Summary of the studies done on the mediator-less
Microbial fuel cells (MFCs) are fascinating bioelectrochemical devices that use the catalytic activity of living microorganisms to draw electric energy from organic matter present naturally in the environment or in the waste. Yeasts are eukaryotic microorganisms, classified as members of the fungus kingdom. Several yeast strains have been studied as biocatalysts in MFC with or without external mediator such as Saccharomyces cerevisiae, Candida melibiosica, Hansenula anomala, Hansenula polymorpha, Arxula adeninvorans and Kluyveromyces marxianus. In this chapter, we will focus on the use of yeast as a biocatalyst in the anode of microbial fuel cells (MFCs). How different yeast strains transfer electrons to the anode of the microbial fuel cells, advantages and challenges of the use of yeasts in MFCs, how to improve the performance and sustainability of the yeast-based MFCs through the modification of the anode electrode surface, and the application of the yeast-based MFCs in continuous wastewater treatment were discussed.
- microbial fuel cell
- electron transfer
Microbial production of energy and/or chemicals from renewable carbohydrate feedstocks, and other organic-based wastes such as wastewater, is an attractive alternative to the current common fossil fuels. Microbial fuel cells (MFCs) are among the fast-growing microbial electrochemical systems (MESs) that offer a promising way for simultaneous wastewater treatment and electricity production [1–3]. Although MFCs showed promising features such as simultaneous wastewater treatment and electricity generation, low sludge production, wide range of substrates and operating at room temperature, the low power output and high cost especially that of the Pt cathode are the main challenges facing their commercialization [4–6].
In MFCs, the exo-electrogenic microorganisms act as biocatalysts in anaerobic oxidation of the organic materials that exist in different wastes, liberating electrons that can be collected by a conductive electrode, i.e., anode, generating an external power-producing circuit, and protons transferred through an electrolyte to a cathode surface. At the cathode, electrons react with protons and oxygen producing water [7–9]. The exo-electrogenic microorganisms that can be used in MFCs can be a prokaryote or eukaryote. Although prokaryotic microorganisms showed promising results in the MFCs and a lot of research has been carried out using them due to their ease in the electron transfer mechanism, yeast, as a eukaryote, attracted researchers’ attention and was extensively studied as a biocatalyst in MFCs [4–6].
2. Microbial fuel cells: structure, components and mechanism
Microbial electrochemical systems (MESs) are innovative technology, recently implemented for numerous applications [10–15] such as (i) the simultaneous wastewater treatment and electricity production by MFCs, (ii) bio-hydrogen and/or other chemical production by microbial electrolysis cells (MECs), (iii) water desalination by microbial dialysis cells (MDCs) and (iv) electricity production in sediments or plant MFCs.
In case of MFCs, microorganisms oxidize organic matter, producing electrons that travel through a series of respiratory enzymes in the cell and make energy for the cell in the form of ATP. The electrons are then released to a terminal electron acceptor (TEA) that becomes reduced. Many TEAs such as oxygen, nitrate, sulfate and others readily diffuse into the cell where they accept electrons forming products that can diffuse out of the cell. However, it is now known that some microorganisms can transfer electrons exogenously (i.e., outside the cell) to a TEA such as metal oxides like iron oxide. This is the case of bacteria called exo-electrogens, which can be used to produce electricity in MFC .
Figure 1 shows a schematic diagram of an air-cathode MFC that consists of anode and cathode electrodes separated by a separator (if needed). The anode compartment composed of anode and carbon source (organic materials), with or without exogenous mediator. At the cathode, an electron acceptor (O2 from air) reacts with protons that pass from the anode to the cathode through the electrolyte, and the electrons produce water.
2.1. Anode material
Anode material is considered as an important parameter that affects the performance of MFCs. The anode of the MFCs should have high electrical, mechanical and chemical stability, be biocompatible and have high surface area . Carbon materials (conventional and nonconventional) are the best materials that are applied as anode in the MFCs showing high power output. The conventional carbon materials such as carbon paper, carbon cloth, carbon brush and carbon felt, and the nonconventional ones such as carbon nanotubes (CNTs), carbon nanofibers and graphene have been extensively applied in MFCs. Little work have been carried out using noncarbonaceous materials such as stainless steel, gold and titanium [17–19], which showed a lower performance compared to that obtained in case of using carbon.
