Optimizing The Hot-Press Procedure of High-Temperature Proton Exchange Membrane Fuel Cells for Adhesion Strength and Conductivity

Abstract

As society considers alternatives to energy systems based on fossil fuel combustion, high-temperature proton exchange membrane (HT-PEM) fuel cells may play an important role in our future energy and transportation portfolio. When compared to low-temperature proton exchange membrane (LT-PEM) fuel cells, HT-PEM technology offers enhanced electrode kinetics, simplified water management, and most notably increased tolerance to fuel impurities such as carbon monoxide. However, compared to more conventional LT-PEM systems, less is known about the durability, performance, and fabrication of HT-PEM materials. Comprised of multiple layers, the membrane electrode assembly (MEA) is a fuel cell’s central component. In a fabrication process similar to LT-PEM MEAs, the proton-conducting membrane, electrodes, gas diffusion layers and sub gaskets are usually hot-pressed together. The parameters surrounding the hot-pressing operation can have a significant impact on the ultimate performance of the fuel cell, in addition to influencing process reliability and repeatability. In the current research, measurements of MEA thickness, impedance, and adhesion strength were obtained for samples over a range of conditions to optimize for the most suitable processing parameters. Testing in this study quantified the effects of hot-press time, temperature and pressure, within ranges of 1-10 minutes, 140 – 200oC, and 7.7 – 92.7 MPa (1120 – 13,400 psi), respectively. After hot pressing, the MEA samples were tested for resistance with a 1260a Solartron impedance gain phase analyzer in tandem with a Solartron Ametek 12962a sample holder, and with a Scott internal bond tester for adhesion strength. Based on the results of these tests the optimal hot-press parameters were established, two 45.2 cm2 MEA were built and tested for performance with post-mortem analysis conducted utilizing scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDX). These measurements enabled interrogation of potential sources of conductivity loss associated with material layer delamination and/or dehydration of phosphoric acid in the polybenzimidazole (PBI) membrane. Figure 1 shows the effect of hot-pressing time and pressure on the resistance of MEA samples for one representative temperature condition. In this case, all tests were conducted at 140oC and the resulting material assembly was allowed to cool to room temperature in a nitrogen glove box prior to executing resistance measurements (i.e., inverse of conductivity). Each point in Figure 1 represents a single hot-pressed sample, with each sample undergoing three consecutive measurements that were then averaged; standard deviations of triplicate measurements ranged from 0.1 to 3.6%. Our results have demonstrated that the optimal conditions for MEA hot-pressing for the HT-PEM system are around 170oC and 23.1 MPa for 5 minutes. Research is ongoing to evaluate the impact of hot-pressing conditions on fuel cell performance and durability with both hydrogen and reformate fuels. Figure 1 – Selected measurements of HT-PEM resistance with variation of hot-press time and pressure at 140oC Figure 1