Electrochemical device comprising thin porous metal sheet
阅读说明：本技术 由薄多孔金属板组成的电化学装置 (Electrochemical device comprising thin porous metal sheet ) 是由 刘伟 于 2019-06-05 设计创作，主要内容包括：使用薄微孔金属板的电化学装置。多孔金属板可以具有小于200μm的厚度,提供孔径在2.0nm至5.0μm范围内的三维互联网絡孔结构,并且是导电的。通过提供反应物/电子大的接触表面积,微孔金属片用于带正电和/或带负电的电极。纳米级催化剂或特征可以添加进含有亚微米级和微米级孔径的多孔金属片的孔内,以增强反应活性和容量。微孔陶瓷材料可以以小于40μm的厚度涂覆在多孔金属板上,以增强多孔金属板的功能。非导电性陶瓷涂层可用作膜分离器。电化学装置可用于分解分子或合成分子,例如从水和氮分子合成氨。(An electrochemical device using a thin microporous metal plate. The porous metal plate may have a thickness of less than 200 μm, provide a three-dimensional inter-network-channel structure with a pore size in the range of 2.0nm to 5.0 μm, and be electrically conductive. Microporous metal sheets are used for positively and/or negatively charged electrodes by providing a large contact surface area for reactants/electrons. Nanoscale catalysts or features may be added into the pores of a porous metal sheet containing submicron and micron pore sizes to enhance reaction activity and capacity. The microporous ceramic material may be coated on the porous metal plate in a thickness of less than 40 μm to enhance the function of the porous metal plate. The non-conductive ceramic coating may be used as a membrane separator. Electrochemical devices can be used to decompose molecules or synthesize molecules, such as ammonia from water and nitrogen molecules.)
1. An electrochemical conversion device comprising:
a positive electrode-carrying assembly;
a negatively charged electrode assembly; and
a membrane separator, a membrane filter,
wherein at least one of the positively charged electrode, the negatively charged electrode or the membrane separator comprises at least one electrically conductive metal-based porous sheet having: the thickness is less than 200 μm, the structure has an interconnected net-like channel structure, the aperture of 50-100% of the channels is less than 5 μm, and the porosity is 20-80%.
2. An electrochemical conversion device according to claim 1, wherein the metal-based porous sheet comprises a layer of microporous ceramic material, the layer having a thickness of less than 40 μm and a surface pore size of less than 100 nm.
3. An electrochemical conversion device according to claim 2, wherein the microporous ceramic material comprises zirconia, yttria-stabilized zirconia, ceria, alumina, silicon carbide, or mixtures thereof.
4. The electrochemical conversion device of claim 2, wherein the microporous ceramic material comprises 0.5-5 wt% of a sintering promoter comprising nickel oxide, cobalt, manganese oxide, silica, or mixtures thereof.
5. The electrochemical conversion device of claim 1, wherein the metal-based porous sheet comprises catalyst particles within the pores in a volume fraction of less than 50% of the pore volume.
6. The electrochemical conversion device according to claim 5, wherein the catalyst particles comprise nano-sized transition metals, metal oxides, metal nitrides, or mixtures thereof.
7. The electrochemical conversion device of claim 1, wherein at least one surface of the metal-based porous sheet comprises a conductive layer coated with an electrolyte and a catalyst, wherein the catalyst comprises nano-and/or micro-sized particles comprising a transition metal, or a transition metal alloy, a transition metal oxide or oxide compound, a metal nitride, a carbon-supported transition metal or alloy, a carbon-supported transition metal and metal oxide, a carbon-supported metal nitride, a transition metal-ceria nanocomposite, or mixtures thereof.
8. An electrochemical conversion device according to claim 7, wherein the nano-sized transition metal oxide catalyst is prepared by using a membrane reactor comprising a continuous injection of one reactant stream through a membrane having a pore size of less than 1 μm to react with another reactant stream.
9. The electrochemical conversion device of claim 1, wherein the metal-based porous sheet comprises carbon nanotubes located inside the pores in a volume fraction of less than 50% of the pore volume.
