Response of anodic biofilm and the performance of microbial fuel cells to different discharging current densities

Accepted Manuscript Response of anodic biofilm and the performance of microbial fuel cells to different discharging current densities Jun Li, Hejing Li, Jili Zheng, Liang Zhang, Qian Fu, Xun Zhu, Qiang Liao PII: DOI: Reference:

S0960-8524(17)30206-7 http://dx.doi.org/10.1016/j.biortech.2017.02.083 BITE 17655

To appear in:

Bioresource Technology

Received Date: Revised Date: Accepted Date:

5 January 2017 16 February 2017 17 February 2017

Please cite this article as: Li, J., Li, H., Zheng, J., Zhang, L., Fu, Q., Zhu, X., Liao, Q., Response of anodic biofilm and the performance of microbial fuel cells to different discharging current densities, Bioresource Technology (2017), doi: http://dx.doi.org/10.1016/j.biortech.2017.02.083

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Response of anodic biofilm and the performance of microbial fuel cells to different discharging current densities *

Jun Li 1, 2, Hejing Li 1, 2, Jili Zheng1,2, Liang Zhang1, 2 , Qian Fu1, 2, Xun Zhu 1, 2, Qiang Liao 1, 2

1

Key Laboratory of Low-grade Energy Utilization Technologies and Systems, Chongqing

University, Ministry of Education, Chongqing 400030, P.R. China 2

Institute of Engineering Thermophysics, Chongqing University, Chongqing 400030, P.R. China

*Corresponding author. Tel.: +86-23-6510-3102; Fax: +86-23-6510-3102; E-mail address: [email protected] (Liang Zhang).

1

Abstract To better understand the responses of anodic biofilm and MFC performance, five identical MFCs started at 100 Ω were operated with different discharging current densities (0.3, 1.6, 3.0, 3.6 and 4.8 A/m2, denoted as MFC-0.3, MFC-1.6, MFC-3.0, MFC-3.6 and MFC-4.8, respectively). It was demonstrated that the discharging current would significantly influence biofilm development and MFC performance. Compared with the original MFC started at 100 Ω, the performance of MFC-0.3 and MFC-1.6 decreased, whereas MFC-3.0 and MFC-3.6 exhibited improved maximum power densities. This was attributed to the reduced charge transfer resistance resulting from the increased active biomass after increasing discharging current. This indicated that the increasing discharging current could enhance active biomass and performance. However, a high discharging current density (4.8 A/m2) caused the exfoliation of carbon particles from the carbon cloth and then the detachment of the anode biofilm, resulting in the cell failure of MFC-4.8.

Keywords: microbial fuel cells, discharging current, performance response, biofilm structure, carbon corrosion

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1. Introduction Microbial fuel cell (MFC) using microorganisms as biocatalyst is a promising device that can recover electricity from the organic matter in domestic or industrial wastewaters (Logan et al., 2006; Oon et al., 2016; Wang et al., 2015). Although significant development has been achieved in recent years, the power generation of MFCs remains insufficient for practical application. In order to promote MFC performance, efforts have been made to enrich more electrochemically active bacteria (Zhang et al., 2011; Wang et al., 2016), to improve reactor configuration (Dong et al., 2015; Zhang et al., 2015; Chang et al., 2017), to identify better electrode materials (Hidalgo et al., 2016; Yang et al., 2017), as well as to optimize process parameters (Jadhav et al., 2009; Wei et al., 2010; Kim et al., 2016). Optimization of the growth condition for the electrochemically active bacteria (EAB) was also an important consideration for improving MFC performance. It was demonstrated optimization of biofilm growth was a useful method to improve the microbial kinetics and then enhance the power generation. Previous studies reported that by optimizing the growth factors (Aelterman et al., 2008; Catal et al., 2011; Jadhav and Ghangrekar, 2009; Rabaey and Verstraete, 2005; You et al., 2007), such as flow rate, pH, temperature and nutrient supply, a high MFC performance and a short start-up time can be achieved. For the MFC startup and operation, three methods such as regulating external resistance, applying anode potential and discharging current are used to control the circuit, which are the most important for optimizing the biofilm growth. The related MFC studies were summarized in the Table S1. The 3

