A New Catalyst System for the Oxygen Reduction Reaction (ORR) at Fuel Cell Cathodes: P-Block Precious Group Metal-Free Tin and Nitrogen-Doped Carbon (SnNC) Catalyst

Field: Chemistry (Physical Sciences)

The sluggish kinetics of the Oxygen Reduction Reaction (ORR) and the consequently large amount of platinum catalyst required for cathode material in Proton Exchange Membrane Fuel Cell (PEMFC) is today’s primary hurdle for commercializing fuel cell technology in automotive application. 1 A class of carbon-doped with metal and nitrogen electrocatalysts (labelled MNC) is promising substitute for platinum-group metals (PGMs) in oxygen reduction reaction (ORR) in PEMFC. 2 Particular attention was placed on a specific family of MNC catalysts derived from 3d transition metal precursors with FeNC and CoNC being the most promising for PEMFC applications requiring high power density. 3-5 In the present contribution, we report on a new MNC catalyst system, different by its chemical nature from all others catalysts previously reported as ORR-active in acidic medium. The highlights of this work are the successful design and synthesis of SnNC as a non-noble metal catalyst for the first time, and the finding that SnNC hosts Sn-based active sites that can achieve the same turnover frequency as Fe-based active sites in FeNC, and much higher turnover frequency than Co-based active sites in CoNC. FeNC and CoNC materials have, until now, been the state-of-art PGM-free catalysts for ORR in acidic medium. The present SnNC material shows also high and similar selectivity for the four-electron ORR pathway as FeNC, much higher than CoNC materials. The prepared SnNC catalysts meet and exceed the FeNC catalysts in terms of hydrogen-air fuel cell power density. The SnNC-NH 3 catalysts displayed a 40-50% higher current density than FeNC-NH 3 at cell voltages below 0.7 V. Added benefits include a Fenton-inactive character of Sn. Among other techniques, density functional theory and 119 Sn Mössbauer spectroscopy were used in combination to investigate SnNC, providing for the first time insights on the structure of its active sites, their rate-determining step for ORR and selectivity for the four-electron reduction pathway. References Holton, O. T.; Stevenson, J. W., The Role of Platinum in Proton Exchange Membrane Fuel Cells. Platinum Metals Review 2013, 57 (4), 259-271. Luo, F.; Wagner, S.; Ju, W.; Primbs, M.; Li, S.; Wang, H.; Kramm, U. I.; Strasser, P., Kinetic Diagnostics and Synthetic Design of Platinum Group Metal-Free Electrocatalysts for the Oxygen Reduction Reaction Using Reactivity Maps and Site Utilization Descriptors. Journal of the American Chemical Society 2022, 144 (30), 13487-13498. Zitolo, A.; Ranjbar-Sahraie, N.; Mineva, T.; Li, J.; Jia, Q.; Stamatin, S.; Harrington, G. F.; Lyth, S. M.; Krtil, P.; Mukerjee, S.; Fonda, E.; Jaouen, F., Identification of catalytic sites in cobalt-nitrogen-carbon materials for the oxygen reduction reaction. Nature Communications 2017, 8 (1), 957. Zitolo, A.; Goellner, V.; Armel, V.; Sougrati, M.-T.; Mineva, T.; Stievano, L.; Fonda, E.; Jaouen, F., Identification of catalytic sites for oxygen reduction in iron- and nitrogen-doped graphene materials. Nature materials 2015, 14 (9), 937-942. Luo, F.; Roy, A.; Silvioli, L.; Cullen, D. A.; Zitolo, A.; Sougrati, M. T.; Oguz, I. C.; Mineva, T.; Teschner, D.; Wagner, S.; Wen, J.; Dionigi, F.; Kramm, U. I.; Rossmeisl, J.; Jaouen, F.; Strasser, P., P-block single-metal-site tin/nitrogen-doped carbon fuel cell cathode catalyst for oxygen reduction reaction. Nature materials 2020, 19 (11), 1215-1223.