Synthesis And Performance Investigation Of Metal-based Catalysts For Electroreduction Of Carbon Dioxide To Formic Acid

The continuously rising level of atmospheric carbon dioxide(CO₂),mainly caused by excessive emission from fossil fuel combustion,has become a severe threat to the environment and energy safety.Utilization and conversion of CO₂ is an effective approach to alleviate the problems.The electroreduction of CO₂ to value-added chemicals driven by sustainable energy sources is a promising way,which can not only relieve the adverse influence caused by CO₂,but also store the intermittent renewable energy into chemical bonds.Liquid products(e.g.formic acid and ethanol)have attracted much attention because of easier transportation and higher energy density,which can also be the feedstocks for fuel cells.Therefore,electroreduction of CO₂ to liquid products is significant to get rid of dependence on fossil fuels and meet the ever-increasing energy.So far,tremendous efforts have been dedicated to the formation of liquid products,while some of the catalysts are hindered by their scarcity,high cost,toxicity or limited activity.Therefore,developing of desirable efficient catalysts with high activity and selectivity is still of paramount importance.This thesis mainly focuses on the liquid products,especially HCOOH production from CO₂ electroreduction through various metal-based electrocatalysts to achieve the highly active,selective and efficient production of HCOOH.The main research contents are as follows:1.Copper(Cu)is the most unique catalyst for CO₂ reduction to form C₂₊productions.Almost all the attentions have been focused on the crystalline Cu metals,however,the amorphous catalysts exhibit peculiar advantages.Therefore,the amorphous copper nanoparticles(a-Cu NPs)are synthesized via a facile while very effective protocol.Unexpectedly,superior electrochemical performances,including high catalytic activity and selectivity of CO₂ reduction to liquid fuels are achieved,that is,a total Faradaic efficiency of liquid fuels could sum up to the maximum value of 59%at-1.4 V(vs.Ag/Ag Cl),with HCOOH and C₂H₆O account for 37 and 22%,respectively,as well as a desirable long-term stability even up to 12 h.The enhanced catalytic performances of a-Cu are attributed to its enhanced CO₂ adsorption ability,abundant activesites,larger active surface,and efficient charge transfer,which should be benefited from more defect sites arising from intrinsic irregular atom structures of amorphous materials.This work opens a new avenue for improved electroreduction of CO₂ based on amorphous metal catalysts.2.To improve the selectivity for HCOOH production,we have focused on the bismuth(Bi)-based catalysts.The ultrafine non-noble bismuth(Bi)nanoparticles anchored on rGO(Bi/rGO)has been synthesized through a facile reduction method.As expected,the Bi/rGO catalyst exhibits excellent electrochemical performance on CO₂ reduction to form HCOOH,with very high Faradaic efficiency(up to 98%),favorable stability(over 12 h),and especially outstanding cathodic energy efficiency(up to 71%).Bi/rGO displays the ultrafine particle size and clean surface of Bi nanoparticles,and moreover,better conductivity of rGO,larger electrochemical active surface area and enhanced CO₂ adsorption ability of Bi/rGO all contribute to the enhanced catalytic activity of Bi/rGO.Further,the DFT calculations are carried out to illustrate that the HCOO^*mechanism is much preferable for the HCOOH production through CO₂ reduction.3.In order to accelerate the HCOOH production rate on Bi-based materials,the bismuth/cerium oxide(Bi/CeO_x)catalyst has been provided from in-situ electroreduciton of bismuth oxycarbonate/cerium oxide(Bi₂O₂CO₃/CeO_x),which shows outstanding current density(149 m A·cm^-2),and unprecedented production rate(2600?mol·h^-1·cm^-2)with high Faradaic efficiency(92%)for HCOOH production.Furthermore,Bi/CeO_x also displays excellent stability(34 h).The amorphous CeO_x could promote the formation of more Bi active sites with a much smaller particle size,and thus result in a higher electrochemical surface area.Besides,the Bi/CeO_x shows stronger adsorption and activation of CO₂,accelerated electron transfer,enhanced formation and stabilization of key intermediates,which contribute to its excellent activity for CO₂ electroreduction.