CN104328456B - Reversible phase transition vanadate electrode material, and preparation method and application thereof - Google Patents

Abstract

_The invention discloses a vanadate electrode material with reversible phase transition and its preparation method and application. Afterwards, a reduced product composed of FeV ₂ O ₄ and Fe is generated, and the reduced product is oxidized to generate the FeVO ₄ again. The cathode material of the invention has high electrical conductivity under reducing atmosphere, and precipitates metal iron catalyst, has high catalytic activity, and has good performance as the cathode material of high-temperature solid oxide electrolytic cell.

Description

A kind of vanadate electrode material with reversible phase transition and its preparation method and application

technical field

The invention relates to the design field of a cathode material for an electrolytic cell, in particular to a vanadate cathode material for a solid oxide electrolytic cell capable of reversible phase transition.

Background technique

The high-temperature solid oxide electrolytic cell is an efficient energy conversion device. At high temperature, water vapor can be electrolyzed to produce hydrogen, and carbon dioxide can be electrolyzed to produce carbon monoxide. It has good kinetic and thermodynamic properties. The main function of the cathode (fuel pole) of the oxygen ion type solid oxide electrolytic cell is to provide a place for the decomposition reaction of water vapor or carbon dioxide and a channel for electron conduction. Therefore, in addition to matching the performance of adjacent components and having chemical compatibility and stability under long-term operation, the fuel electrode must also have good electronic conductivity and electrocatalytic activity, as well as maintain its structure under high temperature conditions. and compositional stability.

The first cathode materials used for catalytic decomposition of water vapor are some noble metals and transition metals, such as Ni, Pt, Co, Ti, etc. These materials have high activity and can remain stable in a reducing atmosphere. However, pure metals have many drawbacks as fuel electrodes in solid oxide electrolytic cells. For example, the thermal expansion coefficient does not match between the metal electrode and the electrolyte material, the electrolytic cell has poor thermal shock resistance, the electrode reaction active sites are few, and high overpotentials are easily generated. In addition, the fuel electrode chamber of the solid oxide electrolytic cell is a high-temperature, high-humidity atmosphere, and the pure metal electrode is prone to sintering, which leads to the performance degradation of the electrolytic cell. The use of pure metals alone as fuel electrode materials for solid oxide electrolytic cells has been greatly restricted and is rarely used now. At present, composite cathode materials are mostly used, such as Ni-YSZ. In Ni-YSZ cermets, Ni and YSZ will not react in a wide temperature range. YSZ as a supporting framework can effectively inhibit the agglomeration and coarsening of Ni particles, and improve the stability of the solid oxide electrolytic cell. At the same time, by adjusting the doping ratio, the thermal expansion coefficient of the composite cathode is similar to that of the YSZ electrolyte, which enhances the chemical stability, and in the Ni-YSZ fuel electrode, the electrode reaction area can extend to a certain depth inside the fuel electrode, thereby increasing the electrode Reactive sites to improve the performance of the electrolytic cell. However, at higher operating temperatures, the metal Ni-based composite cathode is easily oxidized to NiO, thereby losing its electrical conductivity and catalytic performance, which is the main reason for the failure of the Ni-based composite cathode. In addition, due to the high catalytic activity of metal Ni, carbon deposition also occurs, which affects the catalytic activity. Ceramic-based perovskite oxides such as La _0.8 Sr _0.2 TiO ₃ and La _0.75 Sr _0.25 Cr _0.5 Mn _0.5 O ₃ have developed rapidly and have shown a tendency to replace nickel-based electrodes. Ceramic-based La _0.2 Sr _0.8 TiO _3+δ materials have attracted widespread attention due to their high mixed conductivity, catalytic activity, and good redox stability, but La _0.2 Sr _0.8 TiO _3+δ is easily oxidized in the presence of high-temperature electrolysis And the catalytic activity is lower than that of traditional metal electrodes, which restricts its development. Compared with nickel-based composite electrodes, La _x Sr _{1- x} Cr _y Mn _1-y O _3-δ (LSCM) is a highly active and redox-stable material with very small polarization resistance. The field of high-temperature solid oxide electrolytic cells has attracted great attention. However, because LSCM is a p-type conductivity mechanism, under the reduction voltage, LSCM produces a large polarization resistance, which reduces the current efficiency, so further improvement and optimization are needed.

