Patent Application
20220384833
The present invention relates to the field of fuel cells, electrolyzers, solar hydrogen generation devices, redox flow batteries, and other electrochemical synthesis apparatus, as well as dialysis, reverse osmosis, forward osmosis, pervaporation, ion exchange and sensor apparatus, which specifically contain anion exchange membranes (AEMs). The invention provides a composite ion exchange membrane structure that can effectively improve the oxidation resistance and durability of polymeric AEMs known in the art.
Since the introduction of perfluorosulfonic acid polymer membranes, such as Nafion, by Du Pont, Proton exchange membrane (PEM) based low temperature fuel cells (PEMFCs) have received widespread attention for their potential in commercial (EVs, off-road vehicles and backup power, etc.) and military applications that require power ranging from kilowatts to megawatts. However, the acid nature of the proton exchange membrane requires the use of noble metal electrocatalyst, which increases the lifecycle costs and also limits the choice of fuel in the PEMFC to hydrogen, formic acid or methanol.
Alkaline fuel cells (AFCs) were used in NASA space flights throughout 1970s and are still being used in the space shuttle. However, in recent years since the introduction and advancement of PEMFCs, very little attention has been paid to AFC development despite the numerous advantages of alkaline fuel cell system. It is well known that electrochemical oxidation of fuel and electrochemical reductions of oxygen are kinetically more favorable in an alkaline media even at low temperature (T<100° C.), so alkaline electrolyte offers the best prospects for using non-noble metal as the catalyst. At the system level, cathode flooding is less problematical in an AFC since water is produced on the anode side, and methanol, ethanol or borohydride can be used as the fuel; the cross-over issue can also be mitigated with non-precious metal oxygen reduction reaction (ORR) catalysts. In fact, since the pioneering work in alkaline fuel cells by Bacon, in the 1970s and 1980 efforts were made to further develop AFCs for space, electric vehicle (EV) and stationary power applications. The majority of early AFCs used liquid KOH as the electrolyte in either immobilized (in asbestos matrices) or circulated form, due to the very high intrinsic ionic conductivity (>0.6 S cm−1) of the KOH electrolyte (>0.6 S cm−1). However, there are problems encountered in the use of liquid alkaline electrolyte due to: 1) carbonation of the electrolyte by CO2 either from air or from the fuel oxidation products; 2) electrode degradation due to liquid flooding and carbonate precipitation; and 3) altitude sensitivity for the AFC stack. Furthermore, AFCs based on liquid alkaline electrolyte have much lower volumetric power density as compared to modern PEMFCs.
Recently, anion exchange membrane-based fuel cells (AEMFCs) were re-introduced to address these issues. The adoption of AEMFCs offers advantages over PEMFCs in addition to liquid electrolyte, including fast oxygen reduction kinetics, low cost catalyst materials and less corrosion issues. Great effort has been made in developing advanced anion exchange membranes for AEMFCs. Ideal AEMs should have the following characteristics: 1) high ionic conductivity; 2) good chemical and thermal stability in alkaline environment; 3) excellent oxidation stability; 4) low gas or fuel cross-over; and 5) excellent mechanical strength. With technical advancements, ion conductivity has been successfully increased and chemical stability in alkaline electrolyte has also been greatly improved; however, oxidation stability of the AEM has not gained sufficient attention. It is known that hydrocarbon polymer-based AEMs are subject to oxidative attack by the oxidants produced in the electrochemical or photo electrochemical reaction, leading to fast degradation of the membrane and short operating life. In an AEM-based water electrolyzer, oxygen evolution reaction could pose even more severe challenges to the oxidation stability of the AEMs.
For example, US Patent application 2014/0107237 taught a family of cation-strung polymer membrane showing high ionic conductivity (up to 67 mS/cm). Masel et al. in U.S. Pat. No. 9,370,773 taught an ion conductive membrane comprising of copolymer of styrene and vinylbenzyl-Rs (where Rs is selected from the group consisting of imidazolium and pyridiniums). Although copolymers were employed to form the membrane backbone, such hydrocarbon polymer backbone has low oxidation resistance toward the species generated in the electrochemical processes.
