Thermodynamic assessment of carbazole-based organic polycyclic compounds for hydrogen storage applications via a computational approach

Liquid organic hydrogen carriers (LOHCs) are promising candidates for storage and transport of renewable energy due to their reversible reaction characteristics. For the proper assessment of candidate molecules, various thermochemical properties are required, and significant experimental efforts are necessary. In this work, we suggest a systematic method for the estimation of thermochemical properties for LOHC candidate molecules combining Density Functional Theory (DFT) calculations, Conductor-like Screening Model (COSMO) and Molecular Dynamics (MD) simulations. We applied the suggested method for the assessment of previously reported LOHC materials. Based on the analysis, new candidates of carbazole-derivative compounds (N-acetylcarbazole, N-phenylcarbazole, N-benzoylcarbazole, and 4-methyl-4H-benzocarbazole) are suggested, and their properties are estimated and reviewed. Calculation results show that these candidates can provide high theoretical hydrogen uptake capacities above 6 wt% and optimal heats of dehydrogenation in the liquid phase. Analysis on the stereoisomerism showed that the structure-selectivity toward less stable stereoisomers of the hydrogen-rich form is preferable for the dehydrogenation process.     

Boosting the hydrogenation activity of dibenzyltoluene catalyzed by Mg-based metal hydrides

Hydrogenation of dibenzyltoluene (DBT) is of great significance for the application in liquid organic hydrogen carriers (LOHCs). We successfully develop Mg-based metal hydrides (Mg₂NiH₄, MgH₂, and LaH₃) reactive ball-milling for the hydrogenation of DBT. Mg-based metal hydrides milled with 500 min exhibit the best catalytic activity, the hydrogen uptake of DBT can reach 4.63 wt% at the first 4 h and finally achieve 5.70 wt% through 20 h, which is the first time to use hydrogen storage material as a catalyst for the hydrogenation of DBT. The excellent catalytic hydrogenation performance of Mg-based metal hydrides mostly originates from numerous catalytic activity centers formed at the surfaces of Mg₂NiH₄ nanoparticles in the MgH₂ matrix. Inspired by this mechanism, more general metal hydrides can be explored for catalyzing the hydrogenation of LOHCs. The new application of Mg-based metal hydrides is beneficial to developing efficient LOHC based hydrogen storage systems and offers novel insights to hydride-based catalysts.     

Assessment of system variations for hydrogen transport by liquid organic hydrogen carriers

One option to transport hydrogen over longer distances in the future is via Liquid Organic Hydrogen Carriers (LOHC). They can store 6.2 wt% hydrogen by hydrogenation. The most promising LOHCs are toluene and dibenzyltoluene. However, for the dehydrogenation of the LOHCs – to release the hydrogen again – temperatures above 300 °C are needed, leading to a high energy demand. Therefore, a Life Cycle Assessment (LCA) and Life Cycle Costing are conducted. Both assessments concentrate on the whole life cycle rather than just direct emissions and investments. In total five different systems are analysed with the major comparison between conventional transport of hydrogen in a liquefied state of matter and LOHCs. Variations include electricity supply for liquefaction, heat supply for dehydrogenation and the actual LOHC compound. The results show that from an economic point of view transport via LOHCs is favourable while from an environmental point of view transport of liquid hydrogen is favourable.     

Liquid Organic Hydrogen Carrier (LOHC) – Assessment based on chemical and economic properties

Hydrogen can be transported via long distances based on Liquid Organic Hydrogen Carriers (LOHC). Such a transport is realized based on a two-step cycle: (1) loading/storage of hydrogen (hydrogenation) into the LOHC molecule and (2) unloading/release of hydrogen (de-hydrogenation). During the storage period, hydrogen is covalently bound to the respective LOHC. Since the (optimal) LOHC is liquid at ambient conditions and shows similar properties as crude oil based liquids (e.g. diesel, gasoline), it can easily be handled, transported and stored; thus a stepwise implementation using the existing crude oil based infrastructure would be possible. Against this background this paper reviews the current knowledge in hydrogenation and de-hydrogenation of various LOHC. Therefore, a variety of LOHC is evaluated based on their properties and compared to each other. By applying different evaluation criteria representing the requirements of the three different application areas (energy-storage, energy-transport, mobility application), the LOHCs can be assigned to a field they suit best. The analysis shows that the most promising LOHC candidates to date are dibenzyltoluene for energy-transport and energy-storage as well as N-ethylcarbazole for mobility applications. In addition, a use of toluene in the transport sector is also conceivable. Methanol can potentially be applied in all three application fields due to its properties if a compromise between de-hydrogenation temperature and gas flow can be achieved based on further R&D-activities. For future implementation phenazine and formic acid show great potential, but also additional R&D especially regarding catalysis and solvents is necessary.     

