Patent Application
20230317997
This disclosure relates to ion exchange membranes that can facilitate transport of ions with wide applications in electrochemical devices, such as fuel cells, flow batteries, and electrolyzers.
Ion exchange membranes feature positive or negative functional groups that can facilitate selective transport of counter ions, with wide applications in fuel cells, flow batteries, electrolyzers, and more [Ref. 1-4]. As the most widely used proton exchange membrane, Nafion™ features high proton conductivity, good stability, and excellent processability [Ref. 5]. However, its negatively charged sulfonic acid groups restrict its function to acidic environments [Ref. 3,6]. In contrast, anion exchange membranes, particularly hydroxide exchange membranes (HEMs), are operated under alkaline conditions, which enables the use of non-precious metal catalysts, bipolar plates and other stack components thus reducing costs substantially [Ref. 3,7]. For this reason, there has been increasing study of HEMs as an alternative to proton exchange membranes [Ref. 6,8,9], with several candidate materials developed based on polymers that feature cationic functional groups, such as ammonium, imidazolium, and pyridinium, for hydroxide conduction [Ref. 10-13]. However, under harsh basic operating conditions, these cationic groups are still prone to hydroxide attack, which results in the degradation and poor long-term chemical stability of HEM materials [Ref. 14-17]. As a result, it has remained an ongoing challenge to develop HEMs with high hydroxide conductivity and sufficient chemical stability in the harsh alkaline conditions needed for hydroxide exchange.
Researchers are increasingly turning to natural polymers for potential solutions to our energy needs due to their accessibility and enhanced sustainability compared to synthetic polymers. Converting the naturally abundant chitin (widely available in seafood biowaste [Ref. 18,19]) to chitosan produces the only polysaccharide that contains free amino groups, which in their cationic charged state can attract anions (e.g., OH—) for anion exchange applications (see
What is needed therefore are improved ion exchange membranes derived from natural sources that can facilitate transport of anions with wide applications in electrochemical devices, such as fuel cells, flow batteries, and electrolyzers.
The present disclosure addresses the foregoing needs by providing a chitosan material that can act as a hydroxide exchange membrane.
In one aspect, the disclosure provides an ion exchange membrane comprising a plurality of chitosan molecular chains crosslinked with a crosslinking agent selected from the group consisting of multivalent cations and mixtures thereof, wherein the ion exchange membrane has a structure including a crystalline crosslinking zone.
In another aspect, the disclosure provides an electrochemical device comprising: an anode; a cathode; and an ion exchange membrane positioned between the anode and the cathode, wherein the ion exchange membrane comprises a plurality of chitosan molecular chains crosslinked with a crosslinking agent selected from the group consisting of multivalent cations and mixtures thereof.
In yet another aspect, the disclosure provides a method for forming an ion exchange membrane. The method can include the steps of: (a) casting a flowable composition including chitosan on a support to form a chitosan membrane on the support; (b) advancing the support into a region wherein the chitosan membrane is contacted with a crosslinking agent selected from the group consisting of multivalent cations and mixtures thereof to form on the support an ion exchange membrane comprising a plurality of chitosan molecular chains crosslinked with the crosslinking agent; and (c) separating the ion exchange membrane from the support.
These and other features, aspects, and advantages of examples provided in the present disclosure will become better understood upon consideration of the following detailed description, drawings, and appended claims.
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.
Before any embodiments of this disclosure are explained in detail, it is to be understood that the present disclosure is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The presented examples are capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items.
The following discussion is presented to enable a person skilled in the art to make and use embodiments of the disclosure. Various modifications to the illustrated embodiments will be readily apparent to those skilled in the art, and the generic principles herein can be applied to other embodiments and applications without departing from embodiments of the disclosure. Thus, embodiments of the disclosure are not intended to be limited to embodiments shown and described, but are to be accorded the widest scope consistent with the principles and features disclosed herein. Skilled artisans will recognize the examples provided herein have many useful alternatives and fall within the scope of embodiments of the disclosure.
The ion exchange membranes of the present disclosure can be used in a hydroxide exchange membrane fuel cell (HEMFC) such as the non-limiting example HEMFC 100 as shown in
The anode carries out an anode half-reaction which oxidizes fuel releasing electrons to an external circuit and produces oxidized products. The cathode carries out a cathode half-reaction which reduces an oxidizer consuming electrons from the external circuit. The gas diffusion layers 104, 114 serve to deliver the fuel 122 (in this example, H2) and oxidizer 124 (in this example, O2) uniformly across the respective catalyst layers 106 and 116. In the case of the illustrated hydroxide exchange membrane fuel cell (HEMFC), the anode half-reaction consumes fuel and OH− ions and produces waste water (as well as carbon dioxide in the case of carbon containing fuels). The cathode half reaction consumes oxygen and produces OH− ions, which flow from the cathode to the anode through the electrolyte membrane 110. Fuels are limited only by the oxidizing ability of the anode catalyst and typically include hydrogen gas, methanol, ethanol, ethylene glycol, and glycerol. Preferably, the fuel is H2 or methanol.
