Hexagonal-Phase Cobalt Monophosphosulfide for Highly Efficient

Oct 27, 2017 - The rational design and synthesis of nonprecious, efficient, and stable electrocatalysts to replace precious noble metals are crucial t...
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Hexagonal-Phase Cobalt Monophosphosulfide for Highly Efficient Overall Water Splitting Zhengfei Dai,†,‡,⊥ Hongbo Geng,†,‡,⊥ Jiong Wang,§ Yubo Luo,‡ Bing Li,∥ Yun Zong,∥ Jun Yang,‡ Yuanyuan Guo,‡ Yun Zheng,‡ Xin Wang,§ and Qingyu Yan*,‡ †

State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, People’s Republic of China ‡ School of Materials Science and Engineering and §School of Chemical and Biomedical Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798 ∥ Institute of Materials Research and Engineering (IMRE), A*STAR (Agency for Science Technology and Research), 2 Fusionopolis Way, Innovis #08-03, Singapore 138634 S Supporting Information *

ABSTRACT: The rational design and synthesis of nonprecious, efficient, and stable electrocatalysts to replace precious noble metals are crucial to the future of hydrogen economy. Herein, a partial sulfurization/phosphorization strategy is proposed to synthesize a nonstoichiometric pyrrhotite-type cobalt monophosphosulfide material (Co0.9S0.58P0.42) with a hexagonal close-packed phase for electrocatalytic water splitting. By regulating the degree of sulfurization, the P/S atomic ratio in the cobalt monophosphosulfide can be tuned to activate the Co3+/ Co2+ couples. The synergy between the nonstoichiometric nature and the tunable P/S ratio results in the strengthened Co3+/Co2+ couples and tunable electronic structure and thus efficiently promotes the oxygen/hydrogen evolution reaction (OER/HER) processes toward overall water splitting. Especially for OER, the Co0.9S0.58P0.42 material, featured with a uniform yolk−shell spherical morphology, shows a low overpotential of 266 mV at 10 mA cm−2 (η10) with a low Tafel slope of 48 mV dec−1 as well as high stability, which is comparable to that of the reported promising OER electrocatalysts. Coupled with the high HER activity of Co0.9S0.58P0.42, the overall water splitting is demonstrated with a low η10 at 1.59 V and good stability. This study shows that phase engineering and composition control can be the elegant strategy to realize the Co3+/ Co2+ couple activation and electronic structure tuning to promote the electrocatalytic process. The proposed strategy and approaches allow the rational design and synthesis of transition metal monophosphosulfides toward advanced electrochemical applications. KEYWORDS: metal monophosphosulfides, yolk−shell, hydrogen evolution reaction, oxygen evolution reaction, electrocatalysts

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effective approach to improve their electrochemical performance, in which the merits of both might be retained.16,17 A representative example is a MoS2−MoP hybrid system exhibiting higher HER activities than its individual components and holding excellent cycling stability.18 However, the disordered interfaces distributed in the material architecture may impose an obstacle in the clear understanding of the synergetic effect and on-demand design of advanced electrochemical materials.16 A miscible M−P−S phase, namely, metal phosphosulfides, will hold the intriguing promise in this regard, which allows for elegant free-energy diagram modeling and precise tunable catalytic and other physiochemical properties.19,20 Computa-

lectrochemical water splitting offers a promising and sustainable solution to environmental pollution issues and energy shortage.1,2 Currently, the best known electrocatalysts are based on precious metals, such as IrO2/ RuO2 for oxygen evolution reaction (OER) and Pt for hydrogen evolution reaction (HER). However, their scarcity and high cost greatly limit the scalable utilization.3 Hence, searching earth-abundant and efficient catalysts is significant to the future of hydrogen economy.4−6 Transition metal phosphides (TMPs) have been recently indicated as nonprecious electrocatalysts in HER or OER with high activity.7,8 For electrocatalytic application, metal phosphides always exhibit inferior stability in alkaline conditions compared to that in acidic conditions.9−11 Alternatively, transition metal dichalcogenides (TMDs, e.g., MoS2, metallic CoS2) can present better electrochemical stability in spite of lower catalytic activity.12−15 A synergetic alloy between TMPs and TMDs is promising as an © 2017 American Chemical Society

Received: July 18, 2017 Accepted: October 27, 2017 Published: October 27, 2017 11031

