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Microbial Fuel Cell Reaction

Microbial Fuel Cell Reaction


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I found a formula online which explained the reaction which occurred in the anode chamber of a microbial fuel cell but there was a typo

$small NADH + Ferredoxin_{(ox)} (NADH-Ferredoxin Oxidoreductase) longrightarrow NAD^+ + Ferredoxin_{(red)} Ferredoxin_{(red)} (Hydrogenase)#61614;Ferredoxin_{(ox)} + 2H^+ + 2e^-$

I attempted to correct it but I'm not sure if I'm right could you check this and see if it's correct?

$small NADH-Ferredoxin Oxidoreductase longrightarrow NAD^+ + Ferredoxin_{(red)}$

$small Ferredoxin_{(red)} longrightarrow Hydrogenase + Ferredoxin_{(ox)} + 2H^+ + 2e^-$


Anode reaction,

$small Glucose + 2NAD^+ (Embden-Meyerhof pathway) leftrightharpoons 2 Pyruvate + 2NADH$

$small Pyruvate + Ferredoxin_{(ox)} (Pyruvate-ferredoxin Oxidoreductase) leftrightharpoons Acetyl-CoA + CO_2 + Ferredoxin_{(red)}$

$small NADH + Ferredoxin _{(ox)} (NADH-Ferredoxin Oxidoreductase) leftrightharpoons NAD^+ + Ferredoxin_{(red)}$

$Ferredoxin_{(red)} (Hydrogenase)leftrightharpoons Ferredoxin_{(ox)} + 2H^+ + 2e^-$

[Last one: http://en.wikipedia.org/wiki/Ferredoxin_hydrogenase]

Cathode reaction,

$4 H^+ + 4e^- + O_2 leftrightharpoons H_2O$

$C_6H_{12}O_6 + 6O_2 leftrightharpoons 6 CO_2 + 6 H_2O, DG° = - 2870 kj$

Full paper: http://www.altenergymag.com/emagazine.php?issue_number=07.04.01&article=haldia


Microbial Fuel Cell Reaction - Biology

Hydrogen gas can also be produced via fermentation by some bacteria using glucose or cellulose, but the yields are low (a maximum of 4 moles of H2 compared to 12 based on stoichiometry). The Logan Lab at Penn State has done extensive research on fermentation-based hydrogen production, but more recently we have focused on electrochemical hydrogen production in microbial electrolysis cells (MECs). In an MEC, hydrogen gas is produced at the cathode, using exoelectrogenic microorganisms on the anode that convert the organic matter into an electrical current. The voltage that needs to be applied can be as little as 1/10th that needed to split water and make hydrogen gas. The same bacteria that can be used in MFCs to make electricity can be used to generate current in MECs. For a relatively recent and comprehensive review of MFCs and MECs, paper in Science (Logan & Rabaey, 2012)

0.2 to 0.3 V. Thus, we only need to add about 0.2 V or more to make hydrogen gas in the MEC/BEAMR. This voltage is much less than that needed for water electrolysis, which is about 1.8 V in practice. It takes a lot of energy to split water, but “splitting” up organic matter by the bacteria is a thermodynamically favorable reaction when oxygen is used at the cathode. In the MEC process, no oxygen is present and the reaction is not spontaneous for hydrogen production unless a small boost of voltage is added to that produced by the bacteria. Thus, the MEC process is more of an “organic matter electrolysis” procedure (versus water electrolysis).

It is possible to have very high hydrogen production yields and energy efficiencies using MECs. For example, in our paper in the Proceedings of the National Academy of Science (PNAS) (Cheng and Logan, 2007, 104(47): 18871–18873), we have obtained hydrogen gas at yields of 2.01-3.95 mol/mol (50-99% of the theoretical maximum) at applied voltages of 0.2 to 0.8 V using acetic acid as the fuel. At an applied voltage of 0.6 V, the overall energy efficiency of the process was 288% based solely on electricity applied (82% from the energy in acetic acid and electrical energy used). The gas production rate was 1.1 m3-H2 per m3 of reactor per day. Direct high-yield hydrogen gas production was further demonstrated using glucose, several other volatile acids (butyric, lactic, propionic, and valeric) and even cellulose at maximum stoichiometric yields of 54 to 91%, at overall energy efficiencies of 64-82%.

Scaling-up MECs has progressed further than that of MFCs, in part due to the simpler 2-phase reaction (protons in water, electrodes) compared to the 3-phase reaction for MFCs (oxygen in air, protons, electrodes). Laboratory scale systems are typically 28 mL cube reactors, such as that shown in the schematic to the left, but we have built larger laboratory systems such as the 2.5 L reactor shown on the left. We also conducted field tests at the Napa Winery in California, and demonstrated effective treatment of winery wastewater using a 1000 L reactor with a net gain in energy relative to the electrical power input, although methane was produced in that system rather than H2 gas. We are currently working on new designs to more effectively convert cellulose fermentation endproducts to H2 gas at high production rates and recoveries, in a project with the National Renewable Energy Laboratory (NREL), in Golden, CO.

Links and notes

This website contains a number of sections to introduce MFCs and other microbial electrochemical technologies (METs). For example, to see slide shows and videos, go to our Presentations page. To find out more about this and other hydrogen and fuel cell research at Penn State, visit the H2E Center webpage. To read more about microbial fuel cells (MFCs), go the the MFC page.


Microbial fuel cell (MFC) power performance improvement through enhanced microbial electrogenicity

Within the past 5 years, tremendous advances have been made to maximize the performance of microbial fuel cells (MFCs) for both "clean" bioenergy production and bioremediation. Most research efforts have focused on parameters including (i) optimizing reactor configuration, (ii) electrode construction, (iii) addition of redox-active, electron donating mediators, (iv) biofilm acclimation and feed nutrient adjustment, as well as (v) other parameters that contribute to enhanced MFC performance. To date, tremendous advances have been made, but further improvements are needed for MFCs to be economically practical. In this review, the diversity of electrogenic microorganisms and microbial community changes in mixed cultures are discussed. More importantly, different approaches including chemical/genetic modifications and gene regulation of exoelectrogens, synthetic biology approaches and bacterial community cooperation are reviewed. Advances in recent years in metagenomics and microbiomes have allowed researchers to improve bacterial electrogenicity of robust biofilms in MFCs using novel, unconventional approaches. Taken together, this review provides some important and timely information to researchers who are examining additional means to enhance power production of MFCs.

Keywords: Biofilm Electrogenic microorganism Microbial community Microbial fuel cell Synthetic biology.


A microbial fuel cell (MFC) is a system that drives a current by converting chemical energy to electrical energy, using the catalytic activity of microorganisms. The energy crisis makes the circumstances suitable for improvement of MFC energy production, which is a newer source of energy - cheaper, cleaner, and more sustainable. An MFC consists of an anode, a cathode, and to separate these it needs a membrane. An MFC can use different degradable chemicals as the fuel, and works on the same concept as other types of fuel cells, namely an oxidation-reduction reaction. One of the simple designs of MFC is the sediment microbial fuel cell (SMFC). SMFCs are very economical however, their energy production is lower than other types of MFC. SMFCs are also very easy to build and they can be useful in marine floor applications. Developments on MFC technology are very promising this technology might be a significant part of the solution for problems such as the energy crisis and global warming.

