III. RESULTS AND DISCUSSION

As well known, KOH serves as the activation agent to trigger the carbon structure to form the pore; however, the activation mechanism has not been absolutely clarified due to the activation process referring to many reactions. In a generally accepted view, the probable reactions of carbon and KOH are concluded by several equations as follows^{Reference Wang and Kaskel25}:

(1)$$2{\rm{KOH}} \to {{\rm{K}}_2}{\rm{O}} + {{\rm{H}}_2}{\rm{O}}\quad {\rm{,}}$$

(2)$$6{\rm{KOH}} + 2{\rm{C}} \to 2{\rm{K}} + 3{{\rm{H}}_2} + 2{{\rm{K}}_2}{\rm{C}}{{\rm{O}}_3}\quad ,$$

(3)$${\rm{C}} + {{\rm{H}}_2}{\rm{O}} \to {\rm{CO}} + {{\rm{H}}_2}\quad ,$$

(4)$${\rm{CO}} + {{\rm{H}}_2}{\rm{O}} \to {\rm{C}}{{\rm{O}}_2} + {{\rm{H}}_2}\quad ,$$

(5)$${\rm{C}}{{\rm{O}}_2} + {{\rm{K}}_2}{\rm{O}} \to {{\rm{K}}_2}{\rm{C}}{{\rm{O}}_3}\quad ,$$

(6)$${{\rm{K}}_2}{\rm{C}}{{\rm{O}}_3} \to {{\rm{K}}_2}{\rm{O}} + {\rm{C}}{{\rm{O}}_2}\quad ,$$

(7)$${\rm{C}}{{\rm{O}}_2} + {\rm{C}} \to 2{\rm{CO}}\quad {\rm{,}}$$

(8)$${{\rm{K}}_2}{\rm{C}}{{\rm{O}}_3} + 2{\rm{C}} \to 2{\rm{K}} + 3{\rm{CO}}\quad {\rm{,}}$$

(9)$${\rm{C}} + {{\rm{K}}_2}{\rm{O}} \to 2{\rm{K}} + {\rm{CO}}\quad {\rm{.}}$$

As a matter of fact, the KOH activation mechanism is far more complex than these equations embodied. Linares-Solano and co-workers^{Reference Lozano-Castelló, Calo, Cazorla-Amorós and Linares-Solano26} studied the KOH activation process via monitoring the change of CO, CO₂, H₂O, and H₂ by temperature-programmed reaction (TPR) experiments and then combined the theoretical calculation to particularly explain the reactions at different temperatures and stages. As reflected in Eqs. (2) and (5), potassium carbonate (K₂CO₃) is the important intermediate formed at about 400 °C for the activation of carbon materials. Along with the temperature up to 600 °C, KOH will be completely vanished, as shown in Eqs. (1) and (2). The decomposition reaction of the as-formed K₂CO₃ will happen once the temperature is increased to 700 °C in Eqs. (6) and (8). When the temperature increases to 800 °C, K₂CO₃ will be exhausted. At the same time, when the temperature is increased to 700 °C, the generated CO₂ in Eqs. (4) and (6) and the intermediate potassium oxide (K₂O) in Eqs. (1) and (6) can further react with carbon and produce CO and metallic K, conveyed by Eqs. (7) and (9).

According to the reaction in different activation stages, three activation mechanisms of building the pore network of carbon matrix by KOH activation are concluded as follows^{Reference Gong, Li, Luo, Fu and Pan21,Reference Wang and Kaskel25–Reference Otowa, Tanibata and Itoh27} : (i) KOH and multiple potassium compounds (K₂O, K₂CO₃) with good chemical activity work as the chemical activating agent to etch the carbon skeleton, as revealed by Eqs. (2), (8), and (9), which is defined as the chemical activation; (ii) the gas of H₂O and CO₂ produced in Eqs. (1), (4), and (6) can further react with carbon to create more pores, as exhibited in Eqs. (2) and (8), which is named as physical activation [Eqs. (2) and (7)]; (iii) the carbon can be further activated by the insertion of metallic K and K compounds [Eqs. (5), (8), and (9)] within carbon layers, resulting in enlarged carbon layers. After acid treatment, the intercalated metallic K and K compounds will be eliminated, and the inflated carbon lattices with a porous structure of the activated carbon will remain, demonstrating the formation of porous carbon from KOH activation.

