WO2004106241A1

WO2004106241A1 - Diamond electrode for electrochemical applications, and method for the production and use thereof

Info

Publication number: WO2004106241A1
Application number: PCT/EP2004/002449
Authority: WO
Inventors: Andreas Dietz; Lothar SCHÄFER
Original assignee: Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V.
Priority date: 2003-05-30
Filing date: 2004-03-10
Publication date: 2004-12-09
Also published as: DE10324558A1

Abstract

The invention relates to an electrode (1, 8) for electrochemical reactions, which comprises a reaction area made of diamond. Said electrode (1, 8) contains particles (1) which are provided with an external face made of conductive diamond. The invention further relates to a method for producing such an electrode (1, 8), according to which the particles (1) are produced by depositing a conductive diamond layer onto basic members made of a metal, alloy, ceramic material, graphite, or diamond from the gas phase.

Description

Patent Application:

Diamond electrode for electrochemical applications and process for their manufacture and use

Applicant: Fraunhofer Society for the Promotion of Applied

Research e.V.

The invention relates to an electrode for electrochemical reactions, which has a reaction surface made of diamond. Furthermore, a method for producing such an electrode and a method for using it.

Conductive, boron-doped diamond electrodes have become increasingly important in electrochemistry in the past. Diamond electrodes are compared to those from e.g. glassy carbon or graphite awarded in several ways. The special properties include a large range of electrochemical potential in aqueous and non-aqueous media and high chemical and electrochemical stability.

Diamond electrodes can be used in analysis and synthesis. Diamond electrodes can be used particularly advantageously for the purification of waste water and for water disinfection. This is due to the high electrochemical overvoltage in the electrolysis of water. This creates extremely reactive species during operation of the diamond electrodes, such as. B. OH radicals and ozone with high current efficiency.

However, large electrode surfaces are necessary for the large-scale implementation of chemical substances. In order to obtain such large surfaces, electrodes are currently being made different materials, such as metals or silicon, with a conductive, polycrystalline diamond layer. However, such electrodes have the disadvantage of limited chemical stability. Damaged areas or pores in the diamond layer expose the underlying body to the chemical attack of the reactants. In a chemically or electrochemically aggressive environment, this leads to the infiltration of the diamond layer or the dissolution of the less stable base material.

To solve this problem, EP 1036861A1 discloses a diamond-coated electrode with a largely pore-free diamond layer. However, the production comprises several seeding and growth steps, so that the production of large-area electrodes is still very complex. The conversion rates in chemical synthesis are therefore still limited by the available electrode areas.

In M. Yoshimura et al. , Electrochemical characterization of nanoporous honeycomb diamond electrodes, Diamond and Related Materials 10 (2001) 620, it is proposed to use a base body which has a honeycomb structure to enlarge the active surface of the diamond electrode. These honeycombs have a size of 60 x 500 nm and can be obtained by plasma etching of the substrate material. In addition to the additional, complex process step, this method has the disadvantage of increasing the capacitance of the electrode by up to a factor of 400.

The object of the invention is therefore to develop a diamond electrode which has a high chemical resistance. Furthermore, large surfaces of these electrodes are to be provided with a few and simple process steps. The object is achieved according to the invention by an electrode for electrochemical applications which has a reaction surface made of diamond, the electrode containing particles (1) which have at least one outside (2) made of conductive diamond, cf. 1 and 2.

An electrode for electrochemical applications in the sense of this invention is a device which can be connected to an electrical current or voltage source and on whose surface a chemical reaction takes place for analysis or synthesis purposes. A synthesis in this sense is also the cleaning or disinfection of water by electrolysis.

According to the invention, it was recognized that particles of conductive diamond, see FIG. 1c, or particles (3) of a foreign material, which is coated with conductive diamond (2), see FIG. 1a-lb, can be used as an electrode for electrochemical applications , In this case, the large active surface is particularly advantageous compared to a coated, flat diamond electrode. In this case, one also speaks of three-dimensional electrodes.

Furthermore, an electrode formed in this way in reactive media is insensitive to pores or cracks and flaking in the diamond layer, since the destruction of individual particles due to the large number of remaining particles does not significantly impair the function of the electrode.

