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Polydopamine-Graphene Oxide Derived Mesoporous Carbon Nanosheets for Enhanced Oxygen Reduction Received 00th January 20xx, Accepted 00th January 20xx DOI: 10.1039/x0xx00000x www.rsc.org/
Konggang Qu, Yao Zheng, Sheng Dai,* and Shi Zhang Qiao* Composites materials combining nitrogen-doped carbon (NC) with active species represent a paramount breakthrough as the alternative catalysts to Pt for oxygen reduction reaction (ORR) due to their competitive activity, low cost and excellent stability. In this paper, a simple strategy is presented to construct the graphene oxide-polydopamine (GD) based carbon nanosheets. This approach does not need to modify graphene and use any catalyst for polymerization under ambient conditions, and the obtained carbon nanosheets possess adjustable thickness and uniform mesoporous structure without using any template. The thickness of GD hybrids and carbonization temperature are found to play crucial roles on adjusting the microstructure of the resultant carbon nanosheets and, accordingly their ORR catalytic activity. The optimized carbon nanosheet generated by 5 nm thickness of GD hybrid after 900 oC carbonization exhibits superior ORR activity with an onset potential of -0.07 V and a kinetic current density of 13.7 mA cm-2 at -0.6 V . The unique mesoporous structure, high surface areas, abundant defects and favorable nitrogen species are believed to significantly benefit the ORR catalytic process. Furthermore, it also shows remarkable durability and excellent methanol tolerance outperforming those of commercial Pt/C. In view of physicochemical versatility and structural tunability of polydopamine (PDA) materials, our work would shed a new light on the understanding and further development of PDA-based carbon materials for highly efficient electrocatalysts.
1. Introduction The electrochemical efficiency and performance of fuel cells are severely restricted by the sluggish kinetics of the cathodic oxygen reduction reaction (ORR).1 Although precious Pt has been adopted as an effective ORR electrocatalyst,2,3 large-scale commercial utilization has been hindered by its prohibitive cost, limited supply, and poor durability.4 The state-of-the-art catalysts combining nitrogen-doped carbon (NC) and other active species represent a paramount breakthrough as the alternative to Pt because of the competitive activity, low cost and significantly enhanced stability.5-8 The NC materials can be prepared directly by carbonization of nitrogen-containing organic precursors or nitrogen post-doped materials. The precursor choices for preparing NC materials exert particularly crucial influence on the structures, properties and catalytic performance of final products.9,10 The most popular precursors including melamine,8 polypyrrole,11,12 and polyaniline13,14 have been proved to be excellent candidates for NC materials preparation; however, they still suffer from multiple drawbacks such as limited solubility, complicated synthesis
process, low residual yield and weak nanostructural tunability, which have greatly impeded their further potential exploration 15 and commercialization. Polydopamine (PDA) and its derivative materials have been extensively studied for various applications in energy, 16 environmental and biomedical fields over the last decade. PDA can be obtained through the self-polymerization of dopamine (DA) under alkaline condition and can spontaneously deposit on the surface of various solid materials 17 regardless of their chemical nature. As one kind of nitrogen source, PDA displays many striking features surpassing the 8 11,12 above-mentioned precursors of melamine, polypyrrole, 13,14 and polyaniline . The first is its excellent physical properties. DA and PDA can be dissolved in water and most common polar organic solvents, which greatly facilitates its processability. PDA and its derived carbon materials possess excellent 18,19 The second is its structural electrical conductance. tunability. PDA can be facilely controlled to manufacture into any desired nanostructure in terms of the used substrates 20,21 19 including solid and hollow spheres, nanofilms, and 22 nanofibers. Moreover, the thickness of the layer-structured PDA can be easily and precisely tailored at the nanometre scale. The third is the post-modification of PDA. PDA is extremely reactive to amine or thiol functional groups via 17, 23,24 Schiff base or Michael addition reactions. The catechol 25,26 groups of PDA can also react with boric acid easily. These reactions proceed efficiently at room temperature without the need of any harsh reaction condition, and thus are particularly
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powerful for electrocatalytical applications since it enables further introduction of more heteroatoms including nitrogen, sulfur and boron into PDA, and thereby greatly improving the performance of resultant PDA-based carbon materials. Fourthly, PDA displays strong chelation to multivalent metal 27-30 cations owing to its various functional groups; therefore, it can also provide an excellent platform for constructing metalN decorated carbon materials which are appealing substitutes for conventional precious metals as electrocatalysts. Finally, o 21 pure PDA can produce ~ 60 % carbon yield in N2 at 800 C, which is much higher than that of other abovementioned 8,12,31 precursors. In short, the unique and outstanding virtues of PDA render it highly promising as a simple, effective and versatile candidate for the preparation of NC based electrocatalysts. In the recent years, PDA has showed some potentially exciting applications in energy fields including batteries and supercapacitors. However, PDA were mostly serviced as the building block for the construction of metallic or non-metallic composites based on its remarkable physicochemical 32,33 The direct utilization of PDA itself as the versatility. precursor of electro-catalysts has rarely been achieved possibly ascribing to the lack of full awareness of its favourable properties. Up to now, the only reported researches on the eletrocatalytical application of PDA is based on its derived hollow and solid microspheres prepared with and without 34,35 silica as templates. Considering the fascinating advantages of PDA, more detailed and in-depth investigation should be made to discover the unexplored electrocatalytic capacity of PDA-based carbon materials. Herein, we chose graphene oxide (GO) as the substrate to synthesize three kinds of GO-PDA (GD) nanohybrids with different thicknesses using a simple “mix-and-react” method. DA can self-polymerize on the surface of GO to produce a uniform PDA coating layer in which the thickness of the PDA layer can be easily tuned by adjusting the concentration of DA. The carbon nanosheets derived from GD hybrids possess adjustable thicknesses and spontaneously formed uniform mesoporous structures without using any template. The outstanding structural and component features make the newly-prepared carbon nanosheets be excellent ORR electrocatalysts. Moreover, its catalytic activity shows highly selective to the thickness of GD hybrids and the carbonization temperature. This material also possesses high methanol tolerance and excellent stability which are superior to those observed for commercial Pt/C catalyst. To the best of our knowledge, our work is the first report of PDA-graphene derived carbon nanosheets for ORR electrocatalysis. This study would lay a solid foundation for the further exploration and development of PDA-based carbon materials.
