How Relevant is the Reversible Hydrogen Electrode when Voltammetrically Studying the Hydrogen Evolution Reaction?

In this work we address the relevance of the Reversible Hydrogen Electrode (RHE) to the voltammetric study of the Hydrogen Evolution Reaction (HER) under hydrogen-free conditions, this is achieved through experiment, approximate analytical theory, and numerical simulation. Voltammetry is inherently a dynamic electrochemical technique, where the occurrence of the interfacial reaction changes the solution composition near the electrode surface. This work seeks to answer the question: how accurately does the potential of the RHE reflect the voltammetric response of the hydrogen evolution reaction in the presence of a weak acid? Hence, when quantifying the electrocatalytic properties of a material, what is the relevant potential against which we should be referencing our measurements? This paper demonstrates that although the RHE and the voltammetric hydrogen evolution reaction are thermodynamically related, the characteristic potentials (such as the voltammetric mid-point potential) associated with the hydrogen evolution reaction differs from that of the RHE by up to tens of millivolts under regularly used conditions. Further, in contrast to the RHE, the voltammetric response is shown to be sensitive to the buffer concentration. The numerical and experimental results focus on the voltammetric behavior of a stationary macroelectrode; however, as highlighted, the resulting insights are also relevant to studies made under hydrodynamic conditions.