Volume 8 Organic Electrochemistry

8/11/2019 Volume 8 Organic Electrochemistry

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1

1

Methods to Investigate

Mechanisms of Electroorganic

Reactions

Bernd SpeiserInstitut f

..

ur Organische Chemie, Auf der Morgenstelle 18, T..

ubingen, Germany

1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31.1.1 Scope: Methods of Molecular Electrochemistry . . . . . . . . . . . . . . . 31.1.2 Historical Development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.2 Why and How to Investigate Mechanisms of Electroorganic Reactions 41.2.1 Steps of Electrode Reaction Mechanisms. . . . . . . . . . . . . . . . . . . 41.2.1.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.2.1.2 Transport. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

41.2.1.3 Electron Transfer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51.2.1.4 Chemical Kinetic Steps. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51.2.1.5 Adsorption. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61.2.2 Organic Electrode Reaction Mechanisms . . . . . . . . . . . . . . . . . . . 61.2.2.1 Electron Transfer Initiates Chemistry . . . . . . . . . . . . . . . . . . . . . 61.2.2.2 Nomenclature of Electrode Reaction Mechanisms. . . . . . . . . . . . . 61.2.3 Formal Description of Events at an Electrode . . . . . . . . . . . . . . . . 71.2.3.1 Current-Potential-Time Relationships . . . . . . . . . . . . . . . . . . . . . 71.2.3.2 Concentration Profiles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71.2.4 Methods of Mechanistic Electroorganic Chemistry . . . . . . . . . . . . 71.2.4.1 Classification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71.2.4.2 Controlled-Potential Techniques . . . . . . . . . . . . . . . . . . . . . . . . 71.2.4.3 Controlled-Current Techniques

. . . . . . . . . . . . . . . . . . . . . . . . .

111.2.4.4 Hydrodynamic Voltammetry . . . . . . . . . . . . . . . . . . . . . . . . . . . 121.2.4.5 Exhaustive Electrolysis Techniques . . . . . . . . . . . . . . . . . . . . . . . 13

1.3 How to Gain Access to Kinetics, Thermodynamics, and Mechanismsof Electroorganic Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

1.3.1 Qualitative and Quantitative Investigation of Electrode ReactionMechanisms. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

1.3.2 General Recommendations for Mechanistic Analysis. . . . . . . . . . . 14


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2 1 Methods to Investigate Mechanisms of Electroorganic Reactions

1.3.3 Some Mechanistic Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 151.3.3.1 Pure ET Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151.3.3.2 Follow-up Reactions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171.3.3.3 Preequilibria to ETs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181.3.3.4 Catalytic Reactions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

1.4 How to Gain Additional Information about Electroorganic ReactionMechanisms. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

1.4.1 Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191.4.2 Ultramicroelectrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

1.4.3 Electrogravimetry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

201.4.4 Spectroelectrochemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

1.5 Conclusion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21Acknowledgment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21


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3

1.1

Introduction

1.1.1

Scope: Methods of MolecularElectrochemistry

Reaction mechanisms divide the transfor-mations between organic molecules intoclasses that can be understood by well-defined concepts. Thus, for example, the

SN1 or SN2 nucleophilic substitutions areexamples of organic reaction mechanisms.Each mechanism is characterized by tran-sition states and intermediates that arepassed over while the reaction proceeds.It defines the kinetic, stereochemical, andproduct features of the reaction. Reactionmechanisms are thus extremely importantto optimize the respective conversion forconditions, selectivity, or yields of desiredproducts.

Reaction mechanisms are also definedfor electroorganic reactions, induced byor including an electron transfer at anelectrode. Knowledge of such electrodereaction mechanisms includes, prefer-ably but not exclusively, the potential at

which the reaction proceeds, the proofof intermediates, the electron stoichiom-

etry, the kinetics of the various reactionsteps, and the transport properties of

the species involved. Recently, the terms

molecular electrochemistry[1] or dynamicelectrochemistry[2] have been used for thatpart of electrochemistry that studies themechanistic events at or near an electrodeon a molecular level.

There are a large number of methods(often also calledelectroanalytical methods)for such studies of which only the mostimportant ones can be covered in thischapter. Moreover, technical details ofthe methods cannot be described, and

emphasis will be placed on their use inmechanistic electroorganic chemistry.

1.1.2

Historical Development

Although organic electrochemistry hadalready been established in the nineteenthcentury, only the 1960s saw the adventof detailed electroorganic mechanisticstudies.

Most of the techniques employed can be

traced back to polarography, which was al-ready in use in 1925, to determine theconcentrations of organic molecules [3].Technical developments in instrumenta-tion (potentiostats) [4], the use of nonaque-ous electrolytes [5], and the digital controlof experiments [6] led to the spread ofelectroanalytical techniques. For example,cyclic voltammograms are frequently androutinely used today to define the redox


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properties of newly synthesized organiccompounds similar to the use of NMRspectra for structural characterization.

Numerical simulation of the experi-ments [7] became increasingly availableduring the 1980s, and ultramicroelec-trodes [8] opened the way not only toever-faster timescales but also to finerlateral resolutionwhen characterizingelec-trode processes. Finally, combinationswith spectroscopic and mass-sensitive de-vices opened new ways to augment infor-mation available from molecular electro-chemical experiments.

This development contributes to a still-increasing body of knowledge about thefate of organic molecules upon oxidationand reduction.

1.2

Why and How to Investigate Mechanismsof Electroorganic Reactions

1.2.1

Steps of Electrode Reaction Mechanisms

1.2.1.1 General

As heterogeneous reactions at the inter-face electrodeelectrolyte, electrochemicalreactions are intrinsically more complexthan typical (thermal) chemical transfor-mations (Figure 1). We mostly neglect theexact structure of the interface in the fol-lowing description. Transport of the educt

(substrate) from the bulk of the electrolyteto the electrode plays an important, oftenrate-determining role. The electron trans-fer step occurs at the interface. The productof the redox reaction is transported backto the bulk. Purely chemical reactions mayprecede or follow these steps. Specific in-teractions of any species present in theelectrolyte with the electrode surface leadsto adsorption, which may considerably in-

fluence the overall process.

1.2.1.2 Transport

Three types of mass transport are impor-tant at an electrode:

1. Diffusion (along a concentration gradi-ent) is observed if the solution near theelectrode is depleted from a substrate ora product is accumulated. Diffusion ischaracterized by a diffusion coefficientD (typical value: 105 cm2/s) and ex-tends over a diffusion layer (thickness:) that develops from the electrode intothe electrolyte. At the outward bound-ary the concentrations approach theirbulk values.

2. Migration (in the electrical field be-tween the anode and the cathode)contributes to the movementof chargedspecies. In most practical experiments,however, the concentration of support-ing electrolyte ions is much higher(100 1000 : 1) than that of other ions.

P

d

E

Electrontransfer

Adsorption

Chemicalreactions

E E

P P

PElectrode

Transport

Transport

Diffusion layer Bulk

E

Fig. 1 Steps constituting atypical organic electrodereaction; E, E: educt, P, P:product; circles indicateadsorbed molecules.


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1.2 Why and How to Investigate Mechanisms of Electroorganic Reactions 5

Hence, migration of the latter is sup-pressed. On the other hand, migrationbecomes important at modified elec-trodes or in electrolytes of low ionconcentration [9].

3. Convection (of the electrolyte liquidphase as a whole) can be natural (dueto thermal effects or density gradients)or forced (principal mass transportmode in hydrodynamic techniques).

Still, however, close to the electrodesurface a diffusion layer develops.

If we neglect migration, experiments canbe performed underconditionsof minimalconvection, which are thus dominatedby diffusion. Since increases withtime t in such a case, nonstationaryconditions exist. On the other hand, ifconvection dominates in the electrolytebulk, = f(t), and we approachstationaryconditions, as far as diffusion is concerned.

1.2.1.3 Electron TransferThe electron transfer (ET) at the interfacebetween electrode and electrolyte is centralto an electrode reaction. Electrons passthrough the interface. Macroscopically weobserve a currenti .

