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Applied Surface Science 475 (2019) 56–66 Contents lists available at ScienceDirect Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc Full Length Article P-doped hierarchical porous carbon aerogels derived from phenolic resins for high performance supercapacitor T ⁎ Jia Guo, Dongling Wu , Tao Wang, Yan Ma Key Laboratory of Energy Materials Chemistry, Ministry of Education, Key Laboratory of Advanced Functional Materials, Autonomous Region, Institute of Applied Chemistry, Xinjiang University, Xinjiang 830046, PR China A R T I C LE I N FO A B S T R A C T Keywords: Phosphorus-doped Phenolic resins Hierarchical porous carbon Supercapacitor P-doped hierarchical porous carbon aerogels are prepared by carbonizing the phloroglucin-formaldehyde resins in the presence of ZnCl2 and subsequently being activated by KOH. Phosphoric acid is simultaneously used as the polymerization catalyst and P-doped agent in the preparation procedure. Compared with un-doped sample, Pdoped carbon with hierarchical porous structure shows improved electrochemical performance. The prepared sample that activated at 800 °C exhibits good capacitance of 406.2 F g−1 in 6 M KOH at a scan rate of 5 mV s−1. When the scan rate is 500 mV s−1, the speciﬁc capacitance still reaches to 267.4 F g−1, demonstrating good rate capability. When 60 mg of active materials is loaded, the mass speciﬁc capacitance of the prepared electrode reaches to 348.8 F g−1 at a scan rate of 5 mV s−1, and the maximum area capacitance is 11.35 F cm−2. The energy density of the prepared sample is as high as 16.97 Wh kg−1 at a power density of 200 W kg−1 and reaches to 8.52 Wh kg−1 at 2000 W kg−1. Importantly, after 100,000 charging and discharging cycles, the speciﬁc capacitance of the prepared sample is no attenuated, indicating a long-term electrochemical stability. 1. Introduction It is known that phenolic resin can be synthesized by phenolic compounds and aldehydes catalyzed by acid or alkali. Due to its simple production process, it has been widely used in molding compound, foundry resin, friction material, semiconductor packaging, adsorb, electrode materials, etc. [1]. Phenolic resin has been receiving considerable attention in the past few decades. In 1989, Pekala and coworkers ﬁrst synthesized the resorcinol-formaldehyde (RF) organic aerogel via the sol–gel polycondensation of phenolic compounds with formaldehyde, prior to the supercritical drying [1,2]. Subsequently, they also obtained carbon aerogels via pyrolysis of RF aerogels [3]. These pioneer studies resulted in an upsurge of interest in the phenolic resin-based porous carbon aerogels. A large amount of research have been devoted to the improvement of synthesis methods [4–6] and the factors that inﬂuence the structure and performance of these phenolic resin-based porous carbon. These factors such as the species and contents of polymerization catalysts [7–12], soft templates of the template self-assembly method [13,14], other extra additives [12,15,16], the choice of drying method [17–22], as well as the carbonization or activation conditions [23] will inﬂuence the morphology, surface area, pore structure, and the surface physical chemical properties of the prepared samples. Due to its unique structure property such as ⁎ monolithic form, high speciﬁc surface area, through-connected porosity and rich micro- and mesoporous structures, porous carbon aerogel derived from phenolic resin becomes one of alternative carbon materials for carbon electrodes in electric double layer capacitors [24–34]. So far, various physical and chemical activation treatment methods have been developed in order to obtain a rich porous structure with high surface area for phenolic resin-based carbon materials [29,35–38]. However, much of them usually display poor electrochemical performances at large current densities and fast charge-discharge rates [39–41]. This is mainly because that these activation treatments usually lead to abundant micro-pores, which have negative impact on the diﬀusion and transport of electrolyte ions. Even though much eﬀorts have been done to enhance the capacitive performance of phenolic resin-based porous carbon materials, the speciﬁc capacitance, rate capability and mass speciﬁc capacitance are still unsatisfactory. Theoretical and experimental researches have shown that heteroatoms (such as N, P, B and S) can modulate the electronic properties and provide more active sites to the carbon, so as to improve the interaction between porous carbon frameworks and active sites [42–46]. Among them, nitrogen is the most common doping atom, and it has been found that the nitrogen-doped porous carbon materials possess rich active sites and improved capacitance via surface faradaic reactions [42,47,48]. Due to the low electronegativity of phosphorus compared Corresponding author. E-mail address: [email protected] (D. Wu). https://doi.org/10.1016/j.apsusc.2018.12.095 Received 19 July 2018; Received in revised form 13 November 2018; Accepted 11 December 2018 Available online 12 