H. Binder

Mechanism of the electrocatalytic reduction of oxygen on metal chelates

N4 complexes (tetraarylporphyrins, dibenzotetraazaannulenes, phthalocyanines), N2O2 complexes (Pfeiffer complexes), and N2S2 complexes [diacetyldi(thiophenylhydrazones)] have been studied with respect to their catalytic activity for the electroreduction of oxygen in acid electrolyte. Activity has been found with the N4 complexes, of which the phthalocyanines are already known as catalysts. Unfortunately we found these metal chelates not to be completely stable when supported by active carbon or carbon black. Especially the phthalocyanines readily disintegrate in acid, but also the dibenzotetraazaannulenes lose their activity within several days of operation. Only certain tetraarylporphyrins were still active after 300 hr. The N2S2 and N2O2 (monomeric and polymeric) complexes did not show any activity at all although the Pfeiffer complexes are known as reversible oxygen carriers. In contrast to these complexes, the N4 complexes used are not only macrocyclic but also have a conjugated π-electron system, which seems to be a prerequisite for the activation of the oxygen molecule. The mechanism of the activation of the O2 molecule has been explained on the basis of simple MO considerations, which also provide an explanation for the order of activity of central metal ions—Co > Fe > Ni—and for the effect of the support and of substituents of the ligand on the activity and the stability of the chelate.

Investigation into the use of quinone compounds-for battery cathodes

Abstract As organic cathode materials investigated in the past are reduced irreversibly and, therefore, can only be used in primary cells, we investigated quinones/hydroquinones, which are known to act as completely reversible redox couples. Most of the quinones are, however, slightly soluble and partly unstable in electrolyte solution. In order to test their stability electrochemically we mixed the quinones with carbon. The open-circuit potentials of the solid quinone/hydroquinone systems measured in 2 N H2SO4 are very close to the values of the redox potentials measured in alcoholic solutions. Diphenoquinones, which have a potential of about 950 mV (nhe) do not have the required stability. Only tetrachloro-p-benzoquinone (chloranil) and tetramethyl-p-benzoquinone (duroquinone) have been found to be sufficiently insoluble and completely stable, the redox potentials being 668 and 478 mV, respectively. In the case of galvanostatic discharge the potential is almost constant. With chloranil the polarization is only 30 mV at a cd of 60 mA/cm2. Even at a current drain of 600 mA/cm2, 50 per cent of the active material is available for discharge at potentials exceeding 200 mV. In concentrated aqueous ammonium chloride solution the reduction takes place in two one-electron steps separated by approximately 70 mV. This does not happen in zinc chloride solution. The capacity (Ah/kg) of the quinones is in the same range as that of the inorganic depolarizers, but the hydroquinones can be reoxidized with oxygen using hydrophobic electrodes. Thus the quinone electrode is regenerated with air and its capacity is practically unlimited. Regeneration with hydrogen peroxide is also possible. Particular quinones are insoluble in organic solvents used for organic electrolyte solutions and are therefore superior to heavy metal salts as cathode materials in high-energy density secondary batteries.

Electrocatalytic oxygen reduction with thiospinels and other sulphides of transition metals

Abstract Thiospinels and other sulphides were examined systematically with regard to their electrocatalytic activity for oxygen reduction in acid electrolyte. The measurements were carried out by potentiodynamic and galvanostatic methods. The metals cobalt, iron and nickel show the highest activity in sulphur compounds, the best results being obtained by cobalt. Other transition metals reduce the activity. A considerable decrease in activity is observed when sulphur is partly or totally replaced by oxygen, selenium or tellurium; the same effect is observed with disulphides. A simple interpretation of the results obtained is possible considering the geometric and electrostatic conditions in the spinels.

