other types of cells are viewed as having more difficult problems in adapting to light-duty vehicle
requirements. Solid oxide fuel cells, for example, operate at very high temperature (~1000°C),
although their fuel flexibility and high-power density are attractive features. Alkaline cells are
easily poisoned by C0 2 and require pure oxygen, presenting serious challenges for transportation
use. Phosphoric acid fuel cells are relatively advanced, operate at relatively manageable
temperatures of about 160° to 220oC, and can be considered as mature technology for large
stationary source applications. Also, they recently have been adapted for use in buses (see Box 3-
1). Their bulk and low-power density, however, are an important barrier to automotive use. PEM
fuel cells operate below 100°C and are currently widely considered the only fuel cell candidate
likely for car use in the near future, with the phosphoric acid cell being restricted to bus or heavy-
duty truck use. For the longer term, solid oxide fuel cells and fuel cells that can directly transform
methanol into electricity (direct methanol fuel cells) are strong candidates for light-duty vehicular
use.
The PEM cell is essentially a sandwich composed of a hair-thin polymer membrane that serves
as an ion-conducting electrolyte, between thin sheets of a porous, conducting material, coated
with platinum catalyst, that serve as electrodes. One of these electrode/membrane/electrode
assemblies may be less than one millimeter in thickness; these assemblies are stacked to form the
fuel cell. Hydrogen is delivered to the anode, and oxygen (or air) to the cathode. The polymer
membrane/electrolyte conducts protons but seines as a barrier to electrons. At the anode,
hydrogen separates into hydrogen ions and electrons, aided by the platinum catalyst. When an
electrical circuit is connected between anode and cathode, electrons flow through the circuit. The
hydrogen ions flow through the membrane, combining with the returning electrons and oxygen at
the cathode to form water. The cell operates at about 200oF, so that elaborate heat-management
equipment is unnecessary.
A fuel cell system consists of a stack of individual cell “sandwiches,” which produce the
electricity; an air compressor to provide pressurized air to the fuel cell; a cooling system to
manage waste heat; a water management system to keep the polymer membranes saturated and to
remove the water created at the cathode; and a fuel source. The requirement for hydrogen fuel
means that either hydrogen must be carried onboard the vehicle in a storage vessel, or it must be
produced from a “hydrogen-earner” fuel such as methanol. In the latter case, hydrogen is
produced by steam-reforming or partial oxidation of the fuel and the reformer should be
considered as part of the overall system, especially in estimates of cost and system efficiency.
Methanol is the preferred fuel for PEMs because reforming requires only moderate temperatures
of about 300oC or less, whereas other fuels such as ethanol or natural gas require substantially
higher temperatures, implying both higher expense and reduced system efficiency.
Some recent evaluations of PEM fuel cell prospects have been quite optimistic. Allison, for
example, projects that a 60 kW system (60 kW is a reasonable output for a small car), including
the reformer for extracting hydrogen from methanol, should cost about $3,000 in mass
production, or about $46/kW.^95 Although the fuel cell cost does not include the cost of either
95 Allison Gas Turbine Division,General Motors Corp., “Research and Development of Proton-Exchange Membrane (PEM) Fuel Cell System for
Transportation Applications: Initial ConceptualDesign Report,” EDR 16194, U.S.Department of Energy, report prepared for Office of Transportation
Technologies, NOV. 30,1993.