Effects of CO₂ adsorption on proton migration on a hydrated ZrO₂ surface: an ab initio molecular dynamics study

Hydration reactions on a carbonate-terminated cubic ZrO₂(110) surface were analyzed using ab initio molecular dynamics (AIMD) simulations. After hydration reactions, carbonates were still present on the surface at 500 K. However, these carbonates are very weak conjugate bases and only act as steric hindrance in proton hopping processes between acidic chemisorbed H₂O molecules (Zr-OH₂) and monodentate hydroxyl groups (Zr-OH^-). Similar to a carbonate-free hydrated surface, Zr-OH₂, Zr-OH^-, and polydentate hydroxyl groups ([double bond splayed left]OH⁺) were observed, while the ratio of acidic Zr-OH₂ was significantly larger than that on the carbonate-free hydrated surface. A thermodynamic discussion and bond property analysis reveal that CO₂ adsorption significantly decreases the basicity of surface oxide ions ([double bond splayed left]O), whereas the acidity of Zr-OH₂ is not affected. As a result, protons released from [double bond splayed left]OH⁺ react with Zr-OH^- to form Zr-OH₂, leading to a deficiency of proton acceptor sites, which decreases the proton conductivity by the hopping mechanism.