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Controlled Synthesis of a Three-segment Heterostructure for High-performance Overall Water Splitting Lan Hui, Yurui Xue, Dianzeng Jia, Zicheng Zuo, Yongjun Li, Huibiao Liu, Yingjie Zhao, and Yuliang Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16791 • Publication Date (Web): 22 Dec 2017 Downloaded from http://pubs.acs.org on December 26, 2017

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ACS Applied Materials & Interfaces

Controlled Synthesis of a Three-segment Heterostructure for High-performance Overall Water Splitting Lan Hui,†,‡ Yurui Xue,†,* Dianzeng Jia,‡,* Zicheng Zuo,† Yongjun Li,† Huibiao Liu,† Yingjie Zhao,§ Yuliang Li†,* †

Institute of Chemistry, CAS Research/Education Center for Excellence in Molecular

Sciences, Chinese Academy of Sciences, Beijing 100190, P. R. China, University of Chinese Academy of Sciences, Beijing 100049, P.R. China. ‡

Key Laboratory of Energy Materials Chemistry, Ministry of Education; Key laboratory of

Advanced Functional Materials, Autonomous Region; Institute of Applied Chemistry, Xinjiang University, Urumqi 830046, Xinjiang, P. R. China. §

School of Polymer Science and Engineering, Qingdao University of Science and Technology,

Qingdao 266042, P. R. China.

ABSTRACT: Developing earth-abundant, highly active and robust electrocatalysts capable of both oxygen and hydrogen evolution reactions is crucial for commercial success of renewable energy technologies. Here we demonstrate a facile and universal strategy for fabricating transition-metal (TM) sulfides by controlling the atomic ratio of TM precursors for water splitting in basic media. Density functional theory calculations reveal that the incorporation of Fe/Co can significantly improve the catalytic performance. The optimal material exhibits extremely small overpotentials of 208 mV for oxygen evolution and 68 mV for hydrogen evolution at 10 mA cm-2 with robust long-term stability. The optimized material was used as bifunctional electrodes for overall water splitting which delivers 10 mA cm-2 at a very low cell voltage of 1.44 V with robust stability over 80 h at 100 mA cm-2 without ACS Paragon Plus Environment

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degradation, much better than the combination of Pt and RuO2 as benchmark catalysts. The excellent water splitting performance shed light on the promising potential of such sulfides as a high activity and robust stable electrodes.

KEYWORDS: bifunctional electrocatalysts, overall water splitting, heterostructures, earth-abundant elements, noble metal-free

INTRODUCTION Electrochemical water splitting comprising OER on the anode and HER on the cathode provides a promising strategy for sustainable energy conversion, storage and usage (such as renewable hydrogen generation and fuel cells).1˗11 Unfortunately, the kinetically sluggish of these reactions intrinsically limits the overall efficiency of water splitting. Development of highly efficient and robust catalysts are therefore critically important for lowering the kinetic barrier. Currently, noble metal based materials (such as Ru/Ir for OER, and Pt for HER, respectively) are the state-of-the-art electrocatalysts, however, high-cost, scarcity, and poor stability of them greatly limit their wide application in commercial electrolyzers.6,10 Over the past decades, chemists have made great efforts and progress in searching for cost-effective, earth-abundant elements based OER and HER electrocatalysts. But most of the developed electrocatalysts that are efficient in acidic conditions may be inactive or even unstable in alkaline conditions, and vice versa. Such pH mismatch can lead to trouble greatly in integrating both OER and HER catalysts for overall water splitting, resulting in inferior overall performance. To realize practical overall water splitting, OER and HER should be operated in the same electrolyte, especially in alkaline electrolyte, based on a single

