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1Technical Scale ofElectrochemistry

Klans J..uttner

Karl-Winnacker Institute, Dechema e.v., Frankfurtan Main, Germany

1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.2 Advantages and Disadvantages of Electrochemical Processes . . . . . . 4

1.3 Electrochemical Reaction Engineering . . . . . . . . . . . . . . . . . . . . . 5

1.4 Fundamentals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61.4.1 Current Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71.4.2 Space-time Yield . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

1.5 Electrochemical Thermodynamics . . . . . . . . . . . . . . . . . . . . . . . 81.5.1 Energy Yield . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111.5.2 Specific Electrical Energy Consumption . . . . . . . . . . . . . . . . . . . . 12

1.6 Electrochemical Cell Design . . . . . . . . . . . . . . . . . . . . . . . . . . . 12References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Encyclopedia of Electrochemistry. Edited by A.J. Bard and M. StratmannVol. 5 Electrochemical Engineering. Edited by Digby D. Macdonald and Patrik SchmukiCopyright 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. ISBN: 978-3-527-30397-7

3

1.1Introduction

From an academic point of view, electro-chemistry has become a written chapterin science history, manifested, for ex-ample, by the Nernst equation or theButler–Volmer equation, to mention afew of the most common relations. Theinterest of the academic community inelectrochemistry as a discipline of funda-mental importance has gradually declinedin recent years. Electrochemistry todaypresents itself as a strongly diversifiedfield, which one would call ‘‘applied elec-trochemistry’’, ranging from the study ofelectrode processes for synthesis of in-organic or organic compounds, energyconversion, sensor development, electro-analytic devices, electrolytic corrosion tosolid-state ionics, and any kind of systemwhere charge transfer is involved at anelectrified interface between electronic andionic conductors. Research is more andmore specialized on very particular prob-lems, often of a multidisciplinary charac-ter, being in touch with biology, medicine,microelectronics, structuring, and func-tionalizing of surfaces at the nanoscaleas a part of materials and surface science.

Another view on ‘‘applied electroche-mistry’’ is that of the electrochemical

engineer. Electrochemical engineering hasbecome an established discipline of itsown, which deals with the description andoptimization of electrochemical processesbased on the fundamental laws of chem-istry and electrochemistry. Essentially, itis an interdisciplinary field, which cameup in the 1960s after the developmentof chemical engineering, providing thetools of unit operations for the systematicdescription and treatment of chemical pro-cesses in a way that linked the underlyingphysical and physicochemical principles tochemical technology. One of the pioneerswas C. Wagner, who initiated this devel-opment by his article ‘‘The Scope of Elec-trochemical Engineering’’ in ‘‘Advances inElectrochemistry and Electrochemical En-gineering’’ [1]. It soon became apparentthat this kind of treatment can be adoptedand transferred to the field of electro-chemical reaction and process engineer-ing [1–19]. Today, established technicalprocesses include chloralkali electrolysisand related processes of hypochlorite andchlorate formation, electrowinning of met-als like aluminum and magnesium frommolten salt electrolytes, hydrometallurgyfor electrowinning of metals like copper,nickel, zinc, and refining of cast metals,electro-organic synthesis of, for example,adipodinitrile as precursor for polyamide(nylon) production, electroplating in the

Encyclopedia of Electrochemistry. Edited by A.J. Bard and M. StratmannVol. 5 Electrochemical Engineering. Edited by Digby D. Macdonald and Patrik SchmukiCopyright 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. ISBN: 978-3-527-30397-7

4 1 Technical Scale of Electrochemistry

galvanic industry, or electrophoretic coat-ing in the automotive industry. The en-vironmental aspect has become a majorissue and a crucial factor in the devel-opment of industrial processes to meetthe requirements of sustainable devel-opment. In this field, electrochemistryoffers promising approaches due to itsenvironmental compatibility and use ofthe electron as a ‘‘clean reagent’’ [20–28].There is common agreement among sci-entists that electrochemically based pro-cesses will be of increasing importancein the future to meet the economic andsocial challenges resulting from urgentdemands of low-grade raw materials’ uti-lization, energy savings, and protection ofthe environment.

