Electrode having a CoS layer thereon, process or preparation and uses thereof

The present invention relates to an electrode comprising a non-conductive substrate, a first layer and a second layer. The first layer is disposed on the substrate and comprises indium tin oxide or fluorine-doped SnO2. The second layer is disposed on the first layer and comprises CoS. A process for preparing this electrode is also disclosed. Such an electrode is particularly useful in a photovoltaic cell.

TECHNICAL FIELD

The present invention relates to improvements in the field of electrochemistry. In particular, this invention relates to a CoS coated electrode and a process for preparing the same.

BACKGROUND OF THE INVENTION

Electrochemical photovoltaic cells (EPC's) are based on a junction between a semiconductor (p-type or n-type) and an electrolyte containing one redox couple; an auxiliary electrode completes the device. If the semiconductor and electrolyte Fermi levels are different and well suited, a built-in potential will develop at their interface and the device will exhibit diode rectification in the dark. When electrons and holes are photogenerated in the vicinity of the junction, the built-in potential permits separation of the charges. If a n-type material is used, holes (valence band) will migrate to the interface and allow oxidation of reduced species contained in the electrolyte. At the same time, photogenerated electrons (conduction band) will migrate toward the bulk of the semiconductor to reach the auxiliary electrode, via an external circuit, where they will reduce the oxidized species of the electrolyte. If a p-type material is used, the processes are reversed: photoelectrochemical reduction at the semiconductor/electrolyte interface, and electrochemical oxidation at the auxiliary electrode/electrolyte interface. As the reactions involve the same redox couple, there is no net chemical change in the electrolyte (ΔG=0) and therefore the effect of the device illumination is to produce a photocurrent and a photovoltage (photovoltaic effect). Such devices can serve as photodiodes (monochromatic light) and as solar cells (white light). The maximum open-circuit photopotential (Voc) is determined by the difference between the Fermi level of the semiconductor and that of the electrolyte, the latter being fixed by the redox potential.

EPC's are very attractive for the production of electricity and present a number of advantages over the p-n heterojunctions. The latter generally need a doping step and interdiffusion of majoritary carriers between the p and n regions, whereas the semiconductor/electrolyte junction is simply formed by transfer of majoritary carriers from the semiconductor to the electrolyte, on immersion of the semiconductor into the electrolyte. Among other advantages we can stress: (i) the elimination of light energy losses by absorption in one half of the junction if the electrolyte is colorless; (ii) the possibility of using a thin film polycrystalline semiconductor (much less expensive than a single crystal) with only a small decrease in the cell energy conversion efficiency; (iii) the large number of redox couples (and thus of electrolyte Fermi levels) that can be used to vary the junction built-in potential and hence the device photopotential and photocurrent.

There is extensive prior art on EPC's. The direct conversion of solar energy to electricity by using a semiconductor/electrolyte interface has been demonstrated by H. Gerischer and J. Goberecht inBer. Bunsenges Phys. Chem.,80, 327 (1976), and by Ellis et al. inJ. Am. Chem. Soc.,98, 1635 (1976). The Gerischer cell consisted of a n-CdSe single crystal photoanode and of a doped SnO2conducting glass cathode dipped in an aqueous alkaline electrolyte containing the Fe(CN)64−/Fe(CN)63−redox couple. The energy conversion efficiency was 5% but the cell performance decreased rapidly due to decomposition of the illuminated semiconductor electrode. Since that time a major effort has been devoted to the technology of solar energy conversion and to fabrication of various single crystal and polycrystalline semiconductors such as CdS, CdSe, CdTe, WS2, WSe2, MoS2, MoSe2, GaAs, CuInS2, CuInSe2and CuIn1-xGaxSe2. Most of the cells used an aqueous electrolyte (various redox couples were studied: Fe(CN)64−/Fe(CN)63−, I−/I3−, Fe2+/Fe3+, S2−/Sn2−, Se2−/Sen2−, V2+/V3+) and systems exhibiting a good energy conversion efficiency were generally unstable under sustained illumination due to a process called photocorrosion. The use of a solvent-free polymer electrolyte could eliminate the photocorrosion process owing to its larger electrochemical stability window and to the low solvation energy for the ions that compose the semiconductor materials. Furthermore, this medium allows the fabrication of compact devices with no leakage of solvent, giving a lower absorption of visible light by the electrolyte. Few EPC's based on the junction between protected n-Si single crystal and poly(ethylene oxide), PEO, complexed with a mixture of KI and I2, were investigated but their stability has not been demonstrated (T. A. Skotheim,Appl. Phys. Lett.,38, 712 (1981), T. A. Skotheim et al.Journal de Physique,C3, 615 (1983), T. A. Skotheim and O. Inganäs.J. Electrochem. Soc.,132, 2116 (1985)).

A. K. Vijh and B. Marsan inBull. Electrochem.,5, 456 (1989) have demonstrated that the all-solid-state EPC's n-CdSe (polycrystalline)∥high molecular weight PEO-based copolymer (noted as modified PEO) complexed with M2S/xS (M=Li, Na, K; x=1, 3, 5, 7)∥indium tin oxide conducting glass (ITO) are very stable under white light illumination. However, these authors showed that the high series resistance of the cells, mainly attributed to the very low ionic conductivity of the polymer electrolytes, control the device performance.

In order to enhance the conductivity of the solid electrolyte, a cesium thiolate (CsT)/disulfide (T2) redox couple, where T−stands for 5-mercapto-1-methyltetrazolate ion and T2for the corresponding disulfide, was dissolved in modified PEO and studied in an EPC (J. -M. Philias and B. Marsan,Electrochim. Acta,44, 2915 (1999)). It was found that the PEO12−CsT/0.1 T2electrolyte composition, which is transparent to visible light, exhibits the highest ionic conductivity with 2.5×10−5S cm−1at 25° C. (J. -M. Philias and B. Marsan,Electrochim. Acta,44, 2351 (1999)). Under white light illumination, the cell possesses an energy conversion efficiency (0.11% at 50° C.) about 5 times higher than that of the previous configuration. The lower cell series resistance and the more anodic potential of the T−/T2redox couple (0.52 V vs NHE as compared to —0.34 V for the Sn2−/Sn+12−couple) are largely responsible for this improvement. When the EPC is illuminated, thiolate ions (T−) are photooxidized at the n-type semiconductor electrode (forming the S—S bond of the T2disulfide species) whereas T2species are reduced at the conducting glass electrode (with cleavage of the S—S bond). Despite this improvement, the conductivity of the solid polymer electrolyte is still too low, particularly at room temperature, and continues to limit the cell performance. EPC's incorporating a much higher conductive gel electrolyte (˜10−3S cm−1at 25° C.) were reported in the literature, for example by Cao et al. inJ. Phys. Chem.,99, 17071 (1995), and Mao et al. inJ. Electrochem. Soc.,145, 121 (1998). This type of electrolyte consists in the introduction of an aprotic liquid electrolyte in a polymeric matrix. The polymer gives good mechanical properties whereas the liquid electrolyte is responsible for the good conductivity and electrode wetting. Renard et al. inElectrochim. Acta,48/7, 831 (2003) found that the dissolution of the T−/T2redox couple in a mixture of DMF and DMSO, and incorporated in poly(vinylidene fluoride), PVdF, gives transparent and highly conductive gel electrolytes (conductivities up to 7×10−3S cm−1at 25° C.) with very good mechanical properties. However, when this electrolyte replaced the solid ionic membrane PEO12−CsT/0.1 T2in an EPC, the cell conversion efficiency was not improved.

