Insights into electrodeposition and catalytic activity of MoS2 for hydrogen evolution reaction electrocatalysis

MoS_2-x films were electrodeposited cathodically onto copper rod substrates from a solution, containing MoS₄^2- as the common Mo and S ion precursor. The catalyst loading was varied by adjusting electrodeposition conditions – applied potential and deposition time. A typical set of HER electrocatalyst experiments (polarization, Tafel slope analysis) carried out in 0.5 M H₂SO₄ was applied on the deposited MoS_2-x films. Analysis of surface morphology (SEM) and chemical composition (EDS) were also performed. Electrochemical impedance spectroscopy in the same acidic media was used to evaluate the catalyst-solution interface and the interfacial kinetics (by calculating double layer capacitance and charge transfer resistance), as well as characterize the hydrogen adsorption process (adsorption capacitance and resistance). A linear correlation between electrodeposition time and double layer capacitance was observed. However, the charge transfer resistance was found to decrease until it plateaued at longer deposition times. The MoS_2-x film, deposited for 7200 s at -1.0 V (vs. Ag/AgCl), reached 10 mA cm^-2 HER current at -0.18 V (vs. RHE), and represented the best result of this study. Electrochemical impedance spectroscopy (EIS) was further applied to evaluate the subtle changes in the MoS_2-x films’ semiconductor properties after HER stability tests (at -40 mA cm^-2), and to estimate the number of active sites on the material. EIS, in comparison to cyclic voltammetry or roughness factor calculations, is a completely non-destructive method that can be applied to accurately assess the system under investigation.