The morphological features of PTPC samples were unfolded by SEM and TEM. As shown in the SEM images of PTPC-5h [Figs. S1(a) and S1(b), in Supplementary Material], the sample displays some macropores due to KOH activation. However, these SEM images in Figs. S1(a), S1(c), and S1(d) exhibited the as-obtained asymmetrical porous carbon, and some distinct porous carbon established well can be detected (Fig. S2), but a number of carbons without activation as shown in Fig. S1(d) also can be found. It indicates that the activation process of PTPC-5h is asymmetric resulting from the direct heat drying, which causes KOH to be separated out and deposited to a certain extent, leading to KOH uneven distribution around the carbon matrix. The porous carbon prepared by freeze-drying is shown in Figs. 2(a)–2(d). In view of these SEM and TEM images in Figs. 2(a)–2(e), some macropores are clearly observed. According to the SEM images in Figs. 2(a)–2(c), the interconnected framework of PTPC-4, PTPC-5, and PTPC-7 can be certified. As shown in Fig. 2(d), the surface of PTPC is rough, with plenty of small pores. Some similar graphene lamella structure is also found at the edge of fracture in the TEM images [Fig. 2(e)]. The selected electron area diffraction (SAED) pattern [inset in Fig. 2(e)] reveals legible diffraction due to the defect of carbon structure. The porous carbon materials with a homogeneous distribution of plenty micropores can be testified by the HRTEM image in Fig. 2(f). Moreover, some mesopore channel walls are formed owing to the high content of KOH breaking the carbon bones between the micro/mesopores.^{Reference Wang, Yu, Shi, Mao, Chen and Liu13} The N₂ adsorption/desorption measurement was used to comprehensively analyze the pore structure. As shown in the isotherms of PTPC [Fig. 3(a)], the amount of adsorbate rapidly increases at low p/p ₀ (<0.05) and then approaches a limitation, and the isotherm is parallel to the p/p ₀ axis. Therefore, one can deduce that the adsorption type of PTPC belongs to the typical type-I [Fig. 3(b)] of Langmuir adsorption.^{Reference Thommes, Kaneko, Neimark, Olivier, Rodriguez-Reinoso, Rouquerol and Sing28} It indicates that the PTPC samples contain a great quantity of micropores and some narrow mesopores, coinciding with the result of HRTEM. It should be noticed that the adsorption amount of PTPC-7 distinctly increases at high p/p ₀ (>0.9) owing to the condensation of adsorbate at saturated pressure. The hysteresis loops of PTPC-4 were not obvious in Fig. 3(a) but more expansive along with the mass fraction of KOH increased, which showed an emblematical type-H4 owing to capillary condensation in the layer stacked micropores and slits.^{Reference Thommes, Kaneko, Neimark, Olivier, Rodriguez-Reinoso, Rouquerol and Sing28} The PSD of all PTPC is obtained in Fig. 3(b) by the NLDFT based on the adsorption data. From the PSD [Fig. 3(b)] of PTPC-4, PTPC-5, and PTPC-7, all exhibit similar hierarchical porosity, with the content of pores up to the maximum at 0.5 nm, and all the pore diameters are below 3 nm. Meanwhile, the PSD are more intuitionistic reflecting that all PTPC samples show abundant micropores and bits of narrow mesopores on account of the chemical activation of KOH.^{Reference Kierzek, Frackowiak, Lota, Gryglewicz and Machnikowski29} Furthermore, according to the BET results, the specific surface areas of PTPC-4, PTPC-5 and PTPC-7 are calculated as 2275.8, 1914.4, and 1775.3 m²/g, respectively. Moreover, the total pore volume of PTPC-4, PTPC-5, and PTPC-7 calculated is 1.07, 0.90, and 0.89 cm³/g, respectively. It was accounted by pertinent literature that the specific surface area of porous carbon by KOH activation can be augmented as the concentrate of KOH at low content, and then it reached the maximum. Further increase of the KOH concentrate leads to the decrease of specific surface area.^{Reference Ye, Yin, Xin and Guo30} Superabundant concentration of KOH can cause more carbon loss with the ruptured carbon bone and reduce the specific surface area. The average pore diameters of PTPC-4, PTPC-5, and PTPC-7 are 1.88, 1.87, and 2.02 nm, respectively. The mesopore volume of PTPC-4, PTPC-5, and PTPC-7 computed by the BJH method is 0.10, 0.15, and 0.19 cm³/g, respectively. The ratios of the micropore volume to the total volume are 0.90, 0.83, and 0.78 cm³/g for PTPC-4, PTPC-5, and PTPC-7, respectively. Based on the above discussion, one can see that the concentrate of KOH reveals a significant impact on the pore size, and the pore becomes larger with the increase of KOH concentration, resulting in reducing the content of micropores. The micropores are formed in the initial process of KOH activation, and then, the adjacent micropores with the carbon skeleton will be transformed into the mesopores and/or mesopore walls with the increase of the KOH content. With the increased content of KOH, the mesopores and/or mesopore walls increased, leading to the capillary condensation phenomenon by N₂ took off more remarkable, and as a result, the hysteresis loops of the isotherms of porous carbon samples expanded along with the increased mass fraction of KOH.^{Reference Thommes, Kaneko, Neimark, Olivier, Rodriguez-Reinoso, Rouquerol and Sing28} All porous properties of PTPC-4, PTPC-5, and PTPC-7 samples are listed in Table I. Clearly, the PTPC samples reveal hierarchical porous, mesopore walls, interconnected structures, and larger specific surface areas.