The anode catalyst layer 106 and the cathode catalyst layer include a catalyst. The term “catalyst” as used herein to refer to a material that increases the rate of a chemical reaction, e.g., by decreasing the activation energy required for reaction, and the term “electrocatalyst” as used herein refers to a material that increases the rate of an electrochemical reaction including one or more electron transfer steps. Further, with respect to an electrochemical reaction, the catalyst (or electrocatalyst) may decrease the potential required to initiate the reaction, or in other words, decrease the overpotential for the reaction. The type or kind of catalyst is not particularly limited as long as it is sufficiently active for the particular electrochemical application. In some embodiments, the catalyst may be a platinum group based catalyst material or a non-platinum group based catalyst material. Non-limiting examples of the catalyst may include platinum (Pt), rhodium (Rh), ruthenium (Ru), palladium (Pd), iridium (Ir), gold (Au), nickel (Ni), or mixtures thereof.
The use of an anion exchange membrane in the fuel cell 100 creates an alkaline environment inside the cell. The OH− anion species crossing the membrane are generated according to an electrochemical oxygen reduction reaction at the cathode. The OH− are transported to the anode where the fuel 122 (e.g., hydrogen, methanol) in contact with electrocatalysts undergoes an oxidation reaction to generate a molecule of water per single electron. In HEMFC 100, water is generated at the anode and, at the same time, water is a reactant at the cathode. The electrolyte membrane 110 should have the ability to allow the transport of OH− ions while preventing fuel crossover and blocking the transport of electrons to prevent a short circuit. The HEMFC 100 may operate at a pH of 11 or greater, or at a pH of 12 or greater, or at a pH of 13 or greater.
The anode gas diffusion layer 104 and the cathode gas diffusion layer 114 provide support for the corresponding anode catalyst layer 106 and cathode catalyst layer 116. The anode gas diffusion layer 104 and the cathode gas diffusion layer 114 act as a transport channel for reactants and products which take part in the chemical reactions. The anode gas diffusion layer 104 and the cathode gas diffusion layer 114 may be a porous structure of carbon fibers, a non-woven carbon fiber material (i.e., carbon felt), or a woven carbon fiber material.
Chitosan is a natural material that includes a plurality of molecular chains of polysaccharides, more specifically, the molecular chains are a random distributed units of β-(1→4)-linked D-glucosamine (deacetylated unit) and units of N-acetyl-D-glucosamine. According to an aspect of the disclosure herein, the plurality of chitosan molecular chains can be crosslinked with a crosslinking agent to form an ion exchange membrane. The crosslinking agent can be selected from the group consisting of multivalent cations and mixtures thereof. The multivalent cations are selected from the group consisting of ions of copper, zinc, barium, calcium, magnesium, strontium, boron, beryllium, aluminum, iron, cobalt, lead, silver, and mixtures of any of these ions. In a particular embodiment, the multivalent cations are selected from the group consisting of ions of copper. The crosslinking agent coordinates between the chains along at least a portion of the chitosan molecular chains. The multivalent cations are coordinated with amino groups and hydroxyl groups of the chitosan molecular chains.
The plurality of chitosan molecular chains can be crosslinked by contacting the plurality of chitosan molecular chains with the crosslinking agent in an alkaline environment. The alkaline environment can include a solution of LiOH, NaOH, or KOH, for example. In one example, the plurality of chitosan molecular chains are crosslinked by contacting the plurality of chitosan molecular chains with the crosslinking agent as a metallic complex with hydroxyl. In an aspect, the membrane can include 1 wt. % to 10 wt. %, or 4 wt. % to 8 wt. %, of the crosslinking agent based on total weight percent of the membrane.
The long range order of the ion exchange membrane as disclosed herein is different from that of chitosan, as shown in
Upon crosslinking, the chitosan chains twist along the chain axis to accommodate coordination by crosslinking agents such as ions between chains. In one embodiment, the ion exchange membrane can have a trigonal crystal structure. For example, the trigonal crystal structure can have a space group of P3221. In contrast, chitosan chains crystallize in an orthorhombic unit cell and a space group P212121. In this way, the crosslinking provides the ion exchange membrane with a structure having a crystalline crosslinking zone.
