U.S. patent application number 16/714476 was filed with the patent office on 2020-08-20 for ceramic proton-conducting membranes.
This patent application is currently assigned to University of Washington. The applicant listed for this patent is University of Washington. Invention is credited to Anthony William Moretti, Gregory M. Newbloom, Lilo D. Pozzo, Eden Rivers, Aaron West.
Application Number | 20200261856 16/714476 |
Document ID | 20200261856 / US20200261856 |
Family ID | 1000004812627 |
Filed Date | 2020-08-20 |
Patent Application | download [pdf] |
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United States Patent
Application |
20200261856 |
Kind Code |
A1 |
Pozzo; Lilo D. ; et
al. |
August 20, 2020 |
CERAMIC PROTON-CONDUCTING MEMBRANES
Abstract
Disclosed herein are ceramic selective membranes and methods of
forming the ceramic selective membranes by forming a selective
silica ceramic on a porous membrane substrate.
Inventors: |
Pozzo; Lilo D.; (Seattle,
WA) ; Moretti; Anthony William; (Seattle, WA)
; Newbloom; Gregory M.; (Seattle, WA) ; West;
Aaron; (Seattle, WA) ; Rivers; Eden; (Seattle,
WA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
University of Washington |
Seattle |
WA |
US |
|
|
Assignee: |
University of Washington
Seattle
WA
|
Family ID: |
1000004812627 |
Appl. No.: |
16/714476 |
Filed: |
December 13, 2019 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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15796607 |
Oct 27, 2017 |
10537854 |
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16714476 |
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PCT/US2017/016246 |
Feb 2, 2017 |
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15796607 |
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62290053 |
Feb 2, 2016 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B01D 2256/16 20130101;
B01D 39/2068 20130101; B01D 67/0079 20130101; H01M 2300/0094
20130101; H01M 8/0236 20130101; B01D 69/02 20130101; B01D 67/0076
20130101; B01D 2325/14 20130101; B01D 67/0048 20130101; H01M 8/188
20130101; B01D 71/36 20130101; H01M 8/0245 20130101; B01D 71/02
20130101; B01D 2257/102 20130101; B01D 71/024 20130101; B01D 61/025
20130101; B01D 53/228 20130101; B01D 2257/504 20130101; B01D
67/0088 20130101; B01D 71/76 20130101; B01D 69/148 20130101; H01M
8/1016 20130101; H01M 8/1062 20130101; B01D 71/027 20130101; B01D
71/52 20130101; B01D 71/28 20130101; Y02E 60/50 20130101; B01D
71/66 20130101; H01M 8/1053 20130101; B01D 71/34 20130101; H01M
8/106 20130101; B01D 69/12 20130101; B01D 2325/42 20130101; B01D
71/26 20130101; B01D 69/105 20130101; H01M 8/0239 20130101; B01D
2325/26 20130101; B01D 2323/286 20130101 |
International
Class: |
B01D 67/00 20060101
B01D067/00; B01D 39/20 20060101 B01D039/20; B01D 71/02 20060101
B01D071/02; H01M 8/1053 20060101 H01M008/1053; H01M 8/1062 20060101
H01M008/1062; B01D 69/02 20060101 B01D069/02; B01D 69/14 20060101
B01D069/14; H01M 8/106 20060101 H01M008/106; H01M 8/1016 20060101
H01M008/1016; H01M 8/18 20060101 H01M008/18; B01D 69/10 20060101
B01D069/10; B01D 69/12 20060101 B01D069/12; H01M 8/0236 20060101
H01M008/0236; H01M 8/0239 20060101 H01M008/0239; H01M 8/0245
20060101 H01M008/0245; B01D 71/26 20060101 B01D071/26; B01D 71/28
20060101 B01D071/28; B01D 71/76 20060101 B01D071/76 |
Goverment Interests
STATEMENT OF GOVERNMENT LICENSE RIGHTS
[0002] This invention was made with Government support under
Contract No. W911NF-13-1-0166 awarded by the Department of Defense
via the Army Research Office. The Government has certain rights in
the invention.
Claims
1. (canceled)
2. A ceramic selective membrane, comprising: a selective silica
ceramic coating supported by a porous membrane substrate and
functionalized with a silane coupling agent; wherein the porous
membrane substrate has a plurality of pores 10 nm or greater in
diameter; wherein the ceramic selective membrane comprises pores in
the size range of 0.1 nm to 10 nm in diameter; wherein the
selective silica ceramic coating has one or more layers of
selective silica ceramic material formed by chemical gelation of a
precursor sol comprising an alkaline silicate solution; wherein the
ceramic selective membrane has a thickness in the range of 0.1 mm
to 1 mm; and wherein the selective silica ceramic has a thickness
of 0.5 .mu.m to 750 .mu.m disposed on the surface of the porous
membrane substrate.
3. The ceramic selective membrane of claim 2, wherein the porous
membrane substrate is selected from the group of silica filter
paper, polyvinylidene fluoride (PVDF), polyether ether ketone
(PEEK), and polytetrafluoroethylene (PTFE).
4. The ceramic selective membrane of claim 2, further comprising a
compressible polymer edging along at least a portion of an edge of
the porous membrane substrate.
5. The ceramic selective membrane of claim 4, wherein the portion
of the edge is 1 mm or greater in width.
6. The ceramic selective membrane of claim 4, wherein the
compressible polymer comprises a thermoplastic elastomeric
polymer.
7. The ceramic selective membrane of claim 2, further comprising
compressible polymer edging along all edges of the ceramic
selective membrane, defining a gasket.
8. The ceramic selective membrane of claim 2, wherein the selective
silica ceramic comprises a plurality of layers of selective silica
ceramic material.
9. The ceramic selective membrane of claim 2, wherein the ceramic
selective membrane has a mean pore size in the range of 0.5 nm to 2
nm as determined by fitting of a polydispersed fractal model to a
small angle x-ray scattering profile.
