(Invited) Non-Uniformly Polarized Reactive Electrochemical Membrane (REM) for Water Purification
Authors: Ma Jinxing , Yang Kui , Yang Zhifeng 
Published: February 2024

A novel flow-through electrochemical technology (or termed reactive electrochemical membrane, REM) developed from porous electrode materials has recently attracted increasing attention globally in water purification and environment remediation. REM systems compress the boundary diffusion layer on the surface of the electrode via a combination of the convection of the solution and the constraint effect of the micro-pore structure, thus overcoming the mass transfer limitation in traditional flow-by systems. As a result, the kinetics of contaminant separation and conversion in a REM system increases by several to tens of times as compared with the flow-by systems. However, most existing studies are focused on the development of novel electrode materials, the evaluation of the removal efficiency and mechanism for contaminants, and the optimization of the treatment process of a REM system. In comparison, there is a gap in understanding the working principles of the REM electrodes. In contrast to the traditional flow-by system, the convective transport of the solution in the REM system drives the contaminants to the inside of the REM electrode, which activates the inner catalytic zone thereby. Because the electrode features the characteristics of non-uniform polarization, the given potential decays continuously along the depth of the REM electrode, thus causing a non-uniform distribution of the potential inside the REM electrode. As such, the zero-dimensional reaction theory of the electrode in traditional flow-by system (that is, the reaction mainly occurs on the surface of the electrode and is controlled by the potential) is no longer applicable to REM electrodes. It is therefore vital to revisit the reaction mechanism of the REM electrode. This Presentation therefore discusses the non-uniform polarization characteristics of the REM electrodes that are developed by loading catalytic layer on porous substrates. The main contents include:

(1) REM electrode
d
emonstrates non-uniform oxidation characteristics: The key influencing factors (
i.e.
, the pore structure of the REM electrode and the concentration of the electrolyte) and their principles of action to determine the reaction depth of a REM electrode have been identified, by employing (1) a developed mathematical model that could describe the spatial overpotential distribution as a function of depth in the REM electrode and (2) a visualization method to characterize the reactive depth of a REM electrode.

(2) There is a structure-function relationship between the REM electrode pore structure and water purification efficiency
: A series of SnO2-Sb-REMs with pore diameters in a range of 4.7 to 49.4 μm have been prepared to study the influence of the key factor (
i.e.
, pore structure) on the transport-oxidation process of typical antibiotics. The REM electrode performance that could not constantly increase with a decrease in the pore diameter relates to the decreasing reactive depth of the electrode at a smaller pore diameter, which would lead to the compromise of the intrinsic kinetics of a REM electrode.

(3) Direct electron transfer (DET) dominates the oxidation of antibiotics by using REM developed from porous substrates with a catalytic layer: The mechanism of typical antibiotics removal by employing SnO2-Sb-REM,
β
-PbO2-Sb-REM and Ti4O7-REM has been systematically investigated. The non-uniform oxidation characteristics of the REM electrode resulted in the DET rate being much higher than the yield of the hydroxyl radical (•OH), which was insignificantly affected by the electrode materials. The strong DET ability enabled the REM system to quickly and efficiently degrade antibiotics, which also imparted the REM system a stronger resistance to the background matrix (such as Cl–, HCO3
– and natural soluble organic matter).