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manganese-oxidizing bacteria
Electrochimica Acta 55 (2010) 7804–7808
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Power generation from a biocathode microbial fuel cell biocatalyzed by ferro/manganese-oxidizing bacteria Yanping Mao, Lehua Zhang, Dongmei Li, Haifeng Shi, Yongdi Liu, Lankun Cai ∗ School of Resources and Environmental Engineering, East China University of Science and Technology, Meilong Road 130, Shanghai 200237, China
a r t i c l e
i n f o
Article history: Received 30 October 2009 Received in revised form 2 March 2010 Accepted 3 March 2010 Available online 9 March 2010 Keywords: Microbial fuel cells (MFCs) Biocathode Ferro/manganese-oxidizing bacteria Metal oxides modified electrode Power output
a b s t r a c t The abiotic cathodes usually require a catalyst such as Pt to enhance power production, increasing the cost and lowering the operational sustainability. In this paper, the performance of a biocathode microbial fuel cell biocatalyzed by ferro/manganese-oxidizing bacteria was investigated. A scanning electron microscopy with an energy-dispersive spectrometer (SEM-EDS) was used to characterize the cathode and analyze the element of cathode. The amount of ferro/manganese-oxidizing bacteria in the biocathode was examined. In batch-fed systems, the maximum open circuit voltage (OCV) was between 700 and 800 mV and the maximum cell potential difference was higher than 600 mV with an external resistance of 100 . The maximum power density was 32 W m−3 MFC for batch-fed systems (20–40% Coulombic yield) and 28 W m−3 MFC for a continuous system with an acetate loading rate of 1.0 kg COD m−3 day−1 . The results of SEM-EDS clearly showed that cathode was impregnated with iron and manganese. The amount of ferro/manganese-oxidizing bacteria was (7.5–20.0) × 105 MPN mL−1 in the biocathode. Biocathodes alleviate the need to use noble catalysts for the reduction of oxygen, which step forward towards large-scale application of MFCs. © 2010 Elsevier Ltd. All rights reserved.
1. Introduction Microbial fuel cells (MFCs) are capable of converting organic contaminants in wastewater to electricity at high efficiency. Research on MFCs has received an increasing attention as a means to treat wastewater and to produce “green” electricity attributing to environmental pollution and energy crisis all over the world [1]. Conventional MFCs consist of biological anodes and abiotic cathodes, which are half-biological fuel cells. In the anode chamber, organic substrate is biological oxidized to carbon dioxide, producing protons and electrons. Protons migrate from anode to cathode through a proton exchange membrane. Electrons transfer through an electrical circuit from the anode to the cathode. There, oxygen as the electron acceptor is reduced and together with protons to water. The cathode reaction is mainly limited by the unfavorable reaction kinetics of oxygen reduction. Therefore cathodes usually require a catalyst such as platinum or an electron mediator to enhance power generation [2]. However, chemical catalysts such as platinum increase the operating costs and even cause secondary pollution. Such disadvantages could be overcome by cathodes based on biocatalysts [3,4]. The marine sediment MFCs with aerobic biocathodes have been designed and analyzed [5–8]. Freguia et al.
∗ Corresponding author. Tel.: +86 21 64253321; fax: +86 21 64253321. E-mail addresses: [email protected] (L. Zhang), [email protected] (L. Cai). 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2010.03.004
reported that development of a cathodic biofilm achieved a fourfold increase of the current output compared with the non-catalyzed graphite cathode [9]. It has been reported that biological catalysts, combined with redox mediators such as manganese oxides, can facilitate oxygen reduction [10,11]. Rhoads et al. employed the cycle of Mn(IV) reduction and subsequent reoxidation of Mn(II) in the aerobic cathode of a MFC and observed a consistent production of electricity [12]. Electrochemical precipitation of manganese oxides on the cathode electrode decreased the start-up period with approximately 30% versus a non-treated one [13]. Similar to manganese, previous studies have revealed that Fe(II) was oxidized to Fe(III) through microbial activity of Thiobacillus ferrooxidans [14]. An iron-chelated complex Fe–EDTA could effectively be used as an aerated catholyte to generate a maximum power density of 22.9 W m−3 total anode compartment [15]. However, the power production of biocathode MFCs is still lower than that of abiotic cathode MFCs [16]. The procedure of biocathode material preparation is complicated and infeasible in large-scale MFCs. It has been reported that iron played an important role in the metabolism of manganese-oxidizing bacteria in the process of biological manganese oxidation and that biological manganese oxidation is inefficient with a low iron concentration in underwater [17]. Additionally, Tang et al. reported that an electrode modified by iron and manganese showed better electrochemical performances than a manganese electrode in a supercapacitor [18]. Biocathodes co-modified by iron and manganese would be
Y. Mao et al. / Electrochimica Acta 55 (2010) 7804–7808
meaningful for promoting the performance of biocathode MFCs. In this study, a cathode was modified by iron and manganese oxides by a very simple procedure. A biocathode MFC based on the biocatalysis of ferro/manganese-oxidizing bacteria was developed and its performance was investigated. 2. Experimental 2.1. Preparation of cathodic material Granular carbon (diameter 2.5–4.0 mm) sunk in the mixture of manganese and iron chloride solution for 20–24 h. The pH of the mixture solution was adjusted to 10–11 with 3 M NaOH solution before drying at 105 ◦ C. Then the granular carbon was activated at 350 ◦ C for 2 h with the protection of nitrogen gas in a tube-type muffle furnace. Finally, the granular carbon was cooled and stored at 20–25 ◦ C. 2.2. Microbial fuel cell construction The microbial fuel cell consisted of an anaerobic anode and an aerobic biological cathode which were separated by a proton exchange membrane (Nafion 117, DuPont Co., Delaware) (Fig. 1). Anodic and cathodic compartments were filled with pre- and post-modified granular carbon, respectively. In a control reactor, both anodic and cathodic compartments were filled with premodified granular carbon. Every compartment was inserted by a graphite rod (diameter 4.0 mm) as an electrical conductor. The MFC reactor volume (VMFC ) of 150 mL was the sum of the anodic compartment (VAnode ) and cathodic compartment (VCathode ) (VMFC = VAnode + VCathode ). An air pump was used to aerate the cathodic compartment. In the external circuit, a variable-resistance box (0.1–99999.9 ) and a galvanometer were connected in series.
