Phosphonic acid type covalent organic framework proton exchange membrane based on synergistic assisted conduction groups and preparation method thereof

Abstract

The application discloses a preparation method of a phosphoric acid type covalent organic framework proton exchange membrane based on synergistic conduction groups, and first, 2,5-diaminobenzene phosphoric acid is used as a phosphoric acid type amine monomer, the molar concentration of an amine monomer solution is 0.006-0.009 mol mL ‑1 , the molar concentration of an aldehyde monomer solution is 0.004-0.006 mol mL ‑1 , and a phosphoric acid type covalent organic framework nanosheet is synthesized through a single-phase method and a phase transfer interface polymerization method; then a series of phosphoric acid type covalent organic framework proton exchange membranes with a film thickness of 20 um are prepared through a vacuum-assisted self-assembly method. The preparation method is simple in operation and convenient to implement. By changing the number of hydroxyl groups on the aldehyde monomer, the density of oxygen-containing conduction groups in the COF is regulated, the continuity of the proton transfer channel is adjusted, and the proton conduction capacity is strengthened, thereby laying a solid material foundation for the practical application of the phosphoric acid type covalent organic framework proton exchange membrane in electrochemical energy conversion devices such as proton exchange membrane fuel cells.

Description

Technical Field

[0001] This invention belongs to the field of proton exchange membrane material preparation technology. Specifically, it relates to a phosphoric acid-based covalent organic framework proton exchange membrane based on the synergistic effect of co-conduction groups and its preparation. Background Technology

[0002] Hydrogen energy is key to breaking the "impossible triangle" of energy—safety, stability, economic feasibility, and environmental friendliness—and provides a zero-carbon solution for building a clean, low-carbon modern energy system and development. Hydrogen energy conversion utilizes electrochemical processes, offering advantages such as high efficiency, low noise, and zero pollution. Fuel cells, in particular, do not release energy through combustion but instead directly, continuously, and efficiently convert the chemical energy of hydrogen and oxygen into electrical energy through components such as proton exchange membranes (PEMs), becoming the core power unit for transforming the strategic value of hydrogen energy into end-product productivity. As one of the core components of fuel cells, the PEM plays a dual role in conducting protons and preventing gas cross-permeation, significantly impacting the power and lifespan of the fuel cell. Traditional PEMs are mostly polymers, often employing methods such as increasing the number of ionic groups to improve proton conductivity. However, excessive ionic groups weaken inter-chain interactions, resulting in a trade-off effect between proton conductivity and mechanical properties.

[0003] Covalent organic frameworks (COFs), as a new class of crystalline porous organic materials, possess a robust rigid framework, well-ordered nanochannels, and abundant and well-defined transport sites, which is conducive to constructing long-range ordered mass transfer channels. Simultaneously, the monomeric diversity of structural units endows COFs with extremely strong structural designability, enabling them to demonstrate application potential unmatched by traditional materials in many cutting-edge fields such as gas storage and separation, catalysis, photoelectric conversion, and even biomedicine. This "structural designability" advantage of COFs allows functional groups (such as phosphate groups and sulfonic acid groups) to be precisely introduced into their pore walls or framework through covalent bonds, thereby achieving "on-demand customization" of the material's physicochemical properties. Among them, phosphate groups, due to their unique moderately strong acidity and high hydration energy, can effectively promote proton dissociation and migration, thus endowing COF materials with excellent proton conductivity.

[0004] Patent document CN121172201A, published on December 19, 2025, discloses a high-performance covalent organic framework proton exchange membrane, its preparation method, and its applications. The high-performance covalent organic framework proton exchange membrane of this invention is prepared from 1,3,5-tricarboxymethyl phloroglucinol and 3,5-diaminobenzenesulfonic acid, 3,5-diaminobenzoic acid, or 1,3,5-triaminobenzene. The high water absorption capacity of the sulfonic acid groups used results in a sharp drop in the proton conductivity of the system after water evaporation, limiting its application in high-temperature and low-humidity environments.

