Application of Hydrogen Bonded Organic Framework Materials in the Preparation of Proton Exchange Membranes for Fuel Cells

Abstract

The present invention belongs to the technical field of fuel cells, and discloses an application of a hydrogen-bonded organic framework material in the preparation of a proton exchange membrane for a fuel cell. The proton exchange membrane is a composite membrane formed by dispersing the hydrogen-bonded organic framework material in Nafion. The chemical formula of the hydrogen-bonded organic framework material is [(CN<subgt;3< / subgt;H<subgt;6< / subgt;)<subgt;2< / subgt>(C<subgt;10< / subgt>O<subgt;8< / subgt>H<subgt;4< / subgt>). The proton conductivity of the composite membrane composed of the hydrogen-bonded organic framework material of the present invention is higher than that of a commercial Nafion membrane, which helps to realize the large-scale commercial application of proton exchange membrane fuel cells. Moreover, the hydrogen-bonded organic framework material of the present invention is crystalline, which is beneficial to exploring the proton transport mechanism of the proton exchange membrane.

Description

Technical Field

[0001] The present invention belongs to the technical field of fuel cells, and relates to the application of a hydrogen-bonded organic framework material in the preparation of a proton exchange membrane for fuel cells. Background Art

[0002] Fuel cells directly convert renewable chemical energy into electrical energy, with high theoretical efficiency and power density, and are a very promising environment-friendly power generation device. Currently, the main types of fuel cells are: proton exchange membrane fuel cells (PEMFCs); phosphoric acid fuel cells (PAFCs); solid oxide fuel cells (SOFCs); and alkaline fuel cells (AFC). Among them, proton exchange membrane fuel cells are considered to be the most potential candidates to replace traditional energy sources due to their outstanding advantages such as green and efficient, ultra-low emissions, high power density, and fast startup speed. In proton exchange membrane fuel cells, the proton exchange membrane, as the core component of the entire system, needs to meet several conditions: high proton conductivity (> 10 -2 S cm -1 ); good chemical and thermal stability; excellent gas barrier performance; good mechanical properties and processing properties of the membrane (thin film); good compatibility with other components such as bipolar plates and electrode materials; low cost; and easy large-scale production.

[0003] The first to be applied in fuel cells was the Nafion membrane produced by DuPont in 1960, which is a perfluorosulfonated polymer. It has excellent proton conduction performance (10 -1 -10 -2 S cm -1 ), however, its temperature range of use has certain limitations. When the temperature is greater than 80 °C or lower than -5 °C, the proton conduction performance will be damaged. In addition, its high cost, complex synthesis process, and problems such as high-temperature water loss and excessive swelling further limit its application. Currently, the research on proton exchange membranes in proton exchange membrane fuel cells mainly focuses on improving proton conductivity and mechanical properties at high temperatures and low humidities, including high-temperature proton exchange membranes, modified Nafion membranes, and other new proton exchange membranes. Currently, the recasting method is mainly used to develop composite membranes to modify Nafion membranes by adding hydrophilic fillers such as SiO2, TiO2, ZrO2, graphene oxide, and functionalized polymers. However, since changes in the chain structure or assembly environment will change the bicontinuous nanophase structure of the Nafion membrane, thereby reducing its proton conductivity, therefore, simultaneously achieving high proton conductivity and excellent mechanical / chemical stability of the Nafion membrane is the key to developing high-performance Nafion membranes. In addition, as an amorphous material, it is difficult to explore the proton conduction behavior and proton transport path of the Nafion membrane.

[0004] Hydrogen-bonded organic framework materials (HOFs) composed of organic small molecule monomers through hydrogen bonds, π-π stacking, and van der Waals interactions via self-assembly, as a new type of crystalline porous material, have gradually become a powerful branch of porous organic framework materials (POPs) due to their large specific surface area, high porosity, low density, high adsorption properties, etc. Compared with metal-organic frameworks (MOFs) composed of inorganic metals and organic units, HOFs avoid metal nodes and have a lower density and a larger theoretical void volume. On the other hand, compared with covalent organic framework materials (COFs), the weak interactions involved in the assembly of HOFs are conducive to the formation of larger single crystals. Through single-crystal X-ray diffraction structure analysis, detailed structural information of the compound can be obtained, and further more kinetic analyses can be carried out. The preparation of hydrogen-bonded organic framework materials is very simple, avoiding cumbersome synthetic operations. Currently, there are few reports on modifying Nafion membranes with hydrogen-bonded organic framework materials to improve their proton conductivity. Summary of the Invention

[0005] Aiming at the technical problems that most of the modification materials for existing Nafion membranes are hydrophilic materials, which will reduce proton conductivity and are not conducive to exploring their proton conduction behavior and proton transport path, the present invention provides an application of hydrogen-bonded organic framework materials in the preparation of fuel cell proton exchange membranes. The composite membrane composed of hydrogen-bonded organic framework materials and Nafion has a high proton conductivity, which helps to realize the large-scale commercial application of proton exchange membrane fuel cells, and the crystalline hydrogen-bonded organic framework materials are conducive to exploring the proton transport mechanism of proton exchange membranes.