Introduction

The energy crisis is becoming more apparent as global demands for energy are increasing. The costs of fossil fuels have increased and they will continue to grow. Our fossil fuels production is limited, and it will not be able to satisfy our demands. Similarly, costs of other sources of energy are increasing it makes the situation suitable for newer sources of energy that are cheaper, cleaner, and more sustainable. (7)

Figure 1. World GDP Aggregate Weight by World Oil Consumption Shares. Source ITF Interim Report Crude Oil(4)

Microbial Fuel Cell (MFC) technology is one of the newest approaches for electricity production. To generate electricity, an MFC uses biomass, which is a renewable source of energy, economically feasible and vastly available. (2) Furthermore, its consumption of biomass makes water more hygienic for later human consumption. (5)

Several experiments have shown the possibility of power generation from biomass accumulated in the undersea sediments. (3) These sediments contain microbial cultures growing anaerobically, that can oxidize the biomass and release a flow of electrons between anode and cathode reactants. This electron transfer produces a current, and can be used for power generation. Previously, some applications were done using non-oxygen cathodes.

Design of MFC is facing technical challenges as engineers seek usable applications in the real world. Current power densities are feasible, but MFC design has to be more cost effective. The design has to be scalable for larger applications. (3)
The Parts of a Microbial Fuel Cell

In an oxidation-reduction reaction, an electron is provided by one reactant, and consumed by another reactant. But in a fuel cell, electrons cannot directly be transferred between the half reactions. An electron is produced in an oxidation half reaction, where it is called an anode. Then, the released electron moves through a wire to reach the cathode, where it will be used in the reduction half reaction. This electron transfer produces a charge gradient between the cathode and anode. Motivated by the charge gradient, the ion exchange membrane makes the ion transfer possible. This balances the charges in the cathode and the anode. The result of this electron transfer is an electrical current, or in other words, electricity.

node47_micro2
Figure 2. Schematics of an MFC with a Membrane.

The Microbial Fuel Cell has been designed based on the same concept. Consequently, it consists of four elements: anode, cathode, an ion exchange membrane, and a microbial fuel. These four parts function as follows:

Some bacteria serve as anodes, and produce energy by oxidizing organics. Any oxidation reaction requires an electron acceptor, which in this case could be an oxygen molecule, or any other ions present in water that could be reduced. There are many different ways that bacteria can carry their electrons from the oxidation site to the electron acceptor. Some bacteria use the oxygen dissolved in water and reduce it inside their cells. A few others can actually transfer the electrons outside their bodies and donate the electrons to the oxidizing agents. These bacteria can grow in anaerobic environments, since they don¡¯t require oxygen. Oxygen is actually toxic and lethal for some of them. For example, geobacter metallireducens respire on organic compounds using ferric as an electron acceptor, and reduce it to ferrous. In a fuel cell they will donate their electrons to the anode electrode and these electrons will be used on the cathode side. This electron transfer is not only an energy source for the bacterial culture, but it can also produce energy in an external resistance between the anode and the cathode.

There are different types of cathodes, and there are different chemicals used in them. The most economical cathode would use dissolved oxygen as the electron acceptor. Concentration of the oxygen molecules is very low inside the water, so this kind of cathode will not produce a very large driving force in comparison to other types of cathodes. On the other hand, this type of cathode doesn¡¯t need replacement, since the only element it consumes is oxygen, allowing it to perform for a long period of time.

While the cell is performing and producing power, the charge of the cathode and anode becomes unbalanced. Bacterial culture produces protons in the solution, so the anode becomes more positive. Since the oxygen is reduced in the anode, the cathode side becomes negative. In order to keep the cell working there has to be an ion exchange membrane to balance the charges between the anode and the cathode, by moving the ions driven by the charge gradient. The main difficulty is that this membrane can¡¯t be exposed to air, because the anode side has to stay anaerobic. The exchange membrane also puts some resistance on the performance of the overall cell, so it blocks the ions, especially protons, from moving freely through the membrane. (1)

The most economical membrane would be sea sediments. It has been shown that MFCs can be made on the sea floor, where the membrane is only the sea sediments. This kind of MFC is called Sediment Microbial Fuel Cell (SMFC). Unfortunately, SMFCs have very low power densities, because of their high internal resistances. Some of their large internal resistance is due to the inefficient performance of the membrane, and the large distance between the anode and the cathode. (1)

In theory, an MFC can consume any chemical compound that can be oxidized by microorganisms. But research has shown that glucose and acetate are unusually good primary food sources for the microbial cultures that grow on the anode. These organic compounds can be broken down to smaller sugars, carboxylic acids, and alcohols, which are eventually eaten by the microorganisms growing on the anode. This brings up the possibility of using waste water as the food for MFCs and actually reducing the organic contamination of waste water in the waste water treatment plants. (1)
How Does an MFC Operate?

Most biomass chemicals can be broken down to acetate through different catabolic pathways, and this explanation will assume the fuel is acetate. Inside the microorganisms, acetate can simply become Acetyl-CoA. Acetyl-CoA can be consumed in the citric acid cycle, which will produce three molecules of NADH and one FADH2. There are eight electrons produced for each acetate cycle, and they are stored by producing NADH and FADH2. These electrons are released through the electron transport chain, and transferred to the anode electrode. These electrons are used on the cathode side of the reaction, where reduction takes place.
How to Make a Very Simple MFC?

Most microbial cultures growing in undersea sediments have the possibility to transfer their electrons to an anode, so they are suitable to be used in MFC. Also, the sediments that keep the system anaerobic, work as a membrane. Hence, the only work that has to be done is to insert a non-corrosive, conductive anode inside the sediments, and have a larger surface area of the cathode in the aerobic part of the water. Then the cathode and the anode have to be connected by a resistor, depending on their surface area and their performance. This cell will get better after a few days. But if the sediments are in a closed system, they will eventually run out of the food and the voltage will drop. The power production for these types of cells is very limited due to the thickness and high resistance of the membrane. (Figure 3) (6)

Figure 3. Schematic of a SMFC with the dimensions that were experimented.

Conclusion Concerning MFC Potential

SMFCs are not feasible for power production in waste water plants, but they can be a good alternative for batteries in undersea instrument, where it

node47_micro4
Figure 4. Voltage vs. Time Graph of a SMFC

The increase in voltage is due to the growth of microorganisms on the anode side.

is very hard to replace the chemical battery and a very small amount of energy is required for the devises. The SMFCs can perform for an unlimited time, since their system is open, they will never run out of food, and the dissolved oxygen in the water is also unlimited.

Technology of MFC is a promising field of research, which hopefully can solve some of the energy crisis, and reduce the amount of emission gases released into the atmosphere.

Acknowledgement

References

Du, Zhuwei, Haoran Li, and Tingyue Gu. "A state of the art review on microbial fuel cells: A promising technology for wastewater treatment and bioenergy." Biotechnology Advances 25 (2007): 464-82. Web.

Logan, Bruce E. Microbial Fuel Cells. New York: Wiley-Interscience, 2008. Print.

Rinaldi, Antonio, Barbara Mecheri, Virgilio Garavaglia, Silvia Licoccia, Paolo Di Nardo, and Enrico Traversa. "Engineering materials and biology to boost performance of microbial fuel cells: a critical review." The Royal Society of Chemistry (2008): 417-29. Print.

