Dual-Phase Molecular-like Charge Transport in Nanoporous Transition Metal Oxides

The conductivity of porous films in the presence of a large concentration of electrolyte is modeled considering two behaviors: "molecular-like" behavior corresponding to a single electrical phase with localized redox species surrounded by an electrolyte and "dual-phase material-like" behavior corresponding to two distinct conductive electrical phases. In the first case, both simple redox and ion-coupled redox couples are considered. In the second case, a transition from insulating to metallic behavior of the electronic phase is described, showing the interplay between diffusion and migration of charge carriers. The effect of electrochemical doping is systematically investigated, leading to the delineation of the experimental criteria to discriminate between these two types of behaviors in porous films. Application of the model to electrodeposited cobalt oxide films shows that these films, despite their molecular electrocatalytic activity, have a dual-phase material-like conductive behavior arising from nanocrystallites of cobalt oxide surrounded by an electrolyte phase, thus revealing the complexity of electrodeposited catalytic metal oxide materials.