Optimization of the pore structure of nickel/graphite hybrid materials for hydrogen storage

Nickel/graphite hybrid materials were prepared by mixed acid treatment of graphite flakes, following metal nanoparticle deposition. The textural properties were studied by BET surface area measurement and t-plot methods with N₂/77 K adsorption isotherms. The hydrogen storage characteristics of the nickel/graphite at 298 K and 10 MPa were studied using a pressure-composition-temperature apparatus. The pore structure of the materials was studied as a function of processing conditions. In the optimum material, the hydrogen storage capacity was as high as 4.48 wt.%. The total amount of storage was not proportional to the specific surface area or metal content of the adsorbate. A dipole-induced model on nickel/carbon surfaces is proposed for the hydrogen storage mechanism.     

Hydrogen sorption in transition metal modified mesoporous materials

Ni, Rh and Pd incorporated mesoporous MCM-41, MCM-48, HMS and SBA-15 samples were synthesized and were characterized using XRD, ICP/EDX and N₂ adsorption–desorption at 77.4 K. The hydrogen adsorption studies in the synthesized materials were performed at 77.4 K (up to 112 kPa) and 303 K (up to 4000 kPa). The hydrogen adsorption isotherms of pristine and transition metals incorporated mesoporous materials at 77.4 K were completely reversible reflecting physisorption of hydrogen in these materials. The hydrogen adsorption isotherms at 303 K were not reversible showing the chemisorption of hydrogen in these materials at 303 K. Hydrogen sorption studies showed that transition metal modification improved the hydrogen storage capacity of mesoporous materials at 303 K. The desorption of the adsorbed hydrogen by heating up to 500 K from the mesoporous materials were also carried out for studying the recovery of adsorbed hydrogen from transition metal incorporated mesoporous materials.     

Carbon materials for H2 storage

In this work a series of carbons with different structural and textural properties were characterised and evaluated for their application in hydrogen storage. The materials used were different types of commercial carbons: carbon fibers, carbon cloths, nanotubes, superactivated carbons, and synthetic carbons (carbon nanospheres and carbon xerogels). Their textural properties (i.e., surface area, pore size distribution, etc.) were related to their hydrogen adsorption capacities. These H₂ storage capacities were evaluated by various methods (i.e., volumetric and gravimetric) at different temperatures and pressures. The differences between both methods at various operating conditions were evaluated and related to the textural properties of the carbon-based adsorbents. The results showed that temperature has a greater influence on the storage capacity of carbons than pressure. Furthermore, hydrogen storage capacity seems to be proportional to surface area, especially at 77 K. The micropore size distribution and the presence of narrow micropores also notably influence the H₂ storage capacity of carbons. In contrast, morphological or structural characteristics have no influence on gravimetric storage capacity. If synthetic materials are used, the textural properties of carbon materials can be tailored for hydrogen storage. However, a larger pore volume would be needed in order to increase storage capacity. It seems very difficult approach to attain the DOE and EU targets only by physical adsorption on carbon materials. Chemical modification of carbons would seem to be a promising alternative approach in order to increase the capacities.     

In situ deposition of Pd nanoparticles with controllable diameters in hollow carbon spheres for hydrogen storage

A novel in situ synthesis of Pd nanoparticles supported in hollow carbon spheres (HCS) is reported. The size of the nanoparticles can be tuned via application of different Pd precursors. The hydrogen storage properties of Pd supported in HCS under room temperature were examined at partial pressures. We observed significant difference between the storage capacities of two samples containing Pd nanoparticles with different diameter distributions. The results showed that the sample with suitable diameters of Pd nanoparticles was more favorable for the H₂ storage, even lower mass of Pd was used. The maximum hydrogen storage of 0.36 wt % exhibited the sample with Pd nanoparticles with the diameter of 11 nm (measured at 298 K and 24 bar) and it was enhanced by the factor of two in respect to the pristine HCS. The enhanced storage capacity is due to cumulative hydrogen adsorption by HCS and Pd nanoparticles. We also propose the mechanism of hydrogen storage in our material.     

