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Electron transfer mechanisms, characteristics and applications of biological cathode microbial fuel cells – A mini review
⁎Corresponding author. songhailiang@seu.edu.cn (Hai-Liang Song)
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Received: ,
Accepted: ,
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.
Peer review under responsibility of King Saud University.
Abstract
Since the microbial fuel cells (MFCs) research in the laboratory has reached an unprecedented success, it has raised a research upsurge internationally in recent years. However, compared with laboratory studies, the widespread applications of the conventional MFCs were restrained by the limitations of high cost and low efficiency. This stimulates researchers to overcome the obstacles. In this condition, bio-cathodes attracted their great interests. This paper is a brief review about the experimental progress of bio-cathodes in microbial fuel cells with an emphasis on the classification according to the final electron acceptors and the comparison with the traditional abiotic cathode MFCs. Bio-cathodes are feasible in removing nutrient in wastewater treatment and being used as biosensors in bioremediation. Presently, tremendous efforts are being made in investigating appropriate electrodes and dominant strains to achieve the effective practical applications.
Keywords
Microbial fuel cell
Bio-cathode
Final electron acceptor
Wastewater treatment
Biosensors
1 Introduction
After being stricken by energy crises, people have realized the urgency to change the energy structure, which depends too strongly on fossil fuels. Some renewable and sustainable resources are proposed to alleviate the situation, among which bioenergy is considered as the most efficient way. Inspired by Luigi Galvani (1737–1798), an Italian physician and also a physicist, found that frogs tissues are endowed with an intrinsic electricity, which proved the existence of animal electricity (Piccolino, 1998). Michael C. Potter established the first microbial fuel cell (MFC) in 1911, and he demonstrated a current flow between two electrodes emerged in a bacterial culture and in sterile medium (He and Aneenent, 2006). The current design concept of MFC came into existence in 1977 with the work by Karube et al. (1977). However, little was known about how MFCs fecundated at that time. In 1999, Kim et al. successfully found electricity generation uses MFCs, which were regarded as a milestone of the development of MFCs (Kim et al., 1999).
Consisting of an anode chamber and a cathode chamber, which are separated by the proton exchange membranes, a MFC’s power can be generated from the oxidation of organic matter by bacteria at the anode, with reduction of oxygen at the cathode (Logan et al., 2005). According to numerous findings, the configurations of MFCs are quite various, including air–cathode MFCs, aqueous cathodes using dissolved oxygen, and two-chamber reactors with soluble catholytes or poised potentials, tubular packed bed reactors and so on. In these different configurations of MFCs, microbes only exist in the anode chamber. However, some investigations found that bacteria grew quite inevitably in the cathodes and even could increase the power output significantly. Early in 1997, Hasvold et al. found that bacteria colonize on the cathode and catalyze the reduction of oxygen (Hasvold et al., 1997).
And in 2005, Bergel reported the presence of the biofilm on the MFCs’ cathode surface led to efficient electron density (Bergel et al., 2005). These findings encouraged the development of bio-cathode MFC that the bacteria are used as biocatalysts to accept electrons from the cathode electrode. Up to now, bio-cathode MFCs have attracted much attention and have been considered as the promising MFCs (He and Aneenent, 2006). This review aims at classifying the types of the biological MFCs according to their final electron acceptors and analyzing the advantages and disadvantages compared with abiotic MFCs. We anticipate that bio-cathode MFCs would have a wide application in energy recovery and microbial sensor.
2 Classifications of bio-cathodes
With the development of biological cathode, many substances including oxygen, transition metal compounds, inorganic salts and carbon dioxide, are used as the final electron acceptor in bio-cathode. Although the mechanism of biological electron transfer is not completely clear, many researchers proved that microorganisms play an important role in the bio-cathode electron-transfer process.
2.1 Oxygen as the terminal electron acceptor
Because of its high redox potential (+1.229 V) and low-cost to supply, oxygen is the most popular terminal electron acceptor. Microorganisms directly transfer the electrons from the anode to oxygen or they assist the oxidation of transition metal compounds for electron delivery to oxygen.
