Patent Application
20240173678
Flow batteries have attracted great attention as a large-scale energy storage technology because of their unique feature of uncoupling power and energy, which allows the expansion of energy storage by increasing the volume or concentration of the electrolyte. Compared with other electrochemical technologies, such as lead-acid, lithium-ion, and sodium-based batteries, flow batteries have the merits of long life, high safety, and low cost. The ion-selective membrane is an important part of flow batteries because it allows specific ions to pass through to maintain neutral charge in the cell and prevents the crossover of active species to maintain the capacity. The properties of the ion selective membrane significantly determine the performance of the flow battery. Ideal ion-selective membranes should have high ion conductivity and selectivity, high chemical stability, good mechanical strength, and low cost.
Based on the ion-selective mechanisms, ion-selective membranes can be classified into ion-exchange and porous membranes. The ion-exchange membranes contain anion (cation exchange membranes) or cation (anion exchange membranes) groups, which provide ion conductivity and selectivity based on the Donnan exclusion mechanism.
NAFION™ membranes are typical cation exchange membranes that are widely used in flow batteries (NAFION™ is a trademark of The Chemours Company FC, LLC). NAFION™ membranes have polytetrafluoroethylene main chains and side chains with superacidic sulfonic acid groups. Hydrophilic sulfonic acid groups constitute ion-transmission channels that provide excellent cation conductivity. However, the heavy crossover issue owing to the large amount of cation active species and extremely high cost hinders the promotion of NAFION™ membranes.
Sulfonated poly(ether ether ketone) (SPEEK) is an attractive ion exchange membrane with relatively low cost and high stability. Compared with NAFION™, SPEEK has higher ion selectivity owing to the less acidic sulfonic acid groups and hydrophobic PEEK backbone; however the ion conductive channels are relatively tortuous. To achieve sufficient ion conductivity, a high sulfonation degree of SPEEK is needed, which may cause heavy swelling, decrease stability, and reduce ion selectivity.
Porous membranes prepared by the non-solvent induced phase separation (NIPS) method have been adopted as ion selective membranes in flow batteries in the last decade because of their low cost and high ion conductivity. The ion selectivity is mainly based on the pore size exclusion mechanism. These membranes exhibit asymmetric finger-like pores in the vertical direction and thin skin layers on the top and bottom surfaces. The vertical finger-like pores provide excellent ion conductivity, and the ion selectivity is mainly ascribed to the pore morphology in the skin layer. In particular, a dense and thick skin layer should be formed to achieve high ion selectivity; however, this approach also decreases the size and density of the vertical pores, which is detrimental to the ion conductivity of the membrane.
Traditional ion-exchange and porous membranes are based on a single-ion selective mechanism that requires a tradeoff between ion selectivity and conductivity.
Disclosed herein is a double-layer ion-selective membrane, method of production, applications of the double-layer ion-selective membrane as a redox flow cell, a fuel cell, and used for wastewater and air purification. The double-layer ion-selective membrane is comprised of a polyetherimide (PEI) layer and an ultrathin layer comprising porous boron nitride (PBN) flakes enmeshed by a perfluorinated sulfonic acid ionomer resin (NAFION™ resin). The double layer membrane exhibits ion-selectivity and ion-conductivity enabling the membrane to be used in a redox flow cell battery, a fuel cell battery, and to be used in wastewater and air purification applications.
Overall, the double-layer design with a low-cost PEI supporting layer and ultrathin (μm) PBN ion-selective layer significantly reduced the cost compared with the NAFION™ 115 membrane, which is beneficial to the commercialization and promotion of PBN-PEI membranes.
The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.
The foregoing will be apparent from the following more particular description of example embodiments, including those illustrated in the drawings. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating embodiments.
A description of example embodiments follows.
In a first aspect, the application pertains to a double-layer ion-selective membrane, comprising or consisting of: a polyetherimide (PEI) layer having longitudinal unimpeded finger-like pores, and an ultrathin layer comprising porous boron nitride (PBN) flakes defining a tortuous path and enmeshed by a NAFION™ resin that forms proton transfer channels created by the sulfonic acid groups of the NAFION™ resin, the ultrathin layer coated on an open pore end of the PEI layer.
The double-layer ion-selective membrane of the disclosure is inspired by the features of different ion-selective membranes and porous boron nitride (PBN). The double-layer ion-selective membrane possesses a unique PBN bifunctional ion-selective layer and an ion-conductive porous polyetherimide (PEI) layer. The PEI layer is designed to provide a low-cost, yet highly functional conductive layer, while the PBN layer provides an ion selective layer at a low-cost while overcoming ion conductivity and selectivity issues compared to conventional ion-selective membranes.
The PEI layer serves a number of functional roles. It provides a supporting or mechanical layer for the ultrathin PBN layer, and is able to withstand strongly acidic environments, as is necessary for use in redox flow batteries, for example. As such, the thickness of the PEI supporting layer should be from about 10 μm to about 1000 μm, and in some embodiments from about 50 μm to about 200 μm, and yet in other embodiments from about 95 μm to about 105 μm. The thickness of the PEI layer also contributes to the functional properties of the double-layer ion-selective membrane.
Depending upon the intended use of the double-layer ion-selective membrane, the PEI layer can be designed with a thickness that is optimal for the intended use. In some embodiments, the ions conducted by the PEI layer may be protons and vanadium ions.
