U.S. patent application number 16/698620 was filed with the patent office on 2020-04-02 for nanopourous selective sol-gel ceramic membranes.
The applicant listed for this patent is University of Washington. Invention is credited to Ryan Kastilani, Lauren Martin, Gregory M. Newbloom, Lilo D. Pozzo, Jaime Rodriguez, Canfeng Wei, Aaron F. West.
Application Number | 20200101423 16/698620 |
Document ID | / |
Family ID | 67143963 |
Filed Date | 2020-04-02 |
![](/patent/app/20200101423/US20200101423A1-20200402-D00000.png)
![](/patent/app/20200101423/US20200101423A1-20200402-D00001.png)
![](/patent/app/20200101423/US20200101423A1-20200402-D00002.png)
![](/patent/app/20200101423/US20200101423A1-20200402-D00003.png)
![](/patent/app/20200101423/US20200101423A1-20200402-D00004.png)
![](/patent/app/20200101423/US20200101423A1-20200402-D00005.png)
![](/patent/app/20200101423/US20200101423A1-20200402-D00006.png)
![](/patent/app/20200101423/US20200101423A1-20200402-D00007.png)
![](/patent/app/20200101423/US20200101423A1-20200402-D00008.png)
![](/patent/app/20200101423/US20200101423A1-20200402-D00009.png)
![](/patent/app/20200101423/US20200101423A1-20200402-D00010.png)
View All Diagrams
United States Patent
Application |
20200101423 |
Kind Code |
A1 |
Newbloom; Gregory M. ; et
al. |
April 2, 2020 |
NANOPOUROUS SELECTIVE SOL-GEL CERAMIC MEMBRANES
Abstract
Nanoporous selective sol-gel ceramic membranes,
selective-membrane structures, and related methods are described.
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.
Inventors: |
Newbloom; Gregory M.;
(Seattle, WA) ; West; Aaron F.; (Seattle, WA)
; Kastilani; Ryan; (Seattle, WA) ; Wei;
Canfeng; (Seattle, WA) ; Rodriguez; Jaime;
(Seattle, WA) ; Pozzo; Lilo D.; (Seattle, WA)
; Martin; Lauren; (Seattle, WA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
University of Washington |
Seattle |
WA |
US |
|
|
Family ID: |
67143963 |
Appl. No.: |
16/698620 |
Filed: |
November 27, 2019 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
16366598 |
Mar 27, 2019 |
10525417 |
|
|
16698620 |
|
|
|
|
PCT/US2019/012380 |
Jan 4, 2019 |
|
|
|
16366598 |
|
|
|
|
62613719 |
Jan 4, 2018 |
|
|
|
62613712 |
Jan 4, 2018 |
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C04B 35/26 20130101;
B01D 69/02 20130101; B01D 69/12 20130101; C04B 35/111 20130101;
B01D 2323/22 20130101; C04B 35/486 20130101; B01D 71/28 20130101;
B01D 71/82 20130101; H01M 8/188 20130101; C04B 35/16 20130101; B82Y
30/00 20130101; C04B 35/453 20130101; C04B 35/52 20130101; C04B
2235/425 20130101; C04B 35/10 20130101; Y02E 60/50 20130101; B01D
67/0048 20130101; C04B 35/583 20130101; C04B 35/584 20130101; C04B
35/48 20130101; C04B 35/624 20130101; B01D 2325/02 20130101; C04B
35/14 20130101; B01D 2325/26 20130101; C04B 35/565 20130101; C04B
2235/5288 20130101; H01M 8/18 20130101; B01D 71/024 20130101; B01D
2323/46 20130101; C04B 35/46 20130101; B01D 69/10 20130101 |
International
Class: |
B01D 67/00 20060101
B01D067/00; B01D 71/02 20060101 B01D071/02; C04B 35/624 20060101
C04B035/624; C04B 35/14 20060101 C04B035/14; C04B 35/10 20060101
C04B035/10; C04B 35/48 20060101 C04B035/48; B01D 71/28 20060101
B01D071/28; B01D 69/12 20060101 B01D069/12; B01D 71/82 20060101
B01D071/82; B01D 69/02 20060101 B01D069/02; B01D 69/10 20060101
B01D069/10 |
Claims
1. A nanoporous selective sol-gel ceramic membrane, comprising: a
porous support having a plurality of support pores that are 10 nm
or greater in diameter; and a nanoporous composite comprising a
nanoporous sol-gel ceramic composite filling at least a portion of
the porous support; wherein the nanoporous composite comprises a
plurality of nanopores of 5 nm or smaller in radius with a
polydispersity index of 0.7 or lower.
2. The nanoporous selective sol-gel ceramic membrane of claim 1,
wherein the porous substrate comprises a material selected from the
group consisting of a polymeric material, a ceramic material, a
metal, and a combination thereof.
3. The nanoporous selective sol-gel ceramic membrane of claim 1,
wherein the porous substrate comprises a material selected from the
group consisting of polypropylene, polyethylene, polyvinyl
chloride, polystyrene, polyamide, polyimide, polyacetonitrile,
polyvinylacetate, polyethylene glycol, poly ether ketone,
polysulfone, polysulfonamide, polyacrylamide, polydimethylsiloxane,
polyvinylidene fluoride, polyacrylic acid, polyvinyl alcohol,
polyphenylene sulfide, polytetrafluoroethylene, cellulose, and
combinations thereof.
4. The nanoporous selective sol-gel ceramic membrane of claim 1,
wherein the porous substrate comprises silica, titania, zirconia,
germania, alumina, graphite, silicon carbide, silicon nitride,
boron nitride, borosilicate glass, lithium silicate, potassium
silicate, tin oxide, iron oxide, carbon nanotubes, iron, or a
combination thereof.
5. The nanoporous selective sol-gel ceramic membrane of claim 1,
wherein the porous support comprises a material selected from the
group consisting of a nonwoven fabric, a nonwoven mesh, a veil, a
knit fabric, a woven fabric, a woven mesh, an open-cell foam, and a
combination thereof.
6. The nanoporous selective sol-gel ceramic membrane of claim 1,
further comprising a compressible edging along at least a portion
of an edge of the porous support formed by a polymeric material,
wherein the polymeric material infiltrates the porous support by at
least 1 um.
7. The nanoporous selective sol-gel ceramic membrane of claim 6,
wherein the compressible edging is along all edges of the porous
support, defining a gasket.
8. The nanoporous selective sol-gel ceramic membrane of claim 6,
wherein the compressible edging covers 50% or less of a surface of
the nanoporous selective sol-gel ceramic membrane.
9. The nanoporous selective sol-gel ceramic membrane of claim 6,
wherein the polymeric material of the compressible edging comprises
an elastomeric polymer.
10. The nanoporous selective sol-gel ceramic membrane of claim 1,
wherein the nanoporous sol-gel ceramic composite is prepared by
gelling a sol-gel precursor composition comprising one or more
ceramic precursors.
11. The nanoporous selective sol-gel ceramic membrane of claim 10,
wherein the ceramic precursor is selected from the group consisting
of silica, siloxane, silicate ester, silanol, silane, ormosil,
titania, zirconia, germania, alumina, graphite, silicon carbide,
silicon nitride, boron nitride, and combinations thereof.
12. The nanoporous selective sol-gel ceramic membrane of claim 10,
wherein the ceramic precursor comprises tetraalkyl
orthosilicate.
13. The nanoporous selective sol-gel ceramic membrane of claim 10,
wherein the ceramic precursor comprises one or more organosilanes
having the formula R*.sub.2--Si--(OR).sub.2 or R*--Si--(OR).sub.3,
wherein R*, independently at each occurrence, is an optionally
substituted C1-C15 alkyl, optionally substituted C4-C20
heteroalkyl, optionally substituted aryl, or optionally substituted
heteroaryl, and R, independently at each occurrence, is an
optionally substituted C1-C6 alkyl.
14. The nanoporous selective sol-gel ceramic membrane of claim 13,
wherein the organosilane is C.sub.6H.sub.13--Si--(OR).sub.3.
15. The nanoporous selective sol-gel ceramic membrane of claim 10,
wherein the ceramic precursor comprises Ti(OH).sub.x(OR).sub.y,
wherein R, independently at each occurrence, is a C1-C6 alkyl, x is
an integer ranging from 0 to 4, and x is an integer ranging from 0
to 4, and the sum of x and y is 4.
16. The nanoporous selective sol-gel ceramic membrane of claim 10,
wherein the ceramic precursor comprises Al(OR).sub.3, wherein R,
independently at each occurrence, is H or an optionally substituted
C1-C6 alkyl.
17. The nanoporous selective sol-gel ceramic membrane of claim 10,
wherein the sol-gel precursor composition further comprises an
additive selected from the group consisting of a catalyst, an
ion-conducting polymer, electrically conductive particles,
mechanical properties-improving materials, and a combination
thereof.
18. The nanoporous selective sol-gel ceramic membrane of claim 17,
wherein the additive is present in an amount of 10 volume % or less
of the sol-gel precursor composition.
19. The nanoporous selective sol-gel ceramic membrane of claim 1,
wherein the nanoporous selective sol-gel ceramic membrane is of a
type selected from the group consisting of a battery membrane, a
fuel-cell membrane, an electrodialysis membrane, an acid recovery
membrane, a chloro-alkali membrane, a solvent extraction membrane,
an electrodeposition membrane, and electro-deionization membrane, a
nutrient recovery membrane, a food processing membrane, a reverse
osmosis membrane, a gas separation membrane, and a bio-separation
membrane.
20. The nanoporous selective sol-gel ceramic membrane of claim 1,
where in the nanoporous selective sol-gel ceramic membrane has an
ionic area specific resistance (ASR) in the range of 0.01
Ohm*cm.sup.2 to 10 Ohm*cm.sup.2 when measured in 4 M
H.sub.2SO.sub.4 or in the range of 0.1 Ohm*cm.sup.2 to 100
Ohm*cm.sup.2 when measured in 0.5 M NaCl.
Description
CROSS-REFERENCES TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. application Ser.
No. 16/366,598, filed Mar. 27, 2019, which is a continuation of
International Application No. PCT/US2019/012380, filed Jan. 4,
2019, which claims the benefit of U.S. Provisional Application No.
62/613,719, filed Jan. 4, 2018, and U.S. Provisional Application
No. 62/613,712, filed Jan. 4, 2018, the disclosures of which are
expressly incorporated herein by reference in their entirety.
BACKGROUND
[0002] Nanoporous ceramic membranes capable of selectively
filtering molecules offer numerous advantages over their polymeric
counterparts, including enhanced chemical stability and lower
fouling. High-temperature sintering and calcination of ceramic
membranes are used to improve durability and reduce grain
boundaries; otherwise ceramic membranes break under the compression
required to create air- and liquid-tight seals. Unfortunately,
these high-temperature processing steps dramatically increase the
cost of the membrane to the point they often cannot be implemented
commercially.
[0003] The ability to create ceramic selective membranes can be of
especially significant benefit to redox flow batteries (RFB). RFBs
provide a promising solution to grid-scale energy storage needs.
Unlike conventional solid-state batteries, the electrolytes in RFBs
are stored in external tanks and are pumped through the cell stack
of the battery. Thus, the RFB possesses several attractive
qualities, such as ease of scalability, long service life, and the
separation of power and energy. Unfortunately, the current high
cost of RFBs has hindered their ability to be widely
commercialized. The membrane contributes significantly to this high
cost, where it can account for up to 40% of the total capital cost.
In these applications, membranes must be able to transport charge
balancing ions and prevent the crossover of active species. The
poor ion selectivity of commercial membranes has led to an emphasis
on the all-vanadium RFB (VRFB), which can mitigate the detrimental
effect of crossover due to its symmetric electrolyte composition.
Instead of a permanent loss of capacity, crossover only leads to a
loss of efficiency in VRFBs. However, vanadium is expensive and can
result in as much as 50% of the total capital costs of an RFB. The
U.S. Department of Energy has listed a target cost of $100/kWh for
RFBs, yet VRFBs have an estimated capital cost of $447/kWh.
[0004] As the first true RFB, the Iron-Chromium redox flow battery
(ICRFB) is very attractive in terms of its cost effectiveness,
comprising cheap and abundant active materials. It has a standard
overall cell potential of 1.18 V with the reactions occurring
within the battery are listed below.
Positive Electrode:
Fe.sup.2+.revreaction.Fe.sup.3++e.sup.-E.sup.0=+0.77V (1)
Negative Electrode:
Cr.sup.3++e.sup.-.revreaction.Cr.sup.2+E.sup.0=-0.41V (2)
Overall:
Fe.sup.2++Cr.sup.3+.revreaction.F.sup.3++Cr.sup.2+E.sup.0=+1.18- V
(3)
[0005] Despite the cost effectiveness of the ICRFB, it is prone to
significant membrane crossover of the active species that results
in irreversible capacity decay. This same capacity occurs for many
other inexpensive RFB chemistries and has largely prevented their
commercialization.
[0006] There is still a need for a selective sol-gel membrane that
can reduce flow battery costs and enable new flow battery
chemistries.
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, the present disclosure provides a nanoporous
selective sol-gel ceramic membrane, generally including a porous
support having a plurality of support pores that are 10 nm or
greater in diameter; and a nanoporous composite comprising a
nanoporous sol-gel ceramic composite filling at least a portion of
the porous support; wherein the nanoporous composite comprises a
plurality of nanopores of 5 nm or smaller in radius with a
polydispersity index of 0.5 or lower.
[0009] In another aspect, the present disclosure provides a
nanoporous selective sol-gel ceramic membrane, generally including
a porous support having a plurality of support pores that are 10 nm
or greater in diameter; and a nanoporous composite comprising a
nanoporous sol-gel ceramic composite filling at least a portion of
an active area of the porous support; wherein the nanoporous
sol-gel ceramic has a fractal nanoporous structure as determined by
fitting small-angle scattering spectra of the nanoporous sol-gel
ceramic to a mathematical model.
[0010] In yet another aspect, the present disclosure provides a
selective-membrane structure generally including a plurality of
individual selective membranes joined in a planar
configuration.
DESCRIPTION OF THE DRAWINGS
[0011] 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:
[0012] FIG. 1 illustrates a method, in accordance with an
embodiment of the disclosure, for preparing an exemplary nanoporous
selective sol-gel ceramic composite membrane. Step 1: Apply edging
to support. Step 2: Sol-gel coating from precursor solution.
