F834 Journal of The Electrochemical Society, 162 (8) F834-F842 (2015) 0013-4651/2015/162(8)/F834/9/$33.00 © The Electrochemical Society A One-Dimensional Pt Degradation Model for Polymer Electrolyte Fuel Cells Yubai Li, a, * Koji Moriyama, b Wenbin Gu, c Srikanth Arisetty, c and C. Y. Wang a, **, z a Electrochemical Engine Center (ECEC), and Department of Mechanical and Nuclear Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, USA b Honda R&D Co. Ltd., Haga-machi, Haga-gun, Tochigi, 321-3393 Japan c General Motors, Pontiac, Michigan 48340, USA A one-dimensional model is developed and validated to study platinum degradation and the subsequent electrochemical surface area (ECA) loss in the cathode catalyst layer (CL) of polymer electrolyte fuel cells (PEFCs). The model includes two mechanisms of Pt degradation: Ostwald ripening on carbon support and Pt dissolution-re-precipitation through the ionomer phase. Impact of H 2 | N 2 or H 2 | Air operation, operating temperature, and relative humidity (RH) on Pt degradation during voltage cycling is explored. It is shown that ECA loss is non-uniform across the cathode CL with a zone of exacerbated Pt degradation and hence much lower ECA found near the membrane. This non-uniform Pt degradation is caused by consumption of Pt ions by crossover H 2 in both H 2 | N 2 and H 2 | Air systems. An important consequence is that thinning the cathode electrode in a fuel cell would lead to more ECA loss as a higher fraction of the thin CL would fall in this exacerbated degradation zone. We have quantified the effect of thin cathode CLs on Pt degradation for the first time. © 2015 The Electrochemical Society. [DOI: 10.1149/2.0101508jes] All rights reserved. Manuscript submitted March 23, 2015; revised manuscript received April 24, 2015. Published May 7, 2015. Polymer electrolyte fuel cell (PEFC) durability is a key challenge to commercialization of hydrogen fuel cell vehicles. A central issue of PEFC durability is the loss of electrochemical surface area (ECA) over time. Much research has been carried out in the past two decades 1 to understand the fundamental mechanisms of platinum degradation and to suggest effective mitigation strategies. Four processes have been outlined as possible Pt degradation mechanisms 2 : i) Ostwald ripening on carbon support; 3–7 ii) Pt crystal migration and coalescence; 8–10 iii) detachment and agglomeration of Pt nanoparticles induced by carbon corrosion; 11–14 and iv) Pt dissolution, Pt ion transport in the ionomer phase and subsequent re-precipitation by H 2 crossover through the membrane. 4,6,15–18 Although the dominating degradation mechanisms under various fuel cell operating conditions are still under debate, durability test protocols 19 of electrocatalysts, such as those suggested by Fuel Cell Commercialization Conference of Japan (FCCJ), can lend some basic insight into degradation mechanisms. For start/stop durability tests, a triangular-wave potential cycle of 1.0–1.5 V vs. reversible hydrogen electrode (RHE) is chosen and the major degra- dation is carbon corrosion 19 and the ensuing Pt agglomeration and Pt detachment. 20 Suppressing carbon corrosion under such high volt- age would be the first priority. 21,22 On the other hand, a square-wave potential cycle of 0.6–1.0 V vs. RHE is used to simulate the load- cycle tests and Pt degradation induced by carbon corrosion is minor in this case, 19,23 and Ostwald ripening and Pt dissolution followed by re-precipitation could lead to accelerated degradation. 4,24 While the present study is limited to modeling Pt degradation under standard test protocols, the model’s predictability is expected to extend to actual PEFC operating conditions during vehicle driving. According to Pourbaix diagram, Pt dissolution can occur at volt- ages higher than 1.0 V and pH less than 0 at 25 ◦ C. 25 Mitsushima et al. 26 suggested that Pt solubility in acidic media increases with temperature and decreases with pH. It was also observed that the voltage cycling accelerates the dissolution of Pt. 27–29 Since the PEFCs used in auto- motive fuel cells involve an acid membrane, and the cathode potential vs. RHE at open circuit voltage is above 1.0 V, the above mentioned degradation mechanisms occur. 30 The typical operating temperature of an automotive fuel cell could be as high as 90 ◦ C, and the cathode electrode is usually subjected to voltage cycling in a realistic driving cycle. 30 Therefore, Pt dissolution is ubiquitous in automotive PEFCs and hence, ECA loss. 25 Influences of operating conditions on Pt degradation have been scrutinized in the literature. It was reported that higher ∗ Electrochemical Society Student Member. ∗∗ Electrochemical Society Active Member. z E-mail: cxw31@psu.edu temperature 30–35 and higher relative humidity (RH) 32–37 conditions could induce more rapid Pt degradation. The effect of O 2 par- tial pressure on the dissolution of Pt nanoparticles has also been investigated. 37–39 A fundamental study of Matsumoto et al. 38 found 18 times accelerated dissolution of polycrystalline Pt under pure O 2 as compared to under pure N 2 ; however, this acceleration fac- tor decreased to only 1.2 for carbon-supported Pt nanoparticles. Further weakening of the O2 partial pressure effect was experi- mentally observed in catalyst layers made of carbon-supported Pt nanoparticles. 37,39 Indeed, Bi et al. 37 reported that the ECA loss under H 2 | N 2 condition is marginally higher than the loss under H 2 | Air condition. This is likely due to the fact that when the O 2 partial pres- sure increases, the Pt ion sink is pushed farther away from the cathode CL into the membrane, thereby lowering the Pt ion flux dissolved and transported into the membrane. 40,41 Such a weak effect of O 2 partial pressure on Pt degradation and ECA loss in a fuel cell cathode can only be explained by a one-dimensional model to be developed in the present work. Another consequence of the combined Pt Ostwald ripening and Pt dissolution-re-precipitation 42 is non-uniform distribution of degra- dation across the cathode CL. Indeed, under in-situ voltage cycling test conditions, it was observed that most of Pt mass loss occurred near the CL-membrane interface for both H 2 | N 2 4,6 and H 2 | Air 43 conditions. Moreover, Nagai et al. 44 carried out durability tests for Pt catalyst on carbon support with different catalyst loadings and hence different CL thickness, and found that a lower Pt loading CL suffers from more rapid ECA loss but the total Pt mass dissolved remains almost the same. Such an observed trend of ECA loss versus CL thickness could be theoretically explained by the possibility that most of the Pt mass loss occurred near the membrane where the Pt ion sink created by crossover H 2 is located, and Pt mass near the GDL is pre- served by re-deposition. 44 However, accurate measurement of ECA loss distribution across the cathode CL is difficult, if possible at all. Mathematical modeling has been used to aid in deconvoluting Pt degradation mechanisms. Darling and Meyers 45,46 were the first to develop kinetic rate equations for Pt electrochemical dissolution, Pt oxide film formation, and Pt oxide chemical dissolution. Franco and Tembely 47 proposed a transient multiscale model of Pt degradation in the cathode electrode using the multi-layer model 48 of electrochemical interface. Pt degradation under galvanostatic conditions rather than potentiostatic conditions was investigated with this model. 48 Bi and Fuller 49 proposed a one dimensional (1D) bi-modal particle size model to investigate the Pt degradation processes, and the mechanism of Pt ion re-precipitation was included. Holby et al. 50 refined the model of Darling and Meyers 45 by applying hundreds of particle size groups to represent a nearly continuous PSD. Recently, Ahluwalia et al. 35 ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 130.203.224.205 Downloaded on 2015-05-08 to IP