Energy Efficient Water Desalination
- Fig. 1: (A) Electrical double-layer formation at a planar solid-liquid interface between an electrically charged electrode and water with dissolved ions. (B) Scanning electron micrograph cross-section of a pair of electrodes composed of activated carbon and polymer binder, put on graphite foil current collector and separated by a glass fiber separator. This pair of electrodes is part of a multiple stacking inside a CDI laboratory cell (visible inside the optically transparent cell) which is then used for water desalination applications.
- Fig. 2: CDI architectures: flow-through CDI, flow between CDI, flow between membrane CDI, and membrane flow electrode CDI.
- Fig. 3: Three different basic concepts for charge compensation via ioin electrosorption.
Capacitive deionization is an emerging technology for energy efficient water treatment. Having originated from first concept studies in 1960, today's commercial CDI systems target water desalination application ranging from producing potable water from brackish water to the remediation of industrial or mining waste water.
What is Capacitive Deionization?
Capacitive Deionization (CDI) is an emerging technology that can be used for energy efficient removal of dissolved, charged species from water - for example for desalination of brackish or sea water. Yet, CDI is much more than an attractive tool to generate potable water and has been applied also to wastewater remediation and water softening. The high energy efficiency of CDI for the desalination of water with a low salt concentration (typically below 10 g/l) is due to the fact that the salt ions, which are the minority component in the water, are removed from the mixture. By contrast, conventional water desalination systems (such as reverse osmosis, and distillation) instead remove the majority component, the water molecules, from salty feedwater. CDI is typically characterized by an intermittent operation between ion electrosorption (until the electrodes are fully saturated / fully charged) and electrode regeneration (which relates to discharging the electrodes to release the electrosorbed ions). Considering a full CDI cycle, the invested charge for ion removal can be largely recovered during electrode regeneration, enabling energy consumption significantly below 1 kWh/m3 for desalinated brackish water. As a result of the intermittent operation, sequential ion-depleted and ion-enriched stream are generated, yielding a water recovery which can be significantly above 50%, and as high as 90%. Water recovery is an important performance metric for desalination technologies and is defined as the ratio of freshwater volume over inlet volume.
CDI Theory: Ion Electrosorption via Double-Layer Formation
Ion electrosorption is an often encountered phenomenon in nature and technology; it describes the electrostatically induced adsorption of ions or electrically charged molecules at the interface of an electrode surface with opposite charge (Fig.
1A). The high reversibility and energy efficiency of this process is exploited in the operation of high power density and long-lasting electrical double-layer capacitors, also known as supercapacitors. Instead of just exploiting efficient storage and recovery of electrical charges, the process of electrosorption can also be seen from the point of view of the water: when ions are electrosorbed, the formation of an electrical double-layer effectively immobilizes the ions at the fluid-solid interface. Thus, electrosorption can also be used to remove ionic species from aqueous media, in applications such as water desalination, water softening, and wastewater treatment. The structural details of the nanoscale electric double-layer are crucial to electrosorption performance, and determining this structure forms a highly active area of research. Further understanding in this area promises to form the basis for future breakthroughs in electrosorption technologies, such as CDI.
Materials and Setups
The basic element of any CDI cell is a single pair of carbon electrodes (Fig. 1B); CDI systems employ such electrodes with various sizes, thicknesses, and a certain number of pairs. Commonly, activated carbons are used as the active component that affords ion electrosorption by exhibiting a large specific surface area (typically 1200 - 1500 m2/g) and specific pore volume (around 0.8 - 1.0 cm3/g). Polymer binders, such as polyvinylidenfluoride or polytetrafluorethylene, are admixed to the activated carbon powder (5 - 10 wt%) to finally obtain 100 - 250 µm thick film electrodes. To avoid electro-corrosion, such electrodes are commonly applied to graphite foils which effectively acts as a current collector. Finally, between the electrodes, a porous separator (typically glass fiber) is placed to ensure electrical isolation and facile flow of the aqueous solution by the porous carbon materials. The performance of CDI electrodes has been studied intensively, and the ability of the electrodes to remove salt from the feedwater has tripled over the past decade, from roughly 5 mgNaCl, stored/gcarbon to 15 mgNaCl, stored/gcarbon. In addition to their ability to remove salt, electrodes have also significantly improved in the rate of salt removal, achieving up to 2.5 mgNaCl, stored/gcarbon/min. We have not reached fundamental limits in both the salt removal or rate capabilities of CDI electrodes, and so further breakthroughs can be achieved in the coming years.
