JP4754782B2 - Solid acid electrolytes for electrochemical devices - Google Patents

Description

The present invention is an electrolyte for electrochemical devices, relates 及 Beauty electrochemical device incorporating such materials, and more particularly, to an electrolyte made from the novel solid acid material.

  Electrochemical devices rely on proton conducting materials such as proton flow through a membrane, or both proton and electron flow. Thus, protons or materials that conduct both protons and electrons are used as electrolytes or electrodes in many electrochemical devices including fuel cells, electrochemical supercapacitors, sensors, hydrogen separation membranes and membrane reactors.

  One particularly important application for these materials is in fuel cells. Fuel cells are an attractive alternative to combustion engines for a wide variety of applications. This is because fuel cells are more efficient and the pollutant levels produced from these operations are lower. There are three common types of fuel cells that fall under the present invention. 1) a direct hydrogen / air fuel cell system that stores hydrogen and then supplies it to the fuel cell as needed; 2) hydrogen is generated in situ from the hydrocarbon fuel and carbon monoxide is removed therefrom. An indirect hydrogen / air fuel cell that is subsequently fed to the fuel cell; and 3) a direct methanol fuel cell (DMFC) that feeds the methanol / water solution directly to the fuel cell. An example of this latter fuel cell is described, for example, in US Pat. No. 5,559,638. The disclosure of which is incorporated herein by reference.

Regardless of the fuel cell configuration selected, the operating efficiency of the device is limited by the efficiency of the electrolyte during proton transport. Typically, perfluorinated sulfonic acid polymers, polyhydrocarbon sulfonic acid polymers, and composites thereof are used as electrolyte membrane materials for fuel cells. However, such conventional materials utilize hydronium ions (H ₃ O ⁺ ) to facilitate proton conduction. Therefore, these materials must be hydrated and the loss of water immediately leads to degradation of electrolyte conductivity and thus degradation of fuel cell efficiency. Furthermore, such deterioration is irreversible. That is, simply reintroducing water into the system does not restore the conductivity of the electrolyte.

  As a result, fuel cells that utilize these materials require a peripheral system to ensure water recirculation and temperature control so that water does not evaporate. Such a system not only increases the complexity and cost of these fuel cells, but the fuel cell catalyst and other systems must operate at maximum efficiency because the system cannot exceed a temperature of 100 ° C. I can't. Moreover, if the temperature is increased, carbon monoxide poisoning of the fuel cell catalyst is reduced.

It has recently been found that a solid acid such as CsHSO ₄ can be used as an electrolyte in a fuel cell operated at a temperature of 140-160 ° C. The use of this material greatly simplifies the configuration of the fuel cell compared to the polymer electrolyte fuel cell. This is because hydration of the electrolyte is not necessary. Also, since the operating temperature rises, residual CO in the fuel flow can be better tolerated. The high conductivity of CsHSO ₄ and similar materials results from the structural phase transition that occurs at 141 ° C. from an ordered structure to a disordered structure. The ordered structure is based on a chain of SO ₄ groups joined by distinct hydrogen bonds, and in the disordered structure, the SO ₄ groups are free to reorient and easily pass protons between each other. Therefore, disorder in the crystal structure is an important prerequisite for high proton conductivity.

Ultimately, solid acid electrolytes can solve many of the problems faced by prior art polymer-based fuel cells. These problems are that they cannot operate at temperatures above 100 ° C (if they can operate at temperatures above 100 ° C, the CO tolerance of the Pt catalyst will increase), the need for humidification, And methanol permeating across the electrolyte. Therefore, the technical purpose of this study is to simplify fuel cell operation by using alternative electrolytes. However, the lifetime of solid acids based on these sulfates and selenium is insufficient for commercial applications. The short lifetime of both CsHSO ₄ and CsHSeO ₄ under fuel cell operating conditions results from the reduction of sulfur and selenium by hydrogen in the presence of typical fuel cell catalysts. This reduction is:
2 CsHSO ₄ + 4 H ₂ ----> Cs ₂ SO ₄ + H ₂ S + 4H ₂ O
2 CsHSeO ₄ + 4 H ₂ ----> Cs ₂ SeO ₄ + H ₂ Se + 4H ₂ O

  Accordingly, there is a need for a solid acid compound with high proton conductivity that is stable under fuel cell conditions and a processing method that provides a high performance membrane-electrode assembly (MEA) based on such solid acid compound.

Accordingly, the present invention relates to solid acids that can operate for long periods of time at high temperatures by forming hydrogen bonds and being rotationally disordered, such as tetrahedral or octahedral hydrogen-bonding anionic groups; It relates to a stable electrolyte material comprising a solid acid having a protic trigonal disordered phase; and / or operable at a temperature of about 100 ° C. Specifically, the present invention, some improved solid acid electrolyte material classes,及 Beauty relates to an electrochemical device containing such a material.

As an example, this specification describes an electrolyte comprising a solid acid having a superprotonic trigonal phase. In one such example , the solid acid has the general formula: (M _x M ′ _1-x ) ₃ H _3x (PO ₄ ) ₂ (wherein M is any alkali metal having a +1 charge) Or a transition metal or other functional group such as Li ⁺ ... Cs ⁺ , NH ⁴⁺ ; M ′ is any alkaline earth metal or transition metal having a +2 charge, such as Mg ²⁺ ... Ba ²⁺ , Pb ²⁺ ). Alternatively, the solid acid may be M ₃ H _2x [(P _1-x , Si _x ) O ₄ ] ₂ or M ₃ H _2x [(P _1-x , Ge _x ) O ₄ ] ₂ (where M is a + optional alkaline earth metal or transition metal having a second charge, for example, Mg ²⁺ ... Ba ^2+, mixed phosphate compound having a Pb ^2+).

