Hybrid electrode obtained by sol-gel derived Ni0.3Co2.7O4 nanoparticles incorporated into polypyrrole: Electrocatalysis of O2 reduction

A hybrid electrode was formed on glassy carbon (GC) electrode with polypyrrole (PPy) and mixed valence Ni0.3Co2.7O4 nanoparticles to study their behaviour towards the oxygen reduction reaction (ORR). The oxide nanoparticles were prepared by sol-gel route using various parameters and characterized by XRD, SEM, EDX and BET methods. The result showed that desired oxide was obtained and nanocrystallites exhibit a specific surface area ranging from 39 to 73 m/g with diameter varying from 13 to 25 nm. The hybrid electrode was then fabricated by electropolymerization of pyrrole (Py) in the presence of the smallest Ni0.3Co2.7O4 nanoparticles in KCl (0.15 mol/L) at room temperature and characterized by EDX, SEM and LSV methods. Investigation of the hybrid electrode confirmed that O2 reduction mechanism changes by applying potential. Thus, at low overpotential the O2 reduction involves two electrons and provides H2O2, with cathodic transfer coefficients (β) of 0.33 and exchange current density ( j0) of about 8×10 -3 mA/cm, while, at high overpotential H2O2 is further reduced into H2O. The results also show that sol-gel method led to the preparation of Ni0.3Co2.7O4 nanoparticles having a spinel structure with a desired stoichiometry which exhibits a high electrocatalytic activity for reducing oxygen mostly to H2O2.


I. Introduction
Over the last years electrocatalyst particles dispersion throughout the matrix of an electronically conductive polymer (ECP) as electrocatalytic systems for various reactions were widely explored [1][2][3][4][5].Thus, the obtained composite exhibits a three-dimensional distribution of accessible active sites and provides a better charge transport conditions through the polymer film [6].
Among ECP, polypyrrole (PPy) has attracted a great attention in recent years due to its non-toxicity [7], easy fabrication by electropolymerization (which promotes application of aqueous and organic solvents and using chemical and electrochemical methods) [8], high conductivity [9], thermal and environmental stability [10,11].Several recent studies focused on the development of a composite electrode for oxygen reduction reaction (ORR) catalysis and production of H 2 O 2 [12], which can be used as an oxidizer of organic pollutants and bleaching or sterilizing agent in chemical or pharmaceutical industry [13].The spinel Ni x Co 3-x O 4 (0 ≤ x ≤ 1) nanoparticles seem to be very promising electrocatalyst.Thus, Chartier et al. [14,15] reported that the nanoparticles with x = 0.3, synthesized by thermal decomposition of nitrates, had the average size of 35 nm and maximum O 2 reduction activity producing high amount of H 2 O 2 , with an exchange current density ( j 0 ) of 2×10 -3 mA/cm 2 .
Many authors claimed [16,17] that when the catalysts dimensions are reduced, the materials properties might be remarkably changed.It was noticed that the preparation route influences the oxide physicochemical properties [18], including its size [19,20].Among dif- ferent synthesis methods, sol-gel is considered as a lowtemperature soft chemical route [21] frequently used to produce nanosized oxide powders [22] having a strong catalytic activity and relatively high stability [23].
In this study, different Ni 0.3 Co 2.7 O 4 powders were prepared by sol-gel method using two chelating agents (oxalic/citric acids) and different calcination temperatures (300/350 °C).Electrocatalytic activity of the embedded oxide particles in polypyrrole film (being part of a sandwich hybrid electrode GC/PPy/PPy(Ni 0.3 Co 2.7 O 4 )/PPy) has been studied.In order to improve this interesting property, Ni 0.3 Co 2.7 O 4 powder with the smallest particles was selected.The sandwich hybrid electrode was characterized by linear sweep voltammetry (LSV) using rotating disc electrode (RDE) [23].The data were exploited by Koutecky-Levich (K-L) equation to evaluate ORR kinetic parameters that allow us to check out the electrocatalytic performances.Precursor nitrate solution, having Co and Ni cation ratio 9:1, was prepared by mixing cobalt (II) nitrate (Co(NO 3 ) 2 × 6 H 2 O) and nickel (II) nitrate (Ni(NO 3 ) 2 × 6 H 2 O) salts.Two chelating agents (oxalic and citric acids) were separately added to the nitrates mixture and stirred for 24 h under air at 25 °C.Oxalic and citric acids were dissolved in ethanol (95.0%), and distilled water, respectively.The pH value of the nitrate mixtures was adjusted to 0.5 by carefully adding small amount of KOH solution.Two solutions were then stirred for 10 h, heated at 78 °C until formation of wet gels is observed.Subsequently, wet gels were dried at 100 °C, hand ground to a fine powder with an agate mortar and pestle and finally heat treated at 300 °C or 350 °C for 2 h.The synoptic schemes of both approaches are shown in Fig. 1.