Effect of temperature on activated carbon nanotubes for hydrogen storage behaviors

In this work, activated multi-walled carbon nanotubes (Acti-MWNTs) with well-developed pore structures, a highly specific surface area, and higher hydrogen adsorption capacities due to CO₂ activation were prepared. The activation was performed at activation temperatures in the range of 500–1100 °C. The microstructure and crystallinity of the Acti-MWNTs were evaluated with a transmission electron microscope (TEM) and an FT-Raman spectrometer, respectively. The textural properties of the Acti-MWNTs were investigated by using a nitrogen gas sorption analyzer at 77 K. The hydrogen storage capacities of the Acti-MWNTs were investigated by BEL-HP at 298 K/100 bar. The hydrogen storage capacities of the Acti-MWNTs were enhanced to 0.78 wt.% by increasing activation temperatures to 900 °C, which resulted in the formation of a defective structure in the Acti-MWNTs. This result indicated that the CO₂ activation was one of the most effective methods to develop the textural properties, as well as to enhance the hydrogen storage capacities of MWNTs.     

Scale-up activation of carbon fibres for hydrogen storage

In a previous study, we investigated, at a laboratory scale, the chemical activation of two different carbon fibres (CF), their porosity characterization, and their optimization for hydrogen storage [1]. In the present work, this study is extended to: (i) a larger range of KOH activated carbon fibres, (ii) a larger range of hydrogen adsorption measurements at different temperatures and pressures (i.e. at room temperature, up to 20 MPa, and at 77 K, up to 4 MPa), and (iii) a scaling-up activation approach in which the obtained activated carbon fibres (ACF) are compared with those from laboratory-scale activation. The prepared samples cover a large range of porosities, which is found to govern their ability for hydrogen adsorption. The hydrogen uptake capacities of all the prepared samples have been analysed both in volumetric and in gravimetric bases. Thus, maximum adsorption capacities of around 5 wt% are obtained at 77 K, and 1.1 wt% at room temperature, respectively. The packing densities of the materials have been measured, turning out to play an important role in order to estimate the total storage capacity of a tank volume. Maximum values of 17.4 g l⁻¹ at 298 K, and 38.6 g l⁻¹ at 77 K were obtained.     

Investigation of hydrogen storage capacity of various carbon materials

Hydrogen storage capacity of various carbon materials, including activated carbon (AC), single-walled carbon nanohorn, single-walled carbon nanotubes, and graphitic carbon nanofibers, was investigated at 303 and 77 K, respectively. The results showed that hydrogen storage capacity of carbon materials was less than 1 wt% at 303 K, and a super activated carbon, Maxsorb, had the highest capacity (0.67 wt%). By lowering adsorption temperature to 77 K, hydrogen storage capacity of carbon materials increased significantly and Maxsorb could store a large amount of hydrogen (5.7 wt%) at a relatively low pressure of 3 MPa. Hydrogen storage capacity of carbon materials was proportional to their specific surface area and the volume of micropores, and the narrow micropores was preferred to adsorption of hydrogen, indicating that all carbon materials adsorbed hydrogen gas through physical adsorption on the surface.     

Hydrogen storage in cobalt-embedded ordered mesoporous carbon

Ordered mesoporous carbons (OMCs) were synthesized by using ordered mesoporous silica as a template, and chitosan as carbon precursors. A novel process of pre-impregnation is proposed to prepare cobalt-embedded OMC. This process is based on using cobalt chelated chitosan as carbon precursor. The surface functional groups and metal contents were determined by X-ray photoelectron spectroscopy. The bulk cobalt contents in the cobalt-embedded OMCs were measured by an atomic absorption spectrometer. The morphology of the OMCs was observed by small angle X-ray scattering analysis and transmission electron microscope. The OMC texture characteristics were determined by using nitrogen adsorption analysis. Hydrogen capacities of the OMCs were obtained by a volumetric method. The cobalt-embedded OMCs possess obviously higher hydrogen adsorption capacity than that of pure OMC. At 298 K and under 5.5 MPa, the hydrogen capacities of the OMC and OMC–Co-5 are 0.2 and 0.45 wt%, respectively. The H₂/Co ratio of the hydrogen adsorbed on the OMC–Co-5 is 1.54 indicating a Kubas-type interaction between Co and H₂. In addition, the hydrogen spillover effect might occur in parallel.     

