A High-Capacity, Reversible Liquid Organic Hydrogen Carrier: H2

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A High-Capacity, Reversible Liquid Organic Hydrogen Carrier: H2-Release Properties and an Application to a Fuel Cell Munjeong Jang, Young Suk Jo, Won Jong Lee, Byeongsoo Shin, Hyuntae Sohn, Hyangsoo Jeong, Seong Cheol Jang, Sang Kyu Kwak, Jeong Won Kang, and Chang Won Yoon ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04835 • Publication Date (Web): 12 Nov 2018 Downloaded from http://pubs.acs.org on November 13, 2018

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ACS Sustainable Chemistry & Engineering

A High-Capacity, Reversible Liquid Organic Hydrogen Carrier: H2-Release Properties and an Application to a Fuel Cell

Munjeong Jang1,2,§, Young Suk Jo1,§, Won Jong Lee1, Byeong Soo Shin3, Hyuntae Sohn1, Hyangsoo Jeong1, Seong Cheol Jang1, Sang Kyu Kwak4, Jeong Won Kang3, Chang Won Yoon1,2,5,*

1

Fuel Cell Research Center, Korea Institute of Science and Technology (KIST), 5 Hwarang-ro 14-gil,

Seongbuk-gu, Seoul 02792, Republic of Korea 2

Divison of Energy and Environment Technology, KIST School, Korea University of Science and

Technology, 5 Hwarang-ro 14-gil, Seongbuk-gu, Seoul 02792, Republic of Korea 3

Department of Chemical and Biological Engineering, Korea University, Anam-ro 145, Seongbuk-gu,

Seoul 02841, Republic of Korea 4

School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology, 50,

UNIST-gil, Ulsan, 44919, Republic of Korea 5

KHU-KIST Department of Converging Science and Technology, Kyung Hee University, 26

Kyungheedae-ro, Dongdaemun-gu, Seoul 02447, Republic of Korea

Corresponding author: [email protected] §

These authors contributed equally.

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ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract Hydrogen storage in the form of a liquid chemical is an important issue that can bridge the gap between sustainable hydrogen production and utilization with a fuel cell, which is one of the essential sectors in the hydrogen economy. Herein, the application of a potential liquid organic hydrogen carrier, consisting of biphenyl and diphenylmethane, is demonstrated as a safe and economical hydrogen storage material. The presented material is capable of a reversible storage and release of molecular hydrogen with 6.9 wt% and 60 g-H2 L-1 of gravimetric and volumetric hydrogen storage capacities, presenting superior properties as a hydrogen carrier. Equilibrium conversion and the required enthalpies of dehydrogenation are calculated using a density functional theory. Experimentally, dehydrogenation conversion of greater than 99% is achieved, producing molecular hydrogen with greater than 99.9% purity, with negligible side reactions; this is further confirmed by nuclear magnetic resonance spectroscopy. Less than 1% of the material is lost after cyclic tests of hydrogenation and dehydrogenation conducted consecutively nine times. Finally, a dehydrogenation system is designed and operated in conjunction with a polymer electrolyte membrane fuel cell that can generate greater than 0.5 kW of electrical power in a continuous manner, proving its capability as a promising liquid organic hydrogen carrier.

Keywords: Reversible Hydrogen Storage; Liquid Organic Hydrogen Carrier; Biphenyl; Diphenylmethane; Catalytic Dehydrogenation; Fuel Cell


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Introduction Global warming, caused by the increased consumption of fossil fuels, has been a major environmental problem for many decades. Accordingly, significant efforts have been made to reduce the carbon footprint 1-2, and the use of renewable energies is expected to mitigate the current worldwide energy and environmental issues, particularly associated with climate change. Owing to the unpredictable nature of renewable energy supplies, electrical energy produced by renewable energy sources requires a stable storage medium that allows on demand discharge. In this context, hydrogen has been recognized as a clean and sustainable renewable energy carrier that can achieve high efficiencies when used in conjunction with fuel cells 3-5. Whereas hydrogen has a high gravimetric energy density of ca. 33.3 kWh kg-1 6, applications of hydrogen as an energy carrier have been limited because of its low volumetric energy density (2.97 Wh L-1; H2 gas, 273 K, 1 atm), difficulty in storage and delivery, and safety issues such as potential explosiveness and high flammability 7. Therefore, for hydrogen to be applicable to different energy uses, it is necessary to develop a safe and economically viable method for storing mass quantities of hydrogen. To address these issues, not only physical hydrogen storage by compression or liquefaction but also material-based hydrogen storage with metal hydrides 8 and chemical hydrides 9-10 have been extensively developed. Among the proposed storage materials, liquid organic hydrogen carriers (LOHCs), especially those based on aromatic (e.g., toluene 11, naphthalene 12, and dibenzyltoluene 13) and heteroaromatic (e.g., N-ethylcarbazole 14-16) organic compounds have attracted significant attention because the organic hydrides have reasonably high gravimetric hydrogen contents of 5–8 wt% 17, high volumetric hydrogen storage capacities of greater than 60 g-H2 L-1, reversibility under catalytic hydrogenation and dehydrogenation, and high compatibility with the existing gasoline infrastructure

