Preparation of IrO2-Ta2O5 Coatings on Ni Substrate and Oxygen Evolution in Alkaline Aqueous Solution

An electrode consisting of a conductive substrate covered with a noble metal oxide prepared by thermal decomposition is used for an oxygen evolution anode in industrial electrolysis applications such as electrowinning and electrogalvanizing using acidic aqueous solutions. In the cases, titanium is popular as the substrate material because of a superior corrosion resistance to acidic solutions. For oxygen evolution in alkaline solutions such as water electrolysis for hydrogen production and electroplating, nickel alloy electrodes are known to use, because nickel oxyhydroxide or nickel oxide formed on the surface works as a good protective layer. However, the overpotential for oxygen evolution on such nickel alloy electrodes is higher than that of the coated titanium electrode in acidic solution. It seems to be preferable if oxide coatings formed on a cost-effective nickel substrate instead of a titanium one work for oxygen evolution in alkaline solutions with a high catalytic activity and a high durability. In this paper, we investigated the preparation and characterization of IrO₂-Ta₂O₅coatings by thermal decomposition on a nickel substrate and examined oxygen evolution behaviors in alkaline solutions.

IrO₂-Ta₂O₅ coatings were formed on a pretreated nickel plate by thermal decomposition of a precursor solution containing Ir(IV) and Ta(V) ions. The precursor solution was painted on the substrate and heated at temperature 470 ^oC. This process was repeated some times. Some pretreatment procedures of the nickel plate were examined by polishing and chemical etching. The characterization of the obtained coatings was carried out by XRD, SEM, and EDX. Anodic polarization behaviors were measured by cyclic voltammetry using a three-electrode cell with 6.0 mol/L KOH solutions.

Mechanical polishing and chemical etching with some acids such as oxalic acid, sulfuric acid, hydrochloric acid and nitro-hydrochloric acid were used for pretreatment of nickel plates before painting of a precursor solution and found that dipping of the plate into nitro-hydrochloric acid solution gave a rough surface appropriate to paint and hold the precursor solution. Then, IrO₂-Ta₂O₅ coatings were prepared on such the pretreated surface and analyzed by XRD, SEM, and EDX. The XRD results showed some clear diffraction peaks of IrO₂ along with some small peaks of NiO, which suggests that a mixture of crystalline IrO₂ and amorphous Ta₂O₅ is formed on a nickel substrate which is partly oxidized. As shown in Fig. 1, the SEM analysis indicated that the substrate was fully covered with the oxide coating and the surface consisted of flat areas and cracks, which is relatively smooth compared to the coating formed on a titanium substrate, because few aggregated and well-developed IrO₂ particles were seen on the coating on the nickel plate. There was no growth of NiO reaching to the outermost surface of the coating from the results by EDX. Figure 2 shows the cyclic voltammogram of the IrO₂-Ta₂O₅/Ni electrode in 6.0 mol/L KOH solution. The anodic wave (A) is caused by oxygen evolution and located in the same potential range as that previously obtained with an IrO₂-Ta₂O₅/Ti electrode [1]. The result also presented anodic (B, C) and cathodic (D) waves which may correspond to nickel reactions such as the oxidation and reduction of Ni(OH)₂/NiOOH redox couple. From the repeated cyclic voltammograms, the waves of the nickel reactions existed with no increase in the current density during the cycles, suggesting that no nickel reaction was further developed by cyclic voltammetry.

References
1) S. Unoki, T. Zhang, and M. Morimitsu, The 224^th ECS Meeting, Abs#0027, San Francisco (2013).