2.1.1 Oxygen as a direct electron acceptor
Marine sediment MFC is one of the earlier applications of MFCs, which are mainly used for supplying analysis and monitoring equipment. Early in 1997, Hasvold et al. found in the study of seawater battery that marine life affects the cell performance (Hasvold et al., 1997). Bacteria colonize on the cathode surface forming slimes, which catalyze the reduction of oxygen. This increases in catalytic activity of the cathode and results in an increase in the on-load cell voltage from typically 1.2 to 1.6 V. In MFCs system, different types of sludge and sediment were mixed to obtain a cathodic inoculum with sufficient microbial diversity. A MFC converts energy, available in a bio-convertible substrate, directly into electricity. This can be achieved when bacteria switch from the natural electron acceptor, such as oxygen or nitrate, to an insoluble acceptor, such as MFC anode (Fig. 1). These steps will remove organic pollutants and recover energy at the same time. (Clauwaert et al., 2007; Rabaey and Versterarte, 2005).
There are also some efforts to produce oxygen directly in the cathode by applying marine algae. A MFC assembled with an algae cathode and a R. rubrum suspension anode (sandblasted platinum electrodes) gave, after continuous illumination for 21 h, an open-circuit voltage of 0.96 V and a short-circuit density of 75 μA/cm2. A cell free of organisms, operated for comparison, gave a decrease in open-circuit potential over 7 h from 0.19 to 0.03 V. Short-circuit current density decreased from 7.0 to 6.1 μA/cm2. And another control cell which employed organisms and was maintained in the dark gave negligible results, similar to those of the nonbiological control. These data demonstrated the ability of specific microorganisms to convert light energy to electrical energy. The electrical power derived from these cells is estimated to be approximately 0.1–0.2% of that available from the incident radiation. This low efficiency is attributed to both biological and electrochemical inefficiency (Berk and Canfield, 1964).
During recent years, researchers have done a lot of work to prove that biofilm plays an important role in oxygen reduction. In 2005, Bergel conducted the research in a laboratory-scale fuel cell, which was designed with a stainless steel cathode, a platinum anode, and two separated liquid loops (Bergel et al., 2005). The catholic loop was air-saturated, while the anodic loop was hydrogen saturated. Seawater biofilm was previously grown on the stainless steel cathode, and then set up into the fuel cell. The presence of the seawater biofilm on the stainless steel surface led to efficient catalysis of oxygen reduction. When pH of the anode compartment increased up to 12.5, the highest power value (270 mW/m2) was obtained in the presence of biofilm, while a maximum power less than 2.8 mW/m2 was obtained with the cleaned cathode. Moreover, when reducing the surface area of the cathode, the maximum power density supplied by the PEM fuel cell in the presence of biofilm was still increased: 325 mW/m2 were supplied with 1.34 A/m2 current density, and 64 mW/m2 were supplied with 1.89 A/m2. In 2007, Clauwaert et al. (2007) combined the anode of acetate oxidizing tubular MFC with an open-air biocathode for electricity. The maximum power production was 83 ± 11 W/m3 MFC for batch-fed systems (20–40% Coulombic yield) and 65 ± 5 W/m3 MFC for a continuous system with an acetate loading rate of 1.5 kg COD m−3 day−1 (90 ± 3% Coulombic yield).
Biocathodes can substantially increase the viability and sustainability of MFCs has been proved. In 2005, Rabaey et al. found that the electrons flow through a resistor to a cathode, where the electron acceptor is reduced, and the current density up to 1.5 mA/cm2 was obtained (Rabaey and Versterarte, 2005). They also found that some of the axenic culture obtained from the cathode shows an increase in the power output of up to threefold compared to mixed cultures. Decreasing the activation overpotentials and the internal resistance will strongly affect the power output. Parameters influencing the overpotentials are the electrode surface, the electrochemical characteristics of the electrode, the electrode potential, and the kinetics together with the mechanism of the electron transfer and the current of the MFC. Freguia et al. (2008) designed a MFC configuration in which the effluent of an acetate-fed anode was used as a feed for an aerated, biocatalyzed cathode. The development of a cathodic biofilm achieved a fourfold increase of the current output compared with the non-catalyzed graphite cathode, while the pH variation in the cathode compartment was reduced due to the additional transfer of protons via the liquid stream. The sequential anode–cathode configuration also provided for chemical oxygen demand (COD) polishing at the cathode by heterotrophic bacteria, with overall acetate removal greater than 99%. The anode achieved an organic substrate removal of up to 2.45 kg COD/m3 of anode liquid volume per day, at coulombic efficiencies of 65–95%. Electron balances at the cathode revealed that the main cathodic process was oxygen reduction to water with no significant coulombic losses. The maximal power output during polarization was 110 W/m3 cathode liquid volume.
