Proceedings of MATSUS Fall 2023 Conference (MATSUSFall23)
DOI: https://doi.org/10.29363/nanoge.matsus.2023.138
Publication date: 18th July 2023
Low temperature proton exchange membrane fuel cells (PEMFCs) powered by green hydrogen provide a means to sustainable energy production for stationary and transport applications. Their widespread commercialization is limited by the cost of the platinum catalyst at the cathode, where oxygen reduction occurs. Atomic Fe sites within nitrogen-doped carbon (Fe-N-C) offer a cheap and sustainable alternative, exhibiting the most promising non-precious metal activity for oxygen reduction. However, their stability is still below commercial realization owing to several degradation routes. Steps can be taken to minimize these pathways, [1,2] however, demetallation of FeNx active sites is still suggested as a primary performance degradation mechanism in PEMFCs. [3]
To shed new light into Fe demetallation, in this work we focus on monitoring Fe dissolution in 0.1 M HClO4 from our recently reported highly porous and >50% FeNx utilization Fe-N-C catalyst. [4] We compare operando Fe dissolution under O2 (active) and Ar (inert) conditions in both room temperature flow cell (FC) and gas diffusion electrode (GDE) setups, coupled to online inductively-coupled plasma mass spectrometry (ICP-MS). We also monitor the impact of high temperature on Fe dissolution using ex situ rotating disc electrode (RDE) and operando GDE measurements.
Fe dissolution was detected under both atmospheres in FC-ICP-MS configuration, with the amount dependent on potential, electrolyte, and gas atmosphere. After aging test performed at 80oC in O2-saturated electrolyte in RDE post-mortem X-ray absorption spectroscopy and transmission electron microscopy (TEM) analysis show the formation of Fe2O3 nanoparticles. Operando GDE-ICP-MS measurement show reduced Fe dissolution under room temperature O2, but small increase or negligible change at 70oC. Meanwhile, under Ar at room temperature, high Fe dissolution is detected.. Post-mortem TEM and high angle annular dark field scanning TEM shows formation of FexOy nanoparticles under room temperature O2, and limited formation under Ar, in GDE setup. We propose under O2 reduction GDE conditions a degradation mechanism of Fe dissolution from FeNx sites and subsequent reprecipitation into FexOy, which we explore with microkinetic modelling.
The authors acknowledge financial support from the Engineering and Physical Sciences Research Council (EPSRC) (EP/M013812/1, and EP/S023259/1), the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement no. 866402). A. P. thanks the EPSRC Centre for Doctoral Training in the Advanced Characterisation of Materials (grant number EP/L015277/1), the Society of Chemical Industry Scholarship and Turing Global Fellows for financial support.
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