In other embodiments, such as in the air purification, the ions can be removed by the double layer membrane may be Hydrogen ions (H+), Hydronium ions (H3O+), Hydroxide ions (OH−), Ammonium ions (NH4+), Sodium ions (Na+), Chloride ions (Cl−), and Silver ions (Ag+). In the water purification, the ions can be removed by the double layer membrane can be calcium (Ca2+) and magnesium (Mg2+) ions, Chloride ions (Cl−), Nitrate (NO3−) and sulfate (SO42−ions, Sodium (Na+) and potassium (K+) ions, Ammonium (NH4+) ions, Heavy metal ions like copper (Cu2+), lead (Pb2+), chromium (Cr3+/Cr6+), Fluoride (F−) ions.
The PEI layer has unimpeding finger-like pores that extend across the thickness of the layer in the longitudinal plane, as illustrated in for example,
The porous boron nitride (PBN) is an ultrathin layer that is mechanically supported on the PEI layer. Nanoporous boron nitride (PBN) offers a high nanoporosity with superior chemical and thermal stability, high thermal conductivity, and excellent electrical insulating properties. The unique features of PBN provide excellent ion selectivity based on pore size exclusion mechanism and great stability in tough and high oxidation environments and prevent the short circuit in the battery.
An ultrathin layer for purposes of this disclosure should have a thickness of from about 2 μm to about 8 μm. In some embodiments, the PBN layer thickness is about 2 μm, about 3 μm, about 4 μm, about 5 μm, about 6 μm, about 7 μm, about 8 μm. In yet other embodiments the PBN layer thickness is from about 4 μm to about 5 μm. The ultrathin PBN layer is comprised of or consists of porous boron nitride (PBN) flakes that are assembled to create a tortuous path to selectively prevent or permit flow of ions, by size exclusion for example. The manner of assembly of the PBN flakes is shown in exemplary
The PBN layer further comprises or consists of a NAFION™ resin enmeshed with the PBN flakes. The NAFION™ resin has a polytetrafluoroethylene main chains and side chains of super-acidic sulfonic acid groups. An exemplary artistic rendering of the enmeshed NAFION™ resin and PBN flakes is illustrated in
The ratio of PBN flakes to NAFION™ resin can vary depending upon the desired proprieties of the PBN layer, the degree of adherence to the PEI layer, and the degree of bonding of NAFION™ resin to the PBN flakes. In one embodiment, the ratio of PBN to NAFION™ resin is about 25: about 75, about 50: about 50, about 75: about 25 percent by weight. In another embodiment, the ratio of PBN flakes to NAFION™ resin is about 50: about 50 percent by weight. In embodiments, it is desirable to achieve a uniform coating of the PBN flakes and NAFION™ resin layer on the PEI support. This can be achieved by manipulating the ratio of PBN flakes and NAFION™ resin, and/or regulating the thickness of the PBN/NAFION™ layer on the PEI layer, depending upon the intended use of the PBN/NAFION™ layer. High amounts of NAFION™ resin may result in a clogged porous structure, while low amounts of NAFION™ can result in low ion conductivity.
Not being bound to any one theory, the mechanism for the PBN/NAFION™ layer for ion conductivity is based upon the combination of both a vehicle mechanism and the Grotthuss mechanism. The Grotthuss mechanism works by transporting protons through proton hopping, where a proton will move from a proton donor to a proton acceptor within a hydrogen bond network. The NAFION™ resin contains superacidic sulfonic acid (—SO3H) groups which can act as both a proton donor and proton acceptor. Large amounts of these groups are found in NAFION™ and are at the surface when enmeshed with the PBN flakes, which facilitates the ion conductivity in part for the PBN/NAFION™ layer. The vehicle mechanism involves proton diffusion with the carrier to transport acidic media solvated ions and is dependent upon the concentration of ions present in the system. PBN and the surface sulfonic acid groups increase the water absorption by the membrane, enabling a better pathway for proton transfer through the vehicle mechanism. The existence of abundant ion-exchange groups, interconnectivity of the ionic clusters due to the extensive hydrogen bonding, and the acidity of the sulfonic acid groups enhances the proton conductivity.
In some embodiments, the PBN flakes/layer can be functionalized with proton donors and proton acceptors. In some embodiments, the PBN flakes/layer can be functionalized with hydroxyl and amino groups. Incorporation of these additional groups will facilitate proton transfer more rapidly by further interconnecting the hydrogen bonding network.
Together the thickness of the double-layer ion-selective membrane can be from about 12 μm to about 300000 μm, and in some embodiments from about 50 μm to about 500 μm, and yet in other embodiments from about 95 μm to about 115 μm. The thickness of the double-layer ion-selective membrane will ultimately be determined by the intended use. For example, as a membrane in a membrane flow battery, the desired thickness is from about 95 μm to about 115 μm. For use in a fuel cell, the desired thickness is from about 100 mm to about 300 mm. For use in wastewater treatment, the desired thickness is from about 100 μm to about 1000 μm.
In a second aspect, the disclosure pertains to methods of making the double-layer ion-selective membrane, as discussed above. The method comprises or consists of dispersing flakes of porous boron nitrate (PBN) and NAFION™ resin together in a solvent to produce a sprayable suspension of PBN flakes and NAFION™ resin; and spray coating the suspension of PBN flakes and NAFION™ resin in an amount sufficient to coat an ultrathin layer on an open pore side of a polyetherimide (PEI) membrane having longitudinal unimpeded finger-like pores, to produce a double-layer ion-selective membrane. The double-layer ion-selective membrane comprises: a polyetherimide (PEI) layer having longitudinal unimpeded finger-like pores, and an ultrathin layer comprising porous boron nitride (PBN) flakes defining a tortuous path and enmeshed by a NAFION™ resin that forms proton transfer channels created by the sulfonic acid groups of the NAFION™ resin, the ultrathin layer coated on an open pore end of the PEI layer.
