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The effect of Nafion membrane fouling on the power generation of a microbial fuel cell
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The effect of Nafion membrane fouling on the power generation of a microbial fuel cell Sami G.A. Flimban a, Sedky H.A. Hassan a,b, Md. Mukhlesur Rahman c, Sang-Eun Oh a,* a
Department of Biological Environment, Kangwon National University, Gangwondo, Chuncheon, South Korea Department of Botany & Microbiology, Faculty of Science, New Valley Branch, Assiut University, El-Kharga, 72511, Egypt c Department of Animal Science, Bangladesh Agricultural University, Mymensingh 2202, Bangladesh b
article info
abstract
Article history:
Microbial fuel cells (MFCs) are the most useful technologies for energy production and
Received 15 September 2017
wastewater treatment due to their low cost and support of the environment. In this study,
Received in revised form
the membrane fouling and their effects on power generation were investigated using
26 January 2018
scanning electron microscope (SEM). Results demonstrate that proton exchange mem-
Accepted 15 February 2018
brane (PEM) was affected by biofouling in a two-chamber H-type MFC, which would
Available online xxx
significantly affect coulombic efficiencies (CEs), and maximum power densities leading to reduced power generation. The power densities of both rice straw and potato peels were
Keywords:
119.35 mW/m2 and 152.55 mW/m2, respectively. Scanning Electron Microscope (SEM)
Nafion membrane
showed substantial accumulation of bacteria and their end-products forming a thick bio-
Membrane fouling
film on the surface of PEM leading to a decrease, if not, preventing the passage of protons
Microbial fuel cell
from the anode side toward the cathode side. The decline in power generation may result
Bioelectricity
mainly from the biofouling, not of electrodes but, of PEM membrane from both sides
Potato peels
(Anode and Cathode) because of improper regular PEM cleaning.
Rice straw
© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Energy is a fundamental input for social and economic activities [1]. The global demand for energy consumption increased dramatically, especially in developing countries like China, India, and Brazil [2,3]. Even though with a series of issues and challenges to the humanity and the environment that fossil fuels facing, more than 80% of the global energy was met by fossil fuels in 2013 and 70% of the energy supply investment has been related to the fossil fuels sector. Nevertheless, our energy source comes mainly from primary sources either non-
renewable source such as fossil fuels and nuclear power; and/ or renewable source such as biofuels, solar and wind power. These sources originate mostly in the Sun. Electricity is a secondary energy source (or energy carrier), because it is produced by converting primary sources of energy such as coal, natural gas, nuclear energy, solar energy, and wind energy into electrical power. Also, it can be converted to other forms of energy such as mechanical energy or heat. Continual use of fossil fuels is now widely recognized as unsustainable because of their depleting resources and contamination to the environment [4]. Microbial fuel cell (MFC) is a sustainable
* Corresponding author. E-mail address: [email protected] (S.-E. Oh). https://doi.org/10.1016/j.ijhydene.2018.02.097 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Flimban SGA, et al., The effect of Nafion membrane fouling on the power generation of a microbial fuel cell, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.02.097
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technology, which can generate power bioelectrochemically by converting biodegradable organic matter (i.e., rice straw and potato peels) directly into bioelectricity by microorganisms [5e8]. The MFC is composed of an anode, cathode, proton exchange membrane (PEM), biodegradable organic matter and an electrochemically active microorganism [6,9]. Bacterial cross-feeding interactions lead to the complex organic matters degradation and bioenergy generation in microbial electrochemical cells [10e12]. The power generation in MFC depends on many factors such as substrate and its concentration and electrophilic microorganisms [4,12]. Among many challenges facing the MFC technology and limiting the power of this technology are system architecture, types of materials used for electrodes, microbial community and their arrangements on the electrodes, PEM, and separator [13e15]. The separator is one of the most important MFCs components as it separates the anodic and cathodic compartments. The separator material has to be of high proton transfer coefficient to prevent any inhibition of protons transport to the cathode, and of low oxygen transfer coefficient to significantly improve the efficiency of MCFs. Cation exchange membranes (CEMs) such as Nafion, anion exchange membranes (AEM), and ultrafiltration membranes (UFM), have been used in various types of MFCs [4,16]. The most widely used CEM in MFCs is proton exchange membrane (PEM), because of higher cations conductivity and relatively lower internal resistance compared to other membranes material [17,18]. However, membrane fouling always occurs in MFC, as biofilm will undoubtedly be formed on PEM under long-term operation [19]. The effect of biofilm growth on and inside separators has been previously considered [18]. However, to ensure sustainable electricity production, the fouled PEM has to be recovered. Alternatively, it has to be replaced with a new one although the high production cost would be an issue, especially when scaling up for practical application of MFCs. Consequently, the PEM fouling deteriorates the performance of the MFCs.
Rice straw and potato peels are the most common and abundant lignocellulosic biomass residues in the world (about 650e975 million tons of rice straw annually produced) [20,21]. Rice straw contains high carbohydrate such as cellulose (32.47%), hemicelluloses (19.27%) and lignin (5.24%) [8,22,23]. Moreover, out of one ton of potato, 0.73 tons of potato peels is produced [24]. Bacteria in MFCs can utilize such macromolecules for power generation [25]. The rationales of this study were to: a) use rice straw and potato peels as a carbon source in MFC for generating electricity; b) investigate the behaviour of proton exchange membrane during biofouling in a two-chamber MFC system without cleaning for a prolonged period, and investigate the PEM biofouling by scan electron microscopy (SEM).
Materials and methods Microbial fuel cell configuration and operation Fig. 1 demonstrates the schematic diagram of a twochambered H-type microbial fuel cell (MFC) reactor made up of plexiglass. The total working volume of each chamber equipped with an electrode was approximately 200 mL of media with about 100 mL headspace, the total volume 300 mL and that was assembled as described previously by Oh et al. [26]. The actively projected surface area of both anode and cathode electrodes were 10 cm2. New electrodes were immersed in 1 mol L1 HCl to eliminate possible metal ion contamination. Electrical connection to the electrodes was made using copper wire hooked into carbon paper and pasted with catalyst 9 (25 mg) and ECCOBOND solder 56C (1.0 g) (1 part:40 part respectively). The anode and cathode were connected via a resistor with a range of 1000 U. The anode and cathode chambers were separated from each other by installing a proton exchange membrane (PEM) (surface area
Fig. 1 e Major elements of MFC reactor: 1. Proton Exchange Membrane (PEM) selective to Hþ cation only separating the two chambers; 2. Anode chamber under anaerobic conditions; 3. Cathode chamber under aerobic (open air) conditions; 4. Substrate or biomass for bacteria to feed on; 5. Pure or mixed bacterial culture (a) and biofilm (b); 6. Nitrogen gas to remove oxygen and maintain the anaerobic condition; 7. Anode electrode, on which bacterial attachment occurs; 8. Copper wire for transferring electrons to the cathode; 9. U resistor; 10. Cathode electrode to receive the electrons from the anode; 11. Air oxygen; 12. Electrons reducing agent. Please cite this article in press as: Flimban SGA, et al., The effect of Nafion membrane fouling on the power generation of a microbial fuel cell, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.02.097
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3.5 cm2, Nafion™ 117, Dupont Co., USA) in between during fuel cell operation. The PEM, when required, was pretreated and used as mentioned elsewhere [27,28]. Both compartments were filled with 200 mL of mineral nutrient buffer (NMB) solution or MFC media buffer. The anode chamber was flushed with pure N2 gas (N2; 99%) for 5e15 min to remove air and maintain anaerobic condition, and then capped with a rubber stopper. Nonetheless, cathode compartment was left to continuous open air to maintain aerobic condition. The anolyte in the reactor was refilled when the voltage dropped below ~30 mV. On the other hand, catholyte in the reactor was refilled when the solution is evaporated and reached the electrode level. Nevertheless, the catholyte in all experiments contained an unstirred total final concentration of 50 mM potassium ferricyanide. All MFC tests were operated at 1000 U external resistance, the MFC experiments were conducted at a constant temperature of 30 C.
