DE19647534A1 - Electrode for an electrochemical energy converter - Google Patents

Abstract

An electrode (2, 4) for an electrochemical energy converter, especially with a fixed fluid electrolyte (8), consists of a catalyst (2.3, 4.3) on a porous conductive support (2.1, 2.2, 4.1, 4.2) which has two or more pore types of different sizes, the mean pore diameter increasing continuously in the direction from the electrolyte side towards the gas side. Also claimed is an electrochemical energy converter with a fixed fluid electrolyte, a hydrogen electrode and an oxygen electrode, the novelty being that the hydrogen and oxygen electrodes have the above design and that the mean pore radius of the oxygen electrode is greater than that of the hydrogen electrode.

Description

The invention relates to electrodes for electrochemical energy converters, in particular especially with liquid and fixed electrolytes. The invention comes first used in energy converters that generate gases (electrolysis) or consume (fuel cell, gas sensor).

Electrochemical energy converter with solid, liquid or fixed, e.g. B. alka electrolyte are known per se. An electrolyzer with liquid, fixier electrolyte is z. B. Davenport et al. Space water electrolysis: space station through advanced missions in: Journal of Power Sources, 36 (1991) 235-249 be wrote. The electrolyte is in a porous matrix (also called a diaphragm) and in the porous catalyst catalyst located directly adjacent to the matrix coated electrodes fixed. The electrolysis gas development takes place as before in the pores filled with electrolyte. The fixation of the electro lyte under electrolysis is usually achieved by capillary forces. The hydrophilic properties of the electrolyte matrix are formed by the electrolyte is preferably fixed in the matrix and not by the electrodes is suctioned off.

The electrodes comprise a porous carrier material and one attached to it  ordered active layer, hereinafter also as a catalyst or electrocatalyst called gate. So far, have been used as catalysts for hydrogen separation Precious metals, nickel and its alloys, amorphous metals, sintered metals Alloys with lanthanum and nickel and nickel compounds u. a. used. Nickel shows the unpleasant property of aging with formation β-hydrides. For anodic oxygen separation, precious metals, Nickel and its alloys, amorphous alloys, spinels, perovskite, Metal oxides, graphite and. a. used. Most catalysts have none sufficient long-term stability. Many, including RANEY nickel, have one relatively high overvoltage.

The product adjacent to the electrodes, produced with spacers gas chambers are filled with the product gases (for water electrolysis: Oxygen adjacent to the anode, hydrogen adjacent to the cathode) and ent don't hold electrolyte. The phase separation between the product gases and the fixed liquid electrolyte takes place within the porous electrodes instead of. The way it works enables electrolysis without gas separators.

Because of the structure described, each of the two electrodes delimits one side to the electrolyte and the other side to one of the products gases. One therefore speaks of gas-side and electrolyte-side areas an electrode.

Adjacent to the product gas chamber on the cathode side and separated from it by a membrane, the starting material chamber is found with water or an aqueous solution. The educt is fed to the electrodes by gas diffusion. The water vapor diffuses across the membrane through the cathode chamber (H ₂ space) to the cathode due to the water vapor partial pressure drop between the educt chamber and the electrolyte-filled porous cathode. The electrochemically consumed water can be replaced by a circuit that serves both for cooling and for water supply.

In electrochemical energy converters, e.g. B. with liquid and fixed electrical lyten, the pore structure of the electrode essentially determines the available active surface and influences the removal of the product gases (electroly se) or the supply of the reaction gases (fuel cell), the electrical guide ability of the electrode and mechanical stability.

According to our observations, the pore diameter increases active area and thus the efficiency of the electrode process. However is with decreasing pore diameter, the gas flow deteriorates sports in electrolysis operation, while the fi xation of the electrolyte in the pore system of the electrode increases. With one too large pore diameter there is a risk of electrolyte loss.

The object of the invention is therefore to create an electrode

• - with which the largest possible volume of electrolyte can be stored, avoiding loss of electrolyte,
• - and at the same time the best possible educt and product gas transport can be guaranteed.

