Biogenic Melanin-Modified Graphene as a Cathode Catalyst Yields Greater Bioelectrochemical Performances by Stimulating Oxygen-Reduction and Microbial Electron Transfer

Bioelectrochemical systems (BES) can recover energy from organic-bearing waste streams, but their use has been stymied by poor electron transfer from the cathode. Redox-active electron shuttles could stimulate electron transfer provided that they are compatible with the exoelectrogenic bacteria. This work evaluated melanin-modified carboxylated graphene (M/CG) as a novel cathode catalyst in a microbial fuel cell. Biogenic melanin catalysts (i.e., bio-M/CG) significantly increased bioelectricity production due to its abundant pyrrole N, which lowered charge-transfer resistance and, thus, promoted the cathodic oxygen-reduction reaction (ORR). The high content of pyrrole N in the bio-M/CG catalyst also enriched exoelectrogens, such as Azospirillum, Chryseobacterium, and Azoarcus, which accounted for over 50% of the total abundance of bacteria in biofilms on the anode. Moreover, the functional genes of key enzymes involved in microbial electron transfer (MET) were increased by the bio-M/CG catalyst. These data confirm that the bio-M/CG catalyst improved the bioelectrochemical performance via synergetic promotion of cathodic ORR and microbial electron transfer, thus providing a new alternative for advancing BES technology. This work highlights the potential application of melanin in enhancing cathodic oxygen-reduction reaction kinetics and improving microbial electron transfer in BES. This study emphasizes the promising application of melanin in enhancing the ORR kinetics and improving MET in BES, offering exciting prospects for future sustainable and environmentally friendly applications.