High content Fe(III) electrocatalyst for the oxygen reduction and evolution reactions. Spectroscopic, electrochemical, and theoretical insights

X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS), DFT simulations, and standard electrochemical methods were used to analyse the high Fe(III) content of the synthetized Fe phthalocyanine (FePc) axially coordinated to 4-amino-3-nitropyridine (NPy). XPS analysis reveals that NPy exerts a high electron-withdrawing power on the Fe centre causing a shift to a more positive value of E^0’_Fe(III)/(II) contrasted to FePc-CNT with a ΔE^0’ = [E^0’_{FePc-NPy-CNT(III)/(II)} - E^0’_{FePc-CNT(III)/(II)}] = 56 mV, which evidence the possibility to fine-tune this value. XAS and XANES analyses indicate a decreased electron density in the metal centre causing lower interactions with O₂. Consecutively, the Fe²⁺/Fe³⁺ ratio changes from 0.95 to 0.51 for FePc-CNT and Fe-NPy-CNT, respectively. The electrocatalytic studies of the oxygen reduction reaction (ORR) in alkaline media, show better performances than the commonly used Pt 20% electrocatalyst in terms of overpotential (ΔE^0’ = [E^0'_{onset Fe-NPy-CNT} - E^0'_{onset Pt}⁰] = 35 mV) and similar TOF rates at 0.9 V vs. RHE (∼1.00 e⁻ site⁻¹ s⁻¹). DFT calculations show that both FePc-Py-CNT and FePc-NPy-CNT catalysts have nearly equal interactions with O₂ (−0.37 eV and −0.32 eV, respectively). However, the O₂ bond distance in the FePc-NPy-CNT system is 11% greater compared to the FePc-Py-CNT system explaining the superior performance of the FePc-NPy-CNT catalyst for the oxygen reduction reaction (ORR). Additionally, the interaction with H₂O is stronger in the -NPy-configuration, which is consistent with the experimental results.