Data-driven modelling and optimization of hydrogen adsorption on carbon nanostructures

In the present study, using modelling based on experimental data, models for predicting the hydrogen adsorption isotherm were presented. The three Automatic Learning of Algebraic Models (ALAMO), feed-forward artificial neural networks (ANNs), and group method of data handling-type polynomial neural networks (GMDH-PNN) were constructed. The created models were evaluated to predict the equilibrium data of hydrogen storage on carbon nanostructures, including activated carbons doped with palladium (Pd) nanoparticles, fullerene pillared graphene nanocomposites, and nickel (Ni)-decorated carbon nanotubes. The inputs were nanostructure characteristics such as surface area, pore-volume, and thermodynamic conditions such as pressure. The generalization of the trained models was acceptable, and the models successfully predicted the hydrogen adsorption isotherm for new inputs. The relative error percentage for most data points is less than 4%, which demonstrates their applicability in determining adsorption isotherms for any operating conditions. By performing error analysis calculations, it was shown that the ALAMO model has the highest accuracy. Also, sensitivity analysis calculations show that pressure is the most influential parameter in the adsorption process. Besides, by performing Genetic Algorithm (GA) optimization using the ALAMO model, the amount of pressure and adsorbent properties were determined so that the amount of hydrogen adsorption is maximized. According to the optimization results based on the GA, the higher the pressure, the greater the amount of hydrogen adsorption. The nanotubes with a surface area of 194.15 m²/g, a total volume of 1.8 cm³/g, micropore volume of 0.097 cm³/g, and mesopore volume of 0.963 cm³/g, graphene with a surface area of 2977.13 m²/g, a total volume of 1.5134 cm³/g, density of 617.45 kg/m³, and activated carbon at pressures less than 30 bar with a surface of 2546.36 m²/g, a total volume of 1.237 cm³/g, micropore volume of 0.839 cm³/g, and activated carbon at pressures more than 30 bar with a surface of 3027 m²/g, a total volume of 1.343 cm³/g, a micropore volume of 0.9582 cm³/g, and a mesopore volume of 1.23 cm³/g, have the highest amount of stored hydrogen.     

A fast procedure for the estimation of the hydrogen storage capacity by cryoadsorption of metal-organic framework materials from their available porous properties

In order to identify the best porous materials for the cryogenic physisorption of hydrogen, high-throughput calculations are performed starting, i.e., from the collected information in crystallographic databases. However, these calculations, like molecular simulations, require specific training and significant computational cost. Herein, a relatively simple procedure is proposed to estimate and compare hydrogen uptakes at 77 K and pressure values from 40 bar starting from the porous properties of MOF materials, without involving simulation tools. This procedure uses definitions for adsorption and considers the adsorbed phase as an incompressible fluid whose pressure-density change is that for the liquid phase at 19 K. For the 7000 structures from the CoRE MOF database, the average error of the predictions is only of 1% from reference values at 100 bar, with an SD of ±8%. This accuracy is lower than that from simulation tools, but involving lower computational cost and training.     

Hydrogen adsorption with micro-structure deformation in nanoporous carbon under ultra-high pressure

Hydrogen adsorption with micro-structure deformation under ultra-high pressure in nanoporous carbon (NPC) has been studied. This study proposed a new ultra-high pressurization (UHP) method. It produces a gas atmosphere of over 100 MPa utilizing the cold isostatic pressing (CIP) device. NPC materials were pressurized under a hydrogen atmosphere at 100–400 MPa. NPC fabricated from rice husk via KOH activation possesses a high surface area achieving 3500 cm²/g and a micropore volume of over 2.0 cm³/g. The maximum hydrogen uptake reached 3.2 wt% (77 K, 0.1 MPa). Then, NPC materials were treated with 100–400 MPa pressurization in the hydrogen atmosphere. NPC showed a preferred deformation behavior of 1.1–1.2 nm after pressurization, which is the optimum size for hydrogen adsorption. Additionally, the maximum micropore volume increased to 2.51 cm³/g. However, the hydrogen uptake shows a slight decrease to 3.0 wt%. The isosteric heat of adsorption maintained at 8.0–10.3 kJ/mol.     

