bard060402 Photoelectrochemical Solar Energy Storage Cells

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4.2Photoelectrochemical Solar Energy StorageCells

Stuart LichtTechnion – Israel Institute of Technology,Haifa, Israel

4.2.1Introduction

Although society’s electrical needs arelargely continuous, clouds and darknessdictate that photovoltaic solar cells havean intermittent output. A photoelectro-chemical solar cell (PEC) can generatenot only electrical but also electrochem-ical energy and provide the basis for asystem with an energy-storage compo-nent (PECS). Sufficientlyž energetic in-Q1

solation incident on semiconductors candrive electrochemical oxidation/reductionand generate chemical, electrical, or elec-trochemical energy. Aspects include ef-ficient dye-sensitized or direct solar toelectrical energy conversion, photoetch-ing, photoelectrochemical water-splitting,environmental cleanup, and solar energystorage cells. This chapter focuses onphotoenergy storage concepts based onphotoelectrochemical processes, but in-cludes a necessary comparison to othermethods proposed for the conversion andstorage of solar energy. The PEC useslight to carry out a chemical reaction,converting light to chemical energy. Thisfundamental difference of the photovolaticsolar cell’s (PV) solid–solid interface, andthe PEC’s solid–liquid interface has sev-eral ramifications in cell function andapplication. Energetic constraints imposedby single band gap semiconductors havelimited the demonstrated values of photo-electrochemical solar to electrical energyconversion efficiency to 16%, andmultiple

band gap cells can lead to significantlyhigher conversion efficiencies [1, 2a,b].Photoelectrochemical systems may facil-itate not only solar to electrical energyconversion but has also led to investiga-tions in photoelectrochemical synthesis,photoelectrochemical production of fuels,and photoelectrochemical detoxification ofpollutants as discussed in other chaptersin this volume.

4.2.1.1 Regenerative PhotoelectrochemicalConversionIn illuminated semiconductor systems,the absorption of photons generates ex-cited electronic states. These excited stateshave lifetimes of limited duration.Withouta mechanism of charge separation theirintrinsic energy would be lost through re-laxation (recombination). Several distinctmechanisms of charge separation havebeen considered in designing efficientphotoelectrochemical systems. At illumi-nated semiconductor–liquid interfaces, anelectric field (the space charge layer) occursconcurrent with charge–ion redistribu-tion at the interface. On photogenerationof electron-hole pairs, this electric fieldimpedes recombinative processes by op-positely accelerating and separating thesecharges, resulting in minority carrier in-jection into the electrolytic redox couple.This concept of carrier generation is il-

lustrated in Fig. 1(a) (for an n-type PEC)and has been the theoretical basis for sev-eral efficient semiconductor-redox couplePEC cells. Illumination of the electrodesurface with light, whose photon energyis greater than the band gap, promoteselectrons into the conduction band leav-ing holes in the valance band. In the caseof a photoanode, band-bending in the de-pletion region drives any electron that ispromoted into the conduction band intothe interior of the semiconductor andholes

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2 Solar Energy Conversion without Dye Sensitization

Energygain

Counterelectrode

EC

EF

ECD∗

D

EF

EV EV

n-typesemiconductor

Redoxelectrolyte

Counterelectrode

n-typesemiconductor

Dye Redoxelectrolyte

Load Load

E E

Light Light

Energygain

Red Ox+

++

− − −

−

−

−Red Ox+

(a) (b)

Fig. 1 Carrier generation under illumination arising at (a) the semiconductor–liquid interface and(b) the semiconductor–dye sensitizer–liquid interface.

in the valance band toward the electrolyte,where they participate in an oxidation re-action. Electrons through the bulk drive anexternal load before they reach the counterelectrode or storage electrode, where theyparticipate in a reduction process. Underillumination and open circuit, a negativepotential is created in a photoanode and asa result the fermi level for the photoanodeshifts in the negative direction, thus reduc-ing the band-bending. Under illuminationwith increasing intensity, the semiconduc-tor fermi level shifts continually towardnegative potentials until the band-bendingeffectively reduces to zero, which corre-sponds to the flat band condition. At thispoint, a photoanode exhibits its maximumphotovoltage, which is equal to the barrierheight.Excitation can also occur in molecules

directly adsorbed and acting as a medi-ator at the semiconductor interface. Inthis dye sensitization mode, the func-tion of light absorption is separated fromcharge carrier transport. Photoexcitationoccurs at the dye and photogenerated

charge is then injected into a wide bandgap semiconductor. This alternative car-rier generation mode can also lead toeffective charge separation as illustratedin Fig. 1(b). The first high solar to elec-tric conversion efficiency example of sucha device was demonstrated in 1991 [3]through theuse of a novel high surface area(nanostructured thin film) n-TiO2, coatedwith a well-matched trimeric rutheniumcomplex dye immersed in an aqueouspolyiodide electrolyte. The unusually highsurface area of the transparent semi-conductor coupled to the well-matchedspectral characteristics of the dye leadsto a device that harvests a high proportionof insolation.

4.2.1.2 Photoelectrochemical StoragePECs can generate not only electricalbut also electrochemical energy. Figure 2presents one configuration of a PEC com-bining in situ electrochemical storageand solar-conversion capabilities; provid-ing continuous output insensitive to dailyvariations in illumination. A high solar to

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4.2 Photoelectrochemical Solar Energy Storage Cells 3

0.8

m C

sHS

1.8

m C

sHS

1.8

m C

sOH

1.8

m C

sHS

1.8

m C

sOH

CoS

PO

LYS

ULF

IDE

RE

DU

CT

ION

CoS PO

LYS

ULF

IDE

RE

DU

CT

ION

0.8

m C

sOS

1.0

m C

s 2S

4

0.8

m C

sHS

0.8

m C

sOS

1.0

m C

s 2S

4

ME

MB

RA

NE

ME

MB

RA

NE

TIN

SU

LF

IDE

RE

DU

CT

ION

TIN

OX

IDAT

ION

Sn

/Sn

S

Sn

/Sn

S

hn

P P=0S

S

POLYSULFIDEOXIDATION

n-Cd(Se,Te) n-Cd(Se,Te)

LOAD LOADL L

(a) (b)

Fig. 2 Schematic of a photoelectrochemical solar cell combining both solar conversion and storagecapabilities. (a) Under illumination; (b) in the dark.

electric conversion efficiency cell config-uration of this type was demonstrated in1987 and used a Cd(Se,Te)/Sx conversionhalf cell and a Sn/SnS storage system,resulting in a solar cell with a contin-uous output [4]. Under illumination, asseen in Fig. 2(a), the photocurrent drivesan external load. Simultaneously, a por-tion of the photocurrent is used in thedirect electrochemical reduction of metalcations in the device storage half-cell. Indarkness or below a certain level of light,the storage compartment spontaneouslydelivers power by metal oxidation, as seenin Fig. 2(b).

4.2.2Comparative Solar-Storage Processes

4.2.2.1 Thermal Conversion and StorageSolar insolation can be used to directlyactivate a variety of thermal processes; theenthalpy is stored physically or chemicallyand then either directly utilized or released

upon reversal of the storage process.To date, the predominant nonbiologicutilization of solar energy is to heat aworking fluid that is maintained in aninsulated enclosure, storing a portion ofthe incident solar radiation for future use.Limitations of this approach include thelow energy available per unit mass ofthe storage medium and low efficienciesof thermal to mechanical and thermal toelectrical energy conversion. A variety ofpassive and dynamic optical concentratorshave been studied to compensate for theselimitations.The high temperatures generated by

concentrated solar power have been uti-lized to drive highly endothermic re-actions. The reverse reaction releasesthe chemically stored energy as ther-mal energy. Various systems have beeninvestigated such as in 1985 [5]:

SO3 ��! SO2 C 12O2 �H D 98.94

(1)

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4.2.2.2 Photochemical StorageIn photochemical processes, photon ab-sorption creates a molecular excitedstate or stimulates an interband elec-tronic transition in a semiconductorthat induces a molecular change. Com-prehensive reviews, including those byGratzel [6], Kalyanasundaram [7], andHar-riman [8], have discussed various as-pects of photochemical energy conver-sion. A photoactivated molecular excitedstate can drive either (1) photodissociation(2) photoisomerisation or (3) photoredoxreactions. Processes based on semicon-ductors may involve photovoltaic or photo-electrochemical systems.A substantial photochemical effort re-

search has centered on the photoredoxstorage of solar energy.Molecular photore-dox processes use electron transfer fromphotoinduced excited states:

AC hν ��! AŁ (2)

The electron transfer may be eitherdirect

AŁ ��! AC C e� (3)

or a variety of indirect processes asexemplified by either

AŁ ��! B;

followed by

B ��! BC C e� (4)

or AŁ C B ��! AB;

followed by

AB ��! ABC C e� (5)

Solar-activated photodissociation pro-cesses generally involve cleavage of asimple molecule into several energeticproducts. Limitations of this approachinclude the limited absorption of solarenergy by the molecule, low quantum

yield, rapid back reaction, and difficultiesseparating the product species. An exam-ple of storage by such photodissociationprocesses is exemplified by

2NOCl ��! 2NOC Cl2 (6)

In photoisomerization, an absorbedphoton activates molecular rearrangementand conversion of organic molecules intostrained isomers. The products are stored.Despite attractive features including a highheat storage capacity and good thermalstability, most systems tested have poorefficiencies. These systems necessitatetransformation to strained conformationsat high energies; energies consistent withwavelengths below 450 nm. This excludesmuch of the energy inherent in theAM1 ž solar spectra. The stored thermalQ2

energy is released on catalytic-inducedreversion to the starting components. Anexample has been presented by Kutalfor the photocatalytic transformation ofnorbornadiene to quadricyclane [9].Photochemical redox reactions can gen-

erate fuel formation, including H2, CH3,and CH3OH. Because of its availability,the splitting of water to produce H2 hasbeen the focus of particular attention. H2Ois transparent to Wž or visible radiation,Q3

and therefore sensitization is required todrive the water-splitting process. In earlyattempts on photoredox-splitting of waterby Heidt and McMillian, the process wassensitized by using solution redox speciessuch as Ce3C/4C [10].

