Page 280 - Alternative Energy Systems in Building Design

254 FUEL CELL TECHNOLOGY

HYDROCARBON-BASED FUEL RE-FORMATION

In some fuel cell technologies, fuel in the form of liquid hydrocarbons is used. A process
referred to as the steam re-forming of liquid hydrocarbons involves the steam re-formation
of methanol, a process that separates hydrogen from hydrocarbon molecules.
Some of the challenges associated with this type of fuel cell technology include the
control of high temperatures resulting from the reaction, slow startup, frequent replace-
ment of catalysts owing to sulfur poisoning or contamination (which is present in most
liquid fossil fuels), and carbon monoxide removal.
The chemical reactions that take place in the fuel re-formation process are expressed
as follows:

C H + nH O → nCO + (m/2 + n)H 2
m
2
n
CO + H O → CO + H 2
2
2
The thermodynamic efficiency of the hydrocarbon-based fuel cell process, depending
on the purity of the hydrogen product, is between 70 and 85 percent.
One of the most difficult challenges associated with re-former-based systems remains
the fuel cell’s cost and durability. The polymer-electrolyte-membrane catalyst is quite
readily poisoned by carbon monoxide, which cannot be removed during the re-formation
process, resulting in rapid degradation of the catalytic membrane, perhaps one of the
most expensive components of the fuel cell.
The re-former–fuel cell system currently in use commercially uses hydrocarbon
fuels, such as natural gas, gasoline, or diesel fuel. However, owing to the production
of residual pollutants, the use of re-former-type fuel cells in view of global warming
has become a controversial issue.

ELECTROLYSIS

Electrolysis is a manufacturing process used to chemically separate bonded elements
and compounds through the passage of dc current. Electrolysis involves the passage of
an electric current through an ionic substance that is either molten or dissolved in an
aqueous solution, resulting in chemical reactions at the positive and negative elec-
trodes (referred to as the anode and cathode, respectively).
An ionic compound composed of covalently bonded substances or acids is dis-
solved with specific solvents or melted by heat in order to release ions in the liquid.
On the application of an electrical current to the immersed anode and cathode, ions
become attracted to electrodes of the opposite charge. As a result, positively charged
ions, referred to as cations (pronounced “cat-ions”), move toward the cathode, where-
as negatively charged ions, termed anions (pronounced “an-ions”), move toward the
anode. At the anode and cathode probes, electrons are absorbed or released by the
ions, forming a collection of the desired element or compound.

ELECTROLYSIS OF WATER
The electrolysis of water is an electrochemical ion-separation process that results in
the separation of hydrogen and oxygen gases at anode and cathode probes: