Till Kaz

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.

In-situ spin trap electron paramagnetic resonance study of fuel cell processes

A novel method allows the monitoring of radical formation and membrane degradation in-situ in a working fuel cell which is placed in the microwave resonator of an electron paramagnetic resonance (EPR) spectrometer. By introduction of a spin trap molecule at the cathode the formation of immobilized organic radicals on the membrane surface is observed for F-free membranes, revealing the onset of oxidative degradation. For Nafion® there is much less evidence of degradation, and the hydroxyl radical is detected instead. At the anode, free radical intermediates of the fuel oxidation process are observed. No traces of membrane degradation are detected on this side of the fuel cell.

In situ study of methanol oxidation on Pt and Pt/Ru-mixed with Nafion® anodes in a direct methanol fuel cell by means of FTIR spectroscopy

Methanol and carbon monoxide (CO) oxidation on unsupported Pt- and Pt/Ru-mixed with Nafion® anodes in the direct methanol fuel cell (DMFC) were studied by in situ Fourier transform infrared spectroscopy. IR measurements were performed using a specially constructed DMFC by diffuse reflection and on-line transmittance techniques at different potentials applied to anodes under cell operating conditions. The experiments show that the major products of the methanol oxidation are methylformate, carbon monoxide and carbon dioxide. No other products could be discerned. This fact is in contrast with the results reported for smooth/single crystal electrodes in a supporting electrolyte environment, where other products (COHads, formaldehyde, formic acid and dimethoxymethane) were found. This can reflect a difference in the electrochemical conditions in the real DMFC and those which are set on the model electrodes. Inverse and bipolar IR band shapes of the linearly bound CO (COL) were observed. The exact reason for the anomalous shape is not understood, however, the influence of the mixed or supporting material on the optical response of the COL is obvious. The COL experiences reduced influence from the applied potential. The possible reasons are discussed.

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.

New results of PEFC electrodes produced by the DLR dry preparation technique

At DLR Stuttgart a dry production technique has been developed for the preparation of electrodes and membrane electrode assemblies (MEA) for hydrogen PEFC as well as for direct methanol PEFC. Different spray strategies and different mixtures of electrodes and catalysts were tested to improve cell performance and reproducibility. The main advantage of the dry production technique [J. Power Sources 86 (2000) 352, Fuel Cell Bull. 15 (1999) 8] is the solvent-free coating of the electrodes, which allows a continuous production of MEA. In addition, thin layers (<5 μm) can be produced saving expensive catalyst material.

Development of Bifunctional Electrodes for Closed-loop Fuel Cell Applications

Unitized regenerative fuel cells (URFC) in combination with photovoltaic modules are attractive for space missions because they enable extended operation times and low weight. During the planetary day, electrical energy is stored which can be converted into electricity by the fuel cell during the night. All air-independent applications such as spacecraft or space stations would profit signify¬cantly from such energy conversion devices. A unitized regenerative fuel cell is a combined energy con¬version and storage system based on H2 and O2 which combines the advantages of fuel cells and secondary batteries. Substantial advantages of the specific energy density can be expected from the use of a URFC (400-1000 Wh/kg) in comparison to secondary batteries (220-250 Wh/kg for future advanced Li-polymer batteries). An important topic is the function of so-called bifunc¬tional oxygen electrodes which generally require the combination of favourable properties for both operating modes. In particular, different catalysts for oxygen reduction and for oxygen evolution are needed. This contribution investigates various electrode designs with Pt and IrO2 catalysts. For that purpose, the DLR dry spraying technique for the manufacturing of electrodes is applied for by mixing the two different catalysts together (Pt and IrO2) or applying the catalyst on different areas of electrode or even realising different layers with both catalysts. The different options are explained in Fig. 1. Of interest was to compare the simple mixing of the catalysts (option 1), the layered electrode with two compositions (option 2) and the segmented approach with dedicated areas with just one catalyst (option 3).

Dynamic Behavior and In-Situ Diagnostics of PEFCs

During fuel cell operation the electrochemical activity often is not homogenous over the electrode area. This may be caused by an non-uniform water content in the membrane, an inhomogeneous temperature distribution, and reactant gradients in the cell. Consequently a variation of the current density over the cell area occurs which tends to result in inferior performance. For in situ measurements of the current density distribution in fuel cell stacks a segmented bipolar plate was developed. The segmented bipolar plate was first tested in single cells with stack endplates to verify the function of all components. The tests showed that the measurement tool works very reliable and accurate. The insight in an operating fuel cell stack via current density distribution measurement is very helpful to investigate interactions between cells. Results can be used to validate models and to optimise stack components, e.g. flow field and manifold design, as well as to detect the best stack operating conditions. By applying segmented bipolar plates as sensor plates for stack system controls an improved performance, safe operation and longer life cycles can be achieved. The developed segmented bipolar plates with integrated current sensors were used to assemble a short stack consisting of 3 cells; each of them having an active area of 25cm2 divided into 49 segments. The design of the bipolar plate proofed very suitable for easy assembling of single cells and stacks. First measurement results show that different current distributions can appear in the cells and these can vary from cell to cell, depending on the operating conditions of the stack. Electrical coupling between the cells was investigated and found to be only marginal for the assembly used.Copyright