Time-Resolved Nanostructural Analysis of Catalyst Layer Formation Process by Synchrotron X-ray Scattering

Abstract

Polymer electrolyte membrane fuel cell (PEMFC) has been considered as a highly efficient electrochemical power converter with zero emissions of CO2. The oxygen reduction reaction (ORR) occurs at the interface between platinum (Pt) catalyst nanoparticle and ionomer in the catalyst layer of membrane electrode assembly (MEA). The catalyst layer is fabricated by coating of the catalyst ink containing the carbon-supported platinum (Pt/C) catalyst, ionomer, and solvents. In the catalyst layers, ionomer provided an ultra-thin film, coating the Pt catalyst nanoparticles and the carbon support surfaces. These ultra-thin films have a significant influence on the electrochemical activity and transport phenomena that determine the whole cell performances. Recently, precise structural analyses of catalyst inks and catalyst layers have been reported. To the best of our knowledge, however, the dynamics of the catalyst layer formation process remain unclear. Herein, we performed in-situmeasurements of synchrotron radiation grazing-incidence small-angle X-ray scattering (GISAXS) of a catalyst ink during drying process. The ink was coated on a silicone (Si) wafer with a film applicator and the scattering images of the film were acquired as a function of the elapsed time. The catalyst ink was prepared by blending a Pt/C catalyst, ionomer (Nafion®) and solvents (water/1-propanol (NPA) =1:1 in volume ratio). The solid content of the catalyst ink was 10 wt%, and the weight ratio of ionomer to carbon ratio was 1:1. The catalyst ink was coated on a Si wafer using a film applicator and X-ray beam (wavelength λ = 0.100 nm) was irradiated on the film at a small grazing angle (0.15 °) and the scattering images were measured using a photon counting detector (PILATUS3X 2M, DECTRIS Ltd.) at a distance of 2700 mm from the film (BL45XU/SPring-8). Selected 2D GISAXS patterns during catalyst layer formation process are shown in Fig. 1. The 2D GISAXS pattern significantly change with the elapsed time, in particular for the out-of-plane direction. Several scattering peaks appear immediately after the ink coating. With time elapsed, the peak intensities increase and their peak positions shift toward smaller qz . The intensity profiles on the out-of-plane q z axis of GISAXS patterns measured during catalyst layer formation process as a function of the elapsed time (t) are shown in Fig. 2. From t = 0 s, the peak at around q z ~ 0.2 nm−1 shift toward smaller q z with increasing t. Furthermore, at t ~ 50 s, two peaks around q z ~ 0.5 nm−1 appear and combine into one peak. Thereafter, this peak shifts toward smaller q z with increasing t. These results demonstrate that nanostructural evolution of catalyst layer formation process can be traced by utilizing synchrotron X-ray scattering. The peak shifts would be due to the growth of Pt/C agglomeration induced by solvent evaporation. For the precise assignment of scattering peaks, further investigations based on direct observations of the catalyst ink using cryogenic transmission electron microscopy are now in progress. This presentation is based on results obtained from the PEMFC Research and Development Program for “Highly‐Coupled Analysis of Phenomena in MEA and its Constituents and Evaluation of Cell Performance” commissioned by the New Energy and Industrial Technology Development Organization (NEDO). The synchrotron radiation experiments were performed at BL45XU in SPring-8 with the approval of JASRI (Proposal No.2016A1194 and, No.2016B1004). Figure 1