Chemistry Reference
In-Depth Information
ture of gas diffusion layers, electrocatalyst and mem-
brane. The flow field typically consists of narrow
channels machined into the plate material. In addi-
tion, the bipolar plate also may contain channels for
coolant to be used to control the cell stack tempera-
ture because heat is generated by the inherent inef-
ficiency of the electrochemical reactions. The bipolar
plate is required to be resistant to corrosion in the
fuel cell environment and thus typically graphite and
carbon composites are used. Graphite is, however,
expensive to machine, is brittle and porous and thus
has to be relatively thick. Therefore, alternative
materials such as compression-moulded graphite-
polymer resin mixtures and metals such as stainless
steel are used.
(or air) to produce electricity, water and waste heat,
as in a conventional fuel cell. However a 'regenera-
tive' fuel cell also performs the reverse of the cell
reaction, using electricity to form hydrogen and
oxygen from the dissociation of water by electroly-
sis. In principle, the 'closed' system of the regenera-
tive fuel cell has significant attraction because it
could enable the operation of the power system
without requiring a significant hydrogen infrastruc-
ture. A major concern about the regenerative fuel
cell is the cost of making the fuel cell reversible, as
well as achieving long-term operational performance
over many cycles.
An indication of the increasing importance that
electrochemistry plays in the energy sector is the
development of a large-scale energy storage facility
for load levelling, based on a redox fuel cell or
battery called
Regensys
[12]. The Regensys system is
envisaged for use at large-scale power generation
sites or at smaller scale close to the point of use. The
technology is based on two soluble redox couples
Br
-
/Br
3
-
and S
2-
/S
2
2-
in aqueous solution. The process
uses as the basis a bipolar stack of approximately 200
cells, with the charged and discharged electrolytes
stored external to the stacks. The electrodes are
carbon/polymer mixtures made by an extrusion
technique and are divided by cation-permeable
membranes. Mass transport is enhanced by the use
of turbulence promoters. The overall energy effi-
ciency of the system is claimed to be 70%.
Stack operating factors
To produce the final fuel cell system a range of
factors relating to auxillary equipment operating
conditions and control have to be considered. In any
fuel cell system due regard has to be made to the
source and quality of fuel used. Hydrogen therefore
has to be stored appropriately or produced from a
hydrocarbon-based fuel by partial oxidation or steam
reforming. Thus an important issue is the influence
of impurities in the fuel on cell and electrocatalyst
performance over extended periods of operation.
The hydrogen and air must be supplied (pumped)
under some pressure (depending on cell size and
application) to the cell stack, which involves the use
of compressors or air blowers and regulating valves.
The air must be free of contaminants and requires
filtration.
Cell stacks have to be maintained at specified oper-
ating temperatures, as well as the required humid-
ity, to ensure target power performance. Thus the
compressed air is cooled and cell stacks are cooled.
Consequently, energy is expended in supplying com-
pressed air, fuel and cooling medium and for the
electronic controllers for the system. This energy has
to be obtained from the fuel cell and reduces the effi-
ciency of the complete system. The overall operation
of a fuel cell system thus requires a detailed optimi-
sation of all factors to maximise power output while
minimising system cost.
5 Electrochemical Synthesis
Electrosynthesis has been long established as a
method of manufacturing a range of materials and
chemicals—gases, solids and liquids—and in many
cases is the only viable method (see Table 19.3). Elec-
trochemistry can play a strategic part in the minimi-
sation of waste and sustainability in synthesis in that
the oxidation or reduction is via electron exchange
and is not chemical. The benefits to be gained from
the application of electrochemistry in electrosynthe-
sis is due to its many inherent characteristics:
• Mild conditions of operation, e.g. low temperature
and pressure
• Improved selectivity and yield of existing reactions
• Availability of novel chemical transformations
to new molecules and new routes to known
molecules
• Reduction in the number of synthesis steps
• Improved management of potential pollutants
Regenerative fuel cells
A regenerative fuel cell, currently being developed
for utility applications, uses hydrogen and oxygen
