Low operating temperature oxygen electrode contact layer and its manufacturing process

Abstract

An object of the invention is a low operating temperature oxygen electrode contact layer (110) in a fuel cell system or in an electrolyzer cell system providing electrical contact between an oxygen electrode (100) of a solid oxide cell and at least one of the interconnector (114), bipolar plate, and contact structures of the solid oxide stack. The low operating temperature oxygen electrode contact layer (110) comprises an ABO3 perovskite structure containing at least one of La and Ca together with trace amounts of Mg, Sr, and Ba, and La and Sr together with trace amounts of Mg, Ca, and Ba on the A-site and at least one of Mn, Fe, Ni, and Cu and Fe, Ni, Co, and Cu on the B-site to lower the sintering temperature to 500 – 800 °C.

Description

[0001]Low operating temperature oxygen electrode contact layer and its manufacturing process The field of the invention Most of the energy in the world is produced by means of oil, coal, natural gas, or nuclear power. All these production methods have their specific problems as far as, for example, availability and friendliness to the environment are concerned. As far as the environment is concerned, especially oil and coal cause pollution when they are combusted. The problem with nuclear power is, at least, the storage of used fuel. Especially because of environmental problems, new energy sources that are more environmentally friendly and, for example, having a better efficiency than conventional energy sources have been developed. Solid oxide cells operate via a chemical reaction in an environmentally friendly process and are very promising future energy conversion devices. The intermittency of renewable energy sources has introduced challenges for electrical grid stability, calling for increased demand and supply side flexibility and new energy storage and conversion technologies. State of the art An electrochemically active solid oxide cell can be used as a fuel cell or an electrolyser. A fuel cell produces electricity and heat from various fuels, and an electrolysis cell produces chemicals such as hydrogen, methane, ammonia, and carbon monoxide from steam, CO2, and nitrogen, electricity, and heat. Such a cell that operates in both modes, as a fuel cell and electrolyser, is called a solid oxide electrochemical cell (SOEC) or reversible solid oxide cell (rSOC) or simply a solid oxide cell (SOC). Solid oxide cell (SOC) comprises a fuel electrode and an oxygen electrode and an electrolyte layer between them. In solid oxide fuel cells (SOFCs), oxygen is fed to the oxygen electrode and it is reduced to a negative oxygen ion by receiving electrons from the oxygen electrode. The negative oxygen ion is transported through the electrolyte layer to the fuel electrode where it reacts with fuel, producing typically water and carbon monoxide (CO) and carbon dioxide (CO2). Fuel electrode and oxygen electrode are connected through an external electric circuit comprising a load for the fuel cell operating mode withdrawing electrical energy out of the system. The fuel cells also produce heat to the reactant exhaust streams. In electrolysis operating mode, the current flow is reversed, and the solid oxide cells act as a load to which electricity is supplied. Depending on the electrolysis reaction operating conditions, the cell operation can be endothermic, exothermic, or thermoneutral. Fuel cell reactions in the case of methane, carbon monoxide, and hydrogen fuel are shown below: fuel electrode: CH4 + H2O = CO + 3H2 (steam reforming reaction) CO + H2O = CO2+ H2(water gas shift reaction) H2+ O2-= H2O + 2e- (fuel cell anode reaction) oxygen electrode: O2 + 4e- = 2O2-(fuel cell cathode reaction) Net reactions: CH4 + 2O2 = CO2 + 2H2O CO + 1 / 2O2 = CO2 H2+ 1 / 2O2= H2O In electrolysis operating mode the reaction is reversed, i.e., electrical energy from a source is supplied to the cell, where water and often also carbon dioxide are reduced on the fuel electrode, forming oxygen ions that move through the electrolyte layer to the oxygen electrode, where oxidation reaction takes place. It is possible to use the same solid oxide cell in both SOFC and SOEC modes. Prior art solid oxide electrolyser cells operate at temperatures that allow high temperature electrolysis reaction to take place, said temperatures being typically between 500 - 1040 °C, but even temperatures over 1040 °C may be useful. These operating temperatures are similar to those conditions of the solid oxide fuel cells (SOFCs). The net cell reaction produces hydrogen and oxygen gases. The reactions for one mole of water are shown below: fuel electrode: H2O + 2e- ---> 2 H2 + O2-(electrolysis cell cathode reaction) oxygen electrode: O2----> 1 / 2O2+ 2e- (electrolysis cell anode reaction) Net reaction: H2O ---> H2+ 1 / 2O2.In case of co-electrolysis, carbonaceous species are supplied to the cell in addition to steam, typically in proportions favorable for subsequent refining of the resultant gas according to, e.g., the Fischer-Tropsch process. Carbon dioxide can be directly reduced to carbon monoxide or can interact with hydrogen through the reversed water-gas shift reaction to form carbon monoxide and steam. Solid oxide cell can also be used to produce other types of chemicals, either directly