A High Proton Conductive Hydrogen-Sulfate Decorated Titanium

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A High Proton Conductive HydrogenSulfate Decorated Titanium Carboxylate MOF Mohammad Wahiduzzaman, Sujing Wang, Josefine Schnee, Alexandre Vimont, Vanessa Ortiz, Pascal Georges Yot, Richard Retoux, Marco Daturi, Ji Sun Lee, Jong-San Chang, Christian Serre, Guillaume Maurin, and Sabine Devautour-Vinot ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05306 • Publication Date (Web): 24 Jan 2019 Downloaded from http://pubs.acs.org on January 25, 2019

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ACS Sustainable Chemistry & Engineering

A High Proton Conductive Hydrogen-Sulfate Decorated Titanium Carboxylate MOF Mohammad Wahiduzzaman,a,ϯ Sujing Wang,b,ϯ Josefine Schnee,c Alexandre Vimont,c Vanessa Ortiz,a Pascal G. Yot,a Richard Retoux,d Marco Daturi,c Ji Sun Lee,e Jong-San Chang,e Christian Serre,b* Guillaume Maurin,a Sabine Devautour-Vinot,a*

aInstitut

Charles Gerhardt Montpellier UMR 5253 CNRS UM ENSCM, Université

Montpellier, Pl. E. Bataillon, 34095 Montpellier cedex 05, France

bInstitut

des Matériaux Poreux de Paris, FRE 2000 CNRS, Ecole Normale

Supérieure, Ecole Supérieure de Physique et de Chimie Industrielles de Paris, PSL University, 24 rue Lhomond, 75005 Paris, France

cNormandie

Université, ENSICAEN, UNICAEN, CNRS, Laboratoire Catalyse et

Spectrochimie (LCS), 6 Boulevard Marechal Juin, 14000 Caen, France

ACS Paragon Plus Environment

1

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

dNormandie

Page 2 of 45

Université, ENSICAEN, UNICAEN, CNRS, Laboratoire de

Cristallographie et Sciences des Matériaux (CRISMAT), 6 Boulevard Marechal Juin, 14000 Caen, France

eResearch

Technology

Center for Nanocatalyst, Korea Research Institute of Chemical (KRICT),

Sungkyunkwan

University,

Gajeongro

141,

Yuseong,

Daejeon 305-606, Korea

Corresponding Authors * [email protected], +33 4 67 14 33 00

* [email protected], +33 1 44 32 24 63

Keywords MOF; Titanium; green chemistry; post-modification; acidic sites; proton conduction; proton migration mechanism; clean-energy application

Abstract


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A hydrolytically stable titanium carboxylate-based MOF, incorporating hydrogensulfate functions bounded to Ti-metal sites and named MIP-177-SO4H-LT, was prepared using a green and simple synthesis protocol. This solid exhibits a very high proton conductivity of 2.6×10-2 S cm-1 at 298 K and 95% Relative Humidity as evidenced by impedance spectroscopy measurements. This high level of performance maintained over 7 days combined with a very good chemical stability and green synthesis route positions this MOF as a promising candidate for further deployment as a proton-exchange membrane. Ab initio Molecular

Dynamics

simulations

and

advanced

characterization

tools

(IR,

UV/Vis, TGA/MS…) were further coupled to reveal a water-mediated proton transport Grotthuss-like mechanism.

Introduction

Over the past two decades, there has been a growing interest to promote Metal-Organic

Frameworks

(MOFs)

as

novel

proton-conducting


solids

for

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Page 4 of 45

applications in fuel cells, redox flow batteries, supercapacitors and chemical sensors technologies.1–5 Compared to conventional materials, MOFs offer great advantages in terms of tunable functionality, topology and pore size/shape — allowing the design of solids with a large variety of potential proton sources and an ordered arrangement of confined species acting as proton media. In this

context,

conductivity

diverse

strategies

performances

of

have

MOFs

been via

i)

devised the

to

tailor

introduction

the of

proton

functional

hydrophilic ligands with hydroxyl groups or sulfonic, phosphonic and carboxylic acid-based functions as proton sources,6–16 ii) the incorporation of guest molecules in the pores as proton transfer agents e.g., water, protic molecules or strong acids,17–21 iii) the design of anionic MOFs with proton donors as counter-ions in the pores22–24 or iv) the control of the pore size and topology to create a percolated hydrogen bonded network of the guest molecules.25–30 This led the MOF community to discover a series of promising proton conducting materials with conductivity values exceeding 10-3 S cm-1 below 373 K and under

humid

conditions.

