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The first real examples of luminescent solar concentrators (LSCs) based on Ir(III) cyclometalates, are described herein. Two new Ir(III) tetrazole complexes, namely [Ir(ppy)2(iQTZ-PPG)]+ and [Ir(npy)2(iQTZ-PPG)]+, where (C^N) = ppy, 2 phenylpyridine or npy = 2-(naphthalen-2-yl)pyridine and iQTZ-PPG = 1-(2-(prop-2-yn-1-yl)-2H-tetrazol-5-yl)isoquinoline, were synthesized, fully characterized and tested as phosphors for colourless LSCs. Notably, increasing quantities (0.2–1.8 wt %) of the new Ir(III) based phosphors were dispersed in different acrylate polymers like poly(methyl methacrylate), PMMA, poly(benzyl methacrylate), PBzMA and poly(cyclohexyl methacrylate), PCHMA, leading to visible transparent polymeric films exhibiting excellent photostability and bright yellow to orange phosphorescent emissions. The performances as solar collectors of all the Ir(III)-doped polymers were investigated, providing results comparable, or superior, to those obtained from colourless LSC based on organic fluorophores. In fact, the best optical efficiency (ηopt up to 7%, combined to transmittance close to 80% at 390 nm) was displayed by the polymer film obtained from physical dispersion of [Ir(ppy)2(iQTZ-PPG)]+ (1.4 wt %) in PCHMA.

Understanding the transport of sodium ions in ionic liquids is key to designing novel electrolyte materials for sodium-ion batteries. In this work, we combine molecular dynamics simulation and experiments to study how molecular interactions and local ordering affect relevant physico-chemical properties. Ionic transport and local solvation environments are investigated in electrolytes composed of sodium bis(fluorosulfonyl)imide, (Na[FSI]), in N,N-methylpropylpyrrolidinium bis(fluorosulfonyl)imide, [C3C1pyr][FSI], at different salt concentrations. The electrolyte systems are modelled by means of molecular dynamic simulations using a polarizable force field. We show that including polarization effects explicitly in the molecular simulations is required in order to attain a reliable description of the transport properties of sodium in the [C3C1pyr][FSI] electrolyte. The validation of the computational results upon comparison with experimental data allows us to assess the suitability of polarizable force fields in describing and interpreting the structure and dynamics of the sodium salt-ionic liquid system, which is essential to enable the application of IL-based electrolytes in novel energy-storage technologies.

Na-ion batteries are emerging as convenient energy-storage devices for large-scale applications. Enhanced energy density and cycling stability are key in the optimization of functional cathode materials such as P2-type layered transition metal oxides. High operating voltage can be achieved by enabling anionic reactions, but irreversibility of O2-/O2n-/O2 evolution still limits this chance, leading to extra capacity at first cycle that is not fully recovered. Here, we dissect this intriguing oxygen redox activity in Mn-deficient NaxNi0.25Mn0.68O2 from first-principles, by analyzing the formation of oxygen vacancies and dioxygen complexes at different stages of sodiation. We identify low-energy intermediates that release molecular O2 at high voltage, and we show how to improve the overall cathode stability by partial substitution of Ni with Fe. These new atomistic insights on O2 formation mechanism set solid scientific foundations for inhibition and control of this process toward the rational design of new anionic redox-active cathode materials.

Na-ion batteries (NIBs) are emerging as promising energy storage devices for large-scale applications. Great research efforts are devoted to design new effective NIB electrode materials, especially for the anode side. A hybrid 2D heterojunction with graphene and MoS2 has been recently proposed for this purpose: while MoS2 has shown good reversible capacity as a NIB anode, graphene is expected to improve conductivity and resistance to mechanical stress upon cycling. The most relevant processes for the anode are the intercalation and diffusion of the large Na ion, whose complex mechanisms are determined by the structural and electronic features of the MoS2/graphene interface. Understanding these processes and mechanisms is crucial for developing new nanoscale anodes for NIBs with high performances. To this end, here we report a state-of-the-art DFT study to address (a) the structural and electronic properties of heterointerfaces between the MoS2 monolayers and graphene, (b) the most convenient insertion sites for Na, and (c) the possible diffusion paths along the interface and the corresponding energy barrier heights. We considered two MoS2 polymorphs: 1T and 3R. Our results show that 1T-MoS2 interacts more strongly with graphene than 3R-MoS2. In both cases, the best Na host site is found at the MoS2 side of the interface, and the band structure reveals a proper n-type character of the graphene moiety, which is responsible for electronic conduction. Minimum-energy paths for Na diffusion show very low barrier heights for the 3R-MoS2/graphene interface (

