Intermetallic Ni2Ta Electrocatalyst for the Oxygen Evolution Reaction

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Intermetallic Ni2Ta Electrocatalyst for the Oxygen Evolution Reaction in Highly Acidic Electrolytes Jared S. Mondschein,† Kuldeep Kumar,‡ Cameron F. Holder,† Kriti Seth,† Hojong Kim,‡ and Raymond E. Schaak*,† †

Department of Chemistry and Materials Research Institute, The Pennsylvania State University, University Park, Pennsylvania 16802, United States ‡ Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, United States S Supporting Information *

ABSTRACT: The identification of materials capable of catalyzing the oxygen evolution reaction (OER) in highly acidic electrolytes is a critical bottleneck in the development of many water-splitting technologies. Bulk-scale solid-state compounds can be readily produced using high-temperature reactions and therefore used to expand the scope of earth-abundant OER catalysts capable of operating under strongly acidic conditions. Here, we show that high temperature arc melting and powder metallurgy reactions can be used to synthesize electrodes consisting of intermetallic Ni2Ta that can catalyze the OER in 0.50 M H2SO4. Arc melted Ni2Ta electrodes evolve oxygen at a current density of 10 mA/cm2 for >66 h with corrosion rates 2 orders of magnitude lower than that of pure Ni. The overpotential required for pellets of polycrystalline Ni2Ta to produce a current density of 10 mA/cm2 is 570 mV. This strategy can be generalized to include other first-row transition metals, including intermetallic Fe2Ta and Co2Ta systems.

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use in highly acidic electrolytes.30−32 The dearth of OER catalysts comprised of Earth-abundant elements for use in strongly acidic electrolytes, which are often considered to be optimal due to improved component compatibilities and higher achievable current densities,33 is a key bottleneck in this field.8,9 Previous strategies for increasing both OER activity and stability of Earth-abundant materials in acidic solutions have focused primarily on modifying highly studied OER catalysts using alternate processing or electrode preparation methods,34 decoration with TiO2 or other corrosion-resistant materials,35 self-healing through redeposition,36,37 and phase activation.38 However, it is widely acknowledged that the identification of new classes of Earth-abundant OER catalysts that are active and stable in strongly acidic solutions is an increasingly important goal.6,7 Accordingly, we demonstrate that high-temperature metallurgical routes can be used to synthesize electrodes of intermetallic alloys that combine rapidly corroding base metals with refractory, corrosion-resistant early transition metals. These electrodes function as active OER pre-catalyst systems39 having significantly enhanced acid stability relative to their single-element base metal counterparts. Tantalum is several orders of magnitude more abundant in the Earth’s crust than iridium and ruthenium,40 and it forms an acid-stable metal oxide under oxidizing potentials.41 Nickel, which is one of the cheapest and most Earth-abundant elements, catalyzes the OER but rapidly corrodes in acid.42

INTRODUCTION Devices that facilitate water splitting, including photoelectrochemical cells (PECs), are promising solutions for the clean production of hydrogen from sunlight.1−3 These technologies rely on the production of hydrogen gas at the cathode via the hydrogen evolution reaction (HER) and the production of oxygen gas at the anode via the oxygen evolution reaction (OER).1−3 PECs can operate in strongly acidic, strongly alkaline, or neutral electrolytes, and under these conditions, catalysts for the HER and OER must be capable of operating for long periods of time with minimal decrease in activity and little corrosion.4,5 They should also be composed of elements abundant in the Earth’s crust to make widespread commercial use practical.6−9 The discovery of several classes of highly active catalysts for the HER has been achieved through well-known nanoparticle synthetic routes and has yielded high surface area materials capable of operating in a wide range of electrolytes.10−16 For the OER, metal oxide,17−19 metal phosphide,20 metal sulfide,21,22 metal selenide,23 metal telluride,24 and other nanostructured and composite materials25,26 have been reported as active catalysts and catalyst precursors. However, many highly active OER catalysts were not initially accessible as nanoparticles and were instead discovered using bulk solid state and/or thin film techniques. Examples include NiFe-(oxy)hydroxides and cobalt-based perovskites for operation in strongly alkaline electrolytes,27,28 electrodeposited cobaltphosphate thin films in near-neutral electrolytes,29 and ruthenium- and iridium-based pyrochlores and perovskites for © XXXX American Chemical Society

