采用连续电位阶跃方法研究聚吡咯在电解质水溶液中的氧化还原稳定性

田 颖 王晶日 刘 明 史 锟 杨凤林

(1大连交通大学环境与化学工程学院,辽宁大连116028;2大连晟世环境工程有限公司,辽宁大连116600; 3大连理工大学环境科学与工程系,辽宁大连116024)

采用连续电位阶跃方法研究聚吡咯在电解质水溶液中的氧化还原稳定性

田 颖1,*王晶日2刘 明1史 锟1杨凤林3

(1大连交通大学环境与化学工程学院,辽宁大连116028;2大连晟世环境工程有限公司,辽宁大连116600;3大连理工大学环境科学与工程系,辽宁大连116024)

连续电位阶跃方法是一种研究氧化还原稳定性的有效方法.本文采用连续电位阶跃方法研究了以对甲苯磺酸钠为掺杂剂的聚吡咯(ppy)膜的电化学氧化还原稳定性,通过计算聚吡咯在阶跃电位下的还原电量(Qred),还原和氧化电量的比值(Qred/Qox)考察聚吡咯在H2SO4、Na2SO4和NaOH溶液中在不同电位下的氧化还原可逆性.结果发现聚吡咯过氧化的发生强烈依赖支持电解质的pH值和阶跃电位.在H2SO4溶液中,过氧化的起始电位为0.8 V,而在Na2SO4溶液中,过氧化的起始电位为0.5 V.在NaOH溶液中,过氧化在任何电位均可发生,表明溶液中OH-的存在是过氧化发生的直接原因.

聚吡咯;过氧化;氧化还原稳定性;连续电位阶跃

1 Introduction

Redox stability of polypyrrole(ppy)is one of the most important factors in practical applications,such as secondary batteries,capacitors,electromagnetic shutters,and electrochromic devices.1-3It also influences the effect of electrocatalyst,ion exchange,and drug release when ppy is applied in these fields.In order to keep the reversibility property,overoxidation caused by irreversible oxidation at higher potentials must be avoided. It is generally accepted that the nucleophiles,such as OH-,O2, and H2O,could lead to the formation of quinone moieties that disrupt the conjugated double-bond system of ppy.4-10

It is well known that ppy is relatively stable in air.11However,the electrochemical redox stability of ppy in aqueous solutions is completely different,strongly depends on the counterions,supporting electrolytes,and the applied potentials.Many researchers have reported the redox properties and irreversible oxidation of ppy films.12-18Rodriguez et al.19discovered that irreversible oxidation of ppy/NO3occurred at potentials more positive than 0.4 V vs saturated calomel electrode(SCE)in solutions of KNO3,KCl,KBr,and KF by in situ FTIR spectroscopy.Raman spectroscopy was found to be more sensitive tool to monitor the overoxidation reaction and it was found that ppy/ Cl degraded at potentials as low as 0.5 V(vs SCE).20Resistometry,UV-Vis,and Raman spectrometry have been employed by Lewis et al.21for the determination of the overoxidation potential of ppy/NO3.These techniques have indicated that the onset of overoxidation is at potential as low as 0.70-0.75 V(vs SCE).In these studies,in situ UV-Vis,FTIR,and Raman spectrometry are frequently employed techniques.However,the conclusions obtained from these techniques seem inconsistent. Sometimes,it is very difficult to explain the complicated spectra obtained under different conditions.In addition,these stability studies were limited to relatively short-term effects within a high potential regime.Few long-term redox stability studies of ppy within normorlly reversible potential window have been reported.22Pyo et al.23reported the long-term electrochemical switching behavior of ppy films by recurrent potential pulse (RPP)technique cyclied in various organic medias.Travas-Sejdic et al.24have characterized the long-term stability of ppy in aqueous NaPF6electrolyte by RPP experiment.

In order to provide the systematic studies of ppy films,we adopt the RPP technique to detect the long-term redox stability in different electrolyte solutions.The objective of our study is to determine stable potential scopes where ppy processes the property of reversibility in electrolyte solutions of H2SO4, Na2SO4,and NaOH.