Über die anodische oxydation von aktivkohlen in wässrigen elektrolyten

Zusammenfassung Elektroden aus graphitierter Aktivkohle, Aktivkohle, Holzkohle, Russ und Graphit ergeben in 20 %iger Schwefelsaure bei 100°C potentiostatisch bei 1100 mV [reversibles Wasser- stoffpotential = 0 mV] einen anodischen Strom. Bei Russ ist die Stromdichte um den Faktor 10, bei Graphit um den Faktor 100 kleiner als bei den drei erstgenannten Kohlearten. Die anodische Oxydation ist von einer Kohlendioxid-Entwicklung begleitet. Elektrodenkohle mit einer spezifischen Oberflache von 200 m 2 /g erreicht in 20 %iger Schwefelsaure bei 100°C bei einer Stromdichte von 2,5 mA/cm 2 nach einem Verbrauch von etwa 0,5 F/Mol ein Potential von 1070 mV. Bei Erhohung der Stromdichte auf 12,5 mA/cm 2 nimmt die Polarisation um 100 mV zu. Anderseits fallt bei Verringerung der Stromdichte auf 0,25 mA/cm 2 das Potential auf 850 mV. Nach einem Verbrauch von etwa 1 F/Mol nimmt die Polarisation stark zu. Anodische potentiostatische Stromspannungskurven, galvanostatische Potential-Zeit-Kurven, kathodische Reduktion, gravimetrische Umsatzbestimmung, volumetrische Kohlendioxidmessung, chemische Analyse, Rontgenanalyse und Messung des elektrischen Widerstandes wahrend des Stromflusses haben als Hauptergebnis, dass die Elektrodenkohle auf nahezu direktem Wege von etwa 80 Prozent der Elektrizitatsmenge zu Kohlendioxid und von etwa 20 Prozent zu einer Kohlenstoff-Sauerstoff-Verbindung oxydiert wird, und zwar unterhalb des reversiblen Sauerstoffpotentials. Dies geschieht unabhangig von der Temperatur [55–100°C] und der Konzentration der Schwefelsaure (20–70 Prozent) oder der Phosphorsaure (10–85 Prozent). In Kalilauge erfolgt in entsprechender Weise Carbonatbildung. Die Kohlenstoff-Sauerstoff-Verbindung kann bis zu 20 Gew.-Prozent Sauerstoff enthalten. Sie lasst sich oberhalb des reversiblen Wasserstoffpotentials zum grosseren Teil nicht reduzieren, ist thermisch bis mindestens 130°C stabil und enthalt nach dem Trocknen bei dieser Temperatur kein Wasser. Ein kleiner Teil der Kohlenstoff-Sauerstoff-Verbindung ist reduzierbar. Er ist unabhangig vom Oxydationsgrad der Kohle und wird deshalb als Oxiddeckschicht angesehen. Diese reduzierbare Deckschichtmenge ist auch unabhangig davon, ob in Schwefelsaure, Phosphorsaure oder Kalilauge oxydiert wird. Sie ist jedoch nur so gross, dass formal etwa jedes neunte Kohlenstoffatom in der Oberflache eine Ladung tragt. Auf der Oberflache kann sich aber ausserdem noch ein Teil des nicht reduzierbaren Oxids befinden, da die Elektrodenkohle bis zu 0,4 F/Mol an Sauerstoff aufnehmen kann. In diesem Falle truge bei homogener Verteilung des Oxids jedes dritte. Kohlenstoffatom formal eine Ladung, d.h. das Atomverhaltnis Kohlenstoff zu Sauerstoff ware 6:1.

On the mechanism of electrocatalytic oxygen reduction at metal chelates: Part III. Metal phthalocyanines

Abstract With the rotating ring disk electrode the kinetics of electrochemical oxygen reduction was investigated at polymer phthalocyanine-covered electrodes. The RRDE allows the amount of H2O2 formed during the reaction to be determined quantitatively. Therefore the reaction mechanism can be deduced. Polymeric phthalocyanines containing Fe, Co and Ni as central metal were investigated. Polymeric phthalocyanines of Fe and Co were compared with the respective monomeric compounds. The activity of polymeric Fe-phthalocyanines prepared under different conditions was characterised. Rate constants for the reduction of oxygen to H2O and H2O2 in 1 M KOH are provided and the reaction mechanism in strong alkaline solution is discussed.

Adsorption und anodische oxydation von ameisensäure und kohlenmonoxid an platinelektroden mit und ohne schwefeladsorbat

Zusammenfassung Die periodischen Strom-Spannungs-Kurven fur die Oxydation von Ameisensaure zeigen sowohl in alkalischem als auch in saurem Elektrolyten ein ausgepragtes Maximum bei 600–700 mV Bezugsspannung gegen die reversible Wasserstoffelektrode in demselben Elektrolyten; dieses Maximum ist auf die Oxydation eines Chemisorptionsproduktes zuruckzufuhren, das die starke Polarisation der Elektrode schon bei geringer Stromdichte bewirkt. Das Adsorptionsgleichgewicht stellt sich mit Ameisensaure oder Kohlenmonoxid nach 2–3 Stunden ein. Aus der potentiodynamischen Kurve der Oxydation des Chemisorptionsproduktes sowohl von Ameisensaure als auch von Kohlenmonoxid in reinem sauren Elektrolyten erkennt man: Adsorbierter Wasserstoff ist nicht mehr vorhanden; das Chemisorptionsprodukt ist bis ca. 350 mV bestandig; das Oxydationsmaximum liegt bei 70° bei 530 mV und bei 30° bei 650 mV; die Oxydation ist bei 650 mV (70°) beendet; fur die Oxydation des Chemisorptionsprodukts wird eine spezifische Ladung von 1.6 e/Pt benotigt. Das Chemisorptionsprodukt wird als Mischung aus Carboxyl- und Formaldehydradikalen angesehen. In Kaliumhydroxidlosung wird fur die Oxydation des Chemisorptionsprodukts der Ameisensaure eine spezifische Ladung von 0.4–0.5 e/Pt benotigt. Durch Bedeckung der Platinoberflache mit einer monatomaren Schwefelschicht wird die anodische Oxydation in saurem Elektrolyten bei geringer Polarisation stark beschleunigt. Das Maximum liegt bei Ameisensaure bei einem Schwefelbedekkungsgrad von ca. 35% und bei Kohlenmonoxid von 100%. Das Schwefeladsorbat ist bis zu einer Bezugsspannung von 600–700 mV bestandig. Die Menge an Chemisorptionsprodukt von Ameisensaure und Kohlenmonoxid nimmt mit steigendem Bedekkungsgrad der Platinoberflache mit Schwefelab, wobei die spezifische Ladung zunimmt. Am Schwefeladsorbat ist eine Physisorption von Wasserstoff und Kohlenmonoxid wahrscheinlich, nicht jedoch von Ameisensaure, da deren Oxydation durch ein 100%-Schwefeladsorbat vollig unterdruckt wird. Aufgrund der Bestandigkeit des Chemisorptionsproduktes ist anzunehmen, dass sich die anodische Oxydation (auch in Abwesenheit von Schwefel) bis zu einer Bezugsspannung von 350 mV im wesentlichen auf dem Chemisorbat—nicht auf der Platinoberflache—abspielt, und zwar nach einem Elektronen-Radikalmechanismus und nicht nach dem Dehydrierungs-Radikalmechanismus.