1.2Advantages and Disadvantages ofElectrochemical Processes

Applied electrochemistry can provide valu-able cost efficient and environmentallyfriendly contributions to industrial pro-cess development with a minimum ofwaste production and toxic material. Ex-amples are the implementation of electro-chemical effluent treatment, for example,the removal of heavy metal ions fromsolutions, destruction of organic pollu-tants, or abatement of gases. Furtherprogress has been made in inorganicand organic electrosynthesis, fuel celltechnology, primary and secondary bat-teries, for example, metal-hydride andlithium-ion batteries. Examples of innova-tive industrial processes are the membraneprocess in the chloralkali industry andthe implementation of the gas-diffusionelectrode (GDE) in hydrochloric acid elec-trolysis with oxygen reduction instead ofhydrogen evolution at the cathode [28].

Figure 1 shows the cell room of the elec-trolyzer at Bayer Materials Science AG inBrunsbuttel, Germany.

The main advantages of electrochemicalprocesses are as follows:

• Versatility: Direct or indirect oxidationand reduction, phase separation, con-centration or dilution, biocide function-ality, applicability to a variety of mediaand pollutants in gases, liquids, andsolids, and treatment of small to largevolumes from microliters up to millionsof liters.

• Energy efficiency: Lower temperature re-quirements than their nonelectrochem-ical counterparts, for example, anodicdestruction of organic pollutants insteadof thermal incineration; power lossescaused by inhomogeneous current dis-tribution, voltage drop, and side reac-tions being minimized by optimizationof electrode structure and cell design.

• Amenability to automation: The systeminherent variables of electrochemicalprocesses, for example, electrode po-tential and cell current, are particularlysuitable for facilitating process automa-tion and control.

• Cost effectiveness: Cell constructions andperipheral equipment are generally sim-ple and, if properly designed, alsoinexpensive.

Since electrochemical reactions takeplace at the interface of an electronic con-ductor – the electrode – and an ion conduc-tor – the electrolyte – electrochemical pro-cesses are of heterogeneous nature. Thisimplies that, despite the advantages men-tioned earlier, the performance of elec-trochemical processes suffers from mass-transport limitations and the size of thespecific electrode area. A crucial point isalso the stability of cell components in

1.3 Electrochemical Reaction Engineering 5

Fig. 1 Cell room of the hydrochloric acid electrolyzer with GDE in Brunsb..uttel [28]

(photo: Bayer Material Science AG).

contact with aggressive media, in particu-lar, the durability and long-term stabilityof the electrode material and the catalysts.

1.3Electrochemical Reaction Engineering

Electrochemical reaction engineeringdeals with modeling, computation,and prediction of production rates ofelectrochemical processes under realtechnical conditions in a way that technicalprocesses can reach their optimumperformance at the industrial scale. Asin chemical engineering, it centers on theappropriate choice of the electrochemicalreactor, its size and geometry, mode ofoperation, and the operation conditions.This includes calculation of performanceparameters, such as space-time yield,

selectivity, degree of conversion, andenergy efficiency for a given reaction ina special reactor.

Electrochemical thermodynamics is thebasis of electrochemical reaction engineer-ing to estimate the driving forces andthe energetics of the electrochemical reac-tion. Predictions of how fast the reactionsmay occur are answered by electrochemi-cal kinetics. Such predictions need inputfrom fundamental investigations of the re-action mechanism and the microkineticsat the molecular scale [29–50]. While mi-crokinetics describes the charge transferunder idealized conditions at the labora-tory scale, macrokinetics deals with thekinetics of the large-scale process for acertain cell design, taking into accountthe various transport phenomena. The mi-crokinetics combined with calculations ofthe spatial distribution of the potential


Microkinetics

Reactionmechanism

Macrokinetics

Chargetransfer

Masstransport

Heattransfer

Cell design

Fig. 2 Concept ofmicrokinetics and macrokineticsfor electrochemical reactions.

and current density at the electrodesleads to the macrokinetic description ofan electrochemical reactor [3–5, 7, 9, 10,12–18]. The relation between micro- andmacrokinetics is shown schematically inFig. 2.

The phenomenon of charge transport,which is unique to all electrochemicalprocesses, must be considered along withmass, heat, and momentum transport. Thecharge transport determines the currentdistribution in an electrochemical cell,and has far-reaching implications onthe current efficiency, space-time yield,specific energy consumption, and thescale-up of electrochemical reactors.

1.4Fundamentals

In the following, similarities and differ-ences in the fundamental description ofchemical and electrochemical reactions areconsidered with a definition of symbolsand signs [51, 52].

