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1 Efficient hydrogen evolution catalysis using ternary pyrite-type cobalt phosphosulphide Miguel Cabán-Acevedo 1, Michael L. Stone 1, J. R. Schmidt 1, Joseph G. Thomas 1, Qi Ding 1, Hung- Chih Chang 2, Meng-Lin Tsai 2, Jr-Hau He 2 and Song Jin 1 * 1 Department of Chemistry, University of Wisconsin Madison, 1101 University Avenue, Madison, Wisconsin 53706, United States 2 Division of Computer, Electrical and Mathematical Sciences and Engineering King Abdullah University of Science and Technology, Thuwal , Saudi Arabia * Address: jin@chem.wisc.edu NATURE MATERIALS 1

2 Supplementary Figures and Discussion 1. Raman Spectra for a Mixed CoPS and CoS 2 Sample Supplementary Fig. S1. Confocal micro-raman spectrum for a CoPS film on graphite that was synthesized by thermal conversion using a precursor mixture of 1:3 phosphorus to sulfur ratio. The product can be clearly identified as a mixture of CoPS and CoS 2. The peak positions for the E g and A g Raman modes for cobalt pyrite (CoS 2 ) were shown as vertical dash lines for comparison. 2. Cobalt 2p XPS of CoPS in Comparison with CoS 2 Supplementary Fig. S2. X-ray photoelectron spectroscopy (XPS) measurements of cobalt 2p for CoPS and CoS 2 films on graphite. CoPS film shows only one narrow set of Co 2p doublets, 2 NATURE MATERIALS

3 SUPPLEMENTARY INFORMATION while CoS 2 film shows multiplex splitting for the Co 2p doublets. The absence of multiplet splitting for Co 2p in CoPS is indicative of a spin paired electronic configuration for cobalt and an oxidation state of Co 3+ S1,S2. Therefore, XPS supports that the obtained CoPS products are indeed a distinctive ternary alloy phase since the electronic configuration and an oxidation state is different from that of CoS 2 (with unpaired electron; Co 2+ ). The CoS 2 film on graphite was synthesized through thermal conversion S3. 3. Structural Characterization of CoPS Film after HER Test Supplementary Fig. S3. SEM images of the CoPS film on graphite substrate after 48 h HER operation at 10 ma/cm 2 and (b) after subsequent stirring in nanopure water for another 24 h at 2000 rpm, and (c) the corresponding EDS spectra. There was little change in the film morphology throughout this process. The high EDS counts for K, S and O after 48 h HER can be attributed to the formation of a solid electrolyte interface (SEI) composed of double layer ions (such as K + and SO 2-4 ), since their counts dramatically decreased after stirring in nanopure water. Therefore, the apparent higher stoichiometry of sulfur when solving for CoPS from EDS can be attributed to the presence of sulfate ions in this SEI. NATURE MATERIALS 3

4 4. Tafel Plots of Various CoPS Samples after Surface Area Normalization Supplementary Fig. S4. Tafel plots for various CoPS electrodes after normalization by the electrochemically active surface area relative to CoPS film. 4. Optical Absorption of a CoPS Film Supplementary Fig. S5. UV-Vis-NIR integrating sphere measurement for the internal absorbance of CoPS film on borosilicate glass prepared after thermal conversion of 10 nm Co film. The CoPS film thickness was measured to be ~40 50 nm using atomic force microscopy. 6. Surface Co 2p, P 2p and S 2p Chemical Species for CoS 2, CoPS and CoP x. Figure S6 shows XPS measurements of Co 2p, P 2p and S 2p for CoP x in comparison to CoPS and CoS 2 demonstrating that all the surface chemical species for CoPS are different from CoP x 4 NATURE MATERIALS