As another example, this specification describes an electrolyte comprising a solid acid having a tetrahedral anion selected from the group of PO ₄ , PO ₃ F and PO ₃ H. In such an example , the solid acid has the general formula: MH _{1 + x} (PO ₃ A) _1-x (PO ₄ ) _x and M ₃ H _x [(PO ₃ A) _x (PO ₄ ) _1-x ] ₂ (in the formula, M is any alkali metal or other functional group having a + 1 charge, for example, Li ⁺ ... Cs ^+, an NH ^4+, a is a is F or H) consist It may be. Thus, the solid acid species is selected from monofluorophosphate and phosphite species or from mixed monofluorophosphate / phosphate species and mixed phosphite / phosphate species May be. Finally, the solid acid species can also be selected from molecules with mixed chromate / phosphate tetrahedral anionic species. Such molecules have the general formula: MH _{1 + x} (CrO ₄ ) _1-x (PO ₄ ) _x and M ₃ H _x [(CrO ₄ ) _x (PO ₄ ) _1-x ] ₂ (wherein M has any alkali metal or other functional group having a +1 charge, such as Li ⁺ ... Cs ⁺ , NH4 ⁺ ).

As yet another example, this specification describes an electrolyte comprising a solid acid having an octahedral anion. In such an example , the solid acid can be of the general formula: MHSiF ₆ , MHGeF ₆ (M is any alkali metal or other functional group having a +1 charge, such as Li ⁺ ... Cs ⁺ , NH ⁴⁺ ) And fluorosilicates. The solid acid has the general formula: MH _x (P _1-x Si _x ) F ₆ and MH _x (P _1-x Ge _x ) F ₆ (wherein M is any alkali metal having a +1 charge or It can also be selected from mixed phosphosilicates and phosphogermanates of other functional groups, such as Li ⁺ ... Cs ⁺ , NH ⁴⁺ . The solid acid can also be selected from compounds having an aluminum fluoride octahedral anionic group AlF ₆ .

In one embodiment , the present invention relates to a membrane comprising a solid acid electrolyte. In such embodiments, the present invention further includes a structural binder or matrix material to improve mechanical integrity or chemical stability of the membrane. For these embodiments, the binder can comprise any suitable stabilizing material, such as a polymer, ceramic or oxide glass.

As yet another example, the specification describes electron-conducting matrix material, for example, a metal or carbon-based material.

As yet another example, this specification describes a method for synthesizing solid acids.

  In yet another embodiment, the present invention relates to an electrochemical device incorporating a solid acid electrolyte. In such embodiments, the electrolyte is built into protons or any electrochemical device that requires proton and electron flow to function across the membrane, such as a fuel cell, hydrogen separation membrane or electrolyzer. can do.

  These and other features and advantages of the present invention can be better understood with reference to the following detailed description when considered in conjunction with the accompanying drawings.

  The present invention is a stable electrolyte material that forms hydrogen bonds, can be rotationally disordered, and can operate for long periods of time at high temperatures, such as tetrahedral or octahedral hydrogen bonded anionic groups. An electrolyte material comprising a solid acid having a superprotonic trigonal, tetragonal or cubic disordered phase; and / or a solid acid capable of operating at a temperature of 100 ° C. or higher. Such materials are referred to herein as “solid acid electrolytes” or “electrolytes”.

As used herein, the term “solid acid” means a compound having properties intermediate between those of a normal acid such as H ₂ SO ₄ and those of a normal salt such as Cs ₂ SO _4. . In general, the chemical formula of a solid acid of the type used according to the present invention can be shown as a combination of salt and acid, for example CsHSO ₄ (0.5 Cs ₂ SO ₄ * 0.5 H ₂ SO ₄ ). This has already been described in US patent application Ser. No. 09 / 439,377 (filed Nov. 15, 1999). The disclosure content of the above specification is incorporated herein by reference. Solid acids generally include oxyanions such as SO ₄ , SO ₃ , SeO ₄ , SeO ₃ , PO ₄ , PO ₃ F, PO ₃ H, AsO ₄ , SiF ₆ or AlF ₆ . These oxyanions are bonded together through OH ... O hydrogen bonds. Furthermore, the solid acid contains cations for total charge balance. This structure can contain more than one type of oxyanion XO ₄ , XO ₃ , XO ₃ A or XF ₆ groups, and can also contain more than one type of cation M species. Under certain conditions of temperature and pressure, generally at about 50-100 ° C., the crystal structure of these solid acids becomes disordered. Accompanying such a disordered state known as the superprotonic phase, the conductivity of the solid acid often increases by several orders of magnitude to about 10 ⁻³ to 10 ⁻² Ω ⁻¹ cm ⁻¹ . Due to the structure of these solid acids, the proton transport mechanism does not depend on the movement of hydronium ions. As a result, the solid acid does not need to be hydrated to function, and can operate at high temperatures to increase efficiency and reduce the possibility of poisoning of the catalytic medium in the electrochemical device.

Although several conventional superprotic solid acid species have been described above, these materials are generally hydrogenated in the presence of typical fuel cell catalysts, for example:
2 CsHSO ₄ + 4 H ₂ ----> CsHSO ₄ undergoes reduction to H ₂ S based on Cs ₂ SO ₄ + H ₂ S + 4H ₂ O. A similar reduction reaction occurs for CsHSeO ₄ .

In addition, many solid acids, for example CsHSO ₄ and CsHSeO ₄ are water soluble, therefore, no longer life in the presence of liquid water.

  Accordingly, the present invention relates to an improved solid acid structure that does not undergo reduction in the presence of catalytic materials such as Pt and other transition metal elements. In practice, it may operate at a temperature of about 100 ° C. in the presence of a tetrahedral or octahedral hydrogen-bonded anion group; any solid acid having a superprotonic trigonal disordered phase; and / or a typical fuel cell catalyst. Any solid acid that can be used can be utilized as the electrolyte material in the present invention.

For example, Applicants have found that the reduction reaction can be avoided by replacing the superprotonic solid acid sulfur (or Se) with elements such as Si and Ge. The reduction reaction can be avoided because similar reducing compounds such as H ₄ Si and H ₄ Ge are extremely unstable. Accordingly, it is preferred to use superprotic acids such as LaHSiO ₄ , BaH ₂ SiO ₄ and SrH ₂ GeO ₄ in the devices and materials of the present invention.

As an example, the present specification describes a solid acid having a disordered trigonal phase and a tetrahedral anion. For example, it is well known that the compounds Pb ₃ (PO ₄ ) ₂ and Rb ₃ H (SeO ₄ ) ₂ are essentially isostructural at room temperature except that protons are incorporated in the selenate compound. It has been. When both are heated, they are transformed into a disordered trigonal phase. This disorder is accompanied by several orders of magnitude increase in proton conductivity, since protons are built into the selenate.

