By definition, a zinc-air battery is typically a metal–air battery powered by oxidizing zinc with oxygen from the air. These batteries have high energy densities and are relatively inexpensive to produce. Indeed zinc-air batteries are one of the most promising candidates for grid-scale energy storage and electric vehicles. Unfortunately, rechargeable zinc-air batteries suffer from one major shortfall, namely; cycle life improvement, which is limited by issues associated with the Zn and air electrodes. Specifically, the Zn electrode suffers from dendrite formation, shape change, passivation and hydrogen evolution, while the air electrode must contend with slow oxygen reduction reactions and carbonate precipitation. Further, in such systems, electrolytes play a pivotal role in the transport of active species. Previous reports showed that alkaline solutions, ionic liquids and gel polymer electrolytes alone fail to resolve the aforementioned drawback. Recently, hydrogels have been shown to enhance the performance of zinc-air batteries.
Nonetheless, designing a hydrogel network that has chemical and electrochemical stability, high ionic conductivity and good contact with the electrodes is a major challenge for gel polymer electrolytes. On this account, researchers from the Department of Chemical and Materials Engineering at University of Alberta: Thuy Nguyen Thanh Tran (PhD candidate), Professor Hyun-Joong Chung and led by Professor Douglas G. Ivey investigated the behavior of three different hydrogel matrices as model gel polymer electrolytes in potassium hydroxide solutions, which are commonly used for rechargeable zinc-air batteries, and compared their performance. Their work is currently published in the research journal, Electrochimica Acta.
In their work, poly (vinyl alcohol) (PVA), poly (acrylic acid) (PAA) and a charge-symmetric polyampholyte, i.e., poly (4-vinyl-benzenesulfonate-co- [3 (methacryloylamino) propyl] trimethylammonium chloride) (denoted hereafter as PAM for simplicity), were selected as model systems. Generally, the researchers tested various aspects, including: morphology and mechanical strength changes, electrochemical and chemical stability, ionic conductivity, water uptake and battery performance.
The authors reported that PVA, PAA and PAM were chemically stable in 6M concentration alkaline solutions; upon removal of water, the gel polymer electrolytes demonstrated a stable electrochemical window of 2V, which was deemed sufficient for zinc-air battery applications. In addition, the researchers reported that PAA with 6M potassium hydroxide has the highest conductivity with its conductivity increasing with increase in temperature.
In summary, the study demonstrated an in-depth assessment of three different hydrogels based on their ionic conductivities, chemical stability, electrochemical windows and mechanical properties, for application as electrolytes in rechargeable zinc-air batteries. Overall, during full cell testing, it was shown that gel polymer electrolyte could be beneficial to zinc-air battery performance by reducing interfacial and charge transfer resistance. In a statement to Advances in Engineering, Professor Douglas G. Ivey highlighted that their work showcased gel polymer electrolytes to possess considerable promise for use as electrolytes in zinc-air batteries.
Thuy Nguyen Thanh Tran, Hyun-Joong Chung, Douglas G. Ivey. A study of alkaline gel polymer electrolytes for rechargeable zinc-air batteries. Electrochimica Acta, volume 327 (2019) 135021.