Petr Krtil

Orientation Dependence of Charge‐Transfer Processes on TiO2 (Anatase) Single Crystals

Reference LPI-ARTICLE-2000-014View record in Web of Science Record created on 2006-02-21, modified on 2017-05-12

Beyond the volcano limitations in electrocatalysis – oxygen evolution reaction

Oxygen evolution catalysis is restricted by the interdependence of adsorption energies of the reaction intermediates and the surface reactivity. The interdependence reduces the number of degrees of freedom available for catalyst optimization. Here it is demonstrated that this limitation can be removed by active site modification. This can be achieved on ruthenia by incorporation of Ni or Co into the surface, which activates a proton donor-acceptor functionality on the conventionally inactive bridge surface sites. This enhances the actual measured oxygen evolution activity of the catalyst significantly compared to conventional ruthenia.

Lithium Insertion into Mesoscopic and Single‐Crystal TiO2 (Rutile) Electrodes

Electrochemical behavior of single-crystal and mesoscopic TiO{sub 2} (rutile) was studied in propylene carbonate solutions at potentials negative to the flatband potential. In electrolytic solutions containing sodium or tetrabutylammonium (Bu{sub 4}N{sup +}), the injected charge is compensated by protonization of the surface and/or by adsorption of cations in the double layer. In electrolytic solutions containing Li{sup +}, the insertion into the rutile lattice occurs at potentials below 1.5 V (Li/Li{sup +}). At higher potentials, the charge is compensated mainly by a nonfaradaic process. Lithium insertion into rutile proceeds at a potential ca. 0.4 V more negative than the insertion potential into anatase. The maximum insertion capacity of rutile is also lower than that of anatase. The insertion of lithium into rutile is accompanied by an increase of the electrode mass, while the mass/charge relations show hystereses between anodic and cathodic potential sweeps. This behavior is explained in terms of a free convection in the electrode vicinity.

Enhancing Activity for the Oxygen Evolution Reaction: The Beneficial Interaction of Gold with Manganese and Cobalt Oxides

Electrochemical production of hydrogen, facilitated in electrolyzers, holds great promise for energy storage and solar fuel production. A bottleneck in the process is the catalysis of the oxygen evolution reaction, involving the transfer of four electrons. The challenge is that the binding energies of all reaction intermediates cannot be optimized individually. However, experimental investigations have shown that drastic improvements can be realized for manganese and cobalt‐based oxides if gold is added to the surface or used as substrate. We propose an explanation for these enhancements based on a hydrogen acceptor concept. This concept comprises a stabilization of an *OOH intermediate, which effectively lowers the potential needed for breaking bonds to the surface. On this basis, we investigate the interactions between the oxides and gold by using DFT calculations. The results suggest that the oxygen evolution reaction overpotential decreases by 100–300 mV for manganese oxides and 100 mV for cobalt oxides.

Lithium insertion into self-organized mesoscopic TiO2 (anatase) electrodes

Abstract Self-organized nanocrystalline anatase films were prepared by a sol–gel technique with subsequent hydrothermal growth of crystals in basic solutions. Hydrothermal treatment at 190–230°C leads to the formation of compact films with narrow pore size distribution. Materials prepared at higher temperatures show more open structure with lower specific surface area. The electrochemical activity of the self-organized electrodes decreases with decreasing specific surface area. Ordered electrodes showed higher insertion capacity by ca. 15% compared to those of non-ordered electrodes with the same specific surface area. Insertion coefficients, x, exceeding 0.5 were observed for very thin (0.2 mg/cm2) self-organized films with specific surface area higher than 60 m2/g. However, these values were not reproduced for thicker films. The insertion rate depends on the inserted charge with a maximum at x∼0.1. It indicates that only a part of the total electrode–electrolyte interface is available for charge transfer reaction. The total insertion capacity is therefore affected by propagation of the electrochemically active surface within the film.

Particle Size Dependence of the Electrocatalytic Activity of Nanocrystalline RuO2 Electrodes

The effect of nanocrystal size/shape on the electrocatalytic behavior of nanocrystalline oxide electrodes was demonstrated on electrodes based on sol-gel-prepared RuO 2 materials. The typical particle size ranged between 15 and 40 nm. Regardless of the particle size, the nanocrystals were of isometric shape with polygonal prismatic part capped with two pyramids. The prismatic part of the nanocrystals featured {110}- and {100}-oriented faces in the case of small nanocrystals (d 20 nm). The pyramidal parts were formed by {101}-oriented faces. The activity of the RuO 2 nanocrystals towards oxygen evolution reaction decreases with increasing particle size; the activity towards chlorine evolution is insensitive to the particle size/shape. The observed tendency indicates that the oxygen evolution is significantly affected by the crystal edges while chlorine evolution reaction proceeds mainly on crystal faces. The suppression of the oxygen evolution may be connected with the absence of edges separating the {110}-{100} faces on the prismatic part of the bigger nanocrystals. The {110}-{100} edges can be therefore regarded as the preferential site for O 2 evolution.

