Hydrogen reaction analysis in aqueous solutions containing H2S/CO2 at different pressures

•The hydrogen evolution in H2S aqueous solutions at different pressures was studied.•The effect of the presence of CO2 mixed with H2S was evaluated.•An electrochemical process directly attributed to H2S was proposed in this work.•H2S acts on the polarization curves introducing a new electrochemical reaction. The hydrogen evolution reaction (HER) in H2S and H2S/CO2 solutions at different pressures was studied using platinum and stainless steel as working electrodes, with polarization curves and electrohydrodynamic impedance. When Pt was the working electrode, the polarization curves in H2S solutions without CO2 presentedonly one current plateau, while two plateaus were seen when a mixture of H2S/CO2 was used. However, when steel was the working electrode, two plateaus always existed even in the presence of H2S without CO2. HER was interpreted by another source of H2 coming from the H2S reaction coupled to the classical direct reduction of H+.