Theoretical searches for innovative materials for energy harvesting and storage

Resume : Understanding the structure of disordered materials is a prerequisite for further knowledge of their physico-chemical properties. A desirable target is to bridge the gap between fundamental science and potentially useful applications. In this talks , we shall present two examples of studies of ternary glasses prone to technological applications and yet very much interesting from the fundamental point of view because of their controversial structure. Our approach is based on a quantitative prediction of material properties obtained via first-principles molecular dynamics combined to a careful choice of the relevant parts of the theoretical framework (exchange-correlation functionals, inclusion of dispersion forces). The first example deals with the atomic stucture of glassy Ge2Sb2Te5, one of the most established and successful phase change materials. In the second example, devoted to Ga10Ge15Te75, we considered a system studied experimentally because of its exceptionally large infrared transparency window.

Resume : Metal halide perovskite solar cells have rapidly emerged as leading contenders in photovoltaic technology. Compositions with a mixture of cation species on the A-site show the best performance and have higher stability. However, the underlying fundamentals of such enhancement are not fully understood. Here, we report new atomic-scale insights into the local structures and dynamics of mixed A-cation compositions based on formamidimium lead iodide (CH(NH2)2PbI3 ) doped with Cs+, Rb+ and MA+. Our specific findings include the first indication that substitution of low concentrations of smaller cations on the A-site in CH(NH2)2PbI3 results in a global ‘locking’ of the PbI6 octahedra tilting and reduced lattice dynamics. A key impact of this feature is that the rotational or tumbling motion of the CH(NH2)2+ molecular ion in a locked cage is severely restricted. We discuss the impact of locking on the photovoltaic performance and stability. The results presented here provide key fundamental insights into the origin of the significant enhancements in solar cell performance achieved by using mixed A-site species in lead halide perovskites and will have wide impact on future design strategies. Reference : Ghosh et al. ACS Energy Lett. 2017, 2, 2424

Resume : High performance p-type transparent conducting materials (TCMs) must exhibit a rare combination of different properties including high mobility, transparency and p-type dopability. The finding of high-mobility p-type TCMs is very essential for many applications such as solar cells, transparent electronic devices,... Up to now, oxides have been conventionally considered as a promising chemical space to dig out novel p-type TCMs. However, many other non-oxide compounds also show very low hole effective mass and might perform better than traditional p-type TCMs (oxides). In this work, we carried out a high throughput (HT) computational search for a large dataset of more than 30.000 compounds. We show that non-oxides present large opportunities for high performance TCMs and identify BP, CaTe and Li3Sb as very good p-type TCMs candidates. These materials are indirect semiconductors with band gaps smaller than 3.0 (eV) but show high transparency because of indirect and therefore weak optical transitions. Their p-type doping tendencies are verified through defect calculations. Our approach goes beyond the usual analysis based on effective masses and band gap as we also estimated directly carrier mobilities through the computation of electron-phonon coupling and Boltzmann transport theory and obtained very competitive mobilities in comparison with current n-type TCMs.

Resume : This work is focused on understanding the role that ferroelectric domains in MAPbI3 have on perovskite solar cells performance. We study 2D and 3D systems considering different polarization domains patterns, by proposing a polarization model based on the knowledge of the crystalline structure, symmetry considerations and electrical simulations. We compute charge carrier transport by solving a drift-diffusion model, in which the Poisson equation for potential calculation includes the polarization model. We show that the presence of polarization domains has a strong impact on charge separation, thus leading to a decrease of recombination losses and current pathways formation at domains interfaces. In detail, the decrease of Shockley-Read Hall recombination losses improves the open-circuit voltage, while the high current pathways formation lead to an increase of the short-circuit current. The achieved results demonstrate that the presence of ordered ferroelectric domains, even with weak polarization magnitude, can actually affect the performance of the solar cell in terms of power conversion enhancement. Moreover, from the comparison between our results and experimental IV characteristics of MAPbI3(Cl) devices we conclude that the polarization model proposed can effectively reproduce the solar cell operation.

Resume : We have performed a systematic study of hybrid perovskite systems using first principles molecular dynamics on supercell models containing 64 units. Our research includes different cations (MA, FA and Cs), different halide atoms (I, Br and Cl) and different thermodynamics conditions. The picture that emerges is one with several common features across different systems. The high temperature phase displays some degree of anharmonicity in the kinetics of the halide which depends on the structure of the cation, while FAPbI3 behaves close to harmonic at 450 K, the MAPbI3 already show anharmonic modes at this high temperature. Reducing the temperature to 370 K enhances the anharmonic character for all studied systems. For the case of the MA cation, the molecular electric dipole plays an important role that significantly affects simulations performed in 444 supercells. Over correlation of the dipolar interactions across the periodic boundary conditions drives the system to a phase change from cubic to a phase with high dipolar coupling. At the low temperature orthorhombic phases, MAPbX3 (X=I, Br and Cl) behaves as a standard crystal experiencing atomic vibrations but no molecular rotations. As the system is heated above the tetragonal transition temperature the simulations show the activation of molecular degrees of freedom. As a consequence, the static hydrogen bonds of the orthorhombic phase became dynamics and short lived in the tetragonal phase.

Resume : In the past five years, solution-processed organometallic perovskite based solar cells have emerged as a promising thin-film photovoltaic technology. Multilayered 2D Ruddlesden-Popper phases, composed of perovskites layers sandwiched between two layers of large organic cations, have recently demonstrated improved photostability under standard illumination as well as humidity resistance over 2000 hours, affording a conversion efficiency of 12.5 %.(1) For multilayered halide perovskites, intrinsic quantum and dielectric carrier confinements are afforded by the organic inner barriers, which leads to a stable Wannier exciton at room temperature.(2) However, solar cell and LED efficiencies are essentially related to extremely efficient internal exciton dissociation through edge states in layered 2D Ruddlesden-Popper perovskites, as shown from the investigation of both thin films and small single crystals.(3) These materials can be described as unconventional semiconductors. The spin-orbit coupling is giant and shows up in the conduction band, the band gap is direct with the critical wavevector located at one of the edges of the reference Brillouin zone, and among others, excitonic or Rashba-Dresselhaus effects play a crucial role.(4) The electronic band structure can be modeled using either Density Functional Theory calculations. However multilayered halide perovskites are very large systems beyond the possibilities of state of the art DFT codes to include many-body corrections or excitonic effects.(5) Empirical methods based on the effective mass or multiband k.p Hamiltonian are still very useful, provided they are properly combined with DFT results to account for the dielectric and quantum confinements.(6) References 1 H. Tsai et al, Nature 2016; C. M. M. Soe et al, Adv. Ener. Mat. 2 G. Lanty et al, J. Phys. Chem. Lett. 2014; M. Smith et al, Chem. Sci. 2017 ; J. C. Blancon et al, arXiv:1710.07653 3 J. C. Blancon et al, Science 2017, H. Tsai et al, Adv. Mat. 2017 4 J. Even et al, Phys. Rev. B 2012; J. Phys. Chem. C 2015; M. Kepenekian et al, ACS Nano 2015 5 C. Stoumpos et al, Chem 2017 ; C. M. M. Soe et al, J. Am. Chem. Soc. 2017 ; L. Mao et al, J. Am. Chem. Soc. 2017 ; 6 J. Even et al, ChemPhysChem 2014; D. Sapori et al, Nanoscale 2016 ; L. Pedesseau et al, ACS Nano 2016

Resume : The BAC model coupled to k.p theory and Pikus-Bir theory was used to determine the electronic properties of GaNxAs1-x-yBiy strained highly electronegativity alloys. This investigation was performed at room temperature in terms of electronic band structure, energy levels and spin-orbit splitting energies and with a bismuth and nitrogen concentrations range varying from 0 to 12%. The incorporation of a small N or Bi content affects the strain types and induces a large variation in the properties of these semiconductors. As results, we note essentially that, for strained GaAsBi and GaAsN ternaries, a significant reduction of the bandgap energy E_g^c of about 54 meV/%Bi and 99 meV/%N were found, respectively. In addition, the spin-orbit splitting ∆_(0 )^cwas increased by 39meV/%Bi for GaAsBi and decreased by 5meV/%N for GaAsN. On the other hand, for typical concentrations (x = 4% ; y = 5%) of strained GaNxAs1-x-yBiy, the energies E_g^c, ∆_(so ) and the valence band splitting VBS are equal to 0.66 eV, 0.54 eV and 17 meV respectively. Finally, we applied these results in the investigation of GaAs/GaN.03As.90Bi.07/GaAs and GaAs/GaN.02As.93Bi.05/GaAs single quantum well structures under compressive stress emitting at 1.3 and 1.55 μm./ Key words : GaNAsBi/GaAs ; k.p theory ; BAC model; Strain structures ; 1.3 and 1.55 μm emissions

Resume : We investigate the excellent photovoltaic properties, such as suitable bandgap, high optical absorption and long carrier lifetime of organic-inorganic lead halide perovskites (mainly CH3NH3PbI3). To explain the large recombination time we propose the hypothesis that the formation of ferroelectric domains can separate the diffusion pathways of electrons and holes. We have found that a two-dimensional hole confinement in CH3NH3PbI3 is possible under room temperature conditions and is enhanced by the deformation of the inorganic PbI3 sublattice. Beside, to improve their photovoltaic performance, we explore the impact of point defects on their electronic structure through sub-bandgap absorption. We report here the electronic structure of new CH3NH3PbI3 perovskite derivatives, in which narrow band were obtained by replacing Pb atoms by Cr atoms. To deal with the bandgap underestimation problem of common DFT methods, quasiparticle calculations have been applied via the GW approximation. The investigation of the electronic structure of new CH3NH3PbI3 perovskites suggests that the presence of point defects play an important role in the coupling of two low energy photons to achieve a higher energy electron excitation, which would maximize the photovoltaic performance

Resume : A phenomenological model has been developed and is discussed. The model is able to describe the experimentally measured light transmission of nanowires arrays. A slab of nanospheres and rough layers with thickness waviness were combined to simplify the nanowires structure description. This phenomenological description was proven to be feasible by fitting experimental data. As a conclusion, light transmitted through randomly distributed nanowires can be explained by the combination of volume and surface scatterings using respectively Mie theory and rough Fresnel coefficients at the interfaces.

Resume : In this paper, we present three methods to extract physical parameters of single diode electronic circuit modeling photovoltaic solar module. The physical parameters are photocurrent Iph, saturation current Is, ideality factor η, series resistance Rs and shunt conductance Gp. The first method uses the exact solution of module characteristic equation, the second one utilizes the exact expression of dynamic conductance and the third one uses the exact expression of the integral of I-Is. In these methods, we perform fitting of experimental data of current, dynamic conductance and area under I-Is with appropriate analytical expressions to extract model physical parameters in standard test conditions (STC). We then consider these values as initial conditions to derive exact analytical expressions describing the effects of incident solar irradiance and temperature on model physical parameters using temperature coefficients available in module datasheet as well as irradiance coefficients extracted from current-voltage curves set. The analytical expressions derived for model physical parameters are tested on Shell SQ80 photovoltaic module under different conditions of irradiance and temperature and very good agreements between experimental and predictive analytical characteristics are then raised.