It has been demonstrated that the cell performance is actually limited by the very slow reduction kinetics of the oxidized species (T2) at the transparent ITO auxiliary electrode and that the difference between oxidation potential of T−and reduction potential of T2at this electrode is as large as 3.06 V in a PVdF-based gel electrolyte containing 50 mM CsT and 5 mM T2. Other authors previously reported low cathodic charge transfer between ITO and aqueous polysulfide (S2−,S, OH−) (Tenne et al.,Ber. Bunsenges Phys. Chem.,92, 42 (1988)) or polyiodide (I−, I2) solutions (Tenne et al.,J. Electroanal. Chem.,269, 389 (1989)).

Hodes et al. inJ. Electrochem. Soc.,127, 544 (1980) found that the transition metallic sulfides Cu2S and CoSxact as good electrocatalysts for the polysulfide redox reactions. However, the former is mechanically instable in the electrolyte.

U.S. Pat. No. 4,421,835 describes that cobalt sulphide can be deposited on a conducting substrate such as brass. Such a deposition is carried out by first depositing hydrous cobalt hydroxide and then by converting the latter into cobalt sulphide by treating it with a sulphide solution. However, this document does not teach nor suggest how to deposit cobalt sulphide on a non-conducting substrate.

U.S. Pat. No. 4,828,942 describes a thin cobalt sulphide electrode which can be produced by electrodeposition of cobalt onto a brass foil followed by alternating anodic and cathodic treatment in polysulfide solution. However, this document does not teach nor suggest how to deposit cobalt sulphide on a non-conducting substrate.

U.S. Pat. No. 5,648,183 describes an electrocatalytic electrode comprising a porous material such as cobalt sulphide deposited on a porous nickel or porous brass. However, this document does not teach or suggest how to deposit cobalt sulphide on a non-conducting substrate.

It has been shown that deposited Co(II) species can serve as an electrocatalyst for the reduction of Sn2−ions on ITO electrode (Tenne et al.,Ber. Bunsenges Phys. Chem.,92, 42 (1988)) or p-InP photoelectrode (Liu et al.,J. Electrochem. Soc.,129, 1387 (1982)).

A method of depositing cobalt sulfide on ITO has been reported by Tenne et al. inBer. Bunsenges Phys. Chem.,92, 42 (1988). The latter method consists in immersing the substrate for a few minutes in CoCl2solution (≧0.1 M), rinsing in water and then immersing in a separate polysulfide solution for a few minutes; this process can be repeated several times. However, this technique does not allow an adequate control of the COS film thickness.

Hodes et al. inJ. Electrochem. Soc.,127, 544 (1980) reported the preparation of a CoS thin film on stainless steel. The two-steps method involves the electrodeposition, at 25° C. and for few minutes, of Co(OH)2onto the metallic substrate, from an aqueous solution of CoSO4with a potassium biphthalate buffer, at a current density that depends on the pH of the electrolyte. When immersed in a polysulfide solution, Co(OH)2is converted to cobalt sulfide, mainly CoS. However, when the above method is used to form a cobalt sulfide layer on a transparent conducting glass electrode (ITO), metallic cobalt is plated on the substrate (instead of Co(OH)2) during the first step, that cannot be converted to COS by a subsequent immersion in the polysulfide solution.

Thus, it seems to be very difficult to fabricate, on ITO, COS thin films of easily controllable thicknesses (and therefore transparencies).

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to overcome the above-mentioned drawbacks and to provide an electrode having thin layer of CoS thereon and a process of making the same.

According to a first aspect of the invention, there is provided an electrode comprising:a non-conductive substrate;a first layer disposed on the substrate, the layer comprising indium tin oxide or fluorine-doped SnO2; anda second layer disposed on the first layer, the second layer comprising COS.

According to a second aspect of the invention, there is provided an indium tin oxide glass electrode or an indium tin oxide polymer electrode having thereon a layer comprising CoS.

According to a third aspect of the invention, there is provided an electrode comprising:a non-conductive substrate;a first layer disposed on the substrate, the first layer comprising indium tin oxide or fluorine-doped SnO2;a second layer disposed on the first layer, the second layer comprising Co(OH)2; anda third layer disposed on the second layer, the third layer comprising CoS.

According to a fourth aspect of the invention, there is provided an indium tin oxide glass electrode or an indium tin oxide polymer electrode having thereon a layer comprising Co(OH)2and another layer disposed on the layer of Co(OH)2, the other layer comprising CoS.

According to a fifth aspect of the invention, there is provided a process for preparing an electrode, comprising the steps of:a) providing a non-conductive substrate, the substrate having an indium tin oxide and/or a fluorine-doped SnO2layer thereon;b) electrodepositing a layer comprising Co(OH)2on the indium tin oxide or fluorine-doped SnO2layer; andc) converting at least a portion of the layer comprising Co(OH)2into a layer of CoS.

According to a sixth aspect of the invention, there is provided a process for preparing an electrode, comprising the steps of:a) providing an indium tin oxide glass electrode;b) electrodepositing a Co(OH)2layer on the indium tin oxide glass electrode; andc) converting at least a portion of the Co(OH)2layer into a layer of CoS.

According to a seventh aspect of the invention, there is provided a process for preparing an electrode, comprising the steps of:a) providing an indium tin oxide polymer electrode;b) electrodepositing a Co(OH)2layer on the indium tin oxide polymer electrode; andc) converting at least a portion of the Co(OH)2layer into a layer of CoS.

Applicant has found that by preparing an electrode according to the fifth, sixth, and seventh aspects of the invention, it was possible to obtain an ITO electrode or a fluorine-doped SnO2electrodes having thereon a thin and substantially transparent CoS layer. Moreover, the thickness of the CoS layer was substantially controllable. In fact, it has been observed that by using these processes, it was possible to obtain uniform and homogeneous CoS layers. Such a characteristic can also explain the transparency of the obtained electrodes. Thus, the obtained electrodes can be very interesting for uses in photovoltaic cells in view of their properties.

The expression “source of sulfur” as used herein refers to a compound or a blend of compounds capable of converting Co(OH)2into CoS.

The expression “substantially transparent” as used herein, when referring to a layer or a substrate, refers to a layer or a substrate having a transmittance of visible polychromatic light of at least 60%, preferably of at least 70%, more preferably of at least 80%, and even more preferably of at least 90%. A transmittance of at least 95% is preferred.

