WO2019241717A1 - HIERARCHICAL METAL PHOSPHIDE-SANDWICHED Ni5P4-BASED MICROSHEET ARRAYS AS ROBUST PH-UNIVERSAL ELECTROCATALYSTS FOR EFFICIENT HYDROGEN GENERATION - Google Patents

Abstract

Disclosed is an active and durable pH-universal electrocatalyst for catalysis of the hydrogen evolution reaction (HER) from water splitting. The catalysts formed by growing hierarchical metal phosphide nanoparticles as thin skins covering both sides of nickel phosphide (Ni₅P₄) nanosheet arrays to form self-supported sandwich-like Μ_χΡ/Νί₅Ρ₄/Μ_χΡ microsheet arrays are comprised of mesopores and macropores.

Description

HIERARCHICAL METAL PHOSPHIDE-SANDWICHED Ni₅P₄-BASED MICROSHEET ARRAYS AS ROBUST PH-UNIVERSAL ELECTROCATALYSTS

FOR EFFICIENT HYDROGEN GENERATION

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application claims benefit of U.S. Provisional Patent Application No. 62/685,660, filed June 15, 2018, and entitled“Hierarchical Metal Phosphide- Sandwiched Ni₅P₄-Based Microsheet Arrays as Robust PH-Universal Electrocatalysts For Efficient Hydrogen Generation,” which is hereby incorporated herein by reference in its entirety for all purposes.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] This invention was made with Government support under: grant no. DE- SC0010831 awarded by US Department of Energy. The government has certain rights in the invention.

FIELD OF THE INVENTION

[0003] The disclosure herein is related to the field of HER electro-catalysts; and more particularly to hierarchical metal phosphide-sandwiched Ni₅P₄-based microsheet arrays as robust PH-universal electro-catalysts for efficient hydrogen generation, and methods of making the same.

BACKGROUND

[0004] Hydrogen produced by water electrolysis is a clean energy carrier, which can be regarded as a potential alternative to fossil fuels. Water dissociation for hydrogen production via electrolysis requires highly active catalysts to minimize the overpotentials.

[0005] Platinum (Pt)-based materials are the most active electrocatalysts for hydrogen evolution reaction (HER). However, the noble-metal based catalysts are not suitable for large-scale application due to their high cost and limited availability on the earth’s crust.^5,6 For sustainable and clean hydrogen economy, highly active and affordable catalysts based on earth-abundant materials have to be developed. [0006] Noble metal-free materials including metal sulfides, selenides, phosphides, etc., have been widely explored for catalyzing HER.^{7 10} Among them, transition-metal phosphides (TMPs) have been receiving attention due to their promising catalytic activity for HER in water splitting. In the past years, several efficient TMP-based materials have been explored for HER, including Ni₂P,10 Mo-W-P, CoP/CC,CoPS,and Fe_xCo1-_xP.

[0007] Despite considerable achievements, it still remains a challenge to design and develop catalysts consisting of earth-abundant materials with Pt-like HER activity in pH-universal solutions and operational stability at high current densities (> 500 mA cm ²) considering the poor long-term stability of Pt catalysts in both acidic and alkaline media. Indeed, little attention has been given to their structural design to enhance the catalytic activity and long-term durability especially at even higher current densities, which is crucial for large-scale hydrogen production through electro-catalytic water splitting. Many catalysts involve complicated preparation procedures, which are not industrially compatible. What’s more, an ideal electro- catalyst is expected to exhibit outstanding catalytic HER activity over a wide pH range (0-14) like Pt, considering the abundant sources of water on earth, and different water electrolysis technologies with different demands on the pH values of the electrolytes. However, very few non-noble electro-catalysts can be simultaneously robust in catalyzing the HER in both acidic and alkaline media. Thus, developing robust catalysts with pH universality or PH independence, and long-term durability at high current densities remains challenging.

[0008] Electro-catalysts are further prone to diminished catalytic performances with varying electrolytes and pH values, and many electro-catalysts require complicated preparation procedures, which are difficult to reproduce, and therefore unsuited to industrial application.

BRIEF SUMMARY

[0009] A new method of producing robust pH-universal HER catalysts is therefore disclosed herein, which are further suitable for commercial use. In addition, the HER catalysts disclosed provide improved hydrogen generation in acid or base, and are stable at extremely large current densities such as above 500 mA cm ².

[0010] The disclosure herein relates in some embodiments to a three dimensional hydrogen evolution reaction (HER) catalyst as disclosed herein, and comprises a porous Ni foam support; a Ni₅P₄-Ni₂P scaffold positioned on the support; a first layer of a metal phosphide (M_XP) positioned on a first side of the Ni₅P₄-Ni₂P scaffold; and a second layer of the metal phosphate (M_XP) positioned on a second side of the Ni₅P₄- Ni₂P scaffold to form a M_xP/Ni₅P₄/M_xP microsheet array.

[0011] In another embodiment of the catalyst the metal phosphide (M_XP) is selected from the group consisting of Co, Ni, and Fe or a combination thereof, and in a further embodiment x is equal to 1 , 2, or 1/2. In some embodiments of the catalyst the microsheet array comprises CoP/Ni₅P₄/CoP, and in other embodiments the M_xP/Ni₅P₄/M_xP microsheet array comprises mesoporous pores; in further embodiments the mesoporous pores are between 0.001 nm and 50 nm in diameter; and in still further embodiments the M_xP/Ni₅P₄/M_xP microsheet array comprises surface active sites for HER.

[0012] In some embodiments of the catalyst the catalyst is pH independent for catalyzing hydrogen evolution reaction (HER) from water splitting; in other embodiments the catalyst comprises an overpotential of about 71 mv at a current density of about 10 mA cm ² in an alkaline electrolyte; in a further embodiment the catalyst comprises an overpotential of about 33mv at a current density of about 10 mA cm ² in an acid electrolyte; and in some embodiments, the catalyst has at least one of: a low onset potential, large cathode current density, small Tafel slopes, or large exchange current density.

[0013] In an embodiment of the catalyst the M_xP/Ni₅P₄/M_xP microsheet comprises a about to 7.43 ^c 10 ⁷ mol/cm² active sites; and in another embodiment the catalyst comprises TOF values of between 0.453 and 1.220 s ¹ at overpotentials of between 75 and 100 mV.

[0014] Some embodiments herein disclosed provide a method of making a three dimensional hydrogen evolution reaction (HER) catalyst, comprising: positioning a porous Ni foam support, phosphorizing said Ni foam support, and forming a Ni₅P₄- Ni₂P scaffold; soaking said scaffold in a Mc-ink, phosphorizing said Mc-ink; and forming a M_xP/Ni₅P₄/M_xP microsheet array comprising the three dimensional hydrogen evolution reaction (HER) catalyst. In other embodiments of the method phosphorizing of the Ni foam support is in an Ar atmosphere, and in another embodiment phosphorizing the Ni foam support at about 500 °C. In a further embodiment the Mc-ink is Co-ink, and in a still further embodiment the method further comprising dissolving cobalt nitrate hexahydrate [Co(N0₃)₂-6H₂0] in N,N dimethylformamide (DMF) to form said Co-ink. In another embodiment an electrode is disclosed comprising: a three dimensional Hydrogen Evolution Reaction (HER) catalyst, wherein said electrode comprises: a porous Ni foam support; a Ni₅P₄-Ni₂P scaffold positioned on the support; a first layer of a metal phosphide (M_XP) positioned on a first side of the Ni₅P₄-Ni₂P scaffold; and a second layer of the metal phosphate (M_XP) positioned on a second side of the Ni₅P₄-Ni₂P scaffold to form a M_xP/Ni₅P₄/M_xP microsheet array, wherein said catalyst has at least one of: a low onset potential, a large cathode current density, a small Tafel slopes or a large exchange current density, at either of an alkaline or acidic pH. In some embodiments the a low onset potential is between -10 and 200 mV; a large cathode current density is between -10 mV at 10 mA/cm² to about -120 mV at 10 mA/cm²; a small Tafel slopes is between 10 mV/dec to about 100 mV/dec; and a large exchange current density is between 10 to about 1000 pA/cm². In another embodiment of the electrode the catalyst comprises a large 3-D porous surface area.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] For a detailed description of the disclosed exemplary embodiments, reference will now be made to the accompanying drawings, wherein:

[0016] Figure 1 (a-e) depict a synthetic scheme of sandwich-like CoP/Ni₅P₄/CoP electrocatalyst (a and e) show SEM images of Ni foam used as the starting electrode material. (b,f) show SEM images of nickel phosphide nanosheet arrays after the first synthetic step (c) Diagram showing nickel phosphide nanosheets on Ni foam that was soaked in cobalt precursor ink prepared by dissolving cobalt nitrate hexahydrate [Co(N0₃)₂.6H₂0] in DMF. (d,g) show SEM images of hierarchical CoP/Ni₅P₄/CoP microsheet arrays after the third synthetic step;

[0017] Figure 2 (a-f) depict the morphology and chemical composition of CoP/Ni₅P₄/CoP electro-catalyst (a-c) show SEM images of CoP/Ni₅P₄/CoP electrode (d) HRTEM image shows crystalline Ni₅P₄ at the inner structure of CoP/Ni₅P₄/CoP electrode. The Fast Fourier Transform (FFT) in the inset demonstrates the crystal structure of Ni₅P_4. As shown in e, and f. HRTEM images showing amorphous as well as crystalline CoP at the outer structure of CoP/Ni₅P₄/CoP electrode. The FFTs in the inset of (e) and (f) demonstrate the amorphous and crystalline structures of CoP, respectively;

[0018] Figure 3 (a-c) depict the characterization of the CoP/Ni₅P₄/CoP microsheet arrays electrode (a) Typical XRD patterns showing the phase structure of Ni foam, Ni₅P₄-Ni₂P/Ni support and CoP/Ni₅P₄/CoP electrode. Detailed XPS analysis of: (b) Co 2p, (c) P 2p, and (d) Ni 2p spectra;

