This is the pre-peer reviewed version of the following article: Wan K., Luo J., Zhou C., Zhang T., Arbiol J., Lu X., Mao B.-W., Zhang X., Fransaer J.. Hierarchical Porous Ni 3 S 4 with Enriched High-Valence Ni Sites as a Robust Electrocatalyst for Efficient Oxygen Evolution Reaction. Advanced Functional Materials, (2019). 29. 1900315: - . 10.1002/adfm.201900315, which has been published in final form at https://dx.doi.org/10.1002/adfm.201900315. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions. Hierarchical porous Ni3S4 with enriched high-valence Ni sites as a robust electrocatalyst for efficient oxygen evolution reaction Kai Wan, Jiangshui Luo*, Chen Zhou, Ting Zhang, Jordi Arbiol, Xihong Lu*, Bing-Wei Mao, Xuan Zhang*, Jan Fransaer* K. Wan, Prof. J. Luo, Dr. C. Zhou, Dr. X. Zhang, Prof. J. Fransaer Department of Materials Engineering KU Leuven Leuven 3001, Belgium E-mail: jiangshui.luo@gmail.com, xuan.zhang@kuleuven.be, jan.fransaer@kuleuven.be Prof. J. Luo Collaborative Innovation Center of Clean Energy Longyan University Longyan 364012, China E-mail: jiangshui.luo@gmail.com T. Zhang, Prof. J. Arbiol Catalan Institute of Nanoscience and Nanotechnology (ICN2) CSIC and BIST, Campus UAB Bellaterra, 08193 Barcelona, Catalonia, Spain Prof. J. Arbiol ICREA Pg. Lluís Companys 23 08010 Barcelona, Spain Prof. X. H. Lu MOE of the Key Laboratory of Bioinorganic and Synthetic Chemistry The Key Lab of Low-carbon Chem and Energy Conservation of Guangdong Province KLGHEI of Environment and Energy Chemistry School of Chemistry Sun Yat-Sen University Guangzhou 510275, China E-mail: luxh6@mail.sysu.edu.cn Prof. J. Luo, Prof. B.-W. Mao State Key Laboratory of Physical Chemistry of Solid Surfaces and Department of Chemistry College of Chemistry and Chemical Engineering Xiamen University Xiamen 361005, China Keywords: high-valence Ni3+, hierarchical porous structure, Ni3S4, oxygen evolution reaction, durability 1 Electrochemical water splitting is a common way to produce hydrogen gas, but the sluggish kinetics of oxygen evolution reaction (OER) significantly limits the overall energy conversion efficiency of the water splitting. In this work, a highly active and stable, meso-macro hierarchical porous Ni3S4 architecture, enriched in Ni3+ was designed as an advanced electrocatalyst for OER. The obtained Ni3S4 architectures exhibit a relatively low overpotential of 257 mV at 10 mA cm−2, and 300 mV at 50 mA cm−2. Additionally, this Ni3S4 catalyst has excellent long-term stability (no degradation after 300 h at 50 mA cm −2). The outstanding OER performance is due to the high concentration of Ni 3+ and the meso-macro hierarchical porous structure. The presence of Ni 3+ enhances the chemisorption of OH− which facilitates the electron transfer to the surface during OER. The hierarchical porosity increases the number of exposed active sites, and facilitates mass transport. A water-splitting electrolyzer using the prepared Ni3S4 as the anode catalyst and Pt/C as the cathode catalyst achieved a low cell voltage of 1.51 V at 10 mA cm −2. Therefore, this work provides a new strategy for the rational design of highly active OER electrocatalysts with high valence Ni 3+ and hierarchical porous architectures. 1. Introduction Hydrogen is an environmentally friendly and renewable energy carrier for green energy supply and storage.[1-3] Electrochemical water splitting is one way to produce hydrogen. [4, 5] It involves two half-reactions, the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) to produce hydrogen and oxygen, respectively. [6, 7] However, the OER has sluggish kinetics because it is a four-electron transfer process resulting in a high overpotential, which limits the overall energy conversion efficiency of electrochemical water splitting. [8] Currently, IrO2 and RuO2 are considered as the best OER catalysts.[9-11] Nevertheless, the scarcity and high cost of Ir and Ru hamper their applications.[7, 12, 13] Hence, it is highly 2 desirable to develop low cost and earth abundant electrocatalysts with high activity and long- term stability for the OER. In the past years, substantial efforts have been devoted to exploring transition metal (Fe, Co, Ni) based oxides, and hydroxides as noble-metal-alternative electrocatalysts for OER due to their abundance, low-cost, and high catalytic activity. [14-18] However, their electrochemical application is hindered by some severe obstacles: (1) Low content of high valence state metal ions. The high valence state metal ions such as Co3+ and Ni3+ have been confirmed as the active sites for OER,[19-23] while most of the work is focused on Co2+ and Ni2+ based materials.