Method for detecting lithium ions in ion exchange membrane and application

Abstract

The invention belongs to the field of nuclear magnetic resonance technology detection, and particularly relates to a detection method and application of lithium ions in an ion exchange membrane. The detection method comprises the following steps: (1) preparing a sample from a soaked ion exchange membrane, winding the sample on a uniform bar, and loading the sample into the bottom of a nuclear magnetic tube in a screw-in manner; the shape of the sample comprises a polygon with a symmetrical structure and no less than 4 edges; (2) putting the nuclear magnetic tube prepared in the step (1) into a nuclear magnetic resonance spectrometer, selecting a pulse sequence, tuning, shimming, adjusting nuclear magnetic resonance test parameters, and collecting signals; the nuclear magnetic resonance test parameters comprise that the 90-degree pulse width is 10-14 [mu] s; and the power is 30-55W. The pulse width of a nuclear magnetic sample obtained by adopting a specific sample preparation method at 90 degrees is 10-14 microseconds; under the power of 30-55W, 7Li in the ion exchange membrane bulk phase can be detected, and a powerful support is provided for detecting and analyzing structural information in the ion exchange membrane bulk phase.

Description

Technical Field

[0001] The present invention belongs to the field of nuclear magnetic resonance technology detection, and specifically relates to a detection method and application of lithium ions in an ion exchange membrane. Background Art

[0002] Ion exchange membranes are thin films made from polymer materials, typically plastics, adhesives, rubber, or other organic polymers. They include various functional films, such as optical films, water treatment films, battery separators, and conductive films. They are widely used in optical devices, photovoltaic power generation, wastewater treatment, element enrichment, desalination, lithium-ion batteries, and other fields in medicine, food, agriculture, and the chemical industry, offering broad application prospects. Detecting the internal structure, surface morphology, mechanical properties, and electrochemical performance of ion exchange membranes has long been a major research focus in the field. Currently, the main characterization techniques for ion exchange membranes include thermogravimetric analysis, infrared spectroscopy, Raman spectroscopy, atomic force microscopy, and electron microscopy.

[0003] Liquid-State NMR technology has become a core means of chemical structure characterization and dynamic process research due to its excellent resolution (δ≤0.1ppm), non-destructive detection characteristics and multi-dimensional spectral analysis capabilities. However, ion exchange membranes, as a key component of energy conversion and storage systems (such as fuel cells and flow batteries), are functional materials with special microphase separation structures. Their performance is highly dependent on the ion transport channel network and the distribution of hydrophilic and hydrophobic domains in the solid state. If the traditional dissolving liquid NMR detection method is used, its intrinsic structure will be destroyed, resulting in the loss of key microstructural information (such as ion cluster size, connectivity and dynamic behavior). Therefore, how to use liquid NMR to efficiently characterize the microenvironment and ion transport mechanism of ion exchange membranes while maintaining the structural integrity of the membrane material is still a difficulty in the current technical field.

[0004] In addition, lithium ion (Li + ) due to its unique physical and chemical properties, it is widely used in energy storage and battery technology (lithium-ion batteries, solid-state lithium batteries, lithium-sulfur batteries), functional materials and chemical engineering (ion exchange membranes, solid electrolytes, polymer electrolytes), biomedicine and neuroscience (lithium salt drugs for the treatment of mental illness, etc.), nuclear energy and isotope separation, and lithium-ion sensors. Liquid nuclear magnetic resonance technology for lithium ions (Li + ) plays a key role in the fields of energy storage and materials science. However, the Li nuclear spin (I = 3 / 2) has a quadrupole moment broadening effect and is limited by low sensitivity (the gyromagnetic ratio is only 1H, and the signal-to-noise ratio (SNR) is reduced by approximately three orders of magnitude. In asymmetric electric field gradient environments, such as at the binding sites in ion exchange membranes or at solid-state electrode interfaces, the spectral peaks are significantly broadened and the resolution is reduced, significantly limiting its practical application. Therefore, it is urgent to develop an NMR detection method that can maintain the intrinsic microstructure of ion exchange membrane materials. Summary of the Invention

[0005] Therefore, the technical problem to be solved by the present invention is how to detect the nuclear magnetic resonance signal of lithium ions in the ion exchange membrane on the basis of maintaining the intrinsic microstructure of the ion exchange membrane material, thereby providing a method for detecting lithium ions in the ion exchange membrane.