Hydrogen storage in gas reservoirs: A molecular modeling and experimental investigation

Hydrogen is one of the clean energy sources that can be used instead of fossil fuel sources to reduce greenhouse emissions. However, hydrogen supply intermittency significantly reduces the deployment and reliability of this energy resource. Therefore, this work investigates the underground storage of hydrogen in depleted gas reservoirs to avoid seasonal fluctuations in hydrogen supply and assure long-term energy security. The obtained results from molecular simulation (Density Functional Theory) revealed hydrogen is adsorbed physically on calcite (104) and silica (001) surfaces on different adsorption configurations. This conclusion is supported by low adsorption energies (?0.14 eV for calcite and ?0.09 for silica) and by Bader charge analysis, which showed no indication of charge transfer. The experimental results illustrated that hydrogen has a very low adsorption affinity toward carbonate and sandstone rocks in the temperature range of 50–100 °C and pressure up to 20 bar. These results show the potential of depleted gas reservoirs to store hydrogen for s is useful in hydrogen recovery as no hydrogen will be adsorbed to the rock surface of conventional gas reservoirs.     

Modeling of sudden hydrogen expansion from cryogenic pressure vessel failure

We have modeled sudden hydrogen expansion from a cryogenic pressure vessel. This model considers real gas equations of state, single and two-phase flow, and the specific “vessel within vessel” geometry of cryogenic vessels. The model can solve sudden hydrogen expansion for initial pressures up to 1210 bar and for initial temperatures ranging from 27 to 400 K. For practical reasons, our study focuses on hydrogen release from 345 bar, with temperatures between 62 K and 300 K. The pressure vessel internal volume is 151 L. The results indicate that cryogenic pressure vessels may offer a safety advantage with respect to compressed hydrogen vessels because i) the vacuum jacket protects the pressure vessel from environmental damage, ii) hydrogen, when released, discharges first into an intermediate chamber before reaching the outside environment, and iii) working temperature is typically much lower and thus the hydrogen has less energy. Results indicate that key expansion parameters such as pressure, rate of energy release, and thrust are all considerably lower for a cryogenic vessel within vessel geometry as compared to ambient temperature compressed gas vessels. Future work will focus on taking advantage of these favorable conditions to attempt fail-safe cryogenic vessel designs that do not harm people or property even after catastrophic failure of the inner pressure vessel.     

Design and optimization of Isochoric Differential Apparatus (IDA) to reduce uncertainty in H2 sorption process measurements

The quantification of hydrogen absorption and desorption in materials is a crucial step for the assessment of proper storage solutions and their applications. Unfortunately, volumetric instruments are in many cases affected by low accuracy due to several factors such as temperature uncertainty and misleading on calibration proceeding.In this work, we report the superior performance of a new kind of instrumental layout to characterize kinetics and thermodynamics properties of hydrogen storage materials. Hereby presented system is based on differential Sievert measurements, defined as Isochoric Differential Apparatus (IDA). IDA includes two coupled identical Sievert apparatus where pressure values are sampled in differential mode to compensate all temperature transient phenomena and nonlinear effects occurring during the gas expansion step that occurs during the measurements. A physical model to evaluate the sorbed gas at non-isothermal condition has been developed and reported. Detailed error analysis of the kinetic and thermodynamic models has been carried out considering a real gas. Palladium and Magnesium has been utilized as benchmark materials, to test the differential apparatus at ambient and high-temperature values > 300 °C). For both materials, kinetic and thermodynamic properties have been acquired by the differential layout in well agreement with reference data and with a higher accuracy than classic Sievert instrument, involving in identical size of expansion volume. This work demonstrates as the differential layout allows to reduce uncertainty in hydrogen sorption measurement exploiting the full accuracy of equipped transducers. At this level of performance, the impact of calibration procedures and the approach for the estimation of compressibility factor become extremely important to further reduce uncertainty on sorption measurements.     

Effect of long range van der Waals interactions on hydrogen storage capacity and heat of adsorption in large pore silicas

The low temperature hydrogen adsorption capacity of mesopores silica aerogel was investigated and compared with that of other large pore silica based materials (MCM-41, HMS) within a range of surface area and large (2 nm) to very large (20 nm) pore sizes. The hydrogen uptake of the aerogel measured at pressure of 1 bar and a temperature of 77 K is around 2.5 times lower than that of MCM-41, although it has a comparable specific surface area (just 30% smaller). The explanation found is the relation between lower hydrogen heat of adsorption and larger pore size for the investigated materials, which leads to higher surface coverage in the smaller pores.     

