Improving Performance in Alkaline Membrane Fuel Cells through Enhanced Water Management

Abstract

Water management in anion exchange membrane fuel cells (AEMFCs) is more complex than proton exchange membrane fuel cells (PEMFCs). In the PEM system, water is generated at the cathode and otherwise only serves as the membrane proton transport medium. In the hydroxide AEMFC, water is generated at the anode, consumed at the cathode, and is greatly needed for transport of the larger hydroxide ion. The challenge this introduces is the need to provide adequate water to maintain the membrane humidity without flooding the catalyst or gas diffusion layers.1 One method being used to achieve the necessary high membrane water content is utilizing hydrophilic gas diffusion layers without microporous layers.2 Others have used gas diffusion layers with a hydrophobic coated microporous layer while raising the temperature of the humidifiers well above the cell temperature.3 Gas stream dew points above 100% relative humidity and back pressure are common approaches in increasing the membrane water content. However there is instability in the cell performance and conditions that generate high performance levels are not easily repeatable. The problem is the line between proper membrane hydration and flooded catalysts layers is very thin, and non-existent in some cases. In this study, the influence of the membrane, ionomer and gas diffusion layer as well as the flow rate and dew points of the anode and cathode gases on AEMFC performance were explored. Tokuyama A201 membranes and AS-4 ionomer were investigated alongside quaternary-ammonium-functionalized radiation-grafted ETFE alkaline anion-exchange membranes and ionomers.4 Using a hydrophilic gas diffusion layer without a microporous layer increased membrane hydration, but as expected increased the potential for flooding. Manipulating the dew points led to the counter-intuitive discovery that the cell performs better with the humidity higher at the anode than the cathode, despite water generation and electro-osmotic drag towards that electrode. In fact, removing too much water from the anode caused instability in the cell, while increasing the water at the anode decreased the membrane resistivity. Back diffusion likely plays an important role in membrane hydration and hydroxide transport through the membrane. A very high flow rate (1.0 L/min) also increased cell performance, despite being several orders of magnitude above the stoichiometric need. Combining all areas of improvement resulted in a very high performing AEMFC with a maximum current density of 2.2 A/cm2 (at 0.1 V) and max power density of 670 mW/cm2 (880 mW/cm2 iR-corrected) with a membrane resistivity of 75 mOhms*cm2 (Figure 1a). Only a minor drop in the current was observed using air at the cathode with the same 1.0 L/min flow rate as oxygen, giving max current density of 1.7 A/cm2 and a max power density of 580 mW/cm2 with a resistivity of 74 mOhms*cm2 (Figure 1b). This near identical behavior confirms that the amount of reactant present supplied by the higher flow rate is not necessary, but the volumetric flow rate is needed for water management. It is likely that the pressure drop along the single pass cell hardware allows the gas to “jump the bar” only at very high flowrates, which results in better water removal and limits cell flooding in the cell.