[7, 14, 15] (2) Low utilization efficiency of active sites due to the low specific surface area. (3) Relatively low electrical conductivity of the transition metal-based oxides and hydroxides. In order to solve these issues, various nanostructured nickel sulfide-based materials, such as films of NiS microspheres,[24] Ni2S3 nanosheet arrays,[25] Ni3S4 nanosheets,[26] porous Ni9S8,[27] and hierarchical NiCo2S4 nanowires,[28] have been developed to achieve high electrocatalytic performance due to their diverse valence states, high specific surface area, and high electrical conductivity. For instance, Qiao et al. fabricated a Ni9S8/Ni foam electrode for OER which had an overpotential (η) of 340 mV at 30 mA cm−2 with a catalyst loading of 11 mg cm−2.[27] Wang et al. reported a NiS/Ni foam catalyst with an overpotential of 335 mV at 50 mA cm−2 using a catalyst loading of 43 mg cm −2.[24] When Ni3S4 (Ni2+, Ni3+) was used as an electrocatalyst, the current density reached 30 mA cm −2 at η = 330 mV with a relatively low catalyst loading of 1.2 mg cm−2.[26] Shanmugam et al. developed Ni3S2/Ni foam and NiCo2S4/ Ni foam electrodes for OER. The NiCo2S4/Ni foam (η = 260 mV at 10 mA cm−2, current density loss of 15% after 50 h) showed enhanced electrocatalytic activity and long-term stability compared to Ni3S2/Ni (η = 300 mV at 10 mA cm−2, current density loss of 20% after 10 h).[28] Despite the significant progress, the electrocatalytic activities and long-term stability of the most developed nickel sulfides are not yet satisfactory. To this end, the development of alternative electrocatalysts with excellent 3 electrocatalytic performance and durability for OER is highly challenging and intensively pursued. In this work, meso-macro hierarchical porous Ni3S4 architectures enriched in Ni3+ species derived from Ni metal-organic framework (Ni-MOF) is developed as efficient OER electrocatalysts. Metal-organic frameworks (MOFs) with tunable structures and chemical compositions are interesting self-sacrificial templates for the synthesis of nickel sulfides. The as-prepared Ni3S4 architectures enriched in Ni3+ enhance the chemisorption of OH− which facilitates the electron transfer. In addition, the obtained meso-macro porous structure increases the number of active sites and facilitates mass transport. Due to the enhanced chemisorption of OH−, the high number of active sites, and the easy mass transport, the Ni3S4 architectures exhibit a remarkable OER performance. The Ni 3S4 catalyst on Ni foam (NM50- Ni3S4/NF) electrode shows a relatively low overpotential of 257 mV at 10 mA cm −2, and 300 mV at 50 mA cm−2. The NM50-Ni3S4/NF electrode exhibits not only outstanding electrocatalytic activity but also excellent long-term stability even at a high current density (no degradation after 300 h at 50 mA cm−2). Furthermore, a water-splitting electrolyzer using NM50-Ni3S4/NF as the anode catalyst and Pt/C as the cathode catalyst achieves a low cell voltage of 1.51 V at 10 mA cm−2. 2. Results and discussion The Ni3S4 catalysts were synthesized as illustrated in Figure 1a. Firstly, the Ni-MOFs were prepared by dissolving Ni salt and organic linkers in DMF by a solvothermal method. [29, 30] Scanning electron microscopy (SEM) images reveal that the Ni-MOFs consist of large round plates with an average diameter of around 100 μm and an average thickness of around 6 μm (Figure 1b-c and Figure S1, Supporting Information). Disk-like particles with a larger [31, 32] thickness could be obtained by increasing the growth temperature (Figure S1). X-ray diffraction (XRD) patterns of the obtained MOFs exhibit the same diffraction peaks as the theoretical pattern (Figure 1d),[29] indicating the successful synthesis of Ni-MOFs. Next, the 4 Ni-MOFs underwent a solvothermal process to obtain Ni3S4. The as-synthesized NM50-Ni3S4 shows a large nanoflake structure resembling the Ni-MOF template (Figure 1e). Electron energy loss spectroscopy (EELS) spectra (Figure 1f) confirm that nickel sulfide was obtained by this solvothermal method with a uniform distribution of Ni and S. High resolution transmission electron microscopy (HRTEM) images (Figure 1g) indicate that the nickel sulfide has good crystallinity. The lattice fringe distances