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

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. Whereas such mechanical properties have been primarily investigated in one-dimensional (1D) CPs, in this study we successfully fabricated highly flexible free-standing-type CP membranes with a high surface-to-volume ratio, which is beneficial for the increased performance in the foregoing applications. We employed 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 is connected to each other by weak van der Waals forces. Mechanical flexibility was evaluated by bending and tensile tests; flexural and Young’s moduli of the membrane are significantly higher than that of conventional Nafion membranes. Electrochemical impedance spectroscopy revealed that the in-plane proton conductivity of the membrane was almost kept even by applying bending stress. Because the X-ray diffraction analysis indicates that proton-conducting pathway through the hydrogen-bonding network is kept intact during the bending operation, our present study provides a promising strategy for fabricating new and more advanced 2D CPs without using any substrates and addition polymers for protonic devices.