10. A selective membrane comprising a ceramic selective membrane
according to claim 2.
11. The selective membrane of claim 10, wherein the selective
membrane is of a type selected from a battery membrane, a fuel cell
membrane, a food processing membrane, a reverse osmosis membrane, a
gas separation membrane, and a bio-separation membrane.
12. The selective membrane of claim 10, wherein the selective
membrane is an ion-conducting membrane for a flow battery.
13. The selective membrane of claim 10, wherein the selective
membrane is an ion-conducting membrane for a fuel cell.
Description
CROSS-REFERENCES TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. application Ser.
No. 15/796,607, filed Oct. 27, 2017, which is a continuation of
International Application No. PCT/US2017/016246, filed Feb. 2,
2017, which claims the benefit of U.S. Provisional Application No.
62/290,053, filed Feb. 2, 2016, the disclosures of which are
expressly incorporated herein by reference in their entirety.
BACKGROUND
[0003] Fuel cells and redox flow batteries (RFB) are powerful
energy storage technologies that rely on selective transport of
ions across a membrane in order to generate electrical current.
Presently, the standard materials used for membranes in these
technologies are perfluorinated sulfonic acid (PFSA) materials,
such those as marketed under the name NAFION. These PFSA materials
are desirable for their resistance to harsh environments and
ion-conducting properties. However, these materials are relatively
expensive and further improvements in operating properties are
desirable.
[0004] Silicate proton-conducting materials are known but are not
sufficiently developed so as to be commercially viable.
[0005] One common mechanism for proton transport is Grottuss
hopping, wherein hydronium ions "hop" from water molecule to water
molecule. This mechanism requires that water be sufficiently free
to rotate and diffuse in order to promote solvation of a hydronium
ion. Therefore, the design of a silicate proton-conducting material
should have pores of sufficient size to promote the molecular
motion of water (i.e., facilitate hydronium solvation and proton
hopping). This size should be larger than a single water molecule
(i.e., radius of 0.138 nm) but not so large as to facilitate
diffusive transport of other molecules.
[0006] Accordingly, the development of robust, inexpensive
proton-conducting materials is desired, yet significant
compositional and structural limitations presently prevent the use
of silicate materials as replacements for PFSA materials.
SUMMARY
[0007] This summary is provided to introduce a selection of
concepts in a simplified form that are further described below in
the Detailed Description. This summary is not intended to identify
key features of the claimed subject matter, nor is it intended to
be used as an aid in determining the scope of the claimed subject
matter.
[0008] In one aspect, a method of forming a ceramic selective
membrane is provided. In one embodiment, the method includes:
[0009] applying a ceramic precursor sol to a porous membrane
substrate; and
[0010] gelling the ceramic precursor sol, using a sol-gel process,
to form a selective silica ceramic from the ceramic precursor sol,
thereby providing a ceramic selective membrane comprising the
selective silica ceramic supported by the porous membrane
substrate.
[0011] In another aspect, a ceramic selective membrane is provided,
comprising a selective silica ceramic supported by a porous
membrane substrate.
[0012] In another aspect, a selective membrane is provided that
includes a ceramic selective membrane comprising a selective silica
ceramic supported by a porous membrane substrate as previously
described.
[0013] In another aspect, a ceramic selective membrane is provided,
as formed by a method according to any of the disclosed method
embodiments.
DESCRIPTION OF THE DRAWINGS
[0014] The foregoing aspects and many of the attendant advantages
of this invention will become more readily appreciated as the same
become better understood by reference to the following detailed
description, when taken in conjunction with the accompanying
drawings, wherein:
[0015] FIG. 1A is a schematic representation of sol-gel strategies
using alkali silicates to produce ceramic selective membranes in
accordance with certain embodiments disclosed herein.
[0016] FIG. 1B illustrates an exemplary process for producing a
ceramic selective membrane having compressible edging ("elastomeric
edging") in accordance with certain embodiments disclosed herein.
STEP 1: Elastomeric edging is affixed to a pre-sized porous
membrane support. Solvent soluble polymer such as
poly(styrene-isobutylene-styrene) (SIBS) is cast to the size of the
support while also defining the active area. Solvent is placed on
the edges of the support and then sandwiched by the edging. The
solvent partially dissolves the edging which results in strong
adhesion with the support. STEP 2: The support is dipped in a
silica precursor solution. STEP 3: The coated support is dipped in
an acid bath. STEP 4: The membrane is dried at low temperature
(e.g., 60.degree. C.). The process conditions of steps 2-4 can be
varied to produce compressible ceramic selective membranes with
specific performance attributes (e.g., proton conduction,
flexibility, durability, pore size, etc.).
[0017] FIG. 2A is a SEM micrograph of a silica support of the type
usable as a porous membrane substrate in accordance with certain
embodiments disclosed herein. FIG. 2B is a SEM micrograph of a
silica selective membrane formed on a silica support in accordance
with certain embodiments disclosed herein.
[0018] FIG. 3A: Small angle x-ray scattering (SAXS) profile of a
membrane fit with a fractal aggregate model. FIG. 3B: Pore size
distribution based on SAXS modeling. The ideal pore size based on
ionic radii of ions in flow batteries is noted. Ideal pore size
distribution is noted between the radius of H.sub.3O.sup.+ and
VO.sub.2. The known pore size range of NAFION is also noted.
[0019] FIG. 4: SEM images of exemplary ceramic selective membrane
surfaces and cross-sections. The right membrane was dipped in a
methanol/water mixture to reduce capillary stresses prior to
drying. Conceptual depictions of the membrane cross-sections are
also shown.
