Hydrothermally Engineered Mn–Zn Phosphate/rGOComposites: A Synergistic Approach for Asymmetric Solid-State Supercapacitors

Abstract The growing need for highly efficient energy storage devices has accelerated the invention of novel electrode materials. In this context, the current work investigated the incorporation of reduced graphene oxide (rGO) into the synthesis of manganese–zinc phosphate (MZP-C series) heterostructures via a one-step hydrothermal synthesis route. Notably, the inclusion of rGO provided a strongly conductive network that promoted the nucleation and growth of MZP, increasing electrical connectivity and structural integrity. Accordingly, the structural study confirmed the successful formation and high crystallinity of the MZP-C phase. At the same time, the morphological investigation revealed heterogeneous nano/microsheet architectures with a large surface area and porosity, thereby enabling fast ion diffusion and efficient electron transport. This synergistic integration significantly improved the electrochemical efficiency of the composite electrodes. Among the synthesized samples, the MZP-C-4 electrode exhibited an exceptional specific capacitance (capacity) of 1420 F/g (236 mAh/g) at 3 A/g in 1 M KOH and retained approximately 95% of its original capacitance after 10,000 charge–discharge cycles. To further evaluate its practicality, an asymmetric solid-state supercapacitor device (ASSSD) was designed with MZP-C-4 as the cathode and rGO as the anode, operated within a +1.6 V potential range in a PVA-KOH polymeric gel electrolyte. The developed ASSSD provided a maximum specific capacitance (capacity) of 116 F/g (51 mAh/g), a high energy density of 41.6 Wh/kg, and a power density of 3990.4 W/kg. In addition, this device retained about 92% of its capacitance after 10 000 cycles, indicating exceptional long-term endurance. Ultimately, this work shows that integrating conductive carbon frameworks with transition-metal phosphates (TMPOs) produces highly efficient electrode materials with excellent conductivity and biocompatibility, promising for next-generation energy storage and biomedical devices.