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.     

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.     

Molecular insight on unusually high specific hydrogen adsorption over silicon carbide

Hydrogen adsorption over silicon carbide of different nature was investigated. High specific hydrogen adsorption (up to 15 μmol/m²), nontypical for silicon carbide was obtained for the materials prepared by the template method. A more perfect truly "carbide" structure (with a higher content of Si–C bonds in the form of carbide) in this type of materials contributes to an increase of the specific hydrogen adsorption. However, some non-stoichiometricity manifested in the presence of carbon in other (non-carbide) bonds (graphite or terminal and sp³ hybridized C) is required for exceptionally high specific hydrogen adsorption. The influence of the adsorbate nature for several gases (Н₂, N₂, CO) was found to be less prominent for SiC compared to carbon and silica materials. This feature inherent only to silicon carbide exhibiting an increased level of specific hydrogen adsorption, is due to cooperative adsorption, accompanied by an abnormally high loading of the adsorbent surface in the supercritical region.     

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.     

Quantification of “delivered” H2 by a volumetric method to test H2 storage materials

We set up and validated a volumetric method to quantify the amount of hydrogen “delivered” after saturation of a solid material as adsorber at different pressures (up to 100 kg_f/cm²) and temperatures (down to 77 K). This is the practically most relevant datum to quantify the effectiveness of an adsorbent for the present application. A complementary dynamic method has been also developed to take into account the reversibility of adsorption and to assess in at least a semi-quantitative way the strength of interaction between H₂ and the adsorbent. The method has been applied to compare the hydrogen storage capacity of some significant different carbon-based materials (two active carbons and one graphite), as supplied or after thermal treatments under oxidising or reducing conditions. The best results, ca. 7 wt% H₂ “delivered”, were achieved after saturation at 77 K, 20 kg_f/cm² with an active carbon with ca. 3000 m²/g of apparent specific surface area. The thermal treatments, almost always inducing a drop in surface area, showed effective only for saturation at 273 K, in particular the oxidising procedure. This was correlated to the formation of surface oxidised species, likely carboxylic groups, which improved the interaction strength between H₂ and the adsorbent.     

Volumetric hydrogen adsorption capacity of densified MIL-101 monoliths

The hydrogen adsorption isotherms of MIL-101 compressed pellets at 77.3 K are reported. The specific surface area and micropore volume decrease rather sharply when the pellet density approaches the crystal density. Optimum volumetric storage capacity of 40 g L⁻¹ is obtained for monoliths of remarkable mechanical integrity. The X-ray diffraction patterns do not exhibit notable changes with compression up to densities close to the crystal density. However, the infrared spectra show significant modification of the band structure in the range of vibration frequencies characteristic to the carboxylate and phenylene groups, due to the pressure-induced changes in the coordination environment of the metal, close to the adsorption sites. The compression effect on hydrogen adsorption can be correlated with the changes in the nitrogen adsorption isotherms. The results are discussed and compared with the literature results on volumetric hydrogen storage capacity of MOF-5 and MOF-177 monoliths.     

Hydrogen wettability of clays: Implications for underground hydrogen storage

This study investigates the ability of hydrogen (H₂) to wet clay surfaces in the presence of brine, with implications for underground hydrogen storage in clay-containing reservoirs. Rather than measuring contact angles directly with hydrogen gas, a suite of other gases (carbon dioxide (CO₂), argon (Ar), nitrogen (N₂), and helium (He)) were employed in the gas-brine-clay system under storage conditions (moderate temperature (333 K) and high pressures (5, 10, 15, and 20 MPa)), characteristic of a subsurface environment with a shallow geothermal gradient. By virtue of analogies to H₂ and empirical correlations, wettabilities of hydrogen on three clay surfaces were mathematically derived and interpreted. The three clays were kaolinite, illite, and montmorillonite and represent 1:1, 2:1 non-expansive, and 2:1 expansive clay groups, respectively. All clays showed water-wetting behaviour with contact angles below 40° under all experimental set-ups. It follows that the presence of clays in the reservoir (or caprock) is conducive to capillary and/or residual trapping of the gas. Another positive inference is that any tested gas, particularly nitrogen, is suitable as cushion gas to maintain formation pressure during hydrogen storage because they all turned out to be more gas-wetting than hydrogen on the clay surfaces; this allows easier displacement and/or retrieval of hydrogen during injection/production. One downside of the predominant water wettability of the clays is the upstaged role of biogeochemical reactions at the wetted brine-clay/silicate interface and their potential to affect porosity and permeability. Water-wetting decreased from kaolinite as most water-wetting clay over illite to montmorillonite as most hydrogen-wetting clay. Their wetting behaviour is consistent with molecular dynamic modelling that establishes that the accessible basal plane of kaolinite's octahedral sheet is highly hydrophilic and enables strong hydrogen bonds whereas the same octahedral sheet in illite and montmorillonite is not accessible to the brine, rendering these clays less water-wetting.     

