Distribution of Relaxation Time Analysis of Cathode Micro-Patterned PEFC

Abstract

1. Introduction Fuel cells have been attracting worldwide attention because they are a key technology for producing electric energy for a future hydrogen-based society. In particular, polymer electrolyte fuel cells (PEFCs) are important among other fuel cells. This is because they can be operated at relatively low temperatures, which means they have many practical applications, e.g., fuel cell vehicles. However, PEFCs still have some problems. One is their performance because they are not as efficient as other forms of fuel cells. There are various approaches for achieving this, but it is common theory that the cathode chemical process limits the performance of PEFCs. Heinzmann et al. applied electrochemical impedance spectroscopy (EIS) and distribution of relaxation time (DRT) analysis to break down polarization resistance in detail [1]. They clearly showed that the dominant process is the cathode gas diffusion and oxygen reduction reaction (ORR). One way to improve the ORR is to increase the chemical reaction surface. Jeon et al. fabricated a micro-patterned membrane and applied it to membrane electrode assembly (MEA) [2]. As a result, they confirmed that the micro-patterned membrane can improve the performance of the entire PEFC. However, the physical phenomenon of its improvement has yet to be clarified. In detail, these patterns can certainly increase the surface area but may decrease mass transportation, especially gas diffusion. In this study, we performed DRT analysis with micro-patterned MEA to investigate the mechanism of performance improvement from using micro-patterned MEA (Fig. 1).2.Experiment A cathode micro-patterned MEA was fabricated, and then EIS and DRT analysis were performed. The patterned membrane was produced by a thermal nanoimprinting process. A Ni mold with a 5.0-mm line pattern was prepared, and the membrane was pressed onto the mold. Through this process, the pattern of the Ni mold was replicated to a Nafion membrane (NRE-212, Sigma-Aldrich Co. LLC) with a thickness of 50 mm. Then, the catalyst ink was prepared with Pt/C (TEC10V50E, Tanaka Kikinzoku Kogyo K.K.), isopropyl alcohol, water, and a 10% ionomer solution. We sprayed this ink onto the patterned membrane to fabricate the MEA. The catalyst loading was approximately 0.4 mg/cm2. Using a commercial gas diffusion layer (Sigracet®︎ 29BC, SGL carbon GmbH), the cell was assembled, and EIS was operated at 0.1 Hz to 1 MHz and at 0.7, 0.65, and 0.60 V. The acquired data were then applied to DRT analysis using the DRT tools by Wan et al. [3]. Noisy data points were excluded before application because the DRT analysis is very sensitive to noise.3.Results and discussion Figure 2 shows a cross-sectional SEM image of the micro-patterned MEA. Clearly, the membrane is patterned via the designed geometry, and the catalyst layer is fully integrated into the micro-pattern. Figure 3 shows the EIS results. The polarization resistance decreased as the potential was decreased, similar to the general behavior of conventional fuel cells. Figure 4 shows the DRT analysis. Two clear peaks are observable at approximately 10 Hz and 100 Hz, which we call P1 and P2, respectively. P1 remained the same or became slightly larger with decreasing potential. However, P2 decreased as the potential decreased. In addition, P2 of 0.60 V indicated that this peak split into two peaks as the potential decreased. P2 appeared as a combined peak of gas diffusion and the oxygen reduction reaction (ORR) upon comparing it to previous DRT analysis research of commercial cells [1]. We could determine which peak represents what physical phenomena if we perform additional DRT analysis while changing the operating environment, as in previous studies. By comparing the analysis of micro-patterned samples with conventional sample, it will be possible to evaluate the effect of the micro-pattern and its physical phenomena.4.Conclusion We performed DRT analysis with a cathode micro-patterned PEFC. DRT analysis has the ability to clarify the physical phenomena of how a patterned MEA increases cell performance with patterned membranes. Our results indicated that there is a peak at approximately 100 Hz, and it splits into two peaks representing gas diffusion and ORR with a lower potential, such as 0.60 V. As mentioned earlier, further work is required to assign physical phenomena to each peak. A comparison of DRT data using a conventional sample can possibly reveal the mechanism of performance increase owing to membrane patterning.