5 SUPPLEMENTARY INFORMATION and CoS 2. Note that for CoP x, main peaks (orange doublets) and minor peaks (purple doublets) correspond to bulk CoP and surface CoP 2 layer. These XPS results clearly show that the surface of CoPS catalyst does not contain minor CoP or CoP 2 chemical species. Supplementary Fig. S6. X-ray photoelectron spectroscopy (XPS) measurements of (a) Co 2p, (b) P 2p and (c) S 2p peak for CoP x, CoPS and CoS 2. Note that for CoP x, main peaks (orange doublets) and minor peaks (purple doublets) correspond to a bulk CoP and a surface CoP 2 layer. 7. Structural Characterization and Surface Co 2p, P 2p and S 2p Chemical Species for CoP x, CoP x H 2 S, CoP x S, CoS 2 P+H 2. Figure S7a shows the powder X-ray diffraction (PXRD) patterns for CoP x, CoP x H 2 S and CoP x S in comparison with the standard patterns for CoP (Pnma, a = 5.076Å, b = 3.277Å, c = 5.599Å, JCPDS # ), CoP 2 (P2 1 /c, a = 5.61Å, b = 5.591Å, c = 5.643Å, JCPDS # ) and CoS 2 (Pa-3, a = 5.538Å, JCPDS # ). All PXRD peaks for CoP x and CoP x H 2 S can be indexed to CoP and CoP 2 phase. On the other hand, PXRD peaks for CoP x S can be indexed to CoP 2 and CoS 2 phase. NATURE MATERIALS 5

6 Supplementary Fig. S7. (a) PXRD patterns for CoP x, CoP x H 2 S and CoP x S film on borosilicate glass in comparison to the standard pattern for CoP (JCPDS # ), CoP 2 (JCPDS # ) and CoS 2 (JCPDS # ). (Note that CoP 2 is a metastable phase related to the marcasite crystal structure.) (b-d) XPS measurements of (b) Co 2p, (c) P 2p and (d) S 2p peaks for CoP x S and CoP x H 2 S in comparison to Co 2p, P 2p and S 2p peaks for CoPS. The vertical dash lines correspond to the peak positions for the (b) main Co 2p 3/2 chemical species for CoP x, (c) main P 2p 3/2 chemical species for CoP x and (c) bulk S 2p 3/2 species for CoS 2. Figure S7b-d shows the X-ray photoelectron spectroscopy (XPS) measurements of Co 2p, S 2p and P 2p peaks for CoP x, CoP x H 2 S and CoP x S film on graphite. In CoP x S the Co 2p peak fitting is quite complex, however, it can be seen by comparison with Figure S6a that the main Co 2p peak (red doublets) supports the presence of CoS 2 on the surface. The P 2p peak and S 2p peak for CoP x S show a high concentration of metaphosphides (wine doublets in Figure S7c) and polysulfide species (blue doubles in Figure S7d). On the other hand, Co 2p and P 2p species for 6 NATURE MATERIALS

7 SUPPLEMENTARY INFORMATION CoP x H 2 S resemble the chemical species for CoP x (Figure S6a,b), but are slightly shifted towards higher binding energies suggestive of doping. CoP x H 2 S also shows the absence of polysulfide chemical species for the S 2p peak (Figure S7d). Overall, the surface chemical species for CoP x S or CoP x H 2 S does not resemble any chemical species seen for CoPS (Figure S6). 8. Catalytic HER Electrochemical Properties of CoP x, CoP x H 2 S and CoP x S Film and NW Electrodes. We measured the electrochemical characteristics of the CoP x, CoP x H 2 S and CoP x S film and NW electrodes (on graphite) corresponding to the HER catalytic performance using 0.5 M H 2 SO 4 and rotating disk electrode (RDE) at a rate of 2000 rpm, in comparison to CoPS film and CoPS NWs. Figure S8a shows that the HER catalytic performance of CoPS electrodes is superior to CoP x, CoP x H 2 S and CoP x S electrodes for the respective electrode morphology (film or NWs). CoP x H 2 S electrodes show a small improvement in HER catalytic performance in comparison to CoP x electrodes which can be attributed to anion substitution doping. In contrast, CoP x S electrodes show a significant decrease in performance that could be attributed to the presence of CoS 2, metaphosphides and polysulfides on the electrode surface. These results for the HER catalytic performance of CoP x, CoP x H 2 S and CoP x S electrodes (summarized in Table S1) demonstrate that the high catalytic activity of CoPS electrodes cannot be explained by a hypothetical presence of CoP-related minor phases. Furthermore, these results demonstrate that our synthetic approach is a unique strategy for preparing ternary-pyrite cobalt phosphosulfide (CoPS) HER catalyst since the synthesis of this ternary compound cannot be achieved through anion substitution doping strategies for CoP x. NATURE MATERIALS 7

