Electrode system for large-scale reverse electrodialysis: water electrolysis, bubble resistance, and inorganic scaling

Active Control of Irreversible Faradic Reactions to Enhance the Performance of Reverse Electrodialysis for Energy Production from Salinity Gradients

“…Although a CRED stack cannot easily surpass the power density of a conventional RED stack with a reversible redox couple, the main gain for capacitive electrodes is to be achieved in the elimination of potentially irreversible or dangerous faradic reactions. A large-scale RED suffers from undesired results, including fouling and performance decline by irreversible faradic reactions at a high stack voltage, as evidenced by the previous studies. , The current study shows that the CRED system can potentially sustain and stabilize power generation without pH changes, even at high operating voltages.…”

“…Depending on which faradic reactions are adopted, electrode systems can be categorized into two groups. The electrode systems adopting irreversible faradic reactions and electrolytes could generate toxic gas (e.g., Cl 2 ) and explosive gas (e.g., H 2 ). − The gas evolution processes, mainly caused by water electrolysis, consume significant energy, ultimately reducing RED’s net power generation. ,, Moreover, diffusion of chloride ion (Cl – ) toward the electrode rinse solution (ERS) is inevitable, as shielding membranes are never perfectly selective, implying that the ERS needs to be purified frequently to avoid chlorine evolution. The other electrode systems adopting reversible faradic reactions utilize either reactive electrodes (e.g., Cu or Zn) or inert electrodes with homogeneous redox couples (e.g., Fe 2+/3+ or Fe­(CN) 6 3–/4– ). ,,,, One of the main advantages of these electrode systems is that the overall chemical reaction is null, which consumes much lower energy than irreversible faradic reactions during redox reactions.…”

“…Although a RED system adopts a redox couple, it can still generate harmful gases from irreversible faradic reactions when operated at a high stack voltage. Han et al reported that gas evolution took place continuously in the pilot-scale RED system with a pair of inert electrodes and Fe­(CN) 6 3–/4– redox couple due to water electrolysis . This phenomenon took place because the concentration polarization of the redox couple easily occurs locally in large stacks with high stack voltages, which leads to large (local) overpotentials for the Fe­(CN) 6 3–/4– redox couple, exceeding the minimum potential of water electrolysis (i.e., theoretically 1.23 V as the standard electrode potential).…”

“…This phenomenon took place because the concentration polarization of the redox couple easily occurs locally in large stacks with high stack voltages, which leads to large (local) overpotentials for the Fe­(CN) 6 3–/4– redox couple, exceeding the minimum potential of water electrolysis (i.e., theoretically 1.23 V as the standard electrode potential). Blue precipitation was also observed at the anode where oxidation reactions took place, indicating that ferricyanide (Fe­(CN) 6 3– ) was decomposed due to the abrupt pH changes caused by water electrolysis …”

“…8−10 The gas evolution processes, mainly caused by water electrolysis, consume significant energy, ultimately reducing RED's net power generation. 4,11,12 Moreover, diffusion of chloride ion (Cl − ) toward the electrode rinse solution (ERS) is inevitable, as shielding membranes are never perfectly selective, implying that the ERS needs to be purified frequently to avoid chlorine evolution. The other electrode systems adopting reversible faradic reactions utilize either reactive electrodes (e.g., Cu or Zn) or inert electrodes with homogeneous redox couples (e.g., Fe 2+/3+ or Fe(CN) 6 3−/4− ).…”