Efficient and methanol resistant noble metal free electrocatalyst for tetraelectronic oxygen reduction reaction

Electrochimica Acta 326 (2019) 134984

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Efficient and methanol resistant noble metal free electrocatalyst for tetraelectronic oxygen reduction reaction Abhishek Kumar a, *, Josue M. Gonçalves a, Alan R. Lima a, Tiago A. Matias a, Marcelo Nakamura a, Juliana S. Bernardes b, Koiti Araki a, Mauro Bertotti a, ** a b

~o Paulo, Av. Prof. Lineu Prestes, 748, 05508-000, Sa ~o Paulo, SP, Brazil Department of Fundamental Chemistry, Institute of Chemistry, University of Sa National Nanotechnology Laboratory, National Center for Energy and Materials, Rua Giuseppe Maximo Scolfaro, 10.000, Campinas, SP, 13083-970, Brazil

a r t i c l e i n f o

a b s t r a c t

Article history: Received 5 September 2019 Accepted 29 September 2019 Available online 2 October 2019

Efficient dispersion of M-N-C molecular macrocycles on carbon nanomaterials support is an attractive strategy to develop a highly efficient and noble metal-free Oxygen Reduction Reaction (ORR) electrocatalyst for fuel cell devices. Herein, Cobalt Phthalocyanine (CoPc)/reduced graphene oxide (rGO) nanocomposite has been synthesized through non-covalent functionalization of CoPc and GO followed by electrochemical reduction of the resulting hybrid. A comprehensive structural, topographic and morphologic study on the nanocomposite confirmed the GO and CoPc assembly based on p-p interaction and electrochemical reduction leads to enhanced dispersion of CoPc aggregates by breaking its microparticles into nanoclusters. A chemical mapping of the catalysts surface by Confocal Raman Imaging further proved that CoPc and GO phase is homogeneously distributed and impregnated into each other as a result of the electrochemical reduction. The ORR activity of the CoPc/rGO was evaluated through voltammetry and amperometry techniques, which revealed a synergic ORR catalysis at the composite surface highlighted by larger current densities, 300 mV positive shift of the onset potential, and negligible generation of H2O2 as compared to CoPc and GO. In addition, the composite material revealed better stability and resistance to methanol poisoning than a commercial Pt/C catalyst, and such features make it ideal for application in methanol fuel cells. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Graphene oxide Phthalocyanine Noble metal free electrocatalyst Oxygen reduction reaction Fuel cell

1. Introduction Fuel cells have attracted tremendous research interests in recent years as a clean and sustainable source of energy and a promising alternative to minimize the pollution produced by conventional carbon based energy sources [1,2]. Improving the sluggish Oxygen Reduction Reaction (ORR) kinetics [3] is the key to achieve high efficiency and power density in fuel cells. Platinum based catalysts have been conventionally used [4] for that purpose, however the high cost and limited reserve of that noble metal have impeded the wide scale commercialization of such devices. Moreover, Pt-based catalysts suffer with electrode poisoning [5,6] and decline in activity over time. Hence, the development of non-Pt low cost catalysts has become a major focus in recent years for a sustainable

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (M. Bertotti).

(A.

https://doi.org/10.1016/j.electacta.2019.134984 0013-4686/© 2019 Elsevier Ltd. All rights reserved.

Kumar),

[email protected]

clean energy goal. Many research groups have reported that chalcogenides [7], nitrogen doped carbonaceous materials [8e10], conducting polymers [11] metal nitrides [12] and metal chelates [13,14], among others, exhibit ORR catalytic performance comparable or even better than Pt-based materials. Among metal chelates, 2-D Metal Phthalocyanines (MPc) have attracted the attention of many researchers because of their extensive p-conjugated structure, low preparation cost, easy processing into thin films and especially the possibility of tuning their ORR activity by chemical or physical functionalization [15,16] approaches. Recently, M. Mukherjee et al. [17] reported a tetraelectronic ORR mechanism for manganese phthalocyanine microstructures and proposed a model based on theoretical calculations to explain the oxygen interaction with phthalocyanine. However, despite possessing a highly active metal center for ORR process, the MPc activity is often suppressed because of aggregation [18] and poor conductivity [19]. Different approaches have been adopted to overcome these limitations, including polymerization [20], pyrolysis [21], chemical functionalization [22] and immobilization of the macrocycle in the matrix of