DESCRIPTION OF PREFERRED EMBODIMENTS

In the electrode according to the first and third aspects of the invention, the non-conductive substrate can be a polymer substrate or a glass substrate.

In the electrodes of the invention having a polymer substrate, the latter can comprise a polymer selected from the group consisting of polycarbonate, acetate, polyethylene terephthalate, polyethylene naphthalate and polyimide. Preferably, the polymer is selected from the group consisting of polyethylene terephthalate and polyimide.

In the electrode according to the second aspect of the invention, the glass electrode and/or the COS layer can be substantially transparent. The electrode can further includes a layer of Co(OH)2disposed between the indium tin oxide glass electrode and the COS layer.

In the electrodes of the invention the layer comprising CoS can have a thickness of less than about 30 μm. Preferably, the thickness is less than about 15 or preferably less than about 5 μm. A range of about 0.25 to about 4 μm is particularly preferred and a range of about 0.50 to about 2 μm is more preferred. The layer comprising CoS preferably consists of CoS.

In the electrodes of the invention having a layer comprising Co(OH)2, this layer can have thickness of less than about 30 μm. Preferably, the thickness is less than about 15 or preferably less than about 5 μm. A range of about 0.25 to about 4 μm is particularly preferred and a range of about 0.50 to about 2 μm is more preferred. The layer comprising Co(OH)2preferably consists of Co(OH)2.

In the electrodes of the invention, the non-conductive substrate is preferably substantially transparent. Moreover, the CoS layer and/or the Co(OH)2layer are/is also preferably substantially transparent. The electrode is also preferably transparent.

The electrodes of the present invention can have a transmittance of visible polychromatic light of at least 35%, preferably of at least 45%, more preferably of at least 60%, and even more preferably of at least 65%.

According to a preferred embodiment, the electrode comprises a Co(OH)2layer directly disposed on an ITO glass electrode or an ITO polymer electrode, and a CoS layer directly disposed on the Co(OH)2layer.

In the process according to the fifth aspect of the invention, step (b) is preferably carried out by:i) using the substrate of step (a) as a cathode and providing a cobalt electrode as an anode;ii) inserting the cathode and the anode into a cell having therein a solution comprising a cobalt salt and a buffer; andiii) applying a galvanostatic current to the solution thereby forming a layer of Co(OH)2on the substrate or cathode.

The non-conductive substrate can be a glass substrate or a polymer substrate. The substrate of step (a) preferably has a sheet resistance of about 8 to about 15 Ω/□. When the non-conductive substrate is a polymer substrate, the current in step (iii) preferably has a density ranging from about 10 to about 15 mA/cm2. The polymer substrate preferably comprises a polymer selected from the group consisting of polycarbonate, acetate, polyethylene terephthalate, polyethylene naphthalate and polyimide. Preferably, the polymer is selected from the group consisting of polyethylene terephthalate and polyimide. The substrate of step (a) can have a sheet resistance of about 8 to about 15 Ω/□. The substrate in step (a) preferably has an indium tin oxide layer thereon.

In the process according to the sixth aspect of the invention, step (b) is preferably carried out by:i) using the electrode of step (a) as a cathode and providing a cobalt electrode as an anode;ii) inserting the cathode and the anode into a cell having therein a solution comprising a cobalt salt and a buffer; andiii) applying a galvanostatic current to the solution thereby forming a layer of Co(OH)2on the substrate or cathode.

The indium tin oxide glass electrode preferably has a sheet resistance ranging from about 8 to about 15 Ω/□.

In the process according to the fifth and sixth aspects of the invention, a reference electrode can further be used. Preferably, the reference electrode is a Ag/AgCl electrode or a saturated calomel electrode. The solution in step (ii) preferably has a pH of about 6.0 to about 7.5 and preferably from about 6.8 to about 7.5. The solution in step (ii) can further comprises LiCl, NaCl, KCl, or CsCl. The buffer is preferably a NH4Cl/NH4OH buffer. The cobalt salt is preferably selected from the group consisting of cobalt acetate, cobalt chloride, cobalt nitrate, cobalt sulphate and mixtures thereof. Cobalt sulphate is preferred. The current in step (iii) preferably has a density ranging from about 15 to about 30 mA/cm2. The current is preferably applied for a period of time ranging from about 1 to about 120 seconds and more preferably from about 1 to about 30 seconds.

In step (iii) according to the process defined in the fifth aspect of the invention, the layer of Co(OH)2electrodeposited on the substrate is preferably substantially transparent.

In step (iii) according to the process defined in the sixth aspect of the invention, the layer of Co(OH)2electrodeposited on the electrode is preferably substantially transparent.

Step (c) in the process according to the fifth and sixth aspects of the invention, is preferably carried out by contacting the layer of Co(OH)2with a basic solution comprising at least one source of sulfur. The basic solution preferably has a pH of at least 10. The pH is preferably of about 13.0 to about 14.0. The basic solution preferably comprises S together with Li2S, Na2S, K2S or mixtures thereof. More preferably, the basic solution comprises S together with Na2S. The basic solution can include a base selected from the group consisting of LiOH, NaOH, NH4OH, KOH and mixtures thereof. KOH is particularly preferred and more particularly when using a glass substrate. Step (c) is preferably carried out by dipping the electrode so formed in step (b) into the basic solution for a period of time ranging from 5 to 60 minutes. A period of time ranging from 15 to 30 minutes is preferred. When a polymer substrate is used, NH4OH or a similar weak base is particularly preferred.

It will be understood by the person skilled in the art that all the preferred embodiments previously described for the processes of the fifth and sixth aspects of the invention are also valuable, when applicable, to the process of the seventh aspect of the invention.

The electrodes according to any aspects of the invention can be used as an anode or as a cathode. The electrodes of the invention can be used for reducing a disulfide into a corresponding thiolate. Alternatively, they can be used for oxidizing a thiolate into a corresponding disulfide.

The disulfide can be a disulfide of formula (I):
R1—S—S—R2

in which R1and R2are same or different and selected from the group consisting of C2-C20alkenyl, C1-C20alkyl, C2-C20alkynyl, C6-C20aralkyl, C6-C12aryl, C3-C8cycloalkyl and C2-C12heteroaryl comprising 1 to 4 heteroatoms. Preferably, R1and R2are identical. The heteroatoms can be selected from the group consisting of N, O and S.

Preferably, the disulfide is a disulfide of formula (I):
R1—S—S—R2in which R1and R2are same or different and selected from the group consisting of

whereinR3and R4are same or different and selected from the group consisting of a hydrogen atom, halogen atom, —NO2, —OH, —CF3—COR6, —COOH, —COOR6, —NHR5C2-C8alkenyl, C1-C6alkoxy, C1-C8alkyl, C2-C8alkynyl, C6-C20aralkyl, C6-C12aryl, C3-C8cycloalkyl and C2-C12heteroaryl comprising 1 to 4 heteroatoms selected from the group consisting of N, O and S,R5is a C1-C8alkyl, C6-C12aryl, C3-C8cycloalkyl, C2-C12heteroaryl comprising 1 to 4 heteroatoms selected from the group consisting of N, O and S, or any suitable protecting group for a nitrogen atom, such protecting groups are known by the person skilled in the art and are defined in T. W. Green and P. G. M. Wuts,Protective Groupsin Organic Synthesis, 3rd edition, Wiley Interscience, New York, 1999, which is hereby incoparated by reference,R6is a C1-C8alkyl, or a C3-C8cycloalkyl, andX is N, O or S.