17-19

. Recently, we

reported a highly capable and novel hydrogen carrier, a eutectic mixture of biphenyl (C12H10, 35 wt%) and diphenylmethane (C13H12, 65 wt%), and demonstrated its ability to store hydrogen via catalytic hydrogenation of the mixture over an Ru/Al2O3 catalyst at 393 K under 50 bar of H2 (Scheme. 1). The resulting liquid mixture of bicyclohexyl (C12H22) and dicyclohexylmethane (C13H24) further proved to



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have gravimetric and volumetric hydrogen storage densities of 6.9 wt% and ca. 60 g-H2 L-1 (materialbased), respectively

20

. Compared to several potentially promising LOHC candidates, this mixture

shows superior hydrogen storage capacity (Table S1).

Scheme 1. Reversible hydrogenation and dehydrogenation scheme of the biphenyl-diphenylmethane eutectic mixture (H2-lean or dehydrogenated form) with reaction conditions. Whereas other studies have suggested different LOHC candidates 9, the majority of the previous studies have focused mainly on the fundamentals of catalytic hydrogenation and dehydrogenation on a small scale, and only limited reports have demonstrated H2-release kinetics in conjunction with a fuel cell and reversibility of the reaction or purity of the generated hydrogen gas. For example, Fikrt et al. reported a dynamic power supply system integrated with a polymer electrolyte membrane fuel cell (PEMFC) employing hydrogen produced in situ via dehydrogenation of a LOHC, perhydro-dibenzyltoluene over platinum on alumina support (Pt/Al2O3) at temperatures between 563 K and 593 K, further demonstrating a response to the energy demand

21

; the system was reported to

function properly, however 0.2% of the LOHC material was lost via decomposition into methane, toluene, benzene, methyl cyclohexane, and cyclohexane upon dehydrogenation at the operation temperature of ca. 578 K. Further, Wang et al. recently reported a cyclic dehydrogenationhydrogenation using N-ethylcarbazole. However, the selectivity of N-ethylcarbazole was reduced to 72.1% after five cycles of operation 22. For LOHCs to be useful for different practical applications, the


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above mentioned issues must be assessed in a full scope, addressing economically feasible scalability of the chosen LOHC, reaction kinetics of the adopted catalysts, purity of the as-produced hydrogen, and most importantly, reversibility of the material. In this paper, we report the dehydrogenation characteristics of a liquid bicyclohexyl and dicyclohexylmethane mixture where 60 g-H2 L-1 (or 6.9 wt%) of hydrogen is stored by the hydrogenation of a eutectic mixture of biphenyl (35 wt%) and diphenylmethane (65 wt%) (Scheme 1). The dehydrogenation reaction kinetics and product compositions at varying temperatures and feed rates are experimentally studied in detail. Furthermore, equilibrium constants, concentrations, and enthalpies of a stepwise dehydrogenation process are calculated by a density functional theory, supporting the experimental results. A dehydrogenation system capable of powering a 1 kW-class fuel cell is integrated, resulting in 94% conversion of the eutectic mixture (1 kg) over a Pd/C pellet catalyst at 613 K with no apparent decomposition of materials or side reactions. Finally, the reversible H2-release and storage properties, as well as the thermal stability of the eutectic mixture are assessed using proton nuclear magnetic resonance spectroscopy (1H-NMR), gas chromatography-mass spectrometry (GC-MS), and gas chromatography (GC) analyses.

Experimental Materials and Characterization To obtain a hydrogenated form (H2-rich form) from the eutectic mixture, biphenyl (99%, Alfa Aesar) and diphenylmethane (99%, Sigma Aldrich) were used without additional purification. Ruthenium on alumina (Ru/Al2O3, 5 wt%, Sigma Aldrich), a catalyst used for hydrogenation of the eutectic mixture based on the screening results of our previous study 20, was purchased and used as received. For a dehydrogenation reaction, it has been previously determined that Pd and Pt demonstrate superior catalytic activities for heterocycles 9. In our experiments, a palladium on carbon pellet (Pd/C pellet, 10 wt%, Riogen) catalyst was employed for a dehydrogenation reaction following a reduction



using H2 at 573 K for 15 h. A deuterated chloroform (CDCl3, 99.8%, contains 0.05% v/v tetramethylsilane, TMS) for an NMR solvent was purchased from Cambridge Isotope Laboratories. Zeolite (13X, Sigma Aldrich) was used as an adsorbent material for the remaining organics after the liquid-gas separator. The prepared H2-lean and H2-rich liquid mixtures were characterized by 1H-NMR and GCMS. NMR spectra were recorded with a Bruker Avance III 400 MHz FT-NMR spectrometer. GC-MS analysis was performed using an Agilent GC 6890N System equipped with an Agilent J&W GC column DB-5 ms (30 m × 0.25 mm × 0.25 μm) and MS (LECO, Pegasus IV). The purity of the hydrogen gas produced during the dehydrogenation reactions was measured by GC using an Agilent GC 7890A System. Computational methods

