Recently, we have successfully synthesized novel catalysts, IrO₂ nanoparticle catalysts supported on nanocarbon materials with improved specific surface area, surface area per weight, of anode catalysts for PEWE. In this study, we focus on modification of IrO₂ catalysts by doping of hetero atom (N) on supporting materials and characterization its electronic state condition and catalytic activity for OER.

Novel catalyst of IrO₂ nanoparticles supported on nitrogen-doped reduced graphene oxide (N-rGO) was prepared by hydrothermal synthesis. Graphene oxide (GO) prepared by modified Hummers method [3] was mixed with urea and hydrothermally treated at 180 ˚C for 12 h to synthesize N-rGO. For the synthesis of IrO₂ / N-rGO catalysts, mixture of H₂IrCl₆ complex and N-rGO dispersed in ethanol/water mixture solution was stirred at 80 ˚C for 6 h in a flask to attach iridium oxide colloid to N-rGO and heated at 150 ˚C for 4 h in a hydrothermal autoclave to form IrO₂ nanoparticles.

The IrO₂ / N-rGO catalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), energy dispersed X-ray analysis (EDX), X-ray photoelectron spectroscopy (XPS), and electrochemical methods. The OER activity of the IrO₂/ N-rGO was examined in sulfuric acid solution using rotating disk electrode system.

Figure 1 shows TEM image of synthesized IrO₂ / N-rGO catalysts. The IrO₂ nanoparticles with an average particle size of 1.2 nm are uniformly dispersed on surface of the N-rGO substrate. The loading amount of the IrO₂ nanoparticles and doping weight of nitrogen atom of the IrO₂ / N-rGO catalysts are approximately 9 and 7 wt%, respectively, which was estimated from EDX measurements. Figure 2 shows the results of XPS investigation on GO, N-rGO, and IrO₂ / N-rGO. The XPS data reveal nitrogen doping on GO by hydrothermal synthesis of GO with urea. After the reaction between Ir complexes and N-rGO, Ir 4f peak appeared on the spectrum and intensity of O 1s was decreased due to reduction of GO during the hydrothermal reaction. Deconvolution analysis of the N 1s peak of N-rGO indicates doped nitrogen has three different types, pyridinic, pyrrolic, and graphitic type. The binding energy of the Ir 4f peak was shifted to a lower energy state. In addition, XRD pattern of the IrO₂ / N-rGO catalyst suggests that the interlayer distance of graphene sheets decreased during hydrothermal reaction because of reduction of GO. The XRD data suppot the results of the XPS measurement.

Figure 3 shows linear sweep voltammogram of the IrO₂ / N-rGO catalyst obtained in 0.5 M H₂SO₄ solution. Electrochemical measurement reveals high activity of the IrO₂ / N-rGO catalyst for OER, onset potential of the reaction is ca. 1.43 V and mass activity at 1.60 V is ca. 1000 A g^-1. The overvoltage of the IrO₂ / N-rGO catalysts is 70 mV lower than that of conventional IrO₂ powder catalysts. High activity of the IrO₂ / N-rGO catalyst indicates that the IrO₂ / N-rGO catalyst is a promising candidate of anode material for PEWE.

Acknowledgements

This work was supported by JSPS KAKENHI Grant number 17K05969.

Reference

[1] M. Carmo, D. L. Fritz, J. Mergel, D. Stolten, Int. J. Hydrogen Energy 38 (2013) 4901.

[2] E. Antolini, ACS Catal. 4 (2014) 1426.

[3] W. S. Hummers, R. E. Offeman, J. Am. Chem. Soc. 80 (1958) 1339.