A study on process parameters of direct ethanol fuel cell.

Pramanik, H. ; Basu, S.

INTRODUCTION

The electrochemical oxidation of alcohols, especially methanol in DMFC (Direct Methanol Fuel Cell), has been widely investigated and some prototypes were built in the 1960s by the Shell Research Centre in England and by Hitachi Research Laboratories in Japan (Glazebrook, 1982). These studies were abandoned in the mid-1980s due to low performances (20-30 mW/[cm.sup.2] for relatively large Pt loading [approximately equal to] 10 mg/[cm.sup.2]), which corresponds to 2-3 W/g of Pt catalyst of the fuel cell. By 1990, the development of proton exchange membrane (PEM) fuel cell gave a new momentum to further investigate the DMFC using methanol as fuel. The reason behind the selection of methanol was its availability, simplicity of storage, rapid fuelling and high energy density. Many national research programs (in the U.S.A., Japan and Europe) now exist, and the large car companies, such as Daimler-Chrysler, General Motors, Toyota and Nissan, are involved in the development of fuel cells for electric vehicle application and also some companies working on use of fuel cell in portable electronic devices where methanol is the fuel.

Ethanol offers an attractive alternative as fuel in direct ethanol fuel cell (DEFC) because it can easily be produced in great quantity by fermentation from sugar containing biomass resources and thus renewable in nature. Ethanol is less toxic and less volatile compared to methanol. Energy density of ethanol (7.44 kWh/kg) is higher than methanol (6 kWh/kg) (Basu, 2007). All these advantages have propelled ethanol as the best alternative fuel of the future for DAFC (direct alcohol fuel cell).

The electrocatalytic oxidation of ethanol was investigated on different platinum-based electrodes, including Pt-X alloys (with X=Ru, Sn, Mo ...) and among them Pt-Ru and Pt-Sn were the most effective and the least poisoned (Lamy et al., 2001). The case of electrooxidation of ethanol is more difficult than that of methanol with the necessity to break the C-C bond to obtain its complete oxidation. It was observed by infrared reflectance spectroscopy and by gas chromatograph (Lamy et al., 2001) that electrooxidation of ethanol leads to the formation of intermediate products with C-C bond and adsorbed CO poisoning species. In 2002, Lamy et al. (2002) analyzed the detailed electrooxidation reaction mechanism of ethanol and the catalytic role for anode reaction. Spinace et al. (2003) worked on electrooxidation of ethanol on Pt-Ru/C electrodecatalysts prepared from ([eta]-[C.sub.2][H.sub.4])(Cl)Pt([micro]Cl).sub.2]Ru(Cl) [[eta].sub.3] [[eta].sub.3]-[C.sub.10][H.sub.16]). Lamy et al. (2004) developed new Pt-Sn electrode-catalysts for the ethanol oxidation in direct ethanol fuel cell. Recently, Colmati et al. (2006) studied ethanol electrooxidation on carbon supported Pt, P-Ru, [Pt.sub.3]Sn electrode-catalysts in the temperature range 70-120[degrees]C. The DEFC at 70[degrees]C with Pt-Ru/C and [Pt.sub.3]Sn electrode-catalyst showed about the same performance, while for temperature greater than 70[degrees]C the cells with [Pt.sub.3]Sn as anode performed better than that of Pt-Ru as anode. The aim of the present investigation is to study the influence of temperature, ethanol concentration with varied electrodecatalyst loadings at anode (Pt-Ru/C) and cathode (Pt-black). The analyses of the results would give optimized condition in terms of anode and cathode loading, ethanol concentration and temperature required to obtain maximum power density and current density. To recover maximum energy from an alcohol molecule, the oxidation reaction must be complete, that is, it must lead to C[O.sub.2] formation. The complete oxidation of ethanol involves release of 12 electrons per molecule. This is shown as:

[C.sub.2][H.sub.5]OH + 3[H.sub.2]O [right arrow] 2C[O.sub.2] + 12[H.sup.+] + 12[e.sup.-]

The detailed analysis of the reaction products by chromatographic techniques (HPLC, GC) or by DEMS (Hitmi et al., 1994) provide more detailed reaction mechanism of ethanol oxidation on Pt electrodes in acid medium. It involves parallel and consecutive oxidation reactions, as follows:

[FORMULA EXPRESSION NOT REPRODUCIBLE IN ASCII] (1)

[FORMULA EXPRESSION NOT REPRODUCIBLE IN ASCII] (2)

Reaction (1) occurs at higher electrode potentials (E > 0.8 V vs. RHE), where the water molecule is activated to form oxygenated species at the platinum surface. The reaction (2) occurs mainly at lower potentials (E < 0.6 V vs. RHE) (Hitmi et al., 1994).

