Core-Oxidized Amorphous Cobalt Phosphide Nanostructures: An

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Core-Oxidized Amorphous Cobalt Phosphide Nanostructures: An Advanced and Highly Efficient Oxygen Evolution Catalyst Sengeni Anantharaj,†,‡ Pula Nagesh Reddy,∥ and Subrata Kundu*,†,‡,§ †

Academy of Scientific and Innovative Research, CSIR-Central Electrochemical Research Institute (CECRI) Campus, New Delhi, India ‡ Electrochemical Materials Science (ECMS) Division, CSIR-CECRI, Karaikudi 630006, Tamil Nadu, India § Department of Materials Science and Mechanical Engineering, Texas A&M University, College Station, Texas, Texas 77843, United States ∥ CSIR-CECRI, Karaikudi 630006, Tamil Nadu India S Supporting Information *

ABSTRACT: We demonstrated a high-yield and easily reproducible synthesis of a highly active oxygen evolution reaction (OER) catalyst, “the core-oxidized amorphous cobalt phosphide nanostructures”. The rational formation of such core-oxidized amorphous cobalt phosphide nanostructures was accomplished by homogenization, drying, and annealing of a cobalt(II) acetate and sodium hypophosphite mixture taken in the weight ratio of 1:10 in an open atmosphere. Electrocatalytic studies were carried out on the same mixture and in comparison with commercial catalysts, viz., Co3O4-Sigma, NiO-Sigma, and RuO2-Sigma, have shown that our catalyst is superior to all three commercial catalysts in terms of having very low overpotential (287 mV at 10 mA cm−2), lower Tafel slope (0.070 V dec−1), good stability upon constant potential electrolysis, and accelerated degradation tests along with a significantly higher mass activity of 300 A g−1 at an overpotential of 360 mV. The synergism between the amorphous CoxPy shell with the Co3O4 core is attributed to the observed enhancement in the OER performance of our catalyst. Moreover, detailed literature has revealed that our catalyst is superior to most of the earlier reports.

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oxides,19,20 hydroxides,21−24 mixed oxides,25−29 layered double hydroxides (LDH),30−34 chalcogenides,35,36 and phosphides37 are also reported as efficient for the OER in an alkaline electrolyte. In a report of Subbaraman et al., the OER activity trend of 3d metal hydroxides is said to be Mn2+ < Fe2+ < Co2+ < Ni2+, which is inversely related to the bond energetics of M2+OH complexes.38 It is true because the increasing electron population in 3d orbitals of these divalent cations will have strong repulsion between the lone-pair electrons residing on O atoms of the hydroxide ligands. However, this is not true when the oxidation states of the metal centers are different. Although Ni2+ has higher activity than other 3d M2+ ions, catalysts made of Co, Mn, and Fe with various other oxidation states and stoichiometric compositions were shown to have higher activity than a Ni2+-based OER catalyst. Examples for such OER catalysts are the spinel Co3O 4 and the cobalt phosphides.29,39−48 Deep insights into the mechanism of the OER on Co3O4 and other Co-based catalysts were given earlier. Among them, the

INTRODUCTION Increasing importance in the design of high-performance electrochemical oxygen evolution reaction (OER) catalysts out of earth-abundant, non-noble, and cost-effective 3d transition metals is on the peak of an electrocatalytic water oxidation domain among the other forefront areas of applied materials research.1 The electrochemistry of water oxidation involves anodic OER and cathodic hydrogen evolution reaction (HER). Between them, the OER is being studied more frequently than the HER because of various kinetic reasons.2−4 This makes the OER require higher potential, which is termed “overpotential (η)”, which is important in evaluating a catalyst for anodic OER studies. On the other hand, the HER is facile on Pt surfaces,5 and also there are reports other than Pt with significant improvement in the HER activity.5−9 The conventional state-of-the-art OER catalysts comprised of noble metals and their oxides such as Ru/RuO2 and Ir/IrO2 used in acidic water electrolyzers are too expensive and less abundant.10−18 Hence, it is important to find cheaper, abundant, and efficient alternatives to catalyze the OER. Beyond iridium and ruthenium oxides, the compounds of 3d group VIII metals, viz., Fe, Co, and Ni, in various chemical forms such as © XXXX American Chemical Society

Received: December 2, 2016

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DOI: 10.1021/acs.inorgchem.6b02929 Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

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following are the notable ones. In a report by Bell et al., the OER activity of Co3O4 and other Co-based catalysts in comparison with RuO2 was studied by pure density functional theory (DFT) calculations in addition to Hubbard-U correction and showed that Co3O4 and β-CoOOH are better OER catalysts in alkali.49 In another report by Wang et al., similar outcomes were observed.50 These Co4O4 entities are believed to form on the surfaces of Co3O4 catalysts during water oxidation under an applied potential.51 Man et al. derived a universality among all of the oxide surfaces and placed Co3O4 above all.20 The DFT-U study by Mattioli et al. on the OER activity of a Co catalyst also predicted high OER activity for Co3O4.52 Other than these, the reports on Co3O4 and cobalt phosphates by Nocera et al.53−57 and Stahl et al.58 have also shown that Co3O4 could be a better OER catalyst than the other catalysts of other 3d metals. Although it is a known system, with significant improvements, Co-based catalysts are being reported.3,20,25,33,40,52,56,58−70 Enhancement in the catalytic activity is achieved mainly in any one of the following ways. Making composites with other metals28,29,33,44−46,48,71−73 and nanostructured C atoms,42,47,70,74 and thereby tuning their electronic property, is one of the ways to enhance the OER activity of Co catalysts. Examples are the mixed-metal oxides, hydroxides, and sulfides of Co with other metals and/or also with graphene, graphene oxides, heteroatom-doped graphene, carbon nanotubes (CNTs), and polyaniline.42,74 The second and equally important way of enhancing the OER activity of Co-based catalysts is providing them a OER facile environment by designing them with synergistically enhancing ligands, i.e., the phosphate.37,55,56,75−82 In photoelectrocatalytic water oxidation, also cobalt phosphate was employed with photoactive materials such as WO3, BiVO4, and other polyoxometallates.75,79,82−85 Enhancement with a phosphate ligand was attributed to the synergism established by the P atom due to its affinity toward water.37,55,56,75−82 Similar enhancement in the OER activity of IrO2@DNA was also observed where a phosphate environment was provided by DNA.84 Similar enhancement was observed with other metal phosphides also.37,86,87 The role of P in enhancing OER catalysis is also seen with cobalt phosphides. Dutta et al. recently reported the OER performance of surface-oxidized Co2P nanoneedles.88 A report by Liu et al. states the advantageous of using a CoP hollow polyhedron in the OER.89 Hou et al. composited the CNTs with the CoP and used them for total water splitting.90 Another report by Zhu et al. depicted the use of mesoporous CoP nanorods in total water splitting.91 The report by Ryu et al. showed the in situ formed CoP NPs with molecular cobalt oxo/hydroxo units for the OER.92 In all of the reports, one common is the surface oxidation of CoP. Although the CoPs are efficient catalysts in the OER, when they get oxidized to cobalt oxo/hydroxo molecular entities on the surface, they tend loose its activity because the oxo/hydroxo molecular entities have Co2+ that are relatively less active as per the report by Subbaraman et al.38 Hence, the combination of the two most active Co catalysts, viz., Co3O4 and CoP, will be more efficient in the OER. To do so, we report an easy, high-yield wet chemical synthetic route to architect such a highly efficient core-oxidized (Co3O4) amorphous cobalt phosphide nanostructure that has some oxides of P. Thus, architected core-oxidized amorphous cobalt phosphide nanostructures were subsequently characterized and screened for the OER in 1 M KOH, and the results are discussed in subsequent sections.

