Electroactive composites: PANI electrochemical synthesis with GO and rGO for structural carbon fiber coating

Progress in Organic Coatings 138 (2020) 105399

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Electroactive composites: PANI electrochemical synthesis with GO and rGO for structural carbon fiber coating Meriene Gandaraa,b, Emerson Sarmento Gonçalvesa,c,

T

⁎

a

Instituto Tecnológico da Aeronáutica, 12228-900, São José dos Campos, Brazil Instituto de Tecnologia Edson Mororó Moura, 55150-550, Belo Jardim, Brazil c Instituto de Aeronáutica e Espaço, 12228-904, São José dos Campos, Brazil b

A R T I C LE I N FO

A B S T R A C T

Keywords: Polyaniline electrosynthesis Graphene oxide Parallel reactions Electrochemical properties Surface treatment

The use of light structural, thermal and electronic materials is of interest to the aerospace industry. The objective of this paper is to research the most suitable methodology for the surface transformation of carbon fiber with polyaniline (PANI), graphene oxide (GO) and reduced graphene oxide (rGO), through PANI electrosynthesis and dripping of graphene oxides. Four conditions were proposed: (1) PANI electrosynthesis followed by GO dripping electrodeposition; (2) GO dripping followed by PANI electrosynthesis; (3) electrodeposition of PANI and GO from an acid solution of aniline and GO; and (4) electrodeposition of PANI and rGO from an acid solution of aniline and rGO. Morphology, structure, surface electrical resistivity and electrochemical properties were analyzed. Condition 1 showed the lowest electrochemical impedance and surface resistivity, specific capacitance and high current density and, therefore, is the condition that indicated a greater gain of electroactivity in the composite material.

1. Introduction Wang et al. proposed a flexible electrode, prepared by in situ electropolymerization of a polyaniline film on a graphene sheet [1]. Jiang et al. prepared GO films on FTO conductive glass electrodes. A nanocomposite of electrochemically reduced GO (ERGO) was obtained with PANI [2]. Ramezanzadeh proposed a nanocomposite material from polyaniline, graphene oxide and cerium nanoparticles added to the epoxy matrix. The excellent interaction of PANI nanofibers decorating cerium oxide coated GO sheets has created a corrosive inhibition barrier [3]. Ma et al. prepared a composite of PANI/GO/metal hydroxides (Co and Ni) by in situ polymerization with the potential to be used as a supercapacitor [4]. Fernández et al. studied the interaction and electrochemical properties of capacitance and impedance of rGO and PANI by electrodeposition in activated carbon fabric, carried out by separate steps of rGO after PANI [5]. Song et al. proposed a flexible hybrid paper electrode composed of polyaniline/graphene and bacterial cellulose reinforced with carbon fiber by way of chemical synthesis and evaluated the properties by electrochemical characterization with a potential for application in supercapacitors [6]. Chan et al. produced structural dielectric capacitors of graphene oxide film and carbon fiber indicating potential applications in smart vehicles and Unmanned Aerial Vehicles (UAV) [7].

⁎

There are several studies on the combination of polyaniline with graphene oxide and reduced graphene oxide for supercapacitor device applications to meet the new energy demand of new technologies. These low-density materials have high-performance electro-active properties that meet the advanced electronics of automotive, aerospace, power generation systems. Aerospace systems contain various electronic devices responsible for acquiring, transmitting, storing data, controlling devices and supplying power. It is in the aerospace industry's interest to use lightweight structural, thermal and electronic materials and incorporate these properties into the same material. Carbon fibers can be used instead of metallic materials due to their low density, mechanical properties and corrosion resistance [8]. Carbon fiber exhibits electrical conductivity compatible with that of semiconductors [9]. In order to enhance this property of composites, the presence of electric charge carriers is necessary. This can be done, for example, by employing intrinsic conducting polymers [8], which can be synthesized by electrochemical techniques. Polyaniline (PANI) is characterized by its ease of preparation, low cost, and its optical and electrical properties [10]. PANI can be synthesized by electrochemistry way. The electrochemical processes is of interest in innovative processes, for its ease of synthesis, control of electric potential properties and ability to produce thin films in the order of 20.10−9 m [11]. PANI electrosynthesis processes may produce

Corresponding author at: Instituto Tecnológico da Aeronáutica, 12228-900, São José dos Campos, Brazil. E-mail address: [email protected] (E.S. Gonçalves).

https://doi.org/10.1016/j.porgcoat.2019.105399 Received 9 June 2019; Received in revised form 14 October 2019; Accepted 15 October 2019 0300-9440/ © 2019 Elsevier B.V. All rights reserved.

Progress in Organic Coatings 138 (2020) 105399

M. Gandara and E.S. Gonçalves

Condition 2 (C2-G): GO (1.0 g L−1) was dripped on the fiber. The fiber was then dried at room temperature for 24 h and then the PANI electrosynthesis was carried out. Condition 3 (C3PGO): the electrosynthesis fromsuspension of 0.5 mol L−1 aniline solution with 1.0 mol L−1 sulfuric acid and GO 0.1 g L−1 on the carbon fiber was carried out. Condition 4 (C4rGO400): the rGO was first made by graphite oxide thermal treatment up to 400 °C, at a rate of 10 °C min−1. Then the rGO was mixed with 0.5 M aniline and sonicated for 2 h. Subsequently, 1.0 M sulfuric acid was added to this mixture and then preceded to electrosynthesis on the carbon fiber.

material with minimum specific mass, useful to materials of electronic devices and aerospace technologies [12]. PANI is also used as a metallic corrosion inhibiting agent by coating the electrosynthesized film [13]. Another material that may contribute to the improvement of mechanical and electrical properties in composites is graphene oxide (GO). Due to the high functionality of GO by groups such as carbonyl, carboxyl, epoxy, this material has a highly reactive surface. GO can anchor other organic compounds on its surface by electrostatic interactions, hydrogen bonding and Van der Waals forces. With this, the GO sheets can enhance the electroactive properties [14]. A small amount of GO can considerably increase the electrochemical capacity of PANI [15], as in migration processes as in electric charge accumulation, depending on the parameters electrochemical process. The functionality of its particles has potential reactivity and different degrees and forms of interaction with various polymers [16]. GO has high oxygen content, while the reduced graphene oxide (rGO) corresponds to a total or partially reduced structure, in which there is a decrease in the number of oxygenated groups [17]. After the chemical reduction process of graphene oxide, the hybridized carbon sp2, is available in abundance. It is widely used for ease of processing and relatively low cost but can lead to poor thermal and electrical performance due to oxidized carbon atoms. Oxygen groups may be partially reduced due to structural defects that generate vacancies and distortions in the rGO sheet [18]. The combination of carbon fiber with PANI/GO or PANI/rGO forms a supercapacitor material for aeronautical structures. It can be used in power systems to increase energy density and electrical power in a distributed way. In aircraft, for example, such materials can simultaneously contribute to the mechanical strength and transmission of electrical charge that can store and release charge and maintain mechanical integrity for possible applications in electrical panel and sensor connections [19]. The objective of this work is the comparison of different methodologies forcoating of aeronautical structural carbon fiber, through polyaniline electrosynthesis with the addition of graphene oxide, reduced graphene oxide, with high conductivity and relevant values of capacitance.