external resistance is usually used to operate MFCs because it controls the current passing through the electrodes. It was found that bacterial diversity, metabolism pathway and biofilm community in the anode were significantly affected by the external resistance (Jung and Regan, 2011; Rismani-Yazdi et al., 2011; katuri et al., 2011). Zhang et al. also demonstrated that an optimal biofilm microstructure and power generation can be obtained at a proper external resistance (Zhang et al., 2011). Besides the external resistance, the applied anode potential is also widely used since it regulates the theoretical energy gain for EABs. Aelterman et al. reported that by applied an optimal anode potential of -0.2 V vs. Ag/AgCl, an improved current and power generation can be achieved (Aelterman et al., 2008). Carmona-Martínez et al. found that increasing the anode potential from 0.1 to 0.4 V (vs. Ag/AgCl) resulted in an enhanced amount of active biomass on electrode and therefore a higher current generation (Carmona-Martínez et al., 2013). Torres et al. (Torres et al., 2009) and Zhang et al. (Zhang et al., 2013) showed that a low anode potential was beneficial for improving the biofilm conductivity and power production. However, little study was reported on the MFC start-up and operation by using the discharging current method. As directly determining the bio-electrochemical reaction rate, the discharging current was also significant important for MFC startup and operation and it is necessary to investigate its effects on the biofilm growth and MFC performance. In addition, scale-up is another important consideration in enabling a high performance for the MFC practical applications. One of the effective ways for scale-up is the development of MFC stacks. To meet the real application requirements, 4

many individual MFC cells need to be connected to form an MFC stack to sufficiently boost the power output. In principle, a near proportional increase in power generation and voltage output can be achieved by connecting the individual cells in series (Khaled et al., 2016; Wu et al., 2016; Yang et al., 2016; Yazdi et al., 2015). However, the power generation was limited by the performance non-uniformities of the individual cells in the serially connected MFC stacks. It was reported that the difference in internal resistances or operational conditions affected the maximal current of individual cell and caused voltage reversal in series-stacked MFCs (An et al., 2016). Before connection, there is the difference on the maximal current among all the individual cells. After connection in series, a unique current flow through all the individual MFC units having differences in maximal current. To maintain the same current in each individual cell, the biofilm of each individual cell would have response to the current changes and the anode potential of the dismatched individual cells diverged, causing the deviation of the operating conditions from the optimal condition. Since biofilm structure adapts under stimulation by environmental stresses (such as alterations in pH, temperature, external resistance, and anode potential) (Stewart and Franklin, 2008), it is reasonable to postulate that the biased operating conditions would lead to a significant change in the morphology and structure of the biofilm, and therefore cause significant change in the cell performance. Thus, it is expected that, in series-stacked MFCs, the biofilm and performance of the individual cell would be changed by the variable stack currents and then significantly influenced the stack performance. The response of the individual cells to the current changes of 5

stack is significant important to better understand the performance limitation of series stack. In this study, to better understand the response to the current, five identical MFCs were operated with different discharging current densities (0.3, 1.6, 3.0, 3.6 and 4.8 A/m2), respectively. The objectives of the present study were to assess the effects of the discharging current on the active mass content, surface morphology and electrochemical behavior of the biofilm, as well as the electricity generation of the MFC.

2. Materials and methods 2.1 MFC configuration and operation The experiments were conducted using six identical dual-chamber MFCs. Each MFC consisted of a proton exchange membrane (PEM) (7×7 cm, Nafion 117, Dupont), two carbon cloth electrodes (5×5 cm, E-TEK, B-1A, America) and two plexiglass plates (10×10 cm) with a 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. The anode compartments of the six MFCs were simultaneously inoculated with the effluent from a MFC running on acetate. The culture medium contained 0.68 g/L sodium acetate, 50 mM phosphate buffer solution, and 0.1 g/L NH4Cl, 0.5 g/L NaCl, 0.1 g/L MgSO4·7H2O, 15 mg/L CaCl2·2H2O and 1.0 mL/L trace elements solution. A 50mM K3[Fe(CN)6] solution was used as catholyte. Both the anolyte and catholyte 6