The organization of the chains in this way forms nanochannels. The nanochannels in the ion exchange membrane can be polygonal nanochannels as shown in
In a particular embodiment, the ion exchange membrane includes hexagonal nanochannels. For example, the hexagonal nanochannels can have a width in a range of 0.1 to 5 nanometers (see
The ion exchange membrane as disclosed herein can accommodate fast water diffusion, as shown in Table 4. For example, the membrane as disclosed herein can have a water self-diffusion coefficient (Dwater) greater than 10×10−10 m2 s−1 at 90% relative humidity. In another example, the membrane as disclosed herein can have a Dwater greater than 15×10−10 m2 s−1 at 90% relative humidity, or greater than 5×10−10 m2 s−1 .
The ion exchange membrane as disclosed herein has a high ion exchange capacity as compared to previously reported examples of membranes. Furthermore, the ion exchange membrane facilitates anion conductivity. More specifically, the bridging Cu2+ ions in the ion exchange membrane facilitate the transport of anions, for example, the hydroxide anion. In one example, the ion exchange membrane can have a hydroxide conductivity of greater than 10 mS cm−1 at room temperature. Additionally, and alternatively, the ion exchange membrane can have a hydroxide conductivity of greater than 20 mS cm−1 at room temperature, or the ion exchange membrane can have a hydroxide conductivity of greater than 30 mS cm−1 at room temperature, or the ion exchange membrane can have a hydroxide conductivity of greater than 40 mS cm−1 at room temperature, or the ion exchange membrane can have a hydroxide conductivity of greater than 50 mS cm−1 at room temperature. In yet another embodiment, the membrane can have a σIEC of greater than 30 mS g cm−1 mmol1. σIEC is the OH− conductivity normalized to the IEC, which decouples the conductivity from the contribution of the number of OH− molecules and indicates the intrinsic mobility of OH−.
The ion exchange membrane is dense and can take up less water than chitosan. In one embodiment, the ion exchange membrane has a water uptake of less than about 55%, less than 60%, less than 65%, or less than 70%.
The mechanical strength of the ion exchange membrane can be increased by the crosslinking. For example, the crosslinking of the chitosan chains via coordination bonds increases the mechanical properties of the ion exchange membrane in a dry state (
The ion exchange membrane can be used in a direct methanol fuel cell (DMFC) as it has low permeability to methanol. In one embodiment, the diffusion coefficient (P) of the ion exchange membrane is half that of the benchmark Nafion™ 212, indicating the ion exchange membrane can overcome the challenge of fuel (methanol) crossover. In another embodiment, the diffusion coefficient P of the ion exchange membrane is 1.02×10−6 cm2 s−1. In an embodiment, the membrane can have a methanol permeability of less than 1.5×10−6 cm2 s−1.
The ion exchange membrane exhibits stability towards alkaline conditions, as shown in
The XRD profile of the ion exchange membrane indicates the trigonal crystal structure can be maintained after the 1000-hour treatment at 80° C. In an embodiment, the ion exchange membrane is stable in a hydroxide solution exhibiting only 5% conductivity loss at 80° C. after 1000 hours.
According to an aspect as disclosed herein, the ion exchange membrane can be incorporated in an electrochemical device. In an embodiment, the electrochemical device can be an alkaline anion exchange membrane fuel cell.
The electrochemical device can include an anode. The anode can include a catalyst. The electrochemical device can include a cathode, where the cathode includes a catalyst material. The anode catalyst material and the cathode catalyst material can be a platinum group material or a non-platinum group material as described above.
The electrochemical device can include the ion exchange membrane positioned between the anode and the cathode. The ion exchange membrane can include a plurality of chitosan molecular chains crosslinked with a crosslinking agent and having nanochannels as described above. When incorporated in the electrochemical device, the ion exchange membrane has a water self-diffusion coefficient, ion transport properties, methanol permeability, and tensile strength as described above.
The electrochemical device can include a fuel flow path positioned to feed a fuel containing hydrogen and/or methanol into contact with the anode. The device can include an oxidant flow path positioned to feed an oxidant containing oxygen into contact with the cathode.
In an embodiment, the electrochemical device is a direct methanol fuel cell that exhibits a power density of greater than 300 mW cm−2.
The ion exchange membrane can be formed according to the following method. A flowable composition including chitosan can be prepared from chitin, as described above and as shown in
The support including the chitosan membrane can be advanced into a region wherein the chitosan membrane is contacted with a crosslinking agent. This step can include immersing the chitosan membrane on the support in a bath containing the crosslinking agent in an alkaline media. In an example, the chitosan membrane on the support can be immersed in a bath containing the crosslinking agent as a metallic complex with hydroxyl. Additionally, and alternatively, the region can be an open area where the crosslinking agent in an alkaline media can flow over and through the chitosan membrane on the support. The crosslinking agent can be selected from the group consisting of multivalent cations and mixtures thereof. For example, the multivalent cations can be ions of ions of copper, zinc, barium, calcium, magnesium, strontium, boron, beryllium, aluminum, iron, cobalt, lead, silver, and mixtures of any of these ions.
