Robust Proton Conduction against Mechanical Stress in Flexible Free‐Standing Membrane Composed of Two‐Dimensional Coordination Polymer

Abstract

Introduction of mechanical flexibility into proton‐conducting coordination polymers (CPs) is in high demand for future protonic applications such as fuel cells and hydrogen sensors. Although such mechanical properties have been primarily investigated in one‐dimensional (1D) CPs, in this study, we successfully fabricated highly flexible free‐standing CP membranes with a high surface‐to‐volume ratio, which is beneficial for enhanced performance in the aforementioned applications. We fabricated a layered CP, Cu2(NiTCPP) (H4(H2TCPP); 5,10,15,20‐tetrakis(4‐carboxyphenyl) porphyrin), in which a two‐dimensional (2D) square grid sheet composed of tetradentate nickel porphyrins and paddlewheel‐type copper dimers was connected to each other by weak van der Waals forces. The mechanical flexibility was evaluated by bending and tensile tests. The flexural and Young's moduli of the membrane were significantly higher than those of conventional Nafion membranes. Electrochemical impedance spectroscopy analysis revealed that the in‐plane proton conductivity of the membrane was maintained even under applied bending stress. Because the X‐ray diffraction analysis indicates that the proton‐conducting pathway through the hydrogen bonding network remains intact during the bending operation, our present study provides a promising strategy for the fabrication of new and advanced 2D CPs without using substrates or additional polymers for protonic devices.

Hydrogen‐Bonded Metal–Organic Framework Nanosheet as a Proton Conducting Membrane for an H2/O2 Fuel Cell

“…In this method, a minimum thickness of membrane as low as ≈40 μm was achieved (see optical and FE-SEM images of a top view and cross-section/ side view Figure S6, Supporting Information). This free-standing membrane [6,38] was possible due to the high aspect ratio of nanosheets (evident from AFM and FE-SEM) which provided accessible nanoscopic dimension and relatively large orientation of nanosheets. The orientation of the freestanding nanosheet membrane was determined by comparing refraction X-ray diffraction patterns to transmission X-ray diffraction patterns (TXRD).…”

Mechanical Properties of Modulative Undulating Layers in Two-Dimensional Metal–Organic Frameworks

“…2D metal–organic frameworks (2D-MOFs) are modern layered materials that are considered to have applications in various fields. ,− In addition to many synthetic efforts of various researchers for 2D-MOFs to explore their high functionalities, their functionality changes upon mechanical stimuli have sometimes been investigated by preparing free-standing membranes and flakes . However, reports on the mechanical properties of 2D-MOFs remain limited. ,− In particular, the systematicity of anisotropic mechanical properties via chemical tuning of layered structures is largely unexplored, even though the high designability of layered structures using molecular components is an advantage of 2D-MOFs over other 2D materials.…”

Giant anisotropic thermal expansion of copper-cyanido flat layers with flexible copper nodes

“…5,6 Thus, unveiling the relationship between a flexible structure and a response to stimuli for layers is of high significance not only for deepening fundamental insights on 2D-materials but also for the development of future devices utilizing thin films 7,8 and membranes. 9,10…”