Advances in materials process and separation mechanism of the membrane towards hydrogen separation

The increased demand for a reliable and sustainable renewable energy source encourages the hydrogen-based economy. For the same, membrane separation approaches were reviewed as an advantageous process over contemporary techniques due to the environmentally friendly nature, economically viable pathway, and easily adaptable technology. A comprehensive assessment for the advancements in the type of membranes namely, polymeric and mixed matrix membranes (MMMs) has been delineated in the present article with the fabrication methodologies and associated mechanism for hydrogen separation. In hydrogen separation mechanism of the membrane, depends on the morphology of the membrane (dense or porous). The existence of pores in membranes offers various gas transport mechanisms such as Knudsen diffusion, surface diffusion, capillary condensation, molecular sieving mechanisms were observed, depending on the pore size of membranes and in dense membrane gas transport through the solution-diffusion mechanism. In polymer membrane, hydrogen separation occurs mainly due to solubility and diffusivity of gases. The hydrogen separation mechanism in MMMs is very complex due to the combining effect of polymer and inorganic fillers. So, the gas separation performance of MMMs was evaluated using the modified Maxwell model. Moreover, adequate polymeric material and inorganic fillers have been summarised for MMMs synthesis and highlighting the mechanism for gas transport phenomena in the process. Several types of materials implemented with polymeric matrix examined in the literature, amongst these functionally aligned CNTs with Pd-nanoparticles dispersed in polymer matrix were observed to reveal the best outcome for the hydrogen separation membrane due to the uniform distribution of inorganic material in the matrix. Henceforth, the agglomeration gets reduced promoting hydrogen separation.     

Experimental evaluation of graphene oxide/TiO2-alumina nanocomposite membranes performance for hydrogen separation

In recent years, graphene oxide membranes showed interesting performances in terms of high permeating flux and perm-selectivity in several applications of gas separation because of their inherent properties combined to a low energy consumption. In this paper, a graphene oxide layer is coated on modified TiO₂-alumina tubular substrate in order to prepare graphene oxide nanocomposite membranes useful for hydrogen separation. Nanocomposite graphene oxide membrane samples were obtained by using vacuum deep coating method, depositing the graphene oxide solution as single layers on TiO₂-alumina substrate. Temperature and pressure variations were evaluated to achieve high H₂ permeance, high H₂/CO₂ selectivity and membrane performance stability during the experimental tests. Furthermore, it was found that the temperature increase causes a perm-selectivity (H₂/N₂ and H₂/CO₂) decrease, while the transmembrane pressure increase involves a general improvement of the perm-selectivity.     

Highly enhanced proton conductivity of single-step-functionalized graphene oxide/nafion electrolyte membrane towards improved hydrogen fuel cell performance

Acidic group functionalized graphene oxide (GO) as a filler to the state-of-art Nafion electrolytes are regarded as potential materials towards next-generation fuel cell application. However, the tedious synthesis process for GO functionalization, and aggravated chemical durability at high temperatures demands the scientific community to design suitable Nafion-based functionalized GO electrolytes with superior proton conductivity and power density at actual fuel cell conditions i.e., 80 °C and 100% relative humidity (RH). Herein, a potential single-step-phosphorylated graphene oxide (sPGO) modified Nafion (sPGO/NF) is introduced to simultaneously multifold the proton conductivity, chemical durability, and power density of Nafion. Under actual fuel cell conditions, the sPGO/NF exhibits maximum proton conductivity (0.306 Scm⁻¹) which is 1.7-fold and 1.6-fold higher than that of rNF and GO/NF, respectively. Moreover, sPGO/NF achieves the maximum power density of 0.652 Wcm⁻² (80 °C, 100% RH), much higher than the rNF (0.51 Wcm⁻²) and GO/NF (0.53 Wcm⁻²) at same condition.     

