CA2359865C

CA2359865C - Proton conducting membrane using a solid acid

Info

Publication number: CA2359865C
Application number: CA002359865A
Authority: CA
Inventors: Sossina M. Haile; Dane Boysen; Sekharipuram R. Narayanan; Calum Chisholm
Original assignee: California Institute of Technology
Current assignee: California Institute of Technology
Priority date: 1999-01-22
Filing date: 2000-01-21
Publication date: 2006-05-23
Anticipated expiration: 2020-01-21
Also published as: US7125621B2; US6468684B1; AU4795200A; JP2002536787A; EP1171384A4; US20030008190A1; US20070009778A1; CA2359865A1; WO2000045447A2; EP1171384B1; WO2000045447A8; EP1171384A2; WO2000045447A3; WO2000045447A9; JP3938662B2

Abstract

A solid acid material is used as a proton conducting membrane in an electrochemical device. The solid acid material can be one of a plurality of different kinds of materials. A binder can be added, and that binder can be either a nonconducting or a conducting binder. Nonconducting binders can be, for example, a polymer or a glass. A conducting binder enables the device to be both proton conducting and electron conducting.

Description

PROTON CONDUCTING MEMHRANE USING A SOLID ACID
Field The present application describes a proton conducting membrane formed using an solid acid in its solid phase. More specifically, the present application teaches a proton conducting membrane, formed using an solid acid mixed with a supporting binder material, that is impermeable to fluids such as gas and water, can operate without hydration, and has high proton conductivity.

Background Proton conducting materials have a number of applications.
Proton conducting membranes are widely utilized in devices which use a chemical reaction to produce or store electricity, or use electricity to drive a chemical process. Materials which conduct both protons and electrons ("mixed proton and electron conductors") are utilized in related applications.

2 Electrochemical devices depend on the flow of protons, or the flow of both protons and electrons through a proton conducting membrane. Exemplary electrochemical devices include a fuel cell, an electrolysis cell, a hydrogen separation cell, a battery, a supercapacitor, and a membrane reactor. There are other electrochemical devices which also use a proton conducting membrane.
An important use for proton conducting membranes is in fuel cells. Fuel cells are attractive alternatives to combustion engines for the generation of electricity because of their higher efficiency and the lower level of pollutants they produce. A
fuel cell generates electricity from the electrochemical reaction of a fuel e.g. methane, methanol, gasoline, or hydrogen, with oxygen normally obtained from air.
There are three common types of fuel cells used at temperatures close to ambient. A direct hydrogen/air fuel cell system stores hydrogen and then delivers it to the fuel cell as needed.
In an indirect hydrogen/air fuel cell, hydrogen is generated on site from a hydrocarbon fuel, cleaned it of carbon monoxide (CO), and subsequently fed to the fuel cell.
A direct methanol fuel cell ("DMFC"), feeds methanol/water solution directly to the fuel cell, e.g., without any fuel

3 processing. One type of DMFC has been described, for example, in U.S. Patent No. 5,559,638. There are various advantages and disadvantages inherent within all three configurations. All are, to a greater or lesser extent, limited by the performance of the proton conducting membrane.
NafionT'", a perfluorinated sulphonic acid polymer, is often used as a membrane material for fuel cells which operate at temperatures close to ambient. Other hydrated polymers have also been used as proton conductive materials. Membranes of modified perfluorinated sulfonic acid polymers, polyhydrocarbon sulfonic acid polymers, and composites thereof are also known. These and related polymers are used in hydrated form. Proton transport occurs by the motion of hydronium ions, H30+. Water is necessary in order to facilitate proton conduction. Loss of water immediately results in degradation of the conductivity.
Moreover, this degradation is irreversible - a simple reintroduction of water to the system does not restore the conductivity. Thus, the electrolyte membranes of these hydrated polymer-based fuel cells must be kept humidified during operation. This introduces a host of balance-of-plant needs for water recirculation and temperature control.

4 A second limitation derives from the need to maintain water in the membrane. In order to maintain hydration, the temperature of operation cannot exceed 100°C without cell pressurization.
High temperature operation could be desirable, however, to increase catalyst efficiency in generating protons at the anode (in both HZ and direct methanol fuel cells) and to improve catalyst tolerance to carbon monoxide ("CO"). CO is often present in the fuel that is used in the fuel cells. The CO can poison the precious metal catalysts. This is particularly problematic in indirect hydrogen/air fuel cells for which hydrogen is generated on site. High temperatures also benefit the reduction reaction on the cathode.
Another limitation of hydrated polymer electrolytes occurs in applications in methanol fuel cells. These polymers can be permeable to methanol. Direct transport of the fuel (i.e.
methanol) across the membrane to the air cathode results in losses in efficiency.
Alternate proton conducting materials, which do not require humidification, which can be operated at slightly elevated temperatures, and/or which are impermeable to methanol, are desirable for.fuel cell applications.

5 In the field of hydrogen separation, a proton conducting membrane is utilized to separate hydrogen from other gases such as CO and/or C02. Palladium is often used for this application.
Palladium is permeable to molecular hydrogen, but not in general to other gases. There are drawbacks to the use of this material.
It is expensive and the hydrogen diffusion rate is low. It would be desirable to develop new materials which are less expensive and exhibit higher proton/hydrogen transport rates.
In general, materials utilized in other electrochemical devices such as electrolysis cells, batteries, supercapacitors, etc., include liquid acid electrolytes, which are highly corrosive, and solid polymer proton conductors, which require humidification or exhibit insufficient proton conductivity. High conductivity, high chemical and thermal stability solid membranes with good mechanical properties are desirable for all of these applications.
Summary The present specification defines a new kind of material for a proton conducting membrane. Specifically, a proton conducting material is formed using an solid acid. The solid acid can be of the general form MaHb(XOt)~ or MaHb(XOt)~~nH20, where:

