A Desalination Battery Combining Cu3[Fe(CN)6]2 As a Na-Storage Electrode and Bi As a Cl-Storage Electrode Enabling Membrane-Free Desalination
Authors: Nam Do-Hwan , Lumley Margaret Anne , Choi Kyoung-Shin 
Published: February 2020

Lack of access to fresh water is currently one of the most serious global issues affecting our society, with more than half of the world’s population lacking sufficient access to fresh water for at least one month of the year. Desalination of seawater, which separates dissolved salt ions from water, provides the most promising route to meet the continuously growing demand for fresh water. The goal of this work is to construct a desalination battery to enable electrochemical desalination. Desalination batteries offer two major advantages over conventional desalination methods, such as thermal distillation and reverse osmosis (RO). The first is that, using a desalination battery, salt ion removal is coupled with energy generation and storage, reducing the overall energy required to achieve desalination. The second is that our desalination battery can operate without the use of a membrane, unlike RO, which eliminates the need for pre-treatments and post-treatments of water to prevent membrane fouling currently keeping the cost of RO high.
Desalination batteries operate similarly to conventional batteries (e.g. Li-ion) to store and release energy during the charging and discharging processes. To achieve desalination, a Na-storage electrode and a Cl-storage electrode are coupled together to simultaneously remove and release Na+ and Cl- ions while storing and releasing energy. One major barrier for the construction of desalination batteries was the identification of an efficient and inexpensive Cl-storage electrode. Recently, our group discovered that Bi can serve as an effective Cl-storage electrode through the reversible conversion between Bi and BiOCl. In this work, we pair Cu3[Fe(CN)6]2 (CuHCF) as a Na-storage electrode with Bi as a Cl-storage electrode to construct a membrane-free desalination battery. Hexacyanoferrate-based materials have been investigated previously for use in Na-ion batteries and have shown excellent cyclability to perform sodiation/desodiation in acidic media.
In this presentation, we will first discuss the sodiation/desodiation reactions and cycle performance of CuHCF using a half-cell in both acidic and neutral saline solutions (0.6 M NaCl) and provide insight into the chemical reactions that occur under these conditions. This is important in order to evaluate the performance of CuHCF in the presence of Cl- ions. We will then demonstrate the construction of a membrane-free CuHCF/Bi desalination battery and discuss its performance and cycle stability for seawater desalination. We will show that optimization of the solution pH conditions can reduce the overall energy input required for the desalination cycle. Finally, we will conclude that with further optimization of the CuHCF/Bi system, our membrane-free desalination battery will offer new possibilities for seawater desalination. In this presentation, the operating mechanism of our novel membrane-free desalination battery architecture will be described, and we will provide insight into how energy storage systems can be utilized for diverse applications, beyond conventional battery technology.