Article pubs.acs.org/JPCC
Density of States, Carrier Concentration, and Flat Band Potential Derived from Electrochemical Impedance Measurements of N‑Doped Carbon and Their Influence on Electrocatalysis of Oxygen Reduction Reaction Bapi Bera, Arup Chakraborty, Tathagata Kar, Pradipkumar Leuaa, and Manoj Neergat* Department of Energy Science and Engineering, Indian Institute of Technology Bombay (IITB), Powai, Mumbai 400076, India S Supporting Information *
ABSTRACT: Nitrogen-doped carbon (N/C) and graphene (N/G) were synthesized by the established conventional heat-treatment method, and the incorporation of nitrogen into the carbon matrix was confirmed by CHN analysis, X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. Electrochemical impedance spectroscopy (EIS) of the prepared catalysts in argon-saturated 0.1 M KOH was performed in a three-electrode rotating disk electrode (RDE) configuration. The capacitance derived from the lowfrequency region of the EIS patterns was used to estimate the effective density of states [D(EF)] of carbon and its nitrogen-doped counterparts. Moreover, the carrier concentrations (ND) and flat band potentials of the samples were obtained by Mott−Schottky analysis. The metal-free catalyst samples were tested for possible oxygen reduction reaction (ORR) activity in oxygensaturated 0.1 M KOH electrolyte, and the origin of the activity improvement with nitrogen doping of carbon/graphene can be explained on the basis of the effective density of states [D(EF)], carrier concentration (ND), and flat band potential. The results suggest that N/C-900 has the highest carrier concentration and maximum flat band potential and, therefore, the highest activity for the ORR.
1. INTRODUCTION The oxygen reduction reaction (ORR) is one of the most important electrochemical reactions relevant to air-breathing cathodes in fuel cells and metal−air batteries.1−3 Because of sluggish kinetics, precious-metal catalysts (Pt, Pd, etc.) are required to catalyze the ORR.4−6 Because of cost and availability issues, nonprecious-metal and metal-free catalysts have been investigated as alternative ORR catalysts in the past few decades. These alternatives are mostly carbon-based materials and are often doped with transition metals (Fe, Co, Ni, etc.).6,7 Carbon-based materials doped with heteroatoms (N, P, S, B, etc.) are also used extensively for the ORR in alkaline media.8−10 Among them, metal-free N-doped carbon has good catalytic activity in alkaline media. N-doped carbon is mainly synthesized by the heat treatment of carbon with an appropriate nitrogen precursor [polyaniline (PANI), polypyrrole, NH3 gas, melamine, urea, etc.].9−11 Thus, N-doped carbon spheres (NCSs), nanofiber (CNFs), nanotube (CNTs), and so on have been reported to be active toward the ORR. In the literature, the active-site density and turnover frequency (ToF) are descriptors of catalytic activity and its improvement with nonprecious-metal catalysts (NPMCs).12,13 However, there are currently no methods for calculating activesite densities and ToFs for nonmetal catalysts; therefore, the ORR activity and its improvement with metal-free NPMCs are explained qualitatively in many ways. It has been reported that © 2017 American Chemical Society
heat treatment generates significant disorder in carbon-based systems because of extensive nitrogen and oxygen incorporation in the sp2 carbon network, which is known to be beneficial for the ORR.14 Nitrogen-doped carbon has a lonepair electron in the plane of the carbon matrix on the pyridinic N, and it can increase the electron-donor properties of the catalyst. Thus, it weakens the O−O bond through the bonding between oxygen and nitrogen and/or the adjacent carbon atom, facilitating the reduction of oxygen.15 It has been noted that both the basic nature and the catalytic activity increase with doped-nitrogen content in carbon as a result of electron delocalization.16 The strong Lewis basicity of N-doped carbon acts to facilitate reductive O2 adsorption under open-circuit conditions. Although there are several reports in the literature describing the role of the nitrogen content in improving the electrocatalytic activity, an explanation in terms of the electrical properties of the material still remains elusive. Therefore, in this work, the electrochemical properties of nitrogen-doped carbon-based samples are investigated in an alkaline medium (0.1 M KOH). The density of states of each sample is estimated from the capacitance derived from electrochemical impedance spectroscopy (EIS), and subseReceived: July 10, 2017 Revised: August 27, 2017 Published: August 29, 2017 20850
DOI: 10.1021/acs.jpcc.7b06735 J. Phys. Chem. C 2017, 121, 20850−20856
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The Journal of Physical Chemistry C quently, the flat band potential and carrier concentration are calculated from Mott−Schottky analysis.17−21 With the nitrogen doping of carbon, the density of states [D(EF)] and carrier concentration (ND) change, thereby affecting the ORR activity. Thus, through systematic ac (EIS) and dc (voltammetry) analyses of the samples, the origin of the improvement in the ORR activity of N-doped samples is explained, and therefore, the effect of nitrogen doping is established.