DOI: 10.1021/acsnano.7b05050 ACS Nano 2017, 11, 11031−11040

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Figure 1. Phase and chemical state of the Co0.9S (after 2 h sulfurization) and Co0.9S0.58P0.42 (after 2 h sulfurization and 2 h phosphorization) yolk−shell spheres. (a) XRD patterns. Inset shows the structural evolution before and after phosphorization. (b) Co 2p, (c) S 2p, and (d) P 2p XPS spectra.

and tunable electronic structure and thus enhance the electrochemical activities for OER and HER. For OER, the Co0.9S0.58P0.42 material, featured with a uniform yolk−shell spherical morphology, shows a low overpotential of 266 mV at 10 mA cm−2 (η10) with a low Tafel slope of 48 mV dec−1 as well as high stability. Coupled with the high HER activity of Co0.9S0.58P0.42, the Co0.9S0.58P0.42∥Co0.9S0.58P0.42 couple is capable of delivering the overall water splitting at a low operation η10 voltage (1.59 V) with long-term stability. This study shows that phase engineering and composition control can be an elegant strategy to realize the Co3+/Co2+ couple activation to promote the electrocatalytic process.

tional calculations have demonstrated that the substitution of S by less electronegative P atom will lead to the more thermoneutral ion adsorption and thus may bring an enhanced HER activity.21,22 Caban-Acevedo et al. have indicated that pyrite-structured ternary CoPS (cubic crystal system) can achieve a rather low HER overpotential (48 mV at 10 mA cm−2) with tunable electrical properties and rich active sites for HER compared to that of the pyrite CoS2 sample.21 Nevertheless, scientific and engineering challenges still exist regarding the difficulty in controlling their continuous component, by which the electronic structure and band gap can be elegantly regulated to tune the electrocatalytic properties. Moreover, no effort has been demonstrated for investigation of other phases (e.g., hexagonal close-packed, hcp) CoPS for electrocatalysts. 23−25 In other phase metal phosphosulfides, high-valence transition metal ions (e.g., Co3+, Ni3+) have more 3d electron orbits and thus higher electronaccepting characteristics than those of low-valence metal ions (e.g., Co2+ in CoS2), facilitating the electrocatalytic process (e.g., OER, 4OH− → O2 + 2H2O + 4e− in alkaline solution) and resulting in the higher electrochemical activity.26−28 Hence, it is urgently desirable to rationally design and promote cobalt phosphosulfide catalysts on the basis of maximizing the Co3+/ Co2+ couples; phase control should be taken in this direction in future efforts.29,30 Herein, a partial sulfurization/phosphorization strategy is proposed to synthesize a nonstoichiometric cobalt monophosphosulfide material (Co0.9S0.58P0.42) with a hcp phase and tunable P/S atomic ratio. The nonstoichiometric nature and the tunable P/S ratio result in the strengthened Co3+/Co2+ couples

RESULTS AND DISCUSSION Phase and Chemical States. The synthetic process for the cobalt monophosphosulfide (Co−S−P) yolk−shell spheres is illustrated in Figure S1. It starts from the uniform sized cobalt precursor solid spheres (Figure S2) prepared by a hydrothermal process. Thioacetamide was employed as a slow S release precursor for gradually generating the H2S reactant to convert the metal−organic complex precursor to metal sulfides (Figure S1i).31,32 The reaction between the faster outward diffused Co2+ cations and the slower inward diffused S2− ions affords the formation of the cobalt sulfide yolk−shell structures.33,34 The first-step sulfurization process (2 h) leads to the formation of the yolk−shell cobalt sulfide structures. Subsequently, the phosphorization of cobalt sulfide was achieved in the presence of PH3 gas, generated from NaH2PO2 at 320 °C in Ar atmosphere, to form the final yolk−shell Co−S−P products (Figure S1ii).35 Powder X-ray diffraction (XRD) patterns of the 11032

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Figure 2. Structure and morphology of the Co0.9S0.58P0.42 yolk−shell spheres. (a) SEM, (b) expanded SEM view. (c) TEM image. (d) Highresolution TEM image. (e) SAED pattern. (f) STEM image and elemental mapping of a yolk−shell sphere.