More preferably, the disulfide is of formula (I):
R1—S—S—R2in which R1and R2are same and selected from the group consisting of

whereinR3is a C1-C8alkyl or CF3,R5is a C1-C8alkyl or a phenyl, andX is N, O or S.

A particularly preferred disulfide is a disulfide wherein R1and R2are

According to another aspect of the invention, there is provided a photovoltaic cell comprising an electrode as defined in the present invention.

According to still another aspect of the invention, there is provided a photovoltaic cell comprising an anode, an electrolyte and, as a cathode, an electrode as defined in the present invention. The anode can comprise a n-type semiconductor. The n-type semiconductor is preferably n-CdSe. The electrolyte can comprise a redox couple together with a solvent, a polymer, a gel or a combination thereof. The redox couple is preferably R1SM/(R1S)2in which:M is a metal selected from the group consisting of Li, Na, K and Cs;R1S−is a thiolate and (R1S)2is a corresponding disulfide wherein R1is selected from the group consistingof

whereinR3and R4are same or different and selected from the group consisting of a hydrogen atom, halogen atom, —NO2, —OH, —CF3—COR6, —COOH, —COOR6, —NHR5C2-C8alkenyl, C1-C6alkoxy, C1-C8alkyl, C2-C8alkynyl, C6-C20aralkyl, C6-C12aryl, C3-C8cycloalkyl and C2-C12heteroaryl comprising 1 to 4 heteroatoms selected from the group consisting of N, O and S,R5is a C1-C8alkyl, C6-C12aryl, C3-C8cycloalkyl, C2-C12heteroaryl comprising 1 to 4 heteroatoms selected from the group consisting of N, O and S, or any suitable protecting group for a nitrogen atom,R6is a C1-C8alkyl, or a C3-C8cycloalkyl, andX is N, O or S.

More preferably, the redox couple is R1SM/(R1S)2in which:M is a metal selected from the group consisting of Li, Na, K and Cs;R1S−is a corresponding disulfide wherein R1is selected from the group consisting of

whereinR3is a C1-C8alkyl or CF3R5is a C1-C8alkyl or a phenyl, andX is N, O or S.

Even more preferably, R1is

According to another aspect of the invention, there is provided a method for reducing disulfides into thiolates comprising the step of electrochemically reducing the disulfides by means of any one of the electrodes of the present invention.

According to another aspect of the invention, there is provided a method for oxidizing thiolates into disulfides, comprising the step of electrochemically oxidizing the thiolates by means of any one of the electrodes of the present invention.

The electrodes of the invention can also be used for reducing a triiodide (I3−) or iodine (I2) into an iodide (I−). Alternatively, they can be used for oxidizing an iodide (I−) into a triiodide (I3−) or iodine (I2).

The iodide (I−) can be provided from a compound of formula (IIA):
M+I−
in which M+is a metal selected from the group consisting of Li+, Na+, K+, Rb+and Cs+.

The electrodes of the present invention can be used for catalyzing oxidation/reduction reactions for a redox couple. The redox couple can be M+/I3−or M+I−/I2in which M+is a metal selected from the group consisting of Li+, Na+, K+, Rb+and Cs+. The redox couple can be dissolved in liquid organic solvents like ethylene carbonate (>37° C.), propylene carbonate, ethylmethyl carbonate, dimethyl carbonate, diethyl carbonate, methoxyacetonitrile, acetonitrile, N,N-dimethylformamide, dimethyl sulfoxide, methoxypropionitrile, 3-methyl-2-oxazolidinone and mixtures thereof. The electrolytic solution (hereafter called the electrolyte) can also be incorporated in silica nanoparticles or a polymer to form a gel. Examples of such compounds include poly(ethylene glycol), poly(ethylene oxide), poly(acrylonitrile), poly(epichlorohydrin-co-ethylene oxide), poly(methyl methacrylate) and poly(vinylidenefluoride-co-hexafluoropropylene). Another possibility is to incorporate the redox couple in a solvating polymer to form a solid polymer electrolyte. Examples of such compounds include poly(ethylene oxide) and polyphosphazene. The concentration of compound of formula (IIA) is between about 0.05 M and 0.9 M, and iodine is at a concentration of at least 0.005 M. More preferably, the electrolyte is KI/I2(50 mM/5 mM) and is dissolved in N,N-dimethylformamide and dimethyl sulfoxide (60/40).

Also, the iodide (I−) can be provided from a compound of formula (IIB):
T+I−
in which T+is an organic cation and preferably an heterocyclic cation.

Alternatively, the redox couple can thus be T+I−/I3−or T+I−/I2. The compound of formula (IIB) can be provided in the form of an ambient temperature ionic liquid (also known as ambient temperature molten salt or room-temperature ionic liquid (RTIL)), which can consist of an heterocyclic cation based on substituted imidazole and an iodide anion.

The compound of formula (IIB) can be a compound of formula

in which R1and R3are same or different and selected from the group consisting of C1-C9alkyl and benzyl, and R2is a C1-C9alkyl or H.

In a particular embodiment, some of these compounds of formula (III) are not liquid at ambient temperature so they have to be dissolved in organic solvents or ionic liquids comprising an anion that is not iodide. Examples of such anions are halogen atoms, polyiodides (I2−, I3−, I5−, I7−, I9−and I11−), PF6−, BF4−, bis(trifluoromethanesulfonyl)amide, trifluoromethanesulfonate, dicyanamide, AlCl4−, ClO4−, NO3−, CH3COO−, CF3COO−, C4F9SO3−, 2.3 HF, 2,2,2-trifluoro-N-(trifluoromethylsulfonyl)acetamide, CH3SO3−, CH3C6H4SO3−, AsF6−, CF3SO3−and (CF3SO2)3C−. Ionic liquids of this type have many benefits: they can dissolve an enormous range of inorganic, organic and polymeric materials at very high concentrations, are non-corrosive, have low viscosities and no significant vapor pressures.

The compound of formula (IIB) can also be a compound of formula (IV):

in which R4and R5are same or different and represent a C1-C6alkyl.

Preferably, the compound of formula (IV) is 1,3-ethylmethylimidazolium iodide (EMI-I):

The electrolyte can thus be EMI-I/I2(163 mM/10 mM) dissolved in EMI-TFSI (trifluoromethyl sulfonylimide).

When using a compound of formula (III) or (IV) in a solar cell, its concentration is comprised between 0.05 and 0.9 M, when the compound is in a solid form. For iodine, the concentration is between 5 and 100 mM. When the compound is liquid at room temperature, it is used thereof and dissolves a concentration of iodine comprised between 5 and 500 mM.