Figure 1. Stepwise dehydrogenation processes from 3 and 6. X: equilibrium conversion, K: equilibrium constant, C: equilibrium concentration. Shin et al. 23 reported a systematic method for calculating the thermochemical properties of a LOHC using Density Functional Theory (DFT). The computational procedure was employed in the present study to predict the equilibrium constant (Ke) and conversion (Xe) for the stepwise


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dehydrogenations of H2-rich LOHCs (2), bicyclohexyl (3, 35wt%) + dicyclohexylmethane (6, 65wt%), as displayed in Figure 1. Because the dehydrogenation reaction occurs in the gas phase at high temperatures above 573 K, only the thermodynamic properties in the gas phase were considered in the calculation method. In the DFT calculations, the mGGA-TPSS functional 24 and triple numerical plus polarization basis set

25

were employed to calculate the enthalpies of the dehydrogenation in the gas

phase. General computational details are available in a previous report 23. Based on the results, the equilibrium concentrations (Ce) of the 12H-LOHC (3 or 6), 6H-LOHC (4 or 7), and 0H-LOHC (5 or 8) could be obtained as indicated by the block diagram of Figure S1. At the given temperature T, Ke, and Xe, the concentrations of 12H-LOHC, 6H-LOHC, and 0H-LOHC, were calculated by Xe of each reaction step (Figure 1), and new values of the equilibrium constant K(i+1) were updated in Step 3 (Figure S1). The iteration proceeded until δ K was less than a tolerance ε in Step 4. Finally, the converged Ce of 12H-LOHC, 6H-LOHC, and 0H-LOHC were calculated as a function of temperature. Preparation of H2-lean (1) and H2-rich LOHCs (2) For the preparation of the H2-lean LOHC (1), the colorless liquid eutectic mixture (H2-lean form, greater than 1 kg) was prepared by mixing biphenyl (C12H10, 35 wt%, solid at 298 K, 5) and diphenylmethane (C13H12, 65%, liquid at 298 K, 8) with stirring at ambient temperature. The H2-rich form (2), a mixture of bicyclohexyl (C12H22, 3) and dicyclohexylmethane (C13H24, 6) was prepared by catalytic hydrogenation of the as-prepared eutectic mixture (1, ca. 300 g) over 15 g of Ru/Al2O3 (Ru 5 wt%) at 393 K under 50 bar of H2 for 3.3 h in a high-pressure reactor system (Figure S2), yielding a colorless liquid with near full conversion. Dehydrogenation of H2-rich form (2), a mixture of bicyclohexyl (3) and dicyclohexylmethane (6) In a typical dehydrogenation experiment, 12 g of the Pd/C pellet catalysts were packed in a Ushaped reactor (0.45 m, 1/2” stainless steel) installed within a convection oven equipped with a temperature control device. Catalysts were then reduced using H2 at 573 K for 15 h, sufficient time to



yield a desired catalytic activity. The temperature of the convection oven was increased to the temperature at which the dehydrogenation reaction occurs (573 K to 613 K). Further, to prevent the solidification of the product formed after the dehydrogenation reaction (e.g., biphenyl), the temperature of the product line was maintained at 373 K. A dehydrogenation reaction was then initiated by feeding the H2-rich mixture of bicyclohexyl (3, 35 wt%) and dicyclohexylmethane (6, 65 wt%), which was hydrogenated using the eutectic mixture of biphenyl (5, 35 wt%) and diphenylmethane (8, 65 wt%), at a constant flow rate from 0.02 to 4.86 g min-1 via a pump (pump; Series I, SSI LabAlliance) into the Ushaped reactor. The as-produced H2-lean (1) and H2-rich (2) liquid forms are displayed in Figure S3-Sn. In a different set of experiments, the reaction temperature was increased from 573 to 613 K by 10 K increments while maintaining the flow rate of the reactant at 0.54 g min-1. A dehydrogenation conversion was obtained when the flow rate of hydrogen gas achieved a steady state. Note that immediately upon injection of the reactant into the U-shaped reactor, the liquid-state reactant became gaseous because the reaction temperature was greater than the boiling point of the reactant. The gaseous reactant was then dehydrogenated upon contacting the catalyst when passing through the reactor. The product after the dehydrogenation reaction became a liquid when exiting the U-shaped reactor. The liquid products were collected in a flask, and the hydrogen gas produced during the dehydrogenation reaction was passed through the product trap and mass flow meter (MFM, F-201C-FAC-22-V, Bronkhorst). Further, the recorded flow rate of the hydrogen gas was double checked by a bubble flow meter (SF-2U, Horiba) connected to the latter part of the MFM. Large-scale hydrogenation and dehydrogenation with the as-integrated system To demonstrate the H2-release capabilities of 2, a dehydrogenation system was designed and integrated as illustrated in Figure 2 and Figure S4. In the dehydrogenation reactor displayed in Figure S4, Pd/C (Pd 10 wt%) pellet catalysts were employed for a continuous H2-release experiment. The Pd/C pellet catalysts were reduced under a hydrogen atmosphere at 573 K for 15 h prior to the dehydrogenation reaction. The H2-rich form (2) prepared from the batch hydrogenation reaction was provided by a liquid pump (LabM3, SHENCHEN) to the preheater, where the reactants were heated to


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613 K, and then to the dehydrogenation reactor. The reaction temperature was maintained at 613 K and monitored using seven K-type thermocouples installed at different locations of the reactor. Following the reaction, the H2 gas and H2-lean products were separated using a gas-liquid separator. The flow rate of the as-separated hydrogen was measured using a mass flow meter (MFM; F-111CB-2K0-AAD-00V, Bronkhorst). When the flow rate of the produced hydrogen achieved a steady condition with a desired conversion, the H2 gas was fed to a 1 kW-class PEMFC (H-1000W, Horizon Fuel Cell). The fuel cell was loaded by a coupled DC electric loader (ESLL-5K, UNICORN).

