DEGRADATION OF NICKEL ANODES IN ALKALINE FUEL CELLS

Kurzfassung

Alkaline fuel cells (AFC) are an interesting alternative to polymer electrolyte fuel cells (PEFC). AFC have some advantages compared with PEFC: they do need neither expensive electrolyte nor expensive noble metal catalysts. As main the disadvantage one think that AFC are more sensitive to be damaged by overloading and the performance development for opera¬tion with air is lower than for PEFC at present. For using of AFCs the long term behavior of the components is decisive, in particular the behavior of the electrodes, because the electrolyte can be easily changed. The long term behavior of AFC anodes consisting of a mixture of a nickel catalyst, which is formed from a NiAl alloy by dissolving the aluminum with polytetrafluorethylene (PTFE) as organic binder and with added copper powder rolled onto a metal web was investigated by V-i curves. In addition, these electrodes were characterized physically after different opera¬tion times by x-ray photoelectron spectroscopy (XPS), porosimetry measurements by nitrogen adsorption, scanning electron microscopy (SEM) and energy dispersive x-ray analysis (EDX). The electrochemical performance decreases during operation, where in the first phase of ope¬ration the decrease is faster than observed for the long time degradation. The decrease of the electrochemical performance can be displayed by combination of two exponential functions with different time constants, which indicates that at least two different mechanisms control the degradation. With the physical characterization of the electrodes different types of changes in the electro¬des could be distinguished.. The PTFE should be stable under the electrochemical conditions, but the PTFE shows a partial decomposition. The nickel catalyst has a characteristic pore size distribution with a maximum in the number at a pore radius of 2 nm. The specific surface area of the electrode determined by nitrogen adsorption does not significantly change, but the pore number at 2 nm increases strongly during operation. Pores at other pore radii do not appear. In the SEM images a disintegration of the nickel particles is observed, which can be explained by hydrogen induced embrittlement of the nickel. In addition a change of the copper concentration during the operation time can be recorded by XPS. The changes of the physical characteristics will be correlated with the electrochemical performance.