Hydrogen sorption in transition metal modified ETS-10

Hydrogen adsorption studies in nickel, rhodium and palladium exchanged and in situ loaded titanosilicate ETS-10 were performed at 77.4 K using a static volumetric adsorption system up to 1 bar, and 303 K in a gravimetric adsorption system up to 5 bar. The hydrogen adsorption isotherms at 77.4 K were reversible with pressure but chemisorption of hydrogen was noticed at 303 K. Rhodium exchanged ETS-10 showed the highest hydrogen adsorption capacity of 82.6 cc/g at 77.4 K. The hydrogen adsorption isotherm analysis at 303 K was repeated up to three adsorption runs to check the repeatability of hydrogen uptake. At 303 K palladium loaded ETS-10 showed the highest hydrogen uptake capacity of 33.1 cc/g. The DRIFT spectra analysis of ETS-10 samples before and after hydrogen adsorption was conducted, which confirmed that the hydrogen adsorbed in transition metal modified ETS-10 at 303 K was due to the chemical interactions in the form of transition metal hydrides inside ETS-10. The absorbed hydrogen at 303 K can be desorbed by heating the ETS-10 sample up to 413 K.     

Conditioning of hydrogen storage by continuous solar adsorption in activated carbon AX-21 with simultaneous production

The capacity of hydrogen storage by solar adsorption in activated carbon AX-21 and filling rate with simultaneous production have been conditioned under a minimum pressure, to nullify the cost of energy supplied to compressor. A gas accumulator tank connected to electrolyzer and continuous adsorption beds have been proposed in the process scheme. Minimum pressure required for the tank at an ambient filling temperature fixed to 25 °C is only 2 bar. While at atmospheric filling pressure the corresponding value of filling temperature is found to be 5 °C. However, a cooling fluid at low temperature for adsorbent bed during the adsorption process will be an efficient way for increasing the stored amount of hydrogen. Almost 4.5 kg of hydrogen can be stored in an adsorbent mass of 200 kg. The adsorption flow rate has been also modelled to be controlled for being adapted to production rate.     

Volumetric hydrogen sorption measurements – Uncertainty error analysis and the importance of thermal equilibration time

The design of a volumetric measurement apparatus is studied by means of an uncertainty analysis to provide guidelines for optimum hydrogen sorption measurements. The reservoir volume should be as small as possible (i.e., 10 cc) to minimize the uncertainty. In addition, the sample mass loading has a profound effect on the uncertainty and the optimum loading is a function of the sample's intrinsic storage capacity. In general, the higher the sample mass loading the lower the uncertainty, regardless of any other parameter. In cases where the material to be tested is not available in gram quantities, the use of high accuracy pressure and temperature transducers significantly mitigates the uncertainty in the sample's hydrogen uptake. Above all, the thermal equilibration time is an important parameter for high accuracy measurements and needs to be taken into consideration at the start of the measurements. Based on a computational analysis, a 5 min wait time is required for achieving thermal equilibrium when the instrument enclosure temperature is different than the ambient temperature.     

Facilitating analysis of trace impurities in hydrogen: Enrichment based on the principles of pressure swing adsorption

A laboratory-scale gas sampling and impurity enrichment device (GSIED) based on the principles of pressure swing adsorption (PSA) has been designed, fabricated, and tested to show that such a device provides an effective method to enrich trace impurity species in hydrogen by a factor of 10 or more. With the availability of a high pressure sample gas at the hydrogen refueling stations, the device uses only a pressure sequence to enrich the impurities without need of a temperature cycle. Enrichment of the impurities allows the use of simpler and less expensive analytical instruments for hydrogen quality monitoring and certification purposes. A series of experiments was conducted using activated carbon as the PSA sorbent for impurity enrichment in a hydrogen gas containing N₂, CO, CH₄, and CO₂. The enrichment factor varied for the different species according to their affinity of adsorption. The measured impurity enrichment factors agreed well with theoretical analyses, and are functions of the pressure ratio (adsorption/desorption pressures) and adsorption affinity relative to hydrogen (selectivity). Depending on the species of interest and the volume of the enriched sample needed for analysis, the device can be designed to enrich the impurities in hydrogen in 40 min or less.     