A polyetherimide (PEI) membrane was prepared through a non-solvent induced phase separation (NIPS). The NIPS method is performed using a polymer, solvent, and a nonsolvent to fabricate membranes by controlling the interaction between the polymer and solvent(s) of interest. Membranes prepared by NIPS typically show a dense surface with an asymmetric morphology. Using this method enables the finger-like pores to be prepared in the PEI membrane, which may be attributed to improving the flow of protons and hence the high ion conductivity. The thickness of the PEI layer is dependent upon its intended use, for which the thickness is described above.
PBN was synthesized using a one-step template-free method and sonication in isopropanol. A porous boron nitride (PBN) layer was synthesized using boric acid, urea, and water, with further treatments of heating and grinding to prepare pristine PBN. In some embodiments, the PBN was further crystalized by suspending in isopropanol and sonicating. The PBN is combined with NAFION™ resin in uniform suspension, such that the NAFION™ sulfonic acid groups can interact with ions to facilitate ion transfer. The ratios of the PBN to NAFION™ are dependent upon the desired properties of the PBN layer, the degree of adherence to the PEI layer, and the degree of bonding of NAFION™ resin to the PBN flakes. Each of these attributes are described above in the membrane section. The PBN flake/NAFION™ resin suspension is sprayed on top of the PEI membrane using an airbrush until the surface becomes wet, and then treated with acid and heated. This treatment suppresses the movement of the polymer chains and prevents swelling of the membrane.
Compared with the most common ion selective membrane (NAFION™ 115 membrane) the PBN-PEI membrane of the disclosure exhibited lower area resistance (0.165 Ωcm2 Vs 0.210 Ωcm2), lower vanadium permeability (4.27*10−7 cm2/min Vs 9.33*10−7 cm2/min), and high ion selectivity (14.89 107 mS cm−3 min VS 6.48 107 mS cm−3 min). Further, the double-layer design with a low-cost PEI supporting layer and ultrathin PBN ion-selective layer significantly reduced the cost compared with the NAFION™ 115 membrane, which is beneficial to the commercialization and promotion of PBN-PEI membranes.
In another aspect, the double-layer ion-selective membranes of the disclosure can be used in fuel cells. The fuel cell of the disclosure comprises an anode, a cathode, a double-layer ion-selective member and an electrolyte between the anode and the cathode and incorporating fuel sources separately entering on the anode and cathode sides.
A fuel cell is an electrochemical cell that converts chemical energy of a fuel and an oxidizing agent into electricity through a pair of redox reactions. Fuel cells are different from most batteries in requiring a continuous source of fuel and oxygen to sustain the chemical reaction. In some embodiments, the PBN-PEI membrane as described in the membrane section may be used as a component of a proton-exchange membrane fuel cell (PEMFC) as the PBN-PEI membrane allows for the continuous flow of protons through the pores. An electrolyte, along with a cathode and anode will also comprise the PEMFC in order to have the proton exchange occur while using oxygen as an oxidizing agent and a form of chemical energy fuel to generate electricity. The anode will be connected to the membrane on one side with the cathode on the other side of the membrane, with both anode and cathode allowing for fuel flow and oxidant flow, respectively. The anode side has the fuel diffuse to the anode catalyst where it dissociates into protons and electrons. The protons will be conducted through the PBN-PEI membrane to the cathode, but the electrons will travel to an external circuit, which supplies power, as the membrane is electrically insulating. The cathode catalyst reacts oxygen molecules with the electrons and protons to form water. Though not bound to a single theory, this is possible due to the PBN-PEI membrane being hydrolytically stable.
In yet another aspect, the double-layer ion-selective membranes of the disclosure can be used for wastewater treatment and air purification.
In an embodiment, the disclosure pertains to a wastewater purification system, such as a wastewater purification system, comprising the membranes described herein, wherein the membrane separates water from an aqueous solution through forward osmosis to recover water that is purified.
The PBN-PEI membrane as described herein can be used for wastewater treatment. This will be accomplished by connecting an anode and cathode to the PBN-PEI membrane and allowing wastewater to be chemically oxidized, removing organic and some inorganic impurities from the wastewater, and moving the oxidized byproducts into the PBN-PEI membrane. Removal of the impurities in the wastewater releases clean water from the cell. For water purification, the membrane thickness and porosity will be further tuned and adjusted based on the characteristics of water system.
In another embodiment, the disclosure pertains to an air purification system, comprising the membranes described herein, wherein the membrane separates polluted air by trapping impurities in the membrane and releasing purified air. This strategy can be further used for air purification, where the PBN-PEI membrane can capture particulates in the air while allowing air to pass through the membrane, removing impurities in the air. For air purification, the membrane thickness and porosity will be further tuned and adjusted based on the characteristics of the air system.
In another aspect, the double-layer ion-selective membranes of the disclosure can be incorporated into redox flow batteries. The redox flow batteries of the disclosure comprise an anode, a cathode, a double-layer ion-selective member between the anode and the cathode, and an electrolyte that interacts with the anode, cathode and the membrane.
In one embodiment of this aspect, the double-layer ion selective membranes can be incorporated into vanadium redox flow batteries. As described in the exemplification section, the electrochemical performance of the membrane was evaluated in a vanadium redox flow battery (VRFB). The nano-sized and tortuous pores of the PBN flakes can effectively block the crossover of vanadium ions and provide excellent ion selectivity based on the pore size exclusion mechanism. Furthermore, the super-acidic sulfonic acid groups of NAFION™ decorated on the nanoporous structure of PBN provide high-speed proton transfer channels that increase proton conductivity through both Grotthuss and vehicle mechanisms. The vanadium flow battery with the PBN-PEI membrane exhibited higher discharge capacity, Coulombic efficiency, voltage efficiency, and energy efficiency compared with the battery with the most common ion selective membrane (NAFION™ 115 membrane). The membrane achieved a high Coulombic efficiency of about 97.16% and outstanding energy efficiency of about 91.00% at 40 mA cm−2 with a stable cycling performance for over 700 cycles at about 100 mA cm−2.
