Principle

In RED, a concentrated salt solution and a less concentrated salt solution are brought into contact through an alternating series of anion exchange membranes (AEM) and CEM (Fig. 8).

The concentrated and the diluted salt solution are separated by an alternating series of AEMs and CEMs. The AEM contain fixed positive charges and only allow the selective transport of anions toward the anode, whereas the CEM contain fixed negative charges and only allow the selective passage of cations towards the cathode. Both the concentrated

Figure 8 Principle of RED. A is an anion exchange membrane, C a cation exchange membrane, V the potential difference over the applied external load (V), I the electrical current (A) and RLoad the resistance of the external load (O). A redox couple is used at the electrodes to mitigate the transfer of electrons from anode to cathode [34].

Figure 8 Principle of RED. A is an anion exchange membrane, C a cation exchange membrane, V the potential difference over the applied external load (V), I the electrical current (A) and RLoad the resistance of the external load (O). A redox couple is used at the electrodes to mitigate the transfer of electrons from anode to cathode [34].

and the diluted feed compartment contain a spacer to control the hydrodynamics. The electrons released at the anode are subsequently transported through an external circuit containing an external load, to the cathode. In the internal circuit in the stack, charge is carried by ions, while in the external circuit, electrons carry the charge. The ionic current is converted into electrical current by redox reactions that occur at the electrodes at the outer side of the stack. The redox couple is used to mitigate the transfer of electrons. A typical redox couple currently often used for RED is a solution of K4Fe(CN)6 and K3Fe(CN)6 (potassium iron(II) hexacyanoferrate and potassium iron(III) hexacyanoferrate) in a bulk solution of NaCl. At the cathode, the iron(III) complex is reduced and the iron(II) complex is reoxidized at the anode:

Fe(CN)6" + e" 2 Fe(CN)4" E0 = 0.36 V

The solution is recirculated between both electrode compartments to maintain the original iron(III)/iron(II) ratio.

The chemical potential difference between the two salt solutions with different concentrations is the driving force for this process and generates a voltage difference over each pair of membranes. The theoretical value of this potential difference over the membrane for an aqueous monovalent electrolyte (e.g., NaCl) can be calculated using the Nerst equation:

RT fac\

zF \adj where AVtheo is the theoretical membrane potential for a 100% selective membrane (V), R the universal gas constant [8.314J/(molK)], Tthe absolute temperature (K), z the electrochemical valence, F the Faraday constant (96,485 C/mol), ac the activity of the concentrated salt solution (mol/L), and ad the activity of the diluted salt solution (mol/L). For freshwater (0.017 M NaCl, g 7 — 0.878) and seawater (0.5 M NaCl, g + — 0.686), the theoretical voltage difference per membrane is 80.3 mV. The overall, total potential of the system is the sum of the potential differences over each pair of membranes (e.g., 100 membrane pairs provide a voltage difference of 100 x 80.3 — 8030 mV or 8 V).

The power density obtainable from RED (defined as the power generated per unit of total membrane area) is equal to the product of half the current and the potential difference over an external load (comparable to PRO, where the power is equal to the product of the pressure and the flux):

2 2r where pRED is the power density obtainable in RED (W/m2), AV the potential difference over an external load (V), r the area resistance (O m2), and Af the electrochemical potential difference between the two solutions (V). The maximum power density obtainable from RED can be calculated when Eq. (21) is differentiated with respect to the potential difference over the external load. At the maximum power output, dP/dAV is 0 and, as a result, the maximum power output can be obtained when AV is equal to Af/2. In this situation, when substituting this value of A V in Eq. (21), the maximum power density obtainable is equal to pRED = 1Af2 (22)

2r 4

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