Thus, in the example of the solid body acid electrolyte that having a trigonal disordered phase and tetrahedral anions, solid acid electrolyte, the general _{formula: (M 1-x M '} x) 3 H 3 (PO 4) 2 (In this formula, M is any alkaline earth metal or transition metal having a +2 charge, eg Mg ²⁺ ... Ba ²⁺ , Pb ²⁺ ; and M ′ has a +1 charge Selected from phosphate species of any alkali metal or other functional group such as Li ⁺ ... Cs ⁺ , NH ⁴⁺ . When a proton is incorporated in the phosphate compound by substituting an alkali metal or ammonium ion on the alkaline earth site, a superproton highly conductive trigonal disordered phase is generated. The intermediate alkali-alkaline earth acid phosphate as formulated here is Na ₂ CaH ₂ (PO ₄ ) ₂ .

In the second example of a trigonal solid acid with a tetrahedral anion, by replacing P ⁵⁺ with species such as Si ⁴⁺ and Ge ⁴⁺ , a trigonal compound such as Pb ₃ (PO ₄ ) Proton is introduced into ₂ . The built-in proton can then balance the decrease in positive charge. For example, in the case of such an example , the form: M ₃ H _2x [(P _1-x , Si _x ) O ₄ ] ₂ and M ₃ H _2x [(P _1-x , Ge _x ) O ₄ ] (in this formula , M is a suitable solid acid from the group described by any alkaline earth metal or transition metal having a +2 charge, e.g. Mg ²⁺ ... Ba ²⁺ , Pb ²⁺ be able to.

In another example for a solid acid electrolyte material having a tetrahedral anion, a tetrahedral anion group, PO ₃ F, is utilized. It is well known that PO ₃ F is isoelectronic with SO ₄ and shares chemical properties. For example, the compound CsHPO ₃ F is known in the literature and shares some similarities with the superprotonic tetrahedral structure of CsHSO ₄ . The proton content in the monofluorophosphate can be increased by introducing a phosphate anion based on the general chemical formula MH _{1 + x} (PO ₃ F) _1-x (PO ₄ ) _x . These phosphate anions include the simple compound MHPO ₃ F, as well as the known MH _{1 + x} (SO ₄ ) _1-x (PO ₄ ) _x compounds. PO ₃ F anions, not only in the compounds of the general stoichiometric MHSO _4, to replace the SO ₄ even in the compound having a stoichiometric _{M 3 H (SO 4) 2} , the PO ₃ F anions Can be used. Thus, in yet another example of a solid acid having a tetrahedral anion, the solid acid can be represented by the general formula: M ₃ H (PO ₃ F) ₂ and M ₃ H _{1 + 2x} [(PO ₃ F) _1-x ( PO ₄ ) _x ] ₂ , where M is any alkali metal or other functional group having a +1 charge, such as Li ⁺ ... Cs ⁺ , NH ₄ ⁺ It can be selected from species and mixed monofluorophosphate / phosphate species.

Yet another type of tetrahedral group that can be considered as a possible substituent for SO ₄ (or SeO ₄ ) is PO ₃ H. The chemical similarity between SO ₄ and PO ₃ H is slightly lower than the chemical similarity between SO ₄ and PO ₃ F. This is because the PH bond is very clear. Nevertheless, room temperature structures are formed on similar hydrogen bonded tetrahedral units. Several compounds in the MH (PO ₃ H) group are known, including CsH ₂ PO ₃ , KH ₂ PO ₃ , LiH ₂ PO ₃ and NH ₄ H ₂ PO ₃ . Of note compared to compounds such as CsH ₂ PO ₄ is that the oxidation state is +3 instead of +5, and thus phosphite is less likely to undergo reduction during fuel cell operation. . Thus, in yet another example , the tetrahedral solid acid species has the general formula: MH (PO ₃ H), M ₃ H (PO ₃ H) ₂ , MH _{1 + x} (PO ₃ H) _1-x (PO ₄ ) _x and M ₃ H _{1 + 2x} [(PO ₃ H) _1-x (PO ₄ ) _x ] ₂ (wherein M is any alkali metal or other functional group having a +1 charge, such as Li ⁺ ... Cs ⁺ , NH ₄ ⁺ ) and mixed phosphites and phosphite / phosphate species.

In addition, Applicants will note that solid acids having sulfate groups substituted with phosphites will have similar structure, phase transition and proton transport properties as CsHSO ₄ , but advantageously H ₂ It has been found that there is no indication of degradation from the generation of S High temperature testing of the compound CsH (PO ₃ H) (or CsH ₂ PO ₃ , cesium hydrogen phosphite) revealed that this material can initiate a transition to a highly proton conducting phase at 137 ° C. These results are summarized in FIGS. 1a-1e and discussed below. The transition is accompanied by a large amount of transformation heat, ΔH = 58 ± 2 J / g (12.4 ± 0.4 kJ / mol), and shows measurable hysteresis that occurs at 96 ° C on cooling, as shown in Figures 1a-1c . High-temperature X-ray powder diffraction shows that the high-temperature phase is cubic (a _o = 4.896 (1) 、), probably exhibits a CsCl structure, has a Cs atom at the corner of a simple cubic unit cell, and a PO ₃ H group at the center. Showed that. As shown by comparison in FIGS. 1d and 1e, the conductivity in the high temperature phase at 160 ° C. is 5.5 × 10 ⁻³ Ω ⁻¹ cm ⁻¹ and the activation energy for proton transport is 0.40 ± 0.01 eV . These values promote proton transport through rapid reorientation of PO ₃ H groups in the cubic phase of CsH (PO ₃ H), as is known to occur in the high temperature tetragonal CsHSO ₄ phase. It is suggested to be.

