Environ. Sci. Technol. 2008, 42, 8095–8100
An MEC-MFC-Coupled System for Biohydrogen Production from Acetate MIN SUN,† GUO-PING SHENG,† LEI ZHANG,‡ CHANG-RONG XIA,‡ ZHE-XUAN MU,§ XIAN-WEI LIU,† H U A - L I N W A N G , § H A N - Q I N G Y U , * ,† RONG QI,| TAO YU,| AND MIN YANG| Departments of Chemistry and Materials Science & Engineering, University of Science & Technology of China, Hefei, 230026 China, School of Chemical Engineering, Hefei University of Technology, Hefei, 230092 China, and State Key Laboratory of Environmental Aquatic Chemistry, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing, 100085 China
Received June 2, 2008. Revised manuscript received August 12, 2008. Accepted August 13, 2008.
Microbial fuel cells (MFCs) are devices that use bacteria as the catalysts to oxidize organic and inorganic matter and generate current, whereas microbial electrolysis cells (MECs) are a reactor for biohydrogen production by combining MFC and electrolysis. In an MEC, an external voltage must be applied to overcome the thermodynamic barrier. Here we report an MECMFC-coupled system for biohydrogen production from acetate, in which hydrogen was produced in an MEC and the extra power was supplied by an MFC. In this coupled system, hydrogen was produced from acetate without external electric power supply. At 10 mM of phosphate buffer, the hydrogen production rate reached 2.2 ( 0.2 mL L-1 d-1, the cathodic hydrogen recovery (RH2) and overall systemic Coulombic efficiency (CEsys) were 88∼96% and 28∼33%, respectively, and the overall systemic hydrogen yield (YsysH2) peaked at 1.21 molH2 mol-acetate-1. The hydrogen production was elevated by increasing the phosphate buffer concentration, and the highest hydrogen production rate of 14.9 ( 0.4 mL L-1 d-1 and YsysH2 of 1.60 ( 0.08 mol-H2 mol-acetate-1 were achieved at 100 mM of phosphate buffer. The performance of the MEC and the MFC was influenced by each other. This MEC-MFC-coupled system has a potential for biohydrogen production from wastes, and provides an effective way for in situ utilization of the power generated from MFCs.
Materials and Methods
Introduction Hydrogen is recognized as an impermanent renewable energy carrier. Its advantages are numerous: clean, efficient, renewable, and no toxic byproduct. Electrolysis has been used as an efficient way for high-purity hydrogen generation. Electrolysis is usually conducted in alkaline electrolyzer and an * Corresponding author fax: +086-551-3601592; e-mail: hqyu@ ustc.edu.cn. † Department of Chemistry, University of Science and Technology of China. ‡ Department of Materials Science and Engineering, University of Science and Technology of China. § Hefei University of Technology. | Chinese Academy of Sciences. 10.1021/es801513c CCC: $40.75
Published on Web 10/08/2008
extra voltage of 1.8∼2.0 V is put on the electrodes (1). Hydrogen production costs by electrolysis depend mainly on the electricity costs. Since two-thirds of hydrogen production costs are associated with electricity, lowering the needed electricity is essential to electrolysis (2). Microbial electrohydrogenesis has been developed for biohydrogen production from biodegradable wastes (3-10). In this process, the electrolysis and microbial fuel cell (MFC) are combined to accomplish direct conversion of organic materials to hydrogen. An MFC is a device that utilizes microorganisms to capture energy in bioconvertible substrates in the form of electricity. The MFCs have supplied a renewable pathway for producing bioelectricity from organic carbons (11-13). The invention of microbial electrolysis cell (MEC) has expanded the application of MFCs. The MEC is composed of anode and cathode chambers. The exoelectrogens in the anode chamber catalyze the oxidation of organic substances to carbon dioxide via several metabolic reactions. Electrons from these reactions travel through an external circuit and combine with protons migrating through the proton exchange membrane (PEM) to form hydrogen on cathode (3). Hydrogen has been successfully produced from cellulose, glucose, acetate, butyrate, lactate, propionate, and valerate in MECs (6). Theoretically, an applied voltage of 0.14 V is required for hydrogen production through the microbial electrolysis of acetate. Because of overpotentials at the electrodes, a voltage of around 0.22 V should be applied (4). Usually, a voltage of 0.6 V or more was used for a highefficiency hydrogen production (6, 9). Although the electricity supply in MECs is lower than that for alkaline electrolysis, the energy consumption is still high. Thus, reducing electricity supply is one of the key issues for the development and application of efficient and cost-effective MECs (14). Here, we report a modified microbial electrolysis system for biohydrogen production, on the base of the conventional microbial electrolysis proposed by Liu et al. (3). This system was composed of a coupled MEC and MFC: the electrolysis was performed in the MEC designed according to Liu et al. (3), whereas the extra electricity for the electrolysis was supplied by the MFC with an air cathode. This MEC-MFCcoupled biohydrogen-producing system was developed through taking in situ utilization of the power generated from an MFC into account. Since the open circuit voltage of an MFC could reach as high as 0.80 V (15), the extra energy needed for an MEC can be supplied by an MFC. In such an MEC-MFC-coupled system, hydrogen was entirely harvested from substrate in the microbial cells, and the external power supply was saved. Our experimental results demonstrate that this MEC-MFC-coupled system has a potential to produce biohydrogen from wastes.
