FR2780055A1 - Tungsten oxide-coated electrode, especially for water photo-electrolysis or organic waste photo-electrochemical decomposition or for an electrochromic display cell - Google Patents

Abstract

An electrode is provided with an active WO3 film using a colloidal solution of tungstic acid in ethylene glycol, polyethylene glycol, maltose, glycerol, glucose, mannitol or myo-inositol. In the production of an electrode having an active WO3 film by applying a thin layer of a colloidal solution of tungstic acid in an organic material onto a conductive glass sheet and heat treating at \-350 deg C, the organic material is selected from ethylene glycol, polyethylene glycol, maltose, glycerol, glucose, mannitol and myo-inositol. Preferred Features: The colloidal solution is prepared by: (a) mixing a tungstic acid solution with the organic material and forming a colloidal solution from the mixture; or (b) mixing a tungstic acid solution with ethanol, methanol, other volatile alcohol or dimethyl sulfoxide, forming a colloidal solution from the mixture and mixing the colloidal solution with the organic material. The tungstic acid solution is prepared by passage of an aqueous sodium tungstate solution across a cation exchange resin. The colloidal solution is formed by evaporation under reduced pressure. The deposition and heat treatment operations may be repeated up to 12 times.

Description

1 2780055

  METHOD FOR MANUFACTURING AN ELECTRODE COMPRISING

A TUNGSTEN OXIDE FILM

  The present invention relates to a method of manufacturing an electrode comprising an active film of tungsten trioxide (W03). Such electrodes find applications in the production of cells used in various fields, in particular as a photoanode capable of converting solar energy during the photoelectrolysis of water, during the photoelectrochemical degradation of organic waste, or even the production of

  electrochromic display cells.

  For many applications, the electrode must be transparent. This is why a glass plate is used as the substrate.

  io coated with a transparent conductive layer formed of doped tin oxide.

  The method of manufacturing electrodes of this type generally comprises the following steps: - production of a mixture of tungstic acid in solution with an organic material, - formation of a colloidal solution of this mixture, - deposition of the colloidal solution in the form of a thin layer on a conductive glass plate, and - heat treatment of the plate at a temperature of at least

350 C.

  On the occasion of the IUPAC congress (Geneva 1997), the abstract of a poster was published entitled "Preparation and properties of highly transparent anisotropic W03 films", defining the stages of such a process, which consists in : - pass an aqueous sodium tungstate solution through a cation exchange resin which transforms into tungstic acid, - mix the collected acid with an organic material, - form a colloidal solution of this mixture,

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  - deposit the colloidal solution in successive layers on a conductive glass plate, and - heat treat after each deposit (500 to 600 C in a

oxygen-rich atmosphere).

  A mesoporous structure is thus obtained, formed of nanoparticles of crystallized W03 having a diameter of 20 to 50 nm and aggregated with each other, each layer having a thickness of approximately 500 nm. Such a structure makes it possible to produce electrodes which have good

  electrochromic and photoelectrochemical characteristics.

  o These qualities are certainly very appreciable, but an industrial application requires a good adhesion of the W03 to its substrate, failing which the structure quickly disintegrates and makes the electrode unusable. It is also necessary that the colloidal solution is sufficiently stable to be able to

  be usable for several hours, even days.

  The main object of the present invention is to allow the production of films of W03 having excellent adhesion, from a stable mixture.

in time.

  More specifically, the invention relates to a method of manufacturing an electrode comprising a film of W03, of the type comprising the successive operations of: - formation of a colloidal solution comprising a mixture produced from tungstic acid and d '' an organic material, - deposition on a conductive glass plate of a thin layer of the solution, and - heat treatment of said plate at a temperature of at least

350 C.

  The process is mainly characterized in that the organic material used is chosen from ethylene glycol, polyethylene glycol, maltose,

  glycerol, glucose, mannitol and myo inositol.

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  According to an advantageous embodiment of the invention, the operation of forming a colloidal solution comprises the successive operations of: - producing a mixture of tungstic acid in solution with the organic material, and - forming d 'a colloidal solution of the mixture. According to another advantageous embodiment of the invention, the operation of forming a colloidal solution comprises the successive operations of: - producing a mixture of tungstic acid in solution with o10 organic material chosen from among ethanol, methanol, any other volatile alcohol and dimethyl sulfoxide, - formation of a colloidal solution of the mixture, and - preparation of a mixture of the colloidal solution with the organic material. i5 The heat treatment operation is preferably carried out at a

  temperature between 400 and 600 C.