FIG. 2.

SEM images of the as-obtained (a) PTPC-4, (b) PTPC-5, and (c, d) PTPC-7; (e) TEM and (f) HRTEM images of PTPC-5 (the inset for SAED).

FIG. 3.

(a) N₂ adsorption–desorption isotherm and (b) porosity distribution by the density functional theory model of the materials of PTPC; (c) XRD patterns of the PTPC-2; (d) Raman spectra of PTPC-4, PTPC-5, and PTPC-7; fitting result of the Raman spectra of (e) PTPC-4, (f) PTPC-5, and (g) PTPC-7.

TABLE I.

The specific surface area, pore size, and pore volume of PTPC activated with various KOH concentrations.

^a The specific area (S _BET) was calculated by the BET method.

^b V _T represented the total pore volume calculated by NLDFT analysis.

^c The volume of micropore (V _micro) was obtained by subtracting the mesoporous volume from the total pore volume.

^d The volume of mesopore (V _mes) calculated by the BJH method.

Figure 3(c) depicts the XRD pattern of the PTPC-5. It displays two significant peaks at 2θ = 22.3° and 43.3°, corresponding to the (0 0 2) and (1 0 0) diffractions of graphite, respectively. The porous carbon consists of a plenty of defect structures by KOH activation. The interlayer distance (d-spacing) of the graphite layer can be computed by the Bragg equation with the (0 0 2) diffraction peak. The d-spacing of PTPC is 0.398 nm, which is larger than that of graphite with 0.335 nm. As the widely accepted KOH activation mechanism,^{Reference Wang and Kaskel25} additional metallic K and K compounds produced by the activation process could insert into carbon layers and function in the PTPC materials, and thus d-spacing may be expanded. The large distance can function as cushioning for volume expansion during insertion/extraction of sodium ions.^{Reference Wang, Yu, Shi, Mao, Chen and Liu13} As shown in Fig. 3(d), Raman spectra of all samples have two strong peaks. The one is the D-bond of Raman characteristic absorption band at 1350 cm⁻¹, attributing to the disorder or defects of the carbon structure. The other is the G-bond at 1600 cm⁻¹ due to the vibration of the sp ² hybridized carbon framework.^{Reference Qin and Chen3} The fitting result of the Raman spectra of PTPC-4, PTPC-5, and PTPC-7 is displayed in Figs. 3(d)–3(f), respectively. The ratio of the D band to the G band (I _D/I _G) is a significant parameter to judge the crystal degree of the carbon materials that I _D/I _G is susceptible to the change of microstructure, such as lattice defects, amorphous degrees, and heteroatom doping.^{Reference Ou, Yang, Zhang and Xi31} The I _D/I _G of PTPC-4, PTPC-5, and PTPC-7 are 2.42, 2.39, and 2.41, respectively, agreeing with the result of XRD and SAED. The as-achieved pores in carbon materials are largely at the expense of losing the carbon in the carbon matrix with the reaction of C and activator (e.g., KOH, K₂CO₃, K₂O, and CO) proceeding, leading to the larger disorder degree of PTPC.

