Micro-sized Aluminum-air battery based on single-crystal Metal-organic frameworks as air cathode

Abstract

Aluminum-air batteries are promising energy sources due to their high theoretical voltage (2.7 V), high theoretical energy density (8.1 kWh/kg-Al), low cost, and safety and environmental friendliness etc. , , , However Al self-corrosion is limitation in Al-air battery– in aqueous alkaline electrolytes, causing formation of passivating Al(OH)3 layer from Al(OH)4– – is a major obstacle preventing continuous electrochemical reaction during discharge. Moreover, direct contact between Al and water (H2O) results in parasitic hydrogen evolution reaction which, in turn, lowers the operating potential of the battery. Here, we, achieve inhibition of parasitic corrosion by using (i) a polyester anion transport layer with Zn particles embedded in it , , and, most importantly, (ii) a single crystal of rubidium-cyclodextrin-metal-organic framework (Rb-CD-MOF) which not only serves as an air-cathode reducing oxygen but also as a diffuser , for Al(OH)4– and OH– ions to minimize the formation of Al(OH)3 passivation layer on the anode. Furthermore, the Rb-CD-MOF itself exhibits bifunctional electro-catalytic effect , by supporting efficient and stable oxygen reduction reaction (ORR) close to the direct four-electron process, and also oxygen evolution reaction (OER). This ORR/OER bifunctionality allows the battery to efficiently use humid air (i.e., oxygen and water) as “fuel” powering its operation. Because of the inhibition of Al corrosion and the MOF’s bifunctional electro-catalysis, the Al-air battery shows ultra-high discharge capacity. above 15,000 mAh/cm2. The use of single-crystal electrocatalytic MOFs is a novel concept in battery design and, as illustrated by this work, can enhance performance of traditional batteries often limited by interfacial processes.