Electrochemical device comprising a proton-conducting ceramic electrolyte

Abstract

The invention relates to the use of a ceramic of formula Ba.sub.2(1x)M.sub.2xIn.sub.2(1y)M.sub.2yO.sub.4+(OH).sub. where M represents at least one metal cation with an oxidation number II or III or a combination thereof, M represents at least one metal cation with an oxidation number III, IV, V or VI or a combination thereof, 0x1, 0y1, 2 and 0 <2, as solid proton-conducting electrolyte in an electrochemical device, in particular a fuel cell, an electrolytic cell, a membrane separating hydrogen from a gas mixture, or also a hydrogen detector, at an operating temperature of said electrochemical device preferably comprised between 200 C. and 600 C.

Claims

1. A process for producing an electric current comprising the steps of providing a fuel cell comprising an anode compartment with an anode, and a cathode compartment with a cathode, the two compartments being separated by a proton- conducting ceramic electrolyte of formula
Ba.sub.2In.sub.2(1y)M.sub.2yO.sub.4+(OH).sub. where M represents at least one metal cation with an oxidation number III, IV, V or VI or a combination thereof, 0y1,2 and 0<2, and an electrical circuit connecting the anode to the cathode, feeding the anode compartment with hydrogen or with a gas mixture containing hydrogen, and feeding the cathode compartment with oxygen or air, wherein the fuel cell is operated at a temperature of at least 200 C.

2. The process as claimed in claim 1 wherein said ceramic electrolyte has a proton conductivity, measured at 400 C., greater than 10.sup.3 S/cm.

3. The process as claimed in claim 1, wherein M represents a cation of a metal selected from the group consisting of Ga, Sc, Y, Ti, Zr, Hf, Nb, Ta, W, Mo and the elements of the lanthanide series.

4. The process as claimed in claim 3, wherein M represents Ti(IV).

5. The process as claimed in claim 1, wherein 0y0.7.

6. The process as claimed in claim 1, wherein 0y0.3.

7. The process as claimed in claim 1, wherein the ceramic is impermeable to gases.

8. The process as claimed in claim 1, wherein the ceramic is a fritted ceramic material.

9. The process as claimed in claim 8, wherein the ceramic material has a closed porosity and a level of compactness greater than 95%.

10. The process as claimed in claim 9, wherein the closed porosity and the level of compactness greater than 95% can be obtained by fritting at a temperature lower than or equal to 1400 C.

11. The process as claimed in claim 1, wherein the fuel cell is operated at a temperature of between 300 C. and 500 C.

Description

DETAILED DESCRIPTION OF THE DISCLOSURE

(1) FIG. 1 is a schematic view of a fuel cell;

(2) FIG. 2 is a schematic view of an electrolytic cell according to the present disclosure;

(3) FIG. 3 is a schematic view of a selective hydrogen purification membrane according to the present disclosure;

(4) FIG. 4 is schematic view of another hydrogen purification membrane according to the present disclosure;

(5) FIG. 5 is schematic view of a hydrogen detector using a proton-conducting ceramic accordingly to the present disclosure; and

(6) FIG. 6 is a graph showing the conductivity of a compound under humidified air in relation to temperature.

(7) The electrochemical device of the present invention may also be a selective hydrogen purification membrane, one embodiment of which is represented in FIG. 3. Such a membrane comprises a positive electrode (8) and a negative electrode (9) separated by a proton-conducting ceramic (1) of the present invention. The hydrogen contained in a gas mixture (H.sub.2, N.sub.2, CH.sub.4, CO) decomposes into protons and electrons on contact with the positive electrode (8). While the protons formed migrate under the influence of the electric potential through the proton-conducting ceramic (1) to the negative electrode (9), the electrons enter the electrical circuit (10). At the negative electrode, the protons and electrons recombine to form pure hydrogen.

(8) Another embodiment of a hydrogen purification membrane is represented in FIG. 4. Such a membrane must contain, in addition to the proton-conducting ceramic of formula (I), at least one electron-conducting material, such as a metal. The proton-conducting material and the electron-conducting material are generally mixed in the form of powders and compacted and fritted jointly so as to form a percolating mixture, i.e. a mixture allowing the electrons and protons to percolate through a continuous network of the electron- or proton-conducting material. The operating principle of such a membrane is the following: one side of the membrane is brought into contact with a pressurized gas mixture containing hydrogen to be purified.

