Retaining precursor morphology: in-situ growth of Ni₂P and Ni(PO₃)₂ for advanced hybrid supercapacitor electrodes

The field of hybrid supercapacitors is a key area in the race to achieve a decarbonized future and is attracting huge attention from researchers. It is important to develop simple methods for electrode preparation using low-cost material classes. Nickel-based materials have long been of high importance as battery and supercapacitor materials with established commercial relevance. In this study, α-Ni(OH)₂, prepared in our previous work, was used as a precursor to synthesize nickel phosphide (Ni₂P) and nickel metaphosphate (Ni(PO₃)₂). Through an in-situ phosphorisation process, the unique nanoflower morphology of the α-Ni(OH)₂ precursor was largely retained, resulting in nano-porous spherical structures in the final Ni2P and Ni(PO3)2 products. The preservation of the morphologies of the materials effectively enhances their electrochemical performance by improving charge transfer and ion diffusion pathways. The electrode materials were characterised using PXRD, SEM, TEM, and in-situ synchrotron XAS techniques, combined with three-electrode electrochemical measurements to gain a deeper understanding of their properties. Subsequently, the nickel-based materials were assembled with activated carbon (AC) to fabricate hybrid supercapacitors. The devices revealed promising results, with the Ni(PO₃)₂//AC hybrid device achieving 43 Wh kg⁻¹at apower density of 2.4 kW kg⁻¹, outperforming conventional capacitors. Furthermore, the Ni2P//AC device demonstrated excellent cycling stability, retaining 100% of its capacity at 5 A g⁻¹ over 3000 continuous charge-discharge cycles.