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Voltage reversal causes bioanode corrosion in microbial fuel cell stacks
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Voltage reversal causes bioanode corrosion in microbial fuel cell stacks Jun Li a,b, Hejing Li a,b, Qian Fu a,b,*, Qiang Liao a,b, Xun Zhu a,b, Hajime Kobayashi c, Dingding Ye a,b a
Key Laboratory of Low-grade Energy Utilization Technologies and Systems, Chongqing University, Ministry of Education, Chongqing 400030, China b Institute of Engineering Thermophysics, Chongqing University, Chongqing 400030, China c Department of Systems Innovation, Graduate School of Engineering, The University of Tokyo, Tokyo 113-8656, Japan
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abstract
Article history:
A better understanding of voltage reversal phenomenon and its long-term effects on power
Received 21 February 2017
generation are crucial to efficiently improve the voltage of microbial fuel cell (MFC) stacks.
Received in revised form
In this study, six MFCs with imbalanced performances were connected in series. After over
27 May 2017
100 h of operation under voltage reversal conditions, increased turbidity and color were
Accepted 31 May 2017
observed in the anolyte of the reversed MFC. In addition, the cyclic voltammogram of the
Available online xxx
anode changed form a typical catalytic current to a capacitance current. The scanning electron microscopy (SEM) showed that biofilm was dissociated from the anode surface,
Keywords:
indicating that long-term operation under voltage reversal could damage the biofilm and
Microbial fuel cell stacks
ultimately resulted in the failure of MFC stacks. X-ray diffraction (XRD) and SEM equipped
Voltage reversal
with energy dispersive X-ray spectrometry (EDX) analyses showed that the black particles
Carbon corrosion
in the anolyte was mostly carbon exfoliated from the anode, suggesting that carbon
Bioanode failure
corrosion caused the biofilm failure. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Microbial fuel cell (MFC) is a promising energy-harvesting system that generates electricity from organic matters in wastewater, allowing for the integration of renewable energy production and wastewater treatment [1e3]. A typical MFC consists of two electrode chambers separated by a proton exchange membrane, each harboring an anode and a cathode. At the anode, exoelectrogenic bacteria attached on the surface are capable of degrading organic matters, and simultaneously
transferring electrons to the electrode via direct or indirect pathways [4e6]. At the cathode, the electrons came from the anode and the protons permeated from the anode chamber are combined with oxygen, thereby generating electricity. In addition to wastewater treatment, MFCs have wide potential applications on biosensors, bioelectronics, and bioremediation [7e9]. Currently, the implementation of MFCs is largely limited due to the low working voltage. The theoretical maximum voltage that a MFC can create is around 1.1 V with oxygen as
* Corresponding author. Key Laboratory of Low-grade Energy Utilization Technologies and Systems, Chongqing University, Ministry of Education, Chongqing 400030, China. E-mail address: [email protected] (Q. Fu). http://dx.doi.org/10.1016/j.ijhydene.2017.05.221 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Li J, et al., Voltage reversal causes bioanode corrosion in microbial fuel cell stacks, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.221
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electron acceptor and acetate as organic matter [4]. In practice, the working voltage of a MFC at the maximum power density ranges from 0.2 to 0.5 V owing to energy losses (i.e. ohmic loss, activation loss, and mass transfer loss) [10,11]. Such low working voltage has been an obstacle for the application of MFCs, as the voltage required as power sources for electronic devices are usually higher than 1.5 V [12]. Therefore, effective approaches to boost the voltage of MFCs are urgently required. Various approaches have been reported to enhance the voltage of MFCs, including using power management systems equipped with capacitors and inductors [13e15]. Gong et al. used a power management system to increase the voltage of benthic fuel cell to 7 V, which was used as power source of an acoustic modem and a seawater oxygen/temperature sensor system [13]. Donovan et al. used a power management system that enabled a sediment microbial fuel cell to power a remote sensor [15]. However, the system efficiencies were reportedly low in those systems, indicating substantial energy losses. Moreover, the relative complex electronic systems and intermittent operation were also bottlenecks of this approach. Fuel cell “stack”, in which multiple cell units are connected in series, is another approach to boost the voltage and has been widely used in proton exchange membrane fuel cells (PEMFCs) and direct methanol fuel cells (DMFCs) [16,17]. It has been reported that a series connection can boost the voltage of MFCs to the level directly usable as power sources of electronic devices [18e20]. However, the performance of MFC stacks is often severely hindered by the “voltage reversal”, in which the voltage of one MFC in the stack reversed from a positive to a negative value, whereas the voltages of other MFCs maintained at positive values [10,12]. Voltage reversal often occurs in serially stacked MFCs, especially when the MFC stack operated at a high current density [21,22], and can cause a significant reduction of output voltage of the MFC stack, as well as the lifetime of the reversed MFC unit [23]. The imbalanced performance of MFC units in the stack can be magnified by the heterogeneity of reactions on the bioanode of MFC, and is therefore considered as a main reason of voltage reversal [10,24,25]. It has been reported that the heterogeneity of the MFC-units performances could be caused by different anodic reaction rates, and the slow reaction rate on the anode of the MFC with lower performance was responsible for the voltage reversal [12]. Moreover, substrate depletion in the anode chamber could cause a loss of bacterial activity, resulting in voltage reversal [22]. To avoid the voltage reversal, several approaches have been reported, e.g. manipulating critical current density and applying an assistance current with an assistance electrode [10,26]. It has been reported that, upon the voltage reversal, the reversed MFC unit changes from a galvanic cell to an electrolysis cell (i.e. microbial electrolysis cell) [10]. Because a part of energy is used for electrolysis, the energy output of the MFC stack largely reduced. At the anode of the reversed MFC, oxygen production is usually considered as the main reaction [27]. In addition, the positive potential range is also suitable for carbon oxidation, which is a common phenomenon in serially stacked DMFCs and PEMFCs [28,29]. Carbon oxidation (Eq. (1)) was also reported on the anode of a three-electrode bio-electrosynthesis system, which simultaneously produce
H2 and organic compounds on the cathode [30]. However, carbon oxidation has never been reported during the voltage reversal of stacked MFC in series. C þ 2H2 O4CO2 þ 4Hþ þ 4e ; E ¼ 0:118 V vs: SHE
(1)
The scope of this study is to investigate the bioelectrochemical reactions occurred in the reversed MFC unit upon voltage reversal, as well as its long-term effects on the power generation of MFC stack. We firstly started up six MFC units with different external ohmic resistances to construct MFCs with imbalanced performances. Then the six MFC units were stacked in series and operated at a high current density to cause voltage reversal. The bio-electrochemical properties and surface morphology of the anode of the reversed MFC were analyzed before and after voltage reversal. Moreover, black particles accumulated in the anolyte were analyzed using scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. This study provides useful information on the long-term effects of voltage reversal on MFC stacks performance, which will be beneficial to efficiently increase voltage production.