If diamond is used as the base body (3) of the particles (1), the particles are particularly well protected against chemical attacks, since the base body does not dissolve even if the external diamond coating is damaged. The particles according to the invention have a size of approximately 5 μm to approximately 120 μm. The lower limit is chosen such that a sufficient flow of the chemical compounds to be converted through the electrode is still made possible. The upper limit results on the one hand from the fact that the ratio of the surface to the volume decreases with increasing particle size; in the case of approximately spherical particles, this decrease is proportional to 1 / r, where r denotes the radius of the particles. On the other hand, manufacturing costs and costs also increase with increasing size, especially when a diamond body is used. It goes without saying that the particle size can be adapted to the particular synthesis and the flow mechanics of the reactor used.

Particles can be produced in a particularly simple manner by applying a conductive diamond layer to a base body as the core of the particle. Known methods of activated chemical vapor deposition (chemical vapor deposition, CVD) are particularly suitable for this.

Either a metal, an alloy, a ceramic or carbon is suitable as the material for the core. Silicon ceramics are particularly suitable as ceramics. If a carbon core is used, it can consist of both graphite and diamond. Well-available diamond from high-pressure synthesis or natural diamond dust is particularly suitable for a diamond core.

The material used for the core can be in the form of a solid particle or a hollow shape, for example a hollow ball or a cup-shaped body.

The diamond sheathing is applied to this basic body as a protective layer and reaction surface. Conductive diamond layers, such as are obtainable, for example, by doping with boron, are particularly preferred.

In a preferred embodiment, the thickness of the diamond layer is approximately 0.5 μm to 5 μm. The lower limit of the layer thickness results from the requirement that the particles should have a sufficient cross section for the electrical power line. In addition, the diamond layer should be as closed as possible as a protective layer for the base body made of metal, an alloy or a ceramic, see FIG. The upper limit of the layer thickness is either limited by the particle size or by the effort required for diamond deposition.

To ensure the required conductivity of the particles, a conductive base body, for. B. made of niobium or titanium. In M.L. Terranova et al, Electrochemical behavior of electrodes assembled with titanium-containing diamond films, Diamond and Related Materials 10 (2001) 627, describe that in the case of flat diamond electrodes, the inclusion of particles of these

Metals can achieve sufficient conductivity.

The use of a doped, conductive diamond layer is particularly preferred. This conductivity can be achieved, for example, by doping the diamond layer with boron. Preferred boron concentrations are between about 10 ppm and about 10000 ppm. Particularly preferred boron concentrations are between about 100 and about 6000 ppm. Since the boron content of the diamond surface also influences the chemical reaction taking place, the person skilled in the art will choose it according to the desired application.

One way of making electrical contact with the particles (1) is to place the particles in a housing (9) are introduced and means (4, 8) are provided which enable an electrical current to flow via the particles, see FIG. 2. A filter frit or a spatially separated cell is particularly suitable as the housing.

In order to ensure the current flow over the particles, the conductive particles are contacted with a feeder electrode (8). Either a flat steel sheet or several wires that protrude into the particle-filled area are suitable as feeder electrodes. In an electrochemically particularly aggressive environment, contacting with a diamond electrode is also considered, as described for example in EP 1036861A1. In any case, this feeder electrode can be connected to a conventional current or voltage source.

A reactor (9) with two chambers and is particularly preferred

Means (6) for separating these chambers, one of these

Chambers for receiving the chemical compound to be reacted

(7) is formed and contains a working electrode (1,8) and the other chamber contains an electrolyte (5) and a counter electrode (4), one of the diamond electrodes (1) according to the invention being usable as the working electrode.

In such a spatially separated cell, the chambers are separated by a diaphragm or an ion exchange membrane. Membranes with a polytetrafluoroethylene base body with corresponding functional groups are particularly preferred. One chamber contains a counter electrode and an electrolyte, which is adapted to the electrochemical reaction. The second chamber contains the chemical compound intended for the reaction as well as the conductive particles with the diamond reaction surface according to the invention. These particles are electrically contacted by the feeder electrode. After switching on the current flow between The desired electrochemical reaction takes place against the counter electrode and working electrode.

The reaction is advantageously carried out at a current density of less than 2 A / cm 2, particularly preferably less than 1 A / cm 2. A particular advantage of the diamond electrode according to the invention is the large active surface. This means that high absolute currents can be used even at low current densities. The turnover rate can thus be increased in line with washing.

Particularly high turnover rates result from the fact that the chemical compound that is to be converted flows through the electrode. For this purpose, the working chamber of the spatially separated cell can be equipped with a filter insert that retains the diamond particles, but allows the product of the reaction to flow away. Of course, the method according to the invention is not limited to aqueous solutions.