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EXPERIMENTAL SECTION

Synthesis of Core-Oxidized Amorphous Cobalt Phosphide Nanostructures. Materials used are particularized in the Supporting Information (SI). To synthesize a core-oxidized amorphous cobalt phosphide, 100 mL of a water and ethanol mixture (4:1) was taken with 0.249 mg of cobalt(II) acetate (10 mmol) and 0.88 mg of sodium hypophosphite (100 mmol), which was then homogenized by mechanical stirring, during which the color of the solution turned from pink to violet. After 60 min of homogenization, the solvent was evaporated by heating to 120 °C on a hot plate. After evaporation, a dark-brown solid mass was obtained. The obtained dark-brown solid mass was then subjected to annealing at 450 °C in air for 3 h and then allowed to naturally cool to room temperature. After annealing, the brown solid mass was found to be a lustrous black solid mass. Then the same mass was finely powdered and used for further characterization and subsequent OER studies. A graphical sketch of the synthesis sequences is displayed as Scheme 1. Our preliminary

Scheme 1. Schematic Depiction of Synthesis Sequences in Architecting Core-Oxidized Amorphous Cobalt Phosphide Nanostructures

optimization studies showed that the selected molar ratio (1:10) between CoII sources and hypophosphite is essential to succeed in the formation of core-oxidized amorphous cobalt phosphide nanostructures. Because other phosphate-based salts will not cause the reduction of CoII, the hypophosphite as a reductant and P sources are necessary here. In the case where phosphate salts are taken instead of hypophosphite, the formation of cobalt phosphate was predominant and no sign of Co3O4 was detected. Similarly, annealing below 3 h did not result in the formation of Co3O4, annealing beyond 3−4 h at the same temperature led to complete oxidation, and the resultant product was only the spinel Co3O4. All of these preliminary controlled studies indicated that the proposed synthesis route is the most appropriate for the successful synthesis of core-oxidized amorphous cobalt phosphide nanostructures. The specifications of the characterization techniques used are specified in the SI. Electrochemical Studies. A set of essential electrochemical characterizations were performed to evaluate the core-oxidized amorphous cobalt phosphide nanostructures as catalysts for the OER in 1 M KOH. As a comparative study, the OER activity of coreoxidized amorphous cobalt phosphide nanostructures is compared to that of three commercial catalysts such as Co3O4-Sigma, NiO-Sigma, and the state-of-the-art RuO2-Sigma under identical electrochemical conditions with the same normalized loading of 0.205 mg cm−2. A glassy carbon electrode (GCE) of 3 mm diameter (area 0.0732 cm−2) was used as the substrate electrode. An Hg/HgO alkaline reference electrode filled with 1 M KOH was used along with a Pt foil counter electrode in a three-electrode conventional electrochemical cell connected to a CHI6084C electrochemical workstation. All of the measurements were carried out at room temperature (25 °C). Linear sweep voltammograms (LSVs) were obtained in an operating potential window of 0−0.9 V versus Hg/HgO (note: We restricted ourselves below 0.9 V to avoid even any contribution from the substrate GCE to the original catalytic current) at a scan rate of 0.005 V s−1 in 1 M B


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Inorganic Chemistry KOH, and the potential scale is converted into a reversible hydrogen electrode (RHE) scale concomitantly using eq 1.

E RHE = Eobs + Eref 0 + 0.059pH

oxides of P. The following are the Miller indices assigned to the peaks obtained from 10° to 90°: (111), (220), (311), (222), (400), (331), (422), (511), (440), (531), (442), (620), (533), (622), (444), and (711). The observed pattern of the synthesized material matches that of spinel Co3O4 and wellcoincides with the ICDD card data numbered as 065-3103 and also with earlier reports of spinel Co3O4.39−41,46,47,70 Microstructure Analysis with High-Resolution Transmission Electron Microscopy (HR-TEM) and Elemental Color Mapping with Field-Emission Scanning Electron Microscopy (FE-SEM). The microstructure of a material is best analyzed using HR-TEM and FE-SEM techniques. For materials such as nanoarchitectures comprising more than one element/material phase, it is mandatory to ascertain the position of each element/material phase in the material, for which the most reliable tool is the energy-dispersive analysis Xray (EDAX) elemental color mapping in a defined place of the sample specimen. The HR-TEM micrographs captured at low magnification (Figure 2a,b) revealed the morphology of our material, which is an aggregated nanostructured network. Both parts a and b of Figure 2 show that the synthesized material looks aggregated with interweaving contrast and bright parts all over the sample. This indicates that the synthesized material is highly polydisperse in nature. The appearance of bright and contrast parts one over another in almost all places of the sample leaves a clue that there exists more than one nanostructured phase where the brighter parts should be of elements with low Z values or due to some amorphous phases of CoP and the darker parts of the sample should be of elements with higher Z values in the synthesized material. While increasing the magnification, the synthesized material looks to have a crystalline core and an amorphous shell, as seen almost everywhere in the specimen, as shown in Figure 2c−e. Interestingly, it was noticed that the core is highly crystalline, whereas the shell is completely amorphous. Careful calibration of the lattice fringe observed at the core of Figure 2c−e witnessed that the core is spinel Co3O4. Another highmagnification HR-TEM micrograph obtained at the core of the material showed lattice fringes of various d-spacing values that are assigned to the (111), (222), (311), and (220) planes of spinel Co3O4 alone, which are the low-angle diffraction planes. The selected-area electron diffraction (SAED) pattern (inset image of Figure 2b) looks to have dots and rings together, which indicates the polycrystalline nature of the material. The calibrated ring patterns in the obtained SAED resonate well with the results of XRD analysis and confirm the presence of a spinel Co3O4 phase. Besides, it was also found that there was no matching of these d-spacing values with any form of crystalline oxides of P and crystalline cobalt phosphides. This again implies that the core is completely made up of spinel Co3O4 and not accompanied by any other phosphorus oxides. Being puzzled with the observed amorphous shell, we eagerly moved to find out what it was comprised of. EDAX elemental color mapping is the most promising technique that could precisely differentiate various elements by color in an interested spot of the specimen. Figure 3a is the low-magnification FESEM micrograph in which the EDAX spectrum and elemental color mapping were done. In the selected region of the specimen (Figure 3a), the dark core and brighter shells of the material can be best viewed. The corresponding EDAX spectrum obtained (Figure S1) shows peaks for various shells of elements P, O, Co, and C. The peaks of P, O, and Co

