Functionalization of Petroleum Coke-Derived Carbon for Synergistically Enhanced Capacitive Performance
© Zhang et al. 2016
Received: 23 January 2016
Accepted: 17 March 2016
Published: 24 March 2016
Petroleum coke is a valuable and potential source for clean energy storage if it could be modified legitimately and facilely. In the present study, porous carbon with high surface area and abundant oxygen-containing groups was prepared from petroleum coke by chemical activation and modification processes. The as-prepared carbon exhibits a high surface area (1129 m2 · g−1) and stable micrographic structure. It presents a high specific capacitance and excellent rate performance in KOH electrolyte. Even at an ultrahigh current density of 50 A · g−1, the specific capacitance of the prepared carbon can still reach up to an unprecedented value of 261 F · g−1 with a superhigh retention rate of 81 %. In addition, the energy density of this material in aqueous electrolyte can be as high as 13.9 Wh · kg−1. The high energy density and excellent rate performance ensure its prosperous application in high-power energy storage system.
Environment-friendly energy supply is seemed to be one of the biggest concerns right now, which is closely associated with our lives . It is undeniable that fossil energy will continue to play a major role in meeting our energy requirements for an extended period. However, each kilowatt hour of electricity generated by burning coal coproduces an average 1000 g CO2 emission , which will cause global warming. In addition, portable electronic devices and hybrid electric vehicles, with power source of electric energy resource, are growing very fast in recent years. In this regard, efficient electrical energy storage systems have gradually caused extensive concern [1, 3–5]. Supercapacitors are very attractive owing to their superior power density (>10 kW · kg−1), fast charge/discharge rates (within seconds), and long cycle lifetime (>105) compared to other chemical energy storage devices [6, 7]. They can play a vital role in some applications that needed high-power delivery while batteries cannot meet the requirement.
Supercapacitors can be divided into two categories according to the charge storage mechanism . One is electrical double-layer capacitors (EDLCs) which store energy by the adsorption of both anions and cations. The other category is the pseudo-capacitors that store energy through fast surface redox reactions. Carbon-based materials have been widely investigated as electrodes of supercapacitors due to their desirable physical and chemical properties [9–12]. These properties include low cost, ease in processing, controllable porosity, and electrocatalytic active sites for a variety of redox reactions. Large accessible specific surface area and appropriate pore size of the carbon-based electrodes are crucial to ensure a good performance of EDLCs in terms of both power delivery rate and energy storage capacity. Activated carbons (ACs) are often considered as EDLC electrode materials because of their high surface area and somewhat controllable pore size. For example, a mesoporous activated carbon sphere derived from resorcinol-formaldehyde resin has been prepared and chosen as electrode of EDLC . It possesses a high surface area and presents a good electrochemical double-layer capacitive performance. Carbon precursors like such resin are too numerous to mention [14–21]. Among them, petroleum coke (PC) is a better candidate for preparing high surface area AC due to its high carbon content and low ash content. In addition, it is abundant and cheap (less than $100 per ton and usually used as burning fuel) in China. Lee and Choi  prepared a high surface area activated carbon derived from high-sulfur petroleum cokes by chemical activation using KOH. Its surface area can reach up to 1980 m2g−1 with a high KOH to coke ratio of 4. The surface area can even reach up to as high as 3000 m2g−1 when the KOH to coke ratio increases to 10 . It is obvious that PC is very difficult to be activated due to its stable micrographic structure and lack of the initial pores . During the activation of PC, large quantity of KOH was expended to obtain high surface area AC, which was uneconomic and eco-unfriendly. In addition, pure electrochemical double layer (EDL) capacitance from AC is relatively low, resulting in an inferior energy density.
Heteroatom doping seems to be an effective way to solve these problems. It can not only make PC easy to be activated  but also introduce pseudo-capacitance to enhance the overall capacitance of the electrode materials. Lu et al.  studied the oxidization and activation mechanism of PC and found that the oxygen-containing functional groups play an important role during the activation process. Bai et al.  prepared a nitrogen-doped AC from PC by using urea and KOH as activating agent. The as-prepared AC possesses a high level of nitrogen and large surface area, leading to an improved CO2 uptake capacity. Jiang et al.  synthesized ACs from PC by the combination of chemical treatment with HClO4 or H2O2 and chemical activation with KOH. The resulting activated carbon had higher specific surface area and better iodine adsorption value. Unfortunately, the influence of functionalization of PC-derived carbon on its energy storage performance has rarely been studied, limiting its wide use in clean energy storage applications.
Therefore, the purpose of this research is to investigate the effects of oxygen doping of petroleum coke-derived carbon (PCAC) on its capacitive performance. Different oxidants, such as H2O2 or HNO3, were used to tune the surface chemistry property of PCAC. It is found that oxidization by H2O2 can effectively influence the pore structure and capacitive performance of PCAC. The introduced functional groups not only enhance the EDL capacitance but also provide pseudo-capacitance, leading to an enhanced overall capacitance.
A typical Chinese PC from Shengli refinery was used as a raw material. It was ground and sieved to select the grains in the range of 100–149 μm. The obtained powder was used as precursors of active carbon after drying in an oven at 383 K for 3 h. KOH, H2O2, HNO3, and HCl of analytical grade were obtained from Sinopharm Chemical Reagent Co., Ltd. and used without further purification.