Figure 2. Process flow diagram of as-integrated power generation system based on H2-rich form (2).

Results and Discussion Hydrogenation of H2-lean LOHC (1) on a large scale Recently, we discovered a novel, H2-lean LOHC (1), liquid eutectic mixture of biphenyl (C12H10, 35 wt%, solid) and diphenylmethane (C13H12, 65%, liquid) at 298 K, based on material screening with the criteria of (i) hydrogen storage capacity, (ii) thermal stability, (iii) thermodynamic properties associated with hydrogenation and dehydrogenation (e.g., enthalpy of reaction), (iv) physical properties (e.g., melting point, boiling point, viscosity, vapor pressure, etc.), and (v) material costs. The selected H2-lean LOHC (1) revealed its H2 storage properties using an Ru/Al2O3 (Ru 5 wt%) catalyst,



demonstrating over 98% conversion at 393 K under 50 bar of H2 pressure

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20

. To determine its

dehydrogenation properties, the hydrogenation system was first further optimized and newly integrated for practical scale demonstration of the material as a hydrogen carrier (Figure S2). The system proved to be capable of hydrogenating a large quantity of 1 (ca. 283.7 g of LOHC, equivalent to 0.9 MJ of H2 energy when used in conjunction with a fuel cell) at 393 K with H2 (50 bar) at one time (Figure 3a). The content of the molecular hydrogen stored in 1 increased linearly with time and the final H2 storage capacity was 6.9 wt% (material-based). 1H-NMR spectroscopic studies demonstrated that the H2-lean LOHC (1) was fully hydrogenated to form H2-rich LOHC (2), a mixture of bicyclohexyl (3; C12H22, 35 wt%) and dicyclohexylmethane (6; C13H24, 65 wt%), without any detectable byproduct (Figure 3b).

Figure 3. (a) Hydrogen uptake and hydrogen storage capacity of H2-lean form (1) over time, using Ru/Al2O3 catalyst in a large-scale hydrogenation system (Figure S2) and (b) an 1H-NMR spectrum of the H2-rich form (2) confirming a complete hydrogenation of 1.


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Dehydrogenation of the H2-rich form (2) In contrast to the exothermic hydrogenation of cyclic alkanes (e.g., toluene to methylcyclohexane), endothermic dehydrogenation reactions of the corresponding hydrogenated aromatics (e.g., methylcyclohexane to toluene) require enthalpies ranging from 64–69 kJ mol-1 H2

10

and thus require higher temperature than hydrogenation reactions. To estimate the hydrogenation and dehydrogenation enthalpies of the as-employed H2-lean form (1) and H2-rich form (2), we initially calculated equilibrium conversions as a function of temperature using DFT-optimized geometries. Figure 4 reveals the equilibrium concentration of the stepwise dehydrogenation products of biphenyl (5) and diphenylmethane (8) at temperatures ranging from ca. 450 to 625 K. The computational results suggest that diphenylmethane requires a higher temperature of 603 K than biphenyl (568 K) for complete dehydrogenation (greater than 99.99%). Further, diphenylmethane has a lower cross-over concentration of ca. 0.25 between diphenylmethane (8, blue line) and dicyclohexylmethane (6, black line), resulting in a greater intermediate concentration of 0.4 at 548 K (Figure 4b), compared to 0.13 for an intermediate concentration of biphenyl (5) at 518 K (Figure 4a). This further supports the experimental results where concentrations of the intermediate produced from dicyclohexylmethane (6) were greater than those produced from bicyclohexyl (3), even considering the weight ratio of the mixture and reaction temperature above 573 K (detailed compositional analysis is provided in later sections). Therefore, the required greater temperature of dehydrogenation for complete conversion of the H2-rich mixture (2) can be mainly attributed to the thermodynamic equilibrium between 8 + 6H2 and 6. Combining the results, it was determined that bicyclohexyl (3) and dicyclohexylmethane (6) require ca. 568 K and 603 K, respectively, for a complete dehydrogenation, greater than 99.99%, as indicated in Figure 4c.



Figure 4. Theoretically predicted equilibrium concentrations (a) of biphenyl (5), cyclohexylbenzene (4), and bicyclohexyl (3), (b) of diphenylmethane (8), cyclohexylmethylbenzene (7), and dicyclohexylmethane (6) from 450 to 625 K, (c) theoretically predicted equilibrium conversion between 3 and 5, and between 6 and 8.