December 2018 0169-4332/ © 2018 Elsevier B.V. All rights reserved. Applied Surface Science 475 (2019) 56–66 J. Guo et al. 2. Experimental section characterized by scanning electron microscope (SEM, SU4800 Hitachi, Japan) and transmission electron microscope (TEM, JEM-2010). The phase composition of the sample was analyzed by the Powder X-ray diﬀraction D8 using Cu Kα. (λ = 1.5418 Å) radiation. The surface chemical composition of the sample was analyzed by X-ray photoelectron spectroscopy (XPS, ESCALab220i-XL) using a 300 W Al Kα radiation. The microstructure of the sample was analyzed by the Raman spectrometer (SENTERRA, Bruker, Germany). The speciﬁc surface area (SBET) of the sample was measured by the standard BrunauereEmmette-Teller (BET) method in the automatic high pressure physical adsorption instrument ASAP 2020. The pore size distributions, micropore surface areas (Smicro) and pore volumes (Vmicro) were obtained via the DFT method. Fourier transform infrared (FT-IR) spectra were recorded in a range of wavenumbers from 500 to 4000 cm−1. 2.1. Chemicals and materials 2.4. Electrochemical measurements All the chemical reagents in this work are of analytical grade purity and used without further puriﬁcation. Deionized (DI) water was used in all experiments. Phloroglucinol anhydrous was supplied by Aladdin Reagent Co. Ltd., Shanghai, China. Formaldehyde was purchased from Xilong Chemical Co. Ltd., Sichuan, China. Phosphoric acid and ZnCl2 were purchased from Zhiyuan Chemical Reagent Co. Ltd., Tianjin, China. KOH was purchased from Yongsheng Fine Chemical Co. Ltd., Tianjin, China. Ethanol absolute was purchased from Guangfu Science and Technology Development Co. Ltd., Tianjin, China. The mixture of the prepared samples, carbon black and polytetraﬂuoroethylene (PTEF) (binder, suspension, concentration 60 wt%) with a weight ratio of 8:1:1 are dropped onto foamed nickel nets as working electrode for the supercapacitance measurement. The mass loading of active material on each current collector was 2 mg and the area of the prepared electrode is 1 cm2. The electrodes were pressed under a pressure of 10 MPa, followed by being dried at 80 °C overnight. In a three-electrode system, the prepared sample was used as the working electrode, while the platinum foil and standard Hg electrode were used as the counter and reference electrodes, respectively. The measurements were carried out in 6 M KOH aqueous solution. As for the twoelectrode system, the prepared samples were used as the working and counter electrodes, and the two electrodes were separated by a ﬁlter paper, which was fully soaked with electrolyte (6 M KOH and 1 M Na2SO4). The electrochemical properties of the working electrodes were studied by means of cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), electrochemical impedance spectroscopy (EIS) and cyclic stability methods using CHI 660D. The potential window of CV and GCD is −1 to 0 V. The scan rate for CV was varied from 5 to 500 mV s−1, and the current density of GCD was from 1.0 to 50.0 A g−1. EIS measurements were carried out by applying an alternating current with the voltage of 5 mV over a frequency range from 0.01 Hz to 100 kHz. The speciﬁc capacitance to the discharge curves in GCD was calculated by equation [55]: to that of carbon, phosphorus doping in carbon can induced asymmetric charge density, enhanced asymmetric spin density and increased charge delocalization of carbon atoms [49]. Moreover, the high electron-donating ability in phosphorus dopant can remarkably enhance the charge storage and transport capability of porous carbon materials, thus promoting their electrochemical performance [50–54]. Herein, we report a method to prepare P-doped hierarchical porous carbon aerogels (PPC) via carbonization of phloroglucin-formaldehyde resin with ZnCl2, followed by KOH activation. Phosphoric acid was used as the polymerization catalyst and P-doped agent. As the supercapacitor electrode material, the prepared PPC presents superior capacitive property, good rate capability and high cycling stability. 2.2. Synthesis of PPC-x-y In a typical procedure, 0.32 g ZnCl2, and 0.32 g (1 M) phloroglucinol anhydrous (P) was mixed in the DI water/ethanol absolute solution (volume ratio 1:1). After vigorous stirring to form a clear solution, 2 M formaldehyde (F, 37 wt%) was added slowly to form a homogenous solution. Then, 1 mL phosphoric acid (85 wt%) was added to the above solution with consecutive agitation for one minute. The mixture solution was sealed into a 10 mL glass bottle followed by polymerization at 60 °C for 24 h. The polymers were air-dried at 60 °C for 24 h. The prepared PF resin was carbonized at 800, 900 or 1000 °C for 1 h with a heating rate of 5 °C min−1 under N2 atmosphere. After that, the above samples were treated by successive activations. First, the above carbonized samples were washed by HCl (1 M) in order to remove ZnCl2 and other impurities. The products were then added into diﬀerent mass of KOH solution and stirred for few minutes, followed by being air-dried at 80 °C overnight. Finally, the