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2.3. Operational conditions The liquid stream was circulated in an upstream mode through both the anodic and cathodic compartments with the peristaltic pumps (5 mL min−1 ). The medium (50 mM phosphate buffer solution (pH 7.2) with 0.5 g NH4 Cl, 0.1 g MgSO4 , 0.1 g CaCl2 ·2H2 O, 0.1 g KCl, 1 g NaHCO3 and 1 mL trace element) was supplemented with 0.5, 1.0, 2.0, 3.0 and 5.0 g of sodium acetate per liter, respectively, in the batch-fed tests [19]. For the continuous tests, a 1 L anodic recirculation vessel was used (circulating flow of 5 mL min−1 ), and the concentrated acetate was continuously added with a syringe pump with an acetate loading rate of 1.0 kg COD m−3 day−1 . The cathodic liquid was always the same medium as the anodic liquid but without acetate. Aerobic and anaerobic digested sludge from a sewage disposal plant were inoculated in cathode and anode, respectively. 2.4. Electrochemical monitoring and calculation The graphite rods were connected to the external resistor and a data acquisition system (RBH 8223h, China) in parallel. The voltage (U) across an external resistor (Rext ) was measured and recorded by the data acquisition unit every 5 min. Open circuit voltage (OCV) was tested by digital multimeter after 10 min stabilization. Power density is calculated according to Logan et al. as P=
U2 Rext V
(1)
where P is the volumetric power (W m−3 MFC), V is the total MFC reactor volume [20]. In the MFCs, acetate is biologically oxidized at the anodic compartment, producing protons and electrons. Protons migrate from anode to cathode through the Nafion 117 proton exchange membrane. Electrons transfer through the external electrical circuit from the anode to the cathode. The Coulombic yield expresses the recovery of electrons as a current over the total amount of electrons dosed as acetate. A total of 1 kg COD m−3 MFC day−1 equals 140 A m−3 MFC (96485 C mol−1 electrons; 4 mol electrons mol−1 COD). Measurement and determination of the internal resistance were performed according to the power density peak method [21]. 2.5. Ferro/manganese-oxidizing bacteria counting After start-up and continuous experiments, granular carbon in cathode was removed from the compartment and rinsed with cathodic liquid for bacteria counting. The amount of ferro/manganese-oxidizing bacteria in biological cathode was tested according to most probable number (MPN) method [22]. 2.6. Measurement of concentrations of carbonate and bicarbonate At 220 h of inoculation, a new cathodic solution was prepared. Then, at 290 h of inoculation, the concentrations of carbonate and bicarbonate in the cathodic solution were measured in the biocathode MFC and control reactor. 2.7. Microscope
Fig. 1. Schematic of the microbial fuel cell with a biocathode.
A JSM-6360LV scanning electron microscopy (SEM) (JEOL, Japan) was used to characterize the surface of granular carbon. Falcon energy-dispersive X-ray spectroscope (EDS) (EDAX, US) on the SEM was used for elemental analysis of granular carbon which was rinsed with deionized water and stored at 25–30 ◦ C for 48 h before analysis.
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3. Results and discussion
3.2. Start-up of biocathode MFCs
3.1. Cathodic material modified by iron and manganese
The MFC reactor with an iron and manganese oxides comodified biocathode needed a start-up period of 150 h (Fig. 3) to achieve cell potential difference (V) of 0.40 V upon inoculation. Another 250–350 h was generally required to obtain the maximum power output. However, the cell potential difference of the control reactor was less than 0.05 V during the period of 350 h. During the start-up period of biocathode MFC, the OCV was approximately 0.45–0.75 V after 10 min stabilization; the cell potential difference ranged from 0.38 to 0.59 V when operated with a 100 external resistor. The cathode potential ranged between 0.3 and 0.45 V versus standard hydrogen electrode (SHE) after 350 h of inoculation while the dissolved oxygen (DO) concentration in the cathodic solution was 3.0–5.0 mg L−1 . It generally takes 60–100 h to start-up a MFC with an abiotic cathode when an acclimatized anodic consortium is used [23,24]. Clauwaert et al. reported that the reactors with the precipitated
The results of EDS analyses indicated that carbon as well as silicon were major elements while iron and manganese were negligible in the pre-modified granular carbon (Fig. 2a). Iron, manganese and oxygen were observed on post-modified granular carbon surface. It was clear that the granular carbon had been impregnated with iron and manganese after modification. A crystallized layer was found from the scanning electron micrograph of post-modified granular carbon surface, which was probably a layer of iron and manganese oxides (Fig. 2b). After 10 months operation, the crystallized layer changed on the cathodic granular carbon surface but the elements of iron, manganese and oxygen were still observed from EDS test (Fig. 2c). The results elucidate that iron and manganese oxides are sustainable and effective redox mediators in biocathode.
Fig. 2. SEM and EDS analysis of the granular carbon pre-modified, post-modified and after 10 months operation in the cathodic chamber: (a) pre-modify granular carbon; (b) granular carbon co-modified by manganese and iron oxides; (c) granular carbon after 10 months operation in the cathodic chamber.
