Mechanism of the Hydrogen Evolution Reaction in Mildly Acidic Environments on Gold

Abstract

Despite numerous studies investigating the kinetics of the hydrogen evolution reaction (HER) on a gold surface in acidic solutions, the underlying mechanism of this reaction have remained controversial to date. In the present study, the existing mechanisms are reevaluated and found to be inadequate in explaining the steady state polarization behavior of the hydrogen evolution reaction in an extended cathodic potentials and mildly acidic pH range. It was shown that a mechanism including a surface diffusion step of H ads alongside the Volmer, Heyrovsky, and Tafel elementary steps, best describes the experimental data obtained in acidic perchlorate solutions up to pH 5, while the rate determining step changes both with pH and electrode potential. This overall HER mechanism was further verified using a comprehensive mathematical model based on the proposed elementary steps, where a satisfactory agreement with experimental results was obtained. The hydrogen evolution reaction (HER) has been the subject of numerous studies, either as a platform for investigating the theory of electrochemical processes, 1–9 or in terms of hydrogen produc-tion, energy storage, and energy conversion, 10–12 due to its signifi-cance in the alternative energy source framework. This trend had also include extensive investigations of the mechanism of the HER on gold in acidic solutions. 6,8,13–21 However, a literature survey shows no general agreement on the underlying mechanism of this reaction to date. 13–15,17,18,20,21 Besides, the majority of the proposed mechanisms have been developed based on experimental results obtained in highly acidic environments, 6,8,13–21 but were not examined over an extended pH range. The experimental polarization curves obtained on gold in acidic solutions repeatedly reported to have two distinct Tafel slopes with values in the range of 50–70 mV at lower current densities and 100–130 mV at higher current densities. 3,6,16,18,21,22 A number of dif-ferent explanations for the underlying mechanism based on these observed Tafel slopes have been proposed in the literature. In a study by Ives 20 in 0.1 N HCl solutions, the author reported polarization curves with an uncharacterized region at low current densities preced-ing to the 120 mV Tafel slope range. That uncharacterized section of the voltammograms had a significantly lower Tafel slope with values about 50–70 mV, which was extended to the cathodic currents up to about 1 A.m −2 and overpotentials up to about 150 mV. The author associated this lower Tafel slope with the interference of the hydrogen oxidation reaction. 20 However, considering the experimental condi-tions in that study, no significant interference of anodic currents due to hydrogen gas oxidation is expected, especially at the cathodic over-potentials as high as 150 mV. Bockris et al. 14,23 suggested that the apparent change of Tafel slope to ∼60 mV was caused by the change in potential drop across the diffusion double layer. This effect was believed to be most profound at the potentials near the potential of zero charge (PZC). Therefore, authors suggested that for the mechanism with theoretical Tafel slope of ∼120 mV (2.3RT/βF), lower apparent Tafel slopes with a minimum of ∼60 mV can be observed in the vicinity of PZC. At the potentials notably different from the PZC, this effect becomes insignificant, thus the apparent Tafel slope tends to reach the theoretical value (∼120 mV). However, as suggested elsewhere, 18,24 the reported PZC of gold 25,26 is significantly higher than the potentials at which the change in the Tafel slope occurs experimentally. Another mechanism based on the barrierless discharge of H +