A simple method for the production of highly ordered porous carbon materials with increased hydrogen uptake capacities

The synthesis, characterization and hydrogen uptake of porous carbons generated by heat treatment was investigated using various zeolites and mesoporous silicas as hard templates. The effect of heat treatment on the structural order, textural properties and hydrogen uptake capacities of porous carbons templated from the model zeolite EMC-2 in a temperature range of 600–800 °C during chemical vapour deposition were studied in details. The heat treatment improved the structural order of replicated microporous carbons, significantly increased both total and microporous surface area and pore volume, and remarkably increased the hydrogen uptake capacity. The optimized heat treatment conditions were at 900 °C for 3 h. The heat treatment at high temperatures was found to be a simple and general approach to synthesize well-ordered microporous carbons from different zeolite templates, using various carbon precursors and through different synthesis methods. The microporous carbons possessed a high surface area and pore volume with increased microporosity and therefore exhibited improved hydrogen storage capacities up to 5.85 wt% at 20 bar and −196 °C. The heat treatment, however, has no obvious effect on the textural properties and hydrogen uptake capacities for mesoporous carbons templated from mesoporous silicas.     

Synthesis of boron and nitrogen co-doped carbon nanotubes and their application in hydrogen storage

Boron and nitrogen codoped carbon nanotubes (B,N-CNTs) were synthesized by floating catalyst chemical vapor deposition (FCCVD) using ethanol, ferrocene, boric acid and imidazole as carbon source, catalyst, boron and nitrogen precursors, respectively. The samples were analyzed using transmission electron microscopy, Raman spectroscopy, thermogravimetric analysis and X-ray photoemission spectroscopy. 1.5 at% B and 1.34 at% N could be doped in the resultant structure, which has higher length (few μm) with higher thermal stability (621 °C). At pressure 16 bar, hydrogen adsorption for B,N-CNTs was found to be 1.96 and 0.35 wt% at 77 K and 303 K, respectively. Hydrogen storage as function of time was also reported for both the cases. The adsorption process follow pseudo second order kinetics. The present study reveals that the codoping of CNTs aid in tuning properties of CNTs for hydrogen storage application.     

Hydrogen storage ability of porous carbon material fabricated from coffee bean wastes

The hydrogen storage ability at 298 and 77 K of porous carbon materials with microporous structures fabricated from coffee bean wastes through KOH activation was investigated regarding pore structure. The dependence of hydrogen storage ability on the pore structure of porous carbon materials was investigated at 298 and 77 K to clarify the storage mechanism of carbon materials. Hydrogen storage ability at 298 K was increased linearly with increasing of specific surface area increasing. The maximum amount of stored hydrogen was 0.6 wt.% on porous carbon material with 2070 m²/g specific surface area. The hydrogen storage ability at 77 K was 4.0 wt.% on the same sample. The hydrogen storage ability showed a linear relationship with the micro-pore volume size. These changes in the dependence of the hydrogen storage ability on pore size suggested that the storage configuration changed from two- to three-dimensional. The stored hydrogen densities in porous carbon materials calculated from these values were 5.7 and 69.6 mg/cm³ at 298 and 77 K, respectively. The change in density indicated that the state of stored hydrogen in porous carbon materials was filled up aggregational state, which is extremely close to the liquid state, and suggested the realizing of high hydrogen storage ability on carbon materials fabricated from agricultural waste.     

Novel metal graphene framework (MGF) structures for hydrogen storage

Hierarchical novel metal graphene framework (MGF) structures designed, constructed and used for hydrogen storage by adsorption. Three different amounts of pillars used for connecting carboxylate functionalized graphene layers. The three-dimensional atomistic models optimized and then the aqua molecules removed which are coordinated to metal between graphene layers as bridge. Hydrogen storage abilities of aqua free models also investigated. The aqua existence made the structures more effective to store hydrogen according to simulation results. The highest storage capacity calculated for 181C.Aq named structure that includes aqua molecules inside could store 7.745 wt percentage hydrogen at 77 K and 100 bars pressure conditions. Hydrogen storage capacity decreases almost to half by removing aqua molecules for same structure. It is clear the coordinated aqua existence around the bridging atoms provides the spaces between graphene layers. In other word, the spaces between the graphene layers collapse by aqua removal so the hydrogen storage capacities decrease dramatically. Finally, spacing between graphene layers by bridges makes them very effective for hydrogen storage.     

