Stabilization of various phases of MnO 2 for affordable supercapacitor

Abstract

Efficient energy storage is essential to bridge the temporal and spatial gap between energy generation and consumption. In this work, cost-effective transition metal oxide, manganese dioxide (MnO ₂ ), has been synthesized and systematically studied its four polymorphic forms α, β, γ, and δ prepared via hydrothermal/chemical routes by varying precursor concentrations and reaction time. A comprehensive time-concentration phase diagram has been constructed to map the formation of different phases. Remarkably, α-and δ-MnO ₂ were synthesized within 5 min, whereas 100 hours are required to transform into β-MnO ₂ . Various phases have been confirmed using XRD, UV-Vis, FTIR, BET, XPS, SEM, and TEM analyses. Electrochemical measurements revealed that γ-MnO ₂ shows the highest specific capacitance of 306.7 F.g⁻¹ at 1 mV.s⁻¹. A supercapacitor device has further been fabricated using PVA/H 3 PO 4 gel electrolyte, delivering a capacitance of 2.015 F.g⁻¹ at 0.1 A.g -1 , energy density of 2.5 Whkg⁻¹ and power density of 773.3 Wkg⁻¹ and capable of being charged up to 2.0 V using a piezoelectric energy harvester. Four such devices connected in series successfully powered an LED for 260 seconds, demonstrating their potential for self-powered energy storage applications. These devices were charged through battery and discharged by applying a load/LED, validating efficient energy storage material and device thereof.