Experimental and Modelling Analysis of Electrolyte Effects in Zinc-Modified MnO₂/Graphene Symmetric Supercapacitor

Abstract

Aqueous symmetric supercapacitors are promising and sustainable energy storage devices with low cost, low toxicity, operational safety and simple design, using the same active material in both electrodes. Here, we investigate the effect of electrolyte on the interfacial charge storage by testing zinc-doped manganese dioxide/graphene (Zn-MnO 2 /G) nanocomposite electrodes in symmetric cells using KOH and Na 2 SO 4 electrolyte. The composite combined the pseudocapacitive redox activity of MnO 2 with the electronic conductivity and surface accessibility of graphene, and the Zn incorporation was intended to modify the local defect chemistry and charge-transfer behavior. Nitrogen adsorption-desorption analysis suggested mesoporous texture with cumulative pore volume of 0.0453 cm 3 g − 1 , pore surface area of about 13.0 m 2 g − 1 and dominant pore width of about 3.38 nm that can facilitate electrolyte penetration and access to redox-active sites. Electrochemical measurements demonstrated that the studied symmetric cell configuration showed electrolyte-dependent capacitive behavior. The KOH-based device showed lower impedance, more favorable capacitive relaxation and better energy-density retention than the Na 2 SO 4 based device. The rate-capability fitting also showed a smaller rate-induced energy loss in KOH. The improvement in the porous Zn-MnO 2 /G electrode was ascribed to more favorable ion transport, lower interfacial resistance and more efficient utilization of Mn redox-active sites in the alkaline electrolyte. These results suggest that the device-level performance of Zn-MnO 2 /G symmetric supercapacitors is significantly affected by the electrolyte selection.

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