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Enhanced methanol electro-oxidation activity of electrochemically exfoliated graphene-Pt through polyaniline modification
Journal of Electroanalytical Chemistry 858 (2020) 113821
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Enhanced methanol electro-oxidation activity of electrochemically exfoliated graphene-Pt through polyaniline modification ⁎
Jin Zhang, Lirui Nan, Wenbo Yue , Xi Chen Beijing Key Laboratory of Energy Conversion and Storage Materials, College of Chemistry, Beijing Normal University, Beijing 100875, PR China
A R T I C L E
I N F O
Article history: Received 19 August 2019 Received in revised form 4 November 2019 Accepted 3 January 2020 Available online 07 January 2020 Keywords: Electrochemically exfoliated graphene Platinum Polyaniline Surface interaction Methanol oxidation
A B S T R A C T
Pt and Pt-based catalysts are deemed as the popular anode catalysts for direct methanol fuel cells due to their high electrocatalytic activity for methanol oxidation. The structure of the catalyst support is critical to the electrocatalytic performance of the catalysts. In this work, a kind of high-quality graphene (electrochemically exfoliated graphene) with the intact surface is exploited as the catalyst support instead of the popularly used reduced graphene oxide. Pt nanoparticles are successfully synthesized on electrochemically exfoliated graphene and exhibit better methanol electrooxidation activity than those on reduced graphene oxide. To further raise the electrocatalytic performance of Pt catalysts, the substrate surface is modified by conductive polyaniline. It is found that the interaction of electrochemically exfoliated graphene with polyaniline is much different from the interaction of reduced graphene oxide with polyaniline. The intact surface structure of electrochemically exfoliated graphene is favorable for the combination with polyaniline by π-π interactions, which releases more N species for the growth of Pt nanoparticles. The synergistic interaction of Pt and N species can facilitate the dispersion of Pt nanocrystals as well as the electron transport through Pt-based hybrids, thereby further promoting the electrocatalytic properties of Pt catalysts for methanol oxidation. © 2018 Elsevier B.V. All rights reserved.
1. Introduction Compared with traditional fossil fuels, fuel cell is a new clean energy conversion device. Amongst proton exchange membrane fuel cells (PEMFCs), direct methanol fuel cells (DMFCs) are developing rapidly and have attracted a large amount of attention on account of their many advantages such as safe and convenient transportation process, high energy density, cleanliness and environmental protection [1,2]. Methanol oxidation is a six-electron reaction process, including the following steps: 1) electrochemical adsorption of methanol; 2) dissociation of methanol and activation of carbon-hydrogen bonds; 3) adsorption and activation of water molecules; 4) adsorption of oxygen molecules and production of carbon dioxide molecules [3,4]. In the current research on DMFCs, Pt and its alloys are mainly studied as anode catalysts due to their higher electrocatalytic activity than other metal catalysts [5]. In particular, PtRu is regarded as the optimal catalyst with remarkable electrocatalytic activity and stability on account of bi-functional mechanism and electronic interaction [6,7]. Besides, the morphology and structure of Pt catalysts have important impacts on their electrocatalytic properties for methanol oxidation [8,9]. Generally, nano-sized catalysts with large specific surface areas can offer more active ⁎ Corresponding author. E-mail address: [email protected]. (W. Yue).
http://dx.doi.org/10.1016/j.jelechem.2020.113821 1572-6657/© 2018 Elsevier B.V. All rights reserved.
sites to electro-oxidation of methanol and thereby exhibit better electrocatalytic activity than bulk materials [10,11]. However, the high surface energy of nanoparticles makes them prone to aggregate and form large agglomerates, which brings about the performance degradation. Therefore, it is necessary to load Pt catalysts on some conductive materials to improve the dispersibility and utilization of Pt nanoparticles. Carbon materials such as carbon nanotube, carbon nanofiber and porous carbon are widely used as catalyst supports because of their good electrical conductivity, large specific surface area, strong corrosion resistance and low price [12–14]. Graphene is a new type of two-dimensional carbon material which has larger surface area and higher electrical conductivity than other carbon materials [15,16]. Accordingly, graphene-based Pt catalysts usually exhibit better electrocatalytic performance than other Ptloaded carbon materials [17,18]. Graphene with intact structure and high quality can be produced by several methods such as mechanical exfoliation method, chemical vapor deposition (CVD) method and organic synthesis [19–21]. Nevertheless, highly reduced graphene oxide (rGO) which is reduced from graphene oxide (GO) is still a popular graphene substrate used for the synthesis of graphene-based composites because GO can be mass-produced through chemical exfoliation of graphite [22]. In addition, the oxygen-containing groups of GO (e.g., carboxyl and hydroxyl groups) are able to serve as the nucleation sites for Pt catalysts [23]. Although the residual oxygen-containing groups are almost removed during the
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vigorously for 0.5 h. The mixture solution was transferred into a Teflonlined stainless steel autoclave and heated at 120 °C for 24 h. Polyaniline was formed and connected EEG/rGO and Pt nanoparticles under solvothermal conditions. After the temperature of autoclave dropped to the room temperature, the resulting black suspension was centrifuged and washed with ethanol three times to remove DMF. Finally, EEG-PANI-Pt or EEG-PANI-Pt was obtained by drying at 60 °C. EEG-Pt and rGO-Pt were synthesized by a similar method except that aniline and hydrochloric acid were not used.
reduction process of GO [24,25], many defects are still left on the surface of rGO, leading to a decline in the electrical conductivity and mechanical strength. Recently, a kind of high-quality graphene is simply massproduced by a modified electrochemical exfoliation method [26,27]. Electrochemically exfoliated graphene (EEG) shows better electrical and mechanical properties than rGO because few oxygen-containing groups and defects are present on the surface of EEG [28,29]. These characteristics make EEG more suitable as a carrier for building graphene-based hybrids with better electrocatalytic properties than rGO-based hybrids. Nevertheless, lack of nucleation sites makes it difficult to grow or anchor Pt nanocrystals on EEG [30]. In addition, EEG is only dispersed in polar aprotic solvents such as N,N-dimethylformamide (DMF) owing to its hydrophobic surface, which limits the growth conditions of Pt nanocrystals [31]. Modifying the EEG surface by noncovalent functionalization or covalent functionalization is an effective method for promoting the growth of Pt catalysts on graphene. Polyaniline (PANI) as a conductive polymer has attracted a lot of attention owing to its high chemical stability, electrochemical stability, electrical conductivity and hydrophilicity [32]. It is reported that PANI-modified rGO furnishes more active sites for the growth of Pt catalysts and the connection between PANI and Pt is favorable for the dispersion and stability of Pt catalysts on rGO [33,34]. Three kinds of interactions occur between rGO and PANI, including electrostatic interaction, hydrogen bonding and π–π interaction [35]. In view of the structural difference between EEG and rGO, the interaction of EEG with PANI and the effect of PANI-modified EEG on the electrocatalytic performance of Pt catalysts need to be further explored. In this work, Pt nanoparticles are successfully synthesized on EEG (EEGPt) by a solvothermal method. Compared to rGO-based Pt (rGO-Pt) prepared under the same conditions, EEG-Pt shows enhanced methanol electro-oxidation activity because EEG as a catalyst carrier shows better electrical and mechanical properties than rGO. To further raise the electrocatalytic performance of Pt catalysts on EEG and rGO, the substrates are modified by PANI prior to growing Pt nanoparticles (EEG-PANI-Pt and rGO-PANI-Pt). The results show that the methanol electro-oxidation activity of either EEG-PANI-Pt or rGO-PANI-Pt is highly improved by PANI modification. The synergistic interaction of Pt and N species can facilitate the dispersion of Pt nanocrystals on EEG or rGO, and further improves the electron conductivity of EEG-Pt or rGO-Pt, which is favorable for the electrocatalytic performance improvement. More importantly, the interaction of EEG and PANI is much different from the interaction of rGO and PANI owing to the different surface structures of EEG and rGO. Compared to the interactions of traditional rGO with PANI, which mainly include the electrostatic interaction (–N+O−–) and the hydrogen bonding (–O–H⋯N–) [34,35], the π-π interaction is the predominant interaction between EEG and PANI due to the π-conjugated structure of EEG. Therefore, more N species of PANI on EEG can be released for the deposition of Pt nanocrystals, which promotes the electrocatalytic performance of Pt catalysts. On the contrary, the connection of rGO with PANI through the electrostatic interaction and hydrogen bonding prevent nitrogen atoms from becoming nucleation sites for the growth of Pt nanoparticles. Consequently, the π-conjugated structure of graphene is very important to the interactions between graphene and PANI as well as the electrocatalytic performance of Pt catalysts on PANI-modified graphene.
