Effect of Micro-Patterned Membrane PEFC Dependent on Operating Current Density

Abstract

1. IntroductionPolymer electrolyte fuel cells (PEFCs) have been widely recognized as a key technology for a future hydrogen-based society, because of their several practical applications such as in fuel cell vehicles, owing to their good operating capability at low temperatures.It is well known that the mass transport and the chemical reactions at the cathode side of PEFCs limit their performance. Heinzmann et al. used electrochemical impedance spectroscopy (EIS) and distribution of relaxation time (DRT) analysis [1] to show that the dominant processes that occur are cathode gas diffusion and oxygen reduction reaction (ORR).One method to improve the ORR is to increase the surface available for the chemical reaction. Jeon et al. fabricated a micropatterned membrane and applied it to a membrane electrode assembly (MEA) [2]. They confirmed that the use of a micropatterned membrane improved the performance of the PEFC.However, the mechanism of this improvement is yet to be clarified in detail. Micropatterns on the membrane should increase the active reaction area (Fig.1(a)). However, patterns on the membrane surface increase the length of the path from the top to the bottom of the catalyst layer, which should hamper oxygen diffusion (Fig.1(b)). We previously performed DRT analysis with micro-patterned MEA to investigate the mechanism of improvement in PEFC performance using a micro-patterned MEA [3]. In this study, a wider range of EIS was performed, and both the positive and negative effects of using a micro-patterned MEA are discussed herein.2. ExperimentA cathode micro-patterned MEA was fabricated, and EIS and DRT analyses were performed. The catalyst ink was prepared using Pt/C (TEC10V50E, Tanaka Kikinzoku Kogyo K.K.), isopropyl alcohol, water, and a 10% ionomer solution. This ink was sprayed onto a 5.0 mm line-patterned membrane (NRE-212, Sigma-Aldrich Co. LLC) to fabricate the MEA. The catalyst loading was adjusted to 0.4 mg/cm2. The cell was assembled using a commercial gas diffusion layer (Sigracet®︎ 29BC, SGL Carbon GmbH). The operation was carried out galvanostatically with a relative humidity of 80%, gas flow rate of 200 ccm, and oxygen partial pressure of 0.2. EIS was measured at 0.02 to 1.00 A, with frequency ranging between 0.1 Hz and 1 MHz. The acquired data were then subjected to DRT analysis using DRT tools by Wan et al. [4]. Noisy data points were excluded before application because of the high sensitivity of DRT analysis to noise.3.Results and discussionFig.2 shows the polarization curve results. Solid lines with circle markers and broken lines with ‘x’ markers indicate the patterned and flat samples, respectively. In the lower current region, where the ORR is dominant, the patterned sample outperforms the conventional flat sample. On the other hand, in the higher current region, where gas diffusion is dominant, the patterned sample suffers steep drops and the flat becomes relatively better.Fig.3 shows the DRT analysis graph. Fig.3(a) stands for patterned sample result and Fig3.(b) for flat sample. The peaks of DRT indicate the polarization resistance at its time dependency. In the high current density region, the peaks of the patterned and the flat samples are significantly different. In particular, at around 101 to102 Hz, an independent peak appears at 101 Hz for the patterned sample as the current density increases. This shows the polarization resistance of gas diffusion, which indicates that the gas diffusion in the patterned sample is significantly worse than that in the flat sample.The micro-patterned MEA has a negative effect on cell performance because of the longer oxygen diffusion path involved. It can be assumed that they might not perform as well as conventional flat MEA under certain conditions that favor good oxygen transportation, such as high current or low oxygen partial pressure operation. Therefore, it is important to consider the physical properties (catalyst layer porosity, ionomer content, etc.) and operating conditions in order to maximize the potential of the micro-patterned MEA.4. ConclusionWe performed a wide range of galvanostatic EIS and DRT analyses of the data from the operation of a cathode micro-patterned PEFC. The micro-patterned membrane is shown to have both a positive and a negative effect. Comprehensive optimization of parameters such as micropattern geometry, catalyst composition, and operating conditions, are required to take full advantage of the surface-patterning method.