Abstract:
Understanding and controlling the dynamic behavior of molecular electrocatalysts remains one of the central challenges in advancing sustainable energy conversion. Unlike rigid heterogeneous catalysts, molecular systems undergo substantial spin-state transitions, electronic rearrangements, and coordination changes under working conditions—yet these transformations are often elusive due to their transient nature and the lack of techniques that can resolve them operando. This complexity hinders the rational design of catalysts with both high activity and durability. Herein, we address this challenge by showcasing model systems where spin dynamics and orbital reorganization are not only captured but directly linked to catalytic performance. First, we report a spin crossover-driven diiron electrocatalyst for water oxidation, derived from a mononuclear iron precursor. Using a suite of in situ X-ray spectroscopies (XAS, XES, HERFD-XAS, and RIXS), we unravel a sequence of potential-induced transformations that drive dimerization, enhance metal–ligand covalency, and stabilize O–O bond-forming intermediates. The resulting [Fe2(μ-O)(μ-OH)(L1)2], where L1 is a nitrogen-based ligand, structure achieves a turnover frequency of 20.2 s-1 and sustains operation over 1,000 hours at an overpotential of 184 mV. Additionally, we explore an adaptive π-backdonation mechanism in single-atom Fe catalysts where heavier p-block ligands enable orbital reordering and stabilize an elusive η2-peroxo intermediate. By integrating cryogenic tip-enhanced Raman spectroscopy (TERS) and operando HERFD-XAS, we directly observe the dynamic engagement of dxz/dyz orbitals during oxygen reduction, leading to a remarkably low activation barrier and an ultra-high turnover frequency. These findings demonstrate that resolving and leveraging spin-state modulation, orbital reorganization, and dynamic coordination environments are not peripheral issues but central to unlocking new performance regimes in molecular electrocatalysis. Our work provides a blueprint for designing next-generation catalysts by bridging molecular design with operando spectroscopic insight.
Keywords – In situ/operando spectroscopy; Electrocatalysts; Solid-liquid interface