XPS was performed to ascertain their bonding configurations and the elements of carbon materials. As shown in the survey spectrum [Fig. 4(a)], there are two peaks of all samples. One is located at 284.3 eV corresponding to C 1s. The other is centered at 533 eV corresponding to O 1s.^{Reference Cai, Luo, Dong, Zhao, Xiao, Liang, Hu, Liu and Zheng20,Reference Gong, Li, Luo, Fu and Pan21} The O contents of PTPC-4, PTPC-5, and PTPC-7 are 5.15%, 9.00%, and 5.64%, respectively, as depicted in Fig. 4(b). It can be easily found that the highest O content of all samples is from PTPC-5, indicating that the content of O is not directly proportional to the concentration of KOH. The source of the O atom is introduced through the KOH activation process, but the O atom will be enormously lost at high temperatures. As a consequence, adding the concentrate of KOH properly introduces the O atom, and luxuriant KOH leads to the larger extent of activation proportional to the O atom losses because the activation reaction occurs in carbon rings.^{Reference Wang and Kaskel25} The high-resolution C 1s and O 1s spectra of PTPC-4, PTPC-5, and PTPC-7 are graphically depicted in Figs. 4(b)–4(j). The C 1s XPS spectra of all samples have been split into two peaks: sp ³ C–C and sp ² C=C bond graphitic carbon at 284.3 eV and C–O bonds at 286.5 eV [Figs. 4(c), 4(e), and 4(h)].^{Reference Xiong, Li, Bai, Shan, Fan, Wu, Li and Lu32} From the results of fitting the O 1s spectra of all samples in Figs. 4(d), 4(f), and 4(j), only one peak located at 533 eV can be seen, corresponding to C–O.^{Reference Shi, Zhang, Fu and Gan33} The carbon structure only exists in C–O bonds, which is easy to fracture than the C=O bond at high temperature annealing, resulting in the low content of O atom. These oxygen-containing functional groups will introduce more active sites to the electrochemical reaction and enhance the polarity of PTPC to boost the transmit and storage of electrolytes, contributing to advance the electrochemical property of SIBs.^{Reference Deheryan, Cott, Mertens, Heyns and Vereecken34}

FIG. 4.

(a) XPS survey spectra of PTPC-4, PTPC-5, and PTPC-7; (b) atomic percentages of O and C of PTPC-4, PTPC-5, and PTPC-7; high-resolution C 1s spectra of (c) PTPC-4, (e) PTPC-5, and (g) PTPC-7; and high-resolution O 1s spectra of (d) PTPC-4, (f) PTPC-5, and (h) PTPC-7.

