ELECTROCATALYSTS COMPRISING TRANSITION METALS AND CHALCOGEN FOR OXYGEN EVOLUTION REACTIONS (OER) AND MANUFACTURING THEREOF
20210162392 · 2021-06-03
Inventors
Cpc classification
C25B11/091
CHEMISTRY; METALLURGY
C01B19/002
CHEMISTRY; METALLURGY
C25B11/052
CHEMISTRY; METALLURGY
B82Y40/00
PERFORMING OPERATIONS; TRANSPORTING
Y02E60/36
GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
C25B11/093
CHEMISTRY; METALLURGY
B01J37/0018
PERFORMING OPERATIONS; TRANSPORTING
International classification
B01J37/00
PERFORMING OPERATIONS; TRANSPORTING
B01J27/057
PERFORMING OPERATIONS; TRANSPORTING
B01J35/00
PERFORMING OPERATIONS; TRANSPORTING
C25B11/091
CHEMISTRY; METALLURGY
Abstract
The present description relates to metal alloy electrocatalysts, preferably composed of Ni and Co as transition metals and Se as a chalcogen. The electrocatalysts can take the form of nanochalcogenides that can be made using cryogenic milling followed by surfactant-assistant milling. The electrocatalysts can be used in the context of water electrolysis or electroreduction of CO.sub.2 gas into carbon based products.
Claims
1. A process for producing a nanochalcogenide for use in electrocatalysis, comprising: subjecting at least two transition metals and at least one chalcogen to cryogenic milling to produce an alloyed chalcogenide material; subjecting the alloyed chalcogenide material to surfactant-assisted milling to produce a slurry comprising a nanochalcogenide; and separating the nanochalcogenide from the slurry.
2. The process of claim 1, wherein the cryogenic milling comprises cryogenic ball milling.
3. The process of claim 2, wherein the cryogenic milling comprises linear vibrational milling performed at speeds of 25 Hz to 35 Hz.
4. The process of claim 2, wherein the cryogenic ball milling is performed at a ball-to-powder ratio (BPR) of 8:1 to 12:1 on a mass basis.
5. The process of claim 1, wherein the alloyed chalcogenide material produced by cryogenic milling comprises particles having an average size above 1000 nm, measured using DLS or SEM.
6. The process of claim 1, wherein the alloyed chalcogenide material produced by cryogenic milling comprises M.sup.1M.sup.2E and/or (M.sup.1M.sup.2).sub.3E.sub.4, wherein M.sup.1 is a first transition metal, M.sup.2 is a second transition metal and E is a chalcogen, and is a single phase.
7. The process of claim 1, wherein the surfactant-assisted milling comprises surfactant-assisted ball milling performed in the presence of at least one surfactant and a solvent.
8. The process of claim 7, wherein the solvent comprises an alcohol and the least one surfactant comprises diphenylphosphoryl acid (DPPA), or oleic acid, or CRAB surfactant.
9. The process of claim 8, wherein the least one surfactant is present in an amount of 1:3 to 1:5 surfactant-to-powder ratio on a mass basis with respect to the mass of the alloyed chalcogenide material.
10. The process of claim 7, wherein the surfactant-assisted milling is performed at a ball-to-powder ratio (BPR) of 30:1 to 70:1 on a mass basis.
11. The process of claim 1, wherein the surfactant-assisted milling is performed to produce the nanochalcogenide in the form of chalcogenide nanoparticles.
12. The process of claim 11, wherein the surfactant-assisted milling produces the slurry comprising the chalcogenide nanoparticles and a milling liquid, and the separating comprises centrifuging, and wherein the chalcogenide nanoparticles separated by centrifuging have an average size smaller than 100 nm, measured using DLS or SEM.
13. The process of claim 12, wherein the centrifuging comprises a first centrifuging stage to remove larger particles followed by a second centrifuging stage to remove at least a portion of the chalcogenide nanoparticles.
14. The process of 1, further comprising sonicating the slurry prior to the separating.
15. The process of claim 1, wherein the at least two transition metals are selected from Ni, Co, and Fe.
16. The process of claim 1, wherein the at least two transition metals are Ni and Co.
17. The process of claim 16, wherein the at least one chalcogen is Se.
18. The process of claim 1, further comprising activating the separated nanochalcogenide to produce an activated electrocatalyst, wherein the activating comprises: selectively etching the at least one chalcogen out of the nanochalcogenide, leaching the at least one chalcogen out of the nanochalcogenide, or transforming the at least two transitional metals into active oxyhydroxides thereof.
19. An electrocatalyst comprising at least two transition metals and at least one chalcogen that are atomically dispersed and has the form of chalcogenide nanoparticles, and having an average particle size less than 100 nm, measured using DLS or SEM, and wherein the at least two transition metals comprise Ni and Co, and the chalcogen comprises Se.
20. A method for electrolyzing water, comprising: contacting water with an anode and a cathode, wherein the anode comprises an electrocatalyst as defined in claim 19; and applying a voltage to provide a current density to cause the water to be electrochemically split to form oxygen and hydrogen.
21. A method for electrochemical production of a multi-carbon compound from a carbon-containing gas, comprising: contacting the carbon-containing gas and an electrolyte with a cathode comprising a copper containing electroreduction catalyst; contacting the electrolyte with an anode comprises the electrocatalyst as defined in claim 19; applying a voltage to provide a current density to cause the carbon-containing gas contacting the cathode to be electrochemically converted into the multi-carbon compound, and to cause an oxygen evolution reaction at the anode; and recovering the multi-carbon compound.
22. A process for producing a nanochalcogenide for use in electrocatalysis, comprising: subjecting at least two transition metals and at least one chalcogen to a first milling stage to produce an alloyed chalcogenide material having an average particle size above 1 micrometer and a disordered structure including amorphous and nanocrystalline structures; subjecting the alloyed chalcogenide material to a second milling stage to produce a slurry comprising nanochalcogenides having a particle size below 100 nm, measured using DLS or SEM; and separating the nanochalcogenides from the slurry.
Description
DESCRIPTION OF DRAWINGS
[0026]
[0027]
[0028]
[0029]
[0030]
[0031]
[0032]
[0033]
[0034]
[0035]
[0036]
[0037]
[0038]
[0039]
[0040]
[0041]
[0042]
[0043]
[0044]
[0045]
[0046]
[0047]
DETAILED DESCRIPTION
[0048] The present description relates to the production of an electrocatalyst materials. In some implementations, a nanochalcogenide is produced using a two-step process of subjecting at least two transition metals and at least one chalcogenide to cryogenic milling to produce an alloyed chalcogenide material, and then subjecting the alloyed chalcogenide material to surfactant-assisted milling to produce the nanochalcogenide that is part of a slurry. The transition metals can be Ni, Co or Fe, for example, and the chalcogen can be Se or others such as S or Te. The nanochalcogenide can include (NiCo)Se and (NiCo).sub.3Se.sub.4, for example. The nanochalcogenide can then be separated from the slurry as nanoparticles, via centrifugation for example. The separated nanoparticles can then be activated by leaching out the chalcogen and forming oxyhydroxides of the transition metals, thus forming an electrocatalyst material. The electrocatayst material can be disposed on a substrate and used as part of the anode in an electrolysis cell for processes such as water splitting or CO.sub.2 electroreduction or other applications of oxygen evolution reactions (OER) in various electrocatalytic applications.
[0049] One application of the present disclosure is to employ a controlled ball milling process to facilitate cost-effective electrochemical conversion of water, air, CO and CO.sub.2 to fuels and chemicals. Embodiments of the present disclosure include a process that includes two-stage milling and low-cost mass production of highly active and stable nanocatalysts that include a chalcogenide and at least two transition metals, such that the produced catalyst particles are smaller than 100 nm and all elements are atomically dispersed. The two-step milling process can use cryogenic ball milling to mix the elements homogenously at cryogenic temperatures aided by continuous flow of liquid nitrogen around the mixing vessel during linear vibrational milling, for example, while surfactant-assisted ball milling can use surfactant and a wet medium for further size reduction to produce electrocatalyst nanoparticles. The duration and speed of milling, and ball to powder weight and volume ratios, can be varied to control particle size, crystal structure, powder yield and particle morphology. Consequently, amorphous, nanocrytalline, or mixed-structure multi-metal compounds (including oxides, hydroxides and mixed oxide-hydroxides) can be formed electrochemically by cycling the material or during electrochemical reaction, with the chalcogen leaving the initial chalcogenide structure.
[0050] It was found that the use of the chalcogen, such as Se, facilitated the yield of nanoparticles produced by ball milling. Embodiments of the nanochalcogenides exhibited enhanced intrinsic activity when compared to other crystal structures on glassy carbon, with for example 400 hours of stability at 10 mA.Math.cm.sup.−2 on carbon paper and 600 hours of stability at 100, 500, and 1000 mA.Math.cm.sup.−2 on nickel foam.
[0051] The nanochalcogenides can be viewed as pre-catalysts as they can be activated to remove the chalcogen and form oxyhydroxides of the transition metals, e.g., Ni and Co. Activation can be done by leaching or etching of the chalcogen from a deposited layer of the nanochalcogenides on a substrate. The activated catalyst can be in the form of chalcogen-depleted alloyed nanoparticles. It is noted that “chalcogen-depleted” with reference to an activated catalyst as described herein refers to a material where some and preferably most of the chalcogens have been removed from a chalcogenide alloy. Example methods for manufacturing the chalcogenide and for removing the chalcogen to form an activated catalyst are described herein. It is also noted that the chalcogen-depleted activated catalyst has minor or trace amounts of chalcogen atoms remaining in the material; and residual chalcogens in the material can be at levels to contribute to both the mechanical integrity as well as activity characteristics of the catalyst.
[0052] The electrocatalysts and pre-catalysts described herein have applicability in various OER catalyst implementations, and possibly catalyst implementations as well.
[0053] Developing Earth-abundant and stable Ni—Co—Se electrocatalysts for Oxygen Evolution at high current densities is of interest for various applications. In this work, a two-step novel milling process was used to produce Ni,Co-based amorphous nano-electrocatalysts. Cryo-milling (mechanical milling of precursors at cryogenic temperatures to achieve alloying) followed by surfactant-assisted ball milling (SABM for particle size reduction) created stable amorphous alloys with high surface areas and coordinatively unsaturated active sites for the reaction of OER intermediates.
[0054] Two different Ni—Co—Se alloys were milled under various conditions and the structural evolution of the system was monitored using x-ray diffraction (XRD) and electron microscopy. The results confirmed the production of two fully alloyed ternary systems (NiCo).sub.3Se.sub.4 and (NiCo)Se after 6 hours of milling time. The electrocatalytic activity and stability of the catalysts were evaluated by Tafel measurements obtained from linear sweep voltammetry (LSV) and cyclic voltammetry (CV) experiments. It was found that Se in NiCo-based alloys stabilized the amorphous structure by forming non-transitional clusters and significantly facilitated the production of nanoparticles. On flat glassy carbon electrode at 10 mA.Math.cm.sup.−2, this catalyst has demonstrated stable performance at 268 mV overpotential with a Tafel slope of 42 mV.dec.sup.−1 for at least 500 hours. Moreover, the performance of the catalyst at higher current densities on NF was stable for 100 hours while delivering 500 mA.Math.cm.sup.−2 at 320 mV of overpotential. Operando X-ray Absorption Spectroscopy (XAS) was conducted to reveal the role of adding Se on the chemical-structural transformation and bonding environment of surface species during the OER reaction. This work suggests that milling can potentially be used to produce OER catalysts for industrial applications.
[0055] In addition, using a material such as (NiCo).sub.3Se.sub.4 as an OER electrocatalyst and anion exchange membrane, the lowest cell voltage for alkaline water splitting delivering 2 A.Math.cm.sup.−2 at 2 V and for CO.sub.2 reduction delivering 1 A.Math.cm.sup.−2 at 3 V was found.
EXAMPLES & EXPERIMENTATION & FINDINGS
[0056] The following section relates to various experiments that were conducted in the course of this work.
Experimental Setup
Synthesis
[0057] All materials and chemicals used are listed as-purchased in Table 1:
TABLE-US-00001 TABLE 1 List of chemicals and materials used in mechanical alloying electrochemical testing Material Company Purity (%) Particle Size (μm) Ni Alfa Aesar 99.9 74-149 Co Alfa Aesar 99.8 44-149 Se Alfa Aesar 99.5 <44 DDPA Alfa Aesar — — NiCl2•6H2O Sigma 99.9 — Aldrich FeCl.sub.3 Alfa Aesar 98 — (Anhydrous) Ni NPs Alfa Aesar >99 <100 IrO.sub.2-AA Alfa Aesar 99.99 — Nafion-D521 Ion Power — —
[0058] Alloying: Cryo-Milling
[0059] The synthesis of catalysts was carried in two milling stages: milling at cryogenic temperatures (−196° C.) to alloy two or more elements, and surfactant-assisted ball milling (SABM) at room temperature to produce nanoparticles as shown in
[0060] Using cryo-milling, Ni—Co—Se alloy systems were prepared according to quantities and the procedure shown in Table 2 below.
