1,720,968 research outputs found
Influence of supporting electrolytes on the rate of deposition of γ-FeOOH from Fe2+ ions
No full textSimple uni-univalent electrolytes introduce significant modifications in the rate of formation of γ-FeOOH from Fe2+. Kinetic results show that the electrolytes only influence the partition reaction of Fe2+ at the oxide-solution interface. The effect is induced by specific interactions between the oxide and the anions of the electrolytes. © 1988
Oxygen cathode for fuel cells. ORR activity of mesoporous N-modified carbon doped with non-noble metals
No full textFuel cells for low temperature applications are the subject of intensive research because they offer promise of reducing consumption of primary fossil fuels and greenhouse gas emissions. A difficulty comes from the kinetic behaviour of cathodic oxygen reduction that requires some form of effective catalytic promotion to proceed fast enough to meet fuel cell power requirements as for instance in the case of automotive transportation. Platinum and Pt-alloys are at present the best catalysts for oxygen reduction in PEM fuel cells. Nevertheless, due to the natural scarcity and cost of platinum, a very interesting challenge is to find an alternative non-precious, though less catalytically performing, material. Among others, nitrogen-modified carbons doped with non-precious transition centres (Fe, Co, etc.) are of interest because of the flexibility by which composition and morphology can be tailored by preparation.
In this work we present some results on oxygen reduction by various Pt-free catalysts obtained by carbonization and calcination of nitrogen bases/sugar mixtures in the presence of a metal salt and a silica gel as mesopore templating agent.
Catalysts were characterized by physical, chemical and electrochemical methods. Results are given in terms of the influence that sugars, nitrogen bases and metal centres exert on oxygen reduction potential, on the presence of a limiting current and on reaction mechanism. The best materials have been obtained by using fructose, a guanidine derivative and iron acetate. They are characterized by a well-defined limiting current, an onset potential approaching Pt ORR starting potential and a number of exchanged electrons n>3.9
Platinum-free electrocatalysts for oxygen reduction reaction
No full textPlatinum and Pt-alloys are at present the best catalysts for oxygen reduction in PEM fuel cells. Since the precious metal is scarce and expensive, there is a strong effort to find substitute catalysts. Catalysts based on iron and cobalt ions seem to be a good and promising alternative to platinum. Moreover, the nature and behaviour of the catalyst support are also of the utmost importance. Activated carbons are used as catalyst and electrocatalyst supports in many chemical and electrochemical processes. This is due to the fact that through their structural and morphological features (high surface area, controlled porosity with selected pore size dimensions, high adsorption power and complex surface chemistry) they generally improve the intrinsic catalyst behavior. Improved performances of such composite catalyst-substrate materials are desirable and can be obtained by modifying the nature and number of the native functional groups of the carbon surface and by introducing new surface species, as metal centres, heteroatoms, etc.. In this work we present some data concerning electrochemical oxygen reduction onto nitrogen-modified activated carbon also bearing transition metal cations. Different nitrogen-bearing precursors and carbon sources will be considered. Catalysts characterisation was carried out by physical, chemical and electrochemical methods. Results are given in terms of the influence that nitrogen and carbon precursors exert on the electrocatalytic activity of the obtained materials
Surface behaviour of γ-FeOOH : Point of zero charge and specific ionic interactions
No full textThe acid-base dissociation behaviour of the surface of γ-FeOOH is investigated by potentiometric acid-base titrations of the oxide suspensions in aqueous solutions. The zero point of charge (p.z.c.) of the electrical double layer of the reversible oxide solution interface is determined: in KNO3 titrations at various ionic strenghts cross over at pH = 7.6; by coincidence with the pH of the isoelectric point, determined by microelectrophoresis, the cross over of titrations in KNO3 is identified as the point of zero charge of the surface. Specific effects regarding monovalent anions (Cl-, ClO4-) are found to be absent, while SO4-- strongly adsorbs on the γ-FeOOH surface, thus modifying the balance of the positive and negative groups on the surface. The sequence of interaction of monovalent cations with the surface is reported: K+ < Na+ < Li+. Surprising and of particular interest are the strong adsorption of Na+, which has always been reported to interact only electrostatically with oxide surfaces, and the absence of specific interactions with Cl- ions. © 1982
Templating effects onto electrocatalytic properties of Pt-free carbons for oxygen reduction reaction
No full textThe aim of this work was to investigate morphologic effects that templating may exert on the synthesis of nitrogen-doped Pt-free carbons and any resulting variation in the electrochemical behavior of these products for the oxygen reduction reaction (ORR). This latter reaction is kinetically slow and limits the energy efficiency of currently investigated Polymer Electrolyte Membrane Fuel Cells. The starting point of this work was a silica templating method, in which organic precursors reacted at high temperature in the presence of high surface area silica. Other simple templating methods were projected and developed. Among these, some catalyst synthesis procedures required solid supports as shape-imprinting materials. The aim was to obtain a replica of template porosity and surface features in the synthesized products. In other synthesis, auto-templating processes of catalyst precursors, e.g. freeze-drying and in-situ generation of gas, were studied. Synthesized catalysts have been characterized by physico-chemical methods as surface area analysis and cyclic voltammetry, which allowed the study of the kinetics of the electrochemical ORR by rotating disk electrode and rotating ring disk electrode in acidic and alkaline solutions. A relation between pore size distribution and electrochemical activity of the synthesized materials seemingly exists. Overall, a BET porosity analysis suggests that templating procedure can be actually effective to improve the electrocatalytic behaviour for ORR, only if it ensures a homogeneous distribution of macropores and small and medium mesopores (Figure 1). This surface feature is necessary to catalyst performances because it favours mass transport processes to/from the electrode surface
Platinum-free electrocatalysts for Oxygen Reduction Reaction
No full textAt present, platinum and its alloys are the best cathode catalysts for oxygen reduction in PEM fuel cells. However, since this metal is scarce and expensive, there is a strong effort to find alternative catalysts. Catalysts based on iron and cobalt ions seem to be a good and promising alternative to platinum [1]. Moreover, in the literature it is underlined the importance of the nature and behaviour of the catalyst support. Activated carbons can be used to support catalysts and electrocatalysts in many chemical and electrochemical processes. This is due to their structural and morphological features (high surface area and microporosity), high adsorption power and complex surface chemistry. Improved performances of these materials are desirable and can be obtained by modifying the nature and number of the native functional groups of the carbon surface and by introducing new surface species, as metal centres.
In this work we present some results about electrochemical oxygen reduction onto amine-modified activated carbon also bearing metal cations (Co, Fe). Different - diamines were used. Catalysts characterisation was carried out by physical, chemical and electrochemical methods. Results are given in terms of the influence that amines and metal centres exert on the onset of oxygen reduction potential and oxygen reduction currents.
[1] F. Charreteur, F. Jaouen, S. Ruggeri, J.-P. Dodelet, Electrochim. Acta, 53 (2008) 2925
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