WO2001048268A1

WO2001048268A1 - Anode

Info

Publication number: WO2001048268A1
Application number: PCT/GB2000/004900
Authority: WO
Inventors: Michael Leslie Hitchman; Javier Yusta
Original assignee: University Of Strathclyde
Priority date: 1999-12-23
Filing date: 2000-12-19
Publication date: 2001-07-05
Also published as: GB9930277D0

Abstract

An electrode comprising a metal substrate, a tin oxide surface layer and a buffer layer between the surface layer and the substrate, wherein the tin oxide surface layer incorporates molybdenum.

Description

ANODE

The present invention relates to an improved electrochemical anode with increased stability and lifetime; and to a method of producing such an anode.

Electrochemical oxidation of organic compounds is one method whereby industrial and other pollutants may be destroyed. For the application of this process to such pollutants, it is important to make use of stable electrochemical anodes with a high catalytic activity for the destruction of organic compounds.

Currently used electrodes for the electrical oxidation of organic compounds include Pb0₂, C and Pt (D. Pletcher & F.C. Walsh, Industrial Electrochemistry, Blackie, London, 1993) . However, these electrodes suffer the disadvantage of dissolution at the high current densities of between 10 and 100 mA cm^"2 utilised in electrolytic cells. Further, the high cost of platinum and other precious metals limits their potential for use in large area electrodes, or on a large scale.

Tin oxide, Sn0₂, may provide an alternative material for the oxidation of pollutants, particularly as it has an overpotential for oxygen evolution which leads to increased cell efficiencies. However, since Sn0₂, even when doped, has a comparatively low conductivity it is generally used in the form of a thin film deposited on a metal substrate, such as titanium. A range of deposition techniques have been used, such as spray pyrolysis and chemical vapour deposition (CVD) . However, although Sn0₂ is stable at both high and low pH, and has good chemical and mechanical properties, it dissolves at high current densities. The dissolution is usually accompanied by a rise in electrode potential, and is enhanced by an increase in current density. For example, for a Sn0₂ film, doped with Sb, deposited by spray pyrolysis, the lifetime of the film decreases by a factor of 80 (from lOOOh to 12.5h) if the current density is increased from 10 to 100 mA cm^"" (B. Correa-Lozano, Ch. Comninellis & A. De Battisti, J. Appl . Electrochem, 26

(1996) 1; L. Lipp & D. Pletcher, Electrochimica Acta , 42

(1996) 1091) .

The dissolution of Sn0₂ is probably due to both mechanical and chemical processes, and the extent of degradation of such an electrode is illustrated by the experimental observation that for a 3μm thick SnO, layer, half the film dissolves in solution and the remainder becomes detached from the electrode. The table below shows published lifetimes greater than 5h of Sn0₂ electrodes deposited on titanium with several film compositions for a current density of 100 mA cm^"2 in 1 M H₂S0„ solutions. (Data taken from Correa-Lozano et al, 1996; and F. Vincent, E Morallan, C. Quijada, J.L.Vasquez, A. Aldaz & F. Cases, J^". Appl . Electrochem, 28 (1998) 607).

A number of attempts have been made to improve the stability of thin film Sn0₂ electrodes by introducing precious metals into the material during the deposition process. Typical elements which have been used includes Ir and Pt . The use of Ir (F. Cardarelli, P. Taxil, A. Savall, Ch. Comninellis, G. Manolli & O. Leclerc,J^". Appl .

El ectrochem, 28 (1998) 245) is detrimental for pollutant oxidation due to a low oxygen overpotential, but it is used extensively in DSA electrodes for oxygen evolution.

It is among the objects of embodiments of the present invention to provide a long life electrode without the presence of precious metals or electrocatalysts for oxygen evolution. It is further among the objects of the invention to provide an electrode with improved mechanical and electrochemical properties compared with conventional electrodes .

According to a first aspect of the present invention, there is provided an electrode comprising of a metal substrate, a tin oxide surface layer and a buffer layer between the surface layer and the substrate, wherein the tin oxide surface layer incorporates molybdenum.

Preferably the metal substrate is titanium.

Preferably the surface layer is deposited on the electrode as a film. Conveniently, the surface layer is co-deposited by chemical vapour deposition (CVD) . Preferably also the tin oxide of the surface layer is co- deposited with molybdenum. In a preferred embodiment, the surface layer comprises tin oxide and molybdenum at a ratio of around 1% w/w Mo/Sn.

