Unoccupied States Research Articles

Electronic Properties of Functionalized Diamanes for Field-Emission Displays.

Ultrathin diamond films, or diamanes, are promising quasi-2D materials that are characterized by high stiffness, extreme wear resistance, high thermal conductivity, and chemical stability. Surface functionalization of multilayer graphene with different stackings of layers could be an interesting opportunity to induce proper electronic properties into diamanes. Combination of these electronic properties together with extraordinary mechanical ones will lead to their applications as field-emission displays substituting original devices with light-emitting diodes or organic light-emitting diodes. In the present study, we focus on the electronic properties of fluorinated and hydrogenated diamanes with (111), (110), (0001), (101̅0), and (2̅110) crystallographic orientations of surfaces of various thicknesses by using first-principles calculations and Bader analysis of electron density. We see that fluorine induces an occupied surface electronic state, while hydrogen modifies the occupied bulk state and also induces unoccupied surface states. Furthermore, a lower number of layers is necessary for hydrogenated diamanes to achieve the convergence of the work function in comparison with fluorinated diamanes, with the exception of fluorinated (110) and (2̅110) films that achieve rapid convergence and have the same behavior as other hydrogenated surfaces. This induces a modification of the work function with an increase of the number of layers that makes hydrogenated (2̅110) diamanes the most suitable surface for field-emission displays, better than the fluorinated counterparts. In addition, a quasi-quantitative descriptor of surface dipole moment based on the Tantardini-Oganov electronegativity scale is introduced as the average of bond dipole moments between the surface atoms. This new fundamental descriptor is capable of predicting a priori the bond dipole moment and may be considered as a new useful feature for crystal structure prediction based on artificial intelligence.

The electronic structure of β-TeO2 as wide bandgap p-type oxide semiconductor

Wide bandgap oxide semiconductors have gained significant attention in the fields from flat panel displays to solar cells, but their uses have been limited by the lack of high mobility p-type oxide semiconductors. Recently, β-phase TeO2 has been identified as a promising p-type oxide semiconductor with exceptional device performance. In this Letter, we report on the electronic structure of β-TeO2 studied by a combination of high-resolution x-ray spectroscopy and hybrid density functional theory calculations. The bulk bandgap of β-TeO2 is determined to be 3.7 eV. Direct comparisons between experimental and computational results demonstrate that the top of a valence band (VB) of β-TeO2 is composed of the hybridized Te 5s, Te 5p, and O 2p states, whereas a conduction band (CB) is dominated by unoccupied Te 5p states. The hybridization between spatially dispersive Te 5s2 states and O 2p orbitals helps us to alleviate the strong localization in the VB, leading to small hole effective mass and high hole mobility in β-TeO2. The Te 5p states provide stabilizing effect to the hybridized Te 5s-O 2p states, which is enabled by structural distortions of a β-TeO2 lattice. The multiple advantages of large bandgap, high hole mobility, two-dimensional structure, and excellent stability make β-TeO2 a highly competitive material for next-generation opto-electronic devices.

Activity of Metal-Fluorine States upon Delithiation of Disordered Rocksalt Oxyfluorides

The capacity of transition-metal oxides as Li-ion battery cathodes is limited by instabilities that arise when high states of charge are achieved. Oxyfluorides with a disordered rocksalt structure have emerged as attractive alternatives, but the role of F in their electrochemical function, particularly when cationic redox produces high formal oxidation states, remains to be ascertained. Using X-ray absorption spectroscopy, we confirm the existence of Mn–F covalent interactions in Li2MnO2F and Li2Mn2/3Nb1/3O2F. New unoccupied states evolve from hybrid 3d-2p states of both Mn–F and Mn–O bonds when the phases are delithiated, particularly in the presence of Mn(IV). The results challenge the assumption of F as anion whose covalent states with the metal are tapped at very high potentials, providing instead a nuanced picture of redox compensation in oxyfluorides. They suggest the existence of unique knobs of design of battery cathodes by manipulating the covalent interactions between transition metals and two different anions.