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catalyst.12,13 Therefore, searching for cost-effective bifunctional electrocatalysts having high activity and stability towards overall water splitting still remains a potential challenge. Various types of Earth-abundant materials, such as transition metal (TM) sulfides,13-17 oxides,18 hydroxides,19,20 phosphides,21 selenides22-25 and carbon materials26,27 have been identified as potential candidates to precious catalysts for water splitting process. The undercoordinated metal sites on the surface of TM based materials (such as Fe, Co sulfides) are pivotal for OER and HER due to the chemisorption of OH−/oxygen-containing intermediates or high chemisorption capability for hydrogen.28 Though some TM sulfides have been successfully synthesized as HER or OER electrocatalysts, most of them are monometallic species or can catalyze only one kind of reaction (either HER or OER), strongly limiting their practical application in overall water splitting. Bimetallic compounds show better catalytic activity than respective unary ones owing to beneficial synergistic effects between different components4. Recent studies showed that the catalytic performance of metal sulfide could be improved by doping other TMs.20,29 In spite of the discovery of some high-performance catalysts, some key problems remain unresolved in this field. Research and development of bifunctional OER/HER catalysts with ultrahigh catalytic activity and stability, to meet the practical application. In this work, we report our efforts on the structural engineering of the FeCo sulfides grown on Ni foam as an efficient bifunctional OER/HER catalyst in 1.0 M KOH. The nanosheet morphology maximizes the electrochemically active surface area. By modulating the Fe/Co ratios, catalysts with different compositions and catalytic activities were obtained. A comprehensive study of the composition dependent catalytic performance on OER, HER, as well as overall water splitting, is conducted. The relationship between activity and ACS Paragon Plus Environment


composition was obtained. The optimal catalyst exhibits outstanding OER activity, surpassing the benchmark RuO2 and other previously reported state-of-the-art OER electrocatalysts, and excellent HER activity approaching that of Pt foil in 1.0 M KOH. The results of our density function theory (DFT) calculations further demonstrated the relationship between activity and composition. Most notably, when used as both anodic and cathodic catalysts in an alkaline water electrolyzer, it only requires a cell voltage of 1.44 V to achieve current density of 10 mA cm-2, with remarkable stability for over 80 h at 100 mA cm-2, which is even better than RuO2–Pt couple.

EXPERIMENTAL SECTION Materials. All aqueous solutions were prepared with Milli-Q water. Co(NO3)2, FeCl2·4H2O, CO(NH2)2 and NH4F were purchased from Sinopharm Chemical Reagent. Commercial Pt/C (20 wt.% Pt on an activated carbon support) was purchased from Alfa Aesar. Unless otherwise specified, all other reagents were purchased commercially from Sinopharm Chemical Reagent and used without further purification. Synthesis of FeCo–precursors. A facile solvothermal method was used for preparing FeCo– precursors. Ni foam was washed sequentially with water, ethanol, 1 M HCl solution, and water for three times. The dried Ni foam (1 cm × 2 cm) was then immersed into aqueous solution containing deionized water (12 mL), anhydrous ethanol (3 mL), Co(NO3)2 (0.15 mmol), FeCl2·4H2O (0.75 mmol), CO(NH2)2 (4.5 mmol), and NH4F (1.5 mmol) in a polytetrafluoroethylene (Teflon)–lined stainless steel autoclave (20 mL). Subsequently, the autoclave was heated at 120 °C for 9 h, then naturally cooled to room temperature. Afterwards, the products were collected and washed by Milli-Q water and ethanol for several times, and


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dried in a vacuum oven at 60 °C for 10 h, the brown FeCo–precursors were obtained. For special studies, a series of FeCo–precursors with different Fe/Co molar ratios were synthesized by using the same procedure. Synthesis of FeCo–sulfides. The conversion from FeCo–precursors to sulfides was finished through a simple solvothermal process. In typical, an aqueous solution of sodium sulfide (0.2 M) was transferred to the same autoclave, containing FeCo–precursors, and maintained at 100 °C for 10 h. After the autoclave cooled down to room temperature, the resulted FeCo–sulfides was thoroughly washed with deionized water and ethanol, followed by being dried at 60 °C in the vacuum oven. The accurate Fe/Co ratios in products were determined by ICP–MS. Characterization. FESEM (Hitachi Model S–4800) was employed to determine the morphologies. More information on the morphologies, the crystallinity and the elements present in the nanowires was obtained by HRTEM (JEM–2100F, 200 kV). The phase formation was identified by using the powder X–ray powder diffraction (XRD) with a Rigaku D/max–2500 rotation anode x–ray diffractometer equipped with grahite–monochromatized Cu Kα radiation (λ = 1.54178 Å). The chemical composition and element states of the catalysts were determined by X–ray photoelectron spectroscopy (XPS, Thermo Scientific ESCALab 250Xi with 200 W monochromated Al Kα radiation). The obtained banding energies were corrected with reference to C1s (284.8 eV). Electrochemical measurements. All electrochemical experiments were performed in 1.0 M KOH in a three–electrode system controlled by a CHI–760E electrochemical workstation. The as–prepared samples were used as the working electrode; a graphite plate and a saturated calomel electrode (SCE) were used as counter electrode and reference electrode, respectively. The OER activities of all catalysts were measured in O2–saturated 1.0 M KOH, and the HER ACS Paragon Plus Environment


