Macroporous Substrate Research Articles

Developing high-flux dense Janus membrane with robust wetting and scaling resistance: A systematic comparison with Omniphobic membranes

Membrane wetting, scaling and low flux are challenging obstacles for membrane distillation (MD). However, the lack of advanced membranes capable of achieving both high flux and high wetting/scaling resistance constrains MD potential applications. Herein, a dense Janus membrane (JM) was facilely developed by co-depositing a dense PVA/PDA layer on a highly porous PTFE substrate to overcome this dilemma and systematically compared with omniphobic PTFE substrates with varied pore sizes for the first time. Results showed that, unlike flux-resistance “trade-off” observed for PTFE substrates, dense JM effectively resisted 0.4 mM SDS at a high flux of 56.8 LMH and retarded membrane scaling by 20 mM oversaturated gypsum. Robust wetting resistance was primarily attributed to the sieving effect of the sub-nanometer surface layer, which greatly protected substrate from SDS adsorption and feed intrusion, and was less affected by severe surface concentration polarization and relatively low substrate LEP than PTFE substrates. Strongly polar PVA/PDA layer provided a much more thermodynamically unfavorable continuous interface for gypsum adhesion than PTFE substrates, which played a key role in determining anti-scaling behavior. In contrast, gypsum still tended to rapidly cause membrane scaling in stagnant zones within PTFE substrates when flux was increased. This study shall provide new insights in designing high-performance membranes for robust hypersaline wastewater treatment, and highlighting dense JMs with a macroporous substrate and a highly hydrophilic dense surface layer as ideal alternatives.

Building ordered graphene membranes by intermediate layer assisted stacking method for promoted desalination

Laminar separation membranes stacked by graphene and its derivatives hold the merit of controlled mass transfer behaviors. However, precise ion separation is challenged by the disordered assembly of nanosheets on macroporous substrates with complicated surface properties. Herein, we prepare ultrathin ordered graphene membranes by an intermediate layer assisted stacking method. Nanoporous covalent organic framework (COF) nanosheets are first vacuum-filtered on macroporous substrates, followed by deposition and thermal reduction of graphene oxide nanosheets to deliver ultrathin reduced graphene oxide (rGO) layers. The COF-decorated substrates provide a hydrophilic surface with reduced pores and roughness, facilitating the formation of ultrathin rGO layers with ordered stacking patterns and reduced microporous defects. Consequently, the obtained rGO/COF composite membranes can reject ∼90% of Na2SO4, ∼50% of NaCl, and >98% of various organic dyes, which are higher than pure rGO membranes. Meanwhile, the rGO/COF composite membranes can maintain good stability in solutions with different pH values and salt concentrations, and can be long-term operated for at least 10 days. This work is expected to deepen the understanding of the structure of laminar membranes and provides new perspectives to design ordered laminar membranes.

Homoporous membranes of block copolymers: Upscalable preparation by spray coating and performance boosting by quaternization

The remarkable permselectivity of homoporous membranes (HOMEs) confers them with unparalleled advantages in high-precision separation. However, the upscalable fabrication of HOMEs remains a challenging yet crucial task to meet practical demands. Herein, we present a facile and controllable strategy for producing HOMEs through spray coating of block copolymers (BCPs) on macroporous substrates, followed by annealing and selective swelling. By controlling the BCP concentrations and ambient humidity during spray coating, dense BCP layers with adjustable thickness can be formed. The use of suitable pore-filling agents for filling the pores of substrates during spray coating and annealing prevents BCP from penetrating into the substrates. Also, it ensures robust adhesion between the BCP layers and macroporous substrates. Direct spray-coating circumvents the intricate and uncontrollable BCP layer transformation process, thereby enabling the successful fabrication of HOMEs over a large area of 100 cm2. The separation performances of thus-prepared homoporous membranes can be easily regulated by changing the swelling durations. Furthermore, the selective swelling process leads to the enrichment of BCP polarity chains on the surface and pore walls, allowing for further quaternization modification. Consequently, a synchronous promotion in permeability and selectivity of HOMEs is achieved.

