Manganese-cobalt geomimetic materials for supercapacitor electrode

A manganese-cobalt asbolane material synthesized by low-temperature cationic exchange from birnessite in cobalt nitrate solution has been comprehensively characterized and tested for the first time as a massive (with high active mass loading) positive electrode material for to asymmetric aqueous supercapacitors. The structure of this Mn-rich material, which is homologous to the natural asbolanes well known by mineralogists, consists of MnO2-type slabs with partial substitution of Co3+ for Mn; the slabs alternate with Co(OH)2 islands located in the interlayer spacing. This structural arrangement was confirmed through in-depth electronic transmission microscopy analyses, which reveal two interlocking hexagonal sublattices with distinct a lattice-cell parameters but identical c parameters. The electrochemical performance of this geomimetic phase in alkaline electrolytes is highly promising, with specific capacitance of up to 180 F g-1 at moderate current densities and 94 F g-1 at 10 A g-1. Investigation into the charge storage mechanisms indicates effective synergy between the pseudocapacitive properties of the MnO2 slabs and the Co(OH)2 islands, in which protonic conduction is suspected to play a key role. Additionally, long-term cycling and calendar aging tests suggest that the interlayer cobalt gradually migrates to the metal oxide layer upon cycling while maintaining excellent energy storage performance. This study clearly underscores the value of exploring geomimetic minerals as potential electrode materials for energy storage applications.

Kraft Black Liquor as a Carbonaceous Source for the Generation of Porous Monolithic Materials and Applications toward Hydrogen Adsorption and Ultrastable Supercapacitors

High internal phase emulsions (HIPEs) have templated self-standing porous carbonaceous materials (carboHIPEs) while employing Kraft Black Liquor, a paper milling industry byproduct, as a carbon precursor source. As such, the starting emulsion has been prepared through a laboratory-made homogenizer, while native materials have been characterized at various length scales either with Raman spectrometry, X-ray diffraction (XRD), mercury intrusion porosimetry, and nitrogen absorption. After thermal carbonization, specific surface areas ranging from ∼600 m2 g-1 to 1500 m2 g-1 have been reached while maintaining a monolithic character. Despite a poor graphitization yield, the carbonaceous materials offer good electronic transport properties, reaching 31 S m-1. When tested toward energy storage applications, the native unwashed materials revealed a hydrogen storage of 0.07 wt % at 40 bar and room temperature (RT), while hydrogen retention is reaching 0.37 wt % at 40 bar and RT for the washed sample. When employed as supercapacitor electrodes, these carbonaceous foams are able to deliver high capacities of ∼140 F/g at 1 A/g, thereby matching the ones obtained from a commercial carbon reference, while additionally providing a restored remnant capacity of 120 F/g at 2 A/g over 5000 cycle numbers.

Improved Electrochemical Performance for High-Voltage Spinel LiNi0.5Mn1.5O4 Modified by Supercritical Fluid Chemical Deposition

Among candidates at the positive electrode of the next generation of Li-ion technology and even beyond post Li-ion technology as all-solid-state batteries, spinel LiNi_0.5Mn_1.5O₄ (LNMO) is one of the favorites. Nevertheless, before its integration into commercial systems, challenges still remain to be tackled, especially the stabilization of interfaces with the electrolyte (liquid or solid) at high voltage. In this work, a simple, fast, and cheap process is used to prepare a homogeneous coating of Al₂O₃ type to modify the surface of the spinel LNMO: the supercritical fluid chemical deposition (SFCD) route. This process is, to the best of our knowledge, used for the first time in the battery field. Significantly improved performance was demonstrated vs those of bare LNMO, especially at high rates and for highly loaded electrodes.

Composite Mn-Co electrode materials for supercapacitors: why the precursor's morphology matters!