Substituted heterocycles as new candidates for liquid organic hydrogen carriers: In silico design from DFT calculations

A new set of compounds based on N- and S-heterocycles were investigated through Density Functional Theory (DFT) for their use as liquid organic hydrogen carriers (LOHCs). The hydrogenated forms of these compounds could release hydrogen within the most important technical requirements in mobile and stationary applications. In this work, the potential of the 1H-pyrrole/tetrahydro-1H-pyrrole and thiophene/tetrahydrothiophene pairs as possible leader structures to synthesize more sustainable LOHCs from costless oil-refining and oil-hydrotreating by-products is shown. According to DFT-M06-HF results, the 3-allyl-1H-pyrrole/3-allyl-tetrahydro-1H-pyrrole pair presented an adequate theoretical hydrogen storage capacity (3.6 %wt H) and a high theoretical dehydrogenation equilibrium yields (% ε_d = 67.8%) at 453 K. Therefore, this pair is recommended for hydrogen storage stationary applications. On the other hand, the 2-(thiophen-2-yl)-1H-pyrrole/2-(2,3-dihydrothiophen-2-yl)tetrahydropyrrole pair proved to be suitable for both mobile and stationary applications; the storage capacity of this pair was 3.9 %wt H and the theoretical dehydrogenation equilibrium yields at 453 K (% ε_d = 28.1%) was considered moderate.     

Influence of dissolved hydrogen on the viscosity and interfacial tension of the liquid organic hydrogen carrier system based on diphenylmethane by surface light scattering and molecular dynamics simulations

In hydrogenation and dehydrogenation processes of liquid organic hydrogen carriers (LOHCs), molecular hydrogen (H₂) is present, but its influence on the thermophysical properties of the LOHC compounds is still hardly known. This study provides experimental results from surface light scattering and predictions from molecular dynamics simulations on the influence of dissolved H₂ on the liquid viscosity, interfacial tension, and liquid density of the LOHC system based on diphenylmethane at varying degree of hydrogenation, process-relevant temperatures up to 573 K, and pressures up to 7 MPa. First-time measurements of the viscosity of bicyclic hydrocarbon compounds in the presence of dissolved H₂ at saturation conditions reveal a negligible effect of pressure. The interfacial tension decreases independently of the LOHC composition by about 6% at 7 MPa. The simulations can adequately represent the effect of H₂ on the interfacial tension and evidence a weak enrichment of H₂ at the interface.     

Burner-heated dehydrogenation of a liquid organic hydrogen carrier (LOHC) system

For a hydrogen-based economy, safe and efficient hydrogen storage is essential. Compared to other chemical hydrogen storage technologies, such as ammonia or methanol, liquid organic hydrogen carrier (LOHC) systems allow for a reversible storage of hydrogen while being easy to handle in a diesel-like manner. In our contribution, we describe for the first time the successful utilization of the exhaust gas enthalpy of a porous media burner to directly supply the dehydrogenation heat for a kW-scale dehydrogenation of the hydrogen-rich LOHC compound perhydro dibenzyltoluene (H18-DBT). Our setup demonstrates the dynamics of the dehydrogenation unit at a realized maximum hydrogen power of 3.9 kW_th, based on the lower heating value of the released hydrogen. For the intended applications with fluctuating hydrogen demand, e.g. a hydrogen refueling station (HRS) or stationary heating in buildings, a dynamic hydrogen supply from LOHC is important. Methane, e.g. from a biogas plant, is utilized in our scenario as a fuel source for the burner. Hydrogen is released within 30 min after cold start of the system. The dehydrogenation unit exhibits a power density relative to the reactor volume of about 0.5 kW_therm l⁻¹ based on the lower heating value of the hydrogen and a catalyst productivity of up to 0.65 g_H2 g_Pt⁻¹ min⁻¹ for hydrogen release from H18-DBT. An analysis of the by-products and reaction intermediates shows low by-product formation (e.g. maximum 0.6 wt.-% for high boilers and 0.9 wt.- % for low boilers) and uniform distribution of intermediates after the reaction. Thus, a relatively homogeneous temperature distribution and a uniform LOHC flow in the reaction zone can be assumed. Our findings illustrate the dynamics (heating rates of about 10 K min⁻¹) and performance of direct heating of a release unit with a burner and represent a significant step towards LOHC-based hydrogen provisioning systems at technically relevant scales.     

