples, set forth below. The following examples are put forth so as to provide those of ordinary skill in the art with a complete disclosure and description of how the compounds, compositions, articles, devices and/or methods claimed herein are made and evaluated, and are intended to be purely exemplary and are not intended to limit the disclosure. Efforts have been made to ensure accuracy with respect to numbers (e.g., amounts, temperature, etc.), but some errors and deviations should be accounted for. Unless indicated otherwise, parts are parts by weight, temperature is in ° C. or is at ambient temperature, and pressure is at or near atmospheric.

Further, unless expressly stated to the contrary, “or” refers to an inclusive “or” and not to an exclusive “or”. For example, a condition A or B is satisfied by any one of the following: A is true (or present) and B is false (or not present), A is false (or not present) and B is true (or present), and both A and B are true (or present).

Additionally, any examples or illustrations given herein are not to be regarded in any way as restrictions on, limits to, or express definitions of any term or terms with which they are utilized. Instead, these examples or illustrations are to be regarded as being described with respect to one particular embodiment and as being illustrative only. Those of ordinary skill in the art will appreciate that any term or terms with which these examples or illustrations are utilized will encompass other embodiments which may or may not be given therewith or elsewhere in the specification and all such embodiments are intended to be included within the scope of that term or terms. Language designating such nonlimiting examples and illustrations includes, but is not limited to: “for example,” “for instance,” “e.g.,” and “in one embodiment.”

EXAMPLES

Example 1

Material Synthesis

An oxide precursor CoMoO₄ was prepared by dropwise addition of an aqueous solution of CoCl₂ (Alfa Aesar, 99.7%, 40 mL, 0.25 M) into an aqueous solution of Na₂MoO₄ (Alfa Aesar, 15 mL, 0.67 M). The resulting purple suspension was stirred for 1 hour, filtered, washed, and dried at 120° C. overnight. Co₃Mo₃N was synthesized by annealing CoMoO₄ at 750° C. (5° C./min heating rate) for 12 hours under flowing ammonia (50 mL/min) inside a fused quartz tube. Co_0.6Mo_1.4N₂ was obtained by treating Co₃Mo₃N in NH₃ (150 cm³/min) for 1 hour at 400° C. δ₁-MoN was produced by treating MoCl₅ (Alfa Aesar, 99.6%) at 600° C. in NH₃ (150 cm³/mm) for 3 hours.

Example 2

Analytical Methods

X-ray powder diffraction patterns were obtained from a D8 Advance X-ray diffractometer (Bruker, AXS) set to a diffraction radius of 300 mm in Bragg-Brentano geometry, using Cu K_α radiation, and with a 192 channel LynxEye position sensitive strip detector. Scans were collected using a fixed divergence slit width of 0.6°, a 2θ range of 7 to 120° and a collection time of 1.5 s per step. Time-of-flight neutron diffraction measurements were performed on the nanoscale-ordered materials diffractometer (NOMAD) at the Spallation Neutron Source (SNS), Oak Ridge National Laboratory. Approximately 100 mg of powder were loaded into a 2 mm diameter Kapton capillary, with data acquisition time of 2 hours per sample. Data processing of both pair distribution function (PDF) and Bragg diffraction data was done using custom beamline-specific software coded in IDL. The TOPAS software package (Version 4.2, Bruker AXS) was used for Le Bail and Rietveld refinements of both X-ray and neutron diffraction data.

Scanning electron microscopy (SEM) analysis was carried out on a JEOL 7600F high resolution microscope with capabilities for energy-dispersive X-ray spectroscopy (EDX). Transmission electron microscopy (TEM) was performed on a JEOL 1400 microscope operated at an accelerating voltage of 120 kV.

Thermogravimetric analysis (TGA) was performed using a Q5000IR system (TA instruments) to determine sample anion contents. TGA scans were run under flowing O₂ (25 mL/min) with ramp rates of 1° C./min and holds at 500° C. for 10 hours and then 600° C. for 5 hours.

A PHI 3056 X-ray Photoelectron Spectroscopy (XPS) spectrometer with an Al source in a 2×10⁻¹⁰ Ton vacuum chamber was used to characterize sample surfaces properties. The instrument was calibrated before use with gold and silver foils. Samples were pressed into Indium foil (Alfa Aesar) and the foil was attached to the sample holder using carbon tape. High resolution scans were taken with a 5.85 eV pass energy, 0.05 eV energy step, and with 100 repeats to reduce instrument noise. Charging effects were compensated by shifting binding energies based on adventitious carbon 1 s peak (284.8 eV). Reference MoO₂ (Alfa Aesar, 99.95%) and MoO₃ (Alfa Aesar-Puratronic) powders were used to determine the Mo 3d_5/2/Mo 3d_3/2 ratio for the instrument. Peak fits and atomic surface concentration analysis was performed using PHI Multipack software.