[0006] The present invention innovatively provides a method for detecting the nuclear magnetic signals of lithium ions in ion exchange membranes using liquid nuclear magnetic resonance technology. By optimizing sample processing technology and pulse sequence design, it realizes the simultaneous and efficient detection of lithium ion signals and free lithium ion signals in ion exchange membranes for the first time, providing a new method for analyzing the structural performance of ion exchange membranes.

[0007] The present invention provides a method for detecting lithium ions in an ion exchange membrane, comprising the following steps:

[0008] (1) The soaked ion exchange membrane is made into a sample, the sample is wound on a uniform rod, the sample is loaded into the bottom of the nuclear magnetic tube by screwing, and the rod is pulled out or retained in the nuclear magnetic tube; the shape of the sample includes a polygon with a symmetrical structure and a side number of not less than 4; it should be explained that: soaking here refers to soaking the ion exchange membrane in a solution; or, for an ion exchange membrane used in a liquid environment, this process also belongs to soaking; illustratively, the polygon includes a quadrilateral, a pentagon, a hexagon, etc., and the polygon satisfies the requirement that the side number is not less than 4 and has a symmetrical structure; further, the polygon also includes chamfers and / or rounded corners. The present invention has the advantages of a simple nuclear magnetic sample preparation method, good field uniformity effect, high detection sensitivity and spectral resolution during detection. When detecting lithium ions, the present invention requires that the ion exchange membrane will not bend or break when wrapped around the glass rod, and requires that the ion exchange membrane be placed in a regular and orderly manner in the nuclear magnetic tube.

[0009] (2) placing the nuclear magnetic resonance tube obtained in step (1) into a nuclear magnetic resonance spectrometer, selecting a pulse sequence, tuning and shimming, adjusting nuclear magnetic resonance test parameters, and collecting signals; the nuclear magnetic resonance test parameters include: a 90° pulse width of 10 to 14 μs; and a power of 30 to 55 W.

[0010] The principle of using liquid nuclear magnetic resonance to collect lithium ion signals in ion exchange membranes provided by the present invention is: the ion exchange membrane swells after being immersed in a solution. Since the ion exchange membrane can selectively pass ions (for example, including chlorine, sodium, lithium, etc.), ions and water molecules exist simultaneously inside the bulk phase of the ion exchange membrane, that is, the ions and water molecules form countless microscopic liquid environments in the ion exchange membrane. Therefore, the nuclear magnetic signals of lithium atoms in the ion exchange membrane can be successfully detected using liquid nuclear magnetic resonance.

[0011] The cation exchange membrane has the characteristics of cation selective transmission, which can make Li + The lithium ion concentration in the ion exchange membrane is low. The existing liquid NMR detection technology is limited by the inherent thinking of liquid NMR sample preparation and the low lithium ion concentration in the ion exchange membrane phase, which is much lower than that in the liquid. Therefore, it is impossible to detect the lithium ion concentration in the membrane phase. + Accurate analysis. The present invention adopts a specific sampling method to prepare the nuclear magnetic detection sample, and performs the test under specific nuclear magnetic resonance test parameters (90° pulse width is 10-14μs; power is 30-55W). On the basis of not destroying the structural state of the ion exchange membrane, the nuclear magnetic signal of the lithium ions inside the ion exchange membrane bulk phase can be collected, providing strong support for the detection and analysis of the structural information inside the ion exchange membrane bulk phase. The signals collected by the conventional liquid nuclear magnetic sampling method are mainly free atoms, that is, lithium ions in the nuclear magnetic tube solution (located outside the membrane body), and the lithium ion signals on the ion exchange membrane cannot be detected. By adopting the sampling method of the present invention and combining it with the detection method of the present invention, the nuclear magnetic signal of lithium atoms in the ion exchange membrane bulk phase can be detected.

