Water and protons in electrodeposited MnO2 (EMD)

A set of solid/gas techniques have been applied to study protons and proton transport in electrodeposited MnO2 (EMD) which is used in battery cathodes. The concentration of protons has been measured to be 0.4–0.5 mol H+ per mol MnO2 at 24 °C and pH2O=0.022 atm by in situ proton–deuteron exchange using heavy water vapour in a thermobalance. The proton content has furthermore been mapped versus T and pH2O, and it appears that about half of the protons are present in water loosely adsorbed in microporosity in the EMD. Electrical conductivity and transport number measurements show that EMD is mainly an electronic conductor, while a minor proton conductivity, probably mediated by the adsorbed water, increases strongly with decreasing temperature and increasing pH2O and is around 1% of the total conductivity at ambient, humid conditions.