Is the Degradation of Fe-N-C Catalysts in Proton Exchange Fuel Cells Caused By Hydrogen Peroxide Formation?

Abstract

In fuel cells chemical energy is directly converted into electricity. Based on this, fuel cells are attractive for automotive propulsion. However, a major drawback that hinders the widespread use of fuel cells are the costs. A major cost contributor in today’s state of the art fuel cells is the catalyst; namely platinum and its alloys that are used to catalyse the hydrogen oxidation reaction and the oxygen reduction reaction (ORR) [1]. Most of the expensive metal is required for the cathodic ORR. However, Fe-N-C catalysts are a potential alternative to Pt/C as they reach promising activity while the stability still needs to be improved. There seems to be a trade-off between activity and stability for this kind of catalysts. Beside other causes of degradation, it is discussed that hydrogen peroxide formation is one of the driving forces [2]. While molecular FeN4 centres are assumed to reduce oxygen to water, other phases are suggested to contribute to hydrogen peroxide formation. Such side phases are e.g. iron or iron carbide which are found in the catalysts after high temperature pyrolysis or if insufficient acid leaching was applied [3].However, there is no systematic study if under fuel cell conditions indeed hydrogen peroxide formation from these side phases is at the origin of instability or maybe the leaching of iron species (from these side phases) with subsequently induced Fenton’s reaction might cause the degradation. The knowledge on this is required for future catalyst design and therefore given attention in this work.In order to address this point, small quantities of an FeNC catalyst were post-modified with different metals as e.g. Pt, Pd, Ag in order to tune the selectivity for hydrogen peroxide formation of the resulting catalyst while still keeping the iron-related composition the same.The catalysts were characterized by cyclic voltammetry and rotating ring disc electrode experiments in order to see to what extent specific adsorption occurs and to determine the ORR activity and selectivity in aqueous electrolyte (0.1M H2SO4).Besides, polarization curves were measured in H2/O2 fuel cells and potentiostatic stability tests (12h) were performed at U = 0.6 V to identify activity and stability under operating conditions.We will discuss to what extent hydrogen peroxide formation correlates with the degradation of FeNC catalysts in proton exchange fuel cells.