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Shift of voltage reversal in stacked microbial fuel cells
Journal of Power Sources 278 (2015) 534e539
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Shift of voltage reversal in stacked microbial fuel cells Junyeong An a, Bongkyu Kim b, In Seop Chang b, Hyung-Sool Lee a, * a
Department of Civil and Environmental Engineering, University of Waterloo, 200 University Avenue West, Waterloo, Ontario, N2L 3G1, Canada School of Environmental Science and Engineering, Gwangju Institute of Science and Technology (GIST), 261 Cheomdan-gwagiro, Buk-gu, Gwangju, 500-712, Republic of Korea
b
h i g h l i g h t s Voltage reversal occurred in the stacked MFC 1 having sluggish cathode reaction rate. No voltage reversal was observed in the stacked MFC 2 having faster kinetics on the cathode. Change in reaction rates on the cathode shifted voltage reversal between two stacked units. Voltage reversal can be shifted from the anode to the cathode in stacked MFCs.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 November 2014 Received in revised form 5 December 2014 Accepted 22 December 2014 Available online 24 December 2014
We proved that sluggish kinetics on the cathode and the imbalance of cathode kinetics cause voltage reversal in a stacked microbial fuel cell (MFC) equipped with a non-Pt cathode. Catholyte aeration to a unit MFC against passive air diffusion to the cathode in the other unit MFC shifted voltage reversal between the two units, due to improved mass transport and O2 concentration effects in the aerated MFC. The shifted voltage reversal returned to an original status when catholyte aeration was stopped. A Ptcoated cathode increased the rate of oxygen reduction reaction (ORR) by a factor of ~20, as compared to the non-Pt cathode. As a result, the anodic reaction rate that became slower than the rate on the Ptcathode limited current density to overpotential in the stacked MFC equipped with the Pt-cathode. This work shows that dominant kinetic bottlenecks, which are the primary cause of voltage reversal, can be shifted between individual MFCs of stacked MFCs or electrodes depending on relative kinetics. © 2014 Elsevier B.V. All rights reserved.
Keywords: Voltage reversal Sluggish reaction rate Voltage reversal shift Stacked microbial fuel cells Overpotential
1. Introduction Many researchers have attempted to increase voltage by stacking MFCs for their application to energy-efficient wastewater treatment, bio-sensors, and power supplies to small electronic devices [1e3]. Stacking MFCs in series can directly boost voltage and power, but often causes voltage reversal [4e12]. Literature suggests that voltage reversal in stacked MFCs occurs due to significant anode polarization triggered by substrate depletion, the change of microbial community in biofilm anode, or other heterogeneous internal resistance factors [5e12]. The cathode reaction rate in the MFCs that employ efficient noble-metal catalysts (e.g., platinum) or strong liquid oxidants (e.g., potassium ferricyanide) is much faster than the anode reaction rate [5,9,10,13,14]. This phenomenon has resulted in some researchers predicting that
* Corresponding author. E-mail address: [email protected] (H.-S. Lee). http://dx.doi.org/10.1016/j.jpowsour.2014.12.112 0378-7753/© 2014 Elsevier B.V. All rights reserved.
significant anode overpotential leads to voltage reversal in stacked MFCs. Interestingly, the energy loss on the cathode is typically larger than that on the anode in MFCs. Cathode overpotential is as large as 300e500 mV at the maximum current density (0.12e3 mA cm2) in MFCs, while anode overpotential is relatively small at 100e270 mV [15e17]. This substantial cathode energy loss contracts to small cathode overpotentials (100e450 mV) at the extremely high current density of 400e1500 mA cm2 in chemical fuel cells equipped with a 0.1e0.3 mg Pt-cathode similar to that used in MFCs [18,19]. Mass transport limitations and concentration gradients throughout the cathode (hydroxyl ions or O2) that are significant for MFCs can lead to large cathode overpotential even at small current density in MFCs [17,20]. This unique feature of large cathode overpotential relative to small anode overpotential in MFCs implies that slow cathode kinetics would lead to voltage reversal in stacked MFCs. An and Lee [7] recently proved that the imbalance in anode
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kinetics over relatively constant kinetics on the cathode causes voltage reversal in stacked MFCs. They suggested that higher heterogeneity on the anode than on the cathode could readily lead to voltage reversal in stacked MFCs, although different cathode kinetics can drive voltage reversal. It is true that cathodic conditions such as relatively uniform catalysts and a single electron acceptor are homogeneous over the anode. However, heterogeneous features of the cathode cannot be avoided in MFCs. For instance, passive air supply to the cathode lacking intensive mixing will increase heterogeneity in mass transport to the cathode, resulting for instance, in different O2 concentration gradients throughout cathodes for individual MFCs. Heterogeneous mass-transport for protons or hydroxyl ions in the waterecathode interface and the area between the interface and a separator will be substantial due to insufficient advection in air-cathode MFCs [17,20]. This heterogeneous mass transport related to cathodic reaction could limit cathode kinetics, causing voltage reversal in stacked MFCs. Cathode-driven voltage reversal seems to complicate the operation of steady-state stacked MFCs because both anodic and cathodic kinetics can lead to voltage reversal, as suggested by An and Lee [7]. Hence, voltage reversal will readily occur in stacked MFCs caused by relatively sluggish or imbalanced electrode kinetics. This interpretation implies that voltage reversal can be switched between electrodes or individual units in stacked MFCs, exacerbating the performance of stacked MFCs. There are no studies on the voltage reversal caused by cathode kinetics and shift of voltage reversal in stacked MFCs, although understanding them is significant for the success of stacked MFCs. The goals of this work are three-fold. The first is to confirm the phenomenon of voltage reversal led by sluggish cathode reaction rates in a stacked MFC. The second is to further support cathodedriven voltage reversal by showing the shift of voltage reversal between two individual units of the stacked MFC. The final is to demonstrate that sluggish electrode kinetics mainly leads to voltage reversal in the stacked MFC. 2. Materials and