1.1 Low temperature fuel cells 3
Figure 1.2: Potential (U) vs. current density ( j) relationship of a state-of-the-art PEMFC
with a typical ORR catalyst consisting of carbon supported Pt nanoparticles (red line). The
reversible potential U0 is shown for comparison (green dotted line). The kinetics of the ORR
is responsible for most of the potential losses (blue line). Figure adapted from 3.
reversible thermodynamic cell potential for the reaction of equation 1.1 is 1.169 V
at 80 C (a typical operation temperature for PEMFCs), major losses occur. At a
typical current density of 0.7 A cm2 45% of this chemical energy is lost as heat.
According to Gasteiger et al. about 75 % of this efficiency loss can be assigned
to the slow kinetics of the ORR at the cathode 3. The rest is accounted by the
losses due to the kinetics of the HOR, to the omhic resistance and to mass transport
Even the best state-of-the-art catalysts, all based on precious metals such as
Pt or Pd, are still far from optimal and they exhibit considerable overpotentials for
the ORR, which ultimately raises the loading of precious metals in PEMFCs 6, 7.
This means that the catalytic ORR mass activity, defined as the current density
at a given potential per mass of precious metal, must be improved. This project
enters in a broad research for the development of new Pt-based catalysts for the
ORR whose main achievements will be introduced in the following sections of this
chapter. An alternative approach focuses on non-precious metals cathode catalysts.
The challenge here is to find materials that can be stable at the highly oxidizing
acidic conditions (pH 0) of the cathode. Among the metals, only Ir, Pt and Au
are thermodynamically stable at potentials higher than 0.9 V vs. the Reversible
Hydrogen Electrode (RHE) 8 and even Pt corrodes at potentials close to 1 V vs.