Membranes pre-treatment The pre-treatment of the Nafion® 117 was done in steps of 1 h each to take off any impurities [26,27,29]. First of all, the membrane was boiled in distilled water, then for 1hr in 3% hydrogen peroxide (H2O2), washed with deionized distilled water, in 0.5 M of sulphuric acid (H2SO4) and finally, washed with de-ionized distilled water. Then, the PEM was stored in distilled water to maintain the membrane for excellent conductivity (when not in use, the MFC anode and cathode compartments were filled with deionized water).
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curves - produced by sweeping current values covering the range of 0.05e0.65 mA - were obtained. Power (W) was calculated according to the following equations: Power (W) ¼ Current (I) Voltage (V) Voltage (V) ¼ Current (I) External resistance R (U) The power density (PD) was determined as previously described [4], in which the power output is standardized to the projected anode surface area.
Scanning electron microscope (SEM) Employing Variable Pressure Field Emission Scanning Electron Microscope (VP-FESEM, SUPRA55VP, Carl Zeiss), biofilms on the anode surfaces and PEM were examined. The electrode with a bacterial layer attached was removed from the anode chamber, and rinsed with sterile deionized water. A razorsharp knife was then used to cut 20e40 mm beneath the surface of the electrode, allowing the surface to be removed from a thin plate. The thin plate containing electroactive microbial population was fixed in 4% glutaraldehyde (TED PELLA) and 1% paraformaldehyde solution (EMS) in 0.1 M cacodylate (Sigma) buffer (pH 7.4) for 3e4 h. Then samples were dehydrated stepwise in 100% ethanol: 100% isoamyl acetate (2:1, 1:1, 1:2 and 0:1, respectively) 20 min each. The electrode samples were then CO2-critical point dried (CPD) using Hitachi HCP-2, coated with gold/palladium (40/60) and then observed in SEM.
Substrates
Results and discussion Rice straw and potato peels were used as the sole electron donor in the present study and were collected from a local farm in Chuncheon, South Korea. The obtained rice straw was first washed with tap water few times, oven dried for 24 h at 45e60 C, cut into pieces (about 2e3 cm), and then ground in a blender.
Microorganisms and culture media MFC reactor was inoculated with a mixed culture of cellulose degrading bacteria (CDB). The CDB was isolated as described previously [7]. The CDB mixed culture was incubated at 30 C for a 1e2 week and then used as inoculum in the MFCs. Then, these inoculums were grown and fed on rice straw in 30 C incubator. The NMB solution consisted of NH4Cl (0.31 g/L), NaH2PO4H2O (2.49 g/L), KCl (0.13 g/L), Na2HPO4 (4.33 g/L), and 12.5 ml/L of trace metal and vitamin solutions were added as previously described [26].
Data acquisition, calculations, and analysis Each MFC system was monitored using a data acquisition system (2700, Keithley, OH) with a voltage across the resistor and a precise multimeter recorded every 30 min. The voltage of the individual electrodes was measured by an Ag/AgCl reference electrode (0.195; corrected to a standard hydrogen electrode, NHE). Employing IviumStat electrochemical analyzer (IVIUM Technology, the Netherlands), polarization
Start-up and enrichment of the MFCs The highest open circuit voltage (OCV) of 0.7 V was obtained using rice straw as a sole substrate. After attaining a stable, reproducible voltage of the MFCs, the culture medium in the anode chamber was replenished with fresh NMB medium accompanied with 1.0 g/L rice straw as substrate. Fig. 2 displays the working voltage generated using untreated rice straw at the external load of 1000 U (Fig. 2). Moreover, the lag phase observed (Fig. 2) was approximately 80e90 h when untreated rice straw was used in the MFC reactors. The initial lag phase appeared at the beginning, and then it disappeared after successive transfers of new media, suggesting that electricity generation from rice straw was primarily due to direct electron transfer by CDB attached to the anode and did not require accumulation of mediators in the fresh solution. Previous studies have shown similar results that refueling MFCs with new medium results in immediate power generation. The stable period for power generation with rice straw was usually longer than other pure substrates such as glucose, acetate [7,30]. Later, this stability is gradually disappeared with time (Fig. 2).
Electricity generation The MFCs were fed with 1.0 g/L rice straw or potato peels until a reproducible state of the MFCs was achieved. After voltage stability, and as the only electron donor, the MFC reactors
Please cite this article in press as: Flimban SGA, et al., The effect of Nafion membrane fouling on the power generation of a microbial fuel cell, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.02.097
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Fig. 2 e The figure demonstrates two reproducible cycles of power generation giving a reproducible voltage of about 400 mV for about ten days, including the lag phase and bacterial growth curve, using rice straw as a sole electron donor. Continuous power generation lasted for about 300 hours. Onset of changing the substrate and K3Fe(CN)6 are shown in arrows.
were fed with rice straw (Fig. 2). Reproducible voltage was achieved for few cycles for about few days. However, these two cycles are a repetition of many cycles within more than six months operation period of the system. Hassan et al. [8] reported a lag phase period of approximately 110 h after the first inoculation of the anode chamber with CDB and rice straw as substrate. Later, the cell voltage rapidly increased over the next 200 h, reaching an initial maximum voltage of 345 mV and a stable power density of 145.2 mW/m2 was maintained for ten days.
