Lionel Roué

Silicon Composite Electrode with High Capacity and Long Cycle Life

A nanosilicon-based composite electrode that can achieve more than 700 cycles at a high capacity of 960 mAh/g of electrode was prepared using aqueous processing in an acidic medium. The buffering of the aqueous solution is mandatory to promote covalent bonding between Si particles and the carboxymethyl cellulose (CMC) binder. The latter is claimed to allow the formation of mechanically stronger contacts within the composite electrode in addition to the CMC bridging of the Si and carbon black particles.

A low-cost and high performance ball-milled Si-based negative electrode for high-energy Li-ion batteries

A Si-based anode with improved performance can be achieved using high-energy ball-milling as a cheap and easy process to produce Si powders prepared from a coarse-grained material. Ball-milled powders present all the advantages of nanometric Si powders, but not the drawbacks. Milled powders are nanostructured with micrometric agglomerates (median size ∼10 μm), made of submicrometric cold-welded particles with a crystallite size of ∼10 nm. The micrometric particle size provides handling and non-toxicity advantages compared to nanometric powders, as well as four times higher tap density. The nanostructuration is assumed to provide a shortened Li+ diffusion path, a fast Li+ diffusion path along grain boundaries and a smoother phase transition upon cycling. Compared to non-milled 1–5 μm powders, the improved performance of nanostructured milled Si powders is linked to a strong lowering of particle disconnection at each charge, while the irreversibility due to SEI formation remains unchanged. An electrode prepared in acidic conditions with the CMC binder achieves 600 cycles at more than 1170 mA h per gram of the milled Si-based electrode, in an electrolyte containing FEC/VC SEI-forming additives, with a coulombic efficiency above 99%, compared to less than 100 cycles at the same capacity for an electrode containing nanometric Si powder.

Nitrate removal by a paired electrolysis on copper and Ti/IrO2 coupled electrodes - Influence of the anode/cathode surface area ratio

In this study, nitrate removal in alkaline media by a paired electrolysis with copper cathode and Ti/IrO(2) anode enabled the conversion of nitrate to nitrogen. Optimum conditions for carrying out reduction of nitrate to ammonia and subsequent oxidation of the produced ammonia to nitrogen were found. At the copper cathode, electroreduction of nitrate to ammonia was optimal near -1.4 V vs Hg/HgO. At the Ti/IrO(2) anode, a pH value of 12, the presence of chloride and a potential fixed around 2.3 V vs Hg/HgO permitted the production of hypochlorite, leading to the oxidation of ammonia to nitrogen with a N(2) selectivity of 100%. Controlling the cathode/anode surface area ratio, and thus the current density, appeared to be a very efficient way of shifting electrode potentials to optimal values, consequently favoring the conversion of nitrate to nitrogen during a paired galvanostatic electrolysis. A cathode/anode surface area ratio of 2.25 was shown to be the most efficient to convert nitrate to nitrogen.

Electrochemically Activated Copper Electrodes Surface Characterization, Electrochemical Behavior, and Properties for the Electroreduction of Nitrate

A polycrystalline copper electrode was activated by creating a nanostructured and highly electrocatalytic surface through an appropriate electrochemical treatment in 1 M NaOH. It was demonstrated that a thick layer (∼2 μm) of Cu(OH) 2 nanoneedles can be formed on copper substrate after 3000 cycles (scan rate 10 V s -1 ) between -1650 and 1000 mV vs Hg/HgO or by anodization for 15 min at -100 mV. Energy dispersive X-ray analysis, X-ray diffraction, X-ray photoelectron spectroscopy, and in situ Raman spectroscopy analyses revealed that the conversion of orthorhombic Cu(OH) 2 to face-centered-cubic Cu is completed after 20 cycles between -450 and -1650 mV at 20 mV s -1 . However, copper obtained from the reduction of the thickest Cu(OH) 2 nanoneedle films formed by a repetitive fast cycling or by anodization at -100 mV appeared as nanowires, whereas the electrode anodized at 0 or 700 mV recovered a quasi-smooth original copper surface. The presence of high-energy sites on these Cu nanostructures was highlighted by cyclic voltammetry in the pseudo-capacitive potential region, where premonolayer oxidation was observed with an unusually high magnitude at unusually low potentials. As a result, a remarkable improvement of the electrocatalytic activity of the activated Cu electrodes for the nitrate electroreduction was observed.

