R. Reissner

Dry layer preparation and characterisation of polymer electrolyte fuel cell components

Abstract The main problem for future fuel cell commercialisation is the cost of membrane–electrode assemblies (MEAs) satisfying both power density and lifetime requirements. At DLR, low-cost MEA production techniques are being developed. These new MEAs are characterised and investigated with physical and electrochemical methods in order to study the power loss processes, the lifetime, the reaction mechanisms and in support of MEA development. The possibilities for the characterisation methods used will be demonstrated by various examples. At DLR, a new production technique based on the adaptation of a rolling process is developed for fuel cell electrode and MEA preparation. After mixing the dry powder electrode material in a mill, it is blown onto the membrane (or backing) resulting in a uniformly distributed layer. This reactive layer is fixed and thoroughly connected to the membrane by passing them through a calender. In order to produce the second electrode, the same steps are repeated. This procedure is very simple and, as a dry process, avoids the use of any solvents and drying steps. We have achieved a thickness of the reactive layer as low as 5 μm, reducing the amount of catalyst needed and, thus, the costs. Electrochemical investigations have shown a performance comparable to that of commercial electrodes. The degradation of MEA for polymer membrane fuel cell (PEFC) components during the cells lifetime, yields a change in the electrochemical behaviour. The characterisation of PEFC MEA-components after electrochemical operation has given information about the degradation of electrodes and membranes and about the change in the platinum distribution on the anode, whilst on the cathode, the platinum content is unchanged.

Photoelectron study of electrochemically oxidized nickel and water adsorption on defined NiO surface layers

Abstract Electrochemically oxidized nickel and the water adsorption on NiO prepared in UHV were studied by XPS, TPD and UPS. Potassium ions are intercalated in the electrochemically oxidized nickel surface. No molecular water inserted in the electrochemically formed NiOOH layer was observed after transfer to UHV. Two reduction steps can be distinguished during the reduction of the electrochemically oxidized nickel surface in H2 atmosphere. The interaction of water with NiO depends on the surface structure. The TPD signal of the monolayer desorption of H2O on NiO(100) demonstrates two different adsorption states. The water does not dissociate on the smooth NiO(100) surface as well as on the defected NiO(100) surface. In contrast on the polar NiO(111) surface approximately 0.5 ML (monolayer) water dissociate to OH and H after starting water adsorption.

New dry preparation technique for membrane electrode assemblies for PEM fuel cells

A major problem for future fuel cell commercialisation is the cost of membrane electrode assemblies (MEAs) satisfying both power density and lifetime requirements. New, low-cost MEA production techniques are being developed at the DLR (the German national aerospace research centre). These new MEAs have been characterised and investigated using physical and electrochemical methods to support the MEA development. This technique, based on the adaptation of a rolling process, has been developed for fuel cell electrode and MEA preparation. The procedure is very simple, and as a dry process it avoids the use of any solvents and drying steps. We have achieved a thickness of the reactive layer as low as 5 urn, reducing the amount of noble metal catalyst needed to less than 0.05 mg/cm 2 , and thus reducing the costs.

Fully automatic test facilities for the characterisation of DMFC and PEFC MEAs

Abstract Membrane electrode assemblies (MEAs) for polymer electrolyte fuel cells with hydrogen fuel (H 2 -PEFC) and direct methanol fuel cells (DMFC) are under development at DLR. For their characterisation fully automatic test units have been designed and realised to guarantee reproducible test results. The identical oxidant supply at the cathode side of the H 2 -PEFC and DMFC as well as similar test cells and test conditions offer the possibility to realise both modifications in one test unit. The pipework system and all fittings of the cathode supply can be used simultaneously. Different conditions have to be realised particularly in the anode supply. At the anode of the DMFC liquids (methanol/water) and in the H 2 -PEFC gas (hydrogen) are supplied. By integration of an electronic software-supported control unit operating modes can be changed in the test unit depending upon requirement. In order to show the reproducibility of fuel cell operations it is necessary that parameters will be kept within very low deviation limits. An automatic regulation permits impact onto all controllable parameters e.g. pressures, temperatures and mass flow rates. When achieving stationary operating conditions current–voltage-curves can be recorded by automatic change of the electronic cell load. Measured values for current, voltage and all operating parameters are recorded by the software and stored for later interpretation. During data acquisition the parameters are visualised on a graphic interface. It is possible to influence the control at any time. To permit an unguarded long-term experimental operation a sophisticated safety system is necessary. The pre-defined safety parameters are monitored by computer software as well as by an industrial type Programmable Logic Controller (PLC).

Non-Fluorinated Membranes Thickness Effect on the DMFC Performance

Abstract In order to study the influence of the proton exchange membrane thickness on the direct methanol fuel cell (DMFC) performance, sulfonated poly (ether ether ketone) (sPEEK) membranes with a sulfonation degree (SD) of 42% and thicknesses of 25, 40, and 55 µm were prepared, characterized, and tested in a DMFC. These polymeric membranes were tested in a DMFC at several temperatures by evaluating the current-voltage polarization curve, the open circuit voltage (OCV) and the constant voltage current (CV, 35 mV). The CO2 concentration at the cathode outlet was also measured. The thinnest sPEEK membrane proved to have the best DMFC performance, although having lower Faraday efficiency (lower ohmic losses but higher methanol permeation). In contrast, the thickest membrane presented improved properties in terms of methanol permeation (lower methanol crossover). DMFC tests results for this membrane showed 30% global efficiency, obtained with pure oxygen at the cathode feed.

SURFACE SCIENCE STUDY ON THE STABILITY OF VARIOUS CATALYST MATERIALS FOR DMFC

Caused by the electrochemical stresssing in fuel cells the stability of catalysts is a crucial aspect for he life time of fuel cells. In the past several studies of the stability of catalysts in alkaline fuel cells and polymer electrolyte fuel cells were performed. In this paper some investigations by surface science methods of the stability of catalysts for direct methanol fuel cells will be given.

The e-bypass separator: a generic separator constructionwith endless application possibilities

It has been shown that using a double-layer e-bypass separator and rinsing it during electrolysis, with electrolyte containing no dissolved gas (from its internal channel towards the electrode compartments), is a manner to make unprecedented gas qualities, which are independent from AWE’s operational conditions. We are convinced that the new separator may be interesting for improving the functioning of other electrochemical cells as well!