Cation Effects on the Hydrogen Evolution Reaction by Catalysts Based on Cobalt Complexes in Alkaline Electrolytes

This study analyzes the influence of alkaline electrolytes on the hydrogen evolution reaction (HER) of the platinum group metal-free (PGM-free) electrocatalysts based on cobalt tetraphenylporphyrin (CoPor) and cobalt phthalocyanine (CoPc). The effect of pH (11-14), the nature and concentration of alkali metal cations (AMs+), and the role of mass transport are elucidated. It is observed that HER catalyzed by both CoPor and CoPc is apparently a pH-dependent phenomenon where activity improves with an increase in the bulk electrolyte pH, while the overall cation trend is followed in the order of K+>Na+>Li+. Li+ cations support the HER up to pH 14, reaching a saturation level, particularly in the case of CoPor, whereas for CoPc activity continuously improves with the addition of Li+. On the other hand, saturation is achieved at lower pH in the case of Na+ and K+ (at pH 13) with a clear HER inhibition at pH 14, i.e., HER activity decreases if the concentration of AMs+ is further increased. Moreover, with the increase in the electrolyte mass transportation by increasing the rotation rate of the rotating disc electrode (RDE) deposited with either CoPor or CoPc, the corresponding HER activities tend to drop. We ascribe these trends to the interfacial cation concentration and how it is tuned by bulk cation concentration, pH, and mass transport. The estimation of the double-layer capacitance via electrochemical impedance spectroscopy (EIS) confirmed the enhanced concentration of AMs+ at the reaction interface as the bulk pH or the concentration of the electrolyte increased.