Received: February 25, 2018

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DOI: 10.1021/acs.inorgchem.8b00503 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Synthesis. Rods were synthesized via arc melting and a subsequent molding step. Stoichiometric quantities of the elemental precursors were combined and briefly ground together. The mixture was then loaded into the melting chamber of an Edmund Buhler GmbH MAM1 arc melter. The melting chamber was sealed and then evacuated/ purged with ultrahigh purity argon three times. The intermetallics were then arc-melted on a water-cooled copper crucible plate three times to achieve homogeneity. The intermetallics were then molded into a cylindrical shape by replacing the copper crucible plate with a suction casting assembly consisting of a water-cooled copper plate with a 3 mm ID cylindrical mold. The prealloyed sample was placed on the center of this plate, and the melting chamber was sealed. Using a similar evacuation and purging procedure as described in the previous step, an argon environment with a positive pressure (1.1 bar) was formed inside the chamber. The intermetallic was then arc-melted again, and the suction-casting valve was quickly opened to pull the molten intermetallic inside the cylindrical mold. After the mold was allowed to cool to room temperature, the rod-shaped alloy was removed. Ni2Ta pellets were synthesized via solid-state reactions in sealed, evacuated ampules. Ni powder (118.0 mg, 2.0 mmol) and Ta powder (181.0 mg, 1.0 mmol) were ground together for 30 min using a mortar and pestle. Subsequently, the ground powder was pressed into a pellet and loaded into a quartz ampule. The ampule was then flame-sealed under vacuum and heated to 920 °C using a ramp rate of 15 °C/min. After 5 days, the ampule was quenched into a water bath. Co2Ta pellets were synthesized similar to Ni2Ta, except Co powder (118.3 mg, 2.0 mmol) and Ta powder (181.0 mg, 1.0 mmol) were used. Fe2Ta pellets were synthesized in a similar manner. Fe powder (114.5 mg, 2.1 mmol) and Ta powder (186.3 mg, 1.03 mmol) were ground together for 30 min using a mortar and pestle. Subsequently, the ground powder was pressed into a pellet and loaded into a quartz ampule. The ampule was then flame-sealed under vacuum and heated to 920 °C using a ramp rate of 15 °C/min. After 4 days, the furnace was cooled to 750 °C, and the ampule was quenched in a water bath. Ta2O5 pellets were prepared by pressing Ta2O5 powder into a pellet and sintering at 800 °C for 24 h in air. IrO2 pellets were prepared by pressing IrO2 powder into a pellet and sintering at 800 °C for 15 h in air. Electrode Preparation. Rods were electrically connected to the potentiostat via an alligator clamp and then subsequently wrapped with Teflon tape so that ∼1.5 cm2 was exposed. Working electrodes of the Ni2Ta, Fe2Ta, Co2Ta, Ta2O5, and IrO2 pellets were anchored onto a polyvinyl chloride-coated Sn-plated copper wire using Ag paint. All surfaces except for the surface of the pellets were then coated with two-part epoxy. Electrochemical Measurements. All electrochemical measurements were performed using a Gamry Instruments Reference 600 potentiostat or a Reference 1000B potentiostat. A porous graphitic rod was used as the counter electrode, and a saturated calomel electrode (SCE) was used as the reference electrode. Measurements at pH 0.3 were performed in high purity 0.50 M H2SO4 (99.999%) using a onecompartment, three-neck glass cell. Prior to each experiment, the cell was purged with O2 for approximately 15 min. During all measurements, the electrolyte was not stirred. To minimize capacitive current, 2 cyclic voltammograms were collected at a sweep rate of 10 mV/s before obtaining linear sweep voltammograms at a sweep rate of 1 mV/s. The resulting current densities were normalized to the observed background current. Solution resistance was measured and compensated by the potentiostat using the current interrupt method. The electrochemical cell equilibrated for 16 s prior to each experiment. While it is desirable to measure electrochemical surface area via the integration of redox waves, double-layer capacitance (Cdl) values are often utilized when no redox waves are observed,50 as is the case here. Accordingly, the electrochemical surface area was determined via measured Cdl values, as previously described.8 Cyclic voltammetry measurements were performed on Ni2Ta rods during a 10 mA/cm2 galvanostatic experiment. Measurements were performed at ±50 mV of the open circuit potential (−0.02−0.07 V vs SCE). The average midpoint potential was plotted as a function of scan rate, and the

