Polyimide‐Carbonized Membranes for Gas Separation: Structural, Composition, and Morphological Control of Precursors

Energy Application of Graphene Based Membrane: Hydrogen Separation

Hydrogen gas (H2 ) is a viable energy carrier that has the potential to replace the traditional fossil fuels and contribute to achieving zero net emissions, making it an attractive option for a hydrogen-based society. However, current H2 purification technologies are often limited by high energy consumption, and as a result, there is a growing demand for alternative techniques that offer higher H2 purity and energy efficiency. Membrane separation has emerged as a promising approach for obtaining high-purity H2 gas with low energy consumption. Nevertheless, despite years of development, commercial polymeric membranes have limited performance, prompting researchers to explore alternative materials. In this context, carbon-based membranes, specifically graphene-based nanomaterials, have gained significant attention as potential membrane materials due to their unique properties. In this review, we provide a comprehensive overview of carbon-based membranes for H2 gas separation, fabrication of the membrane, and its characterization, including their advantages and limitations. We also explore the current technological challenges and suggest insights into future research directions, highlighting potential ways to improve graphene-based membranes performance for H2 separations.

Strategy to Chemically Decorate Nanopores of a Carbon Membrane for Filtrating Polyphenolics from Ethanol

This study invents a post-pyrolysis modification approach to render the resulting carbon membrane (CM) competent for organic solvent nanofiltration (OSN). A bitumen coating on a porous stainless-steel disk (PSD) serves as the precursor for the intended carbon membrane (CM), which is attained through pyrolysis in Ar. The bitumen coating casts dual-pore networks in the CM because of the dominant asphaltene constituent in bitumen. The subsequent chemical decoration of CM was pursued through the following protocol: dopamine (DA) was deployed in the nanopores of CM via pressurized infiltration and followed by Tris buffer passes through to trigger in situ conversion of DA to polydopamine (PDA), which was affixed over the pore walls to furnish chemical affinity (termed as CM_PDA). Additionally, the catechol moiety of PDA was arranged to chelate with the Zn²⁺ ion, aiming to trim the -OH anchor (termed as CM_PDA-Zn) to probe the effect of chelate on separation. The three membranes (CM, CM_PDA, and CM_PDA-Zn) were thereafter assessed by the separation of ethanol or isopropanol from phenolics [tannic acid (TA)/tetracycline (TC)]. A significantly improved OSN performance [rejection (%) ↔ permeance (L/(m²·h·bar))] of CM vs CM_PDA was observed: (i) for TA feed, 13% ↔ 85 L/(m²·h·bar) vs 83% ↔ 12 L/(m²·h·bar); and (ii) for TC feed, 20% ↔ 78 L/(m²·h·bar) vs 78% ↔ 12 L/(m²·h·bar). Compared to CM_PDA, CM_PDA-Zn further advances the rejection performance (∼89% for TA and ∼80% for TC) over 50 h separation. They are benchmarked by the latest literature results. The performance enhancements can be attributed to the spreading of PDA or PDA-Zn sites in the dual-pore networks, so that they are able to offer H-bonding and steric blocking roles, a chemicomechanical effect, to seize solute molecules over pore walls. It is this interfacial drag effect that sustains the solute rejection.

Effects on Carbon Molecular Sieve Membrane Properties for a Precursor Polyimide with Simultaneous Flatness and Contortion in the Repeat Unit

Carbon molecular sieve (CMS)-based membrane separation is a promising solution for hydrogen separation due to its great advantages on perm-selectivity, thermal stability, and chemical stability. To prepare high-performance CMS membranes, the molecular structure of polymer precursors and their arrangements should be primarily considered. In this work, a benzimidazole-based 6FDA (2,2'-bis(3,4'-dicarboxyphenyl) hexafluoropropane dianhydride)-type polyimide (PABZ-6FDA-PI) is chosen as precursor to prepare the CMS membrane. Effects of chain flatness and contortion in the polyimide precursor on gas-separation performance of CMS membranes were studied in detail by gas adsorption and permeation experiment. The H₂ permeability of CMS is up to 9500 Barrer and ideal selectivity of gas pairs of H₂ /CH₄ and H₂ /CO₂ is up to 3800 and 13, respectively. The comprehensive performance of hydrogen separation including H₂ /CO₂ , H₂ /N₂ , and H₂ /CH₄ gas pairs is located well above previously reported upper bounds for polymers.

Peanut-Shell-like Porous Carbon from Nitrogen-Containing Poly-N-phenylethanolamine for High-Performance Supercapacitor

An efficient soft-template method is proposed for the synthesis of peanut shell-like porous carbon as high-performance supercapacitor electrode materials. The procedure is based on the pyrolysis and chemical activation processes using N-phenylethanolamine as precursor and KOH as activation agent. In a three-electrode system, the resultant carbon material has a specific capacitance of 356 F g(-1) at 1 A g(-1) and a good stability over 1000 cycles. Besides, at a high current density of 30 A g(-1), it has a specific capacitance of 249 F g(-1) and maintains 96% after 10,000 cycles. In two-electrode cell configuration, it delivers about 21.53 Wh kg(-1) at a current density of 20 A g(-1), which is about 7 times higher than the commercial device (<3 Wh kg(-1)). Both high specific capacitance and excellent cycling stabilities guarantee its utilization in supercapacitors.

Effect of membrane-casting parameters on the microstructure and gas permeation of carbon membranes

A study on the controlled fabrication of carbon membranes by varying membrane-casting parameters, including solvent, drying method and pyrolysis temperature.

Deriving a CO2-permselective carbon membrane from a multilayered matrix of polyion complexes

A multilayered assembly consisting of polyion complexes was developed over porous ceramic as a unique precursor for a carbon membrane (CM). This specific layer was attained through in situ polymerization of N-methylpyrrole (mPy) over a prime coating layer of poly(4-styrenesulfonic acid) (PSSA) with an embedded oxidant on the ceramic surface. Extensive ion-pair complexation between the sulfonic acid groups of PSSA and the tertiary amine groups of the resulting poly(N-methylpyrrole) (PmPy) sustains this assembly layer. Incorporating cetyltrimethylammonium bromide (CTAB) into the PSSA is critical in facilitating the infiltration of mPy into the PSSA layer and promoting interfacial contact between the two polymers. Upon pyrolysis, the precursor coating was collectively converted into a carbon composite matrix. Such copyrolysis restrains the grain sizes of the carbonized PmPy, thereby halting defects in the resultant carbonaceous matrix. The gas separation performances of the CMs obtained at various graphitization temperatures showed that the least graphitized carbon matrix exhibited the best selectivity of CO2/CH4 = 167 with a CO2 permeability of 7.19 Barrer. This specific feature is attributed to both imine and imide pendant groups that function as selective adsorption sites for CO2 in the carbon skeleton.

Influence of Pyrolysis Parameters on the Performance of CMSM

Carbon hollow fiber membranes have been prepared by pyrolysis of a P84/S-PEEK blend. Proximate analysis of the precursor was performed using thermogravimetry (TGA), and a carbon yield of approximately 40% can be obtained. This study aimed at understanding the influence of pyrolysis parameters—end temperature, quenching effect, and soaking time—on the membrane properties. Permeation experiments were performed withN2, He, andCO2. Scanning electron microscopy (SEM) has been done for all carbon hollow fibers. The highest permeances were obtained for the membrane submitted to an end temperature of750°Cand the highest ideal selectivities for an end temperature of700°C. In both cases, the membranes were quenched to room temperature.