Abstract
To increase the commercialization of fuel cell electric vehicles, it is imperative to improve the activity and performance of electrocatalysts through combined efforts focused on material characterization and device-level integration. Obtaining fundamental insights into the ongoing structural and compositional changes of electrocatalysts is crucial for not only transitioning an electrode from its as-prepared to functional state, also known as "conditioning", but also for establishing intrinsic electrochemical performances. Here, we investigated several oxygen reduction reaction (ORR) electrocatalysts via in situ and ex situ characterization techniques to provide fundamental insights into the interfacial phenomena that enable peak ORR mass activity and high current density performance. A mechanistic understanding of a fuel cell conditioning procedure is described, which encompasses voltage cycling and subsequent voltage recovery (VR) steps at low potential. In particular, ex situ membrane electrode assembly characterization using transmission electron microscopy and ultra-small angle X-ray scattering were performed to determine changes in carbon and Pt particle size and morphology, while in situ electrochemical diagnostics were performed either during or after specific points in the testing protocol to determine the electrochemical and interfacial changes occurring on the catalyst surface responsible for oxygen transport resistances through ionomer films. The results demonstrate that the voltage cycling (break-in) step aids in the removal of sulfate and fluoride and concomitantly reduces non-Fickian oxygen transport resistances, especially for catalysts where Pt is located within the pores of the carbon support. Subsequent low voltage holds at low temperature and oversaturated conditions, i.e., VR cycles, serve to improve mass activities by a factor of two to three, through a combined removal of contaminants, surface-blocking species (e.g., oxides), and rearrangement of the catalyst atomic structure (e.g., Pt-Pt and Pt-Co coordination).
Original language | English |
---|---|
Journal | ACS Applied Materials and Interfaces |
DOIs | |
State | Published - Nov 6 2019 |
Funding
This work was authored in part by Alliance for Sustainable Energy, LLC, the manager and operator of the National Renewable Energy Laboratory for the U.S. Department of Energy (DOE) under Contract no. DE-AC36-08GO28308. This work was also authored in part by U.S. Department of Energy Office of Science Laboratory operated under Contract no. DE-AC02-06CH11357 by UChicago Argonne, LLC. Funding provided by U.S. Department of Energy Office of Energy Efficiency and Renewable Energy under the Fuel Cell Performance and Durability (FC-PAD) Consortium. Electron microscopy conducted as part of a user project at ORNL’s Center for Nanophase Materials Sciences (CNMS), which is a U.S. DOE Office of Science User Facility. The X-ray scattering measurements were performed at beamline 9-ID and the X-ray absorption experiments at beamline 10-ID, Advanced Photon Source (APS), Argonne National Laboratory, a user facility supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract no. DEAC02-06CH11357. The authors are grateful for the continued collaborative and engaging discussions with Anu Kongkanand, Swami Kumaraguru, and Srikanth Arisetty as well as many others from the GM fuel cell activity program. The authors appreciate the frequent insightful discussions with FC-PAD consortium members Jacob Spendelow, Rod Borup, Adam Weber, Ahmet Kusoglu. The authors would also like to thank Jeremy Kropf, Hemma Mistry, Samantha Medina, and Svitlana Pylypenko for their experimental insights. The authors also wish to thank Ellis Klein and Guido Bender for their dedication and contributions to NREL’s fuel cell program. While we do not endorse any materials presented in this study, we are grateful to Umicore and TKK for the use of their electrocatalysts. The views expressed in the article do not necessarily represent the views of the DOE or the U.S. Government.
Keywords
- conditioning
- electrocatalyst
- electrochemical diagnostics
- fuel cell
- kinetics
- oxygen transport resistances