2.2. Sample characterization X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), ICP atomic emission spectroscopy, field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) were used to characterize samples. Detailed characterization methods were supplied in the Supporting information. 2.3. Electrocatalytic measurements The electrocatalytic performances of samples were measured by using CHI 700E and Gamry Interface 1000 electrochemical workstation. Detailed test methods were supplied in the Supporting information. 3. Results and discussion Fig. 1 shows the synthesis routes for rGO-PANI-Pt and EEG-PANI-Pt. Aniline is first adsorbed on the surface of GO or EEG and then polymerized to PANI layer under solvothermal treatment. Meanwhile, GO is reduced to rGO and H2PtCl6 is reduced to Pt by DMF under solvothermal conditions. The N species of PANI layer can substitute the oxygen-containing functional groups of GO as new nucleation sites for the growth of Pt nanoparticles because of forming a coordination bond between H2PtCl6 and N atom. Accordingly, Pt nanoparticles are more uniformly distributed on rGO or EEG after surface modification by PANI. Three kinds of interactions occur between rGO and PANI, including electrostatic interaction, hydrogen bonding and π–π interaction [35]. However, the lone pair of electrons on N atoms is occupied as forming the electrostatic interaction and hydrogen bonding between rGO and PANI, causing Pt nanoparticles not to form on the N species of PANI. On the contrary, the π–π interaction is the main interaction between EEG and PANI on account of the intact surface structure of EEG, leading to more N species becoming the nucleation sites for the growth of Pt nanoparticles. Therefore, the π-conjugated structure of graphene is very important to induce the function of PANI and further improve the electrocatalytic activity and durability of Pt catalysts for methanol oxidation. The morphologies and structures of rGO-Pt and EEG-Pt with or without PANI modification were characterized by SEM and TEM. SEM images (Fig. 2a,b) show that Pt nanoparticles are successfully deposited on the surface of rGO or EEG. Considering that EEG has good solubility in DMF, this solvothermal method that using DMF as both solvent and reducing agent can expand the synthesis route for EEG hybrids. TEM images (Fig. 2c,d) also confirm the growth of Pt nanocrystals on rGO or EEG. According to the nanoparticle size distribution based on statistical results (Fig. S1), the average size of Pt nanoparticles on rGO (~5.2 nm) is slightly larger than that on EEG (~4.0 nm). In addition, Pt nanoparticles are more uniformly distributed on EEG rather than on rGO. It is inferred that the functional groups and defects of GO may serve as the nucleation sites for the growth of Pt nanocrystals, resulting in the fast growth and aggregation of Pt nanoparticles on rGO. On the contrary, the polymerized precursor molecules are homogeneously adsorbed on the intact surface of EEG by intermolecular interactions and then grow to Pt nanocrystals under solvothermal conditions [28,31]. HRTEM images (Fig. 2e,f) show fine crystalline nanoparticles on rGO and EEG. The spacing of lattice fringes is ~0.226 nm, which coincides with the d-spacing of the (111) plane of the face-centered cubic (fcc) Pt crystal. Furthermore, the lattice spacing of ~0.215 nm corresponding to the
2. Experimental 2.1. Sample preparation EEG and GO were prepared according to the published literatures (see the Supporting information) [22,26]. EEG-PANI-Pt and rGO-PANI-Pt were synthesized by a solvothermal method since EEG cannot be dispersed in water. Briefly, 0.02 g of EEG or GO was uniformly dispersed in 60 mL of N,N-dimethylformamide (DMF) under ultrasonic conditions for 1 h. 0.4 g of aniline and 2 mL of 1 M hydrochloric acid were slowly added into the EEG or rGO solution and stirred for 0.5 h. Subsequently, 0.05 g of chloroplatinic acid was added into the above solution and stirred 2
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Fig. 1. Schematic illustration of the synthesis routes for (a) rGO-PANI-Pt and (b) EEG-PANI-Pt. Three kinds of interactions occur between rGO/EEG and PANI.
XRD and Raman measurements were implemented to study the crystal phase and surface structure of rGO-Pt, rGO-PANI-Pt, EEG-Pt and EEGPANI-Pt. XRD patterns of these four samples (Fig. 4a) show characteristic diffraction peaks at 39.8°, 46.3° and 67.5°, which correspond to the (111), (200) and (220) planes of Pt crystal (JCPDS No. 70-2431), respectively. The broad peaks manifest the small particles size of Pt nanocrystals, which is consistent with the TEM observations. The peak related to the basal spacing of stacked GO (~11°) shifts to ~24.3°, demonstrating the reduction of GO to rGO. The reduced interlamellar spacing means that the oxygen-containing groups of GO are nearly eliminated under solvothermal conditions. The peak belongs to the stacked EEG is centered at a larger diffraction angle (26.5°), implying that the amount of oxygen-containing groups of EEG is even less than that of rGO. The intact surface of EEG was further verified by Raman analysis. The D band at 1352 cm−1 is connected with the vibration of the sp3 carbon atom graphite, and the G band at 1582 cm−1 is connected with the in-plane vibration of sp2 carbon atom [36]. Thus the intensity ratio of ID/IG is used to evaluate the surface integrity of EEG and rGO hybrids. Raman spectra (Fig. 4b) show that the ID/IG value of EEG-Pt (0.43) is much lower than that of rGO-Pt (1.24), reflecting the high surface integrity of EEG even after growth of Pt nanocrystals. On the contrary, in spite of removing most functional groups, the surface defects still result in a large ID/IG value of rGO-Pt. The fast electron transfer through the EEG substrate can effectively accelerate the methanol oxidation reaction on Pt catalysts. On the other hand, the ID/IG values of rGOPANI-Pt and EEG-PANI-Pt are 0.38 and 1.20 respectively, lower than those of unmodified samples. The decreased ID/IG values are attributed to the introduction of PANI, which raises the amount of sp2 carbon atoms. In addition, the PtN bonds replacing the PtC bonds also contribute to the decrease of ID/IG values. FT-IR and XPS characterizations were performed to further examine the composition and chemical bonding of samples. FT-IR spectrum of GO (Fig. S2a) shows several absorption bands corresponding to the CO, C-OH
(100) planes of graphite is also observed in the HRTEM image of EEG-Pt (Fig. 2f), demonstrating the intact surface of EEG with few defects. In contrast, the characteristic hexagonal lattice of graphene is hardly observed in the HRTEM image of rGO-Pt (Fig. 2e) because there are lots of defects on rGO after removing functional groups. The surface defects would reduce the electrical conductivity of rGO, which is not conducive to the electrocatalytic activity of Pt catalysts. To improve the distribution of Pt catalysts on rGO as well as the interaction of Pt catalysts with rGO, the surface of rGO is modified by PANI. For comparison, the surface of EEG is also modified by PANI, which can dramatically raise the nucleation sites of EEG for the growth of Pt catalysts. SEM images (Fig. 3a,b) present the better dispersion of Pt nanoparticles on rGO after PANI modification, while there is no distinct difference in the distribution of Pt nanoparticles on EEG before or after PANI modification. The particle size of Pt nanocrystals on rGO or EEG is also not changed much after PANI modification (Fig. S1). It is inferred that the N species of PANI substitute the functional groups of rGO as nucleation sites for the growth of Pt nanoparticles, leading to the well-distributed of Pt nanocrystals on rGO. However, the growth of Pt nanoparticles on EEG is based on the adsorption of the polymerized precursor molecules on EEG because of the intact surface of EEG. After PANI modification, the ordered structure of PANI also offers dispersed nucleation sites for the growth of Pt nanocrystals. Thus the dispersibility of Pt nanoparticles on PANImodified EEG is not much different from that on EEG. In spite of the high dispersion of Pt nanoparticles on both EEG and PANI-modified EEG, the synergistic interaction of Pt and N species still enhances the electrocatalytic properties of Pt catalysts for methanol oxidation. TEM images (Fig. 3c,d) also confirm the well-dispersed Pt nanoparticles on PANI-modified rGO or EEG, and the particle agglomerates are hardly observed. The lattice fringes of Pt nanoparticles with a spacing of ~0.226 nm are visible in the HRTEM images (Fig. 3e,f), illustrating that Pt nanocrystals can grow on the PANI layer by forming PtN bonds. 3
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Fig. 2. SEM, TEM and HRTEM images of (a, c, e) rGO-Pt and (b, d, f) EEG-Pt.