by electrochemical reactions or through chemical reactions. Such chemicals may include, e.g., methane and ammonia. Methane can be produced when steam and carbonaceous species are fed to a solid oxide electrolysis cell, and ammonia when steam and nitrogen are fed. The reaction rate of the chemical production is dependent on the supplied current, fuel and air side flow rates, fuel and air side gas concentrations, fuel and air side pressures, and fuel and air side temperatures. The state of the art materials for the fuel electrode support are nickel and stabilized zirconia. The state of the art manufacturing methods for the fuel electrode support include tape casting and extrusion. The desired functions of a fuel electrode support, from a chemical and electrochemical reactions perspective, include activity for steam reformation reactions, activity for methanation reaction, activity for water-gas shift reaction, activity for reversed water-gas shift reaction, activity for ammonia cracking, allowing multicomponent gas transfer through the layer, and conductivity for electrons. The desired function of a fuel electrode support from a mechanical perspective, includes providing essential structural support having high flexural strength, compressive strength, and fracture toughness to endure handling, stack assembly, and operation including thermal cycling and accidental redox cycles. Preferred microstructure and morphology, from a chemical and electrochemical functionality perspective, of a fuel electrode support, the fuel electrode support should be porous to allow gas transfer through the layer between a fuel channel and a fuel electrode, has continuous electrical conductivity between a fuel electrode and fuel electrode contact structure, and has large surface area enabling the chemical reactions. The typical thickness of a fuel electrode support is between 0.1 mm to 1 mm. Typical porosity of a fuel electrode support is between 0.3 to 0.7. The state of the art materials for the fuel electrode 104 are nickel and stabilized zirconia. The state of the art manufacturing methods of the fuel electrode 104 include tape casting, dip coating, spray coating, and screen printing. The desired functions of a fuel electrode 104, from a chemical and electrochemical reactions perspective, include activity for electrochemical reactions, activity for steam reformation reactions, activity for methanation reaction, activity for water-gas shift reaction, activity for reversed water-gas shift reaction, activity for ammonia cracking, allowing multicomponent gas transfer through to the layer on to the electrochemically active sites, conductivity for electrons and conductivity for oxygen ions. Preferred microstructure of a fuel electrode is composed so that the electrochemically active sites are maximized, it has porosity which is typically less than for fuel electrode support, it has continuous electrical conductivity properties between fuel electrode support and electrochemically active sites, it has continuous ionic conductivity properties between an electrolyte layer and electrochemically active sites, and it has large surface area enabling the chemical reactions such as reforming and water-gas shift reactions. The state of the art materials for the oxygen electrode 100 include manganites, cobaltites, and ferrites of lanthanum. The state of the art manufacturing methods of the oxygen electrode 100 include dip coating, spray coating, and screen printing. The desired function of the oxygen electrode 100 from a chemical and electrochemical perspective includes activity for electrochemical reactions. The desired function of the oxygen electrode 100 from microstructural perspective is to provide adequate porosity for the diffusion of gaseous species, to provide high surface area to maximize the active sites, provide continuous electrical conductivity properties between active sites and oxygen electrode contact layer 106, and to provide continuous ionic conductivity properties between active sites and the electrolyte layer. The state of the art materials for the electrolyte layer 102 (Fig.1) includes stabilized zirconia. The state of the art manufacturing methods of the electrolyte layer include tape casting, dip coating, spray coating, and screen printing. The desired function of the electrolyte layer 102 is to provide gas tight separation and continuous oxygen conductivity properties between oxygen electrode 100 and fuel electrode 104. The state of the art materials for the cation diffusion barrier layer 108 (Fig.1) includes doped ceria. The state of the art manufacturing methods of the cation ion diffusion barrier layer 108 include tape casting, dip coating, spray coating, and screen printing. The desired function of the cation diffusion barrier layer 108 include excellent chemical stability with the oxygen electrode 100 and electrolyte layer 102, and continuous electrical conductivity properties between oxygen electrode 100 and electrolyte layer 102. The state of the art materials for the oxygen electrode contact layer include manganites, cobaltites, and ferrites of lanthanum. The state of the art manufacturing methods of the oxygen electrode contact layer include dip coating, spray coating, and screen printing. The desired function of the oxygen electrode contact layer from microstructural