RU2446514C1 - Method to manufacture base of electrode of alkaline fuel element of matrix type - Google Patents

Abstract

FIELD: electricity.

SUBSTANCE: method to manufacture a base of a hydrogen electrode of a fuel element of a matrix type includes mixing of a charge from a nickel powder with a pore agent, its application on a carcass grid, formation of a blank and sintering in a recovery medium, with simultaneous removal of the pore agent, besides, the charge composition includes a pore agent in the amount of 7-12 wt %, a fibrous asbestos in the amount of 2-6 wt %, and a nickel powder - balance, sintering is carried out in the recovery medium in two stages, at the temperature of sintering of the first stage from 500 to 700°C with duration from 20 to 50 min., at the temperature of sintering of the second stage from 900 to 1100°C with duration from 5 to 10 min.

EFFECT: increased corrosion resistance of nickel in the electrode base, increased resource of operation of fuel elements battery.

3 cl, 2 ex, 1 tbl

Description

The present invention relates to the field of direct conversion of chemical energy into electrical energy, in particular to a method for manufacturing a matrix of an alkaline fuel cell hydrogen electrode base.

A known method of manufacturing a porous sintered material, comprising mixing a metal powder with a carbonate of a given metal, forming preforms and sintering them at a temperature of 1200-1400 ° C in a reducing environment with the simultaneous removal of a pore former [1] (MP Nerushin and others. "Thin-porous tapes from nickel powders and alloys based on it ”, journal“ Powder metallurgy ”, No. 10, 1977).

The disadvantage of this method is that it does not allow to obtain material with a porosity of more than 55%, while the value of porosity plays an important role in the efficiency of the hydrogen electrode and fuel cell. The higher the porosity at a certain average pore size, the more efficiently the hydrogen electrode works and thus its buffer capacity is greater, which affects the reliability of the battery as a whole.

A known method of manufacturing a porous base of a hydrogen electrode of a chemical current source, including the preparation and rolling of dispersed material, sintering and calibration of rolling, removal of the blowing agent [2] (RF patent No. 2142475, B22F 3/18, priority 06.08.1997).

The disadvantage of this method is that it does not allow to obtain the basis of the electrode with a porosity of more than 60% and an average pore radius of about 1.0 μm. The need to create a porous base with a pore size of about 1.0 μm is explained by the dependence of the buffer capacity of the fuel cell on the ratio of the structures of the active and buffer layers. If the base of the hydrogen electrode (it is a buffer layer) is smaller in structure than the supported catalyst (active layer), the latter remains dry until the hydrogen base is completely flooded, which reduces the electrochemical characteristics of the fuel cell. If the base of the electrode is coarser in structure than the active layer, then the role of the buffer layer decreases as its pore size increases. It is not possible to reduce the pore size of the base of the electrode manufactured by this method, since this will lead to an even greater decrease in the porosity of the base.

A number of methods are known for the manufacture of porous materials of a cellular-frame type. The porous base in them is obtained by filling polyurethane foam with a paste based on metals or their compounds [3] (RF Patent No. 2213645, B22F 3/11, С22С 1/08, С04В 35/56, priority 05.09.2001) or by preparing a reaction mixtures with a gasification additive followed by ignition of this mixture [4] (RF patent No. 2210461, B22F 3/11, B22 F3 / 23, C22 C1 / 08, priority 05.07.2001), or impregnation of the fibrous polymer material with metal salts followed by restoration and plating [5] (RF patent No. 2054758, Н01М 4/80, Н01М 10/28, priority 02/11/1992).

In all cases, the material is obtained with high porosity (more than 60%). However, in terms of pore size, they all repeat the structure of polyurethane foam or felt. The pore size is hundreds and thousands of microns.

Closest to the proposed invention in terms of technical nature and the achieved result is a method of manufacturing a porous base of electrodeless alkaline battery electrodes, including double-sided deposition of nickel powder paste with a binder and blowing agent on a porous substrate tape, drying, compaction and sintering in a reducing atmosphere. The porosity of the base tape is 7-11%, and the ratio of porosity of the base and base tape is from 7 to 9 [6] (RF patent No. 2080694, H01M 4/80, H01M 10/28, priority 07.07.1993).

