Water condensation in gas distribution plates of PEMFCs: flow and removal from the grooved patterns of the plates
Advancing the chemical engineering fundamentals
Electrochemical Engineering (T2-14P)
Keywords: PEMFC, liquid water, bipolar plates, liquid flow, reduced performance
Water produced in PEMFC for the production of heat and electricity can be partly in the form of liquid for high hydration of the inlet gases and high current density. The issue of water management has been investigated for years for the optimal hydration of the membranes, while avoiding the formation of liquid water in the diffusion layers and the grooved structures of the gas distribution plates. Presence of liquid water in too a significant amount can result in reduced activity of some parts of the fuel cell, and to instability of the voltage by the occurrence of slug flow. It is well known that temperature, flow rates and hydration have to be carefully chosen for efficient operation at the nominal current. However, the bipolar plate design also is to exert a significant effect on the water management [1].
The present investigation was aimed at observing the flow of liquid water in the grooved structures of the bipolar plate in emulation experiments. Graphite Electrochem plates with an area of 25 cm2 were used, considering two different designs: (i) The so-called serpentine design actually consisted in the series of seven sets of five parallel channels 47.7 mm long, with collecting and redistribution segments at the edge of the 5x5 cm2 area. (ii) In the column-flow pattern, the gas was distributed through thirteen squared orifices, then circulated around 2.0 mm squares tiles placed regularly on the plate surface, before being collected by the outlet tube.
The bipolar plate considered was simply covered by a Perspex polymeric plate, allowing visual observations, and humidified air was driven into the “half” inert cell. The rear face of the bipolar plate was cooled by tap water allowing partial condensation of water. Gas flow rate and hydration of the gas were adjusted to correspond to operating conditions of real FC tests. High-speed camera allowed continuous observation of the two phase flow.
The serpentine pattern was shown to favour slug flow in the channels. After complete wetting of the graphite surface, droplets of water can be attached at low-shear areas of the plate, before being suddenly removed by a slight overpressure of the gas. The films taken have been interpreted in order to correlate the flow characteristics to the operating conditions, taking into account the calculated temperature profile of the fluid circulating in the cooled structure, and the local amount of liquid water formed. Liquid droplets formed in the column-flow pattern plate are more regularly removed under the pressure of the gas and gravity: droplets falling entrain liquid water in their fall, in sort of avalanche phenomena, and stagnant liquid water can only be observed in the gas collection channel at the outlet of the cell.
[1] J. Scholta et al., Journal of Power Sources, 155 (2006) 66-71
See the full pdf manuscript of the abstract.
Presented Wednesday 19, 13:30 to 15:00, in session Electrochemical Engineering (T2-14P).
