DE19836651A1 - Electrode for electrochemical capacitors has a carbon glass layer applied to a metal sheet support - Google Patents

Abstract

Electrode for electrochemical applications is built up of several conducting layers, in which a carbon glass layer (14) is applied to a metal sheet support (10). An Independent claim is also included for a process for the production of the electrode, comprising applying a carbon glass layer to a support made of metal sheet.

Description

TECHNICAL AREA

The present invention relates to the field of glassy carbon Electrodes. It relates to an electrode for electrochemical applications, which is made up of several conductive layers. It also concerns proceedings for the manufacture of such an electrode and its uses.

STATE OF THE ART

Carbon materials are advantageously used as electrode materials in numerous electrochemical applications. A high specific surface area (100-1000 m ² / gr) must be aimed for so that the electrochemical reactions take place efficiently. Carbon is therefore mainly used in powder form as an electrode. This creates the task of forming thin, sheet-like electrodes from the loose carbon powder. For this purpose, binders are usually used that compact the fine powder; In addition, metal grids are used, into which the carbon powder / binder mixture is pressed. This technology strives for an optimal compromise between, on the one hand, a large specific surface due to loose packing and, on the other hand, good electrical conductivity, which is achieved by packing the powder particles as densely as possible. Only high specific surface areas in combination with sufficient conductivity result in electrodes with high electrochemical efficiency. Numerous methods that lead to the best possible compromise are known.

A material that excellently meets both of the above requirements is glassy carbon (GK). Its density is about 1.5 g / cm ³ , about 65% of graphite (Glassy Carbon, TX Neeman et al. In Polymeric Materials Encyclopedia, Editor in Chief: Joseph C. Salamone, p. 2790, CRC Press, 1996). So there is a pore volume of up to 35% by volume. However, vitreous carbon is liquid and gas tight in its unprocessed form; the pores are therefore isolated from one another and are not accessible from the surface. The electrical conductivity of glassy carbon is 200 (Ωcm) ^-1 . Partial oxidative degradation of the material - either electrochemically or thermally - can make the pores near the surface accessible: With such an activation, a high specific surface area is obtained in the peripheral zone, i.e. on the outside, the remaining framework of the activated zone made of amorphous glassy carbon but maintains its high conductivity. Activated glassy carbon offers a particularly advantageous electrode material.

A disadvantage of the use of glassy carbon in large technical electrochemical cells is its mechanical properties: very high hardness, combined with extreme brittleness. Films with thicknesses between 50 µm and 500 µm are becoming more difficult to handle with increasing surface area; they break extremely easily, especially when there is local mechanical tension. Sufficiently thick plates must therefore be used in electrochemical cells, but they are expensive because of the complex manufacturing process. In addition, their maximum size is then practically limited to approximately 0.1 m ² .

Attempts have already been made to solve these problems, but mostly the electrodes described can neither be economical than bipolar Cell stacks can be built, still a cohesive, electrically conductive Connection between glassy carbon and a carrier are made. This Connection must u. a. be so strong that a subsequent thermal or electrochemical activation of the surface of the glassy carbon is still possible. In addition, there is usually the problem that the connection between metal and Glassy carbon becomes corrosive over time in electrochemical use is attacked, both the conductivity, and the mechanical connection strength reduced.

PRESENTATION OF THE INVENTION

The invention is therefore based on the object of making means available which is the manufacture of thin, large-area electrodes for electrochemical applications Applications using glassy carbon technology allowed.

With an electrode of the type mentioned at the outset, this object is achieved by that a glassy carbon layer is placed on a sheet of metal is brought. In other words, the essence of the invention is an electric one conductive carrier (the metal plate), which is in thin and mechanically stable form is available, with a firmly adhering coating of glassy carbon Mistake. This leads to a combination of mechanically and electrically advantageous properties of the metal sheet and the high specific surface of the Glassy carbon.  

A first preferred embodiment of the invention is characterized in that that the glassy carbon coating is activated thermally or electrochemically is. This leads to an even larger specific surface area of the glassy carbon and thus better electrochemical properties.

Another embodiment is characterized in that the metal sheet is made of a transition metal, preferably from groups 4, 5, 6, or 8 (or egg alloy of these metals), which is more preferably capable of measuring Me to form tall carbide. A corresponding method can be used to achieve between the metal sheet and the glassy carbon coating Forms a layer of metal carbide, which connects metal and glassy carbon det. In addition, this connecting layer is electrically conductive, what the function of the layer structure as an electrode material is essential.

Another embodiment is based on the fact that the glassy carbon layer is made of a polymeric glassy carbon carrier and embedded glassy carbon particles consists. In this way it can be ensured in the manufacturing process that the materials involved metal, metal carbide and glassy carbon do not result as a result different expansions or other differences in the material properties detach from each other.

