Carboxymethyl Carrageenan Based Biopolymer Electrolytes

• The paper highlights the potential of carboxymethyl carrageenan based on iota and kappa to be utilized as host polymer.
• The highest conductivity were achieved up to ∼10−3 S cm−1 by carboxymethyl carrageenan without the addition of plasticizer.
• The electrochemical stability windows of the films were electrochemically stable up to 3.0 V.

A series of biodegradable carboxymethyl carrageenan based polymer electrolytes, which are carboxymethyl kappa carrageenan (sulphate per disaccharide) and carboxymethyl iota carrageenan (two sulphates per disaccharide), have been prepared by a solution casting technique with different ratios of lithium nitrate (LiNO3) salts. Interestingly, the lithium ions tended to interact with the carbonyl group in the different modes of symmetry, as observed from reflection Fourier transform infrared (ATR-FTIR) spectroscopy analysis. In the carboxymethyl kappa carrageenan electrolytes, as the concentration of LiNO3 increased, the asymmetric stretching peak of the carbonyl bond became dominant because it can be observed clearly with the shifting of the peak from 1592 to 1602 cm−1 due to the interaction between the lithium ion and the carbonyl group, while the broad O-H stretching peak became sharp and intense. However, for the carboxymethyl iota carrageenan, the asymmetry stretching mode of the carbonyl group shifted from 1567 to 1599 cm−1, as the salt concentration increased. The shifting of the C-O-C peak also occurred in the iota-based electrolytes. However, the changes in the peak that represented SO42− symmetric stretching were only detected when the ion pair formation was observed. It was proposed that the peak shifting was due to the presence of the lithium ion pathway, forming a dative bond between the lithium and oxygen in the carbonyl group. Accordingly, as more peak shifting was observed, the number of the ion pathways also increased. This hypothesis was supported by the impedance data, which demonstrated that the ionic conductivity of the 20 wt.% LiNO3iota-based electrolytes was highly similar with the 30 wt.% LiNO3kappa-based electrolytes, with values of 5.85 × 10−3 S cm−1 and 5.51 × 10−3 S cm−1, respectively. The lithium transference number for the carboxymethyl kappa carrageenan was higher than for the carboxymethyl iota carrageenan. Additionally, the carboxymethyl iota carrageenan and carboxymethyl kappa carrageenan films were electrochemically stable up to ∼3.0 and 3.1 V, respectively, which indicates that these solid biopolymer electrolytes are promising candidates for utilization in electrochemical devices.