Odysseas Paschos

Electrode–Electrolyte Interface in Li-Ion Batteries: Current Understanding and New Insights

Understanding reactions at the electrode/electrolyte interface (EEI) is essential to developing strategies to enhance cycle life and safety of lithium batteries. Despite research in the past four decades, there is still limited understanding by what means different components are formed at the EEI and how they influence EEI layer properties. We review findings used to establish the well-known mosaic structure model for the EEI (often referred to as solid electrolyte interphase or SEI) on negative electrodes including lithium, graphite, tin, and silicon. Much less understanding exists for EEI layers for positive electrodes. High-capacity Li-rich layered oxides yLi2-xMnO3·(1-y)Li1-xMO2, which can generate highly reactive species toward the electrolyte via oxygen anion redox, highlight the critical need to understand reactions with the electrolyte and EEI layers for advanced positive electrodes. Recent advances in in situ characterization of well-defined electrode surfaces can provide mechanistic insights and strategies to tailor EEI layer composition and properties.

A review on phosphate based, solid state, protonic conductors for intermediate temperature fuel cells

The electrolytes currently used for proton exchange membrane fuel cells are mainly based on polymers such as Nafion which limits the operation regime of the cell to ∼80 °C. Solid oxide fuel cells operate at much elevated temperatures compared to proton exchange membrane fuel cells (∼1000 °C) and employ oxide electrolytes such as yttrium stabilized zirconia and gadolinium doped ceria. So far an intermediate temperature operation regime (300 °C) has not been widely explored which would open new pathways for novel fuel cell systems. In this review we summarize the potential use of phosphate compounds as electrolytes for intermediate temperature fuel cells. Various examples on ammonium polyphosphate, pyrophosphate, cesium phosphate and other phosphate based electrolytes are presented and their preparation methods, conduction mechanism and conductivity values are demonstrated.

Size-Dependent Electrocatalytic Activity of Gold Nanoparticles on HOPG and Highly Boron-Doped Diamond Surfaces

Gold nanoparticles were prepared by electrochemical deposition on highly oriented pyrolytic graphite (HOPG) and boron-doped, epitaxial 100-oriented diamond layers. Using a potentiostatic double pulse technique, the average particle size was varied in the range from 5 nm to 30 nm in the case of HOPG as a support and between <1 nm and 15 nm on diamond surfaces, while keeping the particle density constant. The distribution of particle sizes was very narrow, with standard deviations of around 20% on HOPG and around 30% on diamond. The electrocatalytic activity towards hydrogen evolution and oxygen reduction of these carbon supported gold nanoparticles in dependence of the particle sizes was investigated using cyclic voltammetry. For oxygen reduction the current density normalized to the gold surface (specific current density) increased for decreasing particle size. In contrast, the specific current density of hydrogen evolution showed no dependence on particle size. For both reactions, no effect of the different carbon supports on electrocatalytic activity was observed.

Kinetic Study of Parasitic Reactions in Lithium-Ion Batteries: A Case Study on LiNi0.6Mn0.2Co0.2O2

The side reactions between the electrode materials and the nonaqueous electrolytes have been the major contributor to the degradation of electrochemical performance of lithium-ion batteries. A home-built high-precision leakage current measuring system was deployed to investigate the reaction kinetics between the delithiated LiNi(0.6)Mn(0.2)Co(0.2)O2 and a conventional nonaqueous electrolyte. It was found that the rate of parasitic reaction had strong dependence on the upper cutoff potential of the cathode material. The kinetic data also indicated a change of reaction mode at about 4.5 V vs Li(+)/Li.