[0019] Figure 4 (a-f) depict Electro-catalytic measurements of different electrodes for hydrogen evolution in acid (a) The polarization curves of the CoP/Ni₅P₄/CoP, Ni₅P₄- Ni₂P/Ni, pure Ni foam, and a Pt wire electrodes (b) Tafel plots derived from the curves in (a) (c) EIS Nyquist plots of the CoP/Ni₅P₄/CoP and Ni₅P₄-Ni₂P/Ni electrodes (d) The double-layer capacitance (C_di) of the CoP/Ni₅P₄/CoP and Ni₅P₄- Ni₂P/Ni electrodes (e) Polarization curves of the CoP/Ni₅P₄/CoP electrode after 250 and 1000 cycles (f) Potential testing at constant current densities of 30 mA cm ² and 1 A cm ² on the CoP/Ni₅P₄/CoP electrode;

[0020] Figure 5 (a-d) depict Electrochemical performance of CoP/Ni₅P₄/CoP microsheet arrays electrode in 1.0 M KOH. (a) Polarization curves recorded on CoP/Ni₅P₄/CoP and a Pt wire (b) Tafel plots of the catalysts in (a) (c) Polarization curves of the CoP/Ni₅P₄/CoP electrode after 250 and 1000 cycles (d) Chronopotentiometry tests at 10 and 500 mA cm ²;

[0021] Figure 6 depicts SEM images of CoP/Ni₅P₄/CoP samples prepared with Co- ink concentrations of (a, d) 0.4 g/ml, (b, e) 0.25 g/ml, and (c, f) 0.1 g/ml;

[0022] Figure 7 depicts A typical SEM image showing the sandwich-like structures of CoP/Ni₅P₄/CoP when CoP particles are in-situ grown on the surfaces of nickel phosphide nanosheet arrays. The dark and light arrows indicate the CoP and Ni₅P₄ nanosheet parts, respectively;

[0023] Figure 8 depicts SEM images of samples prepared with annealing in the absence of phosphorus at the third step of synthesis (a, b) before electrochemical test; and (c) after electrochemical test in 0.5 M H₂S0₄;

[0024] Figure 9 depicts at image (a) and (b) SEM images of a sample prepared at 600 °C at the third step of synthesis, showing a high resolution SEM image on the right side;

[0025] Figure 10 depicts a comparison of the SEM morphologies between original Ni₅P₄-Ni₂P/Ni and CoP/Ni₅P₄/CoP catalysts;

[0026] Figure 1 1 depicts the distribution of mesopore sizes of the sandwich-like CoP/Ni₅P₄/CoP electrocatalysts measured by the BJH method;

[0027] Figure 12 depicts EDS elemental mapping images of the as-prepared CoP/Ni₅P₄/CoP. (a) HAADF (b) Co (c) Ni. (d) P and (e) EDS spectra of CoP/Ni₅P₄/CoP; [0028] Figure 13 depicts a comparison of the XRD patterns of the nickel phosphide nanosheets after the 2^nd phosphorization or 1^st phosphorization at 500 °C without cobalt ink, and CoP/Ni₅P₄/CoP using cobalt ink;

[0029] Figure 14 depicts a comparison of XRD patterns between the samples prepared with and without phosphorus source at the third synthetic step;

[0030] Figure 15 depicts a XPS survey spectra of as-prepared CoP/Ni₅P₄/CoP electrode;

[0031] Figure 16 depicts SEM images of the as-prepared CoP/Ni₅P₄/CoP sample. (A- B) showing protruded parts of nickel phosphide nanosheets after phosphorization at the third synthetic step, with a high resolution SEM image on the right side (B);

[0032] Figure 17 depicts an image of a three-electrode setup for electrochemical tests, wherein, in some embodiments a graphite rod or graphite paper was used as the counter electrode;

[0033] Figure 18 depicts plots of the electrochemical performance of CoP/Ni₅P₄/CoP samples prepared with different concentrations of Co-ink, showing (a) polarization curves, and (b) corresponding Tafel plots of the samples in (a).

[0034] Figure 19 depicts plots of the electrochemical performance comparison between samples prepared with annealing in the absence of phosphorus at the third step of synthesis and Ni₅P₄-NiP₂/Ni support in 0.5 M H₂S0_4, (a) shows polarization curves., and (b) shows the corresponding Tafel plots of the samples in (a);

[0035] Figure 20 depicts plots of the electrochemical performance comparison between samples prepared at 500 °C and 600 °C at the third step of synthesis (a) Polarization curves (b) Corresponding Tafel plots of the samples in (a);

[0036] Figure 21 depicts CV curves recorded on the CoP/Ni₅P₄/CoP electrode in the potential ranges between -0.2 V vs RHE and 0.6 V vs RHE in 1 M PBS, wherein the he scan rate was 50 mV s ¹ ;

[0037] Figure 22 depicts TOF values of the CoP/Ni₅P₄/CoP electrode varied with the HER potentials;

[0038] Figure 23 depicts a simplified Randles model used to fit the EIS data;

[0039] Figure 24 depicts the electrochemical measurements of the double-layer capacitance of different electrodes, wherein (a) is Ni₅P₄-Ni₂P/Ni., and (b) is CoP/Ni₅P₄/CoP; [0040] Figure 25 depicts SEM images of as-prepared CoP/Ni₅P₄/CoP sample; (a, and b) are before electrochemical testing, and (c, and d) are after electrochemical test in 0.5 M H₂S0₄, b and d are high resolution SEM images;

[0041] Figure 26 depicts XRD patterns of the CoP/Ni₅P₄/CoP electrode before and after electrochemical test in 0.5 M H₂S0₄;

[0042] Figure 27 depicts high resolution XPS spectra of as-prepared CoP/Ni₅P₄/CoP sample (a-c) are before electrochemical testing, and (d-f) are after electrochemical testing in 0.5 M H₂S0₄; and

[0043] Figure 28 depicts measuring the gas products and determining the corresponding Faradaic efficiency using a gas chromatography (GC) technique. The comparison between the experimental (black) and theoretical (grey) and H₂ amounts in acid (a) and in base (b) media. A volume of 0.3 mL gas sample was injected into the chromatography at different time of electrolysis in acid (a) and base (b), wherein the current density is -50 mA cm ²;

[0044] Figure 29 (A-B) depicts a schematic illustration of (a) the fabrication procedure of the self-supported 3D Ni2(1-x)Mo2xP electrode, and (b) highly porous nanowire arrays for a hydrogen evolution reaction, as described herein;

[0045] Figure 30 (a-i) depicts the morphology and chemical composition analyses of Ni2(1 -x)Mo2xP; and SEM images of (a) NiMo04 xH20 precursor, and (b, c) Ni2(1- x)Mo2xP at different magnifications, (d, e) TEM images of Ni2(1-x)Mo2xP. (f) XRD pattern of Ni2(1 -x)Mo2xP. (g) HRTEM image, and (h) SAED pattern of Ni2(1- x)Mo2xP nanowire (i) shows a STEM image and corresponding elemental mapping of Ni, Mo, and P for Ni2(1-x)Mo2xP nanowires;

[0046] Figure 31 depicts (a) a XPS survey, and a high-resolution XPS spectra of (b) Ni 2p, (c) P 2p, and (d) Mo 3d of the Ni2(1-x)Mo2xP and a Ni2P electrode;

[0047] Figure 32 depicts a HER performance conducted in 1 M KOH, wherein (a) depicts polarization curves, (and b) depicts the current density at the overpotential of 300 mV, and (c) Tafel plots and calculated exchange current density (jO) of the electrodes (d) Polarization curves of the Ni_2(1-X) Mo_2xP/NF electrode before and after 5000 CV cycles (e) Time dependence of the current density for the Ni_2(1-X) Mo_2xP/NF electrode under constant potentials of -1 10, -170, and -220 mV;

[0048] Figure 33 depicts DFT calculations (a) Chemisorption models of H₂0 adsorption, OH adsorption, and H adsorption for the calculated free energies. Here TS represents a transition state of H₂0 activation (b) Adsorption free energy of H₂0 (DEH20), (c) free energy diagram for H₂0 activation (cleavage of O-H bonds of H₂0 molecules), and (d) free energy diagram for H adsorption (AGH) on the (0001 ) surfaces of Ni₂P and NiMoP; and

[0049] Figure 34 (A-D), depicts the overall water splitting performance in 1 M KOH. (a) is a schematic illustration of the electrolyzer using Ni_2(1-X) Mo_2xPand Cu@NiFe LDH as cathode and anode, respectively. Polarization curves at (b) low, and (c) high current density. (The benchmark electrodes of lr02(+)/Pt(-) are tested the same way.) (d) Long-term stability test conducted at constant current densities of 10, 100, and 500 mA cm ².

NOTATION AND NOMENCLATURE

[0050] In the following discussion and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean“including, but not limited to... Also, the term“couple” or“couples” is intended to mean either an indirect, direct, optical or wireless electrical connection. Thus, if a first device couples to a second device, that connection may be through a direct engagement of the devices, through an indirect connection via other intermediate devices and connections, through an optical electrical connection, or through a wireless electrical connection. While various exemplary embodiments have been shown and described, modifications thereof can be made by one skilled in the art without departing from the spirit and teachings of the disclosure. The embodiments described herein are exemplary only, and are not intended to be limiting. Many variations and modifications of the subject matter disclosed herein are possible and are within the scope of the disclosure. Where numerical ranges or limitations are expressly stated, such express ranges or limitations should be understood to include iterative ranges or limitations of like magnitude falling within the expressly stated ranges or limitations (e.g., from about 1 to about 10 includes, 2, 3, 4, etc.; greater than 0.10 includes 0.1 1 , 0.12, 0.13, etc.). For example, whenever a numerical range with a lower limit, R_L and an upper limit, Ru is disclosed, any number falling within the range is specifically disclosed. In particular, the following numbers within the range are specifically disclosed: R=R_L+k^*(Ru-R_L), wherein k is a variable ranging from 1 percent to 100 percent with a 1 percent increment, i.e., k is 1 percent, 2 percent, 3 percent, 4 percent, 5 percent, ... 50 percent, 51 percent, 52 percent, ... , 95 percent, 96 percent, 97 percent, 98 percent, 99 percent, or 100 percent. Moreover, any numerical range defined by two R numbers as defined in the above is also specifically disclosed. Use of the term "optionally" with respect to any element of a claim is intended to mean that the subject element is required, or alternatively, is not required. Both alternatives are intended to be within the scope of the claim. Use of broader terms such as comprises, includes, having, etc. should be understood to provide support for narrower terms such as consisting of, consisting essentially of, comprised substantially of, etc.