2.2. Characterization of PANI and GO/rGO electroactive materials The electrochemical impedance spectroscopy tests were performed in OCP, with potential amplitude of 0.010 V, frequency varying from 10,000 Hz down to 0.010 Hz, in a 0.1 mol L−1 KCl electrolyte. In addition, cyclic voltammetry characterization was performed from −1.05 up to 1.05 V in 0.1 mol L−1 H2SO4, using 0.010 V s−1 as the sweep rate. These measurements were performed using a potentiostat-galvanostat Metröhm Autolab PGSTAT 302, with data analyzer NOVA 1.11. The samples resistivities were obtained by a4 probes experiment, carried out on Jandel Model RM3000 equipment, with a continuous electric current set at 0.001A and a distance between the tips of 0.001 m at room temperature. SEM analyzes were performed using Zeiss 435 VPI equipment, with images obtained by secondary and backscattered electrons; FEG in Tescan Mira 3LM equipment; the AFM was performed in an Agilent 5420 SPM AC mode, 267 kHz frequency unit. The FT-IR was performed using a Perkin Elmer Frontier, by universal attenuated total reflectance (UATR) of diamond crystal with 32 scans. The XRD was generated by PAnalytical, wavelength of the incident ray, and was 0.1541 10−9 m with a scan of 5° ≤ 2Ɵ ≤ 90°. 3. Results 3.1. Cyclic voltammetry of samples preparation

2. Materials and methods

The current density of the first anodic peak of the third cycle for each sample was determined, according to Fig. 1. The first anodic peak refers to the process of oxidative polymerization leading to the formation of emeraldine salt. According to Wei et al., by the fardaic transfer theory, the peak current is directly proportional to the concentration of the sample in irreversible and also reversible effects [20]. Table 1 presents the potential and peak current data for each sample of the current density calculation obtained from the analysis of Fig. 1. The current density is related to the growth rate of PANI on fiber.

All polyaniline (PANI) from this work was obtained by electrosynthesis on structural carbon fiber. A suspension of graphene oxide was added to polyaniline samples obtained under three different conditions. In addition, thermally reduced graphene oxide (rGO) was used in a fourth condition. The precursor graphite oxide of GO and rGO was obtained by the Hummers method. Its sonication in aqueous media led to the formation of GO; its pyrolysis at 400 °C in the inert atmosphere of a tubular furnace led to the formation of rGO. 2.1. PANI electrosynthesis PANI deposition was performed by electrosynthesis in a 3-electrode cell coupled to a Metrohm Autolab PGSTAT 302 potentiostat-galvanostat from 0.5 mol L−1 solution of aniline distilled with 1 mol L−1 sulfuric acid. The working electrode for PANI deposition was a 5 cm cable of Hexcel 282-50 GRAPHITE "plain-weave" carbon fiber with 3000 filaments of 7.1. 10−6 m in diameter; the auxiliary electrode used was platinum. A potential window was also applied in 3 cycles of −0.2 to 1.05 V vs Ag/AgCl at a rate of 50. 10−3 V s−1. The final synthesis potential in all cases was that presented in the first anodic peak (referring to the oxidative polymerization of PANI on the fiber) of the third cycle. Samples were produced only from PANI electrodeposited on the carbon fiber. The concentration of the aniline solution and the parameters of the electrosynthesis process were repeated for all conditions. Condition 1 (C1-P): first the electrosynthesis of PANI on the fiber. This covered fiber was dried at room temperature for 24 h. Then GO (1.0 g L−1) was dripped on the fiber with the electrodeposited PANI.

Fig. 1. Detail on the process end of voltammetric electrosynthesis: C1-P, C2-G, C3PGO, C4rGO400. 2

Progress in Organic Coatings 138 (2020) 105399

M. Gandara and E.S. Gonçalves

Table 1 Potential applied (mV) and current density (mA cm−2). Sample

Potential applied (10−3 V)

Current Density (10−3 A cm−2)

C1-P C2-G C3PGO C4rGO400

278.84 293.43 239.72 264.13

0.43 0.09 0.29 0.08

Fig. 3. Potential applied (10−3 V) and current density (mA cm−2) for all proposed conditions – Tafel Plots. Table 2 Potential applied (mV) and current density (μA. cm−2), Tafel Plots. Sample

Potential applied (10−3 V vs Ag/ AgCl)

Current Density (10−6A cm−2)

C1-P C2-G C3PGO C4rGO400

−44.90 −76.06 −47.89 −67.50

2.46 1.36 2.18 1.70

Fig. 2. Reproducibility of condition 1 material.

the equilibrium exchange current and the equilibrium potential, according to data obtained by the Tafel plots analysis. The oxidation process of aniline is thermodynamically favored in less negative potentials of oxidative polymerization. Then, in this case, the sample C1-P presents a state of equilibrium more favorable to the electrosynthesis of esmeraldine than the others. This behavior is confirmed in the non-equilibrium condition shown in Fig. 1, where its anodic peak current is the highest of the proposed conditions. The peak currents are related to kinetic favoring, and the values are higher for the C1-P, both at the point of greatest current (in the cell) and in the equilibrium condition (transfer current on the carbon fiber electrode). The deposition potential is close to −45 10−3 V and the exchange current density is greater than 210−6 A cm−2. The differences between C1-P and C3PGO are discrete at equilibrium, but well defined at the peak relative to emeraldine polymerization. This is due to the fact that the equilibrium around the carbon fiber electrode occurs between the same species. However, there are serious kinetic obstacles, which negatively interfere in the anodic peak current. It may be due to a previous formation of polyaniline around the particles of GO that compete with the electrode process.

The C2-G condition, in which GO was dripped onto the fiber before electrodeposition, showed a lower current density related to the lower PANI growth, indicating a possible electrical deactivation of the fiber surface by the GO. It is possible that the dielectric and oxygen-filled GO can interact strongly with the surface of the carbon fiber, hindering access of protonated aniline to the fiber. In this case, groups that, for the C1-P sample, would be available to interact with aniline or PANI groups, are engaged in acting as anchoring points for GO on the carbon fiber. In fact, an inactive fiber coating disables it from actively participating in an electrochemical circuit in order to promote processes of growth of polymer films by this technique. However, the fact of exposing this fiber with such a coating indicates the possibility of a high affinity of the fiber with the GO. The possible sites reactive to PANI synthesis become unavailable, generating a low polymerization current. This data can be confirmed in Fig. 9-B SEM. The higher current density can be indicated for the C1-P condition, where the PANI electrosynthesis is performed before the GO dripping. The C1-P synthesis voltammogram presents the highest current density and a highly reproducible material, which can be observed in Fig. 2, with a mean current density of (0.43 ± 0.01) 10−3 cm−2. The C3PGO condition presented the second highest value of current associated to the polymerization process, indicating the same order of magnitude of the growth rate of C1-P. The direct use of reduced graphene oxide (rGO) suspended in an acid solution of aniline was not effective, as indicated by a current of PANI growth peak lower than in the other proposed conditions. In this case, the growth of PANI on fiber was not very pronounced and also on the rGO particles that covered the fiber.

3.2.2. Morphological analysis of kinetic competition The conditions C3PGO and C4rGO400 presented some difficulties related to the process of electrosynthesis. There was precipitation of agglomerated particles of the aniline-GO solution adhesion of particles to the auxiliary electrode, reducing its available area and compromising the transport of charge between electrodes in an irregular way. The difficulty of PANI electrosynthesis using GO or rGO mixed with the electrolytic solution is due to the previous chemical interaction of these components with the solution of aniline and sulfuric acid. Precipitation occurred in the electrolytic cell, which may be a result of a possible former PANI chemical synthesis [21,22]. In the chemical polymerization reaction of the aniline the concentration of oxidizing agent and the aniline/oxidant molar ratio exerts influence on the polymerization oxidation rate [21]. In this case, the GO

3.2. PANI growth assessment 3.2.1. Tafel plots Fig. 3 represents the Tafel plot of the four proposed conditions. It shows that the values of the exchange current density are directly related to the kinetic equation of PANI growth. In Table 2, we highlight 3

Progress in Organic Coatings 138 (2020) 105399

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Fig. 4. Morphology of the GO solution precipitate (0.1 g L−1) + aniline (0.5 M) + H2SO4 (1 M), magnification of 1500× and 15000×.