Pine Instrument Company, Grove City, PA. A standard Ag/ AgCl (saturated KCl) electrode was the reference electrode, and a carbon rod (∼5-mm diameter) was the counter electrode. A catalyst-deposited glassy-carbon rotating disk electrode (RDE; disk area = 0.196 cm2) was the working electrode. A rotating ring−disk electrode (RRDE; disk area = 0.196 cm2, ring area = 0.109 cm2) was used only for the peroxide detection. The catalyst ink was prepared by a method reported in the literature as follows:5 To prepare the catalyst ink, 5 mg of the catalyst was dispersed in 5 mL of DI water, and 10 μL of Nafion in a 25 mL bottle, and the mixture was sonicated for 15 min. Then, 10 mL of isopropanol was added to the dispersion, and the mixture was sonicated for another 15 min to obtain a homogeneous ink. A measured volume of the ink was drop-cast onto a polished glassy-carbon electrode to get a loading of 128 μg cm−2. The surface of the working electrode was then airdried for 2 h prior to the electrochemical measurements. The voltammograms were recorded in argon-saturated 0.1 M KOH electrolyte. The ORR voltammograms were recorded in oxygen-saturated 0.1 M KOH electrolyte with rotation at 1600 rpm. Peroxide was detected at the ring electrode (potential held at 0.2 V vs Ag/AgCl) as the disk electrode potential was scanned from −1.2 to 0.2 V. In all of the experiments, the voltammograms were recorded at a scan rate of 20 mV s−1, and all of the potentials are reported versus standard Ag/AgCl (saturated KCl) in this article. Impedance spectra were recorded using an SP-300 potentiostat [from BioLogic Science Instruments (SeyssinetPariset, France)] in a three-electrode thin-film RDE (TF-RDE) configuration. Before impedance measurements, cyclic voltammograms (CVs) and ORR voltammograms were performed as described above. Impedance spectra were recorded with an ac amplitude of 10 mV rms by sweeping the frequency from 10 kHz to 50 mHz at 10 points per decade. Mott−Schottky analysis was carried out at 100 mHz frequency to calculate the carrier concentration and flat band potential. All data were recorded in a single sine mode.
