Nanoscale carbon material porosity effect on gas adsorption

In this paper, we discussed the potential of hydrogen adsorption by three types of carbon materials (GNF, SWNT and AC) in relation with their surface reactivity due to their nanometric pore dimension. We realized experimental studies of hydrogen adsorption measurements on a high accuracy gravimetric device up to 5 MPa on a series of chemically activated carbons and nanomaterials offering a wide range of porosities. Comparison with a very high pressure volumetric device up to 70 MPa is also given. We used in the mass balance, the material skeleton density determined at 700 K. Samples were characterized by Raman spectroscopy, XRD, TEM and BET methods. Material analysis correlation with macroscopic gas adsorption helps to the phenomenon interpretation at molecular scale. Although gas-solid interactions are weak; the adsorption sites are dense and their accessibility is an important point for optimizing gas uptake by increasing material porosity development. We discuss the limitations of the physical adsorption as a storage tool.     

How to accurately determine the uptake of hydrogen in carbonaceous materials

A volumetric apparatus for adsorption measurement of hydrogen in carbonaceous materials aimed at hydrogen adsorption storage was constructed. The performance of the apparatus was assessed by helium and hydrogen adsorption in one kind of vapor-grown graphitic nanofiber (GNF). The bulk gas amounts determined by the equations of state of the Modified-Benedic-Webb-Rubin (MBWR), the ideal gas and the Soave-Redlich-Kwong (SRK) under the conditions of present study are compared. Two different methods of processing experimental data to determine the residual volume are studied. Experimental results and theoretic analysis showed that the volumetric apparatus and the data processing method described in this paper could accurately determine the Gibbs excess adsorption amount of hydrogen in carbonaceous materials. Although the vapor-grown GNF used in the present study did not show a significant storage capacity of hydrogen, the obtained results would provide favorable reference data for the development of the carbonaceous material for hydrogen adsorption storage.     

Roles of nitric acid treatment on PtRu catalyst supported on graphite nanofibers and their methanol electro-oxidation behaviors

In the present study, the effect of chemical treatment on graphite nanofiber supports (GNFs) with various concentrations of nitric acid was investigated for methanol electro-oxidation. To optimize the electrocatalytic activity, PtRu catalysts were deposited on GNF supports by the impregnation method. The surface and structural properties of the GNF supports were characterized by X-ray photoelectron spectroscopy (XPS), elemental analysis (EA), and X-ray diffraction (XRD). The morphology of the catalysts was characterized by transmission electron microscopy (TEM). The electrocatalytic activity of PtRu/GNF catalysts was investigated by cyclic voltammetry. Oxygen functional groups were introduced on the GNF supports by the addition of nitric acid. Increasing the concentration of nitric acid caused a subsequent increase in the presence of oxygen functional groups, which resulted in smaller catalyst particle size and a higher loading of the catalyst. The electrocatalytic activity of the catalysts for methanol oxidation was also improved with these treatments. Consequently, it was found that chemical treatments could influence the surface properties of the carbon supports, resulting in enhanced electrocatalytic activity of the catalysts for direct methanol fuel cells (DMFCs).     

Energy storage and solidification of paraffin phase change material embedded with graphite nanofibers

Phase change materials (PCMs) are known to be excellent candidates for thermal energy storage in transient applications. However, enhancement of the thermal conductivity of a paraffin-based PCM is required for effective performance, particularly during solidification where diffusion is the dominant heat transfer mode. This study experimentally examines the effect that graphite nanofibers (GNFs), aspect ratio and power density have on both thermal storage and solidification time of a PCM which is embedded between two sets of aluminum fins. Additionally, a figure of merit is introduced in order to quantify the effectiveness of each of these three parameters with respect to solidification time. GNF enhancement was shown to reduce the maximum temperature in the thermal containment unit (TCU) by 48%. It was also found that for aspect ratios of 1, the GNF enhancement shortens solidification time by as much as 61% over the paraffin samples. This research indicates that GNF impregnation into phase change materials is an effective method for the enhancement of the thermal energy storage and the solidification of paraffin-based phase change materials.     

