Framework Doping of Cobalt into Layered Manganese Oxide for Improved Capacitive Behavior

摘要

Manganese oxides, characterized by low cost, abundance, and environmental compatibility, will be served as a low-cost replacement for RUO_2. Capacitive properties of Mn oxides depend on their morphologies, crystal structures, valence states, and defect chemistry. Relationship between the crystallographic structures and their pseudocapacitive properties has been investigated exhaustively by Brousse et al.~1 and Devaraj et al.2 As a conclusion, both research groups pointed out that birnessite of a two-dimensional layered structure with relatively large interlayer separation (~0.7 nm) has higher specific capacitances than one- and three-dimensional crystals, and it can be achieved with smaller BET surface areas than the amorphous counterparts. This feature can be ascribed to the bicontinuous networks of solid and pores, allowing both electrons and cations to move fast and reversibly. Birnessite has so far been prepared chemically in a powder form and used as a pseudocapacitor electrode with conductive and binding additives; i. e., carbon and PTFE, respectively. Thin film is a very attractive form in order to design compact capacitor devices. Moreover, nanostructured materials are also promising for pseudocapacitor application as a result of their reduced diffusion paths and increased specific surface area. If the conductivity is high enough, thin films of nanostructured birnessites will boost applications in electrochemical charge storage devices.