[0013] FIG. 2 illustrates a method, in accordance with an
embodiment of the disclosure, for preparing an exemplary nanoporous
selective sol-gel ceramic composite membrane. Step 1: Apply edging.
Step 2: Pre-coating containing performance enhancing materials.
Step 3: Sol-gel coating from precursor solution.
[0014] FIG. 3 illustrates a method, in accordance with an
embodiment of the disclosure, for preparing an exemplary nanoporous
selective sol-gel ceramic composite membrane. Step 1: Apply edging.
Step 2: Sol-gel coating from precursor solution. Step 3: Dip in an
acid or salt bath to induce further gelation.
[0015] FIG. 4 is a schematic illustration of an H-Cell setup with
CrCl.sub.3 on side 1 and AlCl.sub.3 on side 2 or VOSO.sub.4 on side
1 and MgSO.sub.4 on side 2.
[0016] FIG. 5 is a schematic illustration of a 4-electrode PEIS
setup.
[0017] FIG. 6 is a graph of extrinsic permeability of Cr.sup.3+
ions through Nafion.RTM. membranes and nanoporous selective sol-gel
ceramic composite membranes, in accordance with an embodiment of
the disclosure.
[0018] FIG. 7 is a graph of extrinsic permeability of Fe.sup.3+
ions through Nafion.RTM. membranes and nanoporous selective sol-gel
ceramic composite membranes, in accordance with an embodiment of
the disclosure.
[0019] FIG. 8 is a graph of area specific resistance (ASR) of
Nafion.RTM. membranes and nanoporous selective sol-gel ceramic
composite membranes, in accordance with an embodiment of the
disclosure.
[0020] FIG. 9 is a plot of ASR vs extrinsic permeability of
Cr.sup.3+ ions for Nafion.RTM. membranes and nanoporous selective
sol-gel ceramic composite membranes, in accordance with an
embodiment of the disclosure.
[0021] FIG. 10 is a plot of ASR vs extrinsic permeability of
Fe.sup.3+ ions for Nafion.RTM. membranes and nanoporous selective
sol-gel ceramic composite membranes, in accordance with an
embodiment of the disclosure.
[0022] FIG. 11 shows small-angle x-ray scattering (SAXS) profiles
of nanoporous selective sol-gel ceramic composite membranes, in
accordance with an embodiment of the disclosure.
[0023] FIG. 12 shows SAXS profiles of silica and cellulose porous
supports used in the preparation of nanoporous selective sol-gel
ceramic composite membranes, in accordance with an embodiment of
the disclosure.
[0024] FIG. 13 is a photograph of a nanoporous selective sol-gel
ceramic composite membrane, in accordance with an embodiment of the
disclosure, under stress demonstrating flexibility and toughness of
the nanoporous sol-gel ceramic composite membrane.
[0025] FIG. 14 is a SAXS profile of a nanoporous selective sol-gel
ceramic composite membrane, in accordance with an embodiment of the
disclosure, formed using 50/50 molar mixture of tetraethyl
orthosilicate (TEOS) and hexyl triethoxysilane, along with its
corresponding fit to a core-shell fractal aggregate model.
[0026] FIG. 15 is a plot of ASR and ferric chloride permeability of
exemplary nanoporous selective sol-gel ceramic composite membrane
synthesized using Ludox.RTM. and ethyltriethoxy silane.
[0027] FIG. 16 is a phase diagram of tetraisopropoxide (TTIP),
ethanol, and water.
[0028] FIG. 17 is a photograph of FeCl.sub.3 and CrCl.sub.3
solution passed through membranes, in accordance with embodiments
of the disclosure, and solution color showed on tissue paper.
[0029] FIG. 18 is a series of scanning electron microscopy (SEM)
images showing comparison of polystyrene sulfonate (PSS) TEOS
single-phase molar ratios. The density of the cross-section at both
magnifications increases in relation to increasing PSS ratio.
[0030] FIGS. 19A-19C are SEM images of the top (FIG. 19A) and
cross-section (FIG. 19B) of nine areas in 1:5 PSS:TEOS molar ratio
membrane, superimposed on the imaged area. FIG. 19C show
superimposed pore radii and porosities from SAXS modeling over
their sampled areas.
[0031] FIG. 20 shows a comparison of vanadium (IV) permeability and
proton conductivity for PSS/TEO nanoporous selective sol-gel
ceramic composite membranes, in accordance with embodiments of the
disclosure, created at different ratios.
[0032] FIG. 21 shows comparison of pore radius and proton
conductivity for PSS/TEO nanoporous selective sol-gel ceramic
composite membranes, in accordance with embodiments of the present
disclosure, created at different ratios.
[0033] FIG. 22A shows SAXS profiles of a Nafion.RTM. membrane and
two nanoporous sol-gel ceramic membranes, in accordance with an
embodiment of the disclosure, gelled in different acid conditions
and fit with a fractal aggregate model.
[0034] FIG. 22B is a graph of pore size (radius) distributions of
the Nafion.RTM. membrane and the ceramic membranes of FIG. 22A
based on SAXS modeling.
[0035] FIG. 23 is a graph of proton transport as represented by the
ionic ASR of an a membrane, according to an embodiment of the
disclosure, based on the volume fraction of porosity for 9 mm-thick
nanoporous silica membranes as compared to a 2 mm-thick Nafion.RTM.
membrane (PFSA).
[0036] FIG. 24 is a graph of proton transport as represented by the
ionic area specific resistance (ASR) of a membrane based on the
fractal dimension for 9 mm-thick nanoporous silica membranes, as
compared to a 2 mm-thick Nafion.RTM. membrane (PFSA).
[0037] FIG. 25 is a SAXS profile fitting of a hydrated, nanoporous
Nafion.RTM. membrane with a fractal aggregate model to demonstrate
that commercial membranes have an intrinsically different
nano-structure. This model fitting generates a Chi.sup.2/N.sub.pt
of 830, indicating that the Nafion.RTM. membrane does not have a
fractal structure.
[0038] FIG. 26 is a schematic illustration of a method for
preparing a large grid-like membrane structure comprised of single
smaller membranes, in accordance with an embodiment of the
disclosure.
[0039] FIG. 27 is a schematic illustration of a method for
preparing a large grid-like membrane structure comprised of single
smaller membranes, in accordance with an embodiment of the
disclosure. Step 1: Apply edging. Step 2: Make membrane. Step 3:
Make membrane grid.
[0040] FIG. 28 is a photograph of 2 smaller membranes (250 cm.sup.2
each), in accordance with an embodiment of the disclosure, suitable
for use as a part of a grid structure.
[0041] FIG. 29 is a photograph of a selective-membrane structure
(4000 cm.sup.2) composed of smaller membranes (250 cm.sup.2 each,
box), in accordance with an embodiment of the disclosure.
[0042] FIG. 30A schematically illustrates a selective-membrane
structure, in accordance with an embodiment of the disclosure.
[0043] FIG. 30B schematically illustrates another
selective-membrane structure, in accordance with an embodiment of
the disclosure.
DETAILED DESCRIPTION
[0044] Disclosed herein are nanoporous selective sol-gel ceramic
composite membranes, selective-membrane structures, and a method of
making the nanoporous selective sol-gel ceramic composite membranes
by forming a selective ceramic on a porous membrane support.
Representative nanoporous selective sol-gel ceramic composite
membranes include ion-conductive membranes (e.g., proton-conducting
membranes) and gas selective membranes. Representative uses for the
membranes of the present disclosure include incorporation into fuel
cells and redox flow batteries (RFB) as ion-conducting
membranes.
Nanoporous Selective Sol-Gel Ceramic Composite Membranes
[0045] In one aspect, disclosed herein is a nanoporous selective
sol-gel ceramic membrane, comprising a porous support having a
plurality of support pores that are 10 nm or greater in diameter
and a nanoporous composite comprising a nanoporous sol-gel ceramic
composite filling at least a portion of the porous support; wherein
the nanoporous composite comprises a plurality of nanopores of
about 5 nm or smaller in radius with a polydispersity index of
about 0.5 or lower.
[0046] The nanoporous selective sol-gel ceramic membranes can be
formed without high-temperature processing of the ceramic. The
structure is accomplished, for example, by lining the edge of a
porous support with a compressible polymer (e.g., FIG. 1) and
filling in the porous support with a sol-gel ceramic composite.
This approach enables the membrane active area to be mechanically
decoupled from the compression region, thus providing a route to
use non-sintered and non-calcinated sol-gel ceramic containing
membranes in filtration and separation processes.
[0047] In addition to RFBs, membranes, filters, or separators
prepared using the method disclosed here can be used in
applications such as: fuel cells, lithium ion batteries, other
battery chemistries, electro-dialysis, cross-flow filtration,
dead-end filtration, pharmaceutic purifications, waste water
treatment, reverse osmosis water purification, food processing,
textiles, and others. It should be noted that it is possible to
build nanoporous ceramic composite membranes utilizing other
sol-gel methodologies with key differences.
[0048] Typically, the membranes described comprise three components
that serve distinct purposes: a macroporous support, a nanoporous
composite layer, and a polymeric edge. The macroporous support
structure is capable of wetting a solvent based ceramic
dispersion/solution (e.g., siloxane) in order to create a
nanoporous (i.e., <10 nm) ceramic structure. The nanoporous
composite layer is within the macroporous support structure with
characteristic porosity of <10 nm. The compressible polymer
edge, when present, eliminates or reduces compressive forces on the
nanoporous sol-gel selective ceramic layer while enabling
liquid-tight sealing at the edges. In certain circumstances, the
porous support structures can also contain a pre-coating (i.e.,
prior to the nanoporous sol-gel) to improve mechanical, chemical or
electro-chemical stability. In other circumstances, membranes can
undergo a post treatment chemical bath to further induce
gelation.
[0049] Porous Membrane Supports
[0050] The porous membrane support (sometimes referred to herein
simply as the "porous support" or "support") is the structural
foundation within and/or upon which the nanoporous selective
sol-gel ceramic is formed. The support provides mechanical strength
and a porous structure. Typically, the porous substrates comprise
support pores with an average support pore radius between about 10
nm and about 50 .mu.m. In some embodiments, the support pores have
an average radius of about 10 nm or greater. When the ceramic is
formed on the support, the relatively large pores of the support
are closed and filled with the ceramic until nanometer- or
angstrom-sized pores remain in the final membrane.
[0051] Any suitable organic or inorganic material can be used as a
porous support. In some embodiments, the porous support comprises a
material selected from the group consisting of a polymeric
material, a ceramic material, a metal, and a combination thereof.
The porous substrate can comprise a nonwoven fabric, a nonwoven
mesh, a veil, a knit fabric, a woven fabric, a woven mesh, an
open-cell foam, and combinations thereof. In some embodiments, the
porous membrane support has a chemical surface functionality that
is chemically similar to the ceramic precursor sol used to form the
membrane; for example, a silica mesh can be used as a support for
forming a silica-based sol-gel ceramic membrane of the disclosure.
In other embodiments, the porous membrane support is chemically
different from the ceramic precursor sol used to form the membrane.
For instance, a silica sol can be used to form an exemplary
membrane by filling at least a portion of a polymeric or metal
membrane.
[0052] In some embodiments, the porous support comprises a material
selected from the group consisting of polypropylene, polyethylene,
polyvinyl chloride, polystyrene, polyamide, polyimide,
polyacetonitrile, polyvinylacetate, polyethylene glycol, poly ether
ketone, polysulfone, polysulfonamide, polyacrylamide,
polydimethylsiloxane, polyvinylidene fluoride, polyacrylic acid,
polyvinyl alcohol, polyphenylene sulfide, polytetrafluoroethylene,
cellulose, and combinations thereof. In one embodiment, the porous
support is selected from the group consisting of silica filter
paper, polyvinylidene fluoride (PVDF), polyether ether ketone
(PEEK), and polytetrafluoroethylene (PTFE). In certain embodiments,
the porous support comprises silica, titania, germania, zirconia,
alumina, graphite, silicon carbide, silicon nitride, boron nitride,
borosilicate glass, lithium silicate, potassium silicate, tin
oxide, iron oxide, carbon nanotubes, iron, or a combination
thereof.
[0053] Compressible Edging or Gasket
[0054] In some embodiments, the membranes disclosed herein comprise
a compressible edging or gasket. Compressible edging enables
membranes, in certain embodiments, to be incorporated into
batteries, fuel cells, or other systems where a gasket sealing the
membrane is used. The compressible edging is both mechanically
compressible and also resistant to the heat and/or harsh chemical
environments in which the membranes are utilized.
[0055] Accordingly, in some embodiments, the nanoporous selective
sol-gel ceramic membranes disclosed herein further comprise a
compressible edging along at least a portion of an edge of the
porous support formed by a polymeric material. In some embodiments,
the polymeric material infiltrates the porous support by at least 1
um.
[0056] In some embodiments, the compressible edging is formed along
all edges of the porous support, defining a gasket. In other
embodiments, the compressible edging is formed along at least a
portion of at least one edge of the porous support. In some
embodiments, the compressible edging covers about 50% or less,
about 25% or less, about 10% or less, or about 5% or less of a
surface of the nanoporous selective sol-gel ceramic membrane
disclosed herein. In some embodiments, the edge portion is 1 mm or
greater in width. In some embodiments, the edge portion is 5 mm or
greater in width. In other embodiments, the edge portion is 1 cm or
greater in width.
[0057] The compressible edging can be formed using any suitable
method, prior to or after filling the porous membrane support with
the precursor sol and gelling the precursor sol. In one embodiment,
a compressible edging is formed after formation of the ceramic
selective membrane (i.e., after the sol-gel process). In one
embodiment, the method includes the step of impregnating the edge
portion of the porous membrane substrate with a polymeric material,
for example, impregnating one or more or all edges of the porous
membrane substrate with a compressible polymer, sufficient to form
a gasket bordering the porous membrane substrate.
[0058] In further embodiments, the compressible edging is formed
using ultrasonic welding or hot pressing. In certain embodiments,
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.
[0059] In some embodiments, the polymeric material comprises an
elastomeric polymer, such as a thermoplastic elastomeric polymer.
