Patent Application
20090075138
The present invention relates to an electrochemical device and to a process for manufacturing an electrochemical device.
In particular, the present invention relates to an electrochemical device, more in particular to a solid state electrochemical device, comprising at least one porous supporting electrode, at least one thin electrolyte membrane having a high relative density, and at least one porous counter-electrode.
Furthermore, the present invention also relates to a process for manufacturing an electrochemical device.
Solid state electrochemical devices are often implemented as cells including two porous electrodes, the anode and the cathode, and a dense solid electrolyte membrane which separate the electrodes.
In many implementations such as, for example, in fuel cells and oxygen and syn gas generators, the solid electrolyte membrane comprises a material capable of conducting ionic species such as, for example, oxygen ions, or hydrogen ions, said material having a very low, or even absent, electronic conductivity. In other implementations such as, for example, gas separation devices, the solid electrolyte membrane comprises a mixed ionic electronic conducting material (“MIEC”) . In each case, the solid electrolite membrane must be dense and pinhole free (“gas-tight”) to prevent mixing of the electrochemical reactants.
Solid state electrochemical devices are becoming increasingly important for a variety of applications including energy generation, oxygen separation, hydrogen separation, coal gasification, selective oxidation of hydrocarbons. These devices are typically based on electrochemical cells with ceramic electrodes and electrolyte membranes and have two basic design: tubular and planar. Usually, said electrochemical devices operate at high temperatures, tipically in excess of 900° C. However, such high temperature operation has significant drawbacks with regard to the devices maintainance and the materials available for incorporation into a device, in particular, in the oxidizing environment of an oxygen electrode, for example.
Some recent attempts have been made to develop solid state electrochemical devices which efficiently operate at lower temperature.
For example, U.S. Pat. No. 6,921,557 relates to a process for making a composite article comprising:
Suitable material for said porous substrate are cermets (ceramic and metallic composite materials) such as, for example, lantanium strontium manganese oxide (LSM) incorporating one or more transition metals such as, for example, chromium, iron, copper and silver, or alloys thereof); metals (such as, for example, chromium, silver, copper, iron, nickel); or metal alloys (such as, for example, low-chromium ferritic steel, high-chromium ferritic steel, chrome-containing nickel-based Inconel alloys including Inconel 600). Suitable material for said coating is yttria stabilized zirconia (YSZ). The abovementioned composite article may be incorporated in solid state electrochemical devices. Said solid state electrochemical devices are said to work in a wide range of operating temperatures, in particular of from about 400° C. to about 1000° C.
U.S. Pat. No. 6,605,316 relates to a method of forming a ceramic coating on a solid state electrochemical device substrate, comprising:
Suitable material for the solid state electrochemical device substrate is a porous cermet composed of 50 vol % Al2O3 (e.g., AKP-30) and 50 vol % Inconel 600 with a small amount of binder (e.g., XUS 40303). Suitable material for said coating is yttria stabilized zirconia (YSZ). The abovementioned composite article may be incorporated in solid state electrochemical devices. Said solid state electrochemical devices are said to work in a wide range of operating temperatures, in particular of from about 400° C. to about 1000° C.
International Patent Application WO 2004/106590 in the name of the Applicant, relates to an electrochemical oxygen separator cell including:
The abovementioned electrochemical oxygen separator cell is said to yields surprisingly high performances also in the presence of a cell architecture wherein the supporting element is one of the electrode, thus having a thickness greater than that of the electrolyte membrane (for example, a current density of 3 A/cm2, at 800° C. and at 0.8 V dc operating voltage, is disclosed).
As known in the art, a measure of electrochemical devices performance may the voltage output from said electrochemical devices for a given current density. Higher performance is associated with a higher voltage output for a given current density or higher current density for a given voltage output. Another measure of electrochemical devices performance may be the Faradaic efficiency, which is the ratio of the actual output current to the total current associated with the consumption of fuel in the electrochemical devices. For various reasons, fuel can be consumed in electrochemical devices without generating an output current, such as when an oxygen bleed is used in the fuel stream (for removing carbon monoxide impurity) or when fuel crosses through a membrane electrolyte and reacts on the cathode instead of the anode. A higher Faradaic efficiency thus represents a more efficient use of fuel.
The Applicant has faced the problem of providing an electrochemical device able to operate in a wide range of operating temperature, in particular at relatively low temperatures (i.e., at temperature of from 600° C. to 800° C.) and having improved performances, in particular in term of current density and/or of Faradaic efficiency.
