Enhancement of Proton Conductivity in Nonporous Metal-Organic Frameworks: The Role of Framework Proton Density and Humidity

Simona Pili, Peter Rought, Daniil I. Kolokolov, Longfei Lin, Ivan Da Silva, Yongqiang Cheng, Christopher Marsh, Ian P. Silverwood, Victoria García Sakai, Ming Li, Jeremy J. Titman, Lyndsey Knight, Luke L. Daemen, Anibal J. Ramirez-Cuesta, Chiu C. Tang, Alexander G. Stepanov, Sihai Yang, Martin Schröder

Research output: Contribution to journalArticlepeer-review

32 Citations (Scopus)

Abstract

Owing to their inherent pore structure, porous metal-organic frameworks (MOFs) can undergo postsynthetic modification, such as loading extra-framework proton carriers. However, strategies for improving the proton conductivity for nonporous MOFs are largely lacking, although increasing numbers of nonporous MOFs exhibit promising proton conductivities. Often, high humidity is required for nonporous 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 nonporous 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 the MOF surface. Increased framework proton density in these materials can lead to an enhancement in proton conductivity of up to 4 orders of magnitude. Additionally, we report a comprehensive investigation using in situ 2H NMR and neutron spectroscopy, coupled with molecular dynamic modeling, 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 nonporous MOFs, and directly explains the role of both surface protons and external water in assembling the proton conduction pathways.

Original languageEnglish
Pages (from-to)7593-7602
Number of pages10
JournalChemistry of Materials
Volume30
Issue number21
DOIs
Publication statusPublished - 13 Nov 2018

Keywords

  • COORDINATION POLYMERS
  • WATER-MOLECULES
  • DIFFUSION
  • SORPTION
  • SITES

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