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The participation of oxygen or other anionic species in redox activities in cathode materials for lithium and sodium-ion battery systems is known to play a role in governing the useful capacity of these batteries. Directly probing anionic redox mechanisms is not possible, rather the computational analysis by density functional theory poses the main approach towards gleaning insights into anionic redox activity and harnessing these effects to maximize capacity in future electrode materials. Here we showcase material systems exhibiting this mechanism of ion insertion and removal, and present the key computational considerations in studying anionic redox activities in battery materials. Aided by new computationally derived understandings of the role of anionic redox in emerging battery materials, increasingly greater levels of useable capacities can be extracted through informed materials design.

Using density functional calculations, we examine insertion/extraction of Mg ions in Mg3Bi2, an interesting Mg-ion battery anode. We found that a (1 1 0) facet is the most stable termination. Vacating a Mg2+ ion from the octahedral site is more favourable for both surface and bulk regions of the material. However, the diffusion barriers among the tetrahedral sites are around 3 times smaller than those among octahedral sites. Consequently, during the magnesiation/demagnesiation process, Mg ions first vacate the octahedral sites and then diffuse through the tetrahedral sites. The spin-orbit interaction lowers Mg's vacancy formation energy but has a minor effect on diffusion barriers.

The scarcity of fossil fuels as carbon resources has motivated the steelmaking industry to search for new carbon sources such as end-of-life polymeric products. Using $\textit{ab initio}$ molecular dynamics simulation, we demonstrate that 41% of polycarbonate's carbon content is readily dissolved in molten iron's interface at $T$ = 1823 K which is comparable to graphite with $\sim$ 58% carbon content dissolution. More importantly, we demonstrate that polycarbonate's hydrogen content does not dissolve in molten iron but rather escape in gaseous form. Therefore, waste polycarbonate constitutes a feasible carbon source for steelmaking., Comment: 7 pages, 4 figures, 32 references

Using molecular dynamics simulation, we present a comprehensive study of the volatile thermal degradation of high-density polyethylene (HDPE) across a temperature range of 300 K to 1823 K. We find that degradation at temperatures higher than $\sim$ 1373 K generates significant quantities of reducing gases such as CH$_{\text{n}}$ and hydrogen molecules which are beneficial to the steelmaking industry. Our results provide a new understanding of HDPE's phase transformation from solid to gas that occurs during superheating at steelmaking's electric arc furnace environment offering a new method for eliminating end-of-life HDPE from landfill., Comment: 6 pages, 2 figures, 1 table, 28 references

Through comprehensive density functional calculations, we predict the stability of a rhenium-based ferrite

The significance of the spin-orbit interaction is very well known in compounds containing heavier elements such as the rare-earth Eu ion. Here, through density functional calculations, we investigated the effect of the spin-orbit interaction on the magnetic ground state of Eu doped magnetite ($\mathrm{Fe_3O_4:Eu_{Fe}}$). By examining all possible spin alignments between Eu and magnetite's Fe, we demonstrate that Eu, which is most stable when doped at the tetrahedral site, adapts a spin almost opposite the substituted Fe. Consequently, because of smaller spin cancellation between the cations on the tetrahedral site ($\mathrm{Fe_{Tet}}$ and $\mathrm{Eu_{Tet}}$) and the cations on the octahedral sites ($\mathrm{Fe_{Oct}}$), $\mathrm{Fe_3O_4:Eu_{Fe}}$ exhibits a maximum saturation magnetisation of 9.451 $\mu_B/$f.u. which is significantly larger than that of undoped magnetite (calculated to be 3.929 $\mu_B/$f.u.). We further show that this large magnetisation persists through additional electron doping. However, additional hole doping, which may unintentionally occur in Fe deficient magnetite, can reduce the magnetisation to values smaller than that of the undoped magnetite. The results presented here can aid in designing highly efficient magnetically recoverable catalysts for which both magnetite and rare earth dopants are common materials., Comment: 12 pages, 9 figures, 3 tables

