Nanostructured CuI And Bi Catalysts: In Situ Electroreconstruction And Application In Electrocatalytic Carbon Dioxide Reduction

Excessive CO₂ in the atmosphere,mainly from the usage of fossil fuels such as coal,petroleum oil,can lead to severe greenhouse effects.The conversion of CO₂by electrochemical reduction（CO₂RR）can reduce the major components of greenhouse gases and simultaneously obtain high value-added chemicals.Among the reduced products（CO,CH₄,HCOO^-,CH₃OH,Et OH,C₂H₄,etc.）,multi-carbon products with higher energy density or formic acid/formate with higher economic value attracted our attention.The challenges in electrocatalysis are mainly arised from the facts that:（1）multiple electron and proton transfer steps involved in the process of CO₂RR and thus producing multiple intermediates making the reaction mechanisms ambiguous and various kinds of products;（2）the types of electrolytes,the pretreatments of precursors,and other factors often coupled together and interfere with the understanding of the mechanisms;（3）the competing hydrogen evolution reaction（HER）leads to lower energy efficeincy of the CO₂RR.In this thesis,we addressed Cu-based and Bi-based catalysts.The interplay of Cu⁺,halogen anion and the reconstructed Cu I in performances of electrochemical CO₂ reduction reaction were analyzed.By designing controlled experiments,the in situ reconstruction phenomenon,Cu⁺and halogen anion were demonstrated in CO₂RR.For Bi based catalysts,a series of Bi OX precursors derived efficient Bi nanosheets generated by in situ electroreduction were also investigated and the relationships between their morphologies and electrocatalytic properties were deduced.The main content and results of this thesis are as follows:1.A two-step synthesis method was adopted to obtain mesoporous CuO catalysts,which were used as precursors for CO₂RR in the electrolyte with the presence of iodide ion.At-1.0 V vs.RHE,the total Faraday efficiency（FE）of C₂ reduction products is close to 60%in 1.0 M KI.Our controlled experimental results show that I^-can help suppress HER and induce reconstruction of the catalyst.The reconstructed small Cu I nanoparticles play an important role in promoting the selectivity of C₂products,which is attributed to the stabilization of Cu⁺by I^-anions and the exposure of abundant unsaturated coordination active sites on the reconstructed Cu I surface.2.A series of Bi OX（Bi OX,X=Cl,Br）nanoplates with different thicknesses and sizes were obtained by one-step solvo-thermal method through adjusting the composition of the solvents.Take Bi OCl as an example,two-dimensional（2D）Bi nanosheets electrocatalyst can be obtained by in-situ electrotransformation of Bi OClnanoplates.The effects of the thicknesses and sizes of the precursor on the CO₂RR performaces were studied.Since a large number of unsaturated coordination active sites can be exposed on the 2D Bi nanosheets,the adsorption of intermediates can be enhanced,and simultaneously the electrochemical active areas and electron transfer resistance are optimized.At-0.9 V,the current density is 10.2 m A cm^-2and the maximum FE is close to 92%with 15 h satability.In addition,FEs above 80%at a wide potential window（-0.8～-1.3 V）were obtained.The results show that the Bi OCl nanoplates with a certain range of thicknesses and sizes can maintain the 2D structures during the in-situ electrochemical reduction,thus providing more active sites and lower contact resistance in the CO₂RR.3.A series of Bi OX precursors with different thickness（nanoplates,naosheets,plates）were synthesized by one-step solvothermal method with the adjustment of mannitol/H₂O/surfactants.Taking Bi OI as a typical example,highly efficient 2D nanosheets catalytic structures were obtained by electrochemical in-situ reduction of Bi OI nanoplates in a CO₂-saturated electrolyte.The Bi nanosheets electrocatalyst derived from Bi OI nanoplates precursors（P-nanoplates-Bi）showed high activity and selectivity.It has shown excellent performances in reducing CO₂ to HCOO^-with a maximum FE～95%at-0.9 V and a current density of 13.3±0.6 m A cm^-2.The FEs exceeds 80%in a wide voltage range（-0.8～-1.2 V）.In addition,P-nanoplates-Bi nanosheets were stable for 15 h electrolysis in 0.5 M KHCO₃ aqueous solution.Its excellent performances are mainly attributed to the enhancement of the adsorption strength of CO₂^·-,the enlarged electrochemical active area,the lowered contact resistance,and the acceleration of electron transfer.As the thickness of the precursor increases to 70 nm or decreases to a few nanometers,the in situ electroreduced Bi will aggregate into nanoparticles and appear as uneven sizes and irregular shapes.The catalytic performances are greatly compromised due to the lack of 2D structures.