  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.
