Erich Gülzow

Alkaline Fuel Cells - A Critical View.

Low-temperature fuel cells constitute an important element in pollution-free energy conversion. Although many groups have stopped working in the field of alkaline fuel cells (AFCs) in recent years in Germany, a few groups in Europe continue. Recent important projects are presented here. The mathematical simulation of fuel cells and a few examples of fundamental research on catalysts are shown. It is concluded from these fundamental investigations, that poisoning of electrocatalysts in AFCs with rolled electrodes due to carbon dioxide does not take place. A brief view of calculations of the costs of well known fuel cell systems is also presented. The calculations show that the AFCs are as cheap as other small fuel cell systems. AFCs are as suitable for mobile applications as other low-temperature fuel cells.

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.

Investigation of the degradation of different nickel anode types for alkaline fuel cells (AFCs)

Low temperature fuel cells have the opportunity of becoming important for mobile energy systems as, in contrast to other low temperature fuel cells, the alkaline type requires neither noble metal catalysts nor an expensive polymer electrolyte. In alkaline fuel cells (AFC) nickel is used as anode catalyst in gas diffusion electrodes. The metal catalyst was mixed with polytetraflourethylene (PTFE) as organic binder in a knife mile and rolled onto a metal web in a calandar to prepare the electrode. After an activation process with hydrogen evolution at 5 mA/cm2 for 18 h the electrodes were stressed at constant loading in a half cell equipment. During the fuel- cell operation the electrochemical performance decreased. dDue to changes of the polymer (PTFE) and of the metal particles in the electrode, which is described in detail in another paper. In this study three types of electrodes were investigated. The first type of electrode is composed of pure Raney-nickel and PTFE powder, the nickel particles in the second electrode type werewas selected according to particle size and in the third electrode copper powder was added to the nickel powder not selected by size. The size selected nickel particles shows a better electrochemical performance related to the non-selected catalyst, but due to the electrochemically induced disintegration of the nickel particles the electrochemical performance decreases stronger. The copper powder in the third electrode is added to improve the electronic conductivity of the nickel catalyst, but the copper is not stable under the electrochemical conditions in fuel cell operation. With all three anode types long-term experiments have been performed. The electrodes haves been characterized after the electrochemical stressing to investigate the degradation processes.

XPS analysis of PTFE decomposition due to ionizing radiation

X-ray induced photoelectron spectroscopy (XPS) in combination with depth profiling has been used to investigate the structure and the degradation mechanism of PTFE bonded gas diffusion electrodes (GDE). The XP-spectra of these electrodes show distinctly separated binding states of the C1s electrons at Eb=292 eV and Eb=286 eV. These binding states are related to the carbon in the (CF2)n configuration (C1sCF2) and the graphite (C1sgraphite), respectively. The C1sCF2 signal decreases are induced by both X-ray exposure and ion etching. Simultaneously a decrease of the F1s signal has been approved. The intensity ratio of F1s to C1sCF2 has increased during the experiment. These results indicate a decomposition of PTFE which creates CF fractions, leading to an excess intensity in the energetic range between the C1s binding states of the PTFE and the graphite. Although both the F1s and the C1s spectra are strongly modified by ionizing radiation, samples are comparable, when exposition doses are equal.

Investigation of Membrane Pinhole Effects in Polymer Electrolyte Fuel Cells by Locally Resolved Current Density

In order to increase the reliability of fuel cells, an online diagnostic method for detection of operation malfunctions, as well as the early detection of failures in the fuel cells, is necessary. For this purpose, locally resolved current density measurements are an important tool, but the interpretation of the data related to the detection of malfunctions or failures is not straightforward. Here, segmented cell technology is applied to investigate the current density distributions in the anode and cathode electrodes to ascertain their equivalence due to the strong perpendicular coupling of currents. Current density distributions are further used to determine the signature of pinhole formation in the membrane. Different behavior is observed for membrane leakage under open circuit and under applied load conditions. Whereas the cell at open circuit is characterized by a positive current in the vicinity of the pinhole and small negative currents in the remaining area, an applied load leads to large negative currents at the pinhole. The characteristic behavior can be explained by high crossover rates of hydrogen from the anode to the cathode. The nongeneric signature is used to detect the deterioration of a membrane electrode assembly after a test stand malfunction. A sudden pressure drop associated with vaporation of water and the fast cooling of the cell is assumed to trigger the failure of the membrane. After 48 h, fissures in several positions of the membrane near the edges of the cell holder are observed. Through the evolution of leakages in the fuel cell, a malfunction can be detected at an early stage and thereby catastrophic failure of the whole stack may be avoided or anticipated.

Activation of nickel-anodes for alkaline fuel cells

In alkaline fuel cells (AFC) electrodes containing porous nickel and PTFE can be used as anodes. A low-cost production technique yields passive, polymer covered electrodes, which have to be activated before being used in electrochemical devices. During this activation process the surface structure as well as the chemical composition and the oxidation state of the used materials are changed by a given electrochemical current. The electrodes are investigated in different states during this activation process with X-ray photoelectron spectroscopy, scanning electron microscopy with energy dispersive X-ray spectroscopy and nitrogen adsorption.

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.

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).

Investigation of Locally Resolved Current Density Distribution of Segmented PEM Fuel Cells to Detect Malfunctions

With the aid of PCB technology, current density distribution could be achieved by measuring the local current density inside the fuel cell, which would be more precisely reflecting the local electrochemical reaction. An important application of segmented cell technology was used for in-situ error detection of the evolution of membrane leakage. From the evolution of current density distributions, starting locations of membrane leakages and their spreading could be detected. The effect of operation conditions could be found: the steeply decreased pressure and the fast cooling of the local membrane was the main reason for the degradation of the membrane. It caused large temperature gradient in different local membranes, which resulted in inhomogeneous current density distribution and mechanical strain and stress. With the aid of segmented cell, malfunction could be detected at an early stage and thereby catastrophic failure of the whole stack may be avoided.

Local In-Situ Analysis of PEM Fuel Cells by Impedance Spectoscopy and Raman Measurements

An understanding of the processes inside of low temperature fuel cells on a local scale is required for an effective improvement strategy. For this purpose in situ Raman spectroscopy and local impedance spectroscopy is being developed. The contribution describes the modifications to the cell, and installations of additional devices and the experimental detection systems for integrating both methods into a single cell set up. First results to verify the combined results were carried out and are presented. In the case of the local impedance with segmented cells the additional effort for the simultaneous frequency analysis of all segments is described. The Raman signals of hydrogen, oxygen and water from the channels of the flow field are shown and first measurements of gas composition along the fuel path are analyzed.