Pt Alloys

Of the pure metals, Pt has proven to be the most active and durable catalyst for oxygen reduction. However, at reasonable current densities, Pt still shows overpotentials of over 400 mV from the equilibrium reversible potentials (1.19 V at 80°C). Therefore, great efforts have been made to identify and develop superior catalysts for oxygen reduction.

The development of catalyst technology for the PAFC by Pratt & Whitney (part of UTC) in the 1970s (compare Section 2.7) centered initially on the introduction of carbon-supported Pt catalysts, both for the cathode and anode. This had the advantage of reducing overall PM loading, while increasing available metal surface area. It was found that the surface area of carbon-supported Pt catalysts decreased on operation. However, the expected loss in activity was partly offset by the increase in specific activity of large Pt crystallites (see above). UTC found that carbon-supported Pt alloys prepared by carbothermal reduction had superior activity to Pt catalysts and were stable under PAFC operating conditions for many thousands of hours (Bett, 1992). It was also found that ordered alloy structures were more stable and active than disordered structures. A ternary formulation of Pt, Co, and Cr on graphitized furnace black carbon was selected for commercial systems (Luczak and Landsman, 1987).

It was found that Pt alloys gave an improvement of 40 to 60 mV in performance (at 200 Aft-2 on H2/ air at 190°C) (Landsman and Luczak, 1982; Luczak and Landsman, 1987). Assuming a typical Tafel slope of 90 mV per decade, this implies an improvement in activity of 3 times. However, kinetic activities measured on O2 (at 900 mV) indicated an improvement in catalyst turnover of 1.5-2 times (Buchanan et al., 1992).

Given the enhanced activity in PAFC, carbon-supported Pt alloys have been evaluated in PEMFC to determine whether the benefits observed in PAFC could be found within the PEM environment. Assuming a Tafel slope of 60 mV per decade, an improvement in mass activity of 2 times should show an 18-mV improvement. Early work at Texas A&M University showed that Pt alloys showed improvements in mass and specific activities of 2-3.5 in small PEMFC test cells with O2 at high pressure, consistent with PAFC

0.75

At 538 mA cm-2

Improved PtCr alloy catalyst - Pt catalyst

Ballard Mark 5E single cell.

Cell at 80°C, H2/Air, 30/30 psig, 1.5/2 stoich., internal humidification. Nafion 115 membrane. Low Pt loading MEA (< 1 mgPt cm-2).

0 100 200 300 400 500

Operating Lifetime / hours

FIGURE 6.11 Comparison of carbon-supported PtCr alloy and Pt cathode air performance.

data (Mukerjee and Srinivasan, 1993). More recently, it has been shown that in more practical hardware, Pt alloys do show a kinetic benefit of 25 mV (equivalent to a increase in mass activity of 3 times) on oxygen operation. On operation on air, this benefit can be masked by additional mass transport losses, but these have been overcome by improved catalyst formulation, as shown in Fig. 6.11 (Ralph et al., 1998; Ralph and Thompsett, 1999).

At present, Pt alloys are not widely used in MEAs, although a performance benefit of ca. 25 mV has been proven. It is expected that as the desire to improve cathode performance increases, supported Pt alloys will become the catalysts of choice.

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