Adsorption and compression contributions to hydrogen storage in activated anthracites

We prepared activated carbons (ACs) that are among the best adsorbents for hydrogen storage. These ACs were prepared from anthracites and have surface areas (S_BET) as high as 2772 m² g⁻¹. Anthracites activated with KOH presented the highest adsorption capacities with a maximum of 5.3 wt.% at 77 K and 4 MPa. Non-linearity between hydrogen uptake at 77 K and pore texture was confirmed, as soon as their S_BET exceeded the theoretical limiting value of (geometrical) surface area, i.e., S_BET > 2630 m² g⁻¹. We separated adsorption and compression contributions to total hydrogen storage. The amount of hydrogen stored is significantly increased by adsorption only at moderate pressure: 3 MPa and 0.15 MPa at 298 and 77 K, respectively. Hydrogen adsorption on ACs at high pressure, above 30 MPa at 298 K and 8 MPa at 77 K, has not interest because more gas can be stored by simply compression in the same tank volume.     

Synthesis and hydrogen adsorption properties of a new iron based porous metal-organic framework

A new metal-organic framework [Fe₃O(OOC-C₆H₄-COO)₃(H₂O)₃]Cl·(H₂O)_x was synthesized with a specific surface area of 2823 m²/g and a lattice parameter of 88.61 Å. Isostructural with MIL-101, this compound exhibits similar hydrogen adsorption properties, with maximum adsorption capacity of 5.1wt.% H at 77 K. The adsorption enthalpy of hydrogen for MIL-101 and ITIM-1 (MIL-101Fe) at zero coverage was calculated for a wide temperature range of 77 K ÷ 324 K, considering corrections for the variation of hydrogen gas entropy with the temperature. The resulted adsorption enthalpy is 9.4 kJ/mol for MIL-101, in excellent agreement with the value reported in literature from microcalorimetric measurements, and a value of 10.4 kJ/mol at zero coverage was obtained for ITIM-1 (MIL-101Fe).     

Determination of the enthalpy of adsorption of hydrogen in activated carbon at room temperature

The development of high-performance materials for hydrogen storage by adsorption requires detailed understanding of the adsorbate-adsorbent interactions, e.g., the enthalpy of adsorption ΔH, which measures the interaction strength. The determination of ΔH for a weakly adsorbing gas such as hydrogen in a carbonaceous porous material is difficult experimentally, normally requiring measuring two cryogenic adsorption isotherms. Here we demonstrate a calculation of ΔH based on ca. room temperature adsorption isotherms at 273 K and 296 K using the Clausius-Clapeyron equation. This requires an estimation of the volume of the adsorbed film (~40%, ~12% of the total pore volume at 77 K, 296 K, respectively) obtained from fits of the excess adsorption isotherms to an Ono-Kondo model with the auxiliary use of a fixed point corresponding to the saturation film density (estimated as 100 ± 20 g/L) which appears to be remarkably sample and temperature independent, i.e., a property of the adsorbate. The calculated room temperature enthalpy of adsorption ΔH = 8.3 ± 0.4 kJ/mol is in excellent agreement with the low-coverage cryogenic determination of ΔH. The methodology hereby proposed facilitates reliable calculations of the enthalpy of adsorption at room temperatures for weakly-adsorbing gases.     