Y. Mao et al. / Electrochimica Acta 55 (2010) 7804–7808
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Fig. 3. Cell potential difference during the start-up period when the biocathode MFC operated with 100 external resistance.
manganese oxides needed a start-up period between 10 and 20 days to become fully active upon inoculation, whereas the reactors without manganese treatment could obtain a similar activity after 20–30 days [13]. In the system described here, the OCV of the biocathode MFCs was lower than 0.01 V in the first 120 h. However, biological activities of ferro/manganese bacteria enhanced the start-up of the aerobic cathode and shortened the start-up period to 150 h. Further research is needed to minimize the start-up period by optimizing the doses of iron and manganese compounds. During the polarization curve (Fig. 4), the maximum power production was 13.5 W m−3 MFC. According to Ohm’s law, the maximum power occurred at the point where the internal resistance equals the external resistance. Then the internal resistance of 14 is identified by noting the external resistance that produced the peak output in power density curve. Internal resistance in the MFC was associated to ohmic losses which decrease power output. Recently most studies have focused on reducing internal resistances to promote power production. Modification of reactors’ configuration has aroused interest. Single chamber or membraneless MFCs were constructed to minimize the electrodes spacing [25]. To our knowledge, the relatively low internal resistances with chemical cathodes were about 10–30 [16]. Bacteria in biological cathodes accelerated cathodic electron transition, which is another way to reduce internal resistance. It has been verified that during the growth of bacteria in both compartments, the internal resistance obviously decreased from 40.2 to 14.0 [26]. In this research, internal resistance of 14 implied that
Fig. 5. Discharge curve of batch-fed systems when anodic medium was supplemented with 0.5, 1.0, 2.0, 3.0 and 5.0 g of sodium acetate per liter, respectively.
ferro/manganese-oxidizing bacteria were able to enhance oxygen reduction in cathode and reduce ohmic losses. 3.3. Batch-fed and continuous operation of biocathode MFCs After spike feeding of acetate, the cell was active at its highest cell potential difference for several hours to days (depending on power production), and then the cell potential difference gradually decreased to voltage below 0.05 V. The pH in the anodic electrolyte gradually decreased and was adjusted daily to pH 7.2–7.6 with a 3 M NaOH solution. During the operation, the pH of the cathodic medium increased. A 3 M HCl solution was used daily to adjust the pH to 6.8–7.2. The use of a phosphate buffer and pH correction proved to be necessary for pH control in both the anodic and cathodic solutions. The anodic and cathodic liquids were renewed weekly to remain low concentrations of sodium and chloride. In this case, refreshing the medium did not affect the fuel cell performance. In batch-fed systems, cell potential difference peaked at about 20 h after feeding acetate with a 100 external resistance (Fig. 5). Maximum OCV ranged from 0.70 to 0.80 V depends on different acetate concentrations. The maximum power production was 32 W m−3 MFC. The Coulombic yields were typically between 20 and 40% for batch-fed systems (Table 1). To increase Coulombic yield, the anodic system was operated in a continuous mode. With acetate loading rate of 1.0 kg COD m−3 day−1 , the Coulombic yield achieved 55%, cell potential difference varied between 0.50 and 0.65 V and power density ranged from 16 to 28 W m−3 MFC (Fig. 6). 3.4. Ferro/manganese-oxidizing bacteria counting After 350 h of inoculation, the amount of ferro/manganeseoxidizing bacteria were 11.5 × 105 and 9.5 × 103 MPN mL−1 in the cathodes filled with modified and pre-modified granular carbon, respectively. The amount of ferro/manganese-oxidizing bacteria Table 1 Open circuit voltages and Coulombic yields in batch-fed systems.
Fig. 4. Polarization and power density curve. Cell potential difference () and power density () generated in the biocathode MFC when external resistance on the system is changed from 5 to 300 .
NaAc·3H2 O /g L−1
OCVmax/mV
Coulombic yield/%
0.5 1.0 2.0 3.0 5.0
364 721 817 756 809
23.71 35.97 32.30 34.50 19.88
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Y. Mao et al. / Electrochimica Acta 55 (2010) 7804–7808
cathode. How to explain the cooperation of manganese and iron? It has been reported that iron played an important role in the metabolism of manganese-oxidizing bacteria in the process of biological manganese oxidation [17]. However, the metabolic pathway of ferro/manganese-oxidizing bacteria as well as the relationship between ferro-oxidizing bacteria and manganese-oxidizing bacteria in biocathodes needs to be explored in the further research. 4. Conclusions
Fig. 6. Discharge curve of continuous systems with an acetate loading rate of 1.0 kg COD m−3 day−1 .