Emergence of metallic surface states and negative differential conductance in thin β-FeSi2 films on Si(001)

The electronic properties of the surface of β-FeSi2 have been debated for a long. We studied the surface states of β-FeSi2 films grown on Si(001) substrates using scanning tunnelling microscopy (STM) and spectroscopy (STS), with the aid of density functional theory calculations. STM simulations using the surface model proposed by Romanyuk et al (2014 Phys. Rev. B 90 155305) reproduce the detailed features of experimental STM images. The result of STS showed metallic surface states in accordance with theoretical predictions. The Fermi level was pinned by a surface state that appeared in the bulk band gap of the β-FeSi2 film, irrespective of the polarity of the substrate. We also observed negative differential conductance at ∼0.45 eV above the Fermi level in STS measurements performed at 4.5 K, reflecting the presence of an energy gap in the unoccupied surface states of β-FeSi2.

Universal Band Alignment Rule for Perovskite/Organic Heterojunction Interfaces

In contrast with the band alignment rule governing conventional semiconductor heterojunctions, the Fermi level position (i.e., work function, ϕpsk) of the perovskite and the differences in ionization energies of the organics are shown to dictate the band alignment and band offsets at perovskite/organic heterojunction interfaces. The experimental data show that the interface band alignments follow a universal alignment rule with three different energy windows: pinning of unoccupied molecular orbital states (LUMO) to the perovskite Fermi level, band offsets mediated by perovskite work functions, and pinning of occupied molecular orbital states (HOMO) to the perovskite Fermi level. The surface-passivated perovskites and several common native perovskites are all found to follow the same band alignment rule. Data scattering in measured band offsets, however, is observed on the same type of perovskites without proper surface passivation treatment. Solar cells made on the perovskites without surface passivation are also found to exhibit a large dispersive distribution in energy conversion efficiencies.

Nanoscale chemical and structural investigation of solid solution polyelemental transition metal oxide nanoparticles

Although it has been shown that configurational entropy can improve the structural stability in transition metal oxides (TMOs), little is known about the oxidation state of transition metals under random mixing of alloys. Such information is essential in understanding the chemical reactivity and properties of TMOs stabilized by configurational entropy. Herein, utilizing electron energy loss spectroscopy (EELS) technique in an aberration-corrected scanning transmission electron microscope (STEM), we systematically studied the oxidation state of binary (Mn,Fe)3O4, ternary (Mn, Fe, Ni)3O4, and quinary (Mn, Fe, Ni, Cu, Zn)3O4 solid solution polyelemental transition metal oxides (SSP-TMOs) nanoparticles. Our findings show that the random mixing of multiple elements in the form of solid solution phase not only promotes the entropy stabilization but also results in stable oxidation state in transition metals spanning from binary to quinary transition metal oxide nanoparticles.

Analysis of doping effects on electrical conductivity and interfacial contact resistance of TiO2

In fuel cells, electrolysis cells, etc., titanium have been widely employed due to the high corrosion resistance achieved by the oxide film. But it also leads to low electrical conductivity and reduces the cells efficiency significantly. To illuminate how to improve the conductivity of titanium alloys, first-principles calculations merged with Boltzmann transport equation and Schottky-Mott theory is adopted to investigate the doping effects of thirty-seven elements on the electronic transport property. The results indicate the low electrical conductivity of TiO2 is attribute to the strong ionic bond of Ti-O. Nb, Ta, Sb and Zr can enhance the conductivity to above 1.0×103 S∙cm-1, which can also make the Schottky barrier decline from 1.05 eV to below 0.4 eV. That is mainly due to the Fermi level shifting and the stronger covalent bond formed between the impurity and O atoms. This work also investigated the effect of doping ratio on the conductivity. The covalent bond between impurity and O atoms is found to increase at a higher doped ratio. That results in the extension of the unoccupied state of the valence band maximum and conduction band minimum to the forbidden gap, which further leads to the reduction of the band gap.