Resume : The energy density of many lithium metal oxide battery cathodes is limited by redox reactions associated with the transition metal alone. Anionic redox reactions are affording new opportunities to increase this energy density. Here, we present results from first-principles density functional theory (DFT) calculations aimed at understanding the combined cation/anion redox of Li-rich materials, and designing new materials that will enable high-capacity, reversible cycling with minimal oxygen evolution. We illustrate this approach on three Li-rich compounds: Li5FeO4, Li4Mn2O5, and Li2MnO3. Li5FeO4: Initial removal of Li is accompanied by Fe migration to form a disordered rocksalt structure. A local Li6-O coordination of oxygen, identified by DFT calculations, raises the O 2p band and enables reversible O-/O2- redox behavior, previously unknown in this material. This insight leads to the prediction and subsequent experimental demonstration of anionic and cationic redox reactions with good reversibility and without any obvious O2 gas release. Li4Mn2O5: We study the recently-reported, high-capacity, disordered rocksalt-type Li4Mn2O5 compound and also determine the ground state ordered structure of Li4Mn2O5 via a DFT-based enumeration method. DFT calculations show that the delithiation process occurs via a three-step reaction pathway involving the complex interplay of cation and anion redox reactions: i) Mn3+ ➝ Mn4+ (LixMn2O5, 4 > x > 2), ii) O2− ➝ O1− (2 > x > 1), and iii) Mn4+ ➝ Mn5+ (1 > x > 0) concomitant with Mn migration from the original octahedral site to the adjacent tetrahedral site. Finally, we predict that alloying with M = V and Cr in Li4(Mn,M)2O5 would produce new stable compounds with substantially improved electrochemical properties. Li2MO3: We catalog the family of Li2MO3 compounds as active cathodes or inactive stabilizing agents using high-throughput density functional theory (HT-DFT). With an exhaustive search based on design rules that include phase stability, cell potential, resistance to oxygen evolution, and metal migration, we predict a number of new Li2MIO3–Li2MIIO3 active/inactive electrode pairs, in which MI and MII are transition- or post-transition metal ions, that can be tested experimentally for high-energy-density lithium-ion batteries.

Resume : Today's commercial rechargeable batteries rely on Li-ion technology. Yet the expansion of the battery market towards electric vehicles and large-scale grid storage is likely to lead to a steep increase in the lithium price [Tarascon, 2010]. As a consequence, intense efforts have been devoted to find a suitable alternative. From a chemical perspective, sodium is positioned immediately below lithium in the periodic table and, therefore, it is its natural surrogate. Added to this, the high abundance, environment-friendly nature, and low cost of sodium have made research in Na-based batteries a topic of high interest in recent years [Slater, Kim et al., 2013]. From a Li-based viewpoint, LiFePO4 shows stand-out features such as great stability, remarkable rate capability, and sustained high voltage throughout the whole discharge cycle. In principle, one could expect NaFePO4 to inherit these properties from its isostructural lithium counterpart. Therefore, there has been a recent flurry of interest in NaFePO4 [Landa-Medrano, Li et al., 2016; Saracibar, Carrasco et al., 2016]. However, atomistic simulation of partially sodiated NaFePO4 has been hindered by (i) the lack of interatomic potentials for crystals having simultaneously Fe2+ and Fe3+ (both present in the partially sodiated structure) and (ii) the significant energy barriers associated with diffusion in this system, for which traditional molecular dynamics is not suitable. In this work we tackle both issues by developing a new, polarizable force-field for olivine NaxFePO4 (0 < x < 1) and combining it with an advanced atomistic sampling technique to simulate the diffusion of Na+ through the structure. In order to train the parameters of the proposed force field, we build a DFT-based dataset of configurations at several sodium concentrations. In this regard, our proposal goes beyond all previously suggested interatomic potentials for NaFePO4, which were specifically derived to simulate fully sodiated structures [Whiteside, Fisher et al., 2014]. We show that the new force field outperforms existing alternatives in terms of accuracy and thermal stability. Combining the new force field with a novel, in-house enhanced sampling methodology, the Randomized Shell Mass Generalized Shadow Hybrid Monte Carlo Method (RSM-GSHMC) [Akhmatskaya, Reich, 2008; Escribano, Akhmatskaya et al., 2015], we study with unprecedented level of detail the Na-ion diffusion in Na0.66FePO4. We confirm that the main diffusion mechanism involves single Na-ion hops through the one-dimensional channels along the [010] crystallographic direction. Furthermore, we identify the novel Na-ion diffusion dynamics involving the formation and annihilation of Na/Fe antisite defects, which effectively facilitate the migration of Na-ions between adjacent [010] channels. In principle, similar ion migration mechanisms could operate in other olivine framework compounds as well. The presented study provides an in-depth understanding of Na-ion mobility in olivine NaxFePO4, a promising alternative to the commercially available LiFePO4 cathode material for Na-ion batteries, and paves the way to unveil fundamental aspects of ion dynamics in polyanionic materials, in general. References Akhmatskaya, E.; Reich, S. GSHMC: An efficient method for molecular simulation. Journal of Computational Physics 2008, 227, 4934 – 4954. Escribano, B.; Lozano, A.; Radivojević, T.; Fernández-Pendás, M.; Carrasco, J.; Akhmatskaya, E. Enhancing sampling in atomistic simulations of solid-state materials for batteries: a focus on olivine NaFePO4. Theoretical Chemistry Accounts 2017, 136, 43. Landa-Medrano, I.; Li, C.; Ortiz-Vitoriano, N.; Ruiz de Larramendi, I.; Carrasco, J.; Rojo, T. Sodium–Oxygen Battery: Steps Toward Reality. The Journal of Physical Chemistry Letters 2016, 7, 1161–1166. Saracibar, A.; Carrasco, J.; Saurel, D.; Galceran, M.; Acebedo, B.; Anne, H.; Lep- oitevin, M.; Rojo, T.; Casas Cabanas, M. Investigation of sodium insertion-extraction in olivine NaxFePO4 (0 ≤ x ≤ 1) using first-principles calculations. Phys. Chem. Chem. Phys. 2016, 18, 13045–13051. Slater, M. D.; Kim, D.; Lee, E.; Johnson, C. S. Sodium-Ion Batteries. Advanced Functional Materials 2013, 23, 947–958. Tarascon, J. M. Is lithium the new gold? Nature Chemistry 2010, 2. Whiteside, A.; Fisher, C.; Parker, S.; Islam, M. S. Particel shapes and surface structures of olivine NaFePO4 in comparison to LiFePO4. Phys. Chem. Chem. Phys. 2014, 16, 21788–21794.

Resume : One of the most studied materials for catalysis, sensing and energy applications is the rutile-type RuO2, especially its (110) surface [1]. Structure, chemical composition, and properties of the surface depend on external conditions. Using the evolutionary prediction method (USPEX), we found stable reconstructions of the (110) surface [2]. Two stable reconstructions, RuO4–(2×1) and RuO2–(1×1), were found, and the surface phase diagram was determined. The new RuO4–(2×1) reconstruction is stable in a wide range of environmental conditions, its simulated STM image perfectly matches experimental data, is more thermodynamically stable than previously proposed reconstructions, and explains well pseudocapacitance of RuO2 cathodes. References (1) Over, H. Surface chemistry of ruthenium dioxide in heterogeneous catalysis and electrocatalysis: from fundamental to applied research. Chem. Rev. 112, 3356–3426 (2012). (2) Zakaryan. H. A.; Kvashnin A. G.; Oganov A. R. Stable reconstruction of the (110) surface and its role in pseudocapacitance of rutile-like RuO2. Sci. Rep., 7, 10357 (2017)

Resume : Cluster expansion (CE) [1,2] is an effective method for studying metal alloys and oxides where a thorough exploration of all possible atomic configurations is practically impossible due to the high computational costs of first-principles calculations. CE dramatically reduces the computational cost of the exploring the configuration space by mapping the first-principles calculation results on to a much faster Hamiltonian. Especially with the emerging new cathode materials for Li-ion batteries based on lithium transition metal oxyfluorides [3,4], CE is becoming a relevant method in exploring and evaluating energy materials. This family of new cathode materials also presents new challenges such as a large number of constituting element types (four and a vacancy) and structural deformation upon delithiation, to list a few. We have implemented a CE code as a part of a popular Atomic Simulation Environment (ASE) package [5]. The implemented code includes necessary tools to evaluate complex battery materials while taking a generalized approach to allow the CE modeling of materials in any space group. We demonstrate our approach and its features with the Li2VO2F cathode as an example. 1. J. M. Sanchez, F. Ducastelle, and D. Gratias, Phys. A, 128, 334–350 (1984). 2. J. M. Sanchez, Phys. Rev. B, 81, 224202 (2010). 3. R. Chen et al., Adv. Energy Mater., 5, 1–7 (2015). 4. R. Chen et al., RSC Adv., 6, 65112–65118 (2016). 5. A. H. Larson et al., J. Phys. Condens. Matter, 29, 273002 (2017).

Resume : Titanium doping in lithium manganese oxide spinels was shown to be beneficial for the structural stability of the potential Li-ion battery cathode materials LiTixMn2-xO4, 0.2 < x < 1.5, yet the distribution of Li/Ti/Mn in the structure and the cation oxidation states, both pivotal for the electrochemical performance of the material, are not fully understood. Our work investigates the changes in the local ordering of the ions throughout this series by using a combination of 7 Li NMR spectroscopy and ab initio density functional theory calculations. The 7 Li NMR shifts are first calculated for a variety of Li configurations with different numbers and arrangements of Mn ions in the first metal coordination shell and then decomposed into Li?O?Mn bond pathway contributions to the shift. These Li?O?Mn bond pathways are then used to simulate and assign the experimental NMR spectra of different configurations and stoichiometries beyond those in the initial subset of configurations via a random distribution model and a reverse Monte Carlo approach. This methodology enables a detailed understanding of the experimental 7Li NMR spectra, allowing the variations in the local ordering of the ions in the structure to be identified. A random distribution of Ti4+ ? Mn3+/4+ sites is found at low Ti content (x = 0.2); an inhomogeneous lattice of Mn4+ ? rich and Ti4+ ? rich domains is identified for x = 0.4, and single-phase solid solution is observed for x = 0.6 and 0.8. A mixed Li? Mn2+ tetrahedral and Li?Mn3+/4+ ?Ti octahedral configuration is determined for the x = 1.0 case. A specific cation ordering in the partially inverse LiTi1.5Mn0.5O4 case is found, which transforms into a two-phase network of disordered Mn3+ ? rich and ordered Mn 2+ ? rich domains for x = 1.1?1.4.

Resume : Two-layer freestanding heterostructure consisting of VS2 monolayer and graphene was investigated by means of density functional theory computations as a promising anode material for lithium-ion batteries (LIB). We have investigated lithium atoms’ sorption and diffusion on the surface and in the interface layer of VS2/graphene heterostructure with both H and T configurations of VS2 monolayer. The theoretically predicted capacity of VS2/graphene heterostructures is high (569 mAh/g), and the diffusion barriers are considerably lower for the heterostructures than for bulk VS2, so that they are comparable to barriers in graphitic LIB anodes (∼0.2 eV). Our results suggest that VS2/graphene heterostructures can be used as a promising anode material for lithium-ion batteries with high power density and fast charge/discharge rates.