methods 2.1. MFC configuration and inoculation Two identical sandwich type MFCs (denoted as MFC 1 and MFC 2) were fabricated with Pyrex glass in the Waterloo Engineering Machine Shop at the University of Waterloo. The MFCs consist of an anode and a cathode chamber, and the working volume of each chamber is 29 mL. Non-Pt carbon cloth (CCP40, Fuel Cell Earth, USA) was used for the anode in the MFCs. Pt/C-coated carbon cloth (3 mg cm2 40% Pt, Carbon Cloth Electrode EC34019-2, Fuel Cell Earth, USA) and non-Pt carbon cloth (CCP40, Fuel Cell Earth, USA) were tested for the cathode; two cathodes were compared to prove the shift of voltage reversal in a serially stacked MFC. The projected surface areas of the anode and cathode were identical at 9.6 cm2. Cation exchange membrane (CMI-7000, Membrane International Inc. USA) was positioned between the electrodes in the MFCs. To prevent solution leakage, a rubber gasket manually cut with a sharp blade was inserted between each electrode and chamber. An Ag/ AgCl reference electrode (BAS, MF-2052, USA) was placed at 2 mm distance from the anode in order to monitor electrode potential. MFC 1 and MFC 2 equipped with non-Pt carbon cloth were denoted as MFC-woPt 1 and MFC-woPt 2. In comparison, MFC-wPt indicates the MFC equipped with Pt/C-coated cathode. MFC-woPt 1 and MFC-woPt 2 were inoculated with recycle activated sludge (29 mL) sampled from a wastewater treatment plant (Waterloo, ON, Canada). Tap water was added to the cathode chambers as catholyte. Air was passively provided through a hole of 8 mm diameter on the top of the cathode chambers, which kept the
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concentration of dissolved oxygen (DO) at ~5.42 mg L1 in catholyte during our experiments. We used an external resistor of 120 U during ARB acclimation and fed acetate medium (25 mM acetate) to anode chambers at a flow rate of 0.06 ml min1 using a peristaltic pump (Masterflex, Model 7523-80, USA) over ~3 months; hydraulic retention time (HRT) was fixed at 8.1 h in the anode chambers. The tap water in the cathode was replaced daily with a syringe to compensate for evaporated water, and the pH in the cathode ranged from 7.0 to 7.3. 2.2. Polarization tests for individual and stacked MFCs using non-Pt cathode Polarization tests for MFC-woPt 1 and MFC-woPt 2 were conducted under non-stacked and stacked modes, to prove that voltage reversal can occur due to sluggish cathode reaction rates. Throughout this paper MFC-woPt 1 and -woPt 2 under non-stacked mode were referred to as non-stacked MFC-woPt 1 and MFC-woPt 2 (see Fig. S1(a) in Supplementary material). For stacked mode, MFC-woPt 1 and -woPt 2 connected in series were designated as stacked MFC-woPt, and individual units were denoted as stacked MFC-woPt 1 and stacked MFC-woPt 2, respectively (see Fig. S1(b)). Prior to the polarization tests, individual MFCs connected with an external resister of 120 U were converted to open circuit mode until OCVs in the MFCs became constant. Then, external resistances were sequentially changed from 518 kU to 56.1 U every 2 min. Closed circuit voltage (CCV) in each MFC was monitored using a multimeter (Keithly 2700, Keithley Instruments, Inc. USA) connected to a personal computer. After the polarization tests for non-stacked mode, the two MFCs were fully recharged in open circuit mode and were serially connected for polarization experiments in stacked mode. Stacked-mode polarization tests were carried out in the same manner as with non-stacked MFCs. The polarization tests for non-stacked and stacked mode were conducted in duplicate (see Supplementary material for duplicate test results). External resistances were changed from 581 kU to 56.1 U every 2 min. CCV in stacked mode was monitored between the cathode of stacked MFCwoPt 1 and the anode of stacked MFC-woPt 2, as illustrated in Fig. S1(b). Current was calculated with Ohm's law, i.e., I ¼ Rext/V (I: current (A), Rext: external resistance (U), and V is voltage (V)). In addition, V-current density (j) curves and potential (P)-j curves were built to analyse voltage reversal phenomena for stacked MFCwoPt. 2.3. Aeration tests for the cathode of the MFC-woPt 1 and MFCwoPt 2 under stacked mode: shift of voltage reversal between individual units Aeration tests were carried out for the cathode of stacked MFCwoPt 1 and MFC-woPt 2, respectively, to investigate voltage reversal behaviours in response to different cathode reaction rates: shift of voltage reversal between individual MFC units. Aeration in the cathode can increase dissolved oxygen (DO) concentration, and mass transport (i.e., advection) in catholyte. Both stacked MFC-woPt 1 and 2 were connected with 1.4 U of external resistor, and the voltages in the two MFCs were 13.4 and 14.9 mV, respectively. We aerated the cathode chamber of stacked MFC-woPt 1 using an air blower for 7 min to saturate DO concentration ~6.7 mg L1 in catholyte (see Supplementary material for detailed information on DO measurement). In comparison, air passively diffused to catholyte in stacked MFC-woPt 2 and DO concentration was saturated at ~5.42 mg L1 in the catholyte. Catholyte aeration for stacked MFC-woPt 1 was stopped, while air diffusion to the cathode of stacked MFC-woPt 2 was kept. We then aerated the cathode of stacked MFC-woPt 2 for 7 min in comparison
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to the air-diffusion cathode for stacked MFC-woPt 1. Voltage and electrode potentials were monitored for individual MFCs and Stacked MFC-woPt (see Fig. S1(b)). 2.4. Polarization tests for stacked MFCs using Pt-coated cathode: shift of voltage reversal from cathode to anode To further investigate the shift of voltage reversal caused by different reaction rates on cathodes, we performed polarization tests for stacked MFCs using Pt-coated cathodes: shift of voltage reversal from cathode to anode. For the experiments, we replaced non-Pt cathodes with Pt/C-coated cathodes for two individual MFCs in an anaerobic chamber to protect ARB from oxygen molecules, denoted as MFC-wPt 1 and -wPt 2. Then, we conducted the polarization tests in stacked mode in the identical manner to stacked MFC-woPt. The experiments were conducted in duplicate (see Supplementary material). In this paper, stacked MFC-wPt 1 and -wPt 2 represents the stacked units (i.e., MFC-wPt 1 and -wPt 2) of stacked MFC-wPt. 2.5. Estimation of exchange current density for cathodes To compare ORR kinetics between non-Pt and Pt/C cathodes, we quantified exchange current density (jo) from Tafel plots. Tafel plots were obtained with linear sweep voltammetry (LSV) tests with a three-electrode cell in triplicate (anodes: counter electrodes, cathodes: working electrodes, Ag/AgCl electrode: reference electrodes). For LSV tests, each electrode (9 cm2 non-Pt carbon cloth, 9 cm2 Pt/C-coated carbon cloth) was placed in a beaker having 48 mL tap water (pH: ~7e7.3, dissolved oxygen concentration: ~5.42 mg L1, temperature: ~24.5 C). Cathode potential was swept