In an embodiment, the support can be transported on the roll to a zone where the flowable composition including chitosan is cast on the support as shown in
In a subsequent step, the ion exchange membrane is separated from the support. For example, the ion exchange membrane can be collected on a roll after separating the ion exchange membrane from the support. In another example, the ion exchange membrane can be removed from the mold.
The following Example has been presented in order to further illustrate the aspects of the present disclosure and is not intended to limit the present disclosure in any way. The statements provided in the Example are presented without being bound by theory.
Ion exchange membranes are widely used to selectively transport ions in various electrochemical devices. Hydroxide exchange membranes (HEMs) are promising to couple with lower-cost platinum-free electrocatalysts used in alkaline conditions but are not stable enough in strong alkaline solutions. Herein, we present a Cu2+-crosslinked chitosan (chitosan-Cu) material as a stable and high-performance HEM. The Cu2+ ions are coordinated with the amino and hydroxyl groups of chitosan to crosslink the chitosan chains, forming hexagonal nanochannels (˜1 nm in diameter) that can accommodate water diffusion and facilitate fast ion transport, with a high hydroxide conductivity of 67 mS cm−1 at room temperature. The Cu2+ coordination also enhances the mechanical strength of the membrane, reduces its permeability, and most importantly, improves its stability in alkaline solution (only 5% conductivity loss at 80° C. after 1000 hours). These advantages make chitosan-Cu an outstanding HEM, which we demonstrate in a direct methanol fuel cell (DMFC) that exhibits a high power density of 305 mW cm−2. The design principle of the chitosan-Cu HEM, in which ion transport channels are generated in the polymer through metal-crosslinking of polar functional groups, could inspire the synthesis of many ion exchange membranes for ion transport, ion sieving, ion filtration, and more.
Ion exchange membranes feature positive or negative functional groups that can facilitate selective transport of counter ions, with wide applications in fuel cells, flow batteries, electrolyzers, and more [Ref. 1-4]. As the most widely used proton exchange membrane, Nafion™ features high proton conductivity, good stability, and excellent processability [Ref. 5]. However, its negatively charged sulfonic acid groups restrict its function to acidic environments [Ref. 3,6]. In contrast, anion exchange membranes, particularly hydroxide exchange membranes (HEMs), are operated under alkaline conditions, which enables the use of non-precious metal catalysts, bipolar plates, and other stack components thus reducing costs substantially [Ref. 3,7]. For this reason, there has been increasing study of HEMs as an alternative to proton exchange membranes [Ref. 6,8,9], with several candidate materials developed based on polymers that feature cationic functional groups, such as ammonium, imidazolium, and pyridinium, for hydroxide conduction [Ref. 10-13]. However, under harsh basic operating conditions, these cationic groups are still prone to hydroxide attack, which results in the degradation and poor long-term chemical stability of HEM materials [Ref. 14-17]. As a result, it has remained an ongoing challenge to develop HEMs with high hydroxide conductivity and sufficient chemical stability in the harsh alkaline conditions needed for hydroxide exchange.
Researchers are increasingly turning to natural polymers for potential solutions to our energy needs due to their accessibility and enhanced sustainability compared to synthetic polymers. Converting the natural abundant chitin (wide availability in seafood biowaste [Ref. 18,19]) to chitosan produces the only polysaccharide that contains free amino groups, which in their cationic charged state can attract anions (e.g., OH−) for anion exchange applications (
In this Example, we report a chitosan-based anion conductor, in which ion-conducting nanochannels are formed by crosslinking the chitosan molecular chains with Cu2+ ions. In this chitosan-Cu material, Cu2+ ions coordinate with the amino and hydroxyl groups of chitosan to crosslink adjacent chains, which changes the twofold symmetric structure of the polymer chains to a unique threefold helical conformation. As a result, the orthorhombic crystal structure of chitosan converts to a trigonal crystal structure (FIG. 1A, panel b), in which six chitosan chains are bridged by Cu2+ to form a nanochannel with a diameter of ˜1 nm, oriented along the chain direction. The chelated Cu2+ ions in chitosan-Cu selectively promote the transport of anions (OH− in this Example) within the nanochannels, with a high hydroxide conductivity of 67 mS cm−1 at room temperature and 100% relative humidity. The Cu2+ crosslinking also suppresses the membrane from swelling in water, inhibits fuel permeation, and enhances the mechanical strength. Furthermore, the chitosan-Cu membrane demonstrates excellent chemical stability in harsh alkaline environments, enabling it to serve as a stable HEM. These features allow chitosan-Cu to serve as an effective HEM, which we demonstrate in a DMFC that displays an exceptional power density of 305 mW cm−2—a significant improvement compared to previously reported DMFCs. The chitosan-Cu material and its ion transport nanochannels generated through metal-crosslinking suggest a general approach to achieve high-performance ion exchange polymers.