Towards performance improved anion exchange membrane: Cross-linking with multi-cations oligomer modified graphene oxide

Cross-linking structure has been proven to be an effective approach to address the balance issue between ionic conductivity, dimensional stability and other properties of anion exchange membranes (AEMs). Here, a novel multi-cationic oligomer was synthesized from 1,4-diazabicyclo [2,2,2]octane and 1,6-dibromohexane, and subsequently used to prepare multi-cationic oligomer brushes-decorated graphene oxide (QBGO). The obtained QBGO was employed as the cross-linker to form cross-linked poly (arylene ether sulfone) (QPAES) AEMs by end-cap tertiary amine coupling reaction. Benefiting from the introduction of the multi-cations and flexible long-chain cross-linker structure, the cross-linked QPAES/QBGO membranes formed hydrophilic/hydrophobic phase-separation microstructures and well-defined ionic channels which are responsible for water uptake and ion transfer. As a result, the cross-linked QPAES/QBGO-2.0 membrane exhibited 1.90-fold higher ionic conductivity and better chemical stability than the control QPAES membrane. The QPAES/QBGO-2.0 membrane displayed a higher power density of 75.7 mW cm^?2 than that of the control QPAES membrane (53.1 mW cm^?2) in a H₂/O₂ fuel cell test. In a word, we propose that this novel design strategy holds broad prospects for the design of new polymer electrolyte membrane materials.     

GNs/MOF-based mixed matrix membranes for gas separations

NU-1000 and graphene nanosheet (GNs) with different loadings have been used as fillers to prepare mixed matrix membranes (MMMs) with polyethersulfone (PES). The high performance of the MMMs has been successfully fabricated for the evaluation of gas separation at 1 bar and various temperatures (20, 40, 60 °C). The successful fabrication of the MMMs were confirmed by using SEM, FTIR, AFM, and XRD. The crystalline nature of GNs and NU-1000 in the MMMs are evidenced by XRD, which confirms the successful fabrication of the MMMs. In addition, the thermal stability of the MMMs was enhanced with the increase of the GNs. Separation performance of H₂ was superior to CO₂, N₂ and CH₄ separation on the MMMs which is a critical for producing energy. The best gas separation results in terms of both permeability and selectivity were obtained with 0.03% GNs and 10% NU-1000. PG3N membrane presented maximum H₂/CO₂, H₂/N₂ and H₂/CH₄ selectivity of 5, 4.2, 3.3 at 20 C, respectively. With an increase in temperature, the permeability increased, while the selectivity of all the MMMs decreased. The MMMs exhibited excellent gas separation capability, which offers unique opportunities for potential large-scale practical applications.     

Fabrication and characterization of poly(phenylene oxide)/SBA-15/carbon molecule sieve multilayer mixed matrix membrane for gas separation

A novel multilayer mixed matrix membrane (MMM), consisting of poly(phenylene oxide) (PPO), large-pore mesoporous silica molecular sieve zeolite SBA-15, and a carbon molecular sieve (CMS)/Al₂O₃ substrate, was successfully fabricated using the procedure outlined in this paper. The membranes were cast by spin coating and exposed to different gases for the purpose of determining and comparing the permeability and selectivity of PPO/SBA-15 membranes to H₂, CO₂, N₂, and CH₄. PPO/SBA-15/CMS/Al₂O₃ MMMs with different loading weights of zeolite SBA-15 were also studied. This new class of PPO/SBA-15/CMS/Al₂O₃ multilayer MMMs showed higher levels of gas permeability compared to PPO/SBA-15 membranes. The permselectivity of H₂/N₂ and H₂/CH₄ combinations increased remarkably, with values at 38.9 and 50.9, respectively, at 10 wt% zeolite loading. Field emission scanning electron microscopy results showed that the interface between the polymer and the zeolite in MMMs was better at a 10 wt% loading than other loading levels. The increments of the glass transition temperature of MMMs with zeolite confirm that zeolite causes polymer chains to become rigid.     

