M–N–C electrocatalysts for the oxygen reduction reaction (ORR) have been considered as the most promising alternatives to precious metal catalysts. However, the catalytic activity of M–N–C especially in an acidic medium is still unable to meet the practical requirement, and the corresponding ORR mechanism also remains unclear. Herein, an “MOF-protective-pyrolysis” strategy was adopted to synthesize an Fe–N–C catalyst (P-FeMOF@ZIF-8) with Fe–N4 as the predominant Fe species. The P-FeMOF@ZIF-8 displays high catalytic activity in both acidic and alkaline mediums. The experimental results further reveal the different importance of the Fe species in different mediums. Based on the results, a distinct mechanism of the ORR the in acidic medium was proposed, where the synergistic effect of the Fe–N4 site and carbon in improving the kinetics of the ORR were well explained. The findings and the corresponding perspective may provide a new guidance for the design of highly efficient ORR electrocatalysts.
In this study, a Mn-modified Pt-based catalyst loaded on nitrogen-doped Ketjen black (Mn-Pt/NKB) is prepared using a simple ethylene glycol reduction method. The size of Pt nanoparticles (NPs) is effectively controlled by doping with Mn and N. With the smallest average particle size of 1.7 nm, Mn-Pt/NKB demonstrates half-wave potentials of 0.890 and 0.688 V in the alkaline and neutral electrolytes, respectively, which are superior to those of commercial platinum on activated carbon (Pt/C). When applied as an air cathode in aluminum-air battery, it exhibits ultra-high power densities of 190 (alkaline) and 26.2 mW·cm −2 (neutral). Moreover, the voltage remains stable after 5 h of discharge. The practical application performance of the Mn-Pt/NKB catalyst in an aluminum-air battery is better than that of commercial Pt/C. Furthermore, the oxygen reduction reaction (ORR) mechanism on surfaces with different particle sizes is analyzed using density functional theory. Oxygen cracking is the major pathway on the surface of the small particles with lower energy consumption of 0.5 eV, while water molecule cleavage is the major pathway on the surface of the large particles with higher energy consumption of 0.97 eV. The lower energy consumption of the oxygen cracking pathway further confirms the ORR mechanism for higher activity on small-sized surfaces. This study provides a direction for the rational design of Pt-based catalysts for ORR and sheds light on the commercial development of aluminum-air batteries.
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