Accelerated Stress Test Study on PtRu Anode Catalysts for Reformate PEM Fuel Cells
Authors: Berova Viktoriya , Hengge Katharina , Burger Thomas , Scheu Christina , Jurzinsky Tilman 
Published: September 2023

Proton-exchange membrane fuel cells (PEMFCs) are interesting candidates for heavy duty applications, e.g. in the maritime or bus and truck sector, since they convert renewable fuels into electricity efficiently [1]. Moreover, PEMFCs are flexible with respect to fuels, since they can be operated not only with hydrogen but also with reformate gas, originating from steam reforming of carbohydrates such as renewable methanol [2]. Compared to state-of-the-art carbon-supported Platinum catalysts (Pt), which are severely poisoned by CO, a side product from reforming, carbon-supported Platinum-Ruthenium catalysts (PtRu) prevail in reformate applications due to their promoting effects towards CO oxidation [3]. However, PtRu catalysts are less stable than Pt catalysts due to a higher susceptibility to dissolution [4]. Since the durability of materials in heavy duty applications is crucial to reach long operating times (> 30000 h), major efforts have to be taken to improve the stability of PtRu anode catalysts.
To be able to evaluate the stability of catalysts and to investigate, identify, and understand degradation mechanisms with little time and costs, so-called accelerated stress tests (ASTs) are deployed, which induce fast electrode aging and ideally simulate electrode degradation that can be transferred to real operating conditions. For Pt cathode catalysts in PEMFC application, the U.S Department of energy proposed an AST based on square wave potential cycling between 0.6 V and 0.95 V for 30000 cycles [5]. As Ru and possibly PtRu have different dissolution potentials than Pt and are used on the anode side (therefore exposed to different potentials compared to the cathode), an AST designed for PtRu anode catalysts is required.
In this work, several AST protocols based on square cycling the anode potential for 30000 cycles were developed and tested on cells with the same material composition. The carbon-supported PtRu anode electrodes were manufactured in-house with a patented technique [6], while for membranes, cathode catalysts, and GDLs commercial available products were applied. The ASTs differ by the upper potential limit, which was chosen at 0.2 VRHE, 0.65 VRHE, 0.8 VRHE, and 1.0 VRHE, respectively. The lower potential limit was set at 0.06 VRHE, (cf. Figure 1b). Before, during, and after the AST, electrochemical characterization was conducted via cyclic voltammetry, CO-stripping, and recording U-I curves (cf. Figure 1a). After the AST, post-mortem analyses were performed by means of transmission electron microscopy, X-ray diffraction, and cross-sectional scanning electron microscopy coupled with energy dispersive X-ray spectroscopy to analyze the prominent degradation mechanisms occurring for the different testing conditions.
Strong differences in the degradation of the cells between the different ASTs could be observed. With increasing upper potential limit, the severity of degradation and the observed degradation phenomena increased. For example, Ru leaching occurred at ASTs with upper potentials beyond 0.65 VRHE, and increased with increasing potential, resulting in a reduced CO-tolerance. At the same time, the consequent Ru crossover to the cathode, which lowers the overall cell performance, could be only detected by EDX for ASTs beyond 0.8 VRHE, implying a correlation between Ru crossover and upper potential limit. Moreover, the data indicate that an upper potential limit of 1.0 VRHE may be too high to investigate the stability of Ru in PtRu catalysts, as an additional strong Pt degradation makes the evaluation difficult. Future work will address the correlation of these ASTs to long-term testing at constant potential.
References
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[6] M. Batzer, VERFAHREN ZUR ERZEUGUNG EINER KATALYSATORHALTIGEN ELEKTRODENSCHICHT - European Patent Office - EP 2497141 B1.

Figure 1