Doping is considered to be the main method for improving the thermoelectric performance of layered sodium cobaltate (Na$_{1-x}$CoO$_2$). However, in the vast majority of past reports, the equilibrium location of the dopant in the Na$_{1-x}$CoO$_2$'s complex layered lattice has not been confidently identified. Consequently, a universal strategy for choosing a suitable dopant for enhancing Na$_{1-x}$CoO$_2$'s figure of merit is yet to be established. Here, by examining the formation energy of Gd and Yb dopants in Na$_{0.75}$CoO$_2$ and Na$_{0.50}$CoO$_2$, we demonstrate that in an oxygen poor environment, Gd and Yb dopants reside in the Na layer while in an oxygen rich environment these dopants replace a Co in CoO$_2$ layer. When at Na layer, Gd and Yb dopants reduce the carrier concentration via electron-hole recombination, simultaneously increasing the Seebeck coefficient ($S$) and reducing electric conductivity ($\sigma$). Na site doping, however, improves the thermoelectric power factor (PF) only in Na$_{0.50}$CoO$_2$. When replacing a Co, these dopants reduce $S$ and PF. The results demonstrate how thermoelectric performance critically depends on the synthesis environment that must be fine-tuned for achieving any thermoelectric enhancement., Comment: Six figures and Supplementary Materials

Environmental concerns are the chief drive for more innovative recycling techniques for end-of-life polymeric products. One attractive option is taking advantage of C and H content of polymeric waste in steelmaking industry. In this work, we examined the interaction of two high production polymers, i.e., polyurethane and polysulfide with molten iron using ab initio molecular dynamics simulation. We demonstrate that both polymers can be used as carburizers for molten iron. Additionally, we found that light weight H$_2$ and CH$_x$ molecules were released as by-products of the polymer-molten iron interaction. The outcomes of this study will have applications in the carburization of molten iron during ladle metallurgy and waste plastic injection in electric arc furnace., Comment: 4 figures and 1 table

The water gas shift reaction (WGS) is important and widely applied in the production of H2. Cu modified perovskites are promising catalysts for WGS reactions in hydrogen generation. However, the structure-dependent stability and reaction pathways of such materials remain unclear. Herein, we report catalytically active Cu modified SrTiO3 (nominally SrTi1-xCuxO3) prepared by a modified polymeric precursor method. Microstructural analysis revealed a partially segregated CuO phase in the as-prepared materials. Operando X-ray diffraction and absorption spectroscopy showed the reduction of CuO into a stable metallic phase under conditions of WGS reactions for all compositions. Among the characterized materials, the x = 0.20 composition showed the highest turnover frequency, lowest activation energy, and the highest WGS rate at 300C. According to density functional calculations, the formation of CuO is energetically less favorable compared with SrTiO3, explaining why the segregated CuO phase on the SrTiO3 surface is reduced to Cu during the catalytic reaction, while SrTiO3 remains. For x = 0.20, the size of the segregated CuO phase is optimum for facilitating the catalytic reaction. In contrast, a higher Cu content (x = 0.3) results in an aggregation of smaller CuO particles, resulting in fewer surface active sites and a net decrease in catalytic performance.

Microporous organosilicas assembled from polysilsesquioxane (POSS) building blocks are promising materials that are yet to be explored in-depth. Here, we investigate the processing and molecular structure of bispropylurea bridged POSS (POSS-urea), synthesised through the acidic condensation of 1,3-bis(3-(triethoxysilyl)propyl)urea (BTPU). Experimentally, we show that POSS-urea has excellent functionality for molecular recognition toward acetonitrile with an adsorption level of 74 mmol/g, which compares favourably to MOFs and zeolites, with applications in volatile organic compounds (VOC). The acetonitrile adsorption capacity was 132-fold higher relative to adsorption capacity for toluene, which shows the pores are highly selective towards acetonitrile adsorption due to their size and arrangement. Theoretically, our tight-binding density functional and molecular dynamics calculations demonstrated that this BTPU based POSS is microporous with an irregular placement of the pores. Structural studies confirm maximal pore sizes of around 1 nm, with POSS cages possessing an approximate edge length of approximately 3.16 Angstrom