2008 American Chemical Society
Reactor Construction and Operation. The system was composed of one MEC for hydrogen production and one MFC for extra power supply (Supporting Information, Figure S1). The hydrogen-producing MEC was a two-chamber configuration with the anode and cathode each in a 450 mL chamber separated by a PEM (GEFC-10N, GEFC Co., China). The cathode electrode was made of carbon paper with Pt on it (4 × 4 cm, 2 mg cm-2; purchased from GEFC Co., China), unless otherwise mentioned. The anode was 4 × 4 cm plain carbon paper without wet proof (GEFC Co., China). The two chambers were purged with N2 and then sealed. The electricity-assisting MFC was a one-chamber configuration containing a single side tube 2.5 cm from the reactor bottom. The 2 cm long side tube was connected to a separate single VOL. 42, NO. 21, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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tube of 3 cm length. The two tubes had an inner diameter of 3 cm. The PEM and cathode were held at the joint of the tubes. The cathode was carbon paper with Pt on it (2 × 2 cm, 2 mg cm-2), unless otherwise mentioned. The anode was plain carbon paper (3 × 7.5 cm, not wet proofed). Two ports with inner diameter of 1 cm were arranged at each reactor chamber for sampling. The MEC and MFC were connected in series with a 10 Ω resistor to allow the circuit current measurement. The bioanodes were collected from two conventional MFCs with anaerobic sludge as inoculum and acetate as substrate. The two MFCs had been running over six months, resulting in the natural selection of a consortium of exoelectrogens on the carbon paper. The open circuit voltages of both MFCs were measured as 800 mV. The anode chamber of each reactor was filled with 350 mL of autoclaved anode medium containing (in 1 L of phosphate buffer, pH 7.0): NaAc, 100 mg; NH4Cl, 310 mg; KCl, 130 mg; CaCl2, 10 mg; MgCl2 · 6H2O, 20 mg; NaCl, 2 mg; FeCl2, 5 mg; CoCl2 · 2H2O, 1 mg; MnCl2 · 4H2O, 1 mg; AlCl3, 0.5 mg; (NH4)6Mo7O24, 3 mg; H3BO3, 1 mg; NiCl2 · 6H2O, 0.1 mg; CuSO4 · 5H2O, 1 mg; ZnCl2, 1 mg. The cathode chamber of the MEC was filled with 350 mL autoclaved phosphate buffer of pH 7.0. The concentration of phosphate buffer was 10 mM, except when the phosphate buffers of 50 and 100 mM were used to investigate the effect of solution ionic conductivity on system performance. All batch tests were conducted in duplicate at 30 °C. Analysis and Calculation. The circuit current was calculated from the voltage across the 10 Ω resistor. The resistor voltage was continuously recorded with a potentiostat (660C, CH Instruments, Inc., U.S.) connected to a computer. The current density was calculated based on anode surface of MEC (32 cm-2), with the average currents measured at time intervals of 4 h. The input voltage of MEC was measured using a multimeter with a data acquisition system (UT39A, UNIT Inc., China). Acetate in the solution was measured using a gas chromatograph (model 6890NT, Agilent Inc., U.S.) equipped with a flame ionization detector and a 30 m × 0.25 mm × 0.25 µm fused-silica capillary column (DP-FFAP). The injection volume was 1 µL. The H2 volume was analyzed using another gas chromatograph (model SP-6800A, Lunan Co., China) equipped with a thermal conductivity detector and a 1.5 m stainless-steel column packed with 5 Å molecular sieves (16). The gas was sampled using a gastight syringe of 1 mL (SGE Syringe). The hydrogen amount in 1 mL gas sample was converted to hydrogen volume under standard conditions and was then multiplied by headspace volume of MEC cathode chamber of 100 mL to get the total hydrogen volume. System performance was evaluated in terms of volumetric hydrogen production rate based on the total MEC volume, cathodic hydrogen recovery (RH2), Coulombic efficiency (CE), and hydrogen yield (YH2). RH2 is calculated with the following equation: RH2 )
