ORIGINAL PAPER

Electrochemical deposition and characterizationof polyppyrrole coatings doped with nickel cobaltoxide for environmental applications

Alexandra Banu & Maria Marcu & Elvira Alexandrescu &

Elena Maria Anghel

Received: 20 December 2013 /Revised: 21 March 2014 /Accepted: 25 April 2014# Springer-Verlag Berlin Heidelberg 2014

Abstract Electrochemical depositions of hybrid polypyrrole/nickel cobalt oxide (PPy/NiCoO) coatings onto ferritic stain-less steel surface were carried out with different electrochem-ical techniques from 0.1 M pyrrole (Py) in 0.2-M oxalic acid(OA) solution and less than 150-nanometer-sized NiCoOparticles. The structural properties of the composite wereinvestigated by using different methods such as transmissionelectron microscopy (TEM), scanning electron microscopy(SEM) with energy-dispersive X-ray spectrometer (EDS)and Raman spectroscopy. The embedded NiCoO particles,uniformly distributed onto the surface of the PPy film, havesimilar oxide ratios corresponding to a mixed oxide structure.The electrochemical characterization was done using polari-zation curves and linear sweep voltammetry (LSV) related tooxygen reduction reaction (ORR) in alkaline solution andhydrogen peroxide as an oxygen source. Concerning theexchange current densities for ORR, the obtained values(between 1.06 and 1.45×10−3 mA cm−2 for a totalamount of NiCoO of 0.1 mg cm−2) are comparable withother polymer films with Pt.

Keywords Polypyrrole (PPy) . Electrodeposition . Nickelcobalt oxide (NiCoO) .Microstructure . Oxygen reductionreaction (ORR)

Introduction

Many papers [1–4] have been focused in the last years on thepreparation of the composite electrodes containing nanoparti-cles of noble metals, metal complexes, or metal oxides dis-persed into the matrix of a conductive polymer (CP). A gooddispersion of the catalyst into the polymer matrix provides athree-dimensional repartition of the electrocatalytic sites ac-cessible to the reactant, and hence, good charge transportconditions throughout the polymer are preserved. Hence,properties of these composite depend strongly on electrolyte[5], synthesis methods [6], as well as nature of thesubstrate [7, 8]. The electrochemical formation of polymerpresents some advantages compared with the chemicalmethods of synthesis as direct grafting of the conductingpolymer onto the electrode surface. Among the synthesismethods, the electrochemical ones have been proved to bethe best ways to obtain the composite films on various sub-strates at reduced contact resistance. For instance, the classicalelectrochemical methods (potentiostatic, galvanostatic, andpotentiodynamic methods [9–11]) are widely employed inelectropolymerization of Py while the electrochemical pulsedmethods were reported by some researchers [12–14]. Thepulsed electrodeposition methods allow for an indepen-dent variation of three parameters: potential or current,period, and duty cycle. The main advantages of thepulsed methods over the direct current electrodepositionconsist in the improvement of deposition distribution andadhesion [15]. The electrodeposited film has smoother,compacter, and more homogeneous surface while a moreexpedient composition control is assured.

A. BanuPolitehnica University of Bucharest, Splaiul Independentei 313,060042 Bucharest, Romaniae-mail: alexandrabanu14@yahoo.com

M. Marcu (*) : E. M. AnghelInstitute of Physical Chemistry “Ilie Murgulescu”, SplaiulIndependentei 202, 060021 Bucharest, Romaniae-mail: m_marcu2000@yahoo.com

E. M. Anghele-mail: eanghel@hotmail.com

E. AlexandrescuRomanian Research and Development Institute for Gas TurbinesCOMOTI, Bd. Iuliu Maniu 220, 061126 Bucharest, Romaniae-mail: elviraalexandrescu@yahoo.com

J Solid State ElectrochemDOI 10.1007/s10008-014-2492-1

Despite the intense research activity lately on the develop-ment of the composite systems based on CPs for variousapplications (catalytic electrodes, corrosion protection, solarpanel, fuel cell, and sensors), improving the specific propertiesof these systems is a challenging task.