Pt/CeO2 catalyst synthesized by combustion method for dehydrogenation of perhydro-dibenzyltoluene as liquid organic hydrogen carrier: Effect of pore size and metal dispersion

Liquid organic hydrogen carrier (LOHC) is a chemical hydrogen storage method that stores hydrogen in the form of liquid organics. Dibenzyltoluene (DBT) is a promising LOHC material due to its high storage density, low ignitability, and low cost. In this study, Pt/Al₂O₃ and Pt/CeO₂ catalysts are synthesized using a combustion nanocatalyst synthesis method called the glycine nitrate process (GNP) to obtain high catalytic activity for the dehydrogenation of perhydro-dibenzyltoluene (H18-DBT). Pt/CeO₂ exhibits much faster dehydrogenation than Pt/Al₂O₃, 80.5%/2.5 h versus 3.5%/2.5 h. To investigate the causes of the difference in the dehydrogenation rates, microstructural characterization by N₂ physisorption, CO chemisorption and transmission electron microscopy analysis are conducted, and the catalytic activities are evaluated at various liquid hourly space velocities (LHSVs). The differences in dehydrogenation can be attributed to the mass transport of liquid H18-DBT into the catalyst pores being slow due to the small pores in Pt/Al₂O₃, which is a rarely addressed issue for other LOHC materials. This is because many LOHC materials are dehydrogenated at the gas phase, which has higher diffusivity than that of the liquid phase. Pt/CeO₂ synthesized by the GNP is also compared with a commercial Pt/Al₂O₃ catalyst. The commercial Pt/Al₂O₃ catalyst shows a dehydrogenation of 17.8%/2.5 h, which is much slower than that of Pt/CeO₂ synthesized by the GNP, at 80.5%/2.5 h.     

Benzyltoluene/dibenzyltoluene-based mixtures as suitable liquid organic hydrogen carrier systems for low temperature applications

In this contribution we propose mixtures of the two LOHC systems benzyltoluene (H0-BT)/perhydro benzyltoluene (H12-BT) and dibenzyltoluene (H0-DBT)/perhydro dibenzyltoluene (H18-DBT) as promising hydrogen storage media for technical applications at temperatures below ambient. The mixing of the two LOHC systems provides the advantage of a reduced viscosity of the hydrogen-rich system, for example a 20 wt% addition of H12-BT to H18-DBT reduces the viscosity at 10 °C by 80%. Interestingly, it is also found that the dehydrogenation of such mixture provides a hydrogen release productivity that is 12–16% higher compared to pure H18-DBT dehydrogenation under otherwise identical conditions. This enhanced rate is attributed to a combination of reduced hydrogen partial pressure in the reactor (due to the higher H12-BT vapor pressure), preferred H12-BT dehydrogenation (due to faster H12-BT diffusion) and effective transfer hydrogenation between the two LOHC systems.     

Connected evaluation of polymer electrolyte membrane fuel cell with dehydrogenation reactor of liquid organic hydrogen carrier

Recently, hydrogen energy technologies attract attention as power systems. To develop hydrogen energy systems, hydrogen storage methods with high storage density and good safety are required. Liquid organic hydrogen carrier (LOHC) is one of the novel hydrogen storage technologies. LOHC has advantages of high storage density, good safety, and easy handling. In this study, a polymer electrolyte membrane fuel cell (PEMFC) stack is operated with hydrogen released from LOHC to evaluate the feasibility of the connected operation of the PEMFC stack and LOHC dehydrogenation reactor. Dibenzyltoluene (H0-DBT) is used as a LOHC material, and the dehydrogenation of perhydro dibenzyltoluene (H18-DBT) is conducted at 240–300 °C. Released hydrogen is purified by adsorbent of activated carbon to remove impurities. However, 100–1400 ppm of methane is observed after the purification, and the PEMFC stack power is reduced from 39.4 W to 39.0 W during the operation by hydrogen dilution and physical adsorption of methane. Then, to evaluate the irreversible damage, pure hydrogen was supplied to the PEMFC stack. The stack power is recovered to 39.4 W. It is concluded that the connected operation of the LOHC dehydrogenation reactor and PEMFC stack is feasible, and the activated carbon adsorbent can be a cost-effective purification method for LOHC.     

