Electrodes for Industrial Applications

Electrodes for Industrial Applications
HELMUT VOGT, Beuth University of Applied Technology, Berlin, Germany
SUBRAMANYAN VASUDEVAN, CSIR-Central Electrochemical Research Institute,
Karaikudi, India
1.
2.
3.
Electric Circuiting. . . . . . . . . . . . . . . .
Electrode Qualities . . . . . . . . . . . . . . .
Electrode Materials. . . . . . . . . . . . . . .
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4.
Electrode Wear. . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . .
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1. Electric Circuiting
2. Electrode Qualities
Electrodes are electrically connected unipolar
(monopolar) or bipolar. Such circuiting is generally independent of the particular electrode
shape and the application of a diaphragm or a
membrane separating the anodic compartment
from the cathodic one.
In unipolar circuits of electrodes, the total
electrode is either anode or cathode (Fig. 1A).
The potential of the electrochemical reactors is
that of one cell. The current in the connecting
bus bars to all reactors within the plant are the
sum of the current of each cell and requires
large cross-sectional area of the bars.
In bipolar circuits, the reactor contains
several cells electrically interconnected in
series (Fig. 1B). Each electrode (except for
the outer ones) operates on one side as anode,
on the other side as cathode. The current of
the total reactor is the current of each cell. The
total potential of each reactor is the sum
of the cell potentials (with no regard to possible deviations in ohmic drop). The current in
the bus bars is that of each cell. The potential
at the rectifier attains large values and is
limited by its reverse voltage. A substantial
shortcoming of cells with bipolar electrodes
is the vagrant current in connecting pipes
between the cells of different potential. This
current circumvents cells and does not contribute to an electrochemical reaction. Circuiting several reactors in one plant permits
interconnecting a group of reactors in series,
and another group in parallel to manage the
rectifier blocking voltage.
Certain requisites are applicable to each electrode material as specified below. Various characteristics must be balanced with respect to
smooth process operation, moderate maintenance expenditure and general economic
requirements. Selecting a material meeting
all desired characteristics is often problematic.
In industrial field, the paramount parameter to
appraise electrode performance is the overall
cost per unit product (see Section 3). The
development of electrocatalysis has helped to
define the main requisites of electrodes materials for industrial applications.
# 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
10.1002/14356007
High electrical conductivity. The electrode material should have a high electric
conductivity to keep the ohmic potential
drop within the electrodes at tolerable
values. This requisite is extraordinarily
important for reactors with unipolar electrodes where the current paths through the
electrode are often long, and current density is high.
Efficient electrocatalytic activity of the
electrode material for any particular
reaction generally indicated by a low overpotential plays an important role. The
overpotential depends on the current density and on the electrode material and
often necessitates a compromise with capital costs. Any potential drift with time
towards larger values should be prevented.
Sufficient selectivity. Electrodes must be
selected to enable the desired reaction in
2
Electrodes for Industrial Applications
Figure 1. Electric circuiting of electrodes. A) Unipolar
electrodes; B) Bipolar electrodes
such a way that any competing side
reactions are most widely suppressed.
Long-term mechanical stability is complied with by most electrode materials,
above all by metals. However, some materials such as graphite tend to form a brittle
structure during operation possibly leading to ruptures, under unfavorable conditions to short-circuits.
Corrosion resistance. Competing reactions
may transform the electrode material into
substances solved in the electrolyte liquid
or transferred into the gaseous phase.
The wear of material may result in a loss
of the original electrode shape. A characteristic example is the oxidation of graphite
anodes during operation where apart from
the wanted product traces of oxygen are
generated reacting with the electrode material to form CO2.
Dimensional stability. Inevitable wear of
electrode material may be small enough
to keep the original shape. The so-called
dimensionally stable electrodes compose
of electroactive coating material on an
inert substrate. An adjustment of electrodes becomes dispensable.
Long-term chemical stability. An essential
requisite is low sensitivity to poisoning by
impurities contained in the electrolyte liquid or resulting during operation. They
may form deposits on the electrode surface hampering the desired reaction, i.e.
leading to increased overpotential. The
same effect may result during current
shut down where unwanted reactions
may proceed. In such cases, a small protective current must be applied to the
reactor at any production interruption.