In general, a chemical reaction is writtenas the sum of educts Ai and products Bj

∑i

νAiAi =

∑j

νBjBj (1)

with stoichiometric coefficients νi . Therate of the reaction is defined by

r = 1

νAi

dnAi

dt= 1

νBj

dnBj

dt∀ i, j. (2)

where the stoichiometric coefficients areby definition negative for the educts,νAi

< 0, and positive for the products,νBj

> 0. Taking into account that the molenumbers of the educts are decreasing(dnAi

/dt < 0) and those of the products areincreasing (dnBi

/dt > 0), the reaction rater (mol sec−1) is always a positive number(r > 0).

Electrochemical reactions are charac-terized by at least one electron chargetransfer step taking place at the elec-trode/electrolyte interface. Therefore, theelectron e− appears as a reactant in thereaction equation with its stoichiometriccoefficient νe

∑i

νAiAi + νee

− =∑j

νBjBj , (3)

Typical electrode reactions are given inTable 1.

Applying Eq. (2) to the reactant e−, thereaction rate normalized to the geometrical

1.4 Fundamentals 7

Tab. 1 Electrode reactions and standardpotentials Eo [18]

Electrode reaction StandardpotentialEo/V vs

SHE

Li+ + e− → Li −3.045Na+ + e− → Na −2.711Al3+ + 3e− → Al −1.7062H2O + 2e− → H2 + 2OH− −0.8277Zn2+ + 2e− → Zn −0.763Fe2+ + 2e− → Fe −0.409Ni2+ + 2e− → Ni −0.230Pb2+ + 2e− → Pb −0.1262H3O+ + 2e− → H2 + 2H2O 0Cu2+ + 2e− → Cu 0.340[Fe(CN)6]3− + e− → [Fe(CN)6]4− 0.356O2 + 2H3O+ + 2e− →

H2O2 + 2H2O0.682

Fe3+ + e− → Fe2+ 0.771Ag+ + e− → Ag 0.799O2 + 4H3O+ + 4e− → 6H2O 1.229Cl2 + 2e− → 2Cl− 1.37Ce4+ + e− → Ce3+ 1.443MnO−

4 + 8H3O+ + 5e− →Mn2+ + 12H2O

1.491

O3 + 2H3O+ + 2e− →O2 + 3H2O

2.07

F2 + 2e− → 2F− 2.87

electrode surface area Ae can be written as

r = 1

Aeνe

dne

dt(4)

The number of moles ne transferredwithin a certain time t can be expressed bythe electrical charge Qt = I t = Aeit andthe Faraday constant F = 96 485 As mol−1

ne = Qt

F(5)

The linear relation

r = i

νeF, (6)

which results from (4) and (5) shows thatthe rate of an electrochemical reaction r

(mol cm−2 sec−1) is proportional to thecurrent density i (A cm−2), the sign ofwhich is determined by the sign of thestoichiometric coefficient νe. It follows thatthe current density is negative (cathodic),if the electrode reaction describes areduction process (e.g., Eq. 3), and positive(anodic) for an oxidation process (oppositedirection).

Equation (6) is the differential form ofFaraday’s law. The common expression isobtained by integration of Eq. (2)

1

νP

dnP

dt= 1

νe

dne

dt(7)

when applied to the simplified electrodereaction νRR + νee

− = νpP with a singlereactant R and product P

mP = νPMP

νeFQt or with respect to the

reactant R mR = νRMR

νeFQt (8)

Here mP and mR are the mass andMP and MR the molar mass of R and P,respectively. Owing to the sign conventionof νi , the mass mR in Eq. (8) is negative,indicating that R is consumed during thereaction (Qt < 0).

A prerequisite for the validity of theseequations is that only the consideredreaction takes place at the electrode. Inreality, side reactions can occur in parallel,the individual contribution of which isgiven by their current efficiency �e.

1.4.1Current Efficiency

The classical definition of the currentefficiency �e is based on Faraday’s law andrelates the mole numbers nP of the productto that of the electron ne consumed in the


electrochemical reaction.

�eP = mPνeF

νPMPQt= nP/νP

ne/νe(9)

In that sense, �e is the ‘‘yield ofcharge’’ Qt consumed in the course of thereaction. The current efficiency defined byEq. (9) represents an average value. Thedifferential or point current efficiency isdefined by [17]

�e′P = 1

νP

dnP

dt

/ Aei

νeF(10)

which is consistent with the definition ofthe partial current density of the productP, iP = i�e′

P .

