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

Abstract The field of hybrid supercapacitors is a key area in the race to achieve a decarbonised 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 synthesise nickel phosphide (Ni 2 P) and nickel metaphosphate (Ni(PO 3 ) 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 Ni 2 P and Ni(PO 3 ) 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 powder x-ray diffraction, scanning electron microscopy, transmission electron microscopy, and in-situ synchrotron x-ray absorption spectroscopy 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(PO 3 ) 2 //AC hybrid device achieving 43 Wh kg −1 at a power density of 2.4 kW kg −1 , outperforming conventional capacitors. Furthermore, the Ni 2 P//AC device demonstrated excellent cycling stability, retaining 100% of its capacity at 5 A g −1 over 3000 continuous charge–discharge cycles.