NON-NOBLE OXYGEN ELECTROCATALYSTS FOR ZINC-AIR BATTERIES

Abstract

The fluctuation and intermittence of renewable energies require the development of the large-scale electrical energy storage. Rechargeable metal-air batteries are big attractions due to their high theoretical energy densities. Among the metal-air systems, the rechargeable Zn-air battery has the advantage of being a derivative of the primary Zn-air battery. The abundance and environmental benignity of zinc (which ensure low cost and safe operations), the low equivalent weight of Zn and its stability in alkaline solution (which support high specific energy and durable use) are factors in favor of the rechargeable Zn-air battery being the most viable rechargeable metal-air technologies developed to date. However, the sluggish kinetics of oxygen reduction reaction (ORR) during discharge and oxygen evolution reaction (OER) during recharge should account for the significant energy losses at the air electrode in the charge-discharge process. Efficient oxygen catalysts to mitigate the energy loss are therefore central to the success of rechargeable metal-air battery technology. Platinum group metal (PGM) catalysts have shown good activities for ORR (e.g. Pt) and OER (e.g. Ir and Ru) in alkaline solution. However, the scarcity and the prohibitive high cost of PGMs inhibit their large-scale deployment and more efforts today are focusing on the low-cost alternatives. The thesis presents the designs of several non-PGM oxygen catalysts with good activity for oxygen electrochemistry in alkaline solution. These oxygen catalysts were based exclusively on transition metals (TMs) in the form of oxides, hydroxides and phosphates; and were used unsupported or supported on selective nanocarbons (reduced graphene oxide, rGO). Their performance was evaluated in half cells by standard electrochemical methods and the more promising ones were also evaluated further in customized full-cell Zn-air systems.