High H2O-Assisted Proton Conduction in One Highly Stable Sr(II)-Organic Framework Constructed by Tetrazole-Based Imidazole Dicarboxylic Acid

Solid electrolyte materials with high structural stability and excellent proton conductivity (σ) have long been a popular and challenging research topic in the fuel cell field. This problem can be addressed because of the crystalline metal–organic frameworks’ (MOFs’) high structural stability, adjustable framework composition, and dense H-bonded networks. Herein, one highly stable Sr(II) MOF, {[Sr(H2tmidc)2(H2O)3]·4H2O}n (1) (H3tmidc = 2-(1H-tetrazolium-1-methylene)-1H-imidazole-4,5-dicarboxylic acid) was successfully fabricated, which was structurally characterized by single-crystal X-ray diffraction and electrochemically examined by the AC impedance determination. The results demonstrated that the σ of the compound manifested a positive dependence on temperature and humidity, and the optimal proton conductivity is as high as 1.22 × 10−2 S/cm under 100 °C and 98% relative humidity, which is at the forefront of reported MOFs with ultrahigh σ. The analysis of the proton conduction mechanism reveals that numerous tetrazolium groups, carboxyl groups, coordination, and crystallization water molecules in the framework are responsible for the high efficiency of proton transport. This work offers a fresh perspective on how to create novel crystalline proton conductive materials.


