Theoretical Study Of Surface Micro-Nano Bubbles In Electrochemistry:Nucleaytion,Stabilization And Detachmennt

The chemical industry in the new century primarily revolves around the development and utilization of new energy sources.Among these,electrolysis and electrocatalysis,as core steps in sustainable energy conversion systems,have garnered significant attention.Gases are essential products in many electrocatalytic and electrolytic processes,and the formation of bubbles is often accompanied by electrochemical reactions.However,the presence of micro-nano bubbles on the electrode surface can cover active reaction sites and increase ohmic resistance,thereby reducing electrochemical reaction efficiency.The study of bubble behavior on the electrode surface is of great significance in improving the reaction efficiency.To assist in experimental design and optimize production processes and techniques,this thesis conducts research based on the microkinetics and thermodynamics of electrochemical reaction engineering.Through theoretical and simulation methods,it explores the nucleation,stabilization,and detachment behaviors of micro-nano bubbles on electrode surfaces.It mainly contains the following contents:（1）The nucleation of bubbles at the crevice is studied using the classical nucleation theory（CNT）in the first section,followd by a generalized crevice model proposed in the second one.The classical nucleation theory is first used to calculate the nucleation barrier of the formed bubble at the complex crevice composed of Si O₂ particles and electrodes in typical nanoelectrode experiments,and the theoretical results are in good agreement with the experimental results.In order to simplify the calculation,a chemical equilibrium is added to the classical crevice model,and the generalized crevice model that can be applied to the crevice nucleation in electrochemistry proposed.This model agrces with CNT calculations but is handier and more straightforward.When using CNT to analyze the nucleation at complex crevice,the influence of material properties is explored:the optimal nucleation point of the bubble and the nucleation path are strongly related to the size of the particle.The effect of gas supersaturation is investigated when the generalized crevice model is used to analyze the nucleation.The author found that at certain gas supersaturation levels,bubbles can undergo multi-step nucleation from inside to the outside of the crevice,facilitating bubble growth outwards the crevice.（2）For the critical nuclei formed on the electrode surface,the author proposes a new method based on the nucleation theorem（NT）for the analysis of bubble nucleation in electrochemistry by combining theory and finite element simulation.This method avoids the inherent shortcomings of the CNT-based method,characterized by higher accuracy and wider application range.In detail,it has following improvements:Firstly,the accurate determination of gas oversaturation by finite element simulation during bubble nucleation;Secondly,considering the influence of gas oversaturation on interfacial tension and the compressibility of real gas;Additionally,avoiding the error caused by the transformation between nucleation time and nucleation rate;Lastly,more detailed information of each critical nucleus obtained from curve fitting with better fitting degree to the experimental data.The NT-based method is used to reanalyze the nucleation data of surface nanobubbles reported in the literature,and the results were roughly the same as those obtained from the CNT-based method,which mutually confirms the rationality of the respective models.In addition,it is confirmed that the contact angle of the critical nucleus of the bubble is much larger than Young’s contact angle under high gas oversaturation.（3）For stable nanobubbles on electrode surfaces,the author developed a dynamic equilibrium model for electrochemical surface nanobubbles by combining theory and molecular dynamics simulations and derived a criterion for bubble stability.The dynamic equilibrium model correlates the electrochemical response obtained by experiments with the properties of stable surface nanobubbles and provides a quantitative method for the analysis of electrode surface stable nanobubbles.Through theoretical and simulated analysis of the experimental data,the author found that the electrode size,the type of solvent,and the concentration of solute would lead to different bubble dynamics behaviors:the pinning of bubbles’contact line in aqueous solution and glycol;The instability of bubbles on large electrodes in aqueous solution;The motion of bubbles’contact line in methanol;The oscillation between different pinning states in dimethyl sulfoxide;The existence of a critical solute concentration for bubble stability.And these complex bubble behaviors can be explained by the competition between gas inflow and outflow at the bubble interface.（4）For the detachment of bubbles on the electrode surface,the author theoretically analyzes the structure of the microbubble cloud and microbubble carpet.A theoretical model of the formation of microbubble cloud caused by the deformation of the gas film,and a theoretical model of coalescence-induced detachment of bubbles in the microbubble carpet are established,thereby exploring various influencing factors.The theoretical results reveal that the current variation reported in the experiment is caused by the detachment of the bubbles in the microbubble carpet,and the theoretical prediction is roughly consistent with the experimental data.According to the theoretical model of microbubble cloud generation,the frequent deformation and jumping of the gas film on the electrode surface will cause a microbubble cloud,while the bubbles that do not jump may form a microbubble carpet.Appropriate electrode sizes and the gas films’contact angles are easier to the formation of microbubble clouds.The hydrophilic electrode is more favorable for the formation of microbubble clouds.According to the theoretical model of bubble detachment in the microbubble carpet,when the bubble grows to a suitable height,i.e.,when the microbubble carpet grows to a certain thickness,it can detach from the electrode through coalescence.The hydrophilic electrode is more conducive to bubble escape,while the electrode size has little effect on the bubble escape behavior in the microbubble carpet.