Interagency Task Force on Commodi ty Market s , Inter im Repor t on Crude Oil,,Washington D.C.

Kargi, Fikret, and Serkan Eker. "High power generation with simutaneous COD removal using a circulating colum microbial fuel cell." Wiley Interscience 84th ser. (2009): 961-65. Web.

Donovan, Conrad, Alim Dewan, Deukhyoun Heo, and Haluk Beyenal. "Batteryless, Wireless Sensor Powered by a Sediment Microbial Fuel Cell." Environmental Science and Technology 42 (2008): 8591-596. Print.

MacKay, David JC. Sustainable Energy - without the hot air. CAMBRIDGE: UIT Cambridge Ltd, 2009. Print.


Discussion

The gel prepared as described in the method paragraph, can be considered a solid state electrolyte, even if inside its framework a liquid phase is still present, because it is well accepted by the scientific community considering gels as solid electrolytes 23,24 .

In order to estimate the resistances of ions conductions inside the solid electrolyte, the ohmic resistance has been evaluated. The estimation of the Ohmic losses (RΩ) arising, among other factors, from the resistance of ions conduction into the electrolyte and the membrane and of electrons conduction through the electrodes 25 . It was carried out by means of the current interrupted method (CIM), as described in Methods (MFC operation and characterization). The average value of (12 ± 2) Ω was calculated considering resistances from both the first and the second phase. This result is quite interesting and helps in clarifying that the presence of the new introduced solid electrolyte does not increase the RΩ value. The obtained Ohmic losses are, comparable with those typically measured for liquid anolyte with the same boundaries conditions 26,27 . Once the experiment was over it was possible to notice that the SSA was less compactness than it was at the beginning of the experiment. This phenomenon can be explained by the contribution of two different processes: one can be related to the glucose and fructose consumption due to the bacteria, that causes the reduction of the cross-linking degree among the agar molecular chains, while the other processes can be related to an agar degradation due to the seawater bacteria inoculum, because many marine microorganisms can degrade this algal compound by production of agarase enzyme. In particular the presence of craters on the SSA surface, observed at the end of experiment is a result of the enzymatic digestion of the agar 18 .

The EE, defined as the ratio of energy recovered by the MFC to the heat of combustion of the consumed organic substrate 28 , gives important information on the energetic performance of the device. Moreover, since it takes into account energy losses due to external devices, such as the pumps for feeding and recirculation 28 , it can help appreciating the unique features of the SSA-MFC. Indeed, not needing hydraulic pumping systems for anodic feeding, the SSA-MFC has a zero energy contribution from external systems at the anode, helping to obtain an overall positive energy balance 29 , with an impressive value of EE close to 63%. It is important to say that since this study was mainly focused to analyze the behavior of the new SSA and hence at this stage the repletion of catholyte, in terms of energy costs, has not been taken into account, because the focus is on the anodic chamber energy balance and not that of the whole MFC. The choice of a two-chamber MFC with a liquid chemical catholyte, instead of the open-air cathode architecture is to focus the attention mainly into the new SSA. Considering the difficulty to estimate heat of combustion of wastewater, in literature the results usually show the Coulombic Efficiency (CE) that represents the conversion of organics into electrical charge.

However, CE does not take into account the energy expenditure to run the device and hence, it is not able to give indication of how far MFCs are from an energy-positive device 29 . Data present in literature, show CE values ranging from 1% to up 80% 28,29,30,31,32,33 . These variations mainly depend on the organic substrate but also on design, operative conditions and microbiology.

It is evident, therefore, that, in many cases, if energy consumption of the external devices were taken into account, these efficiency values would be dramatically lower.

The present work fixes a new important step toward the development of a solid phase anolyte and its capability to function as nutrient storage available for bacteria and convertible into useful energy in the time, without any external nutrients addition. It acts as true chemical energy storing system able to release slowly its energy over time and significantly contributes to the autonomy of the resulting system. The easy preparation of SSA and the easy uptake of it from bacteria reveal that solid SSA in an original and convenient fuel for MFC. This is an important step toward the reduction of energy request, bringing a better net energy balance necessary to overcome the biggest bottleneck to marketability of MFCs. The convenience of maintaining a growing community of bacteria thanks to a high-density energy substrate could mean a potentially long-term source of energy with numerous applications: from remote energy source to the waste management practices as treatment of both water and solid waste. Taking into account that many electronic devices or sensors, i.e. temperature sensors, require 1.5 V as operative voltage or even less, i.e. piezoresistive sensor 26 require

1 V, one possibilities is to design a system at least of two MFCs connected in series and eventually with a capacitive storage system. The remote application could take advantage from natural environments, i.e. seawater. There bacteria can found a richer substrate releasing more electrons, permitting to power sensors in remote area. Moreover, the high efficiency of the device (63%) together with the zero contribution for anodic feeding is a crucial step toward a new clear route to portable and autonomous MFC devices for remote applications, cutting down, significantly, operative costs, energy requirements for the system maintenance and reducing hydraulic problems.

It is worthwhile to underline that the anodic chamber, after it was sealed, did not need any more feeding or hydration during the experiment neither reinoculation, giving rise to a really autonomous device from the anodic side.

Next important step could be the integration of air-cathode to replace the chemical catholyte, in order to push the research toward a complete autonomous system. The non-astonishing performance have not to restrain the opportunity of a further development, because SSA shows the way for a new generation of MFCs, that can, for example, be, realistically, employed in devices for remote areas or harsh environments monitoring, powering small devices, such as sensors.


Performance improvement for biomass-fueled MFCs

Operation conditions and optimization

The specific metabolic process in MFCs varies on the type of biomass/organic wastes and the operational conditions of MFCs (Guo et al. 2020). In this section, operation conditions (pH, temperature, and organic loading rate), and optimizations for performance improvement in MFCs are discussed in the following.

pH is one of the essential factors in MFCs that affects both anodic microbial activities and cathodic reactions. Accumulation of protons would cause anolyte acidification, and electrolyte alkalization in the cathode chamber. Hence, reducing pH in the anode chamber due to increased proton concentration results in low power production, which represses the EAMs activation. On the contrary, it causes an increased pH in the cathode chamber that inhibits the oxygen reduction reaction (ORR) (Ivars et al. 2018). Although the use of pH buffers such as phosphate or bicarbonate (pH 7.0) has been suggested for controlling pH at the anolyte, it may increase operating costs and effluent desalination or further phosphorous removal burden (Chen et al. 2019). It was found that an increased anodic pH shall attribute to an increased COD removal and improve the performance of MFCs. The optimal pH strongly depends on the type of microorganisms. Zhang et al. (2011) studied the effect of pH on the performance of MFC and anodic microbial community. Results showed a faster COD removal under acidic pH conditions, in which Simplicispira, Variovorax, Comamonas, and Acinetobacter were the major communities under acidic conditions. Anodic biofilm cracked and cell number greatly decreased at pH ≤ 5.0, and further, MFCs was failed at pH 4.0 due to microbial community composition changes. However, the MFCs could recover optimal electricity generation when pH was further readjusted to 7.0 (Zhang et al. 2011). Optimal pH for maximum power production reported was 8–10 in an air-cathode MFC fueled with acetate (Zhao et al. 2017). Usually, the anodic microbial reaction preferred a neutral pH for optimum cell growth, whereas a weak alkaline pH was more appropriate for cathodic reaction.

