Blagovesta Husein Yemendzhiev, Petko Tanev, Valentin Nenov Journal ofMidyurova, Chemical Technology and Metallurgy, 50, 4, 2015, 543-550
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 E-mail:
[email protected]
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 - 1100ºC. 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 MnO2 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 20210 Ω. The results demonstrate significant improvement of the cathodic reactions and overall MFC performance after introduction of MnO2 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
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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
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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, Gd0.1Ce0.9O1.95). 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 33.13 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 (MnO2) is used [15] instead of Pt. The high redox potential of MnO2 provides a high potential difference between the anode and cathode chambers. It is proved that a MFC with βMnO2 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
Blagovesta Midyurova, Husein Yemendzhiev, Petko Tanev, Valentin Nenov
the influence of MnO2 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 compositionat 100 MPa and firing at high temperature (950ºC-1100ºC). The temperature was gradually increased with 10oC/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]. Table 1. Types of membranes. Ceramic membrane constituents
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 MnO2/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
particle size,
Ratio of constituents
µm Trojan clay*