one located at 130.0 eV (2p1/2) and 129.0 (P 2p3/2) is due to the Co−P bonding, whereas the other at 133.5 and 134.3 eV features the P−O bonding of phosphate. The P−O bonding may be due to the surface oxidation of Co−P under ambient conditions, as previously reported for metal phosphide materials.18,40 Based on the ICP results, the P/S ratio was calculated to be 42:58. Thus, the chemical formula of the sample is denoted as Co0.9S0.58P0.42. In addition, the P/S ratio in the cobalt monophosphosulfide can be adjusted by varying the first-step sulfurization time (0−6 h), as shown in Table S1. Increasing the first-step sulfurization time results in a decrease of phosphorus percentage in the cobalt monophosphosulfide. Except for the 0 h sulfurization sample (CoP, orthorhombic, Figure S3), other samples show a similar cobalt pyrrhotite structure, such as Co0.9S0.83P0.17 (Figure S4a) and Co0.9S0.P0.17 (Figure S4b). Figure S4c (Supporting Information) compares the XRD patterns for all the samples with different stoichiometry ratios. It can be observed that the XRD (330) peaks gradually shift from 54.6° (Co0.9S) to 55.2° (Co0.9S0.58P0.42) with the increased P substitution. According to Bragg’s law, there is a 0.7% reduction of the interplanar distance between (330) planes from Co0.9S to Co0.9S0.58P0.42. Further, we have explored the tunability of Co2+/Co3+ sites with P/S stoichiometry via XPS, Raman, and electron paramagnetic resonance (EPR), as shown in Figure S5. In Figure S5a, characteristic Raman peaks around 186, 299, 460, 599, and 664 cm−1 are attributed to the F2g1, Tg, Eg, F2g2, and A1g modes, respectively.41,42 It is found that all the materials are in the same vibrational modes, and no impurity peak can be observed. The blue shift of Raman peaks from Co0.9S to Co0.9S0.58P0.42 is in accordance with the previous reports on the cobalt phosphosulfide, which means the partial P substitution and possible S−P dumbbell formation.21 This will result in the increase of Co3+ sites.21 On the other hand, the EPR spectra in Figure S5b show a weak signal (at ∼3500 G, marked with the dashed line) from a low-spin Co2+ signal. In Figure S5b, from Co 0.9S to Co0.9S0.58P0.42, we can observe that the peak intensity at ∼3500 G gradually decreases with the increased P substitution, which manifests as the decreased level of low-spin Co2+ with

cobalt precursor after sulfurization and after both sulfurization− phosphorization are illustrated in Figure 1a. After the sulfurization process (2 h), the XRD pattern indicates the formation of polycrystalline nonstoichiometric Co0.90S ht (or β′CoS1.097) phase, known as pyrrhotite-type structure (Co1−xS, x = 0 to 0.2), corresponding to the standard JCPDS card of No. 19-0366 (hexagonal, P63/mmc (194), a = 10.10 Å, b = 10.10 Å, and c = 15.48 Å). The mean crystallite size of the sample was calculated to be about 6.4 nm by the Scherrer equation.36 After the subsequent phosphorization process (2 h), the diffraction pattern is similar, manifesting that phosphorus substitution would not change the crystal structure (the inset of Figure 1a) because of the similar atomic sizes of S and P.22 A slight peak shift (ca. 0.6°) to higher 2θ position means the shrinkage of the cobalt pyrrhotite lattice after phosphorization treatments (a = b = 10.01 Å, c = 15.38 Å). Hence, the pyrrhotite-type cobalt monophosphosulfide yolk−shell spheres have been successfully synthesized. The surface chemical states and material compositions were further investigated. The high-resolution Co 2p X-ray photoelectron spectroscopy (XPS) spectra are shown in Figure 1b. Two sets of doublets can be deconvoluted by Gaussian curve fitting. The first doublet (793.1 and 778.1 eV) and the other one (797.0 and 781.1 eV) are ascribed to Co 2p3/2 and Co 2p1/2, respectively.21,37−39 The shakeup satellites (“Sat.”) located at 783.1 and 785.3 eV correspond to Co2+sat. and Co3+sat., respectively.37,38 The Co 2p spectra of both samples manifest the presence of Co2+ and Co3+, consistent with the nature of nonstoichiometric cobalt sulfides, as marked by the dashed line in Figure 1b. It is also revealed that the increased proportion of the oxidation state (Co3+) is induced by introducing P into the sample.21 In Figure 1c, the S 2p core level spectrum displays three peaks at 161.5, 162.7, and 169.5 eV, in response to S 2p3/2, S 2p1/2, and SO42−, respectively.21,39 The existence of the SO42− components manifests slight surface oxidation of Co0.90S. In the meantime, for the Co−S−P sample, the S 2p XPS spectrum does not show any obvious sulfate components, suggesting P introduction could suppress the oxidation of the cobalt sulfide in air.22 Figure 1d shows the P 2p core level XPS spectrum, displaying two set of doublets. The 11033