To synthesize the chitosan-Cu material, we first derived chitin from crab and shrimp shell waste using a demineralization and deproteinization treatment [Ref. 19]. We then heated the chitin in NaOH solution to conduct the deacetylation process [Ref. 22], during which the acetamide groups of the chitin chains transform to amino groups, producing chitosan. The chitosan was dissolved in acetic acid solution (4 wt. %,
We analyzed the Cu valence and bonding in the chitosan-Cu membrane by X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS). The Cu 2p XPS peaks at 931.6 eV and 951.7 eV and characteristic satellite peaks confirm the presence of Cu2+ in the chitosan-Cu membrane (
We employed 2D X-ray diffraction (XRD) to investigate how Cu2+ crosslinks the chitosan molecular chains and may impact the chitosan structure. As the chitosan-Cu membrane has a low crystalline degree, we used crab tendon, which consists of aligned chitin nanofibers (
In contrast, the aligned chitosan-Cu (
The chitosan-Cu membrane fabricated from dissolved chitosan (
We demonstrate the chitosan-Cu material, with its rich amino groups and unique Cu2+-crosslinking structure, as an excellent HEM with high OH− conductivity. The amino groups of chitosan and coordinated Cu2+ ions introduce positive charges to the chitosan-Cu, as indicated by the increased Zeta potential of 17 mV (
The OH− conductivity (
Moreover, the chitosan-Cu HEM demonstrates superb stability in harsh alkaline conditions, which is important for long lifespan fuel cells [Ref. 16]. We investigated the stability of the chitosan-Cu membrane by examining its room temperature conductivity after storing in 3 M NaOH solution at 80° C. The chitosan-Cu membrane displayed 95% conductivity retention over 1000 hours in the hot, strong basic solution, which is outstanding compared with other HEMs in terms of the OH− conductivity and alkali stability (
According to the ab initio molecular dynamics (AIMD) simulations of the chitosan-Cu (
On top of the high OH− conductivity and alkaline-stability, the chitosan-Cu membrane also displays low fuel permeability. The chitosan-Cu membrane shows a methanol permeability of only 1.02×10−6 cm2 s−1, much smaller than that of the state-of-the-art Nafion™ 212 membrane (2.06×10−6 cm2 s−1) (
The Cu2+ crosslinking of the chitosan chains via Cu—N and Cu—O coordination bonds also increases the mechanical properties of the chitosan-Cu membrane, not only in dry state (
We demonstrated the application of the promising chitosan-Cu HEM in a DMFC. In this fuel cell, 02 is reduced in the cathode to generate hydroxide ions, which transport through the HEM and react with the methanol fuel in the anode (
aOne typical membrane is selected as an example for each type of cation-containing HEM membrane to evaluate its fabrication and cost. Chitosan-Cu was measured in this work. The values of the other four HEM membranes were obtained from references [Ref. 7, 9, 11, 15].
bStability time is the duration that the HEM membrane can remain in alkaline solution while retaining 90% of its initial OH− conductivity. Note that the concentration and temperature of the alkaline solutions vary in different works. The chitosan-Cu in this work was treated in 3M NaOH at 80° C. Cp2Co+-PBI [Ref. 15] and ptmAm-PSf50 [Ref. 11] were treated with 1M KOH at 60° C. PAP-TP-85 [Ref. 9] was treated in 1M KOH at 100° C. HMT-PMPIm [Ref. 7] was treated in 10M KOH at 100° C.
cCost was estimated by the total price of the reagents used to synthesize 1 g of the polymers. The unit prices of the reagents (ACS reagent grade) were based on those sold by Millipore Sigma. The detailed cost calculations for all polymers are listed in Tables 8-12.
All chemicals and solvents were purchased from Millipore Sigma unless otherwise indicated. The copper wire was purchased from McMaster-Carr. The wood pulp was purchased from International Paper. Nafion™ 212 and Fumasep FAB-PK-130 was purchased from Ion Power Inc. and Fuel Cell Inc.
The chitin was prepared from crab and shrimp shell waste, which was collected from Chesapeake blue crab and shrimp (purchased from a local seafood market). The shell waste was first washed and soaked in 1% hydrochloric acid (HCl) to remove inorganics until no more bubbling was observed. The cleaned shell was then treated in 5% NaOH overnight to remove proteins and lipids. [Ref. 22] The obtained pure chitin was then soaked in 6 M NaOH solution, sealed in a Teflon-lined hydrothermal autoclave, and heated at 100° C. overnight to convert chitin to chitosan.
The aligned chitosan was made using the same procedure except that crab tendons were used as the starting material instead of crab/shrimp shells.
