In-Situ Electrochemical Construction of Stable Water Oxidation Catalysts

Abstract

Hydrogen has been regarded as a perfect future energy source alternative to fossil energy because of its wide application in chemical industry and great potential in fuel cells. However, the high overpotential is often required for the anodic reaction (OER) of the water splitting. Presently, the best catalysts for OER are based on precious metals (such as IrO2 and RuO2), however, the high cost and scarcity of these catalysts limit their wide applications. Therefore, non-noble metal catalysts have attracted much attention as electrocatalysts used in alkaline electrolyzer. However, it is more important to improve the stability than the activity of non-noble metal catalysts towards their large-scale application, especially under extreme electrolysis conditions. This work developed an OER electrocatalyst with superlattice structure by in-situ electrochemical process, which shows high electrocatalytic activity and stability in alkaline.Nickel phosphate (NPO) with nanowire morphology was synthesized by hydrothermal reaction firstly. ꞵ-Ni(OH)2 was obtained by in-situ electrochemical scanning the NPO powder coated on carbon papers in alkaline solution. ꞵ-Ni(OH)2-CNTs were also prepared by the same method with the only difference of mixing NPO and CNTs. Catalytic activity of ꞵ-Ni(OH)2 for OER was evaluated by the three-electrode system in 1 M KOH solution. And the catalyst was characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), energy dispersive spectroscopy (EDS), and transmission electron microscope (TEM) analysis.Figure 1a shows the uniform NPO nanowire with a diameter of 5 nm obtained by hydrothermal reaction. Figure 1b-d display that the ꞵ-Ni(OH)2 nanosheets with superlattice structure has been successfully synthesized with the use of NPO nanowires as precursor by in-situ electrochemical method under alkaline condition. High resolution TEM shows that the thickness of ꞵ-Ni(OH)2 and NiOOH layer is 0.9 nm and 0.6 nm, respectively. The ꞵ-Ni(OH)2 and ꞵ-Ni(OH)2-CNTs showed high activity and stability toward the OER with an overpotential of 310 mV and 270 mV to reach the current density of 10 mA cm-2, respectively, and a long-term stability of 100 h showed in Figure 1e-f. This excellent property may be related to the superlattice structure which improves the efficiency of electronic transmission. Figure 1. (a) TEM picture of Ni3(PO4)2; (b) TEM pictures of ꞵ-Ni(OH)2; (c) Elemental mapping of ꞵ-Ni(OH)2; (d) XPS spectra of P 2p of ꞵ-Ni(OH)2; (e) iR-corrected polarization curves of ꞵ-Ni(OH)2, ꞵ-Ni(OH)2-CNTs, IrO2, and RuO2 in 1 M KOH solution; (f) Chronopotentiometry curves of ꞵ-Ni(OH)2 and ꞵ-Ni(OH)2-CNTs at the potential of 1.54 V and 1.50 V, respectively. Figure 1