Hydrogen storage capacity of Al,Ca, Mg,Ni, and Zn decorated phosphorus-doped graphene: Insight from theoretical calculations

In this work, adsorption of molecular hydrogen on five different metals: Aluminum, Calcium, Magnesium, Nickel and Zinc decorated phosphorus-doped graphene have been investigated using density functional theory (DFT) computation at the PBE0-D3BJ/def2svp method. From literature reviews, phosphorus doped graphene are potential candidates for hydrogen storage. Herein, theoretical investigation on the changes in structural and electronic properties of the studied materials was conducted. Natural bond orbital (NBO) analysis was employed to study the intermolecular and intra-molecular interactions arising from chemical bonds in the studied systems. In addition, the density of states (DOS) plots shows notable individual orbital contribution and hybridization between the decorated metals and the phosphorus-doped graphene which is also responsible for the adsorption of hydrogen. Based on the frontier molecular orbital analysis, results indicates that Al and Ni surfaces possess excellent structural and electronic properties with lower values of chemical hardness and ionization with adsorption energy values of 1.924eV and 1.236eV obtained for both surfaces potential indicating better conductivity and excellent H₂ adsorption potential. The obtained results shows the suitability of the Al and Ni decorated phosphorus-doped graphene for hydrogen storage.     

Surface reactivity of graphite materials and their surface passivation during the first electrochemical lithium insertion

The surface passivation of TIMREX^® SLX50 graphite powder was studied as received and after heat treatment at 2500 °C in an inert gas atmosphere by differential electrochemical mass spectrometry in electrochemical lithium half-cells. 1 M LiPF₆ in ethylene carbonate and either a dimethyl carbonate, propylene carbonate or 1-fluoro ethylene carbonate co-solvent was used as electrolyte systems in these half-cells. The SEI-film formation properties of both graphite materials were correlated with their active surface area (ASA), being responsible for the interactions between the carbon and the electrolyte system. The active surface area was determined from the amount of CO and CO₂ gas desorbed at temperatures up to 950 °C from the graphite material surface after chemisorption of oxygen at 300 °C. The structural ordering at the graphite surface increased significantly during the heat treatment of the SLX50 graphite material as indicated by the significant decrease of the ASA value. The increased surface crystallinity was confirmed by krypton gas adsorption, Raman spectroscopy as well as temperature-programmed desorption. This increased structural ordering seemed to be the parameter being responsible for a hindered passivation of the heat-treated SLX50 causing partial exfoliation of the graphite structure during the first electrochemical lithium insertion in the ethylene carbonate/dimethyl carbonate electrolyte. In the case of the ethylene carbonate/1-fluoro ethylene carbonate electrolyte system, primarily the fluoro compound is responsible for the graphite passivation. In this electrolyte system, pristine SLX50 and the less reactive, heat-treated SLX50 graphite showed significantly different SEI-film formation mechanisms. In contrast, no difference in the passivation mechanism could be identified for different graphite surfaces in the ethylene carbonate electrolyte system with propylene carbonate as co-solvent.     

Grand canonical Monte Carlo and molecular dynamics simulations of the structural properties,diffusion and adsorption of hydrogen molecules through poly(benzimidazoles)/nanoparticle oxides composites

Comprehensive structural/molecular simulations have been undertaken to study the poly(benzimidazoles) (PBI) membrane combined with four different nano-oxide materials (ZnO, Al₂O₃, SiO₂ and TiO₂) for purification and production of hydrogen from natural gases. Composite membranes were built with different amounts of nano-oxide materials to investigate the influence of nano-oxide content on the PBI membrane performance. Several structural characterizations such as FFV, WAXD and also a thermal one (glass transition temperature) were done to study the structural properties of all simulated membrane cells. Moreover, MSD and adsorption isotherms tasks were used to estimate the diffusivity and solubility of hydrogen molecules through the latter mixed matrix membranes (MMMs), respectively. Permeability and permselectivity of H₂ penetrate molecules were also carefully calculated using the aforementioned penetrating factors (diffusivity and solubility). Results show a significant improvement in structural and transport properties by increasing the nanomaterials content, which could be due to the growth of penetration pathways through the membranes. Furthermore, membranes with SiO₂ yield the best results compared to other three nano-oxide fillers. H₂ gas yields the best results that help the storage and separation of this precious gas from other gas molecules, which present in natural gases. Compared to the previous studies and literature results, the current results are accurate and reliable to describe the structural and transport properties of PBI/nano-oxides composites.     