Pt catalysts on rGO or EEG. Moreover, the intensity ratio of the C 1s peak to the O 1s peak for rGO-Pt (~4.2) is lower than that for EEG-Pt (~6.6), reflecting that more functional groups are left on rGO. The N 1s peaks emerge in the survey spectra of rGO-PANI-Pt and EEG-PANI-Pt because of the introduction of PANI. The high-resolution C 1s XPS spectra (Fig. 5b) show four peaks related to the CC, CO, CO and O-C=O groups [37]. The peaks related to CO, CO and O-C=O groups are too weak in comparison with the CC peak. By summarizing the XRD, Raman, FT-IR and XPS results, it is concluded that EEG has few functional groups and defects on the surface even after the growth of Pt nanocrystals. In contrast, although most
and CO groups, which are derived from carboxyl, hydroxyl and carbonyl groups [28]. The peaks related to the CO and CO groups vanish in the spectra of rGO-Pt and rGO-PANI-Pt, indicating that GO is highly reduced through solvothermal treatment. The CO and CO peaks are not detected and the C-OH peak becomes weaker in the FT-IR spectrum of EEG (Fig. S2b), revealing that EEG itself possesses few functional groups and defects. In view of the intact surface of EEG, the harsh synthesis conditions such as hydrothermal or solvothermal treatment are not necessary. The peaks of C 1s, O 1s, Pt 4d and Pt 4f peaks emerge in the XPS survey spectra of rGO-Pt and EEG-Pt (Fig. 5a), demonstrating the successful formation of 4
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Fig. 3. SEM, TEM and HRTEM images of (a, c, e) rGO-PANI-Pt and (b, d, f) EEG-PANI-Pt.
resolution N 1s XPS spectra (Fig. 5c) show that the N 1s peak is further deconvoluted into three peaks at 398.8, 399.7 and 400.7 eV, which are assigned to quinonoid imine (=N–), benzenoid amine (–NH–) and positively charged nitrogen (–N+–), respectively [35]. After the growth of Pt nanocrystals on rGO-PANI or EEG-PANI, the intensities of –NH– and – N+– peaks decrease distinctly due to the formation of PtN bond. It is worth noting that the decrease of –NH– and –N+– peak intensities is more obvious in the spectrum of EEG-PANI-Pt than in the spectrum of rGO-PANI-Pt, implying that more amount of Pt catalysts are formed on the PANI layer of EEG-PANI than rGO-PANI. The PANI layer is combined
functional groups are eliminated after solvothermal reduction, rGO still has more functional groups than EEG, and plenty of defects remain on the surface of rGO. The structural difference between the carbon supports would make a great impact on the electrocatalytic activity of Pt catalysts. As shown in Fig. 1, the interactions between rGO and PANI include electrostatic interaction, hydrogen bonding and π–π interaction due to the functional groups and π-conjugated structure of rGO. In contrast, the π–π interaction is the predominant interaction between EEG and PANI because of the intact surface of EEG. The different interactions with PANI could strongly affect the growth of Pt nanocrystals on PANI layers. The high 5
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Fig. 4. (a) XRD patterns and (b) Raman spectra of rGO-Pt, rGO-PANI-Pt, EEG-Pt and EEG-PANI-Pt.
Fig. 5. XPS spectra of rGO-Pt, rGO-PANI-Pt, EEG-Pt and EEG-PANI-Pt: (a) survey, (b) high-resolution C 1s and (d) high-resolution Pt 4f. (c) High-resolution N 1s XPS spectra of rGO-PANI, rGO-PANI-Pt, EEG-PANI and EEG-PANI-Pt.
with EEG mainly through the π-π interaction, which can expose the –NH– and –N+– groups for the growth of Pt catalysts. On the contrary, the – NH– and –N+– groups of PANI are occupied by the functional groups of rGO such as –OH and O=C–O−, making it impossible to combine with Pt. It is well-known that the synergistic interaction of Pt and N species
can facilitate the electrocatalytic activity and durability of Pt catalysts. Accordingly, Pt catalysts are expected to display better electrocatalytic performance on EEG-PANI than rGO-PANI. The high-resolution Pt 4f XPS spectra (Fig. 5d) show two peaks centered at 71.5 eV and 74.8 eV respectively, representing two spin-orbits split of Pt 4f7/2 and Pt 4f5/2. After peak 6
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subdivision, two pairs of peaks belonging to Pt0 (~71.5 eV and ~74.8 eV) and Pt2+ (~72.9 eV and ~76.2 eV) are obtained. The Pt2+ part may be derived from PtO or Pt(OH)2, which did not contribute to the electrocatalytic activity of Pt catalysts. Note that the ratios of peak intensities of Pt0 and Pt2 + are very similar for rGO-Pt (2.3) and EEG-Pt (2.6). However, after combination with PANI, the ratio of Pt0 and Pt2+ parts increases to ~4.9 for EEGPANI-Pt, much higher than that for rGO-PANI-Pt (3.6). This result shows that the PANI layer on EEG also facilitates the formation of metal Pt catalysts. The electrocatalytic activity and durability of rGO-Pt, EEG-Pt, rGOPANI-Pt and EEG-PANI-Pt for methanol oxidation were evaluated by cyclic
voltammetry (CV). The contents of Pt in these four samples are very close (18–20 wt%) based on the ICP analysis (Table S1). CV curves measured in 0.5 M H2SO4 solution (Fig. 6a,b) were first collected to calculate the electrochemically active surface area (ECSA) of Pt catalysts. EEG-Pt shows higher ECSA value (~206.9 cm2 mg−1) than rGO-Pt (~134.6 cm2 mg−1), indicating that EEG is a better catalyst support than rGO. Both rGO-PANIPt and EEG-PANI-Pt show increased ECSA (~274.4 and ~415.4 cm2 mg−1) owing to the PANI modification, which provides abundant N species for the growth of Pt nanoparticles. Methanol oxidation reaction (MOR) measurements were then performed in a solution of 0.5 M H2SO4 and 1 M methanol. CV curves (Fig. 6c,d) show two oxidation
Fig. 6. CV curves of (a) rGO-Pt, EEG-Pt and (b) rGO-PANI-Pt, EEG-PANI-Pt in 0.5 M H2SO4. CV curves of (c) rGO-Pt, EEG-Pt and (d) rGO-PANI-Pt, EEG-PANI-Pt in 0.5 M H2SO4 and 1 M CH3OH. The forward peak current densities of (e) rGO-Pt, EEG-Pt and (f) rGO-PANI-Pt, EEG-PANI-Pt as a function of the cycle number for methanol oxidation. 7
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(~3.9 Ω) than rGO-Pt (~11.3 Ω) due to the intact surface of EEG with few defects. It is interesting to find that the charge transfer resistance of rGO-Pt and EEG-Pt is reduced by PANI modification, i.e., ~9.4 Ω for rGOPANI-Pt and ~2.5 Ω for EEG-PANI-Pt. It is inferred that the introduced N atoms can modify the electronic structure of Pt atoms and thereby improve the electrical conductivity of Pt-based hybrids [40]. In addition, the presence of π-π conjugated system in EEG-PANI-Pt is also favorable for the electron transport.
