First principles calculations of H-storage in sorption materials

A review of various contributions of first principles calculations in the area of hydrogen storage, particularly for the carbon-based sorption materials, is presented. Carbon-based sorption materials are considered as promising hydrogen storage media due to their light weight and large surface area. Depending upon the hybridization state of carbon, these materials can bind the hydrogen via various mechanisms, including physisorption, Kubas and chemical bonding. While attractive binding energy range of Kubas bonding has led to design of several promising storage systems, in reality the experiments remain very few due to materials design challenges that are yet to be overcome. Finally, we will discuss the spillover process, which deals with the catalytic chemisorption of hydrogen, and arguably is the most promising approach for reversibly storing hydrogen under ambient conditions.     

Materials for hydrogen storage

Hydrogen storage is a materials science challenge because, for all six storage methods currently being investigated, materials with either a strong interaction with hydrogen or without any reaction are needed. Besides conventional storage methods, i.e. high pressure gas cylinders and liquid hydrogen, the physisorption of hydrogen on materials with a high specific surface area, hydrogen intercalation in metals and complex hydrides, and storage of hydrogen based on metals and water are reviewed.The goal is to pack hydrogen as close as possible, i.e. to reach the highest volumetric density by using as little additional material as possible. Hydrogen storage implies the reduction of an enormous volume of hydrogen gas. At ambient temperature and atmospheric pressure, 1 kg of the gas has a volume of 11 m³. To increase hydrogen density, work must either be applied to compress the gas, the temperature decreased below the critical temperature, or the repulsion reduced by the interaction of hydrogen with another material.     

Stabilizing the MXenes by Carbon Nanoplating for Developing Hierarchical Nanohybrids with Efficient Lithium Storage and Hydrogen Evolution Capability

The MXenes combining hydrophilic surface, metallic conductivity and rich surface chemistries represent a new family of 2D materials with widespread applications. However, their poor oxygen resistance causes a great loss of electronic properties and surface reactivity, which significantly inhibits the fabrication, the understanding of the chemical nature and full exploitation of the potential of MXene‐based materials. Herein we report a facile carbon nanoplating strategy for efficiently stabilizing the MXenes against structural degradation caused by spontaneous oxidation, which provides a material platform for developing MXene‐based materials with attractive structure and properties. Hierarchical MoS₂/Ti₃C₂‐MXene@C nanohybrids with excellent structural stability, electrical properties and strong interfacial coupling are fabricated by assembling carbon coated few‐layered MoS₂ nanoplates on carbon‐stabilized Ti₃C₂ MXene, exhibiting exceptional performance for Li storage and hydrogen evolution reaction (HER). Remarkably, ultra‐long cycle life of 3000 cycles with high capacities but extremely slow capacity loss of 0.0016% per cycle is achieved for Li storage at a very high rate of 20 A g^?1. They are also highly active HER electrocatalyst with very positive onset potential, low overpotential and long‐term stability in acidic solution. Superb properties highlight the great promise of MXene‐based materials in cornerstone applications of energy storage and conversion.     

2D Transition Metal Carbides (MXenes) for Carbon Capture

Global warming caused by burning of fossil fuels is indisputably one of mankind's greatest challenges in the 21st century. To reduce the ever‐increasing CO₂ emissions released into the atmosphere, dry solid adsorbents with large surface‐to‐volume ratio such as carbonaceous materials, zeolites, and metal–organic frameworks have emerged as promising material candidates for capturing CO₂. However, challenges remain because of limited CO₂/N₂ selectivity and long‐term stability. The effective adsorption of CO₂ gas (≈12 mol kg^?1) on individual sheets of 2D transition metal carbides (referred to as MXenes) is reported here. It is shown that exposure to N₂ gas results in no adsorption, consistent with first‐principles calculations. The adsorption efficiency combined with the CO₂/N₂ selectivity, together with a chemical and thermal stability, identifies the archetype Ti₃C₂ MXene as a new material for carbon capture (CC) applications.     

