Polymer electrolyte fuel cells (PEFCs) are attractive power converters for various applications including electric vehicles and stationary powers without emitting CO₂. The electrochemical reaction to generate electric power occurs at the interface of catalyst nanoparticle and ionomer in a catalyst layer on electrodes. The catalyst layers consist of Pt catalyst nanoparticles supported on carbon (Pt/C) and ionomer as the binder. The fine structure of the catalyst layer such as the connected structure of carbon support, and the interconnected pore structure, and the state of ionomer coverage on carbon support, governs the performance of PEFCs. The catalyst layer is fabricated by coating of the catalyst ink consisting of the Pt/C, ionomer, and solvents. Precise structural analyses of the catalyst ink and the catalyst layer have been reported. However, the determining factor for the catalyst layer formation is still unclear. In this study, we performed in situ grazing-incidence small-angle x-ray scattering (GISAXS) measurements to investigate the aggregation process of Pt/C during the drying process of the catalyst ink.

The catalyst ink was prepared by blending Pt/C (TEC10E50E, Tanaka Kikinzoku Kogyo), ionomer (Nafion^® dispersion, DE2020, Chemours) and mixture of water/1-propanol (NPA) (1/1 v/v) at 6.5/3.5/90 w/w/w. The weight ratio of ionomer to carbon was 1:1. The catalyst ink was coated on a Si wafer using an automatic film applicator at 25 °C. The stainless-steel applicator with a gap of 100 μm height and 40 mm width was used as the blade. The drawing speed was fixed at 20 mm s^-1 for all coatings. In situ GISAXS measurements were carried out at the BL45XU beam line of SPring-8. The X-ray with 0.100 nm in wavelength impinged onto the coating film under an incident angle of 0.12° and the scattering data were collected with 2D detector (PILATUS3X 2M Dectris Ltd.) located at a distance of 3.5 m from the sample.

Figure 1a and 1b show 2D GISAXS patterns collected at several elapsed times (t), and the scattering intensities obtained from vertical line cuts of 2D patterns at q_y = 7.37×10⁻² nm⁻¹ in a width of Δq_y = 6.13×10⁻² and plotted against q_z, respectively. As seen in Figure 1b, the intensity profile at t = 70 s includes a maximum at q_z = 0.2 nm⁻¹, which shifts toward smaller q_z with increasing t from 70 s to 250 s. It suggests that 30-nm-sized Pt/C primary particles aggregate to increase the size and spacing with the solvents evaporate. With further increasing t, the intensity profile did not change, indicating that the nanoscale structure was fixed at t = 250 s. In contrast, the gravimetry for the same process indicates that the decrease of the catalyst ink weight due to solvents evaporation completes at t = 1600 s. These results show that the aggregation of Pt/C particles mostly completes in the first half of drying process

This work was supported by the PEMFC Research and Development Program for “Highly-Coupled Analysis of Phenomena in MEA and its Constituents and Evaluation of Cell Performance” from the New Energy and Industrial Technology Development Organization, NEDO, Japan. The synchrotron radiation experiments were performed at BL45XU beam line in SPring-8 with the approval of JASRI (Proposal No. 2017A1002 and 2017B1006).