Hydrogen storage in activated carbons produced from coals of different ranks: Effect of oxygen content

20 activated carbons (ACs) were prepared by activation of four coals of different ranks (bituminous, low-ash bituminous and sub-bituminous coals, and one anthracite) with potassium hydroxide, in order to evaluate their hydrogen storage capacities at −196 °C. The effect of surface area and oxygen content on hydrogen storage was examined. Oxygen content was determined by temperature-programmed desorption. The significance of oxygen content on hydrogen storage capacity was evaluated by Analysis of Variance (ANOVA). Apparent surface areas higher than 3000 m² g⁻¹ and hydrogen adsorption as high as 6.8 wt.% were obtained. The best results were obtained with ACs from bituminous coals. No significant effect of oxygen content on hydrogen adsorption was observed. We concluded that surface area controls hydrogen storage capacity at −196 °C.     

Preparation of activated rectangular polyaniline-based carbon tubes and their application in hydrogen adsorption

Porous materials, especially porous carbon materials, have the most potential as hydrogen adsorbents. In this research, a series of novel rectangular polyaniline tubes (RPTs) are synthesized using hollow carbon nanosphere (HCNS) templates. By changing mass ratios of ammonium persulfate to HCNSs, the sizes of RPTs can be controlled. Chemical activation with KOH gives rise to a large specific surface area (SSA), ranging from 1680 to 2415 m2 g⁻¹, and big pore volumes that range from 1.274 to 1.550 cm³g⁻¹. These observations demonstrate that activated rectangular polyaniline-based carbon tubes ARP-CTs are promising hydrogen adsorbents. Hydrogen uptake measurements show that the highest hydrogen adsorption reaches 5.2 wt% at 5 MPa/77 K and 0.62 wt% at 7.5 MPa/293 K respectively. Notably, the large pore volume can contribute 2.8 wt% to the total hydrogen storage which has approached 8.0 wt% at 5 MPa/77 K.     

Equilibrium,kinetics and enthalpy of hydrogen adsorption in MOF-177

Metal–organic framework (MOF-177) was synthesized, characterized and evaluated for hydrogen adsorption as a potential adsorbent for hydrogen storage. The hydrogen adsorption equilibrium and kinetic data were measured in a volumetric unit at low pressure and in a magnetic suspension balance at hydrogen pressure up to 100 bar. The MOF-177 adsorbent was characterized with nitrogen adsorption for pore textural properties, scanning electron microscopy for morphology and particle size, and X-ray powder diffraction for phase structure. The MOF-177 synthesized in this work was found to have a uniform pore size distribution with median pore size of 12.7 Å, a higher specific surface area (Langmuir: 5994 m²/g; BET: 3275 m²/g), and a higher hydrogen adsorption capacity (11.0 wt.% excess adsorption, 19.67 wt.% absolute adsorption) than previously reported values on MOF-177. Freundlich equation fits well the hydrogen adsorption isotherms at low and high pressures. Diffusivity and isosteric heat of hydrogen adsorption were estimated from the hydrogen adsorption kinetics and equilibrium data measured in this work.     

Remarkable isosteric heat of hydrogen adsorption on Cu(I)-exchanged SSZ-39

Hydrogen storage capacity on Cu(I)-exchanged SSZ-39 (AEI), -SSZ-13 (CHA) and Ultra stable-Y (US–Y, FAU) at temperatures between 279 K and 304 K are investigated. The gravimetric hydrogen storage capacity values reaching 83 μmol H₂ g⁻¹ (at 279 K and 1 bar) are found to be comparable with the highest adsorption capacity values reported on metal-organic frameworks. The volumetric hydrogen storage capacity values; on the other hand, are found to be more than three times of those reported on metal-organic frameworks (0.57 g/L on Cu(I)-SSZ-39 at 1 bar and 296 K vs. ca. 0.18 g/L on Co₂(m-dobdc) at 1 bar and 298 K (Kapelewski MT, Runčevski T, Tarver JD, Jiang HZH, Hurst KE, Parilla PA et al. Record High Hydrogen Storage Capacity in the Metal-Organic Framework Ni₂(m-dobdc) at Near-Ambient Temperatures. Chem Mater 2018; 30:8179–89)). The isosteric heat of adsorption values are calculated to be between 80 kJ mol⁻¹ and 49 kJ mol⁻¹ on Cu(I)-SSZ-39 and between 22 kJ mol⁻¹ and 15 kJ mol⁻¹ on Cu(I)-US-Y indicating H₂ adsorption mainly at Cu(I) cations located at the eight-membered rings on Cu(I)-SSZ-39 and at six-membered rings on Cu(I)-US-Y. Hydrogen adsorption experiments performed at 77 K showed higher adsorption capacity values for Cu(I)-SSZ-39 at 1 bar, but Cu(I)-US-Y showed potential for hydrogen storage at higher pressure values.     

