Near-surface ion distribution and buffer effects during electrochemical reactions

Abstract

The near-surface ion distribution at the solid-liquid interface during the Hydrogen Oxidation Reaction (HOR)/Hydrogen Evolution Reaction (HER) on a rotating platinum disc electrode is demonstrated in this work. The relation between reaction rate, mass transport and the resulting surface pH-value is used to theoretically predict cyclic voltammetry behaviour using only thermodynamic and diffusion data obtained from the literature, which were confirmed by experimental measurements. The effect of buffer addition on the current signal, the surface pH and the ion distribution is quantitatively described by analytical solutions and the fragility of the surface pH during reactions that form or consume H(+) in near-neutral unbuffered solutions or poorly buffered media is highlighted. While the ideal conditions utilized in this fundamental study cannot be directly applied to real scenarios, they do provide a basic understanding of the surface pH concept for more complex heterogeneous reactions.

Towards Versatile and Sustainable Hydrogen Production through Electrocatalytic Water Splitting: Electrolyte Engineering

“…The interpretation of the influence of pH on the HER current–potential relationship (Figure 6) is still under debate 116, 117, 118, 119, 120. Mayrhofer and co‐workers reported that the Nernst–Planck equation [Eq.…”

“…in which K w is the equilibrium constant of water molecule dissociation into/formation from a proton and hydroxide ion 119, 120. Simply, the fact that the calculation based on the Nernst–Planck equation matches with the experimental observation indicates that the thermodynamics and mass transport are solely responsible for the j – E relationship of the HER/HOR when the kinetically facile Pt electrode is used.…”

“…There have been a couple of studies reported regarding the HER at near‐neutral pH in buffered solutions that function to maintain the local pH level 80, 119, 120. Experimentally, it was clearly demonstrated that the use of a buffered solution as a “supporting electrolyte” successfully prevented the diffusion limitation of hydronium ions at near‐neutral pH levels.…”

“…Notably, when the buffered solution is used as the “supporting electrolyte”, the concentration of the electrolyte has to be carefully selected to at least provide a sufficient buffering capacity to the system. A careful investigation by Mayrhofer and co‐workers clarified the requirement: 10 m m is sufficient to prevent alteration of the local pH up to 1 mA cm −2 119, 120. Other than maintaining the local pH, it should be noted that the presence of buffered species can introduce additional parameters required for describing the electrocatalytic kinetics, as discussed in Section 2.…”

The pH Dependence of Magnesium Dissolution and Hydrogen Evolution during Anodic Polarization

“…22,23 The surface pH should be slightly lower during anodic polarization than at OCP because of the higher rate of production of Mg 2+ , which will hydrolyze to a certain extent, and the lower rate of hydroxide ion formation associated with an expected lower rate of hydrogen evolution. Any pH decrease however, should induce a destabilization of Mg(OH) 2 and cause a disruption of the surface film.…”