Superprotonic Conductivity of UiO-66 with Missing-Linker Defects in Aqua-Ammonia Vapor

High H2O-Assisted Proton Conduction in One Highly Stable Sr(II)-Organic Framework Constructed by Tetrazole-Based Imidazole Dicarboxylic Acid

Solid electrolyte materials with high structural stability and excellent proton conductivity (σ) have long been a popular and challenging research topic in the fuel cell field. This problem can be addressed because of the crystalline metal-organic frameworks' (MOFs') high structural stability, adjustable framework composition, and dense H-bonded networks. Herein, one highly stable Sr(II) MOF, {[Sr(H2tmidc)2(H2O)3]·4H2O}n (1) (H3tmidc = 2-(1H-tetrazolium-1-methylene)-1H-imidazole-4,5-dicarboxylic acid) was successfully fabricated, which was structurally characterized by single-crystal X-ray diffraction and electrochemically examined by the AC impedance determination. The results demonstrated that the σ of the compound manifested a positive dependence on temperature and humidity, and the optimal proton conductivity is as high as 1.22 × 10-2 S/cm under 100 °C and 98% relative humidity, which is at the forefront of reported MOFs with ultrahigh σ. The analysis of the proton conduction mechanism reveals that numerous tetrazolium groups, carboxyl groups, coordination, and crystallization water molecules in the framework are responsible for the high efficiency of proton transport. This work offers a fresh perspective on how to create novel crystalline proton conductive materials.

Tailoring the Electrochemical Responses of MOF-74 Via Dual-Defect Engineering for Superior Energy Storage

Rationally designed defects in a crystal can confer unique properties. This study showcases a novel dual-defects engineering strategy to tailor the electrochemical response of metal-organic framework (MOF) materials used for electrochemical energy storage. Salicylic acid (SA) is identified as an effective modulator to control MOF-74 growth and induce structural defects, and cobalt cation doping is adopted for introducing a second type of defect. The resulting dual-defects engineered bimetallic MOF exhibits a discharging capacity of 218.6 mAh g-1, 4.4 times that of the pristine MOF-74, and significantly improved cycling stability. Moreover, the engineered MOF-74(Ni0.675Co0.325)-8//Zn aqueous battery shows top energy/power density performances for Ni-Zn batteries (266.5 Wh kg-1, 17.22 kW kg-1). Comprehensive investigations reveal that engineered defects modify the local coordination environment and promote the in situ electrochemical reconfiguration during operation to significantly boost the electrochemical activity. This work suggests that rational tailoring of the defects within the MOF crystal is an effective strategy to manipulate the coordination environment of the metal centers and the corresponding electrochemical reconfiguration for electrochemical applications.

Tailored Porous Ferrocene-Based Metal-Organic Frameworks as High-Performance Proton Conductors

Although crystalline metal-organic frameworks (MOFs) have gained a great deal of interest in the field of proton conduction in recent years, the low stability and poor proton conductivity (σ) of some MOFs have hindered their future applications. As a result, resolving the issues listed above must be prioritized. Due to their exceptional structural stability, MOFs with ferrocene groups that exhibit particular physical and chemical properties have drawn a lot of attention. This study describes the effective preparation of a set of three-dimensional ferrocene-based MOFs, MIL-53-FcDC-Al/Ga and CAU-43, containing both main group metals and 1,1'-ferrocene dicarboxylic acid (H2FcDC). Multiple measurements, including powder X-ray diffraction (PXRD), infrared (IR), and scanning electron microscopy (SEM), confirmed that the addition of ferrocene groups enhanced the thermal, water, and acid-base stabilities of the three MOFs. Consequently, their proton-conductive behaviors were meticulously measured utilizing the AC impedance approach, and their best proton conductivities are 5.20 × 10-3, 2.31 × 10-3, and 1.72 × 10-4 S/cm at 100 °C/98% relative humidity (RH), respectively. Excitingly, MIL-53-FcDC-Al/Ga demonstrated an extraordinarily ultrahigh σ of above 10-4 S·cm-1 under 30 °C/98% RH. Using data from structural analysis, PXRD, SEM, thermogravimetry (TG), and activation energy, their proton transport mechanisms were thoroughly examined. The fact that these MOFs are notably easy to assemble, inexpensive, toxin-free, and stable will increase the range of practical uses for them.

Ligand-Functionalized MIL-68-type Indium(III) Metal-Organic Frameworks with Prominent Intrinsic Proton Conductivity

Indium-based metal-organic frameworks (In-MOFs) have now become an attractive class of porous solids in materials science and electrochemistry due to their diverse structures and promising applications. In the field of proton conduction, to find more crystalline MOFs with splendid proton-conductive properties, herein, five three-dimensional isostructural In-MOFs, MIL-68-In or MIL-68-In-X (X = NH2, OH, Br, or NO2) using terephthalic acid (H2BDC) or functionalized terephthalic acids (H2BDC-X) as multifunctional linkages were efficiently fabricated. First, the outstanding structural stability of the five MOFs, including thermal and water stability, was verified by thermal analysis and powder X-ray diffraction. Subsequently, the H2O-mediated proton conductivities (σ) were fully assessed and compared. Notably, their σ evinced a significant positive correlation between the temperature or relative humidity (RH) and varied with the functional groups on the organic ligands. Impressively, their highest σ values are up to 10-3-10-4 S/cm (100 °C/98% RH) and change in this order: MIL-68-In-OH (1.72 × 10-3 S/cm) > MIL-68-In-NH2 (1.70 × 10-3 S/cm) > MIL-68-In-NO2 (4.47 × 10-4 S/cm) > MIL-68-In-Br (4.11 × 10-4 S/cm) > MIL-68-In (2.37 × 10-4 S/cm). Finally, the computed activation energy values under 98 or 68% RHs are assessed, and the related proton conduction mechanisms are speculated. Moreover, after electrochemical testing, these MOFs illustrate remarkable structural rigidity, laying a meritorious material foundation for future applications.

