Sequential enhancement of proton conductivity by aliovalent cadmium substitution and post-synthetic esterolysis in a carboxylate-functionalized indium framework with dimethylaminium templates

Abstract

A sequential improvement strategy has been devised and implemented on a 3D open framework In-BQ showing 2D intersected channels filled by dimethylamine and its protonated cation constructed by –COOCH₃–functionalized anilicate linkers. In situ aliovalent metal substitution and post-synthetic ligand esterolysis led to Cd-BQ-COOH with a doubling of Me₂NH₂⁺ carriers and a great number of residual –COOH groups, resulting in maximum proton concentration and frequent jumping sites. As a result, the modified Cd-BQ-COOH exhibits a 300-fold enhanced value of proton conductivity compared with that of pristine In-BQ, reaching 6.06 × 10⁻² S cm⁻¹. MD calculations reveal that the entire process of proton transportation in Cd-BQ-COOH is achieved by the vehicle mechanism.

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Introduction

Solid-state proton conductors have attracted considerable attention for a wide variety of applications in hydrogen separation, water electrolysis, biological sensors and fuel cells,^1,2 which are currently focused on dramatically improving their proton conductivity and deeply illuminating their intrinsic conducting pathways.^3,4 Compared with other inorganic metal oxides or organic polymers,^5,6 metal–organic frameworks (MOFs) are studied and accepted as preferred candidates for next-generation conducting materials due to their ordered crystalline nature, high internal porosity and tunable modular functionality.^7–10 These unique characteristics are beneficial not only to acquire rich proton sources in a restricted volume to increase proton carrier concentration but also to create abundant hopping sites in a specific alignment to elevate the proton mobility.^11,12 Although most MOF materials show good prospects, it is still required to tactically fine-tune key structural components towards the precise design of high-level performance and long-term durability of proton conduction.^13–15

For this purpose, two distinct strategies involving predesigned methods or post-synthetic modifications^16–19 have been proposed and implemented to draw multiple proton carriers into MOFs: (1) the introduction of protophilic groups (–SO₃H, –PO₃H₂, –COOH, –OH, etc.) on the backbones by retaining residual acidic groups or transforming the precursors into functional groups;^20,21 (2) the incorporation of protic entities (H₂SO₄, H₃PO₄, imidazole, triazole, ammonium cations, etc.) into the channels by balancing charged frameworks or exchanging guest molecules.^22,23 Particularly, there have been some recent synergistic highly proton-conducting materials originating from the aforementioned approaches, for example H₂SO₄@MIL-101-SO₃H,²⁴ BUT-8(Cr)A,²⁵ and PCMOF2½(Tz).²⁶ These reasonable and sequential modifications of the products afford maximum proton donor–acceptor and strong host–guest interactions to establish successive hydrogen-bond networks and efficient proton-transfer pathways, leading to conductivity values surpassing 10⁻¹ S cm⁻¹ below 373 K with humidification, which are comparable to that of commercial Nafion materials.^27–29

Inspired by the feasibility of the above tactics, we attempt to explore enhancing strategies with synergistic effects to optimize proton conduction performance. In a previous report, we proposed an aliovalent metal substitution strategy^30,31 and implemented it on a dimethylaminium-templated compound In-BQ.³¹ This diamond-topology open framework featuring 2D intersected channels filled with dimethylamine and its protonated dimethylaminium constructed by –COOCH₃-functionalized anilicate linkers prompted us to sequentially modify the cooperation of aliovalent Cd(II) substitution and post-synthetic ligand esterolysis. Surprisingly, the modified framework Cd-BQ-COOH has three kinds of proton sources: the first are the attached –OH groups, which could dissociate H⁺ during the coordinate progress; the second are the filled Me₂NH molecule and Me₂NH₂⁺ cation, which could protonate into the doubling of Me₂NH₂⁺ cations through the aliovalent replacement of In(III) by Cd(II); and the third are the functionalized –COOH groups, which are potentially converted from the post-synthetic esterolysis of –COOCH₃ on anilicate ligands. In contrast to the Cd(II)-substituted Cd-BQ and carboxyl-functionalized In-BQ-COOH, which have already boosted the proton conductivities by 100-fold (2.30 × 10⁻² S cm⁻¹, 303 K and 95% RH) and 15-fold (3.54 × 10⁻³ S cm⁻¹, 303 K and 95% RH), respectively, when compared with pristine In-BQ, the sequentially modified Cd-BQ-COOH exhibits a 300-fold enhancement with up to 6.06 × 10⁻² S cm⁻¹ at the same condition.

