Pseudocapacitive behavior of Ti/RhOx + Co3O4 electrodes in acidic medium: application to supercapacitor development

Mixtures of RhO_x+Co₃O₄ have been electrochemically studied by cyclic voltammetry in acid solution as a function of composition. The electrodes were prepared by thermal decomposition at 400 °C of mixtures of nitrate precursors. Their electrochemical behavior shows substantial dependence on the electrode’s composition. The Co site controls the electrochemical behavior of the system in the 5-10 mol.% Rh composition range. A significant increase in the electrodes’ active area is observed for compositions corresponding to more than 10 mol.% RhO_x in admixture with Co₃O₄. Above 10 mol.% Rh, the voltammetric curves become more similar to that for RhO_x and then RhO_x becomes able to stabilize the Co₃O₄ in the mixture. Electrodes of this kind have been found to perform as good materials in electrochemical capacitor applications, exhibiting specific capacitances of 500-800 F g⁻¹ over to 20-60 mol.% RhO_x composition range. The large specific capacitance exhibited by this system arises from a combination of the double-layer capacitance and the pseudocapacitance associated with Rh surface redox-type reactions.