TABLE-US-00002 TABLE 2 Weight percentage of Ni—Co—Se alloy system Element (wt. %) Alloy Ni Co Se Ni—Se 38.5 — 61.5 Co—Se — 38.5 61.5 (NiCo)Se 21.3 21.4 57.3 (NiCo).sub.3Se.sub.4 17.9 18 64.1
[0061] Surfactant-Assisted Ball Milling (SABM)
[0062] In the second stage, powder is added to a 5 mL vial with a BBR of 50:1 to conduct SABM. In addition, anhydrous ethanol with 1:1 ethanol-to-powder mass ratio and Diphenylphosphoryl Acid (DPPA) as surfactant with 1:4 DPPA-to-powder mass ratio were added to the milling vial to produce nanoparticles suspension. SABM was carried out for 10 hours in total (5 cycles of 30 mins and 80 mins of off time between cycles) for all alloy systems in this study. This procedure was followed based on the optimization of parameters conducted in our previous work. To extract nanoparticles from the suspension, the surfactant must be removed after milling. First, ethanol is added to milling vials and the powder-ethanol slurry mixture is extracted by pipetting into centrifuging tubes until the vials are empty. Then, the tubes are sonicated for 30 min before centrifuging times at 3000 rpm for 30 mins, the large particles should be settled at the bottom of the tube. The suspended nanoparticles in solution is extracted by pipetting into new centrifuging tubes. The tubes are sonicated for 30 mins before centrifuging at 10,000 rpm for 30 mins and then the clear solution is washed and replaced with fresh ethanol, this procedure is repeated at least 3 times until all nanoparticles in the solution precipitate at the bottom of the tube. The tubes are then filled with ethanol and the nanoparticles-ethanol mixture is transferred to 20 mL scintillation vials. Scintillation vials are then heated at 70° C. in a furnace under air flow until all ethanol evaporates. The dry nanoparticle powder is then collected from the vials and stored.
Structural and Elemental Characterization
[0063] X-Ray Diffraction (XRD)
[0064] The crystal structure of catalysts was determined using X-ray Diffraction (XRD). A Miniflex 600 (Rigaku, Japan) equipped with D/tex Ultra silicon trip detector and Cu Kα radiation (λ=1.5418 Å) was used. Powders were prepared by mixing with acetone and then dropping a small drop of the mixture to fill a 4 mm diameter×100 μm deep groove in a single crystal silicon holder (zero-background). The angle was varied between 20° to 80° with a step size of 0.050.
[0065] Dynamic Light Scattering (DLS)
[0066] The particle size distribution of catalysts was analyzed using Dynamic Light Scattering (DLS). A LB-500 particle size analyzer (Horiba, Japan) was used. Samples were prepared by dispersing a small amount of the powder in ethanol. Then, solution mixture was loaded into disposable plastic 1.5 mL cuvettes (VWR, U.S.). A 5-mW laser source with a wavelength of 650 nm is directed to the sample in the cuvette to enable the measurement of particles ranging from 3 nm to 6 μm. The incident laser will experience multiple scattering in all directions because of the suspended particles. The scattered light is detected at a specific angle over time to determine the temporal fluctuations, diffusion coefficient, and particle size using Stokes-Einstein equation. Smaller particles will move at higher speeds and therefore will show faster fluctuations than larger particles. The refractive index of the solution and the constitutes elements are required for this analysis. The refractive index of all materials used in this study are shown Table 3 in below.
TABLE-US-00003 TABLE 3 Refractive index of elements used in this work Material Refractive index Reference Ni 2.6-5.83i [83] Co 2.9-5.78i [83] Se 2.89 [83] Ethanol 1.3302 [83]
[0067] Electron Microscopy
[0068] The structural characterization and elemental mapping of the catalysts were done using Scanning Electron Microscopy (SEM) and Transmission Electron Microscope (TEM). TEM experiments were performed in Hitachi HF3300 equipped with a cold field emission electron gun using an accelerating voltage of 300 kV. TEM bright field images were used to determine the size and the shape of nanoparticles. The crystal structure was collected in diffraction mode using a selected area electron diffraction (SAED) aperture. The resultant diffraction pattems (DPs) were analyzed using CrysTBox. Energy X-ray Dispersive Spectroscopy (EDS) detector was used in Scanning Transmission Electron Microscopy (STEM) mode to analyze and quantify the composition of the nanoparticles. Also, Secondary electron (SE) detector was used to collect high resolution images of the morphology of the nanoparticles in the TEM. Powder samples were prepared in ethanol to form an ink, the ink was sonicated for 10 minutes before drop casting 1-2 μL on a 400-mesh copper grid and drying overnight.
[0069] The particle and geometry of larger particles were investigated using Scanning Electron Microscopy (SEM). Imaging was conducted using Hitachi SU3500. The composition of particles was determined using an attached EDS detector. Powder samples were prepared by adhering to carbon tape on a SEM aluminum stub. Compressed air is blown over the stub to loosen excess powder. In some cases, thin carbon or gold coating might be needed for poor conducting samples.
Electrochemical Setup
[0070] For all electrochemical experiments in this work, a BioLogic VSP-300 multi potentiosat was used. All tests were performed in 1 M KOH electrolyte at 30° C. unless indicated otherwise. The electrolyte was pre-electrolyzed using Pt working and counter electrodes at −1.7 V for 48 hours prior using to remove any trace metal impurities in the solution specifically Fe. A cell configuration was used as shown in
[0071] Powder samples were prepared by making inks. The inks were produced by mixing 4 mg of the catalyst with 80 μL of Nafion® D521 (IonPower) and 1.25 mL of water-to-ethanol (4:1) solution. The mixture was sonicated for 30 minutes before dropping 5 μL of the ink on a 3 mm diameter glassy carbon (GC) electrode to produce a thin catalyst layer with a loading of 0.21 mg/cm.sup.−2. The electrode is polished using 0.08 μm colloidal silica every time before applying inks. To load the catalyst on Ni foam (thickness: 1.7 mm, INCO), several drops of the ink are added to achieve a catalyst loading of 2 mg.Math.cm.sup.−2 on 0.5 cm×0.5 cm piece.
[0072] Oxygen Evolution Setup
[0073] A protocol to test the activity and stability of electrocatalysts used for OER was established as shown in Table 7. First, open cell voltage (OCV) is recorded for 30 mins to stabilize the material in the solution. The resistance of the solution is measured by electrochemical impedance spectroscopy (EIS). The frequency is scanned from 1 MHz to 1 kHz with an amplitude of 1 mV. Once EIS is completed, cyclic voltammetry (CV) is performed to clean and activate the surface. Each CV measurement is repeated 3 times at a speed of 50 mV.Math.s.sup.−1 to produce a unique fingerprint of anodic and cathodic peaks in the range from 0-1.6 V vs RHE. This is followed by measuring the electrochemical surface area (ECSA) in a non-Faradaic region using various scanning speeds as explained in the table. Each CV measurement at a specific speed is repeated 3 times. To measure the activity, polarization curves are acquired to conduct Tafel analysis and determine the catalytic properties of OER. This is performed using a Linear Sweep Voltammetry (LSV) with a slow speed 1 mV.Math.s.sup.−1 to allow the surface to stabilize and reflect the true reaction mechanism. In the last step, the stability can be assessed using chronopotentiometry. A fixed current density will be applied to evaluate the stability of the catalyst on different substrates. For quick stability measurement, the catalyst is tested on glassy carbon electrode at 10 mA.Math.cm.sup.−2 for 10 hours. Potential catalysts can then be tested for long-term stability on carbon paper and nickel foam for prolonged time and higher current densities as shown in the table. In this study, we used commercial Ni NPs and IrO.sub.2 as a baseline, the description of the catalyst is listed in Table 4.
TABLE-US-00004 TABLE 4 Reference catalysts materials list Materials Company Purity (%) Particle Size (nm) Ni NPs Alfa Aesar >99 <100 IrO.sub.2-AA Alfa Aesar 99.99 —
TABLE-US-00005 TABLE 5 A protocol to assess the electrocatalysts used for oxygen evolution reaction (OER) # Process Method Parameters 1 Solution Electrochemical 1 MHz-1 kHz Resistance Impedance Amplitude 1 mV Spectroscopy 2 Activation/ Cyclic Cycles = 3 Catalyst Voltammetry Potential = 0-1.6 V vs RHE Fingerprint Speed = 50 mV .Math. s.sup.−1 3 ECSA Cyclic Potential = 0-0.1 V vs RHE Voltammetry Speed = 20, 40, 60, 100 mV .Math. s.sup.−1 Cycles = 3 4 Activity and Linear Sweep Potential = 0-1.6 V vs RHE Mechanism Voltammetry Speed = 1 mV .Math. s.sup.−1 5 Stability Chronopoten- Current = 10, 100, 500, 1000 tiometry mA .Math. cm.sup.−2 Time = 10, 100 and 1000 hours
Results and Discussion
Ni—Co—Se System
[0074] Binary Alloy Systems
[0075] The composition of the alloy systems is systemically controlled by varying milling time, temperature and medium. This will also result in changes in the microstructure and the morphology of the particles. Two control samples were investigated: Ni—Se and Co—Se. Both alloy systems were milled under cryogenic conditions for 3, 6, 9 and 15 hours as shown in XRD patterns in
[0076] Unlike Ni—Se, Co—Se didn't form any phases within the first 6 hours of milling. Instead, the sharp peaks in
[0077] During milling, the particles undergo fracture due to the high kinetic energy produced by the multiple collision events between the steel balls and powder as shown in the SEM micrograph in
[0078] Ternary Alloy System
[0079] Two crystal structures were targeted for the ternary alloy system: MSe and M.sub.3Se.sub.4, where M is any metal combination. In this work, 1:1 ratio Ni:Co was used as a starting point. The precursors were milled for 6 hours under cryogenic conditions. A crystal evolution of binary alloys to ternary alloys is shown in
[0080] Electrochemical Performance
[0081] Electrochemical performance is divided into two sections: activity and stability. For activity test, all powders were prepared in inks and deposited on glass carbon as previously explained in the experimental setup section. Commercial Ni NPs and IrO.sub.2 were used as baseline for all measurements (see experimental setup). Also, NiFeOOH (NiFe) was prepared using Sol-Gel technique reported in previous work to compare benchmark this work's performance. In
TABLE-US-00006 TABLE 6 Summary of some results presented in FIG. 8 Tafel η [mV] η [mV] [mV .Math. Size @1 mA @10 mA dec.sup.−1] [nm] Ni NPs 321 383 62 ± 3 100 NiSe 15 hrs 277 330 53 ± 5 60 ± 30 CoSe 15 hrs 275 332 57 ± 1 80 ± 35 (NiCo)Se 288 353 65 1065 ± 330 6 hrs (NiCo).sub.3Se.sub.4 310 — 79 4055 ± 778 6 hrs
[0082] After SABM, we could reduce the size of the particles to <100 nm as explained in the structural characterization. The electrochemical performance was measured again for both ternary catalysts and compared to IrO.sub.2 and NiFe. Both (NiCo)Se and (NiCo).sub.3Se.sub.4 nanoparticles (after SABM) have shown remarkable improvement in performance from the original bulk material. The overpotential required to draw a current of 10 mA.Math.cm.sup.−2 is 283 and 268 mV for (NiCo)Se and (NiCo).sub.3Se.sub.4 compared to 298 and 272 mV for IrO.sub.2 and NiFe as shown in
[0083] Also, the Tafel slope for (NiCo)Se and (NiCo).sub.3Se.sub.4 decreased to 50 and 42 indicating a change in the reaction mechanism. So far all electrochemical results were reported with respect to the geometric area (GC electrode area=0.0717 cm.sup.2) without considering the actual electrochemically active sites. Electrochemical Surface Area (ECSA) measurements were conducted for all samples to determine the intrinsic activity of the catalysts. The roughness factor (RF), which is basically the ECSA/Geometric Area, was calculated as in Table 7. While monitoring current vs voltage while cycling at different voltage sweeping speeds, the double layer capacitance (C.sub.dl) can be calculated as the slope of linear relationship of current (average difference in anodic and cathodic currents of each cycle) vs sweeping speed. The ECSA is calculated by dividing the Cdl over a specific capacitance value (C.sub.s) that can range between 0.02-0.13 mF.Math.cm.sup.−2. In this case, the work used 0.04 mF.Math.cm.sup.−2 for Ni and Co based alloys. The ECSA results show low RF values compared to other catalysts in the table specifically NiFe. Sol-Gel catalysts form a network of oxides and oxy/hydroxides which boosts the overall surface area. In the case of milling, the catalysts will have minimal surface roughness and modifications as was shown in the structural analysis. This will help in decoupling the actual intrinsic activity produced by the synergistic effects of the elements in the system and surface area contributions.
TABLE-US-00007 TABLE 7 Summary of the results presented in FIG. 9 Tafel η [mV] η [mV] [mV .Math. Size @1 mA @10 mA dec.sup.−1] RF [nm] Ni NPs 321 383 62 ± 3 17.55 ± 7.5 100 IrO.sub.2 250 298 48 ± 1 13.43 ± 0.72 — NiFe 212 272 62 500-100 — (NiCo)Se 288 353 65 — 1065 ± 330 6 hrs (NiCo).sub.3Se.sub.4 310 389* 79 — 4055 ± 778 6 hrs (NiCo)Se 233 283 50 7.60 37 ± 15 6 hrs NP (NiCo).sub.3Se.sub.4 226 268 42 8.13 67 ± 27 6 hrs NP
[0084] In the second step, the stability of the catalysts is tested. In this test, the catalyst is deposited on Carbon paper instead of GC for the reasons discussed further below. The catalysts are first tested using chronopotentiometry (CP) at 10 mA.Math.cm.sup.−2 for 100 hours. As can be seen from the results in
Additional Information
[0085] The present work notably found that activation of cryomilled Ni—Co—Se Nanocrystals can achieve durable electrochemical water oxidation at high current densities. The following description provides information regarding experimentation and findings regarding the production, properties and use of such electrocatalyst materials.
