Tuning Structure and Reactivity on Iron-Sulfur Clusters for Electrocatalytic Water Oxidation

Abstract

Water oxidation (WO) in water electrolysis is thermodynamically and kinetically demanding, severely impeding the overall water splitting kinetics and constituting a primary bottleneck hindering the rapid advancement of water electrolysis technologies. Consequently, developing high-efficiency electrocatalysts represents a crucial strategy to address this challenge. Molecular catalysts have garnered significant attention due to their well-defined active sites, precisely tunable structures, and near-unity atomic utilization efficiency. The combination of iron’s small atomic radius, multiple accessible oxidation states, and diverse spin configurations (high-spin/low-spin) engenders unique catalytic activity and selectivity during redox processes. Iron-based molecular catalysts are exceptional candidate materials for enhancing water oxidation reaction efficiency. In biological systems, iron sulfur (Fe-S) clusters serve as essential cofactors in metalloenzymes, performing critical functions in electron transfer, catalytic transformations, and substrate recognition/activation. Given that iron in these clusters typically resides in lower oxidation states (Fe^II), they are predominantly employed in reductive catalysis and rarely utilized for water oxidation reactions. However, the intrinsic electron transfer capability of Fe-S clusters may exhibit exceptional compatibility and application potential for the four-electron-transfer process inherent to the oxygen evolution. Therefore, this dissertation employs ligand engineering to focus on the rational design and synthesis of Fe-S cluster architectures. Concurrently, the electrocatalytic water oxidation (WO) performance of these clusters and the governing factors were systematically investigated. Through iterative structural optimization, the catalytic activity was progressively enhanced.

1. The author initially investigated the nuclearity effect by synthesizing a series of iron-sulfur complexes. The synthesized complexes were: 2Fe-4S and Fe-2S. For comparative studies on the influence of the initial oxidation state, an iron(III) compound, Fe-PI. Electrocatalytic assessments revealed that 2Fe-4S achieved a turnover frequency (TOF) of 22.1 s^-1 for WO, approximately 7 times higher than that of the mononuclear complex Fe-2S (TOF = 3.3 s^-1). In contrast, Fe-PI exhibited negligible catalytic activity. Mechanistic studies suggested that the first two complexes likely follow distinct catalytic pathways. This finding underscores the significant impact of bimetallic cooperative catalysis on enhancing WO activities, providing crucial guidance for the design of higher-nuclearity catalysts with improved properties.

2. Dinuclear iron-sulfur cluster, Fe-SC, was synthesized through ligand engineering aimed at reducing steric hindrance. Electrocatalytic water oxidation experiments demonstrated that Fe-SC exhibits exceptional catalytic performance, achieving a remarkably high TOF of 454 s^-1. In contrast, the catalytic activities of two counterparts, Fe-MC and Fe-BB, were significantly lower than that of Fe-SC. Comparative analysis of the structure-catalytic property relationships, supported by DFT computational modeling, identified the hexacoordinate iron center as the genuine active site in Fe-SC. Furthermore, EPR spectrum suggest that reversible protonation of the pyridyl groups within the ligand framework, coupled with spin-state changes at the metal centers, plays a decisive role in stabilizing key reactive intermediates.

3. This chapter designed and constructed a linear trinuclear iron-sulfur cluster:[Fe₃(Sip)₄][CF₃SO₃]₂ (Fe₃(Sip)₄). In this complex, the central Fe(II) ion adopts a tetracoordinate S₄ geometry, while the two terminal Fe(II) ions exhibit a coordination environment analogous to Fe-SC (N₄S₂). Consequently, Fe₃(Sip)₄ demonstrated significantly superior electrocatalytic water oxidation activity, achieving a remarkably high TOF of 932 s^-1. DFT calculations revealed that protonation at the pyridyl nitrogen sites during catalysis plays a pivotal role in accelerating the reaction kinetics. This process establishes a pronounced synergistic effect with the low-spin Fe centers, thereby significantly boosting the overall catalytic efficiency. Comparative analysis with the dinuclear Fe-SC complex from the previous chapter and counterpart Fe₃(SNS)₄ indicates that Fe₃(Sip)₄ harbors dual active sites–specifically, the two terminal hexacoordinate iron centers–within a single molecule, and the essential role of the pyridyl nitrogens in the catalytic mechanism.

Date of Award	9 Oct 2025
Original language	English
Awarding Institution	City University of Hong Kong
Sponsors	Xi'an Jiaotong University
Supervisor	Johnny Chung Yin HO (Supervisor), Yanzhen ZHENG (External Supervisor) & Yongquan Qu (External Supervisor)