Heteroatom Polymer-Derived 3D High-Surface ... - ACS Publications

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Heteroatom Polymer-Derived 3D High-Surface-Area and Mesoporous Graphene Sheet-Like Carbon for Supercapacitors Haiyang Sheng, Min Wei, Alyssa D’Aloia, and Gang Wu* Department of Chemical and Biological Engineering, University at Buffalo, The State University of New York, Buffalo, New York 14260, United States S Supporting Information *

ABSTRACT: Current supercapacitors suffer from low energy density mainly due to the high degree of microporosity and insufficient hydrophilicity of their carbon electrodes. Development of a supercapacitor capable of simultaneously storing as much energy as a battery, along with providing sufficient power and long cycle stability would be valued for energy storage applications and innovations. Differing from commonly studied reduced graphene oxides, in this work we identified an inexpensive heteroatom polymer (polyaniline-PANI) as a carbon/nitrogen precursor, and applied a controlled thermal treatment at elevated temperature to convert PANI into 3D high-surface-area graphene-sheet-like carbon materials. During the carbonization process, various transition metals including Fe, Co, and Ni were added, which play critical roles in both catalyzing the graphitization and serving as pore forming agents. Factors including post-treatments, heating temperatures, and types of metal were found crucial for achieving enhanced capacitance performance on resulting carbon materials. Using FeCl3 as precursor along with optimal heating temperature 1000 °C and mixed acid treatment (HCl+HNO3), the highest Brunauer− Emmett−Teller (BET) surface area of 1645 m2g−1 was achieved on the mesopore dominant graphene-sheet-like carbon materials. The unique morphologies featured with high-surface areas, dominant mesopores, proper nitrogen doping, and 3D graphene-like structures correspond to remarkably enhanced electrochemical specific capacitance up to 478 Fg−1 in 1.0 M KOH at a scan rate of 5 mV s−1. Furthermore, in a real two-electrode system of a symmetric supercapacitor, a specific capacitance of 235 Fg−1 using Nafion binder is obtained under a current density of 1 Ag−1 by galvanostatic charge−discharge tests in 6.0 M KOH. Long-term cycle stability up to 5000 cycles by using PVDF binder in electrode was systematically evaluated as a function of types of metals and current densities. KEYWORDS: supercapacitors, high-surface-area carbon, graphene-sheet, heteroatom polymers, carbonization

1. INTRODUCTION An increase in popularity of portable, smart electronic devices, as well as electric vehicles has induced the need for reliable, stable, and sustainable energy storage devices with desired energy and power density. Supercapacitors, an electrochemical double layer capacitor, are a relatively new energy storage technology compared to traditional batteries. Unlike electrochemical pseudocapacitors, which usually contain metal oxides or conducting polymers via faradaic electron transfer during the redox reaction, the electrochemical double-layer capacitors require high-surface areas carbon materials capable of providing much higher capacitance. In addition, upon charging with applied voltage, an electrical double layer is formed at the electrode surfaces with atomic scale charge separation, creating a very large charge storage capacity. Due to the rapid transfer of ions at very short distances (a few nanometers), supercapacitors provide high power density, long life cycles (>10k cycles) and operate in a safe manner.1−5 However, a major limitation to this technology is a much lower energy density (typically 5−10 Wh kg−1), relative to conventional Li-ion batteries around ∼200 Wh kg−1.6 Hence, development of supercapacitors that are © 2016 American Chemical Society

capable of storing as much energy as a battery and can provide sufficient power and long cycle stability would be a revolutionary advancement in all applications of energy storage technologies. Considering a variety of materials to be used for supercapacitor electrodes, activated carbons (ACs), graphene, carbon nanotubes (CNTs),7 certain metal oxides, and conducting polymers have shown their potential applications.8,9 At present, carbon-based materials have attracted substantial attention to prepare supercapacitor electrodes, due to their advantages including lower cost, excellent electrical conductivity, easy processing, higher specific surface area, chemical/electrochemical stability, and wide operating temperature range.7,10 Therefore, high surface area carbon materials with appropriate porosity and surface chemistry are highly desirable for supercapacitors. A commonly used carbon material for this application is activated carbon.11−13 Generally, the specific Received: August 11, 2016 Accepted: October 18, 2016 Published: October 18, 2016 30212