[0086] By way of summary, in the electrocatalytic CO.sub.2 reduction reaction, up to half of the excess electricity input arising due to overpotentials is incurred because of the poor reaction kinetics at the anodic oxygen evolution reaction (OER); this problem worsens over time if the performance of the OER electrocatalyst diminishes during operation. The present work reports the synthesis of electrocatalysts derived from high melting point intermetallics. The approach promotes disorder and generates nanocrystalline structures based on Ni—Co—Se: specifically, (NiCo).sub.3Se.sub.4 and (NiCo)Se were produced via judicious control over cryomilling conditions. Using in situ X-ray absorption spectroscopy (XAS), as well as using electron microscopy, we found that, under OER conditions, the electrochemical leaching of Se ions activates the electrocatalysts, facilitating the transformation of Ni and Co into active and stable oxyhydroxides. On a glassy carbon electrode at 10 mA.Math.cm.sub.geo.sup.−2, activated (NiCo).sub.3Se.sub.4 required only an overpotential of 268 mV in 1M KOH. When operated at higher current densities, the electrocatalyst maintained an overpotential of 279 mV at 0.5 A.Math.cm.sup.−2 and 329 mV at 1 A.Math.cm.sup.−2 for at least 500 hrs, with an average voltage degradation rate of 0.17 mV.Math.h.sup.−1.
[0087] In terms of context, it is noted that affordable electrocatalysis is urgently needed for long-term chemical energy storage necessary to help accelerate the transition from fossil fuels to sustainable energy sources. The sustainable energy market witnessed tremendous growth in the last 10 years, especially in 2017 when power capacity and investments in the market increased at a record-breaking pace. This increase resulted in a significant reduction in renewable energy cost and offered opportunities for the advancement of green technologies.
[0088] The large-scale implementation of sustainable energy requires overcoming intermittency sources, such as solar and wind. The electrocatalytic conversion of CO.sub.2 and water into synthetic fuel and chemical feedstocks, which are readily stored and easily transported, addresses this challenge. The efficiency of electrocatalysis is limited by the sluggish kinetics and high overpotentials, including those associated with the oxygen evolution reaction (OER). A relatively small selection of electrocatalysts meet requirements for OER: low overpotentials (<300 mV) at high current densities (>500 mA.Math.cm.sup.−2) for long-term operation (ultimately >60,000 h). Identifying durable, efficient OER electrocatalysts will support continued progress in the field.
[0089] To date, noble metal OER electrocatalysts, such as RuO.sub.2 and IrO.sub.2, are state-of-art in view of their superior activity and stability; unfortunately, their high price and their scarcity limit their widespread application. Earth-abundant, low cost elements, such as the first-row transition metals Ni, Co, and Fe, can be used as OER electrocatalysts in anion exchange membrane (AEM) systems, which offer an alkaline environment more compatible with non-noble metal based electrocatalysts.
[0090] One of the most efficient non-noble metal electrocatalysts for alkaline solutions is Fe doped NiOOH. While a small amount of Fe can boost the electrocatalytic activity of NiOOH, the nature of the synergistic effect between the two elements is still under debate. Berlinguette et al. found that with high current densities in alkaline solutions, a significant amount of Fe left NiFeO.sub.x and diffused to the cathode. Markovic et al. investigated activity and stability trends for monometallic oxyhydroxides and observed Ni>Co>Fe in stability, but the reverse trend for activity. In the present work, an Fe-free electrocatalyst was developed with a goal of an earth abundant OER catalyst that was both active and stable in alkaline solutions. Among the candidates, Co was selected at least in part because—when used in combination with Ni—it can lower overpotential and improves stability.
[0091] Metal chalcogenides such as sulfides, selenides, and tellurides have demonstrated improved activity compared to oxides. Adding Se and Te to mixed-transition-metal oxides can lead to an increased covalency in the metal-chalcogenide bond due to the smaller associated electronegativity and better conductivity. Selenium can also be used as an alloying element that is selectively leached to increase the nanoporosity of the structure, induce disordering in the lattice, and boost the number of defective sites for electrocatalytic reactions. Se incorporated in the lattice in a compound with transition metals also facilitates partial or complete conversion to active oxyhydroxides.
[0092] The crystal structure of the electrocatalyst can play a relevant role in its activity. Smith et al. demonstrated that amorphous mixed-metal oxides including Fe, Ni, and Co are more active than their crystalline counterparts. Amorphous metals lack long-range structural order and can be produced using a variety of methods: atomization, electrodeposition, electroless deposition, planar flow casting, and solid-state amorphization such as mechanical alloying. Cryomilling, one type of ball milling that involves milling at cryogenic temperatures, can produce a wide compositional range of compounds. Amorphous alloys produced by planar flow casting, for example, are limited to compositions at a near low melting point eutectics. Mechanical alloying has a broader compositional range centered about high melting point intermetallics. During milling, the particles undergo fracture due to the high kinetic energy produced by the multiple collision events between the milling balls and powder. The temperature at which mixing is taking place controls the extent of alloying, the concentration of defects, and local chemical clustering. At high temperatures (≥room temperature), atoms become highly mobile and diffuse into each other forming equilibrium phases, while at low temperatures metastable phases are more stable due to increased defects and reduced atom mobility. Operating at cryogenic conditions can prevent these metastable amorphous-nanocrystalline structures from recrystallizing as a result of the local temperature rise caused by high impact collisions. Targeting high melting Ni—Co—Se intermetallic compounds favors disordering and the formation of abundant defective and coordinatively unsaturated sites (CUS) that are known to be catalytically active. Crystal disordering can be further enhanced by milling under cryogenic temperatures due to reduced welding and increased brittleness of the powder particles at such low temperatures. Unlike conventional high energy milling, amorphization doesn't arise as a result of fast quenching after localized heating by the milling process but rather is believed to be a result of milling-induced disordering followed by crystallite reduction, accumulation of defects and partial or complete amorphization. The improved stability of amorphous alloys is another exciting feature that can significantly enhance the overall catalytic performance of the electrocatalyst.
[0093] In this work, a two-step milling process was used to produce Ni—Co—Se ternary alloys. Then, the electrocatalyst was electrochemically activated by selectively etching Se out of the structure to facilitate the transformation of Ni and Co to active oxyhydroxides. X-ray absorption spectroscopy (XAS) was utilized to identify the role of adding Se on the chemical-structural transformation and bonding environment of active surface species during the OER. The overall performance of Ni—Co—Se as OER electrocatalyst against industrial requirements for water splitting and CO.sub.2R in 5 cm.sup.2 AEM electrolyser.
Synthesis and Activation of the Electrocatalyst
[0094] (NiCo).sub.3Se.sub.4 and (NiCo)Se were synthesized using a two-step solid-state ball milling process including: 1) mechanical alloying of two or more elements at a cryogenic temperature (<−196° C.) to produce disordered crystal structures, and 2) surfactant-assisted ball milling (SABM) at room temperature to tailor the shape and size of the alloyed particles (see
[0095] First, Ni—Se and Co—Se binary compounds were synthesized to study the milling behaviour of Ni and Co with Se. The structural evolution was probed with respect to milling time using XRD (
[0096] Two ternary compounds, each with a composition that lays within the high melting intermetallic composition range of Ni—Se and Co—Se (50-57% at. % Se), were synthesized: (NiCo)Se and (NiCo).sub.3Se.sub.4. After 6 h of milling both ternary compounds were successfully produced as evident by XRD (
[0097] The nanoparticles were dispersed in ethanol and drop-casted on Cu grids and left to dry before imaging by transmission electron microscopy (TEM) (see
[0098] The nanoparticles were electrochemically activated by oxidizing and selectively leaching Se out of the structure. The electrochemical expulsion of Se, as demonstrated in previous studies, can facilitate the complete transformation of transition metals to disordered oxyhydroxide phases (poor crystallinity) rich in defects. According to the Pourbaix diagram of Se, Se is expected to oxidize to soluble SeO.sub.4.sup.2− at high pH while Ni and Co are predicted to convert completely to active oxyhydroxides by lattice strain (see
Electrocatalytic Performance and Stability
[0099] For electrochemical testing, a three-electrode custom-made Teflon cell to avoid contamination from glassware etching was constructed and used 1M Fe-free KOH to rule out the contribution of Fe ions in the electrolyte on the activity and stability of the electrocatalysts (see
[0100] To examine the specific catalytic activity, we normalized the current density by the electrochemical surface area (ECSA) using double layer capacitance measured by cyclic voltammetry (CV). (NiCo).sub.3Se.sub.4 demonstrated ECSA current densities 44 times higher than NiFeOOH (prepared by Sol-Gel) and 4 times higher than commercial IrO.sub.2 using the same catalyst loading of 0.21 mg.Math.cm.sup.−2 for all electrocatalysts. The turnover frequency (TOF) of (NiCo).sub.3Se.sub.4 was calculated as 50.76×10.sup.−3 s.sup.−1 per site, considering only Ni and Co as active sites, ca. two folds higher than IrO.sub.2 28.09×10.sup.−3 and NiFe 23.98×10.sup.−3 s.sup.−1 per site inferring that (NiCo).sub.3Se.sub.4 is intrinsically more active.
[0101] To this point, the description has presented electrochemical current densities normalized by geometric area, to examine the real catalytic activity the study normalized the current density by the electrochemical surface area (ECSA) extracted from cyclic voltammetry (CV) scans (see
[0102] The turnover frequency (TOF) of (NiCo).sub.3Se.sub.4 was calculated as 50.76×10.sup.−3 s.sup.−1 per site, considering only Ni and Co as active sites, ca. two folds higher than IrO.sub.2 28.09×10.sup.−3 and NiFe 23.98×10.sup.−3 s.sup.−1 per site inferring that (NiCo).sub.3Se.sub.4 is intrinsically more active.
[0103] To investigate the active species during OER and study the dynamics of the electrochemical activation of (NiCo).sub.3Se.sub.4, in situ XAS on the K-edge of Ni, Co and Se was conducted at four conditions: dry, open circuit potential (OCP), before OER at 1.1 V, and during OER at 1.5V (see
[0104] Stability is an important metric of performance for OER electrocatalysts. It is common to use rotating disk electrodes (RDE) at 1600 rpm to improve bubble release and reduce the influence of O.sub.2 bubbles attached to the surface on the performance. However, the change in potential during RDE test was found to be not related to electrocatalyst degradation but rather to O.sub.2 micro bubbles accumulating in the pores of electrocatalyst layer and shielding active sites. This is believed to explain the discrepancy between stability observations in RDE tests and membrane electrode assembly (MEA) cells. Hence, in the present work a protocol was proposed to evaluate the stability of the present electrocatalysts first by using an alternating current test (
[0105] The electrocatalyst was then deposited by drop-casting on a 0.5 cm×0.5 cm NF with a mass loading of 0.21 mg.Math.cm.sup.−2. When tested at high current densities, the overpotential was 279 mV at 500 mA.Math.cm.sup.−2.sub.geo and 329 mV at 1000 mA.Math.cm.sup.−2.sub.geo (
AEM Water and CO2R Electrolyser
[0106] To further evaluate the performance of (NiCo).sub.3Se.sub.4 as an OER electrocatalyst for industrial water splitting and CO2R, the work used a 5 cm.sup.2 AEM electrolyser with Pt/C on carbon paper for hydrogen evolution and Cu on PTFE for CO.sub.2R (
AEM Electrolyser Test
[0107] The anode electrode was prepared by spraying (NiCo).sub.3Se.sub.4 and IrO.sub.2 inks on 300 μm Nickel foam (Kunhewuhua™), spraying was controlled to achieve a mass loading of 2 mg.Math.cm.sup.−2. The Nickel foam was degreased and pretreated in HCl before using to remove the passive oxide layer and other contaminants. For hydrogen evolution, 40% Pt/C was sprayed on untreated carbon paper (AvCarb 190, Fuel Cell Store) with a loading of 1 mg.Math.cm.sup.−2, while for CO.sub.2 reduction the Cu electrode on PTFE was prepared following the exact procedure proposed by Gabardo et al. The catalysts were prepared in isopropanol and 5 wt. % Nafion binder before spraying. An anion exchange membrane (Sustanior x37-50) was used to separate the cathode and anode compartments of the electrolyser, each end plate with a 5 cm.sup.2 serpentine flow field. For water splitting, 1 M KOH was fed to both sides of the electrolyser, while for CO.sub.2 reduction only the anode was supplied with 1 M KOH while the cathode was provided with humidified CO.sub.2.
Conclusions and Outlook
[0108] The present work attempted to compare the performance of the electrocatalysts described herein with literature at 10 mA.Math.cm.sub.geo.sup.−2 and high current densities (>100 mA.Math.cm.sub.geo.sup.−2). Over 69 earth-abundant electrocatalysts were listed in Table 10 and mapped using overpotential, Tafel slope, and voltage degradation as performance metrics (
[0109] By way of summary, the present work has demonstrated the synthesis of homogenously dispersed Ni—Co—Se nanocrystals using a two-step milling process. The electrochemical leaching of Se from the structure enabled the transformation of Ni and Co in the electrocatalyst to active oxyhydroxides as validated by SEM, sXAS, and in situ XAS. The (NiCo).sub.3Se.sub.4 electrocatalyst showed a stable OER performance drawing 10 mA.Math.cm.sub.geo.sup.−2 at an initial overpotential of 268 mV in 1M Fe-free KOH for at least 350 hours with a marginal voltage degradation (<10 mV). When operated at higher current densities, the electrocatalyst maintained an overpotential of 290 mV at 0.5 A.Math.cm.sup.−2 and 340 mV at 1 A.Math.cm.sup.−2 for at least 500 hours with an average voltage degradation of 0.17 mV.Math.h.sup.−1. The results encourage the utilization of cryomilling and chalcogenides, such as Se, to mass produce efficient earth-abundant electrocatalysts with tailored nanostructures and long-term durability.
