Research On The Rational Design And Catalytic Mechanism Of High Performance CO₂ Reduction Electrocatalyst

Electrochemical conversion of carbon dioxide(CO₂)to chemicals or fuels has been considered to be an important part of carbon neutrality,as well as energy conversion and chemical energy storage.With this strategy,it can not only effectively promote carbon capture and utilization to reduce the greenhouse gas emission,but also alleviate people's dependence on traditional fossil fuels.In CO₂ reduction technology,the intrinsic properties of the materials will directly determine their catalytic performance in electrocatalytic process.Therefore,the rapid design and preparation of high-performance catalytic materials have become the crucial issue of developing the commercial application of electrocatalytic carbon dioxide reduction reaction(CO₂RR).In this thesis,we focused on the studies of atomically dispersed monometallic or bimetallic catalysts,starting from the metal-porphyrin based single-atom catalysts(TM-SACs).Firstly,we utilized the DFT calculation method to simulate the electrocatalytic CO₂RR process on TM-SACs,and then attempted to correlate the geometric configuration,electronic properties of the catalysts and their CO₂RR catalytic activity.Subsequently,two types of high-performance CO₂RR electrocatalysts,namely bimetallic-atom catalysts(BACs)and single-metal-atom catalyst anchored in covalent triazine frameworks(TM-CTFs),have been proposed to solve the bottleneck problems of relatively high overpotential caused by weak CO₂ chemisorption ability in TM-SACs.The synergistic effect of bimetals in BACs and the steric effect of low-coordinated transition metals in TM-CTFs were used to ameliorate the electronic characteristics of the catalytic unit,and further enchance the reactivity and selectivity of electrocatalytic CO₂RR.Finally,we investigated the CO₂RR catalytic performance of various Ni-SACs with different coordination configurations to provide the efficiency design principle for the development of highly-performance CO₂RR catalysts that are expected to be commercialized.The specific research content and related innovations are as follows:1.Study on the metal-porphyrin based single-atom catalysts(TM-SACs)for electrocatalysis of CO₂RR.We have simulated the processes of catalytic CO₂ reduction on TM-SACs by using density functional theory(DFT)method.Through high-throughput calculations and analysis,it exhibited that most of TM-SACs shown high selectivity to CO products,while other reduction cases(e.g.,HCOOH,CH₄)were inhibited since relatively large overpotential.Moreover,the metal with small atomic radius and large electronegativity was favored to participate in CO₂ chemisorption processes in the form of reduced TM since the constraints of electronic structure and steric effects.Here,we proposed an intrinsic descriptor((37)=V_TM×E_TM/r _TM)to establish the“volcano-shape”relationship with the CO₂RR catalytic ability,from which the reaction activity,product selectivity and catalytic mechanism could be evaluated intuitively.The above predictions have been verified by some reported experimentals,which comfirms the reliability and rationality of the calculations.However,there is often a large energy barrier in the elementary reaction of CO₂chemisorption,which increases their reactive overpotential of TM-SACs.In order to improve the performance of electrocatalytic CO₂RR,we have carried out more in-depth studies on this basis.2.Study on atomically dispersed bimetallic catalysts(BACs)for electrocatalysis of CO₂RR.In this work,we attepted to ameliorate the electronic properties of the catalytic unit through the synergistic effect between the bimetallic atoms,thereby enhancing the adsorption strength of CO₂ and its intermediates to reduce the overpotential.Hence,a class of catalysts with bimetallic active units(TM₁/TM₂-BACs)anchored in covalent-organic framework materials have been designed and used as electrocatalyst to explore their CO₂RR catalytic activity.According to the DFT calculations,we found that the bimetallic active unit could break the scaling relations between the adsorption energies of different intermediates in the electrocatalytic CO₂RR process,which resulting higher catalytic activity of TM₁/TM₂-BACs than traditional noble metals and TM-SACs.In addition,we proposed an intrinsic descriptor((37)=(X_TM1+X_TM2)×(W_TM1+W_TM2)/(E_TM1+E_TM2)),and established a“volcano-shaped”relationship between this descriptor and catalytic performance,from which the reaction activity of different BACs could be predicted reasonably and accurately.3.Study on the single-atom catalysts anchored in the framework of covalent trizines for electrocatalysis of CO₂RR.In this work,we attempted to flexibly regulate the adsorption model of CO₂ and its intermediates through spatial effects,thereby improving their activity of electrocatalytic CO₂RR.Here,we designed a class of transition metal-based single-atom catalysts(TM-CTFs)with a three-coordinated configuration,and simulated the electrocatalytic CO₂RR process on their surfaces using the DFT method.Through systematic and comprehensive calculations and analysis,we found that TM-CTFs with a three-coordinated configuration unit can achieve catalytic performance comparable to bimetallic catalysts(TM₁/TM₂-BACs),and higher than that of TM-SACs with four-coordinated configuration.According to the inner relationship between the geometric configuration of TM-CTFs and their catalytic ability,we proposed an intrinsic descriptor((37)=N/r_TM×n),and established the“volcano-shaped”relationship with the CO₂RR catalytic activity.Through that,the best catalyst with high catalytic activity could be directly predicted,as well as its catalytic selectivity and reaction mechanism.The performance of the above prediction has been verified by some published experimental results,which exhibited that the prediction results are credible and useful.4.Study on the electrocatalytic CO₂ reduction on various of Ni-SACs with different coordination configurations.In this work,we attempted to improve the performance of electrocatalytic CO₂ reduction by regulating the coordination environment.Here,we firstly thoroughly explored the reactivity and selectivity of electrocatalytic CO₂RR on nine different Ni-SACs by using DFT calculations.It exhibited that the CO₂RR catalytic activity of Ni-SACs have been improved as the coordination number of Ni active site descrease,and the catalytic activity of the Ni-C_x unit is higher than that of Ni-N_x.Using these results as design guidelines,we applied the“bottom-up”synthesis strategy to successfully prepare three different Ni-SACs by controlling different annealing temperatures,namely Ni@NCH-800(Ni-N₄),Ni@NCH-900(Ni-N₂V₂-1),Ni@NCH-1000(Ni-C₄).The measure results of electrocatalytic CO₂RR demonstrated the Ni@NCH-1000 catalyst exhibited the best catalytic activity to reduce CO₂ to CO products,while inhibiting the HER side-reaction.At the applied voltage of-0.90 V,the Faraday efficiency(FE^CO)and current density(j _CO)were 94%and-27.0 m A/cm².The activity sequence following:Ni@NCH-1000>Ni@NCH-900>Ni@NCH-800,which is consistent with the predicted results.