ELECTROCHEMICAL SYNTHESIS:FEATURES OF ELECTROCHEMICAL SYNTHESIS

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Chapter - 11

ELECTROCHEMICAL SYNTHESIS

M Helen

INTRODUCTION

Electrochemistry is a branch of science which deals with electrical energy and chemical

change. Spontaneous chemical reactions liberate electrons and they are taken place in

Galvanic or Voltaic Cells. These cells are exploited in batteries and fuel cells to produce

electric power. On the other hand Electrolytic Cells are nonspontaneous where electrical

energy is required to carry out chemical transformations. For an example in the process

called water electrolysis electrical energy is supplied to split water in producing hydrogen

and oxygen. Electrolysis is exploited in electroplating, chlorine gas production and in

refining metals.

Electrochemical synthesis is the production of chemical products or

materials using electricity as the driving or controlling factor. Electrochemical synthesis

is achieved by passing an electric current between two electrodes separated by an

electrolyte. That is, the synthesis takes place at the electrode-electrolyte interface. This

method has significant potential for improving availability of specialty products and for

improving environmental compatibility in several industrial sectors by destroying or

converting unwanted byproducts into useful products. Much progress has been made in

the last few decades in advancing the basic understanding and industrial applications of

electrochemical processes, but many aspects of electrochemical synthesis are still

inadequately understood or explored.

FEATURES OF ELECTROCHEMICAL SYNTHESIS

The features that distinguish electrosynthesis from other synthetic methods are:

The experiments are simple to perform and the instruments required are

inexpensive and readily available.

Electrochemical synthesis takes place at the electrode-electrolyte interface which

has a very high potential gradient of 105 V cm-1.

Under these conditions, the

reactions often lead to products which cannot be obtained in a conventional

chemical synthesis.

An electrochemical synthesis is a redox reaction. By fine-tuning the applied cell

potential, the oxidizing or reducing power can be continuously varied. This

11.2

Electrochemical Synthesis

possibility of continuous variation is not available in conventional chemical

synthesis.

The product is normally deposited on the electrode in the form of a thin film or a

coating.

The film composition can be controlled by varying the bath composition.

Electrochemical synthesis is a low-temperature process limited by the boiling

point of the electrolyte.

Reaction taking place can be controlled kinetically by controlling the current

passed through the cell, while it can be thermodynamically controlled by choosing

the applied potential.

In summary, electrochemical synthesis is a `green' route to produce to fabricate high-

purity materials without any additives.

ELECTROCHEMICAL SYNTHESIS: DESIGNING

Any electrochemical reaction depends on the proper choice of a number of reaction

parameters such as:

(1) Choice of an electrode

(2) Choice of an electrolyte

(3) Choice of temperature, pH, concentration, and composition of the electrolyte

solution

(4) Choice of the cell design - divided or undivided

(5) Mode of electrolysis - potentiostatic or galvanostatic (constant potential or

constant current)

In a typical electrosynthesis, the reactant, which is dissolved in the electrolyte is

deposited as a solid product.

When a metallic salt is dissolved in water it dissociates to

form positively charged ions. The solution that contains these charged ions is referred to

as an electrolyte or a plating solution. By passing electric current through this electrolyte,

one can reduce the metal ions to form solid metal. This process is referred to

electroplating or electrochemical deposition. In Fig. 11.1 electroplating is explained by

taking silver as the anode, fork as the cathode (fork to be plated with silver) and aqueous

solution of silver nitrate as an electrolyte. Both anode and cathode are connected to the

external battery. When the external power is on silver metal at the anode is oxidized to

Synthetic Strategies in Chemistry

11.3

silver ions and moves towards cathode. At the cathode silver ions is reduced to silver and

deposits on the fork. This results in thin covering of silver on the cathode (fork).

