Study OF NEW PROTON-CONDUCTING MATERIALS BASED ON METAL ORGANIC POLYMERS

1Ponomareva V.G., 2Kovalenko K.A., 1Shutova E.S. 2Cheplakova A.M. 2Dybtsev D.N., 2Fedin V.P

1Institute of Solid State Chemistry and Mechanochemistry SB RAS Russia

E-mail: Ponomareva@solid.nsc.ru

2Institute of Inorganic Chemistry SB RAS, Russia

Introduction

Metal organic polymers (MOF) represent a new interesting class of compounds with crystallinity and porosity [1]. The inclusion of various compounds into the pores of the MOF with different size allows the synthesis of functional materials with desired properties. This work is aimed to the studying new proton-conducting materials based on mesoporous MOF and compounds with different acidic properties. Mesoporous Cr3O(H2O)Y(bdc)yHNO3* nH2O(bdc - 1,4-C6H4(COO2)2, Y = F, NO3, y = 0.15, n = 13) so called Cr-MIL-101 was used as a heterogeneous matrix for the synthesis of nanocomposites, as well as CrMIL-100 and CrMIL-53 analogues. Cr-MIL-101 is characterized by high thermal stability, porous structure with a system of cavities ~ 3.8 nm and 2.9 nm and a large pore volume.

Experiments

Phosphoric acid of various concentrations with the low volatility was used as inclusion compound. A number of the acid salts of the MnHm(XO4)p family such as CsH2PO4, CsHSO4, CsH5(PO4)2), as well as imidazolium triflate and benzimidazolium triflate were used to create new highly conductive compounds at medium temperatures (100-250°C). Hybrid materials of various compositions were studied by X-ray diffraction, DSC and TA analysis, IR, Raman, 1H-NMR and impedance spectroscopy at different temperatures and different partial pressures of water vapor. The samples of H3PO4-CrMIL-101 hybrid compounds were synthesized in a wide range of phosphoric acid concentrations by different preparation methods.

Results and Discussion

The X-ray diffraction patterns of all compounds confirm the stability of the CrMIL-101 crystal structure after the repeated conductivity measurements and heating, as well as a long storage. This is also observed for hybrid compounds with a high concentration of H3PO4. The proton conductivity and thermal properties significantly depend on the synthesis conditions of hybrid compounds, H3PO4 content, mesoporous structure of matrix of CrMIL-101, CrMIL-100 and CrMIL-53 and relative humidity. The data of thermal analysis show that only the water molecules release from composites when heated to 250°C and H3PO4 are not removed from pores when heated. The transport of protons is carried out according to the Grotthuss mechanism with the effective activation energy of ~ 0.25 eV. Hybrid compounds H3PO4 - CrMIL-101 are characterized by high proton conductivity (> 3•10–2 S•cm–1 at T=60–80°C), which strongly depends on the acid concentration [2, 3] and atmospheric humidity. A change in relative humidity over a wide range affects both the concentration of water molecules in hybrid compounds and the network of hydrogen bonds for efficient proton transfer and proton mobility. The correlation of proton conductivity and water sorption for different relative humidity was found for H3PO4 - CrMIL-101 and H3PO4 - CrMIL-100 hybrid materials.

The significant nanocomposite effect was shown for different acid salts such as CsHSO4, CsH5(PO4)2) and imidazolium triflates or benzimidazolium triflates with high dispersed nonconducting CrMIL-101. The addition of nonconducting MOFs was shown to increase significantly the proton conductivity up to three orders of magnitude, depending on composition.  The highest proton conductivity σ ~ 10-1 -10-2 S/cm at 230°C was observed. The unusual behavior of salts in the MOFs nanospace and a significant increase in conductivity are associated with the surface interfacial interaction and the formation of a disordered amorphous state of salt in the mesopores. Hybrid compounds with high conductivity are promising as membranes in fuel cells and supercapacitors.

This work was supported by the RFBR grant 18-29-04039.

References

1. Taylor J.M., Mah R.K., Moudrakovski I.L., et.al. JACS. 2010. V. 132. P. 14055.

2. Ponomareva V. G., Kovalenko K.A., Dybtsev, D. N et. al. 2012 JACS. V. 134 P. 15640.

3. Dybtsev, D., Ponomareva, V., Aliev, S., at.el. Applied Materials and Interfaces. 2014. V.6.  P. 5161.