Coordinated water molecule-induced solid-state superprotonic conduction by a highly scalable and pH-stable coordination polymer (CP)

Abstract

The effective design of new efficient crystalline solid-state proton conductors (SSPCs) is relevant to the advancement of clean energy applications. Although it is well known that metal ions enhance the acidity of their coordinated water molecules, proton-conducting coordination polymers (CPs) and metal–organic frameworks (MOFs) in which coordinated water molecules act as sole intrinsic proton sources are largely unexplored. Considering such a design principle, we present a highly scalable and highly robust (stable over a wide pH range of 2–10, in open air for six months, water and boiling water) CP, IITKGP-101, in which Cu₄(μ₃-OH)₄ SBUs are coordinated with ample water molecules and display superprotonic conductivity in both the single crystal and pellet forms. The 1D chains with hydrophilic interlayer spaces within the framework housing abundant coordinated water as the sole proton source, which make an extended H-bonding pathway for efficient proton migration and thus exhibit a conductivity value of 5.2 × 10⁻⁴ S cm⁻¹ in single crystal form and 2.45 × 10⁻⁴ S cm⁻¹ in pellet form at 80 °C and 98% RH. The easy scalability in gram scale in an aqueous medium and highly robust nature make this framework a promising candidate as an SSPC.

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Introduction

The depletion of non-renewable fossil fuels necessitates alternative clean and sustainable energy resources for sustaining human life.^1,2 The proton exchange membrane fuel cell (PEMFC) has received extensive attention as the most favorable candidate for portable energy-related devices and vehicle applications due to its ultra-low emission features and high power density.^3,4 Although the previously developed and commonly used state-of-the-art proton-conducting materials, Nafion and Flemion, displayed superior proton conductivity, the high cost associated with per-fluorination limits their widespread usage.^3,5–7 In addition, the amorphous nature of these traditional polymers is unable to provide accurate structural information for in-depth investigation on proton-conduction mechanisms and pathways. Therefore, the development of crystalline, structurally greatly stable and highly conductive solid-state proton conductors (SSPCs) is in demand.⁵ In this regard, coordination polymers (CPs) and metal–organic frameworks (MOFs) assembled by metal ions and organic linkers having variability of the structural architecture^8–39 and superior tunability^40–61 have displayed excellent proton conductivity, even in ultra-high superprotonic regions (>10⁻² S cm⁻¹ or higher).^6,62 Most importantly, the stronger coordination bonds and highly crystalline nature enable accurate structural characterization, which is beneficial for understanding the plausible proton-conduction mechanism. It should be noted that CPs are comparatively less explored than their MOF counterparts as SSPCs.^{6,8,11,62–73} Again, by taking advantage of their structural superiority, ∼300 proton-conducting CPs have been developed so far; however, CPs displaying conductivity at the superprotonic region (≥10⁻⁴ S cm⁻¹) are even rarer.^6,62

Surprisingly, in most cases, proton conductivity has been analyzed in the pellet form rather than in single crystals.⁷⁴ Due to random orientation during conduction measurement of the pelletized sample, the characterization of anisotropic effects is precluded. To solve this issue, proton conduction measurements on single crystals with ordered and well-defined guest molecules are indispensable. The insufficient crystal length, irregular shape, inadequate stability, and complex sample processing are the major barriers to proton conduction measurements on single crystals.⁷⁴ The proton conduction measurements of single crystals could provide a basis for understanding the essence of complex conductive mechanisms. To the best of our knowledge, there are still very few reports where proton conductivity has been analyzed in the single crystal form or both single crystal and pellet forms,^22,74–84 and thus this demands more attention. On the other hand, CPs are infamous for their instability under water/moist conditions, which limits them in practical applications, especially toward water containing media such as in PEMFCs where a high level of humidification is essential for cyclic operation in FCs. Also, easy bulk-scale synthesis is indeed a prerequisite criterion for successful industrial deployment. Thus, the effective design and development of chemically robust SSPCs with obvious scalability are of significant interest.

