Insight into the bubble-induced overpotential towards high-rate charging of Zn-air batteries

Tremendous efforts are made in designing effective electrocatalysts for rechargeable Zn-air batteries to lower overpotenitals during discharging and charging. When the charging current reaches a critical value, bubble evolution inevitably occurs due to the unreleased oxygen on the air electrode and the possible hydrogen generation on the Zn electrode, whereas the detailed effects on the electrochemical performance are usually overlooked. Herein, the bubble dynamics and the induced overpotential, including activation, ohmic, and concentration overpotentials, during high-rate charging are investigated based on a carbon-free electrode through in-situ observation, mathematical modelling, and numerical simulation. The bubble generation on the air electrode is observed with a three-step process: initial accumulation, transitional growth, and steady evolution. The modelling and simulation results demonstrate that the concentration overpotential is reduced by the bubble-induced convection-disturbance and sink-absorption, while the activation overpotential (4.29 mV at 20 mA cm−2) is an order of magnitude larger than the ohmic component (0.10 mV). Further, a formula to express the whole overpotential is also proposed by the incorporation of potential oscillation from hydrogen bubble evolution on the Zn electrodes. This work elucidates the fundamental relationship between bubble dynamics and electrochemical characteristics, which favors the development of high-performance rechargeable Zn-air batteries.