Ethanol Oxidation on TiOxCy‐Supported Pt Nanoparticles

In recent years, there has been increasing interest in the development of direct ethanol fuel cells (DEFCs). Ethanol, with its relatively high energy density of 7.4 kWh kg , good availability from renewable sources, non-toxicity, and easy storage and transportation, is almost the ideal fuel. However, there is still a lack of catalysts that can completely oxidize ethanol into CO2 at a high rate, which limits its potential application in DEFCs. Catalytic performance can be significantly improved by shifting towards higher operating temperatures, which brings about new challenges concerning the stability of the catalyst, the support materials, and the electrolyte. Another important approach for improving the performance of DEFCs is to perform the oxidation of EtOH in alkaline media, which is possible with various fuels. In proton-exchange-membrane fuel cells (PEMFCs) that are operated at intermediate temperatures, polybenzimidazole (PBI) that is doped with concentrated H3PO4 is currently widely used as membrane, which is a highly corrosive environment. Corrosion of the carbon support has been identified to be the major contributor to catalyst failure for Ptbased catalysts. Other failure modes also contribute to catalyst degradation, such as Pt dissolution (leaching) and precipitation, that is, sintering and agglomeration, all of which cause a decrease in the catalyst’s electroactive surface area. Oxidation of the carbon support into CO2 leads to the separation of Pt nanoparticles from the support and, hence, to a loss in performance. On the anode side, the carbon support can be oxidized in the case of fuel starvation. 8] The full spectrum of durability problems that are related to electrocatalyst–support interactions is still not fully understood. Therefore, it is important to further elucidate degradation phenomena on both the anode and cathode of a fuel cell and to find alternative support materials. Metal-oxide-based supports have received remarkable interest and, among the various supports that have been reported, titania has been found to be a very promising candidate, because it can be easily nanostructured and thermally reduced into electrically conductive Magn li-like phases (TinO2n 1, 4 n 10). Moreover, titania has recently been employed as a catalyst support for the cathode of a PEMFC. A high-temperature carbothermal-reduction treatment was shown to convert TiO2 nanotubes into semimetallic, carbon-rich titanium oxycarbide (TiOxCy) nanotubes with unaltered morphology, which were successfully tested as a support for Pt/Ru catalysts for the electrochemical oxidation of MeOH. In a recent study on the carbo-thermal annealing of planar anodic TiO2, we showed that the conductivity and electrochemical activity towards the hydrogen-evolution reaction (HER) were directly linked to the annealing temperature, which determined the surface morphology, structure, and chemistry. Herein, we investigated planar TiOxCy films as oxide-based model supports for Pt nanoparticles (NPs). The activity of this system towards the EtOH-oxidation reaction (EOR) and its stability in concentrated H3PO4 electrolyte were studied at temperatures ranging from 25 to 80 8C and compared to the performance of glassy carbon (GC)-supported Pt NPs in a half-cell electrochemical (EC) setup. TiOxCy films were prepared according to a literature procedure by the carbo-thermal reduction of compact anodic TiO2 films that were grown on Ti metal at four different annealing temperatures, that is, 750, 850, 950, and 1050 8C. The morphology, composition, structure, and electrochemical properties of TiOxCy have been described previously. TiOxCy mainly consists of titanium carbide (TiC) and TiO2 and shows the presence of titanium suboxides. The ratio of TiC and suboxides to TiO2 in the films increases with decreasing annealing temperature, which leads to an increase in the conductivity for interfacial electron transfer, and it reaches its highest value at 750 8C annealing temperature. Pt NPs were deposited by aerosol-assisted deposition (AAD). The prepared catalyst/support model electrodes are labeled as Pt/TOCxxx, in which xxx denotes the annealing temperature of the support, and as Pt/GC. SEM revealed that the Pt NPs had diameters in the range 6–10 nm (see the Supporting Information, Figure S1). Their morphology and activity remained unaltered on TOC750, whereas the activity on Pt/TOC 850 changed throughout the EC tests. For details on the particle sizes and densities, see the Supporting Information, Table S2. The first focus of this work was the correlation between the conductivity properties of the TOCxxx electrodes and their activity towards the EOR. Therefore, cyclic voltammograms (CVs) were recorded at room temperature in 0.1 m HClO4 with 0.5 m EtOH. The measured current densities (j), which were corrected for the electrochemically active surface area (ECSA) of Pt, are shown in Figure 1 a. The onset potentials for the EOR are about 0.23 V for Pt/TOC750 and Pt/GC (Figure 1 a, inset) versus the standard hydrogen electrode (SHE). The onset of EOR current is associated with the oxidation of adsorbed ethanol. Beyond the onset potential the ethanol adsorbate coverage is lowered and OHad can form which reacts with EOR reaction in[a] C. R diger, J. Brumbarov, F. Wiesinger, S. Leonardi, Dr. O. Paschos, Dr. C. V. Vidal, Dr. J. Kunze-Liebh user Physik Department E19 Technische Universit t M nchen James-Franck-Str.1, 85748 Garching (Germany) Fax: (+ 49) 89-289-12536 E-mail : julia.kunze@tum.de

Fuel Cell Comparison to Alternate Technologies

The actual energy demand and consumption issues make it necessary to critically discuss and compare different energy conversion and storage systems. At present, only one third of the primary energy is converted into end energy, for example, electrical energy. Losses are associated with a high consumption of fossil fuels and large CO2 emissions. They can be avoided by considering important electrochemical processes for energy conversion, using batteries, fuel cells, supercapacitors and electrochemical photovoltaics and by incorporating energy storage, employing rechargeable batteries, supercapacitors, generation of hydrogen via electrolysis, and generation of methanol.