[0051] Accordingly, the scope of protection is not limited by the description set out above but is only limited by the claims which follow, that scope including all equivalents of the subject matter of the claims. Each and every claim is incorporated into the specification as an embodiment of the present disclosure. Thus, the claims are a further description and are an addition to the embodiments of the present disclosure. The discussion of a reference is not an admission that it is prior art to the present disclosure, especially any reference that may have a publication date after the priority date of this application. The disclosures of all patents, patent applications, and publications cited herein are hereby incorporated by reference, to the extent that they provide exemplary, procedural, or other details supplementary to those set forth herein.

DETAILED DESCRIPTION OF THE DISCLOSED EXEMPLARY EMBODIMENTS

[0052] The following discussion is directed to various exemplary embodiments of the invention. One skilled in the art will understand that the following description has broad application, and the discussion of any embodiment is meant only to be exemplary of that embodiment, and not intended to intimate that the scope of the disclosure, including the claims, is limited to that embodiment.

[0053] Highly active catalyst composed of earth-abundant materials performing as efficient as Pt catalysts is crucial for sustainable hydrogen production through water splitting. However, most efficient catalysts are made by complex synthetic methods, making it quite challenging for scale-up ( See Mishra et al., “Hierarchical CoP/Ni₅P₄/CoP Microsheet Arrays as a Robust pH-Universal Electrocatalyst for Efficient Hydrogen Generation”, Energy Environ. Sci. 11 , 2246-2252 (2018), incorporated herein in its entirety by reference).

EXAMPLE 1

[0054] Herein disclosed is method of making an active and durable pH-universal (1- 14) electro-catalyst for hydrogen evolution reaction (HER). This catalyst is a sandwich-like construct where in some embodiments cobalt phosphide (CoP) nanoparticles serve as the thin skin covering both sides of nickel phosphide (Ni₅P₄) nanosheet arrays, and forming self-supported sandwich-like CoP/Ni₅P₄/CoP microsheet arrays with lots of mesopores and macropores.

[0055] The as-prepared electro-catalyst requires an overpotential of 33 mV to achieve the benchmark 10 mA cm ² with a very large exchange current density, and high turnover frequencies (TOFs) in acid media (which is superior to most electro- catalysts made of metal phosphides, well-known MoS₂, and WS₂ catalysts, and performs comparably with the state-of-the-art Pt catalysts). In particular, this electro- catalyst shows impressive operational stability at extremely large current densities of up to 1 A cm ², indicating its suitability for large-scale water electrolysis. Additionally, this electrocatalyst is very active in alkaline electrolyte (71 mV at 10 mA cm ²), which demonstrates its pH universality as a HER catalyst with outstanding catalytic activity. This method does not involve any solvothermal and hydrothermal process, paving a new avenue to design robust non-noble electro-catalysts for hydrogen production toward commercial water electrolysis.

[0056] Highly active catalyst composed of earth-abundant materials performing as efficient as Pt catalysts is crucial for sustainable hydrogen production through water splitting. However, most efficient catalysts are made of nanostructures by complex synthetic methods, making it quite challenging for scale-up. Herein disclosed is an effective strategy to develop a very active and durable pH-universal electrocatalyst for hydrogen evolution reaction (HER). This catalyst is constructed by a sandwich- like structure where cobalt phosphide (CoP) nanoparticles serve as the thin skin covering both sides of nickel phosphide (Ni₅P₄) nanosheet arrays, forming self- supported sandwich-like CoP/Ni₅P₄/CoP microsheet arrays with lots of mesopores and macropores. The as-prepared electrocatalyst requires an overpotential of only 33 mV to achieve the benchmark 10 mA cm ² with very large exchange current density, and high turnover frequencies (TOFs) in acid media, which is superior to most electrocatalysts made of metal phosphides, well-known MoS₂, and WS₂ catalysts, and performs comparably with the state-of-the-art Pt catalysts. In particular, this electrocatalyst shows impressive operational stability at extremely large current densities of 1 A cm ², indicating its possible application toward large- scale water electrolysis. Additionally, this electrocatalyst is very active in alkaline electrolyte (71 mV at 10 mA cm ²), which demonstrates its pH universality as a HER catalyst with outstanding catalytic activity. This simple strategy does not involve any solvothermal and hydrothermal process, paving a new avenue to design robust non- noble electrocatalysts for hydrogen production toward commercial water electrolysis.

[0057] Herein disclosed is a rational design of a highly active electro-catalyst based on a sandwich-like hybrid of cobalt and nickel phosphides for HER in water splitting. The cobalt phosphide (CoP) nanoparticles were designed to cover nickel phosphide (Ni₅P₄) nanosheet arrays, forming self-supported microsheet arrays. Benefiting from the sandwich-like design of active material, the hierarchical CoP/Ni₅P₄/CoP microsheet arrays electrode shows Pt-like activity for HER with pH universality and exceptional stability.

[0058] In some embodiments, in 0.5 M H₂S0₄, the electro-catalyst disclosed herein requires overpotentials of only 33 and 85 mV to achieve current densities of 10 and 100 mA cm ² respectively, with a relatively small Tafel slope of 43 mV dec ¹ and large exchange current density of 1.71 mA cm ², and exhibits large surface area and simultaneous large turnover frequencies (TOF) of 0.453 and 1.22 s ¹ at 75 and 100 mV overpotentials respectively, outperforming most efficient non-noble-metal HER electrocatalysts.

[0059] In particular, this robust catalyst shows excellent durability at high current density of 1 A cm ². Moreover, the as-prepared CoP/Ni₅P₄/CoP microsheet arrays electrode only requires 71 mV to deliver 10 mA cm ² in 1 M KOH, and also exhibits good durability at 30 and 500 mA cm ², demonstrating its pH universality for efficient hydrogen production.

Experimental details

[0060] Chemicals: Cobalt (II) nitrate hexahydrate (Co(N0₃)₂-6H₂0, 98%, Sigma- Aldrich), N,N-Dimethylformamide [(CH₃)₂NC(0)H, anhydrous, 99.8%, Sigma-Aldrich], red phosphorous powder (P, > 97%, Sigma-Aldrich), sulfuric acid (H₂S0₄, ACS 95.0- 98.0%, Alfa Aesar) potassium hydroxide (KOH, 50% w/v, Alfa Aesar), and Ni foam (thickness: 1 .6 mm), graphite foil (Alfa Aesar), and Pt wire (CH Instruments). Deionized water (resistivity: 18.2 MW-cm) was used for the preparation of all aqueous solutions. All the chemicals were of analytical grade and utilized without further purification.

[0061] Synthesis of Ni_fiP4-Ni_?P nanosheet array support on Ni foam. The as-obtained commercial Ni foam was in some embodiments cut into 1 cm² regular pieces, one piece of 1 cm² Ni foam was placed in the tube furnace, which was heated to 500 °C quickly and kept at this temperature for around 1 hour for thermal phosphorization in argon atmosphere. The red phosphorus powder was used as the phosphorus source put in at the upstream. After material growth, shut down of the power of the tube furnace occurred and naturally cooled under argon protection.

[0062] Synthesis of CoP/N PVCoP microsheet arrays electro-catalyst. The cobalt precursor ink was prepared by dissolving cobalt nitrate hexahydrate [CO(N0₃)₂.6H₂0] in N,N dimethylformamide (DMF), then the nickel phosphide nanosheet arrays on Ni foam (Ni₅P₄-Ni₂P/Ni) was soaked in the Co-ink and dried at ambient condition. The dried sample was then thermally phosphorized at 500 °C in a tube furnace with the red phosphorus powder placed at upstream, which resulted in the formation of hierarchical CoP/Ni₅P₄/CoP microsheet arrays.

[0063] Nitrogen adsorption measurements. The electro-catalyst microsheet array samples were firstly dried in vacuum at 373 K for 12 h before measurement. Nitrogen adsorption-desorption isotherms were tested at 77 K by a Quantachrome Autosorb-iQ BET surface analyzer. The specific surface area was evaluated from the BET method, and the distribution of pore sizes was analyzed by the BJH method.

[0064] Electrochemical measurements: were carried out at room temperature via a three-electrode configuration using Gamry Instruments, Reference 600. An 82 ml_ 0.5 M H₂S0₄ was added into the cell for HER as acidic electrolyte, and 100 ml of 1 .0 M KOH was used for HER as alkaline electrolyte. Saturated calomel electrode (SCE) and Hg/HgO electrode were used as the reference electrodes in acidic and alkaline electrolytes, respectively. Graphite foil (Alfa Aesar) was used as the counter electrode and the self-supported catalysts were directly connected with the working electrode. The catalytic performance was studied by collecting the polarization curves under a sweep rate of 2 mV s ¹ with the potentials ranging from 0.050 V to - 0.150 V vs RHE. High-purity anhydrous N₂ gas (Matheson, 99.9999%) was used to purge the system for 30 minutes before any measurements. In acidic electrolyte, the measured potentials vs SCE were converted to RHE by the Nernst equation (E_RHE = E_SCE + 0.0591 pH + 0.245). The catalyst was continuously cycled for over 1000 cycles at a scan rate of 50 mV s ¹ , so as to check its electrochemical stability. Chronopotentiometry test was conducted at cathodic current densities of -30 mA cm ² and -1 A cm ² for more than 25 hours. For the high current densities, durability test at -500 mA cm ² and -1 A cm ², Pt wire was used as counter electrode since graphite was not stable over a long time. In all other measurements, graphite foil was used as the counter electrode. The electrochemical impedance spectroscopy (EIS) spectrum was tested at -0.150 V vs RHE with the frequency changing from 0.1 Hz to 0.1 MHz with a 10 mV AC dither. All the potentials used herein are referred to RHE unless otherwise mentioned. Electrochemical tests were performed at room temperature and the curves were reported with iR compensation. In alkaline electrolyte, the measured potentials vs Hg/HgO were converted to RHE by the Nernst equation (ERHE = ^Hg/HgO + 0.0591 pH + 0.098).