(1)

where ERHE is the potential in the RHE scale, Eobs is the end potential (0.9 V) of the experimental potential window versus Hg/HgO in 1 M KOH, and Eref0 is the reference electrode potential. The ohmic drop due to the uncompensated resistance (Ru) of the system was corrected simultaneously in the electrochemical workstation itself. The constant potential electrolysis studies were carried out without correcting the iR drop at potentials where these four catalysts produce a current density of 10 mA cm−2 for more than 12 h. Similarly, the accelerated degradation (AD) tests on all four catalyst-modified GCEs were performed at a scan rate of 0.200 V s−1 within the experimental potential window for about 250 cycles. The resultant LSVs obtained at regular intervals for all four catalysts were given without iR drop correction, and the increases in the overpotentials at 10 and 20 mA cm−2 were comparatively evaluated. The corresponding Tafel plots were derived from the LSVs obtained with iR correction and given with respective slope values. Electrochemical impedance spectroscopy (EIS) analysis was done on core-oxidized amorphous cobalt phosphide nanostructure-modified GCEs before electrochemical studies and after chronoamperometric and AD analyses at a resting potential with a frequency range of 1 MHz to 0.1 Hz, where the amplitude of the potential oscillation was set at about 0.050 V. The details of the catalyst ink preparation for substrate electrode modification can be found in the SI, along with the sample preparation methods for various material characterizations.

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RESULTS AND DISCUSSION Diffraction Studies Using an X-ray Diffractometer. An X-ray diffraction (XRD) study is the primary tool for qualitatively confirming the expected material formation in materials research. The black powder obtained in our synthesis was directly subjected to XRD analysis, and the obtained pattern (Figure 1) shows various peaks characteristic of spinel

Figure 1. XRD pattern of the synthesized core-oxidized amorphous cobalt phosphide nanostructure.

Co3O4 with less intensities and considerable broadening. The lower intensities are attributed to the hindering effect of amorphous CoxPy shells. This essentially indicates the formation of nanostructured Co3O4. However, it can be perceived that there are noises with very low intensities in the 2θ range of 10−90°. This indicates that there are some amorphous impurities/phases of the desired material in the synthesized material, which may be the cobalt phosphides and C


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Inorganic Chemistry

Figure 2. (a and b) Low-magnification HR-TEM micrographs of core-oxidized amorphous cobalt phosphide nanostructures. The inset image of part b is the respective SAED pattern. (c−f) High-magnification HR-TEM micrographs that show clear lattice fringes of the crystalline core and a distinct amorphous shell.

sample. Having confirmed the presence of spinel Co3O4 as the core and an amorphous shell of Co, P, and O from the above analysis, it is now essential to study the chemical nature, which can give potential information on the shell around the spinel Co3O4 core. Chemical Nature of Elements in Core-Oxidized Amorphous Cobalt Phosphide Material by X-ray Photoelectron Spectroscopy (XPS). With exciting morphological outcomes, we moved forward to analyze the chemical nature of each element present in it. Moreover, the formation of spinel Co3O4 can also be confirmed by XPS analysis. Similarly, it is essential to find out the oxidation states of Co and P in the synthesized sample. This will reveal whether there are any amorphous cobalt phosphides. The shell may be comprised of either the amorphous cobalt phosphides or the oxides of P or even both. The XPS survey scan (Figure S2a) detected all elements, as expected. In-depth information on the oxidation states of Co and P was found from their respective highresolution XPS spectra of the P 2p and Co 2p states, which are provided here as Figure 4a,b. The fitted XPS spectrum of P 2p (Figure 4a) shows peaks corresponding to P with oxidation states of P3+ and P5+, which could possibly be from the amorphous phosphorus oxides that envelope the core. Moreover, in the region of P5+, two more peaks with higher binding energy values are attributed to the trigonal-planar phosphite that requires higher energy to lose electrons than the tetrahedral phosphate though the oxidation states are the same in both. Interestingly, peaks near the characteristic binding energies of elemental P but with still lower binding energy values corresponding to P with negative oxidation states are also observed with considerable intensities. This is a strong indication of the presence of metallic phosphide. Similar P 2p spectra were observed earlier by others for cobalt phosphides.88−92 Similarly, the deconvoluted XPS spectrum of the Co 2p state (Figure 4b) shows a doublet that arose as a consequence of spin−orbit coupling and was separated by 15.32 eV. The 2p3/2 and 2p1/2 states have two peaks

Figure 3. (a) Low-magnification FE-SEM micrograph of core-oxidized amorphous cobalt phosphide nanostructures where the elemental color mapping was performed. (b) EDAX color map of the Co K shell, which indicates the dominant presence of Co only at the core and to some extent in the shell. (c) EDAX color map of the P K shell, which indicates the presence of P almost everywhere but looks denser at the shell. (d) EDAX color map of the O K shell, which indicates equal distribution of O almost everywhere in the sample specimen and implies that maximum content of Co and P are in their oxide forms beyond some cobalt phosphide.

originate from the sample. The peak of C is from the C-coated Cu grid used as the specimen substrate. Parts b−d of Figure 3 are the images that show the positions of the elements Co (blue), P (green), and O (red) with mentioned colors within parentheses. From these three figures, it is evident that Co is present in a major amount at the core and also in some minor amount at the shell of the material. The intense blue color at the core once again proves the presence of Co3O4 witnessed by HR-TEM analysis. On the other hand, P and O are almost everywhere. This certainly proves that the core consists of cobalt oxide (Co3O4 here), whereas the shell could possibly be made up of amorphous cobalt phosphides and phosphorus oxides that enveloped spinel Co3O4 in almost all places in the D


