Energy Fuels 2010, 24, 3329–3333 Published on Web 03/30/2010
: DOI:10.1021/ef901453q
Capacitive Deionization of NaCl Solutions with Modified Activated Carbon Electrodes† Isabel Villar, Silvia Roldan, Vanesa Ruiz, Marcos Granda, Clara Blanco, Rosa Menendez, and Ricardo Santamarı´ a* Instituto Nacional del Carb on, Consejo Superior de Investigaciones Cientı´ficas (CSIC), Apartado 73, 33080 Oviedo, Spain Received December 1, 2009. Revised Manuscript Received March 10, 2010
A pyrolysis product derived from a coal-tar pitch was chemically activated using KOH in a KOH/carbon proportion of 5:1. The activated carbon was used as electrode-active material for capacitive deionization (CDI). Electrochemical parameters, such as current values, charge, specific charge and charge-discharge efficiencies were investigated using a unit cell and solutions of NaCl in different concentrations. The parent activated carbon shows an excellent behavior as electrode-active material in CDI, removing more salt than other carbons previously described in the literature. The activated carbon electrode presents an efficiency higher than 99% after 20 cycles. The parent activated carbon was modified by thermal treatment under nitrogen at different temperatures and by treatment with hydrogen and carbon dioxide. The modified activated carbons were also evaluated as electrode-active material to study the influence of the texture and surface chemistry on the CDI process. The results show the importance of both the texture and surface chemistry of the active material on the CDI process. The best behavior as electrode-active material was obtained for the materials with a high apparent specific surface area and a large quantity of oxygenated functional groups, i.e., the parent activated carbon and the sample modified by hydrogen treatment.
low-cost desalination based on this technology may indeed be achieved, proving that sufficiently stable high surface area electrodes can be produced. Johnson and co-workers6,7 tested a variety of carbons as electrode-active materials. The exhaustive studies undertaken were eventually discontinued, mainly because of the inherent instability of electrodes, particularly in the case of the anode. This demonstrates the importance of a good efficiency in recovering electrodes. Efficient processes could be obtained by employing materials that have a high specific surface area, high electronic conductivity, fast response of the entire surface area to electrosorption-electrodesorption changes, chemical and electrochemical stability over a wide range of pH values, and the ability to tolerate frequent voltage changes.8 It is well-known that there is a relation between the specific surface area and the capacity to remove ions. The importance of the oxygenated surface groups to improve the wettability of the materials is also known.9 However, it is still not known which of these properties has the greatest influence on the CDI process. New carbon materials have been tested as electrode-active materials for the CDI process. Most of these studies suggest that carbon aerogels are the most promising available materials for purposes of CDI.10 However, there are other carbon materials, such as activated carbons, that may also be suitable for this application. The objective of this paper is to find a lowcost material [coal-tar pitch (CTP)-based activated carbon] that can be used as an electrode in CDI processes. With this
Introduction Supplying drinking water to the world population is an important challenge mainly because the demand for water is continuously increasing. Although the amount of water in the world is estimated to be about 1.38 billion km3, only 2.5% of this is drinkable.1 The natural water cycle may lead to the impression that fresh water is a renewable item. In reality, the availability of good-quality fresh water resources is decreasing dramatically because of the increase in pollution, irrational waste, and especially the severe pollution of existing resources.2 For these reasons, the search for new low-cost desalination processes is now a topic of growing research interest. Capacitive deionization (CDI) technology is an electrochemically controlled method for removing salt from aqueous solutions. It takes advantage of the ions adsorbed in the electrical double-layer region at the electrode-solution interface when the electrode is electrically charged by an external power supply. When the electrode has a high specific area, the quantity of ions may increase significantly in terms of grams of salt adsorbed per unit of weight of electrode material. This factor renders the CDI process attractive for water treatment.3,4 A preliminary cost evaluation of the CDI process performed by Johnson et al. in 19715 showed that efficient and † This paper has been designated for the special section Carbon for Energy Storage and Environment Protection. *To whom correspondence should be addressed. E-mail: riqui@ incar.csic.es. (1) Spiegler, K. S.; El-Sayed, Y. M. Desalination 2001, 134, 109–128. (2) Mathioulakis, E.; Belessiotis, V.; Delyannis, E. Desalination 2007, 203, 346–365. (3) Caudle, D. D.; Tucker, J. H.; Cooper, J. L.; Arnold, B. B.; Papastamataki, A. Electrochemical demineralization of water with carbon electrodes. Research Report, Research Institute, University of Oklahoma, Norman, OK, 1996; pp 1-190. (4) Hou, C. H.; Liang, C.; Yiacoumi, S.; Dai, S.; Touris, C. J. Colloid Interface Sci. 2006, 302, 54–61. (5) Johnson, A. M.; Newman, J. J. Electrochem. Soc. 1971, 118, 510–517.