were continuously supplied at the flow rate of 1.5 mL/min. During the startup, all the MFCs were acclimated to relatively small external resistances (100 Ω) instead of 1000 Ω to obtain a high performance, as reported in previous studies (Kargi and Eker, 2007; Katuri et al., 2011; Koók et al., 2016). The MFCs were operated at 100 Ω for 10 days to obtain stable current generation after successful start-up. Then, five individual MFCs were operated with different discharging current densities (0.3, 1.6, 3.0, 3.6 and 4.8 A/m2, denoted as MFC-0.3, MFC-1.6, MFC-3.0, MFC-3.6 and MFC-4.8, respectively) for 10 days while the left one still operated at 100 Ω (MFC/100) was used for comparison. The whole experiment was repeated three times to make the results reliable. All the tests were conducted in a temperature-controlled room at 30 ± 1 °C. 2.2 Measurements and calculations Cell voltage, anode and cathode potentials of the MFCs were monitored by using a data acquisition system (Agilent 34970 A) every 30 s. In the polarization test, the external resistance was varied in a range of 5~1.0 ×10 4 Ω to control the condition of discharging. The power density of MFC was calculated by Eq. (1): P( W⁄m2 ) =

U2 R∙ A

(1)

Where U is the cell voltage (V), R is the external resistance (Ω), and A is the surface area of the electrodes (m2). Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were conducted using an electrochemical workstation (Zahner, Zennium). The anode, the cathode and an Ag/AgCl electrode (+0.197 V vs. SHE) were served as the working 7

electrode, the counter electrode and the reference electrode, respectively. CV measurements were conducted at a scan rate of 1 mV/s in the potential range from -0.6 ~ 0.1 V vs. Ag/AgCl. EIS analysis was carried out at an external resistance of 1000 Ω. The frequency was varied from 100 kHz to 10 mHz with a perturbation of 10 mV. The EIS data was fit using an equivalent circuit to obtain the value of ohmic resistance, mass transfer resistance and charge transfer resistance The biofilm morphologies of the MFCs applied at different discharging currents were analyzed by a scanning electron microscopy (SEM) (3400N, HITACHI instrument). Before the test, the samples were pre-treated according to the following steps: remove a portion of the carbon cloth from different areas from the anode and fix it with phosphate buffer saline (PBS) containing 2.5% glutaraldehyde (pH 7.2– 7.4). Then, incubate the sample at 4℃ for 2 hours in the refrigerator. Next, clean the sample twice with 0.1% saline. Then, the samples were dehydrated a graded series of 30%, 50%, 70%, 80%, 90%, and 100% ethanol for 15 min each. Finally, the samples were blow-dried and sputter-coated with a layer of gold for enhancing the conductivity (Zhang et al., 2011). The quantization of total active biomass was calculated according to the phospholipid method (Findlay et al., 1989). The metabolic energy gain (MEG) for the electrochemically active bacteria was calculated with the following equation described by Wei et al. (Wei et al., 2010). t

MEG = ∫ (Eandoe(t) − E 0` )I (t)dt 0

Where

Eanode(t) and

(2)

I (t ) are the anode potential and current at time t during start-up 8

period respectively. E

0`

is the standard potential of CO2/acetate (-0.29 V). The

suspended solids in the effluent from the MFCs was collected by filtering through 0.2 µm pore-size nylon membrane. Then, the remaining solid on the filter was washed by deionized water and was dried at 105 °C overnight. The compositions of the collected samples were then analyzed by SEM (3400N, Hitachi instrument) equipped with an energy dispersive X-ray Spectrometer (EDS) (Cam-Scan MV 2300).

3. Results and discussion 3.1 MFC startup and operation with discharging currents The whole experiment was repeated three times and similar results were obtained. The average values were presented in the results. The six MFCs inoculated at 100 Ω (MFC/100) exhibited a similar stable current density (I0 in Fig. 1) of 2.5 ± 0.2 A/m2 after a 5.0-day start-up (data not shown). With respect to startup time, it was influenced by several factors such as external resistance (Zhang et al., 2011; katuri et al., 2011), reactor configuration (Zhang et al., 2011; Koók et al., 2016), anode material (Alterman et al., 2008), and feeding mode (Zhang et al., 2011; Koók et al., 2016). The startup time of the present study started up under 100 Ω was approximately 5 days, which was consistent with the startup time of the previous studies (Zhang et al., 2011; Katuri et al., 2011). Performance evaluation of the six MFCs also revealed nearly identical maximum power density (Pmax) and maximum current density (Imax). For clarity, only one representative performance of MFC was shown in Fig. 1. It was observed that MFC/100 showed a Pmax of 2.2 ± 0.1 W/m2 at a 9

