RESUMEN
The performance of structurally and chemically well-defined Ni-free and Ni-modified single-crystalline Co3 O4 (1 1 1) thin-film electrodes in the oxygen reduction and evolution reactions (ORR and OER) was investigated in a combined surface science and electrochemistry approach. Pure and Ni-modified Co3 O4 (1 1 1) film electrodes were prepared and characterized under ultrahigh-vacuum conditions by scanning tunneling microscopy and X-ray photoelectron spectroscopy. Both Ni decoration (by post-deposition of Ni) and Ni doping (by simultaneous vapor deposition of Ni, Co, and O2 ) induced distinct differences in the base cyclic voltammograms in 0.5 m KOH at potentials higher than 0.7â V compared with Co3 O4 (1 1 1) electrodes. Also, all oxide film electrodes showed a higher overpotential for the ORR but a lower one for the OER than polycrystalline Pt. Ni modification significantly improved the ORR current densities by increasing the electrical conductivity, whereas the OER onset of approximately 1.47â VRHE (RHE: reversible hydrogen electrode) at 0.1â mA cm-2 was almost unchanged.
RESUMEN
The process of solid-electrolyte interphase (SEI) formation is systematically investigated along with its chemical composition on carbon electrodes in an ionic liquid-based, Li-containing electrolyte in a combined surface science and electrochemical model study using highly oriented pyrolytic graphite (HOPG) and binder-free graphite powder electrodes (Mage) as model systems. The chemical decomposition process is explored by deposition of Li on a pre-deposited multilayer film of 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([BMP][TFSI]) under ultrahigh vacuum conditions. Electrochemical SEI formation is induced by and monitored during potential cycling in [BMP][TFSI]+0.1 m LiTFSI. The chemical composition of the resulting layers is characterized by X-ray photoelectron spectroscopy (XPS), both at the surface and in deeper layers, closer to the electrode|SEI interface, after partial removal of the film by Ar+ ion sputtering. Clear differences between chemical and electrochemical SEI formation, and also between SEI formation on HOPG and Mage electrodes, are observed and discussed.
RESUMEN
Aiming at a detailed molecular understanding of the initial stage of the solid|electrolyte interphase (SEI) formation in Li-ion batteries, we have investigated the interaction of the battery-relevant ionic liquid (IL) 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([BMP][TFSI]) (solvent/electrolyte) and Li (Li+ ion shuttle) on well-defined Li-poor Li4Ti5O12(111) and Li-rich Li4.3Ti5O12(111) surfaces/electrodes in a combined surface science and electrochemical model study. X-ray photoelectron spectroscopy (XPS) measurements reveal that postdeposition of Li0 under ultrahigh vacuum (UHV) conditions on a Li-poor Li4Ti5O12(111) surface precovered with a molecularly adsorbed [BMP][TFSI] adlayer leads to little IL decomposition at 80 and 300 K. We assume that most of the Li diffuses through the IL adlayer and rapidly inserts into the Li4Ti5O12(111) bulk. More pronounced IL decomposition was obtained upon IL deposition on a Li-rich Li4.3Ti5O12 phase at 80 K and subsequent heating to 300 K. Cyclic voltammograms (CVs) recorded on the Li4Ti5O12(111) electrodes in Li-TFSI/[BMP][TFSI] indicate an almost reversible Li (de-)insertion, with a slight decay of the amount of (de-)inserted Li with increasing cycle number. XPS measurements performed on the electrode after potential cycling show low intensity signals of IL decomposition products, in addition to dominant signals from residual IL electrolyte, which are related to reaction of the adsorbed IL with Li inserted into/extracted from Li4Ti5O12 during the CV. The results indicate a close similarity between IL decomposition products formed under UHV and under electrochemical conditions, underlining the validity of this experimental approach and the potential of such kind of model studies for obtaining detailed understanding of the SEI formation.
RESUMEN
Aiming at a detailed understanding of the interaction between an ionic liquid, O2, and electrodes in Mg-air batteries, we performed a combined differential electrochemical mass spectrometry and in situ infrared spectroscopy model study on the interaction between the ionic liquid (IL) 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl) imide (BMP-TFSI) and a gold film electrode in the presence and absence of O2 and Mg2+ ions in the potential range relevant for the oxygen reduction reaction (ORR) and evolution reaction. Detailed information on the dynamic exchange of adsorbed ions, on the stability/decomposition of the ionic liquid, and on the activity/selectivity/reversibility of the ORR is derived from measurements performed under potentiodynamic and potentiostatic conditions. In neat BMP-TFSI, we find the dynamics of the potential induced exchange of adsorbed ions to depend significantly on the exchange direction. In the presence of O2, the anions formed in the ORR distinctly affect the adsorption characteristics of the IL ions and the exchange dynamics. Furthermore, the ORR changes from reduction to superoxide anions at moderate potentials to reduction to peroxide anion at more negative potentials. In the additional presence of Mg2+ ions, dominant magnesium peroxide and oxide formation result in an irreversible ORR, in contrast to the requirements of an efficient re-chargeable Mg-air battery. In addition, these ions result in the increasing formation of a blocking adlayer, reducing the coverage of adsorbed IL species.
RESUMEN
The role and impact of follow-up processes involving reactive side products in an electrocatalytic reaction is demonstrated for the electrooxidation of methanol at a Pt electrode. By using combined in situ infrared spectroscopy and online mass spectrometry and employing isotope-labeling techniques, it is shown that even small amounts of the incomplete oxidation products formaldehyde and formic acid have pronounced effects on the reaction.
RESUMEN
The morphology and stability of well-ordered, nanostructured Au/TiO2(110) surfaces, prepared by deposition of Au loaded micelles on TiO2(110) substrates and subsequent oxidative removal of the polymer shell in an oxygen plasma, was investigated by noncontact AFM, SEM and XPS. The resulting arrays of Au nanoparticles (particle sizes 1-5 nm) form a nearly hexagonal pattern with well-defined interparticle distances and a narrow particle size distribution. Particle size and particle separation can be controlled independently by varying the Au loading and the block-copolymers in the micelle shell. The oxygen plasma treatment does not affect the size and distance of the Au nanoparticles; the latter are fully metallic after subsequent UHV annealing (400 degrees C). The particles are stable under typical CO oxidation reaction conditions, up to at least 200 degrees C, making these surfaces ideally suited as defined model systems for catalytic studies. Significant changes in the height distributions of the Au nanoparticles are found upon 400 degrees C annealing in O2. For adlayers with small interparticle distances, this leads to a bimodal particle size distribution, which together with the preservation of the lateral order points to Ostwald ripening.