Influence of electrolyte and redox active electrode materials properties on working mechanism and performance of manganese oxide-based supercapacitors

In this work the pseudocapacitors based on graphite foil/manganese oxide powder electrodes, Celgard (TM) separator foil, and different water-based electrolytes were manufactured. Commercially available MnO2 (Sigma Aldrich) with average grain size of 10 mu m, milled - 0.1 mu m, milled and reduced in 5vol.% of H-2 in Ar atmosphere (10 h at 600 degrees C) were used for electrode formation. The influence of electrolytes on the performance of pseudocapacitors was tested using cyclic voltamperometry. The electrolytes were based on cations with different: valence (Na+, K+, Ca2+ and Al3+), activity (a), hydrated ion size (r(h)), specific ionic conductivity of electrolyte (lambda), diffusion coefficient (D) and these parameters were correlated with determined capacitances. The highest capacitances were observed for K+-based electrolytes, which can be explained by the smallest r(h) and the highest D for this cation. Such direct explanation was not possible for other cations as the parameters describing the electrolyte properties are depends on each other and should be analyzed in conjunction. The observed correlations shows that the diffusion coefficient is the most valuable parameter for selection of the electrolyte for pseudocapacitor applications. The influence of electrode materials properties must be also taken into account, as the crystal structure, grain size, morphology or electrical properties influence the mechanism of charge accumulation of different ions, as it was simultaneously observed in this work.