The pH-Induced Increase of the Rate Constant for HER at Au(111) in Acid Revealed by Combining Experiments and Kinetic Simulation.

Origins of pH effects on the kinetics of electrocatalytic reactions involving the transfer of both protons and electrons, including the hydrogen evolution reaction (HER) considered in this study, are heatedly debated. By taking the HER at Au(111) in acid solutions of different pHs and ionic concentrations as the model systems, herein, we report how to derive the intrinsic kinetic parameters of such reactions and their pH dependence through the measurement of j-E curves and the corresponding kinetic simulation based on the Frumkin-Butler-Volmer theory and the modified Poisson-Nernst-Planck equation. Our study reveals the following: (i) the same set of kinetic parameters, such as the standard activation Gibbs free energy, charge transfer coefficient, and Gibbs adsorption energy for Had at Au(111), can simulate well all the j-E curves measured in solutions with different pH and temperatures; (ii) on the reversible hydrogen electrode scale, the intrinsic rate constant increases with the increase of pH, which is in contrast with the decrease of the HER current with the increase of pH; and (iii) the ratio of the rate constants for HER at Au(111) in x M HClO4 + (0.1 - x) M NaClO4 (pH ≤ 3) deduced before properly correcting the electric double layer (EDL) effects to the ones estimated with EDL correction is in the range of ca. 10 to 40, and even in a solution of x M HClO4 + (1 - x) M NaClO4 (pH ≤ 2) there is a difference of ca. 5× in the rate constants without and with EDL correction. The importance of proper correction of the EDL effects as well as several other important factors on unveiling the intrinsic pH-dependent reaction kinetics are discussed to help converge our analysis of pH effects in electrocatalysis.

The Effect of Water on the Quantification of Volatile Species by Differential Electrochemical Mass Spectrometry.

Differential electrochemical mass spectrometry (DEMS) is one of the most powerful online techniques for quantitative determination of volatile species from electrochemical reactions. The products distribution as well as the respective production rate derived from DEMS measurements shed important light on the mechanisms and kinetics of complex reactions. In real measurements, the background mass signal of species to be detected changes with the reaction and the measurement conditions, which interferes the quantification of DEMS analysis. In this study, we analyzed systematically how the background mass signals of species change with the amount of water enters into the vacuum chamber from the electrolytic cell, since water is the dominant species in the cell with aqueous electrolyte. Our results reveal that during DEMS measurement, (1) there is a rather long time(>30 min) for the mass signals of volatile species to reach steady values after the filament for electronic ionization is turned on due to large sampling of water from the aqueous electrolyte; (2) the reaction of water with the hot filament changes the latter's surface state, it also produces H2 and O2, which can interfere the quantification of H2 and O2 produced by electrode reactions; (3) the ionization probabilities of other species are also affected by the change of the filament's surface state, the competition for ionization of water as well as the reaction between ionized water fragments with related species in the ionization chamber. Strategies on how to obtain reliable mass signals purely related to electrocatalytic reactions are provided.