Ce3C(aq)CHC ��! Ce4C(aq)C 12H2

(7)2Ce4C(aq)CH2O ��! 2Ce3C(aq)

C 2HC C 12O2 (8)

These processes have displayed poorquantum yields. As in photoisomerization

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∗Ru(bpy)3 2+

Ru(bpy)3 3+

nνDecomposition

∗TEOA

TEOA

TEOA

Mv+•Pt

Mv++

Mv++

N·(CH3·CH2·OH)3

CH3 — N N — CH3

+

+

H2

H2O

Ru(bpy)3 2+

Sch. 1

processes, these reactions are also gener-ally driven only by high-energy radiation(short wavelength) and cannot efficientlyconvert incident AM1 solar radiation.H2 or O2 generation from water is

a multielectron process. Optimization ofphotoredox-splitting of water necessitatesthe presence of a catalyst to mediatethis complex multielectron transfer. Inone such process, a sacrificial reagenttriethanolamine (TEOA) is consumed ir-reversibly in the process, as denoted inSch. 1 [9]:Direct multielectron processes are rare

and instead incorporate one or more rad-ical intermediate steps. These reactiveintermediates are susceptible to unfavor-able side reactions, resulting in substantiallosses in the energy-conversion process.Kinetically favored back reactions furtherreduce the overall conversion efficiency.The engineering of these complex molec-ular organizations provides a substantialscientific challenge and have generally re-sulted in systems with low conversionefficiencies.

4.2.2.3 Semiconductor PhotoredoxStorageSemiconductor surfaces have been usedas sensitizers to drive photochemical con-version and storage of solar energy. In

principle, this should lead to a higher levelof photon absorption and more effectivecharge separation. Both effects can sub-stantially increase solar photochemical-conversion efficiency, but these systemshave not yet displayed high efficiencies offuel generation or long-term stabilities.Photoredox processes at semiconductor

electrodes generating fuels or productsother than hydrogen, including methanoland ammonia, have been attempted withlow overall yields. The photoelectrolysis ofHI into H2 and I3� at p-InP electrodes hasbeendescribed [11]. TheseH2 and I3� pho-togenerated products are prime candidatesfor a fuel cell. Analogous advanced sys-tems, in which the photoelectrochemicallygenerated fuels have been successfully re-combined to generate electrical energy, arediscussed later in this section.A system exemplifying photoelectro-

chemical synthesis to generate hydro-gen is water photoelectrolysis. An earlydemonstration of water photoelectroly-sis used TiO2 (band gap 3.0 eV) andwas capable of photoelectrolysis at ¾0.1%solar to chemical energy–conversion ef-ficiency [12]. The semiconductor SrTiO3was demonstrated to successfully split wa-ter in a direct photon-driven process byBolts and Wrighton (1976), albeit at lowsolar energy–conversion efficiencies [13].

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The high SrTiO3 band gap, Eg, of 3.2 eVcreates sufficient energetic charge to drivethe photoredox process. This excludesthe longer wavelength photons and corre-sponds to only a small fraction of incidentsolar radiation. To improve the solar re-sponse, Eg has to be lowered; in a singleband gap system, an optimum efficiencycan be expected around 1.4 eV.In photoelectrochemical water-splitting

systems, corrosion of the semiconduc-tor photoelectrodes can pose a significantproblem. Most surface-stabilizing redoxreactions compete with oxygen and hy-drogen generation and must be excludedfrom these systems. To enhance the solarresponse of high band gap materials, tech-niques such as dye sensitization and im-purity sensitization have been attempted,although with little improvement [14].Semiconductor surfaces have been mod-ified to protect low band gap materialsagainst photocorrosion [15, 16]. A self-driven photoelectrochemical cell consist-ing of Pt-coated p-InP andMn-oxide-coatedn-GaAs has been demonstrated to operateat 8.2% maximum efficiency to generateH2 and O2 under simulated sunlight [17],and more recently a two band gap cell in atandemarrangement has beenused to splitwater at 12% efficiency [18]. A multijunc-tion GaAs, Si cell has been recently usedto drive water-splitting at over 18% solar toelectrical-conversion efficiency [19].Colloids and suspensions of semicon-

ductors have been used for the photoredox-splitting of water. The principle advantageof a fine suspension is the large active sur-face area available. Reaction rates of H2

and O2 generation have been enhanced byloading the particles with small deposit ofprecious metals, and although significantprogresswasmade in this direction, a prac-tical system is yet to be demonstrated [11].

4.2.3Modes of Photoelectrochemical Storage

Conversion of a regenerative PEC to a pho-toelectrochemical storage solar cell (PESC)can incorporate several increasingly so-phisticated solar energy conversion andstorage configurations.

4.2.3.1 Two-Electrode ConfigurationsA variety of two-electrode configurationshave been investigated as PESC systems.Important variations of these photoelec-trochemical conversion and storage con-figurations are summarized in Table 1. Ineach case, and as summarized in Fig. 3for the simplest configurations, exposureto light drives separate redox couples anda current through the external load. Thereis a net chemical change in the system,with an overall increase in free energy.In the absence of illumination, the gener-ated chemical change drives a spontaneousdischarge reaction. The electrochemicaldischarge induces a reverse current. Ineach case in Table 1, exposure to lightdrives separate redox couples and currentthrough the external load.Consistent with Fig. 1, in a regenerative

PEC, illumination drives work throughan external load without inducing a netchange in the chemical composition ofthe system. This compares with the two-electrode PECS configurations shown inFig. 3(a) and (b). Unlike a regenerativesystem, there is a net chemical changein the system, with an overall increasein free energy. In the absence of illu-mination, the generated chemical changedrives a spontaneous discharge reaction.The electrochemical discharge inducesa reverse current. Utilizing two quasi-reversible chemical processes, changestaking place in the systemduring illumina-tion can be reversed in the dark. Similar to

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Tab. 1 Important two-electrode photoelectrochemical conversion and storageconfigurations

SCHEME Electrode 1 Electrolyte(s) Electrode 2

I SPE j Redox A Redox B j CEII SPE j Redox A-membrane-Redox B j CEIII SPE j Redox A j Redox BCE-CEIV SPE-Redox ASPE j Redox B j CEV SPE j Redox A-membrane-Redox B j SPE

Note: Components of these systems include a semiconductor photoelectrode (SPE) and acounter electrode (CE). At the electrode–electrolyte interface, redox couples ‘‘A’’ or ‘‘B’’are either in solution (j Redox j), counter electrode–confined (j Redox BCE-CE) or confinedto the semiconductor photoelectrode (SPE-Redox ASPEj).

e−

x−

L

R

O

hν

R′

O′

n-typesemiconductor

e

x−

L

O

R

hνR′

O′

n-typesemiconductor

Membrane Counterelectrode

Storageelectrode

Fig. 3 Schematic diagram of a two-electrode storage cell. The storage electrode (a) aninsoluble redox couple, (b) soluble redox couple.

RCO0hν��! �� OC R0

a secondary battery, the system dischargesproducing an electric flow in the oppositedirection and the system gradually returnsto the same original chemical state.Each of the cells shown in Fig. 3

has some disadvantages. For both bound(Fig. 3a) and soluble (Fig. 3b) redox cou-ples, the redox species may chemically re-act with and impair the active materials ofthe photoelectrode. Furthermore, duringthe discharge process, the photoelectrode

is kinetically unsuited to perform as acounter electrode. In the absence of il-lumination, the photoelectrode P, in thiscase a photoanode, now assumes therole of a counter electrode by supportinga reduction process. For the photoan-ode to perform efficiently during illu-mination (charging), this very same re-duction process should be inhibited tominimize photooxidation back reactionlosses. Hence, the same photoelectrode

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cannot efficiently fulfill the duel role ofbeing kinetically sluggish to reduction dur-ing illumination and yet being kineticallyfacile to the same reduction during darkdischarge. The configuration representedin Fig. 3 has another disadvantage, thedisparity between the small surface areaneeded to minimize photocurrent darkcurrent losses and the large surface areanecessary tominimize storage polarizationlosses to maximize storage capacity [20].