It is to be understood that the terminology used herein is for describing particular embodiments only and is not intended to be limiting. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the disclosure pertains.
Although any methods and materials similar or equivalent to those described herein may be used in the practice for testing of the present disclosure, exemplary materials and methods are described herein.
When a list is presented, unless stated otherwise, it is to be understood that each individual element of that list, and every combination of that list, is a separate embodiment. For example, a list of embodiments presented as “A, B, or C” is to be interpreted as including the embodiments, “A,” “B,” “C,” “A or B,” “A or C,” “B or C,” or “A, B, or C.”
As used in this specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the content clearly dictates otherwise. The conjunctive term “and/or” between multiple recited elements is understood as encompassing both individual and combined options. For instance, where two elements are conjoined by “and/or,” a first option refers to the applicability of the first element without the second. A second option refers to the applicability of the second element without the first. A third option refers to the applicability of the first and second elements together. Any one of these options is understood to fall within the meaning, and therefore satisfy the requirement of the term “and/or” as used herein. Concurrent applicability of more than one of the options is also understood to fall within the meaning, and therefore satisfy the requirement of the term “and/or.”
Unless the context requires otherwise, throughout the specification and claims that follow, the word “comprise” and synonyms and variants thereof such as “have” and “include”, as well as variations thereof, such as “comprises” and “comprising”, are to be construed in an open, inclusive sense, e.g., “including, but not limited to.” The transitional terms “comprising,” “consisting essentially of,” and “consisting of” are intended to connote their generally accepted meanings in the patent vernacular; that is, (i) “comprising,” which is synonymous with “including,” “containing,” or “characterized by,” is inclusive or open-ended and does not exclude additional, unrecited elements or method steps; (ii) “consisting of” excludes any element or step not specified in the claim; and (iii) “consisting essentially of” limits the scope of a claim to the specified materials or steps “and those that do not materially affect the basic and novel characteristic(s)” of the claimed invention. Embodiments described in terms of the phrase “comprising” (or its equivalents) also provide as embodiments those independently described in terms of “consisting of” and “consisting essentially of.”
“About” means within an acceptable error range for the particular value as determined by one of ordinary skill in the art, which will depend in part on how the value is measured or determined, i.e., the limitations of the measurement system. Unless explicitly stated otherwise within the disclosure, claims, result or embodiment, “about” means within one standard deviation per the practice in the art, or can mean a range of ±20%, ±10%, ±5%, ±4, ±3, ±2 or ±1% of a given value. It is to be understood that the term “about” can precede any particular value specified herein, except for particular values used in the Examples. For example, “about” can mean a standard of deviation of ±1 μm, ±2 μm, ±3 μm, ±4 μm, ±5 μm, 6 μm, ±7 μm, ±8 μm, ±9 μm, ±10 μm for the membrane thickness.
“Ultrathin” describes an embodiment with a thickness of 10 nm-10000 nm.
“Tortuous” pores or “tortuosity” is defined as predicting transport properties of porous media such as rocks, soils, and membranes. The term describes pores microstructures and refers to the ratio of the diffusivity in the free space to the diffusivity in the porous medium. The effective diffusivity is proportional to the reciprocal of the square of the geometrical tortuosity. Further, a more “tortuous” path is described to separate mixtures over a longer period of time.
“Enmesh” means to entrap or entangle one material with another material such that the materials are not easily separated. An example of enmeshing includes but is not limited to NAFION™ being intermixed with PBN.
“Bifunctional” means an embodiment that contains multiple purposes or functions. An example of bifunctional includes but is not limited to the mixed PBN and NAFION™ layer that is ion-selective and ion-conductive.
“NAFION™ ” is a brand name for sulfonated tetrafluoroethylene-based fluoropolymer-copolymer, also known as perfluorinated sulfonic acid ionomers. NAFION™ is synthesized by copolymerization of tetrafluoroethylene and a derivative of a perfluoro(alkyl vinyl ether) with sulfonyl acid fluoride, which can be prepared by the pyrolysis of its respective oxide or carboxylic acid to give an olefinated structure. NAFION™ membranes are typical cation exchange membranes that are widely used in flow batteries. NAFION™ has polytetrafluoroethylene main chains and side chains with super-acidic sulfonic acid groups. “Super-acidic” is defined as an acid with an acidity greater than pure sulfuric acid, which has a Hammett acidity function of (H0) of −12. Examples of super-acids include but are not limited to trifluoromethanesulfonic acid and fluorosulfuric acid. NAFION™ membranes are commonly categorized in terms of their equivalent weight (EW) and thickness. For example, NAFION™ 117 indicates an extrusion-cast membrane with 1100 g/mol EW and 0.007 inches (7 thou) in thickness.
All percents are intended to be weight percent unless otherwise specified. The present disclosure is not to be limited in scope by the specific embodiments described herein. Indeed, other various embodiments of and modifications to the present disclosure, in addition to those described herein, will be apparent to those of ordinary skill in the art from the foregoing description and accompanying drawings. Thus, such other embodiments and modifications are intended to fall within the scope of the present disclosure. Further, although the present disclosure has been described herein in the context of a particular implementation in a particular environment for a particular purpose, those of ordinary skill in the art will recognize that its usefulness is not limited thereto and that the present disclosure may be beneficially implemented in any number of environments for any number of purposes. Accordingly, the claims set forth below should be construed in view of the full breadth and spirit of the present disclosure as described herein.