[0005] Reference [1] with DOI number 10.1039 / d3qm00803g suggests that restricting hydrated hydrogen bonds to generate proton channels is a promising strategy to promote intrinsic proton conductivity, and high water-gathering capacity is key to proton-conducting materials. Here, two covalent organic frameworks (COFs) with excellent water absorption capacity (2,6-DHNA-TA and 2,3-DHNA-TA) were rationally designed by pre-installing proton donor groups. 2,6-DHNA-TA has hexagonal pores, with hydroxyl groups anisotropically suspended at the edges of adjacent pores to form independent hydrophilic water areas. 2,3-DHNA-TA presents hexagonal channels, with hydrophilic diketone groups in each compartment forming a wider hydrophilic band. The large canonical Monte Carlo method was used to verify that water molecules preferentially adsorb at hydrophilic sites, and that the water filling process starts at predefined independent anchor points and terminates at domain-restricted mesopores. Free water is enriched in the COF pores and rearranged into short, dense hydrogen bonds, which facilitates proton conduction at low energy barriers. Both COFs exhibit excellent proton conductivity, especially 2,6-DHNA-TA (1.27 × 10⁻³ S cm⁻¹). -1 However, this COF does not contain ionic groups and cannot directly provide freely migrating protons, thus severely limiting its conduction efficiency.

[0006] [References]

[0007] [1]R. Zhu, J. Fu, L. Chen, J. Feng, Z. Gu, Materials Chemistry Frontiers, 2023, 7(23): 5932-5940. [November 20, 2023]. Summary of the Invention

[0008] In view of the above-mentioned prior art, based on the advantage of synergistic enhancement of COF proton conduction by phosphate group transport and oxygen-containing group-assisted transport, this invention provides a phosphate-type covalent organic framework proton exchange membrane based on the synergistic transport of co-conducting groups and its preparation method.

[0009] To address the aforementioned technical problems, this invention proposes a method for preparing a phosphoric acid-based covalent organic framework proton exchange membrane based on the synergistic action of co-conduction groups, comprising the following steps:

[0010] Step 1: Synthesis of phosphoric acid-based covalent organic framework nanosheets: 2,5-Diaminophenylphosphoric acid was dissolved in solvent A to prepare nanosheets with a molar concentration of 0.006~0.009 mol / mL. -1 A solution of amine monomers; dissolving aldehyde monomers in solvent B to obtain a molar concentration of 0.004~0.006 mol / mL. -1The aldehyde monomer solution was slowly transferred to the amine monomer solution using a dropper and reacted at 16-25°C for 7-21 days. The lower aqueous phase solution was removed and placed into a dialysis bag for dialysis in deionized water for 3-5 days. The dialyzed aqueous solution was then removed and diffused in a diffusion cell for 7 days to obtain a phosphoric acid covalent organic framework nanosheet dispersion.

[0011] Step 2, Preparation of Phosphoric Acid Covalent Organic Framework Proton Exchange Membrane: The phosphoric acid covalent organic framework nanosheet dispersion obtained in Step 1 was transferred to a vacuum filtration device; the nanosheets were uniformly deposited onto the surface of a PAN support substrate by vacuum filtration at a volume-to-area ratio of 20 mL / 1.8 cm²; the membrane was then detached from the PAN support substrate by immersion in N,N-dimethylformamide, and then treated with 0.1 mol / L... -1 The detached membrane was soaked in dilute sulfuric acid to obtain a phosphoric acid covalent organic framework membrane with a thickness of 20 μm.

[0012] Further, in the preparation method of the present invention, the amine monomer is 2,5-diaminophenylphosphonic acid, and solvent A is one of dimethyl sulfoxide, sodium carbonate aqueous solution, and sodium benzoate aqueous solution. The aldehyde monomer is one of 1,3,5-trihydroxy-1,3,5-triphenyltricarboxaldehyde (Tp), 2,4-dihydroxy-1,3,5-triphenyltricarboxaldehyde (DTC), and 2-hydroxy-1,3,5-phenyltricarboxaldehyde (TFP); solvent B is dimethyl sulfoxide or n-octanoic acid. The molar ratio of the amine monomer to the aldehyde monomer is 3:2.

[0013] The phosphoric acid-based covalent organic framework proton exchange membrane prepared by this invention exhibits a proton conductivity of 653.35-929.2 mS / cm at 90°C and 100% RH. -1 .