[0012] As an optional embodiment, the nuclear magnetic resonance test parameters also include: pulse sequence is zg; relaxation time: 1 to 5s; number of sampling points: 16k to 32k, k is 1024; sampling time: 1 to 5s; spectrum width is 60 to 120ppm; spectrum range is: -60ppm to +60ppm, cumulative number of times: 4 to 256 times;

[0013] Preferably, the spectrum width is 40 ppm and the spectrum range is -20 ppm to +20 ppm.

[0014] As an optional implementation, the 90° pulse width is 12 μs; and the power is 40 W.

[0015] Exemplarily, the 90° pulse width is 10 μs, 11 μs, 12 μs, 13 μs, or 14 μs; and the power is 30 W, 35 W, 40 W, 45 W, or 55 W.

[0016] It should be noted that after selecting the pulse sequence, the 7Li tuning channel is also selected. Different nuclides have different resonant frequencies, so independent tuning and matching are required. The 7Li tuning channel is in the X channel. For example, using a 600 MHz resonant spectrometer as the detection instrument, the resonant frequency is 233.2 MHz. The resonant frequency of each nuclide in the detection instrument is related to the field strength of the detection instrument and the type of nuclide. The resonance frequency can be determined based on the specific nuclide and the field strength of the detection instrument.

[0017] As an optional embodiment, the lithium is 7Li; and / or,

[0018] The ion exchange membrane is a cation exchange membrane; preferably, the ion exchange membrane comes from a battery.

[0019] The ion exchange membrane may also come from a device provided with an ion exchange membrane, such as a diaphragm in a fuel cell, an electrodialysis device or an electrolysis device.

[0020] As an optional embodiment, the step (2) further comprises adding 5 to 20 μL of a solution containing lithium ions before placing the nuclear magnetic resonance tube into the nuclear magnetic resonance spectrometer;

[0021] Preferably, the concentration of the lithium ion-containing solution is 0.2 to 2.0 mol / L;

[0022] Preferably, the concentration of the lithium ion-containing solution is 1.0 mol / L.

[0023] The method of the present invention can also simultaneously detect the nuclear magnetic resonance signals of lithium ions inside the ion exchange membrane bulk and lithium ions outside the membrane (in a free state). Based on the difference in chemical shifts of the nuclear magnetic signals of lithium atoms inside and outside the cation exchange membrane, the transport performance of the cation exchange membrane for lithium ions can be judged, providing strong support for the structural performance analysis of the cation exchange membrane, and further providing a new idea for screening the special functions of cation exchange membranes used in different fields.

[0024] As an optional embodiment, the solution containing lithium ions includes a soluble lithium salt;

[0025] Preferably, the soluble lithium salt includes at least one of LiCl, Li2SO4, LiNO3, and CHCOOLi.

[0026] As an optional embodiment, the solvent in the lithium ion-containing solution includes a deuterated solvent or a non-deuterated solvent;

[0027] Preferably, the solvent comprises at least one of water, chloroform (CHCl3), dimethyl sulfoxide (DMSO), dichloromethane (CH2Cl2), methanol, and ethanol;

[0028] Preferably, when the solvent in the lithium ion-containing solution includes a deuterated solvent, a field lock step is further performed before field shimming.

[0029] As an optional embodiment, in step (1), the immersion solution can dissociate lithium ions. It should also be noted that during the sample preparation process, the immersion solution of the ion exchange membrane includes a solution that can dissociate Li + A compound capable of dissociating Li + The solution of the compound is selected from any of the above-mentioned lithium ion-containing solutions. The ion exchange membrane soaking solution can be the same as or different from the lithium ion-containing solution, preferably the same. The present invention has no specific requirements for the soaking time, and can be as long as the ion exchange membrane swells and contains ions and water molecules, such as 1 hour, 3 hours, 8 hours, 11 hours, 14 hours, 17 hours, 20 hours, etc.