activities of all catalysts were measured in H2–saturated 1.0 M KOH. Overall water-splitting measurements were conducted in a two–electrode system using the same sample as both the anodic and cathodic electrodes. Electrochemical impedance spectra (EIS) were recorded at corresponding potentials from 100 KHz to 0.01 Hz, with an amplitude of 5 mV. The measured impedance data were fitted using a series R(QR)(QR) equivalent circuit. All potentials used have been converted to the reversible hydrogen electrode (RHE) according to E (RHE) = E (SCE) + E0 (SCE) + 0.05916 × pH. Unless otherwise specified, the potentials of the LSV curves were corrected by measured ohmic resistance of the solution.

Figure 1. In situ growth of FeCo-precursor nanosheets array on Ni foam (1st step) and sulfidation reaction (2nd step) with complete synthesis of FeCo-sulfide nanosheets array on Ni foam via hydrothermal method.


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Figure 2. (a−f) SEM images of FeCo-precursors with the increasing of Co contents; (g−l) SEM images of FeCo-sulfides by sulfidation of corresponding precursors.



RESULTS AND DISCUSSION We first synthesized the FeCo hydroxides nanosheets arrays on a 3D conductive Ni foam (1 cm × 2 cm) via a hydrothermal method (Figure 1), providing the precursors for the preparation of FeCo-sulfide nanosheets. Next, the FeCo hydroxide nanosheets were topotactically converted into FeCo-sulfide nanosheets through an anion exchange reaction (Figure 1, please see the EXPERIMENTAL section for more details). Scanning electron microscopy (SEM) image (Figure 2a-f) shows the uniformly grown of FeCo-precursor nanosheets on NF with a smooth surface, oriented vertically and interconnected with each other. The density of FeCo-precursor nanosheets become higher with the increasing amounts of Co species. After an anion-exchange reaction, all samples retained the morphology of their precursors but with rough surfaces (Figure 2g-l). Inductively coupled plasma mass spectrometry (ICP-MS) analysis shows the accurate atomic ratio of Fe:Co. The crystalline phases of FeCo-sulfides were determined by X-ray diffraction (XRD) analysis as shown in Figure S1. XRD patterns may be ascribed to hexagonal FeS (JCPDS No. 65-9124), cubic Co9S8 (JCPDS No. 02-1459) and hexagonal CoS (JCPDS No. 65−3418), respectively, revealing the formation of well-mixed bimetallic sulfides. We used TEM analysis to further investigate the morphology of the resulted FeCo-sulfide with Fe and Co atomic ratio of 1:0.26, denoted as FeCoS-1 (Figure 3). SEM (Figure 3a-c) and transmission electron microscopy (TEM) images (Figure 3d,e) of FeCoS-1 nanosheet arrays detached ultrasonically from the Ni foam reveal the roughness nature of the FeCoS-1 nanosheets. This could maximize the electrochemically active surface area and high electrode/electrolyte interface area, thus increasing active sites for OER/HER. The SAED pattern (Figure 3f) and high-resolution TEM (HRTEM) image (Figure 3g) reveal the ACS Paragon Plus Environment

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polycrystalline nature of the sample. As shown in Figure 3g, lattice fringes with interplanar distances of 2.06 and 2.64 Å can be indexed to the (102) and (101) planes of hexagonal FeS. Besides, we contributed the 1.96 and 2.99 Å to the diffraction of (102) plane of hexagonal CoS and (311) plane of cubic Co9S8, respectively. The scanning TEM image and EDX mapping images (Figure 3h-k) show the uniform distribution of Fe, Co and S elements within the whole nanosheets.

Figure 3. (a-c) SEM images, (d, e) TEM images, (f) SAED pattern, and (g) HRTEM image of FeCoS-1; (h) STEM image of FeCoS-1 and elemental mapping images of (i) Fe, (j) Co, and (k) S. ACS Paragon Plus Environment


Figure 4. (a) XPS survey spectra of FeCo–sulfides with different Fe:Co ratios (1:0.075, 1:0.26, 1:0.74, 1:1.28, 1:1.80, and 1:2.29), FeS, and CoS, respectively. Wide spectra and high-resolution spectra of (b) Fe 2p, (c) Co 2p, and (d) S 2p, respectively.

