Fig. 5

Possible electron and/or “redox” transfer mechanisms taken place in microbial fuel cells (modified from Schievano et al. 2016). Fermentation systems could be in redox rebalance to have value product generation via different metabolic routes for energy recycling via augmentation of electron shuttles, reduction of electron transfer resistance, improvement of membrane for ion separation, and modification of electrode materials. According to electro-fermentation, the supply of electric current as additional reducing equivalents would create new routes of redox balancing with different reduction potentials. In addition, H₂/H⁺ and redox mediators could act as electron shuttles to stimulate oxidative/reductive fermentation of suspended microbial cultures. As capabilities of ESs strongly affected by operation conditions (e.g., pH values, redox balance), appropriate manipulation of ESs and electric current would provide electrochemically favorable environments for cellular growth and target product generation