Acm2

Figure 2: iR-corrected polarization curves, recorded at 80°C for H2/O2, of all carbon black-based catalysts. Open symbols: Pt-included materials; dark symbols: Pt-supported materials. Broken and dotted lines: ETEK5 and 20%, respectively.

A quick examination of Figs. 1 and 2 shows that the major difference between catalyst samples, particularly in the high current region of interest to fuel cell developers, is the slope of the cathode polarization curves plotted on linear coordinates (E vs i). This linear region is typically controlled by resistive losses within the catalyst layer which are not removed by iR correction. These resistive losses are typically electrode structure related and may be either electronic (contact resistance between graphite or carbon support particles ), ionic or both in nature.

From the linear region of the polarization curves plotted in Fig. 1, one can deduce that: i) a higher support specific area yields higher performance for both supported or incIuded-Pt catalysts; ii) the performance of the included Pt in KS6 (A) is lower than that of the supported Pt on the same material (A). KS6 is a low specific area support (20 m2/g). For HS300, a high specific area graphite (305 m2/g), the contrary is true. Indeed, the catalytic activity of HS300-Pti (O) is much higher than that of HS300-Pts (•). At high current the polarization curve of HS300-Pti departs from that of ETEK5 and even crosses the polarization curve of ETEK20 above 1 A/cm2. It is worth mentioning that HS300-Pti contains only 8.3 wt% Pt compared to 20 wt% Pt for ETEK20 (a 2.4 ratio in the Pt wt%), and that the Pt loading for HS300-Pti is 0.110 rag/cm2 compared to 0.287 mg/cm? for ETEK20 (a 2.6 ratio in the Pt loading of the electrode).

From the linear region of the polarization curves plotted in Fig. 2, one can deduce that: i) considering the Pt loading of the various cathodes, Vu-Pti (A) displays a much better performance than either Vu-Pts (A) or even ETEK20, especially at high currents. Indeed, Vu-Pti contains only 6.1 wt% Pt compared to 20 wt% Pt for ETEK20 (a 3.3 ratio in the Pt wt%); the Pt loading for Vu-Pti is 0.073 mg/cm2 while it is 0.287 mg/cm2 for ETEK20 (a 3.9 ratio in the Pt loading of the electrode); ii) a similar observation can be made for BP-Pti (■). At equivalent Pt wt% and Pt loading on the cathode, BP-Pti performs much better performing than ETEK2. Analysis of Figs. 1 and 2 in this fashion of course assumes that the polarization of the hydrogen anode in the fuel cell is only a few mV.

Table 1

Pt content, Pt cathode loading and specific surface area of Pt-included and Pt-supported catalysts

Table 1

Pt content, Pt cathode loading and specific surface area of Pt-included and Pt-supported catalysts

Type of catalyst

Pt

Ptatthe

Specific surface area

0 0

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