were measured to be 0.536 nm, 0.493 nm, 0.551 and 0.242 nm, respectively, which are reminiscent of the cubic Ni3S4 phase (Table S1, Supporting Information). The magnified detail of the area highlighted in yellow and its corresponding power spectrum reveal that the structure of the selected region is in agreement with the cubic phase of Ni3S4 (space group: Fd3mS) with a = b = c = 0.94570 nm. XRD was employed to characterize the obtained nickel sulfide (Figure 2a, Figure S2, Supporting Information). The typical diffraction pattern of polydymite was observed. The diffraction peaks at 2θ of 26.6°, 31.2°, 37.9°, 47.0°, 49.9°, and 54.7° correspond to the (220), (311), (400), (422), (511), and (440) planes of cubic Ni3S4 (JCPDS: 00-047-1739), respectively.[33] The Ni3S4 sample derived from NM120 precursor shows an additional peak at ca. 30°, which may be due to the incomplete sulfurization caused by its larger thickness (ca. 75 μm). Raman spectra (Figure 2b) reveal that the vibrational bands at 224 cm −1, 286 cm−1, 337 cm−1 and 379 cm−1 in the low wavenumber region are attributed to Ni3S4,[34, 35] confirming that these samples are Ni3S4. Nitrogen ad/desorption isotherm and pore size distribution plots were used to investigate the porous structure of Ni3S4 catalysts (Figure 2c-d). The typical type-IV isotherms at a relative pressure of ca. 0.5-1.0 with an H3 hysteresis loop indicate a hierarchical porous structure and a broad pore-size distribution.[36-38] The specific surface areas of the obtained nickel sulfides are 34, 23 and 14 m2 g−1 for NM50-Ni3S4, NM80- Ni3S4 and NM120-Ni3S4, respectively. X-ray photoelectron spectroscopy (XPS) was employed to investigate the elemental composition and chemical state of the surface of these Ni3S4 samples (Figure 3, Figure S4, 5 and Table S2-S4, Supporting Information). XPS detected C, N, O, S, and Ni. The C, N, and O come from the residue of the Ni-MOFs linkers. The rather high oxygen content in the samples might originate from the surface oxidation of Ni3S4.[39] The Ni 2p spectrum of NM50-Ni3S4 was deconvoluted into eight peaks, which were assigned to the Ni 2p3/2, Ni 2p1/2, and satellite peaks (Figure 3b). For the Ni 2p3/2, the binding energies at 852.99, 852.93, 852.64, 852.64 and 852.80 eV and 856.57, 586.50, 855.59, 855.88 and 856.03 eV were assigned to Ni 2+ and Ni3+, respectively.[40, 41] The peaks at 860.35 ± 0.2 eV and 863.35 ± 0.2 eV were assigned to the satellite peaks (referred as ‘Sat.’ in the figure), which indicate the presence of nickel–oxygen species.[42-44] The peak area of the Ni3+ 2p was much larger than that of Ni2+ 2p, demonstrating the much higher content of Ni3+. Figure 3c summarizes the content of Ni2+ and Ni3+ in these samples, of which the Ni3+ content is ten times higher than that of Ni2+. The NM50-Ni3S4 exhibits the highest Ni content, as well as the highest Ni 3+ content. Ni3+ improves the electrophilicity of adsorbed oxygen and therefore benefits the formation of NiOOH through nucleophilic attack during OER and is generally regarded as the active site for OER.[21, 23] For the S 2p spectrum, the peaks at 161.3 eV, 162.6 eV, and 164.3 eV were attributed to the S 2p3/2 of S2−, S 2p1/2 of S2−, and the bridging S22− (referred to as ‘SO2− 4 ’), respectively (Figure 3d)[39, 45, 46]. The electrocatalytic activity of the Ni 3S4 catalysts for OER was investigated in 1.0 M KOH on a glassy carbon electrode (Figure 4, Figure S5, Supporting Information). The peaks at 1.38 V and 1.28 V vs. RHE refer to the redox reaction of Ni2+ to Ni3+ that goes forward and backward, respectively.[47-49] When scanning to more positive potentials than 1.5 V vs. RHE, the current density rapidly increases due to oxygen evolution. The overpotential (η) at a current density of 10 mA cm−2 was employed to evaluate the electrocatalytic activity of OER. [50] It is seen that all tested Ni3S4 catalysts show a lower overpotential than IrO2 at a current density of 10 mA cm−2 (Figure 4b). The NM50-Ni3S4 shows an overpotential of 0.307 V at a current density of 10 mA cm−2 with a Tafel slope of 67 mV dec−1 (Figure 4c). A Tafel slope between 40 and 120 6 mV dec−1 indicates that the OER on NM50-Ni3S4 may be controlled by the chemisorption of OH− and the formation of *-O− of the Krasil'shcikov path (Supporting Information).