2. EXPERIMENTAL DETAILS 2.1. Materials. Graphite flakes (7−10 μm grain size, 99% purity) from Alfa Aesar; sodium nitrate (NaNO3, 99% purity), sulfuric acid (H2SO4, 98% by weight, G.R.), potassium permanganate (KMnO4, 99% purity), potassium hydroxide (KOH, 99% purity), isopropanol (C3H7OH, 99.5% purity), formaldehyde (CH3CHO), nickel nitrate [Ni(NO3)2·6H2O], hydrochloric acid (HCl), ammonia solution (NH4OH, 30% by weight), and sodium hydroxide (NaOH) from Merck; hydrogen peroxide (H2O2, 30% by weight, A.R.) from Loba Chemie; and hydrazine hydrate (NH2·NH2, 35 wt % solution in water), melamine, and Nafion (5 wt % solution in lower aliphatic alcohols/H2O) from Sigma-Aldrich were used for synthesis and analysis purposes. Dialysis membrane tubing from Hi-Media was used to purify graphene oxide (GO). Deionized (DI) water was obtained from a Direct-Q Millipore deionizer. 2.2. Synthesis of Nitrogen-Doped Carbon Catalyst. The required amount of carbon black (Vulcan-XC 72, 100 mg) was taken in a clean and dry quartz crucible. It was kept in a quartz tubular furnace and the tube was purged with ammoniacal argon gas (argon passed through 30% ammonia solution) for 15 min to flush out the air inside the tube. The gas purging was continued, and the carbon sample in the furnace was heat-treated at the desired temperature for 3 h; thereafter, it was allowed to cool to room temperature. The as-prepared sample was collected, ground well in a mortar with a pestle, and preserved for further characterization. Carbon black samples that were heat-treated in an ammoniacal inert atmosphere at 700, 800, 900, and 1000 °C are denoted as N/C-700, N/C-800, N/C-900, and N/C-1000, respectively, in this article. The details of the syntheses of other catalysts are given in the Supporting Information (SI). 2.3. Physical Characterization. X-ray diffraction (XRD) patterns of the samples were recorded using a Smart Lab X-ray diffractometer from Rigaku, Tokyo, Japan (30 mA, 40 kV), with Cu Kα radiation (λ = 1.5406 Å). Scans were performed from 15° to 70° (2θ) at a scan rate of 10° min−1 with a step size of 0.01. X-ray photoelectron spectroscopy (XPS) was performed with an AXIS Supra instrument from Kratos Analytical (Manchester, U.K.). Peak fit was performed using peak fit 4.1 software. CHN elemental analysis was done with FlashEA 1112 series analyzer from Thermo Finnigan, Monza, Italy. The specific surface area of each sample was calculated from the adsorption branch according to the Brunauer−Emmett−Teller (BET) method (Smart instrument BET surface analyzer, Smart Sorb 92/93). Raman spectra were recorded using an argon-ion laser (514.5 nm) in an HR800 UV spectrometer from Jobin Yvon Horiba, equipped with an Olympus BX41 microscope and a charge-coupled detector from Andor. The data were collected in the wavenumber range of 1000−2000 cm−1. 2.4. Electrochemical Characterization. Electrochemical characterizations were conducted in a conventional threeelectrode system using a Wave Driver 20 Bipotentiostat from
3. RESULTS AND DISCUSSION Although physical characterization (CHN, XRD, Raman, and XPS analyses) of N-doped carbon is well-established in the literature, it is included in this article to establish nitrogen doping in carbon and its trend with temperature. To analyze the amount of nitrogen incorporated into carbon, CHN analysis was performed. The results of CHN analysis are reported in Table 1. From the CHN analysis, it is observed that a small amount of nitrogen is already present in the carbon black as an impurity. With heat-treatment temperature, the nitrogen content increases and it is highest with N/C-900. The BET specific surface area (SBET) increases with heat-treatment temperature, as shown in Table 1. Table 1. Elemental Compositions Obtained from CHN and BET Specific Surface Area (SBET) Analyses of the Catalysts
