Selection of cheap electrodes for two-compartment

Journal of Electroanalytical Chemistry 785 (2017) 235–240

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Selection of cheap electrodes for two-compartment microbial fuel cells Y. Asensio, I.B. Montes, C.M. Fernandez-Marchante, J. Lobato, P. Cañizares, M.A. Rodrigo ⁎ Department of Chemical Engineering, Faculty of Chemical Sciences & Technologies, Universidad de Castilla La Mancha, Campus Universitario s/n, 13071 Ciudad Real. Spain

a r t i c l e

i n f o

Article history: Received 3 October 2016 Received in revised form 6 December 2016 Accepted 27 December 2016 Available online 29 December 2016 Keywords: Microbial fuel cells Anode material Cathode material Carbonaceous materials

a b s t r a c t This work compares the performance of four microbial fuel cells (MFCs) equipped with different cheap electrodic materials during two-month long tests, in which they were operated under the same operating conditions. Despite using sp2 carbon materials (carbon felt, foam and cloth) as anode in the four MFCs, results demonstrates that there are important differences in the performance, pointing out the relevance of the surface area and other physical characteristics on the efficiency of MFCs. Differences were found not only in the production of electricity but also in the consumption of fuel (acetate) and even in the cathodic consumption of oxygen. Carbon felt was found to be the most efficient anode material whereas the worst results were obtained with carbon cloth. Performance seems to be in direct relationship with the specific area of the anode materials. In comparing the performance of the MFC equipped with carbon felt and stainless steel as cathodes, the later shows the worst performance, which clearly indicates how the cathodic process may become the bottleneck of the MFC performance. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Microbial fuel cells (MFC) are energy conversion devices widely studied over the last decades [1,2]. Hundreds of papers have been published recently, pointing out the relevance of the topic for the scientific community [3]. Harvesting energy directly from organic matter as electricity is a promising concept, with very interesting results at small scales which, unfortunately, become difficult to be extrapolated in large facilities [4]. The clarification of the mechanisms involved, with a deeper understanding of the complex interactions between electrochemistry and biotechnology, is the more important handicap to be overcome in the near future and it justifies the research portfolios related to MFC currently carried out by many research groups [5]. In using mixed cultures in MFC, the microbial culture composition is expected to change and acclimate to the operation conditions applied [6,7]. In addition to the carbon source and nutrient composition (fuel of the MFC) [8,9], the values of the solid retention time and temperature [10] are known to be very important, as well as the organic loading rate used [11]. Initially, the electrochemical parameters are expected to show a lower relevance on the performance of the device and almost nil in the microbial composition. In fact, the most important electrochemical input is the choice of the electrode materials [12], because the electrocatalytic properties of these materials influence on the transfer of electrons required to harvest electricity from organic matter and their electric resistance on the voltage vs intensity performance [10]. Obviously, a cheap material exhibiting microbial-compatibility and suitable physical, chemical and electrochemical resistance is always the ⁎ Corresponding author. E-mail address: [email protected] (M.A. Rodrigo).

http://dx.doi.org/10.1016/j.jelechem.2016.12.045 1572-6657/© 2016 Elsevier B.V. All rights reserved.

target, in particular for the anode. According to literature carbonaceous material are the best choice [13,14] and thus, most of the recent works use this type of anode material, which in addition to have high conductivity, they appear to be well suited for bacterial growth [15,16]. However, there are many types of carbonaceous materials with different physical characteristics associated to the sp2-carbon [17] and it is important to determine the main differences between the performance of these materials in order to develop applications of MFC [18]. Within this context, carbon-based electrodes as foam and cloth are very common as electrode materials, exhibiting great advantages over the simpler carbon papers electrodes. Thus, carbon cloth is a flexible material with a greater porosity than carbon paper. It has been used as anode and cathode material with good results, achieving power densities near to 500 mW m−2 and 50% COD removal in single-chamber microbial fuel cell [19] and even higher when combined with activated carbon. Thus, in the treatment of fermented wastewater on a single chamber MFC, this combination achieves a power density of almost 3000 mW m−2 and 93% COD removal [11]. Its main drawback is its relative high cost, as compared to other carbonaceous materials [20]. Opposite to carbon cloth, carbon foams are much thicker and have more space for bacterial fixation, although the transfer of substrate typically limits the growth of microorganisms [12,21]. These materials have not been as extensively used in MFC studies as the paper and cloth materials. Carbon foam has been used as anode in marine benthic microbial fuel cells attaining a maximum power density of nearly 150 mW m−2 and higher values were obtained when carbon foam was modified with urea, attaining almost 260 mW m−2 of maximum power density [22]. Despite being very promising, carbon felt is less used although they have shown to be efficient 3D-electrodes in small mini-MFC [23] exhibiting good stability and fair robustness [24] [25].