Distinct ferrovalley characteristics of the Janus RuClX (X = F, Br) monolayer.

Two-dimensional ferrovalley materials should simultaneously possess three characteristics, that is, a Curie temperature beyond atmospheric temperature, perpendicular magnetic anisotropy, and large valley polarization for potential commercial applications. In this report, we predict two ferrovalley Janus RuClX (X = F, Br) monolayers by first-principles calculations and Monte Carlo simulations. The RuClF monolayer exhibited a valley-splitting energy as large as 194 meV, perpendicular magnetic anisotropy energy of 187 μeV per f.u., and Curie temperature of 320 K. Thus, spontaneous valley polarization at room temperature will be present in the RuClF monolayer, which is nonvolatile for spintronic and valleytronic devices. Although the valley-splitting energy of the RuClBr monolayer was as high as 226 meV with magnetic anisotropy energy of 1.852 meV per f.u., the magnetic anisotropy of the RuClBr monolayer was in-plane, and its Curie temperature was only 179 K. The orbital-resolved magnetic anisotropy energy revealed that the interaction between the occupied spin-up states of dyz and the unoccupied spin-down states of dz2 dominated the out-of-plane magnetic anisotropy in the RuClF monolayer, but the in-plane magnetic anisotropy of the RuClBr monolayer was mostly contributed by the coupling of the dxy and dx2-y2 orbitals. Interestingly, the valley polarizations in the Janus RuClF and RuClBr monolayers appeared in their valence band and conduction band, respectively. Thus, two anomalous valley Hall devices are proposed using the present Janus RuClF and RuClBr monolayers with hole and electron doping, respectively. This study provides interesting and alternative candidate materials for the development of valleytronic devices.

Effects of N-doped concentration in graphene on CO2 adsorption

Solving the problem of the environment related to CO2 emission is an urgent and challenging task to prevent the escalation of climate change, which is due to increasing energy demand from fossil fuels and industrial activities. Using graphene to capture CO2 gas is of great interest to mitigate global warming, where CO2 capture by doped graphene has been shown to significantly improve the CO2 adsorption capacity compared to that of pure graphene. In particular, N-doped graphene is a unique structure that was suggested for CO2 capture by the adsorption phenomenon. However, there is no research available to clarify nitrogen doping concentration in graphene on CO2 adsorption. Therefore, this work has been devoted to elucidating the problem using the density functional theory method considering van der Waals interaction. We showed that increasing the doping content of nitrogen by 3.1, 6.3, 9.4, and 12.5% will increase the CO2 adsorption energy, monotonically for N content below 9.4% and significantly for 12.5%. The most favorable adsorption configuration of the CO2 molecule should be parallel to the surface of the N-doped graphene. The structure of the substrate at 12.5% N distorted upon the CO2 adsorption; therefore, the substrate is more active for CO2 capture. The physical meaning underlying the interaction of the N@graphene system is physisorption, which is due to the contribution of the N pz state and the CO2 pz state along the surface normal of the substrate. Increasing the N-doping content shifts down the unoccupied states of the N pz orbital in the conduction band across the Fermi level to the valence band. Hence, the population of the occupied state at the Fermi level increases as increasing the N-doping concentration, leading to the stronger interaction of the CO2 molecule with the N@graphene substrate. The results should be useful for the rational design of suitable N@graphene substrates for CO2 capture and storage applications.

