Controllable Preparation Of Layered Double Hydroxides And Their Composites For Small Molecule Electrocatalysis

Small molecule(e.g.,water,methanol,ethanol,hydrazine hydrate)electrocatalytic reactions,as the core processes of water splitting,fuel cells and metal-air batteries,have attracted extensive attention.Among them,high-performance electrocatalysts is particularly important.At present,the precious metal materials show excellent electrocatalytic activity for a series of small molecules,but the high cost,low availability and poor stability limit their practical application.Therefore,the development of noble metal-based catalysts with high-mass-activity and non-precious metal-based electrocatalysts with high-efficiency are of great significance.This dissertation take layered double hydroxides(LDHs)as the research object to provide feasible solutions for the above problems.Herein,we design and prepare three types of LDH-based electrocatalysts,including ultrathin NiAl-LDH nanosheets,LDH-supported highly dispersed Pd nanoparticle,and Pt and Ni3S2-containing hollow porous carbon sphere composites derived from LDHs.Their electrocatalytic properties of small molecules(ethanol,hydrazine hydrate,water)were studied,and the structure-activity relationship between catalyst surface structure and electrocatalytic performance was discussed.The specific research contents are as follows:1.Preparation of ultrathin two-dimensional(2D)NiAl-LDH nanosheets to improve their ethanol electrooxidation performance.Ultrathin 2D NiAl-LDH nanosheets(NiAl-LDHNSs)with the thickness of?0.8 nm was successfully prepared by simple hydrothermal synthesis and liquid-exfoliation process.This NiAl-LDHNSs exhibits excellent ethanol electrooxidation performances,including ultrahigh current density(45.80 mA cm-2 at 0.55 V vs.Ag/AgCl;which was 39 times higher than that of bulk NiAl-LDHs),quick turnover frequency and excellent durability.We found that the electronic structure of Ni in the layers of LDH was changed during the liquid-exfoliation process,that is partial Ni(II)was converted to Ni(III)).In addition,we proved that the generated Ni(III)(NiOOH)is true active sites in the electrocatalytic oxidation of ethanol which not only improves the intrinsic activity of the active sites,but also improves the conductivity of the catalyst.Furthermore,we also successfully prepared ultrathin and ultrasmall NiAl-LDH nanosheet(NiAl-LDHNSs-US)with the lateral dimension of only 3?5 nm and the thickness of about 0.8 nm via ultrasonic treatment of the NiAl-LDHNSs.The ultrasmall NiAl-LDHNSs-US provides more edge active sites and enhances the conductivity,which further enhances the ethanol electrocatalytic oxidation performance.2.Design and fabrication of LDH-supported highly dispersed Pd nanoparticles to enhance the electrocatalytic performance toward ethanol and hydrazine hydrate:In this section,we first proposed a bottom-up strategy to realize the one-step synthesize of ultrathin CoNi-LDH nanosheets(CoNi-LDHs-UT).Then,the PdNPs/CoNi-LDHs-UT was prepared by the spontaneous redox reaction between Co2+ on the LDH layer and PdCl42-in the solution.The as-prepared PdNPs/CoNi-LDHs-UT shows excellent ethanol electrooxidation performance,including high mass activity of Pd(2.06 A mgpd-1)and outstanding stability(the oxidation current maintains>90%after 3 h).The excellent performance of PdNPs/CoNi-LDHs-UT was attributed to the high dispersion of PdNPs on the surface of LDHs layers,which improve the utilization of active sites.Moreover,the ultrathin hydroxides nanosheet structure can improve the charge transfer capacity and the stability of the catalyst.In addition,PdNPs/CoAl-LDHNWs were further prepared by using CoAl-LDH nanowall array(CoAl-LDHNWs)as support.The ordered LDHs nanowall structure can provide larger specific surface area for PdNPs location and dispersion.Besides,the solid contact between LDHs and substrate also promote the fast electron transport.Thus,the as-obtained CoAl-LDHNWs showed good electrocatalytic performance of the oxidation of hydrazine hydrate.3.Fabrication of hollow porous carbon spheres containing high-dispersion Pt and Ni3S2 nanoparticles(Pt-Ni3S3/NHCSs)based on the interlayer confinement effect of NiAl-LDHs and investigating the alkaline electrocatalytic HER performance.The Pt-Ni3S2/NHCSs was synthesized through a space-confined synthesis strategy by co-intercalating PtC162-and metanilic anions within the interlayers of the NiAl-LDHs,followed by pyrolysis and acid etching processes.The prepared catalyst has the following structural advantages:(1)the confinement effect of NiAl-LDHs precursors and the unique hollow porous carbon sphere structure guarantee the highly dispersion of Pt nanoparticles(PtNPs);(2)the PtNPs/Ni3S2 heterostructures embedded within the carbon nanoflakes can accelerate the reaction kinetics of the HER in alkaline solution;(3)the active sites are embedded into the carbon nanoflakes of the hollow spheres in situ which can ensure an electrocatalyst with good durability for the HER.Based on the above structural advantages,the Pt-Ni3S2/NHCSs exhibits excellent HER performance with a low overpotential of 42 mV at 10 mA cm-2,and a ultrahigh mass activity of 7.59 A gPt-1 at an overpotential of 70 mV,which is 8.53 times higher than that of commercial Pt/C.