New Type of Nanocomposite CsH2PO4-UiO-66 Electrolyte with High Proton Conductivity

New (1−x)CsH2PO4−xUiO-66 electrolytes with high proton conductivity and thermal stability at 230−250 °C were developed. The phase composition and proton conductivity of nanocomposites (x = 0−0.15) were investigated in detail. As shown, the UiO-66 matrix is thermally and chemically suitable for creating composites based on CsH2PO4. The CsH2PO4 crystal structure remains, but the degree of salt crystallinity changes in nanocomposites. As a result of interface interaction, dispersion, and partial salt amorphization, the proton conductivity of the composite increases by two orders of magnitude in the low-temperature range (up to 200 °C), depending on the UiO-66 fraction, and goes through a maximum. At higher temperatures, up to 250 °C, the conductivity of nanocomposites is close to the superprotonic values of the original salt at low UiO-66 values; then, it decreases linearly within one order of magnitude and drops sharply at x > 0.07. The stability of CsH2PO4-UiO-66 composites with high proton conductivity was shown. This creates prospects for their use as proton membranes in electrochemical devices.

Post-synthesis functionalization of ZIF-90 with sulfonate groups for high proton conduction

Introducing sulfonic acid groups into MOF materials is one of the effective approaches to enhance proton conduction. Here, we attempted to prepare a new post-modified ZIF-90-based material by addition reaction of the aldehyde group with bisulfite to obtain partially functionalized ZIF-90-SO₃Na_(2.3). ZIF-90-SO₃Na_(2.3) exhibits a high proton conductivity of 2.26 × 10^-2 S cm^-1 at 98% RH and 100 °C.

Comparative Studies on the Proton Conductivities of Hafnium-Based Metal-Organic Frameworks and Related Chitosan or Nafion Composite Membranes

Hafnium (Hf)-based UiO-66 series metal-organic frameworks (MOFs) have been widely studied on gas storage, gas separation, reduction reaction, and other aspects since they were first prepared in 2012, but there are few studies on proton conductivity. In this work, one Hf-based MOF, Hf-UiO-66-fum showing UiO-66 structure, also known as MOF-801-Hf, was synthesized at room temperature using cheap fumaric acid as the bridging ligand, and then imidazole units were successfully introduced into MOF-801-Hf to obatin a doped product, Im@MOF-801-Hf. Note that both MOF-801-Hf and Im@MOF-801-Hf demonstrate excellent thermal, water, and acid-base stabilities. Expectedly, the maximum proton conductivity (σ) of Im@MOF-801-Hf (1.46 × 10^-2 S·cm^-1) is nearly 4 times greater than that of MOF-801-Hf (3.98 × 10^-3 S·cm^-1) under 100 °C and 98% relative humidity (RH). To explore their possible practical application value, we doped them into chitosan (CS) or Nafion membranes as fillers, namely, CS/MOF-801-Hf-X, CS/Im@MOF-801-Hf-Y, and Nafion/MOF-801-Hf-Z (X, Y, and Z are the doping percentages of MOF in the membrane, respectively). Intriguingly, it was found that CS/MOF-801-Hf-6 and CS/Im@MOF-801-Hf-4 indicated the highest σ values of 1.73 × 10^-2 and 2.14 × 10^-2 S·cm^-1, respectively, under 100 °C and 98% RH and Nafion/MOF-801-Hf-9 also revealed a high σ value of 4.87 × 10^-2 S·cm^-1 under 80 °C and 98% RH, which showed varying degrees of enhancement compared to the original MOFs or pure CS and Nafion membranes. Our study illustrates that these Hf-based MOFs and related composite membranes offer great potential in electrochemical fields.

Developing a Unique Hydrogen-Bond Network in a Uranyl Coordination Framework for Fuel Cell Applications

Crystalline materials with persistent high anhydrous proton conductivity that can be directly used as a practical electrolyte of the intermediate-temperature proton exchange membrane fuel cells for durable power generation remain a substantial challenge. The present work proposes a unique way of the axial uranyl oxo atoms as hydrogen-bond acceptors to form a dense hydrogen-bonded network within a stable uranyl-based coordination polymer, UO₂(H₂PO₃)₂(C₃N₂H₄)₂ (HUP-3). It exhibits stable and efficient anhydrous proton conductivity over a super-wide temperature range (-40-170 °C). It was also assembled into a H₂/O₂ fuel cell as the electrolyte and shows a high electrical power density of 11.8 mW·cm^-2 at 170 °C, which is among one of the highest values reported from crystalline solid electrolytes. The cell was tested for over 12 h without notable power loss.