Ulrich Stimming

Metallic and non-metallic nanoparticles, usually supported on non-metallic substrates have attracted much interest concerning their application in the field of electrocatalysis. To characterize catalysts with respect to size, morphology, structure and composition (alloys or core-shell) of nanoparticles and their associated electrocatalytic activity, transmission electron microscopy (TEM) is the state of the art method. This investigation shows the advantages of advanced image processing using the local adaptive threshold (LAT) routine.

The interface HgHClO4 · 5.5H2O was investigated by cyclic voltammetry and electrochemical impedance spectroscopy between 170 and 300 K. In this temperature range, phase transitions occur for electrode and electrolyte, such that within 6 K the electrode-electrolyte interface changes from solid-solid over solid-liquid to liquid-liquid or vice versa. The double layer capacity was investigated as a function of potential for

... 221–228. 2 LGJ de Haart, Th. Hauber, K. Mayer and U. Stimming, in B. Thorstensen (ed.), Proc. 2nd Eur. ... 229–235. 3 HP Buchkremer, U. Diekmann, LGJ de Haart, H. Kabs, U. Stimming and D. Stöver, in U. Stimming, SC Singhal, H. Tagawa and W. Lehnert (eds.), Proc. 5th Int. ...

The electrolytes currently used for proton exchange membrane fuel cells are mainly based on polymers such as Nafion which limits the operation regime of the cell to ∼80 °C. Solid oxide fuel cells operate at much elevated temperatures compared to proton exchange membrane fuel cells (∼1000 °C) and employ oxide electrolytes such as yttrium stabilized zirconia and gadolinium doped ceria. So far an intermediate temperature operation regime (300 °C) has not been widely explored which would open new pathways for novel fuel cell systems. In this review we summarize the potential use of phosphate compounds as electrolytes for intermediate temperature fuel cells. Various examples on ammonium polyphosphate, pyrophosphate, cesium phosphate and other phosphate based electrolytes are presented and their preparation methods, conduction mechanism and conductivity values are demonstrated.

In this work, we prepared NH4PO3/MO2 (MSi, Ti) composite materials with various amounts of TiO2 using a sol–gel method, which are potential for application as electrolytes for intermediate temperature fuel cells (150–250°C). The effect of the amount of TiO2 on the conductivities of the composite was investigated systematically by an impedance spectroscopy within the temperature range of 50–275°C under different atmospheres. The composite SiTiO10APP (10mol% TiO2) showed high proton conductivity, 0.001–0.043Scm−1 at 150–250°C. The maximum conductivities are 0.043Scm−1 at 225°C under humid H2 and 0.0085Scm−1 at 175°C under humid air, respectively. A thermogravimetric analysis showed the composite had high thermal stability below 300°C. Moreover, it was found that the microstructure has significant effect on the conductivity of the composites at higher temperature.

ABSTRACT The effect of isotopic exchange (D vs H) on the bulk electric conductivity and the static dielectric constant of acid polyhydrates, having a clathrate structure, were determined between 10 and 228 K (melting point). Protonated and deuterated acids show virtually identical Arrhenius plots of the conductivity above 120 K. Below that temperature they differ, the deuterated compound exhibits a smaller conductivity and a smaller dielectric constant as compared with the protonated form. The hydrogen and the deuterium evolution reactions were investigated at the interface Ag-solid 5.5 hydrated perchloric acid between 140 and 228 K. The gas evolution was found to depend strongly on temperature and isotope (D vs H). At the solid-solid interface the hydrogen evolution occurs faster than the deuterium evolution at the same overpotential. A temperature dependence of the charge transfer coefficient is observed with nα decreasing with decreasing temperature.

... Article. Einfluß von Oxid-Schichten auf Elektrodenprozesse †. Prof. Dr. Joachim Walter Schultze,; Dr. Peter Kohl,; Dr. Manuel Lohrengel,; Dipl.-Chem. Ulrich Stimming. Article first published online: 9 JUN 2004. DOI: 10.1002/cite.330510616. Copyright © 1979 Verlag Chemie, GmbH ...

The electrode performances of the alternative oxides: La0.05Ca0.95Cr0.05Ti0.95O3‐δ‐8YSZ and Ce0.8TM0.2O2‐δ(TM=Mn, Co) for the direct electrochemical oxidation of methane are investigated to assess their potential as anode materials for efficient methane conversion in a SOFC. The electrochemical oxidation of hydrogen was also studied, for comparison. The oxides are characterised electrochemically with impedance spectroscopy in the frequency range from 10 mHz to 1MHz, using a three‐electrode geometry. They are compared to a standard Ni/8YSZ anode for the electrochemical oxidation of hydrogen. It is found that La0.05Ca0.95Cr0.05Ti0.95O3‐δ‐8YSZ demonstrates a poor electrochemical activity in both hydrogen and methane. However, the electrochemical activity of Ce0.8Mn0.2O2‐δ is promising, but the electronic conductivity needs to be increased, e.g., by adding a conducting oxide, before it can be used as an anode material in a SOFC.

