Supporting Information. Direct Observation of Structural Evolution of Metal Chalcogenide in. Electrocatalytic Water Oxidation

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1 Supporting Information Direct Observation of Structural Evolution of Metal Chalcogenide in Electrocatalytic Water Oxidation Ke Fan *,, Haiyuan Zou, Yue Lu *,, Hong Chen, Fusheng Li, Jinxuan Liu, Licheng Sun,, Lianpeng Tong, Michael F. Toney, Manling Sui, Jiaguo Yu *, State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan , P. R. China Institute of Microstructure and Properties of Advanced Materials, Beijing University of Technology, Beijing , P. R. China SSRL, SLAC National Accelerator Laboratory, Stanford University, Menlo Park, California 94025, USA State Key Lab of Fine Chemicals, DUT-KTH Joint Education and Research Center on Molecular Devices, Dalian University of Technology, Dalian , P. R. China Department of Chemistry, KTH Royal Institute of Technology, Stockholm, Sweden School of Chemistry and Chemical Engineering, Guangzhou University, Guangzhou , China * 1

2 Figure S1. XRD patterns of ZIF-67 and the as-prepared CoS x. ZIF-67 shows typical peaks in its XRD pattern, but no obvious peak can be observed in the XRD pattern of the as-prepared CoS x, indicating its amorphous phase. Inset shows the SEM images of ZIF-67 (a) and as-prepared CoS x (b). 2

3 Figure S2. CV curves of CoS x (left) and RuO 2 (right) in 1 M KOH with different scan rates ( mv s -1 ). 3

4 Figure S3. Nyquist plots of CoS x and RuO 2 in 1 M KOH with bias of 350 mv overpotential. 4

5 Figure S4. The near edge X-ray absorption spectra (NEXAS) of Co metallic foil and CoS x active species after OER. 5

6 Figure S5. In situ FTIR of CoS x with 1 ma anodic current in 1 M KOH. 6

7 Figure S6. The XRD pattern (a) and SEM image (b) of the directly synthesized CoOOH. 7

8 Figure S7. The OER performance of the directly synthesized CoOOH. Figure S8. The XRD pattern (a), SEM images of CoSe 2 before (b) and after (c) OER. 8

9 Figure S9. The XPS spectra of Se 3d (a), O 1s (b) and Co 2p (c) of CoSe 2 before and after OER. (d) OER performance of CoSe 2. 9

10 Figure S10. XRD patterns of NGF, ZIF-67/NGF and the as-prepared CoS x /NGF. In the XRD patterns, a broad diffraction peak of NGF is identified at o from the diffraction of reduced graphene oxide, indicating the poor ordering of graphene sheets along their stacking direction and the framework of NFG constructed by few-layer stacked graphene sheets. The obtained ZIF-67/NGF composite shows the both characteristic peaks of ZIF-67 and NGF in its XRD pattern, implying that ZIF-67 has been successfully loaded on NGF. After converting ZIF-67/NGF to CoS x /NGF by hydrothermal treatment, the resultant CoS x /NGF hybrid shows no obvious diffraction peak, suggesting its amorphous phase similar to CoS x. 10

11 Figure S11. (a) SEM image of NGF, inset: SEM of a part of a broken NGF, showing its hollow interior structure. (b) SEM image of ZIF-67/NGF with low magnification. (c) SEM image of ZIF- 67/NGF with higher magnification, inset: SEM image of ZIF-67 loaded on NGF. (d) SEM image of the as-prepared CoS x /NGF, inset: SEM image of CoS x loaded on NGF, the broken ones shows their hollow structure. 11

12 Figure S12. XPS of CoS x /NGF before and after OER measurement. (a) Survey. The appearance of Sn 3d and Ca 2p in the post-oer CoS x /NGF is from the FTO-glass substrate, which is used as the support for post-catalytic CoS x /NGF. The XPS signals of S 2p and S 2s disappear after OER measurement, suggesting that sulfide is almost completely removed from the surface during the electrochemical oxidation process. (b) Co 2p. After OER measurement, the peaks of Co 2p shift to higher binding energies apparently compared to the pre-oer ones, which can be ascribed to the formation of Co-O bond in the electrocatalyst. 12

