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Polymer Electrolyte Fuel Cell Lifetime Limitations: the Role of Electrocatalyst Degradation

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Polymer Electrolyte Fuel Cell Lifetime Limitations: the Role of Electrocatalyst Degradation V.H.1 Polymer Electrolyte Fuel Cell Lifetime Limitations: The Role of Electrocatalyst Degradation (A) Durability Deborah J. Myers (Primary Contact) and (B) Cost Xiaoping Wang (C) Performance Argonne National Laboratory 9700 S. Cass Avenue Lemont, IL 60439 Technical Targets Phone: (630) 252-4261 E-mail: [email protected] This project is conducting fundamental studies of platinum-based PEMFC cathode electrocatalyst DOE Technology Development Manager: degradation mechanisms. Insights gained from these Nancy Garland studies can be applied toward the definition of operating Phone: (202) 586-5673 conditions to extend PEMFC lifetimes and to the E-mail: [email protected] development of cathode electrocatalyst materials that meet the following DOE 2015 electrocatalyst durability Subcontractors: targets with voltage cycling: • Johnson Matthey Fuel Cells, Sonning Commons, United Kingdom • 5,000 hours (<80ºC) and 2,000 hours (>80°C), • United Technologies Research Center, • <40% loss of initial catalytic mass activity, and East Hartford, CT • Massachusetts Institute of Technology, Boston, MA • <30 mV loss at 0.8 A/cm² • University of Texas at Austin, Austin, TX • University of Wisconsin-Madison, Madison, WI Accomplishments Project Start Date: October 1, 2009 • Prepared Ketjen carbon-supported Pt nano-particle Project End Date: September 30, 2012 electrocatalysts (Pt/C) of varying particle size and incorporated these electrocatalysts into the cathodes of membrane-electrode assemblies (MEAs). • Quantified Pt/C catalyst oxygen reduction reaction Objectives (ORR) activity, electrochemically-active surface • Understand the role of cathode electrocatalyst area (ECA), and performance losses in a fuel cell degradation in the long-term loss of polymer as a function of initial Pt particle size and cell electrolyte membrane fuel cell (PEMFC) parameters (relative humidity, temperature, upper performance, potential limit, and cycling protocol). • Establish dominant catalyst and electrode • Developed a kinetic Monte Carlo (KMC) code to degradation mechanisms, predict Pt nano-particle evolution under fuel cell • Identify key properties of catalysts and catalyst conditions. supports that influence and determine their • Determined Pt particle size and particle size degradation rates, distribution evolution during potential cycling as • Quantify the effect of cell operating conditions, a function of initial Pt particle size (1.9, 3.2, 5.7, load profiles, and type of electrocatalyst on the and 9.1 nm) during potential cycling in an aqueous performance degradation, and environment, using anomalous small-angle X-ray scattering. • Determine operating conditions and catalyst types/ structures that will mitigate performance loss and allow PEMFC systems to achieve the DOE lifetime G G G G G targets. Introduction Technical Barriers One of the primary challenges facing the This project addresses the following technical development of PEMFCs for automotive and stationary barriers from the Fuel Cells section of the Fuel power applications is the durability of the fuel cell Cell Technologies Program Multi-Year Research, materials. Though significant progress has been made Development and Demonstration Plan: toward achieving the technical target of 5,000 operating hours, particularly for non-conventional catalyst DOE Hydrogen Program 876 FY 2010 Annual Progress Report Myers – Argonne National Laboratory V.H Fuel Cells / Degradation Studies architectures, the durability status for conventional The results of the experimental efforts will feed stacks is considerably shorter than 5,000 hours [1-3]. into coupled models at various levels of complexity Typical degradation rates for constant load conditions from atomic-level, ab initio oxidation and dissolution are 25-40 μV/h and can be an order of magnitude higher calculations, to catalyst degradation models, to cell when operating under non-steady-state conditions kinetic and transport models. The modeling effort prevalent in the automotive application, including load will also define the experiments necessary to complete and start-stop cycling and extended time at open circuit the cell model. The project can be categorized into [3]. The observed degradation under these conditions three broad and coupled tasks: (1) MEA studies has reversible and irreversible components with one utilizing accelerated stress test protocols, on-line of the most dominant contributors to irreversible electrochemical diagnostics, and post-test microscopic degradation being loss of cathode oxygen reduction and X-ray scattering characterization, (2) mechanistic reaction activity. The subjects of this project are the and physicochemical property studies using aqueous irreversible losses in PEMFC performance, as these are electrochemistry, X-ray spectroscopy/scattering, the most challenging in terms of mitigation strategies. transmission electron microscopy (TEM), scanning Specifically, this project focuses on cathode catalyst transmission electron