impregnated carbons were activated at 700, 800 or 900 °C for 1 h with a heating rate of 2 °C min−1 under N2 atmosphere. The resultant samples were denoted as PPC-x-y (x represents the temperature of 700, 800 or 900 °C, y means the 0, 1, 2, 3 and 4, referring to the mass ratio of PF resin and KOH of 1:0, 1:1, 1:2, 1:3 and 1:4, respectively). The yield of PPC-x-y after activation with KOH is 10.2%. The undoped PC sample was also prepared as reference using HCl as catalyst, with the same above mentioned procedures (Scheme 1). Cs = ∫ (I dV )/(vmΔV ) (1) In this equation, I is the response current (A), ΔV is the diﬀerence of potential during the CV tests (V), v is the potential scan rate (V s−1), and m is the mass of one electrode (g). The Nyquist plots analysed using the following equation [56] of C′ = (Z ″ (ω))/(ω|Z (ω)2| (2) C ″ = (Z′ (ω))/(ω|Z (ω)2| (3) In these equations, Z (ω) is complex impedance, Z' (ω) is real impedance, Z″ (ω) is imaginary impedance, C″ is real capacitance, and C″ is imaginary capacitance. For the two-electrode system, the speciﬁc capacitance, power 2.3. Characterizations The morphology and structure of the prepared samples were Scheme 1. Schematic diagram of the fabrication process of PPC-x-y. 57 Applied Surface Science 475 (2019) 56–66 J. Guo et al. structure is beneﬁcial for further activation as the activation agent can contact with the inside surface. As shown in Fig. 1b–d, the samples were etched by KOH [56]. The sample PPC-800-1 (Fig. 1b) shows that the surface of the connected thick sheets (slices) are full of pores and cavities. As for PPC-800-2 (Fig. 1c), it possesses loose and porous honeycomb structure. To further increase the mass of KOH, the collapsed porous structure and thicker block have been observed for PPC800-3 (Fig. 1d). It suggests that by varying the content of KOH, the pore size and morphologies of the as-prepared samples could be controlled. It is well known that the microscopic morphology and pore structure of electrode materials are vital to the electrochemical performance. The unique porous honeycomb structure of PPC-800-2 can provide eﬃcient electrolyte ions transport channels, thus enhancing the electrochemical performance. The TEM image of PPC-800-2 (Fig. 1e) further reveals well-deﬁned meso- and macro-porous size distributions, which are typically in the range of 48–130 nm. The inserted image in Fig. 1e shows that PPC-800-2 is typically amorphous. Moreover, SEM-EDX elemental mapping analysis of PPC-800-2 was carried out to conﬁrm the elemental distribution of the samples, as shown in Fig. 1f2–4. It can be seen that the C, O and P elements are evenly distributed in PPC-800-2. The homogeneously doped P elements could potentially improve the capacitive performance of PPC-800-2, which is conﬁrmed by the corresponding electrochemical measurements. Finally, we optimized the KOH activation temperature of the serial PPC-x-2 samples, where x represents the temperature. The SEM image (Fig. 2a) shows that the as-prepared sample PPC-700-2 consists of tightly stacked bulk with uniform small pore distribution. When the activation temperature is raised to 800 °C (Fig. 1c), the PPC-800-2 has a loose porous honeycomb structure. However, when the activation temperature is increased to 900 °C, the sample PPC-900-2 prepared at 900 °C (Fig. 2b) is composed of irregularly shaped larger pore, indicating that a severe KOH etching has occurred. Compared with PPC700-2 and PPC-900-2, PPC-800-2 has a richer pore structure and more suitable pore size (see pore size distributions in Fig. 3b), which will facilitate the inﬁltration of electrolyte ions. XRD patterns of PPC-700-2, PPC-800-2 and PPC-900-2 are shown in Fig. 2c. All the samples shows the two broad characteristic (0 0 2) and (1 0 1) peaks at around 2θ = 24° and 43°, respectively, which indicating amorphous structure of carbon. With the increase of activation temperature, the diﬀraction peaks shift to the left. According to the equation 2dsinθ = nλ (d is the density and energy density were calculated according to the equation [55]: Cs = (I ΔV )/(mΔt ) (4) E = (Cs ΔV 2)/8 (5) P = E /Δt (6) where I is the constant discharge current (A), Δt is the discharge time (s), the m is the mass of activate material on one electrode (g) and the ΔV is the potential window for discharging process (V). E is energy density (Wh kg−1) and P is power density (W kg−1). 