In terms of design principles, depending upon the proton sources, CPs are generally categorized into two types: (i) intrinsic and (ii) extrinsic proton conductors.^9,62,63 Although both types of proton sources are efficient to achieve proton-conductivity in the ultra-high superprotonic regions,^6,62 the developed materials having extrinsic proton sources may suffer from a leaching effect, which thus can limit their practical utility.⁵ In these circumstances, the design and development of highly stable CPs having intrinsic proton sources (such as uncoordinated –SO₃H, –PO₃H₂, and –COOH groups flanked toward the pore channels or protic H₃O⁺, Me₂NH₂⁺, NH₄⁺, H₂PO₄⁻, etc. counter-ions present in the pore channels) is of significant interest. Recently, the crucial roles of coordinated water molecules in bringing the protonic conductivity on CPs and MOFs have been highlighted, where these coordinated water molecules are shown to act as the sole intrinsic proton sources by virtue of their enhanced acidity, while bound to the metal centers.^9,63 To construct CPs and MOFs with a higher number of coordinated water molecules and to make the frameworks superprotonic using coordinated water molecules as the only protonic source, exclusively aqueous medium or aqueous DMF solution (usage of minimal DMF to resolve the precursors’ solubility issue) for crystallization has been proposed.⁶³ However, such types of proton-conducting CPs and MOFs are still limited in the literature, among which CPs are even rarer and demand further development.^9,10,16,63 Besides imparting protonic conductivity, such an approach could bring inherent water stability because of the usage of an aqueous crystallization medium, which is again beneficial for being used as an SSPC in real applications.

On the other hand, employing aromatic acid ligands to construct robust CPs is undoubtedly a good choice as they possess strong coordination ability with diverse coordination modes.⁶⁵ It should be noted that the aromatic rings of the constructed CPs have a significant contribution to the framework's overall stability such as (i) π–π stacking interactions between the aromatic rings, which provide extra stabilization energy and (ii) hydrophobicity of the aromatic rings that enhances the water stability. In addition, uncoordinated acid groups (deprotonated carboxylates, sulfonates, and phosphonates) of the ligands do act as proton hopping sites and participate in extensive H-bonding construction, which is further beneficial for achieving proton conductivity of such CPs.

Considering the aforementioned aspects, we have designed and developed a highly scalable and wide range pH-stable (2–10) Cu(II)-bpy chain-based coordination polymer, IITKGP-101, with molecular formula {[Cu₄(μ₃-OH)₄(4,4′-bpy)₂(2,6-NDS)(H₂O)₇]·2,6-NDS·6H₂O}_n (where bpy = 4,4′-bipyridine, 2,6-NDS = disodium 2,6-naphthalenedisulfonate), which displayed superprotonic conductivity in both the single crystal and pellet forms. The extensive H-bonded chains inside the 1D interlayer spaces are constructed among the well-ordered ample coordinated water molecules, lattice water molecules, free lattice 2,6-NDS units, and uncoordinated O atoms of the metal-bound 2,6-NDS units. The maximum proton conductivity was obtained as 5.2 × 10⁻⁴ S cm⁻¹ in the single crystal form at 80 °C and 98% RH, whereas in the pellet form a conductivity of 2.45 × 10⁻⁴ S cm⁻¹ could be achieved under the same conditions. IITKGP-101 was synthesized in an aqueous medium using a room temperature slow evaporation crystallization technique, possessing a high density of coordinated water molecules, and exhibited protonic conductivity driven by ample coordinated water molecules as the sole intrinsic source. Multiple π⋯π stacking interactions between the bpy units along with aromatic 2,6-NDS acid ligands and bpy units not only enhanced the chemical stability (water, boiling water and pH) of the developed framework but also exhibited framework robustness even after treatment under the extreme conditions of conductivity measurement (80 °C and 95–98% RH) for a longer time. Most importantly, the ease of G-scale synthesis, open-air simple handling (stable as long as for six months in an open lab atmosphere), superior stability, and superprotonic conductivity with extended durability made IITKGP-101 promising toward this projected application.

