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 CO2 emission from the cathode, further confirming the ability of the square-wave AST to evaluate the electro-catalyst independently of the support.
| Original language | English |
|---|---|
| Pages (from-to) | F492-F501 |
| Journal | Journal of the Electrochemical Society |
| Volume | 165 |
| Issue number | 7 |
| DOIs | |
| State | Published - 2018 |
Funding
alyst AST with 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. The DOE has adopted this new AST as a tool for evaluating catalyst durability. The adoption of this protocol can be found in Table P.1 of the DOE FCTO Fuel Cells 2016 Multi-Year Research, Development, and Demonstration Plan.1 Moreover these ASTs were independent of the carbon-type used in the MEA, further confirming the ability of these ASTs to evaluate the electrocatalyst independently of the support. The acceleration factors of the square-wave and triangle-wave ASTs were decreased to 10X and 2.5X respectively for the alloy catalysts. For Pt-Co alloy catalysts in the 5 nm particle size range the predominant degradation mechanism is the leaching of transition metal and not the enlargement of the particles. This result was supported by direct carbon corrosion measurements showing little evolved CO2 at the cathode outlet during the catalyst AST. When SOA Pt-Co/HSAC catalyst was subjected to a carbon corrosion AST (1.0–1.5 V cycling), the catalysts demonstrated a minimal change in the Pt-Co particle size and percentage loss of Co from the Pt-Co catalyst particles; however, a significant loss in CCL thickness was observed which resulted in large mass transport performance losses. This further confirmed that the catalyst ASTs with their minimal catalyst layer thickness loss were indeed evaluating the electrocatalyst independently of the support.
