Modulating proton diffusion and conductivity in metal-organic frameworks by incorporation of accessible free carboxylic acid groups.

Peter Rought,Martin Schröder,Sihai Yang,Stephen P Argent,George Whitehead,Victoria García Sakai,Simona Pili,Mark R Warren,Martyn S Brown,Christopher Marsh,Ming Li,Inigo Vitorica-Yrezabal,Ian P Silverwood

doi:10.1039/c8sc03022g

Abstract

Three multi-carboxylic acid functionalised ligands have been designed, synthesised and utilised to prepare the new barium-based MOFs, MFM-510, -511, and -512, which show excellent stability to water-vapour. MFM-510 and MFM-511 show moderate proton conductivities (2.1 × 10-5 and 5.1 × 10-5 S cm-1, respectively) at 99% RH and 298 K, attributed to the lack of free protons or hindered proton diffusion within the framework structures. In contrast, MFM-512, which incorporates a pendant carboxylic acid group directed into the pore of the framework, shows a two orders of magnitude enhancement in proton conductivity (2.9 × 10-3 S cm-1). Quasi-elastic neutron scattering (QENS) suggests that the proton dynamics of MFM-512 are mediated by "free diffusion inside a sphere" confirming that incorporation of free carboxylic acid groups within the pores of MOFs is an efficient albeit synthetically challenging strategy to improve proton conductivity.

Highlights

Proton exchange membrane fuel cells (PEMFCs) are a promising technology for the use of hydrogen-based energy in applications such as transport.[1]
We report an investigation of the proton diffusion and dynamics in MFM-512 via Quasi-elastic neutron scattering (QENS), which con rms the proton conduction in MFM-512 is mediated by the “free diffusion inside a sphere” mechanism
The type I Ba(II) center is bound to three monodentate carboxylates (O1), two terminal aquo ligands (O2) and two formate molecules (O3) which bridge to adjacent type II nodes, the formic acid being formed via the thermal decomposition of DMF during the synthesis

Summary

Introduction

Proton exchange membrane fuel cells (PEMFCs) are a promising technology for the use of hydrogen-based energy in applications such as transport.[1]. Quasi-elastic neutron scattering (QENS) has been shown recently to be a powerful technique for gaining insights into the mechanisn of proton conduction in crystalline MOFs.[15,16]. Such studies remain very rare to date. MFM-511 contains two monodentate carboxylic acid groups which remain protonated, their mobility is highly restricted by strong intramolecular hydrogen bonding to a neighbouring carboxyl oxygen atom, leading to a moderately enhanced conductivity of 5.1 Â 10À5 S cmÀ1 at 99% RH and 298 K. MFM-512, which retains pendant carboxylic acid functionality directed into the unrestricted framework void, shows two orders of magnitude enhancement on proton conductivity (2.9 Â 10À3 S cmÀ1) under the same conditions. Full experimental details of syntheses and techniques used can be found in the ESI.†

Results and discussions

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Conclusions