Environmental Engineering Reference
In-Depth Information
achieve high metal dispersion at relatively low precious metal loading. Then the
main problem of aging of electrocatalysts to be avoided or controlled is the
increase in catalyst particle size, which causes diminution of active surface area
and then of catalytic activity. The coarsening of Pt nanoparticles during PEM fuel
cell operation has been extensively studied, and different mechanisms have been
proposed [
52
], among which the so-called Ostwald ripening is supported by
experimental evidences [
53
]. This mechanism is based on dissolution and diffusion
of Pt particles in the ionomer phase, with successive redeposition on other parti-
cles, leading to their size growth. The same mechanism of dissolution and diffu-
sion of Pt particles into the membrane produces also the effect of a degradation of
proton conductivity properties of the ionomer, caused by platinum reduction
(the presence of hydrogen can determine an effective reducing ambient) and its
precipitation inside the membrane bulk [
54
]. Other possible routes for catalyst
agglomeration have found experimental supports, and they are based on coales-
cence of Pt nanoparticles caused by nanocrystallite migration on the carbonaceous
support surface [
55
].
Also corrosion problems of the carbon support have been considered as a cause
of electrocatalyst durability loss [
32
], in particular carbon oxidation can occur
through electrochemical oxidation at the cathode, with formation of CO
2
(C ? 2H
2
O = CO
2
? 4H
+
? 4e
-
), or through water gas shift reaction, with the
production of CO (C ? H
2
O = CO ? H
2
). Both these routes are catalyzed by Pt
[
56
,
57
] and subtract carbon useful for platinum loading, with consequent metal
sintering and decrease of the electrochemical surface area [
58
].
Since Pt dissolution is favored by high electrode potential, relative humidity,
and temperature, the possibility to limit the risk of electrocatalyst aging is based on
the use of Pt-alloy catalyst instead of pure platinum, at least for the cathode, which
is characterized by higher potential with respect to anode, and by adoption of
operative conditions not too severe in terms of humidity and temperature. While
this last point requires interventions on the membrane structure, the study of
catalyst materials has evidenced that a minor tendency to sintering can be obtained
by the addition of non-noble metals, such as Ni, Cr, or Co, to the Pt cathode
catalyst [
59
,
60
], suggesting a possible pathway for future work. On the other hand
also the potential application of non-platinum catalysts is under study, in particular
transition metal complexes with structures based on porphyrines and related
derivatives have been proposed to substitute noble metals [
61
], but their activity
performance is still far from those of Pt-based catalysts.
The carbon corrosion issues are faced by the study of different carbonaceous
supports, such as carbon black or carbon nanostructures. Recent results evidence
the superiority of graphitized carbon with respect to amorphous carbon black in
terms of corrosion resistance, and the promising characteristics of carbon nano-
cages when Pt sintering effects are considered [
62
].
Another critical issue regarding durability of PEM fuel cells is the reliability of
the electrolytic membrane, which can undergo mechanical, thermal, and chemical/
electrochemical degradations [
63
]. The mechanical degradation consists in
different types of failure (cracks, pinholes, perforations) which are favored by