[51-53] The charge transfer resistance (Rct) of the Ni3S4 catalysts for the OER was evaluated by electrochemical impedance spectroscopy (Figure S6, Supporting Information) at an applied potential of 1.61 V vs. RHE. It can be seen that the NM50-Ni3S4 sample shows the smallest charge transfer resistance (19 Ω), resulting in the highest current density at high overpotential. The Faradaic efficiency of NM50-Ni3S4 for the OER was found to be around 99%, indicating that nearly all of the current was used for the evolution of oxygen (Figure S7, Supporting Information). The NM50-Ni3S4 catalyst was coated on a Ni foam electrode (NM50-Ni3S4/NF) and tested for OER (Figure 4d, Figure S8 and Figure S9 in Supporting Information). The contribution of the Ni foam to the OER can be ignored (Figure 4d). The NM50-Ni3S4/NF electrode shows much higher OER performance than IrO2 and Ni foam. The NM50-Ni3S4/NF electrode reached a current density of 10 mA cm−2, 50 mA cm−2, and 100 mA cm−2 at an overpotential of 0.257 V, 0.300 V, and 0.340 V, respectively, making it one of the best nickel sulfide OER catalysts reported in literature (Table S5, Supporting Information). The NM50-Ni3S4/NF electrode shows excellent long-term stability at 50 mA cm−2, while the overpotential of IrO2 for OER rapidly increased by 73 mV after 4000 s (Figure 4e). Finally, a full water-splitting electrolyzer was fabricated by using NM50-Ni3S4/NF as the anode catalyst and Pt/C as the cathode catalyst. A relatively low cell voltage of 1.51 V was achieved at 10 mA cm −2, and almost no degradation after operation at 20 mA cm−2 for 30 h (Figure 4f). To gain further insights into the electrocatalytic OER activity of the Ni 3S4, density functional theory (DFT) was employed to calculate the chemisorption free energies of OH− on the surface of Ni3S4 (Figure 5). There are two kinds of coordinated Ni ions in the crystal structure of Ni3S4, viz. the tetracoordinated Ni4 (a Ni atom coordinated with four S atoms, Ni2+) and hexacoordinated Ni6 (a Ni atom coordinated with six S atoms, Ni3+) (Figure 5a-b). 7 The (110) plane of Ni3S4 was selected to calculate the chemisorption free energies of OH−, due to the fact that this plane contains both kinds of Ni sites simultaneously. The hexacoordinated Ni6 exhibits an OH− chemisorption free energy of 0.16 eV, which is much lower than 1.01 eV for that of the tetracoordinated Ni4, revealing the strong chemisorption capacity of hexacoordinated Ni6 (Figure 5b). The electron will be transfered from the adsorbed OH− to the surface Ni sites during the OER process. The calculations show that the OH− adsorbed on the hexacoordinated Ni6 exhibits a more positive charge density (red area around OH−) than that of tetracoordinated Ni4, facilitating the electron transfer to the Ni surface sites. The O-H band adsorbed on the hexacoordinated Ni6 shows an enhanced activation thus yielding a shorter Ni-O band distance (1.827 Å) and a longer O-H band (0.983 Å) (Figure 5c-d). Therefore, the outstanding OER performance of the Ni 3S4 electrocatalyst is attributed to the following. Firstly, the Ni3+ is beneficial for the chemisorption of OH− on the surface of Ni3S4 which facilitates the electron transfer from the OH − to the surface Ni sites during OER. Secondly, the hierarchical porous structure provides more active sites per projected surface area and meanwhile enables faster mass transfer. 3. Conclusions A hierarchical porous Ni3+-rich Ni3S4 material was synthesized and tested as an OER electrocatalyst. The Ni3S4 architecture exhibits high electrocatalytic activity for OER (with an overpotential of 257 mV at 10 mA cm−2 and 300 mV at 50 mA cm−2) and excellent long-term stability (no degradation after 300 h at 50 mA cm−2). The outstanding OER performance is attributed to the high concentration of Ni3+ on the surface which favors the adsorption of OH−, and the increased number of active sites and the fast mass transport originating from the hierarchical porous architectures. This work thus provides a strategy for the design of highly active OER electrocatalysts with enriched Ni3+ sites and hierarchical porous architectures, which can be extended to other transition metal sulfides for various applications not limited to 8 OER. 4. Experimental section 4.1. Materials preparation Preparation of Ni-MOF: The Ni(HBTC)(4,4′-bipy)·3DMF metal-organic framework was synthesized by a mild solvothermal method modified from previous work. [29, 30] Typically, 8.73 g Ni(NO3)2·6H2O (99% purity, Acros Organics), 2.11 g trimesic acid (H3BTC, 98% purity, ABCR Germany), and 1.92 g 4,4-dipyridyl (98 % purity, Acros Organics) were dissolved in 200 mL N,N-dimethylformamide (DMF, ≥ 99%, Chem-Lab). This mixture was heated at 50 °C, 80 °C, and 120 °C for 72 h, respectively. The final light green products were washed with DMF and ethanol (EtOH, 99.9% purity, VWR France) and dried in air at 60 °C overnight. The Ni-MOFs synthesized at 50 °C, 80 °C, and 120 °C were denoted as NM50, NM80 and NM120, respectively. Preparation of Ni3S4 catalysts: The Ni-MOFs were converted into Ni3S4 through a solvothermal method in EtOH at 150 °C for 12 h using sodium sulfide (Na2S·9H2O, >98%, Acros Organics) as a sulfur source. The molar ratio of the Ni-MOFs to Na2S·9H2O was 1 : 8. After reaction, the obtained suspension was centrifuged and the solids were washed with EtOH and deionized water, and dried in air at 60 °C overnight. The resulting black powders were denoted as NM50-Ni3S4, NM80-Ni3S4 and NM120-Ni3S4, respectively. 4.2. Physicochemical characterizations X-ray diffraction (XRD) patterns were recorded on a Bruker AXS D8 diffractometer using Cu Kα radiation (λ = 0.15405 nm) and Ni filter with 2θ ranging from 5° to 80° with a step size of 0.02° (1.0 s per step) at 40 kV. Raman spectra were obtained from a LabRAM Aramis spectrometer with a 632.81 nm helium-neon laser. X-ray photoelectron spectroscopy (XPS) was performed with a PHI quantum-2000 (monochromatic Al Kα with 1486.6 eV operating at 15 kV and 300 W). The morphologies and microstructures were observed on a FEI Nova NanoSEM 450 scanning electron microscopy (SEM). High resolution transmission electron 9 microscopy (HRTEM) and scanning transmission electron microscopy (STEM) images were obtained on a FEI Tecnai F20 field emission gun microscope operated at 200 kV with a point- to-point resolution of 0.19 nm, which is equipped with high angle annular dark field (HAADF) and an electron energy loss spectroscopy (EELS) GATAN Quantum detectors. The energy dispersive spectroscopy (EDS) was measured at 15.0 kV. In order to improve the surface electronic conductivity of the MOFs, the MOF-covered metal plate was sputtered with a 10 nm layer of Pt. The pore structures of samples were probed by nitrogen adsorption/desorption isotherms at 77 K (Micromeritics ASAP 2020, US). Prior to the measurements, the samples were outgassed at 100 °C for 12 h. The specific surface area was determined by the Brunauer-Emmett-Teller (BET) model, while the pore size and volume were calculated by the density functional theory (DFT) method of the built-in software. 4.3. Electrochemical measurements All the electrochemical properties were characterized using an Autolab electrochemical workstation at room temperature (25 °C). A three-electrode system with a graphite plate (1 cm × 5 cm) as the counter electrode and a Hg/HgO electrode (Tianjin Aida Hengsheng Technology Co. Ltd., China) as the reference electrode was used. The obtained Ni 3S4 catalysts coated on a glassy carbon disk (5.0 mm in diameter, PINE) or Ni foam (Suzhou Taili Material Technology Co., Ltd., China) was employed as the working electrode. The OER polarization curve of IrO2 (P40V020, particles from Premetek Co.) was recorded for comparison. To evaluate the electrocatalytic activity of the OER, the cyclic voltammograms were recorded at 10 mV s−1 in the potential range between 0.98 and 1.73 V vs. RHE on a rotating disk electrode at a rotating speed of 1600 rpm in oxygen-saturated 1.0 M KOH solution. The long-term stability was carried out with a NM50-Ni3S4/NF electrode (NM50-Ni3S4 coated on Ni foam) by chronoamperometry at 50 mA cm−2 in 1.0 M KOH solution. The NM50-Ni3S4/NF electrode was also used as the anode in a water-splitting electrolyzer using Pt/C on Ni foam as the cathode, operating at 20 mA cm−2. The EIS measurements were recorded in a frequency 10 range from 100 kHz to 0.1 Hz with an amplitude of 5 mV (peak-to-peak) at an applied potential of 1.61 V vs. RHE. Supporting Information Supporting Information is available from the Wiley Online Library or from the author. Acknowledgements K. Wan is grateful to the Oversea Study Program of Guangzhou Elite Project. The Open Project Program of Provincial Key Laboratory of Clean Energy Materials, Longyan University (QJNY-201705) is also acknowledged. X. Zhang is grateful to the China Scholarship Council. J. Luo acknowledges the Research Foundation – Flanders (FWO) for a Research Project (G0B3218N) and a Research Grant (1529816N). Funding from the National Natural Science Foundation of China (No. 21776120), and “Minjiang Scholar” Program and key project (No. JZ160480) of Department of Education, Fujian Province, China is also acknowledged. ICN2 members acknowledge funding from Generalitat de Catalunya 2017 SGR 327 and the Spanish MINECO project VALPEC (ENE2017-85087-C3). ICN2 acknowledges support from the Severo Ochoa Programme (MINECO, Grant no. SEV-2013- 0295) and is funded by the CERCA Programme / Generalitat de Catalunya. Part of the present work has been performed in the framework of Universitat Autònoma de Barcelona Materials Science PhD program. T. 