Outer Electron Research Articles

Reconstruction of electron radiation belts using data assimilation and machine learning

We present a reconstruction of radiation belt electron fluxes using data assimilation with low-Earth-orbiting Polar Orbiting Environmental Satellites (POES) measurements mapped to near equatorial regions. Such mapping is a challenging task and the appropriate methodology should be selected. To map POES measurements, we explore two machine learning methods: multivariate linear regression (MLR) and neural network (NN). The reconstructed flux is included in data assimilation with the Versatile Electron Radiation Belts (VERB) model and compared with Van Allen Probes and GOES observations. We demonstrate that data assimilation using MLR-based mapping provides a reasonably good agreement with observations. Furthermore, the data assimilation with the flux reconstructed by NN provides better performance in comparison to the data assimilation using flux reconstructed by MLR. However, the improvement by adding data assimilation is limited when compared to the purely NN model which by itself already has a high performance of predicting electron fluxes at high altitudes. In the case an optimized machine learning model is not possible, our results suggest that data assimilation can be beneficial for reconstructing outer belt electrons by correcting errors of a machine learning based LEO-to-MEO mapping and by providing physics-based extrapolation to the parameter space portion not included in the LEO-to-MEO mapping, such as at the GEO orbit in this study.

On the Energy‐Dependent Deep (L < 3.5) Penetration of Radiation Belt Electrons

AbstractDeep penetration of outer radiation belt electrons to low L (<3.5) has long been recognized as an energy‐dependent phenomenon but with limited understanding. The Van Allen Probes measurements have clearly shown energy‐dependent electron penetration during geomagnetically active times, with lower energy electrons penetrating to lower L. This study aims to improve our ability to model this phenomenon by quantitatively considering radial transport due to large‐scale azimuthal electric fields (E‐fields) as an energy‐dependent convection term added to a radial diffusion Fokker‐Planck equation. We use a modified Volland‐Stern model to represent the enhanced convection field at lower L to match the observations of storm time values of E‐field. We model 10–400 MeV/G electron phase space density with an energy‐dependent radial diffusion coefficient and this convection term and show that the model reproduces the observed deep penetrations well, suggesting that time‐variant azimuthal E‐fields contribute preferentially to the deep penetration of lower‐energy electrons.

Surface Ti vacancy passivation of Ti3C2O2 MXene via transition metal atoms as high-performance potassium-ion batteries anodes

MXene exhibits tunable interlayer spacing, excellent conductivity and high redox activity, appearing to be ideal intercalated anodes of potassium ion battery. However, defects formed in the preparation process of this material seriously decrease the initial Coulombic efficiency (ICE) and diffusion rate. Herein, transition metals (TMs) passivation is employed to solve this problem based on DFT calculations. TMs filled in Ti vacancy (vTi) is figured out to form strong bonds with surround C and O atoms, which strength is positively-related with the outer electron numbers at d + s orbits of TMs. It enlarged the K adsorption energy difference (ΔEa) between cubic closest packing (ccp)- and hexagonal closest packing (hcp)-site, suggesting larger capacity than Ti3C2O2. ΔEa is revealed negatively linear-correlated with d-band center difference between TM and Ti element, then Fe and Mn would be excellent elements of passivation due to the smallest distance of d-band center to Ti. More importantly, TMs passivation obviously decreases the interaction between vTi and K+, lowering K+ diffusion barrier. This work provides new possibility of surface engineering for MXene performance modification beyond PIBs anodes.

Giant NLO response and ultraviolet transparency of superalakalis decorated C6O6Li6 complexes; a DFT perspective

