R. Messina

Electrochemical behaviour of a silver-exchanged “mordenite”-type zeolite

Abstract The electrochemical behaviour of a silver-exchanged “mordenite”-type zeolite (ZO − Ag + ) has been investigated. It has been found that Ag + ions could be electrochemically reduced in aqueous solutions containing various supporting electrolytes according to ZO − Ag + +M′ + + e − →ZO − M′ + +Ag(0) where M′ + represents the cation of the supporting electrolyte; M′ + =Li + , Na + , K + , Cs + , H + , NH 4 + , etc… Such a reaction requires the transfer of silver ions from the zeolite lattice towards the current collector and simultaneously transfer of M′ + cations from external solution into the zeolite lattice. Two kinds of silver metal particles can be formed: crystallites or dendrites on the current collector which can subsequently be reoxidized, and some small particles which can migrate back into the zeolite. The conditions to obtain dispersed silver deposit in zeolite have been discussed and determined. Hence, electrochemical preparation of some zeolite-supported metal catalysts can be anticipated. The diffusion coefficient of silver ions D Ag + has been calculated from a chronoamperometric study performed in 1 M LiClO 4 .

Mechanism of electrochemical reduction of vanadium oxides

Abstract We specify the different electrochemical processes occurring when V2O5 is electrochemically reduced, yielding insertion products with lithium. Under low current density, V2O5 is of a ternary phase of approximate stoichiometry, V2O5Li0.5. During the second step a further reversible insertion of Li+ occurs, yielding V2O5Li without any important modification of the crystalline structure, thus making the reduction reversible. During the two last steps, Li+ incorporation is much more difficult and rapidly causes an important and irreversible modification of the crystalline structure, thus making the reduction irreversible. V2O3, has nearly the same faradaic capacity as V2O5 but, unlike V2O5, it can be hardly be used in batteries since its discharge occurs in a wide potential range.

Electrochemical behaviour of some transition metal oxides in molten dimethylsulphone at 150°C

In view of the possible application in non-aqueous líthium cells operating at relatively high temperatures, molten dimethylsulphone (DMSO2) has been used as the electrolyte solvent in lithium cells at 150°C. The stability of lithium in molten DMSO2 has been found to be good as compared with that observed in organic solvents such as propylene carbonate, thus indicating that the Li+/Li system can be used as a suitable reference electrode in this medium.The electrochemical behaviour of some transition metal oxides has been investigated in LIClO4 solutions in molten DMSO2. The results obtained from voltammetric and chronopotentiometric measurements have shown a satisfactory behaviour for all the cathodic materials tested. Moreover, electrochemical insertion of Li+ ions into the crystal lattice of these oxides is a very fast process. Thus molten DMSO2 appears to be a very interesting organic solvent usable in high energy density non-aqueous lithium cells.

Electrochemical reduction mechanism of silver chromate in lithium cells

A general mechanism is proposed to explain the charge—discharge features of the LiAg2CrO4 system. For a low discharge rate, the process leads to Ag and Li2CrO4 formation through a dissolution—precipitation, mechanism. Li2 CrO4 is reducible in two steps at lower potentials. For high discharge we show the formation of an intermediate compound, called Ag 2, Li2CrO4 which evolves either to a mixture of metallic Ag and Li2CrO4 or is reduced with the formation of a Cr(V) compound at higher potential than Li2CrO4. This mechanism obtained with experimental electrodes is in excellent agreement with the behaviour of Ag2CrO4 commercial cells.

Electrochemical formation of LiAl alloy in molten dimethylsulfone at 150°C

Abstract The electrochemical behaviour of the Li+/Li couple is examined briefly in molten dimethylsulfone (DMSO2) at 150°C. It has been found that the Li+/Li system is reversible. However, electrodeposited lithium exhibits some instability which severely limits the use of the lithium anode in molten DMSO2 based secondary cells. Therefore we have undertaken the study of the electroformation of the LiAl alloy in 1 mol kg−1 LiClO4/DMSO2 at 150°C by investigating the electrochemical incorporation of lithium into an aluminium electrode by potentiostatic and galvanostatic techniques. The results obtained from electrochemical measurements and X-ray diffraction experiments have proved that incorporation of Li in Al in molten DMSO2 leads to the formation of the β LiAl alloy. Analysis of the chronocoulometric curves has allowed the different processes limiting the rate of incorporation of Li and Al to be specified. Moreover, the diffusion coefficient of lithium into aluminium has been determined from chronoamperometric measurements: 0.7 × 10−10 ⪕ DLi(α) ⪕ 1.4 × 10−10 cm2 s−1. Finally, the galvanostatic study has shown that the β LiAl alloy can be considered a promising anodic material for molten dimethylsulfone-based secondary batteries.

Electrochemical preparation of metallic ions/zeolite catalysts

The electrochemical preparation of metallic ions zeolite catalysts (ZOx− Mx+) is achieved by electroreduction of solid ZO−-Ag+ in aqueous solutions according to: xZO−Ag+ + xe +Mx+ → ZO−xMx+ + xAg° where Mx+ is the metallic cation of supporting electrolyte (Mx+ = Ni2+, Cu2+, etc…). Some thermodynamic aspects of ion exchange between Ag+ and Mx+ are studied, via electrochemistry, for mordenite and faujasite type zeolites. Electroanalytical studies of AglAgo, CullCul and CulCuo zeolite systems in alkaline and ammoniacal solutions allow the determination of ions concentration within zeolites.

Electrochemical study of copper-zinc oxide catalysts

Abstract Cu-Zn oxide catalysts are well known for methanol synthesis. In order to provide further information on the properties of these oxides, we have investigated their electrochemical behaviour, since they contain reducible species. Electrochemical study has been carried out in 1M LiClO 4 -propylene carbonate at room temperature. We observed that Cu(II) electroreduction occurs either at 2.2 V or at 1.2 V against a Li electrode. We have shown that ZnO promotes Cu(II) electroreduction at 2.2 V with copper formation while the 1.2 V process corresponds to CuO electroreduction as occurs in CuO/Li batteries. The addition of aluminium in Cu-Zn oxides enhances the first process at 2.2 V. Zn(II) electroreduction occurs at 0.8 V i.e. after the complete electroreduction of copper. Subsequent to copper formation at 2.2 V, electroreduction of residual water occurs between 1.2 and 1.8 V. Such an activation of water reduction is never observed during electroreduction of CuO or ZnO. This study shows that electrochemical investigations are particularly suitable for providing information about properties of oxide catalysts.

Behaviour of AgBi(Cr2O7)2 as a possible cathode for lithium cells

Abstract The electrochemical reduction of AgBi(Cr2O7)2 electrodes occurs in a nearly continuous way from 3.5 V to 2 V/Li+/Li and leads to a final oxidation state of about III for chromium ions. Three reduction steps can be distinguished: - CR(VI) reduction leading approximately to Cr(V) - Ag(I) reduction to Ag(O) - Further reduction of chromium, leading approximately to CR(III) in a quasi-reversible way. A loss of capacity was observed, however, during repeated charge/discharge cycles.