Resume : Sodium Ion Batteries (SIBs) are the most cost-effective alternative to current generation lithium ion batteries (LIBs), [1-3] but Na is known to deliver very low energy capacity for sodium intercalation compared to Lithium. In order to improve the performance for SIB, we want to understand at the atomistic level how the metal ion changes its coordination to the solvent in positioning itself for insertion into graphite or adding at the metal surface [4,5]. Previous studies (force fields with fixed charges) ignored the coupling of electronic states with the changing charge and solvation. We report here Density Functional Theory molecular metadynamics simulations [6-8] to obtain the free energy landscape including changes in the electronic coupling as a sodium ion in Dimethyl sulfoxide solvent intercalates into graphite, the first step in understanding how the local environment affects the free energy of solvation [9]. We analyze the free energy landscape for all the possible sodium solvation scenarios, while quantifying their free energy barriers [10]. Our simulations indicate that solvent plays an important role in stabilizing the sodium intercalation into graphite through shielding of the sodium while modulating the interaction of the solvent with the graphite sheets. In order to facilitate this intercalation, we propose solvents with negatively charged groups and aromatic cores (e.g., cyclic ethers) that could allow a greater rate of anion exchange to increase Na mobility. References 1. Yabuuchi, N.; Kubota, K.; Dahbi, M.; Komaba, S. Research development on sodium-ion batteries. Chem. Rev. 2014, 114, 11636-11682. 2. Kubota, K.; Komaba, S. Review-Practical Issues and Future Perspective for Na-Ion Batteries. J. Electrochem. Soc. 2015, 162 (14), A2538-A2550. 3. Hwang, J. Y.; Myung, S. T.; Sun, Y. K., Sodium-Ion Batteries: Present and Future. Chem. Soc. Rev. 2017, 46, 3529-3614. 4. Kim, H.; Hong, J.; Yoon, G.; Kim, H.; Park, K. Y.; Park, M. S.; Yoon, W. S.; Kang, K. Sodium Intercalation Chemistry in Graphite. Energy Environ. Sci. 2015, 8, 2963−2969. 5. Jache, B.; Binder, J. O.; Abe, T.; Adelhelm, P. A Comparative Study on the Impact of Different Glymes and Their Derivatives as Electrolyte Solvents for Graphite Co-Intercalation Electrodes in Lithium-Ion and Sodium-Ion Batteries. Phys. Chem. Chem. Phys. 2016, 18, 14299-14316. 6. CP2K version 2.6.2 (Development Version), CP2K is freely available from http://www.cp2k.org 7. Laio, A.; Parrinello, M. Escaping free-energy minima. Proc. Natl. Acad. Sci. 2002, 99, 12562-12566. 8. Iannuzzi, M.; Laio, A.; M. Parrinello, M. Efficient Exploration of Reactive Potential Energy Surfaces Using Car-Parrinello Molecular Dynamics. Phys. Rev. Lett. 2003, 90, 238302. 9. Kachmar, A.; Carignano, M.; Laino, T.; Iannuzzi, M.; Hutter, J. Mapping the Free Energy of Lithium Solvation in the Protic Ionic Liquid Ethylammonuim Nitrate: A Metadynamics Study. ChemSusChem. 2017, 10, 3083-3090. 10. Kachmar, A.; Goddard III, W. A. The Role of Solvent for Sodium Intercalation into Graphite, submitted.

Resume : First, we will describe our recent investigation on the origin of high charge capacity of a composite of transition metal chalcogenides with graphite in lithium ion battery. This is done by crystal structure prediction, ab initio NPT simulation followed by structure optimization, and graphene intercalation. Second, we propose two-dimensional (2D) GeP2 in the tetragonal (T) phase never observed for other group IV-V compounds. Our HSE06 calculation shows that its few-layers are a semiconductors with band gaps in the visible region. Band offset with respect to the Fermi levels of appropriate half-reactions shows that its n-type few-layers can be useful in photocatalyzed CO2 splitting to CO as well as in photocatalyzed water splitting, specifically under acidic conditions. Other recent calculations on the energy applications of 2D materials will be also discussed.

Resume : The search for Si-based anodes capable of undergoing low volume changes during electrochemical operation in rechargeable batteries is ample and active. Here we focus on crystalline Si24 [1], a recently discovered open-cage allotrope of silicon, to thoroughly investigate its electrochemical performance using density functional theory calculations. In particular, we examine the phase stability of NaxSi24 along the whole composition range (0 ? x ? 4), volume and voltage changes during the (de)sodiation process, and sodium ion mobility. We show that NaxSi24 forms a solid solution with minimal volume changes. Yet sodium diffusion is predicted to be insufficiently fast for facile kinetics of Na-ion intake. Considering these advantages and limitations, we discuss the potential usefulness of Si24 as anode material for Na-ion batteries. [1] Arrieta et. al., Scientific Reports, 7, 5350 (2017)

Resume : The Li-ion battery (LIB) technology is expected to play an important role in the future of electrical vehicles for which sufficiently high energy storage capabilities are needed. Research on alternative electrode materials is essential to improve the performance of LIBs, but it is not possible to explore the large available design space only experimentally. Our goal is therefore to simplify the search with the help of computer aided design tools. Ab-initio calculations have become a well-established approach to study the structural properties of electrode materials in LIBs. Here, we will present results for lithiated SnO2 anodes, which stand out due to their high lithium storage capacity. Instead of restricting ourselves to the atomic configuration of Li-SnO2, we will further investigate the electrical transport characteristics of this compound since they also affect the underlying charge/discharge processes. For that purpose, a scheme based on maximally localized Wannier functions and non-equilibrium Green's functions has been developed. With this ab-initio quantum transport method we can not only determine the total current flowing through the considered structures, but also reveal the preferred current trajectories and their dependence on specific atomic alignments. Our study makes it possible to draw precise conclusions on the quality of a material in terms of lithium storage as well as on its limitations when it comes to electron transport.

Resume : IK4-TEKNIKER develop new materials and advanced manufacturing processes for energy conversión and storage systems, offering solutions that start at laboratory scale and can finalize in the industrialization of the product at customer level. In particular: • The phase change materials (PCM) sopported with inorganic matrix with capacity of thermal storage for transport, building applications or automotion. In particular, composite materials with porous support and organic or inorganic PCMs, and development of encapsulation pathways by means of sol gel technology for in situ encapsulation of organic or inorganic PCMs. • Thin layer solar cells with different type of substrates with high efficient energy conversión ratio of solar energy and competitive production processes, as for example, development of new photovoltaic absorvers based con chalcopyrite, or development of transparent outer metallic contacts for solar cells, and finally buffer thin layers (semiconductor type n). • Functional coatings for solar thermal collectors, increasing the yield of electricity generation plants as for example, development of selective coating on steel tubes with high absortive and emisivity, and development of mirrors with high reflexion capability. • Functional coating with high resistance to corrosion for bipolar plates and catalyst based on nanoparticles. • Advanced functional coatings with self cleaning, biocides and anfifouling properties • Nanoadditive materials as for example thermal fluids containing nanofluids, salts for thermal storage and PCM based slurries.

Resume : Due to a continuous and inevitable depletion of fossil fuels, the use of renewable energy sources is gaining an increasing interest. Particularly, novel nanostructured materials are under investigation to enhance the technical and economic feasibility of solar thermal absorption, storage, and transport. In this presentation, we will particularly focus on thermal solar energy at low/medium temperature, and will report on some recent investigations on fundamental modelling and numerical tools for i) designing optically engineered nano-suspensions for sunlight volumetric absorption and for ii) predicting main properties of water sorption nano-porous materials for thermal energy storage applications. First, we show a novel Multi-Scale (MS) model able to predict the stability and aggregation of nanoparticle suspensions in aqueous solution. The strategy developed herein allows the incorporation of nanoscale phenomena into the dynamics of suspended NPs, thereby bridging the challenging gap between nanoscopic and macroscopic scales, and providing the first accurate dynamics of suspended NPs. By synergistically integrating molecular dynamics simulations, stochastic dynamics simulations, and macroscopic theory, the proposed model offers a well-founded platform to predict suspension stability, timescales for NP aggregation, and macroscopic performance indicators of NP suspensions. Second, because of the inconstant availability of solar irradiance, thermal storage devices are becoming crucial to the exploitation of solar source. From the point of view of seasonal storage energy, the most promising technology is represented by sorption thermal batteries, which allow storing energy without heat loss with time. The improvement of thermal batteries design is related to a better understating of heat and mass transport phenomena occurring in the adsorption/desorption phases. In this context, both molecular dynamics and Monte Carlo simulations are used to properly predict the adsorption isotherm and heat of some recently commercialized zeolites with non-trivial behavior.

Resume : The pressure-dependence of the electronic and thermoelectric properties of four ternary chalcogenides with space group Pnma, namely, Cu(Sb,Bi)(S,Se)2 , is investigated up to 8 GPa using density functional theory combined with semiclassical Boltzmann theory. The effects of the van der Waals interaction are included in all calculations, since these compounds have layered structures. They all have indirect band gaps that decrease monotonically with increasing pressure except for CuBiS2 , for which an indirect-indirect band gap transition occurs around 3 GPa leading to conduction band convergence with concomitant increase in the Seebeck coefficient (20%) and power factor (45%). These enhanced thermoelectric properties result from a complex interplay between multivalley and multiband effects as well as band effective masses, driven by hydrostatic pressure. Our results suggest that ongoing developments in high-pressure science may open new opportunities for the discovery of more efficient thermoelectric materials.

Resume : Recent progresses in the synthetic growth of Si nanowires (NWs) [1] have given access to phases such as the hexagonal diamond one that in bulk is only observed under extreme pressure conditions. Hexagonal forms of bulk Si become stable at a pressure of around 16 GPa, while local domains of hexagonal Si in NWs [2] or hexagonal Si shell around a GaP NW core [3] have been reported at atmospheric pressure. A reliable experimental characterization of these polytypes is still lacking, and first-principles calculations play an important role in their prediction. Nevertheless, while several works on the electronic properties exist, no study deals with their thermal properties. Hexagonal Si, however, could be of potential interest for phonon manipulations and thermoelectric applications. This situation is common to several III-V semiconductors that, in NW form, can be selectively grown as cubic zincblende or hexagonal wurtzite. We present a predictive estimate of the thermal conductivity of the cubic and hexagonal polytypes of Si and of a few III-V compound (GaP, GaAs, GaN, InAs), obtained solving the Boltzmann Transport Equation with force constants computed from first-principles density-functional calculations. We report a significantly lower value for the hexagonal crystals and argue that it is a general feature of cubic vs hexagonal polytypes deriving from the enhanced anharmonic scattering due to optical phonons in the mid-frequency range [4]. [1] R. Rurali, Rev. Mod. Phys. 82, 427 (2010) [2] F. J. Lopez et al., ACS Nano 5, 8958 (2011) [3] H. I. T. Hauge et al., Nano Lett. 15, 5855 (2015) [4] M. Raya-Moreno et al., Appl. Phys. Lett. 111, 032107 (2017)

Resume : The semiconductors with the formula Mg2X (X= Si and Sn), have attract attention as potential high-performance thermoelectric materials, and their electronic, and thermal properties have extensively investigated [1-6]. They are an indirect band gap semiconductor. We carried in this work an ab initio study based on the density functional theory to calculate structural, electronic properties of Mg2X(X=Si and Sn). The FP-LAPW method was used with TB-mBJ exchange-correlation potential where give the best result for the lattice parameter, and the best estimation of the band gap energy. The thermoelectric transport properties like electrical conductivity (?), Seebeck coefficient (S), power factor (S2.?) and electronic thermal conductivity (Ke), have been obtained by solving the linearized Boltzmann, our results are in good agreement with experiment and other theoretical results. REFERENCE: 1. Morris R G, Redin R D, Danielson G C. Phys Rev, 1958, 109: 1909. 2. Redin R D, Morris R G, Danielson G C. Phys Rev, 1958, 109: 1916. 3. Hauser J J. Phys Rev B, 1975, 11: 3860. 4. Zaitsev V K, Fedorov M I, Gurieva E A, et al. Phys Rev B, 2006, 74: 045207. 5. Stella A, Brothers A D, Hopkins R H, et al. Phys Stat Sol, 1967, 23: 697. 6. Lott A, Lynch D W. Phys Rev, 1966, 141: 681.

Resume : Superhard materials are found with entropy descriptors.

Resume : PbTe is an important thermoelectric material for power generation applications due to its high conversion efficiency and reliability [1]. PbTe shows a shift of the electronic bandgap with temperature that is opposite to the majority of direct gap semiconductors, i.e. the gap increases with temperature [2]. In this work, we study the temperature dependence of the electronic structure and thermoelectric properties of PbTe. We perform density functional theory and density functional perturbation theory calculations [3] in the local density approximation to calculate electronic and phonon bands. We use Wannier interpolation scheme to calculate electron-phonon matrix elements [4]. Using this information, we build accurate models of electronic and phonon bands, and deformation potentials from first principles. By solving the Boltzmann equation in the momentum relaxation time approximation, we calculate the mobility and thermoelectric transport properties of PbTe. Our results are in good agreement with experiments. We find that the temperature dependence of the gap has a substantial effect on thermoelectric transport in PbTe. [1] Y. Pei, X. Shi, A. LaLonde, et al., Nature 473, 66 (2011). [2] Z. Gibbs, H. Kim, H. Wang, et al., Appl. Phys. Lett. 103, 26 (2013). [3] S. Baroni, S. de Gironcoli, A. Dal Corso, and P. Giannozzi, Rev. Mod. Phys. 73, 515 (2001). [4] F. Giustino, M. L. Cohen, and S. G. Louie, Phys. Rev. B 76, 165108 (2007). F. Giustino, Rev. Mod. Phys. 89, 015003 (2017).