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(6) Johnson, A. M.; Venolia, A. W.; Wilboume, R. G.; Newman, J. The Electrosorb Process for Desalting Water; Marquardt: Van Nuys, CA, 1970; pp 1-31. (7) Johnson A. M. Electric demineralization apparatus. U.S. Patent 3,755,135, 1973. (8) Oren, Y. Desalination 2008, 228, 10–29. (9) Ahn, H.-J.; Lee, J.-H.; Jeong, Y.; Lee, J.-H.; Chi, C.-S.; Oh, H.-J. Mater. Sci. Eng., A 2007, 449-451, 841–845. (10) Yang, K.-L.; Ying, T.-Y.; Yiacoumi, S.; Tsouris, C.; Vittoratos, E. S. Langmuir 2001, 17, 1961–1969.
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Energy Fuels 2010, 24, 3329–3333
: DOI:10.1021/ef901453q
Villar et al.
Table 1. Summary of Experimental Conditions Used for the Modification of the Parent Activated Carbona BM BM600 BM1000 BMH2 BMCO2
treatment
T
Hr
t
atm
none thermal thermal hydrogen carbon dioxide
600 1000 400 700
2.5 2.5 5 5
1 1 4 4
N2 N2 H2 CO2
a T, temperature of treatment (°C); Hr, heating rate (°C min-1); t, residence time (h); atm, atmosphere used.
propose, an activated carbon was subjected to three different treatments (thermal treatment and treatments with hydrogen and carbon dioxide), to modify its characteristics and study the influence of both the texture and the surface chemistry of the active material on the CDI process.
Figure 1. Voltage-current profiles as a function of time for the CDI process. Table 2. Textural Properties of Modified Activated Carbonsa
Experimental Section BM BM600 BM1000 BMH2 BMCO2
Preparation of the Activated Carbon. A commercial CTP was used as raw material for preparing the activated carbon. The parent activated carbon was obtained by a process that involved two consecutive steps: (i) thermal treatment of the CTP at 450 °C for 4 h, under a dynamic nitrogen pressure of 1 MPa using a flow of 40 L h-1, to obtain a semi-coke, followed by (ii) chemical activation of the semi-coke using KOH in a KOH/carbon proportion of 5:1. For this activation, the semi-coke and KOH were mixed in an agate ball mill and afterward treated at 700 °C for 1 h under a nitrogen flow of 62 mL min-l. The resultant material was neutralized with 1 M HCl and washed afterward with distilled water until pH 7. Finally, the activated carbon was dried at 110 °C in a vacuum oven for 24 h and labeled BM. Modification of the Activated Carbon. The parent activated carbon was modified using three different procedures: (i) thermal treatment by heating at 2.5 °C min-1 to 600 and 1000 °C with a residence time of 1 h at each temperature under nitrogen flow, with the modified activated carbons being labeled as BM600 and BM1000, respectively; (ii) treatment with hydrogen, first by heating at 5 °C min-1 to 400 °C under a nitrogen flow of 50 mL min-1, followed by a residence time of 4 h under a hydrogen flow of 50 mL min-1 (BMH2); and (iii) treatment with carbon dioxide, by heating at 5 °C min-1 to 700 °C for 4 h under a CO2 flow of 50 mL min-1 (BMCO2). A summary of these treatments is compiled in Table 1. Characterization of Activated Carbons. Elemental Analysis. The carbon, hydrogen, sulfur, and nitrogen contents of the samples were determined with a LECO-CHNS-932 microanalyzer. The oxygen content was obtained directly using a LECO-VTF-900 furnace coupled to the microanalyzer. The analyses were performed with 1 mg of sample ground and sieved to