Methods and materials Experimental setups Six MFCs with the same configuration as described in previous study were used in this study [31]. Each MFC consists of an anode and a cathode, which are separated by a cation exchange membrane (Ande Membrane INC., China), and two plexiglass plates with a serpentine channel as the frame (Supplemental material, Fig. S1). The effective volume of the serpentine channel in each plexiglass plate is 2.7 mL. Both anode and cathode chambers were equipped with Ag/AgCl reference electrodes. All the electrodes were made of carbon cloth (HCP330, Hesen Co. Ltd., China) with an effective surface area of 25 cm2. To reduce the ohmic resistance of electrodes, carbon clothes were connected to titanium sheet.
Inoculation and operation The effluent of anode chamber in a MFC (working volume of anode chamber: 1.4 L) that has been producing electricity for more than two years in our lab was used as the inoculum. During the start-up process, the inoculum containing suspended exoelectrogenic bacteria and residual anaerobic culture medium (0.68 g CH3COONa, 6.0 g Na2HPO4, 3 g KH2PO4, 0.1 g NH4Cl, 0.5 g NaCl, 0.1 g MgSO4$7H2O, 15 mg CaCl2$2H2O, and 1.0 mL trace elemental solution per liter) continuously flowed through the anode channels of six MFCs in a flow rate of 0.5 mL min1 [32]. 50 mM K3[Fe(CN)6] was used as electron acceptor in the cathode chamber. After the MFCs were successfully started up (i.e., when the current density of all the MFCs become relatively stable), the influent were changed to pre-sterilized anaerobic culture medium as mentioned above. The medium was pre-sterilized at 121 C for 20 min to avoid possible contamination by other unknown bacteria and purged with high concentration N2 (99.99%) to maintain an
Please cite this article in press as: Li J, et al., Voltage reversal causes bioanode corrosion in microbial fuel cell stacks, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.221
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anaerobic condition. All the MFC units were operated in a continuous mode using peristaltic pump (BT 100-1L, Longer Precision Pump Co., Ltd., China). The anolyte and catholyte flow rates were set as 0.5 mL min1. It has been reported that voltage reversal phenomenon can occur when the MFC units in the stack possess imbalanced performance. According to previous study [31,33,34], the MFC started up at a lower external resistance can obtain a better performance than that started up at a higher external resistance. Thus, one MFC was started up with an external resistance of 1000 U (which was denoted as MFC-I) while the other five MFC units were started up with an external resistance of 50 U. To cause voltage reversal in the MFC stack, the six MFCs possessing imbalanced performance were connected in series, and the performance of the MFC stack was measured by reducing the external resistance. To investigate the subsequent processes after voltage reversal, the MFC stack was further operated with an external resistance of 100 U for four days. The anolyte of MFC-I was recycled during the operation to collect the possible exfoliation of anode in the reversed MFC. All the MFC units were operated at a constant temperature of 30 ± 1 C.
Data acquisition and analyses The voltage of each MFC units and electrode potentials were recorded every 30s using a data acquisition unit (34970 A, Agilent, Holland). The performance of each MFC was measured before and after the voltage reversal. During the performance measurement, the external resistance was varied from 8000 to 5 U to control the discharging condition. The power density of MFC was calculated as follows: U2 P W m2 ¼ RA
(2)
where A represents the surface area of the anode (m2), R represents the external resistance (U), and U represents the working voltage (V) of MFCs. Cyclic voltammetry (CV) analyses were performed in MFC-I using an electrochemical workstation (Zennium, Zahner, Germany) with a standard three-electrode system. The anode, cathode and the Ag/AgCl electrode inserted into the anode chamber were acted as the working electrode, counter electrode, and reference electrode, respectively. The parameters for CV were as follows: equilibrium time 99 s, scan rate 1 mV s1, and scanning range 0.4 to 0.1 V vs. SHE. To further investigate the variation of bioanode during the operation condition of voltage reversal, a bioanode that was started up with 1000 U was discharged at a constant current of 8 mA to mimic the condition of voltage reversal. The CVs of the bioanode was measured with a scan rate of 5 mV s1 instead of 1 mV s1 to accurately mirror the activity of bioanode as soon as possible by reducing the scanning time.
Scanning electron microscopy (SEM) analyses The morphology of the anode of MFC-I was analyzed using SEM (3400N, Hitachi instrument, Japan). The sample was first fixed with 2.5% (w/v) glutaraldehyde in 0.1 M phosphate buffer solution (pH 7.4). The samples were washed three times with
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the same buffer and dehydrated stepwise with a graded series of ethanol solutions (30, 50, 70, 80, 90 and 100%) [31]. The samples were then critical-point dried with tert-butyl ethanol, and sputter coated with a thin layer of gold, and finally observed under high vacuum using SEM. The anolyte of MFC-I after long-term operation under a voltage reversal condition was centrifuged at 8000 rpm for 30 min using a centrifuge (GL-21M, Xiangyi Centrifuge Instrument Co. Ltd., China). Insoluble particles in the anolyte were collected by filtration using vacuum suction device (0.22 mm) and dried in an oven at 80 C for 24 h. The collected particles were analyzed using X-ray diffraction (XRD) (PANalytical X'Pert Powder, Spectris Pte. Ltd., Holland) and SEM equipped with Energy Dispersive X-Ray Spectroscopy (EDX, Cam-Scan MV 2300).