The transfer of an electron to (reduc-tion) or from (oxidation) the substrate is anactivated process, characterized by a rateconstant ks, defined as the standard (orformal) potential E0, and the transfer coef-ficient. The three situations mentioned

below can be distinguished:

1. ET much faster than transport (trans-port control). Electrochemical equilib-rium is attained at the electrode surfaceat all times and defined by the electrodepotential E. The concentrations coxandcred of oxidized and reduced forms ofthe redox couple, respectively, followthe Nernst equation (1) (reversibleET)

cox

cred= exp

nF

RT(E E0)

(1)

(n = number of electrons transferred,F= Faraday constant, R= gas con-stant,T= temperature). The current isproportional to the amount of materialtransported to the electrode in a timeunit.

2. ET much slower than transport (ETcontrol). The current follows the But-ler Volmer equation (2)

i= i0

exp

nFRT

(E E0)

exp

(1 )nFRT

(E E0)

(2)

where i0 defines the exchange currentat E= E0 (irreversible ET). A physicalinterpretation of is related to the ETtransition state (see the comprehensivediscussionin ref. [10]). It is furthermore

expected that is potential dependentand important mechanistic conclusionsfollow [11, 12].

3. ET and transport have comparablerates. This mixed-control situation ischaracterized asquasi-reversible.

A given electrode reaction may corre-spond to any of these situations dependingon the experimental conditions, in particu-lar on the external control of mass transfer.

1.2.1.4 Chemical Kinetic StepsMost electrode reactions of interest to theorganic electrochemist involve chemicalreaction steps. These are often assumed tooccur in a homogeneous solution, that is,not at the electrode surface itself. They aredescribed by the usual chemical kineticequations, for example, first- or second-order reactions and may be reversible(chemical reversibility) or irreversible.


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Chemical steps may precede or followthe transport and ET processes. In theformer case, the electroactive species isformed in a preequilibrium. In the lattercase, we produce by ET some reactivespecies, which undergoes a (possiblycomplex) chemical transformation to amore stable product.

1.2.1.5 Adsorption

The involvement of specific attractive in-teractions of molecules with the electrodesurface (adsorption) makes the electrodeprocess even more complex. The inten-sity of such interactions ranges from weak(physisorption) to strong (chemical bondsformed between adsorbate and electrode).

For some common organic electrochem-ical reactions, for example, the Kolbeelectrolysis of carboxylates [13], the adsorp-tion of intermediates has been discussed.

1.2.2

Organic Electrode Reaction Mechanisms

1.2.2.1 Electron Transfer InitiatesChemistry

The majority of organic electrode reactionsis characterized by the generation of areactive intermediate at the electrode by ETand subsequent reactions typical for thatspecies. Thus, the oxidation or reductionstep initiates the follow-up chemistry tothe reaction products (doing chemistrywith electrodes [14]).

Species with electron deficiency (e.g.carbocations), unpaired electrons (e.g.radicals, radical ions), electron excess(e.g. carbanions), or those with unusualoxidation states (e.g. metal complexes withlow- or high-valent central atoms) areproduced at the electrode. Electrochemicalgeneration of such intermediates may beadvantageous because of the mild reactionconditions employed (room temperature,

strong acids or bases are not necessary)and/or the additionalselectivity introducedin controlled-potential experiments.

The reaction mechanisms of organicelectrode reactionsare thus composed of atleast one ET step at the electrode as well aspreceding and follow-up bond-breaking,bond-forming, or structural rearrange-ment steps. These chemical steps maybe concerted with the electron trans-fer [15, 16]. The instrumental techniquesdescribed in this chapter allow the in-vestigation of the course of the reactionaccompanying the overall electrolysis.

1.2.2.2 Nomenclature of ElectrodeReaction Mechanisms

In order to classify the various mech-anisms of organic electrode reactions,a specific nomenclature has been de-veloped [17]. It is often extended in aninformal way to accommodate particularreaction features, and one may find addi-

tional or deviant symbols.Usually, however, electron transfers

at the electrode are denoted by E,while chemical steps not involving theelectrode are denoted by C. The ETmay further be characterized as Er,Eqr, or Ei in the reversible, quasi-reversible, or irreversible case. It is usuallynot indicated how transport occurs. If theC-step is a dimerization, the symbol D iscommon, while an ET between two speciesin a (homogeneous) solution is denoted

SET (for solutionelectrontransfer) [18]or DISP (see, e.g. [19]).For more complex mechanisms, pic-

turesque names such as square, ladder,fence [18] or cubic schemes [20] have beenselected. In redox polymer films, addi-tional transport of counterions, solvation,and polymer reconfiguration are impor-tant and four-dimensional hyper-cubes areneeded to describe the reactions [21].


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C W

R

E

Electrolyte

Potentio/galvanostat

Computer,

Recorder

Functiongenerator

Fig. 2 Schematic representation of experimental set-up forcontrolled-potential experiments; W: working, C: counter, R:references electrodes.

potentiostat in a three-electrode arrange-ment (Figure 2). The current is passedthrough the working (W) and counter(C) electrodes, while E is measured withrespect to a currentless reference (R) elec-

trode. Often, a recording device and afunction generator complement the exper-imental setup.

We will assume a simple reversible one-

electron redox process Ae B in all

cases to introduce the techniques.An important property of the solution

to be investigated is therestor open-circuitpotentialER. This is the potential that theworking electrode develops in the solutionat equilibrium, that is, when no currentflows through the electrode. The value of

ER depends on the components of thesolution and the electrode itself.Chronoamperometry is a technique in

which a potential step is applied to theworking electrode in a quiet solution at t=0 (Figure 3). Initially (t 0, a potential isselected, whichdrivesthe desired electrodereaction. Often, but not necessarily (see,e.g. References [2325]) the latter is in the

transport (diffusion) limited region. Aftersome (pulse) time , E may be switchedback to ER or another appropriate value(double-step chronoamperometry).

Starting at ER guarantees that at t 1from the above procedures, but CV addi-tionally shows the relative thermodynam-ics and depending on the individual E0

values, various shapes of i/E curves are

obtained (Figure 8b). If the two E0

are suf-ficiently different (E0 >100 mV), twoseparated peak couples occur (dash-dottedline). On the other hand, ifE0 decreasesbelow100 mV, the voltammetric signalsmerge (dashed line).

Further, interesting cases are encoun-tered in invertedpotential [54] situations(solid line in Figure 8b, second ET thermo-dynamically easier than the first one), andfor dendrimers with a large number of

0.1 0.030

20

10

0

10

20

3040

0.1 0.2 0.3 0.5 0.60.4

Potential, E

[V]

0.7

Current,i

106

[A]

Fig. 9 Typical cyclicvoltammograms of an ECreaction system; rate offollow-up reaction increasesfrom short-dashed throughdotted, dash-dotted andlong-dashed to solid curve.


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1.3 How to Gain Access to Kinetics, Thermodynamics, and Mechanisms of Electroorganic Reactions 17

redox-active units, which undergo ET atapproximately the same potential [55].

1.3.3.2 Follow-up Reactions

Irreversible follow-up reactions (mostsimple case: EC mechanism) decreasethe concentration of the primary redoxproduct. This is again diagnosed in CV(Figure 9) and also in chronocoulome-try. Timescale variation in CV allows

to modulate the importance of the C-step: at fast v the chemical reaction willhave no influence on the curves, whileat slower v all product has reacted andthe reverse peak disappears. A govern-ing factor is k/a (k= rate constant ofC-step, a= nFv/RT). Thus, for a qual-itative interpretation, the peak currentratio in CV is evaluated as a func-tion of v (and E) in order to calcu-late k [49]. Also, Ep and ip depend onk/a [28].

Reversible follow-up reactions may justshift the entire voltammetric signals (fastequilibration) on the E axis, or lead to ef-fects approaching those of the irreversiblecase [28].

The most important are cases in whichthe product of the C-step is again electroac-tive [ECE mechanism, Reaction (12)]:

Ae B

k Ce D (12)

(for an oxidation; extension to reduction isobvious). In such cases, homogeneous ETs[disproportionation, Reaction (13)] havealso to be considered:

B+ CA+D (13)

where the equilibrium constant is relatedto theE 0 of the two heterogeneous ETs.

Several variants are discussed in theliterature [18, 56, 57]. Figure 10 shows

some cyclic voltammograms. The heightof the second peak depends on the rateof the C-step. In chronoamperometry, theformation of a redox-active product leadsto an increase in the apparent n during theexperiment (e.g. from n = 1 to n = 2). Aplot ofi vs. t1/2 switches from a straightline for n = 1atsmall tto the one for n = 2at larget.

