Impact of Catalyst Ink Stirring Condition on Oxygen Transport within PEFC

Abstract

Polymer electrolyte fuel cells (PEFCs) are regarded as a promising alternative power source for automobiles (fuel cell electric vehicle; FCEV). Cost reduction and increasing power density of PEFCs are the key challenges for commercialization of FCEVs, and reduction of Platinum amount in the catalyst layer (CL) is essential to achieve cost reduction of FCEVs. In order to accomplish the reduction of Platinum loading without sacrificing PEFC performance under high current density operation, it is important to make more effective use of Pt used in the CL of the membrane electrode assembly (MEA), and the mass transport of reactants within the CL should be enhanced significantly. From this background, fabrication process of the CL should be investigated in more detail. The fabrication process of the CL includes an ink preparation process and a coating and dry process. In the former process, the materials composing the CL are mixed and dispersed in the solvent. A blend of water and alcohol is typically used as a dispersion medium for Pt-supported carbon and ionomer. The composition of the dispersion medium, the choice of alcohol, solid contents, and mixing conditions are major controlling parameters. The catalyst ink is deposited on a substrate or directly onto a PEM. The screen printing, die coating, or spray-coating is a major choice for the deposition method. The CL structure is finally formed after drying the deposited CL ink under elevated temperature. Extensive number of studies have been conducted to understand the impact of each parameters on PEFC performance, but the mechanism of structural formation of the CL has not been fully understood. In this study, Impact of stirring condition of the catalyst ink on the CL structure, cell performance and oxygen transport phenomena was investigated by using cyclic voltammetry (CV) measurement, limiting current density measurement, and soft X-ray imaging. To investigate the impact of stirring condition, we prepared two catalyst ink with the same composition but the different stirring condition; stirred with Zr ball (ink A) and without Zr ball (ink B). Figure 1 shows the result of CV measurement, and it showed that the CL prepared from the ink A have larger electrochemical surface area. Because the amount of Pt-supported carbon contained within the MEA should be the same, this result clearly suggested that the stirring condition of the catalyst ink affect the CL structure and its Pt utilization ratio. In addition, the i-V characteristics of these CLs (Figure 2) clearly showed that the CL prepared from the ink A have better performance than the CL prepared from the ink B, and the performance drop of the CL prepared from ink B was serious under the cell temperature of 30 deg.C. To understand this performance drop in more detail, the soft X-ray imaging technique and the oxygen transport resistance measurement were carried out. Results of the soft X-ray imaging (Figure 3) showed that more liquid water was accumulated within the MEA prepared from ink B. In addition, the oxygen transport resistance suggested that the oxygen transport resistance within the MEA prepared by using the ink A is smaller than the MEA prepared by using the ink B. These results clearly showed that the stirring condition of the catalyst ink strongly affect the structure of the CL and the transport phenomena within the MEA. The stirring condition might affect not only the aggregation of the Pt-supported carbon but also the ionomer coverage on it, and their impact on the PEFC performance is still not clear. It is highly important to understand the CL fabrication process more scientifically, and it helps to develop high-performance CL. Acknowledgement: This work was supported by the New Energy and Industrial Technology Development Organization (NEDO), Japan. Figure 1