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carbon nanomaterials. Among carbonaceous materials, carbon black and Vulcan carbon were comprehensively investigated to immobilize MPcs and analogous metalloporphyrins for developing ORR catalysts [23e29]. These composites have shown improved catalytic activity and stability as compared with their MPcs constituents, however further improvement is needed to match the ORR catalytic performance of commercial ORR catalysts like Pt/C. Even though such approaches have led to an enhancement in the ORR catalytic activity of phthalocyanines, combining MPcs with electronic conducting carbon support materials such as graphene and its oxides have clearly demonstrated advantages, as this strategy does not rely on harsh experimental conditions (like high temperature, complex chemical procedures) and saturation of ORR activity was not yet observed [30]. Graphene and its oxides have been the preferred choice among carbon nanomaterials to develop hybrid composites since they can adsorb MPcs on the surface through strong p-p stacking, while exposing their ORR catalytic sites. In addition, graphene derivatives such as graphene oxide (GO) already have high catalytic activity, low cost, facile and large-scale production methods, and can be subsequently reduced into higher conductivity graphene or reduced graphene oxide (r-GO), turning them a preferred choice as support to immobilize poorly conducting molecular catalysts such as MPcs. Moreover, there is a huge avenue to further enhance the ORR activity of GO-MPc assembly by controlling the density of oxygenated defects in GO or by tuning the structure of MPcs. In fact, graphene and its oxides composites with MPc prepared by noncovalent functionalization have shown remarkable ORR activity efficiently catalyzing the 4-electron reduction of oxygen to water, superior stability and higher resistance to methanol poisoning than Pt/C catalysts for potential applications in fuel cells [31e33] and battery type devices [34]. Finally, the covalent functionalization approach has also been adopted to immobilize MPcs on graphene surface leading to synergic improvement in ORR activity, stability and catalyst lifetime as compared with Pt/C [35]. Iron phthalocyanine (FePc) has been used in the majority of such composites developed so far, whereas the highly electrochemically active cobalt phthalocyanine (CoPc) analogues were underexplored for ORR activity. Recently, Adina et al. [36] and Sun et al. [37] achieved improved ORR activity of GO/CoPc hybrids only after high temperature pyrolysis. Grafting ligands in the MPc macrocycle enhances its solubility and dispersion in the graphene support, but the synthesis conditions are very harsh and expensive. Moreover, the majority of substituted phthalocyanines are not as thermally or chemically stable as their unsubstituted counterparts, which limits the lifetime of the composites. On the other hand, unsubstituted CoPc is very cheap, easy to synthesize and a commercially available material. The main drawback for its efficient immobilization on graphene-based materials is the very low solubility in most of the common organic solvents. This leads to the presence of large aggregates in the composite that block the access to the majority of the ORR catalytic active sites. Thus, new innovative strategies are required to enhance the dispersion while preserving the original structure of CoPc, for example by electrochemistry, a simple, fast, inexpensive, environmentally friendly method. Herein, we present a highly efficient approach for large-scale production of ORR electrocatalyst by non-covalent functionalization of GO with unsubstituted CoPc followed by electrochemical reduction of the resulting CoPc/GO hybrid into CoPc/r-GO. The structure of the CoPc/GO composite and the influence of the electrochemical processing on such material were studied by different optical and electronic spectroscopy techniques. Changes in topography, morphology and distribution of CoPc phase in the composite after electrochemical reduction were also studied in depth by atomic force microscopy, scanning electron and confocal Raman

microscopy, respectively. The ORR electrocatalytic activity of CoPc/ r-GO and its constituents was examined through voltammetric and amperometric techniques. The catalyst efficiency towards ORR was further evaluated by rotating ring-disk electrode (RRDE) voltammetry and by the average number of electrons transferred to dioxygen molecule. Finally, long-term stability of the catalyst and its tolerance to poisoning by methanol was investigated to demonstrate its potential application in direct methanol fuel cells. 2. Experimental 2.1. Materials and reagents All chemicals and reagents used in this work were of analytical grade and used without any further purification. Milli-Q ultrapure water (resistivity ~18 MU cm) was used to prepare all aqueous solutions. Cobalt(II) phthalocyanine and Pt/C catalyst (20% platinum on Vulcan Carbon) were purchased from Alfa Aesar. Dimethylformamide (DMF), Dimethylsulfoxide (DMSO), potassium hydroxide (KOH), potassium nitrate (KNO3), sodium sulfate (Na2SO4) and Nafion solution (5 wt% in isopropanol) were purchased from Sigma-Aldrich. Graphite powder was provided by a local chemical ~o Paulo city, Brazil. company in Sa 2.2. Material preparation Graphene oxide was prepared from natural graphite by a modified Hummers’ method. In a typical procedure, concentrated H2SO4 (115 mL) was added to a mixture of graphite flakes (5.0 g) and NaNO3 (2.5 g). Then, KMnO4 (15.0 g) was slowly added into the reaction mixture at 50
C, which was kept under stirring for 6 h. The reaction mixture was carefully diluted with 400 mL of water, then cooled in a water bath before further reaction with 20 mL of H2O2 solution (30% v/v). After 30 min, the reaction mixture was centrifuged (5000 rpm, 10 min), and the supernatant decanted away. The solid material was then successively washed with 800 mL of HCl solution (30% v/v) and centrifuged (5000 rpm for 10 min), and then washed with 400 mL of ethanol. The black solid was re-suspended in 400 mL of diethylether, filtered out with a 0.45 mm PTFE membrane, and dried overnight at room temperature, under vacuum. Graphene oxide (GO) was obtained by dispersing the solid (100 mg) in 100 mL of KOH solution. CoPc dispersed in DMF (0.5 mg mL1) was added in a strongly stirred GO aqueous dispersion, slowly, in a dropwise manner. Stirring was maintained for 24 h, the pH of the solution adjusted to 7, and calcium chloride (CaCl2) was then added leading to formation of a precipitate, which was separated out by centrifugation (5000 rpm for 10 min). The precipitate was washed repeatedly with water until a transparent supernatant was obtained. The black solid thus obtained was dried at a controlled temperature (40
C) in a closed furnace to get a black powder. 2.3. Materials characterization UVevis absorption spectra were recorded in the 190e1100 nm range using a Hewlett-Packard 8453A diode-ray spectrophotometer equipped with a deuterium and a tungsten lamp respectively for UV and visible excitation. Raman spectra were recorded in a WITec 300R Alpha confocal microscope, equipped with a 488 nm Ar laser (power density ¼ 0.06 mW cm2), 100x objective (0.80 NA), 1800 lines mm1 grating, and integration time of 60 s. Raman images were obtained by point-by-point mapping of a 50  50 mm surface area, with a pixel resolution of 200  200 nm, using a piezo-driven XYZ table.

