Crystal CoxB (x = 1–3) Synthesized by a Ball-Milling Method as High

Oct 2, 2017 - A series of CoxB catalysts were prepared by single-step ball-milling ..... before and after 1000 cycles (Figure 5d) was observed for Co2...
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Cite This: ACS Sustainable Chem. Eng. 2017, 5, 10266-10274

Crystal CoxB (x = 1−3) Synthesized by a Ball-Milling Method as HighPerformance Electrocatalysts for the Oxygen Evolution Reaction Xinzhi Ma,†,‡ Jing Wen,‡ Shen Zhang,† Haoran Yuan,† Kaiyue Li,† Feng Yan,† Xitian Zhang,*,‡ and Yujin Chen*,† †

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Key Laboratory of In-Fiber Integrated Optics, Ministry of Education and College of Science, Harbin Engineering University, Harbin 150001, China ‡ Key Laboratory for Photonic and Electronic Bandgap Materials, Ministry of Education and School of Physics and Electronic Engineering, Harbin Normal University, Harbin 150025, China S Supporting Information *

ABSTRACT: Development of noble-metal-free and active electrocatalysts is crucial for the oxygen evolution reaction (OER) in the water-splitting process. Herein, crystal CoxB catalysts (x = 1−3) of the OER are fabricated by a ball-milling method. Among these CoxB catalysts, Co2B exhibits the best OER activity, with a current density of 10 mA cm−2 at an overpotential of 287 mV in 1 M KOH solution. Such OER activity of Co2B is favorably comparable to that of the commercial IrO2 and most recently reported OER catalysts. Furthermore, the Co2B catalyst exhibits excellent stability with a stable current density of 50 mA cm−2 over 12 h of continuous electrolysis operation. X-ray photoelectron spectroscopy and cyclic voltammetry demonstrate that the B in CoxB makes oxidation easier, leading to their enhanced OER activities in comparison to metal Co. In addition, the Co2B electrocatalyst also exhibits high activity in the hydrogen evolution reaction; thus, the catalyst can be used as a bifunctional catalyst for full water splitting. KEYWORDS: Oxygen evolution reaction, Electrocatalysts, CoxB, Ball-milling method

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INTRODUCTION The current rapidly increasing energy demand and the huge depletion of fossil fuels have promoted intense research on lowcost and environmentally friendly energy conversion and storage systems having high efficiency. Photoelectrochemical or electrochemical water-splitting systems provide a promising route to overcome the excessive reliance on fossil fuels.1−3 However, a bottleneck for water splitting is the oxygen evolution reaction (OER), a kinetically sluggish process that proceeds through a complex four-electron redox process and generally requires a high overpotential.4 IrO2 and RuO2 have been considered as the most active OER electrocatalysts. However, their low abundance and high cost hamper their practical application for large-scale water splitting.5,6 Thus, extensive efforts have been undertaken to exploit low-cost OER catalysts with high efficiency. Among OER catalysts, cobalt-based materials have gained considerable attention due to their abundance and moderate activities.7−21 Cobalt oxides exhibited good OER performances over a broad range of pH values;7−11 however, their poor conductivities result in high overpotentials toward the OER.22,23 To overcome the issue, more attention was focused on cobalt chalcogenides, phosphates, and nitrides with higher conductivities than cobalt oxides. For example, Xie and co© 2017 American Chemical Society

workers reported that Co4N nanowires possessed high OER activity with a current density of 10 mA cm−2 at an overpotential of 257 mV, which is due to their metallic character originating from the N atom incorporation.12 Ultrathin CoSe2 nanosheets showed outstanding catalytic activity toward the OER owing to the enriched cobalt vacancies and enhanced conductivity of the nanosheets in comparison to bulk CoSe2.15,16 Co/Co9S8 core−shell structures with S and N codoping exhibited excellent OER activity with a current density of 10 mA cm−2 at an overpotential of 290 mV (0.1 M KOH). The excellent OER performance was attributed to the synergistic effect between Co and Co9S8 as well as the good electrical conductivity of the core−shell structures.19 The covalent coupling and tuned electronic properties due to heteroatom doping were also found in other cobalt sulfide catalysts.24−26 Recently, CoxP nanomaterials were also reported to catalyze the OER efficiently, and the reason for their excellent OER activities was due to the high conductivities of those cobalt phosphides and the large electronegativity of phosphorus.20,21,27−29 In terms of the above reports, it can be Received: July 8, 2017 Revised: September 16, 2017 Published: October 2, 2017 10266

