Enhancement in supercapacitive performance of calcium phosphate nanoneedles through Ni-ion incorporation: Insights into atomic and electronic structures

Phosphates have emerged as promising materials for electrochemical charge storage applications. However, a comprehensive understanding of their storage mechanism remains elusive. The present work reports the impact of nickel ion doping on hydroxyapatite (PH) prepared by a wet-chemical route. X-ray diffraction (XRD) reveals the formation of NiO and Raman and Fourier-transform infrared (FTIR) analyses demonstrate the suppression of phosphate modes on doping. The incorporation of Ni ions induces nanoneedles morphology, effectively engineering the surface area and volume of the material. Notably, specific capacitance significantly improves to 415.35 Fg^-1 at 10 mAg⁻¹ for Ni-incorporated PH (0.1 M of Ni) (0.1NH), a remarkable enhancement compared to 166.77 Fg^-1 at 10 mAg⁻¹ for pristine (PH). X-ray photoelectron spectroscopy (XPS) confirms the mixed valence state of Ni^2+/3+ and the presence of Ni–O bonding on Ni incorporation. Moreover, X-ray absorption spectroscopy (XAS) reveals the formation of Ni–O–Ni clusters, enhanced charge transfer by Ni ions, and optimized coordination of Ni–O bonds. The comprehensive correlation established between the phase, morphology, and electronic and local atomic structures elucidates the excellent electrochemical charge storage performance. This investigation sheds light on the potential of Ni-doped hydroxyapatite as a promising material for electrochemical applications, contributing valuable insights for the design and optimization of advanced energy storage systems.