Availability and low investment cost. Capital expenditure for the initial or repeated
investment of electrodes must meet economic demands. Electrode materials must
be available to a sufficient extent and
available at economically reasonable
price.
Health safety. During storage and operation, the electrode must not release substances harmful for health. A striking
example is mercury, a cathode material
previously used in chlor-akali electrolysis
for long time until health implications
entered the focus of interest.
Unproblematic flow of electrolyte liquid.
To counteract the depletion of electrolyte
in course of operation and to keep the
concentration of electrolyte at a sufficiently high level, a sufficient flow rate
of liquid must be ensured. The shape of
the electrodes contributes essentially to
the flow conditions. Operation and shape
of electrodes evolving gases require particular attention.
3. Electrode Materials
A great variety of electrode materials is
employed in industry. A survey of the most
important materials and applications is given
here. A valuable overview on various electrode
materials and applications including extraordinary ones can be found in [1].
Carbon and graphite have been the main
anodic material in industrial electrochemical
processes for a long time. Graphite is more
widely used as anode material in electrolysis
than carbon, owing to its fair chemical resistance, good conductivity, considerable mechanical strength, satisfactory surface-to-volume
Electrodes for Industrial Applications
ratio, and is rather cheap. But it also undergoes
chemical attack by active oxygen and highly
oxidizing products. Due to the disintegration,
the interelectrode gap continues to increase
with time and this causes a penalty in the total
energy consumption and/or requires discontinuous adjustment of the interelectrode
distance. Moreover, there is an accumulation
of sludge in the cell and disposal becomes a
problem. Sometimes, the disintegrated graphite
sludge causes discoloration of the product or
may lead to short-circuiting. Regarding the
electrocatalytic activity of graphite, it exhibits
higher chlorine overpotential even to platinum
at high current densities. Graphite also exhibits
low oxygen overpotential so that simultaneous
oxygen evolution can occur [2–4].
Platinum is an excellent anode material due
to its extraordinarily small overpotentials. Platinum is stable for most of the reactions. In
compact form it is preferably applied in laboratory experiments. Industrial usage is limited
due to its large capital costs. However, platinum
is a valuable coating material on an inert support material. In the production of perchlorate,
the loss is 5–7 mg/kg of the product. Platinum
coated on titanium used in Cl2 production
lowers wear to 0.5g mg/kg [5].
Magnetite was introduced into electrochemical processes some 90 years ago and has been
used as anode material in aqueous solutions but
exhibits poor acid resistance. Industrial application has suffered from poor electric conductivity (20 times poorer than graphite) and
inherent brittleness. The industrial preparation
results in forms that are poorly compatible with
engineering requisites of modern electrochemical processes [6, 7].
Coatings. Anodes made of thermally prepared oxides of noble metals, so-called as
dimensionally stable anodes (DSA), had better
activities than those of the parent metals, and
triggered a technological revolution in the history of electrode materials. Thus RuO2 with
TiO2 and/or IrO2 became major constituents
of many of the catalytic compositions. Even
though the individual oxides possess extraordinary catalytic activity towards O2 and Cl2 evolution reactions, the loss of activity within time
and corrosion induced the applications of their
mixed oxides with valve metal and nonnoble
metal oxides. Generally, these mixed oxide
3
coating compositions are (i) a stabilizer,
selected from the valve metal oxides, (ii) a
catalyst for the desired reaction, selected
from the noble metal group, (iii) a catalyst
diluent, i.e. dioxides of group 4 and 7 metals,
and (iv) a doping agent to enhance the selectivity and activity of the main catalyst and at the
same time retarding the undesirable reactions.
As examples, coating containing RuO2, TiO2
with IrO2 or SnO2 or PdO2 has been found to
yield longer service life in the production of
chlor-alkali, hypochlorite, chlorate, bromate,
iodate, chlorine substitute organic compounds,
eosin and in cathodic protection with impressed
current. The preparation of DSA is carried out
by taking chlorides of Ru, Ti, Ir, or Sn in an
organic vehicle (like amyl alcohol or isopropyl
alcohol) applying over the pretreated titanium
substrate and then thermally decomposing to
form the oxide. The advantages of these electrodes are: good chemical and mechanical stability,
catalytically highly active, lower Cl2 overpotential and consequently saving in energy, employment of higher current density, maintenance of
same interelectrode gap, purer Cl2, relative
resistance to mercury and increase of diaphragm
life and advent of membrane cell [8, 9].