This method really allows you to make buffer layers with a porosity of more than 65%. However, this method of manufacturing the basis for the positive electrodes of the batteries has a significant drawback - the pore size is much larger than 1 μm. Experimental work on the manufacture of these bases and the subsequent analysis of the porous structure of the samples by mercury porosimetry showed that, at high porosity, the average pore size is in the range from 10 to 30 microns. By lowering the sintering temperature below 1000 ° C, it was possible to reduce the pore size to 5 ÷ 10 μm, but a new problem arose - a decrease in the corrosion resistance of the base nickel during operation under conditions of operation of a low-temperature alkaline fuel cell. Further reduction in pore size was problematic. It was found that the main role in this is played by the binder, which, when straightening the preform before sintering, does not allow nickel and nickel salt powders to come closer to each other. When sintering billets at 1280 ° C, an intensive decomposition of both the nickel salt and the binder occurs. This process leads to an additional increase in pore size.

The objective of the proposed technical solution is to reduce the average pore size of the base of the hydrogen electrode to 1.0 1.5 μm while maintaining porosity of more than 60% and high corrosion resistance.

This problem is solved due to the fact that the basis of the hydrogen electrode of an alkaline fuel cell of the matrix type is made by mixing a mixture of nickel powder with a blowing agent, applying it to a base, for example, on a nickel mesh, forming a preform and sintering in a reducing medium, for example, hydrogen, with the simultaneous removal of the blowing agent, and, according to the claimed technical solution, the third component is introduced into the mixture — fibrous asbestos in an amount of 2-6 wt.%, of the amount of nickel powder, to the amount of the blowing agent is taken 7-12 wt.%. Fibrous asbestos partially plays the role of a binder. Sintering is carried out in furnaces with a reducing atmosphere in two stages, the first stage takes place at a sintering temperature of 500 to 700 ° C and a sintering time of 20 to 50 minutes; the second stage - at a sintering temperature of 900 to 1100 ° C and a sintering time of 5 to 10 minutes.

It was experimentally established that the use of a pore former in an amount of more than 12 wt.% Is undesirable due to the formation of a structure with a large proportion of large (more than 1.5 μm) pores. Such a structure of the base of the hydrogen electrode leads to a decrease in the efficiency of using the buffer layer and, ultimately, to a deterioration in the electrochemical characteristics of the fuel cell. The use of a blowing agent in an amount of less than 7 wt.%, Leads to a decrease in the porosity of the base below 60%.

The introduction of fibrous asbestos into the buffer layer made it possible to increase the sintering temperature of the base of the electrode to 1100 ° C while maintaining its structural characteristics and thereby ensure high corrosion resistance of the base nickel throughout the entire service life. The lower limit on the content of fibrous asbestos, equal to 2.0 wt.%, Due to the significant sintering of the porous nickel layer at lower values. In this case, the porosity of the buffer layer is lower than 60% (the smaller the fraction of asbestos, the lower the porosity). The upper limit on asbestos (more than 6 wt.%) Is due to a decrease in the mechanical strength of the buffer layer. Asbestos fibers interfere with the sintering of nickel powder particles among themselves, and the strength of the porous layer decreases. This leads to a significant defect in the manufacture and deterioration of the electrochemical characteristics of the battery.

The introduction of two-stage sintering is due to the following. The decomposition of the blowing agent is accompanied by the release of carbon dioxide and water vapor. At high temperature, this process occurs very quickly and vigorously. In this case, partial destruction of the electrode base is observed. If sintering is carried out in one stage, then to achieve the desired structural characteristics and acceptable mechanical strength it takes a long time - more than 7 hours. It is economically disadvantageous. To mitigate this process, to provide the necessary mechanical strength of the base and the electrode itself, two-stage sintering was introduced.

