Hydrogen Storage Mechanism Research Articles

Ti/Cr regulated and strategic Ce doped V-Ti-Cr-Mn-Fe high-entropy alloys with extraordinary reversible hydrogen storage properties at ambient temperature

High-entropy alloys (HEAs) are garnering widespread interest due to their exceptional hydrogen storage capabilities at ambient temperatures. Herein, a series of HEAs composed of V-Ti-Cr-Mn-Fe with varying Ti/Cr ratios has been synthesized via arc melting to serve as carrier materials for room-temperature hydrogen storage. Thanks to the composition of a large amount of body centered cubic (BCC) phase V32TixCr58-xMn5Fe5 exhibits excellent hydrogen storage performance, and the hydrogen absorption capacity of these HEAs escalates while the desorption capacity initially rises followed by a decline with an increase in the Ti/Cr ratio. Notably, the HEA with a Ti/Cr ratio of 1.0, namely V32Ti29Cr29Mn5Fe5, demonstrates exceptional performance by absorbing 3.55 wt% H2 within 60 min and rapidly releasing 2.16 wt% H2 within 5 min at 303 K. Further enhancement is achieved with the strategic doping of Ce in the V32Ti29Cr29Mn5Fe5, which leads to the removal of the Ti-rich phase and the incorporation of CeO2, significantly improves the activation performance, allowing the Ce-doped HEA to reach saturation in the 3rd hydrogen absorption cycle. Consequently, the hydrogen ab-/desorption capacities at ambient temperature are further augmented to 3.73 wt% and 2.26 wt%, respectively, and the enthalpies of hydrogen ab-/desorption are significantly reduced to −25.0 and 23.4 kJ/mol H2, marking a new benchmark for V-based HEAs. Dramatically, DFT calculations suggestes that the incorporation of Ce diminishes the structure energy of BCC phase structure for V32Ti29Cr29Mn5Fe4Ce1 as well as elongates the average distance of M−H from 1.955 Å to 1.969 Å, which corroborate the experimental results, offering profound insights into the underlying mechanisms of hydrogen storage within BCC-phase HEAs at ambient temperature.

Prediction of the optimal hydrogen storage in high entropy alloys

Hydrogen is the most abundant element in nature, characterized by its vast resources, recyclability, and environmentally friendly. Also, it can exist in various forms, such as gas, liquid, or solid, to meet different storage needs. In solid-state storage, the hydrogen storage alloys have received widespread attention due to hydrogen is stored in the form of hydrides. In recent years, high entropy alloys (HEAs), as a new type of novel alloy design concept have attracted a lot of attention in many fields including hydrogen storage due to their unique structures and excellent properties. Moreover, the freedom of element selection leads to various types of HEAs, further enhancing the potential for their application in hydrogen storage. This article focuses on the hydrogen storage performance of HEAs and provides a comprehensive overview of the hydrogen storage mechanism, thermodynamics, and kinetics. Additionally, we summarize and compare the hydrogen storage capacities of previously published HEAs and introduce two empirical parameters, valence electron concentration (VEC) and atomic size difference (δ), and their relationship with hydrogen storage capacity. By applying the K-means clustering algorithm, we conclude that the optimal hydrogen storage capacity of HEAs occurs when 4 <VEC < 6 and 5.3 % <δ < 7.5 %. This finding offers valuable guidance for the future design of efficient hydrogen storage HEAs.

Investigation of microstructure, hydrogen storage performance of Re-Mg-based alloy modified by RE2O3 (RE = Dy, Er, Yb)

In order to further study the hydrogen storage mechanism of Mg-based hydrogen storage materials, Mg90Ce5Y5 alloy was used as the basis, and three kinds of heavy rare earth oxides were doped into the alloy by ball milling technology. The microstructure and hydrogen storage properties were characterized by XRD, SEM, TEM and PCI. The results show that the hydrogen absorption rate and discharge rate of modified Mg90Ce5Y5 alloy are significantly increased, and the performance is Er2O3 > Dy2O3 > Yb2O3. The hydrogen storage capacity of Er2O3 catalyzed samples is 5.02 wt%, which is higher than 4.82 wt% and 4.80 wt% of Dy2O3 and Yb2O3 catalyzed samples. The hydrogen absorption saturation rate of the sample catalyzed by Er2O3 for 2 min is ∼90 %, the complete release of hydrogen only takes 50 min at 573 K, and the dehydrogenation activation energy is 76.9 kJ/mol. The hydrogen absorption and emission rate is fast, and the dehydrogenation activation energy is slightly lower than that of the other two catalysts. In the process of hydrogen absorption and desorption, the three catalysts exhibit different phase transitions, namely DyH2↔DyH3, ErH2↔ErH3 and irreversible YbH2. DyH2↔DyH3, ErH2↔ErH3 phase transitions have the "hydrogen pump" effect, which can effectively improve the hydrogen absorption and desorption kinetic rate of Mg90Ce5Y5 alloy. Some nano-rare earth compounds and phase transformation significantly improve the kinetic properties of the alloy. However, the enthalpy change of all three catalytic alloys is around 76 kJ/mol H2, which is considered only a slight improvement in thermodynamic properties.

