Hongqiang
Wang
,
Xin
Li
,
Jiming
Peng
,
Yezheng
Cai
*,
Juantao
Jiang
* and
Qingyu
Li
Guangxi Key Laboratory of Low Carbon Energy Materials, School of Chemistry and Pharmaceutical Sciences, Guangxi Normal University, Guilin, 541004, China. E-mail: yezhengcai@163.com; jtjiang@.gxnu.edu.cn
First published on 2nd July 2021
Rational interface control of porous carbon electrode materials is of significance for achieving efficient supercapacitors. Herein, biomass-derived carbon microspheres with a highly graphitized porous surface and amorphous subsurface were well constructed via a flexible coupled catalysis-activation process. The unique structure not only endows the carbon microspheres with rapid electron transfer but also an ultra-high specific surface area. Owing to the optimized graphitized/amorphous structure, the obtained graphitized and activated starch-derived carbon microspheres display obviously impressive energy storage capability among the reported starch-derived carbon materials, even though they were evaluated in a narrow voltage window. The assembled symmetrical supercapacitor based on the optimized carbon microspheres exhibits a high capacitance of 198 F g−1 at 1 A g−1, a high energy density of 14.67 W h kg−1 at a power density of 4142.80 W kg−1, robust cycle performance, and good rate performance in alkaline aqueous electrolyte. This work provides a strategy for flexible construction of biomass-derived carbon electrode materials, with an optimized graphitized/amorphous and porous structure, for boosted energy storage in supercapacitor applications.
The adopted electrode material directly determines the energy storage capacity of supercapacitors. Given the unique features of good electronic conductivity, low cost, rich pore structures, and good chemical stability, porous carbon-based materials are a class of promising electrode materials for supercapacitors.5–10 Regrettably, carbon-based supercapacitors generally exhibit low energy density (usually below 10 W h kg−1) mainly due to the internal energy storage mechanism in which energy storage occurs only at the carbon electrode surface.11 To overcome this obstacle, increasing the specific capacitance of porous carbon is highly needed according to the energy density equation.12 Exposing more electrochemical active sites by constructing porous carbon with a large specific surface area (SSA) represents a feasible and universal strategy to boost the electrical storage capacity.13 However, achieving a balance between the SSA and electronic conductivity and/or structural stability remains challenging. On the one hand, porous carbon with a large SSA usually contains a considerable part of amorphous carbon, which is not conducive to fast electron transfer and structure maintenance during frequent charging and discharging. On the other hand, porous carbon having an excessive degree of graphitization generally exhibits decreased density of active sites and therefore decreased specific capacitance. A combination of a high graphitization degree and suitable amorphous structure in porous carbon may achieve the target of enhanced energy storage capability.14
Biomass-derived porous carbon microspheres have attracted great interest for use as electrode materials due to their rich sources and excellent electrochemical performance in supercapacitors.1,13,15,16 Flexible construction of biomass-derived carbon microspheres simultaneously possessing a large enough SSA and high graphitization degree is highly desirable, but it is still challenging.17 Herein, by using renewable biomass potato starch as a carbon source, a coupled catalysis-activation process was proposed to construct carbon microspheres with a highly graphitized porous surface and amorphous subsurface. This unique structure endows the carbon microspheres with rapid ion/electron transfer and an ultra-high specific surface area, which can cooperatively improve the energy storage capacity of supercapacitors with increased specific capacitance, higher energy density, and good rate capacity. This work provides an effective strategy for optimizing the rapid ion/electron transfer microenvironment of carbon microsphere electrode materials for boosting energy storage performance.
GASC-Co and GASC-Ni were synthesized in a similar manner to GASC-Fe by replacing FeCl2·4H2O with cobaltous chloride hexahydrate CoCl2·6H2O and NiCl2·6H2O. GASC5, GASC10, and GASC15 with different FeCl2·4H2O/SC mass ratios of 0.05/1.00, 0.10/1.00, and 0.15/1.00 were synthesized in a similar manner to GASC-Fe. For convenient comparison, GASC-Fe was also denoted as GASC20. PS-derived active carbon (AC) microspheres were also synthesized in a similar manner to GASC without the addition of metal precursors for comparison.