[0030] As an optional embodiment, in step (1), the shape of the sample is square; and / or,

[0031] The diameter of the rod is 2 to 3 mm; and / or,

[0032] The rod material includes a rubber rod, a glass rod or a metal rod; and / or,

[0033] The rod is a hollow cylinder. The rod is a hollow cylinder, meaning it can be an inner lining tube, with a hollow interior where liquid can be added. In this case, a deuterated reagent, such as deuterated water, deuterated DMSO, deuterated chloroform, or other common deuterated reagents, can be added to the hollow center of the rod. This allows for a lock field to accurately locate the chemical shift of the atomic nucleus to be measured, while also preventing the deuterated reagent from affecting the ion exchange membrane structure. Furthermore, since deuterated reagents are relatively expensive, a lock field can be achieved using an inner lining tube, saving on the use of deuterated reagents.

[0034] When the sample is square, the length is 4-5 cm. It should be noted that the sample size can be selected based on the rod material, as long as it can wrap around the rod. Square includes square, rectangular, etc.

[0035] As an optional embodiment, the ion exchange membrane comprises a cation exchange membrane, of which several commercial models are listed here, ASTOM Neosepta CXP-S, Fumaep-E-620(K), Fumasep FKS-30, and CMI-7000S.

[0036] The present invention provides the application of the above-mentioned detection method to the detection of diaphragms in fuel cells, electrodialysis devices, or electrolysis devices. This application can detect the lithium ion signal of used ion exchange membranes, providing strong support for the analysis of the structure and performance of ion exchange membranes.

[0037] The technical solution of the present invention has the following advantages:

[0038] 1. The present invention provides a method for detecting lithium ions in an ion exchange membrane, comprising: (1) preparing a sample of the soaked ion exchange membrane, wrapping the sample around a uniform rod, loading the sample into the bottom of a nuclear magnetic resonance tube by a precession method, and extracting the rod or retaining it in the nuclear magnetic resonance tube; the shape of the sample includes a polygon with a symmetrical structure and a number of sides not less than 4; (2) placing the nuclear magnetic resonance tube prepared in step (1) into a nuclear magnetic resonance spectrometer, selecting a pulse sequence, tuning and shimming, adjusting nuclear magnetic resonance test parameters, and collecting signals; the nuclear magnetic resonance test parameters include: a 90° pulse width of 10 to 14 μs; and a power of 30 to 55 W. The nuclear magnetic resonance sample obtained by the present invention using a specific sample preparation method has a good shimming effect, high detection sensitivity and spectral resolution, and can detect 7Li inside the ion exchange membrane bulk phase when the 90° pulse width is 10 to 14 μs and the power is 30 to 55 W, providing strong support for detecting and analyzing structural information inside the ion exchange membrane bulk phase.

[0039] Furthermore, the method of the present invention can also simultaneously detect the nuclear magnetic resonance signals of lithium ions inside the ion exchange membrane bulk and lithium ions outside the membrane (free state). Based on the difference in chemical shift of the nuclear magnetic signals of lithium atoms inside and outside the cation exchange membrane, the transport performance of the cation exchange membrane for lithium ions can be judged, and the membrane structure performance can be analyzed, thereby providing a new idea for screening the special functions of cation exchange membranes used in different fields. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the specific embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0041] Figure 1 This is the NMR spectrum obtained by testing in Example 1 of the present invention;

[0042] Figure 2 This is the NMR spectrum obtained by testing in Example 2 of the present invention.

[0043] Figure 3 This is the nuclear magnetic spectrum obtained by testing comparative example 1 of the present invention.

[0044] Figure 4 This is the state of the ion exchange membrane in the nuclear magnetic tube of Example 1 of the present invention, referred to as the sample schematic diagram;

[0045] Figure 5 This is the state of the ion exchange membrane of Comparative Example 1 of the present invention in the nuclear magnetic tube, referred to as the sample schematic diagram. DETAILED DESCRIPTION

[0046] The following examples are provided for a better understanding of the present invention and are not intended to limit the best mode of implementation. They do not limit the content and scope of protection of the present invention. Any product identical or similar to the present invention obtained by anyone under the guidance of the present invention or by combining the features of the present invention with other prior arts shall fall within the scope of protection of the present invention.

