Study On Synthesis Of Iridium Nanocrystals And Electrochemical Performance For Oxygen Evolution Reaction

Iridium (Ir) and its oxide have showed high electrochemical catalytic activity inboth oxygen evolution reaction and hydrogen evolution reaction. Nanomaterials aregenerally conducive to expose active sites because of their high specific surface area,which can effectively control the usage of precious metal while with good catalyticeffect. In this paper, Ir nanocrystals were prepared by two kinds of solution-phasereduction processes and the relationship between size or shape of the crystals and itsoxygen evolution performance was also studied.Ir nanocrystals were first synthesized through high-temperature solution-phasereduction method. The crystalline phase, morphology and dispersity, and the effect oforganic groups of these synthesized Ir nanocrystals were studied by XRD, TEM, EDS,XPS, FTIR and UV-Vis. And Ir reducing mechanism was obtained preliminary. Theresults show that, Ir sources can significantly control the size of Ir nanocrystals. Whenusing IrCl3and H2IrCl6as Ir sources respectively, the particles size tends to berelatively large, while when using Ir(OAc)3and Ir(acac)3as Ir sources respectively,the particles size tends to be quite small. The mole ratio of oleylamine to Ir in thisreaction system required to provide a proper high reduction condition fornanoparticles growth is9or more. And the Ir nanoparticles growth seems to followthe Ostwald ripening process.Meanwhile, another solvothermal solution-phase reduction method was used tosynthesis Ir nanocrystals. The effects of crystallization temperature and time, Irsources and other surfactant in nanoparticles growth were studied. The results showthat, when the crystallization time is less than28h, only Ir dendrite could be obtained,while for a much longer time such as36h or more, spherical nanoparticles could beproduced. And ethanol, as part of the crystallization solvents, has great effect onparticles growth toward avoiding particles aggregation and over growth, and theappropriate volume ratio of oleylamine, oleic acid and ethanol is1:1:1.Finally, research on oxygen evolution catalytic activity of Ir nanocrystalssynthesized through both high temperature solution-phase reduction method andsolvothermal process were studied. Cyclic voltammograms (CV), polarization curvesand Ac impedance testing were used to compare electrochemical catalytic activity ofIr nanocrystals with different particle size and morphology. The study found that carbon nanotubes (CNTs) can be a good substrate for Ir nanoparticles loading toprevent particles aggregation during calcining process. All Ir nanocrystals showedgood performance in oxygen evolution reaction, and the starting oxygen evolutionpotential is about1.15-1.20V vs. Ag/AgCl. And the ratio of outer charge to totalcharge, q*Outer/q*Total, was about60-70%which proved that active sites can beeffectively exposed after Ir nanocrystallization. Different shape or size of Ir crystalshad different rate control steps in oxygen evolution reaction, which proved that thenanoparticle size can affect the oxygen evolution reaction mechanism directly.