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Response of stacked microbial fuel cells with serpentine flow fields to variable operating conditions
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e8
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Response of stacked microbial fuel cells with serpentine flow fields to variable operating conditions Liang Zhang a,b, Jun Li a,b,*, Xun Zhu a,b, Ding-ding Ye a,b, Qian Fu a,b, Qiang Liao a,b a
Key Laboratory of Low-grade Energy Utilization Technologies and Systems (Chongqing University), Ministry of Education, Chongqing 40003, China b Institute of Engineering Thermophysics, Chongqing University, Chongqing 400030, China
article info
abstract
Article history:
The stacked microbial fuel cell (MFC) is a potential pathway for future applications, and its
Received 11 January 2017
response to variable operating conditions is important for practical operation. In this study,
Received in revised form
a stacked MFC with serpentine flow field was constructed to investigate the stack perfor-
18 April 2017
mance and response to cell number, connection type, variable loads and electrolyte flow
Accepted 19 April 2017
rates. The results showed that the highest maximal power (22.2 mW) was observed in a
Available online xxx
series connection, which was 12.1% and 29.1% higher than the maximal power in the parallel and hybrid connection, respectively. With the increasing number of cells, a grad-
Keywords:
ually decreasing increase in the voltage output was found in the parallel stack and series
Microbial fuel cell
stack at a high load, while the series stack showed first an increase and later a decrease in
Stack
the voltage output at low load. Voltage reversal was observed when switching to a series
Response
connection or decreasing the load, resulting in a decreased stack voltage. The performance
Variable operating conditions
of the stack could be improved to an extent by increasing the electrolyte flow rates.
Voltage reversal
© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Microbial fuel cells (MFCs), which uses electrochemically active bacteria (EAB) as catalysts to directly generate electricity from wastewater, is a potential pathway to a sustainable energy future [1,2]. Although there has been significant development in biofilm enrichment [3,4], reactor configuration [5,6], separator [7,8], electrode materials [4,9], mass transfer [3,10,11] and the optimization of process parameters [3,12], the power output of MFCs remains insufficient for
practical applications. For an individual MFC, the working voltage is usually below 0.5 V, resulting from the thermodynamic and kinetic constraints [1]. To fulfill the real application requirements, many individual MFCs could be connected in parallel, series and hybrid stacks to promote the current, voltage and power [13e15]. In the past decade, most studies of stacked MFCs were conducted on voltage reversal and configuration. It was found that charge reversal could lead to the reverse polarity of cells and a loss of power generation in the MFC series stack [13]. Various electronic components such as capacitors and DC/DC
* Corresponding author. Key Laboratory of Low-grade Energy Utilization Technologies and Systems (Chongqing University), Ministry of Education, Chongqing 40003, China. Fax: þ86 23 6510 3113. E-mail address: [email protected] (J. Li). http://dx.doi.org/10.1016/j.ijhydene.2017.04.205 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Zhang L, et al., Response of stacked microbial fuel cells with serpentine flow fields to variable operating conditions, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.205
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voltage boosters, have been used to alleviate this phenomenon to a certain extent and to further promote the power [14e16]. In addition, it was demonstrated that using the maximum power point tracking devices and hybrid connectivity, an increased power output and voltage reversal avoidance in series connected MFC stacks can be achieved [17]. With respect to the new configuration, several novel stacked MFCs were studied recently. Liter-scale baffled stacking microbial fuel cells were used to generate electricity and treat wastewater containing sulfide and high strength molasses [18,19]. A 10-liter serpentine-type MFC stack constructed with PVC (polyvinylchloride) pipe was developed to treat brewery wastewater [20]. A continuous microbial fuel cell stack with a dual gas diffusion cathode design was developed to treat dark fermentation effluent [21]. A hybrid microbial fuel cell stack based on single and double chamber MFCs was proposed for self-sustaining pH control [22]. Interestingly, untreated urine was used for power generation in a miniature microbial fuel cell stack [23]. Obviously, for future application, more attention should be paid to MFC stacks. A further important consideration for MFC stacks is their response to changes during operation. In future practical application, the MFC stack is used to power multiple devices with varied electric properties and it is necessary to switch the operation modes [24]. Compared with other fuel cells, MFCs had a relatively slow response due to the slow bioelectrochemical reaction [25]. It is well known that the architecture of the biofilm adapts in response to environmental stresses, such as low nutrient availability, high shear forces, unfavorable pH and toxic compounds [3]. In addition to environmental impacts, MFCs also need to adapt in response to the changes of operation in future practical applications. Several MFC studies reported the dynamic behavior in terms of variations in cells or changes in operating conditions and parameters. Katuri and Scott reported a dynamic response of MFC using membrane electrode assemblies design to the external resistance and COD changes, and a corresponding model was developed [25]. Yuan et al. investigated the responses of electrochemical characteristics and performance of anodic biofilms to pH changes in MFCs [26]. For MFC-based sensors, it is significantly important to investigate the dynamic response time and sensitivity after a change of operating parameters [12,27e30]. In addition, a dynamic model was developed to simulate the transient response of MFC voltage to the current step-change [31]. With respect to an MFC stack, few related research was investigated on the response to the variable operating conditions, which is insufficient for future practical operation. It was reported that a MFC stack fed with glycerol was tested to investigate the effects of connection modes on the electricity generation and microbial community [24]. Dynamic reconfiguration as a designing energy harvesting circuitry for MFC stacks can reduce the charging times by allowing storage of the same amount of energy in a shorter period of time [32]. However, more research should be focused on the dynamic response of MFC stack to other key variable operating conditions, proving better understanding of MFC stack operation. In this study, serpentine flow fields were used to enhance mass transfer and four flat plate MFCs with serpentine flow fields were constructed into a stack in parallel, series and
hybrid connection. The objective of the present study was to investigate the response of the stacked MFC to cell number, connection type, variable loads and electrolytes flow rates.