Unveiling reversible hydrogen storage mechanism for the 2D penta-SiCN material

The promise of using 2D materials for hydrogen storage has broad prospects, ascribe to their significant specific surface area and lightweight properties. In this work, the hydrogen storage capability and reversible storage mechanism of 2D penta-SiCN material are investigated based on the first-principles computational method. Thermal stability of penta-SiCN is calculated by the ab-initio molecular dynamics (AIMD) simulation and root-mean-square displacement (RMSD) algorithm. It has been found that penta-SiCN is thermodynamically stable even after adsorbing hydrogen molecules. Taking into account the benchmarks of average and continuous adsorption energies of the adsorption systems, a pristine 2 × 2 × 1 penta-SiCN substrate has the ability to adsorb up to 26H2 molecules, which results in a maximum hydrogen storage capacity of 10.80 wt%. According to the semi-empirical calculation method that based on the thermodynamic analysis, the penta-SiCN adsorption system has a high reversible hydrogen storage capacity of 9.57 wt% within the adsorption and desorption application working conditions. The results proposed in this study demonstrates that penta-SiCN exhibits considerable promise for hydrogen storage with its substantial hydrogen storage capacity, exceptional reversibility, and eco-friendly characteristics.

Solid hydrogen storage with palladium-graphene composites: Synthesis, characterization, and mechanistic insights

Efficient and safe hydrogen storage is a pivotal challenge for the advancement of hydrogen energy technologies. To address this, the engineering of high-performance solid-state hydrogen storage materials through the loading of transition metals (TM) on carbon-based solids has gained considerable interest. However, the mechanics for TM such as Palladium that bolster hydrogen storage have not well addressed yet. This work presents hydrogen absorptions with metal Pd nanoparticles loaded on reduced graphene oxide (Pd-rGO), including material preparation, structural characterization, hydrogen storage performance, and quantum chemical mechanistic insights. A simple material synthesis strategy has been explored and Pd-rGO composites are successfully prepared with graphite powder as raw materials by an easy reduction-oxidation process. Then, Pd-rGO composites are characterized using X-ray diffraction, Fourier-transform infrared spectroscopy, Raman, transmission electron microscopy, and nitrogen adsorption-desorption isotherms methods to clarify the detailed structural characteristics of Pd-rGO with different loading amounts. It is found that Pd nanoparticles with an average particle size of 10 nm are uniformly dispersed on the surface of rGO. The materials have high defect severity and increased specific surface areas with Pd loading. The hydrogen adsorption capacity of Pd-rGO can be increased by 64 % compared with that without Pd loading due to the hydrogen spillover effect. In addition, the intensification mechanism of hydrogen storage of Pd-rGO composites is further understood by using density functional theory calculations and surface interaction analysis.

Machine learning accelerates design of bilayer-modified graphene hydrogen storage materials

To accelerate the exploration of modified graphene hydrogen storage materials, this paper proposes a Data-Driven Exploration Framework (DDEF) based on the best-fitting machine learning (ML) algorithms (Random Forest: Recall = 0.83, Accuracy = 0.83, Gradient boosted regression: R2 = 0.90, RMSE = 0.06) and density functional theory (DFT). The hydrogen storage capacity of bilayer double-deficient graphene (BDG[Li]) doped with B atoms and modified with Li and Ti atoms was predicted using ML, which shortens the design cycle of hydrogen storage materials. The interlayer composite hydrogen storage mechanism of the BDG[Li] structure is revealed by electronic property analysis. For the first time, the calculation method of the interlayer electrostatic force energy (Ej) is proposed, and the equation for calculating the interlayer H2 adsorption energy is optimized. The hydrogen storage data of BDG[Li] and the construction of Ej provide new feature data for subsequent ML calculations. Both DDEF and Ej are computationally verified to have high accuracy and generalizability, which is of research significance for accelerating the design of hydrogen storage materials.