An example of breaking "Ewen's symmetry rule" for olefin catalysis polymerization is proposed by DFT calculations. Catalyst precursors with Cs symmetry are suggested to promote the isotactic propene polymerization by a modification of the active site geometry obtained via coordination with AlH-alkyl species in solution. The origin of stereocontrol in olefin polymerization is due to a dual mechanism dictated by the chiral catalyst. These findings may expand the toolbox for promoting stereoselective olefin polymerization by transition metal catalysts.

Perovskite solar cells (PSCs) represent a promising technology for highly efficient sunlight harvesting and its conversion to electricity at convenient costs. However, a few flaws of current devices undermine the long-term stability of PSCs. Some of them concern the interface between the photoactive perovskite and the hole transport layer (HTL), e.g. undesired charge recombination, polarization barriers and oxidation processes. A strategy to solve this problem is to replace the standard organic HTL (e.g. Spiro-OMeTAD) with a solid-state inorganic layer. Being extensively used in p-type dye sensitized solar cells (DSSCs), nickel oxide (NiO) has been the first choice as an inorganic HTL. Despite the great interests in the application of NiO and other p-type oxides in PSCs, there is no available atomistic model of their interface with a halide perovskite. Here, we address this knowledge gap via a thorough first-principles study of the prototypical PSC perovskite methyl-ammonium lead iodide (MAPI) and two inorganic p-type oxides: NiO and CuGaO2. This copper-gallium delafossite oxide is one of the most promising alternatives to NiO in p-type DSSCs, thanks to its wide optical bandgap and low valence band edge. Here, we characterize the properties of both isolated surface slabs and MAPI/HTL heterostructure models. Besides considering MAPI/NiO and MAPI/CuGaO2 interfaces from the pristine materials, we also address the effects of intrinsic and extrinsic p-type defects in both NiO (Ni vacancy, Ni vacancy with Li and Ag doping) and CuGaO2 (Cu vacancy) using more realistic models. Our study reveals the most convenient interfaces in terms of structural affinities and adhesion energies. From the electronic perspective, we present a detailed analysis on band edge alignments, with direct insights into the key functional parameters of PSCs: hole injection driving force and open circuit potential. Our data show how the presence of defects/dopants is crucial for a convenient hole injection in NiO and CuGaO2. These results provide new science-based design principles for further development of p-type oxides in PSC devices.

Na-ion batteries (NIBs) are attracting widespread interest as a potentially more convenient alternative to current state-of-the-art Li-ion batteries (LIBs), chiefly for large-scale energy storage from renewables. Developing novel active materials is essential for the deployment of NIBs, especially in terms of negative electrodes that can accommodate the larger sodium ions. We focus on TiO2 anatase, which has been proposed as a promising anode material for the overall balance of performance, stability and cost. As the exposed crystal facets in different morphologies of nanostructured anatase can affect the electrochemical performances, here we report a theoretical investigation of Na+ adsorption and migration through (101), (100) and (001) surface terminations, thus explaining the different activities toward sodiation reported in the literature. Energy barriers computed by means of the CI-NEB method at the DFT+U level of theory show that the (001) surface is the most effective termination for Na+ insertion. We also provide a detailed analysis to elucidate that the energy barriers are due to structural modifications of the lattice upon sodiation. From these results we derive new design directions for the development of cheap and effective oxide-based nanostructured electrode materials for advanced NIBs.

The replacement of cobalt in the lattice of lithium-rich layered oxides (LRLO) is mandatory to improve their environmental benignity and reduce costs. In this study, we analyze the impact of the cobalt removal from the trigonal LRLO lattice on the structural, thermodynamic, and electronic properties of this material through density functional theory calculations. To mimic disorder in the transition metal layers, we exploited the special quasi-random structure approach on selected supercells. The cobalt removal was modeled by the simultaneous substitution with Mn/Ni, thus leading to a p-doping in the lattice. Our results show that cobalt removal induces (a) larger cell volumes, originating from expanded distances among stacked planes, (b) a parallel increase of the layer buckling, (c) an increase of the electronic disorder and of the concentration of Jahn–Teller defects, and (d) an increase of the thermodynamic stability of the phase. Overall p-doping appears as a balanced strategy to remove cobalt from LRLO without massively deteriorating the structural integrity and the electronic properties of LRLO.