Temperature effect depends on the nature of anodic EAMs and the characteristics of biomass in MFCs. It has been reported that microorganisms can grow in four classified optimal growth temperature, i.e., psychrophiles (10–15 °C < 20 °C), mesophiles (25–40 °C < 45 °C), thermophiles (50–85 °C), hyperthermophiles (80–113 °C) (Stetter 2006). However, most of the characterized EAMs belong to mesophilic classification. At extremely low temperatures, microbial reactions slow down, and eventually, MFCs cannot be operated in most cases (Ivars et al. 2018). However, MFCs with psychrophiles EAMs can operate at low temperature and attain high CE. Behera et al. (2011) evaluated temperature effects on the performance of dual-chambered mediator-less MFC by adjusting the temperature between 20 and 55 °C. The highest COD removal efficiency of 84% was observed at an operating temperature of 40 °C. Tee et al. (2018) studied the performance of MFC with an adsorption system (MFC-AHS) and palm oil mill effluent as a substrate under various operating temperatures. The optimum operating temperature for such a system was found at 35 °C. Also, results revealed that the maximum current density could increase linearly with the temperature at a rate of 0.1772 mA/m 2 /°C, whereas maximum PD was in a polynomial function (Tee et al. 2018). Larrosa et al. (2010) investigated single-chambered and dual-chambered MFC operation at different temperatures ranging from 4 to 35 °C. The results revealed that the temperature as a crucial factor for COD removal and bioelectricity production, which were obtained 58% COD removal with maximum PD of 15.1 mW/m 3 reactor (8.1 mW/m 2 cathode) at 4 °C, and 94% COD removal with maximum PD of 174 mW/m 3 reactor (92.8 mW/m 2 cathode) at 35 °C (Larrosa-Guerrero et al. 2010).

Organic loading rate (OLR) has a significant impact on anodic biofilm, which primarily depends on the chemical characteristics of wastes. Especially, the fermentation of biomass/organic wastes can result in acidic metabolites production, which affects the anodic electrolyte. Therefore, the OLR fueled MFCs should be carefully optimized to achieve high performance. Further, it is confirmed that PD and CE in MFCs are closely related to OLR, in which an increased or decreased OLR can affect the efficiency of electron transfer. Operation of MFCs at the higher OLRs usually resulted in a decreased CE (Velvizhi and Mohan 2012). In a study reported for an MFC with treating leachate, the increasing OLR from 0.65 to 5.2 kg COD/m 3 /day resulted in a decrease of overall CE from 14.4 to 1.2% (Zhang et al. 2008). Cetinkaya et al. investigated the effect of OLR with changing HRT and leachate COD concentration. The results indicated the COD removal and current density were significantly affected by increasing OLR, although the performance of MFC decreased when HRT was reduced (Cetinkaya et al. 2016).

Ionic conductivity of the electrolyte

Maintaining a suitable pH condition of electrolyte is necessary for obtaining a high PD and CE in MFCs. Cations such as Na + and K + , other than H + are prone to transfer toward the cathode, however, H + mass transport is sluggish, which its accumulation in the anode causes anolyte acidification, and significantly restricts the electricity generation in the MFCs (Ren et al. 2017). Eliminating the anolyte acidification with alkaline catholyte through electrolyte recirculation has relieved the pH decline in MFCs, whilst O2 was likely to be influenced to the anode and restricted the activity of anode biofilm (Zhang et al. 2015). Further, inorganic ions buffers are always indispensable in MFCs to provide certain ionic conductivity and maintain stable pH conditions of the electrolyte (Chen et al. 2019). In addition, inorganic carbons (IC) such as H2CO3 (dissolved CO2), HCO3 − , CO3 −2 could be produced as the final metabolites of MFC, which are considered as endogenous buffers, although their accumulation concentration is insufficient to prevent acidification of anolyte (Ren et al. 2017). To overcome this limitation, Ren et al. (2017) reported a novel buffer-free MFC with anolyte recycling as a feasible strategy that could increase the IC concentration of the anolyte, thoroughly eliminating anolyte acidification and dramatically enhancing the electric power of MFCs.

Electrode modification

Electrode materials should possess good electron conductivity, large surface area and good biocompability for microbial adherence. The surface properties of an electrode, such as roughness, porosity, and surface hydrophilicity can affect the formation of biofilm and subsequently derived electric power in MFCs (Zhao et al. 2017). In recent decades, carbonaceous is the most extensively used anodic material in MFCs. Carbonaceous-based materials such as carbon paper (Hassan et al. 2014), granular graphite (Habibul et al. 2016 Vilajeliu et al. 2017), and graphite rods (Xu et al. 2015) have been identified and widely used in biomass-fueled MFCs. However, the commercial carbon-based electrode showed a smooth surface with low electrochemical activity and biocompatibility. Hence, various strategies for electrode surface modification have been developed. For example, Chen et al. (2018) developed the candle soot modified-CC electrode by inoculating Aeromonas hydrophila NIU01 in MFCs. The modification with 60-s could alter the hydrophobic surfaces of the CC electrodes to super-hydrophilic. Further, the electrochemical measurement of the modified electrode exhibited the highest PD of 19.8 ± 0.2 mW/m 2 with an internal resistance of 619 Ω, which was higher than that of MFCs conducted with the bare electrodes (10.2 ± 0.2 mW/m 2 ) (Chen et al. 2018). In another study, Zhao et al. (2018a, b) thermally modified CF electrodes with a mixed solution of concentrated HNO3 and 30% H2O2 in different volume ratios. The inoculum of MFCs was supplied from local domestic sewage. The modification decreased the anodic charge transfer resistance with a maximum PD of 785.2 mW/m 2 , which was 51.1% higher than the bare electrodes in the MFCs (Zhao et al. 2018a, b). Moreover, modification with conductive polymers such as polypyrrole and polyaniline was demonstrated for improving anodic biofilm formation. The polymer composites can increase electrode surface roughness, and also the presence of cationic nitrogenous groups in their composite structure can enhance cellular adhesion electrostatically (Fogel and Limson 2016). For example, Li et al. (2018) introduced the polypyrrole nanowires coated by graphene oxide (PPy-NWs/GO) using a one-step electrochemical method. The performance of PPy-NWs/GO showed higher PD than PPy-NWs. Besides, the PPy-NWs/GO showed a more extensive biofilm of microbial attachment, which was owing to the GO nanosheet (Li et al. 2018). Razalli et al. (2017) pretreated the extracted crystalline nanocellulose of semantan bamboo with acid hydrolysis to synthesize a polyaniline/crystalline nanocellulose (PANI/CNC) electrode via in situ oxidative polymerization of aniline. The EIS results of PANI/CNC displayed a lower value of RCT (148 Ωcm 2 ) compared to the bare (177 Ωcm 2 ) and PANI (156 Ωcm 2 ) electrodes, which revealed that PANI/CNC incorporation could significantly reduce the charge transfer rate (Razalli et al. 2017). Moreover, nanoparticles, through a combination of the improved electrode surface, alteration of surface chemistry, and the presentation of electroactive moieties to the microbial cells have been used for improvements of the electrical current in MFCs (Fogel and Limson 2016). Ni, Pd, Au, and Fe2O3 nanoparticles have been used to enhance the direct EET for performance improvement of MFCs (Fogel and Limson 2016). Further, carbon-based nanomaterials including carbon nanotubes (CNTs), carbon nanoparticles, and graphene have also been performed for improving cell/electrode interaction, and enhancing EET pathway in MFCs. Besides, the rational inclusion of nanomaterials as electrode material/modifiers could significantly improve the electricity generation of biomass-fueled MFCs. Graphene oxide (GO) with rich hydrophilic functional groups and possessing biocompatibility, superior electrical, mechanical, and optical properties has developed a strong electrochemical performance in MFCs (Yong et al. 2014). GO can react with organic or inorganic chemicals and to remove the oxygen atoms to form proxy groups, which results in reduced graphene oxide (rGO) sheet network. However, apart from chemical reduction of GO, there are many techniques (i.e., hydrothermal reduction, electrochemical reduction, solvothermal reduction, and microbial reduction) that can react with GO and expose the conjugated sp network and degrade the electrical properties of the GO dispersion to form rGO nanosheet (Yong et al. 2014). Nevertheless, the toxicity of some agents cannot be neglected. For example, chemical reduction requires a potent reducing reagent such as hydrazine hydrate (N2H4), which is a highly corrosive material. Hence, the new strategies of GO reduction with biocompatible property and under mild-condition have been considered in recent studies. For example, Goto et al. (2015) reported the effect of GO on SMFCs (sediment) and PMFCs (plant) at different concentrations. Findings revealed a biological GO reduction after 10 days in GO-SMFCs under anaerobic incubation. The highest PD of GO-SMFCs containing 1.0 g/kg of GO was 40 ± 19 mW/m 2 . On the contrary, the GO reduction in PMFCs was much slower than GO-SMFCs, which exhibited a reduction in GO after 27 days of operation time (Goto et al. 2015).

