Search

The problems associated with the global warming due to greenhouse gas emissions from human activities are driving the search for new technologies to reduce CO2 emissions from the fossil fuel combustion. Hydrogen is a clean and efficient energy carrier, so the hydrogen energy is considered as one of the strategic directions for the development of the energy security and sustainability. Although hydrogen can be produced by electrolysis of water, most of the hydrogen produced worldwide currently comes from steam reforming of natural gas, which must be purified to ultra-high purity for economic reasons. Traditional methods based on pressure swing absorption-desorption using cryogenic distillation are energy intensive, which motivates the development of new highly selective and energy efficient H2 purification technologies. Such technologies include rapidly developing membrane technology, which at the moment has not yet reached the level required for its widespread industrial application. In this review article, we analyze the main aspects of current research in the field of hydrogen purification technology using dense metal membranes, with an emphasis on their technological stability due to the selection of their chemical composition (including multicomponent) and optimization of the structural state. We also considered the prospects for further development and use of this technology for economic needs.

The stability of high-entropy alloys, like equiatomic and non-equiatomic micro- and nanostructural ones, is relevant when creating stable multicomponent compositions with improved performance. The implementation of such materials is possible by mechanical alloying, magnetron sputtering, as well as by the electrochemical method using the «top-down and bottom-to-top» strategy at moderate temperatures < 200°C with controlled production of both micro-from 0,5 to 20 μm and nanoscale high-entropy alloys with particles from 2 to 10 nm. The well-studied «structure-property» relationship for classical alloys is not yet completely clear for nano-high-entropy alloys, but it is obvious that it is possible to form excellent mechanical characteristics by selecting chemical compositions and a special heat treatment regime. Regarding the chemical composition, requirements are imposed both on the main components and alloying additives. Preliminarily, not only compositions are selected, but also methods for the synthesis of high-entropy alloys, including ab initio (density functional theory), neural network prediction, and classical molecular dynamic simulation with possible conditions for the formation of model nano- high-entropy alloy samples, as well as derivative options. The resulting descriptions are compared with real methods of high-entropy alloys synthesis, for example, exposure to various synthetic media.

The principles of designing high-entropy alloys related to the selection of elements are analyzed. When selecting elements, a parametric approach is used, including chemical and topological parameters. The main chemical parameter is the enthalpy of mixing of elements, the main topological parameter is the atomic radius. It is emphasized that the use of modified atomic radii (which take into account the local electronic environment) better predicts the formation of either amorphous or crystalline high-entropy alloys. Four main effects that determine the properties of high-entropy alloys are considered: the high entropy effect, the lattice distortion effect, the delayed diffusion effect, and the «cocktail» effect. Obtaining nanosized high-entropy materials based on high-entropy alloys is a new promising direction that allows one to significantly expand their areas of application related to energy (catalysis, energy storage, etc.), nanoelectronics, etc. The article analyzes some methods for the synthesis of nanosized high-entropy alloys and materials based on them. basis, developed as catalysts. The improved performance over conventional catalysts is explained in terms of the effects and features specific to multicomponent systems.

The article presents both molecular dynamics calculations of binary Fe–Ni alloys and experimental studies of Ti and Co alloyed nanocrystalline alloys with a B2–Ti(Fe, Co) matrix structure as well as bcc-(Nb, Ti) and B2– eutectic phases Ti(Fe, Co). The structures of membrane alloys based on Fe–Ni (arrangement of atoms in coordination polyhedra and interatomic distances between atoms), as well as the kinetics of hydrogen - diffusion and permeability have been studied. It is shown that in the membranes of alloyed alloys with the substitution of Ni for cobalt Fe35-XCoXTi35Nb30, with an excess of Fe than for cobalt, mechanical brittleness is manifested in the B2–TiFe phase, and the plasticity of the B2 phase also decreases. At the same time, the resistance to an increase in hydrogen absorption is also weakened, up to mechanical destruction of membranes, so that in high-entropy alloys Fe0,2Ni0,2Cr0,2Co0,2Mn0,2, Fe0,2Co0,2Cr0,2 Ti0,2Al0,2 Fe and Co in equal parts. Other intermetallic alloys are also promising, having more complex compositions with high or moderate entropy, for example, Zr0,2Ti0,2Nb0,2V0,2Co0,2 and Zr0,2Ti0,2Ta0,2V0,2Co0,2, in addition to hydrogen evolution, also have storage properties. Within the framework of molecular dynamics, the effect of strain hardening of membrane HEA alloys is experimentally presented - the mechanism of synergy with multiple deformation. As a result of such hardening, a partial transformation of the austenitic phase into a martensite phase occurs with the formation of twinning in their fcc/hcp grains and the formation of a two-phase matrix structure.

