Investigation On The Application Of Main Group Metals In Electrochemical Catalytic Reduction Reactions Based On Theoretical Calculation

Against the backdrop of worsening energy shortages,environmental pollution,and climate change,electrochemical energy conversion and storage have received extensive attention by researchers due to their advantages of environmental friendliness and sustainability.Topics such as electrolysis of water for hydrogen production,proton exchange membrane fuel cells(PEMFCs),and the reduction of carbon dioxide into valuable industrial products using renewable electricity have received considerable research.In these technologies,the efficient implementation of electrochemical reduction reactions such as hydrogen evolution reaction(HER),oxygen reduction reaction(ORR),and the carbon dioxide reduction reaction(CO2RR)occurring at the cathode is the key to large-scale applications.However,the rapid progress of these electrochemical reduction reactions requires efficient electrocatalysts.Currently,rare noble metal catalysts such as Pt are usually used to catalyze these electrochemical reduction reactions.Therefore,developing efficient and low-cost electrocatalysts for these reactions is a core research topic for the years to come,such as the recently reported non noble transition metal-based catalysts in the literature.However,transition metal-based catalysts also face many problems.For example,commonly used transition metal single atom catalysts(SACs)such as Fe and Co will undergo Fenton reactions during electrocatalytic ORR,causing the difficulty of largescale applications in PEMFCs.Majority of transition metal-based catalysts have high catalytic activity for HER,which is not conducive to some competitive electrocatalytic reduction reactions such as electrocatalytic CO2RR.In order to solve these problems,the main group metal elements have entered our visions due to their rich earth reserves and more environmentally friendliness.However,compared to transition metal elements,the outer electron layers of the main group metal elements have relatively fewer valence electrons.And they have a wide distribution of s and p electrons when forming solid materials,which means that the electronic structure of the main group metal elements is more difficult to regulate as a catalytic active center.However,combined with the initial electronic structure characteristics of the main group metal elements,there are still relevant strategies that can change their electronic structure and make them become the catalytic active centers for electrocatalytic reduction reactions.In this paper,a strategy of embedding the main group metal units coordinated with N,O into graphene substrates was adopted to change the electronic structure of the main group metal atoms.Theoretical calculations were conducted to predict the activity of the group IA metals Li and Na single atom configurations in the s block for electrocatalytic reduction reactions.Corresponding carbon-based catalysts with N,O coordinated main group metal single atom units embedded in graphene were synthesized experimentally,and their catalytic activity for electrocatalytic reduction reactions was evaluated.The specific content is as follows:1.It is found that the vacant 2p orbital of Li can provide great convenience for the electronic structure regulation.Under the guidance of the above electronic structure regulation strategy,a series of N and O coordinated Li embedded graphene models have been designed.In this chapter,the theoretical onset potentials of the constructed models for ORR,HER,and CO2RR have been calculated through density functional theory(DFT)simulation.It is found that the zigzag-type Li-pyridinic-N1-C1 and Li-O2 exhibit remarkable catalytic activity for ORR and CO2RR respectively,suggesting that Li atoms after the electronic structure regulation can become the active centers of electrocatalytic ORR and CO2RR.Further electronic structure analysis shows that there is an s-p orbital hybridization between the s-orbital electrons of Li atom and the porbital electrons of coordinated N,O,and C,and there is a p-π conjugation effect between the 2p orbital of Li atom and the π bond of the graphene substrate.The DFT calculations demonstrate the effectiveness of the electronic structure regulation strategy of anchoring Li metal into graphene substrates through the coordination of N and O.Ab initio molecular dynamics(AIMD)simulation also confirmed the stability of Li in these configurations in aqueous solutions,providing a theoretical basis for the subsequent development of corresponding Li catalysts in experiment.2.Under the guidance of the theoretical calculations in the previous chapter,in this chapter,a carbon based nano catalyst with N,O coordinated Li embedded in graphene substrates was successfully prepared experimentally through a synthesis method similar to high-temperature solid-phase reactions using ZIF-8,a zeolite imidazolate backbone,as a precursor.The relative characterizations prove that Li in the catalyst is coordinated by N and O atoms.The catalyst exhibits excellent acidic ORR catalytic performance and high activity in catalytic reduction of CO2 to CO.The catalyst has a half wave potential of 0.77 V for ORR,a Faradaic efficiency of 98.8%for catalytic CO2 reduction to CO,and a partial current density of 6.93 mA cm-2 for CO production,comparable to some reported transition metal SACs.Meanwhile,the catalyst also showed good stability.Finally,the catalyst can also be used as a potential electrocatalyst for cathode reactions of actual PEMFC.With hydrogen and oxygen as reaction gases,the peak power density can reach 430 mW cm-2,lower than that of the reported benchmark Pt/C and Fe/Co SACs,but higher than that of some reported Mn and Ca SACs.The experimental results in this chapter and the theoretical calculations in the previous chapter show that the use of main group Li elements in electrocatalytic reduction reactions is feasible.3.Using the method of combining theoretical calculations and experiments described in the previous two chapters,this chapter studied the use of the group IA metal element Na in the s block for electrocatalytic CO2RR.Firstly,the initial electronic structure of Na atom is analyzed.The outer 3 s electrons of Na is considered beneficial for the conversion of CO2 to CO,and the p-π conjugation between the 2p electrons of Na and the π electrons of the graphene substrate can be used as an additional parameter for electronic structure regulation.Based on the above analysis,this chapter successfully regulates the electronic structure of the Na metal center by embedding Na single atoms with N,O coordination into the graphene frameworks,enabling it to have efficient electrocatalytic CO2RR activity.DFT calculations have demonstrated the existence of s-p orbital hybridization and p-π conjugation effect,while the embedding of Na-O2 groups at the edge positions can effectively draw electrons from the entire graphene system to the edge doped Na metal center,which is beneficial to the conversion of CO2 to CO.In addition,carbon-based nanomaterials embedded with N and O coordinated Na elements into graphene were prepared experimentally by hightemperature calcination using ZIF-8 as precursor.The catalyst exhibits high selectivity and activity in catalyzing CO2RR,with a Faradaic efficiency of up to 99.9%for CO generation,and a partial current density of 6.68 mA cm-2 for CO production,which is comparable to some reported transition metal SACs.