This object is achieved with the subject matter of claim 1. Beneficial Embodiments of the invention are the subject of dependent claims.  

The electrode comprises an electrically conductive carrier and one or both active layer applied on the side, which is also referred to below as catalyst is not. According to the invention, the porous support of the electrode has at least two pore types of different sizes, the pore types being in are distributed within the carrier that beginning on the electrolyte side and in the direction progressing to the gas side, the average pore diameter increases. The little ones nere pores, which are increasingly present on the electrolyte side, serve to avoid to store (fix) the largest possible volume of electrolyte. The larger pores that are increasingly present on the gas side, serve to ensure the best possible starting material and to ensure product gas transport.

The invention is based on the knowledge that the electrolyte is partially present the smaller pores of the electrolyte-side area of the electrode is pushed by the capillary action of the smaller pores of the electrolyte applied area and into the microporous pore system of the elek trolyte-side area is returned to.

With the electrode according to the invention, an optimal coordination of the ge additional requirements for the active surface of the electrode process on the one hand and gas removal on the other hand.

Furthermore, it is achieved by the invention that without limitation of the Efficiency an electrolyte loss with high security excluded that can.

The porous support according to the invention can for example consist of a single one Layer consist of at least two pore types of different sizes  points. In addition, one of the porous supports can also consist of several layers ten, with each layer having several different pore types Has size.

The porous support may preferably be a metal (such as nickel, titanium or steel) or Be graphite, preferably felts, foams, fabrics are used.

A metal oxide, metal oxide hydrate, metal hydroxide is preferred as the active layer the or mixtures thereof of the elements iridium, ruthenium, palladium Pla tin, osmium, rhodium, tantalum, tin, zinc are used. Are particularly suitable the electron-conducting metal oxides of group VIII of the periodic table. Iridi According to our measurements, dioxide has proven to be particularly stable over the long term sen.

The catalysts mentioned have the following properties:

• - High electrochemical activity with a high specific surface and low resistance,
• - low hydrogen and oxygen overvoltage,
• - Long-term stability at high currents.

They are the known catalysts, particularly with regard to aging and Superior long-term durability. The specified in the introduction to the description Problems of the known catalysts do not occur.

Applying the electroactive layer to the support structure can be advantageous follow the procedure below:  

α) Thermolytic process

The active layer is created by thermal decomposition of iridium salts such as Hexachloroiridium acid on the electrode carrier. The application of the advance fe can be done by dipping, brushing, brushing or a printing process. The Conversion to oxide takes place at approx. 200-500 ° C, preferably at 400 ° C. The Be Layering process is repeated several times. Others can use the iridium salt Components are added, e.g. B. ruthenium, tantalum, tin compounds, to develop specific properties of the active layer.

β) powder process

Metal salt precursors (e.g. hexachloroiridium acid, ruthenium trichloride) become

• - by precipitation with lye in a sol-gel process or
• - by thermal decomposition in a crucible

the metal oxide is produced and applied to the support structure in a second step brought.

The electrode according to the invention is suitable for bifunctional operation means for alternating electrolysis and / or fuel cell operation.

The electrode according to the invention can be used for electrochemical energy converters are generally used or especially for oxygen-producing people housing systems, hydrogen-producing subsystem for exhaust gas aftertreatment as an energy-producing fuel cell, as a compact electrolyser in the Energy industry.

The invention is explained in more detail with reference to FIG. With reference to specific games Ausführungsbei. Show it:

Fig. 1 an electrochemical energy converter according to the invention with two electrodes SEN in a schematic representation;

Fig. 2 bimodal pore distribution of a hydrogen electrode (integrated over all of the electrode layers),

Fig. 3 bimodal pore distribution of an oxygen electrode (integrated over all of the electrode layers),

FIG. 4 shows current-voltage characteristics of coated and uncoated electrodes (abscissa: Current density mA / cm ^2, Ordinate: cell clamping voltage / V),

Fig. 5 Service life behavior of coated and uncoated electrodes at 200 mA / cm ² and 70 ° C for 11,000 hours (abscissa: time / h, ordinate: cell voltage / mV).