13 Figure S13. Upper: CV curves of CoS x /NGF (left) and NGF (right) in 1 M KOH with different scan rates ( mv s -1 ). Bottom: ΔJ (=J a -J c ) of the electrodes plotted against scan rates. The slopes (2C dl ) were used to represent ECSA. The obtained ECSAs of NGF, RuO 2, CoS x and CoS x /NGF are 0.4, 1.8, 10.6 and 45.5 mf cm -2, respectively. 13

14 Figure S14. Nyquist plots of NGF, RuO 2, CoS x and CoS x /NGF in 1 M KOH with bias of 350 mv overpotential. 14

15 Figure S15. The equivalent circuit for the OER catalysts. In the equivalent circuit for the OER catalysts, 1 R Ω presents the uncompensated resistance of solution, C dl describes the double-layer capacitance. The Faradaic OER is treated by R p, R s, and C ϕ. R p (polarization resistance) is related to the overall rate of the OER, including the charge-transfer resistance. R s is related to the ease with which one or more surface intermediates are formed. The Faradaic resistance is simply defined as R far = R p + R s. C Φ presents the change in charged surface species as the OER proceeds. 15

16 Figure S16. The stability of CoS x /NGF with 10 ma cm -2 anodic current density in 1 M KOH electrolyte. 16

17 Figure S17. Schemetic diagram to illustrate the operation of the in situ TEM observation for OER process of CoS x. A piece of carbon grad is fixed on the anode and process OER for different times. Then the carbon grid is took down into a liquid environmental TEM to observe the morphology evolution of CoS x as shown in Figure 4. 17

18 Table S1. Comparison of the OER activity of CoS x /NGF with the reported state-of-the-art Co-based electrocatalysts. η onset : onset overpotential. η 10 : the required overpotential to achieve 10 ma cm -2 catalytic current density. N.A.: not available. Catalysts Mass loading (mg cm -2 ) η onset (mv) η 10 (mv) Tafel slope (mv dec - 1 ) TOF (s -1 ) at η = 300 mv Electrolyte Ref. CoS x /NGF M KOH This work Co 3 S 4 nanosheets M KOH 2 Co 3 O 4 /N-rGO 0.24 N.A N.A. 1.0 M KOH 3 Co nanoparticles 0.20 N.A. 390 N.A. N.A. 0.1 M KOH 4 CoS 4.6 O N.A N.A 1.0 M KOH 5 CoP/rGO N.A 1.0 M KOH 6 Co-P/Pi thin film N.A. 1.0 M KOH 7 Co 3 O M KOH 8 Nanoporous hollow Co 3 S 4 nanosheets N.A N.A. 1.0 M KOH 9 Au@Co 3 O N.A. 0.1 M KOH 10 Co-P/NC M KOH 11 Ni-Co-mixed-oxide nanocages N.A N.A. 1.0 M KOH 12 18

19 Porous Co 3 O M KOH 13 CoFe 2 O N.A M KOH 14 NiCo-LDH 0.07 N.A M KOH 15 Table S2. The fitting parameters of Figure S15 for the OER catalysts at η = 350 mv. Samples C dl (μf cm -2 ) R p (Ω) R s (Ω) C Φ (μf cm -2 ) CoS x /NGF CoS x RuO NGF The fitting results are shown in Table S2. Comparing with the other samples, CoS x /NGF exhibits the smallest R p, indicating its facilitated overall rate of OER and charge transfer ability. Meanwhile, its smallest R s indicates the relatively easily formed surface intermediates, i.e., Co(OH) 2. Therefore, CoS x /NGF possesses the lowest Faradaic resistance R far. The largest C Φ of CoS x /NGF also means the most efficient charge relaxation, indicating the tremendous species change on the surface for OER. These merits render CoS x /NGF the best for OER in our samples. Interestingly, comparing with RuO 2, CoS x has a similar R p, and a slightly smaller R s. This result implies the similar Faradaic resistance between these two samples (273 Ω and 301 Ω for CoS x and RuO 2, respectively), although CoS x shows better OER performance than RuO 2. Apparently, the Faradaic resistance is not the main contributor to the better OER of CoS x. Thus, the better OER from CoS x can be ascribed to the much larger C Φ, which indicates the facile and more efficient charge relaxation for OER than those of RuO 2, favoring the structural evolution on the surface of CoS x during OER measurement. 19

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