(514b) Hybrid Implicit/Explicit Electrolyte Model for the Routine Computation of Kinetic Barriers for Electrocatalytic Processes

The rational design of improved electrocatalysts for water and CO2 electrolysis hinges on understanding how the active site structure influences activity and selectivity. Atomistic simulation of catalytic mechanisms by DFT is a powerful technique for gaining this insight. However, current progress in the application of DFT to electrocatalytic processes is at least a decade behind compared to application of DFT to thermal catalytic processes due to the difficulty in modeling the electrochemical interface. Quantum mechanics explains bond breaking and formation in electrocatalysis, but statistical sampling is crucial due to the vast electrolyte space. This complexity is pronounced when calculating kinetic barriers, as transition states involve charged species transport across interfaces.

Implicit solvation methods bypass the need for statistical sampling. We have enhanced the implicit solvation model in VASP, which we call VASPsol++. While the performance is improved over the original VASPsol implementation by implementing a nonlinear and nonlocal model of the electrolyte, it is still not capable of capturing hydrogen bonding interactions which are important in the solvation of charged species such as hydronium and hydroxide. To address this deficiency, we have developed a hybrid solvation method. This model reproduces well the auto-ionization free energy of water as well as the electron chemical potential of the standard hydrogen electrode.

The utility of the hybrid solvation method was examined by computing the Volmer and Heyrovsky steps of the HER on Pt(111) and also the Volmer step on Cu(100). We compute the barrier as a function of electrode potential at both high and low pH, allowing the construction of computational Tafel plots. We extend our methodology to explore the kinetic barriers associated with the CO2RR on Cu(100). From these applications, we present a framework for using this method for the routine computation of kinetic barriers for electrocatalytic reaction steps.