Is the conductive agent useful in electrodes of graphitized activated carbon?

Abstract

In the making process of supercapacitors, the conductive agent carbon black is used for improving the conductivity of the electrodes and decreasing the resistance of interaction. Is it useful for porous activated carbon? In this research, partly graphitized activated carbon materials are obtained by boron and nitrogen catalysis with renewable fallen leaves as carbon source in a one-step method. The symmetric supercapacitors are fabricated in a conductive agent-free system. The fabrication process is time saving and simple with mixing of activated carbon and binder. Ion adsorption can achieve equilibrium within 2.22 s in the absence of carbon black (CB). The energy density and the power density reach to 48.6 W h kg⁻¹ and 3600 W kg⁻¹ at the potential window of 1.5 V.

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Introduction

Carbon materials have been applied as electrode materials in electrical double-layer capacitors (EDLC).^1–4 Carbon nanotubes, carbon nanofibers and carbon nanospheres with high electrical conductivity and unique pore channels provide a broad passageway for ion transportation. While the low surface area limits the improvement of energy density for supercapacitors.^5–7

Activated carbon materials (ACs) are a good choice for electrical double-layer capacitors (EDLC) due to low cost, high surface areas, abundant, easily acquired and high stability.^8,9 Partially graphitized ACs in a simple and convenient process exhibit improved conductivity. Boron has been proved as the good catalyst of graphitization, by which ACs show the improved oxidation resistance and graphitized effects.^10–12 However, the graphitization of ACs is seldom reported. On the other hand, doping heteroatom in the carbon framework, such as B, N, P, O and so on can increase the ion-accessible surface areas and afford an additional pseudocapacitance.^9,13,14

In addition, manufacturing process of supercapacitors involves using carbon black (CB) as a cheap conductive agent. Yan¹⁵ and Park¹⁶ have reported that CB particles as spacers ensure the high electrochemical utilization of graphene layers as well as the open nano-channels provided by three-dimensional graphene nanosheet/CB hybrid material. That is, CB not only can improve the conductivity of materials, but also can change the structure of carbon particles. Does it have same effect on graphitized ACs? Herein, we use boron and nitrogen graphitizing the ACs via an effective one-step method. The generation of high surface and hierarchical pore structure is based on our previous work with KOH and K₂CO₃ as activators.¹⁷ In order to eliminate the influence of impurities in the carbon resources, the carbonized fallen leaves are washed by 1 M HCl solution. The obtained boron-nitrogen graphitized carbon (BNGC) has good conductivity, a large surface area and hierarchical pore structure. What's more, we explore the roles of carbon black (CB) in supercapacitor to demonstrate the advantage of partly graphitized active carbon. The synthesized BNGC exhibits excellent capacitive property in the absence of CB. Such a low-cost yet high-performance supercapacitor electrode material is potentially useful as a high energy supercapacitors.

Results and discussion

Fig. 1a shows the XRD patterns of samples with two broad diffraction peaks. It indicates a turbostratic structure composed of graphitic-like micro-crystallites.^18,19 Fig. 1b shows the Raman spectroscopy to probe the graphitized degree of ACs. Two featured peaks located at ∼1350 cm⁻¹ (D band) and ∼1590 cm⁻¹ (G band) can be observed. The relative intensity ratio (R) between the D-band and G-band (I_D/I_G) is used to measure the disorder degree or the graphitization of the carbon structure. The R value is 1.005, 0.98 and 0.95 for BGC, NAC and BNGC, respectively. It indicates the deepest graphitization under the joint action of B and N elements.

Fig. 1 XRD patterns (a) and Raman spectra (b) of all the samples.

Fig. 2a shows that BNGC possesses the fluffy morphology. A large number of macropores can be observed as marked in Fig. 2b. Fig. 2c represents high-resolution TEM image. It can be seen that the surface of the carbon is the layered structure with mesopores. Fig. 2d shows that the lattice fringe spacing is about 0.34 nm, which corresponds to the graphite (002) plane, implying a graphitic structure of BNGC. Moreover, the interior of the carbon displays some worm-like nanopores, which is formed by the activation of mixture.

Fig. 2 SEM (a), TEM (b) and HRTEM image under different magnification (c and d) for the sample of BNGC.

Fig. S1a–d† show the nitrogen adsorption/desorption isotherms and pore size distribution of BNGC, NAC, BGC and PAC. It is clear that all of the obtained materials display the hysteresis loops for the desorption branch, indicating the presence of mesopores.^20,21 The BET surface areas for those materials decrease in the order: PAC (757 m² g⁻¹), BNGC (998 m² g⁻¹), BGC (1096 m² g⁻¹) and NAC (1171 m² g⁻¹). Meanwhile, from the curves of PSD (Fig. S1a–d† inserts), there are abundant nanopores below 2 nm, indicating the existence of micropores.