8 Supplementary Fig. S8. (a) J-V curves after IR correction show the catalytic HER performance of the CoP x, CoP x H 2 S and CoP x S film and NW on graphite electrodes (film, open squares; NWs, open triangles) in comparison to CoPS film (red open squares) and CoPS NW (red open triangles) electrodes. (b) Tafel plots for the new data presented in panel a. Table S1. Summary of the electrochemical properties of CoP x, CoP x H 2 S, CoP x S, CoS 2 P and CoS 2 P+H 2 electrodes in comparison with CoPS electrodes. η (mv vs η (mv vs Sample RHE) for J = 10 RHE) for J = 20 R S (Ω cm 2 ) Tafel slope (mv/decade) J 0,geometrical (μa/cm 2 ) ma/cm 2 ma/cm 2 CoPS NWs CoPS film CoP x NWs CoP x Film NATURE MATERIALS

9 SUPPLEMENTARY INFORMATION CoPx H 2 S NWs CoPx H 2 S Film CoPx S NWs CoPx S Film CoS 2 P NWs CoS 2 P Film CoS 2 P+H 2 NWs CoS 2 P+H 2 Film Structural Characterization and Surface Co 2p, P 2p and S 2p Chemical Species for CoS 2 P and CoS 2 P+H 2. Figure S9a shows the powder PXRD patterns for CoS 2, CoS 2 P and CoS 2 P+H 2 in comparison with the standard patterns for CoP (Pnma, a = 5.076Å, b = 3.277Å, c = 5.599Å, JCPDS # ), CoP 2 (P2 1 /c, a = 5.61Å, b = 5.591Å, c = 5.643Å, JCPDS # ), Co 1-x S (P6 3 /mmc, a = 3.38Å, b = 3.38Å, c = 5.185Å, JCPDS # ) and CoS 2 (Pa-3, a = 5.538Å, JCPDS # ). The PXRD pattern for CoS 2 P shows mostly CoS 2 diffraction peaks. While the PXRD pattern for CoS 2 P+H 2 shows mostly CoS 2, and some minor Co 1-x S and CoP diffraction peaks. Figure S9b-d shows the XPS measurements of Co 2p, S 2p and P 2p for CoS 2 P and CoS 2 P+H 2 film on graphite in comparison with CoPS. In CoS 2 P, the Co 2p and P 2p peaks resemble CoP x, but the P 2p peak shows a high concentration of metaphosphides (wine doublets NATURE MATERIALS 9