Resume : Random semiconductor alloys find application in several technological devices, such as light emitting diodes, quantum cascade lasers, multi-junction concentrator solar cells and high-temperature thermoelectrics. Thermal management plays an essential role in these systems and methods able to predict their thermal conductivity are increasingly sought. The combination of first-principles calculations with the Boltzmann transport equation has proved to be an efficient and accurate method in determining the lattice thermal conductivity of semiconductor single crystals. The approaches commonly employed for their alloys consider the disordered mixture as a mass perturbation affecting an underlying non-structural effective medium. While such approximation shows a good predictive power for a limited set of compounds, it fails for general III-V semiconductor alloys. This failure is not surprising as several experimental studies have shown that these materials are characterized by a complex atomic-scale structure. In this contribution, taking In1-xGaxAs as a model material, we analyze the shortcomings of such models and show that a considerable improvement can be achieved by employing a structural description of the alloy and including the effects of local disorder through the introduction of a force-constant perturbation. We then present a first-principles method [arXiv:1712.02577] which implements these ingredients and finally demonstrate its ability to accurately predict the thermal conductivity of general semiconductor alloys.

Resume : Three ab initio approaches to the band structure of ANiSn and ACoSb half-Heusler compounds (A= Ti, Zr,Hf) are compared and their consequences for thermoelectric properties explored. The GW calculations confirm the trend of a smaller band gap (0.75 to 1.05eV) in ANiSn compared to the ACoSb compounds (1.13 to 1.44eV) already expected from the DFT calculations. While in ANiSn materials the GW band gap is 20% to 50% larger than in HSE, the fundamental gap of ACoSb materials is smaller in GW compared to HSE. Using the calculated band structures and scattering rates taking into account the band effective masses, the Seebeck coefficients, thermoelectric power factors and figures of merit ZT are predicted for all six half-Heusler compounds. Comparable performance is predicted for the n-type ANiSn materials, whereas clear differences are found for the p-type ACoSb materials. Using the most reliable GW electronic structure, ZrCoSb is predicted to be the most efficient material with a power factor of up to 0.07 W/(K^2 m) at a temperature of 600K. We find strong variations among the different ab initio methods not only in the prediction of the maximum power factor and ZT value of a given material, but also in comparing different p-type thermoelectric materials. We conclude that the most elaborate, but also most costly GW method is required to perform a reliable computational search for the optimum material.

Resume : Improved energy storage is inevitably needed to improve energy efficiency and to be environmentally friendly to chemical processes. Ionic liquids (ILs) can play a crucial role in addressing these needs due to inherent adjustable properties including low volatility, low flammability, inherent conductivity, wide liquid range, broad electrochemical window, high thermal stability and recyclability. Here, binary mixtures of ILs were prepared with fumed silica nanoparticles and characterized to obtain ILs with conductivity and electrochemical properties optimized for use in energy storage devices. The solutes were prepared by varying the size and the weight percent concentration of the nanoparticles and made up 10 % of the binary mixture by weight. We report on the physical and electrochemical properties of the individual ILs and their binary mixtures.

Resume : Crystalline silicon (c-Si) solar cell power conversion efficiency and lifetime reduces with increase in its operating temperature. In hot countries like India, in summer months solar cell temperature reaches 70oC. If one deposits material like SiO2, Al2O3 and HfO2 on top of c-Si wafer or solar cell than the temperature of it reduces due to passive radiative cooling. Thin films of the above materials are almost transparent in working wavelength range of solar cell and highly emissive in 8-13 µm wavelength range. Due to these properties our simulations show that one can achieve 10 to 15 oC reduction in operating temperature. This implies 4% to 6% increase in power output from an un-encapsulated c-Si solar cell. Based on reports, this temperature reduction will also increase the c-Si solar cell lifetime operating in hot climatic condition by twice. In this present work we have used the combination of above layers to act as 1) passive radiative cooling layer as well as 2) anti-reflection coating layer (ARC). The properties of the ARC layers requires that the thickness of the total combination of ARC layers should be below or equal to 100 nm. Our simulation shows that SiO2 with the combination of HfO2 having higher emission in the range of 9-10µm, similarly Al2O3 with HfO2 shows higher emission in the range of 11-13µm. This shows the possibility to reduce the temperature of the solar cell up to 10-15oC during real time operation with the help of all the above three materials.

Resume : With rapidly increasing demands of high-performance portable electronics and electric vehicles, lithium-ion batteries (LIBs) with high-energy density and high-power density have been intensively pursued. Conventional graphite anodes are able to charge and discharge at very low operating voltage (~0.1 V vs. Li/Li ), however, they have rather small specific capacity (~372 mA h g−1). Alternatively, silicon has long been considered one of the most promising anode materials for replacing graphite in LIBs because of its superior theoretical capacity (~3587 mA h g−1), attractive operating voltage and natural abundance. However, a large volume change of Si during charge/discharge causes severe fading in the reversible capacity. In order to overcome such disadvantages of Si-based electrodes, Si-X compound systems (X = Li-inactive/active element) have been suggested. Although these approaches have exhibited improved electrochemical performances compared to pure-Si electrode, the complex synthetic procedures and high fabrication costs have prevented their commercialization. In this study, we prepared for SiP2/CNT composite by a simple ball milling of silicon and phosphorous powders at room temperature, and we demonstrated its potential to be used as a promising anode for Li-ion batteries. It exhibits not only enhanced rate performance but also superior cycling performance, owing to the formation of electrical conductive buffer matrix. More details will be discussed at the meeting.

Resume : Li-ion batteries (LIBs) are considered the most promising power sources for portable electronic devices, clean transportation, and renewable energy storage facilities. However, conventional graphite anode has a limitation to applications which demand high energy density property. Therefore, to meet the demands of the rapidly increasing market, various high capacity anode materials are being intensively researched. Among various anode materials investigated for use in LIBs, silicon and phosphorous have been widely studied as anode materials in LIBs due to their high theoretical capacities of 3587 mA h g−1 and 2500 mA h g−1 respectively, however, poor cycling stabilities are remained challenging. Recently, SiP2 have been suggested as a new promising anode material which exhibited a high theoretical capacity of 2900 mA h g-1 and has three-step lithiation mechanism. However, its severe fading in the reversible capacity still have prevented their commercialization. In this study, we prepared for SiP2/CNT composite by a simple ball milling of SiP2 and pristine CNT powders at room temperature, and we demonstrated its potential to be used as a promising anode for Li-ion batteries. It exhibits not only enhanced rate performance but also superior cycling performance, owing to the formation of electrical conductive buffer matrix. More details will be discussed at the meeting.

Resume : As for sodium-ion (Na-ion) and potassium-ion (K-ion) are in the similar position in the periodic table, both Na-ion and K-ion batteries share the same ‘‘rocking chair’’ principle of electrochemical operation of Lithium ion battery (LIBs). Thus extensive efforts have been made to sodium-ion battery (SIBs) in the last few years, however, SIBs have relatively high standard hydrogen potential (−2.71 V vs E°) compared to LIBs (−3.04 V vs E°) makes the energy density of SIBs relatively low and limits their potential industrial applications. Comparatively speaking, potassium ion battery (PIBs) have lower standard hydrogen potential of K (−2.93 V vs E°) than that of Na and closer to that of Li, thus PIBs is expected to have the high energy density due to its wide working potential. [ref] Although PIBs have the potential to be an alternative post-LIBs energy storage devices, PIBs are still in the initial stage of development. It becomes significant to explore and develop novel and effective electrode materials for PIBs. Herein, we for the first time report on the electrochemical performance of TiO2 for anode materials for PIBs. To increase the intrinsic low electronic conductivity of TiO2, we incorporate CNT on TiO2 host materials through spray drying method. More details on the synthetic procedure, electrochemical and structural properties will be presented at the meeting.

Resume : Lithium-ion batteries (LIBs) are primarily used for advanced portable electronic devices and automotive electrical energy storage (EES) systems. However, the cost disadvantage and global scarcity of lithium resources restrict the LIBs for widespread and cost efficient large scale applications. Recently, sodium-ion (Na-ion) and potassium-ion (K-ion) batteries have received considerable attention as a potential post-lithium ion energy storage device due to their low cost and abundance. Anatase titanium dioxide (TiO2) is widely used as an anode material in commercial LIBs due to its reasonably low insertion potential (~ 1.7 V vs. Li/Li+), relatively high capacity (335.6 mAh/g) and small volume change (< 4%) upon the Li-ion insertion reaction. And also in NIBs, TiO2 anode materials have achieved a huge success with quiet good cycling stability and relatively high rate performance. Considering TiO2’s encouraging success on Li-ion and Na-ion battery can give inspiration to use of anode materials for potassium-ion batteries. Herein, we for the first time report on the potassium ion storage mechanism of TiO2 for anode materials for PIBs. More details on the synthetic procedure, electrochemical and structural properties will be presented at the meeting.

Resume : Spinel Li4Ti4O12 (LTO) has been demonstrated to be one of the most promising electrode materials for lithium-ion batteries due to relatively high and flat operating voltage of 1.55V (vs. Li/Li+), high reversible capacity (175 mAh g-1), and near-zero structural changes during repeated charge-discharge processes. However, the practical application of LTO is still largely restrained by the low intrinsic electrical conductivity (10-13 S cm-1) and lithium diffusion coefficient (10-13 to 10-10 cm2 s-1) Recently, duplex phase LTO-TiO2 composites have been reported to show better electrochemical performance than pure LTO. The improved electrochemical properties are attributed to the unique advantages of LTO-TiO2 composites as follow: 1) the higher lithium diffusion coefficient of TiO2 phase (~ 10-6 cm2 s-1) could improve lithium insertion/extraction kinetics. 2) The presence of Ti3+ in LixTiO2 during the discharge process could enhance the entire electric conductivity. 3) The high theoretical capacity of TiO2 phase (~336 mAh g-1) could increase specific capacity. In this study, dual phase core-shell structure consisting of LTO core and TiO2 shell were synthesized using chemically treated LTO particles. The fraction of TiO2 shell was controlled by tuning the condition of chemical treatment of LTO and heat treatment. It exhibits not only enhanced rate performance but also higher specific capacity comparing pure LTO. More details will be discussed at the meeting.

Resume : To date, numerous efforts have been devoted to improving the rate capabilities of spinel Li4Ti5O12 (LTO) by decreasing lithium-ion diffusion length and increasing the electronic conductivity. They reported improved rate capability of LTO. However, LTO is not considered the most preferable choice for large-scale applications by the power industries mainly due to severe gassing during charge/discharge cycles and storage, especially at elevated temperatures. It has been reported that the interfacial reactions between LTO and organic electrolyte solution may be the root cause for the gassing behavior. The construction of a barrier layer like a carbon layer is found effective in suppressing the gassing during interfacial reactions. Unfortunately, the carbon materials have high reactivity with electrolyte solutions at elevated temperature, which brings great safety concern. Thereby, finding a coating layer with little interfacial reactivity with electrolyte is urgent for the wide application of LTO in high power LIBs In this study, we developed SiOx coated LTO composite by a chemical process. Since SiOx is an inactive material in LTO operating voltage, SiOx coating layer blocked the interfacial reactions between LTO and organic electrolyte solution resulting in suppressing the gassing. It exhibits ultra-long cycle performance at elevated temperature. More details will be discussed at the meeting.

Resume : As one of the most applicable energy storage devices, lithium-ion batteries (LIBs) have been widely used in hybrid electric vehicles and portable electronic devices because of their high energy density. SnO2 has been studied as a promising candidate for anode material owing to its high theoretical capacity. However, low conductivity and large volume expansion of SnO2 during repeated cycles significantly limit its practical applications to LIBs. One of the most strategies to overcome the limits of the SnO2 is incorporation of graphene to limit aggregation of particles and improve cycle performance by mitigating the internal stress from the volume expansion. Recently, porous graphene have been reported to provide more diffusion channels for Li-ions and provides more edge sites enhancing Li-ion storage. Furthermore, the porous structure of graphene might accommodate the large volume expansion of the SnO2. For these reasons, there have been several reports about incorporation of the SnO2 into porous graphene. However, there is no study about the effects of the pores on the electrochemical activities of the SnO2 or Li-ion storage performance according to the characteristics of pores such as locations of the pores. In this work, we synthesized SnO2/NPG microball which is a unique design of SnO2/graphene structure with nanoperforations located underneath the SnO2 to analyze the effect of nanoperforations on the electrochemical activity of SnO2 or the Li–ion storage performance.