Magnesium-Based Hydrogen Storage Alloys: Advances, Strategies, and Future Outlook for Clean Energy Applications.

Magnesium-based hydrogen storage alloys have attracted significant attention as promising materials for solid-state hydrogen storage due to their high hydrogen storage capacity, abundant reserves, low cost, and reversibility. However, the widespread application of these alloys is hindered by several challenges, including slow hydrogen absorption/desorption kinetics, high thermodynamic stability of magnesium hydride, and limited cycle life. This comprehensive review provides an in-depth overview of the recent advances in magnesium-based hydrogen storage alloys, covering their fundamental properties, synthesis methods, modification strategies, hydrogen storage performance, and potential applications. The review discusses the thermodynamic and kinetic properties of magnesium-based alloys, as well as the effects of alloying, nanostructuring, and surface modification on their hydrogen storage performance. The hydrogen absorption/desorption properties of different magnesium-based alloy systems are compared, and the influence of various modification strategies on these properties is examined. The review also explores the potential applications of magnesium-based hydrogen storage alloys, including mobile and stationary hydrogen storage, rechargeable batteries, and thermal energy storage. Finally, the current challenges and future research directions in this field are discussed, highlighting the need for fundamental understanding of hydrogen storage mechanisms, development of novel alloy compositions, optimization of modification strategies, integration of magnesium-based alloys into hydrogen storage systems, and collaboration between academia and industry.

The study of La and Mg elements on the improved structure and hydrogen storage performance of Ca2MgNi9 alloy

In order to improve the comprehensive hydrogen storage performance of AB3-type (RE/Ca)-Mg-Ni alloys, a new series of Ca(Ca0.75La0.25)2-xMgxNi9 (x = 0.8, 0.9, 1.0, 1.1, 1.2) alloys is designed and prepared. Based on the as-cast Ca2MgNi9 alloy, La is introduced, and the ratio of rare earth (RE) to Mg is adjusted using the induction melting method. This alteration modifies the microstructure of the alloy and significantly enhances its hydrogen storage performance. The effects of microstructure on hydrogen storage performance and mechanism are analyzed. The phase composition and lattice parameters of the main phase are regularly regulated by appropriate element substitution. The lattice parameter of the main phase is increased, the capacity and kinetics of hydrogen storage are significantly improved. The initial hydrogen absorption capacity of the alloy increases from 1.744 wt% for Ca2MgNi9 to 1.975 wt% for Ca(Ca0.75La0.25)1.2Mg0.8Ni9 alloy. Furthermore, the synergistic regulation of La and Mg elements can increase the hydrogen desorption ratio and crystal structure stability of the alloy. This leads to the concurrent enhancement of effective hydrogen storage and cycle performance of the Ca2MgNi9 hydrogen storage alloy. The retention rate of hydrogen absorption capacity for the 50th cycle is increased from 77.13% for Ca2MgNi9 to 84.26% for Ca(Ca0.75La0.25)0.8Mg1.2Ni9. At the same time, owing to the adjustment of the cell volume and phase composition, the platform pressures of hydrogen absorption and desorption of the alloy have also experienced a multi-fold increase between 0.002 and 0.5 MPa at 303 K. This enhancement is advantageous for expanding the potential applications of this type of alloy. This work presents new ideas for enhancing the hydrogen storage performance of Ca2MgNi9 hydrogen storage alloys.

Novel NLi4-BGra/MgH2-based heterojunctions for efficient hydrogen storage and modulation of hydrogen-desorption temperature ranges