Room-temperature hydrogen storage performance of metal-organic framework/graphene oxide composites by molecular simulations

Metal-organic framework/graphene oxide (MOF/GO) composites have been regarded as potential room-temperature hydrogen storage materials recently. In this work, the influence of MOF structural properties, GO functional group contents and different amounts of doped lithium (Li⁺) on hydrogen storage performance of different MOF/GO composites were investigated by grand canonical Monte Carlo (GCMC) simulations. It is found that MOF/GO composites based on small-pore MOFs exhibit enhanced hydrogen storage capacity, whereas MOF/GO based on large-pore MOFs show decreased hydrogen storage capacity, which can be ascribed to the novel pores at MOF/GO interface that favors the enhanced hydrogen storage performance due to the increased pore volume/surface area. By integrating the small-pore MOF-1 with GO, the hydrogen storage capacity was enhanced from 9.88 mg/go up to 11.48 mg/g. However, the interfacial pores are smaller compared with those in large-pore MOFs, resulting in significantly reduced pore volume/surface area as well as hydrogen storage capacities of large-pore MOF/GO composite. Moreover, with the increased contents of hydroxyl, epoxy groups as well as carboxyl group modification, the pore volumes and specific surface areas of MOF/GO are decreased, resulting in reduced hydrogen storage performance. Furthermore, the room-temperature hydrogen storage capacities of Li⁺ doped MOF/GO was improved with increased Li⁺ at low loading and decrease with the increased Li⁺ amounts at high loading. This is due to that the introduced Li⁺ effectively increases the accessible hydrogen adsorption sites at low Li⁺ loading, which eventually favors the hydrogen adsorption capacity. However, high Li⁺ loading causes ion aggregation that reduces the accessible hydrogen adsorption sites, leading to decreased hydrogen storage capacities. MOF-5/GO composites with moderate Li⁺ doping achieved the optimum hydrogen storage capacities of approximately 29 mg/g.     

Investigation of hydrothermal carbonization and chemical activation process conditions on hydrogen storage in loblolly pine-derived superactivated hydrochars

While the challenge of storing hydrogen in inexpensive and renewable adsorbents is relentlessly pursued by researchers all over the world, application of hydrochar derived from biomass is also gaining attention as it can be subsequently chemically activated using activating agents like KOH in order to tailor the development of favorable porosity. However, the synergistic effect of hydrothermal carbonization (HTC) process conditions as well as KOH activating conditions on the development of surface morphology is required to be assessed with the application of such porous superactivated hydrochars in hydrogen storage application. In this study, highly porous superactivated hydrochars were fabricated from inexpensive and abundant loblolly pine. Loblolly pine was hydrothermally carbonized at 180 °C, 220 °C and 260 °C and the hydrochars were then activated at different experimental conditions of 700 °C, 800 °C and 900 °C using solid KOH to loblolly pine hydrochar ratio of 2:1, 3:1 and 4:1 to produce superactivated hydrochars. Superactivated hydrochars as well as loblolly pine and its corresponding hydrochars underwent physicochemical analysis as well as surface morphology analysis by SEM and nitrogen adsorption isotherms at 77 K in order to investigate the effect on BET, pore volume, and pore size distribution due to various process conditions. The superactivated hydrochars were then analyzed to quantify total hydrogen storage capacity of these materials at 77 K and up to pressure of 55 bar. Porosity of superactivated hydrochars were as high as 3666 m²/g of BET specific surface area (SSA), total pore volume of 1.56 cm³/g and micropore volume of 1.32 cm³/g with the hydrogen storage capacity of 10.2 wt% at 77 K and 55 bar. It was conclusive from principal component analysis that higher HTC temperature with moderate activation condition demonstrated favorability in developing porous superactivated hydrochars for hydrogen storage applications.     