at a scan rate of 1 mV s1 from OCP (130 mV for non-Pt carbon cloth, 615 mV for Pt carbon cloth vs. Ag/AgCl) to 370 mV for nonPt cathode and 115 mV for Pt cathode versus Ag/AgCl. Then, jo for the two cathodes was estimated from Tafel slops. 3. Results and discussion 3.1. Performance evaluation of non-stacked MFC-woPt 1 and MFCwoPt 2 Fig. 1(a) and (b) present voltageecurrent density (V-j) and potential-current density (P-j) curves obtained from polarization tests with the individual non-stacked MFCs. The OCVs in nonstacked MFC-woPt 1 and MFC-woPt 2 were 524 mV and 490 mV, respectively. Substantial overvoltage of 497 mV was observed for non-stacked MFC-woPt 1 at the maximum current density of 49.5 uA cm2. A large cathode overpotential of 447 mV was mainly responsible for the overvoltage. Significant overvoltage and cathode overpotential were also found in non-stacked MFC-woPt 2 (see Fig. 1(b)). In comparison, anode overpotentials were as small as 15e25 mV for the two MFCs. Due to the heterogenuity in the cathodes, we could not identify the reasons for different performance of the two identifical MFCs. However, experimental results indicate that sluggish ORR kinetics on the non-Pt cathodes limit MFC performance in our study, due to the lack of efficient metal catalysts on the cathodes. In addition, cathodic reactions would be more heterogeneous than anodic reactions in the MFCs where air is passively diffused to the cathode, based on the larger difference of cathode overpotential than that of anode overpotential between the two MFCs. 3.2. Voltage reversal in the stacked MFC equipped with non-Pt cathode Fig. 2(a) shows the evolutions of voltage and electrode potentials against current density in two individual MFCs of stacked
Fig. 1. The evolutions of voltage and electrode potentials against current density. (a) Vj curves and (b) P-j curves for the non-stacked MFC-woPt 1 and MFC-woPt 2.
Fig. 2. Change of voltage and electrode potentials against current density. (a) Voltage and current density for different external resistors in the stacked MFC-woPt 1, the stacked MFC-woPt 2, and Stacked MFC-woPt. (b) Electrode potentials against current density in the stacked MFC-woPt 1 and MFC-woPt 2.
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MFC-woPt (see Fig. S1(b) in SI). When the current density in stacked MFC-woPt reached 44 mA cm2, the voltage in stacked MFC-woPt 1 was discharged to 0 V, but the voltages of stacked MFC-woPt 2 and stacked MFC-woPt were equal at 52 mV. As shown in Fig. 2(b), the cathode overpotentials at the current density of 44 mA cm2 were significant at 489 mV and 429 mV, respectively, for stacked MFC-woPt 1 and MFC-woPt 2. In comparison, the anode overpotentials of the two MFCs were negligible, less than 20 mV. The voltage in stacked MFC-woPt 1 decreased to 16.2 mV (i.e., voltage reversal) at the current density of 44.8 mA cm2, while the voltage in stacked MFC-woPt 2 still remained positive at 41.1 mV without voltage reversal. Small anode overpotential but large cathode overpotential clearly indicates that the inefficient catalytic function of the non-Pt cathode mainly led to voltage reversal in stacked MFC-woPt 1 of stacked MFC-woPt, as shown in Fig. 2(b), although cathodic conditions were identical between the two MFCs (see Experimental Section). Passive oxygen transfer to catholytes without mixing could lead to heterogeneity in the ORR reaction on the cathode in the two MFCs, such as local concentrations of DO, hydroxyl ions, or protons throughout a boundary layer between bulk liquid and the cathode [17,19]. To confirm voltage reversal dominantly caused by ORR kinetics on the cathode, we conducted catholyte aeration tests.
Fig. 3. Voltage reversal shift driven by catholyte aeration. (a) voltage evolution in the stacked MFC-woPt 1, the stacked MFC-woPt 2, and Stacked MFC-woPt before and after catholyte aeration. (b) change of electrode potential with and without aeration in the three MFCs. (i): no aeration, (ii): aeration in the cathode of the stacked MFC-woPt 1, (iii): aeration stop in the stacked MFC-woPt 1, and (iv): aeration in the cathode of the stacked MFC-woPt 2.
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3.3. Voltage reversal shift between individual MFCs: effect of catholyte aeration Fig. 3(a) presents the voltage evolution in stacked MFC-woPt 1, stacked MFC-woPt 2, and stacked MFC-woPt at Rext 1.4 U (current density of ~44.5 mA cm2) with and without aeration in the cathodes. At stage (i) (passive aeration condition), the negative voltage of 13.4 mV was kept for stacked MFC-woPt 1, while voltage was steady at 14.9 mV for stacked MFC-woPt 2 (see Fig. 3(a)). At stage (ii) where the catholyte of stacked MFC-woPt 1 was aerated, the negative voltage was shifted to 76.3 mV in stacked MFC-woPt 1. Interestingly, the voltage of stacked MFC-woPt 2 was reversed from 14.9 mV to 74.3 mV: the shift of voltage reversal from stacked MFC-woPt 1 to MFC-woPt 2. During stage (ii), the current density increased from 44.5 to 59.0 mA cm2 in stacked MFC-woPt, probably due to improved DO concentration and mass transport in the cathode of stacked MFC-woPt 1. For instance, the DO concentration in the cathode increased from 5.42 to 6.70 mg L1. Once catholyte aeration in the MFC-woPt 1 was stopped at stage (iii), the shifted voltage reversal returned to the initial condition at stage (i): 13.8 mV in stacked MFC-woPt 1 (voltage reversal) and 14.9 mV in stacked MFC-woPt 2. At stage (iv), we aerated the catholyte of stacked MFC-woPt 2, and the voltage reversal was exacerbated in stacked MFC-woPt 1 (from 13.8 mV to 155 mV). The current density in the stacked MFC increased from 46.8 to 68.5 mA/cm2, probably due to the increased DO concentration and mass transport. Fig. 3(b) illustrates the evolutions of electrode potential in the two individual MFCs of stacked MFC-woPt. The change of anode potential is very small at 9e13 mV for the two MFCs during aeration tests (from stage (i) to (iv)), while cathode potential was significantly evolved at 100e150 mV in response to catholyte aeration. These results support that voltage reversal can move from one MFC to another MFC depending on electrode kinetics in individual units of stacked MFCs. Our study proves the shift of voltage reversal between unit MFCs by manipulating cathode kinetics.
