W. Vielstich

Structural effects and reactivity in methanol oxidation on polycrystalline and single crystal platinum

The electrooxidation of methanol on single crystal surfaces is characterized by the formation of strongly adsorbed intermediates. The splitting off of hydrogen atoms is superimposed on hydrogen desorption and gives rise to a well defined peak at polycrystalline and at Pt(100). The role of hydrogen as inhibitor of methanol adsorption is confirmed once more. Adsorbed CO and other hydrogenated adsorbed species like COH, HCOH and H2COH are detected by means of in situ FTIR spectra. The differential adsorption behavior of the three basal planes is emphasized by both the first scan of the respective voltammograms and the ir spectroscopy results. Band center shifts of the CO adsorbate indicate island formation starting at the early stages of adsorption. The band shift can be addressed mainly to chemical coupling between adsorbed CO species. The onset potential for bulk oxidation increases in the order Pt(111) < Pt(110) < Pt(100). While the main reaction pathway for methanol oxidation to CO2 involves adsorbed CO species, a second pathway seems to occur via intermediate formation of formic acid which causes adsorption bands at 1710, 1430 and 1230cm−1.

A sniftirs study of ethanol oxidation on platinum

The oxidation of ethanol at a platinum electrode in perchloric acid solution is followed by in situir reflection-absorption spectroscopy at constant potentials. Depending on the electrode potential different infrared bands were detected and assigned. The faradaic efficiencies for the different products depend on ethanol concentration.

On the study of adsorbed species at platinum from methanol, formic acid and reduced carbon dioxide via in situ FT-ir spectroscopy

Three different coordinated forms of CO adsorbed to the platinum surface were observed for methanol, formic acid and reduced carbon dioxide: linear-, bridge- and multi-bonded CO. Using a new experimental approach absolute absorption bands are observed for the adsorption of methanol and reduced carbon dioxide. Sample spectra are computed with respect to a reference spectrum which is taken before adsorption. In this way the presence of a COH adsorbate is demonstrated. The absolute band procedure must be applied to observe features exhibiting only a small dependence on potential, because in this case using the usual SNIFT-irS procedure (first adsorption and then collection of spectra at two potentials) the signals may cancel out when computing the difference spectra. Using Pt/Ru as a bifunctional catalyst for water splitting and CO/COH oxidation, surface poisoning during the oxidation of methanol is strongly diminished: current-potential relationships of technical interest are obtained.

Investigation of methanol crossover and single electrode performance during PEMDMFC operation: A study using a solid polymer electrolyte membrane fuel cell system

Direct Methanol Fuel Cells (DMFC) employing Polymer Electrolyte Membrane (PEM) electrolytes have recently gained considerable interest because they are attractive candidates for vehicular applications. A manifold of potential sources of performance losses exists for these systems, a very serious one being methanol crossover across the membrane electrolyte. This work is focused on the dependence of crossover on reaction conditions, such as temperature and methanol concentration. Anode performances are also evaluated. Results are given for the establishment of single electrode potentials during cell heat-up and of single electrode performances during operation under various conditions. Additionally, a cyclic voltammetry technique is presented, which allows the evaluation of methanol crossover in a fuel cell under operating conditions.

Distinct performance evaluation of a direct methanol SPE fuel cell. A new method using a dynamic hydrogen reference electrode

Abstract A stable reference electrode to be operated in a proton-exchange membrane direct methanol fuel cell (PEMDMFC) was strongly desired but is still lacking. Conventional reference electrodes known from aqueous systems are difficult to mount and to connect to the membrane electrolyte, and they introduce Donnan potentials into the system. It is shown in this study that a dynamic hydrogen electrode roughly maintains the thermodynamic hydrogen potential when operated at low cathodic current density sufficient to uphold the saturation with hydrogen of the electrode. Such a polarised auxiliary electrode allows the determination of a single electrode potential versus current density curves directly in the fuel cell, even during operation of the DMFC. A distinct determination of single electrode performance and fuel crossover is now possible.

