Comparative Study of the Oxygen Reduction Reaction on Pyrolyzed FePc in Acidic and Alkaline Media

Abstract

Transition‐metal and nitrogen co‐doped carbon materials (M/N/C) have received increasing attention as electrocatalysts for the oxygen reduction reaction (ORR). M/N/C catalysts usually exhibit distinct ORR catalytic activity between acidic and alkaline media. The origin of such pH‐dependent activity is so far unclear. Herein, we investigate the dependence of ORR activity of FePc/C catalysts on the pyrolysis temperature (300–1000 °C) in both acidic and alkaline media to speculate the difference between active sites in the different media. The highest ORR activity is achieved at 800 °C and 500 °C in acidic and alkaline media, respectively. In particular, at 600–800 °C, the ORR activity increases greatly with increasing pyrolysis temperature in acidic medium, whereas it decreases in alkaline medium, indicating different active sites present in the two media. Thermal decomposition analysis of FePc suggests that highly active sites in acidic medium are formed through the decomposition of FePc or Fe‐N4 structures at 600–800 °C after releasing phthalonitrile (C6H5−C2N2), whereas intact FePc or Fe‐N4 incorporated in the carbon matrix are the main active sites in alkaline medium. XPS measurements show that there is a good relationship between the ORR activity and metal N in alkaline medium. This study is of significance to the development of highly active non‐precious metal catalysts.

Molecular Degradation of Iron Phthalocyanine during the Oxygen Reduction Reaction in Acidic Media

“…At the C1 and C2 sites, the radical addition reaction breaks the C–C bond and forms the −CO bond, resulting in the removal of a benzene ring. This possibility is supported by our previous experimental observation that benzene rings were detected as the decomposed products of FePc during pyrolysis . At the C5 sites, the attack by one • OH radical leads to the hydroxylation of the C site.…”

“…This possibility is supported by our previous experimental observation that benzene rings were detected as the decomposed products of FePc during pyrolysis. 69 At the C5 sites, the attack by one • OH radical leads to the hydroxylation of the C site. Similarly, one • OOH radical attack at the C4 site results in the peroxidation of the double-bond carbon.…”

Revealing Energetics of Surface Oxygen Redox from Kinetic Fingerprint in Oxygen Electrocatalysis

“…As summarized in Figure 4B, while many catalysts, especially the Fe/N/C catalysts, have already shown good ORR performance in alkaline media, their activities in acid are significantly lower. 49,53 This problem has been a long-term bottleneck to further improve the efficiency or reduce the cost of proton exchange membrane fuel cells (PEMFCs), whereas the underlying mechanism is still not clear. As a typical example, the poor ORR activity of Au is consistent with its noble chemical property, 54 while the relatively "good" ORR activity in basic environment is difficult to understand.…”

Establishing reactivity descriptors for platinum group metal (PGM)-free Fe–N–C catalysts for PEM fuel cells

“…It should be noted that the multi-peak fitting of the N 1s core level region and the assignment of specific binding energies to functional groups remains a controversial topic of intense research and fitting variability across laboratories. 13,[91][92][93][94] In particular, the assignment and reliable quantification of N atoms involved in Fe-N x moieties remains difficult, as it requires the knowledge of the exact coordination of the sites present in the sample. For example, the number of nitrogen atoms coordinating the Fe centers and the number of carbon atoms in the second coordination sphere can influence the N 1s binding energy.…”