[0020] FIG. 5 graphically illustrates proton conductivity versus
VO.sub.2 permeability for representative ceramic selective
membranes ("SS" is a membrane formed from all sodium silicate; "14
wt % PSS in SS" is a membrane formed with 14 wt % of PSS in a
sodium silicate sol; "NaS-TEOS" is a membrane formed with a
combination of sodium silicate and TEOS to form an aggregate
composite membrane; and "LiS" is a membrane formed in a similar
manner to the NaS membrane but with lithium silicate instead of
sodium silicate) compared to the commercial material NAFION
212.
[0021] FIG. 6A graphically illustrates cycling capacity of an
exemplary all-vanadium RFB using exemplary sol-gel SiO.sub.2;
exemplary sol-gel SiO.sub.2+SIBS composite; and comparative NAFION
115. FIG. 6B illustrates voltage profiles for RFBs including the
membranes of FIG. 6A.
[0022] FIGS. 7A and 7B are SEM images of a TEOS-sodium silicate
membrane in accordance with embodiments disclosed herein. FIG. 7A
is a top view and FIG. 7B is a dense cross-sectional view.
[0023] FIG. 8 graphically illustrates the fractal dimension
extracted from SAXS fitting of representative membranes using the
fractal model (shown in FIG. 3A). The selectivity (proton
conductivity/vanadium ion permeability) is plotted as a function of
the fractal dimension showing higher selectivity for lower fractal
dimensions. NAFION is plotted for reference.
DETAILED DESCRIPTION
[0024] Disclosed herein are ceramic selective membranes and methods
of forming the ceramic selective membranes by forming a selective
silica ceramic on a porous membrane substrate. Representative
ceramic selective membranes include ion-conductive membranes (e.g.,
proton-conducting membranes) and gas selective membranes.
Representative uses for the membranes include incorporation into
fuel cells and redox flow batteries (RFB) as ion-conducting
membranes.
[0025] Prior work on silica membranes focused on thin membranes,
particularly those formed entirely from silica. The membranes
disclosed herein are focused on durability and performance, with an
eye towards commercial relevance (e.g., the replacement of NAFION
membranes in RFBs and fuel cells). Accordingly, the disclosed
membranes are relatively thicker because the precursor sol fills
the support. Generally, it is known that silica materials crack
when coated on top of porous support structures with pore sizes
greater than 100 nm. The disclosed membranes are tailored to reduce
and eliminate cracking in many ways, including both compositionally
(e.g., using polymer additives in the sol that are incorporated
into the ceramic matrix) and by "repair" or "post-processing" by
applying a second (or further) layer of ceramic and/or applying a
non-ceramic coating to "finish" the membrane.
[0026] The methods and many variations will now be discussed in
further detail. Further below the compositions (membranes) formed
by the methods will also be discussed in greater detail.
[0027] Methods of Forming Ceramic Selective Membranes
[0028] In one aspect, a method of forming a ceramic selective
membrane is provided. In one embodiment, the method includes:
[0029] applying a ceramic precursor sol to a porous membrane
substrate; and
[0030] gelling the ceramic precursor sol, using a sol-gel process,
to form a selective silica ceramic from the ceramic precursor sol,
thereby providing a ceramic selective membrane comprising the
selective silica ceramic supported by the porous membrane
substrate.
[0031] Turning first to FIG. 1A, a schematic representation of
sol-gel methods using alkali silicates to produce ceramic selective
membranes in accordance with certain embodiments disclosed herein.
In particular, the sol includes sodium silicate, lithium silicate,
or potassium silicate as the ceramic precursor sol. In certain
embodiments an "optional" additive is added to the ceramic
precursor sol. A "macroporous substrate" (also referred to herein
as a porous membrane substrate) is used as the substrate for the
formation of a sol-gel and eventually a condensate gel of selective
silica ceramic supported by the porous membrane substrate, to
provide a ceramic selective membrane.
[0032] FIG. 1B illustrates an exemplary process for producing a
ceramic selective membrane having compressible edging ("elastomeric
edging") in accordance with certain embodiments disclosed herein.
STEP 1: Elastomeric edging is affixed to a pre-sized porous
membrane support. Solvent soluble polymer such as
poly(styrene-isobutylene-styrene) (SIBS) is cast to the size of the
support while also defining the active area. Solvent is placed on
the edges of the support and then sandwiched by the edging. The
solvent partially dissolves the edging which results in strong
adhesion with the support. STEP 2: The support is dipped in a
silica precursor solution. STEP 3: The coated support is dipped in
an acid bath. STEP 4: The membrane is dried at low temperature
(e.g., 60.degree. C.). The process conditions of steps 2-4 can be
varied to produce compressible ceramic selective membranes with
specific performance attributes (e.g., proton conduction,
flexibility, durability, pore size, etc.).
[0033] Precursors
[0034] The precursor is the solution (sol) from which the ceramic
is formed. One or more additives can be added to the sol in order
to enable the final ceramic membrane with particular properties, as
will be discussed in greater detail below.
[0035] In one embodiment, the ceramic precursor sol comprises an
alkaline silicate solution.
[0036] In one embodiment, the alkaline silicate solution is formed
from a silicate selected from the group consisting of sodium
silicate, lithium silicate, and potassium silicate.
[0037] In one embodiment, the alkaline silicate solution has a
concentration in the range of 5 wt % to 50 wt %. In one embodiment,
the alkaline silicate solution has a concentration in the range of
12 wt % to 30 wt %.
[0038] Porous Membrane Substrates
[0039] The porous membrane substrate (sometimes referred to herein
simply as the "substrate") is the structural foundation upon which
the ceramic is formed. The substrate provides mechanical strength
and a porous structure. When the ceramic is formed on the
substrate, the substrates relatively large pores are closed and
filled with the ceramic until nanometer- or angstrom-sized pores
remain in the final membrane.
[0040] In one embodiment, the porous membrane substrate has a
plurality of pores 10 nm or greater in diameter.
[0041] In one embodiment, the porous membrane substrate has a
chemical surface functionality that is chemically similar to the
ceramic precursor sol.