The chemical states and element compositions were determined by XPS. The survey spectrum reveals the co-existence of Fe, Co, and S elements (Figure 4a). Two spin−orbit doublets corresponding to 2p3/2 and 2p1/2 signals could be observed in Fe 2p-region (Figure 4b). The peaks of Fe at 710.6 and 712.2 eV should arise from FeS.30 For Co 2p (Figure 4c), the fitted peaks at 781.2 and 796.7 eV correspond to 2p3/2 and 2p1/2, respectively, confirming the co-existence of Co3+/Co2+.31 In S 2p spectrum (Figure 4d), the peaks at 161.8 and 162.6 eV correspond to the spin-orbit coupling in metal sulfide.32 The peak at 163.9 eV is typical of ACS Paragon Plus Environment

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metal-sulfur bonds, indicating Fe/Co were covalently assembled in heterostructures. Notably, a positive shift of binding energies (BEs) for Fe 2p and a negative shift of BEs for Co 2p were observed compared with pure FeS and CoS, respectively, implying the successful incorporation of Fe and Co and the strong electron interactions between Fe and Co which benefits to catalytic performances of the catalysts. The catalytic OER activities of catalysts were determined in 1.0 M KOH by cyclic voltammetry (CV) at a scan rate of 5 mV s-1 (Figure 5a). FeS, CoS, RuO2, and Ni foam were employed as references. The overpotentials of FeCo-sulfide ternary composites to reach current densities of 10, 50, and 100 mA cm−2 are lower than that of pure FeS, CoS, RuO2, and Ni foam (Figure 5a,b), suggesting that the incorporation of Fe/Co can effectively improve their OER catalytic activities. The overpotential (ŋ10) at a current density (j) of 10 mA cm−2 was chosen as criteria for comparing activities of various catalysts. Among all FeCo-sulfides, FeCoS-1 (Fe:Co=1:0.26) shows the best OER activity (the lowest overpotential of 208 mV to reach 10 mA cm-2), greatly outperforming RuO2 (318 mV) and almost all reported OER catalysts (Figure 5c, Table S1), such as NF@NC-CoFe2O4/C NRAs (240 mV),33 FeCoNi (325 mV),34 MoS2/Ni3S2 heterostructures (218 mV),28 and Mn-Co oxyphosphide particles (320 mV).37 The OER kinetics were also revealed by Tafel plots (Figure 5d). FeCoS-1 shows the smallest Tafel slope of 37 mV dec-1 among all catalysts, indicative of the most favorable kinetics for the efficient mass and electron transfer. The stability is also very important for practical applications. As shown in Figure 5e, there are no loss in current density occurs after 2500 continuous CV cycles and no noticeable degradation over a 30 h long-term electrolysis test under 100 mA cm-2. SEM images (Figure S2) of FeCoS-1 after 2500 OER cycles clearly show that the morphological integrity of FeCoS-1 were well preserved after cycling tests. ACS Paragon Plus Environment


Seen from the XPS spectra recorded after cycling test, the robust catalysts retain the same Fe, Co, and S as that of freshly-prepared samples (Figure S3). For Fe2p, a peak located at ∼726.5 eV was observed, which suggest the presence of ferric oxides [Fe(III)] phase; for Co 2p spectrum, the new peaks appeared at 783.1 eV and 795.7 eV can be ascribed the oxidized Co species. In addition, an intense peak for O 1s was observed. These results reveal the excellent stability of FeCoS-1. We also assessed the HER performance of the catalysts in 1.0 M KOH. As shown in Figure 6a,b, FeCoS-1 presents Pt-like catalytic activity by exhibiting a very low ŋ of 68 mV at 10 mA cm-2, substantially the lowest among FeS (211 mV@10 mA cm-2), CoS (152 mV@10 mA cm-2), other as-prepared FeCo-sulfides, and most of the reported HER catalysts in both basic [e.g., CoP/CNT (122 mV)36 and Fe0.5Co0.5P (130 mV)37] and acidic electrolytes [e.g., CoPS (128 mV)38 and iron-nickel sulfide (105 mV) 39] (Figure 6c). In 1.0 M KOH, Pt exhibits a lower Tafel slope of 72 mV dec-1. FeCoS-1 shows a small Tafel slope (114 mV dec-1), suggesting a facile kinetics towards HER (Figure 6d). Long-term stability of FeCoS-1 for HER in alkaline condition was also determined. As shown in Figure 6e, the polarization curve of FeCoS-1 after 2500 cycles showed a negligible variation of activity compared with the initial one. Under a cathodic j of -50 mA cm-2 (Figure 6f), no noticeable degradation was observed over 66 h, revealing an excellent HER stability. SEM images (Figure S4) and XPS results (Figure S5) show no distinguishable changes before and after cycling test, implying the ideal structural stability of our material.