The main objective of the present research is to select themost appropriate electrochemical method for the PPy/NiCoOcomposite film synthesis on ferritic stainless steel and studyelectrochemical properties related to stability and its electro-catalytic properties for oxygen reduction reaction (ORR). Theelectrochemical parameters were discussed in terms of micro-structural modifications (scanning electron microscopywith energy-dispersive X-ray spectrometer (SEM-EDS),transmission electron microscopy (TEM), and Raman data).

Experimental

Materials

The material used as support was the ferritic stainless steelwith 17 % chromium due to its availability, low price, andreasonable stability in low aggressive environments. The elec-trolytes were prepared with analytical grade Aldrich reagents:pyrrole (Py), oxalic acid (OA), dodecyl sulfonic acid sodiumsalt (DSASS), and double-distilled water. NiCoO nanopowder(Aldrich) with molecular weight of 192.56 g was also used.

Coating preparation

The polymeric coatingswere prepared by electropolymerizationfrom 0.1MPy, 0.2-MOA solutionwith 30NiCoO nanopowderwith 2.8×10−3 M DSASS, using a Gamry Potentiostat 5.2version. A standard three-electrode cell was used as electro-chemical cell, platinum sheet as auxiliary electrode, and Ag/AgCl as a reference electrode. The working and auxiliaryelectrodes had similar areas and were arranged parallel to oneanother to ensure uniform current distribution. The acquireddata were processed by Gamry Echem Analyst specializedsoftware.

Prior to covering by PPy/NiCoO coatings, the stainlesssteel specimens, cut to a size of 1×1 cm2, were polished to asmooth surface finish and finally ultrasonically cleaned inalkaline solution, followed by warm water cleaning and driedin ethanol. Further, the steel surface was activated by 20-simmersion in 0.1-M HCl solution and transferred in electro-chemical cell without cleaning.

The Ppy and PPy/NiCoO coatings were obtained by thefollowing electrochemical methods:

1. Potentiostatic method. This technique consists ofpotentiostatic polarization at 0.8-V potential values sub-sequently to 5 s of prepolarization step at 0.2 V. The

advantage of applying of the two-step potential techniqueconsists in that it allows obtaining uniform thin films, anddeposited amount of PPy is more easily controlled bychanging the length of steps. The electric charge of 360mC was imposed.

2. Galvanostatic method. The film deposition was per-formed at 2 mA cm−2 current density for 180 s.

3. Potential and galvanic pulse methods. The pulsed tech-niques consist in shifting the potential or current densityrepeatedly between two different values resulting in a seriesof pulses of equal amplitude, duration, and polarity. Eachpulse consists of an ON-time (TON), during which potentialand/or current is applied, and an OFF-time (TOFF), duringwhich zero current is applied. For this, 300 cycles with TONand TOFF=0.6 s, pulse size of 2 mA for galvanic pulse, and0.7 V for potential pulse techniques were used.

The hybrid Ppy/NiCoO coating was prepared in thesame conditions as PPy films previously described; theelectrolysis was performed after 120 min of solutionstirring at 1,000 rpm for homogenization, by using30 % weight mixing ratio between pyrrole and oxides.For a good distribution of oxide phase into the poly-meric coatings, DSASS was chosen as a surfactant [16]at a concentration of 2.8×10−3 M. For each technique,three parallel samples were employed and their datawere averaged arithmetically. According to the prepara-tion method, samples were labeled as presented inTable 1.