Fig. 4. V-j or P-j curves for stacked MFC-wPt 1, MFC-wPt 2, and Stacked MFC-wPt.
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3.4. Voltage reversal by sluggish electrode kinetics: from cathode to anode Fig. 4 shows the evolution of voltages and electrode potentials against current density for two individual MFCs equipped with Pt/Ccoated cathodes, referred to as stacked MFC-wPt 1, stacked MFC-wPt 2, and stacked MFC-wPt (see Fig. S1(b) in SI). The maximum current density of stacked MFC-wPt significantly increased by a factor of 3 (144 mA cm2) as compared to that of stacked MFC-woPt (see Fig. 2 and 4). At the maximum current density, the cathode overpotentials were not large in stacked MFC-wPt, showing 214 mV and 98 mV for stacked MFC-wPt 1 and MFC-wPt 2, respectively, due to improvement of ORR kinetics on Pt-cathode. Again, the heterogenuity in aircathode MFCs would lead to different cathode performance in the two identifical MFCs. Fig. 5 shows the exchange current densities for non-Pt and Pt/C-coated cathodes, which were estimated at 6.4 104 and 1.38 102 mA cm2, respectively. ORR kinetic on the Pt-cathode is ~20 times faster than the non-Pt carbon cloth. Instead of small cathode overpotentials, the anode overpotentials of the MFC 1 and the MFC 2 significantly increased to 415 and 276 mV, respectively, at the maximum current density (see Fig. 4 and 6). As a result, voltage reversal occurred at stacked MFC-wPt 1 that had large anode overpotential, as shown in Fig. 4. This result clearly shows the shift of voltage reversal from the cathode to the anode when sluggish kinetics (small current increase against large overpotential) are switched from the cathode to the anode in the stacked MFC
Fig. 6. Comparison of the voltage evolutions in stacked MFC-woPt and -wPt.
equipped with Pt-coated cathodes; substantial cathode overpotential caused voltage reversal in the stacked MFC equipped with non-Pt cathodes. It implies that primary kinetic bottlenecks (small current increase against large overpotential), which are the main cause of voltage reversal in stacked MFCs, can be shifted from the anode to the cathode or the cathode to the anode, depending on the relative kinetics between the two electrodes. 4. Conclusions In this work, we proved that sluggish kinetics on the cathode leads to voltage reversal in stacked MFCs-woPt. Comparison of increasing kinetics by direct catholyte aeration in an MFC against passive air diffusion to the cathode in the other MFC clearly showed the shift of voltage reversal between two individual units of the stacked MFC. The voltage in stacked MFC-woPt 1 having 13.4 mV (voltage reversal) was shifted to 76.3 mV during direct catholyte aeration, but the voltage in stacked MFC-woPt 2 decreased to 74.3 mV: voltage reversal shift. The voltages in two individual units returned to an original status (13.8 mV for MFC 1 and 14.9 mV for MFC 2) after catholyte aeration was stopped. Direct aeration to the catholyte of stacked MFC-woPt 2 increased the voltage reversal in stacked MFC-woPt 1 (155 mV). Rate-limiting reactions were switched from the cathode to the anode in the MFCs equipped with Pt-cathodes. Slow anode kinetics caused voltage reversal in the stacked MFC having the Pt-cathode: anode-driven voltage reversal. It is concluded that voltage reversal can be shifted from an MFC to another MFC depending on the electrode reaction rates, and dominant kinetic bottlenecks, the primary cause of voltage reversal in stacked MFCs, can be shifted from the anode to the cathode or vice versa, depending on the relative kinetics between the two electrodes. Acknowledgements
Fig. 5. Comparison of jo for non-Pt carbon cloth (a) and Pt/C-coated carbon cloth (b); a: symmetry factor, R: ideal gas constant, F: Faraday constant, T: temperature.
This work was financially supported by the Natural Sciences and Engineering Research Council (NSERC) of Canada, entitled “Development of energy-efficient wastewater treatment technology using
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principles of microbial fuel/electrolysis cells” (NSERC DG #4020452011) and the Ontario Ministry of Economic Development and Innovation, entitled “Development of sustainable anaerobic wastewater treatment technologies: recovery of value-added products.” Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2014.12.112. References [1] B. Min, B.E. Logan, Environ. Sci. Technol. 38 (21) (2004) 5809e5814. [2] I.S. Chang, J.K. Jang, G.C. Gil, M. Kim, H.J. Kim, B.W. Cho, B.H. Kim, Biosens. Bioelectron. 19 (6) (2004) 607e613. [3] Y.H. Gao, J.Y. An, H.D. Ryu, H.S. Lee, ChemSusChem 7 (4) (2014) 1026e1029. [4] S.H. Shin, Y.J. Choi, S.H. Na, S.H. Jung, S. Kim, Bull. Korean Chem. Soc. 27 (2) (2006) 281e285. [5] P. Aelterman, K. Rabaey, H.T. Pham, N. Boon, W. Verstraete, Environ. Sci. Technol. 40 (10) (2006) 3388e3394.
