Rational design of redox active amorphous Ni-Mn phosphate anchored on vertical graphene nanohills (VGNHs) for solid-state energy storage device

Transition metal phosphates (TMPs) have generated a lot of attention due to their high power and energy potential. However, pristine TMPs materials have sluggish electrode kinetics limiting their practical application. In this study, a sustainable electrochemical deposition approach has been employed for the facile fabrication of amorphous nickel-manganese phosphate (VG-NMP series) nanostructure on vertical graphene nanohills (VGNHs) covered carbon cloth (CC) substrate. Among as-prepared electrodes, the VG-NMP-4 electrode showed optimal electrochemical characteristics. The high specific capacity of VG-NMP-4 was calculated to be 1068 C g⁻¹ (5.14 F cm⁻²) at a current density of 3 A g⁻¹ and maintained 97 % capacity for 5000 cycles, which outperformed previously reported TMPs-based electrode materials. The enhanced charge storage capability of VG-NMP was mainly due to the dense edge planes of VGs and many gaps for electrolyte penetration because the exposed edge planes have larger areas than the basal plane surface, due to the synergistic effect between Ni and Mn metal ions. The VG-NMP-4 was further utilized as the cathode and anode in the PVA-KOH gel electrolyte for constructing a symmetric solid-state (SSS) supercapacitor. This SSS device offers great durability of 94 % over 5000 cycles and a supreme energy density of 106.8 Wh kg⁻¹ at a power density of 0.751 kW kg⁻¹. This excellent performance suggests that the amorphous Ni-Mn phosphate@VGNHs-CC nanostructured electrode material would be a promising choice for energy storage appliances.