Materials and methods MFC stack configuration and inoculation In this experiment, the stacked microbial fuel cell with serpentine flow fields was conducted using four identical MFC units, as shown in Fig. 1(a) and (b). Each MFC consisted of a proton exchange membrane (PEM) (Nafion 117, DuPont), two carbon cloth electrodes (E-TEK, B-1A, America) and Plexiglass plates with a serpentine flow channel holding a volume of 2.7 ml. The PEM and electrodes had an apparent surface area of 25 cm2. Both anode and cathode compartments were equipped with Ag/AgCl reference electrodes. For each individual MFC, the anode compartment was inoculated with the effluent from a running MFC fed with an artificial wastewater. And the running MFC was previously inoculated using the activated sludge from the primary clarifier of Tangjiatuo Wastewater Treatment Plant of Chongqing. The artificial wastewater contained 2.7 g/L CH3COONa, 6 g/L Na2HPO4, 3 g/L KH2PO4, 0.1 g g/L NH4Cl, 0.5 g/L NaCl, 0.1 g/L MgSO4$7H2O, 15 mg/L CaCl2$2H2O and 1.0 mL/L of a trace elements solution (500 COD mg/L; pH: 7.04; Conductivity: 15.12 mS/cm). A 50 mM potassium ferricyanide solution was used as the catholyte. The flow rates of each anode and cathode were 1.0 ml/min, unless otherwise noted. After inoculation, the four MFCs were constructed into stacks in parallel, hybrid or series connections for the tests. During the stack test, the stack was operated at different connection modes as shown in Fig. 1(c). All the tests were conducted in a temperature-controlled room at 25 C.
Measurements and calculations Stack and cell voltages (U), anode and cathode potentials, of each MFC were collected every 15 s via an Agilent 34970A data acquisition unit connected to a PC. In the polarization test, the external resistance varied in a range of 5e1.0 104 U to control discharging and to record the voltageecurrent curves. After each change in the external resistance, the MFCs were held until the current and cell voltage reached steady-state with a voltage drift of less than 5 mV/h. The stack current (I) is equal to the voltage divided by the external load and the power (P) is the product of the voltage and the current.
Results and discussion Stacked MFC performance under different connection types To increase the overall stack voltage or current, the four individual MFCs were operated in parallel, series and hybrid connections. As shown in Fig. 2, this resulted in an open circuit potential (OCP) of 3.27 V for the series connection and 1.64 V for the hybrid connection, which was much higher than the OCP (0.82 V) of the parallel connection. With respect to the
Please cite this article in press as: Zhang L, et al., Response of stacked microbial fuel cells with serpentine flow fields to variable operating conditions, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.205
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Fig. 1 e Schematic of stacked microbial fuel cells with serpentine flow fields (a), 3-D schematic (b) and connection modes (c) in the experiment.
maximal power, the highest power (22.2 mW) was observed in the series connection, followed by the parallel connection (19.8 mW) and hybrid connection (17.2 mW). At the maximal power point, a highest voltage output of 2.11 V for the series connection and a highest current output of 44.5 mA for the parallel connection were observed. With respect to the hybrid connection, the maximal power was observed at a voltage of 1.0 V and a current of 16.9 mA. In addition, there was no obvious difference in MFC performance between the hybrid connections using different cell combinations. These results showed that connection type had significant effects on the stack performance. The maximal power density (2.22 W/m2 for series connection and 1.98 W/m2 for parallel connection) of the MFC stack was lower than that of the identical individual MFC (2.6 W/m2) startup under 50 U in our previous study [3]. This indicated that the MFC stack could increase the power but
decreased the power density compared with individual unit, which was in accordance with the previous studies [18,33]. However, a high voltage output or a high current generation could be obtained from the stacked MFCs, providing more practical application of MFCs. In addition, compared with other stacked MFC studies, the maximal power and the maximal power density in the present study were much higher (Table 1). This was due to both the lower Ohmic resistance of the flat-plate structure and the better mass transfer of the serpentine flow field. The reactor structure and mass transfer should be considered in future design of MFC stacks.
Response to the number of connected cells In principle, the power generation of a stack is promoted with the increasing number of individual cell [22,35]. However, this
Fig. 2 e Polarization curves (a) and power curves (b) of the stacked MFC with different connection types. Please cite this article in press as: Zhang L, et al., Response of stacked microbial fuel cells with serpentine flow fields to variable operating conditions, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.205
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Table 1 e Maximal performance of stacked MFCs. Reactor type
Number of individual MFC
Maximal power (mW)
Maximal powder density (MPD, W/m2)
Voltage at MPD (V)
Current density at PMD (A/m2)
References
Cubic MFC Rectangle MFC Baffled MFC Column MFC Tubular MFC Flat-plate MFC Flat-plate MFC
4 2 6 3 5 3 4
2.9 3.9 3.0 e 6.4 10.7 22.2
1.05 0.46 0.20 0.142 0.068 1.43 2.22
0.85 0.53 1.69 0.265 1.1 e 2.11
1.23 0.872 0.12 0.54 0.062 e 1.06
[14] [13] [18] [33] [34] [22] Present study
was not expected in the stacked microbial fuel cell due to the serious voltage reversal caused by the performance difference of the individual cell [36]. Thus, there is an optimal number of cells for an MFC stack. To investigate the response to the number of connected cells, the cells were connected into the series stack or the parallel stack in sequence as shown in Fig. 3. The four cells were in a stable open-circuit condition before being connected. With respect to the parallel stack, the voltage output largely increased from 441.3 mV to 519.7 mV after MFC-2 was connected in parallel with an external load of 30U. It was obvious that the response of voltage consisted of transient state and steady state. Further small increases in voltage output were observed in the parallel stack after connecting MFC-3 and MFC-4, respectively. This showed that a gradually decreasing increase in the voltage output was observed in the parallel stack after increasing the number of cells. However, the increase in voltage output was more than 0.6 V for the connection of each cell except MFC-4 in series stack at an external load of 400 U. After connecting MFC-4, there was no obvious change in voltage output, and MFC-4 showed a voltage drop to 26.0 mV, which might be due to the mass transfer limitation in anode. In addition, at a low external load of 150 U, there was a largely increase in voltage output after connecting MFC-2 while an obvious sharp increase and then gradual decrease in voltage output was observed after the later connection of MFC-3. The decrease in voltage output was attributed to the voltage reversal of MFC-3. The above similar phenomenon was found after connecting MFC-4. The above results indicated that, with the increasing number of cells, the stack in parallel connection showed a gradually decreasing increase in the voltage output. However, the stack in series connection showed an increase in voltage at high external loads, showing first an increase and later a decrease in the voltage output at low external loads. In addition, the series stack at a low load had a relatively slow transient process compared with the parallel stack and the series stack at a high load. The above results indicated that an optimal number of cells would be considered for the future design of an MFC stack.
