The escalating atmospheric CO2 concentrations, primarily due to fossil fuel consumption, necessitate a shift towards carbon-neutral energy sources. Hydrogen, a carbon-free energy carrier, emerges as a promising alternative. However, the hydrogen produced often contains impurities like CO2, N2, and CH4, requiring purification before application. Traditional methods such as distillation and pressure swing adsorption are energy-intensive, prompting the exploration of membrane separation as a more efficient alternative. Polymer membranes, however, face limitations due to the trade-off between permeability and selectivity (Robeson's upper bounds). Two-dimensional (2D) nanosheets, with their nanometer-thin thickness, provide a superior platform for membrane construction. Graphitic carbon nitride (g-C3N4) nanosheets, possessing high-density molecular-sized pores, are particularly attractive for molecular sieving membranes. Their nanometer thickness and high-density nanopores suggest superior performance in H2 separation, supported by theoretical investigations showing H2's lowest diffusion barrier through g-C3N4 nanosheets and predicted high H2 permeance. Despite these theoretical advantages, only a few g-C3N4 nanosheet membranes have been reported for gas separation, primarily due to the challenges in obtaining high-quality nanosheets. Top-down methods, commonly used to exfoliate nanosheets from bulk materials, often lead to structural deterioration and defect formation. These defects negatively impact selectivity and hinder the application of g-C3N4 nanosheets in H2 purification. Additionally, the strong π-π interactions between g-C3N4 nanosheets cause re-stacking, blocking the intrinsic in-plane nanopores. Previous attempts to mitigate re-stacking, such as incorporating polymer chains, compromise the inherent advantages of the g-C3N4 nanosheets. This research addresses these limitations by employing a bottom-up synthesis to produce high-quality g-C3N4 nanosheets, thereby avoiding the structural damage associated with top-down methods. The use of isopropanol as a dispersant further weakens π-π interactions, preventing re-stacking and enabling the formation of high-performance lamellar membranes.

The existing literature highlights the potential of 2D materials, especially g-C3N4, for gas separation applications. Theoretical studies using density functional theory (DFT) and molecular dynamics (MD) simulations have predicted high H2 permeance and selectivity for g-C3N4 membranes. However, experimental realization has been hampered by challenges in producing high-quality g-C3N4 nanosheets free from defects. Top-down methods, while producing nanosheets, often introduce defects and compromise the membrane's performance. Studies have shown that top-down exfoliation, using methods such as thermal oxidation or acid treatment, can create large pores unsuitable for H2 separation. Other researchers have attempted to improve membrane performance by using mixed matrix membranes, incorporating polymers or other materials to prevent re-stacking. These approaches, however, often reduce the performance compared to ideal g-C3N4 nanosheets. This review of the literature underscored the need for a bottom-up synthesis strategy to fabricate high-quality, defect-free g-C3N4 nanosheets for constructing efficient H2 purification membranes.

This study employed a bottom-up approach to synthesize high-quality g-C3N4 nanosheets, eliminating the need for exfoliation and minimizing structural damage. The process began with the self-assembly of melamine and cyanuric acid into layered supramolecular precursors under hydrothermal conditions. An ethanol/glycerol mixture was then intercalated to expand the layers. Subsequent calcination under a nitrogen atmosphere facilitated the formation of g-C3N4 through thermal polycondensation, simultaneously leading to the exfoliation of the layered precursors into nanosheets. This bottom-up method was compared with a traditional top-down approach involving the thermal oxidation of melamine. The synthesized g-C3N4 nanosheets were characterized using various techniques, including 13C solid-state NMR, FTIR, XRD, SEM, AFM, EPR, XPS, and EA to assess their structural integrity and defect concentration. The bottom-up synthesized nanosheets exhibited a thickness of approximately 0.5 nm, showing no apparent nanopore defects, unlike the top-down nanosheets which displayed significant defects. The lower EPR signal intensity and the higher C-N=C to N-(C)3 ratio in the bottom-up nanosheets confirmed a lower defect concentration. To fabricate the membranes, the bottom-up g-C3N4 nanosheets were dispersed in isopropanol to weaken π-π interactions and prevent re-stacking. The resulting suspension was then vacuum-filtered onto porous AAO substrates, creating lamellar membranes with a disordered stacking structure. The membrane morphology and structure were analyzed using SEM, TEM, and XRD, revealing a turbostratic arrangement of the nanosheets. DFT calculations were conducted to study the stacking modes of g-C3N4 nanosheets, showing that the bottom-up nanosheets favored AA stacking, promoting gas permeation. Gas separation performance was evaluated using a Wicke-Kallenbach permeation cell, measuring H2 permeance and selectivity for various gas mixtures. The membrane's long-term stability, under harsh conditions such as temperature swings, wet environments, and prolonged operation, was also assessed. DFT and MD simulations were performed to investigate the gas transport mechanism through the g-C3N4 layer, focusing on the interplay between size exclusion and the interactions between gas molecules and the nanosheets. The experimental gas permeance and selectivity values were compared against the state-of-the-art membranes to demonstrate the advancement. The Wicke-Kallenbach permeation cell, a widely used method for membrane characterization, was used to measure gas permeance and selectivity in both single and mixed gas experiments. Different gas mixtures were employed to ascertain the membrane's selectivity for H2 over other gases. The experimental setup allowed precise control of temperature, pressure, and gas flow rates to accurately determine the permeation parameters. The entire testing process included long-term stability tests to assess the durability of the membrane in various operating conditions.