[0065] Calculation of turnover frequency (TOF). Due to the bulk feature of our catalyst CoP/Ni₅P₄/CoP, the electrochemical methods of Bockris, and Zhou were selected to obtain the active site density at the surface. Herein in one embodiment, nearly all the surface active sites are accessible to the electrolyte, then it is possible to evaluate the TOF values by the equation as follows:

[0066] Herein the physical variables F, n, and / represent the Faraday constant (~ 96485 C/mol), active site density (mol), and the current (A) during hydrogen evolution in 0.5 M H₂S0₄, respectively. The factor 1/2 is because water electrolysis requires two electrons to evolve one hydrogen molecule from two protons. The electrochemical measurements were performed to collect the CV curves in 1 M PBS electrolyte (pH = 7). It is often difficult to assign the observed peaks to a given redox couple, so the surface active sites are nearly in linear relationship with the integrated voltammetric charges (cathodic and anodic) over the CV curves. Assuming a one- electron process for both reduction and oxidation, the upper limit of the active site number (n) can be evaluated according to the follow formula:

[0067] Here F and Q correspond to the Faraday constant and the whole charge of CV curve, respectively. In this case, it can be deduced that the number of active sites for this sandwich-like catalyst CoP/Ni₅P₄/CoP is close to 7.43 ^c 10 ⁷ mol/cm². Thus, the TOF values are calculated to be around 0.453 and 1.220 s ¹ for the CoP/Ni₅P₄/CoP catalyst at overpotentials of 75 and 100 mV, respectively.

[0068] Faradaic efficiency determination. A technique based on gas chromatography (GC)¹ was used to quantify the gas products and then the Faradaic efficiency under a constant current density of -50 mA cm ². Every 10 min, a glass syringe was used to carefully take 0.3 ml_ gas product from the sealed cell and injected it into the GC instrument (GOW-MAC 350 TCD) (Hamilton Gastight 1002). Based on this technique, H₂ gas was found to be the only product in experiment, and at nearly the same amount as that by theoretical calculations, supposing that each electron was utilized for H₂ generation.

[0069] The synthesis of the CoP catalyst, therefore involves in some embodiments a three-step process as illustrated in Figure 1. First, nickel (Ni) foam was thermally phosphorized at 500 °C in a tube furnace using red phosphorous (P) to form nickel phosphide nanosheet arrays as demonstrated by the scanning electron microscopy (SEM) images in Figure 1 b and 1f. Second, the nickel phosphide nanosheet arrays on Ni foam was soaked in the Co-ink and dried at ambient condition (Figure 1 (c)). The cobalt (Co) precursor ink was prepared by dissolving cobalt nitrate hexahydrate [CO(N0₃)₂.6H₂0] in N,N-Dimethylformamide (DMF). The firmly constructed structure (Figure 1 b and 1f) as well as hydrophilic nature of the nickel phosphide nanosheets facilitated to develop a uniform coverage of the nanosheets by Co precursor ink. Finally, the dried sample was thermally phosphorized again at 500 °C, leading to the formation of a unique structure of hierarchical CoP/Ni₅P₄/CoP microsheet arrays electrode, as revealed by the SEM images in Figure 1 d and 1 g.

[0070] Figure 1. shows: a synthetic scheme of sandwich-like Cop/Ni₅P₄/CoP electrocatalyst. (a,e) show SEM images of Ni foam used as the starting electrode material (b, f) show SEM images of nickel phosphide nanosheet arrays after the first synthetic step; (c) is a diagram showing nickel phosphide nanosheets on Ni foam that was soaked in cobalt precursor ink prepared by dissolving cobalt nitrate hexahydrate [Co(N0₃)₂.6H₂0] in DMF; and (d,g) show an SEM images of hierarchical CoP/Ni₅P₄/CoP microsheet arrays after the third synthetic step.

[0071] The surface morphology of CoP/Ni₅P₄/CoP microsheet arrays electrode changes greatly with varying the concentration of precursor Co-ink (Figure 6, ESI†). With precursor ink concentration of 0.25 g/ml, most of the nickel phosphide nanosheets were uniformly covered by CoP after phosphorization (Figure 6 (b, and e) ESI†), leading to an extremely large surface exposure of the cobalt phosphide, and forming a sandwich-like structure between CoP nanoparticles and Ni₅P₄ nanosheet as confirmed by the SEM images (Figure 7, ESI†). When the concentration was increased to 0.4 g/ml, all the interspaces between the nanosheets were filled by cobalt precursor, and the nickel phosphide nanosheets were buried (Figure 6 (a, and d) ESI†), thereby reducing the exposed surface of CoP. On the other hand, at very low concentration of the precursor ink (0.1 g/ml), most parts of the nickel phosphide nanosheets were not covered by CoP (Figure 6 (c), and (f)) ESI†). Since the optimum coverage of the nickel phosphide nanosheets was achieved at Co precursor ink concentration of about 0.25 g/ml, further analyses were carried out using this concentration unless otherwise mentioned.

[0072] In some embodiments, the role of phosphorus was examined, wherein a sample was also prepared by annealing in the absence of phosphorus source at the third step for comparison. A notably different morphology was observed in the absence of phosphorus source (Figure 8 (a, b) ESI†), which in some embodiments may be due to formation of a different chemical compound of cobalt. In addition, the morphology variation with temperature was also investigated by phosphorization at 600 °C at the third step of synthesis. At 600 °C, a prominent change in surface structure of the CoP was demonstrated by SEM images in Figure9 (ESI†), which is due to structural deformation of inner frame of nickel phosphide nanosheets as well as outer cobalt phosphide coverage at higher temperature.

[0073] Figure 2. Shows the morphology and chemical composition of CoP/Ni₅P₄/CoP electro-catalyst (a-c) depicts SEM images of CoP/Ni₅P₄/CoP electrode; (d) depicts an HRTEM image showing crystalline Ni₅P₄ at the inner structure of CoP/Ni₅P₄/CoP electrode, and the Fast Fourier Transform (FFT) in the inset demonstrates the crystal structure of Ni₅P_4. (e, f), HRTEM images showing amorphous as well as crystalline CoP at the outer structure of CoP/Ni₅P₄/CoP electrode. The FFTs in the inset of (e) and (f) demonstrate the amorphous and crystalline structures of CoP, respectively.

[0074] High resolution SEM images in Figure 2a-c display closer views of the surface morphology of the CoP, demonstrating a porous structure of interconnected nanostructures composed of macropores from Ni foam and nickel phosphide nanosheets, and mesopores (size) from CoP particles (Figure 10 and Figurel 1 , ESI†), which may contribute to the high HER catalytic activity by facilitating exposure of numerous active sites, and also offering efficient diffusion channels during H₂ evolution in water splitting. In order to gain further insight into the crystalline structure, the as-prepared electro-catalyst was examined by transmission electron microscopy (TEM) (Figure 2 (d)-(f)). The high-resolution transmission electron microscopy (HRTEM) image in Figure 2 (d) clearly demonstrates that the nickel phosphide nanosheets are highly crystallized with the lattice fringe spacings of 0.207 nm and 0.298 nm corresponding to the (224) and (020) planes of Ni₅P₄ crystals, respectively. Accordingly, the outer coverage of the electrode indicates a mixed structure showing that a small fraction is crystallized corresponding to the (220) and (Ϊ2Ϊ) planes of CoP crystals, along with amorphous CoP as revealed by HRTEM images in Figure 2 (e) and 2 (f). Furthermore, the energy dispersive spectroscopy (EDS) elemental mapping confirms the uniform distribution of Co and P along with a small fraction of Ni, with atomic ratio of Co:P to be 1 :0.93, which is very close to the 1 :1 ratio as CoP, indicating the formation of CoP in the as-prepared electrode (Figure 12, ESI†). The small fraction of Ni in the EDS mapping could be either from Ni₅P₄ that remained while peeling off the cobalt phosphide from the electrode during TEM sample preparation or from diffusion of Ni from inner support during synthesis.

[0075] Powder X-ray diffraction (XRD) was employed to further characterize the phase composition of the samples and to determine the rough atomic ratio of different elements. In some embodiments the XRD patterns from the as-prepared nanosheet arrays (after step 1 ) demonstrate a mixture of nickel phosphides: mainly Ni₅P₄ and minor Ni₂P and a small amount of metallic Ni (Figure 3 (a)), suggesting that the original Ni foam was not fully transformed to nickel phosphide. Interestingly, the XRD pattern of the electrode prepared with re-phosphorization after step 3, shows mostly Ni₅P_4, along with a small shoulder around 48° of the 2-theta position corresponding to the CoP phase. The disappearance of Ni₂P and Ni after re- phosphorization may result from the diffusion of P source into the nickel and nickel phosphide converting them to Ni₅P₄ phase. To support this statement, a second phosphorization of nickel phosphide nanosheet samples in the same conditions as that of CoP growth, was performed. The XRD pattern (Figure 13, ESI†) is the same as that with Ni₅P₄ crystals, confirming the conversion of mixed Ni₅P₄/Ni₂P/Ni phases to a high-purity Ni₅P₄ phase during CoP growth. In some embodiments, the small peak of CoP in XRD patterns indicates the presence of only a small fraction of CoP crystals and the rest in some embodiments may be amorphous CoP, or in some embodiments may be because of the very thin thickness of CoP on top of Ni₅P₄, further supporting the mixed composition of CoP in the as-prepared hierarchical CoP/Ni₅P₄/CoP electrode as revealed by HRTEM images. In addition, compared to the nickel phosphide in the inner support structure, similar phase composition was noticed on the sample prepared by annealing only at the third synthetic step (Figure 14, ESI†), which indicates no further phase change of the nickel rich phosphides in the absence of additional P source. Moreover, absence of shoulder in the XRD patterns around 48° of the 2-theta angular position for the sample prepared with annealing only, further supports that the small shoulder for the sample prepared with re-phosphorization during the third synthetic step is from the CoP crystals.