An analysis of synthesized unique two-dimensional 2D materials with nanolayer hexagonal and pentagonal structures (based on carbon, silicon, tin, binary compounds CN2, BN2, PdSe2 and ternary compounds – BCN, CNP, PdSSe, Zn2C2P2) was carried out. The synthesis of these materials was performed by the chemical vapor deposition or metal epitaxy on pre-prepared substrates. The strength and functional characteristics (electronic, optical) of the created models were also analyzed using the DFT theory in the form of triple monolayers with double-sided deposition of hydrogen on the surface of the p-Si2C4 monolayer: hydrogen/p-Si2C4/hydrogen. It was found that the p-Si2C4-4H layer with its two-sided hydrogen adsorption and good properties was the most dynamically stable. This article also presents relatively recently obtained hexa- and pentagonal two-dimensional materials not only for the elements C, Si, Ge, B, but also for Cu1–xNix, Ti1–xNix alloys and Bi1–xSbx, CN2, BN2, PdSe2, etc. compounds. So, with the new unique materials created – the synthesis of superstrong, thermostable nanocomposites, superconducting layered composites (based on Bi, Hg and Sb), prospects are opening up for the development of nanoelectronics, spintronics, computer technology, as well as the creation of portable strain gauges, pressure sensors, gas sensors and dialysis catalysts for water dialysis with the release of hydrogen and oxygen.

With unique mechanical and functional properties, amorphous, nanocrystalline and matrix duplex microstructure membrane alloys based on group V elements actively contribute to the development of hydrogen energy. There are still not completely resolved problems for these new alloys - their low thermal stability, insufficient mechanical strength (plasticity, hardness), and intermetallic and hydride embrittlement. For effective use, alloys with a triple composition are being developed – which, in addition to the elements of group V, also include nickel and titanium as alloying metals. Not only amorphous and nanocrystalline alloys are obtained that are applicable in electronics and power engineering, as well as membrane alloys with a duplex matrix structure that combines amorphous, nano- and quasicrystalline dendritic-hardening phases strengthening the amorphous matrix. In specialized membrane ternary alloys, NiTi and NiTi2 compounds are formed, which stabilize and protect nano- and crystalline membranes from brittle destruction. It has been found that the intense formation of hydrides in these alternative membrane alloys is as undesirable as for palladium-based compounds. The alloys under consideration actually make it possible to obtain high-purity gaseous hydrogen using new compositions instead of expensive membranes based on Pd-Au/Ag/Cu alloys.

To obtain ultrapure hydrogen by membrane technology, instead of expensive membranes made of Pd alloys, cheaper ones based on metals (Nb-Ni) and (V-Ni) are considered. Due to alloying of these Ti alloys, the performance of the membranes increased – diffusion and permeability of hydrogen, wear resistance and thermal stability, exceeding the Pd alloys. For crystalline analogs, the problem was also solved by increasing the Ti concentration with the formation of eutectic phases in ternary alloy compositions (Nb85-xTixNi15 and V85-xTixNi15). Hydrogen accumulated in membrane matrices forms specific polyhedral eutectic TСР hydrides up to phase transitions, and upon cooling from 673 to 303К under conditions of thermal expansion from 473 to 673К, it increases the temperature of β-hydride formation and forms NiTi and NiTi2 compounds, which stabilize and protect nano- and crystalline membranes from brittle destruction.

An analysis of studies of membrane alloys Ta77Nb23 and Ta94,9W5,1 was carried out with an estimation of their characteristics – thermal stability, strength – by measuring microhardness, matrix structure – by X-ray analysis, as well as the dynamics of hydrogen – diffusion (theoretically) and permeability (experimentally). The efficiency of the hydrogen separation/purification using the Ta77Nb23 and Ta94,9W5,1 alloys was higher than that of palladium, measured under the same test conditions.

