The Stability of Nano-Structured-Thin-Film™ Supported Ir on Pt PEMFC Catalysts Under Rde Simulated Start-up and Shut-Down

The instability of Pt oxygen reduction electro-catalysts is a principle cause of proton exchange membrane fuel cell (PEMFC) performance decline, and the main causes of this instability are load cycling and start-up shut-down (su/sd). The extent of Pt loss is primarily a function of  1) rate of potential change 2) upper potential limit.¹

Work by 3M and partners on PEMFCs with Nano Structured Thin Film (NSTF) supported catalysts have shown minute amounts (2 – 10 µg cm²) of Ir added to Pt does not adversely affect Oxygen reduction reaction (ORR) activity and is effective in protecting Pt (loss < 10%) under simulated su/sd condition² The rotating disk electrode (RDE) can also be used to study the behavior of NSTF supported catalysts.³

Many rotating disk electrode (RDE) durability studies subject candidate PEMFC electro-catalysts to triangular wave potential cycling. Stevens et al. have developed alternative electrochemical techniques to simulate the transient potentials associated with load cycling and su/sd.with the RDE.⁴Figure 1 shows the potential changes of one su/sd “pulse”, designed to simulate run, idle and rest potential with constant potential holds, and simulate start-up and shut-down with constant current holds. This unique approach allows the upper potential limit to change as the test progresses.

Figure 2 shows how ORR activity changes under repeated su/sd “pulses” for various loadings of Ir on NSTF supported Pt (85 µg/cm²). Markers are also colored according to the final potential the constant current hold reaches prior to ORR characterization. The highest Ir loading of 19.1µg/cm² reaches a stable state, where the ORR activity changes very little after 2500 su/sd pulses. For a lower Ir loading (10 µg/cm²) tested at the same current, the ORR activity continually declines. However for the same Ir loading of 10 µg/cm² tested at 2/5^th the current, ORR activity seems to achieve near stability.

The results show an interesting relationship between the stability of Ir on Pt electro-catalysts, and the potential reached by the su/sd simulation current. When potential starts and stays below a threshold of approximately 1.5 V_RHE, the Ir-Pt reaches a stable configuration where ORR and oxygen evolution reaction (OER) activity no longer decline. Achieving stability may be more dependant on the potential reached, or the charge delivered, than the Ir loading. Note that for pure Pt tested at a current an order of magnitude lower than the 19.2 µg/cm² Ir sample, and started at a potential of 1.35 V_RHE, ORR activity continued to decline, indicating that stability of the RDE-NSTF catalyst system requires both low (< 1.5 V_RHE) potential and the presence of Ir.

These results indicate a possibility of indefinitely stable Pt based PEMFC catalysts, and the important role the su/sd potential may play in determining stability of an Ir-Pt electro-catalyst. Work is ongoing to determine Ir loading thresholds for stability.

1. A. A. Topalov, I. Katsounaros, M. Auinger, S. Cherevko, J. C. Meier, S. O. Klemm, and K. J. J. Mayrhofer, Angew. Chem. Int. Ed., 51, 12613–12615 (2012).

2. R. T. Atanasoski, D. A. Cullen, G. D. Vernstrom, G. M. Haugen, and L. L. Atanasoska, ECS Electrochem. Lett., 2, F25–F28 (2013).

3. G. C.-K. Liu, R. J. Sanderson, G. Vernstrom, D. A. Stevens, R. T. Atanasoski, M. K. Debe, and J. R. Dahn, J. Electrochem. Soc., 157, B207 (2010).

4. D. A. Stevens, J. Harlow, R. J. Sanderson, C. W. Watson, T. J. Crowtz, J. R. Dahn, G. D. Vernstrom, L. L. Atanasoska, G. M. Haugen, and R. T. Atanasoski, ECS Trans., 50, 1533–1538 (2013).