The fuel cell differential electrochemical mass spectrometry (FC‐DEMS) measurements were performed for studying the ethanol oxidation reaction (EOR), using alkaline membrane electrode assemblies (MEAs) made up of nanoparticle Pt catalyst and alkaline polymeric membranes. The obtained results indicate that in an alkaline medium, ethanol undergoes significantly more complete electro‐oxidation to CO2 than in an acidic MEA using the same Pt anode. The CO2 current efficiency (CCE) can be compared for acidic and alkaline MEA with similar electrochemical active area on the anode side. The CCE estimated, in case of alkaline MEA with Pt anode, is around 55% at 0.8 V/RHE, 60 °C and 0.1 M ethanol. In comparison, under similar conditions, acidic MEAs using the same anode catalyst show only 2% CCE. This might indicate that the C–C bond scission rates are much higher in alkaline media. However, the mechanism of ethanol oxidation in alkaline media is not exactly known. CO2 produced in electrochemi...

The influence of a high temperature annealing (∼2200 °C) on the microstructure and the electrical properties of BaZr0.9Y0.1O3−δ has been examined. The high temperature annealing was achieved using an optical floating zone furnace. The obtained sample is homogeneous with an average grain size of about 5 μm. No additional phases were found in the XRD pattern and the structure was identified to

ABSTRACT On the basis of a QENS experiment using thermal neutrons on the proton conductor SrCe0.95Yb0.05H0.02O2.985 we extend our two-state model for the proton diffusion in this compound. This model was developed from a recent high resolution QENS experiment using cold neutrons, with the first state representing free diffusion of the protons (jumping from oxygen to oxygen forming OH− ; free state) and the second state representing protons trapped on energetically lowered sites (nearby an Yb3+ ion on a Ce4+ site; trapped state); into this model we integrate a very fast localized motion taking place in the free state as a reorientation of the OH− ions. This motion is visible as a broad quasielastic component in the neutron energy spectra and exhibits a very low activation energy.

... The heat requirement of the reformer during operation is provided by the catalytic combustion of ... cell appears a problem which can be solved from a process engineering point of ... types of fuel cells are currently being discussed for application in mobile systems: the phosphoric ...

This study investigates the improvement of proton exchange membrane fuel cell (PEMFC) carbon monoxide by periodic air dosing. The carbon monoxide in the fuel gas leads to a significant loss in power density due to CO poisoning in the anode. The method involves bleeding air into the anode fuel stream (H2–CO), which contains CO in various concentrations (20ppm, 52.7ppm, and 100ppm). In the transient CO poisoning test, air bleeding is performed for four different periodic air dosing and cell voltage is fixed at 0.6V. The result of a dosing of air for 10s in intervals of 10s is similar to that of continuous air bleeding except for 100ppm CO. The CO tolerance of the fuel cell and cell performance recovery from poisoning can be improved by air bleeding.

ABSTRACT The ethanol electro-oxidation at gas diffusion electrodes made of different catalysts, Pt/C, PtRu(1:1)/C, and PtSn(7:3)/C, were studied by on-line differential electrochemical mass spectrometry in a wide temperature range (30-90 degrees C) as a function of the anode potential, the fuel concentration, and catalyst loading. The CO2 current efficiency (CCE) of the ethanol oxidation reaction (EOR) exhibits a maximum at about 0.6 V and decreases rapidly with further increasing potentials. The CCE for the EOR goes down with the increase in concentration of ethanol. CCE for ethanol oxidation reaction shows a strong increase with increasing catalyst loading. The CCE increases with increasing temperature, exceeding 75% at 90 degrees C, 0.1 M ethanol, and 5 mg/cm(2) Pt catalyst loading. PtSn/C shows high CCE, like Pt/C. But PtRu/C exhibits very small CCE. Of the intermediates, acetaldehyde is quite active for further oxidation. But acetic acid is fairly resistant against further oxidation. Our results indicate that the C-C bond scission observed for the EOR with CCE in excess of 50% has to proceed in parallel with ethanol oxidation to either acetaldehyde or acetic acid, and not sequentially from acetic acid further on, as acetic acid cannot be oxidized any further.

The performance of a proton exchange membrane fuel cell (PEMFC) was evaluated at 80°C in with defined amounts of CO (25 to 250 ppm) and pure oxygen. Membrane electrode assemblies (MEAs) were made using Nafion™ 117 with carbon‐supported dispersed Pt, , and as ...

AJ DICKINSON1, LPL CARRETTE2, JA COLLINS3*, KA FRIEDRICH4 and U. STIMMING5 1 IMRA Europe SAS, Energy and Environment Department, 220 Rue Albert Caquot, 06904 Sophia Antipolis, France 2Advanced Materials Editorial Office, Wiley–VCH, Pappelallee 3, ...