Rusch and Kolde, in U.S. Pat. No. 6,130,175, taught a thin multi-layer composite membrane, in which two different ion-exchange resins were impregnated on the opposite sides of the porous support. The membrane claims to have substantially higher ionic conductivity than other ion exchange membranes.
Bahar and Saltwick, in U.S. Patent Application 2020/0070142A1, taught a multiple layer ionic polymer membrane structure where each layer contains substantially different chemical compositions, such as ionic functionality. The membrane claims to have different functions, such as reducing fuel crossover and imparting substantial hydrophobicity. However, such multiple layer polymer membranes do not provide better oxidation resistance than single layer polymer membranes since they are all made of polymers.
Long term stability of the proton exchange membrane is also critical in numerous applications, such as fuel cells. Bauer, et al., in U.S. Pat. No. 7,108,935, invented a composite ion exchange membrane made of an ionic polymer uniformly filled with zirconium phosphate particles. The composite proton exchange membrane is claimed to have high performance, with decreased methanol crossover in direct methanol fuel cells. Roelofs in U.S. Pat. No. 8,617,765 taught chemically stabilized ionomers containing inorganic fillers. The ionic polymers showed reduced degradation.
Bahar and Gu, in U.S. Patent Application 2020/0406248A1, taught a similar approach by mixing metal oxides with an anionic polymer, which has showed better water retention in the membrane and reduce the gas crossover. However, no information is given for the oxidation resistance.
With these efforts, promising results have been achieved and membrane performance has been substantially improved (e.g. ionic conductivity over 70 mS/cm at room temperature). However, the durability of AEMs is becoming a more critical issue in AEMFC and electrolysis applications, as ionic conductivity of the membrane decreases due to the activity of OH− towards positive charged functional groups through E1, E2 reactions or Nucleophilic attack. Additionally, these hydrocarbon-based polymer backbones can also undergo degradation reaction by oxidative species generated in the electrochemical reaction, leading to performance degradation and/or mechanical failure of the membrane.
While a perfluorinated polymer backbone can substantially improve the oxidation stability, perfluorinated polymer exhibits instability in the alkaline electrolyte and is not ideal polymer backbone for the AEMs.
Therefore, current art needs substantial improvement in oxidation resistance to achieve the long durability of the anion exchange membrane for numerous industrial applications, including AEMFCs, AEM electrolyzers and other applications. There exists a strong motivation to develop an out-of-the-box concept to solve these challenging issues of alkaline and oxidation stability.
The present invention provides a composite anion exchange membrane (AEM) that exhibits substantially improved oxidation resistance and durability. An exemplary AEM may be a composite ion exchange membrane compromising a layer of the state of the art polymeric AEM, an inorganic nanosheet laminar layer on one or both sides of the polymeric AEM layer, a porous layer of oxidation resistance polymer as the binder as illustrated in
Nanosheet layer 204 may be made of two-dimensional nanosheets with thickness ranging from 0.3 nm to 200 nm. For these nanosheet materials, the aspect ratio may be expressed as the ratio of the platelet length to the platelet thickness. Nanosheets used for making nanosheet layer 204 may have an aspect ratio from five-to-one to one hundred thousand-to-one. While not limited by the examples, Table 1 provides an exemplary classification of inorganic layered materials that may form nanosheets through, for example, exfoliation. Nanosheets with aspect ratios over 5 may be beneficial for use in the present invention. Recently, nanosheets have been attracting great attention for their use in making ultrathin gas or ion sieving. They have not been employed as the protection layer for anion exchange membranes.