Polarization curve Within the period of over six months of inducing electroactive consortia, repeatable peak voltages were generated following the periodical addition of substrate every 7e10 days and hence
the performance of the MFC improved to reach a maximum voltage of about 700 mV (OCV). Later, after six months, the voltage started to decrease gradually until it reaches almost zero (Fig. 3). Simultaneously, when the voltage stabilized, polarization experiments were conducted to evaluate the effect of feeding substrate on the performance of MFCs. Since potato peels is well known for producing bioenergy with high efficiency [31], it is worth comparing it with rice straw. The polarization curve and power density versus current density during the batch operation are shown in Fig. 4 for untreated rice straw (Fig. 4A) and for untreated potato peels (Fig. 4B). With a resistance of 1000 U, the maximum power (Pmax) density of 119.35 mW/m2 and 152.55 mW/m2 respectively were reached. The highest power density obtained using twochambered MFCs with rice straw as substrates was much greater than those achieved with wheat straw hydrolysate (0.65e0.124 mW with an initial concentration from 0.25 to 2.0 g COD/L), xylose (38 mW/m2), and glucose (43 mW/m2) due to different organic composition of the rice straw [8]. Furthermore, after the establishment of the lag phase and reproducible power generation of MFC using rice straw as a sole electron donor, another biomass types such as reed, soybean, peanut, and potato peels were used to compare the similarities and differences in power generation against rice straw (data not shown). Unlike other kinds of biomass used, untreated potato peels demonstrated a significant and competitive result (Fig. 4B) compared to untreated rice straw (Fig. 4A).
COD removal and substrate degradation Chemical oxygen demand (COD) removal in the current study increased continuously with time (Fig. 5). Within 5 days, it decreased from 3800 mg/L to 3000 mg/L, and the removal percentage reached 18%. The percentage of COD removal increased gradually untill reaching 43.8% and COD reached to 2136 mg/L after ten days of operation. While at the end of operation reached to 72% (1050 mg/L). These results are
Fig. 3 e Power generation of the MFC reactor for more than six months without cleaning the PEM membrane showed a decline in power generation after the eleventh cycle. Onset of changing the substrate and K3Fe(CN)6 are shown in in arrows.
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Fig. 4 e Polarization curve with Nafion 117 using 1.0 g/L Untreated biomasses; rice straw (RS) (A) and potato skins (PS) (B); Htype MFC; mixed inoculum; 50 mM potassium ferricyanide in the cathode; and 1000 U resistor.
similar to that observed by Li et al., [32]. They reported that the COD removal of artificial wastewater reached to 88% at the end of the operation in baffled MFCs. On the other hand, the corresponding COD removal by the mixed culture of cellulose degrading bacteria using cellulose as a substrate in twochambered MFC was 29.33% after ten days of operation [7].
degradation might be determined by reducing sugar (RS). Fig. 6 shows the concentration of reducing sugars during rice straw degradation for electricity generation by MFC. At the initial stage, the concentration of RS was slightly low, and then the concentration of soluble sugars increased from 45 mg/L at three days to 144.4 mg/L at ten days. After that, it was decreased with the electricity production at a relatively stable level tell the end of operation at fifteen days. Similar results were observed by Hassan et al., [8], they used rice straw as a substrate for electricity generation in double chamber MFC by CDB as a biocatalyst. The RS concentration was low at an
Substrate degradation and reducing sugars Lignocellulosic compounds in rice straw could be hydrolyzed to simple soluble sugars in the anode chamber. The rice straw
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Time (Days) Fig. 5 e Chemical Oxygen Demand (COD) removal increased continuously with time reaching 72% by the end of operation. Please cite this article in press as: Flimban SGA, et al., The effect of Nafion membrane fouling on the power generation of a microbial fuel cell, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.02.097
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initial stage, and increased gradually after five days. After that the amount of soluble sugars was decreased as already consumed for electricity generation.
Nafion membrane fouling Furthermore, the effects of Nafion membrane fouling on power generation in MFCs were investigated. After running the reactor for more than six months, the performance of the MFC reactor dramatically declined and stayed diminutive for more than 200 h (Fig. 3). This indicates, there is something
somehow preventing the protons from being transferred from the anode to the cathode side. Otherwise, the performance of the MFC should be as perfect as shown in Fig. 2. This result leads to an assumption that the membrane might be deteriorated due to biofouling, the factor of which is considered as is a critical factor that leads to graduate but significant decrease in MFC performance and may lead to the cessation of the functionality of the system, the results of which confirmed what other groups found [10,11,28]. The power generation after biofouling is about 37% lower than that before fouling one. Ghasimi et al. demonstrated that the maximum power after biofouling is 40% lower than that before fouling [28]. In addition, they confirmed a very bad influence of biofouling on the PEM for long-running MFC. Moreover, Xu et al., reported severe biofouling of PEM (the layer consisted mainly of microorganisms, microbial extracellular polymers, and inorganic salts) after about 3 months with deterioration and decrease in ion exchange capacity and conductivity by 50% and 81%, respectively [11]. They also stated that the decline in the diffusion coefficient of proton and main cations by 42.9% and >50%, respectively. Subsequently, this lead to a direct cathode potential loss with a sever limitation of charge transport and increased internal resistance, which might be the main attribution to membrane deterioration. Furthermore, Cetinkaya et al. [10] confirmed the finding of the inorganic salts as constituent of the biofouled layer. In order to decrease the biofouling of PEM, it is recommended to clean the PEM every six months to make sure that no biofouling will occur, which in turn will help in increasing the possibility of experiments reproducibility. After six months, the biofouled membrane was cleaned via a physical cleaning. There was a
Fig 7 e SEM of the anode electrode carbon paper. (A) and (B) images represent the control without bacteria, while (C) and (D) images represent the samples with bacterial biofilm showing some physiological structures of the mixed culture bacterial community with Bacillus and coccus forms ((D), headarrows). In addition, some clumps ((D), box) with thread-like structures anchored into the electrode ((D), circle) demonstrating a nice equally distributed biofilm and bacterial attachment. Please cite this article in press as: Flimban SGA, et al., The effect of Nafion membrane fouling on the power generation of a microbial fuel cell, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.02.097
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significant increase in the cell voltage after cleaning (supplementary Fig. S1), supporting that cleaning of the membrane after a long time of operation is very important for preventing biofouling. Choi et al. [33], reported that MFC performance of the biofouled cation exchange membrane (CEM) was improved after cleaning CEM by physical brushing and distilled water.
