Durability of PtCo/C Cathode Catalyst Layers Subjected to Accelerated Stress Testing

Abstract

The cost and durability of Polymer Electrolyte Membrane Fuel Cells (PEMFCs) are the two major barriers to the commercialization of these systems for stationary and transportation power applications.1 Given the stringent performance requirements that PEMFCs have to meet for automotive applications, Pt-alloy catalysts are being considered as cathode catalysts. While PtCox alloy catalysts exhibit better mass activity than Pt/C catalysts and can meet the beginning of life performance requirements, their durability is not proven. Several studies have indicated that the base metal leaches out of these catalysts over time and any benefits due to alloying is lost over the lifetime of the Membrane Electrode Assembly (MEA).2 The U.S. DOE Fuel Cell Tech Team has recommended various ASTs for PEMFC components.3,4 Los Alamos National Laboratory (LANL) has previously reported on the durability of several Pt/C catalysts during both simulated drive cycle testing and accelerated stress tests (ASTs).5 A catalyst AST that involved applying a square wave with 3 sec at 0.6V and 3 sec at 0.95V was determined to be an appropriate AST for evaluating the durability of Pt electrocatalysts.6In this study the performance of PtCo/C cathode catalyst based MEAs subjected to this AST were evaluated. The durability of PtCo/C catalysts was found to be comparable or better than that of the Pt/C based catalysts. Moreover the activity of the PtCo/C catalyst based MEA was also enhanced with respect to the Pt/C MEA. Figure 1 illustrates the evolution of the electrochemical surface area (ECSA) over 30,000 cycles of the square wave AST. The PtCo/C based MEA exhibited little change in ECSA over the course of the test. The ECSA initially increased due to insufficient conditioning and then decreasing due to catalyst particle size increase. In contrast, the Pt/C catalyst showed significant increase in ECSA over the course of the test. Finally the starting ECSA of the PtCo/C catalyst based MEA was significantly lower than that of the Pt/C MEA, while the end of test (EOT) ECSA was almost identical. These results are consistent with the small (≈ 2nm) starting particle size of the Pt and relatively large (≈ 5nm) starting particle size of the PtCo catalyst. The performance of the two MEAs before and after the durability AST is presented in Figure 2. It should be noted that the Pt/C based MEA had a loading of 0.15mg.Pt/cm2 whereas the PtCo/C MEA had an initial loading of 0. 21mg.Pt/cm2. The beginning of test (BOT) performance of the PtCo/C based MEA was significantly better in the kinetic region due to both the alloying effect and higher Pt loading. However, the BOT performance of the Pt/C MEA was better in the mass transport region due to extra mass transport limitations observed in alloy catalysts. The End of test (EOT) performance of the PtCo/C based MEA is only slightly higher than that of the EOT performance of the Pt/C based MEA, consistent with the fact that most of the Co has leached out of the PtCo catalyst. In this presentation the durability implications of using PtCo alloy catalysts will be discussed in detail with results presented from TEM, SEM, XRF, XRD and electrochemical testing.