The bottom-up synthesized g-C3N4 nanosheets exhibited significantly fewer defects compared to those produced via the top-down method. The resulting lamellar membranes displayed superior gas separation performance, with a remarkable H2 permeance of 1.3 × 10−7 mol m−2 s−1 Pa−1 and excellent selectivity for various gas mixtures. The H2/CO2 selectivity reached 41, substantially exceeding the Knudsen selectivity. The high H2 permeance was attributed to the synergistic effect of the high-density sieving channels and the disordered stacking structure of the nanosheets. DFT calculations revealed that the bottom-up nanosheets favored AA stacking, which facilitated gas transport through the interlayer pathways. The membrane displayed exceptional stability, maintaining its performance even after prolonged storage (200 days), exposure to wet conditions (3 vol% water vapor for 100 h), and temperature cycles (25–150 °C). The MD simulations supported the experimental observations, indicating rapid H2 permeation and high selectivity due to the size exclusion effect and strong interactions between CO2 and the g-C3N4 nanosheets. The results demonstrate that the gas separation mechanism is based on the combined effects of size exclusion and the interaction between gas molecules and the g-C3N4 nanosheets. Importantly, the performance of the g-C3N4 membrane surpasses the Robeson's upper bound for H2/CO2 separation, setting a new benchmark for gas separation membranes. The impact of pressure on gas permeance and selectivity was also examined, revealing an increase in permeance with pressure but a decrease in selectivity, which is potentially attributed to the presence of non-selective transport pathways at high pressures. This behavior is common in membranes operating under significant pressure differences.

The findings demonstrate the remarkable potential of bottom-up synthesized g-C3N4 nanosheet membranes for high-performance H2 purification. The significantly enhanced H2 permeance and selectivity compared to state-of-the-art membranes highlight the successful implementation of the bottom-up synthesis strategy and the benefits of the disordered lamellar structure. The excellent long-term stability under diverse and challenging conditions further underscores the practical viability of these membranes for industrial applications. The detailed mechanistic investigations through DFT and MD simulations provide a comprehensive understanding of the gas transport processes, confirming the combined effects of size exclusion and gas-nanosheet interactions. These findings have significant implications for advancing carbon neutrality initiatives, providing a viable pathway for efficient and sustainable H2 production and purification.

This study successfully constructed lamellar g-C3N4 nanosheet membranes exhibiting excellent gas separation performance, surpassing existing state-of-the-art membranes. The bottom-up synthesis strategy resulted in high-quality, defect-free nanosheets, leading to enhanced H2 permeance and selectivity. The disordered stacking structure and the synergistic effect of size exclusion and gas-nanosheet interactions contribute to the superior performance. The exceptional stability and durability of the membranes under harsh conditions demonstrate their potential for practical H2 purification applications, making a significant contribution towards achieving carbon neutrality. Future research could focus on exploring different dispersants to further optimize membrane performance and investigating the scalability of the bottom-up synthesis method for industrial production.

While the study demonstrated excellent membrane performance, certain limitations should be considered. The relatively small scale of the membrane fabrication might affect its direct translation to larger-scale industrial applications. Further investigation into the long-term stability under extreme conditions and with other potential impurities besides the ones tested is necessary for comprehensive assessment. Finally, a detailed economic analysis comparing the costs of this method with existing technologies is needed to fully evaluate its practicality in the broader context of hydrogen purification.