DEMS-cyclic voltammetry investigation of the electrochemistry of nitrogen compounds in 0.5 M potassium hydroxide

Abstract The electro-oxidation and -reduction of 0.05 M ammonia, 0.01 M hydroxylamine, 0.01 M sodium nitrite and 0.05 M sodium nitrate in 0.5 M KOH at Pt-black electrodes has been investigated using a combination of cyclic voltammetry with on-line MS analysis of volatile products (DEMS). All compounds investigated in this study form adsorbates with very similar properties. These adsorbates can be reduced to ammonia and oxidised to nitrogen. Ammonia is adsorbed anodically, while nitrite and nitrate are adsorbed cathodically. In contrast to the adsorbates of the other compounds, adsorbed nitrite can also be reduced to nitrogen oxides in addition to ammonia. The combination of cyclic voltammetry and on-line MS proves that the adsorbates do not consist solely of triple bonded nitrogen. Ammonia and hydroxylamine also form nitrogen oxides in bulk oxidation processes. For the first time, a reaction mechanism which takes into account the interconversion of nitrogen compounds and adsorbate processes is discussed.

Identification of the adsorbate during methanol oxidation

Abstract The formation and oxidation of the methanol adsorbate at platinum in acid solutions is performed under potential control using a flow cell technique. The charges in both processes are equal; this fact confirms the results of Podlovchenko et al. obtained from charging curves after open circuit adsorption. It is shown, using the on-line MS technique and labelled methanol, CD 3 OH, that the adsorbed intermediate is probably COH.

Direct proof of the hydrogen in the methanol adsorbate at platinum — an ECTDMS study☆

The strongly adsorbed intermediate of methanol oxidation at platinum in H2SO4 solution is studied by Electrochemical Thermal Desorption Mass Spectroscopy (ECTDMS). Electrodes with adsorbates are transferred to UHV and exposed to TDMS. The presence of hydrogen in the adsorbate is shown directly. For comparison, the same experiments have been performed with HCOOH and CO.

Reduction of carbon dioxide to methane and ethene—an on-line MS study with rotating electrodes

In this paper, a new type of cell for the detection of volatile products developed during CO2 reduction on copper by on-line mass spectroscopy is described. Bulk copper, electrodeposited copper on glassy carbon and in situ electrodeposited copper on glassy carbon are employed as electrocatalysts. Measurements of the mass signal as function of potential or time are presented. A possible reaction mechanism of the CO2 reduction on copper is discussed.

Methanol oxidation at carbon supported Pt and Pt-Ru electrodes: an on line MS study using technical electrodes

Methanol oxidation at technical carbon based electrodes in 0.05 M H2SO4 has been investigated by cyclic voltammetry using online MS under the conditions of an acid methanol fuel cell (DMFC). 5% Pt on Norit BRX and 30% Pt/Ru (40/60) on Norit BRX were used as catalysts. It is shown that methanol oxidation at technical electrodes can be characterized by a combination of cyclic voltammetry and mass spectroscopy. The onset potentials and potential dependences of the methanol oxidation rate can be determined directly by monitoring the formation of CO2. Onset potentials of 0.5V and 0.25 V/RHE have been measured for Pt and Pt-Ru catalysts, respectively. The onset of methanol oxidation can be shifted to even more cathodic potentials (0.2V) if the Pt-Ru electrode reduces oxygen simultaneously. Carbon monoxide gas was also purged into the methanol containing electroyte during measurement in order to investigate the catalyst performance under more adverse conditions. C13-labelled methanol was used to distinguish between CO2. formed from methanol (m/e = 45) and CO-oxidation (m/e = 44). Without CO the use of C13-labelled methanol enabled a distinction between methanol oxidation and carbon corrosion. The methanol oxidation at the platinum catalyst is severely inhibited by the presence of CO, shifting its onset to 0.65 V/RHE. In contrast the performance of the Pt-Ru electrode is not seriously affected under these conditions. It is concluded that Pt-Ru is an excellent catalyst for a methanol anode in an acid methanol fuel cell (DMFC).