Fe-/Zn-doped and N-doped Carbon ORR Catalysts with Molecular Fe Nx Sites for High Performance Anion-Exchange Membrane Fuel Cells
Patrick Elsaesser a b c, Philipp Veh a d, Esmael Balaghi a b, Severin Vierrath a c, Matthias Breitwieser d e, Anna Fischer a b c f
a Freiburg Center for Interactive Materials and Bioinspired Technologies (FIT), University of Freiburg, Germany
b Institute for Inorganic and Analytical Chemistry, University of Freiburg, Fahnenbergplatz, Freiburg im Breisgau, Germany
c Cluster of Excellence livMatS @ FIT – Freiburg Center for Interactive Materials and Bioinspired Technologies, University of Freiburg, Germany
d Electrochemical Energy Systems, IMTEK-Department of Microsystems Engineering, University of Freiburg
e Hahn-Schickard, University of Freiburg
f Freiburg Materials Research Center (FMF), University of Freiburg, Fahnenbergplatz, Freiburg im Breisgau, Germany
nanoGe Fall Meeting
Proceedings of Materials for Sustainable Development Conference (MAT-SUS) (NFM22)
#SusEnergy - Sustainable materials for energy storage and conversion
Barcelona, Spain, 2022 October 24th - 28th
Organizers: Tim-Patrick Fellinger and Magda Titirici
Poster, Patrick Elsaesser, 346
Publication date: 11th July 2022

Hydrogen technologies such as low-temperature fuel cells are, besides batteries, the most promising technologies for mobility and heavy-duty transport applications. Among the low temperature fuel cells, Anion Exchange Membrane Fuel Cells (AEMFCs) have gained increasing interest as a potent alternative to Proton Exchange Membrane Fuel Cells (PEMFCs) in recent years due to the development and increasing availability of performant and stable hydroxide conductive ionomers. The advantage of AEMFCs over PEMFCs are that they combine the advantages of the PEMFC, like low-temperature operation and high-power density with the low component costs of alkaline fuel cells.1,2 Especially the possibility to use nickel-based materials for the anode and iron-based materials for the cathode, instead of the expensive Pt-based electrocatalysts used in PEMFCs, could significantly decrease the fuel cell costs.1,3,4

Over the last decades, iron- and nitrogen-doped carbons (Fe-N-C) with molecular iron sites (Fe-Nx) were intensively investigated for the oxygen reduction reaction (ORR). Since these materials show comparable catalytic activities and decent stabilities compared to the expensive Pt electrocatalysts in both acidic and alkaline environments, they are the most promising materials to replace Pt as electrocatalyst at the fuel cell cathode.5–7 However, due to the lack of availability and stability of hydroxide conductive ionomers, most of the research focused on the ORR in acidic media and fuel cell measurements in PEMFCs. New commercially available ionomers, e.g., Aemion+® from Ionomr solutions, show excellent stability, enabling the investigation of Fe‑N‑C catalysts for AEMFCs.8

Fe-N-C catalysts are typically synthesized via co-pyrolysis of Fe- N-, and C-sources in an inert atmosphere. In that context, Fe-doped metal-organic frameworks (Fe-MOFs) can be used as single source precursors to achieve Fe-N-C catalysts with large number and high dispersion of Fe-Nx sites, and therefore high ORR activity. The advantage compared to other precursors is the presence of pre-coordinated Fe-Nx motives as well as high porosity and high specific surface area.9

We synthesized Fe‑N‑C catalysts using Fe-, and Zn-doped MOFs (Fe-Zn-MOF) as multicomponent Fe-, Zn-, N-, and C-precursors and pyrolyzed them in the presence of additional nitrogen sources at high temperatures. The resulting Fe-Zn-N-C catalysts revealed high dispersion of molecular Fe and Zn, high specific surface areas (400-600 m2/g), and high porosity as revealed by XRD, XAS, EDX, XPS, and N2 physisorption. Benefiting from the high Fe-dispersion, Fe content, and the large specific surface area, the obtained Fe-Zn-N-C catalysts show high activity towards the ORR in aqueous alkaline electrolyte (0.1 mol/L KOH) as demonstrated by RDE measurements. The best performing Fe‑Zn‑N‑C catalyst featuring a half-wave potential of 0.87 V vs. RHE, outperforming a commercial 50 wt.% Pt/C catalyst (0.83 V vs. RHE).

For further investigations in an AEMFCs, membrane electrode assemblies (MEAs) were prepared with the synthesized Fe-Zn-N-C catalysts at the cathode, a commercial PtRu/C catalyst (40 wt.% Pt, 20 wt.% Ru on carbon black, AlfaAesar) at the anode, and a commercial ionomer (Aemion+) both in the catalyst layer and as a membrane. The best performing Fe‑Zn‑N‑C catalyst revealed a high peak power density of 850 mW/cm², which is among the highest reported peak power densities for non-precious metal cathode catalysts in combination with a commercially available anion exchange ionomer so far. These results represent the huge potential of single site catalysts with molecular Fe‑Nx single sites as cathode catalysts in AEMFCs.

We acknowledge Philip Veh, Dr. Severin Vierrath, and Dr. Matthias Breitwieser for their collaboration and AMEFC measurements.

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