APPLICATION OF CERAMIC MATERIALS TO THE MICROBIAL FUEL CELL DESIGN

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1 Blagovesta Journal of Midyurova, Chemical Technology Husein Yemendzhiev, and Metallurgy, Petko Tanev, 50, 4, 2015, Valentin Nenov APPLICATION OF CERAMIC MATERIALS TO THE MICROBIAL FUEL CELL DESIGN Blagovesta Midyurova, Husein Yemendzhiev, Petko Tanev, Valentin Nenov Department of Water Treatment Technology, Burgas Asen Zlatarov University, 1 Prof. Yakimov Str., Burgas 8000, Bulgaria vnenov@btu.bg Received 05 January 2015 Accepted 24 February 2015 ABSTRACT The microbial fuel cells (MFC) attract scientific interest because of the promising results gained in electricity generation during the biological oxidation of waste organic matter. Recently, the efforts in this technology application refer to the improvement of the reactor design and the exploration of new materials aiming to reach economical and technological efficiency and sustainability. This work reports data on the development of ceramic membranes used as separators and а template for new generation electrodes constructions. The results obtained show that such membranes can be used as a hybrid separator-air-cathode base after specific modification with carbon. Several types of ceramic membranes based on different clays are developed. Different metal meshes are used as support conductive electrode materials. The initial shape of the membranes is obtained by applying 100 MPa pressure to the mold and subsequent firing in a high temperature furnace at 950ºC ºC. Two types of membranes are synthesized and applied as air cathode templates in a single cell MFC. The first ceramic cathodes construction includes layers of Trojan clay, liquid Nafion and steel mesh, while that of the second one includes Trojan clay ceramic membrane containing MnO 2 as a catalyst,carbon cloth and a liquid Nafion layer. The open circuit voltage obtained with these two types of air-cathodes is 303mV and 508mV, respectively. Maximum power density values are observed at external resistance of Ω. The results demonstrate significant improvement of the cathodic reactions and overall MFC performance after introduction of MnO 2 and carbon fibers catalysts to the ceramic electrodes. Keywords: microbial fuel cells, ceramic base membranes, air cathode. INTRODUCTION The phenomenon of current generation by specific bacterium culture was discovered in 1911 by the English scientist Potter. It was vastly explored during 1990s and is recently becoming one of the major research areas worldwide. The microbial fuel cells (MFC) are a powerful tool allowing direct conversion of the chemical energy of organic compounds into electricity. Traditional MFCs are composed of anode and cathode chambers separated by a proton exchange membrane, Fig. 1. In case of organic matter degradation the substrates are oxidized in the anodic compartment and electrons produced can be transferred to the anode by electron mediators or shuttles [1, 2]. The electrons gained by the activity of the electrogenic catalyzing bacteria are directed to the cathode and consumed there. Simultaneously liberated protons during the organic matter degradation process pass through a separator. In most common MFCs in which oxygen is used as an electron acceptor the reaction between electrons, protons and oxygen leads to formation 543