1.4.2Space-time Yield

Another performance criterion, which is ofparticular importance for the calculation ofcapital investment and running costs of anelectrochemical reactor is the space-timeρst yield, which is the amount of productmP produced per unit of time t and reactorvolume Vr

ρst = 1

Vr

dmP

dt(11)

Substitution of mP from Eq. (8) leads tothe point space-time yield

ρst = νPMP

νeF�eaei(t) (12)

which contains a design factor (the specificelectrode area ae = Ae/Vr(cm2/cm3)) anda reaction factor (the product of currentdensity and current efficiency). It isevident that optimization will be directedtoward enlargement of ae and increasein the reaction rate, that is, the currentdensity i. The latter can be achieved,for example, by shifting the reaction

from charge transfer to a mass-transport-controlled regime, where the diffusion-limited current density, as the maximumof the reaction rate, can be reached

id,lim = − νe

νRF kmcR (13)

This leads to the following expression:

ρst = −νP

νRMP�eaekmcR (14)

In this case, ρst is proportional to themass-transport coefficient km, which canbe increased by forced convection, forexample, through agitation of the solu-tion, enhanced electrolyte flow, turbulencepromoters, or setting the electrodes in mo-tion. Equations (12) and (14) are the keyformula for the optimization of electro-chemical cell design. Equation (14) is ofparticular importance for reactions withlow concentrations, for example, recy-cling of metals from spent solutions orwastewater.

1.5Electrochemical Thermodynamics

The Galvani potential difference arisingat the electrode/solution interface, �ϕ =ϕs − ϕe, depends specifically on the en-ergy and density of electronic states inthe two phases in contact. In principle,it is impossible to measure �ϕ at asingle electrode. By combining two elec-trodes 1 and 2 to an electrochemical cell,the difference between the two interfacepotentials can be measured as the cellvoltage Uc = �ϕ1 − �ϕ2. To normalizethis potential measurement, the standardhydrogen electrode (SHE) is used as a ref-erence and the cell voltage Uc is denotedas electrode potential E versus SHE. The

1.5 Electrochemical Thermodynamics 9

electrode potential at equilibrium, Eo, de-pends on the concentrations ci or activitiesai of the reactants according to the Nernstequation

Eo = Eo + RT

νeF

∑i

νi ln ai (15)

where Eo is the standard potential at ai =1, T = 298 K, and pressure p = 1.013 ×105Pa = 1013 bar. Table 1 contains thestandard potentials Eo of the selected elec-trode reactions. The standard potentialsEo versus SHE or cell voltages Uo

c can becalculated from thermodynamic data.

The standard Gibbs free energy of theoverall cell reaction �Go

298 is related to theelectrical work nFUo

c ,

�Go298 = −nF Uo

c (16)

which can be gained under quasireversibleoperating conditions of the cell, that is, atan infinitely low reaction rate.

�Go298 is the stoichiometric sum of the

Gibbs free energies of formation, �Gof,i ,

of the reactants involved in the reaction

�Go298 =

∑νi�Go

f,i (17)

In (16), n is the number of elec-trons transferred in the overall processto maintain electroneutrality. Thermody-namic data for many chemical reactantsand compounds are available as standardenthalpies �H o

f,i and entropies Sof,i from

thermodynamic tables in handbooks, forexample, [53, 54]. For the elements andfor the proton H+ (aq) in aqueous solu-tion (H3O+), the �H o

f,i values are zero bydefinition. Thermodynamic data for sometypical electrochemical reactants are givenin Table 2.

These data are related to the Gibbsfree energy of reaction �Go

298 by theGibbs–Helmholtz equation

�Go298 = �H o

298 − T �So298 (18)

where T �So is the reversible heat ex-changed with the environment. The en-tropy change �So can be either positiveor negative and is directly related to thetemperature coefficient of the cell

(∂Uo

c

∂T

)p

= − 1

nF

∂�Go298

∂T= 1

nF�So

298

(19)

Tab. 2 Thermodynamic data of typical reactants [41]

Reactant State of aggregation �Ho298[kJmol−1] So

298[JK−1mol−1]

