Superprotonic Conductivity of MOFs Confining Zwitterionic Sulfamic Acid as Proton Source and Conducting Medium

Abstract

A few metal-organic frameworks (MOFs), which typically use strong acids as proton sources, display superprotonic conductivity (≈10^-1 S cm^-1 ); however, they are rare due to the instability of MOFs in highly acidic conditions. For the first time, we report superprotonic conductivity using a moderately acidic guest, zwitterionic sulfamic acid (HSA), which is encapsulated in MOF-808 and MIL-101. HSA acts not only as a proton source but also as a proton-conducting medium due to its extensive hydrogen bonding ability and zwitterion effect. A new sustained concentration gradient method results in higher HSA encapsulation compared to conventional methods, producing 10HSA@MOF-808-(bSA)₂ and 8HSA@MIL-101. These MOFs show impressive superprotonic conductivity of 2.47×10^-1 and 3.06×10^-1 S cm^-1 , respectively, at 85 °C and 98 % relative humidity, and maintain stability for 7 days.