ranged between 7.5 × 105 and 20.0 × 105 MPN mL−1 in the biocathode MFC after continuous experiments. It was indicated that a large amount of ferro/manganese-oxidizing bacteria had been inoculated and acclimated in the biocathode MFC. 3.5. Measurement of concentrations of carbonate and bicarbonate The concentrations of carbonate in all samples were undetectable because pH value is about 7.2–7.4. The concentration of bicarbonate was 978 mg L−1 in the new prepared cathodic solution before the tests. The concentrations of bicarbonate in the cathodic solution were 778 and 920 mg L−1 in the biocathode MFC and control reactor, respectively, after 70 h. The results indicate that ferro/manganese-oxidizing bacteria are autotrophic bacteria by which carbonate or bicarbonate are used as carbon resource in the biocathode. 3.6. Biological ferro/manganese oxidation in biocathodes Manganese is an abundant metal that can be easily transformed between its oxidation states. The oxidation of Mn(II) can be accomplished through microbial activity efficiently. Rhoads et al. described a biological manganese shuttling mechanism in the cathodic biofilm where Leptothrix discophora (one kind of manganese-oxidizing bacteria) use Mn(II) as the electron donor and oxygen as the electron acceptor [12]. The first step in the cycle is abiotic in which MnO2 is electrochemically reduced to an intermediate product, MnOOH, by accepting one electron from the cathodic electrode. This is followed by a further reduction of MnOOH to Mn(II) through the acceptance of another electron. The second step is accomplished by Leptothrix discophora, which oxidizes Mn(II) to MnO2 by releasing two electrons to oxygen. It has been found that by adding the cycle of manganese reduction/oxidation to an aerobic biocathode, the maximum power production increased by a factor of more than 40 times. Lopez-Lopez et al. revealed that Thiobacillus ferrooxidans was a kind of ferro-oxidizing bacteria which oxidized Fe(II) to Fe(III) [14]. During the study on underwater purification, the participation of iron has been discovered in the metabolism of manganese-oxidizing bacteria. In this paper, the performance of a biocathode microbial fuel cell based on the biocatalysis of ferro/manganese-oxidizing bacteria was investigated. The results showed that biological ferro/manganese deposition and reoxidation in biocathode were beneficial to enhance power production. Both manganese and iron played an important role in the bio-
Iron and manganese oxides were used as the redox mediators in a biocathode MFC. Electrochemical studies and fuel cell polarization tests showed that a biocathode was favor of reducing the internal resistance to 14 and increasing the maximum power output to 32 W m−3 MFC. The biocathode MFC biocatalyzed by ferro/manganese-oxidizing bacteria needed a start-up period time of 150 h. In batch-fed systems, the maximum OCV ranged from 0.70 to 0.80 V dependent on different acetate concentrations. The maximum power production was 32 W m−3 MFC and the Coulombic yields were typically between 20 and 40%. In a continuous mode, with an acetate loading rate of 1 kg COD day−1 m−3 MFC, the Coulombic yield achieved 55%, cell potential difference varied between 0.50 and 0.65 V and power density ranged from 16 to 28 W m−3 MFC. These findings have interesting possibilities to promote the application of biocathodes, which are crucial to the successful scale up and commercialization of MFCs. Acknowledgements This work was supported by the National Natural Science Foundation of China (NSFC) (20906026), Shanghai Pujiang Program (09PJ1402900) and the Fundamental Research Funds for the Central Universities (WB0914036). References [1] S.A. Cheng, H. Liu, B.E. Logan, Environ. Sci. Technol. 40 (2006) 364. [2] J.C. Biffinger, J. Pietron, R. Ray, B. Little, B.R. Ringeisen, Biosens. Bioelectron. 22 (2007) 1672. [3] B. Erable, I. Vandecandelaere, M. Faimali, M. Delia, L. Etcheverry, P. Vandamme, A. Bergel, Bioelectrochem. 78 (2010) 51–56. [4] B. Virdis, K. Rabaey, Z.G. Yuan, J. Keller, Water Res. 42 (2008) 3013. [5] O. Hasvold, H. Henriksen, E. Melvaer, G. Citi, B.O. Johansen, T. Kjonigsen, R. Galetti, J. Power Sources 65 (1997) 253. [6] A. Bergel, D. Féron, A. Mollica, Electrochem. Commun. 7 (2005) 900. [7] C. Dumas, A. Mollica, D. Féron, R. Basseguy, L. Etcheverry, A. Bergel, Electrochim. Acta 53 (2007) 468. [8] Z. He, H.B. Shao, L.T. Angenent, Biosens. Bioelectron. 22 (2007) 3252. [9] S. Freguia, K. Rabaey, Z.G. Yuan, J. Keller, Water Res. 42 (2008) 1387. [10] Z. He, L.T. Angenent, Electroanalysis 18 (2006) 2009. [11] P. Clauwaert, D. van der Ha, W. Verstraete, Biotechnol. Lett. 30 (2008) 1947. [12] A. Rhoads, H. Beyenal, Z. Lewandowski, Environ. Sci. Technol. 39 (2005) 4666. [13] P. Clauwaert, D. van der Ha, N. Boon, K. Verbeken, M. Verhaege, K. Rabaey, W. Verstraete, Environ. Sci. Technol. 41 (2007) 7564. [14] A. Lopez-Lopez, E. Exposito, J. Anton, F. Rodriguez-Valera, A. Aldaz, Biotechnol. Bioeng. 63 (1999) 79. [15] P. Aeltermana, M. Versichelea, E. Genettello, K. Verbekenb, W. Verstraete, Electrochim. Acta 54 (2009) 5754. [16] Y. Mao, L. Cai, L. Zhang, H. Hou, G. Huang, Y. Liu, Prog. Chem. 21 (2009) 1672. [17] G. Khoe, H.Z. Myint, US Patent 6,558,556 (2003). [18] Z. Tang, X. Geng, Z. Wang, J. Xun, Chin. J. Appl. Chem. 19 (2002) 936. [19] K. Rabaey, W. Ossieur, M. Verhaege, W. Verstraete, Water Sci. Technol. 52 (2005) 515. [20] B.E. Logan, B. Hamelers, R. Rozendal, U. Schrder, J. Keller, S. Freguia, P. Aelterman, W. Verstraete, K. Rabaey, Environ. Sci. Technol. 40 (2006) 5181. [21] B.E. Logan, Microbial Fuel Cells [M], Wiley, 2007, p. 50. [22] D. Li, J. Zhang, H. Wang, L. Chen, H. Wang, J. Water Supply: Res. Technol. 55 (2006) 313. [23] H. Liu, A.S. Cheng, E.B. Logan, Environ. Sci. Technol. 39 (2005) 658. [24] K. Rabaey, P. Clauwaert, P. Aelterman, W. Verstraete, Environ. Sci. Technol. 39 (2005) 8077. [25] S.J. You, Q.L. Zhao, J.N. Zhang, J.Q. Jiang, C.L. Wan, M.A. Du, S.Q. Zhao, J. Power Sources 173 (2007) 172. [26] G.W. Chen, S.J. Choi, T.H. Lee, G.Y. Lee, J.H. Cha, C.W. Kim, Appl. Microbiol. Biotechnol. 79 (2008) 379.