  For some applications, the electrode must also be homogeneous. It is essential in display cells and most often desirable for photoelectrolysis. To this end, the organic material of the colloidal solution is advantageously chosen from ethylene glycol, polyethylene glycol,

  maltose, glucose, mannitol and glycerol.

  In order for the light to reach the electrode-electrolyte interface, it must pass through the electrolyte or the substrate and the active film, one or the other must therefore be transparent. However, the electrolyte is sometimes opaque, or at least absorbent. In this case, the substrate and the active film must be transparent, the light source then being able to operate through the substrate. This object is achieved thanks to the fact that the organic material of the colloidal solution is advantageously chosen from ethylene glycol, the

  polyethylene glycol, mannitol and glycerol.

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  In applications relating to photoelectrochemistry, the efficiency depends on the intensity of the photocurrent of the electrode. To obtain the highest possible intensity, the organic material of the colloidal solution is advantageously chosen from polyethylene glycol, maltose, glucose, mannitol and myo inositol. Experience has shown that the quality of the colloidal solution is highly dependent on the quality of the tungstic acid. This is preferably obtained by passing an aqueous solution of sodium tungstate to the

  through a cation exchange resin.

  0o It was found that, to produce the colloidal solution, it was advantageous

  to carry out an evaporation under reduced pressure.

  The quality of the cell depends in particular on the contact surface between the electrolyte and the active film. As the active film is porous, it is possible to increase this surface by acting on the thickness. To this end, the operations of depositing a thin layer of the colloidal solution on a conductive glass plate and of heat treatment are repeated up to 12 times. Other advantages and characteristics of the invention will emerge from the

  description which follows, made with reference to the appended drawing, in which:

  - Figure 1 shows a conductive glass plate carrying an active film obtained according to the process of the invention, - Figure 2 illustrates the different stages of the process, and - Figure 3 represents the variation of the density of the photocurrent in

according to the potential.

  Referring to Figure 1, we can see, schematically, a conductive glass plate 10 comprising a conductive layer

  transparent 12 and coated with a film of W03 14.

  Such plates are, for example, sold by the house of LOF under the name of conductive glass. Good results have been obtained with a

  quality with a resistance of 10 Q per square.

  The film 14 is formed from an aggregate of particles of W03 having a diameter of 20 to 60 nm, which are rigidly fixed to each other and define a mesoporous structure obtained by alternating deposits of thin layers and heat treatments. The stack advantageously comprises 6 to 9 layers. Depending on the application, this number can be reduced to a single layer or, on the contrary, be increased to 12

layers, for example.

  It will be noted that, in FIG. 1, for obvious reasons, the scales are not respected. In reality, the glass plate has a thickness which is calculated in millimeters while the other dimensions are of the order of a few micrometers, or even a few tens of nanometers, as has been

is explained above.

  The mesoporous structure thus obtained makes it possible to increase

  considerably the contact surface between the electrode and the electrolyte.

  As shown in FIG. 2, the process according to the invention comprises several stages, identified by a capital letter, which are as follows: A. Production of tungstic acid B. Addition of a first organic material C. Concentration to form a colloidal solution D. Introduction of a second organic material E. Deposition of the colloidal solution on a substrate F. Formation of a thin layer of the solution

G. Heat treatment.

  These different steps will now be described in more detail.

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  In the example below, the quantities used correspond to laboratory experiments. It is obvious that in industrial production, the

  volumes used are greater.

  A. We initially have a solution 20 of sodium tungstate (Na2WO4) contained in a first container 21, of distilled water 22 placed in a second container 23, and of cation exchange resin 24, housed in a column 25 and soaked in distilled water. The solution 20 is dosed so as to have 20 ml of 0.5 M sodium tungstate.

  can use as cation exchange resin the product sold by DOWEX under the reference W 50 HCR-2 100-200 mesh.

  The solution 20 is poured into the column 25 and flows through the cation exchange resin 24, repelling before it the water initially contained in the column, which is eliminated. It is itself entrained towards the outlet of the column by distilled water 22, which acts as an eluent. Sodium tungstate 20 is acidified by cation exchange with resin 24, which retains sodium cations and releases protons, thereby forming

tungstic acid 26.

  B. The tungstic acid 26 thus produced is collected at the outlet of the column 25 in a container 27 initially containing 5 to 20 ml of a first organic material 28. An agitator 29 rotates inside the container to ensure mixing tungstic acid 26 and organic material 28, which together form a solution. The acid 26 is allowed to flow until the container 27 contains approximately 50 ml of the solution. This volume is greater than the sum of the volumes of sodium tungstate and organic material. This is due to the fact that water mixes with the tungstate of

  sodium during flow in column 25.

