Propagating Fe-N4 active sites with Vitamin C to efficiently drive oxygen electrocatalysis

Abstract

Free transition metal ions (such as Fe 2+ , Co 2+ ) exposed on the surface of metal organic frameworks (MOFs) would be highly aggregated during the carbonization process, which is not conducive to formation of M-N X active sites, resulting in reduced electrochemically active sites. Accordingly, to effectively suppress the free metal ions on the surface of MOFs and increase Fe-N 4 active sites, Vitamin C (L (+)-ascorbic acid), with an ability to coordinate transition metals, is complexed with ferrous ions. Meanwhile, the acidity of Vitamin C can moderately erode the surface of MOFs materials, further accelerating the generation of holes and defects in the carbonized products. Compared with the control samples without introduction of Vitamin C, the iron-based (VC-MOF-Fe) catalyst, with obviously increased Fe-N 4 active sites, exhibit significantly enhanced oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) performance. When used in rechargeable zinc-air batteries, the peak power density of VC-MOF-Fe (113 mW cm −2 ) is also better than that of 20% commercial Pt/C + RuO 2 . Interestingly, the function of Vitamin C also applies to the cobalt-based catalyst (VC-MOF-Co), evidencing the universality of this strategy. By introducing Vitamin C, the growth of transition metal (Fe, etc.) particles is suppressed, which significantly increases the amount of Fe-N 4 active sites in MOFs-derived catalysts, leading to a significant improvement in oxygen electrocatalytic performance. • Vitamin C suppresses aggregation of free ferrous ions on MOFs, producing more Fe-N 4 active sites. • The prepared VC-MOF-Fe catalyst exhibit significantly enhanced ORR performance in alkaline and acidic media. • It also shows improved OER performance in alkaline media. • It has high open circuit voltage and power density in the application of zinc-air batteries. • It also applies to VC-MOF-Co catalyst, which provides a new route on increasing M-Nx active sites.