The kinetics of hydrogen permeability and diffusion in hydrogenated membrane alloys based on Fe and Ni have been investigated, which have functional characteristics, structural strength and thermal stability, and are a promising alternative to expensive palladium alloys. The strength characteristics, thermal stability and resistance to hydrogen embrittlement have been studied. The mechanisms of structuring, diffusion and permeability of hydrogen in crystalline, amorphous, nanocrystalline states are considered. For membrane alloys, a particularly important aspect is the combination in open structures (amorphous and bcc), while for hydrogen accumulation, in addition to transport channels (diffusion and permeability), high-density intermetallic clusters are also required. It is noted that, during long-term operation, due to the appearance of dense cluster configurations of intermediate ordering, which shorten the diffusion channels, the transport of hydrogen through them begins to decrease.

Using the results of computer modeling and experiment, an analysis has been made of the effect of hydrogen and its kinetics on the nature of the formation of cluster-block configurations in alloys – Laves phases based on Friauf polyhedra and kagome structures that determine the characteristics of new materials with the aim of advancing the ecological and energy-saving technologies for producing ultrapure hydrogen and then its accumulation in the concerning alloys.

Under the conditions of the fast crystallization of melts, the effect of hydrogen on mechanisms of structural transformations in V85Ni15 and V(85-x)TixNi15 alloys was experimentally studied. Structuring of phase compositions, diffusion, and hydrogen permeability were simulated. The regularities of improving the structural characteristics of alloys as a result of alloying with titanium, as well as the effect of hydrogen dynamics on the stability of the formed phases are revealed.

The researchers have conducted a comprehensive analysis of both the original material characteristics and the available information on phase-structural transformations during cooling, glass transition and crystallization of melts, with detailed tracking of their ordering with using the methods as synchrotron X-ray diffraction, scanning electron microscopy, differential scanning calorimetry measurements, and with the interpretation of data by molecular dynamic simulation and thermodynamic calculations.

There have been considered the characteristics and structure forming of amorphous and nanocrystalline binary and ternary alloys based on Pd, as well as high-entropy Zr − Cu (Ni, Ti) compositions based on binary Ni − Ti, Ni − V and ternary Zr − Cu − Ti systems, which are difficult to manufacture, however, can significantly improve the economic efficiency of the processes of separation, purification of hydrogen and its safe storage. The analysis of the formation specificities of stable functional structures of membrane amorphous and nanocrystalline alloys in depending on their ability to hydride formation was also carried out.

In the present article the comparative analysis of results of MD simulation and experiments concerning properties of nanocomposites (metals/graphene, Me: Mg, Al, Cu, Ni, Ti) was carried out. The possibilities of improvement of structural and thermodynamic stability of the metal/graphene interface states, determining the strengthen characteristics of examined nanocomposites, were considered.

Basing on the analysis of the available research data, the short and intermediate order of the structure formations in the amorphous as well as the long order in the nanocrystalline alloys have been revealed for both states of alloys considering with that their structures in corresponding atomic cluster configurations formed not only on the base of icosahedral packing but on the base of helicoid one with fraction symmetry supplying, for the inner structure of nanocrystalline alloys, the mutual coherence of crystalline and non-crystalline coordinations.

In the presented article results of the molecular and dynamic modeling executed with use of many-body potentials. There has been carried out the analysis the thermoactivity of processes of a relaxation, diffusion and formation of the interface structures metal/graphene (is carried out by Me: Cu,Ru/G ), their destruction, as analog of melting in low-dimensional systems.

Analyzing MD simulation data on the thermic evolution of transition metal nanoclusters (transition metals: Pd, Ni, Cu) deposited on graphene substrates specificities have been revealed of such main factors of melting processes and premelting stage including as the nucleation and the kinetic activation of upper cluster shell fluidization and coordination transformation especially in contact interlayer zones of grain transition metals nanocluster/graphene interfaces with different cohesive energies. The following sharp thermostability decreasing of clusters as the consisting and functional interactive units of the whole effective electronic and catalytic systems has been also displayed.

In this article we presented the analysis of molecular dynamic simulations results. The specificities of thermoactivated structure phase changes in 3D Me/C interface forms with grapheme layers and 3D transition metal surface planes have been revealed. The relaxation transition of interface Me-C coordinations and the forming of two-dimensional ring clusters, as the sites of new superlattices have been observed. The correlation of short and long range ordering have been classified.