Oxides containing nickel and tantalum are highly insulating, but their intermetallic alloys are metallic conductors and may therefore be considered as candidates for OER electrocatalysts.27,43,44 Intermetallic alloys of tantalum with nickel, including Ni2Ta, have been widely explored for use as corrosion-resistant coatings.45,46 Ni2Ta, which crystallizes in the MoSi2 structure type, has Ni bonded to Ta in a square pyramidal geometry and Ta bonded to Ni in bicapped square antiprisms (Figure 1a). As is the case for many intermetallic

Figure 1. (a) Crystal structure of Ni2Ta showing the positions of the Ni (blue) and Ta (red) atoms. (b) Photograph, (c) scanning electron micrograph, (d) X-ray diffraction pattern, and (e) X-ray photoelectron spectrum of the Ta 4f region of an arc-melted Ni2Ta rod. In panel d, asterisks (*) and squares (■) correspond to carbon and Ta2O5 impurities, respectively.

compounds, the ordering of the atoms in the crystal structure and the directional bonding give rise to chemical and mechanical properties such as enhanced catalysis and corrosion resistance47,48 that differ from their constituent metallic elements.49 Here, we show that Ni2Ta combines the OER activity of Ni with the corrosion resistance of Ta to yield an active OER catalyst in strongly acidic solutions with a moderate overpotential at device-relevant current densities as well as a dissolution rate that is several orders of magnitude lower than that of Ni. Similar results are achieved for related Co−Ta and Fe−Ta intermetallic systems.

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EXPERIMENTAL SECTION

Materials. Nickel powder (Alfa-Aesar, −325 mesh, 99.8%), tantalum powder (Alfa-Aesar, −325 mesh, 99.97%), iron powder (Sigma-Aldrich, −325 mesh, 97%), tantalum(V) oxide powder (Alfa Aesar, 99.993%), iridium(IV) oxide powder (Alfa Aesar, 99%), and sulfuric acid (Sigma-Aldrich, 99.999%) were used as received without further purification. Nanopure water (18 MΩ) was obtained from a Barnstead Nanopure Analytical Ultrapure water system. Silver paint (Flash-Dry TM) was purchased from SPI supplies, and Sn-plated copper wire and two-part epoxy (HYSOL 9460) were purchased from McMaster-Carr. Nafion 117 was purchased from Fuel Cell Store. B

DOI: 10.1021/acs.inorgchem.8b00503 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry resulting slope gave the double-layer capacitance (Cdl). Cdl was converted to electrochemical surface area by dividing by 0.035 mF/ cm2, a general specific capacitance of metal electrodes in acidic solutions.8 The electrochemical stabilities of the catalysts were examined using galvanostatic measurements where the current density was held at 10 mA/cm2. All galvanostatic measurements were iR corrected using Ru values obtained by measuring the impedance of the cell at 0 V vs SCE with an applied frequency of 100 kHz. The concentrations of dissolved nickel and tantalum ions as a function of time were determined by removing electrolyte aliquots at various time points. Electrolyte samples from tests of the Ni2Ta and Ni rods were then diluted with 2% HNO3 and analyzed via either inductively coupled plasma mass spectrometry (ICP-MS) using a Thermo Fisher Scientific X Series 2 ICP-MS with Collision Cell Technology or inductively coupled plasma optical emission spectroscopy (ICP-OES) using a PerkinElmer Optima 5300 UV. UV/vis spectra of the electrolytes after galvanostatic testing were obtained using an Ocean Optics HR4000 spectrometer equipped with a Mikropack DH-2000-BAL UV−vis-NIR lightsource. Electrolyte samples from tests of the Ni and Ni2Ta polycrystalline powders were diluted with 0.50 M H2SO4 and analyzed with a Shimadzu AA-7000 spectrophotometer. Faradaic efficiency measurements were performed using a custommade 2-compartment electrochemical cell with a Nafion 117 membrane. A Ni2Ta rod was used as the working electrode, and a platinum wire was used as the counter electrode. An anodic current of 20 mA was passed continuously through the anode. Ultrahigh purity argon was bubbled into the electrochemical cell at a rate of 5.0 mL/ min throughout the experiment to dilute the produced O2(g) and to act as a carrier gas. Evolved O2(g) was quantified using a Shimadzu 2014 System GC equipped with a Carboxen 1000 and a HayeSep Q column. Captured gas was injected onto the column from a 1 mL sample loop at a temperature of 120 °C. The oven temperature was initially held at 80 °C for 6 min, then ramped to 200 °C at a rate of 20 °C/min and held for 11 min. After 4.30 min, the flow of Ar carrier gas through the HayeSep Q column was reversed. Approximately 30 min passed between subsequent injections. Materials Characterization. Experimental powder X-ray diffraction (XRD) patterns of the arc-melted rods were collected at room temperature with a PANalytical X’PERT PRO using Cu Kα (λ= 1.5405 Å) radiation and a PIXcel3D detector. XRD patterns of the polycrystalline pellets were collected at room temperature with a Bruker-AXS D8 Advance diffractometer with Cu Kα radiation and a LynxEye 1-D detector. Scanning electron microcopy (SEM) images were obtained using a FEI NanoSEM at an accelerating voltage of 5.0 keV and a working distance of 3.5 mm. Energy dispersive spectroscopy (EDS) data and elemental mapping images were collected at an accelerating voltage of 20.0 keV and a working distance of 5 mm. X-ray photoelectron spectra (XPS) were collected on a PHI VersaProbe II spectrometer, equipped with a scanning monochromatic Al Kα X-ray source (hν = 1486.6 eV). The X-ray gun was set at high-power mode. All spectra were acquired with electron and ion neutralizers active with the analytical chamber pressures in the mid-10−6 Pa range during data acquisition. Survey and high-resolution scans were recorded at pass energies of 117.4 and 93.9 eV, respectively, and the binding energy values were referenced to C 1s at 284.8 eV. All samples were electrically isolated from the plate and fastened to it with 3M doublesided tape.

