Recent advances in and perspectives on pseudocapacitive materials for Supercapacitors–A review

Because of their apparent and intrinsic advantages—including their high-power density and high-rate capability, which result from their high surface areas, appropriate pore distributions, tailored morphologies, heterostructures, and diverse types of composites—pseudocapacitive materials have been identified as versatile electrode materials for supercapacitors (SCs) in energy-storage systems (ESSs). In this review, we first summarize the origin, historical development, and basic principles of pseudocapacitive materials in order to understand their fundamental electrochemical properties. Next, we present synthesis strategies that promote the electrochemical performance of pseudocapacitive materials for SCs by utilizing rational design and fabrication techniques. Then, we highlight the latest advances, focusing on the composition/morphology and structure/electrochemical performance relationships of advanced electrode materials with high-energy densities. Specifically, we discuss the following categories: (i) traditional electrode materials (transition-metal oxides/hydroxides and their composites) and (ii) emerging electrode materials, including niobium pentoxide (Nb2O5), layered double hydroxides (LDHs), MXenes, and metal-organic frameworks (MOFs) and covalent-organic frameworks (COFs). Finally, we provide some of our own insights into the major challenges of and prospective directions for developing pseudocapacitive materials for SCs. We hope this Review will help to provide some guidance in the new era of electrode materials for SCs.