The operating principal of the
proton-exchange-membrane-hydrogen-based fuel cell is relatively simple. The
overall fuel cell as a unit is composed of a number of stacked cells (similar
to the cells of a car battery). Each cell in the stack is composed of a cathode
and anode which are separated by the proton exchange membrane. The PEM serves
as an insulator between the adjacent “half cells” while providing a pathway for
migration of hydrogen protons created during the process. Hydrogen is feed to
the anode side of each cell and air or oxygen to the cathode side. Electrons
are given up by hydrogen on the anode side, directed through an external
circuit and then return to the system on the cathode side where they recombine
with hydrogen ions and oxygen to form water. The process is described in a bit
more detail below.
The process of power generation begins with hydrogen at the anode
side of the cell. The proton exchange membrane is coated with a special
platinum catalyst that causes hydrogen molecules coming in contact with it to
split into two H+ ions (hydrogen ions). The formation of the positive hydrogen
ion requires that each hydrogen atom, from the original hydrogen molecule, give
up an electron. This is where the graphite comes in, as these now “free”
electrons are conducted away by the highly conductive graphite anode and are
directed through an external circuit where they provide energy. At the anode
the hydrogen is oxidized (its electrical charge in increased due to the
shedding of electrons which results in an increased positive charge).The
newly formed hydrogen ions (protons) continue to move through the proton
exchange membrane while on the cathode side of the cell oxygen molecules moving
into the surface of the proton exchange membrane are cleaved into two highly
reactive oxygen atoms(the oxygen molecule, O2, is relatively stable while
oxygen atoms are not).While all of this is occurring the electrons released
from the hydrogen molecules, previously at the anode, return from the external
circuit through the highly conductive graphite cathode. At the interface of the
proton exchange membrane and cathode one oxygen atom, two hydrogen ions, and
two returning electrons combine to form a water molecule. At the cathode the
overall reaction is a reduction (electrons are added to oxygen reducing
its charge).
Overall, in the cell stack, hydrogen is oxidized, oxygen is
reduced, electrons are liberated and reunited, and the “chemical pressure” of
all of this is used to provide power. The by-product of this electrolytic
“combustion” is water, which is vented to the atmosphere. The above process
occurs in each cell of the stack as long as fuel is provided. Each cell is
pumping electrons released from the previous cell resulting in an overall stack
pressure, or voltage, which is equal to the voltage of each cell multiplied
by the number of cells that are in series (the total number of cells in the
stack).