Any suitable elastomeric polymer can be used to form the
compressible edging of the membranes disclosed herein, for example,
suitable materials include styrene isobutylene butadiene polymers,
UV- or heat-curable silicones, and epoxies. In some embodiments,
the polymer is selected from the group consisting of
poly(styrene-isobutylene-styrene) (SIBS), polyvinylidene fluoride
(PVDF), and polydimethylsiloxane (PDMS).
[0060] Sol-Gel Precursors
[0061] In some embodiments, the nanoporous selective sol-gel
ceramic membranes are prepared by coating the porous support with a
sol-gel precursor composition comprising one or more ceramic
precursors and gelling the sol-gel precursor composition to form
nanoporous sol-gel ceramic composite within the porous support.
[0062] Suitable ceramic precursors include silica, siloxane,
silicate ester, silanol, silane, ormosil, titania, zirconia,
germania, alumina, graphite, silicon carbide, silicon nitride,
boron nitride, and combinations thereof. In some embodiments, the
ceramic precursor comprises tetraalkyl orthosilicates, silanols,
silanes, halosilanes, and combinations thereof.
[0063] Typically, the ceramic precursors include small molecules
(i.e., <2 nm radius) and generally account for about 20 volume %
or more of a sol-gel precursor composition. In some embodiments,
the ceramic precursors account for about 40 volume % or more of a
sol-gel precursor composition. In some embodiments, the ceramic
precursors account for about 60 volume % or more of a sol-gel
precursor composition.
[0064] In some embodiments, the ceramic precursor comprises
tetraalkyl orthosilicate of formula Si(OR).sub.4, wherein R is an
optionally substituted C1-C15 alkyl. In some embodiments, the
tetraalkyl orthosilicate is tetraethyl orthosilicate (TEOS).
Preparations of some exemplary membrane using a ceramic precursor
comprising TEOS are described in the Examples below.
[0065] In some embodiments, the ceramic precursor comprises one or
more organosilanes of the formula R*.sub.2--Si--(OR).sub.2 or
R*--Si--(OR).sub.3, wherein R*, independently at each occurrence,
is an optionally substituted C1-C15 alkyl, optionally substituted
C4-C20 heteroalkyl, optionally substituted aryl, or optionally
substituted heteroaryl, and R, independently at each occurrence, is
an optionally substituted C1-C6 alkyl. In some embodiments, the
organosilane is C.sub.6H.sub.13--Si--(OR).sub.3.
[0066] As used herein, the term "alkyl" includes straight-chain,
branched-chain, and cyclic monovalent hydrocarbyl radicals, and
combinations thereof, which contain only C and H when they are
unsubstituted. The term "alkyl," as used herein, includes
cycloalkyl and cycloalkylalkyl groups. Examples include methyl,
ethyl, isobutyl, cyclohexyl, cyclopentylethyl, and the like. The
total number of carbon atoms in each such group is sometimes
described herein, e.g., when the group can contain up to ten carbon
atoms, it can be represented as 1-10C, C1-C10, C.sub.1-C.sub.10,
C1-10, or C.sub.1-10. The term "heteroalkyl," as used herein, means
the corresponding hydrocarbons wherein one or more chain carbon
atoms have been replaced by a heteroatom. Exemplary heteroatoms
include N, O, S, and P. When heteroatoms are allowed to replace
carbon atoms, for example, in heteroalkyl groups, the numbers
describing the group, though still written as e.g. C3-C10,
represent the sum of the number of carbon atoms in the cycle or
chain plus the number of such heteroatoms that are included as
replacements for carbon atoms in the cycle or chain being
described.
[0067] Alkyl groups can be optionally substituted to the extent
that such substitution makes sense chemically. Typical substituents
include, but are not limited to, halogens (F, Cl, Br, I), .dbd.O,
.dbd.NCN, .dbd.NOR, .dbd.NR, OR, NR.sub.2, SR, SO.sub.2R,
SO.sub.2NR.sub.2, NRSO.sub.2R, NRCONR.sub.2, NRC(O)OR, NRC(O)R, CN,
C(O)OR, C(O)NR.sub.2, OC(O)R, C(O)R, and NO.sub.2, wherein each R
is independently H, C1-C8 alkyl, C2-C8 heteroalkyl, C1-C8 acyl,
C2-C8 heteroacyl, C2-C8 alkenyl, C2-C8 heteroalkenyl, C2-C8
alkynyl, C2-C8 heteroalkynyl, C6-C10 aryl, or C5-C10 heteroaryl,
and each R is optionally substituted with halogens (F, Cl, Br, I),
.dbd.O, .dbd.NCN, .dbd.NOR', .dbd.NR', OR', NR'.sub.2, SR',
SO.sub.2R', SO.sub.2NR'.sub.2, NR'SO.sub.2R', NR'CONR'.sub.2,
NR'C(O)OR', NR'C(O)R', CN, C(O)OR', C(O)NR'.sub.2, OC(O)R', C(O)R',
and NO.sub.2, wherein each R' is independently H, C1-C8 alkyl,
C2-C8 heteroalkyl, C1-C8 acyl, C2-C8 heteroacyl, C6-C10 aryl or
C5-C10 heteroaryl. Alkyl groups can also be substituted by C1-C8
acyl, C2-C8 heteroacyl, C6-C10 aryl or C5-C10 heteroaryl, each of
which can be substituted by the substituents that are appropriate
for the particular group.
[0068] "Aromatic" or "aryl" substituent or moiety refers to a
monocyclic or fused bicyclic moiety having the well-known
characteristics of aromaticity; examples of aryls include phenyl
and naphthyl. Similarly, "heteroaromatic" and "heteroaryl" refer to
such monocyclic or fused bicyclic ring systems which contain as
ring members one or more heteroatoms. Suitable heteroatoms include
N, O, and S, inclusion of which permits aromaticity in 5-membered
rings as well as 6-membered rings. Typical heteroaromatic systems
include monocyclic C5-C6 aromatic groups such as pyridyl,
pyrimidyl, pyrazinyl, thienyl, furanyl, pyrrolyl, pyrazolyl,
thiazolyl, oxazolyl, and imidazolyl, and fused bicyclic moieties
formed by fusing one of these monocyclic groups with a phenyl ring
or with any of the heteroaromatic monocyclic groups to form a
C8-C10 bicyclic group such as indolyl, benzimidazolyl, indazolyl,
benzotriazolyl, isoquinolyl, quinolyl, benzothiazolyl,
benzofuranyl, pyrazolopyridyl, quinazolinyl, quinoxalinyl,
cinnolinyl, and the like. Any monocyclic or fused ring bicyclic
system which has the characteristics of aromaticity in terms of
electron distribution throughout the ring system is included in
this definition. It also includes bicyclic groups where at least
the ring which is directly attached to the remainder of the
molecule has the characteristics of aromaticity. Typically, the
ring systems contain 5-14 ring member atoms. Typically, monocyclic
heteroaryls contain 5-6 ring members, and bicyclic heteroaryls
contain 8-10 ring members.
[0069] Aryl and heteroaryl moieties can be substituted with a
variety of substituents including C1-C8 alkyl, C2-C8 alkenyl, C2-C8
alkynyl, C5-C12 aryl, C1-C8 acyl, and heteroforms of these, each of
which can itself be further substituted; other substituents for
aryl and heteroaryl moieties include halogens (F, Cl, Br, I), OR,
NR.sub.2, SR, SO.sub.2R, SO.sub.2NR.sub.2, NRSO.sub.2R,
NRCONR.sub.2, NRC(O)OR, NRC(O)R, CN, C(O)OR, C(O)NR.sub.2, OC(O)R,
C(O)R, and NO.sub.2, wherein each R is independently H, C1-C8
alkyl, C2-C8 heteroalkyl, C2-C8 alkenyl, C2-C8 heteroalkenyl, C2-C8
alkynyl, C2-C8 heteroalkynyl, C6-C10 aryl, C5-C10 heteroaryl,
C7-C12 arylalkyl, or C6-C12 heteroarylalkyl, and each R is
optionally substituted as described above for alkyl groups. The
substituent groups on an aryl or heteroaryl group can be further
substituted with the groups described herein as suitable for each
type of such substituents or for each component of the substituent.
Thus, for example, an arylalkyl substituent can be substituted on
the aryl portion with substituents described herein as typical for
aryl groups, and it can be further substituted on the alkyl portion
with substituents described herein as typical or suitable for alkyl
groups.
[0070] "Optionally substituted," as used herein, indicates that the
particular group being described can have one or more hydrogen
substituents replaced by a non-hydrogen substituent. In some
optionally substituted groups or moieties, all hydrogen
substituents are replaced by a non-hydrogen substituent (e.g., a
polyfluorinated alkyl such as trifluoromethyl). If not otherwise
specified, the total number of such substituents that can be
present is equal to the number of H atoms present on the
unsubstituted form of the group being described. Where an optional
substituent is attached via a double bond, such as a carbonyl
oxygen or oxo (.dbd.O), the group takes up two available valences,
so the total number of substituents that may be included is reduced
according to the number of available valences. As used herein,
optional substituents include negatively charged groups, negatively
chargeable groups, positively charged groups, positively chargeable
groups, hydrophilic groups, and hydrophobic groups. In some
embodiments, optional substituents include a group oxidizable to a
sulfonic acid group, a thiol group (i.e., S--H), an alkylthiol
group, sulfonic acid group, carboxylic acid group, amino group, and
ammonium group.
[0071] In some embodiments, the ceramic precursors comprise groups,
e.g., optional substituents, which functionalize the membranes. For
example, in some embodiments, the ceramic precursor comprises 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. Other embodiments include a silane with a
long alkane group to improve durability or reduce pore size.
Exemplary silanes include triethoxy(hexyl)silane.
[0072] In some embodiments, the ceramic precursor comprises
colloidal ceramic particles. Exemplary colloidal ceramic particles
include colloidal silica particles, for example, Ludox.RTM.
particles. In some embodiments, colloidal silica particles, e.g.,
Ludox.RTM. particles, are mixed with a bifunctional
(R*.sub.2--Si--(OR).sub.2) or a trifunctional organosilane
(R*--Si--(OR).sub.3). Suitable colloidal particles include
Ludox.RTM. SM-30, Ludox.RTM. HS-40, and Ludox.RTM. CL. The type of
organosilane used in combination with the colloidal silica
particles depends on the membrane application. For example, to
prepare a membrane for use in flow batteries, organosilane
comprising alkyl groups can be used to aid selectivity and
organosilane comprising sulfonic groups can be used to aid proton
conductivity. Preparation of an exemplary membrane using a ceramic
precursor comprising colloidal particles is described in the
Examples below.
[0073] Ceramic precursors comprising elements other than silicon
(Si) can be used in the preparation of the nanoporous selective
sol-gel ceramic membranes disclosed herein. In some embodiments,
the ceramic precursor comprises a titanium compound of the formula
Ti(OH).sub.x(OR)y, wherein R, independently at each occurrence, is
an optionally substituted C1-C6 alkyl, x is an integer ranging from
0 to 4, and x is an integer ranging from 0 to 4, and the sum of x
and y is 4. In some embodiments, the ceramic precursor comprises
titanium alkoxides, including tetraisopropoxide (TTIP) and/or its
partially hydrolyzed species, for example, as illustrated in the
Examples below.
[0074] In certain embodiments, the ceramic precursor comprises an
aluminum compound Al(OR).sub.3, wherein R, independently at each
occurrence, is H or an optionally substituted C1-C6 alkyl. Aluminum
alkoxides, including aluminum isopropoxide (AIP), are some of the
exemplary compounds suitable for use as ceramic precursors of the
membranes disclosed herein.
[0075] In some embodiments, the ceramic precursor comprises a
germanium alkoxide. Suitable germanium alkoxides include, but are
not limited to, mon-, di-, tri-, and tetraalkoxy germanane, such
as, tetraethoxygermane, tetramethoxygermane, tetrapropoxygermane,
and tetrabutoxygermane. The germania-based sol-gel precursor can
also be hydrolyzed germanium alkoxide monomers, dimers, and/or
trimers. In some embodiments, ceramic precursors comprise
tetraalkyl orthogermanate Ge(OR).sub.4. In some embodiments,
ceramic precursors comprising mixtures of tetraethyl orthosilicate,
Si(OC.sub.2H.sub.5).sub.4 and tetraethyl orthogermanate,
Ge(OC.sub.2H.sub.5).sub.4 can be useful in preparation of the
nanoporous membranes disclosed herein.
[0076] In some embodiments, sol-gel precursor composition
comprising multiple components described above can also be used as
independent precursors or as composite precursors such as
core-shell particles (e.g. alumina-coated silica nanoparticles).
The precursor can be a pure material or a solution or dispersion in
water and/or one or more other solvents. Further, the ceramic
precursors can be applied as an emulsion or dispersion in water or
other suitable solvents.
[0077] Additives
[0078] In certain embodiments, additives are added to a sol-gel
precursor composition in order to enable specific desirable
properties of the nanoporous selective sol-gel ceramic membrane
when formed.
[0079] In some embodiments, the sol-gel precursor composition
further comprises an additive selected from the group consisting of
a selectivity additive configured to increase ion transport
properties of the nanoporous selective sol-gel ceramic membrane, a
durability additive configured to improve durability of the
nanoporous selective sol-gel ceramic membrane, and a catalyst
additive configured to add catalytic properties to the nanoporous
selective sol-gel ceramic membrane. Suitable additives include
catalyst, an ion-conducting polymer, electrically conductive
particles, mechanical properties-improving materials, and a
combination thereof.
[0080] In certain embodiments, 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 sol-gel precursor composition. Furthermore, they
must be able to handle the harsh environments or be protected from
degradation by the oxide.
[0081] In some embodiments, 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 nanoporous selective sol-gel
ceramic membrane.
[0082] 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 sol-gel precursor composition 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.
[0083] In one embodiment, the additive is a catalyst additive
selected from the group consisting of catalytic particles added to
the sol-gel precursor composition and catalytic particles formed
within the sol-gel precursor composition. 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. In an embodiment, the catalyst additive is suitable to
handle the harsh environments to which the membrane is exposed (if
contained externally) or is protected from degradation by the
ceramic membrane (if contained internally). In one embodiment, the
catalyst additive is 10 vol % or less of the nanoporous selective
sol-gel ceramic membrane.
