Jiri Divisek

Components for PEM fuel cell systems using hydrogen and CO containing fuels

Proton exchange membrane fuel cells (PEMFC) show a significant performance drop in CO containing hydrogen as fuel gas in comparison to pure hydrogen. The lower performance is due to CO adsorption at the anode thus poisoning the hydrogen oxidation reaction. Two approaches to improve the cell performance are discussed. First, the use of improved electrocatalysts for the anode, such as PtRu alloys, can significantly enhance the CO tolerance. On the other hand, CO poisoning of the anode could be avoided by the use of non-electrochemical methods. For example, the addition of liquid hydrogen peroxide to the humidification water of the cell leads to the formation of active oxygen by decomposition of H2O2 and the oxidation of CO. In such a way a complete recovery of the CO free cell performance is achieved for H2/100 ppm CO.

Process engineering of the direct methanol fuel cell

A direct methanol fuel cell (DMFC) model has been developed and experimentally verified, with which fundamental calculations of the DMFC were carried out. Modelling comprises the mass transport of the gases in the diffusion layers and catalyst layers, mass transport in the membrane, as well as the reaction and the potential distribution in the catalyst layers. The performance of the fuel cell is adversely influenced by methanol permeation from the anode to the cathode. Moreover, the formation of a mixed potential is possible both at the anode and cathode and has a large negative effect on the energetic performance of the fuel cell. The model provides information concerning the impact of methanol permeation through the membrane on energy and mass yield, and on the influence of the operating and structural parameters.

Recent developments of the measurement of the methanol permeation in a direct methanol fuel cell

The performance and the efficiency of direct methanol fuel cells (DMFCs) are affected by the methanol permeation from the anode to the cathode. A widely used method to measure the methanol permeation in a DMFC is the analysis of the carbon dioxide content of the cathode exhaust. During the operation of a DMFC large amounts of carbon dioxide are produced in the anodic catalyst layer which can diffuse partially to the cathode. As a consequence the carbon dioxide in the cathodic exhaust gas stream is expected to consist of two fractions: the carbon dioxide resulting from the oxidation of the permeating methanol and the carbon dioxide diffusing from the anode to the cathode. In this work we describe a way to separate the distribution of the two fractions under real DMFC operating conditions. As a results we found that with low methanol concentrations (<1 M) and high current densities the amount of carbon dioxide passing from the anode to the cathode can even be higher than the amount of carbon dioxide formed at the cathode by methanol oxidation.

Two‐Dimensional Simulation of Direct Methanol Fuel Cell. A New (Embedded) Type of Current Collector

A two-dimensional numerical model of the direct methanol fuel cell with gas fuel is developed. Simulation of the cell with current collectors of conventional geometry reveal the formation of fuel-depleted, shaded regions in the cathode and anode catalyst layers. These regions are positioned in front of current collectors, farther from the gas channel windows. Another disadvantage of the conventional geometry is the concentration of electron current at the edges of current collectors. Based on the simulation results, a new design of current collectors is suggested. It is beneficial to position current collectors inside the backing and catalyst layers, parallel to the flow of the fuel. These embedded collectors do not produce shaded regions in the catalyst layers. Two plausible geometries of such collectors are considered: of rectangular and circular shape. Simulations show that depending on the transport properties of the backing and catalyst layers the embedded current collectors may significantly improve the performance of the fuel cell. This conclusion is valid also for hydrogen-oxygen fuel cells.

Performance Improvement of a PEMFC Using Fuels with CO by Addition of Oxygen‐Evolving Compounds

A new method is described to improve the performance of a proton exchange membrane fuel cell (PEMFC) using reformed methanol or H{sub 2}/CO as fuels. The addition of liquid hydrogen peroxide to the humidification water for the fuel gas leads to a heterogeneous decomposition of H{sub 2}O{sub 2} and formation of active oxygen. In this way adsorbed CO is oxidized nonelectrochemically to CO{sub 2} and the blocking of the hydrogen oxidation reaction at the anode can be avoided. It is demonstrated that a complete recovery of the CO-free performance is achieved for H{sub 2}/100 ppm CO.

The kinetics of electrochemical reactions on high temperature fuel cell electrodes

The rates of electrochemical reactions relevant for use in high-temperature solid oxide fuel cells (SOFC) has been investigated as a function of electrode potential, temperature and composition of the gas mixture. From Arrhenius plots, apparent activation energies, Ea, and apparent pre-exponential factors, A, were calculated for the oxygen-reduction and oxygen-evolution reactions at La0.84Sr0.16MnO3 cathodes. At low overpotentials (|η| ⩽ 0.2 V), both apparent activation energies and apparent pre-exponential factors are much higher in the temperature range T = 800−1000 °C (Ea ≈ 160−210 kJ/mol, log A ≈ 6−9) compared with those in the range T = 500−800 °C (Ea ≈ 80−110 kJ/mol, log A ≈ 2−4). For oxygen reduction, reaction orders of ze = 1 at pO2 > 0.2 bar and ze = 0.5 at pO2 < 0.2 bar were obtained. These values may be related to either oxygen adsorbed as molecules or atoms as the reacting species. From impedance spectroscopy, it follows that the rate of the oxygen-exchange reaction is determined not only by charge transfer, but also by another process, possibly the adsorption or surface diffusion of intermediates. For the nickel zirconia cermet anode fabricated by wet powder spraying (WPS), an increase in sintering temperature to 1400 °C results in an increase in current density. A current density of 0.27 A cm−2 at an overvoltage of 0.1 V may be achieved. From Arrhenius plots, an energy of activation of 130 ± 10 kJ mol−1 was determined.

Oxygen evolution at nickel anodes in concentrated alkaline solution

Abstract Oxide coated nickel electrodes were investigated in concentrated alkaline solutions. Their electrochemical behaviour was studied in the potential regime between the double layer capacity region and the oxygen evolution, at 20, 50 and 80°C. Cyclic voltammograms and quasi-steady-state current-potential curves change remarkably with temperature, correspondingly, the impedance of the system changes strongly with potential, but also with temperature. The experimental results do not support the Krasilshchikov mechanism suggested by many authors.

New preparation technique and characterisation of nanostructured catalysts for polymer membrane fuel cells

Abstract A novel preparation technique for catalyst layers used in Proton-Exchange-Membrane Fuel Cells is presented. This method enables a selective electrochemical deposition of platinum nanoparticles on those surface areas of the carbon support which are in contact with the proton and electron conducting phases and simultaneously with the gaseous or liquid fuel. Model electrodes consisting of sprayed layers with a Platinum precursor on glassy carbon substrates were used for the experiments. Size and size distribution of the platinum nanoparticles are determined by high resolution transmission electron microscopy and X-ray line shape analysis; the electrochemical activity of the catalyst layers is investigated by cyclic voltammetry.

Energy balance of D2O electrolysis with a palladium cathode: Part I. Theoretical relations

On the basis of reliable thermochemical data, the relations for the calculation of the thermoneutral voltage, Etn, of the D2O electrolysis from an approx. 0.1 M LiOD+D2O solution with a Pd cathode with PdDn formation have been derived for stationary and non-stationary courses of electrolysis, as a function of reaction temperature, total pressure and current efficiencies of 02(g) and D2(g) evolution. Due to the lack of thermochemical data for PdDn formation with n > 0.8, the accuracy of the calculated thermoneutral voltage with PdDn formation under non-stationary conditions is lower than for the case of stationary electrolysis.