Patent Application
20110183221
The present invention relates to the field of catalysis and electrocatalysis applied to energy production, gas separation and processing of basic chemicals, specifically, the invention focuses on the field of high temperature fuel cells and membranes for oxygen separation at a high temperature.
Fuel cells are electrochemical devices that make it possible to obtain electricity directly from chemical energy contained in fuels such as hydrogen or hydrocarbons, in contrast to thermal processes of energy production from fossil fuels. Fuel cells enable a highly efficient use of fuel chemical energy, it being possible to reach up to 70% efficiency, which, is an improvement of up to 200% compared to current combustion-based technologies. This increased use of energy resources can consequently reduce CO2 emissions per kWh produced, this aspect being increasingly more significant from a socio-economic standpoint. In this sense, the use of fuel cells can reduce the environmental impact exerted by energy generation processes, it being possible to virtually eliminate emissions of SOx and NOx while reducing CO2 production depends on the type of fuel used. Moreover, another major advantage of fuel cells is their silent operation (no moving parts), which allows the reduction of noise pollution in urban or independent generator sets.
Among the different concepts of fuel cells, the cells based on solid oxide (SOFC) had the lowest levels of electrochemical and mechanical degradation, since the electrolyte is a stable solid element (an ion conductor). Equally important is the fact that these cells achieve best efficiency, and because of the operating conditions, it is possible to integrate them with microturbines in high-performance cogeneration systems. These cells have other advantageous aspects compared to other battery designs, such as reduced degradation over time of operation, the possible use of different fuels such as hydrogen, ethanol or natural gas, and tolerance of gases such as CO or NOx. In view of the operating temperatures (600° C.-1,000° C.), use is restricted to large vehicles and stationary energy production, reaching typical power values ranging from 5 kW to 10 MW.
In the past two years, the technology required for mass production and operation for long periods has reached a significant level of maturity. However, the high cost of energy production systems means that they are only launched onto the market for specific applications where price is not the limiting factor. Thus, efforts in research and development of this technology have focused on (a) the reduction of costs of materials and manufacturing processes of these devices and (b) the reduction of the operating temperature, so as to reduce prices of the construction materials, maintenance costs and the deterioration rate. These objectives would increase the efficiency of fuel cells, i.e. the power density per cell area (kW/m2), which would in turn reduce the price and size of the module, or alternatively lower the final cost of kilowatts per hour.
The main electrochemical limitation on the efficiency of solid oxide fuel cells can be found in the phenomena occurring at the cathode, where hydrogen is used as fuel. Thus, the total power of the cell can be substantially increased if such physical and chemical processes are improved. The presently used porous cathodes are composed of electrical conductor ceramic materials and, in some cases, oxygen conductors in solid state, thus materials preferred for their stability and catalytic activity are mixed oxides with a perovskite structure and derived structures, such as La0.6Sr0.4Fe0.8Co0.2O3-δ -LSFC-, and materials composed of mixtures of such perovskite particles and particles of the material composing the electrolyte. The processes that occur at the cathode during operation of the electrochemical cell are (S. B. Adler, Chemical Reviews 104 (2004) 4791-4843): (a) adsorption of O2 on the surface of the particles, (b) dissociation of O2, (c) (catalytic) reduction of atomic oxygen with two electrons to give O2−, (d) transport of anion O2− to the electrolyte through the internal crystal lattice of the particles and (e) jump of the O2− anion to the solid electrolyte. Consequently, the materials making up the cathode must have high electrical and ionic conductivity, good oxygen adsorption capacity, catalytic activity for the dissociation and reduction of O2, and compatibility and good adhesion with the electrolyte. Of the various methods to improve the overall activity of the cathode, the following should be highlighted: (a) improving the conductivity and catalytic activity by studying different crystal structures and the elemental composition of glass, (b) modifying or activating the surface of crystalline material particles (perovskite), and (c) improving the specific area of the cathode available for electrochemical processes, reducing both ohmic and polarization resistances.
