Synthesis and electrochemical supercapacitive performance of copper cobalt spinel hollow spheres

Supercapacitor is a rapidly growing industry that is revolutionizing the energy storage technology today. Their high power density and excellent low temperature performance have made them the technology of choice for back-up power, cold starting, flash cameras and regenerative braking. They also play an important role in the progress of hybrid and electric vehicles, consumer electronics, military and space applications [1]. Supercapacitors are expected to have more industrial applications if their energy density could be improved. This has triggered tremendous research efforts in order to develop new electrode materials that are capable of providing a huge amount of energy in a short period of time [2]. Metal oxides have a higher capacitance/energy than carbonaceous materials due to Faradaic redox reactions [3]. Among them, binary metal oxides such as spinels are more efficient than single metal oxides owning to their richer redox reactions [4]. Herein, for the first time, we report mesoporous copper cobaltite (CuCo2O4) hollow spheres prepared by a facile method as a high-performance supercapacitor electrode material. Mesoporous CuCo2O4 hollow spheres was prepared as follow: 0.5 mmol of Co(NO3)2·6H2O, 0.25 mmol of Cu(NO3)2·6H2O, and 16 mL of glycerol were dissolved into 80 mL of isopropanol. The solution was then transferred to a Teflon-lined stainless steel autoclave and kept at 180 °C for 6 h. After cooling, the brown precipitate (CuCo-glycerate) was centrifuged, washed with several times with ethanol and dried in an oven at 80 °C. In order to obtain the CuCo2O4 hollow spheres, the as-synthesized CuCo-glycerate was annealed at 350 °C for 2 h. XRD, XPS, SEM and TEM images confirm the preparation of CuCo2O4 hollow spheres. Also, BET and BJH isotherms confirm mesoporous structure of sample. The electrodes were prepared by the painting of mixing active material, acetylene black, and polyvinylidene fluoride (PVDF) (with a mass ratio of 85:10:5) on the Ni foams. The half-cell electrochemical performance of the electrodes were 274 investigated in a 6 M KOH solution. The Mesoporous CuCo2O4 hollow spheres electrodes exhibit excellent electrochemical performance, including an ultra-high specific capacitance of 1212 F g−1 at 2 A g−1 and 605 F g−1 at ultra-high current density of 60 A g−1 (50% retention) and excellent cycle life (only 7 % capacitance loss after 5000 cycles at current density of 5 A g−1).This outstanding electrochemical performance of these electrodes can be attributed to their hollow sphere morphology with unique properties, such as mesoporous structure, high surface area and nano-scale shell thickness. Mesoporous structures minimize the diffusion distance from the external electrolyte to the interior surfaces by acting as ion-buffering reservoirs which enhances the accessibility of electrolyte not only at its surface but also throughout the entire material [5]. High specific surface area improves the utilization of the active material by providing more active sites with larger interfacial area between the electrode material and the electrolyte ions [6]. Nanosized shell thickness lead to faster kinetics and higher conductivity by providing short transport pathways for electrons [5]. We envision these electrodes to be useful in a broad range of applications such as supercapacitors, lithium ion batteries, fuel cells, gas sensors, biosensors, catalysis, and other electronic devices.