Proton‐conducting materials in the solid state have received immense attention for their role as electrolytes in proton‐exchange membrane fuel cells. Recently, crystalline materials—metal–organic frameworks (MOFs), hydrogen‐bonded organic frameworks (HOFs), covalent organic frameworks (COFs), polyoxometalates (POMs), and porous organic crystals—have become an exciting research topic in the field of proton‐conducting materials. For a better electrolyte, a high proton conductivity on the order of 10−2 S cm−1 or higher is preferred as efficient proton transport between the electrodes is ultimately necessary. With an emphasis on design principles, this Concept will focus on MOFs and other crystalline solid‐based proton‐conducting platforms that exhibit “ultrahigh superprotonic” conductivities with values in excess of 10−2 S cm−1. While only a handful of MOFs exhibit such an ultrahigh conductivity, this quality in other systems is even rarer. In addition to interpreting the structural–functional correlation by taking advantage of their crystalline nature, we address the challenges and promising directions for future research.

Recently, proton conduction has been a thread of high potential owing to its wide applications in fuel‐cell technology. In the search for a new class of crystalline materials for protonic conductors, three metalo hydrogen‐bonded organic frameworks (MHOFs) based on [Ni(Imdz)6]2+ and arene disulfonates (MHOF1 and MHOF2) or dicarboxylate (MHOF3) have been reported (Imdz=imidazole). The presence of an ionic backbone with charge‐assisted H‐bonds, coupled with amphiprotic imidazoles made these MHOFs protonic conductors, exhibiting conduction values of 0.75×10−3, 3.5×10−4 and 0.97×10−3 S cm−1, respectively, at 80 °C and 98 % relative humidity, which are comparable to other crystalline metal‐organic framework, coordination polymer, polyoxometalate, covalent organic framework, and hydrogen‐bonded organic framework materials. This report initiates the usage of MHOF materials as a new class of solid‐state proton conductors.

Self-assembly of bent dicarboxylate linker 4,4'-sulfonyldibenzoic acid (HSDB) and flexible N,N-donor spacer 1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene (L) with Co(NO)·6HO forms a twofold interpenetrated {[Co(SDB)(L)]·(HO)·(DMF)}, (IITKGP-6) network via solvothermal synthesis with sql(2,6L1) topology, which is characterized by Fourier transform infrared spectroscopy, thermogravimetric analysis, elemental analysis, powder X-ray diffraction (XRD), and single-crystal XRD. The framework is microporous with a solvent-accessible volume of 25.5% and forms a one-dimensional channel along [1-1 0] direction with the dimensions of ∼3.4 × 5.0 Å. As the stability of metal-organic frameworks (MOFs) in the presence of water is a topic of significant importance while considering them for practical applications, this framework reveals its high stability toward water. The desolvated framework shows modest uptake of CO (50.6 and 37.4 cm g at 273 and 295 K under 1 bar pressure, respectively), with high selectivity over N and CH. Ideal adsorbed solution theory calculations show that the selectivity values of CO/N (15:85) are 51.3 at 273 K and 42.8 at 295 K, whereas CO/CH (50:50) selectivity values are 36 at 273 K and 5.1 at 295 K under 100 kPa. The high CO separation selectivity over N and CH along with its water stability makes this MOF a potential candidate for CO separation from flue gas mixture and landfill gas mixture as well.

Accumulation of high concentrations of Al(III) in body has a direct impact on health and therefore, the trace detection of Al(III) has been a matter for substantial concern. An anionic metal organic framework ({[Me2NH2]0.5[Co(DATRz)0.5(NH2BDC)] ⋅ xG}n; 1; HDATRz=3,5‐diamino‐1,2,4‐triazole, H2NH2‐BDC=2‐amino‐1,4‐benzenedicarboxylic acid, G=guest molecule) composed of two types of secondary building units (SBU) and channels of varying sizes was synthesized by employing a rational design mixed ligand synthesis approach. Free −NH2 groups on both the ligands are immobilized onto the pore surface of the MOF which acts as a superior luminescent sensor for turn‐on Al(III) detection. Furthermore, the large channels could allow the counter‐ions to pass through and get exchanged to selectively detect Al(III) in presence of other seventeen metal ions with magnificent luminescence enhancement. The observed limit of detection is as low as 17.5 ppb, which is the lowest among the MOF‐based sensors achieved so far. To make this detection approach simple, portable and economic, we demonstrate MOF filter paper test for real time naked eye observation.

As an important type of crystalline solid proton-conducting material, metal-organic frameworks (MOFs) have received immense attention in the scientific community. In general, such MOFs are designed with intrinsic (H3O+, NH4 +, Me2NH2 +, H2PO4 –, OH– located in pores or dangling −SO3H, −COOH, and −PO3H2 groups as integral parts of the backbone) or extrinsic proton sources (imidazole, triazole, histamine, H3PO4, H2SO4 as guests). It has long been known that metal ions enhance the acidity of coordinated H2O molecules; however, such metal ion-bound water molecules as sole intrinsic proton sources for the construction of MOF-based conductors are largely unexplored. By exploiting such a design principle, herein, we report the proton-conduction properties of Mg-OBA MOF with the molecular formula {[Mg2(OBA)2(H2O)6]·2H2O} n [OBA = 4,4′-oxybis­(benzoic acid)], where half of the octahedral coordination sites around Mg­(II) centers are occupied by water molecules. The 2D layered structure with hydrophilic interlayer spaces within the framework having abundant water coordination as the sole proton sources makes extended H-bonding pathway for efficient protons migration and thus exhibiting ultrahigh conductivity with the value of 1.27 × 10–2 S cm–1 at 80 °C and 95% relative humidity (RH). The activation energy (Ea = 0.13 eV) obtained is one of the lowest of MOF-based proton conductors suggesting facile proton transfer. Besides, this material is quite robust and easy to scale up in the aqueous medium.