Reversible Desorption Research Articles

Double alkali metal salts synergistic activation coupled with S-doped cotton derived honeycomb porous carbon for enhanced zinc ion storage capability

Biomass-derived carbon materials are ideal electrode materials due to their inherent eco-friendliness, easily available and unique micro-morphology. Herein, zinc ion capacitors (ZICs) are designed utilizing S doped honeycomb porous carbon nanosheet cathode, which is obtained via synergistic activation from sodium thiosulfate and potassium carbonate. The honeycomb microstructure is result from the pore-forming effect of double alkali metal salts, which can promote the electrolyte ion fast migration in the cathode. Besides, the S doping in carbon array can regulate the electron distribution within the carbon framework and lower the energy barrier for the chemical absorption of Zn2+/H+, which is demonstrated by density functional theory calculation and expected to produce additional capacitance. Further, the ex-situ X-ray photoelectron spectroscopy reveals the consistent changes in C-O-H and C-O-Zn throughout the charging and discharging cycles, which demonstrate the S doping can promote the reversible chemical adsorption and desorption reaction of Zn2+/H+. As a result, the S doping honeycomb porous carbon nanosheet cathode can achieves a high discharge capacity of 147.2 mAh g−1 and high energy density of 92.7 Wh kg−1. Additionally, the obtained material exhibits good cycling stability at 20 A g−1, retaining 99.9 % of its initial capacity after 2000 cycles. This research presents a versatile approach to synthesizing heteroatom-doped carbon materials from biomass waste for ZICs.

Effect of La–Ni@3DG catalyst concentration on the hydrogen storage properties of MgH2

This research paper explores the influence of varying the quantity of La–Ni@3DG catalyst on the hydrogen storage capabilities of MgH2. La–Ni@3DG catalyst was prepared using lanthanum chloride heptahydrate and nickel chloride hexahydrate as raw materials combined with three-dimensional graphene, then mixed with MgH2 via ball milling to prepare MgH2 + x wt.% La–Ni@3DG (x = 0, 3, 5, 7) composite alloy. X-ray diffraction analysis showed that the composite alloy was mainly composed of the MgH2 phase, with the presence of Mg and (La, Ni) phases detected. With increased La–Ni@3DG addition, the broadening of the MgH2 diffraction peak significantly improved, indicating the catalyst promoted alloy phase structure uniformity and crystallinity. The composite alloy forms new phases such as LaH3, LaNi3H6, and Mg2NiH4 during hydrogen absorption. These phases cooperate with in-situ formed LaH3 and Mg2Ni to significantly improve the reversible hydrogen absorption and desorption properties of MgH2. Adding La–Ni@3DG catalyst has a significant positive effect on the hydrogen absorption and desorption of MgH2, leading to a considerable improvement. At 553 K, the composite alloy reaches hydrogen absorption equilibrium faster than pure MgH2, and the temperature for hydrogen absorption is significantly reduced to 373 K. The dehydrogenation performance is also improved, with 5 wt.% La–Ni@3DG is showing the best dehydrogenation performance at 553 K. The initial dehydrogenation temperature commences at approximately 505 K, while the dehydrogenation activation energy attains its minimum value: a remarkably low 109.91 kJ/mol H2. Additionally, the addition of catalyst reduces the pressure of the hydrogen absorption and desorption platform, effectively reducing the hysteresis effect during hydrogen absorption and desorption when the catalyst content is 5 wt.%, the enthalpy change and entropy change of the composite alloy reach a minimum, −70.45 kJ/mol H2 and 74.38 kJ/mol H2 (hydrogen absorption), −101.93 J/K/mol H2 and 103.68 J/K/mol H2 (hydrogen desorption), respectively.

Enhancing ethanol/water separation in UTSA-280 by a pore engineering strategy

Dehydration of ethanol is the key issue for the production of bioethanol fuel. Based on energy-efficient water-adsorptive mechanism and size sieving effect of ultramicroporous metal–organic framework, UTSA-280 is a promising candidate for high purity ethanol production. But it is still a challenge to improve the water adsorption capacity of UTSA-280 without compromising the separation selectivity. Here, we present a post-synthetic pore engineering strategy towards coordinated water adjustment for UTSA-280 to address this challenge. Partial coordinated water molecules have been successfully removed in UTSA-280·xPCW. By reversible desorption and resorption of water to Ca ions, the saturated water adsorption of UTSA-280·xPCW increased up to 2.6 mol/mol (exceeding 60 % that of original UTSA-280). The enhancing effect were supported by both kinetics and thermodynamics mechanisms. Dynamic vapor breakthrough results showed that UTSA-280·41PCW produced the largest amount of pure ethanol (22.46 mmol/g) and highest water/ethanol separation factor (15.9), providing a synchronized enhancement of water adsorption and ethanol/water separation. UTSA-280·xPCW are promising candidates for the dehydration of ethanol.