Characterization of anodic electrode and membrane For understanding why there was a sudden dramatic decrease in the power generation of the reactor, biofouling and its degree has to be investigated. Therefore, the surface of the anodic electrode and PEM membrane was observed using SEM. After drying the Nafion 117 membrane, the SEMs were obtained. Fig. 7 demonstrates the status of anode carbon paper electrode on the onset of the experiment. Fig. 7A and B are representatives of carbon paper electrode with no bacterial attachment and Fig. 7C and D are representatives of carbon paper electrode with a reasonable accumulation of bacteria forming the essential element of MFC power generation (biofilm). The control carbon paper showed a surface as a structural network of carbon strands. However, the covered carbon paper with biofilm clearly demonstrates an equal distribution of mixed culture inoculated into the anode
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chamber. Moreover, the formation of biofilm on the electrode surface confirms a pronounced quorum sensing among the bacterial community (Fig. 7D, circled), the factor of which is important in biofilm formation and the resulting characteristics of the biofilm community. Furthermore, the attached microorganisms on the carbon paper electrode surface were both bacillus and coccus forms (Fig. 7D, arrowhead), and sometimes packed heavily and attached to each other (Fig. 7D, boxed). The biofilms were composed of highly populated bacterial forms attached to each other and to the anode electrode's surface forming a multilayer biofilm, which has a notable role in the electrons transfer from the bacteria (as a biocatalyst) to the electrode. A study group compared the electrochemical performance of the active biocatalyst at the anode surface, analyzing a developed mixed culture biofilm using cyclic voltammetry (CV) under the same conditions [28]. They found that before incubating the anode chamber with the mixed culture, no oxidation or reduction peaks were obtained. Fig. 8 shows SEM of Nafion117 (control) against the fouled PEM (Fig. 8B, C, and D). The samples show accumulation of bacteria and their end-products as bulkyclumps (Fig. 8D, boxed) adhering to each other forming a cracked wall leading to membrane biofouling (from the anode side) and may be blockage of the pores preventing proton transfer. Interestingly, filamentous or fibrous microscopic
Fig 8 e SEM images of Nafion 117. Anode side view showing the nature of the control membrane (A) compared to the biofouled sample (B) blocking the protons passage from the anode to the cathode compartment. Boxed area (C) shows clumps in bulk with heavy masses all around. Also, some branched structures are forming a network ((D) arrowheads) on the membrane, which seems to be some kind of fungi. Please cite this article in press as: Flimban SGA, et al., The effect of Nafion membrane fouling on the power generation of a microbial fuel cell, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.02.097
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Fig. 9 e SEM images of Nafion 117. Cathode side view showing a complete biofouling of the membrane blocking the way for the protons to be freely transferred from the anode to the cathode compartment.
structures of rice straw (Fig. 8D, arrowhead), and extracellular microbial byproducts, tend to grow on the PEM facing the anode side. This finding in our study was confirmed by another study testing a long-term performance ion exchange membrane held by Ping et al. when they found different shapes of microorganisms with a fungi-like deposit or filamentous organisms on the surface of membrane, the finding of which they explained as it might be a mixture of bacteria and fungi [24]. Furthermore, even with this high degree of membrane biofouling, the power generation is expected to be lower. Hence, the cathode side of the PEM membrane was checked under SEM microscope to confirm any damage might take place to the membrane upon handling or during the process of pretreatment and cleaning. An unexpected finding was experienced, as shown in Fig. 9. Biofouling was highly detectable covering the membrane (from cathode side) almost completely. Together, the biofouling of both anodic and cathodic sides of the membrane might be the reason of decreasing in the power generation, which was suddenly observed after about six months of repeatable, stable power generation using MFC.
Conclusions Although the biofilm of the anode electrode was perfectly distributed and there was no biofouling, but there was on the PEM membrane in both (anodic and cathodic) sides, which might be the primary reason for the decrease in power generation probably due to improper cleaning. Since the biofouling of the PEM could occur six months after the starting up, it is recommended to clean the PEM membrane every six or seven months to make sure that no biofouling will occur, which in turn will help in increasing the possibility of experiments reproducibility. Unfortunately, this could be done on a laboratory scale but not on large scales. Therefore, there should be another way to clean or avoid cleaning without affecting the
MFC performance if we are about to use two-chambered MFC otherwise, another configuration has to be used instead. In this study, we demonstrated that the PEM biofouling is a major factor that might affect the MFC performance. Therefore, cleaning, if not replacement, of the membrane is essentially required to maintain an efficient and producible power generation.
Acknowledgments This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2017R1D1A1A09000676) and by 2015 Research Grant from Kangwon National University (No. D1000100-01-01).
Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.ijhydene.2018.02.097.
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[5] Ulusoy I, Dimoglo A. Electricity generation in microbial fuel cell systems with Thiobacillus ferrooxidans as the cathode microorganism. Int J Hydrogen Energy 2018;43:1171e8. [6] Ghasemi M, Wan Daud WR, Hassan SHA, Jafary T, Rahimnejad M, Ahmad A, et al. Carbon nanotube/polypyrrole nanocomposite as a novel cathode catalyst and proper alternative for Pt in microbial fuel cell. Int J Hydrogen Energy 2016;41:4872e8. [7] Hassan SH, Kim YS, Oh S-E. Power generation from cellulose using mixed and pure cultures of cellulose-degrading bacteria in a microbial fuel cell. Enzym Microb Technol 2012;51:269e73. [8] Hassan SHA, Gad El-Rab SMF, Rahimnejad M, Ghasemi M, Joo J-H, Sik-Ok Y, et al. Electricity generation from rice straw using a microbial fuel cell. Int J Hydrogen Energy 2014;39:9490e6. [9] Mohamed HO, Obaid M, Khalil KA, Barakat NAM. Power generation from unconditioned industrial wastewaters using commercial membranes-based microbial fuel cells. Int J Hydrogen Energy 2016;41:4251e63. € € rog lu EO, Demir NM, Baysoy DY, Ozkaya [10] C¸etinkaya AY, Ko B, C¸akmakc¸ı M. Electricity production by a microbial fuel cell fueled by brewery wastewater and the factors in its membrane deterioration. Chin J Catal 2015;36:1068e76. [11] Xu J, Sheng G-P, Luo H-W, Li W-W, Wang L-F, Yu H-Q. Fouling of proton exchange membrane (PEM) deteriorates the performance of microbial fuel cell. Water Res 2012;46:1817e24. [12] Mohan Y, Manoj Muthu Kumar S, Das D. Electricity generation using microbial fuel cells. Int J Hydrogen Energy 2008;33:423e6. [13] Rinaldi A, Mecheri B, Garavaglia V, Licoccia S, Di Nardo P, Traversa E. Engineering materials and biology to boost performance of microbial fuel cells: a critical review. Energy Environ Sci 2008;1:417e29. [14] Logan BE, Wallack MJ, Kim K-Y, He W, Feng Y, Saikaly PE. Assessment of microbial fuel cell configurations and power densities. Environ Sci Technol Lett 2015;2:206e14. [15] Liu J, Liu L, Gao B, Yang F, Crittenden J, Ren N. Integration of microbial fuel cell with independent membrane cathode bioreactor for power generation, membrane fouling mitigation and wastewater treatment. Int J Hydrogen Energy 2014;39:17865e72. [16] Kim JR, Cheng S, Oh S-E, Logan BE. Power generation using different cation, anion, and ultrafiltration membranes in microbial fuel cells. Environ Sci Technol 2007;41:1004e9. € der U. Selectivity versus mobility: [17] Harnisch F, Schro separation of anode and cathode in microbial bioelectrochemical systems. ChemSusChem 2009;2:921e6. [18] Zhang X, Cheng S, Wang X, Huang X, Logan BE. Separator characteristics for increasing performance of microbial fuel cells. Environ Sci Technol 2009;43:8456e61.