perspective is to provide adequate porosity for the diffusion of gaseous species and to provide continuous electrical conductivity properties between oxygen electrode 100 and interconnector plate or bipolar plate. In Solid Oxide Fuel Cell (SOFC) and Solid Oxide Electrolyser (SOE) stacks, where the flow direction of the fuel electrode gas is relative to the oxygen rich side gas internally in each cell as well as relative to the flow directions of the gases between adjacent cells, stacks are combined through different cell layers of the stack. Further, the fuel electrode gas, or the oxygen electrode gas, or both can pass through more than one cells before it is exhausted, and a plurality of gas streams can be split or merged after passing a primary cell and before passing a secondary cell. These combinations serve to increase the current density and minimize the thermal gradients across the cells and the whole stack. The high operating temperature in the SOC cells and system introduces material related challenges with respect to thermomechanical forces, material properties, chemical stability, and uniformity of operating conditions. These aspects place practical constraints on feasible SOC cell, stack, and module sizes. Scaling the technology for large installations, typical to SOEC application, will thus primarily rely on the multiplication of cells, stacks, and SOC modules. Minimizing the cost of each multiplying unit at all levels is thus crucial for reducing the overall cost. A SOFC delivers, in normal operation, a voltage of approximately 0.5 – 1.2 V and a typical operation voltage of a SOE cell is about 0.9 – 1.6 V. In order to increase the total voltage output, the SOCs are usually assembled in stacks in which the cells are electrically connected via flow field plates (also: interconnector plates, bipolar plates). The desired level of voltage determines the number of cells needed. Bipolar plates separate the anode and cathode sides of adjacent cell units, and at same the time enable electron conduction between anode and cathode. Interconnects, or bipolar plates, are normally provided with a plurality of channels for the passage of fuel gas on one side of an interconnect plate and oxidant gas on the other side. The flow direction of the fuel gas is defined as the substantial direction from the fuel inlet portion to the fuel outlet portion of a cell unit. Likewise, the flow direction of the oxidant gas (i.e., air) is defined as the substantial direction from the inlet portion to the outlet portion of a cell unit. Conventionally, the cells are stacked one on top of each other with a complete overlap resulting in a stack with, for instance, co-flow having all fuel and oxidant inlets on one side of the stack and all fuel and oxidant outlets on the opposite side. One feature affecting the temperatures of the structure in operation is steam reformation of the fuel that is fed into the cell. Steam reformation is an endothermic reaction and cools the fuel inlet edge of the cell. Due to the exothermicity of the electrochemical process, the outlet gases leave at higher temperature than the inlet temperature of the inlet gases. When endothermic and exothermic reactions are combined in an SOFC stack, a significant temperature gradient across the stack is generated. Simultaneously flow rates are wanted to be minimized both at the fuel and oxygen rich side in order to maximize overall system efficiency. Large thermal gradients induce thermal stresses in the stack which are highly undesired, and they entail difference in current density and electrical resistance. The performance and lifetime of a SOC stack can be maximized when as uniform temperature profile as possible over the entire stack can be maintained. Therefore, the problem of thermal management of an SOFC stack exists in reducing thermal gradients enough to avoid unacceptable stresses and to maximize electric efficiency through homogenous current density profile. A SOC module comprises tens up to hundreds of SOC stacks, support structures, thermal insulation, reactant conveying and distribution structures, instrumentation as well as electrical and reactant interfacing towards the application or other modules. As high temperature interfaces are costly, space-consuming and may constitute an ignition source, it is also beneficial to include heat exchanging within the module to lower the temperature of the reactant interfaces. Furthermore, the SOC module needs internal or external means to facilitate safe start-up and shutdown. The advantage of the SOC is its high performance in the conversion processes. In a small, unit cell tests, such a structure can produce electrical power densities exceeding 2 W / cm2and still having extremely high energy conversion efficiency compared to competitive technologies such as other electrochemical cell technologies based on low temperature solutions such as polymer electrolyte or alkaline technologies, or other conversion technologies based on Carnot cycles such as turbine or engine technologies. The cell generates heat power densities in the fuel cell mode that are equivalent to the electrical power densities. The current problem with SOC technology to reach this high electrical power densities is that the heat has to be dissipated to the fuel and oxidant streams or transferred via