Implications of Testing a Zinc-Oxygen Battery with Zinc Foil Anode Revealed by Operando Gas Analysis.

Zinc-oxygen batteries are seen as promising energy storage devices for future mobile and stationary applications. Introducing them as secondary battery is hindered by issues at both the anode and cathode. Research efforts were intensified during the past two decades, mainly focusing on catalyst materials for the cathode. Thereby, zinc foil was almost exclusively used as the anode in electrochemical testing in the lab-scale as it is easy to apply and shall yield reproducible results. However, it is well known that zinc metal reacts with water within the electrolyte to form hydrogen. It is not yet clear how the evolution of hydrogen is affecting the performance results obtained thereof. Herein, we extend the studies and the understanding about the evolution of hydrogen at zinc by analyzing the zinc-oxygen battery during operation. By means of electrochemical measurements, operando gas analysis, and anode surface analysis, we elucidate that the rate of the evolution of hydrogen scales with the current density applied, and that the roughness of the anode surface, that is, the pristine state of the zinc foil surface, affects the rate as well. In the end, we propose a link between the evolution of hydrogen and the unwanted impact on the actual electrochemical performance that might go unnoticed during testing. Thereof, we elucidate the consequences that arise for the working principle and the testing of materials for this battery type.