microscopy (STEM), and in situ degradation, because the degradation of this component TEM, and quartz crystal microbalance measurements, has the most profound impact on cell performance. The and (3) model development, verification, and project’s primary focus is elucidation of the effects of implementation. All of these techniques have been catalyst and support physicochemical properties and demonstrated to provide important and complementary cell operating conditions on the rates and mechanisms information regarding catalyst degradation mechanisms. of cathode catalyst degradation, with a secondary focus on the impact of catalyst degradation on the transport Results properties of the cathode. To establish the background for studies of advanced Approach classes of catalysts, our initial studies have focused on determining the effects of cell operating parameters The project approach is to perform systematic and initial Pt particle size on cell performance and cell degradation tests, in situ and ex situ structural performance degradation. Catalysts were prepared characterization of the catalysts, fundamental out-of- containing 40 wt% Pt nano-particles with a mean cell studies, and theoretical modeling to identify the diameter of 1.9 nm on high-surface-area Ketjen black degradation modes and factors contributing to cathode carbon support (Pt/C). This material was heat treated catalyst degradation. The catalysts to be studied to form catalysts with mean particle sizes of 3.2, 7.1, are benchmark Pt on carbon supports with varying and 12.7 nm, respectively. These catalysts were studied properties, Pt alloys with varying oxophilicity, and three in an aqueous electrochemical environment (0.1 M classes of Pt catalysts having the highest reported oxygen HClO4 electrolyte) using anomalous small angle X-ray reduction activity. Specifically, our approach is to utilize scattering, to determine the evolution of Pt particle size accelerated stress tests of MEAs containing various and particle size distribution as a function of potential catalysts and supports and in situ and ex situ dissolution, cycling using the DOE cycling protocol (0.6 V to microscopic, structural, and chemical characterization 1.0 V triangle wave; 50 mV/s). These catalysts were of these catalysts. To elucidate the effect of particle size, also incorporated into the cathodes of MEAs and we will systematically vary the particle size of Pt and one subjected to the DOE cycling protocol in the fuel cell Pt alloy on a standard support. To elucidate the effect environment. Cell diagnostics of cathode catalyst ECA, of catalyst type and catalyst oxophilicity, we will study ORR mass activity, and air and oxygen polarization four classes of catalysts: Pt, Pt alloys, acid-leached alloys, curves were performed after 1,000, 3,000, 5,000, and and core-shell catalysts with approximately the same 10,000 voltage cycles. Studies were also performed particle size and particle size distributions. To elucidate on the effect of various fuel cell operating parameters support effects, we will study Pt on carbon supports with (relative humidity [RH], temperature, cycling profile, systematically varied surface area, pore size, and relative and upper potential limit) on the degradation of the proportions of micro- and mesopores. In addition, to cathode electrocatalyst performance on membrane- study the role of carbon degradation in ECA loss we will electrode assemblies containing the 3.2 nm Pt/C as the assess the effect of surface area of carbon exposed to the cathode catalyst. The results of these initial studies are electrolyte and Pt contact area with the carbon using a summarized below. series of Pt/C catalysts with differing Pt loading. We will also determine the effects of catalyst precursor impurities Summary of MEA and aqueous cell particle size study on degradation rates by post-synthesis doping of a Pt/C results: catalyst with varying levels of precursor impurities (e.g., Cl, Na, K, and F). • Cycling to 1.0 V has minimal impact on ECA and oxygen reduction reaction mass activity of catalysts FY 2010 Annual Progress Report 877 DOE Hydrogen Program V.H Fuel Cells / Degradation Studies Myers – Argonne National Laboratory 80 25 #5 (1.9 nm) #6 (3.2 nm) Initial 70 #7 (7.1 nm) #8 (12.7 nm) 20 After 1,750 Cycles 60 Pt) - g / 50 15 2 (m 40 A 10 C 30 Frequency E 20 5 10 0 0 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 8 9 10 Particle Size (nm) (x1000) Potential Cycle # 25 Initial ASAXS PSD FIGURE 1. Electrochemically-active surface area as a function of cycle 20 number (50 mV/s triangle wave between 0.4 and 1.0 V) for Pt/C cathode After 1,750 Cycles electrocatalysts in a membrane-electrode assembly with initial mean Pt particle sizes of 1.9 nm (■), 3.2 nm (▲), 7.1 nm (◊), and 12.7 nm (♦). 15 10 with large initial particle size (7.1 and 12.7 nm; see Frequency Figure 1). 5 • Cycling to 1.0 V degrades the ORR mass activity of catalysts with small initial particle size (1.9 and 0 3.2 nm) toward that of larger particles (7.1 and 0 5 10 15 20 12.7 nm). Particle Size (nm) • Beginning of life fuel cell performance of MEAs with smaller cathode catalyst particle size (1.9 nm and 3.2 nm) is highest, but voltage cycling degrades FIGURE 2.
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