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Inorganic Chemistry

phosphides88−92 and metal phosphates.94,95 The O 1s highresolution XPS spectrum (Figure S 2d) has various peaks between 528 and 532 eV, which indicates the presence of O atoms with different chemical environments such as the O atom in the metallic lattices (Co3O4) of Co in two different oxidation states and also in oxides of P in the P3+ and P5+ oxidation states. The observed features of the O 1s spectrum resonate well with earlier reports.44−48 The detailed XPS analysis has confirmed the formation of spinel Co3O4 along with cobalt phosphide and oxides of P. Finally, the formation of architecting a catalyst system comprised of Co3O4, CoP, and oxides of P was successful and was confirmed. Comparative Electrocatalytic OER Studies. The morphologically exciting core-oxidized amorphous cobalt phosphide nanostructures were examined for alkaline water oxidation. As given in the Experimental Section and under the prescribed electrochemical conditions, LSVs obtained for all four catalysts including core-oxidized amorphous cobalt phosphide, Co3O4-Sigma, NiO-Sigma, and RuO2-Sigma are given without and with iR drop compensation, as seen in Figure 5a,b. Within the polarized potential limit, all four catalysts have shown good catalytic activity. However, the catalytic OER activity of our core-oxidized amorphous cobalt phosphide nanostructures is superior to the other three commercial catalysts in terms of lower overpotential (η) for a current density of 10 mA cm−2. The overpotentials at 10 mA cm−2 required by core-oxidized amorphous cobalt phosphide nanostructures, Co3O4-Sigma, NiO-Sigma, and RuO2-Sigma are 302, 371, 360, and 340 mV without iR compensation, respectively. Such a low overpotential at a defined current density of 10 mA cm−2 was not ever seen before for a Co-based OER catalyst without iR compensation. At the same experimental conditions, the commercial Co3O4 catalyst and the best OER catalyst in alkaline water electrolyzers NiO and the state-of-the-art RuO2 failed to compete with our catalyst. The difference in the overpotentials required by Co3O4 and our catalyst is as high as 69 mV at 10 mA cm−2. This implies that a significant enhancement in the OER catalytic activity was brought out because of the catalyst design, which is provided with a shell continuum of amorphous cobalt phosphide and oxides of P over the spinel Co3O4 catalysts. It could also be noted that when Co3O4 alone was examined, the OER activity in terms of the overpotential was lower than those of both the best alkaline OER catalyst (NiO) and the state-of-the-art RuO2. However, when provided with an environment of P and its oxides accompanied by the catalytically more active cobalt phosphide in amorphous nature, the OER activity is highly enhanced. Similar trends are observed in the onset overpotentials too for all four catalysts, which are 205, 265, 254, and 170 mV for core-oxidized amorphous cobalt phosphide nanostructures, Co3O4-Sigma, NiO-Sigma, and RuO2-Sigma, respectively. When the same spinel Co3O4 was provided with the amorphous cobalt phosphide and an environment of phosphorus oxides, the onset overpotential was dragged down to 205 mV from 265 mV. This is mainly due to the synergism established between the cobalt phosphide and spinel Co3O4, which are the catalytically more active forms of a Co catalyst. Because it is essential to correct the iR drop for an electrocatalyst to analyze the kinetics of OER by Tafel analysis, we have done iR drop compensation simultaneously with the electrochemical workstation itself. The iR-corrected LSVs of all four catalysts (Figure 5b) illustrate their corresponding iRdrop-free overpotentials at 10 mA cm−2, which are 287, 354,

Figure 4. High-resolution XPS spectra of the (a) P 2p and (b) Co 2p states in the synthesized core-oxidized amorphous cobalt phosphide nanostructures.

characteristics to that of Co3+ at 781.50 and 796.76 eV with two more peaks that correspond to Co2+ at 783.59 and 798.91 eV. This certainly proves that the material formed contains the Co3O4 and not any Co2O3 or hydroxides of Co. Interestingly, as observed with the P 2p scan, two low intense peaks corresponding to the Co 2p3/2 state of metallic Co at 776.9 eV and the Co 2p1/2 state of metallic Co at 796.2 eV were also seen. This confirmed the existence of cobalt phosphide in the synthesized sample, which could be a part of the amorphous shell along with the oxides of P. Similar P 2p and Co 2p XPS spectra were observed in earlier reports of cobalt phosphides too.88−92 The observed XPS Co 2p spectrum was in agreement with earlier reports of spinel Co3O4 as well.71,93 The formation of Co3O4 with some amorphous cobalt phosphides was further confirmed from the high-resolution XPS spectra of Co 3p (Figure S2b) and Co 3s (Figure S2c). Both Co 3p and Co 3s spectra also have two peaks corresponding to spinel Co3O4 and cobalt phosphide. The broadness observed in these peaks is due to the presence of Co2+ and Co3+ ions in a 1:2 ratio in the synthesized spinel Co3O4. The puzzle laid by the amorphous layer was finally solved by the high-resolution XPS spectra of the synthesized samples for the P 2p, Co 2p, Co 3p, and Co 3s states. These results indicate that the amorphous shell layer, as confirmed by the EDAX elemental color mapping, consists of both cobalt phosphide and the oxides of P. These observations were in agreement with earlier reports on various metal E


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Inorganic Chemistry

Figure 5. (a and b) LSVs without and with iR drop corrections. (c) Plot of j versus η. (d) Tafel plots of core-oxidized amorphous cobalt phosphide and other commercial catalysts studied here. (e) Tafel plot of core-oxidized amorphous cobalt phosphide by chronoamperometry and RDE experiments. (f) Plot of the mass activity versus overpotential.

and pictured as Figure 5c. All four catalysts have shown negligible deviations from the mean values. Moreover, because OER reports on a new catalyst are often seen in comparison with IrO2 also, we have compared the LSV and Tafel features of IrO2-Sigma under identical experimental conditions, with the same catalyst loading as that provided in Figure S3a,b in the SI. Although IrO2-Sigma showed lower onset overpotential and attains 10 mA cm−2 with very little difference in the overpotential (3 mV), for higher current densities such as 20 mA cm−2, the difference in the overpotential between our catalyst and IrO2-Sigma is quite large (27 mV) and higher for even higher current densities. This indicates that although IrO2-