DOI: 10.1021/acssuschemeng.7b02281 ACS Sustainable Chem. Eng. 2017, 5, 10266−10274

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ACS Sustainable Chemistry & Engineering

Figure 1. (a) XRD patterns of CoB, Co2B, Co3B and pure cobalt. Crystal structure models of (b) CoB, (c) Co2B, and (d) Co3B. Low-magnification SEM images of (e) CoB, (f) Co2B, and (g) Co3B.

unchanged, tetragonal Co2B (JCPDS no. 25-0241), with the unit cell structure shown in Figure 1c, would be fabricated. As RCo/B was changed to 2:1 and the rotational speed was set to be 1000 rpm, orthorhombic Co3B (JCPDS no. 12-0443) with the unit cell structure shown in Figure 1d could be formed. Furthermore, diffraction peaks from impurities are not detected, suggesting high crystal purities for these CoxB catalysts. SEM images (Figure 1e−g) show that the as-prepared CoxB catalysts exhibit particle-like shapes, with sizes ranging from several hundreds of nanometers, as confirmed by lowmagnification TEM images (insets in Figure 2a−c).40 HRTEM images taken from the regions highlighted by the red circles in the low-magnification TEM images display clear lattice fringes, as shown in Figure 2a−c, revealing the crystalline nature of all the CoxB catalysts. The detailed measurements for the adjacent lattice spacings in these HRTEM images confirm the crystal structures of these CoxB catalysts. For example, the labeled lattice spacings in the HRTEM image of CoB (Figure 2a) are 0.237 and 0.214 nm, corresponding to the (101) and (120) planes of orthorhombic CoB, respectively; the labeled lattice spacings in the HRTEM image of Co2B (Figure 2b) are 0.210 and 0.251 nm, which can be assigned to the (002) and (200) crystal planes of tetragonal Co2B, respectively; the labeled lattice spacings in the HRTEM image of Co3B (Figure 2c) are 0.236 and 0.199 nm, which can be assigned to the (021) and (103) planes crystal planes of orthorhombic Co3B, respectively. In order to determine the elemental distribution in these CoxB catalysts, energy dispersive X-ray spectrometry (EDX) elemental mapping images were taken. As shown in Figure 2d−f, the Co and B distribute uniformly across the particles, further confirming the successful preparation of CoxB catalysts by the facile ball-milling method. X-ray photoelectron spectroscopy (XPS) was utilized to analyze the chemical surface states and compositions of these CoxB catalysts. The survey XPS spectrum of CoxB catalysts [Figure S1, Supporting Information (SI)] indicates that the surfaces of our CoxB catalysts contain B, Co, and O elements, in which the existence of the O element is due to the exposure of the catalysts to air. According to the XPS spectra, the contents of B and Co can be estimated (the inset in Figure S1,

concluded that the types of nonmetals in the cobalt compounds have an important effect on the OER catalytic activities of Cobased electrocatalysts.30−33 However, the electrochemical activities of cobalt borides have been studied scarcely to date.34−39 Furthermore, the effect of the atomic ratio of metal Co to nonmetal in the cobalt compounds on the OER activity has not been identified. One major reason is that it is difficult to fabricate metal borides through common methods, such as the solvothermal/hydrothermal method, chemical vapor deposition, and electrochemical deposition. Herein we develop a ball-milling method to fabricate a series of cobalt borides (CoxB, x = 1−3), and their OER activities are investigated. Among these cobalt borides, Co2B can drive a current density of 10 mA cm−2 at an overpotential of 287 mV in 1.0 M KOH solution, which is superior to CoB (340 mV) and Co3B (312 mV). Furthermore, compared to most other OER catalysts reported previously, Co2B also exhibits favorable OER catalytic activity. In addition, Co2B in alkaline media has good stability as an OER catalyst, showing a very stable current density at 50 mA cm−2 over 12 h of continuous electrolysis operation. Furthermore, the effect of B in the CoxB catalysts and the reason for the superior OER activity of Co2B are also discussed in terms of X-ray photoelectron spectroscopy and the cyclic voltammetry measurements.

