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The oxygen reduction reaction (ORR) mechanism for electrochemical production of hydrogen peroxide is investigated for an Au-Pd catalyst with 25 at% palladium. The sample was prepared by chemical reduction of a precursor solution on a carbon-based substrate. Rotating ring-disk electrode (RRDE) measurements were performed for electrochemical characterization exhibiting H2O2 current efficiencies of about 75-78% at low disk potentials. The focus of this article is on the mathematical evaluation of the experimental data using the analytical method from Damjanovic et al. [J. Chem. Phys. 45, 4057 (1966)] and a powerful numerical data fitting technique based on RRDE simulations in MATLAB, which is capable of investigating reaction mechanisms with different structures. The latter method is presented in detail and the electro-kinetic parameters of the ORR mechanism at the considered catalyst resulting from both methods are compared quantitatively. It is found that the oxygen reductions via the 2-electron and the 4-electron pathways occur in parallel, whereas the hydrogen peroxide reduction pathway is found apparently inactive.

Recently, the use of alkaline anion exchange membranes has yielded some enhancement of direct ethanol fuel cells at least with respect to the achieved performances. However, so far reports have been made for cells operated with a large excess of KOH and mostly without cycling the anolyte. Under real fuel cell conditions with an anode loop, an accumulation of carbonate ions and a reduction of pH are to be expected. In order to evaluate the effect of these changes on the ethanol oxidation reaction, the ethanol oxidation was studied at polycrystalline platinum thin film electrodes in electrolyte solutions with different pH values and varying carbonate ion concentrations. No severe detrimental effects of high carbonate ion concentration were found. The pH of the electrolyte has an effect depending on the concentration of ethanol. At high ethanol concentrations relevant for fuel cells an optimum value would be around pH 12.

In order to be tax free, the ethanol for use in direct ethanol fuel cells (DEFC) needs to be denatured. The electrochemical behavior of a number of potential denaturing agents was therefore studied with respect to their compatibility with DEFC operation. Tests comprised the investigation of the denaturing agents' oxidation at platinum model electrodes, their influence on the ethanol oxidation at platinum model electrodes and at fuel cell catalysts as well as the analysis of the products which get formed. Cyclic voltammetry, chronoamperometry and differential electrochemical mass spectrometry were used as methods. Eventually, a mixture of ethyl-tert-butyl ether and Bitrex(r) was found which should qualify as denaturing agent and did not show any severe detrimental effect on electrochemical reactions important for DEFC operation.

The oxidation of ethanol and ethylene glycol at platinum as model catalyst was investigated in potassium hydroxide solution. Experiments comprised the investigation of formed adsorbates by stripping voltammetry of pre-adsorbed alcohols and the investigation of the bulk oxidation as function of the alcohol bulk concentration by cyclic voltammetry and chronoamperometry. For all measurements online mass spectrometry was used to detect the volatile reaction products. For both alcohols it was found that during stripping experiments only the C1 products are formed. For the bulk oxidation of ethanol the CO2 current efficiency rapidly decreases with increasing ethanol concentration showing hardly any CO2 formation at technical relevant concentrations. Although CO2 current efficiencies also dropped for the ethylene glycol oxidation with increasing concentration,the effect was much less pronounced. CO2 formation was thus also observed at technical relevant ethylene glycol concentrations even after longer times under potentiostatic conditions.

An ethanol derivative and fusel oil were investigated regarding their possible use as denaturing agents for ethanol for DEFC in 0.5 M sulphuric acid and 0.1 M potassium hydroxide solution at platinum by means of cyclic voltammetry and differential electrochemical mass spectroscopy. It was found that fusel oil is not suitable as denaturing agent for DEFC due to strong adsorption to platinum surfaces and the hindrance of the ethanol conversion. On the other hand, the investigated ethanol derivative does not hinder the ethanol conversion as much and therefore seems to be a promising candidate for a denaturing agent to use with ethanol in DEFC.

In the framework of a project funded by the Fraunhofer Gesellschaft, a direct ethanol fuel cell has been developed. As part of the development, a new catalyst concept for the anodic oxidation of ethanol at an increased CO2 efficiency by using two different catalysts for two steps in the reaction process has recently been presented. Furthermore, new membranes based on an organic-inorganic mixed-matrix which exhibit reduced ethanol cross-over at an increased selectivity for water diffusion, have been developed. In this paper we will describe the developments which were made to integrate these two approaches into membrane electrode assemblies. The obtained membrane electrode assemblies showed better performance in a DEFC compared to membrane electrode assemblies using Nafion® 115, in particular at low temperature and low cathode airflow conditions which are typical for portable applications.