Environmental Engineering Reference
In-Depth Information
Fig. 3.9 Effect of air com-
pressor consumption for a
500 W PEM stack
compressor consumption (about 10% of stack power) is obtained for high output
power (from 200 W up to maximum load) maintaining the stoichiometric ratio not
higher than 2, while at low loads the necessity to maintain a minimum air flow rate
implies that the energy losses due to the compressor represent a large fraction of
the power supplied by the stack.
3.4 Durability of PEM Fuel Cells
Durability is a fundamental and necessary feature for PEM fuel cells to see a wide
diffusion as a practical power source in any application field. The requirements for
fuel cell lifetime vary with the specific application, in particular a duration of at
least 5000 h is mandatory for use on cars, while even longer periods are needed for
bus and stationary employments (for most applications an acceptable degradation
rate is considered to be comprised in the range 2-10 lV/h [
51
]). However, the wide
variability of operative conditions usually encountered in automotive applications,
such as dynamic driving cycles, startup/shutdown phases, and freeze/thaw, makes
also the target for car very difficult to be met with the current technologies.
Since the concept of durability decay is strictly connected to that of aging, in
the sense of a progressive and not reversible loss of performance, a general
examination of this feature for PEM fuel cells has to include some considerations
about the main processes that can cause the degradation of materials used for
components of these systems (electrocatalysts, membranes, GDL, and bipolar
plates), which is mainly related to working conditions of high humidity and
temperature, oxidizing or reducing environment, and dynamic operation.
As discussed before the amount of platinum catalyst used in electrocatalysts of
a PEM fuel cell is supported on carbon in the form of nanoparticles, in order to