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catalyst
nitrogen content (wt.%)
BET surface area (SBET, m2 g−1)
carbon black N/C-700 N/C-800 N/C-900 N/C-1000
0.2 0.9 1.1 1.6 NA
222 221 277 462 559 DOI: 10.1021/acs.jpcc.7b06735 J. Phys. Chem. C 2017, 121, 20850−20856
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reported previously that pyridinic N sites have the highest ORR activity among various types of doped N.23 With increase in heat-treatment temperature, pyrrolic N transforms to pyridinic N and graphitic N. At very high temperature pyridinic N is also converted to graphitic N.24 From Table 2, N/C-900 has the highest percentage of pyridinic N and perhaps therefore the highest activity toward ORR (discussed below). Electrochemical data shows that N/C-900 has highest ORR activity. Raman spectra of carbon and N-doped carbon (N/C-900) are shown in Figure S3 (SI). The intensity of the D band of the Raman spectra of both samples at ∼1344 cm−1 is due to induced disorder, and it is ascribed to structural defects in the graphitic plane of carbon. The G band at ∼1580 cm−1 is commonly observed for all graphitic structures, and it is attributed to the E2g vibrational mode present in sp2-bonded graphitic carbon.14,22,25 The extent of the defects in a graphite material can be qualitatively estimated from the intensity ratio of the D and G bands (ID/IG). The nitrogen-doped carbon heat-treated at 900 °C is found to have an ID/IG ratio (1.35) higher than that of carbon black (1.07). The higher ID/IG ratio for the N/C synthesized at 900 °C is a result of the structural defects caused by nitrogen incorporation into carbon.14,22 The XRD patterns of the N-doped carbon and carbon black samples are shown in Figure S4. A broad peak at ∼25° (2θ), observed in all of the XRD patterns, is attributed to the reflection from the graphite (002) plane, and the second broad peak at ∼43° is attributed to the graphite (004) plane of the Vulcan XC-72 carbon.23 To evaluate the surface electrochemistry and ORR activity of the nitrogen-doped carbon samples synthesized at various temperatures, cyclic voltammograms (CVs) were recorded in argon- (or oxygen-) saturated 0.1 M KOH electrolyte. Figure S5 shows the CVs of carbon and N-doped carbon in argonsaturated 0.1 M KOH solution in the potential range from −1.2 to 0.2 V at a scan rate of 20 mV s−1. Neither carbon black (Vulcan XC-72) nor the N/C samples have any well-defined redox peaks because of the absence of faradic processes; the only process occurring is double-layer charging. Bare carbon black shows the lowest double-layer capacitance, and a systematic increase in the capacitance is observed with increasing nitrogen content. Sample N/C-900 shows the highest double-layer capacitance, which might be due to the higher extent of nitrogen doping occurring at elevated temperature. Another important feature that can be observed with the heat-treated carbons (N/C-800 and N/C-900) is the appearance of a minor redox peak at ∼−0.4 V in the CVs. The peak at ∼−0.4 V is attributed to the generation of oxygenated species on the carbon surface, which shows a redox transformation with potential cycling.23 The charges and specific capacitances due to the double-layer area were calculated for the C and N/C samples. Specifically, the specific capacitances were calculated according to the equation
The N 1s XPS spectra of carbon black and the heat-treated samples are shown in Figure 1. The N 1s spectra of the heat-
Figure 1. High-resolution N 1s XPS spectra of (a) N/C-700, (b) N/ C-800, (c) N/C-900, and (d) N/C-1000.