Fig. 5. Morphology of the precipitate of the rGO solution (0.1 g L−1) + aniline (0.5 M) + H2SO4 (1 M), magnification of 1000×.

synthesis competes with that electrochemical because the acid that collaborates with the protonation of PANI and the GO is itself available as an oxidizing agent. That is, a heterogeneous non-electrochemical catalytic process can naturally compete with the process at the electrode. An aspect not less important is the formation of precipitates that compromise the area of the auxiliary electrode, further decreasing the profile of the faradaic current of the polymerization process. Also, Table 1 shows the lower values of current density, implying a less pronounced growth of PANI for these electrosynthesis conditions (C3 and C4), due to competition within situ chemical polymerization. GO may be a catalyst for chemical polymerization of PANI even after C1-P electrosynthesis, providing a possible (albeit partial) reduction of GO. The adsorption of aniline molecules into GO particles occurs due to the oxygen-rich functional groups interacting with the aniline on the surface and at the edges of the structure, together with the effect of π–π interactions between PANI and GO, creating nucleation during polymerization [27]. In the case of rGO, previously reduced GO, the elimination of part of the oxygenated groups makes it hydrophobic and tends to minimize contact surface area, by structural modifications, minimizing the adsorption of possible dispersed oligomers and system moisture [27,28]. Fig. 5 shows a different morphology of the interaction of rGO with the acid solution of aniline with respect to GO, due to differences in the degree of oxidation that these graphene derivatives provide for the formation of PANI oligomers. According to Almeida et al., the morphology of rGO consists of an aspect of wrinkled leaves that, when mixing the PANI, forms a composite of fine and rough morphology [29].

may have acted as an oxidizing agent. GO has been used as a heterogeneous catalyst for various oxidation reactions, and efficient results have been obtained in the preparation of aldehydes or ketones, by oxidation of alcohols into acid, and also of various aldehydes and primary amines [22]. In the example of aniline polymerization with GO, Zheng et al. produced nanocomposites of graphene oxide and polyaniline and obtained a PANI/GO nanowire composite by in situ polymerization of aniline with ammonium persulfate in the presence of GO, for the construction of some chemical sensors and biosensors [23]. The precipitate of the solution was analyzed by Scanning Electron Microscopy (SEM) and the appearance of particles with pointed formations was observed (Fig. 4). These particles suggest the onset of PANI formation. The chemical oxidation of the aniline is of an autocatalytic character and occurs by heterogeneous nucleation, being the central oligomeric products and residual monomers of nucleation and the formational order of PANI micro and nano structures. [24]. Elongated or pointed forms are strongly associated with the onset of chemical synthesis of PANI. GO has a typical crumpled morphology; fine GO sheets are wrinkled and stacked on top of each other forming crystallites [25]. The interaction of PANI with GO reveals a morphology with rough appearance, typically porous and spongy, meaning that PANI was modified by GO [26]. The use of this solution for electrosynthesis purposes is something that can compromise not only the accuracy of the data, but also create a concurrent route of chemical synthesis in the solution. The chemical 4

Progress in Organic Coatings 138 (2020) 105399

M. Gandara and E.S. Gonçalves

they are not enough to act as amidic site formers. The same region suffers from the more pronounced bands of oxygenated groups in GO. The strong shift to 1742 cm−1 may be associated with the formation of cyclic groups [32] of GO with nitrogenated species of aniline. This is also related to the transport of charges and electroactivity, such as the formation of lactam and lactone groups. Probably, the GO offers greater electroactivity to the PANI (because of its amide bonds) than the rGO that is obtained thermally. In the case of rGO, there is less availability of carbonyl groups, reducing the reactivity to the amide coupling. In both C3 and C4 conditions, the composites obtained by electrochemical synthesis have very similar bands in the FT-IR spectrum, with the wave number very close to the sample of the acid solution of aniline and GO. The same functional groups were found, confirming that there was also chemical synthesis of PANI when adding GO and rGO to the electrolytic cell. The bands corresponding to the wave number 1496 cm−1 and 1589 cm−1 are benzenoid and quinoid groups, respectively. According to ZHAO and WANG, if there is confirmation of these groups, it is valid to affirm the obtaining of polyaniline [12]. Therefore, in both C3 and C4 conditions, PANI was obtained by precipitation (chemical conversion) and by electrosynthesis of the composites an (electrochemical conversion). Therefore, electrochemical performance of structures such as GO and rGO and the suppression of structural defects with the combination of conducting polymers by different methodologies is extensively studied in the literature. Conductive polymers can restore edge defects, interacting with vacancies and distorted structures. This structural addition increases the electrochemical performance of rGO and GO. Due to the pseudo-capacitance effect of these polymers and the electrochemical capacitance of graphene, the double-layer effect is added and the defective structures of the sp2 hybridized carbon atoms formed during the graphite exfoliation are partially repaired by π-conjugate interactions of conducting polymers due to the π–π interactions facilitating the movement of electrons in the formed composites [15].

Fig. 6. FT-IR: solution of aniline and sulfuric acid + GO and C3PGO.

The rGO acts as a less effective oxidizing agent compared to GO, which has more oxygen groups in its structure favoring the chemical stability and growth of PANI [28]. Therefore, in C3PGO and C4rGO400, the drop in current density in PANI electrosynthesis on carbon fiber is associated to the fall in availability of monomer in the electrolytic solution. Probably the solution is being chemically consumed by polymerization prior to the electrochemical process. 3.2.3. Qualitative chemical analysis of kinetic competition Figs. 6 and 7 depicts the FT-IR spectra in comparison with the precipitate of Figs. 5 and 6 (competitive chemical route) with the C3PGO composite (ideally main electrochemical route). Fig. 7 shows the comparison of the precipitate (chemical synthesis) with the composite (C4rGO400), showing the similarity between its bands described in Table 3. The asymmetric and symmetrical stretch of NeH in amine groups belonging to PANI may be related to the formation of an amine bond of aniline with carbonyl, one of the oxygen groups from graphene [15]. It is possible to observe a change of bands in the region; in NeH bonds these are noticed more intensely and more often as bands of 3408 cm−1 and 3396 cm−1. This indicates that, since GO has more oxygen groups than rGO, greater tensioning of PANI amine bonds occurs. This is a possible indication of greater electroactivity by the transfer of charges between these groups [15]. In the samples containing GO referring to condition 3 (C3PGO), the band 1711 cm−1 of C]O, oxygenated group in this characteristic case of GO is not pronounced in the sample spectrum of condition 4 (C4).It may be related to a lack of carbonyl groups in the rGO used [27], and so