CDI cells can be constructed in various architectures (Fig. 2). As ions need to be removed from the in-fed water stream, it is highly desirable to maximize the interaction volume between solution and electrode pores, and a flow-through setup is perhaps the most direct way to achieve this feat. In such a cell, the electrode is constructed from a material with open porosity (such as carbon aerogel) and the saline water flows perpendicular to the electrodes and through the electrode stack. A more conventional setup with flow between electrodes has the benefits of dense current collectors and more facile water flow between the electrodes. Such a flow between setup was the original CDI design in the 1960s and is still today the standard geometry used in commercial devices.
The efficiency and salt sorption capacity of CDI cells can be significantly improved by placing ion exchange membranes in front of the carbon composite electrodes. To understand why membranes allow for performance enhancements, we have to briefly revisit the concept of ion electrosorption. CDI only affords the removal of ions by electrosorption counter-balancing an electrical charge at the electrode. Yet, this process of electric charge compensation can be accomplished, in theory, in three ways: either by counter-ion adsorption, co-ion expulsion, or a combination thereof, namely ion swapping (Fig. 3). Counter-ions have a charge opposite to the electric charge at the electrode and their preferential electrosorption effectively depletes the feedwater of salt ions. Charge accommodation can also be accomplished by expulsion of co-ions, or ions with the same charge sign as the electrode, a process which increases the salt concentration in the feedwater flow. It is important to consider the contribution of both effects when evaluating the salt sorption capacity. The last ideal case, pure co-ion desorption, will not occur under normal operation, rather CDI is typically characterized by some co-ion desorption and significantly more counterion adsorption. The main benefit of adding membranes to the CDI cell is the reduction of the detrimental effect of co-ion expulsion, as the membranes effectively block co-ions from carrying parasitic current, and can thus increase the salt storage capacity of the electrode.
A new Trend in CDI: Flow Electrode
Very recently, a new architectural class for CDI was demonstrated employing carbon flow electrodes which can be pumped through electrode compartments; the latter are separated from the feed water stream typically by ion exchange membranes, although also porous separator configurations are possible. Such a flowable setup has several advantages. First, the feed water flowing through a single cell can be continuously desalinated, as the discharge of the active carbon particles (regeneration) can occur as a separate process downstream of the cell. In contrast, in all previous CDI architectures based on static electrodes, the cell can only desalinate for a limited period of time until the EDLs of the porous electrodes have been fully charged, and then desalination must cease while the cell is discharged to enable subsequent desalination cycles. This intermittent operation also can require complicated fluidic handling as desalinated stream (during charging) and brine streams (during discharging) emerge, at different times, from the same spacer between the electrodes. A second major benefit is that FCDI, by continuously introducing uncharged carbon particles into the charging cell, can effectively increase the capacitance available for desalination above that of static electrode CDI systems. Thus, FCDI can desalinate higher salinity streams than static CDI systems, and the complete desalination of high salinity feeds, such as 32 g/l salt concentration, roughly that of sea water, has been reported. The desalination of sea water was previously not practically attainable by static electrode CDI systems, and thus flow electrode systems are an exciting advance.
Conclusions and Outlook
The field of capacitive deionization has seen a tremendous growth over the last 5 years with an increasing number of articles being published, a conference series on the topic has been established (2015 Saarbrücken, Germany; 2017 Haifa, Israel), and an international working group on the topic (www.cdi-electrosorption.org) which began its work in 2014. The exponentially increasing interest in CDI is motivated by several unique advantages. In contrast to many conventional desalination techniques, CDI operates at low pressures (i.e., sub-osmotic), at room temperature, and requires only small applied voltages (typically 1 V per one pair of electrodes). Thus, the technology does not require high pressure pumps or heat sources. In addition to exhibiting low amounts of energy for salt removal, a large fraction of the invested charge is recovered during electrode regeneration by simply, without chemical agents, discharging the cell.