Powder characterization
Phase purification of the as-prepared products was analysed by X-ray powder diffraction (XRD) with an X-ray diffractometer D008 Siemens using Cu Kα radiation λ = 1.5418Å. XRD profiles were measured in the 2θ range 10-90°, using a scan rate of 0.05°and collection time of 10 s.Plane indexation was realized in agreement with data of the ASTM power diffraction file.The unit cell parameter a was evaluated using a leastsquares program on basis of observed inter-planar d spacing and Miller indexes hkl.The specific surface area of the powders (S p ) was measured by nitrogen adsorption at 77.3 K using a Micromeritics ASAP 2010 gas sorption analyser and estimated by the BET equation.Morphology and composition of the synthesized powders were characterized with SEM and EDX, recorded using Siemens XL30 equipment.

Preparation of composite electrode
The composite electrode, consisting of three layers (GC/PPy, PPy(Ni 0.3 Co 2.7 O 4 ) and PPy), was prepared in three consecutive steps, which will be described below.
Preparation of GC/PPy: Electropolymerization of pyrrole (Py, Aldrich) was carried out in a classical electrochemical cell.The working electrode was a glassy carbon (GC) rotating disk with a cross-section area of 0.2 cm 2 .The counter electrode was a platinum plate of 1 cm 2 area.The reference electrode was a saturated Ag/AgCl electrode.Pyrrole (Py) was purified by distillation to remove oligomers and oxidized forms constantly present in the as-received product.The PPy film was deposited on GC electrode from a solution containing 0.1 mol/L of Py and 0.15 mol/L KCl at room temperature.Prior every trial, the solution was deaer-ated by nitrogen bubbling for 30 min and maintained under it during the whole experiment.Electrodeposition was performed with a constant current density of 1 mA/cm 2 during 60 s using a Potentiostat/Galvanostat Autolab PGSTAT 30 driven by FRA 4.9 Software and controlled by a PC.The same equipment and software were used for all electrochemical experiments.
Preparation of GC/PPy/PPy(Ni 0.3 Co 2.7 O 4 ): The prepared GC/PPy was immersed in a solution containing 0.1 mol/L Py, 0.15 mol/L KCl and 7 g/L Ni 0.3 Co 2.7 O 4 .The nanoparticles were incorporated in PPy layer during the galvanostatic electropolymerization process with a current density of 1 mA/cm 2 during 150 s, and the composite thin film was obtained.
Preparation of GC/PPy/PPy(Ni 0.3 Co 2.7 O 4 )/PPy: The prepared GC/PPy/PPy(Ni 0.3 Co 2.7 O 4 ) electrode was covered with a protective thin PPy film, which is obtained by electrolysis at 1 mA/cm 2 for 20 s using a solution containing 0.1 mol/L of Py and 0.15 mol/L KCl.

Characterization of composite electrode
SEM and EDX were used to characterize the modified polymer surface and its composition.The electrochemical characterization was performed at room temperature in KCl (0.15 mol/L) solution.The reduction current was normalized to the GC electrode crosssection area.

Electrokinetic parameters
The rotating disc electrode (RDE) was used to evaluate electrokinetic parameters of the oxygen reduction reaction (ORR) by performing LSV at different rotational speeds.The aqueous solution was saturated by oxygen throughout the experiment.The obtained data were then analysed by K-L equation: where j is the measured current density, j k is the kinetic current density, ω is the electrode rotational speed, n is the transferred electrons number per one oxygen molecule, F is the Faraday constant, C is oxygen bulk concentration (1.2×10 -6 mol/cm 3 ) [24], D is oxygen coefficient diffusion (1.76×10 -5 cm 2 /s) [25] and ν is the solution kinematic viscosity (0.01 cm 2 /s).The plot of the inverse of the current density at a given potential vs. ω −1/2 gives the inverse of kinetic current density j −1 k and the total number of electrons n as the ordinate intercept at infinite ω and slope, respectively.
The overpotential is related to the kinetic current density according to equation 2: where j i is the limiting current density and b is the Tafel constant and can be determined by: For high overpotential, j −1 k approaches to j −1 i .In this case, the limiting current can be estimated and then used to calculate the kinetic parameters of ORR (β, j 0 ) according to equation 4 [26]: Indeed, the plot of η or E = f (ln( j k /( j i − j k ))) gives b, therefore β from the slope and j 0 from the ordinate intercept.