The compound of formula (IIB) can also be a compound of formula (V):

in which R6and R7are same or different and selected from the group consisting of a hydrogen atom and a C1-C7alkyl.

The compound of formula (IIB) can also be a compound (pyrrolinium cation) of formula (VI) which lies between the fully saturated pyrrolidinium cation and the semi-aromatic imidazolium cation:

in which R8and R9are same or different and selected from the group consisting of hydrogen atom and C1-C4alkyl.

The compound of formula (IIB) can also be a compound of formula (VII):

in which R10, R11and R12are same or different and represent a C1-C12alkyl. Preferred compounds of formula (VII) are: (Et2MeS)I, (Bu2MeS)I and (Bu2EtS)I, which are in liquid form at room temperature.

The compound of formula (IIB) can also be a compound of formula (VIII):

in which R13is selected from the group consisting of a hydrogen atom, a halogen atom and a C1-C18alkyl.

The compound of formula (IIB) can also be a compound of formula (IX):

in which R14and R15are same or different and represent a C1-C3alkyl.

The compound of formula (IIB) can also be a compound of formula (X):

in which R16to R19are same or different and selected from the group consisting of hydrogen atom, C1-C12alkyl (preferably isopropyl), C2-C6alkoxyalkyl (preferably methoxymethyl and ethoxymethyl), C3alkenyl and C3alkynyl.

In the photovoltaic cell of the invention comprising redox couple, the latter can also be a redox couple comprising an iodide of formula (IIA) or (IIB) as previously defined, with I3−or I2.

According to another aspect of the invention, there is provided a method for reducing a triiodide (I3−) or iodine (I2) into an iodide (I−) comprising the step of electrochemically reducing the triiodide or iodine by means of any one of the electrodes of the present invention.

According to another aspect of the invention, there is provided a method for oxidizing an iodide (I−) into a triiodide (I3−) or iodine (I2), comprising the step of electrochemically oxidizing the iodide by means of any one of the electrodes of the present invention.

According to another aspect of the invention, there is provided a method for catalyzing oxidation and reduction reactions of a redox couple of formula R1SM/(R1S)2, as previously defined, comprising the step of submitting the redox couple to an electrical current between at least two electrodes wherein at least one of the electrodes is an electrode as defined in the present invention.

According to another aspect of the invention, there is provided a method for catalyzing oxidation and reduction reactions of a redox couple of formula M+I−/I3−or M+I−/I2, as previously defined, comprising the step of submitting the redox couple to an electrical current between at least two electrodes wherein at least one of the electrodes is an electrode as defined in the present invention.

According to another aspect of the invention, there is provided a method for catalyzing oxidation and reduction reactions of a redox couple of formula T+I−/I3−or T+I−/I2, as previously defined, comprising the step of submitting the redox couple to an electrical current between at least two electrodes wherein at least one of the electrodes is an electrode as defined in the present invention, and wherein T+is as previously defined.

The following non-limiting examples further illustrate the invention.

EXAMPLES

Indium tin oxide on glass electrodes and indium tin oxide on polymer electrodes having thereon a layer of Co(OH)2and a layer of CoS have been prepared according to the following method.

Prior to electrodeposit a layer of Co(OH)2on ITO on glass obtained from LIBBEY OWENS FORD (trade-mark), the latter is cleaned with soap and water, rinsed with water and dried by means of acetone. Then, the electrode is sonicated in dichloromethane for a period of 5 minutes prior to be air dried. Finally, the electrode is connected to a copper clip prior to the electrodeposition.

The electrodeposition is carried out in a cell having three electrodes (by means of a potentiostat) by applying a constant current (galvanostatic mode). The cell contains 25 mL of a solution comprising 20 g/L of CoSO4and 1 to 2 M of NaCl. The solution also comprises 100 μL of a NH4Cl/NH4OH buffer in order to maintain the pH in a range of about 6.8 to about 7.5. The buffer contains 1.6875 g of NH4Cl and 3.575 g of NH4OH.

The ITO on glass electrode, which has a surface area of about 0.1 to about 0.5 cm2exposed to the solution, is used as a cathode and a cobalt electrode is used as an anode. The cobalt anode has a surface area of about 8 cm2and is located at about 3 cm from the cathode. A Ag/AgCl reference electrode is also utilized.

The density of the cathodic current preferably ranges from 15 to 30 mA/cm2. By using such densities of current, the layer of electrodeposited Co(OH)2is of good quality, and can be thin and transparent.

Such an electrodeposition can be performed by using, as a cathode, an electrode comprising a polymeric material (or polymer substrate) having a layer of ITO thereon. When such an electrode is used, it is first cleaned with soap and water in an ultrasonic cleaner for a period of 15 minutes, rinsed with water, sonicated in water for another period of 15 minutes and air dried. The electrodeposition is carried out on a sample having a surface area of about 0.1 to about 1 cm2exposed to the solution and using the same type of cell than that utilized for the electrodeposition on an ITO on glass electrode. The cathodic current preferably ranges from 10 to 15 mA/cm2. It has been noted that the Co(OH)2layers tend to better adhere to polymeric materials than to a glass material. Moreover, since the polymeric materials used are generally foldable, the electrodes of the invention which include a polymeric material substrate can be folded or rulled up, which makes them particularly interesting for the manufacture of low-cost solar cells.

Various electrodeposition times have been investigated in order to provide optimized electrodes. Interesting results have been obtained by electrodepositing the layer of Co(OH)2over a period of time ranging from 1 to 90 seconds. Preferably, the period of time ranges from 1 to 30 seconds. Such periods of time permit to better control the thickness of the layer and hence its transparency.

The thickness of the Co(OH)2layer has been measured by means of a micrometer and confirmed using a scanning electron microscope (SEM). The values range preferably from about 0.25 to about 4 μm.

2) Conversion of at Least a Portion of the Co(OH)2Layer into a CoS Layer

This second step is carried out by dipping the ITO on glass/Co(OH)2electrode obtained in step (1) into a solution comprising 1 M Na2S, 1 M S and 1 M KOH. This solution is prepared by successively dissolving in water KOH, Na2S and S. The dipping of the electrode is carried out over a period of time of about 30 minutes. When a CoS layer is formed, the color of the surface of the electrode changes from blue-green to black. After the conversion, the electrode is rinsed with nanopure water and then dried under vacuum for a period of about 12 hours.

Such a conversion can also be performed by dipping an electrode comprising a polymeric material having a layer of ITO and a layer of Co(OH)2thereon into a solution comprising 0.1 M Na2S, 0.1 M S and 0.1 M NH4OH. When such an electrode is used, the dipping is carried out over the same period of time (30 minutes) and after the conversion, the electrode is rinsed and dried under the same conditions than those given above.

When the Co(OH)2layer is very thin on the glass substrate or on the polymeric substrate, it is possible to obtain a substantially complete conversion of this layer into a CoS layer.

