Electrochemical analysis of Na-Ni bimetallic phosphate electrodes for supercapacitor applications

Bimetallic sodium-nickel phosphate/graphene foam composite (NaNi₄(PO₄)₃/GF) was successfully synthesized using a direct and simple precipitation method. The hierarchically structured composite material was observed to have demonstrated a synergistic effect between the conductive metallic cations and the graphene foam that made up the composite. The graphene served as a base-material for the growth of NaNi₄(PO₄)₃ particles, resulting in highly conductive composite material as compared to the pristine material. The NaNi₄(PO₄)₃/GF composite electrode measured in a 3-electrode system achieved a maximum specific capacity of 63.3 mA h g^-1 at a specific current of 1 A g^-1 in a wide potential range of 0.0-1.0 V using 2 M NaNO₃ aqueous electrolyte. A designed and fabricated hybrid NaNi₄(PO₄)₃/GF//AC device based on NaNi₄(PO₄)₃/GF as positive electrode and activated carbon (AC) selected as a negative electrode could operate well in an extended cell potential of 2.0 V. As an assessment, the hybrid NaNi₄(PO₄)₃/GF//AC device showed the highest energy and power densities of 19.5 W h kg^-1 and 570 W kg^-1, respectively at a specific current of 0.5 A g^-1. The fabricated device could retain an 89% of its initial capacity with a coulombic efficiency of about 94% over 5000 cycling test, which suggests the material's potential for energy storage devices applications.