[0006] To achieve the above object, the present invention adopts the following technical solutions:

[0007] The present invention provides an application of hydrogen-bonded organic framework materials in the preparation of fuel cell proton exchange membranes. The proton exchange membrane is a composite membrane formed by dispersing the hydrogen-bonded organic framework materials in Nafion. The chemical formula of the hydrogen-bonded organic framework materials is [(CN3H6)2(C 10 O8H4)].

[0008] In one technical solution, the preparation method of the hydrogen-bonded organic framework materials includes the following steps: 1,2,4,5-benzenetetracarboxylic acid, guanidine hydrochloride, and distilled water are added to a reaction kettle, and heated at 160 °C for 72 h, cooled to room temperature, filtered, and dried to obtain colorless strip-shaped crystals.

[0009] In one technical solution, the molar ratio of 1,2,4,5-benzenetetracarboxylic acid to guanidine hydrochloride is 1:2.

[0010] The chemical formula of the hydrogen-bonded organic framework materials prepared by the above method is [(CN3H6)2(C10 O8H4)] belongs to the monoclinic system, space group P21 / n, and the unit cell parameters are as follows: α = 90.00°, β = 100.405(12)°, γ = 90.00°. The asymmetric unit of the hydrogen-bonded organic framework material contains a guanidinium cation CN3H6 + and half a carboxylic acid anion C 10 O8H4 2- The three-dimensional structure of the hydrogen-bonded organic framework material is as follows: Each carboxylic acid anion C 10 O8H4 2- contains two types of intramolecular O-H…O hydrogen bonds and is connected to four adjacent guanidinium cations CN3H6 + through eight intermolecular N-H…O hydrogen bonds to form a one-dimensional chain. The one-dimensional chains are further extended into a three-dimensional hydrogen bond framework structure through intermolecular hydrogen bonds N3-H3A…O1 and N1-H1A…O2.

[0011] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0012] The powder proton conductivity of the [(CN3H6)2(C 10 O8H4)] compound prepared in the present invention reaches 2.83×10 -3 S cm -1 under the conditions of 358K and 98% RH. The proton conductivity of the composite membrane composed of it and Nafion membrane reaches 1.68×10 -2 S cm -1 , which is higher than the proton conductivity of commercial Nafion membrane. Moreover, the compound has simple synthesis conditions and high yield. Its stable three-dimensional framework structure helps to improve the mechanical stability of Nafion membrane and contributes to the large-scale commercial application of proton exchange membrane fuel cells. In addition, the present invention studies that the proton transport mechanism of the crystalline hydrogen-bonded organic framework material follows the Grotthuss and vehicle mixed mechanism, while the proton transport mechanism of the composite membrane follows the Grotthuss mechanism. Brief Description of the Drawings

[0013] Figure 1 is the structure of the [(CN3H6)2(C 10 O8H4)] compound of the present invention, where Figure 1 a is the asymmetric unit of the compound; Figure 1 b is the one-dimensional chain structure of the compound; Figure 1 c is the three-dimensional structure of the compound, Figure 1 d is the topological simplified structure of the compound.

[0014] Figure 2 is the [(CN3H6)2(C 10XRD patterns of the [(CN3H6)2(C

[0015] Figure 3 O8H4)] compound at different humidities. 10 Crystal planes of the single crystal of the [(CN3H6)2(C

[0016] Figure 4 O8H4)] compound determined by the single crystal X-ray diffractometer of the present invention. 10 Results of single crystal proton conductivity tests of different crystal planes of the single crystal sample of the [(CN3H6)2(C Figure 4 a is the Nyquist plot in the

[100] direction; Figure 4 b is the Nyquist plot in the

[010] and

[001] directions.