Response to connection types For future practical operation, it is necessary to investigate the stack response after switching the connection type according to the actual demand. It is reported that, in a series connection, the cell voltage reversal would deteriorate the stack performance. The response of the stack from a parallel or
hybrid connection to series connection was shown in Fig. 4. The stack voltage was dropped from 620.0 mV to 387.5 mV at an external load of 30 U after switching from the parallel to the series connection, which resulted from the voltage reversal of MFC-4. In addition, a long lag period was found after switching to series connection, which was in accordance with the reported study [24]. This was probably due to the adaption of anodic microorganisms to the new micro-environment resulted from the new connection mode. In addition, there was no noticeable change on the voltage output after using a different cell combination in a series stack. As shown in Fig. 4(b), a similar phenomenon was found after switching hybrid to series connection. With respect to transient process, the reversal MFC-4 had a relative long time compare with other MFCs. The above results indicated that the voltage reversal of the cell would result in an obvious decrease in the voltage output when switching from a parallel or hybrid connection to a series connection.
Response to variable loads In addition to connection types, external loads would be another key parameter for the practical operation of the stack. As shown in Fig. 3(b) and (c), external load had a significant effect on the stack in the series connection. In the series connection, the stack was operated under 1000, 400, 100 and 50 U in sequence as shown in Fig. 5. At a high external load (1000 U), the stack had a high voltage of 2.77 V and the four MFCs had similar voltage. After operating under 400 U, the stack voltage dropped to 2.08 V, while MFCs-1~3 maintained a similar cell voltage (approximately 0.60 V). It was found that MFC-4 dropped to a very low voltage (25.8 mV). The stack voltage (1.83 V) showed a 245.0 mV decrease and the four MFCs showed small decrease in voltage when the external load dropped to 100 U. After making a further decrease of external load (50 U), the stack voltage firstly dropped to 0.274 V, sharply rose to 0.788 V and then gradually decreased to 0.389 V. MFC-1 and MFC-4 showed minor changes in voltage. The voltage of MFC-2 first dropped to 0.453 V and then quickly rose to 0.576 V, while the voltage of MFC-3 first dropped, later increased and then gradually decreased to 0.664 V. This indicated that, after switching the external load from 100 to 50 U, the voltage reversal first occurred in MFC-2 for the short term and then occurred in MFC-3 for the long term. With respect to transient process, MFC stack had a much longer response time after switching to a lower external load. This was probably related the fact that a relatively low external load for the MFC operation led to a high current
Please cite this article in press as: Zhang L, et al., Response of stacked microbial fuel cells with serpentine flow fields to variable operating conditions, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.205
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single MFC (data not shown). The above results suggested that voltage reversal of a cell would occur in the series stack, especially when operating at a low external load, limiting the performance improvement of the stack.
Response to the flow rates of electrolytes
Fig. 3 e Response of SMFC voltages to the increasing number of connected cells in parallel connection with an external load of 30 U (a), in series connection with an external load of 400 U (b) and 150 U (c).
It is well known that mass transfer is one of crucial factors that affects the electricity generation of MFCs [3,37]. To solve the possible mass transfer limitation, serpentine flow fields were used in a stacked MFC. In this part, the stack responses to different flow rates (1, 10, 20 and 30 ml/min) were investigated both in a parallel stack with an external load of 10 U and in series stacks with an external load of 50 U. As shown in Fig. 6(a), the parallel stack had a voltage of 0.441 V at a current of 44.1 mA when the flow rates of both the anode and cathode were 1 ml/min. After increasing the flow rate (10 ml/min), the stack voltage sharply increased to 0.543 V. It was found that the anode potential and the cathode potential were largely decreased and increased, respectively. This was attributed to the enhanced mass transfer in both the anode and the cathode resulting from the increased flow rates. Thus, the mass transfer resistance was reduced and a higher voltage output was obtained. Minor improvement on the stack performance was observed when the flow rates were further increased to 20 and 30 ml/min. This suggested that mass transfer would not be the main dominating factor for the performance improvement [38]. With respect to the stack in series, at a flow rate of 1 ml/ min, the stack had a voltage of 0.614 V and a current of 12.3 mA. As shown in Fig. 6(b), the voltages of MFCs-1~4 were 0.560, 0.503, 0.548 and 0.868 V, respectively. It was obvious that the voltage reversal occurred in MFC-4, which might be partly due to the mass transfer limitation. After the flow rate was increased to 10 ml/min, it was found that the stack voltage was increased to 0.885 V with a current of 17.7 mA and MFCs-1~4 had large voltage increases. This was due to the enhanced mass transfer and reduced mass transfer resistance, promoting voltage output. A further improvement (60 mV) on the stack voltage was found when the flow rate was 20 ml/min, which mainly resulted from the voltage increase of MFC-4. After making a further increase in the flow rate (30 ml/ min), there was no obvious improvement on the voltage of stack and MFCs-1~4, suggesting the mass transfer would not be the limiting factor at a high flow rate [38]. The stack voltage was returned to a similar voltage (0.623 V), and MFC-4 had a reversal voltage (0.904 V) again when the flow rate was reduced to 1 ml/min. The above results indicated that the stack performance improved by increasing the flow rates to a certain extent, which was in accordance with the previous unbuffered tubular MFC using sequential anodeecathode configuration [38].
