Fabrication of a High-Performance Hybrid Supercapacitor Based on Hydrothermally Synthesized Highly Stable Cobalt Manganese Phosphate Thin Films

In the present study, cobalt manganese phosphate (H-CMP-series) thin films with different compositions of Co/Mn are prepared on stainless steel (SS) substrate via a facile hydrothermal method and employed as binder-free cathode electrodes in a hybrid supercapacitor. The XRD study reveals a monoclinic crystal structure, and the FE-SEM analysis confirmed that H-CMP-series samples displayed a nano/microarchitecture (microflowers to nanoflakes) on the surface of SS substrate with excess available surfaces and unique sizes. Interestingly, the synergy between cobalt and manganese species in the cobalt manganese phosphate thin film electrode demonstrates a maximum specific capacitance of 571 F g^-1 at a 2.2 A g^-1 current density in 1 M KOH. Besides, the nano/microstructured cobalt manganese phosphate was able to maintain capacitance retention of 88% over 8000 charge-discharge cycles. More importantly, the aqueous/all-solid-state asymmetric supercapacitor manufactured with the cobalt manganese phosphate thin film as the cathode and reduced graphene oxide (rGO) as the anode displays a high operating potential window of 1.6 V. The aqueous asymmetric device exhibited a maximum specific capacitance of 128 F g^-1 at a current density of 1 A g^-1 with an energy density of 45.7 Wh kg^-1 and a power density of 1.65 kW kg^-1. In addition, the all-solid-state asymmetric supercapacitor device provides a high specific capacitance of 37 F g^-1 at 1 A g^-1 with 13.3 Wh kg^-1 energy density and 1.64 kW kg^-1 power density in a polymer gel (PVA-KOH) electrolyte. The long cyclic life of both devices (87 and 84%, respectively, after 6000 cycles) and practical demonstration of the solid-state device (lighting of a LED lamp) suggest another alternative choice for cathode materials to develop stable energy storage devices with high energy density. Furthermore, the aforementioned study paves the way to investigate phosphate-based materials as a new class of materials for supercapacitor applicability.