Hendrik Dohle

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.

Interaction between the diffusion layer and the flow field of polymer electrolyte fuel cells—experiments and simulation studies

The flow distribution in fuel cells has an important influence on both the power density and efficiency of fuel cell systems. In order to effectively utilize the area, flow distribution should be as homogeneous as possible. In addition, pressure losses should be minimized with regard to the power demand of auxiliary components as pumps and compressors. In polymer electrolyte fuel cells (PEFCs) and direct methanol fuel cells (DMFCs) the flow field is in direct contact with the diffusion layer. The main task of the diffusion layer is to distribute the reactants from the flow field towards the catalyst layer. To prevent diffusion overvoltages, the diffusion layer is in general highly porous and provides high fluxes of the reactants. Consequently, the flow distribution in the flow field can be superpositioned by a flow in the diffusion layer. In this paper, we discuss the interaction between the diffusion layer and the flow field. Experimentally, we characterized different diffusion layers with regard to their diffusion properties as well as different flow fields. Additional simulation studies help to understand the processes and to determine suitable combinations of flow fields and diffusion layers.

Heat and power management of a direct-methanol-fuel-cell (DMFC) system

In this paper, we describe the heat and the power management of a direct methanol fuel cell system. The system consists mainly of a direct methanol fuel cell stack, an anode feed loop with a heat exchanger and on the cathode side, a compressor/expander unit. The model calculations are carried out by analytical solutions for both mass and energy flows. The study is based on measurements on laboratory scale single cells to obtain data concerning mass and voltage efficiencies and temperature dependence of the cell power. In particular, we investigated the influence of water vaporization in the cathode on the heat management of a direct-methanol-fuel-cell (DMFC) system. Input parameters were the stack temperature, the cathode pressure and the air flow rate. It is shown that especially at operating temperatures above 90 °C, the combinations of pressure and air flow rate are limited because of heat losses due to vaporization of water in the cathode.