Powder characterization
X-ray diffraction spectra, presented in Fig. 2, confirmed the crystalline nature of the prepared Ni 0.3 Co 2.7 O 4 nanoparticles.The characteristic hkl lines: 111, 220, 311, 400, 422, 511 and 440, indicated on the pure cubic spinel structure.Similar results were obtained by Chartier et al. [27] for the powders prepared by thermal decomposition of nitrates.The estimated crystallites cell parameter a is about 8.11 ± 0.015 Å.Some small discrepancies between the experimental a values for different Ni 0.3 Co 2.7 O 4 powders appeared probably due to the differences between synthesis procedures.However, both synthesis strategies lead to the formation of the pure spinel structure.The crystallites average size d p(XRD) was estimated by Scherrer formula and it was ranging from 13 to 25 nm (Table 1).
The specific surface area of the powders, S p , was estimated using BET equation and used for determination of particles size d p(BET ) : where ρ ox is the sample density (7.6 g/cm 3 ).It can be seen from Table 1 that all four Ni 0.3 Co 2.7 O 4 powders have fine structure with nanometric particle size, which is even lower than that reported by Chartier et al. [27].
The average size of the oxide particles increases while its specific surface area decreases with an increase of calcination temperature.Besides, at a given calcination temperature, somewhat smaller particles were obtained when citric acid was used.Thus, the smallest oxide average size was obtained using citric acid and calcination temperature of 300 °C.SEM images of the synthesized powders are shown in Fig. 3.It is clear that the obtained Ni 0.3 Co 2.7 4 is constituted of faceted primary nano-crystallites agglomerates.The particles obtained with oxalic acid are less homogeneous than those obtained with citric acid.Composition, i.e.Co and Ni cation ratio, of the powders prepared with oxalic and citric acids and calcined at 350 °C was determined by EDX.As both powders have very similar EDX spectrum, only one was presented in Fig. 4 and the corresponding Co and Ni cation ratios are listed in Table 2.The obtained results suggest that the formed nanopowders exhibit the desired stoichiometry.The same conclusions, from EDX results, were obtained by other authors [27] for the nanopowders synthesized by nitrates thermal decomposition method.

Characterization of composite electrode
The smallest Ni 0.3 Co 2.7 O 4 nanoparticles obtained with citric acid and calcined at 300 °C were selected for preparation of composite electrode.All three electrodeposition steps were analysed by chronopotentiogram.Thus, Fig. 5 shows the chronopotentiograms of the electropolymerization of Py in KCl solution, under nitrogen bubbling with and without Ni 0.3 Co 2.7 O 4 particles.The chronopotentiometric curves of Py (steps i, iii) are characterized by decrease of the potential during which the electrode is covered with a PPy layer.The succeeding plateau refers to further polymerization on already formed polymer [28].The same plateau was reached during the formation of PPy(Ni 0.3 Co 2.7 O 4 ) on GC/PPy (step ii), however the initiation stage is characterized by an increasing electrode potential.This could indicate that the presence of nanoparticles could facilitate PPy formation [29].
Morphology of the hybrid composite films was investigated by SEM (Fig. 6).SEM images of the PPy film (Figs.6a,b) and nanocomposite film (PPy/PPy(Ni 0.3 Co 2.7 O 4 ) (Figs. 6c,d) show a typical, the so called "cauliflower" structure as reported in literature [30].It can be seen that Ni 0.3 Co 2.7 O 4 nanoparticles aggregates are inhomogeneously dispersed on the surface of the latter film (Figs 6c,d).EDX microanalysis confirms the presence of Ni 0.3 Co 2.7 O 4 in the composite films (not shown here).

Oxygen reduction reaction on composite electrode
Static electrode: Since PPy films exhibit no significant electroanalytical activity [31], the inserted oxide particles impact toward ORR was investigated.Therefore, linear sweep voltammetry (LSV) was recorded by scanning the potential from 0.2 to −0.8 V vs. Ag/AgCl with a rate of 5 mV/s.Three different experimental conditions were explored, i.e. aerated, deaerated, and oxygen saturated solutions (Fig. 7).It can be seen that in the absence of oxygen, no significant current densities  Total current density j −1 is plotted in function of ω −1/2 at different potentials (Fig. 9).The straight line for each electrode potential indicate the first order reaction against dissolved oxygen [34].Thus, for each scatter straight line, the total number of electrons n was calculated and the resulting values are reported versus electrode potentials (Fig. 10).For small applied potential (i.e.small overpotential), the total number of electrons was found to be close to n = 2, indicating the O 2 reduction into H 2 O 2 [35].By decreasing the applied potential (i.e.high overpotential), the total number of electrons is gradually increased up to n = 4. So, H 2 O 2 is further reduced into H 2 O.This behaviour based on potential was also reported by Chartier et al. [27] and attributed to a change in the mechanism with potential, which is a characteristic of the ORR on mixed valency spinel oxides.
From the K-L plot (Fig. 9), the ordinate-intercepts (ω → ∞) give the inverse of the kinetic current density for each potential.The limiting current density j i was obtained from Fig. 11 while the parameters j 0 and  β are deduced from the curve E = f (ln( j k /( j i − j k ))) (Fig. 12) and summarized in Table 3.

