Supporting Information

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1 Supporting Information Influence of the Fe : Ni Ratio and Reaction Temperature on the Efficiency of (Fe x Ni 1-x ) 9 S 8 Electrocatalysts Applied in the Hydrogen Evolution Reaction Stefan Piontek a, Corina Andronescu b,c, Aleksandr Zaichenko d, Bharathi Konkena b, Kai junge Puring a,e, Bernd Marler f, Hendrik Antoni g, Ilya Sinev h, Martin Muhler g, Doreen Mollenhauer d, Beatriz Roldan Cuenya h,i, Wolfgang Schuhmann b, Ulf-Peter Apfel a,e* a Ruhr-Universität Bochum, Inorganic Chemistry I, Universitätsstrasse 150, D Bochum (Germany) b Ruhr-Universität Bochum, Analytical Chemistry - Center for Electrochemical Sciences (CES), Universitätsstrasse 150, D Bochum (Germany) c University Politehnica of Bucharest, Department of Bioresources and Polymer Science, 1-7 Gh. Polizu Street, , Bucharest (Romania) d Justus-Liebig-Universität Gießen, Institute of Physical Chemistry, Heinrich-Buff-Ring 17, D Gießen (Germany) e Fraunhofer UMSICHT, Osterfelder Strasse 3, D Oberhausen f Ruhr-Universität Bochum, Department of Geology, Mineralogy and Geophysics, Universitätsstrasse 150, D Bochum (Germany) g Ruhr-Universität Bochum, Laboratory of Industrial Chemistry, Universitätsstrasse 150, D Bochum (Germany) h Experimental Physics IV, Ruhr-Universität Bochum, Universitätsstrasse 150, D Bochum (Germany) i Department of Interface Science, Fritz-Haber Institute of the Max Planck Society, Berlin, Germany Author Information Corresponding Author * ulf.apfel@rub.de ORCID Corina Andronescu: Martin Muhler: Beatriz Roldan Cuenya: X Wolfgang Schuhmann: Ulf-Peter Apfel:

2 Figure S1. Electrode preparation from (FexNi1-x)9S8 (1), grinded powder (2), compressed pellet (3) to a bulk electrode (4). The pellets have a diameter of 3 mm.

3 Figure S2. (a) Potential difference between E Ag/AgCl sat. KCl at X C (X = 0-90) and at 25 C and (b) E Ag/AgCl sat. KCl as temperature function.

4 Figure S3. SEM images of (a) compressed pentlandite and (b-c) polished pellet with different magnification (20 µm and 200 nm). All samples reveal only low roughness.

5 Figure S4. SEM images of the ground powder of the different composites (Fe x Ni 1-x ) 9 S 8.

6 Fe Ni S Fe 3 Ni 6 S 8 Fe 4 Ni 5 S 8 Fe 4.5 Ni 4.5 S 8 Fe 5 Ni 4 S 8 Fe 6 Ni 3 S 8 Figure S5. SEM EDS elemental mapping of (Fe x Ni 1-x ) 9 S 8 samples of the prepared pellets. The images reveal a homogeneous distribution of the elements on the surface.

7 Figure S6. Electron dispersive X-ray (EDX) spectra of (Fe x Ni 1-x ) 9 S 8 reveal that the expected nominal composition was achieved.

8 Table S1. Chemical composition of the samples calculated from EDX spectra and ICP-OES. Sample Composition of the samples according to EDX Fe : Ni ratio according to EDX Fe : Ni ratio according to ICP-OES Ideal Fe : Ni ratio Fe 3 Ni 6 S 8 Fe 2.9 Ni 5.8 S Fe 4 Ni 5 S 8 Fe 4.2 Ni 4.9 S Fe 5 Ni 4 S 8 Fe 5.3 Ni 3.8 S Fe 6 Ni 3 S 8 Fe 6.4 Ni 2.6 S Fe 4.5 Ni 4.5 S 8 Fe 4.2 Ni 4.1 S

9 Figure S7. Mössbauer spectra of (Fe x Ni 1-x ) 9 S 8 recorded at 298 K. Mössbauer spectra were recorded at K using a 57Co radiation source in a Rh matrix in a SeeCo constant acceleration spectrometer. The spectrometer is equipped with a temperature controller (JANIS) maintaining temperatures within ± 0.1 K. Isomer shifts are referred to α-fe metal at room temperature. Data were fit with a sum of Lorentzian quadrupole doublets using a least-square routine with the WMOSS program.

10 Figure S8. Thermal analysis of FeNi-sulfides via differential scanning calorimetry (DSC) with 10 K/min. (a) DSC thermograms form RT to 1000 C and (b) vice versa revealing two significant phase transitions at 600 C and 860 C.

11 Ni 2p Counts per second (arb. units) O KLL O 1s Fe LMM Fe 2p Ni LMM C 1s S 2s S 2p Binding energy (ev) Figure S9. Survey X-ray phototelectron spectrum (XPS) of Fe 3 Ni 6 S 8 with main photoemission and Auger lines indicated.

12 O KLL Ni 2p Intensity (a.u.) Fe LMM Fe 2p O 1s Ni LMM S 2s C 1s S 2p Binding energy (ev) Figure S10. Survey X-ray phototelectron spectrum (XPS) of Fe 4.5 Ni 4.5 S 8 with main photoemission and Auger lines indicating a carbon content of ~ 8 % in the sample.