High-pressure hydrogen storage on microporous zeolites with varying pore properties

Hydrogen storage on microporous zeolites was examined using a high pressure dose of hydrogen at 30 °C. The roles of the framework structure, surface area, and pore volume of the zeolites on hydrogen adsorption were investigated. The largest hydrogen storage was obtained on the ultra stable Y (USY) zeolite (0.4 wt%). The hydrogen adsorption isotherms on the zeolites reached a maximum after a hydrogen pressure of 50 bar. The amount of hydrogen adsorption on Mordenite (MOR) zeolites increased with increasing Si/Al molar ratio, which was achieved by dealumination. The amount of hydrogen adsorption increased linearly with increasing pore volume of the zeolites. The hydrogen adsorption behavior was found to be dependent mainly on the pore volume of the zeolites.     

Facile synthesis and hydrogen storage application of nitrogen-doped carbon nanotubes with bamboo-like structure

This paper reports a facile method for the preparation of nitrogen-doped carbon nanotubes (N-doped CNTs) that shows enhanced hydrogen storage capacity. The synthesis method involves simple pyrolysis of melamine using FeCl₃ as catalyst in tube furnace. The materials were characterized by scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, elemental analysis, Raman spectroscopy, and nitrogen adsorption–desorption analysis. The results indicated that the prepared N-doped CNTs have a bamboo-like structure with thin compartment layers. The nitrogen doping concentration, specific surface area, and total pore volume of the N-doped CNTs were determined to be 1.5 at%, 135 m²/g, and 0.38 cm³/g, respectively. The hydrogen adsorption measurements at 77 K showed that the N-doped CNTs exhibits gravimetric hydrogen uptake of 0.21 wt% at 1 bar and 1.21 wt% at 7 bar. At room temperature, hydrogen uptake as high as 0.17 wt% at 298 K and 19 bar is achieved, which is among the highest data reported for the N-doped carbon materials under the same condition.     

Experimental study and proposed power correlation for laminar burning velocity of hydrogen-diluted methane with respect to pressure and temperature variation

The aim of the present work is to contribute to the better understanding of the combustion process and the laminar flame properties of methane/hydrogen-air flames at elevated temperatures and pressures. The heat flux method provides an accurate and direct measurement of laminar burning velocities (LBV) at elevated temperatures, while the constant volume chamber method provides measurements at elevated pressures. In the present work, a database of more than 250 experimental points for the range of temperature (298–373 K) and pressure conditions (1–5 bar) for mixtures up to 50% hydrogen in methane was generated using these two methods. Comparison with the sparse literature data shows quite good agreement. A power-law correlation for temperature and pressure is proposed for methane/hydrogen-air mixtures, which has a practical application in estimating the LBV of a natural gas/hydrogen mixture intended to replace pure natural gas in different processes. The power-law temperature exponent, α, and the pressure exponent, β, show inverse trends. The former decreases almost linearly and the latter increases approximately linearly when the hydrogen content is increased. The power-law exponents are highly affected by the mixture equivalence ratio, ?, showing a parabola like trend. However, for the pressure exponent this trend becomes almost linear for 50% H₂ in the mixture. The power-law correlation has been validated against experimental data for a wide range of temperature (up to 573 K), pressure (1–7.5 bar), equivalence ratios (? between 0.7 and 1.3) and H₂ contents up to 50%.     

