Impact of the number of hydrogen bonds on proton conductivity in metallo-hydrogen-bonded organic frameworks: the more the number of hydrogen bonds, the better the proton conductivity at the maximum relative humidity

Abstract

The rapid development of new proton conductors based on metallo-hydrogen-bonded organic frameworks (MHOFs) and rational regulation of their performance are highly desired for proton exchange membrane fuel cells. As we all know, hydrogen bonds are an indispensable part of proton conduction as conducting mediators. However, the effect of hydrogen bonds, especially the number of hydrogen bonds, on proton conductivity is not yet clear. Herein, we focus on clarifying the relationship between proton conductivity and the number of hydrogen bonds in MHOFs. To this end, two MHOFs with analogous host-framework structures and different hydrogen-bonded networks, [Ni(HCi)₂(NH₃)₄] (Ni–Ci–NH₃) and [Ni(HCi)₂(H₂O)₄] (Ni–Ci–H₂O) (H₂Ci = 1H-indazole-5-carboxylic acid), were designed by controlling coordination states between the same metal ion and different hydrogen bond donors and act as targeting models to study their proton-conducting properties. Notably, Ni–Ci–NH₃ shows a high proton conductivity of 6.22 × 10⁻⁴ S cm⁻¹ for the powder sample at ∼97% relative humidity (RH) and 303 K, which is superior to that of Ni–Ci–H₂O (1.62 × 10⁻⁴ S cm⁻¹) under the same condition. The difference in proton conductivity at ∼97% RH is closely related to the number of hydrogen bonds, as evidenced by water adsorption, water contact angle and dielectric loss measurements. Furthermore, frequency- and temperature-varied dielectric constant curves indicate that their proton transfer processes follow the Grotthuss proton-hopping mechanism. Meanwhile, a good correlation between the number of hydrogen bonds and activation energy was obtained, i.e. the more the number of hydrogen bonds, the lower the activation energy. This work contributes to the further understanding of the essential role of hydrogen bonds in the proton conductivity and proton transfer mechanisms.