Characterization of samples

Electrochemical characterization

The electrochemical behavior of the electrodes modified withPpy and PPy/NiCoOwas studied by cyclic voltammetry (CV)

Table 1 The labels of the samples and the corresponding electrodeposi-tion coating methods

Samples Coating type Electrodeposition method

1 Polypyrrole Potentiostatic

2 Polypyrrole Galvanostatic

3 Polypyrrole Pulse potential

4 Polypyrrole Galvanic pulse

5 Polypyrrole/NiCoO Potentiostatic

6 Polypyrrole/NiCoO Galvanostatic

7 Polypyrrole/NiCoO Pulse potential

8 Polypyrrole/NiCoO Galvanic pulse

9 Polypyrrole/NiCoO Potentiostatic

J Solid State Electrochem

in 0.5-M Na2SO4 solution, within a −0.2 to +0.8 V range, at ascanning rate of 50 mV s−1, under de-aerated condition.

To evaluate the electrocatalytic activity of theseelectrodes for the ORR, cathodic polarization curveswere recorded in the chosen potential range of +0.1to −0.7 V versus Ag/AgCl, in a 0.5-M KOH solution,with the scanning rate of 1 mV s−1 (quasi-stationarystate), in both de-aerated and aerated electrolytes. Thede-aerated and aerated electrolytes were obtained bybubbling argon and oxygen for 30 min, respectively.All the electrochemical measurements were performedat 25 °C and atmospheric pressure. The potential wasreferred to the Ag/AgCl.

Structural and morphological characterization

The morphology of the samples was investigated bySEM with a high-resolution microscope Quanta 3DFEG with Everhart–Thornley secondary electron (SE)detector, equipped with EDS analysis. The TEM speci-mens were prepared by extracting micrometric fragmentsfrom the sample surface and mounting these fragmentson holey carbon grids for TEM. Jeol 200CX and IN-SPECT S FEI electron microscopes were used for inves-tigations, and the image processing was performed withAnalysis Olympus software for phase analysis. Roomtemperature Raman spectra were recorded by means ofa LabRam HR spectrometer (Jobin-Yvon–Horiba) over50–1,900 cm−1 range. The 514-nm line of an Ar+ laserexcitation of about 4 mW on the illuminated volume wasused as exciting radiation through a ×50 objective of anOlympus microscope in backscattering geometry and at aconfocal hole of 200 μm.

The thickness of the layers was controlled by cou-lometry and was determined by the difference betweenweight of the specimen before and after deposition onan analytical balance with accuracy of 10−4 g andcalculated from Faraday’s law, and also by SEM andTEM measurements in cross section. The results arepresented in Table 4.

Results and discussions

Electrochemical deposition of the coatings

The chronoamperograms of the PPy coatings in Fig. 1show that electrochemical polymerization occurs usuallyin three stages as reported in literature [17, 18]. Theinitial drop of current (A) can be attributed to theelectroadsorption of the electrolytes and monomers atelectrical double layer, followed by an increase in thecurrent (B) which is caused by the dissolution of steel.

The last stage represents the film growth process (C).The polarization scan of naked ferritic stainless steel in0.2-M OA solution (Fig. 2) reveals a typical passivitybehavior with transpassive anodic branch starting atabout +700 mV. Thus, at the beginning of PPy electro-deposition process at 800 mV, the current density in-crease is due to both transpassive metal dissolution aswell as oxalate formation. Tallman et al. [17] suggestthat the active dissolution of iron during polypyrroleelectroformation from OA solution occurs along withformation of a Fe2+ oxalate interlayer.

The subsequent decrease in the current in I-t tran-sient is due to the deposition of polymeric coatingonto the substrate, the steady-state current density is0.006 A at 0.8-V deposition potential, about 17 timeshigher compared with the steady-state current densityof naked steel potentiostatic polarized at the samepotential (inset of Fig. 1). That allows for neglectingthe latter effect.

Only 30 s elapsed until the stainless steel substratewas fully covered by PPy. Once the coating depositionwas progressing, its color changed from green to black.However, the PPy coating obtained at a deposition timeless than 180 s has a very poor adhesion onto stainlesssteel substrate, and that is correlated with the fact thatthe coating is too thick.

Also, if the coating is too thick, it may act as a physicalbarrier instead of a chemical or electronic diffusion barrier[19].