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.     

Platinum and palladium nano-particles supported by graphitic nano-fibers as catalysts for PEM water electrolysis

Platinum and palladium nano-particles supported by graphitic nano-fibers (GNFs) have been prepared and used as cathodic electrocatalysts in proton-exchange membrane (PEM) water electrolysis cells for the hydrogen evolution reaction (HER). Raw GNF structures have been synthesized by chemical vapor deposition (CVD). Noble metal nano-particles have been deposited at the surface of GNFs using an impregnation-reduction method. Structural properties and electrochemical performances of the GNF-supported catalysts have been determined using TEM analysis and cyclic voltammetry. Current-voltage polarization curves have also been recorded using a PEM cell (7 cm²). The performances obtained with GNF-supported catalysts were found more efficient than those obtained with catalysts supported with conventional carbon black (Vulcan^® XC-72). In particular, a reduced electrolysis cell voltage (1.67 instead 1.72 V at 1 A.cm⁻² and 90 °C) has been obtained using Pt/GNF cathodes in place of Pt/XC-72 at the cathode and with similar platinum contents (40 wt.%).     

Grand canonical Monte Carlo simulations of hydrogen adsorption in carbon aerogels

Hydrogen storage plays a fundamental role in the future hydrogen energy system, and carbon aerogel is one of the most potential hydrogen storage materials because of its high gravimetric and volumetric density on hydrogen adsorption. In this paper, the amorphous structure of carbon, obtained by a numerical simulation process by using the molecular dynamic and Monte Carlo methods, as well as the primary unit method, was intercepted as a sphere structure for numerical annealing to build a carbon nanosphere, which serves as the basic unit to reconstruct the carbon aerogel's skeleton by the Diffusion Limited Cluster Aggregation (DLCA) method. The hydrogen adsorption in carbon aerogel was simulated by using the self-coding parallel grand canonical Monte Carlo (GCMC) method. The influences of particle diameter, density, temperature, pressure, and specific surface area on the hydrogen adsorbing capacity in carbon aerogel were analyzed in detail. The results showed that the carbon aerogel's hydrogen storage capacity with a specific surface area of 2680 m²/g could reach 4.52 wt % at 77 K and 3.0 MPa.     

Hydrogen generation from hydrolysis of aluminum/graphite composites with a core–shell structure

Hydrogen generated by hydrolysis of metal aluminum with water is promising for portable fuel cell applications. However aluminum would not react with water to yield hydrogen at ordinary conditions due to the passive oxide film formed on its surface. In the present investigation, the aluminum/graphite composite were prepared by a ball milling process in an attempt to improve the reactivity of aluminum, using sphere-shape aluminum particles and laminate graphite as the initial materials and 2 wt% NaCl as the milling-assisted agent. The TEM observation showed that the Al particles are covered by graphite to form a core–shell structure. Such a Al/graphite composite material exhibited a pronounced hydrolysis reactivity with tap water to generate hydrogen while Al alone did not react with water. The presence of graphite could lower the hydrogen generation reaction temperature below 45 °C. Increasing the reaction temperature could obtain an increased hydrogen generation rate and the maximum hydrogen generation rate of 40 cm³ min⁻¹ g⁻¹ Al was obtained when the reaction temperature was increased to 75 °C. Prolonging milling time could also improve the Al hydrolysis reactivity in the composite particularly at a relatively low temperature. The XRD results identified that the hydrolysis byproducts are bayerite (Al(OH)₃) and boehmite (AlOOH). The microstructure-related hydrolysis reaction mechanism was finally proposed.     

Hydrogen adsorption on carbon materials

Studies are focused on the hydrogen adsorption on carbon materials at ambient temperature. The hydrogen adsorption from the gas phase was measured by isothermal gravimetric analysis, using a microbalance at hydrogen pressures up to 125 bar at 23°C. In this work, the hydrogen adsorption reached values of approximately 1.5 wt.% at ambient temperature and 125 bar.     