Another interesting additional application of catalytic materials with oxygen and electron mixed conduction properties is their use as catalysts for oxygen activation in dense membranes for oxygen separation. These membranes allow the separation of oxygen from air at high temperatures. Its application in thermal power stations (oxy-fuel process) would enable natural gas (or coal) combustion with pure oxygen, obtaining only CO2 and water as products. Given the absence of N2 in combustion gas, CO2 separation is very simple, it being possible to later liquefy and store it. This concept of thermal power station would minimize CO2 emissions, as stipulated in the Kyoto protocol. However, the separation efficiency of current membranes cannot reach the levels required for industrial implementation (FO2>10 ml/min). The main part of the oxygen membrane is a gas-impermeable film and preferably with a thickness under 50 μm. This film is composed of a mixed conductive material of electrons and oxygen ions in the solid state. Materials commonly used for said application are perovskites with compositions such as La0.68Sr0.3Fe0.8Co0.2O3-L or Ba0.5Sr0.5Fe0.2Co0.8O3-L. This thin film is deposited on a porous support, compatible from a chemical and mechanical viewpoint, therefore, materials such as doped cerium oxide or iron-based perovskites are commonly used. As in the case of SOFC fuel cells, the maximum flow of oxygen through the membrane is limited by surface processes (adsorption, dissociation and oxygen reduction) for thicknesses under 50 μm and temperatures above 600° C. Consequently, efficient electrodes in oxygen activation in fuel cells at a high temperature can be directly applied to membranes to improve oxygen exchange processes on one or both surfaces of the membrane, thereby increasing the flow of pure O2 obtained through the membrane.
The present invention relates to a catalytic porous layer for oxygen activation that can be used in solid oxide fuel cells, (SOFC) and dense ceramic membranes for oxygen separation at a high temperature. This porous layer is mainly composed of a mixed electron and oxygen ion conductive material and has a structure selected from among simple perovskite or double perovskite structures or perovskite-related structures, i.e. structures of the Ruddlesden-Popper, Dion-Jacobson and Aurivillius type. The composition of this crystalline phase has the following general formula:
[(BaaA′bA″cA′″1-a-b-c)1-xBx]N[Fe1-y-zCyDz]M OW
where:
A′, A″ and A′″ are elements selected from La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and Li
and provided fulfilment of the condition A′≠A″≠A′″
B is an element selected from Sr, Ca and combinations thereof.
C is an element selected from Co, Cu, Ni, Mn, Zr, Ti, Cr and combinations thereof,
D is an element selected from Pd, Ag, Rh, Pt and combinations thereof,
0.05≦a≦0.75;
0.05≦b≦50.75;
0≦c≦0.75;
and provided fulfilment of the condition a+b+c≦1
0.2<x<0.8;
0.1≦y≦1;
0≦z≦0.1;
and provided fulfilment of the condition y+z≦1
N/M≧0.98;
W≧2.5
The porous layer comprises crystalline particles with particle sizes ranging from 0.1 to 3 microns, while the pore size distribution is wider, ranging from 0.05 microns to 20 microns. The specific surface area of the porous catalytic coating is in the range of 0.1 to 80 m2/g.
In addition, the catalytic porous layer has a thickness of between 5 and 80 microns, preferably between 10 and 40 microns.
An advantageous aspect of this invention is the improved catalytic activity of crystalline material as regards the state of the art. In the case of the application of the solid oxide fuel cells as a cathode, numerous electrochemical activities have been described which use materials such as La0.6Sr0.4Fe0.8Co0.2O3-δ (Patent Application US2007/0148529A1 and U.S. Pat. No. 6,794,075) or Ba0.5Sr0.5Fe0.2Co0.8O3-δ (M. Shao and S. M. Haile, Nature 431 (2004) 170). Double perovskites of the type GdBaCo2O5+δ have also been disclosed (A. Tarancón et al, Journal of Power Sources 174 (2007) 255-263) and perovskite materials of the Ruddlesden-Popper series, Nd2NiO4+δ (F. Mauvy et al., Solid State Ionics 158 (2003) 17-28). One of the novelties of this invention is the simultaneous combination of three or more metals with a large ionic radius, greater than 0.09 nm with coordination 12 in the perovskite structure, which have interesting catalytic properties in oxygen activation at a high temperature. The effect of mixing several metals with coordination 12 in the perovskite and of redox activity of the metals Fe, C and D gives a favourable activity for oxygen activation at temperatures ranging from 450 to 850° C. Especially interesting is the potential application of the catalytic layer of the present invention at the so-called intermediate temperatures between 450 and 750° C. for the operation of solid oxide cells, because it allows to extend the scope of this type of cell to electric power and heat production from hydrogen and hydrocarbons, while reducing both production and operational prices and the degradation rate of the system.
