Effect of the Nature of the Electrolyte on the Behavior of Supercapacitors Based on Transparent ZnMn 2 O 4 Thin Films.

Transparent ZnMn 2 O 4 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 Na 2 SO 4 electrolyte in a three-electrode electrochemical cell. Analysis of the cycled electrodes with 1.0 M Na 2 SO 4 revealed a local loss of electrode material; this loss increases when electrodes are used in SCCs. To avoid this drawback, solid polyvinylpyrrolidone-LiClO 4 (PVP-LiClO 4 ) and quasi-solid polyvinylpyrrolidone-ionic liquid (PVP-ionic liquid) electrolytes were tested in SSCs as substitutes for aqueous Na 2 SO 4 . An improvement in capacitance retention without a loss of electrode material was observed for the PVP-ionic liquid and PVP-LiClO 4 electrolytes. With these non-aqueous electrolytes, the tetragonal structure of the ZnMn 2 O 4 spinel was maintained throughout the cyclic voltammetry (CV) cycles, although changes occurred in the stoichiometry from ZnMn 2 O 4 to Mn-rich Zn 1-x Mn 3-x O 4 . In the case of the electrolyte 1.0 M Na 2 SO 4 , the loss of Zn 2+ led to the formation of MnO 2 via Zn 1-x M 3-x O 4 . 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.