Catalyst inks for electrochemical testing were prepared by adding a mixture of 2 mg sample and 2 mg carbon black (Vulcan XC72) to a solution of 500 μL Milli-Q water, 500 μL isopropyl alcohol (70% v/v, Aldrich), and 50 μL Nafion (5 wt %, Aldrich), and then sonicating for 30 min to disperse the catalysts in the ink. Afterwards, 25 μL of fresh catalyst ink was dropped onto a glassy carbon (GC) disk electrode (0.196 cm² geometrical area, Pine Research Instrument) and dried at room temperature. All electrochemical measurements were conducted in a three electrode conventional glass cell with an electrolyte solution of 0.1 M HClO₄, an Ag/AgCl reference electrode, and a Pt foil counter electrode. Rotating disc electrode (RDE) measurements were carried out to evaluate its activity. For HER measurements on GCE, RDE data (1600 rpm) were collected in Ar saturated solutions from 0.2 to −0.4 V vs. RHE at a scan rate of 5 mV/s. For measurements taken on Toray carbon paper, data were collected in a H₂-saturated solution.

Example 3

Structure

Referring to FIG. 1, X-ray powder diffraction patterns of Co₃Mo₃N, Co_0.6Mo_1.4N₂ and δ₁-MoN are shown in FIG. 1a. The impurity peak of cobalt metal is indicated by the asterisk. Powder X-ray diffraction studies of Co_0.6Mo_1.4N₂ suggest that it is isostructural with δ₁-MoN (FIG. 1a), with a=2.85 Å, c=2.75 Å, and space group symmetry P-6m2 (#187). Indexing of neutron diffraction data indicates that Co_0.6Mo_1.4N₂ crystallizes in a 1×1×4 supercell (a ˜2.85 Å, c ˜11.01 Å) of δ₁-MoN. Different crystal structures previously reported for transition metal nitrides were tested via Rietveld refinement, The Rietveld refinements of X-ray diffraction for Co_0.6Mo_1.4N₂ showing observed data superimposed on the calculated pattern, and the difference curve (bottom line), are shown in FIG. 1c. The insert in FIG. 1c shows the neutron and X-ray diffraction data in the d-spacing range between 1.0 and 3.0 Å. The asterisks indicate the superstructure peaks detected by neutron diffraction. It was found that only the Li_0.67NbS₂ structure type with P6₃/mmc (#194) space group symmetry is effective in describing the Co_0.6Mo_1.4N₂ structure. In this structure, N ions are found in close packed layers with a repeating AABB stacking sequence while all transition metal ions are found between the N layers at the coordinates directly above/below the unoccupied C layer positions of nitrogens. This leads to alternating layers of trigonal prismatic and octahedral coordination for the transition metals (FIG. 1b).

Structural analogies to compounds such as Fe_0.8Mo_1.2N₂ suggest that Mo prefers the trigonal prismatic site (2a Wykoff position) while Co prefers the octahedral site (2b Wykoff position). Although synthesis reactions were initiated with equimolar amounts of Co and Mo, and though there were no indications of the loss of Co through volatilization during the reactions, the presence of impurity peaks in the reaction product assigned to Co metal suggests that cobalt molybdenum nitride phase is Mo-rich. The final crystallographic refinement was therefore carried out with a mixture of Co/Mo on the octahedral site. No evidence was found for non-stoichiometry on the trigonal prismatic Mo site nor on the nitrogen site during refinement, though vacancies on these sites cannot be conclusively ruled out. Similarly, if excess nitrogen in defect sites exists, it could not be readily detected in our Rietveld refinements. The cobalt molybdenum nitride was therefore assigned the stoichiometry of Co_0.6Mo_1.4N₂. This formula is consistent with the weight fraction of Co metal (˜20%) present as a second phase obtained from Rietveld refinements.