However,

only

a

few


of

them

have

shown

4

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concomitantly competitive performances compared to Nafion®; i.e., σ > 10-2 S cm-1

under

high

relative

stability.10,11,15,16,21,23,31–34

humidity

These

(RH)

combined

and

features

a

good are

water/chemical

clearly

a

crucial

prerequisite to ensure that the proton conductivity performances can be maintained over cycles. Another key point is to leverage the synthesis of environment-friendly MOFs with the minimum use and generation of hazardous substances. A significant number of potential proton-conducting MOFs still suffers from this drawback, since they result from a multi-step synthetic route along with the use of costly and environmentally unfriendly precursors, which has hampered their development for practical applications so far.

Very recently, some of us have discovered a water-stable porous threedimensional (3D) mdip-based Ti MOF,35 namely MIP-177-LT (LT stands for Low Temperature

form,

mdip

for

3,3’,5,5’-tetracarboxydiphenylmethane

and

MIP

stands for Materials from Institute of porous materials from Paris). This material was obtained using a green and simple synthesis protocol, i.e. the preparation and activation steps involve simple processes and less harmful chemicals than


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those typically used for the syntheses of other reported Ti-MOFs. The crystal structure

of

this

MOF

with

the

following

chemical

formula

Ti12O15(mdip)3(formate)6, built up from Ti12O15 oxoclusters connected through mdip ligands and formate groups, delimits an array of hexagonal 1D channels with ca. 1.1 nm of free diameter (see Figure 1a). Here, we have designed a novel derivative of MIP-177-LT phase, incorporating intra-framework –SO4H Brønsted proton species directly bounded to the inorganic building block that are

expected

to

confer

to

this

material

remarkable

proton

conduction

performances. This strategy offers an alternative to the standard -SO3H ligand functionalization route that has been envisaged so far to enhance the proton conduction

performances

of

MOFs.12,15,36,37

In

this

context,

advanced

characterization tools, including PXRD, TGA-MS, UV-Vis and FTIR, evidenced that the treatment of MIP-177-LT with a sulfuric acid solution maintained the structural integrity of the pristine solid and resulted in the substitution of the formates pointing towards the nano-sized channel by Brønsted acid hydrogensulfate sites that can act as additional proton sources. A structure model of the


6

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derivative was further proposed using a computational approach. AC impedance spectroscopy revealed that the newly water stable derived MIP-177-LT, denoted as MIP-177-SO4H-LT, exhibits an exceptional conductivity σ = 2.6×10-2 S cm-1 at 298 K and 95% RH. FTIR further revealed that a proton transfer takes place from the Brønsted acid hydrogen-sulfate sites to the adsorbed water molecules, while ab initio Molecular Dynamics simulations elucidated the proton transport mechanism in this water-mediated proton conducting MOF at the atomic level.


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Figure 1 (a) Crystal structure of MIP-177-LT and (b) fully hydrated MIP-177SO4H-LT showing the substitution of formates by hydrogen-sulfate species and the preferential arrangement of the guest molecules resulting an extended

hydrogen-bonded network. Formates of the pristine solid are marked in blue while protons in the HSO4- moieties are highlighted with green spheres. C, O, H and S atoms are represented in grey, red, white and yellow, respectively. Bottom panel shows characteristic moves of a typical proton hopping from the pore wall towards the channel of MIP-177-SO4H-LT as captured from the AIMD simulations performed at 298 K: (c) formation of a strong hydrogen bond between the proton of a HSO4- group and a neighboring water, (d) proton

transfer to the water molecule and formation of a hydronium ion (H3O+), (e)

subsequent formation of a Zundel species (H5O2+), and (f) interconversion of a

Zundel species to another hydronium.