Yet another type of solid acid electrolyte material with tetrahedral anions is formed by K ₂ CrO ₄ . K ₂ CrO ₄ demonstrates the exchangeable nature of sulfate and chromate. Chromate has also been shown to exhibit the same tetrahedral disorder at high temperatures that sulfate does. Thus, the tetrahedral solid acid species has the general formula: MH _{1 + x} (CrO ₄ ) _1-x (PO ₄ ) _x (where M is any alkali metal or other functional group having a +1 charge) For example, Li ⁺ ... Cs ⁺ , NH ⁴⁺ ). Mixed sulfate / phosphate compounds have been synthesized at a superprotonic phase transition temperature lower than that of the compound end member. For example, in the case of _{Cs 2 HSO 4 H 2 PO 4} , super proton phase transition temperature is the case 141 ° C. for CsHSO _4, compared it to the a 230 ° C. For CsH ₂ PO _4, is 90 ° C.. As the phase transition temperature in similar chromate / phosphate compounds is lowered, this helps to prevent nuisance dehydration of the phosphate at temperatures above the superprotonic phase transition temperature. By replacing the salt with chromate, the reduction of sulfate to H ₂ S in the fuel cell environment is avoided.

As another example, this specification describes solid acid electrolyte compounds containing octahedral polyanions. For this class, materials such as KPF ₆ form structures that remain disordered at slightly elevated temperatures. However, such materials do not contain protons. By replacing the PF ₆ polyanion with SiF ₆ or GeF ₆ , protons can be introduced for charge balance reasons. Also noteworthy in this case is that, unlike their sulfate and phosphate equivalents, these salts and acids are insoluble in water. Thus, the solid acids are of the general formula: MHSiF ₆ and MHGeF ₆ (wherein M is any alkali metal or other functional group having a +1 charge, eg Li ⁺ ... Cs ⁺ , NH ₄ ⁺ ) From fluorosilicates and germanates.

Alternatively, use the well-known structural transition to disordered states in compounds like KPF ₆ for proton transport by chemical substitution of the PF ₆ anion partially (but not completely) with SiF ₆ or GeF ₆ You can also Since proton incorporation is accompanied by substitution, high conductivity in the high-temperature disordered phase can be obtained. Thus, for this alternative example of a solid acid, the solid acid electrolyte material has the general formula: MH _x (P _1-x Si _x ) F ₆ and MH _x (P _1-x Ge _x ) F ₆ (where , M may be selected from mixed phosphosilicates and phosphogermanates of any alkali metal or other functional group having a +1 charge, e.g. Li ⁺ ... Cs ⁺ , NH ₄ ⁺ it can.

In yet another example , the solid acid may have an octahedral anionic group based on AlF ₆ . Examples of such compounds are MH ₂ AlF ₆ and M ₂ HAlF ₆ (wherein M is any alkali metal or other functional group having a +1 charge, such as Li ⁺ ... Cs ⁺ , NH ₄ ⁺ of); or M'HAlF ₆ (in this formula, M 'is + 2 optional alkaline earth metal or transition metal having an electric charge, for example, Mg ²⁺ ... Ba ^2+, is Pb ²⁺ ). Other examples also include mixtures of the above types with MHSiF ₆ , MHGeF ₆ and MPF ₆ .

  The preferred material for any specific electrochemical device depends on its application. For example, the preferred species may depend on device requirements such as high conductivity, low cost, thermal stability or chemical stability.

Although the conductivity of the solid acid is described purely with respect to protons, these solid acid electrolytes are electrons depending on the choice of the M cation and the X element in the anion XO ₄ , XO ₃ or XF ₆ moiety of the above chemical formula. These solid acid electrolytes can be formed to conduct both protons and protons. For example, by using a given amount of a variable valence element, such as Pb or In for M, or Cr or Mn for X, a solid acid can be formed to conduct electrons as well as protons. .

The present specification also describes a method for forming the superprotonic solid acid electrolyte material. The application for the synthesis of stable highly conductive solid acid compounds is that degradation of conventional solid acid materials such as CsHSO ₄ occurs through the production of H ₂ S by the reduction of sulfur under hydrogen Derived from human perception. This reduction reaction is usually slow even at the fuel cell operating temperature of about 100 ° C., but is significantly accelerated in the presence of typical fuel cell catalysts.

However, the Applicant has discovered that such a reduction reaction can be eliminated by replacing the conventional superprotonic solid acid S (or Se) with elements such as Si and Ge. The reduction reaction can be eliminated in this way because H ₄ Si and H ₄ Ge are extremely unstable and the possibility of their formation is low. Thus, target analogs of CsHSO ₄ and CsH ₂ PO ₄ are, for example, LaHSiO ₄ and BaH ₂ SiO ₄ .

If an alkaline earth metal or rare earth metal is incorporated rather than an alkali metal, an additional advantage of such a compound is that the resulting compound is water insoluble. If an alkali metal is utilized, a new stoichiometry, such as Cs ₂ H ₂ SiO ₄ , can be investigated.

In general, however, acidic silicates and acidic germanates are more difficult to synthesize than similar sulfates and phosphates. This is because the former compound groups are water-insoluble, and these compound groups have (Si-O-Si or Ge-O-Ge bonds) rather than crystallizing with separated XO ₄ groups. This is because there is a tendency to form a polymerized structure.

Thus, for the first example , one method of synthesizing such a solid acid electrolyte material begins with the synthesis of known acidic silicates and acidic germanates. Most such compounds contain small alkali or alkaline earth ions. The same synthetic procedure is followed to prepare crystalline compounds containing large cations (necessary for high proton conductivity). However, in this case, acidic cesium silicate or acidic cesium germanate is produced by changing one or more of the reaction materials, for example, by substituting NaOH with CsOH.

  For the second example approach to large cation silicates and germanates, ion exchange reactions are performed on known compounds. That is, by immersing the original material in molten CsOH, Na can be replaced with Cs in known acidic silicates.

In a third example approach, a compound containing Li rather than H is formed, and then an ion exchange reaction is performed in an acidic medium. For example, the compound LaLiSiO ₄ is known, and lithium can be replaced with protons using an acid that does not dissolve the base material.

According to the above method, several chemically stable superproton solid acid examples were synthesized including NaCaHSiO ₄ , BaH ₂ SiO ₄ , SrH ₂ GeO ₄ and Na ₂ H ₂ SiO ₄ .xH ₂ O. The synthetic methods employed for each are as follows:

Reference example 1
CsH (PO ₃ H): Compound CsH (PO ₃ H) was synthesized from an aqueous solution of cesium carbonate and phosphoric acid. The Cs: PO ₃ H molar ratio was fixed at 1: 1. Add complete water to ensure complete dissolution, then gently heat the solution to induce H ₂ O evaporation and precipitate the product. The resulting material was separated from the mother liquor by filtration and rinsed with acetone.

