Substituent effects in carbon-nanotube-supported diiron monophosphine complexes for hydrogen evolution reaction

Abstract

In order to explore substituent effects of PR3-phosphine ligands of diiron dithiolato complexes on the catalytic performances of [FeFe]-hydrogenase mimics for hydrogen evolution reaction (HER) in an aqueous medium, three new diiron monophosphine complexes [{(μ-SCH2)2N(C6H4CH2CH2OH)}Fe2(CO)5{P(C6H4R-4)3}] (labeled as FePR; R = F, H, and Me) were prepared and can be further linked covalently into carbon nanotube (CNT) to construct the target CNT-supported hybrids denoting as CNT-f-FePR. The molecular structures of diiron complexes FePR are well characterized through element analysis, fourier transform infrared spectroscopy (FT-IR), nuclear magnetic resonance (NMR) and X-ray crystallography, whereas the formations of target hybrids CNT-f-FePR have been confirmed by using X-ray photoelectron spectroscopy (XPS), Raman and FT-IR. Notably, the electrochemical HER performances of target hybrids CNT-f-FePR (R = F, H, and Me) are studied and compared in 0.1 M H2SO4 aqueous solution by means of linear sweep voltammetry (LSV), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and density functional theory (DFT) calculation. Among these hybrids, the CNT-f-FePF hybrid exhibits a more efficient HER activity in an aqueous media based on their electrochemical observations that the CNT-f-FePF hybrid with F-substituted phosphine has lower applied overpotential, smaller Tafel slope, larger electrochemical active surface area, smaller charge transfer resistance, and lower hydrogen chemisorption free energy relative to its analogues CNT-f-FePH and CNT-f-FePMe with H- or Me-substituted phosphines.