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 (Fe N C), even though Fe-N x groups have been confirmed as a type of dominating active site for most Fe N C systems. Herein, a facile and practical surface engineering process is adopted to transform metallic iron and iron trioxide nanoparticles into Fe 2 N nanocrystals, meanwhile to make their coated graphitic carbon shells interrupted and broken. This sufficiently facilitates the elimination of embedded Fe 2 N with a subsequent acid-etching process, eventually obtaining AA-Fe 2 N@NC sample. Based on various physicochemical characterizations, it is found that the relative concentrations of defective carbon and pyridinic N for AA-Fe 2 N@NC are clearly enhanced, compared to its counterpart (Fe 2 N@NC sample). Electrochemical measurements demonstrate that AA-Fe 2 N@NC instead of Fe 2 N@NC 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 Fe N C system rather than Fe 2 N compounds or Fe-N x groups actually serve as the dominant active sites to afford a high-performance output of ORR activity.