nH2 nTh
(1)
where nH2 is the moles of hydrogen harvested and nTh is the moles of hydrogen that could be calculated as follows: nH2 ) nTh )
VH2 RT
∫
t
t)0
(2)
Idt
2F
(3)
where I is the circuit current, VH2 is the measured hydrogen volume, R is the gas constant (0.08206 atm L mol-1 K-1), T 8096
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is the absolute temperature (303 K), and F is Faraday’s constant (96,485 C/electron equivalent). The Coulombic efficiency (CEMEC) and hydrogen yield (YMECH2, mol-H2 mol-acetate-1) of MEC are calculated based on the acetate consumption in the MEC, and the Coulombic efficiency (CEMFC) of the MFC was calculated on the acetate consumption in the MFC. The overall systemic Coulombic efficiency (CEsys) and overall systemic hydrogen yield (YsysH2) were calculated on the overall acetate consumption in this coupled system: CEMEC )
nTh 4VL∆CMEC ⁄ MA
(4)
CEMFC )
nTh 4VL∆CMFC ⁄ MA
(5)
nTh 4VL(∆CMEC + ∆CMFC) ⁄ MA
(6)
CEsys )
YMECH2 ) YsysH2 )
nH2 VL∆CMEC ⁄ MA nH2
VL(∆CMEC + ∆CMFC) ⁄ MA
(7)
(8)
where MA is the molecular weight of acetate, VL is the volume of liquid in the anode chamber (350 mL), and the ∆CMEC and ∆CMFC are the decreases in acetate concentration in the MEC and the MFC. The CEs are calculated based on the assumption that 8 moles of electrons are produced from 1 mol of acetate (3).
Results Hydrogen Production and System Performance. Hydrogen was continuously produced in the cathode chamber of the MEC, whereas no hydrogen was detected on the anode. As shown in Figure 1, the average hydrogen-producing rate reached 2.2 ( 0.2 mL L-1 d-1 during the initial 7 days, and then this rate slightly declined. At the end of the batch test, the hydrogen concentration in the headspace reached 15%. The RH2 was 88∼96%, indicating the high efficiency of the cathodic hydrogen recovery from current. The CEMEC and CEMFC were 53∼64% and 49∼75%, respectively, whereas the overall CEsys was only 28∼33%. The YMECH2 and YsysH2 were 2.03∼2.33 and 0.94∼1.21 mol-H2 mol-acetate-1, respectively. The circuit current densities fluctuated from 43 to 82 mA m-2 (Supporting Information, Figure S2). As shown in Figure 2, the circuit current was immediately generated with an initial maximum current of 0.25 mA in 10 h. Thereafter, the current fluctuated and slowly declined to 0.21 mA in the subsequent 150 h. Then, the current declined more rapidly to 0.09 mA over the next 115 h (160-272 h), attributed to the acetate depletion. Finally, the current reduced substantially to a negative value, implying the substrate exhaustion. As shown in Figure 3, acetate was consumed at similar rates in both MEC and MFC. For example, on day seventh the acetate concentration reduced to 32.0 ( 2.0 mg L-1 in the MEC, and 33.2 ( 0.8 mg L-1 in the MFC. Acetate became completely exhausted after 11.5-day operation. The scanning electron microscope (SEM) and denaturing gradient gel electroporesis (DGGE) analyses were carried out to compare the exoelectrogens in both MEC and MFC. The SEM images (Supporting Information, Figure S3) show that the microorganisms colonized on the electrode surface in both MEC and MFC and displayed mainly in rods. The DGGE bands in the two cells are in the same positions (Supporting Information, Figure S4). The B1, B2, B3, and B4 are more prevalent than other bands in both MEC and MFC. These
FIGURE 1. Performance of the coupled system at 10 mM of phosphate buffer: (A) hydrogen volume (calculated at 1 atm and 30 °C); (B) cathodic hydrogen recovery (RH2); (C) Coulombic efficiency (CE); and (D) hydrogen yield (YH2). Error bars are based on the duplicate batch experiments.