Engineering 2D Nanofluidic Li‐Ion Transport Channels for Superior Electrochemical Energy Storage

Rational surface engineering of 2D nanoarchitectures‐based electrode materials is crucial as it may enable fast ion transport, abundant‐surface‐controlled energy storage, long‐term structural integrity, and high‐rate cycling performance. Here we developed the stacked ultrathin Co₃O₄ nanosheets with surface functionalization (SUCNs‐SF) converted from layered hydroxides with inheritance of included anion groups (OH^?, NO₃^?, CO₃^2?). Such stacked structure establishes 2D nanofluidic channels offering extra lithium storage sites, accelerated Li‐ion transport, and sufficient buffering space for volume change during electrochemical processes. Tested as an anode material, this unique nanoarchitecture delivers high specific capacity (1230 and 1011 mAh g^?1 at 0.2 and 1 A g^?1, respectively), excellent rate performance, and long cycle capability (1500 cycles at 5 A g^?1). The demonstrated advantageous features by constructing 2D nanochannels in nonlayered materials may open up possibilities for designing high‐power lithium ion batteries.     

Comparative experimental study of methane adsorption on multi-walled carbon nanotubes and granular activated carbons

In this study, methane storage capacity of granular activated carbons (GACs) and two types of multi-walled carbon nanotubes (MWCNTs) was investigated and compared. An experimental apparatus consisting a dual adsorption vessel was set up for measurement of equilibrium adsorption of methane on adsorbents using volumetric technique at pressure range of 0–50?bar at different temperatures. The first type of MWCNs has shown lower methane uptake (4.5?mmol?g^?1) compared to GACs (6.5?mmol?g^?1) at the temperature of 283.15?K and the pressure of 50?bar, while 33?mmol?g^?1 of methane storage capacity was achieved using the second type of MWCNTs that is much higher than methane storage on GACs at the same operating conditions. The superior uptake performance for the second type of MWCNTs can be attributed to its specific characteristics such as smaller pore size and higher pore volume. The experimental data of adsorption were almost equally well described by Langmuir, Freundlich and Sips equations to determine the model isotherms. The isosteric heat of methane adsorption on the adsorbent was calculated based on Clausius–Clapeyron and the Sips isotherm model using the experimental data at different temperatures. Results revealed that the isosteric heat of methane adsorption on MWCNTs was lower than the heat of methane adsorption on GACs. Low values obtained for isosteric heats of adsorption indicated dominance of physisorption mechanism for all adsorbents. In general, the obtained data indicated that some well-structured MWCNTs with uniform and narrow size distribution as well as higher pore volume are potential materials for methane storage and deserve further study.     

A Universal Strategy for Intimately Coupled Carbon Nanosheets/MoM Nanocrystals (M = P,S, C,and O) Hierarchical Hollow Nanospheres for Hydrogen Evolution Catalysis and Sodium‐Ion Storage

Intimately coupled carbon/transition‐metal‐based hierarchical nanostructures are one of most interesting electrode materials for boosting energy conversion and storage applications owing to the strong synergistic effect between the two components and appealing structural stability. Herein, a universal method is reported for making hierarchical hollow carbon nanospheres (HCSs) with intimately coupled ultrathin carbon nanosheets and Mo‐based nanocrystals. The in situ and confined reaction of the synthetic strategy can not only allow the aggregation of the nanocrystals to be impeded, but also endows extremely intimate coupled interaction between the conductive carbon nanosheets and the nanocrystals MoM (M = P, S, C and O). As a proof of concept, the as‐prepared MoP/C HCSs exhibit extraordinary hydrogen evolution reaction electrocatalytic activity with small overpotential and robust durability in both acidic and alkaline solutions. In addition, the unique sheet‐on‐sheet MoS₂/C HCSs as an anode demonstrate high capacity, great rate capabilities, and long‐term cycles for sodium‐ion batteries (SIBs). The capacity can be maintained at 410 mA h g^?1 even after 1000 cycles even at a high current density of 4 A g^?1, one of the best reported values for MoS₂‐based electrode materials for SIBs. The present work highlights the importance of designing and fabricating functional strongly coupled hybrid materials for enhancing energy conversion and storage applications.     