[0017] Figure 5 Results of powder proton conductivity tests of the [(CN3H6)2(C 10 O8H4)] compound of the present invention, where Figure 5 a is the Nyquist plot at different relative humidities at 298K; Figure 5 b is the Nyquist plot at different temperatures at 98% RH; Figure 5 c is the ln(σT) vs 1000T -1 curve; Figure 5 d is the PXRD pattern before and after the proton conductivity test.

[0018] Figure 6 Characterization results of the proton exchange composite membrane of the present invention, where Figure 6 a is a picture of the composite membrane; Figure 6 b is the SEM picture of the composite membrane; Figure 6 c is the infrared spectrum of the composite membrane; Figure 6 d is the XRD pattern of the composite membrane.

[0019] Figure 7 Proton conductivity test results of the proton exchange composite membrane of the present invention, where Figure 7 a is the Nyquist plot at different relative humidities at 298K; Figure 7 b is the Nyquist plot at different temperatures at 98% RH; Figure 7 c is the ln(σT) vs 1000T -1 curve; Figure 7 d is the comparison of proton conductivities of the composite membrane and the Nafion membrane used at different temperatures at 98% RH. Detailed implementation manners

[0020] The following examples are used to illustrate the present invention, but are not intended to limit the scope of protection of the present invention. Unless otherwise specified, the technical means used in the examples are conventional means well known to those skilled in the art. The test methods in the following examples are all conventional methods unless otherwise specified.

[0021] Example 1 Preparation and Characterization of Hydrogen Bonded Organic Framework Materials

[0022] A mixture of 1,2,4,5-benzenetetracarboxylic acid (0.2 mmol, 0.050 g), guanidine hydrochloride (0.4 mmol, 0.038 g) and 6 mL of distilled water was placed in a 25 mL polytetrafluoroethylene stainless steel autoclave, heated at 160 °C for three days, cooled to room temperature, filtered and dried to obtain colorless strip crystals (yield: 61.7%, calculated based on guanidine hydrochloride).

[0023] 1) Elemental analysis (%): Theoretical values: C 38.71, N 22.57, H 4.33, Experimental values: C 38.75, N 22.41, H 4.25. Infrared spectrum data (KBr, cm -1 ) : 3423(m), 3210(m), 2820(w), 2724(w), 2217(w), 1665(m), 1583(m), 1457(m), 1361(m), 1133(w), 1082(w), 748(vs), 692(s), 581(w).

[0024] 2) A single crystal of appropriate size was selected for X-ray diffraction analysis on a Bruker Apex-II CCD diffractometer at a test temperature of 185(2) K. The graphite monochromated MoKα ray (λ = 0.071073 nm) was used to collect the crystal diffraction point data, and the data was reduced and absorption corrected by the direct method. The structure was analyzed and refined by the SHELXS-2014 and SHELXL-2014 programs. The coordinates of non-hydrogen atoms in the structure were corrected for anisotropic temperature factors by the full matrix least squares method, and the coordinates of hydrogen atoms were obtained by the difference Fourier synthesis method. The crystallographic data of the compound are shown in Table 1.

[0025] Table 1 Crystallographic data of [(CN3H6)2(C 10 O8H4)] compound

[0026]

[0027]

[0028] X-ray single crystal diffraction analysis shows that the compound crystallizes in the monoclinic system, space group P21 / n. As Figure 1As shown in a, the asymmetric unit of the compound contains a guanidinium cation CN3H6 + and half of a carboxylate anion C 10 O8H4 2- ; As Figure 1 shown in b, there are two kinds of O-H…O intramolecular hydrogen bonds in each carboxylate anion C 10 O8H4 2- , and it is connected to four adjacent guanidinium cations CN3H6 + through eight N-H…O intermolecular hydrogen bonds to form a one-dimensional chain; As Figure 1 shown in c, the one-dimensional chains are further extended into a three-dimensional hydrogen bond framework structure through two kinds of N-H…O intermolecular hydrogen bonds (N3-H3A…O1, N1-H1A…O2). At the same time, along the a direction, there is a π-π stacking interaction between the benzene ring molecules of the C 10 O8H4 2- anion, and the centroid distance between the rings is This π-π stacking interaction makes the three-dimensional framework of the compound more stable.

[0029] To understand the hydrogen bond network in the compound more clearly, a topological analysis of the structure was carried out. As Figure 1 shown in d, the carboxylate anion C 10 O8H4 2- acts as a 6-connected point, and the guanidinium cation CN3H6 + is a 3-connected point. Then the three-dimensional hydrogen bond network of the compound can be simplified to an rtl topology.