If, for an oxidation step, the chemicalreaction of B leads to the oxidized form ofthe second redox couple B (and not thereduced one as in the earlier case) and asecond chemical transformation from A

leads back to A [reaction (14)], we arriveat a square scheme (Figure 11), whichforms the basis for many important redoxsystems [18, 58]. Again SET steps

A+ BA + B (14)

can be involved, resulting in rather un-usual voltammograms under certain con-ditions [18, 59].

Fig. 10 Typical cyclicvoltammograms of ECEreaction systems; rate of C-stepincreases from dash-dottedthrough dashed and dotted tosolid curve.

0.10.030

20

10

0

10

20

30

40

0.1 0.2 0.3 0.5 0.80.70.60.4

Potential, E

[V]

Current,i

106

[A]

0.9


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e

A

A

B

B

e Fig. 11 The square scheme

reaction mechanism.

1.3.3.3 Preequilibria to ETs

The square scheme discussed above al-ready includes a further common motifin electroorganic mechanisms: reactionAA forms a preequilibrium to bothETs in the scheme. The response of sucha system in CV depends particularly onthe equilibrium constantK= [A]/[A] andthe rate constantskAA and kAA. If thek are large (reaction at equilibrium), onlythat ET will occur, which is thermody-namically easier (smallerE 0). All materialconsumed by that ET will immediately bereplenished through the equilibrium re-action. On the other hand, if the k aresmall, two peaks will be observed withtheir relative heights proportional to theequilibrium concentrations of A and A,thus allowing determination ofK .

Both partners of the preequilibrium arenot always electroactive (CE mechanism).

Kinetics and thermodynamics will influ-ence the exact appearance of the concen-tration profiles. Figure 12 shows some CEvoltammograms. In particular, chronopo-tentiometry was used for analyses[60, 61],since for highi

i1/2 =

2 nFAc0

D

K

1+K (15)

(with total concentration c0

= c0

A +c0A ) [62]. Furthermore, hydrodynamictechniques were also employed [63, 64].

1.3.3.4 Catalytic Reactions

In some reactions the product of anET at the electrode reacts back to the

starting compound: Ae B

k A.This mechanistic motif is found in me-diated electrode reactions [65] or in sen-sor applications [66]. The reformation of

0.1 0.01.9

1.4

0.9

0.4

0.1

0.6

1.1

1.6

0.1 0.2 0.3 0.5 0.60.4

Potential, E

[V]

0.7

Current,i

106

[A]

Fig. 12 Typical cyclicvoltammograms ofpreequilibrium systems; kineticsof preequilibrium becomeslower from dotted throughdashed to solid curve.


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1.4 How to Gain Additional Information about Electroorganic Reaction Mechanisms 19

0.0100

0

100

200

300

400

500

(a)

0.2 0.6 0.80.4

Time, t

[s]

1.21.0 0.1 0.010

01020304050607080

0.1

(b)

0.2 0.3 0.5 0.60.4

Potential, E

[V]

0.7

Current,i

106

[A]

Current,i

106

[A]

Fig. 13 Typical (a) chronoampero- and (b) cyclic voltammogram of a catalytic system.

the electroactive A leads to an increasein current and a decrease of diffu-sional effects. Thus, in chronoamperom-etry, i reaches a nonzero limiting value(Figure 13a), while in CV the peak disap-pears in favor of an S-shaped i/E curve(Figure 13b). From the limiting CV cur-rent, the rate constant k is accessiblefrom [28, 67]

i=

nFAc0

Dk (16)

1.4

How to Gain Additional Information aboutElectroorganic Reaction Mechanisms

1.4.1

Simulation

A simulation (Volume 3, Chapter 3.1) isthe reproduction of an electroanalyticalexperiment in the form of a set of math-

ematical equations and their solutions,usually on a digital computer [7]. Theequa-tions express a physical model of the realexperiment. Thus, the main steps of theelectrode process (see Section 1.2.1) areincluded.

Various numerical techniques are em-ployed, and commercial programs areavailable, mostly for the CV technique [7].For the elucidation of electrode reaction

mechanisms, simulation is an indispens-able tool for both types of analyses de-scribed in Section 1.3.1. For a simulation,one needs a mechanistic hypothesis that insome programs is translated into the gov-erning equations automatically [45,68, 69].There are various parameters defining thereaction steps in detail, for example, rateconstants or formal potentials. One solvesthe equations for given values of these

parameters and compares the results to ex-perimental curves in an iterative process,

until a best fit is obtained. Automaticfitting is also available [45]. Alternatively,it is illustrating to see how variations inmechanism and/or parameters change theresulting curves.

It is of particular importance to followthe guidelines provided in Section 1.3.2in comparing experiments and simula-tions.

1.4.2Ultramicroelectrodes

In previous sections we have implicitly as-sumed that diffusion occurs perpendicularto the electrode surface (semi-infinite lin-ear diffusion). If we decrease the size of

the electrode to values roughly in the orderof the size of diffusion layers, this assump-

tion becomes invalid. Now, additional


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Quartz crystal withelectrodes

R

WElectrolyte

Oscillator

Computer,Recorder

Functiongenerator

C

Potentio/galvanostat

Frequencycounter

Fig. 14 Set-up of an electrogravimetric experiment with anelectrochemical quartz crystal microbalance.

diffusion components parallel to the sur-

face become important. Thus, the currentdensities are increased. It is common to

call disk electrodes with radii 20 m ul-tramicroelectrodes (UMEs) [8].

UMEs decrease the effects of non-Faradaic currents and of the iR drop.

At usual timescales, diffusional transportbecomes stationary after short settling

times, and the enhanced mass transportleads to a decrease of reaction effects.

On the other hand, in voltammetry veryhigh scan rates (v up to 106 Vs1)become accessible, which is importantfor the study of very fast chemical steps.

For organic reactions, minimization ofthe iR drop is of practical value andhighly nonpolar solvents (e.g. benzene

or hexane [8]) have been used with low

or vanishing concentrations of supportingelectrolyte. In scanning electrochemical

microscopy (SECM [70]), the small sizeof UMEs is exploited to localize electrode

processes in the m scale.

1.4.3

Electrogravimetry

If the electrode process results in thedeposition of some product at the electrodesurface, or in changes of composition of aprecipitate or film on the electrode, masschanges are coupled to the ET. Usually,these changes are small (ngg) andspecial techniques are necessary for theirexact determination.

A technique for such measurementsis the electrochemical quartz crystalmicrobalance (EQCM; Figure 14) [71].Here, the working electrode is part of a

quartz crystal oscillator that is mountedon the wall of the electrochemical celland exposed to the electrolyte. Theresonance frequency fof the quartz crystalis proportional to mass changes m:f m. With base frequencies around10 MHz, the determination ofm in theng range is possible.

Electrogravimetric experiments lead toa mechanistic understanding of polymer


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1.5 Conclusion 21

film formation on electrodes, supportthe study of film morphology and thediffusional as well as the migrationaltransport into and within such films [72].

1.4.4

Spectroelectrochemistry

Although the instrumental techniques de-scribed here give detailed mechanistic

information, they do not provide an in-sight into the structure of intermediates.If we, however, combine electrochemicaland spectroscopic methods, this is ad-vantageously accomplished (spectroelec-trochemistry) [73]. Various spectroscopieshave been coupled with electrochemicalexperiments, among them ESR [74], opti-cal [75], and NMR spectroscopy [76, 77], aswell as mass spectrometry [78, 79].

Three types of spectroelectrochemicalexperiments are useful for mechanisticstudies:

Spectral resolution records spectra atdifferent potentials, for example, duringa CV scan. This allows structuralcharacterization of intermediates.

Temporalresolution records the inten-sity of a spectroscopic signal with t,giving access to formation and decaykinetics.

Spatialresolution [80] leads to informa-tion on the distribution of species withinthe diffusion layer. Distinction between

alternative mechanisms has been re-ported [81].

1.5

Conclusion

This chapter discussed some of themore important electroanalytical tech-niques with particular emphasis on their

use in electroorganic chemistry. Thesetechniques greatly help determine andunderstand the mechanistic course ofelectrode reactions in a qualitative andquantitative way. Besides briefly describ-ing the methods themselves, the chapterprovides examples for their applicationfor some frequently encountered reactionmechanisms. In particular, cyclic voltam-metry is probably the most often usedof these techniques, but other methodsshould also be applied if necessary, andextensions, as discussed in Section 1.4, areexpected to gain additional importance inthe future.