Ni2Ta as the primary phase, along with impurities of carbon (from the arc-melting and molding process) and Ta2O5. The relative intensities of the (00l) peaks of Ni2Ta are higher than predicted based on the simulated XRD pattern, indicating that the grains in the Ni2Ta rod are preferentially oriented in the [001] direction, as expected given the layering of Ni and Ta planes along the c axis of the crystal structure. Similar preferential orientation of crystalline grains has been observed previously in samples arc melted and subsequently molded into shapes such as cylinders.51,52 The EDS spectrum, shown in Figure S2, indicates the presence of both tantalum and nickel. Corresponding XPS data, shown in Figure 1e, confirms the presence of both Ta and Ta5+, consistent with a mixture of Ni2Ta and Ta2O5. No significant Ni signal is observed by XPS, indicating that the surface layers of the as-synthesized sample contain predominantly Ta species. XPS survey scans did not detect any noble metal impurities on the surface (Figure S3), and EDS also showed no detectable noble metal signals. Figure 2 shows plots of potential vs time for Ni2Ta and Ni working electrodes, producing an anodic current density of 10

Figure 2. Galvanostatic measurements of a (a) Ni rod and (b) a Ni2Ta rod at 10 mA/cm2 in 0.50 M H2SO4.

mA/cm2 over more than 3 days in 0.50 M H2SO4. Pure Ta metal anodes were unable to produce 10 mA/cm2 due to the immediate formation of insulating Ta2O5 upon exposure to highly oxidizing potentials, as indicated by the XPS spectra in Figure S4. Analogous Ni electrodes required an overpotential of 610 mV to achieve a current density of 10 mA/cm2. However, as expected based on the known solubility of Ni in sulfuric acid,41 the Ni metal anodes quickly corroded, producing an electrolyte solution that was visibly green due to the presence of dissolved Ni2+ (Figure S5). Consistent with this observation, the rate of Ni dissolution from the Ni electrode was measured by ICP-OES to be 43.6 μg/min (Figure 3). Relative to Ni, the Ni2Ta electrode required a higher overpotential of 980 mV to produce a current density of 10 mA/cm2, normalized to the geometric area, but the rate of Ni dissolution decreased relative to the Ni control by over 2 orders of magnitude to 0.393 μg/ min (Figure 3b). The corrosion rate of the Ni2Ta anode in 0.50 M H2SO4 is still several orders of magnitude faster than the reported corrosion rates of the best precious metal OER catalysts, IrO2 and RuO2, under similar conditions,53 but is similar to the dissolution rate of emerging base metal OER catalysts, such as Co3O4 films on fluorinated tin oxide substrates,34 under comparably acidic conditions. In contrast to the Ni electrode, the electrolyte solution in contact with the Ni2Ta electrode remained colorless, and no significant UV− visible absorption peak associated with dissolved Ni2+ was detected (Figure S5). The rate of Ta dissolution was only 0.15 ng/min (Figure 3b), which suggests that dissolution of Ni was