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[19] Chae KJ, Choi M, Ajayi FF, Park W, Chang IS, Kim IS. Mass transport through a proton exchange membrane (Nafion) in microbial fuel cells. Energy Fuels 2007;22:169e76. [20] Binod P, Sindhu R, Singhania RR, Vikram S, Devi L, Nagalakshmi S, et al. Bioethanol production from rice straw: an overview. Bioresour Technol 2010;101:4767e74. [21] Gadde B, Bonnet S, Menke C, Garivait S. Air pollutant emissions from rice straw open field burning in India, Thailand and the Philippines. Environ Pollut 2009;157:1554e8. JC. Autohydrolysis of [22] Garrote G, Domınguez H, Parajo corncob: study of non-isothermal operation for xylooligosaccharide production. J Food Eng 2002;52:211e8. [23] Saha B. Hemicellulose bioconversion. J Ind Microbiol Biotechnol 2003;30:279e91. [24] Ping Q, Cohen B, Dosoretz C, He Z. Long-term investigation of fouling of cation and anion exchange membranes in microbial desalination cells. Desalination 2013;325:48e55. rez J, Munoz-Dorado J, de la Rubia T, Martinez J. [25] Pe Biodegradation and biological treatments of cellulose, hemicellulose and lignin: an overview. Int Microbiol 2002;5:53e63. [26] Oh S, Min B, Logan BE. Cathode performance as a factor in electricity generation in microbial fuel cells. Environ Sci Technol 2004;38:4900e4. [27] Oh S-E, Logan B. Proton exchange membrane and electrode surface areas as factors that affect power generation in microbial fuel cells. Appl Microbiol Biotechnol 2006;70:162e9. [28] Ghasemi M, Wan Daud WR, Ismail M, Rahimnejad M, Ismail AF, Leong JX, et al. Effect of pre-treatment and biofouling of proton exchange membrane on microbial fuel cell performance. Int J Hydrogen Energy 2013;38:5480e4. [29] Rahimnejad M, Ghasemi M, Najafpour GD, Ismail M, Mohammad AW, Ghoreyshi AA, et al. Synthesis, characterization and application studies of self-made Fe3O4/ PES nanocomposite membranes in microbial fuel cell. Electrochim Acta 2012;85:700e6. [30] Huang L, Zeng RJ, Angelidaki I. Electricity production from xylose using a mediator-less microbial fuel cell. Bioresour Technol 2008;99:4178e84. [31] Abanoz K, Stark B, Akbas M. Enhancement of ethanol production from potato-processing wastewater by engineering Escherichia coli using Vitreoscilla haemoglobin. Lett Appl Microbiol 2012;55:436e43. [32] Li Z, Yao L, Kong L, Liu H. Electricity generation using a baffled microbial fuel cell convenient for stacking. Bioresour Technol 2008;99:1650e5. [33] Choi M-J, Chae K-J, Ajayi FF, Kim K-Y, Yu H-W, Kim C-w, et al. Effects of biofouling on ion transport through cation exchange membranes and microbial fuel cell performance. Bioresour Technol 2011;102:298e303.
Please cite this article in press as: Flimban SGA, et al., The effect of Nafion membrane fouling on the power generation of a microbial fuel cell, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.02.097
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The effect of Nafion membrane fouling on the power generation of a microbial fuel cell Sami G.A. Flimban a, Sedky H.A. Hassan a,b, Md. Mukhlesur Rahman c, Sang-Eun Oh a,* a
Department of Biological Environment, Kangwon National University, Gangwondo, Chuncheon, South Korea Department of Botany & Microbiology, Faculty of Science, New Valley Branch, Assiut University, El-Kharga, 72511, Egypt c Department of Animal Science, Bangladesh Agricultural University, Mymensingh 2202, Bangladesh b
article info
abstract
Article history:
Microbial fuel cells (MFCs) are the most useful technologies for energy production and
Received 15 September 2017
wastewater treatment due to their low cost and support of the environment. In this study,
Received in revised form
the membrane fouling and their effects on power generation were investigated using
26 January 2018
scanning electron microscope (SEM). Results demonstrate that proton exchange mem-
Accepted 15 February 2018
brane (PEM) was affected by biofouling in a two-chamber H-type MFC, which would
Available online xxx
significantly affect coulombic efficiencies (CEs), and maximum power densities leading to reduced power generation. The power densities of both rice straw and potato peels were
Keywords:
119.35 mW/m2 and 152.55 mW/m2, respectively. Scanning Electron Microscope (SEM)
Nafion membrane
showed substantial accumulation of bacteria and their end-products forming a thick bio-
Membrane fouling
film on the surface of PEM leading to a decrease, if not, preventing the passage of protons
Microbial fuel cell
from the anode side toward the cathode side. The decline in power generation may result
Bioelectricity
mainly from the biofouling, not of electrodes but, of PEM membrane from both sides
Potato peels
(Anode and Cathode) because of improper regular PEM cleaning.
Rice straw
© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Energy is a fundamental input for social and economic activities [1]. The global demand for energy consumption increased dramatically, especially in developing countries like China, India, and Brazil [2,3]. Even though with a series of issues and challenges to the humanity and the environment that fossil fuels facing, more than 80% of the global energy was met by fossil fuels in 2013 and 70% of the energy supply investment has been related to the fossil fuels sector. Nevertheless, our energy source comes mainly from primary sources either non-
renewable source such as fossil fuels and nuclear power; and/ or renewable source such as biofuels, solar and wind power. These sources originate mostly in the Sun. Electricity is a secondary energy source (or energy carrier), because it is produced by converting primary sources of energy such as coal, natural gas, nuclear energy, solar energy, and wind energy into electrical power. Also, it can be converted to other forms of energy such as mechanical energy or heat. Continual use of fossil fuels is now widely recognized as unsustainable because of their depleting resources and contamination to the environment [4]. Microbial fuel cell (MFC) is a sustainable
* Corresponding author. E-mail address: [email protected] (S.-E. Oh). https://doi.org/10.1016/j.ijhydene.2018.02.097 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Flimban SGA, et al., The effect of Nafion membrane fouling on the power generation of a microbial fuel cell, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.02.097
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technology, which can generate power bioelectrochemically by converting biodegradable organic matter (i.e., rice straw and potato peels) directly into bioelectricity by microorganisms [5e8]. The MFC is composed of an anode, cathode, proton exchange membrane (PEM), biodegradable organic matter and an electrochemically active microorganism [6,9]. Bacterial cross-feeding interactions lead to the complex organic matters degradation and bioenergy generation in microbial electrochemical cells [10e12]. The power generation in MFC depends on many factors such as substrate and its concentration and electrophilic microorganisms [4,12]. Among many challenges facing the MFC technology and limiting the power of this technology are system architecture, types of materials used for electrodes, microbial community and their arrangements on the electrodes, PEM, and separator [13e15]. The separator is one of the most important MFCs components as it separates the anodic and cathodic compartments. The separator material has to be of high proton transfer coefficient to prevent any inhibition of protons transport to the cathode, and of low oxygen transfer coefficient to significantly improve the efficiency of MCFs. Cation exchange membranes (CEMs) such as Nafion, anion exchange membranes (AEM), and ultrafiltration membranes (UFM), have been used in various types of MFCs [4,16]. The most widely used CEM in MFCs is proton exchange membrane (PEM), because of higher cations conductivity and relatively lower internal resistance compared to other membranes material [17,18]. However, membrane fouling always occurs in MFC, as biofilm will undoubtedly be formed on PEM under long-term operation [19]. The effect of biofilm growth on and inside separators has been previously considered [18]. However, to ensure sustainable electricity production, the fouled PEM has to be recovered. Alternatively, it has to be replaced with a new one although the high production cost would be an issue, especially when scaling up for practical application of MFCs. Consequently, the PEM fouling deteriorates the performance of the MFCs.