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Energy Mater. 2016, 6, 1601198. 15 Figure 1. a) schematic illustration of the synthesis method. b) SEM images of NM50. c) EDS elemental mapping images of the NM50. d) X-ray diffraction (XRD) patterns of the obtained Ni-MOFs. e) TEM images of NM50-Ni3S4. f) EELS chemical composition maps of NM50- Ni3S4 obtained from the area highlighted in red in the STEM micrograph. g) HRTEM micrograph of NM50-Ni3S4 (with the selected area highlighted in yellow) and its corresponding power spectrum. 16 Figure 2. a) X-ray diffraction (XRD) patterns of the Ni3S4 catalysts. b) Raman spectra of the Ni3S4 catalysts. c) Nitrogen ad/desorption isotherms of the Ni3S4 catalysts and d) the corresponding pore size distribution. 17 Figure 3. a) XPS survey spectra of the Ni3S4 catalysts. b) Ni 2p peaks and the peak fitting results of the NM50-Ni3S4 catalyst. c) Ni2+ and Ni3+ content extracted from XPS results. d) S 2p peak and the peak fitting results of the NM50- Ni3S4 catalyst. 18 Figure 4. a) Cyclic voltammograms of the Ni3S4 in 1.0 M KOH at a scan rate of 10 mV s−1 at 1600 rpm. b) The corresponding overpotentials at a current density of 10 mA cm −2 extracted from the backward scan from 1.73 V to 0.98 V vs. RHE. c) The corresponding Tafel plots of the Ni3S4 catalysts. d) Cyclic voltammograms of Ni 3S4 on Ni foam in 1.0 M KOH at a scan rate of 10 mV/s. e) Long-term stability test at 50 mA cm−2 in 1.0 M KOH (inset: photograph of the anode during O2 evolution and chronopotentiometric curves of IrO2 on Ni foam). f) Full water splitting at 20 mA cm−2 in 1.0 M KOH (anode: NM50-Ni3S4/NF, cathode: 10 wt. % Pt/C; insert: a photograph of anode and cathode during water splitting). 19 Figure 5. a) The crystal structure of Ni3S4. b) Chemisorption free energies of OH− on the (110) plane of Ni3S4; Insert: low-valence (Ni2+) tetracoordinated Ni4 and high-valence (Ni3+) hexacoordinated Ni6. c) The electron density difference of OH− adsorbed on Ni3+. d) The electron density difference of OH− adsorbed on Ni2+. 20 The Ni3S4 enriched with high-valence Ni3+ enhance the chemisorption of OH−, which facilitates the electron transfer from the OH− to the surface Ni sites during OER. The hierarchical porous structure enables high number of active sites, and faster mass transfer. High-valence Ni3+ K. Wan, J. Luo*, C. Zhou, T. Zhang, J. Arbiol, X. H. Lu*, B.-W. Mao, X. Zhang*, J. Fransaer* Hierarchical porous Ni3S4 with enriched high-valence Ni sites as a robust electrocatalyst for efficient oxygen evolution reaction 21 Copyright WILEY-VCH Verlag GmbH & Co. KGaA, 69469 Weinheim, Germany, 2018. Supporting Information Hierarchical porous Ni3S4 with enriched high-valence Ni sites as a robust electrocatalyst for efficient oxygen evolution reaction Kai Wan, Jiangshui Luo*, Chen Zhou, Ting Zhang, Jordi Arbiol, Xihong Lu*, Bing-Wei Mao, Xuan Zhang*, Jan Fransaer* 22 1. Electrode preparation The catalyst dispersion was prepared as follows: 5.0 mg of the catalyst was dispersed in 400 μL EtOH + 40 μL 5 wt. % Nafion (Nafion® DE 520, Sigma-Aldrich) + 60 μL deionized water by sonification for 30 min. For the GC electrode, 10 μL of the catalyst dispersion was dropped onto the glassy carbon electrode (GCE) by using a pipette and then dried at room temperature. The catalyst loading of the electrode was 500 μg cm−2. For the Ni foam electrode, 300 μL of the catalyst dispersion was dropped on a Ni foam (1 cm × 1 cm) to reach a catalyst loading of 3.0 mg cm −2. The Ni foam was sonificated successively in ethanol, acetone and diluted hydrochloric acid (4.0 M) for 10 min, respectively, to remove the possible grease and oxide on the surface. Then it was washed with deionized water to be neutral and subsequently dried under vacuum at 60 oC. The IrO2 and 10 wt. % Pt/C catalysts dispersions were also dropped onto the GCE or Ni foam with the same method of Ni3S4. The catalyst loading for GC electrode: 500 μg cm−2 (IrO2); for Ni foam electrodes: 3.0 mg cm−2 (IrO2, 10 wt. % Pt/C 3.0 mg cm−2). 