Investigation on transition metal ions doped hydroxyapatite for electrochemical charge storage applications

Abstract

Design and synthesis of cost-effective electrode materials and enhancing their functionalities are prioritized in energy storage and conversion. Hydroxyapatite, Ca10(PO4)6(OH)2 (HAp) is an interesting candidate belonging to the family of calcium phosphate with weak electrical conductivity. Doping metal ions in the matrix of HAp can enhance the active sites responsible for electrochemical charge storage, paving way for the development of phosphate based energy storage system. Owing to the different major role of Sn, Fe and Ni elements in the HAp matrix for energy storage, the optimal structural adjustment, with respect to the fine-tuned morphology, electrochemical active sites, and active oxidation state for charge storing capacity is obtained from the composites with the different concentrations of doping. The results indicates that the crystallite size of the doped samples were reduced with increase of doping concentration. In Fe doped sample the crystallite size was reduced to 9.56 nm from 26.82 nm. In Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) analyses showed that the dopant alters the morphology from rod to agglomerated spheres and needles with reduced particle size. The surface area and pore diameter are enhanced at low Sn2+ ions (11.29 %) also the surface area of low concentration Fe doped sample is 152.17 m2/g nearly 2.95 times higher compared with HAp. The enhanced electrochemical behaviour is also observed for different metal ion doping such as, 231 Fg-1 at 3 mAg-1 with 100 % cyclic stability up to 800 cycles for Sn doping, 351.3 F/g at 30 mA/g and the energy density 9.41Wh/kg with cyclic stability is up to 1000 cycles. Interestingly the Ni-Doped HAp sample exhibited the enhanced specific capacity of about 415.35Fg-1 at 10 mAg-1 which is much higher than hydroxyapatite. Notably the correlated parameters such as structural distortion, vacancies and defects in the crystal system due to doping of metal ions. Thus, the overall results demonstrate the solution for improving