Battery

Ag+

Anode

Fork Cathode

aq.AgNO3

Fig. 11.1 Representation of silver electroplating

Two parameters determine the course of the reaction i) the deposition current and (ii) the

cell potential. Of the two, any one of them can be controlled as a function of time during

the reaction.

In a galvanostatic synthesis (Fig. 11.2), a constant current is applied through the

electrolytic cell leading to deposits with good adhesion and a controlled morphology.

However the cell potential drifts as the activity (concentration) of the reactant is

decreased.

The drift in the cell potential may lead to a multiplicity of products.

A potentiostatic synthesis is carried out with a three-electrode electrolytic cell (Fig.

11.3). The synthesis is carried out by polarizing the electrode to a desired potential with

respect to a reference electrode. The cell current usually decays rapidly as the reaction

proceeds, both due to low rates of diffusion of the reactant molecules from the bulk to the

electrode surface as well as due to decrease in the activity of the reactant. The reaction

yields a pure single-phase product selected for by the applied potential.

11.4

Electrochemical Synthesis

W.E

C.E

Fig. 11.2. Galvanostatic Synthesis,

G, galvanostat; V, voltmeter; WE, working electrode; CE, counter electrode;1,

electrochemical cell; 2, electrolyte; 3, Lugin capillary

W.E

C.E

R.E

Fig. 11.3. P, potentiostat; R, recorder; RE, reference electrode; WE, working electrode;

CE, counter electrode

Synthetic Strategies in Chemistry

11.5

VARIOUS TECHNIQUES FOR ELECTROCHEMICAL SYNTHESIS

Various techniques are employed in electrosynthesis. A list a few of them and the nature

of products obtained from each are given in Table 11.1.

Table 11.1. Summary of the Electrosynthetic Techniques

Technique

Product

Application

Anodic oxidation

Coatings/films/powders/

Synthesis of compounds with

conducting polymers

high oxidation state, corrosion

control, electrochromism

Cathodic reduction

Coatings/films/powders

Synthesis

electrode

materials for energy systems,

fabrication

hydroxide

films/coatings

Electrolysis

Single crystals, pure metals Crystal growth at moderate

of fused salts, water

and gases

temperature,

pure

gas

production

Electromigration

Polycrystalline powders/

Electrode

materials

for

of reactant species

single crystals/carbon

batteries and electrochromism

Layer-by-layer

Synthesis of composites/solid

Alternate

films/coatings

solutions

voltage/current

synthesis

Electrospraying

Micro or nanoparticles

Biomedical applications

Electrospinning

Micro or nanofibres

Drug delivery, tissue

engineering, sensors

During anodic oxidation a metal ion in a lower oxidation state or a monomer like pyrrole

or aniline is oxidized to a higher oxidation state or polymer anodically.

The anodic

oxidation technique is especially suited for the synthesis of compounds with metal ions in

unusual high oxidation states. In Table 11.2, important anodic syntheses are listed.

Table 11.2. Oxides and Polymers Synthesized by Anodic Oxidation

Compound

Application

Al2O3

Template, molecular filters

Electrochromic devices, lithium ion

CoO2.nH2O

batteries, supercapacitors, and the

protection film of cathodes in molten

carbonate fuel cells

FeO and MnO2

Electrode material

Ta2O5

Protective coating for chemical equipment,

11.6

Electrochemical Synthesis

electronic and sensor devices

Alkaline water electrolysis

NiO(OH), Co2O3

Battery, Organic degradation

PbO2

Magnetic devices, electrode material

Fe3-xLixO4

WO3 with Co, Cr, Fe, Mo, Ni, Ru, and Zn

Electrochromic devices

RuO2

Supercapacitor

Photocatalyst, humidity sensor

TiO2

V2O5 nanofibers

Battery material, active materials in

electrochromic and chemochromic devices

Polypyrrole, polyaniline, polythiophenes,

Sensors, Electroluminescence, Protective

polyacetylenes, polyindole

coating, FET, Organic semiconductors,

batterries

In cathodic reduction electric current is passed through a metal salt solution and hence the

metal is deposited at the cathode.

