Phase conversion effect on cobalt phosphate for asymmetric supercapacitor applications

Abstract

Cobalt phosphate and cobalt ammonium phosphate were prepared via a microwave irradiation method without changing the basic precursor, but rather by varying the pH (10 and 12) value with the help of ammonium solution. The prepared material has been employed as an effective electrode for supercapacitors. From the XRD results, the pH-10 results indicate that the obtained cobalt phosphate is in the form of a monoclinic phase structure and the pH-12 results indicate that the cobalt ammonium phosphate is in the form of an orthorhombic phase structure. SEM studies revealed identical flower-like cobalt phosphate and non-uniform distribution of rectangular plates for cobalt ammonium phosphate. FTIR and Raman spectra were used for the analysis of the fundamental modes of vibration as observed and the oxidation and binding energies of cobalt phosphate were evaluated from XPS study revealing Co³⁺ and Co²⁺ as the cobalt oxidation states. UV-Vis spectroscopy was used to analyze the band gap of both materials. From the obtained band gap, theoretical calculations to determine electron concentration and hole concentration were performed. Thus, the obtained results show that the prepared material was a p-type semiconductor. From the electrochemical analysis, the prepared cobalt phosphate delivered 77.55 mAh g⁻¹ at 0.5 A g⁻¹ in 2 M H₂SO₄ electrolyte. The CoPO//activated carbon device performance showed 24.02 Wh kg⁻¹ energy density and 299.21 W kg⁻¹ power density for the asymmetric device. The DFT simulation optimized structure, band structure, and total density of states of the prepared material were studied. The prepared cobalt phosphate is an efficient, cost-effective material for supercapacitors, paving the way for their practical application.