Biocathode

Biocathode offers biocompatible, cost-effective, and promising material for many applications such as heavy metal removal and waste treatment. The use of biocathodes eliminates the need for expensive construction material and potentially toxic chemicals as catholyte, and further the necessity for their recycling and safe disposal (Gude et al. 2013). Algae play a crucial role in nitrogen and phosphorus cycles in waters. The use of algae to produce oxygen is being considered for exploiting its feasibility as an oxygen supplier for cathodic reaction in MFCs. In order, the produced CO2 by anode through biomass oxidation would be transferred to the cathode as a carbon source for algae growth by the photosynthesis process. Cui et al. (2014) utilized Chlorella vulgaris as biocathode. The maximum PD, and CE at 2500 mg COD/L could be obtained 1926 ± 21.4 mW/m 2 and 6.3 ± 0.2%, respectively. The use of biocathode would significantly reduce the cost of MFC and expand its applications (e.g., CO2 fixation, microalgae cultivation) in biomass-fueled MFC.

Genetically engineered microorganisms

Molecular biology techniques helped to clarify the pathways for electron transfer steps and also provide the possiblity to engineer microorganisms to use biomass as fuels for electricity generation. So far, various technical approaches including random approaches (i.e., directed evolution of redox enzymes, and silver/gold coating of cells), rational design (i.e., heterologous gene expression, engineering of metabolic processes, and engineering of bacterial pili), and de-novo design (i.e., bacterial surface display of redox proteins, yeast surface display of redox proteins, and hybrid MFC-enzyme based fuel cells) have been investigated for the genomic engineering of a novel or optimized biocatalysis in MFCs (Alfonta 2010 Zhao et al. 2020). Recently, Li et al. (2019) designed a bioengineered microbial consortium of Klebsiella pneumoniaS. oneidensis for efficiently harvesting of the electricity from corn stalk hydrolysate. The eliminating of the ethanol and acetate pathway via deleting pta (phosphotransacetylase gene) and adhE (alcohol dehydrogenase gene) genes could reinforce the lactate production in K. pneumoniae. Also, a biosynthesis and delivery system for transporting lactate was assembled in this strain through expressing ldhD (lactate dehydrogenase gene) and lldP (lactate transporter gene). Thus, the engieered K. pneumoniae could ferment the hydrolysate to lactate as fuel for electricity generation by S. oneidensis. Furthermore, to improve the EET efficiency of S. oneidensis, a heterogenous flavins synthesis pathway from Bacillus subtilis was expressed in S. oneidensis. The genetically engineered microbial consortia showed high efficiency for electricity generation from biomass hydrolysate (Li et al. 2019). These findings demonstrate genetic engineered microorganisms would be promising to be adapted for biomass-fueled MFCs.

Coproduction of electricity with other energy products

It is well known that biorefinery of biomass only can harvest a limited fraction of the energy in the biomass, leading to low energy efficiency and a large quantity of wastes. It is a highly innovative approach in MFCs that electricity can be coproduced with other energy products, which can substantially improve the overall energy efficiency for biomass conversion with MFCs. It is reported that MFCs can further recovery the energy from the waste of biomass biorefinery (Offei et al. 2019). For example, a novel biorefinery approach involved the coproduction of bioethanol and electricity production from tropical seaweeds has been developed (Offei et al. 2019). The seaweed biomass was first used for bioethanol fermentation and then the bioethanol production residues were employed for bioelectricity generation in MFC. This combination process achieved coproduction of bioethanol as high as 5.1 g/100 g dry biomass and 0.5 W/m 3 power density, which also reduced waste to 24.4% from 69 to 79% for seaweed bioethanol production alone. More recently, the strategy of isolation and acclimation of a new exoelectrogenic yeast strain (Cystobasidium slooffiae JSUX1) could produce significant bioelectricity and biohydrogen production simultaneously with rapid xylose (secondary dominant sugar derived from biomass) metabolism in MFC, in which the produced electrons were harvested from H2 fermentation with xylose (Moradian et al. 2020). The coproduction of electricity and H2 is of great interest for application as these two energy products are obtained in a single operated MFC, which would significantly reduce the cost of reactor and operation.


Microbial Fuel Cell Reaction - Biology

Sample Research Paper on Microbial Fuel Cell

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Sample Research Paper on Microbial Fuel Cell

Microbial Fuel Cell

In the contemporary world, germs or bacteria are strongly repelled at all costs. Children are advised to wash their hands with disinfectants and housekeepers advised to disinfect every item in the home to keep bacteria away. Industries also take caution of germs and are required to adhere to ISO standards of cleanliness. This means bacteria are never welcomed anywhere and is a wish for many that they cease to exist. However, bacteria are useful in a number of ways, and one of them is being harnessed to produce electricity. The production of electricity is effected through microbial fuel cell (MFC).

MFC is a budding technology with a promising future. It is effected through the use of bacteria from waste products to break down the waste products to generate power. The power realized from MFCs is clean and sustainable, and can be produced at a low cost. MFCs have other advantages, whereby apart from generating electric power, they assist in the reduction of pollution, as well aiding in water treatment procedures (Davis & Higson, 2007). Currently, MFCs are harnessed for water treatment and production of electric power concurrently. This paper will explore this technology in details.