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RESULTS AND DISCUSSION Structural Characterization. The morphologies and the structures of the CoxB catalysts were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and the transmission electron microscopy (TEM). Figure 1a shows the XRD patterns of metal cobalt precursor, CoB, Co2B, and Co3B. The diffraction peaks of metal cobalt precursor at 2θ 44.2° and 51.5° can be assigned to (111) and (200) crystal planes of cubic Co. After ball-milling with B powder under different conditions, CoxB catalysts with different crystal structures can be obtained. As the mole ratio of Co and B (RCo/B) was 1:1 and the rotational speed was set to be 1000 rpm, orthorhombic CoB (JCPDS no. 03-0959) with the unit cell structure shown in Figure 1b could be obtained. When the rotational speed decreased to 900 rpm and RCo/B was kept 10267

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peaks at around 191.6 eV correspond to the high-valence state of B, such as boron-oxo species.32 OER Activities of CoxB Catalysts. The homogeneous signals discussed above suggest that crystal cobalt borates were successfully fabricated by the ball-milling method. To assess the OER catalytic activity, the electrochemical OER activities of CoxB catalysts, commercial cobalt, and IrO2 powers were measured using a three-electrode configuration in 1.0 M KOH at a scan rate of 2 mV s−1. The OER activity of the Co3O4, obtained through heating the commercial cobalt powder at 500 °C for 5 h under air atmosphere, was also evaluated. The crystal Co3O4 catalyst was confirmed by the XRD measurement (Figure S2a, SI). SEM images indicate that the Co3O4 (Figure S2b, SI) surfaces become rough in compared to those of the commercial cobalt powers (Figure S2c, SI). These catalysts were loaded on the carbon fiber papers with the same loading mass of 2.56 mg. A carbon rod and Ag/AgCl (KCl saturated) were used as counter and reference electrodes, respectively. The Ohmic potential drop (iR) losses caused by electrolyte resistance were compensated for in the measurements. As shown in Figures 4a and S3 (SI), typical linear scan voltammetry (LSV) curves indicate that these CoxB catalysts exhibit higher OER activity than the pure cobalt and Co3O4. For example, to drive a current density of 10 mA cm−2, metal cobalt and Co3O4 require overpotentials of 373 and 377 mV, whereas CoB, Co2B, and Co3B only need 340, 287, and 312 mV, respectively; to drive a current density of 100 mA cm−2, metal cobalt and Co3O4 require overpotentials of 507 and 493 mV, whereas CoB, Co2B, and Co3B only need 464, 389, and 428 mV, respectively. Furthermore, it can be found that Co2B shows the best OER activity in the tested potential range among these CoxB catalysts. Remarkably, the OER activity of Co2B is superior to that of IrO2 at overpotentials larger than 289 mV (Figure 4a). For example, to deliver a current density of 100 mA cm−2, IrO2 requires an overpotential of 400 mV, while Co2B needs 389 mV. Even in 0.1 M KOH solution, the Co2B can drive 10 mA cm−2 at 292 mV overpotential (Figure S4, SI). The OER activity of Co2B in both 1 and 0.1 M KOH solutions compares favorably to that of the-state-of-the-art OER catalysts recently reported, such as CoSe/Ti, CoP films, CoP3 nanoneedle arrays, and other borate-based catalysts measured in 0.1 M potassium borate solution (Table S1, SI). Additionally, the electrochemical OER properties of those catalysts were also measured in 0.1 M KOH solutions using a rotating disk electrode (RDE) made of glassy carbon, Pt foil, and AgCl/Ag/ KCl (3 M) as working electrode, counter electrode, and reference electrode, respectively. The scan rate was set to 10 mV s−1 and the rotation rate was 1600 rpm. The loading weight for all the catalysts is about 0.2 mg cm−2. LSV curves show that the Co2B exhibits the best OER activity among these catalysts (Figure S5, SI). To drive a current density of 10 mA cm−2, the Co2B only needs 371 mV overpotential, lower than that of IrO2 (378 mV), CoB (405 mV), Co3B (378 mV), and Co (466 mV). Compared to the catalysts reported previously, the Co2B also exhibits favorably comparable or superior OER activity under similarly measured conditions (Table S2, SI). The OER kinetics of these catalysts was evaluated by the Tafel plots according to the Tafel equation, η = a + b log|j|, where b is the Tafel slope. As shown in Figure 4b, the Tafel slope of Co2B is 50.7 mV dec−1, lower than that of Co (56 mV dec−1), CoB (63 mV dec−1), and Co3B (53 mV dec−1). The Tafel slope of Co2B is lower than that of IrO2 (51.5 mV dec−1) and those of the-state-of-the-art OER catalysts recently

Figure 2. HRTEM images of (a) CoB, (b) Co2B, and (c) Co3B. The insets in parts a−c show the corresponding low-magnification TEM images. Corresponding EDS elemental mapping images of (d) CoB, (e) Co2B, and (f) Co3B.

