Enhancing the hydrogen evolution activity of diiron molecular electrocatalyst by modulating substituent effect of carbon nanotubes

Abstract

Designing molecular catalysts to enhance hydrogen evolution activity presents both great significance and challenges. In hydrogenases, the redox group (i.e., [4Fe4S] subcluster) near catalytic active center (i.e., [2Fe2S] subcluster) plays a crucial role in modulating the enzyme's activity. Inspired by this biological strategy, three carbon-nanotube-supported diiron dithiolato hybrids, which are labeled as CNT-X-ADT (X = N, C, O), were designed. The activity of diiron catalytic center was regulated through side chain substituents (i.e., substituent effects) of CNT. Notably, a bioinspired diiron molecular compound {(μ-SCH₂)₂N(CH₂CO₂C₆H₄CHO-p)}Fe₂(CO)₆ (1), which was used to mimic diiron catalytic active center of hydrogenase enzyme, was first synthesized and then covalently attached to carbon nanotubes to form three target hybrids CNT-X-ADT (X = N, C, O). The side chain substituents, designed to mimic the activity control group, were linked to CNTs through an amination reaction. Significantly, the hydrogen evolution reaction (HER) properties of the CNT-X-ADT hybrids were systematically investigated and compared using various electrochemical techniques. Compared with CNT-C-ADT that lacks side chain regulatory ability, the average turnover frequency (TOFH₂) of CNT-N-ADT is nearly twice and gets 0.175 s⁻¹ after 5-h electrolysis, and the corresponding turnover number (TONH₂) for H₂ generation reaches 3.1 x 10³. In contrast, the CNT-O-ADT hybrid, due to its electron-withdrawing alkoxy side chain, reduces the electron density of the catalytic center, resulting in the poorest HER performance. Overall, this activity modulation by different side-chain substituents holds great significance for the development and design of metal molecular catalysts.