Iron-nitrogen-carbon species for oxygen electro-reduction and Zn-air battery: Surface engineering and experimental probe into active sites

It still requires further investigations on the status of iron-based compounds and actual active sites in iron and nitrogen modified carbon based oxygen reduction catalysts (FeNC), even though Fe-Nx groups have been confirmed as a type of dominating active site for most FeNC systems. Herein, a facile and practical surface engineering process is adopted to transform metallic iron and iron trioxide nanoparticles into Fe2N nanocrystals, meanwhile to make their coated graphitic carbon shells interrupted and broken. This sufficiently facilitates the elimination of embedded Fe2N with a subsequent acid-etching process, eventually obtaining AA-Fe2[email protected] sample. Based on various physicochemical characterizations, it is found that the relative concentrations of defective carbon and pyridinic N for AA-Fe2[email protected] are clearly enhanced, compared to its counterpart (Fe2[email protected] sample). Electrochemical measurements demonstrate that AA-Fe2[email protected] instead of Fe2[email protected] has optimal onset potential (72 mV positive shift), half-wave potential (45 mV positive shift) and limiting current density values (increased by 0.97 mA cm−2). When used as an air cathode of rechargeable Zn-air battery, it delivers a high power density of 168.15 mW cm−2, meanwhile a long-life cycling durability (60 h operation with an increased voltage gap of only 70 mV) is realized at 5 mA cm−2. Combined with the calculation of TOF values and SCN- inhibition test, we make a conclusion and highlight that, in alkaline electrolyte, carbon defects and pyridinic N for this FeNC system rather than Fe2N compounds or Fe-Nx groups actually serve as the dominant active sites to afford a high-performance output of ORR activity.