Theoretical analysis of zirconium oxynitride/water interface using neural network potential

TL;DR

This study uses neural network potentials to analyze the microscopic structure of zirconium oxynitride/water interfaces, revealing how oxygen vacancies influence water adsorption and potentially enhance catalytic activity.

Contribution

It introduces a combined ab initio and neural network approach to simulate defective Zr oxynitride/water interfaces at an atomic level, providing new insights into defect effects on catalytic activity.

Findings

Oxygen vacancies alter water adsorption patterns.

Water molecules prefer Zr atoms near vacancies, avoiding the vacancies themselves.

Vacancies modify water layering, affecting transport properties.

Abstract

Zr oxides and oxynitrides are promising candidates to replace precious metal cathodes in polymer electrolyte fuel cells. Oxygen reduction reaction activity in this class of materials has been correlated with the amount of oxygen vacancies, but a microscopic understanding of this correlation is still lacking. To address this, we simulate a defective Zr$_7$O$_8$N$_4$/H$_2$O interface model and compare it with a pristine ZrO$_2$/H$_2$O interface model. First, ab initio replica exchange Monte Carlo sampling was performed to determine defect segregation at the surface in the oxynitride slab model, then molecular dynamics accelerated by neural network potentials was used to perform 1000 of 500 ps-long simulations to attain sufficient statistical accuracy of the solid/liquid interface structure. The presence of oxygen vacancies on the surface was found to clearly modify the local adsorption structure: water molecules were found to adsorb preferentially on Zr atoms surrounding oxygen vacancies, but not on the oxygen vacancies themselves. The fact that oxygen vacancy sites are free from poisoning by water molecules may explain the activity enhancement in defective systems. The layering of water molecules was also modified considerably, which should influence the proton and O$_2$ transport near the interfaces which is another parameter that determines the overall activity.