# Coupling of surface chemistry and electric double layer at TiO$_2$ electrochemical interfaces

## TL;DR

This study uses advanced density functional theory simulations to explore how surface chemistry influences the electric double layer and capacitance at TiO₂ interfaces, revealing pH-dependent microscopic mechanisms affecting electrochemical behavior.

## Contribution

It provides the first atomistic-level insights into the coupling of surface chemistry and electric double layer at TiO₂ interfaces using finite-field molecular dynamics.

## Key findings

- Water molecules fluctuate more at high pH, increasing capacitance.
- Proton transfers at low pH significantly raise capacitance.
- Results match trends observed in titration experiments.

## Abstract

Surfaces of metal oxides at working conditions are usually electrified due to the acid-base chemistry. The charged interface compensated with counterions forms the so-called electric double layer. The coupling of surface chemistry and electric double layer is considered to be crucial but poorly understood because of lacking the information at the atomistic scale. Here, we used the latest development in density functional theory based finite-field molecular dynamics simulation to investigate pH-dependence of the Helmholtz capacitance at electrified rutile TiO$_2$ (110)-NaCl electrolyte interfaces. It is found that, due to competing forces from surface adsorption and from electric double layer, water molecules have a stronger structural fluctuation at high pH and this leads to a much larger capacitance. It is also seen that, interfacial proton transfers at low pH increase significantly the capacitance value. These findings elucidate the microscopic origin for the same trend observed in titration experiments.