High Performance Membrane Electrode Assembly For High Temperature Polymer Electrolyte Membrane Fuel Cell

Abstract

High–Temperature polymer electrolyte membrane fuel cells (HT–PEMFCs) operate above more than 120 ℃. So, it uses a polybenzimidazole (PBI) electrolyte membrane impregnated with phosphoric acid (PA) instead of Nafion® membrane. Because of PA electrolyte, the fuel cell system can be simplified due to the omission of the humidifier. Although, HT-PEMFCs show lower performance and use much more Pt catalyst than LT-PEMFC due to PA flooding and poisoning in the catalyst layer (CL). Therefore, decreasing the coverage of PA on the platinum (Pt) is a challenge for the HT-PEMFCs membrane electrode assembly (MEA). In this study, to develop the low Pt and high performance electrode, the hydrophobic binder is applied to prevent the PA flooding and covering in CL for anode and cathode respectively. In the HT-PEMFCs cathode, Pt catalyst makes oxygen reduction reaction (ORR) in the triple-phase boundary (TPB) meeting with the PA and oxygen gas. However, the oxygen diffusion is decreased due to the low oxygen permeability of PA when it is in excess. Polytetrafluoroethylene (PTFE) is applied as a hydrophobic binder that can mitigate the flooding of PA. Alcohol solvents like ethanol (EtOH) and tert-butyl alcohol (TBA) are investigated instead of isopropanol (IPA), the common solvent for PTFE, as an alternative solvent for fabricating the catalyst slurry. The cathode was fabricated with Pt loading of 1.0 mgpt/cm2 using bar coating method, which can quickly coat the CL with uniform thickness. The fabricated gas diffusion electrode (GDE) was compared with voltage at 0.2 A/cm2 from the singles cells. As a result, the performance based on EtOH and TBA showed a similar value as 0.66 V but slightly increased compared to IPA. It is also confirmed by the close values of the total resistance of the MEA from the slurries using different alcohol in impedance spectroscopy. The contact angle and surface of the electrode image were compared to observe the physical characteristics. The size of the electrode cake and the width of the crack were changed according to the vaporization rate of water/alcohol mixed solvent. GDE based on IPA shows the largest cake and crack with high hydrophobicity due to the fastest drying speed. The distribution of PTFE binder using energy dispersive spectrometry (EDS) shows that IPA makes the most uniform dispersion of PTFE. Finally, it was indicated that EtOH and TBA can replace the IPA as a solvent of the PTFE binder. The operating environment of HT-PEMFCs causes PA to migrate to the anode, which leads to PA rich environment and flooding in anode CL. Excessive PA covers the surface of Pt catalyst and blocks the pores in CL causing the decrease of performance. Therefore, the spray coating method is applied to suppress the crack generation on the surface while using a PTFE binder to give hydrophobicity to the anode. The response surface methodology, one for the design of experiments, was used to analyze the interaction between the concentration of PTFE in CL and heat treatment temperature. And the optimum anode fabrication condition was confirmed. The optimized anode shows 0.636 V with a Pt loading of 0.2mgpt/cm2, which is only 20 % of the Pt used in commercial electrodes. In additional research, the low Pt high performance anode was fabricated using a gas diffusion layer without microporous layer. Sprayed anode with a Pt loading of 0.2mgpt/cm2 shows high performance as 0.656 V at 0.2A/cm2, while decreasing the amount of PA leakage compared to the commercial anode.