First-principles calculations to investigate doping effects on electrical conductivity and interfacial contact resistance of TiO2

In fuel cells, electrolysis cells, etc., titanium have been widely employed due to the high corrosion resistance achieved by the oxide film. But it also leads to low electrical conductivity and reduces the cells efficiency significantly. To illuminate how to improve the conductivity of titanium alloys, first-principles calculation merged with Boltzmann transport theory and Schottky-Mott theory is used to study the doping effects of thirty-seven elements on the electrical conductivity and the Schottky barrier. The effect of doped concentration on the conductivity was also investigated. The results indicate the low electrical conductivity of TiO2 is attribute to strong ionic bond of Ti-O. The covalent bond between dopant and O atoms is found to enhance at a higher doped concentration. That can lead to the extension of the unoccupied state of valence band maximum and conduction band minimum to the forbidden gap, which further leads to the reduction of the band gap. In addition, Nb, Ta, Sb and Zr can enhance the conductivity to above 1.0 × 103 S cm−1, which can also make the Schottky barrier decline from 1.05 eV to below 0.4 eV. The mechanism of doped modifications was further studied to guide experimental works.

Nanocrystalline Ag-doped cobalt oxide as a flexible electrode material for high performance supercapacitor application

Co3-xAgxO4 (x = 0 ~ –0.4) nanocomposite with a spherical shape and porous surface has been synthesized using a simple, fast and scalable combustion method. The highly stable Ag-doped Co3O4 nanocomposite particles were found to be 20‐–50 nm in size with high surface area, crystallinity, and porosity. The specific capacitance value of the Co2.7Ag0.3O4 (CA3) electrode was observed to be higher than other electrodes. Doping of Ag provides a large Ag/Co3O4 interface which enhanced the electrical conductivity of the electrode by moulding ohmic contact. This contributes to a stable and straight passage for rapid charge transfer. The calculated specific capacitance value of CA3 nanocomposite was 648 F g‐−1 at a current density of 0.25A g‐−1 which was 6-fold greater than the undoped Co3O4 and 2‐–4-fold greater than Ag-doped Co3O4 nanocomposites prepared at other concentrations. The electrodes exhibited excellent cyclic stability with capacitance retention of 93% after 10,000 cycles having an energy density of 88.23 Wh kg‐−1 at 1.75 kW kg‐−1 power density indicating a promising material for supercapacitor application. Fabricated symmetric solid-state flexible device of Ag-doped Co3O4 also demonstrated reliable charge storage performance with a maximum energy density. The theoretical study also revealed that Ag doping into Co3O4 introduces additional localized states around the fermi level, between the highest occupied states and lowest unoccupied states. The presence of higher localized states near the Fermi level and smaller bandgap than Co3O4 enhanced the specific capacitance of the system, providing support to the experimental observation on improvement in energy charge storage.

Photovoltaic performance of heteroatom-doped boron nitride quantum dots in quantum dot photovoltaic cells

The photovoltaic performance of C/O/S-doped hexagonal boron nitride (h-BN) quantum dots (QDs) is studied using density functional theory. Doping leads to occupied or unoccupied midgap states in h-BN QDs, resulting in a redshift in their absorption spectra. C-doping provides better charge transfer capability than S/O-doping. In addition, C-doping reduces the open-circuit voltage, light collection efficiency, fill factor, and driving forces of electron injection and reduction of h-BN QD. However, the fast non-radiative recombination deteriorates the energy conversion of C-doped h-BN QDs. The current study provides evidence for the rational design of photocatalytic devices based on h-BN QDs.

An oxygen vacancy defect and chalcogens impurities in earth-abundant lead-free calcium zirconate perovskite for non-toxic solar cells: First-principles calculations