The lower temperature limit of 500 ° C in the first stage is due to the low strength characteristics of the electrode base during sintering at lower temperatures. The upper limit for sintering temperature at the first stage of 700 ° C is due to an increase in the rate of decomposition of the blowing agent and a violation of the uniformity of the porous structure of the electrode base, up to the partial destruction of the electrode base.

The lower limit for sintering time at the first stage of heat treatment, equal to 20 min, is associated with a decrease in the strength characteristics of the base and a violation of the integrity of the layer. The upper limit of 50 min is due to the fact that the decomposition of nickel salt by this time is already complete, the porous layer acquires sufficient mechanical properties and therefore further exposure is impractical.

The lower limit is 900 ° C and a time of 5 minutes. in the second stage of sintering due to the need to obtain high corrosion resistance of the buffer layer. The electrode base at a lower sintering temperature does not acquire sufficient corrosion resistance, which leads to premature deterioration of the characteristics of the fuel cell.

The upper limit in temperature of 1100 ° С and sintering time of 10 min is due to the formation of a more coarse-porous structure of the electrode base, which affects the size of its buffer capacity and uniformity of characteristics of different fuel cells, if we are talking about a module or battery of several fuel cells.

The optimization of the amount of blowing agent, the amount of fibrous asbestos in the preform, the temperature and time of the second sintering stage was carried out by analyzing the porous structure of the resulting buffer layer by mercury porosimetry.

The lower and upper limits in temperature and time of the first stage of sintering were determined visually by the integrity of the porous preform.

The corrosion resistance of the base of the matrix alkaline fuel cell electrode was evaluated after life tests as part of the fuel cell by X-ray phase analysis for the presence of nickel oxidation products, as well as by the rate of change of the buffer capacity in the resource.

The characteristics of the fuel cell were evaluated during both operational and resource electrochemical tests of individual elements and modules.

The table presents some data on the dependence of the porous structure and mechanical porosity of the bases of the electrodes of an alkaline fuel cell of the matrix type on the sintering mode and the composition of the charge. The results were averaged over three parallel experiments.

Table Base No. Mass fraction,% 1st sintering 2nd sintering Porosity,% Effective pore radius, microns blowing agent Fibrous asbestos temperature, ° С time min temperature, ° С time min one Ni carbonate 10 one 650 35 1050 6 56.3 2.5 2 Ni carbonate 10 7 650 35 1050 6 63,2 0.95

Base No. Mass fraction 1st sintering 2nd sintering Porosity,% Effective pore radius, microns blowing agent Fibrous asbestos temperature, ° С time min temperature, ° С time min 3 Ni carbonate 10 3 650 45 1050 6 60.0 1,1 four Ni carbonate 5 3 650 35 1050 6 55.3 1.05 5 carbonate Ni, 15 3 650 35 1050 6 63,4 1.7 6 Ni carbonate 10 3 450 35 1050 6 crumbled 7 Ni carbonate 10 3 750 35 1050 6 61.3 1,2 8 Ni carbonate 10 3 450 fifteen 1050 6 crumbled 9 Ni carbonate 10 3 750 55 1050 6 60.5 1,1 10 Ni carbonate 10 3 650 35 850 6 61.6 1,1 eleven Ni carbonate 10 3 650 35 1150 12 55,4 1.7 12 Ni carbonate 10 3 650 35 850 four 62.5 1.05 13 Ni carbonate 10 3 650 35 1150 fourteen 59.7 1.05 fourteen Ni carbonate 10 3 650 35 1150 6 58.0 1.19 fifteen Ni carbonate 10 3 650 35 1050 6 58.7 1.15 16 Ni carbonate 10 3 650 35 850 12 60.1 1.12 17 oxalate Ni, 10 3 650 35 1050 6 59.5 1,07

As can be seen from the table, the desired values for the porous structure and hydrogen resistance of the base of the matrix alkaline fuel cell electrode are obtained with the selected manufacturing conditions and the established composition of the charge. Example 1