In this study, a novel superalkali NLi4 decorated Boron doped graphene/magnesium hydride heterojunction NLi4-BGra/MgH2 was designed using density functional theory (DFT). Molecular dynamics (MD) confirmed that this material can release H2 adsorbed near NLi4 and H stored within MgH2 at a fixed hydrogen-desorption temperature (Td) of 298 K and 570 K, respectively. Subsequently, 132 magnesium alloy hydrides Mg1-XMeXH2 with various hydrogen-storage capacities and hydrogen-desorption temperatures were predicted by DFT combined with machine learning (ML). Among them, Mg0.875Li0.125H2 (Td = 400K) and Mg0.875Be0.125H2 (Td = 500K) were selected to form NLi4-BGra/Mg0.875Me0.125H2 (Me = Li, Be). The successful validation of Root Mean Square Deviation, diffusion coefficient and hydrogen-desorption trajectories to qualify the hydrogen-desorption capability of NLi4-BGra/Mg0.875Me0.125H2 (Me = Li, Be) (Td = 400, 500K) confirms that the strategy of using Mg1-XMeXH2 to substitute MgH2 in NLi4-BGra/MgH2 enables the effective modulation of the hydrogen-desorption temperature ranges of NLi4-BGra/MgH2-based heterojunctions. In particular, partial density of state, electron localization function, and charge density difference were comprehensively analyzed to understand the hydrogen-storage mechanism and the nature of the hydrogen-desorption temperature variation.

Theoretical investigation on hydrogen storage into N-ethylcarbazole catalyzed by electron-rich Ni4/TiO2-Al2O3

Liquid organic hydrogen strorage (LOHC) technique is a promising safe long-term storage and long-distance transportation at ambient conditions whose bottleneck is the development of catalyst with outstanding hydrogen storage performance and relative low cost. Here, we report a promising Ni4 clusters supported on TiO2-Al2O3 composite oxides (Ni4/TA) as the hydrogen storage catalyst. The effect of TiO2 on the geometry and electron structures of the catalyst, the adsorption behaviors of N-ethylcarbazole (NEC) and its intermediates as well as the hydrogen storage mechanism are investigated via DFT calculations. The results indicate that the electron density around the loaded Ni4 clusters accumulated and the d-band center of Ni4 cluster is modulated due to the excellent electron migration performance of TiO2 species. Thus strengthened the capture and adsorption of NEC at the active region but weakened the adsorption strength of 4H-NEC and 8H-NEC which benefit the desorption-resorption transformation process. A declination of about 60 kJ·mol−1 in the required energy for the rate controlling step demonstrate TiO2 profoundly promoted the startup of hydrogen storage reaction. Moreover, the protonation effect caused by introduction of TiO2 makes it possible to further storage hydrogen into the second aromatic ring of NEC, thus makes the fully hydrogen storage into NEC easy and controllable. This study provides a theoretical guidance for the developing of non-noble metal supported hydrogen storage catalyst with excellent performance.

Hydrogen adsorption and diffusion behavior in kaolinite slit for underground hydrogen storage: A hybrid GCMC-MD simulation study

Underground hydrogen storage (UHS) in reservoirs offers the opportunity for large-scale and long-term storage of hydrogen. In order to understand the fundamental mechanisms of hydrogen storage and transportation, it is crucial to study the adsorption and diffusion behavior of hydrogen in reservoirs. By utilizing a hybrid GCMC and MD simulation procedure, we investigated the adsorption and diffusion behavior of hydrogen in kaolinite slit with pore sizes ranging from 1 to 20 nm at pressures up to 30 MPa and temperatures ranging from 303 to 423 K. Hydrogen distribution, excess adsorption, diffusion coefficient, and gas–solid interaction energy were analyzed in relation to pore size, temperature, pressure, and mineralogy. When pores are larger than 5 nm, most of the hydrogen is located in the bulk phase, resulting in insignificant hydrogen loss due to adsorption. Therefore, reservoirs with pores larger than 5 nm are suitable for UHS. Reservoirs with low temperatures and high hydrogen pressures are conducive to UHS, but the hydrogen mobility is reduced. The mineralogy of the formation results in pore surfaces with different charge properties, which significantly affects hydrogen storage. Although van der Waals interaction dominates the gas–solid interaction, Coulomb interaction remains important. The negatively charged pore surface mitigates the gas–solid Coulomb interaction, thereby preventing hydrogen loss through adsorption. This study enhances our understanding of hydrogen storage mechanisms in subsurface porous media and elucidates the influence of various factors. Consequently, it provides a theoretical framework for selecting UHS sites.

