Investigation of Manufacturing Process Related Structure and Performance of the Fuel Cell Electrode Thanks to Small Angle Neutron Scattering
Authors: Stando Martha , Morin Arnaud , Gebel Gerard , Porcar Lionel , Cotte Stephane , Drnec Jakub 
Published: February 2024

With climate change and fossil fuel sustainability concerns faced globally, proton exchange membrane fuel cell (PEMFC) based electric vehicles have been recognized as a promising alternative. However, large-scale commercialization requires progress in performance, cost and durability, for which the electrode (catalyst layer) is the limiting component. The electrode is composed of nanoparticles of platinum (Pt) catalyst on a carbon (C) support, embedded in a proton conductive polymer (ionomer). It is obtained from a slurry (catalyst ink) after evaporating solvent consisting of a water and alcohol mixture. The structural properties of the slurry, including the ionomer and carbon dispersions, catalyst aggregation and ionomer coverage onto the Pt/C composite strongly influence the performance and durability of the final electrode. Nevertheless, currently little is known of these structures, particularly with regards to the dispersion of the ionomer component onto the surface of porous carbon, despite the fact it is now established that it plays a crucial role in the operation of the fuel cell. The lack of information leaves research and development to be heavily reliant on a trial and error basis. Developing a detailed understanding of the dispersion and its correlation to fuel cell performance is key in advancing the PEMFC electrode.
However, the heterogeneous nature of these inks, with only short-range order observed on both micro- and nano- scales, makes them difficult to study by conventional laboratory techniques. Small angle neutron scattering (SANS) has been identified as a suitable method for the study of the electrodes as it is not hindered by the ink opacity as in optical techniques (commonly used to study dispersions) and allows to probe the bulk structures in the 1-100 nm scale relevant for each component. In particular, contrast variation (CV) SANS exhibits a major advantage in studying such complex systems, where scattering signal from multiple components can be deconvoluted by matching the scattering length density of the solvent to that of a desired component. Nevertheless, data on the ink itself is missing and it has only been investigated in a handful of studies by SANS.1-3 Shibayama et al.1 proposed a structural model of Pt/C aggregates coated by an ionomer shell in a water-based ink. Yoshimune et al.2 demonstrated that the amount of Pt on the carbon surface alters the thickness and density of said ionomer layer. Harada et al.3 were able to distinguish adsorbed and deposited ionomer on the composite surface as well as quantize their distribution.
Yet although ink composition is known to impact the dispersion greatly, no systematic investigation linking its many variables has been conducted. For example, the ionomer will conform differently in a water/alcohol solvent used commercially in contrast to a water-based one. The porosity of the carbon support will impact distribution of Pt on its surface as well as the degree of ionomer penetration into the support. Additionally, although the fundamental understanding of the ink structure is imperative, it has seldom been further linked to the performance of the electrode itself.
Therefore, the aim of this project is to utilize CV-SANS to study the impact of each catalyst ink component on the structure of the dispersion, and their relation to each other, through altering only a single variable at a time. The variables include the components as well as their proportions, such as the carbon concentration or the ionomer to carbon ratio. The performance of the electrode manufactured from said ink compositions will then be evaluated using electrochemical studies. Recently we performed CV-SANS studies on the ink, investigating the extent of ionomer adsorption on the Pt/C composite as a function of both carbon type and presence of Pt in the system. Both carbon supports studied, high surface area carbon with surface nano-pores accessible to the ionomer, and graphitized carbon, which has high internal porosity, yet inaccessible to the ionomer, pose an interest in industrial applications. Indeed, from these preliminary results it is evident platinum plays a significant role in the ink structure, particularly in the case of graphitized carbon (figure 1). The ink system will be quantitatively characterized using a singular value decomposition method already employed in literature to give insight into parameters such as the amount of ionomer adsorbed onto the carbon surface or the structure of the layer.

References

1 M. Shibayama, T. Matsunaga, T. Kusano, K. Amemiya, N. Kobayashi and T. Yoshida, J Appl Polym Sci, , DOI:10.1002/app.39842.
2 W. Yoshimune and M. Harada, https://doi.org/10.1246/cl.190017, 2019, 48, 487–490.
3 M. Harada, S. I. Takata, H. Iwase, S. Kajiya, H. Kadoura and T. Kanaya, ACS Omega, 2021, 6, 15257–15263.

Figure 1