Acknowledgment

Theauthors thank Kai Ludwigfor technicalassistance in preparing Figures 6 and 7.

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10. J. OM. Bockris, Z. Nagy, J. Chem. Educ.1973,50, 839 843.


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12. S. Antonello, F. Maran, J. Am. Chem. Soc.1999,121, 96689676.

13. E. Klocke, A. Matzeit, M. Gockeln et al.,Chem. Ber. 1993,126, 16231630.

14. D. H. Evans, Acc. Chem. Res. 1977, 10,313319.

15. J.-M. Saveant, Adv. Electron Transfer Chem.1994,4, 53 116.

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17. E. Vieil, G. Cauquis, J. Electroanal. Chem.1983,148, 183 200.

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1.5 Conclusion 23

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71. M. D. Ward, Principles and applicationsof the electrochemical quartz crystal mi-crobalance in Physical Electrochemistry (Ed.:I. Rubinstein), Marcel Dekker, New York,1995, pp. 293338.

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73. W. Plieth, G. S. Wilson, C. Gutierrez de laFe,Pure Appl. Chem. 1998,70, 13951414.

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75. W. R. Heineman, Anal. Chem. 1978, 50,390A402A.

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25

2

Practical Aspects of Preparative

Scale Electrolysis

Jakob J..

orissenUniversit

..

at Dortmund, Dortmund, Germany

2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

2.2 Target and Scale of the Investigations . . . . . . . . . . . . . . . . . . . . . 30

2.3 Principles of Electrochemical Cell Operation . . . . . . . . . . . . . . . . 312.3.1 Essential Definitions for Electroorganic Reactions. . . . . . . . . . . . . 312.3.2 Controlling of the Electrochemical Reaction Rate by Electrode Potential

and Cell Current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

312.3.2.1 General Correlations between Electrode Potential and Current Density 312.3.2.1.1 Equilibrium Potential. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 322.3.2.1.2 Overvoltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

Charge transfer overvoltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33Concentration overvoltage (reaction overvoltage and diffusionovervoltage) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

2.3.2.1.3 Limiting Current Density . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 332.3.2.1.4 Side-reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 332.3.2.1.5 Possible Problems in Electroorganic Reaction Systems. . . . . . . . . . 342.3.2.1.6 Overvoltage Due to Electrolyte and Cell Separator Resistance . . . . . 342.3.2.1.7 Cell Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 352.3.2.2 Operation with Constant Cell Current (Galvanostatic Operation) . . . 352.3.2.3 Operation with Constant Electrode Potential (Potentiostatic Operation) 362.3.3 Undivided or Divided Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . 372.3.4 Batch Operation or Flow-through Cells . . . . . . . . . . . . . . . . . . . . 38

2.4 Components of Electroorganic Reaction Systems . . . . . . . . . . . . . 382.4.1 Electrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 392.4.1.1 Examples of Electrode Materials. . . . . . . . . . . . . . . . . . . . . . . . . 402.4.1.1.1 Anode Materials: General Requirements . . . . . . . . . . . . . . . . . . . 402.4.1.1.2 Cathode Materials: General Requirements . . . . . . . . . . . . . . . . . . 40


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26 2 Practical Aspects of Preparative Scale Electrolysis

2.4.1.1.3 Platinum, Platinum Metals or their Alloys, and Other Noble Metals . 41Anode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41Cathode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

2.4.1.1.4 Nickel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41Anode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41Cathode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

2.4.1.1.5 Iron, Stainless Steel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42Cathode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

2.4.1.1.6 Lead . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42Anode (lead dioxide). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

Cathode. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

422.4.1.1.7 Mercury. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42Cathode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

2.4.1.1.8 Carbon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42Anode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43Cathode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

2.4.1.1.9 Ceramic Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 442.4.1.1.10 Coated Electrodes and Carrier Materials . . . . . . . . . . . . . . . . . . . 44

Titanium as a carrier metal . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44Metal oxide coatings. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45Dimension stable anodes (DSA) . . . . . . . . . . . . . . . . . . . . . . . . 45Diamond coating (boron doped) . . . . . . . . . . . . . . . . . . . . . . . . . 45

2.4.1.2 Examples of Electrode Types and their Special Properties. . . . . . . . 45

2.4.1.2.1 Smooth or Porous Electrodes. . . . . . . . . . . . . . . . . . . . . . . . . . .

452.4.1.2.2 Gas Evolving Electrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 462.4.1.2.3 Gas Diffusion Electrodes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 462.4.1.2.4 Sacrificial Anodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 472.4.2 Electrolytes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 472.4.2.1 Solvents. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 472.4.2.2 Supporting Electrolytes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 482.4.2.3 Examples of Electrolytes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 492.4.2.3.1 Aqueous Electrolytes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 492.4.2.3.2 Electrochemistry Using Emul-sions . . . . . . . . . . . . . . . . . . . . . . 502.4.2.3.3 Electrolytes Based on Nonaqueous Protic Solvents. . . . . . . . . . . . . 502.4.2.3.4 Electrolytes Based on Aprotic Solvents. . . . . . . . . . . . . . . . . . . . . 50

2.4.2.3.5 Molten Salts as Electrolytes. . . . . . . . . . . . . . . . . . . . . . . . . . . .

512.4.2.3.6 Liquefied or Supercritical Gases as Solvents for Electrolytes . . . . . . 512.4.2.3.7 Solid Polymer Electrolyte Techno-logy. . . . . . . . . . . . . . . . . . . . . 512.4.3 Cell Separators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 512.4.3.1 Porous Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 522.4.3.2 Ion-exchange Membranes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53


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2 Practical Aspects of Preparative Scale Electrolysis 27

2.5 Electrochemical Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 542.5.1 Requirements in Electrochemical Cells . . . . . . . . . . . . . . . . . . . . 542.5.1.1 Uniform Current Density . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 552.5.1.2 Uniform Mixing and Mass Transfer . . . . . . . . . . . . . . . . . . . . . . 552.5.1.3 Temperature Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 562.5.1.4 Construction Materials. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 562.5.1.5 Mass and Charge Balancing . . . . . . . . . . . . . . . . . . . . . . . . . . . . 592.5.1.6 Electrode Potential Measurement. . . . . . . . . . . . . . . . . . . . . . . . 612.5.1.6.1 Reference Electrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 612.5.1.6.2 Diffusion Potentials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

2.5.1.6.3 Luggin Capillaries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

622.5.2 Examples of Electrochemical Cells . . . . . . . . . . . . . . . . . . . . . . . 642.5.2.1 H-cell. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 642.5.2.2 Beaker Glass Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 652.5.2.3 Flow-through Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 652.5.2.4 Industrial Scale Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 672.5.2.4.1 Parallel-plate and Frame Cells (Filter Press Cells) . . . . . . . . . . . . . 672.5.2.4.2 Capillary Gap Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 682.5.2.4.3 Swiss Roll Cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 692.5.2.4.4 Innovative Cell Constructions . . . . . . . . . . . . . . . . . . . . . . . . . . 70

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70


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29

2.1

Introduction

The success of an electrolysis processdepends on the choice of a suitableelectrochemical cell and optimal operationconditions because there is a widespreadvariety of electrolyte composition, cellconstructions, electrode materials, andelectrochemical reaction parameters.

The objective of this chapter is to study

some essential practical aspects, whichhave to be considered. First, as neces-sary background information, the differentalternatives for electrochemical cell opera-tion are discussed in general. Then followsan overview of properties of electrode ma-terials, electrolyte components, and cellseparators. Finally, examples of cell con-structions are shown.

A precondition for an appropriate de-cision in the planning of a preparativeelectroorganic synthesis is sufficient infor-

mation about the electrochemicalreaction.As far as possible, knowledge about theinfluence of parameters such as tempera-ture, solvent, pH value, and stirring rateshould be included. Electroanalytical stan-dard methods to acquire such data havebeen discussed in Chapter 1: cyclovoltam-metry as an especially valuable tool and itscombination with the rotating disk elec-trode method for additional knowledge. At

the beginning, literature data about com-parable reactions are very helpful. A wide

overview about reported electroorganic re-actions is given as a basic informationsource in the following chapters.

