Fundamental And Scale-up Research On Enhanced Electrochemical Reduction Of Carbon Dioxide By Ionic Liquid Electrolyte

With the adverse problems caused by carbon dioxide（CO₂）generated by fossil fuel combustion on the ecological environment,energy saving and CO₂ emission reduction are important for energy,ecological environment,national security and sustainable development,and the efficient conversion and utilization of CO₂ has become a research hotspot worldwide.Therefore,CO₂ electrocatalytic reduction has the advantage combining with various sustainable resources such as wind,solar and tidal energy,which provides new opportunity for future industrialization.Most of the existing CO₂ electrocatalytic reduction electrolytes are aqueous salt solution systems,which suffer from poor product selectivity,severe hydrogen evolution reactions,and poor stability.Ionic liquids are widely used in CO₂ electrocatalytic reduction due to the advantages of wide electrochemical window,reduced reaction overpotential and good CO₂ affinity which have the advantages of enhancing the selectivity of target products and increasing current density,providing a new idea for efficient conversion of CO₂.In this study,aromatic nitrile-functionalized ionic liquids were designed and synthesized as electrolytes for electrochemical reduction of CO₂ to oxalate,and the activation mechanism of the new ionic liquids on CO₂ conversion was investigated.In addition,a scale-up device for the electrocatalytic reduction of CO₂ was established,and the first study of CO₂ electroreduction using ionic liquids as electrolyte was conducted to investigate the effect of operating conditions on product selectivity and to examine the stability,which provides implications for the scale-up of CO₂ electroreduction.The main contents and results of this thesis are discussed as follows.（1）An H-type double chamber electrolytic cell device was built for the study of oxalate production by CO₂ electroreduction.The standard curves of gas products such as CO,H₂ and CH₄ were established,and the gas products were detected and analyzed by gas chromatography.The oxalic acid standard curve was established by compounding different concentrations of standard oxalic acid samples with acetonitrile.The oxalate product was detected by high performance liquid chromatography（HPLC）,and the formic acid standard curve was established with dimethyl sulfoxide（DMSO）as the internal standard.The content of formic acid in the analysis product was characterized by nuclear magnetic H spectrum.（2）Novel aromatic nitrile-functional ionic liquid was designed and synthesized for CO₂ electroreduction to oxalate,the Faraday efficiency of the target product oxalate reached 84.9%at a current density of 5 m A·cm^-2.Meanwhile,this study revealed the reaction mechanism of the electroreduction of oxalate in aromatic nitrile ionic liquid electrolyte.Firstly,CO₂ gets electron activated by[TEP][4-CN-Ph O]to form[TEP][4-CN-Ph O]-2·CO₂^-intermediate to reduce the activation energy of the reaction,and then the oxygen（or nitrogen for the nitriles）-carbon bond formed by the intermediate,which successively breaks homolytically generating the parent nitrile and the anion radical of·CO₂^-,which eventually dimerizes to oxalate.（3）The CO₂ electrochemical upgrading and amplification device suitable for ionic liquid electrolyte system was established for the first time.The effects of applied voltage,ionic liquid compositions,gas flow rate and electrolyte circulation flow rate on CO products were obtained,and the highest Faraday efficiency of CO was 83.9%.The stability experiment for 10 hours showed that the device can keep stable with CO products of 75%CO Faraday efficiency and 1.7L/h generation rate.Moreover,¹HNMR results of post-electrolyte showed no noticeable change in structures compared to the fresh-electrolyte,confirming good stability of the IL-based electrolyte in CO₂electroreduction reaction.which provides important data for the industrialization of CO₂ electrochemical reduction.