Hydrogen Adsorption in Porous Geological Materials: A Review

The paper adopts an interdisciplinary approach to comprehensively review the current knowledge in the field of porous geological materials for hydrogen adsorption. It focuses on detailed analyses of the adsorption characteristics of hydrogen in clay minerals, shale, and coal, considering the effect of factors such as pore structure and competitive adsorption with multiple gases. The fundamental principles underlying physically controlled hydrogen storage mechanisms in these porous matrices are explored. The findings show that the adsorption of hydrogen in clay minerals, shale, and coal is predominantly governed by physical adsorption that follows the Langmuir adsorption equation. The adsorption capacity decreases with increasing temperature and increases with increasing pressure. The presence of carbon dioxide and methane affects the adsorption of hydrogen. Pore characteristics—including specific surface area, micropore volume, and pore size—in clay minerals, shale, and coal are crucial factors that influence the adsorption capacity of hydrogen. Micropores play a significant role, allowing hydrogen molecules to interact with multiple pore walls, leading to increased adsorption enthalpy. This comprehensive review provides insights into the hydrogen storage potential of porous geological materials, laying the groundwork for further research and the development of efficient and sustainable hydrogen storage solutions.

Nanoscale engineering of solid-state materials for boosting hydrogen storage.

The development of novel materials capable of securely storing hydrogen at high volumetric and gravimetric densities is a requirement for the wide-scale usage of hydrogen as an energy carrier. In recent years, great efforts via nanoscale tuning and designing strategies on both physisorbents and chemisorbents have been devoted to improvements in their thermodynamic and kinetic aspects. Increasing the hydrogen storage capacity/density for physisorbents and chemisorbents and improving the dehydrogenation kinetics of hydrides are still considered a challenge. The extensive and fast development of advanced nanotechnologies has fueled a surge in research that presents huge potential in designing solid-state materials to meet the ultimate U.S. Department of Energy capacity targets for onboard light-duty vehicles, material-handling equipments, and portable power applications. Different from the existing literature, in this review, particular attention is paid to the recent advances in nanoscale engineering of solid-state materials for boosting hydrogen storage, especially the nanoscale tuning and designing strategies. We first present a short overview of hydrogen storage mechanisms of nanoscale engineering for boosted hydrogen storage performance on solid-state materials, for example, hydrogen spillover, nanopump effect, nanosize effect, nanocatalysis, and other non-classical hydrogen storage mechanisms. Then, the focus is on recent advancements in nanoscale engineering strategies aimed at enhancing the gravimetric hydrogen storage capacity of porous materials, reducing dehydrogenation temperature and improving reaction kinetics and reversibility of hydrogen desorption/absorption for metal hydrides. Effective nanoscale tuning strategies for enhancing the hydrogen storage performance of porous materials include optimizing surface area and pore volume, fine-tuning nanopore sizes, introducing nanostructure doping, and crafting nanoarchitecture and nanohybrid materials. For metal hydrides, successful strategies involve nanoconfinement, nanosizing, and the incorporation of nanocatalysts. This review further addresses the points to future research directions in the hope of ushering in the practical applications of hydrogen storage materials.

Hydrogen production, storage, and transportation: recent advances.

One such technology is hydrogen-based which utilizes hydrogen to generate energy without emission of greenhouse gases. The advantage of such technology is the fact that the only by-product is water. Efficient storage is crucial for the practical application of hydrogen. There are several techniques to store hydrogen, each with certain advantages and disadvantages. In gaseous hydrogen storage, hydrogen gas is compressed and stored at high pressures, requiring robust and expensive pressure vessels. In liquid hydrogen storage, hydrogen is cooled to extremely low temperatures and stored as a liquid, which is energy-intensive. Researchers are exploring advanced materials for hydrogen storage, including metal hydrides, carbon-based materials, metal-organic frameworks (MOFs), and nanomaterials. These materials aim to enhance storage capacity, kinetics, and safety. The hydrogen economy envisions hydrogen as a clean energy carrier, utilized in various sectors like transportation, industry, and power generation. It can contribute to decarbonizing sectors that are challenging to electrify directly. Hydrogen can play a role in a circular economy by facilitating energy storage, supporting intermittent renewable sources, and enabling the production of synthetic fuels and chemicals. The circular economy concept promotes the recycling and reuse of materials, aligning with sustainable development goals. Hydrogen availability depends on the method of production. While it is abundant in nature, obtaining it in a clean and sustainable manner is crucial. The efficiency of hydrogen production and utilization varies among methods, with electrolysis being a cleaner but less efficient process compared to other conventional methods. Chemisorption and physisorption methods aim to enhance storage capacity and control the release of hydrogen. There are various viable options that are being explored to solve these challenges, with one option being the use of a multilayer film of advanced metals. This work provides an overview of hydrogen economy as a green and sustainable energy system for the foreseeable future, hydrogen production methods, hydrogen storage systems and mechanisms including their advantages and disadvantages, and the promising storage system for the future. In summary, hydrogen holds great promise as a clean energy carrier, and ongoing research and technological advancements are addressing challenges related to production, storage, and utilization, bringing us closer to a sustainable hydrogen economy.