2. Experimental section 2.1 Chemicals and reagents Graphite flakes, sulfuric acid (H2SO4, 95-98%), potassium permanganate (KMnO4, 99%), phosphorous acid (H3PO4, 85%),
hydrogen peroxide (H2O2, 30%), dopamine hydrochloride, and disodium hydrogen phosphate (Na2HPO4) were purchased from Sigma-Aldrich. All chemicals were used as received without further purification. Milli-Q water (18.2MΩ) was used throughout the experiments. 2.2 Materials preparation Preparation of graphene oxide (GO). GO was synthesized from 36 natural graphite flake by an improved Hummers’ method. The detailed procedures were shown in the Supporting Information. Preparation of GO-PDA (GD) nanohybrids. In a typical experiment, 85mL GO aqueous dispersion (2 mg/mL) was mixed with 250 mg DA dissolved in 10 mL DI water and another 130 ml DI water. The mixture was sonicated for 5 minutes, 25 mL PBS buffer ( 0.4 M, pH=8.5) was added, and then the mixture was stirred for 24 hours at room temperature. The container was covered with aluminum foil with several small holes. The GD hybrids were obtained by centrifugation and washing with water for three times. The control over the thickness of GD hybrids can be easily achieved by adjusting the amount of DA in the reaction solution. Here, 125, 250 and 500 mg of DA can produce 2.5, 5 and 10 nm GD hybrids, respectively. Carbonization of GD hybrids. The as-prepared GD hybrids were carbonized in a temperature-programmable tube furnace o under N2 atmosphere at 400 C for 2 h with a heating rate of 1 o -1 o C min , which was followed by further treatment at 900 C o o -1 (700, 800 or 1000 C ) for 3 h with a heating rate of 5 C min . 2.3 Sample characterizations Fourier transform infrared (FTIR) spectra were collected on the transmission module of a Thermo Nicolet 6700 FTIR -1 spectrometer at 2 cm resolution and 64 scans. TEM images were acquired on a JEM-2100 microscopy. TEM elemental mapping was obtained through the EDAX detector attached to the JEM-2100. AFM images were acquired under ambient conditions with a Ntegra Solaris AFM (NT-MDT) operating in a tapping mode. The Raman spectra were collected on iHR550 from HORIBA Scientific equipped with a 532 nm solid laser as the excitation source. X-Ray diffraction (XRD) was performed on the Miniflx-600 (Rigaku Ltd.) under ambient conditions using a Cu Kα X-ray. XPS analysis was conducted on Axis Ultra spectrometer (Kratos Analytical Ltd.) with monochromated Al -9 Kα radiation at ca. 5×10 Pa. Thermogravimetric analysis (TGA) was conducted on the TGA/DTA system (STARe, MettlerToledo) -1 at 25-1000 ºC in a 15 ml min N2 flow and a ramp rate of 5 °C -1 min . Nitrogen adsorption-desorption isotherm was collected on the Tristar II (Micrometrics) at 77 K. Pore size distribution was obtained by Barrett-Joyner-Halenda (BJH) model using the adsorption branch on the isotherm. The specific surface areas of the materials were calculated using the adsorption data at pressure range of P/P0 = 0.05-0.3 by Brunauer-Emmett-Teller (BET) model. For the electrochemical tests, 2 mg of the catalyst was dispersed in 1 ml DI water. The mixture was slightly ultrasonicated to obtain a homogenous catalyst ink. To prepare the
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working electrode for electrochemical measurements, 20 μl of the ink was dipped on a mirror polished glass carbon electrode. Then, 5 μl of 0.5 wt. % Nafion aqueous solution was dipped on the electrode and dried at room temperature. After that, the working electrode was inserted into the three-electrode cell setup, which is composed of a platinum counter electrode, an Ag/AgCl/KCl (4 M) reference electrode in a glass cell containing 100 ml 0.1 M KOH aqueous electrolyte. A flow of O2 was maintained over the electrolyte solution (0.1 M KOH) during recording electrochemical data in order to ensure its continued O2 saturation. Cyclic voltammogram (CV), linear sweep voltammogram (LSV), and rotating disk electrode (RDE) tests were carried out using a glassy carbon rotating disk electrode. The scan rate of CVs was kept as 50 mV s-1 while that for the LSVs and RDE tests was 5 mV s-1. The data were recorded using an electrochemical analysis workstation (CHI 760C, CH Instruments, USA). The Koutecky-Levich plots were obtained by linear fitting of the reciprocal rotating speedversus reciprocal current density collected at different potentials form -0.4 to -0.8 V. The overall electron transfer numbers per oxygen molecule involved in a typical ORR process were calculated from the slopes of the Koutecky-Levich plots using the following equation: 1/jD=1/jk + 1/ Bω1/2
(1)
where jk is the kinetic current in amperes at a constant potential, ω is the electrode rotating speed in rpm, and B is the reciprocal of the slope, which was determined from the slope of Koutecky-Levich plots based on Levich equation as followed: B=0.2 nFAν–1/6CO2DO22/3
3. Results and discussion 3.1 Materials synthesis and characterization
As illustrated in Figure 1A, GD hybrids were facilely synthesized by mixing a given amount of DA with GO in PBS buffer (pH=8.5). DA can self-polymerize to form a PDA thin film directly on the surface of GO. In this study, three GD nanohybrids with different PDA thicknesses were synthesized through tuning the concentrations of DA. It is noteworthy that PBS but not Tris buffer was chosen because the primary amine group of Tris can interact covalently with PDA which may introduce extra 17, 24 nitrogen source. The as-prepared GD hybrids were washed, dried and carbonized at different temperatures. The resulting GD derived carbon materials were labeled as GDx-T, where x represents the thickness of GD hybrids and T is the final o carbonized temperature (700, 800, 900 and 1000 C). FTIR spectra shown in Figure S1 identify the functional groups of GD hybrids; the revealed peaks at 1502 and 1616 -1 cm are consistent with the indole or indoline structures of 37,38 PDA as reported earlier. The typical transmission electron microscopy (TEM) image of GD5 reveals the rougher surface than that of GO (Figures 1B and 1C) owing to the deposition of PDA, but GD5 still keeps excellent dispersibility and sheet morphology. The atomic force microscopy (AFM) analyses (Figure 1D and Figure S2) further confirm that PDA can form a uniform coating layer on the surface of GO; moreover, through adjusting the concentration of DA, three GD hybrids with the thickness of 2.5±0.25 nm, 5.0±0.10 nm, 10.0±0.05 nm are obtained.
(2)
where n is the number of electrons transferred per oxygen molecule, F is the Faraday constant (96,485 C mol-1), DO2 is the diffusion coefficient of O2 in 0.1 M KOH (1.9×10-5 cm s-1), v is the kinetic viscosity, and CO2 is the concentration of O2 (1.2×103 mol L-1). The constant 0.2 is adopted when the rotating speed is in rpm. Rotating ring-disk electrode (RRDE) voltammogram measurements were conducted on an RRDE configuration (Pine Research Instrumentation, USA) with a 320 μm gap Pt ring electrode. The linear sweep voltammograms were recorded in O2 saturated 0.1 M KOH at 1600 rpm. The disk was set to scan at 5 mV s-1 from 0.2 to -0.8 V and the ring was set at 0.5 V. The collecting efficiency of the RRDE (N) was 0.37. The peroxide yield (HO2- %) and the electron transfer number (n) were calculated as follows: HO2- % = 200×Ir/N/(Id+Ir/N) n = 4×Id/(Id+Ir/N)
(3)
(4)
where Id is the disk current and Ir is the ring current. The materials’ resistance to methanol crossover effect and stability were tested in the same setup as for the RDE test in the O2 saturated 0.1 M KOH aqueous electrolyte and were performed at a static potential of -0.3 V and -0.5 V, respectively, for the chronoamperometry at room temperature.
Figure 1. (A) illustration of the synthesis of GO-polydopamine derived carbon nanosheets. TEM images of (B) GO, (C) GD5, (E) GD5-900 and (F) the magnified square area shown in E; AFM images of (D) GD5 and (G) GD5-900.
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The morphology and structure of GD hybrids after carbonization were investigated by TEM and AFM. Figure 1 (E, F) and Figure S3 show the typical TEM images of GDx-900. It can be seen that three kinds of GDx-900 materials still inherit the sheet morphology of GD hybrids even after high temperature carbonization, however, small degrees of aggregation happen for GD2.5-900. More intriguingly, GD5-900 has much rougher surface and owns distinctive and regular porous structure with the pore size of about 3-5 nm while GD2.5-900 and GD10-900 only possess some crinkles on their surfaces. It should be nothworthy that the uniform porous structure is spontaneously formed during carbonization without involving any template, and this kind of porosity in the PDA derived carbon materials can be observed distinctly by TEM.21,35 The TEM images of the other GDx-T materials are shown in Figure S4, and only GD5-800 has similar porous structure like GD5-900. The AFM analyses (Figure 1G and Figure S2) reveal three lamellar GDx-900 materials (x=2.5, 5 and 10) have the thicknesses of 1.3 ± 0.15, 2.7±0.12, 5.5 ± 0.07 nm, respectively, which is about 52-55 % of the GDx without carbonization. In addition, the thermogravimetric analysis (TGA) curve of GD5 hybrid indicates that it can retain a carbon yield of 59.8 % after 900 oC carbonization (Figure S5). Nitrogen adsorption isotherms were used to investigate the surface areas and pore structures of as-prepared hybrid materials. The specific surface area, pore volumes and pore sizes are summarized in Table S1. As shown in Figure 2A and Table S1, both the Brunauer-Emmett-Teller (BET) surface areas and the total pore volumes of different GD5-T materials increase with increasing carbonization temperature from 700 to 900 oC and then decrease at 1000 oC. For the GD5-800 and GD5-900, their isotherms display distinct hysteresis loops at relative pressures (P/P0) from 0.45 to 1.0, which coincide with
Figure 2. (A) nitrogen adsorption-desorption isotherms and (B) pore size distribution curves of different GD5-T materials; (C) XRD patterns and (D) Raman spectra of GO, GD5 and GD5-900, the inset in D presents the changes of ID/IG ratios for different GDx-T materials as a function of carbonization temperature.
