Research On The Electrode Interfacial Process Kinetics And Mass Transport Effects In Environmental Solution System

The features of environmental electrochemistry lie in two aspects:first,the involved reactions are often multistep and multielectron irreversible electrode reaction process,where the kinetic theory is the weak link in conventional electrochemistry;second,the environmental solution medium is usually a dilute solution,involving various of mass transport effects.Thus,aiming at the electrode interfacial process in environmental solution system,this paper established a series of current?potential(j?E)electrode reaction equations by progressively studying on the electron transfer process,reactant mass transport process,and H⁺/OH^-transport process.These equations can serve to develop the j?E deciphering methods.Further,we focus on transport effects of oxygen transport process and H⁺/OH^-transport process.The main contents include three parts:(1)Multistep and multielectron electrode reaction equationsThe electrode reaction equations are the kinetic fundamentals of deciphering j?E relationship and obtaining key parameter values.The lack of irreversible electrode reaction equations has impeded the studies and applications of the electrode reactions in environmental solution system.i)A general j?E equation form for the multistep and multielectron electrode reactions was developed,on basis of a simple multistep process with a sole rate limiting step.Application of the this equation to irreversible case obtains a new irreversible electrode reaction equation involving mass transport effect.This irreversible equation is a basic form to unify the conventional Butler?Volmer equation,Tafel equation and Koutecky–Levich equation.ii)Two kinetic analysis methods of steady-state polarization curve were proposed on basis of the irreversible electrode reaction equation.One is the nonlinear fitting method which fits the whole polarization curve region and obtains?,j₀ and K_Ox parameter values.The?and j₀ outputs are superior to those of Tafel fitting,meanwhile,the K_Ox vs.w^-1/2 linear relationship is a better substitute for Koutecky–Levich fitting.The K_Ox vs.w^-1/2 fitting can serve to the electrode structure associated mass transport effect analysis and discovers the positive mass transport effect of porous electrode structure.This method provides a reliable approach for analyzing the interfacial electron transfer and mass transport processes.The other is the derivative-extremum analysis method of polarization curve.It is found that the extremum point in the derivative curve of polarization curve was a special kinetic state point where the activation effects of electron transfer and mass transport reach a balance.The peak potential of this point is the half-wave potential,and the peak value is a new feature parameter designating activation rate.By virtue of this treatment,the electrochemical relationships of the two parameters are derived,and the physical meaning of irreversible half-wave potential is clarified.(2)Kinetic mechanism of ORR and oxygen transport effectOxygen reduction reaction(ORR)is one of the most popular multistep and multielectron irreversible electrode reaction in environment.Oxygen transport effect is an important factor restraining the ORR performance.i)A H₂O₂-mediated pathway model of ORR,where H₂O₂ is a key intermediate,was developed on basis of the irreversible electrode reaction equation.The current ratio of H₂O₂ formation and reduction is defined as the ORR tendency parameter.This model demonstrates that the H₂O₂ is an inevitable intermediate product and the H₂O₂production is regulable.These features are experimentally verified by:1)the Pt-based catalysts are able to produce and largely produce H₂O₂,conforming that H₂O₂ is an inevitable product during ORR process;2)the ORR tendency can be regulated through catalyst loading density and specific ion adsorption,which are reasonably accounted for by the H₂O₂-mediated pathway model,namely,the activity of H₂O₂reduction is more dependent on catalyst loading density or active site amount than that of H₂O₂ formation.Therefore,this model provides a regulatory strategy besides catalyst for the selective application of ORR.ii)For the air cathode,one of the most promising ORR cathode form,two experimental setups were designed to investigate the correlation between gas permeability and electrode performance,one measures gas permeability and the other evaluates the pressure-dependent performance.Meanwhile,an oxygen transport model of air cathode was established.The results show that the rate-limiting process of oxygen transport,which can be controlled by gas permeability,is a key factor to determine the air cathode performance.In low gas permeability,the gas transport is the rate-limiting process,and the performance is limited by oxygen transport;Once gas permeability surpasses a threshold value,the dissolved oxygen transport becomes the rate-limiting process,and the oxygen transport limitation is eliminated.Furthermore,a waterproof breathable membrane assembled air cathode was proposed.(3)The H⁺/OH^-transports of electrode interfacial process and the p H gradient effectThe H⁺/OH^-transport on electrode surface can cause a p H gradient and influence the reaction rate and process due to the weak acid and base feature of environmental solution medium.The extra overpotential caused by this transport process is called the p H overpotential.Differing from the oxygen transport effect,the H⁺/OH^-transport effect is a basic dilute solution effect existing in all the interfacial reactions that involve H⁺/OH^-participation.Centering on this dilute solution effect,this paper firstly established the transport equations of H⁺/OH^-ion transport process and buffer matter transport process,and then clarified the detailed p H gradient effect in unbuffered solution where only the H⁺/OH^-ion transport exists.i)A H⁺/OH^-transport equation describing the j?p H relationship in unbuffered solutions was derived.Then,A j?p H diagram was charted to describe the basic regularity of H⁺/OH^-transport process.Cathodic reaction and anodic reaction present symmetrical j?p H relationship.The j?p H curves in the p H range of 5–9 are overlapped,revealing that the polarization curves in this range should be the same.Combination of j–p H diagram and Pourbaix's E–p H diagram forms a panoramic graphic view of p H function for the extensive interfacial reactions.ii)A buffer transport equation was derived,describing the quantified relationship between transport factors and reaction rate.The maximum transport rate determined by buffer concentration,diffusion coefficient and diffusion condition characterizes the transport slowness of a buffer.The buffer dissociation constant(p K_a)and the solution p H co-determines the buffering state of a buffer in the solution.Combination of the H⁺/OH^-transport equation and buffer transport equation constitutes the buffer medium transport model.Through the relationships of buffer transport limiting current with solution p H and buffer concentration,two key transport parameters,p K_a and m _HA,can be experimentally estimated.The results were well in line with the derived model relationships.iii)The relationship between p H gradient and p H overpotential was quantitatively studied,and the electrode reaction equations involving p H overpotential was developed.The p H gradient effect was revealed to involve two parts:i)the Nernst p H overpotential accounting for the common Nernst relationship with p H,and ii)the p H-dependent function of electron transfer coefficient(?_{p H}).The?_{p H} function effect can be availably assessed as an apparent constant(?_app)instead,and a nonlinear fitting method is proposed for?_app estimation.In summary,this paper established a general j?E equation form of multistep and multielectron electrode reactions,enabling to break through the theoretical limitation of the electrochemical study in environmental solution system.On this basis,the ORR kinetic mechanism,oxygen transport effect and the p H gradient effect in dilute solution were studied.The outcomes of this paper eventually provide theoretical supports for the kinetic analysis of electrode interfacial process and the medium regulation in environmental solution system.