peaks that are associated with the oxidation of methanol (the forward peak) and the oxidation of the residual carbonaceous species (mainly CO) formed during the forward-scan (the backward peak). The forward peak current density of EEG-Pt (~278.5 mA mg−1 Pt ) is much higher than that of rGO-Pt (~167.2 mA mg−1 Pt ), demonstrating that EEG-Pt has higher electrocatalytic activity toward methanol oxidation than rGO-Pt. The electrocatalytic activity of rGO-Pt and EEG-Pt is further improved by PANI modification; for instance, higher forward peak current densities are achieved for rGO-PANI-Pt −1 (~313.3 mA mg−1 Pt ) and EEG-PANI-Pt (~443.7 mA mgPt ). The synergistic interaction between Pt and PANI can facilitate the dispersion of Pt nanoparticles, stabilize Pt nanoparticles during cycling, and improve the electrical conductivity of Pt-based hybrids, which is favorable for the electrocatalytic performance improvement. Moreover, the combination of EEG with PANI would offer more N species for the growth of Pt nanocrystals, which can further promote the electrocatalytic performance of Pt catalysts. The MOR performance comparison of EEG-PANI-Pt with reported graphene through polyaniline modification supported Pt hybrids is listed in Table S2. In addition, the CO-tolerant ability of catalysts can be estimated by the ratio of the forward and backward peak currents (If/Ib). The If/Ib values of rGO-Pt and EEG-Pt (1.11 and 1.17) decrease from 1.11 and 1.17 to 1.03 and 0.98 after PANI modification. The decreased CO tolerance of Pt catalysts is ascribed to the high electrocatalytic activity of Pt catalysts induced by PANI, which produces more amount of residual CO. Introduction of catalyst promoters such as SnO2 can effectively enhance the tolerance of Pt catalysts to CO poisoning [38]. All parameters related to the electrocatalytic performance are summarized in Table S1. The long-term electrocatalytic stability of rGO-Pt, EEG-Pt, rGO-PANI-Pt and EEG-PANI-Pt are also tested and represented in Fig. 6e,f. Although the If value of EEG-Pt drops to ~174.1 mA mg−1 Pt after 200 cycles, it is still higher than that of rGO-Pt after 200 cycles (~83.4 mA mg−1 Pt ), and the catalytic activity retention of EEG-Pt (63%) is also higher than that of rGO-Pt (50%). After PANI modification, the catalytic activity retentions of rGO-PANI-Pt and EEG-PANI-Pt increase to 64% and 80%, respectively. The enhanced anti-poisoning ability of Pt catalysts is attributed to the synergistic interaction between Pt and PANI, and the formed PtN bonds can immobilize Pt nanoparticles on PANI to prevent agglomeration and detachment during cycling. Since the structure of EEG-PANI is beneficial to exposing more N species of PANI for immobilizing Pt catalysts, EEG-PANI-Pt exhibits better electrocatalytic durability for methanol oxidation. The electrical conductivity of Pt-based anodes is an important factor for the electrocatalytic performance of Pt catalysts. Thus EIS measurements were conducted on these four samples. Nyquist plots (Fig. 7) show a semicircle in the high frequency range and an oblique line in the low frequency range. The diameter of the semicircle is equal to the charge transfer resistance (Rct) of samples, which can be calculated by using the equivalent circuit model (Fig. S3) [39]. It can predict that EEG-Pt has a lower Rct value
4. Conclusions We proposed a novel high-quality EEG to replace the popularly used rGO as the catalyst support for Pt catalysts. Pt nanocrystals are successfully deposited on EEG by a solvothermal method despite lack of nucleation sites on the surface of EEG. Compared to rGO-Pt, EEG-Pt exhibits superior electrocatalytic activity and durability for methanol oxidation on account of the outstanding properties of EEG. Moreover, the electrocatalytic performance of rGO-Pt and EEG-Pt can be further improved by PANI modification. The synergistic interaction of Pt catalysts and PANI can hinder the aggregation and detachment of Pt catalysts during cycling and raises the electrical conductivity of Pt-based hybrids, which is favorable for the electrocatalytic performance improvement. More importantly, in view of the different interactions of rGO and EEG with PANI, PANI-modified EEG can expose more N species for the growth of Pt nanocrystals, which can further promote the electrocatalytic performance of Pt catalysts. Therefore, the πconjugated structure of graphene is very significant for the electrocatalytic performance improvement of Pt catalysts by fully releasing the function of PANI.
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was financially supported by National Natural Science Foundation of China (21573023 and 21975030). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jelechem.2020.113821.
Fig. 7. Electrochemical impedance spectra of (a) rGO-Pt, EEG-Pt and (b) rGO-PANI-Pt, EEG-PANI-Pt in 0.5 M H2SO4 and 1 M CH3OH. 8
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Journal of Electroanalytical Chemistry 858 (2020) 113821 [21] N.G. Shang, P. Papakonstantinou, S. Sharma, G. Lubarsky, M.X. Li, D.W. McNeill, A.J. Quinn, W.Z. Zhou, R. Blackley, Controllable selective exfoliation of high-quality graphene nanosheets and nanodots by ionic liquid assisted grinding, Chem. Commun. 48 (2012) 1877–1879. [22] W.S. Hummers, R.E. Offeman, Preparation of graphitic oxide, J. Am. Chem. Soc. 80 (1958) 1339. [23] X.L. Li, H.L. Wang, J.T. Robinson, H. Sanchez, G. Diankov, H.J. Dai, Simultaneous nitrogen doping and reduction of graphene oxide, J. Am. Chem. Soc. 131 (2009) 15939–15944. [24] L.R. Nan, W.B. Yue, Y. Jiang, Fabrication of graphene-porous carbon-Pt nanocomposites with high electrocatalytic activity and durability for methanol oxidation, J. Mater. Chem. A 3 (2015) 22170–22175. [25] L.R. Nan, Z.T. Fan, W.B. Yue, Q. Dong, L.S. Zhu, L. Yang, L.Z. Fan, Graphene-based porous carbon-Pd/SnO2 nanocomposites with enhanced electrocatalytic activity and durability for methanol oxidation, J. Mater. Chem. A 4 (2016) 8898–8904. [26] S. Yang, S. Brüller, Z.S. Wu, Z.Y. Liu, K. Parvez, R.H. Dong, F. Richard, P. Samorì, X.L. Feng, K. Müllen, Organic radical-assisted electrochemical exfoliation for the scalable production of high-quality graphene, J. Am. Chem. Soc. 137 (2015) 13927–13932. [27] S. Yang, M.R. Lohe, K. Müllen, X.L. Feng, New-generation graphene from electrochemical approaches: production and applications, Adv. Mater. 28 (2016) 6213–6221. [28] Z.X. Xu, W.B. Yue, R. Lin, C.-Y. Chiang, W.Z. Zhou, Direct growth of SnO2 nanocrystallites on electrochemically exfoliated graphene for lithium storage, J. Energy Storage 21 (2019) 647–656. [29] Z.X. Xu, W.B. Yue, X. Yuan, X. Chen, Exceptional anodic performance of Sb-doped SnO2 nanoparticles on electrochemically exfoliated graphene for lithium-ion batteries, J. Alloys Compd. 795 (2019) 168–176. [30] K. Parvez, Z.-S. Wu, R.J. Li, X.J. Liu, R. Graf, X.L. Feng, K. Müllen, Exfoliation of graphite into graphene in aqueous solutions of inorganic salts, J. Am. Chem. Soc. 136 (2014) 6083–6091. [31] Z.X. Xu, P. Zhang, J.L. Chen, W.B. Yue, W.Z. Zhou, Growth and growth mechanism of oxide nanocrystals on electrochemically exfoliated graphene for lithium storage, Energy Storage Mater. 