Experiencia del usuario 46

This chapter has been published as:

Ter Heijne, A.; Strik, D.P.B.T.B.; Hamelers, H. V. M.; Buisman, C. J. N. 2010. Cathode potential and mass transfer determine performance of oxygen reducing biocathodes in Microbial Fuel Cells. Environ. Sci. Technol. 44, 7151-7156.

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Abstract

The main limiting factor in Microbial Fuel Cell (MFC) power output is the cathode, because of the high overpotential for oxygen reduction. Oxygen reducing biocathodes can decrease this overpotential by the use of microorganisms as a catalyst. In this study, we investigated the factors limiting biocathode performance. Three biocathodes were started up at different cathode potentials and their performance and catalytic behaviour was tested by means of polarization curves and cyclic voltammetry. The biocathodes controlled at +0.05 V and +0.15 V vs Ag/AgCl produced current almost immediately after inoculation, while the biocathode controlled at +0.25 V vs Ag/AgCl produced no current until day 15. The biocathode controlled at +0.15 V vs Ag/AgCl reached the highest current density of 313 mA/m2_{. Cyclic} voltammetry showed clear catalysis for all three biocathodes. The biocathodes were limited by both mass transfer of oxygen and by charge transfer. Mass transfer calculations show that the transfer of oxygen poses a serious limitation for the use of dissolved oxygen as an electron acceptor in MFCs.

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5.1Introduction

The Microbial Fuel Cell (MFC) is an emerging technology to produce electricity from biodegradable waste materials. An MFC consists of an anode and a cathode, usually separated by a membrane. At the anode, microorganisms oxidize biodegradable materials into CO2, protons and electrons. At the cathode, usually oxygen is reduced to water (Logan et al., 2006), but also many other reactions are possible (Hamelers et al., 2010).

MFC technology at this moment is not cost effective enough for practical application. The main factor limiting MFC performance is the cathode. Oxygen has a high overpotential when uncatalyzed electrodes are used, but at the same time oxygen is the most practical electron accepter because of its unlimited availability. Biological catalysts are an attractive alternative to chemical catalysts to decrease the overpotential for oxygen reduction, because of their low cost and sustainability (He et al., 2006). Until now, different biological catalysts have been tested for oxygen reduction, both in the form of enzymes (Schaetzle et al., 2009) and in the form of microorganisms. Microorganisms were shown to catalyze the oxygen reduction reaction via mediating compounds like manganese (Rhoads et al., 2005) and Fe2+_/Fe3+ _(Ter Heijne et al., 2007), but also directly in seawater (Bergel et al., 2005) and freshwater (Clauwaert et al., 2007; Freguia et al., 2008; Freguia et al., 2010). These biocathodes show better

performance than uncatalyzed materials, however, they are still limiting MFC performance. At this point, it has not been investigated what are the main factors limiting biocathode

performance.

The objective of this study was to investigate the factors that are limiting biocathode performance. Performance of biocathodes is reflected in the combination of cathode potential and current density. Cathode potential regulates the energy that the microorganisms can gain from transferring the electrons from the electrode to oxygen, because it determines the energy level at which the electrons are released. Electrode potential can also have effects on microbial cell surface properties and enzyme activity (Liang et al., 2009) and therefore affects activity of the biofilm. The measured current reflects the rate of oxygen reduction. It is important to study cathode polarization curves, showing the relationship between current density and cathode potential, because the combination of cathode potential and current density determines the maximum power that can be gained from the cathodic part of the bioelectrochemical system.

In this study, three biocathodes were started up at three different cathode potentials. Performance of these biocathodes was investigated via polarization curves, and catalytic

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behavior was analyzed using cyclic voltammetry. The biocathodes were studied on flat (2D) electrodes, enabling us to calculate mass transfer rates and limiting current density.

5.2 Materials and methods

5.2.1 Electrochemical cell setup

The experimental setup consisted of three electrochemical cells. The electrochemical cells contained two flow channels with a projected surface area of 22 cm2 _{and a channel depth of} 1.5 cm as described in (Ter Heijne et al., 2008). The flow channels were separated by a cation exchange membrane (Fumasep FTCM-E, Fumatech, Braunschweig, Germany). The cathodes were rough graphite plates (AlO2 blasted) (Müller & Rössner GmbH & Co, Troisdorf, Germany), while the anodes were flat graphite plates (Müller & Rössner GmbH & Co, Troisdorf, Germany).

5.2.2 Electrochemical cell operation

The catholyte was inoculated with nitrifying biomass from the wastewater treatment plant in Ede, the Netherlands. Nytrifying biomass was chosen because of the presence of autotrophic microorganisms. Each of the three cells was inoculated on day 1 with 100 mL nitrifying biomass. The catholyte had a total volume of 1 L and consisted of phosphate buffer (pH 7, 0.02 M), and macro- and micronutrients (10 mL/L and 1 mL/L) as described in (Ter Heijne et al., 2008). The anolyte also had a total volume of 1 L and was a 0.05 M potassium ferrocyanide solution in 0.02 M phosphate buffer at pH 7.

Biocathodes were started up at three different potentials: +0.05 V vs Ag/AgCl, +0.15 V vs Ag/AgCl, and +0.25 V vs Ag/AgCl with a multi-channel potentiostat (Bank Electronik – Intelligent Controls GmbH, Pohlheim, Germany). To control cathode potential, a cell voltage was applied between anode and cathode, and this cell voltage was manually adjusted until the desired cathode potential was reached. This was checked every two or three days, and the deviation from the desired potential was <0.01 V during >95% of the experiment.

During all experiments, the catholyte was aerated with ambient air, resulting in an oxygen concentration of 6.5 mg/L. Oxygen concentration was measured with a hand-held oxygen meter (Hach Lange NV, Mechelen, Belgium). Catholyte pH was manually controlled at pH 7. A maximum deviation in pH of 0.5 was allowed; pH was adjusted back to 7.0 with NaOH or HCl. Anolyte and catholyte were recirculated at a rate of 12 L/h. Both anode and cathode compartments were equipped with Ag/AgCl, 3 M KCl reference electrodes (+0.205 V vs NHE). Potentials of anode and cathode were measured versus their reference electrodes and

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