10. An electrochemical conversion device according to claim 1, wherein the electrochemical conversion comprises electrolysis of water.
11. An electrochemical conversion device according to claim 1, wherein electrochemical conversion comprises synthesis of ammonia from water and nitrogen.
12. The electrochemical conversion device of claim 1, wherein the device comprises a battery.
13. An electrochemical conversion device according to claim 1, wherein the device comprises a fuel cell.
14. The electrochemical conversion device of claim 1, wherein the device comprises a capacitor.
15. The electrochemical conversion device of claim 1, wherein the metal-based porous sheet comprises nickel, a nickel alloy, or stainless steel.
16. An apparatus for electrochemically producing ammonia, comprising:
a positively charged electrode assembly comprises a conductive metal-based porous plate, wherein the thickness of the conductive metal-based porous plate is less than 200 mu m, an interconnection network channel structure comprises 50-100% of pores, the pore diameter of the pores is less than 5 mu m, and the porosity is within the range of 20-80%;
a membrane separator comprising a porous ceramic material having a thickness of less than 40 μm; and
a negatively charged electrode comprising a nanoscale iron catalyst, a mixture of carbon and nanoscale iron and/or iron oxide, carbon-supported nanoscale iron oxide, carbon-supported nanoscale iron nitride, a mixture of carbon and iron nitride, or a mixture thereof.
Technical Field
Embodiments of the present invention are generally directed to electrochemical systems and their operation, and in particular to electrochemical systems comprised of thin porous metal plates.
Background
The membrane separator, the positive electrode and the negative electrode are three basic components of an electrochemical conversion device, such as an electrochemical production for electrolyzing water, ammonia water, a fuel cell and a battery. The negatively charged electrode provides electrons for the electrochemical reactions occurring in the electrode. At the positive electrode, electrons are removed from the electrochemical reaction. For example, the following chemical reactions occur when water is electrolyzed:
on the negatively charged electrode:
2e-+2H2O→2OH-+H2
on a positively charged electrode:
2OH-→2e-+0.5O2+H2O
the overall reaction is:
H2O→H2+0.5O2
when operating a hydrogen fuel cell in a hydroxide electrolyte, the following reactions occur at the electrodes:
on the negatively charged electrode:
2e-+H2O+0.5O2→2OH-
on a positively charged electrode:
H2+2OH-→2e-+2H2O
the overall reaction is:
H2+0.5O2→H2O
electrochemical conversion is used in many important industrial processes and is becoming increasingly important for storing electrical energy from renewable or intermittent sources such as wind, solar and hydro. Hydrogen and oxygen are two widely used gases, and electrolysis of water in an electrolytic cell can produce. Conversely, the combustion of hydrogen with oxygen in a fuel cell can produce electrical energy. Rechargeable batteries are widely used in electronic devices and electric vehicles, involving reversible electrochemical reactions. Electrochemical reactions play a role in some emerging processes, such as the production of ammonia from water and nitrogen using electrical energy. Small electrochemical devices can be used to produce ammonia on demand, such as providing ammonia for selective catalytic reduction of nitric oxide in vehicle exhaust. The large-scale electrochemical device can be used for producing ammonia by utilizing wind energy or solar energy and is used for chemical fertilizers or chemical energy storage media.
A common requirement of these electrochemical devices is an electrode that provides a high specific surface area for electrons to react with the reactant. Much research and development effort has been devoted to finding new reaction chemistries, improving electrolyte materials, improving catalysts and formulations. However, improvements to the electrode stem structure are limited by the existing materials. The embodiments described herein fill this gap.
In addition to the support structure for the electrodes, the various embodiments described below provide a new material structure for the separator. The separator, as an integral part of the electrochemical conversion device, has one or more of the following functions: i) keeping the positively and negatively charged electrodes insulated from each other to avoid short circuits, ii) allowing the electrolyte to pass, and iii) separating the reaction products of the two electrodes.
