Functionalization Techniques for the Development of Metal-Oxide/Hydroxide-Based Supercapacitors

Supercapacitors are surface charge storage devices. Based on storage mechanisms, supercapacitor materials are classified into two categories: electric double-layered capacitive (EDLC) and pseudocapacitive materials. EDLC materials are non-faradic (redox inactive) materials, and it can store charges by electrostatic attraction between the electrode and electrolytic ions. Pseudocapacitive materials are faradic active (redox-active) materials, and they store charges via reversible valence change redox reactions with electrolyte ions. Most of the metal-oxide/hydroxides are redox-active materials exhibiting pseudocapacitive nature. Its charge storage capacity can be improved by functionalizing it in three different ways, such as intrinsic modification (such as morphologies, defect chemistries, crystal structure/orientation, and metal atoms), extrinsic modification (construction of composites and atomistic doping with other materials), and proper incorporation with a current collector. Alterations in the crystal structure (intrinsic property) will alter its electronic conductivity. Atomistic doping (extrinsic property) will create defective sites, which enhances the charge storage. This chapter mainly focuses on a complete description of all functionalization techniques for developing metal-oxide/hydroxide-based supercapacitors and a comprehensive report on the recent developments of functionalized metal-oxide/hydroxide-based supercapacitors.