Construction and Comparative Research of Two MOFs Proton Conducting Materials Containing Nitro Groups

In field of electrochemistry, there has been a growing interest in the potential applications of proton-conducting metal-organic frameworks (MOFs). Therefore, how to design and synthesize MOFs with high proton conductivity is considered crucial. In this study, two examples of nitro-containing Cd-based MOFs, MOF-1 {[Cd₃(TIPE)_1.5(NO₃)₅Cl(H₂O)₂] ⋅ 17H₂O}_n and MOF-2 {[Cd(TIPE)_0.5(nip)] ⋅ 10H₂O}_n (TIPE=1,1,2,2-tetrakis(4-(1H-imidazole-1-yl)phenyl)ethene, H₂nip=5-Nitroisophthalic Acid), had been successfully designed and synthesized, and their proton-conducting properties were thoroughly investigated. Notably, both materials displayed peak proton conductivity at 98 % RH and 90 °C, exhibiting values of 9.13×10^-3 and 3.00×10^-3 S cm^-1 for MOF-1 and MOF-2, respectively. The plausible proton conduction pathways and mechanisms were elucidated through structural analyses, water vapor adsorption studies, and the determination of activation energy (E_a) values. It was found that the difference in proton conductivity between MOF-1 and MOF-2 was mainly associated with the different water absorption rates of the samples. The uniqueness of this study was that for the first time conducted an in-depth study of the role of nitrate in proton conduction, providing new ideas for designing materials with excellent proton conductivity.