18 (2019) 174–181. [32] G. Ćirić-Marjanović, Recent advances in polyaniline research: polymerization mechanisms, structural aspects, properties and applications, Synth. Met. 177 (2013) 1–47. [33] S.P. Luo, Y. Chen, A.J. Xie, Y. Kong, Y.W. Tao, Y.L. Pan, C. Yao, Synthesis of PtNFs/ PANI/NG with enhanced electrocatalytic activity towards methanol oxidation, Ionics 21 (2015) 1277–1286. [34] Y.L. Wang, Z.S. Li, S.H. Xu, Y.X. Xie, S. Lin, Highly dispersive platinum nanoparticles supporting on polyaniline modified three dimensional graphene and catalyzing methanol oxidation, Mater. Res. Bull. 102 (2018) 172–179. [35] G.G. Kumar, C.J. Kirubaharan, S. Udhayakumar, C. Karthikeyan, K.S. Nahm, Conductive polymer/graphene supported platinum nanoparticles as anode catalysts for the extended power generation of microbial fuel cells, Ind. Eng. Chem. Res. 53 (2014) 16883–16893. [36] R.L. Li, W.B. Yue, X. Chen, Fabrication of porous carbon-coated ZnO nanoparticles on electrochemical exfoliated graphene as an anode material for lithium-ion batteries, J. Alloys Compd. 784 (2019) 800–806. [37] S.T. Liu, W.B. Yue, C. Zhang, D.J. Du, X.J. Yang, Enhanced lithium storage properties of graphene-based metal oxides by coating with amorphous TiO2 nanofilms, J. Alloys Compd. 769 (2018) 293–300. [38] L.N. Gao, W.B. Yue, S.S. Tao, L.Z. Fan, Novel strategy for preparation of graphene-Pd, Pt composite, and its enhanced electrocatalytic activity for alcohol oxidation, Langmuir 29 (2013) 957–964. [39] W. Wei, G. Wang, S. Yang, X.L. Feng, K. Müllen, Efficient coupling of nanoparticles to electrochemically exfoliated graphene, J. Am. Chem. Soc. 137 (2015) 5576–5581. [40] C.-S. Liu, X.-C. Liu, G.-C. Wang, R.-P. Liang, J.-D. Qiu, Preparation of nitrogen-doped graphene supporting Pt nanoparticles as a catalyst for oxygen reduction and methanol oxidation, J. Electroanal. Chem. 728 (2014) 41–50.
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Contents lists available at ScienceDirect
Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem
Enhanced methanol electro-oxidation activity of electrochemically exfoliated graphene-Pt through polyaniline modification ⁎
Jin Zhang, Lirui Nan, Wenbo Yue , Xi Chen Beijing Key Laboratory of Energy Conversion and Storage Materials, College of Chemistry, Beijing Normal University, Beijing 100875, PR China
A R T I C L E
I N F O
Article history: Received 19 August 2019 Received in revised form 4 November 2019 Accepted 3 January 2020 Available online 07 January 2020 Keywords: Electrochemically exfoliated graphene Platinum Polyaniline Surface interaction Methanol oxidation
A B S T R A C T
Pt and Pt-based catalysts are deemed as the popular anode catalysts for direct methanol fuel cells due to their high electrocatalytic activity for methanol oxidation. The structure of the catalyst support is critical to the electrocatalytic performance of the catalysts. In this work, a kind of high-quality graphene (electrochemically exfoliated graphene) with the intact surface is exploited as the catalyst support instead of the popularly used reduced graphene oxide. Pt nanoparticles are successfully synthesized on electrochemically exfoliated graphene and exhibit better methanol electrooxidation activity than those on reduced graphene oxide. To further raise the electrocatalytic performance of Pt catalysts, the substrate surface is modified by conductive polyaniline. It is found that the interaction of electrochemically exfoliated graphene with polyaniline is much different from the interaction of reduced graphene oxide with polyaniline. The intact surface structure of electrochemically exfoliated graphene is favorable for the combination with polyaniline by π-π interactions, which releases more N species for the growth of Pt nanoparticles. The synergistic interaction of Pt and N species can facilitate the dispersion of Pt nanocrystals as well as the electron transport through Pt-based hybrids, thereby further promoting the electrocatalytic properties of Pt catalysts for methanol oxidation. © 2018 Elsevier B.V. All rights reserved.
1. Introduction Compared with traditional fossil fuels, fuel cell is a new clean energy conversion device. Amongst proton exchange membrane fuel cells (PEMFCs), direct methanol fuel cells (DMFCs) are developing rapidly and have attracted a large amount of attention on account of their many advantages such as safe and convenient transportation process, high energy density, cleanliness and environmental protection [1,2]. Methanol oxidation is a six-electron reaction process, including the following steps: 1) electrochemical adsorption of methanol; 2) dissociation of methanol and activation of carbon-hydrogen bonds; 3) adsorption and activation of water molecules; 4) adsorption of oxygen molecules and production of carbon dioxide molecules [3,4]. In the current research on DMFCs, Pt and its alloys are mainly studied as anode catalysts due to their higher electrocatalytic activity than other metal catalysts [5]. In particular, PtRu is regarded as the optimal catalyst with remarkable electrocatalytic activity and stability on account of bi-functional mechanism and electronic interaction [6,7]. Besides, the morphology and structure of Pt catalysts have important impacts on their electrocatalytic properties for methanol oxidation [8,9]. Generally, nano-sized catalysts with large specific surface areas can offer more active ⁎ Corresponding author. E-mail address: [email protected]. (W. Yue).
http://dx.doi.org/10.1016/j.jelechem.2020.113821 1572-6657/© 2018 Elsevier B.V. All rights reserved.
sites to electro-oxidation of methanol and thereby exhibit better electrocatalytic activity than bulk materials [10,11]. However, the high surface energy of nanoparticles makes them prone to aggregate and form large agglomerates, which brings about the performance degradation. Therefore, it is necessary to load Pt catalysts on some conductive materials to improve the dispersibility and utilization of Pt nanoparticles. Carbon materials such as carbon nanotube, carbon nanofiber and porous carbon are widely used as catalyst supports because of their good electrical conductivity, large specific surface area, strong corrosion resistance and low price [12–14]. Graphene is a new type of two-dimensional carbon material which has larger surface area and higher electrical conductivity than other carbon materials [15,16]. Accordingly, graphene-based Pt catalysts usually exhibit better electrocatalytic performance than other Ptloaded carbon materials [17,18]. Graphene with intact structure and high quality can be produced by several methods such as mechanical exfoliation method, chemical vapor deposition (CVD) method and organic synthesis [19–21]. Nevertheless, highly reduced graphene oxide (rGO) which is reduced from graphene oxide (GO) is still a popular graphene substrate used for the synthesis of graphene-based composites because GO can be mass-produced through chemical exfoliation of graphite [22]. In addition, the oxygen-containing groups of GO (e.g., carboxyl and hydroxyl groups) are able to serve as the nucleation sites for Pt catalysts [23]. Although the residual oxygen-containing groups are almost removed during the
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vigorously for 0.5 h. The mixture solution was transferred into a Teflonlined stainless steel autoclave and heated at 120 °C for 24 h. Polyaniline was formed and connected EEG/rGO and Pt nanoparticles under solvothermal conditions. After the temperature of autoclave dropped to the room temperature, the resulting black suspension was centrifuged and washed with ethanol three times to remove DMF. Finally, EEG-PANI-Pt or EEG-PANI-Pt was obtained by drying at 60 °C. EEG-Pt and rGO-Pt were synthesized by a similar method except that aniline and hydrochloric acid were not used.
