channel electrode presentation

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Channel Electrodes – A Review Jonathan A. Cooper and Richard G. Compton*

Physical and Theoretical Chemistry Laboratory, Oxford University,

South Parks Road, Oxford, OX1 3QZ, UK

Submitted by Ganesh Parajuli

Roll No: 31

M. Sc. Second Year

Central Department of Chemistry

Tribhuvan UniversityKirtipur, Kathmandu



Contents:

Abstract

Introduction

ExperimentalTheory

Applications

References

Acknowledgement



Abstract:

Recent progress in the development of channel electrode

voltammetry is reviewed. Both experimental and fundamental

aspects are addressed, and the power of the approach is illustrated

with a variety of applications.



Introduction:

A channel electrode consists of an electrode embedded smoothly inone wall of a rectangular duct through which electrolyte flows. Such

electrodes offer many advantages in diverse application.

First, in analytical procedures where their flow-through nature

facilitates continuous monitoring. For e.g. Chromatographic

Separation



Second their well-defined hydrodynamics permits rigorous

mechanistic investigations of electrode processes via voltammetric

methods.

Third ,they can readily be used for spectroelectrochemistry with little

modification.

Fourth, the possibility of irradiating the electrode surface through the

solution permits photoelectrochemical studies to be readily

undertaken in the channel electrode.



Merits of channel electrodes

Channel electrodes offer several clear merits as compared to

alternative possible electrode systems.

The rate of mass transport is controllable over a very wide range.For example volume flow rates over three orders of magnitude from

10 –4 to over 10 –1 cm3s –1 are readily attainable. the mass-transport

coefficient can be controlled by altering the cell depth, h, and in

particular the electrode length, x e, comparison the range of masstransport rates attainable at alternative hydrodynamic electrodes

such as rotating disks is much more limited.



Under laminar flow conditions the flow is well defined and calculable

so that electrode processes can be accurately simulated so as to

permit discrimination between candidate Mechanisms.This permitsthe easy interpretation of electrochemical data in terms of a variety of

possible mechanisms.

The channel electrode is immediately compatible withspectroelectrochemical methods, permitting the ready detection of

intermediates and products of electrolytic reactions.

The cells are easy to construct for a wide diversity of electrode

materials.



Experimental:The practical design of many channel flow cells, as shown in

Figure1,

1



It consists of first a base-plate. The base-plate has outlet tubes for

connecting (normally) to silicone or other tubing, through which theelectrolyte solution flows, usually either under gravity or via pumping,

but it can also be driven by pressurized gas.

The base plate is most commonly made of an inert material

such as Teflon, but it can be made of silica for spectroelectrochemicalapplications. Second there is a top plate of similar dimensions.

The top plate can also be silica, as this not only allows for

photochemistry to be carried out by shining light onto the surface of

the electrode.



The electrode(s) are embedded either into the base-plate or

attached to the cover-plate where it can be connected to a lead,either design is Practicable. The electrode itself is situated in the

center of the cell, with a slight gap of at least ca. 1mm between it

and the edge of the cell to negate any ‘edge-effects’ and ensure a

constant velocity across the width (w) of the cell.The reference electrode is not usually situated directly in the

flowing solution, but rather in a compartment with electrolytic

connection to the main flow, upstream of the working electrode. The

counter electrode is situated downstream, to avoid contamination of the working electrode by any reaction products formed.



TheoryThe channel flow cell lends itself particularly well to theoretical

modeling since there is a precisely defined, parabolic hydrodynamicflow regime and the dimensions of the cell are accurately

determinable. The convention is that the direction of flow is

designated as the x-direction and the y-direction is normal to the

electrode (see Fig. 1).



Appropriate dimensions are easily and practically designed so that

certain conditions (see below) which allow useful simplifications to

be made with no significant loss of accuracy. These are introduced

later. We start the theoretical discussion with macroelectrodes. Thegeneral equation describing diffusion and convection for a species A

at an electrode of any dimensions in a channel flow cell is as follows

where a is the concentration of A, D is the diffusion coefficient, x, y

and z are the coordinates as defined above and in Figure 1 and v x ,

v y and v z are the solution velocity profiles in those directions.

The equation can be simplified in a number of ways.

First at steady state time dependence is removed.

Second in case of macroelectrode axial diffusion is neglected.



It can be seen that at faster flow rate, the diffusion layer is smaller.

Last, the Le´veˆque approximation may be made which

approximates the parabolic flow to a linear one near the electrode

surface, where the concentration changes are taking place. For a

parabolic flow,

where v 0 is the velocity at the centre of the channel, and h is the half-

height of the cell. The quadratic y is removed making the flow linear.

Solution of the Mass-Transport Equation

The convective-diffusion equation can now be solved analyticallyfor the case of a transport-limited oxidation/reduction with

no coupled kinetics.



