Optimization of Al-doped NiCo2O4 hybrid nanostructures and their electrochemical activation feature in supercapacitors

The performance of supercapacitors majorly depends on electrode materials, while optimizing the composition and microstructure of materials is the key for boosting electrochemical activity. Herein, Al-doping and electrochemical activation strategy are combined to boost the electrochemical activity of NiCo2O4. The influence of Al dopant dosage on the performance is discussed firstly. With an increasing Al3+ dosage, Al-doped samples show a morphology transition from nanoneedles to hybrid structures of nanowire/nanosheet. Optimized Al1.0-NiCo2O4 sample with hybrid nanostructures delivers the highest areal capacitance of 8.22 F cm- 2 at 1 mA cm- 2, much higher than that of pristine NiCo2O4 and other Al-doped samples. Serving as a working electrode, the activation of Al1.0-NiCo2O4 is conducted to induce the reconstruction of microstructure and composition. When activated for 200 cycles, expansive rod structures consisted of vertical nanosheets are generated, which presents a high areal capacitance of 9.93 F cm- 2. Under the same operating conditions, the activation is also proceeded on the Al0.5-NiCo2O4 sample with only nanowires. The maximum capacitance reaches 6.49 F cm- 2, much lower than that of activated Al1.0-NiCo2O4, due to the absence of nanosheets in Al0.5-NiCo2O4. The activation process is confirmed as the structure transition from nanoparticles to thin nanosheets, with the composition transition from initial Al-doped NiCo2O4 to NiCoAl-OH/OOH. When applied in an asymmetric supercapacitor, the NF/Al1.0NiCo2O4 electrode presents a superior electrochemical performance and a charge/discharge activation feature.