Figure 1. SEM images of (a) nano CaCO3, (b) cellulose, (c) cellulose/CaCO3-0.5, (d) cellulose/CaCO3-1 membranes and (e, f) corresponding high magnification images.
Hydrolysis treatment is beneficial to enhance the electrolyte wettability of porous membranes, leading to good electrochemical performance. The reason is that acetyl groups in the CA molecular chain are hydrolyzed under strongly alkaline conditions, and the hydrophobic acetyl groups disappear to form hydrophilic hydroxyl groups.[34] The FTIR was used to verify the complete hydrolysis of CA and the successful introduction of calcium carbonate. The spectra of CA and cellulose/CaCO3 membrane are presented in Figure 2a . It is obvious that the CA membrane exhibits a weak -OH peak at 3450 cm-1 and -CH3 stretching vibrations peaks at 2962 cm-1 and 2872 cm-1.[35] Besides, the C=O and C-O characteristic peaks can be seen at 1750 cm-1 and 1180 cm-1, respectively. Compared to the CA membrane, all cellulose membranes show a strong and broad -OH peak at 3361 cm-1 and the C=O absorption peak at 1743 cm-1 disappeared, demonstrating the complete hydrolysis of CA. What’s more, with the addition of CaCO3, the bending vibration peak of carbonate appears at around 874 cm-1 and the intensity of this peak increases as the content of CaCO3 elevates.[36] From all above, the CaCO3 was introduced to the CA membrane successfully.
Figure 2b shows the XRD patterns of porous membranes. The peaks at 20.4° and 21.8° correspond to the diffraction peaks of cellulose. The crystalline peak of CaCO3 locates at 29.5°. With the increasing content of CaCO3, the crystalline peak at 29.5° became sharper, and the corresponding CaCO3 peaks at 35.9° as well as 39.5° appeared.[37] However, the introduction of CaCO3 had little effect on the crystallinity of cellulose.