Influence of the time deposition on the performances(stability) of the coatings obtained by galvanostatic con-d i t ions (2 mA cm− 2 ) were tes ted us ing cyc l icvoltamograms in 0.5-M Na2SO4 solution (Fig. 3). At shortelectrolysis time (less than 1 min), stored charge was more

Fig. 1 The electrodepositing diagrams of PPy (curve 1) and PPy/NiCoO(curve 2) on ferritic stainless steel from 0.2 M oxalic acid +0.1-M pyrrolesolution at +0.8 V, t=180 s. Chronoamperometric curve of naked ferriticstainless steel (inset)

J Solid State Electrochem

uniformly distributed than that for long time periods. Itis noted that the formation of PPy causes an increase ofthe voltammetric current due to the increased electro-chemical active surface and “pseudocapacitive” behaviorof the organic film. At longer deposition times, thecathode load was generally slightly higher than thatcorresponding to the anode, although the overallvoltammetric behavior was reversible. This behaviorcan be attributed to the partial oxidation of the polymerchain, a process that gave PPy-type doping steps thusobtained [20, 21]. Furthermore, by increasing the elec-trolysis time from 180 s to 300 s, it did not change theshape of voltamograms, and so, we decided to obtain all

the layers by coulometry of 360 mC cm−2 at +800 mVor by current density control of 2 mA cm−2 for 180 s.

The pulsed techniques consist in shifting the potentialof current density repeatedly between two differentvalues which gives a series of pulses of equal ampli-tude, duration, and polarity. Theoretically, when a coat-ing is formed under controlled potential, a chargedbarrier, negative or positive, around the electrode sur-face is formed, and the movement of ions toward sur-face is hindered or slowed [22]. By applying potentialor current pulses, this electrical layer discharges and theelectroactive species reach the surface and hence a moreuniform coating is formed. On the other hand, after fewmoments of electrolysis, the concentration of activespecies (organic in our case) drops off; the organicmolecules are bigger than inorganic ions; their move-ment toward surface is slow; and the diffusion polari-zation component of total electrode polarization is im-portant. During TOFF sequence, this effect is compensat-ed. For all obtained coatings, the total electrodepositiontime was calculated to be similar to that in constantelectrodeposition parameter situation, Q=360 mC cm−2.

The Ppy/NiCoO coatings were obtained in same con-ditions with PPy coatings, and the inorganic nanopowderdid not seem to modify the electropolymerization curve(Fig. 1). That enables attributing all amount of electricityonly to PPy formation, the oxides being physically in-corporated into polymeric coatings.

Structure and microstructure of Ppy and Ppy/ NiCoO coatings

The hybrid PPy/NiCoO coatings obtained by all electro-chemical techniques presented from aqueous solutionwithout surfactant contain small amounts of agglomeratedoxides. The Raman spectra of two hybrid PPy/NiCoOmaterials, samples 5 and 9, along with PPy film andNiCoO precursor are illustrated in Fig. 4a. Due to thethermal instability of the NiCoO above 400 °C [23],Raman spectrum of the NiO nanopowder was illustratedin Fig. 4a for comparison reasons. The Raman mode at557 cm−1 for the NiCoO precursor in Fig. 4a, not attrib-utable to the NiO, CoO, or Co3O4, signals the presence ofa mixed NiCoO rather than the NiCo2O4 spinel with amain Raman band at about 690 cm−1 [23, 24]. Windischet al. [24] reported a mixed NiCoO structure for a Co/(Ni+Co) ratio bellow 0.67 while over this threshold, thespinel structure (NiCo2O4) prevailed. The Co/(Ni+Co)limit of 0.67 corresponds to a double molar content ofCoO than NiO. According to this finding, the NiCoOpowder employed here has the Co/(Ni+Co) ratio bellow0.67 and hence a mixed oxide structure. The nanosize

Fig. 2 Anodic polarization curve of ferritic stainless steel in 0.2-M oxalicacid solution performed between −500 and +1,200 mVwith a scan rate of2.5 mV s−1

Fig. 3 Cyclic voltamograms obtained in 0.5-M Na2SO4 solution on PPycoatings with different thicknesses (different electrolysis time 1–30, 2–60, 3–180, and 4–300 s)

J Solid State Electrochem

powder of the NiO was supported by the lack of the two-magnon scattering (2 M) at about 1,490 cm−1 [25, 26].