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The spectra of pure materials were used as standards for image deconvolution. The contribution of cosmic rays was removed by subtracting the base line from each crude spectrum. The use of low power density (0.06 mW cm2) in the imaging process ensured prevention of any thermal degradation of the composites, and this was also confirmed by taking into account that the intensity of the Raman spectral peaks before and after exposition of the composite surface to the laser for an hour was similar. Infrared spectra were recorded using a Thermo Nicolet 5700 FTIR spectrometer operated in transmission and ATR mode in the 400 to 4000 cm1 range, with resolution of 4 cm1. The surface morphology of CoPc/GO, CoPc/r-GO and GO was investigated using a JEOL JSM-FEG 7401F SEM equipment. Images were recorded at accelerating voltage of 2 kV and different magnifications while scanning the surface. Chemical surface analyses of the nanocomposites were carried out by X-ray photoelectron spectroscopy (XPS), using a Ka X-ray photoelectron spectrometer (Thermo Fisher Scientific, UK), equipped with a hemispherical electron analyzer and an Al-Ka microfocused monochromatized source with resolution of 0.1 eV. Survey (full-range) and high-resolution spectra for carbon and cobalt were acquired using a spot size of 400 mm2 and pass energy of 200 and 50 eV, respectively. The data were analyzed using the Thermo Avantage Software (Version 5.921). Energy dispersive X-ray fluorescence measurements, EDXRF, were carried out using an EDX720 instrument from Shimadzu, equipped with a X-ray tube with Rh target and a Si(Li) detector, working at 5e15 kV. A calibration curve prepared by changing the weight percentage of CoPc in boric acid was employed to determine the mass of CoPc in the composite, by carefully dispersing the CoPc/ GO composite in that matrix and measuring the corresponding X ray fluorescence signals. Atomic Force Microcopy (AFM) images were obtained using a LensAFM equipment with Nanosurf C3000 controller, operating at intermittent contact AFM using TAP e Al G 190 cantilevers (Budget Sensors). The tip radius was less than 10 nm.

2.4. Electrochemical measurements Electrochemical measurements were performed with an Autolab PGSTAT-208 and PGSTAT-128N working station (Metrohm). A conventional three-electrodes cell was used including glassy carbon electrode (GCE), Ag/AgCl (sat. KCl) and platinum wire respectively as working, reference and counter electrode. After each experiment, the GCE electrode was cleaned by polishing with alumina slurry (0.05 mm) and further sonication for 10 min. All electrochemical experiments were carried out at room temperature. The catalytic activity of the samples was evaluated by cyclic voltammetry (CV) and rotation ring-disk electrode (RRDE) voltammetry techniques in N2-saturated or O2-saturated 0.1 mol L1 KNO3 solution, at a scan rate of 50 mV s1. The catalyst dispersion was prepared by mixing 5 mg of the catalyst and 50 mL of Nafion solution diluted in a mixture of 500 mL of water and 500 mL of ethanol followed by ultrasonication for 1 h. Then, modified GC electrodes were prepared by transferring 10 mL of the catalyst dispersion onto the surface and letting it dry in air. The mass loading of all catalysts on the electrode was 0.2 mg cm2. Electrochemical reduction of the composite casted on the GCE was performed in N2 saturated 0.1 mol L1 Na2SO4 solution by carrying out 20 CV cycles in the 0.2 V to 1.3 V range, at a scan rate of 5 mV s1. Linear sweep voltammetry (LSV) experiments were performed in O2-saturated 0.1 mol L1 KNO3 solution, at a scan rate of 5 mV s1 and rotation rates of 100, 225, 400, 625, 900 and 1600 rpm. The amperometric test was conducted at a constant potential of 0.25 V.