Materials having nonlinear optical (NLO) properties are the demand of optics and optoelectronics fields because of their widespread applications. Keeping in view the applications of NLO materials, some new superalkalis doped C6O6Li6 complexes are introduced in this study having remarkable NLO response. Thermodynamic stability of nonlinear optical materials (NLO) is prime requirement of experimentalists for practical applications. All the considered complexes have high thermodynamic stability. Furthermore, these superalkali doped complexes reveal fabulous electronic properties. Superalkalis shift their outer shell electrons towards the C6O6Li6 nanocluster, which act as diffuse excess electrons. The HOMO-LUMO gaps (H-L Egap) of superalkali doped complexes are effectively reduced due to shifting of electrons that reveals their conducting properties. NBO charge analysis confirmed electron charge transfer from superalkalis to C6O6Li6 surface. Linear and NLO properties of these complexes are estimated from first order polarizability (α o) and hyperpolarizability (β o) parameters. Among all complexes, C6O6Li6.K3O has the highest first hyperpolarizability (β o = 6.02 × 105 au). TD-DFT analysis confirmed the ultraviolet transparency of doped complexes. Two level model is used to rationalize the trend of first hyperpolarizability and the internal factors responsible for the enhancement of NLO response.

How Accurately Can DFT Describe Non-valence Anions?

Weakly bound non-valence anions are molecular systems where the excess electron stabilizes in a very diffuse orbital whose size, shape, and binding energy (∼1-100 meV) are governed by the long-range electrostatic potential of the molecule. Its binding energy comes mainly from charge-dipole or charge-multipole interactions or dispersion forces. While highly correlated methods, like coupled cluster methods, are considered to be the state of the art for describing anionic systems, especially when the electron lies in a very diffuse orbital, we consider here the possibility to use DFT-based calculations. In such molecular anions, the outer electron experiences long-range exchange and correlation interactions. We show that DFT can describe long-range bound states provided that a correct asymptotic exchange and correlation potential is used, namely, that from a range-separated hybrid functional. This opens an alternative to the computationally demanding highly correlated method calculations. It is also suggested that the study of weakly bound anions could help in the construction of new DFT potentials to study systems where nonlocal effects are significant.

Building Up an “Elemental Property—Adsorption Energy Descriptor—Decomposition Barrier” Three‐Tier Model for Screening Biatom Catalysts in Sodium–Sulfur Batteries

AbstractDeveloping highly efficient catalysts for the Na2S redox process and sodium polysulfide anchoring is becoming increasingly important for high‐performance sodium–sulfur (Na–S) batteries. The recently emerged graphene‐supported biatom catalysts (G‐BACs) exhibit great potential for providing high activity in both discharging and charging processes. However, the fast screening of promising G‐BACs for Na–S batteries is hindered by the formidable computational cost for calculating Na2S decomposition barriers (Eb). Herein, this work develops an “elemental property—adsorption energy descriptor—decomposition barrier” three‐tier model to accelerate this process and elucidates the origin of catalytic activity. It is found that Eb during the charging process is linearly correlated with the adsorption energy difference between the initial and final states of Na2S decomposition (Ediff) for both homonuclear and heteronuclear transition metal G‐BACs. This work further correlates Ediff with intrinsic properties of metal elements by machine learning approaches and unravels the most significant elemental feature to be the outer electron number. This work not only accelerates the design of highly efficient G‐BACs in Na–S batteries based on the structure–activity relationship, but also provides a feasible strategy for the fast screening of catalysts for other electrochemical reactions for potential experimental design.

More than the interface: Synergistic engineering of the interfacial and its subjacent layers followed by self-triggering for high efficient bioelectricity harvesting

The practical application of microbial fuel cell (MFC) is severely impeded by its low output power density stemming from the low bacterial loadings and the sluggish extracellular electron transfer (EET) kinetics. Thus, the current collector designed to greatly enhance the electrocatalytic performance of microbial electrodes ought to possess both high hydrophilicity and high conductivity, which are, however, often mutually incompatible. Herein, to resolve this dilemma, a composite bioelectrode consisting of the poly(ionic liquid) functionalized single-walled carbon nanotube bundles (SWCNT-PIL) layer at the biotic-electrode interface and its subjacent hydrophilic graphene oxides (GO) layer is prepared. Bacterial adhesion is significantly enhanced, resulting from the positive charges of SWCNT-PIL and the hydrophilic GO. After self-reduction of GO by attached electricigens, the as-prepared mrGSP bioelectrode exhibits a maximum output power density of 4.808 W m−2, which is more than one order of magnitude larger than that of conventional carbon-based electrode (0.319 W m−2), because the SWCNT-PIL layer is capable of not only accelerating the flavin-based mediated electron transfer (MET), but also upregulating the membrane protein expression from 14.64% to 18.01% induced by the enhanced electric field, promoting the outer membrane c-type cytochromes-based direct electron transfer (DET). This work provides a new concept of synergistic engineering the interfacial and its subjacent layers to solve the dilemma between high hydrophilicity and high conductivity of the current collector surface for microbial electrocatalysis.