Resume : As one of the most applicable energy storage devices, lithium-ion batteries (LIBs) have been widely used in hybrid electric vehicles and portable electronic devices because of their high energy density. SnO2 has been studied as a promising candidate for anode material owing to its high theoretical capacity. However, low conductivity and large volume expansion of SnO2 during repeated cycles significantly limit its practical applications to LIBs. One of the most strategies to overcome the limits of the SnO2 is incorporation of graphene to limit aggregation of particles and improve cycle performance by mitigating the internal stress from the volume expansion. In this work, we synthesized SnO2/NPG microball which is a unique design of SnO2/graphene structure with nanoperforations located underneath the SnO2 to analyze the effect of nanoperforations on the electrochemical activity of SnO2 or the Li–ion storage performance. This unique design of SnO2/NPG microball has several advantages in lithium storage performance: I) Improvement in rate capability by decreasing the diffusion length of Li-ions and increased reaction site of SnO2 through introduction of nanoperforations underneath the SnO2 and II) increase in capacity through insertion/extraction of Li-ion from the edge sites, III) advancement in cycle performance by accommodating the volume expansion of SnO2.

Resume : Ni - MH batteries are characterized by important features such as light weight, good thermal performance, long cycle life, configurable design, low maintenance and high power. In fact, Hydrogen storage alloys are crucial in Ni - MH battery power service life and inn defining the battery electrochemical characteristics. LaNi5, having CaCu5 crystal structure, is characterized by almost all these properties and especially fast hydrogen kinetics. In recent years, CaNi5 has been considerably investigated because of its low cost, good kinetic properties and important hydrogen storage capacity. Nevertheless, the major limitation of its application is its extremely low cycle life. [1-4] Our study aims at providing a more stable structure with improved electrochemical capacity by the different milling time. In this work, the hydrogen storage characteristics of the single substituted CaNi4.7Mn0.3, prepared by applying Ni partial substitution, were valuated methodically at various milling times (2, 10, 20, 30, 40, 50 and 60 hours). Obviously, the cells, assembled by employing the obtained active material as anode, were cycled for both charging and discharging. Consequently, the measurement of the hydrogen storage capacity, ranging from 76 to120 mAh/g, was carried out electrochemically. However, compared to pure CaNi5, none of the various milling time improved noticeably the cycling stability. Besides, the alloys X-ray diffraction patterns showed that the hexagonal CaCu5 structure of a true AB5 alloy was maintained just in some cases. References [1]C. Khaldi, H. Mathlouthi, J. Lamloumi,A comparative study, of 1 M and 8 M KOH electrolyte concentrations, used in Ni–MH batteries, Journal of Alloys and Compounds, 469, 464–474, (2009). [2] Xiangrong Chen, Haiyan Wang, Shuping Zhu, Fangfang Li, Yougen Tang, Zuming Liu, Effect of magnetic field on the microstructure and electrochemical performance of rapidly quenched La0.1Nd0.075Mg0.04Ni0.65Co0.12 alloy, Journal of Alloys and Compounds, 617, 722–728,(2014). [3] H Mathlouthi, C. Khaldi, M Ben Moussa, J Lamloumi, A Percheron-Guégan, Electrochemical study of mono-substituted and poly substituted intermetallic hydrides, Journal of Alloys and Compounds, 375, 297–304, (2004). [4] Y. Dabaki, S. Boussami, C. Khaldi, H. Takenouti, O. ElKedim, N. Fenineche, J. Lamloumi, The effect of ZnO addition on the electrochemical properties of the LaNi3.55Mn0.4Al0.3Co0.2Fe0.55 electrode used in nickel metal-hydride batteries, Journal of Solid State Electrochemistry, 21, 1157-1164, (2017). Keywords: Ni-MH batteries, hydrogen storage materials, CaNi5.

Resume : Methylammonium lead tri-iodide (MAPbI3) is a polymorphic material with a remarkable potential for solar energy conversion. It is characterized by two temperature-induced phase transitions at about 165 K and 327 K, accompanied by an orthorhombic-to-tetragonal and a tetragonal-to-cubic lattice modification, respectively. Understanding the origins of these transitions as well as their implications on the crystal structure of the material could be important for its technological exploitation. Here, we use ab initio molecular dynamics to study the phase transitions of MAPbI3. We argue that the orthorhombic-tetragonal transition is characterized by an antiferroelectric to ferroelectric ordering through the partial rearrangement of the organic cations, which locally relaxes the stress arising from the thermal movement of atoms [1,2]. Besides, we propose a macroscopic model for the tetragonal phase that consists of rotated noncentrosymmetric domains where the MA ions are quasi-two-dimensionally confined around the a-b crystallographic plane. We finally show that the gradual disordering of the MA ions at higher temperatures triggers the tetragonal-to-cubic transition, resulting in a loss of the ferroelectric characteristics. [1] I. Deretzis, B.N. Di Mauro, A. Alberti, G. Pellegrino, E. Smecca, A. La Magna, Sci. Rep. 6, 24443 (2016) [2] I. Deretzis, A. La Magna, Nanoscale 9, 5896-5903 (2017)

Resume : Graphene is a two-dimensional material that exhibits unique electrical, mechanical and optical properties. Derivatives of graphene recently became of large interest because of the possibility of adding new functionalities to graphene and tune its electronic properties. Among the possible derivatives, graphane - a graphene structure fully-hydrogenated on both sides of the lattice plane - and graphene oxide have attracted a great deal of attention in the last years. In this work we demonstrate via ab initio calculations that the formation of regular and short-period hydroxylated graphane is possible under suitable thermodynamic conditions. An application is shown where a graphene based derivative directly synthesized by chemical vapour deposition is used as hole transport layer in organic photovoltaic cells. Possibly, hydroxylated graphane could be applied as functional material in the fields of energy generation and storage. References [1] F. Buonocore, A. Capasso and N. Lisi, An ab initio study of hydroxylated graphane, Journal of Chemical Physics vol. 147, issue 10, 104705 (2017). [2] A. Capasso, L. Salamandra, G. Faggio, T. Dikonimos, F. Buonocore, V. Morandi, L. Ortolani, and Nicola Lisi, Chemical Vapor Deposited Graphene-Based Derivative As High-Performance Hole Transport Material for Organic Photovoltaics, ACS Appl. Mater. Interfaces 8 (36), pp 23844–23853 (2016).

Resume : The new generation of solar cells based on the intermediate bands within the energy gap of a semiconductor has been proposed as a way to increase photovoltaic solar cell efficiency. In this paper, we present a theoretical study of one intermediate-band solar cell (IBSC) formed by a multi-junction of InxGa(1-x)N/InN quantum dot supracrystals arrayed in the i layer of the p-i-n type structural cell. Based on Zhang's model [1], it is proposed to place the IB before the valence band (VB). Therefore we took into account electron-hole transitions. In order to determine the position and width of IB derived from the discrete quantized energy levels originating from QDs, the Kronig-Penny model was adopted to solve the Schrödinger equation. By taking into account the built in electric field generated at the junctions, we analyze the influence of the concentration x, mean size L and interdot spacing H of QDs on the position and width of IB. Thus the cell characteristic parameters such as IBSC efficiency, open circuit voltage (Voc), and short circuit current density (Joc) of InxGa (1-x) N/InN QD-IBSCs with single IB became clear. [1]: Q. Zhang, W. Wei, Appl Phys A (2018).

Resume : Energy storage system with high efficiency and capacity has attracted a great attention due to rapidly rising demand of an electric automobile, portable electric devices and internet of things. Lithium ion battery (LIB) has been used as a portable power supply device with high capacity. However, the problem including stability of electrode materials and limited supply of lithium sources has been raised because of chemical reactivity of Li and a limited resource reserves. Recently, alternative energy storage systems have been demanded. Among them, rechargeable aluminum-ion battery(AIB) is spotlight as an advanced-LIB. Aluminum metals can be used as an anode material because aluminum is relatively inexpensive from abundant reserve, high energy density with three-electron redox properties. Generally, ionic liquid is used as electrolyte for AIB, which exhibit high thermal stability, low vapor pressure, wide electrochemical window, high ion conductivity. However, ionic liquid could hinder the migration of charge carriers due to high viscosity. It causes formation of aluminum dendrite on the surface of Al electrode and unstable cycling performance at high current rates. In this work, we tried to figure out the correlation between viscosity and conductivity in electrolyte by adding benzene as organic solvent. According to Nernst-Einstein and Stokes-Einstein equations, conductivity is proportional to the number of charge carriers and inversely proportional to viscosity in electrolyte. But, addition of organic solvent decreases viscosity and reduces number of charge carriers as well. Accordingly, it is very important to set the suitable amount of organic solvent added to the ionic liquid. We prepared various electrolytes according to addition ratio of organic solvent and analyzed by electrochemical methods. (We used ionic liquid (AlCl3 / [EMIM]Cl) as electrolyte, and anhydrous benzene as additive to ionic liquid.) The reversible deposition and dissolution of aluminum and active reaction were investigated through cyclic voltammogram. Galvanostatic charge/discharge profiles were carried out using graphite as cathode for AIB. As a result, electrolyte with organic solvent is effective on stable cycling performance at a high current rate of 5Ag–1. We suggest that addition of organic solvent to ionic liquid can provide improved performance for rechargeable aluminum-ion battery.

Resume : We have theoretically investigated the 1.55 µm p-i-n GaNAsBi-based multiple quantum wells (MQWs) using a self-consistent calculation combined with 16x16 BAC model. Their performances are evaluated in terms of optical gain and radiative current density J_rad. We have found that J_rad reduces by increasing the well thickness〖 L〗_ω. The quantum confined Stark effect as well as the doping effect on spontaneous emission and radiative current density in ideal lasers are also discussed. The optical properties of the heterostructure are improved when the MQWs are doped. The optimization of well parameters can be used as a basis for GaNAsBi-based lasers intended for optical fiber telecommunication wavelength.

Resume : TbFe2 based magnetostrictive materials (owing to its larger magnetostriction value of 2500 ppm at RT) are of significant research interest due to their applications in defence in the form of microactuator, micromotor, sensor etc. However, its quality degradation due to oxidation poses an enormous challenge from being scaled up in a device. Understanding the microstructure of these oxide films is crucial as this would indicate about the loss of the constituent(s) from the bulk of the compound, thereby, giving direct information about the loss in the magnetic properties. In this context, a detailed study employing both theoretical and experimental methods was performed to understand the oxide-film overgrowth due to dry and thermal oxidation of TbFe2. The theoretical formalism predicted the overgrowth of Tb2O3, when there is no segregation of iron on the substrate, which is also experimentally validated. However, when the original surface of the TbFe2 substrate gets fully covered with iron atoms, formation of α − Fe2O3 become thermodynamically preferred over all other iron oxides. Experimentally, this was also found to co-exist with α − Fe2O3 at higher annealing temperatures. Moreover, both the model and the experimental data suggest segregation of Fe on TbFe2 substrate surface when annealed at higher temperatures. Understanding developed from this study would help in tuning the substrate constituents for higher temperature application, where oxidation of the compound is a major concern.

