(286a) Quantifying Kinetic Barriers in Alkaline Hydrogen Electrocatalysis through Single-Crystal Voltammetry and Kinetic Isotope Effects

Authors: 
Tang, M. H., Drexel University
Snyder, J., Drexel University
Intikhab, S., Drexel University
Rebollar, L., Drexel University
The sluggish hydrogen reaction kinetics observed in alkaline media compared to acidic media remain a fundamental challenge in modern electrocatalysis. Despite numerous efforts, the key parameters needed to accurately describe the slow hydrogen reaction kinetics at high pH are still unknown. Thorough understanding of the reaction mechanism and other relevant kinetic parameters is crucial for good electrocatalyst design needed for efficient energy conversion applications. In this work, we undertake an experimental and computational study to determine the effect of solvent dynamics in the kinetics of the hydrogen evolution and hydrogen oxidation reactions (HER and HOR).

A possible explanation for the decreased reaction kinetics arises when considering the role of hydroxide on HER/HOR in alkaline media. The Markovic [1,2] group showed that HER/HOR activity can be significantly improved by decorating the catalyst surface with oxophilic sites (such as Ru or Ni(OH)2 clusters on Pt), suggesting a “bifunctional” reaction pathway where hydroxide adsorbed on these oxophilic sites is an active participant in HER and HOR. Our past studies [3,4], however, indicate that a bifunctional mechanism where adsorbed hydrogen and hydroxide react is thermodynamically inconsistent, and that kinetic parameters must also be considered for explaining the sluggish HER and HOR kinetics in alkaline media.

Other recent efforts have focused on the impact of such oxophilic clusters on the interfacial electric field and water network. The Koper and Feliu [5,6] groups showed that the introduction of Ni(OH)2 clusters onto the Pt surface also decreases the ordering of the water network at the interface to accommodate charge transfer through the double layer, the energetics of which are controlled by how strongly water interacts with the interfacial electric field. The introduction of Ni(OH)2 clusters therefore decreases the interfacial electric field strength and allows for a more dynamic water network near the interface. In another study, Lia et. al. [7] showed the importance of alkali cations for anchoring reactive species in the double layer region, in accordance with other reports by the Markovic and Koper groups [8,9]. While these studies highlight the profound impact that the interfacial water network rigidity and alkali cations have on the system, the fundamental role of solvent dynamics on adsorption processes and overall reaction kinetics is yet to be deciphered.

In this work, we examine through the means of single-crystal voltammetry and microkinetic modeling the effect of solvent dynamics on the overall alkaline hydrogen reaction kinetics via a kinetic isotope effect. By comparing experimentally measured differences between H2O and D2O, we are able to probe the influence of interfacial water dynamics and electrolyte cation choice on adsorption processes (e.g. the Volmer step) and overall reaction kinetics. In addition, we focus on the reaction’s surface sensitivity by comparing Pt(111) and Pt(110) single crystals.

Our preliminary results indicate that the HER and HOR kinetics are slower in D2O by approximately 20%, further showing that hydrogen and hydroxide binding energies are not the sole descriptors for HER and HOR in alkaline media, and that solvent reorganization kinetics also limits activity. Peak-splitting measurements quantify the relative importance of the Volmer and Heyrovsky reactions to identfy the rate-limiting-step. Comparisons with our microkinetic model suggest that adsorbed hydroxide slows Volmer by competing for surface sites, but facilitates Heyrovsky by decreasing the activation energy and increasing the Arrhenius prefactor. Future work will quantitatively relate the relative changes to rotational and vibrational dynamics of H2O vs D2O via molecular simulations. The results presented herein reinforce the importance of local barriers and interfacial solvent dynamics for adsorption processes and overall reaction kinetics.

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