To make the chitosan membrane, typically 1 g of chitosan flakes was dissolved in 200 ml of 4 wt. % acetic acid aqueous solution at room temperature by stirring overnight. After removing the undissolved flakes by filtration, a homogeneous and transparent chitosan solution was obtained. The chitosan solution (0.5 wt. %) was cast on a petri dish or a concentrated chitosan solution (˜4 wt. %) was cast using a doctor-blade on a polyethylene terephthalate (PET) film, followed by drying in air to obtain the chitosan membrane.
To prepare chitosan-Cu, a blue Cu2+-saturated NaOH solution (Na2Cu(OH)4) was first prepared by immersing excess Cu wires in 20 wt. % NaOH solution for 1 week. The concentration of saturated Cu2+ in the NaOH solution was ˜2 wt. %. The chitosan materials were immersed in this Na2Cu(OH)4 solution for 4 days until the blue color of the chitosan materials no longer changed. The chitosan-Cu materials were obtained, after rinsing with excess water to remove the physically adsorbed Cu ions and NaOH, followed by drying in air.
Cellulose nanofibrils were first produced by treating wood pulp using the (2,2,6,6-tetramethylpiperidin-1-yl)oxidanyl-oxidation method [Ref. 44], and then immersing the material in Na2Cu(OH)4 solution, producing a suspension of cellulose-Cu nanofibrils. The cellulose-Cu membrane (50 μm) was made by vacuum filtering the cellulose-Cu nanofibril slurry, followed by drying in air.
2D XRD was conducted on a Xenocs Xeuss SAXS/WAXS system with a Cu Kα (λ=1.5418 Å) micro-focus source and Detris Pilatus 300k detector in transmission mode in vacuum. XPS of the membranes was performed on a Thermo ESCALAB 250. The C—C peak at 284.8 eV was used as a charge correction reference. The zeta potential of the chitosan-Cu and chitosan suspensions was measured using a Zetasizer Nano S90 (Malvern Instrument). SEM was conducted at 5 kV on a Hitachi SU-70 with EDS analysis at 10 kV. FTIR was conducted with a Thermo Nicolet NEXUS 670 FTIR with an Attenuated Total Reflectance (ATR) accessory.
The X-ray absorption spectroscopy measurement at the Cu-K edge (8,979 eV) was performed on the bending-magnet beamline of the X-ray Science Division (9-BM-C) at the Advanced Photon Source, Argonne National Laboratory. The radiation was monochromatized by a Si (111) double-crystal monochromator. Harmonic rejection was accomplished with a rhodium-coated flat mirror. The energy was calibrated with Cu foil at the Cu—K edge. All the spectra were collected in fluorescence mode by a 4-element Vortex detector.
Diffusion nuclear magnetic resonance (NMR) measurements were performed on a 300 MHz NMR spectrometer operating at a magnetic field of 7 T for 1H NMR using a DOTY Z-spec pulsed field gradient (PFG) NMR probe. A simple spin echo diffusion pulse sequence was used to measure the diffusion coefficients. The signal was accumulated by varying the gradient strength from 100-2050 G/cm in 16 increments. The diffusion time and the diffusion pulse length were in the range of 0.004-0.006 s and 0.0012-0.002 s, respectively. The diffusion coefficients were calculated using the Stejskal-Tanner equation [Ref. 45]. The samples for NMR measurement were prepared by packing vacuum-dried chitosan-Cu membranes into 5 mm NMR tubes, injecting proper amounts of deionized water (75%—90% of the mass of the dry membrane) and finally equilibrated inside the sealed NMR tubes at room temperature for at least 48 hours.
The Cu-ion content of the chitosan-Cu membrane and Na2Cu(OH)4 solution were measured by PerkinElmer NexION 300D ICP-MS, where 63Cu standard solutions were used to construct a calibration curve. For ICP measurement, the chitosan-Cu membrane was dissolved in 2 wt. % nitric acid as the testing solution, the Cu2+-0 concentration of which was used to calculate the Cu content in chitosan-Cu.
The tensile strain-stress of the chitosan and chitosan-Cu membranes were tested by a tabletop model testing system (Instron, USA) with a running speed of 0.5 mm min−1. Rectangular films (thickness: 40 μm) were pretreated by immersing in water for 1 hour before testing the tensile performance of the resulting water-soaked chitosan and chitosan-Cu membranes.
The IEC of the chitosan-Cu membrane was measured using the back-titration method. The membrane was immersed in a 0.01 M HCl standard solution (50 ml) for 24 hours and then titrated by a standard 0.01 M NaOH solution until pH=7.0. The IEC was calculated by
where mdry is the mass of the dry chitosan-Cu membrane, CHCl and CNaOH are the concentrations of the HCl and NaOH solutions, respectively, and VHCl and VNaOH are the volumes of the HCl and NaOH solutions, respectively.
