Reverse electrodialysis: evaluation of suitable electrode systems - ITM

1472 J Appl Electrochem (2010) 40:1461–1474Brogioli succeeded in the reversal of the capacitivedeionization (CD) technique [55]. The capacitor is formedby two activated carbon electrodes, immersed in the feedsolution, alternately salt and fresh water. When filled withsalt water, it is charged, and with fresh water it is discharged.Charging and discharging are promoted by thediffusion of ions and consequently there is a net energygain.e Systems with an electrode rinse solution containingcarbon particles and/or ion exchange resin particlesThese systems are described by Kedem et al. for use innormal electrodialysis and patented [56, 57]. The electroderinse solution contains about 1% active carbon and 1%cation exchange resin in a NaCl solution of about 2%. Theauthors suggest that this system is based on the reversiblesystemH þ þ e 1 = 2 H 2The H 2 gas, as evolved at the cathode is adsorbed by thecarbon and transported to the anode where the reversereaction proceeds. The involved H ? is transported by theion exchange resin:RH þ Na þ RNa þ H þwhere RH is the resin in the acid form and RNa the resin inthe salt form.The net reaction at the cathode and at the anode istherefore:RH þ Na þ þ e RNa þ 1 = 2 H 25 ConclusionsIn this work, self-developed electrode systems and electrodesystems described in literature are compared onsafety, health, environment, technical feasibility and economics.A robust system of judging the different systems ispresented. Applying this system to the described and usedsystems, a ranking was made of the different electrodesystems. It is shown that the applicability of an electrodesystem is related to the generating system. Three types ofgenerating systems can be distinguished: systems with (i)obligatory, (ii) facultative and (iii) forbidden interchangeof the feed water inlets. The highest rankings for all thethree generating systems are found for the Fe 2? /Fe 3?couple in a NaCl–HCl supporting electrolyte with bidirectionalRu/Ir covered titanium electrodes.This system is not ideal and may be improved forapplication in commercial operating, large scale RED.There are five interesting topics: (i) improved stable metalelectrodes (with longer lifetime, suitable for currentreversal and high overpotential for gas evolvement), (ii) anew homogeneous redox couple (e.g. an iron complex),(iii) a system with capacitive electrodes, (iv) a system withan electrolyte, containing carbon particles and/or ionexchange resin particles, and (v) the Fe 2? /Fe 3? couple incombination with graphite electrodes. The last improvementseems to be the most realistic alternative; however,duration test should be performed under real RED conditionsto prove that it has sufficient mechanical and electrochemicalstability.Acknowledgement This work was performed in the TTIW-cooperationframework of Wetsus, centre of excellence for sustainablewater technology (www.wetsus.nl). Wetsus is funded by the DutchMinistry of Economic Affairs, the European Union Regional DevelopmentFund, the province of Fryslân, the city of Leeuwarden and theEZ/Kompas program of the ‘‘Samenwerkingsverband Noord-Nederland’’.The first author would like to thank the Noordelijke HogeschoolLeeuwarden who facilitated this research by detaching him toWetsus and the participants of the theme ‘‘Energy’’ for their fruitfuldiscussions and their financial support.Open Access This article is distributed under the terms of theCreative Commons Attribution Noncommercial License which permitsany noncommercial use, distribution, and reproduction in anymedium, provided the original author(s) and source are credited.AppendixApplied criteria.The used marks are: good = 5; reasonable = 4;acceptable = 3; poor = 2; bad = 1 and unacceptable = 0.In some cases minus signs are used. These mean that theirvalue should be subtracted from the maximal score of 5.A Technical feasibilityA 1 Reversal of the electrical currentscore: Already given in Table 2A 2 Stability of the anolyte/catholyteScoreNo instable ions 5[Fe(CN) 6 ] 4- , [Fe(CN) 6 ] 3- 4A 3 Gas formation at the electrodesScoreNo gas 5O 2 formation -1Cl 2 formation -2H 2 formation -1A 4 Participating electrodesScoreNo participating electrodes 5Participating without problems (Cu) 4123