reduction process of GO [24,25], many defects are still left on the surface of rGO, leading to a decline in the electrical conductivity and mechanical strength. Recently, a kind of high-quality graphene is simply massproduced by a modified electrochemical exfoliation method [26,27]. Electrochemically exfoliated graphene (EEG) shows better electrical and mechanical properties than rGO because few oxygen-containing groups and defects are present on the surface of EEG [28,29]. These characteristics make EEG more suitable as a carrier for building graphene-based hybrids with better electrocatalytic properties than rGO-based hybrids. Nevertheless, lack of nucleation sites makes it difficult to grow or anchor Pt nanocrystals on EEG [30]. In addition, EEG is only dispersed in polar aprotic solvents such as N,N-dimethylformamide (DMF) owing to its hydrophobic surface, which limits the growth conditions of Pt nanocrystals [31]. Modifying the EEG surface by noncovalent functionalization or covalent functionalization is an effective method for promoting the growth of Pt catalysts on graphene. Polyaniline (PANI) as a conductive polymer has attracted a lot of attention owing to its high chemical stability, electrochemical stability, electrical conductivity and hydrophilicity [32]. It is reported that PANI-modified rGO furnishes more active sites for the growth of Pt catalysts and the connection between PANI and Pt is favorable for the dispersion and stability of Pt catalysts on rGO [33,34]. Three kinds of interactions occur between rGO and PANI, including electrostatic interaction, hydrogen bonding and π–π interaction [35]. In view of the structural difference between EEG and rGO, the interaction of EEG with PANI and the effect of PANI-modified EEG on the electrocatalytic performance of Pt catalysts need to be further explored. In this work, Pt nanoparticles are successfully synthesized on EEG (EEGPt) by a solvothermal method. Compared to rGO-based Pt (rGO-Pt) prepared under the same conditions, EEG-Pt shows enhanced methanol electro-oxidation activity because EEG as a catalyst carrier shows better electrical and mechanical properties than rGO. To further raise the electrocatalytic performance of Pt catalysts on EEG and rGO, the substrates are modified by PANI prior to growing Pt nanoparticles (EEG-PANI-Pt and rGO-PANI-Pt). The results show that the methanol electro-oxidation activity of either EEG-PANI-Pt or rGO-PANI-Pt is highly improved by PANI modification. The synergistic interaction of Pt and N species can facilitate the dispersion of Pt nanocrystals on EEG or rGO, and further improves the electron conductivity of EEG-Pt or rGO-Pt, which is favorable for the electrocatalytic performance improvement. More importantly, the interaction of EEG and PANI is much different from the interaction of rGO and PANI owing to the different surface structures of EEG and rGO. Compared to the interactions of traditional rGO with PANI, which mainly include the electrostatic interaction (–N+O−–) and the hydrogen bonding (–O–H⋯N–) [34,35], the π-π interaction is the predominant interaction between EEG and PANI due to the π-conjugated structure of EEG. Therefore, more N species of PANI on EEG can be released for the deposition of Pt nanocrystals, which promotes the electrocatalytic performance of Pt catalysts. On the contrary, the connection of rGO with PANI through the electrostatic interaction and hydrogen bonding prevent nitrogen atoms from becoming nucleation sites for the growth of Pt nanoparticles. Consequently, the π-conjugated structure of graphene is very important to the interactions between graphene and PANI as well as the electrocatalytic performance of Pt catalysts on PANI-modified graphene.
2.2. Sample characterization X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), ICP atomic emission spectroscopy, field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) were used to characterize samples. Detailed characterization methods were supplied in the Supporting information. 2.3. Electrocatalytic measurements The electrocatalytic performances of samples were measured by using CHI 700E and Gamry Interface 1000 electrochemical workstation. Detailed test methods were supplied in the Supporting information. 3. Results and discussion Fig. 1 shows the synthesis routes for rGO-PANI-Pt and EEG-PANI-Pt. Aniline is first adsorbed on the surface of GO or EEG and then polymerized to PANI layer under solvothermal treatment. Meanwhile, GO is reduced to rGO and H2PtCl6 is reduced to Pt by DMF under solvothermal conditions. The N species of PANI layer can substitute the oxygen-containing functional groups of GO as new nucleation sites for the growth of Pt nanoparticles because of forming a coordination bond between H2PtCl6 and N atom. Accordingly, Pt nanoparticles are more uniformly distributed on rGO or EEG after surface modification by PANI. Three kinds of interactions occur between rGO and PANI, including electrostatic interaction, hydrogen bonding and π–π interaction [35]. However, the lone pair of electrons on N atoms is occupied as forming the electrostatic interaction and hydrogen bonding between rGO and PANI, causing Pt nanoparticles not to form on the N species of PANI. On the contrary, the π–π interaction is the main interaction between EEG and PANI on account of the intact surface structure of EEG, leading to more N species becoming the nucleation sites for the growth of Pt nanoparticles. Therefore, the π-conjugated structure of graphene is very important to induce the function of PANI and further improve the electrocatalytic activity and durability of Pt catalysts for methanol oxidation. The morphologies and structures of rGO-Pt and EEG-Pt with or without PANI modification were characterized by SEM and TEM. SEM images (Fig. 2a,b) show that Pt nanoparticles are successfully deposited on the surface of rGO or EEG. Considering that EEG has good solubility in DMF, this solvothermal method that using DMF as both solvent and reducing agent can expand the synthesis route for EEG hybrids. TEM images (Fig. 2c,d) also confirm the growth of Pt nanocrystals on rGO or EEG. According to the nanoparticle size distribution based on statistical results (Fig. S1), the average size of Pt nanoparticles on rGO (~5.2 nm) is slightly larger than that on EEG (~4.0 nm). In addition, Pt nanoparticles are more uniformly distributed on EEG rather than on rGO. It is inferred that the functional groups and defects of GO may serve as the nucleation sites for the growth of Pt nanocrystals, resulting in the fast growth and aggregation of Pt nanoparticles on rGO. On the contrary, the polymerized precursor molecules are homogeneously adsorbed on the intact surface of EEG by intermolecular interactions and then grow to Pt nanocrystals under solvothermal conditions [28,31]. HRTEM images (Fig. 2e,f) show fine crystalline nanoparticles on rGO and EEG. The spacing of lattice fringes is ~0.226 nm, which coincides with the d-spacing of the (111) plane of the face-centered cubic (fcc) Pt crystal. Furthermore, the lattice spacing of ~0.215 nm corresponding to the
2. Experimental 2.1. Sample preparation EEG and GO were prepared according to the published literatures (see the Supporting information) [22,26]. EEG-PANI-Pt and rGO-PANI-Pt were synthesized by a solvothermal method since EEG cannot be dispersed in water. Briefly, 0.02 g of EEG or GO was uniformly dispersed in 60 mL of N,N-dimethylformamide (DMF) under ultrasonic conditions for 1 h. 0.4 g of aniline and 2 mL of 1 M hydrochloric acid were slowly added into the EEG or rGO solution and stirred for 0.5 h. Subsequently, 0.05 g of chloroplatinic acid was added into the above solution and stirred 2
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Fig. 1. Schematic illustration of the synthesis routes for (a) rGO-PANI-Pt and (b) EEG-PANI-Pt. Three kinds of interactions occur between rGO/EEG and PANI.