According to the claimed technical solution, in the manufacture of the base of the matrix alkaline fuel cell electrode, a buffer layer is first formed. For this, a mixture is prepared by mixing 60.0 g of carbonyl nickel powder, 6.0 g of a blowing agent, for example, nickel carbonate, and 1.8 g of fibrous asbestos, for example, AXO-MT brand. The resulting mixture is divided into two equal parts, applied on both sides of the base, for example, a nickel mesh, and pressed under pressure of 20 MPa into billets with a thickness of 0.35 mm to 0.38 mm. The obtained preforms are sintered in a reducing atmosphere, for example, hydrogen, in a conveyor furnace at a temperature of 650 ° C for 35 minutes. The second heat treatment is carried out in a conveyor furnace, also in a reducing atmosphere, for example, hydrogen, at a temperature of 1050 ° C with holding in a hot zone for 5 minutes. The result is the basics of the matrix alkaline fuel cell electrode with the following characteristics:

- thickness, mm ........................ 0,35;

- porosity,% ...................... 58.7;

- the average pore size, microns ......... 1.15.

Example 2

To make the base of the matrix-type alkaline fuel cell electrode, 60.0 g of carbonyl nickel powder are mixed as a blowing agent, for example, 6.0 g of nickel oxalate and 1.8 g of fibrous asbestos, for example, AXO-MT grade. This mixture is divided into two equal parts, applied on both sides of the base, for example, a nickel mesh, and pressed under pressure of 20 MPa into billets with a thickness of 0.35 mm to 0.38 mm. The obtained preforms are sintered in a reducing atmosphere (hydrogen) in a conveyor furnace at a temperature of 650 ° C for 35 minutes. The second heat treatment is carried out in a conveyor furnace in a reducing atmosphere, for example, hydrogen, at a temperature of 1050 ° C with holding in a hot zone for 5 minutes. The result is the basics of the matrix alkaline fuel cell electrode with the following characteristics:

- thickness, mm ......................... 0.35;

- porosity,% ...................... 59.5;

- the average pore size, microns ........ 1.07.

Using these fundamentals, hydrogen electrodes were manufactured that passed life tests (858 hours) as part of a six-cell fuel cell battery. Certification of hydrogen electrodes after defect of the module by the X-ray phase method showed the absence of nickel compounds in the electrode base.

The use of the inventive method of manufacturing the base of the hydrogen electrode of the matrix-type fuel cell in production allows, while maintaining the desired initial structural characteristics of the base of the hydrogen electrode, to significantly increase the corrosion resistance of nickel in the base of the electrode and thereby increase the guaranteed battery life of the fuel cell.

Information sources

1. Authors: MP Nerushin et al. “Fine-porous tapes of nickel powders and alloys based on it”, the journal “Powder metallurgy”, No. 10, 1977

2. RF patent №2127475, B22F 3/18, priority 06/08/1997

3. RF patent No. 2213645, B22F 3/11, С22С 1/08, С04В 35/56, priority, September 5, 2001.

4. RF patent No. 2210461, B22F 3/11, B22F 3/23, C22C 1/08, priority 05.07.2001.

5. RF patent No. 2054758, Н01М 4/80, Н01М 10/28, priority 11.02.1992

6. RF patent №2080694, Н01М 4/80, Н01М 10/28, priority 07.07.1993

Claims (3)

1. A method of manufacturing a matrix of a hydrogen element of a matrix type fuel element, comprising mixing a mixture of nickel powder with a blowing agent, applying it to a wire mesh, forming a workpiece and sintering in a reducing medium while removing the blowing agent, characterized in that the mixture contains a blowing agent in an amount 7-12 wt.%, Fibrous asbestos in an amount of 2-6 wt.%, And nickel powder - the rest, sintering is carried out in a reducing medium in two stages, at a sintering temperature of the first stages from 500 to 700 ° C for a duration of 20 to 50 minutes, at a sintering temperature of the second stage from 900 to 1100 ° C for a duration of 5 to 10 minutes 2. A method of manufacturing the basis of the hydrogen electrode of a matrix-type fuel cell according to claim 1, characterized in that nickel carbonate is used as a blowing agent. 3. A method of manufacturing the basis of the hydrogen electrode of the matrix-type fuel cell according to claim 1, characterized in that oxalic nickel is used as a blowing agent.