[0076] Table 1 : Detailed analysis of XPS binding energies of different elements for CoP/Ni₅P₄/CoP catalyst.

[0077] X-ray photoelectron spectroscopy (XPS) was utilized to study the surface chemical composition of the samples. According to the high-resolution XPS spectra of the hierarchical CoP/Ni₅P₄/CoP arrays electrode, therefore providing clear identification of the presence of Co, P, and Ni (Figure 3 (b-d), Figure 15 and Table 1 , ESI†). The Co core level peaks (Figure 3b) appear at binding energies (BEs) of 779.6 eV and 794.6 eV, corresponding to Co 2p^3/2 and 2p^1/2 of cobalt phosphide, respectively.

[0078] In the XPS spectra of Co 2p, some contribution from oxidized component of cobalt phosphide is also observed at BEs of 782.7 eV and 798.6 eV, corresponding to the 2p^3/2 and 2p^1/2, respectively. The P core level peaks (Figure 3 (c))are located at 129.6 eV and 130.5 eV, which correspond to the P 2p^3/2 and 2p^1/2 of CoP, respectively. In the XPS spectra of P 2p, peaks at BEs of 134.3 eV and 135.1 eV correspond to the oxidized state 2p^3/2 and 2p^1/2, respectively, originating from the surface oxidation of the CoP. In addition, Ni core level peaks (Figure 3d) are also observed at the BEs of 857.7 eV and 875.0 eV, originating from the Ni 2p^3/2 and 2p^1/2 of surface oxidized nickel phosphide, respectively, which is possibly from the protruded nickel phosphide nanosheets (Figure 16, ESI†).

[0079] The protruded part in some embodiments is resulted from the formation of P- rich nickel phosphide (Ni₅P₄) phases by diffused P during re-phosphorization, and such feature is observed only in a few regions of the electrode. Furthermore, the BE of Co 2p centered at 779.6 eV is positively shifted from the position of elemental Co (778.1-778.2 eV), and that of P 2p centered at 129.6 eV is negatively shifted from the position of elemental P (129.9 eV), which imply that the Co carries a partial positive charge (d⁺) and the P carries a partial negative charge (d ) in CoP. This metal center Co (d⁺) and pendant base P (d ) in CoP resembles with hydrogenases and other metal complex HER catalysts, indicating the similar catalytic mechanism of CoP with them.

[0080] Figure 3. Characterization of the CoP/Ni₅P₄/CoP microsheet arrays electrode (a) Typical XRD patterns showing the phase structure of Ni foam, Ni₅P₄-Ni₂P/Ni support and CoP/Ni₅P₄/CoP electrode. Detailed XPS analysis of: (b) Co 2p, (c) P 2p, and (d) Ni 2p spectra.

[0081] The electrochemical HER performance of the as-prepared electro-catalyst was first assessed in 0.5 M H₂S0₄, and a scan rate of 2 mV s ¹ was set to collect the polarization curves by linear sweep voltammetry in a three-electrode setup (Figure 17, ESI†). All the potentials used here were converted to the reversible hydrogen electrode (RHE). Figure 4 (a) presents the relevant polarization curves of different electrodes including hierarchical CoP/Ni₅P₄/CoP microstructured arrays, pure Ni foam, a Pt wire, and the Ni₅P₄-Ni₂P/Ni nanosheet arrays support.

[0082] The CoP/Ni₅P₄/CoP electrocatalyst shows a fast increase of cathodic current density with increasing overpotentials, suggesting CoP/Ni₅P₄/CoP as a high- performance 3D cathode for hydrogen generation from water splitting. At a geometric current density of 10 mA cm ², the as-prepared hierarchical CoP/Ni₅P₄/CoP 3D electrode requires an overpotential of only 33 mV, which is very close to that of Pt (31 mV), and much lower than 293 mV for pure Ni foam and 79 mV for the Ni₅P₄- Ni₂P/Ni support. [0083] Compared to the samples prepared with 0.4 and 0.1 g/ml concentration of precursor Co-ink at the second synthetic step, the sample prepared with 0.25 g/ml exhibits much better electrocatalytic performance (Figure 18, ESI^†), which is possibly due to the greater exposure of active surface as revealed by SEM images in Figure 6 (b) and (e). In contrast, the catalytic activity of the sample prepared by annealing only at the step 3 of synthesis shows a poorer HER performance (Figure 19) similar to the Ni₅P₄-NiP₂/Ni support, possibly due to the formation of cobalt oxide rather than cobalt phosphide in the absence of P source. This oxide is not stable during water electrolysis as supported by the SEM images taken after electrochemical test (Figure 8 (c)), which shows that the outer layer of cobalt compound was washed away by highly corrosive acidic electrolyte during electrochemical potential cycling, leaving behind the nickel phosphide nanosheet arrays skeleton. In some embodiments, the phosphorization temperature at step 3 of synthesis plays a role in the morphologies of the hierarchical CoP/Ni₅P₄/CoP, which accordingly has great effects on the electrocatalytic HER activity, confirming that in some embodiments that 500 °C is the optimal temperature for growing this sandwich-like CoP/Ni₅P₄/CoP electrocatalyst (Figure 20), wherein in some further embodiments, the electro-catalytic performance of CoP/Ni₅P₄/CoP grown at 500 °C is improved as compared to many of the highly efficient HER electrocatalysts reported recently (Table 2), including nanostructured Ni₂P (105 mV), Mo-W-P/carbon cloth (100 mV), WS_{2(i -x)}Se_2x/NiSe₂ (88 mV), MoS_{2(1. x)}Se_2x/NiSe₂ (69 mV), CoP/carbon cloth (67 mV),¹Ni/Ni₅P₄/NiP₂ (61 mV), CoNiP (60 mV), CoPS (48 mV), Fe0_.5Co0_.5P (37 mV), etc.

[0084] Table 2: tabulates the catalytic performance of the as-obtained HER catalyst in comparison with other available non-precious catalysts in the literatures. Here, h^_, /7-I 00, and /7-1000 are denoted as the overpotentials at current density of 10, 100, and 1000 mA cm ², respectively, and j₀ is the exchange current density. Electrolyte: 0.5 M H₂S0₄.

[0085] NA: Not applicable since not reported. The symbol“” means that the value is extracted on the basis of the Tafel slope and overpotential at 10 mA cm ².

[0086] In addition, a Tafel slope of 43 mV dec ¹ (Figure 4 (b), derived from the polarization curve, indicates that the HER process by this CoP/Ni₅P₄/CoP electrode proceeds through Volmer-Heyrovsky mechanism. Moreover, from the relevant Tafel plot, herein the exchange current density to be 1.708 mA cm ² for the CoP/Ni₅P₄/CoP catalyst (Table 2, ESI†), which is significantly larger than most of the reported active catalysts based on transition metal chalcogenides including CoSe₂, WP₂ nanowire/CC, and transition metal phosphides including CoP, Ni₂P, and MoP particles can be extracted. Finally, to further explore the superior catalytic performance of this sandwich-like catalyst, its TOF values at different overpotentials using a normal electrochemical method (Figure 21 ) were quantified. This parameter is a relevant performance index representing the intrinsic catalytic activity of the catalyst. As shown in Table 3, the TOF values are evaluated to be around 0.453 and 1.220 H₂S ¹ (the unit seems wrong) at overpotentials of 75 and 100 mV, respectively.

[0087] Table 3: shows the comparison of the catalytic parameters among the catalyst and other robust non-precious catalysts of embodiments of the electro- catalysts disclosed herein. C_di, ECSA, TOF₇₅, and TOF_10o represent double-layer capacitance, electrochemical surface area, and turnover frequencies at overpotentials of 75 and 100 mV, respectively. Electrolyte: 0.5 M H₂S0₄.

[0088] NA: Not applicable since not reported. The symbol“” means that the value is extracted on the basis of the data provided in the references. The C_di and ECSA values were calculated by the method proposed in our previous paper, supposing that the flat electrode has a specific capacitance of 45 pF cm ² in 0.5 M H₂S0₄. Also, the active sites of metal phosphides were calculated according to this reference once the capacitance was provided in the relevant references.

[0089] In some embodiments, the TOF value of the electro-catalysts disclosed herein increases to 4 H₂s ¹ at only 135 mV (Figure 22, ESI†). Thus, this sandwich-like CoP/Ni₅P₄/CoP is found to have a very low overpotential (33 mV), small Tafel slope (43 mV dec ¹), extremely large exchange current density (1 .708 mA cm ²), and very large TOF of 4 H₂s ¹ at 135 mV,_suggesting its exceptional H₂-evolving efficiency.

[0090] Further, Figure 4. depicts electro-catalytic measurements of different electrodes for hydrogen evolution in acid (a) The polarization curves of the CoP/Ni₅P₄/CoP, Ni₅P₄-Ni₂P/Ni, pure Ni foam, and a Pt wire electrodes (b) Tafel plots derived from the curves in (a) (c) EIS Nyquist plots of the CoP/Ni₅P₄/CoP and Ni₅P₄-Ni₂P/Ni electrodes (d) The double-layer capacitance (C_di) of the CoP/Ni₅P₄/CoP and Ni₅P₄-Ni₂P/Ni electrodes (e) Polarization curves of the CoP/Ni₅P₄/CoP electrode after 250 and 1000 cycles (f) Potential testing at constant current densities of 30 mA cm ² and 1 A cm ² on the CoP/Ni₅P₄/CoP electrode.

[0091] The physical origin of the electrode kinetics was further examined by electrochemical impedance spectroscopy (EIS) at the potential of - 0.150 V vs RHE, and the EIS Nyquist plots (Figure 4(c)) can be well fitted by a simplified Randles circuit (Figure 23, ESI^†). The series ( R_s ) and charge-transfer resistances (R_ct) are extracted from the fitted plots. Obviously, the R_ct of the CoP/Ni₅P₄/CoP microsheet arrays electrode is 1.2 W, meaning very fast charge transfer between the electrolyte and the catalyst. Also, there is a small R_s (2.0 W) for this catalyst, which reflects strong electrical integration of the catalyst to its support. To unveil the difference of intrinsic catalytic activity between Ni₅P₄-Ni₂P/Ni support and CoP/Ni₅P₄/CoP microsheet arrays (Table 4, ESI†), a cyclic voltammetry (CV) method (Figure 4 (d), Figure 24) to derive the electrochemical double-layer capacitance (C_di), which is directly related to the electrochemically active surface area (ECSA) of the electrocatalysts was adopted.

