RU2128385C1 - Solid-oxide fuel cell electrode and its manufacturing process - Google Patents

Abstract

FIELD: electrochemical power engineering. SUBSTANCE: modified electrode abuts against solid electrolyte and is composed of solid electrolyte particles having ionic or compound conductivity and porous conducting sections contacting solid electrolyte surface layer and solid electrolyte particles. Porous conducting sections of fuel cell electrode have columnar crystalline structures of electricity conducting materials contacting solid electrolyte particles and solid electrolyte surface through layer of conducting clusters. Magnetron evaporation process used for electrode manufacture includes supply of portions of solid electrolyte fractions to discharge in definite time intervals and their evaporation onto substrate in magnetron discharge while changing evaporation angle in definite time intervals. Evaporation conditions in magnetron provide for condensation of sprayed material on surfaces of fragments supplied to discharge during their movement in discharge to substrate surface. EFFECT: improved capacity of fuel cells. 6 cl

Description

 The invention relates to the field of electrochemical energy - the direct conversion of the chemical energy of hydrogen-containing fuel into electrical energy using solid oxide fuel cells (SOFC).

 SOFCs are more efficient producers of electricity, while providing less pollution than other sources of electricity and other types of fuel cells. With the full commercialization of SOFC, which is projected to happen within 10 years, they will be used as domestic, transport and industrial sources of electricity.

Fundamentally, SOFC consists of porous cathodic (air) and anode (fuel) conductive electrodes adjacent on both sides to a gas-tight layer of an ion-conducting solid electrolyte. Air oxygen diffuses through the pores of the cathode electrode to the surface of the solid electrolyte, dissociating and reducing to O ^2– . The O ^2– ion migrates through the solid electrolyte to the fuel electrode, where it oxidizes the fuel diffused through the pores of the anode. This generates heat and electricity. The reaction products are removed through the pores of the anode, and the electric current is removed by conductive electrodes.

 In order to maintain the integrity of the solid electrolyte layer during operation, it is necessary to have close values of the linear thermal expansion coefficients of the solid electrolyte and SOFC electrodes. In this regard, SOFC electrodes in most cases consist of solid electrolyte particles interconnected with each other and with the surface of the solid electrolyte by porous sections of materials with high electronic or mixed conductivity.

The most widely used compositions and structures of cermet SOFC electrodes are described in [1] and [2]. In particular, as the cathode electrode applied sintered mixture of powders 0,92ZrO ₂ • 0,08Y ₂ O ₃ - and a solid electrolyte oxygen-0,8CeO ₂ • 0,2SmO _1,5 - up to 50 vol% ceramic mixed conductor. and as an anode electrode - a sintered mixture of powders of 0.92 ZrO ₂ • 0.08 Y ₂ O ₃ and nickel - up to 50 vol.%.

 The disadvantage of the described cermets is their shrinkage during operation and, as a consequence, a decrease in the diffusion fluxes of reagents, leading to a decrease in the resource power of SOFC. Along with this, used cermets have a reduced catalytic activity and an increased value of electrical resistance. This is due to the fact that areas of catalytic activity (three-phase boundaries "gas - solid electrolyte - electrically conductive material") are formed during sintering spontaneously. Similarly, sections of through electrical conductivity and through porosity are formed. As a result, a large part of the electrode volume is used inefficiently, and the formed interfacial electrical contacts due to the small surface development have an increased electrical resistance.

 The catalytic activity of cermet electrodes was increased by the technical solution described in [3], by using cyclic redox processes during sintering. The disadvantage of SOFC electrodes described in the prototype of the present invention [3], remains a high content of electrically conductive components having a lower softening temperature than a solid electrolyte. The shrinkage of an electrode that occurs during operation and is coupled to a layer of solid electrolyte can lead to the destruction of SOFC or to a local decrease in the flows of oxidizing agent and reducing agent and making it difficult to remove reaction products from three-phase boundaries and, accordingly, to reduce resource power characteristics.

 The present invention solves the technical problem of reducing the volume fraction of the electrically conductive component with a low total cermet resistance and the creation of developed interphase boundaries "gas - solid electrolyte - electrically conductive material".

 The essence of the invention lies in the fact that the electrode of a solid oxide fuel cell consists of particles of a solid electrolyte having mixed or ionic conductivity and porous sections of an electrically conductive material consisting of crystalline columnar structures of electrically conductive materials distributed across the volume of the electrode in contact with the solid electrolyte particles and the layer surface solid electrolyte through a layer of electrically conductive clusters.

 The increase in SOFC power when using the inventive electrodes is due to the development of three-phase boundaries formed by electrically conductive crystalline clusters bordering on the volume of gas pores and solid electrolyte. The cluster size is the minimum for crystalline formations. In addition, the through electron conductivity of the electrode due to the continuous grid of columnar conductive structures surrounding solid electrolyte particles that contact the layers of conductive clusters at the electrode-solid electrolyte interface, leads to a decrease in electrode resistance and an additional increase in SOFC power.