In another embodiment, in addition to polymer layer 202, nanosheet layer 204, and AEM layer 206, membrane 210 may further include an additional polymer layer 212 and nanosheet layer 214, which may be identical or similar to polymer layer 212 and nanosheet layer 204, respectively.
In one embodiment, non-ion exchangeable nanosheets, including but not limited to boron nitride, graphene or graphene oxide, MXene, or metal dichalocogenides, may be employed to make an ultrathin laminar layer on one or both sides of the polymeric AEMs of the art.
More preferably, ion exchangeable nanosheets, including cation exchangeable nanosheets and anion exchangeable nanosheets, may be employed to make an ultrathin laminar layer on one or both sides of the polymeric AEMs of the art.
Most preferably, anion exchangeable nanosheets, including but not limited to LDH, hydroxide salts or other cation functionalized nanosheets, may be employed to make the ultrathin laminar layer on one or both sides of the polymeric AEMs of the art.
In another embodiment, zircornium hydrophosphate (a-ZrP) and its derivatives may be functionalized to make a variety of materials including proton conductors; however, no one has attempted to make the cation functionalized ZrP. In this embodiment, ZrP nanosheet may be functionalized with silane condensation reaction followed by ammonium ion functionalization (e.g., as shown in
In an embodiment, anion exchangeable nanosheets, including but not limited to LDH nanosheets may be employed to make the laminar layer on one or both sides of AEMs. The general formula of LDHs can be expressed as [M2+1−xM3+x(OH)2]x+[An−x/n]x−.mH2O. LDHs are anion exchangeable lamellar inorganic materials, whose structure is based on brucite-like layers containing a divalent M2+ cation coordinated by six hydroxyl group. Replacing the divalent M2+ (e.g., Mg2+, Zn2+, Ca2+, Ni2+) with trivalent M3+ ions (e.g., Al3+) gives rise to the positive charge on the layered structure (
Moreover, LDHs can be readily exfoliated, forming a colloid dispersion of single layer or ultrathin nanosheets in various media solutions with or without sonication (
In one embodiment, oxidation resistant polymers, including but not limited to, Nafion, PVdF, or other ionic or non-ionic fluoropolymer solution, may be used to coat such nanosheet laminar layers to enhance the adhesion of the composite AEMs.
The ionic conductivity of the membranes in either Cl− or OH− form was measured using 3-probe AC electrochemical impedance spectroscopy at a fixed temperature with a Gamry Instrument. Due to its air stability, most of the ionic conductivities were measured in the Cl− form of AEMs.
Alkaline stability was carried out by immersing the membranes (1 cm×3 cm) in a 1M KOH solution at 80° C. in a seal container for certain period time. The membrane samples were taken from the KOH solution, neutralized with 0.1 M HCl solution and rinsed with deionized water several times. Although the membrane test is in OH− form, it was exchanged to Cl− for impedance measurement for more reliable results (since membrane in its OH− form can react with CO2 in the air leading to the change of ionic conductivity while measuring).
Oxidative stability was measured according to literature. Specifically, the membrane samples (1 cm×3 cm) were immersed in Fenton's reagent (4 ppm of iron(II) sulfate heptahydrate in 3% H2O2) at 80° C. for 24 hr. The oxidative stability was evaluated by comparing the weight losses of the dried membrane samples.
A monomer solution was prepared using 94.4 wt % vinylbenzyl chloride (VBC), 5.6 wt % divinylbenzene (DVB) and 1.76 wt % thermal initiator. The ratio of monomer and crosslinker can be varied to make different precursor polymer film. A porous polypropylene (PP) substrate was impregnated with prepared monomer solution for 1 hour at room temperature. After the monomer sorption process, polymerization proceeds at 90° C. for 3 hours, leading to the formation of membrane precursor film.
