Controllable Construction Of Ru-and Ir-based Electrocatalysts And Their Water Splitting Performance

Electrochemical water splitting can convert renewable energy sources(solar energy and wind energy etc.)into hydrogen energy,which is a green way to produce hydrogen.Electrochemical water splitting consists of two half reactions including anodic oxygen evolution reaction(OER)and cathodic hydrogen evolution reaction(HER).Its large-scale development is limited by slow kinetics,high overpotential and poor stability.Therefore,it is imperative to develop catalysts with high activity,low cost and high stability.Ru-and Ir-based catalysts are promising candidates for the future.However,Ru as HER catalyst has high energy barrier for water dissociation and unsatisfactory hydrogen adsorption/desorption,which severely limits HER activity.Ir-based materials as OER catalysts have poor activity and insufficient stability due to the formation of IrO6 under acidic and oxidation conditions.To overcome these problems,a series of excellent catalysts for electrochemical water splitting have been constructed by strategies including heterogeneous engineering manipulation,geometric and electronic structure regulation and crystal phase engineering modulation.The structure of catalysts is analysed by X-ray powder diffraction,scanning electron microscope,(high resolution)transmission electron microscope,spherical aberration corrected transmission electron microscope,Raman spectroscopy,X-ray photoelectron spectroscopy and X-ray absorption spectroscopy.The catalytic behaviour is studied by electrochemical test.The catalytic mechanism is elucidated by electrochemical experiments combined with density functional theory(DFT),and the structure-activity relationship is well clarified.The specific researches are as follows:(1)Three different hetero-interfaces have been obtained by anchoring Ru nanoparticles on different MoOx(MoO2,MoO2/MoO3,and MoO3)nanotube(NT).MoO2@Ru NT shows excellent HER activity with overpotentials of 89 and 131 mV at 500 and 1000 mA cm-2,respectively,which are higher than the other two heterointerfaces,commercial Ru/C and Pt/C catalysts.MoO2@Ru NT even achieves an exceptionally high current density of 4000 mA cm-2 at an overpotential of 322 mV.Moreover,MoO2@Ru NT shows excellent stability at 1000 mA cm-2.Kinetic experimentals and theoretical calculations show that the moderate electron transfer from Ru to MoO2 enhances the water dissociation kinetics and optimizes the hydrogen adsorption of MoO2@Ru NT,thus promoting HER kinetics.In addition,the anion exchange membrane electrolyzer assembled by MoO2@Ru NT as cathode electrocatalyst requires only 1.78 V to reach 1000 mA cm-2 with no activity decay in 200 h test at 1000 mA cm-2.(2)Trace amount of Ir(1.0 at%)is introduced into hierarchical Co nanorod arrays(IrCo NRAs)for pH-universal OER performance.The results manifest that trace Ir not only adjusts morphology,but also changes electronic structure,thus significantly improving OER activity.Specifically,hierarchical IrCo NRAs require overpotentials of 257.3,296.9 and 376.1 mV at 10 mA cm-2 in alkaline,acidic and neutral conditions,respectively.Moreover,hierarchical IrCo NRAs also exhibit excellen stability over a wide pH range.The DFT calculation indicates that the introduction of Ir can reduce energy barrier of the rate-determining step for OER,thus promoting OER process.(3)Amorphous/crystalline IrO2 nanosheets(a/c-IrO2 NSs)have been successfully synthesized by crystal phase engineering regulation for acidic OER.Amorphous/crystalline heterophase provides Ir coordination unsaturated environment and oxygen defects for a/c-IrO2 NSs,which enhances OH*adsorption and thus promotes OER activity.a/c-IrO2 NSs exhibit an overpotential of 260 mV at 10 mA cm-2,which is much lower than amorphous IrO2 NSs,crystalline IrO2 NSs,and commercial IrO2.Moreover,a/c-IrO2 NSs maintain almost unchanged performance after 2500 and 250 h at 10 and 100 mA cm-2,respectively,which is attributed to the unchanged valence state of Ir and oxygen defect concentration.(4)The oxygen vacancy-rich IrO2 nanoparticles anchored on WO3 nanorods array(WO3@IrO2 NRAs)as acidic OER catalyst has been prepared by hydrothermalelectrodeposition-annealing method.The results indicate WO3 can regulate the electronic structure of IrO2 and induce the formation of abundant oxygen defects,thus improving the intrinsic activity of catalyst.WO3@IrO2 NRAs exhibit an overpotential of 238.6 mV at 10 mA cm-2.Moreover,the reduction of Ir valence state caused by electron transfer from W to Ir prevents Ir from over-oxidation and dissolution in acidic media,thus improving the stability of OER.WO3@IrO2 NRAs maintain durability for 500 h with negligible performance degradation.(5)Ir quantum dots@CeO2 nanorods(Ir QDs@CeO2 NRs)as highly efficient acidic OER catalysts have been obtained by low temperature liquid phase-annealingpolyol reductionmethod.The uniformly dispersion Ir QDs and a large number of hetero-interfaces NRs provided by CeO2 surface not only change the electronic structure of Ir,but also expose a large number of active sites,thus increasing the intrinsic activity.Ir QDs@CeO2 NRs exhibit an overpotential of 251.4 mV to deliver a current density of 10 mA cm-2 and maintain durability for 700 h.