Ch. Fabjan

The vanadium redox-battery: an efficient storage unit for photovoltaic systems

The ‘all vanadium redox flow system’ is a promising candidate for the storage of photovoltaic energy. The reversible cell voltage of 1.3–1.4 V in charged state is well established at various electrode materials in particular carbon based substrate. The kinetics and mechanism were studied for the V2+/V3+ and VO++/VO2+ (V4+/V5+) couples and a one-electron transfer identified as the rate-determining step at smooth surface. The use of activation layers (carbon cloth, felt, etc.) decisively reduced the polarization. Catalysts, which are required for an increase of the reaction rate and the elimination of undesired side reactions, e.g. Ru(O)2 improved the behavior of the positive electrode. The influence of the separator material on mass transfer phenomena (diffusion, migration) and the charge–discharge characteristics were investigated. The requirements to be met as stand alone batteries for the energy supply of users in combination with photovoltaic plants considering the solar irradiation conditions in south Portugal were discussed and the future development goals defined.

In situ FTIR spectroscopy of the Zn–Br battery bromine storage complex at glassy carbon electrodes

In situ reflection absorption FTIR spectroscopy (in situ FTIR) and small spot X-ray electron emission spectroscopy (XPS) of emersed electrodes were used to examine the electrochemical double layer on glassy carbon (GC) and the anodic storage reactions of the zinc bromine battery which are the formation of a non-aqueous N-methyl-ethyl-pyrrolidinium (MEP + ) and/or N-methyl-ethyl-morpholinium (MEM + ) polybromide phase. Oxidative conversion of the GC surface to C-OH, C=O and COO - is observed during potential cycling between the hydrogen and oxygen evolution. The extrapolation of the intensity of the bending vibration signal due to adsorbed water allowed the determination of the point of zero charge (pzc). The pzc of GC is more than 1 V more negative than that of gold in the presence of MEP-Br or MEM-Br. Above the pzc, up to the bromide oxidation potential, MEM + shows a much stronger chemical affinity to GC than MEP + because the MEM morpholinium bridge oxygen shows a stronger interaction with the oxidised carbon surface species than the pure CH-structure of the pyrrolidinium species. This study suggests that the battery charging reaction involves mainly a heterogeneous electron transfer from Br - followed by a homogeneous chemical reaction leading to MEP-polybromide, whereas the conversion of specifically adsorbed MEM-Br ion pairs to polybromide is a slow process.

Development of high performance cathodes for hydrogen production from alkaline solutions

Abstract Ni-based mixed catalysts were tested as hydrogen generating cathodes in alkaline solutions. Low activation energies and low overvoltage was found on binary Ni/TiH catalysts indicating high catalytic activity. The best results under technical operating conditions were obtained on activated plasma-sprayed Raney-alloy coatings and sintered ternary Ni/B/TiH, and are rather a consequence of optimized surface structure parameters and a possible change of mechanism of the H2-evolution reaction than of a further increase of catalytic activity.

Reduction of oxygen from electrolyte emulsions with high oxygen contents

The kinetics of oxygen reduction from electrolyte emulsions containing oxygen carriers was studied with respect to fuel cell applications. Electrochemical experiments including stationary and nonstationary measurements under defined hydrodynamic conditions at a rotating platinum disk electrode revealed that diffusion-controlled limiting currents can be increased significantly at high oxygen concentrations. Additionally, the limiting currents show a strong correlation with the composition and the structure of the emulsions. In particular, the average droplet size of the organic phase and its distribution depends on the nature of the emulsifying agents used for stabilisation. The complex hydrodynamic behaviour of two-phase electrolyte emulsions and the influence of different surfactants seem to outweigh the effect of the total oxygen concentration on the transport kinetics of oxygen.

Die kathodische Sauerstoffreduktion an pyrolytisch abgeschiedenem Kohlenstoff in peroxidhältigen alkalischen Elektrolyten

AbstractThe kinetics of the system O2/HO2−, OH− were studied at pyrolytic carbon in alkaline electrolytes. The rest potentials are close to the reversible values. They decrease by 30 mV when the HO2−-concentration is increased by a factor 10.CathodicTafel lines displayb-values between 70 and 95 mV. The exchange current densities are evaluated by extrapolation ofTafel lines to zero overvoltage and from the charge transfer resistance.Two different succeeding charge transfer reactions occur in course of the overall process, the first of which is the rate-determining step. A cathodic reaction order of zero is obtained with respect to HO2−.Theb values of anodicTafel lines are between 60 and 80 mV, the corresponding reaction order concerning the HO2− concentration is found to be +0.5.The kinetic studies prove the reversibility of the system O2/HO2−, OH− at carbon electrodes. The reaction mechanism is:

Die Kathodische reduktion von ozon an iridium in sauren elektrolyten

\begin{array}{*{20}c} {O_2 + e^ - \rightleftarrows O_2 } \\ {O_{2^ - } + H_2 O \rightleftarrows HO_2 + OH - } \\ {HO_2 + e^ - \rightleftarrows HO_{2^ - } } \\ \end{array}

Untersuchungen über den Mechanismus und die Katalyse der kathodischen Sauerstoffreduktion an verschiedenen Metallen

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On the Electrocatalysis of the Oxygen Reduction in Acid Electrolyte

The method is based on the measurement of the diffusion controlled limiting current obtained with the cathodic reduction of ozone according to O3 + 2H+ + 2e− → H2O + O2 in sulphuric acid. The electrolyte is vigorously mixed with the gas containing ozone and circulated through the electrolytic cell by the ascending gas flow operating as a gas-lift pump. The solution saturated with ozone moves along a cylindric electrode consisting of a smooth platinum foil in laminar flow. A constant potential is maintained at the electrode by means of a potentiostat in such a way, that the electrochemical reaction is proceeding under limiting current conditions. A lead anode of high capacitiy is used as a counter- and reference electrode. The current recorded continuously is directly proportional to the concentration of ozone in gas.ZusammenfassungDie vorgeschlagene Methode zur Ozonbestimmung beruht auf der Messung des Diffusionsgrenzstromes bei der kathodischen Reduktion von Ozon nach O3 + 2H+ + 2e− → H2O + O2 in Schwefelsäure. Der Elektrolyt wird in der elektrochemischen Meßzelle intensiv mit dem ozonhaltigen Gas durchmischt und durch den aufsteigenden Gasstrom nach dem Prinzip der Mammutpumpe kontinuierlich im Kreislauf transportiert. Die mit Ozon gesättigte Lösung fließt in laminarer Strömung über eine zylindrische Meßelektrode aus glattem Platin. An dieser Elektrode wird mittels eines elektronischen Potentiostaten ein solches Potential konstant eingestellt, daß die elektrochemische Reaktion stets im Grenzstrombereich abläuft. Als Gegen- und Bezugselektrode wird eine Bleianode mit hoher Kapazität eingesetzt. Der mit einem Schreiber registrierte Strom ist dem Ozongehalt im Gas direkt proportional.