Implication of the results and outlook generation. However, in short time, the present mass transfer could not maintained the fast reaction rate and thus resulted in a high voltage loss. After a long time, the mass transfer was improved and the voltage of MFCs except the reversal MFC gradually raised. It is noted that the response of voltage to external loads in the parallel stack was similar to that of the
Stacked MFC is an important consideration for the scale-up and practical application. The accumulating studies were mainly focused on the reactor configuration and the phenomenon of voltage reversal [13e23]. With respect to voltage reversal, several methods have been reported effective to
Please cite this article in press as: Zhang L, et al., Response of stacked microbial fuel cells with serpentine flow fields to variable operating conditions, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.205
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Fig. 4 e Response of SMFC and cell voltages from parallel connection (a) or hybrid connection (b) to series connection.
Fig. 5 e Response of voltages to the decreasing external loads in SMFC with a series connection.
alleviate or avoid this phenomenon [14e17] and this study did not focus on this topic. However, for the practical application, the changes in the operation might be intermittently required and it is necessary to better understanding the response to
variable operating conditions. This study demonstrated that the connection mode significantly influenced the stack performance, indicating that the switching of the connection mode are needed according to the required electric properties for the devices. With respect to the design, an optimal number of cells should be considered. During the variable operating, voltage reversal still occurred and deteriorated stack performance. More attentions should be paid on this phenomenon in the practical operation of MFC stack. The enhancement of mass transfer could improve the stack performance to an extent. The present study demonstrated the changes in the operation significantly influenced stack performance and more variable operating conditions are necessary to investigate for the better understanding of MFC stack operation. It should be noted that, the change of operation mode could be regarded as changing external resistance or discharging current. It was reported that the external resistance and discharging current not only influenced power generation but also affected the biofilm structure and microbial communities. Thus, the response of biofilm structure and microbial communities to the change of operation mode should be further investigated for better understanding the stack operation in future application. It was reported that the operating temperature and the substrate quality and concentration can play determining roles in microorganism growth [39] and then
Fig. 6 e Response of voltage and electrode potentials of the stack in parallel connections (a) and in series connections (b) to the increasing flow rates of the electrolytes. Please cite this article in press as: Zhang L, et al., Response of stacked microbial fuel cells with serpentine flow fields to variable operating conditions, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.205
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it is necessary to study the response of MFC stacks to other variable operating conditions such as temperature, substrate kinds and concentrations. In addition, large-scale reactors are needed to be developed and better studied, which is significantly important for the future practical application. Last but not the least, the voltage reversal during the variable operating should be further studied and the possible methods are necessary to alleviate this phenomenon, improving the stack performance.
Conclusions In this study, four flat-plate MFCs with serpentine flow fields were constructed into a stack to investigate the stack performance and response to variable operating conditions. The results showed that the highest maximal power (22.2 mW) was observed in the series connection followed by the parallel connection (19.8 mW) and the hybrid connection (17.2 mW). With an increasing number of cells, the parallel stack showed a gradually decreasing increase in the voltage output. The stack in series showed an increase in the voltage at a high load while showed first increase and later a decrease in the voltage output at a low load. An obvious decrease in voltage output was observed when switching from both parallel and hybrid connections to the series connection due to the voltage reversal of cell. A similar phenomenon was found in the series connection after decreasing the load from 100 to 50 U. A large increase in the stack performance was observed in parallel and series connections after increasing the flow rate from 1 ml/min to 10 ml/min while no obvious improvement was shown after a further increase in the flow rate.
Acknowledgements This work was supported by the National Science Foundation for Young Scientists of China (No. 51606022), the National Natural Science Funds for Distinguished Young Scholar (No. 51325602), the National Natural Science Funds for Outstanding Young Scholar (No. 51622602), and the Fundamental Research Funds for the Central Universities (106112016CDJXY145504).
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cell through a transient response analysis of cell voltage. Biosens Bioelectron 2010;25:1629e34. Papaharalabos G, Greenman J, Stinchcombe A, Horsfield I, Melhuish C, Ieropoulos I. Dynamic electrical reconfiguration for improved capacitor charging in microbial fuel cell stacks. J Power Sources 2014;272:34e8. Yazdi H, Alzate-Gavria L, Ren Z. Pluggable microbial fuel cell stacks for septic wastewater treatment and electricity production. Bioresour Technol 2015;180:258e63. Zhuang L, Zheng Y, Zhou SG, Yuan Y, Yuan HR, Chen Y. Scalable microbial fuel cell (MFC) stack for continuous real wastewater treatment. Bioresour Technol 2012;106:82e8. Wu SJ, Li H, Zhou XC, Liang P, Zhang XY, Jiang Y, et al. A novel pilot-scale stacked microbial fuel cell for efficient electricity generation and wastewater treatment. Water Res 2016;98:396e403. An J, Lee YS, Kim T, Chang IS. Significance of maximum current for voltage boosting of microbial fuel cells in series. J Power Sources 2016;323:23e8. Torres CI, Marcus AK, Rittamnn BE. Proton transport inside the biofilm limits electrical current generation by anoderespiring bacteria. Biotechnol Bioeng 2008;100:872e81. Zhang L, Li J, Zhu X, Ye DD, Liao Q. Effect of proton transfer on the performance of unbuffered tubular microbial fuel cells in continuous flow mode. Int J Hydrogen Energy 2015;40:3953e60. k L, Kumar G, Bakonyi P, Zhen G, Sivagurunathan P, Koo Kim S, et al. Microbial electrochemical systems for sustainable biohydrogen production: surveying the experiences from a start-up viewpoint. Renew Sust Energ Rev 2017;70:589e97.
