Chemistry Reference
In-Depth Information
In the structure of a three-dimensional electrode,
current flows in both electrolyte and electrode
phases. The respective conductivities of the phases
determine the associated distribution of electrode
potential or reaction rate. An adverse effect of poor
electrolyte conductivity is that current will tend to
favour the electronic pathway provided by the elec-
trode material rather than the ionic path through the
solution. Thus, as shown in Fig. 19.5, the distribu-
tion of potential (and current) is not uniform and the
'penetration' of current into the structure is incom-
plete, thus the reaction may not fully use the avail-
able surface area. Thus generally the thickness of the
electrode in the direction of current flow is limited
to less than a few centimetres.
Three-dimensional electrodes can be divided into
two general categories, depending upon whether the
electrode material is stationary or in motion. The
former are either porous, where the electrode ma-
terial is in the form of a continuous matrix, or
packed, where the bed is constructed from 'loosely'
contacting material. The 'moving beds' cover a range
of designs where the particulate electrode itself is in
motion, either by mechanical means or hydrauli-
cally due to electrolyte flow: fluidised, circulating or
moving beds, slurries, rotating and tumbling beds,
E
f
M
f
M
-
f
S
is too small for
electron transfer
f
S
x/h
0
1
0
X
X
THREE DIMENSIONAL
CATHODE
Separator
Feeder
Effective
bed depth
Fig. 19.5
Schematic diagram of current
distribution in three-dimensional
electrodes.
0
x/h
1