Resume : In this study, we used quantum chemistry calculations in order to determine some kinetic parameters of the isomerization reaction of the substituted icosadeca-ene. The studied molecules are: (C20H20, C20H10F10, C20H10Cl8, C20H10Br8 and C20H10I8) Cis and Trans. One of the adopted ways to access these parameters (activation energy, rate constant, etc ...) is looking for the transition state that is based on the exploration of intermediaries during the passage of Cis-Trans isomerization process. The study of a ten molecules series gives the following results: * The trans conformer is more stable than the Cis. * The activation energy changes very greatly depending on the size and nature of the substituent according to the reaction profile. * The constants of the isomerization reaction rates are in the following order: kC20H22 >> k C20H10F10 >> k C20H10Cl10 >>k C20H10Br10 >> k C20H10I10. * The geometrical parameters vary considerably according to intermediate products The calculation methods are DFT (TD-B3LYP) and Ab-initio methods at STO-3G*. Keywords: substituted icosadeca-ene, kinetics; isomerisation, HF (AM1+PM6), DFT

Resume : In commercial lithium ion batteries the most common anode material is graphite with a theoretical specific capacity is approximately 372 mAh/gm, but their low energy density and rate performance is poor. Therefore we have to improve the energy density as well as the specific capacity of anodes for powering electric vehicles. Most of the anode materials suffer from volume expansion and contraction during Li ion intercalation and de-intercalation processes, resulting in pulverization and poor stability of the electrodes in spite of having high storage capacity. In this investigation, we report the systematic study and synthesis of a novel perovskite Nickel Titanate (NiTiO3) by Sol–gel method. Nanosized powders annealed at 900 °C for 3 hours to form a crystalline structure and eliminate organic impurities. The phase and morphological properties were characterized by X-ray diffraction, Raman spectroscopy measurements, scanning electron microscopy, and transmission electron microscopy techniques. The pure-NTO electrode comprising a hexagonal crystal structure give a discharge capacity of approximately 550 mAh g−1 that corresponds to a coulombic efficiency of 67% in the first cycle, which further improved to ∼98% in the following cycles, at an applied specific current of 10 mAg−1, and stable cycling performance for 100 cycles. In the present work we demonstrate the importance of metal titanates, especially NTO, in terms of their applicability as an anode material for lithium ion batteries. Besides by engineering the microstructure and by making composites with carbonaceous materials these can also deliver effective lithiation/delithiation processes with better electrochemical performance.

Resume : The film composite "Si nanocrystals in the matrix of amorphous Si" (nc-Si) is considered as a promising material for the next generation of solar cells (SC) at quantum dots. Among the main problems hampering the practical realization of the benefits of nc-Si is the lack of development of technology for controlling the size and concentration of Si nanocrystals at economically feasible film formation rates. One of the promising ways in this direction is the use of the phenomenon of metal induced crystallization (MIC) of amorphous silicon. An important role in the formation of nanocrystals in this structure is the distribution of temperature profiles that arises during laser irradiation. The temperature and time parameters of the MIC in the system a-Si-Sn are determined, as well as the prospects for using pulsed laser radiation to control the size and concentration of nanocrystals with induced tin crystallization of amorphous silicon. For this purpose, the finite element method was used to calculate the temperature distribution in the structure of a-Si-Sn-a-Si under the influence of different types of laser irradiation. The distribution of temperature profiles according to the intensity of pulsed radiation in the range of 15 KW/cm2 to 85 MW/cm2 with a duration of a single laser pulse of 10 ns and 150 μs (a laser with a wavelength of 535 nm and 1070 nm) is calculated. The difference between the temperature profiles in the structure with irradiated laser light with a length of 535 nm and 1070 nm is established.

Resume : The results of ab initio calculations of polar YAlO3 (001) surfaces by means of a hybrid B3PW exchange-correlation functional as it is implemented in the CRYSTAL computer code are presented. Both polar YO and AlO2-terminations of the cubic YAlO3 (001) surface were considered. We performed relaxation of atoms on upper three layers of both YO and AlO2-terminated YAlO3 (001) surfaces using slabs containing 22 and 23 atoms and 9 layers, respectively. The Al-O chemical bond covalency change near the AlO2-terminated YAlO3 (001) surface relative to the YAlO3 bulk is discussed. YO and AlO2-terminated YAlO3 (001) polar surface energies are calculated and compared with ABO3 perovskite (001) surface energies [1,2]. Our B3PW calculated electronic band gaps near the YO and AlO2-terminated YAlO3 (001) surfaces are compared with the YAlO3 bulk band gap. 1. E. Heifets, R.I. Eglitis, E.A. Kotomin, J. Maier and G. Borstel, Phys. Rev. B 64, 235417 (2001) 2. R.I. Eglitis and A.I. Popov, J. Saudi Chem. Soc. (2018), in press

Resume : Dilute bismide-nitride compound GaNAsBi is a promising candidate for use in GaAs-based optoelectronics devices in the near-infrared region. In this work, we report results of the electronic band structure of GaNAsBi/GaAs strained multiple quantum wells (MQWs). The band structure was modeled within the 16-band anti-crossing model, envelop function formalism and Bir-Pikus theory in conjugation with k·p Hamiltonian method. This investigation was used to describe the optoelectronics properties behavior of these structures such as band offsets, subband energies, strength of inter-band transitions and absorption coefficient spectra. We show that absorption coefficient spectra of GaN.04As.91Bi.05/GaAs strained single QWs has two maxima respectively located at 5,24 104 cm-1 for Τ_(e1-hh1) and 6, 37 104 cm-1 for〖 Τ〗_(e1-lh1). In addition, we have presented the results of the GaNAsBi-based strained double QWs (DQWs). We have deduced that the combined effects of strained and coupling between two wells enhance considerably the absorption coefficient compared to uncoupled DQWs. Keywords GaNAsBi-based strained QWs, Double QWs and coupling, Bir-Pikus theory, Absorption coefficient Optoelectronics properties

Resume : With the rapid development of wearable electronics and sensor networks, batteries cannot meet the sustainable energy requirements due to their limited lifetime, size and degradation. Ambient energies such as wind have been considered as an attractive energy source due to its copiousness, ubiquity, and feasible nature. With miniaturization leading to high-power and robustness, triboelectric nanogenerators (TENG) have been conceived as a promising technology by harvesting mechanical energy for powering small electronic systems. In this work, a state of the art TENG based on a wind venturi system is demonstrated for use in any complex environment. With the introduction of wind into the air channel of the TENG system, a thin flexible based film repeatedly contacts with and separates from the electrodes. Compound stacking not only amplifies the output power but also enables powering multiple sensor networks in remote area utilization. The system converts ambient mechanical energy to electricity with a 1.2kV peak amplitude by rapid super capacitor charging for powering an environmental sensor network. Different thin polymer materials are investigated by comparing the output power and mechanical stability performance. An autonomous sensor network system has been setup to monitor various environmental parameters and send real time signals for periodic analysis. The study also includes the influence of humidity and wind force under different environmental conditions. By considering these merits of simple fabrication, robust characteristics and outstanding performance, the TENG system could provide an autonomous detection system allowing clean and uninterrupted energy production.

Resume : Fuel cell powered by hydrogen from renewable sources is a clean and efficient way of energy utilization and has been developed rapidly over the past fifteen years. Fuel cells and electrolysers depend on electrocatalysis towards oxygen reaction and hydrogen reaction on their anode and cathode. However, the Oxygen Reduction Reaction (ORR) of a Fuel Cell and Oxygen Evolution Reaction (OER) of an electrolyser are kinetically sluggish. Hence, the loading of Platinum(Pt) electrocatalyst at the cathode is 10 times higher compared to that at the anode. The high cost and low earth availability has limit their application. Hence, this research aims to develop a clean and facile method for synthesis some non-precious metal phosphides as electrocatalysts for fuel cells and electrolysers. In this works, three different transition metal phosphides (Tungsten Phosphides, Chromium Phosphides and Tin Phosphides) will be synthesized by carbothermal synthesis and hydrothermal synthesis as catalyst supports, which will be followed by dispersion of Pt and Ir nanoparticles. Their electrochemical properties towards Hydrogen Reaction and Oxidation Reaction will be measured to investigate whether these catalysts can be applied to Fuel Cell efficiently.

Resume : A direct measure of genuine entanglement in tripartite systems based on the volume of the negative part of Wigner function is proposed. We analyze comparatively this quantity and the dierent types of tripartite entanglement for two principal classes (GHZ and W class ) formed in coherent state basis.

Resume : With the aim of finding an interpretation for the isomerization reaction of icosadeca-ene by quantum methods, we have studied a series of three molecules giving the following results: ? The studied segments (C20H20, C20H10F10, C20H10Cl8, C20H10Br8 and C20H10I8) are very stable. This stability is justified by the HOMO-LUMO found energy gap. However, examination of the stability of several conformations shows that the trans conformer is more stable than the cis conformer in the general assembly. ? According to the study of different reaction profiles, we noticed that the size and nature of the dopant plays a very important role on the evolution of the activation energy. ? From the obtained values of the activation energy, we find that the speed constants of the isomerization reaction are in the order: kC20H22 >> k C20H10F10 >> k C20H10Cl10 >> k C20H10Br10 >> k C20H10I10 ? The search for intermediate products during the transition Cis-Trans shows that the geometric parameters (angles and dihedral angles) are the most varied settings, this remark has been observed in the case of substituted and non-substituted icosadeca-ene. ? The methods of calculations performed in this work are the Ab-initio and DFT methods, with the bases (6-31G, 3-21G **). All these calculations are performed with the Hyperchem software, where parameters obtained are in a closer order to those obtained with the Gaussian 03W software ? Examination of different molecules obtained during the Cis-Trans isomerization reaction shows that the total energy of the resulting intermediate product is of the order of -10487.05 eV, corresponding to a 0.87eV activation energy (23.67 kcal / mol). ? With the same HF method (6-31G and 3-21 G**), a close geometry was obtained for the intermediate product in the isomerization reaction with a total energy of 0.93 eV (25.30 kcal/mole), which shows that the different values of the activation energy obtained by the HF and DFT methods at the 6-31G level can be compared to those obtained by Ito, Montaner and Bernier. Keywords: Ab-initio; DFT; kinetics; isomerisation; substituted icosadeca-ene

Resume : The discovery of 112-type Iron-based superconductors Ca1-xRExFeAs2 (RE= rare-earth elements) have attracted huge attention because of their various anomalous physical properties [1,2]. The temperature dependence of upper critical field for Ca0.83La0.17FeAs2 and Ca0.8La0.2Fe0.98Co0.02As2 single crystals are found to related to the two-band superconducting properties. To further understand the mechanism, the upper critical field for superconducting Ca0.83La0.17FeAs2 and Ca0.8La0.2Fe0.98Co0.02As2 crystals using the two band Ginzburg-Landau theory is calculated and analyzed. A two-parameter variables approach is utilized to obtain the upper critical field in arbitrary direction. The temperature and angular dependences of the upper critical field are carefully considered. It is found that the simulated two-band electronic structure for superconductors Ca1-xRExFeAs2 is well-fit with the experimental data in a broad temperature range [3]. The effect of impurity on the anomalous physical properties at superconductors Ca1-xRExFeAs2 (RE= rare-earth elements) is also reported. [1]. Yakita, H. et al. A New Layered Iron Arsenide Superconductor: (Ca,Pr)FeAs2. J. Am. Chem. Soc. 136, 846?9 (2014). [2]. Katayama, N. et al. Superconductivity in Ca1-xLaxFeAs2: A Novel 112-Type Iron Pnictide with Arsenic Zigzag Bonds. J. Phys. Soc.Jpn. 82, 123702 (2013). [3]. Xiangzhuo Xing et al. Two-band and pauli-limiting effects on the upper critical field of 112-type iron pnictide superconductors. Scientific reports. 7, 45943 (2017)

Resume : Currently many investigations focus in finding materials suitable as electrodes for the development of high capacity energy storage devices such as the lithium-ion batteries, specially germanium nanostructures have been considered as an attractive alternative for anodes due to its high theoretical charge capacity. To this end, one of the most promising nanostructures is the porous Germanium (PGe) since it overcomes limitations of crystalline bulk Ge such as the drastic volume expansion. However, the theoretical investigations of this material are scarce. In this work the effect of superficial Li on the electronic properties of PGe was investigated through the density functional theory and the supercell technique. The pores were modeled by removing columns of atoms from crystalline bulk Ge along [001] direction. The surface dangling bonds were passivated with H. To model the effects of superficial Li, H atoms were replaced by Li atoms until full coverage of the pore surface is achieved. Results shows that as the Li concentration increases at the surface, the electronic band gap decreases, favoring electronic conduction in these nanostructures which could be beneficial for use in lithium-ion batteries. Acknowledgment This work was supported by the computing facilities of LANCAD-CONACYT.