3. Result and discussion As shown in Scheme 1, phloroglucinol, formaldehyde, ZnCl2 and H3PO4 were mixed in the ethanol/water solution. P-doped porous carbon aerogels were obtained via carbonizing the phloroglucin-formaldehyde (PF) resins in the presence of ZnCl2, followed by KOH activation. Fig. S1 shows the XRD and Raman patterns of the carbon aerogels prepared at diﬀerent carbonization temperature. The diﬀraction peaks centered at around 2θ = 24° (0 0 2) and 42.5° (1 0 0) were strong and sharp, and the ratio of the relative intensity of the D peak and G peak (ID/IG) value decreased gradually, suggesting the increased degree of graphitization with increasing the carbonization temperature. As a result, the electrochemical performance has been accordingly enhanced. The speciﬁc capacitance of the sample prepared at 1000 °C is higher than that of the other two samples, although the values of the three samples are rather small (Table S1). SEM images indicate that the sample prepared at 800 °C is enrichment of pore-structure (Fig S2). So, the carbonization temperature of 800 °C is chosen for further activation. Furthermore, since the sample prepared with phloroglucinol and ZnCl2 at the mass ratio of 1:1 presented the best electrochemical performance among the prepared samples, it has been used for further study. Furthermore, the eﬀects of activation conditions, including the mass of KOH being as the activating agent, and the activation temperature, on the morphology and structure of the samples have been investigated (Fig. 1). Fig. 1a shows the SEM image of PPC-800-0, it could be observed that the nearly circular macro pores and holes unevenly distributed on the surface of bulk carbon, which might be induced by the ZnCl2 pretreatment prior to the KOH activation. The special porous Fig. 1. SEM images of the PPC-800 activated by diﬀerent mole ratio KOH (a) PPC-800-0, (b) PPC-800-1, (c) PPC-800-2, (d) PPC-800-3, (e) TEM images of the PPC800-2, SEM image (f1) and mapping (f2–4) of PPC-800-2. 58 Applied Surface Science 475 (2019) 56–66 J. Guo et al. Fig. 2. SEM images of PPC-700-2 (a) and PPC-900-2 (b), XRD (c) and RAMAN (d) of PPC-x-2 (x = 700, 800, 900 °C, respectively). crystal plane spacing, θ is the diﬀraction angle, n is diﬀraction series, λ is the incident wave length), the diﬀraction angle θ decreases and the space between the crystal faces increases by d. For the samples of PPC700-2, PPC-800-2 and PPC-900-2, the calculated d is 1.87, 2.03 and 2.04 Å, respectively). With the increase of the activation temperature, the defect of the prepared samples is enhanced. Raman spectra (Fig. 2d) are taken to identify the degree of graphitization of the as-prepared three samples. All the spectra exhibits two main peaks at about 1340 and 1581 cm−1 corresponding to the D and G band, respectively. It is known that the D-band and G-band represent the breathing mode of κpoint phonons with A1g symmetry which is related to the defective graphitic structure and disordered carbon (sp3-rich phase), and E2g phonon vibrations of sp2-bonded carbon atoms [46]. The intensity ratios of ID/IG are 2.27, 2.46 and 2.49 for PPC-700-2, PPC-800-2 and PPC900-2, respectively, which indicates that defects and structural distortions in these samples enhanced with increase the activation temperature. On the contrary, the ID/IG value of the prepared PPC-800, PPC-900 and PPC-1000 samples without KOH activation are only 0.96, 0.94 and 0.89, respectively (Fig. S1b), much smaller than the samples with KOH activation. These results suggest that (1) defects and structural distortions in these samples enhanced with increased the activation temperature. (2) KOH activation can remarkably increase the defects and structural distortions of the prepared samples. The pore properties of the PPC-x-2 were studied by N2 adsorptiondesorption isotherms (Fig. 3). All curves of PPC-x-2 display Type IV isotherms and at a high relative pressure (0.4 < P/P0 < 0.99), all samples shows a H4 hysteresis loops (Fig. 3a), which indicates the existence of mesopores. At the relative pressure below 0.01, the N2 adsorption of the PPC-x-2 samples increased sharply, indicating the existence of micropores. Fig. 3b shows the pore size distributions of PPCx-2 calculated by using the DFT method. As displayed in Table 1, with increasing the activation temperature from 700 to 900 °C, the speciﬁc surface area of the PPC-x-2 was continuously increased from 1433 to 1595 m2 g−1, and pore volume was increased from 0.81 to 1.08 cm3 g−1. It is notable that PPC-800-2 has a wider pore size distribution than PPC-700-2 and PPC-900-2 (Fig. 3b). The PPC-800-2 has a speciﬁc surface area of micropores of 794.05 m2 g−1 accounting for 50.22% of the total speciﬁc surface area. The pore size distribution is also similar with Fig. 3. N2 adsorption/desorption isotherms (a) and pore size distributions (b) of PPC-x-2 (x = 700, 800, 900 °C, respectively). 59 Applied Surface Science 475 (2019) 56–66 J. Guo et al. electrochemical performance of the sample could be improved. The electrochemical performance of PPC-700-2, PPC-800-2, PPC900-2 and PC-800-2 have been tested by using CV, GCD and EIS techniques with a three-electrode system in 6 M KOH electrolyte. Fig. 5a–d shows the CV behavior of PPC-700-2, PPC-800-2, PPC-900-2 and PC800-2 at diﬀerent scan rates (from 5 to 50 mV s−1) with the voltage window of −1.0 to 0.0 V. The CV curves of PPC-700-2, PPC-800-2, PPC-900-2 and PC-800-2 at diﬀerent large scan rates (from 100 to 500 mV s−1) are shown in Fig. S6. It can be seen that all curves exhibit the approximately rectangular shape, characteristic of the double-layer capacitance. But the curves are asymmetrical due to the synergistic eﬀects of double-layer capacitance and pseudocapacitance caused by the existence of a small