Ruthenium oxide–niobium hydroxide composites for pseudocapacitor electrodes

A simple solution-based method has been developed to vary the composition of redox active ruthenium oxide with highly proton-conducting niobium hydroxide to create stable, high capacitance electrodes at elevated temperatures. This method presents a dramatic departure from most other ruthenium oxide systems, which are prepared through annealing of hydrous ruthenium oxide. Typically RuO2 processed at high temperature only exhibits high electrical conductivity and suffers from poor proton conduction, giving low overall capacitances. Here, the optimized Ru/Nb oxide composition can be used to achieve high power densities, high capacitances, and stabilized electrodes while significantly reducing ruthenium content. Extensive materials characterization including high-resolution cross-sectional TEM, elemental mapping, XRD, electrochemical impedance spectroscopy, and proton NMR were used to evaluate the structure of the material system. The electrochemically inert niobium oxide serves as a network former enhancing accessibility to redox active ruthenium oxide. The dispersion of RuO2 in the NbO(OH)x matrix results in reduced RuO2 particle size, as observed via TEM and XRD, while also increasing the proton concentration in the material. Interconnected RuO2 particles provide electrically conducting pathways, even at low Ru contents, where percolation networks remain intact. Ruthenium is more efficiently utilized in the Ru/Nb composites and ruthenium content can be significantly reduced without decreasing capacitive performance. In addition, the composite electrodes, with the fine mixing of Ru and Nb, give higher power performance than for RuO2 alone.

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