Chen et al. (2010) investigated microbial community dynamics and its electron transfer process within a biocathode in a MFC. It was found that Gammaproteobacteria were the most abundant division among all clone types with a percentage of 48.86% in the cathode compartment. They further confirmed that nitrate and oxygen reduction in the cathode compartment could be significantly enhanced by the presence of microbes, which are able to excrete metabolites to assist the electron transfer process either in the anode or in the cathode compartment. Further research is required to identify these microbial excreted metabolites.
2.1.2 Oxygen as an indirect electron acceptor
To achieve high electron transfer efficiency, manganese and iron are used as oxygen transfer mediators under aerobic conditions. The manganese oxides and iron salts are first reduced by the cathode (abiotically) and then reoxidized by bacteria. During the process, oxygen just works as an indirect electron acceptor.
Manganese is a common transition metal that is abundant in the environment and valence state transformation (Mn (IV), Mn (II)) can easily occur with the biocatalysis. Rhoads et al. employed the cycle of Mn (IV) reduction and subsequent reoxidation of Mn (II) in the cathode of a MFC and observed a consistent production of electricity (Rhoads et al., 2005). The whole cycle began with the reduction of MnO2 to an intermediate product, MnOOH, by accepting one electron from the cathode electrode. It was followed by a further reduction of MnOOH to Mn (II) through the acceptance of another electron, which results in the release of manganese ions. Then the release of the divalent manganese occurs in close proximity to the MOB-colonized electrode surface, the divalent manganese was immediately reoxidized to manganese dioxide by the MOB, and the cycle continued (Fig. 2). They demonstrated that biomineralized manganese oxides are superior to oxygen when used as cathodic reactants in MFCs. The current density delivered by using biomineralized manganese oxides as the cathodic reactant was almost 2 orders of magnitude higher than that delivered by using oxygen. Apart from this, Shantaram et al. (2005) applied MFCs for wireless sensor power supply, using Mn (II) involved in the response of the air-biological cathode, and got the voltage up to 211 V. Some of the graphite felts were electrochemically pretreated to contain manganese oxide. Electrochemical precipitation of manganese oxides on the cathodic graphite felt decreased the start-up period with approximately 30% versus a non-treated graphite felt (Clauwaert et al., 2007).
Similar to manganese compounds, iron reduction suggests that organisms with such metabolic abilities play important roles in coupling the oxidation of organic carbon to metal reduction under anaerobic conditions (Nealson and Saffarini, 1994). The cycle of Fe (III) reduction and subsequent reoxidation of Fe (II) can also be used in the cathode of a MFC. However, many researchers demonstrated that Iron compounds have been used as electron mediators in abiotic cathodes. In fact, previous studies have revealed that Fe (II) is oxidized to Fe (III) through microbial activity by Thiobacillus ferrooxidans (Fig. 3). Further studies are required to examine whether Fe (II) compounds can be oxidized by Thiobacillus ferrooxidans with the electrons originating from an anodic bioreaction rather than an external power supply.
Compared to using oxygen as the direct cathode, bio-cathode MFCs with oxygen as the indirect cathode are more favorable for two reasons. First, the diffusion of oxygen from cathode to the anode and the anode potential can both be reduced so that electromotive force of MFCs increased. Second, it can also improve the mass transfer efficiency of electron acceptor, which was controlled by the mass transfer resistance.
2.2 Inorganic salts as the electron acceptor
Under anaerobic conditions, nitrate and sulfate directly accept electrons from a cathode electrode through microbial metabolism. With a low content of oxygen, the oxygen is inhibited from spreading to the anode, and oxygen consumption (electron transport) is reduced that may lead to the loss of coulomb efficiency.