In an embodiment, the separator is a membrane insulator that is non-conductive while allowing ions to pass through. For example, the separator allows OH in an alkaline electrolyte-Ion passage, H in proton electrolyte3O+Ion passage, Li in lithium batteries+The ions pass through. In order to have a high power and energy density, the membrane separator is preferably as thin as possible. However, the membrane separator must be thick enough to prevent shorting of the two electrodes. Because the reaction occurs periodically, the membrane separator must be strong enough to maintain structural integrity. Therefore, in order to increase the power and/or energy density of electrochemical devices, thin, durable, high permeability membrane separators are needed.
The desire for more efficient electrodes and membrane separators has resulted in the development of electrochemical conversion processes for producing ammonia from nitrogen and hydrogen or nitrogen and water. Today, ammonia is mainly produced by the thermally catalyzed reaction of nitrogen and hydrogen at high temperatures (350-. Such processes, commonly referred to as Haber-Bosch (Haber-Bosch) processes, are capital and energy intensive and are widely used for high-scale production. On the other hand, this reaction process is thermodynamically favorable at low temperatures.
ΔH＝-92.4kJ·mol-1
Us patent No. 7,314,544B2(2006) discloses an electrochemical process for the synthesis of ammonia from an electrolyte with nitrogen and hydrogen, the electrolyte consisting of a lithium hydride + lithium nitride molten salt. Without the membrane separator, a mixed product stream of anode and cathode reactions would be produced. Us patent No. 8,916,123b2 (2014) discloses the production of ammonia from nitrogen and water in a lithium nitride electrolyte using a lithium ion conducting membrane, wherein nitrogen reacts lithium hydroxide to form lithium nitride, which is reconverted to lithium hydroxide, while reacting with water to form ammonia. Practical examples of ammonia production from nitrogen and water at temperatures lower than the Haber-Bosch (Haber-Bosch) process are disclosed in U.S. patent publication 2016/0138176a1(2016), U.S. patent publication 20170037521(2017), and U.S. patent 9540737B2 (2017). These three patents disclose the use of alkaline electrolytes including aqueous alkaline hydroxide solutions, alkaline hydroxides and salts, and eutectic melting alkaline hydroxides. However, membrane separators are not described. Since the mixture of hydrogen and oxygen gases may be explosive, separators are required to maintain separation of the oxygen and hydrogen generated from the respective electrodes. Furthermore, even if the mixture concentration is controlled outside the explosive limit, the cost of separating hydrogen and oxygen is high. The literature reports low temperature synthetic ammonia separators (Lan and others) using proton conducting electrolyte (PEM) membranes as air and water, "direct ammonia synthesis from air and water at ambient temperature and pressure", scientific report, 3(2013) 1145. However, these electrolyte (PEM) membranes typically use noble metal catalysts. Ceramic type ion-conducting membranes have been explored (e.g., Lan et al "Synthesis of ammonia directly from humid air at moderate temperatures" application catalysis b: "Environmental" 152: (Environmental) 153(2014) 212-.
In summary, in order to realize practical applications of nitrogen and water for ammonia production, a more efficient and higher-productivity electrochemical device is required.
Disclosure of Invention
Embodiments described herein teach the use of high surface area porous metal sheets as a framework structure for electrodes and/or membrane separators in electrochemical conversion devices. Fig. 1 shows a metal plate 100 having a thickness of less than about 200 μm with an interconnected pore structure 102 having pore sizes in the micron and submicron range. Which may be used as the structural material 106 for the positively and negatively charged electrodes 104,106 in fig. 2A, the only positively charged electrode 104 in fig. 2B, or the only negatively charged electrode 106 in fig. 2C, 2C. The microporous metal plate 100 is conductive and provides a large specific surface area for electrons to react with reactants within the pores. The interconnected porous structures 102 allow reactants, electrolytes and products to move into and out of the electrode body. The chemical composition and/or pore structure of the microporous metal sheet 100 may be adjusted to meet the specific application requirements of a given electrode reaction.