[0110] At higher current densities (0.1-1 A.Math.cm.sup.−2), the electrocatalyst exhibited a stability of 0.17 mV.Math.h.sup.−1 comparable with state-of-the-art IrO.sub.2 (0.21 mV.Math.h.sup.−1). Using (NiCo).sub.3Se.sub.4 as OER electrocatalyst in AEM electrolyser, the work demonstrated outstanding water splitting performance delivering 2 A.Math.cm.sup.−2 at 2 V and 1 A.Math.cm.sup.−2 at 3 V. The results encourage the utilization of cryomilling and chalcogenides such as Se to mass produce efficient earth-abundant electrocatalysts with tailored nanostructures and long-term durability.
[0111] The following references are hereby incorporated herein by reference in their entirety: [0112] 1 IRENA, Electricity storage and renewables: Costs and markets to 2030, 2017. [0113] 2 P. P. Sharma and X. D. Zhou, Electrocatalytic conversion of carbon dioxide to fuels: a review on the interaction between CO2 and the liquid electrolyte, Wiley Interdiscip. Rev. Energy Environ., DOI:10.1002/wene.239. [0114] 3 P. De Luna. C. Hahn, D. Higgins, S. A. Jaffer, T. F. Jaramillo and E. H. Sargent. What would it take for renewably powered electrosynthesis to displace petrochemical processes?, Science (80-.). DOI:10.1126/science.aav3506. [0115] 4 C.-T. Dinh, G. Kibria, A. Seifitokaldani, A. Kiani, O. S. Bushuyev, C. Zou, R. Quintero-Bermudez. E. H. Sargent, T. Burdyny, C. M. Gabardo. J. P. Edwards. Y. Pang. D. Sinton and P. De Luna, CO2 electroreduction to ethylene via hydroxide-mediated copper catalysis at an abrupt interface, Science (80-.). 2018, 360, 783-787. [0116] 5 J. R. SGalán-Mascarós, Water Oxidation at Electrodes Modified with Earth-Abundant Transition-Metal Catalysts, ChemElectroChem, 2015, 2, 37-50. [0117] 6 H. Zhou, F. Yu, Q. Zhu, J. Sun, F. Qin, L. Yu, J. Bao, Y. Yu, S. Chen and Z. Ren, Water splitting by electrolysis at high current densities under 1.6 volts Energy Environ. Sci., 2018, 11, 2858-2864. [0118] 7 M. Yu, G. Moon, E. Bill and H. TOysz, Optimizing Ni—Fe Oxide Electrocatalysts for Oxygen Evolution Reaction by Using Hard Templating as a Toolbox, ACS Appl. Energy Mater., 2019, 2, 1199-1209. [0119] 8 M. Gong, Y. Li, H. Wang, Y. Liang, J. Z. Wu, J. Zhou, J. Wang, T. Regier, F. Wei and H. Dai, An advanced Ni—Fe layered double hydroxide electrocatalyst for water oxidation, J. Am. Chem. Soc., 2013, 135, 8452-8455. [0120] 9 C. Liang, P. Zou, A. Nairan, Y. Zhang, J. Liu, K. Liu, S. Hu, F. Kang. H. J. Fan and C. Yang, Exceptional performance of hierarchical Ni—Fe oxyhydroxide@NiFe alloy nanowire array electrocatalysts for large current density water splitting, Energy Environ. Sci., DOI:10.1039/c9ee02388g. [0121] 10 F. D. Speck, K. E. Dettelbach, R. S. Sherbo, D. A. Salvatore, A. Huang and C. P. Beringuette, On the Electrolytic Stability of Iron-Nickel Oxides, Chem, 2017, 2, 590-597. [0122] 11 D. Y. Chung, P. P. Lopes, P. Farinazzo Bergamo Dias Martins, H. He, T. Kawaguchi, P. Zapol, H. You, D. Tripkovic, D. Strmcnik, Y. Zhu, S. Seifert, S. Lee, V. R. Stamenkovic and N. M. Markovic, Dynamic stability of active sites in hydr(oxy)oxides for the oxygen evolution reaction, Nat. Energy, 2020, 5, 222-230. [0123] 12 M. Wang, Z. Dang. M. Prato, D. V. Shinde, L. De Trizio and L. Manna, Ni—Co—S—Se Alloy Nanocrystals: Influence of their Composition on their in-situ Transformation and Electrocatalytic Activity for the Oxygen Evolution Reaction, ACS Appl. Nano Mater., 2018, acsanm.8b01418. [0124] 13 H. Shin, H. Xiao and W. A. Goddard, In Silico Discovery of New Dopants for Fe-Doped Ni Oxyhydroxide (Ni1-xFexOOH) Catalysts for Oxygen Evolution Reaction, J. Am. Chem. Soc., 2018, 140, 6745-8748. [0125] 14 K. Akbar, J. H. Jeon, M. Kim, J. Jeong, Y. Yi and S. H. Chun, Bifunctional Electrodeposited 3D NiCoSe 2/Nickle Foam Electrocatalysts for Its Applications in Enhanced Oxygen Evolution Reaction and for Hydrazine Oxidation, ACS Sustain. Chem. Eng., 2018, 6, 7735-7742. [0126] 15 Y. Zhang, C. Zhang, Y. Guo, D. Liu, Y. Yu and B. Zhang, Selenium vacancy-rich CoSe2 ultrathin nanomeshes with abundant active sites for electrocatalytic oxygen evolution, 2019, 2536-2540. [0127] 16 K. Guo, S. Cui, H. Hou, W. Chen and L. Mi, Hierarchical ternary Ni—Co—Se nanowires for high-performance supercapacitor device design, Dalt. Trans., 2016, 45, 19458-19465. [0128] 17 C. Tang, N. Cheng, Z. Pu, W. Xing and X. Sun, NiSe Nanowire Film Supported on Nickel Foam: An Efficient and Stable 3D Bifunctional Electrode for Full Water Splitting, 2015, 9351-9355. [0129] 18 I. H. Kwak, H. S. Im, D. M. Jang, Y. W. Kim, K. Park, Y. R. Lim, E. H. Cha and J. Park, CoSe2 and NiSe2 Nanocrystals as Superior Bifunctional Catalysts for Electrochemical and Photoelectrochemical Water Splitting, ACS Appl. Mater. Interfaces, 2016, 8, 5327-5334. [0130] 19 X. Cao, E. Johnson and M. Nath, Identifying high-efficiency oxygen evolution electrocatalysts from Co—Ni—Cu based selenides through combinatorial electrodeposition, J. Mater. Chem. A, 2019, 7, 9877-9889. [0131] 20 X. Cao, Y. Hong, N. Zhang, Q. Chen, J. Masud, M. A. Zaeem and M. Nath, Phase Exploration and Identification of Multinary Transition-Metal Selenides as High-Efficiency Oxygen Evolution Electrocatalysts through Combinatorial Electrodeposition, ACS Catal., 2018, 8, 8273-8289. [0132] 21 F. Lyu, Q. Wang, S. M. Choi and Y. Yin, Small, 2019,15, 1-17. [0133] 22 W. Li, D. Xiong, X. Gao and L. Liu, The oxygen evolution reaction enabled by transition metal phosphide and chalcogenide pre-catalysts with dynamic changes, Chem. Commun., 2019, 55, 8744-8763. [0134] 23 W. Li, X. Gao, D. Xiong, F. Wei, W. G. Song, J. Xu and L. Liu, Hydrothermal Synthesis of Monolithic Co3Se4 Nanowire Electrodes for Oxygen Evolution and Overall Water Splitting with High Efficiency and Extraordinary Catalytic Stability, Adv. Energy Mater., 2017, 7, 1-7. [0135] 24 R. Gao. G. D. Li, J. Hu, Y. Wu, X. Lian, D. Wang and X. Zou, In situ electrochemical formation of NiSe/NiOx core/shell nano-electrocatalysts for superior oxygen evolution activity, Catal. Sci. Techno., 2016, 6, 8268-8275. [0136] 25 R. D. L. Smith, M. S. Prévot, R. D. Fagan. Z. Zhang, P. A. Sedach, M. K. J. Siu, S. Trudel and C. P. Berlinguette. Photochemical route for accessing amorphous metal oxide materials for water oxidation catalysis. Science, 2013, 340, 60-3. [0137] 26 L. Schultz, Formation of amorphous metals by mechanical alloying, Mater. Sci. Eng., 1988, 97, 15-23. [0138] 27 W. L. Johnson, Thermodynamic and kinetic aspects of the crystal to glass transformation in metallic materials, Prog. Mater. Sci., 1986, 30, 81-134. [0139] 28 A. H. Alami, A. Alketbi, J. Abed and M. Almheiri, Assessment of Al—Cu—Fe compound for enhanced solar absorption, Int. J. Energy Res., DOI:10.1002/er.3468. [0140] 29 T. Pas, A. Bergonzi, E. Michiels, F. Rousseau, J. Schymkowitz, R. Koekoekx, C. Clasen, B. Vergauwen and G. Van Den Mooter, Preparation of Amorphous Solid Dispersions by Cryomilling: Chemical and Physical Concems Related to Active Pharmaceutical Ingredients and Carriers, Mol. Pharm., 2020, 17, 1001-1013. [0141] 30 K. M. Cole, D. W. Kirk and S. J. Thorpe, In Situ Raman Study of Amorphous and Crystalline Ni—Co Alloys for the Alkaline Oxygen Evolution Reaction, J. Electrochem. Soc., 2018, 165, J3122-J3129. [0142] 31 S. Ghobrial. D. W. Kirk and S. J. Thorpe. Solid state amorphization in the Ni—Nb—Y system by mechanical alloying, J. Non. Cryst. Solids, 2018, 502, 1-8. [0143] 32 M. S. El-eskandarany, Cyclic phase transformations of mechanically alloyed Co 75 Ti 25 powders, 2002, 50, 1113-1123. [0144] 33 G. Fan, F. Guo, Z. Hu, M. Quan and K. Lu, Amorphization of selenium induced by high-energy ball milling, Phys. Rev. B—Condens. Matter Mater. Phys., 1997, 55, 11010-11013. [0145] 34 C. Suryanarayana, Mechanical Alloying And Milling, CRC Press, 2004, illustrate. 2004. [0146] 35 K. Fan, H. Zou, Y. Lu, H. Chen, F. Li, J. Liu, L. Sun, L. Tong, M. F. Toney, M. Sui and J. Yu, Direct Observation of Structural Evolution of Metal Chalcogenide in Electrocatalytic Water Oxidation, ACS Nano, 2018, 12, 12369-12379. [0147] 36 B. Akabayov, C. J. Doonan, I. J. Pickering, G. N. George and I. Sagi, Using softer X-ray absorption spectroscopy to probe biological systems, J. Synchrotron Radiat., 2005, 12, 392-401. [0148] 37 M. Al Samarai, A. W. Hahn, A. Beheshti Askari, Y. T. Cui, K. Yamazoe, J. Miyawaki, Y. Harada, O. ROdiger and S. Debeer, Elucidation of Structure-Activity Correlations in a Nickel Manganese Oxide Oxygen Evolution Reaction Catalyst by Operando Ni L-Edge X-ray Absorption Spectroscopy and 2p3d Resonant Inelastic X-ray Scattering, ACS Appl. Mater. Interfaces, 2019, 11, 38595-38605. [0149] 38 Y. Liang, Y. Li, H. Wang. J. Zhou, J. Wang, T. Regier and H. Dai, Co3O4 nanocrystals on graphene as a synergistic catalyst for oxygen reduction reaction. Nat. Mater., 2011, 10, 780-786. [0150] 39 D. Meyers, S. Mukherjee, J. G. Cheng, S. Middey, J. S. Zhou, J. B. Goodenough, B. A. Gray, J. W. Freeland, T. Saha-Dasgupta and J. Chakhalian, Zhang-Rice physics and anomalous copper states in A-site ordered perovskites, Sci. Rep., 2013, 3, 3-7. [0151] 40 S. Klaus, Y. Cai, M. W. Louie, L. Trotochaud and A. T. Bell, Effects of Fe electrolyte impurities on Ni(OH)2/NiOOH structure and oxygen evolution activity, J. Phys. Chem. C, 2015.119, 7243-7254. [0152] 41 C. Xiao, X. Lu and C. Zhao, Chem. Commun., 2014, 50, 10122-10125. [0153] 42 K. Lian, D. W. Kirk and S. J. Thorpe, Electrocatalytic behaviour of Ni-base amorphous alloys, Electrochim. Acta, 1991, 36, 537-545. [0154] 43 M. Farmand, A. T. Landers, J. C. Lin, J. T. Feaster, J. W. Beeman, Y. Ye, E. L. Clark, D. Higgins, J. Yano, R. C. Davis, A. Mehta, T. F. Jaramillo, C. Hahn and W. S. Drisdell, Electrochemical flow cell enabling: Operando probing of electrocatalyst surfaces by X-ray spectroscopy and diffraction. Phys. Chem. Chem. Phys., 2019, 21, 5402-5408. [0155] 44 G. Li. L. Anderson, Y. Chen, M. Pan and P.-Y. Abel Chuang, New insights into evaluating catalyst activity and stability for oxygen evolution reactions in alkaline media, Sustain. Energy Fuels, 2018, 2, 237-251. [0156] 45 T. Shinagawa, A. T. Garcia-Esparza and K. Takanabe, Insight on Tafel slopes from a microkinetic analysis of aqueous electrocatalysis for energy conversion. Sci. Rep., 2015, 5, 1-21. [0157] 46 B. G. Amin, A. T. Swesi, J. Masud and M. Nath, CoNi2Se4 as an efficient bifunctional electrocatalyst for overall water splitting, Chem. Commun., 2017, 53, 5412-5415. [0158] 47 P. F. Wang, Y. J. Guo, H. Duan, T. T. Zuo, E. Hu, K. Attenkofer, H. Li, X. S. Zhao, Y. X. Yin, X. Yu and Y. G. Guo, Honeycomb-Ordered Na3Ni1.5M0.5BiO6 (M=Ni, Cu, Mg, Zn) as High-Voltage Layered Cathodes for Sodium-Ion Batteries, ACS Energy Lett., 2017, 2, 2715-2722. [0159] 48 B. Das, M. V Reddy and B. V. R. Chowdari, X-ray absorption spectroscopy and energy storage of Ni-doped cobalt nitride, (Ni 0.33 Co 0.67)N, prepared by a simple synthesis route, 2013, 1961-1966. [0160] 49 J. T. Olegario, N. Yee. M. Miller, J. Sczepaniak and B. Manning, Reduction of Se(VI) to Se(-II) by zerovalent iron nanoparticle suspensions. J. Nanoparticle Res., 2010, 12, 2057-2068. [0161] 50 S. R. Mellsop, A. Gardiner, B. Johannessen and A. T. Marshall, Structure and transformation of oxy-hydroxide films on Ni anodes below and above the oxygen evolution potential in alkaline electrolytes, Electrochim. Acta, 2015, 168, 356-364. [0162] 51 G. Zhao, H. Hu, W. Chen, Z. Jiang, S. Zhang, J. Huang and Y. Lu, Ni2O3-Au+ hybrid active sites on NiO x@Au ensembles for low-temperature gas-phase oxidation of alcohols, Catal. Sci. Technol., 2013, 3, 404-408. [0163] 52 D. Friebel, M. W. Louie, M. Bajdich, K. E. Sanwald, Y. Cai, M. Wise, M. Cheng, D. Sokaras, T. Weng, R. Alonso-mori, R. C. Davis, J. R. Bargar, J. K. Nørskov, A. Nilsson and A. T. Bell, Identification of Highly Active Fe Sites in (Ni, Fe) OOH for Electrocatalytic Water Splitting, J. Am. Chem. Soc, 2015, 137, 1305-1313. [0164] 53 H. Y. Wang, Y. Y. Hsu, R. Chen, T. S. Chan, H. M. Chen and B. Liu, Ni+-induced formation of active NiOOH on the spinel Ni—Co oxide surface for efficient oxygen evolution reaction, Adv. Energy Mater., 2015, 5, 1-8. [0165] 54 J. Huang, J. Chen, T. Yao, J. He, S. Jiang, Z. Sun, Q. Liu, W. Cheng, F. Hu, Y. Jiang, Z. Pan and S. Wei, CoOOH Nanosheets with High Mass Activity for Water Oxidation, Angew. Chemie—Int. Ed., 2015, 54, 8722-8727. [0166] 55 C. I. Pearce, V. S. Coker, J. M. Chamock, R. A. D. Pattrick, J. F. W. Mosselmans, N. Law, T. J. Beveridge and J. R. Lloyd, Microbial manufacture of chalcogenide-based nanoparticles via the reduction of selenite using Veillonella atypica: An in situ EXAFS study, Nanotechnology, DOI:10.1088/0957-4484/19/15/155603. [0167] 56 H. A. El-Sayed, A. Weiß, L. F. Olbrich. G. P. Putro and H. A. Gasteiger, OER Catalyst Stability Investigation Using RDE Technique: A Stability Measure or an Artifact?, J. Electrochem. Soc., 2019, 166, F458-F464. [0168] 57 A. C. Garcia and M. T. M. Koper, Effect of Saturating the Electrolyte with Oxygen on the Activity for the Oxygen Evolution Reaction, ACS Catal., 2018, 8, 9359-9363.
