From supercapacitors to photo-assisted supercapacitors: The development of transition metal oxides in synthesis strategies, mechanisms, and challenges

Transition metal oxides (TMOs) and their composites with other metal oxides have attracted considerable interest as electrode materials for supercapacitors due to their rich redox chemistry, high theoretical capacitance, and structural versatility. In the emerging field of photo-assisted supercapacitors, the semiconducting nature and tunable band gaps of TMOs make them ideal materials for light-harvesting and storage. The purpose of this review is to summarize recent advances in the design and fabrication of metal oxides, composites, and heterostructured architectures. A combination of conventional synthesis routes and advanced fabrication techniques is demonstrated herein, which allows precise control of morphology, which is in turn responsible for ion diffusion pathways, accessibility of electroactive sites, and overall electrochemical performance, as well as adjusting light absorption and storage characteristics critical for photo-assisted supercapacitors. Additionally, the review highlights challenges associated with materials, including low conductivity, structural instability, and limited light absorption efficiency, as well as those associated with device components and long-term stability. Our final section discusses future research directions, morphology-performance correlations, and scalable processing routes to unlock the full potential of TMOs and oxide-based composites for the production of high-performance supercapacitors and photo-assisted supercapacitors.