SI). According to the B and Co contents, the atomic ratios of Co:B in CoB, Co2B, and Co3B calculated from the XPS data about 0.99:1, 2:1.25, and 3.00:1. As shown in Figure 3a, the main binding energies of Co 2p 3/2 and Co 2p 1/2 core peaks at 777.9 and 793.2 eV for CoB, 778.1 and 793.4 eV for Co2B, and 777.8 and 792.8 eV for Co3B are assigned to Co species in cobalt borides.41 The differences between the binding energies of Co species is due to the different electronic environments of Co in these CoxB catalysts. Besides, the Co 2p 3/2 and Co 2p 1 /2 peaks at around 780.5 and 796.4 eV for all cobalt borates can be ascribed to the Co2+ oxidation state [CoO or Co(OH)2] due to air exposure.42,43 The corresponding B 1s spectra, shown in Figure 3b, are all deconvoluted into two distinct peaks. The peaks at 188.4, 188.2, and 188 eV are assigned to the lowvalence B in CoB, Co2B, and Co3B, respectively, while the 10268

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Figure 3. XPS spectra of CoxB: (a) Co 2p core level spectra of CoB, Co2B, and Co3B and (b) B 1s core level spectra of CoB, Co2B, and Co3B.

Figure 4. (a) Polarization curves and (b) Tafel plots of Co and CoxB elctrodes.

and IrO2, respectively. Figure 5a shows the TOF data of the Co-based catalysts. It can be found that the Co2B catalyst has the largest TOF values at all tested potential ranges among the Co-based catalysts and IrO2. Typically, for the given TOF = 0.5 s−1, the cobalt, CoB, Co3B, and IrO2 catalysts require overpotentials of 422, 413, 370, and 375 mV, respectively, significantly higher than that for Co2B catalyst (328 mV). Thus, the trend in intrinsic catalytic activity of these Co-based catalysts is identified as Co2B > Co3B > CoB, consistent with the result determined by LSV curves. To understand the differences among these CoxB catalysts in the OER activities, the effective active areas of catalysts were estimated by measuring the electrochemical double-layer capacitances (Cdl) using a simple CV method.48−50 Figure S7 (SI) shows the CV curves of these CoxB catalysts at different scan rates. As shown in Figure 5b, the Cdl is 1.17, 3.7, and 1.52 mF cm−2 for CoB, Co2B, and Co3B, respectively. The trend of the Cdl values of

reported (Table S1, SI), suggesting the more favorable OER kinetics of Co2B. Thus, Co2B has very promising application in large-scale water splitting. Those slopes near 60 mV dec−1 suggest that a chemical step subsequent to the first electron transfer step is the rate-determining step for these CoxB catalysts.44 Reasons for the Differences between CoxB Catalysts in the OER Activities and the Stability of Co2B toward OER. To further study the intrinsic catalytic activity of these CoxB catalysts, the turnover frequency (TOF), reflecting all the active sites involved in the electrochemical reaction, was determined by a previously reported method.45−47 The number of active sites (n) was first determined by cyclic voltammetry cycling in pH 7 phosphate buffer with a scan rate of 50 mV s−1. The voltammetric charges (Q) were determined by deducting their corresponding blank values (Figure S6, SI), about 0.0309, 0.0444, 0.0343, 0.0430, and 0.1259 C for Co, CoB, Co2B, Co3B 10269

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Figure 5. (a) The TOF data. (b) The differences in current density (ΔJ = Ja − Jc) for the potential range of 0.86−0.96 V vs RHE plotted against the scan rate fitted to a linear regression allow for the estimation of Cdl for CoB, Co2B, and Co3B, respectively. (c) Nyquist plots at 1.586 V vs RHE. The inset in part c shows the corresponding equivalent circuit. (d) Comparisons of the 10th and the 1000th CV curves for Co2B and IrO2 catalysts with the scan rate of 50 mVs−1 and the inset showing the time-dependent current density curves for Co2B and IrO2 catalysts at an overpotential of 345 mV.

