Jon P. Owejan

Water Transport Mechanisms in PEMFC Gas Diffusion Layers

Understanding how water produced in the cathode catalyst layer is removed during proton exchange membrane fuel cell (PEMFC) operation is critical for optimization of materials and model development. The present work combines in situ and ex situ experiments designed to elucidate the dominant water discharge mechanism when considering capillary and vapor transport at normal PEMFC operating conditions. The flux of water vapor driven by the thermal gradient in the cathode diffusion layer can alone be sufficient to remove product water at high current densities even with saturated gas in the delivery channels. The role of an intermediate microporous layer and its impact in vapor vs liquid transport is also considered. We propose that the primary role of the microporous layer is to prevent condensed water from accumulating on and blocking oxygen access to the cathode catalyst layer. .

Accumulation and Removal of Liquid Water in Proton Exchange Membrane Fuel Cells

Removal of liquid water from proton exchange membrane fuel cells is critical for efficient performance, low temperature operation, and robustness for start-up under freezing conditions. The present work investigates the three-dimensional location of liquid water during steady-state operation and the governing mechanisms that control its accumulation and removal. A combination of ex situ and in situ methods with neutron imaging diagnostics is used to investigate water transport in the bulk materials and flow field channels. These results provide experimental evidence of the mechanisms by which water vapor condenses in the anode during operation and the transport resistance for its removal during shutdown purge. Based on these results, a one-dimensional model is proposed that can be applied to calculate the effectiveness of cathode purge starting from a wide range of fuel cell shutdown conditions.

Through-Plane Water Transport Visualization in a PEMFC by Visible and Infrared Imaging

In this study, water transport and thermal profile in the through-plane direction of a proton exchange membrane fuel cell (PEMFC) gas diffusion layer (GDL) are reported. Direct optical and infrared access to both cathode and anode GDLs are provided in a typical 50 cm test section. Dynamic visualization (pixel resolution of 0.6 lm at 28 Hz) of liquid water transport and emergence in the gas distribution channels and diffusion layers are reported and the underlying transport processes are discussed. The temperature distributions across the anode and cathode GDL are also measured with a high resolution infrared camera with a pixel resolution of 5 lm at 30 Hz. VC 2011 The Electrochemical Society. [DOI: 10.1149/1.3560163] All rights reserved.

Investigation of Fundamental Transport Mechanism of Product Water From Cathode Catalyst Layer in PEMFCs

Understanding how water produced in the cathode catalyst layer is removed during PEMFC operation can be critical for materials optimization and representative models. The present work combines in-situ and ex-situ experiments to determine the dominant water discharge mechanism when considering capillarity and vapor transport. Water flux of vapor driven by the thermal gradient in the cathode diffusion layer is shown to be sufficient to remove product water at high current densities with saturated gas in the gas delivery channels. The role of an intermediate microporous layer and its impact in vapor versus liquid transport is also considered. Through novel experimental techniques and capturing all key physical interactions it is concluded that vapor diffusion is the fundamental mechanism by which water is removed from the cathode catalyst layer.Copyright

WATER TRANSPORT VISUALIZATION AND TWO-PHASE PRESSURE DROP MEASUREMENTS IN A SIMULATED PEMFC CATHODE MINICHANNEL

Two-phase flow and water transport in a 1.08 mm hydraulic diameter by 25-cm long gas-transport minichannel are investigated. High-speed side-view images are obtained of water droplets moving through gas diffusion media (GDM) and into a gas channel. This system simulates water transport and the flow of air and water in a polymer electrolyte membrane fuel cell (PEMFC) cathode gas channel. Advancing and receding contact angles and departure droplet diameters are measured with respect to superficial gas velocity for two GDM samples. Pressure drop is measured and compared to two-phase pressure drop correlations for three different water flow and five different airflow rates, and channel-water and GDM-water interactions are described.

Hydrogen-Water Flow Regime Transitions Applied to Anode Flow Phenomena in a PEMFC

Although water is produced on the cathode side of a polymer electrolyte membrane fuel cell (PEMFC), it is known to migrate to the anode, where the two-phase hydrogen-water interactions become critical to keep the channels clear for effective reactant delivery. Hydrogen-water flow regime transitions were experimentally investigated and compared to air-water transitions in a 1.0 mm square channel. Gas superficial velocities were evaluated for an anode stoichiometric ratio of 2.0 over a current density range from 0.1 A/cm2 to 2.0 A/cm2 . Liquid superficial velocities were controlled at the corresponding cathodic water production rate. It is shown that the annular transition in a hydrogen system occurs at as much as twice the gas velocity required for the same transition in an air system. At the low liquid flux expected in the anode channels of a PEMFC, a transition from slug to annular two-phase flow will occur at an unobtainable velocity for efficient fuel cell operation.© 2005 ASME

Through-Plane Water Transport Visualization in an Operating PEM Fuel Cell by Visible and Infrared Imaging

In this study, the liquid water emergence and transport within anode and cathode gas diffusion layers in an operating fuel cell was observed from the cross-section with a digital microscope and a high resolution infrared camera. In the cathode, water droplets were found to be formed on the channel side of the GDL cross-section, while little water was detected in the vicinity of the microporous layer. This finding suggested that water was condensing inside the GDL and may imply the existence of a condensation front, which has been predicted by numerical simulation but previously has not been verified experimentally. The anode results indicate that water was transported through anode GDL dominantly in the vapor form. The temperature distributions across the anode and cathode GDL were also measured with a high resolution infrared camera. These visualizations provided critical information on the water transport across a fuel cell.

Two-Phase Flow Considerations in PEMFC Design and Operation

A proton exchange membrane fuel cell (PEMFC) must maintain a balance between the hydration level required for efficient proton transfer and excess liquid water that can impede the flow of gases to the electrodes where the reactions take place. Therefore, it is critically important to understand the two-phase flow of liquid water combined with either the co-flowing hydrogen (anode) or air (cathode) streams. In this paper, we describe the design of an in-situ test apparatus that enables investigation of two-phase channel flow within PEMFCs, including the flow of water from the porous gas diffusion layer (GDL) into the channel gas flows; the flow of water within the bipolar plate channels themselves; and the dynamics of flow through multiple channels connected to common manifolds which maintain a uniform pressure differential across all possible flow paths. These two-phase flow effects have been studied at relatively low operating temperatures under steady-state conditions and during transient air purging sequences.Copyright

Effects of Flow Field and Diffusion Layer Properties on Water Accumulation in a PEM Fuel Cell

Water is the main product of the electrochemical reaction in a proton exchange membrane (PEM) fuel cell. Where the water is produced over the active area of the cell, and how it accumulates within the flow fields and gas diffusion layers, strongly affects the performance of the device and influences operational considerations such as freeze and durability. In this work, the neutron radiography method was used to obtain two-dimensional distributions of liquid water in operating 50 cm2 fuel cells. Variations were made of flow field channel and diffusion media properties, to assess the effects on the overall volume and spatial distribution of accumulated water. Flow field channels with hydrophobic coating retain more water, but the distribution of a greater number of smaller slugs in the channel area improves fuel cell performance at high current density. Channels with triangular geometry retain less water than rectangular channels of the same cross-sectional area, and the water is mostly trapped in the two corners adjacent to the diffusion media. Also, it was found that cells constructed using diffusion media with lower in-plane gas permeability tended to retain less water. In some cases, large differences in fuel cell performance were observed with very small changes in accumulated water volume, suggesting that flooding within the electrode layer or at the electrode-diffusion media interface is the primary cause of the significant mass transport voltage loss.Copyright