From organic ligand to metal-organic coordination polymer, and to metal-organic coordination polymer-cocrystal composite: a continuous promotion of proton conductivity of crystalline materials
Improving proton conductivity is of great importance for the further application of metal-organic frameworks (MOFs) or metal-organic coordination polymers (MOCPs) in proton exchange membrane fuel cells. In this paper, we proposed a new approach to raise proton conductivities in MOCPs commencing from organic ligand, e.g. coordination inducement and forming MOCPs-cocrystal composite strategy which is based on the potential function of organic ligand. Employing the approach, we have successfully synthesized and characterized three crystalline compounds, PPA (1), [Cu(PPA)I] (2) and [Co(PPA)2(BDC)(H2O)2·(PPA)2(H2BDC)2(H2O)] (3) (PPA= 4-(3-pyridinyl)-2-amino pyrimidine, H2BDC = 1, 4-benzenedicarboxylic acid). These compounds show high water stabilities and different proton conducting behaviours, especially for compound 3 with a proton conductivity of 2.29 × 10-4 S cm-1 at 325 K and ~97% RH, which is higher than or comparable to those of some reported compounds. More importantly, a persistent increase in proton conductivity is observed from 1 to 3, which is about one order of magnitude. Furthermore, single crystal analyses show that coordination and hydrogen-bonding interactions play a crucial role in improving proton-conductive properties for these compounds. The study reveals that it is practical to boost proton conductivity of crystal materials by means of their intrinsic merits and the strategy of molecular design.