Metal−organic frameworks (MOFs) provided a versatile platform for the development of new solid protonic electrolytes but faced great challenges regarding their low chemical stability and poor moisture retention capacity. Herein, we presented the proton-conducting study for zirconium-based MOF-802, revealing that MOF-802 possessed excellent features of extra aqueous and acidic stabilities and room-temperature superprotonic conduction with a proton conductivity of 1.05 × 10 −2 S cm −1 at 288 K under 98% relative humidity (RH). Unfortunately, due to the liberation of water molecules from pores/channels, the proton conductivity of MOF-802 dropped significantly at the temperature above 318 K. To solve this issue, for the first time, MOF-802 was hybridized with poly(vinyl alcohol) (PVA) to form MOF-802@PVA hydrogel composites, where the moisture retention capacity of MOF-802 was greatly improved, giving the high room-temperature proton conductivity over 10 −3 S cm −1 under ambient humidity. This work paves a new way to improve the moisture retention capacity and proton-conducting performances of porous proton conductors.

The development of fast proton-conducting materials above 100 °C with high chemical stability is very challenging but of great significance for proton-exchange membrane fuel cells (PEMFCs). Herein, we demonstrate the first example of benzimidazole-linked COF (PBI-COF) exploited as a fast proton conductor above 100 °C toward PEMFC application. Our PBI-COF integrates the outstanding chemical stability of polybenzimidazoles (PBIs) and the ordered channel structure of COFs to provide a powerful platform to confine and stabilize H 3 PO 4 molecules for proton transport. Upon impregnation with H 3 PO 4 , H 3 PO 4 @PBI-COF exhibits high proton conductivity (>10 −1 S cm −1 above 100 °C), low water swelling, and good long-term durability that is superior to many reported proton conductors. Furthermore, PEMFC with H 3 PO 4 @PBI-COF as a solid electrolyte achieves 0.936 V open-circuit voltages and 125 mW cm −2 maximum power densities.

Crystalline porous materials, such as metal−organic frameworks (MOFs) and covalent organic frameworks (COFs), have been demonstrated to be versatile material platforms for the development of solid proton conductors. However, most crystalline porous proton conductors suffer from decreasing proton conductivity with increasing temperature due to releasing water molecules, and this disadvantage severely restricts their practical application in electrochemical devices. In this work, for the first time, hydrophilic carbon dots (CDs) were utilized to hybridize with high proton conductivity MOF-802, which is a model of MOF proton conductors, aiming to improve its water-retention capacity and thus enhance proton conduction. The resultant CDs@MOF-802 exhibits impregnable proton conduction with increasing temperature, and the proton conductivity reaches 10 −1 S cm −1 , much superior to that of MOF-802, making CDs@MOF-802 one of the most efficient MOF proton conductors reported so far. This study provides a new strategy to improve the water-retention capacity of porous proton conductors and further realize excellent proton conduction.

Antifreezing and highly proton-conductive hydrogels show promising applications in flexible electrochemical devices owing to their inherent stretchability and safety. In this study, we chose an ethylene glycol/water (EG/H2O) binary mixture as the solvent, sulfonated chitosan (CS-SO3H) as the proton-conducting component, and the blend of CS-SO3H and poly­(vinyl alcohol) (PVA) as a gelator to prepare double-network hydrogels, CS-SO3H@PVA-X (X represents the mass ratio of CS-SO3H and PVA with a value of 0, 0.5, 1.0, and 1.5), which are characterized by different techniques, including microanalysis, IR, 1H and 13C NMR spectra, TG, PXRD, and so on. CS-SO3H@PVA-X hydrogels exhibit excellent tensile strength, toughness, and a freezing-tolerant feature. Importantly, CS-SO3H@PVA-1.5 hydrogel displays not only high proton conduction in a wide range of temperatures from −35 to 70 °C, with proton conductivities of 7.2 × 10–4 S cm–1 at −35 °C and 4.56 × 10–2 S cm–1 at 70 °C and ambient humidity, but also exceptional mechanical performance, with a tensile strength of 3.11 MPa and an elongation at break of 423%, indicative of a potential application in electrochemical devices relying on proton transport and operating at extreme conditions. It is also discovered for the first time that the double-network micelles are entangled together to form the spiral crimped texture in hydrogels CS-SO3H@PVA-X (X = 1.0 and 1.5).

Mixed ion−electron conductors have a wide range of important applications in devices relying on mixed ion−electron transport, mainly including conducting polymer composites and ceramics. Herein, we present a new type of mixed ion−electron conductor, an open-framework chalcogenide, (H 3 O)KCu 6 Ge 2 S 8 •nH 2 O (1), with a three-dimensional (3D) framework and two types of one-dimensional (1D) channels, occupied by hydronium ions and potassium ions, respectively. Thermogravimetric (TG) and powder X-ray diffraction (PXRD) measurements suggest that the hydrated proton species in the channels of 1 are in the form of H 2n+1 O n + (n > 1). The impedance measurements reveal that 1 is an intrinsic mixed proton− electron conductor. In the N 2 atmosphere, the conductivity (σ) of 1 increases with release of water molecules and increasing temperature with σ = 0.71 S cm −1 at 123 K and 1.61 S cm −1 at 473 K in the first cooling run, and in this case, the electron conduction is much higher than the ion conduction. At 98% relative humidity (RH), however, the situation is opposite, and the proton conductivity of 1 is higher than its electron conduction with an ion conductivity (σ i )/electron conductivity (σ e ), σ i /σ e , of 1.99 × 10 −3 /6.31 × 10 −4 S cm −1 at 298 K and 2.49 × 10 −2 /2.52 × 10 −3 S cm −1 at 343 K. To the best of our knowledge, 1 is the first example of an open-framework material with mixed proton−electron conduction, and this study demonstrates that open-framework materials, including chalcogenides, MOFs, and COFs, are good candidates for mixed proton− electron conductors.