Metal‐Tuned Acetylene Linkages in Hydrogen Substituted Graphdiyne Boosting the Electrochemical Oxygen Reduction

Different from graphene with the highly stable sp²‐hybridized carbon atoms, which shows poor controllability for constructing strong interactions between graphene and guest metal, graphdiyne has a great potential to be engineered because its high‐reactive acetylene linkages can effectively chelate metal atoms. Herein, a hydrogen‐substituted graphdiyne (HsGDY) supported metal catalyst system through in situ growth of Cu₃Pd nanoalloys on HsGDY surface is developed. Benefiting from the strong metal‐chelating ability of acetylenic linkages, Cu₃Pd nanoalloys are intimately anchored on HsGDY surface that accordingly creates a strong interaction. The optimal HsGDY‐supported Cu₃Pd catalyst (HsGDY/Cu₃Pd‐750) exhibits outstanding electrocatalytic activity for the oxygen reduction reaction (ORR) with an admirable half‐wave potential (0.870 V), an impressive kinetic current density at 0.75 V (57.7 mA cm^?2) and long‐term stability, far outperforming those of the state‐of‐the‐art Pt/C catalyst (0.859 V and 15.8 mA cm^?2). This excellent performance is further highlighted by the Zn–air battery using HsGDY/Cu₃Pd‐750 as cathode. Density function theory calculations show that such electrocatalytic performance is attributed to the strong interaction between Cu₃Pd and C?C bonds of HsGDY, which causes the asymmetric electron distribution on two carbon atoms of C?C bond and the strong charge transfer to weaken the shoulder‐to‐shoulder π conjugation, eventually facilitating the ORR process.     

Strong Charge Transfer at 2H–1T Phase Boundary of MoS2 for Superb High‐Performance Energy Storage

Transition metal dichalcogenides exhibit several different phases (e.g., semiconducting 2H, metallic 1T, 1T′) arising from the collective and sluggish atomic displacements rooted in the charge‐lattice interaction. The coexistence of multiphase in a single sheet enables ubiquitous heterophase and inhomogeneous charge distribution. Herein, by combining the first‐principles calculations and experimental investigations, a strong charge transfer ability at the heterophase boundary of molybdenum disulfide (MoS₂) assembled together with graphene is reported. By modulating the phase composition in MoS₂, the performance of the nanohybrid for energy storage can be modulated, whereby remarkable gravimetric and volumetric capacitances of 272 F g^?1 and 685 F cm^?3 are demonstrated. As a proof of concept for energy application, a flexible solid‐state asymmetric supercapacitor is constructed with the MoS₂‐graphene heterolayers, which shows superb energy and power densities (46.3 mWh cm^?3 and 3.013 W cm^?3, respectively). The present work demonstrates a new pathway for efficient charge flow and application in energy storage by engineering the phase boundary and interface in 2D materials of transition metal dichalcogenides.     

Boosting Sodium Storage in TiO2 Nanotube Arrays through Surface Phosphorylation

Sodium‐ion batteries (SIBs) offer a promise of a scalable, low‐cost, and environmentally benign means of renewable energy storage. However, the low capacity and poor rate capability of anode materials present an unavoidable challenge. In this work, it is demonstrated that surface phosphorylated TiO₂ nanotube arrays grown on Ti substrate can be efficient anode materials for SIBs. Fabrication of the phosphorylated nanoarray film is based on the electrochemical anodization of Ti metal in NH₄F solution and subsequent phosphorylation using sodium hypophosphite. The phosphorylated TiO₂ nanotube arrays afford a reversible capacity of 334 mA h g^?1 at 67 mA g^?1, a superior rate capability of 147 mA h g^?1 at 3350 mA g^?1, and a stable cycle performance up to 1000 cycles. In situ X‐ray diffraction and transmission electron microscopy reveal the near‐zero strain response and robust mechanical behavior of the TiO₂ host upon (de)sodiation, suggesting its excellent structural stability in the Na⁺ storage application.     