In order to better characterize the above-mentioned electrodes, several cyclic voltammetry experiments have been carried out.FIG. 1represents cyclic voltammograms comparing an ITO on glass electrode having an area of 0.50 cm2with an ITO on glass/Co(OH)2/CoS electrode according to a preferred embodiment of the invention. A non-aqueous Ag/Ag+(1 M AgNO3) electrode was used as reference electrode. The ITO on glass/Co(OH)2/CoS electrode has an area of 0.40 cm2. The Co(OH)2layer was electrodeposited on an ITO on glass electrode at a current density of 20 mA/cm2during 90 seconds. The reference and tested electrodes were immersed in a DMF/DMSO (60/40)/0.1 M TBAP solution comprising 50 mM of CsT and 5 mM of T2(redox couple), and the scanning speed was 100 mV/s. T−and T2are represented by the following formulae:

As it can be seen fromFIG. 1, an ITO on glass electrode has been compared with an ITO on glass/Co(OH)2/CoS electrode in order to determine the electrocatalytic properties of the latter. The comparison shows that the CoS in the ITO on glass/Co(OH)2/CoS electrode acts as a very good electrocatalyst for the reduction of T2. In particular, the reduction of T2is favored by 0.84 V and the oxidation of T−is favored by 1.12 V when using the CoS electrode instead of the ITO on glass electrode. The Epcand Epaof the CoS electrode are respectively −0.82 V et 0.25 V vs Ag/Ag+. The ΔEpof the latter is thus 1.07 V.

FIG. 2represents cyclic voltammograms comparing a Pt electrode having an area of 0.025 cm2with an ITO on glass/Co(OH)2/CoS electrode according to a preferred embodiment of the invention. In this figure, the current relative to the Pt electrode was multiplied by a factor of 10 (Pt×10). A non-aqueous Ag/Ag+electrode was used as reference electrode. The ITO on glass/Co(OH)2/CoS electrode has an area of 0.40 cm2. The Co(OH)2layer was electrodeposited on an ITO on glass electrode at a current density of 20 mA/cm2during 90 seconds. The reference and tested electrodes were immersed in a DMF/DMSO (60/40)/1 0.1 M TBAP solution comprising 50 mM of CsT and 5 mM of T2, and the scanning speed was 100 mV/s.

As it can be seen fromFIG. 2, a Pt electrode has been compared with an ITO on glass/Co(OH)2/CoS electrode in order to determine the electrocatalytic properties of the latter. This figure shows that the oxidation potential of the two electrodes is similar, whereas the CoS electrode is slightly more electrocatalytic (by 90 mV) for the reduction process.

FIG. 3represents cyclic voltammograms comparing an ITO on glass electrode having an area of 0.50 cm2with an ITO on glass/Co(OH)2/CoS electrode according to a preferred embodiment of the invention. An Ag wire was used as reference electrode. The ITO on glass/Co(OH)2/CoS electrode has an area of 0.40 cm2. The Co(OH)2layer was electrodeposited on an ITO on glass electrode at a current density of 20 mA/cm2during 1 second. Both electrodes were immersed in gel comprising 20% of PVdF and 80% of DMF/DMSO (60/40). The gel also comprises 50 mM of CsT and 5 mM of T2, and the scanning speed was 100 mV/s. The gel electrolyte was prepared as described by Renard et al. inElectrochim. Acta,48/7, 831 (2003).

FIG. 4represents cyclic voltammograms comparing a Pt electrode having an area of 0.025 cm2with an ITO on glass/Co(OH)2/CoS electrode according to a preferred embodiment of the invention. In this figure, the current relative to the Pt electrode was multiplied by a factor of 10 (Pt×10). An Ag wire was used as reference electrode. The ITO on glass/Co(OH)2/CoS electrode has an area of 0.40 cm2. The Co(OH)2layer was electrodeposited on an ITO on glass electrode at a current density of 20 mA/cm2during 1 second. Both electrodes were immersed in gel comprising 20% of PVdF and 80% of DMF/DMSO (60/40). The gel also comprises 50 mM of CsT and 5 mM of T2, and the scanning speed was 100 mV/s.

As it can be seen fromFIGS. 3 and 4, the ITO on glass/Co(OH)2/CoS electrode has better electrocatalytic properties in a gel than in a liquid medium. The results obtained inFIGS. 3 and 4are summarized in Table 1.

FIG. 5represents the current-potential curve, obtained in darkness, of two Electrochemical Photovoltaic Cells: n-CdSe∥PVdF(20%)/DMF/DMSO DMF/DMSO (60/40)/1.34 M CsT/0.13 M T2∥ITO on glass and n-CdSe∥PVdF(20%)/DMF/DMSO (60/40)/1.34 M CsT/0.13 M T2∥ITO on glass/Co(OH)2/CoS. The n-CdSe electrodes were prepared as described by Philias and Marsan inElectrochim. Acta,44, 2915 (1999). The gel electrolyte was prepared as described by Renard et al. inElectrochim. Acta,48/7, 831 (2003), but using the above redox species concentrations. EPC's were prepared in a glovebox. 60 μL of the gel electrolyte were put into the hole (2 cm2) of a 100 μm thick paraffin film set at the surface of an ITO on glass or an ITO on glass/Co(OH)2/CoS electrode. The n-CdSe electrode was then put in contact with the gel and a piece of glass was put on top of the assembly. Finally, the cell was sealed using epoxy glue. The Co(OH)2layer was electrodeposited on an ITO on glass electrode at a current density of 20 mA/cm2during 1 second. The potential measured (V) is the potential applied to the n-CdSe electrode and the scanning speed was 1 mV/s.

As it can be seen fromFIG. 5, the rectification ratio (|i−|/i+) at 0.8 V increases from 1.0 (ITO on glass electrode) to 12.0 (ITO on glass/Co(OH)2/CoS) which confirms that the quality of the junction is improved when using a CoS electrode.

FIG. 6represents the current-potential curve, obtained under a polychromatic light (incident power density from a tungsten-halogen lamp: 100 mW/cm2) of two Electrochemical Photovoltaic Cells: n-CdSe∥PVdF(20%)/DMF/DMSO (60/40)/1.34 M CsT/0.13 M T2∥ITO on glass and n-CdSe∥PVdF(20%)/DMF/DMSO (60/40)/1.34 M CsT/0.13 M T2∥ITO on glass/Co(OH)2/CoS. The Co(OH)2layer was electrodeposited on an ITO on glass electrode at a current density of 20 mA/cm2during 1 second. The potential measured (V) is the potential applied to the n-CdSe electrode, the scanning speed was 10 mV/s and the light area was 1.3 cm2.

As it can be seen fromFIG. 6, the important increase of the photocurrent, when using the CoS electrode in the cell, means that the reduction of T2is improved in a significant manner. These results demonstrate that the use of a CoS electrode improves the catalytic performance of the cell.