2 Journal of Chemical Technology and Metallurgy, 50, 4, 2015 Fig. 1. Schematic diagram of the dual-chamber MFC structure. of water [3]. The MFC technology is in a phase of development with proved future in wastewater and sludge treatment and promising electrical energy yield. However, besides the obvious success in power density improvement, few pilot and industrial scale MFC units are put in practical application. It is well recognized that the MFC performance can be boosted by a proper choice of electrodes [4]. Such a statement is valid to a large extent for the selection of a cathode because its material and design determine mainly [5] the performance of MFC. The usage of carbon based materials (carbon cloth, carbon felt, carbon fibers) as a supporting substructure of MFC air-cathode [6] is quite common. Ceramic membranes are promising as an alternative of the costly proton exchange membrane (PEM) like Nafion. The interest in implementing ceramic membranes is increased due to the large variety in the types and methods of synthesis which gives unique opportunities for their application. They may be based on various ceramics (corundum, zircon, etc.), minerals (zeolites) and specially synthesized composites. Recently, low cost composite materials based on clay are proposed [7] for separation of the anodic from the cathodic chamber. One of the first studies of ceramic materials applied as a separator in MFC is that of Zhang et al. [8]. The authors use hollow fiber ceramic membrane as an ion separator. They prove that this clay is a better material for such application because of its large porosity. There are several evidences that the ceramic material based on clay minerals possess a certain electrical conductivity and offer high rate of cation-exchange transfer [9, 10]. It is expected on this basis that the clay membranes can play the role not only of a physical separator but also that of a proton exchange membrane separating the anodic and the cathodic compartments. Xia C. et al. [11] use a dense ceramic membrane based on Ce cerium (GDC, Gd 0.1 Ce 0.9 O 1.95 ). They find that the thickness of such membrane can be easily controlled by the amount of ceramic powder compressed. Ajayi et al. [12] use a terracotta pot material for making a single chamber MFC after coating the outer surface of the ceramic membrane by conductive graphite paint. The coulombic efficiency demonstrated is 21 ± 5 %, while the power density is mw m -2. Another direction of MFC air-cathode development is based on the usage of metal mesh covered by different PEM materials. Within this area of development You et al. [13] use a fine carbon polytetrafluoroethylene (PTFE, Teflon) emulsion which is pasted on stainless steel mesh. A carbon/pt catalyzing ink covers the liquid side of the electrode while the air side is a carbon cloth pressed on the mesh. This is a low cost alternative offering a high power density (951.6mW m -2 ). Zhang et al. [14] study stainless steel meshes (30-120) with two layers of polydimethylsiloxane/carbon cloth on the air side of the cathode, while the solution side is covered by Vulcan/ Pt/ Nafion. The largest power density is obtained by membrane with 30 meshes and 50 meshes. The values obtained are 1616 mw m -2 and 1415 mw m -2, respectively. Many researchers use cheaper metal oxides as catalysts in MFCs. Manganese dioxide (MnO 2 ) is used [15] instead of Pt. The high redox potential of MnO 2 provides a high potential difference between the anode and cathode chambers. It is proved that a MFC with β- MnO 2 produces electricity with higher current density higher by 64.1 % when compared to that obtained in a chamber with no catalyst [16]. One of the topics of studies related to the behavior of the electrodes used in MFC is the influence of the externals resistance. Aelterman et al. [17] determine the ability of different types of electrodes (graphite, carbon cloth and carbon non-woven fabric) at applying various values of external resistance. The authors find an increase of the concentration polarization using resistors of 10.5 Ω, 25 Ω and 50 Ω. The data reported shows that maximum current density is obtained when the external resistance is at least equal to the internal resistance of microbial fuel cells. We discuss in our study the performance of some ceramic membranes as elements of a MFC cathode and 544

