Ultrathin, protective coatings of poly(o-phenylenediamine) as electrochemical proton gates:: Making mesoporous MnO2 nanoarchitectures stable in acid electrolytes

We have created hybrid organic-inorganic nanoarchitectures by electrodepositing ultrathin (<10-nm-thick) polymer coatings onto nanostructured MnO2 birnessite-type electrodes with surface areas in excess of 200 m(2) g(-1). By choosing a self-limited growth process, based on the electropolymerization of o-phenylenediamine, the resulting polymer conformally coats the oxide nanoscale network without disrupting the continuous mesoporosity of the initial MnO2 nanoarchitecture. These polymer coatings serve as pinhole-free physical barriers to external, acidic electrolyte, specifically, H2O and hydrated protons, and protect the underlying MnO2 nanoarchitecture from dissolution. The underlying metal oxide remains electrochemically addressable via an electrochemical proton-gating mechanism in which charge-compensating unsolvated protons are transported through the polymer coating. The 3D-templated electrochemical fabrication of polymer at an electrified metal oxide nanoarchitecture provides a new model for the development of electrochemical capacitors based on hybrid configurations using low-cost metal oxides such as MnO2.