Earth’s magnetotail variability during supersubstorms (SSSs): A study on solar wind–magnetosphere–ionosphere coupling

Supersubstorm (SSS) events are associated with extremely intense westward auroral electrojet currents (the minimum SuperMAG AL or SML <-2500nT) resulting from the solar wind-magnetosphere coupling. We present, for the first time, variability of the Earth’s magnetotail and coupling from the solar wind to magnetotail, geosynchronous orbit and auroral ionosphere during the SSS events. Five intervals with multiple SSSs were identified when the Cluster spacecraft was well inside the inner magnetotail. The SSS events are found to be characterized by turbulent magnetotail plasma sheet and injection of energetic (∼100eV to ∼100keV) electrons and protons. Injection of energetic protons from the plasma sheet causes electromagnetic ion cyclotron waves that lead to pitch angle scattering of the outer radiation belt MeV electrons and loss to the atmosphere during the SSSs. The SSS events are found to be associated with interplanetary magnetic clouds characterized by slowly varying interplanetary magnetic fields. An overall increase in the turbulence is recorded from the solar wind to the inner magnetosphere-ionosphere system during the SSSs. From the wavelet and cross-wavelet analyses, the inner magnetosphere and ionosphere are found to respond quasi-periodically at ∼1.5-4hours, and sporadically at shorter timescales of ∼0.5-1.5hours.

Nanotechnology connecting copper metabolism and tumor therapy

AbstractCopper (Cu) is an essential trace element in the human body that is involved in the formation of several natural enzymes, such as superoxide dismutase and cyclooxygenase. Due to the high density of the outer electron cloud of Cu, which allows the transfer of multiple electrons, Cu is often used as the catalytic center in various metabolic enzymes. However, both deficiency and excessive accumulation of Cu can result in irreversible damage to cells. Therefore, strategies to regulate Cu metabolism, such as Cu exhaustion and Cu supplementation, have emerged as attractive approaches in anticancer therapy, due to the potential damages caused by Cu metabolism disorders. Notably, recent advancements in nanotechnology have enabled the development of nanomaterials that can regulate Cu metabolism, making this therapy applicable in vivo. In this review, we provide a systematic discussion of the physical and chemical properties of Cu and summarize the applications of nanotechnology in Cu metabolism‐based antitumor therapy. Finally, we outline the future directions and challenges of nano‐Cu therapy, emphasizing the scientific problems and technical bottlenecks that need to be addressed for successful clinical translation.

Stability of the state of two-electron atoms near to critical nuclear charge

Critical stability of the metastable bound state of two-electron atoms near the critical nuclear charge Z c (smaller than which the bound state ceases to exist) is investigated by employing the explicitly correlated Hylleraas-configuration interaction basis function. Our numerical calculation shows that, when the nuclear charge decreases to Z c , the ionization energy of the system approaches zero linearly and this behavior is fully consistent with the Hellmann–Feynman theorem. Radial and angular physical quantities as well as the inner and outer electron radial density distributions are calculated to reveal the classical geometric structure of the two-electron atom. The inner electron can be well modeled by a hydrogenic model in the 2 p state and its density distribution does not change much as the nuclear charge is changed. The outer electron, which is still localized in a finite region near the nucleus before the system transforms into a well-defined resonance, is shifted notably into a far-distance region. The critical stability of the system near the bound-continuum limit is therefore dominated by the outer electron.