High-performance thermoelectric oxides could offer a great energy solution for integrated and embedded applications in sensing and electronics industries. Oxides, however, often suffer from low Seebeck coefficient when compared with other classes of thermoelectric materials. In search of high-performance thermoelectric oxides, we present a comprehensive density functional investigation, based on GGA$+U$ formalism, surveying the 3d and 4d transition-metal-containing ferrites of the spinel structure. Consequently, we predict MnFe$_2$O$_4$ and RhFe$_2$O$_4$ have Seebeck coefficients of $\sim \pm 600$ $\mu$V K$^{-1}$ at near room temperature, achieved by light hole and electron doping. Furthermore, CrFe$_2$O$_4$ and MoFe$_2$O$_4$ have even higher ambient Seebeck coefficients at $\sim \pm 700$ $\mu$V K$^{-1}$. In the latter compounds, the Seebeck coefficient is approximately a flat function of temperature up to $\sim 700$ K, offering a tremendous operational convenience. Additionally, MoFe$_2$O$_4$ doped with $10^{19}$ holes/cm$^3$ has a calculated thermoelectric power factor of $689.81$ $\mu$W K$^{-2}$ m$^{-1}$ at $300$ K, and $455.67$ $\mu$W K$^{-2}$ m$^{-1}$ at $600$ K. The thermoelectric properties predicted here can bring these thermoelectric oxides to applications at lower temperatures traditionally fulfilled by more toxic and otherwise burdensome materials., Comment: 11 pages, 6 figures, 1 table, 2 supplementary files

The interplay between covalency and magnetism is non-trivial and can be harnessed for designing new functional magnetic materials. Based on a survey using density functional calculations, we show that $TM\unicode{x2013}O$ bond covalency can increase the total magnetic moment of spinel compounds of $TMFe_2O_4$ composition ($TM = V-Ni, Nb-Pd$) which are isomorphic to the much-researched magnetite. Accordingly, $PdFe_2O_4$ was found to exhibit the highest magnetic moment of 7.809 ${\mu}_B$ per formula unit which is approximately twice that of $Fe_3O_4$ with $T_c$ predicted to be well above ambient. We further propose a practical method for synthesising $PdFe_2O_4$., Comment: 5 pages, three figures. 2 tables

Molecular dynamics simulation is used to study vacancy cluster formation in $\beta$- and $\alpha$-$Si_3N_4$ with varying vacancy contents (0 - 25.6 at%). Vacancies are randomly created in supercells, which were subsequently heat-treated for 114 nanoseconds. The results show that both $\beta$ and $\alpha$ can tolerate vacancies up to 12.8 at% and form clusters, confirming previous experimental data indicating 8 at% vacancy in $\alpha$-$Si_3N_4$. However, 25.6 at% vacancy in $\beta$ results in complete amorphization, while the same amount in $\alpha$ results in a transformation of a semi-amorphous $\alpha$ phase to a defective $\beta$ phase, leading to the removal of the clusters in newly formed $\beta$. This clearly explains why cluster vacancies are not experimentally observed in $\beta$, considering that $\beta$-$Si_3N_4$ ceramics are produced from $\alpha$. Furthermore, the lattice parameters of both modifications increase with increasing vacancy content, revealing the cause of different lattice constants that were previously reported for $\alpha$-$Si_3N_4$., Comment: 6 pages, 4 figures, 1 table, journal article

Designing high potential cathodes for Na-ion batteries, which are comparable in performance to Li-ion cathodes, remains a challenging task. Through comprehensive density functional calculations, we disentangle the relationship between the cathode potential and the ionicity of $TM-O$ bonds in $O3$ $NaTMO_2$ compounds in which TM ions is a fourth- or fifth-row transition metal. We demonstrate that the magnetic exchange interaction and the local distortions in the coordination environment of TM ions play more significant roles in determining the cathode potential of the $TM^{3+}$ $\to$ $TM^{4+}+ e^-$ reaction than the ionicity of the $TM-O$ bonds in these compounds. These results indicate that designing cathode materials solely based on empirical electronegativity values to achieve high potential may not be a feasible strategy without taking into account a detailed structural assessment., Comment: 5 pages, 3 figures, 1 table, journal article

Using density functional theory, we calculated the formation energy of native point defects (vacancies, interstitials and antisites) in MAX phase $Ti_2GeC$ and $Ti_3GeC_2$ compounds. Ge vacancy with formation energy of 2.87 eV was the most stable defect in $Ti_2GeC$ while C vacancy with formation energy of 2.47 eV was the most stable defect in $Ti_3GeC_2$. Ge vacancies, in particular, were found to be strong phonon scattering centres that reduce the lattice contribution to thermal conductivity in $Ti_2GeC$. In both compounds, the reported high thermal and electrical conductivity is attributed to the electronic contribution that originates from the high density of states at the Fermi level., Comment: 7 Pages, 4 Figures, 2 Tables

