Microwave-hydrothermally synthesized Ru_xNiCo_2-xO₄ spinel oxide nanoparticles for high-performance pseudocapacitor...

A series of Ru_xNiCo_2-xO₄ (RNCO; x = 0.00-0.10) spinel oxides at nanoscale were synthesized using a microwave hydrothermal (M-H) process. Structural, morphological, and electrochemical characteristics were investigated by various characterizations. The X-ray diffraction results confirm the formation of a single-phase cubic spinel structure with an Fd-3 m space group for all compositions. The lattice parameter decreases linearly from 8.089 Å (x = 0.00) to 8.085 Å (x = 0.10). Field emission scanning microscopy images reveal nanoparticles with elongated and spherical morphologies, with average grain sizes ranging from 32 nm (x = 0.06) to 55 nm (x = 0.10). Fourier transform infrared spectra exhibit characteristic metal-oxygen vibrational bands around 656 cm^-1, confirming spinel oxide formation. X-ray photoelectron spectroscopy analysis verifies the presence of mixed valence states of Ni²⁺/Ni³⁺, Co²⁺/Co³⁺, and Ru³⁺/Ru²⁺. Electrochemical studies demonstrate enhanced pseudocapacitive behavior with Ru incorporation. The results were corroborated to Randles's, Dunn's and impedance analysis. Notably, the Ru_0.06NiCo_1.4O₄ (RNCO-4) electrode delivers a superior specific capacitance of 1541 F g^-1, and an energy density of 53.35 Wh kg^-1 at a power density of 2233 W kg^-1. These results indicate that Ru-substituted NiCo₂O₄ spinel oxides synthesized via the Microwave-Hydrothermal (M-H) route resulted in novel electrode materials for supercapacitor applications.