Capacitive deionization with asymmetric electrodes: Electrode capacitance vs electrode surface area

Significance Statement

Removing salt from Water

Removing charged species from aqueous media is of interest in diverse applications including desalting of saline water. Capacitive deionization (CDI) devices governed by the principles of a supercapacitor use two conductive electrodes which are preferably nanostructured to provide large surface area for adsorption of ions. Practical capacitive deionization systems operate at very low voltages, lower than the dissociation potential of water and unlike Reverse Osmosis, which is popularly used for desalination, capacitive deionization has been shown to be energetically favorable for desalting brackish water.

While it is generally accepted that the specific surface area of the electrode is the primary factor governing the salt removal capacity of the electrode, researchers at Sultan Qaboos University have broken the myth and demonstrated that in practical applications, it is the specific capacitance of the electrode and not the surface area which regulates electrode performance in a capacitive device. Additionally the work highlights the effect of asymmetry in terms of specific capacitance between the two electrodes and proposes a simple electrical model and its dependencies to qualitatively assess the desalting performance. The results show that the electrode with smaller capacitance is the limiting factor, indicating that anode-cathode capacitance should be matched for practical capacitive deionization units to achieve maximum desalting capacities. 

Capacitive deionization with asymmetric electrodes Electrode capacitance vs electrode surface area.renewable energy global innovations

About the author

Karthik Laxman is currently a Post Doctoral researcher at the Water Research center in Sultan Qaboos University in Muscat, Oman. He finished his B.E. in Industrial Electronics from University of Pune (2005), MS in Nanotechnology from Asian Institute of Tech., in Thailand (2009) and PhD from Sultan Qaboos University in 2015. His research experiences include nanostructure synthesis and applications in the fields of quantum dot solar cells, photocatalysis and capacitive deionization. His is currently focusing on electrode optimization, parametric dependence, modeling and device optimization of a capacitive deionization cell for ground water desalination and disinfection and working on developing a prototype for field applications. 

About the author

Laila Al Gharibi is currently a research assistant at Water Research center in Sultan Qaboos University in Muscat, Oman.  She finished her Bachelors’ (BSc) in Chemistry from Sultan Qaboos University. Her experiences include synthesis and characterization of nano-structures and nano-materials for water desalination and purification. She also has extensive exposure in the field of electrochemical analysis of materials along with a host of other characterization techniques. 

About the author

Prof. Joydeep Dutta is the Chair of Functional Materials division at KTH Royal Institute of Technology, Stockholm, Sweden. He was Chair Professor in Nanotechnology for Water Desalination in Sultan Qaboos University, Oman until recently. Earlier, he was the Vice President (Academic Affairs), Director of the Center of Excellence in Nanotechnology and a Professor in Nanotechnology at the Asian Institute of Technology (AIT), Bangkok, Thailand, whose faculty he joined in April 2003 (until October 2011). He completed his Ph.D in 1990 from the Indian Association for the Cultivation of Science, India (Calcutta University). In 1991 and 1992 he did Post Doctoral work at the Electrotechnical Laboratory (ETL, Japan) and at Ecole Polytechnique (France) before moving to Switzerland in 1993 where he was associated with the Swiss Federal Institute of Technology (EPFL), Lausanne, Switzerland until 2003. From 1997-2001 he worked in technical and managerial qualities in high technology industries in Switzerland before returning back to academia in 2002.

His current research interest encompasses visible light photocatalysis, electrocatalysis of waste water (with focus on degradation of produced water and hospital waste), desalination (both membrane and capacitive deionization) and alternate energy sources (rainbow solar cells and hydrogen production from methanol steam reforming).

 

Journal Reference

Electrochimica Acta, Volume 176, 2015, Pages 420–425.

Karthik Laxman1,2, Laila Al Gharibi,1, Joydeep Dutta1,3

Show Affiliations
  1. Chair in Nanotechnology, Water Research Center, Sultan Qaboos University, PO Box 17, Al- Khoudh, Muscat 123, Oman
  2. Department of Electrical and Computer Engineering, College of Engineering, Sultan Qaboos University, PO Box 33, Al-Khoudh, Muscat 123, Oman
  3. Functional Materials Division, School of Information and Communication Technology, KTH Royal Institute of Technology, Isafjordsgatan 22, SE-16440 Kista, Stockholm, Sweden

Abstract

Asymmetry of electrodes on the equilibrium salt adsorption capacity in a capacitive configuration was studied. Experiments were carried out by using activated carbon cloth (ACC) with a specific surface area and specific capacitance of ∼1000 m2/g and 44 F/g as the anode and ACC coated with zinc oxide nanorods (ZnO NR) with a specific surface area and specific capacitance of 637 m2/g and 57 F/g as the cathode. The electrodes were characterized electrically and their salt adsorption capacities measured for various anode-cathode configurations to conclude that for multimodal electrodes, specific capacitance and not specific surface area regulates the salt adsorption capacity. The adsorption trends were analyzed and equated to an electrical model to qualitatively predict the equilibrium salt adsorption capacity, where the smaller capacitance was observed to be the limiting factor. The results in this work are especially useful for practical capacitive deionization units, where anode-cathode capacitance should be matched to achieve maximum salt removal efficiency.

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