Computational evaluation of Mg-decorated g-CN as clean energy gas storage media

Ab initio studies were conducted to evaluate the performance of hydrogen storage by Mg-decorated graphite carbon nitride (g-CN, heptazine structure). In our calculations, we found that each unit of this material can accommodate one Mg atom. Partial charges from Mg were transferred to the pristine material, making itself more electropositive. This is favorable for hydrogen storage, as the adsorbed H₂ molecules can be easily polarized, and the electrostatic interactions can be enhanced. The configurations of the Mg-decorated g-CN with multiple adsorbed H₂ molecules were presented in this study, and the related adsorption mechanisms were also discussed in details. Each unit can adsorb at most 7 H₂ molecules with adsorption energies ranging from −0.276 eV to −0.130 eV. In addition, besides Mg, we also noticed that the nitrogen atoms also perform well in hydrogen adsorption. For this novel material, its highest capacity of hydrogen storage can reach to 7.8 wt%, highly surpassing the target value of 5.5 wt% set by the U.S. department of energy (DOE)[1]. The computational results provided in this study indicates a promising prospect for alkali metal functionalized 2D materials in energy storage; and through decent explorations, the performance of this class of materials can be largely improved.     

A highly adaptable Ni catalyst for Liquid Organic Hydrogen Carriers hydrogenation

Reducing the cost of hydrogenation/dehydrogenation catalysts and improving the catalytic activity are essential steps to promote the commercial application of Liquid Organic Hydrogen Carriers (LOHCs) technology. We reported a series of highly adaptable 70 wt% Ni supported catalysts prepared by a facile co-precipitation method. The as-prepared catalysts were used in the hydrogenation of several promising LOHCs candidates, including benzene, N-propylcarbazole, N-ethylcarbazole and dibenzyltoluene. By adjusting the ratio of Al and Si, the Ni₇₀/AlSiO-1/1 catalyst with Al and Si in a molar ratio of 1:1 presents highest catalytic activity for hydrogenation of the above LOHCs, indicating the catalyst is highly adaptable for different LOHCs. The characterization results proved that the presence of SiO₂ could significantly weaken the interaction between metal and carrier and decrease the formation of NiAl₂O₄ species, which is beneficial to the reducibility of Ni. The introduced Al₂O₃ can inhibit the agglomeration of Ni and increase the dispersion of the metal. Besides, the Ni₇₀/AlSiO-1/1 catalyst was used to hydrogenate N-propylcarbazole by 5 cycles. In the fifth cycle, the hydrogen uptake reached the theoretical hydrogenation storage within 1.5 h, which suggested the excellent stability of the catalyst. Because of its low cost, high efficiency, high adaptation and highly stable, the self-made Ni catalyst has potential prospect in large-scale LOHCs application.     

Role of vacancy defects on the dehydrogenation properties of the ternary hydride ZrNiH3: Ab-initio insights

ZrNi is considered a promising candidate for hydrogen storage and nickel-metal hydride rechargeable batteries (Ni-MH). The effect of creating zirconium and nickel vacancy defects on the dehydrogenation properties of ZrNiH₃ is investigated by means of first-principles calculations. The results indicate that nickel vacancy is energetically more favorable to form in ZrNiH₃ than zirconium vacancy, because of the lesser formation energy of Ni-vacancy. For both Zr and Ni vacancy defects, the formation enthalpy decreases with increasing the concentration of vacancy and, vice versa. In particular, it is found that with ~2.4% of zirconium vacancy defects or with ~4.5% of nickel vacancy defects in ZrNiH₃, the formation enthalpy is around - 40 kJ/mol.H₂, which is recommended by the U.S. Department of Energy (DOE). It is worth noting also that with slightly higher vacancy defects ~2.8 of Zr-vacancy or ~5.3% of Ni-vacancy in ZrNiH₃, it becomes harder to store hydrogen in these systems without cooling. Moreover, the density of states (DOS) analysis indicates that the stability of ZrNiH₃ decreases with increasing Zr-vacancy and Ni-vacancy concentrations, through the shrinkage in the size of the total DOS and shifting in the valence bands near to Fermi level.     