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EXPERIMENTAL

Material

The catalysts used to prepare the anode and cathode were Pt/Ru (40%:20% by wt.)/C and Pt-black high-surface area procured from Johnson Matthey Inc., U.K. The carbon paper (Lydall 484C-1, USA) was used as a substrate for catalyst powder. Nafion[R] (SE-5112, DuPont USA) dispersion was used to cast the proton exchange membrane. Mixture of Nafion[R] and PTFE dispersion (E.I. DuPont India Pvt. Ltd.) was used as a binder. Ethanol (E. Merck) was used as fuel. Air or pure oxygen (99.99% vol) stored in cylinder was used as oxidant. Hydrogen peroxide and [H.sub.2]S[O.sub.4] (E. Merck) was used for cleaning the membrane.

Membrane Preparation

Solid electrolyte, perflurosulphonic acid membrane was cast from Nafion[R] dispersion (SE-5112, DuPont USA) containing 5 wt. % Nafion ionomer. Isopropanol and Nafion dispersion were mixed in a 1:3 volume ratio and then set in an oven for 4 h in vacuum atmosphere until all solvent evaporated and ionomers polymerized to form solid polymer membrane. The membrane film was treated for 1 h in boiling 3 vol. % [H.sub.2][O.sub.2] solutions and for 1 h in 1 M [H.sub.2]S[O.sub.4]. Finally it was rinsed in boiling water for 1 h. These treatments were done to remove the organic and metallic impurities from the caste membrane. The membrane thickness was measured as 145 [micro]m.

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Preparation of Anode, Cathode and Membrane Electrode Assembly (MEA)

Electrodes for DAFC are porous in nature to ensure the liquid fuel (ethanol) diffusion through anode and gas (oxygen from air) diffusion through cathode active zones. The anode was prepared from Pt-Ru/C electrode-catalysts with variable loadings of 0.6 to 1.5 mg/[cm.sup.2], activated carbon and mixture of Nafion ionomer (SE-5112, DuPont) and PTFE dispersion, which acted as binder. The anode electrode-catalysts slurry was prepared by dispersing the required quantity of electrode-catalysts powder in Nafion[R] solution with few drops of PTFE dispersion for 30 min using an ultrasonic water bath to obtain electrodecatalyst slurry. The slurry was painted on a carbon diffusion layer using a paintbrush uniformly in the form of continuous wet film. Then it was dried in an oven for 1 h at a temperature of 80[degrees]C. The cathode was prepared using similar compositions with Pt-black high surface area as electrode-catalysts (loading 0.6 to 1.5 mg/[cm.sup.2]). The dried anode and cathode were sintered at a temperature of 300[degrees]C in a hot oven. The sintered electrodes were placed on either side of the cast Nafion membrane and hot pressed at a pressure of 10 kg/[cm.sup.2] for 2 min at 90[degrees]C temperature to prepare MEA (membrane electrode assembly). The area of MEA was 5 [cm.sup.2].

SEM of Electrodes

The sintered electrodes (anode and cathode) of different loadings were visually observed in scanning electron microscope (SEM) to determine the surface morphology of the electrodes. The SEMs of anode using three different loading of Pt-Ru/C is shown in Figures 1 to 3 and that for Pt-black is shown in Figure 4.

Experimental Set-Up and Method

The tests on direct ethanol fuel cell (DEFC) were performed with a single cell design (Figure 5). The cell was fitted with a membrane electrode assembly (MEA) clamped between two stainless steel blocks with parallel flow channels of 2 x 2 [mm.sup.2] size for ethanol and oxygen/air flow. The cell was held together between two MS plates using a set of retaining bolts positioned around the periphery of the cell. PTFE sheet and tape were used for isolation and leakage prevention. The electrical heaters with control system were placed behind each stainless steel block in order to heat the cell to the desired operating temperature. The ethanol concentration of 1M, 2M and 3M solution was fed at anode at the rate of 1.2 ml/min using a peristaltic pump. Oxygen was supplied from a cylinder in cathode side and the pressure was maintained at 1 bar. The over-potential losses decrease with use of pure oxygen at the cathode. For different concentrations of ethanol, temperatures and loadings of electrode-catalysts, the current and voltage were recorded using multimeters (Sanwa) at variable electronic load.