amount of surface oxygen groups and P dopants, which is consisted with the XPS analyses. The loop area of PPC800-2 is the highest (Fig. S6e), meaning the highest capacitance. The PPC-800-2 electrode has a high speciﬁc capacitance of 406.2 F g−1 at a low scan rate of 5 mV s−1. Even at a high scan rate of 50 mV s−1, the speciﬁc capacitance still remains 357.9 F g−1. As shown in Fig. 5e, the speciﬁc capacitances for PPC-700-2, PPC-900-2 and PC-800-2 are 308.0, 265.4 and 336.0 F g−1, respectively, calculated by Eq. (1) at the scan rate of 5 mV s−1. Table 2 summaries the speciﬁc capacitances of all samples measured at diﬀerent scan rates (from 5 to 500 mV s−1). The galvanostatic charge-discharge curves of all samples are presented in Fig. S7. The speciﬁc capacitance values at diﬀerent current densities calculated by Eq. (4) are listed in Table S2. The coulomb eﬃciency for PPC-700-2, PPC-800-2, PPC-900-2 and PC-800-2 at the current density of 1 A g−1 is 104.4, 101.3 and 98.1% respectively. At a current density of 70 A g−1, the coulomb eﬃciency of PPC-800-2 is 100.1%. The superior capacitive performance of PPC-800-2 can be ascribed to the carbon that provides double-layer capacitance. Meanwhile, the oxygen at the edge and P dopants provide the pseudocapacitance. Moreover, the capacitance of PPC-800-2 is also far superior to those of various carbon materials in recent literatures (Table 3) [57–66]. To evaluate the rate capability of all samples, the capacitance retention ratio versus the scan rate from 5 mV s−1 to 500 mV s−1 are plotted in Fig. 5e. As can be observed that 59.4, 65.8, 57.6 and 50.7% of the initial capacitance for PPC-700-2, PPC-800-2, PPC-900-2 and PC-800-2 can be retained, respectively. EIS was carried out from 0.01 Hz to 100 kHz in three-electrode system. The Nyquists plot and the ﬁtting equivalent circuit model for all electrodes (PPC-700-2, PPC-800-2, PPC-900-2 and PC-800-2) are shown in Fig. 5f. Nyquist plots are consisted of high frequency region corresponding to a charge-transfer process and a line in the low-frequency region corresponding to a diﬀusion process. The semicircle of the PPC800-2 in the high-frequency region is smaller, and the straight line in the low-frequency region has a steeper slope in comparison with the other three samples. This indicates that PPC-800-2 has lower charge transfer and higher ion transmission rate. Rs is the sum of electrolyte resistance, internal resistance of electrode materials and contact resistance between electrode materials and electrolyte. The internal resistance of PPC-700-2, PPC-800-2, PPC-900-2 and PC-800-2 electrode materials is 0.42, 0.39, 0.44 and 0.40 Ω, respectively. The real capacitance (C′), as a function of the frequency of all samples, is deﬁned as the eﬀective capacitance that the material can provide, as shown in Fig. 5g. C′ remained almost constant before 0.1 Hz and fell sharply between 0.1 Hz and 10 Hz. The electrode shows the resistance behavior in the high frequency region, and the capacitive behavior is reﬂected in the low frequency region. Obviously, PPC-800-2 shows the best capacitance performance. Furthermore, the relaxation time (τ0 = 1/ƒ), the reciprocal of the frequency (ƒ), can be used to estimate the charge and discharge rate of the sample. A lower relaxation time (τ0) means a faster charged and discharged capability. Fig. 5h is the curve of imaginary capacitance (C″) and frequency. When the curve reaches the maximum capacitance loss, which is the vertex of the curve, the corresponding frequency is larger, indicating that the shorter the relaxation time (τ0) that is the sample has faster charging and Table 1 Porosity parameters and XPS of Chemical composition of PPC-800-2. Sample PPC-700-2 PPC-800-2 PPC-900-2 PC-800-2 SSA (m2 g−1) Pore Volume (cm3 g−1) XPS (at.%) SBET Smicro Smicro/SBET VTotal Vmicro C O P 1433 1581 1595 1618 786.12 794.05 971.99 1006.91 54.86% 50.22% 60.94% 62.23% 0.81 0.84 1.08 1.00 0.33 0.34 0.44 0.45 81.54 83.84 89.03 89.77 16.22 14.11 9.38 10.23 2.23 2.04 1.59 – the micropore distribution on centred at below 5 nm and the meso/ micropore distributions in the broad range between 1 and 130 nm as indicated by pore size distribution and the TEM image. The adsorption and desorption isotherm and pore size distribution of PC-800-2 are shown in Fig. S3e and f. More detailed speciﬁc pore parameters are summarized in Table 1. The hierarchical porous textures and large speciﬁc surface area of PPC-800-2 can improve its electrochemical properties. It favors the fast diﬀusion and adsorption of the potassium ion on the large electrode surface, the mesoporous are advantageous to the ion transport and at the same time can make sample with better inﬁltration between electrolyte. And the microporous can improve the contact area of samples with the electrolyte. Thus, the PPC-800-2 may have the most eﬃcient electrochemical double layer capacitance and pseudocapacitance performance when applied in supercapacitor. As for the PC-800-2, the larger speciﬁc surface area of this sample is attributed to the contribution of micropores. The