Dopant Effects of Gd3+ on the Electrochemical Pseudocapacitive Characteristics of Electroactive Mesoporous NiO Electrodes for Supercapacitors

Undoped and gadolinium doped nanostructured mesoporous materials, such as NiO, Ni0.98Gd0.02O, Ni0.95Gd0.05O, and Ni0.92Gd0.08O, were synthesized by a facile hydrothermal route using urea as the hydrolysis controlling agent and were studied for supercapacitor applications. The thermal stability of the synthesized samples was identified by thermogravimetric analysis. The phase structure of the as-synthesized and calcined materials was characterized by using powder X-ray diffraction. The average crystallite size of the oxide materials was found to be in the range of 8.2–11.3 nm. Fourier transform infrared spectroscopy revealed the metal–oxygen bond in the compounds. The analyzed morphological phenomenon of the prepared samples confirms the mesoporous flake-like shape. The N2 adsorption/desorption isotherms were performed to examine the surface area and pore-size distribution. The elemental composition and charge states analyses were obtained by energy-dispersive X-ray and X-ray photoelectron spectroscopy, respectively. Cyclic voltammetry, galvanostatic charge/discharge, and electrochemical impedance spectroscopic measurements were applied in an aqueous electrolyte to investigate the electrochemical performances of the active electrodes. Among the four electrodes, Ni0.98Gd0.02O exhibits the highest surface redox reactivity and shows optimum high specific capacitance of 1190 F/g at a current density of 2 A/g. The cycling lifespan of Ni0.98Gd0.02O with capacitance retention of 81.43% was inspected over 3000 cycles at a current density of 3 A/g.