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Current Density (mA/cirf)

Figure 5. Polarization of Pt-RuPc/VuIcan Catalyst in PEMFC Before and After 200 hr LifeTest

ELECTRODE POROSITY AND EFFECTIVE ELECTROCATALYST ACTIVITY IN ELECTRODE-MEMBRANE-ASSEMBLIES (MEAs) OF PEMFCs

A. Fischer, H. Wendt Institut für Chemische Technologie der TH Darmstadt, Petersens». 20, D - 64287 Darmstadt, Germany

Introduction

New production technologies of membrane^Iectrode-assemblies for PEMFCs which ensure almost complete catalyst utilization by "wetting" the internal catalyst surface with the iono-meric electrolyte, (1) to (4), allow for a reduction of Pt-loadings from prior 4 mg cm'2 to now less than 0.5 mg cm"2. Such electrodes are not thicker than from 5 to 10 pm.

Little has been published hitherto about the detailed micromoiphology of such electrodes and the role of electrode porosity on electrode performance. It is well known, that the porosity of thicker fuel cell electrodes, e. g. of PAFC or AFC electrodes is decisive for their performance. Therefore the issue of this investigation is to measure and to modify the porosity of electrodes prepared by typical MEA production procedures and to investigate the influence of this porosity on the effective catalyst activity for cathodic reduction of oxygen from air in membrane cells. It may be anticipated that any mass transfer hindrance of gaseous reactants into porous electrodes would manifest itself rather in the conversion of dilute gases than in the conversion of pure gases (e.g. neat oxygen). Therefore in this investigation the performance of membrane cell cathodes with non pressurized air had been compared to that with neat oxygen at cathodes which had a relatively low Pt-Ioading of 0.15 mg cm'2.

Experimental

The manufacturing procedures for MEAs initially developed by Wilson, Gottesfeld and coworkers (1) to (3) and by Srinivasan (4) and Escribano and coworkers (5) were used. Addition of (i) volatile but homogeneously dissolved pore forming materials (ammonium salts) or (ii) particles of sparcely soluble non volatile but leacheable fillers (lithium carbonate) allowed to adjust the fine porosity by method (i) and coarse porosity by method (ii) within the limits from 25 to 65 vol. %. Additionally to Wilson's and Gottesfeld's hot pressing method a hot spray method was used (2) to (4) by spraying the cold catalyst ink on a heated membrane. The immediate vaporization of the solvents, isopropanole and water, creates - depending on the process temperature - a fine-porosity of from 25 to 35 vol. %. The catalyst loading determined by the amount of 30 wt % Pt on Vulcan XC-72 was kept constant (0.15 mg Pt per cm2) at the anode and cathode for all experiments. The porosity of the electrodes was determined pycnometrically by flooding the dry, symmetrically constructed MEA with toluene at reduced pressure and calculating from the weight gain the flooded void and porosity by accounting for the measured electrode thickness (~ 10 to 20 pm). Current voltage curves were measured under the following standardized conditions in 25 cm2 cells:

H2 flow rate: 300 Nml/min vapour saturated at 85 °C, corresponding to a limiting current density of 1.86 A cm'2, oxygen flow rate: 150 Nml / min and air flow rate: . 300 Nml/min, corresponding to an 02-supply limited current density of 0.74 A cm"2.

Results

Table 1 collects the porosity data and the internal conductivities of the electrodes fabricated by five different production procedures, beginning with hot pressed electrodes proceeding to hot sprayed electrodes which had been produced with a catalyst ink containing the leachable filler, which produces coarser pores. One observes a steady increase of the porosity P, which varies from 20 up to 65 vol %. Simultaneously the electronic conductivity of the electrodes decreases from 2.4 to 0.44 S cm'1 by a factor of 5.5, which is more than proportional as (1-P) decreases only by a factor of 2.3.

Fig. 1 compares the current voltage curves of hot pressed electrodes with that of three different hot sprayed electrodes. Hot sprayed electrodes prepared without any additive are already better than hot pressed ones. But still the performance is improved by forming a more porous structure with up to 65 % void. At a cell voltage of 0,5 V the current density increases by a factor of 1.7 from 220 to 380 mA cm"2 and at 0.4 V from 300 to 380 mA cm'2 by a factor of approx. 1.3.

Discussion

Contraiy to the general, little disputed, opinion that thin film MEA-cathodes are not really porous or that their porosity does not matter much, our results show that the porosity of MEA-cathodes which are operating on non-pressurized air is significant for obtaining high current and power densities. Simple model calculations show that the higher resistivity of the more porous electrodes cannot affect these data as the current is collected by relatively well conducting carbon paper, which covers the electrode evenly, so that the current has to pass through no more than several micrometers of the low conducting electrode matrix generating at current densities of 1 A cm'2 no more than 2 to 3 mV ohmic voltage losses. A detailed evaluation of our data shows, that increasing the porosity by a factor of three leads to an increase of the mass transfer limited current density by a factor of 1.3. The greater part of improved current and power densities in the cell voltage range of from 0.4 to 0.7 V, however, seems to be due to an increased utilization of the catalyst as it is manifested in increasing apparent io values up to -values of approx. 3.7 pA cm"2.

References

(1) M.S. Wilson, S. Gottesfeld, Thin film catalyst layers for polymer electrolyte fuel cell electrodes, J. Appl. Electrochemistry, 22 (1992), 1-7

(2) M.S. Wilson, Membrane catalyst layer for fuel cells, US Pat, 5,234,777 (1993)

(3) M.S. Wilson, Membrane catalyst layer for fiiel cells, US Pat 5,211,984 (1993)

(4) M.S. Wilson, High performance catalyzed membranes of ultra low Pt loadings for polymer electrolyte fuel cells, J. Electrochem. Soc. 139 (1992) L28 - 30

(5) S. Srinivasan, A.C. Ferreira, R. Mosdale, S. Mukerjee, J. Kim, s. Hirano, S.-M. Lee, F.N. Buchi, A.J. Appleby, "Proton exchange membrane fuel cells for space and electric vehicle application" in Program and Abstracts, Fuel Cell Seminar 1994 San Diego, USA, p. 424-7

(6) S. Escribano, S. Miachon, P. Aldebeit, "Low platinum loading wide electrodes for internal humidification hydrogen/oxygen polymer electrolyte membrane fuel cells" in New materials for fuel cell systems 1, Ed. O. Savadogo, P.R. Roberge, T.N. Veziroglu, Editions de l'École Polytechnique de Montréal, p. 135 - 43, Montreal 1995

Table 1

Porosity and intrinsic electronic conductivity of MEA-electrodes obtained by different productive procedures

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