When the microporous metal plate 100 is used for one electrode (fig. 2B or fig. 2C), the other electrode preferably includes a paste or coating layer filled with catalyst particles having a particle diameter ranging from several nanometers to several tens of micrometers. Examples of catalyst particles include nano-and/or micro-sized transition metals (Fe, Ni, Co and alloys thereof), transition metal-ceria nanocomposites, carbon-supported transition metals and metal oxides, transition metal nitrides, and carbon-supported metal nitrides. Some conductive material, such as carbon black, may be added to the slurry to enhance the electronic conductivity of the filled particle layer. Some binder may be added to the layer of filler particles to hold the particles in place.
Another embodiment includes integration of the electrode with a membrane separator, as shown in fig. 3, where a membrane layer is deposited on a porous metal plate. The microporous metal sheet supporting the membrane layer can be used as a positive electrode or a negative electrode. Microporous metal sheets can be an effective support for fragile membrane materials that are difficult to prepare as self-supporting thin (<40 μm) membranes. For example, ceramic-type materials are difficult to prepare into porous sheets for use because of their easy fracture. Processing these ceramic-based materials into films typically requires reaction and/or processing at such high temperatures (e.g., >300 ℃) that most organic polymeric support materials become unstable. The microporous metal sheet of the present invention is suitable as a support for these membranes due to the high mechanical strength and temperature resistance of the metal material and the uniform surface pore structure.
One embodiment is directed to an electrochemical conversion device comprising an assembly of a positively charged electrode, a negatively charged electrode, and a membrane separator. At least one of a positively charged electrode, a negatively charged electrode or a membrane separator comprises at least one electrically conductive metal-based porous sheet having: less than 200 μm thick, a pore structure having 80-90% porosity of less than 5 μm in size, and a porosity of 20-80%.
Another embodiment relates to an apparatus for the electrochemical production of ammonia comprising a positively charged electrode assembly comprising a conductive metal-based porous sheet comprising a thickness of less than 200 μm, an interconnected network pore structure with 80-90% of the pores having a pore diameter of less than 5 μm and a porosity of 20-80%, a membrane separator comprising a porous ceramic material having a thickness of less than 40 μm, and a negatively charged electrode comprising a nanoscale iron catalyst, a mixture of carbon and nanoscale iron and/or iron oxide, carbon-supported nanoscale iron oxide, carbon-supported nanoscale iron nitride, a mixture of carbon and iron nitride or mixtures thereof.
Drawings
Fig. 1 is a perspective view of a microporous metal plate suitable for use in a skeletal structure of an electrochemical converter according to an embodiment.
Fig. 2A-2C are schematic diagrams of embodiments using the microplates of fig. 1 in an electrochemical conversion device.
FIG. 3 is a perspective view of a membrane separator supported on a microporous metal plate according to an embodiment.
Fig. 4A-4D are micrographs illustrating features of a microporous metal plate structure, according to an embodiment.
Fig. 5A and 5B are diagrams illustrating a pore size distribution of an electrode of a porous metal plate according to an embodiment.
Fig. 6 is a schematic illustration of loading a nanoscale catalyst in porous metal plate pores according to an embodiment.
FIGS. 7A and 7B are SEM micrographs showing examples of a porous ceramic coating on a microporous metal plate electrode with one coating and two coatings, respectively.
Fig. 8A and 8B show comparative graphs of water electrolysis using a nickel plate for a commercial metal mesh according to the example.
Fig. 9A and 9B are comparative diagrams of the electrolysis of nickel sheets according to another embodiment and a commercial metal grid.
FIG. 10 is a graph comparing water electrolysis of nickel flakes with a commercial metal grid according to another embodiment.
Fig. 11A and 11B are SEM micrographs illustrating the microstructure of the ceramic coating according to the examples.
FIG. 12 is a topographical view of a film sample after 24 hours of heat treatment according to an example.
FIG. 13A is a schematic diagram illustrating a test cell configuration according to an embodiment.
Fig. 13B is an EIS curve of a film according to an embodiment.
FIG. 14 is a schematic of water electrolysis using ceramic coated porous nickel sheet electrodes according to an embodiment.