In the energy storage field, an electrode material must possess both good ionic and electronic conductivities to perform well, especially when high power is needed. In this context, the development of composite electrode materials combining an electrochemically active and good ionic conductor phase with an electronic conductor appears as a perfectly adapted approach to generate a synergetic effect and optimize the energy storage performance. In this work, three layered MnO₂ phases with various morphologies (veils, nanoplatelets and microplatelets) were combined with electronic conductor cobalt oxyhydroxides with different platelet sizes (∼20 nm vs. 70 nm wide), to synthesize 6 different composites by exfoliation and restacking processes. The influence of precursors' morphology on the distribution of the Mn and Co objects within the composites was carefully investigated and correlated with the electrochemical performance of the final restacked material. Overall, the best performing restacked composite was obtained by combining MnO₂ possessing a veil morphology with the smallest cobalt oxyhydroxide nanoplatelets, leading to the most homogeneous distribution of the Mn and Co objects at the nanoscale. More generally, the aim of this work is to understand how the size and morphology of the precursor building blocks influence their distribution homogeneity within the final composite and to find the most compatible building blocks to reach a homogeneous distribution at the nanoscale.

Particle nanosizing and coating with an ionic liquid: two routes to improve the transport properties of Na3V2(PO4)2FO2

Na₃V₂(PO₄)₂FO₂ is a promising candidate for practical use as a positive electrode material in Na-ion batteries thanks to its high voltage and excellent structural stability upon cycling. However, its limited intrinsic transport properties limit its performance at fast charge/discharge rates. In this work, two efficient approaches are presented to optimize the electrical conductivity of the electrode material: particle nanosizing and particle coating with an ionic liquid (IL). The former reveals that particle downsizing from micrometer to nanometer range improves the electronic conductivity by more than two orders of magnitude, which greatly improves the rate capability without affecting the capacity retention. The second approch dealing with an original surface modification by applying an IL coating strongly enhances the ionic mobility and offers new perspectives to improve the energy storage performance by designing the electrode materials' surface composition.

Controlled Nanostructuration of Cobalt Oxyhydroxide Electrode Material for Hybrid Supercapacitors

Nanostructuration is one of the most promising strategies to develop performant electrode materials for energy storage devices, such as hybrid supercapacitors. In this work, we studied the influence of precipitation medium and the use of a series of 1-alkyl-3-methylimidazolium bromide ionic liquids for the nanostructuration of β(III) cobalt oxyhydroxides. Then, the effect of the nanostructuration and the impact of the different ionic liquids used during synthesis were investigated in terms of energy storage performances. First, we demonstrated that forward precipitation, in a cobalt-rich medium, leads to smaller particles with higher specific surface areas (SSA) and an enhanced mesoporosity. Introduction of ionic liquids (ILs) in the precipitation medium further strongly increased the specific surface area and the mesoporosity to achieve well-nanostructured materials with a very high SSA of 265 m2/g and porosity of 0.43 cm3/g. Additionally, we showed that ILs used as surfactant and template also functionalize the nanomaterial surface, leading to a beneficial synergy between the highly ionic conductive IL and the cobalt oxyhydroxide, which lowers the resistance charge transfer and improves the specific capacity. The nature of the ionic liquid had an important influence on the final electrochemical properties and the best performances were reached with the ionic liquid containing the longest alkyl chain.

Monitoring the Crystal Structure and the Electrochemical Properties of Na3(VO)2(PO4)2F through Fe3+ Substitution

We here present the synthesis of a new material, Na₃(VO)Fe(PO₄)₂F₂, by the sol-gel method. Its atomic and electronic structural descriptions are determined by a combination of several diffraction and spectroscopy techniques such as synchrotron X-ray powder diffraction and synchrotron X-ray absorption spectroscopy at V and Fe K edges, ⁵⁷Fe Mössbauer, and ³¹P solid-state nuclear magnetic resonance spectroscopy. The crystal structure of this newly obtained phase is similar to that of Na₃(VO)₂(PO₄)₂F, with a random distribution of Fe³⁺ ions over vanadium sites. Even though Fe³⁺ and V⁴⁺ ions situate on the same crystallographic position, their local environment can be studied separately using ⁵⁷Fe Mössbauer and X-ray absorption spectroscopy at Fe and V K edges, respectively. The Fe³⁺ ion resides in a symmetric octahedral environment, while the octahedral site of V⁴⁺ is greatly distorted due to the presence of the vanadyl bond. No electrochemical activity of the Fe⁴⁺/Fe³⁺ redox couple is detected, at least up to 5 V, whereas the reduction of Fe³⁺ to Fe²⁺ has been observed at ∼1.5 V versus Na⁺/Na through the insertion of 0.5 Na⁺ into Na₃(VO)Fe(PO₄)₂F₂. Comparing to Na₃(VO)₂(PO₄)₂F, the electrochemical profile of Na₃(VO)Fe(PO₄)₂F₂ in the same cycling condition shows a smaller polarization which could be due to a slight improvement in Na⁺ diffusion process thanks to the presence of Fe³⁺ in the framework. Furthermore, the desodiation mechanism occurring upon charging is investigated by operando synchrotron X-ray diffraction and operando synchrotron X-ray absorption at V K edge.

