Water-assisted proton conduction regulated by hydrophilic groups in metallo-hydrogen-bonded organic frameworks: “like-attracts-like” between hydrophilic groups and water molecules

Abstract

The exploration of proton-conducting solid-state electrolytes operating under humidified conditions is currently of considerable significance for fuel-cell technology. However, the effect of the hydrophilicity of various hydrophilic groups in materials on the proton conductivity remains unknown. Metallo-hydrogen-bonded frameworks (MHOFs) provide a promising alternative for solving this difficult problem due to their versatile functional groups, easy-to-identify structures, tunable functionalities and good chemical stabilities. In this paper, we designed and synthesized two MHOFs with similar chemical structures, [Co(PPA)₂(H₂O)₂(NCS)₂·2H₂O] (Co-2PPA) and [Co(PPA)₄(NCS)₂] (Co-4PPA) (PPA = 4-(3-pyridinyl)-2-amino pyrimidine), to elucidate the issue. Indeed, Co-2PPA and Co-4PPA show remarkably distinct proton conductivities due to the presence of different hydrophilic groups, which are 2.96 × 10⁻⁴ and 2.78 × 10⁻⁵ S cm⁻¹ at 333 K and ∼97% RH, respectively, approaching one order of magnitude. We found that the difference in proton conductivities between Co-2PPA and Co-4PPA is primarily correlated with the different water uptake of the samples. The results originate from the different hydrophilic affinity between hydrophilic groups (H₂O and –NH₂) in MHOFs and adsorbed water molecules, i.e., the more similar the structure between hydrophilic groups and water molecules, the greater the hydrophilicity, the higher the water uptake, the better the proton conductivity, following the “like-attracts-like” rule. Furthermore, frequency- and temperature-dependent dielectric spectroscopy measurements reveal that there exists proton transfer along hydrogen bonds in Co-2PPA and Co-4PPA, obeying the Grotthuss proton-hopping mechanism. Notably, different changes in hydrogen bond lengths of Co-2PPA and Co-4PPA with increasing temperature are responsible for the various E_a values for proton conduction. This work gives an attractive idea for designing solid-state proton conductors and investigating the proton-conducting mechanism.