Results Performance of individual MFC units before being stacked As expected, MFC-I took less time (around 15 h) to produce a stable current, in comparison with other five MFCs, which required more than 35 h (supplemental materials Fig. S2). Fig. 1 shows the power density and polarization curves of MFCs before being stacked in series. The open circuit voltages of all the individual MFCs were similar to each other (around 840 mV). The maximum power density of MFC-I was around 790 mW m2, which was lower than those (around 1900 mW m2) of the other five MFCs. Correspondingly, the maximum current density of MFC-I (around 1450 mA m2) was also lower than that (4000 mA m2) of the other five MFCs. It was consistent with the results in the previous study that using a lower external resistance during the startup process could obtain a higher performance than that using a higher external resistance [31]. In addition, an overshoot phenomenondthe doubling back of power density curve (after the peak power) towards lower current densities rather than the expected higher current densities [35]dwas observed in the power and polarization curve of MFC-I. The anodic and cathodic potential of each individual MFC during the measurement of power and polarization curve were shown in Fig. 1C. The cathodic potential of MFC-I maintained relatively stable, while the anodic potential drastically dropped when the current density was larger than the critical current, demonstrating that the low electricity-generating capacity of bioanode contributed to the overshoot.
Voltage reversal in MFC stack Fig. 2 shows the power and polarization curve of the MFCs stacked in series, as well as the voltage and electrode potential of each MFC during the measurement. The open circuit voltage of the MFC stack was around 5 V, equal to the summation of open circuit voltage of the six individual MFC units. The maximum power of the MFC stack was around 20 mW, obtained at a current of 6.4 mA and a voltage of 3.2 V. Remarkably, the voltage of the MFC stack drastically dropped from 3.2 to 1.7 V when the current increased to 7.0 mA, indicating the occurrence of voltage reversal. Fig. 2B shows the
Please cite this article in press as: Li J, et al., Voltage reversal causes bioanode corrosion in microbial fuel cell stacks, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.221
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Fig. 1 e Performance evaluation of MFCs before being stacked: (A) power density curves; (B) polarization curves; (C) anodic and cathodic potential of each individual MFC.
working voltage of each individual MFC during the measurement of polarization and power density curve of the MFC stack. It can be observed that the voltage of MFC-I drastically dropped from around 0.41 to 0.8 V at the current of 7.0 mA, while the voltage of other MFCs dropped slowly in low constant slopes and remained positive (ranging from 0.47 to 0.57 V) at the current of 7.0 mA. Correspondingly, the anodic potential of MFC-I increased from 0.22 to 1.0 V vs. SHE, while
Fig. 2 e Performance evaluation of MFC stack and each individual MFC in the stack: (A) polarization curve and power density of MFC stack; (B) polarization curve of each single MFC in the stack; (C) anodic and cathodic potential of each single MFC in the stack.
the anodic of other MFC units changed slowly and remained negative (ranging from 0.14 to 0.20 V) (Fig. 2C). It demonstrated that the voltage reversal was mainly due to the limited electricity-generating capacity of anode in MFC-I. As shown in Fig. 3, the anodic potential of MFC-I significantly increased from 0.22 to 1.2 V vs. SHE when the external resistance of MFC stack decreased from 1000 to 100 U, resulting in the voltage reversal of MFC-I and, consequently,
Please cite this article in press as: Li J, et al., Voltage reversal causes bioanode corrosion in microbial fuel cell stacks, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.221
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further.investigate the subsequent bio-electrochemical reactions in MFC-I after voltage reversal, MFC-I was continuously operated under the voltage reversal condition for more than 100 h. As shown in Fig. S3 (supplemental materials), the voltage of MFC-I kept relatively constant at 1.2 V, while the voltage of other five MFCs kept at the original level during the 100 h. In addition, interestingly, with the increase of operation time, increased turbidity and color of the anolyte in MFC-I was observed (as shown in Fig. 4), while the anolyte of other five MFCs remained similar appearances (i.e. similar to the fresh medium).
Electrochemical analyses of bioanode in MFC-I To investigate the long-term effects of voltage reversal on the bioanode of the reversed MFC, CV of the anode in MFC-I before and after a long-term operation under the voltage reversal condition were conducted. As shown in Fig. 5, the CV of the anode before voltage reversal showed a representative sigmoidal shape, with an onset potential of around 0.2 V vs. SHE and a midpoint potential of around 0.15 V vs. SHE. After a long-term operation under the voltage reversal condition, the CV of the anode showed a large capacitive current instead of a catalytic current. In contrast, the current of the noninoculated carbon cloth was negligible. Fig. S4 (as shown in supplemental materials) shows the variation of CVs of a bioanode that was discharged at a constant current of 8 mA to mimic voltage reversal condition in this study. It can be observed that the CV of the anode at 0 min showed a typical catalytic current, which began to decrease after 5 min operation. Notably, the oxidative current became obscure after 10 min operation under this condition, and the catalytic current began to become capacitive current, which increased with the increase of operating time and became relatively stable after 28.5 h operation.
Morphological analyses of bioanode Fig. 6 shows the SEM images of the anode in MFC-I before and after the long term operation under the voltage reversal condition. It can be observed that a thick biofilm was formed on the anode surface before the voltage reversal (Fig. 6B). On the other hand, only a part of carbon fibers was covered by the
Fig. 3 e Voltage (potential) e t curve with the external resistance changing from 1000 to 100 U: (A) voltage e t curve of the MFC stack under different external resistance; (B) voltage e t curve of each single MFC in the stack under different external resistance; (C) anodic and cathodic potential of each single MFC in the stack under different external resistance.
voltage drop in the MFC stack. In contrast, the anodic potential of the other five MFCs only increased a little due to the decrease of external resistance, and the voltage of the other five MFCs remained positive (ranging from 0.34 to 0.5 V). To
Fig. 4 e The anolyte of MFC-I during the operation under voltage reversal (Sample 1 represents fresh anolyte; Sample 2, 3, 4, 5 respectively represent the anolyte after one, two, three, and four days operation under voltage reversal).
