Constructing an inorganic macrocycle-based host-guest system remains a formidable synthetic challenge. Herein, by tuning the coordination modes of the pyrimidine-5-carboxylic acid (Pym-5-CA) and the assembly of [Mo₂O₂S₂]²⁺ units, we can obtain either an organic-templated inorganic ring architecture (H₄[(Mo₂O₂S₂)₆(OH)₁₂(C₅H₃O₂N₂)₂]·6DMF·5H₂O, denoted as Mo₁₂-Pym-5-CA, (1)) or an inorganic-host-organic-guest complex ([(Mo₂O₂S₂)₅(OH)(H₂O)₅(Pym-5-CA)]·24H₂O, Mo₁₀@Pym-5-CA (2)). By employing a similar strategy, but with another organic guest molecule, 3-pyridine-boronic acid (3-PyBA), another host-guest complex H[(Mo₂O₂S₂)₅(OH)(H₂O)₅(3-PyBA)I]·2DMF·16H₂O, Mo₁₀@3-PyBA (3) can be obtained. The host-guest structures have been characterized by multiple techniques, such as powder X-ray diffraction (PXRD), single-crystal X-ray diffraction (SC-XRD), and hydrogen nuclear magnetic resonance spectroscopy (¹H NMR). Proton conduction measurements at 70 ℃ and 90% RH reveal that complex 2 exhibits a proton conductivity of 8.76 × 10^-3 S·cm^-1, approximately 2.1 times higher than that of complex 1 and 1.7 times higher than that of complex 3. This superior performance stems from the synergistic effect of its intact carboxyl groups, which serve as strong proton donors, and its well-defined hydrogen bond network. Potentiometric measurements under varying pH conditions further reveal that complex 2 exhibits a slope of 37 mV dec^-1, higher than those of complex 1 (28 mV dec^-1) and complex 3 (32 mV dec^-1), indicating an enhanced proton-responsive behavior. By elucidating the distinct roles of supramolecular and coordination assembly in dictating the structures and proton conduction of polyoxothiometalates (POTMs), this work establishes a direct correlation between proton transport and interfacial response. Beyond demonstrating their promise as proton-selective electrochemical interfaces, these findings provide valuable insights for the rational design next-generation proton-conductive materials.

Developing novel, low-cost, and high-performance proton exchange membranes (PEMs) to replace commercial Nafion is of great significance. By grafting quaternary ammonium cations onto the side chains of polybenzimidazole, abundant charge carriers and proton hopping sites are provided for proton transport, accelerating proton migration and thereby enhancing the proton conductivity of composite membranes made from this material. Furthermore, polyoxometalates (POMs) are introduced through electrostatic interactions, and their exceptional proton-conducting properties further improve the proton conductivity of the composite membranes, with the maximum proton conductivity reaching 0.123 S·cm⁻¹. This work demonstrates that POM-based materials can serve as excellent proton carriers, expanding the selection of active materials for composite membranes and providing new references for the fabrication of novel proton exchange membranes.

The development of novel proton exchange membranes (PEMs) with high proton conductivity and good mechanical performance as alternatives to Nafion is crucial. Polyoxometalates (POMs), a type of solid-state nanoclusters, possess high proton conductivity and good structural stability, making them suitable functional inorganic fillers to improve the performance of PEMs. Herein, the Keggin-type POM H₃PW₁₂O₄₀·nH₂O (PW₁₂) was introduced into sulfonated polyaryletherketone (SPAEK) with closely packed and flexible side chains to construct hybrid membranes (SPAEK-PW₁₂-x%, x = 5, 10, 13, 15). Because of its structural characteristics, the nanosized PW₁₂ induced precise hybridization of the nanophase structure of this ionomeric polymer and the formation of proton transport channels. Additionally, the hydrogen-bonding networks formed by PW₁₂ and sulfonic acid groups increased the proton conductivity and mechanical strength of the resulting hybrid PEMs and improved the close-packed structure of the PEMs to achieve an appropriate balance between conductivity and fuel permeation. In particular, SPAEK-PW₁₂-13% achieved an enhanced proton conductivity of 0.167 S∙cm⁻¹ at 80 °C, which was 2.2 times greater than that of the pristine membrane. Moreover, the mechanical properties, chemical stability, resistance to methanol penetration, and ionic selectivity of the hybrid membrane were significantly improved upon addition of a moderate amount of PW₁₂. This work provides an approach for the design and development of new-generation organic–inorganic hybrid membranes through precise hybridization of POMs.