The quaternized films were prepared by the immersion of the precursor film in a 30 wt % solution of the corresponding heterocyclic amine, e.g., N-methylpiperidine (MP) as the benchmark AEM at 60° C. for 24 h. The quaternized films were washed with distilled water a number of times. The resulting membrane is an anion exchange membrane in Cl− form. This polymeric membrane is used for comparison purpose. Other polymeric AEMs procured from commercial sources (hereby denoted Commercial F AEM and Commercial X AEM) were also used for comparison purposes.
In an exemplary embodiment, α-ZrP powders were synthesized following procedure by Clearfield et al. A sample of 10 g of ZrOCl2.8H2O (Alfa Aesar) was mixed with 100 mL of 12M H3PO4 (Sigma Aldrich) in a sealed Teflon-lined pressure vessel, and the mixture was reacted at 200° C. for 48 h. α-ZrP microcrystals were then dispersed in deionized water (i.e., 0.1 g in 6.7 ml water) followed by exfoliation by adding TBAOH (Sigma Aldrich) in ice bath for 1˜2 hours. The solution was taken out from ice bath and treated with ultra-sonication. The solution was then centrifuged at a speed of 6000 rpm and 10000 rpm for 15 and 20 minutes respectively. Upper solution was collected as exfoliated nanosheets. TEM image of a typical nanosheet shown in
By changing the concentration of H3PO4 (from 15M to 6M), nanosheets with different aspect ratios may be obtained and used for making nanosheet membrane. SEM and XRD analysis were also conducted on both ZrPs (shown in
The dispersion may also be washed with acetone three times to remove water, followed by toluene wash for three times. This process may lead to the dispersion of ZrP nanosheets in toluene for further synthesis of modified ZrP nanosheets.
As illustrated in the schemes below, three approaches were employed in this embodiment to make the functionalized α-ZrP nanosheets.
ATR/FT-IT data suggested that two silane compounds (i.e. N-[3-(Trimethoxysilyl)propyl]ethylenediamine and Triethoxy-3-(2-imidazolin-1-yl)propylsilane) can functionalize the ZrP nanosheet (denoted ZrP(A) and ZrP(I)) through condensation reaction. Such functionalized ZrP nanosheets may be used to coat on one or both sides of AEMs.
Mg3Al-LDH nanosheets with composition of Mg3Al(OH)2—CO3 were synthesized through a two-step procedure, slightly modified from Cermelj's method
150 mmol of MgCl2 6H2O, 50 mmol AlCl3 6H2O and 1000 mmol of urea were dissolved in 800 mL distilled water to form Solution A. 48 mmol of stearic acid was dissolved in 200 mL distilled water at 80° C. and the pH was adjusted to 10 by dropwise addition of 4 M NaOH to form Solution B. Solution A was added to Solution B dropwise over the period of 1 h with vigorous stirring. The resulting reaction mixture was aged on a hot plate heated at 95° C. for 18 h with stirring. After aging, the Mg3Al-LDH products were washed three times with a mixture of ethanol and DI water (1:1 by weight) at 80° C., followed by drying overnight in an oven at 60° C.
20 grams of the dried Mg3Al-LDH powders prepared by the above homogeneous precipitation was dispersed in 50 mL of ethanol via mixing at 2400 rmp for 5 minutes. The dispersion was then transferred into a 100 mL Teflon liner within a stainless steel outer vessel. The solvothermal treatment was carried out at 150° C. for 48 h. After cooling to the room temperature, Mg3Al-LDH product was washed with 200 mL of ethanol, followed by drying overnight in an oven at 60° C.
LDH nanosheet dispersion with other composition may also be prepared with similar processes. Dispersion of Mg3Al-LDH nanosheets can be done but not limited to the following procedure. 1) 50 mg of the above dried Mg3Al-LDH nanosheets was added to 10 mL of N-Methyl-2-pyrrolidone (NMP) in a glass vial and sonicated for 5 minutes. The dispersion shows translucent and is stable for at least 6 hours. 2) 50 mg of the above dried Mg3Al-LDH nanosheets was added to 10 mL of 1-propanol in a glass vial and sonicated for 5 minutes. The dispersion shows milky and is stable for at least 2 hours. 3) 50 mg of the above dried Mg3Al-LDH nanosheets was added to 10 mL mixture of N-Methyl-2-pyrrolidone (NMP) and ethanol (1:9 by weight) in a glass vial and sonicated for 5 minutes. The dispersion shows translucent and is stable for at least 3 hours.