IV. Discussion
The j 0 value is 4 times higher than that reported by Chartier et al. [15] for the NixCo3 − xO 4 having particle size of about 35 nm and obtained by thermal decomposition method.So, it seems that the sol-gel nanoparticles, due to their smaller size, exhibit better electrocatalytic activity toward ORR.Chen et al. [36] reported the same catalyst behaviour and suggested that Au nanoparticles exhibit much higher coordination number of the surface atoms, which promote O 2 adsorption.Therefore, its surface becomes more active.In fact, oxygen adsorption on catalyst is the first step in the electroreduction process and consequently it is more efficient when its surface allows adsorption of higher amount of oxygen [37].Re-cent theoretical studies [38] have also shown that with decreasing core size of Au, the d bands become narrowed and shift towards the Fermi level.This finding suggests that smaller Au nanoparticles are energetically more favourable for O 2 adsorption.
As the electrocatalysis depends on both geometrical and electronic factors previously exposed, the former are those related to the extension of the actual surface area or actual site density.The latter one is that related to the surface electrical conductivity.Thus, the enhanced catalytic activity observed in our study may be ascribed, at least in part, to the Ni 0.3 Co 2.7 O 4 particle size which is smaller (higher surface area), or to its conductivity improvement.

V. Conclusions
In the present study, spinel Ni 0.3 Co 2.7 O 4 nanoparticles were successfully synthesized using sol-gel route.Particle sizes ranging from 13 to 25 nm were achieved with two chelating agents and calcination temperatures of 300 and 350 °C.The characteristic XRD peaks and the EDX peaks of Ni, Co and O confirmed the formation of the pure spinel Ni 0.3 Co 2.7 O 4 structure.Moreover, particle sizes depend more on calcination temperature than on the chelating agent, however its specific surfaces area and morphologies depend on both synthesis parameters.
The powder with the smallest particles was used to form hybrid electrode (PPy(Ni 0.3 Co 2.7 O 4 ) film sandwiched between two PPy layers on glassy carbon.The investigation of the hybrid electrode confirmed that O 2 reduction mechanism changes by applying potential.Indeed, ORR involves two electrons to form H 2 O 2 , with cathodic transfer coefficient equals 0.33 and exchange current density of about 8×10 -3 mA/cm 2 at small overpotentials.Therefore, compared to thermal decomposition, sol-gel method is considered as interesting and promising method.It enabled us to obtain fine nanoparticles, with large specific surface area and high electrocatalytic activity.It produced a larger amount of H 2 O 2 making it promising for many environmental applications, including electrochemical treatment involving oxygen reduction reaction.

Figure 1 .
Figure 1.Synoptic scheme of the sol gel synthesis of Ni 0.3 Co 2.7 O 4 nanoparticles with oxalic acid and citric acid as chelating agents Ni 0.3 Co 2.7 O 4 powders were synthesized by sol-gel method using nickel and cobalt nitrates and two different chelating agents.The powder with the smallest particles was then dispersed in PPy matrix (to form PPy(Ni 0.3 Co 2.7 O 4 ) film) and sandwiched between two PPy layers, leading to the formation of hybrid electrode [GC/PPy/PPy(Ni 0.3 Co 2.7 O 4 )/PPy] used for the investigation of its oxygen reduction ability.2.1.Preparation of Ni 0.3 Co 2.7 O 4

Figure 2 .
Figure 2. X-ray diffractograms of Ni 0.3 Co 2.7 O 4 powders prepared with oxalic and citric acids and calcined at 300 and 350 °C

Figure 10 .Figure 11 .Figure 12 .
Figure 10.Variation of the total number of electrons n vs. electrode potential (E)

Table 2 .
The atomic Co/Ni ratio in the oxides type obtained with different chelating agents and calcined at 350 °C for 2 h