a requirement that implies some minimum water pumping--and power--requirements. Auto
manufacturers believe that the focus of research to date has been on basic R&D for the stack but
that not much has been done on system integration and in engineering the PEM fuel cell to adapt
to the car. Even with a well-engineered hydrogen fueled PEM cell, manufacturers expect to attain
system average efficiencies over the FTP driving cycle of only about 50 percent or less when
installed in a car. This implies that “balance of plant” efficiency will be about 80 percent.
Efficiency will be still lower if methanol or another fuel is used instead of hydrogen. Current
PEM fuel cells displayed by Mercedes and Ballard do use pure hydrogen as a fuel, but this
arrangement creates important storage difficulties. The alternative of making hydrogen on board
from methanol is also the subject of continuing research sponsored by the DOE. Large-scale
hydrocarbon reformers are well developed technologically. The thermodynamics of methanol-
steam reactions indicate that a minimum of 25 percent of the energy content of methanol is
required for conversion to hydrogen and carbon dioxide.lO1 The energy requirement is associated
with the heat required for steam generation, methanol vapor generation, and reformer reaction
heat. This heat can be supplied by the heat rejected by the fuel cell stack, however, so that it need
not reduce overall system efficiency. Control of heat flows is a major challenge in designing a
compact on-board reformer. In addition, the reformer introduces a lag in system response, as
hydrogen must be supplied at a rate that varies with the power demand from the fuel cell.
Although a battery can provide power for transient power demands in addition to providing
instantaneous vehicle power from a cold start, this adds weight and complexity to the system.
Reforming occurs over a catalyst that operates best at about 250° C, but this implies that the
catalyst must be preheated before the reformer supplies hydrogen.
Another problem posed by the reformer is pollution created by the reforming reaction; some
untreated methanol and CO will exit from the reformer and must be removed to avoid
contaminating the fuel cell stack. Removing these gases is difficult and expensive, however.
Typically, two packed catalyst beds are used to reduce these contaminants to very low levels.
However, CO concentrations remain over 0.25 percent even after catalytic treatment,^102 and PEM
cells are poisoned even by 10 ppm of CO. Further control is by a preferential oxidation (PROX)
unit, where air is mixed with the reformer output and passed over a platinum-alumina oxidation
catalyst. It is not yet clear whether the PROX unit can control CO to very low levels over a wide
range of flow rates and demonstrate the durability required for vehicle use. Strategies such as an
air bleed into the fuel mixture appear to prevent poisoning, but at some loss in efficiency. Alloy
catalysts more resistant to CO poisoning are under development.
In summary, the use of a methanol-based system, instead of using pure hydrogen as a fuel
introduces a range of difficulties. First, the system efficiency is degraded owing to the increased
stack inefficiency as well as greater needs for the “balance-of-plant.” Second, the time lag between
power demand and hydrogen production indicate that a battery system will be required to provide
power for transient accelerations, further adding to weight and complexity. The battery system
will also be required to power the vehicle if instantaneous response from cold start is desired.
101 R.D. Sutton and N.E. Vanderborgh, “Electrochemical Engine System Modeling and Development” paper presented at the Automotive
Technology DevelopmentContractors Coordination Meeting, U.S.Department of Energy, October 1993.
102 Ibid