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[6] J. An, B. Kim, J.K. Jang, H.S. Lee, I.S. Chang, Biosens. Bioelectron. 59 (2014) 28e34. [7] J. An, H.S. Lee, ChemSusChem 7 (6) (2014) 1689e1695. [8] Y. Kim, M.C. Hatzell, A.J. Hutchinson, B.E. Logan, Energy Environ. Sci. 4 (11) (2011) 4662e4667. [9] S.E. Oh, B.E. Logan, J. Power Sour. 167 (1) (2007) 11e17. [10] M.H. Osman, A.A. Shah, F.C. Walsh, Biosens. Bioelectron. 26 (2010) 953e963. [11] L. Zhuang, Y. Zheng, S.G. Zhou, Y. Yuan, H.R. Yuan, Y. Chen, Bioresour. Technol. 106 (2012) 82e88. [12] P. Ledezma, J. Greenman, I. Ieropoulos, Bioresour. Technol. 134 (2013) 158e165. [13] Y.Z. Zhang, G.Q. Mo, X.W. Li, J.S. Ye, J. Power Sour. 197 (2012) 93e96. [14] S.E. Oh, B. Min, B.E. Logan, Environ. Sci. Technol. 38 (18) (2004) 4900e4904. [15] Q. Deng, X.Y. Li, J.E. Zuo, A. Ling, B.E. Logan, J. Power Sour. 195 (4) (2010) 1130e1135. [16] S.L. Chen, H.Q. Hou, F. Harnisch, S.A. Patil, A.A. Carmona-Martinez, S. Agarwal, Y.Y. Zhang, S. Sinha-Ray, A.L. Yarin, A. Greiner, U. Schroder, Energy Environ. Sci. 4 (4) (2011) 1417e1421. [17] S.C. Popat, D. Ki, B.E. Rittmann, C.I. Torres, ChemSusChem 5 (6) (2012) 1071e1079. [18] W. Gu, D.R. Baker, Y. Liu, H.A. Gasteiger, Handbook of Fuel Cells, Willy, 2009. [19] Y. Oh, S.K. Kim, S. Lim, D.H. Jung, D.H. Peck, Y. Shul, Int. J. Hydrog. Energy 37 (22) (2012) 17268e17274. [20] J.B.A. Arends, W. Verstraete, Microb. Biotechnol. 5 (3) (2012) 333e346.
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Shift of voltage reversal in stacked microbial fuel cells Junyeong An a, Bongkyu Kim b, In Seop Chang b, Hyung-Sool Lee a, * a
Department of Civil and Environmental Engineering, University of Waterloo, 200 University Avenue West, Waterloo, Ontario, N2L 3G1, Canada School of Environmental Science and Engineering, Gwangju Institute of Science and Technology (GIST), 261 Cheomdan-gwagiro, Buk-gu, Gwangju, 500-712, Republic of Korea
b
h i g h l i g h t s Voltage reversal occurred in the stacked MFC 1 having sluggish cathode reaction rate. No voltage reversal was observed in the stacked MFC 2 having faster kinetics on the cathode. Change in reaction rates on the cathode shifted voltage reversal between two stacked units. Voltage reversal can be shifted from the anode to the cathode in stacked MFCs.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 November 2014 Received in revised form 5 December 2014 Accepted 22 December 2014 Available online 24 December 2014
We proved that sluggish kinetics on the cathode and the imbalance of cathode kinetics cause voltage reversal in a stacked microbial fuel cell (MFC) equipped with a non-Pt cathode. Catholyte aeration to a unit MFC against passive air diffusion to the cathode in the other unit MFC shifted voltage reversal between the two units, due to improved mass transport and O2 concentration effects in the aerated MFC. The shifted voltage reversal returned to an original status when catholyte aeration was stopped. A Ptcoated cathode increased the rate of oxygen reduction reaction (ORR) by a factor of ~20, as compared to the non-Pt cathode. As a result, the anodic reaction rate that became slower than the rate on the Ptcathode limited current density to overpotential in the stacked MFC equipped with the Pt-cathode. This work shows that dominant kinetic bottlenecks, which are the primary cause of voltage reversal, can be shifted between individual MFCs of stacked MFCs or electrodes depending on relative kinetics. © 2014 Elsevier B.V. All rights reserved.
Keywords: Voltage reversal Sluggish reaction rate Voltage reversal shift Stacked microbial fuel cells Overpotential
1. Introduction Many researchers have attempted to increase voltage by stacking MFCs for their application to energy-efficient wastewater treatment, bio-sensors, and power supplies to small electronic devices [1e3]. Stacking MFCs in series can directly boost voltage and power, but often causes voltage reversal [4e12]. Literature suggests that voltage reversal in stacked MFCs occurs due to significant anode polarization triggered by substrate depletion, the change of microbial community in biofilm anode, or other heterogeneous internal resistance factors [5e12]. The cathode reaction rate in the MFCs that employ efficient noble-metal catalysts (e.g., platinum) or strong liquid oxidants (e.g., potassium ferricyanide) is much faster than the anode reaction rate [5,9,10,13,14]. This phenomenon has resulted in some researchers predicting that
* Corresponding author. E-mail address: [email protected] (H.-S. Lee). http://dx.doi.org/10.1016/j.jpowsour.2014.12.112 0378-7753/© 2014 Elsevier B.V. All rights reserved.
significant anode overpotential leads to voltage reversal in stacked MFCs. Interestingly, the energy loss on the cathode is typically larger than that on the anode in MFCs. Cathode overpotential is as large as 300e500 mV at the maximum current density (0.12e3 mA cm2) in MFCs, while anode overpotential is relatively small at 100e270 mV [15e17]. This substantial cathode energy loss contracts to small cathode overpotentials (100e450 mV) at the extremely high current density of 400e1500 mA cm2 in chemical fuel cells equipped with a 0.1e0.3 mg Pt-cathode similar to that used in MFCs [18,19]. Mass transport limitations and concentration gradients throughout the cathode (hydroxyl ions or O2) that are significant for MFCs can lead to large cathode overpotential even at small current density in MFCs [17,20]. This unique feature of large cathode overpotential relative to small anode overpotential in MFCs implies that slow cathode kinetics would lead to voltage reversal in stacked MFCs. An and Lee [7] recently proved that the imbalance in anode
