Effect of Surface Ionization of Doped MnO on Capacitive Deionization Efficiency.

Associating MnO with carbonaceous supports profoundly enhances capacitive deionization (CDI) efficiency. A fundamental question of how the surface chemistry of MnO itself influences CDI efficiency is not yet fully understood. In this study, the effect of surface ionization on the CDI efficiencies of Fe-, Co-, and Ni-doped α-MnO (<0.1 mol %) as a model cathode material was studied. A pattern that CDI efficiency decreased with increasing negative surface charge density resulting from surface deprotonation was noted. This is likely attributed to the appreciable co-ion expulsion occurring at a highly ionized surface in the mesopores of MnO. It is thus concluded that the combination of surface charge modification and a microporous environment would be important for CDI efficiency enhancement by minimizing co-ion exclusion effect. In the former case, structural stress adjustment by doping elements would be a practical route to regulate the p K and p K values and consequently the degree of surface ionization of MnO.