[0014] Compared with the prior art, the beneficial effects of the present invention are:

[0015] In the preparation method of this invention, the density and distribution of phosphoric acid-type iCOF co-conducting groups are controlled by changing the number of hydroxyl groups in the aldehyde monomer, forming a structure with alternating and orderly arrangement of ionic groups and co-conducting groups. This constructs a more continuous hydrogen bond network, improves the connectivity of the proton transport pathway, and enhances proton conductivity. Under the synergistic effect of phosphate groups and oxygen-containing co-conducting groups, the ionic covalent organic framework membrane (iCOFMs) self-assembled from nanosheets can achieve rapid proton migration and exhibit excellent and stable proton conductivity, laying a solid material foundation for its application in high-performance energy devices such as proton exchange membrane fuel cells. Attached Figure Description

[0016] Figure 1This is a graph showing the change in proton conductivity of the membrane prepared in Example 1 as a function of temperature.

[0017] Figure 2 This is a graph showing the change in proton conductivity of the membrane prepared in Example 2 as a function of temperature.

[0018] Figure 3 This is a graph showing the change in proton conductivity of the membrane prepared in Example 3 as a function of temperature.

[0019] Figure 4 The graph shows the change in proton conductivity of the membrane prepared in Comparative Example 1 as a function of temperature.

[0020] Figure 5 This is a stress-strain curve of the membrane prepared in Example 1;

[0021] Figure 6 This is a graph showing the performance of the fuel cell prepared in Example 1 as a function of humidity. Detailed Implementation

[0022] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, but the following embodiments are by no means intended to limit the present invention.

[0023] Example 1

[0024] In this embodiment, the phosphate-type amine monomer is 2,5-diaminophenylphosphonic acid, and the aldehyde monomer is 1,3,5-trihydroxytriphenylcarbazaldehyde (Tp).

[0025] In this invention, the steps for synthesizing 2,5-diaminophenylphosphine are as follows: 4 g of 2-bromophenyl-1,4-diamine and 1.0 g of tetrakis(triphenylphosphine)palladium are weighed and uniformly dispersed in a mixed solvent prepared with 8.96 mL of diethyl phosphonite, 8.8 mL of triethylamine, and 4 mL of toluene. The mixture is reacted at 90 °C for 56 h. After the reaction, the solvent is removed by vacuum evaporation, and the mixture is purified by chromatography using alkaline silica gel to obtain a purple liquid crude product. 0.12 g of this purple crude liquid is accurately measured and added to a reaction tube along with 6 mL of anhydrous acetonitrile. After adding 0.66 mL of trimethylbromosilane, the mixture is stirred under a nitrogen atmosphere for 16 h. The system gradually forms a white suspension, to which 1.0 g of trimethylbromosilane is added. An orange solid precipitates after being exposed to mL of anhydrous methanol. Unreacted raw materials are removed by extraction, and recrystallization is performed using the difference in solubility to remove trace impurities and improve product purity. Finally, residual solvent is completely removed by filtration, washing, and constant temperature drying to obtain 2,5-diaminophenylphosphoric acid, denoted as Pa-PO3H2.

[0026] The 2,5-diaminophenylphosphoric acid (Pa-PO3H2) synthesized by the above method was used in subsequent examples and comparative examples.

[0027] The steps for preparing a phosphoric acid-based covalent organic framework proton exchange membrane based on the synergistic action of a conduction-promoting group are as follows:

[0028] Step 1: Synthesis of phosphoric acid-based covalent organic framework nanosheets: 14.1 mg of Pa-PO3H2 was dissolved in 10 mL of dimethyl sulfoxide to obtain a Pa-PO3H2 molar concentration of 0.0075 mol / mL. -1 A solution of the amine monomer. 10.5 mg of Tp was dissolved in 10 mL of dimethyl sulfoxide to obtain a Tp molar concentration of 0.005 mol / mL. -1 An aldehyde monomer solution was prepared. The aldehyde monomer solution was slowly added dropwise to the amine monomer solution using a dropper, and the reaction was carried out at 25°C for one week. The nanosheet solution was then removed and placed in a dialysis bag for dialyzing in deionized water for 5 days. The dialyzed aqueous solution was then removed and diffused in a diffusion cell for one week to obtain a phosphoric acid-type covalent organic framework nanosheet dispersion, denoted as TpPa-PO3H2.