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Y. Asensio et al. / Journal of Electroanalytical Chemistry 785 (2017) 235–240

Regarding the cathode material, the complexity is even higher than that shown for MFC anode materials [26,27]. At this point, it is worth to take in mind that in abiotic cathode MFC, the electrochemical reduction of oxygen to water is sometimes the bottleneck of the electricity production in MFC. This fact explains the great effort made in the recent years in the search for efficient cathodes, which includes not only the use of platinum and other catalyst but also special cell design such as the air-cathode microbial fuel cells [26,28]. The main drawback of using catalyst on the cathode is the usual operation temperature of MFC, for which typical catalyst are not very efficient. Hence, more conventional materials are in focus nowadays and within this context, stainless steel have already proved to be efficient in MFC attaining very high current densities (20.5 A m−2) have been achieved with a pure culture of an electrogenic bacteria. In addition, stainless steel cathodes exhibited high catalytic properties for oxygen reduction under this condition [29]. This study shows the influence of cheap anodic and cathodic materials on the performance of several MFCs operated over long periods. To do this, three carbonaceous materials (felt, foam and cloth) were evaluated as anodes in combination with two materials that were tested as cathodes (felt, stainless steel). No catalysts (such as platinum) were added because they may increase the prize and make the technology unfeasible from the view point of costs. The MFC were fed with a highly concentrated solution of acetate and nutrients, used as synthetic fuel and hydraulic retention time (HRT) was kept in 3.2 days over all the test. Two-compartment MFCs were used to evaluate the performance of the different electrode materials and a proton exchange membrane was placed to separate the anode and cathode compartments. The four cells monitored were seeded with the same mixed culture, fed with the same fuel solution and kept within the same operation conditions. Hence, changes are expected to depend only on the electrode material used. 2. Materials and methods 2.1. Microbial fuel cell set-up The set-up used in this work consisted of a MFC with two chambers (4 cm3 volume each one) separated by a proton exchange membrane,

PEM (Sterion®), which has a high ionic conductivity (0.02– 0.90 meq g− 1) and low electronic conductivity (8 × 10− 2 S cm− 1) and has been used previously in PEMFCs with good results [30]. Each MFC is formed by two HDL (high pressure laminate) plates and two silicon plates to improve the mechanical properties and avoid liquid losses. The electrode spacing between the anode and the cathode (1.0 cm) was minimized in order to reduce as much as possible the internal electrical losses from the system. The two electrodes (3 cm2 each) were connected by an external resistance (Rext) of 120 Ω; this low value was chosen to prevent activation losses and facilitate electron transfer during the acclimation period [31]. A fishery compressor that can provide a flow rate of 1.6 L min− 1 and a maximum pressure of 1.2 m of water-column was connected to the cathodic chamber to oxygenate the liquid. Each cell was equipped with two reservoirs (110 cm3) connected, respectively, to its anodic and cathodic compartment. Peristaltic pumps were used to circulate an HCl solution (pH 3.5) from the cathodic reservoir through the cathode chamber of the MFC at 25 cm3 min−1 and to circulate the analyte with a flow of 25 cm3 min−1. The experiments have been carry out at room temperature (23 ± 2 °C) which was kept constant by means of an air conditioning system.