Titanium dioxide (TiO2) has been extensively studied as a suitable material for a wide range of fields including catalysis and sensing. For example, TiO2-based nanoparticles are active in the catalytic conversion of glucose into value-added chemicals, while the good biocompatibility of titania allows for its application in innovative biosensing devices for glucose detection. A key process for efficient and selective biosensors and catalysts is the interaction and binding mode between the analyte and the sensor/catalyst surface. The relevant features regard both the molecular recognition event and its effects on the nanoparticle electronic structure. In this work, we address both these features by combining two first-principles methods based on periodic boundary conditions and cluster approaches (CAs). While the former allows for the investigation of extended materials and surfaces, CAs focus only on a local region of the surface but allow for using hybrid functionals with low computational cost, leading to a highly accurate description of electronic properties. Moreover, the CA is suitable for the study of reaction mechanisms and charged systems, which can be cumbersome with PBC. Here, a direct and detailed comparison of the two computational methodologies is applied for the investigation of d-glucose on the TiO2 (100) anatase surface. As an alternative to the commonly used PBC calculations, the CA is successfully exploited to characterize the formation of surface and subsurface oxygen vacancies and to determine their decisive role in d-glucose adsorption. The results of such direct comparison allow for the selection of an efficient, finite-size structural model that is suitable for future investigations of biosensor electrocatalytic processes and biomass conversion catalysis.

In this contribution, we demonstrate the presence of high-spin Fe3+ in Fe-substituted ZrO2 (FexZr1−xO2−δ), as deduced from X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure (NEXAFS), and 57Fe Mössbauer spectroscopy measurements. The activity of this carbon-supported FexZr1−xO2−δ catalyst toward the oxygen reduction reaction (ORR) was examined by both rotating (ring) disk electrode (R(R)DE) method and single-cell proton exchange membrane fuel cells (PEMFCs). DFT calculations suggest that the much higher ORR mass activity of FexZr1−xO2−δ compared to Fe-free ZrO2 is due to the enhanced formation of oxygen vacancies: their formation is favored after Zr4+ substitution with Fe3+ and the oxygen vacancies create potential adsorption sites, which act as active centers for the ORR. H2O and/or H2O2 production observed in RRDE measurements for the Fe0.07Zr0.93O1.97 is also in agreement with the most likely reaction paths from DFT calculations. In addition, Tafel and Arrhenius analyses are performed on Fe0.07Zr0.93O1.97 using both RRDE and PEMFC data at various temperatures.

Photocatalysis based technologies have a key role in addressing important challenges of the ecological transition, such as environment remediation and conversion of renewable energies. Photocatalysts can in fact be used in hydrogen (H2) production (e.g., via water splitting or photo-reforming of organic substrates), CO2 reduction, pollution mitigation and water or air remediation via oxidation (photodegradation) of pollutants. Titanium dioxide (TiO2) is a “benchmark” photocatalyst, thanks to many favorable characteristics. We here review the basic knowledge on the charge carrier processes that define the optical and photophysical properties of intrinsic TiO2. We describe the main characteristics and advantages of TiO2 as photocatalyst, followed by a summary of historical facts about its application. Next, the dynamics of photogenerated electrons and holes is reviewed, including energy levels and trapping states, charge separation and charge recombination. A section on optical absorption and optical properties follows, including a discussion on TiO2 photoluminescence and on the effect of molecular oxygen (O2) on radiative recombination. We next summarize the elementary photocatalytic processes in aqueous solution, including the photogeneration of reactive oxygen species (ROS) and the hydrogen evolution reaction. We pinpoint the TiO2 limitations and possible ways to overcome them by discussing some of the “hottest” research trends toward solar hydrogen production, which are classified in two categories: (1) approaches based on the use of engineered TiO2 without any cocatalysts. Discussed topics are highly-reduced “black TiO2”, grey and colored TiO2, surface-engineered anatase nanocrystals; (2) strategies based on heterojunction photocatalysts, where TiO2 is electronically coupled with a different material acting as cocatalyst or as sensitizer. Examples discussed include TiO2 composites or heterostructures with metals (e.g., Pt-TiO2, Au-TiO2), with other metal oxides (e.g., Cu2O, NiO, etc.), direct Z-scheme heterojunctions with g-C3N4 (graphitic carbon nitride) and dye-sensitized TiO2.