Results and discussion

Room-temperature crystallization of Cu(NO₃)₂·3H₂O, 4,4′-bpy, and disodium salt of 2,6-NDS under an aqueous medium yielded block-shaped blue crystals of IITKGP-101 (Fig. S1, ESI†). It crystallizes in the triclinic crystal system with the P1̄ space group (Table S1, ESI†) in which a high density of coordinated water molecules containing Cu₄(μ₃-OH)₄ SBUs are connected via 4,4′-bpy bridging ligands and construct a 1D polymeric infinite chain (Fig. 1a). The asymmetric unit consists of two halves of the Cu₄(μ₃-OH)₄ SBU coordinated with seven water molecules, two bridging 4,4′-bpy units, and one coordinated and one lattice 2,6-NDS unit along with six lattice water molecules (Fig. S2, ESI†). Among the four Cu(II) centers, three (Cu2, Cu3, and Cu4) are coordinated with two water molecules each whereas the remaining one (Cu1) is coordinated with one water molecule and one deprotonated 2,6-NDS unit (Fig. 1b). Each of the Cu(II) centers possesses a distorted octahedral coordination environment: coordinated with two crystallographically independent water molecules (except one Cu1-center that is coordinated with one water and one 2,6-NDS unit) along with one 4,4′-bpy unit and three oxygen atoms from the three different μ₃-OH bridges. In Cu₄(μ₃-OH)₄ SBUs, the Cu–O bond distances are significantly shorter and lie in the range of 1.943(8)–1.994(1) Å, whereas the Cu–O bond distances in the mono-coordinated 2,6-NDS unit are relatively higher (2.682(1) Å). The Cu–N bond distances for bidentate bridging bpy ligands lie in the range of 2.010(1)–2.044(1) Å, whereas for coordinated water molecules the Cu–O bond distances lie in the range of 1.973(1)–2.395(1) Å (Table S2, ESI†). As mentioned earlier, aromatic acid ligands play a significant role in the framework's stability through π⋯π stacking interactions; in IITKGP-101, the coordinated 2,6-NDS units also display strong π⋯π stacking interactions with the bridging bpy units having planar separation distances of 3.747 Å (Fig. S3, ESI†). In addition, the bidentate bridging bpy units in two consecutive layers also exhibit π⋯π stacking interactions with a centroid-to-centroid distance of 3.653 Å, as displayed in Fig. S3 (ESI†). Moreover, each 1D chain is further interconnected by H-bonding interactions among the coordinated water molecules, coordinated and lattice 2,6-NDS units, along with lattice water molecules, thus leading to the formation of an extensive H-bonded supramolecular architecture. The overall H-bonding pattern is displayed in Fig. S4 (ESI†) and all such H-bonding interactions are listed in Table S3 (ESI†). Besides extensive H-bonding, several non-bonding interactions are also present throughout the framework, as displayed in Table S4 (ESI†). It should be noted that the hydrophilic channels between the 1D layers are extensively occupied by lattice water molecules and lattice 2,6-NDS units, which enabled the construction of continuous H-bonding throughout the framework. The lattice 2,6-NDS units could connect two different 1D chains via H-bonding interactions, as displayed in Fig. S5 (ESI†). The two oxygen atoms (O18 and O21) of lattice 2,6-NDS units are hydrogen bonded with the hydrogens of the coordinated water molecules (H5A, H23B), whereas another oxygen atom (O17) along with O21 constructed H-bonds with the hydrogens (HW6A and HW1B) of the lattice water molecules.