the type IV isotherm, suggesting the presence of mesopores. The pore size distribution (PSD) curves (Figure 2B) further confirm highly uniform mesoporous structures of the GD5-800 and GD5-900 with pore size centered at 4.6 and 3.9 nm. GD5800 and GD5-900 display similar pore volumes of 0.27 and 0.28 3 -1 3 cm g , which are much larger than that of GD5-700 (0.12 cm -1 3 -1 g ) and GD5-1000 (0.06 cm g ). Most importantly, it was o found that the holding at 400 C for first 2 hours during the carbonization process is necessary for the formation of porous o structures, and the direct heating to 800 or 900 C cannot produce any porous-structured materials (see Experimental section). The formation of porous structure is highly dependent on not only the carbonization process but also the thickness of GD hybrid layer. As shown in Figure S6, the isotherm of GD2.5-900 displays a hysteresis loop of type-IV, but it has a low pore volume (0.09 m2 g-1). As for the GD10-900, no porous structure is observed. Given the inconclusive molecular structure of PDA,39,40 the origin of this porosity behavior remains unclear, nevertheless, this mesoporous architecture is expected to facilitate the exposure of more active sites and diffusion of reactants during the catalytic process. The typical structural evolution during the hybrid synthesis was investigated by XRD and Raman spectra. As shown in Figure 2C, the characteristic sharp peak at 9.7o of GO corresponds to an interlayer distance of 0.91 nm, the broad and weak peak centered at 25o of GD5 may be attributed to ππ stacking interaction of the aromatic rings of coated PDA and/or GO.23,37 For the GD5-900, its XRD pattern exhibits two broad peaks at 25o and 43o which correspond to the (002) and (100) crystal faces, reflecting the successful carbonization of PDA.41 Raman spectra (Figure 2D) dispaly the typical D bands resulting from the defects and disordered structure and G bands from sp2-hybridized graphitic carbon atoms at roughly 1350 and 1580 cm-1, respectively.42 The intensity ratio of D peak to G peak (ID/IG) has been conveniently used to estimate the amount of defect and disordered structure. The inset in Figure 2D depicts the change of ID/IG ratios as a function of carbonization tempareture. Interestingly, for each kind of GDx hybrids, the GDx-900 always has the largest ID/IG ratio, moreover, the ID/IG ratio of GD5-900 is larger than that of GD2.5-900 and GD10-900. Meanwhile, GD5-800 reveals similar ID/IG ratio as that of GD10-900. This abnormal trends are consistent to the results of nitrogen adsorption and can be ascribed to the formation of porous structure which can decompose the graphitic structure and lead to more defects and disordered structure. The compositions of the as-prepared materials were then analyzed by TEM elemental mapping and X-ray photoelectron spectroscopy (XPS). The typical elemental mapping images of GD5-900 (Figure S7) illustrate the presence of C, N, and O elements which are uniformly distributed on the carbon nanosheets. The chemical status of these elements were evaluated by XPS as shown in Figure 3 and Figure S8. XPS survey scans indicate the presence of carbon, oxygen and nitrogen elements on surface. Additionally, the high-resolution N 1s spectra can be deconvoluted into two peaks locating at 398.0 eV and 400.8 eV which are assigned to pyridinic N and
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graphitic N, respectively. This indicates the pyrrolic N (399.9 eV) of PDA has been completely transformed into pyridinic and 43 graphitic N after carbonization. As shown in Figure 3C, the total nitrogen content is found to decrease as the carbonization temperature increases. More specifically, the percentage of graphitic nitrogen first inceases slightly as the o temperature is raised from 700 to 900 C and then decreases, while that of pyridinic nitrogen keeps a gradual decrease beacuse the pyridinic nitrogen is not thermally stable and can convert to graphitic nitrogen at higher carbonization temperature. The total nitrogen content inceases undoubtedly with the increase of the thickness of GD hybrid materials (Figure 3D); however, GD5-900 has the highest percentage of pyridinic nitrogen among the GDx-900 materials because pyridinic nitrogen generally exists at the edge and defect positions and GD5-900 owns highest surface area and most defect structure verified by nitrogen adsorption analyses and Raman spectra. Compared with previously reported methods to prepare nitrogen-doped graphene-based carbon nanosheets,44-46 the strategy presented here is a simple “mix-and-react” process which does not need any catalyst, high temperature, special atmosphere and pre-modification of graphene. The thicknesses of resultant carbon nanosheets can be precisely adjustable by controlling ration of GO and DA. Especially, our results show that the uniform mesoporous structure can be spontaneously formed without using any template and the formation of mesoporous structure is highly selective to the thickness and carbonization process.47-49 It should be noted that two-component (graphene and PDA) integrated materials have excellent sturctural stability arising from the highly strong adhesive effect of PDA.50 Additionally, the resulting carbon nanosheets own large surface area (up to 272.3 m2 g-1),
Figure 3. (A) XPS survey scan and (B) the corresponding highresolution N 1s spectrum of GD5-900; (C) the content of different N species present in different GD5-T materials; (D) the percentages of graphitic N and pyridinic N in different GDx-900 materials.
