How Do Hydrocarbon (HC)-Based Membrane Electrode Assemblies (MEAs) Compare with Perfluorosulfonic Acid (PFSA)-MEAs Above 80 °C?
Authors: Butori Martina , Liljenberg Marcus , Eriksson Björn , Lagergren Carina , Lindbergh Göran , Lindström Rakel Wreland 
Published: December 2024

The ongoing debate around the sustainability of per- and polyfluoroalkyl substances (PFASs) might restrict their use and the production in many sectors [1]. Particularly, in the field of proton exchange membrane fuel cells (PEMFCs), perfluorosulfonic acid (PFSA) ionomers, such as Nafion, represent the current state-of-the-art for membranes and ionomer in the catalyst layers (CLs). In fact, they provide an overall unmatched combination of properties such as ionic conductivity and stability [2,3]. In contrast, hydrocarbon (HC)-based polymers are an attractive alternative to PFSA-based materials, as they have been proven to be valid choices from a performance-standpoint and could potentially overcome the thermal stability limitations of PFSAs [4,5]. However, it has been reported that the conductivity of HC-based polymers is not surpassing that of PFSA-based polymers, especially in the CL, even upon optimization of the formulation [4,6]. Additionally, only a few studies report characterization of HC-MEAs above 80 °C, which is the targeted temperature range for some fuel cell applications [4,7].

In this work we used perfluorosulfonic acid (PFSA)- and sulfonated polyphenylene-based (HC)-polymers as ionomer in the CL and membrane in a fuel cell run above 80 °C. By spray-coating HC-based CLs onto either PFSA- or HC-membranes, we separated the contribution of the membrane from those of the electrodes and we analyzed the behaviour of each component individually. The electrochemical tests were performed between 80 and 120 °C, in a wide range of humidity and pressure levels. They included polarization curves and electrochemical impedance spectroscopy (EIS) in H2/air, similarly to our previous publications [8]. Additionally, we performed cyclic voltammetry and EIS to characterize the activity and conductivity of the cathode CL. Finally, by using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX), we analyzed cross-sections of sprayed samples before and after the test, to investigate possible changes in the structure.

A SEM/EDX cross-section of a sample is shown in Figure 1. The MEA consisted of a PFSA-based membrane (with a mechanical reinforcement) and HC-based CLs. Results suggested that the membrane thickness is unchanged after spraying and that the catalyst layers had a quite compact structure and a thickness of about 9 and 6 μm, for respectively the CL on the left and the one on the right. The EDX elemental analysis confirmed that the fluoride signal was solely present in the membrane, where a PFSA-based polymer was employed, while no appreciable signal was detected in the electrodes given the fluoride-free polymer chosen as ionomer.

By methodically analyzing the performance losses of different polymers in the CL and in the membrane, we contribute to directing the research where the greatest limitations are still present, above 80 °C.

Figure 1 SEM/EDX cross-section of an in-house sprayed sample (left) and a zoom on the line scan with the corresponding EDX spectra for Pt, F and C (right). While the membrane is PFSA-based, the CL is HC-based.

[1] European Chemical Agency, Per- and polyfluoroalkyl substances (PFAS), Regist. Restrict. Intentions until Outcome. (2023). https://echa.europa.eu/registry-of-restriction-intentions/-/dislist/details/0b0236e18663449b (accessed April 10, 2024).

[2] Y. Prykhodko, K. Fatyeyeva, L. Hespel, S. Marais, Progress in hybrid composite Nafion®-based membranes for proton exchange fuel cell application, Chem. Eng. J. 409 (2021) 127329. https://doi.org/10.1016/j.cej.2020.127329.

[3] D.A. Cullen, K.C. Neyerlin, R.K. Ahluwalia, R. Mukundan, K.L. More, R.L. Borup, A.Z. Weber, D.J. Myers, A. Kusoglu, New roads and challenges for fuel cells in heavy-duty transportation, Nat. Energy. 6 (2021) 462–474. https://doi.org/10.1038/s41560-021-00775-z.

[4] H. Nguyen, F. Lombeck, C. Schwarz, P.A. Heizmann, M. Adamski, H.F. Lee, B. Britton, S. Holdcroft, S. Vierrath, M. Breitwieser, Hydrocarbon-based PemionTM proton exchange membrane fuel cells with state-of-the-art performance, Sustain. Energy Fuels. 5 (2021) 3687–3699. https://doi.org/10.1039/d1se00556a.

[5] I. Innovations, Ionomr Innovations’ Pemion® hydrocarbon-based proton exchange membrane and polymer exceed industry durability targets, (n.d.). https://ionomr.com/wp-content/uploads/2023/01/Pemion-Durability-Data_News-Release-and-Technical-Backgrounder_For-Release-Jan19.pdf (accessed April 8, 2024).

[6] A. Strong, B. Britton, D. Edwards, T.J. Peckham, H.-F. Lee, W.Y. Huang, S. Holdcroft, Alcohol-Soluble, Sulfonated Poly(arylene ether)s: Investigation of Hydrocarbon Ionomers for Proton Exchange Membrane Fuel Cell Catalyst Layers, J. Electrochem. Soc. 162 (2015) F513–F518. https://doi.org/10.1149/2.0251506jes.

[7] Hydrogen and Fuel Cell Technologies Office Energy.gov, Fuel Cell 2016 Multi-Year Research, Development and Demonstration Plan, 2016. https://www.energy.gov/sites/default/files/2017/05/f34/fcto_myrdd_fuel_cells.pdf.

[8] M. Butori, B. Eriksson, N. Nikolić, C. Lagergren, G. Lindbergh, R. Wreland Lindström, R.W. Lindström, The Effect of Oxygen Partial Pressure and Humidification in Proton Exchange Membrane Fuel Cells at Intermediate Temperature (80 - 120 ◦C), J. Power Sources. 563 (2023). https://doi.org/10.1016/j.jpowsour.2023.232803.

Figure 1