Fig. 1 (a) 1D polymeric infinite chain of IITKGP-101 (color codes: Cu, cyan; O, red; S, yellow; N, blue; C, gray; and H, white); (b) representation of Cu₄(μ₃-OH)₄ SBUs displaying a high-density of coordinated water; (c) comparison the PXRD plots of IITKGP-101 for the as-synthesized, water-treated, pH-treated and bulk-scale synthesized samples.

As displayed in Fig. 1c, the PXRD pattern of the as-synthesized IITKGP-101 is in good agreement with the simulated pattern obtained from the single-crystal structure and confirms the phase purity of the as-synthesized sample. To verify the preliminary stability of IITKGP-101 in open air, the as-synthesized sample was left open in the laboratory atmosphere for a prolonged time and the crystallinity was monitored by PXRD analysis. No noteworthy change even after six months of open-air exposure signifies an open atmosphere easy accessibility of the developed material (Fig. S6, ESI†). Furthermore, to confirm the water stability of the developed framework, IITKGP-101 was immersed not only in distilled water but also treated with boiling water for 24 h. The excellent retention of the PXRD peak positions in both cases confirms the structural integrity of IITKGP-101 (Fig. 1c). Besides hydro-stability, the chemical stability of the developed framework was also evaluated. No significant change in PXRD patterns even after immersion in pH 2 and pH 10 aqueous solutions for 24 h proves the wide range of chemical stability of IITKGP-101 (Fig. 1c). Such high framework robustness could be attributed to the presence of ample stronger metal–N coordination (soft–soft acid–base combination) with a large number of π⋯π stacking and extensive H-bonding interactions throughout the framework. It may be noted that, although, quite a few MOFs have been reported to show a wide range of pH stability by now and the reports are gradually increasing, while considering their potential industrial usage such numbers are considerably limited for CPs. The bulk material could be easily synthesized in gram scale through a simple room temperature crystallization method from a green aqueous medium. Most importantly, the crystallinity and phase purity of the G-scale as-synthesized sample could also be confirmed by PXRD analysis, as depicted in Fig. 1c. The characteristically strong and broad peak at ∼3500–3000 cm⁻¹ in the FT-IR spectrum demonstrates the presence of water molecules (Fig. S7, ESI†). Thermogravimetric analysis (TGA) under an N₂ atmosphere has been performed to verify the thermal stability of the developed material. IITKGP-101 was thermally stable up to 250 °C with an initial wt loss of 7.3% (calcd 7.5%) at 100 °C followed by 7.9% wt loss (calcd 8.7%) at 240 °C, which could be attributed to the expelling of both crystallized and coordinated water molecules, respectively (Fig. S8, ESI†). Differential scanning calorimetry (DSC) measurements ruled out the possibility of phase transitions under the proton conduction measurement conditions (Fig. S9, ESI†). To have more information on the framework robustness, a VTPXRD (variable-temperature powder X-ray diffraction) study was performed, which displays that IITKGP-101 maintains its crystallinity until 175 °C (Fig. S10, ESI†).

During the synthesis of IITKGP-101, the disodium salt of 2,6-NDS was used; thus, it is essential to confirm that the developed material does not contain any Na⁺ ions that can lead to ionic conductivity. The EDS analysis revealed the complete absence of sodium ions in the synthesized sample and confirmed that the obtained conductivity solely arises because of the protons (Fig. S11a, ESI†). On the other hand, the field emission scanning electron microscopy (FESEM) image displays the dispersed phase crystalline nature of the IITKGP-101 sample (Fig. S11b, ESI†).

