Results And Discussion

Effects of the PFSI content on the pore volume distribution in the membrane/electrode assemblies and modeling the internal structures of the catalyst layer. —The specific pore volume distributions of the M&E assemblies with various PFSI content and an experimental acetylene black (AB18, Sc 835 m2/g, Denkikagaku Kogyo) showed three pore zones which had critical boundaries at ca. 0.04 and 1.0 pm, and had peaks in pore-size distribution from 0.02 to 0.04 pm. The specific pore volume from 0.04 to 1.0 pm decreased with an increase of the PFSI content. Pores larger than 1.0 pm are from the carbon paper. We define the pores f rom 0.02 to 0.04 pm to be the "primary pores", and the pores from 0.04 to 1.0 pm, the "secondary pores" in the PEFC. The volume of the secondary pores linearly decreased with the increase of the PFSI content but that of the primary pores remained unchanged. The particle size distribution of the PFSI colloid consisted chiefly of the primary particles with a mean diameter about 0.043 pm. The schematic internal structures of the catalyst layer with Pt-C agglomerate are illustrated in Fig. 1, one with a liquid electrolyte (Fig. la) and the other with a polymer electrolyte (Fig. lb). From these results, it should be emphasized that the PFSI added to the catalyst layer exists only in the secondary pores -s.

and not in the primary pores (Fig. lb). We proposed that the secondary pores behaved as reaction sites in the PEFC, because the electrolyte (PFSI) existed only in there. Watanabe et al. 2 reported that the primary pores work as a "reaction volume" and the secondary pores as main gas channels in fuel cells with liquid electrolytes (e.g. PAFC) (Fig. la). As for this difference between our results and Watanabes', the former was a polymer and the latter liquid. It seemed that the PFSI was not able to penetrate into the primary pores (< ca. 0.04 pm) in contrast to phosphoric acid, because a particle size of the PFSI (ca. 0.04 pm) was larger than a molecular size of phosphoric acid. This suggested that the Pt particles loaded inside the agglomerate did not take part in the reaction, since the inside Pt particles were out of contact with the polymer electrolyte.

Pt supported carbon (Pt-C)

agglomerate agglomerate

Pt supported carbon (Pt-C)

agglomerate agglomerate

primary pore filled with liquid electrolyte secondary pore vacant for gas channel polymer electrolyte (PFSI)

primary pore filled with liquid electrolyte secondary pore vacant for gas channel polymer electrolyte (PFSI)

(a) liquid electrolyte

(b) polymer electrolyte

Fig. 1. Schematic of the internal structure of the catalyst layer.

Effect of the carbon supports on the cell performance. —The effects of the carbon supports on the polarization curves are shown in Fig. 2. The influence of the carbon blacks on PEFC performance was remarkable. Cells with acetylene blacks had better performance than those with oil-furnace blacks. Differences in the electrochemical characteristics of the cells with the furnace blacks were large; cells with Ketjen black 600JD, N330, #3950, and Black pearls 2000 hardly discharged. Their adhesive strengths between membrane and electrode were very weak; some electrodes peeled off the membrane. Those electrodes simultaneously caused floodings in their gas channels.

Effects of the carbon supports on the pore-volume distribution in the catalyst layer. — The relationship between the specific pore volume in the pore zone of from 0.04 to 1.0 pm and the current densities of the cells at 850 mV (IR-free) in a charge transfer control region are shown in Fig. 3. Data with the PFSI content 1.0 mg/cm2 were plotted with the legend •, and the data with increased PFSI content (shown in figure) were with the legend ©. Performance of the cells with the PFSI content of 1.0 mg/cm2 increased with the specific pore volume up to 0.070 cm3/g, and then decreased, while performance with the PFSI content from 1.2 to 2.0 mg/cm2 increased with specific pore volume. Current density increased linearly with specific pore volume. It seems that the reaction area > comprised of PFSI and Pt increased S ordinarily with pore volume, but excessive pore volume caused shortage of PFSI even when the content was 1.0 mg/cm2. A proper addition of PFSI to the catalyst layer regained the continuity of PFSI and thus the interface between the PFSI and the Pt, and then the reaction area increased. From these results, it should be emphasized that the improvement of the PEFC performance was achieved by the optimal carbon support with larger pore volume able to distribute the PFSI over the

Fig. 2. Effects of the carbon supports on the polarization curves of the cells comprised of Hemion PFSI 1.0 mg/cm2. '

Fig. 2. Effects of the carbon supports on the polarization curves of the cells comprised of Hemion PFSI 1.0 mg/cm2. '

rra «imt u bital

5.00 0.02 0.04 0.0S 0.0S 0.10 Specific pare Tolame (from 0.04 to l.Qim) / cn? -

Pt inside the agglomerate.

Fig. 3. Relationship between the specific pore volume in the pore zone (0.04 to 1.0 pm) and the current densities at 850 mV : (•) PFSI content 1.0 mg/cm2 and (©) 12, 15,2.0 mg/cm2.

Effect of the carbon supports on the pore-volume distribution on the surface of the carbon blacks. — The specific pore volume distributions of the carbons were measured by Ni adsorption and were calculated by the BJH method. Pore volumes of the carbon blacks consisted chiefly of pores smaller than 8 nm. The primary particle diameter of the carbon blacks ranged from 10 to 40 nm. Almost all the pores of the carbons seemed to be in the primary particles. Pore volume increased with the surface area of carbon black. Most of their surface area was in pores smaller than 8 nm. The Pt in these pores (< 8

nm) is considered not to contribute to the reaction for the PEFC stated above, because the particles of the PFSI are larger than the pore diameters and Pt can not contact the PFSI. The relationship between the specific pore volume in the pores smaller than 8 nm of the carbon blacks and the current densities of the cells with those carbons at 850 mV (IR-free) are shown in Fig. 4. Current density at 850 mV decreased with an increase of specific pore volume (< 8 nm). A tendency for the reaction area of the cell to decrease with increase the specific pore volume seems real. AB had both the smallest specific pore volume and the smallest specific surface area. The Pt particles were smaller than the pore diameter (< 8 nm) at the entrance could be adsorbed on the very surfaces of the pores. The rate of Pt in the pores seemed generally to increase with the specific pore volume, because the Pt loading was fixed at 0.5 mg/cm2. Consequently, it should be emphasized that improvement of the PEFC performance was achieved by optimal carbon support with smaller pore volume on the surface of the carbon primary particles, like acetylene blacks, so as to decrease the Pt absorbed in the small pores which the PFSI could not soak into.3

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