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RESULTS AND DISCUSSION Ni2Ta electrodes were fabricated by arc melting stoichiometric quantities of Ni and Ta and subsequently molding them into cylinders with diameters and lengths of 3 mm and 3 cm, respectively (Figure 1b). Tantalum and nickel electrodes with similar dimensions were also synthesized as controls (Figure S1). The SEM image of the Ni2Ta rod, shown in Figure 1c, reveals a flat, polycrystalline surface. The powder XRD pattern in Figure 1d confirms the formation of crystalline, MoSi2-type C

DOI: 10.1021/acs.inorgchem.8b00503 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 3. (a) ICP-OES data characterizing the time-dependent dissolution of Ni and Ta from a Ni rod and Ni2Ta rod. (b) Timedependent dissolution data for the Ni2Ta rod, magnified from the plot in panel a.

Figure 4. Linear sweep voltammograms of a Ni2Ta rod and polycrystalline powders consisting of Ni2Ta, Fe2Ta, and Co2Ta.

overpotential was correlated with a small increase in ECSA to 0.84 cm2. Furthermore, the polycrystalline electrode demonstrated improved corrosion resistance relative to a Ni electrode prepared in a similar fashion when tested galvanostatically at 10 mA/cm2 (Figure S9). Additionally, preliminary evidence suggests that tantalumbased intermetallic alloys of other rapidly corroding but OERactive base metals can also function as OER catalysts with enhanced stability in strongly acidic solutions. For example, a pellet containing Co2Ta produced a current density of 10 mA/ cm2 at an overpotential of 600 mV (Figure 4, Figure S7). Similarly, a pellet containing Fe2Ta required an overpotential of 770 mV to produce a current density of 10 mA/cm2 (Figure 4, Figure S7). These preliminary results indicate that the targeting of intermetallics that combine the OER activity of one metal with the corrosion resistance of a second metal is a promising strategy for the development of new classes of water oxidation catalysts for use in acidic electrolytes. While the corrosion rates and required overpotentials to achieve 10 mA/cm2 exhibited by the tantalum-based intermetallics introduced here are still higher than desired for direct technological applications, it is important to consider how these materials further the development of acid-stable OER catalysts. Efforts to reduce the noble metal loading have resulted in bulk iridium- and ruthenium-based perovskites and pyrochlores that are much more active toward electrocatalytic water splitting than rutile IrO2 and RuO2.30−32 These materials, however, still contain significant amounts of low-abundance elements and likely suffer from non-negligible corrosion rates.56 Introduction of new classes of materials into the scope of known OER catalysts is limited, with a few recent examples providing important advances in this field.57−59 The identification of Ni2Ta, Co2Ta, and Fe2Ta as OER catalyst systems in acidic electrolytes further expands the number of Earth-abundant materials available for further modification and optimization.60

accompanied by the formation of tantalum oxide rather than forming of dissolved Tan+ species. Consistent with this observation, alloys of tantalum exposed to high oxidative potentials in strongly acidic electrolytes are known to form surfaces enriched with tantalum oxide.46 The occurrence of these competing non-Faradaic processes resulted in an experimentally determined Faradaic yield of 85% (Figure S6). Oxide formation during galvanostatic experiments led to a concomitant decrease in the electrochemical surface area (ECSA) as a function of time (Table S1). During a 77 h galvanostatic experiment at 10 mA/cm2, the ECSA decreased from 0.70 to 0.11 cm2. This anodically formed tantalum oxide layer, which is stable in acidic electrolytes and known to serve as a passivating oxide coating, likely allows for subsurface atoms to continue catalyzing water oxidation while limiting their dissolution. However, we cannot rule out that the active catalyst species is within the oxide surface layer. The overpotential of 980 mV required for the arc-melted Ni2Ta rods to produce a current density of 10 mA/cm2 is high relative to the overpotentials of 340−360 mV for ruthenium and iridium oxides and 570 mV for cobalt oxides to achieve comparable current densities in 0.50 M H2SO4. However, the surfaces of bulk arc-melted rods are flat and have low electrochemically active surface areas relative to nanoparticles and films that are polycrystalline or porous.8 Given the refractory nature of Ni2Ta,54,55 and the difficulty in readily producing nanoparticles of tantalum or tantalum-based alloys, the production of higher surface area Ni 2 Ta is not straightforward and will require additional future synthetic efforts. However, as a first step toward decreasing the overpotential through increasing surface area, we attempted to synthesize polycrystalline Ni2Ta by reacting stoichiometric quantities of Ni and Ta powders at 920 °C for 5 days in a quartz ampule sealed under vacuum. Powder XRD indicated that the product contained crystalline Ni2Ta (Figure S7) along with Ni3Ta. Figure 4 shows a plot of current density vs potential for intermetallic Ni−Ta electrodes constructed by pressing and annealing pellets of the polycrystalline powder. For comparison, electrodes composed of polycrystalline pellets of IrO2, the benchmark OER catalyst, and Ta2O5, were tested under similar conditions (Figure 4, Figure S8). In 0.50 M H2SO4, the IrO2 electrode required an overpotential of 290 mV to produce 10 mA/cm2, and the highly insulating Ta2O5 electrode displayed baseline activity, as expected. The polycrystalline intermetallic Ni−Ta electrode required a significantly lower overpotential, relative to that of the Ni2Ta rod, of 570 mV to reach 10 mA/cm2. This decrease in