 The stability of the resource power characteristics of SOFC is increased due to a more efficient use of the volume of the electrode, which allows to reduce the content of the electrically conductive component in comparison with the electrodes obtained by sintering. This reduces the shrinkage of the material during its operation, as it creates a solid frame of particles of high-temperature solid electrolyte.

 As we found in impedance studies of a solid electrolyte with platinum electrodes deposited by sintering and magnetron sputtering methods, in the latter case, the contact transition resistance is much lower, and the specific catalytic activity is higher. In this case, with a decrease in the temperature of the electrodes, this difference in resistance increases. The resistance values of electrochemical reactions (increased SOFC current density) were also significantly lower.

 An additional increase in the catalytic efficiency of SOFC electrodes is achieved in that the columnar structures and clusters of the electrically conductive component contain clusters of catalysts for electrode chemical reactions. Fine crystalline reaction catalysts significantly accelerate the speed of catalytic reactions by reducing the activation energy of reactions.

 The permeability of SOFC electrodes for gas reaction components can be increased in that the particle size of the solid electrolyte and / or the pore size increase in the direction from the solid electrolyte layer.

By controlling the pore size in the SOFC electrodes, an increase in the permeability of the reaction components is achieved and the reaction rate increases, and, therefore, the SOFC power
By controlling the particle size distribution of the solid electrolyte in the SOFC electrode, it is possible to further increase the permeability of the electrodes and further reduce the shrinkage of the SOFC electrodes.

Numerous methods for manufacturing SOFC electrodes are known:
- colloidal deposition and sintering [4];
- rolling and sintering [5];
- impregnation and sintering [6] and others.

All of the above methods consist of the following operations: the manufacture of fine powders, the formation of cermet layers on a binder, high-temperature sintering, impregnation with salts and other compounds, high-temperature processing of cermet. The disadvantage of these methods is the use of high-temperature (more than 1200 ^o C) technological processes, as well as, in accordance with [3], the use of redox media. This may lower the characteristics of one of the SOFC electrodes in the manufacture of the other.

These disadvantages are eliminated in the magnetron method of manufacturing SOFCs described in US patent [7]. SOFC electrodes were deposited by co-magnetron sputtering in a mixture of oxygen and argon of the following materials:
cathode - silver, zirconium, yttrium
anode - nickel, zirconium, yttrium
The advantage of this method is the low temperature of the manufacture of layers (about 200 ^o C). However, the method did not make it possible to obtain layers with adjustable pore sizes, did not provide a decrease in shrinkage during operation, and did not solve the problem of matching the coefficients of linear thermal expansion of SOFC layers.

 The invention solves the technical problem of obtaining magnetron sputtering SOFC electrodes containing a rigid frame of solid electrolyte particles, with adjustable porosity of the conductive sections.

 In our proposed method for magnetron sputtering of SOFC electrodes, solid electrolyte particles are introduced into the discharge. The sprayed conductive component in the form of clusters condenses on the substrate and on solid electrolyte particles deposited on the substrate. The sprayed conductive component forms columnar conductive structures whose porosity is controlled. The number of particles supplied to the discharge is consistent with the deposition rate of columnar structures so that the sprayed electrode has high electronic conductivity.

The essence of the invention lies in the fact that the sputtering of porous columnar layers of SOFC by magnetron sputtering is carried out at the top location of the magnetron above the substrate so that the axis of the beam of atomized particles forms an angle α with the plane of the substrate, and with a cylindrical substrate, angle α with the generatrix of the cylindrical surface. After a given time interval, a change in the angle of inclination of the beam axis by 90 ^o and 180 ^o -α. At the same time, portions of fractions of solid electrolyte particles are fed into the discharge at certain time intervals, the size of which increases with increasing layer thickness. An increase in pore size is achieved by decreasing the angle α and increasing the time interval until the substrate is transferred from position α to position 90 ^° and 180 ^° -α. The substrate potential is kept lower than the magnetron anode potential.

 To increase the rate of cluster formation on the surface of particles introduced into the discharge, an inert gas pressure and discharge current density are increased until the process of condensation of the sprayed material onto the surface of the particles supplied to the discharge during their movement in the discharge to the substrate surface occurs.

EXAMPLE OF PERFORMANCE AND MANUFACTURE OF SOFC ELECTRODE APPLICABLE TO PLANE AND CYLINDRICAL SURFACES
Application of the SOFC anode electrode is carried out, for example, on a carrier plate of cathode material, La _x Sr _1-x MnO ₃ perovskite coated with a solid electrolyte coating of 0.92 ZrO ₂ • 0.08 Y ₂ O ₃ .

 In the event that a solid electrolyte is selected as the carrier, a cathode electrode is also applied in the manner described below.

 The 0.15 mm thick SOFC anode electrode consists of layers of solid electrolyte powder with particle sizes of 2 and 4 μm, as well as clusters and columnar structures sprayed with magnetron sputtering of nickel.

