Achieving Zn-air batteries with high voltage output and high energy efficiency

Abstract

Zn-air batteries (ZABs) are promising candidates for next-generation energy storage and conversion technologies, owing to their high energy density, inherent safety, and cost-effectiveness. Regretfully, their market presence is limited due to their inability to meet the power requirements of electronic devices, which typically require a minimum of 1.5 V voltage output for normal operation, and the energy efficiency requirements of energy storage, which typically requires at least 65% to minimize energy wastage. Currently, ZABs only offer a 1.2 V operating voltage and 60% energy efficiency. This thesis, therefore, aims to develop ZABs capable of exceeding 1.5 V output and achieving over 65% energy efficiency. First, an all-in-one acid-alkaline hydrogel electrolyte design strategy is proposed to achieve high-voltage flexible ZABs. Pluronic® F127 hydrogels, the key material in this strategy, with a unique sol-gel transition property, facilitate the integration and decoupling of acid and alkaline, eliminating the need for expensive bipolar membranes. This design results in an intimate electrolyte-electrolyte and electrolyte/electrode interface, enabling the flexible ZAB to reach a good rate performance and a large area capacity. Most importantly, the acid hydrogel part enhances the potential of cathodic oxygen reduction reaction (ORR), thereby enabling flexible ZABs to achieve a 2 V output. To further enhance the energy efficiency of ZABs, a reaction modifier (RM), specifically KI, is introduced in this thesis. This RM alters the oxidation pathway from the traditionally sluggish oxygen evolution reaction (OER) to a faster iodide oxidation reaction (IOR) with a lower oxidation potential. Therefore, the battery charging voltage is lowered and energy efficiency is improved. The study delves into the IOR process, investigating its electrochemical-chemical nature and the catalytic mechanism involved. The concept of bifunctional ORR/IOR catalysis is also proposed, identifying Pt/C and commercial XC72R carbon black as high-performance bifunctional ORR/IOR catalysts, which facilitate ZABs to achieve energy efficiency of 76.5% and 65.3%, respectively. Moreover, it is also confirmed the decreased charging voltage can alleviate the carbon corrosion of the air cathode, endowing ZABs an extended lifetime. Lastly, the thesis proposes a novel three-chamber Zn-air flow battery (ZAFB) configuration design to simultaneously achieve high output and energy efficiency. This configuration takes advantage of the decoupled acid-alkaline electrolyte during discharge and the fast IOR process during charging. The reduced charging voltage mitigates carbon corrosion, Zn dendrite growth, and hydrogen evolution side reactions, leading to an elongated battery lifetime. Coupled with near 100% energy efficiency and a 76% power density increase, this ZAFB shows promise as a long-duration energy storage technology.