No abstract

Pursuing new proton-conducting materials has become a key issue to improve the performance of proton exchange membrane fuel cells for clean energy. As newly emerging materials, metal–organic frameworks (MOFs) have been attracting wide attention in this regard. Herein, using stable UiO-66 as a platform, through the modification of different functional groups of −SO3H, −2COOH, −NH2, and −Br in ligands, we explore the strategy to tune the proton conductivities of MOFs. With the highly acidic and strong hydrophilic functional groups −SO3H and −COOH, UiO-66-SO3H and UiO-66-2COOH show quite high proton conductivities of 0.34 × 10–2 and 0.10 × 10–2 S cm–1 at 303 K and ∼97% relative humidity, respectively while the −NH2, −H, and −Br represent comparatively low conductivities under the same conditions. Furthermore, water molecules adsorbed in the pores are proved to contribute greatly to the proton conductivities of these MOFs. Thermogravimetry-mass spectrometry (TG-MS) and molecular simulations are then used to analyze the interactions between the water molecules and MOFs. TG-MS analyses show two water molecule loss processes in UiO-66-SO3H and UiO-66-2COOH, but one in UiO-66-NH2, UiO-66, and UiO-66-Br during heating, which indicates the stronger affinity of −SO3H and −COOH functionalized UiO-66 toward water molecules than those with −NH2 and −Br, as well as UiO-66 itself. The isosteric heats (−Q st) of water adsorption and radial distribution functions (RDFs) in these MOFs are also evaluated by molecular simulations. It was found that UiO-66-SO3H and UiO-66-2COOH have higher −Q st of 86.50 and 52.10 kJ mol–1, whereas UiO-66-NH2, UiO-66, and UiO-66-Br have lower ones, 34.03, 19.04, and 36.36 kJ mol–1, respectively. The RDFs reveal the formation of hydrogen-bonding networks in UiO-66-SO3H, UiO-66-2COOH, UiO-66-NH2, and UiO-66, but not in UiO-66-Br.

A highly stable porphyrinic MOF with free-carboxylic group functionalized pore surfaces exhibits a high proton conductivity of 3.9 × 10−2 S cm−1 at 80 °C and 97% RH, which is comparable to most of the reported proton conductive MOFs.

A diaminotriazine-decorated porphyrin-based microporous hydrogenbonded organic framework has been successfully prepared and characterized using single crystal X-ray diffraction analysis. Its activated phase exhibits permanent porosity, gas separation, and proton conductivity under humid conditions.

The polydimethylsiloxane (PDMS) coating penetrated into the underneath ZIF-8 polycrystalline membrane not only blocking the inter-crystalline defects but also hindering the flexibility of the ZIF-8 framework, resulting in an unusual and highly desired increase in the separation selectivity of the CH/CH mixture under high feeding pressures.