This principle is widely used to obtain metal coatings.

But depending upon the deposition potential, choice of the anion and the pH of the

solution various other reactions take place at the cathode. Switzer in 1987 introduced this

technique for the first time as a synthetic route to obtain oriented ceramic films as well as

Polycrystalline CeO2 powder was synthesized from a cerous

polycrystalline powders.

nitrate solution. Various oxide materials are synthesised by using electrogeneration of

base by cathodic reduction and their typical applications are listed in Table 11.3.

During electrolysis of fused salts, a low-melting salt containing the transition metal

oxide, is melted and electrolyzed at elevated temperatures using an inert Pt electrode or a

reactive metal electrode such as Fe, Co, or Ni depending on the desired product.

During

water electrolysis pure oxygen and hydrogen is collected at the anode and cathode

respectively.

Synthesis by electromigration, is based on kinetic control over the reaction by

electrochemistry. This technique involves intercalation or deintercalation of a guest ion

in a host lattice by applying an electric potential between the electrodes.

During pulsed electrolysis, the working electrode is alternately polarized anodically for a

length of time, t1(called the on-time) and then cathodically for time, t2 (called the off-

time).

Voltage as well as current pulses were used to obtain oxide films in the Pb-Tl

Synthetic Strategies in Chemistry

11.7

system using a stainless steel electrode and a mixed Pb-(II)-Tl(I) bath.

At low current

densities, the films were Tl-rich while at high current densities the films were Pb-rich.

Cathodic deposition of the precursor film for the YBaCuO system has been carried out

from a mixed metal nitrate bath containing KCN as well as a complexing agent.

The

superconductive films obtained from a cyanide bath show a Tc ~ 92 K which is greater

than all other reported values for this class of superconductors, obtained by

electrochemical techniques.

High intensity pulsed electric fields is an interesting

alternative to traditional techniques like thermal pasteurization in preservation of liquid

foods such as fruit juices or milk.

Conventional preservation methods such as heat

treatment often fail to produce microbiologically stable food at the desired quality level.

High intensity pulsed electric fields processing can deliver safe and shelf-stable products

with high nutritional value.

Pulsed electrodeposition is an effective method to prepare

nano materials of different morphologies.

Table 3. Oxides Synthesized by Electrogeneration of Base by Cathodic Reduction

(reproduced from [1])

Compound

Application

Gas sensor, fuel cells

CeO2

La1-xMxCrO3 (M = Ca, Sr or Ba)

Electronic conductor

ZrO2

Ionic conductor

Dielectric components

BaTiO3

LaFeO3

Oxide coating

Corrosion-protective coating

Al2O3, Cr2O3, Ln2Cr3O12.7H2O

Electrode material

PbO2

Mo1-xMxO3 (M = Co, Cr, Ni, W or Zn)

Optical light modulators

TiO2

Photocatalyst

Nd2CuO4

Superconductor

Optical and electronic devices

ZnO

Giant magnetoresistance (GMR)

LaMnO3

WO3

Electrochromic devices

11.8

Electrochemical Synthesis

MICRO- AND NANOPARTICLE PRODUCTION BY ELECTROSPRAYING

Electrospraying is a process of liquid atomisation by electrical forces. It is a dynamic

process of droplet generation and simultaneously charging by means of an electric field.

Droplets spraying out of a capillary nozzle, produced by electrospraying are highly

charged. Charged droplets are self dispersing in space, resulting in the absence of droplet

coagulation. The deposition efficiency of a charged spray on an object is higher than for

an un-charged spray. This feature can be advantageous in thin-film formation or in

surface coating. Generation of the droplet and its size can be controlled by controlling

the flow rate of the liquid and the voltage at the nozzle. Nearly monodisperse size

distribution is achievable.

The size of the electrospray droplets can range from

micrometers to nanometers.