current density of 4.0 ± 0.1 A/m2 and a Imax of 4.4 ± 0.2 A/m2 at 40 Ω. It was noted that power overshoot occurred in the MFCs when the external resistance was less than 40 Ω. To study the response to long-term discharging, five MFCs were operated with different discharging current densities for 10 days while the left one was still operated at 100 Ω. The five discharging current densities were selected as 0.3, 1.6, 3.0, 3.6 and 4.8 A/m2, respectively, corresponding to the dashed Line A to E in Fig. 1. During 10 days’ operation with discharging currents, it was found that MFC-0.3 and MFC-1.6 had a higher voltage output and MFC-3.6 had a lower voltage output compared with MFC/100. However, it was interesting that MFC-4.8 had a large negative voltage (less than -0.9 V).

3.2 Response of MFC performance The performance of the MFCs after operated at the selected discharging current densities were shown in Fig. 2. There was noted that no obvious change was found in MFC/100 performance (data not shown). However, after 10 days’ discharging operation, diverged MFC performances were observed in these five MFCs (Fig. 2). The details of the MFC performances were summarized in Table 1. As shown in Fig. 2a and Table 1, when the MFCs were operated at the current densities below I0 (e.g., 0.3 and 1.6 A/m2), lower Pmax and Imax values were noted as compared with MFC/100. On the other hand, operating the MFCs at a current density above I0 resulted in a higher performance than that of the original MFCs. For example, MFC-3.6 exhibited 10

a Pmax of 2.6 ± 0.2 W/m2, which was ~18% higher than that of MFC/100. More importantly, Fig. 2a reveals that the power overshoot phenomenon of MFC-3.6 was eliminated after long-term operation at the high current densities. Table 1 also indicates that the Imax values of MFC-3.0 and MFC-3.6 increased to 6.0 ± 0.1 and 11.3 ± 0.3 A/m2, respectively. Due to the elimination of the power overshoot, the Imax of MFC-3.6 was 2.6 times higher than that of MFC/100. In addition, it is interesting to note from Fig. 2a that the power output of MFC-4.8 decreased to near zero when the discharging current density (4.8 A/m2) is higher than Imax of MFC/100. The cell failure of MFC-4.8 suggested irreversible damages on the biofilms established on the anode surface of MFC-4.8. Furthermore, polarization curves of the MFCs in Fig. 2(c) show that the difference in power production of the MFCs was due to the different anode performance rather than the cathode. Thus, the electrochemical activity, the active biomass and the biofilm morphology of the biofilms should be discussed.

3.3 Response of MFC anode biofilm 3.3.1 EIS analysis of MFC anodes The above results indicated that the differences in the anode performance induced the diverged performance of MFCs. EIS tests were conducted to find the key factors for the differences in the anode performance by evaluating the internal resistance composition of anode. The anode internal resistance (Rin) is consisted of charge transfer resistance (Rct), ohmic resistance (Rohm) and mass transfer resistance (Rt). The 11

EIS data were fit using an equivalent circuit, and the fitting results are shown in Table 1. There was noted that a strange Nyquist spectra resulted in an unsuccessful fit for MFC-4.8 (date not shown). As seen in Table 1, the Rohm for different MFCs anode was similar due to the identical cell configuration and electrolyte. It was observed that the Rct value of the MFCs decreased with the increasing discharging current density. The Rct value of the anode of MFC-0.3, MFC-1.6, MFC-3.0 and MFC-3.6 was 18.4, 16.2, 13.4and 12.1 Ω, respectively. MFCs operated at high discharging current densities resulted in a low Rct, which might due to the beneficial effects of more active bacteria attachment and better biofilm growth at high current densities for promoting the electron transfer rate. 3.3.2 Electrochemical activity of anode biofilm The bioelectrochemical activity of the biofilms were examined by CV test under turnover condition. It was observed that the CV curves of the anode biofilms expect MFC-4.8 showed similar sigmoidal catalytic waves but with different levels of bio-catalytic current. All CV curves of the anodes exhibited oxidation peaks nearly at -0.28 (V vs. Ag/AgCl) (Fig. 3), which could be attributed to the oxidation of acetate. In addition, different peak current responses appeared under various applied current densities. The oxidation peak currents of MFC-0.3, MFC-1.6, MFC-3.0, MFC-3.6 and MFC/100 were 2.0, 3.1, 4.9, 5.6 and 4.5 A/m2 respectively, implying that the electrochemical activity of the biofilm for acetate oxidation was enhanced at a higher discharging current density. By comparison, the CV of MFC-4.8 anode changed noticeably with the appearance of electric double layer, which was significantly 12

