(308e) Understanding of Anion Effect on Iron Redox Behavior and Oxygen Reduction Activity of Fe-N-C Electrocatalysts

Fe-N-C catalysts are the most promising platinum group metal (PGM) free electrocatalysts for oxygen reduction reaction (ORR). Significant progress has been made in understanding the active site structure and activity and improving the ORR performance. It is generally accepted that the Fe-N-C sites are associated with FeN_x centers embedded in a defective nitrogen-doped graphene. Furthermore, experimental characterization, such as in-situ X-ray absorption spectroscopy (XAS), has elucidated the redox behavior of Fe²⁺ to Fe³⁺ during the reaction. Interestingly, in different acidic environment, the ORR activity exhibits differently. For instance, in comparison between H₂SO₄ and HClO₄ solutions at the same pH value, the Fe redox happens at lower potential in H₂SO₄; however, the right shift of half-wave potential (E_1/2) of ORR in H₂SO₄, compared with HClO₄, indicates higher ORR activity. This indicates that the electrolyte systems have significant impact on ORR activity. To further understand this, density functional theory (DFT) calculations utilizing explicit solvation effect were carried out to study the Fe redox behavior and ORR activity of FeN₄ sites in both H₂SO₄ and HClO₄ solutions. As presented by DFT calculations, the anions in solutions can interact with FeN₄ site differently. SO₄^2- binds strongly with the FeN₄ center, while HSO₄^- and ClO₄^- interact rather weakly with FeN₄ site. Furthermore, mechanistic study shows that the transformation of Fe²⁺ to Fe³⁺ happens at 0.52 V in H₂SO₄, which is lower than that in HClO₄, 0.69V. More interestingly, the limiting potential (activity) of ORR in H₂SO₄ is higher than that in HClO₄ (0.94 vs 0.68 V). DFT simulation indicated that the anions in electrolyte can impact the Fe redox behavior and ORR performance, which agrees well with the experimental investigation. This work also provides significant guidance to further improve ORR activity of Fe-N-C catalysts.