Before proton conductivity evaluation, the moisture stability of IITKGP-101 was analyzed by exposing the as-synthesized sample under 97% RH for 3 days, which displayed complete retention of the crystallinity. Furthermore, the as-synthesized sample was also exposed to 80 °C and 98% RH for 16 h. No significant change in PXRD patterns under extreme measurement conditions confirmed the excellent moisture stability with extended durability of the developed material (Fig. S12, ESI†). These are the preliminary criteria to be fulfilled for a crystalline material for further evaluation of its proton conduction properties. Such high stability coupled with the presence of several proton-conducting species (abundant coordinated and lattice water molecules, cationic [Cu₄(μ₃-OH)₄(4,4′-bpy)₂(2,6-NDS)(H₂O)₇]²⁺ units and anionic charge balancing 2,6-NDS units) bearing copious proton hopping sites interconnected via a well-defined H-bonding network as previously described prompted us to analyze the proton conductivity of IITKGP-101. As mentioned earlier, the metal center enhances the acidity of the coordinated water molecules, which can act as efficient proton sources and thus may lead to higher proton conductivity. A proton conduction study was performed via alternating-current (AC) impedance analysis in both the single crystal and pellet forms (Fig. S13, ESI†). Proton conduction measurements were performed by varying both the temperature (30–80 °C) and humidity (40–98%). At ambient temperature (30 °C) in the single crystal form, IITKGP-101 displayed a proton conductivity of 4.25 × 10⁻⁵ S cm⁻¹ under 98% RH (Table S5, ESI†), which gradually increased with the increase of temperature and reached the superprotonic region with the value of 5.22 × 10⁻⁴ S cm⁻¹ at 80 °C (Fig. 2). The higher proton conductivity at higher temperatures could be attributed to the generation of more active proton-conducting species due to the bond cleavage and the mobility increment of the protonic species. In the Nyquist plots, two different components were observed in which semicircles attribute the bulk resistance of the material, whereas the tails of the curves represent the accumulation of protons at the electrodes (Fig. 2). At 80 °C under lower humidity (40%), IITKGP-101 exhibited a relatively lower proton conductivity of 6.22 × 10⁻⁸ S cm⁻¹ (Table S5, ESI†); however, the conductivity increased significantly with the increase of RH and reached the superprotonic region at 98% RH with the value of 5.22 × 10⁻⁴ S cm⁻¹ (Fig. S14 and 15, ESI†). It should be noted that the proton conductivity increased by four orders of magnitude from low RH (40%) to high RH (98%). Such a significant rise in conductivity with the increment of RH demonstrates that the adsorbed water molecules act as bridges between the donor–acceptor molecules and lead to water-mediated proton diffusion efficiently into the structure. For a better understanding of the proton-conducting property, an ambient temperature water-sorption study was performed by activating the as-synthesized material at 373 K temperature under a vacuum. As displayed in Fig. S16 (ESI†), the steep water uptake capacity even at lower relative humidity signifies the hydrophilic nature of the developed framework. At a lower relative pressure (P/P₀ = 0.1), the H₂O sorption was 34.7 cc g⁻¹ (corresponding to uptake of two water molecules per formula unit), which increased significantly with the increase of relative pressure and reached the maximum of 100.1 cc g⁻¹ at P/P₀ = 0.83 (corresponding to ∼5.4 water molecules per formula unit). The absorbed water molecules further act as efficient proton carriers in the channel and displayed a superprotonic conductivity at higher RH. For a better understanding of this phenomenon, we have correlated the proton conductivity with the number of absorbed water molecules present in the system (Fig. 3). The lower proton conductivity of 6.22 × 10⁻⁸ S cm⁻¹ at low humidity (40% RH) could be attributed to the lower number of absorbed water molecules (4.42) per formula unit, which increased with the increase of humidity and reached the maximum of ∼5.4 water molecules per formula unit at 98% RH to lead the superprotonic conductivity. The maximum proton conductivity of 5.22 × 10⁻⁴ S cm⁻¹ at 80 °C and 98% RH is comparable to those of benchmark MOFs and CPs in which proton conductivity has been tested in the single crystal form (Table S7, ESI†). Although the proton conductivity value of IITKGP-101 is slightly lower than those of Co-MOF-74 (6.1 × 10⁻⁴ S cm⁻¹ at 90 °C, 95% RH),⁸⁴1·3H₂O (1.43 × 10⁻³ S cm⁻¹ at 80 °C, 95% RH),⁸²PCC-72 (1.2 × 10⁻² S cm⁻¹ at 95 °C, 98% RH),⁸¹CFA-17 (2.1 × 10⁻³ S cm⁻¹ at 22 °C, 95% RH),⁷⁴In(5-Hsip)₂ (1.25 × 10⁻³ S cm⁻¹ at 25 °C, 40% RH),⁸³CPM-103a (5.8 × 10⁻² S cm⁻¹ at 22.5 °C, 98.5% RH),⁸⁰Pt₂(MPC)₄Cl₂Co(DMA)(HDMA)·guest (2.2 × 10⁻² S cm⁻¹ at 60 °C, 95% RH),⁶¹CoCa·nH₂O (1 × 10⁻³ S cm⁻¹ at 25 °C, 95% RH),⁷⁹ and In(imdcH)(ox) (1.11 × 10⁻² S cm⁻¹ at 22.5 °C, 98.5% RH),²² the conductivity is higher than those of CoLa-II (3.05 × 10⁻⁴ S cm⁻¹ at 25 °C, 95% RH),⁷⁶Zn(H₂PO₄)₂(TzH)₂ (1.1 × 10⁻⁴ S cm⁻¹ at 130 °C),⁷⁵ and Eu₂(CO₃)(ox)₂ (9.02 × 10⁻⁵ S cm⁻¹ at 80 °C).⁷⁷