3.3. Scanning electron microscopy (SEM) Fig. 8 shows the morphology presented by the micrographs of each material proposed under conditions C1-P, C2-G, C3PGO, C4rGO400. The coating of fiber by the conducting polymer is well demonstrated, being more homogeneous and uniform in condition 1 (deposition of PANI on the fiber before GO). In the other conditions, especially in condition 2, the non-uniform growth of PANI stands out and there is still a very pronounced PANI on PANI growth. This is due to the effect of the GO coating on the fiber surface previously discussed (Section 3.2). In condition 3 (mixture of PANI and GO in electrosynthesis), there was a slightly higher dispersion of PANI, but there was plate formation on the fiber instead of a thinner and more homogeneous film, which may have occurred through the electrostatic interaction between GO and PANI. Finally, under condition 4, rGO also demonstrated an effect similar to condition 2, and PANI did not cover the whole electrode. Obtained by FEG, Fig. 9, shows the cross-section of the carbon fiber and its rough and porous surface morphology. These irregularities may contribute positively as sites, both for the formation of conducting polymer, as well as for their physical or chemical accommodation. This is valid for PANI electrosynthesis and the C1-P sample. In addition, they increase the surface area of the substrate, making it more susceptible to the formation of regions of charge accumulation. In the case of sample C2-G, the GO can find important anchor points in the fiber. Referring to Fig. 10, it is possible to observe the cross-section of the carbon fiber with only electrodeposited PANI under the same electrosynthesis conditions used for the above conditions. It is possible to observe the formation of a thin layer of polymeric

Fig. 7. FT-IR: solution of aniline and sulfuric acid + rGO400 and C4rGO400. 5

6

1495 cm−1

1461 cm−1 1296 cm−1

1142 cm−1

–

1496 cm−1

1467 cm−1 1296 cm−1

1148 cm−1

1109 cm−1

797 cm

−1

791 cm

−1

1015 cm

1600 cm−1 1589 cm−1

1601 cm−1 1589 cm−1

1017 cm

1634 cm−1

1638 cm−1

−1

–

1711 cm−1

–

−1

3185 cm−1 –

3185 cm−1 –

3191 cm−1 1742 cm−1

–

1017 cm

– −1

1147 cm−1

1467 cm−1 1291 cm−1

1494 cm−1

1601 cm−1 1589 cm−1

1638 cm−1

3374 cm−1

3396 cm−1

3408 cm−1

Bands of Aniline + H2SO4 + rGO suspension

Bands of C3PGO composite

Bands of Aniline + H2SO4 + GO suspension

–

1011 cm

– −1

1129 cm−1

1467 cm−1 1327 cm−1

1496 cm−1

– 1568 cm−1

1636 cm−1

–

3184 cm−1 –

3369 cm−1

Bands of C4rGO composite

Asymmetric and symmetrical NeH stretch in amine groups belonging to PANI. It may also be related to OeH, since it comprises the bands of 3200–3500 cm−1 and the precursor group GO and rGO NH stretching of quinoid group inchain termination The band 1742 cm−1, refers to C]O of lactone, lactam, ester and anhydride groups. Lactams (cyclic amides) are very likely because of interaction with nitrogen from PANI. The band 1711 cm−1 refers to the elongation of C]O, the oxygen groups are related to the aromatic fixation plane (GO), but without structural node tension, such as lactams. (1) π–π stacking or (2) deformation of NeH in amides. However, the effects do not seem to separate themselves at that wavenumber. The fact that the wavenumber does not vary may be more related to the first effect. Deformation NeH. C]N stretching next to aromatic ring C]C, quinoid ring elongation (N]Q]N) related to conduction in PANI oligomers. It varies from1565 a 1595 cm−1 1495–1498 cm−1: the variation may be attributed to the benzenoid e quinoid ring. Transformation of amino groups into imino groups. C]C in the emeraldine salt, oxidation degree plus conductive of PANI Mode of vibration of eNH+ in the protonated emeraldine phase of PANI and delocalization of π electrons. can also be C]NeC stretching, vibration of the quinoid or benzenoid rings, in QBcisQ, QBB and BBQ units. A band of 1327 cm–1 is related to units of the quinoid group From 1170 to 1140 cm–1, the vibration mode Q]NH+eB or BeN+HeB, related to the charge transport property, is obtained. Polaronic semiquinoid structure, possiblymore conductive. From 1156 to 1124 cm–1, refers to the mode CeH, Q]N+HeB ou BeNHeB The 1109 cm−1 band may have been detached and related to the 1115 cm−1 band. π–π stacking occurs and hydrogen bonds with oxygen groups belonging to GO. Displaced bands of 1010 cm−1, corresponding to disubstituted CeH 1,4 or monosubstituted ring CeH, out-of-plane vibrations, given the head-tail coupling of the polymer chains

Attributions

Table 3 FT-IR bands: suspension of aniline, sulfuric acid and GO vs C3PGO composite; suspension of aniline, sulfuric acid and rGO400 vs C4rGO composite.

[35]

[34]

[33]

[30,33,34]

[34] [27,33,34]

[21]

[30] [30]

[15,33]

[27,32]

[30] [27,32]

[30,31]

References

M. Gandara and E.S. Gonçalves

Progress in Organic Coatings 138 (2020) 105399

Progress in Organic Coatings 138 (2020) 105399

M. Gandara and E.S. Gonçalves

Fig. 8. Micrographs with 5000× of magnification for (A) C1-P; (B) C2-G; and 1000x of magnification of the samples (C) C3PGO; and(D)C4rGO400.

to the increase of composite electroactivity. They are associated with high orientation (allowing greater conductive effect) and effects of charge balance between plates (enabling capacitive effect). PANI over fiber and PANI over PANI are competing formations. This is highlighted in the first peaks in the analysis of the diffractograms. This growth type occurs under all conditions, with different intensities, distribution along the fiber, homogeneity, morphologies due to interactions with GO and rGO, as observed in Fig. 9-C and -D. Fig. 11 refers to the morphology of carbon fiber under two conditions: pure and coated only with PANI. The growth morphology of PANI on carbon fiber is represented by a very thin film on its surface and between the monofilaments. This phenomenon connects them in order to guide the fiber structure without spaces between them; therefore, the growth of PANI over PANI also occurs in smaller quantities in a heterogeneous way in few regions of the fiber. This morphology of polymer growth on the electrode is important to evaluate the behavior of its structure. Their arrangement can influence the interaction between PANI, GO, rGO and carbon fiber, as well as the transport of charges and the conductivity of the material. Some additional aspects can be better evaluated by X-ray diffraction analysis.

3.4. X-ray diffraction (XRD) Fig. 9. Micrographs with 13000x magnification of carbon fiber (CF).

The arrangement of the polymer chains as well as their structure is important for the determination of methodologies and final properties of the material. The "doping" effect of PANI may increase its crystallinity and electrical conductivity [36]. As was evidenced from the FT-IR analysis, emeraldine, which is more conducive, was obtained. It can be observed through the presented X-ray pattern that PANI assumes polycrystallinity. This aspect is modified by GO or rGO in some peaks Fig. 12 shows the X-ray pattern performed for each proposed condition. SESHADRI and BHAT [37] proposed several XRD patterns for polyaniline synthesized electrochemically. Thus, an approximation of the structure obtained from PANI given the morphology shown in Fig. 10, was performed by comparing the proximity of the positions (2θ) of some peaks presented by literature. The diffraction peaks (Table 4) compare to the peaks obtained only to PANI [37]. Given the proximity of the peak positions obtained from the PANI crystalline structure, the Literature allows the assertionthat its crystalline structure is orthorhombic [37]. There are several peaks of PANI reported in the Literature, which may undergoshifts according to its doping and substrate, for example [12]. With the presence of GO and rGO in the material, it is possible to evaluate peak intensity ratios that record changes in the orientation of the peaks in relation to PANI electrosynthesis. This factis probably due to the possible structural deformation of PANI structures by GO particles (edge group effects [28,30]), through the possible electrostatic interactions discussed above [13,31,33]. Peak 1 (001) represents the growth of PANI directly over fiber; peaks 2 (010) and 3 (012) represent the growth of PANI over PANI [38]. These are the most pronounced peaks in this analysis. From their relative intensities, it was possible to calculate ratios among them, providing an idea of structure modification through these proportions, by adding GO or rGO to PANI on fiber. These ratios were calculated

Fig. 10. Micrographs with 4000x magnification of PANI on cf.

film that covers the surface and interconnects the monofilaments of the fiber. This interconnection presents a varied morphology of PANI, such as elongated crystals and parallel plates. In fact, these effects contribute 7

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M. Gandara and E.S. Gonçalves

Fig. 11. Micrographs with 1000x increase of the samples: (A) Carbon fiber (CF) and (B) PANI.