2 Experimental

Polypyrrole films were prepared at room temperature in aqueous solution containing 0.2 mol·L-1sodium p-toluenesulfonate(Tianjin Bodi Chemical Engineering Co.Ltd.,China) and 0.14 mol·L-1pyrrole(Shanghai Lihua Chemical Reagent Co.Ltd.,China)under nitrogen atmosphere.The polymerization was performed under a constant potential of 0.8 V with a total charge of 5 C.A Φ 13 mm stainless steel sheet,a Φ 20 mm stainless steel sheet,and saturated calomel electrode (SCE)were employed as working,counter,and reference electrodes,respectively.The working electrode was polished with water-proof sand paper and rinsed in acid solution and distilled water prior to each experiment.

Recurrent potential pulse for redox stability measurement was carried out with potentials subsequently stepped between V1and V2specially stated in the text below and held for 50 s at each potential in H2SO4,Na2SO4,and NaOH solutions,respectively.A Potentiostat/Galvanostat Model 263A(Princeton Applied Research,USA)was used for electropolymerization and recurrent potential pulse measurements.

All potentials were measured and were reported against SCE.All reagents were reagent grade,and deionized water was used.Pyrrole was freshly distilled prior to use.All other chemicals were used as received.

3 Results and discussion

3.1 Redox stability in H2SO4solution

In order to investigate the stability in acid solutions,potentials were subsequently stepped between-0.1 and 0.4 V and held for 50 s for each potential,monitoring the charge passage in the reduction process(Qred),and the ratio of the charges consumed in reduction and oxidation processes(Qred/Qox).Fig.1a shows Qredand Qred/Qoxover cycle number.It can be seen that Qredvalues increase gradually due to the swelling of the polymer film,leading to the current increase over the long periods of time.25Since the reduction and oxidation processes are diffusion-controlled,26the concentration of cations in electrolyte solutions plays an important role in cations diffusion into/out the film during reduction/oxidation process.Therefore,the values of Qredare larger in 1.0 mol·L-1H2SO4due to the high concentration of cations than those in 0.1 mol·L-1H2SO4.

It can be observed that the ratios of Qred/Qoxare near to one, which indicates that the reduction and oxidation processes in each cycle are reversible.18In diluted acid of 0.1 mol·L-1H2SO4,the ratios of Qred/Qoxare more close to 1 than in 1.0 mol· L-1H2SO4,indicating that the extent of reversibility in 0.1 mol· L-1H2SO4is higher.As mentioned above,the reduction and oxidation processes are diffusion-controlled.Diffusion of cations is easier to enter or leave the polymer in diluted solution than in strong solution.26As a result,the reduction and oxidation processes in 0.1 mol·L-1H2SO4are more reversible than in 1.0 mol·L-1H2SO4.

Similar results are obtained as shown in Fig.1b with the potentials switched from-0.4 to 0.4 V.The ratios of Qred/Qoxfor both solutions of 0.1 mol·L-1H2SO4and 1.0 mol·L-1H2SO4are still close to one,indicating the reduction and oxidation processes are still reversible at the applied potentials.

Fig.1c shows the result when potentials are subsequently stepped between-0.8 and 0.4 V.In 1.0 mol·L-1H2SO4,a large amount of gaseous hydrogen was produced at the negative potential of-0.8 V,which consumed large charges.The charges consumed for hydrogen evolution is increased with elapsed time.Because long-time gaseous evolution could accelerate the H+ions diffusion,thus the values of Qredincrease drastically with the cycle number.The ratios of Qred/Qoxincrease signifi-cantly from 1 to 8.As for 0.1 mol·L-1H2SO4,no hydrogen evolution was generated.The values of Qredare assigned to the reduction of ppy film itself and almost remain constant during the whole reduction/oxidation cycles.Since the negative potential is as low as-0.8 V,the positive potential is 0.4 V,therefore,the ratio of Qred/Qoxis slightly bigger than one.