[0030] 3) By measuring the X-ray powder diffraction (PXRD) of the [(CN3H6)2(C 10 O8H4)] compound at a series of different humidities, as Figure 2 shown, its PXRD pattern is consistent with the theoretical value obtained by X-ray single crystal diffraction, indicating that the [(CN3H6)2(C 10 O8H4)] compound has good stability at different humidities.

[0031] 4) Single crystal proton conductivity test of the [(CN3H6)2(C 10 O8H4)] compound.

[0032] Select a single crystal strip of appropriate size, and determine its three crystal planes as

[100] ,

[010] and

[001] through an X-ray single crystal diffractometer, as Figure 3 shown. And AC impedance (AC) tests of different crystal planes were carried out, as Figure 4 shown. It can be seen that under the conditions of 298K and a relative humidity of 98%, the single crystal sample of the compound shows high proton conduction behavior in the

[100] direction, and the proton conductivity is 1.78×10-2 S cm -1 ( Figure 4 a), the proton conductivity in this direction is 3 - 5 orders of magnitude higher than those in the other two directions

[010] and

[001] , which are 3.06×10 -7 S cm -1 and 1.98×10 -5 S cm -1 respectively. Figure 4 b), exceeding the proton conduction performance of most single-crystal samples of HOFs and MOFs.

[0033] 5) Powder proton conductivity test of [(CN₃H₆)₂(C 10 O₈H₄)] compound.

[0034] The crystal of the compound was ground into powder. A 10 mg powder sample was placed on a press die with a diameter of 3 mm and pressed for about two minutes under a pressure of 0.5 MPa to obtain a wafer with a thickness of 0.265 mm. Conductive silver paste was evenly coated on the upper and lower cross-sections of the wafer, and it was fixed on the sample stage with a gold wire for testing. The instrument used for the alternating current impedance (AC) test of the compound was a high-precision impedance gain phase analyzer (Solartron 1260 / 1296). The measured frequency range was 0.01 - 10 MHz, the input voltage was 100 mV, the measured temperature range was from 298 K to 358 K, and the relative humidity range was from 55% RH to 98% RH. The conductivity σ (S cm -1 ) was calculated by the formula σ = L / (RA), where L (cm) and A (cm 2 ) represent the thickness and cross-sectional area of the wafer respectively, and R (Ω) is the resistance value of the sample, which was obtained by simulating the impedance data in the Nyquist plot using the Zview equivalent circuit method. The conductivity of the compound was calculated from the resistance in the Nyquist plot. The activation energy (E a ) was obtained by the Arrhenius formula σT = σ o exp(-E a / kT), where k is the Boltzmann constant (eV / k) and T (K) is the temperature.

[0035] Nyquist plots at different relative humidities at 298 K, as shown in Figure 5 a. At 298 K, as the relative humidity increased from 55% to 98%, the conductivity of the compound continuously increased, from 3.49×10 -9 S cm -1 to 3.54×10 -5 S cm -1 , an increase of four orders of magnitude, indicating that humidity has a great influence on the test of proton conductivity.

[0036] Nyquist plots at different temperatures with a relative humidity of 98%, as Figure 5 shown in Fig. b. When the relative humidity is 98% and the temperature is continuously increased to 358 K, the proton conductivity of the compound reaches 2.83×10 -3 S cm -1 , which is higher than the proton conductivities of existing metal-organic framework materials and hydrogen-bonded organic framework materials, and it is a very promising proton-conducting material.

[0037] To further explore the proton transport mechanism, the temperature-dependent proton conductivity was fitted using the Arrhenius equation, and a plot of [ln(σT) vs 1000 / T -1 was made ( Figure 5 Fig. c), and the activation energy E a of this compound was calculated to be 0.46 eV, following the Grotthuss and vehicle mixed mechanism. In addition, the sample after proton conductivity testing was subjected to X-ray powder diffraction (PXRD) testing, as Figure 5 shown in Fig. d. The positions of the peaks in the XRD pattern are basically consistent with the theoretical peaks of the XRD pattern simulated from single-crystal data, indicating that the structure of the compound did not change after testing.

[0038] Example 2 Preparation and Characterization of Proton Exchange Composite Membrane

[0039] 2 mL of 5 wt% Nafion solution (from DuPont Nafion membrane solution D520) was evaporated and concentrated to 1 mL at 50 °C, then 2 mL of DMF was added, and it was evaporated and concentrated to 1.5 mL at 60 °C to form a mixture. Subsequently, 0.1 g of the [(CN3H6)2(C 10 O8H4)] compound ground into powder was uniformly dispersed in 1.5 mL of the mixture, stirred for 3 h, transferred to an evaporating dish with a diameter of 35 mm, and placed in a vacuum drying oven at 60 °C to obtain a composite membrane. The composite membrane was boiled in 3 wt% H2O2 solution, then soaked in 1 mol / L H2SO4 solution, and then washed with distilled water and dried at room temperature. The content of the compound in the composite membrane is 53%. The appearance picture of the composite membrane is as Figure 6 shown in Fig. a.

