Preparation Of Flexible Self-supporting Carbon Fiber Electrode And Study On Electrocatalytic Reduction Carbon Dioxide

The electrochemical CO₂ reduction reactions（CO₂RR）into low-carbon fuels or other chemicals,driven by renewable energy,can not only reduce the concentration of CO₂ in the atmosphere,but also alleviate the problem of energy shortage.Because of the strong thermodynamic stability of CO₂,it is generally necessary to use a catalyst to activate it,so looking for a highly efficient CO₂ electroreduction catalyst has become one of the hotspots in current research.Researchers at home and abroad have done a lot of work in this field and achieved a series of remarkable outstanding results.However,most of the CO₂electroreduction catalysts reported so far are powdered structures,which need to be made into usable electrodes using adhesives such as Nafion and PVDF.In this process,the adhesive can block a large number of electron,ion,and gas transmission channels,on the one hand,reduce the conductivity of the catalyst,on the other hand,the catalytic active sites are wrapped inside the catalyst,reduce the number of effective active sites that can participate in CO₂electroreduction,and thus reduce the catalytic activity.Therefore,the introduction of catalysts with self-supported structures is of great significance to promote the theoretical research and practical application of CO₂ electroreduction.In addition,most of the CO₂ electroreduction catalysts still use H-type electrolytic cells for performance testing.The poor solubility of CO₂in aqueous solutions and the lack of special gas diffusion layers in H-type electrolytic cells,greatly limit the overall reaction rate,making the current density difficult to meet the requirements of commercial applications(>200 m A cm^-2).In order to achieve commercial CO₂RR,the design of new CO₂RR reaction devices is gradually becoming a research hotspot.In view of this,this thesis focuses on the synthesis of a series of carbon-based self-supported electrodes and"gas diffusion layer/catalyst layer"integrated gas diffusion electrode（GDE）by electrospinning technology,and systematically studies its application in electrocatalytic CO₂reduction.The specific research contents are as follows:（1）Nitrogen-doped carbon Nano fibers embedded with platinum nanoparticles are directly electrospun onto a flexible carbon cloth,and formed in one step to obtain a Pt-NPs@NCNFs@CC composite material,which can be directly used as CO₂RR working electrode without adhesive.The active substance Pt-NPs@NCNFs of this material is in close contact with the carbon cloth substrate,and it is not easy to fall off from the carbon cloth.At the same time,the network structure formed by the overlap of Pt-NPs@NCNFs ensures that sufficient electrolyte enters the active site and reduces the contact resistance.Through comparative experiments,it was found that the ultrafine Pt-NPs embedded in NCNFs coexist with pyridine nitrogen species can reduce CO₂ to formic acid with high selectivity.The Faraday efficiency of formic acid can reach 91%at-0.5 V_RHE.After long 30 h electrolysis,the current density was remained around 40 m A cm^-2,demonstrating the good stability of the Pt-NPs@NCNFs@CC material.（2）A kind of self-supported,cross-linked and high-yield cobalt single atoms doped hierarchical porous carbon nanofibers（Co SA/HCNFs）material was constructed using electrospinning technology.The obtained material is an independent and complete membrane,which is directly used for the CO₂RR performance test.Co SA/HCNFs through the hierarchical porous structure can increase electrochemical active surface areas,expose more active sites,and improve the utilization efficiency of single atom active sites.At the same time,the presence of hierarchical porous increases the amount of CO₂ adsorption,which results in the accumulation of more CO₂ on the catalyst surface,which is beneficial to the activation and reduction of CO₂.Density functional theory（DFT）calculations show that the Co-N₄ structure formed by N atoms anchoring cobalt single atoms is the main active site for electrocatalytic CO₂ reduction,and can convert CO₂ to CO with high selectivity.In the H-type cell,Co SA/HCNFs can efficiently reduce CO₂ to CO in a wide potential range.At-0.6 V_RHEpotential,the Faraday efficiency of CO is as high as 97%,which is one of the best results ever reported by single-atom catalysts so far.In addition,Co SA/HCNFs were continuously electrolyzed at-0.9 V_RHE potential for 50 h,the current density was maintained around 65 m A cm^-2 with negligible degradation,indicating excellent stability of the self-supported structure.（3）Designed and synthesized a kind of flexible self-supported Ni single atoms doped porous carbon nanofiber membrane（Ni SA/PCFM）material that is easy to large-scale produce.Ni SA/PCFM has three-dimensional interconnected nanofiber and hierarchical porous structure,and a large number of Ni single atoms are anchored on the carbon nanofiber,showing excellent CO₂RR catalytic activity.In a typical H-type cell,the Faradaic efficiency of CO up to 96%by Ni SA/PCFM at-0.7 V_RHE.Moreover,Ni SA/PCFM has excellent mechanical strength and flexibility,combined with Ni single atoms uniformly anchored on carbon nanofibers.Ni SA/PCFM forms one architecture in which gas diffusion layer and catalyst layer are integrated,providing a dynamic and stable three-phase interface,greatly improving the current density of the reaction and achieving efficient electrocatalytic CO₂reduction.The Ni SA/PCFM was directly used as gas diffusion electrode to test on the GDE flow cell,the partial current density of CO was as high as 308.4 m A cm^-2 at-1.0 V_RHE,reaching the commercially-relevant current density.After long 120 h electrolysis,the Faraday efficiency of CO is still 88%,showing excellent stability.The study found that the DFT calculations based on*COOH intermediate provide a molecular understanding of the observed high efficiency on single-atom Ni sites:a large number of Ni single atoms anchored on carbon nanofibers form a Ni-N₄-C structure,which has a smaller the Gibbs free energy barrier can reduce CO₂ to CO with high selectivity.