Comparative synthesis of ammonium manganese phosphate (NH4MnPO4) and its electrochemical performance enhancement using ammonium metavanadate in sulfuric acid for supercapacitor applications

Abstract

This work investigates the electrochemical behavior of ammonium manganese phosphate electrodes synthesized via a microwave method using two different precursors, MnSO₄ and Mn(CH₃COO)₂, for supercapacitor applications. From the Tauc plot, the NH₄MnPO₄–MnSO₄ and NH₄MnPO₄–Mn(CH₃COO)₂ band gaps were estimated to be 3.11 eV and 3.00 eV, respectively. SEM analysis revealed that the Mn(CH₃COO)₂-derived electrode forms sheet-like structures with higher surface area and porosity, promoting efficient ion diffusion and electron transport, which leads to superior electrochemical nature. Cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD) studies demonstrated combined electric double-layer and pseudocapacitive behavior. NH₄VO₃, as a redox additive in 2 M H₂SO₄ electrolyte, enhanced reversible vanadium redox reactions, increasing the specific capacity by 54.4% and improving the energy density, rate capability, and long-term cycle stability. Full-cell tests with the NH₄MnPO₄–Mn(CH₃COO)₂//AC device showed excellent performance, retaining 99.4% capacity after 5000 cycles and highlighting the promising potential of ammonium manganese phosphate-based electrodes with redox-active electrolytes for supercapacitor applications.