MFCs with NO3− as electron acceptors in the electrode reaction were initially applied to the electrode-biofilm reactor (BER). The basic principle is with imposing a certain current, the microbes utilizing electron transferred or hydrogen produced from cathode to finish the reduction of nitrate to nitrogen gas. This reaction helps realizing denitrification under low-carbon or no carbon source and avoiding adding more carbon source in the treatment process. Holmes et al. found that both groups of enriched organisms are involved in nitrogen transformations such as ammonia oxidation and denitrification, and proved the presence of nitrogen cycling at the cathode (Holmes et al., 2004). Gregory et al. showed that electrodes may serve as a direct electron donor for nitrate reduction to nitrite (Gregory et al., 2004). Park et al. detected nitrate diffuse from the bulk solution into the cathode biofilm and then be biologically reduced to nitrogen gas by the hydrogen produced with the electrolysis of water in the biofilm (Park et al., 2005). Virdis et al. designed and demonstrated a novel process configuration that achieves both carbon and nitrogen removal using MFC (Virdis et al., 2008). Although redox potential of sulfate is very low and its ability to receive electronic receptor is much weaker than the nitrate, the researchers are still optimistic about the potential that sulfate can be used as the cathode electron acceptor because its reduction does not require strict anaerobic conditions. Cord-Ruwisch and Widdel found that metallic iron without application of an external electron-motive force is in fact used as a source of reducing equivalents for dissimilatory reduction of sulfate (Cord-Ruwisch and Widdel, 1986). The reducing equivalents were obviously molecular hydrogen formed by the cathodic reaction of iron with protons from water. Goldner et al. investigated the nutrient requirement of sulfate-reducing bacteria when they were applied to biocathodes in a MFC with an active metal anode (Goldner et al., 1963). Their results demonstrated that hydrogen oxidation was accomplished by an enriched culture of D. desulfuricans in seawater, containing a small amount of yeast extract and ammonium ions.
2.3 Others
Apart from the above ones we are talking about, many compounds such as urea, fumarate and carbon dioxide can be used as electron acceptor. Two MFCs were specially designed to increase the metabolism efficiency and energy conservation (Park et al., 1999; Park and Zeikus, 1999). The electrochemical bioreactor (ECB) system was separated into anode and cathode compartments by a cation-selective membrane septum (diameter [φ] = 22 mm for type I and [φ] = 64 mm for type II); 3.5 Ω cm−2 in 0.25 N NaOH). The anode and cathode were set up by woven graphite felt (6 mm thick; 0.47 m2 g−1 available surface area). A platinum wire (φ = 0.5 mm; <1.0 Ω cm−2) was attached to the graphite felt with graphite epoxy (<1.0 Ω cm−2). The electrical resistance between anode and cathode was < 1.0 kΩ. The weight of both electrodes was adjusted to 0.4 g (surface area, 0.188 m2) for system I and 3.0 g (surface area, 1.41 m2) for system II. Park et al. found that in the MFC with cathode of constant potential balance, microbes (respectively, methanogen and Actinobacillus succinogenes) reduced carbon dioxide to methane and fumarate to succinate. However, the redox potential of CO2/CH4 and fumaric acid salt/succinic acid salt (respectively, −0.24 V and +0.03 V) was very small, and this stimulated them to do further research. They investigated the growth of Actinobacillus on glucose plus electrically reduced neutral red (NR) in an ECB system versus on glucose alone, and found that electrically reduced NR enhances glucose consumption, growth, and succinate production by about 20% while it decreases acetate production by about 50%. The rate of fumarate reduction to succinate by purified membranes was twofold higher with electrically reduced NR than with hydrogen as the electron donor. Thus, NR appears to enable Actinobacillus succinogenes to utilize electricity as a significant source of metabolic reducing power.
Despite the voltage output was quite small and it could not be applied into practical system, Park et al. pointed out that Anaerobiospirillum and Actinobacillus species produce high levels of succinate (35 and 95 g/l, respectively) during glucose fermentation because hydrogen serves as an additional electron donor for metabolism. This could be exploited as an important application of this kind of biocathode MFC because succinic acid has many industrial uses (Samuelov et al., 1990).
3 Advantages and disadvantages
Achievements with bio-cathode MFCs over the past 3–4 years have been particularly impressive. The versatility of bio-cathode enables us to use not only metal ions but also contaminants as possible electron acceptors. However, there still exist some problems which may discourage the practical application. This section reviews the advantages and disadvantages of bio-cathode MFCs compared with abiotic MFCs.
