# The nanoscale structure of the Pt-water double layer under bias revealed

## TL;DR

This study uses advanced ab initio simulations to reveal the nanoscopic structure and dynamic response of the Pt-water double layer under bias, challenging traditional capacitor models and providing new insights into electrochemical interfaces.

## Contribution

First ab initio simulation of the Pt-water double layer under bias, revealing detailed nanoscopic structure and response to potential in realistic conditions.

## Key findings

- Electrode polarization involves charge oscillation and water densification, not captured by traditional models.
- Interface dipole depends on charging state and density, not just water reorientation.
- Water reorientation occurs mainly in the second layer, not the first.

## Abstract

The nanoscopic mass and charge distribution within the double layer at electrified interfaces plays a key role in electrochemical phenomena of huge technological relevance for energy production and conversion. However, in spite of its importance, the nanoscopic structure of the double layer and its response to an applied potential is still almost entirely unknown, even for Pt-water, the most fundamental electrochemical interface. Using a general ab initio methodology which advances previous models towards a dynamic and more realistic description of an electrode/electrolyte interface, we simulate for the first time the nanoscopic structure of the Pt-water double layer and its response to an applied potential, in realistic solution conditions. We reveal that the nanoscopic metal/surface structure and charging are not captured by traditional capacitor models, as the electrode polarization is associated with a charge oscillation within the double layer and a densification of the water layer in contact with the electrode, both of which strongly depend on the applied potential. Furthermore, we demonstrate that the interface dipole is not determined by the reorientation of the first water layer in contact with the electrode, but by its charging state in combination with its number density, while water reorientation becomes relevant only in the second water layer. Our findings will be essential to develop highly realistic models for the catalytic processes at the Pt-water interface.