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3. Results and discussion 3.1. Synthesis of the CoPc/rGO composite material Metallophthalocyanines are very well-known for their very low solubility in all solvents such that their dispersion is possible only using high shearing techniques. Even considering such high energy conditions, more or less large agglomerates are present such that the interaction with another material generates quite coarse composites. Such expectation matched with the materials obtained by interaction with GO, where the morphology was strongly dependent on the CoPc particles size distribution in the dispersion. However, our goal was the preparation of a CoPc/GO composite based on non-covalent binding of CoPc macrocycle on GO through p-p interactions (Fig. 1), thus preserving the intrinsic properties of constituents, while imparting enhanced ORR electrocatalytic properties to the composite material. Accordingly, several solvents were tested in order to get the finest dispersion as possible of the macrocyclic compound. Among them, DMF was found to be the best one leading to a dark blue colored solution upon processing using an ultrasonic bath. This solution was then mixed with a GO suspension in water to get a black solid in equilibrium with a blue CoPc solution. The resultant precipitate was separated by centrifugation and mixed with suitable amounts of Nafion to get an ink for preparation of modified glassy carbon electrodes (GCEs). Then, the above described process was repeated changing the proportion of CoPc and GO in order to optimize the electrocatalytic activity for the oxygen reduction reaction (ORR). Such optimization process is described in detail in the supporting information (Figs. S1 and S2). The presence of both components in the composite was initially confirmed by comparing the UV-vis spectrum (Fig. 2a) of GO (aqueous), CoPc (in DMF) and CoPc/GO composite (in DMF þ water) in the 200e900 nm range. CoPc displayed the characteristic Q (at 658 nm) and B (328 nm) bands of phthalocyanine p-p* HOMOLUMO transitions, where the weak satellite band at 594 nm was attributed to h-type aggregation. The spectrum of GO presented an absorption band at 235 nm corresponding to the p-p* electronic transition. The absorption bands of CoPc were relatively weak and red shifted in the composite spectra, which may indicate a strong p-p interaction between GO and CoPc with concurrent decrease in the HOMO-LUMO gap in the phthalocyanine [38]. FTIR spectrum also shows the characteristic CoPc and GO peaks in the composite (Fig. 2b). The broad peak around 3420 cm1 in the GO spectrum was attributed to OeH stretching of adsorbed water molecules and corresponding oxygenated defects. Vibrational modes associated with other oxygenated defects such as C]O

Fig. 1. Scheme showing the preparation of the CoPc/GO composite and the p-stacking interaction of CoPc and GO.

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Fig. 2. UV-Vis (a) and Infrared (b) spectra of GO (in black), CoPc (in blue) and CoPc/GO (in yellow) composite. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

stretching in carbonyl or carboxylic acid, CeOeC stretching in epoxy and CeOH stretching were observed at around 1700 cm1, 1032 cm1 and 1400 cm1, respectively [39]. The peak at 1580 cm1 was assigned to skeletal modes of graphitic domains. The representative peaks of CoPc at 1089 cm1, 1121 cm1, 1425 cm1, 1522 cm1 and 1600 cm1 were assigned to in plane bending of CeH, symmetric vibration of isoindole, CeC stretching of isoindole, CeN]C and C]C stretching modes of benzene, respectively. Vibrational bands of oxygenated defects in GO appeared as intense peaks in the IR spectrum of the composite, whereas CoPc fingerprint peaks appeared slightly red shifted because of the p-p interaction with GO. The ink prepared with the best GO/CoPc composite material was drop-casted on the GCE surface and electrochemically reduced by performing 20 CV cycles in the 0.2 to 1.3 V range, in nitrogen saturated 0.1 mol L1 Na2SO4 solution (pH~7), in an attempt to get reduced graphene oxide (rGO) and rGO/CoPc, a composite with improved electrocatalytic activity. The CVs shown in Fig. 3 exhibit much larger capacitive current as a function of the number of redox cycles, as expected for increasing electric conductivity and surface area as GO is converted into rGO. Enhancement in surface area of the composite after electrochemical reduction was further confirmed by the increased electrochemical surface area of CoPc/ rGO as compared with CoPc/GO (calculation details given in supplementary information in Fig. S3). Moreover, the reduction current at 1.3 V decreases with increasing the number of CV cycles, and this can be attributed to the reduction of oxygenated groups in GO at negative potentials [40]. Additionally, the evolution of a reversible pair of peaks at 0.6 V associated with the Co(II)Pc/Co(I)Pc redox process [41,42] is also consistent with the occurrence of that process. In addition, such Faradaic process generates the negatively charged Co(I)Pc species exhibiting much higher solubility, that can eventually help breaking the aggregates into smaller ones under the influence of negative electrode potentials. This assumption was shown to be true, as demonstrated in the next sections.

Fig. 3. 20 CV cycles of CoPc/GO modified GCE electrode in the 0.2 to -1.3 V range in nitrogen saturated 0.1 mol L1 Na2SO4 solution (pH~7), at scan rate of 5 mV s1.

3.2. Microstructural characterization of the composite The topography of GO/CoPc and rGO/CoPc composite films was investigated by AFM in order to confirm the possible disaggregation of the CoPc microaggregates into smaller nanoclusters as a result of the electrochemical reduction. The AFM images and the height profile (Figs. S4a and S4b) were consistent with the presence of approximately 1 nm thick flat sheets as expected for GO monolayers [43] dispersed in the respective ink. A contrasting profile was found for the GO/CoPc composite (Fig. 4a), whose sheets are larger (10 mm) and with increased height (Fig. S5a), whereas such dimensions were reduced to nanometer scale after 20 redox process in the 1.3 to 0.2 V range (Fig. S5b), confirming the dramatic

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Fig. 4. AFM images of CoPc/GO (a) and CoPc/rGO (b) composites immobilized on mica.