[0042] In one embodiment, the porous membrane substrate is selected
from the group consisting of silica filter paper, polyvinylidene
fluoride (PVDF), polyether ether ketone (PEEK),
polytetrafluoroethylene (PTFE).
[0043] Compressible Edging
[0044] Compressible edging enables membranes in certain embodiments
to be incorporated into batteries, fuel cells, or other systems
where a gasket sealing the membrane is required. Accordingly, the
compressible edging is both mechanically compressible and also
resistant to the heat and/or harsh chemical environments in which
the membranes are utilized.
[0045] In one embodiment, the method further includes a step of
impregnating an edge portion of the porous membrane substrate with
a compressible polymer prior to the step of applying the ceramic
precursor sol to the porous membrane substrate.
[0046] In one embodiment, the step of impregnating the edge portion
of the porous membrane substrate with the compressible polymer
comprises impregnating all edges of the porous membrane substrate
with the compressible polymer, sufficient to form a gasket
bordering the porous membrane substrate.
[0047] In one embodiment, a compressible polymer edging is formed
after formation of the ceramic selective membrane (i.e., after the
sol-gel process). In a further embodiment, the compressible polymer
edging is formed using ultrasonic welding or hot pressing.
[0048] In one embodiment, the edge portion is 1 mm or greater in
width. In one embodiment, the edge portion is 5 mm or greater in
width. In one embodiment, the edge portion is 1 cm or greater in
width.
[0049] In one embodiment, the compressible polymer comprises a
thermoplastic elastomeric polymer. In one embodiment, thermoplastic
elastomeric polymer is selected from the group consisting of
poly(styrene-isobutylene-styrene) (SIBS), polyvinylidene fluoride
(PVDF), and polydimethylsiloxane (PDMS).
[0050] In one embodiment, impregnating the edge portion of the
porous membrane substrate with the compressible polymer comprises a
method selected from the group consisting of melting, solution
deposition, and in situ reaction.
[0051] Chemical Gelation
[0052] After the sol is coated on the membrane, the sol must be
gelled. In one embodiment, gelling the ceramic precursor sol
comprises chemical gelation.
[0053] In one embodiment, chemical gelation comprises exposing the
ceramic precursor sol to an acid solution. In one embodiment, the
acid solution is greater than 0.001 N. In one embodiment, the acid
solution is greater than 1 N. In one embodiment, the acid solution
is greater than 3 N.
[0054] In one embodiment, the acid solution is selected from the
group consisting of sulfuric acid, nitric acid, acetic acid,
hydrochloric acid, methane sulfonic acid, and phosphoric acid.
[0055] In one embodiment, the step of chemical gelation is
completed in less than 1 hour. In one embodiment, the step of
chemical gelation is completed in less than 24 hours In one
embodiment, the step of chemical gelation is completed in less than
96 hours.
[0056] In one embodiment, the step of chemical gelation further
comprises, after exposing the ceramic precursor sol to an acid
solution, exposing to a temperature in the range of 20.degree. C.
to 100.degree. C. Keeping the temperature low in this embodiment is
important, such that no sintering or calcination occurs, which
damage the durability of the final ceramic selective membrane.
[0057] Heating Gelation
[0058] In one embodiment, gelling the ceramic precursor sol
comprises exposing to a temperature in the range of 20.degree. C.
to 100.degree. C. Keeping the temperature low in this embodiment is
important, such that no sintering or calcination occurs, which
damage the durability of the final ceramic selective membrane. In a
further embodiment, the step of gelling does not include exposure
to acid.
[0059] In one embodiment, the ceramic selective membrane comprises
pores in the size range of 0.1 nm to 10 nm in diameter. The pore
size is the final pore size of the ceramic selective membrane,
including all layers of ceramic and any post-processing layers
(e.g., a "finishing" alkyl layer). In one embodiment, the ceramic
selective membrane comprises pores in the size range of 0.1 nm to 5
nm in diameter. In one embodiment, the ceramic selective membrane
comprises pores in the size range of 0.1 nm to 1 nm in diameter. In
one embodiment, the ceramic selective membrane comprises pores in
the size range of 0.5 nm to 1 nm in diameter.
[0060] In one embodiment, the ceramic selective membrane has a
thickness in the range of 0.1 mm to 1 mm. In one embodiment, the
ceramic selective membrane has a thickness in the range of 0.1 mm
to 0.5 mm. In one embodiment, the ceramic selective membrane has a
thickness in the range of 0.2 mm to 0.4 mm.
[0061] In certain embodiments, multiple layers of ceramic are
deposited to provide a thicker ceramic layer on the support and/or
improve durability of the final membrane. In one embodiment two
coatings of ceramic are performed. In a further embodiment, three
coatings of ceramic are performed. Multiple coatings result in a
denser and more defect-free membrane structure. As an example, at
27 wt % sodium silicate solution, a single coating of ceramic may
suffice to produce a sufficiently dense and defect-free coating.
More than one coating is required for 15 wt % and 5 wt % sodium
silicate concentration when using a 250 .mu.m substrate.
[0062] In one embodiment, the method further includes a step of
depositing at least one additional layer of ceramic by:
[0063] applying the ceramic precursor sol to the selective silica
ceramic supported by the porous membrane substrate; and
[0064] gelling the ceramic precursor sol to provide a double-coated
selective silica ceramic supported by the porous membrane
substrate.
[0065] In a further embodiment, the step of depositing at least one
additional layer of ceramic includes repeating for a second time
the step of depositing at least one additional layer of ceramic, to
provide a triple-coated selective silica ceramic supported by the
porous membrane substrate.
[0066] Additives
[0067] As previously discussed, additives are added to the sol in
order to enable specific desirable properties of the membrane when
formed.
[0068] In one embodiment, the ceramic precursor sol further
comprises an additive selected from the group consisting of a
selectivity additive configured to increase ion transport
properties of the ceramic selective membrane, a durability additive
configured to improve durability of the ceramic selective membrane,
and a catalyst additive configured to add catalytic properties to
the ceramic selective membrane.