XRD and Raman measurements were implemented to study the crystal phase and surface structure of rGO-Pt, rGO-PANI-Pt, EEG-Pt and EEGPANI-Pt. XRD patterns of these four samples (Fig. 4a) show characteristic diffraction peaks at 39.8°, 46.3° and 67.5°, which correspond to the (111), (200) and (220) planes of Pt crystal (JCPDS No. 70-2431), respectively. The broad peaks manifest the small particles size of Pt nanocrystals, which is consistent with the TEM observations. The peak related to the basal spacing of stacked GO (~11°) shifts to ~24.3°, demonstrating the reduction of GO to rGO. The reduced interlamellar spacing means that the oxygen-containing groups of GO are nearly eliminated under solvothermal conditions. The peak belongs to the stacked EEG is centered at a larger diffraction angle (26.5°), implying that the amount of oxygen-containing groups of EEG is even less than that of rGO. The intact surface of EEG was further verified by Raman analysis. The D band at 1352 cm−1 is connected with the vibration of the sp3 carbon atom graphite, and the G band at 1582 cm−1 is connected with the in-plane vibration of sp2 carbon atom [36]. Thus the intensity ratio of ID/IG is used to evaluate the surface integrity of EEG and rGO hybrids. Raman spectra (Fig. 4b) show that the ID/IG value of EEG-Pt (0.43) is much lower than that of rGO-Pt (1.24), reflecting the high surface integrity of EEG even after growth of Pt nanocrystals. On the contrary, in spite of removing most functional groups, the surface defects still result in a large ID/IG value of rGO-Pt. The fast electron transfer through the EEG substrate can effectively accelerate the methanol oxidation reaction on Pt catalysts. On the other hand, the ID/IG values of rGOPANI-Pt and EEG-PANI-Pt are 0.38 and 1.20 respectively, lower than those of unmodified samples. The decreased ID/IG values are attributed to the introduction of PANI, which raises the amount of sp2 carbon atoms. In addition, the PtN bonds replacing the PtC bonds also contribute to the decrease of ID/IG values. FT-IR and XPS characterizations were performed to further examine the composition and chemical bonding of samples. FT-IR spectrum of GO (Fig. S2a) shows several absorption bands corresponding to the CO, C-OH
(100) planes of graphite is also observed in the HRTEM image of EEG-Pt (Fig. 2f), demonstrating the intact surface of EEG with few defects. In contrast, the characteristic hexagonal lattice of graphene is hardly observed in the HRTEM image of rGO-Pt (Fig. 2e) because there are lots of defects on rGO after removing functional groups. The surface defects would reduce the electrical conductivity of rGO, which is not conducive to the electrocatalytic activity of Pt catalysts. To improve the distribution of Pt catalysts on rGO as well as the interaction of Pt catalysts with rGO, the surface of rGO is modified by PANI. For comparison, the surface of EEG is also modified by PANI, which can dramatically raise the nucleation sites of EEG for the growth of Pt catalysts. SEM images (Fig. 3a,b) present the better dispersion of Pt nanoparticles on rGO after PANI modification, while there is no distinct difference in the distribution of Pt nanoparticles on EEG before or after PANI modification. The particle size of Pt nanocrystals on rGO or EEG is also not changed much after PANI modification (Fig. S1). It is inferred that the N species of PANI substitute the functional groups of rGO as nucleation sites for the growth of Pt nanoparticles, leading to the well-distributed of Pt nanocrystals on rGO. However, the growth of Pt nanoparticles on EEG is based on the adsorption of the polymerized precursor molecules on EEG because of the intact surface of EEG. After PANI modification, the ordered structure of PANI also offers dispersed nucleation sites for the growth of Pt nanocrystals. Thus the dispersibility of Pt nanoparticles on PANImodified EEG is not much different from that on EEG. In spite of the high dispersion of Pt nanoparticles on both EEG and PANI-modified EEG, the synergistic interaction of Pt and N species still enhances the electrocatalytic properties of Pt catalysts for methanol oxidation. TEM images (Fig. 3c,d) also confirm the well-dispersed Pt nanoparticles on PANI-modified rGO or EEG, and the particle agglomerates are hardly observed. The lattice fringes of Pt nanoparticles with a spacing of ~0.226 nm are visible in the HRTEM images (Fig. 3e,f), illustrating that Pt nanocrystals can grow on the PANI layer by forming PtN bonds. 3
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Fig. 2. SEM, TEM and HRTEM images of (a, c, e) rGO-Pt and (b, d, f) EEG-Pt.
Pt catalysts on rGO or EEG. Moreover, the intensity ratio of the C 1s peak to the O 1s peak for rGO-Pt (~4.2) is lower than that for EEG-Pt (~6.6), reflecting that more functional groups are left on rGO. The N 1s peaks emerge in the survey spectra of rGO-PANI-Pt and EEG-PANI-Pt because of the introduction of PANI. The high-resolution C 1s XPS spectra (Fig. 5b) show four peaks related to the CC, CO, CO and O-C=O groups [37]. The peaks related to CO, CO and O-C=O groups are too weak in comparison with the CC peak. By summarizing the XRD, Raman, FT-IR and XPS results, it is concluded that EEG has few functional groups and defects on the surface even after the growth of Pt nanocrystals. In contrast, although most
and CO groups, which are derived from carboxyl, hydroxyl and carbonyl groups [28]. The peaks related to the CO and CO groups vanish in the spectra of rGO-Pt and rGO-PANI-Pt, indicating that GO is highly reduced through solvothermal treatment. The CO and CO peaks are not detected and the C-OH peak becomes weaker in the FT-IR spectrum of EEG (Fig. S2b), revealing that EEG itself possesses few functional groups and defects. In view of the intact surface of EEG, the harsh synthesis conditions such as hydrothermal or solvothermal treatment are not necessary. The peaks of C 1s, O 1s, Pt 4d and Pt 4f peaks emerge in the XPS survey spectra of rGO-Pt and EEG-Pt (Fig. 5a), demonstrating the successful formation of 4
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Fig. 3. SEM, TEM and HRTEM images of (a, c, e) rGO-PANI-Pt and (b, d, f) EEG-PANI-Pt.
resolution N 1s XPS spectra (Fig. 5c) show that the N 1s peak is further deconvoluted into three peaks at 398.8, 399.7 and 400.7 eV, which are assigned to quinonoid imine (=N–), benzenoid amine (–NH–) and positively charged nitrogen (–N+–), respectively [35]. After the growth of Pt nanocrystals on rGO-PANI or EEG-PANI, the intensities of –NH– and – N+– peaks decrease distinctly due to the formation of PtN bond. It is worth noting that the decrease of –NH– and –N+– peak intensities is more obvious in the spectrum of EEG-PANI-Pt than in the spectrum of rGO-PANI-Pt, implying that more amount of Pt catalysts are formed on the PANI layer of EEG-PANI than rGO-PANI. The PANI layer is combined
functional groups are eliminated after solvothermal reduction, rGO still has more functional groups than EEG, and plenty of defects remain on the surface of rGO. The structural difference between the carbon supports would make a great impact on the electrocatalytic activity of Pt catalysts. As shown in Fig. 1, the interactions between rGO and PANI include electrostatic interaction, hydrogen bonding and π–π interaction due to the functional groups and π-conjugated structure of rGO. In contrast, the π–π interaction is the predominant interaction between EEG and PANI because of the intact surface of EEG. The different interactions with PANI could strongly affect the growth of Pt nanocrystals on PANI layers. The high 5
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Fig. 4. (a) XRD patterns and (b) Raman spectra of rGO-Pt, rGO-PANI-Pt, EEG-Pt and EEG-PANI-Pt.