covered by the oxidation-reduction current response (Fig. 3). It was reported that the appearance of the electric double layer in CV curves was due to the introduction of active groups (such as carboxyl and alcoholic hydroxyl) in the treatment of carbon paper by electrochemical methods (Momma et al., 1996; Tang et al., 2011; Zhou et al., 2012). This indicated that the electrochemical behavior of MFC-4.8 was probably resulted from the carbon cloth in anode instead of active biomass. Thus, there was probably no active biomass on MFC-4.8 anode. 3.3.3 Active biomass and energy gain during discharging Considering the direct relationship with current generation in the MFCs, the active biomass and the energy gain during the discharging were discussed in Fig. 4. As can be seen in Fig. 4, the energy gains for microorganism growth during discharging increased in the following order: MFC-0.3
samples developed under different discharging current densities were scanned. There is note that, SEM images at different parts of each biofilm sample were obtained. With respect to different samples, it was obvious that the significant differences were the surface coverage rate of biofilm on the anode, which verified the biomass results. However, for each biofilm sample, the surface coverage rates of biofilm at different parts of the anode were similar although the biofilm coverages were different. Thus, for each biofilm sample, one SEM representative was used to show the surface coverage rate of biofilm. It can be clearly seen that the coverage of the biofilm on the anode surface increased as the current densities increased from 0.3 to 3.6 A/m2. This result was consistent with the biomass results. For MFC-3.6, the entire anode surface was totally covered by the biofilm. The SEM image at a higher magnification provides further details of the biofilm morphology on the MFC-3.6 anode. As can be seen, extensive voids were found within biofilm. Similar phenomena was observed when the biofilms were established at a low external resistance (Zhang et al., 2011), and can be attributed to the adaption of biofilm to the environmental stresses resulting from the higher proton production rate at higher current densities (Torres et al., 2008). The existence of large amounts of voids within the biofilms would lead to the formation of water channels that facilitate the substrate and buffer supply, as well as proton removal, and thereby would be beneficial for the improvement of MFC performance. No microbes was observed on the MFC-4.8 anode, which supported the results in above sections. It was also interesting to note that, compared with the fibers on the 14

blank carbon cloth, numerous micro-cavities were found on the fiber surface of the carbon cloth used in MFC-4.8. When the discharging current densities (4.8 A/m2) was higher than Imax (4.4 A/m2), electrons produced by acetate electro-oxidation in the biofilms were not enough to meet the needs of the high current. Under this circumstance, carbon on the anode is prone to be oxidized to provide the required electrons for the high current, according to the following equation (Liang et al., 2009): C + 2H O → CO + 4H + 4e   = 0.207

(3)

As a consequence, the anode potential of MFC-4.8 increased to a value of 1.4 V (vs. Ag/AgCl) in 2 hours, resulting in a cell voltage of -1.2 V. It is speculated that aggravation of carbon corrosion would lead to the exfoliation of carbon particles from the carbon cloth, resulting in not only the formation of micro-cavities on the fibers of carbon cloth, but also the gradual detachment of the biofilm. To further verify carbon corrosion occurring on MFC-4.8 anode, the exfoliation of carbon particles from the carbon paper electrodes at 4.8 A/m2 was further confirmed by the color change of MFC effluents. The images of the effluent from MFC-4.8 and other MFCs was compared. The effluent from MFC-4.8 turned black after 10 days operation, while the others remained clear and colorless. In addition, a further evidence was the SEM and EDX analysis on the suspended solids in the effluent from MFC-4.8. SEM analysis of the suspended solids reveals irregular shaped particles with a random size distribution. From the EDS results, the amount of C and O were evaluated as 60.8 and 59.8 wt%, respectively. Beside these elements, slight amount of Na (0.7 wt%) and P (0.6 wt%) were also detected. The existence of a large amount of 15

C confirmed the exfoliation of carbon particles from the carbon cloth of MFC-4.8. The above results proved the carbon corrosion occurring on MFC-4.8 anode applied a high current.