2.2. Characterization techniques A digital multimeter (Keithley 2000 Multimeter) was connected to the system to monitor continuously the value of the cell voltage at the value of the external load (120 Ω). Chemical oxygen demand (COD) was determined using a Velp ECO-16 digester and a Pharo 100 Merck spectrophotometer analyzer and pH, conductivity and dissolved oxygen were measured with a GLP22 Crison pH meter, a Crison Cm 35 conductivity meter and an Oxi538 WTW oxy meter, respectively. Polarization curves have been recorded periodically and obtained by replacing the external resistance with different loads. Three important parameters were evaluated: the open circuit voltage (OCV) or the maximum allowable MFC voltage, the maximum intensity and the maximum power density of the MFC. In addition, the shape of curves gives important information about the limiting processes, which control the performance of the cell (Fig. 1).

Fig. 1. Experimental setup used for each MFC.


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Table 1 Wastewater composition. Composition

Synthetic wastewater (g dm−3)

NaCH3COO NaHCO3 (NH4)2SO4 KH2PO4 MgCl2 CaCl2 (NH4)Fe(SO4)2

8.05 2.77 1.85 1.11 0.92 1.25 0.07

2.3. Inoculum Activated sludge from a municipal wastewater treatment plant (Ciudad Real, Spain) was used as the inoculum for the anodic compartment. Activated sludge was introduced into the anodic compartment with raw wastewater in a 1:2 ratio during three days, after this period MFC were fed with synthetic wastewater (5000 mg O2 dm−3) and sodium acetate as carbon source (Table 1). 2.4. Electrode materials Different carbonaceous materials were used including Carbon cloth Panex® 30 Graphitized Spun Yarn Carbon Fabric, Carbon felt Sigracell® GFA6EA and Carbon foam Duocel® Reticulated Vitreous Carbon Foam with defined properties. In three of the cells, cathode electrodes were made of carbon felt Sigracell® GFA6EA while in the other of stainless steel AISI316. Characteristics of carbonaceous materials are summarized in Table 2. Areal weight, porosity and roughness were provided by suppliers. The specific surface area was determined by the BET method with N2 adsorption at 77 K.

Fig. 2. Influence of the electrode material on the consumption of COD in the anode chambers of the MFCs. Part a: Time-course of the tests. (□) anode: Carbon foam; cathode: Carbon felt, (●) anode: Carbon felt; cathode: Carbon felt, (Δ) anode: Carbon cloth; cathode: Carbon felt, (*) anode: Carbon felt cathode: Stainless steel. Part b: steady-state COD consumption rate. Average temperature: 23 °C.

3. Results and discussion In evaluating a MFC as an energy conversion device, two parameters are worth to be monitored: COD in the outlet of the MFC (effluent) and current density produced. The first is related to the fuel consumed, whereas the second stands for the electricity harvested. A high value as well as a high ratio between both parameters are indicatives of an optimum performance of the microbial fuel cell. Fig. 2 compares the performance in terms of COD consumption of several MFC equipped with different electrode materials over 2-month operating test. Part a focuses on the time course of the COD at the outlet of the MFC (operated in semi continuous mode) whereas part b shows the COD consumption rate calculated by a mass balance once the COD was stabilized. As it can be seen, in less than two weeks of operation, the COD measured at the outlet of the MFC stabilizes in a steady-state value, for which only the typical fluctuations associated to the complexity of biological processes are observed. Unexpectedly, not the same behavior but significant changes are observed in comparing the steady state values reached by the four MFCs. Initially, these differences can only be explained in terms of the performance of electrogenic microorganisms, because all the cells were started and operated simultaneously, keeping the same operation conditions, including feeding composition and rate,

Table 2 Main characteristics of the carbonaceous materials evaluated. Parameter 2

−1

Specific area (m g ) Areal weight (g m−2) Specific area (m2 m−2) Porosity (%) Roughness (μm)