2.2. Cathode material
Cathode material has a significant impact on the overall cell voltage and it should have a high redox potential. Carbon materials such as carbon paper and carbon cloth modified with high active catalyst such as Pt catalyst are among the most common cathodes of the MFCs . Although modifying the carbon cloth and/or carbon paper with Pt significantly decreased the oxygen reduction activation energy and increased the reaction rate, the high cost and scarcity of the Pt are the main challenges facing the application of such cathode. Recently, a wide range of non–Pt-based catalysts were investigated as cathodes in MFCs and showed promising results that gave them a potential to replace Pt catalyst in the near future such as carbon nitrogen alloys and metal carbides [18, 20–29].
As anode is working under anaerobic conditions, while cathode is working under aerobic conditions, the addition of separator with high ionic conductivity and low permeability could improve the MFC performance . A large number of separators have been extensively studied in MFCs such as anion and cation exchange membranes, salt bridge, glass fibers, microfiltration membrane, porous fabrics, and coarse-pore filters [31–37]. It is worth mentioning that some MFCs showed better performance even without using the separator .
2.4. Microbes and electron transfer in microbial fuel cells
Microorganisms are generally divided into two main categories, prokaryotes and eukaryotes. Prokaryotes are simpler (no distinct nucleus) and smaller in size (around 1 μ in diameter) compared to eukaryotes that have larger size (5–10 μ or more) and are complex (possessing a distinct nucleus and subcellular organelles such as plastids and mitochondria) [4, 6]. All microorganisms that are capable of exo-cellular electron transfer (exo-electrogens) can be effectively used in MFCs without adding soluble exogenous mediators [4, 22, 30–38].
The possible electron transfer mechanisms in MFCs are shown in Figure 2 and can be summarized in the following:
Direct electron transfer (DET) whether by direct cell attachment or through nanowires (pili)
DET requires a direct contact between the anode surface and the outer membrane of the microorganism. Pili are nanowires that are formed out to connect the microorganism’s membrane to the anode surface. The merits of the pili formation that multiple layers biofilm microorganisms can participate in the electron transfer while bulk ones do not participate in the electron transfer [4, 39–43].
Indirect electron transfer through external or internal mediators
In this type, a redox active material (mediator) is responsible for the electron transfer between the microorganism and the anode surface. This redox can either be exerted naturally by the microorganisms (internal) or can be added from outside (external). These mediators whether internal or external will be responsible for the electron transfer from the bulk microorganisms to the anode surface. The electron transfer in the mediated electron transfer is higher than that in the DET [4, 44–51].
Internal mediators have several advantages over the external ones such as they are cheap as they are exerted by the microorganism and have no toxic effect on the microorganism. Figure 3 shows a schematic diagram of the disadvantages of external mediators and some types of the internal and external mediators.
Several external mediators have been investigated in MFCs such as methylene blue (MB), methyl red, methanyl yellow, methyl orange, bromocresol purple, bromocresol green (BcG), romothymol blue, bromophenol blue, Congo red, cresol red, eriochrome black T, murexide, neutral red (NR), yeast extract, etc.
3. Yeast as a biocatalyst in MFCs
Yeast is a eukaryote with cell compartmentalization and has more complicated architecture compared to prokaryotes. Yeast is considered as an ideal biocatalyst for microbial fuel cell applications as most strains are nonpathogens, can metabolize wide range of substrates, are robust, and are easily handled. The bio-catalytic activity of the yeast would be related to the existence of different natural electron shuttles, mediators, such as azurin, ferredoxin and cytochromes, which could be used by redox enzymes for electron transfer from the yeast cells to the anode surface. This is in addition to the high extent of proteins in the yeast cell membrane, which is an important characteristic of electroactive species [4, 6]. Yeast cells also have a thick (100–200 nm) cell wall constructed of polysaccharides and proteins [43, 52]. Yeast cytochromes are located in the mitochondria, and transmembrane proteins (tPMETs) are located in the cell membrane, which are enclosed by the cell wall. Hence, to obtain an electrochemical response from the yeast cells, it has been assumed that a mediator must traverse the cell wall and interact with the membrane and/or internal redox sites such as NAD+/NADH [41, 42], or that the response originates from the soluble electroactive species exported from the cell [4, 45].
