Tuning the spin state of the iron center by FePc/Mg(OH)₂ heterojunction boosting oxygen reduction performance

Iron phthalocyanine (FePc) is a promising non-noble metal catalyst for oxygen reduction reaction (ORR). While, with the plane-symmetric FeN₄ site, the ORR activity of FePc is generally low due to its low ability to adsorb and activate O₂. Herein, we anchor FePc on Mg(OH)₂/N-doped carbon nanosheets building the ternary plate-like catalyst FePc/Mg(OH)₂/NC. Theoretical and experimental results show that due to the formation of FePc/Mg(OH)₂ heterojunction, the electron spin state of the monodisperse iron active site ranges from medium spin (MS, t_2g⁵e_g¹) to low spin (LS, t_2g⁶e_g⁰), enhancing the adsorption with oxygen-containing intermediates, thereby improving the dynamics of the oxygen reduction reaction. As a result, FePc/Mg(OH)₂/NC catalyst exhibits an outstanding performance (E₀ = 1.02 V, E_1/2 = 0.91 V) for ORR, superior to the commercial Pt/C electrode (E₀ = 1.01 V, E_1/2 = 0.85 V) and FePc/NC (E₀ = 0.97 V, E_1/2 = 0.87 V) under the alkaline conditions. This work offers a new way for the rational design of effective FeNC catalysts with the support of metal oxides/hydroxides.