Effect of Fuel Cell Electrode Fabrication on Ionomer Nanostructure in Correlation with Electrochemical Performances

Abstract

Proton Exchange Membrane Fuel Cell (PEMFC) electrodes, seat of the electrochemical reactions, rule the whole operation and are critical components in terms of cost, performance and durability. Their improvement will monitor the large-scale commercialization of PEMFC.These porous electrodes are made of catalyst, composed by platinum nanoparticles (2-5 nm) supported on electrically conductive carbon particles (~30-50 nm) which are bound by an ionomer. The ionomer plays a crucial role as a binder and proton conductor upon its hydration. It can be in the form of aggregates within the electrode pores or of nanofilms on the surface of the catalyst as shown by Atomic Force Microscopy (AFM)1 and Small angle neutron scattering (SANS)2.The electrode is obtained by coating and drying an ink made of catalyst and ionomer dispersed in a solvent. Electrode performances depend on the ionomer content3 and on fabrication parameters such as the solvent4 as it can be seen in Figure 1(a). It was shown that the solvent changes the ionomer organization into the ink5. Thus, it will affect its nanostructure within the electrode. The distribution of the ionomer at the nanometer scale influences the water sorption and reactants (protons and gases) transport properties of the electrode6. However, little is known about the ionomer in the electrode because of the complexity of its characterization.SANS is used to characterize the ionomer within the electrode because its contribution is clearly visible2 but neutron sources are rarer and less available than X-ray sources. In addition, Small angle X-ray scattering (SAXS) can be performed with laboratory equipment and therefore does not necessarily require the use of synchrotron facilities. SAXS have already been performed on electrodes but rather to investigate the carbon or platinum nanoparticle size distributions7,8. The ionomer contribution in a SAXS profile of an electrode is hardly distinguishable because its contribution is hidden by the large scattering of the Pt nanoparticles due to its strong interaction with X-rays.In this work, SAXS is used to characterize the structure of the ionomer within the electrode that is crucial to a better understanding of the correlation between the electrodes fabrication and their electrochemical performances. The performance of the electrodes was characterized thanks to tests under H2/Air at 1.5 bara. Electrochemical tests and SAXS measurements were performed on electrodes equilibrated at several relative humidities. A data processing method for SAXS profile of electrode is proposed in order to subtract the platinum contribution and to highlight the contributions of carbon, ionomer and water. This method allows the extraction of information about the ionomer nanostructure and the hydration of the electrode. Figure 1(b) shows 1D SAXS profiles obtained for an electrode made from a water/ethanol mix based ink and another one made from a triethyl phosphate (TEP) based ink. After data processing, the curves show differences with increasing humidity. The electrode made with TEP shows a peak around 0.2 Å-1 (Figure 1(d)) which can be assimilated to an ionomer peak and therefore to aggregates. It has been shown using SANS that if the ionomer is in the form of aggregates within the electrode, it behaves as bulk ionomer, thus presents an ionomer peak2. The electrode made with water/ethanol mix does not show this peak (Figure 1(c)), thus the ionomer is more efficiently dispersed within it. The better dispersion of the ionomer seems to lead to better electrochemical performances compared to TEP as shown in Figure 1(a).(1) Morawietz, T. & al. Fuel Cells 2018, 18 (3), 239–250. https://doi.org/10.1002/fuce.201700113.(2) Chabot, F. & al. ACS Appl. Energy Mater. 2023, 6 (3), 1185–1196. https://doi.org/10.1021/acsaem.2c02384.(3) Suzuki, T. & al. International Journal of Hydrogen Energy 2011, 36 (19), 12361–12369. https://doi.org/10.1016/j.ijhydene.2011.06.090.(4) Kim, T. & al. International Journal of Hydrogen Energy 2017, 42 (1), 478–485. https://doi.org/10.1016/j.ijhydene.2016.12.015.(5) Tarokh, A. & al. Macromolecules 2020, 53 (1), 288–301. https://doi.org/10.1021/acs.macromol.9b01663.(6) Kobayashi, A. & al. ACS Appl. Energy Mater. 2021, 4 (3), 2307–2317. https://doi.org/10.1021/acsaem.0c02841.(7) Povia, M. & al. ACS Catal. 2018, 8 (8), 7000–7015. https://doi.org/10.1021/acscatal.8b01321.(8) Wang, M. & al. ACS Appl. Energy Mater. 2019, 2 (9), 6417–6427. https://doi.org/10.1021/acsaem.9b01037. Figure 1