Carbon felt

Carbon cloth

Carbon foam

35.3 500 17,700 95 30

31.3 120 3500 90 15

30.0 217 6500 97 10

hydraulic retention time, external electrical load and temperature. Hence, the only difference is the pair anode-cathode installed in each MFC, which should explain the differences in the performance observed. Metabolisms pathways based on the use of electron-acceptors can be influenced by these materials, if they change the electric current (electrons transfer rate), whereas anaerobic metabolic pathways are not expected to be affected by these materials. As it is known, COD is consumed by both, electrogenic and non-electrogenic microorganisms. The anode is the only electrode in direct contact to the microorganism contained in the bioreactor because the MFCs are divided into two compartments by a membrane that prevents crossing of microorganisms to the cathode compartment [32]. In addition, in the cathode, operating conditions are not suitable to keep microorganisms viability, not only because of the extreme value of pH (near 3.0) but also because of the lack of nutrients and carbon sources. Nevertheless, the role of the cathode is envisaged to be less important and, initially, it is only expected to perform a sink of the electrons exchanged in the anodic microbial-driven reactions, by its use in the electrochemical reduction of oxygen to water. Hence, this cathode compartment is abiotic, but it can limit the rate of the processes developed in the anode by electrogenic microorganisms and, in an extreme case, it may prevent the efficient performance of the cell. This is what it is observed in the comparison of the carbon felt and stainless steel cathodes. Thus, in comparing cathodes it can be stated that despite the higher electrical conductivity of stainless steel, the COD reduction in this MFC is much lower. This observation can only be explained in terms of a less efficient transfer of electrons from the anode to the cathode surface because of the lower electrocatalytic activity of the stainless steel towards the reduction of oxygen. In this case, this behavior is limiting the performance of the anodic chamber in terms of oxidation of COD by electrogenic microorganisms and it may be the bottleneck in the production of electricity.

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Regarding the anode, in addition to its expected role, that is to serve as a surface to exchange electrons either by direct or mediated mechanisms, the electrode can also be used as supporting material for the developing of fixed culture biomass [33]. Obviously, this fixed culture can be electrogenic, but it can also include not electrogenic microorganisms and, in the extreme case, it could be a completely non-electrogenic culture that event may prevent the performance of the bioreactor as electrochemical cell. This explains that a higher efficiency in the degradation of COD should not only be explained by the metabolisms of electrogenic microorganisms, but it can also be associated to nonelectrogenic microorganisms. According to the Fig. 2, the most effective anode in terms of COD removal rate is carbon felt and the worst is the carbon cloth. In comparing characteristics of the anodes, it can be stated that the main difference among them is the specific area (m2 m−2), 2.7 times higher in the case of felt and 1.9 times higher in the case of foam as compared to the carbon cloth. Then, this parameter increases in the sequence cloth-foam-felt, just the same sequence in which it is observed the increase in the COD removal rate. Changes in the COD consumption have to be reflected on the production of electricity [34] and even the differences in this last parameter become greater among the different MFC, as it is clearly shown in Fig. 3, which shows the changes in the current density produced by the cells over the 2-month tests. Electric current production is associated in the anode to the activity of electrogenic microorganisms and in the cathode to the electrochemical reduction of oxygen (abiotic). As in every electrochemical process, both reactions are coupled and one limits the other. Initially, for MFC it is expected that the anode reaction acts as the limiting process because conditions kept in the cathode (in particular the acidic pH) favor the reduction of oxygen to water very efficiently. In comparing MFC with different anodes, the worst performance is obtained by the cell operated with the carbon cloth (the cell with the lowest specific area) and the best with carbon felt (MFC with the largest specific area). Regarding the cathodes, stainless steel exhibits a much worse performance than carbon felt. Both results, related to the

Fig. 3. Influence of the electrode material on the production of electricity. Time-course of the tests. (□) anode: Carbon foam; cathode: Carbon felt, (●) anode: Carbon felt; cathode: Carbon felt, (Δ) anode: Carbon cloth; cathode: Carbon felt, (*) anode: Carbon felt; cathode: Stainless steel. Average temperature: 23 °C.

