Hydrogen - Energy Policy Analysis: A Conceptual Framework - page 197

Figure 12.2 Hydrogen Fuel Cell

O 2-

Water

O 2-

O 2-

Carbon Dioxide O 2-

O 2-

Air

E
L
E
C
T
R
O
L
Y
T
E

Fuel: Natural gas, coal
gas, biogas, or
hydrogen

C
A
T
H
O
D
E

A
N
O
D
E

Electricity

Source: USDOE 2011a.

than 20 percent efficient at converting chemical energy in gasoline into power that moves the
vehicle. Hydrogen fuel cell vehicles, which use electric motors, are more energy-efficient, using
40 to 60 percent of the fuel's energy—corresponding to more than a 50 percent reduction in fuel
consumption, compared to a conventional vehicle with a gasoline internal combustion engine. Fuel
cells operate quietly and have fewer moving parts that may need maintenance or repair (USDOE
2011a). Their size, flexibility, and corresponding electrical output make fuel cells ideal for a wide
variety of applications, from a few kilowatts to power a laptop computer to several megawatts at
a central power generation facility (USEIA 2008b).
A single fuel cell consists of an electrolyte sandwiched between two electrodes, an anode and
a cathode, as illustrated in Figure 12.2. Bipolar plates on either side of the cell help distribute
gases and serve as current collectors. In a polymer electrolyte membrane (PEM) fuel cell, which
is widely regarded as the most promising for light-duty transportation, hydrogen gas flows through
channels to the anode, where a catalyst causes the hydrogen molecules to separate into protons
and electrons. The membrane allows only protons to pass through it. While protons are conducted
through a membrane to the other side of the cell, a stream of negatively charged electrons follows
an external circuit to a cathode. This flow of electrons is electricity that can be used to do work,
such as power a motor (USDOE 2011a).
On the other side of the cell, oxygen gas, typically drawn from outside air, flows through
channels to the cathode. When electrons return from doing work, they react with oxygen and