10 in Figure S9c). The bulk S 2p species (red doublets in Figure S9d) for CoS 2 P are significantly shifted towards lower binding energy in comparison to CoS 2. On the other hand, Co 2p, P 2p and S 2p species for CoS 2 P+H 2 suggest the formation of a surface CoP x layer, primarily due to the similarities between the main P 2p species for CoS 2 P+H 2 and the minor P 2p species for CoP x (see purple doublets for CoP x in Figure S6b or for CoP x H 2 S in Figure S7c). Overall, the surface chemical species for CoS 2 P and CoS 2 P+H 2 do not resemble those seen for the ternary compound CoPS (Figure S6). Supplementary Fig. S9. (a) PXRD patterns for CoS 2 P and CoS 2 P+H 2 film on borosilicate glass in comparison with the standard patterns for CoP (JCPDS # ), CoP 2 (JCPDS # ), Co 1-x S (JCPDS # ) and CoS 2 (JCPDS # ). (b-d) XPS measurements of (b) Co 2p, (c) P 2p and (d) S 2p peaks for CoS 2 P and CoS 2 P+H 2 in comparison with the Co 2p, P 2p and S 2p peaks for CoPS. The vertical dash lines correspond to the peak positions for the (b) bulk Co 2p 3/2 species for CoS 2, (c) main P 2p 3/2 species for CoP x and (d) bulk S 2p 3/2 species for CoS NATURE MATERIALS

11 SUPPLEMENTARY INFORMATION 10. Catalytic HER Electrochemical Properties of CoS 2 P and CoS 2 P+H 2 Film and NW Electrodes. Lastly, we measured the electrochemical characteristics of the CoS 2 P+H 2 and CoS 2 P film and NW electrodes (on graphite) corresponding to the HER catalytic performance using 0.5 M H 2 SO 4 and rotating disk electrode (RDE) at a rate of 2000 rpm, in comparison with CoPS film and CoPS NWs. Figure S10a shows that the HER catalytic performance of CoPS electrodes is superior to CoS 2 P+H 2 and CoS 2 P electrodes for the respective electrode morphology (film or NWs). CoS 2 P+H 2 electrodes show HER catalytic performance very similar to the CoP x electrodes which can be attributed to the presence of CoP x phases at the surface. In contrast, CoS 2 P electrodes show significantly lower HER catalytic performance in comparison to CoS 2 P+H 2 electrodes, which can be attributed to the high concentration of metaphosphides on the electrode surface. These results for the HER catalytic performance of CoS 2 P+H 2 and CoS 2 P electrodes (summarized in Table S2) also indicate that the high catalytic activity of CoPS electrodes cannot be explain by a hypothetical presence of CoP-related minor phases. Furthermore, these results demonstrate that our synthetic approach is a unique strategy for preparing the outstanding ternary-pyrite cobalt phosphosulfide HER catalyst since the synthesis of this ternary compound cannot be achieved readily through anion substitution doping strategies for CoS 2. NATURE MATERIALS 11

12 Supplementary Fig. S10. (a) J-V curves after IR correction show the catalytic HER performance of the CoS 2 P and CoS 2 P+H 2 film and NW on graphite electrodes (film, open squares; NWs, open triangles) in comparison to CoPS film (red open squares) and CoPS NW (red open triangles) electrodes. (b) Tafel plots for the new data presented in panel a. References: S1 S2 S3 Moulder, J. F., Stickle W, F., Sobol, P. E. & Bomben, K. D. Handbook of X-ray Photoelectron Spectroscopy. (Perkin-Elmer Corp, 1992). Biesinger, M. C., Lau, L. W. M., Gerson, A. R. & Smart, R. S. C. Resolving surface chemical states in XPS analysis of first row transition metals, oxides and hydroxides: Sc, Ti, V, Cu and Zn. Appl. Surf. Sci. 257, (2010). Faber, M. S., Dziedzic, R., Lukowski, M. A., Kaiser, N. S., Ding, Q. & Jin, S. High- Performance Electrocatalysis Using Metallic Cobalt Pyrite (CoS 2 ) Micro- and Nanostructures. J. Am. Chem. Soc. 136, (2014). S4 Xia, X., Chao, D., Qi, X., Xiong, Q., Zhang, Y., Tu, J., Zhang, H. & Fan, H. J. Controllable Growth of Conducting Polymers Shell for Constructing High-Quality Organic/Inorganic Core/Shell Nanostructures and Their Optical-Electrochemical Properties. Nano Lett. 13, (2013). 12 NATURE MATERIALS