3 Blagovesta Midyurova, Husein Yemendzhiev, Petko Tanev, Valentin Nenov the influence of MnO 2 additive as a catalyzing agent. Besides, the attention is focused on testing the effect of different external resistances on the current density. EXPERIMENTAL Ceramic membranes development The technological properties of the clay minerals depend mainly on their degree of dispersion. Particle size distribution of the clay affects the properties such as density, porosity, etc. For the synthesis of the ceramic membranes, different ratio of raw materials was used. The initial shape of membranes was obtained by pressing the selected composition at 100 MPa and firing at high temperature (950ºC-1100ºC). The temperature was gradually increased with 10 o C/min and upon reaching the maximal temperature the samples were hold for 60 min. The cooling process was conducted by keeping the heated samples for 24 hours at room temperature [18]. The types of ceramic membranes developed are shown in Table 1. Microbial fuel cell design and operation Two different electrodes (cathodes) were developed. The first one is composed of 5wt. % of Nafion liquid (Nafion, per fluorinated resin solution, Aldrich Sigma) and steel mesh electrode with pore size < 42µm (Fig. 2D). Three Nafion layers were applied on the liquid side of the steel mesh membrane. The second one consisted of a ceramic membrane (Trojan clay) impregnated with MnO 2 /carbon cloth treated with Nafion solution and steel mesh with three layers of Nafion (Fig. 2C and D). In both cases the prepared cathodes were dried for 3 hours at room temperature and then were incorporated in the cell. The experiments with the electrodes developed were performed in a separate single-chamber MFCs in a batch mode. The principle scheme of the electrode assembly is shown in Fig. 2 (A). The above mentioned Table 1. Types of membranes. Ceramic membrane constituents particle size, µm Ratio of constituents Trojan clay* < Trojan clay/mno 2 < :10 Trojan clay /electro corundum** < :20 Trojan clay /electro corundum < :30 Kaolin***/PAN fiber/carbon/ dextrin < :1:5:7 Trojan clay/pan fiber/carbon/ dextrin/sio 2 < :1:5:7:5:5 Zeolite**** < Zeolite/ glass < :2 Trojan clay/pan fiber < :1 Electro corundum < Kaolin/PAN fiber/carbon/ dextrin/sio 2 < :1:5:7:5 Trojan clay/pan fiber/tio 2 /MnO 2 < :1:5:5 Trojan clay /electro corundum < :15 Trojan clay /electro corundum < :15 *Trojan clay constituents (in %): SiO , Al , Fe 2-6.0, Na 2 O -0.9, K 2 O -3.0, CaO-8.2, MgO-1.5; **Corundum is in its crystalline form of aluminum oxide, Al 2 (it contains also traces of iron, titanium and chromium); ***Kaolin clay constituents (in %): SiO , Al 2 33, Fe 2 1, TiO 2 0.2, Na 2 O 0.1, K 2 O 0.8, CaO 0.7, MgO 0.7; ****Zeolite constituents (in %): SiO %, Al %, Fe %, TiO %, MgO - 0,06 %, CaO - 2,8 %, Na 2 O - 0,22 %, K 2 O - 2,9 % [19]. 545

4 Journal of Chemical Technology and Metallurgy, 50, 4, 2015 (A) (B) (C) (D) (E) Fig. 2. A schematic MFC (A); a laboratory-scale prototype of the MFC (B); ceramic (C) membrane (Trojan clay and MnO 2 ); Nafion /steel (D) mesh; different types of ceramic membranes (E). membranes (electrodes) were clamped between two PVC elements by using rubber rings. MFCs consisted of a single cylindrical PVC chamber (length of 3 cm, diameter of 4.5 cm; cell volume of 48 ml) containing carbon cloth (anode) with a surface area of m 2. The anodic compartment had one port for input and output flows. It was filled with granular activated carbon to provide biofilm formation and electron transduction to the electrode. The anode and cathode were connected with an external electrical circuit through different resistances. The electrogenic microorganisms were isolated from the bottom sediment of Yasna Polyana Dam near Burgas. The enrichment of the mixed culture was performed under anaerobic conditions by inoculation of 0.5 ml sediment in 20 ml nutrient medium of ph of 7containing glucose (15g/dm 3 ), tryptone (10 g dm -3 ), yeast extract (5 g dm -3 ) and NaCl (5 g L -1 ). After 96 hours of cell growth the enriched culture was washed and re-suspended in fresh nutrient medium of the same composition but in absence of glucose. The voltage generated was measured and recorded by Auto ranging digital Multimeter Model MY-66. Electrochemical monitoring and calculation In order to study the influence of external resistances on the performance of the microbial fuel cell, 546