3.4. Implication of the results This study demonstrated that, besides the startup period, the anodic biofilm and performance of MFCs would have a slow response to the changes during operation. Besides the environmental stresses, the control parameters such as discharging current would also dynamically influenced the anodic biofilm and then MFC performance. This would be helpful for the better understanding of future practical application under varying operating conditions. With respect to the discharging operation, a discharging current beyond the stable current during operation would be beneficial for microorganism growth due to the high energy gain and then result in high active biomass. A discharging current below the stable current would result in the opposite results. It was also found that the maximal current was changed after discharging operation. However, carbon corrosion occurred when the discharging current was higher than the maximal current of MFCs at the present condition. This indicated that electrons produced by acetate electro-oxidation in the biofilms were not enough to meet the needs of the high current and carbon on the anode is prone to be oxidized to provide the required electrons for the high current. Based on these results and recalling that voltage reversal of one individual cell occurred in series stack at a relatively high 16

current, one could assumed that carbon corrosion probably occurred in the parts of the anode of the reversal cell, which seriously limited the stack performance. In this case, the initial transient current of series stack was higher than the maximal current of the reversal MFC and led to the very quickly carbon corrosion and biofilm detachment. This irreversible process resulted in the extra high anode potential and then voltage reversal. The next research of our team would be focused on the above assumption and more attentions should be paid to the stack research, promoting MFC practical application.

4. Conclusions In this paper, the responses of the anodic biofilm and MFC performance to different discharging current densities were studied. The results showed that, MFC applied a high discharging current density induced the increase in active biomass and the electrochemical activity of biofilm, as well as the reduction of charge transfer resistance, leading to a promotion of MFC performance. However, when the discharging current density was higher than the Imax of the original MFC, cell failure occurred due to the detachment of the anode biofilm caused by the exfoliation of carbon particles from the carbon paper.

Acknowledgements This work was supported by the National Natural Science Funds for Outstanding Young Scholar (No. 51622602), the National Natural Science Funds for 17

Distinguished Young Scholar (No. 51325602), the National Science Foundation for Young Scientists of China (No. 51606022), and the Fundamental Research Funds for the Central Universities (106112016CDJXY145504).

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at:

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Table Caption Table 1. Pmax, Imax and the composition of the internal resistance of the MFCs operated at different charging current densities and 100 Ω.

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Figure Captions Fig. 1. Polarization and power curves for MFCs started up at 100 Ω (The arrow indicates the operating point of MFC/100 at 100 Ω and the dashed lines indicate the selected current densities during discharging) Fig. 2. Power density (a), voltage output (b) and electrode potentials (c) of MFCs operated at different discharging current densities after 10 days’ discharging Fig. 3. Cyclic Voltammetry tests on the anodic biofilms of MFC-0.3, 1.6, 3, 3.6 and MFC/100 under turnover condition at a potential sweep rate of 1 mV/s (Insert: MFC-4.8) Fig. 4. Active biomass after discharging and energy gain for microorganism growth during discharging

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Table 1. Pmax, Imax and the composition of the internal resistance of the MFCs operated at different charging current densities and 100 Ω. MFC sample

Pmax

I max

(W/m2)

(A/m2)

Rohm (Ω)

Rct (Ω)

MFC-0.3

1.3 ± 0.1

3.2 ± 0.1

1.0

18.4

MFC-1.6

1.8 ± 0.1

3.7 ± 0.1

0.8

16.2

MFC/100

2.2 ± 0.1

4.4 ± 0.2

0.8

14.1

MFC-3.0

2.3 ± 0.1

6.0 ± 0.1

0.8

13.4

MFC-3.6

2.6 ± 0.2

11.3 ± 0.3

0.9

12.1

MFC-4.8

--

--

--

--

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Rin (Ω)

Highlights

•

Different discharging current densities were applied to five identical MFCs.

•

Increasing discharging currents enhanced active biomass and performance of MFCs.

•

Cell failure occurred when the discharging current density was too high.

•

A high discharging current induced carbon corrosion and then biofilm detachment.

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