Phosphate-based polyanionic insertion materials for oxygen electrocatalysis

Electrocatalyst-based energy storage technologies such as alkali metal-air batteries, fuel cells, and water splitting devices are the new holy grail in the next-generation energy storage landscape as they deliver higher energy densities than Li-ion/Na-ion batteries (LIBs/SIBs). The new chemistries of energy storage such as metal-air batteries under aqueous or non-aqueous conditions will complement existing LIBs/SIBs owing to the increasing requirement for batteries with high energy density in the present era. Phosphate-based polyanionic frameworks have long been known for their ability to (de)intercalate alkali metal ions. Because of their innate oxygen electrocatalytic activity, these insertion cathode materials have lately emerged as air electrodes in metal-air battery systems. In this review, the present status of phosphate-based polyanionic insertion materials for oxygen reduction and oxygen evolution reaction (ORR and OER) electrocatalysis is summarized. Factors influencing electrocatalytic activity in these materials, such as the presence of different types of alkali metal cations, transition metals, and the type of ligand/mixed anion as well as coordination around the transition metals are discussed. Finally, the development of metal-air batteries derived from phosphate-based polyanionic insertion materials as air electrodes is discussed. Electrocatalyst-based technologies: alkali metal-air batteries, fuel cells, and water splitting devices are the new holy grail in the next-generation energy storage landscape as they deliver higher energy densities than Li+/Na+ batteries (LIBs/SIBs).