[0084] The additives disclosed herein can be present in the final
membranes in any suitable amounts, which are specific to the
additive used. For example, for PSS & PDDA, it is typically
advantageous to have a final membrane loading (i.e., dry with
water/solvent removed) of between about 3 wt % and about 40 wt %,
between about 3 wt % and about 20 wt %, between about 3 wt % and
about 10 wt %. In certain embodiments, the additive is present in
an amount of 10 volume % or less of the sol-gel precursor
composition.
[0085] Solvents
[0086] In some embodiments, the sol-gel precursor composition
further comprises one or more organic solvents. Any suitable
organic solvent can be included in the sol-gel precursor. In some
embodiments, the organic solvent is a C1-C5 alcohol or a C6
arylene. Exemplary solvents include methanol, ethanol, isopropanol,
propanol, butanol, toluene, xylene, and mixtures thereof. The
organic solvent is typically added in the amount specific to the
sol-gel precursor composition used. For example, for compositions
comprising TEOS, water, and an organic solvent, the molar ratios of
the TEOS:Water: Organic Solvent components are: 1:1-4:1-2 or
1:1-3:1. For example, in some embodiments, when ethanol is used as
an organic solvent, the sol-gel precursor composition includes
TEOS:Water:Ethanol in the molar ratios of 1:1:1, 1:2:1, 1:3:1, or
1:4:1. In other embodiments, when isopropanol is used, the sol-gel
precursor composition includes TEOS:Water:Isopropanol in the molar
ratios of 1:1:1, 1:2:1, 1:3:1 or 1:4:1. Typically, water with a pH
in the range of 0-4 is used in the sol-gel precursor
compositions.
[0087] In some embodiments, the sol-gel precursor compositions do
not include an organic solvent. In some embodiments, the sol-gel
precursor compositions further include an acid or base suitable to
catalyze the hydrolysable gelation of the ceramic precursor. In
some embodiments, a component of the ceramic precursor comprises a
basic group or an acidic group that can serve as a gelation
catalyst. For example, PSS comprises sulfonic acid groups suitable
to act as an acid catalyst for gelation of sol-gel precursor
compositions comprising TEOS and aqueous solutions of PSS, as
demonstrated in the Examples.
[0088] In certain embodiments, the sol-gel precursor composition
comprises (a) a ceramic precursor, such as silica (e.g., siloxane),
ormosils, titania, germania, zirconia, alumina, graphite, silicon
carbide, silicon nitride, boron nitride or others, and (b)
optionally a solvent, such as an alcohol (e.g., methanol, ethanol,
isopropanol, butanol, etc.) or an aromatic (e.g., toluene, xylene,
etc.). The mixture of these two components typically accounts for
about 30 volume % or less of the sol-gel precursor composition,
about 20 volume % or less of the sol-gel precursor composition, or
about 10 volume % or less of the sol-gel precursor composition. In
some embodiments, the sol-gel precursor composition further
comprises water in the amount of about 40 volume % or less of the
sol-gel precursor composition, about 30 volume % or less of the
sol-gel precursor composition, or about 20 volume % or less of the
sol-gel precursor composition.
[0089] In some embodiments, the sol-gel precursor composition or a
pre-treatment composition is a solution, gel or slurry comprising
water and/or solvent: such as alcohols (e.g., methanol, ethanol,
isopropanol, butanol, etc.) or aromatics (e.g., toluene, xylene,
etc.) present in the amount of about 80 volume % or less of the
composition, about 60 volume % or less of the composition, or about
50 volume % or less of the composition. Additionally, the sol-gel
precursor composition or a pre-treatment composition comprises a
colloidal suspension or a nanoparticle dispersion of ceramics (e.g.
silica, titania, germania, alumina, etc.), present in the amount of
about 50 volume % of the composition, about 40 volume % of the
composition, or about 20 volume % of the composition.
[0090] In some embodiments, a post-treatment composition, e.g.,
chemical bath for dipping the sol-gel membrane comprises water and
an acid, such as sulfuric acid, hydrochloric acid, phosphoric acid,
nitric acid, methansulfonic acid, polystyrene sulfonic acid, acetic
acid, or a mixture thereof. The acid concentrations typically are
between about 10 M and about 0.1 M, or between about 2.5 M and
about 0.5 M. The post-treatment composition can further comprise
salts containing cationic groups such as sodium, calcium, lithium,
ammonium, or magnesium and anionic groups such as chloride,
bromide, carbonate, sulfate, sulfonate, iodide, phosphate, nitrite,
nitrate, chlorate, borate, thiocyanate, thiosulfate and sulfide.
Typically, the salt concentration is between about 1 M and about
0.01 M or between about 1 M and about 0.1 M.
Methods of Preparing Nanoporous Selective Sol-Gel Ceramic
Membranes
[0091] Any suitable gelation method can be used to form the
membranes disclosed herein from the components described above. For
example, in some embodiments, gelling the sol-gel precursor
compositions comprises chemical gelation, including hydrolyzing
chemical gelation, non-hydrolyzing chemical gelation, and
combinations thereof.
[0092] In one embodiment, chemical gelation comprises exposing the
sol-gel precursor composition 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. Suitable acids
can be selected from the group consisting of sulfuric acid, nitric
acid, acetic acid, hydrochloric acid, methane sulfonic acid, and
phosphoric acid.
[0093] FIGS. 1-3 outline the steps of exemplary methods of
preparation of the membranes disclosed herein.
[0094] Referring to FIG. 1, a polymer is added to a porous support
such that the polymer infiltrates the macroporous support by at
least 1 .mu.m to ensure adhesion between the support and polymer.
This step forms an edging that surrounds the active area. The
polymer can be added to the edge of a macroporous support that is
cut to size such that the polymer extends beyond the edge of the
macroporous support by at least 1 .mu.m, at least 1 mm, or at least
1 cm. In some embodiments, the polymer or compressible edging does
not extend past the outer edge of the macroporous support. The edge
of the polymer support that extends beyond the membrane must form a
liquid tight seal with the polymer on the other side of the
support. The polymer can be applied as a film/sheet using heating,
solvent or radiation or the polymer can be applied from a liquid
phase as a solution or dispersion using common coating techniques
(e.g., dip, spray, drop, blade, screen). In some embodiments, the
polymer can be formed in situ by applying a composition comprising
a monomer or an oligomer capable of in situ polymerization reaction
and then polymerized to form compressible edging.
[0095] In some embodiments, the macroporous support not covered by
polymer can represent >50% of the grid surface area, ideally
>95%. Additionally, the area of the support not covered by
polymer is typically between 1 cm.sup.2 to 10 m.sup.2. Following
application of the polymer, a sol-gel based coating can be applied
within the porous support to create a nanoporous membrane
structure. The membrane formed by the above described process
typically has an average pore size of about 5 nm or less with a
polydispersity index of about 0.5 or less or about 0.25 or less.
The sol-gel can be applied using standard coating processes such as
dip, spray, blade, screen printing, jet, brush, etc. In some
embodiments, the steps of application of the edging and the
application of the sol-gel ceramic precursor described above can be
reversed.
[0096] After coating with the sol-gel precursor, a reaction
proceeds such that the membrane is allowed to sit for at least 1
minute. In some embodiments, the reaction is allowed to proceed for
up to 48 hours to induce self-assembly of the sol-gel solution to
produce the desired nanoporosity. In some embodiments, the
self-assembly process can take place in a controlled atmosphere
(e.g., humidity and temperature). This self-assembly process can
take place with the membrane against a flat solid surface, against
a flat porous surface or with the membrane hung vertically. In some
embodiments, it can be advantageous to coat the macroporous support
multiple times. In some embodiments, the interval between two
coatings is about 30 minutes, about 20 minutes, or about 10
minutes.
[0097] In some embodiments, it is advantageous to add a third
optional step of dip coating (or drop coating, blade coating, spray
coating) the active area of the membrane into a liquid of lower
surface tension than the sol-gel precursor composition. This step
can provide membranes with reduced cracking of the membrane if
drying is desired. Suitable liquids include alcohol, water, and
alcohol-water mixtures. A small concentration of surfactant can be
added to the liquids.
[0098] In some embodiments, it is advantageous to add a fourth
optional step which is to dry the membrane. Typical drying can be
done at temperatures up to about 400.degree. C., up to about
150.degree. C., or up to about 80.degree. C. In some embodiments,
the membranes are dried to 5-10% water content, such that the
sol-gel collapses but it does not become brittle or crack during
handling.
[0099] In some embodiments, the membrane can be further heated to a
higher temperature after the sol-gel coating dries at room
temperature. The temperature can be in the range of about
250.degree. C. to about 450.degree. C. In some embodiments, the
membrane is heated at the target temperature for about 2 hours. In
some embodiments, the membrane is heated at the target temperature
for up to about 48 hours. In some embodiments, the additional
heating produces a more compact membrane.
[0100] Referring to FIG. 2, a polymer is added to a porous support
such that the polymer infiltrates the macroporous support, as
described for FIG. 1. A pre-coating solution, gel, or slurry of a
pre-coating material is then applied within or on top of the porous
support to enhance the mechanical, chemical or electro-chemical
stability of the support structure. The pre-coating solution, gel,
or slurry can be applied using standard coating processes such as
dip, spray, blade, screen printing, jet, brush, or a combination
thereof.
[0101] After the pre-coating, the porous support is dried to remove
about 90 to about 100 volume % of the liquid phase of the
pre-coating solution, gel, or slurry such the pre-coating materials
adhere to the porous support. Drying generally takes about 24 hours
or less. In some embodiments, drying takes about 5 minutes. The
drying times generally depend on the nature of the liquid phase
used in the pre-coating solution, gel, or slurry. Drying can take
place at any temperature between room temperature and about
100.degree. C. This step can be repeated up to three times using
the same or different pre-coating compositions in order to
sufficiently enhance the properties of the macroporous support
prior to sol-gel coating.
[0102] Following the pre-coating step, the composite comprising a
nanoporous sol-gel ceramic composite filling at least a portion of
the porous support can be formed as described above for the method
depicted in FIG. 1. Further post-treatment steps can be used as
described above.
[0103] Referring to FIG. 3, a polymer is added to a porous support
such that the polymer infiltrates the macroporous support, as
described for FIG. 1, and a pre-coating step can be optionally
performed as described in FIG. 2 above. After sol-gel precursor
composition is applied, the membrane is exposed to an acid or salt
solution for a period of at least 10 seconds, greater than about 4
hours, or about 24 hours. This step can be utilized to control the
pore size, shape and structure of sol-gel coating by changing the
kinetics of gelation. The formed membrane can be subjected to the
post-treatment steps as described above.
[0104] In some embodiments, the membranes disclosed herein are
subjected to a post-treatment step after gelation of the sol-gel
precursor compositions. Post-treatment provides 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. For example, silanes can be
used to functionalize the membranes disclosed herein. This includes
a silane with a sulfonic acid group to improve proton conductivity
or molecular selectivity. Other embodiments include a silane with a
long alkane group to improve durability or reduce pore size.
Exemplary post-treatment reagents include
3-trihydroxysilyl-1-propanesulfonic-acid and
triethoxy(hexyl)silane.
[0105] In some embodiments, a step of applying a pretreatment to
the porous membrane support prior to the step of applying the
sol-gel precursor compositions to the porous membrane support is
included in the preparation of the nanoporous selective sol-gel
ceramic membranes. For example, the porous membrane support can be
coated with a polymer (e.g., polystyrene sulfonic acid).
[0106] The nanoporous selective sol-gel ceramic membranes described
herein are suitable for use in many applications, for example, as
battery membranes, fuel-cell membranes, an electrodialysis
membranes, an acid recovery membranes, chloro-alkali membranes,
solvent extraction membranes, electrodeposition membranes,
electrodeioniziation membranes, nutrient recovery membranes, food
processing membranes, reverse osmosis membranes, gas separation
membranes, and a bio-separation membranes. In some embodiments, the
nanoporous selective sol-gel ceramic membrane is an ion-conducting
membrane for a flow battery. In other embodiments, the nanoporous
selective sol-gel ceramic membrane is an ion-conducting membrane
for a fuel cell. In yet other embodiments, the nanoporous selective
sol-gel ceramic membrane is an ion-conducting membrane for
electrodialysis or an ion-conducting membrane for chloro-alkali
processes.
Characterization of the Nanoporous Selective Sol-Gel Ceramic
Composite Membranes
[0107] The membranes disclosed herein comprise a plurality of
nanopores. In some embodiments, the membrane have porosity between
about 10 vol % and 90 vol %, between about 50 vol % and 90 vol %,
or between about 70 vol % and 90 vol %. 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
nanoporous selective sol-gel ceramic membrane, including all layers
of ceramic and any pretreatment or post-treatment layers. 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.
[0108] In some embodiments, the nanoporous composite of the
membranes disclosed herein comprises a plurality of nanopores of
about 5 nm or smaller in radius with a polydispersity index of
about 0.7 or lower. In some embodiments, the plurality of nanopores
has a polydispersity index of about 0.6 or lower, about 0.5 or
lower, about 0.3 or lower, or about 0.25 or lower.
[0109] Typically, the membranes disclosed herein have a thickness
between 5 .mu.m and 1 mm. In one embodiment, the nanoporous
selective sol-gel ceramic 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.
[0110] Advantageously, the membrane disclosed herein experiences
about 5% or less linear expansion in the x-, y- or z-directions
according to the ASTM D756 standard practice for determination of
weight and shape changes of ion exchange membranes. In one
embodiment, the membranes' linear expansion in the x-, y- or
z-directions according to the ASTM D756 standard practice is about
1% or less. In another embodiment, the membranes' linear expansion
in the x-, y- or z-directions according to the ASTM D756 standard
practice is about 0.5% or less.