3.1 Advantages of bio-cathode
3.1.1 Lower expense
The cost of construction and operation of bio-cathode MFCs are lower than abiotic MFCs. The most common types of catalysts of non-biological cathode are Pt including Pt-coated metals, transition metal elements and ferricyanide. Pt is such a kind of efficient metal that the catalytic reaction with the Pt-catalyzed electrodes can improve the electrical properties of a MFC nearly 4-fold. However, Pt is expensive, greatly increases the cost of constructing a MFC, and is not suitable for large-scale application. Transition metals such as iron and cobalt are also available electron mediators because they can switch among their redox states rapidly. Cheng et al. indicated that the cathode potential was reduced slightly (20–40 mV) when Pt loadings were decreased from 2 to 0.1 mg/cm2 (Cheng and Logan, 2006). They also found that when the Pt loading on cathode was reduced to 0.1 mg/cm2, the maximum power density of MFC was reduced on average by 19% (379 ± 5 to 301 ± 15 mW/m2; Nafion binder). Power densities with CoTMPP were only 12% (369 ± 8 mW/m2) lower over 25 cycles than those obtained with Pt (0.5 mg/cm2; Nafion binder). Precious metal Pt is a kind of ideal abiotic cathode catalyst. However, its wide application is restricted by the high cost. Another cobalt material CoTMPP faces the same problem. The abiotic cathode catalysts need to be replaced continually and would increase maintenance costs. Due to the same reason that compared with microorganisms, exorbitant price of metal has been a crucial factor constraining MFC’s commercial application. Fortunately, biological cathode MFC does not need metal catalysts or artificial electron mediators because microorganisms can function as catalysts to assist the electron transfer.
3.1.2 Improved sustainability
Bio-cathodes can improve MFCs sustainability because problems with sulfur poisoning of platinum or consumption replenishment of electron mediator will be eliminated (Bergel et al., 2005). Take Pt for instance, Pt loading in both catalysts below 0.1 mg/cm2 is more prone to lead to catalyst poisoning (Trapananti, 2008); therefore, its application has been limited. Moreover, the oxide layer (PtO) after a long-time running would reduce the electrode activity. When it comes to the ferricyanide, troubles still exist. People utilize the circle that when [Fe (CN)6]3− is reduced to [Fe (CN)6]4− as soon as it receives electrons, it will be oxidized to [Fe (CN)6]3− by oxygen. However, this circle is not ideally substance conservation because [Fe (CN)6]4− cannot totally be oxidized (Jang et al., 2004). In other words, electrolyte needs to be replaced regularly. These obstacles can be overcome easily by bio-cathodes, which is environmental friendly and sustainably. In the system of bio-cathode MFCs, the heavy metal catalysts are replaced by microbes with high catalytic efficiency. With the aging of biofilm, the microbes’ metabolic rates get lower and new membranes grow automatically, which can take place of the old ones. And there is no need to add extra electrolytes regularly due to this way.
3.1.3 Easy improvement
Many factors including cathode material, electrode surface, electrode spacing, configuration, buffer solution properties and concentration of substrate have been proved to be associated with the performance of MFCs’ power output. For instance, increasing cathode surface area and retaining a relatively small area of the anode have been recommended widely to achieve the cathodic reaction rate in abiotic cathode MFCs (Fan et al., 2007; Logan, 2009). When it comes to bio-cathode MFCs, people can adopt more methods and easier actions to increase the production efficiency. In bio-cathode MFCs, increasing cathode surface area would apparently make room for more quantity of catalyst bacteria to live on, which would result in the decline of electrode over potential, improving cathode potential and power output (Kawai et al., 2000). Moreover, the characteristics of surface functional group and surface roughness have a significant influence on the initial adhesion and subsequent colonization processes (Tang et al., 2007).