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Experimental Section Materials. MIP-177-LT was synthesized under easily-handled mild reflux and atmospheric pressure, without any toxic and complicated reaction systems (such as DMF, HF, additives, prepared cluster precursor, inert atmosphere), according to the following procedure already reported.35 To a 500 mL round bottom flask, H4mdip (4 g, 11.6 mmol) and formic acid (200 mL) were added and stirred at room temperature until the solid dispersed uniformly. Then Ti(iPrO)4 (8 mL, 26.4 mmol) was added dropwise. Afterward, the reaction mixture was heated under reflux for 24 h. After cooling to room temperature, the white solid product of MIP-177LT was filtered, washed with ethanol and air dry. MIP-177-LT powder (400 mg) was dispersed in H2SO4 (6M) aqueous solution at room temperature with stirring for 24 hours. Afterwards, centrifuge was applied to collect all the products. Then acetone (20 mL) was added to wash away the H2SO4 molecules attached to the product surface and helps the sample to dry quickly under vacuum at room temperature. The excess of H2SO4 molecules was


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eliminated by washing with a large amount of water at room temperature, generating the product denoted as MIP-177-SO4H-LT.

Characterization techniques. X-Ray Powder diffraction (XRPD) patterns of the pristine MIP-177-LT and the derivative MIP-177-SO4H-LT were collected at room temperature on a high-throughput Bruker D8 Advance diffractometer working on transmission mode and equipped with a focusing Göbel mirror producing CuKα radiation (λ = 1.5418 Å). Nitrogen sorption measurements were performed with a BEL Japan Belsorp Mini apparatus at 77 K after the sample being fully activated (BEL Japan, BELSORP Prep). For in situ IR analysis, MIP-177 LT and MIP-177-SO4H-LT samples were pressed (100 bar) into a self-supported disc (2 cm2 area, 7-10 mg cm-2). They were placed in a quartz cell equipped with KBr windows. A movable quartz sample holder permitted the adjustment of the pellet in the infrared beam for spectra acquisition and to displace it into a furnace at the top of the cell for thermal treatments. IR spectra were recorded at 4 cm-1 resolution on a Nicolet Nexus spectrometer equipped with an extended KBr beam splitting device and a mercury cadmium telluride (MCT)


10

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cryo-detector. The crystal size and morphology of the samples were observed using transmission electron microscopy (TEM). The Energy Dispersive Analysis and Electron Microscopy observations were performed at room temperature on 200 kV JEOL 2010 FEG electron microscopes (tilt ± 42°) equipped with an EDS (Energy Dispersive Spectrometer, Si/Li detector) EDAX and fitted with double tilt sample holder. For the sample preparation for TEM observations, small flakes of diluted suspensions of MIP-177-SO4H-LT nanomaterials were deposited on a holey carbon film, supported by a copper grid. The chemical composition of the observed sample was determined by EDS point analysis. The

diffuse

reflectance

UV-vis

spectra

were

measured

using

a

diffuse

reflectance accessory (Praying Mantis). The UV-vis spectra (200 - 800 nm) were recorded by a Cary 4000 spectrophotometer (Agilent Corp.) using MgO as reflectance standard. Thermogravimetric coupled with Mass Spectrometry (TGAMS) measurements were collected on a STA 449 F1 Jupiter - QMS 403D Aëolos

