Multiphase CO ₂ Dissolution, Diffusion, Absorption, and Electrochemical Conversion in Aluminum–CO ₂ Batteries

The rechargeable aluminum–carbon dioxide (Al–CO2) battery is a promising energy storage system, offering electrochemical conversion and utilization of CO2, while providing high discharge capacity. Aluminum is abundant, inexpensive, and safe, with a high theoretical specific energy density of 2980 Ah/kg, making Al–CO2 battery systems very attractive in comparison to other metal–CO2 battery chemistries. In this work, the dissolution, diffusion, absorption, and electrochemical conversion of CO2 were investigated to establish its influence on battery performance. CO2 and the redox mediator, namely, AlI3, were found to participate directly in the cathode–electrolyte interface, highlighting the importance of maintaining sufficient redox mediator and active cathode material to enhance cycle stability and discharge capacity of these batteries. Gas decay experiments combined with pressure transducer monitoring results revealed equilibrium dissolution values, confirming a CO2 diffusion coefficient of 1.08 × 10–9 m2/s while verifying that active cycling accelerates CO2 transport. The gas diffusion electrode (GDE) used in these experiments, referred to as KBGR311, was developed by mixing Ketjenblack and graphene at a certain ratio. The KBGR311 GDE also exhibited superior CO2 uptake, leveraging the high surface area of the material. Furthermore, evaluations of various electrolyte compositions revealed the influence of AlI3 in the system, improving reversibility by stabilizing intermediates during the CO2 reduction reaction. Finally, characterization techniques using 27Al NMR, Fourier transform infrared (FT-IR), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) confirmed aluminum oxalate (Al2(C2O4)3) as the primary discharge product in the system. These findings provide comprehensive importance of the electrode structure, electrolyte composition, and CO2 utilization in the battery system.