Method for producing an air electrode, the electrode thus obtained and its uses

Abstract

This invention relates to a method for preparing an air electrode based on Pr.sub.2-xNiO.sub.4 with 0≦x<2, comprising a step consisting in sintering a ceramic ink comprising Pr.sub.2-xNiO.sub.4 and a pore-forming agent at a temperature above 1000° C. and below or equal to 1150° C. This invention also relates to the air electrode thus obtained and its uses.

Claims

1. Method for preparing an air electrode based on Pr.sub.2-xNiO.sub.4 with 0≦x<2, comprising sintering a ceramic ink comprising Pr.sub.2-xNiO.sub.4 and a pore-forming agent at a temperature above 1000° C. and below or equal to 1150° C., wherein, before the sintering step, said ceramic ink is brought to the sintering temperature following a temperature rise gradient of between 50 and 400° C./h.

2. Method according to claim 1, wherein the quantity of Pr.sub.2-xNiO.sub.4 powder with 0≦x<2, expressed by mass relative to the total mass of said ink is comprised between 30 and 70%.

3. Method according to claim 1, wherein said pore-forming agent is in the form of a powder the particles of which have a median diameter (d50), determined using the Malvern particle size analyser, below 4 μm.

4. Method according to claim 1, wherein said pore-forming agent is selected from the group consisting of acetylene black, polystyrene, polymethyl methacrylate (PMMA), starch, polyethylene, cyclodextrine, a wax, monosaccharide, oligosaccharide and polysaccharide.

5. Method according to claim 1, wherein the mass ratio Mp/(Mc+Mp) where Mp is the mass of pore-forming agent and Mc is the mass of Pr.sub.2-xNiO.sub.4 with 0≦x<2 is between 1.5 and 10%.

6. Method according to claim 1, wherein said ceramic ink comprises at least one solvent and at least one binder and wherein the mass ratio Ml/(Ml+Ms) where Ms is the mass of solvent and Ml is the mass of binder, is between 1 and 10%.

7. Method according to claim 1, wherein said ceramic ink is subjected to the sintering temperature for a duration of between 1 and 5 h.

8. Method for preparing an air electrode based on Pr.sub.2-xNiO.sub.4 with 0≦x<2, comprising sintering a ceramic ink comprising Pr.sub.2-xNiO.sub.4 and a pore-forming agent at a temperature above 1000° C. and below or equal to 1150° C., wherein, subsequent to the sintering step, said ceramic ink is brought to a temperature of between 10 and 40° C., the layer being brought from the sintering temperature to the cooled temperature along a cooling gradient of between 50 and 400° C./h.

9. Air electrode based on Pr.sub.2-xNiO.sub.4 with 0≦x<2, obtained by sintering a ceramic ink containing Pr.sub.2-xNiO.sub.4 and a pore-forming agent at a temperature above 1000° C. and below or equal to 1150° C., wherein, before the sintering step, said ceramic ink is brought to the sintering temperature following a temperature rise gradient of between 50 and 400° C./h, and wherein the air electrode has a polarisation resistance below 0.06 Ω.Math.cm.sup.2 at 800° C. in air.

10. Air electrode according to claim 9, wherein the air electrode has an open porosity between 30 and 70%, the percentage being expressed as a volume relative to the total volume of the air electrode.

11. Electrochemical half-cell comprising an air electrode based on Pr.sub.2-xNiO.sub.4 with 0≦x<2, obtained by sintering a ceramic ink containing Pr.sub.2-xNiO.sub.4 and a pore-forming agent at a temperature above 1000° C. and below or equal to 1150° C., wherein, before the sintering step, said ceramic ink is brought to the sintering temperature following a temperature rise gradient of between 50 and 400° C./h, and wherein the air electrode has a polarisation resistance below 0.06 Ω.Math.cm.sup.2 at 800° C. in air.

12. The electrochemical half-cell according to claim 11, wherein the air electrode has an open porosity between 30 and 70%, the percentage being expressed as a volume relative to the total volume of the air electrode.

13. The electrochemical half-cell according to claim 11, comprising: i) a dense electrolyte; ii) a barrier layer made from stabilised zirconia and/or from doped ceria, and deposited on said electrolyte; and iii) the air electrode deposited on said barrier layer.