FIGURE 2. Current profiles of the coupled system at 10 mM of phosphate buffer in a batch test. observations demonstrate that there was no significant difference in bacterial community on the MEC and the MFC anodes. System Performance at Different Phosphate Buffer Concentrations. Table 1 shows the system performance at different phosphate buffer concentrations. No hydrogen was produced if the catalyst Pt was removed from the cathode in the MFC. This is because the low power supply of 5.10 ( 0.00 mW m-2 was not able to overcome the thermodynamic constraints (6). If the Pt catalyst was removed from the cathode in the MEC, hydrogen was produced with a very low rate and efficiency. In this case, the hydrogen-producing rate was 0.3 ( 0.0 mL L-1 d-1 and YsysH2 was 0.20 ( 0.07 mol-H2 mol-acetate-1. A higher ionic strength has been found to enhance the current production in MFCs because it reduces the electrolyte ohmic resistance (17). When the phosphate buffer concen-
FIGURE 3. Evolution of acetate concentrations in the MEC and MFC in a batch test at 10 mM of phosphate buffer. tration in the MEC or MFC was increased to 50 mM and the phosphate buffer concentration in another cell was kept at 10 mM, the hydrogen-producing rates were 5.1 ( 0.1 or 5.5 ( 0.1 mL L-1 d-1, respectively, and the circuit currents were 122 ( 17 and 147 ( 2 mA, respectively, nearly 2 times of those at 10 mM of phosphate buffer. Meanwhile, the CEs and YH2 had no distinct improvement. When the phosphate buffer concentration in both MEC and MFC were increased to 50 mM, the hydrogen-producing rate, CE and YH2 all increased. At 100 mM of phosphate buffer, the system performance was further improved (Table 1). Input Voltage and Power Density of the HydrogenProducing MEC. When the system was operated without acetate in the MEC, the current generation was at a low level of 0.7 mA m-2 anode surface and no hydrogen was produced in the cathode chamber. In this case, the MEC did not work VOL. 42, NO. 21, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Performance of the MEC-MFC-Coupled System in Phosphate Buffer Solutions at Different Concentrations buffer concentration (mM)
hydrogenproducing rate
MEC
MFC (mL L-1 d-1)
10 10 10b 50 10 50 100
10 10a 10 10 50 50 100
2.2 ( 0.2 N
0.3 ( 0.2 5.1 ( 0.1 5.5 ( 0.1 10..9 ( 0.0 14.9 ( 0.4
RH2
YH2 (mol-H2 mol-acetate-1)
CE (%)
(%)
CEMEC
CEMFC
CEsys
94 ( 7 N 47 ( 14 104 ( 2 95 ( 0 96 ( 0 94 ( 0
53 ( 8 N 21 ( 7 53 ( 3 45 ( 3 75 ( 2 86 ( 6
63 ( 11 N 27 ( 11 49 ( 4 56 ( 2 82 ( 3 84 ( 0
30 ( 2 N 11 ( 3 25 ( 2 25 ( 1 39 ( 1 43 ( 2
YMEC
Ysys
1.98 ( 0.14 1.10 ( 0.08 N N 0.36 ( 0.09 0.20 ( 0.07 2.02 ( 0.04 0.95 ( 0.03 1.79 ( 0.00 1.00 ( 0.02 2.88 ( 0.01 1.57 ( 0.01 3.24 ( 0.16 1.60 ( 0.08
current density
input voltage of MEC
input power of MEC
(mA/m2)
(mV)
(mW m-2)
59 ( 13 12 ( 2 15 ( 1 122 ( 17 147 ( 2 285 ( 11 404 ( 2
364 ( 8 254 ( 0 534 ( 18 245 ( 4 429 ( 17 335 ( 13 348 ( 10
21.48 ( 3.61 5.08 ( 0.00 8.01 ( 0.60 29.89 ( 4.19 63.06 ( 2.64 98.04 ( 5.37 140.59 ( 4.10
a Carbon paper without Pt was used as the MEC cathode. b Carbon paper without Pt was used as the MFC cathode. The current density and power input density of the MEC were calculated based on the anode surface area of the MEC (32 cm2).