DOI: 10.1021/acsami.6b10099 ACS Appl. Mater. Interfaces 2016, 8, 30212−30224

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ACS Applied Materials & Interfaces

Figure 1. Synthesis of M-PANI-Cs. (A) Polymerization of aniline by adding APS in the presence of metal precursors. (B) Mixing of high-surface-area carbon support with the suspension of PANI and metal precursors. (C) Heat treatment in N2 atmosphere. (D) Acid leaching, removal of metal aggregates.

derived from an inexpensive heteroatom (e.g., polyanilinePANI) via a high-temperature carbonization in the presence transition metal precursors (M: Fe, Co, or Ni). In addition to the catalyzing function, the metal precursors served as selfsacrificing agents that were in situ formed during carbonization, occupied their positions, and then were eliminated by acid leaching, thus creating porous structures. With the highest specific surface area (1645 m2 g−1), an optimal sample (FePANI-C) was derived from PANI and Fe and exhibited a high capacitance of 478 F g−1 at a scan rate of 5 mV s−1 determined by using cyclic voltammetry in 1.0 M KOH. These newly prepared carbon materials demonstrated smaller series resistance and charge transfer resistance than those of Black Pearl 2000 (BP 2000). Furthermore, in subsequent galvanostatic charge−discharge tests, two-electrode real supercapacitors were fabricated. Although the determined capacitance is reduced to certain degree when shifting from glassy carbon electrode to real supercapacitor electrode with an increased loading of 3−4 mg cm−2 and using PVDF as a binder, M-PANICs presented ideal cycle stability with sufficient capacitance retention.

capacitances of carbon materials exhibit the supercapacitor performance with values in the range of 75−200 Fg−1 for aqueous electrolytes and 40−150 Fg−1 for organic electrolytes.14−16 The current issue for low capacity supercapacitors lies within the electrode and its inability to provide sufficient electrochemically accessible surface area, or inability to provide optimal morphology for mass/charge transfer. For example, as for one activated carbon (∼500 m2 g−1), typically only about 10−20% of the “theoretical” capacitance can be achieved on the activated carbon electrode. This is due to the high degree of microporosity and insufficient hydrophilicity, which result in sigifnicant wetting deficiencies of electrolytes on the electrode surface. Attention has been shifted from traditional carbon electrodes to other advanced materials with increased surface areas and enhanced accessibility of ions. Nanotechnology has opened up new frontiers by offering unique enabling technologies and new materials for energy storage. In particular, graphitic carbon nanomaterials, including carbon nanotubes (CNTs),17−22 carbon nanofibers,23−25 and reduced graphene oxides,6,26−30 have played an important role in the development of highperformance supercapacitors.31−35 Bing et al.36 developed a two-step etching route to ultrathin carbon nanosheets for high performance supercapacitors showing the specific capacitance of 220 F g−1 at 0.5 A g−1. An et al.37 developed a single-walled carbon nanotube electrode, exhibiting a high specific capacitance (180 Fg−1) as well. However, these nanocarbonbased materials still suffer from insufficient surface areas and charge transfer at the electrode/electrolyte interfaces. Therefore, the development of novel high-surface-area carbon with optimal morphology and nanostructures would be beneficial for enhanced supercapacitor performance. To overcome these issues, in this work we developed novel 3D high-surface area carbon materials dominant with graphenesheet-like morphologies (not graphene or reduced graphene oxide itself), demonstrating remarkable capacitance for supercapacitors. This synthetic method is facile and low cost, and is

2. EXPERIMENTAL SECTION 2.1. Materials Synthesis. With our typical approach to chemically polymerize aniline for in situ preparation of polyaniline (PANI),26,38−42 2.0 mL of aniline was dispersed in 100 mL 2.0 M hydrochloride acid (HCl), followed by the addition of 15 g metal precursors (FeCl3, CoCl2·6H2O, or NiCl2·6H2O). Then 5.0 g ammonium peroxydisulfate (APS) was dissolved in another 100 mL 2.0 M HCl. The well dissolved APS solution was added into the predispersed suspension of aniline and metal precursor to initialize the polymerization. Black Pearl 2000 (BP2000) was used as the carbon support. 0.4 g BP2000 was ultrasonically dispersed in 20 mL Milli-Q water with 2 mL isopropyl alcohol (IPA). The suspension was sonicated for 2 h before it was added to the PANI suspension. Constant stirring of the suspension for 24 h is crucial for allowing fully polymerization of PANI, which promotes uniform mixing of metal precursors and polyaniline, and both polyaniline chain and metal precursors covering the carbon support. Next, the suspension 30213