Contents of the invention

The present invention aims at the current metal electrodes being easily oxidized and agglomerated and the catalytic activity of ceramic electrodes insufficient, etc., using FeVO that can undergo reversible phase transitions as the cathode material of a solid oxide electrolytic cell, utilizing the reversible phase transition of _{FeVO 4 ,} in reducing atmosphere It can be decomposed into metal Fe catalyst and FeV ₂ O ₄ to ensure that the material has sufficient catalytic activity, and the generated material maintains a stable structure in a reducing atmosphere, and then obtains a solid oxide electrolytic cell cathode material with better electrochemical performance.

Concrete technical scheme of the present invention is:

The vanadate electrode material with reversible phase transition of the present invention is characterized in that its composition chemical formula is FeVO ₄ .

The vanadate electrode material with reversible phase transition of the present invention is also characterized in that: the vanadate electrode material can undergo a reversible phase transition, that is, the vanadate electrode material _FeVO4 is reduced under a reducing atmosphere at 700-750 ° C, Generate a reduced product composed of FeV ₂ O ₄ and Fe, and oxidize the reduced product in an oxidizing environment at 700-800°C to generate the vanadate electrode material FeVO ₄ again; the volume percentage of H ₂ in the reducing atmosphere is 5%, and the balance is Ar (5% H ₂ /Ar); the oxidation environment is in air atmosphere.

The vanadate electrode material of the invention is prepared by a solid phase reaction method using ferric oxide and vanadium pentoxide as raw materials.

The concrete steps that material of the present invention synthesizes with solid phase reaction method are:

A, take ferric oxide and vanadium pentoxide as reaction raw material according to stoichiometric ratio;

B, said reaction raw material is packed in the ball mill jar of ball mill, in ball mill jar, add agate ball as grinding medium, then add acetone as ball mill dispersant, acetone is added to cover 2/3rds of ball mill jar volume;

c, ball milling for 15 minutes at a ball milling speed of 1000 rpm, so that ferric oxide and vanadium pentoxide are evenly mixed to obtain a mixture;

d. Dry the mixture and press it into tablets, then calcined at 750°C for 10 hours to obtain calcined tablets;

f. Grinding the calcined sheet into powder in a mortar to obtain the vanadate electrode material FeVO ₄ .

The drying in step d is carried out in an air atmosphere at 150° C. for 0.5 to 1.5 hours.

The tablet pressing in step d is to press the dried mixture into a tablet with a diameter of 12-18 mm and a thickness of 1-3 mm under a pressure of 4 MP.

The present invention further provides the application of the vanadate electrode material _FeVO as the cathode material of a high-temperature solid oxide electrolytic cell. When it is working, the electrode is in a reducing atmosphere. At this time, the FeV ₂ O ₄ phase structure is stable, and Fe has High metal catalytic activity can promote electrode activation, reduce polarization resistance, and improve electrode performance. However, when the electrode is in an oxidizing environment, the electrode material will not agglomerate due to oxidation like Ni and other metal electrodes, which will cause the electrode to lose its activity. Fe and FeV ₂ O ₄ react again to form FeVO ₄ . Therefore, the redox reversibility of FeVO ₄ is applied to the cathode material of the electrolytic cell, which has great advantages.

Using _FeVO of the present invention As the cathode material, the method for preparing a symmetrical battery of a high-temperature solid oxide electrolytic cell is: mixing the synthesized cathode powder with Ce _0.8 Sm _0.2 O _2-δ (SDC) in a mass ratio of 65:35, adding Ethylcellulose terpineol was used to create pores to prepare composite electrode materials. Grind it thoroughly, apply it uniformly on the surface of dense yttrium-stabilized zirconia (YSZ) electrolyte, and calcinate at 1000°C for 3 hours. Coat silver paste on the surface of the electrode as the current collector layer, and use silver wire as the wire to make a symmetrical battery.

