Effects of P-doping on the electrochemical properties of atomically dispersed Fe-N-C electrocatalysts for oxygen reduction reaction

Abstract

Development of highly-active and cost-effective electrocatalyst as a replacement to platinum for oxygen reduction reaction (ORR) is of great importance for pervasively using electrochemical devices such as fuel cells and metal-air batteries. Fe-N-C have attracted much attention owing to their high activity attributed to Fe-$N_x$ motifs, however, is still struggling to develop the catalyst with enhanced activity in a harsh acidic as well as alkaline electrolytes. Herein, we report a P-doped Fe-N-C catalyst derived from additional phosphorus doping process toward Fe-N-C synthesized from zeolitic imidazolate framework-8 (ZIF-8) and revealed the effects of phosphorus doping which has never reported before. The P-Fe-N-C exhibited outstanding ORR activity with a half-wave potential ($E_{1/2}$) of 0.862 $V_{RHE}$ and the kinetic current density ($J_K$ @ 0.8 $V_{RHE}$) of 32.89 mA/$cm^2$, which outperformed Fe-N-C (E1/2 of 0.843 $V_{RHE}$ and $J_K$ of 17.32 mA/$cm^2$) and commercial Pt/C ($E_{1/2}$ of 0.837 $V_{RHE}$ and JK of 18.11 mA/$cm^2$) in 0.1 M KOH. Moreover, P-Fe-N-C showed higher mass activity ($J_M$ @ 0.7 $V_{RHE}$) of 8.83 A/$mg_{Fe}$ than Fe-N-C (4.92 A/$mg_{Fe}$) and commercial Pt/C (1.17 A/$mg_{Pt}$) in acidic media. Through systematic investigation of XAFS analysis and DFT calculation, it is revealed that the improvement is attributed to the change of active sites configuration from $FeN_4$ to $FeN_3P$. Density functional theory (DFT) calculations further revealed that change of charge distribution around Fe increases $O_2$ adsorption energy with nearly unchanged OH detachment on the active sites. This work not only provides insight into the ORR activity of ZIF-8 derived $Fe_{SA}$-N-C but also offers new opportunities to enhance the performance of ORR energy devices by rationally designing electrocatalysts.