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

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(HL 1 )], (H 4 L 1 = biphenyl-4,4′-diphosphonic acid; M = La, Ce, Nd, Sm, Gd, Ho), MFM-550(Ba), [Ba(H 2 L 1 )], and MFM-555(M), [M(HL 2 )], (H 4 L 2 = 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 2 H 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.

Characterization of Proton Dynamics for the Understanding of Conduction Mechanism in Proton Conductive Metal‐Organic Frameworks

“…Another strategy for stabilizing the proton transfer pathway is to exploit more acidic hydroxyl groups on the framework surface. Pili et al (2018), [67] ], (H 4 L 2 = benzene-1,4-diphosphonic acid; M=La, Ce, Nd, Sm, Gd, Ho) that were characterized with 2 H NMR and NS. MFM-555(Ho) has the highest conductivity value (1 0 À 4 S cm À 1 ), among these MOFs and its proton-conduction mechanism was investigated.…”

“…Currently, the application of the INS to probe the proton vibrational states in MOF systems is scarce. However, in a recent study by Pili et al in 2018, [67] INS was applied to probe the presence of the hydroxyl groups in the metal phosphonate non-porous MOF MFM-555(Ho). The INS spectrum showed the well-resolved peaks that were assigned to the vibrational modes of the phosphonate moiety, by means of the DFT calculations: The OÀ PÀ OÀ H wagging and PÀ OÀ H stretching modes were assigned to the peaks at 203 and 208 cm À 1 , respectively, and the PÀ O stretching modes were assigned to the peaks at 878 and 979 cm À 1 .…”

“…Pili et al. (2018), [67] on a series of non‐porous proton‐conductive MOFs (MFM‐550(M), [M(H 4 L 1 )], (H 4 L 1 =biphenyl‐4,4′‐diphosphonic acid; M=La, Ce, Nd, Sm, Gd, Ho), MFM‐550(Ba), [Ba(H 2 L 1 )], and MFM‐555(M), [M(H 4 L 2 )], (H 4 L 2 =benzene‐1,4‐diphosphonic acid; M=La, Ce, Nd, Sm, Gd, Ho) that were characterized with 2 H NMR and NS. MFM‐555(Ho) has the highest conductivity value (∼10 −4 S cm −1 ), among these MOFs and its proton‐conduction mechanism was investigated.…”

“…The PFG NMR method operates on long time and length scales and allows direct tracking of the diffusion coefficients of the mobile protons. The main idea behind the method was first introduced by Stejskal and Tanner [67,68] and revolved about measuring the intensity loss of the signal within a modified spin‐echo sequence with a symmetric pulse gradient before and after the refocusing pulse(s) (a 180 pulse for the usual spin‐echo sequence and two 90 pulses spaced by a delay for the stimulated echo sequence). In the absence of diffusion, the second gradient pulse should perfectly rephase the spins, and hence no additional intensity loss should be observed.…”

Metal‐Organic Frameworks for Hydrogen Energy Applications: Advances and Challenges

“…In most of the studies, the Grotthuss mechanism is suggested for many MOFs basing on the activation energy values. Recently, deuterium NMR spectroscopy and quasielastic neutron scattering (QENS) techniques have been used to get some insight into proton dynamics …”