The effect of crystal phase of manganese oxide on the capacitive deionization of simple electrolytes

MnO₂ is a common material for the fabrication and design of capacitive deionization (CDI) devices but there is little information on the role of MnO₂ crystal phase on CDI performance. A series of MnO₂ (α, β, γ, and δ phase) were synthesized and fabricated as cathodes for studying the CDI performance as affected by pH in simple batch mode experiments. Our results revealed that the deionization efficiency decreased with increased negative surface charge as a result of the deprotonated surface. Importantly, this correlation was pH independent and the surface heterogeneity due to different MnO₂ phase was likely responsible for the different degree of surface ionization and consequently the CDI efficiency. Results of electrochemical impedance spectroscopy analyses further implicated that a highly ionized surface would result in a diffusion layer with a great resistance that conversely inhibited the access of co-ions in the CDI process. This indicated the applied potential was mainly responsible for driving ions transporting through the double layer resistance instead of accommodating them (electrosorption). Based on our results, the surface heterogeneity as a result of different spatially distributed MnO₆ octahedral would be accounted for the varying degree of surface ionization and consequently the discrepancy in CDI efficiency among different MnO₂ phases.