Guidelines for Stable Operation of a Polymer Electrolyte Fuel Cell with Self-Humidifying Membrane Electrolyte Assembly

Abstract

The desired performance of a polymer electrolyte fuel cell is always hindered by mismatching of operating conditions ͑such as temperature, flow rates of fuel and oxidant, level of humidification of reactants, etc.͒ with parameters associated with the cell performance ͑such as electrolyte conductivity, net back diffusion of water behavior in the membrane electrolyte assembly, etc.͒. This paper presents some design guidelines for polymer electrolyte fuel cells by establishing relationships among critical parameters of the fuel cell and is particularly useful for the developmental stage of mini/microfuel cells operating under nonpressurized condition and without external humidification of fuel.Polymer electrolyte fuel cells or widely named as proton exchange membrane fuel cells ͑PEMFCs͒ have made tremendous strides in the past few years. Research and development in fuel cells, in particular for micro/mini power sources, focus on miniaturization of the cell component and its component microstructure by employing micro/nanomachining technologies. In addition to researching on cell stacking methodology, flow patterning design based on simulation and combined conceptual design and trial-and-error approaches are also popular. Nguyen and Chan 1 reviewed the recent progress of the development of micromachined membrane-based fuel cells for both H 2 -fed and direct methanol-fed fuel cells. The review focused on the choice of materials and the design consideration of the components in the miniature fuel cell. Performances of the microfuel cells with different designs and under different operating settings were also compared.As is known, to achieve a stable and high-performance PEMFC there must be sufficient water in the polymer membrane, because the proton conductivity is directly related to the water content in the membrane. However, the amount of water should not be too excessive causing cathode flooding, thus blocking the pores in the electrodes for air diffusion. A good water balance is thus an important consideration in PEMFC. The dehydration of membrane comes from two possible sources: electro-osmotic drag pulling water molecules from the anode to the cathode during the migration of protons, 2,3 and loss of water to unsaturated flow streams by ͑force͒ convection. The first source is an inherent problem which can be worse under high overpotential in a poor design PEMFC. The understanding of the second source is important for self-humidifying fuel cell design, which is the case for micro/minifuel cell where introducing water by means of external humidification of reactants is impractical. Thus, water which forms in the cathode should properly be managed so as to keep the membrane at the correct level of hydration but not too excessively causing electrode flooding. Though water balance can be determined by computational fluid dynamics ͑CFD͒ modeling approach, most models developed for such a purpose are over simplified due to the extremely complicated electrochemical processes and transport phenomena involved in the porous...

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“…This model is widely used in literature to describe the water uptake of ionomeric materials since it captures the sigmoidal shape of their sorption isotherm [2,47]. The downward concavity at low vapor water activity is associated to the formation of monolayers of bounded water specifically interacting with the hydrophilic groups of Nafion and filler particles.…”

Synthesis and characterization of novel hybrid polysulfone/silica membranes doped with phosphomolybdic acid for fuel cell applications

“…Currently, the most commonly used polymer electrolyte in PEMFC is based on expensive perfluorinated membranes, such as Nafion s , which present high proton conductivity, good mechan ical strength and excellent stability. However, these membranes show several drawbacks since to ensure good proton conduction they must be fully hydrated, limiting their working temperature range below 80 1C and 100% relative humidity [2,3]. Moreover, this type of membrane exhibits high methanol permeability and high cost of production, which limits its further application and future commercialization.…”

Relative humidity control in polymer electrolyte membrane fuel cells without extra humidification

“…In Refs. [12][13][14], the net water flux and other parameters are considered under several operational conditions.…”

“…The air stoichiometry ( ) is calculated according to the following equation: where RH des is the desired relative humidity to maintain saturated condition, normally between 80 and 100% [13].…”

“…And most likely, the smaller plant the bigger humidifier. Also, some researches have reported other forms to avoid extra humidification [13,21,22].…”