Experimental section

Chemicals and materials

The starting materials and solvents were purchased and utilized from commercial sources without further purification. In-BQ and Cd-BQ were prepared according to our previously described procedure.³¹ To maintain the integrity of the skeletons and reduce the possibility of losing molecules/cations in the pores as much as possible, a mild and reversible acid hydrolysis was employed. In-BQ (0.2 g) and Cd-BQ (0.2 g) were separately hydrolyzed with 1 M H₂SO₄ solution (20 mL) in a 50 mL round-bottom flask. The suspensions of the solid were stirred for 10 h at 40 °C. After cooling to about 30 °C, the resulting powdered solids were isolated by centrifugation and washed with H₂O several times until the upper fluid was neutral. The morphological changes in both materials before and after esterolysis are shown in Fig. S1.†

Physical characterization

Scanning electron microscopy (SEM) images were recorded using a Thermofisher APR 20 emission scanning electron microscope with an accelerating voltage of 2–10 kV. Powder X-ray diffraction (PXRD) was measured on a Rigaku D/max-2550 diffractometer using Cu Kα radiation with an angle range (2θ) from 5° to 50°. Fourier transform infrared spectra (FT-IR) were obtained on KBr pellets with a PERKIN-ELMER 100-IR spectrometer (400–4000 cm⁻¹) at 298 K. Thermal gravimetric analysis (TGA) curves were collected using a JUPITER STA 449F3 instrument in an air atmosphere with a heating rate of 10 °C min⁻¹. Nuclear magnetic resonance (NMR) data were analysed using a BRUKER AVANCE 400 spectrometer. Electrospray ionization-mass spectrometry (ESI-MS) was performed on a BRUKER SolanX 70 FT-MS. The H₂O vapor adsorption isotherms were recorded using a BSD-VVS gravimetric vacuum steam adsorber at room temperature.

Proton conductivity measurement

The proton conductivity was tested using a Princeton ParStat-4000 electrochemical analyzer with a two-probe system at an AC voltage of 100 mV in the frequency range of 1 MHz to 0.1 Hz. The as-synthesized samples were pressed into pellets of 6.0 mm diameter and 1–2 mm thickness at a pressure of 20 MPa. The measured slice pellet was sandwiched between two stainless steel wafers and clamped with an electrode clip. The system temperature and humidity were controlled by a BLUEPARD BPS-50L programmable incubator. The proton conductivity (σ) was calculated according to eqn (1), where σ = proton conductivity (S cm⁻¹), L = the thickness (cm), A = surface area (cm²), and R = impedance (Ω).E_a is the activation energy, which was determined from the fitted slope of the Arrhenius eqn (2).

Computational details

The initial and fully hydrolyzed In-BQ-COOH and Cd-BQ-COOH structures were built and optimized on the basis of the In-BQ and Cd-BQ crystal structures, respectively. Subsequently, a computational unit involving a couple of Me₂NH₂⁺ (or one Me₂NH₂⁺ and one Me₂NH) and associated framework molecules was chosen as a theoretical model. The positions of the O atoms and C atoms of the framework were highly fixed to avoid crumbling or large structural distribution. Meanwhile, all atoms of H and the dimethylaminium templates remained free, allowing proton carriers with H⁺ to perform transportation or exchange. The entire model was fixed in a simulation box with a vacuum area of 10 ų. The quantum molecular dynamics (MD) were simulated using the SIESTA package³² with the GGA-PBE functional³³ and Troullier–Martins norm-conserving pseudopotentials.³⁴ A double-ζ polarized (DZP) basis set was employed for the valence electronic orbitals of all atoms and the mesh cut-off was set as 250 Ry. The Monkhorst–Pack type of k-point sampling with a (1 × 1 × 1) mesh was used for the In-BQ-COOH and Cd-BQ-COOH structure models. The MD simulation was controlled by the Nosé³⁵ method employing 2000 steps at a time step of 1 fs at 298 K.