Supplementary Information
[0169] The following supplementary information is also provided and relates to various findings and experiments that were performed in the context of this work.
Materials and Methods
[0170] Two-Stet Milling Synthesis
[0171] Milling in both stages was performed in a Retsch Cryomill at a vibrational milling speed of 30 Hz. The elemental powder precursors in addition to two 7 mm diameter stainless steel balls, weighing together 2769 mg on average, were all placed in a 5 mL stainless steel vail to maintain a ball-to-powder ratio (BPR) of 10:1. Cryo-milling was performed in several cycles of 30 minutes each. Between cycles, the vails were pre-cooled by flowing liquid nitrogen (LN2) for 5 minutes to sustain cryogenic temperatures during the entire ball milling process. Before milling, the vials were sealed under Argon environment in a glovebox.
[0172] Using cryo-milling, Ni—Co—Se alloy systems were prepared according to quantities and the procedure shown in Table 2 above.
[0173] In the second stage, powder was added to a 5 mL vial with a BBR of 50:1 to conduct SABM. In addition, anhydrous ethanol with 1:1 ethanol-to-powder mass ratio and dodecylphosphonic acid (DPPA) as surfactant with 1:4 DPPA-to-powder mass ratio were added to the milling vial to produce nanoparticles suspension. SABM was carried out for 10 hours in total (5 cycles of 30 mins and 80 mins of off time between cycles) for all alloy systems in this study. This procedure was followed based on the optimization of parameters conducted in previous work (see S. Ghobrial, D. W. Kirk and S. J. Thorpe, Amorphous Ni—Nb—Y Alloys as Hydrogen Evolution Electrocatalysts, Electrocatalysis, 2019, 10, 243-252). To extract nanoparticles from the suspension, the surfactant should be removed after milling. First, ethanol is added to milling vials and the powder-ethanol slurry mixture is extracted by pipetting into centrifuging tubes until the vials are empty. Then, the tubes are sonicated for 30 min before centrifuging times at 3000 rpm for 30 mins, the large particles should be settled at the bottom of the tube. The suspended nanoparticles in solution is extracted by pipetting into new centrifuging tubes. The tubes are sonicated for 30 mins before centrifuging at 10,000 rpm for 30 mins and then the clear solution is washed and replaced with fresh ethanol, this procedure is repeated at least 3 times until all nanoparticles in the solution precipitate at the bottom of the tube. The tubes are then filled with ethanol and the nanoparticles-ethanol mixture is transferred to 20 mL scintillation vials. Scintillation vials are then heated at 70° C. in a furnace under air flow until all ethanol evaporates. The dry nanoparticle powder is then collected from the vials and stored.
[0174] Sol-Gel Synthesis
[0175] NiFe oxyhydroxides were synthesized using a modified aqueous sol-gel technique (see Ref. 2 below). Anhydrous NiCl.sub.2.6H.sub.2O (2.0 mmol) and FeCl.sub.3 anhydrous (0.7 mmol) were first dissolved in ethanol (4 mL) in a vial. Deionized water (DI) (0.21 mL) was mixed with ethanol (2 mL) in a separate vial. All above solutions were cooled in an ice bath for 2 h. The Ni and Fe precursors were then mixed with an ethanol-water mixture to form a clear solution. To this solution, propylene oxide (=1 mL) was then slowly added, forming a dark gel. The NiFe wet-gel was aged for 1 day to promote network formation, immersed in acetone, which was replaced periodically for 5 days before the gel was dried in vacuum oven.
Characterization
[0176] XRD
[0177] The crystal structure of catalysts was determined using X-ray Diffraction (XRD). A Miniflex 600 (Rigaku, Japan) equipped with D/tex Ultra silicon trip detector and Cu Kα radiation (λ=1.5418 Å) was used. Powders were prepared by mixing with acetone and then dropping a small drop of the mixture to fill a 4 mm diameter×100 μm deep groove in a single crystal silicon holder (zero-background). The angle was varied between 20° to 80° with a step size of 0.05° every 2 seconds.
[0178] Using Scherrer's equation:
where D is the mean crystallite size which can be equal to or less than the particle size, λ is 0.154 nm for Cu X-ray source, K is shape factor has a typical value between 0.9-1.
[0179] The analysis was carried on (NiCo)Se because, unlik2e (NiCo).sub.3Se, it has fewer overlapping peaks in its XRD pattern, which can complicate the analysis. Also, the analysis was carried out on planes in the 2θ range of 30-50 to avoid broadening caused by the instrumentation at higher 2θ values.
TABLE-US-00008 Plane d-spacing (nm) 2θ FWHM (rad) D (nm) (101) 2.711 33.01 0.084 1.92 (102) 2.032 44.55 0.0910 1.83 (110) 1.820 50.08 0.099 1.71
[0180] DLS
[0181] The particle size distribution of catalysts was analyzed using Dynamic Light Scattering (DLS). A LB-500 particle size analyzer (Horiba, Japan) was used. Samples were prepared by dispersing a small amount of the powder in ethanol. The solution mixture was then loaded into disposable plastic 4 mL cuvettes (VWR, U.S.). A 5-mW laser source with a wavelength of 650 nm is directed to the sample in the cuvette to enable the measurement of particles ranging from 3 nm to 6 μm.
[0182] Electron Microscopy
[0183] The structural characterization and elemental mapping of the catalysts were done using Scanning Electron Microscopy (SEM) and Transmission Electron Microscope (TEM). TEM experiments were performed in a Hitachi HF3300 equipped with a cold field emission electron gun using an accelerating voltage of 300 kV. TEM bright field images were used to determine the size and the shape of nanoparticles. Energy X-ray dispersive spectroscopy (EDS) detector was used in scanning transmission electron microscopy (STEM) mode to analyze and quantify the composition of the nanoparticles. Also, secondary electron (SE) detector was used to collect high resolution images of the morphology of the nanoparticles in the TEM. Powder samples were prepared in ethanol to form an ink, the ink was sonicated for 10 minutes before drop casting a 1-2 μL drop on a 400-mesh copper grid and drying overnight.
[0184] The particle and geometry of larger particles were investigated using Scanning Electron Microscopy (SEM). Imaging was conducted using Hitachi SU3500. The composition of particles was determined using an attached EDS detector. Powder samples were prepared by adhering to carbon tape on a SEM aluminum stub. Compressed air is blown over the stub to loosen excess powder. In some cases, thin carbon or gold coating might be needed for poor conducting samples.
Electrochemical Testing
[0185] For all electrochemical experiments in this work, a BioLogic VSP-300 multi potentiosat was used. All tests were performed in 1 M Fe-free KOH electrolyte at 30° C. The electrolyte was pre-electrolyzed using a two-electrode set-up Pt working and counter electrodes at −1.7 V for 48 hours prior using to remove any trace metal impurities in the solution specifically Fe. The cell is placed in a water bath to control the temperature and the electrolyte is purged with Ar for an hour before the test to remove O.sub.2.
[0186] Sample preparation for electrocatalyst on glassy carbon: Powder samples were prepared by making inks. The inks were produced by mixing 4 mg of the catalyst with 80 μL of Nafion® D521 (IonPower) and 1.25 mL of water-to-ethanol (4:1) solution. The mixture was sonicated for 30 minutes before drop casting a 5 μL of the ink on a 3 mm diameter glassy carbon (GC) electrode to produce a thin catalyst layer with a loading of 0.21 mg.Math.cm.sup.−2. The electrode is polished using 0.08 μm colloidal silica every time before applying inks. To load the catalyst on Ni foam (thickness: 1.7 mm, INCO), several drops of the ink are added to achieve a catalyst loading of 2 mg.Math.cm.sup.−2 on 0.5 cm×0.5 cm piece.
[0187] Sample preparation for electrocatalyst on carbon paper: The electrocatalyst were prepared in inks by mixing with a solution of 4:1 water to ethanol and Nafion binder. The ink was then deposited on one side of a 0.5 cm×0.5 cm untreated carbon paper and left to dry in air.
[0188] Sample preparation for electrocatalyst on nickel foam: The electrocatalyst were prepared in inks by mixing with a solution of 4:1 water to ethanol and Nafion binder. The ink was deposited by drop-casting on one side of a 0.5 cm×0.5 cm nickel foam piece with a mass loading of 2 mg.Math.cm.sup.−2.
[0189] Activity and Stability Protocols
[0190] A protocol to test the activity and stability of electrocatalysts used for OER was established as shown in Table 5 above. First, the open cell potential (OCP) was recorded for 30 mins to stabilize the material in the solution. The resistance of the solution was measured by electrochemical impedance spectroscopy (EIS). The frequency was scanned from 1 MHz to 1 kHz with an amplitude of 1 mV. Once EIS is completed, cyclic voltammetry (CV) is performed to clean and activate the surface. Each CV measurement was repeated 3 times at a speed of 50 mV.Math.s.sup.−1 to produce a unique fingerprint of anodic and cathodic peaks in the range from 0-1.6 V vs RHE. This was followed by measuring the electrochemical surface area (ECSA) in a non-Faradaic region using various scanning speeds as explained in Table 5. Each CV measurement at a specific speed is repeated 3 times. To measure the activity, polarization curves were acquired to conduct Tafel analysis and determine the catalytic properties of OER. This was performed using a Linear Sweep Voltammetry (LSV) with a slow speed 1 mV.Math.s.sup.−1 to allow the surface to stabilize and reflect the true reaction mechanism. In the last step, the stability can be assessed using chronopotentiometry. A fixed current density will be applied to evaluate the stability of the catalyst on different substrates. Electrode potentials were measured with respect to Hg/HgO reference electrode. The Hg/HgO reference electrode was experimentally calibrated in a two-electrode setup using three different standard calomel electrodes (SCE) yielding in E(Hg/HgO)=E(SCE)+144 mV. The potentials were converted to the RHE using E(RHE)=E(SCE)+0.241V+0.059*pH.