FIGS. 15A-15D are SEM micrographs illustrating the synthesis of nano molybdenum nitride catalyst particles within the pores of a nickel plate according to an example.
16A-16F are SEM micrographs illustrating carbon nanotubes grown within pores of a nickel plate according to an embodiment.
FIG. 17 is a schematic diagram of a membrane reactor for continuous synthesis of nano-metallic particles according to an embodiment.
FIGS. 18A-18C are photographs of a slurry of nano-iron particles produced by the membrane reactor shown in FIG. 17.
Fig. 19A-19C are TEM micrographs of nano-sized iron particles made according to one example of nano-iron oxide particles made by conventional techniques.
FIG. 20 is a schematic of an electrode assembly for producing ammonia from water and nitrogen, according to an embodiment.
Fig. 21 is a graph demonstrating ammonia yield at different voltages and temperatures on a porous nickel plate electrode according to an example.
Fig. 22A is a photograph of micro-and nano-structures of an N-Ni catalytic porous sheet at 50,000X magnification according to an embodiment.
Fig. 22B is a photograph of the porous sheet shown in fig. 22A at 500X magnification, including sample points for atomic composition analysis.
FIG. 23A is a photograph of micro-and nanostructures of a 10,000X magnification Fe/C (2M) catalyst, according to an embodiment
Fig. 23B is a photograph of the porous sheet shown in fig. 23A at 500X magnification, including sample points for atomic composition analysis.
FIG. 24A is a photograph of micro and nano structures of a N-Fe/C (1M) catalyst at 50,000 Xmagnification according to one embodiment.
Fig. 24B is a photograph of the porous sheet shown in fig. 24A at 500X magnification, including sample points for atomic composition analysis.
Fig. 25A is a photograph of micro-and nanostructures of an N-Fe catalyst at 50,000X magnification according to an embodiment.
Fig. 25B is a photograph of the porous sheet shown in fig. 25A at 500X magnification, including sample points for atomic composition analysis.
Detailed Description
Morphological and structural features of microporous metal sheet electrodes with various embodiments of 20cm x 50 μm porous nickel sheets are shown in fig. 4. The thin flat plate 100 provides a smooth and uniform surface (fig. 4A), similar to a metal foil. Its uniform microstructure was shown under a Scanning Electron Microscope (SEM) (fig. 4B and 4C). The surface includes micron and sub-micron pores 110 formed between metal particles 112, the surface being porous in pore structure at higher porosity. The three-dimensional pore interconnect structure 102 is clearly visible in cross-sectional view (fig. 4D), with pores interconnected throughout the thickness.
Preferably, the metal plate 100 is an electrical conductor that distributes or collects electrons from the entire plate 100 to the galvanic connection points. The metal material itself may be used as an electrochemical reaction catalyst. Or a catalyst may be deposited on the surface of the electron conducting metal particles 112 inside the pores 110. The heterogeneous reaction zone enhancement factor is defined as the ratio of the electron/reactant reaction zone in the porous metal sheet electrodes 104,106 to its geometric surface area. The relationship between heterogeneous reaction zone enhancement factor and pore size and porosity can be described by the following formula:
wherein F is an enhancing factor, SArArea of electron/chemical reaction zone, SAgGeometric surface area, δeThickness of sheet, epsilon porosity, dpThe pore diameter.
Thus, for a dense metal foil based electrode, the enhancement factor is 1. Using a thickness (. delta.) of 50 μme) A porous metal plate 100 with 50% porosity (. epsilon.) and an average pore diameter (dp) of 0.5 μm, and a reinforcement factor of 200. The degree of influence of porosity on the enhancement factor is limited. The low porosity (0.35) differs from the high porosity plate (0.70) by only a factor of 2. For constant porosity, the effect of pore size is significant. The reinforcing coefficient of the 0.1 μm pore sheet may be 10 times that of the 1 μm pore sheet and 100 times that of the 10 μm pore sheet. The effect of sheet thickness on the enhancement factor is limited because the transport resistance of reactants into the interior of the electrode and products out of the electrode increases proportionally with sheet thickness.