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According to previous studies, noble metal catalysts are known to be highly active towards the dehydrogenation of cyclohexanes 9. Based on the preliminary tests using transition metal-based catalysts, the Pd/C (Pd 10 wt%) pellet catalyst was chosen for additional experiments for continuous dehydrogenation. When 2 was supplied continuously with a rate of 0.02 g min-1 into the U-shaped reactor heated at 613 K, the system released molecular hydrogen, increasing linearly with time until 10 min. The amount of H2 produced then became constant (15 mL min-1) over 1 h with conversion close to 100% (Figure 5a). Next, the feeding rate of the reactant was varied at the temperature of 613 K, and as depicted in Figure 5b, the conversion decreased from 100% to 17% as the LOHC supply rate increased from 0.02 g min-1 to 4.86 g min-1. The initial conversions with a feeding rate of 0.02 g min-1 were virtually identical at 573 K and 613 K, however the conversion difference became greater with an increased supply rate. Then, the gap decreased again at high feeds with low conversion. The feed rate was maintained at 0.54 g min-1 and the H2-release amount was observed with increasing reaction temperature, resulting in higher conversion as expected (Figure 5c). At varying temperatures and feeding rates, the error range of the gas flow measurements using a film flow meter was less than ± 2.5%.



Figure 5. (a) Flow rate of hydrogen from continuous dehydrogenation reaction at 613 K with a feed rate of 0.02 g min-1 over time, (b) Dehydrogenation conversions as functions of feed rate of reactant and temperature, (c) degree of H2-release over reaction temperature with a feed rate of 0.54 g min-1. Figure 6 displays the 1H-NMR spectra obtained after dehydrogenation of 2 with a supply rate of 0.02 g min-1 at 573 K and 613 K, respectively, presenting the formation of 1 in addition to H2-release. A small amount of unreacted 2 remained in the product mixture following dehydrogenation at 573 K (Figure 6), whereas no reactant remained with the formation of small amounts of byproducts, toluene(9) and benzene(10), after the dehydrogenation reaction at 613 K. The byproduct formation results from the dissociation of the C-C bond from the formed biphenyl(5) and/or diphenylmethane(8). The


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infinitesimal quantities of toluene and benzene can also store hydrogen to form methylcyclohexane and cyclohexane, which can further release molecular hydrogen. Therefore, the total hydrogen storage capacity of the mixture is virtually constant. Even if benzene and toluene have high vapor pressures, they do not have a significant effect on the total vapor pressure because they are small amount. Note that the reactants, cyclohexane and methylcyclohexane, were not detected, indicating that no C-C bond scission of bicyclohexyl(3) and dicyclohexylmethane(6) occurred. Further characterization with GCMS confirmed the formation of 1 after H2-release from 2 (Figure 7). Consistent with the NMR results (Figure 6), 2 (i.e., 3 + 6) and partially dehydrogenated products, 4 and 7, were formed upon heating at 573 K whereas the dehydrogenation reaction completed at 613 K.

Figure 6. 1H-NMR spectra of dehydrogenation reaction of 2 with a feed rate of 0.02 g min-1 at temperatures 573 and 613 K with enlarged spectra between  = 3.0–0.6 ppm.



Figure 7. Concentration profile of dehydrogenated products at 573 K with a feed rate of 0.02 g min-1 via GC-MS and at 613 K with feed rate of a 0.02 g min-1 via GC-MS (detailed GC-MS profiles presented in Figure S5). Hydrogen purity is one of the key factors considered when developing LOHC materials feasible to a fuel cell application. In particular, organic byproducts possibly decomposed at high temperatures of greater than 523 K could be present and must be sequestrated. In this context, the purity of the hydrogen released via dehydrogenation of 2 was monitored by GC. When 2 was dehydrogenated at 613 K in the presence of a Pd/C pellet catalyst, GC peaks at ca. 0.98 min corresponding to molecular hydrogen were apparently observed with small peaks at ca. 3.95, 5.99, 6.59, 6.70, 7.55, and 8.32 min attributed to an infinitesimal quantity of 1 and unreacted 2 (Figure S6a). These peaks were observed even in the blank test, by feeding a mixture of 1 and 2 at 298 K without a catalyst, suggesting that the vapor pressures of the organic materials cause a small fraction of these compounds. Although a liquidgas separator was installed at latter part of the dehydrogenation reactor, these peaks remained. These GC signals, corresponding to the organic materials, were then completely removed after passing 13X zeolite as an adsorbent material (Figure S6b). Therefore, the purity of the hydrogen following


