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Enhancement of proton conductivity in non-porous metal-organic frameworks: the role of framework proton density and humidity

DOI: 10.1021/acs.chemmater.8b02765 DOI Help

Authors: Simona Pili (University of Manchester) , Peter Rought (University of Manchester) , Daniil I. Kolokolov (Novosibirsk State University; Boreskov Institute of Catalysis, Siberian Branch of Russian Academy of Sciences) , Longfei Lin (University of Manchester) , Ivan Da Silva (ISIS Pulsed Neutron and Muon Source) , Yongqiang Cheng (Spallation Neutron Source) , Christopher Marsh (University of Manchester) , Ian P. Silverwood (ISIS Pulsed Neutron and Muon Source) , Victoria GarcĂ­a-sakai (ISIS Pulsed Neutron and Muon Source) , Ming Li (University of Nottingham) , Jeremy J. Titman (University of Nottingham) , Lyndsey Knight (University of Nottingham) , Luke L. Daemen (Spallation Neutron Source) , Anibal J. Ramirez-cuesta (Spallation Neutron Source) , Chiu C. Tang (Diamond Light Source) , Alexander G. Stepanov (Novosibirsk State University; Boreskov Institute of Catalysis, Siberian Branch of Russian Academy of Sciences) , Sihai Yang (University of Manchester) , Martin Schroeder (University of Manchester)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Chemistry Of Materials

State: Published (Approved)
Published: September 2018
Diamond Proposal Number(s): 13247

Abstract: Owing to their inherent pore structure, porous metal-organic frameworks (MOFs) can undergo post-synthetic modification, such as loading extra-framework proton carriers. However, strategies for improving the proton conductivity for non-porous MOFs are largely lacking, although increasing numbers of non-porous MOFs exhibit promising proton conductivities. Often, high humidity is required for non-porous MOFs to achieve high conductivities, but to date no clear mechanisms have been experimentally identified. Here we describe the new materials MFM-550(M), [M(HL1)], (H4L1 = biphenyl-4,4'-diphosphonic acid; M = La, Ce, Nd, Sm, Gd, Ho), MFM-550(Ba), [Ba(H2L1)], and MFM-555(M), [M(HL2)], (H4L2 = benzene-1,4-diphosphonic acid; M = La, Ce, Nd, Sm, Gd, Ho), and report enhanced proton conductivities in these non-porous materials by (i) replacing the metal ion to one with a lower oxidation state, (ii) reducing the length of the organic ligand, and (iii) introducing additional acidic protons on MOF surface. Increased framework proton density in these materials can lead to an enhancement in proton conductivity of up to four orders of magnitude. Additionally, we report a comprehensive investigation using in situ 2H NMR and neutron spectroscopy, coupled with molecular dynamic modelling, to elucidate the role of humidity in assembling interconnected networks for proton hopping. This study constructs a relationship between framework proton density and the corresponding proton conductivity in non-porous MOFs, and directly explains the role of both surface protons and external water in assembling the proton conducting pathways.

Subject Areas: Chemistry, Materials

Instruments: I11-High Resolution Powder Diffraction

Other Facilities: ISIS

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