[0111] The nanoporous selective sol-gel ceramic membranes have an
ionic area specific resistance (ASR) in the range of 0.01
Ohm*cm.sup.2 to 10 Ohm*cm.sup.2 when measured in 4 M
H.sub.2SO.sub.4 or in the range of 0.1 Ohm*cm.sup.2 to 100
Ohm*cm.sup.2 when measured in 0.5 M NaCl, when ASR measurements are
performed using a galvanodynamic sweep from 0 to 0.5 Amps in an
h-cell with luggin capillaries and the cell ionic resistance is
subtracted to obtain the ASR. The method and setup can be found in
S. Slade et al., "Ionic Conductivity of an Extruded Nafion.RTM. 110
EW Series of Membranes", J. Electrochemical Society, 149 (12),
A1556-A1564 (2002). In one embodiment, membranes have an ASR in the
range of between about 0.08 Ohm*cm.sup.2 and about 0.5 Ohm*cm.sup.2
when measured in 4 M H.sub.2SO.sub.4. In one embodiment, membranes
have an ASR in the range of between about between about 0.6
Ohm*cm.sup.2 and about 15 Ohm*cm.sup.2 when measured in 0.5 M
NaCl.
Nanoporous Selective Sol-Gel Ceramic Membranes with Fractal
Nanoporous Structure
[0112] In an aspect, the present disclosure provides a nanoporous
selective sol-gel ceramic membrane comprising a porous support and
a nanoporous composite disposed on the porous support wherein the
nanoporous composite comprises a nanoporous sol-gel ceramic having
a fractal nanoporous structure. As discussed further herein, such a
fractal nanoporous structure can be determined by fitting
small-angle scattering spectra of the nanoporous sol-gel ceramic to
a mathematical model. In an embodiment, the nanoporous selective
sol-gel ceramic membrane is a nanoporous selective sol-gel ceramic
membrane as described elsewhere herein.
[0113] As discussed further herein, in an embodiment, the porous
support defines a number of pores. In an embodiment, the pores have
an average diameter of 10 nm or greater. In this regard, as the
nanoporous composite comprising the nanoporous sol-gel ceramic
composite fills at least a portion of the porous support the
nanoporous composite defines, at least in part, an active area of
the nanoporous selective sol-gel ceramic membrane. As discussed
further herein, such an active area is suitable to selectively
allow passage of fluid and, for example, ions through the
nanoporous selective sol-gel ceramic membrane, such as for
filtration, ion exchange, flow batteries, and the like.
[0114] In the present aspect, small-angle scattering spectra of the
nanoporous sol-gel ceramic may be fit to a mathematical model. In
an embodiment, the mathematical model is a fractal aggregate model.
As an example, the fractal aggregate model may be a fractal
aggregate model as described in J Teixeira, J. Appl. Cryst., 21
(1988) 781-785, which is incorporated herein by reference in its
entirety. In this regard, the nanoporous sol-gel ceramic composite
has a fractal structure defined, at least in part, by a fractal
dimension, D.sub.f, and a correlation length, .xi., as discussed
further herein.
[0115] The scattering spectra which may be fit to the mathematical
model can include small-angle scattering spectra. Such small-angle
scattering spectra are suitable to provide information about the
inner structure of the nanoporous selective sol-gel ceramic
membrane including, particularly, the nanoporous sol-gel ceramic.
Analysis of such small-angle scattering spectra, particularly of
elastic scattering of particles off of, for example, the nanoporous
sol-gel ceramics provide information regarding pore size, particle
arrangement, and the like.
[0116] In an embodiment, the small-angle scattering spectra
includes small-angle scattering spectra selected from the group
consisting of small-angle x-ray scattering spectra, small-angle
neutron scattering spectra, small-angle light scattering spectra,
and combinations thereof. In an embodiment, the small-angle
scattering spectra include small-angle x-ray scattering
spectra.
[0117] In an embodiment, the small-angle scattering spectra of the
nanoporous sol-gel ceramic fit closely to the mathematical model.
In this regard, a least squares regression fit of a de-smeared,
1-dimensional small-angle scattering spectra to the fractal
aggregate model provides a .chi..sup.2/N.sub.pt value of less than
10, wherein .chi..sup.2 is a squared sum of an intensity difference
between the fractal aggregate model and small-angle scattering
spectra data, and N.sub.pt is a number of small-angle scattering
data points over a model fitting range. Such a relatively low
.chi..sup.2/N.sub.pt value is indicative of a good fit between the
small-angle scattering spectra and the fractal aggregate model. In
an embodiment, .chi..sup.2/N.sub.pt value is less than 5. In an
embodiment, .chi..sup.2/N.sub.pt value is less than 1. As discussed
further herein with respect to FIG. 22A, the small-angle scattering
spectra of the nanoporous sol-gel ceramic generally have a
.chi..sup.2/N.sub.pt value less than 10, and frequently less than
5. This is in contrast to the porous structures, such as
Nafion.RTM., that do not have a fractal structure. As discussed
with respect to FIG. 22A, Nafion.RTM., for example, has a
.chi..sup.2/N.sub.pt value of 830, far greater than corresponding
values of the nanoporous sol-gel ceramic of the present
disclosure.
[0118] In an embodiment, the fractal aggregate model is a measure
of scattering intensity, I, as a function of a scattering vector,
q. In an embodiment, the fractal aggregate model is according to an
equation:
I(q).dbd.P(q)S(q)+bck
[0119] S(q) is a network or fractal structure that defines an
organization or configuration of building blocks of the nanoporous
sol-gel ceramic. Bck defines background scattering, such as from a
scattering particle source and/or inelastic scattering of
scattering particles off of the nanoporous sol-gel ceramic. In an
embodiment, S(q) may be fined by the following equation:
S ( q ) = 1 + D f .GAMMA. ( D f - 1 ) [ 1 + 1 / q .xi. ) 2 ] ( D f
- 1 ) / 2 sin [ ( D f - 1 ) tan - 1 ( q .xi. ) ] ( qR 0 ) D f
##EQU00001##
[0120] wherein
[0121] R.sub.o is a radius of the building blocks,
[0122] .rho..sub.solvent is a scattering length density of a
solvent,
[0123] .rho..sub.block is a scattering length density of the
building blocks,
[0124] D.sub.f is a fractal dimension,
[0125] .xi. is a correlation length, and
[0126] .GAMMA. is the standard mathematical gamma function.
[0127] P(q) is a form factor that defines a structure of the
building blocks of the nanoporous sol-gel ceramic as a function of
q. Such form factors may take on a variety of shapes, such as
simple geometric shapes like spheres, cubes, ovals, and the
like.
[0128] In an embodiment, the building blocks are defined as
homogeneous building blocks, such as homogeneous spheres. In that
regard, in an embodiment, P(q) is defined by the following
equation:
P(q)=scale.times.V(.rho..sub.block-.rho..sub.solvent).sup.2F(qR.sub.0).s-
up.2,
[0129] wherein
F ( x ) = 3 [ sin ( x ) - x cos ( x ) ] x 3 , V = 4 3 .pi. R 0 3 ,
##EQU00002##
[0130] scale is a volume fraction of building blocks of the
measured nanoporous sol-gel ceramic,
[0131] In an embodiment, the form factor defines a spherical
core-shell building block.
[0132] In that regard, P(q) may be defined by the following
formula:
P ( q ) = scale V s [ 3 V c ( .rho. c - .rho. s ) [ sin ( qr c ) -
qr c cos ( qr c ) ] ( qr c ) 3 + 3 V s ( .rho. s - .rho. block ) [
sin ( qr s ) - qr cos ( qr s ) ] ( qr s ) 3 ] 2 + bkg
##EQU00003##
[0133] wherein
[0134] scale is a volume fraction of building blocks of the
measured nanoporous sol-gel ceramic,
[0135] V.sub.c is a volume of the core,
[0136] V.sub.s is a volume of the shell,
[0137] .rho..sub.c is a scattering length density of the core,
[0138] .rho..sub.s is a scattering length density of the shell,
[0139] .rho..sub.block is a scattering length density of the
building blocks,
[0140] r.sub.c is a radius of the core,
[0141] r.sub.s is a radius of the shell, and
[0142] bck is background scattering.
[0143] In an embodiment, one or more surfaces of the nanoporous
sol-gel ceramic are coated with a coating. Such a coating can
comprise a material selected from the group consisting of an alkyl
group, a sulfonic acid, a carboxylic acid group, ammonium, and
combinations thereof. A core-shell model, such as the core-shell
fractal aggregate model may be suitable to characterize core-shell
particle building blocks, such as ceramic particles coated with an
alkyl or other hydrocarbon shell.
[0144] As above, various embodiments of the form factors, P(q), of
the fractal aggregate models used to characterize small-angle
scattering spectra include a factor including a difference between
scattering length densities.
[0145] In an embodiment, scattering length densities in the above
equations are defined by the materials that make up the components
of the nanoporous sol-gel ceramic membranes. Generally, larger
differences between scattering length densities of scattering
sources, such as pores, and surrounding ceramic materials provide
larger scattering contrast. Accordingly, in an embodiment,
small-angle scattering data is generated from nanoporous sol-gel
ceramics that have been dried to remove solvent or other liquid
from the pores, thus providing a greater difference in scattering
length densities compared to a nanporous sol-gel ceramic with pores
filled with a liquid solvent.
[0146] In an embodiment, the scattering length densities are
defined in units of .ANG..sup.-2 (inverse angstroms squared). The
scattering length density is defined as the sum of the bound
coherent scattering length of each atom normalized by the molecular
volume. For example, the x-ray scattering length density of air is
roughly 0 .ANG..sup.-2 while x-ray scattering length density of
amorphous silica is roughly 18.8 .ANG..sup.-2.
[0147] In an embodiment, small-angle scattering data is generated
from nanoporous sol-gel ceramics that have been rinsed to remove
residual ions, chemical reactants, and the like. In this regard,
the small-angle scattering data is more representative of the
nanoporous sol-gel ceramics and the pores it defines, rather than
such a structure further including the residual ions.
[0148] In fitting small-angle scattering spectra to a fractal
aggregate model includes fitting the small-angle scattering spectra
over a range of q values that exceeds an order of magnitude, such
as over an order of magnitude where q is in units of .ANG..sup.-1.
Such a relatively wide fitting range ensures that the data is fit
to the fractal aggregate model over a range of sizes commensurate
in scope with, for example, a size scale of pores of the nanoporous
sol-gel ceramic, in addition to providing data sufficient to fit
with the fractal aggregate model. In an embodiment, fitting
small-angle scattering spectra to a fractal aggregate model
includes fitting the small-angle scattering spectra over a range of
q values in a range of about 0.01 .ANG..sup.-1 to about 1
.ANG..sup.-1. In an embodiment, fitting small-angle scattering
spectra to a fractal aggregate model includes fitting the
small-angle scattering spectra over a range of q values in a range
of about 0.02 .ANG..sup.-1 to about 0.8 .ANG..sup.-1.
[0149] As above, scale corresponds to a volume fraction of pores in
the nanoporous sol-gel ceramic. In embodiment, scale corresponds to
membrane porosity when the small-angle scattering spectra are in
intensity units of 1/cm and the scale is less than 0.7. In this
regard, the scale corresponds to a number of pores, normalized by a
size of the sample. In an embodiment, the nanoporous sol-gel
ceramic has a porosity volume fraction in a range of about 0.01 to
about 0.7. In an embodiment, the nanoporous sol-gel ceramic has a
porosity volume fraction in a range of about 0.15 to about
0.35.
[0150] D.sub.f is a fractal dimension of some embodiments of the
fractal aggregate models described herein. In an embodiment,
D.sub.f corresponds to a shape and/or configuration of pores within
the nanoporous sol-gel ceramic. Generally, D.sub.f is in a range of
about 1 to about 3. Where D.sub.f is close to or at 1, the pores
may be generally characterized as 1-dimensional tunnels. Where
D.sub.f is close to or at 3, the pores may be generally
characterized as open spheres.
[0151] In an embodiment, it is advantageous to have a nanoporous
sol-gel ceramic defining pores having a tortuous or indirect route
through the nanoporous sol-gel ceramic. For an ion or other
particle in fluid communication with a tortuous pore, it is less
likely that the ion or other particle will traverse the membrane as
a size the ion or other particle approaches that of the tortuous
pore than compared to a less tortuous pore. In that regard, such
nanoporous sol-gel ceramics defining tortuous pores are suitable to
provide, for example, more selective filtration than, for example,
nanoporous sol-gel ceramics defining pores of the same size, but
that provide a more direct path through the nanoporous sol-gel
ceramic. In this regard, D.sub.f may be in a range of about 1.5 to
about 2.5. In an embodiment D.sub.f is in a range of 1.8 to about
2.0. Such D.sub.f ranges describe or characterize nanoporous
sol-gel ceramics having relatively tortuous pores having a form
factor somewhere between a straight line and an open sphere.
[0152] In an embodiment, the fractal aggregate model is constrained
to have pore sizes within a particular range. As above, the porous
support defines pores that are greater than 10 nm. Likewise, in an
embodiment, the pores of the nanoporous sol-gel ceramic are
constrained to 10 nm or less. In an embodiment, the fractal
aggregate model is constrained to have pore sizes or 5 nm or less.
As discussed further herein, nanoporous sol-gel ceramics defining
pores having average radii of less than, for example, 5 nm are
generally suitable for use in ion exchange, filtration, flow batter
membranes, and the like. Likewise, the methods described in the
present disclosure are suitable to make such nanoporous sol-gel
ceramics defining pores in such a size range. Accordingly, by
constraining fractal aggregate models used to fit small-angle
scattering data, a good fit between the fractal nanoporous sol-gel
ceramic and the fractal aggregate model can be obtained.
[0153] As above, the correlation length, .xi., is a length over
which the fractal pattern of the nanoporous sol-gel ceramic repeats
itself. Higher-quality nanoporous sol-gel ceramics will repeat the
fractal pattern over a relatively large size scale. In this regard,
such higher-quality nanoporous sol-gel ceramics define regular,
ordered pores over a relatively large size scale, which corresponds
to improved functional properties, such as filtration, ion
exchange, and the like. Analogously, a fractal pattern generally
cannot extend to size scales smaller than a size scale of building
blocks of the nanoporous sol-gel ceramic, such as smaller than
molecules or atoms. Accordingly, in an embodiment, the correlation
length, .xi., is constrained to a value of greater than 1 nm. In an
embodiment, the correlation length, .xi., is constrained to a value
of greater than 50 nm. In an embodiment, the correlation length,
.xi., is constrained to a value of greater than 100 nm. In an
embodiment, the correlation length, .xi., is constrained to a value
of about a thickness of the nanoporous sol-gel ceramic. In an
embodiment, the nanoporous sol-gel ceramics have a correlation
length, .xi., greater than 1 nm, such as greater than 50 nm or
greater than 100 nm.