Synthesis of Surface-modified Activated Carbon Derived from PC
The samples were prepared from PC with a combination of KOH activation and chemical modification using hydrogen peroxide or nitric acid as oxidizing agent.
At a KOH/PC mass ratio of 2:1, 10.0 g PC was mixed with KOH. The mixture was transferred into a crucible and carbonized at 850 °C for 2 h with a heating ramp of 5 °C min−1 in argon. The carbonized sample was washed and filtered with HCl aqueous solution and distilled water successively until the pH of the filtrate was 7. After rinsing, the sample was dried overnight at 120 °C and named as PCAC.
In a typical procedure, 3.0 g PCAC power was first added into 30 mL 20 % H2O2 or 1 M HNO3 solution, then the solution was transferred to autoclave and stirred for 8 h at 353 K. After that, the sample was thoroughly washed with distilled water to remove excess H2O2 or HNO3. The obtained materials were dried at 373 K in an oven for 12 h. The samples via modification with H2O2 and HNO3 are denoted as PCHO and PCNC, respectively.
The as-prepared carbons were characterized by Raman spectroscopy (JY Labram HR 800, HORIBA Jobin Yvon, λ = 514 nm) for structure detection. The surface chemical properties of the samples were characterized by Fourier transform infrared spectroscopy (FT-IR) (Nicolet 6700, Thermoscientific). An elemental analyzer (ANTEK, ANTEK 9000, USA) was used to analyze the elemental content of the synthesized samples. The microscopic morphologies of the samples were observed using scanning electron microscope (SEM, S-4800 Hitachi) and transmission electron microscopy (TEM, JEM-2100 JEOL) techniques. Nitrogen adsorption-desorption measurements were performed on a Tristar 3000 analyzer (Micromeritics, USA) to obtain specific surface area and pore structure parameters of the samples. The specific surface area of the samples was calculated by the Brunauer-Emmett-Teller (BET) method in the relative pressure (P/P 0) between 0.06 and 0.3, and the total pore volume was determined from the amount of nitrogen uptake at a relative pressure (P/P 0) of 0.99.
where C is the specific capacitance (F · g −1), I is the discharge current (A), ∆t is the discharge time (s), ∆V is the potential change during discharge process (V), and m is the mass of active material in a single electrode (g).
Results and Discussion
Textural and Surface Properties of Samples
Textural properties of the carbon materials
S BET a [m2/g]
S Micro b [m2/g]
S Meso c [m2/g]
V Total d [cm3/g]
V Micro b [cm3/g]
V Meso c [cm3/g]
D e [nm]
The intensity ratios of I D/I G in Raman spectra of carbon samples and the size of their graphitic carbon
I D/IG a
The elemental composition of samples
Element content (%)
Electrochemical Capacitive Performance
Electrochemical impedance spectroscopy was further used to analyze the charge transfer and ion transport of the materials. Figure 5c shows Nyquist plots in the frequency range from 100 kHz to 0.01 Hz. The near vertical line over the low-frequency ranges suggests excellent capacitive behavior. It is well known that high slope value means the fast ion transport . Obviously, PCHO presents a bigger slope than PCAC and PCNC. In addition, PCHO also shows a smaller semicircle than PCAC at high-frequency region of the Nyquist plot. This is because that the higher degree of graphitization of PCHO, as illustrated by Raman results, is in favor of fast electron transfer. The energy density and power density at an operating voltage of 1.2 V for PCAC-, PCHO-, and PCNC-based ECs are plotted in Fig. 5d. It can be found that the energy density and power density of PCHO are much higher than those of PCAC and PCNC. At a very high power density of 10 kW · kg−1, PCHO can still deliver a high energy density of 9.2 Wh · kg−1, suggesting the promising application of PCHO in some scenarios requiring both high power and high energy density.
The high specific capacitance and excellent rate performance could be attributed to the following reasons: (i) The high surface area provides a large adsorption interface for electrolyte to form the EDL, leading to a great electrochemical double layer capacitance. (ii) The introduction of oxygen-containing groups can improve the wettability of PCHO in aqueous electrolyte. They can not only increase the accessibility of electrolyte ions to make full use of the surface area but also provide considerable pseudo-capacitance to enhance the overall capacitance. (iii) The stable micrographitic structure cannot be damaged during the chemical activation by moderate amount of KOH. Thus, the ordered sp2 carbon domains can enhance the transfer of electric charges, which reduces the internal resistance and also helps to maintain high rate performance of the supercapacitor.
To promote the application of petroleum coke in clean energy storage, we have synthesized an oxygen-doped porous carbon via KOH activation and chemical modification of petroleum coke. The as-prepared carbon possesses a high surface area and abundant oxygen-containing groups, leading to large EDL capacitance and pseudo-capacitance. The introduced oxygen-containing groups can also improve the accessibility of electrolyte ions; thus, the ions can get to the surface easily even at ultrahigh current density. In addition, the stable micrographic structure of the material makes the electron transfer fast. Furthermore, the facile and efficient synthesis process could be easily scaled up, which makes the practical application of petroleum coke in energy storage possible.
This work was financially supported by National Natural Science Foundation of China (21476264), Distinguished Young Scientist Foundation of Shandong Province (JQ201215), Taishan Scholar Foundation (ts20130929) and Fundamental Research Funds for the Central Universities (15CX05029A, 15CX08009A, 15CX06038A).
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