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RESULTS AND DISCUSSIONS

SEM Observations

Figure 2 shows even distribution of Pt-Ru/C electrode-catalysts over the surface of the electrode (1 mg/[cm.sup.2]) compared to that in Figure 1 (0.6 mg/[cm.sup.2]). Figure 3 is the SEM for anode loading of 1.5 mg/[cm.sup.2] Pt-Ru/C, which shows that the catalysts layer is compact and less porous. The BET porosity measurement results show large decrease in porosity with the increase in loading from 1 mg/[cm.sup.2] to 1.5 mg/[cm.sup.2]. Thus, the anode loading of 1 mg/[cm.sup.2] would possibly give higher performance than that for 0.6 and 1.5 mg/[cm.sup.2] of loading. The SEM for cathode loading of 1 mg/[cm.sup.2] Pt-black shows similar surface morphology to that obtained for anode at the same loading.

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Polarization Curves and Power Density Curves

Effect of electrode-catalysts loading

Figure 6 shows the polarization curves and power density curves for different anode (Pt-Ru/C) loadings and 0.6 mg/[cm.sup.2] of cathode (Pt-black) loading at 42[degrees]C using 1 M ethanol fuel. It is seen that the power density increases with the increase in anode loading. Figure 6 shows that the maximum current density of 17 mA/[cm.sup.2] and power density of 5.4 mW/[cm.sup.2] is obtained for 1 mg/[cm.sup.2] Pt-Ru/C anode. However, the DEFC performance decreased to current density of 14 mW/[cm.sup.2] and power density of 4.9 mW/[cm.sup.2], when loading is increased to 1.5 mg/[cm.sup.2] Pt-Ru/C at anode. Figure 7 shows the polarization curves and power density curves for different anode loadings when the cathode (Pt-black) loading is increased to 1 mg/[cm.sup.2] and ethanol concentration to 2 M. It is seen that the anode loading of 1 mg/[cm.sup.2] produces the maximum current density (22.3 mA/[cm.sup.2]) and power density (7.83 mW [cm.sup.2]). Figure 8 shows the effect of anode loading on DEFC performance when the operating temperature of the cell is increased from 42[degrees]C to 90[degrees]C anode and 60[degrees]C cathode. Here, the maximum DEFC performance is obtained for anode loading of 1 mg/[cm.sup.2]. This shows irrespective of ethanol concentration, cell temperature and cathode loading, the maximum power density is obtained for anode (Pt-Ru/C) loading of 1 mg/[cm.sup.2]. Figure 9 shows the polarization and power density curves for different cathode (Pt-black) loadings with anode loading of 1 mg/[cm.sup.2] and 2 M ethanol feed at 90[degrees]C anode and 60[degrees]C cathode. It is seen that the maximum power density is obtained for 1 mg/[cm.sup.2] of cathode loading. The reason for the decrease in DEFC performance at 1.5 mg/[cm.sup.2] of anode and cathode loadings may be because of the compacting of the electrode-catalysts in a limited space, which results in decrease in porosity of the electrode-catalysts layer and as well as diffusion of fuel and oxidant through the electrodes. In conclusion, the maximum current density of 27.9 mA/[cm.sup.2], power density of 10.30 mW/[cm.sup.2] and OCV of 0.815 V were obtained for 2 M ethanol, at a temperature of 90[degrees]C anode and 60[degrees]C cathode with 1 mg/[cm.sup.2] anode (Pt-Ru/C) and cathode (Pt-black) loadings.