conduction or radiation from the electrochemically active sites in order not to overheat the SOC structure. The heat transfer challenge is pronounced at stack level in which the main heat transfer mechanism is the convective heat transfer to the fuel and air fluids. A SOC stack is composed of a SOC, a flow field plate structure for the fuel, a flow field plate structure for the oxidant, electric contacting structures for the fuel and oxidant, protective coatings for the flow field plate structures and sealing solution preventing assuring reactant gases to flow over the electrochemically active SOC areas. In a SOC stack, a flow field plate that serves fuel to a SOC on its other side, can supply oxidant to an adjacent SOC in the stack. Also, the other flow field plate that supplies oxidant to the same SOC, can supply fuel to a third SOC. This kind of a structure is called a single repeat unit (SRU) structure. In such structural SOC stack typical power densities are between 0.1 – 0.4 W / cm2. Heat generation together with limited heat transfer properties produce temperature increase and large temperature gradients in the electrochemically active SOC structure. The maximum power density is typically limited in a stack containing not only ceramic cells but also steal based interconnect parts, coatings and sealing solutions by the maximum operation temperature of these parts and materials. The absolute temperature as the gradient can be harmful for the SRU because of issues related to corrosion, chemical stability, decreased mechanical strength properties, increased stress levels etc. The unit cell however could deliver power densities exceeding today’s stack values and up to 2 W / cm2have been reported in the open literature. If the power density could be increased, this would bring down the cost of produced power or per produced cubic meter of chemical species. SOC in electrolysis mode can be connected with existing industrial processes such as pulp and paper production, steel production, energy production, etc., that produce high temperature exhaust gases. These exhaust gases may be used to produce steam from liquid water and to superheat the same to the operational temperature of an electrolysis stack. The same exhaust gases may also be used to heat up effectively the SOC stack to its operational temperature and to use in providing heat for the endothermic electrolysis reactions. The exhaust gases typically contain such high concentration of impurities such as sulfur and halogenides that these gases cannot be supplied directly to the electrochemically active elements without extensive gas cleaning processes. Fuel electrode supported solid oxide fuel cells (SOFCs) are receiving considerable interest since they are suited for operation at lower temperatures. By lowering the operating temperature of the SOFCs to around 700°C, conventional stainless steel can be used for interconnectors or auxiliary components with which high mechanical reliability of a cell-stack and lower manufacturing costs can be achieved. For the anode-supported cell, it is advantageous that the ohmic resistance is lower than that of the electrolyte-supported cell because much thinner electrolyte films / membranes can be employed enabling lower operating temperature and higher performance. The lower operating temperature greatly reduces degradation (related to the corrosion of the ancillary reactant delivery equipment and adverse effects on the cell), thereby increasing lifetime. Additionally, this design has a thin cathode layer that provides an opportunity for tailoring the cathode material formulation to enhance performance or provide chemical resistance for various applications. Independent of the solid oxide cell structure and shape, the oxygen electrode contact layer is a critical component of the solid oxide cell which provides electrical contact between the cell and the interconnector, the bipolar plate, or the contacting structure of the solid oxide stack. The desired functions of the oxygen electrode contact layer include good stability in oxidizing atmosphere, high electrical conductivity in the oxidizing atmosphere, matching thermal expansion coefficient with solid oxide stack components, easy processing, better sintering properties to provide good mechanical interlocking or mechanical bonding between the cell and the interconnector, bipolar plate, or the contacting structures, less reactivity towards the oxygen electrode and interconnet or bipolar plate materials, and good reactivity towards the oxygen electrode poisoning impurities such as Si, Cr, and S. The prior art materials of the oxygen electrode contact layer include at least LaCoO3, La1-xSrxCoO3, Sm1-xSrxCoO3, La1-x-ySmxSryCoO3, LaCrO3, La1-xSrxCrO3, La1-xCaxCrO3, La1-xSrxMnO3, La1-xSrxCo1-yFeyO3, La1-xSrxCo1-yMnyO3, La1-xSrxCu1-yFeyO3, La1-xSrxCo1-y-zMnyFezO3, La1-xNi1-yFeyO3, La1-xCo1-yNiyO3, La1-xSrxFeO3, La2Ni1-yCuyO4, La2-xSrxNiO4, Nd2-xCexCuO4, Sm2-x-yBaxSryCo2O5, GdSrCo2O5, LaBaCo2O5, and YBaCo2O5. Prior art oxygen electrode contact layer materials suffer from high reactivity and poor mechanical contact with the interconnector or bipolar plate materials. Sintering temperatures of said prior art materials are –above 900 °C. Brief description of the invention An object of the invention is to lower sintering temperatures and