Further embodiments of the electrode result from the dependent An sayings.

In addition, a method for producing an electrode consisting of a Glass carbon layer and a metal sheet presented. In this procedure the metal sheet from a transition metal of groups 4, 5, 6, or 8 (or one Alloy of these metals) with a layer of preferably activated glass carbon coated, with the preferred procedure being that between an intermediate layer of Me. the metal layer and the glassy carbon layer tall carbide forms.  

Another embodiment of the above method is that the Me sheet metal, before it is coated with glassy carbon, is roughened to a Ensure integral connection between metal and glassy carbon.

Another embodiment of the method is characterized in that the Glassy carbon layer is made by a mixture of a glassy carbon substance-forming, essentially monomeric organic substance and glass carbon Len lenparticles or carbon particles on the roughened metal sheet gen, and then the mixture at elevated temperatures in the air for is polymerized for several hours, and then the coated Me tallblech is heated in a vacuum oven to 700 to 1200 °. This is how one is formed very strong, integral and electrically conductive connection layer made of Me tall carbide. In particular, bipolar cell stacks can be eco with this method be made nomically.

Further embodiments of the method result from the dependent An sayings.

In addition, uses of the above electrode are preferred as bipolar elec trochemical capacitors proposed.

BRIEF EXPLANATION OF THE FIGURES

The invention is intended to be described below using exemplary embodiments hang explained with the drawings.

Fig. 1 is a schematic representation showing a detail of the construction of a multilayer glass carbon electrode, and

Fig. 2 shows the various manufacturing steps of a multilayer glass carbon electrode.

WAYS OF CARRYING OUT THE INVENTION

The exemplary embodiments described are based on the following two facts:
First, certain organic substances, e.g. B. furfuryl alcohol, in a conventional manner, polymerizable, and the solid polymer formed converts by pyrolysis between 700 ° C and 2000 ° C in a vacuum to glassy carbon. The resultant glassy carbon is then further activated by heating in air at 400-600 ° C. Partial oxidation creates a highly porous edge zone, the specific surface of which is 100-1000 m ² / gr and which is highly electrochemically active.

On the other hand, it is also known that the reaction occurs with certain carbide-forming transition metals

Metal + carbon - <metal carbide

takes place. Furthermore, the carbides formed in this way (e.g. TiC, WC, MoC, VC, TaC, HfC), mostly storage carbides, electrically conductive, and have a metal-like, high conductivity.

Experiments have shown that there is contact between glassy carbon material and titanium at 700-1000 ° C a reaction zone of TiC is formed, the one on the one hand, both materials are mechanically firmly connected, and on the other hand, a very good one creates conductive electrical contact between the two substances.

It also shows that the desired, technically usable connection is created in a special way if two measures are taken:

• 1. A composite should be used as the starting material for the glassy carbon (GK) layer already pyrolyzed GK powder or active carbon powder and liquid furfu Ryl alcohol can be used.
• 2. The surface of the metal sheet on which the GK raw material is applied is should be appropriately roughened, e.g. B. by sandblasting.

With this procedure, the manufacturing process proceeds as follows (for example, titanium is chosen as metal): The viscous mixture of GK powder and furfuryl alcohol, to which a suitable catalyst for promoting the polymerization has been added, is applied as a layer with a thickness of 10-200 µm on the roughened Ti surface, e.g. B. by spraying with a paint spray gun. The liquid part of the mixture penetrates the roughened structure of the Ti sheet. It polymerizes after approx. 1/2 hour at 100 ° C, and forms a solid composite of polyfurfuryl and GK particles. Already at this stage, but especially during the subsequent pyrolytic decomposition at 700 ° C to 1000 ° C to GK, there is a significant linear shrinkage of the pure polyfurfuryl. Without the admixture of already pyrolyzed GK powder, the layer would shrink more than 20% linearly and therefore u. U. detach from the Ti sheet. This is prevented in the procedure described, on the one hand because the shrinkage is greatly reduced due to the high volume fraction of powder, and on the other hand because the polymer is and remains effectively anchored in the cavities of the Ti surface. The electrical conductivity of the finished GK composite layer is high. With σ = 100-200 (Ωcm ^-1 ) it corresponds approximately to the conductivity of monolithic GK. B. from polymerized furfuryl alcohol without the addition of pulverischem GK.

At the beginning of the process, the liquid part of the mixture wets the roughened The contact between the titanium surface and the course of the polymerization is maintained. While in vacuum with increasing temperature from the polymer by pyro lysis gradually releases pure carbon, the reaction with Ti begins at the same time  tan to TiC. The reaction is accompanied by diffusion processes of the components into each other, and therefore the reaction zone becomes a material, mechanical fixed connection between GK and titanium. Also between the polymer and the GK powder filler arises from the initial wetting by the subsequent pyrolysis a material connection with a firm bond between the Carbon phases.