348, and 330 mV for core-oxidized amorphous cobalt phosphide nanostructures, Co3O4-Sigma, NiO-Sigma, and RuO2-Sigma, respectively. It is necessary to emphasis here that iR compensation will not affect the onset overpotential of a catalyst, as can be seen in our case too. An iR-drop-free overpotential of just 287 mV is the lowest one for a single 3d transition-metal-based OER catalyst so far. There are very few reports with better activity than this report, where more than one transition metal is employed together with high catalyst loading such as metal selenides and LDH materials.30−32 Standard errors in the overpotentials for all four catalysts at defined current densities are calculated from the first 25 runs F


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Inorganic Chemistry

activity of our catalyst was 265.65 A g−1, whereas the commercial catalysts have shown relatively poor mass activities, viz., 54.39, 71.31, and 62.87 A g−1 respectively for Co3O4Sigma, NiO-Sigma, and RuO2-Sigma. The mass activity trend advocated the superior OER activity of our catalyst over the commercial catalysts. To have a reasonable comparison, a detailed literature survey was done, and the reported Co-based OER catalysts with high activity are benchmarked against our core-oxidized amorphous cobalt phosphide catalyst in terms of the overpotential at 10 mA cm−2 and provided as Figure 6,

Sigma showed better activity than RuO2-Sigma in catalyzing OER under these conditions, the poor kinetics in alkaline conditions also allowed this one to fail in competing with our catalyst. The same trend was also observed with Tafel analysis (Figure S3b), where IrO2-Sigma is shown to have a very large Tafel slope of 133 mV dec−1, and this is closer to the one observed with RuO2-Sigma (137 mV dec−1). These results again indicate that, whether with IrO2 or RuO2, our catalyst is superior in alkaline conditions. The kinetics of an OER on an electrocatalyst surface is judged by its corresponding Tafel slope. The Tafel plots of core-oxidized amorphous cobalt phosphide nanostructures, Co3O4-Sigma, NiO-Sigma, and RuO2-Sigma are provided together in Figure 5d. As observed in the LSV characteristics, the core-oxidized amorphous cobalt phosphide nanostructures have shown a Tafel slope of 0.070 V dec−1, which directly implies that the kinetics of the OER on its surface is comparatively more facile than that on the other three commercial catalysts. The Tafel slopes of Co3O4-Sigma, NiOSigma, and RuO2-Sigma are 0.080, 0.137, and 0.100 V dec−1. As observed in the overpotential at 10 mA cm−2, the Tafel slope of Co3O4 was also brought down from 0.080 to 0.070 V dec−1. This once again emphasizes the synergistic enhancement by the spinel Co3O4 core and the amorphous cobalt phosphide in the shell. The Tafel slopes have also revealed that, although RuO2 is the state-of-the-art OER catalyst in acid, it is poor in alkaline solutions. This is the reason for the observed large Tafel slope (0.100 V dec−1) for the same mixture. Moreover, the best alkaline OER catalyst (NiO) showed a higher Tafel slope (0.137 V dec−1) than our catalyst (0.070 V dec−1), indicating the superior catalytic activity of our catalyst once again under the deployed experimental conditions. Moreover, to ascertain the observed Tafel slopes obtained by interconverting the LSV acquired at a scan rate of 5 mV s−1, the Tafel slopes of our catalyst were determined by two other methods, namely, chronoamperometry and a rotating disk electrode (RDE) experiment. If the electrocatalytic process was affected by the mass-transfer overpotential during the OER in 1 M KOH, these two experiments should give slopes significantly different from the one observed before. By the chronoamperometry method, the steady-state currents are obtained at various potentials in the OER region by running chronoamperometry for 200 s. The logarithmic steady-state currents are plotted against the overpotential to get the respective slope. Similarly, to obtain the Tafel slope by the RDE experiment, an LSV at a very low scan rate of 0.5 mV s−1 was obtained with a rotation rate of 1600 rpm. Both Tafel curves are shown as Figure 5e. The observed slopes by chronoamperometry and the RDE experiments are 69.2 and 69.5 mV dec−1. These are very close to the value obtained earlier and indicate that the concentration overpotential has no significant role in affecting the kinetics of the OER in 1 M KOH, where the availability of the analyte (OH−/H2O) is plenty. The mass activity of all four catalysts studied here is calculated in the overpotential range from 300 to 440 mV (1.53 to 1.67 V vs RHE) at an interval of 20 mV. The calculated mass activity values (see the SI for detailed calculation) are plotted against their respective overpotentials and illustrated as Figure 5f. From Figure 5f, the superior catalytic performance of core-oxidized amorphous cobalt phosphide nanostructures can best be witnessed. Having the same mass loading of 0.205 mg cm−2, our catalyst has shown better catalytic OER activity than the other three commercial catalysts. At an overpotential of 355 mV, the calculated mass

Figure 6. Benchmarking core-oxidized amorphous cobalt phosphide nanostructures in terms of the overpotential at 10 mA cm−2 against other Co-based OER catalysts. The symbol # indicates that the catalyst loading was the same (0.2 mg cm−2) as ours in those reports, the symbol $ indicates a lesser loading of 0.1 mg cm−2, and the symbol @ indicates a very high catalyst loading of ∼1 mg cm−2. With other reports, the loading details are not provided.

from which the enhanced OER activity of our catalysts could be proven again. The results of the overall comparative electrocatalytic OER study are tabulated as Table 1. Moreover, our results are also compared with other Co-based OER reports and others in Table 2.75,86,96−99 As mentioned earlier, from the first discovery of cobalt phosphate with enhanced OER activity by Nocera and co-workers,53−57 people have observed such enhancement in the catalytic activity of Co and other OER active metal-based catalysts in various forms by providing them a P environment.37,85 The enhancement observed in an environment of P is mainly attributed to the high water affinity of P and its oxides in all pH values that assists the coordination water to the electroactive site, which brings down the threshold potential of water splitting and also enhances the overall OER activity.53,85 Although there are reports for Co with P environments, there are no data for such systematic architecting of a core-oxidized amorphous cobalt phosphide that is morphologically favorable for the OER. The highly enhanced OER performance by a core-oxidized amorphous cobalt phosphide-nanostructured catalyst is sketched as Scheme 2. Having found an exceptionally enhanced OER performance for our core-oxidized amorphous cobalt phosphide-nanostructured catalyst as a consequence of synergistic assistance established by the amorphous cobalt phosphide shell, we moved to the next level of examining the stability of our catalyst in comparison with the other three commercial catalysts by chronoamperometry analysis and by potential cycling at higher scan rates, which is otherwise known as the “AD test”. The potentiostatic endurance test for more than 12 h was examined on a core-oxidized amorphous cobalt phosphide-nanostrucG