[0069] In one embodiment, the additive is a selectivity additive
selected from the group consisting of an ionic-conducting polymer
and a gas conducting polymer. To improve selectivity, in certain
embodiments a polymer is an additive used to facilitate selective
ion transport. For example, proton conducting polymers such as
polystyrene sulfonate (PSS), polydiallyldimethylammonium chloride
(PolyDADMAC), sulfonated nanocrystalline cellulose, sulfonated poly
ether ether ketone (SPEEK), sulfonated polybenzimidazole (S-PBI) or
perfluorosulfonic acid (PFSA). In other embodiments, the additive
is a polymers (i.e., polyvinyl alcohol, polyacrylic acid,
polyacrylamide, polyethylene glycol and others) promoting selective
transport of other molecules (e.g., gases or other ions). Any
additive polymers are soluble or dispersible in the alkaline
ceramic solution. Furthermore, they must be able to handle the
harsh environments or be protected from degradation by the
oxide.
[0070] In one embodiment, the additive is a durability additive
selected from the group consisting of a low Young's modulus polymer
configured to provide increased flexibility to the ceramic
selective membrane and a high Young's modulus polymer configured to
provide increased durability to the ceramic selective membrane.
[0071] In certain embodiments, the durability additive is a
polymer. A low Young's modulus polymer additive will lead to
flexibility of the final membrane or a high Young's modulus will
lead to improved durability of the final membrane. These are
soluble or dispersible in the alkaline ceramic solution able to
handle the harsh environments defined earlier or be protected from
degradation by the oxide. Representative durability polymer
additives include polyvinyl alcohol, polyacrylic acid,
polyacrylamide, and polyethylene glycol, as well as combinations
and copolymers thereof.
[0072] In one embodiment, the additive is a catalyst additive
selected from the group consisting of catalytic particles added to
the ceramic precursor sol and catalytic particles formed with in
the ceramic precursor sol. The catalyst additive is selected from
the following schemes: (1) the addition of catalytic nano- or
micro-particles to the sol; (2) forming catalytic particles within
the sol (e.g., prior to gelation/self-assembly); (3) forming
catalytic particles during the sol-gel; and (4) applying/coating
the surface of the active area with catalytic particles after it is
cured. Platinum is an example a catalyst additive. The catalyst
additive must be able to handle the harsh environments that the
membrane is exposed to (if contained externally) or be protected
from degradation by the ceramic membrane (if contained internally).
In one embodiment, the catalyst additive is 10 vol % or less of the
membrane.
[0073] Post-Treatment of the Membrane
[0074] Post-treatment of the membrane is another route to providing
specific properties to the membrane. Post treatment can be used
along with additives, or instead of additives, to generate a
membrane with specific properties.
[0075] In one embodiment, the method further includes depositing a
finishing layer on the selective silica ceramic supported by the
porous membrane substrate to provide a finished-coated selective
silica ceramic supported by the porous membrane substrate.
[0076] In one embodiment, the step of depositing a finishing layer
comprises treating the selective silica ceramic supported by the
porous membrane substrate with a silica-based compound with a
hydrolyzable group (e.g., tetraethyl orthosilicate).
[0077] In one embodiment, the step of treating the selective silica
ceramic supported by the porous membrane substrate by applying a
silica-based compound with a hydrolyzable group further comprises a
step of exposing the silica-based compound with a hydrolyzable
group to water after applying the silica-based compound with a
hydrolyzable group to the selective silica ceramic supported by the
porous membrane substrate.
[0078] In one embodiment, the method further includes treating the
ceramic selective membrane with an agent selected from the group
consisting of an ion-conductivity enhancer, a molecular-selectivity
enhancer, a catalyst, and a durability enhancer.
[0079] In one exemplary embodiment, a sulfonated
poly(styrene-isobutylene-styrene) (S-SIBS) coating has been shows
to improve molecular selectivity of membranes in a flow battery
environment. S--SIBS is referred to in FIGS. 1B, 6A, and 6B.
S--SIBS can be applied using any coating method, (e.g., drop
coating). In an exemplary method, the outside of a dry (fully
formed) membrane was coated with <100 .mu.m of S-SIBS polymer to
reduce defects.
[0080] It is also possible to functionalize the membranes using
standard silane coupling agents. This includes a silane with a
sulfonic acid group to improve proton conductivity or molecular
selectivity. Exemplary compounds include
3-trihydroxysilyl-1-propanesulfonic-acid and
triethoxy(hexyl)silane.
[0081] Other embodiments include a silane with a long alkane group
to improve durability or reduce pore size. Exemplary silanes with
an alkane group are tetraethyl orthosilicate (TEOS) and
triethoxy(hexyl)silane.
[0082] In an exemplary method, the outside of a membrane dipped in
the sol was coated with TEOS and allowed to dry at room
temperature.
[0083] In another exemplary method, a dry membrane was coated with
a polystyrene sulfonate (PSS) solution and allowed to dry.
[0084] In one embodiment, the method further includes a step of
exposing the ceramic selective membrane to a lower-surface-tension
liquid, after gelling the ceramic precursor sol. This step helps to
reduce the amount of surface cracking in the final membrane due to
lower capillary stresses.
[0085] In such embodiments, instead of placing the membrane in an
oven (or at room temperature) after exposing to acid after the sol
is applied, the membrane is exposed to a lower surface tension
fluid. In one method there is only a single exposure step into
water, methanol or a 50/50 water/methanol mixture. In another
variation on the method, sequential dips into water, 50/50
water/ethanol mixture and then methanol are used.
[0086] Pretreatment of the Porous Membrane Substrate
[0087] In one embodiment, the method further includes a step of
applying a pretreatment to the porous membrane substrate prior to
the step of applying the ceramic precursor sol to the porous
membrane substrate.
