Understanding hydrogen evolution reaction in bicarbonate buffer

Buffers are often employed as electrolytes for their ability to resist to pH changes. Still, they are not just spectators in the electrocatalytic processes, rather they may take a primary role. Here, we attempt to understand the mechanism of the hydrogen evolution reaction (HER) in bicarbonate-containing electrolytes on two distinct materials, Au and Pt. Keeping the bulk pH constant, voltammetric measurements reveal a promotional effect on the HER rate in the presence of bicarbonate buffer, as well as a substantial change in the reaction mechanism with a Tafel Slope in bicarbonate resembling the one in acidic media (ca. 100 and 40 mVdec−1 on Au and Pt, respectively). Nevertheless, because of the increase in surface pH due to the concomitant water reduction (2H2O + 2e−→ H2 + 2 OH−), bicarbonate is consumed by acid-base reactions and the bicarbonate branch of HER becomes significant only in the presence of large bulk buffer concentrations. Bicarbonate and water reduction appear intrinsically different, as they exhibit opposite dependence on the cation identity and scale differently with the specific area of the electrode. Through microkinetic modelling, we exclude that HER in bicarbonate can be explained via the generation of a proton by solution acid-base reactions. Rather, we suggest a direct bicarbonate reduction pathway, which, due to the negatively charged reactant, is strongly dependent on the cation concentration.