[0092] Table 4. provides a summary of the catalytic activities from the Ni₅P₄-Ni₂P/Ni and CoP/Ni₅P₄/CoP electrodes yo, _normalized is the normalized current density by relative surface area. The electrochemical surface area (ECSA) was calculated by dividing double-layer capacitance (C_di) by the specific capacitance (C_s = 45 pF cm ²) of flat electrodes in 0.5 M H₂S0₄.

[0093] In some embodiments of the electro-catalyst there is a large electrochemical surface area and roughness factor due to the presence of mesopores, nanosheet arrays and macropores of the catalyst (Table 3, ESI†). Then the exchange current density (/o_{, normalized}) is further normalized by the C_di values, which is a useful parameter to compare the intrinsic catalytic activity. After normalization (Table 4), it was found herein that the normalized exchange current density of this CoP/Ni₅P₄/CoP catalyst is 625.6 mA cm ², which is still far larger than that (366.1 mA cm ²) of the Ni₅P₄-Ni₂P/Ni support, demonstrating that the CoP/Ni₅P₄/CoP catalyst has a higher intrinsic catalytic activity than Ni₅P₄-Ni₂P/Ni nanosheet arrays-based catalyst. The exchange current density of the as-prepared CoP/Ni₅P₄/CoP catalyst compares favorably to most of the highly efficient electrocatalysts reported so far (Table 2).

[0094] Thus, in some embodiments, and according to the C_di measurements and EIS analysis, the CoP/Ni₅P₄/CoP electrode has a much higher electrode kinetics toward hydrogen evolution, which could be related to the following factors: (1 ) the strong contact of CoP with inner Ni₅P₄ support that enables good mechanical and electrical connection, providing an easy pathway for electrons to flow during cathodic polarization; (2) increased and improved electrical conductivity of CoP, facilitating fast charge transport; and (3) the 3D structure along with highly porous feature of the interconnected nanostructures that enables greater exposure of active sites for hydrogen adsorption as well as easy diffusion pathways for the electrolyte and gaseous products.

[0095] Durability is another important factor to assess the electro-catalysts, which was studied by conducting accelerated cyclic voltammetry between potentials of 0.050 V and -0.150 V vs RHE at a scan rate of 50 mV s ¹ for 1000 potential cycles. No significant reduction in the current densities (Figure 4e) demonstrated the high operational stability of the as-prepared catalyst. Additionally, chronopotentiometry test at two different current densities of 30 mA cm ² and 1 A cm ² for over 25 hours (Figure 4 (f)) was performed. No dramatic change of the potential is detected, further confirming the exceptional operational stability of the CoP/Ni₅P₄/CoP catalyst in acid at a very high current density. Additionally, the structure, phase and surface chemical composition of the electrocatalyst after performing 1000 CV cycles polarization test by SEM, XRD, and XPS (Figure 25-27) was examined. Evidently, almost no obvious changes are found in the SEM images and XRD patterns, meaning that there is no change of the crystal structure and phase of the catalysts during HER testing. The XPS spectra of Co 2p and P 2p core level peaks of CoP/Ni₅P₄/CoP electrode after electrochemical test showed almost diminished peaks from oxides, possibly due to the reduction of surface oxidation during potential cycles. The CoP/Ni₅P₄/CoP electrocatalyst also demonstrates promising HER activity under alkaline condition. Figure 5 shows the HER performance of CoP/Ni₅P₄/CoP in 1.0 M KOH (pl-M4). The hierarchical CoP/Ni₅P₄/CoP requires only 71 and 140 mV of overpotentials to obtain the current densities of 10 and 100 mA cm ², respectively, along with the Tafel slope of 58 mV dec ¹. The catalytic activity of as-prepared CoP/Ni₅P₄/CoP at a benchmark current density of 10 mA cm ² in alkaline media compares favorably to the recently reported efficient non-noble-metal based HER catalysts, including NiS₂/MoS₂ (204 mV), CoMoP (81 mV), (Co_1-xFe_x)₂P (79 mV), transition metal phosphides (Table 5).

[0096] Table 5. Comparison of the catalytic HER performance among as-obtained catalyst and other available metal phosphide catalysts in the literatures. Here, h^_, and /?io₀ are denoted as the overpotentials at current density of 10, and 100 mA cm ², respectively. Electrolyte: 1 M KOH.

[0097] NA: Not applicable since not reported. The symbol“” means that the value is extracted on the basis of the Tafel slope and overpotential at 10 mA cm ².

[0098] Disclosed herein is a further investigation of the electrochemical performance at large current densities (Figure 5 (a)). The CoP/Ni₅P₄/CoP electrode shows more efficient catalytic activity than the Pt wire at current density higher than 91 mA cm ², which is important for large scale hydrogen production via water electrolysis, wherein the CoP/Ni₅P₄/CoP shows pH-universal HER activity in a wide pH range. In some embodiments, it is noticed that metal phosphosulfides are promising electrocatalysts for hydrogen generation for water splitting in acid or base, however, the best of these prior art catalysts still require an overpotential of 48 mV to reach the benchmark current density 10 mA cm ² in acid, which is not a efficient as the CoP/Ni₅P₄/CoP catalysts described herein (33 mV), and such catalysts have much lower Tafel slope, and faster increase of the geometric current density with the overpotential, lower overpotential (71 mV) to reach 10 mA cm ², for hydrogen generation at wide pH ranges. Furthermore, the reasonable operational stability at high current density in highly alkaline medium (Figure 5d), is another important feature of the as-prepared electrode that may be of great potential to be implemented as a sustainable pH universal hydrogen evolving electrode. Finally, the gas products and relevant Faradaic efficiency were evaluated in acid and base electrolytes by the gas chromatography-based technique (Figure 28). The efficiency, which reflects the conversion of the electrons involved in the catalytic reaction, is determined to be nearly 100%, meaning that nearly all the electrons are utilized for generating H₂ during water electrolysis.

[0099] Hence, disclosed herein in one embodiment is a highly efficient HER electrocatalyst developed by a facile synthetic approach. The as-prepared hierarchical CoP/Ni₅P₄/CoP microsheet arrays electrode is binder free, self- supported 3D architecture that can be directly used as a cathode for HER. The CoP/Ni₅P₄/CoP microsheets arrays electrode shows Pt-like activity for HER catalysis with reasonable operational stability at high current density in acidic as well as alkaline electrolytes. The outstanding HER catalytic activity of this electrode is related to the good mechanical and electrical connection between CoP catalyst and Ni₅P₄ support, numerous active sites and high intrinsic catalytic activity of the sandwich-like CoP/Ni₅P₄/CoP electrode. Herein disclosed is a new self-supported 3D robust HER catalysts for large-scale hydrogen production via water splitting.

EXAMPLE 2

[0100] In another embodiment, an efficient HER catalyst functions in an alkaline environment under a large current density. It is of immense importance to accelerate gas releasing from the catalyst surface for large-current-density water splitting, which makes way for more active sites. This can be achieved by integrating 1 D nanorods or nanowires (NWs) into 3D hierarchical architectures on conductive substrates which not only enhances the gas releasing but also maximizes the surface area and accelerates the electron transfer. Meanwhile, it also avoids the use of polymer binders and ensures robust contacts. Furthermore, porous materials are extremely favorable for catalytic reactions due to their large surface area and rich active sites. Hence, a novel 3D hierarchical catalyst consisting of highly porous Ni_2(i-x)Mo_2xP nanowire arrays on Ni foams for efficient HER under large-current-density in alkaline electrolyte are also disclosed herein. As illustrated in Figure 29, the well-aligned nanowire arrays on the Ni foam make the ternary Ni_{2(1 -x)}Mo_2xP catalyst as an integrated 3D electrode, where the Ni foam works as an efficient current collector and the highly conductive Ni_2(1-x)Mo_2xP nanowires provides a continuous pathway for electron transfer. Additionally, the highly porous nanowires with a rough surface offer a large active surface area with numerous active sites and rapid release of H₂. More importantly, density functional theory (DFT) calculations determine that the free energy of water activation and hydrogen adsorption is optimized for the Ni_{2(1 -x)}Mo_2xP catalyst.

[0101] The fabrication procedure of 3D Ni_{2(1 -x)}Mo_2xP catalysts involves two steps, which is illustrated in Figure 29. First, NiMo0₄ xH₂0 nanowire arrays were grown on Ni foams (NF) via a hydrothermal method. Then the as-prepared NiMo0₄ xH₂0 nanowire arrays were transformed to ternary Ni_2(1-x)Mo_2xP through a one-step phosphorization reaction. Before hydrothermal reactions, the NF was treated in HCI solution (3 M) for several minutes, which generates a rough surface for strong adhesion of active materials, as the scanning electron microscopy (SEM) image shown in Figure 30 (a) which shows the SEM images of NiMo0₄ xH₂0 precursor, revealing uniform coverage of vertical nanowire arrays on the entire NF. X-ray diffraction (XRD) pattern and energy dispersive X-ray (EDX) spectrum on the sample demonstrated the pure phase and composition of NiMo0₄ xH₂0. In some embodiments a series of Ni_2(1-x)Mo_2xP nanowire arrays were synthesized on the NF under different temperatures and phosphorus (P) amounts to optimize the phosphorization process. Figure 30 (b and c) show the SEM images of Nί_2(1-c)Mo_2cR sample prepared under 500 °C with 100 mg P at low and high magnifications, respectively. From Figure 30 (b), it can observe that the bimetallic phosphide maintains the vertically aligned nanowire morphology and integration feature of the precursor after phosphorization. The well aligned nanowire arrays grown on the NF will offer efficient diffusion pathways for electrolyte and open channels for gaseous products to be released. Figure 30 (c) reveals that the nanowires present a cuboid shape with a rough surface. The cross-sectional SEM images further show that the nanowires are vertically grown on the Ni foam, and the thickness of the nanowire layer are ~10 pm. Transmission electron microscopy (TEM) images in Figure 30 (d and e) further reveal that the nanowires are highly porous with many tiny pores inside. Figure 30 (e) also exhibit the extremely rough surface of the nanowires. Such highly porous and rough nanostructures are beneficial to catalytic reactions by providing more exposed active sites.