4.2.3.2 Three-Electrode ConfigurationsSeveral of the two-electrode configura-tion disadvantages can be overcome byconsidering a three-electrode storage cell

configuration as shown in Fig. 4. In Fig. 4,the switches E and F are generally alter-nated during charge anddischarge.Duringthe charging, only switch E may be closed,facilitating the storage process, and dur-ing discharge, E is kept open while Fis closed. In this case, chemical changesthat took place during the storage phaseare reversed, and a current flow is main-tained from the storage electrode to a third(counter) electrode that is kept in the firstcompartment. To minimize polarizationlosses during the discharge, this third elec-trode should be kinetically fast to the redoxcouple used in the first compartment.Still an improved situation would be to

E

F

L•

• •

•

hν

R R

EL

R′

O′O

O

P A M S

First compartment Second compartment

Fig. 4 Schematic diagram for a storage system with a third electrode(counter electrode) in the photoelectrode compartment. P D Photoelectrode,A D Counter electrode, M D Membrane, S D Storage electrode,EL D Electrolyte, E, F D Electrical switch, and L D Load.

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have both switches closed all the time. Inthis case, electric current flows from thephotoelectrode to both counter and stor-age electrodes. The system is energeticallytuned such that when insolation is avail-able, a significant fraction of the convertedenergy flows to the storage electrode. Inthe dark or diminished insolation, thestorage electrode begins to discharge, driv-ing continued current through the load.In this system, a proper balance shouldbe maintained between the potential ofthe solar energy–conversion process andthe electrochemical potential of the stor-age process. There may be residual electricflow through the photoelectrode duringdark cell discharge, as the photoelectrodeis sluggish, but not entirely passive, to areduction process. This can be correctedby inserting a diode between the photo-electrode and the outer circuit.

4.2.4Optimization of PhotoelectrochemicalStorage

The power obtained is the product of volt-age and current, and consideration of thephotocurrent is as important as the photo-voltage. If the band-bending is sufficientlylarge, then the minority carrier redox re-action, which is essential to maintainthe photocurrent, can compete effectivelywith the recombination of photogeneratedelectron hole pairs. This recombinationrepresents a loss of absorbed photo en-ergy. Therefore, an objective is to maintaina high band-bending and at the same timea significant photovoltage. A photoanodecreates a negative photovoltage under il-lumination, which results in reducing theband-bending. In principal, one way to ac-complish high band-bending is to choose avery positive redox couple in the electrolyte.The converse is true for a photoanode.

Improvements relating to its stability andconversion efficiency are of paramountimportance.

4.2.4.1 Improvements of thePhotoresponse of a PhotoelectrodeTo improve the solar response of aphotoelectrode, a proper match betweenthe solar spectrum and the band gap ofthe semiconductor should be maintained.When a single band gap semiconductor isused, a band gap in the vicinity of 1.4 eVis most desirable from the standpoint ofoptimum solar-conversion efficiency. Animportant criterion is that the minoritycarrier that is driven toward the semi-conductor–electrolyte interface should notparticipate in a photocorrosion reactionthat is detrimental to the long-term stabil-ity of the photoelectrode. Photocorrosioncan be viewed in terms of either kineticor thermodynamic considerations and thereal cause may be a mixture of both. Fromthermodynamic perspective, a photoanodeis susceptible to corrosion if the fermi levelfor holes is at a positive potential with re-spective to the semiconductor corrosionpotential [21]. The corrosion can be pre-vented or at least inhibited by choosing aredox couple that has its Eredox more nega-tive than that for the corrosion process [22,23]. The kinetic approach has been to allowanother desired redox process to occur at amuch faster rate than the photocorrosionreaction [13]. Other attempts to minimizethe photocorrosion has been to coat thephotoelectrode surface with layers such asSe [24] and protective conductive polymerfilms [25], and to search for alternate lowband gap semiconductors [26]. Extensivereviews on the performance and stabilityof cadmium chalcogenides include thoseby Cahen and coworkers, 1980 [27] andHodes, 1983 [28]. Etching of photoelec-trode surface has been recognized and

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widely used as an important treatmentto achieve high-conversion efficiency [29].This effect is mostly attributed to removalof surface states that may act as trap-ping centers for photogenerated carriers.A related procedure called photoetching,initially developed for CdS and then ap-plied to a wide variety of semiconductors,improves the photoelectrode performanceand preferentially removes the surface de-fects acting as recombination centers [30].In addition to the variety of etching pro-

cedures, several other surface treatmentshave been used to improve photoelectrodeperformance. Examples include aGa3C iondip on CdSe [31], ZnCl2 dip on thin filmCdSe [32], Ru onGaAs [33], Ru on InP [34],and Cu on CdSe [34]. Reasons explainingthe effectiveness of these dips range from adecrease of dark current to electrocatalysisby surface-deposited metal atoms. Solu-tion phase chemistry of the electrolyte is animportant parameter that has been shownto dramatically influence photoeffects. Theequilibrium position of the redox cou-ple will affect equilibrium band-bending.A photoanodic system with a solutioncontaining a more positive potential re-dox couple causes a greater band-bending,which in turn leads to a higher photovolt-age and efficient carrier separation undernormal experimental conditions.

4.2.4.2 Effect of the ElectrolyteSemiconductor photoeffects in a complexredox electrolyte are largely affected by thesolution properties such as solution re-dox level, interfacial kinetics (adsorption),conductivity, viscosity, overall ionic activ-ity, solution stability, and transparencywithin a crucial wavelength region. Re-dox electrolytes are known to inhibitunfavorable phenomena such as surfacerecombination and trapping [35]. In addi-tion, solution redox couple may develop

a favorable influence on the PEC sys-tem by improved charge-transfer kineticsleading to improved stability of the pho-toelectrode [36]. Additives incorporated inredox electrolytes are known to enhancethe performance of PEC systems. Additionof small concentrations of Se in polysulfideelectrolyte is known to improve the sta-bility of CdSe (single crystal)/polysulfidesystem [22]. In this case, Se improves thePEC performance by reducing S/Se ex-change and by increasing the dissolutionof the photooxidized product S, which isthe rate-determining step in the oxidationof sulfide at the anode. Addition of Cu2Cinto the I�/I3� electrolyte is known toimprove the stability of CuInX2 photoelec-trode considerably [37], and in the sameelectrolyte, tungsten and molybdenumdichalcogenide photoelectrochemistry canbe substantially improved by addition ofAgC, or other metal cations, and shift ofthe I�/I3�Eredox [38].In the case of CdSe/polysulfide system,

solution activity, conductivity, efficiencyof the photoanode (fill factor), charge-transfer kinetics at the interface, and thestability of the photoelectrode are knownto exhibit improvements in the trendLi > Na > K > Cs > for alkali polysulfideelectrolyte. This trend is explained in termsof the secondary cation effect on electro-chemical anion oxidation in concentratedaqueous polysulfide electrolytes [39]. Inthe case of Cd(Se,Te)/polysulfide system,the efficiency of light energy conversionis improved by using a polysulfide elec-trolyte without added hydroxide becauseof the combined effect of increasing thesolution transparency, relative increase ofS42�, and decrease in S32� in solution.For the same photoelectrode–electrolytesystem, an optimum photoeffect was ob-served for a solution containing a sul-fur–sulfide ratio of 1.5 : 2.1 with l : 2 molal

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potassium sulfide concentrations becauseof the combined effect of optimized solu-tion viscosity, transparency, activity, andshift in solution redox level [40]. Stabil-ity of the polysulfide redox electrolyte,which is another parameter that deter-mines the long-term performance of aPEC cell, has been shown to increase withsulfur and alkali metal sulfide concentra-tion and to decrease with either increasing–OH� concentration or at high ratio ofadded sulfur to alkali metal sulfide [39,41]. The combined polysulfide electrolyteoptimization can substantially enhancecadmiumchalcogenide photoelectrochem-ical conversion.Chemical composition of the electrolyte

is a particularly important parameterin PEC systems based on complexelectrolytes, such as polysulfide or ferro/ferricyanide. In the latter redox couple,replacement of a single hexacyano ligandstrongly changes the photoelectrochemicalresponse of illuminated n-CdSe [42], andaddition of the KCN to the electrolyte canincrease n-CdSe and n-CdTe photovoltageby 200 mV [43].

4.2.4.3 Effect of the Counter ElectrodeIn a photoanodic system, even at mod-erate current densities, the occurrenceof sluggish counter electrode kinetics forthe cathodic process will cause significantpolarization losses and diminish the pho-tovoltage. Minimization of these kineticlimitations necessitates a counter electrodewith good catalytic properties. For exam-ple, as shown by [34], CoS on stainlesssteel or brass electrodes exhibits elec-trocatalytic properties toward polysulfidereduction and overpotentials as low as1 mV cm2 mA�1 has been realized. Com-position of a particular redox electrolytemay have a bearing on the extent of counterelectrode polarization [32, 39].

