Environmental Engineering Reference
In-Depth Information
varying operative conditions of fuel cells, especially when used for transportation
applications. In particular the possibility of degradation is mainly related to dif-
ferent RH conditions, whose effect can range from membrane shrinkage in the
absence of external humidification to swelling and in-plane compression at high
RH levels. Any type of mechanical failure is very dangerous for stack life,
especially pinholes and perforation, because they can allow the crossover of
reactant gases with direct hydrogen combustion on the catalyst surface, consequent
strong heat release, and local hot spot generation. This initiates a cyclic process of
increasing crossover and perforations which can lead to the fast stack degradation.
The thermal degradation of the membrane is connected not only to diminution
of proton conductivities properties at high temperatures and low RH levels, but
also to decomposition of Nafion in proximity of local hot spots. Moreover, the
frequent temperature changes typical of automotive applications can produce
severe limitations to proton conductivity, gas impermeability, and mechanical
resistance of Nafion membranes, due to phase transformations associated with the
wide range of temperatures encountered (from freezing to about 80C). The
mitigation strategies for this degradation cause consist essentially in removal of
residual water by gas purging.
The chemical/electrochemical degradation is related to the attack on the
membrane and catalysts by peroxide and hydroperoxide radicals, produced by
anode and cathode reactions. The formation of these species and the consequent
membrane aging is accelerated in OCV and low-RH conditions. The overcoming
of this problem requires the developing of new materials able to resist against
peroxy radicals, either by addition of free-radical inhibitors (or peroxide decom-
position catalysts [
64
]) during the membrane fabrication or by modifications of its
structure [
65
].
The degradation of the GDL is not clearly assessed because of the difficulties in
separating the effects from contiguous elements, such as catalysts and bipolar
plates. While some decrease of GDL conductivity and hydrophobicity has been
associated with loss of PTFE and carbon, due to temperature and electrochemical
surface oxidation during fuel cell operation [
66
], a recent study suggests that
hydrophobicity changes of GDL can be considered negligible in 10000 h tests
under high humidification [
67
].
References
1. Mahan BM (1968) University chemistry. Addison-Wesley Publishing Company, Reading
2. Lange NA (1967) Handbook of chemistry. McGraw Hill, New York
3. Zemansky MW, Abbott MM, Van Hess HC (1975) Basic engineering thermodynamics.
McGraw Hill, New York
4. Lide DR, Frederikse HPR (eds) (1996) CRC handbook of chemistry and physics. CRC Press,
Boca Raton
5. Atkins P, de Paula J (2002) Physical chemistry. Oxford University Press, Oxford
6. Weaver G (ed) (2002) World fuel cells—an industry profile with market prospect to 2010.
Elsevier Advanced Technology, Kidlington, Oxford
