Stabilisation of various phases of MnO2 for an affordable supercapacitor

Abstract

Efficient energy storage is essential to bridge the temporal and spatial gap between energy generation and consumption. In this work, a cost-effective transition metal oxide, manganese dioxide (MnO₂), has been synthesised and systematically studied in its four polymorphic forms, α, β, γ, and δ, prepared via hydrothermal/chemical routes by varying the precursor concentrations and reaction times. A comprehensive time–concentration phase diagram has been constructed to map the formation of different phases. Remarkably, α- and δ-MnO₂ were synthesised within 5 min, while 100 hours were required to transform α-MnO₂ into β-MnO₂. The various phases have been confirmed using XRD, UV-Vis, FTIR, BET, XPS, SEM, and TEM. Electrochemical measurements revealed that γ-MnO₂ shows the highest specific capacitance of 306.7 F g⁻¹ at 1 mV s⁻¹. A supercapacitor device has further been fabricated using a PVA/H₃PO₄ gel electrolyte, delivering a capacitance of 2.015 F g⁻¹ at 0.11 A g⁻¹, energy density of 2.5 W h kg⁻¹ and power density of 773.3 W kg⁻¹, and it is capable of being charged up to 2.0 V using a piezoelectric energy harvester. Four such devices connected in series have successfully powered an LED for 260 seconds, demonstrating their potential for self-powered energy storage applications. These devices have been charged through a battery and discharged by applying a load/LED, validating them as efficient energy storage materials and devices thereof.