change induced by the electrochemical processing (Fig. 4b). In this case, micrometer large particles as well as many nanometric sized particles are clearly seen in the image. The enhanced dispersion of CoPc agglomerates in the film and the formation of CoPc/rGO nanoclusters as a consequence of the electrochemical reduction are expected to change the morphology of the composite on the electrode surface. At this point it is important to say that GO forms relatively smooth films on GCE, as shown in the SEM image of Fig. S6, such that changes in surface morphology, as those clearly visible in Fig. 5a and b, can be associated with the presence of CoPc agglomerates, as in the previous case. The SEM image of the CoPc/GO composite film on GCE surface seems to reveal many large elongated particles deposited on

smaller particles embedded in GO layers, probably as a result of a kind of segregation taking place during the film formation process, which is not present in the material after electrochemical processing. In addition, the CoPc/rGO film apparently was densified and the large elongated particles were broken into smaller ones, making the surface smoother. The presence of CoPc nanoclusters on graphene sheets was further confirmed by transmission electron microscopy image of the CoPc/rGO, as depicted in Fig. S7 as dark spots. A more quantitative information about the CoPc particles size before and after the electrochemical processing was obtained by analyzing the SEM images taken at 15 different locations of the composite film. By inspection of the distribution map of particles

Fig. 5. SEM images of CoPc/GO (a) and CoPc/rGO composite deposited onto GCE surface, and respective average particle size distribution estimated from the SEM images of 15 different locations of CoPc/GO (c) and CoPc/rGO modified GC electrodes.

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size depicted in Fig. 5c and d it is clearly seen that the number of large CoPc aggregates was reduced and that of smaller particles was enhanced in the composite after electrochemical reduction. This study seems to confirm that large CoPc aggregates can have their size reduced upon electrochemical processing, leading to improved dispersion of the macrocycle in the GO support. To get further insight on the microstructure, the distribution of CoPc and GO phases, and eventual chemical interactions taking place in the composite, the materials were investigated by Raman spectroscopy and confocal Raman imaging. Raman spectrum of GO is characterized by two broad peaks centered at 1347 cm1 and 1604 cm1 (Fig. 6a), respectively, corresponding to the D band associated with out of plane vibrations of sp3 carbons, and the G band assigned to in-plane vibrational modes of sp2 carbon network. The ratio of D and G bands intensities (ID/IG), generally used as a measure of the average amount of defects in the graphene sheets, was estimated to be 1.13 [44] suggesting a quite large concentration of defective sites. The composite material presented all phthalocyanine Raman peaks superimposed to the broad GO peaks, confirming that both materials preserved their chemical structure. Among them, the most intense ones were found at 1540 cm1 (CeNeC bridge vibration), 1342 cm1 (CeC stretching mode of pyrrolic ring), and 1150 cm1. Moreover, the GO G-band in the composites experienced a red shift to 1594 cm1, which was ascribed to p-p interactions of CoPc and GO. More interestingly, the change in the ID/ IG ratio from 1.15 to 1.07 indicates that the density of GO defects in

the composite was reduced by the electrochemical processing, as expected for an increase in the sp2 domains consistent with the formation of reduced graphene oxide [45]. In order to visualize the influence of the electrochemical processing on the distribution of GO and CoPc in the composite microstructure, they were imaged by confocal Raman microscopy. The imaging process consists in collecting Raman spectra for each pixel (200 nm  200 nm) while scanning a selected area (50 mm  50 mm) of the composite material film, hence providing a chemical map of the surface. The image is generated by weighing each pixel according to the contribution of the components, which was color coded accordingly (green for CoPc and red for GO) to clearly present the composition (Fig. 6b and c). The CoPc/GO material exhibits a large amount of large phthalocyanine particles, that become significantly smaller upon electrochemical processing, confirming the previous results obtained by AFM and scanning electron microscopy. The image shown in Fig. 6c also reveals bigger dark regions associated with lower Raman intensity, corresponding to uneven height distributions in CoPc/GO film because of larger aggregates that become smaller upon electrochemical processing in Fig. 6b depicting a homogeneous distribution of CoPc on rGO support. More interestingly, there was an extraordinary increase in the amount of small dark spots, consistent with the presence of small amounts of CoPc deposited on GO, which can be associated with the dispersion of small aggregates throughout the carbon material. This is a clear evidence that the reduction of GO/CoPc composite at

Fig. 6. Raman spectra of GO (in red), CoPc (in green), CoPc/GO (in blue) and CoPc/r-GO (in purple) deposited on GCE (a). Confocal Raman image of CoPc/rGO (b) and CoPc/GO (c) deposited onto the GCE surface. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

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potentials as negative as 1.3 V can break CoPc aggregates, generating much smaller nanoparticles. Such structures can bind strongly on the GO surface by p-stacking interactions, as also evidenced in the deconvoluted confocal Raman images (Figs. S8 and S9), generating more homogeneous composite materials. 3.3. Structure and elemental analysis of composites The simultaneous formation of rGO during the electrochemical processing was demonstrated by monitoring the average concentration of oxygenated defects by comparing the XPS analysis of the CoPc/GO composite before and after the electrochemical processing. Additionally, XPS spectroscopy can assess the degree of purity and can reveal any chemical change that may take place during the composite formation. The survey XPS spectrum of CoPc, GO, CoPc/ GO and CoPc/rGO are shown in Fig. S10. The presence of O1s (associated with the oxygenated defects and adsorbed water in GO) and N1s (associated with CoPc) peaks in the composites spectra at 532 eV and 399 eV with no shift in the binding energy relative to the peaks of pure GO and CoPc, respectively, confirm the formation of composites without any change in GO and CoPc structure. Moreover, the oxygen content (intensity of O1s peak) was significantly reduced in the electrochemically processed composite, as expected for the removal of oxygenated defects in order to generate

