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.

http://onlinelibrary.wiley.com/doi/10.1002/anie.201404164/full