Proton Conductivities in Functionalized UiO-66: Tuned Properties, Thermogravimetry Mass, and Molecular Simulation Analyses

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 (−Qst) 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 −Qst 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.