Opposite role of humic acid in capacitive desalination system using intercalation electrode: Effect of electrode crystal structure

Abstract

Capacitive deionization (CDI) is accelerating application for purification of saline streams with continuously progressive desalination performances due to the development of advanced intercalation materials. However, issues such as the sustained desalination performances in the presence of naturally occurring organic matters (NOM) are still not thoroughly understood, especially the relationship between intercalation electrode properties and desalination performances in the presence of NOM remains unclear. This study makes an initial attempt to fill up the research gap by elucidating the dependence of desalination performance of intercalation electrodes on their crystal microstructures with the occurrence of humic acid (HA) using tunnel and layered sodium manganese oxides (T-NMO and L-NMO) as representatives. As observed, both T-NMO and L-NMO electrodes suffered a capacity fading of around 10 % after 25 cycles in pure NaCl solution. However, after exposed to 500 mg/L HA, the capacity fading of T-NMO electrode aggravated to 64 %, while interestingly the capacity of L-NMO electrode increased to 106 % of initial capacity. These results demonstrate the diametrically opposite role of HA on the ion storage performances of T-NMO and L-NMO electrodes. The significant decay in ion storage capacity of T-NMO electrode caused by HA insertion is attributed to the crystal lattice contraction, hinderance of ions diffusion and amplification of electrode dissolution which result in less exposure and utilization of active sites. Instead, the insertion of HA into the interlayers of L-NMO electrode enlarges the interlayer distance, facilitates ion diffusion and diminishes electrode dissolution, resulting in the improvement of both total and available active sites and thus the enhancement of ion storage capacity. The acquired results in this study highlight the contrasting impact mechanisms of HA by which the desalination performances of intercalation electrode are affected by the crystal structure and will provide important guidance for the design and deployment of CDI in real-world applications.