Understanding the mechanism of hydrogenated NiCo2O4 nanograss supported on Ni foam for enhanced-performance supercapacitors

Abstract

Spinel NiCo₂O₄ is considered a promising supercapacitive material because of its high theoretical capacity (greater than 3000 F g⁻¹), nontoxicity, and safety. Here, we report that electrodes of porous NiCo₂O₄ nanograss grown in situ and supported on Ni foam achieved remarkable enhancement in electrochemical performance through facile hydrogenation at 300 °C for time periods of 1–4 h. The electrodes synthesized via 3 h of hydrogenation (H-NiCo₂O₄-3h) exhibited superior comprehensive electrochemical performance compared with the pristine pattern (air-annealed). The peak value of the area capacitance improved from the pristine 0.88 F cm⁻² (338.5 F g⁻¹) to 2.1 F cm⁻² (807.7 F g⁻¹) of H-3h, an increase of ∼240%. Additionally, the capacity retention from 1 to 30 mA cm⁻² improved to a value of 71% (H-NiCo₂O₄-3h), which was superior to that of non-hydrogenated samples (54%). Furthermore, the long-cycling performance at 10 mA cm⁻² exhibited a capacitance activation in H-NiCo₂O₄-3h within the first 1000 cycles, from 2.4 (923 F g⁻¹) to 3.2 F cm⁻² (1230 F g⁻¹), and declined to 1.5 F cm⁻² (577 F g⁻¹) after another 2000 cycles; the last value is still greater than that of the pristine pattern (1.3 F cm⁻² (500 F g⁻¹)). The prominent electrochemical capacitive properties of hydrogenated NiCo₂O₄ are attributed to the enhancement in the electrical conductivity observed by an in situ TEM electrical test, resulting from the formation of oxygen vacancies in disordered surface layers (∼5 nm) observed in the hydrogenated samples based on in situ transmission electron microscopy characterization. Our findings provide a scientific explanation for the remarkable hydrogenation-induced electrochemical performance of cobalt oxide or binary nickel cobaltite compounds and offer a new route for the large-scale production of high-performance supercapacitor electrodes.