(9) On contact with the membrane, the hydrogen decomposes into protons and electrons. The former pass through the membrane via the proton-conducting material of formula (I) while the latter are led by the electron-conducting material. As the membrane is also impermeable to the other gases of the mixture, the recombining of the protons and electrons leads to the formation of pure hydrogen on the other side of the membrane.

(10) The only motive power of such a membrane is the partial pressure difference of hydrogen on either side of the membrane. The higher this is, the more effective the membrane is.

(11) FIG. 5 represents a hydrogen detector using a proton-conducting ceramic of the present invention. This detector operates essentially according to the same principle as the separation membrane represented in FIG. 3, except that the whole of the surface of the positive electrode (8) is covered by a perforated lid (11). When this lid is brought into contact with a gas mixture containing hydrogen, some of this mixture passes through the opening (12) of the lid (11) and diffuses towards the positive electrode (8). The decomposition of the hydrogen on contact with the positive electrode (8) gives rise to a current, the intensity of which, measured by an ammeter (13) in the electrical circuit (10), is directly proportional to the concentration of H.sub.2 in the gas mixture.

EXAMPLE

(12) Preparation of a Proton-Conducting Ceramic

(13) Barium carbonate BaCO.sub.3, titanium oxide TiO.sub.2 and indium oxide In.sub.2O.sub.3 are mixed in the suitable proportions to obtain a material of formula (I) where M=Ti, x=0 and y=0.2. The powders are placed in a mortar, then mixed while grinding with acetone. After evaporation of the acetone, the mixture of powders is placed in a platinum crucible and heated at a rate of 400 C./h to a temperature of 1200 C., then kept at this temperature for 24 hours. The material is then cooled to room temperature at exactly the same rate at which it was heated, then the product obtained is ground using a mortar so as to obtain a fine powder. This powder is then compacted using a uniaxial press and pressed into pellets. The pellets then undergo a thermal treatment under air atmosphere at 1350 C. for 24 hours (rate of heating and cooling of 140 C./h). This first stage leads to a partially hydrated pure material corresponding to the formula Ba.sub.2In.sub.1,6Ti.sub.0,4O.sub.5,2-/2(OH).sub. (<0.8). This material is then raised to a temperature of approximately 200 C. under a humidified air atmosphere (P.sub.H2O 3%) and kept in these conditions for one week. This hot hydration leads to a material of formula Ba.sub.2In.sub.1,6Ti.sub.0,4O.sub.4,4(OH).sub.1,6.

(14) When this material is heated in a humidified CO.sub.2 atmosphere (P.sub.H2O 3%) a chemical stability vis--vis carbon dioxide up to a temperature of approximately 550 C. is observed. Above this temperature, the material reacts with the carbon dioxide at a rate proportional to the temperature.

(15) The powder obtained at the end of the first stage is then ground for 2 hours using a planetary mill (0.5 g powder in ethanol, 500 r.p.m., 3 beads per jar) then compacted using a uniaxial press. The tablet is subjected to a thermal treatment under an air atmosphere at 1350 C. for 24 hours (rate of heating and cooling 140 C./h). Scanning electron microscopy of the sample obtained reveals a closed porosity lower than 5%. The dense sample, subjected to a thermal cycling between 30 C. and 800 C. under humid atmosphere, in other words to a succession of alternating between hydration and dehydration, does not show any sign of cracking or breaking.

(16) The electrical characterization of the sample was obtained by complex impedance spectroscopy under a controlled oxygen or steam atmosphere. FIG. 6 shows the conductivity of the compound Ba.sub.2In.sub.1,6Ti.sub.0,4O.sub.5,2-/2(OH).sub., under humidified air (.square-solid.) at 3% in relation to the temperature. The conductivity curve of the non-protonated compound Ba.sub.2In.sub.1,6Ti.sub.0,4O.sub.5,2 under dry air is also given, for information only. The conductivity of the hydrated material reaches a value of 2.10.sup.3 S.cm.sup.1 at approximately 400 C., at which temperature is approximately equal to 0.25. This figure illustrates the optimum operating range of the proton-conducting material of the present invention. In fact, at temperatures lower than 200 C., the material is strongly hydrated but the limited mobility of the protons is reflected in insufficient conductivities. Above approximately 550 C. to 600 C., the material is almost completely dehydrated and the conductivity curve determined under humid atmosphere overlaps that determined under dry atmosphere. The conductivity is thus essentially anionic (O.sub.2.sup.).