Room-Temperature Electrochemical Intercalation of Oxygen or Fluorine into La[sub 2]CuO[sub 4] Using Organic Electrolytic Media: Mechanistic Approach

Abstract

The electrochemical intercalation of oxygen into La 2 CuO 4 has been studied in various anhydrous organic media. Several combinations between an organic solvent and a supporting electrolyte have been examined. The addition of an oxygen source such as KO 2 or H 2 O has been found to be unnecessary. Experiments have shown that the electrochemical oxygen insertion mainly implies the decomposition of the solvent leading to the formation of oxygen species able to oxidize the starting material. La 2 CuO 4.10 has been prepared in the DMSO associated with nBu 4 NClO 4 . A superconducting behavior is observed below 44 K and the physical characterizations of the material before and after controlled anodic polarization are reported. It has also been shown that this intercalation reaction is reversible. The mechanism of the oxygen intercalation is tentatively proposed with respect to the organic media. Fluorine has also been intercalated for the first time in such media.Oxides showing high oxidation states of metal cations are usually prepared at high temperatures under high oxygen pressures, which gives the thermodynamically stable form of the compound. Preparing such oxides at lower temperatures ͑20-300°C͒ requires reactions of ''Chimie Douce''-type, 1 often leading to metastable compounds. Besides the use of highly oxidizing reagents, 2,3 electrochemical processes have been used and many studies have shown that the use of the electric field as the driving force for redox reactions can be very beneficial in solid-state chemistry for preparing new materials at about room temperature, as for instance in the field of batteries. Initiated for the first time in 1989, the electrochemical oxidation appeared as an efficient ''Chimie Douce'' technique for intercalating oxygen into oxide host networks. 4 It is carried out in air, at room temperature, in alkaline media, and allows preparing noteworthy materials, some having never been obtained with classic synthesis routes used in solid-state chemistry.In alkaline solution, in the vicinity of the oxygen evolution, under anodic potentials, a very high oxygen activity can be obtained at the electrode surface according toOn metallic electrodes, such a reaction competes with the formation of oxide thin films ͑i.e., Al 2 O 3 , ZnO, CuO, TiO 2 ,...) 5 whereas on appropriate oxide electrodes such as A x M m O y 1 not only the surface is affected but also the bulk. When analyzed in electrochemistry, these materials present (i, E) curves which exhibit a typical plateau at potentials lower than that corresponding to oxygen evolution, which is the signature of an oxidation process according towhere A is an alkaline-earth or a lanthanide ion and M a 3d transition metal and 2␦ ϭ the number of exchanged electrons corresponding to the amount of oxidized metal cations (Cu 3ϩ for La 2 CuO 4 ). 6 Various oxides have thus been prepared and characterized, 1 some of them exhibiting quite original properties such as superconductivity for La 2 CuO 4.09 , 7 charge density ordering in La 2 NiO 4.25 , 8 ferro...

Electrochemical Fluorination of La2CuO4: A Mild “Chimie Douce” Route to Superconducting Oxyfluoride Materials

“…This fluorine content more or less rapidly decreases upon etching with a concomitant increase of the oxygen amount (Figure a,b). DMSO has been shown to be a source of oxygen in such electrochemical treatment, leading to preferential insertion of oxygen as compared to fluorine . The very high amount of fluorine on the outer surface of some materials (see Figure b for example) shows that even such a low current density induces partial surface decomposition, especially with n Bu 4 NF as a salt.…”

“…DMSO has been shown to be a source of oxygen in such electrochemical treatment, leading to preferential insertion of oxygen as compared to fluorine. 38 The very high amount of fluorine on the outer surface of some materials (see Figure 1b for example) shows that even such a low current density induces partial surface decomposition, especially with nBu 4 NF as a salt. A reaction performed at a higher current density (600 μA/cm 2 ) led to a higher degradation rate, with the formation of a fluorinated material identified as LaF 3 down to 200 nm (results not shown).…”

“…Finally, no 19 F NMR signals (except the parasitic probe signal) were observed for the sample La 2 CuO 4.10 prepared electrochemically in DMSO/nBu 4 NBF 4 , which is indeed not expected to be fluorinated. 38…”

“…We have shown, in a previous paper, that organic media without any external source of oxygen (KO 2 or H 2 O) could be used to induce the electrochemical oxidative intercalation of oxygen in the well-known La 2 CuO 4 material. Various electrochemical processes occur, implying not only the decomposition of the “oxygen-bearing” solvents but also that of the supporting salt, leading to the formation of oxygen species as O − or O • , able to oxidize the material.…”

“…The use of fluorine-containing salts during electrochemical insertion led to materials which exhibited electronic properties different from those obtained in fluorine-free salts. Insertion of fluorine in their structure was suggested …”