Capacitive behaviour of nanocrystalline octacalcium phosphate (OCP) (Ca8H2(PO4)6·5H2O) as an electrode material for supercapacitors: biosupercaps

Abstract

Octacalcium phosphate (OCP) is classified as a low-temperature phase of calcium phosphate (CaPs); it is a widely used ceramic material in biomedical applications. Interestingly, this study demonstrated the capacitive behavior of OCP as an electrode material in supercapacitors, alternatively named biosupercaps, for the first time in the literature. OCP powder was synthesized by solution precipitation at pH 5.5 at 60 °C in the presence of succinic acid. X-Ray diffraction (XRD) fully confirmed the OCP phase, with a crystallite size of around 40 nm, as calculated by the Scherrer equation. The FE-SEM micrographs of the OCP powder revealed plate-like morphology with a high surface area/thickness ratio. The surface widths of these layers ranged from about 2 to 100 microns, whereas the thickness of the layers was on the nanoscale (<100 nm). Raman spectroscopy was performed to confirm the microstructural formation of the OCP powder and electrodes according to the Raman spectra. Asymmetric and symmetric capacitors were prepared by various designs using OCP powder as a potential electrode material. The electrochemical performance of each biosupercap containing OCP was analyzed by a potentiostat in terms of current–voltage (CV) curves; each sample presented a typical pseudocapacitive behaviour. The electrochemical impedance spectra (EIS) of the OPC materials confirmed their significant capacitive performance, with up to 6 mA h g⁻¹ specific capacity (SCp); this may be valuable for future medical electronics such as biocompatible energy storage and harvesting microdevices.