Netzsch system, from 303 to 823 K (heating ramp, q = 1 K min-1 )

and under Argon flow (70 mL min-1). Impedance measurements were performed


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on a Broadband Dielectric Spectrometer, Novocontrol alpha analyzer over a frequency range from 1 Hz to 1 MHz with an applied ac voltage of 20 mV. Measurements were collected at room temperature on the hydrated solid, which was introduced into an Espec Corp. SH-221 incubator, to control RH from 40% to 95%. The solid was equilibrated for 24 hours at given RH values, to ensure fixed water content before recording the impedance. The measurements were performed using powders. The consideration of pellets led to similar data (see SI). Resistivity was determined from the semi-circle extrapolation as well as the fitting of the Nyquist plots using equivalent circuit models. As shown in ESI, both procedures converge towards similar conductivity values. Conductivity was calculated considering σ = 1/R×l/S, where R is the resistance (), σ is the conductivity (S cm-1), l and S are the sample thickness (cm) and surface (cm²), respectively. Alternatively, the Bode representation was also considered to determine the bulk conductivity, which equally leads to data consistent with that deduced from the Nyquist plot analysis.


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Modeling Approaches. Periodic Density Functional Theory (DFT) calculations were performed to propose a structure model of MIP-177-SO4H-LT with cell parameters (a = b = 22.553 Å and c = 12.397 Å) as obtained by indexing experimental PXRD data. We used the Quickstep module38 of the CP2K program

39,40

employing the Gaussian Plane Wave (GPW) formalism. The

general gradient approximation (GGA) to the exchange-correlation functional according to Perdew-Burke-Ernzerhof (PBE)41 was used in combination of Grimme’s

DFT-D3

semi

empirical

dispersion

corrections.42,43

Molecularly

optimized Triple-ζ plus valence polarized Gaussian-type basis sets (TZVPMOLOPT) were considered for all atoms, except for the Ti metal centers, where

shorter-range

double-ζ

MOLOPT) were employed.

44

plus

valence

polarization

functions

(DZVP-

The interactions between core electrons and

valence shells of the atoms were described by the pseudopotentials derived by Goedecker, Teter, and Hutter (GTH).45–47 The auxiliary plane wave basis sets were truncated at 400 Ry. Atomic partial charges of MIP-177-SO4H-LT were


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further derived by applying the Restrained Electrostatic Potential (RESP) fitting strategy for the periodic system as implemented in the CP2K code.

Grand Canonical Monte Carlo (GCMC) calculations were then performed at 298K to construct a starting structure model for the fully hydrated MIP-177SO4H-LT. We considered a simulation box made of 8 conventional unit cells (2 × 2 × 2) maintaining atoms at their initial positions. The interactions between the guest water molecules and the MOF structure were described by a combination of site-to-site Lennard-Jones (LJ) contributions and Coulombic terms. A mixed set of universal force field (UFF)48 and DREIDING force field49 parameters were adopted to describe the LJ parameters for the atoms in the inorganic and organic part of the framework. The water molecules were described by the TIP4P/2005 potential model50 corresponding to a microscopic representation of four LJ sites. Following the treatment adopted in other wellknown force fields,51,52 the hydrogen atoms of the acidic –SO4H functional groups interact with the adsorbate water molecules via the Coulombic potential only. Short-range dispersion forces were truncated at a cutoff radius of 12 Å


14

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while the interactions between unlike force field centers were treated by means of

the

Lorentz-Berthelot

combination

rule.

The

long-range

electrostatic

interactions were handled using the Ewald summation technique. Typically, 2×108 Monte Carlo steps have been used for both equilibration and production runs. These MC calculations were performed using the Complex Adsorption and Diffusion Simulation Suite (CADSS) code.53 A GCMC derived snapshot of a structure model corresponding to the fully saturated state was further geometry optimized at the DFT level.

Born-Oppenheimer first-principles MD simulations were performed considering one unit cell of the fully hydrated MIP-177-SO4H-LT and using the CP2K package at the same level of theory and associated settings described above. It is worth mentioning that in aqueous systems the proton shuffling and hopping phenomena usually occur in a timescale of only several femtoseconds and few ps respectively. Indeed, these MD simulations were performed for 17 ps with a time step of 1 fs at 298 K.


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Page 16 of 45

Results and discussion.

The XRPD pattern and N2-adsorption profile of MIP-177-SO4H-LT coincide with that of the parent MIP-177-LT (see Figures S1a, S2a and S2b), evidencing that the crystal structure as well as the porosity are kept after the acid treatment. This observation is consistent with the exceptional chemical stability of MIP-177-LT under strongly acidic conditions.