Reversible hydrogen storage and release mechanism of a B2N monolayer: a first-principles insight.

It was found the physical adsorption could be an efficient strategy for high capacity, high efficiency and high safety in hydrogen storage. In this research, a systematic investigation into the potential of the B2N monolayer as an excellent physical adsorption hydrogen storage material is conducted by utilizing the first-principles calculation method. The findings of the investigation demonstrate that the B2N monolayer has a planar lattice and excellent structural stability. It is possible for H2 molecules to adsorb onto the B2N monolayer spontaneously. Both the individual adsorption and saturation adsorption corresponded to average adsorption energies ranging from -0.221 to -0.194 eV, fulfilling the physical adsorption criteria. In the case of saturation adsorption, a 1 × 2 × 1 B2N supercell can store a total of 24 H2 molecules, with the hydrogen gravimetric density up to 14.511 wt% and volumetric density up to 138 g L-1. A semi-empirical calculation method is used to research the performance of the system in terms of adsorption and desorption with actual temperature and pressure conditions. Under the actual conditions with adsorption carried out at 30 atm/233 K and desorption carried out at 3 atm/358 K, the maximal reversible hydrogen storage capacity of the hydrogen storage system based on the B2N monolayer can still reach 12.157 wt%, which is superior to that of many other boron-nitrogen compounds and metal-free functionalized hydrogen storage materials. The findings of this work indicate that the pristine B2N monolayer is one of the promising physical adsorption materials which could achieve excellent reversible hydrogen storage under defined conditions.

Temperature-dependent hydrogenation behaviour and hydrogen storage thermodynamics of Mg0.9In0.1 solid solution alloy.

To clarify the hydrogen storage mechanism of Mg(In) solid solution, the hydrogenation and dehydrogenation characteristics of Mg0.9In0.1 alloy are systematically studied in this work. It is found that the Mg0.9In0.1 solid solution is first hydrogenated to Mg3In and MgH2 under a hydrogen atmosphere. The polymorphism of Mg3In leads to different hydrogenation features of the solid solution in various temperature ranges. Consequently, the reversible dehydrogenation reactions have somewhat distinct enthalpy changes due to the different crystal structures of Mg3In. When the hydrogenation temperature is not lower than 340 °C, Mg3In can be further hydrogenated to (Mg1-xInx)3In and MgH2. The hydrogenation reactions of both β'-Mg3In and β-Mg3In are also reversible although they have sloping hydrogenation and dehydrogenation plateaus in pressure-composition isotherms. This work provides new insights into the hydrogen storage mechanism of Mg(In) solid solution.

Advances in metal-organic frameworks for efficient hydrogen storage

This article addresses environmental and energy issues by proposing the use of hydrogen as an alternative energy source. However, metal-organic frameworks (MOFs) were chosen as a medium for hydrogen storage and transport because of their large capacity, favourable reversibility, suitable reaction conditions, and relatively low density. The study also introduces the hydrogen storage mechanism of MOFs, and factors affecting their hydrogen storage capacity, and discusses some ways to improve the storage. also introduces the hydrogen storage mechanism of MOFs, factors affecting its hydrogen storage capacity, and discusses some ways to improve the storage The study also introduces the hydrogen storage mechanism of MOFs, factors affecting its hydrogen storage capacity, and discusses some ways to improve the storage features of MOFs. Furthermore, a few of the newest technologies that aid in the creation of MOFs are discussed. This makes it possible to find potential MOFs fast, which·h saves money and effort.1 Introduction.

