Development of the Soaking Method for Manufacturing Polymer Electrolyte Fuel Cells with a High-Aspect-Ratio Microstructure

Abstract

1. IntroductionPolymer electrolyte fuel cells (PEFCs) are one of the major devices utilizing hydrogen. Owing to its small size and capacity of low-temperature operation, PEFCs are used in practical applications such as fuel cell vehicles.The power generation efficiency of PEFC is relatively low. Heinzmann et al. suggested that the main cause of low efficiency is the chemical reactions at the cathode [1]. Therefore, having a large triple phase boundary (TPB), where an oxygen reduction reaction occurs, is crucial. Bea et al. reported that the performance of PEFCs was improved by introducing micro-patterns on the surface of a Nafion membrane [2]. Organizing a high-aspect-ratio microstructure on its surface is more effective for increasing the size of the TPB.In the powder molding field, a typical problem exists: particles in solution form bridge structures because Van der Waals forces are dominant at the micrometer scale. Thus, fabricating a high-aspect-ratio microstructure on the surface of a Nafion membrane is challenging.2. ExperimentLine-and-space patterns with three different dimensions—width/spacing/height of 1/1/2 μm, respectively—were introduced onto the cathode surface via thermal nanoimprint lithography (120 °C; 5 MPa). The catalyst ink was prepared using Pt/C (TEC10V50E, Tanaka Kikinzoku Kogyo K.K.), deionized water, and 10 wt.% ionomer solution. Two samples (samples A and B) were fabricated by spraying the catalyst ink on the micro-patterned surface with a 1-cm2 mask attached at 80 °C and 30 °C, respectively. Moreover, the time between one push and another was controlled at 5 s and 0.5 s, respectively. The same amount of catalyst ink was sprayed on both samples under the same relative pressure (0.1 MPa).After drying, the cross-sections of both samples were investigated using scanning electron microscopy (SEM).3. Results and discussionFigure 1 shows the appearance of each sample immediately after spraying. As shown in Figure 1 (a), the ink on sample A had already dried, whereas, as shown in Figure 1 (b), the Nafion membrane of sample B was soaked in the ink. Figure 2 shows the SEM images of the cross-sections of both samples. As shown in Figure 2 (a), the catalyst layer formed a bridge structure in sample A. On the contrary, as shown in Figure 2 (b), the catalyst layer goes deep into the spaces of line-and-space patterns without forming a bridge structure in sample B.Figure 3 shows the mechanism of bridge structure formation. Water accounted for 99 wt.% of the catalyst ink; therefore, the ink is incompatible with the water-repellent Nafion membrane. The reason for forming a bridge structure when normally sprayed as sample A can be that water, which is a solvent, rapidly evaporates before the catalyst ink enters the spaces of line-and-space patterns. On the contrary, the reason for preventing bridge structure from forming when soaking can be that water evaporates after the catalyst ink goes deep into the spaces.4. ConclusionsBy using the original soaking method, a high-aspect-ratio microstructure was successfully arranged in the cathode without the formation of a bridge structure. When a cell is manufactured, the surface of its cathode with micro patterns will first be soaked and dried as the process of making sample B. Subsequently, catalyst ink was normally sprayed on both the cathode and anode surfaces as much as needed. Optimization of the soaking process parameters is required to manufacture a higher-aspect-ratio microstructure and to increase the size of the TPB.