6

7 PCT/US00/01783 M is one or more of the species in the group consisting of Li, Be, Na, Mg, K, Ca, Rb, Sr, Cs, Ba, T1 and NH4+ or Cu+;
X is one or more of the species in the group consisting of Si, P, S, As, Se, Te, Cr and Mn; and a, b, c, n and t are rational numbers.
Solid acids do not rely on the presence of hydronium ions for proton transport, thus they do not require hydration for use as proton conductors.
A preferred solid acid used according to this specification is a solid phase solid acid that exhibits a superprotonic phase, a phase in which the solid has disorder in its crystal structure and a very high proton conductivity.
An embodiment uses a structural binder or matrix material to enhance the mechanical integrity and/or chemical stability of the membrane. That structural~binder can be many different kinds of materials in the different embodiments. In particular, the structural binder can be a polymer, a ceramic, or an oxide glass.
Another embodiment uses an electronically conducting material as a matrix. This creates a membrane which conducts both protons and electrons.

The resulting material can be used for a proton conducting material in a device that relies on the flow of protons or the flow of both protons and electrons across a membrane, herein an "electrochemical" device e.g. a fuel cell, a hydrogen separation membrane, or a electrolysis cell.
In a specific aspect the invention provides a proton conducting membrane for use in the presence of H20 in a device selected from the group consisting of fuel cells, proton transport membranes, membrane reactors and sensors, comprising: a solid acid material of the general formula:
MaHb(XOt)~ wherein M is a ration having a charge from +1 to +2, X is selected from the group consisting of S, Se, P, As, Si and Te, and a, b, c and t are non-zero integers, wherein the solid acid material is capable of conducting protons in a solid state through a superprotonic mechanism.
Brief Description of the Drawings Figure 1 shows an exemplary hydrogen/air fuel cell using a solid acid supported by a binder as its proton conducting membrane;
Figure 2 shows an exemplary direct methanol fuel cell using a solid acid supported by a binder as its proton conducting membrane;
Figure 3 shows a hydrogen separation membrane for the removal of CO and other gases from hydrogen;
Figure 4 shows another type of hydrogen separation membrane made of a proton conducting composite; and Figures 5 and 6 show a membrane reactor.

8 Detailed Description The present application teaches using a solid acid as a proton conducting membrane.
A solid acid can be of the general formula MaHb ( XOt ) c ' riH2O .
8a where:
M is one or more of the species in the group consisting of Li, Be, Na, Mg, K, Ca, Rb, Sr, Cs, Ba, Tl and NH4+;
X is one or more of the species in the group consisting of Si, P, S, As, Se, Te, Cr and Mn; and a, b, c, n and t are rational numbers; with t preferably being 3 or 4, and where t~0..
The solid acids used herein are compounds, such as CsHS04, whose properties are intermediate between those of a normal acid, such as HZS04, and a normal salt, such as Cs2S04. In general, the chemical formula of the solid acids of the type used according to the present specification can be written as a combination of the salt and the acid.
In general, solid acids are comprised of oxyanions; for example 504, 503, Se04, Se03, Si04, P04 or As04, etc. , which are linked together via O-H.... O hydrogen bonds. The structure may contain more than one type of X04 or X03 group, and may also contain more than one type of M species.
Certain solid acids are solid materials at room temperature.

9 Many different solid acids are contemplated by this specification. One example of a material that can be used as the solid acid is CSHS04, which is intermediate between Cs2S04 (a normal salt) and HzS04 (a normal acid). In this case, the solid acid can be written as 0.5 Cs2S04 * 0.5 HZS04. Another example, using the same salt and the same acid, is 1.5 Cs2S04 * 0.5 HzS04, to give Cs3H ( S04 ) 2 .
Other examples are:
CsH2PO4, Cs5 (HSO4) 3 (HzP04) 2~ Cs2 (HSO4) (HaP04) ~ Cs3 (HSO4) 2 (HzP04) , CS3 (HSOg ) 2 (H1 .5 ( S0. 5P0.5 ) ~4 ) i CSSH3 ( S~4 ) 4 ' XH2O, T1HS04 , CsHSeO4 , Cs2 (HSeO4) (HzP04) , Cs3H (SeO4) a (NH4) sH (SO4) 2, (NH4) a (HSO4) (HzP04) , Rb3H (SO4) 2~ R.bsH (SeO4) 2, CSi.sLli.sH (SO4) 2, Cs2Na (HSO4) 3. T1H3 (SeO3) 2~
CsHzAs04 (NH4) 2 (HS04) (HZAs04) , CaNaHSi04 The preferred material for any specific electrochemical device depends on the application. For example, Cs2 (HS04) (HZP04) may be preferred for electrochemical devices where high conductivity is critical. (NH4)3H(S04)Z may be preferred where low cost is critical. CaNaHSi04 may be preferred where chemical stability is critical.

Solid acids have certain characteristics that can be advantageous when used as a proton conducting membrane. The proton transport process does not rely on the motion of hydronium ions, thus solid acids need not be humidified and their conductivity is substantially independent of humidity. Another advantage is that solid acids are generally stable against thermal decomposition at elevated temperatures. The thermal decomposition temperature for some of the solid acids described in this specification, e.g., CaNaHSi04, can be as high as 350°C.
Since solid acids need not be humidified, solid acid based membranes can be operated at elevated temperatures, e.g.
temperatures above 100°C.
The conductivity of solid acids may be made purely protonic, or both electronic and protonic depending on the choice of the X element in the chemical formula MaHb (X04 ) ~ ~ nH20 or MaHb (X03 ) ~ ~ nH20. That is, by using a given amount of a variable valence element such as Cr or Mn for X, the solid acid can be made to conduct electrons as well as protons.

