C. Lamy

Recent advances in the development of direct alcohol fuel cells (DAFC)

Abstract Recent developments into technology of proton exchange membrane fuel cells (PEMFC) now allow serious consideration to be given to a direct alcohol fuel cell (DAFC) based on a PEMFC, in which alcohol is used directly as the fuel. This is particularly advantageous for mobile applications, since this will avoid the use of a bulky and expensive reformer. However, the relatively complex reaction mechanism, leading to a low electroreactivity of most alcohols, even methanol, needs the investigation of new platinum-based electrocatalysts, particularly active for breaking the CC bond when alcohols other than methanol are to be used. Moreover, in order to overcome the deleterious effect of alcohol crossover through the proton exchange membrane, it is necessary to develop new oxygen reduction electrocatalysts insensitive to the presence of alcohols.

Electrocatalytic oxidation of aliphatic alcohols: Application to the direct alcohol fuel cell (DAFC)

The electrooxidation of some low molecular weight alcohols, such as ethanol, ethylene glycol and n-propanol, is discussed in terms of reaction mechanisms and catalytic activity of the anode material. Some examples of a single cell, using a proton exchange membrane (PEM) as electrolyte, are given to illustrate interesting results, particularly for the direct electrooxidation of ethanol. This alcohol may replace methanol in a direct alcohol fuel cell.

In situ FTIRS study of the electrocatalytic oxidation of carbon monoxide and methanol at platinum–ruthenium bulk alloy electrodes

The electro-oxidation of carbon monoxide and methanol on platinum–ruthenium bulk alloys was investigated by cyclic voltammetry and in situ FTIR reflectance spectroscopy. Electrochemical measurements revealed that these two oxidation reactions are dependent on the ruthenium composition in the alloy. For CO oxidation, the alloy with an Ru concentration of 50 at.% allows this reaction to occur at lower potentials than on pure platinum or pure ruthenium. Conversely, methanol electro-oxidation gives higher current densities on alloys, containing 10–15 at.% Ru. With FTIRS measurements, it was found that, during methanol oxidation on Pt–Ru alloys, CO, but also small amounts of HCHO and HCOOH, are formed, the surface distribution of which depend on the alloy composition. The COads absorption bands are quite absent in spectra corresponding to alloys with the optimum composition (15–20 at.% Ru). On the other hand, during the oxidation of gaseous CO on alloys with the optimum composition (Pt0.5Ru0.5), the CO2 formed is not adsorbed on the electrode surface, the opposite of the other compositions. These results suggest strongly that the rate determining step for both reactions is the formation of adsorbed OH on the ruthenium surface.

Investigation of Ternary Catalysts for Methanol Electrooxidation

The electrochemical oxidation of methanol was investigated on Pt–Ru–X ternary metallic catalysts (with X=Au, Co, Cu, Fe, Mo, Ni, Sn or W). The catalysts were prepared by electrochemical deposition and dispersed in a conductive three-dimensional matrix, an electronic conducting polymer, polyaniline (PAni). A comparative study of the behaviour of several ternary catalysts towards the electro-oxidation of methanol shows that PAni/Pt–Ru–Mo is the most efficient anode at potentials up to 500mV vs RHE. This latter ternary electrocatalyst leads to current densities up to 10 times higher than those measured with PAni/Pt–Ru in this potential range. Moreover, the catalyst appears to be stable for potentials lower than 550mV vs RHE. According to EDX analysis, the good behaviour of the Pt–Ru–Mo ternary catalyst seems to result from the presence of a small amount of the third metal, at an atomic ratio close to 5%. This set of encouraging results has also been confirmed by preliminary measurements in a single cell direct methanol fuel cell (DMFC) containing a home made PAni/Pt–Ru–Mo anode. The ternary catalyst leads to higher power densities than the PAni/Pt–Ru binary catalyst under the same experimental conditions.

