Void Space versus Surface Functionalization for Proton Conduction in Metal-Organic Frameworks

Void space and functionality of the pore surface are important structural factors for proton-conductive metal-organic frameworks (MOFs) impregnated with conducting media. However, no clear study has compared their priority factors, which need to be considered when designing proton-conductive MOFs. Herein, we demonstrate the effects of void space and pore-surface modification on proton conduction in MOFs through the surface-modified isoreticular MOF-74(Ni) series [Ni₂ (dobdc or dobpdc), dobdc=2,5-dihydroxy-1,4-benzenedicarboxylate and dobpdc=4,4'-dihydroxy-(1,1'-biphenyl)-3,3'-dicarboxylate]. The MOF with lower porosity with the same surface functionality showed higher proton conductivity than that with higher porosity despite including a smaller amount of conducting medium. Density functional theory calculations suggest that strong hydrogen bonding between molecules of the conducting medium at high porosity is inefficient in inducing high proton conductivity.