Results and discussion

In the host backbone of In-BQ, each In(III) ion is eight-coordinated with O atoms belonging to four anilicate ligands in the chelate mode, and each linker is connected to two In(III) ions in a bis-bidentate fashion, building a 3D diamond-topology structure containing 2D intersected channels. The channels are decorated by coordinated –OH groups and residual –COOCH₃ groups, as well as filled with a Me₂NH molecule and a Me₂NH₂⁺ counterion. Accompanied with the in situ aliovalent replacement of In(III) by Cd(II), the original Me₂NH/Me₂NH₂⁺ in In-BQ protonate into the doubling of Me₂NH₂⁺ cations in Cd-BQ for charge balance. Moreover, the absolute freedom of the –COOCH₃ groups on the anilicate linkers offers an uncommon opportunity for the –COOH ligand functionalization to confer this material with advanced proton conduction performance. Through the post-synthetic esterolysis of In-BQ and Cd-BQ by H₂SO₄ (1 M) aqueous solution, the modified frameworks have been generated, denoted as Cd-BQ-COOH and In-BQ-COOH, respectively (Fig. 1).

Fig. 1 The formation processes for In-BQ-COOH and Cd-BQ-COOH.

Notably, the hydrogen bonds in Cd-BQ-COOH [N–H⋯O (2.86–3.10 Å) and O–H⋯O (2.61–3.69 Å)] are much shorter than those in In-BQ-COOH [N–H⋯O (2.97–3.65 Å) and O–H⋯O (2.71 Å)] (Table S1† and Fig. 2), and are much richer than those in Cd-BQ and In-BQ as well (Fig. S2†). Apparently, the multi-step route of Cd(II) substitution and –COOH functionalized modification gives rise to an expanded hydrogen-bond network, and endows Cd-BQ-COOH with progressively boosted proton conductivity.

Fig. 2 Comparison of the H-bond arrangements of (a) In-BQ-COOH and (b) Cd-BQ-COOH. Note: The H atoms of –OH, –COOH, and Me₂NH/Me₂NH₂⁺ are highlighted in blue, yellow, and green, respectively. The blue and orange dashed lines denote the N–H⋯O and O–H⋯O bonds, respectively.

The PXRD profiles before and after the proton-conducting tests of In-BQ-COOH and Cd-BQ-COOH are matched well with the simulated and as-synthesized patterns of the In-BQ and Cd-BQ frameworks, respectively, demonstrating the existence of the crystalline structures after the entire post-synthetic treatments and impedance measurements (Fig. S3†). Additionally, the –COOH functionalization frameworks were confirmed by FT-IR (Fig. S4†). The absorption peaks of the –COOCH₃ group located at 1720 and 1150 cm⁻¹ in In-BQ and 1730 and 1300 cm⁻¹ in Cd-BQ, respectively, are weakened in the spectra of In-BQ-COOH and Cd-BQ-COOH, and several peaks appear near 1710 and 900 cm⁻¹ stemming from the C=O and O–H stretching vibrations, respectively. To further confirm the production of the –COOH group, we performed ESI-MS and NMR analyses (Fig. S5 and S6†). The ESI-MS spectrum after hydrolysis shows high intensity peaks at m/z values of 227.45 and 256.24, while the spectrum before hydrolysis only shows one intensity peak at 256.77. The new peak position fits well with the mass of the expected carboxylic fragment. In the ¹H-NMR spectra, where dibromide was used as a reference, the integral area of –CH₃ was obviously diminished after hydrolysis, indirectly demonstrating the formation of –COOH. Both frameworks after hydrolysis show almost identical proportion of losing weight from In-BQ and Cd-BQ at the first step in the range of 30–150 °C, suggesting that the guest molecules are retained without much damage (Fig. S7†). The different hydrophilicity of the inner pores has been characterized by H₂O vapor adsorption at 298 K (Fig. S8†). The H₂O vapor uptakes of In-BQ-COOH and Cd-BQ-COOH are 140 wt% and 102 wt%, respectively, much higher than the 58 wt% of In-BQ and 38 wt% of Cd-BQ.³¹ This typical increase is associated with the effective transition of hydrophobic groups into hydrophilic groups hanging in the inner pore wall.