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Power generation from a biocathode microbial fuel cell biocatalyzed by ferro/manganese-oxidizing bacteria Yanping Mao, Lehua Zhang, Dongmei Li, Haifeng Shi, Yongdi Liu, Lankun Cai ∗ School of Resources and Environmental Engineering, East China University of Science and Technology, Meilong Road 130, Shanghai 200237, China
a r t i c l e
i n f o
Article history: Received 30 October 2009 Received in revised form 2 March 2010 Accepted 3 March 2010 Available online 9 March 2010 Keywords: Microbial fuel cells (MFCs) Biocathode Ferro/manganese-oxidizing bacteria Metal oxides modified electrode Power output
a b s t r a c t The abiotic cathodes usually require a catalyst such as Pt to enhance power production, increasing the cost and lowering the operational sustainability. In this paper, the performance of a biocathode microbial fuel cell biocatalyzed by ferro/manganese-oxidizing bacteria was investigated. A scanning electron microscopy with an energy-dispersive spectrometer (SEM-EDS) was used to characterize the cathode and analyze the element of cathode. The amount of ferro/manganese-oxidizing bacteria in the biocathode was examined. In batch-fed systems, the maximum open circuit voltage (OCV) was between 700 and 800 mV and the maximum cell potential difference was higher than 600 mV with an external resistance of 100 . The maximum power density was 32 W m−3 MFC for batch-fed systems (20–40% Coulombic yield) and 28 W m−3 MFC for a continuous system with an acetate loading rate of 1.0 kg COD m−3 day−1 . The results of SEM-EDS clearly showed that cathode was impregnated with iron and manganese. The amount of ferro/manganese-oxidizing bacteria was (7.5–20.0) × 105 MPN mL−1 in the biocathode. Biocathodes alleviate the need to use noble catalysts for the reduction of oxygen, which step forward towards large-scale application of MFCs. © 2010 Elsevier Ltd. All rights reserved.
1. Introduction Microbial fuel cells (MFCs) are capable of converting organic contaminants in wastewater to electricity at high efficiency. Research on MFCs has received an increasing attention as a means to treat wastewater and to produce “green” electricity attributing to environmental pollution and energy crisis all over the world [1]. Conventional MFCs consist of biological anodes and abiotic cathodes, which are half-biological fuel cells. In the anode chamber, organic substrate is biological oxidized to carbon dioxide, producing protons and electrons. Protons migrate from anode to cathode through a proton exchange membrane. Electrons transfer through an electrical circuit from the anode to the cathode. There, oxygen as the electron acceptor is reduced and together with protons to water. The cathode reaction is mainly limited by the unfavorable reaction kinetics of oxygen reduction. Therefore cathodes usually require a catalyst such as platinum or an electron mediator to enhance power generation [2]. However, chemical catalysts such as platinum increase the operating costs and even cause secondary pollution. Such disadvantages could be overcome by cathodes based on biocatalysts [3,4]. The marine sediment MFCs with aerobic biocathodes have been designed and analyzed [5–8]. Freguia et al.
∗ Corresponding author. Tel.: +86 21 64253321; fax: +86 21 64253321. E-mail addresses: [email protected] (L. Zhang), [email protected] (L. Cai). 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2010.03.004
reported that development of a cathodic biofilm achieved a fourfold increase of the current output compared with the non-catalyzed graphite cathode [9]. It has been reported that biological catalysts, combined with redox mediators such as manganese oxides, can facilitate oxygen reduction [10,11]. Rhoads et al. employed the cycle of Mn(IV) reduction and subsequent reoxidation of Mn(II) in the aerobic cathode of a MFC and observed a consistent production of electricity [12]. Electrochemical precipitation of manganese oxides on the cathode electrode decreased the start-up period with approximately 30% versus a non-treated one [13]. Similar to manganese, previous studies have revealed that Fe(II) was oxidized to Fe(III) through microbial activity of Thiobacillus ferrooxidans [14]. An iron-chelated complex Fe–EDTA could effectively be used as an aerated catholyte to generate a maximum power density of 22.9 W m−3 total anode compartment [15]. However, the power production of biocathode MFCs is still lower than that of abiotic cathode MFCs [16]. The procedure of biocathode material preparation is complicated and infeasible in large-scale MFCs. It has been reported that iron played an important role in the metabolism of manganese-oxidizing bacteria in the process of biological manganese oxidation and that biological manganese oxidation is inefficient with a low iron concentration in underwater [17]. Additionally, Tang et al. reported that an electrode modified by iron and manganese showed better electrochemical performances than a manganese electrode in a supercapacitor [18]. Biocathodes co-modified by iron and manganese would be
Y. Mao et al. / Electrochimica Acta 55 (2010) 7804–7808
meaningful for promoting the performance of biocathode MFCs. In this study, a cathode was modified by iron and manganese oxides by a very simple procedure. A biocathode MFC based on the biocatalysis of ferro/manganese-oxidizing bacteria was developed and its performance was investigated. 2. Experimental 2.1. Preparation of cathodic material Granular carbon (diameter 2.5–4.0 mm) sunk in the mixture of manganese and iron chloride solution for 20–24 h. The pH of the mixture solution was adjusted to 10–11 with 3 M NaOH solution before drying at 105 ◦ C. Then the granular carbon was activated at 350 ◦ C for 2 h with the protection of nitrogen gas in a tube-type muffle furnace. Finally, the granular carbon was cooled and stored at 20–25 ◦ C. 2.2. Microbial fuel cell construction The microbial fuel cell consisted of an anaerobic anode and an aerobic biological cathode which were separated by a proton exchange membrane (Nafion 117, DuPont Co., Delaware) (Fig. 1). Anodic and cathodic compartments were filled with pre- and post-modified granular carbon, respectively. In a control reactor, both anodic and cathodic compartments were filled with premodified granular carbon. Every compartment was inserted by a graphite rod (diameter 4.0 mm) as an electrical conductor. The MFC reactor volume (VMFC ) of 150 mL was the sum of the anodic compartment (VAnode ) and cathodic compartment (VCathode ) (VMFC = VAnode + VCathode ). An air pump was used to aerate the cathodic compartment. In the external circuit, a variable-resistance box (0.1–99999.9 ) and a galvanometer were connected in series.