The Raman bands located below 800 cm−1 of thesample 5, illustrated in Fig. 4a, consists in the PPymodes [27, 28] while the NiCoO modes [23, 24] at175, 339, 484, 552, and 637 cm−1 are difficult toidentify. Hence, a film with double NiCoO amount thansample 5 was prepared and named sample 9. Similar tothe spectrum of the NiCoO nanopowder, presence of the537 cm−1 band for the sample 9, assignable to the 1Pscattering of the NiO due to the defects or surface effect[25], points out that a mixed NiCoO and not a NiCo2O4

spinel is embedded in the PPy film. Band at 1,474 cm−1

in the Raman spectrum of the sample 9, due to the 2 Mscattering of the NiO single crystal, might point out tomicron-sized aggregation of the oxide powder in thePPy film at higher NiCoO content. Mao et al. [29] alsosignaled aggregation of the NiO nanopowder in theNiO-PPy materials. The band at 484 cm−1 for the same

powder is attributable to the cobalt oxides [30]. Veryslight modifications of the band profile and intensity forthe bands at 925, 987, and 1,045 cm−1, originating fromthe PPy ring deformation and C–H in plane bending[27, 28, 31], of the sample 5 (PPy/NiCoO composite) incomparison to the sample 1 (PPy film) indicate a weakinteraction of the NiCoO with PPy analogous to thePPy/NiO films [25]. For instance, slight widening ofthe 1,045 cm−1 mode is noticeable on the Raman spec-trum of the sample 5 as a consequence of the NiOinfluence which has the 2LO modes at 1,040 cm−1

[25]. The most intense bands at 1,330 and 1,578 cm−1

of the PPy film, ring stretching and backbone stretchingof the C=C bonds [27], are shifted toward lower fre-quency in sample 5 (the PPy/NiCoO composite)(Fig. 4a).

TEM and X-ray analysis of the NiCoO nanopowderrevealed that the NiO/CoO ratio was about 1:1.57(Fig. 4b), in agreement with molecular weight,192.56 g, of the pristine NiCoO (Aldrich). That isimportant because EDS analysis of the hybrid coatingsrevealed the ratio between nickel and cobalt oxides intopolymeric matrix in this range.

Microstructure of the PPy films obtained by variouselectrochemical methods (potentiostatic, galvanostatic,potential pulse, and galvanic pulse techniques) in thepresence of DSASS as a surfactant was analyzed bySEM-EDS and TEM techniques. Despite of similar cau-liflower fine aspect of all the PPy films investigated(Fig. 5), visible at magnification of ×20,000, a fewdifferences of the film microstructure were recordedaccording to the electrochemical method used. Thus, atthe same small magnification, coatings obtained bypotentiostatic and potential pulse methods revealed acoarse surface (Fig. 5a, b) with cauliflower structure,while the coatings obtained by galvanostatic and gal-vanic pulse methods have a smoother aspect of thesurface (Fig. 5c, d). It is worth to note that the PPyfilms copied the substrate relief.