In this manuscript, we report a detailed physico-chemical comparison between the α- and β-polymorphs of the NaMnO2 compound, a promising material for application in positive electrodes for secondary aprotic sodium batteries. In particular, the structure and vibrational properties, as well as electrochemical performance in sodium batteries, are compared to highlight differences and similarities. We exploit both laboratory techniques (Raman spectroscopy, electrochemical methods) and synchrotron radiation experiments (Fast-Fourier Transform Infrared spectroscopy, and X-ray diffraction). Notably the vibrational spectra of these phases are here reported for the first time in the literature as well as the detailed structural analysis from diffraction data. DFT+U calculations predict both phases to have similar electronic features, with structural parameters consistent with the experimental counterparts. The experimental evidence of antisite defects in the beta-phase between sodium and manganese ions is noticeable. Both polymorphs have been also tested in aprotic batteries by comparing the impact of different liquid electrolytes on the ability to de-intercalated/intercalate sodium ions. Overall, the monoclinic α-NaMnO2 shows larger reversible capacity exceeding 175 mAhg−1 at 10 mAg−1.

We present a first-principles study on the structure-property relationships in MoS2 and WS2 monolayers and their vertically stacked hetero-bilayer, with and without Sulfur vacancies, in order to dissect the electronic features behind their photocatalytic water splitting capabilities. We also benchmark the accuracy of three different exchange-correlation density functionals for both minimum-energy geometries and electronic structure. The best compromise between computational cost and qualitative accuracy is achieved with the HSE06 density functional on top of Perdew-Burke-Ernzerhof minima, including dispersion with Grimme's D3 scheme. This computational approach predicts the presence of mid-gap states for defective monolayers, in accordance with the present literature. For the heterojunction, we find unexpected vacancy-position dependent electronic features: the location of the defects leads either to mid-gap trap states, detrimental for photocatalyst or to a modification of characteristic type II band alignment behavior, responsible for interlayer charge separation and low recombination rates.

The recycling of CO2 back into chemicals via photo-electrochemical cells represents a viable route to mitigate the global climate crisis of current days. In this paper we focus on the copper-iron delafossite oxide, CuFeO2, which has been proven to have suitable physico-chemical properties to photo-catalyze the carbon dioxide reduction to formic acid. The specific electronic and structural features that allow activating the highly stable CO2 molecule are dissected by exploiting state-of-the art first principles methods. The effects of surface oxygen vacancies as well as the different roles of copper and iron on the adsorption and the activation of carbon dioxide at the CuFeO2 most stable surface are analyzed. These results highlight the key role of oxygen defects in favoring the CO2 adsorption and in promoting the formation of a radical anion CO2 − or a carbonate-like adsorbate so providing the scientific grounds for implementing new rational design strategies and improving the performances of CuFeO2-based photoelectrodes for CO2 reduction.

In this study we address the Li-ion de-insertion/insertion mechanisms from/into the lattice of the mixed olivine LiCo1/3Fe1/3Mn1/3PO4 (LCFMP). This mechanism is driven by a subtle interplay of structural, electronic and thermodynamic features. We aim at dissecting this complex landscape that is tightly connected to the long-term electrochemical performance of this material as a positive electrode in lithium-ion cells. To this end, we report advanced structural characterization, based on ex situ synchrotronradiation diffraction on samples at different lithium contents. We couple this analysis with first-principles simulations, for a direct vis-a-vis comparison. Our results show that (1) the mixing of the three transition-metal (TM) cations in the olivine lattice leads to a solid solution, providing the olivine lattice with the necessary flexibility to retain its single-phase structure during cell operation; (2) the electronic features of the three TMs are responsible for the observed electrochemical performance; (3) the de-lithiation of the olivine lattice is a thermodynamically driven process. Last but not least, our integrated experimental and theoretical results reveal the subtle features behind the formation of antisite defects that selectively involve Li–Co couples. In conclusion, our study provides the necessary scientific foundations to understand the structure–property–function relationships in LCFMP olivines, paving the way for further development and optimization of this material for application in Li-ion batteries.