The invention will be explained in more detail below on the basis of exemplary embodiments.

FIRST EMBODIMENT

In a first exemplary embodiment, the particles according to the invention for forming a diamond electrode for electrochemical reactions are produced in a hot wire CVD process. For this purpose, basic bodies made of diamond dust with an average grain size of 50 μm are introduced into a CVD reactor. The storage takes place on a temperature-resistant sheet, e.g. B. made of silicon or molybdenum.

The pressure in the process chamber during diamond deposition is approximately 10 to 100 mbar. The Process gas contains 0.5 - 3% methane and 0.5 - 3% of a gaseous, boron-containing compound, preferably trimethyl borate, as a dopant in order to achieve the desired conductivity of the diamond particles. The rest of the process gas supplied consists of hydrogen. By heating the tungsten wire installed in the chamber, the components of the gas atmosphere in the chamber are activated and diamond is deposited on the base bodies.

A seeding step of the particles preceding the diamond deposition by known processes is also possible, which e.g. are described in DE 4233085 C2 or in EP 1129233 B1.

After the base body has been overgrown with a diamond layer of 1 to 2 μm, the growth process is stopped. The particles thus obtained can be used as electrode material without further treatment.

SECOND EMBODIMENT

In a second embodiment, waste water 7 is to be cleaned of organic pollutants. For this, a

Reactor according to Figure 2 used. An electrolyte 5 flows through the chamber of the counter electrode 4. Dilute sulfuric acid (H ₂ S0 ₄ ) is used as an example.

The second chamber, which contains the diamond powder 1 according to the invention, is separated from the counterelectrode 4 by a cation exchange membrane (KAM) 6. The contaminated wastewater 7 is introduced into this chamber from below. An electrical voltage is applied between the counter electrode 4 and the feeder electrodes 8, so that a current density of approximately 10 amperes per cm ^{2 is} established. The oxidation of water on the diamond electrode forms OH radicals, which are organic substances such as B. phenols and complexing agents, decompose. The wastewater cleaned in this way and the C0 ₂ formed during the reaction leaves the second chamber via an outlet.

Claims

claims

1. Electrode for electrochemical applications, which has a reaction surface made of diamond, characterized in that the electrode contains particles (1) which have an outer side (2) made of conductive diamond.

2. Electrode according to claim 1, characterized in that the conductive particles have a size of about 5 microns to about 120 microns.

3. Electrode according to one of claims 1 or 2, characterized in that the particles consist of a base body (3) made of a metal, an alloy, a ceramic or carbon and a shell (2) made of diamond.

4. Electrode according to claim 3, characterized in that the diamond layer has a thickness of approximately 0.5 μm to approximately 5 μm

5. Electrode according to claim 4, characterized in that the diamond layer has a thickness of approximately 1 μm to approximately 3 μm.

6. Electrode according to one of claims 1 to 5, characterized in that the diamond has a boron content of about 10 ppm to about 10000 ppm.

7. Electrode according to claim 6, characterized in that the diamond has a boron content of about 100 ppm to about 6000 ppm.

8. Electrode according to one of claims 1 to 7, characterized in that the particles in a housing are introduced and means are provided which enable an electrical current to flow via the particles.

9. A method for producing an electrode according to one of claims 1 to 8, characterized in that the conductive particles are produced in that a diamond layer is deposited from the gas phase onto a base body made of a metal, an alloy, a ceramic, graphite or diamond.

10. Reactor (9) which is designed to receive a chemical compound (7) to be reacted and contains means (8) which enable a power supply, characterized in that a diamond electrode (1) according to one of claims 1 to 8 can be used as the working electrode ,

11. Reactor according to claim 10, characterized in that two chambers and means (6) are provided for separating these chambers, one chamber containing a counter electrode (4) and designed to receive an electrolyte (5) and the other chamber a feeder electrode (8) and a diamond electrode (1) according to any one of claims 1 to 8 and is designed to receive a chemical compound to be reacted (7).

12. A method for the electrochemical conversion of a chemical compound, characterized in that an electrode according to one of claims 1 to 8 is brought into contact with the chemical compound and a voltage is applied.

13. The method according to claim 12, characterized in that the chemical compound flows through the electrode.

4. The method according to any one of claims 12 or 13, characterized in that the reaction takes place at a current density of less than 2 A / cm ² , particularly preferably less than 1 A / cm ² .