Characterization of the Interfacial Joule Heating Effect in the Electrochemical Advanced Oxidation Process

The electrochemical advanced oxidation process (EAOP) has gained popularity in the field of water purification. During the EAOP, it is in the boundary layer of the anode-solution interface that organic pollutants are oxidized by hydroxyl radicals (^•OH) produced from water oxidation. Applying current to an anode dissipates heat to the surroundings according to Joule's law, leading to an interfacial temperature that is much higher than that of the bulk solution, which is known as the "interfacial Joule heating" (IJH) effect. The modeling and experimental results show that the IJH effect had an inevitable consequence for the activity of ^•OH, rate constants, and mass transport within the boundary layer. The interfacial temperature could be increased from 25 to 70.2 °C, a value mostly doubling that of the bulk solution (33.6 °C) at the end of a 120 min electrolysis (10 mA cm^-2). Correspondingly, the ^•OH concentration available for oxidation of organic pollutants was much lower than that calculated at a constant temperature of 25 °C probably due to H₂O₂ formation via ^•OH dimerization. The enhanced ^•OH diffusion resulting from strengthened molecular thermodynamic movement and decreased kinematic viscosity of the solution also drove ^•OH to move far from the anode surface and thus extended the maximum thickness of the boundary layer. The oxidation rate was positively correlated to the interfacial temperature, the activation energy, and the number of activated molecules, indicated by a 1.57-2.28-fold increase depending on the target organic compounds. The finding of the IJH effect prompts a re-examination of the literature based on a realistic rather than a constant temperature (e.g., 20-30 °C), the case reflected in a number of prior studies that does not exist virtually, and reconsideration of behaviors that can be attributed to the change in temperature during EAOP.