Hybridization of Fe-O in iron doped hydroxyapatite for superior electrochemical charge storage

Hydroxyapatite (HAp) is an interesting phosphate candidate with a weak electrical conductivity that can be tuned by doping metal ions to enhance the active sites for electrochemical charge storage. HAp and Fe-doped HAp were prepared by a wet chemical method. On doping, the crystallite size reduces to 9.56 nm. The surface area of 0.01FH (0.01% Fe) is 152.17 m2/g which is ∼3 times higher compared to PH. The 0.01FH sample shows the highest specific capacitance of 351.3 F/g at 30 mA/g. While X-ray photoemission spectroscopy shows the oxidation states of Fe in 2+ and 3+, X-ray absorption indicates (i) the enhancement of unoccupied density of states of Fe, and (ii) reduced interatomic distance of Fe-O in 0.01FH. These results show that the augmentation of surface properties and hybridization of Fe-O and the modification of its local atomic structures are responsible for high electrochemical charge storage in 0.01FH