Rational Design And Structure-Property Relationship Of Highly-efficient CO₂/N₂ Electroreduction Catalysts

The increasing consumption of fossil fuels is depleting finite natural resources and leading to the overproduction of greenhouse gas carbon dioxide(CO2).With the intensification of global warming,it has become urgent to develop more effective techniques to reduce CO2 in the atmosphere.Among several feasible CO2 utilization strategies(photochemistry,biochemistry,thermochemistry and electrochemistry),electrochemical conversion or reduction of CO2(CO2RR)into valuable hydrocarbon fuels under ambient condition is particularly attractive since it is not limited by the Carnot cycle to realize the recycling of carbon element.In addition,nitrogen cycle is also a vital part of the earth’s ecosystem as carbon cycle.Electrocatalytic reduction of nitrogen to ammonia(N2RR)can greatly reduce energy consumption and carbon emissions of ammonia synthesis process.However,the bottlenecks restricting the large-scale practical application of CO2/N2 electroreduction technology lie in the low reaction current density,poor selectivity of reduction products,large overpotential,and more kinetically preferred H2 evolution reaction(HER).Therefore,how to improve the reduction rate and yield of target products through rational design and controllable synthesis of more efficient catalysts,combined with understanding and regulation of catalytic mechanism and structure-property relationship,is still the key scientific problem to be urgently tackled.Given the above research status,the design and regulation principle of CO2/N2 electroreduction catalysts in this paper is mainly divided into three parts.The first part focuses on regulation of crystal face and atomic layer thickness of two-dimensional(2D)metal materials,the second part focuses on controllable synthesis of metal single atomic and adjustment of coordination environment,and the third part focuses on the structural design and defect engineering of carbon-based metal-free materials.The detailed contents are as follows:Part I:2D metal have ordered structure,and their surface atoms are almost exposed safely,which can provide abundant reactive sites.Compared with bulk materials,atom utilization is greatly improved.(1)Controllable preparation of Bismuthene for CO2RR to formate:We show the first simple large-scale synthesis of free-standing single-atomic layer Bismuthene by wet-chemical method,and demonstrate its high thermostability(400℃)and CO2RR activity for formate formation(FEHCOO-～99%and onset overpotential down to 90 mV)due to the unique compressive strain of Bismuthene(111)facet.However,the exposed(011)facet of thicker Bi nanosheets has a strong binding ability to reaction intermediates(OCHO*),leading to poor performance.This work helps to understand the relationship between atomic layers thickness and crystal face of 2D materials with CO2RR performance.(2)CO2 reduction mechanism of gold nanoprisms:We report an unexpected observation that defect-graphene supported Au nanoprisms with(111)facet exposed can efficiently catalyze the electroreduction of CO2 to ethanol.Significantly,the catalyst also presents highly efficient ethanol formation(the partial current density jethanol reached 212 mA cm-2)in an acid polymer electrolyte membrane-based CO2 electrolyzer.The 13CO2 isotope labeling NMR experiments accurately and reliably confirmed the formation of 13CH313CH2OH.Experiments and DFT calculations suggest that the carbon defects on graphene support can accelerate the dimerization of CO intermediate on atomically flat Au(111)facet.This study helps to understand the mechanism and path of CO2RR to C2+products by materials other than copper.Part Ⅱ:To further improve atomic utilization,metal components are dispersed as single-atomic,which will inevitably exhibit different activity,selectivity and stability from 2D metal catalysts.(3)The structure-property relationship of ZnN4-based Single-Atom catalyst for CO2 reduction:We report a type of carbon-supported nitrogen-anchored Zn single atom catalyst(ZnN4/C)for highly efficient CO2RR to form CO with high Faradaic efficiency(95%)and a large turnover frequency(TOF)of 9969 h-1,and the activity rank is ZnN4/C>N-C>Zn-C.The correlation between the property of samples obtained at different temperatures and the contents of different N-based active sites on ZnN4/C confirm that the synergistic effect between Zn and N,and free energy barrier for rate-limiting step of the formation*COOH was greatly decreased in the whole CO2RR on such active site.(4)Activity Modulation of Fe single-atomic catalyst for N2RR:We develop a new low-temperature pyrolysis strategy to synthesize an oxygen defect-anchored Fe single-atom on two-dimensional porous carbon(FeSA-O-C),and demonstrate its high electrocatalytic efficiency for ammonia formation from N2RR(FENH3～28.6%,yield rate～51.8 μgh-1 mgcat-1).For the first time,we designed a type of acid membrane N2 hydrogenation electrolyzer with a steady ammonia yield rate of 375.8 μg h-1 mgcat-1.Operando XAFS and DFT calculations reveal that the real active site during N2RR process is not the original 5-coordination structure(FeSA-OH-O4-C4),but a stable 4-coordination structure(FeSA-OH-O3-C4)after the initial electrocatalysis process.It represents an important step forward to the commercialization of electrochemical N2 utilization.Part III:Compared with metal catalysts,the structure and morphology of carbon-based materials are easier to control.The activity can be modulated by defect engineering,does not depend on the intrinsic characteristics of elements.(5)Nature of oxygen-containing groups on carbon for high-efficiency CO2RR:We synthesize a metal-free single-layer graphene nanodisks material(GNDs)with rich oxygen defects by a simple hydrothermal reaction,and demonstrate its high activity of CO2 reduction to formate(FE-86%).While significantly,experiments and DFT calculations demonstrate that the observed high CO2RR activity originates not from the solo carboxyl or other oxygen-containing groups,but from the synergistic effect between carboxyl groups and adjacent other types of groups(namely,hydroxyl,epoxide,and carbonyl)on GNDs.In-situ ATR-SEIRAS show clearly that the carboxyl(-COOH)groups can exist stably on GNDs surface during long-term CO2RR process.Based on it,we further find that its electrocatalytic activity can be regenerated cyclically via electro-oxidation method to regenerate the surface carboxyl groups.Such work deepens our understanding to the role of oxygen-containing groups in catalysis and provides a new strategy for the design of high-performance metal-free carbon-based catalysts.In addition,we unexpectedly found that amine-functionalization afGNDs show high-efficiency photoluminescence performance,the quantum yield(QY)reaches 43.4%,and have been used for in-vitro bioimaging with HeLa cells.Further study found that the fluorescence intensity is positively correlated with the surface content of amino and amide groups,and the contribution is C-NH2>-CONH2.