All-solid-state batteries are promising candidates for the future generation of energy storage materials. An ideal solid-state electrolyte should have the advantages of excellent compatibility with electrodes and decent ionic conductivity. Nevertheless, the inherent low ionic conductivity of polyethylene oxide (PEO)-based electrolytes leads to low capacity, which significantly limits their wide commercial application. In this study, Dawson-type Li₆P₂Mo₁₈O₆₂ (LPM) or Li₆P₂W₁₈O₆₂ (LPW) was selected as lithium salt, combined with ionic liquids (ILs) with ether oxygen chains, and incorporated into a polymer matrix blended with PEO and polyvinylidene fluoride as fillers. A polymer electrolyte film with a smooth surface and uniform filler distribution was prepared using a mechanical co-blending method. The challenge of polyoxometalates as ion-conducting materials is attributed to the strong binding ability of their anion clusters to cations. One prominent benefit of this study is that the dissociation of Li⁺ from LPM or LPW is facilitated by ILs and relies on the ether oxygen chains in ILs for transport, yielding composites with favorable conduction properties. This study demonstrates the vast potential of polyoxometalates in the field of ionic conductivity.

Releasing cations from highly negatively charged polyoxometalates (POMs) is never an easy task. Herein, by using a zwitterion (1-sulfopropyl-3-methylimidazolium salt, MIMPS) to dissociate POMs, the proton conductivity of POM electrolytes was enhanced and the capacitive performance of solid-state supercapacitors (SCs) based on polyaniline was further improved. MIMPS can promote the dissolution and dissociation of POMs in polymer solutions, releasing more mobile protons, which is conducive to rapid proton transport. The MIMPS-modified SCs have higher capacitive performance, with an areal capacitance of 13 F·cm⁻² at a current density of 0.5 mA·cm⁻², compared to SCs without MIMPS (6.4 F·cm⁻²). In addition, the MIMPS-modified SCs have lower interfacial impedance, indicating that MIMPS can improve the proton conductivity and interfacial conduction. This work provides a new strategy for improving the overall performance of SCs by optimizing POM-based electrolytes with a zwitterion.

Solid-state electrolytes have attracted considerable attention in new energy-related devices due to their high safety and broad application platform. Polyoxometalates (POMs) are a kind of molecular-level cluster compounds with unique structures. In recent years, owing to their abundant physicochemical properties (including high ionic conductivity and reversible redox activity), POMs have shown great potential in becoming a new generation of solid-state electrolytes. In this review, an overview is investigated about how POMs have evolved as ion-conducting materials from basic research to novel solid-state electrolytes in energy devices. First, some expressive POM-based ion-conducting materials in recent years are introduced and classified, mainly inspecting their structural and functional relationship. After that, it is further focused on the application of these ion-conducting electrolytes in the fields of proton exchange membranes, supercapacitors, and ion batteries. In addition, some properties of POMs (such as inherent dimension, capable of forming stable hydrogen bonds, and reversible bonding to water molecules) enable these functional POM-based electrolytes to be employed in innovative applications such as ion selection, humidity sensing, and smart materials. Finally, some fundamental recommendations are given on the current opportunities and challenges of POM-based ion-conducting electrolytes.

In order to sustainably transform N₂ to ammonia (NRR) using electrocatalysts under mild ambient condition, it is urgent to design and develop non-nobel metal nanocatalysts that are inexpensive and suitable for mass-production. Herein, a calcium metalate catalyst CaCoO_x with oxygen vacancies was synthesized and used as an electrocatalyst for NRR for the first time, whose morphology can be controlled by the calcination temperature and the heating rate. Under the optimal conditions, the CaCoO_x catalyst achieved the yield of nitrogen conversion to ammonia of 16.25 μg·h^-1·mg_cat.^-1 at the potential of -0.3 V relative to the reversible hydrogen electrode (RHE) with a Faraday efficiency of 20.51%. The electrocatalyst showed good stability even after 12 times recyclability under environmental conditions and neutral electrolyte. Later, the electrocatalytic nitrogen reduction performance of CaFeO_x, CaNiO_x, CaCuO_x was investigated. These earth-rich transition metals also exhibited certain NRR electrocatalytic capabilities, which provided a door for further development of inexpensive and easily available transition metal as nitrogen reduction electrocatalysts.