From irreversible transformation of VO2 to V2O5 electrochromic films

Since its discovery, electrochromism, known as a modulation of the optical properties under an applied voltage, has attracted strong interest from the scientific community and has proved to be of significant utility in various applications. Although vanadium dioxide (VO2) has been a candidate of extensive research for its thermochromic properties, its intrinsic electrochromism has scarcely been reported so far. In this study, multi-electrochromism is described from VO2 thick films. Indeed, VO2 opaque film, doctor bladed from homemade monoclinic VO2 powder, shows a pronounced color modulation from orange to green and blue associated with an amorphization-recrystallization phenomenon upon cycling in lithium-based electrolyte. The strong memory effect allows to follow the coloration mechanism by combining various ex-situ and in-situ characterizations addressing both structural and electronic aspects. Upon cycling the multi-chromism of VO2 finds its origin in a transformation of VO2 into orange-V2O5 on oxidation while in reduction the blue color lithiated state illustrates a mixed vanadium oxidation state

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 massive positive electrode material dedicated to asymmetric aqueous supercapacitors. The structure of this Mn-rich material, homologous of natural asbolanes well known from mineralogists, consists of MnO2 type slabs, with partial substitution of Co 3+ for Mn, alternating with Co(OH)2 islands located in the interlayer spacing. This structural arrangement has been confirmed through in-depth electronic transmission microscopy analyses, revealing two interlocking hexagonal sublattices with distinct a lattice cell parameter but an identical c parameter. The electrochemical performance of this geomimetic phase exhibit great promise in alkaline electrolytes, with specific capacitance up to 180 F/g at moderate current densities and 94 F/g at 10 A/g. Investigation into the charge storage mechanisms puts in evidence an effective synergy between the pseudocapacitive properties of the MnO2 slabs and the Co(OH)2 islands, in which the protonic conduction is suspected to bring a key role. Additionally, long-term cycling and calendar aging tests suggest that the interlayer cobalt gradually migrates upon cycling to the metal oxide layer while maintainng excellent energy storage performance. The study clearly underscores the value of exploring geomimetic minerals as potential electrode materials for energy storage applications

Induced circular dichroism from helicoidal nano substrates to porphyrins: The role of chiral self-assembly

International audienceBecause the combination of chiral and magnetic properties is becoming more and more attractive for magneto-chiral phenomena, we here aim at exploring the induction of chirality to achiral magnetic molecules as a strategy for the preparation of magneto-chiral objects. To this end, we have associated free baseand metallo-porphyrins with silica nano helices, using a variety of elaboration methods and studied them mainly by electronic natural circular dichroism (NCD) and magnetic circular dichroism (MCD) spectroscopies. While electrostatic or covalent surface grafting uniformly yielded very low induced CD (ICD) for the four assayed porphyrins, a moderate response was observed when the porphyrins were incorporated into the interior of the double-walled helices, likely due to the association of the molecules with the chirally-organized gemini surfactant in the helices' interior. A generally stronger, but more variable, ICD was observed when the molecules were drop casted onto the helices immobilised on a quartz plate, likely due to the different capacities of the porphyrins to aggregate into chiral assemblies. Electronic spectroscopy, electron microscopy and IR spectroscopy were used to interpret the patterns of aggregation and their influence on ICD and MCD. No enhancement of MCD was observed as a result of association with the nanohelices except in the case of the free base, 5,10,15,20-tetra-(4-sulfonatophenyl)porphyrin (TPPS). This nanocomposite demonstrated a large ICD in the Soret region and a large MCD in the Q-region due to J-aggregation. However, no induced MChD was observed, possibly due to the spectral mismatch between the ICD and MCD peaks