In a two electrodes system (coin cells (CR2025)), GCD curves at operating voltage of 0–1 V (BNGC-1), 0–1.2 V (BNGC-1.2) and 0–1.5 V (BNGC-1.5) are recorded in the 6 M KOH aqueous electrolyte and at the current density of 1 A g⁻¹, respectively. The charge/discharge profiles of carbon electrodes exhibit almost the isosceles triangle curves, indicating excellent coulomb efficiency (Fig. S2a†). The capacitance values of all the samples are displayed in Fig. S2b† calculated with the equation C = 2IΔt/(mΔV), where m is the mass of active carbon (g), ΔV is potential difference (V), I is the electric current density (A), Δt is the time of discharge (s). BNGC achieves 208 F g⁻¹ at 50 mA g⁻¹ and decreases gradually to 153 F g⁻¹ at 1 A g⁻¹ and 140 F g⁻¹ at 10 A g⁻¹. However, the capacitance of PAC is 123 F g⁻¹ at 50 mA g⁻¹ and 121 F g⁻¹ at 1 A g⁻¹. Otherwise, the discharge time for BNGC-1.2 and BNGC-1.5 is longer than that of BNGC-1.

Fig. S2c† shows a Ragone plot calculated from the GCD curves. We can observe a decreasing trend in power density with the increase of energy density for all cells. BNGC possesses very promising energy power combinations: 28.9 W h kg⁻¹ at the power density of 180 W kg⁻¹, and maintains at 19.5 W h kg⁻¹ even the power density reached 3600 W kg⁻¹. The energy density decreases with the increase of power density attributed to the complex resistance and tortuous diffusion pathway within the porous textures.²² The cycle lifetime for the electrode of material is an important indicator of the electrochemical test. As shown in Fig. S2d,† all the electrodes of materials almost no decay up to 2000 cycles, indicating the excellent cycling stability. While, the stability at the potential of 1.2 V and 1.5 V maintains 97% and 74%, respectively. Comparing with the references, the prepared BNGC shows the excellent performance under the potential of 1.2 V.²³

The CV curves and the specific capacitance at different current density calculated by GCD curves for PAC (no adding CB) and PAC/CB (adding CB) are shown in Fig. 3a. CB possesses a profound effect on the capacity character. When adding CB into PAC, the specific capacitance increases from 190 F g⁻¹ to 240 F g⁻¹. The CV curve of PAC/CB displays the increased area in symmetric shape comparing with that of PAC (Fig. 3a (inset)). While, BNGC has an opposite results from PAC (Fig. 3b), the addition of CB decreases the specific capacitance from 238 F g⁻¹ to 144 F g⁻¹, and the CV curves are shrink accordingly (Fig. 3b (inset)). However, the addition of CB could assist BNGC adapting for larger scan rate, but with the capacity decreasing (Fig. 3c). From the phenomenon above, we speculate that CB can enter the pores of BNGC and accelerate the transmission velocity of ions. In the meantime, injecting CB could block the pores keeping the ions out of the inner pores.^15,24 It is well known that ACs have lower electrical conductivity due to the amorphous structure and abundant surface groups, so adding CB to improve the conductivity is necessary to some degree. While, adding CB in BNGC leads to the capacity decreased. Fig. 3d shows the influence of CB for PAC and AC in two electrode system, which is similar and opposite for BNGC. Moreover, the addition of CB decreases the capacity with the increasing of current density which may due to the blocking of pores by CB and the transportation of electrolyte ions are limited during fast charging. Electrolyte ions do not have enough time to reach the surface of micropores at high current density, therefore the specific capacitance at high current density is lower than that at low current density. Such observation is obvious in micro–mesoporous carbon materials.²⁵ While, the carbon materials with abundant macropores represent a decreased capacity when incorporation of CB.

Fig. 3 The CV curves and GCD curves for the influence of CB for the sample of PAC (a) and BNGC (b); the CV curves of BNGC/CB at different scan rate (c); the specific capacitance for the samples at different current density (d).