Please cite this article in press as: Li J, et al., Voltage reversal causes bioanode corrosion in microbial fuel cell stacks, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.221
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Fig. 5 e CVs of the anode in MFC-I before and after voltage reversal.
biofilm after 20 h of operation under voltage reversal (Fig. 6C). Notably, no biofilm was attached on the anode surface after 100 h operation under the voltage reversal condition, corresponding to the CV (Fig. 5, red line (in the web version)) in which no catalytic current was observed. Interestingly, the anode surface after 100 h operation under the voltage reversal condition showed uneven morphology with fragmented edges
(Fig. 6D). Moreover, carbon fibers of the anode were thinner than that of the non-inoculated control (Fig. 6A). These results suggested that the long-term operation under voltage reversal caused damage on both the anode biofilm and electrode, and the turbidity in the anolyte of MFC-I (Fig. 4) was attributed to the dispersion of fragmented carbon fibers caused by electrode corrosion. To further investigate the effects of voltage reversal on the anode in MFC-I, the insoluble particles in the anolyte after 100 h operation under the voltage reversal condition were analyzed using XRD and SEM equipped with EDX. The EDX analyses of the particles after voltage reversal showed that main element of those particles was carbon and oxygen, but there was no carbon element in the particles before voltage reversal (Supplemental material, Fig. S5). The XRD analyses of the particles showed a similar peak with that of XC-72 carbon black (Fig. 7). Those results suggested that those particles were fragmented the carbon fiber released from the anode.
Discussion In this study, six MFC units with imbalanced electricitygenerating capacity were started up and connected in series. With the increase of operating current of the MFC stack, the voltage reversal was occurred at the MFC with the weakest
Fig. 6 e Scanning electron microscopy images of electrode: (A) fresh carbon electrode, (B) bioanode of MFC-I before voltage reversal, (C) bioanode of MFC-I after voltage reversal at 20 h, (D) bioanode of MFC-I after 100 h operated under voltage reversal. Please cite this article in press as: Li J, et al., Voltage reversal causes bioanode corrosion in microbial fuel cell stacks, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.221
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Fig. 7 e XRD patterns of black particles in the anolyte of MFC-I and Vulcan XC-72 carbon black.
electricity-generating capacity (MFC-I) in the stack, supporting that the imbalanced performance between the MFC units was the main cause for voltage reversal. Considering the variation of anodic potential relative to the stable cathodic potential, the voltage reversal was mainly due to the imbalanced performance caused by heterogeneous kinetics on the bioanodes. Therefore, it is suggested that drawing excessive current from the weakest MFCdmore than the original chemical reaction on the bioanode can producedcan result in voltage reversal: the anode potential changes from negative to positive to meet the high current in the MFC stack. In previous study, oxygen production is considered as the main reaction at the anode after the voltage reversal [27]. As the theoretical potential for oxygen production was 0.83 V vs. SHE and the anodic potential of MFC-I after voltage reversal was around 1.2 V vs. SHE, oxygen was likely a product at the anode. The dissolved oxygen concentration in the anolyte of MFC-I was also measured. However, no obvious oxygen production was detected (data not shown). One possible reason was that the oxygen was produced and immediately consumed by microorganisms. It is also likely that the oxygen was not produced due to the large overpotential (especially without catalyst at the anode). In addition, the power output of six MFCs showed that the MFC-I has turned into an electrolysis cell from a galvanic cell and consumed electric energy that produced by other MFCs after the voltage reversal (Supplemental materials Fig. S6). After a long-term operation under voltage reversal conditions, the CV of anode in MFC-I changed from a typical catalytic current to a typical capacitive current. However, the CV of anode in MFC (MFC-Ⅱ) started up with 50 U shows a typical sigmoidal shape (Supplemental material, Fig. S7). In addition, after long-term voltage reversal, comparing the SEM images of the anode of MFC-I with the one (MFC-Ⅱ) started up with 50 U (Supplemental material, Fig. S8), it showed that the biofilm of MFC-I was dissociated from the anode while the MFC-Ⅱ was unaffected. Moreover, carbon fibers of the MFC-I electrode were fragmented, suggesting that the long-term operation under voltage reversal caused unrecovered damage on the
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biofilm and electrode itself. As for the capacitive current, it was probably due to the formation of functional groups (such as alcohol and carboxyl groups) on the anode surface [36]. One interesting phenomenon was the increased turbidity and color change in the anolyte in the reversed MFC along with the operating time. A similar phenomenon has also been observed in a three-electrode bioelectrochemical system (BES) [30]. In that case, it was suggested that carbon oxidation occurred on the anode to produce graphene oxide with positive potential (þ1.6 V vs. SHE) applied on the anode. In this study, the particles contained in the anolyte of MFC-I were likely fragmented carbon fibers from the electrode. Given that the anodic potential of MFC-I was around 1.2 V vs. SHE, it was likely that carbon oxidation occurred at the anode, causing the fragmentation of the electrode and thereby severely hindering performance of the MFC stack. Overall, the findings in this study provide useful information on the long-term effects of voltage reversal on MFC stack performance, which will be beneficial to the manipulation of MFC stacks.
Conclusions In this study, we investigated the subsequent reaction on the anode of the reversed MFC in the MFC stack and its effects on the power generation under long-term operation of voltage reversal conditions. Increased turbidity and color change was observed in the anolyte of MFC-I, and SEM analyses showed that biofilm thoroughly dissociated from the anode after longterm operation under voltage reversal condition. Combined with XRD and SEM analyses of insoluble particles in the anolyte, it was concluded that carbon corrosion was occurred on the anode after long-term operation under voltage reversal and resulted in the failure of bioanode.
Acknowledgment The authors are grateful for the financial support provided by National Natural Science Funds for Outstanding Young Scholar (No. 51622602), National Natural Science Funds for Distinguished Young Scholar (No. 51325602), National Science Foundation for Young Scientists of China (No. 51506017), and Natural Science Foundation of Chongqing, China (No: cstc2015jcyjA90017).