Another advantage is caused by the structure of the solid acids themselves. Since solid acids are dense, inorganic materials, they are impermeable to gases and other fluids that may be present in the electrochemical environment, e.g., gases and hydrocarbon liquids.
The materials are also relatively inexpensive.
This combination of properties: good conductivity in dry environments, conductivity which can be controlled to be either purely proton conducting or both electron and proton conducting, impermeability to gases and hydrocarbon liquids, serviceability at elevated temperatures, e.g. temperatures over 100°C and relatively low cost, render solid acids as useful materials for use as membranes in electrochemical devices.
Solid acids exhibit another advantageous property for applications in proton conducting membranes. Under certain conditions of temperature and pressure, the crystal structure of a solid acid can become disordered. Concomitant with this disorder is an high conductivity, as high as 10-3 to 10-2 S2-lcm-1.
Because of the high proton conductivity of the structurally disordered state, it is known as a superprotonic phase. The proton transport is believed to be facilitated by rapid X04 or X03 group reorientations, which occur because of the disorder.

Many solid acids enter a superprotonic state at a temperature between 50 and 150°C at ambient pressures. The transition into the superprotonic phase may be either sharp or gradual. The superprotonic phase is marked by an increase in conductivity, often by several orders of magnitude. At temperatures above the transition temperature, the solid acid is superprotonic and retains its high proton conductivity until the decomposition or melting temperature is reached.
Solid acids that undergo a superprotonic transition include:
1~ CsHSO4, Csz (HSO4) (HzPO4) , Cs3 (HSO4) z (HzPO4) , CS3 (HSOq) 2 (H1.5 (SO.SP0.5) ~4) i CS5H3 (S04) 4'XHzO, CSHSeOg, CS3H (SeOq) 2, (NH4)aH(S04)z. RbaH(Se04)z.
The superprotonic phases of solid acids have increased conductivity. An interesting embodiment is a solid acid operated at a temperature above the superprotonic transition temperature, and below the decomposition or melt temperature.
Despite the many advantageous properties of solid acids, problems can be encountered in trying to implement them in electrochemical devices because many solid acids are water soluble. They can also be difficult to process into large area membranes, and they often have poor mechanical properties. Some solid acids, such as CaNaHSi04 and other silicates, are not soluble in water.

Because of these difficulties, a disclosed embodiment includes a composite comprised of an solid acid embedded in a supporting matrix. The solid acid part of the composite provides the desired electrochemical activity, whereas the matrix provides mechanical support and also may increase chemical stability.
Different materials are contemplated herein for use as the supporting matrix.
In light of the properties of solid acids outlined above, the preferred embodiment is a composite material comprised of a solid acid embedded in a supporting matrix and operated at a slightly elevated temperature. In such a composite, the solid acid is in its superprotonic phase, exhibits high conductivity, and provides the desired electrochemical functions; the support matrix may provide mechanical support, and it may also serve to protect the solid acid from water in the environment. A high temperature of operation can render the solid acid into its superprotonic state. A high temperature of operation can also ensure that any water present in the electrochemical device will be present in the form of steam rather than liquid water, making the H20 less likely to attack the solid acid.
Hydrogen/Air Fuel Cells A hydrogen/air fuel cell is shown in Figure 1, in which the proton conducting membrane is a solid acid/matrix composite of the type described herein. Because the membrane need not be humidified, the fuel cell system can be simpler than one which uses a hydrated polymer membrane. The humidification system normally required for fuel cell utilizing a Nafion or related polymer membrane can be eliminated in Figure 1. Hence, less rigid temperature monitoring and control may be used in the solid acid based system as compared with Nafion based fuel cell systems. These differences allow a less-costly fuel cell system.
Because the membrane need not be humidified, the fuel cell shown in Fig. 1 can be operated at temperatures above 100°C. The tolerance of the Pt/Ru catalysts to carbon monoxide CO
poisoning increases with increasing temperature. Thus, a fuel cell such as shown in Fig. 1, operated at a temperature above 100°C may withstand higher concentrations of CO in the hydrogen fuel than a Nafion based fuel cell which is typically operated at a temperature lower than 100°C.

The high temperature of operation also enhances the kinetics of the electrochemical reactions, and can thereby result in a fuel cell with higher overall efficiency than one based on Nafion or other hydrated polymers.
Direct Methanol Fuel Cells A direct methanol fuel cell is shown in Figure 2. The proton conducting membrane is a solid acid/matrix composite of the type described herein. Because the membrane need not be humidified, the fuel cell system is much simpler and thus less costly than state of the art direct methanol fuel cell systems.
The humidification system normally required for fuel cell utilizing a Nafion or related polymer membrane is eliminated in Figure 2. Furthermore, temperature monitoring and control in the solid acid based system does not need to be as tight as in Nafion based fuel cell systems. Because the solid acid based membrane need not be humidified, the fuel cell may be operated at elevated temperatures. High temperatures can enhance the kinetics of the electrochemical reactions. This can result in a fuel cell with very high efficiency.

Another significant advantage of the fuel cell shown in Figure 2 over state of the art direct methanol fuel cells results from the decreased permeability of the membrane to methanol. In state of the art direct methanol fuel cells, in which Nafion or another hydrated polymer serves as the membrane, methanol cross-over through the polymeric membrane lowers fuel cell efficiencies. The impermeability of a solid acid membrane can improve this efficiency.
Hydrogen Separation Membranes The Ru/Pt catalyst in a hydrogen/air fuel cell is sensitive to CO poisoning, particularly at temperatures close to ambient.
Therefore, in an indirect hydrogen/air fuel cell, the hydrogen produced by the reformer is often cleaned, of e.g. CO to below 50ppm, before it enters the fuel cell for electrochemical reaction.
In Figure 3, a hydrogen separation membrane is shown for the removal of CO and other gases from hydrogen. The hydrogen separation membrane is made of a mixed proton and electron conducting membrane, as described herein. Hydrogen gas, mixed with other undesirable gases, is introduced onto one side of the membrane. Clean hydrogen gas is extracted from the other side of the membrane.