Reference example 2
NaCaHSiO ₄ : NaOH, Ca (OH) ₂ and SiO ₂ (molar ratio 3: 2: 2) were combined with a few drops of water and placed in a thermal bomb at 280 ° C. for 48 hours. (BG Cooksley and HFW Tayer, Acta. Cryst. B30 (1974) 864-867)

Example 3
BaH ₂ SiO ₄ : Dissolve NaOH and Na ₂ SiO ₃ in hot deionized water, then add aqueous BaCl ₂ (molar ratio Ba: Si = 1.2: 1), thereby precipitating the desired product Triggered. (G. Kruger and W. Wieker, Z. Anorg. Allg. Chem. 340 (1965) 227-293)

Example 4
SrH ₂ GeO ₄ : Sr (OH) ₂ and GeO ₂ are combined (molar ratio 1: 1) and placed in a thermal bomb at 280 ° C. for several days; or Sr (OH) ₂ and GeO ₂ are heated Mixed in water (1: 1 molar ratio), this induced the formation of the desired product. (H. Nowotny and G. Szekely, Monat. Chem. Ver. Teile Wissenshaft. 161 (1952) 568-582)

Reference Example 5
Na ₂ H ₂ SiO ₄ .5H ₂ O: By dissolving NaOH and Na ₂ SiO ₃ in hot deionized water (molar ratio Na: Si = 4.65: 1) and slowly evaporating excess water, The expected product crystal growth was induced. (PB Jamieson and LS Dent Glasser, Acta Cryst. 20 (1966) 373-376)

The resulting material diffraction pattern was then used to confirm that the expected product was obtained. However, it should be understood that in some cases a small amount of impurity phase will be present. Two examples of dihydrogen silicates, BaH ₂ SiO ₄ and Na ₂ H ₂ SiO ₄ .5H ₂ O, were examined for thermal behavior. These compounds have been found to be stable up to ~ 320 ° C (decomposition) and 70 ° C (melting), respectively. (G. Kruger and W. Wieker, Z. Anorg. Allg. Chem. 340 (1965) 227-293; J. Flesche, B. Ketterer and RL Schmid, Thermochim. Acta 77 (1984) 109-121). The remaining two compounds, the NaCaHSiO ₄ and SrH ₂ GeO _4, characterized by a thermal method (thermogravimetric analysis and calorimetry) by the applicant, and tested all four compounds by impedance spectroscopy, thereby The suitability as a fuel cell electrolyte was evaluated. In the case of acidic sodium calcium silicate, the nature of the charged species was established by additionally performing H / D ion exchange followed by conductivity measurements.

The results in FIG. 2 show the thermogravimetric analysis curves of NaCaHSiO ₄ and SrH ₂ GeO ₄ and the silicate compound gradually loses weight by ˜1% from room temperature to 460 ° C. and then rapidly decomposes Thus, the weight is further lost by 3.5% around 560 ° C., and the maximum weight loss rate is 505 ° C. The total weight change corresponds to 0.42 mol H ₂ O (per mole of NaCaHSiO). This value is close to the expected value of 0.5 mol. In contrast to this, SrH ₂ GeO ₄ loses weight rapidly around 315 ° C. This weight loss begins at 295 ° C and ends essentially at around 335 ° C. A total weight change of 9.6% corresponds to 1.2 moles of H ₂ O (per mole of SrH ₂ GeO ₄ ). This value is greater than the expected value of 8.0% (1 mol H ₂ O / SrH ₂ GeO ₄ ). This suggests that an initial weight loss of 1.6% by weight occurring over the temperature range 70-295 ° C. corresponds to a loss of surface water. These results indicate that the NaCaHSiO ₄ compound exhibits relatively good thermal stability up to ˜400 ° C. and the SrH ₂ GeO ₄ compound up to 280 ° C.

  Calorimetry (results not shown) showed that none of these compounds undergo a structural phase transition. That is, no sharp transition to the superprotonic phase was observed. On the contrary, a wide range of thermal events corresponding to dehydration as established from gravimetric analysis were obtained.

The conductivity of the compounds in Reference Examples 1 and 2 and Example 3 is shown in FIG. As illustrated, sodium silicate, but it could not be only measured over a limited temperature range, however, the four highest conductivity near ambient temperature, that is at 50 ℃ 1.3 x 10 ^-6 Ω ^{- 1} cm ^-1 was indicated.

In the case of NaCaHSiO ₄ compounds, the possibility that charge transport occurs at least by the movement of sodium ions cannot be ruled out in advance. Comparing the conductivity of the as-synthesized material with the conductivity of the deuterated analog (exposed to high temperature D ₂ O for several hours), as shown in Figure 4, the deuterated material Is a very poor conductor. This means that proton / deuteron is actually a mobile species. In fact, a single-digit difference between the two conductivities that is larger than can be easily explained as a change in jump frequency or zero point energy eliminates the possibility of measurable alkali ion involvement in the conductance. Is done. Furthermore, it was confirmed that structural change was not performed by examining the material after deuteration treatment by X-ray powder diffraction.

What is more prominent than conductivity is the stability of these silicates and germanates. Compare the thermogravimetric behavior of NaCaHSiO ₄ , SrH ₂ GeO ₄ and CsHSO ₄ under flowing hydrogen gas and in the presence of Pt catalyst material as shown in FIG. Sulfate is dramatically lost in weight under these conditions, whereas it is immediately apparent that silicates and germanates are extremely stable. As shown in FIG. 6, similar results were obtained (in the presence of a Pt—Ru catalyst) under methanol vapor, NaCaHSiO ₄ showed good stability, and CsHSO ₄ showed immediate weight loss.

The greatest success of the direct synthesis route has been in the preparation of new single alkali silicates, specifically silicates containing cesium. By reacting CsOH with SiO ₂ (molar ratio 4: 1) at 900 ° C., a highly crystalline product was produced. However, the product was unlikely to contain acidic protons. Reacting these two starting materials (2: 1 molar ratio) in a 280 ° C. thermal cylinder also yielded a highly crystalline product. However, this second product has a distinctly different diffraction pattern from the first product, and the synthesis conditions suggest that the material contains acidic protons. The thermogravimetric analysis curves of these two materials, cesium silicate # 1 and cesium silicate # 2, are shown in FIG.