Aluminum substitution for vanadium in the Na3V2(PO4)2F3 and Na3V2(PO4)2FO2 type materials

Among the positive electrode materials for Na-ion batteries, Na3V2(PO4)2F3 is considered as one of the most promising and generates high interest. Here, we study the influence of the sol-gel synthesis parameters on the structure and on the electrochemical signature of the partially substituted Na3V2-zAlz(PO4)2(F,O)3 materials. We demonstrate that the acidity of the starting solution influences the vanadium oxidation state of the final product. For the first time we report on the possibility of controlling the double Al/V and O/F substitution that leads to the preparation of the Na3V2-zAlz(PO4)2F1+zO2-z solid solution.

Ionic liquids to monitor the nano-structuration and the surface functionalization of material electrodes: a proof of concept applied to cobalt oxyhydroxide

This paper reports on an innovative and efficient approach based on the use of ionic liquids to govern the nano-structuration of HCoO₂, in order to optimize the porosity and enhance the ionic diffusion through the electrode materials. In this work, we show that (1-pentyl-3-methyl-imidazolium bromide (PMIMBr) and 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF₄)) ionic liquids (ILs) used as templates during the synthesis orientate the nanoparticle aggregation which leads to increase of the porosity and the average pore size of the electrode material. It is also demonstrated that the ILs are strongly bonded to the HCoO₂ surface, leading to surface functionalized HCoO₂ materials, also called nanohybrids. This surface tailoring stabilizes the material upon cycling and shifts the oxidation potential linked to the Co(iii)/Co(iv) redox couple to lower voltage in an alkaline 5 M KOH electrolyte. The surface and porosity optimizations facilitate the ionic diffusion through the material, improve the electron transfer ability within the electrode and lead to greatly enhanced specific capacity in both alkaline 5 M-KOH and neutral 0.5 M-K₂SO₄ aqueous electrolytes (66.7 mA h g^-1 and 47.5 mA h g^-1 respectively for HCoO₂-PMIMBr and HCoO₂-EMIMBF₄ compared to 18.1 mA h g^-1 for bare HCoO₂ in 5 M-KOH at 1 A g^-1).

Identification and optical features of the Pb4Ln2O7 series (Ln = La, Gd, Sm, Nd); genuine 2D-van der Waals oxides

We report on the identification and survey of the Pb4Ln2O7 series (Ln = La, Gd, Sm and Nd) which turn out to be real van der Waals 2D oxides. In the neutral layers, strong covalent Pb-O bonds together with external stereoactive Pb2+ lone pairs, which act as sensitizers, lead to an ideal matrix for enhanced and tunable luminescence by lanthanide emitters, tested here for Sm3+ and Eu3+ doping. DFT calculations and preliminary ex-solution experiments validate the weak bonding between the layers separated by 3.5 Å and suggest a indirect to direct crossover realized by isolating the layers.

Monitoring the solvation process and stability of Eu2+ in an ionic liquid by in situ luminescence analysis

Ionic liquids (ILs) offer the remarkable possibility of the direct synthesis of Eu2+-doped nanophosphors in solution, under atmospheric conditions, without the necessity of a high-temperature post-synthetic reduction from its trivalent oxidation state. This work uses for the first time in situ luminescence measurements for monitoring the solvation process of Eu2+ from the solid salt to the IL and its stability against oxidation under atmospheric conditions. Upon the addition of EuBr2 to 1-butyl-3-methyl-imidazolium tetrafluoroborate, the formation of the solvation shell is detected by the shift of the emission band at approximately 24 100 cm−1 assigned to the 5d→4f electronic transitions of Eu2+ within EuBr2 to approximately 22 000 cm−1, assigned to Eu2+ within BminBF4, tracking the time-dependent influence of the Eu2+ coordination environment on the crystal field splitting of its d orbitals. Even though the solubility of EuBr2 was demonstrated to be improved by reducing the concentration and increasing the temperature to 60°C, the performance of reactions at room temperature is recommended for future synthesis of Eu2+ materials in ILs due to the slight oxidation to Eu3+ observed upon heating.

