EAGER: Engineering the Ionic Polymer Phase Surface Properties in a PEM Fuel Cell Catalyst Layer

University Of Kansas Center For Research Inc, Lawrence KS

Investigators

Abstract

Nguyen - 1518755 Proton exchange membrane fuel cells, also known as polymer electrolyte membrane (PEM) fuel cells, are being developed for transport, stationary fuel cell and portable fuel cell applications. A PEM fuel cell transforms the chemical energy liberated during the electrochemical reaction of hydrogen and oxygen to electrical energy, as opposed to the direct combustion of hydrogen and oxygen gases to produce thermal energy. The structure of their membrane assembly consists of an anode and a cathode that is separated by an electrolyte that contains a catalyst. A stream of hydrogen is delivered to the anode side where it is catalytically split into protons and electrons. The newly formed protons permeate through the polymer electrolyte membrane to the cathode side. The electrons travel along an external circuit to the cathode side creating the current output of the fuel cell. Meanwhile, a stream of oxygen is delivered to the cathode side where oxygen molecules react with the protons permeating through the polymer electrolyte membrane and the electrons arriving through the external circuit to form water molecules. Such fuel cells operate at relatively low temperatures, are small and lightweight, making them ideal as potential replacements for internal combustion engines that use fossil fuels as their energy source. This project is aimed at developing better PEM fuel cells. High power density operation in PEM fuel cells is often limited by high liquid water saturation levels in the catalyst layer due to high wettability of the ionic polymer phase that is added to provide three dimensional ionic access to the catalyst sites. In this EAGER project a simple approach is proposed based on a hypothesis generated from recent discoveries on interfacial properties of fluorocarbon-based ionic polymers. It is that heat treatment at high relative humidity leads to a polymer surface with a high fraction of sulfonic acid groups and a high degree of hydrophilicity, while heat treatment at dry conditions leads to a polymer surface with a small fraction of sulfonic acid groups and a very high fraction of the hydrophobic PTFE phase. Based on these observations, it is hypothesized that if the external surface of the ionic polymer coating located in the pores of the catalyst layers could be made hydrophobic, any water that is produced at the catalyst surface and diffuses through the polymer coating to the polymer coating/gas phase interface would bead and move away without forming a layer over the polymer coating. Therefore, oxygen and hydrogen gases would have direct access to the ionic polymer phase and catalyst surface. If this morphology could be achieved the fuel cell performance is expected to be greatly increased. To test this hypothesis, two tasks are proposed: 1) Validating the hypothesis that heat treating an ionic polymer (e.g., Nafion) in a dry or low RH condition will lead to a polymer film with a permanent hydrophobic external surface and determine the heat treatment conditions that are optimal for the formation of a permanent hydrophobic external surface; and 2) Confirming that modifying the membrane?s outer surface wetting property does not affect the proton conductivity within the polymer film, nor does it adversely affect the polymer/catalyst interface. If this approach is successful, it could transform water management in PEM fuel cells and lead to the development of higher performance, lower cost, and more durable fuel cells. This could accelerate the commercialization of fuel cell powered vehicles, an energy efficient and environmentally friendly transportation system. The PI has been actively recruiting students from different ethnic, racial, gender, geographical, economic and family backgrounds and has been preparing these students for the global market. The PI will also organize visits to local elementary schools and middle schools to host demonstration workshops on fuel cell and other renewable energy sources.