[0191] iR Correction
[0192] We used electrochemical impedance spectroscopy (EIS) to determine uncompensated resistance (Rs). At all potentials tested on GCE, the potential was corrected using Ohm's law:
E.sub.corrected=E.sub.applied−iR
E.sub.applied is applied potential, i is the current measured, and compensating for 100% of the resistance.
[0193] Intrinsic Activity
[0194] Electrochemical Surface Area (ECSA) measurements were conducted to determine the intrinsic activity of the catalysts. The roughness factor (RF), which is equal to ECSA/geometric area, was calculated. While cycling at different scanning speeds, the double layer capacitance (C.sub.dl) can be calculated from the slope of the current (average difference between anodic and cathodic values for each cycle) vs scanning speed. ECSA is calculated by dividing Cd over a specific capacitance value (C.sub.s) that can range between 0.02-0.13 mF.Math.cm.sup.−2. This work used 0.04 mF.Math.cm.sup.−2 for all Ni and Co based alloys.
Alternating Stability Test
[0195] A special electrochemical protocol was designed to test the stability of the (NiCo).sub.3Se.sub.4 in Fe-free 1M KOH. The sample was polarized at constant current density 10 mA.Math.cm.sup.−2.sub.geo for 10 hours. Then, open circuit potential (OCP) for 1 h, linear sweep voltammetry (LSV) and cyclic voltammetry were carried out to evaluate the change in performance with time. This was repeated 35 times to evaluate the stability for 35 hours of operation.
Turnover Frequency Calculations
[0196]
[0197] Here, j is the measured current density (mA.Math.cm.sup.2), S is the geometric surface area of the electrode, and n is the moles of the active atoms in the loaded catalyst. Also, assuming that all metallic sites are active (lower limit of TOF).
TABLE-US-00009 j.sub.geo @ j.sub.ECSA @ TOF 300 mV 300 mV 10.sup.−3 s.sup.−1 Catalyst mA .Math. cm.sup.−2 mA .Math. cm.sup.−2 RF per site (NiCo).sub.3Se.sub.4 25.04 3.08 8.13 50.76 (NiCo)Se 16.64 2.19 7.60 28.09 IrO.sub.2 9.94 0.74 13.43 27.49 NiFe Sol-Gel 19.25 0.07 275 23.98 Ni 0.70 0.04 17.55 0.51
X-Ray Absorption Spectroscopy
[0198] sXAS
[0199] Ex-situ sXAS measurements were performed at the Spherical Grating Monochromator (SGM) beamline of the Canadian Light Source. All samples were scanned from in 0.1 eV steps for the Surface sensitive absorption spectra were recorded using total electron yield (TEY) and partial fluorescence yield (PFY) simultaneously.
[0200] In Situ XAS
[0201] In situ X-ray absorption spectroscopy (XAS) measurements for the K-edge of Ni and Co were carried out at 9-BM using quick XAS (30 secs per spectrum), while Se was measured at 20BM in the advanced photon source (APS, IL, USA). An in-house custom-made electrochemical cell with a typical three-electrode configuration was used to track XAS changes in situ. A platinum mesh and Ag/AgCl electrode were used as counter electrode and reference electrode, respectively. The catalyst was prepared on one side of a carbon paper while the other side was stuck to a thin Kapton tape (polyimide film with silicone adhesive) and connected electrically by Cu tape. Then, the sample was mounted on the window of the electrochemical cell so that the back side of the sample is facing the beam and the catalyst (front side) is in direct contact with the electrolyte. All measurements were done in air and under ambient conditions. In situ experiments were conducted at 4 conditions: Dry, OCP, 1.1V (Before OER) and 1.5V (during OER) in 1M KOH. Data was collected in fluorescence mode using a Passivated Implanted Planar Silicon (PIPS)/Lytle detector placed at 45° degrees. Several spectra were collected and averaged at each condition to improve the quality of the data and increase the signal to noise ratio. Data post-processing and fitting was done entirely using Demeter software package.
[0202]
TABLE-US-00010 TABLE 9 Summary of electrochemical performance of all catalysts in this study. η [mV] η [mV] Tafel @1 @10 [mV .Math. Size mA mA dec.sup.−1] RF [nm] Ni NPs 321 ± 2 383 ± 3 62 17.55 <100 IrO.sub.2 250 ± 1 298 ± 1 48 13.43 <100 NiFe 212 ± 3 272 ± 5 62 275 — NiSe 277 ± 1 330 ± 5 53 — 60 ± 30 CoSe 275 ± 1 332 ± 1 57 — 80 ± 35 (NiCo)Se Cryo 288 ± 2 353 ± 3 65 — 1065 ± 330 (NiCo).sub.3Se.sub.4 Cryo 310 ± 3 389* 79 — 4055 ± 778 (NiCo)Se 233 ± 2 283 ± 1 50 7.60 37 ± 15 SABM (NiCo).sub.3Se.sub.4 SABM 226 ± 2 268 ± 2 42 8.13 67 ± 27 *overpotential was extrapolated using Tafel slope.
TABLE-US-00011 TABLE 10 OER Performance table from literature Activity Loading Tafel Durability Elec- Elec- [mg .Math. η.sup.(b) [mV .Math. (Electrode) Time # Catalyst trolyte trode.sup.(a) cm.sup.−2] [mV] dec.sup.−.sup.1] Test.sup.(c) [hours] Reference 1 FeP/Ni.sub.2P 1M KOH NF 8 154 22.7 CP @500 mA .Math. cm.sup.−2 40 Nat. Comm., 2018.sup.6 2 (Ni,Fe)OOH 1M KOH NF 4.0 154 41.5 CP @1 A .Math. cm.sup.−2 42 Ener. Env. Sci., 2018.sup.7 3 Core-shell NiFeCu 1M KOH NF 10.2 180 33 CP @10 mA .Math. cm.sup.−2 20 Nat. Comm., 2018.sup.8 4 Ni.sub.3S.sub.2 nanorods 0.1M KOH NF 37 187 159.3 CA 10 mA .Math. cm.sup.−2 10 Ener. Env. Sci., 2013.sup.9 5 Co(OH).sub.2 TCNQ.sup.(d) 1M KOH Cu — 200 188 CA @450 mV 25 Adv. Mat., 2018.sup.10 Foam 6 NiFe nanosheets 1M KOH NF — 215 32 CP @100 mA .Math. cm.sup.−2 10 Nat. Comm., 2015.sup.11 CP @500 mA .Math. cm.sup.−2 2 7 (Ni,Co).sub.0.85Se@NiCo- 1M KOH CC 6 216 77 CP @10 mA .Math. cm.sup.−2 24 Adv. Mat., 2015.sup.12 LDH η = +11% 8 G-FeCoW 1M KOH GC 0.21 223 37 (NF/Au) CP @10 500 Science, 2016.sup.13 mA .Math. cm.sup.−2 9 Co—NiSe.sub.2 1M KOH Ti 1.67 226 94 CP @10 mA .Math. cm.sup.−2 10 Nanoscale, 2016.sup.14 plate 10 (Ni.sub.0.75F.sub.0.25)Se.sub.2.sup.(d) 1M KOH CC — 230 47.2 CP @35 mA .Math. cm.sup.−2 28 ACS App. Mat. & Int., 2016.sup.15 11 FeOOH/Co/FeOOH.sup.(d) 1M NaOH NF 0.5 237 32 CP @200 mA .Math. cm.sub.−2 50 Angew. Chem., 2016.sup.16 12 P—Co.sub.3O.sub.4.sup.(d) 1M KOH NF — 240 60 CA @300 mV 25 ACS Catalysis, 2018.sup.17 13 NiSe Nanowires.sup.(d) 1M KOH NF 2.77 250 64 CP @100 mA .Math. cm.sup.−2 12 Angew. Chem., 2015.sup.18 14 CoAl—NS 1M KOH Graph- 0.05 252 36 CP @70 mA .Math. cm.sup.−2 30 Adv. Mat., 2016.sup.19 ene 15 a-CoVO.sub.x/NF 1M KOH NF — 254 35 CP @10 mA .Math. cm.sup.−2 15 ACS Catalysis, 2018.sup.20 16 (NiCo).sub.0.85Se 1M KOH CC 5 255 79 CP @10 mA .Math. cm.sup.−2 24 Adv. Mat., 2015.sup.12 η = +8% 17 Co.sub.4N nanowire 1M KOH CC 0.82 257 44 CA 75 mA .Math. cm.sup.−2 12 Angew. Chem., 2015.sup.21 18 NiFe-LDH/CNT.sup.(d) 1M KOH GC 0.2 264 31 (GC) CP @10 mA .Math. cm.sup.−2 1 JACS, 2013.sup.22 (NF) CP @20 mA .Math. cm.sup.−2 108 19 NiFe LDH/Fe Plate 1M KOH Fe — 269 48.3 CP @1000 mA .Math. cm.sup.−2 5000 Nat. Comm., 2018.sup.23 Corrosion Engineering Plate 20 CoO hexagrams 1M KOH GC 0.28 269 64.4 (ITO) CP @10 mA .Math. cm.sup.−2 10 Chem. Sci., 2018.sup.24 21 CoO—MoO.sub.2 1M KOH NF — 270 36.7 CP @20 mA .Math. cm.sup.−2 10 Nanoscale, 2015.sup.25 22 Fe.sub.7S.sub.8 nanosheets 1M KOH GC 0.143 270 43 CP @10 mA .Math. cm.sup.−2 25 ACS Cent. Sci., 2017.sup.26 23 Ni.sub.0.2Co.sub.0.8Se 1M KOH CC 0.2 280 86.8 CA j = −17.44% 10 Nano-Micro Lett., 2019.sup.27 24 S|NiN.sub.x 1M KOH GF 0.15 280 45 CP @10 mA .Math. cm.sup.−2 10 Nature Comm., 2019.sup.28 25 CoSe.sub.2 UNM 1M KOH GC 0.28 284 46.3 CP @10 mA .Math. cm.sup.−2 20 J. of Mat. Chem. A, 2019.sup.29 26 Ni.sub.2P 1M KOH GC 0.14 290 47 CP @10 mA .Math. cm.sup.−2 10 Ener. Env. Sci., 2015.sup.30 27 A-CoS.sub.4.6O.sub.0.6 1M KOH GC 0.8 290 67 CP @10 mA .Math. cm.sup.−2 1.25 Angew. Chem., 2017.sup.31 28 Co.sub.3O.sub.4C—NA 0.1M KOH Cu 0.2 290 70 CP @10 mA .Math. cm.sup.−2 30 JACS, 2014.sup.32 foil 29 NiCo.sub.2O.sub.4 1M NaOH NF 1 290 53 CP @10 mA .Math. cm.sup.−2 32 Angew. Chem, 2016.sup.33 30 a-Co.sub.4Fe(OH).sub.x 1M KOH GC 0.28 295 52 CA 10 mA .Math. cm.sup.−2 3 JACS, 2017.sup.34 31 γ-CoOOH nanosheet 1M KOH GC 0.15 300 38 — Science, 2017.sup.35 32 NiFe-nanosheet 1M KOH GC 0.07 300 40 — — JACS, 2005.sup.36 33 IrO.sub.2 1M KOH GC 0.250 300 48 — — Sus. Ener. & Fuels, 2017.sup.37 34 Ni.sub.0.75V.sub.0.25-LDH 1M KOH NF 0.25 300 50 CP @10 mA .Math. cm.sup.−2 25 Nat. Comm., 2016.sup.38 35 FeCo—ONS 0.1M KOH GC 0.36 308 36.8 CA j = +11.2% 2.8 Adv. Mat., 2017.sup.39 36 CoO—MoO.sub.2 1M KOH GC 0.5 312 70 CP @10 mA .Math. cm.sup.−2 15 Adv. Fun. Mat., 2017.sup.40 37 CoSn(OH).sub.6 1M KOH GC 0.094 313 — CA @313 mV 11 Ener. Env. Sci., 2016.sup.41 38 NiCo-r 1M KOH GC 0.285 320 30 CV 1000 cycles — Angew. Chem., 2015.sup.42 39 CoSe.sub.2 nanosheet 0.1M KOH GC 0.142 320 44 CV 1000 cycles — JACS, 2014.sup.43 40 Fe.sub.3O.sub.4@Co.sub.9S.sub.8/ 1M KOH GC 0.25 320 54.5 CA @320 mV 6 Adv. Fun. Mat., 2016.sup.44 rGO-2 j = −12% 41 Ag—CoSe.sub.2 nanobelts 0.1M KOH GC 0.2 320 56 CV 1000 cycles — Angew. Chem., 2017.sup.45 42 NiV-LDH 1M KOH GC 0.143 320 — — — Nat. Comm., 2016.sup.38 43 CoMn LDH 1M KOH GC 0.142 324 43 CA @300 mV 14 JACS, 2014.sup.46 44 Co.sub.0.85Se 1M KOH CC 4.3 324 85 — — Adv. Mat., 2015.sup.12 45 Fe—CoOOH/G 1M KOH GC 0.2 330 37 CP @10 mA .Math. cm.sup.−2 5.5 Adv. Ener. Mat., 2017.sup.47 46 α-Ni(OH).sub.2 0.1M KOH GC 0.2 331 42 CA @350 mV 25 JACS, 2014.sup.48 47 nNiFe LDH/NGF 0.1M KOH GC 0.25 337 45 CA @350 mV 3.3 Adv. Mat. 2015.sup.49 48 NiCo.sub.2S.sub.4 1M KOH GC 0.07 337 64 CP @10 mA .Math. cm.sup.−2 30 Green Chemistry, 2017.sup.50 49 Co.sub.3O.sub.4/NiCo.sub.2O.sub.4 1M KOH NF 1 340 88 — — JACS, 2015.sup.51 50 CoSe 1M KOH CC 0.2 345 89.3 — — Nano-Micro Lett., 2019.sup.27 51 CoFe.sub.35 LDH 0.1M KOH GC 0.25 350 49 (NF) CP @10 mA .Math. cm.sup.−2 48 ChemSusChem, 2016.sup.52 52 CoTe.sub.2 nanofleeces 0.1M KOH GC 0.25 357 32 CV 2000 cycles — Angew. Chem., 2017.sup.53 53 PrBa.sub.0.5Sr.sub.0.5Co.sub.1.5Fe.sub.0.5O.sub.5 0.1M KOH GC 0.202 358 52 CP @10 mA .Math. cm.sup.−2 12 Nat. Comm., 2017.sup.54 54 a-CoVO.sub.x/GC 1M KOH GC 0.14 358 51 — — ACS Catalysis, 2018.sup.20 55 Ni.sub.0.5Co.sub.0.5Se 1M KOH CC 0.2 360 98 — — Nano-Micro Lett., 2019.sup.27 56 Co.sub.3S.sub.4 nanosheets 0.1M KOH GC 0.283 363 90 CV 1000 cycles — ACS Nano, 2014.sup.55 57 NG-CoSe.sub.2 0.1M KOH GC 0.2 366 40 CV 2000 cycles — ACS Nano, 2014.sup.56 η = +6% @10 mA .Math. cm.sup.−2 58 NiCo LDH 1M KOH CP 0.17 367 40 CP @20 mA .Math. cm.sup.−2 6 Nano Lett., 2015.sup.57 59 NiCo.sub.2O.sub.4 1M KOH GC 0.07 377 91 — Green Chemistry, 2017.sup.50 60 CoO/hi-Mn.sub.3O.sub.4 1M KOH GC — 378 61 CA @370 mV 2.2 Angew. Chem., 2017.sup.58 61 Ba.sub.0.5Sr.sub.0.5Co.sub.0.8Fe.sub.0.2O.sub.x RDE 0.25 380 60 — — Science, 2011.sup.59 62 α-Co(OH).sub.2 1M KOH GC 0.28 380 67 CP @5 mA .Math. cm.sup.−2 7 Dalton Trans., 2017.sup.60 63 CoS-βCo(OH).sub.2 1M KOH NF 0.2 380 68 CP @10 mA .Math. cm.sup.−2 28 Adv. Fun. Mat., 2016.sup.61 @aMoS.sub.2+x 64 NiSe 1M KOH CC 0.2 390 225.6 — — Nano-Micro Lett., 2019.sup.27 65 Co.sub.3O.sub.4 nanowire 1M KOH GC 0.136 400 72 CA @420 mV 1.7 Adv. Ener. Mat., 2014.sup.62 j = −31% 66 Co.sub.9 S.sub.8@MoS.sub.2/CNFs 1M KOH GC 0.212 430 61 CV 1000 cycles — Adv. Mat., 2015.sup.63 .sup.(a)GC: glassy carbon, CP: carbon paper, NF: nickel foam, CC: Carbon Cloth, GF: Graphite Foil .sup.(b)Taken @10 mA .Math. cm.sup.−2 .sup.(c)CP chronopotentiometry, CA: chronoamperometry, CV: cyclic voltammetry .sup.(d)η obtained using Tafel slope.