H2 g 0 130.74Cl2 g 0 223.09H3O+ aq 0 0Cl− aq −167.54 55.13O2 g 0 205.25H2O l −285.25 70.12Zn0 s 0 41.65Zn++ aq −152.51 −106.54HCl g −92.53 186.79C s 0 5.69CO g −110.5 198.0

(g: gas; l: liquid; s: solid; aq: aqueous solution of activity one)


The �Go298 values are calculated from

the stoichiometric sum of the enthalpiesof formation �H o

f,i and the entropies Sof,i

of the species involved

�Go298 =

∑i

νi�H of,i − T

∑i

νiSof,i

(20)

If �Go298 is negative, the reaction is ex-

ergonic and should occur spontaneously,otherwise it is endergonic and does notoccur or occurs in the reverse directiononly. The negative sign in Eq. (16) impliesthat the cell voltage or standard potentialof an exergonic reaction must be positive.This calls for a definition of the cell voltagemeasurement. According to IUPAC [51],the cell voltage is the potential differencebetween the right half cell I and the lefthalf cell II, as shown in Fig. 3. The direc-tion of the electronic current in the outerelectronic circuit is defined from left toright, which means that cathodic reductionmust take place in half cell I and anodicoxidation in half cell II.

This is demonstrated by combining anoxygen electrode and a hydrogen electrodeas the two half cells of an electrochemicalcell. The half cell reactions are commonlyformulated as reductions:

Oxygen electrode:

1

2O2 + 2H+ + 2e− = H2O with

EoO2/H2O = +1.229 V versus SHE

(21)

Hydrogen electrode:

H+ + e− = 1

2H2 with Eo

H+/H2

= 0.0 V versus SHE (22)

If the oxygen electrode is placed on theright and the hydrogen electrode on the leftin Fig. 3, according to IUPAC convention,the electrode reactions must be written

1

2O2 + 2H+ + 2e− = H2O (23)

H2 = 2H+ + 2e− (24)

so that the sum yields the overall cellreaction and the electrons are cancelled.

1

2O2 + H2 = H2O (25)

For the example considered, the overallelectron-transfer number in Eq. (16) isn = 2.

Oxidation Reduction

e−

e− e− Half cell IHalf cell II

e−

− +

Cell voltage Uc[V]

Fig. 3 Electrochemical cell forthe definition of cell voltagemeasurement according to theIUPAC convention [51].

1.5 Electrochemical Thermodynamics 11

The cell voltage under standard con-ditions is the potential difference be-tween the right and left half cell: Uo

c =Eo

O2/H2O − EoH+/H2

= 1.229 V. Since Uoc

is positive, the Gibbs free energy ofthe reaction is negative according toEq. (16)

�Go298 = −2.96485

· (1.299 − 0)VAs mol−1

= −237.160 kJ mol−1.

The cell reaction (25) occurs sponta-neously and the cell delivers electrical worknFUo

c , which is utilized in H2/O2 fuel cells(see also Chapter 8). Calculation of �Go

298from thermodynamic data in Table 2 yields

�Go298 = (�H o

H2Ofl−�H o

H2,g− 1

2�H o

O2,g)

− T (SoH2Ofl

− SoH2,g

− 1

2So

O2,g)

(26)

�Go298 = (−285.25 − 0 − 0) − 298

· (70.12 − 130.74 − 1

2205.25)

= −236.603 kJ mol−1

The reversible heat T �So =−48.647 kJ mol−1 is negative. It is releasedby the system and cannot be exploited aselectrical work.

If the two half cells are arranged oppositewith the hydrogen electrode on the rightand the oxygen electrode on the left, thecorresponding reactions are reversed andthe overall process refers to the electrolysisof water with hydrogen and oxygen as theproducts. In this case, the cell voltageUo

c is negative and �Go298 is positive.

The reversible heat T �So is positive andis taken up from the environment; thecell cools down in the course of the

reaction. This effect can be compensatedif the cell is operated at an increasedcell voltage generating the correspondingamount of Joule’s heat in the system. Thecorresponding cell voltage is denoted asthe thermoneutral voltage

Uth = − 1

nF�H o

298 with

�H o298 = �Go

298 + T �So298.

(27)

It is interesting to note that, in contrast tochemical reactions, electrochemical cellscan operate in principle against a positive�G if an outer current source is applied,driving the electrons in the oppositedirection. This is the principle of thesecondary batteries and electrochemicalpower sources, which can be chargedand discharged to accumulate and releaseelectrical energy.

