Studies On The Fabrication Of Carbon-based Single-atom Copper Electro-catalysts For Oxygen Reduction Reaction

Oxygen reduction reaction（ORR）is an important electrochemical reaction,utilized in various energy conversion devices including fuel cells and metal-air batteries.Due to the kinetic sluggishness of ORR,catalysts are required to accelerate the reaction rate,and the state-of-the-are catalyst which has already been commercialized is precious metal Pt.In order to promote the utilization of abovementioned energy conversion devices,wide research and explorati on on ORR catalysts are demanded,to achieve higher activity,more sytasfying stability and lower costs.Guided by these goals,this thesis contribute to innovating the fabrication process and optimizing the carbon substrate,in order to fabricate single-atom Cu catalysts with both high activity and stability.Comprehensive studies have been carried out on the formation mechanism of single atom Cu sites,the structure of metal active sites and factors correlated to the activity expression of such active sites.A novel self-initiated dispersion protocol is employed to fabricate single-atom Cu on graphene（Cu-G）using dicyandiamide as N source and Cu foil as metal source.The analysis on the effect of precursors and pyrolysis temperature on the resultant composition,combined with further evidence from in-situ TG-MS detection on the decomposition product,suggests that the transfer of Cu is assisted by the decomposition product of C₃N₄via gas phase.Element analysis on Cu in Cu-G reveals the average oxidation state close to+1.The characteristics of XAS spectra and spherical aberration corrected STEM observation confirm the presence of single atom Cu in Cu-G,whose detailed coordination structure is identified as Cu N ₂C₂according to XAF fitting.DFT computation suggests that the over strong adsorption of ORR intermediates on Cu is greatly regulated by the atomization of Cu,which results in satisfied ORR activity.Cu-G presents the ORR half-wave potential as high as 0.847 V,and outperforms Pt/C in Zn-air battery test even at the same catalyst loading.A three-dimensional hierarchical carbon with high graphitic level is fabricated as carbon support using polystyrene and metal oxide as dual templates and hollow polyaniline sphere as carbon source.According to the phase analysis at different fabrication stages,the thermal decomposition of polyaniline releases various reductive gases,which reduces metal oxide into metals,enabling metal nanoparticle templated carbon growth.Because excess iron oxide nanoparticles are served as templates,the grown graphene sphere are mostly broken,leading to the complete leaching out of Fe and the formation of hierarchical material NHC.The ultrahigh surface to bulk ratio of NHC promotes the loading level of Cu via gas phase,which contributes to the high ORR activity,and the high graphitic nature of carbon structure presents high resistance to oxidation and thus high electrochemical stability.Carbon nanotubes of different sizes are utilized as carbon substrate for single-atom Cu,obtaining a series of Cu-CNT samples.Using XAS analysis,the detailed coordination structure of Cu in these Cu-CNT samples are identified as Cu N₂C₂,the same as one another,and as Cu-G as well.DFT computation suggests that changes in geometry structure of Cu N₂C₂site is apparently varied on different substrates.When the O-containing species adsorb on Cu atom,Cu is dragged out axially from its original position,distorting the original planar quadrilateral structure.Such distortion is more obvious on CNT substrate with diatemer of 4 nm than that on graphene.Analysis on the electron structure suggests that the distortion of active site on the one hand strengthens the bonding of Cu with O,on the other hand weakens the original bonding of Cu with C/N.The substrate with moderate distortion will have the most balanced bonding,the most sufficient amount of electron transferred from catalyst to adsorbed O₂and highest ORR activity.The correlation of substrate-induced local curvature to the distortion degree is proposed,which well explains the ORR activity variation up to six-fold of Cu N₂C₂sites on different carbon substrate,and shows reference value for the future design of single atom catalysts as a structure-function relationship.