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CONCLUSION The intermetallic compound Ni2Ta functions as an active OER catalyst in strongly acidic solutions, having a rate of Ni corrosion that is more than two orders of magnitude slower than that of a comparable Ni electrode. Similar preliminary results were obtained for Co2Ta and Fe2Ta intermetallic systems, indicating that alloying 3d transition metals with tantalum significantly enhances acid stability under oxidizing potentials and suggesting that other catalytic materials may also exist in these and related intermetallic systems. Future efforts to increase the surface area of this family of compounds are D

DOI: 10.1021/acs.inorgchem.8b00503 Inorg. Chem. XXXX, XXX, XXX−XXX

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Catalysts for the Hydrogen-Evolution Reaction. Chem. Mater. 2016, 28, 6017−6044. (11) Jaramillo, T. F.; Jørgensen, K. P.; Bonde, J.; Nielsen, J. H.; Horch, S.; Chorkendorff, I. Identification of Active Edge Sites for Electrochemical H2 Evolution from MoS2 Nanocatalysts. Science 2007, 317, 100−102. (12) Popczun, E. J.; McKone, J. R.; Read, C. G.; Biacchi, A. J.; Wiltrout, A. M.; Lewis, N. S.; Schaak, R. E. Nanostructured Nickel Phosphide as an Electrocatalyst for the Hydrogen Evolution Reaction. J. Am. Chem. Soc. 2013, 135, 9267−9270. (13) Popczun, E. J.; Read, C. G.; Roske, C. W.; Lewis, N. S.; Schaak, R. E. Highly Active Electrocatalysis of the Hydrogen Evolution Reaction by Cobalt Phosphide Nanoparticles. Angew. Chem., Int. Ed. 2014, 53, 5427−5430. (14) Callejas, J. F.; McEnaney, J. M.; Read, C. G.; Crompton, J. C.; Biacchi, A. J.; Popczun, E. J.; Gordon, T. R.; Lewis, N. S.; Schaak, R. E. Electrocatalytic and Photocatalytic Hydrogen Production from Acidic and Neutral-pH Aqueous Solutions Using Iron Phosphide Nanoparticles. ACS Nano 2014, 8, 11101−11107. (15) Callejas, J. F.; Read, C. G.; Popczun, E. J.; McEnaney, J. M.; Schaak, R. E. Nanostructured Co2P Electrocatalyst for the Hydrogen Evolution Reaction and Direct Comparison with Morphologically Equivalent CoP. Chem. Mater. 2015, 27, 3769−3774. (16) Zou, X.; Huang, X.; Goswami, A.; Silva, R.; Sathe, B. R.; Mikmeková, E.; Asefa, T. Cobalt-Embedded Nitrogen-Rich Carbon Nanotubes Efficiently Catalyze Hydrogen Evolution Reaction at All pH Values. Angew. Chem., Int. Ed. 2014, 53, 4372−4376. (17) Park, J.; Sa, Y. J.; Baik, H.; Kwon, T.; Joo, S. H.; Lee, K. IridiumBased Multimetallic Nanoframe@Nanoframe Structure: An Efficient and Robust Electrocatalyst toward Oxygen Evolution Reaction. ACS Nano 2017, 11, 5500−5509. (18) Zhao, Y.; Vargas-Barbosa, N. M.; Hernandez-Pagan, E. A.; Mallouk, T. E. Anodic Deposition of Colloidal Iridium Oxide Thin Films from Hexahydroxyiridate(IV) Solutions. Small 2011, 7, 2087− 2093. (19) Cui, B.; Lin, H.; Li, J.; Li, X.; Yang, J.; Tao, J. Core-Ring Structured NiCo2O4 Nanoplatelets: Synthesis, Characterization, and Electrocatalytic Applications. Adv. Funct. Mater. 