Introduction
With the growth of the population and the advancement of science and technology, the problem of fossil energy scarcity and environmental pollution will arrive sooner than expected.As a result, all governments now prioritize the development of renewable clean energy.The proton exchange membrane fuel cell (PEMFC), a mature renewable energy consumption device, has been extensively explored [1][2][3].The proton exchange membrane (PEM), as the "heart" of PEMPC, not only allows protons to pass through the membrane and links the circuit, but it also regulates the intensity of the reaction, allowing control over the battery's power and life [4][5][6].Nafion and its derivative membranes are currently the most common commercial PEMs, with a proton conductivity (σ) of up to 10 −2 S/cm.Although the Nafion membrane has a large commercial market, its high cost, difficult preparation, and tight operating conditions make it unsuitable for most people's needs.Its proton conduction mechanism is particularly challenging to characterize due to its amorphous form [7].As a result, much effort has been directed into the creation of novel alternative proton-conductive materials.
Organic ligands and their functional groups have long been recognized as important in forming proton-conductive MOFs.For instance, carboxylic acid groups carried by organic ligands, can not only coordinate with metal ions to constitute a stable MOF framework but can also act as proton sources and carriers, as well as participate in the construction of Hbonds within the framework to provide proton transport pathways [22][23][24][25][26][27][28][29].Moreover, it has also been discovered that the electron absorption induction of N-heterocyclic groups such as imidazole and triazole increases the acidity of the carboxyl group, thereby improving the structural stability of MOFs, resulting in high water resistance and chemical inertia, laying the groundwork for future practical applications [18, [30][31][32][33].For example, in 2012, the Banerjee group used In(III) and Cd(II) metal salt to react with 5-triazoline phenylic acid (5-TIA) to generate two proton-conducting MOFs, In-5TIA and Cd-5TIA [34].Under 301 K and 98% relative humidity (RH), their σ values were as high as 5.35 × 10 −5 and 3.61 × 10 −3 S/cm.Our research group has used substituted imidazole dicarboxylic acid ligands to assemble some structurally stable MOFs for proton conduction investigation.We have obtained some exciting findings, such as the optimal proton conductivity of up to 10 −3 S/cm [35][36][37][38][39].At the same time, free N-heterolytic small molecules can be employed as a proton source or carrier in a MOF's framework, resulting in high-efficiency proton transmission [40][41][42][43].However, these post-modified molecules are not anchored firmly enough in the MOF framework, resulting in leakage defects during use.
Given the above, we intend to introduce multiple nitrogen heterocyclic groups directly into organic carboxylic acid compounds via pre-design and then self-assemble with metal ions as multifunctional bridging ligands to prepare structurally stable MOFs containing both carboxylic and N-heterocyclic groups for proton conduction research.
Herein, the ligand of 2-(1H-tetrazolium-1-methylene)-1H-imidazole-4,5-dicarboxylic acid (H 3 tmidc) bearing imidazole and tetrazole and carboxylate units was selected to react with strontium chloride hexahydrate, obtaining a new Sr(II) MOF, {[Sr(H 2 tmidc) 2 (H 2 O) 3 ]•4H 2 O} n (1), which was characterized structurally by single-crystal X-ray diffraction.Consequently, its high thermal, H 2 O, and chemical stabilities were confirmed by thermogravimetric (TG) analysis and a powder X-ray diffraction (PXRD) test.Furthermore, the compound's σ was investigated in a humid environment.The results revealed that its proton conductivity boosted with rising temperature or RH and reached the highest point at 100 • C and 98% RH, up to 1.22 × 10 −2 S/cm, which is at the forefront of known high-performance protonconductive MOFs.Finally, crystal structure analysis and estimated activation energy values illustrate the framework's proton transport mechanism.
The dihedral angle between the imidazole and the tetrazole planes is 73.345(6) • , and the dihedral angles between the imidazole unit and the two carboxyl planes are 4.48 (7) • and 9.68(9) • , respectively.Sr-N distances are 2.8083 (12) and 2.8084(12) Å, respectively, while Sr-O distances vary from 2.545(2) to 2.7606(12) Å (Table 1).These Sr-N/O distances are within the normal range and are consistent with the corresponding chemical bond lengths in other Sr(II) complexes reported in the literature [44,45].The dihedral angle between the imidazole and the tetrazole planes is 73.345( the dihedral angles between the imidazole unit and the two carboxyl planes are and 9.68(9)°, respectively.Sr-N distances are 2.8083(12) and 2.8084(12) Å, respe while Sr-O distances vary from 2.545(2) to 2.7606( 12) Å (Table 1).These Sr-N/O d are within the normal range and are consistent with the corresponding chemic lengths in other Sr(II) complexes reported in the literature [44,45].The adjacent Sr(II) ions were joined to each other by the bridging ligand H2tm form a long chain extending indefinitely along the c-direction, where the distance b  The adjacent Sr(II) ions were joined to each other by the bridging ligand H 2 tmidc − to form a long chain extending indefinitely along the c-direction, where the distance between adjacent Sr(II) ions is 8.8476(3) Å (Figure 2).In addition to the intramolecular O-H• • • O H-bonding link between carboxyl units, there are O-H• • • N H-bonds between coordination water and tetrazolium rings between chains, and O-H• • • O H-bonds between carboxyl units, crystalline water, and coordination H 2 O (please see Figure 3 for an array of these H-bonds).As a result, the chains are linked together by the seven kinds of hydrogen bonds mentioned above (Table 2), forming the two-dimensional layered structure (Figure 3). Figure 4 depicts a three-dimensional structure of 1 with layers joined by two kinds of O-H• • • N hydrogen-bonding connections.Furthermore, there is a significant π-π interaction between the neighboring imidazole units, and the distance between the centroids of the adjacent imidazole rings is 3.5776 (7) Å.This adds stability to the threedimensional solid structure.mentioned above (Table 2), forming the two-dimensional layered structure (Figure 3). Figure 4 depicts a three-dimensional structure of 1 with layers joined by two kinds of O-H⋯N hydrogen-bonding connections.Furthermore, there is a significant π-π interaction between the neighboring imidazole units, and the distance between the centroids of the adjacent imidazole rings is 3.5776 (7) Å.This adds stability to the three-dimensional solid structure.mentioned above (Table 2), forming the two-dimensional layered structure (Figure 3). Figure 4 depicts a three-dimensional structure of 1 with layers joined by two kinds of O-H⋯N hydrogen-bonding connections.Furthermore, there is a significant π-π interaction between the neighboring imidazole units, and the distance between the centroids of the adjacent imidazole rings is 3.5776 (7) Å.This adds stability to the three-dimensional solid structure.

D-H•
0.85 2.31 3.144(3) 167.5 Symmetry modes: #3: To summarize, compound 1 has a large number of free tetrazolium groups, imidazole groups, carboxyl oxygen atoms, coordination, and crystalline H 2 O units.The complicated interaction between these groups will generate a dense and rich hydrogen bond network, ensuring the compound's efficient proton transport.To summarize, compound 1 has a large number of free tetrazolium groups, imidazole groups, carboxyl oxygen atoms, coordination, and crystalline H2O units.The complicated interaction between these groups will generate a dense and rich hydrogen bond network, ensuring the compound's efficient proton transport.