Where R is the ratio of the peaks, I1 is the intensity of peak 1 (001), I2 is the intensity of peak 2 (010) and I3 is the intensity of peak 3 (012). For PANI, R = 1.26, the ratio value being proportional to the growth of PANI over PANI. For C1-P R = 0.97, under which condition the GO deposited on PANI grown on fiber reoriented the surface structure of the polymer. In the C2-G sample, R was 0.74, GO covered the major part of the surface, providing few sites for PANI growth that can be observed by its morphology in Fig. 8-B. This aspect had a great influence on its orientation. The C3PGO sample ratio is 1.16, indicating low influence of GO in the orientation of the PANI crystallographic growth planes, since this value is very close to that relative to PANI grown on fiber. In the sample C4rGO, peak 2 is absent and there is only a relationship between peaks 1 and 3, decreasing this ratio and generating growth of PANI on fiber in isolated points (see Fig. 8-D in Section 3.3, or on the particles of the same rGO). Table 5 summarizes the ratios and their implications in the structural plan of PANI growth, given the influence of its interaction with GO. 3.5. Fourier transform infrared spectroscopy (FT-IR) Fig. 13 shows the bands referring to carbon fiber (CF), PANI only on fiber and C1-P and C2-G conditions, PANI electrosynthesis and GO drip, and GO drip and PANI electrosynthesis, respectively. In Table 6, there is a description of bands found in fiber and composites. In the spectra, it is possible to observe bands assigned to quinoid and benzenoid groups, respectively, of the conditions C1-P (1589 and 1494 cm−1), C2-G (1595 and 1494 cm−1), C3PGO (1589 and 1495 cm−1) e C4rGO400 (1568 and 1496 cm−1). Eq. (2) shows the ratio of the intensities of the quinoid and benzenoid bands that represents the degree of oxidation (y) of PANI, being IQ (quinoid intensity) and IB (benzenoid intensity).

Fig. 12. X-ray patterns: PANI, C1-P, C2-G, C3PGO and C4rPGO.

Table 4 Comparison between the X-ray pattern PANI peak positions of the Literature vs this work. Literature - 2θ [32]

9.8° (001)

14.91° (010)

20.51° (100)

25.4° (110)

27.01 ° (111)

PANI - 2θ

9.69°

15.64°

19.52°

25.37°

25.6°

y = IQ/(IQ + IB)

The ratio of the degree of oxidation of the polyaniline is represented by y = 0 (leucoesmeraldine), y = 0.5 (emeraldine) and y = 1.0 (pernigranilin) [12]. The emeraldine phase is the state in which the conductivity of PANI is most favored. The calculated values obtained are shown in Table 7. The emeraldine phase is predominant in all samples, indicating the formation of the most conductive phase of the polymer, generated by the more controlled process of electrosynthesis.

from Eq. (1). R= (I2 + I3)/ I1

(2)

(1)

Table 5 Influence of GO on PANI crystallographic growth directions. Samples

Ratio

Effect of GO/rGO - PANI

PANI C1-P C2-G C3PGO C4rGO400

1.26 0.97 0.74 1.16 0.49

Growth of PANI on PANI GO reoriented the polymer structure Predominanton surface, GO provided few sites for PANI growth. Little influence of the GO on the orientation of PANI's crystallographic plans Peak 2 is absent and there is only a relationship between peaks 1 and 4, decreasing that ratio by generating a PANI over fiber growth at isolated points or on rGO particles

8

3.6. Electrical resistivity by 4-probe tests

For the qualitative analysis of conductivity measurements of the carbon fiber (CF)samples, PANI and the different proposed conditions, a 4-probe test was performed and the surface resistance of the PANI film and its derivations with GO and rGO, on the carbon fiber. Measurements were made along the entire length of the fiber at 5 different points, as shown in Fig. 14; it is possible to compare the resistance of the films and, therefore, to infer which condition has a

9

–

1015 cm−1

–

1017 cm−1

1048 cm−1

–

– 1142 cm−1

– 1159 cm−1

1230 cm−1 –

3185 cm−1 – – 1698 cm−1 1634 cm−1 – 1589 cm−1 – 1494 cm−1 – 1296 cm−1

3390 cm

– 3060 cm−1 – – 1636 cm−1 1605 cm−1 1568 cm−1 – 1488 cm−1 – 1289 cm−1

3407 cm

−1

Bands of C1-P

– – 1729 cm−1 – – – – 1505 cm−1 – 1450 cm−1 –

3453 cm

−1

Bands of PANI

1016 cm−1

–

– 1150 cm−1

– 3097 cm−1 – 1699 cm−1 1635 cm−1 – 1595 cm−1 – 1494 cm−1 – –

3410 cm

−1

−1

+

The range between 3500–3000 cm corresponds to NeH in the presence of polymeric inter-chains, hydrogen bonds from NH and NH . The bands at 3410, 3407 cm−1 are attributed to NH stretching, next to ring of the emeraldine salt form. It can also be related to OeH, since it comprises the bands of 3200–3500 cm−1 and, being a group belonging tothe GO structure and the carbon fiber. The band 3185 cm−1 is related to NeH stretching of quinoid group in chain termination. It represents shorter chains as oligomers or low molar mass polymers From 3100 to 2800 cm−1, bands may be related to CeH vibrations, next to aromatic ring. The 1729 cm−1 band, refers to CeOH of carboxylic acid groups on the fiber surface 1699 and 1698 cm−1 bands may be related to elongation overlap C]C and π-πstacking between PANI and GO π–π Stacking 1605 cm−1 band is related to C]N stretching next to aromatic ring. C]C quinoid ring elongation (N]Q]N), may range from 1565 to 1595 cm−1 The 1505 cm−1 band is related to a central methylene group on the carbon fiber. The 1494 cm−1 band represents CeH deformation, vibration in the benzenoid ring The 1450 cm−1 band is related to CeH deformation in methyl and methylene groups (carbon fiber). Mode of vibration of eNH+ in the protonated emeraldine phase of PANI and delocalization of π electrons. can also be C]NeC stretching, vibration of the quinoid or benzenoid rings, in QBcisQ, QBB and BBQ units. From 1280 to 1180 cm−1 deformation of the CeC(]O)eC groups occurs on carbon fiber surface. From 1170 to 1140 cm–1, the vibration mode Q = NH+-B or B-N+H-B, related to the charge transport property, is obtained. Polaronic semiquinoid structure, possibly more conductive. From 1156 to 1124 cm–1, refers to the mode CeH, Q]N+HeB ou BeNHeB The 1109 cm−1 band may have been detached and related to the 1115 cm−1 band. π–π stacking occurs and hydrogen bonds with oxygen groups belonging to GO. The 1010 cm−1 band is related to CeH deformation in 1,4-dissubstituted or monosubstituted ring

Attributions

Fig. 13. FT-IR: CF, PANI and conditions C1-P and C2-G.