Preparation of mixed matrix composite membrane for hydrogen purification by incorporating ZIF-8 nanoparticles modified with tannic acid

The incompatibility between nanofillers and polymer, caused by the agglomeration of nanoparticles and their weak interaction with each other, is still a challenge to develop mixed matrix composite membrane. Herein, we introduced the ZIF-8-TA nanoparticles synthesized by in situ hydrophilic modification into the hydrophilic poly(vinylamine) (PVAm) matrix to prepare composite membranes for H₂ purification. The dispersion of ZIF-8 in water was improved by tannic acid modification, and the compatibility between ZIF-8 particles and PVAm matrix was enhanced by chemical crosslinking between the quinone groups in oxidized tannic acid (TA) and the amino groups in PVAm. Moreover, the compatibility between hydrophobic polydimethylsiloxane (PDMS) gutter layer and hydrophilic separation layer was achieved by the adhesion of TA-Fe³⁺ complex to the surface of PDMS layer during membrane preparation. The interlayer hydrophilic modification and the formation of separation layer were accomplished in one step, which simplified the preparation process. The experimental results indicated that when the TA addition used for modification was 0.5 g and the ZIF-8-TA_0.5 content in membrane was 12 wt%, the prepared membrane showed the best separation performance with the CO₂ permeance of 987 GPU and the CO₂/H₂ selectivity of 31, under the feed gas pressure of 0.12 MPa.     

Sorption properties of hydrogen-selective PDMS/zeolite 4A mixed matrix membrane

The present study explores the fundamental science of estimating sorption of gases in membranes comprised of inorganic porous fillers within a polymer matrix with a novel semi-empirical correlation. The sorption properties of H₂, C₃H₈, CO₂ and CH₄ were determined in polydimethylsiloxane (PDMS)/zeolite 4A mixed matrix membranes (MMMs) to assess the viability of these membranes for hydrogen purification and natural gas sweetening. Zeolite filling in MMMs results an increase in solubility over neat PDMS membrane. In addition, incorporation of zeolite 4A to PDMS membrane improved H₂ permeation and H₂/CH₄ selectivity. The results confirmed that zeolite 4A can significantly improve the separation properties of poorly H₂-selective PDMS membrane from 0.7 up to 11 and this overcomes the Robeson upper-bound limitation. This improvement was explained referring the Flory–Huggins interaction parameter within MMMs.     

Synthesis and characterization of transition metal mixed oxide doped graphene embedded durable electrocatalyst for hydrogen evolution reaction

A promising electrocatalyst containing variable percentage of V₂O₅–TiO₂ mixed oxide in graphene oxide support was prepared by embedding the catalyst on Cu substrate through facile electroless Ni–Co–P plating for hydrogen evolution reaction. The solvothermal decomposition method was opted for tuning the crystalline characteristics of prepared material. The optimized mixed oxide was well characterized, active sites centres were identified and explained by X-ray diffraction, high resolution tunnelling electron microscopy, scanning electron microscopy coupled with energy dispersive X-ray and X-ray photon spectroscopy analysis. The structural and electronic characteristics of material was done by fourier transform infrared spectroscopy and the electrochemical behaviour of the prepared material was evaluated by using Tafel plot, electrochemical impedance analysis, linear sweep voltammetry, open circuit analysis and chronoamperometry measurements. The results show the enhanced catalytic activity of Ni–Co–P than pure Ni–P plate, due to synergic effect. Moreover, the prepared mixed oxide incorporated Ni–Co–P plate has a high activity towards HER with low over potential of 101 mV, low Tafel slope of 36 mVdec^?1, high exchange current density of 9.90 × 10^?2 Acm^?2.     