Li Insertion into Li-Ti-O Spinels: Voltammetric and Electrochemical Impedance Spectroscopy Study

The insertion of Li into nanocrystalline Li-Ti-O spinel electrodes was studied using cyclic voltammetry combined with electrochemical quartz crystal microbalance, open circuit potential measurements, and electrochemical impedance spectroscopy studies. Insertion characteristics of spinel were compared with those of titania polymorphs. The insertion into spinel occurs at potential 300 mV more negative to that of insertion into anatase, and about 40 mV positive to that into rutile. The specific capacity (160 mAh/g) is comparable with that of TiO 2 polymorphs Due to the relatively negative potential of the insertion and significantly lower tendency to self-discharge, Li-Ti-O spinel is more convenient for use in a 2 V lithium-ion battery than are other active phases in the Li-Ti-O system. The behavior of the spinel electrodes in equilibrium can be described by the Frumkin insertion isotherm model. The value of interaction parameter, g, below -4 indicates that the insertion process leads to a first-order phase transition. Diffusion coefficient of Li in spinel ranges between 10 -15 and 10 -16 cm 2 s 1 ; the charge transfer kinetics depends on the redox composition. Heterogeneous rate constant ranges between I × 10 8 and 4 × 10 -10 cm s -1 that is ca. two orders of magnitude higher than that reported for anatase.

Dramatically enhanced cleavage of the C-C bond using an electrocatalytically coupled reaction.

This paper describes a generalized approach for the selective electrocatalytic C-C bond splitting in aliphatic alcohols at low temperature in aqueous state, with ethanol as an example. We show that selective C-C bond cleavage, leading to carbon dioxide, is possible in high pH aqueous media at low overpotentials. This improved selectivity and activity is achieved using a solution-born co-catalyst based on Pb(IV) acetate, which controls the mode of the ethanol adsorption so as to facilitate direct activation of the C-C bond. The simultaneously formed under-potentially deposited (UPD) Pb and surface lead hydroxide change the functionality of the catalyst surface for efficient promotion of CO oxidation. The resulting catalyst retains an unprecedented ability to sustain the full oxidation reaction pathway on an extended time scale of hours as opposed to minutes without addition of Pb(IV) acetate.

Oxidation of Acetonitrile‐Based Electrolyte Solutions at High Potentials An In Situ Fourier Transform Infrared Spectroscopy Study

The oxidation of LiAsF{sub 6}/acetonitrile and LiClO{sub 4}/acetonitrile solutions containing 0.003--0.05M H{sub 2}O was studied on platinum and glassy carbon electrodes by in situ Fourier transform infrared spectroscopy. Both the solvent and the solute are unstable at potentials positive to ca. 2.2 V vs. SCE. The main oxidation product is CO{sub 2}. Other products detected are: via a reaction of acetonitrile with trace water as substantiated by the occurrence of an isotopic shift of the CO{sub 2} stretching vibration when using H{sub 2}{sup 18}O. The rate of the CO{sub 2} formation is increased by the presence of perchlorate anions and the Pt surface.

Electrochemical activity of hydrothermally synthesized Li-Ti-O cubic oxides toward Li insertion

Li-Ti-O oxides prepared by hydrothermal synthesis are active hosts for lithium insertion. Annealing of synthesized samples at temperatures above 250°C leads to structural water removal, which causes narrowing of voltammetric peaks and. subsequently, shift of the insertion potential toward more negative potentials. An increase of annealing temperature also improves coulombic efficiency and stability against sell-discharge reactions. On the other hand, the lithium insertion/extraction kinetics slows down with increasing annealing temperature. Hydrothermally synthesized Li-Ti-O spinels show more favorable charge transfer kinetics as well as lithium transport comparing with spinels prepared at 800°C. Thermodynamics of Li insertion into hydrothermally prepared phases can he described by the Frumkin insertion isotherm. The low temperature prepared spinels seem to he less susceptible to a lithium-insertion triggered phase transition than the high temperature synthesized materials.