Probing The Redox -Mediation of Transition Metal Phthalocyanine in Metal-Air Battery

Metal air batteries have garnered significant attention as a potential alternative to Li-ion batteries due to their high theoretical energy. In lithium-air batteries, during the oxygen reduction reaction (ORR), O₂ is reduced to solid insulating LiO₂, Li₂O₂ and in the reverse charge process, which is the oxygen evolution reaction (OER), discharge products convert back to O₂ and Li⁺. However, the practical implementation of Li-O₂ batteries has been hindered by several challenges, primarily due to the sluggish kinetics of ORR and OER. In recent times, a liquid-based redox mediator (@ electrocatalyst) has been demonstrated as an effective approach to increase battery efficiency and modify the reaction pathways to alleviate side reactions. In this presentation, we have systematically explored the solution-based redox mediation behaviour of first–row transition metal phthalocyanines for lithium-oxygen batteries. Our findings, based on experiment and theory, convincingly demonstrate that d⁵ (Mn), d⁷ (Co), and d⁸ (Ni) configurations function better compared to d⁶ (Fe) and d⁹ (Cu) in redox mediation of the discharge and charge step. The d¹⁰ configuration (Zn) and non-d analogs (Mg) do not show any redox mediation because of the inability to binding to O₂. During the discharge process, the Li₂O₂ coordinate with metal phthalocyanines and leads to a solution phase reaction pathway and reverse process controlled by the RM, thus confirming a bifunctional redox mediator. Apart from the reaction pathways predicted based on thermodynamic considerations, density functional theory calculations also reveal interesting effects of electrochemical perturbation on the redox mediation mechanisms and the role of the transition-metal centre. In addition, to account for the interaction between RM mediator and the discharge products (KO₂(LiO₂₎, Li₂O₂) and parasitic products (Li₂CO₃ and LiOH), further spectroscopic and theoretically investigations were conducted. Our finding reveals that discharge/parasitic product coordinated to metal centre with the unfilled d orbital and oriented in between to M-N bond.in the case of filled d system, the discharge/parasitic products along the M-N bond and interacting with the imidazole moiety of the phthalocyanine ring leads to the poisoning the catalytic activity in battery performance.