Fig. 4. Oxygen consumption rate in the cathode chamber of the MFC. Average temperature: 23 °C.

production of electricity, are in agreement to the results observed in the consumption of fuel (COD consumption rate) and point out the significance of the electrogenic microorganisms on the performance of the device. Electrode materials have a definitive influence on the performance of the cell and the higher specific area of carbon felt as compared to the other anode materials, as well as the improve catalytic properties as compared to stainless steel, can explain the differences observed [13, 35]. In literature, other works reach the same conclusions [36,37] showing that increasing the specific surface area of carbon materials enhance the power output of MFCs. Hence, taking into account results shown in this work, an ideal biofilm should be one that can be attached firmly on carbon surface and, simultaneously, can provide electron transfer paths to anode. In fact, it is reported [38] that rough anode surface not only provides proper sites to accommodate bacteria but also stimulates bacteria to produce their nanowires that help bacteria to firmly bond each other and provide the electron transfer bridges. The oxygen consumption rate in the cathode compartment is a direct measurement of the reduction capacity of the cathode in the MFC in which it is installed. This can be easily understood if it is taken into account that electrons supplied for the reduction can only be provided by the processes that occur on the corresponding anode surface. The average value for each MFC tested in this study measured in the steady-state is shown in Fig. 4 [39]. In comparing this parameter in the cells with different anode materials, it can be stated that for the same cathode different oxygen consumption rates are obtained with the different anode

Fig. 5. Maximum power densities measured in the MFC during the tests. (□) cathode: Carbon foam; anode: Carbon felt, (●) cathode: Carbon felt; anode: Carbon felt, (Δ) cathode: Carbon cloth; anode: Carbon felt, (*) cathode: Carbon felt; anode: Stainless steel. Average temperature: 23 °C.


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Fig. 6. Maximum intensity and open circuit potential (□) cathode: Carbon foam anode: Carbon felt, (●) cathode: Carbon felt; anode: Carbon felt, (Δ) cathode: Carbon cloth; anode: Carbon felt, (*) cathode: Carbon felt; anode: Stainless steel. Average temperature: 23 °C.

materials and that the oxygen consumption rates are directly related to the electricity produced and COD consumed. This observation confirms that anode material is a limiting step in the performance of the MFC over the experiments carried out in this work and hence the higher the extension of the anode processes, the higher is the oxygen depletion rate. Likewise, the lower value obtained in the MFC equipped with a stainless steel cathode remarks again that this cathodic process can become the bottleneck in the operation of a MFC [27]. Over the four tests carried out in this work, polarization curves were obtained in order to evaluate the changes in the maximum power density that can be attained. Results of this parameter are shown in Fig. 5. Values of the open circuit voltage and maximum intensity recorded in the polarization curves are also shown in Fig. 6. Opposite to OCV, which does not show any trend but simply a random distribution (with values slightly higher for the foam and felt than for the cloth in comparing anodes performance), a marked trend is observed for maximum current and maximum power densities attainable in each cell, in particular for the carbon felt. Maximum power density of the cell equipped with carbon felt anode and cathode is close to 1000 mW m−2, almost 40% over the value attained by the carbon foam anode and more than one fold over the value obtained by using the carbon cloth. These results are in agreement with the higher effective area of the carbon felt used as it was shown in Table 2 and with the performance results shown in the previous figures. The maximum value of the power density reached in this work compares favorably with the values found in other found in the literature. Thus, it is reported [40] that MFC using carbon foam electrode (20 pore per inch) as cathode and anode may reach a maximum power density of 170 mW m−2. Using carbon felt as anode values of 430 [25], 606 [38] and 420 [41] mW m−2 were reached. In this work, when using carbon foam (100 pore per inch) as anode and carbon felt as cathode and the maximum power density obtained was 881 mW m−2 and with a carbon felt anode 1062 mW m−2 were reached. Obviously, differences can be explained not only in terms of the anode materials but also because of the reactor configuration, inoculation sources and operation conditions. On the other hand, the influence of the cathode is clearly demonstrated by comparing results of the MFC equipped with carbon felt cathode and stainless steel, which shows a much lower value in this latter case which indicates that cathodic reaction becomes the bottleneck in that MFC. It is important to point out the great advantage of carbon felt in terms of transient response, because the performance parameters improve all over the test, while for the other materials the steady-state is reached in a shorter time. All these results are in agreement with the time-course performance of the different cells and point out the significance of the physical characteristics of the electrode material in the performance of MFC. This highlights the relevance of the use of carbon felt as anode and cathode, because of the outstanding behavior obtained, explained in terms of its higher specific area. This property, in addition to its lower cost as

