Oxygen and Hydrogen Evolution Activities of IrO2 Nanocrystals Synthesized By Molten-Salt Method

Tuesday, 30 May 2017: 14:30
Grand Salon A - Section 3 (Hilton New Orleans Riverside)
Y. Mao, S. Mohan, and J. Ahmed (University of Texas Rio Grande Valley)
The storage of renewable energy using hydrogen “fuel” has been viewed as a viable mechanism by electrolysis of water into oxygen and hydrogen fuels, but is hampered by the slow kinetics of the oxygen evolution reaction (OER). Hence there is a broad effort to improve performance of currently used materials and develop new materials. Iridium oxide (IrO2) is an effective stable electro-catalyst which can lower the over-potential and keep a remarkable current to improve the efficiency of fuel generation technologies. To further improve its activity with novel nanostructures synthesized by facile and reliable methods, here ultrafine iridium oxide nanocrystals (IrO2 NCs) were successfully synthesized using a facile one-step molten salt synthesis process at 650 oC in air. To tune the size and morphology of these IrO2 NCs, three molar ratios of IrCl4, NaCl, and KCl were employed, specifically 1:10:10, 1:30:30, and 1:60:60. The structural and morphological characterizations of these IrO2 NRs were carried out by powder X-ray diffraction, Raman spectroscopy, Infrared spectroscopy, X-ray photoelectron spectroscopy, and electron microscopic techniques. The electrocatalytic performance of these IrO2 NCs for OER in acidic media was compared with that of commercial IrO2 nanoparticles (NPs) and the annealed derivative in terms of specific capacitance, total charge, most accessible charge, electrochemically active surface area, and roughness factor. The as-synthesized IrO2 NCs show enhanced electrocatalytic OER activity in 0.5 M H2SO4 media compared to the commercial IrO2 NPs before and after annealing. Among all five IrO2 samples, IrO2 NCs synthesized with the ratios of IrCl4:NaCl:KCl = 1:60:60 and 1:30:30 demonstrated better OER activity than commercial IrO2 NPs while the annealed commercial IrO2 NPs showed the worst performance. Moreover, compared to commercial IrO2 NPs and previous reports, the IrO2 nanorods show enhanced electrocatalytic activity to oxygen and hydrogen evolution reactions by passing either N2 or O2 gas in a 0.5 M KOH electrolyte before electrochemical measurements, including cyclic voltammetry, chronoamperometry and electrochemical impedance spectroscopy. Our results are comparable with, and in most cases, higher than reported data in the literature. Therefore, the current study reports a novel, reliable and scalable synthetic process from iridium tetrachloride salt for IrO2 nanostructures, but also a type of highly electrocatalytic efficient IrO2 nanostructures. It is expected that these IrO2 NCs can serve as a benchmark in the development of active OER and HER (photo)electrocatalysts for various applications.