Electronic State Modulation Of Catalysts Towards Carbon Dioxide Electroreduction

At present,the structure of energy resource in the development of human society is dominated by fossil fuels including coal,oil,and natural gas.As the increase of the productive forces,our dependence on fossil fuels increases simultaneously.Nevertheless,the massive consumption of fossil fuels has accelerated the depletion of resources and brought in a series of environmental issues.Hence,it is of pivotal significance to adjust the current energy structures.Although the renewable energy exhibited the characteristics of environmental friendliness,it is difficult to realize continuous and high-density centralized utilization due to its regional and discontinuous nature.As such,the intermittent renewable energy can be converted into electric energy,heat energy,etc.,which could be further applied to the chemical-bond energy storage for the convenient transportation and recycle.In the field of chemical energy storage,the utilization of carbon dioxide(CO2)can not only achieve high-density chemicals,but also alleviate environmental issues.However,due to the high stability of non-polar C=O double bonds in CO2,the efficient activation and bond breaking is the crucial process for CO2 utilization.Therefore,the exploiting of the catalysts with efficient CO2 activation is the hotspot towards CO2 reduction.Herein,we developed three kinds of highly efficient electrocatalysts to promote CO2 activation towards CO2 electroreduction.The effect of surface electronic states on the activation of CO2 and the catalytic performance of CO2 electroreduction was investigated via engineering the structure of catalysts.In addition,we also gained in-depth understanding on the catalytic reaction mechanism from both experimental and theoretical perspectives.The specific contents were listed as follow:1.We investigated the effect of the electronic structure of ZnO nanosheets on the activation of CO2 by constructing the oxygen vacancy on the surface of ZnO nanosheets.The introduction of oxygen vacancy on the surface of ZnO nanosheets induced the enrichment of local electrons,promoting electron transfer from ZnO to the anti-bonding orbital of CO2 and thus enhancing the activation of CO2.The experimental results revealed that ZnO nanosheets with rich oxygen vacancy exhibited Faraday efficiency for CO of 83%with partial current density for CO of 16.1 mA cm-2 towards CO2 electroreduction.The reaction kinetics analysis and density functional theory(DFT)calculation further suggested that the introduction of oxygen vacancy accelerated the reaction kinetics and reduced the reaction barrier,thus improving the catalytic activity for CO2 electroreduction into CO.2.The coordination activation process of CO2 in molecular catalysts was explored via exploring the effects of conjugated salophen and non-conjugated salen ligands on the active Co sites.Due to the large conjugated structure of salophen,the electron on Co sites was averaged in Co-salophen in comparison with that in Co-salen.Both quasi-situ experiments and DFT calculations unraveled that Co sites with high-valence state were more conducive to the formation of side-on coordination mode of CO2,thus enhancing CO2 activation.3.We investigated the activation of CO2 on Co-salophen by the substitution of spherical H with halogen atoms(Cl,Br,and I).The halogen-substituted Co-salophen exhibited basically the same valence state for Co species,whereas the electron occupation on Co site was redistributed due to the introduction of halogen atoms.Br substituted Co-salophen promoted the transition of the spin state of Co from low spin to high spin.Both experimental and theoretical results uncovered that the electrons occupied at 3dz2 and 3dx2-y2 at the Co sites with high spin state promoted the activation of CO2,improving the selectivity for CO towards CO2 electroreduction with the faraday efficiency for CO up to 98.5%.