Recent progress and applications of niobium-based nanomaterials and their composites for supercapacitors and hybrid ion capacitors

Niobium (Nb)-based materials show great potential in the field of supercapacitors (SCs) and hybrid ion capacitors (HICs) because of their unique crystal structure, rapid ion diffusion ability, excellent chemical durability, and high safety, although they are rare and expensive. However, pure niobium pentoxide has poor conductivity, so the preparation of composite materials and doped materials can effectively enhance its conductivity, thus improving its rate capability. In this review, we summarize the research advances in Nb-based oxides and composites, niobium nitrides, niobium carbides, niobium selenides, and niobium sulfides in recent years. We compare the effect of crystal structures, synthesis methods, and morphology control on the electrochemical performance of niobium-based materials. The energy storage mechanism of Nb-based materials is discussed through corresponding electrochemical reactions and advanced characterization technologies. In addition, the feasible strategies to improve the electrochemical performance of Nb electrodes are briefly introduced. Finally, the future challenges and outlooks of Nb-based materials in energy storage devices are also discussed.