abundant defect sites (ID/IG ratio ~1.15) and favorable nitrogen species (only pyridinic and graphitic N) transformed from inherent pyrrolic N of PDA, all of these features would make these materials particularly beneficial to the electrocatalytic applications. 3.2 Electrocatalytic analysis
The ORR was employed as a probe reaction to investigate the intrinsic advantages of these hybrid carbon nanosheets. Since GDx-900 (x = 2.5, 5 and 10) has obvious structure advantages among all synthesized GDx materials, their active ORR catalyst performance is presented and discussed in this work. Firstly, the cyclic voltammograms (CVs) and the linear sweep voltammograms (LSVs) of GO, GD5 and GD5-900 were examined in O2-saturated 0.1 M KOH solution. As shown in Figures 4A and 4B, both CVs and LSVs of GO and GD5 show their negligible ORR catalytic activities. However, the CV curve of the GD5-900 displays a well-defined characteristic ORR peak at -0.26 V and its LSV curve shows a high ORR onset potential of -0.07 V with a current density of 3.50 mA cm-2 at -0.6 V. These results apparently indicate the synthesized carbon nanosheets can function as efficient ORR catalysts. To obtain additional insight about the effect of three carbon nanosheets with different thicknesses on ORR activities, the LSVs of GDx-900 (x = 2.5, 5 and 10) were recorded and compared in O2-saturated electrolyte on a rotating disk electrode (RDE) at 1600 rpm (Figure 4C). Remarkably, GD5-900 exhibits an ORR onset potential of -0.07 V, which is much more positive than that of GD2.5-900 and GD10-900 (-0.10 V and -0.14 V, respectively). On the other hand, the ORR current density of GD5-900 is also higher than those of the other two materials over the whole potential range. At -0.6 V, GD5-900 shows a current density of 3.50 mA cm-2, surpassing GD2.5-900 and GD10-900 with their current densities of 2.82 and 3.23 mA cm-2. The most positive onset potential and highest current density of GD5-900 further reveal that this material has clearly best catalytic performance, which is most likely due to its larger surface areas and more accessible catalytic active sites resulting from its unique and abundant mesoporous structure. The electrochemical active surface areas (ECSA) of various GDx900 catalysts were estimated, and the thickness dependence of the ECSA for these catalysts follows similar trend as the BET results (Figure S9). Compared with previously reported ORR catalytical materials, the onset potential of GD5-900 is much more positive than those of single-doped graphene including N-graphene and S-graphene and can be comparable to those of graphitic C3N4 and dual-doped graphene including B, Ngraphene and N, S-graphene;8,51,52 and its current density is also comparable to those of graphitic C3N4, Fe3O4/N-graphene and CoO/carbon nanotube.8,53,54 Although this GD5-900 cannot outperform the Pt/C, considering the versatile physicochemical properties of PDA, it has much room for the improvement of catalytic activity through further introduction of more heteroatoms and/or nonprecious metal elements. To quantitatively understand the ORR activities of these -1 -1/2 GDx-T catalysts, the Koutecky-Levich (K-L) plots (J vs ω ) were calculated from the LSVs and compared at -0.6 V and
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Figure 4. (A) CV curves and (B) LSVs at a sweep rate of 5 mV s-1 of GO, GD5 and GD5-900; (C) LSVs at a sweep rate of 5 mV s-1, (D) the calculated K-L plots and (E) kinetic current density (Jk) and electron transfer number (n) of GD2.5-900, GD5-900, GD10900 and Pt/C; (F) H2O2 yield (solid, primary Y-axis) and the corresponding electron transfer number (dash, second Y-axis) of GD2.5-900, GD5-900 and GD10-900. various rotating speeds (Figure 4D); the K-L plots were also obtained at other potentials as presented in Figure S10, which display excellent linearity at different rotating speeds. Noticeably, GD5-900 shows the highest ORR current density, followed by GD2.5-900 and GD10-900. Furthermore, the electron transfer numbers (n) and kinetic current density (JK) could be obtained from the slopes and the intercepts of linear K-L plots on the basis of the K-L equation (Figure 4E). GD5-900 shows the largest n value of 3.7 and the highest JK value of 13.7 mA cm-2 at -0.6 V, much closer to those of Pt/C (4.0 and 16.4 mA cm-2) as compared to those of GD2.5-900 (3.1 and 7.95 mA cm-2) and GD10-900 (3.0 and 13.0 mA cm-2). These results explicitly suggest that the efficient four-electron reduction dominates the catalytic process on GD5-900, whereas the ORR process for GD2.5-900 and GD10-900 is a combined pathway of 2e- and 4e- reduction. Apparently, the largest electron transfer number and highest kinetic current density displayed on GD5900 confirm its best ORR performance. The rotating ring-disk electrode (RRDE) technique was used to further verify the ORR pathway by monitoring the formation of intermediate peroxide species during the ORR process (Figure 4F). Notably, the ORR on GD5-900 yields about 17.326.6% H2O2 over a wide potential range from -0.2 to -0.8 V with n ranging from 3.47 to 3.65. On the other hand, GD2.5-900, its n ranges from 3.03 to 3.35, gives ∼32.6-48.4% H2O2 under
identical conditions. GD10-900, its n ranges from 2.74 to 3.19, gives even higher amount of H2O2 (∼40.6-63.0%) in the reaction. These results are in accordance with the RDE measurements, indicating the highest electrocatalytic efficiency of GD5-900. Further CV and LSV studies were carried out to evaluate the influence of carbonization temperature on materials’ electrocatalytic activities (Figure S11). Based on the onset potentials obtained from LSV curves, we can see that electrocatalytic activities of GDx catalysts share a similar trend with increasing carbonization temperature, where the ORR electrocatalytic activity first improves as the carbonization temperature rises from 700 to 900 °C but subsequently drops as the temperature further increases from 900 to 1000 °C. It can be explained by the most defect sites possessed by the GDx-900 for different GDx hybrids (Figure 2D and S9), which can provide most electrocatalytic active sites. However, the onset potential of GD5 material shows an improvement of 0.2 V when the carbonization temperature rises from 700 to 900 °C, which is much larger than that of GD2.5 and GD10 under the same condition, about 0.05 V and 0.1 V, respectively. Expectedly, the optimized ORR performance was observed on GD5-900, which is more superior to GD2.5 and GD10 irrespective of their carbonization temperature. That is because the generation of unique mesoporous structure on GD5-900 results in largest surface areas and most defect sites, which bring about its largest performance improvement. Meanwhile, GD5800 has the second best electrocatalytic activity which is close to that of GD5-900 on account of their similar structures. These results suggest ORR performance is highly correlated with the nanostructure of catalysts. The stability of GD5-900 was further assessed through the chronoamperometric measurement under a constant cathodic voltage of -0.5 V. For comparison, the activity of Pt/C was also measured in the same way. As shown in Figure 5A, GD5-900 exhibits a high stability with a very slow attenuation over 40 h, retaining 88.1 % of the initial current, whereas Pt/C shows nearly 40 % loss of its initial current over the same time period, confirming a much better stability of GD5-900 over commercial Pt/C in alkaline environment. Additionally, the methanol crossover effect was also evaluated on both GD5-900 and Pt/C
Figure 5. (A) current-time chronoamperometric response of GD5-900 and Pt/C in O2-saturated 0.1 M KOH solution; (B) current-time chronoamperometric response of GD5-900 and Pt/C in O2-saturated 0.1 M KOH solution before and after addition of 3 M methanol.
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(Figure 5B). After adding 3 M methanol to the electrolyte, the original cathodic ORR current of GD5-900 remains almost unchanged, whereas the corresponding current on Pt/C instantaneously shift from a cathodic ORR current to a reversed anodic current owing to the methanol oxidation reaction on Pt/C. The remarkable stability and excellent catalytic selectivity to ORR of GD5-900 make it highly promising as an ORR electrocatalyst for direct methanol fuel cells. The best ORR performance of GD5-900 can be attributed to its incomparable structural and component features. First, the largest surface area and unique mesoporous structure endow GD5-900 better exposure and enhanced utilization of electroactive sites, which greatly contribute to its efficient ORR activity. Moreover, the existence of abundant mesoporous structure can boost the mass transport during catalytic process. Secondly, the abundant defects and N species can afford high amount of catalytic sites for the ORR. Especially, the N species only consist of favorable pyridinic and graphitic N transformed from the inherent pyrrolic N of PDA, which is expected to enhance the ORR catalytic activity of catalysts. It has been evidenced that pyridinic N can improve the onset potential by reducing the adsorption energy of O2 and graphitic N can increase the limiting current density while pyrrolic N has little [43] effect on ORR acticity. . Thirdly, graphene oxide as the substrate can be transformed to highly conductive reduced [50] graphene after carbonization. For GD5-900, the mesoporous structure can further lead to more exposure of graphene surface, and thus improve the overall charge-tranfer capability of GD5-900. On the other hand, the unique and strong adhesive effect of PDA renders the integrated carbon [10,27] nanosheets an excellent stability in long-term ORR tests.
This work was financially supported by the Australian Research Council (ARC) through the Discovery Project program of DP110102877, DP130104459 and DP140104062.
Notes and references 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
4. Conclusions In summary, we have developed a new and facile strategy for preparing mesoporous GO-PDA hybrid carbon nanosheets with different PDA thickness. The mircostructures of resultant carbon nanosheets were in-depthly investigated and found highly correlated with the thickness of GD hybrids and carbonization temperature. The GD5-900 with 5 nm GD o thickness synthesized at 900 C carbonization exhibits distinguished ORR performance, excellent stability and outstanding methanol tolerance, which can be attributed to its unique mesoporous structure, high surface areas, abundant defect sites and effective nitrogen species. By applying the physicochemical versatility and structural tunability of PDA, our work provides new insights into the design and manufacture of various PDA-based heteroatom-doped carbon materials and metal-N decorated carbon materials, which would be highly promising substitutes for the expensive noble metal catalysts in fuel cells, hydrogen production, and photocatalysis applications.