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rGO. Additionally, the presence of Co2p doublet indicates unequivocally the presence of CoPc macrocycle. On the other hand, the intensity of deconvoluted C1s spectra of CoPc/GO and CoPc/rGO (Fig. 7a and b), respectively, relative to the O1s peak clearly revealed the disappearance of oxygenated defects such as carboxylic acid, carbonyl, hydroxyl and epoxy groups, after the electrochemical reduction. Moreover, the relative amount of graphitic sp2 domains was significantly increased in the CoPc/rGO composite as can be observed in Table S1, where the atomic percentage of defects and graphitic domains in the composite before and after electrochemical processing are listed, confirming the electrochemical conversion of GO in rGO. The stability of CoPc during the electrochemical reduction process was further validated by the presence of deconvoluted XPS spectrum associated with NeC]N bridge bond in the phthalocyanine macrocycle in CoPc/GO and CoPc/rGO. It can be noted that the area under the peak assigned to NeC]N remained similar, confirming that CoPc molecules do not degrade under the high negative potentials of the electrochemical processing. In addition, it is evident from the similar Co2p3/2 and Co2p1/2 peak position at binding energy of 780.4 eV and 795.6 eV, respectively, that CoPc is present as the Co(II) species in CoPc, CoPc/GO and CoPc/rGO (Fig. 7c), demonstrating the macrocyclic species is

Fig. 7. C1s spectra of CoPc/GO (a) and CoPc/r-GO (b). Co2p spectra of CoPc (in blue), CoPc/GO (dark yellow) and CoPc/rGO (in red) (a) and deconvolution of the Co2p3/2 peak (d). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

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regenerated after eventual reduction to the negatively charged CoIPc species. A shoulder in the Co2p spectra of CoPc/rGO can be also noticed, which after deconvolution gives a peak at 781.7 eV (Fig. 7d) associated to the superficial oxidation of CoPc [46,47]. Such partial charge development on CoPc can be consequence of improved interaction with the rGO plane [38]. Elemental analysis of the composite by XPS was further complemented by Energy Dispersive X-ray fluorescence spectroscopy (EDXFS) to estimate the percentage of CoPc in the composite. Details of the calculations and EDXFS calibration spectra have been given in SI (Fig. S11). From the spectra, the cobalt wt.% in the CoPc/ GO composite was determined to be 1.23%, from which the amount of CoPc was assessed to be 36.5 wt% in the composite. The value is slightly less than the one obtained by XPS and this can be explained by taking into account that EDXFS analysis is performed over a larger area and gives an average value, whereas XPS provides elemental composition in a much more localized spot. The change in the structural orientation after the composite formation and its further electrochemical reduction were investigated by X-ray diffraction (XRD) at room temperature (Fig. S12). Notably, the characteristic carbon peak (001) in GO appeared at 2q ¼ 9
and is associated with an interplanar distance of 0.96 nm [48]. Such peak almost disappeared after the composite formation with CoPc, confirming the largely amorphous nature of the CoPc/ GO composite. Nonetheless, diffraction peaks from CoPc aggregates can still be seen in the composite spectra. After the electrochemical reduction, new broad diffraction peaks appeared at 2q ¼ 23
, corresponding to the (002) plane of graphitic carbon in reduced GO with an interlayer distance of 0.4 nm [49], thus confirming the

formation of the CoPc/rGO composite. 3.4. Electrochemical characterization of composite materials for ORR activity ORR activity of the GO, CoPc and the CoPc/GO and CoPc/rGO composites was investigated by comparing the cyclic voltammograms in O2 saturated and N2 saturated 0.1 mol L1 KNO3 solution, at scan rate of 50 mV s1 (Fig. 8). CoPc exhibits an irreversible reduction wave at Ecp ¼ 0.4 V and a similar behavior was noticed for the GO modified electrode. The CoPc/GO composite showed a more intense and narrow wave shifted to less negative potentials (0.19 V), as expected for a significantly larger ORR catalytic activity. Interestingly enough, the current intensity for ORR was increased and shifted further to 0.14 V in the CoPc/rGO modified electrode, which also presented a reversible pair of waves characteristic of CoII/IPc process at 0.60 V, as expected for the larger degree of dispersion and stronger p-stacking interation with the rGO substrate. In other words, the voltammograms presented in Fig. 8 clearly demonstrate that the composite materials, especially after electrochemical processing, are better ORR catalysts since the modified electrodes showed about 3-fold enhancement in cathodic current and around 270 mV shift to less negative potentials in comparison to pure GO and CoPc. Such improvement in ORR activity of the composites is a consequence of the strong p-p interactions of GO and especially rGO with CoPc. Hence, much smaller nanometer sized particles strongly bonded on the surface are formed, as demonstrated by confocal Raman microscopy, that must be acting

Fig. 8. Comparison of CVs of (a) CoPc, (b) GO, (c) CoPc/GO and (d) CoPc/rGO modified glassy carbon electrodes in O2 saturated (in red) and N2 saturated (in black) 0.1 mol L1 KNO3 solution, at a scan rate 50 mV s1. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