Please cite this article in press as: Zhang L, et al., Response of stacked microbial fuel cells with serpentine flow fields to variable operating conditions, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.205
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Response of stacked microbial fuel cells with serpentine flow fields to variable operating conditions Liang Zhang a,b, Jun Li a,b,*, Xun Zhu a,b, Ding-ding Ye a,b, Qian Fu a,b, Qiang Liao a,b a
Key Laboratory of Low-grade Energy Utilization Technologies and Systems (Chongqing University), Ministry of Education, Chongqing 40003, China b Institute of Engineering Thermophysics, Chongqing University, Chongqing 400030, China
article info
abstract
Article history:
The stacked microbial fuel cell (MFC) is a potential pathway for future applications, and its
Received 11 January 2017
response to variable operating conditions is important for practical operation. In this study,
Received in revised form
a stacked MFC with serpentine flow field was constructed to investigate the stack perfor-
18 April 2017
mance and response to cell number, connection type, variable loads and electrolyte flow
Accepted 19 April 2017
rates. The results showed that the highest maximal power (22.2 mW) was observed in a
Available online xxx
series connection, which was 12.1% and 29.1% higher than the maximal power in the parallel and hybrid connection, respectively. With the increasing number of cells, a grad-
Keywords:
ually decreasing increase in the voltage output was found in the parallel stack and series
Microbial fuel cell
stack at a high load, while the series stack showed first an increase and later a decrease in
Stack
the voltage output at low load. Voltage reversal was observed when switching to a series
Response
connection or decreasing the load, resulting in a decreased stack voltage. The performance
Variable operating conditions
of the stack could be improved to an extent by increasing the electrolyte flow rates.
Voltage reversal
© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Microbial fuel cells (MFCs), which uses electrochemically active bacteria (EAB) as catalysts to directly generate electricity from wastewater, is a potential pathway to a sustainable energy future [1,2]. Although there has been significant development in biofilm enrichment [3,4], reactor configuration [5,6], separator [7,8], electrode materials [4,9], mass transfer [3,10,11] and the optimization of process parameters [3,12], the power output of MFCs remains insufficient for
practical applications. For an individual MFC, the working voltage is usually below 0.5 V, resulting from the thermodynamic and kinetic constraints [1]. To fulfill the real application requirements, many individual MFCs could be connected in parallel, series and hybrid stacks to promote the current, voltage and power [13e15]. In the past decade, most studies of stacked MFCs were conducted on voltage reversal and configuration. It was found that charge reversal could lead to the reverse polarity of cells and a loss of power generation in the MFC series stack [13]. Various electronic components such as capacitors and DC/DC
* Corresponding author. Key Laboratory of Low-grade Energy Utilization Technologies and Systems (Chongqing University), Ministry of Education, Chongqing 40003, China. Fax: þ86 23 6510 3113. E-mail address: [email protected] (J. Li). http://dx.doi.org/10.1016/j.ijhydene.2017.04.205 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Zhang L, et al., Response of stacked microbial fuel cells with serpentine flow fields to variable operating conditions, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.205
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voltage boosters, have been used to alleviate this phenomenon to a certain extent and to further promote the power [14e16]. In addition, it was demonstrated that using the maximum power point tracking devices and hybrid connectivity, an increased power output and voltage reversal avoidance in series connected MFC stacks can be achieved [17]. With respect to the new configuration, several novel stacked MFCs were studied recently. Liter-scale baffled stacking microbial fuel cells were used to generate electricity and treat wastewater containing sulfide and high strength molasses [18,19]. A 10-liter serpentine-type MFC stack constructed with PVC (polyvinylchloride) pipe was developed to treat brewery wastewater [20]. A continuous microbial fuel cell stack with a dual gas diffusion cathode design was developed to treat dark fermentation effluent [21]. A hybrid microbial fuel cell stack based on single and double chamber MFCs was proposed for self-sustaining pH control [22]. Interestingly, untreated urine was used for power generation in a miniature microbial fuel cell stack [23]. Obviously, for future application, more attention should be paid to MFC stacks. A further important consideration for MFC stacks is their response to changes during operation. In future practical application, the MFC stack is used to power multiple devices with varied electric properties and it is necessary to switch the operation modes [24]. Compared with other fuel cells, MFCs had a relatively slow response due to the slow bioelectrochemical reaction [25]. It is well known that the architecture of the biofilm adapts in response to environmental stresses, such as low nutrient availability, high shear forces, unfavorable pH and toxic compounds [3]. In addition to environmental impacts, MFCs also need to adapt in response to the changes of operation in future practical applications. Several MFC studies reported the dynamic behavior in terms of variations in cells or changes in operating conditions and parameters. Katuri and Scott reported a dynamic response of MFC using membrane electrode assemblies design to the external resistance and COD changes, and a corresponding model was developed [25]. Yuan et al. investigated the responses of electrochemical characteristics and performance of anodic biofilms to pH changes in MFCs [26]. For MFC-based sensors, it is significantly important to investigate the dynamic response time and sensitivity after a change of operating parameters [12,27e30]. In addition, a dynamic model was developed to simulate the transient response of MFC voltage to the current step-change [31]. With respect to an MFC stack, few related research was investigated on the response to the variable operating conditions, which is insufficient for future practical operation. It was reported that a MFC stack fed with glycerol was tested to investigate the effects of connection modes on the electricity generation and microbial community [24]. Dynamic reconfiguration as a designing energy harvesting circuitry for MFC stacks can reduce the charging times by allowing storage of the same amount of energy in a shorter period of time [32]. However, more research should be focused on the dynamic response of MFC stack to other key variable operating conditions, proving better understanding of MFC stack operation. In this study, serpentine flow fields were used to enhance mass transfer and four flat plate MFCs with serpentine flow fields were constructed into a stack in parallel, series and
hybrid connection. The objective of the present study was to investigate the response of the stacked MFC to cell number, connection type, variable loads and electrolytes flow rates.
