First-Principles Study of Hydrogen Trapping in Electrolytic Manganese Dioxide*

Alkaline Zn/MnO$_{2}$ batteries hold great promise for electrical energy storage due to their high energy density, non-toxicity, and low cost. At a low depth of discharge, the reduction reaction in the Zn/MnO$_{2}$ battery cathode is governed by hydrogen trapping in the solid phase of $\gamma$-MnO$_{2}$. We applied ab initio computational methods based on density functional theory to study the mechanism of hydrogen insertion into the pyrolusite and ramsdellite tunnels of $\gamma$-MnO$_{2}$. Our calculations were carried out using the Quantum ESPRESSO electronic structure code combined with Vanderbilt ultrasoft pseudopotentials. We found that the trapped hydrogen initially occupied the 2$\times$1 ramsdellite tunnels of $\gamma$-MnO$_{2}$. Our study showed that the insertion of hydrogen into the 1$\times$1 pyrolusite tunnels induced significant structural distortions leading to the breakdown of the crystal structure of $\gamma$-MnO$_{2}$. These results could explain the presence of groutite and the absence of manganite among the reaction products of partially reduced $\gamma$-MnO$_{2}$.