Microscopic Insights into the Degradation Mechanisms of Electrocatalysts in PEM Electrolyzers
Authors: Yu Haoran , Cullen David A. , Zachman Michael J. , Bonville Leonard J. , Jankovic Jasna , Maric Radenka , Yu Shule , Zhang Feng-Yuan 
Published: December 2020

Water electrolysis has the potential to become a key element in coupling the electricity, mobility, heating and chemical sectors via power-to-liquid or power-to-gas in a future sustainable energy system [1]. While proton exchange membrane electrolyzer cells (PEMEC) offer several advantages over traditional electrolysis technologies, the high cost of catalysts and associated components remain a barrier for their wide industrial commercialization [2]. Fundamental understanding of the PEMEC degradation mechanisms is crucial to elucidate the key parameters for developing new high-performance, low-cost and durable materials. This work aims to reveal degradation mechanisms of two novel material systems as investigation models. Electron microscopy methods, including transmission electron microscopy/scanning transmission electron microscopy (TEM/STEM) and energy dispersive X-ray spectroscopy (EDS) are applied to quantify the degradation of iridium- and platinum-based electrocatalysts in membrane electrode assemblies (MEAs).

The first material system involves an ultra-low catalyst loaded MEA with Pt loading of 0.3 mg cm−2 and Ir loading of 0.08 mg cm−2. A long-term test of ~4500 h at 1.8 A cm−2 and 2.76 MPa differential pressure was performed on the MEA. A mechanism of cathode degradation is proposed using established physical models from two aspects: (1) direct Pt dissolution from nanoparticles due to the Gibbs-Thomson relation and (2) transient Pt dissolution due to rapid Pt oxide reduction during start/stop of operation [3]. Furthermore, as Ir dissolves and migrates through the membrane, Pt-Ir precipitates are formed in the membrane. The degradation mechanisms and the distribution of platinum and iridium across the MEA are provided from this study.

The second material system involves a catalyst-coated liquid/gas diffusion layers (CCLGDLs) to reduce the cost of traditional Ti MEAs [4]. The CCLGDLs are significantly thinner than commercial catalyst-coated membranes (CCMs) and PTLs where the LGDL is less than 100 µm thick and the catalyst layer (CL) is less than 0.3 µm thick. The catalyst coated LGDLs demonstrated excellent PEMEC performance with cell potential less than 1.85 V at 2 A/cm2. This work examines and compares as-prepared and post-test CCLGDLs to investigate their morphology, composition, and stability relative to traditional electrode layers.

In summary, quantitative analysis from microscopic imaging (TEM/STEM) and chemical analysis (EDS) is an effective tool for analyzing catalyst morphology, dissolution, and re-distribution in degraded MEAs. Quantitative microscopic insights into catalyst degradation mechanisms, such as those presented here, help to accelerate research into improved PEMEC performance and durability.

References

[1] Buttler, A., Spliethoff, H., Renewable and Sustainable Energy Reviews, 2018, 82, 2440-2454.

[2] Spori, C., Kwan, J.T.H., Bonakdarpour, A., et al., Angew. Chem. Int. Ed., 2017, 56, 5994-6021.

[3] Yu, H., Bonville, L., Jankovic, J., Maric, R., Appl. Catal. B: Environ., 2020, 260, 118194.

[4] Kang, Z., Mo, J., Yang, F., et al., Energy. Environ. Sci., 2017, 10, 166-175.

[5] This work was supported by the U.S. Department of Energy, Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office under the Electrocatalysis Consortium (ElectroCat) and Award DE-EE0008426. Microscopy performed as part of a user project at ORNL’s Center for Nanophase Materials Sciences (CNMS), which is a DOE Office of Science User Facility.

This manuscript has been authored by UT-Battelle, LLC, under contract DE-AC05-00OR22725 with the US Department of Energy (DOE). The US government retains and the publisher, by accepting the article for publication, acknowledges that the US government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this manuscript, or allow others to do so, for US government purposes. DOE will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).