14. The electrochemical half-cell according to claim 13, wherein the air electrode has an open porosity between 30 and 70%, the percentage being expressed as a volume relative to the total volume of the air electrode.

15. Electrochemical cell comprising an electrochemical half-cell as defined in claim 14 and at least one hydrogen or water electrode and optionally one or more barrier layer(s) separating said hydrogen or water electrode from the electrolyte.

16. The electrochemical half-cell according to claim 13, wherein the dense electrolyte is a dense Yttrium Stabilized Zirconia (YSZ) and the barrier layer is based on Gadolinia Doped Ceria (GDC) or Yttria Doped Ceria (YDC).

17. Electrochemical cell comprising an electrochemical half-cell as defined in claim 13 and at least one hydrogen or water electrode and optionally one or more barrier layer(s) separating said hydrogen or water electrode from the electrolyte.

18. Method for preparing an electrochemical half-cell comprising an air electrode based on Pr.sub.2-xNiO.sub.4 with 0≦x<2, wherein the air electrode has a polarisation resistance below 0.06 Ω.Math.cm.sup.2 at 800° C. in air, comprising: a) depositing a precursor of a barrier layer on a dense electrolyte; b) depositing a ceramic ink comprising Pr.sub.2-xNiO.sub.4 with 0≦x<2 and a pore-forming agent, on said precursor; and then c) applying to the assembly a sintering step at a temperature above 1000° C. and below or equal to 1150° C., wherein, before the sintering step, said ceramic ink is brought to the sintering temperature following a temperature rise gradient of between 50 and 400° C./h.

19. Method according to claim 18, wherein the air electrode has an open porosity between 30 and 70%, the percentage being expressed as a volume relative to the total volume of the air electrode.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows micrographs in Scanning Electron Microscopy (SEM) in secondary electrons mode of the fracture surface of an electrochemical half-cell with an optimised air electrode based on Pr.sub.2NiO.sub.4. FIG. 1A is a general view of the half-cell, showing from bottom to top the electrolyte, the barrier layer and the air electrode according to the invention. FIG. 1B is a more precise view, centred on the upper part of the electrolyte (8YSZ), the barrier layer (GDC) and the air electrode according to the invention (Pr.sub.2NiO.sub.4).

(2) FIG. 2 shows an SEM micrograph in secondary electrons mode of the surface of an optimised air electrode based on Pr.sub.2NiO.sub.4.

(3) FIG. 3 is a Nyquist representation of the complex impedance spectrum obtained at 800° C. in air from the optimised symmetric cell based on Pr.sub.2NiO.sub.4.

DETAILED PRESENTATION OF PARTICULAR EMBODIMENTS

(4) I. Preparation Method According to the Invention.

(5) I.1. Initial Compounds.

(6) This method is based on: an initial Pr.sub.2NiO.sub.4 powder with a specific surface area of ˜4.4 m.sup.2/g and a particle size distribution with the following characteristics: d.sub.50 ˜0.46 μm and d.sub.90 ˜0.97 μm. This oxide is used to make the functional layer of the electrode that also acts as a collecting layer; a so-called <<barrier>> compound based on Gadolinia Doped Ceria (GDC) used as a barrier layer, in the form of a deposit, to limit the chemical reactivity between the compound in the functional layer and the material forming the electrolyte. This powder has a particle size distribution such that d.sub.50 is ˜0.5 μm and d.sub.90 is ˜1.30 μm. Its specific surface area is equal to 12.2 m.sup.2/g. acetylene black or acetylene carbon that is used as a pore-forming agent and has a centred particle size distribution such that d.sub.50 is ˜40 nm for a specific surface area of 62 m.sup.2/g.

(7) I.2. Preparation of Silk-Screen Printing Inks.

(8) The electrode is formed from these compounds, using the screen printing deposition technique. Therefore, the following step is to produce two screen printing inks.

(9) A first ink is made using the GDC powder compound that is added at 50% by mass to a solvent/binder mixture itself composed of 95% by mass of terpineol and 5% by mass of ethylcellulose.

(10) A second ink is made from a mix composed of 97.5% by mass of Pr.sub.2NiO.sub.4 and 2.5% by mass of acetylene carbon for the solid part that is added at 50% by mass to a solvent/binder mixture itself composed of 95% by mass of terpineol and 5% by mass of ethylcellulose.