Volumetric productivity of electrodes is an important performance parameter of electrochemical devices, surface to volume (SA)V) In this regard, the following are described:
the specific surface area increases in the order of the reciprocal as the pore size decreases.
The above discussion highlights the benefit of reducing the pore size of the electrochemical reaction zone in a porous electrode.
The porosity of the perforated plate 100 can be measured as geometric porosity and adsorption porosity as described below.
Wherein epsilongGeometric porosity, pgGeometric density, pmMaterial density ∈lAdsorption porosity, VlAmount of liquid adsorbed, VgGeometric volume.
The geometric porosity is simply measured based on the weight and volume of the porous metal plate relative to its material density. The liquid adsorption porosity can be measured by the amount of liquid a given volume of the porous metal plate 100 can absorb. The liquid fluid used in the measurement should be completely wettable. Therefore, if all the pores 110 are interconnected in the porous metal plate 100, the geometric porosity should be equal to the liquid adsorption porosity. If the adsorption porosity is less than the geometric porosity, some of the pores in the porous metal plate 100 are inaccessible to the liquid. As an effective electrode structure, the porous metal plate 100 should have a liquid-adsorbing porosity that is the same as or very close to the geometric porosity. For the porous metal plate structure shown in fig. 4, the liquid adsorption porosity is the same as the geometric porosity.
The pore size of the porous metal plate 100 can be physically checked under a microscope. For the microporous metal plate 100, the pore size can be better characterized by using the now established analytical procedures. The outer pore size of the porous metal plate 100 may be characterized by mercury porosimetry techniques, as shown in fig. 5A. For two porous metal plates 100 of different thickness, a single narrow peak is shown in the pore size distribution curve, which represents a uniform surface pore structure. The number of pores of 2 μm or more is less than 1% for these two kinds of sheets 100. Capillary flow is another common method of characterizing the pore size distribution of the expanded metal sheet 100. Unlike mercury porosimetry, capillary flow techniques measure pore size based on gas flow path-i.e., aeration flow along the thickness of the sheet. Fig. 5B shows the pore size distribution of the two porous metal plates 100. In comparison with fig. 5A, it is shown that there are substantially smaller pores in the interior of the plate, the pore size being in the range of 0.1 to 0.8 μm, i.e. in the sub-micron range.
The nanocatalyst 120, the stabilizer 122, and/or the promoter 124 may be loaded within the pores 110 of the porous metal plate 100 to catalyze desired reactions and suppress undesired reactions, as shown in fig. 6. The size of the nanocatalyst is preferably smaller than the pore size of the porous metal plate 100, preferably by a multiple of a factor such that the pore openings are maintained in a certain ratio for rapid transport of the reactants and products. Preferably, the catalyst is in intimate contact with the metal particles 112. The nanocatalyst may be loaded into the support pores 110 using several methods, including impregnation and chemical vapor deposition.
Using the impregnation technique, the catalyst precursor is first prepared in solution. The solution is used to fill the pores 102,110 of the porous metal plate 100. The liquid carrier is then removed by drying, for example by heating or by applying a vacuum or both. The dried sample can be activated by heating under a controlled gas environment. For example, salts of transition metals (Fe, Co, Ni, Mn, etc.) may be dissolved in aqueous solutions. After drying, a nano-oxide catalyst is obtained by heating the sample in an oxidizing gas environment, and a nano-metal catalyst is obtained if the dried sample is heated in a reducing gas environment. Alumina, silica or zirconia oxides may be added by impregnation to promote and stabilize the catalytic activity of the nano transition metal (oxide) catalyst.
The porous metal plate 100 is heated in a gaseous environment containing a catalyst precursor using a chemical vapor deposition process such that the catalyst precursor diffuses into the pores 102,110 and grows catalyst within the pores 102, 110.