2. Calibration of reference electrode potentials All potentials are calibrated to the RHE potential based on Nernst equation: ⊖ 𝐸RHE = 𝐸Hg/HgO + 0.0591 × pH (1) ⊖ where 𝐸Hg/HgO is the standard electrode potential of the Hg/HgO electrode (0.098 V vs. SHE), and T is room temperature (298.15 K). 3. iR-corrections All potentials are iR-corrected to compensate for the influence of solution resistances, using the following equation: 𝐸iR−corrected = 𝐸 − 𝑖𝑅 (2) where 𝑖 is the current and R is the uncompensated ohmic electrolyte resistance measured by 23 EIS. 4. Overpotential (η) The overpotential (η) is calculated using the following equation: 𝜂 = 𝐸iR−corrected − 1.23 (3) 5. Tafel slope The Tafel slope is determined by fitting polarization data to the Tafel equation: 𝜂 = a + b ∗ log|𝑗| (4) where η is the overpotential, b is the Tafel slope, and j is the current density. 6. The Krasil'shcikov path: Tafel slopes are usually used to describe the primary activities of OER. Generally, the commonly accepted mechanism for the OER in alkaline media is the Krasil'shcikov path, as shown in equations (5)−(8).[1-3] * + OH− → *-OH + e− Tafel slope: 120 mV dec−1 (5) *-OH + OH− → *-O− + H2O Tafel slope: 60 mV dec−1 (6) *-O− → *-O + e− Tafel slope: 40 mV dec−1 (7) 2*-O → 2* + O2 ↑ Tafel slope: 15 mV dec−1 (8) where * is an active site of the catalyst. 7. Faradaic efficiency The Faradic efficiency of the catalyst for the OER is defined as the ratio of the experimentally measured volume of oxygen evolved divided by the theoretical volume of O 2 based on the ideal gas law the Faraday law. The measurements of the volume of O2 were performed in a home-made electrolytic cell by water drainage method. A constant current of 0.1 A on an electrode with an area of 1 cm2 (1 cm × 1 cm) was applied for 300 s, 600 s, 900 s, and 1200 s, respectively, to produce O2 during the measurements. 24 Figure S1. SEM images of the Ni-MOFs: a-b) NM50, c-d) NM80, e-f) NM120. 25 Figure S2. a) X-ray diffraction (XRD) patterns of the Ni3S4 samples. b) Raman spectra of the Ni3S4 samples. c) Nitrogen ad/desorption isotherms of the Ni3S4 samples. d) The corresponding pore size distribution. In the synthesis process of Ni3S4, the sulfurization conditions were first investigated by using NM50 as the self-sacrificial templates. The molar ratios of the sulfur ion to Ni-MOF were fixed at 1 : 1, 4 :1 , and 8 : 1, and the obtained Ni3S4 were denoted as Ni3S4-1S, Ni3S4-4S, Ni3S4-8S, respectively. It can be seen from Figure S2a that the sample synthesized with a molar ratio of 1 : 1 (sulfur ion vs. Ni salt) exhibits a different XRD pattern, which may be due to the incomplete sulfurization when using a low molar ratio of S vs. Ni. When the molar ratio of sulfur to Ni increased gradually, a typical diffraction pattern of cubic phase of Ni3S4 (JCPDS: 00-047-1739) can be observed. 26 Figure S3. Elemental composition (at. %) of the Ni3S4 obtained from EDS plane scan. The Ni3S4 materials synthesized with a molar ratio (sulfur ion vs. Ni salt) of 4 : 1 and 8 : 1 exhibit almost the same content of O, Ni, and S. Therefore, a molar ratio of sulfur to Ni-MOF of 8 : 1 was selected for the synthesis of Ni3S4, and the resulting Ni3S4 samples are denoted as NM50-Ni3S4, NM80-Ni3S4, and NM120-Ni3S4, respectively. 27 Figure S4. Ni 2p peaks and the peak fitting results: a) NM50-Ni3S4, b) NM80-Ni3S4, c) NM120-Ni3S4. S 2p peak and the peak fitting results: d) NM50-Ni3S4, e) NM80-Ni3S4, f) NM120-Ni3S4. The Ni 2p spectrum was deconvoluted into eight peaks, which are assigned to the Ni 2p 3/2, Ni 2p1/2, and the satellite peaks. For the Ni 2p3/2, the binding energies at 852.99, 852.93, 852.64, 852.64 and 852.80 eV and 856.57, 586.50, 855.59, 855.88 and 856.03 eV were assigned to Ni2+ and Ni3+, respectively.[4, 5] And the peaks at 860.35 ± 0.2 eV and 863.35 ± 0.2 eV were assigned to the satellite peaks (referred as ‘Sat.’ in the figure), which indicate the presence of nickel–oxygen species.[6-8] The S 2p spectrum was deconvoluted into five peaks. The three peaks at around 161.3 eV, 162.6 eV, and 164.3 eV were attributed to the S 2p3/2 of S2−, S 2p1/2 of S2−, and the bridging S22− (referred to as ‘SO2− 4 ’), respectively. [9-11] The peaks at 168.4 eV and 169.7 eV were assigned to the sulfates resulting from the surface oxidation of the Ni 3S4.