The considerations, prior to beginning,

must include special characteristics of elec-trochemical reactions and their practicalconsequences in a preparative scale elec-trolysis:

The first fundamental decision is to useone of the following alternatives

direct electroorganic reaction at an in-

ert or electrocatalytic active electrodesurface, which needs no additional

agent in the electrolyte, indirect electrolysis, that is, the elec-

trochemical regeneration of a conven-tional oxidizing or reducing agent,

application of a mediator, which is

present like a homogeneous catalyst

and is continuously regenerated insitu at an electrode (see Chapter 15).

Many examples of these ways are

shown in this volume, discussing theiradvantages and drawbacks.

The principle of electrochemistry isto replace the direct electron transferbetween atoms or molecules of a con-

ventional redox reaction by separating


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the oxidation at the anode and the re-duction at the cathode. Even thoughin most cases only one of these re-actions is intended (at the workingelectrode), the other one unavoidablyhas to be carried out (at the counterelectrode), at least without detrimen-tal effects on the desired reaction. Inconsequence, the selection of optimal

electrode materials and of a suitable

cell undivided or divided by a sepa-rator will be essential. Recent develop-ments aim at conversions that produceuseful products simultaneously at theanode and cathode (paired electroly-ses, see Chapter 3).

A typical advantage of electrochemistryin comparison to conventional chemi-cal reactions is the possibility to controlthe reaction by electrical parameters.The choice of the alternatives using aconstant cell current (galvanostatic op-

eration) or using a constant electrodepotential (potentiostatic operation) gen-erally has a significant influence on theresults of electroorganic syntheses (seeSect. 2.3.2.2 and 2.3.2.3 and Chapter 3).

An electrochemical reaction needs thetransfer of ions between the electrodes.Therefore, the solution in the cell re-quires usually at least minimal ionconductivity. In most cases, a support-ing electrolyte has to be added, andafter the reaction it is separated and

reused. Electrochemical reactions proceed, in

principle, heterogeneously at the elec-trode surfaces. Hence, the mass transferhas a major influence, especially onthe selectivity of the electrode reactions.Therefore, the mixing conditions in thecell have to be optimized, consideringalso the operation mode as batch or asflow-through reactor.

2.2

Target and Scale of the Investigations

Prior to beginning it is necessary to eval-uate the aim and the scale of the plannedinvestigations because many particularaspects, discussed in this chapter, are de-pendent on this decision. There may be awide range of intentions for preparativeelectrolysis investigations, demonstratedhere by two borderline cases:

If the target is to find new electrochemi-cal conversions perhaps of expensivecompounds then the products onlyhave to be accessible in small amountsfor their identification. A high yield con-cerning the reactants is required buttechnical aspects such as energy con-sumption are not interesting. In thiscase, a small volume will probably bethe most important feature of the elec-trochemical cell.

If the investigations are intended to de-velop an industrial production, the focuswill be to optimize the operation condi-tions and to get base data for scale-up.In this case, the electrochemical prop-erties of the experimental cell have tobe equivalent with the planned techni-cal cell. Thus, it is necessary to carry outexperiments on a sufficient large scale,including lifetime tests of cell compo-nents. For industrial and engineeringaspects, see for example, [1, 2, 3b, 4]

(overview), [5c] (detailed), and [6, 7] (in-cluding theory).

For numerous research intentions, anaverage scale will be chosen, consideringon the one hand, the costs of chemicalsand on the other, the easier experimentalwork and better reproducibility of results,using a cell of medium but not toosmall dimensions.


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2.3 Principles of Electrochemical Cell Operation 31

2.3

Principles of Electrochemical Cell Operation

2.3.1

Essential Definitions for ElectroorganicReactions

Fundamental criteria to evaluate the re-sults of any organic synthesis are theyield, being the fraction of the entiresupplied reactant, which has formed the

product, and the selectivity, being thefraction of the converted reactant, whichhas been used to generate the product.

In addition, the current efficiency(current yield) is typical for an electroly-sis process, the fraction of the electrical cellcurrent or (integrated over the time) thefraction of the transferred charge whichis used to form the product. The theoret-ical charge transfer for one mol productis given by the Faraday constant F, thecharge of one mol electrons, F = 96485

As/mol = 26, 8 Ah/mol, multiplied by thenumber of transferred electrons.

Of general importance for reactionsis the degree of conversion (short:conversion), being the fraction of areactant that has been removed because ofthe reaction. Because the concentrationsof reactants are decreased and that ofproducts increased with rising conversion,the selectivity of the desired reactionmostly becomes smaller during the courseof the reaction owing to a decrease of

the desired reaction of the reactants andenhancement of consecutive reactions ofthe products.

If the reaction conditions are changingwith time, (especially during batch oper-ation, see Sect. 2.3.4), it is necessary foryield, selectivity, and current efficiency todistinguish between the actual values andthe summarized (integrated) values fromthe start to the end of the reaction.

2.3.2

Controlling of the Electrochemical ReactionRate by Electrode Potential and Cell Current

For choosing a suitable cell constructionand optimal reaction conditions in the cell,it is inevitable to consider the fundamentalcorrelations between electrode potentialand cell current and their influence onselectivity and yield of the electrochemicalreactions. Therefore, a simplified overview

is given here. The detailed theory iselucidated in Chapter 1.

The electrochemical reaction rate andthus the speed of production in the cellare proportional to the cell current. Thecurrent density the cell current dividedby the electrode area is dependent on thepotential of the working electrode.

To achieve a large production rate, thecurrent density should be as high as possi-ble. Particularly, industrial cells need a sat-isfactory current density and spacetime

yield, that is, production per time and cellvolume, because the investment costs andconsequently the production costs are en-larged with increasing electrode area andcell volume. But, naturally, the currentdensity is limited by different reasons thathave to be considered.

2.3.2.1 General Correlations betweenElectrode Potential and Current Density

Figure 1 shows typical current den-sity potential curves of an electroorganic

reaction. In this example, the thin linerepresents the anodic oxidation of theelectrolyte without reactants at a higherpotential, here at more than 0.8 V versusNHE. If the reactant 1 is present, it canbe converted according to the thick com-pact lines at lower potentials above 0.2 Vversus NHE, and this selectively can oc-cur up to 0.5 V versus NHE. Over 0.5 Vversus NHE also, an additional reactant


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20

10

00 0.2 0.4 0.6 0.8 1 1.2

Electrode potential[V versus NHE]

Curren

tdens

ity

[mAcm

2]

Reactant 1changing

concentration C1

Reactant 2constant

concentration C2

C2

C2

C2

C1

C12/3

C11/3

Fig. 1 Current density potential curves for the anodic oxidation oftwo various reactants and finally of the solvent. The electrode potentialis measured against a reference electrode (RE), here for example,the normal hydrogen electrode (NHE).

2 can be oxidized, increasing the currentconsistent with the thick dotted lines (aconstant concentration of reactant 2 is as-sumed). An analogous correlation has to

be considered for the counter electrode(here the cathode).

2.3.2.1.1 Equilibrium Potential The min-imum potential, which is necessary to

perform a (reversible) reaction, is the equi-librium potential E, defined for zero cellcurrent. It is typical for a given reaction. Bydefinition, it is related to the NHE, which

represents the potential zero. If the elec-trode reaction is coupled with the reaction2 H+ + 2e H2 at the NHE, theoreti-cally E can be calculated using the free

reaction enthalpy G (Gibbs energy) ofthe total reaction divided by the chargetransfer of the reaction:E = G/(z F)[V] (z = number of transferred electrons,

F = Faraday constant). The equilibriumpotentialE is dependent on the tempera-ture and on the concentrations (activities)of the oxidized and reduced species of thereactants according to the Nernst equation(see Chapter 1). In practice, electroorganicconversions mostly are not simple re-versible reactions. Often, they will include,for example, energy-rich intermediates,complicated reaction mechanisms, and ir-reversible steps.In this case, it is difficult todefineEand it has only poor practical rele-

vance. Then, a suitable value of the redoxpotential is used as a base for the designof an electroorganic synthesis. It can be es-timated from measurements of the peakpotential in cyclovoltammetry or of thehalf-wave potential in polarography (seeChapter 1). Usually, a common RE suchas the calomel electrode is applied (seeSect. 2.5.1.6.1). Numerous literature dataare available, for example, in [5b, 8, 9].