  The first material is thermally stable up to 800 ° C. (except for a small weight loss due to desorption of surface water adsorbed after synthesis), which is consistent with the absence of acidic protons. The second material is only thermally stable up to ˜400 ° C., which is consistent with the presence of acidic protons.

  As shown in FIG. 8, although the conductivity of these materials is not as high as that of acid sulfate, the increased stability of the materials and the relatively high conductivity show great potential as electrolyte materials. .

Similarly, the synthesis of new acidic silicates by ion exchange of alkaline species in NaCaHSiO ₄ yields the results summarized in Table 1 below along with appropriate reaction conditions.

As shown in FIG. 9, in some cases the ion exchange process has little effect on NaCaHSiO ₄ , whereas in other cases, this ion exchange results completely in A new phase is crystallized. In general, the change in conductivity induced by ion exchange was less than an order of magnitude. However, exposure of NaCaHSiO ₄ to molten potassium acetate (IonEx-5) results in a two-digit increase in conductivity, and in this case, no significant change in crystal structure is caused.

The final route for preparing new alkaline acidic silicates and germanates with proton exchange from Li-containing compounds is described for the compounds Li ₄ SiO ₄ and LaLiSiO ₄ . Both of these compounds are in solid state according to the procedures outlined by Pfeiffer (J. Nucl. Mat. 257 (1998) 309-317) and Sato et al. (Solid State Ionics 83 (1996) 249-256), respectively. It is well synthesized by reaction. The synthesis of these and other compounds shows that the direct synthesis of acid silicates requires more extreme hydrothermal conditions, ie higher pressures and temperatures. However, the ion exchange method (which led to a two-digit increase in the conductivity of NaCaHSiO ₄ ) shows excellent results.

While the above discussion focuses on the raw solid acid electrolyte material, it is preferred that the solid acid electrolyte material of the present invention be formed in a membrane (MEA) for integration into an electrochemical device. You should understand that. The present specification thus also describes a method of processing membranes from the solid acid electrolyte materials described herein .

Processing of thin, dense solid acid films generally involves simultaneously applying high temperature and pressure (uniaxial). Applicants have found that a pressure of ~ 700 psi and a temperature of ~ 190 ° C yields a transparent film of CsHSO ₄ as shown in Figures 10a and 10b. Specifically, FIG. 10a shows a hot pressed CsHSO ₄ image attached to a copper plate with a sample thickness of 320 μm under reflected light. FIG. 10b shows this material with illumination from behind that emphasizes translucency.

  While the above discussion has focused on processing pure solid acid electrolyte materials, due to the mechanical difficulties described above, in one embodiment of the present invention, the solid acid electrolyte material is contained within a support matrix. Processed as a composite material embedded in In such composite materials, the solid acid is in its superproton state, exhibits high conductivity, and provides the desired electrochemical properties, whereas the support matrix provides a mechanical support. To do. This support matrix can also serve to protect solid acids from water in the environment. For such embodiments, any suitable matrix material that will provide sufficient mechanical and / or chemical support, such as a polymer, ceramic or oxide glass, may be utilized.

  Thus, in one embodiment, a composite film comprising a solid acid according to the present invention and a simple structural polymer such as polyvinylidene fluoride can be prepared by simple melt processing. In this melt processing, the two components are lightly ground together and then hot pressed at 180 ° C. and 10 kpsi for 15 minutes. The biggest difficulty that this method often faces is to remove the electrolyte from the hot press without introducing cracks, which limits the thinnest film that can be processed by the hot press method to ~ 200 μm. have. In the case of a non-polymeric matrix material, a high density composite film is produced by hot pressing the mixture of the two components, preferably at a temperature at which the solid acid is melted and fluidized.

As another example, this specification describes a method of forming a thinner electrolyte in a stabilizing matrix. For such an example, the film may be a thermosetting polymer provided in the form of monomers or prepolymers, then formed by polymerizing the polymer in the solid acid and in situ. In this method, the two materials are again ground together and a polymerization / crosslinking catalyst is added. Any suitable prepolymer, such as a polyester resin, may be used. As an example , the solid acid was ground and mixed with the prepolymer. A polymerization catalyst was introduced and the mixture was then poured onto a plate and pressed into a thin film. The film thus produced was then cured at 100 ° C. for approximately 2 hours.

In yet another example , the membrane can be formed by suspension casting. In this example , the solid acid and matrix material are dissolved and / or dispersed in a suitable solvent system, such as a water / ethanol solution. The membrane is then formed by casting the suspension and allowing the solvent to evaporate. In addition to the polymer, films containing non-polymeric matrix materials, such as ceramic or oxide glass, can be formed by this method.

  According to the above discussion, a composite material is formed by mixing a solid acid with a support structure that does not react electrochemically. The solid acids of the present invention without careful selection of M and X species are inherently poor electronic conductors, so these materials alone can only be used to allow proton transport. Can not. However, in one embodiment of the invention, the matrix material has conductive properties such that the composite membrane allows both electron transport and proton transport.

  In the case of the first embodiment, by preparing a composite material comprising a solid acid and a second substance having a high electron conductivity through any of the above methods, the electronic conductivity in the solid acid. Is introduced. This second material may be any suitable electronically conductive material, such as a conductive polymer, such as polyaniline, or a typical metal, such as copper or graphite. If the electronically conductive material is a metal, it may be further advantageous to use it as a binder by providing a flexible and inert material, such as a polymer, in the composite material.

  In the second embodiment, electronic conductivity is introduced into the solid acid by direct chemical substitution with the variable valence ions as described above. For example, a portion of phosphorus, silicon, or germanium can be replaced with an ion that exhibits a variable valence state, such as manganese. Similarly, some of the alkali metals or alkaline earth metals can be replaced with large variable valence ions such as thallium, indium, lead and tin. The solid acid thus modified can be used directly in an electrochemical device or it can be combined with a support matrix material as a composite material as described above.

  While the above discussion has focused on the solid acid electrolyte materials of the present invention and methods of forming such materials, the present invention also relates to electrochemical devices that incorporate such materials and films. Several example device embodiments are shown in FIGS.

