DEGRADATION OF NICKEL ANODES DURING OPERATION IN ALKALINE FUEL CELL

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 the main disadvantage it is generally believed that AFC are more sensitive to being damaged by overloading and the power density with air is lower than for PEFC at the present state of development. For practical use 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. At DLR the long term behavior of anodes with nickel catalysts was investigated by electrochemical measurements during operation as well as by physical characterization by x-ray photoelectron spectroscopy (XPS), porosimetry measurements by nitrogen adsorption, scanning electron microscopy (SEM) and energy dispersive x-ray analysis (EDX) before and after electrochemical operation. Long term test with up to few thousand hours operation times were performed .The electrochemical performance decreased during the long term tests, but the electrodes were still active after few thousand hours operation. The electrodes were electrochemically characterized mainly by recording V-i curves during the operation time. The decrease of the electrochemical performance yields not only to a down scaling of the V-i curve, but also the shape of the V-i curves is changed. Different degradation effects in the electrode can be detected by the physical characterization: the chemical composition of the electrode surface as well as the structure of the electrode and catalyst are changed by electrochemical stressing during fuel cell operation, e.g. the PTFE will be partially decomposed, the nickel catalysts will be disintegrated.