The influence of ionothermal synthesis using BmimBF4 as a solvent on nanophosphor BaFBr:Eu2+ photoluminescence

Ionothermal synthesis is a strongly growing research area due to the outstanding physical and chemical properties of ionic liquids (ILs) in the design of new materials, however, the interaction of these IL molecules with optical materials and their effect on the luminescence properties of the materials are poorly known. In this work, it is shown that the imidazolium based BmimBF4 ionic liquid, used as a solvent during the synthesis, tethers at the surface of the nanoparticles and influences the photoluminescence of the nanophosphor BaFBr:Eu2+ materials. In time resolved spectra, two different emissions are observed for the material synthesized by the ionothermal approach, one corresponding to Eu(ii) 5d-4f transitions in the BaFBr host with a decay time of 843 ns and the other with a much faster decay time compared to the ionic liquid BmimBF4.

Facile Ionic Liquid-Assisted Strategy for Direct Precipitation of Eu2+ -Activated Nanophosphors under Ambient Conditions

This work describes a novel ionic liquid (IL)-assisted synthesis strategy for a direct and easy production of Eu²⁺ -doped nanoparticles (NPs), where ILs are also used as fluoride sources to avoid the use of elemental fluorine or toxic hydrofluoric acid. Up to now, the direct synthesis of Eu²⁺ -doped nanophosphors consisted of an enormous challenge, due to the oxidation to Eu³⁺ observed in hydrous solution, which is commonly used for the preparation of NPs, generating lattice defects and undesired particle growth or agglomeration by additional reducing steps at high temperatures. In contrast, ILs, unless containing ClO₄^- or NO₃^- anions, do not present an oxidizing character, allowing the direct precipitation of NPs, e.g., using Eu²⁺ containing starting materials. Here, the undoped and Eu²⁺ -doped BaFCl NPs have been prepared under atmospheric conditions for the first time using ILs as solvents and also as fluoride source, applying sonochemical and microwave-assisted approaches. In general, this method bears an enormous potential for an easy synthesis of fluoride materials compared to inconvenient solid-state methods. In addition, the IL plays the role of a strongly attached protecting shell which represents ≈7-8% of the total NPs weight.

Bonding Scheme and Optical Properties in BiM2 O2 (PO4 ) (M=Cd, Mg, Zn); Experimental and Theoretical Analysis

Luminescence properties of the Bi(M,M')₂ PO₆ (M=Mg, Zn, Cd) series have been rationalized as a function of the M element using optical spectroscopy, as well as empirical and first principles calculations. The latter yielded indirect band gaps for all compounds with energies between 2.64 and 3.62 eV, whereas luminescence measurements exhibit bright warm white emission luminescence even at room temperature assigned to Bi³⁺ transitions with, for example, 22.8 % quantum yield for M=Mg. The energies of the excitation maxima are shifted with the covalent character of the Bi-O bond by inductive effects of the neighboring M-O bonds. This is discussed on the basis of empirical and electronic structure calculations. Strikingly, in all the investigated compounds, an excitation process occurring at energies higher than the band gaps is observed, which seems to be intrinsic to the s² →sp electronic transitions of the Bi³⁺ ions. Concerning the emission process, a direct correlation between the lone pair (LP) activity and the emission energy upon change of the lattice parameters was established governing the LP stereo-activity in the BiMg_2-x Cd_x PO₆ system. As a result, the possibility for tunable optical properties appears realistic in the Bi₂ O₃ -MO-X₂ O₅ (X=P, V, As, etc.) systems taking into account the diversity of reported or novel crystal structures that can be designed using well-established rules of crystal chemistry.

Decay times of the spin-forbidden and spin-enabled transitions of Yb2+ doped in CsCaX3 and CsSrX3 (X = Cl, Br, I)

In this article, a detailed analysis of the structure–property correlation of the decay times of the spin-forbidden and spin-enabled transitions of Yb²⁺ in the halidoperovskites CsCaX₃ and CsSrX₃ (X = Cl, Br, I) is presented.