1.5.1Energy Yield

The costs of electrochemical processes areclosely connected to the energy efficiencyor energy yield, which is either related tothe change of the Gibbs free energy of thereaction �G

γG = �G�e

nFUc= Uc,o

Uc�e, (28)

or to the thermal energy (enthalpy change�H )

γH = �H�e

nFUc= Uth

Uc�e. (29)

Here Uc,o = −�G/nF is the cell voltageat equilibrium, Uc is the actual cellvoltage under current operation, and Uth isthe thermoneutral voltage, Eq. (27), whichpermits isothermal operation.


1.5.2Specific Electrical Energy Consumption

For solid or liquid products, the specificelectrical energy consumed is usually massrelated

Ees = UcνeF

νPMP�e (J kg−1) (30)

1.6Electrochemical Cell Design

Different electrochemical cells have beendeveloped for various applications in thepast. According to the criteria of highmass-transport coefficient km and/or largespecific electrode area ae, as predicted byEq. (14), these cells can be classified as [20]:

• Cells with relatively small electrode areabut improved mass transport to increasekm by setting the electrodes in motionor by applying turbulence promoters.Examples are the pump cell [55, 56], theChemelec cell [57], the ECO cell [58–60],the beat-rod cell [61, 62], and cells withvibrating electrodes or electrolytes [63].

• Cells with enlarged electrode area in asmall cell volume are found in themultiple-cathode cell [64], the Swiss-rolecell [65], the extended surface electroly-sis (ESE) cell [66], and the capillary gapcell [13, 67].

• Cells with three-dimensional electrodesproviding enlarged specific electrode areaand improved mass transport due to thespecific fluid dynamics inside the three-dimensional structure are, for example,the porous flow-through cell [68], theRETEC (RETEC is a trademark ofELTECH Systems Inc., Cardon, Ohio)cell [15], the packed-bed cell [69–71],

the fluidized-bed cell [72–76], and therolling tube cell [77, 78].

Some of them are shown, for example,in Fig. 4(a–c).

The pump cell is the rotating analogue tothe capillary gap cell [67]. Although thiscell has not yet been employed on anindustrial scale, it is interesting becauseof its operation principles. It consists oftwo static disc electrodes with currentconnections and a bipolar rotating diskin between. The electrolyte is pumpedradially from the central tube to theouter circumference by the rotating disk.The advantage of this design is that themass-transfer coefficient can be controlledindependent of the flow rate and theresidence time of the electrolyte.

The Chemelec cell, Fig. 4(b), uses a flu-idized bed of glass spheres as turbulencepromoters to improve the mass transportto the electrodes consisting of a series ofclosely spaced gauze or expanded metalsheet. The residence time and the degreeof conversion per pass are relatively lowbecause the flow rate has to exceed thefluidization velocity of the bed. This cell istherefore suitable for pretreatment or recy-cling operations and is commonly used inthe electroplating industry for maintaininga moderate metal ion concentration in arecirculated wash-water tank.

High mass-transport coefficients areobtained in cells with a rotating cylinderelectrode (RCE) and a small gap betweenthe anode and the cathode, Fig. 4(a). Highrates of mass transport are experienced inthe turbulent flow regime, so that RCEreactors allow metal deposition at highspeed, even from dilute solutions. RCEreactors have been operated at a scaleinvolving diameters from 5 to 100 cm,with rotation speeds from 100 to 1500 rpmand currents from 1 A to 10 kA [79]. It

1.6 Electrochemical Cell Design 13

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ed-

bed

cath

ode

Larg

epa

rtic

le o

utle

t

Flo

w d

istr

ibut

or

Cat

holy

teA

noly

te

−+

−+

Ion-

exch

ange

mem

bran

e

(c)

Fig.

4(C

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.


is possible to design RCE reactors witha scrapping device to remove depositedmetal powder continuously from thecathode surface. The so-called ECO cell,Fig. 4(a), with a cascade of baffle platesaround the rotating cylinder cathode,which prevent back mixing of the solutionto a great extent, attains high degrees ofconversion.

A cell with a special design is the beat-rod cell, Fig. 4(a), which was mainly usedin small electroplating shops. Supportedcathode rods slowly rotate within anannular type of chamber in the electrolysistank. The rods strike one another so thatthe metal deposit is removed and settles asa powder at the bottom.

