Peiyi Wu

“Evaporating” Graphene Oxide Sheets (GOSs) for Rolled up GOSs and Its Applications in Proton Exchange Membrane Fuel Cell

In the present work, we prepare rolled up graphene oxide sheets (GOSs) by evaporating GOSs from their dispersion to a remote aluminum foil surface. The topological structure of the rolled up GOSs on the aluminum foil surface, which is determined by the quantity of the formed Al³⁺ ions from the reaction between the alumina on the aluminum foil surface and the weak acidic condensed vapor of the GOS dispersion, can be easily controlled via simply changing the H₂O content in the original GOS dispersion. Meanwhile, a GO/Nafion composite membrane for proton exchange membrane fuel cell is successfully prepared utilizing the as-obtained hole-like self-assembled structure of the rolled-up GOSs as a supporting material . The resultant composite membrane exhibits excellent proton conductivity compared to that of the recast Nafion membrane, especially under low-humidity conditions. An increase in proton conductivity by several times could be easily observed here, which is mainly attributed to the rearrangement of the microstructures of Nafion matrix to significantly facilitate the proton transport with rolled up GOSs being independently incorporated. The method reported here offers new degrees of freedom to achieve such transformations among the allotropic forms of carbon and/or develop new carbon material/polymer composite materials with excellent properties.

Metal–organic framework–graphene oxide composites: a facile method to highly improve the proton conductivity of PEMs operated under low humidity

This work studies Nafion based proton exchange membranes (PEMs) modified by a metal–organic framework–graphene oxide composite (ZIF-8@GO). The ZIF-8@GO/Nafion hybrid membrane displays a proton conductivity as high as 0.28 S cm−1 at 120 °C and 40% RH, resulting from a synergetic effect of ZIF-8 and GO.

UF membrane with highly improved flux by hydrophilic network between graphene oxide and brominated poly(2,6-dimethyl-1,4-phenylene oxide)

In this work, graphene oxide (GO) was first functionalized with branched polyethyleneimine (PEI). The obtained PEI–GO was incorporated into a brominated poly(2,6-dimethyl-1,4-phenylene oxide) (BPPO) matrix by a covalent bond interaction to form a cross-linking network. Then, a novel ultrafiltration (UF) membrane was prepared via casting and phase-inversion methods. The PEI–GO/BPPO membrane showed highly improved water flux, which was almost 6 times higher than that of the pristine BPPO membrane and 2.5 times higher than that of the GO/BPPO membrane, whereas the rejection of PEI–GO/BPPO membrane was still maintained at a high level. The improvement of membrane performance could be attributed to the special property of PEI–GO and the interactions between PEI–GO and the BPPO matrix. First, a cross-linking network of PEI–GO/BPPO membrane was formed because of the reaction between PEI–GO and the BPPO matrix to provide passageways for water rapidly passing through. Second, the high hydrophilicity of PEI–GO could accelerate the exchange rate between solvent and non-solvent, resulting in a rougher and more hydrophilic surface, higher porosity and a more porous structure. Third, the good dispersion and compatibility of PEI–GO promoted the formation of a uniform structure with fewer defects. Proper molecular weight of PEI was very important for the modification of GO, subsequently resulting in an overall enhancement in membrane performance. Anti-fouling experiments and stability tests of the membranes were also conducted. All of these results were confirmed by various characterizations, such as SEM, TEM, AFM and, etc.

Selective growth of MoS2 for proton exchange membranes with extremely high selectivity.

Proton conductivity and methanol permeability are the most important transport properties of proton exchange membranes (PEMs). The ratio of proton conductivity to methanol permeability is usually called selectivity. Herein, a novel strategy of in situ growth of MoS2 is employed to prepare MoS2/Nafion composite membranes for highly selective PEM. The strong interactions between the Mo precursor ((NH4)2MoS4) and Nafions sulfonic groups in a suitable solvent environment (DMF) probably lead to a selective growth of MoS2 flakes mainly around the ionic clusters of the resultant MoS2/Nafion composite membrane. Therefore, it would significantly promote the aggregation and hence lead to a better connectivity of these ionic clusters, which favors the increase in proton conductivity. Meanwhile, the existence of MoS2 in the ionic channels effectively prevents methanol transporting through the PEM, contributing to the dramatic decrease in the methanol permeability. Consequently, the MoS2/Nafion composite membranes exhibit greatly increased selectivity. Under some severe conditions, such as 50 °C with 80 v/v% of methanol concentration, an increase in the membrane selectivity by nearly 2 orders of magnitude compared with that of the recast Nafion membrane could be achieved here, proving our method as a very promising way to prepare high-performance PEMs. All these conclusions are confirmed by various characterizations, such as (FE-) SEM, TEM, AFM, IR, Raman, TGA, XRD, etc.

