Carbon Dioxide Evolution in Aqueous Zinc Metal Batteries

Gas evolution reactions in aqueous zinc metal batteries (AZMBs) cause gas accumulation and battery swelling that negatively affect their performance. However, previous work often reported hydrogen as the main, if not the only, gas species evolved in AZMBs; the complexity of gas evolution has been overlooked. For the first time, this work found the CO₂ evolution reaction (CER) in AZMBs, pinpointed its sources, and identified electrolyte modulation strategies. Using differential electrochemical mass spectrometry, CER was detected in V₂O₅||Zn full cells, instead of in asymmetric Cu||Zn cells, and it became substantial when being charged to 2.0 V. By using a carbon isotope tracing method, the primary origin of CER was identified as the electrochemical corrosion of conductive carbon at the cathode. Among six representative electrolytes, the weakly solvating electrolyte (3 m Zn(OTf)₂ in acetonitrile/water) presented a high CER resistance by reducing water solvating and disturbing hydrogen bonding. This work sheds light on interfacial parasitic reactions for practical aqueous metal (Zn and Al) batteries.