Analysis of Dispersion State of Catalyst Inks for PEFC
Catalyst ink is manufactured by mixing catalyst supported carbon and ionomers in solvent and the way of mixing may differentiate the size distribution of carbons and ionomers in the catalyst ink, which is one of the most important factors that determine the cell performance of PEFC (Polymer electrode fuel cell). In order to develop high performance and low cost PEFC, it is very important to elucidate the relationship between the cell performance and the method of preparing catalyst ink. In this study, catalyst ink made by different processes, mechanical stirring and the beads-style stirring were investigated by light diffraction method, small angle X-ray scattering and NMR (Nuclear magnetic resonance).
Catalyst supported carbon (TEC10E50E) and ionomer dispersion (Nafion® in propanol) are mixed in different processes, mechanical stirring (denoted as “Ink A”), and the beads-style stirring (denoted as “Ink B”). Particle size distributions are evaluated by light diffraction method and synchrotron small angle X-ray scattering (SAXS). The samples analyzed by light diffraction are 2000 times diluted by propanol. SAXS measurement is conducted at SPring-8 BL08B2. The molecular mobility of ionomers is evaluated by the relaxation time measurement of 19 F NMR.
3. Results and discussions
Figure 1 shows the size distribution obtained by light diffraction method, which mainly reflects the particle size of the aggregated catalyst supported carbon. Ink A possesses the uniform size distribution around several 100 nm, but on the other hand, Ink B shows broad distribution ranging from several 100 nm to several mm. Because the cell performance of PEFC used Ink A was superior to Ink B, the uniform distribution of the aggregated catalyst supported carbon results in better cell performance.
Figure 2 shows SAXS profiles of Ink A and Ink B. In SAXS measurement, the domain size distribution can be analyzed without diluting the samples by propanol, from several nm to 100 nm, which is confined to the realm of far smaller than light diffraction method. From the SAXS profiles, there exists two components with different domain size, one is around 1 to 2 nm which derives from catalyst particles, the other is around 60 to 70 nm which derives from ionomers. In SAXS measurement, two samples show almost the same profiles and there is no significant size difference between two samples in the small size range from several nanometers to 100 nm.
In addition to the size distribution of the sample, the molecular mobility of ionomers is evaluated by the relaxation time measurement of 19F NMR. In Ink B, there exist those ionomers which have high molecular mobility and no interaction with carbon and catalysts.
From the size distribution and molecular mobility measurements stated above, aggregation size of catalyst supported carbon is changed by mixing process and the uniform distribution results in better cell performance. As for ionomers, it is important to have interaction, contact with and adhesion to catalyst supported carbon.