High Efficiency Electrocatalytic Reduction Of Carbon Dioxide Material Design And Performance Enhancement

The rapid consumption of fossil fuels has not only caused the energy crisis but also brought a series of environmental problems.In particular,the release of large amounts of carbon dioxide gas into the atmosphere has led to the increasingly prominent greenhouse effect,leading to global warming,the reduction of polar glacier area,sea level rise and other serious environmental problems.If the CO2 in the atmosphere could be recovered and converted into useful chemicals or fuels,the environmental problems would be solved and part of the energy depletion be alleviated.For this reason,researchers have done a lot of research to address these issues and develope a series of methods in recent decades.Among them,electrocatalytic reduction of CO2 has attracted the attention of researchers due to its advantages such as high selectivity,high efficiency,high stability and the operation powered by clean,renewable energy.Although electrocatalytic reduction of CO2 has been studied for nearly half of a century,there are still many problems to be solved in this field.For example,(1)the reaction requires high kinetics and high overpotential and limited by high symmetry structure in CO2;(2)the mechanisms of the CO2 reduction process were derivated from theoretical simulations,but the progress of the reaction process is slow through the experimental technology;(3)the hydrogen evolution overpotentials of many cheap bulk metal materials are too low,resulting in high selectivity in hydrogen evolution and thus low efficiency in electrocatalytic reduction of CO2;(4)most high efficiency electrocatalytic CO2 reduction systems use precious metal catalysts such as gold,silver,ruthenium and rhodium,with high cost;(5)in the process of electrocatalytic reduction of CO2,only the cathode produces products of economic value,so the utilization efficiency of electricity energy is not satisfactory.Therefore,the development of low overpotential,cheap and efficient electrocatalytic materials has become the top priority in the field of CO2 reduction.In addition,it is necessary to find an appropriate anode reaction that couples with the cathode reaction of electrocatalytic reduction of CO2 to enhance the energy efficiency of the overall reaction.My doctoral thesis aims to,for electrocatalytic reduction of CO2,(1)improve the catalytic activity of bulk metal materials and study their reaction mechanism;(2)develop cheap and efficient catalyst materials to replace noble metal materials;(3)synthesize new copper-based materials,with improved product selectivity and reduced overpotential;(4)try to combine CO2 reduction and anodic oxidation reaction to achieve efficient utilization of electric energy.The specific research content is as follows:1.Electrochemical CO2 reduction provides a desirable pathway to convert greenhouse gas into useful chemicals.It is a great challenge to reduce CO2 efficiently in aqueous solution,especially on commercial bulk metal electrodes.Here,we report substantial improvement in CO2 reduction performance on bulk silver electrode through the introduction of ionic surfactant in aqueous electrolyte.The hydrogen evolution on the electrode surface is greatly suppressed by the surfactant,while the catalytic ability of silver towards CO2 reduction is maintained.The Faradaic efficiency for CO is greatly enhanced from 50%to 95%after the addition of this low-cost surfactant.This study may provide new pathways towards efficient CO2 reduction through the inhibition of proton reduction.2.Development of efficient electrocatalysts towards CO2 reduction is a key step on conversion of greenhouse gas into to useful chemicals.As for the electroreduction of CO2 to CO,noble catalysts(e.g.gold and silver)are well known for their superior selectivity,whereas the high cost is not suitable for scaled-up application.In this work,we show that an inexpensive nano-sized zinc(Nano-Zn)catalyst,prepared by electrochemical modification of commercial Zn foil,produces CO with up to 93%Faraday efficiency in aqueous NaCl solution.Moreover,the activity of the Nano-Zn electrode was stable over several hours,whereas bulk Zn electrode lost activity within 1 h.The involvement of both nanostructure and chloride catholyte in enhancing performance of Nano-Zn catalyst is demonstrated,and functional mechanisms for the catalysis are proposed.This work provides a new direction for developing new cheap catalyst materials in the future.3.Copper catalyst materials are the only electrocatalysts that can reduce CO2 to hydrocarbon and other advanced products at room temperature and pressure in aqueous solution.Lots of research attentions have been placed on copper-catalyzed CO2 electroreduction.However,copper-based catalysts carry out CO2 reduction with the product distribution being of wide range and the selectivity being very low.In addition,there are also problems of low efficiency and high overpotential.Therefore,achieving narrow product distribution,high selectivity,and low overpotential by copper-based electrocatalysts is a major challenge for electrocatalytic reduction of CO2.In this study,we synthesized monatomic copper-supported nitrogen-doped carbon materials by calcining at high temperature.The electrocatalytic reduction of CO2 in NaCl solution at-0.6 V versus RHE showed excellent catalytic activity and product selectivity with CO Faraday efficiency of up to 92.3%.Compared with other previously-reported copper-based catalysts,the product selectivity of CO2 reduction was obviously improved,and the overpotential of the reaction was also reduced.This provides a new idea to enhance the selectivity of copper catalyst material for the product and reduce the overpotential of the reaction.4.Electroreduction of carbon dioxide into value-added chemicals provides a potential approach to mitigate environmental problems induced by excessive emission of CO2,during which the electricity can be provided by renewable clean energy sources like solar power.The development of low-cost and efficient electrocatalysts for the carbon dioxide reduction reaction(CO2RR)is desirable,but remains a great challenge.In this work,a robust and inexpensive electrocatalyst,iron single-atoms loaded on nitrogen-doped carbon(Fe-SAs/N-C),was developed for efficient conversion of CO2 to CO with extremely high faradaic efficiency over 99.6%at a low overpotential of 0.34 V versus RHE.When this cathodic CO2RR was combined with an anodic half-reaction for producing hypochlorite,value-added products(CO and hypochlorite)with selectivity ? 99%could be respectively obtained on cathode and anode sides with the total reaction of CO2+NaCl? CO+NaClO.Impressively,high energy efficiency(81.5%)was achieved for this novel electrochemical reaction at a low cell voltage of 2.0 V with excellent stability over continuous electrolysis.This study paves a way to realize electrochemical reaction with 100%electron economy by coupling CO2RR with other reactions.