C = (I × Δt)/(ΔV × m) | (1) |
The energy density (E, W h kg−1) and power density (P, W kg−1) of the assembled SS were calculated according to eqn (2) and (3).
E = C × ΔV2/(2 × 3.6) | (2) |
P = 3600E/Δt | (3) |
Fig. 1 (a) Schematic of the synthesis of GASC; (b–g) SEM and HRTEM images of GASC-Fe (b and e), GASC-Co (c and f), and GASC-Ni (d and g). |
To reveal the influence of the Fe salt content on the catalytic graphitization effect of the final GASC, GASC5, GASC10, and GASC15 with different mass ratios of Fe salt/SC were synthesized in a similar manner to GASC-Fe, and PS-derived active carbon (AC) microspheres were also synthesized in a similar manner to GASC-Fe without the addition of the Fe precursor. The SEM images (Fig. 2a–d and S3†) indicate that these four samples all display a carbon microsphere shape similar to SC. Note that excess iron salt will destroy the original microspheres and therefore lead to the formation of irregular shape carbon blocks (see the SEM image of GASC15 in Fig. S3c†), suggesting the significance of the Fe salt ratio in tuning the microscopic features. The partial structural collapse of carbon microspheres in GASC15 is mainly due to excessive catalytic graphitization from excess Fe catalyst. As seen in TEM images (Fig. 2e–h), thin carbon sheets with some metal particles were formed in GASC5, GASC10, and GASC15 but not in AC. The HRTEM image (Fig. 2i) indicates that no carbon lattice fringes can be found in AC. In contrast, clear carbon lattice fringes observed in HRTEM images (Fig. 2j–l) of GASC5, GASC10, and GASC15 confirm the enhanced graphitization of the carbon matrix.
Fig. 2 SEM, TEM, and HRTEM images of AC (a, e and i), GASC5 (b, f and j), GASC10 (c, g and k), and GASC15 (d, h and l). |
To understand the crystal structure characteristics, the synthesized GASC and AC samples were examined by XRD. As displayed in Fig. S4a,† the XRD patterns of AC display two gentle peaks at ∼26° and 44° corresponding to the (002) and (100) planes of the carbon material, indicating amorphous structures; however, the intensity of these two peaks obviously increased when Fe salt was introduced as seen from the XRD patterns of GASC-Fe, GASC-Co, and GASC-Ni, suggesting an enhanced graphitization degree due to the catalytic graphitization process of carbon species during heat treatment. Note that the (002) peak of GASC-Fe is sharper than that of GASC-Co and GASC-Ni, implying that Fe is more effective for catalytic graphitization of PS-derived carbon which is consistent with the result of HRTEM (Fig. 1e–g). The XRD patterns (Fig. 3a) of GASC05, GASC10, GASC15, and GASC20 further indicate that these peaks became sharper with the increase of the Fe salt ratio along with lattice contraction (Table S1†), confirming the robust effect of catalytic graphitization by metallic Fe, which was in situ formed from the Fe salt reduced by PS-derived reducing substances. Besides, no additional diffraction peaks can be seen from the XRD patterns of the GASC samples when compared to AC, demonstrating the absence of metal and/or metal oxides. The enhanced graphitization degrees of the GASC samples were further confirmed from Raman spectra (Fig. S4b† and 3b), as evidenced from the decreased ID/IG value (Table S1†), which is the intensity ratio of the D band to the G band, of the GASC samples compared to AC, which is consistent with the XRD result.20,21 Note that the appropriately enhanced graphitization degree benefits the electronic transfer and therefore promotes the electrochemical performance.
