Support-induced morphology and content tailored NiCo₂O₄ nanostructures on temperature-dependent carbon nanofibers with enhanced pseudocapacitive performance

Direct anchorage of metal oxides on carbon nanofibers is challenging, and simultaneous modulation of morphology, content and thus pseudocapacitance of anchored metal oxides has never been achieved. Herein we report direct anchorage of NiCo₂O₄ nanostructures with tailored morphology, content and pseudocapacitance, on temperature-dependent carbon nanofibers (CNFs), which are derived from zinc-trimesic acid fibers at a temperature no more than 600 °C. The temperature-dependent CNFs, including CNF-390, CNF-450 and CNF-600, were first investigated by SEM, TG, FT-IR, XPS and Boehm titration studies. It is found that their textural property, thermal stability and surface functionality vary depending on pyrolysis temperature. As a result, the morphology, content and thus the pseudocapacitance of anchored NiCo₂O₄ nanostructures are also temperature-dependent. It is revealed that as increasing the pyrolysis temperature, the shape of NiCo₂O₄ is transformed from embedded nanoparticles to exposed nanosheets while its content decreases from 95 to 55 wt%. The resultant NiCo₂O₄/CNF-450, bearing highly dispersed nanosheets and optimized NiCo₂O₄ content (72.6%), exhibits the best capacitive performance with a large specific capacitance of 870 F g⁻¹ at a scan rate of 1 mV s⁻¹, and a high energy density of 39.3 Wh kg⁻¹ in the case of power density of 300 W kg⁻¹. The elaborately fabricated NiCo₂O₄/CNF-450 also shows excellent charge-discharge stability (with the capacity retention ratio of 85.7% after 5000 cycles). The improved accessibility for more pseudocapacitive NiCo₂O₄ nanosheets, as well as the strong interaction between NiCo₂O₄ and carbon nanofiber, should be responsible for the enhanced capacitive performance of NiCo₂O₄/CNF-450. This research also provides the concept of support-induced modulation of property and pseudocapacitive performance of anchored metal oxides.