[0029] Step 2, Preparation of Phosphoric Acid Covalent Organic Framework Proton Exchange Membrane: 20 mL of the TpPa-PO3H2 nanosheet dispersion prepared in Step 1 was transferred to a vacuum filtration apparatus; vacuum filtration was performed using a vacuum pump to uniformly deposit the nanosheets onto a PAN support substrate with an effective area of ​​1.8 cm²; the membrane was then detached from the substrate by soaking in N,N-dimethylformamide (DMF), and then treated with 0.1 mol / L... -1 The membrane was soaked in dilute sulfuric acid to obtain a phosphoric acid covalent organic framework membrane, named TpPa-PO3H2 membrane, with an overall thickness of 20 μm.

[0030] After testing, such as Figure 1 As shown, the TpPa-PO3H2 membrane prepared in Example 1 has a proton conductivity of 929.2 mS / cm at 90°C and 100% RH. -1 .

[0031] Example 2

[0032] A phosphoric acid-based covalent organic framework proton exchange membrane was prepared based on the synergistic effect of a co-conduction group. In this embodiment, the phosphoric acid amine monomer was 2,5-diaminophenylphosphonic acid, and the aldehyde monomer was 2,4-dihydroxy-1,3,5-pyromellitic tricarboxaldehyde (DTC). The preparation steps of the membrane are as follows:

[0033] Step 1: Synthesis of phosphate-based covalent organic framework nanosheets: 11.28 mg of Pa-PO3H2 and 1.6 mg of sodium carbonate were dissolved in 10 mL of water to obtain a Pa-PO3H2 molar concentration of 0.006 mol / mL. -1A solution of the amine monomer. 7.77 mg of DTC was weighed and dissolved in 10 mL of octanoic acid, yielding a DTC molar concentration of 0.004 mol / mL. -1 An aldehyde monomer solution was prepared. The aldehyde monomer solution was slowly transferred above the amine monomer solution using a dropper, and the reaction was carried out at 16°C for 3 weeks. The lower aqueous phase solution was collected and placed in a dialysis bag for dialyzing in deionized water for 3 days. The dialyzed aqueous solution was then collected and diffused in a diffusion cell for 1 week to obtain a phosphoric acid-type covalent organic framework nanosheet dispersion, denoted as DTCPa-PO3H2.

[0034] Step 2, Preparation of phosphoric acid covalent organic framework proton exchange membrane: Following the same operation as in Step 2 of Example 1, a phosphoric acid covalent organic framework membrane was finally obtained, named DTCPa-PO3H2 membrane, with an overall thickness of 20 μm.

[0035] After testing, such as Figure 2 As shown, the proton conductivity of the DTCPa-PO3H2 membrane prepared in Example 2 is 754.73 mS / cm under conditions of 90°C and 100% RH. -1 .

[0036] Example 3

[0037] A phosphoric acid-based covalent organic framework proton exchange membrane was prepared based on the synergistic effect of a co-conduction group. In this embodiment, the phosphoric acid amine monomer was 2,5-diaminophenylphosphonic acid, and the aldehyde monomer was 2-hydroxy-1,3,5-benzenetriformaldehyde (TFP). The preparation steps of the membrane are as follows:

[0038] Step 1: Synthesis of phosphoric acid-based covalent organic framework nanosheets: 16.92 mg of Pa-PO3H2 obtained in Step 1 and 4.32 mg of sodium benzoate were weighed and dissolved in 10 mL of water to obtain a Pa-PO3H2 molar concentration of 0.009 mol / mL. -1 A solution of the amine monomer. 10.69 mg of TFP was weighed and dissolved in 10 mL of octanoic acid, yielding a TFP molar concentration of 0.006 mol / mL. -1 An aldehyde monomer solution was prepared. The aldehyde monomer solution was slowly transferred onto the amine monomer solution using a dropper, and the reaction was carried out at 16°C for 3 weeks. The lower aqueous phase solution was collected and placed in a dialysis bag for dialyzing in deionized water for 3 days. The dialyzed aqueous solution was then collected and diffused in a diffusion cell for 1 week to obtain a phosphoric acid-type covalent organic framework nanosheet dispersion, denoted as TFPPa-PO3H2.

[0039] Step 2, Preparation of phosphoric acid covalent organic framework proton exchange membrane: Following the same operation as in Step 2 of Example 1, a phosphoric acid covalent organic framework membrane was finally obtained, named TFPPa-PO3H2 membrane, with an overall thickness of 20 μm.

