In-situ observation of the gas evolution process on the air electrode of Zn-air batteries during charging

Although tremendous efforts are made in developing stable Zn electrodes and bifunctional catalysts in rechargeable Zn–air batteries, the charging process is few reported. Herein, an in-depth investigation into the gas evolution on the air electrode is conducted by in-situ characterization. It is found that the charging process can be divided into three stages: no obvious bubbles, small bubbles owing to the oxygen evolution, and large bubbles owing to oxygen evolution and carbon corrosion. The post analyses illustrate that the severe bubble formation can deteriorate the electrochemical performance of the air electrode. With the catalyst loading increases, the bubbles on the surface become smaller and sparser. At low current densities, the air electrode can keep “self-clean” from bubbles, which is the ideal state for the charge. Whereas with an increase of the current density, the large-bubble stage is enlarged, and more bubbles are attached to the surface, leading to extra charge impedance. As the extent of discharge/charge reduces, the three stages disappear, and only small bubbles can be found on the surface, which is favourable for the stable operation. This work provides a profound understanding of the charging behaviors on the air electrode, facilitating the development of high-performance Zn-air and other metal-air batteries.