|dc.description.abstract||Zinc-air batteries, whose advantages include relatively high energy density (1218 Wh kg-1), abundance of zinc in earth’s crust, and very safe operational characteristics, are promising for applications in consumer electronics, electrified transportation, grid storage, and other fields. At the moment, primary zinc-air batteries are produced for low-drain electronic gadgets such as hearing aids. However, secondary (i.e., electrically rechargeable) zinc-air batteries have eluded widespread adoption due mainly to the slow reaction kinetics of oxygen evolution at the air electrode during recharge. A bifunctional oxygen electrocatalyst that can recharge the batteries more efficiently is required. Moreover, in the presence of aqueous alkaline electrolytes, zinc-air batteries suffer from low durability and performance loss due mainly to the formation of zinc dendrites during charging, the loss of aqueous electrolytes, the detachment of the catalyst layer and the precipitation of carbonates at the air electrode. These persistent issues have motivated a shift in electrolyte design towards efficient hydroxide ion-conductive polymeric electrolytes. A combination of efficient bifunctional oxygen electrocatalysts and polymeric electrolyte improvements may enable zinc-air batteries to be implemented in widespread applications in flexible/lightweight electronic devices and electric vehicles.
In this work, I present a feasible strategy combining material innovations with engineering methods to develop a new type of zinc-air battery, i.e., a flexible, rechargeable polymer electrolyte membrane zinc-air battery (PEMZAB). In the first study, a proof of concept of a film-shaped, rechargeable PEMZAB was conducted by using a KOH-doped poly(vinyl alcohol) (PVA) gel electrolyte, porous zinc electrode and bifunctional air electrode comprising a commercial Co3O4 nanoparticles-loaded carbon cloth. Then, a novel hydroxide ion-conductive polymeric electrolyte membrane and an efficient bifunctional oxygen zinc-air battery performance. Specifically, highly quaternaized cellulose nanofibers were synthesized to produce a hydroxide ion-conductive electrolyte membrane (referred to as QAFC). The QAFC membrane shows advantages of a high ionic conductivity of 21.2 mS cm-1, good chemical stability, mechanical robustness and flexibility, and inhibition of zinc dendrites and carbonations. In addition to the QAFC electrolyte membrane development, a hybrid bifunctional oxygen electrocatalyst, consisting of cobalt oxysulfide nanoparticles and nitrogen-doped graphene nanomeshes (CoO0.87S0.13/GN), was prepared. The defect chemistries of both oxygen-vacancy-rich cobalt oxysulfides and edge-nitrogen-rich graphene nanomeshes lead to a remarkable improvement in electrocatalytic performance, where CoO0.87S0.13/GN exhibits strongly comparable catalytic activity and much better stability than the best-known benchmark noble metal catalysts.
A simple, water-based filtration method for a direct assembly of the QAFC membrane and the CoO0.87S0.13/GN catalyst film was demonstrated with the PEMZAB. Such a fabrication approach enables intimate contact between the solid-solid catalyst-electrolyte interfaces for facile charge transfer. Moreover, benefiting from the performance improvement of the QAFC electrolyte membrane and the CoO0.87S0.13/GN bifunctional catalyst, the resulting battery possesses a higher energy density of 857.9 Wh kg-1 and a more stable cycling performance, over 300 hours of operation at 20 mA cm-2 under ambient conditions, than those of a battery using PVA-KOH gel electrolyte and commercial Co3O4 bifunctional catalysts.
In the last study, the knowledge gained from the hybrid CoO0.87S0.13/GN bifunctional catalyst is transferred to the fabrication of a hybrid catalyst/current collector assembly for the bifunctional air electrode. In this assembly, a hair-like array of mesoporous cobalt oxide nanopetals in nitrogen-doped carbon nanotubes is grown directly on a stainless-steel mesh through chemical vapor deposition and electrodeposition methods. Such integrative design not only ensures a large number of catalytically active sites in a given electrode surface, but also increases the electron transfer between each individual catalyst and the conductive substrate. This advanced air electrode assembly further boosts the PEMZAB performance, with a high peak power density of 160.7 mW cm-2 at 250 mA cm-2 and a remarkable cycling durability: lasting over 600 hours of operation at 25 mA cm-2 under ambient conditions.||en