Preheated self-aligned graphene oxide for enhanced room temperature hydrogen storage

This study explored the hydrogen adsorption capacity of self-assembled aligned graphene oxide at room temperature. The characteristics of as-prepared graphene oxide were determined by scanning electron microscopy, Raman spectroscopy, and X-ray diffractometry techniques. Three different temperatures were taken for preheating, i.e., 25, 250, and 400 °C. The maximum adsorption pressure was given to 20 bar, and we evaluated the hydrogen adsorption competency at room temperature (25 ± 2 °C). The maximum hydrogen storage capacity was achieved ~2.5 wt%, which was found for the graphene oxide sample preheated at 400 °C. This hydrogen storage capacity was 67% and 40% more than the graphene oxide samples preheated at 25 and 250 °C, respectively. Such an enhancement of hydrogen storage capacity in the self-aligned graphene oxide samples at room temperature is attributed to reduced interlayer spacing and increased topological defects in preheated graphene oxide samples at 400 °C.     

Graphene oxide modified non-noble metal electrode for alkaline anion exchange membrane water electrolyzers

This paper reports the performance of a graphene oxide modified non noble metal based electrode in alkaline anion exchange water electrolyzer. The electrolytic cell was fabricated using a polystyrene based anion exchange membrane and a ternary alloy electrode of Ni as cathode and oxidized Ni electrode coated with graphene oxide as anode. The electrochemical activity of the graphene oxide modified electrode was higher than the uncoated electrode. The anion exchange membrane water electrolyzer (AEMWE) with the modified electrode gave 50% higher current density at 30 °C with deionised water compared to that of an uncoated electrode at 2 V. Performance was found to increase with increase in temperature and with the use of alkaline solutions. The results of the solid state water electrolysis cell are promising method of producing low cost hydrogen.     

A poly (ethylene oxide)/graphene oxide electrolyte membrane for low temperature polymer fuel cells

A novel method to prepare poly (ethylene oxide)/graphene oxide (PEO/GO) composite membrane aimed for the low temperature polymer electrolyte membrane fuel cells without any chemical modification is presented in this work. The membrane thickness is 80 μm with a GO content of 0.5 wt%. And SEM images show the PEO/GO membrane is condensed composite material without structure defects. Small angle XRD results for the membrane samples show that the d-spacing reflection (0 0 1) of GO in PEO matrix is shifted from 2θ = 11° to 4.5° as the PEO molecules intercalated into the GO layers during the membrane preparation process. FTIR tests show the typical -COOH vibration near 1700 cm⁻¹. Tensile tests show the resultant PEO/GO membrane tensile strength of 52.22 MPa and Young's modulus 3.21 GPa, and the fractured elongation was about 5%. The ionic conductivity of this PEO/GO membrane increases from 0.086 to 0.134 S cm⁻¹ when the operation temperature increases from 25 to 60 °C with 100% relative humidity. And further tests show the DC electronic resistance of this membrane is higher than 20 MΩ at room temperature with 100% relative humidity. Polarization curves in a single cell with this membrane give a maximum power density of 53 mW cm⁻² at the operation temperature around 60 °C, without optimizing the catalyst layer composition.     

Hydrogen separation and purification with poly (4-methyl-1-pentyne)/MIL 53 mixed matrix membrane based on reverse selectivity

The effect of MIL 53 (Al) metal organic framework on gas transport properties of poly (4-methyl-1-pentyne) (PMP) was determined based on reverse selectivity. Mixed matrix membranes (MMMs) were fabricated considering various weight percent of MIL 53 particles. The reverse MMMs permselectivities were evaluated through measurement of pure CO₂ and H₂ permeation together with calculation of CO₂/H₂ selectivity. The PMP/MIL 53 (Al) MMMs exhibited privileged CO₂/H₂ permselectivity in comparison with the neat PMP. In addition, CO₂ solubility coefficient was significantly increased with increasing the MIL 53 loading, while the H₂ solubility coefficient was almost remained unchanged. Moreover with increasing the feed pressure the permeability of CO₂ and CO₂/H₂ selectivity were dramatically enhanced, especially at higher filler loadings. Therefore, it was observed that the reverse selectivity of MMMs was enhanced so that the Robeson upper bound was overcome. The best yielding membranes (PMP/30 wt.% MIL 53) represented the CO₂ permeability and CO₂/H₂ selectivity of 377.24 barrer and 24.91 for pure gas experiments respectively.     

