transfer mechanisms, remains as insufficient to clearly describe the effectiveness of the EF system (Lai and Lan, 2020; Moscoviz et al., 2016; Xafenias et al., 2017). The interactions between microbial cells and electrodes, particularly EET mechanisms for electron transfer from bacteria to electrode, have been extensively studied for various MXCs, while electron transport from the electrode to microbial cells is relatively unexplored (Gong et al., 2020; Kracke et al., 2018). For EF-based production of some specific value-added products (Table 1.1), an in-depth understanding of cathodic EET mechanisms (e.g., electron uptake) should be further investigated for successful engineering cathodic EF process.
In recent years, the utilization of waste feedstocks for biofuels productions (e.g., bioethanol, biodiesel, etc.) has received significant attention worldwide. Electro-fermentation carries an excellent potential for regulating fermentation pathways and enhancing productivity and yield of target value-added products. However, except for a few studies, most of the EF studies utilized synthetic model compounds rather than real feedstocks. For instance, further research would be needed for developing a sustainable crude glycerol-based EF process. Crude glycerol from the biodiesel production process may contain a considerable amount of impurities (e.g., metals and residual alcohols) (Xafenias et al., 2015); the biodiesel process uses excess methanol to promote product yields (Dhar and Kirtania, 2009). Therefore, the effects of those impurities on EF should be further studied for practical application. Thus, the feasibility of utilizing waste biomass has yet to be demonstrated on a lab-scale to reach a considerable state for further pilot, scale-up, and commercialization. With more future experimental evidence, the EF system can demonstrate such high potential to facilitate the field-scale productions of biofuels and chemicals with only small electrical energy input.
1.5 Acknowledgements
We gratefully acknowledge support from Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery Grant for this research. Tae Hyun Chung acknowledges support from Alberta Innovates Graduate Student Scholarship and NSERC Alexander Graham Bell Canada Graduate Scholarship-Doctoral programs.
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