The proton-conductivities of In-BQ-COOH and Cd-BQ-COOH were measured by performing electrochemical impedance spectroscopy (EIS) on the pelletized powder samples. All resistance values were estimated from the Z′ intercept values due to the imperfect semicircle of the Nyquist plots.^22,36 In order to evaluate the water affinities of these two compounds, under similar testing conditions to those of In-BQ and Cd-BQ, humidity-dependent proton conducting data were measured at 25 °C with RH increasing from 55% to 95% (Table S2 and Fig. S9†). At 55% RH, the conductive value of Cd-BQ-COOH is 4.85 × 10⁻⁶ S cm⁻¹, compared with the 8.27 × 10⁻⁶ S cm⁻¹ of In-BQ-COOH. With rising humidity, the proton conductivity of Cd-BQ-COOH exceeds that of In-BQ-COOH and exhibits an ultrahigh conductive value of 3.03 × 10⁻² S cm⁻¹ at 95% RH, which is nearly 20 times higher than that of In-BQ-COOH (1.57 × 10⁻³ S cm⁻¹) at the same humidity. This increasing trend is consistent with the humidity-dependent results of In-BQ and Cd-BQ, and the proton conductivities are all improved under the parallel comparison. This is mainly attributed to the hydrophilicity of the –COOH groups, where absorbed water molecules acting as proton carriers could increase the efficiency of proton transfer.

To further investigate the potential conducting mechanism, the temperature-dependent proton conductivity, which is a key factor for the calculation of the resulting activation energy (E_a), was tested. Analogous to Cd-BQ and In-BQ, both Cd-BQ-COOH and In-BQ-COOH show better conductive behaviors in the low temperature region,^12,37,38 and thus the temperature-dependent conductive ability of these two compounds was characterized from 10 to 30 °C under 95% RH (Fig. 3, Fig. S10 and Table S3†). At 10 °C, the conducting values of Cd-BQ-COOH and In-BQ-COOH are 2.01 × 10⁻² S cm⁻¹ and 8.75 × 10⁻⁴ S cm⁻¹, respectively. As the temperature increases to 30 °C, the conductivities increase and reach their maximum values of 6.06 × 10⁻² S cm⁻¹ and 3.54 × 10⁻³ S cm⁻¹, respectively. The conductivities of Cd-BQ-COOH and In-BQ-COOH are also compared with other reported MOF-based proton conductors by ligand post-synthetic modification, as shown in Table S4.† Obviously, the 20-fold increased conductivities of the two compounds in the same range are almost maintained. As a comparison, the E_a values of Cd-BQ-COOH and In-BQ-COOH are determined using the Arrhenius equation and found to be 0.41 eV and 0.56 eV, respectively, much lower than the 0.48 eV of Cd-BQ and 0.73 eV of In-BQ.³¹ This result suggests that the energy consumption of the proton transportation in both compounds decreases with the participation of the –COOH groups, and the intrinsic proton migration may be due to the vehicle mechanism.

Fig. 3 Impedance spectra of (a) In-BQ-COOH and (b) Cd-BQ-COOH at 10–30 °C under 95% RH. Comparison of the Arrhenius plots of (c) In-BQ-COOH and (d) Cd-BQ-COOH with In-BQ and Cd-BQ, respectively, at 10–30 °C under 95% RH.

MD calculation has been recognized as a promising method to clearly elucidate the proton transportation mechanism at the atomic scale.^39–43 Based on this, further simulation studies on Cd-BQ-COOH and In-BQ-COOH were performed by first-principles calculations with an assumption that all the ester groups in Cd-BQ and In-BQ were completely replaced by –COOH groups.