Surface Decoration of Amino-Functionalized Metal–Organic Framework/Graphene Oxide Composite onto Polydopamine-Coated Membrane Substrate for Highly Efficient Heavy Metal Removal

A new metal-organic framework/graphene oxide composite (IRMOF-3/GO) with high adsorption capacity of copper(II) (maximal adsorption amount = 254.14 mg/g at pH 5.0 and 25 °C) was prepared. Novel and highly efficient nanofiltration (NF) membrane can be facilely fabricated via surface decoration of IRMOF-3/GO onto polydopamine (PDA)-coated polysulfone (PSF) substrate. After decoration of IRMOF-3/GO, membrane surface potential increased from 6.7 to 13.1 mV at pH 5.0 and 25 °C. Due to the adsorption effect of IRMOF-3/GO and the enhancement of membrane surface potential, the prepared NF membrane (the loading amount of IRMOF-3/GO is ca. 13.6 g/m2) exhibits a highly efficient rejection of copper(II). The copper(II) rejection reaches up to ∼90%, while maintaining a relatively high flux of ∼31 L/m2/h at the pressure of 0.7 MPa and pH 5.0. Moreover, the membrane also presents an outstanding stability throughout the 2000 min NF testing period. Thus, the newly developed NF membrane shows a promising potential for water cleaning. This work provides a worthy reference for designing highly efficient NF membranes modified by metal-organic framework (MOF) relevant materials.

Development of Hybrid Ultrafiltration Membranes with Improved Water Separation Properties Using Modified Superhydrophilic Metal–Organic Framework Nanoparticles

Metal-organic frameworks (MOFs) are being intensively explored as filler materials for polymeric membranes primarily due to their high polymer affinity, large pore volumes, and alterable pore functionalities, but the development of MOF-based ultrafiltration (UF) membranes for water treatment lags behind. Herein, poly(sulfobetaine methacrylate) (PSBMA)-functionalized MOF UiO-66-PSBMA was developed, and incorporated into polysulfone (PSf) casting solution to fabricate novel hybrid UF membranes via phase-inversion method. The resultant UiO-66-PSBMA/PSf membrane exhibited significantly improved water flux (up to 602 L m-2 h-1), which was 2.5 times that of the pristine PSf membrane (240 L m-2 h-1) and 2 times that of UiO-66-NH2/PSf membrane (294 L m-2 h-1), whereas the rejection of UiO-66-PSBMA/PSf membrane was still maintained at a high level. Moreover, UiO-66-PSBMA/PSf membrane exhibited improved antifouling performance. The improvement of membrane performances could be attributed to the well-tailored properties of UiO-66-PSBMA. On one hand, the excellent dispersion and compatibility of UiO-66-PSBMA ensured the formation of a uniform structure with few defects. On the other hand, the superhydrophilicity of UiO-66-PSBMA could accelerate the exchange rate between solvent and nonsolvent, resulting in a more hydrophilic surface and a more porous structure. Besides, UiO-66-PSBMA nanoparticles in the thin layer provided additional flow paths for water permeation through their hydrophilic porous structure as well as the tiny interspace between PSf matrix. This study indicates the great application potential of UiO-66-PSBMA in fabricating hybrid UF membranes and provides a useful guideline to integrate other modified hydrophilic MOFs to design UF membranes for water treatment.