Testing the key processes that accelerate outer radiation belt relativistic electrons during geomagnetic storms

Since the discovery of the Earth’s radiation belts in 1958, it has always been a challenge to determine the dominant physical mechanisms, whether local acceleration by chorus or inward radial diffusion, that leads to outer radiation belt relativistic electron flux enhancements. In this study, we test a chain of processes with several potential successive steps that is believed to accelerate outer belt relativistic electrons. By performing correlation analysis of different part of this chain, including the geomagnetic condition, evolution of source and seed electron fluxes, chorus wave activity, and maximum fluxes (jmax) of relativistic electrons, we aim to identify the critical steps that lead to acceleration of MeV electrons. Based on 5-years of Van Allen Probes observations, our results confirm the repeatable response of both source and seed electrons to the storms, showing a significant flux enhancement during the main phase of storms, followed by either a gradual decay or flux persistence at a stable level. However, it is the intense and prolonged occurrence of substorms that contributes to the long-lasting existence of both source and seed electrons, which is also strongly associated with the jmax of relativistic electrons. The significant correlation (Correlation Coefficient, CC∼0.8) between the seed electron fluxes and jmax reveal that the prolonged and pronounced seed electrons are the prerequisite for the significant flux enhancement of relativistic electrons regardless of the acceleration mechanism. The slightly smaller CC (∼0.5–0.7) between source electron fluxes and jmax of relativistic electrons indicates that while local acceleration by chorus wave plays an important role to accelerate relativistic electrons to jmax, other mechanisms such as inward radial diffusion are still needed in this process. The CC between the source electrons and the chorus wave amplitude increases with increasing levels of substorms, showing (CC)max of ∼0.8, which further supports the crucial role of chorus waves in accelerating the relativistic electrons during intense substroms.

Redox Reactions of Ag(I)/Ag and Ferrocenium/Ferrocene in 1-Butyl-1-methylpyrrolidinium Bis(fluorosulfonyl)amide Ionic Liquid

The electrode reactions of Ag(I)/Ag and ferrocenium/ferrocene (Fc+/Fc) were investigated in an ionic liquid, 1-butyl-1-methylpyrrolidinium bis(fluorosulfonyl)amide (BMPFSA). The potential of Ag(I)/Ag depended on the logarithm of the concentration of Ag(I), as predicted by the Nernst equation, indicating the Ag(I)/Ag can be used as a reference electrode reaction in BMPFSA. The reversible electrode reaction of Fc+/Fc was observed in BMPFSA by cyclic voltammetry. The donor number of BMPFSA was estimated to be 13 from the difference in the formal potentials of Ag(I)/Ag and Fc+/Fc, indicating the coordination ability of FSA– was slightly stronger than that of bis(trifluoromethylsulfonyl)amide (TFSA–). The diffusion coefficients (D) of Fc and Fc+ were (5.7 ± 0.7) and (3.3 ± 0.2) × 10–7 cm2 s–1, respectively. The ratio of D of Fc+ against that of Fc was smaller than those in TFSA–-type ionic liquids, reflecting the higher charge density of FSA–. The standard rate constant (k 0) of Fc+/Fc was estimated to be (5.4 ± 1.1) × 10–3 cm s–1. The apparent activation energy for k 0 was close to the activation energy for D, suggesting the electrode reaction of Fc+/Fc can be regarded as the outer sphere electron transfer reaction with a very small reorganization energy.

Opening the Black Box of the Radiation Belt Machine Learning Model

AbstractMany Machine Learning (ML) systems, especially deep neural networks, are fundamentally regarded as black boxes since it is difficult to fully grasp how they function once they have been trained. Here, we tackle the issue of the interpretability of a high‐accuracy ML model created to model the flux of Earth's radiation belt electrons. The Outer RadIation belt Electron Neural net (ORIENT) model uses only solar wind conditions and geomagnetic indices as input features. Using the Deep SHAPley additive explanations (DeepSHAP) method, for the first time, we show that the “black box” ORIENT model can be successfully explained. Two significant electron flux enhancement events observed by Van Allen Probes during the storm interval of 17–18 March 2013 and non‐storm interval of 19–20 September 2013 are investigated using the DeepSHAP method. The results show that the feature importance calculated from the purely data‐driven ORIENT model identifies physically meaningful behavior consistent with current physical understanding. This work not only demonstrates that the physics of the radiation belt was captured in the training of our previous model, but that this method can also be applied generally to other similar models to better explain the results and to potentially discover new physical mechanisms.