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2.3. Operational conditions The liquid stream was circulated in an upstream mode through both the anodic and cathodic compartments with the peristaltic pumps (5 mL min−1 ). The medium (50 mM phosphate buffer solution (pH 7.2) with 0.5 g NH4 Cl, 0.1 g MgSO4 , 0.1 g CaCl2 ·2H2 O, 0.1 g KCl, 1 g NaHCO3 and 1 mL trace element) was supplemented with 0.5, 1.0, 2.0, 3.0 and 5.0 g of sodium acetate per liter, respectively, in the batch-fed tests [19]. For the continuous tests, a 1 L anodic recirculation vessel was used (circulating flow of 5 mL min−1 ), and the concentrated acetate was continuously added with a syringe pump with an acetate loading rate of 1.0 kg COD m−3 day−1 . The cathodic liquid was always the same medium as the anodic liquid but without acetate. Aerobic and anaerobic digested sludge from a sewage disposal plant were inoculated in cathode and anode, respectively. 2.4. Electrochemical monitoring and calculation The graphite rods were connected to the external resistor and a data acquisition system (RBH 8223h, China) in parallel. The voltage (U) across an external resistor (Rext ) was measured and recorded by the data acquisition unit every 5 min. Open circuit voltage (OCV) was tested by digital multimeter after 10 min stabilization. Power density is calculated according to Logan et al. as P=
U2 Rext V
(1)
where P is the volumetric power (W m−3 MFC), V is the total MFC reactor volume [20]. In the MFCs, acetate is biologically oxidized at the anodic compartment, producing protons and electrons. Protons migrate from anode to cathode through the Nafion 117 proton exchange membrane. Electrons transfer through the external electrical circuit from the anode to the cathode. The Coulombic yield expresses the recovery of electrons as a current over the total amount of electrons dosed as acetate. A total of 1 kg COD m−3 MFC day−1 equals 140 A m−3 MFC (96485 C mol−1 electrons; 4 mol electrons mol−1 COD). Measurement and determination of the internal resistance were performed according to the power density peak method [21]. 2.5. Ferro/manganese-oxidizing bacteria counting After start-up and continuous experiments, granular carbon in cathode was removed from the compartment and rinsed with cathodic liquid for bacteria counting. The amount of ferro/manganese-oxidizing bacteria in biological cathode was tested according to most probable number (MPN) method [22]. 2.6. Measurement of concentrations of carbonate and bicarbonate At 220 h of inoculation, a new cathodic solution was prepared. Then, at 290 h of inoculation, the concentrations of carbonate and bicarbonate in the cathodic solution were measured in the biocathode MFC and control reactor. 2.7. Microscope
Fig. 1. Schematic of the microbial fuel cell with a biocathode.
A JSM-6360LV scanning electron microscopy (SEM) (JEOL, Japan) was used to characterize the surface of granular carbon. Falcon energy-dispersive X-ray spectroscope (EDS) (EDAX, US) on the SEM was used for elemental analysis of granular carbon which was rinsed with deionized water and stored at 25–30 ◦ C for 48 h before analysis.
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3. Results and discussion
3.2. Start-up of biocathode MFCs
3.1. Cathodic material modified by iron and manganese
The MFC reactor with an iron and manganese oxides comodified biocathode needed a start-up period of 150 h (Fig. 3) to achieve cell potential difference (V) of 0.40 V upon inoculation. Another 250–350 h was generally required to obtain the maximum power output. However, the cell potential difference of the control reactor was less than 0.05 V during the period of 350 h. During the start-up period of biocathode MFC, the OCV was approximately 0.45–0.75 V after 10 min stabilization; the cell potential difference ranged from 0.38 to 0.59 V when operated with a 100 external resistor. The cathode potential ranged between 0.3 and 0.45 V versus standard hydrogen electrode (SHE) after 350 h of inoculation while the dissolved oxygen (DO) concentration in the cathodic solution was 3.0–5.0 mg L−1 . It generally takes 60–100 h to start-up a MFC with an abiotic cathode when an acclimatized anodic consortium is used [23,24]. Clauwaert et al. reported that the reactors with the precipitated
The results of EDS analyses indicated that carbon as well as silicon were major elements while iron and manganese were negligible in the pre-modified granular carbon (Fig. 2a). Iron, manganese and oxygen were observed on post-modified granular carbon surface. It was clear that the granular carbon had been impregnated with iron and manganese after modification. A crystallized layer was found from the scanning electron micrograph of post-modified granular carbon surface, which was probably a layer of iron and manganese oxides (Fig. 2b). After 10 months operation, the crystallized layer changed on the cathodic granular carbon surface but the elements of iron, manganese and oxygen were still observed from EDS test (Fig. 2c). The results elucidate that iron and manganese oxides are sustainable and effective redox mediators in biocathode.
Fig. 2. SEM and EDS analysis of the granular carbon pre-modified, post-modified and after 10 months operation in the cathodic chamber: (a) pre-modify granular carbon; (b) granular carbon co-modified by manganese and iron oxides; (c) granular carbon after 10 months operation in the cathodic chamber.
