Time-Resolved Nanostructural Analysis of Thin-Film Formation Process from Nafion Solution by Synchrotron X-Ray Scattering

Abstract

Perfluorosulfonate ionomers such as Nafion consist of a polytetrafluoroethylene backbone having pendant side chains terminating with sulfonic acid groups. These materials have attracted much attention as proton-conducting materials in polymer electrolyte membrane fuel cells (PEMFCs), due to their excellent chemical stability and high proton conductivity. The catalyst layer is fabricated by deposition of catalyst ink containing catalyst, ionomer, and solvents. In the catalyst layers, ionomer provided an ultra-thin film, coating the catalyst and the catalyst 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 ionomer in the solutions and solid-state thin films have been reported. To the best of our knowledge, however, the dynamics of ionomer thin-film formation process remain unclear.Herein, time-resolved measurements of grazing-incidence small- and wide-angle X-ray scattering (GISAXS/GIWAXS) utilizing synchrotron radiation at BL45XU/SPring-8 were carried out to observe the nanostructural evolution of thin film during solidification processes (i.e., spin-coating) from the Nafion solution. GISAXS and GIWAXS are synchrotron-based X-ray techniques that have been used extensively to provide nanostructural insight of polymeric thin films. In the present study, effects of solvent evaporation and centrifugal force on the structural formation behaviors of Nafion were observed for water/1-propranol (NPA) solution of Nafion (3.5 wt% Nafion in water/NPA (1/1 in volume ratio)). The wavelength of the X-ray beam was 0.1 nm, and camera length was 270 mm and 2503 mm for GIWAXS and GISAXS measurements, respectively. The scattering intensity at a certain scattering vector q was measured as a function of time after deposition of Nafion solution on the rotated Si substrate (2000 rpm). The 2D-scattering image was acquired using a photon counting detector (PILATUS3X 2M, DECTRIS Ltd.). In the first stage from t = 0 to 20 s, the 2D GIWAXS profile showed strong solvent peak and crystalline peak arising from the packing of the Nafion backbone (q z ~ 11.7 nm-1, d-spacing is defined as 2π/qz , d ~ 0.5 nm) quickly appeared. After 180 s, the solvent peak disappeared, and only the crystalline peak was observed. This supports that fine crystallites were contained in Nafion solution. The 2D GISAXS profile, on the other hand, significantly changed with time. Selected GISAXS patterns during thin-film formation process are shown in Fig. 1. In the first stage (from t = 0 to 110 s), an interaction peak was observed at q > 0.1 nm-1, the position of which is related to the main first neighbor distance D between the scatter structures in Nafion solution, and the peak position shifted towards larger q values with time. This indicates that solvent evaporation decreases the distance between the scatter structures in Nafion solution. At the second stage (from t = 110 to 135 s), the new peak of q z ~ 1.9 nm-1 (d ~ 3.3 nm) corresponding to so-called ionomer peak appeared. At the third stage (form t = 135 to 180 s), the interaction peak disappeared, and only the peak of qz ~ 1.9 nm-1 was observed, indicating that the ordered structure were fixed in solid-state thin film after the solvent evaporation. This work clearly indicates that second-scale nanostructural evolution of Nafion during the film forming process can be traced by utilizing synchrotron GISAXS/GIWAXS measurements. Further investigations on the effects of process parameters during film forming with a film applicator on nanostructural formation behaviors of Nafion 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.2015B1105). Figure 1