Mxene-supported NbVH nanoparticles as efficient catalysts for reversible hydrogen storage in magnesium borohydride

Bimetallic niobium vanadium hydride nanoparticles (NbVH NPs) anchored on Ti3C2 were synthesized using a facile ball milling method as an effective catalyst for the dehydrogenation kinetics and reversibility of Mg(BH4)2. It was found that the onset dehydrogenation temperature of the Mg(BH4)2 + 30NbVH NPs@Ti3C2 composite was 105.7 °C and released 9.07 wt% of hydrogen at 330 °C, while undoped Mg(BH4)2 only released 4.2 wt% of H2 from 248 °C to 330 °C. Additionally, the Mg(BH4)2 + 30NbVH NPs@Ti3C2 composite achieved remarkable hydrogen release of over 9.44 wt% at a temperature as low as 230 °C, indicating unexpected dehydrogenation kinetics. In the reversible hydrogen desorption tests, Mg(BH4)2 + 30NbVH NPs@Ti3C2 released 5.98 wt% of H2 in the 2nd cycle at 250 °C and maintained a reversible capacity of 4.35 wt% after 10 cycles, demonstrating a remarkable enhancement in reversibility. The enhancement in the dehydrogenation kinetics of Mg(BH4)2 could be attributed to the greatly reduced activation energies from 343 kJ·mol−1 and 138.3 kJ·mol−1 to 103.7 kJ·mol−1 and 124 kJ·mol−1. Investigations on the evaluation of catalyst and Mg(BH4)2 during reversible hydrogen storage have revealed that NbVH NPs actually acted as a “bidirectional hydrogen pump”, facilitating both the hydrogen release and uptake from Mg(BH4)2. This strategy of multi-component hydrides may show a new way to design effective catalysts for solid-state hydrogen storage materials.

Synthesis and characterization of nitrogen-doped-MWCNT@cobalt oxide for nerve agent simulant detection

Organophosphorus nerve agents are toxic compounds that disrupt neuromuscular transmission by inhibiting the neurotransmitter enzyme, acetylcholinesterase, leading to rapid death. A hybrid composite was synthesized using a hydrothermal process for the early detection of dimethyl methyl phosphonate (DMMP), a simulant of the G-series nerve agent, sarin. Quartz crystal microbalance (QCM) and surface acoustic wave (SAW) sensors were used as detectors. Nitrogen-doped multiwalled carbon nanotubes (N-MWCNTs), cobalt oxide (Co3O4), and N-MWCNT@Co3O4 were compared to detect DMMP concentrations of 25–150 ppm. At 25 ppm, the differential frequencies (Δf) of the N-MWCNT, Co3O4, and N-MWCNT@Co3O4 sensors were 5.8, 2.3, and 99.5 Hz, respectively. The selectivity results revealed a preference for the DMMP rather than potential interference. The coefficients of determination (R2) of the N-MWCNT, Co3O4, and N-MWCNT@Co3O4 sensors for detecting 25–150 ppm DMMP were 0.983, 0.986, and 0.999, respectively. The response times of the N-MWCNT, Co3O4, and N-MWCNT@Co3O4 sensors for detecting 100 ppm DMMP were 25, 27, and 34 s, respectively, while the corresponding recovery times were 85, 105, and 181 s. The repeatability results revealed the reversible adsorption and desorption phenomena for the fixed DMMP concentration of 100 ppm. These unique findings show that synthesized materials can be used to detect organophosphorus nerve agents.