In PEC systems, a compromise is main-tained to simultaneously optimize thephotoelectrode efficiency, stability, andelectrolytic properties of the electrolyte.Practical PEC systems often require largeworking and counter electrodes and theirgeometric configuration within the PECsystem will effect mass transport andeffective cell current. In some cases, ad-vantageous use has been made of selectivesluggish counter electrode kinetics towardcertain cathodic processes. For example,carbon is a poor cathode for H2 evolutioncompared to Pt, and the direct hydrogena-tion of anthraquinone at a PEC cathodehas been avoided by using a carbon an-ode [44]. In this case, such hydrogenationrepresents an undesirable side reaction.

4.2.4.4 Combined Optimization of Storageand PhotoconversionAn efficient photoelectrochemical conver-sion and storage system requires not onlyan efficient functional performance of theseparate cell components but also a systemcompatibility. In the combined photoelec-trochemical storage system, simultaneousparameters to be optimized include

1. minimization of light losses reachingthe photoelectrode,

2. high photoelectrode–conversion effi-ciency of solar energy,

3. close potentialmatch between the pho-topotential and the required storage-charting potential,

4. high current and potential efficiencyof the redox storage process,

5. high energy capacity of the redoxstorage,

6. reversibility (large number of charge–discharge cycles of the redox storage),

7. stability of the photoelectrode,8. stability of the electrolyte,9. stability of the counter electrode,

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10. economy and cost effectiveness, and11. reduced toxicity and utilization of

environmentally benign materials.

A photoelectrochemical solar cell implic-itly contains an electrolytic medium. Inthe majority of laboratory PEC configu-rations, incident light travels through theelectrolytic medium before illuminatingthe photoelectrode. The resultant light ab-sorbance by the electrolyte is a significantloss, which is avoided by use of a back cellconfiguration. For example, the substan-tial absorptivity of dissolved polysenidespecies has been avoided in a n-GaAs pho-toelectrochemistry through the use of theback wall cell configuration presented inFig. 5 [45].Thephotoelectrochemical systemshown

in Fig. 4 is a combination of a photoelec-trode, electrolyte, membrane, storage, and

a counter electrode. As an example of chal-lenges that may arise in the combinedphotoconversion and storage system, con-sider an n�CdSe/polysulfide/tin sulfideversion of Fig. 4 and consisting of Cell 1.

Cell: 1. CdSe j HS�, OH�, Sx2�jMembrane jHS�, OH�j SnS j SnWith illumination, the cell exhibits si-

multaneous photoelectrode, counter elec-trode and storage reactions, and equilibriaincluding

Photoanode:

HS� COH� ��! SC 2e� CH2O(9)

Photocompartment equilibria:

SC S2�x ��! S(xC 1)2� (10)

Counter electrode:

SC 2e� ��! HS� COH� (11)

Sapphirewindow

hν

hν

hν

Cu wire

Ag paste

Au grid

1.9 µmthickn-GaAs

Window

Epoxy

Photoelectrodecontact

Photoelectrodecontact

Counterelectrodecontact

Mountedthin-filmPEC

Polyselenide(aq)

Fig. 5 A back wall n-GaAs/aqueous polyselenide photoelectrochemical cell.

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Storage electrode:

SnSCH2OC 2e� ��! SnCHS�

COH� (12)

Unlike the case of the analogous re-generative PEC system, in the precedingequations, sulfur formed at the photoan-ode (and dissolved as polysulfide species,S(xC1)2� for x D 1 to 4) is not balanced bythe reduction that is taking place at thecounter electrode because of the simulta-neous reduction process taking place atthe storage electrode. As a result, sulfuris accumulated in the photoelectrode com-partment and is removed only in the sub-sequent discharge process. This dynamic

variation in electrolyte composition mayhave a profound influence on the stabilityof the photoelectrode and electrolyte andon cell potential. Hence, tominimize theseeffects, either excess polysulfide must beincluded in the photocompartment or alimit must be set to the maximum depthof cell charge and discharge.Another important consideration is the

energy compatibility between the photo-conversion and the storage processes. Thiscompatibility is referred to as the volt-age optimization. Figure 6 presents thecombined IV characteristics for an idealphotoelectrode and current–voltage curvesfor two potential redox processes; processA and process B. Vph is the maximum

Redox process B Redox process A

B:A:

EB EAVPh

Vmax

Potential

Cur

rent

P: IVphoto ISC

Fig. 6 Current–voltage curves forelectrochemical storage processes, A or B.Process A may be charged by the photodrivencurrent–voltage curve P, whereas process B maynot. In the photodriven IV curve P, Vmax is the

voltage corresponding to the point of maximumpower, Pmax, and Isc and Vph are the short-circuitcurrent and open-circuit photopotentials,respectively. EA and EB refer to the redox Q6potentials for storage processes, respectively.

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photovoltage that can be generated. Iscis the short-circuit photocurrent corre-sponding to maximum band-bending. InFig. 6, consider the electrochemical pro-cess represented by curve B. This processis located outside the region of poten-tials generated by the photoelectrode; itdoes not represent a potential storagesystem to be driven by a single photo-electrode. In such a case, a serial combi-nation of more than one photoelectrodewould be necessary. For a redox pro-cess to be a potential candidate for aredox storage system, the storage and pho-todriven current–voltage curves shouldintersect. Whereas Vph and Isc correspondto zero power, the point Pmax shown inFig. 6 corresponds to the maximum powerpoint. Solar energy conversion is accom-plished at its maximum efficiency onlyduring operation in the potential vicinityof Pmax.By adjusting the electrical load L, shown

in Fig. 4, the system can be constrainedto operate near its maximum power effi-ciency. In this case, if the counter electrodeis not polarized, the potential differencebetween photoelectrode and the counterelectrode will be close to Vmax. If onechooses a facile redox process for the stor-age electrode, as indicated by the sharplyrising IV curve for process A in Fig. 6, withEredox in the vicinity of Vmax, then the po-tential during charge and discharge of thestorage process will remain near Vmax. Asa result, the potential will be a highly in-variant current that would be maintainedthrough the load L, regardless of insolationintensity. This situation represents an idealmatch between solar energy conversionand storage processes within a PESC. Non-ideality occurs with poor voltage-matchingor kinetic limitations and polarizationlosses associated with the counter, storage,or photoelectrodes.

Ideally, the membrane used to separatethe two-cell compartments, as indicatedin Fig. 4, must be permeable only to ionsthat will transport charge, but that will notchemically react or otherwise impair anyelectrode. The permeability of membranesis generally less than ideal. Differentmem-branes permit other ions and water to per-meate to a varying degree [46, 47]. Grossmixing of activematerials across themem-brane causes them to combine chemicallyand in the process loose energy. Favorablequalities that a membrane should exhibitare low permeability toward chemicallyreactive ions, low resistivity, mechanicalintegrity, and cost effectiveness.

4.2.5High Efficiency Solar Cells with Storage

4.2.5.1 Multiple Band Gap Cells withStorageA limited fraction of incident solar photonshave sufficient (greater than band gap) en-ergy to initiate charge excitation within asemiconductor. Because of the low frac-tion of short wavelength solar light, wideband gap solar cells generate a high photo-voltage but have low photocurrent. Smallerband gap cells can use a larger fraction ofthe incident photons, but generate lowerphotovoltage. Multiple band gap devicescan overcome these limitations. In stackedmultijunction systems, the topmost cellabsorbs (and converts) energetic photons,but it is transparent to lower energy pho-tons. Subsequent layer(s) absorb the lowerenergy photons. Conversion efficienciescan be enhanced, and calculations predictthat a 1.64-eV and 0.96-eV two–band gapsystem has an ideal efficiency of 38% and50%, for 1 and 1000 sunsž concentration, Q5

respectively. The ideal efficiency increasesto a limit of 72% for a 36–band gap solarcell [48].

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Recently, high solar conversion and stor-age efficiencies have been attained witha system that combines efficient mul-tiple band gap semiconductors, with asimultaneous high capacity electrochem-ical storage [49, 50]. The energy diagramfor one of several multiple band gap cellsis presented in Fig. 7, and several otherconfigurations are also feasible [1, 2a, b].In the figure, storage occurs at a potentialof Eredox D EAC/A � EB/BC. On illumina-tion, two photons generate each electron,a fraction of which drives a load, whereasthe remainder (1/xe�) charges the storageredox couple. Without light – the poten-tial falls below Eredox – the storage couplespontaneously discharges. This dark dis-charge is directed through the load ratherthan through the multijunction semicon-ductor’s high dark resistance.