Fig. 2 (a) Temperature-dependent (30–80 °C) Nyquist plots for the single crystal form of IITKGP-101 at 98% RH; (b) proton conductivity vs. temperature plot for IITKGP-101 in crystal form at 98% RH.

Fig. 3 Proton conductivity vs. water sorption isotherm correlation plots at 80 °C for IITKGP-101 with various RH values.

Besides single crystal proton conductivity measurements of IITKGP-101, the temperature and humidity-dependent proton conductivity was also investigated for the pelletized sample. As the mobility of the proton-conducting species is directly proportional to the temperature, the proton conductivity of IITKGP-101 increases with the increase of temperature. At 98% RH and ambient temperature (30 °C) the pelletized sample of IITKGP-101 displayed a proton conductivity of 3.17 × 10⁻⁷ S cm⁻¹ (Table S6, ESI†), lower than the crystal form (4.25 × 10⁻⁵ S cm⁻¹); however with increasing temperature, the proton conductivity also increased significantly and reached the maximum of 2.46 × 10⁻⁴ S cm⁻¹ at 80 °C and 98% RH (Fig. 4). Although this value was slightly lower than the value obtained from the proton conductivity measurement (5.22 × 10⁻⁴ S cm⁻¹) of the crystal form under the same conditions, it still lay in the superprotonic region (≥10⁻⁴ S cm⁻¹).³ The relatively lower proton conductivity value of the pelletized sample than the single crystal could be attributed to the random orientation of the pelletized samples during conduction measurements and grain boundary effects. Furthermore, the humidity-dependent proton conductivity was measured with the pelletized sample. From Fig. S17 (ESI†), it has been observed that with the increase of R.H., the bulk resistance of the material decreases, which is reflected in the Nyquist plot. Gradually decreasing semicircles with increasing R.H. lead to the maximum conductivity of 2.46 × 10⁻⁴ S cm⁻¹ under 98% R.H. at 80 °C (Table S6, ESI†), which is comparable with those of several well-known crystalline materials (Table S8, ESI†). An attempt was made to measure the proton conductivity under anhydrous conditions of activated IITKGP-101; however, the Nyquist plots showed an arc without a semicircle and the resistance was very high beyond the detection range of the instrument. Therefore the proton conductivity of IITKGP-101 is poor under anhydrous conditions (Fig. S18, ESI†).

Fig. 4 (a) Temperature-dependent (30–80 °C) Nyquist plots for the pellet form of IITKGP-101 at 98% RH (inset: enlarged portions of the high-frequency regions for clear visualization); (b) proton conductivity vs. temperature plot for IITKGP-101 in pellet form at 98% RH.