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Polydopamine-Graphene Oxide Derived Mesoporous Carbon Nanosheets for Enhanced Oxygen Reduction Received 00th January 20xx, Accepted 00th January 20xx DOI: 10.1039/x0xx00000x www.rsc.org/
Konggang Qu, Yao Zheng, Sheng Dai,* and Shi Zhang Qiao* Composites materials combining nitrogen-doped carbon (NC) with active species represent a paramount breakthrough as the alternative catalysts to Pt for oxygen reduction reaction (ORR) due to their competitive activity, low cost and excellent stability. In this paper, a simple strategy is presented to construct the graphene oxide-polydopamine (GD) based carbon nanosheets. This approach does not need to modify graphene and use any catalyst for polymerization under ambient conditions, and the obtained carbon nanosheets possess adjustable thickness and uniform mesoporous structure without using any template. The thickness of GD hybrids and carbonization temperature are found to play crucial roles on adjusting the microstructure of the resultant carbon nanosheets and, accordingly their ORR catalytic activity. The optimized carbon nanosheet generated by 5 nm thickness of GD hybrid after 900 oC carbonization exhibits superior ORR activity with an onset potential of -0.07 V and a kinetic current density of 13.7 mA cm-2 at -0.6 V . The unique mesoporous structure, high surface areas, abundant defects and favorable nitrogen species are believed to significantly benefit the ORR catalytic process. Furthermore, it also shows remarkable durability and excellent methanol tolerance outperforming those of commercial Pt/C. In view of physicochemical versatility and structural tunability of polydopamine (PDA) materials, our work would shed a new light on the understanding and further development of PDA-based carbon materials for highly efficient electrocatalysts.
1. Introduction The electrochemical efficiency and performance of fuel cells are severely restricted by the sluggish kinetics of the cathodic oxygen reduction reaction (ORR).1 Although precious Pt has been adopted as an effective ORR electrocatalyst,2,3 large-scale commercial utilization has been hindered by its prohibitive cost, limited supply, and poor durability.4 The state-of-the-art catalysts combining nitrogen-doped carbon (NC) and other active species represent a paramount breakthrough as the alternative to Pt because of the competitive activity, low cost and significantly enhanced stability.5-8 The NC materials can be prepared directly by carbonization of nitrogen-containing organic precursors or nitrogen post-doped materials. The precursor choices for preparing NC materials exert particularly crucial influence on the structures, properties and catalytic performance of final products.9,10 The most popular precursors including melamine,8 polypyrrole,11,12 and polyaniline13,14 have been proved to be excellent candidates for NC materials preparation; however, they still suffer from multiple drawbacks such as limited solubility, complicated synthesis
process, low residual yield and weak nanostructural tunability, which have greatly impeded their further potential exploration 15 and commercialization. Polydopamine (PDA) and its derivative materials have been extensively studied for various applications in energy, 16 environmental and biomedical fields over the last decade. PDA can be obtained through the self-polymerization of dopamine (DA) under alkaline condition and can spontaneously deposit on the surface of various solid materials 17 regardless of their chemical nature. As one kind of nitrogen source, PDA displays many striking features surpassing the 8 11,12 above-mentioned precursors of melamine, polypyrrole, 13,14 and polyaniline . The first is its excellent physical properties. DA and PDA can be dissolved in water and most common polar organic solvents, which greatly facilitates its processability. PDA and its derived carbon materials possess excellent 18,19 The second is its structural electrical conductance. tunability. PDA can be facilely controlled to manufacture into any desired nanostructure in terms of the used substrates 20,21 19 including solid and hollow spheres, nanofilms, and 22 nanofibers. Moreover, the thickness of the layer-structured PDA can be easily and precisely tailored at the nanometre scale. The third is the post-modification of PDA. PDA is extremely reactive to amine or thiol functional groups via 17, 23,24 Schiff base or Michael addition reactions. The catechol 25,26 groups of PDA can also react with boric acid easily. These reactions proceed efficiently at room temperature without the need of any harsh reaction condition, and thus are particularly
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powerful for electrocatalytical applications since it enables further introduction of more heteroatoms including nitrogen, sulfur and boron into PDA, and thereby greatly improving the performance of resultant PDA-based carbon materials. Fourthly, PDA displays strong chelation to multivalent metal 27-30 cations owing to its various functional groups; therefore, it can also provide an excellent platform for constructing metalN decorated carbon materials which are appealing substitutes for conventional precious metals as electrocatalysts. Finally, o 21 pure PDA can produce ~ 60 % carbon yield in N2 at 800 C, which is much higher than that of other abovementioned 8,12,31 precursors. In short, the unique and outstanding virtues of PDA render it highly promising as a simple, effective and versatile candidate for the preparation of NC based electrocatalysts. In the recent years, PDA has showed some potentially exciting applications in energy fields including batteries and supercapacitors. However, PDA were mostly serviced as the building block for the construction of metallic or non-metallic composites based on its remarkable physicochemical 32,33 The direct utilization of PDA itself as the versatility. precursor of electro-catalysts has rarely been achieved possibly ascribing to the lack of full awareness of its favourable properties. Up to now, the only reported researches on the eletrocatalytical application of PDA is based on its derived hollow and solid microspheres prepared with and without 34,35 silica as templates. Considering the fascinating advantages of PDA, more detailed and in-depth investigation should be made to discover the unexplored electrocatalytic capacity of PDA-based carbon materials. Herein, we chose graphene oxide (GO) as the substrate to synthesize three kinds of GO-PDA (GD) nanohybrids with different thicknesses using a simple “mix-and-react” method. DA can self-polymerize on the surface of GO to produce a uniform PDA coating layer in which the thickness of the PDA layer can be easily tuned by adjusting the concentration of DA. The carbon nanosheets derived from GD hybrids possess adjustable thicknesses and spontaneously formed uniform mesoporous structures without using any template. The outstanding structural and component features make the newly-prepared carbon nanosheets be excellent ORR electrocatalysts. Moreover, its catalytic activity shows highly selective to the thickness of GD hybrids and the carbonization temperature. This material also possesses high methanol tolerance and excellent stability which are superior to those observed for commercial Pt/C catalyst. To the best of our knowledge, our work is the first report of PDA-graphene derived carbon nanosheets for ORR electrocatalysis. This study would lay a solid foundation for the further exploration and development of PDA-based carbon materials.
