The porous electrodes

The porous electrodes used in low-temperature fuel cells consist of a composite structure that
contains platinum (Pt) electrocatalyst on a high surface area carbon black and a PTFE
(polytetrafluoroethylene) binder. Such electrodes for acid and alkaline fuel cells are described by
Kordesch et al. (3). In these porous electrodes, PTFE is hydrophobic (acts as a wet proofing
agent) and serves as the gas permeable phase, and carbon black is an electron conductor that
provides a high surface area to support the electrocatalyst. Platinum serves as the electrocatalyst,
which promotes the rate of electrochemical reactions (oxidation/reduction) for a given surface
area. The carbon black also has a certain degree of hydrophobicity, depending on the surface
properties of the material. The composite structure of PTFE and carbon establishes an extensive
three-phase interface in the porous electrode, which is the benchmark of PTFE bonded electrodes.
Some interesting results have been reported by Japanese workers on higher performance gas
diffusion electrodes for acid fuel cells (see Section 3.1.2).
In MCFCs, which operate at relatively high temperature, no materials are known that wet-proof a
porous structure against ingress by molten carbonates. Consequently, the technology used to
obtain a stable three-phase interface in MCFC porous electrodes is different from that used in
PAFCs. In the MCFC, the stable interface is achieved in the electrodes by carefully tailoring the
pore structures of the electrodes and the electrolyte matrix (LiA1O2) so that the capillary forces
establish a dynamic equilibrium in the different porous structures. Pigeaud et al. (4) provide a
discussion of porous electrodes for MCFCs.