Porous Structure‐Electrochemical Performance Relationship of Carbonaceous Electrode‐Based Zinc Ion Capacitors

AbstractThe porous structure is critical for carbonaceous electrode‐based zinc‐ion capacitors (ZICs) to achieve excellent electrochemical performance, but the corresponding porous structure‐electrochemical performance relationship is yet to be fully understand. Herein, three types of N‐doped carbons with different porous structures are developed to investigate the relationship between the pore size distribution and the electrochemical performance of the devices. The optimized porous carbon (LVCR) exhibits large electrochemical surface area, plentiful oxygen functional groups, and hierarchical porous structure that facilitates electron transfer and ion diffusion. Consequently, the LVCR‐based ZIC exhibits a remarkable peak power density of 31.4 kW kg−1 and an impressive specific energy density of 126.6 Wh kg−1. Moreover, it demonstrates exceptional longevity, retaining the capacitance of 97.7% even after undergoing 50 000 cycles. Systematic characterization demonstrates that the macroporous and mesoporous structures determine the different stages of Zn2+ storage kinetics. The excellent Zn2+ storage and electrochemical performance of LVCR are attributed to the fast ion transport channels provided by the hierarchical porous structure and facilitated reversible chemisorption and desorption. This work not only deepens the understanding of charge storage mechanism, but also provides guidelines for rationally designing carbonaceous materials toward high‐performance ZICs in the view of porous structure‐electrochemical performance relationship.

Electrochemical Investigation of Lithium Perchlorate-Doped Polypyrrole Growing on Titanium Substrate

Lithium perchlorate-doped polypyrrole growing on titanium substrate (LiClO4-PPy/Ti) has been fabricated to act as electroactive electrode material for feasible electrochemical energy storage. A theoretical and experimental investigation is adopted to disclose the conductivity, electroactivity properties and interfacial interaction-dependent capacitance of LiClO4-PPy/Ti electrode. The experimental measurement results disclose that LiClO4-PPy/Ti reveals lower ohmic resistance (0.2226 Ω cm−2) and charge transfer resistance (2116 Ω cm−2) to exhibit higher electrochemical conductivity, a more reactive surface, and feasible ion diffusion to present higher double-layer capacitance (0.1930 mF cm−2) rather than LiClO4/Ti (0.3660 Ω cm−2, 65,250 Ω cm−2, 0.0334 mF cm−2). LiClO4-PPy/Ti reveals higher Faradaic capacitance caused by the reversible doping and dedoping process of perchlorate ion on PPy than the electrical double-layer capacitance of LiClO4/Ti caused by the reversible adsorption and desorption process of the LiClO4 electrolyte on Ti. Theoretical simulation calculation results prove that a more intensive electrostatic interaction of pyrrole N···Ti (2.450 Å) in LiClO4-PPy/Ti rather than perchlorate O···Ti (3.537 Å) in LiClO4/Ti. LiClO4-PPy/Ti exhibits higher density of states (57.321 electrons/eV) at Fermi energy and lower HOMO-LUMO molecule orbital energy gap (0.032 eV) than LiClO4/Ti (9.652 electrons/eV, 0.340 eV) to present the enhanced electronic conductivity. LiClO4-PPy/Ti also exhibits a more declined interface energy (−1.461 × 104) than LiClO4/Ti (−5.202 × 103 eV) to present the intensified interfacial interaction. LiClO4-PPy/Ti accordingly exhibits much higher specific capacitances of 0.123~0.0122 mF cm−2 at current densities of 0.01~0.10 mA cm−2 rather than LiClO4/Ti (0.010~0.0095 mF cm−2, presenting superior electroactivity and electrochemical capacitance properties. LiClO4-PPy/Ti could well act as the electroactive supercapacitor electrode for feasible energy storage.

Understanding the synergistic improving adsorbed and catalytic activity of polysulfide by design of oxygen vacancies and Li+ lithiation/delithiation behavior in Titanium Oxide separator

The commercialization of lithium-sulfur (Li-S) battery is hindered by severe shuttle effect and sluggish electrochemical kinetics, which are induced by the high dissolution of intermediate lithium polysulfides (LiPSs) in cathode side. Here, we describe a strategy towards efficient adsorption and conversion of polysulfides by designing a functional TiO2-based separator. The enhanced overall electrochemical performance of cells is highly related to the synergistic effect of mutually transformational of TiO2 and LixTiO2 and the existence of oxygen vacancies (OVs). During the charging and discharging process of Li-S cell, the in-situ formation of LixTiO2 assigned to lithiation of anatase TiO2 plays a pivotal role in reversible adsorption and desorption of LiPSs. In addition, we also discussed the effect of OVs, which can act as active sites to accelerate electrochemical kinetics and exhibit stronger chemical interaction with LiPSs. The synergy of lithiation and delithiation process of TiO2 with OVs not only offers an effective pathway towards the improvement of LiPSs adsorption and catalytic performance but could also inspire new ideas on developing functional metal oxides modified separator for Li-S battery. We anticipate that this work will provide new avenues for Li-S battery commercialization.

