Reverse electrodialysis: evaluation of suitable electrode systems - ITM

1470 J Appl Electrochem (2010) 40:1461–1474A RED system should fulfill the requirements on safety,health, environment, technical feasibility and economics.In Table 3 the formulated criteria are applied to the differentdescribed electrode systems from Table 1 (for GS 1in bold, for GS 2 regular and for GS 3 in italics). The row‘reversal to the electrical current’ (A 1 ) is split up in threeparts for the combination of the different generating systemswith the electrode system in question. The same goesfor the rows with the end-scores and ranks.Table 3 shows that from the described used systems(column 1–10), the iron system with Ti/Ru,Ir electrodes(column 8) is the best system in combination with each ofthe three generating systems.The hexacyanoferrate system with the same electrodes(column 9) may be useful in laboratory equipment. Thetable also indicates that by application of graphite electrodesthe score of these systems is approached (column 11and 12).4.2 Proposal for improved electrode systemsPossible improvements are:a Use of graphite electrodesAs indicated in Sects. 2.3 and 4.1, the use of graphiteelectrodes in combination with homogeneous redox systemlike Fe 2? /Fe 3? has good prospects.b Improved electrodes of the DSA type– Longer lifetimeFor their experiments Veerman et al. [1, 11, 15] usedtitanium mesh end electrodes, coated with Ru–Ir mixedmetal oxides. These electrodes are reversible (i.e. currentreversal is possible) and therefore usable for all types ofstacks. Lifetime of these electrodes is more than 10 yearsunder the applied current density (4 mA cm -2 ). Animprovement of the lifetime with a factor 5 can beachieved with special Ir-MMO (mixed metal oxide) coatedtitanium anodes and Ru-MMO coated cathodes; anotheroption is to use the extra stability of these electrodes forreducing the cost of these electrodes by decreasing thesurface of the electrodes (the so called partial electrodes).A limitation of this system is that frequent current reversalduring longer times is not possible with these electrodes.– High overvoltages for Cl 2 /O 2 evolutionThe success of DSA type electrodes is due to a combinationof high mechanical and electrochemical stabilitywith good catalytic properties and low overvoltages forspecial electrode reactions. Especially such anodes aredeveloped for Cl 2 and O 2 generation. However, in a REDprocess with a homogeneous redox couple (e.g. the Fe 2? /Fe 3? or the system [Fe(CN) 6 ] 4- /[Fe(CN) 6 ] 3- ), these reactionsare unwanted. Therefore, high overvoltages for gasevolution are favorable to the process. It requires someextra development to realize this with DSA type electrodes.– Specific composite electrodes for REDFor use in normal electrodialysis some systems withcomposite electrodes are patented [37–39]. These systemsallow current reversal with electrodes which are specificallyusable as cathode or anode. In each current directiononly one part of each electrode is used. Eventually theunused part can be protected with a low potential differencebetween this part and the adjacent part in operation.c New homogeneous redox coupleHomogeneous redox systems (e.g. the system[Fe(CN) 6 ] 4- /[Fe(CN) 6 ] 3- ) do not cause net chemicalreactions and therefore the power losses are low. Mostorganic homogeneous redox couples involve protons intheir reactions and special measures should be taken tocontrol the pH. However, there are many iron complexesand possibly candidates are available that satisfy all mentionedcriteria. The use of graphite electrodes may beconsidered again in this case.d Capacitive electrodesThe last decades, special high ion absorbing carbon hasbeen developed for use in super capacitors [47–50] and incapacitive deionization (CDI) [51, 52]. If this material isused as electrode in a RED stack, during a certain time aninternal ionic current can persist. The electrodes behave asa capacitor and during that time there is also an externalelectrical current. After some time, the process should beinverted by changing the sea water and the river water feed.It is obvious that the changing time is related to the residencetime in the system. Assuming that time to be 1 min,then a half period (the time between two feed interchanges)should be at least about 60 min. Current RED stacksoperate at maximum power output with a current density ofabout 4 mA cm -2 or 144,000 C m -2 h -1 . The cathodeshould adsorb then 144,000/F = 1.5 mol Na ? and on theanode the same amount of Cl - (F is the Faraday constant;1 F = 96,485 C mol -1 ).Typical specific capacitances are reported of 100–200 F g -1 carbon in aqueous solution [47–50]. Using thehighest value of 200 F g -1 , for an uploading to 1 V, thereis 200 F g -1 carbon needed. For a 1-h operation, theelectrodes should consist of 720 g carbon per m 2 .Thisseems a realistic value for further development of such asystem.In principle, the method is very clean and sustainable.Carbon aerogel electrodes may have lifetimes of more than10 years [53]. A restriction of this system is the obligatoryinterchange. For optimal performance, the connected loadshould be adapted continuously during one cycle. This ispossible with modern DC–DC converters. For normalelectrodialysis, such a system is patented by Wei et al. [54].123