pore size distribution (Fig. S3f) shown that PC-800-2 has a narrow pore size distribution. The surface functional groups of P doped sample (PPC-800-2) and the undoped sample (PC-800-2) are identiﬁed by FT-IR spectra (Fig. S4). It can be seen that both samples display ﬁve characteristic peaks, i.e. out-of-plane vibration of eCH2 group at ∼956 cm−1, CeH bending vibration at ∼1037 cm−1 and ∼1193 cm−1, C]C stretching vibration at ∼1537 cm−1, and at ∼1706 cm−1 is attributed to C]O group. Comparing with PC-800-2, PPC-800-2 shows a new peak at ∼1005 cm−1 corresponding to the presence of PeO bond. FT-IR spectra conﬁrm that P was successfully doped into PPC-800-2, which is consistent with the SEM-EDX elemental mapping analysis. The surface elemental chemical states of all samples are analyzed by X-ray photoelectron spectroscopy (XPS) (Figs. 4 and S5). Fig. 4a shows three typical peaks for C 1s, O 1s, and P 2p on XPS survey spectrum of PPC-800-2. The phosphorus elements are successfully doped in PPC800-2, which is consistent with the SEM-EDX elemental mapping and FT-IR analysis result. High-resolution XPS spectra of C 1s (Fig. 4b) can be deconvoluted into a main peak of CeC (285.5 eV) and other four peaks corresponding to CeOeH (286.7 eV), CeOeC (288.2 eV), C] OeC (289.9 eV) and C]O (291.6 eV) [10]. The deconvolution of O 1s spectra (Fig. 4c) yields two characteristic peaks. The peak of 531.5 eV could be assigned to C]O and P]O. The peak located at 532.8 eV could be assigned to CeO and CeOeP groups. This result conﬁrms that the PPC-800-2 has some oxygen-containing functional groups, which may generate the pseudocapacitance. As for the P 2p spectra (Fig. 4d), the peak located at 133.0 eV could be assigned to PeC bonds, while another peak located at 134.6 eV could be attributed to the PeO signal that may derive from oxidized P species during air exposure [57]. As shown in Table 1, as the temperature rises from 700 °C to 900 °C, the contents of O and P of the samples gradually decrease. XPS analysis indicated that the binding mode of P in the PPC-700-2, PPC-800-2 and PPC-900-2 samples was PeO bond and PeC bond. It is well known that oxygen-containing functional groups decrease as the calcination temperature increases, and therefore, an increase in temperature causes the PeO bond to break, and the contents of P and O decrease. XPS further conﬁrmed that the P element was doped into the PPC-800-2. The doping of P atoms will change the charge distribution of the carbon skeleton and increase the active site of electrochemical reaction, so the 60 Applied Surface Science 475 (2019) 56–66 J. Guo et al. Fig. 4. The XPS survey spectrum (a), C1s spectrum (b), O1s spectrum (c), and P1s spectrum (d) of PPC-800-2. approximately 95.0% of speciﬁc capacitance after 10,000 cycles. There is no signiﬁcant change of PPC-800-2 at the ﬁrst ﬁve cycles and the ﬁnal end, conﬁrming that the capacitance of the PPC-800-2 has not decreased after the circulation of 10,000 cycles. We examine the capacity loss on PC-800-2 from two sides, the morphology and the structure. Fig. S3a and b shows the SEM images of PC-800 before and after KOH activation. For comparison, the SEM images of samples that using phosphoric acid as catalyst and doping agent are also shown in Fig. S3c and d. Fig. S3a shows that before activation, PC-800-0 sample has no apparent pores and cracks. After KOH activation, the prepared sample PC800-2 displays stacked bulk solid with full of small pores (Fig. S3b). Fig. S3c shows the SEM image of PPC-800-0 (before KOH activation), it could be observed that the nearly circular macro pores and holes unevenly distributed on the surface of bulk carbon. After KOH activation, as is shown in Fig. S3d, PPC-800-2 displays a honeycomb-like porous structure. It is known that the rich porous structure is helpful to improve the capacitance performance of the materials. PC-800-2 is lack of rich porous structure compared to PPC-800-2, and thus its capacitance performance is poor. On the other hand, we tested the speciﬁc surface area and the pore size distribution of PC-800-2. The speciﬁc surface area of PC-800-2 is 1618 m2 g−1, which is larger slightly than that of PPC-800-2 (1581 m2 g−1) (see Table 1). However, the micropores of PC-800-2 account for 62.23% of the total pore volume, while the micropores of PPC-800-2 account for 50.22%. The larger speciﬁc surface area of PC-800-2 is attributed to the contribution of micropores. The pore size distributions (Fig. S3f) shown that PPC-800-2 has a wider pore size distribution than PC-800-2, which is more conducive to electrolyte inﬁltration and ion transport. As well known that the wide pore size distribution can enhance the capacitive performance of the samples. PC-800-2 has a narrow pore size distribution, so its capacitive performance is poor. Furthermore, to further explain the reason for the capacity loss on PC-800-2, the impedance measurement was analyzed. From the EIS analysis that the semicircle of the PPC-800-2 in the highfrequency region is smaller, and the straight line in the low-frequency region has a steeper