Conventional ion-selective membranes, e.g., ion-exchange and porous membranes, are unable to perform high conductivity and selectivity simultaneously due to the contradictions between their ion selecting and conducting mechanisms. In this work, a bifunctional layer was developed via the combination of nanoporous boron nitride (PBN) and ion exchange groups from NAFION™ to achieve high ion conductivity through dual ion conducting mechanisms as well as high ion selectivity. A one-step template-free method was adopted to synthesize PBN flakes. PBN is dispersed in isopropanol and functionalized simultaneously by sonication. PBN is further enmeshed with NAFION™ resin to form the bifunctional layer coated onto a porous polyetherimide membrane. The double-layer membrane exhibits excellent ion selectivity (1.49×108 mS cm−3 min), which is 22 times greater than that of the pristine porous polyetherimide membrane, with maintained outstanding ion conductivity (64 mS cm−1). In a vanadium flow battery, the double-layer membrane achieves a high Coulombic efficiency of 97% and outstanding energy efficiency of 91% at 40 mA cm−2 with a stable cycling performance for over 700 cycles at 100 mA cm−2. PBN with ion exchange groups may therefore offer a potential solution to the limitation between ion selectivity and conductivity in ion-selective membranes.
Introduction. Flow batteries have attracted considerable attention as a large-scale energy storage technology because of their unique feature of uncoupling power and energy, which allows the expansion of energy storage by increasing the volume or concentration of the electrolyte.[1] Compared with other electrochemical technologies, such as lead-acid, lithium-ion, and sodium-based batteries, flow batteries have the advantages of long life, high safety, and low cost.[2] An ion-selective membrane is an essential part of flow batteries. On the one hand, the membrane allows specific ions to pass through, maintaining a neutral charge in the cell.[3] On the other hand, it prevents the crossover of active species to maintain the capacity. The properties of the ion-selective membrane have a significant impact on the flow battery's performance. The ideal ion-selective membranes should have high ion conductivity and selectivity, high chemical stability, good mechanical strength, and low cost.[4]
Based on their ion-selective mechanisms, conventional ion-selective membranes can be classified into ion-exchange and porous membranes.[4-5] The ion-exchange membranes contain anion (cation exchange membranes) or cation (anion exchange membranes) groups, allowing ion conductivity and selectivity based on the Donnan exclusion mechanism.[5b] The NAFION™ membrane is one of the most common cation exchange membranes used in flow batteries. NAFION™ consists of polytetrafluoroethylene main chains and side chains containing superacidic sulfonic acid groups.[6] Hydrophilic sulfonic acid groups constitute ion-transmission channels that provide excellent cation conductivity. However, NAFION™ membranes have a heavy crossover issue due to the large amount of cation active species involved, as well as the high cost of the NAFION™ membranes hindering the commercial application.[3, 7] Sulfonated poly (ether ether ketone) (SPEEK) is an attractive ion exchange membrane with relatively low cost and high stability.[8] In comparison with NAFION™, SPEEK has a higher ion selectivity because of the less acidic sulfonic acid groups and hydrophobic PEEK backbone; however, the ion conductive channels are relatively tortuous.[9] Normally, a high sulfonation degree of SPEEK is required in order to achieve sufficient ion conductivity. However, this can lead to swelling, decreases stability, as well as reduced ion selectivity[6, 9]. Over the past decade, porous membranes prepared through non-solvent induced phase separation (NIPS) have been used as ion-selective membranes in flow batteries because of their low cost and high ion conductivity.[10] The ion selectivity is governed by the pore size exclusion mechanism. These membranes exhibit asymmetric finger-like pores in the vertical direction with thin surface layers on the top and bottom of the membranes. Vertical finger-like pores provide excellent and unhindered ion transfer, and the ion selectivity is mainly attributable to the pore morphology in the surface layer.[10] Ideally, a dense and thick surface layer should be formed to achieve high ion selectivity, but in doing so it also reduces the size and density of the vertical pores, which is detrimental to the ion conductivity of the membrane.[10c] Traditional ion-exchange and porous membranes are both based on a single ion-selective mechanism which requires a trade-off between ion selectivity and conductivity.
Boron nitride (BN), also known as white graphite, exhibits superior chemical and thermal stability, high thermal conductivity, and excellent electrical insulating properties.[11] Further to the advantages of BN, porous BN (PBN) has a unique ability to adjust nanoporosity, thus being useful for multiple applications, such as absorption, separation, and chemical conversion.[12] With its high nanoporosity and stability, PBN has the potential to be an attractive material for ion selective. PBN can be obtained through three bottom-up methods: chemical blowing,[13] template-based,[14] and template-free[15] techniques. The template-free approach has several advantages over chemical blowing or template-based approaches, including a simple synthesis procedure, low costs, and relatively low toxicity.[16] PBN is synthesized by reacting boron-containing and excess nitrogen-containing precursors at high temperatures. Nanopores are formed through the decomposition and release of excess nitrogen precursors during the synthesis process.[15f] The porosity and morphology of PBN can be tailored by varying different types or proportions of precursors and changing reaction conditions.[15b, 15e]
Inspired by the features of different ion-selective membranes and PBN, a double-layer ion-selective membrane with a unique PBN bifunctional ion-selective layer on a low-cost and highly ion-conductive porous polyetherimide (PEI) layer was developed. As a first step, PBN was synthesized using a template-free method with a stacked flake-like nanoporous structure. The hydrophilicity and crystallinity of the PBN flakes were significantly increased after sonication of the PBN flakes in isopropanol, as showed by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA), and contact angle measurements. The nanoporosity of PBN was characterized using the N2 adsorption-desorption technique and calculated using non-local density functional theory (NLDFT). PBN was further mixed with NAFION™ resin and spray-coated onto the porous PEI membrane prepared by the NIPS method to form a PBN-PEI double-layer membrane. The morphology and properties of the PBN-PEI membrane were investigated in order to identify its ion-selective and ion-conductive mechanisms. Furthermore, the electrochemical performance of the membrane was evaluated in a vanadium redox flow battery (VRFB). The unique PBN bifunctional ion-selective layer with the nanoporous structure and ion-exchange groups overcomes the limitation between ion conductivity and selectivity in conventional ion-selective membranes. More importantly, the overlap of the novel bifunctional PBN ion selective layer with the low-cost high ion-conducting PEI layer demonstrated superior performance due to its ability to integrate the advantages of both layers.