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Figure 2. (A) CV curves of Fe-PANI-C prepared from various heating temperature ranging from 850 to1050 °C at a scan rate of 20 mV s−1. (B) Calculated specific capacitance of Fe-PANI-Cs as a function of the temperatures of heat treatment. 1.0 V at different current densities (1−5 A g−1) for 5000 cycles. The working electrodes were prepared by mixing the active materials, polyvinylidene fluoride (PVDF) and acetylene black at a weight ratio of 80:10:10 in N-methyl-2-pyrrolidone (NMP) to form a slurry and pressing the slurry onto a nickel foam substrate, then drying the electrodes under vacuum at 80 °C for 12 h. Then two electrodes with same mass loading of active material were pressed together and separated by a filter paper. Figure S2 demonstrates the general procedure of fabrication of a symmetric supercapacitor.

containing carbon, polymer, and transition metal(s) was moved to a beaker and heated to slowly evaporate the water. The resulting solid mixture was then heat-treated at a high temperature ranging from 850 to 1050 °C in a tube furnace under an inert nitrogen atmosphere for 1 h. The heat-treated sample was leached in various acids including individual 0.5 M H2SO4, 30% HNO3, or mixed acid (30% HNO3 and 2.0 M HCl with a volumetric ratio of 1:3) at 80 °C for 5 h to remove unstable metal aggregates, followed by a thorough wash with Milli-Q water. Finally, the washed samples were dried overnight in a vacuum oven at 80 °C. The as-prepared samples are labeled as follows: MPANI-C-AAA-BBB (M: Fe, Co, or Ni; AAA indicating heating temperatures: 850, 900, 950, 1000, or 1050 °C; BBB labeling the acid for post treatment: H2SO4, HNO3 or Mix Acid). In the latter part of this work, samples are only labeled as M-PANI-C (M: Fe, CO, or Ni) because all the samples were prepared under exactly the same procedure (1000 °C and mixed acids). To further explore the performance of our samples compared with commercial materials, BP 2000 carbon black was chosen as a reference. The scheme and flowchart of synthesis procedure are highlighted in Figure 1 and Figure S1 of the Supporting Information (SI). 2.2. Physical Characterization. The morphologies of samples were visualized through scanning electron microscopy (SEM) by Carl Zeiss AURIGA CrossBeam, operated at 3 kV. The crystalline phases present in each sample were identified using X-ray diffraction (XRD) by a Rigaku Ultima IV diffractometer with Cu K-α X-rays (λ = 1.5406 Å). The Raman spectrum was measured and collected at an excitation wavelength of 514.5 nm by a Renishaw inVia Raman system. BET surface area and N2 adsorption−desorption isotherms were measured on a Micromeritics TriStar II at 77 K. Samples were pretreated at 150 °C for 3 h under vacuum prior to nitrogen physisorption measurements. 2.3. Electrochemical Measurements. Cyclic voltammetry (CV) was performed on a CHI 760E electrochemical workstation using a three-electrode test system in an N2 purging electrolyte. The threeelectrode system includes an active material coated glass carbon electrode (GCE) or nickel foam electrode (NFE) as a working electrode, a Ag/AgCl (KCl saturated) reference electrode and a graphite rod counter electrode immersed in 1.0 M KOH over the potential range of 0−1 V at scan rates of 5−100 mV s−1. The mass loading of active materials on a GCE and a NFE was controlled at ∼0.2 and ∼3 mg/cm2, respectively. Electrochemical impedance spectroscopy (EIS) measurements were performed by Autolab (Metrohm) with the same three-electrode system with an active material coated NFE as the working electrode, with an amplitude of 5 mV and frequencies ranging from 100 kHz to 0.01 Hz. The obtained EIS data was fitted to an equivalent electrical circuit model using Nova 1.11.2 (Metrohm) software. Galvanostatic charge−discharge (GCD) curves of the symmetric super capacitors in a two-electrode configuration were measured on a battery test station (Arbin) with MITS Pro software. The potential window was applied between 0 and