The pore structure features of the GASC and AC (Fig. S4c, d† and 3c) samples were characterized by N2 adsorption–desorption isotherms. AC displayed a type-I isotherm curve indicating a microporous structure. However, all GASC samples exhibited combined characteristics of typical type I and IV isotherms with sharp and gentle adsorption behaviors at low and moderate pressures, indicating the existence of micropores and mesopores.22,23 Note that micropores can promote the exposure of electrochemically active sites and the mesopores can boost the ion transfer. The GASC samples are highly expected to possess enhanced energy storage capability. The specific surface areas of AC and GASC samples calculated by the BET method are sufficiently large for achieving energy storage and the detailed pore features are provided in Table S1.† The tested samples have a vast majority of micropores with some mesopores. Notably, GASC10 (2369 m2 g−1) has the largest BET specific surface area among the tested samples, suggesting that appropriate control of the iron salt ratio benefits the maximum exposure of the specific surface area for promoting the electrochemical performance. A balance between the high graphitization and rich pore structure can be achieved by suitable surface catalytic graphitization.
The surface chemical features of the AC and GASC samples were explored by XPS. Fig. 3d shows the representative XPS survey spectra of AC and GASC10. These two samples only possess C and O elements with C/O atomic ratio of 77.63/22.37 for AC and 82.31/17.69 for GASC10. No other element or Fe species existed on the surface of GASC10, as further confirmed from the EDS spectra in the selected area of AC (Fig. S5†) and GASC10 (Fig. S6†). EDS mappings further reveal the distributions of C and O through the carbon microspheres. The high-resolution C 1s spectra (Fig. 3e–i) of the AC, GASC05, GASC10, GASC15, and GASC20 samples can be fitted into 4 peaks corresponding to C sp2 (284.4 eV), C sp3 (285.6 eV), C–O (286.2 eV) and COO– (288.6 eV), respectively.1,24 The C sp2 peak represents the orderly arranged carbon structure while the C sp3 peak indicates the existence of disorderly arranged carbon. The increase in the C sp2/sp3 area ratio of GASC5, GASC10, GASC15, and GASC20 compared to that of AC indicates the enhanced graphitization degree trend, which is consistent with the above XRD and Raman results. Note that GASC10 exhibits a sharp increase in the C sp2/sp3 area ratio compared to AC and further increasing the Fe salt ratio during the high temperature activation process will not obviously increase the C sp2/sp3 area ratio. Hence, GASC10 has been highly graphitized.
Encouraged by the porous structure with abundant micropores and mesopores, the large specific surface area, and the high graphitization degree, the as-synthesized AC and GASC samples were highly expected to possess desirable energy storage capability. The electrochemical performance of symmetrical supercapacitors (SSCs) using GASC-Fe, GASC-Co, and GASC-Ni samples as electrode materials was first evaluated by CV measurements and compared to that of an AC SSC as shown in Fig. 4a–d. All the CV curves of the AC, GASC-Fe, GASC-Co, and GASC-Ni SSCs exhibit a good quasi-rectangular shape even when the scan rate was increased from 10 mV to 100 mV, indicating the characteristics of good electric double layer capacitance and rapid ion transfer.25 The GCD curves (Fig. 4e–h) displaying symmetrical isosceles triangle features demonstrate good electrochemical reversibility and high Coulomb efficiency. Regrettably, the discharging times of the GASC-Fe, GASC-Co, and GASC-Ni SSCs wereshorter than that of the AC SSC, implying relatively low specific capacitance mainly due to the difference in structural features. Notably, the GASC-Co SSC having a larger specific surface area and lower resistivity than the AC SSC exhibited smaller specific capacitance than AC (Fig. S7†), which can be mainly due to the relatively low graphitization degree (see Table S1†) and the smaller average pore width of GASC-Co (0.752 nm) calculated by the DFT method when compared to GASC-Fe (0.818 nm) and GASC-Ni (0.818 nm). The purpose of catalytic graphitization is to enhance electron transfer and structural stability for benefiting energy storage. However, excessive catalytic graphitization due to the addition of a high metal salt ratio during the high temperature activation process will destroy the mesopores which is unfavorable for ion transfer. This observation also indicates that the optimal pore structure with appropriate amounts of micropores and mesopores is critical in determining the energy storage capability of carbon materials. Therefore, a balance between the specific surface area and graphitization degree should be achieved by controlling the content of interface graphitized/amorphous carbon.