Similar to the PPy coatings, structure of hybrid PPy/NiCoO coating depended on the electrochemical tech-nique used for preparation (Fig. 6a–d). At first approx-imation, it was assumed that the oxide particles act aspolymerization centers and a new block of polymerstarts to build around them [32]. Another interestingaspect in our opinion is correlated with the action ofsurfactant on particle entrapping, as can be seen in theFig. 7 where the oxide particles are surrounded bysurfactant film and appear as bubbles in polymericmatrix. The actions of surfactant are meant to increasethe stability of pyrrole-oxide powder solution suspension

Fig. 4 a Raman spectra on NiCoO nanopowder, NiO nanopowder, PPyfilm, and the PPy/NiCoO hybrid coatings obtained by the potentiostaticmethod (samples 5 and 9). b EDS spectrum on NiCoO nanoparticles(copper signal is from Cu TEM grille)

J Solid State Electrochem

and as a consequence of that to increase the amount ofoxide particles into polymer film. On the other hand,embedding of surfactant into the polymer resulted inincreasing its brittleness which caused film crackingespecially during oxygen bubbling (Fig. 6b).

In order to estimate the amount of NiCoO nanopar-ticles entrapped into the polymeric film, two evaluationtechniques were employed, namely SEM-EDS, to eval-uate the average amounts and distribution of particleson surface, and Faraday’s law, to evaluate the amount ofoxide phase in a polymer bulk. Thus, maps of elementaldistribution by SEM-EDS, neglecting those elements of nointerest (Fe and Cr) as being part of the support, gave anaverage amount over five different points of the oxide phase(Fig. 8 and Table 2). Thus, the Co/Ni atomic ratio increasingin succession (S5 (3.27)<S6 (4.15)<S8 (4.34)<S7 (5.03)) ismuch different than the starting NiCoO powder of 1.6:1 ratio.This behavior can be explained by considering that the Co2+

and Ni2+ ions are either mechanically embedded and/or chem-ically bonded to the PPy as a consequence of different coor-dination preferences of Co2+ and Ni2+ ions [33]. Becauseof different coordination forces of the Co2+ and Ni2+ ions withpyrrole matrix, taking place during mixing and polymerization

processes, partial segregation of the cobalt ions causes alter-ation of the Ni/Co ratio.

The atomic concentrations of the PPy/NiCoO compositederived from analysis of the EDS spectra corresponding tothe cluster areas in Fig. 9 are summarized in Table 3.According to these data, the Co/Ni ratio is almost half incomparison to the ones presented in Table 2.

It is worthy to note that in the cluster areas of the samples 5,6, and 8 in Fig. 9, the CoO/NiO ratio was slightly higher thanthe initial value of the NiCoO nanopowder but lower thanvalue 2. The biggest Co/Ni ratio is recorded for the cluster areain the sample 7 coating, i.e., 3.53, well above the threshold of2 dividing the NiO–CoO system into the mixed NiCoO andspinel structure domains [24].

The X-ray features in Fig. 9 for Fe and Cr haddiminished intensity, while O, Co, and Ni were moreintense. The S contribution, from surfactant molecules,was evenly distributed.

To estimate the amount of NiCoO into the bulk of polymerby Faraday’s law [34, 35], few assumptions were made. Thus,the pyrrole component was assumed to be the only participantin the electrochemical processes, and oxide phase is mechan-ically embedded.

Fig. 5 SEM evaluation of thePPy coatings obtained from 0.2Moxalic acid +0.1-M pyrrolesolution with 2.8×10−3 MDSASS by a by potentiostaticcontrol, b by pulse potentialelectrodeposition, c bygalvanostatic control, and d bygalvanic pulse electrodeposition

J Solid State Electrochem

Because all electrodeposition processes were conductedcoulometrically, at controlled electrical charge of 360 mC,

we considered that the electrical charge had to be attributedto polypyrrole deposition, even in case of hybrid coatingformation, so the difference between coating weight, obtainedin similar conditions, could be attributed to the amount ofoxide phase. The samples were weighed on analytical balancebefore and after deposition. The results are displayed in Ta-ble 4, and there are no differences concerning total amount ofoxide phase embedded in the samples 5–7, with exception forsample 8 with a lower NiCoO content of 0.07 mg cm−2.