3. RESULTS AND DISCUSSION 3.1. Effect of Heating Temperatures. The highly porous carbon materials are converted from PANI via a carbonization process. Therefore, the heating temperature is a critical factor in controlling the materials’ morphologies and surface chemistry. Using the Fe-PANI-C system, materials morphologies and resulting electrochemical double layer capacitance was determined as a function of temperature from 850 to 1050 °C with an incremental step of 50 °C. In order to optimize the temperature, thus we only used a simple acid treatment with 0.5 M H2SO4. Figure 2A shows the CV curves of Fe-PANI Csamples at a scan rate of 20 mV s−1 in 1.0 M KOH electrolyte. All five CV curves exhibit rectangular shapes (rare peaks showing), indicating ideal supercapacitor charge−discharge performance. Furthermore, the measured capacitance reaches the maximum value (158 F g−1) when the temperature of heat treatment is increased to 1000 °C and then starts to drop at a further increased temperature of 1050 °C. Figure 2B shows the correlation of the heating temperature and the resulting capacitance determined from their CV curves, clearly indicating that 1000 °C is the optimal temperature for carbonizing PANI and generates maximum capacitance. Lower temperatures such as 850 °C, 900 °C, and 950 °C may not be sufficient to fully carbonize PANI that has very low surface areas (∼5 m2g−1). This would result in less porous structures which would yield relatively low capacitance, while the excessively high temperature (1050 °C) may cause deconstruction of the porous structure of the sample. As a result, 1000 °C was selected as the optimized temperature for heat treatment in the following sections. 3.2. Effect of Acidic Leaching Treatments. Along with heat treatment, acid leaching was also an important factor to consider in this study. In principle, acid leaching treatment provides two key aspects in further enhancing the performance of carbon materials in supercapacitors. First, the in situ formed metal aggregates can be removed by using the acidic treatment, 30214


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Figure 3. (A) CV curves of Fe-PANI-C-1000 samples treated by using various acids including 0.5 M H2SO4, 30% HNO3 and mixed 30%HNO3+2.0 M HCl at a scan rate of 20 mV s−1. (B) Calculated specific capacitance for these Fe-PANI-C-1000 samples.

Figure 4. (A) N2 adsorption−desorption isotherms and BET surface area of Fe-PANI-C-900-H2SO4, Fe-PANI-C-1000-H2SO4, and Fe-PANI-C1000-Mix Acid. (B) DFT pore size distribution of Fe-PANI-C-900-H2SO4, Fe-PANI-C-1000-H2SO4, and Fe-PANI-C-1000-Mix Acid. (C) Pore volume distribution of Fe-PANI-C-900-H2SO4, Fe-PANI-C-1000-H2SO4 and Fe-PANI-C-1000-Mix Acid.

opening porous structures and exposing increased surface areas. Second, proper acid treatment results in oxygen-containing functional group on the surface, thereby improving the hydrophilicity of carbonized carbon materials. In this work, we applied three different acid solutions for acid leaching FePANI-C samples: 0.5 M H2SO4, 30% HNO3, and mix acid (30% HNO3 and 2.0 M HCl with a volumetric ratio of 1:3). Figure 3A shows the CV curves of Fe-PANI-C samples at a scan rate of 20 mV s−1 as a function of type of acids. Relative to

traditional acid treated Fe-PANI-C-1000-H2SO4 sample, the Fe-PANI-C-1000-HNO3 exhibits slightly better performance. However, a significant improvement is found on Fe-PANI-C1000-Mix Acid, with a specific capacitance of 444 Fg−1 at a scan rate of 20 mV s−1. Figure 3B further summarizes the comparison of capacitance of Fe-PANI-C samples treated by various acids. The mixed acid treatment is able to generate more than two times higher capacitance when compared to the H2SO4 treated sample (158 Fg−1). Therefore, among studied 30215


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Figure 5. (A) CV curves of BP2000 and M-PANI-Cs on a glassy carbon electrode (GCE) with a scan rate of 20 mV s−1. (B) CV curves of Fe-PANIC on a GCE with different scan rates. (C) Specific capacitance of BP2000 and M-PANI-Cs on a GCE as a function of scan rate. (D) Nyquist plot of BP2000 and M-PANI-Cs coated nickel foam electrodes (NFEs). Inset: enlarged Nyquist plots for three M-PANI-C electrodes in high-frequency region.