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sequestration of the infinitesimal amount of organic impurities from the dehydrogenation of 2 was 99.9%. Note that the result here is again consistent with those obtained using the NMR and GC-MS analyses. The initial H2 storage capacity (6.9 wt%, material-based) remained constant after repeated and reversible dehydrogenation-hydrogenation cycles although 2 undesirably decomposed into toluene and benzene upon heating at > 573 K because toluene and benzene can be used as hydrogen storage materials. To analyze the changes in the H2 storage densities of 2 and stability of the mixture under cyclic storage and discharge routines, the dehydrogenation of 2 (over Pd/C at 613 K under 1 bar) and consecutive hydrogenation of the formed 1 (over Ru/Al2O3 at 393 K under 50 bar of H2) were performed multiple times (nine cycles total). As depicted in Figure 8a, the hydrogen storage capacity was virtually constant up to the sixth cycle; then, the H2 storage capacity marginally decreased at the seventh and eighth cycles. The rate of hydrogenation of 1 appeared to be virtually identical throughout the cycles because fresh Ru/Al2O3 catalyst was employed in the batch reactor for every reaction. Conversely, the hydrogen storage capacity marginally decreased from the seventh cycle, presumably due to decreased activity of the Pd/C pellet catalyst packed in the U-shaped continuous reactor, resulting in a decreased dehydrogenation rate. To restore the hydrogen storage capacity, the used Pd/C pellet catalysts were replaced by a fresh Pd/C pellet catalyst at the ninth cycle, which restored the H2 storage capacity to 6.9 wt%, near to the initial capacity (Figure 8a). Further, it was determined that the peaks of aliphatic compounds corresponding to 1 to 3 ppm, which began to increase after the sixth cycle, virtually disappeared at the ninth cycle via 1H-NMR analysis (Figure 8b). These results imply that the observed decrease in hydrogen storage capacity was due to deactivated catalysts rather than decomposition of the eutectic mixture. Under the dehydrogenation condition employed, a small fraction (total less than 2%) of diphenylmethane was decomposed after nine hydrogenation-dehydrogenation cycles. This result suggests that the material has exceptional reversibility, yet, the degradation mechanism, or durability of the catalyst must be studied further.



Figure 8. (a) Hydrogen storage capacities of the eutectic mixture following repeated dehydrogenationhydrogenation cycles, (b) 1H-NMR spectra of product 2 in 1st (blue), sixth (red), seventh (green), ninth (purple) dehydrogenation tests and their enhanced spectra between 2.0–0.8 ppm. Continuous generation of electrical energy from a large scale dehydrogenation reactor integrated with a PEMFC Based on the obtained hydrogen purity and excellent reversibility between 1 + 12H2 and 2, a dehydrogenation system composed of a preheater, a reactor packed with the Pd/C pellet catalyst, a gas-


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liquid separator, and two storage tanks for 1 and 2 was produced and further integrated with a PEMFC to verify its power generation capacity, as depicted in Figure 2 and Figure S4. A dehydrogenation reaction was initiated by supplying gas phase 2, preheated at 613 K, into the reactor following reduction of the Pd/C pellet catalyst at 573 K for 15 h. Reactant flow rates were varied from 1–11 g min-1 at the reaction temperature of 613 K. More than 7.5 L min-1 of hydrogen was generated continuously at a reactant feed rate of 10 g min-1 (Figure S7). A current-voltage curve of the fuel cell exhibited that 500 W of power was produced with 7.5 L min-1 of H2 from the dehydrogenation system (Figure 9a).

Figure 9. (a) Current-voltage and current-power curves and (b) power output of the PEMFC supplied by hydrogen produced from the dehydrogenation system.



Then, the system was operated with a fixed reactant flow rate of 10 g min-1 for 60 mins, continuously producing 500 W of power as illustrated in Figure 9b. At this point, cumulative hydrogen production confirmed reactant conversion of more than 90%, and the 1H-NMR and GC-MS analyses of the product confirmed 94% conversion (Figure 10). GC-MS analysis revealed that the product contained 2.4% of the original reactants and 5.4% of the dehydrogenated intermediates (Table 2), confirming that no reactants or products decomposed into undesired byproducts at 613 K due to the relatively shorter contact time between the reactant and catalyst than that of 0.02 g min-1. Moreover, 4.9% of the dehydrogenated intermediates derived from the diphenylmethane, which is consistent with the theoretically predicted equilibrium concentrations (Figure 4b). Finally, the cumulative conversion of 2 throughout the test at varying reactant flow rates was calculated to be 96.3%.

Figure 10. (a) 1H-NMR spectrum of dehydrogenation reaction product of 2 with feed rate of 1–11 g min-1 at temperatures 613 K and (b) enlarged spectrum between  = 3.0–0.6 ppm.


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Table 1. Composition of products dehydrogenated at 613 K, measured via GC-MS (detailed analysis results in Figure S8).

Molecular Structure

Name

Proportion

Diphenylmethane (8)

61.6%

Biphenyl (5)

30.6%

Benzene, (cyclohexylmethyl)(7)

4.9%

Cyclohexane, 1,1'methylenebis(6)

1.1%

1,1'-Bicyclohexyl (3)

1.3%

Benzene, cyclohexyl(4)

0.5%

Conclusion Different aspects of a newly discovered eutectic mixture of biphenyl and diphenylmethane as a potential liquid organic hydrogen carrier were studied, and its hydrogen storage/release capacities were demonstrated in a practical scale hydrogenation and dehydrogenation system. The eutectic mixture, H2-lean form (1), and H2-rich form (2), proved to have a capability to store and release molecular hydrogen reversibly with excellent gravimetric and volumetric H2 storage capacities of 6.9 wt% and 60 g-H2 L-1 (material-based). 1 was hydrogenated using a Ru/Al2O3 catalyst under 50 bar of H2 at 393 K; 1

H-NMR and GC-MS analyses confirmed near to full conversion. Further, 2 was dehydrogenated using

a Pd/C pellet catalyst in a continuous reactor, releasing high purity H2 with infinitesimal amounts of 1.