Article

Inorganic Chemistry Table 1. Results of the Comparative Electrocatalytic OER Studies catalyst

loading (mg cm−2)

mass activity (A g−1)

overpotential η at 10 mA cm−2 (mV)

overpotential ηiR free at 10 mA cm−2 (mV)

change in the overpotential Δη at 10 mA cm−2 and 20 mA cm−2 after the AD test (mV)

Tafel slope (V dec−1)

Co3O4@PxOy Co3O4-Sigma NiO-Sigma RuO2-Sigma

0.205 0.205 0.205 0.205

265.65 54.39 71.31 62.87

302 371 360 344

287 354 348 330

8 and 19 18 and 36 20 and 33 25 and 51

0.070 0.080 0.135 0.100

Table 2. Results of the Electrocatalytic OER Studies Performed in This Work in Comparison with Other Literature Reportsa catalyst core-oxidized amorphous CoxPy Co3O4-Sigma NiO-Sigma RuO2-Sigma surface-oxidized CoP NRs/C Au@Co3O4 core−shell NPs electrodeposited CoP Au/mesoporous Co3O4 Co3O4 NPs@GCE Co3O4 NPs@Au Co3O4 NPs@Pt Co3O4@MWNTs cobalt corrole sea-urchin-like (Co0.54Fe0.46)P electrodeposited Co3O4 3D crumpled-graphene Co3O4 Au-supported Co3O4 Zn−Co LDH Co@Co3O4@N-doped graphene on Ni foam Co3O4 NPs of 5.9 nm diameter Co3O4 NPs of 21.1 nm diameter Co3O4 NPs of 46.9 nm diameter ZnCo2O4 Co3O4 Li-doped Co3O4 Li−Co−Ni−Fe LDH with varying compositions Co3O4@graphene

loading (mg cm−2)

η at 10 mA cm−2 (mV)

Scheme 2. Graphical Sketch That Illustrates the Exceptional OER Performance of Core-Oxidized Amorphous Cobalt Phosphide Nanostructures

Tafel (mV dec−1)

ref

0.205

287

70

TW

0.205 0.205 0.205 5.000

354 348 330 357

80 135 100 71

TW TW TW 37

0.200 >5.000 0.200

∼370 ∼340 440 483 453 567 390 483 370

60 60 46 120 120 120 65 -

48 65 99 98 98 98 75 97 88

-

410 340

49 75

43 70

0.280 0.420

∼500 507 260

-

74 99 42

1.000

328

47 ± 7

40

1.000

363

47 ± 7

40

1.000

382

47 ± 7

40

0.250 0.100

390 410 ∼350 295−400

46 54 52−60 35−50

27 27 33 34

-

313

56

47

Figure 7. Chronoamperometric analysis carried out for more than 12 h on core-oxidized amorphous cobalt phosphide nanostructures and the other three commercial catalysts.

Sigma and RuO2-Sigma. Similarly, the AD test by potential sweeping at a scan rate of 0.200 V s−1 within the experimental potential window for about 250 cycles was carried out for a core-oxidized amorphous cobalt phosphide-nanostructured catalyst in addition to the other three studied commercial catalysts. The LSVs acquired at the 1st, 100th, 150th, 200th, and 250th cycles on core-oxidized amorphous cobalt phosphide nanostructures and Co3O4-Sigma-, NiO-Sigma-, and RuO2Sigma-modified surfaces are provided (without iR drop compensation) in Figure 8a−d. The increase in the overpotentials after the 250th cycle at current densities of 10 and 20 mA cm−2 were taken as parameters of quantifying stability upon accelerated potential sweeping. For core-oxidized amorphous cobalt phosphide nanostructures, the increase in the overpotentials at 10 and 20 mA cm−2 are 8 and 19 mV. This is a very low event after such harsh potential cycling and indicates

TW represents “this work”. The dashed columns mean that there was no data available in the cited report.

a

tured catalyst in addition to the other three commercial catalysts without iR drop correction. The j−t responses of all four catalysts are given together in Figure 7. In an overview, all four catalysts seem to have comparable stability. However, closer observation revealed that, except our core-oxidized amorphous cobalt phosphide-nanostructured catalysts and NiO-Sigma, the current densities of the other catalysts had fallen slightly below 10 mA cm−2 within the experimental time scale. This emphasizes that our core-oxidized amorphous cobalt phosphide-nanostructured catalyst is as stable as the commercial NiO-Sigma and has better stability than Co3O4H


Article

Inorganic Chemistry

Figure 8. (a−d) LSVs acquired at the 1st, 100th, 150th, 200th, and 250th cycles of the cycling test carried out on core-oxidized amorphous cobalt phosphide nanostructures and the other three commercial catalysts at a sweep rate of 200 mV s−1 in 1 M KOH at room temperature where the catalyst loading was kept constant (0.2 mg cm−2) for all four catalysts.

Figure 9. (a and b) Low-magnification HR-TEM micrographs of core-oxidized amorphous cobalt phosphide nanostructures after the OER studies and stability examination by chronoamperometry and cycling. (c) Corresponding SAED pattern. (d−f) High-magnification HR-TEM micrographs that show the morphological robustness of our catalyst and its ability in retaining the morphology without any noticeable destruction.

I


Article

Inorganic Chemistry

Figure 10. EDAX spectrum (a), STEM micrograph (b), and color maps of O, P, and Co (c−e) of the “post-OER” sample.

chronoamperometry and cycling study, it is now essential to take a look into the material’s properties. Here, the change in the electrochemical properties of our material after all of the above electrochemical characterizations was studied by EIS analysis. Figure S4 in the SI shows the Nyquist plot of our catalyst-modified GCE before electrochemical studies and after chronoamperometric and cycling studies together. The chargetransfer resistance (Rct) after all of these electrochemical characterizations was almost unaffected, and the observed increase (1.31 ohm) is highly negligible. However, the solution resistance (Rs) was increased by 3.91 ± 0.2 Ω after cycling and chronoamperometric analyses, for which we believe that overoxidation of the catalyst and surface passivation of the GCE upon continuous exposure to an anodic potential for more than 12 h could be the reason. The microstructure robustness after these electrochemical characterizations was advocated by the post-OER HR-TEM and SAED analyses. The

the superior stability of our catalyst upon cycling. However, the increase in the overpotentials at 10 and 20 mA cm−2 for Co3O4Sigma, NiO-Sigma, and RuO2-Sigma are 18 and 36 mV, 20 and 33 mV, and 25 and 51 mV, respectively. These increases are huge and imply the comparably poor stability of commercial catalysts upon cycling at higher scan rates. The peaks observed with Co3O4 (Figure 8b) and NiO (Figure 8c) are their respective oxidation peaks associated with the oxide-tooxyhydroxide conversion during anodic polarization. Both chronoamperometry and cycling studies have shown that our catalyst is comparably more robust for alkaline water oxidation. This is mainly attributed to the well-known corrosion inhibition phenomenon of oxides of P that significantly prevented material degradation. Insight into the Catalysts’ Robustness by EIS, HR-TEM, EDAX Color Mapping, and Electron Diffraction Studies. Because the stability of our catalysts was proven by the J