and it had an input voltage of 702 mV. However, in the presence of acetate in the anode chamber, current was generated and the hydrogen production became significant. As shown in Table 1, the input voltage of the MEC declined simultaneously (Supporting Information, Figure S5, with the input voltage evolution of the system with/without Pt in the MEC cathode as an example). Such a voltage decline was attributed to the activation of the MEC and subsequent total system. The input voltage of the MEC changed slightly with the phosphate buffer concentration. When the MFC was operated in the absence of Pt catalyst or at a lower phosphate buffer concentration, the voltage input of the MEC became relatively lower than the original value. However, the voltage input of the MEC was elevated when the MEC performance became deteriorated. The input power densities of the MEC were calculated by multiplying its input voltages and circuit currents. The MEC power input peaked of 143.02 ( 4.10 mW m-2 at 100 mM of phosphate buffer (Table 1).
Discussion Key factors Governing the Hydrogen-Producing Efficiency. The hydrogen- producing reaction in the MEC is as follows: C2H4O2 + 2H2O f 2CO2 + 4H2 In this reaction 4 mol of hydrogen is produced when 1 mol of acetate is consumed in the MEC with an extra energy input. In our system, the overall systemic hydrogen yield is calculated as 3.04 mol-H2 mol-acetate-1, assuming that the energy in acetate is completely converted to hydrogen (upper heating values of 285.83 kJ mole-H2-1 and 870.28 kJ moleacetate-1) (6). The key factors influencing the hydrogen yield include cathodic hydrogen recovery and Coulombic efficiency. The RH2 value reflects the hydrogen recovery from current and is dependent on proton reduction rate on the cathode, which can be catalyzed by Pt. As shown in Table 1, when the Pt catalyst was removed from the MEC cathode, the RH2 was 25∼65% and the actual YsysH2 was only 6% of the theoretical value. The proton reduction was slow at ambient temperatures in the absence of Pt catalyst. As a result, the electrons might react with other electron acceptors such as oxygen that diffused through sampling ports. In addition, hydrogen might be lost via diffusion through membrane or sampling ports. When the hydrogen production was slow, the hydrogen loss became appreciable and would significantly influence the calculated RH2 (4). In order to achieve a high hydrogen yield, the catalyst Pt has been widely used in cathode to accelerate hydrogen production in MEC studies (3-6, 9). The CE value reflects electrons capture in substrates as current in anode, and can be diminished through anodic competitive processes by alternate electron acceptors (18). The CEs were found to increase by applying a higher 8098
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phosphate buffer concentration or promoting the cathodic efficiency. Such a phenomenon has also been observed in other studies and is explained as the diminution of the competitive anodic reactions via decreasing operation time (17). At 100 mM of phosphate buffer, the highest CEs were achieved, with CEMEC of 86 ( 6%, CEMFC of 84 ( 0%, and CEsys of 43 ( 2%. The YMECH2 and YsysH2 were as high as 81 and 53% of the theoretical values. Even with a high hydrogen yield, the hydrogen-producing rate was still lower, compared with the previous studies. In a previous study (6), the hydrogen-producing rate of 1500 mL L-1 d-1 was achieved when graphite granule anode with a specific area of 1320 m2 m-3 of anodic volume was used at an applied voltage of 0.8 V. Call and Logan (9) acquired a maximum hydrogen-producing rate of 3120 mL L-1 d-1 at 0.8 V in a membrane-less system with a 0.22 m2 graphite fiber brush in a 28 mL reactor. In comparison, even at 100 mM of phosphate buffer, the hydrogen-producing rate in our study was only 14.9 mL L-1 d-1. In a conventional MEC, the hydrogen production can be promoted by applying a higher extra voltage. However, in this coupled system, the extra voltage was supplied by the MFC, thus, the power input was limited to the maximum value that the MFC could supply. Since the RH2 was high in the presence of Pt catalyst, the low hydrogen-producing rate was