The opportunity of membrane technology for hydrogen purification in the power to hydrogen (P2H) roadmap: a review

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.

Monolayer Fullerene Membranes for Hydrogen Separation

Hydrogen separation membranes are a critical component in the emerging hydrogen economy, offering an energy-efficient solution for the purification and production of hydrogen gas. Inspired by the recent discovery of monolayer covalent fullerene networks, here we show from concentration-gradient-driven molecular dynamics that quasi-square-latticed monolayer fullerene membranes provide the best pore size match, a unique funnel-shaped pore, and entropic selectivity. The integration of these attributes renders these membranes promising for separating H2 from larger gases such as CO2 and O2. The ultrathin membranes exhibit excellent hydrogen permeance as well as high selectivity for H2/CO2 and H2/O2 separations, surpassing the 2008 Robeson upper bounds by a large margin. The present work points toward a promising direction of using monolayer fullerene networks as membranes for high-permeance, selective hydrogen separation for processes such as water splitting.

Metal-Induced Aminosilica Rigidity Improves Highly Permeable Microporous Membranes via Different Types of Pendant Precursors

In this study, nickel-doped aminosilica membranes containing pendant groups were prepared with 3-aminopropyltriethoxysilane (APTES), trimethoxy[3-(methylamino)propyl]silane (MAPTS), 3 N,N-dimethyl aminopropyltrimethoxysilane (DAPTMS), N-[3-(trimethoxysilylpropyl]ethylene diamine (TMSPED), and 1-[3-(trimethoxysilyl)propyl] urea (TMSPU). Differences in the structures of terminal amine ligands significantly contributed to the formation of a coordinated structural assembly. Ultraviolet-visible spectroscopy (UV-vis), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), and N₂ adsorption isotherms revealed that short and rigid pendant amino groups successfully coordinated with nickel to produce subnanopores in the membranes, while an ion-exchange interaction was suggested for longer and sterically hindered aminosilica precursors. Moreover, the basicity of amine precursors affected the affinity of ligands for the development of a coordinated network. A pristine aminosilica membrane showed low levels of H₂ permeance that range from 0.1 to 0.5 × 10^-6 mol m^-2 s^-1 Pa^-1 with a H₂/N₂ permeance ratio that ranges from 15 to 100. On the contrary, nickel coordination increased the H₂ permeance to 0.1-3.0 × 10^-6 mol m^-2 s^-1 Pa^-1 with H₂/N₂ permeance ratios that range from 10 to 68, which indicates the formation of a microporous structure and enlargement of pore sizes. The strong level of coordination affinity between nickel ions and amine groups induced rearrangement of the flexible pendant chain into a more rigid structure.

Modelling Sorption and Transport of Gases in Polymeric Membranes across Different Scales: A Review

Professor Giulio C. Sarti has provided outstanding contributions to the modelling of fluid sorption and transport in polymeric materials, with a special eye on industrial applications such as membrane separation, due to his Chemical Engineering background. He was the co-creator of innovative theories such as the Non-Equilibrium Theory for Glassy Polymers (NET-GP), a flexible tool to estimate the solubility of pure and mixed fluids in a wide range of polymers, and of the Standard Transport Model (STM) for estimating membrane permeability and selectivity. In this review, inspired by his rigorous and original approach to representing membrane fundamentals, we provide an overview of the most significant and up-to-date modeling tools available to estimate the main properties governing polymeric membranes in fluid separation, namely solubility and diffusivity. The paper is not meant to be comprehensive, but it focuses on those contributions that are most relevant or that show the potential to be relevant in the future. We do not restrict our view to the field of macroscopic modelling, which was the main playground of professor Sarti, but also devote our attention to Molecular and Multiscale Hierarchical Modeling. This work proposes a critical evaluation of the different approaches considered, along with their limitations and potentiality.

Tailor-Made Modification of Commercial Ceramic Membranes for Environmental and Energy-Oriented Gas Separation Applications

Ceramic membranes have been considered as potential candidates for several gas separation processes of industrial interest, due to their increased thermal and chemical stability compared to polymeric ones. In the present study, commercial Hybrid Silica (HybSi^®) membranes have been evaluated and modified accordingly, to enhance their gas separation performance for targeted applications, including CO₂ removal from flue gas and H₂ recovery from hydrogen-containing natural gas streams. The developed membranes have been characterized before and after modification by relative permeability, single gas permeation, and equimolar separation tests of the respective gas mixtures. The modification procedures, involving in situ chemical vapor deposition and superficial functionalization, aim for precise control of the membranes’ pore size and surface chemistry. High performance membranes have been successfully developed, presenting an increase in H₂/CH₄ permselectivity from 12.8 to 45.6 at 250 °C. Ultimately, the modified HybSi^® membrane exhibits a promising separation performance at 250 °C, and 5 bar feed pressure, obtaining above 92% H₂ purity in the product stream combined with a notable H₂ recovery of 65%, which can be further improved if a vacuum is applied on the permeate side, leading to 94.3% H₂ purity and 69% H₂ recovery.