A “H2O donating/methanol accepting” platform for preparation of highly selective Nafion-based proton exchange membranes

For a proton exchange membrane (PEM), the ratio of its proton conductivity to its fuel permeability usually defines the membrane selectivity. Generally, a highly selective PEM is preferred for application in direct methanol fuel cells. Herein, sulfonated SiO2@polystyrene core–shell (SiO2@sPS) nanoparticles were synthesized and then imbedded into a Nafion membrane by a blending–casting method. SiO2@sPS partakes in strong interactions with the Nafion polymer, which benefits its dispersion in the membrane matrix. The as-prepared SiO2@sPS + Nafion composite PEM presents a large increase in its proton conductivity owing to the introduction of additional –SO3H groups and hence has optimized channels for proton transport. Meanwhile, a reduced methanol crossover was also observed for the SiO2@sPS + Nafion composite PEM because of the formation of obstructed transport channels for bulk methanol. Besides this, a deep investigation on further enhancement of the membrane’s performance was conducted by etching the SiO2 core and hence forming well-dispersed uniform hollow spheres inside the membrane matrix. The intact hollow sulfonated PS spheres (h-sPS) acted as water reservoirs which in turn could gradually release water to hydrate the membrane under high-temperature and low-humidity conditions. Therefore, compared to the SiO2@sPS + Nafion membrane, the h-sPS + Nafion one presented a further increased proton conductivity at 100 °C under 40% RH. Meanwhile, h-sPS further suppressed methanol penetration by trapping it inside the hollow spheres. Herein, a “H2O donating/methanol accepting” mechanism was proposed for the first time, providing a promising platform to alleviate critical disadvantages of Nafion membranes and thereby fabricate highly selective Nafion-based PEMs.

β-Cyclodextrin modified silica nanoparticles for Nafion based proton exchange membranes with significantly enhanced transport properties

In the current study, a composite proton exchange membrane (PEM) was prepared by incorporating β-cyclodextrin (β-CD) modified silica nanoparticles (SN-β-CD) into a Nafion matrix. Due to the decoration of β-CD on the SN surface, SN-β-CD possesses excellent compatibility with the Nafion polymer, resulting in a good dispersibility inside the membrane matrix. SN-β-CD brings a better water retention capability for the composite PEM and hence significantly improves the proton conductivity of the composite PEM. Simultaneously, the barrier effect of SN-β-CD which increases the tortuosity of transport channels for bulk methanol leads to an evident reduction in methanol permeability. Herein, a nearly two-order-of-magnitude promotion in membrane selectivity (i.e. the ratio of proton conductivity to membrane permeability) was achieved even under crucial conditions at elevated temperatures or high methanol concentrations.

A multifunctional skin-like sensor based on a 3D printed thermo-responsive hydrogel

An effective and general strategy is developed to prepare a multifunctional and mechanically compliant skin-like sensor by incorporating a 3D printed thermo-responsive hydrogel into a capacitor circuit. The prepared intelligent skin shows a sensitive and stable capacitance–temperature response, and also exhibits very high pressure sensitivity within 1 kPa, allowing it to sense body temperature, gentle finger touches and finger bending motion. This work not only demonstrates that stimuli-responsive hydrogels are promising candidates for artificially intelligent skins, but might also enrich the design of skin-like sensors for future artificial intelligence, wearable devices and human/machine interaction applications.

Preparation of a positively charged nanofiltration membrane based on hydrophilic–hydrophobic transformation of a poly(ionic liquid)

In this work, we report a novel method to prepare a positively charged nanofiltration (NF) membrane by rapid counter-ion exchange of a poly(ionic liquid) (PIL) in aqueous solution, which transforms from being hydrophilic to hydrophobic. A thin PIL layer is deposited on the supporting membrane via a phase separation process induced by an ion exchange reaction along with a self-inhibiting effect, and a series of positively charged NF membranes are obtained. The membrane formation process is mainly dominated by the concentration of the hydrophilic PIL and its counter-ions. In brief, the density of the top layer is predominated both by the PIL and its aqueous counter-ion, and the thickness of the surface layer is mainly determined by the aqueous counter-ion. A streaming potential measurement confirmed that the resultant membrane is positively charged when the pH range is below 11. The pure water flux (PWF) was up to 45.3 L m−2 h−1 under the operating pressure of 0.6 MPa. The rejection to MgCl2 of the membrane reached 84% and decreased in the order of MgCl2, NaCl, MgSO4, and Na2SO4. It also shows a high rejection of about 90% to heavy metallic salts such as CuCl2, NiCl2 and CoCl2. The method based on the hydrophilic–hydrophobic transformation of the PIL provides an alternative way to prepare a charged membrane with high performance.