To have a better insight into the proton transportation mechanism, the activation energy (E_a) of IITKGP-101 was calculated from the perfect linear fitting of the Arrhenius plot (Fig. S19, ESI†). Depending upon the activation energy (E_a), the proton conduction mechanism of the solid-state electrolytes has been classified into two types, (i) Grotthuss mechanism (E_a < 0.4 eV) in which a proton hops through the H-bonding between the hydronium ion and water. Also, the proton transfer and sequential molecular rotation occur simultaneously in the Grotthuss mechanism. (ii) Vehicular mechanism (E_a > 0.4 eV) in which the protic species (H₃O⁺ or NH₄⁺) diffuses by itself as a proton-attached vehicle.⁶⁹ The resulting activation energy of 0.78 eV for IITKGP-101 suggests that proton transportation follows a vehicular-type mechanism in which protons transfer in the hydrated form. At higher humidity, both the coordinated and lattice water molecules interacted with the adsorbed water molecules to construct directional H-bonding networks and the dissociated protons from the coordinated water molecules passed through this network to achieve superprotonic conductivity. The adsorbed water molecules act as bridges between the donor–acceptor molecules and lead to water-mediated proton diffusion efficiently into the structure. As discussed earlier, having ample coordinated water molecules with their enhanced acidity along with high water uptake by IITKGP-101 facilitates the construction of efficient and fast proton transfer pathways, which further leads to such a high proton conductivity. After impedance measurement, the excellent retention of the PXRD patterns of the as-synthesized material confirms the framework stability throughout the proton conduction measurement (Fig. S20, ESI†).

The proton conductivity value reported herein is on the order of only 10⁻⁴ S cm⁻¹, which is considerably low compared to the recently reported MOFs and CPs; however, such a basic concept of applying the enhanced acidity of coordinated water molecules to induce proton conductivity, its wide range pH stability, hydro and boiling water stability, open air stability for as long as six months, ease of scalability through simple crystallization from green solvents like water, and conductivity measurements in both single crystal and pellet forms made this framework truly interesting and of high potential as a solid-state proton conductor.

Conclusions

In summary, taking advantage of coordinated water molecules’ enhanced acidity, we have constructed a pH-stable coordination polymer, IITKGP-101, in which coordinated water molecules act solely as proton sources, thus imparting the proton conductivity. A logical approach of design strategy and synthesis conditions could bring the robustness of the developed framework. The presence of abundant coordinated water molecules along with lattice water molecules, lattice 2,6-NDS units, and uncoordinated O atoms of the metal-bound 2,6-NDS unit in the hydrophilic interlayer spaces of the developed framework constructs an effective proton-conducting pathway responsible for exhibiting superprotonic conductivity. Both the temperature and humidity-dependent conductivity have been measured in single crystal and pellet form as well. The moderate conductivity, framework's ultrahigh robustness, and easy aqueous medium scale-up synthesis process made IITKGP-101 a promising proton conductor. We believe that the design principle presented in this work will mobilize the researchers to develop more proton-conducting MOFs and CPs having ample coordinated water molecules as intrinsic protonic sources and will be beneficial for further development of this burgeoning field.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgements

R. S. thanks CSIR, New Delhi, India, for his Senior Research Fellowship. We acknowledge Prof. Gang Xu (Fujian Institute of Research on the Structure of Matter, CAS) for conductivity measurements. M. C. D. acknowledges the SERB, New Delhi for financial support via the Core Research Grant (CRG/2019/001034).

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Footnotes

† Electronic supplementary information (ESI) available: The Electronic supplementary information contains instrument details, synthesis details, table of crystallographic data, PXRD patterns, TGA profiles, DSC patterns, calculation details, proton conduction results, and proton conduction comparison tables. CCDC 2220922. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3qm00007a

‡ These authors contributed equally to this work.

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