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

•In-situ observation discloses a three-step process during bubble evolution.•Activation and ohmic overpotential is obtained by modelling and experiments.•Concentration overpotential is reduced by bubble convection and absorption.•A formula for voltage oscillation from hydrogen bubble evolution is proposed.•Overall bubble-induced overpotentials in a Zn-air battery are revealed.