Zinc sulfide quantum dots modified 3D Nitrogen-Doped framework carbon enhances the ion transport rate of potassium ion hybrid capacitors

The potassium-ion hybrid capacitor (PIHC) has garnered considerable attention due to its ability to leverage the advantages of both battery-type anodes and capacitor-type cathodes simultaneously. Nevertheless, the slow K+ transfer rate between the PIHC battery-type anode and capacitor-type cathode significantly impedes the overall performance. Herein, we have employed quantum-sized zinc sulfide (ZnS) immobilized within a three-dimensional N-doped porous carbon framework (ZnSQDS@3DNC) as a standalone anode. The corresponding three-dimensional N-doped porous carbon (3DNC) serves as the cathode, thereby constructing a PIHC device. The strong coupling between ZnS quantum dots and N-doped three-dimensional porous carbon material effectively enhances ion/electron transport, facilitating intercalation-conversion-exfoliation reactions and improving K+ transfer rates. The numerous active sites within the 3DNC significantly enhance the capacitance performance of the cathode, facilitating the reversible adsorption and desorption of FSI-. The values of DK+ have all been calculated to be within the range of 10-10 to 10-9 cm2 s−1, indicating the rapid diffusion capability of this ZnSQDs@3DNC structure. More impressively, the assembled PIHC device can achieve high energy densities of 179.9 Wh kg−1 at power densities of 200 W kg−1, with an ultralong cycling life over 10,000 cycles. This study serves to advance the development of metal-ion hybrid capacitors and provides direction for improving ion transport kinetics.

Graphene-supported Cu2O-CoO heterojunctions for high performance supercapacitors

Transition metal oxides are excellent electrode materials for supercapacitors because of their high theoretical capacitance, good stability, environmental friendliness, and tunable structure. In this study, Cu2O-CoO heterojunction materials loaded on graphene oxide (Cu2O-CoO/GO) are prepared via the precipitation, hydrothermal, and heat treatment processes. Through the adsorption of transition metal ions by the oxygen-containing functional groups of GO, followed by alkali titration as well as hydrothermal reaction of Cu2+ and Co2+ ions, the hydroxides of metals are adsorbed on GO. Meanwhile, the reduced GO completes the self-assembly, and the GO with good spatial structure supports the formation of Cu2O-CoO heterojunction structure. The Cu2O-CoO/GO electrode material combines the high conductivity of GO with the excellent internal conductivity of Cu2O-CoO, and its abundant active sites and specific surface area, thereby ensuring good electrochemical performance. Thus, it enables reversible ion adsorption and desorption, exhibits the pseudocapacitance of copper oxide and cobalt oxide, and has higher capacitance performance than Cu2O/GO and CoO/GO electrodes. The specific capacitance of Cu2O-CoO/GO at 1 A g−1 is 723 F g−1. Cu2O-CoO/GO/ASC exhibits an energy density of 44.1 Wh kg−1 at 794 W kg−1 and capacitance retention rate of 89.3 % after 10,000 cycles. This indicates that the material has significant potential for a wide range of supercapacitor applications.

Microcrystalline Nanofiber Electrode with Adaptive Intrinsic Structure and Microscopic Interface.