Preferably the buffer layer includes a metal oxide. In a preferred embodiment, the buffer layer comprises Ti0₂ and TiN. Preferably, the Ti0₂ is in the crystalline rutile form. In an alternative embodiment, the buffer layer may comprise, in part, tin oxide (SnO_x) . Preferably the balance of the buffer layer comprises the same material as the substrate . Preferably the buffer layer comprises around 34% Sn adjacent the surface layer. Preferably also, the tin oxide is implanted in the substrate; that is the SnO_x buffer layer extends beneath the surface of the metal substrate .

According to a second aspect of the present invention, there is provided a method of producing an electrode, the method comprising the steps of: pretreating a metal substrate; depositing a buffer layer including a metal oxide on the substrate; and depositing on the buffer layer a tin oxide surface layer which incorporates molybdenum.

Preferably the metal substrate is titanium.

Preferably the pretreatment comprises abrading the surface of the substrate. Preferably also the pretreatment further comprises cleaning the surface of the substrate. It has been found that such pretreatment increases the lifetime of electrodes produced in this way.

In a preferred embodiment, the buffer layer comprises

Ti0₂ and TiN. Preferably the buffer layer is deposited by reaction of a substrate with NH₃/N₂ at a temperature between

300°C and 1000°C, for a time between 1 minute and 100 minutes .

In an alternative embodiment, the buffer layer comprises, in part, tin oxide (SnO_x) . Preferably the Sn0_x is implanted in the substrate; that is, the Sn0₂ buffer layer extends beneath the surface of the metal substrate. Preferably the balance of the buffer layer comprises the same material as the substrate.

Preferably Sn0₂ and Mo are co-deposited onto the buffer layer. Preferably the codeposition is performed from a mixture of SnCl₂, with a 10% w/w ratio of Mo/Sn. Preferably the deposition is performed at a rate around 0.025μm min^"1, at a temperature of around 480°C. Preferably the final ratio of Mo/Sn in the surface layer is about 1% w/w.

The method may further comprise the step of annealing the electrode at a temperature of around 500°C in an inert atmosphere. Conveniently the inert atmosphere is N₂. Preferably annealing is performed for at least 12 hours. These and other aspects of the present invention will now be described by way of example only, and with reference to the accompanying figures, in which:

Figure 1 shows a schematic illustration of the composition of an electrode in accordance with a first aspect of the present invention;

Figure 2 shows a schematic illustration of the composition of an electrode in accordance with a second aspect of the present invention; and

Figure 3 shows a graph of the lifetime of various test electrodes.

Referring to figures 1 and 2, these show schematically the composition of two alternative embodiments of an electrode, in accordance with the present invention . Each electrode comprises three regions; from left to right on the figures these are a substrate layer, a buffer layer, and a surface layer, respectively. In both examples the substrate comprises titanium, while the surface layer, which acts as the catalytic surface when the electrode is used for oxidation of pollutants, comprises tin oxide and molybdenum, at a ratio of around 1% w/w Mo/Sn, and the buffer layer in each example includes a metal oxide; titanium dioxide in figure 1, and tin oxide in figure 2. Preparation of each electrode is performed as follows.

Titanium substrates are sandblasted for 1 minute with 0.1 mm diameter beads to abrade the surface, and then cleaned in an ultrasonic bath. Finally, the substrates are etched with a 2.67% HF, 46.67% HN0₃, 50.66% H₂0 mixture for 5 seconds.

For the electrode shown in figure 1, a buffer layer of Ti0₂ + TiN was deposited by reaction of the titanium substrate with a flow of 100 seem of NH₃ and 100 seem of N₂ at 750°C for 30 minutes. The Sn0₂ buffer layer of figure 2 was deposited by ion assisted sputtering from an Sn0₂ target .

It can be seen from the figures that the two buffer layers are of somewhat different organisation: the Ti0₂ + TiN buffer is deposited above the Ti substrate, with very little interpenetration of the two layers, and an abrupt transition therebetween. By contrast, the sputtered Sn0₂ buffer layer penetrates the Ti substrate for some distance, with a gradual transition between the layers.

Finally, the surface layer of Sn0₂ + Mo is co- deposited onto either buffer layer as a thin film. Deposition is carried out at a rate of 0.025 μm min^"1 at 480°C, from a mixture of SnCl_: + MoS, at 420°C, with a flow of 0₂ of 600 seem.

