Figure 1 shows the TEM images of Mg(OH)2 templates precursor and LPCs. The TEM image of Mg(OH)2 precursor exhibits a sheet-like structure with the size ranging from 500 to 1000 nm as shown in Fig. 1(a). Figure 1(b-d) shows the TEM of LPC2, LPC2.5 and LPC3, respectively. It can be seen clearly that the as-prepared LPCs exhibits a large lamellar structure with a small portion of wrinkles. This structure can help to increase the specific surface area and electrical conductivity of LPCs. The pores in the lamellar structure of LPCs are short pores, which is important for improving the rate performance of the supercapacitor (He et al. 2016).
The nitrogen adsorption was used to characterize the pore structure. As shown in Fig. 2(a), all the samples show I type and IV type curves with a steep uptakes (P/P0 < 0.01) and has a small hysteresis loop (0.4 < P/P0 < 1.0), indicating the presence of micropores and mesopores in the LPCs(Tang et al. 2017). Figure 2(b) shows the pore size distribution curve of the LPCs, which contains a large number of micropores with diameter of 1–2 nm and a small amount of mesopores with diameter of 2–5 nm. The detailed pore structure parameters of LPCs are listed in Table 1. The average pore size of LPCs is between 2.22 and 2.45 nm. And LPC2.5 exhibits the largest surface area, which is due to the appropriate amount of KOH. The specific surface area of LPC2.5 reaches 1879 m2 g− 1, which is higher than that of LPC2 and LPC3. The specific surface area of the LPCs increases at first and then decreases with the increase of the alkali dosage. When KOH is excessive, it will cause large etching of the carbon material and collapse of the cell walls, which will result in less specific surface area of the material. These results indicate that the pore structure of LPCs can be adjusted by changing the mass ratio of carbon source to active agent appropriately.
Figure 2(c) shows the XPS survey spectrum of three LPCs. It can be seen clearly that the two peaks are Cls and Ols, respectively. Figure 2(d) shows the Ols orbital analysis of the sample of LPC2.5, which mainly contain C = O (531.9 eV) and C-O (533.4 eV) functional groups. The oxygen-containing functional groups are expected to increase the wettability of the materials (Wang et al. 2017).
The electrochemical performance of LPCs as electrode materials for supercapacitor were tested in 6 M KOH electrolyte. Figure 3(a) shows the galvanostatic charge-discharge curves of the LPC electrodes at 1 A g− 1 current density. All the GCD curves show equicrural triangle shape, which shows that LPC as the electrode material has the ideal capacitance behavior and no obvious pseudocapacitance behavior. In addition, the IR drop of LPC2.5 is only 0.0032 V, indicating that the internal impedance of LPC2.5 is small (Zhang et al. 2017). Figure 3(b) shows the variation of the specific capacitance of LPC electrodes vs. the discharge current density. It can be found that the specific capacitance decreases slowly with the increase of current density from 0.05 A g− 1 to 100 A g− 1. Obviously, the LPC2.5 electrode shows the biggest specific capacitance among the LPC electrodes, which is reached 231 F g− 1 at 0.05 A g− 1. When the discharge current density is increased to 100 A g− 1, the capacitance of the LPC2.5 remains 173 F g− 1, indicating an outstanding rate capability. The excellent performance of LPC2.5 is ascribed to its high surface area, well-balanced micropores and mesopores for ion fast transport and storage, and as well as thin layered structures in LPC materials help to achieve good electron conduction. Figure 3(c) shows the Ragone plots of LPC electrodes in 6 M KOH aqueous electrolyte. The energy density of LPC2.5 capacitor reaches 8.03 Wh kg− 1 at 0.05 A g− 1 and 5.35 Wh kg− 1 at 100 A g− 1, with the energy density retention of 66.63%. The high energy retention of LPC2.5 capacitor is attributed to the highest electrical conductivity and the lamellar structure (Choudhary et al. 2017). In addition, LPC2.5-based device also exhibit excellent cycling stability as demonstrated in Fig. 3(d). The capacitance retention of LPC2.5 is up to 95.93% after 10,000 cycles.