High active pd@mil-101 catalyst for dehydrogenation of liquid organic hydrogen carrier

Highly dispersed Pd nanoparticles immobilized in MIL-101 (Pd@MIL-101) were prepared and used for the catalytic dehydrogenation of Liquid organic hydrogen carriers (LOHC). The as-synthesized catalysts were characterized and it was found that 3 wt% of Pd@MIL-101 embodied smaller and highly dispersed Pd NPs. The catalytic activities of as-synthesized catalysts were investigated by the dehydrogenation of a representative LOHC compound, perhydro-N-propylcarbazole (12H-NPCZ). The results indicated that 3 wt% Pd@MIL-101 catalyst exhibited good catalytic activity and good reusability for the dehydrogenation of 12H-NPCZ, which is superior to that of commercial 5 wt% Pd/Al₂O₃ catalyst. This study demonstrates that Pd@MIL-101 is a promising dehydrogenation catalyst for the application of LOHC technology.     

Hydrogen storage using liquid organic carriers: Equilibrium simulation and dehydrogenation reactor design

Liquid organic hydrogen carriers (LOHC) are unsaturated organic compounds used for chemical hydrogen storage. Using an equilibrium model of the LOHC N-ethylcarbazole, we discuss potential efficiency increases of hydrogen storage systems based on N-ethylcarbazole by the integration of low-temperature waste heat. N-ethylcarbazole is well suited for pressure swing operation with heat exchange between hydrogenation and dehydrogenation. We present and discuss kinetic data of the dehydrogenation reaction gathered in a tubular reactor that was mounted in different orientations and flow configurations. Similar maximum values of power density are reached in vertical and in horizontal orientation. Vertical orientation allows the favorable operation with counter-flow of the liquid carrier and the evolved hydrogen gas and radial heat transfer is significantly better than in horizontal orientation. In vertical reactor configurations, catalyst efficiency and operational stability are impaired at high void fractions. This issue can be reduced by dehydrogenation at elevated pressure and intermediate gas separation from the catalyst bed.     

Combined dynamic operation of PEM fuel cell and continuous dehydrogenation of perhydro-dibenzyltoluene

Hydrogen storage in liquid organic hydrogen carriers (LOHC) such as the substance system dibenzyltoluene/perhydro-dibenzyltoluene (H0/H18-DBT) offers a promising alternative to conventional methods. In this contribution, we describe the successful demonstration of the dynamic combined operation of a continuously operated LOHC reactor and a PEM (polymer exchange membrane) fuel cell. The fuel cell was operated stable with fluctuating hydrogen release from dehydrogenation of H18-DBT over a total period of 4.5 h, reaching electrical stack powers of 6.6 kW. The contamination with hydrocarbons contained in the hydrogen after activated carbon filtering did not result in any detectable impairment or degradation of the fuel cell. The proposed pressure control algorithm based on a proportional integral (PI) controller proved to be a robust and easy-to-implement approach to enable the dynamic combined operation of LOHC dehydrogenation and PEM fuel cell.     

Hydrogenation of liquid organic hydrogen carrier systems using multicomponent gas mixtures

The cost of industrial hydrogen production and logistics, and the purity of hydrogen produced from different technologies are two critical aspects for the success of a future hydrogen economy. Here, we present a way to charge the Liquid Organic Hydrogen Carrier (LOHC) dibenzyltoluene (H0-DBT) with industrially relevant, CO₂- and CO-containing gas mixtures. As only hydrogen binds to the hydrogen-lean carrier molecule, this process step extracts hydrogen from the gas mixture and binds it selectively to the carrier. Pd on alumina has been identified as the most promising catalyst system for successfully hydrogenating H0-DBT using model gas mixtures resembling the compositions produced in methane reforming and in industrial coke production (up to 50% CO₂ and 7% CO). Up to 80% of the hydrogen present in the feedstock mixture could be extracted during the LOHC hydrogenation process. 99.5% of the reacting hydrogen was selectively bound to the H0-DBT LOHC compound. The purity of hydrogen released from the resulting perhydro dibenzyltoluene previously charged with the hydrogen-rich gas mixture proved to be up to 99.99 mol%.     