3.2 Disadvantages of bio-cathode
3.2.1 Fluctuation of pH
Due to proton transport through the Nafion seemed to be slower than the proton production rate in the anode chamber and the proton consumption rate in the cathode chamber, a decreasing pH in the anode chamber and an increasing pH in the cathode chamber were happened during the operation of two-chamber MFC (Gil et al., 2003; Rozendal et al., 2007). Improvements in the system will soon result in power generation that is dependent on the capabilities of the microorganisms. As microorganisms’ metabolism activity is directly affected by the pH of the environment, the continuing shift would result in the fluctuation of both voltage output and cathode potential. This troublesome might be solved if using non-membrane bio-cathode MFC and oxygen might diffuse directly into the anode (Liu and Logan, 2004). In their experiment, the MFC consisted of an anode and cathode placed on opposite side in a plastic (Plexiglas) cylindrical chamber 4 cm long by 3 cm in diameter. The anode electrode was made of Toray carbon paper (without wet proofing; E-Tek) and did not contain a catalyst. The carbon electrode/PEM cathode (CE-PEM) was manufactured by bonding the PEM directly onto a flexible carbon-cloth electrode containing 0.5 mg/cm2 of Pt catalyst (E-Tek). The cathode used in the absence of the PEM was a more rigid carbon paper containing 0.35 mg/cm2 of Pt (E-Tek). Platinum wire was used to connect the circuit and results in the potential for a maximum oxygen flux of 0.05 mg/h (CE-PEM). In the absence of the PEM, the oxygen flux could reach 0.187 mg/h (CE). In the presence of the PEM, values obtained for oxygen diffusivities through the PEM are consistent with oxygen diffusion results for hydrogen fuel cells. In the absence of the PEM, however, the rate of oxygen transfer into the anode chamber substantially increased. Some attempts have also been made to enhance current generation from the fuel cell by lowering the oxygen diffusion, including the uses of ferricyanide as a cathode mediator and of a platinum-coated graphite electrode (Jang et al., 2004).
3.2.2 Material of cathode
The material of cathode is another problem. Carbon paper, cloth, graphite, woven graphite, graphite granules and brushes have been used to act as MFC’S cathode (Logan, 2008). Compared with no bio-cathode MFC, ideal bio-cathode materials are bound to have large surface area as well as tough appearance which offer higher possibility for microbes to live on, and which would result in the decline of electrode over potential, improving cathode potential and power output (Logan, 2009). This was also confirmed by You et al.; however, the most suitable material has not yet been determined (You et al., 2009). They provide a demonstration of graphite fiber brush (GFB) used as bio-cathode material in MFC for more efficient and sustainable electricity recovery from organic substances.
3.2.3 Others
In a short few years, power increased to 96 W/m3 using a phosphate buffer, and further to 115 W/m3 using an ammonia-treated electrode. The combined effects of these two treatments boosted power production by 48% compared to previous results using this air–cathode MFC (Cheng and Logan, 2006). However, compared with conventional water treatment technologies, recommended value of 400 W/m3 is not even enough. If bio-cathode MFCs are aimed at large commercial application, the power output ought to be enhanced. Compared with abiotic cathode MFCs, bio-cathode ones are more limited by the activity of microorganisms. Efforts could be made to increase temperature or balance pH to stimulate the highest rate of metabolism of bacteria. Furthermore, new culture of microorganisms with high metabolism and strong survivability might be introduced.
4 Applications
Bio-cathodes are a welcome advancement in the quest to implement MFCs for practical applications, such as wastewater treatment and sediment MFCs because of potential cost savings, waste removal, and operational sustainability. Large amount of researches about abiotic MFCs of being used in wastewater treatment to remove organics, electricity and hydrogen generation have been performed. The investigations up to now suggest that the applications of bio-cathode MFCs mainly relate to simultaneous electricity generation and nitrification.