−1

Bands of CF

Bands of C2-G

The band 3410 cm−1 may also refer to NeH elongation of the amide group (eNHCO−), corresponding to aniline oligomer interacting with GO oxygen groups [15]. The vibrational mode Q]NH+eB or BeN+HeB (1170–1140 cm−1 semiquinoid polaronic structure), to which the degree of electroactivity in the polymer is related, appears in the spectrum in the same range relative to possible interactions of the type stacking π-π and hydrogen bonds belonging to oxygen groups of GO [5]. The vibration in question occurs with lower energy, recorded in the wave number 1142 cm−1. This fact indicates a structural relaxation that favors the transport of electrons to C1-P. Therefore, this also indicates that this condition may be more electroactive than the others. Vibrations of quinoid or benzenoid rings of the QBcisQ type may indicate the formation of non-linear polymer chains (spirals, crosslinking, for example), favoring amorphous character and alveolar morphologies, which may compromise the electroactivity of the polymer. However, the C1-P condition presented a higher vibration energy (1296 cm−1) than pure PANI (1289 cm−1), an indication that disfavored this phenomenon, as well as evidence of increased electroactivity of the C1-P. The presence of the intense band of 1150 cm−1 indicates a high concentration of electrical charges in the material. In cases where graphene oxides are present, this band competes with those of alkoxide (1000–1210 cm−1) [33], whose contribution is less intense in the C1-P sample, which is the most conductive and with a double layer more capacitive among those that were compared. The ease of electron transport via amide and imine groups, through interaction with GO oxygen groups and π-π interactions of the proposed conditions can be related to the low surface resistance of the composite seen in the analysis of 4 probes.

Table 6 FT-IR bands: CF,PANI, C1-P and C2-G.

[34]

[40]

[42] [30,33,34]

[30] [30,39] [39] [27,30] [33] [25] [30] [41] [30] [42] [27,30,33,34]

[31,39,23,40]

References

M. Gandara and E.S. Gonçalves

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from the carbon fiber electrode. Its specific impedance showed a higher order of magnitude. The resistance of the material, from the Z', given at low frequencies, is markedly higher than in the coated fiber. It justifies the addition of PANI and GO to raise the fiber conductivity. In the same way, it is possible to infer the low capacitance of the fiber and the consequent increase of the capacitance, from −Z", when coated. Condition 1 showed the lowest impedance material and higher current density. In fact, it showed a more pronounced growth of PANI, followed by a more conductive and capacitive material. In this case, the GO was dripped after the growth of PANI by electrosynthesis. Electrostatic interaction possibly occurred due to attractive electrostatic interaction between PANI and GO. π cloud stacking [13,33] and transfer through formation of amide groups [15] may favor the transfer of charge and thus decrease the electrical resistance. At higher frequencies, the double-layer phenomena are highlighted by the Randles circle diameter [43]. Through its diameter, it is possible to infer the capacitance of the double layer and the resistance to the transfer of charge. However, as can be seen in Fig. 16, the Randles circle is not very pronounced in the Nyquist Diagram. Fig. 16 shows all the composites with more or less relevant (pseudo) capacitive contribution. These processes are related to the formation of CPE [44]. This effect suggests the consequence of two properties studied above: the nonfrequent interaction between the carbonyl groups remaining in the rGO and the amines or imines of PANI (through FT-IR), and the poor distribution of the polymer in compact plates, on the surface of the carbon fiber as observed in SEM images. Furthermore, it is noticed that the surfaces with presence of PANI or PANI/GO are extremely irregular. Such surfaces, as the non-continuous compact plates and others forms, generate distorted capacitance effects, as is commonly the case in CPE. All these contributions of the other samples contain reflective effects (pseudocapacitives) below 250 Hz, indicating that the transport of charge through morphological cavities and internal structures of PANI and graphene are better favored in these electrochemical systems. At this frequency, the C1-P sample has the property of transmitting almost all the charge accumulated at lower frequencies at the frequency of 250 Hz, so as to present almost immediate charge transfer, given the very low impedance values and their real and complex components associated with this frequency. Therefore, the pseudocapacitive process is converted into a double layer transport process, given the resistive-capacitive semicircle formation sketch, which extends from 15 Hz to above 2000 Hz. In these cases, the range comprises charge delay property due to the formation

Table 7 Degree of oxidation y of PANI vs proposed conditions C1, C2, C3 and C4. Samples

y

PANI CI-P C2-G C3PGO C4rGO400

0.53 0.50 0.52 0.53 0.49

higher conductivity. The measurement is different from the EIS, because the direct current conductive characteristics of the charge transport through only the surface of the fiber or composite is strictly valid. The EIS measurement refers to transverse transport to the surface in alternating current. Thus, in this paper, these are complementary measures. The lowest surface resistance of the polymer films among the conditions performed were the first – PANI electrosynthesis and posterior GO drip and the third: aniline acid solution and GO in electrosynthesis of approximately 8 Ω sq−1. The presence of the graphene derivative has a surface resistance of approximately 13 Ω sq−1, an even lower conductivity than that found in the work of Song et al. [6] with a carbon fiber electrode, polyaniline, graphene and bacterial cellulose of surface resistance 29.7 Ω sq−1. The pure PANI by thin film by electrosynthesis presented low resistivity, which is even lower in the presence of GO using the two different methods proposed in this work. The electroactivity of the composites is facilitated as a result of the interactions of the amine groups of the polymer with the GO oxygenated group, enabling formation of amide groups, which cause vibrational energy displacement, seen in the discussion of FT-IR, favoring electron transport by the polymer. In addition, the π–π stacking is another effect observed as a strong contribution to this action in the conduction.

3.7. Analysis of electrochemical impedance spectroscopy (EIS) data Fig. 15 shows the Electrochemical Impedance Spectroscopy (EIS) measurements performed on the carbon fiber, on the electrosynthesized PANI, and for each PANI@GO nanocomposite formation condition that was proposed in the work, in a comparative manner. Table 8 shows the low and high frequency values for each sample. The highest resistance related to the material, at low frequencies, is

Fig. 14. Surface resistance measurements for CF, PANI, C1-P, C2-G, C3PGO and C4rGO conditions. 10

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Fig. 15. EIS measurements for samples C1-P, C2-G, C3PGO, C4rGO and PANI.

