Mechanisms of the Key Factors Influencing the Degradation of PPy in Electrolyte Solutions and Their Kinetic Models

Author:Jin Zhen Zhen

Supervisor:qiu yu bing

Database:Doctor

Degree Year:2019

Download:18

Pages:186

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Among numerous conducting polymers,polypyrrole(PPy)has become a most promising and attractive conducting polymer due to its ease in preparation,high conductivity,excellent electrochemical and optical properties as well as good biocompatibility,etc.Its wide applications in electrode materials,sensors,electrochemical actuators,catalysts,and protective coatings,etc.,have attracted a lot of attentions in recent years.However,the main drawback of PPy for commercial applications is its poor stability,resulting in the loss of electrochemical activity and reduced functionality.The previous researches on the mechanisms of degradation and aging of PPy have been mainly from the point of view of chemical reactions,according to the changes of PPy in physical and chemical properties,structures and chemical compositions.However,PPy can reach a conductive level as metal materials,which also possesses electrochemical redox activity and electrocatalytic property to electron transfer processes.Therefore,our research group has firstly introduced the electrochemical corrosion mechanism into the degradation process of PPy in which the chemical and electrochemical processes have function of mutual promotion.Based on the chemical and electrochemical degradation mechanisms of PPy,the effects of key factors on the degradation of PPy were investigated,in this dissertation.On this basis,the degradation kinetic model of PPy was established.,which can be used to evaluate its degradation degree and predict its failure time.These results will shed substantial light on new directions and theoretical guidance for the study of PPy degradation mechanism,failure prediction,and methods of improving stability.The main conclusions are as follows:I.Effect of doping anions on the degradation of PPy and its mechanismThe degradation behavior of PPy/A(A:SO42-,pTS-,PSS-,and GO)films in NaOH were investigated by various electrochemical methods and spectroscopic analyses,and the mechanisms involved were discussed.The degradation rates of PPy/A films follow the first-order kinetics.The PPy/SO4 and PPy/GO films have two degradation stages,while the PPy/pTS and PPy/PSS films just show one degradation stage.The PPy/pTS film shows the best stability of electrochemical activity.The doping anions change the degradation rate constants(k)by changing the structure,composition,surface state(e.g.,hydrophilicity)and ion exchange ability of PPy.The degradation rate of PPy film in the electrolyte solution mainly depends on both the exchange ability of the doping anions with OH-and the ability of PPy films to prevent the penetration of OH-,rather than the size of the doping anions.2.Effect of temperature on the degradation of PPy and its mechanismThe degradation behavior of PPy/pTS film in NaOH at various temperatures(25~90 ℃)were investigated by various electrochemical methods and spectroscopic analyses,and the mechanisms involved were discussed.The degradation of electroactivity of PPy/pTS film follows the first-order kinetics in aerated and de-aerated 0.5 M NaOH at 25~80 0C.However,at 90 ℃,the electroactivity of PPy/pTS film remains very good,and its degradation does not conform to the first-order kinetics.Temperature affects the concentration of DO,the open circuit potential(EOCP)of PPy,and the mass transfer process at the PPy/solution interface,resulting in the effects on the degradation kinetics of PPy.In aerated 0.5 M NaOH,an increase in temperature induces a decrease in the concentration of DO,which have opposite effects on the degradation of PPy,resulting in a maximum inkae at 70 ℃.In de-aerated 0.5 M NaOH,the high temperature induces the significantly negative shift of EOCP of PPy to~-1.0 V vs.SCE,and the cathodic process on PPy surface becomes to the hydrogen evolution of water.Thus,the closed anion channels in the PPy film inhibit the degradation process from expanding into the inner film,which has the opposite effect on the degradation of PPy versus the effect of increasing temperature,resulting in a maximum in kde at 50 0C.3.Degradation behavior of free-standing PPy film and its mechanismFree-standing PPy/pTS films with different thicknesses(1.8 and 4.2μm)were prepared and their degradation behavior in NaOH was investigated by various electrochemical methods and spectroscopic analyses,and the mechanisms involved were discussed.The degradation of electroactivity of free-standing PPy/pTS film shows one degradation stage and its rate follows the first-order kinetics.The thick free-standing PPy/pTS film(4.2μm)possesses a more ordered and compact structure,which largely impedes the penetration of OH-into PPy film and the de-doping of pTS-,inhibiting the chemical and electrochemical degradation processes.The degradation of free-standing PPy/pTS film in de-aerated 0.5 M NaOH is mainly chemical process.Its degradation in aerated 0.5 M NaOH is mainly electrochemical process,and the chemical attack from OH-can largely promote the electrochemical degradation process.The degradation of free-standing PPy/pTS film can develop from the two surfaces of the PPy film to facilitate the penetration of OH-and the de-doping of pTS-,increasing the degradation processes significantly.Even though the Pt substrate does not change the first-order degradation kinetics of PPy,it can significantly increase its electrochemical activity and reduces its degradation rate.4.Effect of substrates on the degradation of PPy and its mechanismThe PPy/pTS films onto insulated(Glass)and different types of conductive(Pt and Stainless Steel(SS))substrates were prepared and their degradation behavior in electrolyte solutions were investigated by various electrochemical methods,and the mechanisms involved were discussed.The substrates significantly affect the degradation rate constants of PPy.The substrate can form an occlusion zone with PPy like that of metal corrosion,which makes the media conditions in the inner and outer surfaces of PPy film different,increasing the driving force of the galvanic cells formed in the PPy film to promote the electrochemical degradation process.At the same time,the conductive substrate can provide short-circuited connections for the galvanic cells formed in the PPy film,which will promote the electrochemical degradation process.In aerated and de-aerated 0.5 M NaOH,the occlusion effect dominates the degradation process of PPy.In aerated 0.5 M Na2SO4,the promotion of the conductive substrate on electrochemical degradation process of PPy dominates the degradation process.Compared with free-standing PPy/pTS film,both the insulated(Glass)and the conductive(SS)substrates significantly promote the degradation process of PPy,so that Glass//PPy/pTS and SS//PPy/pTS show larger values of kae and kde.This is mainly due to the occlusion effect of PPy and the substrate,and the promotion of the conductive substrate on electrochemical degradation process of PPy.5.Establishment of degradation kinetic model of PPy.The reaction rates equations of chemical and electrochemical degradation of PPy(vc and ve)in NaOH solution have been established.The relationships between the degradation rates of electroactivity of PPy in aerated and de-aerated solutions(udeg,de and vdeg,ae)and vc and ve are theoretically deduced,respectively.And the corresponding relationships between the degradation rate constants of electroactivity(kde and kde)and degradation rate constants of chemical and electrochemical degradation reactions(kc and ke)are obtained,i.e.,kde=kcCOH-βand(kae-kde)=keCOH-q.Therefore,the values of reaction orders(β and q)and rate constants(kc and k})can be obtained experimentally to establish the degradation kinetic model of PPy.The established model is validated by experimental data.The degradation degree can be evaluated and the failure time of PPy can be predicted according to the model.