Recent advances in rare earth doped metal oxide based nanomaterials for supercapacitors

Greater theoretical specific capacity, nontoxicity, cost-effectiveness, and ease of fabrication of metal oxide nanomaterials make them attractive materials globally for devices used to store energy, like batteries and supercapacitors. Metal oxide nanomaterials-based supercapacitors, commonly known as pseudocapacitors, have been explored exponentially over the past several years. However, these pseudocapacitor materials face issues like low conductivity, limited surface area, and inferior electrochemical activity. Rare earth doping is a powerful approach to generate localized electronic states, promote chemical activity, and improve electronic conductivity and chemical stability in metal oxide nanomaterials, which enhance electrolyte ion diffusion and charge transport in supercapacitors. In this review, a systematic critical review of rare earth doped various metal oxide nanomaterials has been presented. Beginning with an overview of the fundamentals of supercapacitors, different synthesis strategies, the importance and impact of adding rare earth dopants on the crystal structure, surface structure, chemical reactivity and their correlation with electrochemical activity have been discussed thoroughly. We find that the electrochemical performance of metal oxide nanomaterials can be enhanced by doping with appropriate rare earth element or rare earth element oxide hybrid. However, most of the studies are focussed on aqueous supercapacitors in three-electrode configurations and reports on practical solid-state supercapacitor devices are limited. Moreover, theoretical works in support of experimental observations are rarely conducted. Our review of the most recent results including comparative electrochemical performance like capacitance, cyclic stability, etc. of various rare earth doped metal oxide nanomaterials suggests that extensive research in this direction is needed to address the current limitations and develop more practical supercapacitor devices.