Ultra-narrow-band luminescence with spectral width down to 32 nm in bismuth-activated phosphate phosphors with superior thermal stability

Bismuth-activated phosphors have become popular as a replacement for phosphors based on intensively studied lanthanide ions because of their low spectral reabsorption and sustainable utilization. However, it is of a great challenge to yield narrow-band emission from Bi3+ since its ns2-nsp electron transition lacks the shielding of outer electron orbitals. Herein, we report a new ultra-narrow band Bi3+-activated blue-violet phosphor Ca6BaP4O17 (Ca6BaP4O17:Bi3+). Impressively, its emission band width is down to ∼32 nm, which is less than that of all the reported Bi3+-activated ones. Experimental and theoretical calculation results corroborate that such unusual narrow-band emission originates from Bi3+ ions occupying two cation sites, Ca1 and Ca2, in the Ca6BaP4O17 compound. At 423 K, Ca6BaP4O17:0.5%Bi3+ shows a robust thermal quenching resistance with an integrated emission intensity loss of merely 1%. This is attributable to rare structural advantages of the host such as a high Debye temperature (Θ = 632 K) and large bandgap (∼6.1 eV), which could minimize thermally non-radiative relaxation pathways. We fabricate a blue-violet LED prototype by using the resulting phosphors and ultraviolet LED chip, and demonstrate its potential application in porrets growth. These findings provide in-depth insights the on site occupancy of Bi3+ ions and may simulate more research studies on Bi3+-activated phosphors.

Freestanding Mo3N2 nanotubes for long‐term stabilized 2e− intermediate‐based high energy efficiency Li–CO2 batteries

AbstractLi–CO2 batteries are considered one of the promising power sources owing to ultrahigh energy density and carbon fixation. Nevertheless, the sluggish reaction kinetics of 4e− discharged process (Li2CO3) impede its potential application. One of the efficient strategies for developing cathode catalysts is to stabilize 2e− intermediate Li2C2O4 and improve reaction reversibility. However, long‐term catalysts of stabilized Li2C2O4 are barely achieved, whereas cycle stability is far from satisfactory level. Herein, non‐noble metal–based Mo3N2 is synthesized and employed as freestanding cathodes for Li–CO2 batteries. Owing to rich delocalized electrons of Mo2+ and reversible electron localization structure, freestanding Mo3N2 cathodes exhibit a low charge potential (3.28 V) with an ultralow potential gap (0.64 V), high energy efficiency of up to 80.46%, fast rate capability, and outstanding cycle stability (&gt;910 h). In situ experiments and theoretical calculation verify that Mo3N2 stabilizes 2e− Li2C2O4 intermediate by the interaction of Mo2+ as active sites where Mo2+ promotes the transfer of outer electrons to O, prevents its disproportionation to Li2CO3, and promotes reaction kinetics, contributing to high energy efficiency and outstanding cycle reversibility. In addition, the pouch‐cells deliver ultrahigh energy density of up to 6350.7 W h kg−1 based on the mass of cathode materials.

Electrocatalytic Aldehyde Oxidation by a Tungsten Dependent Aldehyde Oxidoreductase from Aromatoleum Aromaticum.

In contrast to their molybdenum dependent relatives, tungsten enzymes operate at significantly lower redox potentials, and in some cases they can carry out reversible redox transformations of their substrates and products. Still, the electrochemical properties of W enzymes have received much less attention than their Mo relatives. Herein we analyse the tungsten enzyme aldehyde oxidoreductase (AOR) from the mesophilic bacterium Aromatoleum aromaticum which has been immobilised on a glassy carbon working electrode. This generates a functional system that electrochemically oxidises a wide variety of aromatic and aliphatic aldehydes in the presence of the electron transfer mediators benzyl viologen, methylene blue or dichlorophenol indophenol. Simulation of the cyclic voltammetry has enabled a thorough kinetic analysis of the system, which reveals that methylene blue acts as a two-electron acceptor. In contrast, the other two mediators act as single electron oxidants. The different electrochemical driving forces imparted by these mediators also lead to significantly different outer sphere electron transfer rates with AOR. This work shows that electrocatalytic aldehyde oxidation can be achieved at a low applied electrochemical potential leading to an extremely energy efficient catalytic process.