TABLE-US-00012 TABLE Interatomic distance of Ni—O (Å) for OER mechanism intermediates for Ni—Se, Ni—Se.sub.vac, Ni—Co—Se, and Ni—Co—Se.sub.vac surfaces with species reacting at Ni and Co sites. Interatomic distance (Å) Structure Ni—O Ni—O Ni—O Ni—O Ni—O Ni—O Ni—Co—Se Ni—Co—Se Ni—Co—Se.sub.vac Ni—Co—Se.sub.vac Ni—Se Ni—Se.sub.vac (Ni Site) (Co Site) (Ni Site) (Co Site) NiSe (001) H.sub.2O 4.323 2.169 4.039 5.247 2.265 4.120 OH 3.486 2.179 3.653 4.994 2.265 3.810 O 3.528 1.942 3.549 5.025 2.042 3.843 OOH 3.466 2.135 3.641 4.918 2.278 3.915 O.sub.2 4.953 2.148 4.933 5.878 2.794 3.859 NiSe (101) H.sub.2O 3.727 3.525 3.5456 4.797 3.266* OH 3.355 1.969 1.947 3.422 1.912 O 3.299 1.970 1.909 3.394 1.797 OOH 3.291 1.995 1.936 3.599 1.910 O.sub.2 4.297 1.986 2.101 4.820 1.811 Interatomic distance of Se—O (Å) for OER mechanism intermediates for Ni—Se, Ni—Se.sub.vac, Ni—Co—Se, and Ni—Co—Se.sub.vac surfaces with species reacting at Ni and Co sites. Interatomic distance (Å) Structure Se—O Se—O Se—O Se—O Se—O Se—O Ni—Co—Se Ni—Co—Se Ni—Co—Se.sub.vac Ni—Co—Se.sub.vac Ni—Se Ni—Se.sub.vac (Ni Site) (Co Site) (Ni Site) (Co Site) NiSe (001) H.sub.2O 3.463 3.203 3.554 3.443 3.338 3.175 OH 1.871 2.926 1.878 1.874 2.879 2.752 O 1.684 2.788 1.684 1.687 2.804 2.884 OOH 1.943 2.892 1.948 1.943 2.890 2.785 O.sub.2 3.354 2.904 3.397 3.319 2.994 2.752 NiSe (101) H.sub.2O 3.552 3.119 3.430 3.572 3.453* OH 1.926 2.338 2.753 1.937 3.031 O 1.702 1.799 1.804 1.694 3.048 OOH 2.057 1.876 2.664 2.056 2.869 O.sub.2 3.231 1.695 2.856 3.120 2.767 Interatomic distance of Co—O (Å) for OER mechanism intermediates for Ni—Co—Se, and Ni—Co—Se.sub.vac surfaces with species reacting at Ni and Co sites. Interatomic distance (Å) Structure Co—O Co—O Co—O Co—O Ni—Co—Se Ni—Co—Se Ni—Co—Se.sub.vac Ni—Co—Se.sub.vac (Ni Site) (Co Site) (Ni Site) (Co Site) NiSe (001) H.sub.2O 5.479 4.089 3.077 2.107 OH 3.432 3.475 2.082 2.129 O 3.493 3.522 1.878 1.726 OOH 3.557 3.485 2.009 2.093 O.sub.2 4.849 4.866 1.889 2.034 NiSe (101) H.sub.2O 4.483 3.964 3.898* OH 3.498 3.594 1.933 O 3.659 3.512 1.735 OOH 3.851 3.583 1.915 O.sub.2 3.720 4.340 1.736 *The distinction between a Ni and a Co site is lost on the Ni—Co—Se.sub.vac (101) surface due to the adsorbed oxygen species filling the Se vacancy and bridging a surface Co and Ni atom.
[0203] The following references are hereby incorporated herein by reference in their entirety: [0204] 1 S. Ghobrial, D. W. Kirk and S. J. Thorpe, Amorphous Ni—Nb—Y Alloys as Hydrogen Evolution Electrocatalysts, Electrocatalysis, 2019, 10, 243-252. [0205] 2 B. Zhang, X. Zheng, O. Voznyy, R. Comin, M. Bajdich, F. P. G. De Arquer, C. T. Dinh, F. Fan, M. Yuan, A. Janmohamed, H. L. Xin and H. Yang, Homogeneously dispersed, multimetal oxygen-evolving catalysts, Science, 2016, 352, 333-338. [0206] 3 G. Li, L. Anderson, Y. Chen, M. Pan and P.-Y. Abel Chuang, New insights into evaluating catalyst activity and stability for oxygen evolution reactions in alkaline media, Sustain. Energy Fuels, 2018, 2, 237-251. [0207] 4 H. Zhang, J. Zheng, Y. Chao, K. Zhang and Z. Zhu, Surface engineering of FeCo-based electrocatalysts non-noble metals for water oxidation †, 2018, 7254-7261. [0208] 5 B. Ravel and M. Newville, ATHENA, ARTEMIS, HEPHAESTUS: Data analysis for X-ray absorption spectroscopy using IFEFFIT, J. Synchrotron Radiat., 2005, 12, 537-541. [0209] 6 F. Yu, H. Zhou, Y. Huang, J. Sun, F. Qin, J. Bao, W. A. Goddard, S. Chen and Z. Ren, High-performance bifunctional porous non-noble metal phosphide catalyst for overall water splitting, Nat. Commun., 2018, 9, 1-9. [0210] 7 H. Zhou, F. Yu, Q. Zhu, J. Sun, F. Qin, L. Yu, J. Bao, Y. Yu, S. Chen and Z. Ren, Water splitting by electrolysis at high current densities under 1.6 volts, Energy Environ. Sci., 2018, 11, 2858-2864. [0211] 8 P. Zhang, L. Li, D. Nordlund, H. Chen, L. Fan, B. Zhang, X. Sheng, Q. Daniel and L. Sun, Dendritic core-shell nickel-iron-copper metal/metal oxide electrode for efficient electrocatalytic water oxidation, Nat. Commun., 2018, 9, 1-10. [0212] 9 W. Zhou, X. J. Wu, X. Cao, X. Huang, C. Tan, J. Tian, H. Liu, J. Wang and H. Zhang, Ni3S2 nanorods/Ni foam composite electrode with low overpotential for electrocatalytic oxygen evolution, Energy Environ. Sci., 2013, 6, 2921-2924. [0213] 10 D. Wu, Y. Wei, X. Ren, X. Ji, Y. Liu, X. Guo, Z. Liu, A. M. Asiri, Q. Wei and X. Sun, Co(OH)2 Nanoparticle-Encapsulating Conductive Nanowires Array: Room-Temperature Electrochemical Preparation for High-Performance Water Oxidation Electrocatalysis, Adv. Mater., 2018, 30, 1-7. [0214] 11 X. Lu and C. Zhao, Electrodeposition of hierarchically structured three-dimensional nickel-iron electrodes for efficient oxygen evolution at high current densities, Nat. Communications, 2015, 122, 128-136. [0215] 12 C. Xia, Q. Jiang, C. Zhao, M. N. Hedhili and H. N. Alshareef, Selenide-Based Electrocatalysts and Scaffolds for Water Oxidation Applications, Adv. Mater., 2016, 28, 77-85. [0216] 13 B. Zhang, B. Zhang, X. Zheng, O. Voznyy, R. Comin, M. Bajdich, M. Garcia-melchor, L. Han, J. Xu, M. Liu, L. Zheng, F. P. G. De Arquer, C. T. Dinh, F. Fan, M. Yuan, E. Yassitepe, N. Chen, T. Regier, P. Liu, Y. Li, P. De Luna, H. L. Xin, H. Yang, A. Vojvodic and E. H. Sargent, Homogeneously dispersed, multimetal oxygen-evolving catalysts, Science (80-.). 2016, 352, 333. [0217] 14 T. Liu, A. M. Asiri and X. Sun, Electrodeposited Co-doped NiSe2 nanoparticles film: A good electrocatalyst for efficient water splitting, Nanoscale, 2016, 8, 3911-3915. [0218] 15 Z. Wang, J. Li, X. Tian, X. Wang, Y. Yu, K. A. Owusu, L. He and L. Mai, Porous Nickel-Iron Selenide Nanosheets as Highly Efficient Electrocatalysts for Oxygen Evolution Reaction, ACS Appl. Mater. Interfaces, 2016, 8, 19386-19392. [0219] 16 J. X. Feng, H. Xu, Y. T. Dong, S. H. Ye, Y. X. Tong and G. R. Li, FeOOH/Co/FeOOH Hybrid Nanotube Arrays as High-Performance Electrocatalysts for the Oxygen Evolution Reaction, Angew. Chemie—Int. Ed., 2016, 55, 3694-3698. [0220] 17 Z. Wang, H. Liu, R. Ge, X. Ren, J. Ren, D. Yang, L. Zhang and X. Sun, Phosphorus-Doped Co3O4 Nanowire Array: A Highly Efficient Bifunctional Electrocatalyst for Overall Water Splitting, ACS Catal., 2018, 8, 2236-2241. [0221] 18 C. Tang, N. Cheng, Z. Pu, W. Xing and X. Sun, NiSe Nanowire Film Supported on Nickel Foam: An Efficient and Stable 3D Bifunctional Electrode for Full Water Splitting, Angew. Chemie—Int. Ed., 2015, 54, 9351-9355. [0222] 19 J. Ping, Y. Wang, Q. Lu, B. Chen, J. Chen, Y. Huang, Q. Ma, C. Tan, J. Yang, X. Cao, Z. Wang, J. Wu, Y. Ying and H. Zhang, Self-Assembly of Single-Layer CoAl-Layered Double Hydroxide Nanosheets on 3D Graphene Network Used as Highly Efficient Electrocatalyst for Oxygen Evolution Reaction, Adv. Mater., 2016, 28, 7640-7645. [0223] 20 L. Liardet and X. Hu, Amorphous Cobalt Vanadium Oxide as a Highly Active Electrocatalyst for Oxygen Evolution, ACS Catal., 2018, 8, 644-650. [0224] 21 P. Chen, K. Xu, Z. Fang, Y. Tong, J. Wu, X. Lu, X. Peng, H. Ding, C. Wu and Y. Xie, Metallic Co4N porous nanowire arrays activated by surface oxidation as electrocatalysts for the oxygen evolution reaction, Angew. Chemie—Int. Ed., 2015, 54, 14710-14714. [0225] 22 M. Gong, Y. Li, H. Wang, Y. Liang, J. Z. Wu, J. Zhou, J. Wang, T. Regier, F. Wei and H. Dai, An advanced Ni—Fe layered double hydroxide electrocatalyst for water oxidation, J. Am. Chem. Soc., 2013.135, 8452-8455. [0226] 23 Y. Liu, X. Liang, L. Gu, Y. Zhang, G. D. Li, X. Zou and J. S. Chen, Corrosion engineering towards efficient oxygen evolution electrodes with stable catalytic activity for over 6000 hours, Nat. Commun., 2018, 9, 1-10. [0227] 24 Z. Liang, Z. Huang, H. Yuan, Z. Yang, C. Zhang, Y. Xu, W. Zhang, H. Zheng and R. Cao, Quasi-single-crystalline CoO hexagrams with abundant defects for highly efficient electrocatalytic water oxidation, Chem. Sci., 2018, 9, 6961-6968. [0228] 25 B. B. Li, Y. Q. Liang, X. J. Yang, Z. D. Cui, S. Z. Qiao, S. L. Zhu, Z. Y. Li and K. Yin, MoO2-CoO coupled with a macroporous carbon hybrid electrocatalyst for highly efficient oxygen evolution, Nanoscale, 2015, 7, 16704-16714. [0229] 26 S. Chen, Z. Kang, X. Zhang, J. Xie, H. Wang, W. Shao, X. Zheng, W. Yan, B. Pan and Y. Xie, Highly Active Fe Sites in Ultrathin Pyrrhotite Fe7S8 Nanosheets Realizing Efficient Electrocatalytic Oxygen Evolution, ACS Cent. Sci., 2017, 3, 1221-1227. [0230] 27 Z. Qian, Y. Chen, Z. Tang, Z. Liu, X. Wang, Y. Tian and W. Gao, Hollow