Auto-combustion synthesis,structural analysis,and electrochemical solid-state hydrogen storage performance of strontium cobalt oxide nanostructures

Hydrogen storage in transition mixed metal oxides (MMOs) are predicted from their tendency for adsorption-desorption hydrogen. Hydrogen itself requires initial forces pressure for initiation of condensation. MMOs, based on their effective immobilization matrices, are potential nanocatalysts for energy storage. Even various materials are highlighted for hydrogen storage; however, their adsorption capacities are insufficient for real applications. Here we report, for the first time, a novel hydrogen storage MMOs (Sr₂Co₉O₁₄ nanoparticles) potential for physical hydrogen sorption, containing a redox species. This polycrystalline nanoparticle is prepared via a combustion method in the presence of various fuels like glucose, fructose, sucrose, lactose, and maltose. The glucose supports the pure and homogenous formation of Sr₂Co₉O₁₄ nanoparticles consisting the particles less than 100 nm. Interestingly, a maximum discharge capacity of around 950 mA h/g at room temperature has recorded; emphasizing Sr₂Co₉O₁₄ nanoparticles is a potential substrate for hydrogen storage.     

Investigation of hydrogen storage capacity of multi-walled carbon nanotubes deposited with Pd or V

This work presents the synthesis and characterization of multi-walled carbon nanotubes (multi-walled CNTs) deposited with Pd or V and their hydrogen storage capacity measured by Sievert's volumetric apparatus. The CNTs were grown by the CVD method using LPG and LaNi₅ as the carbon source and catalyst, respectively. Pd was impregnated on the CNTs by the reflux method with hydrogen gas as a reducing agent, while V was embedded on the CNTs by the vapor deposition method. The average metal particle size deposited on the CNTs was around 5.8 nm for Pd and 3.6 nm for V. Hydrogen adsorption experiments were performed at room temperature and at −196 °C under a hydrogen pressure of 65 bar. At −196 °C, the treated CNTs had a maximum hydrogen uptake of 1.21 wt%, while the CNTs deposited with Pd (Pd-CNTs) and CNTs deposited with V (V-CNTs) possessed lower surface areas, inducing lower hydrogen adsorption capacities of 0.37 and 0.4 wt%, respectively. For hydrogen sorption at room temperature, the CNTs decorated with the metal nanoparticles had a higher hydrogen uptake compared to the treated CNTs. Hydrogen adsorption capacity was 0.125 and 0.1 wt% for the Pd-CNTs and V-CNTs, respectively, while the hydrogen uptake of the treated CNTs was <0.01 wt%. For the second cycle, only half of the first hydrogen uptake was obtained, and this was attributed to the re-crystallization of the defect sites on the carbon substrate after the first hydrogen desorption.     

Molecular simulation on hydrogen storage properties of five novel covalent organic frameworks with the higher valency

With the methods of density functional theory (DFT) and molecular simulations, we have investigated the structural characteristics and hydrogen storage properties of five new reported boron-phosphorus cube based covalent organic frameworks (BP-COFs) with the higher valency. The structural parameters of five BP-COFs were researched by the numeric Monte Carlo (NMC) method, and the hydrogen adsorption properties were studied with grand canonical Monte Carlo (GCMC) simulations under the pressure of 0.1 bar–100 bar at both 77 K and 298 K. The results reveal that BP-COF-4 and BP-COF-5 possess the higher hydrogen adsorption capacities than BP-COF-1 to BP-COF-3 at both 77 K and 298 K. The possible methods to improve the H₂ adsorption properties of five BP-COFs are also proposed. We hope this study may provide some reference and inspiration for exploring new COFs with the higher valency as high-performance hydrogen storage materials in future.     

Catalyzed hydrogen spillover for hydrogen storage on microporous organic polymers

Catalyzed hydrogen spillover for hydrogen storage on microporous organic materials has been studied in this work. The method, i.e. “preparation of Pt nanoparticle first and then in situ formation of microporous materials” has been developed for the synthesis of microporous hypercrosslinked polymers with highly dispersed Pt nanoparticles. Hydrogen adsorption isotherms are measured at 77.3 K and up to 1.13 bar, and 298.15 K and up to 19 bar. By containing 2 wt % Pt nanoparticles, the hydrogen storage capacity of hypercrosslinked polymers is enhanced to 0.21 wt % at 298.15 K and 19 bar. Compared to the similar materials without Pt nanoparticles, the H₂ adsorption amount has been enhanced by a factor of 1.75.     