Infrared Spectral Analysis
As denoted in Figure S1, the infrared spectra of organic ligand H3tmidc and its equivalent MOF 1 are comparable but not identical, as discussed below.For H3tmidc, the wide absorption band at 3540 cm −1 is attributed to the N-H stretching vibration.The absorption is NH in-plane bending vibration at 1477 cm −1 and C=N stretching vibration at 1551 cm −1 .Continuous strong absorption peaks at 1476-1551 cm −1 manifest the presence of tetrazole rings.The absorption peaks at 1716 and 1476 cm −1 are ascribed to C=O stretching vibration and the in-plane bending of OH, respectively.The absorption peaks at 1369 and 1331 cm −1 are the stretching vibration of C-O.For 1, compared with H3tmidc, the infrared characteristic peaks of νOH expansion vibration are blue-shifted and the absorption peaks are enhanced.That means an O-Sr coordination bond is formed.The blue shift of the absorption peak of νC=N indicates that N participates in the coordination.The above fully indicates that H3tmidc does indeed coordinate with the metal Sr(II).

Stability Analysis
To determine the thermal stability of 1 in an air atmosphere, a certain number of microcrystalline samples were precisely weighed to investigate the thermal degradation behavior.As depicted in Figure S2, the weight loss from 25 to 115 °C is due to the loss of four crystalline H2O units (found 10.71%, calculated 10.46%).As the temperature rises,

Infrared Spectral Analysis
As denoted in Figure S1, the infrared spectra of organic ligand H 3 tmidc and its equivalent MOF 1 are comparable but not identical, as discussed below.For H 3 tmidc, the wide absorption band at 3540 cm −1 is attributed to the N-H stretching vibration.The absorption is NH in-plane bending vibration at 1477 cm −1 and C=N stretching vibration at 1551 cm −1 .Continuous strong absorption peaks at 1476-1551 cm −1 manifest the presence of tetrazole rings.The absorption peaks at 1716 and 1476 cm −1 are ascribed to C=O stretching vibration and the in-plane bending of OH, respectively.The absorption peaks at 1369 and 1331 cm −1 are the stretching vibration of C-O.For 1, compared with H 3 tmidc, the infrared characteristic peaks of ν OH expansion vibration are blue-shifted and the absorption peaks are enhanced.That means an O-Sr coordination bond is formed.The blue shift of the absorption peak of ν C=N indicates that N participates in the coordination.The above fully indicates that H 3 tmidc does indeed coordinate with the metal Sr(II).

Stability Analysis
To determine the thermal stability of 1 in an air atmosphere, a certain number of microcrystalline samples were precisely weighed to investigate the thermal degradation behavior.As depicted in Figure S2, the weight loss from 25 to 115 • C is due to the loss of four crystalline H 2 O units (found 10.71%, calculated 10.46%).As the temperature rises, three coordination water molecules are lost, which continues until 216 • C (found 7.66%, calculated 7.85%).As the temperature rose, the organic components within the framework continued to undergo intense thermal breakdown, which lasted until 795 • C. A constant weight platform then forms, indicating the creation of strontium oxide (found 14.88%, calculated 15.05%).The above test results reveal that, whereas the crystalline water molecules of compound 1 are lost below 100 • C, the coordinated water molecules remain, giving a material basis for the study of proton transport in water vapor.
A high-quality proton-conductive material must be resistant to water and chemically inert, making it suitable for future practical applications.Consequently, we looked into compound 1's chemical and water stability in more detail.Initially, a suitable quantity of microcrystalline samples was immersed in water for either a week at room temperature or a day at boiling temperature.Subsequently, the collected samples were dried and used for PXRD tests to ascertain compound 1's H 2 O stability.As denoted in Figure 5a, the PXRD patterns of samples for 1 before and after H 2 O treatment are consistent with the simulated ones from crystal data, manifesting high water stability.
three coordination water molecules are lost, which continues until 216 °C (found 7.66%, calculated 7.85%).As the temperature rose, the organic components within the framework continued to undergo intense thermal breakdown, which lasted until 795 °C.A constant weight platform then forms, indicating the creation of strontium oxide (found 14.88%, calculated 15.05%).The above test results reveal that, whereas the crystalline water molecules of compound 1 are lost below 100 °C, the coordinated water molecules remain, giving a material basis for the study of proton transport in water vapor.
A high-quality proton-conductive material must be resistant to water and chemically inert, making it suitable for future practical applications.Consequently, we looked into compound 1's chemical and water stability in more detail.Initially, a suitable quantity of microcrystalline samples was immersed in water for either a week at room temperature or a day at boiling temperature.Subsequently, the collected samples were dried and used for PXRD tests to ascertain compound 1's H2O stability.As denoted in Figure 5a, the PXRD patterns of samples for 1 before and after H2O treatment are consistent with the simulated ones from crystal data, manifesting high water stability.Moreover, to demonstrate the chemical stability of compound 1, two suitable microcrystalline samples were submerged in acidic (pH = 3.5) or basic (pH = 10.5)aqueous solutions for 1 d.The samples were collected, washed with H2O, and dried for PXRD pattern testing and comparison.As unveiled in Figure 5b, the overlapped PXRD patterns demonstrated that 1 had good chemical stability.