Investigation of Gas Diffusion Layer Degradation in Polymer Electrolyte Fuel Cell Via Chemical Oxidation

The gas diffusion layer (GDL) is a key part of the membrane electrolyte assembly and is critical for the transport of fuel, oxygen and water. The GDL is made up of two layers: macroporous substrate (MPS) and microporous layer (MPL) (1). To achieve the hydrophobicity required for water management, the two layers are typically treated with polytetrafluoroethylene (PTFE) (2). In many long-term experiments, it has been observed that GDL degradation, such as carbon corrosion and PTFE loss, leads to a deterioration of water management, conductivity and mass transport (3).To solve this durability problem, a thorough and in-depth understanding of the GDL degradation mechanism is required. In this study, GDLs are subjected to an accelerated stress test to investigate degradation by chemical oxidation. GDLs are soaked in Fenton's reagent for 24 hours. The hydrophobic properties of pristine and soaked GDLs are compared based on contact angle measurements, thermogravimetry and scanning electron microscopy. In addition, the chemical composition of the solutions before and after the immersion test is analyzed using a total organic carbon analyzer and an ion chromatograph. The results show that the hydrophobicity of GDL exposed to Fenton reagent is decreased by 5 and 4 degrees for the MPL and the MPS respectively. Fig 1. depicts a decrease in surface contact angle, which is associated with surface oxidation and PTFE deterioration.AcknowledgementThis research work is performed under the project AlpeDHues (AlpeDHues / FFG 889328) which is supported by the Austrian Research Promotion Agency (FFG).References C. Csoklich, R. Steim, F. Marone, T. Schmidt and F. Büchi, ACS Applied Materials & Interfaces, 13, 9908–9918 (2021).A. Ozden, S. Shahgaldi, X. Li and F. Hamdullahpur, Progress in Energy and Combustion Science, 74, 50–102 (2019).Y. Yang, X. Zhou, B. Li and C. Zhang, Applied Energy, 303, 117688 (2021). Figure 1

Ultrahigh-permeance polyamide thin-film composite membranes enabled by interfacial polymerization on a macro-porous substrate toward organic solvent nanofiltration

Organic solvent nanofiltration (OSN) membranes have become a powerful separation platform in a myriad of chemical and pharmaceutical fields owing to their superior merits in high separation efficiency and low energy consuming. Despite extensive progress in exploiting polyamide thin film composite (TFC) membranes for OSN, they are heavily limited by inferior solvent permeability, especially when using conventional low-porous polyimide as TFC substrate. Herein, we report a facile substrate engineering strategy to leverage macro-porous Nylon66 microfiltration membrane to replace the conventional low-porous polyimide substrates for fabricating TFC membranes by interfacial polymerization, enabling high and robust solvent permeability during OSN. Distinct from conventional polyimide, Nylon66 substrates take advantage of their high porosity, macro-porous size and excellent hydrophilicity to realize more monomers storage and uniform monomers distribution, allowing to create crumpled and defect-free polyamide layers. With this method, the polyamide TFC membranes show a 2.1-fold increase in ethanol permeance (up to 16.0 L h−1 m−2 bar−1) meanwhile possessing 98 % rejection of dye, surpassing the most reported TFC membranes. We also demonstrate that the TFC membranes exhibit excellent long-term stability in both polar and non-polar solvent, as well as hold huge promise in the precise separation of dye mixture, pharmaceutical ingredient, and catalyst.

Support-free interfacial polymerized polyamide membrane on a macroporous substrate to reduce internal concentration polarization and increase water flux in forward osmosis

Macroporous substrates have great potential in the development of high performance thin film composite forward osmosis (TFC FO) membranes. This paper reports on the preparation of TFC FO membrane via support-free interfacial polymerization (SFIP) technology on a macroporous substrate. SFIP technology can effectively break the bottleneck of conventional interfacial polymerization (IP) technology, which makes it difficult to form a complete polyamide (PA) film on the macroporous substrate. Briefly, the complete PA film is formed at the free water-organic phase interface and then attached to the macroporous substrate by vacuum filtration. The monomer concentration and filtration pressure were adjusted to form the optimal SFIP-PA film. Meanwhile, we chose a macroporous polyvinylidene fluoride (PVDF) microfiltration substrate and screened the pore size to reduce the structural parameters and then the internal concentration polarization, which caused the improvement of water flux. The performance of SFIP-PVDF membranes was investigated and discussed. Under the AL-FS mode with DI water and 1.0 M NaCl solution as the feed solution and draw solution, respectively, the SFIP-PVDF membrane, which used a macroporous PVDF substrate with a pore size of 0.45 μm exhibited a water flux (20 L/m2·h) and a reverse salt flux (2.5 g/m2·h). Besides, the membrane showed good stability in the 72 h test. The specific salt flux (Js/Jw) of SFIP-PVDF membranes was much lower than that of IP-PVDF membranes prepared via IP technology, which were incomplete and defective, suggesting that SFIP technology was superior to IP technology. This work effectively exploits the potential of macroporous substrates for the development of high performance TFC FO membranes and provides an insight into the high performance FO membrane design.