[0154] The fractal aggregate models used to characterize the
nanoporous sol-gel ceramics may include terms to account for
variability in sizes of scattering sources, such as the pores of
the nanoporous sol-gel ceramics. In that regard, in an embodiment,
the fractal aggregate model includes a polydispersity ratio in a
radius parameter. Accordingly, in an embodiment a radius of a
building block, R.sub.0, is a weighted average rather than a
constant. The weighted average may be according to a number of
mathematical functions, such a Gaussian function, a log-normal
function, a rectangular distribution, and the like. In an
embodiment, the polydispersity ratio is Gaussian and according to
the equation:
f ( x ) = 1 Norm exp ( - ( x - x mean ) 2 2 .sigma. 2 )
##EQU00004##
[0155] wherein
[0156] x.sub.mean is a mean value of the distribution, and
[0157] Norm is a normalization factor determined during numerical
calculation.
[0158] In an embodiment, the polydispersity ratio is lognormal and
according to the equation:
f ( x ) = 1 Norm 1 xp exp ( - ( ln ( x ) - .mu. ) 2 2 p 2 )
##EQU00005##
[0159] wherein
[0160] .mu.=ln(x.sub.med)
[0161] x.sub.med is a median value of the distribution, and
[0162] Norm is a normalization factor determined during numerical
calculation.
[0163] In certain embodiment, lognormal distributions are
advantageous as they are generally not symmetric about x.sub.med.
In this regard, as the polydispersity ratio increases, the lower
tail may not fall into ranges that are aphysical, such as those
which would define a pore size smaller than, for example, atoms,
etc. that physically define the pores.
[0164] In an embodiment, the polydispersity ratio is constrained to
less than 0.7. Polydispersities greater than, for example, 0.7
would define a high polydispersity characterizing a nanoporous
sol-gel ceramic defining pores of highly disperse and variable
diameters. In an embodiment, a polydispersity ratio is in a range
of about 0.1 to about 0.4. In an embodiment, a polydispersity ratio
is in a range of about 0.1 to about 0.2.
[0165] As above, the nanoporous sol-gel ceramics can include any of
the nanoporous sol-gel ceramics described herein. In an embodiment,
the nanoporous sol-gel ceramic comprises greater than 20 mole %
oxygen and greater than 10 mole % inorganic molecules. In an
embodiment, the nanoporous sol-gel ceramic comprises oxygen in a
range of about 20 mole % to about 80 mole %. In an embodiment, the
nanoporous sol-gel ceramic comprises oxygen in a range of about 10
mole % to about 33.3 mole %. In an embodiment, the nanoporous
sol-gel ceramic comprises a material selected from the group
consisting of silica, alumina, titania, germania, zirconia, and
combinations thereof.
Selective-Membrane Structures
[0166] In another aspect, the present disclosure provides a
selective-membrane structure, comprising a plurality of individual
selective membranes. In an embodiment, the individual selective
membranes are joined by a support member in a planar configuration
to allow fluid flow therethrough.
[0167] In an embodiment, the individual selective membrane defines
one or more pores. In an embodiment, the individual selective
membrane comprises a porous material. In an embodiment, the porous
material includes a polymeric material that defines one or more
pores. Such a porous polymeric material can include polymers of
intrinsic porosity (PIM). In an embodiment, the porous material
defines one or more pores having an average diameter in a range of
about 5 nm to about 1,000 nm. In an embodiment, the porous material
defines one or more pores having an average diameter in a range of
about 1 micron to about 100 microns. In an embodiment, the porous
material includes a sol-gel assembled material, such as a sol-gel
assembled polymer, a sol-gel assembled metal, and the like. In an
embodiment, the porous material includes poly(tetrafluoroethylene)
(PTFE), such as porous PTFE and expanded PTFE. In an embodiment,
the porous material includes a porous polyethylene. In an
embodiment, the porous material includes a porous poly(vinylidene
fluoride) (PVDF).
[0168] As above, in certain embodiment, non-sintered and
non-calcinated ceramic-containing membranes are brittle.
Configuring such membranes in a grid-like structure, as described
with respect to the present aspect, solves an important problem
associated with the brittleness of large-area, non-sintered, and
non-calcinated ceramic-containing membranes. By creating smaller
discrete ceramic-containing membranes, which can be combined in a
grid structure, it is possible to create a large-area membrane that
is flexible due to the support structure, including for example a
polymer linkage between ceramic cells. Further, the active area of
the ceramic may be increased by adding additional cells.
[0169] The individual selective membranes can include any of the
nanoporous selective sol-gel ceramic membrane of the present
disclosure. In that regard, in an embodiment, the individual
selective membranes can include a nanoporous composite comprising a
nanoporous sol-gel ceramic composite filling at least a portion of
a porous support, wherein the nanoporous composite comprises a
plurality of nanopores of 5 nm or smaller in radius with a
polydispersity index of 0.5 or lower. In an embodiment, the
individual selective membranes can include a nanoporous composite
comprising a nanoporous sol-gel ceramic composite filling an active
area of the porous support; wherein the nanoporous sol-gel ceramic
has a fractal nanoporous structure as determined by fitting
small-angle scattering spectra of the nanoporous sol-gel ceramic to
a mathematical model.
[0170] In an embodiment, the individual selective membrane
comprises a material selected from the group consisting of
graphene, graphene oxide, and combinations thereof. In an
embodiment, such materials define apertures, such as molecule-sized
holes. In an embodiment, the apertures are formed by puncturing the
graphene and/or graphene oxide using ions to create a nanoporous
membrane structure.
[0171] In an embodiment, the individual selective membranes
comprise graphene, graphene oxide, or combinations thereof where
such materials are in the form of a single atomic layer or sheet of
graphene or graphene oxide. In an embodiment, the individual
selective membranes comprise graphene, graphene oxide, or
combinations thereof in the form of stacked atomic layers or sheets
of graphene or graphene oxide.
[0172] In an embodiment, one or more of the individual selective
membranes comprises graphene, such as graphene filling at least a
portion of a porous support having a plurality of pores that are 10
nm or greater in diameter. While graphene is suitable for use, for
example, as a filtration membrane, it generally does not have
sufficient structural characteristics to act as a stand-alone
filter. However, when individual selective membranes comprising
graphene are joined to and supported by a support member, such
graphene-containing individual selective membranes are suitable to
withstand greater stress than without the support member. In this
regard, a selective-membrane structure comprising
graphene-containing individual selective membranes is suitable, for
example, to selectively filter components of a fluid in contact
with the graphene-containing individual selective membranes.
[0173] In an embodiment, the individual selective membranes
comprise an ion-exchange material, such as Nafion.RTM.. In an
embodiment, ion-exchange material fills at least a portion of a
porous support structure. In an embodiment, such an ion-exchange
material is incorporated into the selective-membrane structure,
such as by infiltrating the porous support structure, after support
members have been coupled to the porous support structure.
[0174] FIG. 30A schematically illustrates a selective-membrane
structure 100, in accordance with an embodiment of the disclosure.
As shown, selective-membrane structure 100 is shown to include a
plurality of individual selective membranes 102 joined in a planar
configuration by a support structure 106. In the illustrated
embodiment, the support structure 106 includes a plurality of
support members 104 disposed adjacent to side edges of individual
selective members 102. In this regard, the plurality of support
members 104 overlaps at least a portion of the plurality of
individual selective membranes 102. As shown, the support members
104 are disposed in a grid configuration to define a plurality of
apertures into which the individual selective membranes 102 are
disposed.
[0175] In an embodiment, the individual selective membranes 102
include an edging material (not shown, see for example FIGS. 26 and
27) disposed about an outer edge of the individual selective
membranes 102. Such an edging material can include an elastic
and/or compressible material. The edging material is suitable to
join the individual selective membranes 102 to one another along
the outer edges. As shown, the selective-membrane structure 100
further includes the support structure 106, which may be configured
to compress the edging material to hold the plurality of individual
selective membranes 102 together in the grid configuration. In this
regard, the grid-like structure is configured to provide a
liquid-tight seal such that liquid does not flow between the
individual selective membranes 102. Rather, the selective-membrane
structure 100 is configured to allow fluid to flow though the
individual selective membranes 102, such as to filter the fluid or
allow ion transport therethrough.
[0176] The edging material may be polymeric, such as made from a
compressible polymer.
[0177] The edging material of one individual selective membrane 102
may be coupled to an edging material of another individual
selective membrane 102 by a method selected from the group
consisting of heating the edging materials, exposing the edging
materials to a solvent, radiation of the edging material, and
combinations thereof.
[0178] In an embodiment, the edging material includes a
compressible polymer and a compressible thermoplastic
elastomer.
[0179] In an embodiment, the support structure 106 is flexible and,
therefore, able to absorb stresses on the selective-membrane
structure 100, such as during use in filtration, ion exchange, and
the like, as fluid is flowed across the selective membrane
structure 100. In this regard, the support structure 106 is
configured to bend, deflect, compress, deform and the like as
stress is applied to the selective-membrane structure 100. Such
stress absorption reduces stress on nanoporous sol-gel ceramics of
the individual selective membranes 102. As discussed further
herein, such nanoporous sol-gel ceramics can be rigid or brittle.
By reducing stress on the nanoporous sol-gel ceramics, they are
less likely to break, thereby increasing a lifetime of the
selective-membrane structure 100 and/or increasing breaking points
of the selective-membrane structure 100.
[0180] In an embodiment, the support structure 106 is polymeric. In
an embodiment, the support structure 106 comprises a material
selected from the group consisting of epoxy, polyurethane,
poly(styrene-isoprene-styrene), poly(styrene-isobutylene-styrene),
polypropylene, polyethylene, polyvinyl chloride, polystyrene,
polyamide, polyimide, polyacetonitrile, polyvinylacetate,
polyethylene glycol, poly ether ether ketone, polysulfone,
polyacrylamide, polydimethylsiloxane, polyvinylidene fluoride,
polyacrylic acid, polyvinyl alcohol, polyphenylene sulfide,
polytetrafluoroethylene, cellulose and its derivatives, and
combinations thereof.
[0181] In an embodiment, the support structure 106 including the
support members 104 includes an ion-exchange material. In this
regard, the support structure 106 contributes to a functional area
of the selective membrane structure 100. The support structure 106
can include materials functional to perform tasks other than or in
addition to ion-exchange, such as those suitable for filtration,
redox flow cell batteries, electro-dialysis, dead-end filtration,
pharmaceutic filtration, lithium ion batteries, reverse osmosis
water purification, waste water treatment, food processing,
textiles, and the like.
[0182] As above, the support members can be disposed in a grid
configuration to define a plurality of apertures into which the
individual selective membranes are disposed. As illustrated in FIG.
30A, the grid is a square grid defining square active areas. While
square active areas and a square grid are illustrated, it will be
understood that other grid configurations are possible, such as
regular tessellating polygons, irregular shapes, circles, and the
like. In that regard, attention is directed to FIG. 30B, in which
another selective-membrane structure 200 is illustrated.
[0183] As shown, the selective-membrane structure 200 includes a
plurality of individual selective membranes 202 joined in a planar
configuration to allow fluid flow therethrough. In that regard, the
selective-membrane structure 200 includes a support structure 206
including a plurality of support members 204 coupling the
individual selective membranes together. In the illustrated
embodiment, the individual selective membranes 202 are disposed in
a tessellated hexagonal configuration.
[0184] Such a configuration may be suitable to increase an active
membrane surface area of the selective-membrane structure 202. In
an embodiment, an active membrane surface area of the
selective-membrane structure including the plurality of individual
selective membranes is greater than 50% of a total surface area of
the selective-membrane structure. In an embodiment, the active
membrane surface area of the selective-membrane structure including
the plurality of individual selective membranes is in a range of
about 50% to about 95%. Various configurations of individual
selective membranes 202 may be combined with functional support
structure 206 materials, as discussed further herein with respect
to FIG. 30A, to further increase an active membrane surface
area.
[0185] In an embodiment, the selective-membrane structures of the
present disclosure include a support structure is disposed
underneath the nanoporous composite comprising a nanoporous sol-gel
ceramic composite. Such a support structure can be, for example,
coupled to the porous support and the nanoporous composite is
formed over the porous support and the support structure. In such a
configuration, with the support structure disposed underneath at
least a portion of the nanoporous composite, the support structure
forms an endoskeleton for the nanoporous composite. In this regard,
the support structure provides physical support to the nanoporous
composite to reduce stresses applied to the selective membrane
structure, as discussed further herein with respect to FIGS. 30A
and 30B.
[0186] The materials of such an endoskeletal support structure can
include those support structure materials described further herein.
In this regard, the endoskeletal support structure may be flexible
and/or compressible. In an embodiment, the endoskeletal support
structure is disposed in a grid configuration defining one or more
apertures to allow fluid flow through the nanoporous composite.
[0187] While each of the elements of the present invention is
described herein as containing multiple embodiments, it should be
understood that, unless indicated otherwise, each of the
embodiments of a given element of the present invention is capable
of being used with each of the embodiments of the other elements of
the present invention and each such use is intended to form a
distinct embodiment of the present invention.
[0188] The referenced patents, patent applications, and scientific
literature referred to herein are hereby incorporated by reference
in their entirety as if each individual publication, patent or
patent application were specifically and individually indicated to
be incorporated by reference. Any conflict between any reference
cited herein and the specific teachings of this specification shall
be resolved in favor of the latter. Likewise, any conflict between
an art-understood definition of a word or phrase and a definition
of the word or phrase as specifically taught in this specification
shall be resolved in favor of the latter.
[0189] As can be appreciated from the disclosure above, the present
invention has a wide variety of applications. The invention is
further illustrated by the following examples, which are only
illustrative and are not intended to limit the definition and scope
of the invention in any way.
EXAMPLES
Example 1
[0190] In this example, methods of structural characterization of
the nanoporous selective sol-gel ceramic membranes are
described.