  11 and 12 show an embodiment of a hydrogen / air fuel cell and a methanol fuel cell 10. In these embodiments, proton conducting membrane 12 is a solid acid electrolyte of the type described herein, sandwiched between two graphite layers 14. Since the membrane 12 does not need to be humidified for operation, this fuel cell system can be very simple. For example, as shown, the humidification system normally required for conventional fuel cells utilizing conventional electrolyte materials, such as Nafion®, is not required. Furthermore, the temperature monitoring and temperature control used in the fuel cell need not be as strict. Since membranes based on solid acids do not require humidification, fuel cells can operate at high temperatures, and such high temperatures can improve the kinetic properties of electrochemical reactions, It can withstand high concentrations of CO in fuel. In contrast, conventional humidified fuel cells cannot operate at temperatures above the boiling point of water (100 ° C.).

  The use of solid acid electrolyte materials in fuel cells is an important application, but such materials can also be incorporated into various membrane reactors. For example, the Pt-based catalyst 16 in the hydrogen / air fuel cell shown in FIG. 11 is extremely sensitive to CO poisoning, specifically at a temperature close to the ambient temperature. Thus, in the case of an indirect hydrogen / air fuel cell, CO impurities are often removed from the hydrogen produced by the reformer before it enters the fuel cell. Thus, in one embodiment schematically illustrated in FIG. 13, the material is shown embedded within a hydrogen separation membrane for removing CO and other gases from hydrogen.

In this embodiment, the hydrogen separation membrane 20 is formed from a proton conducting electrolyte 22 of the type described herein and connected to a current source 24. Hydrogen gas 26 mixed with other unwanted gas 27 is introduced into one side (inlet) 28 of the membrane, and clean hydrogen gas 26a is extracted from the other side of the membrane. In this embodiment, when current is applied, the hydrogen gas is separated into H ⁺ and e ⁻ . Since the membrane conducts only protons, these protons are the only species that can move through the membrane. Electrons travel through the current source to the outlet side 29 of the membrane, where H ⁺ coalesces with electrons coming from the current source, thereby forming hydrogen gas. Such membranes are substantially impermeable to other gases and fluids. This entire hydrogen separation process is driven by the current applied from the current source 24.

  Another type of hydrogen separation membrane is shown in FIG. In this embodiment, the hydrogen separation membrane 30 is formed from a mixed proton / electron conducting membrane 36. As mentioned above, such a film can also be made into a solid acid, for example by including a given amount of variable valence elements, such as Cr or Mn for X. Alternatively, the membrane can be formed as a composite material. In this composite material, the matrix material may be an electronically conductive material, such as a conductive polymer, such as polyaniline, or a typical metal, such as aluminum or copper, or graphite.

Again, hydrogen gas 32 mixed with other unwanted gas 34 is introduced into one side (inlet) of membrane 36 and clean hydrogen gas 37 is extracted from the other side of the membrane. During operation, hydrogen gas is separated on the inlet 38 side of the membrane 36 into H ⁺ and e ⁻ . Since membrane 36 is proton conductive and electronically conductive, both of these species move through the membrane. However, again, the membrane is substantially impermeable to other gases and fluids 34. Thus, CO and other unwanted gases or fluids cannot move in this way. On the other hand, H ⁺ and e ⁻ recombine to form hydrogen gas. The entire process is driven by a hydrogen chemical potential gradient. This potential gradient is high on the inlet side of the membrane and low on the outlet side 39 of the membrane.

Finally, FIG. 15 schematically shows a membrane reactor incorporating the electrolyte material of the present invention. The membrane reactor shown in FIG. 15 uses a mixed proton / electron conducting membrane of the type described above. The general reaction carried out in this reactor has the following form:
Follow A + B-> C + H ₂ .

By using a mixed proton / electron conducting membrane in this reactor, the reaction can be enhanced, providing a yield that exceeds the thermodynamic equilibrium value. On the inlet side of the membrane reactor 40, the reactant 41 forms products C42 and H ₂ 44. Under equilibrium conditions, the hydrogen concentration increases and the positive reaction slows down. By using the mixed proton / electron conducting membrane 46, the hydrogen 44 is rapidly extracted from the reaction region 48 via transport through the membrane.

Examples of such reactions that can increase the yield by using such a membrane reactor include the following 1) -5):
1) Production of synthesis gas by steam reforming of methane according to CH ₄ + H ₂ O-> CO + 3H ₂
2) Production of CO ₂ and H ₂ by steam reforming of CO:
CO + H ₂ O-> CO ₂ + H ₂
3) Decomposition of hydrogen sulfide:
H ₂ S-> H ₂ + S
4) Production of polypropylene by dehydrogenation of propane, and 5) Production of various products by dehydrogenation of alkanes and aromatic compounds.

  Although one embodiment of a membrane reactor has been described above in connection with FIG. 15, the membrane of the present invention may be used for other reactions utilizing proton conducting membranes or mixed proton / electron conducting membranes, such as selective hydrogenation reactions. You can also

