Correcting the Hydrogen Diffusion Limitation in Rotating Disk Electrode Measurements of Hydrogen Evolution Reaction Kinetics

Abstract

Rotating disk electrode (RDE) method is widely employed in studies on the hydrogen oxidation/evolution reaction (HOR/HER) owing to its well-defined mass transport behaviors. While it is accepted that the measured HOR current is controlled by both the electrode kinetics and the diffusion of H 2 , HER is typically assumed to be free of diffusion limitation. Here we demonstrate that HER could also be diffusion limited when the electrode kinetics is fast, as evidenced by the rotation speed dependent HER current on Pt in acid (pH = 1) and the overlap of the HER polarization curve with the concentration overpotential curve. The HER diffusion limitation originates from the insufficient mass transport of produced H 2 from the electrode surface to the bulk electrolyte and the highly reversible nature of HOR/HER. Kinetic analyses based on HER polarization curves on Pt in acid without correcting for the diffusion limitation could lead to inaccurate Tafel slopes and mechanistic interpretations, and significantly underestimated HER activities. A general data analysis protocol based on the reversible Koutecky-Levich equation is developed to obtain accurate kinetic information of HOR/HER even when electrode kinetics is facile. This new method is compared with other existing methods on Pt disk electrodes at different pHs and thin-film electrodes with different Pt loadings.

Electrocatalysts for hydrogen oxidation and evolution reactions

“…The HER performance of Pt is also listed in Table 2 for comparison. Note that due to the ultrafast HOR/HER kinetics of Pt in acid, the overpotential at 10 mA cm −2 only comes from mass transport (Equation (4)) and is not related to the kinetics of the catalyst [36].…”

Size-Dependent Hydrogen Oxidation and Evolution Activities on Supported Palladium Nanoparticles in Acid and Base

“…8 Sun et al reported increased activity with increased particle size for HOR/HER on 2-7 nm Pt nanoparticles in acid, 18 and Ohyama et al reported the same trend for HOR/HER on 2-4 nm Pt nanoparticles in base. 19 However, the rotating disk electrode (RDE) method cannot be used to measure HOR/HER activity on Pt in acid due to the overlapping polarization and concentration overpotential curves, 8,20 and thus the results from Sun et al 18 via RDE measurements might not be reliable. Using the H 2 -pump method in a proton exchange membrane fuel cell configuration, Durst et al characterized the exchange current densities of HOR/HER on 2-9 nm Pt/C and reported no particle size effect.…”

A Stand‐Alone Module for Solar‐Driven H2 Production Coupled with Redox‐Mediated Sulfide Remediation

“…This value is larger than that reported for NiMo/Ni‐foam (30 mV) in the phosphate buffer without the redox ions, which is a standard solution for water splitting in near‐neutral pH conditions . The HER on Pt is known to be kinetically facile, and the observed overpotential is mainly due to the concentration overpotential from the proton donors and hydrogen products . The concentration overpotential is highly dependent on the viscosity of the electrolyte, which alters the diffusion coefficient according to the Stokes–Einstein equation.…”

“…[45] The HER on Pt is known to be kinetically facile, and the observed overpotential is mainly due to the concentration overpotential from the proton donors and hydrogen products. [45][46][47] The concentration overpotential is highly dependent on the viscosity of the electrolyte, which alters the diffusion coefficient according to the Stokes-Einstein equation. The diffusion coefficient decreases in viscous solutions, which makes the mass-transport of the reactants and products difficult and results in the increase in concentration overpotential.…”