reactivity and to improve the electrical and mechanical contact properties and durability of an oxygen electrode contact layer. This is achieved by a low operation temperature oxygen electrode contact layer in a fuel cell system or in an electrolyser cell system providing electrical contact between an oxygen electrode of a solid oxide cell and the interconnector, bipolar plate, or contacting structures of the solid oxide stack. The low operation temperature oxygen electrode contact layer comprises an ABO3 perovskite structure containing at least one of La and Ca together with trace amounts of Mg, Sr, and Ba, and La and Sr together with trace amounts of Mg, Ca, and Ba on the A-site and at least one of Mn, Fe, Ni, and Cu and Fe, Ni, Co, and Cu on the B-site to lower the sintering temperature to 500 – 800 °C. An object of the invention is also a manufacturing process of low operation temperature oxygen electrode contact layer. In the manufacturing process the low operation temperature oxygen electrode contact layer is made of an ABO3perovskite structure containing at least one of La and Ca together with trace amounts of Mg, Sr, and Ba, and La and Sr together with trace amounts of Mg, Ca, and Ba on the A-site and at least one of Mn, Fe, Ni, and Cu and Fe, Ni, Co, and Cu on the B-site to lower the sintering temperature to 500 – 800 °C. The invention is based on that the low operation temperature oxygen electrode contact layer comprises an ABO3perovskite structure containing at least one of La and Ca together with trace amounts of Mg, Sr, and Ba, and La and Sr together with trace amounts of Mg, Ca, and Ba on the A-site and at least one of Mn, Fe, Ni, and Cu and Fe, Ni, Co, and Cu on the B-site. Benefit of the invention is lower sintering temperatures and lower reactivity and improved electrical and mechanical contact properties and durability. The low operation temperature oxygen electrode contact layer according to the present invention is compatible with all structures and shapes of the solid oxide cell. Brief description of the drawings Figure 1 presents a solid oxide cell structure. Figure 2 presents an exemplary structure of a solid oxide cell according to the present invention including interconnector 114 plate and interconnector plate coating layer 112. Figure 3 presents a scanned electron microscope image of the low operation temperature oxygen electrode contact layer sintered at 650 °C. Detailed description of the invention Independent of the solid oxide cell structure and shape, the oxygen electrode contact layer is a critical component of the solid oxide cell which provides electrical contact between the cell and the interconnector, the bipolar plate, or contacting structures. The desired functions of the oxygen electrode contact layer include good stability in oxidizing atmosphere, high electrical conductivity in the oxidizing atmosphere, matching thermal expansion coefficient with solid oxide stack components, easy processing, better sintering properties to provide good mechanical interlocking or mechanical bonding between the cell and the interconnector, the bipolar plate, or the contacting structures, less reactivity towards the oxygen electrode and the interconnector or bipolar plate materials, and good reactivity towards the oxygen electrode poisoning impurities such as Si, Cr, and S. The low operation temperature oxygen electrode contact layer according to the present invention is defined as low operation temperature oxygen electrode contact layer due to its lower sintering temperature. According to the present invention is provided new materials for the oxygen electrode contact layer of the solid oxide cell defined as low operation temperature oxygen electrode contact layer and its materials. The materials for the low temperature oxygen electrode contact layer can comprise an ABO3perovskite structure containing at least one of La and Ca together with trace amounts of Mg, Sr, and Ba, and La and Sr together with trace amounts of Mg, Ca, and Ba on the A-site and at least one of Mn, Fe, Ni, and Cu and Fe, Ni, Co, and Cu on the B-site to lower the sintering temperature to 500 – 800 °C. The low operation temperature oxygen electrode contact layer materials can be defined as A1-x-yAʹxAʹʹyB1-a-b-c BʹaBʹʹbBʹʹʹcO3, where A is La, Aʹ is Sr or Ca, Aʹʹ can be a single element or a mixture of elements including at least one of Mg, Ca, Sr, and Ba, B is Mn or Co, Bʹ is Fe, Bʹʹ is Ni, and Bʹʹʹ is Cu. The electrical, mechanical, and chemical properties of the low operation temperature oxygen electrode contact layer are tailored by varying the stoichiometric composition of the constituting elements in a range where x is 0.1 to 0.5, y is in trace amounts, and a, b, and c are 0.1 to 0.3. The best electrical, mechanical, and chemical properties are obtained when x is 0.2 to 0.5 and c is 0.15 to 0.25. The low operation temperature oxygen electrode contact layer can be directly manufactured onto the oxygen electrode via at least one of screen printing, dip coating, or brush painting methods. For manufacturing purpose, a slurry should be prepared by mixing the low operation temperature oxygen electrode contact layer powder with a solvent including at least one of ethanol, methanol, isopropyl alcohol, toluene, benzene, xylene, acetone, terpinol, and water and a binder including at