slope in comparison with PC-800-2, the internal resistance of PPC-800-2 and PC-800-2 electrode materials is 0.39 and discharging performance. The PPC-700-2, PPC-800-2, PPC-900-2 and PC-800-2 have the highest peak frequency of 0.41, 0.42, 0.38 and 0.37 Hz, corresponding to the characteristic relaxation time τ0 of 2.44, 2.38, 2.62 and 2.72 s, respectively. PPC-800-2 has the smallest relaxation time, indicating that it has the best magniﬁcation performance among all the samples. The reason can be attributed to the proper pore structure and surface chemical state (P atom doping) of PPC-800-2, which plays an active role in the kinetics diﬀusion of electrolyte ions inside the electrode. In order to further evaluate the potential of PPC-800-2 as electrode material in practical application, the inﬂuence of the loading of the active material has been further evaluated for the PPC-800-2. The area speciﬁc capacitance of PPC-800-2 has been calculated based on the cyclic voltammetry (Fig. 6). Even if 60 mg of active material has been loaded, the area speciﬁc capacitance of the prepared electrode can still reach to 348.8 F g−1. Impressively, the capacitance of the PPC-800-2 has barely decayed with increasing the loading from 20 to 60 mg. This can be attributed to the fact that PPC-800-2 has good conductivity and allows a rapid ion diﬀusion, as discussed previously. When PPC-800-2 is used as electrode material, the correlationship of mass loading and area capacitance is linear. The maximum area capacitances is about 11.35 F cm−2. This fairly good rate capability and mass capacitance are beneﬁted from the developed loose and porous honeycomb structure. The larger speciﬁc surface area facilitates electrolyte ions being accessible to the electrodes and leads to the improvement of the electrochemical capacitance. Additionally, the large total pore volume originating from mesoporous will provide abundant short-paths for fast ion transport to further enhance the rate capability. The cycling stability of PPC-800-2 was evaluated by galvanostatic charge–discharge tests in 6 M KOH solution running up to 10,000 cycles and voltage window was 1.0 V at a current density of 4 A g−1 in a threeelectrode system. As is shown in Fig. 7, PPC-800-2 achieves the speciﬁc capacitance no attenuated, and the capacitance retention rate is 100% after 10,000 cycles, indicating that PPC-800-2 has an excellent cycling stability being as the potential electrode material for supercapacitors. The reference PC-800-2 sample decreased in capacity and retained 61 Applied Surface Science 475 (2019) 56–66 J. Guo et al. Fig. 5. CV curve of the capacitors assembled by the samples from 5 to 50 mV s−1 PPC-700-2 (a), PPC-800-2 (b), PPC-900-2 (c) and PC-800-2 (d), respectively. The plots of Cs versus the scan rates of diﬀerent samples (e) and Nyquist plots of diﬀerent samples (f). The frequency response of the real (g) and the imaginary (h) parts of the capacitance. 0.40 Ω, respectively. And the relaxation time (τ0) of PPC-800-2 and PC800-2 are 2.38 and 2.72 s respectively. The EIS curve explains the difference in cycle stability of PC-800-2 (Fig. S3g). For PPC-800-2 and PC800-2, before long-term cycling, they both exhibit the ideal capacitive behaviors with a small semicircle at high frequency region and a highslope line at low frequency region. Small semicircle indicates their small charge transfer resistance, thus beneﬁt for the charge transfer between electrolyte ion and electrode. The approximately vertical curve at low frequency reﬂects good electrolyte ion diﬀusion. However, after the cycle, the EIS curve of PC-800-2 as the active electrode material changed. The inset from the magniﬁed data shows the diﬀusion distance of electrolyte ion increases after cycling (Fig. S3g), the semicircle became bigger and the charge transfer resistance increased. And the straight line in the low-frequency region has a smaller slope, means the 62 Applied Surface Science 475 (2019) 56–66 J. Guo et al. Table 2 The eﬀect of scan rate on the speciﬁc capacitance of diﬀerent samples. Samples PPC-700-2 PPC-800-2 PPC-900-2 PC-800-2 Speciﬁc capacitance (F g−1) 5 (mV s−1) 10 (mV s−1) 20 (mV s−1) 30 (mV s−1) 40 (mV s−1) 50 (mV s−1) 100 (mV s−1) 300 (mV s−1) 500 (mV s−1) 308.0 406.2 265.4 336.0 293.5 393.6 253.5 325.5 282.4 381.3 244.2 312.9 275.8 374.1 239.0 305.0 270.8 368.8 235.0 298.6 266.8 357.9 231.0 293.0 251.0 347.1 215.7 272.4 211.6 301.8 180.5 214.8 182.8 267.4 152.9 170.5 fabricated using PPC-800-2 as electrode materials. The symmetric coin cell preparation method is presented in Fig. 8a. The GCD of two electrodes was measured in 6 M KOH solution. When the current density is 1 A g−1, the speciﬁc capacitance is calculated to be 149.1 F g−1, while the speciﬁc capacitance is 123.9 F g−1 when the current density increases to 10 A g−1. The speciﬁc capacitance of symmetric supercapacitor is no attenuated after 100,000 cycles at the current density of 2 A g−1, demonstrating an excellent long-term stability (Fig. 8a). It again proves that PPC-800-2 is a very promising electrode material. Fig. 8b shows