The stoichiometry of Co_0.6Mo_1.4N₂ can be indirectly determined by thermogravimetric analysis (FIG. 2). Black Co_0.6Mo_1.4N₂ (together with Co) was completely oxidized to purple CoMoO₄ by heating the nitride in an oxygen atmosphere, as judged by powder X-ray diffraction. The thermal response indicates that this is not a direct conversion. Two intermediate transitions were observed at mass gains of ˜6% (˜200° C.) and ˜12% (˜300° C.), and a the maximum mass gain is seen at 500° C., beyond which there is a slight mass decrease to a stable plateau which persists over a wide temperature range (600-800° C.). The product at 500° C. is mainly β-CoMoO₄ while product at higher temperature is a mixture of both α- and β-CoMoO₄. The maximum weight gain is 24.55 wt %, which is nearly the theoretical weight gain of 25.16 wt % for a CoMoN₂ sample (Co_0.6Mo_1.4N₂ with the balance of Co present as Co metal). The smaller mass gain than expected during oxidation is attributed to the presence of a small amount of oxygen (0.61 wt %, 1.54 mol %) in the sample of Co_0.6Mo_1.4N₂, presumably associated with the residual oxygen left in the product from precursor during the nitridation. Residual oxygen is also observed in SEM-EDX analysis.

Additional insights into the crystal structure of Co_0.6Mo_1.4N₂ have been obtained through “small box” fitting of the neutron pair distribution function (PDF) data, shown in FIG. 3. It can be seen that this structural model results in an excellent fit to the observed local structure over the length scale of 1.7-20 Å, suggesting that the average structure provides a very complete description of this compound even on the atomic scale. As such, it is expected that the local environments of Co and Mo within the octahedral layer are indistinguishable, and that both ions are found at the same crystallographic position without clustering or ordering of ions within this layer. In contrast, the simple WC model cannot effectively fit the PDF data, again confirming the importance of the superstructure despite its absence in powder X-ray diffraction data.

Insights into particle size and morphology were obtained from electron microscopy studies. TEM and SEM images (FIGS. 4 and 5) showed that the primary particle size of Co_0.6Mo_1.4N₂ has nanoscale dimensions (<100 nm), and these primary particles are aggregated into larger secondary particles which can be more than a micron across. The secondary particle size is comparable with that of the Co₃Mo₃N precursor used in the syntheses. The primary particles of δ₁-MoN were substantially larger (FIG. 6), suggesting that Co plays a role in minimizing the Co_0.6Mo_1.4N₂ particle size. TEM studies are able to resolve isolated crystallites of Co_0.6Mo_1.4N₂, and find typical primary particles to be obtained without well defined facets, and with a maximum dimension of ˜80 nm. Further imaging of the catalyst inks (CoMoN₂ mixed with carbon black) used for activity tests showed that the nitrides were well dispersed on carbon black after sonication (FIG. 7).

Example 4

Valence and Bonding

The idealized formula of ternary CoMoN₂ is expected to correlate with a 4+ valence of Mo that is raised relative to the 3+ valence of binary MoN, though the non-stoichiometry of Co_0.6Mo_1.4N₂ indicates a more complex situation. The stoichiometry of Co_0.6Mo_1.4N₂ corresponds to an average Mo valence of +3.4 if all Co is divalent, though the presence of cation vacancies in the structure will result in more oxidized Mo ions.

While a bond valence sum (BVS) analysis is often used to gain insights into the valence of oxides, it may be difficult to apply this methodology to the present system as there are not enough close structural analogues with well-defined valence states and well-characterized structures to construct an accurate parameterization. However, meaningful insights into the Mo valence can be obtained by comparing the Mo—N bond distances in the octahedral (2.18 Å) and trigonal prismatic (2.12 Å) sites of Co_0.6Mo_1.4N₂ to those in reference nitride compounds which also have 6-coordinate Mo. MoN is an appropriate reference for 3+ Mo, and has been reported to have Mo—N bond distances of 2.16-2.17 Å in δ₁-MoN, and similar distances were reported for δ₂ and δ₃ polytypes. These distances are in very good agreement with the sum of the Shannon ionic radii Mo³⁺ and N³⁻ with appropriate coordination number. The octahedral sites in Co_0.6Mo_1.4N₂ can therefore be assigned a valence of 3+, while the trigonal prismatic sites must be higher in valence to account for their reduced Mo—N bond lengths.