On the inlet side of the membrane, hydrogen gas is dissociated into H+ and e-. Because the membrane is both proton conducting and electron conducting, both of these species can migrate through the membrane. However, the membrane is substantially impermeable to other gases and fluids. Hence, CO
and other undesirable gases or fluids cannot so migrate. On the outlet side of the membrane, the H+ and e- recombine to form hydrogen gas. The overall process is driven by the hydrogen chemical potential gradient, which is high on the inlet side of the membrane and low on the outlet side of the membrane.

Another type of hydrogen separation membrane is shown in Figure 4. The membrane is made of a proton conducting composite of the type described herein, and is connected to a current source. Hydrogen gas, mixed with other undesirable gases, is introduced onto one side of the membrane and clean hydrogen gas is extracted from the other side of the membrane. Application of a current causes the hydrogen gas to dissociate into H+ and e-.
Because the membrane conducts only protons, these protons are the only species which can migrate through the membrane. The electrons migrate through the current source to the outlet side of the membrane, where the H+ and e- recombine to form hydrogen gas. The membrane is substantially impervious to other gases and fluids. Hence, CO and other undesirable gases or fluids cannot migrate through the proton conducting membrane. The overall process is driven by electric current applied via the current source.
Membrane Reactors In Figure 5 a membrane reactor is shown, in which a mixed proton and electron conducting membrane of the type described herein is utilized. The general reaction is that reactants A + B
react to form products C + D, where D is hydrogen gas. Use of a mixed proton and electron conducting membrane in this reactor can enhance the reaction to give yields that exceed thermodynamic equilibrium values. On the inlet side of the membrane reactor, the reactants form products C + H2. Under equilibrium conditions, the hydrogen concentration builds up and the forward reaction is slowed. With the use of the mixed hydrogen and electron conducting membrane, the hydrogen is immediately extracted from the reaction region via transport through the membrane, and the forward reaction is enhanced. Examples of reactions in which yield could be enhanced by using such a membrane reactor include (1) the steam reformation of methane (natural gas) to produce syngas: CH4 + H20 -> CO + 3H2; (2) the steam reformation of CO to produce C02 and H2: CO + H20 -> C02 +
H2; (3) the decomposition of H2S to H2 and S, (4) the decomposition of NH3 to H2 and N2; (4) the dehydrogenation of propane to polypropylene; and (5) the dehydrogenation of alkanes and aromatic compounds to various products.

In Figure 6 a second type of membrane reaction is shown, again, utilizing a mixed proton and electron conducting membrane of the type described herein. In this case, the general reaction is that the reactants A + B form the products C + D, where B is hydrogen. The hydrogen enters the reaction region via transport through the mixed conducting membrane, whereas the reactant A is introduced at the inlet to the membrane reactor, and is mixed with other species. The manner in which the hydrogen is introduced into the reactant stream (through the membrane) ensures that only the reactant A, and none of the other species reacts with hydrogen. This effect is termed selective hydrogenation.
The mixed proton and electron conducting membranes described herein provide an advantage over state-of-the-art membranes in that the conductivity is high at temperatures as low as 100°C, and the membranes are relatively inexpensive. Selective hydrogenation at temperatures close to ambient may have particular application in synthesis of pharmaceutically important compounds which cannot withstand high temperatures.

According to a first class of materials, the solid acid is mixed with a supporting structure that is electrochemically unreactive, to form a composite. A first embodiment uses a solid acid mixed with a melt-processable polymer as the supporting matrix structure.
The solid acid (CHS) was prepared from aqueous solutions containing stoichiometric amounts of Cs2C03 and H2S04.
Crystalline CsHS04 and a small amount (~ 8 wto) of the related compound Cs5H3(S04)4~xH20 (which also exhibits superprotonic behavior) were obtained upon introduction of methanol into the solution. Composite membranes of the solid acid and poly(vinylidene fluoride) were prepared by simple melt-processing methods. The two components were lightly ground together then hot-pressed at 180 °C and 10 kpsi for 15 minutes. Volume ratios of CHS:PVDF from 1000 CsHS04 to 1000 PVDF were prepared in 10 vol% increments.
Another example of a composite contains a solid acid and a thermoset polymer, which can be mixed in with the solid acid in monomer or prepolymer form, and then polymerized in situ.

The solid acid (CHS) was prepared from aqueous solutions containing stoichiometric amounts of Cs2C03 and HzS04.
Crystalline CsHS04 and a small amount (- 8 wt%) of the related compound Cs5H3(S04)4~xH20 (which also exhibits superprotonic behavior) were obtained upon introduction of methanol into the solution. Composite membranes of the solid acid and the polyester resin marketed under the Trade-mark Castoghas by Buehler, Inc. were synthesized simply by lightly grinding the solid acid and pre-polymer together and then adding the polymerization/crosslinking catalyst. A material with a 50:50 volume ratio was prepared.
Another example of a thermoset polymer - solid acid composite comprises the solid acid (NH3)3H(S04)2 and the polymer poly(dicyclopentadiene) or poly DCPD.
The solid acid, TARS, was prepared from aqueous solutions of (NH4)ZSO9 and HZS04. The solid acid was ground then mixed with the monomer dicyclopentadiene. The polymerization catalyst was introduced into the mixture, which was then poured onto a Teflon plate and pressed into a thin film. The film was cured at 100°C
for approximately 2 hours. Materials with 25 and 17 vol % TARS
were prepared.