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Figure 5. (a) CV curves for FeCo-sulfides (Fe:Co = 1:0.075, 1:0.26, 1:0.74, 1:1.28, 1:1.80, and 1:1.29), Pt, CoS, FeS and Ni foam, respectively; (b) overpotentials of OER at current densities of 10, 50 and 100 mA cm-2 as a function of Fe/Co ratios (1:0.075, 1:0.26, 1:0.74, 1:1.28, 1:1.80, and 1:1.29). (c) comparison of OER overpotentials at 10 mA cm-2 with benchmark electrocatalysts; (d) Tafel slopes for Fe:Co = 1:0.26, Pt, CoS, FeS and Ni foam, respectively; (e) polarization curves before and after OER durability test; (f) long-term



stability test at 100 mA cm-2.

Figure 6. (a) polarization curves for FeCo-sulfides (Fe:Co = 1:0.075, 1:0.26, 1:0.74, 1:1.28, 1:1.80, and 1:1.29), Pt, CoS, FeS and Ni foam, respectively; (b) overpotentials of HER at current densities of 10, 50 and 100 mA cm-2 as a function of Fe/Co ratios (1:0.075, 1:0.26, 1:0.74, 1:1.28, 1:1.80, and 1:1.29). (c) comparison of HER overpotentials at 10 mA cm-2 with benchmark electrocatalysts; (d) Tafel slopes for Fe:Co = 1:0.26, Pt, CoS, FeS and Ni foam,


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respectively; (e) polarization curves before and after HER durability test; (f) long-term stability test at -50 mA cm-2.

Figure 7. (a) CV curves for FeCo-sulfides (Fe:Co = 1:0.075, 1:0.26, 1:0.74, 1:1.28, 1:1.80, and 1:1.29), CoS, FeS, RuO2||Pt, and NF in a two-electrode system; (b) time-dependent current density curves of FeCoS-1 at about 100 mA cm-2 in an alkaline electrolyzer.

The FeCoS-1 as bifunctional electrodes is further used to overall water splitting in a simple two-electrode cell in 1.0 M KOH. As shown in Figure 7a, FeCoS-1||FeCoS-1 exhibited the highest performances for overall water splitting with cell voltages of only 1.440, 1.466, and 1.486 V to reach current densities of 10, 100 and 500 mA cm-2 (Figure S6). These values are much higher than RuO2||Pt couple and other reported electrodes (Table S2) such as Co3O4 microtube arrays (1.687 V@10 mA cm-2),40 and Ni1.5Fe0.5P/CF||Ni1.5Fe0.5P/CF (1.589 V@10 mA cm-2).41 Under 100 mA cm-2, there is negligible degradation during the continuous electrolysis over 80 h (Figure 7b). Such prominent performances of the FeCoS-1 based electrolyzer is higher than commercial requirements for a water-splitting electrolyzer (voltage 1.55 V at 500 mA cm-2).42 Electrical Impedance Spectra (EIS) was performed to evaluate the electron and charge



transfer ability during catalytic process (Figure 8a, Figure S7).43 The parameters obtained by fitting Nyquist plots to R(QR)(QR) equivalent circuit (Figure 8b) were illustrated in Table S3. The smallest solution resistance (Rs = 2.18 Ω) and the charge transfer resistance (R1 = 0.336

Ω) of FeCoS-1 suggest the highest conductivity and charge transfer ability, which are beneficial to catalytic performance. The electrochemical double layer capacitances (Cdl), proportional to the electrochemical surface areas, of the catalysts was further determined by a simple CV method (Figure 8c, Figure S8). It is clear that the Cdl of FeCoS-1 (42 mF cm−2) is higher than that of others (Figure 8d, Table S4), confirming FeCoS-1 has the largest catalytic active surface area, which is beneficial to enhance the catalytic activity. To gain deep perceptions into the relationship between the catalyst composition and catalytic performances, an electrochemical study was performed by only tuning Fe/Co ratios to determine the optimal Fe/Co ratios in catalysts. By controlling the Fe/Co molar ratio in the precursors, a series of FeCoxS nanosheets were synthesized. As discussed above, at 10 mA cm-2, the OER/HER ŋ10 of FeS and CoS are 252/211 and 242/152 mV, respectively. In sharp contrast, the OER/HER activity can be remarkably enhanced with the incorporation of Fe/Co (Table S4, Figures 5-7). The catalytic activities of OER, HER, as well as overall water splitting, vary with the molar ratios of Fe/Co, revealing a component dependent catalytic behavior. The FeCoS-1(Fe:Co = 1:0.26) shows the highest activity for OER, HER, and overall water splitting processes.


