Dilip Natarajan

A Two-Dimensional, Two-Phase, Multicomponent, Transient Model for the Cathode of a Proton Exchange Membrane Fuel Cell Using Conventional Gas Distributors

A two-dimensional, two-phase, multicomponent, transient model was developed for the cathode of the proton exchange membrane fuel cell. Gas transport was addressed by multicomponent diffusion equations while Darcys law was adapted to account for the capillary flow of liquid water in the porous gas diffusion layer. The model was validated with experimental results and qualitative information on the effects of various operating conditions and design parameters and the transient phenomena upon imposing a cathodic overpotential were obtained. The performance of the cathode was found to be dominated by the dynamics of liquid water, especially in the high current density range. Conditions that promote faster liquid water removal such as temperature, dryness of the inlet gas stream, reduced diffusion layer thickness, and higher porosity improved the performance of the cathode. There seems to be an optimum in the diffusion layer thickness at the low current density range. The model results showed that for a fixed electrode width, a greater number of channels and shorter shoulder widths are preferred. The transient profiles clearly showed that liquid water transport is the slowest mass-transfer phenomenon in the cathode and is primarily responsible for mass-transfer restrictions especially over the shoulder.

Three-dimensional effects of liquid water flooding in the cathode of a PEM fuel cell

A two-dimensional model available in the literature for conventional gas distributors was expanded to account for the dimension along the length of the channel. The channel was discretized into control volumes in series that were treated as well mixed. An iterative solution procedure was incorporated in each control volume to determine the average current density and the corresponding oxygen consumption and water generation rates. Downstream channel concentrations were calculated based on stoichiometric flow rates and the solution obtained from the preceding control volumes. Comparison of the model results with experimental data and the existing two-dimensional model showed that accounting for the oxygen concentration variations along the channel and its effect on the current density is critical for accurately predicting the cathode performance. Variations in the current density along the channel were strongly influenced by the changes in oxygen concentration caused by consumption due to reaction and dilution caused by water evaporation. Operating parameters that facilitated better water removal by evaporation like higher temperature and stoichiometric flow rates and lower inlet stream humidity resulted in higher net current. Operating conditions that resulted in minimal loss in oxygen concentrations resulted in a more uniform current density distribution along the channel.

Air-Breathing Laminar Flow-Based Direct Methanol Fuel Cell with Alkaline Electrolyte

Microfuel cells have the potential to achieve higher energy densities than batteries and have thus received intense investigation as a power source for a wide range of portable applications. Extensive research efforts are focused on the development and miniaturization of promising fuel cell technologies, including direct methanol fuel cells DMFCs and polymer electrolyte membrane-based fuel cells PEMFCs, operated with hydrogen/oxygen. 1-3 In most fuel cells, a polymer electrolyte membrane such as Nafion allows protons to diffuse from the anode to the cathode, while trying to prevent fuel molecules from diffusing across and mixing with oxygen at the cathode. Poor performance or a lack of selectivity by the membrane leads to a key performance-limiting process called fuel crossover that has plagued the PEM-based fuel cells. In addition to fuel crossover, cathode flooding and anode dry-out water management due to osmotic drag of water molecules associated with protons diffusing from the anode to the cathode, as well as due to the formation and consumption of water at the cathode and anode, respectively, impedes the performance and commercial implementation of these fuel cells. 4