[0040] The composite membrane was subjected to scanning electron microscopy (SEM) testing, as Figure 6 shown in Fig. b. It can be seen from the figure that the surface of the composite membrane is smooth, indicating that the [(CN3H6)2(C 10 O8H4)] compound is uniformly dispersed in the Nafion solution.

[0041] The potassium bromide tablet method was used to measure in the range of 400 - 4000 cm -1in the range of [(CN3H6)2(C 10 O8H4)] compound and the composite membrane were respectively tested for infrared spectra using a Bruker VERTEX-70 Fourier transform infrared spectrometer. As Figure 6 shown in c, the infrared characteristic peaks of the compound and Nafion were shown in the composite membrane, indicating the successful preparation of the composite membrane. Similarly, by comparing the XRD patterns of [(CN3H6)2(C 10 O8H4)] compound and the composite membrane (as Figure 6 shown in d), 2θ = 17° represents the characteristic peak of Nafion, which further indicates the successful preparation of the composite membrane.

[0042] We also carried out proton conductivity tests on the composite membrane. The size of the composite membrane used for the test was: length × width = 10 mm × 2 mm, and the test method was the same as that in Example 1. As Figure 7 shown in a, the proton conductivity of this composite membrane was 3.77×10 -5 S cm -1 at 298K and 55% RH; with the continuous increase of relative humidity, the proton conductivity reached 1.51×10 -3 S cm -1 at 298K and 98% RH, increasing by two orders of magnitude. As Figure 7 shown in b, at 98% RH, when the temperature was raised to 358K, the proton conductivity of the composite membrane reached 1.68×10 -2 S cm -1 , which was higher than the proton conductivity of the Nafion membrane used ( Figure 7 d). The activation energy E a of the composite membrane was obtained by fitting with the Arrhenius formula as 0.38 eV (as Figure 7 shown in c), following the Grotthuss mechanism.

[0043] The above-described embodiments are only the preferred embodiments of the present invention, which are only used to explain the present invention and do not limit the scope of implementation of the present invention. For those skilled in the art of this technology, of course, other implementation manners can be easily made according to the technical content disclosed in this specification by means of substitution or change. Therefore, all changes and improvements made on the principle of the present invention should be included within the scope of the patent application of the present invention.

Claims

1. Application of a hydrogen-bonded organic framework material in preparing a proton exchange membrane for a fuel cell, characterized in that, The proton exchange membrane is a composite membrane formed by dispersing the hydrogen-bonded organic framework material in Nafion. The chemical formula of the hydrogen-bonded organic framework material is [(CN3H6)2(C 10 O8H4)], and the hydrogen-bonded organic framework material belongs to the monoclinic system, P 21 / n space group. The unit cell parameters are: a = 3.7873(10) Å, b = 20.098(7) Å, c = 10.382(3) Å, α = 90.00°, β = 100.405(12)°, γ = 90.00°; The asymmetric unit of the hydrogen-bonded organic framework material contains one guanidinium cation CN3H6 + and half a carboxylic acid anion C 10 O8H4 2‒ . The three-dimensional structure is as follows: There are two O‒H…O intramolecular hydrogen bonds in each carboxylic acid anion C 10 O8H4 2‒ . It is connected to the adjacent four guanidinium cations CN3H6 + through eight N‒H…O intermolecular hydrogen bonds to form a one-dimensional chain. The one-dimensional chains are further extended into a three-dimensional hydrogen-bonded framework structure through the intermolecular hydrogen bonds N3‒H3A…O1 and N1‒H1A…O2.

2. The application according to claim 1, wherein The preparation method of the hydrogen-bonded organic framework material comprises the following steps: 1,2,4,5-benzenetetracarboxylic acid, guanidine hydrochloride and distilled water are added into a reaction kettle, and heated at 160 °C for reaction for 72 h, cooled to room temperature, filtered and dried to obtain colorless strip crystals.

3. The application according to claim 1, characterized in that The molar ratio of the 1,2,4,5-benzenetetracarboxylic acid to the guanidine hydrochloride is 1:2.

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