Inclusion of pH and potential in atomic-scale simulations of the electrochemical interface

Recent improvements in computational power and theory have allowed for density functional theory calculations on electrochemical systems. Currently, there are two main types of ab initio studies on electrochemical systems. Catalyst screening/optimization studies focus on adsorption free energies of reaction intermediates.

Water and electric fields are often omitted to reduce the computational resources, and the effect of potential is added a posteriori via the computational hydrogen electrode [1]. More advanced studies try to model the entire interface and focus on setting up an explicit electrode potential and electric field at the interface via water layers, excess free charge, counter-ions, and counter electrodes.

No existing approach addresses the effect of pH on the interfacial structure. Electrochemical reaction rates can, however, be strongly affected by solution pH, and there is increasing interest in the development of efficient electrocatalysts for alkaline environments [2]. Consideration of pH is thus a crucial challenge in ab initio simulations.

Here we present a generalization of the computational hydrogen electrode to explicitly capture the respective pH and potential effects on the interface structure and its corresponding free energy. Using simple thermodynamic arguments, the method determines ground state interface structures as a function of pH and potential.

As an example, we apply the method to a set of Pt(111)| water structures and determine the corresponding Pourbaix diagram. Additionally, we draw attention to some of the electronic properties characterizing an electrochemical interface. In particular, we discuss how metal and molecular electronic energy levels must align at the interface in order to enable tuning of the electrode potential [3].

References [1] J. K. Nørskov et al., J. Phys. Chem. B 108 (2004) 17886. [2] R. Subbaraman et al., Science 334 (2011) 1256. [3] M. E. Björketun et al., Chem. Phys. Lett. 555 (2013) 145.