Fig. 1 shows an electrochemical energy converter which comprises two preferred embodiments of the electrode according to the invention as cathode 2 and anode 4 and an electrolyte 8 arranged therebetween.

The two electrodes 2 , 4 each comprise three layers:

• a) a gas-side carrier layer 2.1 , 4.1 with a bimodal pore radius distribution, that is to say two preferred pore radii; the majority of the larger pores face the gas side and the majority of the smaller pores face the electrolyte;
• b) an electrolyte-side carrier layer 2.2 , 4.2 with bimodal pore radius distribution, the majority of the smaller pores lying on the electrolyte side and the majority of the larger pores lying on the gas side,
• c) an active layer 2.3 , 4.3 made of iridium dioxide.

As can be seen from Fig. 1, the larger pores of the electrolytic layer 2.2 or 4.2 are smaller than the larger pores of the gas-side layer 2.1 or 4.1 . Likewise, the smaller pores of the electrolyte-side layer 2.2 or 4.2 are smaller than the smaller pores of the gas-side layer 2.1 or 4.1 .

In the diagram in the lower part of FIG. 1, the basic pore radius distributions of the two bimodal cathode layers 2.1 , 2.2 are shown as examples (abscissa: pore radius, ordinate: share of total pore volume). It can be seen that in each layer 2.1 , 2.2 there are two preferred pore radii (two relative maxima). In the electrode layer 2.2 , which is arranged on the electrolyte side, the proportion of the smaller pores outweighs the larger pores. Complementary to this is the layer 2.1 facing away from the electrolyte.

The larger pores in each layer serve the unimpeded gas transport, the smaller pores of electrolyte fixation.

The layers 2.1 , 4.1 are mainly used for gas transport or gas transport, where the smaller pores prevent the electrolyte from being discharged.

The layers 2.2 , 4.2 serve mainly to fix the electrolyte, where the larger pores allow the entry and exit of educt and product gases.

The electrochemical reaction takes place at the electrolyte-side active layers 2.3 , 4.3 , which can partially penetrate into layers 2.2 or 4.2 .

Without the above-mentioned layer structure of the electrode either electrolyte is discharged or gas transport becomes sensitive different.  

With an electrolysis electrode, larger pores are used for the oxygen electrode than with the hydrogen electrode, because oxygen bubbles are larger than hydrogen bubbles and the size of the oxygen bubbles also increases with increasing current density. As can be seen from FIG. 1, the pores of the anode are therefore larger than the corresponding pores of the cathode.

The production of bimodal layers, as shown in FIG. 1, in which the two types of pores are concentrated on the gas side or on the electrolyte side, is relatively complex. From a production point of view, it can therefore be advantageous to replace a layer according to FIG. 1 with several thinner sublayers, the pore types being evenly distributed within each sublayer, but beginning with the electrolyte side and progressing towards the gas side, the average pore diameter of the individual layers of Layer to layer is increased.

Example 1 (support structure)

The electrodes with fixed alkaline electrolyte comprise exactly two un different carrier layers. The layers become metallic on both sides barrier layer (e.g. sheet metal, felt, fleece, foam or fabric). A nickel felt or graphite felt with two layers underneath is particularly advantageous different porosity and different bimodal pore structure det. The predominates in the electrode layer, which is arranged on the electrolyte side Proportion of smaller pores compared to larger pores. Complementary to that the side facing away from the electrolyte is designed: The electrolysis gases become mostly in the electrolyte-side electrode layer with the smaller pores generates and migrates to the electrode layer facing away from the electrolyte with the size pores not filled with electrolyte and further into the adjacent gas  rooms.

The pore radius distributions for the hydrogen electrode and the oxygen electrode are different:

• - The hydrogen electrode mainly contains smaller pores with distribution maxima at 3-5 µm on the electrolyte side and 20-50 µm on the gas side ( Fig. 2).
• - The oxygen electrode mainly contains larger pores with distribution maxima at 3-5 µm on the electrolyte side and 70-100 µm on the gas side ( Fig. 3).