Porosity Analysis
Under 77 K, the N2 adsorption/desorption isotherm was determined by employing the activated sample of MOF 1 (Figure S3).According to the IUPAC classification, this is a typical type III adsorption/desorption isotherm.As displayed in Figure S3, the N2 uptake boosts with the rising P/P0 and attains the highest value of 96.04 cm 3 /g STP as P/P0 = 0.99.At relative pressures ranging from 0.8 to 0.99, the adsorption and desorption curves exhibit a degree of variation indicating a specific interaction between the nitrogen molecules and the framework components.Its Brunauer-Emmett-Teller (BET) surface area was 27.0 m 2 /g, and the average Barret-Joyner-Halenda (BJH) pore size and pore volume were 18.9 nm and 0.15 cm 3 /g, respectively.Compound 1 may have a small aperture size with small volume and has no prominent pores based on the nitrogen adsorption/desorption results.

Proton-Conducting Properties
The AC impedance spectra of 1 at variable temperatures and humidity were measured using the quasi-four-electrode method.The compound's proton conductivity (Table 3) was then assessed using the AC impedance test findings.Based on this, we investigated Moreover, to demonstrate the chemical stability of compound 1, two suitable microcrystalline samples were submerged in acidic (pH = 3.5) or basic (pH = 10.5)aqueous solutions for 1 d.The samples were collected, washed with H 2 O, and dried for PXRD pattern testing and comparison.As unveiled in Figure 5b, the overlapped PXRD patterns demonstrated that 1 had good chemical stability.

Porosity Analysis
Under 77 K, the N 2 adsorption/desorption isotherm was determined by employing the activated sample of MOF 1 (Figure S3).According to the IUPAC classification, this is a typical type III adsorption/desorption isotherm.As displayed in Figure S3, the N 2 uptake boosts with the rising P/P 0 and attains the highest value of 96.04 cm 3 /g STP as P/P 0 = 0.99.At relative pressures ranging from 0.8 to 0.99, the adsorption and desorption curves exhibit a degree of variation indicating a specific interaction between the nitrogen molecules and the framework components.Its Brunauer-Emmett-Teller (BET) surface area was 27.0 m 2 /g, and the average Barret-Joyner-Halenda (BJH) pore size and pore volume were 18.9 nm and 0.15 cm 3 /g, respectively.Compound 1 may have a small aperture size with small volume and has no prominent pores based on the nitrogen adsorption/desorption results.