A strategy of microcrystalline aggregation is proposed to fabricate energy storage electrode with outstanding capacity and stability. Carbon-rich electrode (BDTG) functionalized with benzo[1,2-b:4,5-b']dithiophene units and butadiyne segments are prepared. The linear conjugate chains pack as microcrystalline nanofibers on nanoscale, which further aggregates to form a porous interpenetrating network. The microcrystalline aggregation feature of BDTG exhibit stable structure during long cycling test, revealing the following advantage in structure and property. The stretchable butadiyne linker facilitates reversible adsorption and desorption of Li with the aid of adjacent sulfur heteroatom. The alkyne-alkene transition exhibits intrinsic structural stability of microcrystalline region in BDTG electrodes. Meanwhile, alkynyl groups and sulfur heteroatoms on the surface of BDTG nanofibers participate in the formation of microscopic interface, providing a stable interfacial contact between BDTG electrodes and adjacent electrolyte. As a proof-of-concept, BDTG-based electrode shows high capacity (1430 mAh g-1 at 50mA g-1) and excellent cycle performance (8000 cycles under 5 A g-1) in half-cell of lithium-ion batteries, and a reversible capacity of 120 mAh g-1 is obtained under the current density of 2 C in full-cell. This work shows microcrystalline aggregation is beneficial to realize adaptive intrinsic structure and interface contact during the charge-discharge process.

Stimuli-Responsive Ion Adsorbents for Sustainable Separation Applications.

Stimuli-responsive ion absorbents (SRIAs) with reversible ion adsorption and desorption properties have recently attracted immense attention due to their outstanding functionalities for sustainable separation applications. Over the past decade, a series of SRIAs that respond to single or multiple external stimuli (e.g., pH, gas, temperature, light, magnetic, and voltage) have been reported to achieve excellent ion adsorption capacity and selectivity while simultaneously allowing for their reusability. In contrast to traditional adsorbents that are mainly regenerated through chemical additives, SRIAs allow for reduced chemical and even chemical-free regeneration capacities, thereby enabling environmentally friendly and energy-efficient separation technologies. In this review, we systematically summarize the materials and strategies reported to date for synthesizing single-, dual-, and multiresponsive ion adsorbents. Following a discourse on the fundamental mechanisms that govern their adsorption and desorption under various external stimuli, we provide a concise discussion of the regeneration capacity and application of these responsive ion adsorbents for sustainable water desalination, toxic ion removal, and valuable ion extract and recovery. Finally, we discuss the challenges in developing and deploying these promising multifunctional responsive ion adsorbents together with strategies to overcome these limitations and provide prospects for their future.

N doping porous carbon embedded in self-assembled three-dimensional reduced graphene oxide networks for electrochemical hydrogen storage

Electrochemical hydrogen storage can realize reversible hydrogen absorption and desorption at room temperature and pressure, which has attracted extensive attention recently. A nanosheet-like reduced graphene oxide (rGO)-porous carbon composite with N-doping defect is prepared via a facile annealing, followed by hydrothermal and freeze-drying process. After N-doping, the porous carbon materials exhibit rich pore structure, and high pyridine nitrogen and pyrrolic nitrogen content. Adding them into the graphene self-assembly process, the aggregation tendency of graphene sheets can be reduced, the composite shows a free-standing 3D porous structure. Benefit from the synthetic effect between the rGO conductive network, the hierarchical porous structure, and the doping of N atoms, the rGO-N800 delivers a high hydrogen storage capacity of 266.3 mAh g−1 at 200 mA g−1 after 100 cycles. Moreover, the reversible capacities of the composite can reach 288.4, and 267.1 mAh g−1 at 1000 mA g−1 and 2000 mA g−1, respectively. The outstanding electrochemical performance of the composite demonstrates a promising direction for design carbon-based hydrogen storage materials.

Effect of surface curvature on the hydrogen storage capacity of the Sc-, Ti-, and V-doped graphene surfaces: Theoretical study

Hydrogen (H2) is an excellent energy carrier and can be produced in an environmentally friendly way. The H2 binding performance of different Sc-, Ti-, and V-doped carbon surfaces (i.e., graphenes, circumcoronenes, and circumtrindenes) is investigated using density functional theory. The calculated H2 binding energies are ranging from −12 kJ mol−1 to −22 kJ mol−1 for one H2 molecule per transition metal (TM) atom. Such binding energies are suitable for reversible H2 adsorption and desorption cycle. Obtained results suggest that the curvature of the TM-doped carbon surfaces enhances their H2 binding ability. TM atom in the curved carbon surfaces, representing an active site, is sterically more easily accessible for H2 binding than in the case of the planar metal-doped graphene surfaces. For comparison, the N2 binding ability of the same set of TM-doped carbon surfaces is studied as well.