The mass of a dry membrane (after drying under vacuum overnight) was measured as the dry mass (mdry). The membrane was then further conditioned in a humidity-controlled chamber for 12 hours at different relative humidity to achieve the wet mass (mwet). The water uptake was calculated by
The apparent methanol permeability of the membranes was measured at 25° C. using synthesized chitosan-Cu and cellulose-Cu membranes. A commercial Nafion™ 212 membrane was used for the control experiment. The thickness of the hydrated chitosan-Cu, cellulose-Cu, and Nafion™ 212 membranes was 110 μm, 81 μm, and 55 μm, respectively. The membranes were clamped between two glass cells of ˜125 ml capacity with an inner area (A) of 4.5 cm2. The left reservoir was filled with 120 ml of methanol with a concentration (CA) of 22 M and the right receiving reservoir was filled with water with a volume (VB) of 120 ml. Solutions in both reservoirs were vigorously stirred at the same speed. The concentration of methanol as a function of time (CB(t)) in the receiving reservoir was determined by measuring the density of the solution, which was repeated five times at certain intervals of time (1-3 hours). A calibration curve of the density versus methanol concentration was obtained prior to the permeation measurements. The apparent methanol permeability (P) was calculated according to the equation:
where L is the membrane thickness and t is the permeating time of methanol through the membranes.
Before the hydroxide conductivity measurement, the chitosan-Cu membranes were immersed in 3 M NaOH for 24 hours and washed with water to remove excess NaOH. The in-plane hydroxide conductivity of the chitosan-Cu membrane was measured by EIS in a humidity-controlled chamber. The EIS was measured using a Biologic electrochemical station with frequency from 100 kHz to 1 Hz. The hydroxide conductivity (a) was calculated by the equation:
where L is the length of the chitosan-Cu membrane, R is the resistance obtained by the EIS test (the real axis value at the high frequency intercept), and S is the cross-sectional area of the membrane.
Commercial PtRu/C (Alfa Aesar, 60% PtRu on Vulkan XC-72, Pt:Ru=2:1) and Acta 4020 (Acta S.p.A) were used as the anode and cathode catalysts, respectively. The electrode ink was prepared by adding catalyst and ionomers (PiperION PAP-TP-100 ionomer for the anode and AS-4 ionomer for the cathode) into isopropanol solvent, followed by sonication for 1 hour in an ice-water bath. The weight ratios of the catalyst and ionomer for the anode and cathode were 9:1 and 8:2, respectively. The hydrophilic PiperION PAP-TP-100 ionomer was used in the anode to facilitate the transport of the aqueous methanol solution [Ref. 41]. The hydrophobic AS-4 ionomer (20 wt. %) was used to alleviate the cathode flooding issue [Ref. 46]. The anode was prepared by spraying PtRu/C catalysts on carbon cloth with a loading of 2 mgPGM cm−2, and the cathode was constructed by spraying catalysts onto chitosan-Cu membranes with a loading of 3 mgActa cm−2. The MEAs with an active area of 5 cm2 were assembled with an anode electrode, a chitosan-Cu membrane coating with a cathode catalysts layer, a gas diffusion layer (SGL 29 BC) in the cathode, two fluorinated ethylene propylene gaskets, two graphite plates with 5 cm2 flow field (ElectroChem), and metal current collectors.
A fuel cell test station was used to collect polarization curves of the prepared DMFCs. The experiments were performed at 80° C. and ambient pressure. The fuel cell temperature was controlled by a feedback loop comprised of electric heating tape and a thermocouple-based thermometer in the end plates. The default testing was performed using 5 ml min−1 of 3 M methanol in 6 M KOH as the anode fuel and 500 ml min−1 O2 as the cathode fuel at 80° C.
A system containing one Cu2+, two OH−, six GLUs (two chitosan chains), and 66 H2O molecules was used to perform the AIMD simulations. The two chitosan chains (each with three GLUs) were connected by one Cu2+ by coordinating with the hydroxyl O atoms and N atoms of the amino groups. Note that the amount of each element (Cu2+, OH− and H2O) was calculated from the Cu2+ ratio, IEC, and water uptake, respectively. Four out of the 66 H2O molecules were replaced by four NaOH to mimic the 3 M NaOH aqueous solution. Another system of uncoordinated Cu2+ in a NaOH aqueous solution was constructed as a control of the coordinated Cu2+.