[0047] If no specific experimental steps or conditions are specified in the examples, the conventional experimental steps or conditions described in the literature in this field can be used. If the manufacturer of the reagents or instruments is not specified, they are all commercially available conventional reagents.

[0048] Example 1

[0049] This embodiment provides a method for detecting lithium ions in an ion exchange membrane, comprising the following steps:

[0050] (1) Ion exchange membrane (Fumasep FKS-30) was immersed in 1 mol / L LiCl aqueous solution (solvent is non-deuterated reagent) for 24 h. After removal, it was wiped dry. The ion exchange membrane was cut into regular square samples with a length of 4 cm. Then, it was wrapped around a glass rod with a diameter of 3 mm. The sample was evenly wrapped around the bottom of the NMR tube by screwing it out. The glass rod was pulled out. The state of the sample in the NMR tube was shown in Fig. Figure 4 .

[0051] (2) Place the NMR tube in the NMR spectrometer, tune and homogenize the field. The detection instrument is a 600 MHz liquid NMR spectrometer with a resonance frequency of 233.2 MHz. The NMR test parameters include: 90° pulse width of 12 μs; power of 40 W; pulse sequence of zg; relaxation time: 1 s; number of sampling points: 32 k, k is 1024; sampling time: 1.7 s; spectral width of 40 ppm; spectral range of -20 ppm to +20 ppm; cumulative number of times: 128 times.

[0052] NMR spectrum see Figure 1 As can be seen from the figure, the chemical shift of 7Li is -0.23ppm. The sample preparation method and the testing method of the present invention can obtain the nuclear magnetic signal of 7Li in the ion exchange membrane with high resolution and high signal-to-noise ratio.

[0053] Example 2

[0054] This embodiment provides a method for detecting lithium ions in an ion exchange membrane, comprising the following steps:

[0055] (1) The ion exchange membrane (same as in Example 1) was immersed in a 1 mol / L LiCl aqueous solution (the solvent was a non-deuterated reagent), removed and dried, and the ion exchange membrane was cut into a regular square sample with a length of 4 cm. The sample was then wrapped around a glass rod with a diameter of 3 mm. The sample was evenly placed at the bottom of the NMR tube by screwing it inward, and the glass rod was removed. 10 μL of the 1 mol / L LiCl aqueous solution was added to the NMR tube.

[0056] (2) Place the NMR tube in the NMR spectrometer, tune and homogenize the field. The detection instrument is a 600 MHz liquid NMR spectrometer with a resonance frequency of 233.2 MHz. The NMR test parameters include: 90° pulse width of 12 μs; power of 40 W; pulse sequence of zg; relaxation time: 1 s; number of sampling points: 32 k, k is 1024; sampling time: 1.7 s; spectral width of 40 ppm; spectral range of -20 ppm to +20 ppm; cumulative number of times: 128 times.

[0057] NMR spectrum see Figure 2 The chemical shift of 7Li inside the membrane is -0.23ppm, and the chemical shift of 7Li outside the membrane is 0.1ppm. In addition, since the movement of 7Li atoms inside the ion exchange membrane bulk is restricted, the transverse relaxation time (T2) of 7Li becomes shorter, and the spectral peak is broadened relative to the outside of the membrane, indicating that the present invention can detect the nuclear magnetic resonance signals of 7Li inside and outside the ion exchange membrane bulk with high resolution and signal-to-noise ratio.

[0058] Comparative Example 1

[0059] This comparative example provides a method for detecting lithium ions in an ion exchange membrane, comprising the following steps:

[0060] (1) The ion exchange membrane (same as in Example 1) was immersed in a 1 mol / L LiCl aqueous solution, taken out and dried, curled and cut into small pieces (about 1 cm long) and then inserted into the NMR tube. The ion exchange membrane appeared as irregular clumps in the NMR tube. The sample schematic is shown in FIG. Figure 5 .

