Novel Dual-Ion Capacitive Deionization System

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polymers Article A Novel Dual-Ion Capacitive Deionization System Design with Ultrahigh Desalination Performance Yuxin Jiang 1, Zhiguo Hou 2, Lvji Yan 1, Haiyin Gang 1, Haiying Wang 1,3,4,* and Liyuan Chai 1,3,4 1 2 3 Abstract: Capacitive deionization is an emerging desalination technology with mild operation con- ditions and high energy efficiency. However, its application is limited due to the low deionization capacity of traditional capacitive electrodes. Herein, we report a novel dual-ion capacitive deion- ization system with a lithium-ion battery cathode LiMn2O4/C and a sodium-ion battery anode NaTi2(PO4)3/C. Lithium ions could enhance the charge transfer during CDI desalination, while NaTi2(PO4)3/C provided direct intercalation sites for sodium ions. The electrochemical capacities of the battery electrodes fitted well, which was favorable for the optimization of the desalination capacity. The low potential of the redox couple Ti3+/Ti4+ (−0.8 V versus Ag/AgCl) and intercala- tion/deintercalation behaviors of sodium ions that suppressed hydrogen evolution could enlarge the voltage window of the CDI process to 1.8 V. The novel CDI cell achieved an ultrahigh desalination capacity of 140.03 mg·g−1 at 1.8 V with an initial salinity of 20 mM, revealing a new direction for the CDI performance enhancement. Keywords: capacitive deionization; desalination; dual-ion; battery electrode 1. Introduction Desalination is an inevitable choice for solving the global freshwater scarcity crisis, which has been a hotspot of scientific research over recent years [1–3]. A number of tech- niques have been put into industrial application, such as multi-effect distillation, multi-stage flash, membrane distillation, reverse osmosis, nano/ultrafiltration, etc. [4–9]. However, these methods are mainly thermal-driven or pressure-driven, which are energy-intensive. On the other hand, capacitive-deionization (CDI), as an emerging energy-efficient desalina- tion technology that employs electrode materials to adsorb salt from saline water, could operate under mild conditions [10,11]. Moreover, energy could be stored simultaneously during desalination in CDI [12], which is a mechanism similar to a capacitor [13]. Yet, the CDI technology has already been scaled up by only a few commercial com- panies (EST Water and Technologies in China, Current Water Technologies in Canada, etc.) [14]. The main obstacle hindering the wide application of CDI is the low desali- nation capacity [15]. Traditional electrode materials in CDI are mainly carbon-based materials. These materials adsorb salt with the mechanism of electric double layers; thus, the capacity is strongly limited by the specific surface area of the carbonaceous material (<15 mg·g−1) [16,17]. Nevertheless, battery electrode materials with larger electrochemical capacities, which could store salt ions not only on the surfaces but also in the inner crystal structures, have shown great potential in CDI desalination [18,19]. Numerous battery electrodes have been employed in CDI to obtain high desalination capacities. Lee [20] used a Na4Mn9O18 electrode in a hybrid CDI (HCDI) system with activated carbon (AC) as the counter electrode, and a deionization capacity of 31.2 mg·g−1 was achieved. Ahn [21] School of Metallurgy and Environment, Central South University, Changsha 410083, China School of Chemistry and Materials, University of Science and Technology of China, Hefei 230026, China Chinese National Engineering Research Center for Control and Treatment of Heavy Metal Pollution, Changsha 410083, China Water Pollution Control Technology Key Lab. of Hunan Province, Changsha 410083, China 4 * Correspondence: haiyw25@csu.edu.cn Citation: Jiang, Y.; Hou, Z.; Yan, L.; Gang, H.; Wang, H.; Chai, L. A Novel Dual-Ion Capacitive Deionization System Design with Ultrahigh Desalination Performance. Polymers 2022,14,4776. https://doi.org/ 10.3390/polym14214776 Academic Editor: Shuang Luo Received: 19 October 2022 Accepted: 31 October 2022 Published: 7 November 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Polymers 2022, 14, 4776. https://doi.org/10.3390/polym14214776 https://www.mdpi.com/journal/polymers

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