Such an electrode structure is particularly favorable for the hydrogen electro lysis because oxygen bubbles are larger than hydrogen bubbles. It can be advantageous adhesive, but can also be used for fuel cell electrodes.

Example 2 (Preparation of the Carrier)

On a nickel felt of 90-600 g / m ² one-sided sieved fractions of a nickel powder (z. B. INCO 123) are sintered, so that a macroporous elec trode structure is formed, which is used on the electrolyte side. The gas side of the electrode is produced from a coarser grain nickel powder (e.g. INCO 244) by a sintering process.

Pore formers (e.g. 25% fumaric acid or ammonium carbonate) can be used. The resulting pore structure works favorably on the current-voltage curve of the oxygen electrode.

The specific surface of the support structure is between 20 and 60 m ² / g, the average pore diameter between 4 and 17 µm. Maxima of the pore diameter distribution are 5-7 µm and 15-20 µm. There are also large pores with a diameter of 95-125 µm.

Example 3 (active layer)

The following coating process has proven to be particularly advantageous:

• - nickel felts are degreased in organic solvents (such as acetone), etched in 20% sulfuric acid, carefully rinsed in water.
• - A solution of hexachlororidic acid in water / isopropanol is applied to the nickel felts by brushing, dipping or printing techniques.
  The coating solution can contain additives of polyalcohols and cellulose ethers, such as those used in painting technology.
• - The electrodes are then heated in the oven with the entry of air.
  When using the above solution, the calcination is carried out at approx. 325 ° C. for approx. 30 min. long. At this temperature, especially conductive layers with low hydrogen and oxygen overvoltage are maintained. The oxide obtained is not a stoichiometric IrO ₂ , as previously described in the literature, but an oxide hydrate, the approximate composition between IrO (OH) and IrO ₂ .2 H ₂ O, with iridium in the oxidation states III and IV.
• - The coating and calcining process is carried out at least once carried out.
• - After-treatment is then carried out with the coated electrodes.
  The electrodes are anodically polarized at an elevated temperature of approx. 60 ° C. This pre-polarization can also take place in electrolysis mode.

The pre-polarization removes impurities in the electrodes or converts them to a stable oxidation state (including Ni ² ⁺ in Ni (IV)). As a result, he will keep particularly long-term stable corrosion-resistant electrodes.

Service life in electrolysis operation at 80 ° C and 10 bar of over 15,000 hours without any noteworthy degradation was demonstrated. Due to the activity of the electrode material and the low upper voltage, the electrolyzer can be operated at operating voltages below 1.6 V ( FIG. 4). As a result, corrosive reactions on the nickel supports and electrodes no longer play a role; the electrolyzer can also be operated at lower pressures and lower temperatures than was previously possible with other systems. Overall, an increase in safety and service life is achieved with lower operating costs.

Fig. 4 shows that the active layer, the cell voltage versus pure nickel in the alkaline electrolysis by about one half volt lowered (at 200 mA / cm ^2, 70 ° C).

Example 4 (Electrode for a Fuel and / or Electrolysis Cell)

From an aqueous solution of hexachlororidium acid by adding Alkali iridium oxide hydrate precipitated, filtered off and washed: the ideal precipitate lungs pH is between pH 9 and 14. The iridium hydrate is ge at 90 ° -450 ° C dries, powdered and sintered at 325 ° C on metallic supports. Alterna tiv is the application without temperature influence with the help of a paste that Nafion® contains, possible.

Example 5 (Electrode for a Fuel Cell)

The iridium oxide hydrate is dried or undried with up to 5% PTFE powder processed into a paste. The paste can also contain portions of coal powder included. The paste is applied to the carrier by brushing, Rolling, pressing or sintering.  

Example 6 (Electrode for a Fuel and / or Electrolysis Cell)

A mixture of RuCl ₃ and H ₂ IrCl ₆ and sodium nitrate is converted to the electrode material at 400 to 450 ° C, carefully washed out and sintered onto the carrier at 325 ° C. The electrode can additionally contain portions of activated carbon or graphite and platinum. The platinum metals mentioned occur in the oxidation stages III, IV (and VI).

