Mn- and N- doped carbon as promising catalysts for oxygen reduction reaction: Theoretical prediction and experimental validation

Abstract

Development of platinum group metal (PGM)-free as well as iron-free electrocatalysts is imperative to achieve low-cost and long-term durability of polymer electrolyte membrane fuel cells. Here, we combined computational and experimental studies to investigate the mechanism, activity, and durability of Mn and N co-doped carbon (denoted as Mn-N-C) as promising catalysts for oxygen reduction reaction (ORR) in challenging acid medium. The first-principles density functional theory calculations predict that it is favorable for O2 to be reduced into H2O via four-electron pathway on MnN4 sites embedded in carbon layer. Using the reaction energies calculated from DFT, microkinetic analysis predicts that the MnN4 sites could catalyze ORR with a half-wave potential only 60 mV lower than that of Pt (111) and 80 mV lower than that of the FeN4 sites embedded in carbon layer, assuming the same density of active sites in the catalysts. Motivated by the computational prediction, we synthesized a Mn-N-C catalyst using a polymer (i.e., polyaniline-PANI) hydrogel precursor via a high temperature approach. Structural characterization indicates that atomically dispersed Mn sites coordinated with N are very likely formed in the catalyst. Electrochemical measurements show that the synthesized Mn-N-C catalyst can promote four-electron ORR with a catalytic activity in acids comparable to that of the Fe-N-C catalyst prepared using the same procedure. More importantly, the Mn-N-C catalyst exhibits superior potential cyclic stability, only losing 20 mV after 10000 cycles (0.6 to 1.0 V in O2 saturated electrolyte). In comparison, the Fe-N-C catalyst would loss 80 mV after only 5000 cycles under the same testing conditions. Our computational and experimental results strongly suggest that the Mn and N co-doped carbon could be promising high-performance catalysts for ORR in acidic medium.