A Study of Pt and Pt-Alloy Fuel Cell Catalyst Degradation in Aqueous and Membrane Electrode Assembly Environments Using In Situ Anomalous Small-Angle X-Ray Scattering

Polymer electrolyte membrane (PEM) fuel cells are an attractive energy conversion technology due to their high energy efficiency resulting from direct conversion of chemical energy into electrical energy. However, their cost effective implementation, especially for automotive power, has been hindered by degradation of the electrochemically-active surface area (ECA) of the Pt nanoparticle electrocatalysts.  While numerous studies using ex-situ post-mortem techniques have provided insight into the effect of operating conditions on ECA loss, the governing mechanisms and underlying processes are not fully understood.  Toward the goal of elucidating the electrocatalyst degradation mechanisms, we have successfully followed Pt nanoparticle growth during potential cycling of the electrocatalyst in an aqueous acidic environment and in a membrane electrode assembly (MEA) environment using in-situ anomalous small-angle X-ray scattering (ASAXS).¹

Several catalysts were utilized in this study with varying mean particles sizes; a commercial carbon-supported platinum (20 wt% Pt on XC-72 Vulcan carbon, E-Tek) with a mean particles size of ~2 nm, along with 40 wt% Pt and Pt₃Co nano-particles supported on Akzo Nobel Ketjen EC300J carbon black (provided by Johnson Matthey) with a mean particle size of ~3 nm and ~6 nm, respectively. Various potential cycling protocols (triangle, square, and trapezoidal) were applied to aqueous and MEA environments with anodic potential limits in the range of 1.0 to 1.1 V.

As an example of the results that will be discussed, ASAXS patterns were analyzed and found that the dominant Pt surface area loss mechanism was found to be preferential dissolution or loss of the smallest particles with varying extents of re-precipitation of the dissolved species onto existing particles in both environments. Correlation of ASAXS-determined particle growth and dissolved Pt measurements in the aqueous environment with both calculated and voltammetrically-determined oxide coverages.²This demonstrates that the oxide coverage is playing a key role in the dissolution process and in the corresponding growth of the mean Pt nanoparticle size and loss of ECA.

After extended cycling, the MEA environment showed accelerated degradation as compared to the aqueous stagnant environment (23% to 10% geometric surface area loss after 1,000 potential cycles). Introduction of electrolyte flow to the aqueous cell helped accelerate the Pt growth process to become similar to that observed in the MEA (24% geometric surface area loss after 1,000 potential cycles).

These results along with others including those with the Pt₃Co catalyst will be discussed in order to shed light on the Pt degradation mechanisms in all environments.

Acknowledgements: The authors would like to acknowledge Sarah Ball, Jonathan Sharman, Brian Theobald, and Graham Hards from Johnson Matthey Fuel Cells for supplying the catalysts and catalyst coated membranes. The authors also acknowledge funding from the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Fuel Cell Technologies Program (Nancy Garland, Technology Development Manager) and the U.S. Department of Energy, Office of Basic Energy Sciences for support of the Advanced Photon Source.

References

1. T. Narayanan, Soft Matter: Scattering, Imaging, and Manipulation. 1st ed.; Springer: Berlin, 2007.

J. Gilbert, N. Kariuki, R. Subbaraman, A.J. Kropf, M. Smith, E. Holby, D. Morgan, D. Myers, 2012, J Amer Chem Soc, 134, 36.