Cell: 2 In Fig. 8, an operational form ofthe solar conversion is presented and a

storage cell described by the Fig. 7 en-ergy diagram. The single cell containsboth multiple band gap and electrochem-ical storage, which unlike conventionalphotovoltaics, provides a nearly constantenergetic output in illuminated or darkconditions. The cell combines bipolar Al-GaAs (Eg D 1.6 eV) and Si (Eg D 1.0 eV)and AB5 metal hydride/NiOOH storage.Appropriate lattice-matching between Al-GaAs and Si is critical to minimize darkcurrent, provide ohmic contact without ab-sorption loss, andmaximize cell efficiency.The NiOOH/MH metal hydride storageprocess is near ideal for the AlGaAs/Si be-cause of the excellent match of the storageand photocharging potentials. The electro-chemical storage processes utilizesMHox-idation and nickel oxyhydroxide reduction:

Q6

MHCOH� ��! MCH2OC e�;EM/MH D �0.8 V vs SHEž (13)

EAredox

EBredox

Ele

ctro

cata

lyst

ano

de

Ele

ctro

cata

lyst

cat

hode

p pn nWide gap Small gap

EVW

Ohmicjunction

EFermi(pw)

EFermi(nw=ps)

EFermi(ns)

hν > EGW

hν

⇒ h+

⇒ h+

EGw

FCw

EVsEGw > hν > EGs

ECs

Vw

Storage bipolar MPEC

Semiconductor

(1–1/x)e–

1/xe–

Rloade–

e–

EGs

ElectrolyteElectro-catalyst

Vs

V = Vw + Vo

ηA

ηB

1/xA → 1/xA+

1/xB → 1/xB+hν

Fig. 7 Energy diagram for a bipolar band gap indirect ohmic storage multiple bandgap photoelectrochemical solar cell (MBPEC).

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Load

curr

ent

Pho

tocu

rren

t

Sto

rage

curr

ent

1/xe–

(1−1/x)e-

e–

50 n

m

300

nm

1.0

µm

1.7

nm

20 n

m

10 n

m

1.0

µm

350

µm80

0 nm

(1 ×

1018

cm–3

)

(1 ×

1018

cm–3

)

(2 ×

1017

cm–3

)

(1 ×

1018

cm–3

)

(1 ×

1019

cm–3

)

(8 ×

1015

cm–3

)

(4 ×

1019

cm–3

)

Rload

P+-A

l 0.8

Ga 0

.2A

s

P+-G

aAs

P+-A

l (0.3

-0.1

5)G

a (0.

7-0.

85)A

s

MH

x–1

+ H

2O +

e–

MH

x +

OH

–

Ni(O

H) 2

+ O

H–

NiO

OH

+ H

2O +

e–

N+-A

l (0.1

5)G

a (0.

85)A

s

N-A

l (0.1

5)G

a (0.

85)A

s

Au-

Zn/

Au

N-G

aAs G

aAs(

buffe

r la

yer)

Nic

kel c

atho

de

Met

al h

ydrid

e an

ode

Sep

arat

or w

ith K

OH

ele

ctro

lyte

Au-

Sb/

Au

P+-S

i

N-S

i

N+-S

i

AR

coa

ting

[ZnS

(50n

m)/

MgF

2(70

nm)]

⇒2hν

Illu

min

atio

n

Fig. 8 The bipolar AlGaAs/Si/MH/NiOOH MBPEC solar cell.

NiOOHCH2OC e�

��! Ni(OH)2 COH�;ENiOOH/Ni(OH)2 D 0.4 V vs SHE (14)

As reported [49] and as shown inFig. 9, the cell generates a light vari-ation insensitive potential of 1.2–1.3 Vat total (including storage losses) so-lar–electrical energy conversion efficiencyof over 18%.A long-term indoor cycling experiment

was conducted to probe the stability ofthe AlGaAs/Si metal hydride storage solarcell [50]. Unlike the variable insolation ofFig. 9, in each 24-hour cycle, a constantsimulated AM0 (135.3 mW cm�2) illumi-nation was applied for 12 hours, followedby 12 hours of darkness, and the cell po-tential, and storage (charge and discharge)currents monitored as a function of time

over approximately an eight-month period.Figure 10 presents representative resultsfor two-day periods occurring 0, 40, 140,and 240 days into the experiment. As sum-marized in the lower curves of the figure,the load potential is again nearly constant,despite a 100% variation in illumination(AM0/dark) conditions. Over a 24-hour pe-riod, the loadpotential increases by¾2%asthe cell chargeswith illumination, followedby a similar decrease in potential as storedenergy is spontaneously released in thedark. The cycles exhibited in Fig. 4 are rep-resentative, and as observed exhibit littlevariation on the order of weeks, and exhibita variation of¾1%over a period ofmonths.In this figure, photopower is determinedas the product of the measured cell po-tential and measured photocurrent. Powerover load is determined as the product of

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Generated VCell insensitive to illumination variation

Illumination Illumination

AlGa/Si MH multiple band gap storage solar cell

Area = 0.22 cm2

Dark Dark

Charge Charge

Discharge Discharge

insitu AB5/NiOOHMetal hydride storage

0 6 12 18 24 30 36 42 48

Time, [h]

−2

0

2

0

2

4

0.5

1.0

1.5

ISto

rage

, [m

A]

IPho

to[m

A]

Load

pot

entia

l[m

A]

Fig. 9 Two days measured conversion and storage characteristics of theAlGaAs/Si/MH/NiOOH MBPEC solar cell.

the measured cell potential and measuredload current.Under constant 12-hour (AM0) illumi-

nation, the long-term indoor cycling cellgenerated a nearly constant photocurrentdensity of 21.2 (constant to within 1% orš0.2 mA cm�2), and as seen in the topcurves of Fig. 10, a photopower that var-ied by š3%. The cell’s storage componentexhibits the expected increase in chargingpotential with cumulative charging, whichmoves the system to a higher photopoten-tial. The observed increase in photopowerduring 12 hours of illumination is be-cause of this increase in photopotentialwith cumulative charging. A majority ofthe photogenerated power drives the redox

cell, and the remainder consists of thepower over load during illumination, asillustrated in Fig. 10. In the dark, inclusiveof storage losses, the stored energy is spon-taneously released and this power overload during both 12-hour illumination and12-hour dark periods is also summarized.The cell is a single physical–chemical de-vice generating load current without anyexternal switching.

4.2.5.2 PECS Driving an External Fuel CellIn the early 1980s, Texas Instruments,Inc. developed an innovative programbased on a hybrid photovoltaic storage thatused imbeddedmultilayer photoanode andphotocathode silicon spheres and was

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00

1

2

3

4

5

6

7

0

1

2

3

4

5

6

7

2 40 42 140 142 240 242

Time, days

Potential over loadPhotopowerPower over load

Illumination cycle:12 hour 135.5 mW cm−2 & 12 hour darkArea: 0.22 cm2

AlGaAs/Si MH bipolar gap storage solar C2/3 year summary

Lo

ad p

ote

nti

al[V

]

Po

wer

[mW

]

Fig. 10 Eight-months photopotential and power characteristics of the AlGaAs/Si/MH/NiOOHsolar cell under fully charged AM0 conditions. Each day, the cell is illuminated for 12 hours andis in the dark for 12 hours.

designed to provide a close match betweentheir maximum power point voltage and asolution phase bromine oxidation processin acidic solution. The program wasdiscontinued, but the system has severalattractive features.

Cell: 3 [51]. In the bromine-imbedded Sisphere system, energy stored as bromineis recovered in an external hydrogenbromine fuel cell. The conversion andstorage reaction and cell configuration aresummarized by

2HBrCH2OC hν ��! H2 C Br2CH2O (15)

Photo anode:Contact metal j Ohmic contact j n-Si j

p-Si j Surface metal j Solution

Photo cathode:Contact metal j Ohmic contact j p-Si j

n-Si j Surface metal j Solution

A description of the Texas Instrument cellaction is provided in Fig. 11.

4.2.6Other Examples of PhotoelectrochemicalStorage Cells

Photoelectrochemical storage cells of dif-ferent configurations have been suggested,designed, and tested for their performance,under sunlight or artificial illumination.Although none of these configurations hasattained the high solar to electrical conver-sion and storage efficiency of the system inthe earlier sections, they are of significant

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Hydrogenstorage

Fuel cellconverter

Storageand heat

exchanger

Electrical energy Thermal energy

Glass panelcoverSeparator

e

e

p p ppn n nnh

Conductive plane

Hydrogenbromidecathode

HBranode

e−

e−

H2

1/2 H21/2 Br2

Br2

Br

Glass

H

h

h

hMetal

Fig. 11 Schematic diagram of the Texas Instruments Solar EnergySystem. Illumination occurs through the electrolyte to produce hydrogengas that can be stored as a metal hydride and bromine that can be storedas aqueous tribromide. A hydrogen-bromine fuel cell is used to convertchemical to electrical energy and regenerate the hydrogen bromideelectrolyte in a closed loop cyclic system. Thermal energy can be extractedthrough a heat exchanger.

scientific interest and form a solid basisfor further development toward futuresystems. The following three sections, cov-ering PECS with either a solution, solid,or intercalation storage redox processes,provide a brief summary of many of theseinvestigations, with a particular emphasison their performance.

4.2.6.1 PECS Cells with Solution PhaseStorageCell: 4 [52]. This is an example of the useof photoexcitable absorbers to promote aredox process using the following reactionsequence:

THC C 2Fe2C C 3HC ��! TH42C

C 2Fe3C (THC D Thionene acetate)(16)

The back reaction between Leucoth-ionene (TH4

2C) and Fe3C is slow. Leucoth-ionene is oxidized at the SnO2 electrode.