conditions, as shown in Fig. 17, consistent with CV data. Sample C1-P indicates more intense electric charge migration, both in the double layer region and in the material involving the interaction of PANI with GO. As discussed in Section 3.5, this interaction is strong and occurs by forming amide-like groups. Such interactions favor the transport of charges in the film, as well as pseudo-capacitive effects, confirmed by the cyclic voltammetry test (Section 3.8). This condition has the second highest specific capacitance value, given the peaks related to redox phenomena in the energy storage process. The CPE 1 and CPE 2 values indicate the difference in the degree of relevance between double-layer storage and the pseudocapacitive effects of the material. In fact, CPE 2 (pseudo-capacitive effect) has 6 orders of magnitude more than CPE 1 (double layer effect). These phenomena make the C1-P sample more conductive, as seen by 4-point surface resistivity analysis (Fig. 14). Through it, low surface resistance of the formed film is observed. This conductive effect is more pronounced than the charge transfer to the electrolyte, indicating some degree of "metallic character" for the film obtained through this condition. Condition 2 (C2-G) shows the highest double-layer capacitance values among the conditions studied. The sample has double-layer capacitance 100 times greater than carbon fiber and pseudocapacitance of the same order as C1-P. In general, this data can be confirmed by the CV test, observing the highest specific capacitance value among the materials. Fig. 8-B shows a discontinuous PANI growth. This behavior may be related to low charge transfer when compared to samples C1-P and C3PGO. There is a suspicion that the dripped GO is anchored by the fiber and sizing chemical groups, creating a concentric cylinder with each filament. This hypothesis raises the possibility of forming intense double-layer effects (CPE 1) on GO polarizable groups [31]. In addition, an important capacitive contribution may come from the spacing between the GO layer and the fiber surface (C1). In fact, this value has the same order of magnitude presented by CPE 3 of sample C1-P. The small particles of PANI are scattered on this surface. In them there is also interaction of PANI with GO (CPE 2), also of the same order of magnitude of the PANI formed in C1-P. Resistance (R1) to electric charge transfer is more pronounced than to C1-P. Also the film has higher resistance (R2), as can be supported from surface resistivity data. Condition 3 (C3PGO) generates a double layer with one of the most capacitive behaviors, with the lowest load transfer resistance. There is

Table 8 Low and high frequency values for each sample obtained by EIS. Samples

High Frequencies (Hz)

Low Frequencies (Hz)

CF PANI C1-P C2-G C3PGO C4rGO

3162.21 2154.80/63.15 251.20 3980.9/63.15 2511.86/15.84 3980.99

0.12 0.11 0.12 0.012 0.12 0.012

of double layer capacitance, although incipient, indicating the possibility of acting as an electrochemical capacitor. The C2-G sample tends to present two relevant mechanisms of charge accumulation: both by pseudocapacitance and by the formation of a double layer. Fig. 16 shows the electrical circuits related to each material and Table 9 shows the values obtained from each element of the circuits. The electrical circuit representation of each material was calculated by the least squares method (CNLS) [45,46]. Carbon fiber is surrounded by a protective layer of mechanical damage from processing, handling and direct chemical attack, which is commercially called sizing. [47]. This layer may vary in thickness and chemical composition. It has higher strength than carbon fiber. Thus, elements R2 and CPE2 (Fig. 16-A) represent the uneven, rugged relief surface on each fiber monofilament. Surface cavities may also be noted on the fiber surface (Fig. 9), represented in Fig. 16-A by elements R3 and C3. These indicate a small electrical charge storage capacity associated with low resistance (such as a locally peeled fiber). The double layer is also marked by low resistance (R1) and low load storage capacity (C1). Thus, the fiber in its commercial condition (as received) has only incipient double layer capacitive behavior. Low electrical charge retention can also be observed by the cyclic voltammetry test (Section 3.8) which analyzes the capacitance of the material. The smallest electric charge storage capacity is from cf. PANI data show that the effects related to the electrode-electrolyte interface are quite permissive in terms of charge transfer. The double layer has the lowest storage capacity among all samples. On the other hand, the film has the lowest capacitance among the four treatment 11

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M. Gandara and E.S. Gonçalves

Fig. 16. Representation of the electrical circuits of each material: A-CF, B-PANI, C-C1-P, D-C2-G, E-C3PGO, F-C4rGO400.

film material, confirmed by surface resistivity measurements, provides conductance similar to C1-P. The fact that the material has one of the worst film capacitance values indicates that the non-expressive pseudocapacitive effect, almost all storage is due to double-layer processes,

probably a certain degree of GO accumulation (possibly reduced after interaction with PANI) on the film surface. This layer probably induces greater charge transfer to the electrolyte, represented by the low resistance R1. In addition, the low resistance (R2) relative to PANI-GO Table 9 Values of each element of the presented circuits of the obtained materials. RS 2

R1 (Ω cm ) R2 (Ω cm2) R3 (Ω cm2) Y0CPE1 (S cm−2) Y0CPE2 (S cm−2) Y0CPE3 (S cm−2) CCPE1 (F cm−2) CCPE2 (F cm−2) N1 N2 N3 C1 (F cm−2) C2 (F cm−2)

CF

PANI

C1-P

C2-G

C3PGO

C4rGO400

189.53 2.03 106 116.18 7.28 10−6 – – 3.73 10−4 – 0.03 – – 2.69 10−6 7.47 10−8

176.41 53.54 – 2.01 10−3 1.21 10−3 – 4.71 10−5 1.21 10−3 0.216 0.96 – 1.97 10−7 –

401.55 94.76 – 1.08 10−6 1.89 10−2 7.06 10−4 1.19 10−9 4.93 10−4 0.02 0.03 0.04 – –

969.07 220.31 – 2.07 10−5 8.93- 10−4 – 0.05 7.88 10−4 6.78 10−3 0.03 – 2.88 10−4 –

265.29 231.56 – 5.71 10−3 2.54 10−4 1.66 10−3 6.76 10−3 7.28 10−7 0.03 0.01 0.03 – –

7.98 4.12 – 9.11 2.07 – 8.14 5.53 0.03 0.02 – – –

12

103 104 10−5 10−4 10−5 10−4

Progress in Organic Coatings 138 (2020) 105399

M. Gandara and E.S. Gonçalves

it is only better than C1-P) and the possible carbon fiber surface clusters or exposed areas where the effect is similar to other treatments. Generally speaking, this is an underperforming sample. The interaction is reduced graphene oxide with PANI, unlike the other conditions that carbonyl functional groups chemically bond with PANI increasing the effect of charge transfer and storage capacity. This effect is not observed with rGO, precisely because of the reduction of oxygenated groups that could bind to PANI. The resistance of the solution is approximately 15 Ω in all cases. 3.8. Characterization by cyclic voltammetry The materials were characterized by cyclic voltammetry, according to Fig. 17 and, from Eq. (3), where Ca is the specific capacitance per area, the integral of I.dV function is the area under the cyclic voltammetry curve, A is the electrode area, v is sweep rate (10 mV s−1) and ΔV is the variation of the potential window [48]. The pseudo-capacitance measurement of each material was calculated and includes aspects of PANI and its structural modifications by the addition of GO and rGO: Fig. 17. Characterization by cyclic voltammetry at 10 mV s−1 of the materials: CF, PANI, C1-P, C2-G, C3PGO and C4rGO400.

