Cation Vacancies in MnO2 and Their Influence on Electrochemical Reactivity

Two types of protons must be distinguished in the cation‐deficient lattice: localized (Ruetschi) protons, associated with vacancies, and mobile (Coleman) protons, associated with electrons forming Mn 3+ ions. The localized protons provide "proton bridges" for chain‐type proton transfer. Thermogravimetric experiments reveal that the cation vacancy fraction remains invariant during proton (plus electron) insertion and that the cation‐deficient lattice becomes increasingly unstable with increasing degree of (topotactical) reduction. The activation energy of proton (plus electron) diffusion is of similar magnitude as the activation energy of electronic conductivity, while the activation energy of cation (vacancy) diffusion is several times larger.