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kinetics over relatively constant kinetics on the cathode causes voltage reversal in stacked MFCs. They suggested that higher heterogeneity on the anode than on the cathode could readily lead to voltage reversal in stacked MFCs, although different cathode kinetics can drive voltage reversal. It is true that cathodic conditions such as relatively uniform catalysts and a single electron acceptor are homogeneous over the anode. However, heterogeneous features of the cathode cannot be avoided in MFCs. For instance, passive air supply to the cathode lacking intensive mixing will increase heterogeneity in mass transport to the cathode, resulting for instance, in different O2 concentration gradients throughout cathodes for individual MFCs. Heterogeneous mass-transport for protons or hydroxyl ions in the waterecathode interface and the area between the interface and a separator will be substantial due to insufficient advection in air-cathode MFCs [17,20]. This heterogeneous mass transport related to cathodic reaction could limit cathode kinetics, causing voltage reversal in stacked MFCs. Cathode-driven voltage reversal seems to complicate the operation of steady-state stacked MFCs because both anodic and cathodic kinetics can lead to voltage reversal, as suggested by An and Lee [7]. Hence, voltage reversal will readily occur in stacked MFCs caused by relatively sluggish or imbalanced electrode kinetics. This interpretation implies that voltage reversal can be switched between electrodes or individual units in stacked MFCs, exacerbating the performance of stacked MFCs. There are no studies on the voltage reversal caused by cathode kinetics and shift of voltage reversal in stacked MFCs, although understanding them is significant for the success of stacked MFCs. The goals of this work are three-fold. The first is to confirm the phenomenon of voltage reversal led by sluggish cathode reaction rates in a stacked MFC. The second is to further support cathodedriven voltage reversal by showing the shift of voltage reversal between two individual units of the stacked MFC. The final is to demonstrate that sluggish electrode kinetics mainly leads to voltage reversal in the stacked MFC. 2. Materials and methods 2.1. MFC configuration and inoculation Two identical sandwich type MFCs (denoted as MFC 1 and MFC 2) were fabricated with Pyrex glass in the Waterloo Engineering Machine Shop at the University of Waterloo. The MFCs consist of an anode and a cathode chamber, and the working volume of each chamber is 29 mL. Non-Pt carbon cloth (CCP40, Fuel Cell Earth, USA) was used for the anode in the MFCs. Pt/C-coated carbon cloth (3 mg cm2 40% Pt, Carbon Cloth Electrode EC34019-2, Fuel Cell Earth, USA) and non-Pt carbon cloth (CCP40, Fuel Cell Earth, USA) were tested for the cathode; two cathodes were compared to prove the shift of voltage reversal in a serially stacked MFC. The projected surface areas of the anode and cathode were identical at 9.6 cm2. Cation exchange membrane (CMI-7000, Membrane International Inc. USA) was positioned between the electrodes in the MFCs. To prevent solution leakage, a rubber gasket manually cut with a sharp blade was inserted between each electrode and chamber. An Ag/ AgCl reference electrode (BAS, MF-2052, USA) was placed at 2 mm distance from the anode in order to monitor electrode potential. MFC 1 and MFC 2 equipped with non-Pt carbon cloth were denoted as MFC-woPt 1 and MFC-woPt 2. In comparison, MFC-wPt indicates the MFC equipped with Pt/C-coated cathode. MFC-woPt 1 and MFC-woPt 2 were inoculated with recycle activated sludge (29 mL) sampled from a wastewater treatment plant (Waterloo, ON, Canada). Tap water was added to the cathode chambers as catholyte. Air was passively provided through a hole of 8 mm diameter on the top of the cathode chambers, which kept the
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concentration of dissolved oxygen (DO) at ~5.42 mg L1 in catholyte during our experiments. We used an external resistor of 120 U during ARB acclimation and fed acetate medium (25 mM acetate) to anode chambers at a flow rate of 0.06 ml min1 using a peristaltic pump (Masterflex, Model 7523-80, USA) over ~3 months; hydraulic retention time (HRT) was fixed at 8.1 h in the anode chambers. The tap water in the cathode was replaced daily with a syringe to compensate for evaporated water, and the pH in the cathode ranged from 7.0 to 7.3. 2.2. Polarization tests for individual and stacked MFCs using non-Pt cathode Polarization tests for MFC-woPt 1 and MFC-woPt 2 were conducted under non-stacked and stacked modes, to prove that voltage reversal can occur due to sluggish cathode reaction rates. Throughout this paper MFC-woPt 1 and -woPt 2 under non-stacked mode were referred to as non-stacked MFC-woPt 1 and MFC-woPt 2 (see Fig. S1(a) in Supplementary material). For stacked mode, MFC-woPt 1 and -woPt 2 connected in series were designated as stacked MFC-woPt, and individual units were denoted as stacked MFC-woPt 1 and stacked MFC-woPt 2, respectively (see Fig. S1(b)). Prior to the polarization tests, individual MFCs connected with an external resister of 120 U were converted to open circuit mode until OCVs in the MFCs became constant. Then, external resistances were sequentially changed from 518 kU to 56.1 U every 2 min. Closed circuit voltage (CCV) in each MFC was monitored using a multimeter (Keithly 2700, Keithley Instruments, Inc. USA) connected to a personal computer. After the polarization tests for non-stacked mode, the two MFCs were fully recharged in open circuit mode and were serially connected for polarization experiments in stacked mode. Stacked-mode polarization tests were carried out in the same manner as with non-stacked MFCs. The polarization tests for non-stacked and stacked mode were conducted in duplicate (see Supplementary material for duplicate test results). External resistances were changed from 581 kU to 56.1 U every 2 min. CCV in stacked mode was monitored between the cathode of stacked MFCwoPt 1 and the anode of stacked MFC-woPt 2, as illustrated in Fig. S1(b). Current was calculated with Ohm's law, i.e., I ¼ Rext/V (I: current (A), Rext: external resistance (U), and V is voltage (V)). In addition, V-current density (j) curves and potential (P)-j curves were built to analyse voltage reversal phenomena for stacked MFCwoPt. 2.3. Aeration tests for the cathode of the MFC-woPt 1 and MFCwoPt 2 under stacked mode: shift of voltage reversal between individual units Aeration tests were carried out for the cathode of stacked MFCwoPt 1 and MFC-woPt 2, respectively, to investigate voltage reversal behaviours in response to different cathode reaction rates: shift of voltage reversal between individual MFC units. Aeration in the cathode can increase dissolved oxygen (DO) concentration, and mass transport (i.e., advection) in catholyte. Both stacked MFC-woPt 1 and 2 were connected with 1.4 U of external resistor, and the voltages in the two MFCs were 13.4 and 14.9 mV, respectively. We aerated the cathode chamber of stacked MFC-woPt 1 using an air blower for 7 min to saturate DO concentration ~6.7 mg L1 in catholyte (see Supplementary material for detailed information on DO measurement). In comparison, air passively diffused to catholyte in stacked MFC-woPt 2 and DO concentration was saturated at ~5.42 mg L1 in the catholyte. Catholyte aeration for stacked MFC-woPt 1 was stopped, while air diffusion to the cathode of stacked MFC-woPt 2 was kept. We then aerated the cathode of stacked MFC-woPt 2 for 7 min in comparison