The combination of metals with interesting catalytic properties results in a highly increased catalytic activity and far higher than that expected by simple linear combination of the catalytic activity by separate metals (J. M. Serra et al. Journal of the Electrochemical Society 55 (2008), B207-B214). Therefore, the object of this invention is novel because it has required extensive experimental work, due to the complexity of multicomponent interactions and synergistic effects of different “catalytic” metals with coordination 12 in the perovskite structure and the grain boundaries during the catalytic process of oxygen activation at a high temperature.
Preferred catalytic metals in coordination position 12 in the perovskite are: (1) barium, since it reduces the electrode activation energy and improves molecular oxygen adsorption and dissociation processes, (2) lanthanum, given its ability of forming superoxide and peroxide species, and (3) metals such as praseodymium, cerium, samarium, europium, gadolinium, terbium and ytterbium, combining a redox couple and a suitable ionic radius for accommodation in the perovskite structure. On the other hand, preferred catalytic metals for the octahedral position of the perovskite are iron, cobalt, copper and palladium.
In another embodiment of the present invention, the catalytic porous layer comprises the multicomponent crystalline material generally defined above by the formula [(BaaA′bA″cA′″1-a-b-c)1-xBx]N[Fe1-y-zCyDz]MOW and also a second crystalline material, physically mixed, which has predominantly pure ionic conduction properties. This second crystalline material is preferably a pure ionic conductive oxide. A selection of these materials may be, without being limited thereto, the following:
In another embodiment of the present invention, the crystalline material or materials of the catalytic porous layer for oxygen activation is impregnated with one of the following metals Pd, Ag, Rh, Pt and combinations thereof at a rate less than or equal to 1% by weight.
The crystalline material defined
[(BaaA′bA″cA′″1-a-b-c)1-xBx]N[Fe1-y-zCyDz]MOW
can be prepared by the following manufacturing methods: (a) solid state reaction between simple oxides or simple carbonates of different metals at high temperatures (700-1300° C.), (b) the process of metal complexation in aqueous or organic solution followed by drying and optionally a polymerization reaction, and finally calcined at a temperature above 600° C. to remove traces of organic matter and allow crystallization of the mixed oxide in the perovskite or perovskite family type structure (U.S. Pat. No. 3,330,697 and U.S. Pat. No. 6,752,979), (c) hydrothermal synthesis in an aqueous medium or with organic solvents, using convection, radiation, induction or microwave heating furnaces, (d) physical vapour deposition (PVD) or chemical vapour deposition (CVD), (e) reactive grinding and (f) spray drying.
Appropriate methods for the deposition of the catalytic porous layer under the present invention include: silkscreen printing, electroplating, thermal spray, electro-spray, plasma spray, spin-coating, tape-casting, dip-coating, buffering, physical vapour deposition (PVD) or chemical vapour deposition (CVD).
A preferred embodiment of the present invention is a porous layer between 20 and 40 microns thick, composed of grains with sizes in the range of 0.1-0.8 microns and a composition [(BaaLabPrc)1-xSrx]N[Fe1-y-zCoyPdz]MOW with simple perovskite structure. A preferred method for the manufacture of the layer is silkscreen printing, whereby the silkscreen ink of the compound [(BaaA′bA″cA′″1-a-b-c)1-xBx]N[Fe1-y-zCyDz]MOW is deposited on a solid electrolyte ionic conductor and is then baked in air in the temperature range from 900 to 1100° C. for a variable time between 1 and 12 hours using a heating ramp range of 2 K/min.
Another object of this invention is the use of the porous layer as a cathode of a solid oxide fuel cell when deposited on a pure solid electrolyte ionic conductor: The electrolyte can be a conductor of oxygen ions or protons, preferably oxygen ions. The electrolyte can be supported on a porous anode substrate, such as a composite material of the type YSZ/NiO, CGO/NiO and LSGM/NiO if the thickness is less than 25 microns. Likewise, if the electrolyte is thick (a thickness greater than or equal to 100 microns), it can act in itself as a support for the electrodes: the cathode described above according to the present invention and the anode with compositions such as YSZ/Ni, CGO/Ni, doped lanthanum chromites, double perovskites based on Sr2Mg1-xMnxMoO6-x, YSZ/Cu, CGO/Cu, etc. The electrolyte can be composed of several layers of different materials with ionic conductivity, as the use of protective coatings or contact layers between the electrolyte and electrodes is common (S. Uhlenbruck et al., Thin Solid Films 515 (2007) 4053-4060). The materials used as electrolytes of the fuel cell on which the cathode presented herein is deposited include at least one of the following materials:
Another preferred embodiment of the present invention is the deposition of the porous catalytic layer with a composition [(BaaA′bA″cA′″1-a-b-c)1-xBx]N[Fe1-y-zCyDz]MOW on a pure ionic electrolyte with the configuration and composition described above which is covered with a dense and thin layer (0.1-2 μm) of a material with mixed ionic and electronic conductivity. Preferably, the composition of the dense coating of the electrolyte has the same composition as the cathode, i.e.
