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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX-XXX
Scalable Approach to Highly Efficient and Rapid Capacitive Deionization with CNT-Thread As Electrodes Maku Moronshing and Chandramouli Subramaniam* Department of Chemistry, Indian Institute of Technology Bombay, Powai-Mumbai 400076, India S Supporting Information *
ABSTRACT: A scalable route to highly efficient purification of water through capacitive deionization (CDI) is reported using CNT-thread as electrodes. Electro-sorption capacity (qe) of 139 mg g−1 and average salt-adsorption rate (ASAR) of 2.78 mg g−1min−1 achieved here is the highest among all known electrode materials and nonmembrane techniques, indicating efficient and rapid deionization. Such exceptional performance is achieved with feedstock concentrations (≤1000 ppm) where conventional techniques such as reverse osmosis and electrodialysis prove ineffective. Further, both cations (Na+, K+, Mg2+, and Ca2+) and anions (Cl−, SO42− and NO3−) are removed with equally high efficiency (∼80%). Synergism between electrical conductivity (∼25 S cm−1), high specific surface area (∼900 m2 g−1), porosity (0.7 nm, 3 nm) and hydrophilicity (contact angle ∼25°) in CNT-thread electrode enable superior contact with water, rapid formation of extensive electrical double layer and consequently efficient deionization. The tunable capacitance of the device (0.4−120 mF) and its high specific capacitance (∼27.2 F g−1) enable exceptional performance across a wide range of saline concentrations (50−1000 ppm). Facile regeneration of the electrode and reusability of the device is achieved for several cycles. The device demonstrated can desalinate water as it trickles down its surface because of gravity, thereby eliminating the requirement of any water pumping system. Finally, its portable adaptability is demonstrated by operating the device with an AA battery. KEYWORDS: capacitive deionization, desalination, CNT-thread, electrosorption capacity, average salt-adsorption rate
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porosity, hydrophilicity, and chemical inertness. Accordingly, the prominent choice of electrodes for CDI are carbon based materials such as activated carbon (AC),20−26 carbon nanotubes (CNTs)27−31 and graphene32−36 in the form of aerogels, sponges and sheets. There exist three major limitations of employing such nanocarbons as CDI electrodes. First, all nanocarbons are inherently hydrophobic causing ineffective electrode-water interface for desalination. The hydrophobicity also leads to aggregation of the nanocarbons in an aqueous environment resulting in degradation of the effective surface area available for desalination. Such materials, when employed with binders or additives, results in substantial degradation of the properties and decreases the efficiency and kinetics of CDI.37,38 Second, the processing and immobilization of the nanocarbons leads to change in the porosity, because of which the capacitance decreases and never reaches theoretically predicted values. Third, large scale deployment of such materials has never been realized thus far. Thus, the design of the CDI electrode should incorporate a suitable, mechanically robust and scalable substrate with the optimal porosity for supporting and immobilizing the nanocarbons. Therefore, we
resh water forms a miniscule fraction (3%) of the total, constant volume of water that is nonuniformly distributed across the earth. Anthropogenic activities cause an immense stress on such fresh water sources, often leading to irreversible damage.1−4 This has assumed alarming proportions with nearly 3.5 billion (∼65%) of the world population facing acute water scarcity by 2025.1 Although industrial wastewater predominantly contains Mg2+ and Ca2+ and causes its hardness, brackish water is rich in Na+ and K+. Therefore, a singular material that can work for removing multiple ions is preferred.5,6 Conventional techniques for desalination include membranebased techniques such as reverse osmosis (RO),7 electrodialysis (ED)8,9 and thermal separations methods (TSM)10,11 that are predominantly based on separating bulk water from dissolved salts. Considering that ∼97% (v/v) of any feedstock consists of pure water, capacitive deionization offers an alternative by removing ions from water feedstock.12−18 The viability of this technique demands electrode materials that are highly efficient and rapid in deionization, are biocompatible, recyclable and scalable, with low energy consumption. The combination of highly efficient and rapid deionization is particularly challenging to achieve, as seen from the Ragone plot for various CDI materials.19 Fundamentally, this translates to an electrically conductive material with high specific surface area, optimal © XXXX American Chemical Society
Received: August 9, 2017 Accepted: November 7, 2017 Published: November 7, 2017 A
DOI: 10.1021/acsami.7b11866 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Letter
ACS Applied Materials & Interfaces
Figure 1. (a) Schematic representation of the fabrication of electrode and its assembly to form the final functional prototype. The photograph of the electrode and assembled device is provided. SEM images of (b) pristine thread (left) and CNT thread (right), (c) CNT thread with EDS image based on carbon (inset). Scale bar of inset is 10 μm. SEM images of the surface of CNT thread (d) before and (e) after capacitive deionization experiments. (f) Plot of electrosorption efficiency for various cations as a function of residence time. (g) Comparison of qe of CNT thread with other reports27,29,36,50−61 (h) Nitrogen sorption isotherms and pore-size distribution (inset) of CNT thread.