Figure 6. HRTEM images of (a) CoB, (b) Co2B, and (c) Co3B after the OER process at a current density of 50 mA cm−2 for 12 h. The insets in parts a−c show the coresponding low-magnification TEM images.

stability of Co2B and IrO2 was evaluated by CV sweeping in N2saturated 1.0 M KOH solution at a high scan rate of 50 mV s−1. A negligible difference in the CV curve profiles before and after 1000 cycles (Figure 5d) was observed for Co2B; in contrast, a macroscopically larger potential shift occurred for the IrO2 electrode. The long-term durability of Co2B toward OER was further assessed by potentiostatic measurements at a given overpotential of 345 mV. As shown in the inset in Figure 5d, the initial current density was around 50 mA cm−2, and there was a negligible decrease in the current density over 12 h electrolysis for Co2B; however, a 24.4% current density loss happened on the IrO2 electrode. Thus, the Co2B catalyst not only exhibited high activity toward the OER but also excellent long-term durability toward the OER.

CoxB catalysts is consistent with their trend in OER activity. Therefore, the high OER activity of Co2B is attributed to its high effective active area. The transfer resistance (Rct) has an important effect on the catalytic activity of the catalyst. Thus, the electrochemical impendence spectroscopy (EIS) was measured to determine the Rct for these CoxB catalysts. Figure 5c shows the Nyquist plots of these catalysts at 356 mV overpotential. According to the equivalent circuits (the inset in Figure 5c), Rct is 1.09 Ω cm−2 for Co2B, smaller than that of Co (1.52 Ω cm−2), CoB (1.44 Ω cm−2), and Co3B (1.16 Ω cm−2). The small Rct facilitates charge transport during OER process and thus improves the OER activity of Co2B. Besides the catalytic activity, the stability of the catalysts is also an important criterion for their practical application. The 10270

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Figure 7. (a) XPS spectra of the Co 2p core level for CoxB electrodes after the OER process at the current density of 50 mA cm−2 for 12 h. (b) CV curves of CoxB and cobalt electrodes measured in N2-saturated 1.0 M KOH solution at a scan rate of 2 mV s−1.

The Mechanism of CoxB Catalysts in the OER. To further analyze the OER mechanism and study the structural changes of the CoxB catalysts, XRD and SEM measurements of the CoB, Co2B, and Co3B catalysts after the OER process for 12 h at a current density of 50 mA cm−2 were carried out. XRD patterns (Figure S8, SI) show that there are almost no obvious changes in the crystal phases of these CoxB catalysts after the long-term OER operation, while the diffraction peaks at 2θ 42.4°, 44.5°, and 54.5° in all CoxB patterns come from the carbon paper substrates (JCPDS no. 08-0415). However, SEM images (Figure S9, SI) indicate that thin films with nanosheetlike shapes are formed on the surfaces of these catalysts, as further confirmed by low-magnification TEM images (the insets in Figure 5).41,51,52 HRTEM images (Figure 6) show that the outermost nanosheets are polycrystalline and enriched in defects. In Figure 6a, the labeled lattice distances are 0.309, 0.261, and 0.176 nm, corresponding to the (120), (130), and (150) planes of CoOOH, respectively; in Figure 6b, the labeled lattice distances are 0.31, 0.201, and 0.223 nm, corresponding to the (120), (140), and (200) planes of CoOOH, respectively; in Figure 6c, the labeled lattice distances are 0.229, 0.224, and 0.258 nm, corresponding to the (111), (200), and (130) crystal planes of CoOOH, respectively. On the basis of the HRTEM analyses, the outermost nanosheets of the CoxB catalysts after OER processes are mainly composed of CoOOH species. Notably, the outermost nanosheets are preferentially contacted by the electrolyte, and thus, they are the real active sites for OER. In order to further determine the chemical composition of the nanosheets, XPS analyses of these CoxB catalysts after the long-term OER operation were carried out. As shown in Figure 7a, all the fitted XPS spectra show that the CoxB surfaces are composed of Co3O4 and CoOOH, which dominate the catalyst surfaces.43 On the other hand, B in all the CoxB

catalysts is oxidized, as evidenced by the enlarged peaks of boron-oxo species and the disappearance of the boron species (Figure S10, SI). Thus, the real active sites of these CoxB catalysts are CoOOH species, consistent with previous reports.32 On the basis of the discussion above, we can conclude that the following reactions occur during the oxygen evolution process of these CoxB catalysts:53,12 Co + 2OH− → Co(OH)2 + 2e−