The electron transfer during the metabolism of the organic materials in the yeast cell is shown in Figure 4. Electrons liberate during the oxidation of the substrate into pyruvate in the glycolysis process, which takes place in the cytosol of the cell. These electrons received by the NAD+ forming NADH, which is recycled through its oxidation by the liberation of the electrons to the anode surface whether directly through the tPMETs or through the mediator to form NAD+ again — cycle of NADH to NAD+. In mitochondria, oxidation of pyruvate into organic acids is associated with the liberation of the electrons that are received by the NAD+ forming NADH, which in turn are oxidized by releasing electrons to the mediator to form the NAD+ again. The reduced form of the mediator lost electrons to the anode surface to complete the cycle [38, 46].
Several yeast strains have been studied as biocatalysts in MFC with or without external mediator such as
Baker’s yeast (
3.1.1. Mediator-less MFC
Mediator-less MFCs are those that operate without the addition of any external mediator. Sayed et al.  studied the mechanism by which
The same conclusions for the direct electron transfer and no role of the mediator in the electron transfer of the
In another study, the performance of air-cathode MFC using
Anode modification .
Immobilization of the yeast cells on carbon nanotube .
Yeast surface display of dehydrogenases .
184.108.40.206. Enhancement of electron transfer in a mediator-less MFC
The electrical conductivity of the anode plays an important role in the performance of the MFCs. The effect of the modification of carbon paper with thin layer of different transition metals, i.e., cobalt and gold, on the performance of air-cathode MFCs using
|Ref.||Max. power||Anode chamber (WV)||Separator||Cathode||Anode material||Carbon source||MFC type|
|6||3.||17||84 mL (70 mL WV)||NRE 212||O2 (air)||Pt/C over carbon paper||Carbon paper||Glucose||Air cathode|
|42||12.9||(70 mL WV)||Nafion 117||O2 (air)||Pt/C over carbon paper||Carbon paper||Glucose||Air cathode|
|20.2||Co sputtered carbon paper|
|2||Au-sputtered carbon paper|
|38||25.51||350 mL (320 mL WV)||Nafion 117||O2 (air)||Graphite plate||Graphite plate||Synthetic wastewater||Air cathode|
|52||2.7||8–10 mL (5 mL WV)||Nafion 117||O2 (air)||A graphite plate||A graphite plate/MWCNT||Lactose||Dual chamber|
|46||40||500 mL||Nafion 117||Potassium ferricyanide||Reticulated Vitreous carbon||Reticulated Vitreous carbon||Glucose||Dual chamber|
|47||28||850 mL (760 mL WV)||Nafion 117||-||Graphite plates||Graphite plates||Glucose||Dual chamber|
The electron transfer of
The performance of
3.1.2. Mediated yeast-based MFC
Several studies have been carried out to enhance the electron transfer through the addition of an external mediator. A candidate external mediator must satisfy several requirements such as being electrochemically active, fast release of electrons on the electrode surface, biocompatible to the microorganisms, soluble and chemically stable in the anolyte media, easily penetrate the cell membrane, and has a prober redox potential that is sufficiently positive to provide fast electron transfer from microorganisms to the anode while not too strong to avoid a big loss of potential [2, 14, 16]. Different mediators such as MB, NR, thionine, yeast extract, and others enhanced the electron transfer in
|Ref.||Max. power||Mediator||Anode chamber|
|Separator||Cathode||Anode material||Carbon source||MFC type|
|59||22||850 × 103||2-hydroxy-1,4-|
|WV, 7.5 cm3||Gore-Tex, 30 μm||K3[Fe(CN)6]||Carbon|
|Carbon rods and carbon fiber bundles||Glucose||Dual-chamber|
|50||80||MB||(70 mL WV)||Nafion 117||O2 (air)||Pt/C over carbon paper||Carbon paper||Glucose||Air cathode|
|148||Co-sputtered carbon paper|
|120||Au-sputtered carbon paper|
|45||150||MB||10 mL||Nafion||Potassium ferricyanide||Carbon felt||Carbon felt||Glucose||Dual chamber|
|46||146.71 ± 7.7||MB||500 mL||Nafion 117||Potassium ferricyanide||Reticulated vitreous carbon||Reticulated vitreous carbon||Glucose||Dual chamber|