Nanocages of NixCo1-xSe for Efficient Zinc-Air Batteries and Overall Water Splitting, Nano-Micro Left., DOI:10.1007/s40820-019-0258-0. [0231] 28 Y. Hou, M. Qiu, M. G. Kim, P. Liu, G. Nam, T. Zhang, X. Zhuang, B. Yang, J. Cho, M. Chen, C. Yuan, L. Lei and X. Feng, Atomically dispersed nickel-nitrogen-sulfur species anchored on porous carbon nanosheets for efficient water oxidation, Nat. Commun., 2019, 10, 1-9. [0232] 29 Y. Zhang, C. Zhang, Y. Guo, D. Liu, Y. Yu and B. Zhang, Selenium vacancy-rich CoSe2 ultrathin nanomeshes with abundant active sites for electrocatalytic oxygen evolution, 2019, 2536-2540. [0233] 30 L. A. Stem, L. Feng, F. Song and X. Hu, Ni2P as a Janus catalyst for water splitting: The oxygen evolution activity of Ni2P nanoparticles, Energy Environ. Sci., 2015, 8, 2347-2351. [0234] 31 P. Cai, J. Huang, J. Chen and Z. Wen, Oxygen-Containing Amorphous Cobalt Sulfide Porous Nanocubes as High-Activity Electrocatalysts for the Oxygen Evolution Reaction in an Alkaline/Neutral Medium, Angew. Chemie—Int. Ed., 2017, 56, 4858-4861. [0235] 32 T. Y. Ma, S. Dai, M. Jaroniec and S. Z. Qiao, Metal-organic framework derived hybrid Co3O4-carbon porous nanowire arrays as reversible oxygen evolution electrodes, J. Am. Chem. Soc., 2014, 136, 13925-13931. [0236] 33 X. Gao, H. Zhang, Q. Li, X. Yu, Z. Hong, X. Zhang, C. Liang and Z. Lin, Hierarchical NiCo2O4 hollow microcuboids as bifunctional electrocatalysts for overall water-splitting, Angew. Chemie—Int. Ed., 2016, 55, 6290-6294. [0237] 34 H. Jin, S. Mao, G. Zhan, F. Xu, X. Bao and Y. Wang, Fe incorporated α-Co(OH).sub.2 nanosheets with remarkably improved activity towards the oxygen evolution reaction, J. Mater. Chem. A, 2017, 5, 1078-1084. [0238] 35 Z. W. Seh, J. Kibsgaard, C. F. Dickens, I. Chorkendorff, J. K. Nerskov and T. F. Jaramillo, Combining theory and experiment in electrocatalysis: Insights into materials design, Science (80-.). 2017, 355, 4998. [0239] 36 Z. Liu, R. Ma, M. Osada, K. Takada and T. Sasaki, Selective and controlled synthesis of α- and β-cobalt hydroxides in highly developed hexagonal platelets, J. Am. Chem. Soc., 2005, 127.13869-13874. [0240] 37 G. Li, L. Anderson and Y. Chen, New insights into evaluating catalyst activity and stability for oxygen evolution reactions in alkaline media †, Sustain. Energy Fuels, 2017, 2, 237-251. [0241] 38 K. Fan, H. Chen, Y. Ji, H. Huang, P. M. Claesson, Q. Daniel, B. Philippe, H. Rensmo, F. Li, Y. Luo and L. Sun, Nickel-vanadium monolayer double hydroxide for efficient electrochemical water oxidation, Nat. Commun., 2016, 7, 1-9. [0242] 39 L. Zhuang, L. Ge, Y. Yang, M. Li, Y. Jia, X. Yao and Z. Zhu, Ultrathin Iron-Cobalt Oxide Nanosheets with Abundant Oxygen Vacancies for the Oxygen Evolution Reaction, Adv. Mater., DOI:10.1002/adma.201606793. [0243] 40 F. Lyu, Y. Bai, Z. Li, W. Xu, Q. Wang, J. Mao, L. Wang, X. Zhang and Y. Yin, Self-Templated Fabrication of CoO—MoO2 Nanocages for Enhanced Oxygen Evolution, Adv. Funct. Mater., 2017, 27, 2-9. [0244] 41 F. Song, K. Schenk and X. Hu, A nanoporous oxygen evolution catalyst synthesized by selective electrochemical etching of perovskite hydroxide CoSn(OH)6 nanocubes, Energy Environ. Sci., 2016, 9, 473-477. [0245] 42 J. Bao, X. Zhang, B. Fan, J. Zhang, M. Zhou, W. Yang, X. Hu, H. Wang, B. Pan and Y. Xie, Ultrathin Spinel-Structured Nanosheets Rich in Oxygen Deficiencies for Enhanced Electrocatalytic Water Oxidation, Angew. Chemie—Int. Ed., 2015, 54, 7399-7404. [0246] 43 Y. Liu, H. Cheng, M. Lyu, S. Fan, Q. Liu, W. Zhang, Y. Zhi, C. Wang, C. Xiao, S. Wei, B. Ye and Y. Xie, Low overpotential in vacancy-rich ultrathin CoSe2 nanosheets for water oxidation, J. Am. Chem. Soc., 2014, 136, 15670-15675. [0247] 44 J. Yang, G. Zhu, Y. Liu, J. Xia, Z. Ji, X. Shen and S. Wu, Fe.sub.3O.sub.4-Decorated Co9S8 Nanoparticles In Situ Grown on Reduced Graphene Oxide: A New and Efficient Electrocatalyst for Oxygen Evolution Reaction, Adv. Funct. Mater., 2016, 26, 4712-4721. [0248] 45 X. Zhao, H. Zhang, Y. Yan, J. Cao, X. Li, S. Zhou, Z. Peng and J. Zeng, Engineering the Electrical Conductivity of Lamellar Silver-Doped Cobalt(II) Selenide Nanobelts for Enhanced Oxygen Evolution, Angew. Chemie—Int. Ed., 2017, 56, 328-332. [0249] 46 F. Song and X. Hu, Ultrathin cobalt-manganese layered double hydroxide is an efficient oxygen evolution catalyst, J. Am. Chem. Soc., 2014, 136, 16481-16484. [0250] 47 X. Han, C. Yu, S. Zhou, C. Zhao, H. Huang, J. Yang, Z. Liu, J. Zhao and J. Qiu, Ultrasensitive Iron-Triggered Nanosized Fe—CoOOH Integrated with Graphene for Highly Efficient Oxygen Evolution, Adv. Energy Mater. DOI:10.1002/aenm.201602148. [0251] 48 M. Gao, W. Sheng, Z. Zhuang, Q. Fang, S. Gu, J. Jiang and Y. Yan, Efficient water oxidation using nanostructured α-nickel-hydroxide as an electrocatalyst, J. Am. Chem. Soc., 2014, 136, 7077-7084. [0252] 49 C. Tang, H. Sen Wang, H. F. Wang, Q. Zhang, G. L. Tian, J. Q. Nie and F. Wei, Spatially Confined Hybridization of Nanometer-Sized NiFe Hydroxides into Nitrogen-Doped Graphene Frameworks Leading to Superior Oxygen Evolution Reactivity, Adv. Mater., 2015, 27, 4516-4522. [0253] 50 J. Jiang, C. Yan, X. Zhao, H. Luo, Z. Xue and T. Mu, A PEGylated deep eutectic solvent for controllable solvothermal synthesis of porous NiCo2S4 for efficient oxygen evolution reaction, Green Chem., 2017, 19, 3023-3031. [0254] 51 H. Hu, B. Guan, B. Xia and X. W. Lou, Designed formation of Co3O4/NiCo2O4 double-shelled nanocages with enhanced pseudocapacitive and electrocatalytic properties, J. Am. Chem. Soc., 2015, 137, 5590-5595. [0255] 52 F. Yang, K. Sliozberg, I. Sinev, H. Antoni, A. Bahr, K. Ollegott, W. Xia, J. Masa, W. Grünert, B. R. Cuenya, W. Schuhmann and M. Muhler, Synergistic Effect of Cobalt and Iron in Layered Double Hydroxide Catalysts for the Oxygen Evolution Reaction, ChemSusChem, 2017, 10, 156-165. [0256] 53 Q. Gao, C. Q. Huang, Y. M. Ju, M. R. Gao, J. W. Liu, D. An, C. H. Cui, Y. R. Zheng, W. X. Li and S. H. Yu, Phase-Selective Syntheses of Cobalt Telluride Nanofleeces for Efficient Oxygen Evolution Catalysts, Angew. Chemie—Int. Ed., 2017, 56, 7769-7773. [0257] 54 B. Zhao, L. Zhang, D. Zhen, S. Yoo, Y. Ding, D. Chen, Y. Chen, Q. Zhang, B. Doyle, X. Xiong and M. Liu, A tailored double perovskite nanofiber catalyst enables ultrafast oxygen evolution, Nat. Commun., 2017, 8, 1-9. [0258] 55 W. Zhao, C. Zhang, F. Geng, S. Zhuo and B. Zhang, Nanoporous hollow transition metal chalcogenide nanosheets synthesized via the anion-exchange reaction of metal hydroxides with chalcogenide ions, ACS Nano, 2014, 8, 10909-10919. [0259] 56 M. R. Gao, X. Cao, Q. Gao, Y. F. Xu, Y. R. Zheng, J. Jiang and S. H. Yu, Nitrogen-doped graphene supported CoSe2 nanobelt composite catalyst for efficient water oxidation, ACS Nano, 2014, 8, 3970-3978. [0260] 57 H. Liang, F. Meng, M. Cabin-Acevedo, L. Li, A. Forticaux, L. Xiu, Z. Wang and S. Jin, Hydrothermal continuous flow synthesis and exfoliation of NiCo layered double hydroxide nanosheets for enhanced oxygen evolution catalysis, Nano Lett., 2015, 15, 1421-1427. [0261] 58 C. Guo, Y. Zheng, J. Ran, F. Xie, M. Jaroniec and S. Z. Qiao, Engineering High-Energy Interfacial Structures for High-Performance Oxygen-Involving Electrocatalysis, Angew. Chemie—Int. Ed., 2017, 56, 8539-8543. [0262] 59 J. Suntivich, K. J. May, H. a Gasteiger, J. B. Goodenough and Y. Shao-horn, A Perovskite Oxide Optimized for Molecular Orbital Principles, Science (80-.). 2011, 334, 2010-2012. [0263] 60 F. Lyu, Y. Bai, Q. Wang, L. Wang, X. Zhang and Y. Yin, Phase-controllable synthesis of cobalt hydroxide for electrocatalytic oxygen evolution, Dalt. Trans., 2017, 46, 10545-10548. [0264] 61 T. Yoon and K. S. Kim, One-Step Synthesis of CoS-Doped β-Co(OH)2@Amorphous MoS2+x Hybrid Catalyst Grown on Nickel Foam for High-Performance Electrochemical Overall Water Splitting, Adv. Funct. Mater., 2016, 26, 7386-7393. [0265] 62 Y. Wang, T. Zhou, K. Jiang, P. Da, Z. Peng, J. Tang, B. Kong, W. Bin Cai, Z. Yang and G. Zheng, Reduced mesoporous Co3O4 nanowires as efficient water oxidation electrocatalysts and supercapacitor electrodes, Adv. Energy Mater., 2014, 4, 1-7. [0266] 63 H. Zhu, J. Zhang, R. Yanzhang, M. Du, Q. Wang, G. Gao, J. Wu, G. Wu, M. Zhang, B. Liu, J. Yao and X. Zhang, When Cubic Cobalt Sulfide Meets Layered Molybdenum Disulfide: A Core-Shell System Toward Synergetic Electrocatalytic Water Splitting, Adv. Mater., 2015, 27, 4752-4759.