Reversibility tests involving nine consecutive hydrogenation and dehydrogenation cycles confirmed minimal decrease in the hydrogen storage capacity. Finally, a dehydrogenation system was integrated and operated in conjunction with a PEMFC, generating greater than 0.5 kWh of energy in a continuous manner. The as-explored LOHC was determined to have a number of advantages including high gravimetric and volumetric hydrogen storage capacities, excellent reversibility of hydrogen storagerelease cycles, ability to produce high purity hydrogen via dehydrogenation, and inexpensive material cost with excellent scalability, undoubtedly supporting the as-explored eutectic mixture as a promising liquid hydrogen carrier for different fuel cell applications including H2-based energy storage systems, off-grid power generation systems, distributed power production systems, and H2 refueling stations. Supporting Information I. Figure S1-S8 S1: A block diagram for calculation procedure of equilibrium concentrations. S2: A hydrogenation system equipped with a batch-type hydrogenation reactor, a reference cell, and a pressure-sealed mechanical stirrer (left) and a process flow diagram of the hydrogenation system (right). S3: H2-lean form (1, left) and H2-rich form (2, right). S4: A dehydrogenation system fueled by the H2-rich LOHC (2). S5: GC-MS raw data of products effluent from dehydrogenation of 2 with a feed rate of 0.02 g min1

at the temperature of: (a) 300 ℃ and (b) 340 ℃. S6: Purities of H2 via dehydrogenation of H2-rich

LOHC (2), characterized GC. S7: Feed rate of the reactant, H2-rich LOHC (2), and flow rate of the produced hydrogen over time. S8: GC-MS raw data of dehydrogenation products from a large scale system at 340 ℃. Table S1: Representative liquid organic hydrogen carriers and their hydrogen storage properties. II. 1H and 13C-NMR spectra of the H2-lean and H2-rich LOHCs III. 1H and 13C-NMR spectra of the LOHCs (continued) IV. Analysis of residual LOHCs on adsorbent


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V. References

Acknowledgment This work was supported by the Technology Development Program to Solve Climate Changes of the National Research Foundation (NRF) funded by the Ministry of Science, ICT & Future Planning (2015M1A2A2074688), the New & Renewable Energy Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (20153030041030), as well as the KIST institutional program funded by the Korea Institute of Science and Technology (2E28272).

References 1.

Zhang, T.; Low, J.; Koh, K.; Yu, J.; Asefa, T., Mesoporous TiO2 Comprising Small, Highly

Crystalline Nanoparticles for Efficient CO2 Reduction by H2O. ACS Sustainable Chem. Eng. 2017, 6 (1), 531-540, DOI 10.1021/acssuschemeng.7b02827 2.

Zhang, T.; Low, J. X.; Huang, X. X.; Al-Sharab, J. F.; Yu, J. G.; Asefa, T., Copper-Decorated

Microsized Nanoporous Titanium Dioxide Photocatalysts for Carbon Dioxide Reduction by Water. Chemcatchem 2017, 9 (15), 3054-3062, DOI 10.1002/cctc.201700512 3.

Turner, J. A., Sustainable hydrogen production. Science 2004, 305 (5686), 972-4, DOI

10.1126/science.1103197 4.

Dunn, S., Hydrogen futures: toward a sustainable energy system. Int. J. Hydrogen Energy 2002,

27 (3), 235-264, DOI 10.1016/s0360-3199(01)00131-8 5.

Singh, S.; Jain, S.; Ps, V.; Tiwari, A. K.; Nouni, M. R.; Pandey, J. K.; Goel, S., Hydrogen: A

sustainable fuel for future of the transport sector. Renewable Sustainable Energy Rev. 2015, 51, 623633, DOI 10.1016/j.rser.2015.06.040 6.

Preuster, P.; Papp, C.; Wasserscheid, P., Liquid Organic Hydrogen Carriers (LOHCs): Toward



a

Hydrogen-free

Hydrogen

Economy.

Acc.

Chem.

Res.

Page 24 of 27

2017,

50

(1),

74-85,

DOI

10.1021/acs.accounts.6b00474 7.

Schlapbach, L., Technology: Hydrogen-fuelled vehicles. Nature 2009, 460 (7257), 809-11,

DOI 10.1038/460809a 8. storage:

Sakintuna, B.; Lamaridarkrim, F.; Hirscher, M., Metal hydride materials for solid hydrogen A

review☆.

Int.

J.

Hydrogen

Energy

2007,

32

(9),

1121-1140,

DOI

10.1016/j.ijhydene.2006.11.022 9.

Zhu, Q.-L.; Xu, Q., Liquid organic and inorganic chemical hydrides for high-capacity

hydrogen storage. Energy Environ. Sci. 2015, 8 (2), 478-512, DOI 10.1039/c4ee03690e 10.