Specifying High Altitude Electrons Using Low‐Altitude LEO Systems: Updates to the SHELLS Model

Specifying High Altitude Electrons Using Low‐Altitude LEO Systems: Updates to the SHELLS Model

Analysis of Experimental Results of Defect Concentraions in the Surface Layer of Acceptor Doped Ceria

Recent experimental results of the dependence on oxygen partial pressure, P(O2), of the small polarons concentration in the monomolecular surface layer of doped ceria, are interpreted theoretically. Two different, experimental conditions are examined. First, the highly reduced surface of Sm0.2Ce0.8O1.9 – δ (SDC) is treated and the [Ce3+]s--P(O2) relations are evaluated. The experimental data are explained by a metallic surface layer, approximately neutral, i.e. with negligible transfer of charge to the bulk. The surface is metallic as the outer electrons on Ce3+ ions, which are small polarons when under low concentration, lose their localization under high concentration. Second, the surface of Pr0.1Ce0.9O2 – δ (PCO) is examined. The experimental data of [Pr3+]s--P(O2) relations are explained by a semiconducting, negatively charged, monomolecular surface layer forming a double layer with a positive bulk. In both SDC and PCO the surface defect concentration dependence on P(O2) is different from that of the bulk.

Evidence of H+‐Band EMIC Waves in the Inner Radiation Belt Observed by Van Allen Probes During Magnetic Storms

AbstractElectromagnetic ion cyclotron (EMIC) waves play an important role in the losses of ring current ions and outer radiation belt relativistic electrons. While EMIC waves have been reported to occur frequently in the outer belt and slot region, there is little information about the occurrence of EMIC waves in the inner radiation belt. On basis of the magnetometer observations from Van Allen Probes, we analyze two representative events of H+‐band EMIC waves that occurred in the inner zone on December 21 and June 22, 2015. We find that these waves occurred synchronously accompanying with 5–35 keV proton dispersive injections at the dawn and night sectors. During the periods of EMIC waves, the perpendicular temperature of injected protons increased, while the corresponding parallel component decreased. By analyzing the relation between the temperature anisotropy and parallel plasma beta for the two cases, we suggest that the evident enhancements of the temperature anisotropy (T⊥/T∥) of 5–35 keV protons may provide the energy source to excite the H+‐band EMIC waves. A total of 16 left‐hand H+‐band EMIC wave events have been found in the inner radiation belt during the 4‐year (2013–2016) interval and all occur during magnetic storms. The statistical results implicated that left‐hand H+‐band EMIC waves in the inner radiation belt is associated with storm‐time ion injections.

Through the Self-Optimization process to achieve high OER activity of SAC catalysts within the framework of TMO3@G and TMO4@G: A High-Throughput theoretical study

High-throughput DFT calculations are performed to explore the oxygen evolution reaction (OER) catalytic activity of a series of 2D graphene-based systems with TMO3 or TMO4 functional units. By screening the 3d/4d/5d transition metal (TM) atoms, a total of twelve TMO3@G or TMO4@G systems had extremely low overpotential of 0.33 ∼ 0.59 V, in which the V/Nb/Ta atom in VB group and Ru/Co/Rh/Ir atom in VIII group served as the active sites. The mechanism analysis reveals that the filling of outer electrons of TM atom can play an important role in determining the overpotential value by affecting the ΔGO* value as an effective descriptor. Especially, in addition to the general situation of OER on the clean surface of the systems containing the Rh/Ir metal centers, the self-optimization process of TM-sites was carried out, and it made most of these single-atom catalysts (SAC) systems to have high OER catalytic activity. All these fascinating findings can contribute to an in-depth understanding of the OER catalytic activity and mechanism of the excellent graphene-based SAC systems. This work will facilitate the design and implementation of non-precious and highly efficient OER catalysts in the near future.