Resume : The rate of water evaporation as fundamental phase-change phenomenon is critically important to thermal processes in various industrial and manufacturing applications. With the development of nanotechnology, significant acceleration of evaporation rate process is now potentially feasible, which can lead to much higher efficiency, for example for thermal desalination. Recently, hollow and porous nanostructures have exhibited promising potential in the evaporation enhancement due to their capillary effect. However, the mechanism of water vapor transport through a capillary media at the nanoscale and the effect of surface properties remain unexplored. The present study investigates the evaporation behavior of water through the capillary channel of a graphene bilayer using molecular dynamics simulations, with tunable surface wettability by changing the surface charge states. The effects of both structural and environmental parameters, including the capillary channel distance and temperature, are also examined in details. The results show that significant enhancement of evaporation occurs when the graphene bilayer is brought into contact with the water surface. It is also found that the evaporation behavior is mainly controlled by two factors, namely, the morphology of the liquid-gas interface and the interaction energy between the water molecules and the graphene layer.

Resume : The growing concern about CO2 impact on the environment pushes the researchers to the development of new materials with improved performances for CO2 capture and separation. Graphene oxide (GO) received recently a great attention and is recognized as one of the most promising materials for application in several fields such as solar cells, batteries, optoelectronic devices, energy storage, gas sorption and separation, gas sensing, etc.. The design of the GO structure and of its functionalization is of extreme importance for tailoring GO performances according to the requested applications. In this work we investigated the properties of graphene oxide obtained by modified Hummer’ method supported by CaO and not supported for application in CO2 capture.

Resume : Light Driven Organic Transformations to Enhance Hydrogen Production The water splitting reaction is a reaction that is kinetically and thermodynamically challenging to carry out. Alternative Hydrogen production involves the use of organic materials instead of water to facilitate hydrogen production. With the aid of Photo induced and Transient Absorption Spectroscopy along with the previous knowledge regarding the oxidation of methanol, the mechanism of the oxidation of organic waste is investigated. Using the oxidation of methanol to formaldehyde on a hematite electro catalyst under the irradiation of 365nm light as a platform to work from, we look to see if similar oxidation reactions of alcohols, glycerol and formation of epoxides work in a similar manner. Upon examination of the reactions of these organic substrates, one substrate will be selected to carry out further reactions. During these further reactions a pseudo steady state condition will be reached and then removed allowing for examination of rate of reaction and rate constants using Photo induced absorption spectroscopy. From this we can then seek to shine some light into the mechanism of this oxidation process. 1. Kinetics of Photoelectrochemical Oxidation of Methanol on Hematite Photoanodes Camilo A. Mesa, Andreas Kafizas, Laia Francàs, Stephanie R. Pendlebury, Ernest Pastor, Yimeng Ma, Florian Le Formal, Matthew T. Mayer, Michael Grätzel, and James R. Durrant Journal of the American Chemical Society 2017 139 (33), 11537-11543 DOI: 10.1021/jacs.7b05184 2. Kinetic Analysis of an Efficient Molecular Light-Driven Water Oxidation System Laia Francàs, Roc Matheu, Ernest Pastor, Anna Reynal, Serena Berardi, Xavier Sala, Antoni Llobet, and James R. Durrant, ACS Catalysis 2017 7 (8), 5142-5150, DOI: 10.1021/acscatal.7b01357 3. Jia, Jieyang, Linsey C Seitz, Jesse D Benck, Yijie Huo, Yusi Chen, Jia Wei Desmond Ng, Taner Bilir, James S Harris and Thomas F Jaramillo. (2016). "Solar water splitting by photovoltaic-electrolysis with a solar-to-hydrogen efficiency over 30%." Nature Communications 7: 13237.

Resume : In this work, we employed density functional theory [1] to study the effect of ZnO thickness on the physical properties of ZnO/TiO2 core/shell nanostructures and their practical impacts when these structures are used in photocatalytic or photovoltaic systems [2]. We varied the thickness of the core from 4 to 16 A. The effect of the thickness on the electronic and mechanical properties was analyzed at GGA and GGA+U levels. The results show a non-uniform variation in the energy gap of the core/shell nanostructure suggesting a compensating mechanism due to the electronic defect states [3]. A larger energy gradient between the conduction band of ZnO and the conduction band of TiO2 was observed and favors the transfer of the excited electrons from ZnO to TiO2, in agreement with experiments [4]. However, the heterostructure shows a good resistivity even for a very fine core thickness. With such core/shell structures, the ratio of cells in surface area can be increased and demonstrate a good life time when they are used in dye sensitized solar cells (DSSCs) [2]. [1] W. Kohn and L. J. Sham, Phys. Rev. 140, A1133 (1965). [2] B. O. Regan and M. Graetzel Nature, 353, 737 (1991). [3] Haffad, arXiv:1702.02545v1 [physics.comp-ph]. [4] Fatma Kayaci, Sesha Vempati, Cagla Ozgit-Akgun, Inci Donmez, Necmi Biyikliab, and Tamer Uyar, Nanoscale, 6, 5735-5745 (2014).

Resume : The ionization equilibrium in n-type indium antimonide under external quantizing magnetic field is considered. The electroneutrality condition has the form: n(↑) + n(↓) + N(−1) = N(+1) = N − N(0,↑) − N(0,↓), where n(↑) + n(↓) is the concentration of c-band electrons which have parallel (↑) and antiparallel (↓) to the direction of magnetic field spin orientation, N = N(0,↑) + N(0,↓) + N(+1) is the concentration of all donors, N(0,↑) + N(0,↓) is the concentration of electrically neutral donors with “optical” electron, N(+1) and N(−1) are the concentrations of ionized donors and compensating acceptors. The calculation of electron concentrations n(↑) and n(↓) and electrically neutral donors concentrations N(0,↑) and N(0,↓) takes into account: 1) quantizing and spin-splitting of orbital movement energy (Landau levels) of c-band electrons; 2) spin-splitting of energy levels of impurity band; 3) c-band tail and distribution of impurity energy levels in the band gap; 4) electrostatic screening of impurity ions, percolation threshold and exchange interaction of c-band electrons [1]. The solution of electroneutrality condition gives the dependences of n(↑), n(↓), N(0,↑) and N(0,↓) on magnetic field, which are consistent with the theoretically unexplained experimental data on CW electron spin resonance in n-InSb [2]. The work was supported by the EU Programme Horizon 2020 (Grant Nos. H2020-MSCA-RISE-2015-691010 HUNTER and H2020-MSCA-RISE-2015-690968 NANOGUARD2AR). [1] N.A. Poklonski, S.A. Vyrko, A.I. Kovalev, A.N. Dzeraviaha. J. Phys. Commun. 2, 015013 (2018). [2] M.V. Kondrat’ev. Sov. Phys. Semicond. 20 (1986) [Fiz. Tekh. Poluprovodn. 20, 1485 (1986)].

Resume : Chemical bonding and stability of simple metal hydrides and transition metal doped MgH2 have been assessed by means of charge density topology analysis on the both local (Bader concept of atoms in molecules) and integral (concept of non covalent interactions) level. Trends in macroscopic properties (melting temperatures and overall thermodynamic stability, elastic properties etc.) observed in compound series such as alkali hydrides and halides, could be attributed to existence of subtle differencies in charge density topologies. These differencies on local level can be described using different number and distribution of charge density stationary (critical) points. On the broader scale, regions of bonding attraction and repulsion can be identified using non-covalent interactions concept. The local concept is insufficient to descibe topological transition between LiH ? NaH ? KH. Although LiH and NaH belong to different topological classes, non-local approach reveales that NaH is actually transition case, with attractive H-H charge concentrations not sufficient to fullfil bonding condition. Complex structure of NCI reduced charge density gradient in ionic MgH2 with first and second neighbor bonded atoms is replaced with more pronounced directional first neighbor bonding in transition metal doped MgH2.

Resume : In the present work, we suggest on highlighting simulation study of a new structure ZnO/Si/Cu2O solar cell using Analysis of Microelectronic and Photonic Structures (AMPS-1D) computer simulator under solar spectrum AM1.5G illuminated from n-ZnO side. The inconveniences of using p-metal-oxide/n-metal-oxide (p-MO/n-MO) as a traditional PN junction in solar cells are more interface states for cell-based heterojunctions and high defects within MO layers which induce serious problems such as recombination and tunneling and hence deteriorate the cell performances. Considering widespread, robust and reliable Si materials in solar cell application, are performed to find optimized design structures based on n-ZnO and p-Cu2O MOs and crystalline p-type Si as a middle layer. The J-V characteristics as well as electric fields and quantum efficiencies (QE) of the new structure are investigated based on device design and input parameters such as band offsets, layers thickness and doping densities. Mutually optimization of the structure showed an appreciable improvement of conversion efficiency with an estimated value of ~13.4% compared to ~10.6% for n-ZnO/p-Cu2O two-layer structure. Key words: p-Si absorber, Cu2O, solar cell, AMPS-1D, J-V characteristics.

Resume : The development of efficient means of hydrogen storage is essential for the adoption of hydrogen as fuel, since it would be cleaner and more available than fossil fuels. Storage by solid state materials provides advantages over compressed gas or liquefaction, and germanium and 2D materials have proven to be good adsorbents. In this work, we study the adsorption of H2 molecules in a germanene supercell, aided by a lithium, sodium or potassium atom decorating the monolayer. The analysis of the energies was carried out from calculations within density functional theory. It was discovered from geometrical optimization and energy calculations that all systems studied are structurally stable, and alkali atoms are chemically bonded with germanene. Up to seven H2 molecules can be physisorbed in the decorated germanene. Various adsorption sites for the molecules were analyzed. The results show that the site above the center of the hexagons of the lattice is the most energetically favorable for the adsorption of a H2 molecule, in all cases. The adsorption energy increases when the H2 is adsorbed through the alkaline atoms, K being the strongest adsorbent. As the H2 molecules are adsorbed on the K atom, the adsorption energy per molecule improves. The opposite happens in the cases of Li and Na. These results may be significant in the development of hydrogen storage systems based on solid state materials. Acknowledgements: Computing resources for this work were given by LANCAD project.

Resume : Electrocatalysts are essential to two key electrochemical reactions, oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) in renewable energy conversion and storage technologies such as regenerative fuel cells and rechargeable metal-air batteries. Density functional theory calculations were used to measure the free energy profile for OER on 2D boron monolayer, metal-free material, at room temperature in this work. Moreover, to rationally search for the best catalyst of boron sheet materials, we calculated the free energy and overpotential for elementary reactions of OER for all the possible active sites on 2D boron sheet doped with p-block elements. These calculations suggest that these materials have good performance of OER, which provide to a new class of low-cost, metal-free and efficient OER catalysts.

Resume : The search for novel efficient semiconductors for water splitting, that is photoelectrochemical conversion of water into O2 and high energetic H2, is an important and still growing field of research. To be efficient such semiconductors need to fulfil several requirements: Their bandgap (Eg) must be between 1.8 and 2.2 eV for efficient solar light absorption allowing sufficient photovoltage for water oxidation and reduction. layered materials, intensively studied for thermoelectric applications, would be excellent candidates if it wasn’t for their too small bandgap (at most 1.1eV for BiCuOS). To overcome to this issue, two options will be emphasized in this communication: In one hand, the lowering of valence band, driven by CuS layer, was investigated by simulating BiAgOS compound, with DFT calculations based on the HSE06 range-separated hybrid functional with inclusion of spin-orbit coupling. Resulting structure and electronic properties, in close agreement with the experimental values, confirmed the band gap increase. In the other hand, the conduction band was elevated by modifying BiO layer. To do so we designed Bi1-xRExCuOS solid solutions (RE = Y, La, Gd and Lu) that opened the door to bandgap tuning between pure BiCuOS value (Eg~1.1 eV) to pure RECuOS value (Eg~2.9 eV), reproducing experimental evidence for similar systems. The effect of Bi/RE mixing on the band positions and width, as well as charge transport properties, will be discussed in this presentation.