The Electrode/Electrolyte Interface in MXene-Based Electrochemical Capacitors

The operating voltage of electrochemical capacitor (EC) in aqueous medium is considerably limited due to the theoretical stability of water (1.23V). Hence, the delivered energy density of such systems is restricted compared to that of batteries. A traditional approach to tackle this issue is to focus on improving the capacitance rather than the working voltage of ECs. Especially, combining traditionally used carbons with pseudocapacitive materials has been investigated by many researchers. Adopting this strategy improves the delivered capacity of the system, but key metrics such as a power performance and cycle life of ECs may face a serious failure. The slow nature of faradaic reactions affects the frequency response, while the parasitic activities downgrades the favorable cyclability of the device. Hence, a realistic pathway to improve the energy performance of ECs must be taken. Transition metal carbides, carbonitrides, and nitrides (MXenes) are a new family of 2D layered materials. They have the general formula of Mn+1Xn, in which M indicates the transition metal, like Ti, Mo, Nb, etc., and X is carbon or nitrogen. Considering favorable properties such as a high conductivity and the ability of cation intercalation, many ECs designs based on MXenes have been reported. Nevertheless, there is a lack of clear elucidation of the charge storage mechanism in these materials. Here, the interface of electrode-electrolyte was studied to gain a better insight into the hydrogen storage mechanism and overpotential of MXenes (Ti3C2Tx) in acidic and neutral media. It has been found that MXene-based ECs are suffering from an unbalanced capacitive performance between negative and positive electrodes. Especially, a low capacitance in the positive potential range limits the cell voltage to 0.8V and 1.3V in 1M H2SO4 and 1M Li2SO4, respectively. To extend the cell voltage, self-standing composites based on 3D graphene and Ti3C2Tx were designed. Interestingly, it was observed that during the negative polarization, a significant redox couple activity was observed at the surface of 3DG/ Ti3C2Tx composite electrode. The working potential of the positive electrode was enlarged using micro/mesoporous Black Pearl (BP2000) as electrode material. The realized asymmetric cell was operated in a wider potential window. A higher expansion was realized after balancing the charge of both electrodes. The addition of redox-active salt (KI) to the electrolyte medium improved the capacitive performance of the positive electrode, due to the activity of iodide/iodine species. Excessive increase of the positive electrode capacity was further equalized by balancing the mass of electrodes. In the neutral medium, an asymmetric design of the cell based on Ti3C2Tx (negative electrode) and BP2000 (positive electrode) allowed the cell to operate in a broad range of voltage (up to 2V) with a stable cycling (more than 7000 cycles). The prepared electrodes were characterized using different physiochemical techniques, including XRD, XPS, BET, and SEM analyses. For the electrochemical characterization in two and three-electrode cells, electrodes in the form of pellets or self-standing discs were utilized. The performance of EC cells was studied by cyclic voltammetry, galvanostatic charge-discharge, and electrochemical impedance spectroscopy.

A DFT study on defects and N doping to enhance hydrogen storage in Mg-decorated graphene

In this study, first-principles calculations were used to conduct in-depth studies on multiple key aspects of N-doped defective graphene modified with Mg atoms, including structure, electronic property, thermal stability, as well as hydrogen storage performance and mechanism. The research results show that after the introduction of Mg, N atoms and vacancy defects, pristine graphene transforms from semi-metallic property to metallic property. And the AIMD results indicate excellent thermodynamic stability of the substrate. It can adsorb up to seven H2 molecules, and the adsorption energy values is between −0.15 and −0.21 eV, which is in the stable energy range (-0.15 to −0.60 eV). The hydrogen storage mechanism is mainly attributed to the van der Waals force caused by the polarization of H2 molecules and orbital hybridization between H atoms and the substrate. The calculation of the desorption temperature shows that this substrate can become a potential candidate for reversible hydrogen storage materials at temperatures above 206 K. Compared with graphene decorated with Mg atoms, the introduction of N atoms and vacancy defects significantly improved the hydrogen storage performance.

Application of MOFs in Hydrogen Storage

This research analyzes the current energy problems and environmental problems and proposes to use hydrogen energy as a clean alternative energy source, which has high energy density and less pollution. After comparing the advantages and disadvantages of various hydrogen storage solutions, metal-organic frameworks (MOFs) are selected as the storage and transportation medium because of its large capacity, favourable reversibility, appropriate reaction conditions and relatively low density. The hydrogen storage mechanism of MOFs and the factors affecting its hydrogen storage capacity are introduced and discuss some methods to improve the storage features of MOFs. Three promising, MOFs, MOF-5, MIL-101 and NU-1501, are listed and their storage performance and specific structure are introduced. The current problems in the application and processing of partial hydrogen storage MOFs are proposed, and some feasible solutions and processing methods are introduced. Finally, the application prospects of MOFs are prospected and suitable development directions are proposed, which will be helpful for future research.