Materials and methods MFC stack configuration and inoculation In this experiment, the stacked microbial fuel cell with serpentine flow fields was conducted using four identical MFC units, as shown in Fig. 1(a) and (b). Each MFC consisted of a proton exchange membrane (PEM) (Nafion 117, DuPont), two carbon cloth electrodes (E-TEK, B-1A, America) and Plexiglass plates with a serpentine flow channel holding a volume of 2.7 ml. The PEM and electrodes had an apparent surface area of 25 cm2. Both anode and cathode compartments were equipped with Ag/AgCl reference electrodes. For each individual MFC, the anode compartment was inoculated with the effluent from a running MFC fed with an artificial wastewater. And the running MFC was previously inoculated using the activated sludge from the primary clarifier of Tangjiatuo Wastewater Treatment Plant of Chongqing. The artificial wastewater contained 2.7 g/L CH3COONa, 6 g/L Na2HPO4, 3 g/L KH2PO4, 0.1 g g/L NH4Cl, 0.5 g/L NaCl, 0.1 g/L MgSO4$7H2O, 15 mg/L CaCl2$2H2O and 1.0 mL/L of a trace elements solution (500 COD mg/L; pH: 7.04; Conductivity: 15.12 mS/cm). A 50 mM potassium ferricyanide solution was used as the catholyte. The flow rates of each anode and cathode were 1.0 ml/min, unless otherwise noted. After inoculation, the four MFCs were constructed into stacks in parallel, hybrid or series connections for the tests. During the stack test, the stack was operated at different connection modes as shown in Fig. 1(c). All the tests were conducted in a temperature-controlled room at 25 C.
Measurements and calculations Stack and cell voltages (U), anode and cathode potentials, of each MFC were collected every 15 s via an Agilent 34970A data acquisition unit connected to a PC. In the polarization test, the external resistance varied in a range of 5e1.0 104 U to control discharging and to record the voltageecurrent curves. After each change in the external resistance, the MFCs were held until the current and cell voltage reached steady-state with a voltage drift of less than 5 mV/h. The stack current (I) is equal to the voltage divided by the external load and the power (P) is the product of the voltage and the current.
Results and discussion Stacked MFC performance under different connection types To increase the overall stack voltage or current, the four individual MFCs were operated in parallel, series and hybrid connections. As shown in Fig. 2, this resulted in an open circuit potential (OCP) of 3.27 V for the series connection and 1.64 V for the hybrid connection, which was much higher than the OCP (0.82 V) of the parallel connection. With respect to the
Please cite this article in press as: Zhang L, et al., Response of stacked microbial fuel cells with serpentine flow fields to variable operating conditions, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.205
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Fig. 1 e Schematic of stacked microbial fuel cells with serpentine flow fields (a), 3-D schematic (b) and connection modes (c) in the experiment.
maximal power, the highest power (22.2 mW) was observed in the series connection, followed by the parallel connection (19.8 mW) and hybrid connection (17.2 mW). At the maximal power point, a highest voltage output of 2.11 V for the series connection and a highest current output of 44.5 mA for the parallel connection were observed. With respect to the hybrid connection, the maximal power was observed at a voltage of 1.0 V and a current of 16.9 mA. In addition, there was no obvious difference in MFC performance between the hybrid connections using different cell combinations. These results showed that connection type had significant effects on the stack performance. The maximal power density (2.22 W/m2 for series connection and 1.98 W/m2 for parallel connection) of the MFC stack was lower than that of the identical individual MFC (2.6 W/m2) startup under 50 U in our previous study [3]. This indicated that the MFC stack could increase the power but
decreased the power density compared with individual unit, which was in accordance with the previous studies [18,33]. However, a high voltage output or a high current generation could be obtained from the stacked MFCs, providing more practical application of MFCs. In addition, compared with other stacked MFC studies, the maximal power and the maximal power density in the present study were much higher (Table 1). This was due to both the lower Ohmic resistance of the flat-plate structure and the better mass transfer of the serpentine flow field. The reactor structure and mass transfer should be considered in future design of MFC stacks.
