Inhibition of Redox Behaviors in Hierarchically StructuredManganese Cobalt Phosphate Supercapacitor Performance by Surface TrivalentCations

摘要

The stability and performance of supercapacitor devices are limited by the diffusion-controlled redox process occurring at materials’ surfaces. Phosphate-based metal oxides could be effectively used as pseudocapacitors because of their polar nature. However, electrochemical energy storage applications of Mn–Co-based phosphate materials and their related kinetics studies have been rarely reported. In this work, we have reported a morphology-tuned MnxCo3–x(PO4)2·8H2O (MCP) spinel compound synthesized by a one-step hydrothermal method. Detailed physical and chemical insights of the active material coated on the nickel substrate are examined by X-ray diffraction, field-emission scanning electron microscopy, field-emission transmission electron microscopy, and high-resolution X-ray photoelectron spectroscopy analyses. Physiochemical studies reveal that the well-defined redox behavior usually observed in Co²⁺/Ni²⁺ surface-terminated compounds is suppressed by reducing the divalent cation density with an increased Co³⁺ and Mn³⁺ surface states. A uniform and dense leaflike morphology observed in the MnCo2 phosphate compound with an increased surface area enhances the electrochemical energy storageperformance. The high polar nature of P–O bonding formed atthe surface leads to a higher rate of polarization and a very lowrelaxation time, resulting in a perfect square-shaped cyclic voltagramand triangular-shaped galvanostatic charge and discharge curve. Wehave achieved a highly pseudocapacitive MCP, and it can be used asa vital candidate in supercapacitor energy storage applications.