TH42C ��! THC C 2e� C 3HC (17)

Ferric cation is reduced at a Pt electrodein a second compartment.As a result, the concentration ratio of

Fe3C/Fe2C is increased in the first com-partment and decreases in the secondcompartment, which is equivalent to a dif-ference in chemical potential. The systemreturns to its original uncharged state bydischarging in the dark. Only 60 mV of po-tential difference is equivalent to a decadechange of the Fe3C/Fe2C concentration ra-tio. Hence, this concentration cell doesnot generate a significant potential andthe power density is low. The cell has theconfiguration

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SnO2 j 0.0001 M Thionene Acetate,0.01 M FeSO4, 0.001 M Fe2(SO4)3H2SO4, pH = 1.7–2 j Ion-exchangemembrane j 0.01 M FeSO4 0.001 MFe2(SO4)3 j PtStudied cell characteristics: Polycrys-

talline photoelectrode, Pt counter elec-trode, Eredox of Fe3C/Fe2C D 0.77 V vsSHE, illumination 40–50 mW cm�2 bya tungsten lamp, initial current duringdischarge D 9.1 µA, and initial voltage D10.9 mV.

Cell: 5 [53]. The photoelectrode used inthis investigation was a powder-pressedsintered pellet. The high band gap ofthe semiconductor used (n-Pb3O4; Eg D2.1 eV) and electron hole recombinationsat grain boundaries have contributed to anobserved low-conversion efficiency, whichdrives the overall cell reaction:

I� C 6OH� C 6Fe3CDark��! ��Light

IO3�

C 3H2OC 6Fe2C (18)

A saturated salt bridge is used betweenthe cell compartments to minimize mem-brane IR loss, but it allows the active redoxspecies to mix and chemically combineacross the junction. The cell has the form

n-Pb3O4 j 0.1 M Fe3C, SaturatedFe2C j Salt bridge j 0.1 M IO3

�,0.1 M I� j Pt

Studied cell characteristics: Polycrys-talline photoelectrode, Pt counter elec-trodes, three-electrode configuration as inFig. 4., Eredox of Fe3C/Fe2C D 0.77 V vsSHE, Eredox of IO3

�/I� D �0� 26 V vsSHE, Illumination 60 mW cm�2 quartzhalogen lamp, conversion efficiency D0.09%, FF D 0.38, photopotential D172 mV, Vmax D 172 mV, charge effi-ciency of the battery D 74%, potential

difference between Pt electrodes beforecharging D 720 mV, and after charging D840 mV.

Cell: 6 [54]. In this cell, a wide gapsemiconductor (Eg D 3.3), BaTiO3 is used,capable of absorbing only near UV ra-diation and comprising less than 5% ofavailable solar energy. This limits its prac-tical use for solar energy conversion. Thefollowing storage couple is used:

Ce3C C Fe3CLight��! ��Dark

Ce4C C Fe2C (19)

The storage system operates well awayfrom the maximum power point of thesemiconductor device, and therefore stor-age and discharge efficiency is poor. Thecell uses a salt bridge between the twocompartments and is of the form

Single crystal j n-BaTiO3 j 0.1 M Ce2(SO4)3, 0.005 M Ce(SO4)2 j Salt bridge j0.1 M Fe2(SO4)3, 0.005 M FeSO4 j PtStudied cell characteristics: Pt counter

electrodes, three-electrode configurationas in Fig. 4, Eredox of Ce4C/Ce3C D1.45 V vs SHE, Eredox of Fe3C/Fe2C D0.77 V vs SHE, illumination sunlight,conversion efficiency D 0.01%, FF D 0.26,photopotential D 730 mV, Vmax D 0.33 V,charge efficiency 15%, potential across twoPt terminals of the charged cell D 0.60,and short-circuit current D 0.12 mA.

Cell: 7 [55]. In this study, attempts havebeen made to improve the behavior of aMoS2 electrode in HBr electrolyte. Equili-bration between the electrode andHBr hasbeen improved by subjecting the electrodeto a dark anodic potential. A ratio of 10between the areas of counter and work-ing electrode is another favorable featurein this study to minimize the polariza-tion resistance at the counter electrode.Nafion-315 membrane contributes only a

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moderate resistance of 20 ohm cm2. Thestorage reaction and cell configuration are

2Br� C 3I2Light��! ��Dark

Br2 C 2I3� (20)

n-MoSe2 j 0.1 M HBr, 0.01 M Br2 jNafion 315 j 1 M KI, 0.18 M I2 j PtStudied cell characteristics: Single

crystal photoelectrode, Pt counterelectrodes, cell configuration as in Fig. 4,Eredox of Br/Br� D 1.087 vs SHE, Eredoxof I3�/I� D 0.534 V vs SHE. Illumination200 mW cm�2 Xe lamp, conversionefficiency D 6.2%, potential across two Ptterminals of the charged cell D 0.49, andshort-circuit current D 0.5 mA.

Cell: 8 [55]. The next cell also uses aNafion membrane, but makes use of n-CdSe to drive a polysulfide–polyselenidestorage couple. Low-output power densityis the biggest drawback in this cell. Thestorage reaction and cell configuration are

S0 C Se2�Dark��! ��Light

S2� C Se0 (21)

n-CdSe j 1 M in Na2S, S, NaOH j Nafion315 j 1 M in M Na2Se, Se, NaOH j PtStudied cell characteristics: Polycrys-

talline photoelectrode, Pt counter elec-trodes, cell configuration as in Fig. 4,Eredoxof polysulfide electrolyte D �700 mV vsaSCEž Eredox of Se22�/Se2� D �800 mV vsQ7

SEC, illumination 100 mW cm�2 XenonQ8

lamp, conversion efficiency 4%, FF D0.45, photovoltage D �400 mV, chargedcell has an open-circuit voltage of 60 mV,and initial current across a 100 ohm acrossPt electrode D 0.5 mA.

Cell: 9 [44]. This study uses organic re-dox species for energy-storage purposes.Stability of the n-WSe2 photoanode iniodine electrolyte and the stability ofanthraquinone redox couple have been

demonstrated in this study. Any H2 evolu-tion would carry out direct hydrogenationof AQ and associated side reactions, andtherefore a carbon electrode has been se-lected because of the H2 over potentialon this electrode. The cell underwent sev-eral deep charge and discharge cycles withreproducible performance. The storage re-action and cell form are

2I� C AQC 2HCLight��! ��Dark

AQH2 C I2 (22)

n-WSe2 j 1 M KI, 0.1 M Na2SO4, 0.5 MH2SO4 j Saturated KCl bridge j 5ð 10�2M AQ, 0.5 M H2SO4 j C

Studied cell characteristics: Single-crys-tal photoelectrode, C counter electrodeduring charging, Pt during discharg-ing. The cell configuration is similarto that in Fig. 4. Eredox of I3�/I� D0.534 V vs SHE, Eredox of AQ/AQH2, il-lumination 150 mW cm�2 He–Ne laser(632.8 nm), conversion efficiency D 9%,discharge across a 10 ohm ž produce a cur- Q9

rent 1 mA cm�2, and open-circuit voltage200 mV.

Cell: 10 [44]. This study uses a p-WSe2photocathode rather than n-WSe2. Duringthe cell discharge, oxidation of AQH2 atthe surface of p-WSe2 indicates that theelectrode has the duel role of being acathode during the charging and being theanode during the discharge. As discussedearlier, this limits the activity and low-current densities were observed. Thestorage reaction and cell configuration are

AQC 2HC C 2ILight��! ��Dark

AQH2 C I2 (23)

p-WSe2 j 5ð 10�2 M AQ j Saturated j1 M KI, 0.5 M H2SO4 j PtSingle crystal j 0.5 M H2SO4 j Saltbridge j 0.5 M Na2SO4 j

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Cell: 11 [55]. The theoretical band gap ofWSe2 provides a near ideal single band gapmatch for the solar spectrum. But the fol-lowing cell has some disadvantages. Theseinclude the low solubility of the storageredox couple employed, MV2C and MVCž

and the possibility of undesirable side re-actions of the radical ion MVCž. Usingdual (n-type and p-type) photoelectrodes ex-pands the potential regime one can accessfor the redox-storage couple. The storagereaction and cell configuration are

2I� C 2MV2CLight��! ��Dark

2MVCž C I2 (24)

n-WSe2 j I� jMV2C j p-WSe2

4.2.6.2 PECS Cells Including a SolidPhase–Storage CoupleThe earlier experimental investigations,Cells 2–9, use only solution phase redoxcouples. However, as indicated in thefollowing examples, a solid phase–storagecouple may also be employed, whichin principle tends to increase the cell’sstorage capacity.

Cell: 12 [57]. Having at least onecomponent in insoluble form may addcompactness into the cell configuration,although low conductivity of the insolubleactive component may cause significantpolarization losses associated with thestorage electrode, as exemplified by thelow conductivity of silver (chloride) in oneof the next cells. The next four cells usea TiO2 polycrystalline photoelectrode. Inthe first cell, the storage reaction and cellconfiguration are

2H2OC 4AgCLight��! ��Dark

4HC C 4AgCO2

(25)TiO2 j 1 M HNO3, 1 M KNO3 j AnionSpecific Membrane j 1 M AgNO3, 1 MKNO3 j Ag

Studied cell characteristics: Polycrys-talline photoelectrode, Pt counter elec-trodes, cell configuration is similar toFig. 4, Eredox of O2, HC/H2O couple D1.23 V vs NHE at pH D 1, Eredox ofAg/AgC D 0.80 V vs NHEž, illumination Q10

500 W Hg lamp, conversion efficiency D1%, photopotential D 0.28 V vs NHE,open-circuit voltage of the chargedcell D 0.28 V, and short-circuit current D0.3 mA cm�2.

