Porosity Changes during Reduction of γ-MnO2 for Aqueous Alkaline Applications

Abstract

In this work a structural and porosity analysis of a series of chemically reduced electrolytic manganese dioxide (EMD) samples has been carried out. EMD, or , for use in aqueous alkaline cathodes possesses an orthorhombic unit cell that progressively increases in size as reduction continues. This unit cell expansion is nonuniform, with dimensional changes being different in all three dimensions . The unit cell expansion is the direct result of the reduction process, with the discharge products ( and ) being larger than the corresponding ions in the host lattice ( and ). Gas adsorption isotherms on the same series of reduced EMD samples demonstrated that the Brunauer–Emmett–Teller surface area decreased considerably over the entire discharge compositional range. The calculated pore size distribution showed that microporosity was eliminated with reduction, but the mesopore volume increased. The origin of these changes in porosity is discussed in terms of the structural expansion, as is their effect on electrochemical performance.