Page 54 - Introduction to Transfer Phenomena in PEM Fuel Cells

Introduction to Hydrogen Technology 43
The electrode typically consists of a mixture of an ionomer and
suspended black platinum on coal particles (Pt/C); the charge catalyst is
–2
usually between 0.1 and 0.4 mg/cm for both the anode and the cathode
[MIK 07].
The reaction mechanisms in the active layers are relatively complex.
These catalytic layers are often the thinnest of the fuel cell, with a thickness
of 5 to 30 microns. They are also the most complex because they must
contain several types of gas, water and facilitate chemical reactions. These
catalytic layers are made of a porous carbon (granular porous materials),
platinum or platinum/ruthenium material [MIK 01]. The reactions that occur
in the catalytic layer are exothermic; it is therefore necessary to remove heat
from the fuel cell. The heat can be removed by convection through the
channels and conduction into the solid part of the catalytic layer, the gas
diffusion layer and the bipolar plates. Since water is produced in PEMFC,
condensation and evaporation of this water affects the heat transfer. This is
why the condensation and evaporation of water and the temperature of the
fuel cell are intimately linked [APP 88].
In all cases, these layers, also called “active” layers, must always be
accessible to gas, but they must also be able to ensure the transfer of protons
via the membrane and the transfer of electrons to the external electrical
circuit from the anode to the cathode; the manufacture of electrodes must
consider the best compromise to respect these rules of operation. Over time,
there may be a decrease in catalyst activity, which, although not consumed
during the reaction, gradually decompose and no longer provide contact with
the electrolyte. In addition, the electrodes are very sensitive to carbon
monoxide, which may be one of the products of the reforming of hydrogen.
A few ppm of CO is sufficient to poison active sites, resulting in a decrease
in potential [NGU 10, RAM 05].

Operation in pure oxygen or oxygen-enriched air substantially improves
the performance of the fuel cell. The diffusion of oxygen through the
diffusers and especially through the pores of the electrode is facilitated.
However, this improvement should be considered alongside the additional
costs and difficulties related to the complexity of the system [RAM 05].