Fe, Co, N-doped carbon nanotubes as bifunctional oxygen electrocatalysts

Carbon nanotubes were modified towards oxygen reduction and evolution reactions by incorporation of cobalt and/or iron phthalocyanines (CoPc and FePc, respectively). The obtained monometallic samples were physically mixed in different ratios to assess the importance of each phthalocyanine. The increase of FePc content enhanced the electroactivity towards the oxygen reduction reaction, especially the limiting current density and reaction mechanism, for which two relationships were found by varying the CoPc:FePc mass ratio. Samples with a mass ratio (FePc:CoPc) ≥ 1 surpassed the electroactivity of the current commercial electrocatalyst (Pt/C) and presented similarly high long-term durability (82–85%). Interestingly, a synergetic effect between both types of metallic active centers was observed for the oxygen evolution reaction. A 1:1 CoPc:FePc mass ratio provided the highest oxygen evolution reaction electroactivity, displaying a potential of 1.67 V at 10 mA cm−2, higher than that of the commercial RuO2 catalyst (1.58 V). This sample also presented the lowest potential gap (0.81 V), which is much lower than that of Pt/C (1.20 V) and RuO2 (1.41 V), demonstrating the high potential of these materials as substitutes of noble metal electrocatalysts.