Fig.1 Qredand Qred/Qoxcalculated from recurrent potential pulse experiments in 0.1 and 1.0 mol·L-1H2SO4solutions held for 50 s at each potentialstepped potentials:(a)-0.1-0.4 V,(b)-0.4-0.4 V,(c)-0.8-0.4 V,(d)-0.4-0.8 V; -△-Qred/Qoxin 0.1 mol·L-1H2SO4,-○-Qred/Qoxin 1.0 mol·L-1H2SO4,-▲-Qredin 0.1 mol·L-1H2SO4,-●-Qredin 1.0 mol·L-1H2SO4

When potentials are subsequently stepped between-0.4 and 0.8 V,the results obtained from the experiments are shown in Fig.1d.As we known,overoxidation occurs through C＝O functional groups in the polymer backbone at sufficiently positive potential.Formation of these species disrupts the conjugated structure of ppy and leads to the loss of electroactivity capacity.12,19,20It can be seen from Fig.1d that the responses of Qredboth in 0.1 and 1.0 mol·L-1acid solutions decrease significantly assigned to overoxidation at positive potential of 0.8 V.As Qoxat 0.8 V declines more quickly than the response of Qredat-0.4 V,therefore,the ratio of Qred/Qoxincreases and closes to one in the latter cycles.However,the reversibility of ppy has been disrupted which can be deduced from the ratios of Qred/Qoxmuch lower than 1 in the initial period of potential cycles.

From the results mentioned above,we find that when negative potential is lower enough to produce hydrogen,the response of Qredwill increase with the cycle number and the ratio of Qred/Qoxis much higher than 1,which means that the redox reactions on ppy films are irreversible.It can be observed hydrogen evolutions generated at-0.8 V for 1.0 mol·L-1H2SO4and-1.2 V for 0.1 mol·L-1H2SO4have made ppy films swell significantly and even peel off the substrate.When positive potential is up to 0.8 V,overoxidation occurs for ppy film in 0.1 and 1.0 mol·L-1H2SO4solutions,which leads to the rapid drop of Qredand Qoxdue to the irreversible loss of electroactivity.

3.2 Redox stability in Na2SO4solution

Fig.2a shows Qredand Qred/Qoxcalculated from the reduction and oxidation processes obtained by the potential switches stepped from-0.4 to 0.4 V in Na2SO4solution.It can be seen that the response of Qredreaches to a constant after equilibrium was established at the first forty cycles.The ratios of Qred/Qoxare close to 1,indicating the redox reactions are reversible at the applied potentials.Similar to that in acid solutions,the values of Qredin 1.0 mol·L-1Na2SO4are larger than that in 0.1 mol·L-1Na2SO4indicates that ppy is more reversible in 0.1 mol·L-1Na2SO4than in 1.0 mol·L-1Na2SO4,since the ratios of Qred/Qoxin 0.1 mol·L-1Na2SO4are much closer to 1.

Fig.2b is the variation of Qredand Qred/Qoxwith cycle number, for which potentials stepped subsequently between-0.8 and 0.4 V.No hydrogen evolution was observed at negative potential of-0.8 V in both electrolyte solutions.The values of Qreddecrease slightly with increasing cycle numbers after the first twenty cycles,but the ratios of Qred/Qoxare still close to 1,indicates that overoxidation is very subtle under this condition.Ob-vious overoxidation could be observed at positive potential of 0.5 V for 0.1 and 1.0 mol·L-1Na2SO4solutions(Figure is omitted).

Fig.2 Qredand Qred/Qoxcalculated from recurrent potential pulse experiments in 0.1 and 1.0 mol·L-1Na2SO4solutions held for 50 s at each potentialstepped potentials:(a)-0.4-0.4 V,(b)-0.8-0.4 V,(c)-0.4-0.8 V; -△-Qred/Qoxin 0.1 mol·L-1Na2SO4,-○-Qred/Qoxin 1.0 mol·L-1Na2SO4, -▲-Qredin 0.1 mol·L-1Na2SO4,-●-Qredin 1.0 mol·L-1Na2SO4

Fig.2c shows Qredand Qred/Qoxover cycle number with potentials stepping subsequently from-0.4 to 0.8 V.It can be noted that the values of Qreddecrease rapidly in the two neutral electrolyte solutions.This response is associated to the strong overoxidation,irreversible structural change within the polymer chains at sufficient positive potential of 0.8 V.This can be further verified by the ratio of Qred/Qoxmuch less than 1 at the initial stage.At latter stage,the ratios of Qred/Qoxincrease and reach to 1,which can be explained that the values of Qoxdecline more drastically than that of Qred.In this case,it can be concluded that overoxidation of the polymer becomes more serious over the cycle number.