水平井地质模型-数值模型-FSD耦合分段压裂优化研究

为解决水平井分段改造和均衡层间产气差异的难题,从岩石细观力学角度出发,建立了岩石渗流-应力-损伤耦合模型(FSD模型);考虑岩石的非均质性,应用弹性有限元法作为应力分析工具,计算分析岩石的应力场和位移场,对水压裂缝的萌生、扩展过程中渗透率演化规律及其渗流-应力耦合机制和渗流与损伤耦合机制进行模拟分析。采用基于储层特征、完井方式、流体性质及油藏数值模拟等综合信息的Multi Well Simulator开放式软件平台,通过定义损伤断裂延伸准则和FSD模型方程,可以实现油藏数值模拟和FSD动态耦合分析。以D403-2H井为例,建立地质模型-数值模型-FSD模型,对射孔参数、措施层段、分段工具位置等进行优化,对合理划分措施井段和调整产气剖面进行研究。现场应用结果表明,该技术能够有效解决层间矛盾突出的水平井的层间储量动用不均衡的问题。     

Hierarchical porous graphene-based carbons prepared by carbon dioxide activation and their gas adsorption properties

Hierarchical porous graphene-based carbons (HPGCs) have been prepared by a simple carbon dioxide activation of graphite oxide. The effects of activation temperature on the structural and textural properties as well as gas adsorption capacities of the resultant carbons have been investigated. The HPGCs showed hierarchically micro-meso-macroporous structures, high specific surface areas of up to 532 m² g⁻¹, and large pore volumes of up to 1.67 cm³ g⁻¹. Moreover, the HPGC materials were demonstrated to be efficient for CO₂ and H₂ adsorption. The HPGC-850, which was obtained after two hours of activation at 850 °C, exhibited the highest adsorption capacities of 7.74 wt% (1.76 mmol g⁻¹) at 273 K and 1 bar for CO₂ and of 0.75 wt% (3.76 mmol g⁻¹) at 77 K and 1 bar for H₂.     

On the nature, capability and reversibility of hydrogen storage in novel carbon nanomaterials for mobile power units

On the basis of the thermodynamic analysis of most representative recent data on hydrogen storage in graphite and novel carbon-based nanomaterials of sp² hybridization, the open questions have been studied of the nature, capability and reversibility of such storage, particularly, for mobile power units. For an interpretation of the thermodynamic and kinetic (diffusion) characteristics obtained in such a way, the known results have been used of ‘ab initio’ molecular orbital study of adsorption of atomic hydrogen on graphite and carbon nanostructures.     

Hydrogen storage properties of various carbon supported NaBH4 prepared via metathesis

Sodium borohydride nanoparticles prepared via the metathesis reaction between LiBH₄ and NaCl were successfully deposited on various carbon supporting materials such as graphite, graphene oxide and carbon nanotubes. The X-ray diffraction analyses were conducted to identify the phase of NaBH₄ deposited on various carbon supporting materials. The transmittance electron micrograph analyses were also conducted to investigate the particle size and dispersion of NaBH₄ within carbon supporting materials. The particle size and size distribution of NaBH₄ on graphite were observed to be larger and broader than of other two supporting materials, graphene oxide and CNT due to the lower surface energy as compared to GO and CNT. The bonding state of NaBH₄ was confirmed by the Fourier-transformed infrared spectroscopy analysis. The TG and PCT results show that the hydrogen desorption of the NaBH₄ deposited on carbon supports takes place at temperature (130 °C～) significantly lower than that of pure NaBH₄ (above 500 °C) and the amount of desorption was in the order of graphene oxide (12.3 mass %) > CNT (9.8 mass %) > graphite (5.7 mass %). The reversibility of hydrogen adsorption after five cycles of adsorption-desorption showed that NaBH₄/GO and NaBH₄/CNT were much better than that of pure NaBH₄ due to excellent structural stability.     