Meaning of Microbial Fuel Cell

Microbial fuel cells utilize the power of bacteria, whereby the technology involves converting the energy from metabolic reaction into electric energy. The cell consists of two electrodes divided by a semi-permeable membrane. The electrodes are submerged in an electrolyte maintained at a certain level. A typical setup MFC used in the laboratory for experiments is shown in the diagram below:

Figure 1: A typical setup MFC (Logan, 2008, p. 2)

The two electrodes are connected using a wire and the negative electrode or anode has some bacteria growing on it. The bacteria at the anode break down waste products from food to yield electricity. The process that takes place in an MFC cell is self-sustaining because bacteria are the main agent of the process. As long as food for nourishing the bacteria on the anode are available, they will continue to replicate, and this implies that the system is self-sustaining and simple to maintain. At the same time, MFCs are very effective because they do not require specialized fuels. Simple food and sewage waste products are used as a source of fuel. Mostly a catalyst is applied to speed the reaction. Based on the mode of working, an MFC is, therefore, a system that facilitates the conversion of chemical energy to electric energy in the presence of a catalyst (Davis & Higson, 2007).

How a Microbial Fuel Cell Works

To comprehend how an MFC system works, the diagram in figure 2 will be used, as shown below:

Figure 2: how MFC system works (Logan, 2008, p. 4)

From the diagram above, the MFC is divided into aerobic and anaerobic chamber separated by a semi-permeable membrane. The aerobic chamber consists of a positive electrode, and has oxygen gas bubbled through it in the same way a fish tank works. The other aerobic section of the chamber has no oxygen and allows the negative electrode to be an electron receiver for bacterial process. The semi permeable membrane prevents oxygen from reaching the anaerobic chamber, but allows hydrogen ions to pass through (Logan, 2008).

To understand how the system works, the notations in diagram in figure 2 will be used to explain the working principle of the system. The notations 1, 2, 3, 4, and 5 illustrate each process that occurs in the sequence during the working process of the battery. Each step is defined below:

  1. Anode bacteria breakdown organic matter to release electron and hydrogen ions
  2. Electrons migrate from bacteria to the anode assisted by a mediator molecule.
  3. The electrons flow to the cathode from the anode through the wire. During the process of flow of electrons, electricity is generated. The generated electricity can be used to perform work, and thus, it is harnessed to the load.
  4. The hydrogen ions from the anode flow through a semi-permeable membrane to the cathode. This process is facilitated by electrochemical gradient, whereby the high concentration of hydrogen ions near the anode naturally flows to the cathode region where there is a low concentration.
  5. The electrons released from the cathode link with dissolved oxygen and hydrogen ions to form pure water (Logan, 2008).

In the anaerobic compartment, bacteria food in solution is circulated. Such food consists of compounds found in sewage and food waste, including glucose and acetate. The bacteria’s main role is to breakdown the food into useful products that are used in the production of electric power. In the first place, the part of the food molecule is broken down into carbon dioxide, hydrogen ions, and electrons (Logan, 2008). The procedure is displayed in the figure below:

Figure 3: The electron transport chain (Mercer, 2014, p. 25)

From the diagram, it can be seen that bacteria utilizes electrons to produce energy through electron transport chain. The electron transport chain is disrupted by a mediator molecule as a way of shuttling electrons to the anode. This means MFCs are an extension of a chain of electron transport, whereby the final step takes place outside the cell of the bacteria to facilitate harvesting of energy. This final step involves the formation of water from the combination of electrons, oxygen, and hydrogen ions (Mercer, 2014).

The electron transport chain shown in figure 3 can be explained using the notations 1, 2, 3, 4, and 5 as shown on the diagram. Each notation is explained below:

  1. The electron transport chain process commences with a biological transport molecule labeled NADH on the diagram. NADH releases an electron (e-) from a high energy level. It also releases a proton inform of hydrogen ion (H+).
  2. During the second stage, the electrons flow through a path denoted by a red line on the diagram. The red path passes through the large blobs (colored black in the diagram) in the membrane of the mitochondria.
  3. During the process of electron flow through each blob, hydrogen ions are pumped through the membrane.
  4. In a typical cell of bacteria, the electron proceeds along the red dotted line in the figure above and combines with oxygen to make water, but this is not the case in a microbial cell.
  5. In MFCs, the electron proceeds along the solid red path indicated in the diagram in figure 3, whereby it is picked by a mediator molecule and transported to the anode. This completes the process (Davis & Higson, 2007).
The Chemical Reaction

The reaction producing electricity in MFCs can be represented by an equation. First, it ought to be noted that microorganism produces water and carbon dioxide when they consume sugar in the presence of oxygen. However, in the absence of oxygen, the microorganism produces electrons, protons, and carbon dioxide. The process can be represented by the equation given below:

From the reaction above, it can be seen that the metabolic reaction process produces free electrons and hydrogen ions. The hydrogen ions created are united with oxygen to form water. The free electrons produced, on the other hand, are responsible for the producing of electricity in the cell. Microbial Fuel Cell utilizes mediators, explained in the previous section, to redirect the electrons produced into the conductor (Mercer, 2014).

History of a Microbial Fuel Cell

Professor M. C. Potter from the University of Durham is credited with the MFC concepts. As a botany professor, Potter was obsessed with the idea of obtaining electric energy from bacteria in 1911. Potter discovered that the electric energy was produced when organic matter decomposed and thus sort to investigate the phenomena in details. Potter, therefore, looked for ways of harnessing this energy for commercial use. In addition, little was recognized about the metabolic process of bacterial. Therefore, Potter came up with a primitive MFC that was later improved (Davis & Higson, 2007).

Since 1911, when MNC was first discovered, there was little improvement in the design for harnessing the energy for the commercial process until 1980. In 1980, Peter Bennetto and MJ Allen from Kings College in London improved the original MFC design by Potter. Bennetto and Allen were motivated by the quest to provide an inexpensive source of power to the developing nations. Therefore, they incorporated new development in the electron transport chain to understand the process. Also, they depended on the progression in technology to improve the physical design of the Microbial Fuel Cell, and the design they produced is still in the market today (Logan, 2008).

Although the advancement in MFCs was meant to improve developing countries, the technology is still in its pilot state in most countries. The main problem has been ways of simplifying the design to enable the rural population to use it. Nevertheless, there is an amplified interest in the growth of the technology among scientists (Mercer, 2014).

When scientists started working on the microbial fuel cell, one main issue that occupied their minds was how to transfer electrons from the electron transfer chain to the anode. When focusing on this issue, B-H Kim, a scientist from the Institution of Technology and Science in Korea realized that some bacteria species were active electrochemically. Such bacteria did not require one to use mediator molecule to facilitate the transportation of electrons to the electrodes. As a result, a new Microbial Fuel Cell was discovered. This MFC was cheap and eliminated the use of toxic and expensive mediators (Logan, 2008).

The current efforts in microbial fuel cells design and development is focused on optimizing electrode materials, bacteria combination, and the transfer of electrons in the cell. Although this technology was discovered 100 years ago, effort to utilize it on a commercial scale only started in 2000s. At the same time, the full understating of the process involved in the technology is a 21 st century innovation (Logan, 2008).