[0061] (2) Place the NMR tube in the NMR spectrometer, tune and homogenize the field. The detection instrument is a 600 MHz liquid NMR spectrometer with a resonance frequency of 233.2 MHz. The NMR test parameters include: 90° pulse width of 12 μs; power of 40 W; pulse sequence of zg; relaxation time: 1 s; number of sampling points: 32 k, k is 1024; sampling time: 1.7 s; spectral width of 40 ppm; spectral range of -20 ppm to +20 ppm; cumulative number of times: 128 times.

[0062] NMR spectrum see Figure 3 ,Depend on Figure 3It can be seen that although the ordinary sample preparation method can detect the 7Li NMR signal in the membrane bulk phase, the uniform field effect is deteriorated, the spectrum is broadened, and the spectrum resolution is reduced, which has no guiding significance for judging the structural performance of the ion exchange membrane.

[0063] Obviously, the above embodiments are merely examples for clarity of explanation and are not intended to limit the implementation methods. Those skilled in the art will appreciate that other variations or modifications can be made based on the above description. It is not necessary and impossible to enumerate all implementation methods here. Obvious variations or modifications arising therefrom remain within the scope of protection of the present invention.

Claims

1. A method for detecting lithium ions in an ion exchange membrane, characterized in that: The following steps are involved: (1) preparing a sample of the soaked ion exchange membrane, winding the sample onto a uniform rod, inserting the sample into the bottom of a nuclear magnetic resonance tube by screwing, and extracting the rod or retaining it in the nuclear magnetic resonance tube; the shape of the sample includes a polygon having a symmetrical structure and a side number of not less than 4; (2) placing the nuclear magnetic resonance tube obtained in step (1) into a nuclear magnetic resonance spectrometer, selecting a pulse sequence, tuning, shimming, adjusting nuclear magnetic resonance test parameters, and collecting signals; the nuclear magnetic resonance test parameters include: a 90° pulse width of 10 to 14 μs; The power is 30~55W.

2. The detection method according to claim 1, wherein The NMR test parameters also include: pulse sequence zg; relaxation time: 1 to 5s; number of sampling points: 16k to 32k, k is 1024; sampling time: 1 to 5s; spectrum width: 60 to 120ppm; spectrum range: -60ppm to +60ppm, cumulative number of times: 4 to 256 times; Preferably, the spectrum width is 40 ppm and the spectrum range is -20 ppm to +20 ppm.

3. The detection method according to claim 1, wherein The 90° pulse width is 12μs; the power is 40W.

4. The detection method according to any one of claims 1 to 3, characterized in that: The lithium is 7Li; and / or, The ion exchange membrane is a cation exchange membrane; preferably, the ion exchange membrane comes from a battery.

5. The detection method according to claim 1, wherein The step (2) further comprises adding 5 to 20 μL of a solution containing lithium ions before placing the nuclear magnetic resonance tube into the nuclear magnetic resonance spectrometer; Preferably, the concentration of the lithium ion-containing solution is 0.2 to 2.0 mol / L; Preferably, the concentration of the lithium ion-containing solution is 1.0 mol / L.

6. The detection method according to claim 5, characterized in that The lithium ion-containing solution includes a soluble lithium salt; Preferably, the soluble lithium salt includes at least one of LiCl, Li2SO4, LiNO3, and CHCOOLi.

7. The detection method according to claim 6, characterized in that The solvent in the lithium ion-containing solution includes a deuterated solvent or a non-deuterated solvent; Preferably, the solvent comprises at least one of water, chloroform, dimethyl sulfoxide, dichloromethane, methanol, and ethanol; Preferably, when the solvent in the lithium ion-containing solution includes a deuterated solvent, a field lock step is further performed before field shimming.

8. The detection method according to claim 1, wherein In the step (1), the immersion solution can dissociate lithium ions.

9. The detection method according to claim 1, wherein In the step (1), the sample is in a square shape; and / or, The diameter of the rod is 2 to 3 mm; and / or, The rod material includes a rubber rod, a glass rod or a metal rod; and / or, The rod is a hollow cylinder.

10. Use of the detection method according to any one of claims 1 to 9 in detecting a diaphragm in a fuel cell, an electrodialysis device or an electrolysis device.

Images