Enhancing Supercapacitive Swing Adsorption of CO2 with Advanced Activated Carbon Electrodes

AbstractGlobal warming due to anthropogenic CO2 emissions argues for the rapid development of efficient carbon capture technologies. Supercapacitive swing adsorption (SSA) is a gas separation technology that relies on the reversible charge and discharge of supercapacitor electrodes to absorb and desorb CO2 highly selectively and reversibly. However, thus far SSA only showed low sorption capacity, and slow sorption kinetics. Herein, it is shown that the sorption capacity can be substantially increased via the use of carbons with a higher specific capacitance. The highest gravimetric sorption capacity is measured with electrodes made from garlic‐roots derived activated carbon valuing 273 mmol kg−1 having a specific capacitance of 257 F g−1. In addition, the overall adsorption rate and productivity are improved. Cycling the electrodes for over 100 h showed highly reproducible, reversible CO2 adsorption and desorption behavior. A preliminary technoeconomic and sensitivity analysis is provided to demonstrate the potential of SSA for commercial applications.

Targeted Synthesis of an Highly Stable Aluminium Phosphonate Metal-Organic Framework Showing Reversible HCl Adsorption.

A concept for obtaining isoreticular compounds with tri- instead of tetravalent metal cations using highly acidic reaction conditions was developed and successfully applied in a high throughput study using N,N'-piperazinebis(methylenephosphonic acid) (H4PMP), that resulted in the discovery of a new porous aluminium phosphonate denoted CAU-60 ∙ 6 HCl. The high-throughput study was subsequently extended to other trivalent metal ions. Al-CAU-60 ∙ 6 HCl demonstrates reversible desorption of HCl (18.3 wt% loading) with three distinct compositions observed with zero, four or six HCl molecules per formula unit. Structural changes were followed in detail by powder X-ray diffraction, EDX analysis as well as IR spectroscopy. Rapid desorption of HCl in water within minutes and subsequent adsorption from the gas phase and from aqueous solution are shown. Furthermore, it is possible to adsorb HBr into the guest free Al-CAU-60 framework, demonstrating the high stability of this compound.

Targeted Synthesis of a Highly Stable Aluminium Phosphonate Metal–Organic Framework Showing Reversible HCl Adsorption

AbstractA concept for obtaining isoreticular compounds with tri‐ instead of tetravalent metal cations using highly acidic reaction conditions was developed and successfully applied in a high throughput study using N,N′‐piperazinebis(methylenephosphonic acid) (H4PMP), that resulted in the discovery of a new porous aluminium phosphonate denoted CAU‐60⋅6 HCl. The high‐throughput study was subsequently extended to other trivalent metal ions. Al‐CAU‐60⋅6 HCl demonstrates reversible desorption of HCl (18.3 wt % loading) with three distinct compositions observed with zero, four or six HCl molecules per formula unit. Structural changes were followed in detail by powder X‐ray diffraction, EDX analysis as well as IR spectroscopy. Rapid desorption of HCl in water within minutes and subsequent adsorption from the gas phase and from aqueous solution are shown. Furthermore, it is possible to adsorb HBr into the guest free Al‐CAU‐60 framework, demonstrating the high stability of this compound.

Methane pyrolysis and carbon formation mechanisms in molten manganese chloride mixtures

Molten manganese chloride (MnCl2) is attractive as a high-temperature liquid catalyst for methane pyrolysis (CH4 → C + 2H2). Nevertheless, the mechanism of this reaction for continuously producing high-purity hydrogen and graphitic carbon from CH4 has not been fully elucidated. In this work, we investigated the reaction kinetics and mechanism of CH4 pyrolysis in mixtures of molten MnCl2 with various monovalent or divalent chlorides. The apparent activation energies of the molten MnCl2 mixtures for CH4 pyrolysis (<230 kJ mol−1) were much lower than that of the uncatalyzed reaction (400 kJ mol−1). Among the MnCl2 mixtures, only MnCl2–KCl showed a lower apparent activation energy than pure MnCl2 (152 and 172 kJ mol−1, respectively). In addition, MnCl2–KCl produced the largest amount of CH2* and the final solid carbon product with the highest crystallinity, suggesting that this system has unique CH4 dehydrogenation and carbon formation pathways. Density functional theory calculations also predicted high concentrations of CH2* in MnCl2–KCl, as confirmed by CH2 formation being more thermodynamically favorable in MnCl2–KCl than in MnCl2. Furthermore, the generation of C2+ unsaturated intermediates from CHx* (x < 3) was thermodynamically favored, suggesting a more facile pathway for the formation of graphitic carbon layers in molten MnCl2–KCl. In addition, an NVT ab initio molecular dynamics simulation suggested that gas-phase C–C coupling was facilitated in MnCl2–KCl by the frequent reversible desorption and adsorption of CHx* intermediates. These fundamental insights into the origins of the enhanced CH4 pyrolysis performance in MnCl2–KCl could aid in the development of molten salt catalysts with enhanced reactivity.