[0102] Figure 30 (f) shows the XRD pattern of Ni2(1-x)Mo2xP, which is consistent with the standard Ni2P pattern (JCPDS No. 89-2742; the tiny peak around 53.8° was attributed to the NiMo04 precursor.) High-resolution TEM (HRTEM) image in Figure 30 (g) reveals apparent lattice fringes with an inter-planar spacing of 0.225 nm, which is assigned to the (1 1 1 ) plane of Ni_2(i-x)Mo_2xP. The corresponding selected- area electron diffraction (SAED) pattern in Figure 30 (h) further verifies the diffraction spots of different planes of the well crystallized Ni_2(i-x)Mo_2xP. Figure 1 (i) shows the scanning TEM (STEM) image and corresponding EDX elemental mapping images of Ni, Mo, and P, confirming the existence and uniform distribution of the three elements in the whole nanowires. The atomic ratio of Ni: Mo: P was determined to be 1 : 0.97: 0.91 by the inductively coupled plasma optical emission spectrometry (ICP- OES) (See: Luo et al.,“Ternary Ni_{2(1 -x)}Mo_2xP Nanowire Arrays toward Efficient and Stable Hydrogen Evolution Electrocatalysis under Large-Current-Density”, Nano Energy 53, 492-500 (2018) incorporated herein in its entirety by reference). For comparison, pure Ni₂P was also synthesized on the Ni foam via the same phosphorization method of Ni(OH)2 under 500 °C with 100 mg P. X-ray photoelectron spectroscopy (XPS) measurements were then conducted to investigate the surface composition and chemical valence states of the Ni_2(1-x)Mo_2xP and Ni₂P samples. As shown in Figure 31 (a), the XPS survey spectra confirm the presence of Ni, Mo, and P elements for Ni_2(1-x)Mo_2xP, while only Ni and P elements for Ni₂P. In Figure 30 (b), the high-resolution XPS spectra of Ni 2p show two spin- orbit doublets along with two shakeup satellites (identified as“Sat.”) for the two samples. For the pure Ni₂P, the two spin-orbit doublets are located at binding energies (BEs) of 854.6 and 872.4 eV, which are assigned to the Ni 2p3/2 and Ni 2p1/2 of Ni-P, respectively. However, the two spin-orbit doublets slightly shift to the lower BEs at 854.1 and 871.9 eV for the Ni2(1-x)Mo2xP, suggesting an electronic structure change due to Mo substitution. The two small peaks at BEs of 856.4 and 874.8 eV for the Ni2P correspond to the Ni-POx due to the surface oxidation of Ni₂P in air. In the P 2p region of Ni₂P (Figure 2c), the two peaks at BEs of 129.8 and 130.6 eV are consistent with P 2p3/2 and P 2p1/2 from Ni2P, respectively, and the peak at BEs of 135.4 eV is attributed to the oxidized P species. Nevertheless, only two shifted peaks at BEs of 130.1 and 131 .2 corresponding to the P 2p3/2 and P 2p1/2 of Ni-P are observed for the Ni_2(i-x)Mo_2xP, which is consistent with the XPS spectra of Ni 2p. For the Mo 3d spectrum of Ni_2(1-x)Mo_2xP (Figure 2d), the two spin- orbit doublets at BEs of 233.1 and 236.2 eV are assigned to the Mo 3d5/2 and Mo 3d3/2, respectively.

[0103] The HER activity of as-prepared catalysts was evaluated in 1 M KOH in a typical three-electrode cell comprising a graphite foil as the counter electrode. The activity of various Ni_2(1-x)Mo_2xP electrodes prepared under different temperatures and P amounts were first assessed.

[0104] From the HER polarization curves the sample prepared under 500 °C with 100 mg P performs the best was identified then compared with the NiMo0₄ (obtained by annealing of Ni- Mo0₄ xH₂0 precursor at 500 °C without any P), pure Ni₂P, and commercial Pt wire. The inset in Figure 3a displays the HER performance at current densities up to 100 mA cm ², showing that the ternary Ni_2(1-x)Mo_2xP catalyst at disclosed herein yields current densities of 10 and 100 mA cm ² at overpotentials of 72 and 162 mV, respectively, exhibiting a much higher activity than that of NiMo04 (263 and 446 mV) and Ni2P (167 and 279 mV).

[0105] This outperforms most of the reported TMPs catalysts including MoP₂ (194 and 260 mV) [45], (Co1 -xFex)2P (79 and 178 mV) and CoMoP (81 and 165 mV), TMDs catalysts including MoS₂/Ni₃S₂ (98 and 191 mV), NixCo3-xS4/Ni3S2 (136 and 258 mV), Cu@CoS_x (134 and 267 mV), and other non-noble metal catalysts for the alkaline HER.

[0106] For commercial applications, the performance at large-current-density was herein disclosed at Figure 31 (a). Embodiments of the 3D Ni_2(i-x)Mo_2xP electrode presented herein exceeds the Pt wire when the current density is larger than 188 mA cm ², and it delivers current densities of 500 and 1000 mA cm-2 at overpotentials of 240 and 294 mV, respectively.

[0107] Figure 31 (b) presents a comparison of current density at overpotential of 300 mV for the electrodes. The 3D Ni_2(1-x)Mo_2xP electrode achieves a current density of 1077 mA cm ² at overpotential of 300 mV, which is about 58 and 7.7 times that of NiMo04 (18.5 mA cm ²) and Ni2P (140 mA cm ²), respectively, as well as 90% higher than that of commercial Pt wire (566 mA cm-2). The Tafel plots of Figure 31 (c) shows that the 3D Nί_2(ΐ-_c)Mo_2cR electrode has a Tafel slope of 46.4 mV dec-1 , which is close to that of the Pt wire (36.9 mV dec-1 ) and much smaller than that of NiMo0₄ (106 mV dec-1 ) and Ni2P(89.6 mV dec-1 ).

[0108] This value is also much lower than most of the reported HER catalysts in 1 M KOH electrolyte. What's more, based on the intercept of the Tafel plots at the thermodynamic redox potential (zero overpotential, the exchange current density (jO) was calculated to be 537 mA cm ² for the ternary Ni_2(1-x₎Mo_2xP catalyst, which is not only 10.7 times that of NiMo0₄ (50.1 pA cm ²) and 2.7 times that of Ni₂P (199 pA cm ²), but also much larger than most of the reported non-noble metal HER catalysts in 1 M KOH electrolyte. It was also determined herein that the turnover frequency (TOF) for Ni_2(i-X)Mo_2xP and Ni₂P catalysts, and the Ni_{2(i -X)}Mo_2xP catalyst showed TOF values of 0.038, 0.452, and 1 .965 s ¹ at overpotentials of 100, 200, and 300 mV, respectively, which were much larger than that of Ni₂P. The small Tafel slope, large jO and TOF reveal the inherent excellent HER activity of embodiments of the ternary Ni_2(1-X)Mo_2xP catalyst in the strong base. Thus, these results strongly demonstrate that embodiments of the disclosed 3D ternary Ni_{2(1 -X)}Mo_2xP electrode are highly efficient HER catalyst in alkaline media, especially at large-current density.

[0109] Stability is also a pivotal criterion in evaluating the performance of an electrocatalyst, especially for large-scale water electrolysis. To this end, we examined the stability of our 3D N ^Mo^P electrode by cyclic voltammetry (CV) sweeps between 0.02 V to -0.25 V vs. RHE at a scan rate of 50 mV s-1 in 1 M KOH.

[0110] As shown in Figure 32 (d)), the polarization curve after 5000 CV cycles is almost identical with the initial one, showing no apparent degradation of cathodic current densities. Besides, the long-term electrochemical stability tests were also conducted at different potentials. As shown in Figure 32 (e), the current densities of the Ni_2(1-X)Mo_2xP electrode disclosed herein remain stable at given potentials of -1 10, -170, and -220 mV for over 160h. Further investigation of the morphology and composition change of the Ni_2(1-x)Mo_2xP electrode after stability test indicate that the morphology of vertically aligned nanowire arrays is well maintained after the longterm stability test. Also the XRD pattern of Ni_2(1-x)Mo_2xP after stability tests match well with the initial images indicating no phase change after electrolysis. Moreover, XPS spectra are also almost the same with those from the fresh samples, meaning the surface composition of Ni_{2(1 -x)}Mo_2xP catalyst has not changed after the stability test. [0111] Therefore, embodiments of the 3D ternary catalyst disclosed herein are extremely stable for HER in the alkaline media. In an attempt to investigate the origins of performance of the disclosed Ni_{2(i -x)}Mo_2xP catalyst, the electrochemical active surface area (ECSA) for the as-prepared catalysts were determined. This was done by conducting cyclic voltammetry measurements and calculating the double- layer capacitance (Cdl), which is proportional to the ECSA . The Ni_2(1-x)Mo_2xP electrode possesses a Cdl of 51.2 mF cm ², which is nearly 1 .3 times that of Ni₂P (39.4 mF cm ²) and 3.22 times that of NiMo0₄ (16.2 mF cm ²), indicating a large ECSA with more exposure of active sites. However, the difference of ECSA between the three catalysts cannot fully account for the significant difference in HER performance (Figure 32 (a) and (b)), thus, further normalizing of the current density by the ECSA occurred, wherein Ni_2(1-x)Mo_2xP still shows much larger current densities than the other two catalysts at the same overpotentials, indicating the intrinsically high activity of Ni_{2(1 -x)}Mo_2xP.