[0191] Membrane Permeability
[0192] To measure ion diffusion across the membrane, the basic
H-cell setup was utilized (FIG. 4). The left side of the cell
contained 10 mL of 1M FeCl.sub.3 or 1M CrCl.sub.3, both in 3 M HCl
OR 1 M VOSO.sub.4 in 2.5 M or H.sub.2SO.sub.4. The right side
contained 10 mL of 1 M AlCl.sub.3 or MgSO.sub.4 to balance out the
osmotic pressure and ionic strength. As the ions began to diffuse
across the membrane, 1 mL from the permeate side was extracted and
placed into a cuvette, with the date and time of extraction
recorded. Then, 1 mL of fresh AlCl.sub.3 or MgSO.sub.4 solution was
placed back into the permeate side. After the collection of
sufficient samples, a Thermo Scientific Evolution 300 UV Vis
Spectrophotometer was used to measure the absorbance of each sample
and consequently calculate the concentration using Beer's law
below, where Abs is the absorbance, e is the extinction coefficient
of the FeCl.sub.3, CrCl.sub.3, or VOSO.sub.4 solution, l is the
path length of light that travels through the cuvette, and c is the
concentration.
Beer's Law: Abs=.epsilon.lc (1)
[0193] The concentration of the ions is plotted against time, and
fit to the concentration profile obtained in the H-Cell using a
pseudo-steady diffusion approximation Once the value for the
diffusion process time (t.sub.p) is obtained, the extrinsic
permeability is obtained using equation 3, where A is the area of
the membrane and V is the volume of solution in the cell.
C ( t ) = C 0 2 ( 1 + e - t t p ) ( 2 ) ##EQU00006##
[0194] Area Specific Resistance (Asr)
[0195] FIG. 5 shows a schematic of the setup utilized to measure
ASR by 4-electrode potentiostatic electrochemical impedance
spectroscopy (PEIS). The instrumentation used was a Gamry Reference
600 Potentiostat. The setup consists of a modified H-cell with
Luggin capillaries. The volume inside the cell holds 50 mL. An AC
potential of 5 mV was applied at a frequency of 10-1,000,000 Hz. A
small AC potential excitation (1-10 mV) is common in the literature
to maintain pseudo-linearity of the system.
[0196] Structural Characterization
[0197] Small angle x-ray scattering (SAXS) was utilized to
characterize the nanostructure of the sol-gel ceramic composite
membranes. SAXS techniques have been extensively used in the
literature to investigate the nanostructure of sol-gel materials.
SAXS measurements were performed using an Anton Paar SAXess
instrument in slit collimation. Data corrections for background
(e.g., quartz capillary) are applied when necessary and data is put
in absolute scale using a high density polyethylene (HDPE) standard
as described by Fan and coworkers (Fan, M. Degen, S. Bendle, N.
Grupido, J. Ilavsky, The absolute calibration of a small angle
scattering source instrument with a laboratory x-ray source, J.
Phys: Conf. Series, 247 (2010), 012005).
Example 2
[0198] In this Example, four variations of exemplary membranes were
prepared as follows.
[0199] PSS TEOS Silica Membrane
[0200] This membrane was processed using the method described in
FIG. 1. For step 1, a non-woven silica macroporous substrate (1
.mu.m nominal pore size, 330 .mu.m thickness, 70% porosity) was
used in combination with a SIBS polymer edging (Kraton D1170). The
SIBS was 400 .mu.m thick and had a 2 mm overlap region with
macroporous support and a 9 mm non-overlapping region in which the
SIBS on either side of the support could seal to itself. SIBS was
melted into the macroporous support using a heat press with
pressure <100 kPa and temperatures between 300.degree.
F.-400.degree. F. for times of 1-10 minutes. This ensured
sufficient infiltration of the SIBS polymer into the macroporous
support. To further ensure a complete seal, 50 .mu.L of toluene was
dropped onto the boundary region of the macroporous support and
polymer edging. For step 2, a sol-gel solution containing a 1:1
volume ratio of 98 weight % tetraethyl orthosilicate (TEOS) and 18
weight % polystyrene sulfonic acid in water (PSS) was prepared. The
edged supports from step 1 were soaked in the sol-gel solution for
15 minutes and subsequently removed. After this, they were allowed
to dry as an optional 3.sup.rd step in order to induce gelation
(i.e., self-assembly) and remove as much of the liquid content from
the membrane (i.e., condensation/evaporation). Membranes were dried
for 12-24 hours at ambient pressure and temperature (i.e., 1 atm
and .about.23.degree. C.) on a teflon plate.
[0201] SS TEOS Silica Membrane
[0202] This membrane was processed using the method described in
FIG. 2. For step 1, a non-woven silica macroporous substrate (1
.mu.m nominal pore size, 150 .mu.m thickness, 75% porosity) was
used in combination with a SIBS polymer edging (Kraton D1170). The
SIBS was 400 .mu.m thick and had a 2 mm overlap region with
macroporous support and a 9 mm non-overlapping region in which the
SIBS on either side of the support could seal to itself. SIBS was
melted into the macroporous support using a heat press with
pressure <100 kPa and temperatures between 300.degree.
F.-400.degree. F. for times of 1-10 minutes. This ensured
sufficient infiltration of the SIBS polymer into the macroporous
support. To further ensure a complete seal, 50 .mu.L of toluene was
dropped onto the boundary region of the macroporous support and
polymer edging. For step 2, the edged macroporous support was
soaked into a 27% sodium silicate (SS) solution in water for 15
minutes. The SS aids in fully filling the substrate. For step 3, 40
.mu.L of a sol-gel solution containing 98 weight % tetraethyl
orthosilicate was drop-casted onto the SS infiltrated substrate.
The 40 .mu.L was enough to cover the entire unedged potion of the
substrate. After the sol-gel coating, membranes were allowed to dry
as an optional 4.sup.th step in order to induce gelation (i.e.,
self-assembly) and remove as much of the liquid content from the
membrane (i.e., condensation/evaporation). Membranes were dried for
12-24 hours at ambient pressure and temperature (i.e., 1 atm and
.about.23.degree. C.) on a teflon plate.
[0203] SS TEOS Cellulose Membrane
[0204] This membrane was processed using the method described in
FIG. 2. For step 1, a non-woven cellulose macroporous substrate 1
.mu.m, 190 .mu.m thickness, 75% porosity) was used in combination
with a SIBS polymer edging (Kraton D1170). The SIBS was 400 .mu.m
thick and had a 2 mm overlap region with macroporous support and a
9 mm non-overlapping region in which the SIBS on either side of the
support could seal to itself. SIBS was melted into the macroporous
support using a heat press with pressure <100 kPa and
temperatures between 300.degree. F.-400.degree. F. for times of
1-10 minutes. This ensured sufficient infiltration of the SIBS
polymer into the macroporous support. To further ensure a complete
seal, 50 .mu.L of toluene was dropped onto the boundary region of
the macroporous support and polymer edging. For step 2, the edged
macroporous support was soaked into a 27% sodium silicate (SS)
solution in water for 15 minutes. The SS aids in fully filling the
substrate. For step 3, 40 .mu.L of a sol-gel solution containing 98
weight % tetraethyl orthosilicate was drop-casted onto the SS
infiltrated substrate. The 40 .mu.L was enough to cover the entire
unedged potion of the substrate. After the sol-gel coating,
membranes were allowed to dry as an optional 4.sup.th step in order
to induce gelation (i.e., self-assembly) and remove as much of the
liquid content from the membrane (i.e., condensation/evaporation).
Membranes were dried for 12-24 hours at ambient pressure and
temperature (i.e., 1 atm and .about.23.degree. C.) on a teflon
plate.
[0205] SS PSS TEOS Silica Membrane
[0206] This membrane was processed using the method described in
FIG. 2. For step 1, a non-woven silica macroporous substrate (1
.mu.m nominal pore size, 330 .mu.m thickness, 70% porosity) was
used in combination with a SIBS polymer edging (Kraton D1170). The
SIBS was 400 .mu.m thick and had a 2 mm overlap region with
macroporous support and a 9 mm non-overlapping region in which the
SIBS on either side of the support could seal to itself. SIBS was
melted into the macroporous support using a heat press with
pressure <100 kPa and temperatures between 300.degree.
F.-400.degree. F. for times of 1-10 minutes. This ensured
sufficient infiltration of the SIBS polymer into the macroporous
support. To further ensure a complete seal, 50 .mu.L of toluene was
dropped onto the boundary region of the macroporous support and
polymer edging. For step 2, the edged macroporous support was
soaked into a 27% sodium silicate (SS) solution in water for 15
minutes. The SS aids in fully filling the substrate. For step 3, a
sol-gel solution containing a 1:1 volume ratio of 98 weight %
tetraethyl orthosilicate (TEOS) and 18 weight % polystyrene
sulfonic acid in water (PSS) was prepared. The SS infiltrated
substrates from step 2 were soaked in the sol-gel solution for 15
minutes and subsequently removed. After the sol-gel coating,
membranes were allowed to dry as an optional 4.sup.th step in order
to induce gelation (i.e., self-assembly) and remove as much of the
liquid content from the membrane (i.e., condensation/evaporation).
Membranes were dried for 12-24 hours at ambient pressure and
temperature (i.e., 1 atm and .about.23.degree. C.) on a teflon
plate.
[0207] The exemplary membranes were characterized as follows. FIGS.
6 and 7 show extrinsic permeabilities of the exemplary membranes
for Cr.sup.3+ and Fe.sup.3+ ions. It should be noted that a lower
permeability is desired for the membranes. Two of the most common
types of perflourosulfonic membranes commonly used in industry,
Nafion.RTM. 115 and 212, served as a benchmark for the exemplary
sol-gel ceramic composite membranes. The PSS TEOS Silica membrane
processed according to FIG. 1 had the highest permeability. Next,
the SS TEOS Silica and Cellulose membranes, processed according to
FIG. 2, had the next highest permeabilities. There was not a
significant difference between these two membranes based on the
variation between the silica based and cellulose based macroprous
supports. However, it makes sense that the cellulose membranes had
a slightly higher permeability compared to those with silica paper,
due to the fact that the cellulose paper is less dense and thinner.
Interestingly, the SS PSS TEOS Silica membrane, processed again
according to FIG. 2, achieved the lowest permeability amongst the
sol-gel ceramic composite membranes. Even more noteworthy, this
membrane formulation was able to match and reduce the Cr.sup.3+ and
Fe.sup.3+ permeabilities, respectively, of both Nafion.RTM.
membranes. This is believed to be due to the benefits of both the
PSS catalyst and the dense SS solution present in the same
membrane. It should also be mentioned that these are very small
ions permeating through the membranes. While the SS TEOS
Silica/Cellulose and PSS TEOS Silica membranes did not perform as
well for this specific purpose (i.e., ICRFB), they should not be
discarded for other types of battery membranes or filtration
purposes that use larger molecules. These indeed could also serve
to be a great option.
[0208] The area specific resistance (ASR) was calculated for these
membranes. After performing potentiostatic electrochemical
impedance spectroscopy (PEIS), the resistance of the membrane can
be trivially found by subtracting the resistance of the cell with
the membrane (R.sub.1) from the resistance of the cell without a
membrane (R.sub.o), and finally multiplying by the cross-sectional
area of the membrane (A). The results are seen below in FIG. 8. Not
surprisingly, the Nafion.RTM. membranes possessed the lowest ASR,
as this is a quality for which it is highly revered and is a large
reason why it is so popular despite its high cost.
ASR=(R1-Ro)*A (4)
[0209] The ASR of the sol-gel ceramic composite membranes were all
higher than Nafion.RTM.. In addition, a well-known hurdle in
membrane manufacturing is the relationship between the ASR and
permeability. In general, membranes with lower permeabilities will
generally have higher ASR and vice-versa. This can be seen
explicitly in FIG. 8 where the SS PSS TEOS Silica membrane had the
highest ASR, whereas previously it was seen that it possessed the
lowest permeability. The other sol-gel ceramic composite membranes
followed a similar trend. This trend is highlighted further in
FIGS. 9 and 10 where the extrinsic permeability is plotted against
the ASR of these membranes.
[0210] An inverse relationship between the ASR and permeability of
the sol gel ceramic composite membranes can be noticed. It should
be mentioned however that an ASR of 1 .OMEGA.cm.sup.2 is deemed an
acceptable value for the ICRFB. It is desirable to have as low of
an ASR as possible and this can be achieved with further
modifications and selections of precursors for this sol gel
process.
[0211] FIGS. 11 and 12 shows the results of the SAXS experiments.
FIG. 12 is the scattering of the bare silica and cellulose
macroporous supports as a reference. A broad peak in the low Q
region of the PSS TEOS Silica membrane is seen. This membrane had a
very different scattering pattern compared to the other three
membranes which utilized the SS pre-coat as described earlier and
in FIG. 2. From this it is evident that the pre-coat had some sort
of significant effect on the structure of the sol-gel ceramic
composite membranes. This was also noticed previously in the
permeability results, as all the membranes that contained the SS
pre-coat performed better than the membrane processed according to
FIG. 1 that did not contain the pre-coat. Overall, the results
presented here show the potential for sol-gel ceramic composite
membranes processed via this method to serve as a replacement for
Nafion.RTM. and other expensive membranes and filters.
Example 3
[0212] The following Example details the synthesis of porous and
flexible silica-based membrane with various desired functional
groups inside the pores. These membranes were processed either
using the method described in FIG. 1 or FIG. 2. Both methods have
the same first step, and differ only in the filling of the
macroporous support. In step 1, a non-woven silica macroporous
substrate (1 .mu.m nominal pore size, 220 .mu.m thickness, 70%
porosity) was used in combination with a SIBS polymer edging
(Kraton D1170). The SIBS was 150 .mu.m thick and had a 5 mm overlap
region with macroporous support and a 15 mm non-overlapping region
in which the SIBS on either side of the support could seal to
itself. SIBS was melted into the macroporous support using a heat
press with pressure <100 kPa and temperatures between
300.degree. F.-400.degree. F. for times of 1-10 minutes. This
ensured sufficient infiltration of the SIBS polymer into the
macroporous support. Depending on the formulation, some membranes
were tough enough to be used without the SIBS edging.
