Synthesis of Highly Active Catalysts for Oxygen Evolution Reaction: Iridium Ruthenium Oxide Nanoparticles Supported on Heteroatom-Doped Reduced Graphene Oxide

Abstract

Environmental issues of global warming and depletion of fossil fuels have become significant topics in recent years. Effective utilization of renewable energies such as solar and wind energy is a critical issue to overcome the environmental problems. A large amount of effort has been invested in researches for conversion and storage of surplus energies. One of the prospective candidates is production of hydrogen via water electrolysis in polymer electrolyte water electrolyzer. However, catalytic activity, durability, and cost of water electrolysis systems are inadequate for practical application due to low kinetics and high over-potential of electrode reactions, especially, oxygen evolution reaction (OER) on the anode [1,2]. As OER catalyst, iridium oxide (IrO2) particles have been employed because of its high activity and durability. However, high cost and low abundance of iridium require reduction of iridium loading amount on the catalyst. Recently, we have succeeded to synthesize novel IrO2 nanoparticle with large surface area of IrO2 catalyst, which are supported on carbon nanotubes [3] and heteroatom-doped graphene [4], to improve OER activity. For further improvement of OER activity, we also develop alloy catalyst of IrO2 with other metals, such as ruthenium oxide (RuO2) to enhance the specific activity of the catalyst for OER.In this study, we have synthesized novel alloy nanoparticle catalysts, IrRuOx, supported on the heteroatom (N or B) doped reduced graphene oxide (rGO), IrRuOx/N-rGO and IrRuOx/B-rGO, and evaluated its catalytic activity as active catalysts for OER.The IrRuOx/N-rGO and IrRuOx/B-rGO catalyst were synthesized by hydrothermal method. Briefly, nitrogen- or boron-doped reduced graphene oxide (N-rGO, B-rGO) were prepared by thermal treatment of mixture of urea or boric anhydride and graphene oxide (GO), which was prepared by modified Hummers’ method, at 800 and 1000˚C, respectively, for 60 min. Then, required amounts of metal complexes, H2IrCl6 and RuCl3, and N-rGO, or B-rGO were dispersed in ethanol/water mixture and heated at 80˚C for 6 h. Finally, the mixture was heated at 150˚C in hydrothermal autoclave for 4 h to form IrRuOx nanoparticles. The synthesized catalysts were characterized by transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), and electrochemical methods. The OER activity of the catalysts was examined in 0.5 M H2SO4 solution by linear sweep voltammetry (LSV) using a rotating disk electrode (RDE) system.A TEM image of the IrRuOx/N- rGO catalyst is shown in Figure 1. The IrRuOx nanoparticles were dispersed uniformly on the surface of N-rGO substrate. An average size of the nanoparticles was ca. 1.7 nm. The loading amount of the IrRuOx catalyst is estimated to be approximately 20.1 wt% by EDX measurements. In the case of IrRuOx/B- rGO, average particle size was also ca. 1.7 nm, and the loading amount of the catalyst is 11.7 wt%. In addition, the EDX analysis reveals that the ratio of Ir to Ru of the IrRuOx nanoparticles is proportional to the molecular ratio of starting material compounds, H2IrCl6 to RuCl3. We confirmed the composition of the catalysts by XPS, as shown in Figure 2, and the results consisted with EDX measurement. Furthermore, the XPS results reveal that binding energy of Ir 4f peak on both catalysts was shifted to 0.1 eV lower energy state than that on the catalyst without alloying. The shift of the binding energy of Ir 4f peak reflects the modification of the electronic state of Ir, which affects adsorption energy of O and the catalytic activity for OER.Figure 3 shows mass activity of the prepared catalysts for OER estimated from LSV in 0.5 M H2SO4 solution. Electrochemical measurement reveals that the mass activity of the IrRuOx catalyst is higher than that on the catalyst without alloying. In addition, the catalyst supported on B-rGO has higher activity than the catalyst on non-doped or N-doped GO supports. High activity of the IrRuOx / B-rGO catalyst indicates that alloying IrO2 with RuO2 and heteroatom doping on the support materials improve the catalytic activity for OER. In summary, we successfully prepared the alloy catalysts of IrO2 and RuO2 supported on N-rGO and B-rGO substrate, and the catalysts can be promising candidates as anodes for water electrolysis.