A Two-in-One Strategy to Simultaneously Boost the Site Density and Turnover Frequency of Fe−N−C Oxygen Reduction Catalysts

Site density (SD) and turnover frequency (TOF) are the two fundamental kinetic descriptors that determine the oxygen reduction activity of iron-nitrogen-carbon (Fe−N−C) catalysts that represent the most promising alternatives to precious and scarce platinum. However, it remains a grand challenge to simultaneously optimize these two parameters in a single Fe−N−C catalyst. Here we show that treating a typical Fe−N−C catalyst with ammonium iodine (NH₄I) vapor via a one-step chemical vapor deposition process not only increases the surface area and porosity of the catalyst (and thus enhanced exposure of active sites) via the etching effect of the in situ released NH₃, but also regulates the electronic structure of the Fe−N−C moieties by the iodine dopants incorporated into the carbon matrix. As a result, the NH₄I-treated Fe−N−C catalyst possesses both high values in the SD of 2.15×10¹⁹ sites g⁻¹ (×2 enhancement compared to the untreated counterpart) and TOF of 3.71 electrons site⁻¹ s⁻¹ (×3 enhancement) that correspond to a high mass activity of 12.78 A g⁻¹, as determined by in situ nitrite stripping technique. Moreover, this catalyst exhibits an excellent oxygen reduction activity in base with a half-wave potential (E_1/2) of 0.924 V and acceptable activity in acid with E_1/2 =0.795 V, and superior power density of 249.1 mW cm⁻² in a zinc-air battery.