Importance of clay-H2 interactions for large-scale underground hydrogen storage

Underground hydrogen storage is considered an option for large-scale green hydrogen storage. Among different geological storage types, depleted oil/gas fields and saline aquifers stand out. In these cases, hydrogen will be prevented from leaking back to the surface by a tight caprock seal. It is therefore essential to understand hydrogen interactions with shale-type caprocks. To this end, natural pure montmorillonite clay was exposed to hydrogen gas at different pressures (0–50 bar) and temperatures (77, 195, 303 K) to acquire data on its adsorption capacity related to UHS and caprock saturation. Montmorillonite was chosen because of its large specific surface area enabling quantification of the adsorption process. Hydrogen adsorption was successfully fitted with a Langmuir isotherm model and yielded small partition coefficients indicating that hydrogen does not preferentially adsorb to the clay surface. Adsorption on montmorillonite goes back to weak physisorption as inferred from minor negative changes in the enthalpy of reaction (−790 J/mol), derived from an Excel Solver approach to the van't Hoff equation. Based on own as well as literature values, adsorption capacities, which were originally reported as mol/kg or wt%, are recast as hydrogen volume adsorbed per specific surface area (μL/m²). The acquired range is surprisingly narrow, with values ranging from 3 to 6 μL/m², and indicates the normalised volume of hydrogen that can be expected to remain in the shale-type caprock after injected hydrogen migrated upwards through the porous reservoir. This ‘residual’ caprock saturation with hydrogen can be further restrained by considering the geothermal gradient and its effect on the molar volume of hydrogen. The experimental results presented here recommend injecting hydrogen deeper rather than shallower as pressure and temperature work in favour of increased storage volumes and decreased hydrogen loss through clay adsorption in the caprock.     

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.     

Accurate gas – Zeolite interaction measurements by using high pressure gravimetric volumetric adsorption method

In this work, we discuss the purification of hydrogen by physical adsorption on zeolites Li–LSX exchanged 78%, 83% and 99%. A newly developed adsorption device is applied to the gas–solid adsorption measurements. Isotherms of hydrogen adsorption are gravimetric volumetrically measured at 293.15 K up to 5 MPa. The accuracy of this new device is compared to NIST gas density data's of hydrogen and nitrogen at 293.15 K. Further the real density of the zeolites is obtained by helium skeleton density measurements at high temperature (650 K). The paper provides an interpretation of hydrogen adsorption capacities according to the gas-surface interaction. Further the isosteric heat of adsorption is obtained for the studied materials and analysed in relation to the zeolite cations exchange rate. Moreover, we discuss the influence of the ratio of cation exchange on hydrogen gas adsorption.     

Cryo-adsorptive hydrogen storage on activated carbon. I: Thermodynamic analysis of adsorption vessels and comparison with liquid and compressed gas hydrogen storage

This paper presents a thermodynamic analysis of cryo-adsorption vessels for hydrogen storage. The analysis is carried out with an unsteady lumped model and gives a global assessment of the behavior of the storage system during operation (discharge), dormancy and filling. The adsorbent used is superactivated carbon AX-21™. Cryogenic hydrogen storage, either by compression or adsorption, takes advantage of the effect of temperature on the storage density. In order to store 4.1 kg H₂ in 100 L, a pressure of 750 bar at 298 K is necessary, but only 150 bar at 77 K. The pressure is further reduced to 60 bar if the container is filled with pellets of activated carbon [7]. However, adsorption vessels are submitted to intrinsic thermal effects which considerably influence their dynamic behavior and due to which thermal management is required for smooth operation. In this analysis, among energy balances for filling and discharge processes, the influence of the intrinsic thermal effects during vessel operation is presented. Hydrogen losses during normal operation as well as during long periods of inactivity are also considered. The results are compared to those obtained in low-pressure and high-pressure insulated LH₂ and CH₂ tanks.     

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.     

Thermochemical method for controlling pore structure to enhance hydrogen storage capacity of biochar

Developing new carbon-based hydrogen storage materials can significantly promote solid-state hydrogen storage technology. Biochar with high hydrogen storage capacity can be prepared by KOH melt activation, which has a high proportion of micropores (96.56%) compared with the porous carbon in the existing literature. Its specific surface area and pore volume are 2801.88 m²/g and 1.44 cm³/g, respectively. The size of the nanopores is affected by the activation ratio, but the temperature has little effect at the low activation ratio. SEM results show that the KOH activation process gradually shifts from the biochar's inside to the outside. A low KOH/char ratio (less than 2:1) can promote the formation of small aromatic rings. Due to its high specific surface area and microporosity, the absolute adsorption capacity of hydrogen in biochar is 2.53 wt% at −196 °C and 1 bar, rising to 5.32 wt% at 50 bar. The hydrogen adsorption process conforms to the Langmuir model. Microporous, mesoporous, and macroporous exhibit different hydrogen adsorption characteristics in various pressure ranges. However, ultramicroporous (<0.7 nm) always plays a decisive role in the hydrogen storage of biochar.