35

The FTIR spectra depicted in

Figure 2a and b show that the area of both C-H bands characteristic of formate present at 2868 and 2750

cm-1 in the pristine MIP-177-LT,54 drastically

diminish by half for MIP-177- SO4H-LT, while two new bands appear at 1072 and 1128 cm-1 which are assigned to the (S-O) vibrational modes.55 This supports that half of the formates are replaced by sulfate like-species, consistent

with

the

chemical

Ti12O15(mdip)3(formate)3(sulfate-like

formula

of

species)3,

the

MIP-177-SO4H-LT

determined

by

sample,

Transmission

Electron Microscopy with Energy Dispersive X-ray microanalysis (see Figure


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S3).

Further

UV-Vis

measurements

revealed

the

presence

of

an

extra-

absorption band centred at 345 nm for MIP-177-SO4H-LT (see Figure S4), which is typically attributed to the absorption of titanyl sulfate ion pair species.56 Furthermore, analysis of the TGA-MS data evidenced that in MIP-177-SO4H-LT, Ti4+ forms strong complexes with the sulfate-like species (see Figure S6), since the weight loss, which coincides with the release of SO and SO2 fragments (i.e. m/z = 48 and 64 g mol-1), occurs at high temperature (> 673 K). Moreover, FTIR data collected upon adsorption of pyridine used as a probe molecule showed the presence of broad (NH) absorption bands at 2400 and 3350 cm-1 only for MIP-177-SO4H-LT. This feature relates to the formation of pyridinium species on strong Brønsted acid sites (Figure S8).57 This assumption is confirmed by the appearance of the characteristic 8a and 8b bands of lutidinium species centered at 1648 and 1630 cm-1,58 once lutidine (2,6dimethyl-pyridine) is adsorbed on MIP-177-SO4H-LT (Figure S9).

Indeed, this

whole set of experimental characterizations supports the substitution of half of the formates by bridging acidic hydrogen-sulfate groups, leading to the chemical


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formula Ti12O15(mdip)3(formate)3(SO4H)3. Two types of environments exist for the formates in equal concentration in the pristine MIP-177-LT: (i) a position that bridges pairs of adjacent Ti ions in the Ti12O15 oxocluster, pointing towards the channel and (ii) a position that ensures the linkage of two consecutive building units along the channel axis. Due to steric restriction, one can assume that the formates pointing towards the pore interspace are the most likely species removed by the H2SO4 treatment, generating strong Brønsted acid sites accessible to confined guest molecules through the hexagonal channel.


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Figure 2 FTIR spectra of MIP-177-LT (curve-1) and MIP-177-SO4H-LT (curve-2) after the activation under vacuum at 393 K overnight presented in the regions containing the bands attributed to formate species (a), and to hydrogen-sulfate species (b). Part (c) and (d) in the bottom panel present FTIR spectra of the hydrated MIP-177-SO4H-LT collected after a thermodesorption under a nitrogen flow (2 °C min-1) at 298 K (curve-1), 303 K (curve-2), 344 K (curve-3), 398 K (curve-4) and 438 K (curve-5).

AC impedance spectroscopy was further employed to examine the proton conductivity of the pristine MIP-177-LT and MIP-177-SO4H-LT at 298 K and 95% RH. Their corresponding Nyquist plots are presented in Figures 3a and 3b. Compared to the response of the pristine MOF, MIP-177-SO4H-LT exhibits an extremely lower resistance in such a way that the semi-circle was not


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observed. The corresponding signal results in a linear tail end of the semi-circle at high frequency followed by a subsequent capacitive tail due to the ionicblocking electrodes at lower frequencies. Noteworthy, the resulting proton conductivity of MIP-177-SO4H-LT (σ298K/95%RH = 2.6×10-2 S cm-1, see table S2) is roughly 104-fold higher than the value of the pristine solid (σ298K/95%RH = 4.3×10-6 S cm-1, see table S2). This confirms that the H2SO4 treatment is a very efficient way for the creation of highly acidic proton sources in MIP-177LT. The exceptional proton conductivity obtained at room temperature and under 95% RH exceeds that of the benchmark material (10-2 S cm-1) and it is among the top performances reported to date for purely water mediated proton conducting MOFs under similar conditions.7,11,12,15,16,31,33,34,59


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Figure 3 Nyquist plots of (a) MIP-177-LT and (b) MIP-177-SO4H-LT recorded at 298 K and 95% RH.