Biniwale, R.; Rayalu, S.; Devotta, S.; Ichikawa, M., Chemical hydrides: A solution to high

capacity hydrogen storage and supply. Int. J. Hydrogen Energy 2008, 33 (1), 360-365, DOI 10.1016/j.ijhydene.2007.07.028 11.

Alhumaidan, F.; Cresswell, D.; Garforth, A., Hydrogen Storage in Liquid Organic Hydride:

Producing Hydrogen Catalytically from Methylcyclohexane. Energy Fuels 2011, 25 (10), 4217-4234, DOI 10.1021/ef200829x 12.

Hodoshima, S., Catalytic decalin dehydrogenation/naphthalene hydrogenation pair as a

hydrogen source for fuel-cell vehicle. Int. J. Hydrogen Energy 2003, 28 (11), 1255-1262, DOI 10.1016/s0360-3199(02)00250-1 13.

Müller, K.; Stark, K.; Emel’yanenko, V. N.; Varfolomeev, M. A.; Zaitsau, D. H.; Shoifet, E.;

Schick, C.; Verevkin, S. P.; Arlt, W., Liquid Organic Hydrogen Carriers: Thermophysical and Thermochemical Studies of Benzyl- and Dibenzyl-toluene Derivatives. Ind. Eng. Chem. Res. 2015, 54 (32), 7967-7976, DOI 10.1021/acs.iecr.5b01840 14.

Crabtree, R. H., Hydrogen storage in liquid organic heterocycles. Energy Environ. Sci. 2008,

1 (1), 134-138, DOI 10.1039/b805644g 15.

Yang, M.; Dong, Y.; Fei, S.; Ke, H.; Cheng, H., A comparative study of catalytic

dehydrogenation of perhydro-N-ethylcarbazole over noble metal catalysts. Int. J. Hydrogen Energy 2014, 39 (33), 18976-18983, DOI 10.1016/j.ijhydene.2014.09.123


Page 25 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


16.

Sotoodeh, F.; Huber, B. J. M.; Smith, K. J., The effect of the N atom on the dehydrogenation

of heterocycles used for hydrogen storage. Appl. Catal., A 2012, 419-420, 67-72, DOI 10.1016/j.apcata.2012.01.013 17.

He, T.; Pei, Q.; Chen, P., Liquid organic hydrogen carriers. J. Energy Chem. 2015, 24 (5), 587-

594, DOI 10.1016/j.jechem.2015.08.007 18.

Teichmann, D.; Arlt, W.; Wasserscheid, P.; Freymann, R., A future energy supply based on

Liquid Organic Hydrogen Carriers (LOHC). Energy Environ. Sci. 2011, 4 (8), 2767-2773, DOI 10.1039/c1ee01454d 19.

Teichmann, D.; Arlt, W.; Wasserscheid, P., Liquid Organic Hydrogen Carriers as an efficient

vector for the transport and storage of renewable energy. Int. J. Hydrogen Energy 2012, 37 (23), 1811818132, DOI 10.1016/j.ijhydene.2012.08.066 20.

Han, D. J.; Jo, Y. S.; Shin, B. S.; Jang, M.; Kang, J. W.; Han, J. H.; Nam, S. W.; Yoon, C. W.,

A Novel Eutectic Mixture of Biphenyl and Diphenylmethane as a Potential Liquid Organic Hydrogen Carrier: Catalytic Hydrogenation. Energy Technol. 2018, DOI 10.1002/ente.201700694 21.

Fikrt, A.; Brehmer, R.; Milella, V.-O.; Müller, K.; Bösmann, A.; Preuster, P.; Alt, N.; Schlücker,

E.; Wasserscheid, P.; Arlt, W., Dynamic power supply by hydrogen bound to a liquid organic hydrogen carrier. Appl. Energy 2017, 194, 1-8, DOI 10.1016/j.apenergy.2017.02.070 22.

Wang, B.; Yan, T.; Chang, T.; Wei, J.; Zhou, Q.; Yang, S.; Fang, T., Palladium supported on

reduced graphene oxide as a high-performance catalyst for the dehydrogenation of dodecahydro- N ethylcarbazole. Carbon 2017, 122, 9-18, DOI 10.1016/j.carbon.2017.06.021 23.

Shin, B. S.; Yoon, C. W.; Kwak, S. K.; Kang, J. W., Thermodynamic assessment of carbazole-

based organic polycyclic compounds for hydrogen storage applications via a computational approach. Int. J. Hydrogen Energy 2018, 43 (27), 12158-12167, DOI 10.1016/j.ijhydene.2018.04.182 24.

Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J., Ab-Initio Calculation of

Vibrational Absorption and Circular-Dichroism Spectra Using Density-Functional Force-Fields. J Phys Chem-Us 1994, 98 (45), 11623-11627, DOI 10.1021/J100096a001 25.

Delley, B., Ground-state enthalpies: evaluation of electronic structure approaches with



emphasis on the density functional method. J. Phys. Chem. A 2006, 110 (50), 13632-9, DOI 10.1021/jp0653611


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Abstract Graphic

Synopsis Eutectic mixture of liquid organic materials work as high capacity reversibly hydrogen carriers applicable to a fuel cell.


A High-Capacity, Reversible Liquid Organic Hydrogen Carrier: H2

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