[0088] In one embodiment, the pretreatment is selected from the
group consisting of an acid and a polymer. In an exemplary
embodiment, the substrate is not coated first with the silica sol
but is instead coated with acid or polymer (polystyrene sulfonic
acid). In one method, this pre-coating is at low temperature
(T<100.degree. C.) and the newly coated substrate is then taken
through the standard sol/sol-gel process as disclosed herein. In an
alternative method, the pre-coating is not dried and is instead
dipped directly into the silica sol.
[0089] Ceramic Selective Membrane Compositions
[0090] In addition to the methods of forming ceramic selective
membranes, the membranes themselves, as compositions, will now be
discussed. Essentially, any membrane formed by the disclosed
methods is considered an embodiment of the disclosed aspects.
Accordingly, the compositions and features of the membranes flow
from the previous discussion of the methods.
[0091] In another aspect, a ceramic selective membrane is provided,
comprising a selective silica ceramic supported by a porous
membrane substrate.
[0092] In one embodiment, the porous membrane substrate is selected
from the group consisting of silica filter paper, polyvinylidene
fluoride (PVDF), polyether ether ketone (PEEK),
polytetrafluoroethylene (PTFE).
[0093] In one embodiment, the porous membrane substrate has a
plurality of pores 10 nm or greater in diameter.
[0094] In one embodiment, the ceramic selective membrane further
includes a compressible polymer edging along at least a portion of
an edge of the porous membrane.
[0095] In one embodiment, the ceramic selective membrane further
includes compressible polymer edging along all edges of the ceramic
selective membrane, defining a gasket.
[0096] In one embodiment, the edge portion is 1 mm or greater in
width. In one embodiment, the edge portion is 5 mm or greater in
width. In one embodiment, the edge portion is 1 cm or greater in
width.
[0097] In one embodiment, the compressible polymer comprises a
thermoplastic elastomeric polymer. In one embodiment, thermoplastic
elastomeric polymer is selected from the group consisting of
poly(styrene-isobutylene-styrene) (SIBS), polyvinylidene fluoride
(PVDF), and polydimethylsiloxane (PDMS).
[0098] In one embodiment, the selective silica ceramic comprises a
plurality of layers of selective silica ceramic material.
[0099] In one embodiment, the ceramic selective membrane further
includes a finishing layer coating the selective silica
ceramic.
[0100] In one embodiment, the finishing layer comprises an
alkyl-containing compound.
[0101] In one embodiment, the selective silica ceramic comprises an
additive selected from the group consisting of a selectivity
additive configured to increase ion transport properties of the
ceramic selective membrane, a durability additive configured to
improve durability of the ceramic selective membrane, and a
catalyst additive configured to add catalytic properties to the
ceramic selective membrane.
[0102] In one embodiment, the selective silica ceramic comprises a
selectivity additive selected from the group consisting of an
ionic-conducting polymer and a gas conducting polymer.
[0103] In one embodiment, the additive is a durability additive
selected from the group consisting of a low Young's modulus polymer
configured to provide increased flexibility to the ceramic
selective membrane and a high Young's modulus polymer configured to
provide increased durability to the ceramic selective membrane.
[0104] In one embodiment, the additive is a catalyst additive
selected from the group consisting of catalytic particles and
catalytic compounds incorporated into the selective silica
ceramic.
[0105] In one embodiment, the ceramic selective membrane further
includes a surface treatment layer on the selective silica ceramic
comprising an agent selected from the group consisting of an
ion-conductivity enhancer, a molecular-selectivity enhancer, a
catalyst, and a durability enhancer.
[0106] Ceramic Selective Membrane Characteristics
[0107] The characteristics of the ceramic selective membranes are
unique and enable use of the membranes in batteries, fuel cells,
and the like.
[0108] In one embodiment, the ceramic selective membrane has a mean
pore size in the range of 0.5 nm to 2 nm as determined by fitting
of a polydispersed fractal model to a small angle x-ray scattering
profile.
[0109] In one embodiment, the fractality of the pore structure is
about 3 as determined by fitting of a polydispersed fractal model
to a small angle x-ray scattering profile (see, e.g., FIG. 8). In
one embodiment, the fractality of the pore structure is in the
range of about 1.4 to about 3.
[0110] In one embodiment, the galvanodynamic proton conductivity as
measured in an h-cell in 4M H.sub.2SO.sub.4 is in the range of
0.001 to 1 S/cm. In one embodiment, the galvanodynamic proton
conductivity as measured in an h-cell in 4M H.sub.2SO.sub.4 is in
the range of 0.05 to 1 S/cm.
[0111] In one embodiment, the permeability of a vanadium(IV)
sulfate hydrate ion across the ceramic selective membrane is
measured to be in the range of 1.times.10-8 to 8.times.10-4
cm.sup.2/min. In one embodiment, the permeability of a vanadium(IV)
sulfate hydrate ion across the ceramic selective membrane is
measured to be in the range of 1.times.10-6 to 1.times.10-5
cm.sup.2/min.
[0112] In one embodiment, the proton/vanadium ion selectivity of
the ceramic selective membrane is defined by the ratio: proton
conductivity/vanadium ion permeability and measured to be 3,500 to
58,000 Smin/cm.sup.3. In one embodiment, the proton/vanadium ion
selectivity of the ceramic selective membrane is defined by the
ratio: proton conductivity/vanadium ion permeability and measured
to be 15,000 to 30,000 Smin/cm.sup.3.
[0113] In one embodiment, the ceramic selective membrane comprises
pores in the size range of 0.1 nm to 10 nm in diameter. The pore
size is the final pore size of the ceramic selective membrane,
including all layers of ceramic and any post-processing layers
(e.g., a "finishing" alkyl layer). In one embodiment, the ceramic
selective membrane comprises pores in the size range of 0.1 nm to 5
nm in diameter. In one embodiment, the ceramic selective membrane
comprises pores in the size range of 0.1 nm to 1 nm in diameter. In
one embodiment, the ceramic selective membrane comprises pores in
the size range of 0.5 nm to 1 nm in diameter.