The experimental lifetime of the electrode of figure 2 is illustrated in column 5 of the graph of figure 3, with a surface layer thickness of 3 μm. The graph of figure 3 also shows experimental lifetimes of various other test electrodes, prepared as described in the examples below. It can be seen that the most significant increase in electrode lifetime is that obtained from addition of molybdenum to the surface film (shown in the transition from column 4 to column 5) . The larger increase shown from column 5 to column 6 was obtained by increasing the thickness of the film sixfold.

The examples below give further details of the preparation of the electrodes and the testing results shown in figure 3.

Example 1 : Substrate Choice and Preparation A number of studies of a given Sn0₂ film thickness

(3μm) on substrates with different surface preparations were carried out with a current density of 100 mA cm^"2 and with 1M H₂SO^ as the electrolyte. These conditions allowed a ready comparison with literature results. The substrates investigated were of different geometries such as wires, sheets, meshes, expanded meshes and three dimensional structures . Four different substrate materials were studied: industrial grade Ti, Cu, Ni and Al . Sn0₂ films deposited on Cu came off easily due to the growth of a thick copper oxide film during the oxidative deposition process. Films deposited on Ni and Al were shown by EDX to have a content of these elements as high as 80% and the metals dissolved readily in the acid electrolyte as shown by AAS of the test solutions; in other words, electrodes of Sn0₂ film deposited on Ni and Al behaved essentially as electrodes of the substrate metals. On the other hand, Sn0₂ films deposited on Ti had less than 0.1% Ti content in a 3 μm thick film (as shown by SNMS) and had a good adherence to the underlying substrate. Therefore, only Ti was used as substrate material for all further tests of electrode characteristics of Sn0₂ layers .

Surface treatment methods investigated were sandblasting of Ti substrates with bead sizes ranging from 0.01 mm to 1 mm and a subsequent HF/HN0₃ treatment with an HF concentration between 1 and 100% and etching times between Is and 330 min, mechanical polishing, HCl etching and electropolishing . The treatment of Ti substrates which showed films with the longest lifetimes was that involving sandblasting of the industrial grade Ti for 1 min with 0.1 mm beads to abrade the surface. After sandblasting, surface cleaning was carried out with a degreasing solution in an ultrasonic bath for 5 mins , and then further ultrasonic cleaning, first in acetone and second with isopropanol with 5 mins for each solvent. Between each stage, the substrates were rinsed with deionised water (18.2 MΩ) . After the isopropanol clean, HF etching was carried out with a 2.67% HF, 46.67% HN0₃ and 50.66% water mixture for 5s. No water rinse was given to the substrates after the HF etch in order to prevent excessive hydration of the surface and subsequent growth of a thick Ti0₂ film. This surface preparation treatment was used prior to all the film depositions on Ti . A typical lifetime improvement for a 3μm thick film was a factor of 8 compared to any of the other surface treatment methods. Film lifetimes were about 3 h. (See columns 1 and 2 on figure 3) .

Example 2 : Deposition of Buffer Layers Prior to Sn0₂

Film Deposition

Before deposition of a Sn0₂ layer, a buffer layer was formed on the titanium substrate. The purpose of the buffer layer was to modify the surface to try and enhance the compatibility between an Sn0₂ deposit and the substrate surface. Two different types of buffer layers were investigated: a Ti0₂ + TiN layer and SnO, implants. The Ti0₂ + TiN layer was chosen for two reasons. First, the Ti0₂ has a crystal lattice structure similar to that of Sn0₂. But Ti0₂ is insulating so TiN, which also has a similar structure was incorporated to provide a conducting film. The rationale of using an SnO_v implant was to try and obtain a region in the titanium with a transition from pure titanium to SnO, in order to facilitate the adherence of the CVD Sn0₂ layer on the surface.

The Ti0₂ + TiN layers were deposited by reaction of the sandblasted Ti substrate with a stream of NH₃/N₂ with flows between 10 and 3000 seem at deposition temperatures in the range 300 to 1000°C for reaction times between 1 and 100 min. The thickness of the buffer layer thus produced was between 0.01 and lOμm. The final condition decided upon for buffer layer growth were : • Flushing of the reactor for 5 min in a 500 seem flow of N to remove oxygen from the reaction chamber.

