FeCo alloy encapsulated in N-doped carbon shell for efficient oxygen reduction reaction

Abstract

This work reports the synthesis of FeCo alloy encapsulated in N-doped carbon shell for efficient oxygen reduction reaction (ORR). The FeCoNC-0.3compound were synthesized by high temperature pyrolysis of ZIF-L with the assistance of different Fe salts (i.e., FeCl3·6H2O, FeSO4·7H2O, K3Fe(CN)6, Fe(NH4)2·(SO4)2·6H2O and (NH4)3·Fe(C2O4)3·3H2O). With the presence of FeCl3·6H2O, FeSO4·7H2O, Fe(NH4)2·(SO4)2·6H2O and (NH4)3·Fe(C2O4)3·3H2O during the pyrolysis process, FeCo alloys are formed and more nitrogen elements are remained after high temperature pyrolysis compared with that in CoNC derived from pure ZIF-L. Differently, the introduction of K3Fe(CN)6 cannot lead to the generation of FeCo alloy and less nitrogen elements are remained after high temperature pyrolysis in comparison with that in CoNC. Linear relationship between the ORR onset potential and the ID/IG value of the FeCoNC-0.3compound catalysts are observed. Besides, the onset potential also increases linearly with the increase of the total content of pyridinic-N + graphitic-N + Fe(Co)-Nx. These results suggest that high content of carbon defects and high content of pyridinic-N, graphitic-N and Fe(Co)-Nx are beneficial for the improvement of the catalytic activities of the FeCoNC-0.3compound catalysts. Among all the as-prepared samples, FeCoNC-0.3FeCl3 demonstrates the highest catalytic activity towards ORR due to its unique core–shell structure with a carbon layer as the shell and FeCo alloy as the core, hierarchical channels, large pore volume, high content of carbon defects and high total content of pyridine-N, graphite-N and Fe(Co)-Nx. FeCoNC-0.3FeCl3 displays an onset potential of 0.97 V and half-wave potential of 0.82 V, which are comparable to the commercial Pt/C (onset potential of 1.05 V, half-wave potential of 0.86 V). Moreover, FeCoNC-0.3FeCl3 demonstrates much higher durability and methanol tolerance compared with Pt/C. Zn-air battery with FeCoNC-0.3FeCl3 as the cathode catalyst demonstrates maximum power density of 105 mW cm−2, which is close to that with Pt/C as the cathode catalyst (127 mW cm−2).