(11) Once these mixtures have been prepared, they are homogenised and deaerated if necessary.

(12) I.3. Making Screen Printing Deposits.

(13) The electrode is then made on the surface of the electrolyte by deposition using a screen printing machine.

(14) A first deposit about ˜5 μm thick is made on the surface of the electrolyte (8YSZ) using GDC ink. This deposit is then dried in a drying oven with temperature increased to 140° C. for 15 min.

(15) A second deposit is made using ink containing Pr.sub.2NiO.sub.4 and the pore-forming agent, about ˜15 μm thick on the previous deposit once the previous deposit is dry.

(16) When this final deposit has been made, a new drying step at 140° C. for 15 min is necessary in a drying oven before the partial sintering treatment.

(17) I.4. Sintering the Electrode.

(18) A final heat treatment is necessary to eliminate the pore-forming agent and to partially sinter the electrode. The purpose of partial sintering is to create sintering bridges or necks between Pr.sub.2NiO.sub.4 particles while maintaining sufficient porosity to allow free gas circulation.

(19) The optimum treatment that consists of a 3 h plateau at 1100° C. is as follows:
25° C.-150° C./h.fwdarw.1100° C./3 h-150° C./h.fwdarw.25° C.

(20) The electrode based on Pr.sub.2NiO.sub.4 is ready for use at the end of this heat treatment.

(21) II. Microstructural Characterisations of the Electrode.

(22) The heat treated electrode has a homogenous microstructure as shown on the SEM micrographs in FIG. 1.

(23) On these micrographs, a distinction can be made between the GDC layer at the interface with the electrolyte and the Pr.sub.2NiO.sub.4 layer above it (FIG. 1B).

(24) The thickness of the GDC layer is about 3 μm while the thickness of the Pr.sub.2NiO.sub.4 layer is about 8 μm. These two layers are porous and due to partial sintering, the particles forming them have approximately maintained their initial size.

(25) The open porosity of the Pr.sub.2NiO.sub.4 layer represents ˜50% of its volume.

(26) This microstructural homogeneity of the porosity and particle size can also be seen by observation of the electrode surface by SEM (FIG. 2).

(27) III. Electrochemical Characterisations of the Electrode.

(28) Symmetric electrode//electrolyte//electrode cells have been prepared based on these optimisations, and tested by complex impedance spectroscopy in air at 800° C.

(29) The results obtained are shown in the spectrum in FIG. 3. Using this spectrum, a polarisation resistance of ˜0.013 Ω.Math.cm.sup.2 at 800° C. in air can be determined, which is a value ˜4.6 times below the value given in the literature [2]. This result demonstrates the importance of the microstructural optimisation defined herein. Values of polarisation resistance in air were measured at different temperatures and are summarised in Table 1.

(30) TABLE-US-00001 TABLE 1 Values of polarisation resistance (Rp) for an optimised electrode based on Pr.sub.2NiO.sub.4 at different temperatures. T (° C.) Rp (Ω .Math. cm.sup.2) 1000/T (K) 700 0.037 1.02775 750 0.022 0.97752 800 0.013 0.93197

REFERENCES

(31) [1] Skinner and Kilner, 2000. <<Oxygen diffusion and surface exchange in La.sub.2-xSr.sub.xNiO.sub.4+d>>, Solid State Ionics, vol. 135, pages 709-712. [2] Mawdsley and al., 2007, Electrode Materials Development for High Temperature Steam Electrolysis, Abstract n° 334, 211.sup.th ECS Meeting transaction. [3] Kim and al., 2001. <<Characterization of LSM-YSZ composite electrode by ac impedance spectroscopy>>, Solid State Ionics, vol. 143, pages 379-389. [4] Taillades and al., 2010. <<Intermediate temperature anode-supported fuel cell based on BaCe.sub.0.9Y.sub.0.1O.sub.3 electrolyte with novel Pr.sub.2NiO.sub.4 cathode>>, Fuel Cells, vol. 1. pages 166-173. [5] Meng et al., 2008, <<Characterization of Pr.sub.1-xSr.sub.xCo.sub.0.8Fe.sub.0.2O.sub.3-δ (0.2≦x≦0.6) cathode materials for intermediate temperature solid oxide fuel cells>>, Journal of Power Sources, vol. 183, pages 581-585.