Optimal fabrication of carbon paper by different lengths of chopped carbon fibers and its enhanced performance in proton exchange membrane fuel cell

Carbon paper as a macroporous substrate of gas diffusion layer in proton exchange membrane fuel cells directly impacts the output performance of cells. In this study, we present a straightforward strategy to improve the overall performance of carbon paper by mixing short/long (6mm/10 mm) chopped carbon fibers at an optimal concentration of phenolic resin. The results show that incorporating longer carbon fibers can increase the porosity, conductivity, gas flux, and mechanical properties of carbon paper. The membrane electrode assembly achieved a peak power density of 1182.61 mW cm−2 at 60% RH using carbon paper with a long carbon fiber content of 40 wt% and an impregnation concentration of 10 wt%. This outperforms commercially available carbon paper. Based on electrochemical impedance spectroscopy results, it was confirmed that our carbon paper had a lower mass transfer resistance of only 32.17 mΩ cm−2 under conditions of 2 A cm−2 and 60% RH. This was due to its sparser three−dimensional network-like pore structure which was created by mixing different lengths (6mm/10 mm) of carbon fibers. This work provides new insights into preparing high−performance carbon papers.

Investigation of Gas Diffusion Layer Degradation in Polymer Electrolyte Fuel Cell Via Chemical Oxidation

The gas diffusion layer (GDL) enables and influences the internal transport of fuel, oxygen, electricity, heat and water. The GDL is made up of the macroporous substrate and the microporous layer. To achieve the hydrophobicity required for water management, the two layers are typically treated with polytetrafluoroethylene (PTFE). Degradation of GDL, including carbon corrosion and PTFE loss, affects water management, conductivity and mass transport. GDLs were subjected to accelerated stress tests by immersing them in Fenton's reagent for 24 hours. Analysis of hydrophobic properties through contact angle measurements, thermogravimetry, and energy dispersive X-ray spectroscopy indicated that the hydrophobicity of the GDL exposed to Fenton's reagent decreased. This loss of hydrophobicity is associated with surface oxidation and PTFE degradation.

Experimental and numerical study on the two-phase flow inside a cracked gas diffusion layer of PEMFC

The presence of cracks in the gas diffusion layer (GDL) can have a significant impact on the flow of two phases within Proton Exchange Membrane Fuel Cells (PEMFCs). This study focuses on investigating the transport process of two-phases flow in a GDL sample using both experimental and numerical methods. The porous structure of a hydrophobic carbon paper, serving as the macro-porous substrate (MPS) of the GDL, is examined using synchrotron radiation X-ray phase contrast Computed Tomography (CT). A 3-D model with various crack geometries is then created, and a Lattice Boltzmann Method (LBM) simulation is employed to analyze the flow behavior within the GDL model. The findings reveal that the water intrusion mode in the MPS primarily depends on its hydrophobic nature, but the width and depth of the cracks also significantly influence both the rate and path of water intrusion. Increasing the crack width from 7 μm to 28 μm results in a 30% increment in water intrusion rate, while the intrusion path remains unchanged. However, when the crack width reaches 42 μm, the crack itself becomes the primary path for water intrusion, doubling the intrusion rate. Furthermore, an increase in crack depth leads to a higher invasion rate of the liquid phase, consequently increasing the liquid storage volume within the GDL. This research provides a novel approach for analyzing cracked GDLs and demonstrates the potential to optimize the water management process of PEMFCs through GDL microstructure design.

