Effect of the Nature of the Electrolyte on the Behavior of Supercapacitors Based on Transparent ZnMn₂O₄ Thin Films

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 Zn_1−xMn_3−xO₄. In the case of the electrolyte 1.0 M Na₂SO₄, the loss of Zn²⁺ led to the formation of MnO₂ via Zn_1-xM_3-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.