The effects of an oxygen vacancy (Ov), and chalcogens impurities on electronic, optical properties, thermodynamic stability, and photocatalytic hydrogen production form water splitting ability of non-toxic lead-free calcium zirconate perovskite CaZrO 3 are investigated using first-principles calculations-based Density Functional Theory and LDA+TB-mBJ functional. It is found that an Ov itself in CaZrO 3 compound results of free electrons inside the unoccupied states in CB which changed the type of electrical conductivity from p to n-type. Besides, the wide forbidden band energy of CaZrO 3 was effectively reduced when chalcogens impurities doped CaZrO 3 with the presence of an Ov. Furthermore, the optical properties show that the percentage of the transmittance reduced while the absorption coefficient’s percentage increased along the visible spectrum in all structures having an Ov, suggesting that Ov creates free charge carriers in the CaZrO 3 compound. In Addition, the formation energy confirms that all studied structures are thermodynamically stables. Finley, S/Se@OI/OII/OIII+Ov structures are promising photocatalysts to drive water splitting reactions as they are distinguished by their appropriate forbidden bands, visible light absorption, and band edge positions. We believe that our study could be used to give CaZrO 3 the opportunity to be used as a semiconductor for photovoltaic and photocatalytic applications.

Correlation between Electronic Descriptor and Proton-Coupled Electron Transfer Thermodynamics in Doped Graphite-Conjugated Catalysts.

Graphite-conjugated catalysts (GCCs) provide a powerful framework for investigating correlations between electronic structure features and chemical reactivity of single-site heterogeneous catalysts. GCC-phenazine undergoes proton-coupled electron transfer (PCET) involving protonation of phenazine at its two nitrogen atoms with the addition of two electrons. Herein, this PCET reaction is investigated in the presence of defects, such as heteroatom dopants, in the graphitic surface. The proton-coupled redox potentials, EPCET, are computed using a constant potential periodic density functional theory (DFT) strategy. The electronic states directly involved in PCET for GCC-phenazine exhibit the same nitrogen orbital character as those for molecular phenazine. The energy εLUS of this phenazine-related lowest unoccupied electronic state in GCC-phenazine is identified as a descriptor for changes in PCET thermodynamics. Importantly, εLUS is obtained from only a single DFT calculation but can predict EPCET, which requires many such calculations. Similar electronic features may be useful descriptors for thermodynamic properties of other single-site catalysts.

Strong Electron Coupling Effect at the CoO/CeO2 Interface Enables Efficient Oxygen Evolution Reaction

CeO2 has played an important role in improving the oxygen evolution reaction (OER) performance of transition metals-based catalysts whether as a hybrid, substrate, or interface. The high OER activity is ascribed to the optimized transition metals and/or the formed oxygen vacancies. In this work, the interface effect between CoO and CeO2 is reported to be the reason for the excellent OER performance of CoO/CeO2. Compared with sole CoO or CoO/CeO2-L (larger CoO nanosheets), CoO/CeO2 has exhibited higher OER performance due to the faster kinetics and higher OER intrinsic activity because of the large amount of Co/Ce interfaces. Theoretical calculations reveal the generation of the reconstructed active center of the Co–O–Ce configuration due to the strong electron coupling effect between CeO2 and CoO. The Co–O–Ce hybridization at the interface brings in the unoccupied oxygen states, acting as the acceptor-like states and optimizing the bonding energy with OER intermediates. The findings give an avenue to design excellent CeO2-containing OER catalysts through interface control.

Monitoring Electron Flow in Nickel Single-Atom Catalysts during Nitrogen Photofixation.

An efficient catalytic system for nitrogen (N2) photofixation generally consists of light-harvesting units, active sites, and an electron-transfer bridge. In order to track photogenerated electron flow between different functional units, it is highly desired to develop in situ characterization techniques with element-specific capability, surface sensitivity, and detection of unoccupied states. In this work, we developed in situ synchrotron radiation soft X-ray absorption spectroscopy (in situ sXAS) to probe the variation of electronic structure for a reaction system during N2 photoreduction. Nickel single-atom and ceria nanoparticle comodified reduced graphene oxide (CeO2/Ni-G) was designed as a model catalyst. In situ sXAS directly reveals the dynamic interfacial charge transfer of photogenerated electrons under illumination and the consequent charge accumulation at the catalytic active sites for N2 activation. This work provides a powerful tool to monitor the electronic structure evolution of active sites under reaction conditions for photocatalysis and beyond.