treated samples (Figure 1) are deconvoluted, and the peaks at 398.5, 400, 401, and 403 eV are assigned to the presence of pyridinic nitrogen, pyrrolic nitrogen, graphitic nitrogen, and oxidized pyridinic nitrogen, respectively.7,9,11,18 The graphitic nitrogen corresponds to N atoms that are linked to three carbon atoms in the graphene basal plane, replacing the C atoms in a hexagonal ring. Pyrrolic N present in the fivemembered ring can contribute to the conjugated system in the graphite layers with two electrons. Pyridinic N present in the six-membered ring can donate one electron to the aromatic system (shown in Figure S1). The surface atomic concentrations of the different nitrogen-containing functional groups, estimated from XPS, are listed in Table 2. The maximum N doping obtained is ∼2 wt.% when the nitrogen precursor used for the synthesis is ammonia solution. Therefore, to increase the nitrogen content in N-doped carbon and N-doped graphene, melamine was used as the nitrogen precursor. XPS spectra of N/C-melamine and N/G-1050 are shown in the SI (Figure S2). Both of these samples have higher percentages of pyridinic nitrogen compared to the others, and oxidized pyridinic N is absent in N/C-melamine because it was synthesized at lower temperature (700 °C). The electrocatalytic activity toward ORR is reported to increase significantly with N-doping. The N doping introduces structural defects, and doped-nitrogen with lone pair electron provides negative charge. The carbon atoms adjacent to nitrogen with high positive charge density (because of electron withdrawing nitrogen atoms) increases their interaction with anions which results in an enhanced ORR activity.22 It has been
Table 2. Concentrations of Different Nitrogen-Containing Functional Groups on the Surface of N/C Catalysts concentration (atom %) catalyst
pyridinic N (at BE 398.5 eV)
pyrrolic N (at BE 400 eV)
graphitic N (at BE 401 eV)
oxidized pyridinic N (at BE 403.2 eV)
N/C-700 N/C-800 N/C-900 N/C-1000
22 32 38 21
55 34 19 19
6 15 17 21
17 19 26 40
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slight increase in current is observed toward lower potentials. The highest current observed in this region is ∼6.0 mA cm−2 with N/C-900. The rotating ring−disk electrode (RRDE) technique is employed to measure the amount of HO2− generated at the disk electrode by holding the Pt ring at 0.2 V. The observed ring current has the same onset potential as the ORR. As can be seen in Figure 2b, the peroxide generation current is inversely proportional to the ORR current; the peroxide generation is higher with catalysts synthesized at lower temperatures (with lower N contents), and the peroxide generation decreases with the samples heat-treated at higher temperatures. Thus, N/C900 has the highest ORR activity and the lowest peroxide generation current. The percentage of peroxide generation was calculated using the equation26
specific capacitance (F g −1) =
∫ I dV (scan rate) × (potential range) × (loading)
(1)
The results are reported in Table 3. The N-doped carbon synthesized by the heat treatment at 900 °C shows the highest Table 3. Charges and Specific Capacitances of the Catalysts Estimated from the Voltammograms catalyst
charge (mC)
specific capacitance (F g−1)
carbon black N/C-700 N/C-800 N/C-900 N/C-1000
1.06 1.74 2.23 3.08 1.52
29.8 48.9 62.7 86.6 42.8
X H2O2 = 2
specific capacitance of ∼86.6 F g−1. Figure S6 shows the voltammograms of carbon black, reduced graphene oxide (RGO), N/G-1050, and N/C-melamine in 0.1 M KOH solution at a scan rate of 20 mV s−1. These voltammograms do not have any well-defined redox peaks, and the only redox couple in the higher potential region might be due to the presence of a redox functional group in the graphene matrix. Graphene has the highest capacitance among all the samples. Figure 2 shows the ORR voltammograms of the carbon and N-doped carbon samples in oxygen-saturated 0.1 M KOH
IR N
⎛ IR ⎞ ⎜I + ⎟ ⎝ D N⎠
(2)
where IR, ID, and N are the ring current, disk current, and collection efficiency, respectively. The ring current starts at ∼−0.2 V, reaches a maximum at ∼−0.4 V, and decreases gradually thereafter. In the reverse scan, a peak is observed in the ring current at −0.8 V. Figure S7 shows backgroundcorrected ORR voltammograms of carbon black, RGO, N/G1050, and N/C-melamine in 0.1 M KOH solution at a scan rate of 20 mV s−1. Both N-doped carbon and graphene synthesized using melamine have comparable ORR activities . Figure 3 shows the EIS patterns of carbon black, N/C-700, N/C-800, N/C-900, and N/C-1000 at −300 mV potential in
Figure 3. EIS patterns of carbon black, N/C-700, N/C-800, N/C-900, and N/C-1000 in argon-saturated 0.1 M KOH solution. The values of the density of states and capacitance of each catalyst at −300 mV and 100 mHz frequency are also shown. The inset shows the equivalent circuit.
