Comprehensive review on MXenes, transition-metal oxides, MOFs, and COFs for supercapacitors: Progress, challenges, and future solutions

Pseudocapacitive nanostructured materials have emerged as versatile electrode candidates for supercapacitors owing to their large surface areas, tailored morphologies, engineered heterostructures, optimized pore distributions, and diverse composite architectures. These intrinsic attributes endow them with high power density and rapid charge-discharge capability, positioning them as crucial materials for next-generation energy storage devices. This review begins by tracing the origins, historical development, and fundamental principles of pseudocapacitive behavior, providing the foundation for understanding their electrochemical properties. We then examine synthesis strategies aimed at enhancing electrochemical performance, focusing on rational design and advanced fabrication methods. Particular emphasis is placed on the relationships between composition, surface modification, structural morphology, and resulting electrochemical behavior, which collectively govern capacitance and energy density. The discussion addresses two broad classes: traditional materials such as transition-metal oxides (TMOs) and hydroxides and their composites, and emerging systems including MXenes, metal-organic frameworks (MOFs), and covalent-organic frameworks (COFs). The review concludes with an outlook on key challenges and future opportunities, highlighting the importance of scalable synthesis, structural engineering, stability, and sustainability. This article provides a critical perspective and roadmap for the development of advanced pseudocapacitive materials that bridge the performance gap between conventional supercapacitors and batteries.