Compared with prior art, the beneficial effect of the present invention is:

1. The present invention provides a reversible phase transition vanadate electrode material FeVO ₄ and its preparation method. The preparation method is simple, and the prepared FeVO ₄ can undergo a reversible phase transition, so that it can effectively promote the electrode as a cathode material. Activation, reducing polarization resistance, thereby improving electrode performance.

2. The present invention uses FeVO ₄ as the cathode material of the high-temperature solid oxide electrolytic cell, which can generate Fe metal element and spinel structure FeV ₂ O ₄ in a reducing atmosphere, and has redox reversibility; high-temperature solid oxidation When the cathode of the electrolytic cell is working, the electrode is in a reducing atmosphere, and the phase structure of FeV ₂ O ₄ generated at this time remains stable, and Fe has high metal catalytic activity, which can promote electrode activation and reduce polarization resistance, thereby improving electrode performance. ; When the electrode is in an oxidizing environment, Fe and FeV ₂ O ₄ react again to form FeVO ₄ , which prevents the agglomeration of the catalyst and prevents the electrode from losing its activity under oxidizing conditions.

3. When the FeVO ₄ of the present invention is in a reducing atmosphere, due to the presence of precipitated metal elemental Fe, the symmetrical battery electrodes assembled with this material and YSZ as the electrolyte have excellent performance.

Description of drawings

Fig. 1 is the XRD collection of illustrative plates of embodiment 1 gained product;

Fig. 2 is the XRD spectrum of the product FeVO obtained in Example 1 after being reduced at 750°C for ₅ hours in a 5% H ₂ /Ar environment;

Fig. 3 is the XRD spectrum of embodiment 1 gained FeVO ₄ reduction product calcined in air atmosphere for 5 hours;

Figure 4 shows the polarization resistance of FeVO ₄ at 800 °C under different hydrogen partial pressure atmospheres in symmetrical cells with YSZ as the electrolyte.

detailed description

Example 1: Synthesis of 0.04mol FeVO ₄ material, characterization of post-redox phase and test of electrochemical AC impedance of symmetric battery.

According to the chemical formula ratio of material, take by weighing 3.1938g Fe ₂ O ₃ (analytical pure) and 3.64g V ₂ O ₅ as reaction raw material; Put the reaction raw material into the ball mill jar that volume is 100ml, use agate ball as grinding medium, Add acetone (adding to cover two-thirds of the ball mill jar) as a dispersant, and ball mill it for 15 minutes at a speed of 1000 revs/min in a ball mill to make it evenly mixed to obtain a mixture; the mixture is placed in an air atmosphere, Dry at 150°C for 1 hour, then take an appropriate amount of powder and press it into a tablet with a diameter of about 15mm and a thickness of about 2mm under a pressure of 4MP, and finally calcinate the pressed tablet at 750°C for 10 hours, and place the calcined tablet in a mortar Grinding into powder in medium to obtain the target product, FeVO ₄ , the vanadate electrode material.

The spectrum of the prepared powder is measured by XRD powder diffraction method, and refined by GSAS (refinement is the process of making the theoretical structure model and experimental data consistent through the correction of related variables, and the crystal constant can be obtained more reliably from the refined results .), and the results are shown in Figure 1. The XRD pattern of the actual sample in Figure 1 basically coincides with the theoretical model, indicating that the crystal phase is better, and the crystal constants of the sample in the oxidation state are also obtained through refinement.

Take part of the prepared powder, place it in 5% H ₂ /Ar environment, and reduce it at 750°C for 5 hours. After the reduction, XRD test is carried out, which proves that FeVO ₄ decomposes to generate FeV ₂ O ₄ and elemental Fe. The obtained XRD is refined, as shown in Figure 2 As shown, the formed FeV ₂ O ₄ has a spinel structure with good crystallinity. When the 2Θ is about 44.6°, the peak of simple iron appears, indicating that two phases of FeV ₂ O ₄ and simple Fe are formed after reduction.