Design and synthesis of Ti-peroxo/phosphorus heterostructures for enhanced photocatalytic hydrogen evolution

For the heterojunction composite photocatalyst, the contact interface is the key to charge carrier separation conditions. In order to present novel research through the interaction between these interfaces, the Ti based peroxo complex (TP)/red phosphorus (RP) composite system was introduced and designed to improve carrier separation and transport properties during photocatalytic hydrogen evolution. In this study, we have successfully synthesized TP/RP by facile solution process through stirring at room temperature and pressure. Regarding the specific surface area, which is one of the important factors in the photocatalytic activity, it was confirmed that the specific surface area of the TP (166.4 m²/g) and TP/RP (281.4 m²/g) samples was dramatically improved as the particle surface was oxidized based on TiH₂ (0.613 m²/g), the precursor. And the photo-induced charge carrier life time of the TP/RP was extended by approximately 60% compared to the conventional TP. Finally, excellent research results were obtained in which the photocatalytic hydrogen evolution efficiency (17.05 μmol/h) under visible-light irradiation (200 W Xenon lamp) was improved by about 3 times than that of the conventional TP sample (5.26 μmol/h).     

Remarkable promotion of photocatalytic hydrogen evolution from water on TiO2-pillared titanoniobate

TiO₂-pillared titanoniobate TiO₂/HTiNbO₅ as an efficient photocatalyst was prepared via an exfoliation–restacking route. The as-prepared nanohybrid is mesoporous with a high specific surface area of 171 m²/g and a gallery height of 1.55 nm. Under a 300 W Xe lamp irradiation, the nanohybrid exhibited a high photocatalytic activity of 219 μmol/h/(g cat) in splitting water into hydrogen, which is 12 times as high as its parent HTiNbO₅ (18 μmol/h/g) and 24 times as TiO₂ (9 μmol/h/g). Enlarged surface area and effective electronic coupling between the host and the guest components contribute to the high photocatalytic activity of TiO₂/HTiNbO₅. Its photocatalytic activity was further improved through platinizing, and 5 wt% Pt-loaded TiO₂/HTiNbO₅ gave a remarkable hydrogen evolution rate of 4735 μmol/h/g. A photoexcitation model of the semiconductor–semiconductor pillared photocatalyst was proposed based on the results of XPS and UV–vis.     

Preparation of a porous carbon from ferrocene-loaded polyaniline and its use in hydrogen adsorption

A porous carbon made of polyaniline with different ferrocene loadings was prepared through carbonization and thermal chemistry activation with KOH. The ferrocene served as a pore-forming agent and a resource of iron nanoparticles. N₂ adsorption/desorption measurements showed that the specific surface area and pore volume ranged from 2681 to 3246 m² g⁻¹ and from 1.56 to 2.06 cm³ g⁻¹, respectively, with increasing ferrocene loadings. Similarly, hydrogen adsorption also increased from 5.3 to 6.2 wt% at 77 K/5 MPa and 0.6 wt% to 0.85 wt% at 293 K/8 MPa. Scanning electron microscopy, X-ray diffraction and energy dispersive X-ray analysis showed that iron nanoparticles were embedded in the carbon matrix or dispersed on the surface. The large specific surface area and big pore volume improved the original hydrogen adsorption heat up to 7.2 kJ mol⁻¹ for the best sample.     

Polyacrylonitrile-based highly porous carbon materials for exceptional hydrogen storage

In this paper, a common low-cost chemical material-polyacrylonitrile (PAN) is transformed into porous carbon with excellent specific surface area (2564.6–3048.8 m² g⁻¹) and highly concentrated micropore size distribution (0.7–2.0 nm). Benefit to the unique structure, the as-prepared materials show appealing hydrogen adsorption capacity (4.70–5.94 wt % at 20 bar, 7.15–10.14 wt % at 50 bar), demonstrating a promising prospect of practical application. This work also confirmed that the narrow and deep ultramicropore (<0.7 nm) could facilitate adsorption of hydrogen molecules significantly at atmospheric pressure, and the volume increase of supermicropore (0.7–2.0 nm) could lead to hydrogen capacity promotion at relative high pressure (>20 bar), which provides valuable guidance for the construction of ideal porous adsorbent for efficiency hydrogen storage.