Fig. 11.4. Schematic for production of particles of uniform size by electrospraying

(reproduced from [2])

Electrospraying technique is a single-step, low-energy, and low-cost material processing

technology, which can deliver products of unique properties.

Electrospraying is

exploited in many industrial processes such as painting, fine powder production, or

micro- and nanothin film deposition. It is also employed in microfluidic devices and

nanotechnology.

Spraying solutions or suspensions allows production of particles,

Synthetic Strategies in Chemistry

11.9

ranging from micro to nanometer size.

Production of particles of uniform size can be

accomplished by using pulsed or ac superimposed on dc bias voltage for liquid jet

excitation.

By tuning the frequency of ac voltage, uniform size of droplets are formed

by disintegrating the liquid jet. Various parameters can control the droplet size they are

the bias and ac voltage magnitudes, ac voltage frequency, and volume flow rate of the

liquid.

Uniform droplets can be achieved when ac and dc voltages are adjusted such

that the droplets are formed and detached at the voltage peaks.

A schematic diagram of

a system for harmonic spraying is shown in Fig. 11.4.

Electrospraying is exploited for the generation of micro/nanospheres for biomedical

applications since the process has an advantage of not making use of any external

dispersion/emulsion phase which often involves ingredients that are undesirable for

biomedical

applications.

Chitosan

micro/nanospheres

were

synthesized

electrospraying with acetic acid solution is exploited for drug delivery applications. This

technique is also used to prepare polycaprolactone (PCL) polymer particles with a

different microstructure by the evaporation of solvents during the electrospraying

process.

MICRO- AND NANOFIBRES PRODUCTION BY ELECTROSPINNING

Electrospinning is a process in which a high voltage electric field is applied to a melt or

polymer solution in order to attain charge repulsion on the liquid surface. This overcomes

surface tension there by a thin liquid jet is ejected. Narrow jet diameter is attained due to

the electrostatic repulsion caused between the charges on the liquid surface and the

collector that has a different electric field. Solid or polymer fibres ranging from 10 m to

10 nm is attainable. As shown in Fig. 9.5, a typical electrospinning setup consists of a

syringe with a capillary nozzle through which the liquid to be electrospun is forced; a

high voltage source with positive or negative polarity to charge the liquid jet and a

ground collector.

Electrospun textiles are exploited in preparing filters, semi-permeable membranes, as

scaffolding for tissue engineering and in drug delivery. Electrospinning has flexibility in

selecting materials for drug delivery applications.

Either biodegradable or non-

degradable materials can be used to control whether drug release occurs via diffusion

alone or diffusion and scaffold degradation. Due to the flexibility in material selection a

11.10

Electrochemical Synthesis

number of drugs can be delivered including: antibiotics, anticancer drugs, proteins, and

DNA.

Syringe

Sample

Voltage generator

(

Non-woven fiber

Collector

Ground

Fig. 11.5. Schematic of an electrospinning system

CONCLUSION

Electrochemical techniques such as anodic oxidation, cathodic reduction, alternating

current pulsing, electrospraying and electrospinning provide simple, cost effective

alternative routes for the production of micro or nanomaterials, thin films, coating,

composites having unique properties and applications. It is hoped that the ease and

versatility of this technique will find will find it a permanent place in synthetic chemistry.

REFERENCES

1. G. H. A. Therese and P. V. Kamath, Chem. Mater., 12 (2000) 1195.

2. A. Jaworek, Powder Technol., 176 (2007) 18.

3. A. Jaworek, A.T. Sobczyk J. Electrosta., 66 (2008) 197.

4. N. Arya, S. Chakraborty, N. Dube, D.S Katti, J Biomed Mater Res B Appl

Biomater., 2008 (In Press)

5. S. Tan, X. Feng, B. Zhao, Y. Zou, X. Huang, Mater. Lett., 62 (2008) 2419.

6. T. J. Sill, H. A. von Recum, Biomaterials, 29 (2008) 1989.

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