2008, 18, 1440−1447. (20) Dutta, A.; Pradhan, N. Developments of Metal Phosphides as Efficient OER Precatalysts. J. Phys. Chem. Lett. 2017, 8, 144−152. (21) Chen, S.; Kang, Z.; Zhang, X.; Xie, J.; Wang, H.; Shao, W.; Zheng, X.; Yan, W.; Pan, B.; Xie, Y. Highly Active Fe Sites in Ultrathin Pyrrhotite Fe7S8 Nanosheets Realizing Efficient Electrocatalytic Oxygen Evolution. ACS Cent. Sci. 2017, 3, 1221−1227. (22) Wiltrout, A. M.; Read, C. G.; Spencer, E. M.; Schaak, R. E. Solution Synthesis of Thiospinel CuCo2S4 Nanoparticles. Inorg. Chem. 2016, 55, 221−226. (23) Gao, M.; Zheng, Y.; Jiang, J.; Yu, S. Pyrite-Type Nanomaterials for Advanced Electrocatalysis. Acc. Chem. Res. 2017, 50, 2194−2204. (24) Gao, Q.; Huang, C.; Ju, Y.; Gao, M.; Liu, J.; An, D.; Cui, C.; Zheng, Y.; Li, W.; Yu, S. Phase-Selective Syntheses of Cobalt Telluride Nanofleeces for Efficient Oxygen Evolution Catalysts. Angew. Chem., Int. Ed. 2017, 56, 7769−7773. (25) Zhang, L.; Xiao, J.; Wang, H.; Shao, M. Carbon-Based Electrocatalysts for Hydrogen and Oxygen Evolution Reactions. ACS Catal. 2017, 7, 7855−7865. (26) Wang, J.; Liu, D.; Huang, H.; Yang, N.; Yu, B.; Wen, M.; Wang, X.; Chu, P. K.; Yu, X. In-Plane Black Phosphorus/Dicobalt Phosphide Heterostructure for Efficient Electrocatalysis. Angew. Chem., Int. Ed. 2018, 57, 2600−2604. (27) Burke, M. S.; Zou, S.; Enman, L. J.; Kellon, J. E.; Gabor, C. A.; Pledger, E.; Boettcher, S. W. Revised Oxygen Evolution Reaction Activity Trends for First-Row Transition-Metal (Oxy)hydroxides in Alkaline Media. J. Phys. Chem. Lett. 2015, 6, 3737−3742. (28) Suntivich, J.; May, K. J.; Gasteiger, H. A.; Goodenough, J. B.; Shao-Horn, Y. A Perovskite Oxide Optimized for Oxygen Evolution Catalysis from Molecular Orbital Principles. Science 2011, 334, 1383− 1385.

anticipated to further reduce the overpotential required to achieve target current densities. Intermetallic compounds that combine corrosion-resistant elements with acid-soluble but OER-active base metals are therefore promising candidate materials for evaluating as Earth-abundant OER catalysts in acidic solutions.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00503. Experimental details and additional XRD, SEM, XPS, UV/vis, AAS, and electrochemical data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Raymond E. Schaak: 0000-0002-7468-8181 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS J.S.M., C.F.H., K.S., and R.E.S were supported by the U.S. National Science Foundation Center for Chemical Innovation in Solar Fuels (Grant CHE-1305124). K.K. and H.K. acknowledge the American Chemical Society Petroleum Research Fund (Grant 54898-DNI10) for support. SEM, XRD, and XPS data were acquired through the Materials Characterization Laboratory at the Penn State University Materials Research Institute. The authors thank Dr. An Nguyen for technical support.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.8b00503 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.8b00503 Inorg. Chem. XXXX, XXX, XXX−XXX