Fig. 4 CV curves at different scan rates and charge–discharge curves at different current densities of AC (a and e), GASC-Fe (b and f), GASC-Co (c and g), and GASC-Ni (d and h). |
To optimize catalytic graphitization and the pore structure, Fe-catalyzed GASC05, GASC10, and GASC15 samples with different Fe salt ratios were synthesized in a similar manner to GASC-Fe. The CV curves of the AC, GASC05, GASC10, GASC15, and GASC20 samples in Fig. 5a indicate that GASC10 has the largest area among the tested samples, suggesting superior capacitance mainly due to the largest BET specific surface area (Table S1†). The CV curves of GASC10 at different scan rates indicate good electrical double-layer capacitive (EDLC) behavior, as evidenced from the maintenance of a rectangular-like shape even when the scan rate was increased from 10 mV to 100 mV (Fig. 5b).26 The GCD curves (Fig. 5c) of these five samples at a current density of 1 A g−1 showing symmetrical isosceles triangles suggest rapid electron transfer and desirable reversibility.27 Remarkably, the GASC10 SSC exhibited the longest discharging time, confirming enhanced energy storage capability compared to the GASC20 SSC (here GASC20 is equal to GASC-Fe). The GCD curves (Fig. S8a–c†) of the AC, GASC05 and GASC10 SSCs at different current densities display a nearly symmetrical triangle shape indicative of EDLC behavior without pseudocapacitance, low internal resistance, and high Coulomb efficiency. However, the GCD curves (Fig. S8d and e†) of the GASC15 and GASC20 SSCs exhibited a slight ohmic drop mainly due to the decrease in specific surface area and mesopore ratio derived from excessive graphitization. The corresponding specific capacitance results calculated from the GCD curves displayed in Fig. 5d indicate that GASC10 exhibited higher specific capacitance than the other samples at different current densities, confirming superior energy storage capability. The enhanced energy storage capability of GASC10 can be attributed to the large specific surface area of GASC10 (2369 m2 g−1) compared to the other samples, such as GASC5 (1954 m2 g−1), GASC15 (2119 m2 g−1), and GASC20 (1744 m2 g−1). Besides, the specific capacitance retention of GASC10 (198 F g−1) reached up to 83.3% when the current density was increased from 1 A g−1 to 10 A g−1, indicative of good electrochemical reversibility.4 The kinetic behavior of the electrochemical reactions was explored by EIS measurements and the corresponding Nyquist plots are presented in Fig. 5e. The GASC samples exhibit a smaller semicircle than AC implying smaller charge transfer resistance. The charge transfer resistance decreases in the order of AC < GASC05 < GASC10 < GASC15 < GASC20, confirming that the catalytic graphitization can effectively improve the electronic conductivity of GASC samples. The resistivity of the sample was also tested on a four-probe powder conductivity tester and the result is provided in Table S1.† GASC10 (0.187 Ω cm) showed a sharp decrease in resistivity compared to AC (1.536 Ω cm), further confirming improved electronic transfer. The long-life stability of the GASC SSCs was also evaluated and compared to that of an AC-based supercapacitor (Fig. 5f). All the assembled SSCs can maintain their initial specific capacitances well after 1000 cycles at a current density of 1 A g−1. Particularly, the GASC10 SSC retains 99.0% of its initial specific capacitance, which was still much higher than that of the AC SSC, exhibiting excellent cycle stability and promising application potential. To more realistically evaluate the energy storage performance of the supercapacitors, the Ragone plot reflecting the relationship between the energy density and power density of the GASC-based supercapacitors is presented in Fig. 6. The as-optimized GASC10 SSC manifests a high energy density of 17.6 W h kg−1 at a power density of 390.18 W kg−1 at 1 A g−1. Remarkably, a high energy density of 14.67 W h kg−1 can be maintained at a high power density of 4142.80 W kg−1 at 10 A g−1, suggesting impressive energy storage capability and rate performance compared with previously reported related materials.13,22,28–32 Therefore, we can conclude that this outstanding energy storage performance of GASC-based supercapacitors is the result of improvement in the carbon electrode material. The highly graphitized carbon microspheres with a large specific surface area and rich pore structure benefit the electron and mass transfer.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1na00262g |
This journal is © The Royal Society of Chemistry 2021 |