Co-electrodeposition of RuO2–MnO2 nanowires and the contribution of RuO2 to the capacitance increase

Abstract

A wide range of metal oxides have been studied as pseudocapitors, with the goal of achieving higher power than traditional batteries and higher energy than traditional capacitors. However, most metal oxides have relatively low conductivity, and the few exceptions, like RuO₂, are prohibitively expensive. Mixed metal oxides provided an opportunity to incorporate small amounts of expensive materials to enhance the performance of a less expensive, poorer performing material. Here, by homogeneously co-depositing a small amount of energy dense and conductive RuO₂ into MnO₂ nanowires, we demonstrate an improvement in specific capacitance. Importantly, we also demonstrate that this improvement is not primarily provided by redox activity of RuO₂, but rather by improvement of the composite conductivity. A series of RuO₂–MnO₂ composite nanowires with different RuO₂ loading percentages have been synthesized by performing co-electrodeposition in a porous alumina template. The structure of these RuO₂–MnO₂ nanowires is characterized by TEM and SEM. EDS mapping shows that RuO₂ is well distributed in MnO₂ matrix nanowires. The chemical constituents and the phase of these composite nanowires are confirmed by X-ray photoelectron and Raman spectroscopy. The amount of RuO₂ is controlled by varying the concentrations of RuCl₃ and MnAc₂ in the deposition solution. The precise masses of MnO₂ and RuO₂ are determined by ICP-AES elemental analysis. MnO₂ nanowires with 6.70 wt% RuO₂ demonstrate a specific capacitance of 302 F g⁻¹ at 20 mV s⁻¹, compared to 210 F g⁻¹ for pristine MnO₂ nanowires. Investigation of the RuO₂ loading amount effect was conducted by electrochemical impedance spectroscopy (EIS) and deconvolution of capacitances, using methods previously reported by both Dunn and Transsiti. The RuO₂–MnO₂ nanowires studied here demonstrate a simple, straighforward method to overcome the intrinsically poor conductivity of MnO₂, and clarify the source of RuO₂'s contribution to the improved performance.