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2017.05.221.
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Voltage reversal causes bioanode corrosion in microbial fuel cell stacks Jun Li a,b, Hejing Li a,b, Qian Fu a,b,*, Qiang Liao a,b, Xun Zhu a,b, Hajime Kobayashi c, Dingding Ye a,b a
Key Laboratory of Low-grade Energy Utilization Technologies and Systems, Chongqing University, Ministry of Education, Chongqing 400030, China b Institute of Engineering Thermophysics, Chongqing University, Chongqing 400030, China c Department of Systems Innovation, Graduate School of Engineering, The University of Tokyo, Tokyo 113-8656, Japan
article info
abstract
Article history:
A better understanding of voltage reversal phenomenon and its long-term effects on power
Received 21 February 2017
generation are crucial to efficiently improve the voltage of microbial fuel cell (MFC) stacks.
Received in revised form
In this study, six MFCs with imbalanced performances were connected in series. After over
27 May 2017
100 h of operation under voltage reversal conditions, increased turbidity and color were
Accepted 31 May 2017
observed in the anolyte of the reversed MFC. In addition, the cyclic voltammogram of the
Available online xxx
anode changed form a typical catalytic current to a capacitance current. The scanning electron microscopy (SEM) showed that biofilm was dissociated from the anode surface,
Keywords:
indicating that long-term operation under voltage reversal could damage the biofilm and
Microbial fuel cell stacks
ultimately resulted in the failure of MFC stacks. X-ray diffraction (XRD) and SEM equipped
Voltage reversal
with energy dispersive X-ray spectrometry (EDX) analyses showed that the black particles
Carbon corrosion
in the anolyte was mostly carbon exfoliated from the anode, suggesting that carbon
Bioanode failure
corrosion caused the biofilm failure. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Microbial fuel cell (MFC) is a promising energy-harvesting system that generates electricity from organic matters in wastewater, allowing for the integration of renewable energy production and wastewater treatment [1e3]. A typical MFC consists of two electrode chambers separated by a proton exchange membrane, each harboring an anode and a cathode. At the anode, exoelectrogenic bacteria attached on the surface are capable of degrading organic matters, and simultaneously
transferring electrons to the electrode via direct or indirect pathways [4e6]. At the cathode, the electrons came from the anode and the protons permeated from the anode chamber are combined with oxygen, thereby generating electricity. In addition to wastewater treatment, MFCs have wide potential applications on biosensors, bioelectronics, and bioremediation [7e9]. Currently, the implementation of MFCs is largely limited due to the low working voltage. The theoretical maximum voltage that a MFC can create is around 1.1 V with oxygen as
* Corresponding author. Key Laboratory of Low-grade Energy Utilization Technologies and Systems, Chongqing University, Ministry of Education, Chongqing 400030, China. E-mail address: [email protected] (Q. Fu). http://dx.doi.org/10.1016/j.ijhydene.2017.05.221 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Li J, et al., Voltage reversal causes bioanode corrosion in microbial fuel cell stacks, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.221
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electron acceptor and acetate as organic matter [4]. In practice, the working voltage of a MFC at the maximum power density ranges from 0.2 to 0.5 V owing to energy losses (i.e. ohmic loss, activation loss, and mass transfer loss) [10,11]. Such low working voltage has been an obstacle for the application of MFCs, as the voltage required as power sources for electronic devices are usually higher than 1.5 V [12]. Therefore, effective approaches to boost the voltage of MFCs are urgently required. Various approaches have been reported to enhance the voltage of MFCs, including using power management systems equipped with capacitors and inductors [13e15]. Gong et al. used a power management system to increase the voltage of benthic fuel cell to 7 V, which was used as power source of an acoustic modem and a seawater oxygen/temperature sensor system [13]. Donovan et al. used a power management system that enabled a sediment microbial fuel cell to power a remote sensor [15]. However, the system efficiencies were reportedly low in those systems, indicating substantial energy losses. Moreover, the relative complex electronic systems and intermittent operation were also bottlenecks of this approach. Fuel cell “stack”, in which multiple cell units are connected in series, is another approach to boost the voltage and has been widely used in proton exchange membrane fuel cells (PEMFCs) and direct methanol fuel cells (DMFCs) [16,17]. It has been reported that a series connection can boost the voltage of MFCs to the level directly usable as power sources of electronic devices [18e20]. However, the performance of MFC stacks is often severely hindered by the “voltage reversal”, in which the voltage of one MFC in the stack reversed from a positive to a negative value, whereas the voltages of other MFCs maintained at positive values [10,12]. Voltage reversal often occurs in serially stacked MFCs, especially when the MFC stack operated at a high current density [21,22], and can cause a significant reduction of output voltage of the MFC stack, as well as the lifetime of the reversed MFC unit [23]. The imbalanced performance of MFC units in the stack can be magnified by the heterogeneity of reactions on the bioanode of MFC, and is therefore considered as a main reason of voltage reversal [10,24,25]. It has been reported that the heterogeneity of the MFC-units performances could be caused by different anodic reaction rates, and the slow reaction rate on the anode of the MFC with lower performance was responsible for the voltage reversal [12]. Moreover, substrate depletion in the anode chamber could cause a loss of bacterial activity, resulting in voltage reversal [22]. To avoid the voltage reversal, several approaches have been reported, e.g. manipulating critical current density and applying an assistance current with an assistance electrode [10,26]. It has been reported that, upon the voltage reversal, the reversed MFC unit changes from a galvanic cell to an electrolysis cell (i.e. microbial electrolysis cell) [10]. Because a part of energy is used for electrolysis, the energy output of the MFC stack largely reduced. At the anode of the reversed MFC, oxygen production is usually considered as the main reaction [27]. In addition, the positive potential range is also suitable for carbon oxidation, which is a common phenomenon in serially stacked DMFCs and PEMFCs [28,29]. Carbon oxidation (Eq. (1)) was also reported on the anode of a three-electrode bio-electrosynthesis system, which simultaneously produce
H2 and organic compounds on the cathode [30]. However, carbon oxidation has never been reported during the voltage reversal of stacked MFC in series. C þ 2H2 O4CO2 þ 4Hþ þ 4e ; E ¼ 0:118 V vs: SHE
(1)
The scope of this study is to investigate the bioelectrochemical reactions occurred in the reversed MFC unit upon voltage reversal, as well as its long-term effects on the power generation of MFC stack. We firstly started up six MFC units with different external ohmic resistances to construct MFCs with imbalanced performances. Then the six MFC units were stacked in series and operated at a high current density to cause voltage reversal. The bio-electrochemical properties and surface morphology of the anode of the reversed MFC were analyzed before and after voltage reversal. Moreover, black particles accumulated in the anolyte were analyzed using scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. This study provides useful information on the long-term effects of voltage reversal on MFC stacks performance, which will be beneficial to efficiently increase voltage production.
