Multivariate Synergistic Flexible Metal-Organic Frameworks with Superproton Conductivity for Direct Methanol Fuel Cells.

Improving proton conductivity and fabricating viable metal-organic frameworks (MOFs) based proton exchange membranes (PEMs) are central issues exploiting electrolyte MOFs. We aim to design multivariate flexibility synergistic strategy to achieve Flexible MOFs (FMOFs) with high conductivity at a wide range of humidity. In situ powder X-ray diffraction (PXRD) and temperature-dependent Fourier transform infrared spectra (FT-IR) prove the synergistic self-adaption between dynamic torsion of alkyl sulfonic acid and dynamic breathing of FMOF, forming a continuous hydrogen-bonding networks to maintain high conductivity. Based on the convincing proton conductivity, we construct a series of long-term durable MOF-based PEMs that serve as a bridge between MOF and fuel cell. Consequently, the membrane electrode assembly (MEA) of the flexible PMNS1-40 exhibits a maximum single-cell power density of 34.76 mW cm-2 and hopefully opens doors to evaluate the practical application of proton-conducting MOFs in direct methanol fuel cells.

Construction of Proton Transport Highways Induced by Polarity-Driving in Proton Exchange Membranes to Enhance the Performance of Fuel Cells.

The approach to constructing proton transport channels via direct adjustments, including hydrophilia and analytical acid concentration in hydrophilic domains, has been proved to be circumscribed when encouraging the flatter hydrophilic-hydrophobic microphase separation structures and reducing conductivity activation energy. Here, we propose a constructive solution by regulating the polarity of hydrophobic domains, which indirectly varies the aggregation and connection of hydrophilic ion clusters during membrane formation, enabling orderly self-assembly and homogeneously distributed microphase structures. Accordingly, a series of comb-shaped polymers were synthesized with diversified optimization, and more uniformly distributed ion cluster lattices were subsequently observed using high-resolution transmission electron microscopy. Simultaneously, combining with density functional theory calculations, we analyzed the mechanism of membrane degradations caused by hydroxyl radical attacks. Experimental results demonstrated that, facilitated by proper molecule polarity, beneficial changes of bond dissociation energy could extend the membrane lifetime more than the protection from side chains near ether bonds, which were deemed to reduce the probability of attacks by the steric effect. With the optimal strategy chosen among various trials, the maximum power density of direct methanol fuel cell and H2/air proton exchange membrane fuel cell was enhanced to 95 and 485 mW cm-2, respectively.

Preparation of a Cross-Linked Sulfonated Poly(arylene ether ketone) Proton Exchange Membrane with Enhanced Proton Conductivity and Methanol Resistance by Introducing an Ionic Liquid-Impregnated Metal Organic Framework.

A novel ionic liquid-impregnated metal-organic-framework (IL@NH2-MIL-101) was prepared and introduced into sulfonated poly(arylene ether ketone) with pendent carboxyl groups (SPAEK) as the nanofiller for achieving hybrid proton exchange membranes. The nanofiller was anchored in the polymeric matrix by the formation of amido linkage between the pendent carboxyl group attached to the molecule chain of SPAEK and amino group belonging to the inorganic framework, thus leading to the enhancement in mechanical properties and dimensional stability. Besides, the hybrid membrane (IL@MOF-1) exhibits an enhanced proton conductivity up to 0.184 S·cm-1 because of the incorporation of ionic liquid in the nanocages of NH2-MIL-101. Moreover, the special structure of NH2-MIL-101 contributes to a low leakage of ionic liquid so as to retain the stable proton conductivity of hybrid membranes under fully hydrated conditions. Furthermore, as a result of a cross-linked structure formed by inorganic nanofiller, the IL@MOF-1 hybrid membrane shows a lower methanol permeability (7.53 × 10-7 cm2 s-1) and superior selectivity (2.44 × 105 S s cm-3) than the pristine SPAEK membrane. Especially, IL@MOF-1 performs high single-cell efficiency with a peak power density of 37.5 mW cm-2, almost 2.3-fold to SPAEK. Electrochemical impedance spectroscopy and scanning electron microscopy indicated that the nanofiller not only contributed to faster proton transfer but also resulted in a tighter bond between the membrane and catalyst. Therefore, the incorporation of IL@NH2-MIL-101 to prepare the hybrid membrane is proven to be suitable for application in direct methanol fuel cells.

Enhanced Proton Conductivity of Sulfonated Hybrid Poly(arylene ether ketone) Membranes by Incorporating an Amino-Sulfo Bifunctionalized Metal-Organic Framework for Direct Methanol Fuel Cells.

Novel side-chain-type sulfonated poly(arylene ether ketone) (SNF-PAEK) containing naphthalene and fluorine moieties on the main chain was prepared in this work, and a new amino-sulfo-bifunctionalized metal-organic framework (MNS, short for MIL-101-NH2-SO3H) was synthesized via a hydrothermal technology and postmodification. Then, MNS was incorporated into a SNF-PAEK matrix as an inorganic nanofiller to prepare a series of organic-inorganic hybrid membranes (MNS@SNF-PAEK-XX). The mechanical property, methanol resistance, electrochemistry, and other properties of MNS@SNF-PAEK-XX hybrid membranes were characterized in detail. We found that the mechanical strength and methanol resistances of these hybrid membranes were improved by the formation of an ionic cross-linking structure between -NH2 of MNS and -SO3H on the side chain of SNF-PAEK. Particularly, the proton conductivity of these hybrid membranes increased obviously after the addition of MNS. MNS@SNF-PAEK-3% exhibited the proton conductivity of 0.192 S·cm-1, which was much higher than those of the pristine membrane (0.145 S·cm-1) and recast Nafion (0.134 S·cm-1) at 80 °C. This result indicated that bifunctionalized MNS rearranged the microstructure of hybrid membranes, which could accelerate the transfer of protons. The hybrid membrane (MNS@SNF-PAEK-3%) showed a better direct methanol fuel cell performance with a higher peak power density of 125.7 mW/cm2 at 80 °C and a higher open-circuit voltage (0.839 V) than the pristine membrane.