An Investigation On Electrochemical CO₂ Reduction With Highly Dispersed Transition Metal Catalysts

The large amount of CO₂ emitted by burning fossil fuels leads great pressure on the sustainable carbon cycle and seriously challenges the ecological environment.Electrochemical CO₂ reduction using renewable electricity to convert CO₂ into high value-added chemicals or fuels,can not only realize the clean utilization of CO₂,but also the effective storage of renewable energy in chemical forms.It is considered to be one of the most promising strategies for CO₂ emission reduction and utilization,and is of great significance for the solution of energy and environmental problems,aiming at"carbon neutral".The kinetics of CO₂ activation is very slow due to the stable structure of CO₂ molecule and the electrochemical CO₂ reduction reaction faces fierce competition from hydrogen evolution reaction.Besides,the reaction involves multi protons coupled and electrons transfer and has complicated paths and intermediates,resulting in highly selective conversion of CO₂ being a huge challenge.Therefore,the development of excellent selective and stable electrocatalysts is essential to solving the technical bottlenecks.Although noble metals,such as Au and Ag,exhibit high selectivity of electrochemical CO₂ reduction,the high cost and overpotential restrict their large-scale application.To this end,this thesis starts with the adjustment of the material coordination,designs a series of highly dispersed transition metal nitrogen-doped catalysts for electrochemical CO₂ reduction reaction,and systematically investigates the selectivity of products,catalytic performance and reaction mechanism.1.In this paper,a simple synthesis method of highly dispersed transition metal catalysts is developed.By pyrolysis metal-organic framework precursor and the organic molecule-inorganic material hybrid technology to regulate the coordination environment of the metal center,the highly dispersed copper-iron atom（Cu Fe/N-C）and nickel polyphthalocyanine molecular（Ni PPc/CNT）catalysts are successfully prepared,which realize electrochemical reduction of CO₂ to CO with both high selectivity and low overpotential.The results show that the synergistic effect of copper and iron in the Cu Fe/N-C catalyst overcomes the shortcoming of single copper with high overpotential or iron material with H₂ producing and presents the catalytic performance.The overpotential for CO generation is only 90 m V and the highest Faraday efficiency reaches 95.5%,it also shows good durability during 20 hours electrolysis.Theoretical research suggests that the excellent activity of Cu Fe/N-C is attributed to its specific coordination environment,which enhances the protons and electrons transfer.This study provides a new approach for designing of high performance diatomic dispersed catalysts for electrochemical CO₂ reduction.The Ni PPc/CNT catalyst synthesized by a simple one-step method also demonstrates excellent catalytic activity.It not only retains the coordination structure of the nickel phthalocyanine molecule but inhibits the aggregation of organic molecules and improves the electron transfer by extensively cross-linked on the carbon nanotube substrate.The overpotential for CO production is only 255 m V,the Faraday efficiency of CO reaches 92%and the CO current density is as high as-18.6 m A·cm^-2,as well as it exhibits high stability during 16 h electrolysis.This organic molecule-inorganic material hybrid catalyst provides a new strategy for the performance enhancements of electrochemical CO₂ reduction.2.This paper designs an organic synthesis strategy to precisely customize the molecular structure of highly dispersed transition metal catalyst.A new high-dispersed cobalt phthalocyanine molecular（Co Pc3-CNT）catalyst is successfully prepared,which molecular coordination and steric hindrance being improved by introducing specific groups and dispersed on carbon substrate by non-covalent bonding.The electrochemical CO₂ reduction to methanol of molecular catalyst is achieved by CO₂-CO-CH₃OH tandem process,and the selectivity and stability for CH₃OH forming is also improved.The results show that in the first step Co Pc3-CNT converts CO₂ into CO with a maximum Faraday efficiency of 95%,and exhibits continuous production capacity of CO.The second step further converts CO to CH₃OH with the Faraday efficiency is as high as 18.91%,thus realizing efficient electrochemical reduction of CO₂ to CH₃OH fuel.Further analysis found that the improvement of the catalytic activity by Co Pc3-CNT is attributed to the enhanced steric interaction between the positive charge of the group in Co Pc3 and the negative charge of the O atom in CO₂,which promotes the coordination reaction between them and is beneficial to the adsorption of CO₂ molecules on the Co metal center.This study provides an effective method for precise customization of highly dispersed molecular complexes as electrocatalysts for electrochemical CO₂ reduction into liquid fuels.3.This paper proposes a plan for accurately developing new high-dispersed copper-iron structure.Using density functional theory calculation method,a new atom-dispersed copper-iron catalyst（Cu Fe@C₂N）is designed by regulating the monolayer carbon substrate supported copper-iron atoms and optimizing the coordination structure of the metal center.Comprehensive theoretical analysis is carried out to investigate the reaction path and product selectivity of electrochemical CO₂ reduction.Density functional theory calculations show that the structure of Cu Fe@C₂N is stable（the formation energy is-1.65 e V）,the atomically dispersed Cu Fe is precisely anchored on the porous C₂N substrate and the good coordination environment and electronic structure exhibits excellent electrochemical activity.The unique copper-iron coordination structure enhances the adsorption strength of CO₂ molecular and other intermediates（adsorption energy of CO₂ reaches-1.73 e V）,which is conducive to the electrochemical CO₂ reduction reaction while inhibiting the hydrogen evolution reaction.The Gibbs free energy change of the reaction path to C₂H₅OH（-1.27 e V）is lower than other products,and the formation of*CHOHCO intermediate by the C-C coupling is the most feasible thermodynamic step（ΔG=-0.71 e V）,thereby achieving high selectivity of C₂H₅OH generation and effectively further reducing CO₂ to multi-carbon products.This study provides theoretical guidance for more precise design and prediction of high-efficiency catalysts for electrochemical CO₂ reduction reaction.4.In this paper,a self-designed electrochemical CO₂ reduction flow cell is used to establish a CO₂reduction evaluation system.The effect of CO₂ solubility in the aqueous solution on the catalytic performance of Co Pc3-CNT catalyst is evaluated.Compared with H-type electrolytic cell,the activity of the Co Pc3-CNT catalyst for electrochemical CO₂ reduction to CO is significantly improved using the gas diffusion electrode in flow cell,which can not only break through the limitation of low solubility and poor diffusion of CO₂ in aqueous solution but also build a stable gas-liquid-solid three-phase reaction interface.By optimizing the experimental conditions,a process model for the efficient and stable operation of the electrochemical CO₂reduction flow cell system was established.The results show that the Faraday efficiency of CO production is85%,the current density of CO is as high as-140.3 m A·cm^-2,and it also exhibits excellent stability during115 hours electrolysis at a large current density of-50 m A·cm^-2.Through the enlightenment of the effective operation of the electrochemical CO₂ reduction evaluation system,a new method of optimizing the catalytic electrode and flow cell for electrochemical CO₂ reduction is revealed.This dissertation systematically studies the application of highly dispersed transition metal catalysts for electrochemical CO₂ reduction reaction,and actively explores the development of environmentally friendly CO₂ emission reduction technologies with high selectivity and low cost.