An ion-permeable, non-conductive coating 108 may be deposited on one surface of the microporous metal sheet 100, integrating the membrane separator 108 with one or both of the electrodes. In one embodiment, the membrane thickness is preferably about 2 to 10 times the pore size on the coated microporous metal surface so that the metal particle surface is completely covered by the continuous coating 108. Complete coverage of the metal particles 112 is required to eliminate possible short circuits. The pore size in the coating 108 may be determined by the nature of the material being coated and may be an order of magnitude smaller than the opening of the microporous metal sheet 100. Suitable materials for the membrane separator 108 include, but are not limited to, metal oxides, ceramics, glass and metal oxide + polymer mixed or composite phases. The membrane separator 108 may include one coating of the same material composition and/or structure (fig. 7A), or multiple layers of different materials and/or structures (fig. 7B). The film thickness can be determined by physically examining the cross-section under a microscope, as shown in fig. 7A and 7B. Coating thickness can also be evaluated by area loading density, which can be calculated from the following equation based on experimental measurements:
wherein wc,sArea load density (g/cm)2)，WcWeight of material coated on support plate (g), SAcArea of the coated support plate (cm)2)，δC,SCoating thickness (cm), pc,pThe packing density of the coating material (g/cc).
Most metal oxides have a packing density in the range of 1-3 g/cc. The coating thickness is preferably less than 2x10-3cm (20 μm) corresponds to 1-6X10-3Coating load of g/cc.
The film layer 108 may be formed on the porous metal plate 100 by hydrothermal growth. The seeds of the molecular sieve may be distributed on the support sheet surface, and then the seed-loaded sheet is immersed in a growth solution, under certain conditions of temperature and time, crystals of the molecular sieve grow from the seeds and fill the inter-crystalline voids, forming a continuous film layer 108.
In one embodiment, the membrane layer 108 may be made by wet chemical coating and post-treatment. Ceramic powders such as zirconia having particle sizes of 200-20nm can be dispersed in a liquid carrier to form a uniform coating slurry or solution. The coating solution is applied to the sheet metal support 100 to form a continuous packing of ceramic particles. The coated sheet may then be heated in an appropriate gas environment such that the ceramic particles sinter to form a durable coating 108 the sintering conditions (gas environment, temperature and time) may be selected to achieve the desired degree of sintering without causing damage to the porous metal support 100. For example, at a temperature higher than 500 ℃, an inert or reducing gas is preferably used if preferred to prevent oxidation of the metal plate 100.
In another embodiment, the coating is applied by vapor deposition. The material to be coated may be sputtered in the vapor phase or under vacuum into small particles or chips, which are deposited on the porous metal sheet 100 support surface to form a continuous layer 108.
One function of the membrane separator 108 in an electrochemical conversion device is to keep the oppositely charged electrodes 104,106 insulated from each other. Therefore, durable insulating materials such as zirconia, ceria, alumina and silica are preferred. For a given material, the resistance of the membrane separator 108 to shorting or electrical penetration increases with thickness. However, the thick film layer 108 is preferably avoided in the disclosed embodiments. One problem with the thick film layer 108 is that the thick film 108 on the porous metal plate support 100 is prone to cracking and delamination. Therefore, a thickness of less than 40 μm is preferable. Another problem with thick film layer 108 is increased mass transfer resistance. Allowing some ions in the electrolyte 118 to permeate is another function of the membrane separator. However, mass transfer resistance increases in proportion to the film thickness.
As a membrane separator 108, the coated material may have an inherent ionic conductivity, or may provide a path for the electrolyte 118 to move back and forth. For example, the zirconia material does not have ionic conductivity at low temperatures, but the porous zirconia film 108 can carry the alkaline hydroxide electrolyte solution by capillary forces. Capillary pressure is related to surface tension, contact angle and pore radius by the following equation.
Wherein Δ PcCapillary pressure, σ, surface tension of a liquid, θ, contact angle, rpRadius of pore
The KOH solution had a surface tension of about 0.067N/m at 37 ℃. If the solution is fully wettable in the pores, the capillary pressure is about 26 bar for pores of 50nm radius. If the membrane material does not provide ionic conductivity, the porous membrane layer 108, which is made of a material completely wettable to the electrolyte solution, controls the pore diameter to be less than 100nm to firmly hold the electrolyte solution in the pores.