(401c) Reversible CO2 Reduction Via Homogeneous Aluminum Iodide for the Secondary Al-CO2 Battery

Metal-CO₂ batteries have emerged as a promising approach to simultaneously enhancing energy storage technology and capturing/concentrating carbon dioxide. We previously demonstrated an Al-CO₂ battery employing a homogeneous iodine-based redox mediator, enabling reversible discharge and charge of the battery with an ultra-low overpotential of 0.05V (Fetrow et al., ACS Appl. Mater. Interfaces 2023, 15, 10, 12908-12914). In the absence of any additive, the battery shows a negligible discharge capacity of 0.03 mAh/g when discharged at 20 mA/g to 0.5V, which increases to 3,557 mAh/g upon introducing aluminum iodide. The capacity enhancement is observed at a very low aluminum iodide concentration of 0.05M and displays low concentration dependence, indicating that the enhancement results from a catalytic mechanism. As shown in Figure 1, initial electrochemical analysis of the battery suggests that the solvated iodine forms a two-state system, as opposed to the previously observed three-state system (Karpinski and Osteryoung, J. Electroanal. Chem. 1984, 178 (2), 281-294.) We identify the electrochemical mechanism of the battery, and the specific catalytic role of iodine, using ex-situ Nuclear Magnetic Resonance spectroscopy as well as in-situ UV-Visible and Raman spectroscopy. Our work demonstrates a novel catalytic pathway for reversible CO₂ reduction and facilitates the informed design of an effective Al-CO₂ battery.

Figure 1: Cyclic voltammetry studies of acidic ionic liquids with added iodine. A) AlCl₃:butylpyridinium chloride melts with added molecular iodine. 1: 2:1 melt with 3.0 mM iodine. 2: 1.1:1 melt with 3.8 mM iodine. Collected at 20 mV/s in a cell with aluminum and tungsten electrodes.¹ B) A 1.3:1 AlCl₃:1-ethyl-3-methylimidazolium chloride melt with added 50 mM AlI₃. Collected at 0.1 mV/s in a cell with aluminum and nanostructured carbon electrodes.²

1. Karpinski, Z. J.; Osteryoung, R. A., Electrochemical studies of iodine in an aluminum chloride-butylpyrdinium chloride ionic liquid: Part I. Acidic solvent compositions. Journal of electroanalytical chemistry and interfacial electrochemistry 1984, 164 (2), 281-298.
2. Fetrow, C.; Carugati, C.; Yu, X.; Zhou, X.-D.; Wei, S., A Secondary Al–CO2 Battery Enabled by Aluminum Iodide as a Homogeneous Redox Mediator. ACS Applied Materials & Interfaces 2023.