Efficient Piezocatalytic CO₂ Reduction Reaction: Design,Piezocatalytic Mechanism,Application And Outlook

Piezoelectric materials have been developed over a century.Their non-centrosymmetric crystal structure has built a bridge between mechanical and electrical energy.From the discovery of piezoelectricity in quartz crystals by the Curie brothers to the investigation of the hysteresis effect between force and electrical effect in potassium sodium tartrate.Piezoelectric materials have attracted a great deal of attention in a wide range of applications due to their unique piezoelectric properties.In catalysis,however,piezoelectric materials just begin.Compared to thermocatalysis,and the relatively new photocatalysis and electrocatalysis,piezocatalysis still has its unique advantages.In contrast to thermocatalysis,which requires a large amount of energy from the environment and causes considerable environmental pollution from industrial waste.piezocatalysis uses the mechanical energy of the environment and thus saves energy to a great extent.To compare with photocatalysis,which requires the use of sacrificial agents that increase costs and cause secondary pollution.Piezoelectric materials with polarisation properties to avoid the use of sacrificial agents.The mechanical energy used in piezocatalysis allows the reaction to take place in the absence of light.The balance between the cost of electricity used in electrocatalysis and the added value of the current product is a matter of concern.For the same product,the mechanical energy used in piezocatalysis has an energy cost advantage.However,piezocatalysis also has a number of problems that need to be solved.Firstly,from the piezocatalysis studies,researchers focus on the piezoelectric properties of piezoatalysts but negelect the selection of active sites.This led most piezocatalysis studies have focused on relatively easy reactions such as piezoelectric degradation and hydrogen precipitation.Secondly,the current mechanism of piezocatalysis is focused on the piezoelectric effect and energy band theory.However,they also have some limitations,which limit the understanding of piezocatalysis.Finally,current research in piezocatalysis is in the laboratory,where the catalysts are mostly in powder form and the reaction volume is small,which does not allow for further scale-up.As a result,no one has yet explored in the direction of practical applications,reaction landing,etc.To address the above issues,this paper summarises the relevant studies.Combined with rational experimental design and simulation calculation,it provides innovative ideas and powerful exploration on the rational design of high-efficiency piezocatalysts,the in-depth understanding of piezocatalytic mechanism and the application of piezocatalysis in multiple scenarios.The main contents of the thesis are as follows.（1）We propose to design and prepare piezocatalysts by selecting suitable piezoelectric carriers and introducing suitable active sites,providing a new idea for the design of piezocatalysts in order to improve the activity and selectivity of piezocatalysis.The Co-N-C piezocatalysts with atomically dispersed active sites were obtained by roasting using Co-ZIF as the precursor.The piezoelectric properties of Co-N-C were found for the first time through a series of piezoelectric performance tests.It was also applied to the CO₂ reduction reaction for the first time and good piezocatalytic activity was achieved.The CO yield was 34.59μmol g^-1h^-1 and H₂ yield was 3.3μmol g^-1h^-1under ultrasonic vibration at 50 k Hz,and a CO selectivity of 91%was achieved.It also achieves a good level of performance in comparison to related studies of CO₂RR in photocatalysis.This method provides a powerful exploration and reference for the development of new piezoelectric materials and the application of MOF materials in the field of piezocatalysis.（2）In order to further explore the mechanism of piezocatalysis and improve its piezocatalytic performance.By using the classical piezoelectric material barium titanate as the piezoelectric provider and Co-N-C with atomic level active sites as the active sites Co-N-C@BTO piezocatalysts with gram-scale yields were prepared for the first time using the ball milling method.Excellent catalytic activity was demonstrated in the piezocatalytic reduction of CO₂,with a CO yield of up to 261.8μmol g^-1h^-1 at an ultrasonic frequency of 50 k Hz and a CO selectivity of 93.8%,which is superior to most reported photocatalytic CO₂RR and piezoelectric catalytic CO₂RR.In terms of the piezocatalytic mechanism,displacement current theory and piezoelectric nano generator principle combined with conventional piezoelectric effect and energy band theory.The piezocatalytic mechanism has been verified by means of friction nanogenerators,energy band analysis and finite element simulation.When a piezocatalyst is subjected to mechanical forces,a displacement current is formed inside the catalyst,which triggers a time-varying electric field on the surface of the piezoelectric material and drives the charge generated by the piezoelectric effect to the active site to combine with the reactants to complete the catalytic reaction.This mechanism provides a more comprehensive insight into the process of piezocatalysis.It explains how piezocatalysts can be sustained to create stronger displacement currents,thereby enhancing the time-varying electric field,how to accelerate the transfer of electrons,and how to select the right active site for the reaction to occur.The proposed mechanism provides a reference from the source for both the design of piezocatalysts and piezocatalysis reactions.（3）In order to give a strong push to the further development of piezocatalysis for application in practical production.We have prepared Co-N-C@BTO piezoelectric thin films and Co-N-C@BTO piezoelectric aerogel.The highest ethane and hydrogen yields of Co-N-C@BTO piezoelectric films reached 1.161μmol g^-1h^-1 and 3.403μmol g^-1h^-1.Not only that,in this paper,ethane products were obtained using Co-N-C@BTO piezoelectric gels in a gas phase reduction reaction of CO₂ for the first time.Despite the low ethane yields,this is the first time that C₂ products have been prepared in the current field of piezoelectric catalysis.Given the weak mechanical energy in the natural environment,the application of both two-dimensional Co-N-C@BTO piezoelectric film structures and three-dimensional Co-N-C@BTO piezoelectric gels can absorb and transform the mechanical energy in the real environment to greater extent,and greatly exploit the potential of piezocatalysis for applications in multiple scenarios.It provides a strong reference for the development and application of piezocatalysis.