Competitive Effect of Surface Hydrophobicity–Hydrophilicity and the Number of Accessible Protons on Water-Assisted Proton Conductivity in Metal–Organic Frameworks: Dielectric σ-Relaxation Triggered by Disordered Guest Ionic Migration and Its Mechanism

The rapid upsurge of metal-organic frameworks (MOFs) has sparked profound interest in their potential as proton conductors for proton exchange membrane fuel cells. However, proton-conducting behaviors of hydrophobic MOFs remain poorly understood compared with their hydrophilic counterparts, largely due to the absence of a microscopic phase separation structure akin to that found in Nafion membranes. Herein, we demonstrate a strategy for regulating the structures and proton conductivities of MOFs by separately incorporating hydrophobic -C(CF3)2- group alongside hydrophilic -O- and -SO2- groups into organic ligands as linkers. Three analogous MOFs, {[Zr6(obba)4(μ3-O)4(μ3-OH)4(OH)3(H2O)3(HCOO)]·3.5DMF·3CH3COCH3}n (Zr-obba, H2obba = 4,4'-oxybis(benzoic acid)), {[Zr6(sdba)4(μ3-O)4(μ3-OH)4(OH)2(H2O)2(HCOO)2]·2(CH3)2NH2+·2HCOO-·7(CH3)2NH·6.5H2O}n (Zr-sdba, H2sdba = 4,4'-sulfonyldibenzoic acid), and {[Zr6(hfipbba)4(μ3-O)4(μ3-OH)4(OH)4(H2O)4]·7.5(CH3)2NH2+·7.5HCOO-·3.5(CH3)2NH}n (Zr-hfipbba, H2hfipbba = 4,4'-(hexafluoroisopropylidene)bis(benzoic acid)), were prepared and characterized. Interestingly, we observed a trend in proton conductivity: Zr-obba > Zr-sdba > Zr-hfipbba, which can be attributed to the competitive effect of surface hydrophobicity-hydrophilicity and the availability of accessible protons. More importantly, we identified dielectric σ-relaxation in Zr-sdba and Zr-hfipbba, which is linked to the thermally activated migration of disordered guest ions within the MOFs, highlighting the significant relationship between proton conduction and dielectric relaxation. This finding contributes to a deeper understanding of the critical roles that hydrophobicity-hydrophilicity play in proton conduction and the dielectric σ-relaxation mechanism in crystalline MOFs.