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2.3.2.1.2 Overvoltage To obtain a cellcurrent, an overvoltage, a potentialdifference additional to the equilibriumpotential, has to be applied. Thus, theovervoltage consisting of different com-ponents is the deciding parameter tocontrol the speed of an electrochemicalreaction. The energy demand due to theovervoltages at both electrodes is lost com-pletely as heat.

Charge transfer overvoltage The chargetransfer overvoltage is necessary to over-come the kinetic hindrance of the elec-trochemical reaction, that is, to surpassthe activation energy of the charge trans-fer at the electrode. The current densityincreases exponentially with this overvolt-age (see the thin line for the electrolytein Fig. 1), frequently by a factor of aboutten with additional 120 mV overvoltage(for one electron in the transfer step;

this is quantified analogous to the Ar-rhenius law of chemical reactions by theButlerVolmer equation, see Chapter 1).

Concentration overvoltage (reaction over-

voltage and diffusion overvoltage) If asignificant current is flowing, the concen-trations of reactants will be lower andthose of products higher at the elec-trode surface than in the bulk electrolyte.Hence, consistent with the Nernst equa-tion, the electrode potential is shifted by

the concentration overvoltage. Partially,it can be caused by slow chemical reactionsteps before and/or after the charge trans-fer (reaction overvoltage). Additionally,an unavoidable part of the concentrationovervoltage is the diffusion overvoltagedue to concentration differences in thediffusion layer that is formed in the elec-trolyte adjacent to the electrode surface.In this layer, a mass transfer is possible

only by diffusion and not by convection(see Chapter 1). Usually, these concentra-tion differences increase proportional tothe current density, according to the firstFicks law.

2.3.2.1.3 Limiting Current Density Thediffusion overvoltage hinders the currentdensity to rise continuously with increas-ing potential, especially in case of low

reactant concentrations. The limiting cur-rentdensityforareactionisreachedwhenthe current density becomes equivalentto the maximally accessible diffusion rateof a required reactant, see the horizontalsections of the thick curves in Fig. 1. Here,the reactant concentration at the electrodesurface tends to zero and the diffusionovervoltage can reach very high values.The limiting current density usually inconsequence of the first Ficks law is pro-portional to the reactant concentration, as

shown for reactant 1 by the different thickcompact curves in Fig. 1. Thus, the lim-iting current density can be improved byan increased reactant concentration,for ex-ample,due to choosing a reduced degree ofconversion. There are further methods toenhance the diffusion rate, such as inten-sified stirring (i.e. thinner diffusion layer),elevated temperature, and/or reduced vis-cosity (i.e. increased diffusion coefficient).For the electrolyte decomposition(thinlinein Fig. 1), the diffusion overvoltage is neg-ligible in the considered range of current

density due to the excess concentration ofthe solvent, and no limiting current densityis observed.

2.3.2.1.4 Side-reactions As soon as thecell current density surpasses the limitingcurrent density of one reaction, theelectrode potential rises until additionallyanother reaction takes place (in Fig. 1


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potential and current density. Additionalovervoltages are caused by the resistance ofthe electrolyte and of the separator in a di-vided cell. Although these overvoltages are

not parts of the electrode potentials, theyhave to be surpassed in order to enable a

current in the cell and they enhance theheat evolutionin the cell. They increase lin-early with rising current due to the ohmicresistance (in case of strong gas evolu-

tion, the increase may be steeper becausethe resistance increases). These overvolt-ages can be lowered by taking electrolyteswith higher conductivity, for example, sol-

vents with higher dielectric constants andthe use of sufficiently dissociated support-

ing electrolytes. These overvoltages canbe furthermore lessened by decreasingthe distance between the electrodes andby using diaphragms of medium to highporosity (see Sect. 2.4.3.1).

2.3.2.1.7 Cell Voltage Figure 2 showsschematically the cell voltage as summa-tion of the above discussed equilibrium

potentials and overvoltages and of theohmic voltage drops in the electrodes (elec-tron conductors) and in the electrolytes,including cell separators (ion conductors).

2.3.2.2 Operation with Constant CellCurrent (Galvanostatic Operation)Constant current electrolysis is an easyway to operate an electrochemical cell.Usually, it is also applied in industrialscale electrolysis. For laboratory scaleexperiments, inexpensive power suppliesfor constant current operationare available(also a potentiostat normally can work ingalvanostatic operation). The transferredcharge can be calculated directly bymultiplication of cell current and time (nointegration is needed).

The electrode potentials(exactly the over-voltages) are dependant on the currentdensity. Thus, using the galvanostatic op-eration mode, optimal results are attainedonly if a well-defined current density can

be chosen with a clear difference be-tween desired and undesired reactions,as in Fig. 1. This precondition is favored

Anode current feeder

Anode

Anodic equilibrium potential

Charge transferreaction diffusion

Overvoltage anode

Charge transferreaction diffusion

Overvoltage cathode

Cathodic equilibrium potential

Anolyte(possibly increased by gas bubbles)

Catholyte(possibly increased by gas bubbles)

Cell separator

Cathode Ohmic voltage drop(electron conductors)

Ohmic voltage drop(electron conductors)

Ohmic voltage drop

(ion conductors)

Cathode current feeder

Anode

potential

Cathode

potential

Cellvoltage

+

Fig. 2 Composition of the cell voltage (not in real scale).


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especially at constant concentrations ow-ing to continuous addition of reactant andremoval of product in a flow-through cell(steady state).

For batch operation (see Sect. 2.3.4), thelimiting current density is going to zero forincreasing degree of conversion (see reac-tant 1 in Fig. 1). Here, the galvanostaticoperation may only be acceptable if exclu-sively unproblematic side reactions occur,such as water electrolysis as solvent decom-position. In all other cases, better resultscan be expected using the potentiostaticoperation (see next section).

In some problematic cases, there will beno obvious limits available for the choiceof the current density in galvanostatic op-eration. Concurrent reactions take place,resulting in a poor selectivity. But herethe potentiostatic operation also cannotdemonstrate its advantages, and proba-bly the simpler galvanostatic operationmay be applied. To find relatively suitable

operation conditions, an experimental op-timization of the current density should becarried out, perhaps including parameters

such as concentrations of reactants andproducts, degree of conversion, tempera-ture, and so on.

2.3.2.3 Operation with Constant ElectrodePotential (Potentiostatic Operation)As discussed in Sect. 2.3.2.1, electroor-ganic reactions can often be selectivelycontrolled by a constant potential of theworking electrode, even at decreasing reac-tant concentrations (see Fig. 3). A precon-dition of this operation mode is a suitablepotential-measuring equipment in the cell(special practical aspects of potential mea-surement are discussed in Sect. 2.5.1.6).The optimal potential can be chosen us-ing a current density potential curve (seeFig. 1), available by cyclovoltammetry witha very low scan rate.

A potentiostat is relatively expensive,especially if high power is needed. Acheaper method is to use the galvanostaticoperation and to measure continuously

the potential and to adapt the cell currentmanually (or using a computer dataacquisition system) in order to adjust the

Potentiostat

DirectcurrentsourcemA

mV

Con

tro

linpu

t

Work

inge

lectro

de

Re

ference

elec

trode

Coun

tere

lec

trode

RE

Diap

hragm

Luggincapillary

Fig. 3 Scheme of potentiostaticoperation for a preparative electrolysis,using in principle a simplifiedcyclovoltammetry equipment. Thepotential of the working electrode ismeasured by a Luggin capillary, coupledwith a reference electrode (RE, seeSect. 2.5.1.6). The control circuit in thepotentiostat adjusts the cell current untilthe potential of the working electrode isequal to the voltage at the control input.


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electrode potential as accuratelyas possibleat the desired value. This technique maybe applicable even in cases where it isdifficult to measure the potential correctly,for example, in nonaqueous solvents ofpoor conductivity. Then, the control circuit

of a potentiostat cannot properly work, butitis mucheasier to measure thanto controla potential.

Because the current is not constant

during the potentiostatic operation, it hasto be integrated during the experimentfor calculating the charge transfer andthe current efficiency. Coulometers orelectronic integrators are commerciallyavailable. If a computer data acquisitionsystem is used, the current integration ispossible by software.

In principle, a further inexpensivemethod is to work at constant cell volt-

age. But here the potentials of the workingand of the counter electrode, and all volt-

age drops of the electrolytes and of the cellseparator are included (see Fig. 2). Thus,in most cases, clearly defined conditions atthe working electrode cannot be adjustedusing this operation mode (nevertheless,because of its uncomplicated realization,it is applied in most technical electroly-ses to achieve approximately the desiredcell current).

2.3.3

Undivided or Divided Cells

Because of the low-cost construction andsimple operation, an undivided cell is al-

ways desired but it cannot be realized in allcases. A precondition for electrolysis in an

undivided cell is that disadvantageousreac-tions and reaction products at the counterelectrode can be avoided, for example, byselection of the electrode material and/orof the electrolyte composition.

For instance, graphite has a higher hy-drogen overvoltage than platinum and itscatalytic activity for hydrogenation is low.Thus, a graphite counter electrode maybe useful for hydrogen evolution withoutfurther electroorganic reactions. Anotherexample is the addition of a depolarizer,which enables an innocuous reaction atthe counter electrode before an essential

compound in the solution can be at-

tacked. Special depolarized electrodes aregas diffusion electrodes (GDE), knownfrom fuel cells, or sacrificial electrodes,which are dissolved during the reaction(see Sect. 2.4.1.2).

A typical counter electrode reactionis the electrolysis of water. Here thecathodic evolution of hydrogen is coupledwith the formation of base, the anodicdevelopment of oxygen produces acidadditionally. Frequently, acid and baseformation at both electrodes will be

balanced. Otherwise, a buffer solutionor a (continuous) base/acid addition, forexample, by a pH-controlling system, canenable the application of an undivided cell.

In many cases, it will be impossible toprevent unwanted reactions at the counterelectrode. Then a separation of the anolyteand catholyte is needed. An optimal com-promise has to be found for the separatorbetween separation effectiveness and ionconductivity, that is, minimized electricalresistance and low energy consumption.

Moreover, chemical, thermal, and me-chanical stability and price of the separatorhave to be considered. Naturally, a com-

plete separation is impossible, because aslight diffusion rate is inevitable. In labo-ratory scale experiments, probably a highcell voltage is acceptable in order to realizea maximal separation.

Two basically different types of cellseparators are available: porous separators


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with unselective ion transport and ion-exchange membranes, which support theselective transport of either cations oranions (see Sect. 2.4.3).

2.3.4

Batch Operation or Flow-through Cells

Batch operation is the simplest way ofelectrolysis and, therefore, mostly appliedfor electroorganic syntheses. The reac-tant concentration decreases with risingdegree of conversion (see reactant 1 inFig. 1). The selectivity of the reaction canbe maintained in spite of a decline ofthe limiting current density by potentio-static cell operation. Usually, the reactionis carried out up to a selected conversionor transferred charge, respectively. Owingto the continuously changing conditions,much information about the reaction isavailable by analysis of samples, extractedin suitable intervals during the exper-

iment. A plot of all concentrations ofreactants and products versus time ortransferred charge givesinformation aboutreaction rate, yield, selectivity, current ef-ficiency, and also about any by-productformation in parallel and/or consecutivereactions.

Constant process conditions as wellof concentrations as of other parame-ters are realized using a flow-throughcell in steady state operation. Into thecell continuously reactants are added and

products are removed to maintain constantconcentration and conversion. Additionalexpenses, especially pumps, are needed,however. This continuous operation willbe applied, for example, if optimal resultsonly are achievable using well-defined pro-cess conditions. Another example is theapplication of cell components such asion-exchange membranes that need con-stant concentrations and a long time after

start-up for optimal working. Large-scale

industrial cells are often operated under

steady state conditions.Batch operation in a larger scale in

laboratory or even industrial applica-

tions frequently is realized using a flow-

through cell with optimized flow charac-teristics, which is coupled by circulating

pumps with reservoirs that contain thereaction solutions.

2.4

Components of Electroorganic ReactionSystems

The following short overview can only give

an impression of some usual or innovative

cell components and materials (a more

detailed overview is given, for example,in [3a, 3b, 10, 11]).

Particular attention should be paid totoxic materials. Electroorganic synthesis

will become increasingly of interest in

the preparation of speciality chemicals,for example, food additives and pharma-

ceuticals. Thus, toxic materials should be

avoided as far as possible, for example, for

electrodes, solvents, or supporting elec-

trolytes. At least, it has to be guaranteed

that toxic materials in the products canbe separated or removed below the official

threshold values.

Precondition of a successful electroor-ganic synthesis is an optimal arrangement

of all incorporated components. Therefore,all available information from literature,

supplemented, if possible, by results of

own experiments, should be considered.

The best way to get actual informa-tion about suppliers of materials and

equipment that probably may be very

quickly changing is the Internet via a

search engine.


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2.4 Components of Electroorganic Reaction Systems 39

2.4.1

Electrodes

The electrodes are the typical and mostimportant components of an electrochem-ical cell especially the working elec-trode which usually decide about thesuccess of an electroorganic synthesis.Electrode materials need a sufficient elec-tronic conductivity and corrosion stabilityas well as, ideally, a selective electrocat-

alytic activity which favors the desiredreaction. The overvoltages for undesiredreactions should be high, for example, forthe decomposition of the solvent waterby anodic oxygen or cathodic hydrogenevolution. But, additionally, the behav-ior of electrodes can show unexpectedand incomprehensible effects, which willcause difficulties to attain reproducibleresults.

The electrode reaction typically in-cludes a lot of steps, such as adsorp-

tion and desorption, one or several elec-tron transfer steps, preceding, and/or

subsequent chemical reactions. All thesesteps, and consequently the selectivityof the reactions, will be influenced byproperties of the electrode surface, forexample, by chemical composition, mor-phology, and porosity, which may also bedependent on the history of the electrode.Usually, there is a significant interdepen-dencybetween the electrode propertiesandthe electrolyte composition, that is, reac-tants, products, solvents, and supportingelectrolytes, including impurities.

This shall be elucidated by two exam-ples for the influence of the electrodematerial on the product spectrum of well-known electrochemical reactions [4], seeScheme 1 below.

A special problem can be the passiva-tion of the electrode surface by insulatinglayers, for example, formation of oxideson metals at a too high anodic poten-tial or precipitation of polymers in aproticsolvents from olefinic or aromatic com-

pounds by anodic oxidation. As a result,the effective surface and the activity of the

Pt smooth[R ] RR (Kolbe reaction) [12, 13]

In water PbO2RCOO ROH, CO, CO2

e, CO2 Graphite[R+] Resulting products [14]

Cathodic reduction of acrylonitrile in aqueous solution

Anodic oxidation of carboxylates in aqueous solution via decarboxylation

Pt, NiCH3CH2CN (via Had) [15]

Hg, Pb, C, Cd NC(CH2)4CN

and CH3CH2CN

In water [16]CH2=CHCN via

+ePb CH2=CHCH2NH2 [CH2=CHCN]

Strongly acidic [17]

Sn Sn(CH2CH2CN)4

[18]

Scheme 1 Influence of the electrode material on the product spectrum of an electrochemicalreaction.


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electrode are diminished. To avoid an in-creasing of the local current density andconsequently a reduced reaction selectiv-ity, the cell current should be decreased(automatically achieved by potentiostaticoperation). A periodical change of the po-larity of the electrodes for regeneration ofthe activity can be helpful (then a sym-metrical construction of the cell will besuitable). Also, additives to the electrolyte

with better dissolving power for polymerscan be beneficial.Electrodes may consist of a homoge-

neous material frequently, with an insitu formed active layer on the surface orof a carrier material with an active coat-ing. A proper connection to the currentfeeder and a suitable assembling of thecell must be enabled; often, a leakproofinstallation of electrodes in the cell body isrequired. Therefore, the mechanical prop-erties of the electrode material have to passpractical selection criteria:

strength, hardness, elasticity, brittle-ness, and so on.

possibility to be converted to wires,sheets, grids, expanded metal sheets,porous plates, such as sintered metal,felt, or foamed material, and so on.

possibilities of cutting, machining,welding, or soldering, and so on.

