such as neutral red (Choi et al., 2012), methyl viologen (Kim and Kim, 1988), or ferricyanide (Xafenias et al., 2017) are required during the fermentation to impact the extracellular ORP (Choi et al., 2012; Kim and Kim, 1988; Sturm-Richter et al., 2015). When the redox mediators are introduced, they can first, be oxidized or reduced by the fermentative bacteria, then they are recycled or recovered electrochemically by the anode or cathode electrodes (Moscoviz et al., 2016). In this context, the redox mediators are used as electron shuttles, and this process is known as the mediated electron transfer (Gong et al., 2020; Rabaey and Rozendal, 2010; Thrash and Coates, 2008). Furthermore, other studies demonstrated another way to add a redox mediator in CEF, such as using produced H2 at the cathode that can be further used as a one-way electron shuttle (Gong et al., 2020; Xafenias et al., 2015; Zhou et al., 2013; Zhou et al., 2015).
On the other hand, metabolically engineered fermentative bacterial strains are another feasible option, for instance, by adding the property of electroactivity (Moscoviz et al., 2016). This approach has been confirmed by adopting the strains (e.g., c-type cytochromes CymA, MtrA, STC) from electroactive bacteria (Shewanella oneidensis) to fermentative bacteria (Escherichia coli), where the electron transfer process can be greatly improved (e.g., by 183%) (Sturm-Richter et al., 2015). Alternatively, electroactive bacterial species (e.g., Shewanella oneidensis) can also be engineered to utilize a variety range of substrates and organic wastes to further aid the whole EF processes (Flynn et al., 2010).
1.3 Value-Added Products from Electro-Fermentation
To date, electro-fermentation has been investigated for a wide variety of value-added products, including carboxylates, alcohols, biopolymers, and other platform chemicals (see Table 1.1). This section reviews the studies related to EF for producing different value-added products.
Table 1.1 Summary of electro-fermentation and electro-selective fermentation for value-added bioproducts.
| Product | Feedstock | Inoculum | System configuration | Total working volume (L) | Temperature (°C)/initial pH | Applied voltage/potential | Working electrode | Reference |
|---|---|---|---|---|---|---|---|---|
| Butanol | Glucose | C. pasteurianum | Dual chamber | 900 | 37/6.7 | 0-2.6 V | Cathode | (Mostafazadeh et al., 2016) |
| Butanol | Glucose | Clostridium pasteurianum DSM 525 | Dual chamber | 900 | 37/6.5 | +0.045 V vs. SHE | Cathode | (Choi et al., 2014) |
| Ethanol | Glycerol | Clostridium cellobioparum, + G. sulfurreducens | Dual chamber | 190 | 30/6 | 0.24 V vs. Ag/AgCl | Anode | (Speers et al., 2014) |
| Ethanol | Glycerol | Escherichia coli | Dual chamber | 50 | 37/7.4 | −44 mV vs. SCE | Anode | (Sturm-Richter et al., 2015) |
| Ethanol | Cellobiose | G. sulfurreducens+ Cellulomonas uda | Single chamber | 1000 | 30/6.97 | 0.24 V vs. Ag/AgCl | Anode | (Awate et al., 2017) |
| Ethanol | Food waste | Mixed culture | Single chamber | 400 | 30/6.8 | - | - | (Chandrasekhar et al., 2015) Acetone-Butanol- |
| Ethanol (ABE) | Glucose | C. acetobutylicum | Dual chamber | 240 | 37/6.8 | -600 mV vs. Ag/AgCl | Anode | (Engel et al., 2019) |
| 1,3-propanediol | Glycerol | Mixed-culture + G. sulfurreducens pre-colonized cathode. | Dual chamber | 900 | 37/7 | -900 mV vs. SCE | Cathode | (Moscoviz et al., 2018) |
| 1,3-propanediol | Glycerol | Mixed culture | Dual chamber | 520 | 21/6.9 | −0.80 V to −1.10 V vs. SHE | Cathode | (Xafenias et al., 2015) |
| 1,3-propanediol | Glycerol | Clostridium pasteurianum DSM 525 | Dual chamber | 900 | 37/6.5 | +0.045 V vs. SHE | Cathode | (Choi et al., 2014) |
| Butyric acid | Glucose | Mixed culture | Dual chamber | 540 | 25/5.5 | -700 mV vs. SHE | Cathode | (Paiano et al., 2019) |
| 3-hydroxypropionic acid | Glycerol | Recombinant Klebsiella pneumoniae L17 | Dual chamber | 620 | 37/6 | +0.5 V vs. Ag/AgCl | Anode | (Kim et al., 2017) |