Electron localization in recrystallized models of the Ge2Sb2Te5 phase-change memory material

Understanding the relation between the structural disorder in the atomic geometry of the recrystallized state of phase-change memory materials and the localized states in the electronic structure is necessary not only for technological advances, but also essential to achieve a fundamental understanding of these materials. In this computational study, hybrid density-functional theory simulations are employed to ascertain the impact of antisite defects on the spatial localization of the electronic states in the bottom of the conduction band in recrystallized models of the prototypical phase-change material ${\mathrm{Ge}}_{2}{\mathrm{Sb}}_{2}{\mathrm{Te}}_{5}$. $\mathrm{Te}\text{\ensuremath{-}}\mathrm{Te}$ homopolar bonds are the local defective atomic environments mainly responsible for the electron localization of the conduction-band-edge states in the simulated structures, while $\mathrm{Sb}\text{\ensuremath{-}}\mathrm{Te}$ chains can also induce spatial localization. Unoccupied defect-related electronic states can emerge in the band gap during a crystallization event, while $\mathrm{Sb}\text{\ensuremath{-}}\mathrm{Sb}$ homopolar bonds have been identified in the defect environment of a deep localized state.

Synthetic Engineering in Na2MSn2(NCN)6 (M = Mn, Fe, Co, and Ni) Based on Electronic Structure Theory.

Quaternary transition metal cyanamides Na2MSn2(NCN)6 with M = Mn, Fe, Co, and Ni were prepared via solid-state metathesis reactions between Na2Sn(NCN)3 and binary transition metal fluorides MF2 in a 2:1 molar ratio. All phases crystallize isotypically in [NiAs]-derived structures (P3̅1m) with inter- and intra-layer cation ordering over the octahedral sites. This leads to a highly asymmetric coordination of the NCN2- anion, resulting in a strong degree of cyanamide character, which is confirmed via IR measurements. Intriguingly, the optical properties of Na2MSn2(NCN)6 phases change markedly as the nature of the divalent transition metal is varied, and UV-vis measurements evidence a band gap reduction from Mn (3.43 eV) via Fe (1.90 eV) to Co (1.75 eV), which broadly mirrors the DFT+U calculated energetic interval from the Fermi level to the unoccupied 3d states. Mott-Schottky analysis then goes on to characterize Na2FeSn2(NCN)6 and Na2CoSn2(NCN)6 as n-type semiconductors with flat-band potentials of 0.46 and -0.24 eV, respectively, vs RHE. This study demonstrates the utility of transition metal substitutions, within a flexible cyanamide framework, to electronically tune this growing family of pseudo-oxides.

Quantitative Evaluation of the Activity of Nickel Ion Site of LiNi0.5Mn1.5O4 Spinel for Oxygen Evolution Reaction