[(BaaA′bA″cA′″1-a-b-c)1-xBx]N[Fe1-y-zCyDz]MOW.
A preferred embodiment includes the deposition of a cathode according to this invention on a YSZ electrolyte of 10 microns on an anodic support of YSZ/NiO, and a protective layer between the YSZ electrolyte and gadolinium-doped cerium oxide cathode, preferably with a thickness of 0.5 microns and with a density greater than 95%.
According to another particular embodiment, the porous layer object of the present invention can act as a catalytic coating activating the oxygen exchange in oxygen ion-permeable dense ceramic membranes. The operating temperature of such membranes according to the present invention is between 500 and 950° C. The catalytic coating may be deposited (a) on the membrane side with a greater partial pressure of oxygen (b) on the membrane side with a lower partial pressure of oxygen, or (c) on both sides. The application of the catalytic layer can increase the oxygen exchange rate, which is particularly relevant when the thickness of the oxygen ion-permeable dense layer is below 100 microns. In this situation, the total flow of oxygen is usually limited by the oxygen exchange, mainly on the membrane side with a lower partial pressure.
The material the dense membrane on which the activation catalytic layer is deposited consists of a mixed conductor of oxygen ions and electrons and is generally a crystalline solid, which may comprise one of the following materials or physical mixtures thereof (J. Sunarso et al. Journal of Membrane Science 320 (2008) 13-41):
The oxygen ion-permeable dense membrane and catalytically activated with the porous layer described herein finds application in (1) the separation of oxygen at a high temperature, (2) the activation of hydrocarbon as a membrane reactor for reactions such as selective oxidation of methane, dehydrogenation of ethane to ethylene and of propane to propylene, methane and ethane aromatization, selective oxidation of ammonia, Andrussow reaction to produce HCN from methane and ammonia, etc.
The following examples are intended to illustrate aspects of the invention but in no way to limit the scope thereof.
Using the complexation-polymerization method, five crystalline materials were prepared with the following compositions:
(1) La0.6Sr0.4Fe0.8Co0.2O3-δ
(2) Ba0.6Sr0.4Fe0.8Co0.2O3-δ
(3) Pr0.6Sr0.4Fe0.8Co0.2O3-δ
(4) (Ba0.25La0.75)0.6Sr0.4Fe0.8Co0.2O3-δ, according to the present invention
(5) (Ba0.25La0.375Pr0.375)0.6Sr0.4Fe0.8Co0.2O3-δ, according to the present invention.
The materials were prepared as follows: stoichiometric amounts of nitrates of the different metals of the compound to be synthesized were dissolved in water. Then citric acid was added in a 2:1 molar ratio with respect to the total moles of cations, and thereafter ethylene glycol was added in a 4:1 molar ratio with respect to total moles of cations. The final solution was heated gradually to 150° C. until a viscous and spongy gel is obtained. Finally, the gel is pyrolyzed at 1000° C. for 2 hours using a heating ramp rate of 2 K/min.
Using the five inks, the relevant porous layers were deposited on dense electrolytes composed of cerium oxide compounds doped with gadolinium (Ce0.8Gd0.2O1.9) with a thickness of 800 microns. The deposited porous layers were dried at 60° C. for 4 hours and then baked at 1060° C. for 2 hours.
The electrochemical characterization was performed by impedance spectroscopy using cells in a symmetrical configuration. To this end, Ce0.8Gd0.2O1.9 electrolytes were coated on both sides with each of the five screen printing inks under the previous process. The study was carried out under air flow for temperatures of 450, 500, 550, 600 and 650° C. The scanning frequency range was 0.01 to 1000000 Hz and the electrical contact was made using a platinum mesh. This study allows the evaluation of the catalytic activity of the layers both for use as solid oxide fuel cell cathodes and for the activation of oxygen ion-permeable dense membranes.
| Number | Date | Country | Kind |
|---|---|---|---|
| P200802001 | Jun 2008 | ES | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/ES09/70255 | 6/25/2009 | WO | 00 | 3/28/2011 |
