Evolution of Ionomer Coverage during Accelerated Stress Tests in Polymer Electrolyte Fuel Cells

Despite the current readiness level and the market availability of the polymer electrolyte fuel cells (PEFCs) durability is still a technical challenge that the technology, has to resolve to become competitive with internal combustion engines (ICE). In the automotive sector, the ICE engines typically reach lifetimes of more than a decade. In the nearest future PEFCs can become competitive in the medium to heavy-duty vehicles sector, where the space constraints and the power density requirements are more relaxed, but cell lifetime becomes of critical importance for such vehicles, that drive for up to 100,000 miles per year. Accelerated stress tests (ASTs) are used to assess the lifetime of a PEFC in a limited amount of time. the US Drive Fuel Cell Technical Team of the US DOE adopted several protocols targeting specific components of the cell or ageing phenomena (platinum dissolution or carbon corrosion). In the PEFC cathode electrode ionomer is added to enable proton transport to the electrocatalyst. Ionomer forms thin films around Pt/C agglomerates. The SO3 - groups (from the side chains of PFSA-based polymer ionomers) adsorb onto the platinum surface and reduce its overall specific activity. Additionally, when Nafion ionomer that has crystalline backbone aligns parallel to the catalyst surface it impedes local oxygen transport1 . Recent study did not show direct correlation between SO3 - group coverage and specific mass activity, when the coverage was varied from 0.16 to 0.262 . This may be because even with 0.16 coverage Pt has already been ‘poisoned’ and further addition of ionomer can actually have a positive effect enhancing proton accessibility. Understanding the behavior of the ionomer in the electrode and its evolution during the cell lifetime is of great importance, but at the present the literature does not show durability studies focused on this aspect. In this contribution, we aim to investigate and understand the coverage of the ionomer on Pt and carbon and SO3- groups adsorption in a PEFC during ASTs. CO stripping and displacement were used to probe the ionomer coverage as described by Garrick et al.3 Two types of cells were investigated using carbon corrosion AST: the first one with traditional carbon material and the second one with start/stop tolerant catalyst support. While the tolerant support led to small performance decrease and ECSA loss, the cell without start/stop tolerant support saw a sharp decrease in the ionomer coverage in the electrode between 100 and 500 cycles, that corresponded to a considerable loss in polarization. The ECSA was not changed between 100 to 500 cycles indicating that Pt loss was not a reason for polarization decrease and therefore we attribute the cause of the cell failure mainly to the loss of ionomer. Catalyst AST was also investigated, and the ionomer coverage was measured both in dry and wet conditions to separate the contribution of the interfaces between platinum/ionomer and platinum/water as in Iden and Ohma work4. By comparing the ECSA in dry and wet conditions, we see that while ECSA in wet condition decreases (that corresponds to the total ECSA reachable by protons) the dry ECSA (that considers only the Pt portion contacted by ionomer) remains constant. This behavior, coupled with an ionomer coverage that remains rather constant during the cycling, suggests that Pt dissolution is accelerated in the regions where Pt is not covered by ionomer. This work highlights the importance of the ionomer coverage and its evolution during aging. It suggests the role of ionomer is critical for catalyst stability in both carbon corrosion and catalyst ASTs. References: Shinozaki, K. et al. Special Feature : Popularizing Fuel Cell Vehicles : Designing and Controlling Electrochemical Reactions in the MEA Oxygen Reduction Reaction Activity Measurement for Fuel Cell Catalysts Using the Rotating Disk Electrode Technique. 49, 33–46 (2018). Van Cleve, T. et al. Dictating Pt-Based Electrocatalyst Performance in Polymer Electrolyte Fuel Cells, from Formulation to Application. ACS Appl. Mater. Interfaces 11, 46953–46964 (2019). Garrick, T. R., Moylan, T. E., Yarlagadda, V. & Kongkanand, A. Characterizing Electrolyte and Platinum Interface in PEM Fuel Cells Using CO Displacement. J. Electrochem. Soc. 164, F60–F64 (2017). Iden, H. & Ohma, A. An in situ technique for analyzing ionomer coverage in catalyst layers. J. Electroanal. Chem. 693, 34–41 (2013).