Investigating the Effect of Metals (M = Ni, Co, Fe) on the Energy Storage Performance of Porous Phosphonates for Interdigital In-Plane Microsupercapacitors

The most challenging topic at the moment is energy demand, which rises as the world's population grows. The current study provides a productive method for investigating the impact of transition metal centers (Ni, Co, and Fe) for energy storage applications toward supercapacitance studies in different metal phosphonates (NiPPA, CoPPA, and FePPA) that are hydrothermally synthesized utilizing phenylphosphinic acid as an organophosphorus ligand. Notably, NiPPA manifests the highest capacitance of 3792 F g–1 with a battery-type redox charge storage mechanism studied by Dunn's method. The better redox reaction with prominent oxidation and reduction peaks in NiPPA compared to CoPPA and FePPA estimated a higher specific capacitance value with enhanced charge storage performance. An asymmetric two-electrode coin cell (NiPPA//AC) was fabricated using NiPPA (cathode) and activated carbon (anode), and the electrochemical activities were investigated for implementation in practical use. Additionally, to improve NiPPA's internal conductivity, an asymmetric planar interdigital microsupercapacitor device (MSC) was designed using a low-cost vacuum filtration method. The interdigital structure facilitates faster ionic migration due to the shorter diffusion channel in the same plane of the fabricated device. The microelectrode demonstrates an areal capacitance of 75.5 mF cm–2 with proper flexibility at different bending states for wearability purposes.