least one of poly(vinyl butyral) , poly(vinyl alcohol), poly(vinyl acetate), poly(vinyl pyrrolidine), poly(acrylic acid), poly(ethyl acrylate), poly(methacrylic acid), poly(methyl methacrylate), poly(alkylene carbonate), methyl cellulose, ethyl cellulose, hydroxyethyl cellulose, hydroxypropyl methyl cellulose, and hydroxybutyl methyl cellulose by at least one of ball milling and high shear mixing methods. In order to obtain a proper slurry, additives such as dispersant including at least one of fish oil, triethanolamine, terpinol, ammonium polyacrylate, and ammonium poly(methacrylate), a plasticizer including at least one of dibutyl phthalate, benzyl butyl phthalate, dibutyl phosphate, glycerol, poly(ethylene glycol), and poly(propylene glycol), a surfactant including at least one of triethanolamine, triton, 4-(1,1,3,3-tetramethylbutyl)phenyl-polyethylene glycol and t-octylphenoxypolyethoxyethanol, pine oil, and 4-(1,1,3,3- Tetramethylbutyl)phenyl-polyethylene glycol, and a pore former including at least one of graphite powder, carbon black, activated carbon, starch, dextrose, gelatine, polyethylene, polypropylene, polystyrene, poly(methyl methacrylate), and cellulose acetate, can be added during the mixing process. The low operation temperature oxygen electrode contact layer can also be manufactured by at least one of tape casting, air spray coating, ultrasonic spray coating, electrostatic spray coating, spray pyrolysis, high velocity oxy- fuel spray, high velocity air fuel spray, combustion flame spray, plasma spray, vacuum plasma spray, warm spray, gas dynamic cold spray, vacuum assisted dip coating, spin coating, inkjet printing, ALD, PLD, PVD, and CVD techniques. In the embodiments according to the present invention is referred to exemplary figures 1 and 2. The low operation temperature oxygen electrode contact layer 110 comprises an ABO3 perovskite structure containing at least one of La and Ca together with trace amounts of Mg, Sr, and Ba, and La and Sr together with trace amounts of Mg, Ca, and Ba on the A-site and at least one of Mn, Fe, Ni, and Cu and Fe, Ni, Co, and Cu on the B-site to lower the sintering temperature to 500 – 800 °C. The low operation temperature oxygen electrode contact layer 110 material can preferably comprise La on the A-site in a range of 50 to 80 mole percent, and Cu on the B-site in a range of 15 to 25 mole percent to provide good electrical and mechanical contact properties and desired sintering properties. The low operation temperature oxygen electrode contact layer material 110 properties can be configured to even eliminate the oxygen electrode contact layer sintering step during the cell manufacturing process. This is based on the lower sintering temperature the low operation temperature oxygen electrode contact layer 110 which sinters at the SOC operating temperatures (500 – 900 °C) and provide excellent electrical and mechanical contact between the oxygen electrode 100 and the interconnector 114, the bipolar plate, and / or the contacting structures. Figure 3 presents a scanned electron microscope image of the low operation temperature oxygen electrode contact layer sintered at 650 °C. Sintering of the low operation temperature oxygen electrode contact layer 110 can also be assisted by the current drawn (SOFC) or supplied (SOEC) during the SOC operation. In a manufacturing process, the low operation temperature oxygen electrode contact layer 110 can be directly manufactured on the oxygen electrode 100 of the solid oxide cells via at least one of screen printing, dip-coating, brush painting, tape casting, air spray coating, ultrasonic spray coating, electrostatic spray coating, spray pyrolysis, high velocity oxy-fuel spray, high velocity air fuel spray, combustion flame spray, plasma spray, vacuum plasma spray, warm spray, gas dynamic cold spray, vacuum assisted dip coating, spin coating, inkjet printing, ALD, PLD, PVD, and CVD techniques. The low operation temperature oxygen electrode contact layer 110 material can be configured to enhanced mechanical interlocking and mechanical bonding between oxygen electrode 100 and interconnector 114, bipolar plate, and / or the contacting structures during the stack operation at the operating temperatures such as from 500 to 900 °C. In embodiments according to the present invention the low operation temperature oxygen electrode contact layer 110 can comprise matching thermal expansion coefficient with the solid oxide stack components. The low temperature oxygen electrode contact layer 110 materials can be configured to being unreactive towards the state-of-the-art oxygen electrode 100 materials such as lanthanum strontium cobaltite (LSC), samarium strontium cobaltite (SSC) lanthanum strontium magnetite (LSM), and lanthanum strontium cobalt ferrite (LSCF). Other oxygen electrode materials that can be use with the low temperature oxygen electrode contact layer can be at least one of single metal oxide, composite metal oxides, single metal, or an alloy of at least one of Mn, Co, Fe, Ni, Cu, Mo, W, Nb, Ru, Pd, Pt, Ag, Au, La, Ce, Pr, Pm, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu, ABO3-δ, A2B2O6-δ, and A2BO4-δwhere A can be a single element or a mixture of elements including at least one of La, Ce, Pr, Pm, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Mg, Ca, Sr, Ba, and Y, and