At the initial situation in Cd-BQ-COOH (Fig. 4a), the Me₂NH₂⁺ cation mainly spins and glides around the Cd1(a) atom through the breakage and reestablishment of H-bonds. Then, at ∼500 fs (Fig. 4b ), the H1(a) and H1(b) atoms on the Me₂NH₂⁺ cation keep on moving towards the central region of the channel, building almost equal lengths of H-bonds with O1(a) and O1(b). Accompanied by the continuous movement of the Me₂NH₂⁺ cation at 650 fs, the H-bonds between H1(a) and H1(b) with O1(b) and O2(b) are re-established, as shown in Fig. 4c, which lead to the protons transferring near Cd1(b). In the final process (Fig. 4d), the Me₂NH₂⁺ cation migrates and returns near Cd1(a) by means of the H-bonds established with O2(a) and O3(a). It can be clearly seen that in the whole progress of proton transmission, the Me₂NH₂⁺ cation acts as a whole to undergo self-rotation and migration in the counterclockwise direction of “Cd1(a)–Cd1(b)–Cd1(a)”.

Fig. 4 Migration trajectory of Me₂NH₂⁺ in Cd-BQ-COOH at (a) the initial state; (b and c) the intermediate states; and (d) the later state. (e and f) The pathway of proton hopping between the –OH group and –COOH group in In-BQ-COOH. (g) A strong interaction is established between the H⁺ and Me₂NH. (h) Formation of a protonated Me₂NH₂⁺. Note: The Me₂NH/Me₂NH₂⁺ and the hopping protons are highlighted in green. The blue and orange dashed lines illustrate the changes of N–H⋯O and O–H⋯O bonds, respectively.

Besides, in In-BQ-COOH, an interesting consecutive proton reorientation occurs on the Me₂NH molecule and finally protonates the Me₂NH₂⁺ cation. Fig. 4e represents the situation at 50 fs, where the H3 atom on the –COOH group jumps from the O3 atom to the O4 atom following the intramolecular H-bond. Subsequently at ∼90 fs, the O3 atom acting as a hopping site accepts the H2 atom coordinated with the O2 atom belonging to –OH moieties (Fig. 4f). This intermediate stage of the H2 proton on O3 is estimated to be ∼100 fs. Then with the self-rotation and vibration of the Me₂NH molecule, the H2 atom jumps from the O3 atom to the N2 atom at 200 fs, which corresponds to an O⋯N distance of 2.58 Å and an O3–H2⋯N2 angle of 162(08)° (Fig. 4g). In the later process, the protonated Me₂NH₂⁺ cation as a whole continues to migrate (Fig. 4h). Overall, the entire process of proton migration involves the moving sequence of “O2–O3–N2”, which is jointly achieved by the hopping mechanism and the vehicle mechanism.

Conclusions

In summary, by sequentially realizing in situ aliovalent Cd(II) substitution and post-synthetic ligand esterolysis on a 3-D diamond-like open framework In-BQ, we have successfully acquired the modified framework Cd-BQ-COOH, which possesses the doubling of Me₂NH₂⁺ proton carriers and a great number of residual –COOH groups. These proton sources and hopping sites are beneficial to establish abundant hydrogen-bond networks and strong host–guest interaction, which lead to a 300-fold enhanced conductivity compared with that of pristine In-BQ, reaching 6.06 × 10⁻² S cm⁻¹ at 303 K and 95% RH. This work not only confirms the effectiveness and feasibility of the aliovalent metal substitution strategy extended to ligand modification but also provides a promising route to maximize the proton conduction performance of MOF materials.

Author contributions

Hui Gao: Conceptualization, investigation, formal analysis, visualization, writing-original draft. Ying-Xia Wang: Investigation, formal analysis, writing-review & editing. Yan-Bin He: Conceptualization (computations), investigation (computations). Xian-Ming Zhang: Conceptualization, project administration, funding acquisition, supervision, writing-review & editing.

Conflicts of interest

The authors declare that they have no conflict of interest. The authors declare no competing financial interests.

Acknowledgements

We thank the support of NSFC (21871167), 1331 Project of Shanxi, Shanxi Province Science Foundation for Youths (201901D211391), research project supported by Shanxi Scholarship Council of China (2020-088), and the Technology Innovation Team (CX201904).

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: Proton-conducting measurement, H₂O adsorption, IR, PXRD, TGA, MS, NMR data (PDF). ESI Videos of Cd-BQ-COOH and In-BQ-COOH are showing the proposed conduction mechanism (MP4). See DOI: https://doi.org/10.1039/d2qi00407k

‡ These authors contribute equally to this work.

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