Methods and materials Experimental setups Six MFCs with the same configuration as described in previous study were used in this study [31]. Each MFC consists of an anode and a cathode, which are separated by a cation exchange membrane (Ande Membrane INC., China), and two plexiglass plates with a serpentine channel as the frame (Supplemental material, Fig. S1). The effective volume of the serpentine channel in each plexiglass plate is 2.7 mL. Both anode and cathode chambers were equipped with Ag/AgCl reference electrodes. All the electrodes were made of carbon cloth (HCP330, Hesen Co. Ltd., China) with an effective surface area of 25 cm2. To reduce the ohmic resistance of electrodes, carbon clothes were connected to titanium sheet.
Inoculation and operation The effluent of anode chamber in a MFC (working volume of anode chamber: 1.4 L) that has been producing electricity for more than two years in our lab was used as the inoculum. During the start-up process, the inoculum containing suspended exoelectrogenic bacteria and residual anaerobic culture medium (0.68 g CH3COONa, 6.0 g Na2HPO4, 3 g KH2PO4, 0.1 g NH4Cl, 0.5 g NaCl, 0.1 g MgSO4$7H2O, 15 mg CaCl2$2H2O, and 1.0 mL trace elemental solution per liter) continuously flowed through the anode channels of six MFCs in a flow rate of 0.5 mL min1 [32]. 50 mM K3[Fe(CN)6] was used as electron acceptor in the cathode chamber. After the MFCs were successfully started up (i.e., when the current density of all the MFCs become relatively stable), the influent were changed to pre-sterilized anaerobic culture medium as mentioned above. The medium was pre-sterilized at 121 C for 20 min to avoid possible contamination by other unknown bacteria and purged with high concentration N2 (99.99%) to maintain an
Please cite this article in press as: Li J, et al., Voltage reversal causes bioanode corrosion in microbial fuel cell stacks, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.221
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anaerobic condition. All the MFC units were operated in a continuous mode using peristaltic pump (BT 100-1L, Longer Precision Pump Co., Ltd., China). The anolyte and catholyte flow rates were set as 0.5 mL min1. It has been reported that voltage reversal phenomenon can occur when the MFC units in the stack possess imbalanced performance. According to previous study [31,33,34], the MFC started up at a lower external resistance can obtain a better performance than that started up at a higher external resistance. Thus, one MFC was started up with an external resistance of 1000 U (which was denoted as MFC-I) while the other five MFC units were started up with an external resistance of 50 U. To cause voltage reversal in the MFC stack, the six MFCs possessing imbalanced performance were connected in series, and the performance of the MFC stack was measured by reducing the external resistance. To investigate the subsequent processes after voltage reversal, the MFC stack was further operated with an external resistance of 100 U for four days. The anolyte of MFC-I was recycled during the operation to collect the possible exfoliation of anode in the reversed MFC. All the MFC units were operated at a constant temperature of 30 ± 1 C.
Data acquisition and analyses The voltage of each MFC units and electrode potentials were recorded every 30s using a data acquisition unit (34970 A, Agilent, Holland). The performance of each MFC was measured before and after the voltage reversal. During the performance measurement, the external resistance was varied from 8000 to 5 U to control the discharging condition. The power density of MFC was calculated as follows: U2 P W m2 ¼ RA
(2)
where A represents the surface area of the anode (m2), R represents the external resistance (U), and U represents the working voltage (V) of MFCs. Cyclic voltammetry (CV) analyses were performed in MFC-I using an electrochemical workstation (Zennium, Zahner, Germany) with a standard three-electrode system. The anode, cathode and the Ag/AgCl electrode inserted into the anode chamber were acted as the working electrode, counter electrode, and reference electrode, respectively. The parameters for CV were as follows: equilibrium time 99 s, scan rate 1 mV s1, and scanning range 0.4 to 0.1 V vs. SHE. To further investigate the variation of bioanode during the operation condition of voltage reversal, a bioanode that was started up with 1000 U was discharged at a constant current of 8 mA to mimic the condition of voltage reversal. The CVs of the bioanode was measured with a scan rate of 5 mV s1 instead of 1 mV s1 to accurately mirror the activity of bioanode as soon as possible by reducing the scanning time.
Scanning electron microscopy (SEM) analyses The morphology of the anode of MFC-I was analyzed using SEM (3400N, Hitachi instrument, Japan). The sample was first fixed with 2.5% (w/v) glutaraldehyde in 0.1 M phosphate buffer solution (pH 7.4). The samples were washed three times with
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the same buffer and dehydrated stepwise with a graded series of ethanol solutions (30, 50, 70, 80, 90 and 100%) [31]. The samples were then critical-point dried with tert-butyl ethanol, and sputter coated with a thin layer of gold, and finally observed under high vacuum using SEM. The anolyte of MFC-I after long-term operation under a voltage reversal condition was centrifuged at 8000 rpm for 30 min using a centrifuge (GL-21M, Xiangyi Centrifuge Instrument Co. Ltd., China). Insoluble particles in the anolyte were collected by filtration using vacuum suction device (0.22 mm) and dried in an oven at 80 C for 24 h. The collected particles were analyzed using X-ray diffraction (XRD) (PANalytical X'Pert Powder, Spectris Pte. Ltd., Holland) and SEM equipped with Energy Dispersive X-Ray Spectroscopy (EDX, Cam-Scan MV 2300).
