We report a F-doped FeNC catalyst with improved ORR performance. The enhanced performance is associated with the large BET surface area, abundant single Fe atoms, and strong electron-withdrawing F-doping.

Zinc‐air batteries have several advantages in comparison with the lithium‐ion technology as they enable the use of earth‐abundant elements, work at low cost, are lightweight, and are also much safer in application. In addition to the chemistry related to the zinc electrode, efficient and stable bifunctional catalysts are required for oxygen reduction reaction (ORR, for discharging) and oxygen evolution reaction (OER, for charging) on the air‐electrode side. Herein, a family of non‐precious metal catalysts is investigated as possible bifunctional composite: metal–nitrogen–carbon (MNC) catalysts for ORR, and metal oxyhydroxides as OER catalysts (Ox). The effect of transition metal and metal loading in these composite MNC + Ox catalysts on ORR and OER activities in half‐cell measurements is discussed. The catalysts were characterized using X‐ray diffraction and Raman spectroscopy to identify their phase composition. For the most active material, a potential gap of 0.79 V between OER and ORR was obtained, respectively. In a zinc‐air cell, this catalyst moreover showed a peak power density of 62 mW cm−2 and a charge–discharge gap of 0.94 V after 26 h of charge–discharge cycling.

FeNC catalysts are promising substitutes of platinum-type catalysts for the oxygen reduction reaction (ORR). While previous research disclosed that high pyrolysis temperatures are required to achieve good stability, it was identified that a trade-off needs to be made regarding the active site density. The central question is, if a good stability can also be reached at milder pyrolysis conditions but longer duration retaining more active sites, while enabling the defect-rich carbon to heal during a long residence time? To address this, a variation of pyrolysis temperatures and durations is used in FeNC fabrication. Carbon morphology and iron species are characterized by Raman spectroscopy and Mössbauer spectroscopy, respectively. Fuel cell (FC) activity and stability data are acquired. The results are compared to ORR activity and selectivity data from rotating ring disc electrode experiments and resulting durability in accelerated stress tests mimicking the load cycle and start-up and shut-down cycle conditions. It is discussed how pyrolysis temperature and duration affect FC activity and stability. But, more important, the results connect the pyrolysis conditions to the required accelerated stress test protocol combination to enable a prediction of the catalyst stability in fuel cells.

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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.References:[1] B. D. James, D. A. DeSantis et al., Department of Energy, December (2018), Strategic Analysis.[2] C. H. Choi, F. Jaouen et al., Energy Environ. Sci. 11 (2018) 3176-3182. [3] U. I. Kramm, S. Wagner et al., Adv. Materials. 31 (2019) 1805623.