The reduction in the trigonal prismatic average Mo—N bond length by 0.06 Å relative to the octahedral site suggests a substantial increase in valence. The Shannon ionic radii suggest a decrease in ionic radius of 0.04 Å on moving from Mo³⁺ to Mo⁴⁺, though it should be noted that the ionic radii of these ions were assigned based on a very limited data set (two halide structures for 3+, one halide structure for 4+). The Mo valence on the trigonal prismatic site should be similar to that which is observed for MnMoN₂ (2.12 Å Mo—N bond length) and Fe_0.8Mo_1.2N₂ (2.13 Å) and is probably near 4+ based on these analogies. This is larger than the +3.6 valence of the trigonal prismatic site for the Co_0.6Mo_1.4N₂ stoichiometry obtained from Rietveld refinements of neutron diffraction data (assuming that the octahedral site is a mixture of divalent Co and trivalent Mo), though this valence could be raised to +4 if there are ˜20% vacancies on the octahedral cation site. The compound LiMoN₂ is expected to have a 5+ Mo valence, and has been reported to have Mo—N bond length of 2.09 Å which is substantially shorter than observed for Co_0.6Mo_1.4N₂. An analysis of crystal field levels (FIG. 8) suggests that Mo⁴⁺ (d²) can benefit from the distortions associated with a trigonal prismatic environment while Mo³⁺ (d³) cannot, and that the observed structure of Co_0.6Mo_1.4N₂ does an excellent job of satisfying the bonding preferences associated with the partial oxidation of Mo³⁺.

Further insights into the valence of transition metals at the Co_0.6Mo_1.4N₂ catalyst surface were obtained through XPS measurements (FIG. 9). Although Co species may not be directly involved in the HER reaction mechanism, knowledge of the Co valence will be helpful in understanding the Mo valence through charge balance arguments, and the results of the XPS are summarized in Table 1.

TABLE 1
XPS analysis of Co3Mo3N and Co0.6Mo1.4N2.
	Mo 3d5/2	Mo 3d5/2 and 3d3/2	Co 2p3/2
Sample	Mo0	Mo2/3+	Mo3/4+	Mo6+	Co0	Co—O/N	Co—OH	Co2+ satellite
Co3Mo3N %	227.77	228.86	231.09	232.25	778.39	781.16	783.54
	26.7	28.6	39.9	4.8	24.0	51.7	24.3
CoMoN2 %		228.89	230.34	232.01	778.49	781.12	783.55	786.96
		55.4	22.2	22.4	12.0	49.0	22.3	16.7
	N 1s	O 1s
Sample	Mo 3p3/2	Co/Mo—N	N—H	Co/Mo—O	Co/Mo—OH or C—O
Co3Mo3N %	393.81	395.64	397.73	399.79	530.46	533.52
			83.7	16.3	90.0	10.0
Co0.6Mo1.4N2 %	392.67	394.50	396.75	398.98	531.10
			77.0	23.0	100

The composition determined from the XPS analysis suggests that the sample surface is enriched in Co relative to Mo (1.8:1 Co:Mo ratio, vs. 1:1 ratio of starting materials in synthesis). The dominant peak in the Co 2p_3/2 XPS at 781.1 eV (49%) is from Co²⁺ coordinated to O or N ions, and is attributed to the Co_0.6Mo_1.4N₂ phase. The impurity phase of metallic Co can be seen at 778.5 eV (12%) and Co³⁺-O/N bond at 784 eV (22%) is probably due to the surface oxidation of metallic Co. The remainder of the contribution from this phase is observed at 787 eV (17%) associated with Co—OH moieties resulting from the reaction of Co metal (or even oxides) with moisture in air. The assignment of a bulk Co valence of 2+ in Co_0.6Mo_1.4N₂ catalyst is supported by this XPS data despite the small amount of Co³⁺, which is believed to only occur through surface oxidation

The analysis of the Co_0.6Mo_1.4N₂ Mo 3d XPS spectra is more complex due to a spin-orbit coupling feature that splits the 3d response into separate 3d_5/2 and 3d_3/2 peaks. MoO₃ was therefore used as a reference sample to determine the separation and relative intensities of Mo 3d_5/2 and 3d_3/2 peaks. Fitting of the Co_0.6Mo_1.4N₂ data reveals the three Mo 3d_5/2 species: Mo^2/3+ (229 eV, 55%), Mo^3/4+ (230 eV, 22%) and Mo⁶⁺ (232 eV, 22%). In the context of the analysis of bond distances, the first peak is ascribed to Mo³⁺, the second peak is assigned to Mo⁴⁺ in the prismatic layer, and the final peak Mo⁶⁺ is attributed to surface Mo species that have been oxidized upon air exposure. There is clearly a strong ionic character in Co_0.6Mo_1.4N₂, unlike the precursor compound Co₃Mo₃N which exhibits a substantial fraction of Mo⁰ and Co⁰ character in its XPS spectra. The observed mixture of valence states in Co_0.6Mo_1.4N₂ suggests that the HER electrocatalysis is associated with either the 3+/4+ or 4+/6+ pairs of valence states.