essential for such material are low power consumption, reusability, biocompatibility, and scalability. Developing such a comprehensive electrode material forms an immediate demand for practical realization of wastewater recovery from both industrial and brackish sources. Addressing this immediate demand, we report an electrically conductive, CNT thread electrode exhibiting the highest electrosorption capacity (83, 139, and 200 mg g−1 for 250, 500, and 1000 ppm, respectively) and highest ASAR (1.72, 2.78, and 4.00 mg g−1 min−1 for 250, 500, and 1000 ppm respectively) among any non-membrane material. With high electrical conductivity (∼25 S cm−1), high specific surface area (900 m2 g−1), optimal porosity (0.7 and 3 nm), hydrophilicity (contact angle ∼25°) and mechanical robustness, the design of the CNT thread captures the best properties of both the CNTs and the cellulose platform on which they are immobilized. A single CNT thread exhibits a maximum absolute capacitance of 4 mF (specific capacitance ∼27.2 F g−1) that is scalable to 120 mF by increasing the number of windings of the electrode, similar to the functioning of DC motor. Importantly, the device operates at ultralow power (0.15 mW), is portable and miniaturized. Additionally, its versatility is demonstrated by operating the device with an AA battery, using only gravitational water flow for on-demand water purification, especially in geographically remote area with inefficient electrical power. Further, the performance (qe and ASAR) is completely retained over several (∼5) continuous cycles indicating its reusability and facile regeneration. Finally the easy-to-fabricate electrode material overcomes the fundamental
rationalized that biocompatible cellulose yarns as optimal substrates for immobilizing the CNTs. The natural porosity of the cellulose and its hydrophilicity would symbiotically assist the CNTs during CDI. In addition to scalability, such CNTimmobilized yarns would also be adaptable to a variety of geometries. The electrosorption capacity (qe) and average salt adsorption rate (ASAR) are two important parameters relating to efficiency and rate of capacitive deionization, respectively.18,19,39 Although qe is well-documented to scale linearly with ionic concentration of feedstock solution, there are only a few reports addressing ASAR (Figure S1).18,19,36,39−43 Capacitive deionization is more effective at higher concentrations (∼5000 ppm) pertaining to brackish water. The ineffectiveness of CDI, RO and ED at lower concentrations is due to (a) the solution approaching ideality at infinite dilution resulting in the activity coefficient nearing unity, and (b) lower electrolyte conductivity causing its greater electrical resistance. Thus, these are serious scientific and technological challenges while dealing with solutions with concentrations ≤1000 ppm. Importantly, this concentration range (50−1000 ppm) assumes greater significance since World Health Organization stipulates a maximum permissible limit of 2 nm pores facilitates rapid formation of extensive electrical double layer and is therefore desired for high salt-adsorption capacity.43,63−65 In contrast, the pure cellulose yarn exhibits negligibly small specific surface area (13 m2 g-1) with nanoporosity (
