In Situ Aliovalent Nickle Substitution and Acidic Modification of Nanowalls Promoted Proton Conductivity in InOF with 1D Helical Channel

Abstract

Proton-conductive materials have attracted increasing attention because of their broad explorations in chemical sensors, water electrolysis, fuel cells, and biological systems. Especially, metal–organic frameworks (MOFs) have been demonstrated to be extremely promising candidates as proton-exchange membrane (PEM) fuel cells. Compared with other configurations, MOFs with one-dimensional (1D) channels have the characteristics of enhancing the host–guest interaction and promoting the anisotropic motion of proton carriers in restricted volume, which are beneficial for acquiring rich proton sources and forming successive hydrogen bonds to improve proton conductivity. We are endeavored to screen and find a helical three-dimensional (3D) framework InOF-1, namely, [In2(OH)2(BPTC)]·6H2O (BPTC4– = 3,3′,5,5′-biphenyl tetracarboxylate), as a typical 1D-channel MOF, which is pristinely grafted with spirally distributed −OH groups on the channel surface. Accompanied by an aliovalent substitution Ni­(II) for In­(III), isostructural NiOF-1 ([Ni2(BPTC)­(HCOOH)2]·3H2O) is successfully prepared and massive formic acids are anchored at interior walls, which are interacted with adsorbed water molecules via the formation of stronger O–H···O bonds. This interaction between host–guest molecules and dynamics of lattice water has already led to a remarkable conductivity of InOF-1 (σ = 7.86 × 10–3 S/cm at 328 K under 95% RH). The synergistic effect of the acidic-modified nanowall, contracted volume, and enhanced adsorption of water molecules in the NiOF-1 channel contributes to a high conductivity value of 3.41 × 10–2 S/cm (at 328 K under 95% RH). Moreover, the proton conduction mechanism is further visually presented by molecular dynamic (MD) simulation. In contrast to InOF-1, aliovalent-substituted and acidic-modified NiOF-1 has a stronger host–guest interaction and more abundant hydrogen-bond networks, resulting in shorter proton migration distances and more frequent proton hopping, in agreement with the experimental results.

Solid-State Proton Conduction Driven by Coordinated Water Molecules in Metal–Organic Frameworks and Coordination Polymers

“…that, besides coordinated water molecules, other protic solvents such as methanol (pK a = 15.5), ethanol (pK a = 15.9), and formic acid (pK a = 3.75) could also be coordinated to the metal centers to impart proton conductivity; however, such reports are significantly less, in fact restricted to only two (one for coordinated alcohol and one for coordinated formic acid). 53,54 As expected, the alcohols-coordinated MOFs showed much lower conductivity than their water-coordinated counterparts because of their higher pK a values compared to those with the coordinated water molecules. 53 The following considerations may be useful to design more efficient coordinated-water-driven solid-state proton conductors:…”

“…While this Perspective offers discussion and further scope of solid-state proton conduction driven by coordinated water molecules in MOFs and CPs by taking advantage of the enhanced acidity of the coordinated water molecules, such examples may be extended to other solid-state crystalline materials, such as polyoxometalates (POMs) and metallo hydrogen-bonded organic frameworks (MHOFs). ,,,, However, coordinated-water-driven proton conduction on these crystalline platforms is either rarely explored (for POMs) or completely unexplored thus far (for MHOFs) and therefore demands attention from researchers. It may be noted that, besides coordinated water molecules, other protic solvents such as methanol (p K a = 15.5), ethanol (p K a = 15.9), and formic acid (p K a = 3.75) could also be coordinated to the metal centers to impart proton conductivity; however, such reports are significantly less, in fact restricted to only two (one for coordinated alcohol and one for coordinated formic acid). , As expected, the alcohols-coordinated MOFs showed much lower conductivity than their water-coordinated counterparts because of their higher p K a values compared to those with the coordinated water molecules …”

Three isostructural MOFs based on different metal cations: proton conductivities and SC–SC transformation leading to magnetic changes

“…This viewpoint was also reported in some MOF conductors. 51 Based on the above analyses, the coordination water molecules in compounds 1-3 may be considered acidic molecules, the stronger the coordination bond energy, the more acidic the water molecule. It is well known that metal cations with smaller radii are more likely to accept lone pair electrons of the ligand and form stronger coordination bonds.…”

Coordinated water molecule-induced solid-state superprotonic conduction by a highly scalable and pH-stable coordination polymer (CP)

“…Therefore, the development of crystalline, structurally greatly stable and highly conductive solid-state proton conductors (SSPCs) is in demand. 5 In this regard, coordination polymers (CPs) and metal–organic frameworks (MOFs) assembled by metal ions and organic linkers having variability of the structural architecture 8–39 and superior tunability 40–61 have displayed excellent proton conductivity, even in ultra-high superprotonic regions (>10 −2 S cm −1 or higher). 6,62 Most importantly, the stronger coordination bonds and highly crystalline nature enable accurate structural characterization, which is beneficial for understanding the plausible proton-conduction mechanism.…”