[0114] In one embodiment, the ceramic selective membrane has a
thickness in the range of 0.1 mm to 1 mm. In one embodiment, the
ceramic selective membrane has a thickness in the range of 0.1 mm
to 0.5 mm. In one embodiment, the ceramic selective membrane has a
thickness in the range of 0.2 mm to 0.4 mm.
[0115] In one embodiment, the selective silica ceramic has a
thickness of 0.5 .mu.m to 750 .mu.m disposed on the surface of the
porous membrane substrate.
[0116] Selective Membranes Incorporating a Ceramic Selective
Membrane
[0117] In another aspect, a selective membrane is provided that
includes a ceramic selective membrane comprising a selective silica
ceramic supported by a porous membrane substrate as previously
described.
[0118] The disclosed membrane can be used in any existing or
future-developed system where separation is desired in a manner in
which the membrane can be configured (e.g., based on pore size,
selectivity of transport across the membrane, etc.). In one
embodiment, the selective membrane is of a type selected from the
group consisting of a battery membrane (e.g., a RFB membrane), a
fuel cell membrane, a food processing membrane (for example, purify
glucose from starch, to clarify fruit juices, separation of gelatin
from proteins, and separation of curds and whey), a reverse osmosis
membrane, a gas separation membrane (for example, separation of
nitrogen from air, separation of carbon dioxide from natural gas,
and separation of hydrogen from light petroleum products), and a
bio-separation membrane (for example, a dialysis membrane, the
purification of viruses/bacteria, the separation of plasma out of
blood, and the purification of pharmaceuticals to demineralize and
concentrate antibiotics).
[0119] In one embodiment, the selective membrane is an
ion-conducting membrane for a flow battery.
[0120] In one embodiment, the selective membrane is an
ion-conducting membrane for a fuel cell.
[0121] Membranes Produced by the Methods Described
[0122] In another aspect, a ceramic selective membrane is provided,
as formed by a method according to any of the disclosed method
embodiments.
[0123] In another aspect, a selective membrane is provided that
includes a ceramic selective membrane according to any of the
disclosed embodiments.
[0124] In one embodiment, the selective membrane is of a type
selected from the group consisting of a battery membrane, a fuel
cell membrane, a food processing membrane, a reverse osmosis
membrane, a gas separation membrane, and a bio-separation
membrane.
[0125] In one embodiment, the selective membrane is an
ion-conducting membrane for a flow battery.
[0126] In one embodiment, the selective membrane is an
ion-conducting membrane for a fuel cell.
[0127] The following examples are included for the purpose of
illustrating, not limiting, the described embodiments.
Examples
[0128] The methods of forming exemplary membranes and the
characterization of the membranes are disclosed below.
[0129] An exemplary membrane is produced using the process outline
in FIG. 1A. It includes dipping a macroporous support (e.g., silica
filter substrate) into a 27 wt % sodium silicate solution for 30
seconds to promote wicking. That soaked support is then dipped in
3N sulfuric acid for 8 hours before being removed and dried in an
oven at 70.degree. C. for 2 hours. The process is then repeated at
least once to further reduce pore size and reduce cracking, while
improving durability.
[0130] Referring to FIG. 1B, an exemplary membrane production
process is shown in 4 steps. STEP 1: Elastomeric edging is affixed
to a pre-sized ceramic macroporous support. Solvent soluble
polymer, such as poly(styrene-isobutylene-styrene) (SIBS), is cast
to the size of the membrane while also defining the active area.
Solvent is placed on the edges of the membrane and then sandwiched
by the polymer edging. The solvent partially dissolves the edging
which results in strong adhesion with the support. STEP 2: Membrane
is dipped in a silica precursor solution. STEP 3: Membrane is
dipped in an acid bath. STEP 4: Membrane is dried at low
temperature (e.g., 60.degree. C.). The process conditions of steps
2-4 can be varied to produce compressible ceramic proton conducting
membranes with specific performance attributes.
[0131] FIG. 2A is a SEM micrograph of a silica support of the type
usable as a porous membrane substrate in accordance with certain
embodiments disclosed herein (specifically of the type disclosed
above with regard to FIG. 1A). FIG. 2B is a SEM micrograph of a
silica selective membrane formed on a glass fiber support in
accordance with certain embodiments disclosed herein (specifically
of the type disclosed above with regard to FIG. 1A). FIGS. 2A and
2B are SEM images of the surface of materials that have been coated
with a 10 nm layer of Au/Pd to reduce charging during imaging. The
glass fiber support is produced by Pall Corp and is a borosilicate
glass fiber support without binder ("Type A/C"). The nominal pore
size is 1 micron and the thickness is 254 microns.
[0132] FIG. 3A: Small angle x-ray scattering (SAXS) profile of a
membrane fit with a fractal aggregate model. FIG. 3B: Pore size
distribution based on SAXS modeling. The ideal pore size based on
ionic radii of ions in flow batteries is noted. Ideal pore size
distribution is noted between the radius of H.sub.3O.sup.+ and
VO.sub.2. The known pore size range of NAFION is also noted. These
samples were all processed using the method of FIG. 1 as discussed
above. The SAXS profile in FIG. 3A is for a membrane formed using
nitric acid. In FIG. 3B the only difference between the curves is
acid type. The following mean radius (in Angstroms) was found for
the corresponding acids: 4.5=CH.sub.3COOH (Acetic), 5.0=HCl,
6.0=CH.sub.3SO.sub.4 (Methane Sulfonic), 7.2=H.sub.3PO.sub.4,
8.1=HNO.sub.3. These FIGURES demonstrate that we can achieve
various pore sizes depending on processing conditions. NAFION is
included for reference in FIG. 3B. In view of these data, the ideal
pore size (radius) of the ceramic selective membranes is 3 nm and
below.