Proton-Conducting Properties
The AC impedance spectra of 1 at variable temperatures and humidity were measured using the quasi-four-electrode method.The compound's proton conductivity (Table 3) was then assessed using the AC impedance test findings.Based on this, we investigated the relationship between σ and temperature and humidity and the mechanism of proton conduction.First, by varying the temperature at a constant relative humidity, the relationship between compound 1's proton conductivity and temperature was examined.We set five humidity conditions (68%, 75%, 85%, 93% and 98% RHs) to test the effect of temperature on σ.The determined AC impedance spectra at a fixed RH and variable temperatures were exhibited in Figure 6a and Figures S4-S7, respectively.The Nyquist diagram for compound 1 at 98% RH, as shown in Figure 6a, has a slope in the low-frequency zone and a semi-arc in the high-frequency region.As temperature rises, the semicircle gets smaller and eventually turns into a full slope.This implies that the compound's framework exhibits conventional proton conduction behavior [46][47][48][49], and the compound's AC impedance lowers as temperature increases, increasing proton conductivity.Similarly, the same changes in the Nyquist plots with increasing temperature are visible at other relative humidity levels (Figures S4-S7).As listed in Table 3, the σ values vary from 1.62 × 10 that at each set humidity, the σ of 1 soars dramatically with temperature, with the difference between the lowest and greatest values being 2-3 orders of magnitude.Figure 6b illustrates the propensity of proton conductivity to grow fast as temperature rises.The following explanations can be used to explain compound 1's positive association between temperature and proton conductivity.First, as temperature rises, more energy can be absorbed by the coordination and crystalline water molecules in the framework or by water molecules adsorbed from the outside, which causes more hydrated protons to dissociate and increases the number of proton sources.Second, as temperature rises, more hydrated protons vibrate faster, which causes more orientational changes that are more favorable for the efficient transmission of protons and, ultimately, a sharp rise in proton conductivity.
Molecules 2024, 29, x FOR PEER REVIEW 7 of 13 the relationship between σ and temperature and humidity and the mechanism of proton conduction.First, by varying the temperature at a constant relative humidity, the relationship between compound 1's proton conductivity and temperature was examined.We set five humidity conditions (68%, 75%, 85%, 93% and 98% RHs) to test the effect of temperature on σ.The determined AC impedance spectra at a fixed RH and variable temperatures were exhibited in Figures 6a and S4-S7, respectively.The Nyquist diagram for compound 1 at 98% RH, as shown in Figure 6a, has a slope in the low-frequency zone and a semi-arc in the high-frequency region.As temperature rises, the semicircle gets smaller and eventually turns into a full slope.This implies that the compound's framework exhibits conventional proton conduction behavior [46][47][48][49], and the compound's AC impedance lowers as temperature increases, increasing proton conductivity.Similarly, the same changes in the Nyquist plots with increasing temperature are visible at other relative humidity levels (Figures S4-S7).As listed in Table 3, the σ values vary from 1.62 × 10 −5 It has been discovered that at each set humidity, the σ of 1 soars dramatically with temperature, with the difference between the lowest and greatest values being 2-3 orders of magnitude.Figure 6b illustrates the propensity of proton conductivity to grow fast as temperature rises.The following explanations can be used to explain compound 1's positive association between temperature and proton conductivity.First, as temperature rises, more energy can be absorbed by the coordination and crystalline water molecules in the framework or by water molecules adsorbed from the outside, which causes more hydrated protons to dissociate and increases the number of proton sources.Second, as temperature rises, more hydrated protons vibrate faster, which causes more orientational changes that are more favorable for the efficient transmission of protons and, ultimately, a sharp rise in proton conductivity.Additionally, at a given temperature, the relationship between compound 1's proton conductivity and relative humidity was examined.As manifested in Figure 7a, from 40 to 100 • C, the σ values boost rapidly with the rising RHs.For instance, under 40 • C, the σ values are from 4.84 × 10 −4 (68% RH) to 1.62 × 10 −5 S/cm (98% RH).Under 100 • C, the σ values are from 3.02 × 10 −6 (68% RH) to 1.22 × 10 −2 S/cm (98% RH) (Table 3).Proton conductivity exhibits a rapid increase with increasing humidity, increasing by 3-4 orders of magnitude at different temperatures.This phenomenon suggests that an increase in the number of water molecules in the external