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 MnO 2 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 MnO 2 nanoarchitecture. These polymer coatings serve as pinhole-free physical barriers to external, acidic electrolyte, specifically, H 2 O and hydrated protons, and protect the underlying MnO 2 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 MnO 2 .