• Heating up the reactor to a temperature of 750°C in 100 seem N₂. • Flowing 100 seem of NH₃ + 100 seem of N₂ through the reaction chamber for 30 min: these reaction conditions grew a buffer layer of about O.lμm.

• Cooling down of the reactor in 100 seem N₂ flow. X- ray diffraction (XRD) analysis showed that the main component of the buffer layer was highly crystalline rutile

Ti0₂ with less crystalline TiN incorporated into the Ti0₂.

The Sn0_v implants were obtained by sputtering from an

SnO*-. target. This was prepared by spray pyrolysis of a 0.2

M solution of SnCl_4.5H₂0 in 9:1 v/v methanol water. The deposition process was carried out under two different conditions: non ion-assisted and oxygen ion assisted deposition. For both sets of conditions a sputtering voltage between 0.1 keV and 10 keV, a current density between 0.1 and 10mA cm^"2 and a working pressure between 10^"2Pa and K^Pa were used. The working pressure was the contribution from Ar and 0₂ partial pressures. Using these conditions deposition rates between 1 and 10 x 10¹⁴ ions cm^": were obtained. For the ion assisted condition the substrates were also bombarded with oxygen at voltages between 10 and 500 eV. The thickness of the implanted films from this technique were between 5 and lOOnm.

The optimum conditions for the implant of the Sn0_x films were found to be as follows:

• Sputter cleaning of the substrates with 0.3 mA cm^"2 and a voltage of 800eV of Ar for 10 min.

• Ion assisted deposition from a target of Sn0₂ on borosilicate glass. The sputtering beam of Ar was 1 keV at 2 mA cm^"2 with a neutraliser placed at 45°. The working pressure in the chamber was 2.5 x 10^"2Pa of which 1.5 x 10^"2 was from Argon through the sputtering source and 1 x 10^"2 of oxygen directly into the chamber.

The films were bombarded with oxygen at 300eV at 45°, with an ion atom arrival of 3:1.

The composition and deposition rate of the deposited films were determined by Rutherford back scattering (RBS) with a 2 MeV He⁺ at normal incidence and with a 168° scattering angle. The thickness of the film was estimated to be 28-30 nm by assuming a density of 6.9 kg m^"3. The film composition was 34% Sn, 0.3% Fe, 65% 0 and 0.07% Al .

Figures 1 and 2 show schematic illustrations of the composition of various regions of electrodes with buffer regions of, respectively, Ti0₂ and TiN, and Sn0_x. It can be seen from the figures that while the transition between layers is abrupt for the TiN + Ti0₂ buffer layer, that for the Sn0₂ buffer layer is less so, there being considerable implanting of SnO, in the Ti substrate layer.

Sn0₂ films with a thickness of 3μm deposited on either of the above buffer layers showed lifetimes three times longer (9.1 h) than those obtained with films deposited on bare titanium. (See Column 3 on figure 3) .

Example 3 : Sn0₂ Deposition Rates

Film deposition rates were varied between 1 and

O.OOlμm min^"1. It was found that for a 3μm Sn0₂ thick film the lifetime increased from 9. lh up to 27.5h in decreasing the deposition rate to the lowest level. (See column 4 on figure 3 ) .

Example 4 : Addition of Metallic Elements to Sn0₂ Films

The incorporation of other metallic or pseudo-metallic elements into the Sn0₂ surface film was investigated. In particular, the following elements were co-deposited with

Sn0₂ onto the titanium substrate/buffer layer: Mo, P, Ni,

Co, Fe, Cu and W. Of these elements only Mo gave significant improvements in electrode lifetime. Deposition of the mixed Mo + Sn0₂ films was carried out by chemical vapour deposition (CVD) from a mixture of

SnCl₂ + MoS₂ in a 10% w/w Mo/Sn ratio from an evaporator at

420°C. Film deposition was carried out at a rate of 0.025 μm min^"1 at a temperature of 480°C. The deposition process was as follows:

• The reactor was flushed with 500 seem of N₂ for 5 min .

• The N₂ flow was decreased down to 100 seem and the substrate was heated to the deposition temperature of 480 °C .

• A N₂ flow of 500 seem was flowed through the evaporator which was heated up to the evaporation temperature of 420°C. • Once this temperature had been achieved, a flow of 0₂ of 600 seem was passed through the reactor to start the deposition process.

