Study on polypyrrole as an electrode material in neutral solutions

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Corrosion Science\ Vol[ 39\ No[ 00\ pp[ 0762Ð0770\ 0887 Þ 0887 Elsevier Science Ltd[ All rights reserved[ Printed in Great Britain[ 9909Ð827X:87:,Ðsee front matter

Pergamon

PII] S9909!827X"87#99974!6

STUDY ON POLYPYRROLE AS AN ELECTRODE MATERIAL IN NEUTRAL SOLUTIONS S[ BIALLOZOR and T[ ZALEWSKA Department of Chemical Technology\ Technical University of Gdansk\ 79!841 Gdan sk\ Narutowicza\ 00:01\ Poland Abstract*A study of the polypyrrole "PPy# as an electrode material for cathodic reduction of dioxygen in water solutions depending on the substrate material was carried out[ It has been found that the electroactivity of a PPy _lm strongly depends on the nature of a substrate\ especially if the thickness of the PPy _lm is low[ The e.ciency coe.cient of the dioxygen reduction on the Ni:PPy electrode is higher than the one on the Pt:PPy electrode and achieves 73)[ The electroactivity of PPy _lm falls as a result of cathodic polarization\ but it can be regenerated by short anodic polarization pulse[ The stability of the PPy _lm in 9[0 M NaCl is higher than in 9[0 M Na1 SO3[ Þ 0887 Elsevier Science Ltd[ All rights reserved Keywords] water deoxidation^ polypyrrole nickel^ oxygen reduction

INTRODUCTION Electrochemical water deoxidation is one of the ways of diminishing its corrosive activity towards iron[0\1 The e.ciency of such process depends on the correct choice of the cathode material\ which ought to have electrocatalytic properties towards the electroreduction reaction of O1[ The electronically conductive polymers "ECP# have been recently a subject of a large number of investigations\ what is connected with a wide possibility of their practical applications as electrode materials\ e[g[ in chemical sources of energy and fuel cells[2 It has been found3Ð7 that the polypyrrole has electrocatalytic properties towards the dioxygen reduction reaction in acidic solutions[ In our previous publication\8 the possibility of using electrodes covered by PPy layer as a cathode for the reduction of O1 in the 9[0 M Na1SO3 solutions has been con_rmed[ It has been found that the current e.ciency "WOH# of the reaction on GC:PPy electrode is much lower than on the Pt:PPy or Ni:PPy[ The results of the continuation of these studies in neutral solution of 9[0 M Na1SO3 and 9[0 M NaCl are presented below[ The purpose of this study was to check if the support material in~uences the elec! trochemical properties of PPy used as a cathode for dioxygen reduction in neutral solutions[ EXPERIMENTAL PROCEDURE The electroreduction of O1 was studied in the aerated 9[0 M Na1SO3 or 9[0 M NaCl solutions at 1920>C[ The solution was prepared from crystalline salts\ 88) purity "POCH\ Manuscript received 04 December 0886 0762

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Poland# and triply distilled water[ The PPy layer was obtained electrochemically3\4 in Ar atmosphere from acetonitrilic solution\ containing 9[0 M or 9[1 M "on Ni# of pyrrole "Aldrich# freshly distillated and 9[1 M Et3NClO3 "puri_ed by recrystallization#[ The PPy _lm was deposited galvanostatically "jc  39 A m−1\ Ea ca 9[81 V vs NHE#\ and its thickness was controlled by the amount of passed charge[3 Electrode with freshly deposited PPy _lm was immersed in 9[0 M Na1SO3 "or NaCl# solution for 04 min[ to achieve the equilibrium and to remove the monomer molecules[ Pt and Ni sealed in the Te~on tube were used as substrates[ The platinum electrode was polished with Al1O2 "9[94 mm granulation#\ rinsed with distilled water\ immersed into a concentrated H1SO3 solution for 0 min[ and _nally\ it was washed carefully with triply distilled water[ The nickel electrode was thoroughly cleaned mechanically by polishing with emery paper up to the grade of 0999\ then degreased and washed with triply distilled water[ A platinum sheet of a surface area approximately 099 times larger than the area of the working electrodes was used as a counter electrode[ The Hg:Hg1SO3 in 9[0 M Na1SO3 and Ag:AgCl electrodes were used as a reference electrode\ but the potentials given in the paper were recalculated vs[ NHE[ All experiments\ except the pH change measurements\ were carried out in one compartment cell[ The change of catholite pH was measured in two compartment cells as it was described earlier[8 The cyclic voltammetry "CV# and chronoamperometry "CA# techniques were employed[ The regeneration of the PPy _lm after its cathodic reduction were curried out at potential 9[78 V vs[ NHE[ More details about the experimental conditions were given in our previous paper[8

