Technology Status of Hydrogen Road Vehicles

difficulties must be overcome:
! High capital cost, probably subject to steep economies of production, but saddled with precious catalysts
both scarce and expensive.
! Low current densities at electrodes, increasing bulk and weight, decreasing useful load and decreasing
vehicle efficiency at the wheels.
! Hydrogen storage.
Although these drawbacks make FCs impractical for industrial use today, the space applications (the Shuttle
has three 12-kW FCs) and increasing transport demonstrations (see Tables 1 and 2) are putting them in better
perspective: the potential economies of future mass production are undeniable, and Pt loadings at the
electrodes have decreased by one to two orders of magnitude in the past 10 years; current densities are
increasing rapidly, and have improved threefold in recent years; while hydrogen storage can be roughly half
that of ICE vehicles with the same range, and significant progress is being made as discussed in Chapter 3.
If we compare this perspective with that of the ICE at a similar stage in its development--say 100 years ago-there
is every reason to be optimistic about FC development, provided it receives the concentrated industrial
attention that the ICE enjoyed. This being said, one item could halt the large-scale deployment of FCs: their
use of strategic and scarce catalysts. If the relative loadings cannot be brought to acceptable levels, not only
the price but ultimately the unavailability of the present favorites (Pt, Pd, etc.) could be decisive.
A3.2 Types of Fuel Cells
The main distinguishing feature of FCs is the type of electrolyte.
Ideally, the electrolyte should allow easy passage of the hydrogen ions without allowing electrons through.
At the moderate temperatures preferred for transport applications, a strong acid or strong alkali solution is
indicated and led to earlier choices of phosphoric acid or potassium hydroxide alkali as electrolytes. Known
respectively as PA FC and A FC, they have found some limited application. However, the actual experience
has not been favorable and they are being abandoned for what is becoming the almost unique choice: an acidtype
in the form of a solid polymer known then as a SPE FC, but now increasingly called the PEM FC.
The solid polymer electrolyte of the PEM FC comes in very thin sheets (only a few tenths of a micron) and,
although it must be kept humid, it is not subject to liquid loss. The sheets are under proprietary trademarks
or patents, are very expensive considering the bulk plastics industry behind them, and are presently marketed
by Asahi, Ballard, De Nora, Dow, DuPont and probably others. Since only these thin sheets and relative
humidity paths separate the electrodes, the stacking density (cells/cm) is relatively high but a limit is posed
by the electrode thickness which demands inversely-proportional catalyst loading. The current density (A/cm 2 )
of these sheets is relatively high, and appears to be increasing rapidly. The result is -and the evolution is
strongly in favor of the PEM FC- that it provides power densities (W/kg) over 260, as compared to about 100
for PA FC or 65 for A FC, all at efficiencies of 50%-55%.
Nonaqueous electrolytes are also being developed for FCs, the most important being molten carbonates, which
operate at 650°C, and solid oxides, which operate at 1000°C. The high temperatures encourage the
electrochemical reactions, and catalysts can be dispensed with, but other problems can arise on road vehicles
and, so far, no project has emerged. Considerable interest is shown in these high-temperature FCs for
stationary power generation.
Attention will be restricted in the following to the PEM FC, but much will be applicable to others, especially
the aqueous low-temperature ones.
A3.3 FC System Optimization
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