Self-assembled CoS/S-rGO dual-functional composite, and reliable hydrogen evolution performance in both acidic and alkaline media

Developing an amphoteric-functional hydrogen evolution electrode that is useable in both acidic and alkaline electrolytes is an innovative future energy strategy. Here, CoS/S-rGO/CP electrode is obtained by direct hydrothermal growth of CoS loaded on S-rGO onto carbon paper (CP). The introduction of S reduces defects in the hexagonal carbon rings in rGO. CoS/S-rGO/CP electrode is slightly more stable in acidic one (0.5 M H2SO4) than in alkaline (1.0 M KOH) electrolytes. This is because S ion elution from CoS or S-rGO is suppressed due to the presence of SO42− ion, a common ion, in the acid electrolyte. Gibbs free energy calculations suggest that the overall rate in the alkaline electrolyte is thermodynamically slightly slower than in the acidic electrolyte due to the lower H∗ coverage. In CoS/S-rGO/CP, S in S-rGO donates electrons to Co2+ in CoS, while S in CoS repels electrons to C=S in S-rGO, resulting in there is a strong synergy between CoS and S-rGO. Eventually, CoS/S-rGO/CP electrode exhibits faradaic efficiencies of 97.85% and 97.01% in acidic and alkaline electrolytes with overpotentials of −0.20 and −0.22 V, and Tafel slopes of 89.2 and 96.1 mV dec−1, respectively. In particular, the electrode performs almost as well as commercial Pt/CP under the same reaction conditions, highlighting its future commercialization prospects as a noble-metal-free HER electrode. [Display omitted] •Dual-functional HER electrode useable in both acidic and alkaline electrolytes.•The reduced hexagonal carbon ring defects in the rGO by introduction of S.•Superior HER performance of CoS/S-rGO/CP in acidic electrolyte.•Faradaic efficiencies of 97.8% with overpotentials of −0.21 V in acidic electrolyte.