Types of Microbial Fuel Cell

Based on the historical account of MFCs and other the working principle of the technology, it can be seen that MFCs are divided into two categories mediator-less and mediator microbial fuel cells.

Mediator Microbial Fuel Cell

In this type of microbial fuel cell, the microbial cells used are inactive electrochemically. As a result, mediators are used for the electron transfer. Common mediators used on the market today include humic acid, methyl viologen, thoinine, and neutral red among others. These mediators are toxic and expensive (Logan, 2008).

Mediator-free MFCs

Mediator-free MFCs use bacteria that are active in terms of the electrochemical transfer. This means electrons are carried directly to the electrode from the respiratory enzyme of bacteria. There are various bacteria species with electrochemical qualities, and they include Aeromonas hydrophila and Shewanella Putrefaciens among others. Studies into mediator-less MFCs are new and still ongoing (Cai, Zheng, Qaisar, & Xing, 2014). Mediator-less MFCs have some limitations. Their optimal operations depend on a number of factors, and the first one is the condition of the system. Conditions, such as high temperature, varying pH and the concentration level of the electrolyte affects the way in which bacteria works. Another way through which mediator-free MFCs are affected is through the strain of bacteria is used. Little information is available on how bacteria can be enhanced to reduce the strain during the operation of the cell. At the same time, the type of membrane used is also an issue because a number of factors, which have not been realized, affect ion exchange through the membrane (Mercer, 2014).

Besides operating on wastewater, mediator-less MFCs can derive energy from certain plants. A system called plant MFC is used in this process. A sample system setup for a plant MFC is shown below:

Figure 4: plant MFC (Logan, 2008, p. 4)

A number of species of plants are used for operating this system, and they include tomatoes, algae, reed sweet grass, rice, and cord grass among others. The energy production in this case is in situ-energy, whereby the production of energy is done onsite. This means the process is used to the advantage of the environment (Logan, 2008).

Another type of mediator-less microbial fuel cell is the MEC or the microbial electrolysis cells. MECs works by reversing the process of MFCs, whereby it results in the production of methane rather than water. In this case, carbon dioxide is reduced by bacteria with the aid of electric energy to form compounds of carbon (Logan, 2008).

Application of Microbial Fuel Cell

Microbial Fuel Cell technology is applied in a number of cases as discussed below:

Power Generation

MFCs are used for harnessing electric power in various situations. However, the energy is produced on a low scale. The energy is also applied in a situation, whereby it is expensive to replace batteries in use. For instance, in wireless sensor networks, MFC is applied because it is effective and easy to maintain. MFCs are applied in various situations to replace battery power because they do not need one to recharge them, but are rather self-sustaining (Davis, & Higson, 2007).

Education

Microbial fuel cells that are based on plants are vital education tools. In the first place, their construction process is facilitated through a number of considerations drawn from various disciplines. Such disciplines include geochemistry, microbiology, and electric engineering among others. They are preferred because they can be constructed from the common or local material from the laboratories. Various kits have been designed for classroom use, as well as for corporate use. Most research tools in microbiology employ this technology (Davis & Higson, 2007).

Biosensor

Municipal wastewater is a common nuisance to the municipal governments because they must be treated and released in water. Microbial fuel cells are essential in this process because it involves waste digestion. Microbial fuel cells operate on the principle that is harnessed to measure the concentration of wastewater before being released to the water. As a biosensor, microbial fuel cells are utilized based on the principle that the current generated by the cell is directly proportional to the energy content of the wastewater. In this case, is it possible to ascertain the concentration of the solute in wastewater? The concentration of municipal wastewater is ascertained through the biochemical oxygen demand or BOD (Logan, 2008). A BOD sensor utilizing Microbial Fuel Cell technology is efficient and fast that conventional BOD sensors. According to a study by Zhang, Qiao, Miao, Yang, Li Xu, and Ying (2014), conventional BOD sensors take about five days to ascertain the BOD of waste waster compared to Microbial Fuel Cell BOD sensors, which takes less than five days.

Sewage treatment

According to a study by Zejie, Taekwon, Bongsu, Chansoo, and Joonhong (2014), wastewater in the sewage plant can be biologically treated using Microbial Fuel Cell. Studies into microbial fuel cells indicate that the system can digest the amount of organic components in the sewage for up to about 80%. The wastewater is cleaned to remove some toxins, and thereafter, taken through the bioreactor for microbial fuel cells treatment process. During the process of treatment, the waste is converted to electric energy and water. The electricity created is used to counterbalance the high cost of treating water and thus the process is made cheaper and justified.

Treatment of Brewery Wastewater

Wastewaters from brewing plants are rich in organic compounds, and thus, it is easy for one to treat it using MFCs. Wastewater from the brewery has constant substrate concentration and thus it is easy use MFCs to clean it. In the setup, all the wastewater is channeled to the chamber containing the system of microbial fuel cells. Electricity is generated from the process, and this ensures that the plant gains twice. In the first place, the plant obtains power to use in the plant. Thereafter, the company also ensures that its wastewater is cleaned for easy disposal. Australia is among the leading nations that have applied microbial fuel cell technology to treat brewery wastewater (Wu et al., 2013).

Production of Hydrogen

Microbial Fuel Cells technology can be applied in the creation of hydrogen that in turn can be used as fuel. However, the process is supported by external source of power for it to be effective. The external power source is used for converting the organic materials into hydrogen gas and carbon dioxide. The system used for producing hydrogen gas has two chambers just like the microbial fuel cell system. However, both chambers are made anaerobic, and enhanced by electric power of about 0.25 volts. According to a study by Li, Cheng, and Wong (2013), it was discovered that more than 90% of electrons and protons generated by bacteria at the anode are converted to hydrogen gas. The conventional methods used to produce hydrogen gas require electricity that is ten times higher than the energy used by microbial fuel cells for generating electricity.

Conclusion

Waste products, especially wastewater from breweries and municipal plants can be a nuisance to the environment. However, this paper has established that such waste products are useful because they are harnessed for producing electricity. Microbial fuel cells are an essential innovation because they have opened up opportunities for researchers in various disciplines. MFCs are simple to construct and are not expensive. MFCs have other advantages, whereby apart from generating electric power, they assist in the reduction of pollution, as well aiding in water treatment procedures.

References

Cai, J., Zheng, P., Qaisar, M., & Xing, Y. (2014). Effect of operating modes on simultaneous anaerobic sulfide and nitrate removal in microbial fuel cell. J Ind Microbiol Biotechnol , 41, 795–802.

Davis, F., & Higson, S. J. (2007). Biofuel cells—Recent advances and applications. Biosensors & Bioelectronics, 22(7), 1224-1235. doi:10.1016/j.bios.2006.04.029

Li, X., Cheng, K., & Wong, J. C. (2013). Bioelectricity production from food waste leachate using microbial fuel cells: Effect of NaCl and pH. Bioresource Technology, 149452-458. doi:10.1016/j.biortech.2013.09.037

Logan, B. (2008). Microbial Fuel Cells. New York: Wiley-Interscience.

Mercer, J. (2014). Microbial Fuel Cells: Generating Power from Waste. A review of engineering, 15 (2), 1-14.

