Fluidized Electrocatalysis And Electrosynthesis Based On Nanoparticle Impact Electrochemistry

In recent years,nanoparticle-impact electrochemistry（NIE）,based on the stochastic collisions of individual nanoparticles onto an inert ultramicroelectrode from Brownian motion,has been rapidly progressed.NIE has been utilized to characterize a variety of single nanoparticles and unveil the intrinsic informat ion at nano-confined domains masked in the average measurements in ensemble electrochemistry.Apart from serving as a powerful analytical tool,NIE exhibits characteristics intrinsically different from their ensemble counterpart.Firstly,for individual random walk nanoparticles in NIE,comparably well-defined radial diffusion regimes will be developed,greatly promoting the mass transport efficiency as compared to the drop-cast nanoparticles.Secondly,the random-walk nanoparticles collide the electrode surface and return to the solution,breaking the continuum of the reaction.Thirdly,in impact events,the nanoparticles are,in most cases,temporarily electrically contacted,resulting in a drastically reduced reaction time scale.Recently,our group reported the strategy of fluidized electrochemistry based on NIE.Instead of being fixed on the electrode surface,the catalytic nanoparticles are now fluidized in the electrolyte.The catalytic reaction can happen only when the nanoparticles impact with the electrode.The fluidized strategy has been proved to greatly eliminate catalyst fatigue problem.The development of fluidized electrochemistry based on NIE is still in its infancy,and no relevant studies have been conducted on the kinetics and fluidized electrosynthesis.In this thesis,the fluidized electrochemistry was first utilized for the investigation of the oxygen evolution reaction,where greatly improved reaction kinetics and catalytic efficiency can be found compared to the fixed reaction.Furthermore,the fluidized electrochemistry was further extended to highly efficient and controllable fluidized electrosynthesis of metal organic frameworks（MOFs）including nanoparticles and nanofibers.Finally,an in situ modification technique was proposed to tackle the nanparticle aggregation issues usually occurring in the traditional drop-cast approach.Main reserch work in this thesis includes:1.Fluidized nanoparticles catalyzed oxygen evolution reaction.In this work,OER is examined by a fluidized working mode using RuO₂ NPs and Fe-Ni bimetallic MOF NPs（Fe-Ni-MOF）as two model catalysts,which represent different particle-electrode interactions（contact/reaction time）.Both fluidized RuO₂ NPs and Fe-Ni-MOF NPs display much higher catalyst stability after long-term electrochemical test over their immobilized counterpart.Moreover,a much favored reaction kinetics is for the first time demonstrated for fluidized reactions along with the greatly elevated electroactivity.These benefits should be attributed to the efficient mass transport,drastically shortened reaction time scale and the spatially and temporally decoupled electron transfer step fr om other relatively slower steps（adsorption,desorption,and mass transport）due to the recycling mode of the fluidized reaction.This work increases the generality of the fluidized electrochemistry to the reaction time scale of hundreds of milliseconds,and expands its advantages in promoting reaction kinetics as well as the electrocatalytic activity.2.Highly efficient and controllable electrosynthesis of nanosized MOFs via a fluidized approach.In this study,copper nanoparticles are agitated by magnetic stirring in the electrolyte and are force to collided with a glass carbon plate electrode（1 V vs.SCE）to be oxidized into Cu²⁺,serving as metal ions in the synthesis.The hydrogen evolution reaction occurring at another working electrode connected to a dual potentiostat promotes the dissociation of the ligand.The solution in the electrochemical cell turns blue within a short re action period of 500 s.By using X-ray diffraction,scanning electron microscopy,Raman spectroscopy,X-ray photoelectron spectroscopy,nitrogen adsorption desorption,and thermogracimetric technology,the structure,morphology,functional groups,valence of the material elements,surface area,pore size and thermal stability properties of the prepared blue substance are investigated,suggesting the successful sysnthsis of the nanoscaled MOF particles using the fluidized strategy.Furthermore,the size of the MOF particles can be further tuned by simply adjusting the size of starting copper nanoparticles.Even more,MOF nanofiber materials can also be successfully prepared by the assistance of ultrasonication.This work provides a highly efficient and controllable synthetic route for the nanoscaled MOFs with different morphology.3.In situ modified electrode.Inspired by NIE,this work presents a new in situ modification technique instead of the routine drop-casting method by immersing a commonly used glassy carbon electrode in an alkaline solution including diluted RuO₂ NPs.Over a period of time individual RuO₂ NP entities randomly and continuously collide with the electrode surface following exactly the same behavior of NIE,forming a layer of low coverage of RuO₂ NPs due to the sticking of a small amount of NPs during collisions.The morphology measurement and the electrochemical testing at both ensemble and single nanoparticle levels reveal that the in situ modified electrodes exhibit negligible NP aggregation and significantly improved NP-average activity.The efficiently new electrode modification method results in spatial isolation of the catalytic particles on the electrode surface,creating a much higher mass transport,which not only significantly promotes the utilization of the catalysts but also provides accurate measurements of the activity usually ambiguated by nanoparticle aggregation occurring to the drop casting technique.