Response to the number of connected cells In principle, the power generation of a stack is promoted with the increasing number of individual cell [22,35]. However, this
Fig. 2 e Polarization curves (a) and power curves (b) of the stacked MFC with different connection types. Please cite this article in press as: Zhang L, et al., Response of stacked microbial fuel cells with serpentine flow fields to variable operating conditions, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.205
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Table 1 e Maximal performance of stacked MFCs. Reactor type
Number of individual MFC
Maximal power (mW)
Maximal powder density (MPD, W/m2)
Voltage at MPD (V)
Current density at PMD (A/m2)
References
Cubic MFC Rectangle MFC Baffled MFC Column MFC Tubular MFC Flat-plate MFC Flat-plate MFC
4 2 6 3 5 3 4
2.9 3.9 3.0 e 6.4 10.7 22.2
1.05 0.46 0.20 0.142 0.068 1.43 2.22
0.85 0.53 1.69 0.265 1.1 e 2.11
1.23 0.872 0.12 0.54 0.062 e 1.06
[14] [13] [18] [33] [34] [22] Present study
was not expected in the stacked microbial fuel cell due to the serious voltage reversal caused by the performance difference of the individual cell [36]. Thus, there is an optimal number of cells for an MFC stack. To investigate the response to the number of connected cells, the cells were connected into the series stack or the parallel stack in sequence as shown in Fig. 3. The four cells were in a stable open-circuit condition before being connected. With respect to the parallel stack, the voltage output largely increased from 441.3 mV to 519.7 mV after MFC-2 was connected in parallel with an external load of 30U. It was obvious that the response of voltage consisted of transient state and steady state. Further small increases in voltage output were observed in the parallel stack after connecting MFC-3 and MFC-4, respectively. This showed that a gradually decreasing increase in the voltage output was observed in the parallel stack after increasing the number of cells. However, the increase in voltage output was more than 0.6 V for the connection of each cell except MFC-4 in series stack at an external load of 400 U. After connecting MFC-4, there was no obvious change in voltage output, and MFC-4 showed a voltage drop to 26.0 mV, which might be due to the mass transfer limitation in anode. In addition, at a low external load of 150 U, there was a largely increase in voltage output after connecting MFC-2 while an obvious sharp increase and then gradual decrease in voltage output was observed after the later connection of MFC-3. The decrease in voltage output was attributed to the voltage reversal of MFC-3. The above similar phenomenon was found after connecting MFC-4. The above results indicated that, with the increasing number of cells, the stack in parallel connection showed a gradually decreasing increase in the voltage output. However, the stack in series connection showed an increase in voltage at high external loads, showing first an increase and later a decrease in the voltage output at low external loads. In addition, the series stack at a low load had a relatively slow transient process compared with the parallel stack and the series stack at a high load. The above results indicated that an optimal number of cells would be considered for the future design of an MFC stack.
Response to connection types For future practical operation, it is necessary to investigate the stack response after switching the connection type according to the actual demand. It is reported that, in a series connection, the cell voltage reversal would deteriorate the stack performance. The response of the stack from a parallel or
hybrid connection to series connection was shown in Fig. 4. The stack voltage was dropped from 620.0 mV to 387.5 mV at an external load of 30 U after switching from the parallel to the series connection, which resulted from the voltage reversal of MFC-4. In addition, a long lag period was found after switching to series connection, which was in accordance with the reported study [24]. This was probably due to the adaption of anodic microorganisms to the new micro-environment resulted from the new connection mode. In addition, there was no noticeable change on the voltage output after using a different cell combination in a series stack. As shown in Fig. 4(b), a similar phenomenon was found after switching hybrid to series connection. With respect to transient process, the reversal MFC-4 had a relative long time compare with other MFCs. The above results indicated that the voltage reversal of the cell would result in an obvious decrease in the voltage output when switching from a parallel or hybrid connection to a series connection.
Response to variable loads In addition to connection types, external loads would be another key parameter for the practical operation of the stack. As shown in Fig. 3(b) and (c), external load had a significant effect on the stack in the series connection. In the series connection, the stack was operated under 1000, 400, 100 and 50 U in sequence as shown in Fig. 5. At a high external load (1000 U), the stack had a high voltage of 2.77 V and the four MFCs had similar voltage. After operating under 400 U, the stack voltage dropped to 2.08 V, while MFCs-1~3 maintained a similar cell voltage (approximately 0.60 V). It was found that MFC-4 dropped to a very low voltage (25.8 mV). The stack voltage (1.83 V) showed a 245.0 mV decrease and the four MFCs showed small decrease in voltage when the external load dropped to 100 U. After making a further decrease of external load (50 U), the stack voltage firstly dropped to 0.274 V, sharply rose to 0.788 V and then gradually decreased to 0.389 V. MFC-1 and MFC-4 showed minor changes in voltage. The voltage of MFC-2 first dropped to 0.453 V and then quickly rose to 0.576 V, while the voltage of MFC-3 first dropped, later increased and then gradually decreased to 0.664 V. This indicated that, after switching the external load from 100 to 50 U, the voltage reversal first occurred in MFC-2 for the short term and then occurred in MFC-3 for the long term. With respect to transient process, MFC stack had a much longer response time after switching to a lower external load. This was probably related the fact that a relatively low external load for the MFC operation led to a high current
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single MFC (data not shown). The above results suggested that voltage reversal of a cell would occur in the series stack, especially when operating at a low external load, limiting the performance improvement of the stack.
Response to the flow rates of electrolytes
Fig. 3 e Response of SMFC voltages to the increasing number of connected cells in parallel connection with an external load of 30 U (a), in series connection with an external load of 400 U (b) and 150 U (c).