RESULTS 2[0[ Study on the oxy`en reduction The examples of CV curves recorded on Pt:PPy and Ni:PPy electrodes in aerated and deaerated solutions are shown in Figs[ 0 and 1[ The cathodic and anodic maxima which appeared on the CV curves are connected with the reduction and reoxidation of a polypyrrole _lm\ as well as with the reduction of an O1 molecule dissolved in electrolyte[ The shapes of voltammograms in aerated and deaerated solutions are the same\ and no maxima exist on the CV curves\ which could indicate that electroreduction of dioxygen proceeds on the electrode[ Thus one can conclude that the reduction of O1 on cathode is the result of a side chemical reaction between O1 and PPy _lm\ after its electroreduction[ As it is known09 the gaseous O1 oxidises PPy\ so the reaction of O1 with the freshly electroreducted PPy layer seems probable[ The possible scheme describing this process may be following] 1ðPPyŁ1¦¦3e:1PPy¦O1 ¦1H1 O:1ðPPyŁ1¦¦3OH− Previously it was shown by Barendrecht et al[ ð4Ł that the electroreduction of dioxygen on the electrode covered by PPy layer in 9[4 M H1SO3 solution takes place on the metal! solution boundary[ Also other authors con_rmed this during their studies of various red!ox processes in the same solution[00Ð02 The contact between metal and the solution realizes mainly in the polymer _lm pores\ although it cannot be excluded that a discharge reaction proceeds on the metal surface as a result of di}usion "or migration# of electroative substances through the polymer layer[ All the above cited studies were carried out on the Pt electrode covered by very thin "³0 mm# polymer _lm[ Such thin polypyrrole _lm is very porous[ In this case\ the area ratios of the metal!solution and

Study on polypyrrole as an electrode material in neutral solutions

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Fig[ 0[ Example of the voltammetry curve on the Pt:0 mm PPy electrode in 9[0 M nondeaerated NaCl solution\ scanning rate] v9[0 Vs−0[

the polymer!solution boundary are close to each other[ It may be assumed that the reduction of O1 molecules can proceed on the two parallel paths] as an electroreduction on the metal surface and as a chemical reaction with polypyrrole "according to above given scheme#[ When the polymer layer thickness grows\ access to the metal surface rapidly diminishes\ due to diminishing of the polymer _lm porosity and transport velocity of the active substances through the _lm[ Moreover the contact time between the dioxygen and PPy becomes longer during the transport of O1 molecules across the polymer layer\ when it is thicker[ Thus\ in the case of su.ciently thick PPy layers "×0 mm# one can suppose that electrodic reaction will proceed mainly on the polymer surface[ In order to compare the rate of the dioxygen reduction on investigated electrodes\ the potentiostatic polarization was carried out at the limited time and the pH changes of the catholite were measured[ As it is known\03 the mechanism of the dioxygen electroreduction process is very complicated and depends on the conditions in which reaction takes place[ The _nal products of the reduction process may be H1O1 or OH−[ However\ it has been found previously8 that on the Pt:PPy and Ni:PPy electrodes the only stable products of O1 reduction are OH− ions[ Thus one may assume that the change of the pH of the catholite after polarization _nishing\ can be used as a measure of the O1 amount which has been reduced[ The measurements were carried out at two potentials] a little more positive and more negative than the cathodic peaks potential[ The results are given in Table 0[ This table contains data of the values of total charge

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Fig[ 1[ Examples of the voltammetry curves on the Ni:0 mm PPy electrode in 9[0 M NaCl!non! deaerated "solid line# and deareated "dash line#\ scanning rate] v9[0 Vs−0[