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Effect of ethanol concentration

Figure 10 shows polarization and power density curves for optimum anode (Pt-Ru/C) and cathode (Pt-black) loading of 1 mg/[cm.sup.2] using different ethanol concentrations at a temperature of 42[degrees]C. The power density of DEFC increases with the increase in ethanol concentration. This is because the activation over-potential is reduced with increase in ethanol concentration. Figure 10 indicates that 2 M ethanol results in higher current density (16 mA/[cm.sup.2]) and power density (8 mW/[cm.sup.2]) compared to that for 3 M ethanol (16 mA/[cm.sup.2] and 6.4 mW/[cm.sup.2]). However, the higher concentration (3 M) of ethanol reduces the cell performance because the active sites of the electrode-catalysts are blocked by the ethanol spices hindering water molecule to reach the active sites. Although not shown here, a similar result on ethanol dependence is obtained for different catalyst loadings and at different temperatures.

Effect of temperatures

Figure 11 illustrates the polarization and power density curves for 2 M ethanol at different anode and cathode temperatures. Figure 11 shows that the increase in temperature increase the current density and power density at an optimum electrode-catalysts loading of anode and cathode of 1 mg/[cm.sup.2]. The temperature combination of 90[degrees]C anode, 60[degrees]C cathode and cell temperature 79[degrees]C produce maximum current density of 27.9 mA/[cm.sup.2] and power density of 10.30 mW/[cm.sup.2]. The increase cell performance with the increase in temperature is due to decrease activation overpotential and faster reaction kinetics. While at the temperature of 120[degrees]C anode, 88[degrees]C cathode and 112[degrees]C cell temperature, the DEFC gives poor performance (22.56 mA/[cm.sup.2] and 7.73 mW/[cm.sup.2]) mainly because of low proton conductivity in PEM at higher temperature as the membrane is dehydrated.

CONCLUSIONS

The maximum OCV at a temperature of 90[degrees]C anode, 60[degrees]C cathode and with 2 M ethanol with pure oxygen supply at cathode was 0.815 V. Initially the cell performance increases with the increase in electrode-catalysts loading and decreases with further increase in electrode-catalyst loading. The optimum anode and cathode loading is 1 mg/[cm.sup.2]. The increase in concentration of ethanol fuel increases the current density and power density. The DEFC performance is increased up to 2 M ethanol concentration and it decreased with the further increase in ethanol concentration. The DEFC performance increases with the increase in cell temperature with optimized anode and cathode loading and ethanol concentration. However, the temperature 120[degrees]C anode and 88[degrees]C cathode, the DEFC performance decreases.

ACKNOWLEDGEMENT

The authors acknowledge the funding received from MNRE for the research work on direct ethanol fuel cell (102/01/2002-NT).

Manuscript received March 5, 2007; revised manuscript received July 15, 2007; accepted for publication August 8, 2007.

REFERENCES

Basu, S., (Ed.), "Recent Trends in Fuel Cell Science and Technology," Springer New York /Anamaya New Delhi (2007).

Colmati, F., E. Antolini and E. R. Gonzalez, "Effect of Temperature on Mechanism of Ethanol Oxidation on Carbon Supported Pt, PtRu and [Pt.sub.3]Sn Electrocatalysts," J. of Power Sources 157, 98-103 (2006).

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Lamy, C., S. Rousseau, E. M. Belgsir, C. Coutanceau and J.-M. Leger, "Recent Progress in the Direct Ethanol Fuel Cell: Development of New Platinum-Tin Electrocatalysts," Electrochimica. Acta. 49, 3901-3908 (2004).

Spinace, E. V., N. A. Oliveira and M. Linardi, "Electro-Oxidation of Ethanol on PtRu/C Electrocatalysts Prepared from ([eta]-[C.sub.2][H.sub.4])(Cl)Pt[([micro]Cl).sub.2] Ru(Cl) ([[eta].sub.3], [[eta.sup.3], [[eta].sup.3]-[C.sub.10] [H.sub.16])," J. of Power

Sources 124, 426-431 (2003).

H. Pramanik and S. Basu *

Department of Chemical Engineering, Indian Institute of Technology Delhi, New Delhi, 110016 India

* Author to whom correspondence may be addressed. E-mail address: [email protected]

Figure 11. Comparison of current
density versus cell voltage and current
density versus power density for 2 M
ethanol at different anode and cathode
temperatures. A = anode temperature;
C= cathode temperature; F = cell
temperature.

A C F

 42[degrees]C 42[degrees]C 42[degrees]C
 70[degrees]C 50[degrees]C 63[degrees]C
 90[degrees]C 70[degrees]C 79[degrees]C
120[degrees]C 88[degrees]C 112[degrees]C