Saharan, B., Sharma, D., Sahu, R., Sahin, O., & Warren, A. (2013). Towards algal biofuel production: a concept of green bio-energy development. Innovative Romanian Food Biotechnology, 121-21.

Thepsuparungsikul, N. N., Ng, T. C., Lefebvre, O. O., & Ng, H. Y. (2014). Different types of carbon nanotube-based anodes to improve microbial fuel cell performance. Water Science & Technology, 69(9), 1900-1910. doi:10.2166/wst.2014.102


Abstract

Microbial fuel cells (MFCs) that remove carbon as well as nitrogen compounds out of wastewater are of special interest for practice. We developed a MFC in which microorganisms in the cathode performed a complete denitrification by using electrons supplied by microorganisms oxidizing acetate in the anode. The MFC with a cation exchange membrane was designed as a tubular reactor with an internal cathode and was able to remove up to 0.146 kg NO3 - -N m -3 net cathodic compartment (NCC) d -1 (0.080 kg NO3 - -N m -3 total cathodic compartment d -1 (TCC)) at a current of 58 A m -3 NCC (32 A m -3 TCC) and a cell voltage of 0.075 V. The highest power output in the denitrification system was 8 W m -3 NCC (4 W m -3 TCC) with a cell voltage of 0.214 V and a current of 35 A m -3 NCC. The denitrification rate and the power production was limited by the cathodic microorganisms, which only denitrified significantly at a cathodic electrode potential below 0 V versus standard hydrogen electrode (SHE). This is, to our knowledge, the first study in which a MFC has both a biological anode and cathode performing simultaneous removal of an organic substrate, power production, and complete denitrification without relying on H2-formation or external added power.


3 Application-based models for MFCs

Although the mechanism-based models are able to optimise MFCs from basic principles, the computing time and complexity always limit the practical application. Hence, for engineering designers, some models based on the electrical characters of MFCs expressed by equivalent circuit (EC) model [ 38-41 ] and are used to achieve the electrical application. There are also some models developed from several typical reactions and experimental data of MFCs to acquire the best parameters and maximum power [ 42-46 ].

3.1 Electrical model

In electrical models, the EC model and power output curve are presented by classical electrical parameters of MFCs. The electrical model provides a novel analytical method particularly aims at the application in electronic and electrical way. For example, the EC model is applied to design the post-electrical circuit to achieve the energy harvesting or the maximum power tracking of MFCs (e.g. [ 47 , 48 ]).

The internal resistance influences the output performance of MFCs to a large extent has been reported and the most direct impact is reflected in the ohm losses [ 49 , 50 ]. To figure out the effect of internal resistance, electrochemical impedance spectroscopy (EIS) is an effective tool to measure the value of internal resistance of an MFC [ 51 ]. However, the material of electrode is one of the factors that determine the internal resistance. Hernández-Flores et al. [ 52 ] introduced a numerical relationship between the volumetric power of MFCs and specific surface area of anode, related the anode region to the output power, as well as internal resistance in a succinct expression based on Tafel equation. It is noticeable that the model was firstly proposed as alternative way to analyse the electrical performance of MFCs with anode surface area. Sindhuja et al. [ 38 ] proposed two EC models for different materials of electrode. A lumped EC model for activated charcoal electrode and fitting EC model for graphite electrode were established both through the EIS measurement of cathode and anode and the calculation of anode overpotential. Yin et al. [ 39 ] modelled the MFC operated under static magnetic field (MF) that contributed to induce several biological reactions in a microbial system [ 53 ]. The EIS was used to investigate the electrochemical reactions of the MFC and presented the EC model of anode, cathode, and entire system, respectively. The work of [ 41 ] improved the defect of EIS so that the cell disconnect to a potentiostat for the duration of measurement, which was a novel method to determine the value of charge transfer resistance and double-layer capacitor in MFCs. An EC model was introduced with the effect in terms of capacitor within electrode considered. Coronado et al. [ 40 ] conducted the experiment that the MFC connected to the PWM external resistance (R-PMW). The existence of fast and slow dynamic component of the MFC was obtained via the analysis of the result, thus utilising the capacitor and resistance to establish the EC model. Although the electrochemical reactions were negligible in this model, it provided the dynamic process of relevant electrical performance.

3.2 Learning model

As the increasingly development of artificial intelligence and modern control theory, some models are established without much in-depth understanding of mechanism of MFCs instead, a few significant reactions and/or massive experimental data plays an essential role in modelling. These models facilitate designers to maximum the MFCs output and configure the MFCs with less bio-electrochemical knowledge, which contributes to improve the development of MFCs application.

In fact, MFCs have the slow dynamics character due to much electrochemical and biochemical reactions occurred, which requires a long period to acquire the experimental result. The experiment design approach has been therefore proposed to shorten the time of experiments. Fang et al. [ 42 ] developed a modelling approach to optimise the power generation of MFCs with multivariable. The experiment was carried out by the uniform design method which determined four input variables with five levels and regarded the power density and coulombic efficiency as output variables. The model adopted relevance vector machine to acquire a mathematical model between the input and output variables through the result from 16 experiments. To speed up the rate of optimisation of model's parameters, accelerating genetic algorithm was selected to seek the maximum output variables and optimal reacting conditions. Another experimental design approach: the response surface design methodology (RSM), which was also employed to design the experiment of MFCs in [ 46 ]. The work introduced a Box–Behnken design approach to optimise the temperature, external resistance, the concentration of feed fuel, and the pH in anode chamber, maximised output power density simultaneously. The results from experiments could derive a quadratic mathematical model for an MFC by RSM meanwhile, the output power density achieved the maximum. To improve the shortcoming of the model that has long computational time, He and Ma [ 43 ] proposed a model based on data-driven Gaussian process regression for MFCs. The experiment design approach was firstly used to obtain a series of experiment data as the initial sample. The model applied the concentration of substrate, anodic feed rate, and current density as the input variables while the voltage as the output variable, and the accuracy and reliability of the model was validated by operating both in online and offline mode with different requirement of training samples and executive procedures.

Besides, the state estimation method was introduced with state-space model as well. A control-oriented model based on state space equation of an MFC was presented in [ 44 ]. The model set up the substrate concentration, biomass concentration, hydrogen ion concentration, and bicarbonate ion concentration as the state variables, combined the mass balances and voltage equation to establish the state-space equations. It was worth noting that the approach offered a way to deduce the model that is likely to employ the state estimation method (e.g. Kalman filter) to observe the parameters. In view of the more complexity of the MFC model, the more difficult it is for the process controlling and the parameter estimation, the model in [ 45 ] proposed a model combined the previous model mentioned competitive substrate consumption in different microbial population [ 6 ] and the EC model [ 38 ] into a bioelectrochemical–electrical model which could be applied to both the fast calculation process and the slow dynamic simulation.


The potassium hexacyanoferrate (III) solution is light-sensitive and should therefore be stored in a light-proof bottle or in a bottle wrapped in aluminium foil. It should not be kept for more than six months.

You may wish to store the cation exchange membrane in a bottle of distilled water so that it is ready to use. The water should be replaced from time to time if the membrane is stored for an extended period.

Dried yeast, even in a sealed container, has a limited shelf life. The supplier’s ‘best before’ date should therefore be observed.


Watch the video: Microbial fuel cell MFC Class 12 Zoology (May 2022).