[11, 12] 28 Figure S5. Cyclic voltammograms of the Ni3S4 samples in 1.0 M KOH at a scan rate of 10 mV s−1 at 1600 rpm, showing the OER polarization curves (the molar ratios of the sulfur ion to Ni-MOF were fixed at 1:1, 4:1 and 8:1, and the obtained Ni3S4 were denoted as Ni3S4-1S, Ni3S4-4S, Ni3S4-8S, respectively.). 29 Figure S6. Nyquist plots and fitting results of the Ni 3S4 catalysts in the frequency range from 100 kHz to 0.1 Hz with an amplitude of 5 mV (peak-to-peak) at 1.61 V vs. RHE. Inset is the equivalent circuit, where Rs is the resistance of the electrolyte, Rct is the charge transfer resistance, and CPE the constant phase element. 30 Figure S7. The theoretical and experimental amounts of O2 evolved during the OER in 1.0 M KOH solution at 100 mA cm−2 as a function of time. The Faradaic efficiency of NM50-Ni3S4 for the OER was found to be around 99%, indicating that nearly all of the current was used for the evolution of oxygen. 31 Figure S8. SEM images: a-c) Ni foam, d-f) the NM50-Ni3S4/NF electrode. Clearly, the catalyst was coated on the surface of the Ni foam successfully. 32 Figure S9. Staircase chronopotentiometric curves of NM50-Ni3S4/NF electrode for OER recorded at different current densities in 1.0 M KOH. The staircase chronopotentiometric curves of a NM50-Ni3S4/NF electrode for OER at current densities from 10 to 200 mA cm−2 were recorded. The potentials at 10 mA cm−2 exhibit a small decrease even after the chronopotentiometric test at 200 mA cm −2, which may be due to the oxidation of the Ni foam thus increasing the OER activity. 33 Figure S10. a) EELS chemical composition maps of NM50-Ni3S4 after OER (100 h test at 50 mA cm−2) obtained from the area highlighted in red of the STEM micrograph; b) HR-TEM micrograph of NM50-Ni3S4 after OER, details of the area highlighted in orange and its corresponding power spectrum. It can be seen that the sample derived from theNi3S4 catalyst after the long duration test shows sheet-like nanostructures. The EELS mapping images indicate the existence of Ni and O after OER test. This result confirms the loss of S under oxidative environments of the OER, which is consistent with the previous reports.[13-16] The HR-TEM result reveals that the selected nanocrystal structure is in agreement with the NiO(OH) monoclinic phase (space group: C12/m1) with a = b = c = 8.1140 Å. The NiO(OH) lattice fringe distances were measured to be 0.217 nm, 0.209 nm and 0.245 nm at 56.00º and 66.36º which could be interpreted as the monoclinic NiO(OH) phase, visualized along its [010] zone axis. The high content of high-valence Ni3+ in the Ni3S4 catalyst facilitates the formation of active NiOOH, which benefits the chemisorption of OH− during OER. 34 Table S1. Bulk plane spacing distances and the angles between planes: experimental vs. theoretical. Spot Experimental (nm) Ni3S4 (Fd3mS) [011̅] 1 0.536 0.546 (111) 2 0.493(54.9º vs. Spot 1) 0.473 (54.74º) (200) 3 0.551 (112.35º vs. Spot 1) 0.546 (109.47º) (11̅1̅) 4 0.242 (54.09º vs. Spot 1) 0.236 (54.74º) (400) From the crystalline domain, the Ni3S4 lattice fringe distances were measured to be 0.536 nm, 0.493 nm, 0.551 and 0.242 nm, the latter three of which were at angles of 54.91º, 112.35º and 54.09º, which could be interpreted as the cubic Ni 3S4 phase, visualized along its [011̅] zone axis. 35 Table S2. Elemental composition (at. %) of the resultant nickel sulfides measured by XPS. Sample C N O S Ni NM50-Ni3S4 21.1 0.9 50.5 18.3 9.2 NM80-Ni3S4 19.3 0.9 54.2 17.3 8.3 NM120-Ni3S4 34.2 0.5 45.2 14.1 6.0 36 Table S3. The content of each Ni component (%) of the obtained Ni3S4 catalysts based on Figure S4, indicating the greater exposure of Ni3+ sites over Ni2+ sites at the surface of Ni3S4. Ni−O Ni2+ Ni3+ Sample species 2p1/2 2p3/2 Total 2p1/2 2p3/2 Total Total NM50-Ni3S4 1.49 2.99 4.48 16.80 33.61 50.41 45.11 NM80-Ni3S4 1.42 2.84 4.26 16.10 32.19 48.29 47.45 NM120-Ni3S4 1.06 2.12 3.18 16.85 33.71 50.56 46.26 37 Table S4. The content of each S component (%) of the Ni3S4 catalysts based on Figure S4. S2− S22− S−O species Sample 2p1/2 2p3/2 Total Total Total NM50-Ni3S4 12.15 24.29 36.44 8.74 54.82 NM80-Ni3S4 6.63 13.26 19.89 5.46 74.65 NM120-Ni3S4 4.40 8.79 13.19 3.52 83.29 38 Table S5. Comparison of the OER catalytic performance of the NM50-Ni3S4/NF electrode and the metal sulfides reported in literature (on Ni foam in 1.0 M KOH). 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