2. Experimental section 2.1 Chemicals and reagents Graphite flakes, sulfuric acid (H2SO4, 95-98%), potassium permanganate (KMnO4, 99%), phosphorous acid (H3PO4, 85%),
hydrogen peroxide (H2O2, 30%), dopamine hydrochloride, and disodium hydrogen phosphate (Na2HPO4) were purchased from Sigma-Aldrich. All chemicals were used as received without further purification. Milli-Q water (18.2MΩ) was used throughout the experiments. 2.2 Materials preparation Preparation of graphene oxide (GO). GO was synthesized from 36 natural graphite flake by an improved Hummers’ method. The detailed procedures were shown in the Supporting Information. Preparation of GO-PDA (GD) nanohybrids. In a typical experiment, 85mL GO aqueous dispersion (2 mg/mL) was mixed with 250 mg DA dissolved in 10 mL DI water and another 130 ml DI water. The mixture was sonicated for 5 minutes, 25 mL PBS buffer ( 0.4 M, pH=8.5) was added, and then the mixture was stirred for 24 hours at room temperature. The container was covered with aluminum foil with several small holes. The GD hybrids were obtained by centrifugation and washing with water for three times. The control over the thickness of GD hybrids can be easily achieved by adjusting the amount of DA in the reaction solution. Here, 125, 250 and 500 mg of DA can produce 2.5, 5 and 10 nm GD hybrids, respectively. Carbonization of GD hybrids. The as-prepared GD hybrids were carbonized in a temperature-programmable tube furnace o under N2 atmosphere at 400 C for 2 h with a heating rate of 1 o -1 o C min , which was followed by further treatment at 900 C o o -1 (700, 800 or 1000 C ) for 3 h with a heating rate of 5 C min . 2.3 Sample characterizations Fourier transform infrared (FTIR) spectra were collected on the transmission module of a Thermo Nicolet 6700 FTIR -1 spectrometer at 2 cm resolution and 64 scans. TEM images were acquired on a JEM-2100 microscopy. TEM elemental mapping was obtained through the EDAX detector attached to the JEM-2100. AFM images were acquired under ambient conditions with a Ntegra Solaris AFM (NT-MDT) operating in a tapping mode. The Raman spectra were collected on iHR550 from HORIBA Scientific equipped with a 532 nm solid laser as the excitation source. X-Ray diffraction (XRD) was performed on the Miniflx-600 (Rigaku Ltd.) under ambient conditions using a Cu Kα X-ray. XPS analysis was conducted on Axis Ultra spectrometer (Kratos Analytical Ltd.) with monochromated Al -9 Kα radiation at ca. 5×10 Pa. Thermogravimetric analysis (TGA) was conducted on the TGA/DTA system (STARe, MettlerToledo) -1 at 25-1000 ºC in a 15 ml min N2 flow and a ramp rate of 5 °C -1 min . Nitrogen adsorption-desorption isotherm was collected on the Tristar II (Micrometrics) at 77 K. Pore size distribution was obtained by Barrett-Joyner-Halenda (BJH) model using the adsorption branch on the isotherm. The specific surface areas of the materials were calculated using the adsorption data at pressure range of P/P0 = 0.05-0.3 by Brunauer-Emmett-Teller (BET) model. For the electrochemical tests, 2 mg of the catalyst was dispersed in 1 ml DI water. The mixture was slightly ultrasonicated to obtain a homogenous catalyst ink. To prepare the
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working electrode for electrochemical measurements, 20 μl of the ink was dipped on a mirror polished glass carbon electrode. Then, 5 μl of 0.5 wt. % Nafion aqueous solution was dipped on the electrode and dried at room temperature. After that, the working electrode was inserted into the three-electrode cell setup, which is composed of a platinum counter electrode, an Ag/AgCl/KCl (4 M) reference electrode in a glass cell containing 100 ml 0.1 M KOH aqueous electrolyte. A flow of O2 was maintained over the electrolyte solution (0.1 M KOH) during recording electrochemical data in order to ensure its continued O2 saturation. Cyclic voltammogram (CV), linear sweep voltammogram (LSV), and rotating disk electrode (RDE) tests were carried out using a glassy carbon rotating disk electrode. The scan rate of CVs was kept as 50 mV s-1 while that for the LSVs and RDE tests was 5 mV s-1. The data were recorded using an electrochemical analysis workstation (CHI 760C, CH Instruments, USA). The Koutecky-Levich plots were obtained by linear fitting of the reciprocal rotating speedversus reciprocal current density collected at different potentials form -0.4 to -0.8 V. The overall electron transfer numbers per oxygen molecule involved in a typical ORR process were calculated from the slopes of the Koutecky-Levich plots using the following equation: 1/jD=1/jk + 1/ Bω1/2
(1)
where jk is the kinetic current in amperes at a constant potential, ω is the electrode rotating speed in rpm, and B is the reciprocal of the slope, which was determined from the slope of Koutecky-Levich plots based on Levich equation as followed: B=0.2 nFAν–1/6CO2DO22/3
3. Results and discussion 3.1 Materials synthesis and characterization
As illustrated in Figure 1A, GD hybrids were facilely synthesized by mixing a given amount of DA with GO in PBS buffer (pH=8.5). DA can self-polymerize to form a PDA thin film directly on the surface of GO. In this study, three GD nanohybrids with different PDA thicknesses were synthesized through tuning the concentrations of DA. It is noteworthy that PBS but not Tris buffer was chosen because the primary amine group of Tris can interact covalently with PDA which may introduce extra 17, 24 nitrogen source. The as-prepared GD hybrids were washed, dried and carbonized at different temperatures. The resulting GD derived carbon materials were labeled as GDx-T, where x represents the thickness of GD hybrids and T is the final o carbonized temperature (700, 800, 900 and 1000 C). FTIR spectra shown in Figure S1 identify the functional groups of GD hybrids; the revealed peaks at 1502 and 1616 -1 cm are consistent with the indole or indoline structures of 37,38 PDA as reported earlier. The typical transmission electron microscopy (TEM) image of GD5 reveals the rougher surface than that of GO (Figures 1B and 1C) owing to the deposition of PDA, but GD5 still keeps excellent dispersibility and sheet morphology. The atomic force microscopy (AFM) analyses (Figure 1D and Figure S2) further confirm that PDA can form a uniform coating layer on the surface of GO; moreover, through adjusting the concentration of DA, three GD hybrids with the thickness of 2.5±0.25 nm, 5.0±0.10 nm, 10.0±0.05 nm are obtained.
(2)
where n is the number of electrons transferred per oxygen molecule, F is the Faraday constant (96,485 C mol-1), DO2 is the diffusion coefficient of O2 in 0.1 M KOH (1.9×10-5 cm s-1), v is the kinetic viscosity, and CO2 is the concentration of O2 (1.2×103 mol L-1). The constant 0.2 is adopted when the rotating speed is in rpm. Rotating ring-disk electrode (RRDE) voltammogram measurements were conducted on an RRDE configuration (Pine Research Instrumentation, USA) with a 320 μm gap Pt ring electrode. The linear sweep voltammograms were recorded in O2 saturated 0.1 M KOH at 1600 rpm. The disk was set to scan at 5 mV s-1 from 0.2 to -0.8 V and the ring was set at 0.5 V. The collecting efficiency of the RRDE (N) was 0.37. The peroxide yield (HO2- %) and the electron transfer number (n) were calculated as follows: HO2- % = 200×Ir/N/(Id+Ir/N) n = 4×Id/(Id+Ir/N)
(3)
(4)
where Id is the disk current and Ir is the ring current. The materials’ resistance to methanol crossover effect and stability were tested in the same setup as for the RDE test in the O2 saturated 0.1 M KOH aqueous electrolyte and were performed at a static potential of -0.3 V and -0.5 V, respectively, for the chronoamperometry at room temperature.
Figure 1. (A) illustration of the synthesis of GO-polydopamine derived carbon nanosheets. TEM images of (B) GO, (C) GD5, (E) GD5-900 and (F) the magnified square area shown in E; AFM images of (D) GD5 and (G) GD5-900.
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The morphology and structure of GD hybrids after carbonization were investigated by TEM and AFM. Figure 1 (E, F) and Figure S3 show the typical TEM images of GDx-900. It can be seen that three kinds of GDx-900 materials still inherit the sheet morphology of GD hybrids even after high temperature carbonization, however, small degrees of aggregation happen for GD2.5-900. More intriguingly, GD5-900 has much rougher surface and owns distinctive and regular porous structure with the pore size of about 3-5 nm while GD2.5-900 and GD10-900 only possess some crinkles on their surfaces. It should be nothworthy that the uniform porous structure is spontaneously formed during carbonization without involving any template, and this kind of porosity in the PDA derived carbon materials can be observed distinctly by TEM.21,35 The TEM images of the other GDx-T materials are shown in Figure S4, and only GD5-800 has similar porous structure like GD5-900. The AFM analyses (Figure 1G and Figure S2) reveal three lamellar GDx-900 materials (x=2.5, 5 and 10) have the thicknesses of 1.3 ± 0.15, 2.7±0.12, 5.5 ± 0.07 nm, respectively, which is about 52-55 % of the GDx without carbonization. In addition, the thermogravimetric analysis (TGA) curve of GD5 hybrid indicates that it can retain a carbon yield of 59.8 % after 900 oC carbonization (Figure S5). Nitrogen adsorption isotherms were used to investigate the surface areas and pore structures of as-prepared hybrid materials. The specific surface area, pore volumes and pore sizes are summarized in Table S1. As shown in Figure 2A and Table S1, both the Brunauer-Emmett-Teller (BET) surface areas and the total pore volumes of different GD5-T materials increase with increasing carbonization temperature from 700 to 900 oC and then decrease at 1000 oC. For the GD5-800 and GD5-900, their isotherms display distinct hysteresis loops at relative pressures (P/P0) from 0.45 to 1.0, which coincide with
Figure 2. (A) nitrogen adsorption-desorption isotherms and (B) pore size distribution curves of different GD5-T materials; (C) XRD patterns and (D) Raman spectra of GO, GD5 and GD5-900, the inset in D presents the changes of ID/IG ratios for different GDx-T materials as a function of carbonization temperature.