|52||39||MB (0.1 M)||25 mL||O2 (air)||Pt/C over carbon cloth||Graphite plate||d-xylose||Air cathode|
|44||400||MB||32 mL||Nafion 115||Reticulated vitreous carbon||Reticulated vitreous carbon||Dextrose||Dual chamber|
|45||1500||MB||10 mL||Nafion||Potassium ferricyanide||Carbon felt,||Carbon felt||Glucose||Dual chamber MFC|
|46||145||MB||500 mL||Nafion 117||Potassium ferricyanide||Reticulated vitreous carbon,||Reticulated vitreous carbon||Glucose||Dual chamber MFC|
(760 mL WV)
|Nafion 117||-||Graphite plates||Graphite plates||Dual chamber|
|Nafion 117||Pt/C over carbon paper||Carbon paper||Glucose||Air cathode|
|70||Au-plated carbon paper|
Using copper electrodes and a sulfonated polyether ether ketone (SPEEK) as proton exchange membrane, Permana et al.  studied the performance of dual chamber
The effect of the anode modification on the performance of the mediated
MB was also used in air-cathode MFC that used modified
Compared to MB, NR showed promising results in a two-compartment
Thionine is another mediator that worked effectively in
Yeast extract, which is one of the main components of the biological cultivating media, was effectively used as a mediator in
|Ref.||Max. power||Electron transfer|
|Anode chamber (WV)||Separator||Cathode||Anode material||Carbon source||MFC type|
|mW/m2||mW/m3||Mediator less||Mediator||Electron acceptor||Electrode|
|53||60||Mediator less||100 mL||Salt bridge||Potassium ferricyanide||Graphite rods,||Graphite rods||Fructose,||Dual chamber|
|54||640||MB||13 mL||Nafion 117||Potassium ferricyanide||Carbon felt||Carbon felt||YPfru||Two chamber|
|55||36||Mediator less||13 mL||Nafion 117||Potassium ferricyanide||Carbon felt||Carbon felt|
|720||Ni-nanomodified carbon felts|
galvanostatic pulse deposition (GME)
|56||390||Mediator less||13 mL||Nafion 117||Potassium ferricyanide||Carbon felt||Ni-nanomodified carbon felts|
potentiostatic pulse technique (PME)
The effect of the mediator type, i.e., bromocresol green (BcG), bromocresol purple, romothymol blue, bromophenol blue, Congo red, cresol red, eosin, eriochrome black T, methyl red, methanyl yellow, MB, methyl orange, murexide and NR on the performance of
The performance of
3.3. Other yeast strains
The catalytic activity of
The electron transfer pathways between the cytosolic redox enzymes of
The biocatalytic activity of the nonconventional yeast
Kaneshiro et al.  have investigated the catalytic activity of six different yeast strains in a dual chamber MFC with glucose as the substrate including
4. Large-scale yeast-based MFC
A novel yeast-based MFC stack that composed of 4 units of total capacity of 1840 mL was designed and operated using glucose as the carbon source, graphite plates as the electrodes and Nafion 117 as the separator . The stack was operated under continuous mode with a hydraulic retention time of 6.7 h. Single cell and cells connected in parallel and/or series connections were investigated to achieve the best operating conditions. A maximum current of 6447 mA/m2 and maximum power of 2003 mW/m2 were obtained. A Columbic efficiency of 22% was obtained in the parallel connection. Figure 11 showed that the stack could be operated for more than 3 days with stable voltage and power output. The results obtained in this study proved the potential of yeast for scaling up. Table 4 showed summary of the materials and operating conditions used in the stack.
|MFC type||MFCs stack composed of 4 anodes and 3 cathodes compartments|
|Anode||Graphite plates, size of 40 × 60 × 1.2 mm|
|Cathode||Graphite plates, size of 40 × 60 × 1.2 mm|
|Membrane||Nafion 117.32 cm2|
|Catholyte||Potassium permanganate (400 μmol/L)|
|Anode media||Yeast (|
|Fuel||Glucose, 30 g/L|
|Anode chamber (volume)||460 mL|
|Working volume||350 mL|
|Current collector||Copper wire|
|Mode||Continuous up flow mode|
5. Conclusions and recommendations
Yeast is successfully used as a biocatalyst in MFC, which exhibits different electron transfer mechanisms according to its strains. In