Recent Advances in Catalyst Accelerated Stress Tests for Polymer Electrolyte Membrane Fuel Cells

Abstract

The U.S. Department of Energy (DOE) set the 2020 durability target for polymer electrolyte membrane fuel cell transportation applications at 5000 hours. Since it is impractical to test every fuel cell for this length of time, there is ever increasing interest in developing accelerated stress tests (ASTs) that can accurately simulate the material component degradation in the membrane electrode assembly (MEA) observed under automotive operating conditions, but over a much shorter time frame. In this work, a square-wave catalyst AST was examined that shows a 5X time acceleration factor over the triangle-wave catalyst AST and a 25X time acceleration factor over the modified wet drive-cycle catalyst durability protocol, significantly decreasing the testing time. These acceleration factors were correlated to the platinum (Pt) particle size increase and associated decrease in electrochemical surface area (ECSA). This square-wave AST has been adopted by the DOE as a standard protocol to evaluate catalyst durability. We also compare three catalyst-durability protocols using state-of-the-art platinum-cobalt catalysts supported on high surface area carbon (SOA Pt-Co/HSAC) in the cathode catalyst layer. The results for each of the three tests showed both catalyst particle size increase and transition metal leaching. Moreover the acceleration factors for the alloy catalysts were smaller due to Co leaching being the predominant mechanism of voltage decay in ∼5 nm PtCo/C catalysts. Finally, an extremely harsh carbon corrosion AST was run using the same SOA Pt-Co/HSAC catalyst. This showed minimal change in particle size and a low percentage Co loss from the cathode catalyst particles, despite a significant loss in catalyst layer thickness and cell performance. The carbon corrosion rates during these various ASTs were directly measured by monitoring the CO 2 emission from the cathode, further confirming the ability of the square-wave AST to evaluate the electro-catalyst independently of the support. Polymer electrolyte membrane fuel cells (PEMFCs) have the po-tential to be excellent alternative energy conversion devices; however, one of the biggest obstacles to their commercialization is the dura-bility of the materials comprising the cathode catalyst layer (CCL). The U.S. Department of Energy (DOE) Fuel Cell Technologies Office (FCTO) has set a 2020 durability target of 5000 hours for PEMFCs used in transportation applications and 60,000 hours for PEMFCs used in stationary systems. 1 To meet the automotive target, the U.S. DOE-FCTO, in conjunction with the U.S. DRIVE Fuel Cell Techni-cal Team (FCTT), developed a 5000 hr drive cycle durability protocol to simulate the different degradation mechanisms a fuel cell expe-riences under normal transportation operating conditions (Table P.7 from Reference 1). This drive cycle is used to determine the dura-bility of the entire membrane electrode assembly (MEA) and not the individual fuel cell components. The drive cycle varies operating con-ditions including potential, load, and humidity cycling; conditions that are known to degrade the catalyst, catalyst support, membrane, and bipolar plates. The U.S. DRIVE-FCTT drive cycle durability protocol is shown schematically in Figure 1. It is not realistic to test every new fuel cell component that is devel-oped for the full 5000 hrs. Thus, over the past several years, accelerated stress tests (ASTs) have been developed and refined to simulate the different materials/component degradation mechanism(s) experienced within the MEA of a PEMFC during operation. In particular, dura-bility of the electro-catalyst is a key topic of interest for recent AST development efforts. Platinum (Pt) dissolution and re-precipitation, and the resulting increase in the catalyst particle size coupled with the leaching of the transition metal out of an alloy catalyst particle, and migration of Pt and/or alloying elements out of the CCL into the membrane, that occur during potential cycling, are the primary degradation mechanisms. 2,3