Article

Inorganic Chemistry TEM specimens were prepared after the electrochemical characterizations as follows: about 68.5 μL of the catalyst ink was cast onto a fluorine-doped tin oxide substrate electrode with an area of 1 cm−2. After chronoamperometry and the AD test, the coating was gently scratched with the use of a glass slide and the catalyst material was transferred to 1 mL of water and sonicated for complete homogenization. A drop of the resultant solution was cast onto a TEM grid for post-OER microstructural studies. Parts a and b of Figure 9 are the HRTEM micrographs obtained after the electrochemical characterizations with regular increasing magnification that show aggregated and interconnected morphology similar to that seen before OER studies. Figure 9c is the corresponding SAED pattern, which also almost retained the same pattern features as those observed before the OER studies. This indicates that both the morphological and solid-state properties of the materials were unaffected. The observed patterns are calibrated accordingly and assigned to their respective diffraction planes such as (111), (220), (311), (400), (511), and (440) in spinel Co3O4. With a further increase in the magnification (Figure 9d−f), we found that the core−shell morphology had also been retained even after such a harsh electrochemical treatment at high anodic overpotentials, which is highly significant and interesting in terms of the material’s stability. The observed lattice fringes at the crystalline core in these figures (Figure 9d−f) were assigned to their corresponding diffraction planes based on the measured d-spacing values. Almost all of the planes were corroborating to the lower-angle planes such as (111), (222), and (311) of spinel Co3O4, as observed before the OER studies. These observations clearly tell us that our core-oxidized amorphous cobalt phosphide-nanostructured catalyst is highly stable in terms of the microstructure morphology (post-OER HR-TEM and SAED results) and also of the electrochemical properties (EIS results). To have further information on the elemental composition on the postOER sample, EDAX elemental color mapping in scanning transmission electron microscopy (STEM) mode was done and is provided in Figure 10a−e, along with the respective EDAX spectrum. Figure 10a is the EDAX spectrum acquired in the selected region of color mapping that had shown various peaks for Co, P, O, Na, and K. Figure 10b is the corresponding STEM micrograph. Parts c−e of Figure 10 are the color maps of O, P, and Co. The uniform distribution of all of these elements can be seen everywhere in the maps shown. Besides, there is no sign of S, C, and F in both the EDAX spectrum and color maps. This mainly indicates that there is no impurity because of the binder used in the post-OER sample, which could possibly be dissolved back to water when homogenized by sonication. Moreover, to gain insight into the chemical nature of elements P, Co, and O, XPS analyses were carried out on post-OER samples, and the resultant P 2p and Co 2p spectra after deconvolution are provided in Figure 11a,b. Figure 11a reveals peak features totally different from those observed before. However, the P 2p peak that corresponds to cobalt phosphide is still observed at its respective binding energy region of 127.8−129.8 eV. In contrast to the P 2p spectrum obtained before the OER, the region that corresponds to various phosphorus oxides had shrunk, and we found only two peaks upon deconvolution. This implies that the phosphorus(III) oxides and the trigonal phosphite with the P5+ oxidation state have now completely been converted to the tetrahedral phosphate, which is so obvious because we have subjected the catalyst to a high anodic potential for a long time. Figure

Figure 11. High-resolution XPS spectra of the P 2p and Co 2p states acquired on the “post-OER” sample.

11b shows us almost similar peak features for various cobalt species such as metallic Co (from cobalt phosphides) and Co3+ and Co2+ from spinel Co3O4 at their corresponding binding energy values. Interestingly, we found another less intense peak next to that spinel Co3O4, which corresponds to cobalt hydroxide. This cobalt hydroxide formation is also obvious because the catalytic material was kept in contact with the highly alkaline electrolyte in addition to the applied potential. All of these spectra primarily resemble those obtained before the electrochemical characterizations with some expected variations in their peak feature. This post-OER XPS analysis strongly implies the chemical stability of our catalyst in addition to the EIS and microstructure analyses. Having such an advantageous and exceptional stability with incredibly enhanced OER performance, our material could be an efficient, cheaper, and long-lasting alternate OER catalyst for water electrolyzers.

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CONCLUSION The most active OER catalysts of Co, viz., Co3O4 and CoxPy, were systematically architected together at a nanoscale that had shown an incredibly enhanced OER performance in a 1 M KOH solution. The comparative electrochemical characterizations carried out by taking our catalyst with the other three commercial catalysts had revealed the superior OER performance of our catalyst. The lowest overpotential (287 mV) at 10 mA cm−2 among all of the Co-based OER catalysts along with a lower Tafel slope (0.070 V dec−1) showed our catalyst’s glory in K


