To compete with well-established fossil fuel-based power sources and to maximize their potential as environmental friendly alternative, proton exchange membrane (PEM) fuel cells have to meet several technical criteria, as described by the US Department of Energy’s Multi Year Research, Development and Demonstration Plan (MYRD&D) [1]. Those criteria include long-term durability under normal operation conditions and stability to external disturbances, both of which are strongly related to the aging processes of fuel cell electrocatalysts. Proposed degradation mechanisms for the most commonly used platinum group metal nanoparticles supported on a carbon support material include [2]: Ostwald ripening, coalescence via particle migration, particle detachment from the carbon support, and Pt dissolution into the electrolyte. These catalyst aging processes significantly depend on the robustness of the metal nanoparticles and support material as well as the stabilizing particle – carbon interactions [3, 4]. As all of the degradation processes result in a loss of electrochemically active surface area and often appear simultaneously, differentiating between these aging processes is a highly complicated task which we track in our comprehensive project.

In our work, we provide a deeper understanding about the degradation mechanisms occurring on carbon-supported platinum nanoparticles (Pt/C) as electrocatalyst for PEM fuel cells. Thin film electrodes made from Pt/C were electrochemically analyzed by exposing them to accelerated stress test (AST) protocols using a rotating disc electrode (RDE) setup. Our protocols include (i) potential cycling between 0.5 – 1.0 V vs. RHE (10,000 cycles) to resemble normal operation conditions in a fuel cell, (ii) 0.5 – 1.5 V vs. RHE (2,000 cycles) to simulate more extreme conditions e.g. during start-up and shutdown of the cell, and finally (iii) 1.0 – 1.5 V vs. RHE (2,000 cycles) to analyze corrosion effects on the carbon support material. As indicators for the catalyst degradation we monitored the electrochemically active surface area (ECSA), as well as the specific and mass activities at 0.9V vs. RHE of the Pt nanoparticles over the course of the AST experiments. Furthermore, the catalysts were characterized using various techniques such as transmission electron microscopy (TEM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). The losses of ECSA and ORR activity were then correlated to the structural changes such as particle size and carbon structure as well as to the experimental parameters such as number of cycles and potential range of the AST protocols. We observed a strong decrease in ECSA by up to 70% during simulated “start/stop” experiments, while “lifetime” and “carbon corrosion” protocols showed relatively small losses of ~30% and ~10%, respectively. These observations were accompanied by a strong growth in particle size during “start/stop” and less particle growth during “lifetime” and “carbon corrosion” protocols. Since the degradation rates of the Pt/C altered during the runs of these AST protocols in different manners, we performed intermediate stops to monitor the changes in structure and morphology of the Pt nanoparticles as well as of the carbon.

Based on our results, we developed a model to clarify the degradation mechanisms during different types of accelerated stress tests and to identify the critical parameters for maintaining the catalytic performance of the Pt/C catalysts under operating fuel cell conditions.

References:

[1] https://www.energy.gov/eere/fuelcells/downloads/fuel-cell-technologies-office-multi-year-research-development-and-22

[2] Y. Shao-Horn et al., Top. Catal., 46, 285–305 (2007).

[3] F. Hasché, M. Oezaslan, and P. Strasser, ChemCatChem, 3, 1805–1813 (2011)

[4] F. Hasché, M. Oezaslan, and P. Strasser, Phys. Chem. Chem. Phys., 12, 15251 (2010)