Effect of in-situ boron doping on hydrogen adsorption properties of carbon nanotubes

Floating catalyst chemical vapor deposition method was used for the synthesis of boron doped carbon nanotubes (BCNTs) using ethanol, triethyl borate and ferrocene as carbon source, boron source and catalyst precursor, respectively. The synthesized BCNTs were characterized by transmission electron microscopy, Raman spectroscopy, thermogravimetric analysis and X-ray photoelectron spectroscopy (XPS). The hydrogen adsorption activity was studied for BCNTs along with undoped single walled and multi walled carbon nanotubes. Significant enhancement in the hydrogen storage value was found in doped CNTs as compared to the other undoped CNTs. Hydrogen storage for BCNTs was found to be 2.5 wt% at 10 bar and 77 K. In-situ doped BCNTs gives higher hydrogen adsorption as compared to ex-situ doped BCNTs. The Langmuir adsorption isotherm was found to be suitable for describing the adsorption isotherm as compared with Freundlich isotherm. Maximum adsorption capacity was about 9.8 wt% at 77 K. Pseudo second order kinetics was followed by BCNTs for hydrogen adsorption.     

Static and dynamic hydrogen adsorption on Pt/AC and MOF-5

Hydrogen adsorption has been studied by static and dynamic methods on activated carbon (AC), platinum/activated carbon (Pt/AC), metal organic frameworks (MOF-5), and Pt/AC_MOF-5.The static method showed that all of adsorbents used in this study exhibited a Langmuir (type I) adsorption isotherm at 77 K and a linear function of hydrogen partial pressure at 298 K. The dynamic method produced breakthrough curves, indicating (i) slow rate of hydrogen diffusion in the densely packed activated carbon and Pt/AC beds and (ii) high rate of hydrogen diffusion in the loosely packed bed with large MOF-5 crystallites. Temperature variable adsorption resulted in the higher hydrogen uptake on Pt/AC than other adsorbents. The results suggested that temperature variable adsorption enhanced the hydrogen storage process by (i) initiating hydrogen dissociation at high temperature and (ii) facilitating spillover at low temperature on Pt/AC.     

Improved hydrogen adsorption of ZnO doped multi-walled carbon nanotubes

Hydrogen storage is still one of the most important problems to improve hydrogen energy usage widespread. New materials capable of storing hydrogen with high efficiency must be introduced to overcome this problem. In recent years, addition of metals or inorganic compounds to multiwalled carbon nanotubes (MWCNTs) has been generally used for hydrogen uptake studies to enhance adsorption property of the nanotubes. In this study, Zinc oxide (ZnO) nanoparticles doped MWCNTs (ZnO-MWCNTs) have been produced as new reversible hydrogen storage materials, and we have investigated characterization of ZnO-MWCNTs by XRD, SEM, TGA, TEM and BET analyses. The functionalized MWCNTs and ZnO doped MWCNTs were subjected to hydrogenation step by dynamic gas sorption analyser under pressure of 5–50 bar. The hydrogen uptake capacities of the materials under different pressures were measured gravimetrically. It was indicated that by controlling the pressures for hydrogenation of ZnO-MWCNTs induces the spillover of ZnO nanoparticles in the layer of MWCNTs which in return with high hydrogen adsorption capacity. Consequently, the hydrogen adsorption of the functionalized MWCNTs (f-MWCNTs) and the ZnO-MWCNTs were achieved to be 1.05 wt% and 2.7091 wt% under pressure of 50 bar as maximum.     

Copper-doped activated carbon from amorphous cellulose for hydrogen,methane and carbon dioxide storage

The transition away from fossil fuel and ultimately to a carbon-neutral energy sector requires new storage materials for hydrogen and methane as well as new solutions for carbon capture and storage. Among the investigated adsorbents, activated carbons are considered especially promising because they have a high specific surface area, are lightweight, thermally and chemically stable, and easy to produce. Moreover, their porosity can be tuned and they can be produced from inexpensive and environmentally friendly raw materials. This study reports on the development and characterization of activated carbons synthesized starting from amorphous cellulose with and without the inclusion of copper nanoparticles. The aim was to investigate how the presence of different concentrations of metal nanoparticles affects porosity and gas storage properties. Therefore, the research work focused on synthesis and characterization of physical and chemical properties of pristine and metal-doped activated carbons materials and on further investigation to analyze their hydrogen, methane and carbon dioxide adsorption capacity. For an optimized Cu content the microporosity is improved, resulting in a specific surface area increase of 25%, which leads to a H₂ uptake (at 77 K) higher than the theoretical value predicted by the Chahine Rule. For CH₄, the storage capacity is improved by the addition of Cu but less importantly because the size of the molecule hampers easy access of the smaller pores. For CO₂ a 26% increase in adsorption capacity compared to pure activated carbon was achieved, which translated with an absolute value of over 48 wt% at 298 K and 15 bar of pressure.