Regulation and Control of Nano-structured Electrocatalyts for Efficient Electrochemical Reactions

Author:Zheng Zuo Zuo

Supervisor:zeng jie wu xiao jun


Degree Year:2019





The non-renewability of fossil energy along with the enormous consumption by human society aggravates the exhaustion of non-renewable resources.At the same time,the environmental problems caused by excessive emissions of greenhouse gases from traditional fuels burning also make it urgent to develop cheap,clean and renewable energy.In recent years,the development of various low-pollution energy conversion technologies is on the rise.Among them,the electrochemical technology method has attracted much attention due to the mild reaction conditions,good controllability(directly controlling the surface free energy of the electrode by the applied potential)and environmental friendliness.The emerging new energy technologies includes electrocatalytical water splitting and carbon dioxide electroreduction.These new energy technologies are based on electrochemical catalytic processes,which require catalysts to reduce the energy barrier of the reaction process to drive the reaction.At present,highly efficient catalysts for various electrocatalytic reactions are still dominated by noble metals.However,these precious metal catalysts are expensive and have low earth reserves,which greatly restrict the industrial application of electrocatalysis.Under this circumstance,it is urgent to design a new type of high-performance catalyst.The requirements of new-generation catalysts encompass reducing the amount of precious metal,increasing catalytic activity,especially the unit mass activity,and maintaining a good stability.In response to these challenges,a variety of catalytic materials have been developed to replace noble metal catalysts,such as metal oxides,metal chalcogenides,heteroatom-doped carbons,and molecular complexes.Despite the remarkable achievements,the activities and selectivities of these electrocatalysts still fall short of standards of high throughput,low energy consumption and large-scale production in practical applications.Therefore,further exploration of new strategies is highly desired for exploring ideal electrocatalysts which can drive the reaction at low overpotential and possess high activity and stability,but nevertheless remains challenging.This paper aims to design and develop inexpensive and efficient nanocatalysts and explore their related electrocatalytic properties.In this paper,we optimize the activity of catalysts by constructing a two-dimensional structure,through defect engineering,or introducing single atomic sites,to optimize the electronic structure of the electrocatalysts.The key process and regulation mechanism of electrocatalytic reaction are revealed at the atomic and molecular level through advanced characterization techniques and density functional theory simulation.These findings have guiding significance for the construction of highly-efficient nanoelectrocatalysts and the understanding of the structure-activity relationship of catalysts,and have a propelling effect on the practical application of electrocatalytic technology.This paper mainly includes the following aspects:1.We systematically explored the effect of oxygen deficiencies on the electronic structure of perovskite-like 2D-structured tungsten oxides and the correlations between the defects and hydrogen evolution reaction performance.By calculating the energy band structure,we found that after introducing oxygen defects,the WO3 material was transformed from semiconductor to degenerate semiconductor,which result in the enhancement of electrical conductivity.By calculating the hydrogen adsorption free energy of tungsten oxide with different amount of oxygen defects,it was found in the W24O67 model,the free energy of the adsorption of hydrogen of W atoms is closest to zero.Finally,we we experimentally verified that 2D-morphology endows WO3 with increased active sites.Moreover,existence of oxygen defects induced the enhanced electrical conductivity of WO3,resulting in the great improvement of hydrogen evolution performance.2.By chemical lithium intercalation,we activated the otherwise inert bismuth selenide to be highly active for both hydrogen evolution and water oxidation reaction in a wide pH range.Further studies have found that lithium intercalation makes the original IrSe2 more porous,with a larger specific surface area,and also introduces a large number of tri-coordinate Se defects.Theoretical calculations show that the loss of the tricoordinate Se optimizes the hydrogen adsorption free energy of the neighboring Ir,which is beneficial to the hydrogen evolution reaction.For water oxidation,Se defects facilitate the formation of active Ir oxide intermediates,thereby improving the performance of OER.We further integrated Li-IrSe2 into the water-splitting electrolyzer as both anode and cathode catalyst,respectively,which achieved excellent performances at full pH.This study is instructive for the design of high-efficiency catalysts and advances the industrialization of electrocatalytic water decomposition.3.We synthesized a single-atom catalyst in which Ni single atoms uniformly dispersed on two-dimensional graphene,and proved that Ni single atoms as the active sites have high activity and high selectivity for CO2-to-CO reaction.We have also found that other transition metals single atoms,such as Co,Fe and Mn,undergoes different reaction pathways.Especially for Co,hydrogen is the main products.Density functional theory calculations show that Ni single atoms have weaker CO adsorption energies and higher HER energy barriers,thus exhibiting higher selectivity for CO product.4.We have introduced a simple method of large-scale synthesis of nickel single atom catalyst.In the catalyst synthesis,commercially available and inexpensive carbon blacks which have been surface activated were used as supports to anchor single atoms.The 3-dimensional morphology of the activated carbon blacks can promote the diffusion of CO2 through the gas diffusion layer to ensure a high local concentration of the reactants.As a comparison,the graphene nanoplates which stacked layer by layer can block gas diffusion.By integrating this single atom catalyst into a membrane electrode assembly unit achieved a record high 8.3 A CO2 electrolysis current with CO selectivity of up to 99%.This work is of great significance for promoting the industrialization of CO2 electroreduction.