Computer Simulations of Proton Discharge from Aqueous Solutions on Metal Electrodes

A reactive trajectory approach for the study of proton discharge from aqueous environments on charged metal surfaces is reviewed. It is based on an extension of a minimalistic empirical valence bond (EVB) model to study proton transfer in the bulk. Extensive quantum mechanical density functional theory calculations were parametrized for the EVB force field [1]. The model is used to investigate reactive (discharging) proton trajectories which were started in the bulk of a water film adsorbed on charged metal electrodes. The results indicate a transition between a reaction-dominated regime at moderate negative charges, where the rate constant increases exponentially, to a “transport limited'' regime where the transfer rate is almost independent of the surface charge density (at highly negative surface charge densities) [2,3].

Extensions of the model to electrolyte solutions are discussed along corresponding results of trajectory calculations that introduce background electrolytes with and without specific ion adsorption are presented. In NaCl solutions, e.g., it is found that the discharge rate is slowed down relative to that in pure water. We discuss how this phenomenon is associated with adsorption site-blocking by ions from the electrolyte, and with the reduced number of water pathways for Grotthuss style proton hops as a consequence of the presence of hydration shells containing water molecules in unfavourable arrangements for proton transfer. Further mechanistic insight can be gained from temperature dependent studies [4].

[1] F. Wilhelm, W. Schmickler, R. R. Nazmutdinov, and E. Spohr, “A model for proton transfer to metal electrodes,” J. Phys. Chem. C, vol. 112, pp. 10814–10826, 2008.

[2] F. Wilhelm, W. Schmickler, and E. Spohr, “Proton transfer to charged platinum electrodes. A molecular dynamics trajectory study,” J. Phys.: Condens. Matter, vol. 22, p. 175001, 2010.

[3] F. Wilhelm, W. Schmickler, R. Nazmutdinov, and E. Spohr, “Modeling proton transfer to charged silver electrodes,” Eletrochim. Acta, vol. 56, pp. 10632–10644, 2011.

[4] W. Schmickler, F. Wilhelm, and E. Spohr, “Probing the temperature dependence of proton transfer to charged platinum electrodes by reactive molecular dynamics trajectory studies”, Electrochim. Acta 101, 341, 2013.

This work was supported by Deutsche Forschungsgemeinschaft through Collaborative Research Group FOR1376 “Elementary Reaction Steps in Electrocatalysis: Theory meets Experiment” and through the Cluster of Excellence RESOLV (EXC 1069).