(c) Relative Humidity (RH) dependence of the

conductivity recorded at 298 K for the MIP-177-SO4H-LT.

Propitiously, this outstanding conductivity performance is maintained over seven days of experiments performed at 298 K and 95% RH (see Figure S15). Furthermore,

the

PXRD

patterns

recorded

before

and

after

impedance

measurements remain unchanged (see Figure S1b). This observation highlights


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the stability of the MOF under the operating conductivity T/RH conditions,35 as expected regarding the solid stability when it was soaked in boiling water (see Figure S1d). This point constitutes a key feature that promotes the MIP-177SO4H-LT as a viable proton electrolyte material at room temperature. The correlation between the proton conductivity of MIP-177-SO4H-LT and RH was further examined at 298 K (see Figure 3c). As the RH increases from 40 % to 95 %, the conductivity goes up from a very negligible σ of 7.2×10-7 S cm-1 to an impressive value of 2.6×10-2 S cm-1 (cf Figure S13 and Table S2). This humidity-dependent proton conductivity trend is typically observed for waterassisted proton conducting MOFs. This suggests that the guest water molecules are likely to increase the proton concentration and/or mobility by i) facilitating acid dissociation and/or ii) creating an effective and expanded proton transport pathway. To support these hypotheses, FTIR experiments were performed on the fully hydrated MIP-177-SO4H-LT upon heating (see Figure 2c and 2d). The FTIR spectra evidenced typical features of the guest water molecules. The intensity of the large band in the 2500-3500 cm-1 region, assigned to the OH


22

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stretching vibration of water ((OH)), progressively decreases with increasing temperature and this corresponds to the temperature-induced desorption of water

from

MIP-177-SO4H-LT.

It

is

worth

noting

that

the

corresponding

wavenumber of the bending mode of the adsorbed water molecules is observed near 1700 cm-1, a position up-shifted compared to that observed for the liquid phase (1630 cm−1) and similar to that reported for protonated water confined in strong Brønsted acid solids, like Nafion®60 or tungsto-heteropoly acid.61 This reveals the presence of hydronium (H3O+) and/or Zundel (H5O2+) species, in line with the H+ transfer from the titanyl hydrogen-sulfate proton source towards a neighbouring adsorbed water molecule.

Molecular simulations were further performed to shed light on this efficient water-mediated proton transport at the atomistic scale. To this purpose, a structure model of MIP-177-SO4H-LT was constructed assuming the presence of bridging SO4H moieties (see Figure 1b), as evidenced in the FTIR experiments. Subsequently, the resulting model was geometry optimized at the DFT level keeping the cell parameters fixed as derived from the experimental PXRD data.


23


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Ab initio Molecular Dynamics (AIMD) simulations were conducted at 298 K on the fully hydrated MIP-177-SO4H-LT to unveil the preferential arrangements of the confined water molecules within the pores, their interactions with –SO4H moieties and the proton migration mechanism. These AIMD calculations first evidenced the formation of hydrogen bonds between water and the hydrogen sulfate groups as well as among the water molecules themselves. The respective intermolecular Ow-Ow and Ow-Osulfate radial distribution functions (see Figure S16) revealed the presence of a principle peak at 2.7 Å, characterizing s strong hydrogen bond. Such interactions are at the origin of a massive 3D percolating hydrogen-bonded network making bridge between the acidic –SO4H groups pointing towards the hexagonal channel through an infinite chain of interconnected water molecules in the pore channel (Figure 1b). Such spatial distribution of guest molecules coherently offers an ideal scenario for the longrange transfer of the acidic protons throughout the pore channel via the water molecules as charge carriers. The AIMD simulations further manifested that the –SO4H groups clearly serve as proton sources, the acidic protons being


24

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transferred from these hydrogen-sulfate functions to the water molecules through the hydrogen-bonded network as evidenced in Figures 1c to 1f.