Synthesis of Uniquely Structured Yolk–Shell Metal Oxide Microspheres Filled with Nitrogen‐Doped Graphitic Carbon with Excellent Li–Ion Storage Performance

Novel structured composite microspheres of metal oxide and nitrogen‐doped graphitic carbon (NGC) have been developed as efficient anode materials for lithium‐ion batteries. A new strategy is first applied to a one‐pot preparation of composite (FeO_x‐NGC/Y) microspheres via spray pyrolysis. The FeO_x‐NGC/Y composite microspheres have a yolk–shell structure based on the iron oxide material. The void space of the yolk–shell microsphere is filled with NGC. Dicyandiamide additive plays a key role in the formation of the FeO_x‐NGC/Y composite microspheres by inducing Ostwald ripening to form a yolk–shell structure based on the iron oxide material. The FeO_x‐NGC/Y composite microspheres with the mixed crystal structure of rock salt FeO and spinel Fe₃O₄ phases show highly superior lithium‐ion storage performances compared to the dense‐structured FeO_x microspheres with and without carbon material. The discharge capacities of the FeO_x‐NGC/Y microspheres for the 1st and 1000th cycle at 1 A g^?1 are 1423 and 1071 mAh g^?1, respectively. The microspheres have a reversible discharge capacity of 598 mAh g^?1 at an extremely high current density of 10 A g^?1. Furthermore, the strategy described in this study is generally applied to multicomponent metal oxide–carbon composite microspheres with yolk–shell structures based on metal oxide materials.     

Formation of N‐Doped Carbon‐Coated ZnO/ZnCo2O4/CuCo2O4 Derived from a Polymetallic Metal–Organic Framework: Toward High‐Rate and Long‐Cycle‐Life Lithium Storage

Metal–organic frameworks (MOFs) are very promising self‐sacrificing templates for the large‐scale fabrication of new functional materials owing to their versatile functionalities and tunable porosities. Most conventional metal oxide electrodes derived from MOFs are limited by the low abundance of incorporated metal elements. This study reports a new strategy for the synthesis of multicomponent active metal oxides by the pyrolysis of polymetallic MOF precursors. A hollow N‐doped carbon‐coated ZnO/ZnCo₂O₄/CuCo₂O₄ nanohybrid is prepared by the thermal annealing of a polymetallic MOF with ammonium bicarbonate as a pore‐forming agent. This is the first report on the rational design and preparation of a hybrid composed of three active metal oxide components originating from MOF precursors. Interestingly, as a lithium‐ion battery anode, the developed electrode delivers a reversible capacity of 1742 mAh g^?1 after 500 cycles at a current density of 0.3 mA g^?1. Furthermore, the material shows large storage capacities (1009 and 667 mAh g^?1), even at high current flow (3 and 10 A g^?1). The remarkable high‐rate capability and outstanding long‐life cycling stability of the multidoped metal oxide benefits from the carbon‐coated integrated nanostructure with a hollow interior and the three active metal oxide components.     

Twinned Growth of Metal‐Free,Triazine‐Based Photocatalyst Films as Mixed‐Dimensional (2D/3D) van der Waals Heterostructures

Design and synthesis of ordered, metal‐free layered materials is intrinsically difficult due to the limitations of vapor deposition processes that are used in their making. Mixed‐dimensional (2D/3D) metal‐free van der Waals (vdW) heterostructures based on triazine (C₃N₃) linkers grow as large area, transparent yellow‐orange membranes on copper surfaces from solution. The membranes have an indirect band gap (E _g,opt = 1.91 eV, E _g,elec = 1.84 eV) and are moderately porous (124 m² g^?1). The material consists of a crystalline 2D phase that is fully sp² hybridized and provides structural stability, and an amorphous, porous phase with mixed sp²–sp hybridization. Interestingly, this 2D/3D vdW heterostructure grows in a twinned mechanism from a one‐pot reaction mixture: unprecedented for metal‐free frameworks and a direct consequence of on‐catalyst synthesis. Thanks to the efficient type I heterojunction, electron transfer processes are fundamentally improved and hence, the material is capable of metal‐free, light‐induced hydrogen evolution from water without the need for a noble metal cocatalyst (34 µmol h^?1 g^?1 without Pt). The results highlight that twinned growth mechanisms are observed in the realm of “wet” chemistry, and that they can be used to fabricate otherwise challenging 2D/3D vdW heterostructures with composite properties.     