Elucidating the Mass Transportation Behavior of Gas Diffusion Layers via a H2 Limiting Current Test.

The gas diffusion layer (GDL), as a key component of proton exchange membrane fuel cells (PEMFCs), plays a crucial role in PEMFC's polarization performance, particularly in mass transport properties at high current densities. To elucidate the correlation between GDLs' structure and their mass transport properties, a limiting current test with the H2 molecular probe was established and employed to investigate three representative GDLs with and without the microporous layer (MPL). By varying humidity and back pressure, the mass transport resistance of three GDLs was measured in an operating fuel cell, and an elaborate analysis of H2 transport was conducted. The results showed that the transport resistance (RDM) of GDLs was affected by the thickness and pore size distribution of the macroporous substrate (MPS) and the MPL. In the process of gas transport, the smaller pore size and thicker MPL increase the force of gas on the pore wall, resulting in an increase in transmission resistance. Through further calculation and analysis, the total transport resistance can be divided into pressure-related resistance (RP) and pressure-independent resistance (RNP). RP mainly originates from the transport resistance in both MPLs and the substrate layers of GDLs, exhibiting a linear relationship to the pressure; RNP mainly originates from the transport resistance in the MPLs. 29BC with thick MPL shows the largest RNP, and T060 without MPL shows the RNP = 0. This methodology enables in situ measurements of mass transport resistances for gas diffusion media, which can be easily applied for developing and deploying PEMFCs.

A review on gas diffusion layer in proton exchange membrane fuel cell: Materials and manufacturing

A promising alternative to fossil fuel energy is the fuel cell technology. Proton exchange membrane (PEM) fuel cells are widely used in electric vehicle and mobile power generation fields. There are several components in the PEM fuel cell, namely the gas diffusion layer (GDL), PEM, catalyst layer (CL) and bipolar plate (BPP). The GDL is one of the key components of a PEM fuel cell and acts as a distributor of reagents, current conductor, water manager and a CL support. This paper presents an analysis of the materials and fabrication technologies of the GDL in the PEM fuel cell. A summary of the main materials for manufacturing the macroporous substrate (MPS) and microporous layer (MPL) of GDL was presented. Advanced manufacturing methods were described as single layer manufacturing, double layer manufacturing and multilayer manufacturing. A brief overview of the materials and manufacturing processes of BPP and CL were also presented. Future research directions have also been proposed.

Regulation of dopamine forward osmosis membrane via inorganic salt and macro-porous substrates for pharmaceutical concentration

Dopamine (DA) with good designability could self-polymerize into polydopamine (PDA) with various polymerization states in alkaline solution. However, the poor structural controllability of PDA leads to serious PDA hyper-polymers, making DA difficulty to be used as sole aqueous phase monomer during conventional interfacial polymerization (IP) for membrane preparation. In this work, the organic solvent forward osmosis (OSFO) membranes were fabricated via substrate-floating interfacial polymerization (SFIP) method with DA as sole aqueous phase monomer and macro-porous polyether sulfone (PES) membrane with pore size of 0.1–0.22 μm as substrates. Inorganic salt such as NaCl was employed as aqueous phase additive, which not only effectively regulated the oligomeric polymerization state of PDA but also promoted DA diffusion. The macro-porous substrate could enlarge the reaction contact area and relieve the internal concentration polarization in support layer. The resulting OSFO membranes exhibited an excellent ethanol flux of ∼10.70 L m−2 h−1 and lower reverse LiCl flux of ∼0.53 g m−2 h−1, as well as a high tetracycline concentration efficiency of >15 % for 1 h run whatever the tetracycline concentration in the feed (500–2000 mg/L). This work provides new insights into the fabrication of high-performance OSFO membrane for solvent recovery and pharmaceutical concentration.