Transition-metal (TM) layered oxides have been attracting enormous interests in recent decades because of their excellent functional properties as positive electrode materials in lithium-ion batteries. In particular LiCoO2 (LCO), LiNiO2 (LNO) and LiMnO2 (LMO) are the structural prototypes of a large family of complex compounds with similar layered structures incorporating mixtures of transition metals. Here, we present a comparative study on the phase stability of LCO, LMO and LNO by means of first-principles calculations, considering three different lattices for all oxides, i.e., rhombohedral (hR12), monoclinic (mC8) and orthorhombic (oP8). We provide a detailed analysis&mdash, at the same level of theory&mdash, on geometry, electronic and magnetic structures for all the three systems in their competitive structural arrangements. In particular, we report the thermodynamics of formation for all ground state and metastable phases of the three compounds for the first time. The final Gibbs Energy of Formation values at 298 K from elements are: LCO(hR12) &minus, 672 &plusmn, 8 kJ mol&minus, 1, LCO(mC8) &minus, 655 &plusmn, LCO(oP8) &minus, 607 &plusmn, LNO(hR12) &minus, 548 &plusmn, LNO(mC8) &minus, 557 &plusmn, LNO(oP8) &minus, LMO(hR12) &minus, 765 &plusmn, 10 kJ mol&minus, LMO(mC8) &minus, 779 &plusmn, LMO(oP8) &minus, 780 &plusmn, 1. These values are of fundamental importance for the implementation of reliable multi-phase thermodynamic modelling of complex multi-TM layered oxide systems and for the understanding of thermodynamically driven structural phase degradations in real applications such as lithium-ion batteries.

International audience; Dye-sensitized photoelectrochemical cells form an emerging technology for the large-scale storage of solar energy in the form of (solar) fuels because of the low cost and easy processing of their constitutive photoelectrode materials. Such hybrid photoelectrodes consist of molecular dyes grafted onto transparent semiconducting metal oxides in combination with catalytic centers. The optimization of the performances of such hybrid photoelectrodes requires a detailed understanding of the light-driven electron transfer processes occurring first at the interface between the semiconducting material and the dye and then between the dye and the catalytic center. Here we address the first of these issues and use quantum chemistry to determine the structural and electronic features of the interfaces between a push-pull dye and the p-NiO (100) surface. We show that these calculations are in good agreement with transient absorption spectroscopic measurements on a prototypical dye-sensitized photocathode system able to evolve hydrogen in the presence of a cobaloxime catalyst in solution.

International audience; The development of NiO-based molecular photocathodes is attracting growing interest in thefield of dye-sensitized photoelectrochemical cells (DS-PEC) for efficient conversion of sunlightinto fuel. For this purpose different strategies are developed to assemble the molecularcomponents together in order to build functional devices. Here, an original dye-catalystsupramolecular assembly was designed and obtained via axial coordination of a cobalt-basedH2-evolving catalyst, i.e. a cobaloxime complex, to a pyridyl-functionalized rutheniumdiacetylide photosensitizer. The new supramolecular assembly was successfully employed forthe construction of efficient NiO-based photocathodes for solar hydrogen production. We reporta joint experimental and theoretical study of the new photocatalytic system, includingelectrochemical and XPS analyses. Photo-electrochemical generation of H2 under pertinentaqueous conditions eventually led to a faradaic efficiency of 27 %

In the quest for a sustainable society, energy storage technology is destined to play a central role in the future energy landscape. Breakthroughs in materials and methods involving sustainable resources are crucial to protect humankind from the most serious consequences of climate change. Rechargeable batteries of all forms will be required to follow the path. Elements that are eligible to harmonically contribute to the development of a sustainable ecosystem and fulfil the demands of high energy density batteries include Na, K, Ca, Mg, Zn, and Al. Numerous research efforts are underway to explore new battery chemistries based on these elements and, depending on the field of application, different elements inherit different advantages and challenges. Full sustainability implies that the environmental friendliness of these systems must be characterized by a “cradle-to-grave” approach. In this context, the pursuit of global environmental and economical sustainability from mass production, raw materials, and technical challenges is discussed herein for the most recent battery concepts based on monovalent and multivalent metal anodes. A perspective on strategies and opportunities particularly around the development of all-solid-state system configurations is provided, and the most important obstacles to overcome in search of a more sustainable future for electrochemical energy storage are addressed.