Correlations between Mass Activity and Physicochemical Properties of Fe/N/C Catalysts for the ORR in PEM Fuel Cell via ⁵⁷Fe Mössbauer Spectroscopy and Other Techniques

Abstract

The aim of this work is to clarify the origin of the enhanced PEM-FC performance of catalysts prepared by the procedures described in Science 2009, 324, 71 and Nat. Commun. 2011, 2, 416. Catalysts were characterized after a first heat treatment in argon at 1050 °C (Ar) and a second heat treatment in ammonia at 950 °C (Ar + NH3). For the NC catalysts a variation of the nitrogen precursor was also implemented. (57)Fe Mössbauer spectroscopy, X-ray photoelectron spectroscopy, neutron activation analysis, and N2 sorption measurements were used to characterize all catalysts. The results were correlated to the mass activity of these catalysts measured at 0.8 V in H2/O2 PEM-FC. It was found that all catalysts contain the same FeN4-like species already found in INRS Standard (Phys. Chem. Chem. Phys. 2012, 14, 11673). Among all FeN4-like species, only D1 sites, assigned to FeN4/C, and D3, assigned to N-FeN2+2 /C sites, were active for the oxygen reduction reaction (ORR). The difference between INRS Standard and the new catalysts is simply that there are many more D1 and D3 sites available in the new catalysts. All (Ar + NH3)-type catalysts have a much larger porosity than Ar-type catalysts, while the maximum number of their active sites is only slightly larger after a second heat treatment in NH3. The large difference in activity between the Ar-type catalysts and the Ar + NH3 ones stems from the availability of the sites to perform ORR, as many sites of the Ar-type catalysts are secluded in the material, while they are available at the surface of the Ar + NH3-type catalysts.

Influence of the Structure-Forming Agent on the Performance of Fe-N-C Catalysts

“…The Tafel slopes of the more-active catalysts show values of about 70 mV dec −1 . This value is in good agreement with the values that were observed for other catalysts prepared by the (oxalate supported) pyrolysis of porphyrins [15,24,29] as well as alternatively prepared catalysts (without the use of a reactive gas heat-treatment) [19]. When the fraction of oxalic acid goes down the Tafel slope increases, getting closer to 100 mV dec −1 .…”

“…While not intended, this might be related to an interaction of the catalyst with ammonia (from DCDA decomposition) during the heat-treatment. A larger Tafel slope value for catalysts that involve ammonia in their preparation was also previously observed [19]. The lowest value of 64 mV dec −1 is obtained for the catalyst with exclusively oxalic acid as SFA.…”

“…In addition to this, the use of sulfur strongly affected the morphology and performance of the catalysts [16,17]. Sulfur usually enables a higher porosity and lower graphitization [16,18], but more important it was found to prevent carbide formation that was assigned to active site destruction by works of us and of others [17,19,20]. That indeed excess iron was at the origin of active site destruction was further confirmed through an alternative strategy to prevent active site destruction [21].…”

Influence of the Structure Forming Agent on the Performance of Fe-N-C Catalysts

“…The Tafel slopes of the more-active catalysts give values of about 70 mV dec -1 . This value is in good agreement with the values that were observed for other catalysts prepared by the (oxalate supported) pyrolysis of porphyrins [15,[24][25] as well as alternatively prepared catalysts (without the use of a reactive gas heat-treatment) [19]. When the fraction of oxalic acid goes down the Tafel slope increases, getting closer to 100 mV dec -1 .…”

Progress in Oxygen Reduction Reaction Electrocatalysts for Metal-Air Batteries

“…最近, Choi 等 [97] 结合实验表征和理论计算考察了 [96] . 他们采用高微孔结构的 Black Pearl 2000 为载体, 通过将碳载体、1,10-邻菲罗啉和醋酸亚铁 球磨, 并先后在氩气和氨气气氛下热处理而得到了催化 活性较高的 Fe 基催化剂, 研究发现氨气气氛处理后催 化活性有了很大提高, 通过进一步分析得出拥有较多 Fe-N 4 /C (N 为吡啶型和吡咯型)结构的催化剂具有较高 的催化活性 [98] . 除 N 原子能够提高催化活性外, S、P、B 等原子的 掺杂也有报道 [99,100] .…”