Spontaneous Aerobic Ageing of Fe-N-C Materials and Consequences on Oxygen Reduction Reaction Kinetics

Tremendous progress in the beginning-of-life oxygen reduction reaction (ORR) activity of iron nitrogen carbon (Fe–N–C) catalysts holds the promise to replace platinum-group metals in proton exchange membrane fuel cells cathode. Improving the understanding of their degradation mechanisms as well as their practical durability are the next two grand challenges. Here, we report on a spontaneous aerobic degradation phenomenon of Fe–N–C materials that takes place upon storage under atmospheric conditions (air, room temperature), and depreciates their electrocatalytic activity towards the ORR. Our study covers a period of 47 months and involves six catalysts, which were synthesized by different laboratories and different methods (sacrificial metal organic framework, silica templating, aerogel-derived, wet impregnation of high surface area carbon black) and which feature distinct morphology, structure and density of active sites. The results from electron and X-ray based techniques indicate that a fraction of the single Fe atoms spontaneously transforms into Fe or Fe-oxide aggregates over time, in line with the decrease in the active site density measured by in situ nitrite stripping. Along with these structural changes, a strong decrease in ORR turnover frequency was also observed. These adverse effects can be mitigated using storage under dry and oxygen-free atmosphere.