WO1999067181A1

WO1999067181A1 - Method for making an electrode comprising a tungsten oxide film

Info

Publication number: WO1999067181A1
Application number: PCT/CH1999/000270
Authority: WO
Inventors: Jan Augustynski; Martine Ulmann; Clara Santato
Original assignee: Universite De Geneve
Priority date: 1998-06-22
Filing date: 1999-06-21
Publication date: 1999-12-29
Also published as: FR2780055A1

Abstract

The invention concerns a method for making an electrode comprising a WO3 film comprising the following successive steps: forming a colloidal solution comprising a mixture produced from tungstic acid and an organic material; depositing on a conductive glass plate a thin film of said solution; performing a heat treatment on said plate at a temperature not less than 350 °C. The invention is characterised in that the organic material is selected among ethylene glycol, polyethylene glycol, maltose, glycerol, glucose, mannitol and myo inositol. The invention is applicable in photoelectrochemistry and electrochromism.

Description

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 (WO ₃ ). 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 cells. electrochromic display.

For many applications, the electrode must be transparent. This is why a glass plate coated with a transparent conductive layer formed from doped tin oxide is used as substrate.

The method of manufacturing electrodes of this type generally comprises the following steps:

- making a mixture of tungstic acid in solution with an organic material, - forming a colloidal solution of this mixture,

depositing the colloidal solution in the form of a thin layer on a plate of conductive glass, 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 WO ₃ films", defining the stages of such a process, which consists at:

- pass through an cation exchange resin an aqueous solution of sodium tungstate which is transformed into tungstic acid,

- mix the acid collected with an organic material, - form a colloidal solution of this mixture,

- deposit the colloidal solution in successive layers on a plate of conductive glass, and

- heat treatment after each deposit (500 ° to 600 ° C in an oxygen-rich atmosphere).

A mesoporous structure is thus obtained, formed of crystallized WO ₃ nanoparticles having a diameter of 20 to 50 nm and aggregated together, 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.

These qualities are certainly very appreciable, but an industrial application requires a good adhesion of WO ₃ to its substrate, failing which the structure quickly disintegrates and makes the electrode unusable. It is furthermore necessary that the colloidal solution is sufficiently stable to be usable for several hours, even several days.

The main object of the present invention is to allow the production of WO ₃ films having excellent adhesion, from a mixture which is stable over time.

More specifically, the invention relates to a method of manufacturing an electrode comprising a film of WO ₃ , of the type comprising the successive operations of:

- formation of a colloidal solution comprising a mixture produced from tungstic acid and an organic material,

- depositing a thin layer of the solution on a conductive glass plate, 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. According to an advantageous embodiment of the invention, the operation of forming a colloidal solution comprises the successive operations of:

- making a mixture of tungstic acid in solution with the organic material, and - forming 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:

production of a mixture of tungstic acid in solution with an organic material chosen from ethanol, methanol, any other volatile alcohol and dimethyl sulfoxide,

- formation of a colloidal solution of the mixture, and

- making a mixture of the colloidal solution with the organic material. 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, polyethylene glycol, mannitol and glycerol. 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 through a cation exchange resin. It has been 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, given with reference to the appended drawing, in which: - Figure 1 shows a plate of conductive glass carrying an active film obtained according to the method of the invention ,

FIG. 2 illustrates the different stages of the process, and

- Figure 3 shows the variation of the density of the photocurrent as a function of the potential. Referring to FIG. 1, we can see, schematically, a conductive glass plate 10 comprising a transparent conductive layer 12 and coated with a film of WO ₃ 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 having a resistance of 10 Ω per square.

The film 14 is formed from an aggregate of WO ₃ particles having a diameter of 20 to 60 nm, which are rigidly fixed to each other and define a mesoporous structure obtained by alternating deposition 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 explained above.

The mesoporous structure thus obtained makes it possible to considerably increase 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 an uppercase 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. In the example below, the quantities used correspond to laboratory experiments. It is obvious that in industrial production, the volumes used are greater.

A. Initially, a solution 20 of sodium tungstate (Na ₂ WO ₄ ) 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. The product sold by DOWEX under the reference W 50 HCR-2 100-200 mesh can be used as cation exchange resin.

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. The sodium tungstate 20 is acidified by cation exchange with the resin 24, which retains the sodium cations and releases protons, thus 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 sodium tungstate during the 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, until a colloidal solution is obtained. 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 WO ₃ 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 extended 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 WO ₃ crystallizes on the substrate into a porous structure.

As already mentioned, the deposition operations (E), formation of a thin layer (F) and passage through the oven (G) can be repeated up to a dozen times, to obtain a thickness which can reach more than 5 μm.

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. The first organic material 28 is then chosen from products a) and b). In this case, the second organic material 32 is chosen from 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 then no longer necessary to then 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. 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 most interesting results are summarized in the table below.

A = adhesion 1) Homogeneous but not as much as with PEG H = homogeneity 2) Transparent if we put at most 3-4 layers 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.

More precisely, the first column defines the organic material used, the second the concentration of the colloidal solution in WO ₃ and the third, the weight ratio between WO ₃ 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 examining this table, it can be seen that the concentration of the colloidal solution of WO ₃ 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 weight ratio between WO ₃ 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 make it possible, at the concentrations tested, to obtain an adherent film.

On the contrary, ethylene glycol, polyethylene glycol and mannitol give good results from all points of view.

The electrodes obtained from a solution containing polyvinyl alcohol (molar mass of 15,000) are, on the contrary, of little interest.

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 WO ₃ / 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 advantageous physical characteristics.

When it is absolutely necessary to have a homogeneous film of WO ₃ , it is therefore possible to use ethylene glycol, polyethylene glycol, maltose, glucose, mannitol and glycerol. 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, mannitol and myo inositol.

As an example, we can see, in FIG. 3, the variation of the photocurrent density as a function of the voltage applied to the electrode, measured with a photoanode comprising a film of WO ₃ of 5 μm thickness, made 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 0.1 M methanol 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

1. Method for manufacturing an electrode comprising an active film of WO ₃ , comprising the successive operations of:

depositing a thin layer of said solution on a conductive glass plate, 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 of forming a colloidal solution comprises the successive operations of:

making 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 of 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 - Making 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 ° C.

5. Method according to one of claims 1 to 4 for obtaining a homogeneous WO ₃ film, 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 transparent film of WO ₃ , 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 WO ₃ 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 the passage of an aqueous solution of sodium tungstate through a cation exchange resin.

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 operations of depositing on a conductive glass plate a thin layer of said solution and heat treatment of said plate are repeated up to 12 times.