(1)

−

3Co(OH)2 + 2OH → Co3O4 + 4H 2O + 2e −

Co3O4 + H 2O + OH → 3CoOOH + e

−

−

(2) (3)

CoOOH + OH− → CoO2 + H 2O + e−

(4)

CoO2 → Co* + O2

(5)

Summary: 4OH− → 2H 2O + O2 + 4e−

(6)

In short, CoO or Co(OH)2 on the catalyst surfaces undergoes oxidation and then Co3O4 and/or CoOOH appear gradually, depending on the electrode potential in basic solutions.54,55 During the reaction, Co3O4 might favor the oxidation from Co2+ to Co3+ by the typical cationic occupancies of Co2+ and Co3+ on the tetrahedral and octahedral sites, and this facilitates the further oxidation to CoOOH and CoO2,56 with CoO2 being more efficient for the OER and generated by the oxidization of CoOOH.57 In addition, the effect of B on the OER activities of the CoxB catalysts is studied by the CV measurements at a scanning rate of 2 mV s−1. As shown in Figure 7b, there is an oxidation peak at about 1.45 V for metal Co before oxygen evolution. However, upon B introduction, positions of the oxidation peaks for the CoxB catalysts shift toward the low potential region, suggesting that the oxidation 10271

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develop high performance, inexpensive electrocatalysts for large-scale water-splitting.

of Co in the CoxB catalysts becomes easier than for metal Co. As a result, the CoxB catalysts show lower onset potentials (corresponding current density: 1 mA cm−2) toward OER in comparison to metal Co. For example, the onset potentials for CoB, Co2B, and Co3B are 1.51, 1.46, and 1.49 V, respectively, 30, 80, and 50 mV lower than that of metal Co (1.54 V). Thus, B elements in CoxB catalysts have positive effects on the OER activities. In addition, the catalytic activity of Co2B in the hydrogen evolution reaction (HER) was assessed. The Co2B powder mixed with 10 wt % carbon nanotubes was loading on carbon fiber paper with a loading mass of about 2.56 mg, and the HER activity of the Co2B was examined in 1.0 M KOH solution with a scan rate of 2 mV/s. Figure 8a shows the polarization curve of

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METHODS

Chemicals. Cobalt (purity, >99.99%; diameter, ∼0.5 μm) and boron powder (purity, >99.9%) were purchased from Aladdin Chemical Co., Ltd. It is worth noting that the potassium hydroxide aqueous solution was further purified as the electrolyte. Experimental Methods. All purchased materials were used without further treatment. The synthesis of high-performance materials was mainly through a ball-milled method. The mixing mole ratio of Co and B (RCo/B) is 1:1 for both CoB and Co2B and is 2:1 for Co3B. All the samples were ball-milled using a planetary ball mill (Fritsch Pulverisette 4) with the ball-to-power weight ratio of 20:1. Air in the grinding bowls was removed by an Ar (99.99%) inflation−deflation process, and the pressure of bowls was vacuumed to 95 KPa. Rotational speeds for CoB, Co2B, and Co3B were kept at 1000, 900, and 1000 rpm, respectively. The reaction time was set to be 10 h for all samples. Structure Characterizations. XRD data were collected on a Rigaku D/max-2600/PC with Cu Kα radiation (λ = 1.5418 Å). The morphology and size of samples were characterized by a scanning electron microscope (Hitachi SU70) and an FEI Tecnai-F20 transmission electron microscope equipped with a Gatan imaging filter (GIF). XPS analyses were carried out by using a spectrometer with Mg Kα radiation (PHI 5700 ESCA System). The binding energy was calibrated with the C 1s position of contaminant carbon in the vacuum chamber of the XPS instrument (284.6 eV). Electrochemical Measurements. Electrochemical measurements of individual electrodes were conducted on a VMP3 electrochemical workstation (Bio Logic) with a standard three-electrode electrochemical system using CoxB catalysts on carbon paper as the working electrode; a graphite rod and an Ag/AgCl electrode were used as the counter and the reference electrodes, respectively. The carbon paper working electrode was prepared as described below: the catalysts were dispersed in N-methyl-2-pyrrolidone solvent containing 7.5 wt % PVDF under sonication, in which the weight ratio (Wcatalyst/WPVDF) was 8/1, without conductive agents employed. Then the slurry was coated onto a piece of carbon paper, on which the loading density of the catalyst was about 2.56 mg cm−2. The asprepared catalyst film was vacuum-dried at 90 °C. Electrochemical measurements of the catalysts were measured in 1 M KOH solution after purging the electrolyte with N2 gas for 30 min. For each catalyst, LSV curves were first recorded from 0 to 0.8 V vs Ag/AgCl at a scan rate of 2 mV s−1. The activation of catalysts was carried out under a potential range from 0 V to that corresponding to 100 mA cm−2 for several cycles until the curves remained unchanged. Then, the activity of the catalysts toward the OER was evaluated by recording a linear scan voltammogram from 0 to 0.75 V vs Ag/AgCl at a scan rate of 2 mV s−1. Polarization curves were obtained using LSV. The long-term stability test was carried out using chronopotentiometric measurements. All potentials measured were calibrated to RHE using the following equation: E(RHE) = E(Ag/AgCl) + 0.21 V + 0.059 × pH. All current densities presented are corrected against Ohmic potential drop. TOF Calculation. The number of active sites (n) was first examined by employing cyclic voltammograms with phosphate buffer (pH 7) at a scan rate of 50 mV s−1. Then the number of the voltammetric charges (Q) could be determined after deduction of the blank value. The number of moles (n) could be determined with the equation n (mol) = Q/4F, where F is Faraday’s constant. TOF (s−1) could be calculated with the equation TOF (s−1) = I/4nAF, where I (A) was the current of the polarization curve obtained by LSV measurements and A is the geometric area of the electrodes.

