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.