compared with catalyst-based electrodes makes carbon felt a very good choice of electrode for MFC. 4. Conclusions From this work, it can be concluded that electrode materials exhibit a high influence on the performance of microbial fuel cells, not only in the production of electricity but also in terms of the degradation of COD attained. Results obtained in the four cells tested (under exactly the same operating condition) regarding anodic COD consumption, cathodic oxygen consumption and current produced are in agreement and indicate that carbon felt is a very good material for both, anode and cathode material and it can reach a maximum power of 1 W m−2, despite not using any catalyst like platinum. In comparing carbon felt, cloth and foam, regardless of having the same chemical composition the larger specific area of felt can explain the higher efficiency obtained. As in the anode, the use of this carbon material for the cathode is advisable. Thus, cathodic reduction of oxygen has become the bottleneck of the MFC performance with stainless steel and not with carbon felt. Acknowledgements Financial support from Spanish Ministry of Economy and Competitiveness (MINECO) through project CTQ2013-49748-EXP (Explora Program) is gratefully acknowledged. References [1] F. Davis, S.P.J. Higson, Biofuel cells - recent advances and applications, Biosens. Bioelectron. 22 (7) (2007) 1224–1235. [2] K. Rabaey, W. Verstraete, Microbial fuel cells: novel biotechnology for energy generation, Trends Biotechnol. 23 (6) (2005) 291–298. [3] B.E. Logan, Essential data and techniques for conducting microbial fuel cell and other types of bioelectrochemical system experiments, ChemSusChem 5 (6) (2012) 988–994. [4] B.E. Logan, B. Hamelers, R.A. Rozendal, U. Schrorder, J. Keller, S. Freguia, P. Aelterman, W. Verstraete, K. Rabaey, Microbial fuel cells: methodology and technology, Environ. Sci. Technol. 40 (17) (2006) 5181–5192. [5] S. Bebelis, K. Bouzek, A. Cornell, M.G.S. Ferreira, G.H. Kelsall, F. Lapicque, C.P. de Leon, M.A. Rodrigo, F.C. Walsh, Highlights during the development of electrochemical engineering, Chem. Eng. Res. Des. 91 (10) (2013) 1998–2020. [6] A.G. del Campo, J.F. Perez, P. Canizares, M.A. Rodrigo, F.J. Fernandez, J. Lobato, Characterization of light/dark cycle and long-term performance test in a photosynthetic microbial fuel cell, Fuel 140 (2015) 209–216. [7] Y. Asensio, C.M. Fernandez-Marchante, J. Lobato, P. Cañizares, M.A. Rodrigo, Influence of the fuel and dosage on the performance of double-compartment microbial fuel cells, Water Res. 99 (2016) 16–23. [8] J. Md Khudzari, B. Tartakovsky, G.S.V. Raghavan, Effect of C/N ratio and salinity on power generation in compost microbial fuel cells, Waste Manag. (New York, N.Y.) 48 (2016) 135–142. [9] A. Gonzalez del Campo, J. Lobato, P. Canizares, M.A. Rodrigo, F.J. Fernandez Morales, Short-term effects of temperature and COD in a microbial fuel cell, Appl. Energy 101 (2013) 213–217. [10] M.A. Rodrigo, P. Canizares, J. Lobato, R. Paz, C. Saez, J.J. Linares, Production of electricity from the treatment of urban waste water using a microbial fuel cell, J. Power Sources 169 (1) (2007) 198–204.

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