FeNi/N Doped Carbon Coating by Cathodic Plasma Electrolytic Deposition and Its Electrocatalytic Oxygen Production

The work aims to improve the catalytic activity and stability of the anodic electrocatalyst for oxygen evolution reaction (OER) and reduce the energy consumption of hydrogen production from water electrolysis. In this paper, FeNi/ N-doped carbon coatings on TC4 titanium alloy were deposited by cathodic plasma electrolytic deposition technique in organic system electrolyte containing urea, formamide and triethanolamine. The phase composition, morphology and surface elemental valence of the synthesized materials were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman and X-ray photoelectron spectroscopy (XPS). FeNi alloy and a spot of iron oxide was also found by XRD pattern. SEM image showed large number of pores and large particles on the coating surface were formed during cathodic plasma electrolytic oxidation, and there were many irregular pores in the coating, suggesting its porous structure. The thickness of the coating was about 145 μm, and it was closely bound to the substrate. Raman confirmed the formation of carbon. The deposited carbon was from high temperature pyrolysis of organic phase while FeNi alloy was derived from anodic oxidation dissolution and subsequent cathodic reduction. The surface element composition and valent state was characterized by the XPS spectra. The surface of as-obtained sample was composed of C, O, Fe and Ni, consistent with the energy dispersive spectra (EDS) result. Fe 2p high-resolution XPS spectra showed divalent and trivalent iron on the sample surface due to the spontaneous oxidation of FeNi alloy. N doping was in the form of pyridinic N (398.6 eV), pyrrolic N (399.8 eV) and graphitic N (401.2 eV), confirmed by the C 1s high-resolution XPS spectrum. With three-electrode system, i.e., the as-synthesized coating as the working electrode, the platinum wire and the saturated calomel electrode (SCE) as the counter and reference electrodes, respectively, the electrocatalytic activity for oxygen production and long-term stability of the as-synthesized materials were evaluated by linear sweep voltammetry (LSV), Tafel curve, electrochemical impedance spectroscopy and chronopotentiometry in 1.0 mol/L KOH solution. The phase composition of the as-synthesized coating was mainly composed of FeNi and N-doped carbon, and the surface showed rough and porous structure. The OER overpotential at 10 mA/cm2 of FeNi/N-doped carbon obtained by electrolytic deposition for 70 min was only 0.20 V, which was significantly lower than that obtained at 10 min, 40 min and 100 min (0.22 V). More importantly, the OER performance of this sample was superior to that of the precious metals IrO2 and RuO2. Meanwhile, this sample showed lower charge transfer resistance (1.75 Ω), Tafel slope (38.3 mV/dec). The overpotential of this catalyst had no obvious change even running 48 000 s at 20 mA/cm2, indicating robust long-term stability. The rough and porous structure of the coating surface can effectively enhance the mass transfer and provide abundant active sites for electrocatalytic oxygen production, thus improving OER performance. Additionally, the simple preparation method and self-supporting structure of the material can simplify the preparation cost of the electrode, thus shows a potential application prospect in water electrolysis for green hydrogen production. © 2023 Chongqing Wujiu Periodicals Press. All rights reserved.