Transparent ZnMn2O4 thin films on indium tin oxide (ITO) were prepared through spray pyrolysis and implemented as electrodes in symmetric supercapacitors (SSCs). A specific capacitance value of 752 F g-1 at 0.5 A g-1 and a 70% retention over 3000 galvanostatic charge-discharge (GCD) cycles were reached with a 1.0 M Na2SO4 electrolyte in a three-electrode electrochemical cell. Analysis of the cycled electrodes with 1.0 M Na2SO4 revealed a local loss of electrode material; this loss increases when electrodes are used in SCCs. To avoid this drawback, solid polyvinylpyrrolidone-LiClO4 (PVP-LiClO4) and quasi-solid polyvinylpyrrolidone-ionic liquid (PVP-ionic liquid) electrolytes were tested in SSCs as substitutes for aqueous Na2SO4. An improvement in capacitance retention without a loss of electrode material was observed for the PVP-ionic liquid and PVP-LiClO4 electrolytes. With these non-aqueous electrolytes, the tetragonal structure of the ZnMn2O4 spinel was maintained throughout the cyclic voltammetry (CV) cycles, although changes occurred in the stoichiometry from ZnMn2O4 to Mn-rich Zn1-xMn3-xO4. In the case of the electrolyte 1.0 M Na2SO4, the loss of Zn2+ led to the formation of MnO2 via Zn1-xM3-xO4. The location of the three SCCs in the Ragone plot shows supercapacitor behavior. The electrochemical results prove that the pseudocapacitance is the major contributor to the electrode capacitance, and the SCCs can therefore be considered as pseudocapacitors.
Keywords: ZnMn2O4; electrolyte; energy storage; supercapacitor; thin film; transparent.