Results and discussions.
1Calculated according to the price listed on Quatzy.com.
2 The yield of PBN is calculated by 50%
3Calculated according to the price from Ion Power Inc. for 0.09 m2 NAFION ™ 115 membrane
For the preparation of the low-cost and highly ion-conductive PEI membrane using the NIPS method, 75 wt. % 1-methyl-2-pyrrolidinone was added as the non-solvent phase. In addition, polyvinylpyrrolidone (PVP) was added to further improve ion conductivity.[10d] The porous PEI membrane presented a smooth and flat surface (
PBN was synthesized by the template-free method with a precursor of boric acid and excess urea through the reactions as shown in Equations (1), (2), and (3):[12c, 15c, 15f]
2H3BO3≙B2O3+3H2O↑ (1)
NH2CONH2≙NH3↑+HNCO↑ (2)
B2O3+NH3≙2BN+3H2O↑ (3)
The nanoporosity is attributed to the generation and release of gaseous products during the reaction.[15f] As shown in the SEM image (
PBN flakes were obtained by grinding and sonicating pristine PBN in isopropanol, followed by centrifugation and filtration. In addition, PBN was also surface functionalized during the sonication process. PBN was compared before and after treatment, using SEM, XRD, FTIR, and TGA results to investigate the effects and mechanisms of treatment.
According to the FTIR spectra shown in (
TGA was performed to further verify the functionalization of PBN. Compared to commercial hBN, pristine PBN showed an approximate weight loss of 4.8% as a result of the thermal degradation of some amorphous BN after 300° C. Nevertheless, the treated PBN lost 4.9% of its weight loss before 300° C., indicating the functional groups had successfully been introduced (
The pore size distribution (PSD) and porosity of the PBN were evaluated by the N2 adsorption-desorption method. Based on the standards of the International Union of Pure and Applied Chemistry (IUPAC), both pristine and treated PBN exhibited type I and IV isotherms with type H3 and H4 hysteresis loops (
PBN was further mixed with NAFION™ resin as a binder and ion exchange group supplier to form the bifunctional ion-selective layer, as shown in
PBN-PEI membranes were characterized to determine their composition, hydrophilicity, water uptake, swelling ratio, ion conductivity, ion selectivity, and mechanical strength. Based on the FTIR spectrum (
The mechanical properties of the membrane strongly influenced long-term stability. Good mechanical properties such as high tensile strength and ductility help to prevent membrane deterioration resulting from structural damage during use. The stress-strain curves for the pristine PEI, PBN-PEI, and NAFION™ 115 membranes are shown in
Ion conductivity and selectivity are the most significant characteristics of ion-selective membranes, which determine the area resistance and ion permeability of the membrane, thereby affecting the electrochemical performance of the flow battery. The high-frequency impedances of all membranes were measured using electrochemical impedance spectroscopy (
Using the morphology and properties of the PBN-PEI membranes, a relationship between the PBN ratio and ion conductivity was examined. NAFION™ resin with super acidic sulfonic acid groups was successfully introduced into the PBN structure in all the PBN layers. It was found that when the PBN ratio was too low, i.e., with a high NAFION™ content, the NAFION™ resin clogged the porous structure. Consequently, the rigid structure of PBN could not provide additional space for the sulfonic acid groups to uptake water, resulting in limited proton conductivity.[23] With a medium PBN ratio, the PBN structure was still well enmeshed by the NAFION™ resin with sufficient space for sulfonic acid groups to uptake water. This resulted in excellent proton conductivity based on both the vehicle and Grotthuss mechanisms.[17] With an excessively high PBN ratio, the limited sulfonic acid groups and hydrophobicity did not provide high proton conductivity. Due to the nonporous layers in the NAFION™-PEI membrane, proton transfer channels based on pores were blocked, resulting in a decrease in ion conductivity. Hence, the PBN layer with an appropriate PBN ratio was able to efficiently utilize the ion-exchange groups from the NAFION™ resin and maintained its porous structure to provide excellent ion conductivity.