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Figure 8. (a) Nyquist plots of samples in 1.0 M KOH electrolyte. The inset displays the magnified plot. (b) The equivalent circuit model for EIS analysis. (c) CV curves of FeCoS-1 (Fe:Co = 1:0.26) in potential range of 0.638~0.738 V versus RHE at different scan rates; (d) the capacitive currents at 0.688 V versus RHE with a scan rate of 40, 80, 120, 140, 180, 200, and 220 mV s−1 in 1.0 M KOH.



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Figure 9. Free energy profiles for the OER over (a) FeS, (b) CoS and (c) FeCoS-1 (Fe:Co=1:0.26); (d) DFT-optimized model structure of a FeCoS-1 sheet.

Theoretical investigation based on the density functional theory is further carried out to show the high performance of FeCoS-1 ternary composites for OER. The Vienna Ab Initio Simulation Package (VASP)44,45 is employed to show the free energy curves of OER on the FeS, CoS, and FeCoS-1 respectively. The details about the calculation are shown in Supporting Information, and the optimized structures for OER process are illustrated in Figure S9. Generally, OER follows a four electron step process in an alkaline electrolyte shown as follows46: OH

－

+ * → HO* + e

－

(1)

－

HO* + OH → H2O + O* + e －

O* + OH → HOO* + e

－

－

(2) (3)


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－

HOO* + OH → O2* + e

－

(4)

For the free energy curves in Figure 9, it is seen that some elementary reactions are positive free energy change at zero potential and equilibrium potential at 1.23 V. As the electrode potential increases, these reactions become less endothermic. All the elementary reactions become downhill until the electrode potential increases to 2.51 V on FeS, 1.95 V on CoS and 1.63 V on FeCoS-1. This suggests the overpotential is 1.28, 0.72 and 0.40 V on FeS, CoS and FeCoS-1 for the OER process compared with the equilibrium potential (1.23 V), respectively. This results is in good agreement with the experimental conclusion that the FeCoS-1 catalyst shows higher OER activity than those of FeS and CoS catalysts. These findings strongly reveal that elaborate structural engineering can effectively improve their catalytic performances. According to previous reports, this could be due to the moderate free energy or the balance of orbital occupation and electron transfer between metal species.37,43 The beneficial synergistic effects among different components offer a better combination of activity and stability than respective monometallic form. Therefore, the modulating electronic structures by changing metal ratios plays a critical role in improving OER/HER activity. Conclusion In summary, we presented a facile and universal approach for controllable synthesis of highly efficient bifunctional catalysts for overall water splitting by directly changing metal proportions. Experimental results show that the elaborate modulation of metal ratios could effectively improve the catalytic activity. The optimal catalyst (FeCoS-1) exhibited outstanding OER and HER performances with low overpotentials, Tafel slopes, and robust stability in alkaline media. Remarkably, when applying as bifunctional overall water splitting, ACS Paragon Plus Environment


it needs only 1.44 V to achieve 10 mA cm-2, outperforming the RuO2–Pt couple (1.54 V). DFT calculations further demonstrate that the change in composition can significantly improve their catalytic activity. In addition to a new insight into the correlation between composition and catalytic activity, this work also contributes to fundamental guidelines for controllable synthesis of cost-effective and efficient bifunctional catalysts for practical applications.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Computational details about the free energy of FeCo-based catalysts, XRD patterns and EIS Nyquist plots of the samples, SEM profiles and XPS data of FeCoS-1 after stability tests, overpotentials of FeCoS-1 for overall water splitting at current densities, CV curves of as-prepared catalysts in potential range of -0.43~-0.33 V at different scan rates, optimized structures of HO*, O* and HOO* adsorbed on FeS, CoS, and FeCoS-1, comparison of the OER performances (PDF).

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (Y.X.) *E-mail: [email protected] (D.J.) *E-mail: [email protected] (Y.L.)


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Notes The authors declare no competing financial interest.

Acknowledgements This work was supported by the National Nature Science Foundation of China (21790050 and 21790051),

the

National

Key

Research

and

Development

Project

of

China

(2016YFA0200104), and the Key Program of the Chinese Academy of Sciences (QYZDY-SSW-SLH015).

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