| Oxygen evolution reaction（OER）is widely used in clean energy process such as water splitting,metal-air battery,and carbon dioxide reduction.However,OER shows large overpotential because of the sluggish four-electron transfer process,which limits the energy efficiency of the whole water splitting.Therefore,designing highly active and stable OER electrocatalysts is highly meaningful.Layered double hydroxides（LDHs）are cheap and show high electrocatalytic performance,which has emerged as a potential OER electrocatalyst for industry use.Nevertheless,there are still great challenges in identifying and modifying the intrinsic activity of LDHs-based OER electrocatalysts.This thesis focuses on highly active NiFe-LDH,delve into the relationship between facet and activity,coupled with the probe molecule method,demonstrating that the edge facet is the active OER facet of NiFe-LDH.Furthermore,by adsorbing anions selectively on edge facets,NiFe-LDH`s OER activity is further enhanced.Specific research contents are as follows:（1）Active facet determination of NiFe-LDH for oxygen evolution reactionNiFe-LDHs with varied edge facet area ratios are synthesized by tuning hydrothermal time.It is found that onset potential decreases with increasing edge facet area ratio,while Tafel slope increases with increasing edge facet area ratio.This different trend results from the difference in crystallinity.NiFe-LDH with a high edge facet area ratio experienced shorter hydrothermal treatment,resulting in lower stability,and thus higher Tafel slope in LSV curve.To eliminate the influence of crystallinity,reverse microemulsion with same reaction time and varied Rw values was used for synthesizing NiFe-LDH with same crystallinity,and different Rw values enable us to obtain NiFe-LDH with different edge facet area ratio.The overpotential and Tafel slope both exhibit a decreasing trend with increasing edge facet area ratio,demonstrating that the edge facet is the active facet.Furthermore,it is found that cyanate anion could act as a probe molecule that adsorbs on the edge facet of NiFe-LDH and inhibit the OER activity,a phenomenon also corroborating the active edge facet.Multiple characterization support that cyanate anion adsorbs on edge facet of NiFe-LDH.FT-IR after interlayer anion exchange proves that cyanate is not located in interlayer but on basal plane.XPS confirms that cyanate anion bonding with metal via N atom.EPR supports that almost all edge sites are adsorbed by cyanate anion.Elemental analysis verifies that cyanate anion`s coverage on the inside of basal plane is very low（5.6%）.These characterizations corroborates the selective adsorption of cyanate anion on the edge facet of NiFe-LDH,further demonstrating that the major OER activity of NiFe-LDH is from its edge facets.（2）The effect of anion adsorption on oxygen evolution reaction of NiFe-LDH.In this chapter,we optimize the hydrothermal synthesis method used in the previous chapter by changing the base-into-salt process into a salt-into-base process,and NiFe-LDH with uniformly distributed elements are obtained.Moreover,NiFe-LDH`s electrosynthesis method is also improved by using pulsed deposition.Solution becoming turbid occuring in previously reported potentiostatic method is alleviated and dilute acid immersion suppress the formation of insulating oxide film on Ni foam,which allow metal elements distribute more uniformly.Then the effects of nucleophilic anions（sulfate and carbonate）in electrolyte on NiFe-LDH`s OER were investigated.We hypothesize that adsorbed anion only takes part in a portion of catalytic cycle.The adsorption of sulfate on NIFe-LDH was confirmed by semi in-situ Raman spectroscopy.So we deduce that there is competitive adsorption between sulfate and hydroxide anion.If the catalyst`s affinity toward sulfate could be increased,the enhancement of sulfate on OER could be further improved.Based on these pieces of knowledge,we rationally incorporate“softer acid”cation(Zn2+)into NiFe-LDH and successfully enhanced anion`s adsorption,largely improving NiFe-LDH`s OER activity in1 M OH-.In contrast,doping hard acid such as Ti4+could weaken added anion`s adsorption and decrease its improvement on OER.These findings demonstrated our hypothesis that surface sites`chemical hardness on a catalyst can influence the competitive adsorption between added anions and hydroxide anions. |
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