attributed to the poor anodic performance. The small relative anode surface area was one of the main limiting factors responsible for the anodic performance. A larger electrode accommodates more exoelectrogens. As a result, electron transfer and substrate oxidation can be accelerated. The anodic area was only 7.1 m2 m-3-anodic volume in our study, with a 2 magnitude lower volume than those reported in the previous studies (6, 8). To achieve a higher hydrogen-producing rate, efforts should be made to improve the anodes in both MEC and MFC, e.g., using electrodes with a high surface area. Since the MEC and MFC share many similar characteristics, findings for improving electricity generation on MFCs are expected to be applicable for MECs as well (19). Interactions Between the MEC and MFC in the Coupled System. As shown in Figure 4, in this MEC-MFC-coupled system, four half-reactions are involved: the acetate oxidation in the anodes of the two cells; the oxygen reduction in the MFC cathode; and proton reduction in the MEC cathode. The above four half-reactions are affected by each other and presumed to reach a balance for a stabilized system. Under the balanced conditions, the protons needed for oxygen reduction come from acetate oxidation in the MFC and electrons from the MEC. On the other hand, the protons needed for hydrogen production come from the MEC, while the electrons from the MFC. Therefore, a constant circuit current means that the electron flow from the MFC is the same as that from the MEC.
FIGURE 4. Working principles of the MEC-MFC-coupled system. The CE is an indicator for the MFC ability to recover electrons stored in substrates as electricity (19). Comparison among the CEs under various operating conditions (Table 1) demonstrates that the hydrogen-producing MEC and the electricity-assisting MFC were influenced by each other. Compared with the system at 50 mM of buffer in both cells, the CE of another cell was also reduced at a lower buffer concentration of 10 mM in the MEC or the MFC. In the absence of Pt catalyst poor MFC performance was founded and no hydrogen was produced. When the Pt catalyst was removed from the MEC, both CEMEC and CEMFC declined. These results demonstrate that, if the MEC or the MFC performance becomes deteriorated, another cell in the system will also be depressed. The interactions between the MEC and the MFC can also be interpreted from the input power densities of the MEC. The deteriorated anodic/cathodic performance of the MEC resulted in the declined power efficiency of the MFC. As a consequence, the MEC input power densities decreased. It is proposed that a stable system requires the high efficiencies of four half-reactions, and that any reaction at a low efficiency will have a negative effect on the overall system performance. When the Pt catalyst was removed from the MEC or the MFC, the bottleneck was the proton reduction or oxygen oxidation, resulting in poor system performance. In the systems with different ionic strengths in the MEC and the MFC, the substrate oxidation in the anode at a lower ionic strength governs the system performance. In a conventional MEC, the hydrogen production was in proportional with the MEC input voltage. The current density could be increased through increasing input voltage (19). But, it is a different case for the MEC-MFC-coupled system. The MEC input voltage is mainly controlled by the relative internal resistance of the MEC to the MFC. For the MEC, when its anodic or cathodic reaction is suppressed, the internal resistance increases and thus more voltages are distributed to the MEC. By comparison, the MEC input voltage decreases if the reactions in the MFC are suppressed. Therefore, in the MEC-MFC-coupled system, a high MEC input voltage does not necessarily mean good system performance, but a higher hydrogen production could be achieved at a higher circuit current. Significance of the MEC-MFC-Coupled System. The biocatalyzed electrolysis concept proposed by Liu et al. (3) has opened a new path for biohydrogen recovery from organic wastes. Hydrogen can be produced at ambient temperatures and pressures, and the hydrogen yield is much higher than