Suntivich et al. found a volcano relationship between the eg occupancy in perovskite oxide and OER activity. eg filling of ~ 1.2 is the optimum value1. Nevertheless, some materials do not follow this trend. A typical example is oxides containing a high valence Ni(4+) state. To elucidate the effect of high-valence Ni on OER activity, it is necessary to evaluate the OER activity of materials with stable Ni in a high oxidation state (4+). However, in case of the perovskite family, the formation of Ni4+ requires calcination under high oxygen partial pressures (60-200 bar). In contrast, LiNi0.5Mn1.5O4 (LNMO) with a spinel structure is known that lithium ions are desorbed by oxidation to produce tetravalent nickel ions quantitatively. Therefore, by using this material as an oxygen-evolving electrode, it is possible to quantitatively evaluate the activity of tetravalent nickel ions. Herein, we report the OER activity of nickel-manganese spinel, in which Ni4+ was stable up to the bulk. LiNi0.5Mn1.5O4 powder was synthesized as follows. Stoichiometric amounts of LiNO3, Ni(NO3)2.6H2O, and Mn(NO3)2.6H2O were dissolved in ultrapure water. A four-to-five-fold molar excess of citric acid was added while stirring. This solution was heated at 400 °C to obtain the precursor. The obtained powder was calcined at 800 °C for 12 h and at 700 °C for 48 h under pure O2.Various levels of chemical delithiation were achieved by reacting LiNi0.5Mn1.5O4 with nitronium tetrafluoroborate in acetonitrile solution for 48 h at 25 ºC. The degree of delithiation was controlled by changing the ratio of LiNi0.5Mn1.5O4 to NO2BF4. The suspension of LiNi0.5Mn1.5O4 and NO2BF4 was stirred for 48 h to obtain a uniform particle composition. Subsequently, the suspension was filtered, thoroughly washed with acetonitrile at least three times, and dried overnight in a vacuum oven. Then, chemical composition was determined using inductively coupled plasma optical emission spectrometry, X-ray diffraction (XRD) patterns were collected in BL02B2 at SPring-8, Japan. XAS was conducted for Mn and Ni K-edges, XAS for Mn, Ni L-edges, and O K-edge was also conducted. This study focused on the electrocatalytic activity of LixNi0.5Mn1.5O4 spinels with different lithium contents. The lithium (x) content in LixNi0.49Mn1.51O4 was controlled by topochemical delithiation using NO2BF4 in an acetonitrile solution. Upon applying this approach, nickel changed from the divalent to tetravalent state, while the tetravalent state of manganese was maintained. The OER activity was found to increase with decreasing x. x = 0.00 showed the highest OER activity. The Tafel slopes indicated that the RDS changed from OH− adsorption to proton transfer from OH*; thus, the OER kinetics was enhanced. The XAS spectra of Ni and O indicated that the presence of Ni4+ caused a downshift in the unoccupied Ni 3d state and decreased the energy barrier of charge transfer from the oxygen intermediate species to the metal active site; hence, the OER activity was enhanced. This study revealed the mechanism underpinning the high OER activity of high-valence Ni(4+) and also provided a approach for the development of highly active catalyst materials, namely, topochemical delithiation. Acknowledgement This work is based on results obtained from a project (JPNP14021) commissioned by the New Energy and Industrial Technology Development Organization (NEDO) of Japan. References Suntivich, J.; May, K. J.; Gasteiger, H. A.; Goodenough, J. B.; Shao-Horn, Y., Science 2011, 334, 1383-1385. Ren Y. et al., ACS Appl. Energy Mater. 2021 4 (10), 10731-10738

Compositional effects on the hydrogen storage properties in a series of refractory high entropy alloys

The possible combinations in the multidimensional space of high entropy alloys are extremely broad, which makes the incremental experimental research limited. As a result, establishing trends with well-known empirical parameters (lattice distortion, valence electron concentration etc.) and predicting effects of the chemical composition change are vital to guide future research in the field of materials science. In this context, we propose a strategy to rationalize the effect of chemical composition change on the hydrogen sorption properties in a series of high entropy alloys: Ti0.30V0.25Zr0.10Nb0.25M0.10 with M = Mg, Al, Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo, Ta and ∅ (corresponding quaternary alloy). All materials are bcc alloys and absorb hydrogen at room temperature forming fcc or pseudo-fcc dihydride phases. The maximum hydrogen storage capacity at room temperature strongly depends on the valence electron concentration (VEC) of the alloys: the capacity is high (1.5–2.0 H/M) for low values of VEC (<4.9) whereas, a drastic fading is observed for VEC ≥4.9 which is the case for alloys with M being a late 3d transition metal. The structural analysis suggests that steric effects might not be responsible for this trend and electronic reasons may be invoked. Increasing the VEC by alloying with late 3d transition metals will fill the unoccupied valence states and the electrons from interstitial hydrogens can no longer be accommodated, which is unfavorable for hydrogen storage. Moreover, the onset temperature of desorption increases almost linearly with VEC for this composition series. These findings suggest that alloys with low VEC are more likely to become promising candidates for hydrogen storage.