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Figure 3. Electrocatalytic performance of different catalysts for OER in 1 M KOH. (a) Polarization curves. (b) Tafel plots. (c) Polarization data for Co−S0.58−P0.42 before and after 3000 cyclic voltammetry cycles. (d) Chronopotentiometry curve under 1.5 V for 20 h.

phosphorus introduction.43,44 In addition, the Co 2p spectra of cobalt monophosphosulfides were also examined (Figure S5c), which obviously illustrates the increase of the Co3+/Co2+ couple ratio with P introduction. Hence, the Co3+/Co2+ ratio in the cobalt monophosphosulfides can be tuned via the P/S stoichiometry. Morphology and Microstructure. The morphologies of the cobalt monophosphosulfide were further investigated. The first-step sulfurization process (2 h) leads to the formation of the yolk−shell cobalt sulfide structures (Figure S6). The subsequent phosphorization will complete the formation of the cobalt monophosphosulfide phase (Co0.9S0.58P0.42). Figure 2a shows the scanning electron microscopy (SEM) image of the Co0.9S0.58P0.42 sample. The cobalt monophosphosulfide is featured with homogeneous spherical morphology with a mean size of 450 nm (Figure S7). An expanded view of the broken spheres (marked with arrows in Figure 2b) clearly displays their hollow structure. As evidenced from the transmission electron microscopy (TEM) investigation (Figure 2c), the cobalt monophosphosulfide spheres are yolk−shell with a shell thickness as thin as 20 nm and a core size of 240 nm. A fringe spacing of 0.25 nm (indicated in Figure 2d) matches the (220) lattice plane of the pyrrhotite-type cobalt monophosphosulfide (JCPDS No. 19-0366). The well-defined rings in the selected area electron diffraction (SAED) pattern (Figure 2e) further confirm that the yolk−shell sample is polycrystalline. An observed (102) plane with a larger fringe spacing of 0.542 nm was clearly recognized (Figure 2d), and the interfacial angle between (102) and (220) planes was calculated as 55° based on eq S1 in the Supporting Information in a hexagonal crystal system (Co0.90S ht), which is in

accordance with the actual measured value indicated in Figure 2d. The scanning TEM (STEM) image of a representative yolk−shell sphere and the elemental mappings (Figure 2f) illustrate the homogeneous distribution of Co, S, and P through the entire hollow structure. The specific surface area of the cobalt monophosphosulfide yolk−shell sphere has been tested to be 61.9 m2 g−1 by the Brunauer−Emmett−Teller (BET) method (Figure S8). OER Activity and Mechanism. The OER performance of Co−S−P yolk−shell structures is first characterized under a three-electrode cell. Polarization curves in Figure 3a demonstrate that the Co0.9S0.58P0.42 sample exhibits an OER onset potential (0.19 V, 1 M KOH) lower than those of Co0.9S0.83P0.17 (0.21 V), Co0.9S0.71P0.29 (0.22 V), Co−S (0.34 V), Co−P (0.30 V), IrO2/C (0.26 V), and Co3O4 (0.33 V). The IrO2/C and Co3O4 nanoparticles are commercially purchased. Their operating overpotential (η10, at 10 mA cm−2) were further investigated. The Co0.9S0.58P0.42 electrode can deliver a low η10 of 266 mV (1.496 V vs RHE), smaller than the η10 of Co0.9S0.83P0.17 (330 mV), Co0.9S0.71P0.29 (291 mV), Co−S (440 mV), CoP (399 mV), IrO2/C (360 mV), and Co3O4 (380 mV). In addition, the OER electrocatalytic kinetics were examined in Figure 3b. The Tafel slope of Co0.9S0.58P0.42 (48 mV/dec) is lower than those of Co0.9S0.83P0.17 (72 mV/dec), Co0.9S0.71P0.29 (53 mV/dec), Co−S (109 mV/dec), Co−P (111 mV/dec), IrO2/C (82 mV/dec), and Co3O4 (61 mV/dec). Moreover, the outstanding OER activity of Co0.9S0.58P0.42 is comparable with those of recently developed excellent transition-metal-based OER electrocatalysts, as listed in Table S2 (Supporting Information). 29,30,45 To assess the durability of the Co0.9S0.58P0.42 electrode, linear sweeps were processed repeat11034