Resume : Photocatalytic water splitting on semiconductor surfaces is considered a promising route to produce clean hydrogen fuel. Since the discovery of this reaction by Fujishima and Honda using TiO2 under UV irradiation, various materials have been examined as visible light photocatalysts. Perovskite structured oxides hold particular promise due to their structural and chemical flexibility. Oxynitride perovskites additionally benefit from a smaller band gap compared to pure oxides, making them more promising for photocatalytic water splitting under visible light. In this work, we compare the surface chemistry of oxides and oxynitrides by investigating the oxygen evolution reaction with four proton-coupled electrons transfer steps on NaTaO3 (113) and SrTaO2N (001) surfaces. All free energy differences between reaction intermediates (OH*, O*, OOH*) are calculated using density functional theory (DFT) following Nørskov’s approach. Initially, we consider adsorption on a single reactive site. Our results predict a similar rate-determining step (formation of OOH*) for both surfaces, with a larger overpotential for the oxide than for the oxynitride. Following this, we establish the stable adsorbate coverage of each surface under photochemical conditions (pH, U) by computing the Pourbaix diagram and study the water oxidation steps under these conditions.

Resume : Metal-semiconductor (M-Sc) junctions have been a subject of wide scientific interest for their leading applications in electronics and photovoltaics. Over the last decade, these interfaces have also been exploited as a crucial component in nanoheterostructures, specifically for photocatalytic water-splitting and its important half-reaction of hydrogen evolution (HER). These semiconductor nanostructures have two physically crucial interfaces: one with the aqueous solution, and the other with the metal end. In our earlier work on the adsorption of water molecule-CdS surface [1], we had shown interesting cation vacancy motifs, which strongly chemisorb the water molecule. In the current work, we are focusing on the CdS-metal interface, which is solely responsible for enhancing the quantum efficiency of the photocatalytic mechanism, by physically separating the photogenerated charge carriers of the semiconductor. It is at this metal deposited end, that the important reduction reaction of obtaining hydrogen gas from liquid water takes place at room temperature. In our study, we have investigated the M-Sc interfaces of CdS (a very successful HER photocatalyst) with some of the relevant co-catalyst metal depositions using spin-polarised density functional theory calculations [2]. In the sub-nanometer size regime, many of the transition and noble metal clusters show unpaired electron density, for which a spin-polarized treatment becomes essential. These M-Sc interfaces were studied within the standard GGA-PBE functional, where we have exploited the fact that GGA-PBE already gives very accurate predictions of conduction band minima, and hence is suitable for predicting n-type Schottky barrier heights of these interfaces. Depositions of 13-atom and 55-atom clusters of Ni, Pd, Pt and Au were studied on the (1010) nonpolar CdS surfaces. Although these cluster sizes are in the non-scalable cluster regime, they were often shown to give optimal catalytic yield in HER experiments. For each metal, we selected a robust set of at least five metal clusters from the literature, with a wide range of (effective) coordination numbers, reactivity and stability, to study their isolated and interfacial properties. Depending on the chemical nature of the metal deposited on the CdS surface, interesting insights on cluster relaxation, atomic inter-diffusion, adhesion energies, n-type barrier heights could be made. Using the Sabatier principle for the HER, a criterium was drawn for the quality of the studied metal co-catalyst and CdS photocatalyst interfaces. Along with this, we also studied epitaxial depositions, which are also important as they resemble the various facets of the polycrystalline metal caps deposited on these nanostructures. The CdS-Metal interfaces with epitaxial mismatches smaller than 6% were studied. With such epitaxial modelling, we investigated the most stable epitaxial interface for different metal facets determining the initial stages of their epitaxial growth. In some cases, upon deposition of the metal overlayer on the substrate, spontaneous formation of extended row reconstructions were found to be the lowest-energy states, which can be experimentally tested. The barrier heights and the dipole strengths of the epitaxial interfaces were compared with the clustered depositions. From all these simulations, it becomes clear that the electronic properties and chemical reactivity of clustered or epitaxial CdS-metal interfaces are strongly dependent on both the size and morphology of metal clusters and on the epitaxial mismatches of the various metallic overlayers. Furthermore, in some cases, distinctly different behaviour was found when comparing the various metal depositions. This extensive modelling of the M-Sc interfaces with realistic sub-nanometre cluster sizes and extended epitaxial depositions has brought new insights into the physics and chemistry of these efficient nanoscale photocatalysts. [1] S.S. Gupta and M.A. van Huis, J. Phys. Chem. C, 2017,121 (18), pp 9815–9824 [2] S.S. Gupta, M.A. van Huis (in preparation).

Resume : Solar water splitting is a promising process for green and renewable production of H2 as an energy carrier. Economic viability of this process however requires the discovery and design of materials with suitable photocatalytic properties. Oxides with the perovskite structure were shown to be potential candidates for photoelectrodes. However, their wide band gap restricts their photocatalytic activity to the UV part of the light spectrum. Substituting oxygen with less electronegative nitrogen narrows the band gap due to higher-energy nitrogen states, making oxynitrides suitable materials for solar water splitting. In addition, layered perovskites such as Ruddlesden-Popper (RP) phases are reported to have better photocatalytic properties than non-layered perovskites. This raises the question if layered oxynitrides will have an even better performance as photocatalysts than their non-layered counterparts. Although there are studies on layered bulk oxynitrides, the electronic features of their surfaces have not been studied to date. In present work, we investigate the structural and electronic properties of RP Sr2TaO3N bulk as well as its (001) and (100) surfaces by density functional theory (DFT) calculations. We focus on the electronic features of the surfaces and we show that the (001) TaON-terminated surface potentially exhibits promising photocatalytic properties due to the formation of nitrogen surface states that suppress electron-hole recombination.

Resume : Perovskite oxynitrides are a promising class of material for photocatalytic water splitting under visible light, mainly due to their small band gaps. It has been shown for different oxynitrides that epitaxial strain can engineer the anion order as well as induce ferroelectricity, opening new possibilities to reach smaller overpotentials for the overall water splitting reaction. Using density functional theory, we analyse for the case of LaTiO2N as well as SrTaO2N the relationship between the anion order and polarity. We find a correlation between the two, which is caused by the larger ionic radii of nitrogen compared to oxygen. Further, we investigate changes in bandgaps for epitaxial strained oxynitrides with different anion orders in both the polar and nonpolar phase. We find the change in bandgap with epitaxial strain for oxynitrides to be intrinsically different compared to oxides. Most marked is the difference between polar oxides and oxynitrides: While for oxides polarity can increase the band gap up to 0.1 eV in a strain range of 4%, in oxynitrides polarity can increase the band gap in the same strain range by almost 1 eV. Therefore, especially for SrTaO2N, which we find to be polar already at zero strain, suppression of ferroelectric distortions, for example by increasing temperature, might significantly decrease the band gap and lead to a large increase in the photocatalyst's efficiency.

Resume : Seeking ideal heterostructured photocatalyst components is extremely time-consuming and ineffective way because of many requirements for being good photocatalysts in heterojunctions. In this talk, computational methods to select ideal photocatalytic materials using DFT calculations will be introduced. In addition, our recent successful computational and experimental works where predictions from DFT calculation guided real materials syntheses in cost-effective way.

Resume : The Inorganic Crystal Structure Database (ICSD) is the world's largest database of fully evaluated and published inorganic crystal structure data mostly obtained from experimental results. However, the purely experimental approach is no longer the only route to discover new compounds and structures. In the past decades numerous computational methods for simulating and predicting structures of inorganic solids have emerged. On the other hand there are numerous problems with storing theoretical data, e.g. the great amount of (un)published work, discarding criterion, variety of methods/codes, no standardization, etc. Starting with the last year ICSD release, the scope of the database is extended to include theoretical structures. We would like to highlight that each theoretical structure has been carefully evaluated according to the several criteria and published in a peer review journal, while the theoretical CIF file has been upgraded and standardized, completely coherent with the experimental one. Furthermore, we show a complete systematics of theoretical methods for the first time, including additional categories (remarks) used for comparison of the experimental and theoretical data, e.g. predicted structures, excellent tool for synthesis planning, or optimized structures, excellent tool for properties investigation. Finally, we present keyword option, which in combination with previous features is another excellent tool in computational and experimental material science.

Resume : Among all the synthesized nano titania, high curvature nano-systems, e.g. nanospheres (NSs) or nanorods, are of prominent interest for TiO2 functionalization in photoapplications and biomedicine. TiO2 NSs are fundamental building blocks of technologies that operate in an aqueous environment, thus understanding their interaction with water is of extreme importance. Despite the great interest on TiO2 nanoparticles and their interaction with water, quantum chemical studies have been, so far, limited to low index TiO2 surfaces, due to the large size of realistic TiO2 NSs. However, titania flat surfaces are far to be representative of the real highly curved surface of nanospheres. We present an innovative computational approach, which allows to tackle the complexity of realistic TiO2 nanospheres (with a diameter of 2.2 nm) interacting with water multilayers, keeping a quantum chemical level of theory. First, we use an highly performing DFT-based semiempirical method, namely density functional tight-binding theory (SCC-DFTB), to obtain globally optimized NSs structure in vacuum. Second, we set up new SCC-DFTB parameters to simulate the interaction between TiO2 and water with an accuracy comparable to DFT. Finally, the static and dynamic interaction of the NSs with water is studied at both DFT (only static) and SCC-DFTB (static and dynamic) level of theory. This study gives a precious atomistic insight on the processes occuring at the TiO2 NS/water interface and on the influence of the aqueous medium on the NS structural and electronic properties.

Resume : Understanding hot carrier dynamics is crucial for the development of photovoltaic and optoelectronic devices. Electron scattering by phonons is one of the major processes that determine the relaxation dynamics iof hot carriers. Recently, we have developed a computational method, based on density functional theory and on interpolation of the electron-phonon matrix elements in Wannier space, for the calculation of the electron-phonon coupling in polar materials [1]. This method allowed us to successfully interpret the dynamics of hot electron relaxation in bulk GaAs, in excellent agreement with time- and angle- resolved photoemission experiment by the group of K. Tanimura (University of Osaka, Japan). We have demonstrated, for the relaxation of hot carriers in GaAs, the existence of two distinct relaxation regimes, one related with the momentum, and the other with energy relaxation [2]. Interestingly, the energy relaxation times become faster at lower energies. Furthermore, we will present our new results on hot electron relaxation dynamics in Ge and in InSe, which we compare to recent ARPES and transient optical spectroscopy experiments. [1] J. Sjakste, N. Vast, M. Calandra, and F. Mauri, Phys. Rev. B 92, 054307 (2015) [2] H. Tanimura, J. Kanasaki, K.Tanimura, J. Sjakste, N. Vast, M. Calandra, F. Mauri Phys. Rev. B 93, 161203(R) (2016)

Resume : Functional materials that intelligently adapt to changing environmental conditions, have long been the focus of scientific investigation because of the never-ending demand for smaller, cheaper, faster, and energy efficient devices. In this talk, I will first discuss basic concepts and mechanisms of several intelligent materials, focusing mainly on ferromagnetic shape memory alloys. I will present recent in silico studies that reveal the underlying physics behind these novel features. Thereafter, I will discuss the atomic and electronic structure of next-generation photovoltaic (PV) materials, such as hybrid-perovskites, kesterites, and all-inorganic perovskites solar cell materials. To elucidate the electronic, magnetic, optical and chemical bonding properties of these materials, we have employed first-principles density functional theory. The phononic properties (i.e. lattice dynamics) were investigated using first principles density functional perturbation theory and a direct, supercell force-constant approach. To treat anharmonic effects beyond perturbation theory, we have used our inhouse code, Self-consistent force constants (SCFCS). This talk will illustrate how theoretical results can highlight physical materials properties that might be important for the development of intelligent next generation functional materials.