The geometry optimizations and AIMD simulations under NVT ensemble (constant number of atoms, volume, and temperature) were performed using the CP2K package (version 8.1). The Gaussian and plane waves (GPW) method as implemented in the QUICKSTEP module [Ref. 47] was adopted. The Goedecker-Teter-Hutter pseudopotentials along with DZVP basis set [Ref. 48] were used and truncated at 280 Ry. The BLYP functional [Ref. 49] with D3 dispersion correction [Ref. 50] was used, which works very well for liquid water systems [Ref. 51]. A Nose-Hoover thermostat [Ref. 52] was used for controlling the temperature at 353 K (or 80° C.). The time step was set to 1.0 fs and the total simulation time was 25.0 ps. The first 10.0 ps of the AIMD simulations were for equilibration, and the last 15.0 ps were taken for energy analyses. Total energies taken from the last 15.0 ps for the two systems were compared, which can be used to check if Cu2+ tends to be bound with chitosan or dissolve in solution. For the system in which Cu2+ is coordinated with chitosan, the bond distances between Cu and its associated O and N atoms from GLUs were monitored to confirm its stability.
A system including two glucosamine, two glucose units or four H2O molecules and one Cu2+ were used to perform DFT calculations of the binding energies of model systems of chitosan-Cu, cellulose-Cu, and Cu-water, respectively. Binding energy is defined as Eb=2×Eunit+ECu
DFT calculations were performed using the Gaussian 09 code (Revision D.01). The hybrid PBEO functional [Ref. 53] and the basis set 6-311+G** were used for geometry optimizations and energy calculations. The D3 version of Grimme's dispersion with Becke-Johnson damping [Ref. 50] was adopted to correct for the weak interactions. The implicit solvation model SMD [Ref. 54] was used. The dielectric constant was set to 78.4 to represent the water system.
In summary, we have developed a Cu2+-coordinated chitosan material and demonstrated its excellent performance as an HEM. The chitosan-Cu was fabricated from the cationic polymer chitosan using a facile and scalable solution-based approach. The process converts the orthorhombic crystal structure of chitosan to a trigonal crystal structure, comprising crosslinked chitosan chains through the coordination of Cu2+ with the —NH2 and —OH groups of chitosan. Due to the unique structure of the Cu2+-crosslinked chitosan chains, in which six chains are connected by Cu2+ to form a -1 nm wide hexagonal nanochannel, chitosan-Cu achieves fast OH− transport for high OH− conductivity (67 mS cm−1), in addition to low methanol crossover and good structural strength. The strong bonding of Cu—N and Cu—O in chitosan-Cu ensures the material's structural stability, even under harsh alkaline conditions. These features make chitosan-Cu an excellent ion exchange membrane for fuel cells, which we demonstrate in a DMFC with a high power density of 305 mW cm−2. This concept of using metal ions to crosslink polymers to form new HEM materials suggests an avenue for the development of high-conductivity and alkali-stable anion exchange membranes, as well as the revalorization of naturally abundant biomaterial in value-added systems.
29. Zadok, I., Dekel, D. R. & Srebnik, S. Effect of ammonium cations on the siffusivity and structure of hydroxide ions in low hydration media.J. Phys. Chem. C 123, 27355-27362, (2019).
31. Zadok, I. et al. Unexpected hydroxide ion structure and properties at low hydration. J. Mol. Lip. 313, 113485, (2020).
The citation of any document is not to be construed as an admission that it is prior art with respect to the present disclosure.
Thus, examples of the present disclosure provide ion exchange membranes that can facilitate transport of ions with wide applications in electrochemical devices, such as fuel cells, flow batteries, and electrolyzers.
In light of the principles and example embodiments described and illustrated herein, it will be recognized that the example embodiments can be modified in arrangement and detail without departing from such principles. Also, the foregoing discussion has focused on particular embodiments, but other configurations are also contemplated. In particular, even though expressions such as “in one embodiment”, “in another embodiment”, “in an embodiment”, “in some embodiments”, “in a further embodiment”, or the like are used herein, these phrases are meant to generally reference embodiment possibilities, and are not intended to limit the disclosure to particular embodiment configurations. As used herein, these terms may reference the same or different embodiments that are combinable into other embodiments. As a rule, any embodiment referenced herein is freely combinable with any one or more of the other embodiments referenced herein, and any number of features of different embodiments are combinable with one another, unless indicated otherwise.
Although the present disclosure has been described in considerable detail with reference to certain embodiments, one skilled in the art will appreciate that aspects of the present disclosure can be used in alternative embodiments to those described, which have been presented for purposes of illustration and not of limitation. Therefore, the scope of the appended claims should not be limited to the description of the embodiments contained herein.
This application claims priority to U.S. Patent Application No. 63/362,358 filed Apr. 1, 2022, which hereby is incorporated by reference in its entirety.
This invention was made with government support under 70NANB15H261 awarded by the National Institute of Standards and Technology (NIST). The government has certain rights in the invention.
| Number | Date | Country | |
|---|---|---|---|
| 63362358 | Apr 2022 | US |