the symmetric supercapacitor using PPC-800-2 as the electrode material, and the LED light of the point. The energy density of PPC-800-2 is tested in neutral electrolyte (1 M Na2SO4). As shown in Fig. 8c, the potential window of PPC-800-2 can reach to 1.6 V, with an increases of 0.6 V than being tested in alkaline electrolyte. It is well known that the energy density of a supercapacitor is determined by voltage and capacitance. In the neutral electrolyte, the potential window of PPC-800-2 is greatly expanded, which is beneﬁcial to improve the energy density of the supercapacitor. The energy density and power density of PPC-800-2 are shown in Fig. 8d. The energy density is 16.97 Wh kg−1 at a power density of 200 W kg−1 and 8.52 Wh kg−1 at a power density of 2000 W kg−1. It has a higher energy density than phenolic carbon (3.5 Wh kg−1) [67], N/O-co doped phenolic base carbon (9.1 Wh kg−1) [68] and phenolic activated carbon (10.9 Wh kg−1) [69]. The PPC-800-2 as the electrode material has excellent performance due to the following points: (1) the large speciﬁc surface area and the hierarchical porous structure are conducive to the inﬁltration of the electrolyte and to reduce the diﬀusion resistance of the electrolyte ion, Table 3 Comparison of capacitance performance of carbon materials. Electrode material N, O, P decorated porous carbons N-doped hollow carbon spheres Core/shell hierarchical porous carbon N-doped hollow mesoporous carbon spheres Nitrogen doped carbon spheres Microporous graphene frameworks Yolk-shelled carbon spheres Hollow mesoporous carbon spheres Mesoporous activated carbon spheres N、P co-doped hollow carbon microspheres p-doped hierarchical porous carbons Cs (F g−1) Electrolyte References 6 M KOH 6 M KOH 6 M KOH [57] [58] [59] 6 M KOH [60] 6M 6M 6M 6M 2M KOH KOH KOH KOH KOH [61] [62] [63] [64] [65] 200 (0.5 A g−1) 6 M KOH [66] 406.2 (5 mV s−1) 6 M KOH This work −1 206 (0.1 A g ) 170 (1 A g−1) 183.1 (5 mV s−1) 159 (1 A g−1) 203 286 120 157 196 −1 (0.5 A g ) (10 mV s−1) (0.5 A g−1) (0.5 A g−1) (1 A g−1) Warburg impedance increased. So, the speciﬁc capacitance PC-800-2 decreased. In short, compared with PPC-800-2, the capacity loss of PC800-2 can be ascribed to its morphology and structure. According to the above-mentioned structural and morphological results, the reason for the good electrochemical performance of PPC-800-2 could be contributed to (1) the large speciﬁc surface area and suitable pore structure, (2) the successful doping of P atoms alternating the sample surface chemistry, (3) the good conductivity. Finally, two-electrode symmetric electrochemical device has been Fig. 6. Mass speciﬁc capacitance of PPC-800-2. 63 Applied Surface Science 475 (2019) 56–66 J. Guo et al. Fig. 7. The cycle performance under 4 A g−1 of the PC-800-2 and PPC-800-2. Fig. 8. Cycling stability of PPC-800-2 at a current density of 2 A g−1 in a two-electrode system (a), the practical application of symmetrical supercapacitor (b), CV curve of PPC-800-2 in diﬀerent potential windows at scan rate is 50 mV s−1 in 1 M Na2SO4 electrolyte (c), Ragone plot of the symmetric supercapacitor (d). capacitance attenuation after cycling 100,000 cycles. The mass speciﬁc capacitance of PPC-800-2 electrode can still reach to 348.8 F g−1 even 60 mg sample has been loaded for test. The correlationship of mass loading and area capacitance is linear and the maximum area capacitance is 11.35 F cm−2. The assembled symmetric supercapacitor also exhibits excellent cyclic stability, and its energy density could reach to 16.97 Wh kg−1 at a power density of 200 W kg−1 in the neutral electrolyte. (2) the successful doping of heteroatom (P) is beneﬁcial to change the surface chemical state of electrode material and provide active site to improve electrochemical performance. 4. Conclusion P-doped hierarchical porous carbon aerogels are prepared by carbonizing the phloroglucin-formaldehyde resins in the presence of ZnCl2, followed by KOH activation. Beneﬁting from the surface area, hierarchical porous structure, P dopants, the as-prepared electrode exhibits very good electrochemical performance as supercapacitor electrode. PPC-800-2 shows the speciﬁc capacitance of 406.2 F g−1 at 5 mV s−1 and the capacitance retention of 267.4 F g−1 at 500 mV s−1. Importantly, it shows excellent cycling stability with no obvious Acknowledgements This work was supported by the University Research Program of the Xinjiang Uygur Autonomous Region (XJEDU2016I014), the Key Laboratory Open Research Foundation of the Xinjiang Autonomous 64 Applied Surface Science 475 (2019) 56–66 J. Guo et al. Region (No. 2018D04007), National Natural Science Foundation of China (No. 21561029 and 21763023). Thanks to the Center of Testing and Analysis, Xinjiang University for the XPS characterization. 121–133. [28] E.J. Zanto, A.M. And, J.A. Ritter, Sol−gel-derived carbon aerogels and xerogels: design of experiments approach to materials synthesis, Indust. Eng. Chem. Res. 41 (2002) 3151–3162. [29] C. Ma, X. Chen, D. Long, J. Wang, W. Qiao, L. 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