Novel polyamide forward osmosis membrane prepared via synergistically substrate-floating interfacial polymerization and salt regulation for efficient solvent recovery

Obtaining an ultrathin and high-quality skin layer on a macro-porous substrate is considered as an obstacle for the desalted membrane fabrication, although macro-porous substrate seems nearly ideal for the forward osmosis (FO) membranes. In this work, a novel substrate-floating interfacial polymerization (SFIP) method was proposed to realize the fabrication of a defect-free polyamide skin layer on a macro-porous substrate. Meanwhile, inorganic salt such as NaCl was added into aqueous phase to cause local differences in interface tension during SFIP process, promoting the MPD transfer from the bulk to surface of aqueous solution via Marangoni convection so as to diffuse more MPD molecules. The resulting FO membrane exhibited an excellent ethanol flux (JV)/reverse LiCl flux (JS) value of 277.17 g L−1, which was 2–10 times higher than the data reported in previous literature. Besides, our SFIP method was also employed to fabricate the polyamide skin layer on the hydrophobic PVDF substrate, and found that the hydrophilicity/hydrophobicity of substrate could control the forming position of polyamide skin layer via SFIP method. This work provides a feasible strategy for fabricating high-quality desalted membranes, and demonstrates their potential in organic solvent recovery for energy conservation and emission reduction.

Fabrication of novel thin-film composite membrane based on ultrathin metal-organic framework interlayer for enhancing forward osmosis performance

To improve operation efficiency, an interlayered thin-film composite forward osmosis (iTFC-FO) membrane was designed by introducing an ultrathin and porous interlayer based on aluminum tetra-(4-carboxyphenyl)porphyrin (a stable metal−organic framework nanosheet, Al-MOF). Surface characterization results revealed that Al-MOF spread evenly in the macro-porous substrate, and provided a flat and smooth reaction interface with moderate hydrophilicity and uniform small aperture. The resultant polyamide (PA) layer had a thin base (without intrusion into substrate) and crumpled surface (with abundant leaves). The leaves size and cross-linking degree of PA layer firstly increased and then decreased with the Al-MOF loading. Compared to the original membrane, the iTFC-FO showed an enhanced water permeability and a reduced reverse sodium flux in both modes of active layer facing feed solution (AL-FS) and active layer facing draw solution (AL-DS). To be specific, the specific reverse sodium flux (reverse sodium flux/pure water flux) decreased from 0.27 g/L to 0.04 g/L in the AL-FS mode, while from 1.36 g/L to 0.23 g/L in the AL-DS mode with 2 mol/L NaCl as DS. Moreover, the iTFC-FO maintained high stability and high permeability under high-salinity and contaminated environment. This study offers a new possibility for the rational fabrication of high-performance TFC-FO membranes.

Gas Diffusion Layer for Proton Exchange Membrane Fuel Cells: A Review.

Proton exchange membrane fuel cells (PEMFCs) are an attractive type of fuel cell that have received successful commercialization, benefitted from its unique advantages (including an all solid-state structure, a low operating temperature and low environmental impact). In general, the structure of PEMFCs can be regarded as a sequential stacking of functional layers, among which the gas diffusion layer (GDL) plays an important role in connecting bipolar plates and catalyst layers both physically and electrically, offering a route for gas diffusion and drainage and providing mechanical support to the membrane electrode assemblies. The GDL commonly contains two layers; one is a thick and rigid macroporous substrate (MPS) and the other is a thin microporous layer (MPL), both with special functions. This work provides a brief review on the GDL to explain its structure and functions, summarize recent progress and outline future perspectives.

Large-area homoporous membranes (HOMEs) enabled by multiple annealing

Homoporous membranes (HOMEs) featuring through pores with almost identical apertures, are highly desired for high-precision separations. It remains a major challenge to produce large-area HOMEs because the microstructure control and defect annihilation become increasingly difficult when membrane sizes are dramatically increased. Herein, we propose a multiple annealing strategy to fabricate large-area HOMEs. Polystyrene-block-poly(2-vinyl pyridine) (PS-b-P2VP) films are first coated on smooth and large substrates. Being annealed by chloroform vapor, perpendicularly aligned P2VP cylinders are formed within the films, and further converted into through pores via the mechanism of selective swelling-induced pore generation. It is shown that the conventional single-time annealing is inadequate to create the ordered through pores, because P2VP chains may have no sufficient time to locomote to the right position as required for the ideal hexagonally arranged ordering. By virtue of the multiple annealing, well-defined through pores can be generated after consecutive annealing for three times, as more P2VP chains aggregate into the ideally ordering position. The films are transferred and composited onto macroporous substrates, producing composite membranes with an effective area of up to ∼100 cm2. The resultant membranes exhibit excellent separation performances. Importantly, HOMEs can be fabricated on low-cost substrates via this strategy, highlighting their great promise in real-world applications of high-precision separations.

