Towards The Structural And Catalytic Evolution In Transition-metal-centered Single-site Electrocatalysts: Controlled Synthesis And Electrocatalytic Properties

Facing the environmental challenges and energy crisis which are induced by the usage of foil fuels,development of the fuel cells,electrolyzers,and metal-air batteris remains to be of great significance.Electrolysis is a critical process,which enables the operation of these devices,accordingly,the electrocatalysts R & D dominates the future of these clean energy conversion/storage devices.To replace the expensive noble metal catalysts,lots of efforts have been dedicated on the transition-metal-based electrocatalysts in the past several decades.Especially,a promising synthetic approach towards noble-metal-free electrocatalysis is developed,which comprise of a high-temperature pyrolytic method to realize the integration of the cobalt and/or iron species into nitrogen-doped carbons.The catalysis reaction mainly takes place on the coordination unsaturated sites on the top surface of traditional supported catalysts.For the molecular catalysts,such as metal porphyrins and metal phthalocyanines,they also show electrocatalytic activity towards oxygen reduction,while the critical bottleneck of low selectivity and/or structural instability still holds.Recently,the single-site/single-atom catalysts with full atom utilization and special coordination structure as well as electronic properties appear as a new frontier in the fileds of catalysis and energy.Therefore,developing high-performance transition-metal-centered single-atom catalysts through high-temperature pyrolysis approaches is believd to be an enabler for the combination of supported solid catalysts and molecular catalysts,which shows great prospects in many fields,like electrocatalysis and organic synthesis.Previous efforts devoted in this field mainly focused on the catalysts synthesis or the active sites elucidation.However,achieving molecular insights into the the synthetic chemisty in transition-metal-centered electrocatalysts,and understanding their superiority over supported solid catalysts and molecular catalysts,as well as probing the effect of the chemical states of metal sites on their electrocatalytic properties are believed to be the critical steps towards the further optimization of transition-metal-centered electrocatalysts.Herin,using the oxgen reduction reaction and hydrogen evolution reaction as example,the roles of non-planar coordination structure in single-atom electrocatalysts,and the anions in the metal precursors,as well as the involved supports on the active sites formation and electrocatalytic performances are systematically investigated in this thesis.The main results are summaried as follows.1)With a combination of materials design,and electrochemical studies,and synchrotron-based X-ray absorption characterization,a non-planar coordinaton engineering strategy towards the decrease of energy position of the 3d unoccupied electronic orbitals and the resultant enhancement on the adsorption of oxygen and intemediates on metal sites is developed.The as-designed cobalt-based single-site electrocatalysts show high activity and selectivity,as well as good cycling stability,therefore overcoming the inadequacies of the molecular catalysts of cobalt porphyrins and cobalt phthalocyanine.The eluciation of the active site in such active catalysts indicates a media-dependent catalytic pathway and different main active sites in acidic and alkaline media,respectively.The obtained new knowledge in this study provides a new insight for the molecular design of single-atom electrocatalysts and their catalytic tuning.2)In an effort to understand the synthetic chemistry in the pyrolytic approach,the neglected role of involved anions on the formation of transition metal sites is originally revealed.An anion-regulated selective generation of metal sites in nitrogen doped carbon strategies is developed,where the chlorides are found to be really beneficial for the construction of the atomically dispersed metal sites.The electrocatalytic performances of single-atom metal sites and the encased metal nanoparticles towards oxygen reduction and hydrogen evolution are further studied and compared,indicating the cobalt-site-dependent performances.Our study shows the superiority of the catalytic contributions from the atomically dispersed metal sites over the metal-particles-based materials.3)Aiming at a better understanding towards the synthetic chemistry in single-site electrocatalytic sites construction,the strong effects of supports on the electrocatalytic activity of single-site catalysts are originally revealed with two mocular catalysts as precursors.A combination of the synchrotron-based aborption spectroscopic and electrochemical studies shows that the precursors are subject to a reconstruction of the coordination structure in the support-assisted pyrolytic synthesis.It is such structural reconstruction that discriminates the single-site electrocatalysts from normal molecule catalysts,accounting for the significantly enhanced activity and stability.And our original studies on the influence of the coordination numbers around the single-atom sites on the activity and structural stability show that an enhanced activity is obtained through the decreased coordination numbers of the coordination ligands(pyridinic nitrogen)around metal centers,while accompanied by the decreased structural stability in electrochemical conditions.Therefore,our study highlights the significance of balancing catalytic activity and electrochemical stability in trastion-metal-centered single-atom electrocatalysts design.These studies provide valuable insights towards the rational design and controlled synthesis of single-atom electrocatalysts,also pathing a new direction for better understanding on the catalytic mechanisms and reaction pathways in electrocatalysis.