that in the fermentation process. However, in the MEC, a considerable amount of electricity is still required. The energy efficiency is reduced because of the relatively high electricity demand in the electrolysis (14). Compared with the conventional biocatalyzed electrolyzer, the MEC-MFCcoupled system have two following advantages: (i) hydrogen is completely harvested from the substrate in an MEC and MFC without extra electricity supply; and (ii) the electric energy of an MFC is in situ utilized, and this saves equipment for electricity storage and diminishes the power loss. To become a matured biohydrogen-producing system, further efforts should be made to improve this coupled system. First of all, the hydrogen production is operated under unstable conditions, resulting in the fluctuation of the circuit current and hydrogen production in the process. There are many factors influencing the stability of an MFC, such as substrate type and concentration, catalytic activity of the anode microorganisms, and internal resistance, etc. In the conventional MEC, the extra voltage was fixed, thus the system was relatively stable. In the coupled system, the MEC and MFC have an influence on each other, thus, even a slight change of these related factors may cause instability of the system. Therefore, efforts should be pursued to increase the long-term stability of MFCs/MECs. Second, any measures aimed at increasing the anodic performance of MEC/MFC, such as new designs of configurations (20-23) and modification of electrodes and membrane (24-26), are expected to enhance the system performance. Finally, optimization of the process parameters should be made to elevate the hydrogen production. In summary, the MEC-MFC-coupled system described in this paper was able to save the extra electricity supply for biohydrogen production. Our experimental results demonstrate that, for the first time, the power generated from an MFC could be utilized for hydrogen production in an MEC in situ. Extensive studies are required for make this technique more effective and applicable.
Acknowledgments We thank the CAS (KSCX2-YW-G-001), the NSFC (50625825), the NSFC-JST Joint Project (20610002), the National Basic Research Program of China (2004CB719602), and the National 863 Program of China (2006AA06Z340) for the partial support of this study.
Supporting Information Available This material is available free of charge via the Internet at http://pubs.acs.org.
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(18) Logan, B. E.; Hamelers, B.; Rozendal, R.; Schroder, U.; Keller, J.; Freguia, S.; Aelterman, P.; Verstraete, W.; Rabaey, K. Microbial fuel cells: methodology and technology. Environ. Sci. Technol. 2006, 40, 5181–5192. (19) Logan, B. E., Microbial Fuel Cells, Vol 127; Wiley: New York, 2007; pp 48. (20) Schroder, U.; Niessen, J.; Scholz, F. A. A generation of microbial fuel cells with current outputs boosted by more than one order of magnitude. Angew. Chem. 2003, 115, 2986–2989. (21) Aelterman, P.; Rabaey, K.; Pham, H. T.; Boon, N.; Verstraete, W. Continuous electricity generation at high voltages and currents using stacked microbial fuel cells. Environ. Sci. Technol. 2006, 40, 3388–3394. (22) Freguia, S.; Rabaey, K.; Yuan, Z. G.; Keller, J. Sequential anodecathode configuraiton improves cathodic oxygen reduction and effluent quality of microbial fuel cells. Water. Res. 2008, 42, 1387–1396. (23) Cheng, S.; Liu, H.; Logan, B. E. Power densities using different cathode catalysts (Pt and CoTMPP) and polymer binders (Nafion and PTFE) in single chamber microbial fuel cells. Environ. Sci. Technol. 2006, 40, 364–369. (24) Bond, D. R.; Lovely, D. R. Electricity production by Geobacter sulfurreducens attached to electrodes. Appl. Environ. Microbial. 2003, 69, 1548–1555. (25) Logan, B. E.; Cheng, S.; Watson, V.; Estadt, G. Graphite fiber brush anodes for increased power production in air-cathode microbial fuel cells. Environ. Sci. Technol. 2007, 41, 3341–3346. (26) Kim, J. R.; Cheng, S.; Oh, S. E.; Logan, B. E. Power generation using different cation, anion, and ultrafiltration membranes in microbial fuel cells. Environ. Sci. Technol. 2007, 41, 1004–1009.
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