Structural design of microporous layer to mitigate carbon corrosion in proton exchange membrane fuel cells

Unsteady-state operating conditions of proton exchange membrane fuel cells (PEMFCs) will cause local ultrahigh potential, resulting in carbon corrosion of the gas diffusion layer (GDL). To mitigate performance degradation, this work prepared a novel wavy microporous layer (MPL) by gelatinous MPL slurry on the surface of GDL. By morphology characterization, GDL containing common MPL (com-GDL) exhibited serious structural destruction. In contrast, GDL containing wavy MPL (wavy-GDL) revealed high structural stability. Due to the wavy shape and rough surface, the corrosion region of GDL was transferred from MPL to the macroporous substrate. In the electrochemical testing of the samples, the performance of fresh wavy-GDL was higher than that of fresh com-GDL. After being corroded, the performance of the com-GDL was reduced by more than 20%. While the degradation of the wavy-GDL was less than 10%, realizing effective mitigation of corrosion. In summary, this paper pointed out a novel strategy to improve the durability of PEMFCs. The findings are helpful to promote the carbon corrosion mitigation strategy for other devices at high potential.

Recently emerging advancements in polymeric cryogel nanostructures and biomedical applications

Cryogels are interlinked macroporous substrates or materials which have undergone processing from a monomeric solution below zero temperatures. Due to their exceptional features including biocompatibility, sensitivity, and physical inhibition, cryogels appear as injectable gels, powders, column, monolithic, beads, spheres, and membrane forms and are utilized in biomedical applications. Therefore, this paper elucidates recently emerging trends in cryogels structures for biomedical and pharmaceutical applications including drug delivery, cell transplantation, tissue engineering, therapeutic, immunotherapy, cell transplantation, and so on.

Dual-layered covalent organic framework/MXene membranes with short paths for fast water treatment

MXenes are receiving growing attention in wastewater treatment, but the preparation of ultrathin MXene membranes with short water pathways on macroporous substrates for improving flux is challenging due to the deformation of the flexible MXene nanosheets. Herein, we demonstrate the fabrication of ultrathin MXene membranes by constructing an intermediate porous covalent organic framework (COF) layer. COF nanosheets are first vacuum filtrated on macroporous substrates to form a porous COF layer, followed by the vacuum deposition of MXene nanosheets to produce the dual-layered COF/MXene composite membranes. The COF layer provides a smooth surface with rich pores and good hydrophilicity for depositing MXene nanosheets, leading to the well-assembled MXene layer as thin as 8 nm. The ultrathin MXene layer and the porous COF layer enable the short water transport pathways, and the well-assembled MXene nanosheets guarantee the narrow interlayer nanochannels. Membranes thereby are endowed with high permeance of 563 L/(m2·h·bar) and high rejection to congo red of 99.6%, outperforming other reported COF- or MXene-based membranes. This work suggests a facile construction strategy of ultrathin MXene-based composite membranes with high separation performances, which are highly desired in water treatment.

Rational design of carbon network structure in microporous layer toward enhanced mass transport of proton exchange membrane fuel cell

Microporous layer (MPL) is commonly applied to gas diffusion layer to improve water management performance of a proton exchange membrane fuel cell (PEMFC). In this work, we introduce a new type of MPL comprising whisker-like carbon nanotubes (WCNTs) and Ketjen Blacks (KBs) onto the macroporous substrate to optimize the pore structures. Scanning electron microscopy images show that KBs populate overlapping networks formed by WCNTs, leading to an increased proportion of mesopores (about 34.87%) of crack-free MPL. This optimal structure of MPL with a superhydrophobic surface ensures the maximum power density up to 2.046 W cm−2 at 100% relative humidity. The electrochemical impedance spectroscopy verifies that the mass transfer resistance is significantly lower, which means rapid drain of excess water and supply of reactant gas, and it is further confirmed by theoretical analysis of pore characteristics. A mechanism of gas-water transport balance promoted by the network structures of MPL constructed by WCNTs/KBs is proposed, which allows enhancing PEMFC performance at high humidity conditions.