The result is the Levich equation

where, in addition to the quantities defined earlier, n is the number

of electrons transferred, F the Faraday constant, abulk the bulk

concentration of species A, v f the solution volume flow rate, and x ethe length of the electrode

The reaction scheme for this mechanism in case of an electro-

reduction is given below



We can define an effective number of electrons transferred, N eff as follows

Under steady state neglecting axial diffusion, and making Le´veˆque

approximation the equation tends to

Where,



the response of a channel electrode of arbitrary geometry at any flow

rate by a working curve which plots theoretical values of N eff as a

function of the normalized parameter K norm. Since b and c depends

only on χ , ξ and K norm. the general DISP process

If the second step [k(1)] in the above scheme is rate determining thenit is a DISP1 process, whilst if the fourth [k(2)] is rate determining

then it is a DISP2 process.



First the limiting current is measured, and from this N eff is determined

by comparison with the current expected for a simple one-electron

process. The working curves then gives the corresponding values of

K norm for candidate mechanisms, which are assessed by plotting

K norm against the flow rate raised to an appropriate power, depending

on the precise nature of the expression for K norm. For example, for a

homogeneous ECE or DISP 1 process, K norm is plotted against vf

–2/3

. A direct dependence consistent with Equation 20 is supportive of the

proposed mechanism. If the plot is not a straight line then alternative

mechanisms need to be explored. This procedure is repeated until a

‘fit’ of experimental and simulated data is obtained and the kineticsand mechanism of the system are ascertained. The rate constant can

be determined from the slope of the relevant K norm /flow rate plot.



Some Applications of the Channel Flow Cell

Voltammetric WaveshapesIn the above the use of voltammetric half-wave potentials to give

kinetic information was outlined for the specific case of an ECreaction. In other cases the full voltammetric wave shape contains

additional useful information about coupled homogeneous kinetics.



Diffusion Coefficients of Electrogenerated Species

Diffusion coefficients of electroactive species can be readilymeasured in a number of ways voltammetrically, including via the

channel flow cell, but measuring the diffusion coefficients of

electrochemically generated species, is more difficult. A modified

channel flow cell has recently been developed for this purpose. Thecell is designed to fit inside an ordinary UV/vis spectrometer, and it is

possible to set up a flow system to incorporate the cell. The

experiment involves a potential step from a potential where no

current flows to one where the limiting current would flow, and themonitoring of an absorbance in the spectrum of the electrogenerated

product as a function of time, starting at the time when the step is

made.

S



SpectroelectrochemistryChannel flow cells can easily be made from optically transparent

materials, often silica, to facilitate infra-red or UV/vis spectrometry.

One of the simplest applications is to measure the UV/vis spectrumof a radical species.

Photoelectrochemistry As described above, channel flow cells can be constructed to allow

irradiation of the electrode area with UV or visible light. The addition

of light to excite species involved in electrochemical processes can

often promote further reactions, including electron transfer, bonddissociation, possibly followed by further reduction/ oxidation,

disproportionation, or simply enhanced reactivity. Entirely new

reaction pathways can be opened up as a result, and novel

intermediates and products generated and characterized.



Initial analysis in terms of a DISP2 scheme was unsuccessful, due to

the inverse dependence of K norm on the concentration of 4-

chlorobiphenyl, as determined from plots of Knorm against (vf) –2/3 as

discussed generally above. A pure DISP2 scheme would be expectedto have a linear dependence on the concentration. Detailed

numerical simulation led to the conclusion that if all the steps in the

above apply, and that k5[A]>>k7 and k4[A]>>k6, then theory and

experiment matched up well.



Acknowledgement:

I am very grateful to respected Asst. Prof. Dr. Amar Prasad

Yadav, CDC, for providing me the required articles and for giving me

valuable suggestions to complete this work.I am thankful to the Central Department of Chemistry for

giving me this opportunity to attend in the seminar.

I would also like to acknowledge my dearest friends Mr. Bimal

Koirala and Mr. Sanjip Kumar Sapkota for their best efforts in editingand designing this paper.

f



References

[1] A.M. Waller, R.G. Compton, Comprehensive Chemical Kinetics

1989,29, 297.

[2] R.A.W. Dryfe, R.G. Compton, Prog. Reaction Kinetics 1995, 20,

245.

[3] R.G. Compton, J.C. Ball, J.A. Cooper, J.Electroanal. Chem. in

press.

[4] S.G. Weber, J. Electroanal. Chem. 1987, 222, 117.

[5] P.R. Unwin, J. Electroanal. Chem. 1991, 297, 103.[6] K. Tokuda, J. Electroanal. Chem. 1988, 84, 2155.

[7] R.G. Compton, P.R. Unwin, J. Electroanal. Chem. 1986, 206, 57.

[8] K. Aoki, K. Tokuda, H. Matsuda, J. Electroanal. Chem. 1986, 209,

247.



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