Rice straw and potato peels are the most common and abundant lignocellulosic biomass residues in the world (about 650e975 million tons of rice straw annually produced) [20,21]. Rice straw contains high carbohydrate such as cellulose (32.47%), hemicelluloses (19.27%) and lignin (5.24%) [8,22,23]. Moreover, out of one ton of potato, 0.73 tons of potato peels is produced [24]. Bacteria in MFCs can utilize such macromolecules for power generation [25]. The rationales of this study were to: a) use rice straw and potato peels as a carbon source in MFC for generating electricity; b) investigate the behaviour of proton exchange membrane during biofouling in a two-chamber MFC system without cleaning for a prolonged period, and investigate the PEM biofouling by scan electron microscopy (SEM).
Materials and methods Microbial fuel cell configuration and operation Fig. 1 demonstrates the schematic diagram of a twochambered H-type microbial fuel cell (MFC) reactor made up of plexiglass. The total working volume of each chamber equipped with an electrode was approximately 200 mL of media with about 100 mL headspace, the total volume 300 mL and that was assembled as described previously by Oh et al. [26]. The actively projected surface area of both anode and cathode electrodes were 10 cm2. New electrodes were immersed in 1 mol L1 HCl to eliminate possible metal ion contamination. Electrical connection to the electrodes was made using copper wire hooked into carbon paper and pasted with catalyst 9 (25 mg) and ECCOBOND solder 56C (1.0 g) (1 part:40 part respectively). The anode and cathode were connected via a resistor with a range of 1000 U. The anode and cathode chambers were separated from each other by installing a proton exchange membrane (PEM) (surface area
Fig. 1 e Major elements of MFC reactor: 1. Proton Exchange Membrane (PEM) selective to Hþ cation only separating the two chambers; 2. Anode chamber under anaerobic conditions; 3. Cathode chamber under aerobic (open air) conditions; 4. Substrate or biomass for bacteria to feed on; 5. Pure or mixed bacterial culture (a) and biofilm (b); 6. Nitrogen gas to remove oxygen and maintain the anaerobic condition; 7. Anode electrode, on which bacterial attachment occurs; 8. Copper wire for transferring electrons to the cathode; 9. U resistor; 10. Cathode electrode to receive the electrons from the anode; 11. Air oxygen; 12. Electrons reducing agent. Please cite this article in press as: Flimban SGA, et al., The effect of Nafion membrane fouling on the power generation of a microbial fuel cell, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.02.097
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3.5 cm2, Nafion™ 117, Dupont Co., USA) in between during fuel cell operation. The PEM, when required, was pretreated and used as mentioned elsewhere [27,28]. Both compartments were filled with 200 mL of mineral nutrient buffer (NMB) solution or MFC media buffer. The anode chamber was flushed with pure N2 gas (N2; 99%) for 5e15 min to remove air and maintain anaerobic condition, and then capped with a rubber stopper. Nonetheless, cathode compartment was left to continuous open air to maintain aerobic condition. The anolyte in the reactor was refilled when the voltage dropped below ~30 mV. On the other hand, catholyte in the reactor was refilled when the solution is evaporated and reached the electrode level. Nevertheless, the catholyte in all experiments contained an unstirred total final concentration of 50 mM potassium ferricyanide. All MFC tests were operated at 1000 U external resistance, the MFC experiments were conducted at a constant temperature of 30 C.
Membranes pre-treatment The pre-treatment of the Nafion® 117 was done in steps of 1 h each to take off any impurities [26,27,29]. First of all, the membrane was boiled in distilled water, then for 1hr in 3% hydrogen peroxide (H2O2), washed with deionized distilled water, in 0.5 M of sulphuric acid (H2SO4) and finally, washed with de-ionized distilled water. Then, the PEM was stored in distilled water to maintain the membrane for excellent conductivity (when not in use, the MFC anode and cathode compartments were filled with deionized water).
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curves - produced by sweeping current values covering the range of 0.05e0.65 mA - were obtained. Power (W) was calculated according to the following equations: Power (W) ¼ Current (I) Voltage (V) Voltage (V) ¼ Current (I) External resistance R (U) The power density (PD) was determined as previously described [4], in which the power output is standardized to the projected anode surface area.
Scanning electron microscope (SEM) Employing Variable Pressure Field Emission Scanning Electron Microscope (VP-FESEM, SUPRA55VP, Carl Zeiss), biofilms on the anode surfaces and PEM were examined. The electrode with a bacterial layer attached was removed from the anode chamber, and rinsed with sterile deionized water. A razorsharp knife was then used to cut 20e40 mm beneath the surface of the electrode, allowing the surface to be removed from a thin plate. The thin plate containing electroactive microbial population was fixed in 4% glutaraldehyde (TED PELLA) and 1% paraformaldehyde solution (EMS) in 0.1 M cacodylate (Sigma) buffer (pH 7.4) for 3e4 h. Then samples were dehydrated stepwise in 100% ethanol: 100% isoamyl acetate (2:1, 1:1, 1:2 and 0:1, respectively) 20 min each. The electrode samples were then CO2-critical point dried (CPD) using Hitachi HCP-2, coated with gold/palladium (40/60) and then observed in SEM.
Substrates
Results and discussion Rice straw and potato peels were used as the sole electron donor in the present study and were collected from a local farm in Chuncheon, South Korea. The obtained rice straw was first washed with tap water few times, oven dried for 24 h at 45e60 C, cut into pieces (about 2e3 cm), and then ground in a blender.
Microorganisms and culture media MFC reactor was inoculated with a mixed culture of cellulose degrading bacteria (CDB). The CDB was isolated as described previously [7]. The CDB mixed culture was incubated at 30 C for a 1e2 week and then used as inoculum in the MFCs. Then, these inoculums were grown and fed on rice straw in 30 C incubator. The NMB solution consisted of NH4Cl (0.31 g/L), NaH2PO4H2O (2.49 g/L), KCl (0.13 g/L), Na2HPO4 (4.33 g/L), and 12.5 ml/L of trace metal and vitamin solutions were added as previously described [26].
