(560ck) pH Effects in the Hydrogen Oxidation Reaction: A DFT/MD Approach to Understand Adsorbed Hydroxyl on Platinum

Authors: 
Wilson, J., University of Delaware
Yan, Y., University of Delaware
Caratzoulas, S., University of Delaware
Vlachos, D. G., University of Delaware
Transitioning from proton-exchange membrane to hydroxide-exchange membrane hydrogen fuel cells has been proposed as an approach for cutting overall stack costs. Therefore, understanding the slow kinetics of the hydrogen oxidation/evolution reaction on catalysts in alkaline media is an important thrust in fuel cell research. Of the catalysts studied, platinum is the most widely utilized. Here, we have used a computational approach to study pH effects on model platinum surfaces.

Adsorbed hydroxyl has been implicated in strongly affecting the hydrogen oxidation/evolution reaction kinetics, but there is still an ongoing debate about the exact role of OH on the surface electrode. Previous authors have shown that adsorbed hydroxyl coverage is likely higher in base on certain platinum sites. We argue that, on account of surface dipoles, surface hydroxyl should increase the work function of the electrode, making it more oxidative. Indeed, we present density-functional theory calculations which show that the work functions of Pt (100), Pt (110) and Pt (111) increase with OH surface coverage.

On that basis, we further argue that the positive shift in the underpotential hydrogen deposition features with pH on stepped Pt can be explained by changes in the electrostatic environment at the surface due to adsorbed OH which is accompanied by changes in water adsorption. We employ molecular dynamics simulations to compute potentials of mean force for hydroxyl, hydrogen, potassium and water binding to Pt at varying electrolyte concentrations and assess two competing hypotheses: (a) at high pH, Pt-OH bonds are weakened by adsorbed cations in the electrolyte solution, shifting the equilibrium of competitive adsorption between H and OH [1]; or (b) at high pH, the hydrogen binding energy relative to the binding energy of water increases (effective hydrogen binding energy hypothesis) [2]. In order to account for solvent re-organization phenomena, we perform umbrella sampling calculations in which we control the distance of the adsorbate species from the electrode and the solvent re-organization energy. Redox events are modelled with the Anderson-News Hamiltonian and we assume bilinear coupling between the solvent collective coordinate (viz. re-organization energy) and the electronic degrees of freedom (i.e., occupancy of ion’s electronic orbital involved in the charge transfer)[3].

1 M.J. Janik, I.T. McCrum, and M.T.M. Koper, J. Catal. 367, 332 (2018).

2 S.A. Giles, J.C. Wilson, J. Nash, B. Xu, D.G. Vlachos, and Y. Yan, J. Catal. 367, 328 (2018).

3 J.B. Straus, A. Calhoun, and G.A. Voth, J. Chem. Phys. 102, 529 (1995).