The result of the procedure according to the invention is shown schematically in FIG. 1. On the roughened titanium 10 is initially a titanium carbide layer 11 , which is well interlocked with the titanium 10 and connected. On the titanium carbide layer 11 there is then the glassy carbon layer 14 , which is composed of the polymeric glassy carbon body 12 and the glassy carbon particles 13 embedded therein.

Both when shrinking during the pyrolysis of the polymer, as well as by Di dimensional changes in the formation of crystalline TiC from the components, can high mechanical stresses between the substrate titanium and the Be stratification arise. In the embodiment described here, the No detachment of the GK composite from titanium has been found when the process conditions can be selected as described. Apparently the tensions already largely degraded while the carbon-carbon composite trains. The remaining residual stresses then exceed the adhesive strength the Ti / TiC / GK reaction zone no longer. The adhesive strength is both sufficient the stresses resulting from the different thermal expansions of titanium and GK to survive cooling from 700-1000 ° C to room temperature as well as when reheating to 400-600 ° C in air for the subsequent, thermal oxidative activation.

The following embodiment will now be described in detail. Fig. 2 the successive steps is characterized by graphically in a flow chart.

Titanium sheets with the dimensions 100 × 100 mm, thickness 100 µm are sandblasted on the surfaces with silicon carbide powder so that a uniform roughness depth of 15 µm is created. Subsequent tempering at 800 ° C between flat plates made of Al ₂ O ₃ removes the dent caused by sandblasting; you get completely flat sheets. The following mixture is used for coating:

• - 50 gr furfuryl alcohol,
• - 50 gr high molecular weight poly furfuryl alcohol molecular weight 300 (supplied by Great Lakes Chemical Corporation, West Lafayette IN 47906, USA, Handelsna me: Quacorr 1300 Resin),
• - 300 gr methyl butyl ketone (or another organic solvent e.g. Iso propyl alcohol) in which 2 grams of paratoluenesulfonic acid was dissolved as a catalyst;

500 gr GK powder is added with a particle diameter distribution cha characterized by 0.4 µm <d <12 µm and mixed with a for 10 minutes slow running stirrer. The resulting mixture has the consistency of Paint and is applied in layers in a spray chapel with a paint spray gun the titanium sheets are applied until the layer thickness is 10-100 µm.

The coated sheets are at 40 ° C in a well ventilated oven for 5 hours. stored, then the oven at 20 ° C / hour. brought to 100 ° C. The emerged NEN polymer layers then adhere firmly to the titanium sheets.

The polymer layers are then pyrolyzed to GK. The coated sheets are placed in a vacuum oven, the residual pressure of which is p ≦ 10 ^-5 Torr. At 100 ° C / hour the oven is brought to 900 ° C. and held there for 60 minutes. After cooling, 30-50 µm thick composite layers of GK particles in a GK matrix firmly adhere to the titanium sheets. The volume fraction of the GK matrix is approx. 15% of the volume of the particles. The specific electrical conductivity, measured with a four-contact method on a sample produced in the same process step on insulating Al ₂ O ₃ ceramic, was determined to be σ = 110 (Ωcm) ^-1 .

The specific contact resistance between GK composite layer and titanium sheet was determined to be approx. Ρ _k = 10 ^-4 Ωcm ² . The GK electrodes are now activated by bringing them to 450 ° C in air at 200 ° C / hour, holding them there for 2 hours and letting them cool down again. The finished electrodes have an approximately 20 µm thick active zone on the surface, the specific surface of which was determined to be 453 m ² / g, using the method of N ₂ absorption according to Brunauer, Emmet and Tel ler, also called the BET method ( S. Brunauer, PH Emmet, and E. Teller, J. Am. Chem. Soc. 60, 309, 1938).

Reference list

10th

Titanium foil

11

Titanium carbide layer

12th

Polymeric glassy carbon body

13

Glassy carbon particles

14

Glassy carbon layer

Claims (25)