Cell: 13 [57]. Of the four TiO2 Cells10–13, the following cell 3.2ž exhibited Q11

the highest short-circuit discharge currentand voltage. However, during the chargingprocess, a stationary concentration ofCe4C was observed in the photoanodecompartment. This suggest the existenceof competing process that consumes theoxidized species Ce4C. The later is knownto participate in photochemical reactionsunder illumination [10]. Considering thelow concentration of the reduced form ofactivematerials used with the photoanode,there is a possibility that the wateroxidation becomes the dominant processduring charging. In this study, it wasobserved that with a passage of a chargeof 10 coulomb during charging, Ce4Cpresent in the photoanode compartmentaccounted for only 22% of the charge. Inthis second TiO2 photoelectrode cell, thestorage reaction and cell configuration are

Ce3C C AgCLight��! ��Dark

Ce4C C Ag (26)

TiO2 j 1 M HNO3, 0.05 M Ce2(SO4)3,0.1 M Ce(SO4)2 j Anion Specific Mem-brane j 1 M AgNO3, 1 M KNO3 j Ag

Studied cell characteristics: Polycrys-talline photoelectrode, Pt counter elec-trode, cell configuration illumination etc.are similar to the earlier cell, Eredox

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of Ce4C/Ce3C vs SHE, charge effi-ciency of the cell without stirring D18%, open-circuit voltage of the chargedcell D 0.76, and initial short-circuit cur-rent 1.3 mA cm�2.

Cell: 14 [57]. In this third TiO2 photo-electrode cell, the storage reaction and cellconfiguration are

2Fe2C C Cu2CLight��! ��Dark

2Fe3C C Cu (27)

TiO2 j 1 M KNO3, 0.01 M FeSO4 j AnionSpecificMembrane j 0.025 MCuSO4, 1 MKNO3 j CuStudied cell characteristics: Poly-

crystalline photoelectrode, Eredox ofCu2C/Cu D 0.34 V vs NHE, Eredox ofFe3C/Fe2C D 0.77 vs NHE, open-circuitvoltage of the charged cell D 0.3, and short-circuit current 1.5 mA cm�2.

Cell: 15 [57]. The wide band bag ofTiO2 is not an appropriate match tothe solar spectrum. In this fourth TiO2

photoelectrode cell, the storage reactionand cell configuration are

Fe2C C AgClLight��! ��Dark

Fe3C C AgC Cl�

(28)

TiO2 j 0.2 M KCl, 0.01 M FeCl2 jAnionSpecificMembrane j 0.2 MKCl j AgCl j PtStudied cell characteristics: Polycrys-

talline photoelectrode, cell configurationand illumination are the same as in theprevious cell, Eredox Fe3C/Fe2C D 0.77 Vvs NHE, Eredox of AgCl/AgC D 0.22 V vsNHE, open-circuit voltage of the chargedcell D 0.39 V, and short-circuit current D0.4 mA cm�2.

Cell: 16 [58]. In this next cell, Ni isdeposited during charge at 80% chargeefficiency. Losses may be because of the

competing reaction of H2 evolution. Cellvoltage of the charged cell is higher thanthe photovoltage available, which indicatesthe possible influence of another redoxcouple Ni(OH)2/NiOH� occurring at ahigher redox potential. Only about 55% ofthe charge stored can be recovered duringdischarge. The possibility of self-dischargereactions because of imperfect permeabil-ity of the membrane has been cited as apossible cause, and is further complicatedby the complex ferro/ferricyanide equilib-ria that is known to occur (Licht, 1995).In this cell, the storage reaction and cellconfiguration are

2Fe(CN)64� CNi2C

Light��! ��Dark

Fe(CN)63�

CNi (29)

n-GaP j 0.2 M K2SO4, pH D 6.7, 0.05 MK3Fe(CN)6, 0.05 M K4Fe(CN)6 j AnionSpecificMembrane j 0.05 M K2SO4, 0.2 MNiSO4, 0.06 M NiCl2 j PtStudied cell characteristics: Single crys-

tal photoelectrode, Pt counter electrodes,cell configuration is similar to Fig. 4.Eredox of Fe(CN)63�/Fe(CN)64C is 0.36 Vvs NHE, Eredox of Ni2C/Ni D �0.25Vvs NHE, illumination 500 W Hg lamp,conversion efficiency 13% for 450–540-nm region, photovoltage D 0.63 V, open-circuit voltage of the charged cell D 0.75 V,short-circuit current D 4.3 mA cm�2, andcharge efficiency D 55%.

Cell: 17 [55]. The conversion efficiencydata in the following cell reflect thepoor quality of the GaAs material thatwas used, although in other studies,there has been higher efficiency GaAsPEC (without storage). In this study,significant polarization was observed andperformance data of the storage cell wasnot reported. The storage reaction and cell

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configuration are

CdC Se22� C 2OH�Dark��! ��Light

Cd(OH)2

C 2Se2� (30)

n-GaAs j 0.1 M Na2Se, 0.1 M Se, 1 MNaOH j Nafion j 2 M NaOH j CdStudied cell characteristics: Single crys-

tal photoelectrode, Pt counter electrode,cell configuration is similar to Fig. 4, Eredoxof Se2�/Se22� D �800 mV vs SEC, Eredoxof Cd/Cd(OH)2 D �1050 mV vs SCE.Illumination 100 mW cm�2 Xe lamp,conversion efficiency D 4%, FF D 0.53,photopotential D �500 mV, and short-circuit discharge current of the storagecell in the dark using Pt electrodes D14.6 mA cm�2.

Cell: 18 [55]. The next two cells use apolycrystalline n-CdSe photoanode. Thefollowing cell exhibited steady current-time and voltage-time curves during thephotoelectrochemical charging and darkdischarging. The flat discharge curve pre-vailed until the capacity of the sulfideelectrolyte is exhausted. The storage re-action and cell configuration are

CdC SC 2OH�Dark��! ��Light

Cd(OH)2 C S2�

(31)

n-CdSe j 0.1 M in NaOH, Na2S, 1 M in S,Na2Se, Se j Nafion j 2 M NaOH j CdStudied cell characteristics: Polycrys-

talline photoelectrode, Pt counter elec-trodes, cell configuration is similar toFig. 4. Eredox of Sx2/S2� D �700 mV vsSCE, Eredox of Cd(OH)2/Cd D �1050 mVvs SCE, illumination 100 mV cm�2 Xelight, conversion efficiency D 4%, FF D0.45, photovoltage D �400 mV during

discharge through Pt and Cd electrodeswith a 100-ohm load, and a current of8.3 mA cm�2 flowed at cell voltage close to175 mV.

Cell: 19 [59]. In this study, the possi-bility of using organic semiconductors todrive storage processes is demonstrated.The process is in principle similar toa concentration cell. During photocharg-ing, Prussian Blue (PB, Fe4[FeII(CN)6]3) isreduced at the photocathode and PB is ox-idized at the anode. In the dark, the redoxprocess involving PB is reversed producingan electron flow. Process ability, stability,and lack of photocorrosion make theselow band gap organic materials very attrac-tive for photoelectrochemical applications.However, they are defect-based systems,and the very low conversion efficienciesand self-discharge appear to outweighthese benefits. The storage reaction andcell configuration are

bilayer electrode: FeII4 [FeII(CN)6]3

4�

C 4hCLight��! ��Dark

FeIII4 [FeII(CN)6]3 (32)

counter electrode: FeIII4 [FeII(CN)6]3 C 4e�Light��! ��Dark

FeII4 [FeII(CN)6]3

4� (33)

ITO j P3MT j PB j 0.2 M KCl, 0.1 MHCl j PB j ITOStudied cell characteristics: Illumination

500 W Xenon lamp, the ITO/P3MT elec-trode has open-circuit voltage D 0.44 V,short-circuit photocurrent 0.09 µA cm�2,and charge efficiency of the storage cell D40%.

