Yu Haoran, Zachman Michael J, Li Chenzhao, Hu Leiming, Kariuki Nancy N, Mukundan Rangachary, Xie Jian, Neyerlin Kenneth C, Myers Deborah J, Cullen David A
Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States.
Department of Mechanical and Energy Engineering, Purdue School of Engineering and Technology, Indiana University-Purdue University Indianapolis, Indianapolis, Indiana 46202, United States.
ACS Appl Mater Interfaces. 2022 May 11;14(18):20418-20429. doi: 10.1021/acsami.1c23281. Epub 2022 Mar 1.
The recent surge in interest of proton exchange membrane fuel cells (PEMFCs) for heavy-duty vehicles increases the demand on the durability of oxygen reduction reaction electrocatalysts used in the fuel cell cathode. This prioritizes efforts aimed at understanding and subsequently controlling catalyst degradation. Identical-location scanning transmission electron microscopy (IL-STEM) is a powerful method that enables precise characterization of degradation processes in individual catalyst nanoparticles across various stages of cycling. Recreating the degradation processes that occur in PEMFC membrane electrode assemblies (MEAs) within the aqueous cell used for IL-STEM experiments is vital for generating an accurate understanding of these processes. In this work, we investigate the type and degree of catalyst degradation achieved by cycling in an aqueous cell compared to a PEMFC MEA. While significant degradation is observed in IL-STEM experiments performed on a traditional Pt catalyst using the standard accelerated stress test potential window (0.6-0.95 V), degradation of a PtCo catalyst designed for heavy-duty vehicle use is very limited compared to that observed in MEAs. We therefore explore various experimental parameters such as temperature, acid type, acid concentration, ionomer content, and potential window to identify conditions that reproduce the degradation observed in MEAs. We find that by extending the cycling potential window to 0.4-1.0 V in an electrolyte containing Pt ions, the degraded particle size distribution and alloy composition better match that observed in MEAs. In particular, these conditions increase the relative contribution of Ostwald ripening, which appears to play a more significant role in the degradation of larger alloy particles supported on high-surface-area carbons than coalescence. Results from this work highlight the potential for discrepancies between aqueous experiments and MEA tests. While different catalysts may require a unique modification to the AST protocol, strategies provided in this work enable future and identical-location experiments that will play an important role in the development of robust catalysts for heavy-duty vehicle applications.
近期，质子交换膜燃料电池（PEMFC）在重型车辆领域的关注度激增，这使得对燃料电池阴极所用氧还原反应电催化剂耐久性的要求提高。这使得旨在理解并随后控制催化剂降解的工作成为优先事项。同位置扫描透射电子显微镜（IL-STEM）是一种强大的方法，能够精确表征单个催化剂纳米颗粒在循环各个阶段的降解过程。在用于IL-STEM实验的水性电池中重现PEMFC膜电极组件（MEA）中发生的降解过程，对于准确理解这些过程至关重要。在这项工作中，我们研究了与PEMFC MEA相比，在水性电池中循环所导致的催化剂降解类型和程度。虽然在使用标准加速应力测试电位窗口（0.6 - 0.95 V）对传统Pt催化剂进行的IL-STEM实验中观察到了显著降解，但与MEA中观察到的情况相比，专为重型车辆设计的PtCo催化剂的降解非常有限。因此，我们探索了各种实验参数，如温度、酸的类型、酸浓度、离聚物含量和电位窗口，以确定能够重现MEA中观察到的降解的条件。我们发现，在含有Pt离子的电解质中将循环电位窗口扩展到0.4 - 1.0 V时，降解后的粒径分布和合金组成与MEA中观察到的情况更匹配。特别是，这些条件增加了奥斯特瓦尔德熟化的相对贡献，奥斯特瓦尔德熟化在高比表面积碳负载的较大合金颗粒降解中似乎比聚结起着更重要的作用。这项工作的结果突出了水性实验和MEA测试之间存在差异的可能性。虽然不同的催化剂可能需要对加速应力测试协议进行独特的修改，但这项工作中提供的策略能够实现未来的同位置实验，这将在开发用于重型车辆应用的耐用催化剂方面发挥重要作用。
