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

Free transition metal ions (such as Fe2+, Co2+) 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-NX active sites, resulting in reduced electrochemically active sites. Accordingly, to effectively suppress the free metal ions on the surface of MOFs and increase Fe-N4 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-N4 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 + RuO2. Interestingly, the function of Vitamin C also applies to the cobalt-based catalyst (VC-MOF-Co), evidencing the universality of this strategy.