environment can greatly promote the adsorption of more water molecules in the framework of 1, increasing the number of proton sources in the pore and a denser hydrogen bond network in the framework, allowing for faster proton transfer.Note that although proton conductivity typically increases rapidly with increasing humidity, it increases slowly between 75% and 85% RHs or at specific temperatures.This could be owing to inadequate water molecules adsorbed by the framework at 75-85% RHs or temperature circumstances that prevent the complete completion of water molecular deprotonation.Additionally, at a given temperature, the relationship between compound 1's proton conductivity and relative humidity was examined.As manifested in Figure 7a, from 40 to 100 °C, the σ values boost rapidly with the rising RHs.For instance, under 40 °C, the σ values are from 4.84 × 10 −4 (68% RH) to 1.62 × 10 −5 S/cm (98% RH).Under 100 °C, the σ values are from 3.02 × 10 −6 (68% RH) to 1.22 × 10 −2 S/cm (98% RH) (Table 3).Proton conductivity exhibits a rapid increase with increasing humidity, increasing by 3-4 orders of magnitude at different temperatures.This phenomenon suggests that an increase in the number of water molecules in the external environment can greatly promote the adsorption of more water molecules in the framework of 1, increasing the number of proton sources in the pore and a denser hydrogen bond network in the framework, allowing for faster proton transfer.Note that although proton conductivity typically increases rapidly with increasing humidity, it increases slowly between 75% and 85% RHs or at specific temperatures.This could be owing to inadequate water molecules adsorbed by the framework at 75-85% RHs or temperature circumstances that prevent the complete completion of water molecular deprotonation.Excitingly, compound 1 has an optimal σ of 1.22 × 10 −2 S/cm under 98% RH/100 °C, which is comparable to the excellent proton conductivity of MOF materials reported in the literature [50][51][52] and far greater than the σ of some MOFs built by other N-heterocyclic carboxylic acid ligands [35][36][37][38][39], demonstrating the significant construction advantages of the ligands used in this paper.
To gain insight into the proton-conducting mechanism of MOF 1, based on the measured proton conductivity data and the Arrhenius equation, we performed linear fitting, estimated the corresponding activation energy data (Figure 7b) at 98% and 68% RH, and then predicted the proton transport behavior within the framework using the crystal structure data.As denoted in Figure 7b, the assessed activation energy (Ea) values under 98% and 68% RHs are 1.19 and 0.73 eV, showing that the Ea values of 1 are much greater than 0.4 eV at both high and low humidity, which indicates that proton transfer in the framework mainly obeys the vehicular mechanism [8,53].The reason for this phenomenon could be that the existence of a great number of strong intramolecular H-bonds in the framework of compound 1 weakens the construction of the continuous dense intermolecular hydrogen bond network, preventing protons from completing the "jump" through the hydrogen bond network, which can only be completed via hydrated proton-directed migration.Nonetheless, based on our examination of crystal structure and hydrogen bond Excitingly, compound 1 has an optimal σ of 1.22 × 10 −2 S/cm under 98% RH/100 • C, which is comparable to the excellent proton conductivity of MOF materials reported in the literature [50][51][52] and far greater than the σ of some MOFs built by other N-heterocyclic carboxylic acid ligands [35][36][37][38][39], demonstrating the significant construction advantages of the ligands used in this paper.
To gain insight into the proton-conducting mechanism of MOF 1, based on the measured proton conductivity data and the Arrhenius equation, we performed linear fitting, estimated the corresponding activation energy data (Figure 7b) at 98% and 68% RH, and then predicted the proton transport behavior within the framework using the crystal structure data.As denoted in Figure 7b, the assessed activation energy (E a ) values under 98% and 68% RHs are 1.19 and 0.73 eV, showing that the E a values of 1 are much greater than 0.4 eV at both high and low humidity, which indicates that proton transfer in the framework mainly obeys the vehicular mechanism [8,53].The reason for this phenomenon could be that the existence of a great number of strong intramolecular H-bonds in the framework of compound 1 weakens the construction of the continuous dense intermolecular hydrogen bond network, preventing protons from completing the "jump" through the hydrogen bond network, which can only be completed via hydrated proton-directed migration.Nonetheless, based on our examination of crystal structure and hydrogen bond data, we conclude that the proton transfer process in the framework is inextricably linked to the hydrogen bond network, implying that the Grotthuss mechanism is accompanied.