Figure 2. (a) ORR voltammograms of carbon black, N/C-700, N/C800, N/C-900, and N/C-1000 recorded in oxygen-saturated 0.1 M KOH solution at 20 mV s−1 with rotation at 1600 rpm. (b) Peroxide oxidation currents recorded parallel to the ORR voltammograms. The solid lines indicate forward scans, and the dotted lines indicate backward scans. The inset in panel (a) shows the percentage of HO2− formation during oxygen reduction.
argon-saturated 0.1 M KOH solution. Typical carbon features with equivalent series resistance (ESR), equivalent distributed resistance (EDR), and capacitance are shown in Figure 3. ESR originates mainly from the solution resistance and electrical connections. The arc in the Nyquist plot in the high-frequency (HF) region is due to limitations on ion (H+, K+, OH−) transport through the carbon/Nafion matrix and is called EDR.27−29 After the HF region, the straight line parallel to the y axis with a tilt toward x axis, observed in the mid-frequency (MF) and low-frequency (LF) regions, is attributed to the double-layer capacitance. The EIS patterns of carbon black, N/
solution in the potential range from −1.2 to 0.2 V at a scan rate of 20 mV s−1 with rotation at 1600 rpm. From Figure 2a, it can be observed that the half-wave potential of the ORR shifts to higher values with N-doped catalysts, and the maximum shift occurs with the catalyst heat-treated at 900 °C in ammonia atmosphere. There is no well-defined mass-transfer-limited region; in the potential range between −1.2 and − 0.3 V, a 20853
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The Journal of Physical Chemistry C C-melamine, RGO, and N/G-1050 at −300 mV potential in 0.1 M KOH solution are shown in Figure S8. RGO has the highest EDR value among these samples because of its layered stack structure. The total double-layer capacitance (C) of each system can be calculated from the imaginary impedance using the equation18,30 C = −1/(2πfZimg )
(3)
where f is the frequency and Zimg is the imaginary impedance. Calculations of capacitance from the impedance data are shown in Figure S9 (SI). The space charge capacitance (CSC) can be calculated using the equation31 1 1 1 = + C CH CSC (4) where CH is Helmholtz capacitance. The density of states [D(EF)] can be calculated using the equation18,30 CSC =
εε0D(E F) ×e
Figure 4. Capacitance−potential relationships of carbon black, N/C700, N/C-800, N/C-900, and N/C-1000.
(5)
where ε, ε0, and e are the dielectric constant, vacuum permittivity, and electronic charge, respectively. Both the density of states and capacitance increase with increasing nitrogen content in the carbon samples prepared by the heat treatment of carbon in an ammoniacal atmosphere, as reported in Table 4. These data show that N/C-900 has the highest
CSC
capacitance (mF)
carbon black N/C-700 N/C-800 N/C-900 N/C-1000
0.52 0.68 0.75 1.63 0.60
density of states (cm−3 eV−1) 5.98 1.81 8.54 4.12 6.02
× × × × ×
(6)
where N, V, and Vfb are the carrier concentration, electrode potential, and flat band potential, respectively. k and T have their usual meaning of Boltzmann’s constant and absolute temperature, respectively. When a sample is undoped or an intrinsic semiconductor, there is no excess electron density at the surface. Therefore, with negative potential, capacitance increases exponentially according to eq 6, and the same occurs with higher positive potentials. However, there is a sharper change in capacitance at higher positive potential because of the oxygen evolution reaction at the electrode−electrolyte interface in aqueous media. When carbon is doped with nitrogen at higher temperatures, there is a higher electron density at the surface of the carbon, and the carbon behaves as an n-type semiconductor. For an n-type