We have systematically investigated the $Na_xCoO_2$ system doped with Cu, Y, Sn, W, Au and Bi for $x$ = 0:5; 0:75 and 1.00 using density functional theory. Sn, W, and Bi always substitute a Co while Au always substitutes a Na regardless of Na concentration. However, for Cu and Y, the substitution site depends on Na concentration. When compared to the available experimental data, we find that thermoelectric performance is enhanced when the dopants substitute a Na site. In this case, surprisingly, resistivity decreases despite the reduced hole concentration caused by carrier recombination. We propose improved carrier mobility to be the cause of observed reduced resistivity., Comment: 5 Pages, 4 Figures

$Na_xCoO_2$ that comprises of alternating Na and $CoO_2$ layers has exotic magnetic and thermoelectric properties that could favorably be manipulated by adding dopants or varying Na concentration. In this work, we investigated the structural and electronic properties of Sr and Sb doped $Na_xCoO_2$ ($x$ = 0.50; 0.625; 0.75 and 0.875) through comprehensive density functional calculations. We found that Sr dopants always occupy a site in the Na layer while Sb dopants always substitute a Co ion in the host lattice regardless of Na concentration. This conclusion withstood when either generalized gradient approximation (GGA) or GGA+$U$ method was used. By residing on the Na layer, Sr dopants create charge and mass inertia against the liquid like Na layer, therefore, improving the crystallinity and decreasing the electrical resistivity through better carrier mobility. On the other hand, by substituting Co ions, Sb dopants reduce the electrical conductivity and therefore decrease the Seebeck coefficient., Comment: 9 pages, 6 figures

We theoretically investigated the electronic, electrochemical and magnetic properties of Sb doped $Na_xCoO_2$ ($x = 1, 0.75$ and $0.50$). $Sb_{Co}$ dopants adopt +5 oxidation state in $Na_xCoO_2$ host lattice for all Na concentrations ($x$). Due to high oxidation states, $Sb^{5+}$ strongly repels Na ions and therefore it decreases the electrochemical potential (vs. Na/Na$^+$). The electrons introduced by $Sb^{5+}$ localize on nearby Co ions creating $Co^{2+}$ species which are absent in undoped $Na_xCoO_2$. $Co^{2+}$ ions reduce the spin entropy flow decreasing the Seebeck coefficient in the Sb doped compounds. The results can be generalized to other dopants with high oxidation state., Comment: 6 pages, 3 figures, 2 tables

The hydrophobicity of CeO2 surfaces is examined here. Since wettability measurements are extremely sensitive to experimental conditions, we propose a general approach to obtain contact angles between water and ceria surfaces of specified orientations based on density functional calculations. In particular, we analysed the low index surfaces of this oxide to establish their interactions with water. According to our calculations, the CeO2 (111) surface was the most hydrophobic with a contact angle of {\Theta} = 112.53{\deg} followed by (100) with {\Theta} = 93.91{\deg}. The CeO2 (110) surface was, on the other hand, mildly hydrophilic with {\Theta} = 64.09{\deg}. By combining our calculations with an atomistic thermodynamic approach, we found that the O terminated (100) surface was unstable unless fully covered by molecularly adsorbed water. We also identified a strong attractive interaction between the hydrogen atoms in water molecules and surface oxygen, which gives rise to the hydrophilic behaviour of (110) surfaces. Interestingly, the adsorption of water molecules on the lower-energy (111) surface stabilises oxygen vacancies, which are expected to enhance the catalytic activity of this plane. The findings here shed light on the origin of the intrinsic wettability of rare earth oxides in general and CeO2 surfaces in particular and also explain why CeO2 (100) surface properties are so critically dependant on applied synthesis methods.

Copper dopants are varyingly reported to enhance photocatalytic activity at titanium dioxide surfaces through uncertain mechanisms. In order to interpret how copper doping might alter the performance of titanium dioxide photocatalysts in aqueous media we applied density functional theory methods to simulate surface units of doped anatase (101) planes. By including van der Waals interactions, we consider the energetics of adsorbed water at anatase surfaces in pristine and copper doped systems. Simulation results indicate that copper dopant at anatase (101) surfaces is most stable in a 2+ oxidation state and a disperse configuration, suggesting the formation of secondary CuO phases is energetically unfavourable. In agreement with previous reports, water at the studied surface is predicted to exhibit molecular adsorption with this tendency slightly enhanced by copper. Results imply that the enhancement of photoactivity at anatase surfaces through Cu doping is more likely to arise from electronic interactions mediated by charge transfer and inter-bandgap states increasing photoexcitation and extending surface-hole lifetimes rather than through the increased density of adsorbed hydroxyl groups.