Fabrication and characterisation of the graphite bi-polar plates used in A 0.25 kW PAFC stack

The graphite bi-polar plates were fabricated using lamination technique with polyether sulfone (PES) films (50 μm) and graphite foils (400 μn) in between the two porous graphite plates (CBC) by keeping in a specially designed and fabricated fixture with stainless steel plates at the top and bottom. The fixture was then kept in an hydraulic hot press, at loads of 10–20 tons, and heat treated at 410 °C for 30 min. Then these graphite plates were sized to 30 cm × 20 cm × 0.64 cm, leaving 0.4 mm thick graphite foil at the centre of the plate, to avoid the intermixing of the hydrogen and oxygen/air. While, the gas permeability (cm²/sec) of the plates was determined, with N₂ gas using differential pressure method, their electrical resistivity (mΩm) was measured using milliohmmeter and passing DC current to the graphite plates, at loads from 1–5 kgs. The values of permeability and electrical resistivity of the plates are found to be lower than 0.01 cm²/sec and 4–14 mΩm respectively. A stack with 6 cells was assembled using the in house developed graphite bi-polar plates, anodes and cathodes with matrix, to generate a DC power of 0.25 kW (3.6 V × 71.0 amps). It was operated for 300 h successfully using H₂ and Air, 1 bar, at 175 °C. In this paper, the detailed fabrication method of graphite bi-polar plates and their characteristics of gas permeability, electrical resistivity and the results of the 0.25 kW PAFC stack operation are presented.     

Solar hydrogen production from the thermal splitting of methane in a high temperature solar chemical reactor

This study addresses the single-step thermal decomposition (pyrolysis) of methane without catalysts. The process co-produces hydrogen-rich gas and high-grade carbon black (CB) from concentrated solar energy and methane. It is an unconventional route for potentially cost effective hydrogen production from solar energy without emitting carbon dioxide since solid carbon is sequestered.A high temperature solar chemical reactor has been designed to study the thermal splitting of methane for hydrogen generation. It features a nozzle-type graphite receiver which absorbs the solar power and transfers the heat to the flow of reactant at a temperature that allows dissociation. Theoretical and experimental investigations have been performed to study the performances of the solar reactor. The experimental set-up and effect of operating conditions are described in this paper. In addition, simulation results are presented to interpret the experimental results and to improve the solar reactor concept. The temperature, geometry of the graphite nozzle, gas flow rates, and CH₄ mole fraction have a strong effect on the final chemical conversion of methane. Numerical simulations have shown that a simple tubular receiver is not enough efficient to heat the bulk gas in the central zone, thus limiting the chemical conversion. In that case, the reaction takes place only within a thin region located near the hot graphite wall. The maximum CH₄ conversion (98%) was obtained with an improved nozzle, which allows a more efficient gas heating due to its higher heat exchange area.     

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 uptake of reduced graphene oxide and graphene sheets decorated with Fe nanoclusters

Graphene oxide (GO) has been prepared by employing modified Staudenmaier's method through thermal exfoliation of graphite oxide. High pressure hydrogen sorption isotherms up to 50 bar of GO, reduced by thermal reduction (TR-GO), chemical reduction (CR-GO) and graphene sheets decorated with Fe nanoclusters (Fe-GS) have been investigated. Thermal reduction of GO at 623 K under high vacuum yields TR-GO. Chemical reduction of GO using hydrazine forms CR-GO. Fe-GS was synthesized through arc-discharge between the ends of two graphite rods with one rod carrying Fe nanoparticles. The surface areas of these graphene samples were determined from the nitrogen adsorption isotherm employing Brunauer, Emmett and Teller (BET) method. Kelvin's equation was used to determine the pore size distribution of all graphene based samples. Hydrogen pressure-composition isotherms (PCI) were determined at 300 K and at 77 K, between 0.1 and 50 bar. Further, in this paper, we present a comparative adsorption isotherm analysis of hydrogen and helium on TR-GO. This reveals that the volume of hydrogen and helium adsorbed by TR-GO is nearly equal. The similar uptake volume determined for both hydrogen and helium indicates the possibility of monolayer adsorption of hydrogen and also nearly similar binding energy between TR-GO and H₂/He.