• Energy Research

Conversion coating modified by alumina has been studied as a way for improving the resistance to thermal oxidation of an austenitic stainless steel. Conversion coating, characterized by a particular morphology and strong interfacial adhesion with the substrate, facilitate the electrochemical deposition of ceramic layers and enhance their adhesion to the substrate. The influence of the current density and treatment time on alumina deposit was studied using statistical experimental designs like Doehlert uniform shell design. After heating, coatings present a continuous composition gradient with refractory compounds at the surface. The behavior at high temperature (1000 °C) of the alumina coating was investigated. The presence of alumina increases the oxidation resistance of an austenitic stainless steel at 1000 °C. The morphology and the chemical composition of the deposit are analyzed. Results on the thermal stability of coating on austenitic stainless steel are presented.

Highly dispersed crystalline/amorphous LiFePO4 (LFP) nanoparticles encapsulated within hollow-structured graphitic carbon were synthesized using an in situ ultracentrifugation process.

Abstract Biomass waste recently emerged as efficient precursors for hard carbon anode preparation for Na-ion batteries. Despite their very complex microstructure (organic/inorganic) there is a lack of knowledge about their impact on carbon formation. In this paper, the influence of inorganic impurities of three raw local biomass wastes (asparagus, grape and potato) on the hard carbon properties and on their electrochemical performance is investigated, by performing a washing step either before or after the thermal treatment (TT) at 1300 °C. When washing was done after the TT (with HCl), most of crystalline inorganic impurities (K, Ca, Si, Mg - based compounds) could be significantly removed. This triggered the increase of ultramicroporosity along with mesoporosity formation and graphite interlayer space (d002) contraction. Such observations are less pronounced on grape derived carbon due to the inorganic's catalytic induced local graphitization during pyrolysis. The oxygen content in the pristine carbons was high, owing to the presence of inorganic metal oxides and carbonates, and could be diminished after washing along with the amount of defects. Thus, the carbon content and the electronic conductivity of the materials were enhanced. The electrochemical performance improvement after washing was limited since the positive effect brought by impurities removal was negatively compensated by the changes occurring in the materials, particularly the increase in the specific surface area. Diffrently, the washing done before the TT (with water) induced only fewer changes on the materials porosity and structure and slightly improved capacity (from 215 to 230 mAh g−1). Furthermore, higher pyrolysis temperature (1400 °C) on washed HCs afforded a better reversible capacity up to 280 mAh g−1. This comprehensive study opens the door for green and mild synthesis approach to be further explored for sustainable fabrication of hard carbon for Na-ion batteries.

Lithium cation insertion and extraction in LiFePO4 were electrochemically studied with a cavity microelectrode (CME). Cyclic voltammetry measurements were used to characterize the kinetics of the material. LiFePO4 was successfully cycled from 0.1 mV s–1 up to 1 V s–1 and is therefore a suitable material to be used in high power applications, such as asymmetric hybrid supercapacitors. Several kinetic behaviors were observed depending on the sweep rate. The LiFePO4 was found to follow different kinetics behaviors depending of the sweep rate. The charge storage mechanisms were investigated for Liþ extraction/insertion.

International audience ; The capacitance of the electrochemical interface has traditionally been separated into two distinct types: non-Faradaic electric double-layer capacitance, which involves charge induction, and Faradaic pseudocapacitance, which involves charge transfer. However, the electrochemical interface in most energy technologies is not planar but involves porous and layered materials that offer varying degrees of electrolyte confinement. We suggest that understanding electrosorption under confinement in porous and layered materials requires a more nuanced view of the capacitive mechanism than that at a planar interface. In particular, we consider the crucial role of the electrolyte confinement in these systems to reconcile different viewpoints on electrochemical capacitance. We propose that there is a continuum between double-layer capacitance and Faradaic intercalation that is dependent on the specific confinement microenvironment. We also discuss open questions regarding electrochemical capacitance in porous and layered materials and how these lead to opportunities for future energy technologies.

Capacitive mixing (CapMix) and capacitive deionization (CDI) are currently developed as alternatives to membrane-based processes to harvest blue energy—from salinity gradients between river and sea water—and to desalinate water—using charge-discharge cycles of capacitors. Nanoporous electrodes increase the contact area with the electrolyte and hence, in principle, also the performance of the process. However, models to design and optimize devices should be used with caution when the size of the pores becomes comparable to that of ions and water molecules. Here, we address this issue by simulating realistic capacitors based on aqueous electrolytes and nanoporous carbide-derived carbon (CDC) electrodes, accounting for both their complex structure and their polarization by the electrolyte under applied voltage. We compute the capacitance for two salt concentrations and validate our simulations by comparison with cyclic voltammetry experiments. We discuss the predictions of Debye-Hückel and Poisson-Boltzmann theories, as well as modified Donnan models, and we show that the latter can be parametrized using the molecular simulation results at high concentration. This then allows us to extrapolate the capacitance and salt adsorption capacity at lower concentrations, which cannot be simulated, finding a reasonable agreement with the experimental capacitance. We analyze the solvation of ions and their confinement within the electrodes—microscopic properties that are much more difficult to obtain experimentally than the electrochemical response but very important to understand the mechanisms at play. We finally discuss the implications of our findings for CapMix and CDI, both from the modeling point of view and from the use of CDCs in these contexts.