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Co0.9S0.58P0.42 material presents a higher linear slope at 101.1 mF/cm2, compared to those of Co0.9S0.83P0.17 (70.1 mF/cm2), Co0.9S0.71P0.29 (58.9 mF/cm2), Co−S (47.6 mF/cm2), Co−P (29.1 mF/cm2), IrO2/C (27.9 mF/cm2), and Co3O4 (19.4 mF/ cm2). The higher ECSA is due to the enhanced anion exchangeability between the catalytic active sites and electrolyte, which will lead to the significant improvement of OER properties for Co0.9S0.58P0.42. Furthermore, the mass activity (MA, normalized to loaded catalysts mass) and specific activity (SA, normalized to BET surface area) were also calculated, as shown in the bar plot of Figure 4c.46 Among them, the Co0.9S0.58P0.42 sample shows the highest MA (40 mA mg−1) and SA (0.26 mA cm−2) values. The enhanced OER performance could be easily noticed by the higher Co3+ site of Co0.9S0.58P0.42 catalyst. As is well understood, the OER activity mainly depends on the surface chemisorption free energy of intermediates (e.g., OH*, O*, and OOH*) in its four elemental steps.47−49 Regarding the entire OER process, 4OH− → O2 + 2H2O + 4e−, Co3+ ions play important roles in promoting the OER activity, due to more valence 3d electron orbits and higher electron-accepting characteristics than those of Co2+.50,51 The abundant Co3+ ions here are attributed to not only the P substitution but also the nonstoichiometric phase. Compared with the pyrite-type CoPS (Figure S9) converted from the CoIIS2, our pyrrhotitetype Co0.9S0.58P0.42 converted from nonstoichiometric Co0.9S (contain both Co2+ and Co3+) shows superior catalytic performance for OER, highlighting the effective strategy of Co3+/Co2+ couple activation via phase control. Further, XPS spectra of Co0.9S0.58P0.42 before and after the OER test are examined to understand the OER mechanism. The Co 2p spectra in Figure 5a reveal the increased proportion of Co3+ induced in the OER testing processes, whereas the chemical state of sulfur does not change obviously according to the S 2p spectra (Figure 5b). In Figure 5c, the near disappearance of the P 2p phosphide peak (129.1 eV) and relatively high intensity of oxidized phosphate species at 133.6 eV represent the oxidation of Co0.9S0.58P0.42.52 In the O 1s core level XPS spectrum (Figure 5d), the intense bands at 530.4 and 532.3 eV reveal the abundant lattice oxygen and surface hydroxyls generated on the surface of the Co0.9S0.58P0.42 electrode. It can be related to the formation of the cobalt oxides/hydroxides in the OER process, which are highly OER active.53 The formation of an oxidation layer is further confirmed by the “post mortem” XRD investigation in Figure 5e. It is found that the material underwent the partial oxidization and formed the Co(OH)2 (JCPDS No. 30-0443) on the surface in the OER process (Figure 5e). In spite of the partial oxidization, the main peaks still match well with the hexagonal Co0.9S phase. Raman spectra in Figure 5f also did not show any new peaks after OER testing after 1000 potential cycles. All of these confirm the good electrochemical stability of the Co0.9S0.58P0.42 catalyst in the OER process under alkaline media. According to the previous reports,54,55 some non-oxide Co-based catalysts can tolerate the strongly oxidizing OER reaction conditions because the surface oxidation layer will gradually reach the steady state with the prolonged OER tests and protect the Co-based core from further oxidation. An interface between sulfides and oxides has been demonstrated to be beneficial to enhance the OH− adsorption and reduce the Gibbs free energy of the reaction intermediate, which further improves the OER catalytic activity.56,57 This could be the

edly upon the anode (Figure 3c). After 3000 cycles, only a slight OER current loss was observed for Co0.9S0.58P0.42 electrode at a 50 mV s−1 scanning rate. The I−t measurement further reveals a good OER activity retention over 20 h testing in alkaline solution (Figure 3d), suggesting its high stability of the Co0.9S0.58P0.42 for OER. The electrochemically active surface area (ECSA) was also evaluated via a cyclic voltammetry (CV) method in Figure 4a, where ECSA could be represented by the linear slope (twice of the electrochemical double-layer capacitance, Cdl). The