It is well known that mass transfer is one of crucial factors that affects the electricity generation of MFCs [3,37]. To solve the possible mass transfer limitation, serpentine flow fields were used in a stacked MFC. In this part, the stack responses to different flow rates (1, 10, 20 and 30 ml/min) were investigated both in a parallel stack with an external load of 10 U and in series stacks with an external load of 50 U. As shown in Fig. 6(a), the parallel stack had a voltage of 0.441 V at a current of 44.1 mA when the flow rates of both the anode and cathode were 1 ml/min. After increasing the flow rate (10 ml/min), the stack voltage sharply increased to 0.543 V. It was found that the anode potential and the cathode potential were largely decreased and increased, respectively. This was attributed to the enhanced mass transfer in both the anode and the cathode resulting from the increased flow rates. Thus, the mass transfer resistance was reduced and a higher voltage output was obtained. Minor improvement on the stack performance was observed when the flow rates were further increased to 20 and 30 ml/min. This suggested that mass transfer would not be the main dominating factor for the performance improvement [38]. With respect to the stack in series, at a flow rate of 1 ml/ min, the stack had a voltage of 0.614 V and a current of 12.3 mA. As shown in Fig. 6(b), the voltages of MFCs-1~4 were 0.560, 0.503, 0.548 and 0.868 V, respectively. It was obvious that the voltage reversal occurred in MFC-4, which might be partly due to the mass transfer limitation. After the flow rate was increased to 10 ml/min, it was found that the stack voltage was increased to 0.885 V with a current of 17.7 mA and MFCs-1~4 had large voltage increases. This was due to the enhanced mass transfer and reduced mass transfer resistance, promoting voltage output. A further improvement (60 mV) on the stack voltage was found when the flow rate was 20 ml/min, which mainly resulted from the voltage increase of MFC-4. After making a further increase in the flow rate (30 ml/ min), there was no obvious improvement on the voltage of stack and MFCs-1~4, suggesting the mass transfer would not be the limiting factor at a high flow rate [38]. The stack voltage was returned to a similar voltage (0.623 V), and MFC-4 had a reversal voltage (0.904 V) again when the flow rate was reduced to 1 ml/min. The above results indicated that the stack performance improved by increasing the flow rates to a certain extent, which was in accordance with the previous unbuffered tubular MFC using sequential anodeecathode configuration [38].
Implication of the results and outlook generation. However, in short time, the present mass transfer could not maintained the fast reaction rate and thus resulted in a high voltage loss. After a long time, the mass transfer was improved and the voltage of MFCs except the reversal MFC gradually raised. It is noted that the response of voltage to external loads in the parallel stack was similar to that of the
Stacked MFC is an important consideration for the scale-up and practical application. The accumulating studies were mainly focused on the reactor configuration and the phenomenon of voltage reversal [13e23]. With respect to voltage reversal, several methods have been reported effective to
Please cite this article in press as: Zhang L, et al., Response of stacked microbial fuel cells with serpentine flow fields to variable operating conditions, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.205
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Fig. 4 e Response of SMFC and cell voltages from parallel connection (a) or hybrid connection (b) to series connection.
Fig. 5 e Response of voltages to the decreasing external loads in SMFC with a series connection.
alleviate or avoid this phenomenon [14e17] and this study did not focus on this topic. However, for the practical application, the changes in the operation might be intermittently required and it is necessary to better understanding the response to
variable operating conditions. This study demonstrated that the connection mode significantly influenced the stack performance, indicating that the switching of the connection mode are needed according to the required electric properties for the devices. With respect to the design, an optimal number of cells should be considered. During the variable operating, voltage reversal still occurred and deteriorated stack performance. More attentions should be paid on this phenomenon in the practical operation of MFC stack. The enhancement of mass transfer could improve the stack performance to an extent. The present study demonstrated the changes in the operation significantly influenced stack performance and more variable operating conditions are necessary to investigate for the better understanding of MFC stack operation. It should be noted that, the change of operation mode could be regarded as changing external resistance or discharging current. It was reported that the external resistance and discharging current not only influenced power generation but also affected the biofilm structure and microbial communities. Thus, the response of biofilm structure and microbial communities to the change of operation mode should be further investigated for better understanding the stack operation in future application. It was reported that the operating temperature and the substrate quality and concentration can play determining roles in microorganism growth [39] and then
Fig. 6 e Response of voltage and electrode potentials of the stack in parallel connections (a) and in series connections (b) to the increasing flow rates of the electrolytes. Please cite this article in press as: Zhang L, et al., Response of stacked microbial fuel cells with serpentine flow fields to variable operating conditions, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.205
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it is necessary to study the response of MFC stacks to other variable operating conditions such as temperature, substrate kinds and concentrations. In addition, large-scale reactors are needed to be developed and better studied, which is significantly important for the future practical application. Last but not the least, the voltage reversal during the variable operating should be further studied and the possible methods are necessary to alleviate this phenomenon, improving the stack performance.
Conclusions In this study, four flat-plate MFCs with serpentine flow fields were constructed into a stack to investigate the stack performance and response to variable operating conditions. The results showed that the highest maximal power (22.2 mW) was observed in the series connection followed by the parallel connection (19.8 mW) and the hybrid connection (17.2 mW). With an increasing number of cells, the parallel stack showed a gradually decreasing increase in the voltage output. The stack in series showed an increase in the voltage at a high load while showed first increase and later a decrease in the voltage output at a low load. An obvious decrease in voltage output was observed when switching from both parallel and hybrid connections to the series connection due to the voltage reversal of cell. A similar phenomenon was found in the series connection after decreasing the load from 100 to 50 U. A large increase in the stack performance was observed in parallel and series connections after increasing the flow rate from 1 ml/min to 10 ml/min while no obvious improvement was shown after a further increase in the flow rate.
Acknowledgements This work was supported by the National Science Foundation for Young Scientists of China (No. 51606022), the National Natural Science Funds for Distinguished Young Scholar (No. 51325602), the National Natural Science Funds for Outstanding Young Scholar (No. 51622602), and the Fundamental Research Funds for the Central Universities (106112016CDJXY145504).
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Please cite this article in press as: Zhang L, et al., Response of stacked microbial fuel cells with serpentine flow fields to variable operating conditions, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.205