First-principles study on the dehydrogenation properties and mechanism of Al-doped Mg2NiH4

Mg₂NiH₄, with fast sorption kinetics, is considered to be a promising hydrogen storage material. However, its hydrogen desorption enthalpy is too high for practical applications. In this paper, first-principles calculations based on density functional theory (DFT) were performed to systematically study the effects of Al doping on dehydrogenation properties of Mg₂NiH₄, and the underlying dehydrogenation mechanism was investigated. The energetic calculations reveal that partial component substitution of Mg by Al results in a stabilization of the alloy Mg₂Ni and a destabilization of the hydride Mg₂NiH₄, which significantly alters the hydrogen desorption enthalpy ΔH_des for the reaction Mg₂NiH₄ → Mg₂Ni + 2H₂. A desirable enthalpy value of ∼0.4 eV/H₂ for application can be obtained for a doping level of x ≥ 0.35 in Mg_2−xAl_xNi alloy. The stability calculations by considering possible decompositions indicate that the Al-doped Mg₂Ni and Mg₂NiH₄ exhibit thermodynamically unstable with respect to phase segregation, which explains well the experimental results that these doped materials are multiphase systems. The dehydrogenation reaction of Al-doped Mg₂NiH₄ is energetically favorable to perform from a metastable hydrogenated state to a multiphase dehydrogenated state composed of Mg₂Ni and Mg₃AlNi₂ as well as NiAl intermetallics. Further analysis of density of states (DOS) suggests the improving of dehydrogenation properties of Al-doped Mg₂NiH₄ can be attributed to the weakened Mg-Ni and Ni-H interactions and the decreasing bonding electrons number below Fermi level. The mechanistic understanding gained from this study can be applied to the selection and optimization of dopants for designing better hydrogen storage materials.     

Study on reversible hydrogen uptake and release of 1,2-dimethylindole as a new liquid organic hydrogen carrier

Indole derivatives have been considered as promising liquid organic hydrogen carriers (LOHCs) for onboard hydrogen storage applications. Here a new member of indole family, 1,2-dimethylindole (1,2-DMID), was reported as a potential liquid organic hydrogen carrier with a hydrogen storage content of 5.23 wt%, a meting point of 55 °C and a boiling point of 260 °C. Full hydrogenation and dehydrogenation of 1,2-DMID can be achieved with fast kinetics under mild conditions. The hydrogenation of 1,2-DMID followed the first order kinetics with an apparent activation energy of 85.1 kJ/mol. Dehydrogenation of fully hydrogenated product, octahydro-1,2-DMID was conducted over 5 wt% Pd/Al₂O₃ at 170–200 °C. The stored hydrogen can be completely released at 180 °C in 3 h and at 200 °C in 1 h. The energy barrier of dehydrogenation of octahydro-1,2-DMID was calculated to be 111.9 kJ/mol 3 times cycles of hydrogenation and dehydrogenation were employed to test the recycle ability of 1,2-DMID. The structures of intermediates were also discussed by means of Material Studio calculations.     

Li decorated penta-silicene as a high capacity hydrogen storage material: A density functional theory study

The H₂ adsorption characteristics of Li decorated single-sided and double-sided penta-silicene are predicted via density functional theory (DFT). The orbital hybridization results in Li atom strongly bind onto the surface of the penta-silicene with a large binding energy and it keeps the decorated Li atoms from aggregation. Moreover, Li decorated double-sided penta-silicene can store up to 12H₂ molecules with the average hydrogen adsorption energy of ?0.220 eV/H₂ and hydrogen uptake capacity of 6.42 wt%, respectively. The ab initio molecular dynamics (AIMD) simulations demonstrate the H₂ molecules are released gradually from the substrate material with the increasing simulation time and the calculated desorption temperature T_D is 281 K in the suitable operating temperature range. Our explorations confirm that Li decorated penta-silicene can be regarded as a promising hydrogen storage candidate for hydrogen storage applications.