semiconductor, when negative potential is applied, there is an accumulation of electrons at the surface, and the capacitance increases exponentially according to eq 6. This region is called the accumulation region. When positive potential is applied, there is a depletion of electrons at the surface, and the capacitance curve become flat in this region; this is called the depletion region. With further increase in positive potential, the hole density dominates over the electron density, and the capacitance increases exponentially. This region is called the inversion region. These three regions are clearly evident for samples N/C-800 and N/C-900. From −0.25 to 0.4 V, the capacitance curve is flat, and beyond this range, the capacitance increases exponentially. There is a hump at −0.4 V for N/C-800 and N/C-900 that is due to the pseudocapacitance (quinone−hydroquinone redox couple) behavior of the sample (which is shown in Figure S3 at a potential of −0.4 V). Memming and Schwandt also reported a similar type of hump in the capacitance versus potential plot in a certain potential range for germanium doped with impurities (e.g., copper, silver, or gold ions); these ions give rise to a surface state capacitance.35 However, N/C-1000 exhibits a parabolic curve similar to that of carbon black. At very high temperatures (above 900 °C), nitrogen escapes from the carbon matrix. The n-type semiconductor behavior of the carbon black, N/ C-700, N/C-800, N/C-900, and N/C-1000 samples is verified from the C−2 versus V curve, as shown in Figure 5. During EIS measurements for an M−S plot, the frequency is chosen to be
Table 4. Capacitances and Densities of States of N-Doped Carbon Catalysts catalyst
⎛ 2εε e 2N ⎞1/2 ⎡ (V − Vfb)e ⎤ ⎟ cosh⎢ =⎜ 0 ⎥ ⎣ 2kT ⎦ ⎝ kT ⎠
1021 1022 1021 1022 1020
capacitance as well as the highest density of states. The order of the density of states for the N-doped samples is similar to the order of the ORR activities. In the literature, it has been reported that N-doped carbon behaves as a semiconductor.17−19 Nitrogen has an extra lonepair electron that goes near the conduction band in nitrogendoped carbon to make the latter an n-type semiconductor. The semiconductor−electrolyte interface can be described using Mott−Schottky (M−S) analysis. The level of N doping and the flat band potential of the catalyst can be calculated from the Mott−Schottky analysis. The capacitance versus potential plots of carbon black, N/C-700, N/C-800, N/C-900 and N/C-1000 generated from their respective EIS measurements are shown in Figure 4. Carbon black exhibits a parabolic trend in the plot wherein the capacitance is highest at 0.8 V, reaches a minimum at 0.4 V, and rises again with potential thereafter until −1.2 V.17 Zhou et al.17 and Gerischer et al.32 reported similar types of parabolic curves for undoped highly oriented pyrolytic graphite (HOPG) and pyrolytic graphite, respectively. Memming reported a capacitance−potential curve with a parabolic nature for an intrinsic semiconductor (germanium electrode) in aqueous solution.33 The decrease in capacitance with positive potential is characteristic of n-type semiconductor behavior.17 For a pure intrinsic semiconductor, the space charge capacitance, Csc, should exhibit a dependence on potential given by17,33,34 20854
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Table 5. Carrier Concentrations (ND) and Flat Band Potentials of N-Doped Carbon Catalysts catalyst
carrier concentration (cm−3)
flat band potential (V)
carbon black N/C-700 N/C-800 N/C-900 N/C-1000
1.10 × 1021 4.46 × 1021 2.82 × 1021 8.7 × 1021 4.59 × 1020
−0.774 −1.127 −0.892 −0.735 −0.891
potential (−0.735 V), which means that it requires a lower overpotential to start the charge-transfer reaction.
