Symmetry related proton conductivity tunability via aliovalent metal substitution in imidazolium templated stable metal-organic framework hybrid membranes

Proton-conducting materials have gained popularity owing to their extensive applications in biologic/chemical sensors, supercapacitors, proton sieving, and proton-exchange-membrane fuel cells. To date, the most commercially used polymer membrane has been the Nafion series that exhibits conductivity exceeding 0.1 S cm −1 , however, this series is expensive, has poor dimensional stability, and requires a complex synthesis process. The key criterion for selecting Nafion alternatives is to achieve the systematic integration of high proton conductivity with high stability through a simple and efficient approach. In this study, we used an aliovalent metal substitution strategy to design serial metal–organic frameworks (MOFs), including tetragonal T -Cd-BTC (CH 3 NH 2 CH 3 ) 2 [Cd(BTC)](H 2 O) and quasi-cubic quasi- C -In-BTC (C 4 H 7 N 2 )[In(BTC)] and Im@quasi- C -In-BTC (C 3 H 5 N 2 ) 2 [In(BTC)] frameworks, with 2-methylimidazolium and imidazolium cations as templates, respectively. Because of the aliovalent substitution of In(III) for Cd(II), both the metal–oxygen bond strength and unit cell symmetry gradually increased, resulting in an increase in the thermal stability of quasi- C -In-BTC and Im@quasi- C -In-BTC at temperatures of up to 700 K. Compared with in situ loaded 2-methylimidazolium quasi- C -In-BTC , Im@quasi- C -In-BTC prepared by incorporating the imidazolium cation into the pores of activated quasi- C -In-BTC exhibited a higher proton conductivity of 7.1 × 10 −2  S cm −1 at 338 K and 95 % relative humidity. Thus, Im@quasi- C -In-BTC demonstrated real-life application. This result was confirmed by integrating Im@quasi- C -In-BTC with a poly(vinyl pyrrolidone)-poly(vinylidene fluoride) polymer matrix. Density functional theory simulations indicated that Im@quasi- C -In-BTC was strongly acidic and had high water–adsorption capacities, which contributed to extensive hydrogen-bond networks and strong host–guest interactions, in accordance with the experimental finding.