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Ion Exchange Membranes

Ion exchange membranes are commonly employed to separate the anodic and cathodic electrolytes in a double-compartment PEC reactor to avoid mixing the produced hydrogen and oxygen and the back reactions (i.e., cathodic 02 reduction), thus causing low productivity of the PEC water splitting. Three types of ion exchange membranes have been explored in the PEC systems, including proton exchange membranes, anion exchange membranes, and bipolar membranes.

Proton Exchange Membrane (PEM)

The most commonly used PEM in PEC water splitting is NationĀ®, a perfluorosul- fonic acid membrane with polytetrafluoroethylene as support, though it is known for the high cost [118-120]. In fact, as one of the best proton conductive membranes (H+ conductivity of about 0.1 S/cm at room temperature), Nafion has some other advantages, such as the low gas permeability and exceptional mechanical and chemical stability [119,120]. Therefore, Nafion has been widely used in traditional fuel cells and electrolyzers.

Recently, some other proton exchange membranes (such as sulfonated polyether- sulfone [121]) have also been developed and investigated for PEC water splitting, but are not widely used yet. This can be attributed to the following factors: (i) most of these membranes are high cost, which is an especially important issue under the low current density condition because it requires a large area and (ii) besides protons, the proton exchange membranes also allow the transport of other cations (e.g., K+ or Na+) [118], which will partly account for the charge transport through the membrane when they dominate the concentration of protons [118]. In the long run, the pH at the cathode will increase, while the pH at the anode will decrease. As a result, the required voltage for PEC water splitting will increase [122].

Anion Exchange Membrane (AEM)

Different from PEMs, AEMs only allow the transport of anions, such as OH~. Recently, AEMs have also been investigated for PEC water splitting cells as an alternative for PEMs [123]. Among various AEMs, Selemion is a popular commercial one that is prepared from styrene, Chloromethyl styrene, and divinylbenzene containing quaternary ammonium groups, which has a high efficiency in alkaline media [119]. The main drawback of Selemion is the lower ion conductivity than Nafion, which limits the PEC efficiency (though it may not be an issue at the current density of ~10 mA cm-2) [118,119]. Additionally, AEMs suffer from the limited chemical stability in strong alkaline environments and the limited selectivity that can lead to cation crossover instead of OH- transport [119]. Therefore, the design of stable, selective, and possibly transparent AEMs suitable for PEC devices is still an enormous challenge.

Bipolar Membrane (BPM)

Most recently, the application of BPMs in PEC water splitting has emerged owing to their additional advantages in guaranteeing the reactions in each chamber to proceed under different electrolyte conditions that best fit their exigencies [124]. BPMs can dissociate water into H+ and OH" and block the anion transport from the cathode compartment to the anode compartment through the cation-exchange layer and the cation transport from the anode compartment to the cathode compartment through the anion-exchange layer, which allows maintaining a different pH at either side of the membrane [119]. Therefore, the employment of a BPM can significantly expand the possibilities for practical PEC cell designs.

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