CR-2032 typed coin cells of all PTPC samples were assembled to test their electrochemical performance in the voltage of 0.01–3 V versus Na/Na⁺. In Fig. 5(a), the CV curves of PTPC-5 was acquired at a scanning rate of 0.1 mV/s to analyze the electrochemical reaction during charge/discharge cycles. In the first cycle, a shape peak at 1.1 V can be observed, owing to the irreversible reaction between the electrolyte and surface active functional groups and then forming the irreversible solid electrolyte interface (SEI).^{Reference Ou, Yang and Xi35} It is obvious that the reduction peak at 1.1 V only appears in the first cycle, implying the PTPC electrode with a low initial Coulombic efficiency. Moreover, a pair of redox peaks at a lower potential (about 0.1 V versus Na/Na⁺) can be discerned, in accordance with the insertion/extraction of Na⁺ into/from the porous carbon materials, analogous to the behavior of Li⁺ for carbon-based materials. As shown in Fig. 5(a), the reduction peak at 0.1 V is weak in the 2nd cycle related to that in the first cycle. The grievous irreversible insertion of Na⁺ results in high capacity fading in the first charge/discharge cycle. After the first cycle, the CV curves approximate to the rectangular shape in a wide voltage range, which also appears in porous carbon materials as the anode for LIBs. It can be seen that the PTPC electrode showed the approximate rectangular shape in a wide voltage range, revealing the existence of abundant pores associated with the high specific surface area. It promotes the electron transport and sodium ion diffusion in SIBs. This capacitance effect of porous carbon materials is similar to its CV curves for the supercapacitor.^{Reference Simon and Gogotsi36} Apart from the first cycle, the CV curves of PTPC-5 almost overlap, illustrating the stable electrochemical reaction of the PTPC electrode. The 1st, 2nd, 4th, 20th, and 100th charge/discharge profiles of the KOH-activated carbon electrode at a constant current density of 50 mA/g is reflected in Fig. 5(b). PTPC-5 reveals a high initial discharge specific capacity of 696 mA/g originating from high surface area, large carbon layer spacing, and oxygen-containing functional group. Sodium ion can be inserted into the interspaces between parallel carbon interlamination as well as the micropores. However, it should not be ignored that the initial charge specific capacity of the PTPC electrode decreases to 169 mA h/g, with the initial Coulombic efficiency of only 24%. It is a normal weakness that high-surface-area carbon materials deliver a low initial Coulombic efficiency, whether used for sodium storage, lithium storage, or other energy storage appliance. The reason can be due to the irreversible side reaction, irreversible Na⁺ insertion into the carbon network, and the SEI formed on the surface of the carbon electrode in the first cycle.^{Reference Wang, Yu, Shi, Mao, Chen and Liu13} In the next cycle, the increase of the Coulombic efficiency is extremely significant. In the 5th cycle, the charge and discharge capacity is 184 and 197 mA/g, respectively, delivering the Coulombic efficiency of 93.4%. The reversible specific capacity can be maintained at 179 mA h/g after 100 cycles, with the Coulombic efficiency of 96%, testifying that the PTPC electrode offers good reaction reversibility and structural stability for sodium storage. It is in good coincidence with the conclusions of CV curves mirrored. Figure 5(c) compares the cycle performance of PTPC-4, PTPC-5, and PTPC-7 at the current density of 50 mA/g for 100 cycles. Figure 5(c) reveals that the initial discharge capacities of PTPC-4 (618 mA h/g), PTPC-5 (696 mA h/g), and PTPC-7 (520 mA h/g) suffer high irreversible behavior with the charge capacity of only 147, 169, and 142 mA h/g, respectively. In the first 5 cycles, the capacity fade of all samples by the different concentration of KOH activation is obvious, particularly, the specific capacities of PTPC-4, PTPC-5, and PTPC-7 are only 185.9, 152.5, and 140.6 mA/g after 5 cycles, respectively. It indicates that all PTPC materials reveal larger capacity fading in the initial cycles, due to the larger irreversible reaction and SEI formation, which is consistent with the CV results with keen-edged irreversible reduce peaks in the first cycle. Fortunately, the capacity fading is puny after the 5th cycle, proving that the PTPC materials possess good stability. After 100 cycles, the reversible specific capacities of PTPC-4, PTPC-5, and PTPC-7 can be maintained at 149, 179, and 132 mA h/g, respectively, which exhibit high capacity retention. To demonstrate the remarkable structure stability of the PTPC, the rate capability of cells was tested at diverse current densities from 0.05 to 1 A/g. The rate capability of PTPC-4, PTPC-5, and PTPC-7 is exhibited in Fig. 5(d). The charge/discharge capacities of PTPC-5 under varied current densities, successively at 0.05, 0.1, 0.2, 0.5, and 1 A/g, deliver 197, 174, 148, 118, and 100 mA h/g, respectively. At the higher current density of 1 A/g, PTPC-4, PTPC-5, and PTPC-7 can be maintained at 70, 100, and 57 mA h/g. Meanwhile, when current density recovered to 0.05 A/g after cycling at high current densities, the reversible specific capacity of PTPC-4, PTPC-5, and PTPC-7 can be stabilized at 154, 181, and 137 mA h/g, compliant with the result of long cycles at 0.05 A/g. As a result, the PTPC-5 samples exhibit the bestrate capability, compared with the commercial activated carbon of high-surface-area (2050 m²/g) with only a capacity of 40 mA h/g at a high current density of 1 A/g.^{Reference Wang, Yu, Shi, Mao, Chen and Liu13} The storage Na performance improvement can be attributed to high specific surface areas, pore shapes, and PSDs, large interlayer distances, and interconnected structures.

FIG. 5.