Another method for preparing solid acid/polymer composites is suspension coasting. For this, CsHS04 was dissolved in a water/ethanol solution. The polymer PVDF was then dispersed into this solution. A composite membrane was formed by casting the suspension and allowing the solvents to evaporate. Composite membranes comprised of a solid acid and a non-polymeric matrix material, such as a ceramic or an oxide glass can be prepared in the following manner. The solid acid is synthesized form aqueous solution and the matrix material is synthesized separately. The two components are mixed and ground together. The mixture is then hot pressed, preferably at a temperature which causes the solid acid to melt and flow, to yield a dense composite membrane.
The nature of the chemical bonding in solid acids of general formula MaHb (X04 ) ~ ~ nH20 or MaHb (X03 ) ~ ~ nH20 where M is one or more of the species in the group consisting of Li, Be, Na, Mg, K, Ca, Rb, Sr, Cs, Ba, T1 and NH4+;
X is one or more of the species in the group consisting of Si, P, S, As, Se, and Te; and a, b, c, and n are rational numbers, and n can be zero.
leads to materials which are inherently poor conductors of electrons. These compounds can be used in devices which require both proton and electron transport directly through the membrane if a mechanism for electron transport is introduced.

The first approach for introducing electronic conductivity into solid acid based materials is to prepare a composite comprised of the solid acid and a second substance which has a high electronic conductivity. This second substance may be an electronically conducting polymer, such as poly(aniline), or a typical metal, such as aluminum or copper. Where the electronically conducting component is a metal, it may be advantageous to introduce a chemically and electrically inert polymer into the composite simply to serve as a binder and provide the membrane with good mechanical properties. The processing methods described above may be used to prepare such composite membranes.

The second approach for introducing electronic conductivity into solid acid based materials is to perform direct chemical substitutions with variable valence ions. For example, a portion of the sulfur in CsHS04 may be replaced by chromium, which can be present in an oxidation state of anywhere from 2+ to 6+.
Similarly, manganese may be introduced on the sulfur site, as this ion exhibits valence states anywhere between 2+ and 7+.
Chemical substitution may also be performed with respect to the cesium in a compound such as CsHS04. Large ions with variable valence, such as thallium, indium, lead and tin can be used for these substitutions. The solid acid so modified may be used in an electrochemical device directly, or may be combined with a supporting matrix material as described above.

In the Figure 1 embodiment, a membrane-electrode assembly (MEA) is prepared from the CHS-PVDF composite film in which the solid acid to polymer volume ratio is 50:50. The electrodes are formed of graphite paper which is impregnated with a complex slurry of platinum powder, PVDF, the solid acid, and Nafion, suspended/dissolved in a water and isopropanol solution. After evaporation of the solvents, the electrodes so prepared are hot-pressed onto the composite membrane. The MEA is placed in a fuel cell test station at 140°C and hydrogen is introduced at the anode and oxygen at the cathode. The open cell voltage (OCV) obtained in this manner was 0.88 V. The same type of MEA may also be used in the Fig 2 embodiment.
VII. Examples Example 1 A Cs based solid acid such as CsHS04, CsHSe04 or Cs5H3(S04)4~xH20 is ground and mixed with a melt-processable polymer binder, such as poly(vinylidene fluoride), and hot-pressed. The result forms a solid composite membrane which is proton conducting even in dry atmospheres. The composite membrane, being comprised of two components whicha re substantially impermeable to fluids " may be less permeable than Naf ionTM .

Example 2 A Cs based solid acid such as Cs3 (HSO9) 2 (Hl,s (So.spo.s) 04) .
Cs3 (HS04) 2 (H2P04) , Css (HS04) 3 (HZP04) z or Cs2 (HS09) (HzP04) is ground and mixed with a melt-processable polymer binder, such as poly(vinylidene fluoride), and hot-pressed. The result forms a solid composite membrane which is proton conducting even in dry atmospheres. The membrane is also less permeable to fluids than Naf ions"' l0 Example 3.
A NH9 based solid acid such as (NH4 ) 3H (S04 ) 2 or (NH4 ) 3H ( Se04 ) 2 is ground and mixed with a melt-processable polymer binder, such TM
as Crystar 101 thermoplastic, and hot-pressed. The result forms IS a solid composite membrane which is proton conducting even in dry atmospheres. The membrane is less permeable to fluids than Nafion""' and is also less expensive.
' Example 4.

An solid acid silicate of general formula MaHbSi04, such as CaNaHSi04, Cs3HSi04, (NH4)3HSi04, is used as a membrane. Some of these materials are water insoluble and may have sufficient structural integrity that a binder is not required in some applications.
Example 5.
A Cs or NH4 based solid acid, such as CsHS04, Cs2 ( HSO4 ) ( HZPO4 ) , CsSH3 ( SO4 ) 4 ~ X HzO Or (NH4 ) 3H ( SO4 ) z is mixed Wlth the prepolymer of a resin such as "castoglas", a commercial product from Buehler, Inc. The polymerization/crosslinking catalyst is added to the mixture, and a solid composite membrane so formed. The in situ polymerization/crosslinking can lead to a higher impermeability than composites formed by melt-processing.
Example 6.

A Cs or NHg based solid acid, such as CsHSOg, Cs2 (HSOg) (H2POg) , Cs5H3 (SOg) g ~XHZO or (NHg) 3H (SOg) 2 is mixed Wlth a monomer such as dicyclopentadiene. A polymerization catalyst is then added to the mixture, and a solid composite membrane comprised of the solid acid and poly(dicyclopentadiene) is formed. The in situ polymerization of the polymer can lead to a higher impermeability than composites formed by melt-processing.
Use of a NHg based solid acid can result in an inexpensive membrane.
Example 7.
A Cs or NHg based solid acid, such as CSHSOg, Csz (HSOg) (HZPOg) , Cs5H3 (SOg) g xH2O or (NHg) 3H (SOg) 2 1S dlSSOlVed In water, and added to a suspension of an insoluble polymer such as poly(vinylidene fluoride) suspended in a fluid such as ethanol.
The mixture is cast and the liquids (water and ethanol) allowed to evaporate. This procedure yields a composite membrane which is proton conducting even in dry atmospheres. The casting step can produce very thin membranes, with thicknesses on the order of one hundred microns.