  C. The mixture thus obtained is placed in a rotary evaporator 30 to concentrate it by evaporation under reduced pressure. This operation is carried out at a temperature of 40 to 70 C, typically 55 C, up to

obtain a colloidal solution.

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  D. A second organic material 32 is added to the colloidal solution 31 contained in a container 33, stirred by means of the stirrer 29 to form a mixture 34. This mixture then has a W03 concentration of between 0.1 and 0.9 M E. The mixture 34 of the colloidal solution 31 and of the organic material 32 is deposited in the form of a drop 35 by means of a pipette 36 on the

  conductive layer 12 of a conductive glass plate 10.

  F. The drop 35 is spread on the plate 10 to form a ribbon, then drawn

  over its entire surface by means of a glass plate 37.

  G. An oven 38 makes it possible to treat a set of plates thus obtained in an oxidizing atmosphere at a temperature between 350 and 650 C, preferably between 400 and 600 C, for 15 to 60 minutes, preferably between 30 to 60 minutes. During this operation, the organic materials burn and volatilize while the W03 crystallizes on the

substrate in a porous structure.

  As already mentioned, the deposition operations (E), formation of a thin layer (F) and passage to the oven (G) can be repeated up to a dozen

  times, to obtain a thickness of up to more than 5 gIm.

  The following organic materials have been used successfully: a) ethanol, methanol and other volatile alcohols, b) dimethyl sulfoxide, c) ethylene glycol, d) polyethylene glycol 200, 300, 600 and 1000, e) maltose and glucose, f) glycerol, g) mannitol

h) myo inositol.

  In the embodiment of the invention which has just been described, a first organic material 28 was used to collect the tungstic acid and

  a second organic material 32 was mixed with the colloidal solution.

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  The first organic material 28 is then chosen from products a) and b).

  In this case, the second organic material 32 is chosen from the products

c) to h).

  According to a variant of the method, the first organic material 28 is chosen from products c) to h). It is no longer necessary then to introduce the

second organic material 32.

  Among the many organic materials tested, the products a) to h) from the above list are those which have given the desired results. The exact reasons why the method according to the invention makes it possible to obtain an adherent mesoporous structure are not however fully explained. It will be noted that the tests carried out did not show large differences in the characteristics of the electrodes depending on whether the products c) to h) were mixed before or after the concentration operation to obtain the colloidal solution 1s. However, when tungstic acid is collected in dimethyl

  sulfoxide, the stability of the colloidal solution is greatly increased.

  Tests have also been carried out using unsatisfactory products. In order to have a good overview, the results

  more interesting are summarized in the table below.

  Material Concentration Organic ratio (WO3) [M] WO3 / mat. A H T I I _____ organic Ethylene 0.4 0.3; 0.9 X 1) 2) low glycol PEG 200 0.1-0.5 0.1-0.3 X _ very good and 0.1-0.5 0.2-0.9 X X X very good PEG 300 2 0.3 0.8-2.1 X _ very good <0.3 "X X X very good

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  PEG 600 0.5 0.25 X - good 0.5 X X X good PEG 0.35 0.35 --- good 1000 0.7 X X - good 1.4 - good

PVA 0.5 1 --...

  15000 3 X 3) - weak 4 X 4) --- weak

---.....

maltose 0.65 0.5 X X - Very good

0.8 0.25-1 X X _

glucose 0.65 0.5 X X --- Very good

0.8 0.25 --- -

  0.5 XX - Very good mannitol 0.55 0.25-0.5 XXX Very good 1 XX - Very good glycerol 0.4 0.5 XX 2) Poor myo 0.4 0.4 X - Very good acid inositol 0.65 0.5 --- --- --- Poor tartaric maltitol 0.65 0.5..Good A = adhesion 1) Homogeneous but not as much as with PEG H = homogeneity 2) Transparent if we put at most 3-4 layers s T = transparency 3) Maximum 4 layers I = photocurrent 4) Maximum 2 layers This table has seven columns. The first three define the composition of the colloidal solution while the other four indicate the

  characteristics of the films obtained.

  l0 More precisely, the first column defines the organic material used, the second the concentration of the colloidal solution in W03 and the

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  third, the weighted relationship between WO3 and organic material. The fourth column gives an indication of the quality of the adhesion of the deposit, the fifth of its homogeneity, the sixth of its transparency and the seventh of the intensity of the photocurrent measured when the polarized electrode is exposed to simulated sunlight. On examination of this table, it can be seen that the concentration of the colloidal solution of WO3 is between 0.1 M and 0.8 M. This parameter does not seem to play a preponderant role in the quality of the electrodes obtained. For its part, the weighted ratio between WO3 and organic material

  plays a more or less important role.