FIG. 7shows still further cyclic voltammograms demonstrating the influence of the deposition time of the Co(OH)2layer in an ITO on glass/Co(OH)2/CoS electrode according to a preferred embodiment of the invention. The Co(OH)2layers were electrodeposited on an ITO on glass electrode at a current density of 20 mA/cm2. The ITO on glass/Co(OH)2/CoS electrodes have an area of 0.10 cm2. An Ag wire was used as reference electrode. All the electrodes were immersed in a DMF/DMSO (60/40)/0.1 M TBAP solution comprising 50 mM of CsT and 5 mM of T2, and the scanning speed was 100 mV/s.

As it can be seen fromFIG. 7, in comparison withFIG. 1, the ΔEpof the ITO on glass/Co(OH)2/CoS electrode has been improved from 1.07 V (deposition time of 90 seconds) to 0.99 V (deposition time of 60 seconds), a difference of 80 mV. It can be seen that the ΔEpis influenced by the deposition time of Co(OH)2. These results are shown in Table 2.

FIG. 8shows an XPS sulphur analysis comparison between a first electrode (an ITO on glass/Co(OH)2electrode) and a second electrode (an ITO on glass/Co(OH)2/CoS electrode). The Co(OH)2layers were electrodeposited on an ITO on glass electrode at a current density of 20 mA/cm2during 30 seconds. The only peaks present in case of the first electrode are probably due to traces of sulphate used in the preparation of the latter. However, in the case of the second electrode, a peak at 162.4 eV (S 2p3/2transition) and a peak at 163.5 eV (S P1/2transition) are observed and correspond to CoS. This analysis thus proves the existence of the CoS layer in the second electrode.

FIGS. 9 and 10show, respectively, an X-ray diffraction pattern of an ITO on glass/Co(OH)2electrode and an X-ray diffraction pattern of a Co(OH)2powder. The ITO on glass/Co(OH)2electrode was prepared by electrodepositing a Co(OH)2layer on an ITO on glass electrode at 20 mA/cm2during 90 seconds. The Co(OH)2powder was obtained by reacting together Co(NO3)2and KOH. These analyses have been performed in order to determine if the Co(OH)2layer electrodeposited on the ITO on glass electrode had different characteristics than Co(OH)2obtained in powder form. The results obtained inFIGS. 9 and 10are summarized in tables 3 and 4, respectively.

According toFIG. 10and table 4, it can be seen that the d-spacings obtained concerning the Co(OH)2powder are similar to those reported in the scientific literature. Also, a preferential orientation of the (100) plane was noted. It can be concluded fromFIG. 10that Co(OH)2has a hexagonal phase.

InFIGS. 9 and 10the position of the peaks is identical, which demonstrate that the positions are the same even if Co(OH)2is prepared according to different methods. It can also be seen from the latter two figures that the peaks of the ITO on glass/Co(OH)2electrode (FIG. 9) are generally more intense and narrower than the peaks of the Co(OH)2powder (FIG. 10). This difference indicates that electrodeposited Co(OH)2is more cristalline than the Co(OH)2powder. As example, the (101) peak inFIG. 9has an intensity of 790 Cps, whereas the (101) peak inFIG. 10has an intensity of 510 Cps. By comparing the peaks ofFIGS. 9 and 10, and by using the Scherrer relation it is possible to quantitatively establish that the crystal grains in the electrodeposited Co(OH)2 are bigger.

The visible absorption spectra of an ITO on glass electrode (FIG. 11), different ITO on glass/Co(OH)2/CoS electrodes (FIG. 12) and of a PVdF(20%)/DMF/DMSO (60/40)/1.34 M CsT/0.13 M T2gel electrolyte having a thickness of 100 μm (FIG. 13) are analyzed in table 5, which give the percentage of transmitted visible polychromatic light as obtained using a radiometer.

As it can be seen from table 5, visible light is transmitted up to 68% (transmission of 90% for the Co(OH)2/CoS layers) when using an ITO on glass/Co(OH)2/CoS electrode, which has been prepared by electrodepositing Co(OH)2over a period of 1 second. FromFIG. 12it can also be seen that the maximum absorbance is very low, i.e. almost nonexistent. These results demonstrate that the Co(OH)2deposition time strongly influences the light transmission of the prepared electrode and that optimal results are obtained with a deposition time of 1 second at 20 mA/cm2. These results also demonstrate that these specific Co(OH)2and CoS layers have a high degree of transparency.

When using an ITO on polymer/Co(OH)2/CoS electrode, the polychromatic visible light transmitted is substantially the same than that in the case of an ITO on glass/Co(OH)2/CoS electrode. It is to be noted, however, as pointed out above, that the current density preferably ranges from 15 to 30 mA/cm2and from 10 to 15 mA/cm2when using ITO on glass and ITO on polymer, respectively.

FIG. 14represents cyclic voltammograms comparing an ITO on glass electrode having a surface area of 0.1 cm2with an ITO on glass/Co(OH)2/CoS electrode according to a preferred embodiment of the invention. In this figure, the current (I) relative to the ITO on glass/Co(OH)2/CoS electrode was divided by a factor of 3.5 (I/3.5). A silver wire was used as a reference electrode. The ITO on glass/Co(OH)2/CoS electrode has a surface area of 0.1 cm2. The Co(OH)2layer was electrodeposited on an ITO on glass electrode at a current density of 20 mA/cm2during 90 seconds using a solution containing 1 M of NaCl. The reference and the tested electrodes were immersed in a DMF/DMSO (60/40)/0.1 M TBAP solution comprising 50 mM of KI and 5 mM of 12 (redox couple), and the scanning speed was 100 mV/s.

As it can be seen fromFIG. 14, an ITO on glass electrode has been compared with an ITO on glass/Co(OH)2/CoS electrode in order to determine the electrocatalytic properties of the latter. The comparison shows that the ITO on glass/Co(OH)2/CoS electrode acts as a very good electrocatalyst for the reduction of triiodide. In particular, the reduction of I3−is favored by 0.86 V and the oxidation of I−is favored by 0.78 V when using the CoS electrode instead of the ITO on glass electrode. The Epc1and Epa1of the CoS electrode are respectively 0.25 V and 1.01 V vs Ag. The ΔEp1of the latter is thus 0.76 V instead of 2.40 V for ITO on glass.

FIG. 15shows cyclic voltammogramms demonstrating the influence of the NaCl concentration in the electrodepositing solution used for the electrodeposition of the Co(OH)2layers on ITO on glass, to prepare ITO on glass/Co(OH)2/CoS electrodes. In this figure, the currents (I) relative to the ITO on glass/Co(OH)2/CoS electrodes prepared using a solution containing 1.5 and 2 M NaCl were multiplied by a factor of 1.5 (I×1.5). The Co(OH)2layers were electrodeposited on ITO on glass electrodes at a current density of 20 mA/cm2during 90 seconds. The ITO on glass/Co(OH)2/CoS electrodes have a surface area of 0.1 cm2. A Ag wire was used as a reference electrode. All the electrodes were immersed in a DMF/DMSO (60/40)/0.1 M TBAP solution comprising 50 mM of KI and 5 mM of I2, and the scanning speed was 100 mV/s.