Example 8.
A Cs or NHg based solid acid, such as CsHSOg, Cs2 (HSOg ) (H2POg ) , Cs5H3 ( SOg ) g ~XHZO Or (NHg ) 3H ( SOg ) 2 1S ground and mixed with a ceramic, such as A1z03, or an oxide glass, such as amorphous Si02. The mixed powders are compressed by hot-pressing. The resulting composite membrane may be stable to higher temperatures than those in which the binder is a polymer.
Example 9.
A Cs or NHg based solid acid, such as CsHSOg, CS2 (HSOg) (HzPOg) , Cs5H3 (SOg) g ~XH2O Or (NHg) 3H (SOg) 2 1S dlSSOlVed In water. The solution is introduced into a porous membrane comprised of an inert binder such as TeflonTM, Si02, or A1203.
The water is allowed to evaporate, leaving the solid acid to fill the pores of the binder. The result is a composite membrane which is proton conducting even in dry atmospheres.
Example 10.

WO 00/45447 PCT/iJS00/01783 A Cs or NH4 based solid acid, such as CsHS04, Cs2 (HS04) (HZP04) , Cs5H3 (S04) 4 ~xHzO or (NH4) 3H (S04) 2, which is only proton conducting, is ground and mixed with an electronically conducting polymer such as poly(anylene). The composite membrane formed can conduct both protons and electrons.
Example 11.
An solid acid silicate of general formula MaHbSi04, such as CaNaHSi04, Cs3HSi04 or (NH4) 3HSi04, is ground and mixed with an electronically conducting polymer such as poly(anilene). The composite membrane formed can conduct both protons and electrons.
Example 12.
A proton conducting solid acid, such as CsHS04, Csz (HS04) (HZP04) , (NH4) 3H (S04) 2 or CaNaHSi04, and a metal, such as Ag, Au, or Cu, are ground and mixed. The mixed powders are compressed by hot-pressing. The composite membrane formed can conduct both protons and electrons, and may be stable to higher temperatures than a composite in which the electron conducting component is a polymer.
Example 13.

A proton conducting solid acid, such as CsHS04, Cs2 (HS04) (H2P04) , (NH4) 3H (S04) z or CaNaHSi04, and a metal, such as Ag, Au, or Cu, are ground and mixed. A polymeric material is also added. A solid composite membrane is prepared either by hot-pressing, if the polymer is melt-processable such as poly(vinylidene fluoride), or by in situ polymerization, if the polymer is in situ polymerizable such as poly(dicyclopentadiene).
The composite membrane is both proton and electron conducting, and may have superior mechanical properties to a composite containing only a solid acid and a metal.
Example 14.
A mixed electron and proton conducting solid acid, such as CsHCrXSl_X04 or (NH4) 3H (CrXSl_X04) 2 in which one of the X elements has a variable valence, is mixed with an inert polymeric binder.
If the polymer is melt-processable, such as poly(vinylidene fluoride), a membrane is formed by hot-pressing. If the polymer can be polymerized in situ, a membrane is formed by mixing the solid acid, the monomer and the polymerization catalyst. The resulting membrane conducts both protons and electrons, and may be more stable in oxidizing atmospheres than a composite containing metal particles.

Example 15.
A Cs or NH4 based solid acid, such as CsHS04, Cs2 ( HS04 ) ( HZ P04 ) , Cs5H3 ( S04 ) 4 ~xH20 or ( NH4 ) 3H ( S04 ) 2 i s prepared f rom aqueous solution, ground, and then pressed into a thin membrane.
The membrane is used in an electrochemical device at a temperature above the superprotonic transition temperature and above 100°C, so that the proton conductivity of the solid acid is high and any Hz0 that may be present in the device exists in the form of steam rather than liquid water.
Example 16.
A mixed electron and proton conducting solid acid, such as CsHCrXSl_X04 or (NH4) 3H (CrXSl_X04) 2 in which one of the X elements has a variable valence, is prepared from aqueous solution or by solid state reaction. The powder is then ground and pressed into a thin membrane. The membrane is used in an electrochemical device at a temperature above the superprotonic transition temperature and above 100°C, so that the conductivity of the solid acid is high and any H20 that may be present in the device exists in the form of steam rather than liquid water.
Example 17.

A composite comprised of one or more of the solid acids listed in Table 1 and one or more of inert binders listed in Table 2. If one or more of the components in the composite is electronically conducting, the composite membrane will be capable of conducting both protons and electrons. Electronically conducting substances are indicated.
Table 1. Solid acid compounds.
Sulfates and selenates and silicates sulfate-phosphates selenate phosphates CsHS04 CsHSe04 CaNaHS i04 Cs3H ( S04 ) z Cs3H ( Se04 ) z CaH2Si04 CssH3 ( SO4 ) 4 ' xHzOCssH3 ( SeO4 ) 4 ' CsH3S1O4 xHzO

Cs3 (HS04) z (Hi.s Csa (HSe04) z (Hi.s CszH2Si04 (So.sPo. (Seo.sP
5) 04) 0.5) ~4) Cs3 (HSO4) z (HzPO4) Cs3 (HSeO4) z (HzPO4) CS3HSlO4 Csz (HS04) (H2P04) Csz (HSe04) (H2P04) NH4H3Si04 CSS (HSO4) 3 (H2P~4) CS5 (HSeOg) 3 (H2P04) (NH4) 2H2SlOg CsHzP04 (NH4) 3HSi04 NH4HS04 NH4HSe04 RbH3Si04 (NH4) sH (SO4) z (NH4) 3H (SeO4) 2 RbzHzSlOg (NH4 ) sHa ( SO4 ) (NH4 ) sH3 ( SeO4 ) Rb3HSiO4 4 ' xHzO 4 ' xHzO

(NH4) z (HS04) (HzP04)(NH4) z (HSe04) (HZP04)KH3Si04 (NHg) HzPOg KZHzSiOg RbHSOg RbHSeOg K3HS iOg Rb3H ( SOg ) 2 Rb3H ( SeOg ) 2 NaH3Si0g Rb5H3 ( SOg ) g ' Rb5H3 ( SeOg ) g ' Na2HzSi0g xH20 xHzO

