Heme-Based Composites as Cathode Catalysts for Lithium-Oxygen Batteries and Analysis of the Reaction Mechanism

Abstract

Despite the high theoretical energy density of lithium-oxygen (Li-O2) batteries, the charge/discharge efficiency is unsatisfactory. To overcome this critical problem, we synthesized heme-graphene composites (HEME-GO) as catalysts for Li-O2 batteries. The introduction of graphene produced π-π interactions with the heme matrix, resulting in a composite with enhanced ORR/OER catalytic activity. The free energy diagram of the redox reaction was calculated using density functional theory(DFT) for HEME-GO based on the four-electron reaction pathway, and it was demonstrated that HEME-GO has the lowest ORR overpotential (0.67 V) and OER overpotential (0.53 V). The catalytic mechanism of HEME-GO was also quantitatively described by calculating the adsorption energy of intermediates in the rate determining step. In addition, the Li-O2 batteries with the composite catalyst exhibited better cycling performance, discharge capacity (7770 mAh∙g-1), and lower overpotential due to the ability of heme to scavenge superoxide radicals and thus protect the electrode. These results contribute to the understanding of the redox process of Li-O2 batteries for organic systems and suggest innovative ideas for the design of environmentally friendly batteries.