Theoretically, ZrP nanosheet could be coated on the precursor film followed by quaternization or on the AEM after quaternization. Preliminary tests suggest that the precursor film is hydrophobic and does not work well with 0.5% ZrP nanosheet dispersion in water. Most of our coating will be carried out on AEM directly after quaternization. In our coating process, dilute ZrP solution was casted on one side of the AEM in a fixture followed by slow drying in a vacuum oven at 65° C. for 24 hrs. Other processes such as spray coating may also be used to obtain laminar nanosheet coating layer. Unlike particular particles, nanosheets may stack on the substrate forming multi-layered laminar structure.
Commercial F AEM and Commercial X AEM were procured. Their oxidation resistances of these AEMs along with our benchmark polymeric AEM (before coating) were measured using the procedure as shown in oxidation stability test section. As shown in Table 2, commercial F AEM lost 35%˜40% of its weight due to Fenton reagent oxidation and commercial X AEM lost over 70% of its weight due to Fenton reagent oxidation, while uncoated benchmark polymeric AEM lost 20% of its weight due to Fenton reagent oxidation. All polymeric AEMs exhibited substantial oxidation weight loss, which corresponds to short durability in applications such as AEMFCs and AEM electrolyzers.
After the coating process was successfully developed. AEM samples were coated with Pristine ZrP nanosheet layer followed by Nafion coating with ultrasonic spray nozzle. Each coating layer thickness can be well controlled by the number of coatings.
As discussed in Section 7.3 below, such ZrP-AEM exhibited excellent oxidation resistance.
Similarly, functionalized ZrP nanosheets were coated on the polymer AEM, which also demonstrated superior oxidation resistance as shown on Table 3.
The oxidative stability of both polymer AEM and composite AEM was evaluated under an accelerated degradation test using a Fenton's solution at 80° C. During oxidation test, hydroxyl radicals attack side chains and functional groups first, which could lead to degradation of the functional groups. Some researchers suggested that it is possible that polymer backbone could be subject to oxidative degradation as well.
When compared with the pristine ZrP nanosheet (NS) coated AEM, the functionalized ZrP coated AEM showed the better oxidation stability. The functionalized ZrP coated AEM (one side or both sides) exhibited much higher oxidation stability (6% and 2% weight loss). As compared with a benchmark polymeric AEM, coated AEM exhibits far superior oxidation resistance (2% weight loss vs. 20% weight loss).
Dispersion of Mg3Al-LDH nanosheets prepared in example 3 was employed for coating AEM on single side and both sides followed by polymer binder coating. The resulting composite AEM exhibited much improved oxidation resistance as shown in Table 4 in the Fenton reagent oxidation weight loss test. The results confirmed our invention that the inorganic nanosheet layer does provide strong oxidation resistance, and can provide long durability in various applications, such as AEMFCs, AEM electrolyzer and other photoelectrochemical apparatus.
Persons skilled in the art will also appreciate that the present invention is not limited to only the embodiments described. Persons skilled in the art will also appreciate that the membranes and apparatuses of the present invention may be implemented in other ways than those described herein. Such modifications are within the scope of the present invention.
This application claims the benefit of U.S. Provisional Application No. 63/133,545, filed on Jan. 4, 2021, the disclosure of which is incorporated herein by reference in its entirety for all purposes.
The invention was made with the U.S. government support under N00014-12-C-0565 and DE-SC0020599. The Federal government may have certain rights in the invention.
| Number | Date | Country | |
|---|---|---|---|
| 63133545 | Jan 2021 | US |
