Technology Status of Hydrogen Road Vehicles

Appendix 3. Fuel Cells for Hydrogen Vehicles
The FC is in effect an electrolyzer operating in reverse: instead of an applied DC voltage splitting water into
hydrogen at the cathode and oxygen at the anode of the electrolyzer, the FC provides a DC voltage when the
electrodes, separated by a suitable electrolyte, are supplied with these gases. Water is produced by
electrochemical reaction of the hydrogen and oxygen; no other waste products arise.
Another way to regard the FC is as a storage battery: a DC voltage is supplied by the latter from intermittent
charging, while the former does so continuously in the presence of hydrogen and oxygen at the electrodes.
The waste products of a battery are, however, often objectionable; an FC emits only water, which can even
be used for recycling to the cell.
A3.1 Fundamentals of Fuel Cells
An example of a basic FC consists of two electrodes (hydrogen at the anode and oxygen at the cathode)
separated by an electrolyte. The electrolyte blocks the electrons which are released during the oxidation of
the hydrogen at the anode. The electrons are forced to travel through the external circuit to the cathode, where
the oxygen is reduced. The electrolyte allows easy passage of hydrogen ions (protons), enabling the hydrogen
to react with the reduced oxygen at the cathode to form water. As will be seen later, the hydrogen must
presently be supplied in a pure form at the anode, but normal air is satisfactory at the cathode.
The open-circuit voltage, i.e., without external load, of an FC is about 1.2 V at ambient conditions. This,
however, is an ideal value, and in practice drops to about 1 V at no load, giving a maximum attainable
efficiency of practical devices reaching 80%. When the load is increased, the electrochemical reactions
proceed rapidly at the anode, but the inherently slower oxygen reactions at the cathode--even when highly
catalyzed with precious metals and with increased air flow and pressure--together introduce irreversibility into
the electrochemical reactions, and the efficiency drops continuously. Increasing resistive losses in the
electrodes and electrolyte cause further losses. The extent of the drop depends on FC type, temperatures,
catalysts, current densities, etc., but 50% at full load can be taken as representative of today’s technology.
Losses in ancillary systems for fuel handling, vehicle services, etc., inevitably cause further drops in overall
efficiency.
This deterioration from ideal to practical use is typical of any real-life process. The FC was promoted from
a laboratory curiosity to a vital function with the advent of manned space flight, which requires hydrogen and
oxygen on-board for propulsion and life-support systems, making the FC particularly attractive for local power
needs.
Compared to thermal machines, FCs have outstanding potential:
! Efficiencies are not limited by the Carnot one, and they increase with decreasing load; together these
characteristics combine to give a doubled efficiency over a driving cycle.
! Increased flexibility in use, since the electrochemical reactions are practically instantaneous: in practice,
fast startup and shutdown with low-temperature devices, and smooth load change with electrical drive.
! No pollution of a chemical or acoustical nature, except from the air blowers as presently used.
! The modular construction confers geometrical ease of adaptation to given volumes, and strong potential
for capital cost reductions on scale-up to industrial production.
Compared to storage batteries, the FC has the great advantage of one to two orders of magnitude improvement
in energy density (kWh/kg).
Despite these advantages, before FCs can be considered realistic for large-scale urban transport the following
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