Page 270 - Alternative Energy Systems in Building Design

246 FUEL CELL TECHNOLOGY

In general in fuel cells, the reactant, or fuel, is introduced into a chamber that is
exposed to an electrode referred to as the anode. For example, a reactant such as hydro-
gen, when placed in the intake chamber, diffuses to the anode catalyst, where it later
dissociates into protons and electrons. The protons then react with oxidants such as
oxygen. In the process, protons are conducted through the membrane to the cathode.
However, electrons are forced to travel in an external circuit, supplying power to a load.
On the cathode catalyst, oxygen molecules from air react with the hydrogen molecules
that have been reconstructed by the recombination of electrons that have traveled
through the external circuit with the protons. The chemical combination of oxygen and
hydrogen results in the formation of water molecules in the form of water vapor as the
waste product. In addition to electric power, the chemical reaction also produces a con-
siderable amount of heat energy that can be used for steam cogeneration and many other
industrial processes. In addition to hydrogen as a fuel, hydrocarbon fuels such as methanol
and many other chemical hydrates are used in various types of fuel cells that produce
electricity and different types of waste products.

CONSTRUCTION OF A LOW-TEMPERATURE POLYMER-
ELECTROLYTE-MEMBRANE FUEL CELL
Polymer-electrolyte-membrane fuel cells (PEMFCs) are constructed from bipolar or
dual electrode plates. These plates have an in-milled, or grooved, gas-channel structure
fabricated from conductive plastic that use carbon nanotubes to enhance conductivity.
A reactive layer constructed from porous carbon paper is adhered to a polymer mem-
brane to promote conductivity. Small quantities of platinum in the membrane are also
used as the catalytic component to promote electron and proton separation. Carbon
paper is used to separate the electrodes from the electrolyte.
Materials used in the construction of bipolar electrode plates include various met-
als, such as nickel, and carbon, which are coated with a catalyst such as platinum or
nano iron powder. Palladium electrolytes could also be used to construct ceramic arti-
ficial membranes. Gold wires embedded within the PEMFCs allow for electric current
collection.
A typical PEMFC produces from 0.6 to 0.7 V at a full-rated load. Power output varies
owing to voltage and current fluctuations. Power loss, or efficiency, in fuel cells is the
result of several factors, including activation loss, ohmic loss owing to voltage drop in
interconnecting conductive material, and mass-transport loss, which results from the
depletion of reactants at catalyst sites under high current demand and causes rapid
voltage drops.
To deliver the desired amount of energy, fuel cells, like conventional batteries, are
combined in series and parallel circuits. Series design yield higher voltages, and par-
allel designs allow for a larger amount of electric current circulation. Parallel and
series combinations of fuel cells are referred to as fuel cell stacking. Figure 7.1 is a
diagram of a PEMFC, and Fig. 7.2 illustrates PEMFC condensed water extraction
from an air channel.