A very important aspect is corrosion,concerning a possible contamination of

electrolyte and products particularly incase of toxic materials and with respectto the electrode lifetime. Last, not least,the price can be decisive, especially if acommercial application is planned.

In the following section, examplesof electrode materials for applicationas anode and/or as cathode, and thensome electrode types of practical interestare discussed. A comprehensive overview

about electrodes is given, for example,in [10, 11].

2.4.1.1 Examples of Electrode Materials

2.4.1.1.1 Anode Materials: General Re-

quirements A major problem and thusa decisive factor for the choice of anodematerials is corrosion, except when the dis-solution of a metal is the desired reaction

(sacrificial anodes, see Sect. 2.4.1.2.4).The stability of anode materials is ex-tremely dependent on the composition ofthe anolyte (e.g. pH value, aqueous or non-aqueous medium, temperature, presenceof halogenides, etc.).

In aqueous electrolytes, frequently theoxygen overvoltage is an essential aspect.If the anode is the working electrode and astrong oxidation power is desired, specialmaterials with a high oxygen overvoltageare needed in order to reduce concurrent

oxygen production. If the anode is theoxygen evolving counter electrode it ismostly difficult to avoid disadvantageousoxidation reactions, due to the highpotential of the oxygen formation, furtherincreased by the oxygen overvoltage. Thereare no anode materials with a real lowoxygen overvoltage. Thus, in most cases,the anode as counter electrode has towork in a divided cell within a separatecompartment, unless a depolarized anodecan be used (see Sect. 2.3.3).

2.4.1.1.2 Cathode Materials: General Re-

quirements Cathodes usually have nocorrosion problem. If the cathode is theworking electrode, a main selection cri-terion of the materials is the hydrogenovervoltage, that is, the accessible reduc-tion power, which may vary in a wide range(e.g. hydrogen overvoltage at 1 mA cm2

very lowH < 0.1 V: Pt, platinum metals;


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2.4.1.1.5 Iron, Stainless Steel

Cathode Iron is a very cheap cathodematerial with a relatively low hydrogenovervoltage (e.g. [25]). It is of interestfor industrial applications. In order toavoid corrosion during interruption of thecurrent, stainless steel may be suitable,especially in laboratory cells where the in-creased electrical resistance and hydrogenovervoltage are irrelevant.

2.4.1.1.6 Lead About lead much litera-ture is available due its technical applica-tion in the lead-acid-battery. Pure leadis very soft and has a poor mechanical sta-bility. Therefore, often it is applied as acoating on a carrier or alloys are used, forexample, with antimony (type metal).

Anode (lead dioxide) On lead as anodematerial in aqueous solutions (for exam-

ple, [26]) usually a lead dioxide layer isformed and continuously regenerated athigh anodic potentials, but in longer oper-ation the lead is destroyed by this process.PbO2 is an anode material with a veryhigh oxygen overvoltage and therefore itoffers a high anodic potential and a strongoxidation effect in aqueous acidic solu-tions, especially in sulfuric acid (industrialutilization e.g. for chromic acid regenera-tion [27]). Because PbO2is a strong chem-ical oxidation agent too, its application asanode material is restricted if a sponta-neous reaction with a reactant is possible.

PbO2 is also applicable as a coat-ing on a suitable carrier material (seeSect. 2.4.1.1.10). PbO2-coated titanium an-odes with good stability are commerciallyavailable. On platinum or platinum-coatedtitanium a coating of PbO2 for laboratoryuse can easily be prepared electrochemi-cally (e.g. [28]).

The corrosion rate of PbO2 often en-hanced by mechanical erosion is rela-tively high and may be a problem due tothe toxicity of lead. PbO2can be stabilizedby modification with, for example, silver,antimony, tin, cobalt oxides (or by alloyingof the lead base metal with these metals,respectively) [29].

Cathode The hydrogen overvoltage on

lead is especially high, but this can be real-ized only with very pure lead. Other metalswith a lower hydrogen overvoltage mustnot be present, for example, in the elec-trolyte or from the anode. In the negativeelectrode of lead-acid-batteries, antimonyas alloying metal is replaced with calciumin order to achieve a high hydrogen over-voltage (maintenance-free battery) [30].

2.4.1.1.7 Mercury

Cathode Mercury is a classical cathodematerial for electroorganic reductions dueto its extraordinarily high hydrogen over-voltage in aqueous solutions. Because it isliquid, it needs special cell constructions(see e.g. Fig. 8). By stirring, its surface canbe continuously renewed so that reducedmetals including even alkali metals froma supporting electrolyte will be dissolvedas amalgam and thus will not decrease thehydrogen overvoltage at the surface. Formany applications also, an amalgamatedmetal electrode such as copper, which is

easier to handle, may be sufficient. Thetoxicity of mercury restricts its applicabilityfor technical syntheses.

2.4.1.1.8 Carbon Carbon is a commonelectrode material (e.g. [31]) that is muchcheaper than noble metals. Its conductivityis by a factor of about 100 lower than that ofmetals, but this will be no problem using


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2.4 Components of Electroorganic Reaction Systems 43

suitable electrode constructions. Its tech-nical applications include current feedersof batteries, anodes for chlorine evolu-tion, bipolar electrodes for industrial scaleelectroorganic syntheses (see Fig. 12), andanodes in the molten-salt electrolysis foraluminum winning.

Carbon is available in a large varietyof qualities. Electrode carbon is producedwith different percentages of crystallinegraphite (as a consequence, for example,different conductivities and stabilities areresulting). It is usually highly porous andnot leakproof against gases and fluids.It can also be impregnated with resins,even using fluorinated resins, in order torealize tight materials of high chemicalstability. All these materials can be easilymachined but their solidity is limited andthey are somewhat brittle. Graphite fiberreinforced graphite with a very high me-chanical strength is obtainable for specialapplications (e.g. SGL-Carbon Group).

Graphite is offered moreover in flexiblesheets (e.g. Sigraflex of the SGL-CarbonGroup). This is primarily a sealing mate-rial, but it is suitable also as electrode orcorrosion resistant current feeder (erosionin case of gas evolution is possible).

Carbon fiber or graphite fiber materials,available, for example, as felt, clothes, orpaper, and so on, are state of the artfor realizing conductive diffusion zonesin fuel cells but also they can be used aselectrodes. They attain a very high porosity

(free space volume up to 80%) and asurprisingly good elasticity.Glassy carbon (vitreous carbon) is a

further modification. It is a smooth andtight material of high corrosion resistanceand for many reactions it may be asuitable alternative of platinum. It isrelatively expensive, but much cheaperthan noble metals. It is very hard and brittleand allows only limited possibilities for

machining (diamond tools necessary). It isadditionally available as a foamed material.

Carbon- or graphite-filled polymers ora carbon paste, which can easily beregenerated by removing a surface layer,are also possible electrode materials.

Generally, carbon materials show a lowelectrocatalytic activity. But often it is verydependent on the history of the electrode,for example, due to removing of a surface

coverage or due to roughening of thesurface. Thus it may be difficult to getreproducible results, and frequently a longtime of changing conditions after start ofthe electrolysis is observed.

Anode In aqueous anolytes, under con-ditions which favor the oxygen evolution,carbon is attacked under carbon dioxideformation, this is increasingly encoun-tered with more porous materials. Glassycarbon will be relatively stable. A low pH

value may retard the oxygen reaction, butcarbon remains a problematic anode ma-terial in aqueous solutions. Additionally,it can be attacked because of intercalationof anions.

In nonaqueous media, carbon (graphite)frequently is an optimal anode material,for example, for methoxylation reactions,even in an industrial scale (capillary gapcell, see Fig. 12). It shows an appreciableovervoltage for the oxidation of methanolcompared to platinum.

Cathode The hydrogen overvoltage ofcarbon is relatively high. Thus, if thecathode is the working electrode, a lotof reduction reactions is enabled. Onthe other hand, the catalytic activityfor hydrogenation reactions is low. Thisis advantageous if the cathode is thecounter electrode: cathodic side-reactionsare avoided besides hydrogen evolution,


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and therefore carbon can be a useful

cathode material, even though the energy

consumption is increased because of the

hydrogen overvoltage.

2.4.1.1.9 Ceramic Materials An example

of a sufficiently conductive metal oxide is

magnetite Fe3O4, which has been used,for

example, in the past as corrosion resistant

anode material for industri

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