Optimization of the cathode material for nitrate removal by a paired electrolysis process.

Ni, Cu, Cu(90)Ni(10) and Cu(70)Ni(30) were evaluated as cathode materials for the conversion of nitrate to nitrogen by a paired electrolysis process using an undivided flow-through electrolyzer. Firstly, corrosion measurements revealed that Ni and Cu(70)Ni(30) electrodes have a much better corrosion resistance than Cu and Cu(90)Ni(10) in the presence of chloride, nitrate and ammonia. Secondly, nitrate electroreduction experiments showed that the cupro-nickel electrodes are the most efficient for reducing nitrate to ammonia with a selectivity of 100%. Finally, paired electrolysis experiments confirmed the efficiency of Cu(70)Ni(30) and Cu(90)Ni(10) cathodes for the conversion of nitrate to nitrogen. During a typical electrolysis, the concentration of nitrate varied from 620ppm to less than 50ppm NO(3)(-) with an N(2) selectivity of 100% and a mean energy consumption of 20kWh/kg NO(3)(-) (compared to ∼35 and ∼220kWh/kg NO(3)(-) with Cu and Ni cathodes, respectively).

Influence of carbon on the electrode characteristics of MgNi prepared by mechanical alloying

Abstract The influence of carbon addition on the characteristics of MgNi alloy prepared by mechanical alloying and used as metal hydride electrode has been studied. Our results indicate that, despite its low proportion, carbon addition has a major deleterious effect on the electrode performance, i.e. the initial discharge capacity decreases exponentially from 522 to 332 mA h g −1 with carbon content increasing from 0 to 3.5 wt.%. This exponential decay cannot be only explained by a decrease in the H solubility due to the dissolution of C atoms in the hydrogen interstitial sites. The cycle life of the electrode is not influenced by the carbon content. The particle morphology and the amorphous structure of the material are unmodified by the carbon addition. In addition, our results indicate that the hydrogen diffusivity in MgNi is unchanged by carbon addition ( D H / a 2 =3.2×10 −5 s −1 ). In contrast, the exchange current density I 0 decreases from 92 to 62 mA g −1 with increasing carbon content in the alloy. This results indicate that carbon limits the charge-transfer reaction at the surface of the alloy, which must have an influence on the initial discharge capacity of the alloy. In addition, the electrochemical PCT curves reveal that the slopes of the isotherms become steep with C addition indicating a wider distribution of energy levels for hydrogen. This is considered as the major reason for the observed decrease in the initial discharge capacity of the MgNiC x alloys with increasing x .

XPS surface study of nanocrystalline Ti–Ru–Fe materials

Abstract The surface properties of Ti:Ru:Fe (2− x :1+ x /2:1+ x /2) (with x =0, 0.25, 0.5, 0.75, and 1) and Ti:Ru:Fe:O (2:1:1: w ) (with w =0.0, 0.5, 1.0, 1.5, and 2.0) have been determined by X-ray photoelectron spectroscopy (XPS) in both their as-milled state and after being in contact with a chlorate oxidizing solution. The O surface concentrations of both sets of samples are almost identical, indicating that the O-free samples readily react with air. All samples in their as-milled state have an elemental Ti, Ru and Fe surface contents that closely follow that expected from their bulk composition, indicating that there is no surface enrichment in any of the elements. In the as-milled state, more than 90% of Ti and Fe atoms are in the highest possible oxidation state, while Ru is in the metallic state. Following immersion of the samples in an oxidizing chlorate electrolyte, the Ru surface concentration decreases by a factor of two. This is also accompanied by an increase in the oxidation state of the Ru atoms left at the surface from 0 to +4. From a comparison between the Ru 3 d 5/2,3/2 core level spectra of the electrodes with those of crystalline and hydrated RuO 2 , it is postulated that dissolution and re-deposition of Ru in the form of hydrated RuO 2 occurs at open circuit potential in the chlorate electrolyte. The consequences of these findings for the electrocatalytic activity of the electrodes in chlorate electrolyte are finally discussed.