4. CONCLUSIONS Carbon and N-doped carbon-based samples were investigated using EIS measurements and Mott−Schottky analysis to understand the effects of nitrogen doping on the electrochemical properties. EIS measurements were recorded in 0.1 M KOH solution in the frequency range from 10 kHz to 50 mHz to calculate the capacitance, density of states, carrier concentration, and flat band potential of each sample. The positive slopes of the M−S plots confirm the n-type semiconductor behavior of the samples. Nitrogen doping in the carbon-based samples introduces more electron that are positioned above the Fermi level and that can facilitate the oxygen reduction reaction. The nitrogen content increases with heat-treatment temperature up to 900 °C, whereas it decreases at higher temperatures. Both the capacitance and density of states are functions of N-content in the carbon sample. Among the N-doped carbon samples, N/C-900 was found to have the highest carrier concentration and flat band potential. With increasing carrier concentration, it was found that the ORR activity increases and the peroxide generation decreases, which supports the highest ORR activity for sample N/C 900.
Figure 5. Mott−Schottky plots of carbon black, N/C-700, N/C-800, N/C-900, and N/C-1000. The inset shows the same plot for a restricted potential range from −0.4 to 0.0 V.
high enough that the time scale is too short to allow for the effective filling and unfilling of surface states or for the buildup of a double-layer capacitance. For highly crystalline materials (e.g., silicon, germanium, etc.) frequency is in the range of 1− 20 kHz.21 In this work, the measurements were carried out at 100 mHz because carbon black is amorphous in nature.18,30 From the Bode phase plot (inset of Figure S9), it can be seen that the capacitance behavior is predominant after 100 mHz. The potential window was restricted to the range from −0.4 to 0 V so that the C−2 versus V plot gives a straight line (calculations and M−S plot are provided in the SI).17 The inset of Figure 5 shows the positive slope of the M−S plot, which indicates the n-type semiconductor behavior of the sample. The n-type behavior of the sample is associated with the valence electrons of nitrogen, and thus, the additional p electron of nitrogen causes π-conjugation, turning the sample into an ntype semiconductor.17 The variation of the C−2 value from 0.2 to −1.2 V (double-layer region) is a maximum for carbon black, whereas it is a minimum for N/C-900, suggesting that the N/ C-900 has a higher capacitance than carbon black. Similar trends are observed when N-doped carbon and graphene are synthesized using melamine as a precursor. The capacitance versus potential and Mott−Schottky plots of RGO, N/G-1050, and N/C-melamine are shown in Figures S10 and S11, respectively. The values of the capacitance, flat band potential, and density of states for N/G-1050 and N/Cmelamine are reported in Table S1. According to Mott−Schottky theory, the capacitance and potential are related by the equation ⎞⎡ ⎛ ⎛ 1 2 kT ⎞⎤ ⎟⎥ ⎟⎢V − ⎜Vfb + =⎜ 2 2 ⎝ e ⎠⎦ C ⎝ εε0A eND ⎠⎣
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b06735. Catalyst synthesis procedures; XPS and Raman spectra and XRD patterns of N-doped samples; CVs and ORR voltammograms in 0.1 M KOH electrolyte; and calculations of the density of states, flat band potential, and carrier concentration (PDF)
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +91 22 2576 7893. Fax: +91 22 2576 4890. E-mail:
[email protected]. ORCID
Manoj Neergat: 0000-0003-1106-079X Notes
(7)
The authors declare no competing financial interest.
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where A is the BET surface area of the catalyst.19,20 (For an ntype semiconductor, eq 6 become eq 7.) The carrier concentration (ND) and the flat band potential (reported in Table 5) for each of the catalysts were calculated from the slope of Mott−Schottky plot and the corresponding straight line intercept with the x axis (Figure 5), respectively (see Figure S12). The data show that carbon black has the lowest carrier concentration and N/C-900 has the highest carrier concentration. N/C-900 also has highest flat band
ACKNOWLEDGMENTS The Department of Science and Technology (DST), India, is acknowledged for the financial support of this project through Grant SR/S1/PC-68/2012. The Sophisticated Analytical Instrument Facility (SAIF), Department of Physics, and Department of Metallurgical Engineering and Materials Science, all at IIT Bombay, are acknowledged for physical characterization of the samples. 20855
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DOI: 10.1021/acs.jpcc.7b06735 J. Phys. Chem. C 2017, 121, 20850−20856