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Fig. 3. Cell potential difference during the start-up period when the biocathode MFC operated with 100 external resistance.
manganese oxides needed a start-up period between 10 and 20 days to become fully active upon inoculation, whereas the reactors without manganese treatment could obtain a similar activity after 20–30 days [13]. In the system described here, the OCV of the biocathode MFCs was lower than 0.01 V in the first 120 h. However, biological activities of ferro/manganese bacteria enhanced the start-up of the aerobic cathode and shortened the start-up period to 150 h. Further research is needed to minimize the start-up period by optimizing the doses of iron and manganese compounds. During the polarization curve (Fig. 4), the maximum power production was 13.5 W m−3 MFC. According to Ohm’s law, the maximum power occurred at the point where the internal resistance equals the external resistance. Then the internal resistance of 14 is identified by noting the external resistance that produced the peak output in power density curve. Internal resistance in the MFC was associated to ohmic losses which decrease power output. Recently most studies have focused on reducing internal resistances to promote power production. Modification of reactors’ configuration has aroused interest. Single chamber or membraneless MFCs were constructed to minimize the electrodes spacing [25]. To our knowledge, the relatively low internal resistances with chemical cathodes were about 10–30 [16]. Bacteria in biological cathodes accelerated cathodic electron transition, which is another way to reduce internal resistance. It has been verified that during the growth of bacteria in both compartments, the internal resistance obviously decreased from 40.2 to 14.0 [26]. In this research, internal resistance of 14 implied that
Fig. 5. Discharge curve of batch-fed systems when anodic medium was supplemented with 0.5, 1.0, 2.0, 3.0 and 5.0 g of sodium acetate per liter, respectively.
ferro/manganese-oxidizing bacteria were able to enhance oxygen reduction in cathode and reduce ohmic losses. 3.3. Batch-fed and continuous operation of biocathode MFCs After spike feeding of acetate, the cell was active at its highest cell potential difference for several hours to days (depending on power production), and then the cell potential difference gradually decreased to voltage below 0.05 V. The pH in the anodic electrolyte gradually decreased and was adjusted daily to pH 7.2–7.6 with a 3 M NaOH solution. During the operation, the pH of the cathodic medium increased. A 3 M HCl solution was used daily to adjust the pH to 6.8–7.2. The use of a phosphate buffer and pH correction proved to be necessary for pH control in both the anodic and cathodic solutions. The anodic and cathodic liquids were renewed weekly to remain low concentrations of sodium and chloride. In this case, refreshing the medium did not affect the fuel cell performance. In batch-fed systems, cell potential difference peaked at about 20 h after feeding acetate with a 100 external resistance (Fig. 5). Maximum OCV ranged from 0.70 to 0.80 V depends on different acetate concentrations. The maximum power production was 32 W m−3 MFC. The Coulombic yields were typically between 20 and 40% for batch-fed systems (Table 1). To increase Coulombic yield, the anodic system was operated in a continuous mode. With acetate loading rate of 1.0 kg COD m−3 day−1 , the Coulombic yield achieved 55%, cell potential difference varied between 0.50 and 0.65 V and power density ranged from 16 to 28 W m−3 MFC (Fig. 6). 3.4. Ferro/manganese-oxidizing bacteria counting After 350 h of inoculation, the amount of ferro/manganeseoxidizing bacteria were 11.5 × 105 and 9.5 × 103 MPN mL−1 in the cathodes filled with modified and pre-modified granular carbon, respectively. The amount of ferro/manganese-oxidizing bacteria Table 1 Open circuit voltages and Coulombic yields in batch-fed systems.
Fig. 4. Polarization and power density curve. Cell potential difference () and power density () generated in the biocathode MFC when external resistance on the system is changed from 5 to 300 .
NaAc·3H2 O /g L−1
OCVmax/mV
Coulombic yield/%
0.5 1.0 2.0 3.0 5.0
364 721 817 756 809
23.71 35.97 32.30 34.50 19.88
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cathode. How to explain the cooperation of manganese and iron? It has been reported that iron played an important role in the metabolism of manganese-oxidizing bacteria in the process of biological manganese oxidation [17]. However, the metabolic pathway of ferro/manganese-oxidizing bacteria as well as the relationship between ferro-oxidizing bacteria and manganese-oxidizing bacteria in biocathodes needs to be explored in the further research. 4. Conclusions
Fig. 6. Discharge curve of continuous systems with an acetate loading rate of 1.0 kg COD m−3 day−1 .
