High-efficiency MOF-based Fe-Sn-N-C catalysts for the ORR in HT-PEM fuel cells

Abstract

Metal-nitrogen-carbon (M-N-C) compounds are promising catalysts for the oxygen reduction reaction (ORR) at the cathode of high-temperature proton exchange membrane fuel cells (HT-PEMFC) due to their promising activity and their tolerance to poisoning by phosphoric acid electrolyte.[1] However, low volumetric activities compared to commonly used Pt nanoparticles on carbon blacks (Pt/C) and insufficient stabilities have been observed so far.[2] Therefore, the activity and stability of M-N-Cs and M M-N-Cs with Fe and Sn based on metal-organic frameworks (MOFs) are investigated in this study. The MOF precursor ZIF-8 provides a unique platform to control metal and nitrogen doping of the carbon. Iron and tin are used in different ratios to assess the influence of metal ratio on catalytic activity and stability towards ORR. Stable and highly active catalysts were obtained and therefore the presentation will highlight the beneficial aspects of introducing the second metal tin for the ORR. The catalysts are physically and electrochemically characterized using the rotating ring disk electrode (RRDE) technique. Fe Sn-N-C catalysts show excellent mass activities (MA) of 8.2 A g-1 (Fe Sn N-C 1:1) and 19.3 A g-1 (Fe Sn N-C 1:0.3), exceeding those of the commercial Fe N-C catalyst (PMF D14401, Pajarito Powder) and even surpassing the MA of a commercial Pt/C catalyst (Tanaka) (Figure 1a) and current Fe-Sn N C catalysts reported in literature (4.8 A g-1, 0.1 mol L-1 HClO4, Luo et al.).[3] In contrast, the MA of the Sn N C without incorporated iron is significantly lower at 0.5 A g-1, which implies that sufficient activity of only Sn-containing catalysts is not given. Furthermore, an accelerated stress test (AST) is applied to evaluate the stability of the catalysts and reveals promising results for the Fe Sn N-C with a Fe:Sn ratio of 1:1 (Figure 1b). The decreased stability of the Fe-Sn-N-C (1:0.3) shows the stability enhancing effects of Sn due to the lower amount of Sn. Future studies will examine the Fe Sn bonding situation to discuss the synergistic effects of the metals and also testing under HT-PEMFC conditions.