The Applicant has now found that by using an electrochemical device having a specific cell architecture, a porous supporting electrode with a specific composition as better defined hereinbelow, and a thin electrolyte membrane having a high relative density, it is possible to obtain said improved performances, in particular in term of current density. Moreover, said improved performances are maintained in a wide range of operating temperature, in particular at realtively low temperatures (i.e., at temperature of from 600° C. to 800° C.). Furthermore, an improved Faradaic efficiency is also obtained.
According to a first aspect, the present invention relates to an electrochemical device comprising:
For the purpose of the present description and of the claims which follows the relative density has to be intended as the value obtained as follows: experimental density/theoretical density. Said experimental density may be measured according to techniques known in the art such as, for example, by means of Scanning Electron Microscopy (SEM).
For the purpose of the present description and of the claims which follow, except where otherwise indicated, all numbers expressing amounts, quantities, percentages, and so forth, are to be understood as being modified in all instances by the term “about”. Also, all ranges include any combination of the maximum and minimum points disclosed and include any intermediate ranges therein, which may or may not be specifically enumerated herein.
According to one preferred embodiment, said porous supporting electrode has a porosity higher than or equal to 10%, preferably of from 20% to 50%. Said porosity may be measured according to techniques known in the art such as, for example, by means of Scanning Electron Microscopy (SEM), or of Hg-porosimetry.
According to one preferred embodiment, said electrochemical device may be used as:
According to a more preferred embodiment, said electrochemical device may be used as an electrochemical oxygen separator cell.
According to one preferred embodiment, said porous supporting electrode may be either the anode or the cathode.
According to a further preferred embodiment, in the case said electrochemical device is used as an electrochemical oxygen separator cell, said porous supporting electrode is the anode.
According to a further preferred embodiment, in the case said electrochemical device is used as solid oxide fuel cell (SOFC), said porous supporting electrode is the cathode.
According to one preferred embodiment, said porous supporting electrode comprises:
According to one preferred embodiment, said electronically conducting material may be selected, for example, from conductive metal alloys including conductive metal oxides such as the rare earth perovskites having the following general formula (I):
A1-aA′aB1-bB′bO3-δ (I)
wherein:
Specific examples of rare earth perovskites having general formula (I) which may be advantageously used according to the present invention are: La1-aSraMnO3-δ(LSM) wherein 0≦a≦0.5; Pr1-aSraMnO3-δ(PSM) wherein 0≦a≦0.6; Pr1-aSraCoO3-δ wherein 0≦a≦0.5; La1-aSraCo1-bFebO3-δ (LSCFO) wherein 0≦a≦0.4 and 0≦b≦0.8; La1-aSraCo1-bNibO3-δ wherein 0≦a≦0.6 and 0≦b≦0.4; La1-aSraCrO3-δ wherein 0≦a≦0.5; La1-aCaaCrO3-δ wherein 0≦a≦0.5.
According to one preferred embodiment, the rare earth perovskites having general formula (I) may be selected, for example, from: La0.8Sr0.2MnO3 (hereinafter referred to as LSMO-80), La0.6Sr0.4Co0.2Fe0.8O3 (hereinafter referred to as LSCFO-80), or mixtures thereof. LSCFO-80 is particularly preferred.
According to one preferred embodiment, said ionically conducting material may be selected, for example, from: gadolinium-doped ceria (CGO), samarium-doped ceria (SDC), mixed lanthanum and gallium oxides, or mixtures thereof. Gadolinium-doped ceria (CGO) is particularly preferred. Ce0.8Gd0.2O190 (hereinafter referred to as CGO-20) is still particularly preferred.
According to one preferred embodiment, said electrolyte membrane comprises an ionically conducting material having a ionic conductivity, at 800° C., not lower than or equal to 0.005 S/cm−1, preferably of from 0.01 S/cm−1 to 0.1 S/cm−1.
According to a further preferred embodiment, said ionically conducting material may be selected, for example, from: gadolinium-doped ceria (CGO), samarium-doped ceria (SDC), mixed lanthanum and gallium oxides, or mixtures thereof. Gadolinium-doped ceria (CGO) is particularly preferred. Ceo0.8Gd0.2O1.90 (hereinafter referred to as CGO-20) is still particularly preferred.
According to one preferred embodiment, said counter-electrode has a porosity higher than or equal to 10%, preferably of from 20% to 50%. Said porosity may be measured by techniques known in the art such as, for example, by means of Scanning Electron Microscopy (SEM), or of Hg-porosimetry.