Through comprehensive density functional calculations, the crystallographic, magnetic and electronic properties of $Na_xCoO_2$ ($x$ = 1, 0.875, 0.75, 0.625 and 0.50) were investigated. We found that all Na ions in $NaCoO_2$ and $Na_{0.875}CoO_2$ share the basal coordinates with O ions. However, as $x$ decreases, some of Na ions move within the basal plane in order to reduce the in-plane Na$-$Na electrostatic repulsion. Magnetically, there was strong tendency for type A antiferromagnetism in the $Na_{0.75}CoO_2$ system, while all other Na deficient systems had a weaker ferromagnetic tendency. The results on magnetism were in excellent agreement with the experiments., Comment: 4 Pages, 3 Figures, 1 Supplementary PDF

In this work, we investigated the behaviour of Sb dopants in $Na_{x}CoO_{2}$ for Na concentrations of $x = 0.75, 0.875$ and $1.00$ by density functional theory. We chose $Na_{x}CoO_{2}$ with higher Na concentration of $x > 0.75$ because it has excessively higher thermo-power thus it is appealing for practical applications. The rationale for choosing Sb was its exceedingly higher atomic mass than all elements of the host crystal which enable Sb to rattle phonons considerably., Comment: 1st Kansai Nano-scale and Nanotechnology International Symposium

We carried out density functional theory calculations to model the electronic structure and the magnetic interactions in copper doped anatase and rutile titanium dioxide in order to shed light on the potential of these systems as magnetic oxides using different density functional schemes. In both polymorphs copper dopant was found to be most stable in substitutional lattice positions. Ferromagnetism is predicted to be stable well above room temperature with long range interactions prevailing in the anatase phase while the rutile phase exhibits only short range superexchange interaction among nearest neighbouring Cu ions. Additionally, energetic evaluation of dopants in scattered and compact configurations reveals a dopant clustering tendency in anatase TiO2., Comment: 13 pages, five figures, one table

$Na_xCoO_2$ has a fascinating and complex magnetic phase diagram that can be further manipulated by doping. Here, we investigated the effect of electron doping on the magnetic coupling among $Co^{4+}$ ions in $Na_xCoO_2$ using density functional theory based on Hartree-Fock hybrid functional. We found that electron doping through substitutional $Sb_{Co}$ dopants flip the in-plane ferromagnetic coupling among $Co^{4+}$ ions in the undoped compound to antiferromagnetic. Electron doping through interstitial $Cu_{Int}$, however, does not have a similar effect as $Cu_{Int}$ dopant leaves the compound ferromagnetic, just like the case of the undoped compound. The results demonstrate the critical dependence of magnetic phase in $Na_{0.5}CoO_2$ on the dopant and its incorporation site., Comment: 5 pages, 5 figures

Engineered $Na_{0.75}CoO_2$ is considered a prime candidate to achieve high-efficiency thermoelectric systems to regenerate electricity from waste heat. In this work, three elements with outmost electronic configurations, (1) an open d shell (Ni), (2) a closed d shell (Zn), and (3) a half filled f shell (Eu) with maximum unpaired electrons, were selected to outline the dopants' effects on electronic and crystallographic structures of $Na_{0.75}CoO_2$. Systematic $ab$ $initio$ density functional calculations with $DMOL^3$ package showed that the Ni and Zn were more stable when substituting Co with formation energy $-2.35$ eV, $2.08$ eV when Fermi level equals to the valence band maximum. While Eu is more stable when it substitutes Na having formation energy of $-2.64$ eV. As these results show great harmony with existing experimental data, they provide new insights into the fundamental principle of dopant selection for manipulating the physical properties in the development of high-performance sodium cobaltate based multifunctional materials., Comment: 8 pages, 9 figures

We report room temperature ferromagnetism in as-implanted Eu doped ZnO (ZnO:Eu). To address the origin of ferromagnetism ab initio calculations of ZnO:Eu system are performed. Results show that the ferromagnetism is induced by ZnO point defects as Eu ions in perfect ZnO tend to align antiferromagnetically., Comment: 4 pages,2 figurs, one table