The vanadium (IV) permeabilities of all the membranes were calculated based on the results of the vanadium penetration test (
The ion selectivity of the membranes was calculated, as shown in
Since the excellent properties of the 50% PBN-PEI membrane, the comprehensive electrochemical performance of the membrane was further evaluated in the VRFB and compared with those of the PEI, NAFION™-PEI, and NAFION™ 115 membranes. The discharge capacities are shown in
The Coulombic efficiency (CE), voltage efficiency (VE), and energy efficiency (EE) of the PEI, 25% PBN-PEI, 50% PBN-PEI, 75% PBN-PEI, NAFION™ 115, and NAFION™-PEI are listed in Table 3. The rate performance of 25% PBN-PEI and 75% PBN-PEI membranes were plotted in
In order to investigate the electrochemical performance of the 50% PBN-PEI membrane, the charge and discharge profiles at different current densities were plotted (
In order to investigate the stability of the 50% PBN-PEI membrane, a battery assembled with the membrane was continuously cycled for over 700 cycles at a current density of 100 mA cm−2. As shown in
Conclusion. A highly efficient double-layer ion-selective membrane was obtained by integrating the unique properties of porous boron nitride (PBN) with the advantages of the porous polyetherimide (PEI) membranes and NAFION™ resin. High-nanoporosity PBN flakes were synthesized by a scalable template-free method through dispersion and functionalization by sonication in isopropanol. During sonication, the B—N bonds near the edges or defects in the amorphous part of PBN were attacked by the solvent molecules and broken into new edges with the hydroxyl and amino groups, which increased the hydrophilicity and crystallinity of PBN. The pore size distribution characterization revealed an ultrahigh mesopore volume (0.76 cm3 g−1) of PBN, whereby more than 37% of the pores were smaller than 5 nm, which ensured its high ion selectivity. PBN was further mixed with NAFION™ resin to form a bifunctional ion-selective layer, which combined the nanoporous structure with the ion-exchange groups. Meanwhile, the inorganic rigid PBN structure suppresses the swelling of conventional organic ion-exchange membranes. Through a simple spray-coating process, a PBN ion-selective layer was deposited on a highly ion-conductive and low-cost porous PEI membrane prepared by the NIPS method. The 50% PBN-PEI membrane demonstrated an excellent ion selectivity (1.49×108 mS cm−3 min) compared with the pristine PEI membrane (6.71×106 mS cm−3 min) while maintaining its high ion conductivity (64 mS cm−1). The 50% PBN-PEI membrane achieved superior performance than the NAFION™ 115 membrane in VRFB with higher CE, VE, EE, and capacity at all current densities and high stability with a lower capacity fading rate (0.17% per cycle vs. 0.74% per cycle) at 100 mA cm−2. The 50% PBN-PEI membrane also demonstrated a stable operation in VRFB at a current density of 100 mA cm−2 over 700 cycles. Therefore, the PBN exhibits high ion selectivity based on its nanoporous structure. The introduction of the ion-exchange group to the unique PBN nanoporous structure resulted in excellent ion selectivity and conductivity. The work presented the remarkable performance of the PBN bifunctional layer in terms of ion conductivity and selectivity; the combination with the low-cost high ion-conductive porous PEI membrane demonstrated a great potential for commercialization of the PBN-PEI double-layer membrane.
Materials: Boric acid (H3BO3, 99.8%, Alfa Aesar) and urea (Certified ACS, Fisher Chemical) were used to synthesize BN. Polyetherimide (PEI, Sigma-Aldrich), polyvinylpyrrolidone (PVP, MW. 40000, Alfa Aesar), 1-methyl-2-pyrrolidinone (NMP, ≥99%, Sigma-Aldrich) were used for base membrane fabrication. Vanadium (IV) sulfate oxide hydrate (VOSO4, 99.9%, Alfa Aesar) and sulfuric acid (H2SO4, 98.0%, Sigma Aldrich) were used to prepare electrolytes. All chemicals described here were used as received. NAFION™ perfluorinated resin solution (5 wt %, Sigma-Aldrich) diluted to 1 wt % by isopropanol (IPA, 99.5%, Acros) used for binder in the spray process. The graphite felt (GFD 2, 5 EA, Sigracell) was treated at 400° C. for 30 h in the air and cut into 2.3×2.2 cm2 used as the electrode. NAFION™ 115 membrane (Ion Power Inc.) was orderly pretreated in 5 wt. % hydrogen peroxide, deionized water, and 1 M sulfuric acid for one hour of each liquid at 80° C. and then stored and stored in 1M sulfuric acid over one day before use.
Fabrication of porous PEI membrane: 22.5 g PEI and 2.5 g PVP were mixed and dissolved in 75 g NMP solvent at 120° C. for 5 hours with magnetic stirring.[10d] The solution was cast on a glass plate at room temperature using the film coater (MSK-AFA-I, MTI) and doctor blade with a thickness of 150 μm. The cast membrane was then immersed in deionized water for 24 h to complete the phase-inversion process and remove the solvent completely.
Synthesis and treatment of PBN: 0.1 mol boric acid and 0.5 mol urea were solved in 100 ml deionized water and dried in the oven overnight at 105° C. The dried intermediate was further ground into powders and placed in the tube furnace (OTF-1200X, MTI) heated to 1050° C. (10° C./min ramp rate) under nitrogen gas flow (0.05 NI/min) and held for 3.5 h.[15b] The furnace was then allowed to cool naturally under a nitrogen atmosphere. Pristine PBN was collected after synthesis. 2 g pristine PBN was further ground into powders and then dispersed into 200 ml IPA by sonicating for 4 hours. The dispersion was centrifuged for 10 min at 2500 rpm by Sorvall T1 Centrifuge (Thermo Scientific). The PBN in the suspension was further collected by the PTFE filter through vacuum filtration.
Preparation of PBN-PEI membrane: Different amounts of PBN (e.g., 9.0 mg) were dispersed in 4 ml IPA by sonicating for 1 hour and then mixed with different amounts (e.g., 90 mg) of 1wt % NAFION™ perfluorinated resin solution by sonicating an additional 10 min to form a uniform suspension. The suspensions were uniformly sprayed on the top surface (waterside in the casting process) of the PEI membrane by airbrush until the surface became wet, and then the membrane was heated on the hotplate (Cimarec+™, Thermo Scientific) at 60° C. until the surface became dried. These two processes were repeated until all suspensions were sprayed. All membranes were further treated in deionized water and 1M sulfuric at 80° C. for one hour for each liquid and stored in IM sulfuric acid over one night before use.