capacitance measured with the low-temperature treated samples (Fe-PANI-C-900-H2SO4). Figure 4B further compares the pore size distribution among these three samples. All samples exhibit two dominant pore distribution centering at 1.0 and 3.0 nm, belonging to the micropore and the mesopore region, respectively. Figure 4C summarizes their pore volume distribution. Generally, all three studied samples are composed of a dominant fraction of mesopore. Compared to the 900 °C treated sample, an increase in temperature to 1000 °C leads to increased micropores and macropores, due to a high degree of carbonization at higher temperatures associated with micropores. In addition, the increased temperature leads to generation of larger particles of metallic aggregates and more open macropore structures after acid leaching treatments. Meanwhile, the mixed acid treatment does not change the various pore size distribution, except for slight increases of micro- and mesopores. Therefore, compared to acidic post treatment, heating temperature is the foremost factor to govern the overall porous structures of carbon samples. Table S1 compares more detailed information regarding the values of BET surface area and pore volume distribution of these studied samples. 3.3. Effect of Transition Metals during the Carbonization. In addition to Fe, other two transition metals (Co, Ni) were also used during the synthesis due to their ability to catalyze graphitized carbon during the carbonization. As a result, Co-PANI-C and Ni-PANI-C samples were prepared using the optimal heating temperature of 1000 °C and acid leaching using mixed acid. Thus, in this section, three metalderived carbon samples along with the BP 2000 reference were

acids, the mixed acid is the most efficient to remove the metal species enclosed in carbon, create more pores inside the sample structure, and generate appropriate surface functional groups, when compared to individual 0.5 M H2SO4 and 30% HNO3. The combination of HCl and HNO3 makes the mixed acid with strong oxidizing properties. Under the acid leaching process at 80 °C for 5 h, the mixed acid removes most of the leftover metals and simultaneously introduces a large amount of oxygen-containing functional groups (e.g., hydroxyl, epoxy, and carboxyl) to the surface of carbon structures, which improves the hydrophilicity of the materials and provides greater pore volume accessible to the electrolyte. Therefore, the mixed acid combination of HCl and HNO3 leads to an increase of electrochemically accessible surface areas, thereby increasing the capacitance. To better understand how the optimal 1000 °C and the mixed acid can generate the highest capacitance, three samples including Fe-PANI-900-H2SO4, Fe-PANI-1000-H2SO4, and FePANI-1000-Mix acid were selected for subsequent N 2 adsorption−desorption tests in terms of their porosity and BET surface area. Figure 4A shows the N2 adsorption− desorption isotherms and BET surface areas of these selected samples. Compared with Fe-PANI-C-1000-H2SO4 (957 m2 g−1), consistent with the higher supercapacitor performance as observed in Figure 3A, Fe-PANI-C-1000-Mix Acid exhibits a much larger BET surface area (1645 m2 g−1), which indicates more efficient removal of metal species to open pore structures by using mixed acid. However, Fe-PANI-C-900-H2SO4 shows smaller BET surface area (601 m2 g−1) than Fe-PANI-C-1000H2SO4, which agrees with the result shown in Figure 2, low 30216


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During the carbonization process, PANI directly converted into carbon materials. Meanwhile, nitrogen atoms originally in PANI are able to dope into carbon structures during the thermal treatment.44,45 XPS is a powerful technique to determine nitrogen doping, including its doping content and position according to corresponding binding energy. As shown in Figure 6, the peak at relatively low binding energy around