Crystal Structure Measurement
A light yellow high-quality single crystal of compound 1 with a size of 0.22 × 0.21 × 0.16 mm 3 is glued to a thin glass fiber.A molybdenum target microfocal source (λ = 0.71073 A) was tested on a Bruker D8 VENTURE PHOTON double microfocal X-ray single crystal diffractometer at T = 300(2) K. Data were collected in the range of 2.709-27.505degrees using ω and φ scanning.SHELXT software (v6.2) package was used to obtain the initial model of the crystal structure by a direct approach, and then the SHELXL crystallography software package [55,56] was used to refine the coordinates of all non-hydrogen atoms by the total matrix least squares method based on F 2 .The H on C and N is obtained via theoretical hydrogenation, and the hydrogen on H 2 O is measured in the light of the residual peak and the direction of the H bond.All non-H atoms undergo anisotropic refinement, while hydrogen atoms undergo isotropic refinement.The crystallographic parameters of 1 are listed in Table 4. CCDC no. is 2353666.

Stability Determinations
The general procedure for testing the water and chemical stability of compound 1 is as follows: weigh the appropriate weight of the microcrystalline sample of 1, soak it in room temperature or boiling water for seven days or one day, respectively, or soak it in acidic (HCl) or alkaline (NaOH) aqueous solutions for one day, then centrifuge the solids, dry them in the air, and perform PXRD analysis.

Proton-Conducting Determinations
The Princeton 2273 electrochemical Station was used in a quasi-four-electrode manner to test compound 1's AC impedance profile (AC voltage: 100 mV; 1-10 6 Hz).
Because it is hard to obtain a single-crystal sample of a suitable size, only microcrystal samples of 1 can be used for electrochemical testing via the pressing tablet method.The disc sample was manufactured as described previously [36,37]: approximately 35 mg of the microcrystalline sample was placed into a disc mold and allowed to rest for 3 min at a pressure of 3.5 Mpa, and then the disc was collected.Vernier caliper was used to precisely measure the radius and thickness.Then, it was sandwiched between two silver electrodes, suspended in a humid environment, and balanced for 18 h, and the AC impedance map was recorded.A constant temperature and humidity box (Shanghai Yi-Heng BPS-50CL (Shanghai YiHeng Co. LTD.Shanghai, China)) regulates the 68-98% RHs and 30-100 • C levels.

Conclusions
This study reports the successful preparation of a one-dimensional chain strontium(II)based MOF with good thermal stability, water stability, and chemical inertia utilizing a multi-functional organic bridge ligand of dicarboxylic acid comprising tetrazolium and imidazole groups.These lengthy chains produce a three-dimensional solid-structure with a dense hydrogen bond network as a result of intermolecular and intramolecular hydrogen bonding and π-π packing force.The compound demonstrated excellent proton conductivity with the best value of 10 −2 S/cm, which was significantly higher than the σ of MOFs built

Figure 2 .
Figure 2. The infinite chain of 1 along the c-axis.

Figure 3 .
Figure 3.The two-dimensional sheet of 1 bearing H-bonds.

Figure 2 .
Figure 2. The infinite chain of 1 along the c-axis.

Figure 2 .
Figure 2. The infinite chain of 1 along the c-axis.

Figure 3 .
Figure 3.The two-dimensional sheet of 1 bearing H-bonds.

Figure 3 .
Figure 3.The two-dimensional sheet of 1 bearing H-bonds.

Figure 5 .
Figure 5. PXRD patterns of 1 before and after H2O treatment (a) or acidic and basic solutions treatment (b).

Figure 5 .
Figure 5. PXRD patterns of 1 before and after H 2 O treatment (a) or acidic and basic solutions treatment (b).

Figure 6 .
Figure 6.Nyquist spectra of 1 under 98% RH and different temperatures (a).Dependence diagram of σ for 1 and temperature at the fixed RHs (b).

Molecules 2024 , 13 Figure 6 .
Figure 6.Nyquist spectra of 1 under 98% RH and different temperatures (a).Dependence diagram of σ for 1 and temperature at the fixed RHs (b).

Figure 7 .
Figure 7. Dependence diagram of σ for 1 and RH at the fixed temperatures (a).The evaluated Ea values of 1 under 68% and 98% RHs (b).

Figure 7 .
Figure 7. Dependence diagram of σ for 1 and RH at the fixed temperatures (a).The evaluated E a values of 1 under 68% and 98% RHs (b).

Table 3 .
The σ values (S/cm) of 1 under variable temperatures and RHs.

Table 3 .
The σ values (S/cm) of 1 under variable temperatures and RHs.

Table 4 .
The crystallographic parameters of 1.