3D 1T‐MoS2/CoS2 Heterostructure via Interface Engineering for Ultrafast Hydrogen Evolution Reaction

Metallic phase (1T) MoS₂ has been regarded as an appealing material for hydrogen evolution reaction. In this work, a novel interface‐induced strategy is reported to achieve stable and high‐percentage 1T MoS₂ through highly active 1T‐MoS₂/CoS₂ hetero‐nanostructure. Herein, a large number of heterointerfaces can be obtained by interlinked 1T‐MoS₂ and CoS₂ nanosheets in situ grown from the molybdate cobalt oxide nanorod under moderate conditions. Owing to the strong interaction between MoS₂ and CoS₂, high‐percentage of metallic‐phase (1T) MoS₂ of 76.6% can be achieved, leading to high electroconductivity and abundant active sites compared to 2H MoS₂. Furthermore, the interlinked MoS₂ and CoS₂ nanosheets can effectively disperse the nanosheets so as to enlarge the exposed active surface area. The near zero free energy of hydrogen adsorption at the heterointerface can also be achieved, indicating the fast kinetics and excellent catalytic activity induced by heterojunction. Therefore, when applied in hydrogen evolution reaction (HER), 1T‐MoS₂/CoS₂ heterostructure delivers low overpotential of 71 and 26 mV at the current density of 10 mA cm^?2 with low Tafel slops of 60 and 43 mV dec^?1, respectively in alkaline and acidic conditions.     

Composition and Architecture Design of Double‐Shelled Co0.85Se1−xSx@Carbon/Graphene Hollow Polyhedron with Superior Alkali (Li,Na, K)‐Ion Storage

The exploration of materials with reversible and stable electrochemical performance is crucial in energy storage, which can (de) intercalate all the alkali‐metal ions (Li⁺, Na⁺, and K⁺). Although transition‐metal chalcogenides are investigated continually, the design and controllable preparation of hierarchical nanostructure and subtle composite withstable properties are still great challenges. Herein, component‐optimal Co_0.85Se_1?xS_x nanoparticles are fabricated by in situ sulfidization of metal organic framework, which are wrapped by the S‐doped graphene, constructing a hollow polyhedron framework with double carbon shells (CoSSe@C/G). Benefiting from the synergistic effect of composition regulation and architecture design by S‐substitution, the electrochemical kinetic is enhanced by the boosted electrochemistry‐active sites, and the volume variation is mitigated by the designed structure, resulting in the advanced alkali‐ion storage performance. Thus, it delivers an outstanding reversible capacity of 636.2 mAh g^?1 at 2 A g^?1 after 1400 cycles for Li‐ion batteries. Remarkably, satisfactory initial charge capacities of 548.1 and 532.9 mAh g^?1 at 0.1 A g^?1 can be obtained for Na‐ion and K‐ion batteries, respectively. The prominent performance combined with the theory calculation confirms that the synergistic strategy can improve the alkali‐ion transportation and structure stability, providing an instructive guide for designing high‐performance anode materials for universal alkali‐ion storage.     

Engineering the Electronic Structure of 2D WS2 Nanosheets Using Co Incorporation as CoxW(1‐x)S2 for Conspicuously Enhanced Hydrogen Generation

Transition metal dichalcogenides (TMDs), as one of potential electrocatalysts for hydrogen evolution reaction (HER), have been extensively studied. Such TMD‐based ternary materials are believed to engender optimization of hydrogen adsorption free energy to thermoneutral value. Theoretically, cobalt is predicted to actively promote the catalytic activity of WS₂. However, experimentally it requires systematic approach to form Co_xW_(1?x)S₂ without any concomitant side phases that are detrimental for the intended purpose. This study reports a rational method to synthesize pure ternary Co_xW_(1?x)S₂ nanosheets for efficiently catalyzing HER. Benefiting from the modification in the electronic structure, the resultant material requires overpotential of 121 mV versus reversible hydrogen electrode (RHE) to achieve current density of 10 mA cm^?2 and shows Tafel slope of 67 mV dec^?1. Furthermore, negligible loss of activity is observed over continues electrolysis of up to 2 h demonstrating its fair stability. The finding provides noticeable experimental support for other computational reports and paves the way for further works in the area of HER catalysis based on ternary materials.     

