| With the exhaustion of fossil energy and environmental problems increasingly prominent,it has become very urgent to seek and develop a clean,cheap,and efficient renewable energy source.In the nature,there are a large number of easily available small molecules,such as H2,O2,N2,CO2,H2O,etc.The utilization of targeted transformation of small molecules to synthesize value-added chemicals,fuels and fertilizers have attracted widespread attention.This research has important practical significance for the sustainable development of energy and green chemistry,but it is still extremely challenging.Electrocatalytic conversion technology can realize efficient conversion between electrical energy and chemical energy in much renewable energy storage and utilization equipment,thereby providing a huge opportunity for the utilization and conversion of small molecules under mild conditions.However,the chemical properties of these small molecules are relatively stable.How to design and prepare cheap and efficient catalysts is of vital importance for scientific research and industrialization in the field of energy and catalysis.Single-atom catalysts have been widely used in the forefront of electrochemical energy conversion and storage due to their 100%atomic utilization and unique electronic properties.It not only can provide an ideal model and research platform for understanding the mechanism of catalytic reactions at the atomic scale,but also can be expected to become a novel industrial catalyst with great application potential.Among many small molecules,H2 and NH3 have important application value in industrial production.H2,as one of the important renewable clean energy sources,has been favored by governments and research institutions all over the world for its advantages of high energy density,zero pollution of combustion products,and abundant resources.It has been regarded as a potential candidate for a low-carbon,clean and sustainable energy source in the future.NH3,as the second most important chemical substance in industrial production,has been widely used in pharmaceuticals,synthetic fiber,fertilizer production,and energy conversion systems.Compared with other energy carriers,NH3 is considered an ideal storage medium for hydrogen because it contains 17.6 wt.%hydrogen and exists in liquid form.What's more,ammonia is an important carbon-free energy carrier that does not emit any carbon dioxide when it is decomposed.The rational utilization and high-efficiency directional conversion of hydrogen and ammonia can open up a feasible new way to solve the increasingly serious energy crisis and environmental problems today.In view of these,this thesis has carried out research on the electrocatalytic synthesis of H2 and NH3 based on single-atom catalysts.The general synthesis strategy of single-atom catalysts was discussed in detail.The electrocatalytic activation and directional transformation of small molecules on the surface of single-atom catalysts were systematically studied.The structure-activity relationship between the structure of single-atom catalysts and their electrocatalytic performances was discussed in depth.To achieve efficient guidance for the rational design of single-atom catalysts for electrocatalytic conversion of small molecules,the reaction mechanism and regulation rules were explored and revealed.The specific research contents are as follows:(1)The development of cost-effective and high-activity catalysts for acidic HER is highly desirable,but still faces huge challenges.Herein,single-atomic Ru sites anchored onto Ti3C2Ox MXene nanosheets(Ru-SA/Ti3C2Ox)are first served as trifunctional electrocatalysts for simultaneously catalyzing acidic hydrogen evolution reaction(HER)?oxygen evolution reaction(OER)and oxygen reduction reaction(ORR).Based on the Ru-SA/Ti3C2Ox catalyst,a half-wave potential of 0.80 V for ORR and small overpotentials of 70 m V and 290 m V for HER and OER,respectively,at 10m A cm-2 are achieved.Acidic OWS based on Ru-SA/Ti3C2Ox catalyst only needs 1.56V battery potential to reach a current density of 10 m A cm-2.The maximum power density of an H2-O2 fuel cell using the as-prepared catalyst can reach as high as 941m W cm-2.Theoretical calculations revealed that isolated Ru-O2 sites can effectively optimize the adsorption of reactants/intermediates and lower the energy barriers for the potential-determining steps,thereby accelerating the HER,ORR,and OER kinetics.(2)Based on the highly dispersed Rh single-atom on the oxygen-functionalized Ti3C2Ox MXene nanosheets(Rh-SA/Ti3C2Ox),we designed and prepared a novel self-driven dual hydrogen production system for high-efficient hydrogen production.The bifunctional Rh-SA/Ti3C2Ox catalyst exhibits excellent catalytic activity for both p H-universal hydrogen evolution reaction and hydrazine oxidation reaction.Based on the Rh-SA/Ti3C2Ox catalyst,a self-driven dual hydrogen production system was assembled by combining a Zn-H2 battery and an overall hydrazine splitting unit.The hydrogen production rate achieved by this system is as high as 45.77 mmol h-1.Density functional theory studies have shown that atomically dispersed Rh single atoms can not only make the free energy of hydrogen adsorbed in the HER reaction have higher thermal neutrality,but also greatly reduce the free energy barrier for dehydrogenation of the*NHOH intermediate.(3)The electro-synthesis of NH3 under ambient conditions through electrocatalytic technology N2 is an important subject to replace the traditional Haber-Bosch process.However,the guideless search for electrocatalysts cannot efficiently promote NH3 yield rates in the N2 reduction reaction.Herein,our first-principles calculations reveal that the successive emergence of vertical end-on *N2 and oblique end-on *NNH admolecules on single metal sites is key to high-performance N2 reduction reaction.By targeting the admolecules,single Ag sites with the Ag-N4 coordination are found and synthesized massively.Under ambient conditions,the catalyst exhibit a record-high NH3 yield rate(270.9?g h-1 mg-1cat.or 69.4 mg h-1 mg Ag-1)and a desirable Faradaic efficiency(21.9%)in HCl aqueous solution.The generation rate of NH3 is stable during20 consecutive reaction cycles,and the reduction current density is almost constant for60 h.(4)To realize the strategy of electrochemical NO reduction to synthesize NH3,we synthesize a family of electrocatalysts by assembling single metal atoms(Al,Mn,Fe,Cu and Nb)on B and N co-doped carbon nanotubes based on the 100%atomic utilization and unique electronic characteristics of single-atom catalysts.Electrochemical studies have found that the catalytic family exhibits high NO reduction reaction activities for the ambient electrosynthesis of NH3.Particularly,the single-atomic-site Nb catalyst delivers an NH3 yield rate of 8.2×10-8 mol cm-2 s-1,which is two orders of magnitude larger than those of the best-reported N2 reduction reaction catalysts and approaches the target of industrial electrosynthesis proposed by the US Department of Energy.Theoretical calculations show that the high-efficiency performance for the synthesis of NH3 via NO reduction reaction comes from the single NbB2N2 site in the Nb-SA/BNC catalyst.It not only promotes the adsorption of NO molecules,but also effectively reduces the energy barrier of the final step. |
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