Data acquisition, calculations, and analysis Each MFC system was monitored using a data acquisition system (2700, Keithley, OH) with a voltage across the resistor and a precise multimeter recorded every 30 min. The voltage of the individual electrodes was measured by an Ag/AgCl reference electrode (0.195; corrected to a standard hydrogen electrode, NHE). Employing IviumStat electrochemical analyzer (IVIUM Technology, the Netherlands), polarization
Start-up and enrichment of the MFCs The highest open circuit voltage (OCV) of 0.7 V was obtained using rice straw as a sole substrate. After attaining a stable, reproducible voltage of the MFCs, the culture medium in the anode chamber was replenished with fresh NMB medium accompanied with 1.0 g/L rice straw as substrate. Fig. 2 displays the working voltage generated using untreated rice straw at the external load of 1000 U (Fig. 2). Moreover, the lag phase observed (Fig. 2) was approximately 80e90 h when untreated rice straw was used in the MFC reactors. The initial lag phase appeared at the beginning, and then it disappeared after successive transfers of new media, suggesting that electricity generation from rice straw was primarily due to direct electron transfer by CDB attached to the anode and did not require accumulation of mediators in the fresh solution. Previous studies have shown similar results that refueling MFCs with new medium results in immediate power generation. The stable period for power generation with rice straw was usually longer than other pure substrates such as glucose, acetate [7,30]. Later, this stability is gradually disappeared with time (Fig. 2).
Electricity generation The MFCs were fed with 1.0 g/L rice straw or potato peels until a reproducible state of the MFCs was achieved. After voltage stability, and as the only electron donor, the MFC reactors
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Fig. 2 e The figure demonstrates two reproducible cycles of power generation giving a reproducible voltage of about 400 mV for about ten days, including the lag phase and bacterial growth curve, using rice straw as a sole electron donor. Continuous power generation lasted for about 300 hours. Onset of changing the substrate and K3Fe(CN)6 are shown in arrows.
were fed with rice straw (Fig. 2). Reproducible voltage was achieved for few cycles for about few days. However, these two cycles are a repetition of many cycles within more than six months operation period of the system. Hassan et al. [8] reported a lag phase period of approximately 110 h after the first inoculation of the anode chamber with CDB and rice straw as substrate. Later, the cell voltage rapidly increased over the next 200 h, reaching an initial maximum voltage of 345 mV and a stable power density of 145.2 mW/m2 was maintained for ten days.
Polarization curve Within the period of over six months of inducing electroactive consortia, repeatable peak voltages were generated following the periodical addition of substrate every 7e10 days and hence
the performance of the MFC improved to reach a maximum voltage of about 700 mV (OCV). Later, after six months, the voltage started to decrease gradually until it reaches almost zero (Fig. 3). Simultaneously, when the voltage stabilized, polarization experiments were conducted to evaluate the effect of feeding substrate on the performance of MFCs. Since potato peels is well known for producing bioenergy with high efficiency [31], it is worth comparing it with rice straw. The polarization curve and power density versus current density during the batch operation are shown in Fig. 4 for untreated rice straw (Fig. 4A) and for untreated potato peels (Fig. 4B). With a resistance of 1000 U, the maximum power (Pmax) density of 119.35 mW/m2 and 152.55 mW/m2 respectively were reached. The highest power density obtained using twochambered MFCs with rice straw as substrates was much greater than those achieved with wheat straw hydrolysate (0.65e0.124 mW with an initial concentration from 0.25 to 2.0 g COD/L), xylose (38 mW/m2), and glucose (43 mW/m2) due to different organic composition of the rice straw [8]. Furthermore, after the establishment of the lag phase and reproducible power generation of MFC using rice straw as a sole electron donor, another biomass types such as reed, soybean, peanut, and potato peels were used to compare the similarities and differences in power generation against rice straw (data not shown). Unlike other kinds of biomass used, untreated potato peels demonstrated a significant and competitive result (Fig. 4B) compared to untreated rice straw (Fig. 4A).
COD removal and substrate degradation Chemical oxygen demand (COD) removal in the current study increased continuously with time (Fig. 5). Within 5 days, it decreased from 3800 mg/L to 3000 mg/L, and the removal percentage reached 18%. The percentage of COD removal increased gradually untill reaching 43.8% and COD reached to 2136 mg/L after ten days of operation. While at the end of operation reached to 72% (1050 mg/L). These results are
Fig. 3 e Power generation of the MFC reactor for more than six months without cleaning the PEM membrane showed a decline in power generation after the eleventh cycle. Onset of changing the substrate and K3Fe(CN)6 are shown in in arrows.
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Fig. 4 e Polarization curve with Nafion 117 using 1.0 g/L Untreated biomasses; rice straw (RS) (A) and potato skins (PS) (B); Htype MFC; mixed inoculum; 50 mM potassium ferricyanide in the cathode; and 1000 U resistor.
similar to that observed by Li et al., [32]. They reported that the COD removal of artificial wastewater reached to 88% at the end of the operation in baffled MFCs. On the other hand, the corresponding COD removal by the mixed culture of cellulose degrading bacteria using cellulose as a substrate in twochambered MFC was 29.33% after ten days of operation [7].
degradation might be determined by reducing sugar (RS). Fig. 6 shows the concentration of reducing sugars during rice straw degradation for electricity generation by MFC. At the initial stage, the concentration of RS was slightly low, and then the concentration of soluble sugars increased from 45 mg/L at three days to 144.4 mg/L at ten days. After that, it was decreased with the electricity production at a relatively stable level tell the end of operation at fifteen days. Similar results were observed by Hassan et al., [8], they used rice straw as a substrate for electricity generation in double chamber MFC by CDB as a biocatalyst. The RS concentration was low at an
Substrate degradation and reducing sugars Lignocellulosic compounds in rice straw could be hydrolyzed to simple soluble sugars in the anode chamber. The rice straw
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Time (Days) Fig. 5 e Chemical Oxygen Demand (COD) removal increased continuously with time reaching 72% by the end of operation. Please cite this article in press as: Flimban SGA, et al., The effect of Nafion membrane fouling on the power generation of a microbial fuel cell, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.02.097
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initial stage, and increased gradually after five days. After that the amount of soluble sugars was decreased as already consumed for electricity generation.
