1 Korean J. Chem. Eng., 38(1), 1-7 (2021) DOI: 10.1007/s11814-020-0677-0 REVIEW PAPER pISSN: 0256-1115 eISSN: 1975-7220 INVITED REVIEW PAPER † To whom correspondence should be addressed. E-mail: ashrafi@iust.ac.ir Copyright by The Korean Institute of Chemical Engineers. A brief review on the recent achievements in flow-electrode capacitive deionization Seyed Nezameddin Ashrafizadeh † , Ardalan Ganjizade, and Amin Navapour Research Lab for Advanced Separation Processes, Department of Chemical Engineering, Iran University of Science and Technology, Narmak, Tehran 16846-13114, Iran (Received 23 June 2020 • Revised 7 September 2020 • Accepted 11 September 2020) AbstractThe recent studies, achievements and advances in Flow-Electrode Capacitive Deionization (FCDI), known as a relatively new desalination method, are briefly reviewed. First, the geometry and configuration of membranes and electrodes, and their effects on the performance of FCDI are studied. The effect of feeding procedure of slurries on the operation of FCDI is then reviewed. The mode of applying the electric field to the system as another factor influencing the FCDI performance is also discussed. The effect of slurries and the membrane properties on desalination is another subject which is reviewed. Along with all the above mentioned investigations, it is attempted to introduce the new pat- ents related to FCDI and to show an outlook for such a desalination process. It is finally concluded that, despite some shortcomings, due to the high number of studies dedicated to the FCDI and according to recent advancements in this field, not only can FCDI become a mature and applicable desalination process in the future but also play an important role in the other engineering fields. Keywords: Capacitive Deionization, Flow-electrode, Electrochemical Separation, Desalination INTRODUCTION As the accessible volume of fresh potable water is limited, and production of sweet water is crucial for various industrial activi- ties, desalination of water has become an inevitable challenge. Besides the traditional methods engaged in heat transfer and/or membrane barriers, capacitive deionization (CDI) [1], membrane capacitive deionization (MCDI) [2] and flow-electrode capacitive deioniza- tion (FCDI) [3] have recently attracted researchers’ attention. It is said that due to their energy efficiency, environmental friendliness and ion removal efficiency, such processes are better alternatives for the common existing methods like reverse osmosis, multistage flash, and multi-effect desalination. In CDI, a low DC electric field is applied across two fixed porous carbon electrodes between which the water to be treated is pumped. In such a process, water desalination occurs by the electrically- driven migration of ions and formation of electric double layers (EDL) in the vicinity of the electrodes (Fig. 1(a)). However, ion migration and desalination terminate as the electrodes become satu- rated. Hence, for CDI, one must consider a step in which a reverse or zero potential is applied to remove the adsorbed ions from the electrodes and regenerate them. Thus, CDI must be performed in charge-discharge cycles. As a result, it can be said that CDI cannot desalinate water continuously. The effort to improve the efficiency and capacity of CDI resulted in the introduction of MCDI. MCDI (Fig. 1(b)) is different from CDI only in the cation and anion ex- change membranes placed next to the cathode and anode elec- trodes, respectively [2,4]. As the membranes do not let the co-ions leave the electrodes during the adsorption process, and due to the lower input energy needed for performing desalination by means of MCDI, this process may be more advantageous than CDI. How- ever, the fact that the desalination of water cannot be performed continuously (due to saturation of the electrodes) is still a draw- back of MCDI. Jeon et al. [3] suggested a modified form of MCDI, named as FCDI, wherein two aqueous carbon slurries, known as flow electrodes (FEs), are pumped in the spaces between the cur- rent collectors and membranes. In such systems, carbon particles play the same role as the fixed porous electrodes in MCDI (Fig. 1(c)). The inserted electric field between current collectors thus makes the ions transport through the membranes (electrodialysis) and become adsorbed on the EDLs of moving suspended carbon particles (capacitive mechanism) [5]. Despite CDI and MCDI, in some modes of operations, FCDI is capable of simultaneously remov- ing the salts and regenerating the FEs, and consequently desalinating the water continuously [4,6,7]. The superiority of FCDI desalina- tion over the two other named processes is depicted in Fig. 1(d). On one hand, new patents, somehow related to FCDI, are emerg- ing one after another (Fig. 1(e)) [8-12]. On the other hand, although there are some published researches generally reviewing the recent studies dealing with capacitive deionization methods [13,14], to the best of our knowledge, no study has deeply focused on FCDI. Here, we are not only reviewing the latest studies on FCDI, but also briefly reporting the recent advances in this field and present- ing an outlook for this novel method. AN OVERVIEW ON THE ADVANCES OF FCDI Thanks to recent developments, FCDI has found, or will find, its way into different fields of science, including chemical engineer- ing [15], environmental sciences [3], and energy [16]. However, there still exist some shortcomings with such a process, such as the relatively high energy consumption [17], inefficient charge transport