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

Abstract

A desalination battery is an attractive route for seawater desalination because it couples ion removal with energy storage. In this work, we paired Cu 3 [Fe(CN) 6 ] 2 •nH 2 O as the Na-storage electrode with Bi as the Cl-storage electrode to construct a novel desalination battery that enables membrane-free desalination. Most current desalination technologies, with the exception of thermal distillation, rely on the use of membranes. Eliminating the need for a membrane can significantly simplify the construction and maintenance of desalination systems. After carefully examining the sodiation/desodiation reactions and cycle performance of Cu 3 [Fe(CN) 6 ] 2 •nH 2 O in both acidic and neutral saline solutions (0.6 M NaCl), we combined Cu 3 [Fe(CN) 6 ] 2 •nH 2 O with Bi, which was previously identified as a promising Cl-storage electrode, to construct a Cu 3 [Fe(CN) 6 ] 2 •nH 2 O/Bi desalination battery. The Cu 3 [Fe(CN) 6 ] 2 •nH 2 O/Bi desalination battery generates an electrical energy output during desalination, which is equivalent to discharging, and requires an electrical energy input during salination, which is equivalent to charging. We investigated optimum pH conditions to perform salination to minimize the energy necessary for charging so that the desalination/salination cycle could be achieved with a minimum overall energy input. The results obtained in this study suggest that with further optimization the Cu 3 [Fe(CN) 6 ] 2 •nH 2 O/Bi desalination battery will offer new possibilities for practical seawater desalination.

Faradaic Electrodes Open a New Era for Capacitive Deionization

“…In addition, redox-active polymers are also promising for water desalination, as they can show either strong interactions with Na + (such as redox-active polyimide) [56] or Cl − (such as polymers with [Fe(CN) 6 ] 4− ) [44] depending on the tunable redox active moieties. In addition to the aforementioned novel ion-selective Na + /Cl − capture electrode materials [47,57,58] and various cell designs, [43,46,49] some recent promising developments of Faradaic electrodes have been achieved, such as research on the influences of operational parameters on CDI performance metrics, [41,43,59] and some typical scientific and practical application. [40,[60][61][62][63] However, it appears that Faradaic electrodes used in CDI cells are still not mature enough to meet the requirement for practical implementation and commercialization, which can be attributed to major challenges such as not fully understanding the ion capture mechanisms and behaviors of materials, matching issues between the Na + capture cathode and Cl − capture anode, and the need to establish standardized test conditions, etc.…”

“…For the former, a common trend for CDI cells employing Faradaic electrode materials is that an increase in influent salt concentration increases the salt removal capacity, and the trend remains even when the salinity is as high as seawater in some studies. [59,96] This could be due to the following reasons. First, a higher influent salt concentration reduces the ionic resistance of the electrolyte, thus enhancing the electrochemical activity of the electrodes.…”

“…3D insertion materials have open 3D framework structures, such as NASICON-type phosphates [41,132] or metal hexacyanometalates. [59,133] These materials provide 3D transport channels for ion diffusion, and accommodate inserted ions into specific crystal structural sites of 3D framework structures. The structural features and corresponding insertion mechanism of each insertion materials are detailed in Section 3.…”

“…To be specific, both electrodes in Faradaic CDI cells, such as Na 0.44 MnO 2 //BiOCl cell, [47] Na 3 V 2 (PO 4 ) 3 //AgCl cell, [94] and NaTi 2 (PO 4 ) 3 //Bi cell, [43] should be either ion donors (Na-rich and Cl-rich materials) or ion acceptors (Na-poor and Cl-poor materials) to ensure the normal running of CDI system. [43,47,59,94,95] While for NID cells, one electrode should be Na + donor and another be Na + acceptor to ensure an optimal CDI process. [50,52] Practicability: Both operational/economic feasibility and safety should be optimal for practical CDI applications.…”

“…Hence, metal hexacyanometalates are considered as promising candidates for CDI electrodes. [53,59,230] Since metal hexacyanometalates are generally synthesized through low-temperature solution-phase methods, the prepared products tends to contain crystal water occupying [R′(CN) 6 ] vacancies as shown in Figure 19a (right), forming a defective compound that can be expressed as A x R[R′(CN) 6 ] 1−y •nH 2 O (0 < y < 1). The water content varies with the preparation conditions, but it is difficult to achieve completely final dehydrated states due to the rapid kinetics of the precipitation process.…”