Article

Inorganic Chemistry

(2) Lyons, M. E. G.; Floquet, S. Mechanism of Oxygen Reactions at Porous Oxide Electrodes. Part 2Oxygen Evolution at RuO2, IrO2 and IrxRu1−xO2 Electrodes in Aqueous Acid and Alkaline Solution. Phys. Chem. Chem. Phys. 2011, 13, 5314−5335. (3) Anantharaj, S.; Ede, S. R.; Sakthikumar, K.; Karthick, K.; Mishra, S.; Kundu, S. Recent Trends and Perspectives in Electrochemical Water Splitting with an Emphasis on Sulfide, Selenide, and Phosphide Catalysts of Fe, Co, and Ni: A Review. ACS Catal. 2016, 6, 8069− 8097. (4) Rossmeisl, J.; Qu, Z. W.; Zhu, H.; Kroes, G. J.; Nørskov, J. K. Electrolysis of Water on Oxide Surfaces. J. Electroanal. Chem. 2007, 607, 83−89. (5) Anantharaj, S.; Karthik, P. E.; Subramanian, B.; Kundu, S. Pt Nanoparticle Anchored Molecular Self-Assemblies of DNA: An Extremely Stable and Efficient HER Electrocatalyst with Ultralow Pt Content. ACS Catal. 2016, 6, 4660−4672. (6) Chang, K.; Chen, W. l-Cysteine-Assisted Synthesis of Layered MoS2/Graphene Composites with Excellent Electrochemical Performances for Lithium Ion Batteries. ACS Nano 2011, 5, 4720−4728. (7) Chen, W.; Iyer, S.; Iyer, S.; Sasaki, K.; Wang, C.; Zhu, Y.; Muckerman, J. T.; Fujita, E. Biomass-derived Electrocatalytic Composites for Hydrogen Evolution. Energy Environ. Sci. 2013, 6, 1818−1826. (8) Du, H.; Liu, Q.; Cheng, N.; Asiri, A. M.; Sun, X.; Li, C. M. Template-assisted Synthesis of CoP Nanotubes to Efficiently Catalyze Hydrogen-Evolving Reaction. J. Mater. Chem. A 2014, 2, 14812− 14816. (9) Feng, J. X.; Ding, L. X.; Ye, S. H.; He, X. J.; Xu, H.; Tong, Y. X.; Li, G. R. Co(OH)2 @PANI Hybrid Nanosheets with 3D Networks as High-Performance Electrocatalysts for Hydrogen Evolution Reaction. Adv. Mater. 2015, 27, 7051−7057. (10) Anantharaj, S.; Jayachandran, M.; Kundu, S. Unprotected and interconnected Ru0 nano-chain networks: advantages of unprotected surfaces in catalysis and electrocatalysis. Chem. Sci. 2016, 7, 3188− 3205. (11) Kwon, S. J.; Fan, F.-R. F.; Bard, A. J. Observing Iridium Oxide (IrOx) Single Nanoparticle Collisions at Ultramicroelectrodes. J. Am. Chem. Soc. 2010, 132, 13165−13167. (12) Li, G.; Yu, H.; Wang, X.; Sun, S.; Li, Y.; Yi, B.; Shao, Z. Highly Effective IrxSn1−xO2 Electrocatalysts for Oxygen Evolution Reaction in the Solid Polymer Electrolyte Water Electrolyser. Phys. Chem. Chem. Phys. 2013, 15, 2858−2866. (13) Nakagawa, T.; Bjorge, N. S.; Murray, R. W. Electrogenerated IrOx Nanoparticles as Dissolved Redox Catalysts for Water Oxidation. J. Am. Chem. Soc. 2009, 131, 15578−15579. (14) Nong, H. N.; Gan, L.; Willinger, E.; Teschner, D.; Strasser, P. IrOx Core-shell Nanocatalysts for Cost- and Energy-Efficient Electrochemical Water Splitting. Chem. Sci. 2014, 5, 2955−2963. (15) Sadakane, M.; Rinn, N.; Moroi, S.; Kitatomi, H.; Ozeki, T.; Kurasawa, M.; Itakura, M.; Hayakawa, S.; Kato, K.; Miyamoto, M.; Ogo, S.; Ide, Y.; Sano, T. Preparation and StructuralCharacterization of RuII-DMSO and RuIII-DMSO-substituted α-Keggin-type Phosphotungstates, [PW11O39RuIIDMSO]5− and [PW11O39RuIIIDMSO]4−, and Catalytic Activity for Water Oxidation. Z. Anorg. Allg. Chem. 2011, 637, 1467−1474. (16) Sartorel, A.; Carraro, M.; Scorrano, G.; Zorzi, R.; Geremia, S.; Mcdaniel, N. D.; Bernhard, S.; Bonchio, M. Polyoxometalate Embedding of a Tetraruthenium(IV)-oxo-core by Template-Directed Metalation of [γ-SiW10O36]8−: A Totally Inorganic Oxygen-Evolving Catalyst. J. Am. Chem. Soc. 2008, 130, 5006−5007. (17) Stoerzinger, K. A.; Qiao, L.; Biegalski, M. D.; Shao-horn, Y. Orientation-Dependent Oxygen Evolution Activities of Rutile IrO2 and RuO2. J. Phys. Chem. Lett. 2014, 5, 1636. (18) Yagi, M.; Tomita, E.; Sakita, S.; Kuwabara, T.; Nagai, K. SelfAssembly of Active IrO2 Colloid Catalyst on an ITO Electrode for Efficient Electrochemical Water Oxidation. J. Phys. Chem. B 2005, 109, 21489−21491. (19) Lyons, M. E. G.; Brandon, M. P. The Oxygen Evolution Reaction on Passive Oxide Covered Transition Metal Electrodes in

alkaline OER. The incredible enhancement was brought out by the catalyst’s design. For the first time, we report an incredible enhancement in the OER activity as a result of the synergism between Co3O4 and amorphous cobalt phosphide by making a core−shell nanostructure with an environment of phosphorus oxides with mixed oxidation states of P that varied from 3+ to 5+. The existing reports on cobalt phosphides for OER catalysis have one thing in common and that is the surface oxidation, and the same was attributed to the effect of anodic polarization and exposure to atmosphere. In our case, we controllably designed the catalyst system by a distinguished synthesis route that enabled oxidation of the core in a cobalt phosphide catalyst, which is rare and was never observed before. Having cobalt phosphide, which is more metallic, on the surfaces, we realized a better OER performance than that of the surfaceoxidized cobalt phosphides. Moreover, the same synthesis method can also be chosen for the synthesis of other such metal oxide/phosphide nanocatalyst systems. All of the above encouraging findings have certainly testified that our catalyst could be an efficient, cheaper, and long-lasting alternative OER catalyst to noble-metal oxides such as IrO2 and RuO2.

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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.6b02929. Details on the materials used, methods of preparing samples for various characterizations, technical details of the instruments used to characterize and study the material, calculation of the time-of-flight and mass activity, figures related to EDAX, a XPS survey spectrum, high-resolution XPS spectra of Co 3p, Co 3s, and O 1s, LSVs, and Tafel curves of our catalysts in comparison with those of IrO2, and Nyquist plots (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] or [email protected]. Phone: +91-4565-241487. Fax: +91-4565-227651. ORCID

Sengeni Anantharaj: 0000-0002-3265-2455 Subrata Kundu: 0000-0002-1992-9659 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge Dr. Vijayamohanan K. Pillai for his continuous support and encouragement. S.A. acknowledges CSIR, New Delhi, India, for the award of a Senior Research Fellowship. The authors acknowledge Dr. B. Subramanian, Sr. Scientist, ECMS Division, for his kind help during electrochemical studies. Continual support and help from A. Rathishkumar, V. Prabhu, J. Kennedy (TEM, FESEM, and XPS in-charges, CIF-CECRI) and other faculties of CIFCECRI are thankfully acknowledged.

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REFERENCES

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