A plausible explanation for these weight differences of theelectrodeposited polymer under potentiostatic and galvanosta-tic conditions at the same electrical charge should be ex-plained, in our opinion, by different polymerization mecha-nisms. Therefore, under galvanostatic conditions (current den-sity of 2 mA cm−2), the potential reached +0.65 V (curve notshown), while under potentiostatic conditions, the potentialwas +0.8 V. That means that the same structural unit (polaron)can carry different electrical charges and finally differentquantities of monomer to the electrode at different potentialvalues. This hypothesis is to be verified. The coating thicknessformed by electropolymerization can be determined under thesame conditions. The nominal density for PPy films was takenas 1.5 g cm−3 [36]. Thus, applying a 360 mC cm−2 chargedensity, a coating with an average thickness of 0.7 μm grew

Fig. 7 SEM aspect of entrapped NiCoO particles into polymeric filmobtained by current density control at 2 mA cm−2 (detail from Fig. 6clabeled zone)

Fig. 6 SEM evaluation of hybridPPy/NiCoO coating structureobtained from 0.2 M oxalic acidand 0.1-M pyrrole solution with2.8×10−3 M DSASS by apotentiostatic, b pulse potential, cgalvanostatic, and d galvanicpulse methods. Details fromlabeled zone for NiCoO particlesvisualization (inset)

J Solid State Electrochem

on the support. Figure 10 presents the coating thicknessesmeasured by TEM and SEM techniques, of 0.71–0.83 μm,which were very close to the calculated values.

ORR performance

In order to assess the electrocatalytic activity of NiCoOparticles for ORR, potentiostatic measurements underquasi-stationary conditions (scan rate 1 mV s−1) were

carried out on the stainless steel electrodes coated withPPy and Ppy/NiCoO films.

As noticeable on the Fig. 11, presence of NiCoOparticles in PPy matrix causes an increasing of thereduction current of dissolved oxygen by ∼85 %. Itshould be emphasized that this behavior cannot be sole-ly due to the enhancement of the active surface, whichrepresents ∼25 % (the cyclic voltammetry measurementsin the inset of Fig. 11), but also due to the catalyticactivity for oxygen reduction of the NiCoO. This con-clusion is also supported by the fact that the oxygenreduction process starts at lower electrode potentialwhen the NiCoO is present in the electrocatalyst incomparison to the pristine PPy film.

According to the data illustrated in Fig. 11, it can beassumed that the increasing of the cathodic current in thepresence of NiCoO in the PPy/NiCoO films is due to eithercatalyzed oxygen reaction, or other reduction reaction on thesurface which involves nickel or cobalt species. It is worthmentioning that the catalytic activity of PPy for this reaction issomehow negligible (see curve 1 in Fig. 11). To elucidate this

Fig. 8 Images of the distributionof relative intensity of X-raydetected on microarea from figurehigh left side, for characteristicelements (C, O, S, Cr, Fe, Co,and Ni) Kα, coating obtainedby potentiostatic control

Table 2 The average estimated values of atomic concentration (%),calculated as the average of measurements in five points, from EDSmeasurements

Samples C N O S Co Ni

5 64.83 12.65 17.45 0.80 3.27 1.00

6 76.16 – 18.01 1.19 3.74 0.9

7 59.58 14.84 19.78 0.97 4.03 0.80

8 59.19 12.36 22.46 1.02 4.04 0.93

J Solid State Electrochem

point of view, all electrodes coated with Ppy/NiCoO filmswere investigated for the ORR by determining the quasi-steady-state cathodic polarization curves at the scan rate of1 mV s−1 in an argon- or oxygen-saturated 0.5-M KOHsolution as earlier reported by Sulub et al. [37].

Since the observed current density (jAr) at the PPy/NiCoOelectrode in the argon atmosphere was not negligible, it wassubtracted from the corresponding value of the current densitydetermined at the same electrode in oxygen atmosphere inorder to obtain the current density j (jOx−jAr) produced by theO2 reduction only. The jAr values in Fig. 12 recorded during

the cathodic polarization study were found to be within 0.15−0.25 mA cm−2 range, at E=−0.7 V for the samples 5–8 inTable 1. The ORR begins at about −0.1 V for all samplespresented in Fig. 12, proving the electrocatalytic behavior ofthese PPy-NiCoO composite materials. However, the curveshapes in Fig. 12 suggest that the ORR occurs at significantrates only at electronegative potentials lower than −0.4 V.