Electrochemical Reduction of Nitrate on Pyrolytic Graphite-Supported Cu and Pd–Cu Electrocatalysts

The electrochemical reduction of nitrate and nitrite on Cu- and Pd-Cu-modified pyrolytic graphite electrodes was studied in neutral and alkaline media. The beneficial effect of Pd on the intrinsic electrocatalytic performance of the modified graphite electrode was demonstrated. In fact, the presence of Pd on the catalytic surface causes a positive potential shift in the onset of nitrate reduction current and a remarkable increase of the faradaic current. The activation energy of the nitrate electroreduction was estimated for the Cu/graphite and Pd-Cu/graphite electrodes at about 32 and 18 kJ mol -1 , respectively. The reaction rate constants were equal to 0.75 X 10 -4 and 2.23 X 10 -4 s -1 for the Cu and Pd-Cu surface, respectively, reflecting a more facile electron-transfer process on the latter surface. The beneficial effect of Pd on the catalytic stability is probably related to the existence of a protective Pd layer enveloping the Cu core in the electrode surface structure. The selectivity of the modified electrodes depends on the electrolysis time and potential. In neutral medium, the nitrate electroreduction mainly led to the formation of nitrite at -1.1 V and ammonia at -1.5 V vs Ag/AgCl. The reaction selectivity was also studied in alkaline medium at -1.1 V for the Pd-Cu/graphite electrode only. In this case, the maximal selectivity for the production of N 2 was 70% and was achieved with a surface composition of 95 atom % Pd-5 atom % Cu.

Nanostructured Palladium Thin Films Prepared by Pulsed Laser Deposition Structural Characterizations and Hydrogen Electrosorption Properties

Nanostructured palladium thin films <30 nm thick were prepared by pulsed laser deposition. X-ray diffraction investigations show that the Pd films are plastically deformed. Lattice strains are anisotropic and induced by the film growth mechanism as well as by the anisotropy of Youngs modulus of palladium. In addition, a lattice contraction is observed near the surface. X-ray photoelectron spectroscopic analyses display a distinctive photoemission assigned to a less-ordered phase in addition to the common crystalline phase, which could be related to the grain boundaries. As a result, the electrochemical behavior of such Pd films clearly differs from that of common coarse-grained Pd. H-sorption and Pd oxide formation/removal processes greatly influence the subsequent film hydriding behavior, suggesting the occurrence of structural rearrangement/relaxation in the film. Hydride formation is favored when the film thickness decreases, which reflects the major role played by the subsurface layer on the hydriding process. The Pd films have a good stability upon hydrogen charge/discharge cycling which may be related to the lack of an abrupt α-to-β phase transition.

Electrolytic Production of Aluminum Using Mechanically Alloyed Cu–Al–Ni–Fe-Based Materials as Inert Anodes

Cu 92-x Al x Ni 5 Fe 3 materials with x varying from 0 to 20 (wt %) were prepared by mechanical alloying. For x ≤ 10, the as-milled Cu 92-x Al x Ni 5 Fe 3 was made of an α-phase, whereas γ-Cu 9 Al 4 was formed at x = 20. Upon consolidation, a small amount of κ Ni/Fe-rich Al phase with a B2 structure is formed for x = 6 and 10, whereas no new phase is formed in the other compounds. Aluminum electrolysis tests conducted at an anode current density of 0.5 A cm -2 for 20 h in a low temperature (700°C) KF-AlF 3 electrolyte showed that the electrode stability and aluminum purity are strongly dependent on the alloy composition. The lowest values of cell voltage (4.1 V) and Cu contamination (0.8 wt %) were obtained for x = 10 wt %. This relatively lower contamination is due to the formation of a dense and adherent CuAl 2 O 4 oxide scale between the outermost Cu 2 O oxide layer and the substrate. In comparison, a hot-rolled C63000 commercially available alloy with the same composition but lower chemical/ microstructural homogeneity gave a higher Cu contamination level (1.4 wt %).