• After the film deposition the evaporator furnace and then the deposition furnace were switched off. The effect of annealing of the films was also investigated. Annealing at 650°C and above increased the film resistivity, as shown by 4 point probe measurements. This was probably due to the diffusion of the buffer layer oxygen into the upper film. This treatment was found to decrease drastically the electrode lifetime. On the other hand, annealing of the films for 13 h at 500°C in a N, atmosphere gave a 5% increase in electrode lifetime.

Analysis of the film composition by EDX showed a concentration of about 1% w/w Mo/Sn in the films. With the addition of the non-precious metal Mo into the films the lifetime of a 3μm thick film increased from 27.5 h to 90.1 h (see column 5 on figure 3) .

Deposition of mixed Mo/Sn films on Ti meshes produced similar results to those obtained on Ti sheets. Example 5 : Increasing the (Sn0₂ + Mo) Layer

Thickness

The effect of a composite layer thickness between 3μm and 18μm was investigated. For the highest thickness of 18 μm a lifetime of 449h was obtained. There is no reason why co-deposited films should not be considerably thicker than 18 μm. Films as thick as 100 μm or even greater could be envisaged (see column 6 on figure 3) .

Electrodes at various stages of production, as described above, were tested in order to determine their lifetime. The results are shown in figure 3. Conclusion

Deposition of buffer layers prior to Sn0₂ film deposition increased lifetime of the electrodes by a factor of three. Buffer film compositions investigated include sputtered and implanted SnO_x and mixed composite layers of Ti0₂ + TiN. The co-deposition of metallic elements, particularly Mo, also increased the lifetime of the films by a factor of three.

The accelerated test for the lifetime was oxygen evolution in 1M H₂S0₄ at a current density of 100mA cm^"2. The lifetime obtained under these conditions with an 18μm thick electrode was 449 h. However, under industrial conditions common current densities range between 10 and 40 mA cm^"2. Experiments at 30 mA cm^"2 on 3μm thick films showed an increase in electrode lifetime by a factor of 30: i.e. from 90.1 h to 2670 h. It has been found that there is a linear relationship between electrode lifetime and electrode thickness. Therefore, lifetime extrapolation for an 18μm thick film gives an expected lifetime of about 13500 h. It should be possible to readily grow films even thicker than 18μm and to obtain even longer electrode lifetimes . All the film layers can be readily deposited in an inexpensive atmospheric CVD reactor with readily available inorganic precursors. Also no precious metals have been used in the development of the electrode. It can be seen that the Sn0₂ based electrode described herein does not contain precious metals but has a lifetime long enough to be considered for commercial electrolytic cells for the destruction of organic compounds.

Claims

1. An electrode comprising a metal substrate, a tin oxide surface layer and a buffer layer between the surface layer and the substrate, wherein the tin oxide surface layer incorporates molybdenum.

2. An electrode as claimed in claim 1, wherein the metal substrate is titanium.

3. An electrode as claimed in any preceding claim, wherein the tinoxide and molybdenum of the surface layer are co-deposited and the surface layer comprises tin oxide and molybdenum at a ratio of around 1% w/w Mo/Sn.

4. An electrode as claimed in any preceding claim, wherein the buffer layer includes a metal oxide.

5. An electrode as claimed in claim 4, wherein the buffer layer comprises Ti0₂ and TiN.

6. An electrode as claimed in claim 5, wherein the Ti0₂ is in the crystalline rutile form.

7. An electrode as claimed in claim 4, wherein the buffer layer comprises around 34% Sn adjacent the surface layer .

8. A method of producing an electrode, the method comprising the steps of : pretreating a metal substrate; depositing a buffer layer including a metal oxide on the substrate; and depositing on the buffer layer a tin oxide surface layer which incorporates molybdenum.

9. The method of claim 8, wherein the pretreatment comprises abrading the surface of the substrate.

10. The method of claim 8, wherein the buffer layer is deposited by reaction of a substrate with NH₃/N₂ at a temperature between 300°C and 1000°C, for a time between 1 minute and 100 minutes.

11. The method of any one of claims 8 - 10, wherein the surface layer is co-deposited by chemical vapour deposition (CVD) at a rate of about 0.025 μm min^"1 at a temperature of about 480°C.

12. The method of any one of claims 11 - 13, including the step of annealing the electrode at a temperature of around 500°C in an inert atmosphere.