An efficient way of fitting large planarelectrodes in a small volume is the designof the Swiss-role cell, Fig. 4(b). Thin metalfoils are separated by a plastic mesh andwrapped around a central core. The ohmicdrop within the thin metal foil is largelycompensated by contacting the foils onthe opposite side. The electrolyte flowsaxially through the electrode pack. Largespace-time yields are realized and mass-transport conditions are favorable sincethe separator also functions as a turbu-lence promoter. Mesh electrodes wrappedaround a perforated winding core lead toanother version of the Swiss-role cell withradial flow. Cells with mesh electrodesinstead of foils are known as extendedsurface electrode ESE cells. A commercialsystem of this type is the RETEC cell,which contains 6–50 three-dimensionalcathodes as flow-through metal spongeelectrodes [15]. The cell is used in closedwater recycling systems in the electroplat-ing industry with applications similar tothose of the Chemelec cell, Fig. 4(b).

A simple three-dimensional electrode isobtained by using a packed bed of conduc-tive particulate material through which the

electrolyte is pumped. Numerous versionsof packed-bed cells have been described inthe literature [13, 68, 80–84]. The poten-tial distribution and optimum bed depthfor metal deposition, Mez+ + ze− = Me,was calculated as a function of the ef-fective particle and solution conductivities(κp, κs) [13, 20]. For rectangular cell geom-etry and fully developed diffusion limitedcurrent density within the particle bed,the following relation was found for theoptimal bed depth:

hopt =√

2εκs�η

aezFkmcMez+

(condition κp � κs) (31)

where ε is the voidage of the 3-D electrodestructure and �η the overvoltage range ofthe prevailing limiting diffusion currentdensity in the microkinetic polarizationcurve. According to Eq. (31), the optimalbed depth increases with decreasing metalion concentration by an inverse square rootdependence. This has led to the enViro-cell design depicted in Fig. 4(c), wherethe bed depth widens with decreasingconcentration along the flow direction ofthe solution.

The principle of fluidized-bed electrol-ysis originates from Fleischmann andGoodridge [73]. It has been examined fordifferent applications [72, 85, 86] and wasapplied on an industrial scale for metal re-covery [75, 76]. The electrolyte flows frombottom to top through a bed of fluidizedparticles, which are charged via a feederelectrode. The advantage is that the parti-cles are held in suspension, thus avoidingbed blockage by the deposited metal. Be-cause the height of the cell is restricted toabout 2 m for hydraulic reasons and thefluidization velocity has to be maintainedat a relatively high level, short residencetimes and thus only a limited degree of


conversion per pass can be achieved. Con-tinuous operation with recirculation of thesolution and cascade arrangement of cellshas therefore been used in practical appli-cations.

Moving and circulating bed electrodes, alsodenoted as a spouted bed, are describedin the literature [87]. As an example, therolling tube cell is shown in Fig. 4(c) [77, 78].In principle, this cell resembles the well-known plating process for piece goods,using slowly rotating barrels. The rotatingdrum is only partially filled with parti-cles to achieve thorough agitation. Thiscell has proved to be especially useful forsilver and gold recovery. Further develop-ments of moving bed electrodes and theirapplication can be found elsewhere [88].

In Fig. 5, the specific electrode areaae (cm−1) of various electrochemical cellsis shown as a function of a characteristiclength (interelectrode distance or parti-cle diameter). It can be seen that theelectrochemical cells differ significantlyin their specific area and characteristiclength by orders of magnitude. At a current

efficiency of �e = 1, the space-time yieldis mainly determined by the product

aei = I

Vr= j∗. (32)

The quantity j∗ (A cm−3) can be con-sidered as a volumetric current densityor a current concentration, also shown inFig. 5 as a function of the characteristiclength. Extremely high current densities,for example, 10 kA cm−2 in the chloralkalielectrolysis, can be applied only in a limitednumber of cases, the number of cells withhigh specific electrode area is restricted tocases of low current densities [13].

