Catalytic production of hydrogen peroxide and its fuel cells with metalloporphyrins, metallophthalocyanines, and analogs

Metalloporphyrins, metallophthalocyanines, and analogs are known to act as efficient catalysts for the reduction of dioxygen in the presence of an acid in solution. Whether dioxygen is reduced to hydrogen peroxide (H2O2: two-electron reduced species of O2) or water (H2O: four-electron reduced species of O2) is changed depending on metals and ligands. This mini-review focuses on the catalytic production of hydrogen peroxide by a combination of 2e-O2 reduction and 2e-or 4e- H2O oxidation with the use of metalloporphyrins and metallophthalocyanines as photocatalysts. H2O2generated by the photocatalytic H2O oxidation with O2 can be used as a fuel in hydrogen peroxide fuel cells in which metalloporphyrins, metallophthalocyanines, and analogs are employed as the cathode catalysts. Metalloporphyrins are also employed as efficient electrocatalysts to exhibit significantly improved performance in a Zn-air battery that has a much higher energy density than that of lithium-ion batteries.