Understanding N 1 s XPS peaks is important for nitride compounds, but can only be accomplished after accounting for the effects of partially overlapping Mo 3p_3/2 peaks. This allows two N 1 s peaks to be resolved. The major species is N—Co/Mo (397 eV, 84%) which confirms that the surface of Co_0.6Mo_1.4N₂ remains a nitride even after air exposure. This is consistent with the composition calculated by analyzing peak areas (Table 1), which suggests that the majority of anions at the surface are nitrogens. The N—Co/Mo binding energy is closer to the value expected for Mo—N bonds (396.7 eV) than for Co—N bonds (398.1 eV). The other N 1 s species is assigned to NH groups (399 eV, 17%) which demonstrates that H species are abundant at the sample surface. Although the origin of the surface H is likely due to an adventitious process such as incomplete reaction with NH₃ or reaction with moisture, this signal does indicate that N ions at the sample surface are able to strongly interact with H species, and that a large number of N—H moieties should be available to participate in the HER reaction mechanism if such a pathway is energetically accessible.

Example 5

Electrochemical Measurements

Typical HER activity polarization curves of Co_0.6Mo_1.4N₂ in acidic media (0.1 M HClO₄) are shown in FIGS. 10 and 11, together with data collected on alternative systems including Co₃Mo₃N (FIG. 10), Pt (FIG. 11), δ₁-MoN (FIG. 11), and bulk MoS₂ (FIG. 11). It can be seen that Co_0.6Mo_1.4N₂ has a very promising HER activity, including a low onset potential (˜100 mV) and a current density which reaches 10 mA cm⁻² at a potential of −0.2 V vs. RHE. This activity is clearly enhanced relative to that of δ₁-MoN, and is only a little lower than that of Pt. The observed Co_0.6Mo_1.4N₂ activity at the low loadings tested in this study (0.243 mg/cm²) is competitive with the best values observed to date for non-noble metal alternatives such as MoS₂, Ni—Mo—N, Mo₂C, and Ni₂P, as summarized in Table 2:

TABLE 2
Summarized HER activity of non-noble metal
catalysts tested in acidic electrolyte.
		Current	Corresponding	Tafel
	Loading	density	Overpotential	Slope
Catalyst	(mg/cm2)	(mA/cm2)	(mV vs. RHE)	(mV/dec)
Co0.6Mo1.4N2	0.243	10	~190	60
Pt	0.46 × 10−3			50
	3.44 × 10−3			30
Ni2P	1	20	130	46~81
MoS2/RGO	0.28	10	150	41
MoS2/MoO3		2	200	50~60
Bulk Mo2C		20	~240	88
Bulk MoB		20	~240
Mo2C/CNT	2	10	~150	55
Ni—Mo—N	0.25	3.5	~200	36
nanosheet
Ni—Mo	3	10	80
nanopowder

Tafel plots are presented in the inset of FIG. 11. In the low overpotential range, the Tafel slopes for δ₁-MoN, Co_0.6Mo_1.4N₂, and low-loading Pt are 100, 60 and 50 mV per decade, respectively, though these values are approximate because of limitations of the rotating disc electrode method for assessing the Tafel behavior of hydrogen evolution catalysts. It may be difficult to effectively measure the Tafel slope at high overpotential ranges because the slope rises rapidly due to the ohmic resistance of the electrode and solution. Moreover, the generated hydrogen bubbles can cover the glassy carbon electrode, resulting in a current difference for the forward and backward voltage sweeps. Therefore, Toray carbon paper was used as an alternative to the glassy carbon electrode as carbon paper has a fibrous and porous feature that will not trap evolved hydrogen. The results for Co_0.6Mo_1.4N₂ is shown in FIG. 12. As seen in FIG. 12, the current difference in cycles is smaller than the current differences in cycles for the glassy carbon electrode. In order to eliminate the effect of ohmic resistance that caused non-linearity in the Tafel plot of uncorrected data, an iR correction was applied using parameters determined from electrochemical impedance spectroscopy resulting in good linearity (inset to FIG. 12). The ohmic resistance was measured to be 2.3 Ωcm² for electrode of Co_0.6Mo_1.4N₂ and 4.1 Ωcm² for that of δ₁-MoN. This fact demonstrates that Co_0.6Mo_1.4N₂ has a higher conductivity and an improved electron transfer is approached. The corrected Tafel slope of 120 mV/dec indicates that nanostructured Co_0.6Mo_1.4N₂ is an active non-noble metal electrocatalyst for the HER materials, and functions effectively even at high current densities.

