Precious Metal-Free Electrocatalysts for Anion-Exchange Membrane Fuel Cell Cathodes via Magnesium-Based Template-Assisted Synthesis
Authors: Sarapuu Ave , Sajjad Umber , Kisand Kaarel , Juvanen Silver , Kozlova Jekaterina , Piirsoo Helle-Mai , Kikas Arvo , Treshchalov Alexey , Käärik Maike , Aruväli Jaan , Paiste Päärn , Leis Jaan , Kisand Vambola , Kukli Kaupo , Tamm Aile , Douglin John C. , Dekel Dario R , Tammeveski Kaido 
Published: December 2024

Anion-exchange membrane fuel cells (AEMFCs) represent a viable alternative to proton-exchange membrane fuel cells, as they allow for the utilization of less expensive cell components and non-precious metal electrocatalysts on the electrodes. Transition metal-containing nitrogen-doped carbon materials (M-N-C catalysts) have shown very high activity toward the oxygen reduction reaction (ORR) in alkaline solution and promising performance as cathodes in AEMFC tests.1 Such materials are often prepared via high-temperature pyrolysis of carbon precursors in the presence of nitrogen and metal sources, in which nitrogen doping occurs during the carbonization. For the fuel cell application, hierarchical porous structure of the catalysts is desirable. To tailor the porous structure of carbon materials, hard templates are often used in the pyrolysis, typically silica nanoparticles, which need harsh conditions for removal. More sustainable alternatives to these are various other inorganic nanoparticles, e.g. some salts or oxides, which can be dissolved in dilute acids or water. These nanoparticles can also be derived in situ during the pyrolysis from thermally unstable precursors, for instance, MgO particles can be obtained from Mg acetate or citrate.2

This study introduces a simple method for preparing mesoporous M-N-C catalysts (M= Fe or both Fe and Co) for the ORR by pyrolysis of organic precursors, dicyandiamide and transition metal salts.3 Lignin, mixture of alkylresorcinols, and rapeseed press cake were used as sustainable carbon sources, whereas Mg acetate served as a precursor for a sacrificial template and was leached post-pyrolysis in a dilute acid solution.
The physicochemical analyses of the catalyst materials revealed that these consisted of porous sheet-like carbon structures. The presence of the template precursor in the synthesis significantly increased the specific surface area of the catalysts and the amount of smaller pores (d<10 nm). Nitrogen was uniformly distributed on the catalyst, while transition metal was partly distributed, presumably as atomically dispersed metal centers, and partly forming metal-rich nanoparticles. These particles consisted of Fe carbide or FeCo alloy and were encapsulated in carbon layers. Rotating disc electrode (RDE) tests conducted in alkaline solution indicated slightly higher ORR activity for the catalysts prepared with the template. The activity of both Fe-containing and bimetallic (FeCo) catalysts was similar to that of a commercial Pt/C catalyst. In addition, the ORR activity of catalysts derived from different organic precursors was also rather similar. In short-time stability tests the catalysts showed promising durability. In the membrane-electrode assembly tests at AEMFC conditions, the bimetallic catalyst derived from lignin surpassed the Fe-containing counterpart, achieving a promising peak power density of 833 mW cm-2 at 80 °C. These findings underscore the potential to produce highly active precious metal-free cathode catalysts for AEMFCs by a straightforward high-temperature pyrolysis from various sustainable carbon sources.

Acknowledgments

This work was financially supported by the Estonian Research Council (grants PRG4, PRG723, PRG753, and PRG1509) and by the Estonian Ministry of Education and Research (TK141 and TK210).

References

A. Sarapuu, J. Lilloja, S. Akula, J. H. Zagal, S. Specchia and K. Tammeveski, ChemCatChem, 2023, 15, e202300849.

T. Morishita, Y. Soneda, T. Tsumura, and M. Inagaki, Carbon, 2006, 44, 2360-2367.

K. Kisand, A. Sarapuu, J. C. Douglin, A. Kikas, A. Treshchalov, M. Käärik, H. M. Piirsoo, P. Paiste, J. Aruväli, J. Leis, V. Kisand, A. Tamm, D. R. Dekel, and K. Tammeveski, ACS Catal., 2022, 12, 14050-14061.

Figure 1