The Nyquist plot in Fig. S3a† displays that the ESR of PAC/CB is slightly lower than PAC, indicating that the conductivity of PAC is improved by the addition of CB. The Warburg resistance also is shorter, representing the better diffusion of ions into the electrodes and the improved conductivity. AC is similar to PAC, and the Warburg resistance increases more obvious through the addition of CB. The condition for BNGC is opposite that the ESR and Warburg resistance are bigger with the addition of CB. The value of ESR for each sample is listed in Table 1. This proves that the extra CB increases the resistance and prevents the ion transportation into the pores. The frequency response of capacitance reveals the important influence for the hierarchical porous structures on the rate of ion transport. Fig. S3b† represents the capacitance–frequency dependency, reflecting the penetration of the alternating current into the bulk pores of the electrode material. This can illustrate how many solvated ions reach to the pore surfaces at a specific frequency. The curve is root in Bode plot and the capacitance values are calculated by the following equation:²⁶ C = −1/2πfZ_im, where f is the applied frequency (Hz), and Z_im is the imaginary part of impedance (Ω). Clearly, the capacitance of all the samples gradually decreases with the increase of frequency, indicating that the limitation of ion transportation at high frequency. The capacity value of BNGC is much higher than that of BNGC/CB, but the addition of CB increases the values for AC and PAC (Table 1). We also see that the conductivity can improve the capacity at low frequency, in which the sample is not graphitized. Instead, the capacity of graphitized sample decreases with the addition of the CB. As for the frequency response, the capacitance of BNGC electrode nearly approaches saturation at around 0.45 Hz, which implies that ions adsorption can achieve the equilibrium within 2.22 s. It is faster than that in commercial devices (generally in the range of 10–100 s).²⁷ The BNGC/CB is similar to BNGC, but AC and PAC need longer time (about 10–100 s) to achieve equilibrium. Comparing the frequencies at which the capacitance is 50% of the maximum value f_0.5, we clearly see that BNGC electrodes exhibit the fastest frequency response with f_0.5 ≈ 2.67 Hz, corresponding to the characteristic relaxation time constant of less than 0.37 s. The faster performance of these samples correlates with their slightly better conductivity. The operating frequency of BNGC also is higher than that of micropores carbons, ordered mesopores carbons and hierarchical porous carbons which reported by others.^27–29 Wang and co-workers have reported a 3-dimensional hierarchical porous graphitic carbon material with macroporous cores, mesoporous walls and micropores for high rate supercapacitor applications.³⁰ The existed macropores serve as ion-buffering reservoirs; the wall of graphitic mesopores provides excellent electrical conductivity and overcomes the primary kinetic limits of electrochemical process in porous electrodes and the presence of micropores can enhance the charge storage. At a time of 2 s, the values of the energy and power densities reach to 5.7 W h kg⁻¹ and 10 kW kg⁻¹.

Table 1 The electrochemical data of the prepared electrodes measured in two-electrode system

Samples	ESR/Ω	Capacity/F	Response time^a/s (Hz)	Frequency f0.5^b/Hz
a The response time is taken at saturated frequency.b The frequencies at 50% of the maximum capacitance (f0.5).
PAC	0.069	81	3.72(0.27)	1.13
PAC/CB	0.053	112	4.76(0.21)	0.81
AC	0.061	71	10.12(0.1)	0.45
AC/CB	0.060	167	6.25(0.16)	0.45
BNGC	0.014	138	2.22(0.45)	2.67
BNGC/CB	0.021	77	2.22(0.45)	2.67

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The added boron not only reduces the number of carbon reactive site, leading to achieve a proportional reduction of carbon reactivity, but also suppresses the intrinsic reactivity (or turnover frequency) of these sites, achieving an exponential reduction of carbon reactivity.³¹ The wrapup mechanism of prepared BNGC is shown in Fig. 4. The original carbon has a turbostratic structure leading to the inferior conductivity. After the activation, AC possesses the hierarchical pore structure. Meanwhile, B catalyzes carbon forming graphitic structure on the surface of carbon to increase the conductivity effectively, but the active sites also be decreased. As compensation, the addition of N can increase the active site and improve the delocalization effect of electron for the electrochemical characters.

Fig. 4 The scheme for the roles of B, N and activators in the process of activation and graphitization.

Experimental

Materials and methods

The carbon source is the fallen leaves from Fraxinus chinensis, which was carbonized (denoted as FLC) as our previous report.¹⁷ The FLC was soaked in 1 M HCl for 10 h in order to eliminate the influence of the ash composition, and then washed with deionized water till neutral. After dried at 100 °C in oven, it was stirred in the solution of KOH, K₂CO₃, H₃BO₃ and urea with the mass rate of 1 : 1 : 2 : 0.02 : 0, 1 : 1 : 2 : 0.02 : 10 and 1 : 1 : 2 : 0 : 10 (sample was denoted as BGC, BNGC and NAC, respectively) at room temperature for 10 h, and dried at 100 °C in oven. Then the mixture was heated in a tube resistance furnace at 800 °C for 2 h with a heating rate of 10 °C min⁻¹ in a nitrogen flow of 40 ml min⁻¹. The obtained control sample without B and/or N was named PAC. Furthermore, commercial active carbon (AC) was tested as a contract.

Conclusions

Partly graphitized active carbon is a new route to improve the conductivity of ACs. BNGC keeps the hierarchical porous structure during the partly graphitization. Such unusual structure is favorable for ion transport in the absence of conductive agent. The partly graphitized ACs provides excellent electronic conductivity that promotes fast ion and electron transport, which ensures high rate capability. The addition of N can remedy the lost surface active sites deduced by graphitization of boron. These advantageous features result in a high capacity, superior rate capability, long-term stability, and high energy and power densities in absence of CB, whose conductivity is redundant and block the pores of BNGC on the contrary. Hence, the obtained BNGC possesses great potential in the construction of high performance supercapacitors in absence of conductive agent.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21076056), Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry, the Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT, IRT1059), and Hebei Provincial Key Lab of Green Chemical Technology & High Efficient Energy Saving, School of Chemical Engineering & Technology, Hebei University of Technology.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra18246a

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