In-Operando Mapping of pH Distribution in Electrochemical Processes.

In aqueous electrochemical processes, the pH evolves spatially and temporally, and often dictates the process performance. Herein, a new method for the in-operando monitoring of pH distribution in an electrochemical cell is demonstrated. A combination of pH-sensitive fluorescent dyes, encompassing a wide pH range from ≈1.5 to 8.5, and rapid electrochemically coupled laser scanning confocal microscopy is used to observe pH changes in the cell. Using electrocoagulation as an example process, we show that the method provides new insights into the reaction mechanisms. The pH close to the aluminium electrode surface is influenced by the applied current density, hydrolysis of aluminium cations, and gas evolution. Through quantification of the pH at the anode, along with gas analysis, we find that hydrogen is evolved at the anode due to a non-Faradaic chemical reaction. This leads to increased production of coagulant, which may open new routes to enhance the process performance. This method for in-operando dynamic visualization of pH paves the way for studies of electrochemical processes, including other water treatment, electrosynthesis, and batteries.