the type IV isotherm, suggesting the presence of mesopores. The pore size distribution (PSD) curves (Figure 2B) further confirm highly uniform mesoporous structures of the GD5-800 and GD5-900 with pore size centered at 4.6 and 3.9 nm. GD5800 and GD5-900 display similar pore volumes of 0.27 and 0.28 3 -1 3 cm g , which are much larger than that of GD5-700 (0.12 cm -1 3 -1 g ) and GD5-1000 (0.06 cm g ). Most importantly, it was o found that the holding at 400 C for first 2 hours during the carbonization process is necessary for the formation of porous o structures, and the direct heating to 800 or 900 C cannot produce any porous-structured materials (see Experimental section). The formation of porous structure is highly dependent on not only the carbonization process but also the thickness of GD hybrid layer. As shown in Figure S6, the isotherm of GD2.5-900 displays a hysteresis loop of type-IV, but it has a low pore volume (0.09 m2 g-1). As for the GD10-900, no porous structure is observed. Given the inconclusive molecular structure of PDA,39,40 the origin of this porosity behavior remains unclear, nevertheless, this mesoporous architecture is expected to facilitate the exposure of more active sites and diffusion of reactants during the catalytic process. The typical structural evolution during the hybrid synthesis was investigated by XRD and Raman spectra. As shown in Figure 2C, the characteristic sharp peak at 9.7o of GO corresponds to an interlayer distance of 0.91 nm, the broad and weak peak centered at 25o of GD5 may be attributed to ππ stacking interaction of the aromatic rings of coated PDA and/or GO.23,37 For the GD5-900, its XRD pattern exhibits two broad peaks at 25o and 43o which correspond to the (002) and (100) crystal faces, reflecting the successful carbonization of PDA.41 Raman spectra (Figure 2D) dispaly the typical D bands resulting from the defects and disordered structure and G bands from sp2-hybridized graphitic carbon atoms at roughly 1350 and 1580 cm-1, respectively.42 The intensity ratio of D peak to G peak (ID/IG) has been conveniently used to estimate the amount of defect and disordered structure. The inset in Figure 2D depicts the change of ID/IG ratios as a function of carbonization tempareture. Interestingly, for each kind of GDx hybrids, the GDx-900 always has the largest ID/IG ratio, moreover, the ID/IG ratio of GD5-900 is larger than that of GD2.5-900 and GD10-900. Meanwhile, GD5-800 reveals similar ID/IG ratio as that of GD10-900. This abnormal trends are consistent to the results of nitrogen adsorption and can be ascribed to the formation of porous structure which can decompose the graphitic structure and lead to more defects and disordered structure. The compositions of the as-prepared materials were then analyzed by TEM elemental mapping and X-ray photoelectron spectroscopy (XPS). The typical elemental mapping images of GD5-900 (Figure S7) illustrate the presence of C, N, and O elements which are uniformly distributed on the carbon nanosheets. The chemical status of these elements were evaluated by XPS as shown in Figure 3 and Figure S8. XPS survey scans indicate the presence of carbon, oxygen and nitrogen elements on surface. Additionally, the high-resolution N 1s spectra can be deconvoluted into two peaks locating at 398.0 eV and 400.8 eV which are assigned to pyridinic N and
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graphitic N, respectively. This indicates the pyrrolic N (399.9 eV) of PDA has been completely transformed into pyridinic and 43 graphitic N after carbonization. As shown in Figure 3C, the total nitrogen content is found to decrease as the carbonization temperature increases. More specifically, the percentage of graphitic nitrogen first inceases slightly as the o temperature is raised from 700 to 900 C and then decreases, while that of pyridinic nitrogen keeps a gradual decrease beacuse the pyridinic nitrogen is not thermally stable and can convert to graphitic nitrogen at higher carbonization temperature. The total nitrogen content inceases undoubtedly with the increase of the thickness of GD hybrid materials (Figure 3D); however, GD5-900 has the highest percentage of pyridinic nitrogen among the GDx-900 materials because pyridinic nitrogen generally exists at the edge and defect positions and GD5-900 owns highest surface area and most defect structure verified by nitrogen adsorption analyses and Raman spectra. Compared with previously reported methods to prepare nitrogen-doped graphene-based carbon nanosheets,44-46 the strategy presented here is a simple “mix-and-react” process which does not need any catalyst, high temperature, special atmosphere and pre-modification of graphene. The thicknesses of resultant carbon nanosheets can be precisely adjustable by controlling ration of GO and DA. Especially, our results show that the uniform mesoporous structure can be spontaneously formed without using any template and the formation of mesoporous structure is highly selective to the thickness and carbonization process.47-49 It should be noted that two-component (graphene and PDA) integrated materials have excellent sturctural stability arising from the highly strong adhesive effect of PDA.50 Additionally, the resulting carbon nanosheets own large surface area (up to 272.3 m2 g-1),
Figure 3. (A) XPS survey scan and (B) the corresponding highresolution N 1s spectrum of GD5-900; (C) the content of different N species present in different GD5-T materials; (D) the percentages of graphitic N and pyridinic N in different GDx-900 materials.