As it can be seen fromFIG. 15, the ΔEp1of the ITO on glass/Co(OH)2/CoS electrode has been improved from 0.76 V (NaCl 1 M) to 0.56 V (NaCl 2 M), a difference of 0.20 V. This difference is mainly due to the less positive value of Epa1associated to the electrode prepared using NaCl at a higher concentration. The same downward trend is observed for ΔEp2, going from 0.55 V (NaCl 1 M) to 0.28 V (NaCl 2 M), a difference of 0.27 V. It can be seen that both ΔEpare influenced by the sodium chloride concentration in the electrodepositing solution. These results are shown in Table 7.

FIG. 16represents cyclic voltammograms comparing a Pt electrode having a surface area of 0.02 cm2with an ITO on glass/Co(OH)2/CoS electrode according to a preferred embodiment of the invention. In this figure, the current (I) relative to the ITO on glass/Co(OH)2/CoS electrode prepared using a solution containing 2 M NaCl was multiplied by a factor of 1.5 (I×1.5), and that of the Pt electrode was multiplied by a factor of 6.5 (I×6.5). The Co(OH)2layer was electrodeposited on an ITO on glass electrode at a current density of 20 mA/cm2during 90 seconds. The concentration of NaCl in the electrodepositing solution was 2 M. The ITO on glass/Co(OH)2/CoS electrodes have a surface area of 0.1 cm2. A Ag wire was used as a reference electrode. All the electrodes were immersed in a DMF/DMSO (60/40)/0.1 M TBAP solution comprising 50 mM of KI and 5 mM of I2, and the scanning speed was 100 mV/s.

As it can be seen fromFIG. 16, a Pt electrode has been compared with an ITO on glass/Co(OH)2/CoS electrode in order to determine the electrocatalytic properties of the latter. According to this figure and Table 8, the iodide oxidation potential for the two electrodes (the redox process that occurs at the most cathodic potential) is similar, whereas the CoS electrode is more electrocatalytic (by 90 mV) for the reduction of I3−. The ΔEp1of the latter is 110 mV smaller than the one for Pt. Inversely, for the most anodic redox process (A2/C2), the reduction potential for the two electrodes is similar, whereas the Pt electrode is more electrocatalytic (110 mV) for the oxidation process. The ΔEp2of the latter is 60 mV smaller than the one for the CoS electrode.

FIG. 17represents cyclic voltammograms comparing an ITO on glass electrode having a surface area of 0.07 cm2with an ITO on glass/Co(OH)2/CoS electrode (0.09 cm2) according to a preferred embodiment of the invention. A silver wire was used as a reference electrode. The Co(OH)2layer was electrodeposited on an ITO on glass electrode at a current density of 20 mA/cm2during 30 seconds using a solution containing 1 M of NaCl. The reference and the tested electrodes were immersed in a EMI-TFSI solution comprising 0.163 M of EMI-I and 10 mM of I2. The scanning speed was 100 mV/s.

As it can be seen fromFIG. 17, an ITO on glass electrode has been compared with an ITO on glass/Co(OH)2/CoS electrode in order to determine the electrocatalytic properties of the latter. The comparison shows that the ITO on glass/Co(OH)2/CoS electrode acts as a very good electrocatalyst for the reduction of triiodide. In particular, the reduction of I3−is favored by 0.74 V and the oxidation of I−is favored by 0.77 V when using the CoS electrode instead of the ITO electrode. The Epc1and Epa1of the CoS electrode are respectively 0.19 V and 0.40 V vs Ag (Table 9). The ΔEp1of the latter is thus 0.21 V instead of 1.72 V for ITO.

FIG. 18shows other cyclic voltammograms comparing a Pt electrode having a surface area of 0.025 cm2with an ITO on glass/Co(OH)2/CoS electrode (0.09 cm2) according to a preferred embodiment of the invention. A Ag wire was used as a reference electrode. The Co(OH)2layer was electrodeposited on an ITO on glass electrode at a current density of 20 mA/cm2during 30 seconds. The NaCl concentration in the electrodepositing solution was 1 M. All the electrodes were immersed in a EMI-TFSI solution comprising 0.163 M of EMI-I and 10 mM of I2. The scanning speed was 100 mV/s.

As it can be seen fromFIG. 18, a Pt electrode has been compared with an ITO on glass/Co(OH)2/CoS electrode in order to determine the electrocatalytic properties of the latter. This figure shows that the oxidation and reduction potentials of the two electrodes are similar (see also Table 10). However, the CoS electrode demonstrates a higher current density than Pt, which is of a great interest.

FIG. 19shows still further cyclic voltammograms demonstrating the influence of the deposition time of the Co(OH)2layer on ITO on glass, to prepare ITO on glass/Co(OH)2/CoS electrodes according to a preferred embodiment of the invention. The Co(OH)2layers were electrodeposited on an ITO on glass electrode at a current density of 20 mA/cm2. The ITO on glass/Co(OH)2/CoS electrodes have a surface area of 0.09 cm2for electrodeposition time of 30 and 60 seconds, and 0.06 cm2for an electrodeposition time of 90 seconds. A Ag wire was used as a reference electrode. All the electrodes were immersed in a EMI-TFSI solution comprising 0.163 M of EMI-I and 10 mM of I2. The scanning speed was 100 mV/s.

As it can be seen fromFIG. 19, the ITO on glass/Co(OH)2/CoS electrodes, which have been prepared by electrodepositing Co(OH)2for 60 or 90 seconds, show higher current densities than the electrode prepared using an electrodeposition time of 30 seconds. These results demonstrate that the roughness factor increases with the electrodeposition time. Regarding the oxidation and reduction potentials, a similarity is observed for the three electrodes (Table 11).

FIG. 20 and 21represent cyclic voltammograms comparing an ITO on polymer (polyethylene terephthalate) electrode having a surface area of 0.05 cm2(FIG. 20) with an ITO on polymer (polyethylene terephthalate)/Co(OH)2/CoS electrode (0.05 cm2) (FIG. 21) according to a preferred embodiment of the invention. A silver wire was used as a reference electrode. The Co(OH)2layer was electrodeposited on an ITO on polymer electrode at a current density of 15 mA/cm2during 90 seconds using a solution containing 1 M of NaCl. The reference and the tested electrodes were immersed in a EMI-TFSI solution comprising 0.163 M of EMI-I and 10 mM of I2. The scanning speed was 100 mV/s.

As it can be seen fromFIG. 20 and 21, an ITO on polymer electrode has been compared with an ITO on polymer/Co(OH)2/CoS electrode in order to determine the electrocatalytic properties of the latter. The comparison shows that the ITO on polymer/Co(OH)2/CoS electrode acts as a very good electrocatalyst for the reduction of triiodide. In particular, the reduction of I3−is favored by 1.1 V and the oxidation of I−is favored by 0.65 V when using the CoS electrode instead of the ITO electrode. The Epc1and Epa1of the CoS electrode are respectively 0.70 V and 1.15 V vs Ag (Table 12). The ΔEp1of the latter is thus 0.45 V instead of 2.20 V for ITO.