High-performance pseudocapacitor electrode materials: cobalt (II) chloride–GQDs electrodes

This paper reports an effective synthesis, for the first time, of cobalt (II) chloride (CoCl₂)-doped graphene quantum dots (cobalt (II) chloride–GQDs). Excellent results were obtained by using cobalt (II) chloride powder and graphene oxide as source materials in a facile hydrothermal process. The prepared materials were characterized by using high-resolution transmission electron microscopy (HRTEM), Raman spectroscopy, ultraviolet–visible spectroscopy, Fourier transform infrared spectroscopy, powder X-ray diffraction and photoluminescence (PL). HRTEM images suggest that the diameter of the majority of the cobalt (II) chloride–GQDs is in the narrow range of 2·5–7 nm, much smaller than that of GQDs, which are approximately 15 nm. By using different excitation energies in PL, the appearance of a peak introduces an additional energy level between π and π*, reinforcing the cobalt (II) chloride doping process. The authors observed that cobalt (II) chloride electrodes can exhibit a reversible redox reaction of Co²⁺ ↔ Co⁴⁺ and show specific pseudocapacitance of ∼300 F/g. The new charge storage mechanism using cobalt (II) chloride electrodes is easily scalable and highly economical for future pseudocapacitors and can be extended to a large variety of commercial inorganic salt electrodes.