compared systematically in terms of their electrochemical properties using CV, EIS, and galvanostatic charge−discharge (GCD) with a three-electrode system and a two-electrode system. CV curves (Figure 5A) obtained from active materials reveal a notable difference of capacitance among BP2000 and M-PANI-Cs. At a scan rate of 20 mV s−1, BP 2000 shows the lowest specific capacitance (77 F g−1). Once again, the FePANI-C shows the highest specific capacitance 444 F g−1 followed by Ni-PANI-C (207 F g−1) and Co-PANI-C (175 F g−1). In addition, the measured double layer capacitance currents of Fe-PANI-C increases linearly with the scan rates (Figure 5B), indicating an ideal supercapacitor behavior and perfect accessibility to electrolytes. It should be noted that the highest capacitance measured at the lowest scanning rate of 5 mV s−1 reaches 478 Fg−1 that is the highest value everreported for carbon-based electrode for supercapacitors. Figure 5C gives an overall comparison of the specific capacitance for BP2000 and M-PANI-Cs on a GCE as a function of different scan rates. Fe-PANI-C has the highest specific capacitance at each scan rate but drops faster than the other samples at higher scan rates. Co-PANI-C and Ni-PANI-C maintain more stable performance at higher scan rates. Most likely, this is due to a relatively larger fraction of macropore. In addition, specific capacitances for all of samples at different scan rates are listed in Table S2. The charge transfer behavior of BP2000 and M-PANI-Cs based nickel foam electrodes (NFEs) was investigated using EIS. The representative Nyquist plots are shown in Figure 5D. The equivalent circuit model is shown in Figure S3. RS is the series resistance; RCT is the charge-transfer resistance. RS is derived from the high-frequency intersection of the Nyquist plot in the real axis, including ionic resistance of electrolyte, intrinsic resistance of substrate, and contact resistance at the interface of the active material/current collector. The diameter of the semicircle represents RCT at the electrode/electrolyte interface in the middle-frequency region. In the low frequency region, the slope of the curve represents the Warburg resistance, ZW, which represents the diffusion of ions into the porous electrode.43 In the high frequency region, M-PANI-Cs based NFEs presents perfect semicircles, indicating the ideal supercapacitor performance, whereas BP2000 shows an extreme small fraction of circle with large curvature. In the low frequency region, M-PANI-C-based NFEs present sharply increased and linear lines, which are characteristic of pure capacitive behavior, whereas BP2000 has a slowly increased curve (Figure 5D). RS and RCT of BP2000 and M-PANI-C based NFEs were obtained by fitting the Nyquist plots using the equivalent circuit model, which are also listed in Table S2. Details about fitted Nyquist plots of BP2000 and M-PANI-Cs based NFEs can be found in Figure S4. BP 2000-based NFE has both the highest RS (2.86 Ω) and RCT (7.22 Ω), which are responsible for the slow charging and discharging rates. FePANI-C, Co-PANI-C, and Ni-PANI-C have similar RS values (1.77, 1.80, and 1.83 Ω respectively), indicating almost the same series resistances. Fe-PANI-C based NFE has an RCT of 2.07 Ω, which is slightly larger than those of Co-PANI-C- (1.60 Ω) and Ni-PANI-C- (1.22 Ω) based NFEs, indicating a larger charge transfer resistance. This could be credited to the relatively higher graphitization degree and high content of nitrogen doping at edge sites (pyridinic N) obtained from Coand Ni-PANI-C samples, corresponding to fast charge transfer. This interesting difference associated with the various nitrogen doping content and the relative content of pyridinic nitrogen will be discussed in the XPS characterization.

Figure 6. Nitrogen doping in these M-PANI-C samples determined using XPS: (A) Fe-PANI-C, (B) Co-PANI-C, and (C) Ni-PANI-C. Two major types of doping were identified as pyridinic N at edge sites and graphitic N at center of carbon planes.

398.6 eV is due to the pyridinic N, in which nitrogen atoms are doped at the edge sites of the carbon plane. At higher binding energy, another dominant peak around 401.2 eV usually can be detected, which is associated with nitrogen atoms doped at the center of carbon planes labeled as graphitic N. It should be noted that transition metals play an important role in affecting 30217


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Figure 7. SEM images for (A-B) Fe-PANI-C before heating treatment, (C−F) Fe-PANI-C after heating treatment at 1000 °C and acid leaching in mixed acid.

improving hydrophilicity.45 This is partially responsible for the significant increase in electrochemical surface areas of Fe-PANIC, associated with the highest capacitance determined using CV. Therefore, we believe the favorable nitrogen doping plays an important role in enhancing the charge transfer for the nonfaradaic process associated with supercapacitors. However, the overwhelmingly high surface areas obtained from Fe-based carbon materials, as discussed below, lead to the largest capacitances. In our ongoing effort, we are trying to combine Fe and Ni to prepare bimetallic carbon materials with an aim to optimize the surface areas and nitrogen doping simultaneously. Among the studied M-PANI-Cs, Fe-based carbon materials exhibit the highest BET surface areas (1645 m2 g−1) and the largest capacitance up to 478 Fg−1. These remarkable

the nitrogen doping during the PANI carbonization. Among the studied metals (e.g., Fe, Co, and Ni), Ni induces the highest nitrogen doping content (8.9 at%) with the relatively highest pyridinic nitrogen. Oppositely, Fe yields nitrogen doping with the lowest content (4.5 at%) as well as the smallest pyridinic fraction. This suggests that Ni is the most effective to induce nitrogen doping at the edge site of carbon during the PANI carbonization. Oppositely, Fe is favorable for doping nitrogen at the center of carbon planes. In general, pyridinic N often provides active sites for ion adsorption thereby improving ion diffusion and transfer at the interface. This is in agreement with the lowest charge transfer resistance measured with the NiPANI-C samples as shown in Figure 5D. However, graphitic N dominant in Fe-PANI-C tends to modify the electron distribution on the carbon plane, resulting in polarization and 30218