Rb2 ( HSOg ) ( HZ Rb2 ( HSeOg ) ( HzPOg Na3HSiOg POg ) ) RbH2POg BaCsHSiOg Table 2. Binder or matrix materials Polymer ceramic/oxide metal or glass semiconductor poly(vinylidene fluoride) Si02 Ag*

poly(dicyclopentadiene) A1203 Au*

poly(tetraflouroethelyne) Mg0 Cu*
[Teflon]

poly(ether-ether ketone) cordierite Al*

poly(ether sulfone) Ni*

Silicones [dimethyl Fe*
siloxane polymers]

poly(pyrrole)* Zn*

poly(aniline)* graphite*

silicon*

electronically conducting Other modifications are within the disclosed embodiment.
For example, the above has described the materials having a superprotonic transition upon heating. Certain materials may have their superprotonic transition temperature below room temperature. Thus, there may be no apparent superprotonic transition and the material would be disordered at room temperature. These solid acids with structural disorder even prior to heating are also contemplated.

Claims

CLAIMS:

1. A proton conducting membrane for use in the presence of H2O in a device selected from the group consisting of fuel cells, proton transport membranes, membrane reactors and sensors, comprising:
a solid acid material of the general formula:
M a H b(XO t)c wherein M is a cation having a charge from +1 to +2, X is selected from the group consisting of S, Se, P, As, Si and Te, and a, b, c and t are non-zero integers, wherein the solid acid material is capable of conducting protons in a solid state through a superprotonic mechanism.

2. A membrane as in claim 1, wherein M is one or more of the species selected from the group consisting of Li, Be, Na, Mg, K, Ca, Rb, Cs, Ba, Tl, NH4+ and Cu.

3. A membrane as in claim 1, wherein t is 3 or 4.

4. A membrane as in claim 1, wherein said solid acid material is of the general form Cs a H b(XO t)c.

5. A membrane as in claim 1, wherein X is silicon.

6. A membrane as in claim 3, wherein M is Cs.

7. A membrane as in claim 3, wherein M is NH4+.

8. A membrane as in claim 3, wherein said solid acid material is of the general formula M a H b(XO t)c.cndot.nH2O, wherein n is a rational number.

9. A membrane as in claim 3, wherein X is P.

10. A membrane as in claim 1, wherein said solid acid material is CsH2PO4.

11. A membrane as in claim 1, wherein said solid acid material is Cs5(HSO4)x(H2PO4)y, wherein x and y are rational numbers.

12. A membrane as in claim 1, wherein said solid acid material is Cs2(HSO4)X(H2PO4)Y), wherein x and y are rational numbers.

13. A membrane as in claim 1, wherein said solid acid material is Cs3(HSO4)2(H1.5(S0.5P0.5)O4).

14. A membrane as in claim 1, wherein said solid acid material is Cs5H3(SO4)4.cndot.xH2O, wherein x is a rational number.

15. A membrane as in claim 1, wherein said solid acid material is TlHSO4.

16. A membrane as in claim 1, wherein said solid acid material is CsHSeO4.

17. A membrane as in claim 1, wherein said solid acid material is Cs2(HSeO4)(H2PO4).

18. A membrane as in claim 1, wherein said solid acid material is (NH4)3H(SO4)2.

19. A membrane as in claim 1, wherein said solid acid material is (NH4)2(HSO4)(H2PO4).

20. A membrane as in claim 1, wherein said solid acid material is Rb3H(SO4)2.

21. A membrane as in claim 1, wherein said solid acid material is Rb3H(SeO4)2.

22. A membrane as in claim 1, wherein said solid acid material is Cs1.5Li1.5H(SO4)2.

23. A membrane as in claim 1, wherein said solid acid material is Cs2Na(HSO4)3.

24. A membrane as in claim 1, wherein said solid acid material is TlH3(SeO3)2.

25. A membrane as in claim 1, wherein said solid acid material is CsH2ASO4.

26. A membrane as in claim 1, wherein said solid acid material is (NH4)2(HSO4)(H2ASO4).

27. A membrane as in claim 1, wherein said solid acid material is CaNaHSiO4.

28. A membrane as in any one of claims 1 to 27, further comprising an electrochemical device, using said membrane for proton transport.

29. A membrane as in claim 1, wherein M comprises one or more cations selected from the group consisting of Li, Be, Na, Mg, K, Ca, Rb, Sr, Cs, Ba, Tl, and NH4+, and X is selected from the group consisting of Si, P, S, As, Se, Te and a combination thereof.

30. A method of conducting protons across a barrier, comprising:
forming a membrane as in any one of claims 1 to 29; and using the membrane to conduct protons.

31. A method as in claim 30, wherein said solid acid material undergoes a superprotonic transition between a first temperature and a second temperature, the method further comprising:
operating said membrane as a proton conducting membrane at a temperature between said first and second temperatures.

32. A method as in claim 30 or 31, wherein M is Cs.

33. A method as in claim 30 or 31, wherein M is NH4+.

34. A method as in any one of claims 30 to 33, wherein X comprises silicon.

35. A method as in any one of claims 30 to 34, wherein said protons are conducted in a fuel cell.

36. A method as in any one of claims 30 to 34, wherein said protons are conducted in a hydrogen separator.

37. A method as is in any one of claims 30 to 34, wherein said protons are conducted in an electrolysis cell.

38. A method as in any one of claims 30 to 34, wherein said protons are conducted in a battery.

39. A membrane as in any one of claims 1 to 29, further comprising:
a structural binder for said solid acid material, forming a membrane with said solid acid material.