High Performance Lithium‐Ion Batteries Using Layered 2H‐MoTe2 as Anode

The major challenges faced by candidate electrode materials in lithium‐ion batteries (LIBs) include their low electronic and ionic conductivities. 2D van der Waals materials with good electronic conductivity and weak interlayer interaction have been intensively studied in the electrochemical processes involving ion migrations. In particular, molybdenum ditelluride (MoTe₂) has emerged as a new material for energy storage applications. Though 2H‐MoTe₂ with hexagonal semiconducting phase is expected to facilitate more efficient ion insertion/deinsertion than the monoclinic semi‐metallic phase, its application as an anode in LIB has been elusive. Here, 2H‐MoTe₂, prepared by a solid‐state synthesis route, has been employed as an efficient anode with remarkable Li⁺ storage capacity. The as‐prepared 2H‐MoTe₂ electrodes exhibit an initial specific capacity of 432 mAh g^?1 and retain a high reversible specific capacity of 291 mAh g^?1 after 260 cycles at 1.0 A g^?1. Further, a full‐cell prototype is demonstrated by using 2H‐MoTe₂ anode with lithium cobalt oxide cathode, showing a high energy density of 454 Wh kg^?1 (based on the MoTe₂ mass) and capacity retention of 80% over 100 cycles. Synchrotron‐based in situ X‐ray absorption near‐edge structures have revealed the unique lithium reaction pathway and storage mechanism, which is supported by density functional theory based calculations.     

Nickel Promoted Palladium Nanoparticles for Electrocatalysis of Carbohydrazide Oxidation Reaction

Carbohydrazide is a potential alternative to toxic hydrazine for fuel cell applications to overcome the challenges of storage and transportation of hydrogen. In this work, Ni‐alloyed Pd nanoparticles (NPs) with varied Pd–Ni ratios supported on carbon black (PdNi_x/C) are prepared and their catalytic performance for the carbohydrazide electro‐oxidation reaction is investigated. The catalytic performance of PdNi_x/C NPs is significantly improved in comparison to Pd/C NPs. The current density of PdNi_x/C NPs with optimized Pd–Ni atom ratio can reach 3.26 A mg^?1_metal at a potential of 0.4 V (vs reversible hydrogen electrode), which is an increase of 2.4 times compared to that of Pd/C. The density functional theory calculation indicates the enhanced catalytic activity is caused by the change of adsorption energy of carbohydrazide molecules on the metal surface. It exhibits a volcano relationship between the adsorption energy and the catalytic current density of PdNi_x/C with varied Pd–Ni atom ratios.     

Hierarchical SnO2 hollow nanotubes as anodes for high performance lithium-ion battery

Nanostructured transition metal oxides are promising anode materials for lithium-ion batteries. Nevertheless, the problem of high volume expansion rate limits its further application. In this paper, we present a 3D hierarchical SnO₂ hollow nanotubes material by calcining C@SnS₂ materials in the air. This structure combines the advantages of both the hollow nanotubes and the outer staggered nanosheets structure, in which the hollow nanotube can provide more lithium ion transport channels, the space between the tubes can buffer the volume change, and the staggering nanosheets structure can effectively improve the relative specific surface area of the material and improve the storage capacity. As a result, the SnO₂ hollow nanotubes anode exhibits the highly reversible capacity of 1079 mAh g^?1 at a current density of 100 mA g^?1, while the reversible specific capacity of 770 mAh g^?1 was obtained after 100 cycles. The research results obtained in this work provide a feasible strategy for synthetic nanoscale transition metal oxide as high-performance lithium anode material.