Design, Preparation, and High Intrinsic Proton Conductivity of Two Highly Stable Hydrazone-Linked Covalent Organic Frameworks

Abstract

Assembling crystalline materials with high stability and high proton conductivity as a potential alternative to the Nafion membrane is a challenging topic in the field of energy materials. Herein, we concentrated on the creation and preparation of hydrazone-linked COFs with super-high stability to explore their proton conduction. Fortunately, two hydrazone-linked COFs, TpBth and TaBth, were solvothermally prepared by using benzene-1,3,5-tricarbohydrazide (Bth), 2,4,6-trihydroxy-benzene-1,3,5tricarbaldehyde (Tp), and 2,4,6-tris(4-formylphenyl)-1,3,5-triazine (Ta) as monomers. Their structures were simulated by Material Studio 8.0 software and confirmed by the PXRD pattern, demonstrating a two-dimensional framework with AA packing. The presence of a large number of carbonyl groups as well as −NH−NH 2 − groups on the backbone is responsible for their super-high water stability as well as high water absorption capacity. AC impedance tests demonstrated a positive correlation between the waterassisted proton conductivity (σ) of the two COFs and the temperature and humidity. Under 100 °C/98% RH, the highest σ values of TpBth and TaBth can reach 2.11 × 10 −4 and 0.62 × 10 −5 S•cm −1 , which are among the high σ values of the reported COFs. Their proton-conductive mechanisms were highlighted by structural analyses as well as N 2 and H 2 O vapor adsorption data and activation energy values. Our systematic research affords ideas for the synthesis of proton-conducting COFs with high σ values.

Synthesis of Phosphovanadate-Based Porous Inorganic Frameworks with High Proton Conductivity

“…Porous polyoxometalates have garnered significant attention owing to their diverse structures, abundant terminal and bridging oxygen, and large pore sizes. − Polyoxovanadates (POVs) represent a significant subclass of POMs and are widely applied in various fields such as proton conductors, magnetism, ion batteries, and catalysts. − Notably, vanadium exhibits multiple valence states and presents diverse coordination modes, such as {VO 4 } in a tetrahedral configuration, {VO 5 } in a pentahedral configuration, and {VO 6 } in an octahedral configuration. − Introducing the Brønsted acid group as a means to construct POMs is recognized as one of the strategies for enhancing proton conduction performance. , In 2012, Shimizu replaced sulfonate with phosphate to obtain a homogeneous three-dimensional (3D) MOF, and the proton conduction rate was improved to 2.1 × 10 –2 S·cm –1 at 85 °C and 90% RH . In 2020, the Tiemann group compared two 3D Ba 2+ -based MOFs and found that TMOF-2 with –SO 3 H had higher proton conduction due to the hydrophilicity of sulfonate .…”

“…24−27 Introducing the Brønsted acid group as a means to construct POMs is recognized as one of the strategies for enhancing proton conduction performance. 28,29 In 2012, Shimizu replaced sulfonate with phosphate to obtain a homogeneous threedimensional (3D) MOF, and the proton conduction rate was improved to 2.1 × 10 −2 S•cm −1 at 85 °C and 90% RH. 30 In 2020, the Tiemann group compared two 3D Ba 2+ -based MOFs and found that TMOF-2 with −SO 3 H had higher proton conduction due to the hydrophilicity of sulfonate.…”