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to the air-diffusion cathode for stacked MFC-woPt 1. Voltage and electrode potentials were monitored for individual MFCs and Stacked MFC-woPt (see Fig. S1(b)). 2.4. Polarization tests for stacked MFCs using Pt-coated cathode: shift of voltage reversal from cathode to anode To further investigate the shift of voltage reversal caused by different reaction rates on cathodes, we performed polarization tests for stacked MFCs using Pt-coated cathodes: shift of voltage reversal from cathode to anode. For the experiments, we replaced non-Pt cathodes with Pt/C-coated cathodes for two individual MFCs in an anaerobic chamber to protect ARB from oxygen molecules, denoted as MFC-wPt 1 and -wPt 2. Then, we conducted the polarization tests in stacked mode in the identical manner to stacked MFC-woPt. The experiments were conducted in duplicate (see Supplementary material). In this paper, stacked MFC-wPt 1 and -wPt 2 represents the stacked units (i.e., MFC-wPt 1 and -wPt 2) of stacked MFC-wPt. 2.5. Estimation of exchange current density for cathodes To compare ORR kinetics between non-Pt and Pt/C cathodes, we quantified exchange current density (jo) from Tafel plots. Tafel plots were obtained with linear sweep voltammetry (LSV) tests with a three-electrode cell in triplicate (anodes: counter electrodes, cathodes: working electrodes, Ag/AgCl electrode: reference electrodes). For LSV tests, each electrode (9 cm2 non-Pt carbon cloth, 9 cm2 Pt/C-coated carbon cloth) was placed in a beaker having 48 mL tap water (pH: ~7e7.3, dissolved oxygen concentration: ~5.42 mg L1, temperature: ~24.5 C). Cathode potential was swept
at a scan rate of 1 mV s1 from OCP (130 mV for non-Pt carbon cloth, 615 mV for Pt carbon cloth vs. Ag/AgCl) to 370 mV for nonPt cathode and 115 mV for Pt cathode versus Ag/AgCl. Then, jo for the two cathodes was estimated from Tafel slops. 3. Results and discussion 3.1. Performance evaluation of non-stacked MFC-woPt 1 and MFCwoPt 2 Fig. 1(a) and (b) present voltageecurrent density (V-j) and potential-current density (P-j) curves obtained from polarization tests with the individual non-stacked MFCs. The OCVs in nonstacked MFC-woPt 1 and MFC-woPt 2 were 524 mV and 490 mV, respectively. Substantial overvoltage of 497 mV was observed for non-stacked MFC-woPt 1 at the maximum current density of 49.5 uA cm2. A large cathode overpotential of 447 mV was mainly responsible for the overvoltage. Significant overvoltage and cathode overpotential were also found in non-stacked MFC-woPt 2 (see Fig. 1(b)). In comparison, anode overpotentials were as small as 15e25 mV for the two MFCs. Due to the heterogenuity in the cathodes, we could not identify the reasons for different performance of the two identifical MFCs. However, experimental results indicate that sluggish ORR kinetics on the non-Pt cathodes limit MFC performance in our study, due to the lack of efficient metal catalysts on the cathodes. In addition, cathodic reactions would be more heterogeneous than anodic reactions in the MFCs where air is passively diffused to the cathode, based on the larger difference of cathode overpotential than that of anode overpotential between the two MFCs. 3.2. Voltage reversal in the stacked MFC equipped with non-Pt cathode Fig. 2(a) shows the evolutions of voltage and electrode potentials against current density in two individual MFCs of stacked
Fig. 1. The evolutions of voltage and electrode potentials against current density. (a) Vj curves and (b) P-j curves for the non-stacked MFC-woPt 1 and MFC-woPt 2.
Fig. 2. Change of voltage and electrode potentials against current density. (a) Voltage and current density for different external resistors in the stacked MFC-woPt 1, the stacked MFC-woPt 2, and Stacked MFC-woPt. (b) Electrode potentials against current density in the stacked MFC-woPt 1 and MFC-woPt 2.
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MFC-woPt (see Fig. S1(b) in SI). When the current density in stacked MFC-woPt reached 44 mA cm2, the voltage in stacked MFC-woPt 1 was discharged to 0 V, but the voltages of stacked MFC-woPt 2 and stacked MFC-woPt were equal at 52 mV. As shown in Fig. 2(b), the cathode overpotentials at the current density of 44 mA cm2 were significant at 489 mV and 429 mV, respectively, for stacked MFC-woPt 1 and MFC-woPt 2. In comparison, the anode overpotentials of the two MFCs were negligible, less than 20 mV. The voltage in stacked MFC-woPt 1 decreased to 16.2 mV (i.e., voltage reversal) at the current density of 44.8 mA cm2, while the voltage in stacked MFC-woPt 2 still remained positive at 41.1 mV without voltage reversal. Small anode overpotential but large cathode overpotential clearly indicates that the inefficient catalytic function of the non-Pt cathode mainly led to voltage reversal in stacked MFC-woPt 1 of stacked MFC-woPt, as shown in Fig. 2(b), although cathodic conditions were identical between the two MFCs (see Experimental Section). Passive oxygen transfer to catholytes without mixing could lead to heterogeneity in the ORR reaction on the cathode in the two MFCs, such as local concentrations of DO, hydroxyl ions, or protons throughout a boundary layer between bulk liquid and the cathode [17,19]. To confirm voltage reversal dominantly caused by ORR kinetics on the cathode, we conducted catholyte aeration tests.
Fig. 3. Voltage reversal shift driven by catholyte aeration. (a) voltage evolution in the stacked MFC-woPt 1, the stacked MFC-woPt 2, and Stacked MFC-woPt before and after catholyte aeration. (b) change of electrode potential with and without aeration in the three MFCs. (i): no aeration, (ii): aeration in the cathode of the stacked MFC-woPt 1, (iii): aeration stop in the stacked MFC-woPt 1, and (iv): aeration in the cathode of the stacked MFC-woPt 2.