A kinetic analysis of the electro-oxidation of ethanol at a platinum electrode in acid medium

In order to establish the kinetic laws which govern the oxidation process of ethanol on smooth platinum electrodes in acid medium, electrolyses of ethanol were carried out under different experimental conditions. The influence of the initial ethanol concentration and of the potential plateau of electrolysis were investigated and allowed us to improve our understanding of the reaction mechanism. In addition, some ir reflectance spectroscopy experiments were performed to identify the adsorbed intermediates and the reaction products.

Structural effects in electrocatalysis: A comparative study of the oxidation of CO, HCOOH and CH3OH on single crystal Pt electrodes

Abstract The electrocatalytic oxidation of CO, HCOOH and CH3OH is compared on Pt(100), Pt(110) and Pt(111) single crystals. The similarity of behaviour of CH3OH and CO on the three single crystal planes leads to the conclusion that the oxidation of both compounds involves CO-like intermediates. On the other hand, HCOOH behaves quite differently, particularly on Pt(100) and Pt(111) electrodes, i.e its oxidation involves different adsorbed intermediates, some of them being poisoning species formed in the hydrogen region.

In situ UV visible reflectance spectroscopic investigation of the nickel electrode-alkaline solution interface

Abstract An in situ uv visible reflectance spectroscopic technique is used to investigate the nickel electrode-alkaline solution interface. It is shown that by recording the change in relative reflectivity of the surface vs the potential applied to the electrode, at various fixed wavelengths, it is possible to get the spectra of superficial layers. Reflection-absorption spectra were thus obtained respectively for the hydroxide αNi(OH) 2 (two maxima at 360 and 500 nm) and the oxyhydroxide NiOOH (broad band with a maximum at 500–550 nm), providing new insights concerning the reversibility of some of the electrochemical processes which occur on the electrode surface when varying the potential.

Electrocatalytic oxidation of methanol on platinum-based binary electrodes

Abstract Four platinum-based binary catalysts (Pt+Sn, Pt+Pb, Pt+Re, Pt+Ru) used for the electrochemical oxidation of methanol were investigated by linear and cyclic voltammetry. Except for the Pt+Ru system, which always gives an enhanced electrocatalytic activity when compared to pure metals, thus leading to a synergistic effect, the other ad-atom+Pt systems show an increased activity only for some concentration ranges of the precursor salts, and for some ranges of electrode potentials. Outside of these experimental conditions, the three metal ad-atoms behave as relatively strong catalytic poisons. An explanation of these effects is suggested on the basis of a competitive adsorption between the second-metal ad-atoms, the adsorbed hydrogen and the strongly bound residue of methanol adsorption.

Electrocatalytic reduction of dioxygen at platinum particles dispersed in a polyaniline film

Platinum based electrocatalysts for the oxygen reduction reaction were prepared by electrodeposition of Pt particles in a polyaniline film acting as a convenient matrix to achieve low Pt loadings (from 11 to 600 μg cm−2). Using a rotating disc electrode or a rotating ring disc electrode allowed us to separate the different contributions in the overall reaction : diffusion of molecular oxygen in the electrolyte solution, diffusion inside the PAni film, adsorption process, and electron transfer. Koutecky–Levich analysis made possible to evaluate the kinetics parameters (total number of exchanged electrons, limiting current density, Tafel slope and exchange current density). These parameters vary with the Pt loading, displaying a steep increase at around 200 μg.cm−2, above which the behavior of bulk platinum is found. At lower Pt loadings, the kinetics is controlled by a Temkin isotherm, and a large amount of hydrogen peroxide is formed.

Electro-oxidation of ethanol on gold : analysis of the reaction products and mechanism

Although the electrocatalytic properties of gold are not as good as those of platinum, the oxidation of small alcohol molecules, such as ethanol, is possible at gold both in acid and alkaline media. By using programmed potential electrolysis coupled with chromatographic analysis of the products formed, the nature and the variation of their concentrations as a function of pH and time have been determined. In alkaline medium, the only product detected was acetic acid. On the other hand, in perchloric acid medium, the reaction products are highly dependent on the value of the oxidation potential, with the following successive steps: ethanol→acetaldehyde→acetic acid. A general tentative mechanism is proposed and discussed.