5 Blagovesta Midyurova, Husein Yemendzhiev, Petko Tanev, Valentin Nenov the following resistances were applied: R 1 = 10 Ω; R 2 = 99.3 Ω; R 3 = 976 Ω; R 4 = 5.03 kω; R 5 =9.89 kω; R 6 = kω; R 7 = Ω; R 8 = kω; R 9 = 503 kω; R 10 = 510 kω; R 11 = 998 kω; R 12 = MΩ). The corresponding voltages generated were measured and recorded every 10 min after stabilizing the values. The power was calculated: P = I.V, where (P) power, (I) current, and (V) voltage. Two parameters, namely the current density and the power density were measured to establish the resistance required for the maximum capacity performance of MFC : Power density = P/surface area of anode, mw m -2 Current density = I / surfacearea of anode, ma m -2 RESULTS AND DISCUSSION We measured the open circuit voltage (OCV) as well as the current and power densities at different electric resistances in the external circuit to evaluate the MFC operation. The OCV generated by the first cell (steel mesh Nafion layer cathode) was 303 mv. This value is slightly lower but still comparable to that of reactors equipped with chemical cathodes employing potassium hexacyanoferrate as a terminal electron acceptor [20]. In view of this result the air cathode designed provides effective reduction potential in regard to the atmospheric oxygen. This is an important prerequisite for successful bio-electrochemical process. The related power and current densities are shown in Fig. 3. The maximal power density (0.23 mw m -2 ) is observed at external resistance between 10 kω and 20 kω. Under these conditions the cell produces an electric current of 6.67 ma m -2 density. Taking into account the standard electrode potential of the elements involved in MFC electrochemistry, the theoretically possible maximal OCV can reach values of 500 mv to 600 mv [21]. The lower values are usually an implication of the cathodic potential insufficiency. That is why the next stage of this study was aimed at modification of the existing air cathode in order to intensify the cathodic reactions and to obtain the highest OCV possible. It is well known that the cathodic reactions are one of the major limiting factors in the performance of microbial fuel cells [22]. One of the methods to improve MFC cathodes is the application of catalysts. The most popular one is platinum. This option is suitable for laboratory experiments and research but in case of scaling up and design of pilot or even industrial scale reactors searching of alternative catalysts is crucial. Actually, this is one of the biggest challenges to overcome prior to this technology commercialization. For the purposes of this study the air cathode is modified by using a ceramic membrane containing MnO 2 and carbon fibers covered by Nafion layer was applied. The choice of these materials is based on our previous results demonstrating improved electrical parameters of MFC with ceramic membrane separators containing manganese oxide and carbon black powder [23]. These preliminary studies results (presented in Figs. 4 and 5) show 16 % higher OCV and nearly 50 % higher power and current densities in the reactors containing MnO 2. It is recognized that the higher concentrations of manganese are toxic, especially for the anaerobic microorganisms [24, 25]. They can also cause a breakdown in the MFC electrochemistry due to the higher electrode Fig. 3. Current and power densities vs. electric resistance in MFC equipped with Nafion layer /steel mesh air cathode. 547

6 Journal of Chemical Technology and Metallurgy, 50, 4, 2015 Fig. 4. Current and power density recorded at a Trojan clay ceramic membrane. Fig. 5. Current and power density recorded at a ceramic Trojan clay + MnO 2 membrane. potential. The main purpose to use a MnO 2 catalyst as a component included in the ceramic membrane is to avoid these effects which can occur after a contact between the manganese ions and the anodic solution containing the electrogenic microorganisms. The inorganic matrix of the ceramic membrane holds the catalyst particles and thus it can not express a negative influence on the biology and electrochemistry of the anode. The results obtained show that the modification applied is successful and we observe 66 % higher OCV (508 mv) in MFC with a modified air-cathode. Since the anode design and the content of the anodic solution (growth medium and the microbial culture) is not changed during this experiment it is obvious that the improvement observed is a result of the increased cathodic potential. CONCLUSIONS The study confirms the attractive behavior of MFC with a ceramic separator/carrier containing MnO 2 in comparison with a ceramic membrane based on clay with no MnO 2. It is also shown that the composite electrode consisting of MnO 2 loaded ceramic membrane, carbon cloth and steel mesh treated with Nafion possesses higher OCV than the simple metal mesh/ Nafion electrode. The innovation which is currently explored provides reduction of the spacing between the electrode and membrane. The results are a step for increasing MFC efficiency in waste management. Acknowledgements This work is support by grant No. IZEBZO_143004/1, Innovative P-recovery from waste sludge (INNOVA P-recovery) from the Bulgarian-Swiss Research Programme and the University of Applied Sciences and Arts of Northwestern Switzerland (FHNW), BSRP

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