Example 7 (Hydrophobic Pores)

To prevent or penetrate the electrolyte from the porous electrode structure To prevent the electrode from being hydropholized on one or both sides. To do this, the electrode surface is covered with a solution of a polymer such as PTFE or Nafion® treated.

Example 8 (Stacked Beam)

Carbon fiber mats or fabrics of different types are used as supports Porosity layered, with the porosity of the layers from the gas space decreases towards the electrolyte. It can a. commercially available graphite papers are used. The layers can be glued.

Claims (16)

1. Electrode ( 2 , 4 ) for electrochemical energy converters, in particular energy converters with liquid, fixed electrolyte ( 8 ), with a porous conductive carrier ( 2.1 , 2.2 , 4.1 , 4.2 ) and a catalytic converter ( 2.3 , 4.3 ) arranged thereon, thereby characterized in that the porous carrier ( 2.1 , 2.2 , 4.1 , 4.2 ) has at least two types of pores of different sizes, and starting on the electrolyte side and progressively increasing in the direction of the gas side, the average pore diameter increases. 2. Electrode according to claim 1, characterized in that the carrier comprises a plurality of layers ( 2.1 , 2.2 , 4.1 , 4.2 ), each layer having at least two pore types of different sizes. 3. Electrode according to claim 2, characterized in that within a layer ( 2.1 , 2.2 , 4.1 , 4.2 ) beginning on the electrolyte side and progressing towards the gas side, the average pore diameter increases. 4. Electrode according to claim 2 or 3, characterized in that the average pore diameter of the individual layers on the electrolyte side starting and progressing towards the gas side from layer to layer Layer increases. 5. Electrode according to one of claims 2 to 4, characterized in that at least one of the layers ( 2.1 , 2.2 , 4.1 , 4.2 ) has hydrophilic Po ren. 6. Electrolytes according to claim 5, characterized in that the gas-side layer ( 2.1 , 2.2 ) has hydrophobic pores. 7. Electrode according to one of the preceding claims 2 to 6, characterized characterized in that it is an electrically conductive gas and liquid density-tight layer (barrier layer), which is between two of the layer th of the carrier is arranged. 8. Electrode according to claim 7, characterized in that the lock layer is a nickel sheet or a graphite foil. 9. Electrode according to one of the preceding claims, characterized ge indicates that they have layers of felt, fabric or foam contains a metal such as nickel or carbon. 10. Electrode according to one of the preceding claims, characterized in that the carrier ( 2.1 , 2.2 , 4.1 , 4.2 ) consists of metal such as nickel, titanium, steel or graphite. 11. Electrode according to one of the preceding claims, characterized in that the catalyst ( 2.3 , 4.3 ) is a metal oxide, metal oxide hydrate, metal hydroxides or mixtures thereof of the elements iridium, ruthenium, palladium platinum, osmium, rhodium, tantalum, tin, zinc . 12. Electrode according to claim 11, characterized in that the catalyst ( 2.3 , 4.3 ) is applied by a thermolytic and a subsequent anodic forming process, ionic stages at temperatures below 500 ° C on the carrier to form oxides / hydroxides and / or mixed oxides decomposed and then post-treated with an anodic current density of up to approx. 200 mA / cm ² . 13. Electrode according to claim 11, characterized in that the catalyst ( 2.3 , 4.3 ) is produced by a sol-gel process or precipitation process from metal salts of the elements iridium, ruthenium, palladium platinum, Osmi um, rhodium, tantalum, tin, zinc is. 14. Electrode according to claim 11, 12 or 13, characterized in that the catalyst ( 2.3 , 4.3 ) is in the oxidation states III and IV. 15. Electrode according to one of the preceding claims, characterized ge indicates that the electrochemical converter is an electrolyser or is a fuel cell or alternately as an electrolyser and Fuel cell can be operated. 16. Electrochemical energy converter with liquid fixed electrolyte and hydrogen electrode and oxygen electrode, thereby ge indicates that hydrogen electrode and oxygen electrode are constructed according to one of the preceding claims, wherein the average pore radius of the oxygen electrode is larger than the average Pore radius of the hydrogen electrode.