Gas Permeability, Fractional Free Volume and Molecular Kinetic Diameters: The Effect of Thermal Rearrangement on ortho-hydroxy Polyamide Membranes Loaded with a Porous Polymer Network

Mixed-matrix membranes (MMMs) consisting of an ortho-hydroxy polyamide (HPA) matrix, and variable loads of a porous polymer network (PPN) were thermally treated to induce the transformation of HPA to polybenzoxazole (β-TR-PBO). Two different HPAs were synthesized to be used as a matrix, 6FCl-APAF and tBTpCl-APAF, while the PPN used as a filler was prepared by reacting triptycene and trifluoroacetophenone. The permeability of He, H2, N2, O2, CH4 and CO2 gases through these MMMs are analyzed as a function of the fraction of free volume (FFV) of the membrane and the kinetic diameter of the gas, allowing for the evaluation of the free volume. Thermal rearrangement entails an increase in the FFV. Both before and after thermal rearrangement, the free volume increases with the PPN content very similarly for both polymeric matrices. It is shown that there is a portion of free volume that is inaccessible to permeation (occluded volume), probably due to it being trapped within the filler. In fact, permeability and selectivity change below what could be expected according to densities, when the fraction of occluded volume increases. A higher filler load increases the percentage of inaccessible or trapped free volume, probably due to the increasing agglomeration of the filler. On the other hand, the phenomenon is slightly affected by thermal rearrangement. The fraction of trapped free volume seems to be lower for membranes in which the tBTpCl-APAF is used as a matrix than for those with a 6FCl-APAF matrix, possibly because tBTpCl-APAF could approach the PPN better. The application of an effective medium theory for permeability allowed us to extrapolate for a 100% filler, giving the same value for both thermally rearranged and non-rearranged MMMs. The pure filler could also be extrapolated by assuming the same tendency as in the Robeson's plots for MMMs with low filler content.

Recent Advances in Catalysts and Membranes for MCH Dehydrogenation: A Mini Review

Methylcyclohexane (MCH), one of the liquid organic hydrogen carriers (LOHCs), offers a convenient way to store, transport, and supply hydrogen. Some features of MCH such as its liquid state at ambient temperature and pressure, large hydrogen storage capacity, its well-known catalytic endothermic dehydrogenation reaction and ease at which its dehydrogenated counterpart (toluene) can be hydrogenated back to MCH and make it one of the serious contenders for the development of hydrogen storage and transportation system of the future. In addition to advances on catalysts for MCH dehydrogenation and inorganic membrane for selective and efficient separation of hydrogen, there are increasing research interests on catalytic membrane reactors (CMR) that combine a catalyst and hydrogen separation membrane together in a compact system for improved efficiency because of the shift of the equilibrium dehydrogenation reaction forwarded by the continuous removal of hydrogen from the reaction mixture. Development of efficient CMRs can serve as an important step toward commercially viable hydrogen production systems. The recently demonstrated commercial MCH-TOL based hydrogen storage plant, international transportation network and compact hydrogen producing plants by Chiyoda and some other companies serves as initial successful steps toward the development of full-fledged operation of manufacturing, transportation and storage of zero carbon emission hydrogen in the future. There have been initiatives by industries in the development of compact on-board dehydrogenation plants to fuel hydrogen-powered locomotives. This review mainly focuses on recent advances in different technical aspects of catalytic dehydrogenation of MCH and some significant achievements in the commercial development of MCH-TOL based hydrogen storage, transportation and supply systems, along with the challenges and future prospects.

PEEK-WC-Based Mixed Matrix Membranes Containing Polyimine Cages for Gas Separation

Membrane-based processes are taking a more and more prominent position in the search for sustainable and energy-efficient gas separation applications. It is known that the separation performance of pure polymers may significantly be improved by the dispersion of suitable filler materials in the polymer matrix, to produce so-called mixed matrix membranes. In the present work, four different organic cages were dispersed in the poly(ether ether ketone) with cardo group, PEEK-WC. The m-xylyl imine and furanyl imine-based fillers yielded mechanically robust and selective films after silicone coating. Instead, poor dispersion of p-xylyl imine and diphenyl imine cages did not allow the formation of selective films. The H2, He, O2, N2, CH4, and CO2 pure gas permeability of the neat polymer and the MMMs were measured, and the effect of filler was compared with the maximum limits expected for infinitely permeable and impermeable fillers, according to the Maxwell model. Time lag measurements allowed the calculation of the diffusion coefficient and demonstrated that 20 wt % of furanyl imine cage strongly increased the diffusion coefficient of the bulkier gases and decreased the diffusion selectivity, whereas the m-xylyl imine cage slightly increased the diffusion coefficient and improved the size-selectivity. The performance and properties of the membranes were discussed in relation to their composition and morphology.

Ultrafast H2-selective nanoporous multilayer graphene membrane prepared by confined thermal annealing

H₂ selective dense pores are generated in a graphene oxide (GO) layer by thermal-decomposition of oxygen-functional groups under high pressure. The nanoporous GO membrane shows H₂/CO₂ selectivity of 12.1 and H₂ permeability of 10360 Barrer.