The decomposition product obtained by reduction was calcined in air at 750°C for 5 hours, and the obtained powder was subjected to XRD test, which proved the reversibility of phase transition, as shown in Figure 3, indicating that FeVO ₄ produced FeV ₂ O ₄ and elemental substance after reduction Fe, after the product is oxidized again, can be reacted to obtain FeVO ₄ , without the formation of other impurity phases. Taking advantage of the reversibility of the phase transition, the material can generate a stable phase of spinel structure and Fe with metal catalytic activity under reducing conditions, and transform into a stable structure of FeVO ₄ again after oxidation.

The synthesized cathode powder and Ce _0.8 Sm _0.2 O _2-δ (SDC) were mixed at a mass ratio of 65:35, and ethyl cellulose terpineol was added to create pores to prepare a composite electrode material. After fully grinding, the obtained composite electrode slurry is evenly coated on the surface of the dense YSZ electrolyte, and calcined at 1000°C for 3 hours. Coat silver paste on the surface of the electrode as the current collector layer, and use silver wire as the wire to make a symmetrical battery. The AC impedance test was carried out with a symmetrical battery, and the polarization resistance under different hydrogen partial pressures at 800°C was obtained, as shown in Figure 4. As the partial pressure of hydrogen increases, the polarization resistance decreases continuously. Under the atmosphere of 80%H ₂ /20%N ₂ , the polarization resistance is 2.55Ω·cm ² . With the increase of hydrogen partial pressure, FeVO ₄ is reduced to FeV ₂ O ₄ and the amount of elemental Fe will also increase. Due to the increase in the content of metal Fe catalyst, the activation of the electrode can be promoted, thereby reducing the polarization resistance and improving the electrode performance. .

Claims (5)

1. a kind of application of vanadate electrode material it is characterised in that：Described vanadate electrode material is used for as high-temp solid Oxidate electrolytic cell cathode material；

The chemical formula of described vanadate electrode material is FeVO₄；

Described vanadate electrode material can carry out reversible phase in version, and described reversible phase in version refers to：By described vanadic acid salt electrode Material FeVO₄Reduce under 700-750 DEG C of reducing atmosphere, generate by FeV₂O₄The reduzate constituting with Fe, will be described also original After thing aoxidizes under 700-800 DEG C of oxidation environment, generate described vanadate electrode material FeVO again₄；H in described reducing atmosphere₂ Percentage by volume is 5%, balance of Ar；Described oxidation environment is in air atmosphere.

2. according to claim 1 application it is characterised in that：Described vanadate electrode material is with iron sesquioxide and five V 2 O is raw material, is prepared by solid reaction process.

3. according to claim 1 and 2 application it is characterised in that：Described vanadate electrode material adopts solid state reaction Method synthesizes, and concretely comprises the following steps：

A, weigh iron sesquioxide and vanadic anhydride according to stoichiometric proportion as reaction raw materials；

B, described reaction raw materials are loaded in the ball grinder of ball mill, ball grinder adds agate ball as abrasive media, so Add acetone as ball milling dispersant afterwards, acetone adds to and covers 2/3rds of ball milling tank volume；

C, with 1000 revs/min of ball milling speed ball milling 15 minutes, make iron sesquioxide and vanadic anhydride mix homogeneously, obtain Compound；

D, by compound be dried, tabletting, then at 750 DEG C calcine 10 hours, obtain calcining piece；

E, by described calcining piece pulverize in mortar, obtain final product vanadate electrode material FeVO₄.

4. according to claim 3 application it is characterised in that：

Described in step d, drying is in air atmosphere, and 0.5～1.5h is dried at 150 DEG C.

5. according to claim 3 application it is characterised in that：

Tabletting described in step d be under 4MPa pressure by drying after compound be pressed into a diameter of 12～18mm, thickness be 1～3mm Piece.