Lead Dioxide (PbO2). Lead is an easily
available and cheap electrode material. Substitutes for the noble metal anodes for industrial
applications have led to the development of
lead dioxide anodes, especially in the form of
graphite or titanium substrate lead dioxide.
They are relatively stable from mechanical,
chemical, and electrochemical point of view.
The interesting aspect of a lead dioxide anode is
that the anodic oxygen evolution takes place at
more positive potential. Although lead dioxide
could be deposited anodically from different
baths, lead nitrate bath containing copper
nitrate (to prevent the deposition of lead at
the cathode) is preferred for getting an adherent
and smooth deposit. In order to get pore-free
deposit, different techniques (i) rotation,
(ii) fluidizing an inert material with flow of
electrolyte, (iii) ultrasonic, and (iv) addition of
surfactants, both ionic and nonionic have been
employed. These anodes allow the preparation
of light-weight electrodes of desired dimensions to handle large currents on a commercial
scale. However, lead dioxide is a poor catalyst
both for O2 and Cl2 evolution [10].
4
Electrodes for Industrial Applications
Low-carbon steel electrodes exhibit low
capital costs and low hydrogen overpotentials
in hydrogen formation. They are widely used as
cathodes in industrial reactors for production of
chlorine in membrane or diaphragm cells and in
production of chlorate. At any reactor shutdown, the electrodes must be kept at a sufficient
catholic potential to prevent corrosion.
Mercury is an extraordinary liquid electrode
material at all industrial operation temperatures. Its high overpotential in hydrogen
evolution allows decomposition of sodium in
chlor-alkali electrolysis with subsequent formation of an amalgam to be easily released from
the reactor. However, the large quantities of
mercury required for each reactor and the enormous capital costs together with environmental
and health problems have favored the competing diaphragm and membrane process rid of
mercury.
4. Electrode Wear
All electrode materials exhibit more or less
undesired wear. In some cases, wear in course
of industrial application is small enough to
consider the electrodes stable or insoluble, in
industrial use particularly referring to suitable
coatings. In other applications, wear is tolerable
or may be intended.
The dissolution of metal anodes is widely
employed in electroplating and inherent in
electroforming and electrorefining [11, 12] to
balance anode and cathode current efficiencies.
Cadmium is employed as soluble anode in
protective electroplating from either cyanide
bath or fluoroborate bath although in extraction
and recovery of the metal, sulfate and chloride
baths are employed. Copper anodes are used in
electroplating of copper from either acid sulfate
bath or alkaline cyanide bath.
In cathodic protection, the sacrificial anodes,
which are selected to have a minimal tendency
to passivity and mainly to protect the steel in
various environments, are limited to zinc, aluminum, and magnesium [12].
Aluminum, magnesium, or iron are preferred as soluble anodes in reactions where
electroflotation principle is adopted. Thus, in
the removal of inorganic contaminants from
water aluminum, magnesium, or its alloys as
anode are employed dissolving to form aluminum/magnesium hydroxide.
In synthesis of very pure chemicals, application of soluble anodes is attractive. Thus,
high purity potassium stannate, silver nitrate,
antimony trioxide, lead and tin fluoroborates,
aluminum hydroxychloride are manufactured
in a membrane cell by the anodic dissolution of
tin, copper, antimony, lead tin, silver, and aluminum, respectively.
The inevitable wear as stated above must be
distinguished from the intended wear as with
carbon in the Hall–Heroult electrolysis for production of aluminum. In this case, the anodic
generation of CO and CO2 provides continuous
consumption of the anodes and necessitates
their frequent replacement but provides a
decrease in cell potential. Anode consumption
is substantial and requires frequent replacement
of prebaked carbon anodes or self-baking
S€oderberg anodes but contributes to lower the
cell potential.
References
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Handbook, Springer Verlag, Berlin 2008.
2 M.M. Jaksic, J. Appl. Electrochem. 3 (1973) 219–225.
3 V.I. Eberil, L.M. Elina, Sov. Electrochem. 6 (1970) 758–761.
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577–587.
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