(a) CV of PTPC-5 electrodes in various cycles at a scan rate of 0.1 mV/s, (b) galvanostatic charge/discharge profiles, and (c) cycle performance of PTPC-4, PTPC-5, and PTPC-7 electrodes at a current density of 0.05 A/g; (d) rate capability of PTPC-4, PTPC-5, and PTPC-7 electrodes at various current densities in the current density range of 0.05–1.00 A/g in a voltage range of 0.01–3.0 V (versus Na/Na⁺). (e) Nyquist plots and fitting data of PTPC-5 electrodes with an amplitude of 5.0 mV over the frequency ranging from 100 KHz to 0.01 Hz; (f) equivalent circuit used to fit EIS curves.

To understand improved cycling performance, the electrochemical impedance spectra (EIS) of the PTPC-5 sample was measured. As Fig. 5(e) reflected, the impedance spectra during various cycles consist of two portions, in which one is a semicircle in the middle–high frequency range and the other is a sloping straight line in the low frequency. The equivalent circuit in Fig. 5(f) was utilized to fit the Nyquist plots of PTPC-5. Normally, R _s, representing the inherent resistance of the electrolyte and the active substances, and the contact resistance at the interface between the active electrode and the current collector, is described by the intercept with the X-axis in high frequency. R _ct represents the charge transfer resistance, reflected by the semicircle in the middle frequency range. CPE represents the capacitance of the double layer and passivation film. The Warburg impedance (Z _w), reflected by the sloping line in high to medium frequency, is dependent on the Na⁺ diffusion in the electrodes.^{Reference Liu, Jia, Sun, Cao, Chen, Zhu, Wu, Qiao and Xu37} The values of the R _s, R _ct, and Z _w during various cycles are listed in Table S1. Before cycling, R _ct is 585.9 Ω. The value of R _ct decreases with the charge–discharge cycling from 189.1 Ω in the 1st cycle to 166.5 Ω in the 15th cycle, and the value of R _ct increases to 171.5 Ω after 50 cycle. For high surface area porous carbons, the increase of R _ct with cycling implies that sectional porous structure is disintegrated.^{Reference Wang, Yu, Shi, Mao, Chen and Liu13}

As shown in Figs. 5(c) and 5(b), there is no inevitable connection between the specific capacity and specific surface area, conforming to the relevant report.^{Reference Ye, Yin, Xin and Guo30} The electrochemical property of porous carbon materials is bound up with several significant parameters: (i) the heteroatoms (such as O, P, S, B, and N), exist in carbon materials, can enhance the polarity of porous carbon electrode, thus contributing to adsorption of atoms and ion. Moreover, heteroatom doping contributes to the pseudocapacitance to improve the capacity of batteries. The heteroatom doping with destructing faultless carbon network is detrimental to the conductivity. (ii) Extent graphitization, determining the carbon materials conductivity, can be advanced by high temperature carbonization with “self-healing” of the graphite lattice. However, the carbon materials containing heteroatoms will be decomposed more easily, thereby lessening the content of heteroatoms at high temperature treatment to impair carrier concentrate. (iii) A larger surface area can support more active sites and storage space; however, it also can increase the side reactions and form more SEI on the activation material surface, thus leading to a high irreversibility, with a low Coulombic efficiency. (iv) Pore shape usually includes cylindrical, slits, conical, spherical, ink bottle, interstices, etc. (v) Pore size distribution: The micropores (<2 nm) provide large surface areas and storage space for the active substance to enhance capacity, and the mesopores (2–50 nm) serves as the aisle for the various kind of ions and electrons rapidly arriving at the active surface to accelerate reaction as well as the bridge between micropores and macropores. The macropores (>50 nm) resemble the ion-buffer reservoir to ensure that the electrode materials sufficiently penetrated into the electrolyte.^{Reference Qiao, Chen, Wang and Yuan38} Hence, the electrochemical performance of porous carbon materials may not be measured by only or a few independent factors. For all PTPC materials in this study, taking into consideration various factors, such as the transfer and storage of Na⁺, the transfer and permeation of the electrolyte, and all reaction (reversible and irreversible reaction) charge–discharge cycling, the PTPC-5 synthesized via the mass rate of 1:5 KOH activation exhibits the best performance as the anode for SIBs.