40. A membrane as in claim 39, wherein said structural binder is a polymer.

41. A membrane as in claim 40, wherein said polymer is a melt processable polymer.

42. A membrane as in claim 40, wherein said polymer is an in-situ polymerized polymer.

43. A membrane as in claim 39, wherein said structural binder is a ceramic.

44. A membrane as in claim 39, wherein said structural binder is a glass.

45. A membrane as in any one of claims 39 to 44, wherein said structural binder is electronically insulating.

46. A membrane as in claim 39, wherein said structural binder is an electrically conducting material.

47. A membrane as in claim 46, wherein said electrically conducting material is a conducting polymer.

48. A membrane as in claim 46, wherein said electrically conducting material is a metal.

49. A membrane as in claim 48, wherein said metal is mixed with a polymer.

50. A membrane as in claim 46, wherein said electrically conducting material is formed by direct chemical substitution with variable valence ions.

51. A membrane as in claim 39, wherein said structural binder comprises silicon.

52. A membrane as in claim 39, wherein said structural binder is a polyester binder.

53. A membrane as in claim 39, wherein said structural binder is electrochemically unreactive.

54. A membrane as in any one of claims 39 to 53, wherein M is Cs.

55. A membrane as in any one of claims 39 to 53, wherein M is NH4+.

56. A membrane as in any one of claims 39 to 55, wherein X is Si.

57. A membrane as in claim 45, wherein said solid acid material is processed to include variable valence elements.

58. A method of operating an electrochemical device, comprising:
providing a fuel to a membrane as in any one of claims 1 to 29; and carrying out an electrochemical reaction at said membrane, without hydrating said membrane.

59. A method as in claim 58, wherein said electrochemical reaction is carried out at a temperature of 100° C or higher.

60. A method as in claim 59, wherein said membrane comprises a binder.

61. A membrane as in any one of claims 1 to 29, wherein M has a variable valence and said membrane conducts both protons and electrons.

62. A membrane as in claim 61, where X comprises silicon.

63. A membrane as in claim 61 or 62, further comprising a structural binder.

64. A membrane as in claim 63, wherein said binder comprises a conducting material.

65. ~A membrane as in claim 64, wherein said conducting material comprises a conductive polymer.

66. ~A membrane as in claim 64, wherein said conducting material comprises a metal.

67. ~A method of separating H2 from other materials, comprising:
chemically reacting H2 at a surface of a membrane as in any one of claims 61 to 66, to decompose H2 into H+
and e-; and using said membrane to allow said H+ and e- to pass while blocking other materials from passing.

68. ~A method as in claim 67, wherein the other materials comprise CO.

69. ~A membrane as in any one of claims 1 to 29, wherein the solid acid material comprises:
a Cs based solid acid material, the membrane further comprising:
a conductive melt processable polymer binder.

70. ~A membrane as in claim 69, wherein said Cs based solid acid material is selected from the group consisting of CS3(HSO4)2(H1.5(S0.5P0.5)O4), CS3(HSO4)2(H2PO4), CS5(HSO4)3(H2PO4)2 or Cs2(HSO4)(H2PO4) CsHSO4, CsHSeO4 and Cs5H3(SO4)4.cndot.xH2O, wherein x is a rational number.

71. ~A membrane as in claim 69 or 70, wherein said membrane is formed by hot pressing.

72. ~A membrane as in any one of claims 1 to 29, wherein the solid acid material comprises:

a NH4+ based solid acid material, the membrane further comprising:
a structural binder.

73. ~A membrane as in claim 72, wherein said structural binder is a melt processable polymer.

74. ~A membrane as in claim 72 or 73, wherein said solid acid material is selected from the group consisting of Cs3H(SeO4)2(NH4)3H(SO4)2, (NH4)2(HSO4)(H2PO4) and CsH2AsO4(NH4)2(HSO4)(H2AsO4).

75. ~A membrane as in any one of claims 1 to 29, wherein the solid acid material is a silicate of the general formula M A H B SiO4, wherein A and B are rational numbers.

76. ~A membrane as in claim 75, further comprising a~
structural binder.

77. ~A membrane as in claim 75 or 76, wherein said solid acid material is selected from the group consisting of CaNaHSiO9, Cs3HSiO9 and (NH9)3HSiO4.

78. ~A membrane as claimed in any one of claims 1 to 29, wherein the solid acid material comprises:
a Cs or NH4 based solid acid, the membrane further comprising:
a structural binder selected from the group consisting of a ceramic and glass.

79. ~A membrane as in claim 78, wherein said binder is porous.

80. ~A method of operating an electrochemical device, comprising:~

forming a membrane according to any one of~
claims 1 to 29; and using said membrane within the electrochemical device to conduct protons.

81. ~A method as in claim 80, wherein the forming a membrane step further comprises heating said solid acid material to a temperature at which it undergoes a superprotonic transition.

82. ~A method as in claim 80 or 81, wherein said solid acid material is a sulfate or sulfate phosphate type solid acid.

83. ~A method as in claim 80 or 81, wherein said solid acid material is a selenate or selenate phosphate type solid~
acid.

84. ~A method as in claim 80 or 81, wherein said solid acid material is a silicate.

85. ~A method as in any one of claims 80 to 84, wherein said forming a membrane step comprises adding a binder to said solid acid material.

86. ~A method as in claim 85, wherein said binder is a polymer.

87. ~A method as in claim 85, wherein said binder is a ceramic/oxide glass.

88. ~A method as in claim 85, wherein said binder is a conducting metal or semiconductor.

89. ~A method of operating an electrochemical device comprising:

providing a fuel to a membrane as in any one of claims 1 to 29, in the presence of carbon monoxide; and carrying out an electrochemical reaction at said membrane, without removing said carbon monoxide.

90. A method of forming a membrane-electrode assembly, comprising:
forming a composite film comprising a polymer and a membrane as in any one of claims 1 to 29;
forming said composite film onto a backing;
forming electrodes on said backing; and hot pressing said composite film to form the assembly.

91. A method as in claim 90, wherein the solid acid material to polymer volume ratio is 50/50.

92. A method as in claim 90 or 91, wherein said backing is graphite paper.

93. A method as in claim 30, wherein protons are conducted in a supercapacitor.