A special cell concept combining easeof construction and scale-up is the capil-lary gap cell, which is mainly applied inelectro-organic synthesis, where low con-ductivities of the electrolytes are a majorproblem, Figs 6 and 7, [13, 28, 67, 89].The cell consists of circular disk elec-trodes with a small interelectrode gap(1–2 mm) to minimize the ohmic volt-age drop in the electrolyte. The electrolyte

100

10

1.0

0.100.1 0.1 1 10

Spe

cific

ele

ctro

de a

rea

a e[c

m−1

]

Porous flow-through cell

Fluidized-bed cell

Swiss-roll cell

Packed-bed cell

Plate-and-frame cell

Chloralkali cell(mercury process)

Characteristic length[cm]

100

0.10

0.01

0.00100.1 0.1 1 10

Cur

rent

con

cent

ratio

n j*

[A c

m−3

]

Porous flow-through cell

Fluidized-bed cell

Swiss-roll cell

Packed-bed cell

Plate-and-frame cell

Chloralkali cell(mercury process)

Characteristic length[cm]

Fig. 5 Specific electrode area ae and volumetric current concentration j∗ as a function of thecharacteristic length of different electrochemical cells [13].


Current collector

Cell hood

Contact plate graphite

Graphite disks

Electrolyte flow

Base plate/connector

Inlet

Outlet

Product + H2

“Capillary gap”(1–2 mm)

Fig. 6 Scheme of the capillary gap cell of BASF AG [28].

Fig. 7 Cell room with capillary gap electrolyzers [28] (photo: BASF AG).

enters via a central channel and is radi-ally distributed between the circular diskelectrodes. In the bipolar configuration,the disks are isolated from each other andconnected at the end plates to the volt-age supply. To avoid reconversion of theproduct at the counterelectrodes in theundivided cells, the disk geometry andflow velocity must be carefully tuned toensure that the diffusion layer thickness,

which increases along the radius, does notreach the counterelectrode. Although thescale-up of the stack height and numberof disks is simple, the outer disk radiuscannot be enlarged. A number of electro-organic processes with methoxylation re-actions for production of, for example,anisaldehyde and tert-butylbenzaldehyde,were recently developed at BASF [90, 91]using a somewhat modified cell with


graphite felt electrodes. They were alsosuccessful in developing ‘‘paired elec-trolysis’’ with the simultaneous forma-tion of valuable products at the anodeand at the cathode. A methoxylationat the anode was combined, for exam-ple, with the reduction of phthalic aciddimethylester to phthalide, a valuableprecursor in pest management chem-istry [92].

Anode :R-C6H4-CH3−4e−+2 CH3OH

−−−→ R-C6H4-CH(OCH3)2 + 4 H+

Cathode :C6H4(COOCH3)2+4e−+4 H+

−−−→ C8H8O2 + 2 MeOH

Another cell design strictly directed to-ward minimization of the ohmic dropin the electrolyte, especially if gases aredeveloped at the electrodes, is the zero-gap cell, shown in Fig. 8 [17, 93, 94]. Theperforated electrodes are pressed directlyonto the diaphragm by the current col-lectors providing optimum contact acrossthe whole electrode area. However, uneven

current distribution and voltage drop inthe electrolyte as well as in the diaphragmcannot be avoided, as illustrated in Fig. 8.Zero-gap cells have been successfully usedfor alkaline water electrolysis [93, 94].

Another design that is used in chlo-ralkali electrolysis, water electrolysis, andelectro-organic synthesis [95–97] is thesolid polymer electrolyte (SPE) cell, wherean ion exchanger membrane, for exam-ple, Nafion, serves as the electrolyte,Fig. 9. The microporous catalytic reac-tion layers are pressed directly onto themembrane with porous current collectorsallowing transport of dissolved reactantsand gaseous products into and out of thereaction layer.

The advantages of SPE technology areas follows: electro-organic synthesis with-out an additional supporting electrolyte,reduced energy demand for separationand recycling processes, and eliminationof side reactions with the electrolyte atmoderate reaction conditions with ease ofprocess control.

The structure of the SPE electrolysiscell is similar to that of the membrane

Electrodes

Gas bubbles Schematicdistributionof currentlines

Diaphragm

Fig. 8 Gas evolution andcurrent distribution atperforated zero-gapelectrodes [13].


Fig. 9 Solid polymer electrolyte(SPE) cell with cation exchangermembrane Nafion [13].

Anode withcatalyst layer

Anodic currentcollector

Cathodic currentcollector

Nafion 120

Cathode withcatalyst layer

Metal mesh

Bipolarplate

electrode assembly in polymer electrolytemembrane (PEM) fuel cell technology,which is described in detail in Chapter 8.

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