Results Performance of individual MFC units before being stacked As expected, MFC-I took less time (around 15 h) to produce a stable current, in comparison with other five MFCs, which required more than 35 h (supplemental materials Fig. S2). Fig. 1 shows the power density and polarization curves of MFCs before being stacked in series. The open circuit voltages of all the individual MFCs were similar to each other (around 840 mV). The maximum power density of MFC-I was around 790 mW m2, which was lower than those (around 1900 mW m2) of the other five MFCs. Correspondingly, the maximum current density of MFC-I (around 1450 mA m2) was also lower than that (4000 mA m2) of the other five MFCs. It was consistent with the results in the previous study that using a lower external resistance during the startup process could obtain a higher performance than that using a higher external resistance [31]. In addition, an overshoot phenomenondthe doubling back of power density curve (after the peak power) towards lower current densities rather than the expected higher current densities [35]dwas observed in the power and polarization curve of MFC-I. The anodic and cathodic potential of each individual MFC during the measurement of power and polarization curve were shown in Fig. 1C. The cathodic potential of MFC-I maintained relatively stable, while the anodic potential drastically dropped when the current density was larger than the critical current, demonstrating that the low electricity-generating capacity of bioanode contributed to the overshoot.
Voltage reversal in MFC stack Fig. 2 shows the power and polarization curve of the MFCs stacked in series, as well as the voltage and electrode potential of each MFC during the measurement. The open circuit voltage of the MFC stack was around 5 V, equal to the summation of open circuit voltage of the six individual MFC units. The maximum power of the MFC stack was around 20 mW, obtained at a current of 6.4 mA and a voltage of 3.2 V. Remarkably, the voltage of the MFC stack drastically dropped from 3.2 to 1.7 V when the current increased to 7.0 mA, indicating the occurrence of voltage reversal. Fig. 2B shows the
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Fig. 1 e Performance evaluation of MFCs before being stacked: (A) power density curves; (B) polarization curves; (C) anodic and cathodic potential of each individual MFC.
working voltage of each individual MFC during the measurement of polarization and power density curve of the MFC stack. It can be observed that the voltage of MFC-I drastically dropped from around 0.41 to 0.8 V at the current of 7.0 mA, while the voltage of other MFCs dropped slowly in low constant slopes and remained positive (ranging from 0.47 to 0.57 V) at the current of 7.0 mA. Correspondingly, the anodic potential of MFC-I increased from 0.22 to 1.0 V vs. SHE, while
Fig. 2 e Performance evaluation of MFC stack and each individual MFC in the stack: (A) polarization curve and power density of MFC stack; (B) polarization curve of each single MFC in the stack; (C) anodic and cathodic potential of each single MFC in the stack.
the anodic of other MFC units changed slowly and remained negative (ranging from 0.14 to 0.20 V) (Fig. 2C). It demonstrated that the voltage reversal was mainly due to the limited electricity-generating capacity of anode in MFC-I. As shown in Fig. 3, the anodic potential of MFC-I significantly increased from 0.22 to 1.2 V vs. SHE when the external resistance of MFC stack decreased from 1000 to 100 U, resulting in the voltage reversal of MFC-I and, consequently,
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further.investigate the subsequent bio-electrochemical reactions in MFC-I after voltage reversal, MFC-I was continuously operated under the voltage reversal condition for more than 100 h. As shown in Fig. S3 (supplemental materials), the voltage of MFC-I kept relatively constant at 1.2 V, while the voltage of other five MFCs kept at the original level during the 100 h. In addition, interestingly, with the increase of operation time, increased turbidity and color of the anolyte in MFC-I was observed (as shown in Fig. 4), while the anolyte of other five MFCs remained similar appearances (i.e. similar to the fresh medium).
Electrochemical analyses of bioanode in MFC-I To investigate the long-term effects of voltage reversal on the bioanode of the reversed MFC, CV of the anode in MFC-I before and after a long-term operation under the voltage reversal condition were conducted. As shown in Fig. 5, the CV of the anode before voltage reversal showed a representative sigmoidal shape, with an onset potential of around 0.2 V vs. SHE and a midpoint potential of around 0.15 V vs. SHE. After a long-term operation under the voltage reversal condition, the CV of the anode showed a large capacitive current instead of a catalytic current. In contrast, the current of the noninoculated carbon cloth was negligible. Fig. S4 (as shown in supplemental materials) shows the variation of CVs of a bioanode that was discharged at a constant current of 8 mA to mimic voltage reversal condition in this study. It can be observed that the CV of the anode at 0 min showed a typical catalytic current, which began to decrease after 5 min operation. Notably, the oxidative current became obscure after 10 min operation under this condition, and the catalytic current began to become capacitive current, which increased with the increase of operating time and became relatively stable after 28.5 h operation.
Morphological analyses of bioanode Fig. 6 shows the SEM images of the anode in MFC-I before and after the long term operation under the voltage reversal condition. It can be observed that a thick biofilm was formed on the anode surface before the voltage reversal (Fig. 6B). On the other hand, only a part of carbon fibers was covered by the
Fig. 3 e Voltage (potential) e t curve with the external resistance changing from 1000 to 100 U: (A) voltage e t curve of the MFC stack under different external resistance; (B) voltage e t curve of each single MFC in the stack under different external resistance; (C) anodic and cathodic potential of each single MFC in the stack under different external resistance.
voltage drop in the MFC stack. In contrast, the anodic potential of the other five MFCs only increased a little due to the decrease of external resistance, and the voltage of the other five MFCs remained positive (ranging from 0.34 to 0.5 V). To
Fig. 4 e The anolyte of MFC-I during the operation under voltage reversal (Sample 1 represents fresh anolyte; Sample 2, 3, 4, 5 respectively represent the anolyte after one, two, three, and four days operation under voltage reversal).
