MOF-Derived Fe-Zn-N-C Catalysts for Precious Metal Free Cathodes Showing High Performance in Anion-Exchange Membrane Fuel Cells

Abstract

Hydrogen technologies such as low-temperature fuel cells are, besides batteries, the most promising technologies for mobility and transport applications. Currently, Proton Exchange Membrane Fuel Cells (PEMFCs) are, in terms of low-temperature fuel cells, the state-of-the-art technology achieving high power densities and reasonable stabilities. With the recent development and increasing availability of stable and performant hydroxide conductive ionomers, Anion Exchange Membrane Fuel Cells (AEMFCs) have gained increasing interest in recent years as they combine the advantages of PEMFCs, like low-temperature operation and high-power density with the low component costs of alkaline fuel cells.1,2 Especially the possibility of replacing the expensive Pt-based electrocatalysts used in PEMFCs with cheaper electrocatalysts like nickel-based materials for the anode and iron-based materials for the cathode could significantly decrease the fuel cell costs.1,3,4 Although the oxygen reduction reaction (ORR) at the cathode is favored in alkaline media compared to acidic media, the ORR is still a challenging reaction in AEMFCs.5,6 Over the last decades, various materials were investigated in order to replace the expensive Pt electrocatalysts at the cathode. Among these materials, iron- and nitrogen-doped carbons (Fe-N-C) with molecular iron sites (Fe-Nx) show comparable catalytic activities to Pt and decent stabilities.7–9 Fe-N-C catalysts can be obtained by co-pyrolysis of Fe- N-, and C-sources and subsequential acid leaching. One way to achieve Fe-N-C catalysts with high dispersion of Fe-Nx sites is by using Fe-doped metal-organic frameworks (Fe-MOFs) as precursors. These combine the presence of pre-coordinated Fe-Nx motives as well as high porosity and high specific surface area.10 To produce Fe-N-C catalysts, we synthesized 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 Fe and Zn, high specific surface areas (400-600 m2/g), and high porosity as revealed by XRD, EDX, HAADF STEM, and N2 physisorption. Depending on the pre-treatment of the Fe-Zn-MOF, the Fe content of the resulting Fe-N-C catalyst could be varied. Benefiting from the high Fe-dispersion and the high specific surface areas, the best performing Fe-Zn-N-C catalyst with high Fe content shows high activity towards the ORR in alkaline media (0.1 mol/L KOH) as demonstrated by RDE measurements featuring a high half-wave potential (0.87 V vs. RHE) and high mass activity (47 mA/mgcat) and thereby outperforming a commercial 50 wt.% Pt/C catalyst in terms of half-wave potential (0.83 V vs. RHE). To investigate the performance of the Fe-Zn-N-C catalysts in 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 as membrane and catalyst layer ionomer. The AEMFC with 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. 1 D. R. Dekel, J. Power Sources, 2018, 375, 158–169. 2 R. O’Hayre, S.-W. Cha, W. Colella and F. B. Prinz, Fuel Cell Fundamentals, John Wiley & Sons, Inc, Hoboken, NJ, USA, 2016. 3 X. Ge, A. Sumboja, D. Wuu, T. An, B. Li, F. W. T. Goh, T. S. A. Hor, Y. Zong and Z. Liu, ACS Catal., 2015, 5, 4643–4667. 4 E. S. Davydova, S. Mukerjee, F. Jaouen and D. R. Dekel, ACS Catal., 2018, 8, 6665–6690. 5 M. Hren, M. Božič, D. Fakin, K. S. Kleinschek and S. Gorgieva, Sustain. Energy Fuels, 2021, 5, 604–637. 6 N. Ramaswamy and S. Mukerjee, Adv. Phys. Chem., 2012, 2012, 1–17. 7 X. Ren, B. Liu, X. Liang, Y. Wang, Q. Lv and A. Liu, J. Electrochem. Soc., 2021, 168, 044521. 8 L. Osmieri, L. Pezzolato and S. Specchia, Curr. Opin. Electrochem., 2018, 9, 240–256. 9 M. M. Hossen, K. Artyushkova, P. Atanassov and A. Serov, J. Power Sources, 2018, 375, 214–221. 10 C. Li, D. H. Zhao, H. L. Long and M. Li, Rare Met., 2021, 40, 2657–2689.