Carbon composite support improving catalytic effect of NbC nanoparticles on the low-temperature hydrogen storage performance of MgH2

Ultrafine carbon-based transition metal compounds have been widely investigated as efficient catalysts for enhancing the hydrogen storage performance of magnesium hydride. In this work, the carbon thermal shock method is applied to synthesize the ultrafine carbon-encapsulated NbC nanoparticles with an average grain size of 17.3 nm. The MgH2–10 wt% NbC/C composites show excellent low-temperature hydrogen storage performance with the onset dehydrogenation temperature of 196.1 °C, which is 92.2 °C and 98 °C lower than that of MgH2–10 wt% NbC and undoped MgH2, respectively. Specifically, MgH2–10 wt% NbC/C can absorb 6.71 wt% H2 at 100 °C within 30 min around and retain almost 100% reversible hydrogen desorption capacity after 10 cycles. For the catalytic mechanism, the electron transfer process between multi-valence Nb cations of in-situ formed NbHx and Mg, H atoms can greatly improve the cyclic de/rehydrogenation kinetics of MgH2-NbC/C. Besides, the enhancement of dehydrogenation kinetics can also be ascribed to MgH2 particle refinement by NbC nanoparticles, and destabilization of the Mg-H bond caused by carbon substrate. This investigation not only proves that carbon-encapsulated NbC nanoparticles can greatly enhance the hydrogen storage performance of MgH2 but provides an idea of preparing carbon-based transition metal carbides as effective catalysts for magnesium-based hydrogen storage materials.

Hydrogen Absorption Reactions of Hydrogen Storage Alloy LaNi5 under High Pressure.

Hydrogen can be stored in the interstitial sites of the lattices of intermetallic compounds. To date, intermetallic compound LaNi5 or related LaNi5-based alloys are known to be practical hydrogen storage materials owing to their higher volumetric hydrogen densities, making them a compact hydrogen storage method and allowing stable reversible hydrogen absorption and desorption reactions to take place at room temperature below 1.0 MPa. By contrast, gravimetric hydrogen density is required for key improvements (e.g., gravimetric hydrogen density of LaNi5: 1.38 mass%). Although hydrogen storage materials have typically been evaluated for their hydrogen storage properties below 10 MPa, reactions between hydrogen and materials can be facilitated above 1 GPa because the chemical potential of hydrogen dramatically increases at a higher pressure. This indicates that high-pressure experiments above 1 GPa could clarify the latent hydrogen absorption reactions below 10 MPa and potentially explore new hydride phases. In this study, we investigated the hydrogen absorption reaction of LaNi5 above 1 GPa at room temperature to understand their potential hydrogen storage capacities. The high-pressure experiments on LaNi5 with and without an internal hydrogen source (BH3NH3) were performed using a multi-anvil-type high-pressure apparatus, and the reactions were observed using in situ synchrotron radiation X-ray diffraction with an energy dispersive method. The results showed that 2.07 mass% hydrogen was absorbed by LaNi5 at 6 GPa. Considering the unit cell volume expansion, the estimated hydrogen storage capacity could be 1.5 times higher than that obtained from hydrogen absorption reaction below 1.0 MPa at 303 K. Thus, 33% of the available interstitial sites in LaNi5 remained unoccupied by hydrogen atoms under conventional conditions. Although the hydrogen-absorbed LaNi5Hx (x < 9) was maintained below 573 K at 10 GPa, LaNi5Hx began decomposing into NiH, and the formation of a new phase was observed at 873 K and 10 GPa. The new phase was indexed to a hexagonal or trigonal unit cell with a ≈ 4.44 Å and c ≈ 8.44 Å. Further, the newly-formed phase was speculated to be a new hydride phase because the Bragg peak positions and unit cell parameters were inconsistent with those reported for the La-Ni intermetallic compounds and La-Ni hydride phases.