  On the other hand, the results obtained differ considerably depending on the organic material chosen. Some, such as tartaric acid and maltitol, do not

  do not allow, at the concentrations tested, to obtain an adherent film.

  On the contrary, ethylene glycol, polyethylene glycol and mannitol give

  good results in every way.

  The electrodes obtained from a solution containing alcohol

  polyvinilic (molar mass of 15,000) are, on the contrary, not very interesting.

  Maltose, glucose and myo inositol make it possible to obtain films with good adhesion and have a very good photocurrent. On the other hand, transparency is weak. Glucose only seems usable for a W03 / organic material ratio of around 0.5. The use of myo inositol leads to

inhomogeneous electrodes.

  In other words, by an adequate choice of the organic material associated with the colloidal solution, it is possible to give the electrodes

  particularly interesting physical characteristics.

  When a homogeneous WO3 film is imperative, ethylene glycol, polyethylene glycol, maltose, glucose,

mannitol and glycerol.

  il A transparent electrode can be obtained by choosing the organic material from ethylene glycol, polyethylene glycol, mannitol and glycerol. In applications where the intensity of the photocurrent plays an important role, for example during the photoelectrolysis of water or the photoelectrochemical decomposition of organic compounds, particularly good results are obtained using polyethylene glycol, maltose, glucose, the

mannitol and myo inositol.

  By way of example, we can see, in FIG. 3, the variation in the density of the phtocurrent as a function of the voltage applied to the electrode, measured with a photoanode comprising a film of WO 3 of 5 μm thick, produced at starting from a colloidal solution with PEG 300 ,. The potentials are given relative to a reversible hydrogen electrode (ERH) in the same solution. The electrolyte is formed from 1 M perchloric acid. The measurement was carried out with simulated sunlight illumination according to the "1 sun AM 1.5" standard and a variation of the applied potential of 5 mV / s. More precisely, the curve identified by solid circles corresponds to a photoelectrochemical degradation process of methanol

0.1 M in the electrolyte.

  The curve identified by hollowed out circles shows the variation of the photocurrent density during the photoelectrolysis of water, with release of oxygen.

Claims (10)

  1. Method for manufacturing an electrode comprising an active film of W03, comprising the successive operations of: - formation of a colloidal solution comprising a mixture produced from tungstic acid and an organic material, - deposition on a glass plate conducting a thin layer of said solution, and - heat treatment of said plate at a temperature of at least 350 C,   characterized in that said organic material is chosen from ethylene glycol, polyethylene glycol, maltose, glycerol,   glucose, mannitol and myo inositol.   2. Method according to claim 1, characterized in that the operation for forming a colloidal solution comprises the successive operations of: - production of a mixture of tungstic acid in solution with said organic material, and   - Formation of a colloidal solution of the mixture produced.   3. Method according to claim 1, characterized in that the operation for forming a colloidal solution comprises the successive operations of: - production of a mixture from tungstic acid in solution and an organic material chosen from ethanol, methanol, any other volatile alcohol and dimethyl sulfoxide, - formation of a colloidal solution of the mixture produced, and - production of a mixture of said colloidal solution with said organic material.   4. Method according to one of claims 1 to 3, characterized in that   the heat treatment operation is carried out at a temperature between 400 and 600 OC.   5. Method according to one of claims 1 to 4 for obtaining a film   of homogeneous W03, characterized in that said organic material is chosen from ethylene glycol, polyethylene glycol, maltose,   glucose, mannitol and glycerol.   6. Method according to one of claims 1 to 4 for obtaining a film   of transparent W03, characterized in that said organic material is chosen from ethylene glycol, polyethylene glycol, mannitol and glycerol.   7. Method according to one of claims 1 to 4 for obtaining a film   of W03 which has a high photocurrent, characterized in that said organic material is chosen from polyethylene glycol, maltose,   glucose, mannitol and myo inositol.   8. Method according to one of claims 1 to 7, characterized in that the   tungstic acid solution is obtained by passing an aqueous sodium tungstate solution through an exchange resin cations. 14 2780055   9. Method according to one of claims 1 to 8, characterized in that the   colloidal solution is obtained by evaporation under reduced pressure.   10. Method according to one of claims 1 to 9, characterized in that the   deposition operations on a conductive glass plate with a thin layer of said solution and heat treatment of said plate are repeated up to 12 times.