A nickel target with a diameter of 50 mm is made of a 1 mm thick plate and bonded to a diamagnetic conductive substrate with a thickness of about 2 mm to maintain the required magnetic field in front of the nickel plate. The target is installed in a magnetron sputtering unit. Nickel deposition was carried out in the following mode:
- argon pressure (1-2) -10 ^-2 Torr;
- voltage 1.2 kV;
- discharge current 0.1 A;
- negative offset voltage of the substrate relative to the anode is 100 V;
- nickel deposition rate of 6 μm / hour.

 Initially, the substrate was covered with a shutter and for 10 minutes the target was cleaned by sputtering in argon in the specified mode.

Then, for 1-1.5 minutes, a cluster sublayer was sprayed, then the fraction of solid electrolyte with a particle size of 2 μm was fed into the discharge. The feed of this fraction was conducted to a layer thickness of 0.05 mm. For this purpose, the following mode was chosen for spraying:
- supply of particles in portions, ensuring the formation of a monolayer of particles;
- deposition of Nickel at angles α = -60 ^o , 0, 60 ^o in relation to the normal to the substrate. The angle was changed for each monolayer of particles.

 The thickness of the nickel layer sprayed onto the monolayer of particles is 0.2 μm. The pore perimeter L was estimated by the formula L = 3 • v • Δt, where v is the deposition rate (μm / min), Δt is the layer dusting time (min). In our case, v = 0.1 μm / min, Δt = 2 min, L = 0.6 μm.

Initially, 25 monolayers of solid electrolyte particles with a particle size of 2 μm were sprayed with a 2-minute exposure to dust each monolayer (50 minutes, a layer thickness of about 50 μm),
Next, we proceeded to supply successively 25 monolayers of particles of solid electrolyte with a size of 4 μm with a shutter speed of 3 minutes for dusting each monolayer (the total spraying time of the layer is 75 minutes, the thickness of the layer is about 100 μm). The total electrode manufacturing time is 125 minutes. When this heating the substrate is not more than 200 ^o C.

Sources of information
1. T. Kenjo at al. High temperature air cathodes containing ion conductive oxides // J.Electrochem.Soc., V. 138 (1991), N 2, p. 349.

 2. A. Isenberg at al. Sulfur tolerant composite cermet electrodes for solid oxide electrochemical cells // Patent USA N 4702971, May 28, 1986.

 3. A. Isenberg at al. High performance cermet electrodes // Patent USA N 4582766, Apr. 15, 1986.

 4. S. de Souza at al YSZ Thin-film electrolytes for high power reduced temperature solid oxide fuel cells // An Electr. Power Research Institute and Gas Research Institute, Fuel Cell Workshop on Fuel Cell Technology Research and Development, 1996.

 5. Minh N., Monolithic fuel cell having improved Interconnect layer // Patent USA N 5356700, Oct. 18, 1994.

 6. Ruka R. J. at al. Modified cermet fuel electrodes for solid oxide electrochemical cells // Patent USA N 5021304, Jun. 4, 1991.

 7. Barnett at al. Solid-oxide fuel cells // Patent USA N 5395704, May 7, 1995.

Claims (5)

 1. The electrode of a solid oxide fuel cell deposited on a layer of solid electrolyte, consisting of particles of solid electrolyte having mixed or ionic conductivity, and porous sections of an electrically conductive material, characterized in that the porous electrically conductive sections consist of crystalline columnar structures of electrically conductive materials in contact with solid particles electrolyte and the surface of the solid electrolyte layer through a layer of electrically conductive clusters.  2. The electrode according to claim 1. characterized in that the columnar structures and clusters contain clusters of catalysts for electrode chemical reactions.  3. The electrode according to claim 2, characterized in that the particle size of the solid electrolyte increases in the direction from the layer of solid electrolyte.  4. The electrode according to claim 3, characterized in that the pore size, consisting of crystalline columnar structures, increases in the direction from the solid electrolyte layer. 5. A method of manufacturing an electrode of a solid oxide fuel cell, which consists in magnetron sputtering on a substrate, characterized in that the deposition of porous columnar layers is carried out at a substrate potential lower than the anode potential, when the magnetron is located above the substrate so that the beam axis forms an angle α with the plane substrate, and the cylindrical substrate at the angle α with a generatrix with a change after a predetermined time interval by an angle 90 ^o and 180 ^o -α, wherein a discharge at predetermined time intervals Fractions fed batch second solid electrolyte particles, the size of which increases with increasing layer thickness, the pore size increase is achieved decreasing angle α and increasing the time interval to the substrate transfer position angle 90 ^o and 180 ^o -α.
6. The method according to claim 5, characterized in that during spraying, the inert gas pressure and the discharge current density are increased until the process of condensation of the sprayed material onto the surface of the particles supplied to the discharge during their movement in the discharge to the substrate surface occurs.