Metal-coordinated water-assisted enhanced proton conductivity of robust Ni(II)-based 3D/2D metal-organic frameworks

MOFs are considered as frontline support materials for obtaining high proton conduction under humid conditions. This study deals with a 3D and a 2D single-crystal Ni-MOFs. The open-network 3D Ni-MOF (Ni-NDC-BPYH), constructed from 2,6-napthalenedicarboxylic acid (2,6-NDC) and 1,2-bis(pyridin-4-ylmethylene)hydrazine as linkers, displayed a monoclinic crystal system with the C2/c space group. Whereas the 2D Ni-MOF (Ni-NDC-BPY) was synthesized using 2,6-NDC and bipyridine as linkers displays a triclinic crystal system with the P-1 space group. Both MOFs demonstrated high crystallinity, a wide range of pH stability, and high thermal stability. The pristine Ni-NDC-BPY and Ni-NDC-BPYH MOF pellets exhibited proton conductivity values of 8.92 × 10 −4 and 2.18 × 10 −3 S/cm at 80 °C and 98% RH conditions. While under the same conditions, the phosphoric acid (PA)-doped Ni-NDC-BPY and Ni-NDC-BPYH MOF pellets achieved superior proton conductivity values of 4.27 × 10 −3 and 1.67 × 10 −2 S/cm, respectively. Therefore, PA-doping to the MOF increases proton conductivity by an order of magnitude compared to pristine MOFs. This study demonstrates a systematic approach to the design of superior ion-conducting solid-state materials for future electrochemical applications.