According to a further preferred embodiment, said counter-electrode has a thickness lower than or equal to 100 μm, preferably of from 10 μm to 50 μm.
The composition of the counter-electrode will be different depending on the use of the electrochemical device.
As already reported above, in the case of said electrochemical device is used as an electrochemical oxygen separator cell, said counter-electrode is the cathode. Said cathode may comprise at least one electronically conducting material and, optionally, at least one ionically conducting material, said ionically conducting material preferably having a ionic conductivity, at 800° C., not lower than or equal to 0.005 S/cm−1, preferably of from 0.01 S/cm−1 to 0.1 S/cm−1. Preferably, said cathode comprises at least one electronically conducting material. Both, said electronically conducting material and said ionically conducting material, may be selected from those above reported.
Examples of counter-electrodes which may be advantageously used in the case of said electrochemical device is used as an electrochemical oxygen separator cell may be found, for example, in International Patent Application WO 2004/106590 above disclosed.
On the contrary, as already reported above, in the case of said electrochemical device is used as solid oxide fuel cell (SOFC), said counter-electrode is the anode. Preferably, said anode comprises nickel (Ni) cermets (ceramic and metallic composite materials) . More preferably, said anode comprises a ceramic material and an alloy comprising nickel and at least a second metal selected from: aluminum, titanium, molybdenum, cobalt, iron, chromium, copper, silicon, tungsten, niobium, said alloy having, preferably an average particle size not higher than 20 nm. The ceramic material of said anode may be selected from gadolinium-doped ceria (GCO), samarium-doped ceria (SDC), mixed lanthanum and gallium oxide.
Examples of counter-electrodes which may be advantageously used the case of said electrochemical device is used as solid oxide fuel cell (SOFC) may be found, for example, in International Patent Application WO 2004/038844 in the name of the Applicant.
As already reported above, the electrochemical device according to the present invention, is able to operate in a wide range of operating temperature, in particular at realtively low temperatures (i.e., at temperature of from 600° C. to 800° C.). In particular, the electrochemical device according to the present invention, provides a current density of 1 A/cm2, at 800° C. and at 0.025 V dc operating voltage.
In a further aspect the present invention relates to a process for manufacturing an electrochemical device, said process comprising the following steps:
Preferably, said step (d) is carried out by means of infrared rays.
The terms “green supporting electrode”, “green bilayered structure” (i.e., green supporting electrode+green elecrolyte membrane),. “green counter-electrode”, indicate that the materials from which they are made have not yet been fired to a temperature sufficiently high to sinter said materials. As it is known in the art, sintering refers to a process of forming a coherent mass, for example from a metallic powder, by heating without melting.
The powder comprising at least one electronically conducting material and at least one ionically conducting material of step (a) may be made by processes known in the art. For example, said powder may be made by a process comprising the following steps:
The homogeneous suspension of at least one lonically conducting material of step (c) of the above disclosed process, may be made by processes known in the art. For example, said homogeneous suspension of at least one ionically conducting material may be made by:
The counter-electrode of step (g) may be made according to processes known in the art. For example, said counter-electrode may be made by means of the process disclosed in International Patent Applications WO 2004/038844, or WO 2004/106590 above disclosed. Alternatively, said counter-electrode may be made according to the process for making a porous supporting electrode disclosed above.
The step (g) of the process above reported may be carried out according to techniques known in the art such as, for example, by spraying. Further details regarding said techniques may be found, for example, in International Patent Applications WO 2004/038844, or WO 2004/106590 above disclosed.
The present invention will now be illustrated in further detail by means of the attached
The present invention will be further illustrated below by means of a number of preparation examples, which are given for purely indicative purposes and without any limitation of this invention.
An electrochemical oxygen separator cell having the following architecture and composition was prepared and tested.