Conventional wastewater treatments are aimed at removing the impurities with external forces but ignoring the environmental self-recovery. Therefore, the treatment process is not a sustainable or economic one. Fortunately, this disadvantage could be overcome if bio-cathode MFCs were applied. Inoculating nitrifying bacteria in the cathode chamber could achieve simultaneous nitrification and electricity production. Nitrification process and the power production process in the same area not only take full advantage of dissolved oxygen, saving aeration energy consumption, but also the nitrification produces extra proton, effectively avoiding the cathode’s pH fluctuation caused by the electric production process. Xie et al. confirmed that the maximum nominal current and maximum power density were 47 mA and 45.50 W/m3, respectively (Xie et al., 2010). They also constructed a coupled MFCs system for simultaneous removal of carbon and nitrogen from a synthetic wastewater by using oxic/anoxic – bio-cathode MFCs (Xie et al., 2011). The system composed of an O-MFC and an A-MFC. Each MFC was made up of a middle anode and a two-sideward cathode, and the two-sideward cathode was placed at the two sides of the anode chamber. Synthetic wastewater was fed into the O-MFC and A-MFC anode chambers using two peristaltic pumps, respectively. The effluents from the two anode chambers were subsequently directed into the two cathode side chambers of the O-MFC for aerobic nitrification. Oxygenation of the cathodic vessel was supplied by an air pump, and the airflow was adjusted with an airflow rotameter. The effluent from the O-MFC cathode chambers was fed into the A-MFC cathode chambers for electrochemical denitrification. Part of the effluent from the A-MFC cathode chamber was recirculated into the O-MFC cathode chamber for deep nitrification. A maximum nominal current of 52.1 ± 0.9 mA was obtained by the A-MFC when a 5 Ω. was applied on the reactor, while 22.7 ± 0.1 mA was instead obtained at 20 Ω. The specific currents were 26.1 ± 0.5 A/m3 net cathodic compartment (NCC) and 11.4 ± 0.0 A/ m3 NCC, respectively. The A-MFC produced a maximum power production of 6.8 ± 0.2 W/m3 NCC at the resistance of 5Ω. The specific current and specific power density of the O-MFC were 19.4 ± 0.1 A/m3 NCC and 15.1 ± 0.0 W/m3 NCC, respectively. In addition, the coupled MFC system achieved a maximum COD, NH4+–N and TN removal of 98.8%, 97.4% and 97.3%, respectively. Moreover, operation parameters, especially external resistance and influent flow ratio, which have great effects on nitrogen removal, can be accommodated according to the COD/N ratio of wastewater.
5 Prospect
Bio-cathode MFCs have the advantages of low cost, uninterrupted operation and reduction of secondary pollution, etc. However, few studies have described bio-cathodes completely and comprehensively. And the efforts are always being made to improve the performance of bio-cathodes. First of all, it would be important in the future to look for cheap and efficient electrode materials. Researching and developing electrodes appropriate for bacteria living and comparatively of low resistance would be favorable for large-scale application of bio-cathode MFCs. Secondly, the dominant bacteria with sustainable electrochemically active should be cultured and acclimated for wastewater treatment. Thirdly, it should be admitted that the power output of bio-cathode MFC is impossible to weigh against conventional chemical fuel cells, which means there is a huge potential that bio-cathodes could be improved. We are confident that the application of bio-cathodes in MFCs will prosper in the fields of practical wastewater treatment to enhance the sustainable and profitable as well as sediment MFCs to power small electronic sensors in the abysmal sea where electrical energy is hard to supply.
Acknowledgments
The authors would like to acknowledge the financial support from the Provincial Natural Science Foundation of Jiangsu, China (BK20141117) and the National Science Foundation of China (Grant No. 51109038).
References
- Catalysis of oxygen reduction in PEM fuel cell by seawater biofilm. Electrochem. Commun.. 2005;7:900-904.
- [Google Scholar]
- Bioelectrochemical energy conversion canfield. Appl. Environ. Microbiol.. 1964;12:10-12.
- [Google Scholar]
- Microbial community dynamics and electron transfer of a biocathode in microbial fuel cells. Korean J. Chem. Eng.. 2010;27:1513-1520.
- [Google Scholar]
- Ammonia treatment of carbon cloth anodes to enhance power generation of microbial fuel cells. Electrochem. Commun.. 2006;9:492-496.
- [Google Scholar]
- Open-air biocathode enables effective electricity generation with microbial fuel cells. Environ. Sci. Technol.. 2007;41:7564-7569.
- [Google Scholar]
- Corroding iron as a hydrogen source for sulphate reduction in growing cultures of sulphate-reducing bacteria. Appl. Microbiol. Biotechnol.. 1986;25:169-174.
- [Google Scholar]
- Enhanced coulombic efficiency and power density of air–cathode microbial fuel cells with an improved cell configuration. Power Sour.. 2007;171:348-354.
- [Google Scholar]
- Sequential anode cathode configuration improves cathodic oxygen reduction and effluent quality of microbial fuel cells. Water Res.. 2008;42:1387-1396.
- [Google Scholar]
- Operational parameters affecting the performance of a mediator-less microbial fuel cell. Biosens. Bioelectron.. 2003;18:327-334.
- [Google Scholar]
- Application of bacteriological processes to the generation of electrical power. Microbiol. 1963;4:70-80.
- [Google Scholar]
- Graphite electrodes as electron donors for anaerobic respiration. Environ. Microbiol.. 2004;6:596-604.