Claims

1. A catalyst for promoting a hydrogen evolution reaction, the catalyst comprising a metal nitride having a formula:

M′xM″yNz

wherein M′ is selected from the group consisting of Ag, Al, Ca, Co, Cr, Cu, Fe, Ga, In, Li, Mg, Mn, Na, Ni, Sc, Ti, V, Y, Zn, and mixtures thereof;

wherein M″ is selected from the group consisting of Hf, Mo, Nb, Re, Ru, Ta, W, Zr, and mixtures thereof;

wherein x is a number from 0 to 1;

wherein y is a number from 1 to 2;

wherein z is a number greater than 1.8 and less than 2.2; and

wherein the metal nitride comprises a hexagonal lattice with a four-layered stacking sequence that comprises two formula units of mixed close packed structure with alternating layers of M′ metals in trigonal prismatic coordination and; M′, or M′ and M″ metals in octahedral coordination.

2. The catalyst of claim 1, wherein M′ is selected from the group consisting Ag, Ca, Co, Cr, Fe, Mg, Mn, Ni, Zn, and mixtures thereof.

3. (canceled)

4. The catalyst of claim 2, wherein M″ is Mo.

5. The catalyst of claim 1, wherein the metal nitride comprises lattice parameters of about 2.85×2.85×11.0 Å.

6. The catalyst of claim 2, wherein x is a number greater than 0.4 and less than 0.8

7. (canceled)

8. The catalyst of claim 6, wherein y is a number greater than 1.2 and less than 1.6.

9. (canceled)

10. The catalyst of claim 8, wherein z is a number greater than 1.9 and less than 2.1.

11. (canceled)

12. The catalyst of claim 1, wherein the metal nitride occupies a P63/mmc space group.

13. The catalyst of claim 1, wherein M′ is Co, M″ is Mo, x is about 0.6, y is about 1.4, and z is about 2.

14. The catalyst of claim 1, wherein the catalyst promotes the hydrogen evolution reaction with a low onset potential between about −1 mV and about −200 mV under acidic conditions.

15. The catalyst of claim 14, wherein the onset potential is between about −75 mV and about −150 mV.

16. A method of producing hydrogen, the method comprising

(a) providing a one chamber electrochemical cell comprising: an anode; supporting electrolytes and a reagent solution; a cathode comprising a hydrogen evolution reaction catalyst, wherein the catalyst comprises a metal nitride having a formula: M′xM″yNz wherein M′ is selected from the group consisting of Ag, Al, Ca, Co, Cr, Cu, Fe, Ga, In, Li, Mg, Mn, Na, Ni, Sc, Ti, V, Y, Zn, and mixtures thereof; wherein M″ is selected from the group consisting of Hf, Mo, Nb, Re, Ru, Ta, W, Zr, and mixtures thereof; wherein x is a number from 0 to 1; wherein y is a number from 1 to 2; wherein z is a number greater than 1.8 and less than 2.2; and wherein the metal nitride comprises a hexagonal lattice with a four-layered stacking sequence that comprises two formula units of mixed close packed structure with alternating layers of M″ metals in trigonal prismatic coordination; and M′, or M′ and M″ metals in octahedral coordination; and

(b) applying current to the electrochemical cell, whereby hydrogen is produced at the cathode.

17. The method of claim 16, wherein the reagent solution has a pH of about 0 to 3.

18. The method of claim 16, wherein M′ is selected from the group consisting Ag, Ca, Co, Cr, Fe, Mg, Mn, Ni, Zn, and mixtures thereof.

19. (canceled)

20. The method of claim 18, wherein M″ is Mo.

21. The method of claim 16, wherein the metal nitride comprises lattice parameters of about 2.85×2.85×11.0 Å.

22. The method of, wherein x is a number greater than 0.4 and less than 0.8, y is a number greater than 1.2 and less than 1.6, and z is a number greater than 1.9 and less than 2.1.

23-27. (canceled)

28. The method of claim 16, wherein the metal nitride occupies a P63/mmc space group.

29. The method of claim 16, wherein M′ is Co, M″ is Mo, x is about 0.6, y is about 1.4, and z is about 2.

30. The method of claim 16, wherein the catalyst promotes the hydrogen evolution reaction with a low onset potential between about −1 mV and about −200 mV under acidic conditions.

31-32. (canceled)