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Figure 8. Two TEM images (A) and (B) showing the highly porous and 3D morphology for the Fe-PANI-C samples.

Figure 9. (A) XRD patterns and (B) Raman Spectra of BP 2000 and M-PANI-Cs.

morphologies were further characterized using electron microscopy. Figure 7 represents the overall morphologies for the Fe-PANI-C before and after the carbonization process. The in situ polymerized PANI exhibits uniform and porous fiber-like network morphologies with a fiber diameter of 100 nm (Figure 7A and B). After 1000 °C treatment in N2 atmosphere, these well-defined fiber networks change to porous graphene-sheet like morphologies (Figure 7C−F). The transparent bubble-like structure suggests thin-layered carbon planes. As a reference, SEM images for well-known reduced graphene oxide are shown in Figure S5, indicating a significant morphology difference. It would be an interesting method to generate thin-layered carbon sheet structures via high-temperature decomposition of polymer, instead of traditional physical and chemical methods for exfoliation of graphite. In addition, the highly porous 3D graphene sheet like structures were further justified using TEM images (Figure 8). The thin-layered carbon sheet tangled and twisted together in a porous structure is responsible for the highest surfaces area with improved mass transfer determined for the Fe-PANI-C carbon materials. Furthermore, Figure S6 shows the morphologies of BP 2000 and other M-PANI-C samples. BP 2000 presents the amorphous and porous structures with a uniform distribution of carbon particles. Both Co-PANI-C and Ni-PANI-C exhibit less porous structures. A small fraction of graphitized structure

was detected, covered by some metal aggregate particles. In particular, Ni-PANI-C contained some short carbon nanotubes that were encapsulated and surrounded by a large amount of amorphous carbon. X-ray diffraction (XRD) results show the crystalline structure of BP 2000 and M-PANI-C (Figure 9A). Similar to the peaks of BP 2000 at 25° and 44° associated with carbon,46 there are no Fe aggregates identified from the Fe-PANI-C catalysts, suggesting that the mixed acid treatment is able to remove all of the metallic species, leaving porous carbon. Unlike the FePANI-C, in addition to the broad peak around 25° assignable to carbon (002), the distinct XRD peaks associated with metal aggregates in Ni-PANI-C and Co-PANI-C (shown by green and blue stars) could be assigned to nickel carbide (Ni0.98C0.02, JCPDS No. 01-074-5561) and Co9S8 (JCPDS No. 00-0560002), respectively. The sulfur sources originates from the APS that was used as an effective oxidant to polymerize aniline during the material synthesis. The carbide could be formed during the heat treatment of Ni-PANI-C. The presence of nickel carbide (Ni0.98C0.02) and Co9S8 indicated that acid leaching cannot entirely remove all the metal species, especially for those enclosed into graphitized carbon shells. This is in agreement with the observation of SEM images, showing graphitic carbon in Co and Ni-derived samples. Also, relative to the major Fe aggregates (FeS) in Fe-PANI-C before acid 30219


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Figure 10. (A) N2 adsorption−desorption isotherms, (B) DFT pore size distribution, and (C) pore volume distribution of BP2000 and M-PANI-Cs.

for their low surface areas. Also, BP2000 contains the largest volume of marcopores (>50 nm), evidenced by its isotherm plot with a sharp increase of P/P0 in the region close to P/P0 = 1.0. This feature was not observed for M-PANI-C samples. DFT pore size distribution of BP2000 and M-PANI-Cs were shown in Figure 10B. BP2000 showed a PSD centered at 0.85 and 1.2 nm belongs to the micropore (0−2 nm) region along with the mesopore region (∼3 nm and larger than 10 nm). However, all pores detected in M-PANI-C were located in a relatively small diameter region (

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