To obtain the structural information of the cotton-derived carbon samples, XRD, Raman, BET and FT-IR techniques were carried out, respectively. The XRD patterns in
Fig. 2a have two broad peaks located at about 24° and 44°, which can be assigned to the planes of (002) and (100) in graphitic carbon (JCPDS No. 26-1079). It should be noted that the (002) peaks of OSC, H—O—C, and H—OS—C samples show slight shift to low angles, compared with that of C sample. This means that the S-doping and hydrolyzation strategies could broaden the d-spacing of (002) plane [
41,
42], which are beneficial to the storage of potassium ions. The low peak intensity and broad peak width suggest the amorphous structure, which is corresponding to the TEM image shown in
Figs. 1e and
f. As can be seen from the Raman spectra shown in
Fig. 2b, there exist two peaks, namely, D peak (about 1330 cm
−1) and G peak (about 1590 cm
−1). The D peak is caused by the disordered aromatic structure of saturated carbons, meaning the defect-sites, while the G peak is due to the str
etching vibration mode E
2g for sp
2 carbons [
43]. All the four Raman spectra have the similar sharp, a higher D peak and a lower G peak, suggesting the low crystallinity and rich defect-sites. The intensity ratios (
ID:
IG) of OC, OSC, H—O—C, and H—OS—C samples are 1.08, 1.09, 1.13 and 1.11, respectively. It means the H—O—C and H—OS—C have more defect-sites than OS—C and O—C samples, because the hydrolyzation will bring more oxygen groups (−OH, −COOH). Defect sites can play the role of channel for the diffusion of potassium ions, and more defect sites could bring a better rate performance for potassium storage.
Fig. 2c shows the nitrogen adsorption/desorption isotherms of the four samples. Obviously, the H—OS—C has the largest specific surface area of 612 m
2/g. The profiles of OSC, H—O—C, and H—OS—C three samples present the same type, the type-Ⅳ adsorption/desorption hysteresis, which suggests the mesoporous structure. On the contrary, the C sample has the smallest specific surface area of 427 m
2/g and the different type of adsorption/desorption hysteresis. The pore sizes of the above four samples are centered at about 1.88 nm, indicating the mesoporous structure, as shown in Fig. S3 (Supporting information). As shown in
Fig. 2d, the FT-IR spectra prove the existences of CC, C–O, and −OH bonds. In detail, all the four samples have the broad peak at 3480 cm
−1 and the small peak at 1640 cm
−1, which can be attributed to the str
etching vibrations of −OH group and the str
etching vibrations of CC bond in aromatic rings. The existence of −OH group means O-defect sites and the CC bonds suggests the successful carbonization. Only the two hydrolyzed cotton-derived samples have the sharp peaks at about 1105 cm
−1.