13 Table S2. Overall sample surface composition in atomic % measured by XPS. Sample C O S Fe Ni Fe 3 Ni 6 S Fe 4 Ni 5 S Fe 5 Ni 4 S Fe 6 Ni 3 S Fe 4.5 Ni 4.5 S

14 Figure S11. a) Nyquist plots at HER overpotentials (η=400 mv) in 0.5 M H 2 SO 4 for all (Fe x Ni 1x ) 9 S 8 composites b) Armstrong equivalent circuit. Nyquist plots from Figure S11a were fitted with the Armstrong equivalent circuit (Fig. S11b) which is usually used to fit EIS spectra registered under hydrogen evolution reactions. In the model, R s is the electrolyte resistance, R ct - charge transfer resistance, CPE dl and Cp double layer capcaitance and R p - the Faradaic resistance of the electrodesorption and/or recombination reactions. In this case, the circuit was modified, for CPE dl we used a constant phase element instead of a capacitor. [1] According to the values presented in Tabel S3, Ni 4.5 Fe 4.5 S 8 shows lower values for both R ct and R p, compared with the other compounds. Tabel S3. Parameters obtained by fitting the Nyquist spectra using the Armstrong equivalent circuit. Sample R s (Ω) CPE dl (mmho) n dl R ct (Ω) C p (µf) R p (Ω) Fe 3 Ni 6 S Fe 4 Ni 5 S Fe 4.5 Ni 4.5 S E Fe 5 Ni 4 S Fe 6 Ni 3 S E

15 Figure S12. Electrochemical surface area (ECSA) plots of FeNiS composites at varying scan rates from mv s -1 in 0.5M H 2 SO 4. Cyclic voltammograms of (a) Fe 3 Ni 6 S 8, (b) Fe 4 Ni 5 S 8, (c) Fe 5 Ni 4 S 8 and (d) Fe 6 Ni 3 S 8.

16 Figure S13. Example of estimation for the electrochemical surface area (ECSA) at Fe 4.5 Ni 4.5 S 8 in 0.5M H 2 SO 4.

17 During the activation process, an increased catalytic activity was registered and we investigated if the possible Pt contamination of the surface is causing this. Therefore, similar experiments were performed using glassy carbon electrode or gold plate as counter electrode. As presented in Figure S14, similar increase in activity for the Fe 4.5 Ni 4.5 S 8 was observed also when instead of Pt, a gold counter electrode or glassy carbon electrode was used during the application of 0.8 V overpotential. Similar increase of the catalyst activity was observed also when a similar experiment was performed in a twocompartment cell separated with a NAFION membrane. Figure S14. Comparison of controlled potential coulometry (CPC) measurements of a Ni 4.5 Fe 4.5 S 8 electrode at an applied potential of -0.8 V using a Pt grid (blue) or Au plate (red) as counter electrode (a) and at an applied potential of -0.6 V using GC as counter electrode (b) in one-cell compartment. Measurements longer than 2 hours were not performed with the GC electrode due to deterioration of the electrode surface. Controlled potential coulometry experiments at -0.6 V vs RHE of Ni 4.5 Fe 4.5 S 8, using Pt as counter electrode, in two compartment cell separated with a NAFION membrane (c). While the scan quality is influenced by the formation of hydrogen bubbles under HER conditions, no significant effect of the counter electrode material is observed on the HER activity of the pentlandite electrode. This behavior suggests that Pt-deposition is not the reason for the changes observed herein.

18 a Pt 4d C 1s b S 2s Cu 3p S 2p Pt 4f Cu 3s Intensity (Counts per sec.) Cu 2p Ni 2p Cu LMM Fe 2p O 1s C 1s N 1s Binding Energy (ev) Figure S15. Survey X-ray phototelectron spectrum (XPS) of bulk Fe 4.5 Ni 4.5 S 8 after 20 h activation at applied potential -0.6 V vs RHE in 0.5M H 2 SO 4 with main photoemission and Auger lines indicated. Magnification of Pt 4d region without attendance of Pt (a, b). Note: contrary to experiments performed with iron/nickel sulfide powders, a solid piece of Fe 4.5 Ni 4.5 S 8 was utilized for the experiments to allow for a sufficiently high surface to be investigated. The presence of Cu stems from the contacting of the bulk material (stone) with a Cu-band (b).

19 Figure S16. SEM image of the electrode surface of Fe 4.5 Ni 4.5 S 8 after 20 h activation at applied potential of -0.5V vs RHE in 0.5M H 2 SO 4 (a), EDS images of Fe, Ni and S of the activated electrode surface revealing no specific topology (b), EDX analysis of the activated electrode surface at different positions reveal no Pt (red labeling) (c) and elemental ratios calculated from EDX data show loss of sulfur at the surface (d).

20 After V vs RHE Pentlandite reference Intensity (a.u.) Theta (degrees) Figure S17. Powder X-ray analysis of Ni 4.5 Fe 4.5 S 8 pellets after 20 hours of activation at -0.5 V vs RHE reveals no changes. References (1) Jiao, P.; Duan, N.; Zhang, C.; Xu, F.; Chen, G.; Li, J.; Jiang, Li. Int. J. Hydrog. Energy 2016, 41,

N-doped Graphite Carbon Derived from Carbon Foam for Efficient Hydrogen Evolution Reaction

Electronic Supplementary Material (ESI) for ChemComm. This journal is The Royal Society of Chemistry 2018 Ruthenium @ N-doped Graphite Carbon Derived from Carbon Foam for Efficient Hydrogen Evolution Reaction