(141b) Water Management Strategies for PEM Fuel Cells - a Review

Authors: 
Bajpai, H., Purdue University
Revankar, S. T., Purdue University
Water management is an integral part of PEM fuel cell operation. Water has to be adequately balanced inside a fuel cell between hydration, diffusion and electro-osmotic drag processes, as an imbalance can lead to either flooding or drying, which can result in degradation of current density and output voltage. This paper reviews the operating parameters and design of fuel cell components for efficient water balance.

It is very crucial to optimize operating parameters viz. pressure, humidity of inlet streams, temperature, flow rates and stoichiometry in a fuel cell. An increase in temperature, pressure and reactant flow rate improves performance. Generally, a pressure gradient (Pc > Pa) exists between anode and cathode, to remove excess water from cathode. Also, high or low pressure pulses can aid in the removal of water molecules. External humidification of gases is preferred as internal humidification lowers the performance by 40%. After flooding, flushing can be facilitated by dehumidification of one or both inlet streams. The optimum values of relative humidity and flow rates are decided by a bargain between proton hydration and water imbalance.

The design enhancement strategies can significantly augment the performance of fuel cells. Interdigitated channels are better than conventional parallel flow fields and can double the power density. The horizontal orientation of serpentine channels with inlet manifold located on upper side results in gravity aided water removal. Also, the use of hydrophilic substances in diffusion layers can be effective, as they lead to uniform distribution of water molecules. A microporous layer with hydrophobic substances can improve water diffusion from electrode assembly into the diffusion medium. Poly dimethyl siloxane (PDMS) proved to have better hydrophobicity than poly tetra fluoro ethylene (PTFE) and fluorinated ethylene propylene (FEP) for microporous layers. The higher in-plane permeability of gas diffusion layer leads to a better gas transport and reduces retention volume of water. The impregnation of substances on cathode that only allows oxygen to permeate and not water can also be used. There has been development of novel methods that enable water management. An asymmetric membrane has been used with hydrophilic silica on anode and hydrophobic silica on cathode to improve proton conductivity at low humidity conditions. An innovative down-slanted channel design improved hydration and conductivity compared to rectangular channels.

Thus, fuel cell performance is strongly linked to the adopted strategy.

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