(Invited) Novel Insights in the Activity, Selectivity and Durability of Fenc, Mn-Oxides and Fenc/Mn-Oxide Composites for ORR Catalysis in Alkaline Electrolyte and AEMFC

Abstract

While high power densities are reached with proton-exchange membrane fuel cells (PEMFCs), several drawbacks hold their large-scale commercialization. In particular, the sluggish oxygen reduction reaction (ORR) at the cathode requires significant amount of platinum. Among PGM-free catalysts, iron-nitrogen-carbon catalysts have hitherto shown the highest initial activity toward ORR and especially those that have been subjected to NH3 pyrolysis. However their performance in PEMFC decays too fast. In contrast, prospects for active and durable Fe-N-C catalysts competitive to Pt catalysts are good in alkaline environment, and the advent of high performance anion exchange membrane (AEM) now offers the possibility to investigate this in AEM fuel cell (AEMFC). This presentation will first report on the ORR activity, stability and operando spectroscopic identification of Fe-N-C catalysts in alkaline electrolyte, with materials comprising only FeNx moieties. To further increase the ORR selectivity toward water formation, the activity and stability of different Mn-oxide polymorphs for ORR and hydrogen peroxide electro-reduction (HPRR) were studied. The properties of a composite comprising the most stable Mn-oxide and Fe-N-C material were also studied, confirming that HPRR is facilitated when Mn2O3 is added to Fe-N-C. At the same time, however, we show that HO2 - is harmful to the MnOx catalyst itself, increasing the manganese dissolution. FeNC and FeNC/Mn2O3 cathodes were then tested in AEMFC based on radiation-grafted AEM and AEI and compared to performance obtained with a Pt-based cathode. The results show high and comparable ORR activity at 0.9 V between cathodes based on 1.0-1.5 mgFeNC cm-2 and 0.4-0.5 mgPt cm-2. Peak power densities in the range of 1.1-1.7 W·cm-2 were reached in O2-fed AEMFC with such FeNC cathodes, depending on the operating conditions. Results from stability testing are also very promising, and in strong contrast to instability of the same FeNC material in PEMFC. While some performance gap still remains at high power density between highly Pt-loaded cathode and such FeNC cathode, the results show a promising application of FeNC for high performance AEMFC. Acknowledgements: The project CREATE leading to these results has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 721065.