Page 96 - Introduction to Transfer Phenomena in PEM Fuel Cells

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Mass Transfer Phenomena

3.1. Introduction

A fuel cell should be supplied continuously with fuel (hydrogen) and an
oxidant (oxygen) to ensure the production of electricity. On the contrary, the
products of the reaction in the heart of the cell should also be released
continually for maximum efficiency, as well as to improve its lifetime.
Studying the mass transfer of the different species involved in the fuel cell is
essential. In fact, poor mass flow management can lead to significant failures
in the performance of the battery.

3.2. Flow of matter

Concentrations of reactants and products adjacent to the catalyst layers
have a strong influence on determining the performance of the cell.
Decreases in concentration can be minimized by optimizing the upstream
transport of these species, from flow channels to electrodes and diffusion
layers (GDL). Table 3.1 summarizes the role of mass transfer in each cell
component of a PEMFC [SPI 07].

Two main mass transfer phenomena are encountered in the PEMFC,
diffusion in the diffusion layers (GDLs) and electrodes, and convection in
the flow structures (flow channels); this can be related to the difference in
dimensions. Indeed, the length of the flow structures is in millimeters, or
even centimeters, and the structure is a well-defined set of channels. For the
diffusion layers and electrodes, the porous structures are in micrometers or
even nanometers, and the tortuous structures trap the molecules of the