Electronic Structure Engineering In Layered Double Hydroxides For Electrocatalysis

The industrialization of society has brought about increasing energy consumption,however,traditional fossil fuels,including coal,petroleum and natural gas,have limited reserves and at the same time,extensive use of traditional fossil energy has brought unavoidable environmental pollution.Therefore,it is a worldwide urgent problem to find a clean and renewable energy with high energy density.Aiming at lowering the barrier of oxygen evolution reaction(OER)in the preparation of clean and pollution-free energy(hydrogen)from water electrolysis,a finely modulation of electronic structure of efficient OER catalysts(based on layered double hydroxides,LDH)was conducted to prepare superior OER catalysts.Furthermore,the relationship between electronic structure of metal sites in LDH with corresponding electrocatalytic perforamcnes was further established.The research contents and conclusions are shown as following:1.LDH intercalated by differnet redox-active anions were prepared by co-precipitation method,and the corresponding OER catalytic performances in alkaline media were further studied.It was found that the redox-active anions in the interlayer of LDH had no obvious effect on the morphology or crystallinity of LDH,however,the electronic structure of metal sites in LDH was tightly connected to the redox activity of anions.LDH interclated by anions with electron-donating/reducing ability(SO32-,H2PO2-etc.)had superior OER catalytic activity to carbonate-intercalated LDH,while the intrinsic catalytic activity of LDH interecatled by anions with electron-withdrawing/oxidizing(ClO-,ClO2-etc.)was poor.The difference in catalytic activity of LDH mainly originated from the difference in electronic structure of metal sites in it.Metal sites with electron-rich structure had high affinity and strong binding strength to hydroxide in electrolyte,thus promoting the deprotonation step and facilitating the OER process,however,metal sites with electron-deficient structures can hardly adsorb hydroxide in the electrolyte,which hindered the deprotonation step.2.Several methods were put up to enhance the OER performance of LDH.Oxygen vacancies were introduced into NiFe-LDH by rapid reducing flame calcination.The taliored electron-rich structure of metal sites enhanced the adsorption of LDH to intermediates,and ultimately improved the intrinsic OER activity of LDH.The LDH showed low onset potential(1.42 V,vs.RHE)and satisfying stability after being treated by reducing flame for 30 s.Doping metal ions with weaker electronegativity(Fe2+,Mn2+,etc.)into the laminates of LDH can also tune the electronic structure of parent metal sites,which activated both edge-sited metal sites and inert in-plane metal sites together,improving the intrinsic activity and the number of active sites in LDH.Furthermore,the tensile strain had been introduced into LDH via mechanochemistry method.The electronic structure of metal sites and binding strength were jointly tailored by tensile strain.The enhanced binding strength to oxygenated intermediates on electron-rich metal sites contributed to the OER onset potential as low as 1.43 V,and it required only 270 mV overpotential to reach working current density of 10 mA/cm2.3.NiFe-LDH nanoarray with gradient doping effect was prepared by gradient Fe doping via hydrothermal method.Gradient doping effect led to continuous changes in metal-oxygen bond length and electronic structure of metal sites in NiF-LDH nanoarray,which maximized the interfical effect and enhanced intrinsic OER activity of NiFe-LDH nanoarray.Furthermore,the valence band structure of the gradient material was changed due to the gradient doping of Fe.It was found that the gradient doping effect brought about the favorable electron transfer in NiFe-LDH nanoarray,which suppressed the backward electron transfer.Therefore,the doping gradient effect improved intrinsic activity and facilitated electron transfer in NiFe-LDH nanoarray together.4.Based on the research above,a series of LDH/graphene composites were prepared by in-situ growth method.The electronic structure of metal sites in LDH/graphene composites was further tailored by doping foreign metal ions or sulfurization.The prepared LDH/graphene colloidal composites illustrated an array-like structure,which benefitted the exposure of active sites,facilated mass and electron transfer.By further assembly in prototype rechargeable zinc-air batteries,the composite materials showed satisfying bifunctional oxygen reactivity with excellent OER activity,which can achieve low charging potential and high working current density.The study here may provide implications for the further development of rechargeable zinc-air batteries.