On-Site H₂O₂ Synthesis Using a Series of Ni(II) Furan-2-thiocarboxylate Complexes Integrated on Multi-Walled Carbon Nanotubes: Soft Donor Sites Stimulated Selective Two-Electron Oxygen Reduction

A series of Ni(II) complexes [Ni(tmeda)(fCOS)2] (1a), [Ni(2,2′-bipy)(fCOS)2] (1b), [Ni(1,10-Phen)(fCOS)2] (1c), [Ni(opda)2(fCOS)2] (2), [Ni(dppe)(fCOS)2] (3a), and [Ni(dppee)(fCOS)2] (3b) (fCOS = furan-2-thiocarboxylate) have been synthesized. All the complexes, except 1b, were characterized by single-crystal X-ray diffraction analysis and spectroscopic techniques. In the solid state, the geometry around the Ni(II) center is square planar in the cases of 3a and 3b, while the same is octahedral (with varying degrees of distortion) in all other complexes. The complexes were integrated separately on multi-walled carbon nanotubes (MWCNTs) to prepare their respective composites. Electrochemical characteristics and oxygen reduction response of all the composites were studied. All composites exhibited a two-electron oxygen reduction process. The composite of complex 2 (2@MWCNTs) having S and N donor sites shows the highest onset potential and the highest rate of oxygen reduction. Composites of complexes having P and S coordination sites (3a@MWCNTs and 3b@MWCNTs) show relatively high selectivity toward the two-electron oxygen reduction. The composites of complexes having S, N, and O coordination sites (1a@MWCNTs, 1b@MWCNTs, and 1c@MWCNTs) show moderate selectivity and activity for the two-electron oxygen reduction. 3a@MWCNTs show the optimum combination of activity and selectivity for H2O2 production. Remarkably, all the composites show high durability and may be suitable for the long-term oxygen-reduction process to synthesize H2O2.