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The POM capsule functions as a redox-active inorganic crown ether. Specifically, Cs+ ions are captured in the crown-ether-like pores {Mo3Fe3O6}, which exist on the surface of the capsule, and electrons are stored in the Mo atoms of the capsule.

A polymerizable ionic-liquid cation having methacryl group and imidazolium moieties (denoted as MAImC1) was successfully hybridized with octamolybdate (Mo8O264−, Mo8) to obtain inorganic–organic hybrid crystals containing monovalent metal cations. The hybrid crystals were initially prepared as sodium ion-containing hybrid crystal of MAImC1–Na–Mo8. The sodium cations in MAImC1–Na–Mo8 were exchangeable to other monovalent metal cations such as silver or potassium to form hybrid crystals of MAImC1–Ag–Mo8 or MAImC1–K–Mo8. Their molecular and crystal structures were clearly revealed by single crystal X-ray analyses. The MAImC1–Na–Mo8 hybrid crystal worked as the hybrid monomer, and was found polymerizable by a radical polymerization using AIBN as an initiator to obtain inorganic–organic polymers (P-MAImC1–Na–Mo8). The conductivities of the hybrid monomer and polymer consisting of MAImC1–Na–Mo8 were investigated.

Anionic metal-oxygen clusters known as polyoxometalates (POMs) have been widely researched as components of proton conductors. While proton conduction under non-humidified intermediate-temperature (100-250 °C) conditions is advantageous from the viewpoint of kinetics, few solid-state materials, not to mention POM-based crystals, show truly effective proton conduction without the aid of water vapor. In this context, non-volatile proton-conductive polymers have been confined into POM-based frameworks, while fast proton conduction was infeasible. Herein, we demonstrate a new strategy to synthesize POM-polymer composites exhibiting fast proton conduction under non-humidified intermediate-temperature conditions. Specifically, a molecular design approach utilizing poly(ethylene glycol)s (PEGs) of different terminal groups or chain lengths controls the proton carrier density, and a crystal engineering approach using a large Dawson-type POM ([α-P2W18O62]6-) with an anisotropic molecular shape and alkali metal ions as counter cations fine-tunes the mobility of the confined PEGs as proton carriers. By integrating these approaches, proton conductivity over 10-4 S cm-1 at 150 °C, comparable to the well-known highly proton-conductive solid-state materials, is achieved. The proton conduction mechanism is discussed with alternative current impedance spectroscopy jointly with specific heat capacity measurements and solid-state NMR spectroscopy.

Deprotonatable ionic-liquid surfactant cation of 1-dodecylimidazolium (C12im) was successfully hybridized with polyoxovanadate anion to obtain conductive inorganic-organic hybrid crystal. Single crystal X-ray analysis revealed that the hybrid crystal contained the alternate stacking of decavanadate (V10) layers and interdigitated C12im layers. Four C12im cations were associated with one diprotonated V10 anion, and the two diprotonated V10 anions formed a dimer species. Anhydrous proton conductivity derived from the dissociative protons of the hybrid crystal was estimated to be 4.6 × 10−5 S cm−1 at 373 K by alternating current (AC) impedance spectroscopy.

Proton-conducting materials working under anhydrous or less-humidified atmosphere are significant for improving the energy efficiency of fuel-cell batteries. Here, proton-containing inorganic-organic hybrid crystals were successfully constructed as single crystals with polyoxovanadate cluster and structure-directing surfactants. Decavanadate ([V10O28]6‒, V10) anion having proton affinity was employed to hybridize with alkyltrimethylammonium ([CnH2n+1N(CH3)3]+, Cn (n = 12, 14, 16)) cations. All the hybrid crystals were homologues composed from alternate stacking of V10 monolayers and Cn interdigitated bilayers. Inside the V10 monolayers, diprotonated V10 anions were arranged to form one-dimensional infinite chains connected through O‒H•••O hydrogen bonds. These hybrid crystals exhibited anhydrous proton conductivity at intermediate temperatures (373‒433 K (100‒160 °C)). A moderate anhydrous conductivity of 2.9 × 10–4 S cm–1 was observed for C16-V10 hybrid crystals at 373 K (100 °C).

Synthetic bilayers of double-chained surfactants were successfully constructed by using polyoxomolybdates as inorganic–organic hybrid crystals. Dialkyldimethylammonium ([(CnH2n+1)2N(CH3)2]+, (Cn)2; n = 10, 12, 14, 16) cations were employed to hybridize with octamoybdate ([Mo8O26]4–, Mo8) anion. The obtained crystals were homologues which possessed distinct layered structures composed from alternate stacking of Mo8 monolayers and (Cn)2 bilayers. The layered distances were controllable with an increase in the alkyl chain length of the (Cn)2 cation. Each alkyl chain of (Cn)2 was interdigitated to result in rather unique conformations for the (Cn)2 cations packed in the solid states. Inside the inorganic monolayers, the Mo8 anions were associated with sodium ions to form two-dimensional network, which could be promising for sodium-ion conductive materials.

Proton-conductive solid electrolytes are significant for fuel-cell battery technology. Especially for use in motor vehicles, proton conductors which work at intermediate temperatures (373–673 K) under an anhydrous atmosphere are desired to improve the fuel cell stability and efficiency. Inorganic–organic hybrid supramolecular architectures are a promising option for the realization of highly conductive proton conductors. Here, a hybrid layered crystal was synthesized for the first time by using an proton-containing decavanadate (V10) anion and a heterocyclic surfactant cation. A simple ion-exchange reaction led to the formation of an inorganic–organic hybrid of V10 by using dodecylpyridazinium (C12pda) as the heterocyclic surfactant. Single crystal X-ray analyses revealed that four C12pda cations were associated with one V10 anion, which was a diprotonated species forming a one-dimensional infinite chain structure through hydrogen bonds. Anhydrous proton conductivity was investigated by alternating current (AC) impedance spectroscopy in the range of 313–393 K, exhibiting a maximum value of 1.7 × 10−5 S cm−1 at 373 K.

Solid electrolytes are crucial materials for lithium-ion or fuel-cell battery technology due to their structural stability and easiness for handling. Emergence of high conductivity in solid electrolytes requires precise control of the composition and structure. A promising strategy toward highly-conductive solid electrolytes is employing a thermally-stable inorganic component and a structurally-flexible organic moiety to construct inorganic-organic hybrid materials. Ionic liquids as the organic component will be advantageous for the emergence of high conductivity, and polyoxometalate, such as heteropolyacids, are well-known as inorganic proton conductors. Here, newly-designed ionic liquid imidazolium cations, having a polymerizable methacryl group (denoted as MAImC1), were successfully hybridized with heteropolyanions of [PW12O40]3− (PW12) to form inorganic-organic hybrid monomers of MAImC1-PW12. The synthetic procedure of MAImC1-PW12 was a simple ion-exchange reaction, being generally applicable to several polyoxometalates, in principle. MAImC1-PW12 was obtained as single crystals, and its molecular and crystal structures were clearly revealed. Additionally, the hybrid monomer of MAImC1-PW12 was polymerized by a radical polymerization using AIBN as an initiator. Some of the resulting inorganic-organic hybrid polymers exhibited conductivity of 10−4 S·cm−1 order under humidified conditions at 313 K.