Multi-walled carbon nanotube-supported Ruthenium bis(terpyridine) complexes as ORR catalyst for zinc-air batteries

Developing efficient and stable oxygen reduction reaction (ORR) electrocatalysts is crucial for enhancing the core process of electrochemical energy conversion and storage technologies. In this work, Ruthenium(ii) bis(terpyridine) complex (MetpyRutpy) were covalently grafted onto multi-walled carbon nanotubes (MWCNTs) via esterification reactions. The successful graft of Ruthenium complexes on the surface of MWCNTs was confirmed by Fourier transform infrared spectroscopy (FT-IR), Ultraviolet visible spectroscopy (UV/Vis), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and thermogravimetric analysis (TGA). Specifically, each 280 carbon atoms of MWCNTs were covalently grafted via an ester group with one MetpyRutpy functional molecule. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) characterizations reveal that the grafted ruthenium complexes are uniformly distributed on the surface of MWCNTs without damaging the tubular structure of MWCNTs. Electrochemical tests showed that the hybrid material exhibited excellent ORR catalytic activity in alkaline medium (0.1 M KOH), with a half-wave potential of 0.715 V vs. RHE and a maximum diffusion current density of 4.47 mA cm −2 . The reaction pathway is consistent with a 4-electron transfer mechanism (average number of transferred electrons n ≈ 3.56). Compared with MetpyRutpy and MWCNTCOOH, the MetpyRutpyMWCNT composite exhibits a significant positive shift in the onset potential of the ORR. The open circuit voltage and the peak power density for the zinc air battery, assembled with MetpyRutpyMWCNT as the air cathode, reach 1.47 V and 47.1 mW cm 2 respectively. At a current density of 10 mA cm −2 , the specific capacity of the battery reaches 722.4 mAhg zn −1 . Such superior performance is attributed to the outstanding ORR property of the Metpyrutpy enity, the excellent conductivity of MWCNTs, and the synergistic catalytic effect between the two. This study provides a new design strategy for developing high-efficiency non‑platinum-based ORR electrocatalysts and broadens the application scope of Ruthenium complexes in energy-conversion devices.