Fig. 9 EDS results of cluster analysis for PPy/NiCoO hybrid coatings of samples 5, 6, 7, and 8

Table 3 The maximum estimated values of atomic concentration (%)(zone 1) from EDS measurements

Samples C N O S Co Ni Co/Ni

5 61.05 10.98 18.93 0.54 5.75 2.75 2.09

6 61.44 – 24.39 0.8 8.77 4.6 1.90

7 58.67 11.74 22.69 1 4.6 1.3 3.50

8 51.81 7.43 25.43 0.68 9.69 4.96 1.90

Table 4 The amount ofoxidic NiCoO powderentrapped into polymercoatings, determined byweight measurements

Samples Δm(mg cm−2)

mNiCoO

(mg cm−2)

1 0.3 –

2 0.2 –

3 0.3 –

4 0.2 –

5 0.4 0.1

6 0.3 0.1

7 0.4 0.1

8 0.27 0.07

9 0.5 0.2

J Solid State Electrochem

Fitting the polarization curves or plotting the overpotentialversus log (i) give the Tafel slope and the exchange currentdensity. We fitted the polarization curves for all PPy/NiCoOelectrodes (sample 5–8) at high current density range(>0.2 mA cm−2). Concerning the exchange current densities

for the ORR, the obtained values increasing in succession S5(1.45×10−3 mA cm−2)>S6 (1.20×10−3 mA cm−2)>S7(1.12×10−3 mA cm−2)>S8 (1.06×10−3 mA cm−2) for a totalcontent of NiCoO of 0.1 mg cm−2 and are comparable with theones reported by Coutanceau et al. [1] for the PANI/Pt films.Despite the smaller oxide content of about 4.27 %, sample 5has the best efficiency for ORR, i0=1.45 μA cm−2, probablydue to the coarser surface.

Conclusions

The PPy/NiCoO hybrid films were prepared by variouselectrochemical methods. The hybrid materials obtainedin the presence of DSASS contain a larger quantity ofthe oxide particles and are uniform concerning oxidedistribution onto surface for all electrochemical tech-niques. Among the electrodeposition techniques, thepotentiostatic method under charge control is the mostappropriate one since the pulse techniques gave smooth-er surface which hindered the electrode reactions (thesmallest Q discharge values and exchange current den-sity for ORR) despite of a larger quantity of ceramicphase embedded. The higher NiCoO content embeddedin the PPy matrix, the more aggregated NiCoO powder(micron-sized aggregates) is found in the hybrid films.These materials are potential candidates for corrosioncontrol and fuel cell purposes. The catalytic efficiencyfor ORR of hybrid potentiostatically obtained material iscomparable with that reported by other authors [1] forplatinum-containing materials.

Acknowledgments Raman measurements: Support of the EU (ERDF)and Romanian government, which allowed the acquisition of the researchinfrastructure under POS-CCE O 2.2.1 project INFRANANOCHEM Nr.19/01.0.3. 2009, is gratefully acknowledged.

Fig. 11 Polarization curves of oxygen reduction on electrode modifiedwith Ppy (1) and PPy/NiCoO (sample 5) under argon (2) and oxygen (3)saturated 0.5-M KOH solution. Scan rate 1 mV s−1

Fig. 10 a TEM image for polypyrrole coating thickness measurementsfor sample 1. b SEM image for hybrid coating thickness measurementsfor sample 5

Fig. 12 Variation of oxygen reduction reaction current with potential ofelectrodes modified with PPy/NiCoO in 0.5-M KOH solution (samples5–8 in Table 1)

J Solid State Electrochem

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