Ball milling, for 1 hour, at room temperature (23° C.), 3.0 g of CGO-20 (primary particle size of 28 nm, BET surface area of 7.84 m2/g; from Praxair), 7 g of LSCFO-80 (primary particle size of 9 nm, BET surface area of 4.12 m2/g; from Praxair), 1 g of polyvinyl alcohol (molecular weight range: 13000-23000) previously dissolved in 20 ml water at 60° C. as a binding agent and 1 g of carbon (Timrex KS4, BET surface area of 25 m2/g; from Timcal) as a pore-former. The obtained slurry was dried at 90° C., by infrared rays, for 3 hours. 30 ml of ethanol was then added to the dried slurry which was subsequently ball milled, for 14 hours, at room temperature (23° C.), and then was dried at 90° C., by infrared rays, for 3 hours. Then, the dried slurry was grinded in a agata mortar obtaining a powder. The obtained powder was subsequently placed in a pressing die having a cylindrical shape (φ=16 mm, d=1 mm) and was subjected to a uniaxial pressure of 2 MPa, at a temperature of 10° C., for 5 min, obtaining a green supporting anode.
CGO-20 powder (10 g) was mixed with ethanol (20 ml) in a ball mill, at room temperature (23° C.), for 14 hours, to give a slurry. Said slurry was dried at 90° C., by infrared rays, for 3 hours and was subsequently placed in an ultrasonic bath, for 15 minutes obtaining a homogeneous suspension. The resulting suspension was sprayed by an aerograph device onto the green supporting anode obtained as disclosed above, operating at the following conditions:
A green bilayered structure (green supporting anode+green electrolyte membrane) was obtained which was subsequently dried at 90° C., by infrared rays, for 3 hours and was then subjected to an uniaxial pressure of 200 MPa, at room temperature (23° C.), for 20 min.
Subsequently, the green bilayered structure was removed from the pressing die and was fired, to burn out the pore-former and the binding agent and to sinter the structure, according to the following conditions: heating at 1° C./min to 350° C., held 2 hours, heating at 1° C./min to 1150° C., held 6 hours, cooling at 2° C./min to 25° C.: a sintered bilayered structure (sintered supporting anode+sintered electrolyte membrane) was obtained.
CGO-20 powder (10 g) was mixed with ethanol (20 ml) in a ball mill, at room temperature (23° C.), for 14 hours to give a slurry. Said slurry was dried at 90° C., by infrared rays, for 3 hours and subsequently placed in an ultrasonic bath, for 15 minutes obtaining a homogeneous suspension. The resulting suspension was sprayed by an aerograph device onto the sintered bilayered structure obtained as above disclosed, operating at the following conditions:
A trilayered structure (sintered supporting anode+sintered electrolyte membrane+green cathode) was sintered operating at the following conditions: heating at 15° C./min to 950° C., held 2 hours, cooling at 5° C./min to room temperature (23° C.) obtaining the desired electrochemical oxygen separator cell.
The characteristics of the electrochemical oxygen separator cell obtained as disclosed above, were the following:
The polarization measurement was carried out by potentiometric measurement [by applying a voltage (V) and measuring the current density (A/cm2)] by means of an electrochemical oxygen separator cell according to the schematic drawing of
The measurement was carried out by an AUTOLAB Ecochemie potentiostat/galvanostat and impedance analyzer, at 800° C., by fluxing He (20 cc/min) at the anode side and maintaining static air at the cathode side. The results are set forth in
Moreover, a Faradaic Efficiency was measured. To this aim, the expected flux of oxygen produced by an electrochemical oxygen separator cell according to the schematic drawing of
wherein I is the electrical current (A), t is time (sec), F is the Faraday constant (i.e. 96485.3 C/eq) and 4 is the number of electrons exchanged in the electrochemical reaction: 20−2→4e−+O2, eq/mol.
On the other end, the real flux of oxygen produced by said electrochemical oxygen separator cell was measured, at the anode side, by fluxing He (20 cc/min) and by recovering the oxygen produced which was further analyzed by a gas cromatography. The Faradaic efficiency was of 98±2%.
An electrochemical oxygen separator cell having the architecture and composition as disclosed in Example 1 was prepared and tested, the only difference being in the anode preparation: the pore-former was not used.
The characteristics of the electrochemical oxygen separator cell obtained as disclosed above, were the following:
The polarization measurement and the Faradaic efficiency measurement was carried out as described in Example 1.
The results of polarization measurement are set forth in
The Faradaic efficiency was of 98±2%.
An electrochemical oxygen separator cell having the architecture and composition as disclosed in Example 1 was prepared and tested. The differences in the preparation were the following:
The characteristics of the electrochemical oxygen separator cell obtained as disclosed above, were the following:
The polarization measurement and the Faradaic efficiency measurement was carried out as described in Example 1.
The results of polarization measurement are set forth in
The Faradaic efficiency was of 45±2%.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2006/002340 | 3/14/2006 | WO | 00 | 9/30/2008 |