- [Google Scholar]
- Application of bacterial biocathodes in microbial fuel cells. Electroanalysis. 2006;18:2009-2015.
- [Google Scholar]
- Microbial communities associated with electrodes harvesting electricity from a variety of aquatic sediments. Microb. Ecol.. 2004;48:178-190.
- [Google Scholar]
- Improvement of cathode reaction of a mediatorless microbial fuel cell. Microbiol. Biotechnol.. 2004;14:324-329.
- [Google Scholar]
- Biochemical cells utilizing immobilized cells of Clostridium butyricum. Biotechnol. Bioeng.. 1977;19:1727-1733.
- [Google Scholar]
- Effect of surface roughness of porcelain on adhesion of bacteria and their synthesizing glucans. Prosthet. Dent.. 2000;83:664-667.
- [Google Scholar]
- A microbial fuel cell type lactate biosensor using a metal-reducing bacterium, Shewanella putrefaciens. J. Microbiol. Biotechnol.. 1999;9:365-367.
- [Google Scholar]
- Electricity generation using an air–cathode single chamber microbial fuel cell in the presence and absence of a proton exchange membrane. Environ. Sci. Technol.. 2004;38:4040-4046.
- [Google Scholar]
- Microbial Fuel Cell. John Wiley & Sons Inc; 2008.
- Exoelectrogenic bacteria that power microbial fuel cells. Nat. Rev. Microbiol.. 2009;7:375-381.
- [Google Scholar]
- Electricity generation from cysteine in a microbial fuel cell. Water Resour.. 2005;39:942-952.
- [Google Scholar]
- Iron and manganese in anaerobic respiration: environmental significance, physiology and regulation. Ann. Rev. Microbiol.. 1994;48:311-343.
- [Google Scholar]
- Utilization of electrically reduced neutral red by Actinobacillus succinogenes: Physiological function of neutral red in membrane-driven fumarate reduction and energy conservation. Bacteriology. 1999;181:2403-2410.
- [Google Scholar]
- Microbial utilization of electrically reduced neutral red as the sole electron donor for growth and metabolite production. Appl. Environ. Microbiol.. 1999;65:2912-2917.
- [Google Scholar]
- Nitrate reduction using an electrode as direct electron donor in a biofilm-electrode reactor. Process Biochem.. 2005;40:3383-3388.
- [Google Scholar]
- Animal electricity and the birth of electrophysiology: the legacy of Luigi Galvani. Brain Resour. Bull.. 1998;46:386-407.
- [Google Scholar]
- Microbial fuel cells: novel biotechnology for energy generation. Trends Biotechnol.. 2005;23:291-298.
- [Google Scholar]
- Microbial fuel cell using anaerobic respiration as an anodic reaction and biomineralized manganese as a cathodic reactant. Environ. Sci. Technol.. 2005;39:4666-4671.
- [Google Scholar]
- Effects of membrane cation transport on pH and microbial fuel cell performance. Environ. Sci. Technol.. 2007;40:5206-5211.
- [Google Scholar]
- Microbial decarboxylation of succinate to propionate: kinetic studies. Ann. NY Acad. Sci.. 1990;589:697-704.
- [Google Scholar]
- Wireless sensors powered by microbial fuel cells. Environ. Sci. Technol.. 2005;39:5037-5042.
- [Google Scholar]
- Effect of surface modification of silicone on Staphylococcus epidermidis adhesion and colonization. Biomed. Mater. Res. Part A. 2007;80A:885-894.
- [Google Scholar]
- Trapananti, A., 2008. Experiment title: Novel Pt–Cs–W electrocatalysts for PEM fuel cell investigated by in situ X-ray absorption spectroscopy.
- Microbial fuel cells for simultaneous carbon and nitrogen removal. Water Res.. 2008;42:3013-3024.
- [Google Scholar]
- Simultaneous electricity generation and nitrification in a microbial fuel cell with aerobic biocathode. Environ. Sci. Technol.. 2010;31:1601-1606.
- [Google Scholar]
- Simultaneous carbon and nitrogen removal using an oxic/anoxic-biocathode microial fuel cells coupled system. Bioresour. Technol.. 2011;102:348-354.
- [Google Scholar]
- Power generation and electrochemical analysis of biocathode microbial fuel cell using graphite fiber brush as cathode material. Fuel Cells. 2009;5:588-596.
- [Google Scholar]