Design,Synthesis And Synchrotron Radiation Characterization Of Low-Dimensional Cobalt-based Electrocatalysts

With the accelerating economic development process all over the world,human production activities are increasingly demanding for energy.The exposured drawbacks during using traditional fossil energy,such as the greenhouse effect,smog and other environmental issues have attracted the many attentions in the past few decades.In addition,the large-scale consumption of fossil energy resources with limited reserves has exacerbated the energy crisis.In order to cope with the current energy crisis and environmental issues and achieve sustainable development,the development of efficient and renewable clean energy is urgent.The development of clean energy through electrocatalytic energy conversion is considered to be an important way to replace fossil energy and be environmentally friendly.The electrocatalyst is considered as the core of electrocatalytic energy conversion,and efficient electrocatalyst can play an important role in improving conversion efficiency.Electrocatalysts based on precious metals such as platinum,iridium,and ruthenium are traditionally considered to have high electrocatalytic activity.However,low reserves and high cost greatly limit their large-scale application in actual production.Therefore,the development of high-efficiency electrocatalysts based on non-precious metals is of great significance.3d transition metals(iron,cobalt,nickel,etc.)based compounds have been widely studied as popular candidates for electrocatalysts due to their large crustal reserves and relatively high conductivity.In fact,the electrocatalytic performance of traditional transition metal compounds still has a large gap compared with precious metals.Nanomaterials with low-dimensional structure have large specific surface area,unique atomic structure and fast carrier transport capacity,which brings new opportunities for the design and application of electrocatalysts.In this thesis,we reported a series electrocatalysts based on cobalt-based nanostructures via rationally design and controllable synthesis.Synchrotron radiation X-ray absorption spectroscopy(XAS)techniques were used to characterize the atomic,coordination and electronic structure information for deeply understanding the relationship between structural information and catalytic performance.The specific research content and innovation point of this thesis includes the following aspects:(1)The uniformly dispersed Co nanoparticles(Co NPs)enwrapped in nitrogen-doped carbon(Co-C-Nnanosheets electrocatalyst was synthesized by using ultrathin Co-based metal organic frameworks(Co-MOFs)as templates,combined with a self-polymerized dopamine coating followed by high-temperature pyrolysis preparation strategy.Characterization results showed that the nitrogen-doped carbon layer formed by the pyrolysis of polydopamine coating can not only effectively maintaining the nanosheet structure of Co-MOFs,but also limiting the growth of large-sized Co NPs.Finally,a sheets like Co-C-N electrocatalyst was obtained.Further characterization of the Co-C-N structure by XAS indicate that the Co-N coordination was formed between Co NPs and the nitrogen-doped carbon coating layer,showing obvious atomic structure distortion and unsaturated atom coordination,thus prompting Co-C-N exhibiting comparable electrocatalytic ORR activity to the commercial platinum/carbon(Pt/C).(2)Based on ultrathin Co hydroxide nanosheets,a facile one-step co-precipitation method was developed to synthesize well-dispersed Ir incorporated Co hydroxide nanosheets as highly efficient OER electrocatalyst.The Ir species incorporated in the defect-riched hydroxide nanosheets are clusters and single atoms through the formation of Co-Ir species,as revealed by XAS characterizations combining with high-angle annular dark-field scanning transmission electron microscopy measurement.Further characterizations toward the catalyst after undergoing OER process indicate that unique Co oxyhydroxide and high valence Ir species with low-coordination structure are formed due to the high oxidation potentials.The synergistic effect of Co oxyhydroxide and high valence Ir oxide makes this bimetallic electrocatalyst exhibits superior OER performance over commercial IrO2 in both neutral and alkaline solution.(3)A simple high temperature pyrolysis strategy was developed to synthesize a new type of composite with in situ growth of Co NPs encapsulated nitrogen doped carbon nanotubes(N-CNTs)among conductive Ti3C2Tx MXene matrix(Co/N-CNTs@Ti3C2Tx)as bifunctional electrocatalyst toward ORR and OER.The strong interface coupling and electron transfer between Co/N-CNTs and Ti3C2Tx were verified by SEM and XAS analysis.Benefits from encapsulated small Co NPs,robust N-CNTs and the strong coupling between the high conductive CNTs and Ti3C2Tx MXene to induced the electron transfer,the Co/N-CNTs@Ti3C2Tx composite shows good bifunctional electrocatalytic activity and cycling stability for ORR and OER in alkaline electrolyte.