ranged between 7.5 × 105 and 20.0 × 105 MPN mL−1 in the biocathode MFC after continuous experiments. It was indicated that a large amount of ferro/manganese-oxidizing bacteria had been inoculated and acclimated in the biocathode MFC. 3.5. Measurement of concentrations of carbonate and bicarbonate The concentrations of carbonate in all samples were undetectable because pH value is about 7.2–7.4. The concentration of bicarbonate was 978 mg L−1 in the new prepared cathodic solution before the tests. The concentrations of bicarbonate in the cathodic solution were 778 and 920 mg L−1 in the biocathode MFC and control reactor, respectively, after 70 h. The results indicate that ferro/manganese-oxidizing bacteria are autotrophic bacteria by which carbonate or bicarbonate are used as carbon resource in the biocathode. 3.6. Biological ferro/manganese oxidation in biocathodes Manganese is an abundant metal that can be easily transformed between its oxidation states. The oxidation of Mn(II) can be accomplished through microbial activity efficiently. Rhoads et al. described a biological manganese shuttling mechanism in the cathodic biofilm where Leptothrix discophora (one kind of manganese-oxidizing bacteria) use Mn(II) as the electron donor and oxygen as the electron acceptor [12]. The first step in the cycle is abiotic in which MnO2 is electrochemically reduced to an intermediate product, MnOOH, by accepting one electron from the cathodic electrode. This is followed by a further reduction of MnOOH to Mn(II) through the acceptance of another electron. The second step is accomplished by Leptothrix discophora, which oxidizes Mn(II) to MnO2 by releasing two electrons to oxygen. It has been found that by adding the cycle of manganese reduction/oxidation to an aerobic biocathode, the maximum power production increased by a factor of more than 40 times. Lopez-Lopez et al. revealed that Thiobacillus ferrooxidans was a kind of ferro-oxidizing bacteria which oxidized Fe(II) to Fe(III) [14]. During the study on underwater purification, the participation of iron has been discovered in the metabolism of manganese-oxidizing bacteria. In this paper, the performance of a biocathode microbial fuel cell based on the biocatalysis of ferro/manganese-oxidizing bacteria was investigated. The results showed that biological ferro/manganese deposition and reoxidation in biocathode were beneficial to enhance power production. Both manganese and iron played an important role in the bio-
Iron and manganese oxides were used as the redox mediators in a biocathode MFC. Electrochemical studies and fuel cell polarization tests showed that a biocathode was favor of reducing the internal resistance to 14 and increasing the maximum power output to 32 W m−3 MFC. The biocathode MFC biocatalyzed by ferro/manganese-oxidizing bacteria needed a start-up period time of 150 h. In batch-fed systems, the maximum OCV ranged from 0.70 to 0.80 V dependent on different acetate concentrations. The maximum power production was 32 W m−3 MFC and the Coulombic yields were typically between 20 and 40%. In a continuous mode, with an acetate loading rate of 1 kg COD day−1 m−3 MFC, the Coulombic yield achieved 55%, cell potential difference varied between 0.50 and 0.65 V and power density ranged from 16 to 28 W m−3 MFC. These findings have interesting possibilities to promote the application of biocathodes, which are crucial to the successful scale up and commercialization of MFCs. Acknowledgements This work was supported by the National Natural Science Foundation of China (NSFC) (20906026), Shanghai Pujiang Program (09PJ1402900) and the Fundamental Research Funds for the Central Universities (WB0914036). References [1] S.A. Cheng, H. Liu, B.E. Logan, Environ. Sci. Technol. 40 (2006) 364. [2] J.C. Biffinger, J. Pietron, R. Ray, B. Little, B.R. Ringeisen, Biosens. Bioelectron. 22 (2007) 1672. [3] B. Erable, I. Vandecandelaere, M. Faimali, M. Delia, L. Etcheverry, P. Vandamme, A. Bergel, Bioelectrochem. 78 (2010) 51–56. [4] B. Virdis, K. Rabaey, Z.G. Yuan, J. Keller, Water Res. 42 (2008) 3013. [5] O. Hasvold, H. Henriksen, E. Melvaer, G. Citi, B.O. Johansen, T. Kjonigsen, R. Galetti, J. Power Sources 65 (1997) 253. [6] A. Bergel, D. Féron, A. Mollica, Electrochem. Commun. 7 (2005) 900. [7] C. Dumas, A. Mollica, D. Féron, R. Basseguy, L. Etcheverry, A. Bergel, Electrochim. Acta 53 (2007) 468. [8] Z. He, H.B. Shao, L.T. Angenent, Biosens. Bioelectron. 22 (2007) 3252. [9] S. Freguia, K. Rabaey, Z.G. Yuan, J. Keller, Water Res. 42 (2008) 1387. [10] Z. He, L.T. Angenent, Electroanalysis 18 (2006) 2009. [11] P. Clauwaert, D. van der Ha, W. Verstraete, Biotechnol. Lett. 30 (2008) 1947. [12] A. Rhoads, H. Beyenal, Z. Lewandowski, Environ. Sci. Technol. 39 (2005) 4666. [13] P. Clauwaert, D. van der Ha, N. Boon, K. Verbeken, M. Verhaege, K. Rabaey, W. Verstraete, Environ. Sci. Technol. 41 (2007) 7564. [14] A. Lopez-Lopez, E. Exposito, J. Anton, F. Rodriguez-Valera, A. Aldaz, Biotechnol. Bioeng. 63 (1999) 79. [15] P. Aeltermana, M. Versichelea, E. Genettello, K. Verbekenb, W. Verstraete, Electrochim. Acta 54 (2009) 5754. [16] Y. Mao, L. Cai, L. Zhang, H. Hou, G. Huang, Y. Liu, Prog. Chem. 21 (2009) 1672. [17] G. Khoe, H.Z. Myint, US Patent 6,558,556 (2003). [18] Z. Tang, X. Geng, Z. Wang, J. Xun, Chin. J. Appl. Chem. 19 (2002) 936. [19] K. Rabaey, W. Ossieur, M. Verhaege, W. Verstraete, Water Sci. Technol. 52 (2005) 515. [20] B.E. Logan, B. Hamelers, R. Rozendal, U. Schrder, J. Keller, S. Freguia, P. Aelterman, W. Verstraete, K. Rabaey, Environ. Sci. Technol. 40 (2006) 5181. [21] B.E. Logan, Microbial Fuel Cells [M], Wiley, 2007, p. 50. [22] D. Li, J. Zhang, H. Wang, L. Chen, H. Wang, J. Water Supply: Res. Technol. 55 (2006) 313. [23] H. Liu, A.S. Cheng, E.B. Logan, Environ. Sci. Technol. 39 (2005) 658. [24] K. Rabaey, P. Clauwaert, P. Aelterman, W. Verstraete, Environ. Sci. Technol. 39 (2005) 8077. [25] S.J. You, Q.L. Zhao, J.N. Zhang, J.Q. Jiang, C.L. Wan, M.A. Du, S.Q. Zhao, J. Power Sources 173 (2007) 172. [26] G.W. Chen, S.J. Choi, T.H. Lee, G.Y. Lee, J.H. Cha, C.W. Kim, Appl. Microbiol. Biotechnol. 79 (2008) 379.