B can be a single element or a mixture of elements including at least one of Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ta, W, Mo, Nb, Ru, Pd, Al, and Ga. The materials can also be configured to being unreactive towards the coating materials for the interconnect or bipolar plates such as spinel oxides. Spinel oxides are represented by a general formula AxB3-xO4where A and B contain single dissimilar elements or a mixture of Cr, Mn, Fe, Co, Ni, Cu, and Zn also, addition of a single element or a mixture of Li, Na, K, Mg, Ca, Sr, Ba, Sc, Y, Ti, V, Al, Ga, Id, Tl, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu may produce desired electrical and stability characteristics. Also, the materials can be configured to provide protection to the oxygen electrode from oxygen electrode poisoning impurities such as Si, Cr, and S by providing a reactive buffer layer. Although the air should be cleaned prior being supplied to the SOC system, it can cumulate on the oxygen electrode 100 poisoning impurities from the balance of plant equipment operating at high temperatures. In a manufacturing process the low operation temperature oxygen electrode contact layer 110 is made of an ABO3 perovskite structure containing at least one of La and Ca together with trace amounts of Mg, Sr, and Ba and La, and Sr together with trace amounts of Mg, Ca, and Ba on the A-site and at least one of Mn, Fe, Ni, and Cu and Fe, Ni, Co, and Cu on the B-site to lower the sintering temperature to 500 – 800 °C. Preferably, the low operation temperature oxygen electrode contact layer 110 is manufactured onto a solid oxide cell independent of its structure and shape. The low operation temperature oxygen electrode contact layer 110 according to the present invention can be directly manufactured on the oxygen electrode 100 of the solid oxide cells via at least one of screen printing, dip- coating, brush painting, tape casting, air spray coating, ultrasonic spray coating, electrostatic spray coating, spray pyrolysis, high velocity oxy-fuel spray, high velocity air fuel spray, combustion flame spray, plasma spray, vacuum plasma spray, warm spray, gas dynamic cold spray, vacuum assisted dip coating, spin coating, inkjet printing, 3D printing, ALD, PLD, PVD, and CVD techniques. According to the present invention the low operation temperature oxygen electrode contact layer 110 powder can be manufactured, e.g., by at least one of the solid-state reaction, sol-gel processing, chemical combustion, spray pyrolysis, co-precipitation, and hydrothermal methods while using the precursors, such as metal, oxide, nitrate, acetate, and / or carbonate. The low operation temperature oxygen electrode contact layer powder can be processed, e.g., by at least ball milling to obtain a proper particle size distribution ranging from nanometer scale to micrometer scale. Depending on the mechanically supporting layer in the solid oxide cells, they can be arranged into several structures such as, fuel electrode supported 106 solid oxide cell, electrolyte supported solid oxide cell, oxygen electrode supported solid oxide cell, inert porous substrate supported solid oxide cell, and metal supported solid oxide cell. The solid oxide cells can also be manufactured in different shapes, such as planar, tubular, and flat tubular. A planar solid oxide cell can be a square, a polygon, or a circle. Various solid oxide cell structures and shapes provide certain benefits and are chosen based on the solid oxide stack and system requirements. For example, planar fuel electrode supported 106 solid oxide fuel cells are receiving considerable interest since they are suited for operation at lower temperatures. The low operation temperature oxygen electrode contact layer 110 can be manufactured onto a solid oxide cell independent of its structure and shape. Lowered sintering temperature of the low operation temperature oxygen electrode contact layer 110 according to the present invention provides better mechanical interlocking or mechanical bonding between oxygen electrode 100 and interconnector 114, bipolar plate, or the contacting structures during the stack operation at the operating temperatures such as from 500 to 900 °C. The low operation temperature oxygen electrode contact layer 110 provides good electrical conductivity at the solid oxide cell operating temperatures. The low operation temperature oxygen electrode contact layer 110 can also provide matching thermal expansion coefficient with the solid oxide stack components. Thus, while there have been shown and described and pointed out fundamental novel features of the invention as applied to a preferred embodiment thereof, it will be understood that various omissions and substitutions and changes in the form and details of the invention may be made by those skilled in the art without departing from the spirit of the invention. For example, it is expressly intended that all combinations of those elements which perform substantially the same results are within the scope of the invention. Substitutions of the elements from one described embodiment to another are also fully intended and contemplated. It is also to be understood that the drawings are not necessarily drawn to the scale, but they are merely conceptual in nature. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended hereto.