[0112] Electrochemical impedance spectroscopy (EIS) was then used to study the catalytic kinetics of the catalysts. Ni_2(1-x)Mo_2xP electrodes of embodiments herein exhibit a much smaller charge transfer resistance (Ret) of ~1 .5 W, in contrast to 3.1 W of Ni₂P and 13.3 W of NiMo0₄, revealing favorable electron transport and fast catalytic kinetics. Meanwhile, all the three electrodes possess small series resistances (Rs), suggesting good electrical contacts with the substrate. The small Ret was closely related to the excellent conductivity of Ni_{2(1 -x)}Mo_2xP, which was further demonstrated by the electronic band structures calculated by the density function theory (DFT). The adopted structure for the Ni_2(1-x)Mo_2xP and Ni₂P was hexagonal in some embodiments, and in some embodiments both compounds show no band gap, indicating a metallic nature, thus beneficial for electron transfer.

[0113] It is generally accepted that the HER in alkaline media mainly involves two typical processes, namely, the Volmer step (equation 1 a) for H₂0 adsorption and activation (cleavage of O-H bonds to form H atoms adsorbed on surface of catalysts, H^*), and the Heyrovsky step (equation 2a) for H^* combination and H₂ release, which was called Volmer- Heyrovsky pathway:

Cat. + H₂0 + e Cat. - H^* + OH- (1 a)

(Volmer step) H20 + Cat. - H^* + e H2+ Cat. + OH- (2a)

(Heyrovsky step)

[0114] Here Cat. refers to catalysts. Therefore, further DFT calculations were conducted to assess the free energies of H₂0 adsorption, H₂0 activation, OH adsorption, and H adsorption on the (0001 ) surface of Ni₂P and Ni_2(1-x)Mo_2xP to deeply understand the nature of high HER activity of the Ni_2(1-x)Mo_2xP catalyst. The atomic ratio of Ni, Mo, and P in the Ni_{2(1 -x)}Mo_2xP is around 1 :1 :1 , so a NiMoP hexagonal structure was used for calculation, and two possibilities with Ni exposure and Mo exposure were considered.

[0115] For Ni₂P, two adsorption sites are taken for consideration. Four molecular structures were utilized for DFT calculations. Figure 33 (a) further displays the chemisorption models of H₂0 adsorption, OH adsorption and H adsorption on the (0001 ) surface of Ni₂P and NiMoP. As shown in Figure 33 (b), the NiMoP catalyst owns a much lower water adsorption energy (DEH20) compared to the Ni2P, especially for the Mo exposed NiMoP (DEH20 =-0.699 eV), indicating that water adsorption on Ni_2(1-x)Mo_2xP is much easier than that on Ni₂P. Figure 33 (c) shows the H₂0 activation energy for cleavage of O-H bonds on the (0001 ) surfaces of Ni₂P and NiMoP. It is clear that the NiMoP catalyst, especially for the Mo exposed surface, has a much lower H₂0 activation energy than that of Ni₂P. Thus, the Volmer step of H₂0 adsorption and activation is more favorable for the Ni_2(1-x)Mo_2xP catalyst, while relatively sluggish for the Ni₂P. Figure 4d shows the free energy diagram for H adsorption (AGH) on the (0001 ) surfaces of Ni₂P and NiMoP. The Mo exposed NiMoP has a lowest |AGH| of 0.08 eV, which is not only much lower than that of Ni₂P, but also very close to the Pt corresponding to an optimized hydrogen adsorption and desorption capability, thus beneficial for the Heyrovsky step.

[0116] Thereby, both the experimental and theoretical results confirm that embodiments of the ternary Ni_2(1-x)Mo_2xP catalyst disclosed herein is a highly efficient HER electrocatalyst in the alkaline media.

[0117] In another embodiment the OER activity of Ni_2(1-x)Mo_2xP catalysts in 1 M KOH was found to deliver current densities of 50 and 100 mA cm ² at overpotentials of 323 and 346 mV, respectively. OER catalysts of Cu nanowires which support NiFe layered double hydroxide nanosheets (Cu@ NiFe LDH) however had an output current densities of 50 and 100 mA cm ² at overpotentials of 244 and 281 mV, respectively. Therefore, some embodiments combined the disclosed 3D ternary Ni_2(i- _X)MO_2xP electrode with an Cu@NiFe LDH catalyst to build an alkaline electrolyzer for overall water splitting (Figure 34 (a)). As shown in Figure 34 (b), the electrolyzer delivers a current density of 10 mA cm ² at a small voltage of 1.51 V at room temperature, which is lower than the benchmark of Ir02/Pt. Attractively, the performance of Ni_2(i-x)Mo_2xP /Cu@NiFe LDH at large-current-density outdistances the benchmark. As shown in Figure 5c, current densities of 100 mA cm ² at 1 .65 V, 300 mA cm ² at 1.75 V, and 500 mA cm ² at 1.82 V are demonstrated, which is superior to most of the reported hetero-catalysts (two different catalysts for HER and OER, respectively) as well as almost all the bifunctional catalysts for overall water splitting in 1 M KOH. Moreover, as shown in Figure 34 (d), the stability upon long- term tests both under small (10 and 100 mA cm ²) and large current densities (500 mA cm ²) is very good.

[0118] Therefore, the combination of embodiments of the HER catalyst Ni_2(1-x)Mo_2xP and OER catalyst Cu@NiFe LDH disclosed herein are very promising for large-scale water electrolysis. Thus, it has been demonstrated that embodiments of the 3D ternary Ni_2(1-x)Mo_2xP porous nanowire arrays supported on Ni foams as disclosed herein, are highly efficient and stable electrocatalysts for HER in alkaline media. The 3D porous nanowire structures endow the catalyst with abundant active sites and open channels for gas releasing, and the good electrical conductivity of the bimetallic phosphide ensures the fast electron transfer. DFT calculations reveal that the ternary Ni_2(1-x)Mo_2xP owns optimal free energy of water activation and hydrogen adsorption on the surface. Thus, embodiments of the 3D ternary catalyst exhibits outstanding HER performance in alkaline electrolytes, especially at large-current-density, outperforms the commercial Pt wire and almost all the other alkaline HER catalysts. Pairing of the HER catalyst with OER catalyst of Cu@NiFe LDH, the electrolyzer also enables excellent performance for overall water splitting.

Claims

What is claimed is:

1 : A three dimensional hydrogen evolution reaction (HER) catalyst, comprising:

a porous Ni foam support;

a Ni₅P₄-Ni₂P scaffold positioned on the support;

a first layer of a metal phosphide (M_XP) positioned on a first side of the Ni₅P₄-Ni₂P scaffold; and

a second layer of the metal phosphate (M_XP) positioned on a second side of the Ni₅P₄-Ni₂P scaffold to form a M_xP/Ni₅P₄/M_xP microsheet array.

2. The catalyst of claim 1 , wherein the metal phosphide (M_XP) is selected from the group consisting of Co, Ni, and Fe or a combination thereof.

3. The catalyst of claim 2, wherein x is equal to 1 , 2, or 1/2.

4. The catalyst of claim 1 , wherein the microsheet array comprises CoP/Ni₅P₄/CoP.

5. The catalyst of claim 1 , wherein the M_xP/Ni₅P₄/M_xP microsheet array

comprises mesoporous pores.

6. The catalyst of claim 5, wherein the mesoporous pores are between 0.001 nm and 50 nm in diameter.

7. The catalyst of claim 1 , wherein the M_xP/Ni₅P₄/M_xP microsheet array

comprises surface active sites for HER.

8. The catalyst of claim 1 , wherein said catalyst is pH independent for catalyzing hydrogen evolution reaction (HER) from water splitting.

9. The catalyst of claim 1 , wherein the catalyst comprises an overpotential of about 71 mv at a current density of about 10 mA cm ² in an alkaline electrolyte.

10. The catalyst of claim 1 , wherein the catalyst comprises an overpotential of about 33mv at a current density of about 10 mA cm ² in an acid electrolyte.

1 1. The catalyst of claim 1 , wherein the catalyst has at least one of: a low onset potential, large cathode current density, small Tafel slopes, or large exchange current density.

12. A method of making a three dimensional hydrogen evolution reaction (HER) catalyst, comprising:

positioning a porous Ni foam support,

phosphorizing said Ni foam support, and forming a Ni₅P₄-Ni₂P scaffold; soaking said scaffold in a M_c-ink,

phosphorizing said M_c-ink; and

forming a M_xP/Ni₅P₄/M_xP microsheet array comprising the three dimensional hydrogen evolution reaction (HER) catalyst.

13. The method of claim 12, wherein said phosphorizing said Ni foam support is in an Ar atmosphere.

14. The method of claim 12, wherein said phosphorizing said Ni foam support at about 500 °C.

15. The method of claim 12, wherein said Mc-ink is Co-ink.

16. The method of claim 15, further comprising dissolving cobalt nitrate hexahydrate [Co(N0₃)₂.6H₂0] in N,N dimethylformamide (DMF) to form said Co-ink.

17. An electrode, comprising:

a three dimensional Hydrogen Evolution Reaction (HER) catalyst, wherein said electrode comprises:

a porous Ni foam support;

a Ni₅P₄-Ni₂P scaffold positioned on the support;

a first layer of a metal phosphide (M_XP) positioned on a first side of the Ni₅P₄-Ni₂P scaffold; and

a second layer of the metal phosphate (M_XP) positioned on a second side of the Ni₅P₄-Ni₂P scaffold to form a M_xP/Ni₅P₄/M_xP microsheet array, wherein said catalyst has at least one of: a low onset potential, a large cathode current density, a small Tafel slopes or a large exchange current density, at either of an alkaline or acidic pH.

18. The electrode of claim 17, wherein a low onset potential is between -10 and 200 mV; a large cathode current density is between -10 mV at 10 mA/cm² to about -120 mV at 10 mA/cm²; a small Tafel slopes is between 10 mV/dec to about 100 mV/dec; and a large exchange current density is between 10 to about 1000 pA/cm².

19. The catalyst of claim 1 wherein the catalyst comprises a large 3-D porous surface area.

20. The catalyst of claim 1 , wherein the M_xP/Ni₅P₄/M_xP microsheet comprises a about to 7.43 * 10 ⁷ mol/cm² active sites.

21. The catalyst of claim 4, wherein said catalyst comprises TOF values of between 0.453 and 1 .220 s ¹ at overpotentials of between 75 and 100 mV.