Nafion membrane fouling Furthermore, the effects of Nafion membrane fouling on power generation in MFCs were investigated. After running the reactor for more than six months, the performance of the MFC reactor dramatically declined and stayed diminutive for more than 200 h (Fig. 3). This indicates, there is something
somehow preventing the protons from being transferred from the anode to the cathode side. Otherwise, the performance of the MFC should be as perfect as shown in Fig. 2. This result leads to an assumption that the membrane might be deteriorated due to biofouling, the factor of which is considered as is a critical factor that leads to graduate but significant decrease in MFC performance and may lead to the cessation of the functionality of the system, the results of which confirmed what other groups found [10,11,28]. The power generation after biofouling is about 37% lower than that before fouling one. Ghasimi et al. demonstrated that the maximum power after biofouling is 40% lower than that before fouling [28]. In addition, they confirmed a very bad influence of biofouling on the PEM for long-running MFC. Moreover, Xu et al., reported severe biofouling of PEM (the layer consisted mainly of microorganisms, microbial extracellular polymers, and inorganic salts) after about 3 months with deterioration and decrease in ion exchange capacity and conductivity by 50% and 81%, respectively [11]. They also stated that the decline in the diffusion coefficient of proton and main cations by 42.9% and >50%, respectively. Subsequently, this lead to a direct cathode potential loss with a sever limitation of charge transport and increased internal resistance, which might be the main attribution to membrane deterioration. Furthermore, Cetinkaya et al. [10] confirmed the finding of the inorganic salts as constituent of the biofouled layer. In order to decrease the biofouling of PEM, it is recommended to clean the PEM every six months to make sure that no biofouling will occur, which in turn will help in increasing the possibility of experiments reproducibility. After six months, the biofouled membrane was cleaned via a physical cleaning. There was a
Fig 7 e SEM of the anode electrode carbon paper. (A) and (B) images represent the control without bacteria, while (C) and (D) images represent the samples with bacterial biofilm showing some physiological structures of the mixed culture bacterial community with Bacillus and coccus forms ((D), headarrows). In addition, some clumps ((D), box) with thread-like structures anchored into the electrode ((D), circle) demonstrating a nice equally distributed biofilm and bacterial attachment. Please cite this article in press as: Flimban SGA, et al., The effect of Nafion membrane fouling on the power generation of a microbial fuel cell, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.02.097
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significant increase in the cell voltage after cleaning (supplementary Fig. S1), supporting that cleaning of the membrane after a long time of operation is very important for preventing biofouling. Choi et al. [33], reported that MFC performance of the biofouled cation exchange membrane (CEM) was improved after cleaning CEM by physical brushing and distilled water.
Characterization of anodic electrode and membrane For understanding why there was a sudden dramatic decrease in the power generation of the reactor, biofouling and its degree has to be investigated. Therefore, the surface of the anodic electrode and PEM membrane was observed using SEM. After drying the Nafion 117 membrane, the SEMs were obtained. Fig. 7 demonstrates the status of anode carbon paper electrode on the onset of the experiment. Fig. 7A and B are representatives of carbon paper electrode with no bacterial attachment and Fig. 7C and D are representatives of carbon paper electrode with a reasonable accumulation of bacteria forming the essential element of MFC power generation (biofilm). The control carbon paper showed a surface as a structural network of carbon strands. However, the covered carbon paper with biofilm clearly demonstrates an equal distribution of mixed culture inoculated into the anode
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chamber. Moreover, the formation of biofilm on the electrode surface confirms a pronounced quorum sensing among the bacterial community (Fig. 7D, circled), the factor of which is important in biofilm formation and the resulting characteristics of the biofilm community. Furthermore, the attached microorganisms on the carbon paper electrode surface were both bacillus and coccus forms (Fig. 7D, arrowhead), and sometimes packed heavily and attached to each other (Fig. 7D, boxed). The biofilms were composed of highly populated bacterial forms attached to each other and to the anode electrode's surface forming a multilayer biofilm, which has a notable role in the electrons transfer from the bacteria (as a biocatalyst) to the electrode. A study group compared the electrochemical performance of the active biocatalyst at the anode surface, analyzing a developed mixed culture biofilm using cyclic voltammetry (CV) under the same conditions [28]. They found that before incubating the anode chamber with the mixed culture, no oxidation or reduction peaks were obtained. Fig. 8 shows SEM of Nafion117 (control) against the fouled PEM (Fig. 8B, C, and D). The samples show accumulation of bacteria and their end-products as bulkyclumps (Fig. 8D, boxed) adhering to each other forming a cracked wall leading to membrane biofouling (from the anode side) and may be blockage of the pores preventing proton transfer. Interestingly, filamentous or fibrous microscopic
Fig 8 e SEM images of Nafion 117. Anode side view showing the nature of the control membrane (A) compared to the biofouled sample (B) blocking the protons passage from the anode to the cathode compartment. Boxed area (C) shows clumps in bulk with heavy masses all around. Also, some branched structures are forming a network ((D) arrowheads) on the membrane, which seems to be some kind of fungi. Please cite this article in press as: Flimban SGA, et al., The effect of Nafion membrane fouling on the power generation of a microbial fuel cell, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.02.097
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Fig. 9 e SEM images of Nafion 117. Cathode side view showing a complete biofouling of the membrane blocking the way for the protons to be freely transferred from the anode to the cathode compartment.
structures of rice straw (Fig. 8D, arrowhead), and extracellular microbial byproducts, tend to grow on the PEM facing the anode side. This finding in our study was confirmed by another study testing a long-term performance ion exchange membrane held by Ping et al. when they found different shapes of microorganisms with a fungi-like deposit or filamentous organisms on the surface of membrane, the finding of which they explained as it might be a mixture of bacteria and fungi [24]. Furthermore, even with this high degree of membrane biofouling, the power generation is expected to be lower. Hence, the cathode side of the PEM membrane was checked under SEM microscope to confirm any damage might take place to the membrane upon handling or during the process of pretreatment and cleaning. An unexpected finding was experienced, as shown in Fig. 9. Biofouling was highly detectable covering the membrane (from cathode side) almost completely. Together, the biofouling of both anodic and cathodic sides of the membrane might be the reason of decreasing in the power generation, which was suddenly observed after about six months of repeatable, stable power generation using MFC.
Conclusions Although the biofilm of the anode electrode was perfectly distributed and there was no biofouling, but there was on the PEM membrane in both (anodic and cathodic) sides, which might be the primary reason for the decrease in power generation probably due to improper cleaning. Since the biofouling of the PEM could occur six months after the starting up, it is recommended to clean the PEM membrane every six or seven months to make sure that no biofouling will occur, which in turn will help in increasing the possibility of experiments reproducibility. Unfortunately, this could be done on a laboratory scale but not on large scales. Therefore, there should be another way to clean or avoid cleaning without affecting the
MFC performance if we are about to use two-chambered MFC otherwise, another configuration has to be used instead. In this study, we demonstrated that the PEM biofouling is a major factor that might affect the MFC performance. Therefore, cleaning, if not replacement, of the membrane is essentially required to maintain an efficient and producible power generation.
Acknowledgments This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2017R1D1A1A09000676) and by 2015 Research Grant from Kangwon National University (No. D1000100-01-01).
Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.ijhydene.2018.02.097.
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Please cite this article in press as: Flimban SGA, et al., The effect of Nafion membrane fouling on the power generation of a microbial fuel cell, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.02.097