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Fig. 5 e CVs of the anode in MFC-I before and after voltage reversal.
biofilm after 20 h of operation under voltage reversal (Fig. 6C). Notably, no biofilm was attached on the anode surface after 100 h operation under the voltage reversal condition, corresponding to the CV (Fig. 5, red line (in the web version)) in which no catalytic current was observed. Interestingly, the anode surface after 100 h operation under the voltage reversal condition showed uneven morphology with fragmented edges
(Fig. 6D). Moreover, carbon fibers of the anode were thinner than that of the non-inoculated control (Fig. 6A). These results suggested that the long-term operation under voltage reversal caused damage on both the anode biofilm and electrode, and the turbidity in the anolyte of MFC-I (Fig. 4) was attributed to the dispersion of fragmented carbon fibers caused by electrode corrosion. To further investigate the effects of voltage reversal on the anode in MFC-I, the insoluble particles in the anolyte after 100 h operation under the voltage reversal condition were analyzed using XRD and SEM equipped with EDX. The EDX analyses of the particles after voltage reversal showed that main element of those particles was carbon and oxygen, but there was no carbon element in the particles before voltage reversal (Supplemental material, Fig. S5). The XRD analyses of the particles showed a similar peak with that of XC-72 carbon black (Fig. 7). Those results suggested that those particles were fragmented the carbon fiber released from the anode.
Discussion In this study, six MFC units with imbalanced electricitygenerating capacity were started up and connected in series. With the increase of operating current of the MFC stack, the voltage reversal was occurred at the MFC with the weakest
Fig. 6 e Scanning electron microscopy images of electrode: (A) fresh carbon electrode, (B) bioanode of MFC-I before voltage reversal, (C) bioanode of MFC-I after voltage reversal at 20 h, (D) bioanode of MFC-I after 100 h operated under voltage reversal. Please cite this article in press as: Li J, et al., Voltage reversal causes bioanode corrosion in microbial fuel cell stacks, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.221
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Fig. 7 e XRD patterns of black particles in the anolyte of MFC-I and Vulcan XC-72 carbon black.
electricity-generating capacity (MFC-I) in the stack, supporting that the imbalanced performance between the MFC units was the main cause for voltage reversal. Considering the variation of anodic potential relative to the stable cathodic potential, the voltage reversal was mainly due to the imbalanced performance caused by heterogeneous kinetics on the bioanodes. Therefore, it is suggested that drawing excessive current from the weakest MFCdmore than the original chemical reaction on the bioanode can producedcan result in voltage reversal: the anode potential changes from negative to positive to meet the high current in the MFC stack. In previous study, oxygen production is considered as the main reaction at the anode after the voltage reversal [27]. As the theoretical potential for oxygen production was 0.83 V vs. SHE and the anodic potential of MFC-I after voltage reversal was around 1.2 V vs. SHE, oxygen was likely a product at the anode. The dissolved oxygen concentration in the anolyte of MFC-I was also measured. However, no obvious oxygen production was detected (data not shown). One possible reason was that the oxygen was produced and immediately consumed by microorganisms. It is also likely that the oxygen was not produced due to the large overpotential (especially without catalyst at the anode). In addition, the power output of six MFCs showed that the MFC-I has turned into an electrolysis cell from a galvanic cell and consumed electric energy that produced by other MFCs after the voltage reversal (Supplemental materials Fig. S6). After a long-term operation under voltage reversal conditions, the CV of anode in MFC-I changed from a typical catalytic current to a typical capacitive current. However, the CV of anode in MFC (MFC-Ⅱ) started up with 50 U shows a typical sigmoidal shape (Supplemental material, Fig. S7). In addition, after long-term voltage reversal, comparing the SEM images of the anode of MFC-I with the one (MFC-Ⅱ) started up with 50 U (Supplemental material, Fig. S8), it showed that the biofilm of MFC-I was dissociated from the anode while the MFC-Ⅱ was unaffected. Moreover, carbon fibers of the MFC-I electrode were fragmented, suggesting that the long-term operation under voltage reversal caused unrecovered damage on the
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biofilm and electrode itself. As for the capacitive current, it was probably due to the formation of functional groups (such as alcohol and carboxyl groups) on the anode surface [36]. One interesting phenomenon was the increased turbidity and color change in the anolyte in the reversed MFC along with the operating time. A similar phenomenon has also been observed in a three-electrode bioelectrochemical system (BES) [30]. In that case, it was suggested that carbon oxidation occurred on the anode to produce graphene oxide with positive potential (þ1.6 V vs. SHE) applied on the anode. In this study, the particles contained in the anolyte of MFC-I were likely fragmented carbon fibers from the electrode. Given that the anodic potential of MFC-I was around 1.2 V vs. SHE, it was likely that carbon oxidation occurred at the anode, causing the fragmentation of the electrode and thereby severely hindering performance of the MFC stack. Overall, the findings in this study provide useful information on the long-term effects of voltage reversal on MFC stack performance, which will be beneficial to the manipulation of MFC stacks.
Conclusions In this study, we investigated the subsequent reaction on the anode of the reversed MFC in the MFC stack and its effects on the power generation under long-term operation of voltage reversal conditions. Increased turbidity and color change was observed in the anolyte of MFC-I, and SEM analyses showed that biofilm thoroughly dissociated from the anode after longterm operation under voltage reversal condition. Combined with XRD and SEM analyses of insoluble particles in the anolyte, it was concluded that carbon corrosion was occurred on the anode after long-term operation under voltage reversal and resulted in the failure of bioanode.
Acknowledgment The authors are grateful for the financial support provided by National Natural Science Funds for Outstanding Young Scholar (No. 51622602), National Natural Science Funds for Distinguished Young Scholar (No. 51325602), National Science Foundation for Young Scientists of China (No. 51506017), and Natural Science Foundation of Chongqing, China (No: cstc2015jcyjA90017).
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2017.05.221.
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Please cite this article in press as: Li J, et al., Voltage reversal causes bioanode corrosion in microbial fuel cell stacks, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.221