Figure 8. (a) Polarization curve of Co2B on carbon paper for HER in 1.0 KOH solution at a scan rate of 2 mV s−1. (b) Long-term stability of an alkaline water electrolyzer assembled with Co2B powder as the bifunctional catalyst.

the Co2B electrode. The current density of 10 mA cm−2 can be achieved at an overpotential of 109 mV. Considering the high OER activity of the Co2B catalyst, we assembled an alkaline electrolyer with Co2B as a bifunctional catalyst. Figure S11 (SI) shows the polarization curve of the Co2B∥Co2B couple. The Co2B∥Co2B electrolyzer can drive a current density of 10 mA cm−2 at a cell voltage of 1.656 V, lower than those values for bifunctional catalysts such as NiFe LDH/Ni foam∥NiFe LDH/ Ni foam (1.70 V)58 and commercial electrolyzers (above 1.8 V).59 Furthermore, the Co2B∥Co2B couple for full water splitting had a good stability. As shown in Figure 8b, the voltage for driving a current density of 10 mA cm−2 shows no obvious change for 12 h of water splitting. Thus, Co2B has potential as a bifunctional catalyst in full water splitting.

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CONCLUSION In summary, crystal CoB, Co2B, and Co3B catalysts were successful synthesized by a ball-milling method. All the products showed OER performance and benefited from the formation of Co3O4 and CoOOH during the reaction and also from the good conductivities of the alloy CoxB cores. The most optimized catalyst Co2B achieved a current density of 10 mA cm−2 at an overpotential of 287 mV on a carbon paper electrode and showed stability over 12 h of continuous electrolysis operation at a current density of 50 mA cm−2, which was facilitated by suitable B alloying. In addition, Co2B also exhibited a good HER performance with a current density of 10 mA cm−2 at 109 mV. An ingenious design and industrialized strategy would open up new approaches to 10272

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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/acssuschemeng.7b02281. Figures S1−S11 and Table S1 and S2 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*X.Z. e-mail: [email protected]. *Y.C. e-mail: [email protected]. ORCID

Yujin Chen: 0000-0002-6794-2276 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (Grant No. 51572051), the Natural Science Foundation of Heilongjiang Province (E2016023), the Fundamental Research Funds for the Central Universities (GK2110260179 and GK2110260188), the 111 project (B13015) of Ministry Education of China to the Harbin Engineering University, the PhD Student Research and Innovation Fund of the Fundamental Research Funds (HEUGIP201714) for the Central Universities and also the Open Project Program (PEBM201508) of Key Laboratory for Photonic and Electric Bandgap Materials, Ministry of Education, Harbin Normal University, China.

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REFERENCES

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