abundant defect sites (ID/IG ratio ~1.15) and favorable nitrogen species (only pyridinic and graphitic N) transformed from inherent pyrrolic N of PDA, all of these features would make these materials particularly beneficial to the electrocatalytic applications. 3.2 Electrocatalytic analysis
The ORR was employed as a probe reaction to investigate the intrinsic advantages of these hybrid carbon nanosheets. Since GDx-900 (x = 2.5, 5 and 10) has obvious structure advantages among all synthesized GDx materials, their active ORR catalyst performance is presented and discussed in this work. Firstly, the cyclic voltammograms (CVs) and the linear sweep voltammograms (LSVs) of GO, GD5 and GD5-900 were examined in O2-saturated 0.1 M KOH solution. As shown in Figures 4A and 4B, both CVs and LSVs of GO and GD5 show their negligible ORR catalytic activities. However, the CV curve of the GD5-900 displays a well-defined characteristic ORR peak at -0.26 V and its LSV curve shows a high ORR onset potential of -0.07 V with a current density of 3.50 mA cm-2 at -0.6 V. These results apparently indicate the synthesized carbon nanosheets can function as efficient ORR catalysts. To obtain additional insight about the effect of three carbon nanosheets with different thicknesses on ORR activities, the LSVs of GDx-900 (x = 2.5, 5 and 10) were recorded and compared in O2-saturated electrolyte on a rotating disk electrode (RDE) at 1600 rpm (Figure 4C). Remarkably, GD5-900 exhibits an ORR onset potential of -0.07 V, which is much more positive than that of GD2.5-900 and GD10-900 (-0.10 V and -0.14 V, respectively). On the other hand, the ORR current density of GD5-900 is also higher than those of the other two materials over the whole potential range. At -0.6 V, GD5-900 shows a current density of 3.50 mA cm-2, surpassing GD2.5-900 and GD10-900 with their current densities of 2.82 and 3.23 mA cm-2. The most positive onset potential and highest current density of GD5-900 further reveal that this material has clearly best catalytic performance, which is most likely due to its larger surface areas and more accessible catalytic active sites resulting from its unique and abundant mesoporous structure. The electrochemical active surface areas (ECSA) of various GDx900 catalysts were estimated, and the thickness dependence of the ECSA for these catalysts follows similar trend as the BET results (Figure S9). Compared with previously reported ORR catalytical materials, the onset potential of GD5-900 is much more positive than those of single-doped graphene including N-graphene and S-graphene and can be comparable to those of graphitic C3N4 and dual-doped graphene including B, Ngraphene and N, S-graphene;8,51,52 and its current density is also comparable to those of graphitic C3N4, Fe3O4/N-graphene and CoO/carbon nanotube.8,53,54 Although this GD5-900 cannot outperform the Pt/C, considering the versatile physicochemical properties of PDA, it has much room for the improvement of catalytic activity through further introduction of more heteroatoms and/or nonprecious metal elements. To quantitatively understand the ORR activities of these -1 -1/2 GDx-T catalysts, the Koutecky-Levich (K-L) plots (J vs ω ) were calculated from the LSVs and compared at -0.6 V and
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Figure 4. (A) CV curves and (B) LSVs at a sweep rate of 5 mV s-1 of GO, GD5 and GD5-900; (C) LSVs at a sweep rate of 5 mV s-1, (D) the calculated K-L plots and (E) kinetic current density (Jk) and electron transfer number (n) of GD2.5-900, GD5-900, GD10900 and Pt/C; (F) H2O2 yield (solid, primary Y-axis) and the corresponding electron transfer number (dash, second Y-axis) of GD2.5-900, GD5-900 and GD10-900. various rotating speeds (Figure 4D); the K-L plots were also obtained at other potentials as presented in Figure S10, which display excellent linearity at different rotating speeds. Noticeably, GD5-900 shows the highest ORR current density, followed by GD2.5-900 and GD10-900. Furthermore, the electron transfer numbers (n) and kinetic current density (JK) could be obtained from the slopes and the intercepts of linear K-L plots on the basis of the K-L equation (Figure 4E). GD5-900 shows the largest n value of 3.7 and the highest JK value of 13.7 mA cm-2 at -0.6 V, much closer to those of Pt/C (4.0 and 16.4 mA cm-2) as compared to those of GD2.5-900 (3.1 and 7.95 mA cm-2) and GD10-900 (3.0 and 13.0 mA cm-2). These results explicitly suggest that the efficient four-electron reduction dominates the catalytic process on GD5-900, whereas the ORR process for GD2.5-900 and GD10-900 is a combined pathway of 2e- and 4e- reduction. Apparently, the largest electron transfer number and highest kinetic current density displayed on GD5900 confirm its best ORR performance. The rotating ring-disk electrode (RRDE) technique was used to further verify the ORR pathway by monitoring the formation of intermediate peroxide species during the ORR process (Figure 4F). Notably, the ORR on GD5-900 yields about 17.326.6% H2O2 over a wide potential range from -0.2 to -0.8 V with n ranging from 3.47 to 3.65. On the other hand, GD2.5-900, its n ranges from 3.03 to 3.35, gives ∼32.6-48.4% H2O2 under
identical conditions. GD10-900, its n ranges from 2.74 to 3.19, gives even higher amount of H2O2 (∼40.6-63.0%) in the reaction. These results are in accordance with the RDE measurements, indicating the highest electrocatalytic efficiency of GD5-900. Further CV and LSV studies were carried out to evaluate the influence of carbonization temperature on materials’ electrocatalytic activities (Figure S11). Based on the onset potentials obtained from LSV curves, we can see that electrocatalytic activities of GDx catalysts share a similar trend with increasing carbonization temperature, where the ORR electrocatalytic activity first improves as the carbonization temperature rises from 700 to 900 °C but subsequently drops as the temperature further increases from 900 to 1000 °C. It can be explained by the most defect sites possessed by the GDx-900 for different GDx hybrids (Figure 2D and S9), which can provide most electrocatalytic active sites. However, the onset potential of GD5 material shows an improvement of 0.2 V when the carbonization temperature rises from 700 to 900 °C, which is much larger than that of GD2.5 and GD10 under the same condition, about 0.05 V and 0.1 V, respectively. Expectedly, the optimized ORR performance was observed on GD5-900, which is more superior to GD2.5 and GD10 irrespective of their carbonization temperature. That is because the generation of unique mesoporous structure on GD5-900 results in largest surface areas and most defect sites, which bring about its largest performance improvement. Meanwhile, GD5800 has the second best electrocatalytic activity which is close to that of GD5-900 on account of their similar structures. These results suggest ORR performance is highly correlated with the nanostructure of catalysts. The stability of GD5-900 was further assessed through the chronoamperometric measurement under a constant cathodic voltage of -0.5 V. For comparison, the activity of Pt/C was also measured in the same way. As shown in Figure 5A, GD5-900 exhibits a high stability with a very slow attenuation over 40 h, retaining 88.1 % of the initial current, whereas Pt/C shows nearly 40 % loss of its initial current over the same time period, confirming a much better stability of GD5-900 over commercial Pt/C in alkaline environment. Additionally, the methanol crossover effect was also evaluated on both GD5-900 and Pt/C
Figure 5. (A) current-time chronoamperometric response of GD5-900 and Pt/C in O2-saturated 0.1 M KOH solution; (B) current-time chronoamperometric response of GD5-900 and Pt/C in O2-saturated 0.1 M KOH solution before and after addition of 3 M methanol.
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(Figure 5B). After adding 3 M methanol to the electrolyte, the original cathodic ORR current of GD5-900 remains almost unchanged, whereas the corresponding current on Pt/C instantaneously shift from a cathodic ORR current to a reversed anodic current owing to the methanol oxidation reaction on Pt/C. The remarkable stability and excellent catalytic selectivity to ORR of GD5-900 make it highly promising as an ORR electrocatalyst for direct methanol fuel cells. The best ORR performance of GD5-900 can be attributed to its incomparable structural and component features. First, the largest surface area and unique mesoporous structure endow GD5-900 better exposure and enhanced utilization of electroactive sites, which greatly contribute to its efficient ORR activity. Moreover, the existence of abundant mesoporous structure can boost the mass transport during catalytic process. Secondly, the abundant defects and N species can afford high amount of catalytic sites for the ORR. Especially, the N species only consist of favorable pyridinic and graphitic N transformed from the inherent pyrrolic N of PDA, which is expected to enhance the ORR catalytic activity of catalysts. It has been evidenced that pyridinic N can improve the onset potential by reducing the adsorption energy of O2 and graphitic N can increase the limiting current density while pyrrolic N has little [43] effect on ORR acticity. . Thirdly, graphene oxide as the substrate can be transformed to highly conductive reduced [50] graphene after carbonization. For GD5-900, the mesoporous structure can further lead to more exposure of graphene surface, and thus improve the overall charge-tranfer capability of GD5-900. On the other hand, the unique and strong adhesive effect of PDA renders the integrated carbon [10,27] nanosheets an excellent stability in long-term ORR tests.
This work was financially supported by the Australian Research Council (ARC) through the Discovery Project program of DP110102877, DP130104459 and DP140104062.
Notes and references 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
4. Conclusions In summary, we have developed a new and facile strategy for preparing mesoporous GO-PDA hybrid carbon nanosheets with different PDA thickness. The mircostructures of resultant carbon nanosheets were in-depthly investigated and found highly correlated with the thickness of GD hybrids and carbonization temperature. The GD5-900 with 5 nm GD o thickness synthesized at 900 C carbonization exhibits distinguished ORR performance, excellent stability and outstanding methanol tolerance, which can be attributed to its unique mesoporous structure, high surface areas, abundant defect sites and effective nitrogen species. By applying the physicochemical versatility and structural tunability of PDA, our work provides new insights into the design and manufacture of various PDA-based heteroatom-doped carbon materials and metal-N decorated carbon materials, which would be highly promising substitutes for the expensive noble metal catalysts in fuel cells, hydrogen production, and photocatalysis applications.
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