Selective storage and evolution of hydrogen on nafion/NaCl/graphene quantum dot mixed matrix using tensammetry as power electrochemical technique

Hydrogen storage/evolution behavior of nafion/NaCl/graphene quantum dot (GQD) mixed matrix as selective hydrogen capacitor (power source) was evaluated in detail through an electrochemical process at two independent potential ranges. For this purpose, a three-electrode system included Pt disk as counter electrode, Ag/AgCl as reference electrode and GQD-based mixed matrix-modified Pt disk as working electrode. For hydrogen storage, the deposition potential and time were evaluated to ?1.0 V (vs. Ag/AgCl) and 120 s, respectively under high basic solution generated using NaOH (1.0 M) solution, followed by evolution of hydrogen at +0.8 V (vs. Ag/AgCl) during formation of hydrogen bubbles. The main advantage of this system was the occurrence of hydrogen storage and evolution at two independent potential windows. Both mass transfer and adsorption processes were estimated for the tensammetric peak during the evolution step. The mechanism of hydrogen storage and evolution was obeyed from diffusion and tensammetry, respectively. According to Randles–Sevcik equation using 1.0 mM ${{\text{Fe}\left(\text{CN}\right)}_6}^{3?/4?}$, the active surface area of nafion/NaCl/GQD mixed matrix was ～1906 m²g^?1. Based on the CHN analyses, pressure-concentration temperature as well as hydrogen temperature-programmed desorption, the capacity of the synthesized GQDs for hydrogen storage and evolution was estimated to at least 10.1 and 8.6 wt%, respectively. The stability of the electrode was also estimated during 7000s by chronoamperometry during applying at least 40 cycles in the range from ?1.0 to +1.3 V with reproducible tensammetric peak current (relative standard deviation: 2.54%).     

Tungsten-molybdenum oxide nanowires/reduced graphene oxide nanocomposite with enhanced and durable performance for electrocatalytic hydrogen evolution reaction

Hydrogen has attracted huge interest globally as a durable, environmentally safe and renewable fuel. Electrocatalytic hydrogen evolution reaction (HER) is one of the most promising methods for large scale hydrogen production, but the high cost of Pt-based materials which exhibit the highest activity for HER forced researchers to find alternative electro-catalyst. In this study, we report noble metal free a 3D hybrid composite of tungsten-molybdenum oxide and reduced graphene oxide (GO) prepared by a simple one step hydrothermal method for HER. Benefitting from the synergistic effect between tungsten-molybdenum oxide nanowires and reduced graphene oxide, the obtained W-Mo-O/rGO nanocomposite showed excellent electro-catalytic activity for HER with onset potential 50 mV, a Tafel slope of 46 mV decade^?1 and a large cathodic current, while the tungsten-molybdenum oxide nanowires itself is not as efficient HER catalyst. Additionally, W-Mo-O/rGO composite also demonstrated good durability up to 2000 cycles in acidic medium. The enhanced and durable hydrogen evolution reaction activity stemmed from the synergistic effect broadens noble metal free catalysts for HER and provides an insight into the design and synthesis of low-cost and environment friendly catalysts in electrochemical hydrogen production.     

Gas sorption in H2-selective mixed matrix membranes: Experimental and neural network modeling

Robust artificial neural network (ANN) was developed to forecast sorption of gases in membranes comprised of porous nanoparticles dispersed homogenously within polymer matrix. The main purpose of this study was to predict sorption of light gases (H₂, CH₄, CO₂) within mixed matrix membranes (MMMs) as function of critical temperature, nanoparticles loading and upstream pressure. Collected data were distributed into three portions of training (70%), validation (19%), and testing (11%). The optimum network structure was determined by trial-error method (4:6:2:1) and was applied for modeling the gas sorption. The prediction results were remarkably agreed with the experimental data with MSE of 0.00005 and correlation coefficient of 0.9994.     