Single flow battery performance: The flow battery was assembled by sandwiching a membrane between four graphite felts, two on each side, clamped by two pieces of graphite flow fields and gold-coating current collectors. In this case, the coated layer of the double-layer membrane was placed on the anode side. The effective area of the electrode and membrane was 5 cm2. The 1M VOSO4 and 3M H2SO4 solution was charged to V3.5+ electrolyte for both the cathode and anode sides. The cell was firstly fully charged the battery at a constant voltage of 1.65 V until the current dropped below 10 mA (
The characterizations and membrane properties measurements methods are shown in the Supporting Information, including SEM, XCT, PSD, XRD, FTIR, TGA, contact angle measurement, water uptake, and swelling measurements, area resistance and ion conductivity measurements, vanadium (IV) permeability and ion selectivity measurements, and tensile strength measurement.
Supporting Information. Characterization methods used in the Example follow.
Scanning electronic microscopy (SEM): The morphologies of porous BN and membranes were observed using a scanning electron microscope (S4800, Hitachi). The cross-section of the membrane was observed by breaking the membranes in liquid nitrogen. All samples were platinum-coated before observation.
X-ray computed tomography (XCT): The X-ray computed tomography of the porous PEI membrane was conducted on FXI beamline at National Synchrotron Light Source II of Brookhaven National Laboratory. Dragonfly software was used to reconstruct the three-dimensional structure.
Pore size distribution analysis: The nitrogen physisorption isotherms of PBN were measured at 77 K by an accelerated surface area and porosimetry system (ASAP™ 2020, micropolitics). Before the test, all samples were degassed in a vacuum for 12 hours. Pore size distribution was calculated by the non-local density functional theory (NLDFT) method by the QuadraWin software.
X-Ray Diffraction (XRD)): The XRD spectra were collected by PANalytical/Philips X'Pert Pro (PANalytical, Netherlands) with Cu Kα radiation. All samples were ground into powers before the test. The full-width half maximum and crystallinity were calculated by MDI Jade software.
Fourier transformed infrared (FTIR) spectroscopy: The FTIR spectra were recorded by Cary 630 FTIR (Agilent) under the reflectance mode ranging from 4000 cm−1 to 750 cm−1. All samples were ground into powers before the test.
Thermogravimetric analysis (TGA): The TGA of PBN was performed in TA Q50 Thermogravimetric analyzer (TA Instruments) from room temperature to 1000° C. under the nitrogen atmospheres.
Contact angle measurements: The contact angle was investigated by the optical contact angle measuring instrument (SDC-350, SINDIN). The PBN powders were compressed into tablet formats by applying 60 Mpa pressure 10 min in a cylindrical mold with a diameter of 12.6 mm. For the membranes, before the measurement, all membranes were dried at 80° C. for 24 hours to remove water and taped on a glass substrate. The frozen images were taken after dropping 8 μL of deionized water on the membrane surface for 5 seconds.
Water uptake and swelling measurements: The membrane after the pretreatment was immersed in the deionized water over 24 hours to remove the acid and was wiped to remove the water on the surface. The weight and length of the wet membrane were measured and recorded as Wwet and Lwet. The membrane was further dried at 80° C. for 24 h to remove the water from the membrane. The weight and length of the dried membrane were measured and recorded as Wdry and Ldry. The water uptake was calculated by Equation S1:
The swelling ratio was calculated by Equation S2:
Area resistance and ion conductivity measurements: The area resistances of embranes were measured in the flow cell by the electrochemical impedance spectroscopy (EIS) (SP-150, BioLogic) with an effective area of 5 cm2 and 1 M VOSO4 and 3M H2SO4 as electrolytes. The sinusoidal voltage waveform of amplitude was 10 mV, and the frequency range was from 500 kHz to 100 Hz. The area resistances (R) were calculated by Equation S3, where Rs and R0 are the high-frequency intercepts with the horizontal axis (X) with and without the membrane, and A is the effective area.
R=(Rs−R0)×A (S3)
Ion conductivities (c) were calculated by Equation 4, where L is the membrane thickness.
Vanadium (IV) permeability and ion selectivity measurements: A H-cell separated by different membranes was used to evaluate the vanadium permeability. The left-side chamber was filled with 10 mL 1 M VOSO4 in a 3 M H2SO4 solution, and the right-side chamber was filled with 10 mL 1 M MgSO4+3 M H2SO4 solution. 1 mL of solution was collected from the right-side chamber every 2 hours, and the chamber was replenished by fresh 1 M MgSO4+3 M H2SO4 solution to maintain the same volume at each side. To detect the vanadium permeability, the absorbance of each sample was measured at 760 nm wavelength using a UV-vis spectrometer (Agilent 8453, USA), and a calibration curve of VOSO4 was also obtained at 760 nm wavelength. The vanadium permeability rate was then calculated from the concentration equivalent to each measured absorbance using the calibration curve. The permeabilities (P) were calculated by Equation S5 (a pseudo-steady-state order was applied in between the membrane), where L is the membrane thickness, A is the effective area, VR is the volume of the right chamber, CL is the VO2+ concentration in the left chamber (assuming the change of the concentration was negligible during the test), CR(t) is the VO2+ concentration on the right chamber as a function of time, and t is time.
The ion selectivity (S) was calculated by Equation S6, where P was permeability and σ is ion conductivity.
Tensile strength: The tensile strength of the wet membrane was measured by a rotational rheometer (ARES-G2, TA Instruments) with a 100 μm min−1 displacement speed. The sample was cut into the size of 20 mm in length and 5 mm in width and stored in deionized water until the measurement.
The teachings of all patents, published applications and references cited herein are incorporated by reference in their entirety.
While example embodiments have been particularly shown and described, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the embodiments.
This application claims the benefit of U.S. Provisional Application No. 63/385,199, filed on Nov. 28, 2022. The entire teachings of the above application are incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 63385199 | Nov 2022 | US |
