Deciphering biostimulation strategy of using medicinal herbs and tea extracts for bioelectricity generation in microbial fuel cells

This first-attempt study revealed optimal strategy to supplement extracts of polyphenolics-abundant medicinal herbs and Camellia tea as electron shuttles (ESs) for stimulating bioenergy generation in microbial fuel cells (MFCs). Apparently, Camellia sinensis (L.) Kuntze and Syzygium aromaticum were promising electroactive ESs. Moderate temperature (ca. 65 °C) and slightly alkali pHs (∼10) were electrochemically feasible conditions for herbal extraction. Optimal contents of polyphenolics-rich herbs and tea extracts with maximal electrochemical activities could be stably obtained. Power density of MFC supplemented with Camellia green tea extract could significantly increase ca. 176%, suggesting that green tea extract would be the most appropriate ESs. Total phenolic contents and electron shuttling capabilities were all electrochemically associated. In addition, chemical structure strongly affected whether antioxidant activities of polyphenolics-abundant herbal extracts could be reversibly switched to be electron-shuttling capabilities (e.g., substitution patterns). Hydroxyl substiutuents ortho or para to each other were very likely promising for electron-shuttling, but not for meta substituents. Moreover, bioelectrochemical treatment upon medicinal herbal extracts (e.g., cyclic electron-donating and withdrawing processes) might be inevitably needed for toxicity attenuation to fully express bioenergy-shuttling activities.