Numerical Deconvolution of Surface Interrogation Scanning Electrochemical Microscopy Experiments on Platinum During Hydrogen Evolution

Abstract

AbstractIt is experimentally challenging to deconvolute the potential‐dependent adsorption of the different intermediates that occur during the hydrogen evolution reaction (HER) in alkaline media. This difficulty has limited our understanding regarding why the HER kinetics are more sluggish in alkaline media compared to acidic media. Herein, we utilized the surface interrogation mode of scanning electrochemical microscopy (SI‐SECM) to investigate the surface adsorbed species that form during the HER in alkaline media on polycrystalline platinum, Pt(poly). To deconvolute the different adsorbed intermediates, we developed a detailed COMSOL‐based kinetic model to rapidly simulate the SI‐SECM titration reactions under our experimental conditions. Utilization of this rapid‐kinetic model overcomes the limitation of SI‐SECM not having the ability to simultaneously resolve multiple surface adsorbed intermediates. We demonstrate that these numerical simulations can separate the potential dependant formation of the underpotential deposition of hydrogen (H(UPD)) from the overpotential deposition (H(OPD)) of hydrogen. In addition, our simulations show that a spectator species may also exist on the surface during HER potentials. Our simulations also show that at full H2‐producing potentials, the surface of Pt(poly) is fully saturated with intermediates. Comparison between the potential‐dependent adsorption of H(OPD) and Tafel analysis reveal that the Heyrovsky step is likely rate‐determining in alkaline media. However, in alkaline media the Heyrovsky step transitions from first‐order in H(OPD) at low H(OPD) coverage to zero‐order at high H(OPD) coverage, due to surface saturation of adsorbed intermediates. Tafel analysis in acidic media shows that the Heyrovsky step is likely rate‐determining, but remains first‐order in Had over a larger potential range. These fundamental insights reveal that a sluggish Heyrovsky step is a major contributor to the attenuated kinetics of the HER in alkaline media.