[0040] After testing, such as Figure 3 As shown, the proton conductivity of the TFPPa-PO3H2 membrane prepared in Example 3 is 653.35 mS / cm under conditions of 90°C and 100% RH. -1 .

[0041] Comparative Example 1

[0042] A phosphoric acid-based covalent organic framework proton exchange membrane was prepared based on the synergistic effect of a co-conduction group. In this embodiment, the phosphoric acid amine monomer was 2,5-diaminophenylphosphonic acid, and the aldehyde monomer was trimesoaldehyde (TFB). The preparation steps of the membrane are as follows:

[0043] Step 1: Synthesis of phosphoric acid-based covalent organic framework nanosheets: 22.56 mg of Pa-PO3H2 obtained in Step 1 and 5.76 mg of sodium benzoate were weighed and dissolved in 10 mL of water to obtain a Pa-PO3H2 molar concentration of 0.012 mol / mL. -1 A solution of the amine monomer. 12.97 mg of TFB was weighed and dissolved in 10 mL of octanoic acid, yielding a TFB molar concentration of 0.008 mol / mL. -1 The aldehyde monomer solution was slowly transferred above the amine monomer solution using a dropper, and the reaction was carried out at 16°C for 3 weeks. The lower aqueous phase solution was collected and placed in a dialysis bag for dialyzing in deionized water for 3 days. The dialyzed aqueous solution was then collected and diffused in a diffusion cell for 1 week to obtain a phosphoric acid-type covalent organic framework nanosheet dispersion, denoted as TFBPa-PO3H2.

[0044] Step 2, Preparation of phosphoric acid covalent organic framework proton exchange membrane: Following the same operation as in Step 2 of Example 1, a phosphoric acid covalent organic framework membrane was finally obtained, named TFBPa-PO3H2 membrane, with an overall thickness of 20 μm.

[0045] After testing, such as Figure 4 As shown, the proton conductivity of the TFBPa-PO3H2 membrane prepared in Comparative Example 1 was 129.91 mS / cm under conditions of 90 °C and 100% RH. -1 .

[0046] Table 1 Test data of embodiments and comparative examples of the present invention

[0047]

[0048] Based on the preparation conditions of all Examples 1-3 and Comparative Example 1, and Table 1, it can be concluded that from the TFBPa-PO3H2 membrane to the TpPa-PO3H2 membrane, the proton conductivity gradually increases with the increase of the density of oxygen-containing co-conduction groups on the phosphate-type iCOF. The conductivity results indicate that oxygen-containing co-conduction groups are beneficial for constructing more continuous proton transport channels and improving proton conduction performance. Figure 5 As shown, the mechanical tensile strength of the TpPa-PO3H2 membrane prepared in the optimal membrane example 1 is 47.27 MPa. It was assembled into a fuel cell for testing, and the results are as follows... Figure 6 As shown, the maximum power density of the TpPa-PO3H2 membrane under conditions of 90℃ and 100% RH is 1.3543 W / cm². -2 Even when the humidity drops to 40%RH, the maximum power density can still be maintained at 0.52 W / cm². -2 .

[0049] In summary, the series of phosphate nanosheets (TpPa-PO3H2, DTCPa-PO3H2, TFPPa-PO3H2, and TFBPa-PO3H2) in Examples 1-3 and Comparative Example 1 were synthesized via single-phase method and phase transfer interface polymerization. Then, a series of phosphate-type covalent organic framework proton exchange membranes were prepared using a vacuum-assisted self-assembly method. The preparation methods are simple and easy to implement. In the preparation method of this invention, the continuity of the proton transport channel is controlled by changing the number of hydroxyl groups on the aldehyde monomer and regulating the density of oxygen-containing co-conducting groups in the COF, thereby optimizing the proton transport behavior and laying a solid material foundation for its practical application in electrochemical energy conversion devices such as proton exchange membrane fuel cells. In particular, TFBPa-PO3H2, due to the lack of hydroxyl groups, results in the hydrated proton domains surrounding the phosphate groups being isolated, leading to discontinuous proton transport channels and consequently poor proton conductivity.

[0050] Although the present invention has been described above in conjunction with the accompanying drawings, the present invention is not limited to the specific embodiments described above. The specific embodiments described above are merely illustrative and not restrictive. Those skilled in the art can make many improvements and changes under the guidance of the present invention without departing from the spirit of the present invention, and these improvements and changes are all within the protection scope of the present invention.

