pH-Dependent Proton Conducting Behavior in a Metal-Organic Framework Material

A porous metal–organic framework (MOF), [Ni2(dobdc)(H2O)2]⋅6 H2O (Ni2(dobdc) or Ni‐MOF‐74; dobdc4−=2,5‐dioxido‐1,4‐benzenedicarboxylate) with hexagonal channels was synthesized using a microwave‐assisted solvothermal reaction. Soaking Ni2(dobdc) in sulfuric acid solutions at different pH values afforded new proton‐conducting frameworks, H+@Ni2(dobdc). At pH 1.8, the acidified MOF shows proton conductivity of 2.2×10−2 S cm−1 at 80 °C and 95 % relative humidity (RH), approaching the highest values reported for MOFs. Proton conduction occurs via the Grotthuss mechanism with a significantly low activation energy as compared to other proton‐conducting MOFs. Protonated water clusters within the pores of H+@Ni2(dobdc) play an important role in the conduction process. A fuel cell candidate: Treatment of the metal–organic framework [Ni2(dobdc)] with sulfuric acid afforded a new proton conducting framework, H+@Ni2(dobdc). This acidified MOF displays one of the highest proton conductivities reported for MOFs and is characterized by a exceptionally low activation energy of proton transfer.