Figure 4. (a) CV curves in an overpotential windows of 21−118 mV of Co0.9S0.58P0.42 electrode. (b) Charging current density differences at different scan rates. (c) Specific and mass activities of different catalysts at 1.5 V vs RHE. 11035

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Figure 5. Post mortem characterizations of Co0.9S0.58P0.42 catalyst before and after 1000 potential cycling OER. (a) Co 2p, (b) S 2p, (c) P 2p, and (d) O 1s core level XPS spectra. (e) XRD pattern after OER testing and (f) Raman spectra.

enhanced HER performance of Co0.9S0.58P0.42 with a high P/S ratio is attributable to the electronic structure tuning via substituting sulfur with more electron-donating phosphorus, which may facilitate the HER process (H2O + e− → 1/2 H2 + OH−). In addition, the Co0.9S0.58P0.42 catalyst also demonstrated good stability in both basic and acidic media for HER (Figure S11). As the Co0.9S0.58P0.42 samples can effectively promote both OER and HER processes in alkaline solution, the overall water splitting has been demonstrated based on a two-electrode setup by employing Co0.9S0.58P0.42 as both anode and cathode in 1.0 M KOH. It is found that the Co0.9S0.58P0.42∥Co0.9S0.58P0.42 couple is capable of delivering the overall water splitting under a low operation η10 of 1.59 V (Figure 7a), which is comparable with the conventional noble metal based benchmark (1.55 V for IrO2∥Pt).58 The morphology of the electrocatalyst after the water splitting is shown in the inset of Figure 7a, where a spherical morphology is still maintained. Finally, the Co0.9S0.58P0.42∥Co0.9S0.58P0.42 couple was examined at different water splitting operation voltages, as depicted in Figure 7b. No obvious decay is observed in the long-term testing, suggesting the high stability of Co0.9S0.58P0.42 for overall water splitting.

mechanism for the high OER performance of Co0.9S0.58P0.42 with surface oxidation. HER and Overall Water Splitting. The HER activity of the Co−S−P yolk−shell structures is characterized in different solutions. In 0.5 M H2SO4 (Figure 6a), except the 20 wt % Pt/ C benchmark, the Co0.9S0.58P0.42 delivered superior hydrogen production properties with a 44 mV onset potential and a lower operating η10 of 139 mV than those η10 of Co0.9S0.83P0.17 (191 mV), Co0.9S0.71P0.29 (176 mV), Co−S (343 mV), and Co−P (241 mV). The corresponding HER electrocatalytic kinetics were examined in Figure 6b. For the Co0.9S0.58P0.42 sample, the Tafel slope is a lower value around 69 mV dec−1, compared to those of Co0.9S0.83P0.17 (88 mV dec−1), Co0.9S0.71P0.29 (78 mV dec−1), Co−S (201 mV dec−1), and Co−P (113 mV dec−1) in 0.5 M H2SO4. Meanwhile, the same trend has been shown for their HER performance in the alkaline solution (Figure 6c,d). In 1 M KOH, Co0.9S0.58P0.42 exhibited a HER performance (η10 = 141 mV, Tafel slope = 72 mV dec−1) higher than those of controls, such as Co0.9S0.83P0.17 (202 mV, 85 mV dec−1), Co0.9S0.71P0.29 (161 mV, 82 mV dec−1), Co−S (398 mV, 182 mV dec−1), and Co−P (272 mV, 142 mV dec−1) and also showed higher mass activity and specific activity for HER (Figure S10). Although the increased proportion of electronaccepting Co3+ may not be beneficial to the HER process, the 11036

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Figure 6. Electrocatalytic performance of different catalysts for HER in alkaline and acidic electrolytes. (a) Polarization curves and (b) Tafel plots in 0.5 M H2SO4. (c) Polarization curves and (d) Tafel plots in 1 M KOH.

Figure 7. Overall water splitting performance based on the Co0.9S0.58P0.42∥Co0.9S0.58P0.42 electrodes in 1 M KOH. (a) Polarization curves; inset is the catalysts after overall water splitting. (b) Catalyst durability for water splitting at 1.59, 1.62, and 1.64 V.