Vc

∫Va

Specific Capacitance (F m−2)

CF PANI C1-P C2-G C3PGO C4rGO400

3.86 27.86 36.92 37.38 29.62 16.79

(3)

Referring to Table 10, it is possible to observe the specific surface capacitance values calculated for each type of material. It is evident that carbon fiber coating with conductive polymer and graphene increases carbon fiber capacitance. This capacitance becomes even greater when combining PANI and GO by dripping methods, and the CI-P condition also has a higher conductivity. Therefore, it stores energy and discharges it in a greater quantity. This characteristic is typical of material similar to supercapacitors. Although the C2-G condition did not present a higher conductivity, the property related to specific capacitance was one of the higher values. When GO first drips onto the fiber, it may have formed a polarization effect provenient from the relationship of hydroxyl, epoxy, carboxylic groups, among other oxygen groups of the GO, and the surface of the carbon fiber. Then the addition of PANI, although in small amounts and dispersed throughout the material, facilitates the transport of charges in the regions in which it is deposited. In addition to the cylindrical geometry of each monofilament of the fiber, with the coating of the GO forming cylindrical parallel plates of carbon network and a material of groups with possibly polarized charges. Therefore, a structure was created propitious to the storage of charges. Although the condition C4rGO400 has presented specific low capacitance in relation to the other conditions, with the coating of PANI

Table 10 Calculated values of the specific capacitance of each material. Materials

(I . dV )/ A. v. ΔV

getting closer to the pure supercapacitor than the other configurations. The conductance in the film is also of the order of magnitude of C1-P and C2-G, although this is not its most important property. Sample C4rGO has high charge and material transfer resistance values compared to other conditions. PANI and rGO adhere to discontinuous regions of the fiber surface after having suffered the effects described in Section 3.2.2. Figs. 5 and 8D offer some allowances to justify parallel adjustment. Resistance values are relatively high for both the charge transfer and the capacitive film forming surface (or its clusters). Capacitive effects are also low in both the double layer (where

Fig. 18. Morphologic analysis of PANI 2D and 3D, respectively, using AFM. 13

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M. Gandara and E.S. Gonçalves

Fig. 19. Morphologic analysis of C1-P 2D and 3D, respectively, using AFM.

and rGO that grows on the carbon fiber that has aporous surface (seen in the FEG of Fig. 9) and forms a ternary structure of 3D. This structure increases the electrical contact between the electrode and electrolyte by raising the specific capacitance relative to pure carbon fiber, increasing the storage of charges in the material [29].

Acknowledgements The authors are grateful for the financial support ITEMM (Instituto de Tecnologia Edson Mororó Moura), from Brazil. References

3.9. Atomic force microscopy (AFM)

[1] D.W. Wang, et al., Fabrication of graphene/polyaniline composite paper via in situ anodic electropolymerization for high-performance flexible electrode, ASC Nano 7 (2009) 1745–1752, https://doi.org/10.1021/nn900297m. [2] X. Jiang, et al., An easy one-step electrosynthesis of graphene/polyaniline composites and electrochemical capacitor, Carbon 67 (2014) 662–672, https://doi.org/10. 1016/j.carbon.2013.10.055. [3] B. Ramezanzadeh, G. Bahlakeh, M. Ramezanzadeh, Polyaniline-cerium oxide (PAniCeO2) coated graphene oxide for enhancement of epoxy coating corrosion protection performance on mild steel, Corros. Sci. 137 (2018) 111–126, https://doi.org/ 10.1016/j.corsci.2018.03.038. [4] L. Ma, et al., A controllable morphology GO/PANI/metal hydroxide composite for supercapacitor, J. Electroanal. Chem. 777 (2016) 75–84, https://doi.org/10.1016/ j.jelechem.2016.07.033. [5] J. Fernández, J. Bonastre, J. Molina, A. I Del Rio, F. Cases, Study on the specific capacitance of an activated carbon cloth modified with reduced graphene oxide and polyaniline by cyclic voltammetry, Eur. Polym. J. 92 (2017) 194–203, https://doi. org/10.1016/j.eurpolymj.2017.04.044. [6] N. Song, H. Tan, Y. Zhao, Carbon fiber-bridged polyaniline/graphene paper electrode for a highly foldable all-solid-state supercapacitor, J. Solid State Electrochem. 23 (2018) 9–17, https://doi.org/10.1007/s10008-018-4109-6. [7] K.T. Chan, H. Lin, K. Qiao, J. Baohua, K.T. Lau, Multifunctional graphene oxide paper embodied structural dielectric capacitor based on carbon fibre reinforced composites, Compos. Sci. Technol. 163 (2018) 180–190, https://doi.org/10.1016/j. compscitech.2018.05.027. [8] X. Cheng, et al., Electrical conductivity and interlaminar shear strength enhancement of carbon fiber reinforced polymers through synergetic effect between graphene oxide and polyaniline, Compos. Part A: Appl. Sci. Manuf. 90 (2016) 243–249, https://doi.org/10.1016/j.compositesa.2016.07.015. [9] B. Cheng, J. Wang, F. Zhang, Q. Shuhua, Preparation of silver/carbon fiber/polyaniline microwave absorption composite and its application in epoxy resin, Polym. Bull. 75 (2017) 381–393, https://doi.org/10.1007/s00289-017-2035-x. [10] X. Cheng, et al., Highly conductive graphene oxide/polyaniline hybrid polymer nanocomposites with simultaneously improved mechanical properties, Compos. Part A: Appl. Sci. Manuf. 82 (2016) 100–107, https://doi.org/10.1016/j. compositesa.2015.12.006. [11] C. Ning, et al., Electroactive polymers for tissue regeneration: developments and perspectives, Prog. Polym. Sci. 81 (2018) 144–162, https://doi.org/10.1016/j. progpolymsci.2018.01.001. [12] A.K. Poli, R.B. Hilário, A.M. Gama, M.R. Baldan, E.S. Gonçalves, Electrosynthesis of polyaniline on carbon Fiber felt: influence of voltammetric cycles on electroactivity, J. Electrochem. Soc. 164 (2017) 631–639, https://doi.org/10.1149/2.1521709jes. [13] B. Ramezanzadeh, et al., Fabrication of a highly tunable graphene oxide composite through layer-by-layer assembly of highly crystalline polyaniline nanofibers and green corrosion inhibitors: complementary experimental and first-principles quantum-mechanics modeling approaches, J. Phys. Chem. C 121 (2017) 20433–20450, https://doi.org/10.1021/acs.jpcc.7b04323. [14] Y. Hayatgheib, et al., A comparative study on fabrication of a highly effective corrosion protective system based on graphene oxide-polyaniline nanofibers/epoxy composite, Corros. Sci. 133 (2018) 358–373, https://doi.org/10.1016/j.corsci. 2018.01.046.

Referring to Figs. 18 and 19 it is possible to observe the morphology of the condition C1-P (PANI with GO, by drip) and PANI without the presence of GO or rGO. By means of the topology of the materials it was possible to obtain the roughness ratio of the surface by the root mean square of the roughness, with Γrms = 40.52 nm for PANI and Γrms = 732.1 nm for C1-P. According to SHABANI-NOOSHABADI and ZAHEDI, the greater the root mean square value of roughness, obtained through the roughness of the material surface, the greater the interaction between the active sites of the material (electrode) and the electrolyte ions, necessary for supercapacitor materials [49]. The presence of GO interacting with PANI increases the roughness of the material, changing its morphology as observed in Fig. 9-A, which increases the charge storage capacity, given that its specific capacitance is 2 times greater than pure PANI.

4. Conclusions Condition C1-P (GO dripped in PANI) favors the migration of electric charge due to electrostatic interactions, electron interactions and possible chemical bonds of GO over PANI. C1-P presented material with lower electrochemical impedance, lower surface resistance and higher material capacitance value compared to other conditions and uncoated carbon fiber. In condition C2-G is a more resistive material due to the isolation of electroactive sites, but with high capacitance, especially in relation to the electrochemical double layer. The conditions of C3PGO and C4rGO400 presented route competition, chemical and electrochemical polymerization occurring simultaneously, causing low deposition rate and relatively low electroactivity of these materials. Therefore, the composite material with higher electroactive properties and reproducibility is proposed by condition 1.

Declaration of Competing Interest All authors declare that no conflict of interest. 14

Progress in Organic Coatings 138 (2020) 105399

M. Gandara and E.S. Gonçalves

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