Engineering the Ionic Polymer/Gas Interfacial Properties of a Fuel Cell Catalyst Layer and Performance Optimization of Various Reversible Fuel Cells

Abstract

The primary barrier to full-scale commercialization of proton exchange membrane fuel cells (PEMFCs) is their inability to operate at high power density and energy efficiency. High power density operation is currently limited by high liquid water saturation levels in the cathode catalyst layer. Due to the wettability of the ionomer phase, water produced in the cathode catalyst layer wets the ionomer-gas interface and negatively impacts mass transport of oxygen to the catalyst reaction sites. Therefore, proper water management is vital for operating PEMFCs at high power density. Despite fuel cell water management improvements to membranes, gas diffusion layers, microporous layers, and flow field designs, developing and understanding water transport in the fuel cell catalyst layer continues to be an area of great importance. Previous approaches for improving water management in the catalyst layer resulted in more complicated designs and increased costs. The main objective of this doctoral work is to improve water management in a fuel cell cathode catalyst layer to enable a PEMFC to operate with increased power density and energy efficiency. The work involves (1) validating the hypothesis that specific heat treatment conditions lead to a hydrophobic or hydrophilic ionomer/gas interface, (2) developing a process to incorporate these conditions into the fabrication of the membrane electrode assembly (MEA), and (3) characterizing and testing the MEAs to confirm that the desired ionomer interfacial properties were achieved and that they led to improved fuel cell performance. XPS results confirm specific heat treatment conditions lead to a hydrophobic or hydrophilic ionomer interface. Fuel cell test results show that MEAs with hydrophobic ionomer/gas interfaces generate 133% more power over those with conventional MEAs. The remainder of this doctoral work focuses on performance optimization of various reversible fuel cells, including the hydrogen-bromine (H2-Br2), hydrogen-iodine (H2-I2), and hydrogen-vanadium fuel cells. The H2-Br2 and H2-I2 reversible fuel cell systems can be operated in the acidic or alkaline modes. The alkaline versions were evaluated because of the advantages over the acidic systems such as higher cell potential, lower corrosivity, and lower catalyst cost for the hydrogen evolution and oxidation reactions. The results confirmed that the alkaline H2-Br2 and H2-I2 fuel cells have a higher cell voltage than their corresponding acidic systems while maintaining similarly fast electrode reaction kinetics. Hydrogen-vanadium reversible fuel cells were tested to determine the effect of operating and design variables, such as electrolyte flow rate, carbon electrode type, and membrane type and thickness, on the fuel cell performance. Higher performance was observed with higher vanadium flow rate, thinner membranes and carbon nanotube (CNT) vanadium electrode. Peak power density of greater than 540 mW/cm2 was obtained using a Nafion NR212 membrane and CNT vanadium electrode. Finally, a new technique was developed to measure crossover rate in a hydrogen-vanadium reversible fuel cell. Vanadium crossover through the ion exchange membrane in vanadium-based redox flow battery systems results in self discharge and variations in electrolyte concentration. Measuring crossover of electrolyte species directly with a fuel cell, as compared to an idealized dual-chamber system, allows for determining diffusivity under actual fuel cell testing conditions. This new in-situ technique for measuring crossover with a fuel cell is shown to be reliable and easy to use. The crossover measurement method shows consistent results with diffusivities of ~10-7 cm2/s reported in the literature.