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The importance of polymer binders in the overall behavior of RuO2 and RuO2/polyaniline composite electrodes for supercapacitors

Among various materials such as conducting polymers and transition-metal oxides, hydrous ruthenium oxide (RuO2•xH2O) exhibits the most promising pseudocapacitive properties for the potential applications as an active electrode material in supercapacitors. In order to achieve its maximal utilization, different procedures for the preparation of RuO2•xH2O active electrodes were attempted. Most of them are based on the optimization of mixed electronic and proton conduction which is usually achieved by annealing of RuO2•xH2O samples at the temperatures between 150-200 °C. Another approach is to prepare the composite material consisting of RuO2•xH2O incorporated in the conductive matrix which is usually made of different forms of carbon or conducting polymer. In this respect conducting polymers are advantageous since they enable a good inter-particle connectivity providing low-resistance electron pathways between RuO2•xH2O particles and metallic support. In addition, they usually exhibit a high pseudocapacitance contributing to the overall charge of the composite electrode. Assembling of the active electrodes for supercapacitors usually involves the deposition of the paste consisting of active material and binder to current collector. The binders are usually highly fluorinated polymers which provide cohesion of the particles of the active material. In order to maximize the performance of supercapacitor it is of utmost importance to choose and design the inert binder which will enable the flexibility of electrode material, good cohesion between active particles without blocking of the active material surface and provide low-resistive adhesion of active materials to current collector. In this work we studied the electrochemical behaviour of RuO2•xH2O and RuO2•xH2O/polyaniline electrodes in the presence of two polymeric binders: poly(vinylidene fluoride) (PVDF) and NafionTM. The composites were formed at glassy carbon (GC) electrodes by casting a homogenised suspension of different amounts of RuO2 in the solution of Nafion® or PVDF and covered subsequently by various quantities of deposited polyaniline (PANI). The electrochemical properties of the composite electrodes were investigated by cyclic voltammetry and electrochemical impedance spectroscopy techniques. The results show that the electrochemical behaviour and total specific charge extracted from RuO2•xH2O and RuO2•xH2O/polyaniline electrodes considerably depend on the type of binder (ie. difference in their physical and chemical properties) as well as the ratio of RuO2•xH2O/binder. In the case of PVDF, decrease of specific capacitance was obtained with higher ratio of RuO2•xH2O/binder, unlike in the case of Nafion where increase of specific capacitance was obtained with higher ratio RuO2•xH2O/binder. Capacity of prepared composites with PVFD increases with deposition of PANI, but in the case of composites with Nafion, capacity increases only when small amounts of PANI are deposited.

RuO2; Nafion; PVDF; supercapacitors

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