1. Electrode for electrochemical applications, which is composed of several conductive layers, characterized in that a glassy carbon layer ( 14 ) is applied to a base made of a metal sheet ( 10 ). 2. Electrode according to claim 1, characterized in that the glass carbon layer ( 14 ) is activated. 3. Electrode according to claim 2, characterized in that the glass carbon layer ( 14 ) is electrochemically activated. 4. Electrode according to claim 2, characterized in that the glass carbon layer ( 14 ) is thermally activated. 5. Electrode according to one of claims 1 to 4, characterized in that the metal sheet ( 10 ) consists of a transition metal. 6. Electrode according to claim 5, characterized in that the metal sheet ( 10 ) consists of a transition metal from one of groups 4, 5, 6 or 8, or an alloy of these metals and that the transition metal or the transition metal alloy conduct metal-like de carbides can form. 7. Electrode according to claim 6, characterized in that the metal sheet ( 10 ) consists of titanium. 8. Electrode according to one of claims 6 or 7, characterized in that there is a transition metal carbide layer ( 11 ) between the glassy carbon layer ( 14 ) and the metal sheet ( 10 ). 9. Electrode according to claim 8, characterized in that the metal sheet ( 10 ) and the glassy carbon layer ( 14 ) through the transition metal carbide layer ( 11 ) are integrally bonded and electrically conductive ver connected. 10. Electrode according to claim 9, characterized in that the glass carbon layer consists of a polymeric glassy carbon body ( 12 ) with embedded glassy carbon particles ( 13 ). 11. A method for producing an electrode for electrochemical applications, which is composed of several conductive layers, characterized in that a glassy carbon layer ( 14 ) is applied to a base made of a metal sheet ( 10 ), this metal sheet made of a transition metal being one of the Groups 4, 5, 6, or 8 or an alloy thereof, and wherein the transition metal or the transition metal alloy can form metal-like conductive carbides. 12. The method according to claim 11, characterized in that the glass carbon layer ( 14 ) is activated thermally or electrochemically. 13. The method according to any one of claims 11 or 12, characterized in that the coated metal sheet ( 10 ) is exposed to such an elevated temperature that a metal carbide layer ( 11 ) forms between the metal sheet ( 10 ) and the glassy carbon layer ( 14 ), and that the metal sheet ( 10 ) and the glassy carbon layer ( 14 ) are integrally and electrically well connected by the transition metal carbide layer ( 11 ). 14. The method according to any one of claims 11 to 13, characterized in that the Me tallblech ( 10 ) is roughened before applying the glassy carbon layer ( 14 ). 15. The method according to claim 14, characterized in that the Me tallblech ( 10 ) is mechanically roughened by sandblasting, and that the roughness depth in the metal sheet ( 10 ) is 1 to 20 micrometers. 16. The method according to any one of claims 11 to 15, characterized in that the glassy carbon layer ( 14 ) is produced by applying a mixture of a glassy carbon-forming, substantially monomeric organic substance and glassy carbon particles ( 13 ) or carbon particles on the metal sheet , and then the coating is polymerized at temperatures in the range from 50 to 150 ° C. in air for 1 to 5 hours, and the coated metal sheet ( 10 ) is then heated in an oven at 700 to 1200 ° for 15 to 90 minutes . 17. The method according to claim 16, characterized in that the furnace is operated with a residual pressure of less than 10 ^-5 Torr. 18. The method according to claim 16, characterized in that the furnace is flushed with an inert gas, such as in particular He, Ar and / or N ₂ , and that the atmosphere generated by the flushing with respect to one or more of the foreign gases H ₂ O, H ₂ , NH ₃ , and / or O _{2 is} cleaned up to a volume fraction of these foreign gases of less than 10 ^-5 . 19. The method according to any one of claims 17 or 18, characterized in that to activate the glassy carbon layer ( 14 ) at the end of the electrode at 50 to 200 ° per hour in air heated to 400 to 500 ° C and at these temperatures for 0.1 to Is held for 5 hours, an activated zon of 1 to 50 micrometers forming on the glassy carbon layer ( 14 ), the specific surface area of which is 50 to 1000 m ² / gr. 20. The method according to claim 19, characterized in that the glass carbon-forming mixture is a mixture of powder from Glaskoh lenstoffpartikel ( 13 ) with particle diameters in the range of 0.1 to 50 micrometers and liquid furfuryl alcohol, the 0.5 to 4.0 Ge weight percent paratoluenesulfonic acid was added as a catalyst , is that the mixture consists of 10 to 95 volume percent powder and 90 to 5 volume percent furfuryl alcohol, and that this mixture is applied to the metal sheet ( 10 ) with a layer thickness in the range of 10 to 500 micrometers. 21. The method according to claim 20, characterized in that instead of the glassy carbon particles ( 13 ), an electrochemically active carbon powder with a specific surface area of 50 to 2000 m ² / gr is used for the mixture. 22. The method according to any one of claims 20 or 21, characterized records that instead of furfuryl alcohol, a mixture of furfu ryl alcohol and prepolymerized furfuryl alcohol with one molecular weight of 200 to 500 gr / mol is used for the mixture, and that the catalyst dissolved in an organic solvent beige will give. 23. The method according to any one of claims 11 to 22, characterized in net that the metal sheet is made of titanium. 24. Use of an electrode according to one of claims 1 to 10 for electrochemical capacitors. 25. Use of an electrode according to one of claims 1 to 10 for bipolar electrochemical capacitors.