Cell: 20 [60]. Metal ions introduced intoa solidβ-alumina lattice behave like ions insolution. This study illustrates a compactsolid-state storage cell that can be chargedusing solar energy. During charging, Fe

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and Ti change their oxidation state and thecharge balance is maintained by the mi-gration of NaC ions from one phase to theother. In the actual cell design, an n-typesemiconductor is connected to the alu-mina phase containing Ti and p-materialis connected to the phase containing Fe.Limitations are the comparatively slowdiffusion of ions in the solid electrolyteand resistance to ionic movement at var-ious phase boundaries, and lower theenergy output during discharge. In thisdevice, back wall illumination demandsthe use of very thin semiconductor layersto minimize absorption losses and has thegeneral form

n-semiconductor j Na2O.11(AlFeO3) jNa2O.11(Al2O3) j Na2O.11(AlTiO3) jp-semiconductor

Cell: 21 [47]. In this detailed study, selec-tion of a Nafion-315 membrane was doneon the basis of (1) stability in high alkalinesulfide solutions, (2) low IR drop, and (3)lowpermeability to sulfide.Maintaining anarea ratio of 1 : 8 betweenphoto and storageelectrodes has minimized polarization atthe storage electrode. The storage systemwas driven by three semiconductor PECdevices connected in series. Charging wasdone up to 90% of the capacity followedby complete discharge. Overall observedcharge efficiency was 83%. Although thesystem was not fully optimized with re-spect to photoelectrode, electrolyte, andstorage, voltage efficiency of 75% was ob-tained during discharge. Discharge curveswere flat until the stored active materialwas fully consumed. The storage reactionand cell configuration are

S2� C Zn(OH)42� Light��! ��Dark

S0 C Zn

C 4OH� (34)

n� CdSe j 1 M in NaOH, Na2S, S Nafion-315 j 0.1 M ZnO, 1 M NaOH j C

Studied cell characteristics: Polycrys-talline photoelectrode, Ni counter elec-trode, basic cell configuration is basedon Fig. 4, Eredox of Sx2/S2� D 0.500 V vsSHE, Eredox of Zn/Zn(OH)42� D �1.25 Vvs SHE, artificial illumination, conversionefficiency D 3%, photovoltage D �0.50 V,during discharge through 75-ohm loadbetween C and Ni discharge current D10 mA, and voltage D 0.6 V.

Cell: 22 [61]. This cell takes advantageof photocorrosion to drive a storage cell.Under illumination, n-CdSe is decom-posed and p-CdSe is electroplated, andthe reverse occurs during cell discharge.However, photoactivity depends on an op-timized semiconductor surface, and in anenvironment where the surface is changedconstantly, the surface optimization is lost.This and the poor kinetics of the p-typephotoreduction result in a continual dete-rioration of the photoactivity and cause lowphotoefficiency and low-discharge powerdensity. The storage reaction and cell con-figuration are

CdSeC 2hCLight��! ��Dark

Se0 C Cd2C (35)

and the other electrode in photoelectro-plated by Cd

CdTeC 2e�Light��! ��Dark

Cd0 C Te2� (36)

n-CdSe j 0.1 M CdSO4 j p-CdTeCell: 23 [46]. This is a detailed study of a

thin film cell withmoderately high outdoorsolar efficiency, high storage efficiency,and an output that is highly invariant de-spite changing illumination. This studyprovides extensive details of the choice

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of photoelectrode, membrane, and elec-trochemistry of the tin–tin sulfide redoxstorage. Cd(Se,Te) electrodes, comparedto CdSe, improve the band gap match andincrease solar-conversion efficiency. Twophotoelectrodes in series were used to pro-vide a voltage match to the storage redoxcouple in a cell of the form of Fig. 12.The conversion and storage reactions

and cell configuration are presented as

SnSC 2eLight��! ��Dark

S2� C Sn Storage (37)

S2�Light��! ��Dark

SC 2e Photo electrode (38)

n-Cd(Se,Te) j 2 M in NaOH, Na2S, S jRedcadMembrane j 2 M inNaOH,Na2S jSnS j SnStudied cell characteristics: Bipolar se-

ries polycrystalline photoelectrode, CoScounter electrodes, cell configuration isas shown in Fig. 4 without the needof switches E or F. Eredox of S/S2� D�0.48 V vs NHE, Eredox of SnS/Sn,S

2� D�0.94 V vs NHE. Illumination sunlight,500 mWhr cm�2 per day, conversion ef-ficiency 6–7%, photovoltage D �600 mV,and storage efficiency >90%. After twoweeks of continuous operation the over-all solar to electrical efficiency (includingconversion and storage losses) is 2–7%.

Cell: 24 [4]. The earlier cell is improvedby a series of solution phase optimizations(cesium electrolyte with low hydroxideand optimized polysulfide), to providea higher photopotential and improvedstability and also the use of a singlecrystal, rather than thin film, Cd(Se,Te)to also improve photopotential and cellefficiency, as described earlier in Fig. 12.Because of the higher photopotential,only a single photoelectrode is requiredto match the storage potential and highoverall efficiencies are observed. The cell

has the design as shown in beginning ofthe chapter (Fig. 2) and uses conversionand storage reactions described in theearlier cell and a configuration

n-Cd(Se,Te) j 0.8 M Cs2S, 1 M Cs2S4 jRedcadMembrane j 1.8 MCs2S j SnS j SnStudied cell characteristics: The PEChad

a power conversion efficiency of 12.7%under 96.5 mW cm�2 insolation and volt-age at maximum power point was of�1.1 V vs SHE, sufficient to drive theSnS/Sn storage system. Under direct illu-mination, the 0.08 cm2 single crystal pho-toelectrode generated more than 1.5 mAthrough the 3 cm3 SnS electrode drivingSnS reduction while supporting 0.33 mAthrough a 1500 load simultaneously at aphotogenerated 0.495 V. In the dark spon-taneous oxidation drive, the load withstorage efficiency over 95%. The total con-version efficiency, including conversionand storage losses, was 11.8%.

4.2.6.3 PECS Cells IncorporatingIntercalationIn photointercalation, illumination drivesinsertion storage into layer type com-pounds [62]. The photointercalation pro-cess can be characterized as

TX2 C e�(h)C pC(h)

CMsolCTMINX2 C PC (39)

where TX2 is generally a nonintercalatedtransition metal dichalcogenide. For thisprocess to occur without the assistance ofan external power source, a counter elec-trode is driven at an electrode potentialnegative to that of the layer type interca-lating electrode. The process is generallyrestricted to p-type materials. The develop-ment of this concept has been slowbecauseof dearth of materials that are stable semi-conductors and at the same time behave as

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LoadTin storage

AA2

hν hν

B

CoS counter

CoS counterCd Se Te photo Cd Se Te photo

Membrane

Bipolar Cd Se Tel [polysulfide]lCoS PECwith in-situ tini[sulfide]ll[polysulfide]lCos storage

Fig. 12 A bipolar thin film photoelectrochemical solar cell with in situ storage.Compartments A and A2 contain alkali polysulfide solution and compartment Bcontains alkali sulfide solution.

intercalating compounds that are able toexchange guest ions andmoleculeswith anelectrolyte in a reversible manner, and yetthat is not disruptive to photon absorption.

Cell: 25 [63]. In this cell, Eredox of copperthiophosphate is variable depending onthe degree of intercalation. A limitation ofthis system is poor-discharge kinetics andlow-energy density of the discharge. Thecell configuration is given by

Cu3PS4 j 0.02 M CuCl j CH3CN j Cu2S

Studied cell characteristics: Eredox ofCuC/Cu0 D �0.344 vs NHE, illumination117 mW cm�2 Xe lamp, photopotential Dl00 mV, charging current < 50 µA cm�2,and discharge current < 10 µA cm�2.

Cell: 26 [64]. This cell illustrates anotherall solid state design for a thin storagecell. p-CuxS changes its electrode potentialwith changes in its composition. Duringcharging, Cu is oxidized at n-CdS surfacewhile it is reduced at the Cu electrode.Between the two electrodes CuC iontransport process takes place in the solidstate electrolyte. The cell configuration isgiven by

Cu j n-CdS j p-Cu2S j RuCl4I5Cl3.5 j Cu

Cell: 27 [64]. As with the earlier cell,this final cell requires a very thin design

because to reach the junction, light has totravel several layers. The cell functions inthe same manner as the earlier cell, andthe configuration is given by

Conductive Glass j Cu j CuC Conductingsolid electrolyte j p-Cu2Te j n-CdTe jMo

4.2.7Summary

Conversion and storage of solar energyis of growing importance as fossil fuelenergy sources are depleted and stricterenvironmental legislation is implemented.Although society’s electrical needs arelargely continuous, clouds and darknessdictate that photovoltaic solar cells havean intermittent output. Photoelectrochem-ical systems have the potential to notonly convert but also store incident so-lar energy. Design component and systemconsiderations and a number of photoelec-trochemical solar cells with storage havebeen reviewed in this chapter.

Acknowledgment

S. Licht is grateful to Dharmasena Per-amunage and for support by the BMBFIsrael–German Cooperation.

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Author Queries

Query No. Query

1 Please clarify if this is sufficient energetic insolation or sufficientlyenergetic.

2 Please clarify if this is Air Mass 13 Please clarify what W is refered to in this context.4 Please clarify if this is EA and EB similar to that in the figure5 Please clarify if this is a unit.6 Please expand in the first instance7 Please expand SCE8 Please expand SEC9 Au: Please clarify if a word is missing and if the sentence should read

as follows: ‘‘discharge across a 10 ohm produces a current1 mA cm�2, and open-circuit voltage 200 mV

10 Please expand this11 Please clarify if this cell number is correct12,13,14 Please clarify if it is the volume number or page number

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bard060402 Photoelectrochemical Solar Energy Storage Cells