As shown in Figures 1c and 1d, a proton of the acidic –SO4H group is first transferred to a nearby water molecule to form a hydronium ion (H3O+). The residence time of a proton in a hydronium ion was estimated to be ~21 fs. H3O+ then moves towards a second water molecule and this leads to the formation of a Zundel (H5O2+) species (see Figure 1e). This prediction is fully consistent with the conclusions drawn from the FTIR data analysis described above. A further investigation of the frequency distribution of the intermolecular Ow-Ow distances averaged over the full MD trajectory shows that at least 15% of the donor and acceptor oxygen atoms falls below the typical distance reported for an isolated Zundel species (H5O2+) in gas phase62 (2.5 Å, see Figure S19). This H5O2+ moiety then breaks apart, resulting in a proton transfer towards another water molecule to form the next H3O+ as shown in Figure 1f, which will again interconvert into a Zundel species and so forth. Such an illustration provides a microscopic picture of the Grotthuss-like mechanism at


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the origin of the proton transfer in this water-mediated proton conducting MOF in a similar way to that we previously reported in UiO-66(Zr)-(CO2H)2.13,63

Conclusions

In summary, the preparation of MIP-177-SO4H-LT was demonstrated as an alternative strategy to incorporate highly acidic hydrogen-sulfate species into the inorganic building unit of the MOF leading to an exceptional proton conductivity of 2.6×10-2 S cm-1 at 298 K and 95% RH that outperforms the benchmark materials. This outstanding proton conductive performance was further explained at the atomistic scale using ab initio molecular dynamics simulation. These calculations revealed a Grotthuss-like mechanism initiated by a proton transfer from

the

acidic

hydrogen-sulfate

groups

of

MIP-177-SO4H-LT

towards

neighbouring guest H2O molecules subsequently followed by a proton shuttle over longer distances, through the H-bonded network formed by the guest water molecules along the channels. This decorated MIP-177-SO4H-LT offers a


26

Page 27 of 45 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


good compromise in terms of high proton conduction performance and other crucial

features

(water

stability,

green

and

scalable

synthesis,

cheap

chemicals…). This emphasizes the significant advances made by the design of this derivative Ti-based MOF for future processing in real applications.

Supporting Information.

The Supporting Information is available free of charge on the ACS Publications website at DOI:

Methodology

details;

Solid

characterization:

X-Ray

powder

Diffraction,

N2

physisorption, EDS analysis, FT-IR spectroscopy, UV/Vis spectrometry, TGA-MS analysis, Conductivity and molecular simulations.

Author information Corresponding Authors * [email protected], +33 4 67 14 33 00

* [email protected], +33 1 44 32 24 63


27


Page 28 of 45

Author Contributions

S.W. and C.S. synthesized MIP-177-SO4H-LT; J.S., A.V. and M.D. performed IR and UV-Vis investigations; R.R. was in charge of EDX experiments, J.S. L. and J.-S.C. characterized the samples adsorption properties; V.O., P.G.Y. and S.D.V. characterized the samples and performed the conductivity measurements; M.W. and G.M were in charge of the molecular simulations. M.W., G.M. and S.D.V. wrote the manuscript. All authors have implemented, amended and given approval to the final version of the manuscript.

† These authors contributed equally to this work.

Acknowledgment S.D.V. thanks Joël Couve (ICGM) for the TGA-MS measurements. G.M. thanks Institut

Universitaire

de

France

for

its

support.

The

authors

declare

no

competing financial interest.

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Synopsis A new eco-friendly and robust hydrogen-sulfate decorated titanium based MOF as a promising water-mediated proton conductive material for clean-energy related applications.

TOC


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A High Proton Conductive Hydrogen-Sulfate Decorated Titanium

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