Claims

1. A method for preparing a phosphate-based covalent organic framework proton exchange membrane based on the synergistic action of a co-conduction group, characterized in that, Includes the following steps: Step 1: Synthesis of phosphate-based covalent organic framework nanosheets: 2,5-Diaminophenylphosphoric acid was dissolved in solvent A to prepare solutions with molar concentrations ranging from 0.006 to 0.009 mol / mL. -1 A solution of amine monomers; dissolving aldehyde monomers in solvent B to obtain a molar concentration of 0.004~0.006 mol / mL. -1 Aldehyde monomer solution; The aldehyde monomer solution was slowly transferred to the amine monomer solution using a dropper and reacted at 16-25°C for 7-21 days. The lower aqueous phase solution was then removed and placed into a dialysis bag for dialysis in deionized water for 3-5 days. The dialysis aqueous solution was then removed and diffused in a diffusion cell for 7 days to obtain a phosphoric acid covalent organic framework nanosheet dispersion. Step 2, Preparation of phosphoric acid covalent organic framework proton exchange membrane: The phosphoric acid-based covalent organic framework nanosheet dispersion prepared in step 1 was transferred to a vacuum filtration device. At a volume-to-area ratio of 20 mL / 1.8 cm², the nanosheets were uniformly deposited onto the surface of a PAN-supported substrate using vacuum filtration. The substrate was then soaked in N,N-dimethylformamide to detach the membrane from the PAN-supported substrate, and then treated with a 0.1 mol / L solution. -1 The detached membrane was soaked in dilute sulfuric acid to obtain a phosphoric acid covalent organic framework membrane with a thickness of 20 μm.

2. The preparation method according to claim 1, characterized in that, In step 1, the amino monomer is 2,5-diaminophenylphosphonic acid, and solvent A is one of dimethyl sulfoxide, sodium carbonate aqueous solution, and sodium benzoate aqueous solution.

3. The preparation method according to claim 1, characterized in that, In step 1, the aldehyde monomer is one of 1,3,5-trihydroxytriphenyltricarboxaldehyde (Tp), 2,4-dihydroxy-1,3,5-triphenyltricarboxaldehyde (DTC), or 2-hydroxy-1,3,5-triphenyltricarboxaldehyde (TFP); solvent B is dimethyl sulfoxide or octanoic acid.

4. The preparation method according to claim 1, characterized in that, The preparation steps for 2,5-diaminophenylphosphonic acid used in step 1 are as follows: Mixture A was prepared by uniformly dispersing 2-bromophenyl-1,4-diamine and tetrakis(triphenylphosphine)palladium in a mixed solvent at a mass ratio of 4:

1. The mixed solvent was prepared by mixing diethyl phosphonite, triethylamine, and toluene in a volume ratio of 2.24:2.2:

1. Mixture A was reacted at 90 °C for 56 h. After the reaction was completed, the solvent was removed by vacuum evaporation, and the product was purified by chromatography using alkaline silica gel to obtain a purple liquid crude product. The purple liquid crude product and anhydrous acetonitrile were added to a reaction tube at a mass-to-volume ratio of 0.02 g / 6 mL to obtain mixture B. Trimethylbromosilane was added dropwise to mixture B to obtain mixture C, wherein the volume ratio of trimethylbromosilane to mixture B was 0.11:

1. Mixture C was stirred and reacted under a nitrogen atmosphere for 16 h, and a white suspension gradually formed. An orange solid was precipitated after adding anhydrous methanol to the reaction system, wherein the volume ratio of anhydrous methanol to mixture B was 1:

1. Unreacted raw materials were removed by extraction, and recrystallization was performed using the difference in solubility to remove trace impurities and improve product purity. Finally, residual solvent was removed by filtration, washing, and constant temperature drying to obtain 2,5-diaminophenylphosphoric acid.

5. A phosphate-based covalent organic framework proton exchange membrane based on the synergistic action of a co-conduction group, characterized in that, The phosphoric acid-based covalent organic framework proton exchange membrane, prepared according to any one of claims 1 to 4, exhibits a proton conductivity of 653.35-929.2 mS / cm at 90°C and 100% RH. -1 .

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