Material Design and Engineering of Zn–air Batteries

Abstract

The growing environmental deterioration and rapid depletion of fossil fuels, such as coal, oil and natural gas, have led to urgent needs of the electrochemical energy devices. With demands for alternatives to petroleum fuels, the energy conversion and storage system have been developed. Among them, Zn–air batteries are receiving intense interest as the most promising electrical energy storage (EES) systems because of its high specific energy density (ca. 1,084 Wh kg–1), high safety with aqueous electrolytes and low cost of zinc metal anode. However, there are limitations arising from the slow kinetics of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). To date, pyrochlore oxides (A2[B2–xAx]O7–y) have been intensively studied as bi-functional oxygen electrocatalysts among a variety of metal oxides for achieving both high ORR and OER activities. However, the origin of outstanding catalytic activities and structural stabilities of pyrochlore oxides in aqueous Zn–air batteries are not clearly revealed, in part due to the difficulty in identification during electrocatalysis. Herein, we present the highly pure single crystalline pyrochlore nanoparticles including lead ruthenate (Pb2Ru2O6.5) and yttrium ruthenate (Y2[Ru2–xYx]O7–y) as efficient electrocatalysts for rechargeable Zn–air batteries. In addition, recent developments in the field of wearable and portable electronic devices have received the spotlight for practical applications such as smart electronics and flexible devices. With demands for flexible design of Zn–air batteries, we first developed the cable-type flexible Zn–air batteries with high electrochemical performance. In order to achieve the flexible system, we fabricated a freestanding gelatin based gel polymer electrolyte based upon 0.1 M KOH, showing highly improved ionic conductivity. Notably, the cable-type Zn–air batteries are fabricated for outstanding flexible and bendable properties with highly stable operating potential, of which value is identical to that of non-bending condition.