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as ORR electrocatalytic sites. Moreover, the more than twice as large cathodic current as compared to CoPc and GO alone suggests a very fast heterogeneous electron transfer kinetics and a possible change in the oxygen reduction mechanism from a bielectronic to a tetraelectronic process. This may be consequence of the generation of a new more efficient electrocatalyst, resulting from the electrochemically induced formation of smaller CoPc nanoparticles concomitantly with of rGO, as well as their dispersion and enhanced synergic interactions, generating larger amounts of catalytic active sites. To get deeper insight on the mechanism of ORR catalytic activity, rotating ring-disk electrode (RRDE) voltammetry experiments were carried out in an oxygen saturated 0.1 mol L1 KNO3 solution, at rotation rates in the 100e1600 rpm range, and the polarization curves at a rotation rate of 1600 rpm are compared in Fig. 9a. In addition to CoPc/rGO and CoPc/GO composites and their constituents, RRDE measurements were also performed with a commercial Pt/C (20 wt% platinum in Vulcan carbon) catalyst as a control experiment. By analyzing the voltammograms shown in Fig. 9a, it is evident that the onset potential (Eonset) and half-wave potential (E1/ 2) values became less negative in the sequence GO, CoPc, CoPc/GO and CoPc/rGO. Furthermore, a large increase in the limiting current can be observed from CoPc and GO to CoPc/GO as expected for an increase in the surface area, and then to CoPc/rGO, which exhibits twice as large limiting current for reduction of dioxygen, thus demonstrating a synergic improvement of the ORR catalytic activity of the composite materials. The comercial Pt/C catalyst presented the less negative onset potential, even though a sharper increase in the cathodic current was noticed for the CoPc/rGO modified

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electrode for the same amount of catalyst loading. The similar JD values strongly suggests a change from a bi-electronic to a tetraelectronic ORR process for the CoPc/rGO modified electrode. The number of electrons involved in the ORR process is very important to further evaluate the catalytic performance. The ORR process can proceed through a 2e reduction pathway, generating H2O2 as intermediate species, or a 4e pathway producing water, which is the desired process for energy production. From the disk polarization curves recorded at different rotation rates, the number of electrons transferred per oxygen was calculated using the KouteckyLevich (K-L) equation (explained in supporting information). A plot of reciprocal of J vs reciprocal of square root of u (angular rotation speed of the electrode), known as the K-L plot, was obtained as a function of the polarization potential (Figs. S13, S14, S15 and S16) for each catalyst, and the number of electrons transferred in the ORR process was calculated from the slope of the linear regression. The number of electrons transferred as a function of the polarization potential for the relevant electrode materials is shown in Fig. 9b. As expected, Pt/C and CoPc/rGO exhibited very similar behavior and consistent with the 4e reduction of molecular oxygen into water. In contrast, the ORR process largely proceeded via 2e pathway at the CoPc modified electrode, while a mixture of the two pathways was noticed for both GO and CoPc/GO composite electrodes, with a larger contribution from the 4e mechanism for the latter one. The superior catalytic activity of the CoPc/rGO among others was further demonstrated by the Tafel plot analysis, as given in the supporting information (Fig. S17). A Tafel slope of 95 mV/dec was obtained for CoPc/rGO and this value is smaller than those found for other catalysts, and very similar to the

Fig. 9. Comparison of linear sweep voltammograms in oxygen saturated 0.1 mol L1 KNO3 solution at a rotation rate of 1600 rpm for GCE modified with CoPc, GO, CoPc/GO, CoPc/ rGO and Pt/C (a), average number of electrons transferred in the ORR process (b), and percentage of H2O2 generated in the ORR process as a function of potential (c). Scan rate 5 mV s1.

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A. Kumar et al. / Electrochimica Acta 326 (2019) 134984

corresponding one for the commercial Pt/C catalyst (91 mV/dec), in agreement with a previously reported work in neutral medium [50]. The tetraelectronic reduction of oxygen into water further demonstrates the strong synergic interaction of CoPc and rGO, leading to the conversion from a two-electron to a four-electron ORR process, as well as the advantage of the proposed electrochemical processing to generate more efficient composite electrocatalysts. Such a faster and enhanced ORR activity of CoPc/rGO is a consequence of improved dispersion and strong p-stacking interaction of CoPc nanoparticles with the rGO surface, causing increased electronic effects in CoPc which activate the composite material for the tetra-electronic reduction of dioxygen molecules directly to water. The percentage of H2O2 generated during the ORR process for each electrocatalyst was also calculated from the RRDE polarization curves (equation 3 in supporting information) at different polarization potential values, as shown in Fig. 9c. The percentage of peroxide generated was below 5% on the CoPc/rGO modified electrode, similar to the value obtained for the commercial Pt/C catalyst, which is in agreement with the 4e ORR electrocatalytic activity of these materials. 3.5. Methanol tolerance and long term stability of CoPc/reduced GO composite For application of the CoPc/rGO electrocatalyst in devices such as methanol fuel cells, its long term stability and tolerance to poisoning effects are also a necessary requirements in addition to high ORR catalytic activity. The stability of the ORR activity of the

