A Redox-Mediated Zinc-Air Fuel Cell

Abstract

As one of the most promising power sources in converting and storing chemical energy, aqueous zinc-air batteries (ZABs) stand out because of their high theoretical energy density, low cost, high safety of aqueous electrolyte, and environmental friendliness. Three main types of ZABs including primary ZABs, mechanically rechargeable ZABs, and electrically rechargeable ZABs have been developed up to now.[1] Primary ZABs for single use are usually discarded after the Zn anode is consumed or passivated although other parts of battery remain intact, leading to enormous waste. To address it, the depleted anode and failure electrolyte can be physically replaced to recover the capacity in mechanically rechargeable ZABs. But the cumbersome disassembly/reassembly process and the performance deterioration after every recharge process limit its practical application. In addition, it is not feasible to replace zinc in some integrated systems, such as the large-scale ZAB stack. Electrically rechargeable ZABs with bifunctional oxygen reduction reaction (ORR)/oxygen evolution reaction (OER) catalysts could be returned to the original state by electrochemical charging process, but also face some formidable challenges, such as dendritic zinc deposition, electrode deformation, and hydrogen evolution, limiting the long-term cycling performance. Besides, sluggish kinetics of ORR/OER and Zn passivation result in low energy efficiency, limited depth of discharge and power density. Especially in the resource-constrained and off-grid region, there isn’t external electricity source to charge the electrically rechargeable ZABs which limits their applications. Moreover, the reactions inherently take place on the surface of Zn anode and air cathode, in which the storage and conversion capability of the cell would be essentially constrained by the surface area of electrode and the volume of electrode compartment.The concept of redox targeting (RT) of battery materials offers a feasible solution to address these issues by employing redox mediators (RMs). To tackle the above challenges, here we demonstrate a redox-mediated Zn-air fuel cell (RM-ZAFC) based on the RT reactions of cobalt triisopropanolamine complex (CoTiPA) with O2 and 7,8-dihydroxy-2-phenazinesulfonic acid (DHPS) with Zn metal in the catholyte and anolyte, respectively. Both the 4-electron (4 e-) ORR and Zn oxidation reactions are liberated from the electrode surface and occur inside separate reactor tanks resorting to the RT processes. Upon operation, Co(III)TiPA in the catholyte is electrochemically reduced to Co(II)TiPA on the cathode and chemically oxidized back through ORR in the gas diffusion tank (GDT) into which O2 is fed. In conventional ZABs, ORR takes place on the triple-phase reaction interface of the heterogeneous electrocatalyst, and the catalytic performance is closely related to the density of exposed active site, electrical conductivity and reaction energy barrier. As compared, the redox-mediated ORR in GDT occurs homogeneously in the catholyte in which the dissolved RM could provide a large quantity of rection sites with tunable reaction energy barrier. For the negative side, DHPS-2H is firstly oxidized to DHPS on the anode and circulated into anodic tank, where it is regenerated by oxidizing Zn to ZnO. The discharge product ZnO is formed and deposited on the Zn granules loaded in the tank. After fully discharged, the cell can be feasibly refueled by replacing the Zn granules in the tank instead of disassembling and reassembling the whole cell. In this RM-ZAFC, the energy storage unit is decoupled from the power generation unit, thus offering greater operation flexibility and system scalability with modular design than the conventional ZABs. Compared with the commercial hydrogen tank used in hydrogen fuel cells, Zn granules loaded in the anodic tank present much higher volumetric capacity, greater safety and ease for storage and transportation. Therefore, as an energy conversion and storage device, RM-ZAFC would have great promise for automotive, backup, and stationary power applications.[2] In addition, we also propose an efficient and zinc regeneration method by RT reaction between RM and the product ZnO in RM-ZAFC to avoid the waste of ZnO and reduce the cost.