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3.3. Voltage reversal shift between individual MFCs: effect of catholyte aeration Fig. 3(a) presents the voltage evolution in stacked MFC-woPt 1, stacked MFC-woPt 2, and stacked MFC-woPt at Rext 1.4 U (current density of ~44.5 mA cm2) with and without aeration in the cathodes. At stage (i) (passive aeration condition), the negative voltage of 13.4 mV was kept for stacked MFC-woPt 1, while voltage was steady at 14.9 mV for stacked MFC-woPt 2 (see Fig. 3(a)). At stage (ii) where the catholyte of stacked MFC-woPt 1 was aerated, the negative voltage was shifted to 76.3 mV in stacked MFC-woPt 1. Interestingly, the voltage of stacked MFC-woPt 2 was reversed from 14.9 mV to 74.3 mV: the shift of voltage reversal from stacked MFC-woPt 1 to MFC-woPt 2. During stage (ii), the current density increased from 44.5 to 59.0 mA cm2 in stacked MFC-woPt, probably due to improved DO concentration and mass transport in the cathode of stacked MFC-woPt 1. For instance, the DO concentration in the cathode increased from 5.42 to 6.70 mg L1. Once catholyte aeration in the MFC-woPt 1 was stopped at stage (iii), the shifted voltage reversal returned to the initial condition at stage (i): 13.8 mV in stacked MFC-woPt 1 (voltage reversal) and 14.9 mV in stacked MFC-woPt 2. At stage (iv), we aerated the catholyte of stacked MFC-woPt 2, and the voltage reversal was exacerbated in stacked MFC-woPt 1 (from 13.8 mV to 155 mV). The current density in the stacked MFC increased from 46.8 to 68.5 mA/cm2, probably due to the increased DO concentration and mass transport. Fig. 3(b) illustrates the evolutions of electrode potential in the two individual MFCs of stacked MFC-woPt. The change of anode potential is very small at 9e13 mV for the two MFCs during aeration tests (from stage (i) to (iv)), while cathode potential was significantly evolved at 100e150 mV in response to catholyte aeration. These results support that voltage reversal can move from one MFC to another MFC depending on electrode kinetics in individual units of stacked MFCs. Our study proves the shift of voltage reversal between unit MFCs by manipulating cathode kinetics.
Fig. 4. V-j or P-j curves for stacked MFC-wPt 1, MFC-wPt 2, and Stacked MFC-wPt.
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3.4. Voltage reversal by sluggish electrode kinetics: from cathode to anode Fig. 4 shows the evolution of voltages and electrode potentials against current density for two individual MFCs equipped with Pt/Ccoated cathodes, referred to as stacked MFC-wPt 1, stacked MFC-wPt 2, and stacked MFC-wPt (see Fig. S1(b) in SI). The maximum current density of stacked MFC-wPt significantly increased by a factor of 3 (144 mA cm2) as compared to that of stacked MFC-woPt (see Fig. 2 and 4). At the maximum current density, the cathode overpotentials were not large in stacked MFC-wPt, showing 214 mV and 98 mV for stacked MFC-wPt 1 and MFC-wPt 2, respectively, due to improvement of ORR kinetics on Pt-cathode. Again, the heterogenuity in aircathode MFCs would lead to different cathode performance in the two identifical MFCs. Fig. 5 shows the exchange current densities for non-Pt and Pt/C-coated cathodes, which were estimated at 6.4 104 and 1.38 102 mA cm2, respectively. ORR kinetic on the Pt-cathode is ~20 times faster than the non-Pt carbon cloth. Instead of small cathode overpotentials, the anode overpotentials of the MFC 1 and the MFC 2 significantly increased to 415 and 276 mV, respectively, at the maximum current density (see Fig. 4 and 6). As a result, voltage reversal occurred at stacked MFC-wPt 1 that had large anode overpotential, as shown in Fig. 4. This result clearly shows the shift of voltage reversal from the cathode to the anode when sluggish kinetics (small current increase against large overpotential) are switched from the cathode to the anode in the stacked MFC
Fig. 6. Comparison of the voltage evolutions in stacked MFC-woPt and -wPt.
equipped with Pt-coated cathodes; substantial cathode overpotential caused voltage reversal in the stacked MFC equipped with non-Pt cathodes. It implies that primary kinetic bottlenecks (small current increase against large overpotential), which are the main cause of voltage reversal in stacked MFCs, can be shifted from the anode to the cathode or the cathode to the anode, depending on the relative kinetics between the two electrodes. 4. Conclusions In this work, we proved that sluggish kinetics on the cathode leads to voltage reversal in stacked MFCs-woPt. Comparison of increasing kinetics by direct catholyte aeration in an MFC against passive air diffusion to the cathode in the other MFC clearly showed the shift of voltage reversal between two individual units of the stacked MFC. The voltage in stacked MFC-woPt 1 having 13.4 mV (voltage reversal) was shifted to 76.3 mV during direct catholyte aeration, but the voltage in stacked MFC-woPt 2 decreased to 74.3 mV: voltage reversal shift. The voltages in two individual units returned to an original status (13.8 mV for MFC 1 and 14.9 mV for MFC 2) after catholyte aeration was stopped. Direct aeration to the catholyte of stacked MFC-woPt 2 increased the voltage reversal in stacked MFC-woPt 1 (155 mV). Rate-limiting reactions were switched from the cathode to the anode in the MFCs equipped with Pt-cathodes. Slow anode kinetics caused voltage reversal in the stacked MFC having the Pt-cathode: anode-driven voltage reversal. It is concluded that voltage reversal can be shifted from an MFC to another MFC depending on the electrode reaction rates, and dominant kinetic bottlenecks, the primary cause of voltage reversal in stacked MFCs, can be shifted from the anode to the cathode or vice versa, depending on the relative kinetics between the two electrodes. Acknowledgements
Fig. 5. Comparison of jo for non-Pt carbon cloth (a) and Pt/C-coated carbon cloth (b); a: symmetry factor, R: ideal gas constant, F: Faraday constant, T: temperature.
This work was financially supported by the Natural Sciences and Engineering Research Council (NSERC) of Canada, entitled “Development of energy-efficient wastewater treatment technology using
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principles of microbial fuel/electrolysis cells” (NSERC DG #4020452011) and the Ontario Ministry of Economic Development and Innovation, entitled “Development of sustainable anaerobic wastewater treatment technologies: recovery of value-added products.” Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2014.12.112. References [1] B. Min, B.E. Logan, Environ. Sci. Technol. 38 (21) (2004) 5809e5814. [2] I.S. Chang, J.K. Jang, G.C. Gil, M. Kim, H.J. Kim, B.W. Cho, B.H. Kim, Biosens. Bioelectron. 19 (6) (2004) 607e613. [3] Y.H. Gao, J.Y. An, H.D. Ryu, H.S. Lee, ChemSusChem 7 (4) (2014) 1026e1029. [4] S.H. Shin, Y.J. Choi, S.H. Na, S.H. Jung, S. Kim, Bull. Korean Chem. Soc. 27 (2) (2006) 281e285. [5] P. Aelterman, K. Rabaey, H.T. Pham, N. Boon, W. Verstraete, Environ. Sci. Technol. 40 (10) (2006) 3388e3394.
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