[0133] FIG. 4: SEM images of exemplary ceramic selective membrane
surfaces and cross-sections. The right membrane was dipped in a
methanol/water mixture to reduce capillary stresses prior to
drying. Conceptual depictions of the membrane cross-sections are
also shown. This FIGURE shows that we can generate dense silica
structures within the membrane. These may have surface cracks but
do not have bridging cracks that ruin membrane performance.
Furthermore, the surface cracking can be reduced by dipping the
membrane into a water/methanol bath prior to drying. These samples
were made using the standard process previously described with
regard to FIG. 1A. The acid used in both was 3N H.sub.3PO.sub.4 and
they were made using 2 dip cycles (also previously described). The
No-Post Wash sample was dried as normal at 70.degree. C. The
Post-Wash was dipped in a 50/50 v/v water/methanol mixture for 1
hour prior and then dried at 70.degree. C. (this dip was only
performed on the last cycle).
[0134] FIG. 5 graphically illustrates proton conductivity versus
VO.sub.2 permeability for representative ceramic selective
membranes ("NaS" is a membrane formed from all sodium silicate; "14
wt % PSS in NaS" is a membrane formed with 14 wt % of PSS in a
sodium silicate sol; "NaS-TEOS" is a membrane formed with a
combination of sodium silicate and TEOS to form an aggregate
composite membrane; and "LiS" is a membrane formed in a similar
manner to the NaS membrane but with lithium silicate instead of
sodium silicate) compared to the commercial material NAFION
212.
[0135] FIG. 5 illustrates a number of key performance attributes
for flow batteries for formulations that have been discussed
already.
[0136] Proton conductivity is tested in a 4M H.sub.2SO.sub.4
solution with a galvanodynamic sweep from 0 to 200 mA. The
measurement is performed in an h-cell with luggin capillaries,
platinum leads and Ag/AgCl reference electrode.
[0137] The vanadium permeability is also performed in an h-cell
with 1.5M VOSO.sub.4 and 2M H.sub.2SO.sub.4 on one side and 1.5M
MgSO.sub.4 and 2M H.sub.2SO.sub.4 on the other side. Aliquots of
liquid are taken as a function of time to track vanadium diffusion
across the membrane. Vanadium is blue and the concentration can be
determined using UV-vis.
[0138] While the ideal is low permeability and high conductivity,
high conductivity and modest permeability is also valuable because
it enables batteries to charge faster. NaS-membranes are good
candidates because they are similar to NAFION but dramatically less
expensive to produce.
[0139] NAFION 212 is defined as 2 mil thick NAFION
(tetrafluoroethylene-perfluoro-3,6-dioxa-4-methyl-7-octenesulfonic
acid copolymer). The pure NaS is a sodium silicate sample processed
in the same was as described previously with regard to FIG. 1A
(i.e., with 2 coats and a 70.degree. C. drying step).
[0140] The 14 wt % PSS in NaS is polystyrene sulfonate and sodium
silicate. In this case, the PSS is mixed with the sodium silicate
prior to wicking into the substrate. The PSS acts as an acid and so
the acid dip step is not used in this method. Roughly 1 minute
after mixing PSS and NaS, the substrate is dipped and the composite
solution wicks into the macroporous substrate. The membrane is then
removed and gelled/dried at room temperature for 1 week.
[0141] The lithium silicate (LiS) sample is processed the same way
as sodium silicate in FIG. 1A. It is Lithisil.TM. 25 (23% lithium
silicate and 77% water). The acid used was 3N H.sub.3PO.sub.4 for
24 hours. It was not dried, but instead is immediately removed and
tested.
[0142] The NaS-TEOS is processed differently. In this case, the
macroporous substrate is dipped in sodium silicate (which is
slightly basic) for 30 seconds (for wicking) and then placed on a
sheet. The TEOS is then added as a top layer and it reacts with the
water (hydrolysis) and the base acts as a catalyst. The TEOS and
sodium silicate aggregate to form a dense structure within the
support. Some of the TEOS evaporates during gelation/drying. The
gelation/drying process is left for 1 week at room temperature.
[0143] All of these samples tested in FIG. 5 have a .about.1
cm.sup.2 active area and 5 cm.sup.2 total area, including
edging.
[0144] FIG. 6A graphically illustrates cycling capacity of an
exemplary all-vanadium RFB using exemplary sol-gel SiO.sub.2;
exemplary sol-gel SiO.sub.2+SIBS composite; and comparative NAFION
115. FIG. 6B illustrates voltage profiles for RFBs including the
membranes of FIG. 6A.
[0145] FIGS. 7A and 7B are SEM images of a TEOS-sodium silicate
membrane in accordance with embodiments disclosed herein. FIG. 7A
is a top view and FIG. 7B is a dense cross-sectional view.
[0146] FIG. 8 graphically illustrates the fractal dimension
extracted from SAXS fitting of representative membranes using the
fractal model (shown in FIG. 3A). The selectivity (proton
conductivity/vanadium ion permeability) is plotted as a function of
the fractal dimension showing higher selectivity for lower fractal
dimensions. NAFION is plotted for reference. The method used here
is the same as the one described in FIG. 3A and the fractal
dimensions correspond to the following acids:
2.97=CH.sub.3SO.sub.4, 1.6=H.sub.3PO.sub.4, 2.88=CH.sub.3COOH,
2.49=H.sub.2SO.sub.4, 2.67=HNO.sub.3, 2.4=HCl.
[0147] Any approximate terms, such as "about," "approximately," and
"substantially," indicate that the subject can be modified by plus
or minus 5% and fall within the described embodiment.
[0148] While illustrative embodiments have been illustrated and
described, it will be appreciated that various changes can be made
therein without departing from the spirit and scope of the
invention.
* * * * *