Experimental investigation of thermal performance of the graphene oxide–coated plates

This paper experimentally investigates the conductive heat transfer of samples with different materials and coatings. A range of graphene oxide nanoparticle concentration has been employed. Results demonstrate that utilizing nanoparticles leads to enhancement of conductive heat transfer by 10.07% and 8.01% for EK2 and ST14 samples, respectively. The aforementioned nanoparticles also reduce coating thickness and yield an enhanced quality of the surface, in terms of mechanical properties. The convective and radiative methods of heat transfer have been ignored in this study.     

Agent-free synthesis of graphene oxide/transition metal oxide composites and its application for hydrogen storage

Graphene oxide (GO) wrapped transition metal oxide composite materials were synthesized by a very simple route without any additional agents and the hydrogen adsorption properties of the materials were investigated. The morphologies of GO/V₂O₅ and GO/TiO₂ were examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results show that single- or few-layered GO sheets wrapped throughout the V₂O₅ and TiO₂ particles. According to X-ray photoelectron spectroscopy (XPS), the C–OH species of GO and the surface-adsorbed oxygen of the transition metal oxide bond together via a dehydration reaction. The wrapping phenomenon of GO causes the enhancement of hydrogen storage capacity at liquid nitrogen temperature (77 K) compared with those of the pristine transition metal oxides and GO. The enhancement of hydrogen storage capacity of GO-wrapped transition metal oxide composite materials results from the existence of interspaces between the transition metal oxide particles and the thin GO layers.     

Hydrogen separation and purification using crosslinkable PDMS/zeolite A nanoparticles mixed matrix membranes

The transport properties of gases in polydimethylsiloxane (PDMS)/zeolite A mixed matrix membranes (MMMs) were determined based on pure gas permeation experiments. MMMs were prepared by incorporating zeolite 4A nanoparticles into a PDMS matrix using a new procedure. The permeation rates of C₃H₈, CH₄, CO₂, and H₂ were evaluated through a dense homogeneous pure PDMS membrane and PDMS/4A MMMs to assess the viability of these membranes for natural gas sweetening and hydrogen purification. SEM investigations showed good adhesion of the polymer to the zeolite in MMMs. Permeation performance of the membranes was also investigated using a laboratory-scale gas separation apparatus and effects of feed pressure, zeolite loading and pore size of zeolite on the gas separation performance of the MMMs were evaluated. The MMMs exhibited both higher selectivity of H₂/CH₄ and H₂ permeability as compared with the neat PDMS membrane, suggesting that these membranes are very promising for gas separations such as H₂/CH₄ separation.     

Efficient room temperature hydrogen gas sensing based on graphene oxide and decorated porous silicon

In this study, a triple system for a hydrogen gas sensor was fabricated using graphene oxide, palladium nanoparticles, and porous silicon as a substrate. The fabricated sample was investigated by energy dispersive X-ray spectroscopy, field emission scanning electron microscopy, and Raman spectroscopy. Field emission scanning electron microscopy images displayed a relatively uniform distribution of Palladium nanoparticles over porous silicon. In addition, it was observed that the graphene oxide nanosheets accumulated over the Palladium nanoparticles. Hydrogen-sensing measurements demonstrated that the fabricated system can even detect hydrogen at 200 ppm and 15 °C. The formation of palladium hydride was the main mechanism for detection. In fact, this structure caused a change in resistance through the creation of new electron pathways. Furthermore, the H₂ concentration showed a linear function to the reciprocal of the response time; this suggests that the sensing kinetics of the sample depends on the atomization of hydrogen molecules, which occurs via Pd nanoparticles. Moreover, the fabricated sample displayed significant selectivity for hydrogen gas compared to other examined gases.