CONCLUSIONS

overpotential around 140 mV with small Tafel slopes around 70 mV/dec in both alkaline and acidic media. Especially for OER catalytic application, the Co0.9S0.58P0.42 sample shows a low overpotential of 266 mV at 10 mA cm−2 with a small Tafel slope of 48 mV/dec as well as high operation stability. A symmetrical overall water splitting was then demonstrated based on the Co0.9S0.58P0.42∥Co0.9S0.58P0.42 couple. This study shows that phase engineering and composition control can be the elegant means to realize the Co3+/Co2+ couple activation and electronic structures to promote the OER and HER

In summary, the uniform nonstoichiometric cobalt monophosphosulfide (Co0.9S0.58P0.42) yolk−shell spheres were successfully synthesized with an hcp phase based on a partial sulfurization/phosphorization process. In the yolk−shell spheres, Co, P, and S elements are homogeneously distributed throughout the entire structure, and the P/S molar ratio can be tuned (e.g., Co0.9S0.83P0.17, Co0.9S0.71P0.29). The electrochemical properties for HER and OER of Co0.9S0.58P0.42 have been investigated. As for HER, it showed a low operating 11037

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Synthetic scheme; phase and morphology of the Co precursor spheres; phase and structure of the cobalt phosphide spheres; XRD patterns, EPR spectra, Raman spectra, XPS spectra of all sulfides and monophosphosulfides; morphology and structure of the cobalt sulfide; size distribution of Co0.9S0.58P0.42 yolk−shell spheres; nitrogen adsorption−desorption isotherms; OER properties of pyrite-structured CoS|P; specific and mass activities and I−t curves for HER; table for the elemental Co, P, and S atomic percentage for different electrocatalyst; comparison of OER activities of metal sulfides and phosphides catalysts; details for the calculation of interfacial angle; Figures S1−S11 and Tables S1 and S2 (PDF)

process for overall water splitting. Such a facile synthetic method can be also expanded to obtain other metal monophosphosulfides for electrochemical applications, showing an exciting avenue to enrich the family of low-cost, efficient, and stable electrocatalysts.

MATERIALS AND METHODS Materials. Nafion perfluorinated resin solution (5%), iridium oxide (IrO2, 99.9%), and 20 wt % Pt−C, thioacetamide (95%) were purchased from Sigma-Aldrich (Singapore). The isopropyl alcohol (IPA, 99%) and glycerol (99%) were purchased from Alfa Aesar (Shanghai, China). Co(NO3)2·6H2O and NaH2PO2 (99%) were purchased from Aladdin (Shanghai, China). All chemicals and materials were used without further purification. Synthesis of Uniform Co Precursor Spheres. Cobalt precursor was synthesized by reacting cobalt powder with IPA and glycerol. Uniform Co precursor spheres were synthesized through a facile modified hydrothermal reaction. Under vigorous magnetic stirring, Co(NO3)2·6H2O (0.2 M) and glycerol (4 mL) were first dissolved into IPA (20 mL). The transparent solution was subsequently transferred to an autoclave (45 mL) and set aside in an oven. The reaction was conducted at 180 °C for 6 h. The product was then washed with ethanol five times and dried in an oven at 60 °C for 12 h. Synthesis of Co−S−P Yolk−Shell Spheres. The Co−S−P yolk−shell spheres were obtained via a two-step process. First, 20 mg of Co precursor spheres was dispersed into 30 mL of ethanol solution containing 40 mg of thioacetamide (Sigma-Aldrich). This mixture was then shifted to an autoclave (45 mL) and set aside at 180 °C for 2−6 h. After the reaction, the next step was to collect the black precipitate by repetitive washing with ethanol for at least six times before placing it to vacuum-dry at room temperature overnight. The last step is the phosphorization of Co−S in a tube furnace, where NaH2PO2 and the black precipitate were put at the upstream and downstream parts, respectively. The weight ratio of precipitate to NaH2PO2 is 1:5. Then, the NaH2PO2 side was set aside at 320 °C for 2 h under Ar atmosphere. The sulfurization and phosphorization time were varied to synthesize samples with different S/P ratios. The pure cobalt sulfide (Co−S) yolk−shell structures were obtained by hydrothermal sulfurization (at 180 °C for 6 h). The pure cobalt phosphide was obtained by one-step phosphorization (400 °C) in an Ar funace to obtain a crystalline CoP. Characterizations. The phase of the as-prepared products was identified by XRD with Cu Kα radiation (λ = 1.5418 Å). SEM (JEOL, JSM-7600F) operating at 5 kV was used to characterize the morphology and composition. TEM and elemental mapping were conducted on a JEOL JEM-2100F microscope (200 kV). With monochromatic Mg Kα X-ray (1283.3 eV), XPS spectra were measured using KRATOS Axis ultra-DLD X-ray photoelectron spectrometer. A confocal Raman microscopy system (WITec CRM200) was used for Raman spectroscopy measurements with the excitation line of 532 nm by. Room temperature EPR spectra were measured by using a Bruker ELEXSYS EPR system (X-band, ∼9.4 GHz). Electrochemical Measurements. For HER and OER testing, the anode slurry was made with 5 mg of cobalt monophosphosulfide in 1 mL of IPA and 20 μL of Nafion in a three-electrode cell. Hg/HgO and Ag/AgCl (in saturated KCl solution) reference electrodes (RE) were used for HER and for OER testing, respectively. Pt wire and graphite rod were chosen as the counter electrodes for OER and HER, respectively. The electrolytes were 1 M KOH for HER and OER and 0.5 M H2SO4 for HER. Linear sweep voltammetry was conducted at a scan rate of 2 mV/s. In all measurements, Ag/AgCl RE was calibrated: E(RHE) = E(Ag/AgCl) + 0.059 pH + 0.197.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Zhengfei Dai: 0000-0002-3709-8895 Yun Zong: 0000-0001-9934-0889 Xin Wang: 0000-0003-2686-466X Qingyu Yan: 0000-0003-0317-3225 Author Contributions ⊥