[0213] In using the method of FIG. 1, Ludox.RTM. particles were
mixed with bifunctional R*.sub.2--Si--(OR).sub.2 or trifunctional
organosilanes R*--Si--(OR).sub.3. Typically, Ludox.RTM. SM-30 (30%
wt. in water with 7 nm diameter) is used, but other types of
Ludox.RTM. such as Ludox.RTM. HS-40 or Ludox.RTM. CL can be used as
well depending on the application. The type of organosilane used
depends on the application as well. For example, for membranes in
flow batteries, alkyl groups were used to aid selectivity and
sulfonic groups were used to aid proton conductivity The Ludox.RTM.
suspension was mixed with the one or more types of organosilanes;
the mixture varies between 50-95% Ludox.RTM. suspension and 5-50%
organosilanes by volume. The pH was tuned to 2 using concentrated
hydrochloric acid or sodium hydroxide, and the mixture was stirred
vigorously at 60.degree. C. until the mixture became a single phase
solution. Initially, the mixture may not be miscible, but the
release of ethanol and the hydroxyl groups of the organosilanes as
a result of hydrolysis was sufficient to make a single-phase
solution. The solution was aged 0-24 hours. The substrates from
Step 1 were soaked for 30 seconds in the sol-gel solution. The
sol-gel infiltrated substrates were then removed from the solution.
The soaked macroporous support are left to dry on a Teflon plate at
room temperature for 24 hours. The membranes resulting from this
process is tough and flexible. The membrane does not crack upon
maximum compression or bending to about 10 cm radius of
curvature.
[0214] In using the method of FIG. 2, Ludox.RTM. suspension was
tuned to pH 2 using concentrated hydrochloric acid. The substrates
from Step 1 were soaked for 30 seconds in the sol-gel solution. The
sol-gel infiltrated substrates were then removed from the solution.
The soaked macroporous support are left to dry on a Teflon plate at
room temperature for 24 hours. Another batch of sol was then
prepared, where tetrafunctional silane molecular precursors such as
tetraethyl orthosilicate (TEOS) or tetramethyl orthosilicate (TMOS)
are mixed with organosilanes, alcohol (methanol, ethanol,
isopropanol), water, and hydrochloric acid to tune the pH to 2. The
amount of organosilane can range from 0-50% of the total silane
species. The mole ratio of silane species to water and alcohol vary
from 1:2-5:0-4. The components were vigorously stirred at
60.degree. C. until there was sufficient hydrolysis of silane
ethoxy groups and there is no phase separation. The sol was then
aged for 0-24 hours. Finally, the previously filled macroporous
support was immersed in the sol and left to dry on a Teflon plate
for 24 hours.
[0215] The functional groups on the organosilanes point outwards
into the pores rather than getting embedded within the silica
network. SAXS was performed on membranes formed from hexyl
triethoxysilane and TEOS (FIG. 14), and its fit to the fractal core
shell model, where the core is silica and the shell is a
combination of the hydrocarbon and solvent. The fit in FIG. 14
shows a composite membrane with a core silica radius of 5.6 .ANG.
and a hexyl shell of 10.4 .ANG.. The model indicates that even when
R* are hydrophobic alkyl functional groups, these groups are
pointing outwards rather than forming micelles. This structure of
the sols in dispersion, where the R* groups are pointing outwards,
is key to creating a membrane with tunable functional groups in the
pores. Without adding TEOS or Ludox.RTM. into the mixture, however,
the mixture did not gel even after an extended period of time
because the organic groups and not able to hydrolyze and condense.
The minimum amount of TEOS required to gel the sols depends on
organosilane used, but is typically about 30% by mol.
[0216] ASR and permeability measurements (FIG. 15) were also
performed as described in the methods section. The values are
compared against Nafion.RTM. as the benchmark. The results are
promising: both ASR and permeability of the silica based membranes
are comparable or better than Nafion.RTM..
Example 4
[0217] This Example illustrates preparation of a titania-based
exemplary membrane. The chemicals used in titania membranes were
titanium tetraisopropoxide (TTIP), TEOS, 3M hydrochloric acid, DI
water, and pure ethanol (EtOH). 97% TTIP and 99% TEOS were obtained
from Sigma-Aldrich Corporation, and pure ethanol 200 proof was from
Decon Laboratories, Inc. Titanium dioxide was prepared by
hydrolyzing TTIP with small amount of water in ethanol, and the pH
needed to be adjusted by hydrochloric acid to prevent TiO.sub.2
from precipitating. Partially hydrolyzed products are soluble in
ethanol and form polymeric chains through the condensation of
oxygen bridges. The equations are shown below:
Ti(OR).sub.4+xH.sub.2O.fwdarw.Ti(OH).sub.x(OR).sub.4-x+xHOR (1)
Ti(OR).sub.y(OH).sub.x.fwdarw.TiO.sub.z(OR).sub.y.sub.1(OH).sub.x.sub.1+-
(y-y.sub.1)RH (2)
[0218] where x<4, x.sub.1<x, y.sub.1<y,
z=[4-(x.sub.1+y.sub.1)]/2, and z is the number of oxygen bridges
formed per titanium atom.
[0219] The membranes were prepared using the method described in
FIG. 1. In this case, the optional method (2a) in the description
of FIG. 1 was utilized where the sol-gel coating and drying step
take place before adding the polymer edging. For Step 1, chemicals
were mixed in the order of ethanol, HCl, TTIP, and water or TEOS to
prepare the sol-gel solution. The following H.sub.2O:TTIP:EtOH mass
ratio of 1:29:18, 1:24:23, 1:14:32 were utilized. Solution could
also be prepared with TEOS:TTIP:EtOH mass ratio of 1:21:26,
1:17:26, 1:10:26. Glass fiber paper (1 .mu.m nominal pore size and
330 .mu.m thickness) was coated by dipping the paper in the
solution for about two minutes. The paper was coated three times.
The interval between coatings was about ten minutes so that the
surface was not visibly wet. For Step 2, the coated paper naturally
dried out in fume hood for about 24 hours after final coating. The
membranes were sent to F6030C furnace from Thermo Scientific for
sintering. The furnace heated up to 50.degree. C. below the target
temperature at the speed of 5.degree. C./min, and then the
temperature increased to the target temperature at the speed of
1.degree. C./min. The membranes were sintered at the target
temperature (250.degree. C., 350.degree. C., & 450.degree. C.)
for 2 hours and then cooled down to room temperature. For Step 3, a
SIBS polymer edge (Kraton D1170) was added to the membranes by
melting the polymer into the coated paper using a heat press with
temperature about 325.degree. F. and time for about 2 minutes. The
SIBIS polymer extended beyond the edge of the membranes by about 15
mm, and they had an overlap area about 5 mm.
[0220] In order to find appropriate amount of chemicals used in the
reactions, a phase diagram of TTIP, ethanol, and water was explored
(FIG. 16). The phase diagram is based on weight percent. The weight
percent of hydrochloric acid was fixed to 0.022%, and the weight
percent of other chemicals was changed. FIG. 14 shows this phase
diagram. TTIP would precipitate with excess amount of water. Clear
yellow solution was obtained after the reactions with little amount
of water (less than 10%). After the samples dried out, the samples
with less TTIP became transparent yellow crystals, and the samples
with more TTIP became non-transparent yellow crystals. The samples
were found to form yellow gel immediately with 10%-20% of water and
with relative small amount of TTIP. The shaded area in the phase
diagram is the region where yellow transparent crystals formed
after drying out. The transparent yellow color shows that TiO.sub.2
nanomaterial might be obtained. Thus, the shaded area would be
focused for membranes coating.
[0221] Finished membranes were tested by dropping several drops
(about 12.mu.L per drop) of 1M FeCl.sub.3 or 1M CrCl.sub.3 on the
membranes. The membranes then were pressed to tissue paper to check
whether metal ions passed through. FIG. 17 shows the membranes
after this test. Coated paper can prevent several drops of solution
from passing through, and the tissue paper would keep clean. When
the membranes failed, the color of the test solution would show on
the tissue paper. When test solution dropped on empty substrates,
the solution directly penetrated, and the color showed on the
tissue paper. Generally, without sintering, the membranes coated
with larger amount of TTIP would prevent more drops of solution
from penetrating. Thus, the membranes coated with higher
concentration of TTIP might have worse permeability.
[0222] Table 1 compares SAXS results and drop test results among
different membranes. Generally, the sintered membranes show larger
pore radius and lower void fraction than non-sintered membranes.
Compact structure might form after sintering, and shrinkage might
happen to cause larger pore size. The drop test results would not
change a lot after sintering. The membranes coated with water show
worse drop test results and largest pore radius. All fractal
dimensions are close to 3, which suggests that the pores are
organized in a spherically expanding fractal structure.
[0223] The membranes coated with the same composition were sintered
at different temperatures, and Table 2 shows the properties of
these membranes. As sintering temperature increases, the pore size
increases while void fraction decreases. Denser structure might
form at higher sintering temperature. The membranes turned to brown
after they were heated at lower temperature, which might be caused
by the burning of organic contaminants. Organic contaminants might
evaporate at higher sintering temperature so that the membranes
kept the white color. Table 3 shows the WAXS results of the
membranes sintered at 450.degree. C. with different compositions.
The pore size increases as the composition of TTIP increases in
solution. However, void fraction does not show clear trend. The
change of composition does not show critical effect on void
fraction.
TABLE-US-00001 TABLE 1 Comparison of Exemplary Titania Membranes
Pore Composition Void Radius Fractal (Weight %) Sintered? Fraction
(.ANG.) Dimension Drop Test Results TTIP:EtOH:H2O No 0.063 7.6 3.0
3.sup.rd drop passed 60:38:2.1 Yes 0.033 48 3.0 2.sup.nd drop
passed TTIP:EtOH:H2O No 0.040 6.0 3.0 2.sup.nd drop passed
50:48:2.1 Yes 0.024 59 3.1 2.sup.nd drop passed TTIP:EtOH:TEOS No
0.027 6.4 3.0 4.sup.th drop passed 50:48:1.9 Yes 0.0093 14 2.7
3.sup.rd drop passed TTIP:EtOH:TEOS No 0.041 6.7 3.0 3.sup.rd drop
passed 44:54:2.1 Yes 0.011 12 2.8 3.sup.rd drop passed
TABLE-US-00002 TABLE 2 Properties of the Membranes Sintered at
Different Temperatures Composition (Weight %) Void Pore Fractal
TTIP:EtOH:TEOS:44: 54:2.1 Fraction Size (A) Dimension Color Without
Sintering 0.046 5.2 3.0 White Sintering at 250.degree. C. 0.027 9.6
3.0 Brown Sintering at 350.degree. C. 0.018 10 3.0 Brown Sintering
at 450.degree. C. 0.012 14 2.9 White
TABLE-US-00003 TABLE 3 Properties of the Membranes with Different
Compositions Sintered at 450.degree. C. Composition (Weight %) Void
Pore Fractal TTIP:EtOH:TEOS Fraction Size (A) Dimension Color
44:54:2.1 0.019 16 2.9 White 38:59:2.3 0.025 14 3.0 White 28:69:2.7
0.022 12 3.0 White
Example 5
[0224] In this Example, the sol-gel solution is comprised of
volumetric ratios of a 17 wt % polystyrene sulfonate (PSS) in water
solution and pure tetraethyl orthosilicate (TEOS). These membranes
were processed using the method described in FIG. 1. For Step 1, a
non-woven silica macroporous substrate (1 .mu.m nominal pore size,
220 .mu.m thickness, 70% porosity) was used in combination with a
SIBS polymer edging (Kraton D1170). The SIBS was 150 .mu.m thick
and had a 5 mm overlap region with macroporous support and a 15 mm
non-overlapping region in which the SIBS on either side of the
support could seal to itself. SIBS was melted into the macroporous
support using a heat press with pressure <100 kPa and
temperatures between 300.degree. F.-400.degree. F. for times of
1-10 minutes. This ensured sufficient infiltration of the SIBS
polymer into the macroporous support. For Step 2, the sol-gel
solutions were prepared. The following PSS:TEOS ratios of 2:1, 1:1,
1:2, 1:3, 1:5, 1:7, 1:11, 1:18, 1:41 were utilized. Out of these
ratios, the 1:2-1:11 range managed to achieve single phase sols
within 5 hours and showed superior performance. Therefore,
solutions of PSS and TEOS were prepared in PSS:TEOS ratios of 1:2,
1:3, 1:5, 1:7, 1:11, and stirred until the solution achieves a
single phase. The polymer edged substrates from Step 1 were soaked
for 30 seconds in the sol-gel solution. The sol-gel infiltrated
substrates were then removed from the solution. An optional
3.sup.rd Step (i.e., drying) was utilized to induce gelation (i.e.,
self-assembly) and then condensation. Drying was done for 24 hours
at room temperature (.about.23.degree. C.) on Teflon plates to
prevent sticking of the membrane to the drying surface.
[0225] The membrane structure was characterized using Scanning
Electron Microscopy (SEM). SEM was utilized to collect 200.times.
and 2000.times. magnification images of both the top and the
cross-section of the membranes. The images were used to scan for
visible defects on top & cross-section and bridging cracks
within the cross-section only. FIG. 16 shows these SEM images for
the samples produced. As the amount of TEOS increases, the
membranes look more densely filled from the top. However,
cross-sectional images (close-up and zoomed out) shows that all
membrane formulations are filled, which is necessary to prevent
effective membrane operation.
[0226] SAXS data was also collected. SAXS profiles were collected
of 10 minutes per sample using a transmission method and fit to a
fractal aggregate model as described in EXAMPLE 4 above. The
reproducibility of the membrane nanostructure was probed in a 10
cm.times.10 cm membrane using a 1:5 PSS:TEOS ratio using the same
process described above. The membrane was divided into nine zones,
each of which was subjected to SEM and SAXS procedures (FIG. 17).
SEM images indicate no discernible variation across the membranes
in terms of density, defects, or macrostructures. SAXS modeling was
unable to detect any variations across the zones, and a
characteristic pore radius of 4.5 A, and minimum porosity of
2.6%.
[0227] Membranes were also characterized using ASR and permeability
measurements as described in the methods section. This data is
plotted in FIGS. 18 and 19. Ideally, a membrane would have low ion
permeability and high proton conductivity, though a trade-off
between the 2 is common. In this case, FIG. 18 shows this trade-off
with a PSS:TEOS ratio of 1:7, 1:5 and 1:3 showing the best
performance. FIG. 19 shows the relationship between membrane pore
size (assessed by SAXS) and proton conductivity showing an
increasing conductivity with decreasing pore size (and increasing
PSS content).
[0228] 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.
[0229] 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.
* * * * *