Fig. 5. XPS spectra of rGO-Pt, rGO-PANI-Pt, EEG-Pt and EEG-PANI-Pt: (a) survey, (b) high-resolution C 1s and (d) high-resolution Pt 4f. (c) High-resolution N 1s XPS spectra of rGO-PANI, rGO-PANI-Pt, EEG-PANI and EEG-PANI-Pt.
with EEG mainly through the π-π interaction, which can expose the –NH– and –N+– groups for the growth of Pt catalysts. On the contrary, the – NH– and –N+– groups of PANI are occupied by the functional groups of rGO such as –OH and O=C–O−, making it impossible to combine with Pt. It is well-known that the synergistic interaction of Pt and N species
can facilitate the electrocatalytic activity and durability of Pt catalysts. Accordingly, Pt catalysts are expected to display better electrocatalytic performance on EEG-PANI than rGO-PANI. The high-resolution Pt 4f XPS spectra (Fig. 5d) show two peaks centered at 71.5 eV and 74.8 eV respectively, representing two spin-orbits split of Pt 4f7/2 and Pt 4f5/2. After peak 6
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subdivision, two pairs of peaks belonging to Pt0 (~71.5 eV and ~74.8 eV) and Pt2+ (~72.9 eV and ~76.2 eV) are obtained. The Pt2+ part may be derived from PtO or Pt(OH)2, which did not contribute to the electrocatalytic activity of Pt catalysts. Note that the ratios of peak intensities of Pt0 and Pt2 + are very similar for rGO-Pt (2.3) and EEG-Pt (2.6). However, after combination with PANI, the ratio of Pt0 and Pt2+ parts increases to ~4.9 for EEGPANI-Pt, much higher than that for rGO-PANI-Pt (3.6). This result shows that the PANI layer on EEG also facilitates the formation of metal Pt catalysts. The electrocatalytic activity and durability of rGO-Pt, EEG-Pt, rGOPANI-Pt and EEG-PANI-Pt for methanol oxidation were evaluated by cyclic
voltammetry (CV). The contents of Pt in these four samples are very close (18–20 wt%) based on the ICP analysis (Table S1). CV curves measured in 0.5 M H2SO4 solution (Fig. 6a,b) were first collected to calculate the electrochemically active surface area (ECSA) of Pt catalysts. EEG-Pt shows higher ECSA value (~206.9 cm2 mg−1) than rGO-Pt (~134.6 cm2 mg−1), indicating that EEG is a better catalyst support than rGO. Both rGO-PANIPt and EEG-PANI-Pt show increased ECSA (~274.4 and ~415.4 cm2 mg−1) owing to the PANI modification, which provides abundant N species for the growth of Pt nanoparticles. Methanol oxidation reaction (MOR) measurements were then performed in a solution of 0.5 M H2SO4 and 1 M methanol. CV curves (Fig. 6c,d) show two oxidation
Fig. 6. CV curves of (a) rGO-Pt, EEG-Pt and (b) rGO-PANI-Pt, EEG-PANI-Pt in 0.5 M H2SO4. CV curves of (c) rGO-Pt, EEG-Pt and (d) rGO-PANI-Pt, EEG-PANI-Pt in 0.5 M H2SO4 and 1 M CH3OH. The forward peak current densities of (e) rGO-Pt, EEG-Pt and (f) rGO-PANI-Pt, EEG-PANI-Pt as a function of the cycle number for methanol oxidation. 7
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(~3.9 Ω) than rGO-Pt (~11.3 Ω) due to the intact surface of EEG with few defects. It is interesting to find that the charge transfer resistance of rGO-Pt and EEG-Pt is reduced by PANI modification, i.e., ~9.4 Ω for rGOPANI-Pt and ~2.5 Ω for EEG-PANI-Pt. It is inferred that the introduced N atoms can modify the electronic structure of Pt atoms and thereby improve the electrical conductivity of Pt-based hybrids [40]. In addition, the presence of π-π conjugated system in EEG-PANI-Pt is also favorable for the electron transport.
peaks that are associated with the oxidation of methanol (the forward peak) and the oxidation of the residual carbonaceous species (mainly CO) formed during the forward-scan (the backward peak). The forward peak current density of EEG-Pt (~278.5 mA mg−1 Pt ) is much higher than that of rGO-Pt (~167.2 mA mg−1 Pt ), demonstrating that EEG-Pt has higher electrocatalytic activity toward methanol oxidation than rGO-Pt. The electrocatalytic activity of rGO-Pt and EEG-Pt is further improved by PANI modification; for instance, higher forward peak current densities are achieved for rGO-PANI-Pt −1 (~313.3 mA mg−1 Pt ) and EEG-PANI-Pt (~443.7 mA mgPt ). The synergistic interaction between Pt and PANI can facilitate the dispersion of Pt nanoparticles, stabilize Pt nanoparticles during cycling, and improve the electrical conductivity of Pt-based hybrids, which is favorable for the electrocatalytic performance improvement. Moreover, the combination of EEG with PANI would offer more N species for the growth of Pt nanocrystals, which can further promote the electrocatalytic performance of Pt catalysts. The MOR performance comparison of EEG-PANI-Pt with reported graphene through polyaniline modification supported Pt hybrids is listed in Table S2. In addition, the CO-tolerant ability of catalysts can be estimated by the ratio of the forward and backward peak currents (If/Ib). The If/Ib values of rGO-Pt and EEG-Pt (1.11 and 1.17) decrease from 1.11 and 1.17 to 1.03 and 0.98 after PANI modification. The decreased CO tolerance of Pt catalysts is ascribed to the high electrocatalytic activity of Pt catalysts induced by PANI, which produces more amount of residual CO. Introduction of catalyst promoters such as SnO2 can effectively enhance the tolerance of Pt catalysts to CO poisoning [38]. All parameters related to the electrocatalytic performance are summarized in Table S1. The long-term electrocatalytic stability of rGO-Pt, EEG-Pt, rGO-PANI-Pt and EEG-PANI-Pt are also tested and represented in Fig. 6e,f. Although the If value of EEG-Pt drops to ~174.1 mA mg−1 Pt after 200 cycles, it is still higher than that of rGO-Pt after 200 cycles (~83.4 mA mg−1 Pt ), and the catalytic activity retention of EEG-Pt (63%) is also higher than that of rGO-Pt (50%). After PANI modification, the catalytic activity retentions of rGO-PANI-Pt and EEG-PANI-Pt increase to 64% and 80%, respectively. The enhanced anti-poisoning ability of Pt catalysts is attributed to the synergistic interaction between Pt and PANI, and the formed PtN bonds can immobilize Pt nanoparticles on PANI to prevent agglomeration and detachment during cycling. Since the structure of EEG-PANI is beneficial to exposing more N species of PANI for immobilizing Pt catalysts, EEG-PANI-Pt exhibits better electrocatalytic durability for methanol oxidation. The electrical conductivity of Pt-based anodes is an important factor for the electrocatalytic performance of Pt catalysts. Thus EIS measurements were conducted on these four samples. Nyquist plots (Fig. 7) show a semicircle in the high frequency range and an oblique line in the low frequency range. The diameter of the semicircle is equal to the charge transfer resistance (Rct) of samples, which can be calculated by using the equivalent circuit model (Fig. S3) [39]. It can predict that EEG-Pt has a lower Rct value
4. Conclusions We proposed a novel high-quality EEG to replace the popularly used rGO as the catalyst support for Pt catalysts. Pt nanocrystals are successfully deposited on EEG by a solvothermal method despite lack of nucleation sites on the surface of EEG. Compared to rGO-Pt, EEG-Pt exhibits superior electrocatalytic activity and durability for methanol oxidation on account of the outstanding properties of EEG. Moreover, the electrocatalytic performance of rGO-Pt and EEG-Pt can be further improved by PANI modification. The synergistic interaction of Pt catalysts and PANI can hinder the aggregation and detachment of Pt catalysts during cycling and raises the electrical conductivity of Pt-based hybrids, which is favorable for the electrocatalytic performance improvement. More importantly, in view of the different interactions of rGO and EEG with PANI, PANI-modified EEG can expose more N species for the growth of Pt nanocrystals, which can further promote the electrocatalytic performance of Pt catalysts. Therefore, the πconjugated structure of graphene is very significant for the electrocatalytic performance improvement of Pt catalysts by fully releasing the function of PANI.
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was financially supported by National Natural Science Foundation of China (21573023 and 21975030). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jelechem.2020.113821.
Fig. 7. Electrochemical impedance spectra of (a) rGO-Pt, EEG-Pt and (b) rGO-PANI-Pt, EEG-PANI-Pt in 0.5 M H2SO4 and 1 M CH3OH. 8
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