High Proton Mobility with High Directionality in Isolated Channels of MOF-74

Abstract

Isolated one-dimensional (1-D) proton channels in a metal-organic framework, MOF-74, have been reasonably expected to show highly directional proton conductivity, although no evidence has been provided. As a result of dimensional anisotropy of the channels evenly aligned in the c-axis of MOF-74 single crystal, highly directional proton conductivity is demonstrated by using electrochemical impedance spectroscopy. In particular, single crystals treated with sulfuric acid or ammonium hydroxide displays a maximum ∼1200-fold-enhanced c-axis proton conductivity compared to its a-axis conductivity, demonstrating highly directional proton migration through the channels. Very low activation energies (e.g., 0.12 eV) for the c-axis conductivity of MOF-74 also suggest a high proton mobility that arises via Grotthuss proton transfer parallel to the channels.

Humidity-Mediated Anisotropic Proton Conductivity through the 1D Channels of Co-MOF-74

“…(A generally higher conductivity is observed at 30 • C than at 21 • C. The impact of the temperature will be discussed in more detail below.) These findings confirm that the presence of water in the micropores of Co-MOF-74 facilitates protonic conductivity, as frequently observed in MOF materials [30,38,40]. measurements in this study.…”

“…This eliminates potential difficulties that arise from grain boundary effects or from the impact of surface-adsorbed water in bulk powder or pressed pellets. Most importantly, single-crystal specimens also offer an opportunity to study the anisotropy of proton conductance [30,32,[38][39][40][41]. Prior to measurement, the material was dried and activated at 160 • C, to remove adsorbed molecules from the micropores.…”

A Stable Coordination Polymer Based on Rod-Like Silver(I) Nodes with Contiguous Ag-S Bonding

“…The rod-like nodes in (Ag 2 (ASBDC)) are coordinated to three carboxylate linkers at each repeat, giving rise to hexagonal pores with pore diameters of ~9 Å, measured from the carbon atoms of the allyl thioether groups. The structure of (Ag 2 (ASBDC)) bears significant similarities to those of M-MOF-74 (M = Zn, Ni, Co, Mg, Mn, Fe); a family of stable materials formed from 2,5-dihydroxybenzene dicarboxylate (DOBDC) [ 37 , 38 ]. In MOF-74, one-dimensional, infinite, helical M 2 O 2 (COO) 2 SBUs are bridged by the DOBDC linker to form a similar three-dimensional honeycomb structure, with infinite rod-like SBUs [ 39 ].…”

Metal–Organic Frameworks as a Versatile Platform for Proton Conductors

“…Recently, Jeong's group chose Co‐MOF‐74 as mother framework and loaded various proton carries (H 2 SO 4 , H 2 O or NH 4 OH) into the channels to explore its proton conduction behaviors. [ 147 ] It was worth mentioning that single crystal conductivities of Co‐MOF‐74 were increased by 2.4‐times or 7.4‐times along the a ‐axis or c ‐axis direction after the H 2 SO 4 or NH 4 OH treatment (Figure 17b–e). In particularly, the c ‐axis single crystal conductivity (σ c ) of Co‐MOF‐74 was three orders of magnitude higher than the a ‐axis conductivity (σ a ), even over five orders of magnitude higher than the conductivity of pelleted sample.…”