In‐Operando Mapping of pH Distribution in Electrochemical Processes

Abstract

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.

Mitigating voltage losses in photoelectrochemical cell scale-up

“…Finally, we note that our simulation results, which show non-linearity dependence between the electrode area and the total scale-up associated losses, qualitatively agree with experimental results reported in the literature. 21,23,55 Further experiments usingfor examplein situ pH monitoring 56,57 and particle image velocimetry 58 are needed to quantitatively validate our ndings.…”

In situ observation of pH change during water splitting in neutral pH conditions: impact of natural convection driven by buoyancy effects

“…Local pH measurements have been performed by electrochemical approaches, using rotating ring disk electrode (RRDE) and scanning electrochemical microscope (SECM) measurements, or spectroscopic approaches, such as surface-enhanced infrared absorption spectroscopy (SEIRAS) and fluorescence spectroscopy. 32,33,[42][43][44][45][46][47][34][35][36][37][38][39][40][41] Fluorescent dyes, which produce different fluorescence spectra depending on the protonation state of the dye, are often introduced in electrochemical cells to visualize the local pH with the help of a confocal laser scanning fluorescence microscope. 33,35,[37][38][39][40][41][42]45 These efforts are usually targeted to specifically monitor pH at the close vicinity of the electrodes, and ultramicroelectrodes are often used as a model electrode.…”

Electrochemically Generated Interfacial pH Change: Application to Signal‐Triggered Molecule Release

“…[42] Fluorescence microscopy, particularly using a confocal microscope, has also been used to map interfacial pH at different distances from an electrode surface. [11,[43][44][45][46][47][48] Notably, the confocal fluorescence spectroscopy, when appropriate instrumentation is used, allows the in-operando monitoring of interfacial pH variation in real time under variable experimental conditions. [44] Figure 2A shows pH controlled fluorescence output of 5(6)-carboxynaphthofluorescein (CNF) dye demonstrating fluorescence decrease at 567 nm and increase at 667 nm upon pH increase measured in a bulk solution.…”

“…[11,[43][44][45][46][47][48] Notably, the confocal fluorescence spectroscopy, when appropriate instrumentation is used, allows the in-operando monitoring of interfacial pH variation in real time under variable experimental conditions. [44] Figure 2A shows pH controlled fluorescence output of 5(6)-carboxynaphthofluorescein (CNF) dye demonstrating fluorescence decrease at 567 nm and increase at 667 nm upon pH increase measured in a bulk solution. The ratio of the fluorescence intensity measured at 667 nm and 567 nm (Figure 2B) can be used as a calibration function for measuring pH using laser scanning confocal microscope (LSCM) connected to an electrochemical cell, Figure 2C.…”

“…The images were obtained after the pH distribution had reached a steady-state, while applying 8 mA cm À 2 current density. Adapted from Ref [44]. with permission.…”