Elektrochemische oxydation von kohlenwasserstoffen in einer festelektrolytbrennstoffzelle bei temperaturen von 900–1000°C☆

A high-temperature fuel cell has been operated using disks of stabilised cubic-type zirconium-calcium oxide as solid electrolyte. The disks were sintered at 1600°C and had a lattice constant of a0 = 5·130 A. The cell was run on hydrogen, carbon monoxide, propane and hexane as fuels. In order to avoid cracking of the hydrocarbons, they were reformed with water and/or carbon dioxide in an integrated converter prior to the electrochemical reaction. The theoretical e.m.f. was calculated as a function of the number of C atoms in the hydrocarbon molecule and the composition of the gas mixture prior to conversion. The open circuit voltage was measured as a function of the temperature under variation of the ratio: reactant/reaction product. The maximum difference between the experimental results and the theoretical values was one per cent. Further, the cell voltage was measured as a function of the current density up to 100 ma/cm2. At small current densities a minor polarization was observed in addition to the ohmic drop in the electrolyte. With converted hydrocarbons, notably carbon monoxide, similar polarization phenomena were observed, also at large current densities. The conductivity computed from the slope of the current-voltage plots was the same as determined by direct a.c. resistance measurements. At 1000°C we found a conductivity of about 0·02 ohm−1 cm−1.

Evaluation of organic battery electrodes: Voltammetric study of the redox behaviour of solid quinones

The redox behaviour of solid quinones has been examined by potential sweep and cyclic voltammetry. A number of quinones are insoluble in aqueous electrolytes and can be completely discharged and recharged without appreciable polarization, as found, for example, with solid chloranil (tetrachloro-p-benzoquinone) and solid duroquinone (tetramethyl-p-benzoquinone). Hence quinones are reversible redox systems not only in solutions but also in the solid state. During discharge and recharge in dilute sulphuric acid only slight polarization occurs, which is attributed to a pure diffusion overvoltage. Polarization is not much greater in salt solutions. In this case, however, the peaks of the voltammetric curves are broader. In ammonium chloride solutions a double peak corresponding to the two electron transfers is obtained for chloranil both anodically and cathodically; this shows that the semiquinone is stable in ammonium chloride solution. Duroquinone also gives two peaks, but these are observed only in non-stationary measurements so that they do not correspond to the state of equilibrium. The pH dependence of the redox potential is 59 mV per pH unit. Only chloranil in ammonium chloride solution shows two redox potentials, depending on the discharge depth, which are both lower than the potential corresponding to the pH. The possible application of quinones in secondary battery cathodes is discussed.

Comparison of the reaction mechanisms of electrocatalytic oxygen reduction using transition metal thiospinels and chelates

The cathode process of oxygen reduction on thiospinels and other sulphides was examined by potentiodynamic, galvanostatic and rotating ring-disc electrode measurements. The highest activity was found with the sulphur compounds of cobalt and iron. The results are compared with those obtained with iron and cobalt melt chelates of the macrocyclic N4 type (phthalocyanines, dibenzotetraazaannulenes) because knowledge is required about the reaction itself and the mechanism of the reduction of activity of the non-precious metal catalysts. The reaction scheme for the oxygen reduction is discussed and values of the rate constants of the electrochemical reactions are given.

Microgravimetric preparation and characterisation of catalyst candidates for fuel cell electrodes

Abstract In the search for noble-metal-free cathode materials resistant to acid electrolytes and capable of catalysing the conversion of oxygen, thermogravimetric methods have been used to determine the conditions of preparation, to analyse the chemical properties, and to characterise the pore structure of the potential catalysts. The paper comprises results of investigations on inorganic compounds, such as ammonium tungstates, chromates, vanadates, molybdate and permanganate, and on organic polymers, such as polyamide nitriles, other CN polymers and polyacenoquinone pyrolysates.