"Qt# transported by the electrode\ calculated by integration of CA response[ The values of charge density applied for the reduction of O1 "QOH# calculated with the assumption of the four electrons reaction\ are also given in Table 0[ In the last column of Table 0\ the e.ciency coe.cients "WOH# of the process are given[ They were calculated as the ratio of QOH to Qt[ As it is seen\ the Ni:PPy electrode is the most e}ective[ The rate of dioxygen reduction reaction on the Ni:PPy electrode in sulfate electrolyte increases especially in the range of the more negative potentials[ These values of potential can be achived on the Ni:PPy electrode due to the fact that the potential of water reduction on this electrode is much more negative\ than on the Pt:PPy electrode[ The magnitude of the WOH coe.cient on the Ni:PPy electrode positively distinguishes it among others[ On the Ni:PPy electrode in chloride solution\ only one cathodic maximum has appeared\ and WOH in the potential range of this maximum is also very high[ The calculated WOH are much higher than it could be expected on basic of observed decreases of Qc and jpc in dearated solutions[ It con_rms the above mentioned assumption about chemical nature of dioxygen reduction on electroreducted PPy _lm in neutral water solutions[ 2[1[ Study of the electroactivity chan`e of PPy layer Unfortunately cathodic polarization of PPy _lm\ particularly in neutral solutions\ leads to a decrease in its electroactivity and\ as a consequence\ to a diminishing of the total charge transferred by the PPy _lm during cathodic reduction[ An additional e}ect

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Table 0[ The change of the catholyte pOH as the function of cathodic material and potential "Ec#[ Cathodic polarization time tc09 min[ a# in 9[0 M Na1SO3 Electrode

Pt Pt:0 mm PPy Ni Ni:0 mm PPy Ni:2 mm PPy

b# in 9[0 M NaCl Electrode

Ni:0 mm PPy

Epc :V −9[115 −9[115 −9[483 −9[115 −9[456 −9[115 −9[759 −9[115 −9[483 −9[759

Epc :V −9[115 −9[456

D pOH :0 9−3 m−1

9[97 9[27 9[87 9 9[34 9[43 9[72 9[16 9[5 9[57

D

pOH :09−3 m−1 9[55 9[79

Qt :Cm−1

Q OH− :Cm−1

072 598 771 44 311 422 0975 0941 0197 0638

0 3[6 084[7 9[4

Qt :C m−1

Q"OH−# :C m−1

583[1 892[4

118[2 702[5

02[64 803[4 6[2 042 219

WQOH− :Qt:) 9[44 9[66 11[1 9 9[5 1[5 73[1 9[6 01[6 07[2

WQ"OH−# :Qt:) 22[92 89[94

 The geometric surface area has been taken into account to calculate all electrode charge densities and DpOH[ Qt* The total charge density passed during the cathodic polarization^ Q"OH−#*calculated charge density used for the OH− formation^ DpOH*between the pH of the solution before and after polarization\ respectively[

is the destruction of the PPy caused mainly by increase of pH solution "as a result of O1 reduction#\ what leads to changing of the voltammogram shapes and to decreasing of a total charge density transferred by the PPy _lm "QCV#[ This latter phenomenon can be used for measuring of PPy _lm electroactivity change\ as a consequence of its cathodic polarization[ The _lm electroreduction was performed by potentiostatic polarization at Ec −9[115 V[ The total charge density QCV transferred by PPy _lm was calculated by integration of CV response[ The potentiostatic reduction of PPy _lm was carried out for 019 minutes\ but every 29 min[ the reduction process was interrupted and CV curves were recorded\ what allowed to calculate the QCV passed through the PPy _lm during its voltammetric reduction and oxidation[ The results of measurements are presented in Figs[ 2 and 3[ As it is seen\ the activity of the PPy layer in 9[0 M Na1SO3 falls down more quickly than in 9[0 M NaCl[ The thickness of the layer does not in~uence the decrease of the activity rate\ however the total charge transferred by the thicker PPy layer is bigger and is specially large in the case of Ni:2PPy electrode[ The ratio of Qa:Qc\ calculated from the _elds corresponding to anodic and cathodic parts of CV curves\ is

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Fig[ 2[ The dependence of total charge transferred through the Pt:PPy electrode on the cathodic charge which passed by the electrode during its polarization at Ec −9[115 V vs NHE] 0* Pt:0 mm PPy in 9[0 M Na1SO3\ 1*Pt:2 mm PPy in 9[0 M Na1SO3\ 2*Pt:0 mm PPy in 9[0 M NaCl solutions[