Although specific embodiments have been disclosed herein, solid acid electrolytes and electrochemical devices have been developed by those skilled in the art utilizing such materials that fall within the scope of the text under literal or equivalent principles. It is expected to be configurable. Exemplary embodiments of the present invention are listed below.
1.
Represented by the general formula: M′H ₂ AO _4, wherein M ′ is any alkaline earth metal or transition metal having a +2 charge, and A is one of Si or Ge. An electrolyte membrane containing a solid acid.
2.
Electrolyte comprising a solid acid represented by the general formula: M''HAO ₄ (wherein M '' is any transition metal having a +3 charge and A is one of Si or Ge) film.
3.
The electrolyte membrane according to item 1, wherein M ′ is selected from the group consisting of Be, Mg, Ca, Sr, Ba, and Ra.
4).
3. The electrolyte membrane according to item 2, wherein M ″ is selected from the group consisting of Sc, Y, La, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu.
5.
3. The electrolyte membrane according to either item 1 or 2, further comprising a structural binder material.
6).
6. The electrolyte membrane of item 5, wherein the structural binder material is a species selected from the group consisting of graphite, polymer, ceramic, glass and metal.
7).
3. The electrolyte membrane according to either item 1 or 2, further comprising a separate conductive material.
8).
8. The electrolyte membrane according to item 7, wherein the conductive material is selected from the group consisting of a conductive polymer, a metal, and graphite.
9.
3. The electrolyte membrane according to either item 1 or 2, wherein the solid acid contains one or more variable valence elements.
10.
3. The electrolyte membrane according to either item 1 or 2, wherein the electrolyte is water-insoluble.
11.
3. The electrolyte membrane according to either item 1 or 2, wherein the electrolyte is thermally stable at a temperature exceeding 100 ° C.
12
3. The electrolyte membrane according to either item 1 or 2, wherein the electrolyte conducts protons.
13.
3. The electrolyte membrane according to any one of items 1 and 2, wherein the electrolyte has a proton conductivity of 10 ⁻⁵ Ω ⁻¹ cm ⁻¹ or more at an operating temperature .
14
3. The electrolyte membrane according to either item 1 or 2, wherein the electrolyte conducts both protons and electrons.
15.
3. An electrochemical device incorporating the solid acid electrolyte membrane according to item 1 or 2.
16.
16. An electrochemical device according to item 15, wherein the device is selected from the group consisting of a fuel cell, a battery, a hydrogen separation membrane and a membrane reactor.
17.
3. The electrolyte membrane according to either item 1 or 2, wherein A is Si.
18.
3. The electrolyte membrane according to either item 1 or 2, wherein A is Ge.
19.
_2. The electrolyte membrane according to item 1, wherein the solid acid is BaH ₂ SiO ₄ .
20.
Item _2. The electrolyte membrane according to Item 1, wherein the solid acid is SrH ₂ GeO ₄ .
21.
3. The electrolyte membrane according to item 2, wherein the solid acid is LaHSiO ₄ .
22.
The electrolyte according to item 2, wherein M '' is selected from the group consisting of Sc, Y, La, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu, and A is Ge. film.

Is a graph showing the characteristics of the CsH (PO ₃ H) solid acid as an example. Is a graph showing the characteristics of the CsH (PO ₃ H) solid acid as an example. Is a graph showing the characteristics of the CsH (PO ₃ H) solid acid as an example. Is a graph showing the characteristics of the CsH (PO ₃ H) solid acid as an example. Is a graph showing the characteristics of the CsH (PO ₃ H) solid acid as an example. It is a graph which shows the temperature decomposition characteristic under nitrogen of the solid acid electrolyte as an example by this invention. It is a graph which shows the electrical conductivity characteristic in the air of the solid acid electrolyte as an example by this invention. It is a graph which shows the electrical conductivity characteristic of the solid acid electrolyte as an example. 6 is a graph showing a comparison of decomposition characteristics under hydrogen of a conventional (CsHSO ₄ ) and an example solid acid electrolyte according to the present invention. Conventional (CsHSO _4), and is a graph comparing the degradation characteristics of the gas phase methanol of a solid acid electrolyte as an example. It is a graph which shows the temperature decomposition characteristic in nitrogenation of the solid acid electrolyte as an example synthesize | combined based on description of this specification . It is a graph which shows the electrical conductivity characteristic of the solid acid electrolyte as an example synthesize | combined based on description of this specification . It is a graph which shows the electrical conductivity characteristic of the solid acid electrolyte as an example synthesize | combined based on description of this specification . A film photograph which reproduces as an example formed from solid acid electrolyte as an example. A film photograph which reproduces as an example formed from solid acid electrolyte as an example. 1 is a schematic diagram illustrating an example hydrogen / air fuel cell using an example solid acid electrolyte membrane according to the present invention. FIG. 1 is a schematic diagram illustrating an example direct methanol fuel cell using an example solid acid supported by a binder in accordance with the present invention. FIG. It is the schematic which shows the hydrogen separation membrane as another example using the proton-conductive solid acid as an example by this invention. It is the schematic which shows the hydrogen separation membrane as an example using the mixed type electron and proton conduction solid acid as an example by this invention. 1 is a schematic diagram illustrating an example of a membrane reactor using an exemplary solid acid according to the present invention. FIG.

Claims (22)

Represented by the general formula: M′H ₂ AO _4, wherein M ′ is any alkaline earth metal or transition metal having a +2 charge, and A is one of Si or Ge. An electrolyte membrane containing a solid acid. Electrolyte comprising a solid acid represented by the general formula: M''HAO ₄ (wherein M '' is any transition metal having a +3 charge and A is one of Si or Ge) film. M 'is Be, Mg, Ca, Sr, Ba, and Ra or we made is selected from the group, the electrolyte membrane according to claim 1.   The electrolyte membrane according to claim 2, wherein M '' is selected from the group consisting of Sc, Y, La, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu.   The electrolyte membrane according to claim 1, further comprising a structural binder material.   6. The electrolyte membrane according to claim 5, wherein the structural binder material is a species selected from the group consisting of graphite, polymer, ceramic, glass and metal.   The electrolyte membrane according to claim 1, further comprising a separate conductive material.   8. The electrolyte membrane according to claim 7, wherein the conductive material is selected from the group consisting of conductive polymers, metals and graphite.   The electrolyte membrane according to claim 1, wherein the solid acid contains one or more variable valence elements.   The electrolyte membrane according to claim 1, wherein the electrolyte is water-insoluble.   The electrolyte membrane according to claim 1 or 2, wherein the electrolyte is thermally stable at a temperature exceeding 100 ° C.   The electrolyte membrane according to claim 1, wherein the electrolyte conducts protons. The electrolyte membrane according to claim 1, wherein the electrolyte has a proton conductivity of 10 ⁻⁵ Ω ⁻¹ cm ⁻¹ or more at a use temperature.   The electrolyte membrane according to claim 1, wherein the electrolyte conducts both protons and electrons.   An electrochemical device incorporating the solid acid electrolyte membrane according to claim 1.   The electrochemical device of claim 15, wherein the device is selected from the group consisting of a fuel cell, a battery, a hydrogen separation membrane, and a membrane reactor.   The electrolyte membrane according to claim 1, wherein A is Si.   The electrolyte membrane according to claim 1, wherein A is Ge. The electrolyte membrane according to claim 1, wherein the solid acid is BaH ₂ SiO ₄ . The electrolyte membrane according to claim 1, wherein the solid acid is SrH ₂ GeO ₄ . The electrolyte membrane according to claim 2, wherein the solid acid is LaHSiO ₄ .   3. The method of claim 2, wherein M '' is selected from the group consisting of Sc, Y, La, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu, and A is Ge. Electrolyte membrane.

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