It can be observed that when negative potential is lower than-1.5 V for Na2SO4(The difference of hydryogen evolution potentials for 0.1 mol·L-1Na2SO4and 1.0 mol·L-1Na2SO4is very slight),hydrogen evolution will occur.Through the experiments mentioned above,it is noted that negative potentials selected to avoid hydrogen release could not affect the reversible characteristics of ppy film.But sufficient positive potential higher than 0.5 V for 0.1 and 1.0 mol·L-1Na2SO4will lead to an irreversible structural change in the polymer as a result of overoxidation.

Compared the results of RPP experiments conducted in H2SO4and Na2SO4solutions,it can be revealed that the onset of overoxidation is at 0.8 V in H2SO4(The difference of initial overoxidation potential for 0.1 and 1.0 mol·L-1H2SO4is very slight),while it is about 0.5 V in Na2SO4(The difference of initial overoxidation potential for 0.1 and 1.0 mol·L-1Na2SO4is also very slight).This indicates that overoxidation is strongly related to the pH value of the electrolytes.The initial overoxidation potential is higher in solutions with lower pH.As we all know,in neutral or acid solutions,the redox potential(ϕ)for water electrolysis reaction:

can be expressed by

The atomic oxygen was produced at higher potential in the solutions with lower pH value.Oxygen reacting directly with ppy chains could result in the overoxidation,which is also verified by Li and Qian.27

3.3 Redox stability in NaOH solution

Fig.3a presents the change of Qredand Qred/Qoxfor ppy films with potentials stepped subsequently between-0.4 and 0.4 V in NaOH solutions.The values of Qreddecrease significantly with cycle numbers and the ratios of Qred/Qoxare much lower than 1.These results indicate that ppy undergoes strong overoxidation,leading to the interruption of the conjugation and degradation of electroactivity.It has been accepted that overoxidation is closely related to the attack of hydroxide yielding carbonyl groups,which can be demonstrated as follows:

Fig.3 Qredand Qred/Qoxcalculated from recurrent potential pulse experiments in 0.1 and 1.0 mol·L-1NaOH solutions held for 50 s at each potentialstepped potentials:(a)-0.4-0.4 V;(b)-0.8-0.4 V;(c)-0.4-0.8 V; -△-Qred/Qoxin 0.1 mol·L-1NaOH,-○-Qred/Qoxin 1.0 mol·L-1NaOH,-▲-Qredin 0.1 mol·L-1NaOH,-●-Qredin 1.0 mol·L-1NaOH

Similar results are shown in Fig.3b obtained with potentials stepped subsequently between-0.8 and 0.4 V.Since the selected negative potential is lower than the negative potential in the experiment shown in Fig.3a,the values of Qredand Qred/Qoxare higher under this condition.From both results of the Fig.3a and Fig.3b,we can find that the values of Qredare lower in 1.0 mol·L-1NaOH than that in 0.1 mol·L-1NaOH due to the strong overoxidation occurred in high NaOH concentration. This result proves that the strong nucleophile of OH-is the direct reagent for the attack on the conjugated double bonds of the polymer.When the concentration of OH-is higher,the extent of overoxidation is more serious.

