Fe-Triazole coordination compound-derived Fe2O3 nanoparticles anchored on Fe-MOF/N-doped carbon nanosheets for efficient electrocatalytic oxygen evolution reaction

Abstract

Using FeCl₃·6H₂O and 1,2,4-triazole (Htrz) as starting materials, an Fe coordination compound (CC), [FeCl₃(Htrz)₃]·H₂O, was synthesized at room temperature. Fe-CC can be partially transformed into an Fe metal–organic framework (MOF), [FeCl₂(Htrz)], via low-temperature annealing. After sulfurization at 250, 300, and 400 °C, S-doped Fe₂O₃/N-doped carbon (denoted as NC)/Fe-MOF, FeS₂/NC/Fe-MOF, and FeS₂/NC were obtained, respectively. S-doped Fe₂O₃/NC/Fe-MOF shows the best oxygen evolution reaction (OER) catalytic activity in 1 M KOH solution, with overpotentials (η) of 185, 232, and 258 mV required to reach current densities of 10, 30, and 50 mA cm⁻², respectively, outperforming commercial RuO₂ and most transition-metal oxides reported to date; this high performance is associated with the Fe₂O₃ nanoparticles (NPs) anchored on the Fe-MOF/NC nanosheets. The Fe-MOF/NC matrix can act as a support to prevent the agglomeration of Fe₂O₃ NPs. In addition, S-doped Fe₂O₃/NC/Fe-MOF exhibits long-term OER activity at 20 mA cm⁻², which is related to the partial transformation of Fe₂O₃/Fe-MOF into FeOOH. In addition, density functional theory (DFT) calculations show that the rate-determining step of the OER process at the Fe sites of both Fe₂O₃ and FeS₂ is the formation of Fe*–OH, and the Fe₂O₃ sites display a lower Gibbs free energy (ΔG_max) of 1.674 eV and a smaller η value of ∼0.444 V. Bader charge, differential charge density mapping, and density of states (DOS) analysis all reveal more charge accumulation at the Fe sites of FeS₂ than at the Fe sites of Fe₂O₃, which is due to the lower electronegativity of S than of O. As a result, the Fe sites of FeS₂ show weaker affinity for –OH intermediates, giving rise to inferior OER performance compared with Fe₂O₃.