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262 Y. Huang et al. / Bioelectrochemistry 79 (2010) 261–264with the cathode acting as counter electrode (CE) followed by recordingof the spectrum for the cathode with the anode serving as CE.3. Results and discussionFig. 1. A schematic diagram (a) and photograph of MFC (b).was a layer of a carbon/Pt mixture that was applied on the outside ofthe membrane. The carbon/Pt mixture was prepared by two differentmethods: 1) carbon powder containing 10% of Pt (Vulcan XC 72, BASFFuel Cell, Inc., NJ, USA) was mixed with the Nafion solution (Dupont,Nafion® perfluorinated ion-exchange resin, 1 µL for every 1 mg of Pt/Cpowder) and 2) the Nafion solution was replaced by tap water. Afterthe mixture was applied to the membrane, Ni-coated carbon fibers(TenaX®-J, Toho Tenax Co., Ltd., grade HTS 40, Irvine, CA, USA) servingas current collectors and part of the cathode were wrapped around themembrane and air-dried at room temperature overnight. An Ag/AgClreference electrode was placed in the anode chamber of each MFC.Both MFCs were fed continuously with a solution containingNaC 2 H 3 O 2·3H 2 O (3 g/L); yeast extract (0.2 g/L); NH 4 Cl (1 g/L); MgSO 4(0.25 g/L); NaCl (0.5 g/L); CaCl 2 (15 mg/L); trace solution (1 mL/L) [3]and phosphate buffered nutrient medium (100 mL/L) containingNaH 2 PO 4 (50 g/L) and Na 2 HPO 4 (107 g/L). The hydraulic retentiontime was 8 h. Twenty ml of a mixture of anaerobic and aerobic sludge(50/50), which were collected from a wastewater treatment plant(Joint Water Pollution Plant, Los Angeles, CA), was injected into theanode chamber as inoculum. All tests were conducted at roomtemperature (23 °C–25 °C).Fig. 2 shows an example of the current–time curves that wererecorded over extended time periods. The current did not changemuch with time and the WC produced higher current values than theNC. The performance of the NC and the WC was investigated by therecording U c –I curves.Since the area of the anode was not known, the experimentalcurrent and power values were normalized by the liquid volume ofthe anode compartment which was 150 cm 3 . Fig. 3 gives a comparisonof the current concentration and the power concentration producedby the WC and the NC. Fig. 3a shows the current concentration as afunction of the cell voltage for the two cells. The maximum currentconcentration obtained from the WC (26 µA/cm 3 ) was more thantwice that from the NC (12 µA/cm 3 )(Fig. 3a). Fig. 3b shows the powerconcentration as a function of the current concentration. Themaximum power concentration produced by the WC was about1.8 µW/cm 3 compared to 1.2 µW/cm 3 for the NC. The maximumpower concentration was obtained at a cell voltage of about 200 mV inboth cases as can be seen from the P–U c curves in Fig. 3c.At the cell voltage U max where maximum power output P maxoccurs R int =R ext , where R int is the internal resistance of the MFC andR ext is the external resistor that is placed between the anode and thecathode to obtain U max [9]. R int has been defined as [9]:R int = R a p + R c p + R Ω ;where R p a and R p c are the polarization resistance of the anode and thecathode, respectively and R Ω contains the ohmic contributions such asthe solution resistance between the anode and the cathode, themembrane resistance and the electrical resistance of the electrodes,leads, etc. R int of the WC was calculated as 117 Ω, while for the NC avalue of 195 Ω was obtained. As discussed previously, R int depends oncell voltage [9].The impedance spectra obtained for the NC and the WC at theopen-circuit potential (OCP) for the cathode and the anode during theexposure times of 120 and 140 days are shown in Fig. 4 as Bode-plotsin which the logarithm of the impedance modulus |Z| is plotted vs. thelogarithm of the applied frequency f. The impedance spectra of the NCand the WC cell cathodes are in general agreement with the coatingmodel proposed by Mansfeld et al. [10,11] for the analysis ofimpedance data for polymer coated metals. The decrease of theimpedance of the cathode for both MFCs with the exposure time isconsidered to be due to a decrease of the thickness of the polymerð1Þ2.2. Data collectionThe voltage across a R ext =500Ω external resistor was recordedevery 30 s using a computer based data acquisition system (Keithley2700 multimeter). These measurements were only interrupted whenimpedance spectra or cell voltage–current curves were recorded. Cellvoltage (U c )–current (I) curves were collected by applying a potentiodynamicscan at a scan rate of 0.1 mV/s from the open-circuit cellvoltage U oc to zero cell voltage U o using a Gamry reference 600potentiostat. The power concentration P (µW/cm 3 ) based on the liquidvolume of the MFC was calculated from the U c –I curves and plotted as afunction of current concentration (j) or cell voltage. The impedancespectra were collected at the open-circuit potential (OCP) of the anodeand the cathode using a Gamry reference 600 potentiostat (Garmryinstruments, Warminster, PA, USA). An ac voltage signal of 10 mV wasapplied in a frequency range from 100 kHz to 5 mHz. EIS measurementswere conducted by first recording the impedance spectrum of the anodeFig. 2. Current–time curves for the WC and the NC (R ext =500 Ω) for an exposure timeof 34 days.
Y. Huang et al. / Bioelectrochemistry 79 (2010) 261–264263Fig. 4. Bode-plots for the cathode (a) and the anode (b) of the WC and the NC for anexposure time of 65 days.explained at present, however they point to a complex interactionbetween the anode and the cathode for the NC.4. Summary and conclusionsFig. 3. Cell voltage–current concentration (a), power concentration–current concentration(b) and power concentration–cell voltage curves (c) for the WC and the NC foran exposure time of 55 days.layers covering the graphite fibers (increase of high-frequencycapacitance) and increased porosity of these layers (decrease ofpore resistance at intermediate frequencies) (Fig. 4b). The impedanceof the cathode for the NC is higher than that for the WC for bothexposure times due to the polymer layers formed during treatmentwith the Nafion solution.The impedance spectra for the WC anode did not changesignificantly with time indicating stable behavior (Fig. 4a). Theimpedance for fb10 4 Hz is due to the solution resistance and is verylow. The impedance for fb1 Hz is due to the capacitance of the anode.The impedance spectra for the NC anode at 120 days were differentfrom those of the WC at the same exposure time with a significantlylower capacitance value. The spectra changed to a two-time constantsystem for an exposure time of 140 days. These changes cannot beThe performance of tubular MFCs (Fig. 1) with and without Nafionsolution as a binding agent was investigated using different electrochemicaltechniques. The current–time curves (Fig. 2) andthepowerconcentration–current concentration or power concentration–cellvoltage curves that were obtained from the experimental cell voltage–current curves (Fig. 3) have demonstrated that the power production bythe water cell is higher than that of the Nafion cell. The internalresistance determined from the power density–cell voltage curves(Fig. 3c) was higher for the Nafion cell. The impedance spectra for thecathodes of the two MFCs suggest that this higher value is due to thehigher polarization resistance of the Nafion cell cathode. The ohmiccontribution to the internal resistance (Eq. (1)) was very small (Fig. 4).The Nafion polymer layers covering the cathode material (Pt particlesand Ni-coated carbon fibers) apparently block access of oxygen andthereby reduce the rate of oxygen reduction.AcknowledgmentThe authors acknowledge financial support from the ChevronCorporation.
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