Strategic Synthesis of Heptacoordinated Fe^III Bifunctional Complexes for Efficient Water Electrolysis

In this research, highly efficient heterogeneous bifunctional (BF) electrocatalysts (ECs) have been strategically designed by Fe coordination (C_R ) complexes, [Fe₂ L₂ (H₂ O)₂ Cl₂ ] (C1) and [Fe₂ L₂ (H₂ O)₂ (SO₄ )].2(CH₄ O) (C2) where the high seven C_R number synergistically modifies the electronic environment of the Fe centre for facilitation of H₂ O electrolysis. The electronic status of Fe and its adjacent atomic sites have been further modified by the replacement of -Cl^- in C1 by -SO₄ ^2- in C2. Interestingly, compared to C1, the O-S-O bridged C2 reveals superior BF activity with extremely low overpotential (η) at 10 mA cm^-2 (140 mV_OER , 62 mV_HER ) and small Tafel slope (120.9 mV dec^-1 _OER , 45.8 mV dec^-1 _HER ). Additionally, C2 also facilitates a high-performance alkaline H₂ O electrolyzer with cell voltage of 1.54 V at 10 mA cm^-2 and exhibits remarkable long-term stability. Thus, exploration of the intrinsic properties of metal-organic framework (MOF)-based ECs opens up a new approach to the rational design of a wide range of molecular catalysts.