Claims

Claims 1. Low operating temperature oxygen electrode contact layer (110) in a fuel cell system or in an electrolyser cell system providing electrical contact between an oxygen electrode (100) of a solid oxide cell and at least one of the interconnector (114), bipolar plate, and contact structures of the solid oxide stack, characterized in that the low operating temperature oxygen electrode contact layer (110) comprises an ABO3 perovskite structure containing at least one of La and Ca together with trace amounts of Mg, Sr, and Ba and La, and Sr together with trace amounts of Mg, Ca, and Ba on the A-site and at least one of Mn, Fe, Ni, and Cu and Fe, Ni, Co, and Cu on the B-site to lower the sintering temperature to 500 – 800 °C.

2. Low operating temperature oxygen electrode contact layer (110) according to claim 1, characterized in that the low operating temperature oxygen electrode contact layer material comprises La on the A-site in a range of 50 to 90 mole percent, and Cu on the B-site in a range of 10 to 30 mole percent to provide good electrical and mechanical contact properties.

3. Low operating temperature oxygen electrode contact layer (110) according to claim 1, characterized in that the low operating temperature oxygen electrode contact layer (110) material properties are being configured to eliminate the additional sintering step for the oxygen electrode contact layer (110) during the cell manufacturing process.

4. Low operating temperature oxygen electrode contact layer (110) according to claim 1, characterized in that the low operating temperature oxygen electrode contact layer (110) has been directly manufactured on the oxygen electrode (100) of the solid oxide cells via at least one of screen printing, dip-coating, brush painting, tape casting, air spray coating, ultrasonic spray coating, electrostatic spray coating, spray pyrolysis, high velocity oxy- fuel spray, high velocity air fuel spray, combustion flame spray, plasmaspray, vacuum plasma spray, warm spray, gas dynamic cold spray, vacuum assisted dip coating, spin coating, inkjet printing, 3D printing, ALD, PLD, PVD, and CVD techniques.

5. Low operating temperature oxygen electrode contact layer (110) according to claim 1, characterized in that the low operating temperature oxygen electrode contact layer (110) comprises matching thermal expansion coefficient with the solid oxide stack components.

6. Low operating temperature oxygen electrode contact layer (110) according to claim 1, characterized in that the low operating temperature oxygen electrode contact layer (110) material is being configured to enhanced mechanical interlocking and mechanical bonding between oxygen electrode (100) and at least one of the interconnector (114), bipolar plate, and contact structures during the stack operation at the operating temperatures such as from 500 to 900 °C.

7. Low operating temperature oxygen electrode contact layer (110) according to claim 1, characterized in that the low operating temperature oxygen electrode contact layer (110) materials are configured to being unreactive towards the state-of-the-art oxygen electrode materials.

8. Low operating temperature oxygen electrode contact layer (110) according to claim 1, characterized in that the low operating temperature oxygen electrode contact layer (110) materials are configured to being unreactive towards the coating materials for at least one of the interconnector (114), bipolar plate, and contact structures.

9. Low operating temperature oxygen electrode contact layer (110) according to claim 1, characterized in that the low operating temperature oxygen electrode contact layer (110) materials are being configured to provide protection to the oxygen electrode (100) from the oxygen electrode poisoning impurities by providing a reactive buffer layer.

10. Manufacturing process of low operating temperature oxygen electrode contact layer (110) according to any of claims 1-9, characterized in that in the manufacturing process the low operating temperature oxygen electrode contact layer (110) is made of an ABO3 perovskite structure containing at least one of La and Ca together with trace amounts of Mg, Sr, and Ba, and La and Sr together with trace amounts of Mg, Ca, and Ba on the A-site and at least one of Mn, Fe, Ni, and Cu and Fe, Ni, Co, and Cu on the B-site to lower the sintering temperature to 500 – 800 °C.

11. Manufacturing process of low operating temperature oxygen electrode contact layer (110) according to claim 10, characterized in that the low operating temperature oxygen electrode contact layer (110) is manufactured onto a solid oxide cell independent of its structure and shape.

12. Manufacturing process of low operating temperature oxygen electrode contact layer (110) according to claim 10, characterized in that the low operating temperature oxygen electrode contact layer (110) is directly manufactured on the oxygen electrode (100) of the solid oxide cells by at least one of screen printing, dip-coating, brush painting, tape casting, air spray coating, ultrasonic spray coating, electrostatic spray coating, spray pyrolysis, high velocity oxy-fuel spray, high velocity air fuel spray, combustion flame spray, plasma spray, vacuum plasma spray, warm spray, gas dynamic cold spray, vacuum assisted dip coating, spin coating, inkjet printing, 3D printing, ALD, PLD, PVD, and CVD techniques.

13. Manufacturing process of low operating temperature oxygen electrode contact layer (110) according to claim 10, characterized in that the low operating temperature oxygen electrode contact layer (110) powder is synthesized by at least one of the solid-state reaction, sol-gel processing, chemical combustion, spray pyrolysis, co-precipitation, and hydrothermal methods.

14. Manufacturing process of low operating temperature oxygen electrode contact layer (110) according to claim 10, characterized in that thelow operating temperature oxygen electrode contact layer (110) powder is processed via at least ball milling to obtain a proper particle size distribution ranging from nanometer scale to micrometer scale.

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