catalysts was investigated by performing amperometric measurements using a rotating disk electrode polarized at 0.25 V with a rotation speed of 1600 rpm in oxygen saturated 0.1 mol L1 KNO3 solution for 10 h. The corresponding amperometric responses of CoPc/rGO and Pt/C electrodes in O2 saturated 0.1 mol L1 KNO3 solution are shown in Fig. 10a. The response for the CoPc/rGO modified electrode showed to be quite stable at 0.25 V and a decrease of only 4% in the cathodic current was observed, whereas a decrease of up to 29% was measured for the commercial Pt/C catalyst. The resistance of the composite material for possible poisoning by methanol crossover in the fuel cell was also evaluated. Accordingly, similar amperometric measurements were repeated by spiking methanol in the solution and the results are shown in Fig. 10b. The addition of methanol (3 mol L1) increased momentaneously the current of the CoPc/rGO modified electrode generating a spike, but the current returned to the initial value and remained more or less constant decreasing slowly as a function of time up to 6000 s (only 2% decrease was observed). In contrast, the addition of the same amount of methanol decreased instantaneously the current for the Pt/C modified electrode to about half, with a further slow current increase reaching about 2/3 of the initial value. Clearly, the Pt/C electrode was strongly poisoned by methanol, whilst only a minor effect was caused to CoPc/rGO, demonstrating its exceptional robustness and catalytic activity for ORR at neutral conditions and room temperature, thus fulfilling the requirements for application in fuel cell devices. The long-term stability of the CoPc/rGO and Pt/C catalyst was further investigated by Accelerated Durability Test (ADT), in which

Fig. 10. Current-time amperometric response of CoPc/rGO (in red) and Pt/C (in black) modified GCE rotating at a speed of 1600 rpm in oxygen saturated 0.1 mol L1 KNO3 solution before (a) and after addition of methanol (final concentration ¼ 3 mol L1) at applied potential of -0.25 V (b). Comparison of polarization curves of CoPc/rGO(c) and Pt/C (d) modified GCE rotating at a speed of 1600 rpm in oxygen saturated 0.1 mol L1 KNO3 solution before and after 2000 CV cycles. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

A. Kumar et al. / Electrochimica Acta 326 (2019) 134984

2000 CV cycles in oxygen saturated 0.1 mol L1 KNO3 solution were performed. Changes in the polarization curves, at 1600 rpm, before and after the CV cycling, were then evaluated. Notably, as shown in Fig. 10c and d, the CoPc/rGO material exhibited better stability than Pt/C for the ORR process, as only 14 mV increase in potential at E1/2 was observed after 2000 CV cycles, as compared to 23 mV for Pt/C. Moreover, a very small decrease in JD was obtained after 2000 CV cycles in CoPc/rGO materials. Such superior ORR activity of the CoPc/rGO material makes it a potential candidate for the development of a real fuel cell device. 4. Conclusions We successfully achieved an inexpensive, simple and green synthesis of a CoPc/rGO electrocatalyst intended for ORR catalysis in fuel cell devices based on strong p-stacking interaction of a planar CoPc macrocycle on extended GO support. In our approach, the dispersion of CoPc on GO was improved through electrochemical reduction of the CoPc/GO composite, which induced the breakage of CoPc microaggregates into nanoclusters that were redistributed on the extended p-conjugated rGO surface. The structure, topography and morphology of the composite was thoroughly characterized by AFM, SEM and Raman microscopy techniques, which confirmed the breakage of CoPc microparticles into nanoparticles as a result of the electrochemical reduction and p-stacking on GO. Particularly, a chemical mapping of the composite surface before and after electrochemical reduction by confocal Raman imaging clearly established an improved impregnation of a CoPc phase on reduced graphene oxide phase. Spectroscopic characterization of the composite through Raman, UVVis, infrared and XPS confirmed the existence of strong p-p interaction between CoPc and GO, which was improved after the electrochemical reduction. The CoPc/rGO nanocomposite revealed excellent ORR activity demonstrated by high JD, E1/2 and Eonset associated with a tetraelectronic reduction of molecular oxygen into water, which was comparable to the Pt/C commercial catalyst. Moreover, a synergic improvement in ORR catalysis at the CoPc/rGO electrode was confirmed by a significant enhancement of JD, positive shifts in E1/2 and Eonset and negligible H2O2 production as compared to CoPc and GO modified electrodes. Such synergic effect in ORR activity at the CoPc/rGO electrode was correlated with an enhanced transfer of electronic density from rGO to CoPc, making it more reactive for the oxygen molecule. CoPc/rGO as a noble metalfree ORR electrocatalyst demonstrated much superior stability and tolerance to methanol poisoning as compared to the Pt/C catalyst, making it a potentially better alternative to the expensive noble metal based commercial ORR catalysts in fuel cell devices. Declaration of competing interest The authors declare no competing financial interest. Acknowledgment ~o Paulo The authors express their sincere gratitude to FAPESP (Sa State Research Foundation) for providing generous funding through projects (2018/08782-1), (2018/21489-1) and (2016-07461-1); and CNPq 401581/2016-0. Authors are also thankful to Central Analytica at the Institute of Chemistry-USP for letting us use the electron microscopy facility and Dr. Alceu T. S. Junior for his precious help in  M. Gonçalves and Alan Roge rio the EDXRF experiments. Josue Ferreira Lima thank National Council for Scientific and Technological Development (CNPq 402281/2013-6, and CNPq 141853/2015-8) for providing doctoral and postdoctoral fellowships, respectively.

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