Z.D. and H.G. contributed equally.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS We acknowledge the financial support from Singapore MOE AcRF Tier 1 Grants 2016-T1-002-065, RG113/15, Singapore A*STAR Pharos program SERC 1527200 022. We would like to acknowledge the Facility for Analysis, Characterization, Testing and Simulation (FACTS), Nanyang Technological University, Singapore, for use of their electron microscopy (or XRD) facilities. REFERENCES (1) Detchon, R.; Van Leeuwen, R. Bring Sustainable Energy to the Developing World. Nature 2014, 508, 309−311. (2) Bashyam, R.; Zelenay, P. A Class of Non-Precious Metal Composite Catalysts for Fuel Cells. Nature 2006, 443, 63−66. (3) Zhao, Y.; Jia, X.; Waterhouse, G. I.; Wu, L. Z.; Tung, C. H.; O’Hare, D.; Zhang, T. Layered Double Hydroxide Nanostructured Photocatalysts for Renewable Energy Production. Adv. Energy Mater. 2016, 6, 1501974. (4) Zhao, L.; Dong, B.; Li, S.; Zhou, L.; Lai, L.; Wang, Z.; Zhao, S.; Han, M.; Gao, K.; Lu, M.; et al. Interdiffusion Reaction-Assisted Hybridization of Two-Dimensional Metal-Organic Frameworks and Ti3C2Tx Nanosheets for Electrocatalytic Oxygen Evolution. ACS Nano 2017, 11, 5800−5807. (5) Ganesan, P.; Prabu, M.; Sanetuntikul, J.; Shanmugam, S. Cobalt Sulfide Nanoparticles Grown on Nitrogen and Sulfur Codoped Graphene Oxide: An Efficient Electrocatalyst for Oxygen Reduction and Evolution Reactions. ACS Catal. 2015, 5, 3625−3637. (6) Seh, Z. W.; Fredrickson, K. D.; Anasori, B.; Kibsgaard, J.; Strickler, A. L.; Lukatskaya, M. R.; Gogotsi, Y.; Jaramillo, T. F.; Vojvodic, A. Two-Dimensional Molybdenum Carbide (MXene) as an Efficient electrocatalyst for Hydrogen Evolution. ACS Energy Lett. 2016, 1, 589−594. (7) Yang, H.; Zhang, Y.; Hu, F.; Wang, Q. Urchin-Like CoP Nanocrystals as Hydrogen Evolution Reaction and Oxygen Reduction Reaction Dual-Electrocatalyst with Superior Stability. Nano Lett. 2015, 15, 7616−7620.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b05050. 11038

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