When potentials stepped subsequently between-0.4 to 0.8 V,ppy film could be disrupted and even broken out due to the oxygen evolution and strong overoxidation at positive potential of 0.8 V in 1.0 mol·L-1NaOH solution.The RPP experiment could not be carried out in this electrolyte solution.Therefore,

Fig.3c only presents the variation of Qredand Qred/Qoxin 0.1 mol·L-1NaOH solution.It can be seen that the values of Qreddecrease rapidly from 0.27 to 0.05 C,the ratios of Qred/Qoxare much less than 1,reach to only 0.01 at the latter stage of potential switches.The result indicates that overoxidation is remarkable pronounced under this condition.Compared the results of Fig.3a and Fig.3c with the same negative potential of-0.4 V,it can be concluded that the extent of overoxidation is stronger at higher potential.

From the experiments conducted in NaOH solutions,it could be noted that irreversible overoxidation would occur at any potentials,leading to the drastical decline of Qoxand Qred. Higher positive potential and higher OH-concentration lead to more serious overoxidation on ppy film.

4 Conclusions

The redox stability of ppy deposited on stainless steel doped with sodium p-toluenesulfonate in aqueous solutions with different pH values was studied systematically by RPP technique. Qredand Qred/Qoxwere calculated from the experiments.It is found that when negative potential is sufficient low to generate gaseous hydrogen,the values of Qredincrease drastically with the cycle numbers,the ratios of Qred/Qoxare much higher than 1,indicates that the redox reaction at applied potential is irreversible.When positive potential is sufficient high to cause overoxidation,the values of Qredand Qoxwill decrease rapidly, the ratios of Qred/Qoxare lower than 1.In this case,ppy undergoes the irreversible loss of conjugation and electroactivity decay.When potential window exerted on ppy is propriate to avoid hydrogen evolution in negative potential and to avoid generating overoxidation in positive potential,the values of Qredand Qoxalmost keep constant and the ratios of Qred/Qoxare close to 1,which indicates that the redox reactions are reversible for numerous potential cycles.In this way,the potential windows where ppy films possess reversibility property have been determined,it is within-0.8-0.8 V for 1.0 mol·L-1H2SO4,-1.2-0.8 V for 0.1 mol·L-1H2SO4,and-1.5-0.5 V for 1.0 and 0.1 mol·L-1Na2SO4solution.However,in NaOH solution,overoxidation arises at any potential,indicating that the existence of OH-ions is the direct reason for overoxidaton. The results demonstrate that pH value of solutions and switching potentials have profound influences on the oxidation and reduction reactions on ppy films.From our work,it can be proved that RPP is an alternative effective technique for measurement of redox stability for ppy-modified electrode in aqueous electrolyte solutions.

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Redox Stability of Polypyrrole in Aqueous Electrolyte Solutions by a Recurrent Potential Pulse Technique

TIAN Ying1,*WANG Jing-Ri2LIU Ming1SHI Kun1YANG Feng-Lin3
(1College of Environmental and Chemical Engineering,Dalian Jiaotong University,Dalian 116028,Liaoning Province,P.R.China;2Dalian Shengshi Environmental Co.Ltd.,Dalian 116600,Liaoning Province,P.R.China;3Department of Environmental Science and Technology,Dalian University of Technology,Dalian 116024,Liaoning Province,P.R.China)

The recurrent potential pulse(RPP)technique is an alternative and effective technique for redox stability measurement.We investigated the electrochemical redox stability of polypyrrole(ppy)films doped with sodium p-toluenesulfonate by RPP technique in this study.The reduction charge(Qred)and the ratio of reduction and oxidation charges(Qred/Qox)obtained from the switching potentials in aqueous solutions of H2SO4,Na2SO4,and NaOH were calculated to describe the reversibility of ppy at the applied potential windows.We found that the irreversible overoxidation strongly depended on the pH value of the supporting electrolytes and on the switching potentials.The onset of the overoxidation potential is 0.8 V in H2SO4solution while it is only 0.5 V in Na2SO4solution.In NaOH solution,overoxidation occurs at any potential indicating that the existence of OH-ions is directly responsible for overoxidation.

Polypyrrole;Overoxidation;Redox stability;Recurrent potential pulse

O646

∗Corresponding author.Email:greenhusk@126.com;Tel:+86-411-84106746.

The project was supported by the National Natural Science Foundation of China(51078050).国家自然科学基金(51078050)资助项目