equal to ca 44) regardless of the value of a passed charge\ which shows that the reduction of PPy layer is an irreversible process[ As it is seen in Figs[ 2Ð3 in 9[0 M Na1SO3 solution\ in the case of Pt:PPy electrode transfer of the cathodic charge equals to 299 C m−1 "Qc# already leads to loss of its electrochemical activity by ca 49) for 0 mm\ and by ca 24) for 2 mm thick _lm[ However\ on the Ni electrode covered by 2 mm PPy layer the electrochemical activity decreases on ca 34) for Qc0199 C m−1[ The values of charge connected with cathodic processes which proceed in the PPy _lm were always higher on Ni:PPy electrode and\ for both Pt and Ni electrodes\ they were higher in chloride than in sulfate solution[ These results con_rm the large dependence of the PPy electrochemical properties deposited on Pt and Ni electrode[ The most probable reason for such di}erent behaviours of PPy _lm deposited on the nickel electrode\ may be the supposition that in this case some complexes between Ni1¦ and pyrrole rings are formed in the polymer layer[ The results obtained by the ESCA investigation of the PPy _lm deposited on the Ni surface con_rm this assumption[ More details concerning this problem are given in our paper[04 The PPy _lm may be regenerated after its reduction\ by reoxidation at the potential more positive than 9[1 V vs SCE[ The value of the current density in cathodic maximum on CV curve "jpc# is connected with the PPy _lm electroactivity\ if other experimental conditions do not change[ Thus\ the comparison of jpc values after transferring some

Study on polypyrrole as an electrode material in neutral solutions

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Fig[ 3[ The dependence of total charge transferred through the Ni:2 mm PPy electrode on the cathodic charge which passed by the electrode during its polarization at Ec −9[115 V vs NHE in 0*9[0 M Na1SO3 and 1*in 9[0 M NaCl[

charge through the _lm\ gives information about the change of PPy electroactivity[ Table 1 gives the changes of jpc values\ obtained from voltammograms "scan rate 9[0 Vs−0# after anodic reoxidation of the PPy _lm at potential Ea9[78 V[ The information about the total charge which passed through the PPy layer during its polarization before

Table 1[ The data of the PPy layer regeneration anodic pulse followed after cathodic reduction of this _lm "Ea9[78 V\ Ec9[115 V vs NHE#[ Pt:0 mm PPy in 9[0 M NaCl Q:C m−1 jpc:A m−1 9 −280 −235 071 051 045 041

03[7 01[3 09[7 6[5 8[9 01[4 02[9

Ni:0 mm PPy in 9[0 M NaCl Q:C m−1 jpc:A m−1 9 −669 −683 6131 6707 6828

36[9 25[4 27[9 41[4 36[7 36[7

Q*the charge passed trough the _lm during potentiostatic polarization] {−|*cathodic and {¦|*anodic respectively jpc*current density obtained by CV curve "v9[0 V s−0#

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recording the CV curve are marked as {−| for cathodic polarization at Ec −9[115 V\ and as {¦| for anodic regeneration at 9[78 V[ The values of the charge\ which passed through the _lm when CV curves were recorded\ have been neglected[ The _rst value of jpc relates to the freshly deposited PPy _lm before anodic polarization\ whereas the further values of jpc were obtained after reoxidation of the _lm[ The comparison of the jpc values indicates the degree to which the PPy _lm electroactivity has diminished after cathodic polarization\ as well as the possibility of the _lm regeneration by its anodic reoxidation[ As seen\ the anodic regeneration of PPy layer in chloride solution has indeed taken place[ When the PPy _lm was deposited on Ni substrate\ a very large number of Qa passed during reoxidation of the PPy _lm\ which undoubtedly was connected with simultaneous process of Ni anodic dissolution[ Thus\ the roughness of the electrode surface and porosity of the PPy _lm can increase during regeneration process[ It cannot be excluded also that the composition of the PPy layer on nickel electrode may change as a result of increasing of Ni!pyrrole complexes in the deposit\ because in the case of Ni:PPy electrode\ jpc after reoxidation not only reaches its value recorded on the freshly deposited PPy\ but even exceeds it[ Hence\ the application of the Ni:PPy electrode for electrolytical deoxidation of water\ might be very pro_table[ However\ for practical application of such electrodes\ the circulation of the solution has to be used\ in order to avoid degradation of the PPy _lm caused by the increase of the pH in catholite\ and possibility of the ClO − ions appearance in the anolite\ when the solution contains Cl− ions[05 CONCLUSIONS The experimental results indicate that polypyrrole might be used as an electrode material for cathodic reduction of dioxygen in neutral solution[ Particularly usage of a nickel electrode as a base metal\ seems to be very promising\ because the e.ciency of O1 reduction on the Ni:0 mm PPy electrode is the highest among studied electrodes[ Especially good results were obtained in chloride solutions[ The electroactivity of PPy _lm decreases during cathodic polarization\ but it can be regenerated by short anodic pulse[ Thus\ the Ni:PPy electrode may be recommended for further investigations as a perspective outlook for electroreduction process of O1 in neutral solutions[ Acknowled`ements*Authors are grateful to the Polish Research Committee for the _nancial support\ grant KBN No 845971:99[

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