Ultrahigh Proton Conduction in Two Highly Stable Ferrocenyl Carboxylate Frameworks

Nowadays, although research of proton conductive materials has been extended from traditional sulfonated polymers to novel crystalline solid materials such as MOFs, COFs, and HOFs, research on crystalline ferrocene-based carboxylate materials is very limited. Herein, we selected two hydrogen-bonded and π–π interactions-supported ferrocenyl carboxylate frameworks (FCFs), [FcCO­(CH2)2COOH] (FCF 1) and [FcCOOH] (FCF 2) (Fc = (η5-C5H5)­Fe­(η5-C5H4)) to fully investigate their water-mediated proton conduction. Their excellent thermal, water, and chemical stabilities were confirmed by the means of thermogravimetric analyses, PXRD, and SEM determinations. The two FCFs indicate temperature- and humidity-dependent proton conductive features. Intriguingly, their ultrahigh proton conductivities are 1.17 × 10–1 and 1.01 × 10–2 S/cm, respectively, under 100 °C and 98% RH, which not only are comparable to the commercial Nafion membranes but also rank among the highest performing MOFs, HOFs, and COFs ever described. On the basis of the structural analysis, calculated Ea value, H2O vapor adsorption, PXRD, and SEM measurements, reasonable conduction mechanisms are highlighted. Our research provides a novel inspiration for finding new high proton conducting crystalline solid materials. Importantly, the outstanding conducting performance of 1 and 2 suggests their, hopefully, potential in fuel cells and related electrochemical fields.