Observation of spatial variation of pH in capacitive deionization

Capacitive deionization is a promising technique for salt removal in brackish water applications. However, there are technical challenges to the longevity of operation related to the pH. In this study, we present a new technique for visualizing pH using a colorimetric dye injected into the spacer region between the anode and cathode. A purpose-built device in the form of a batch electrochemical microreactor was developed. In the process, many challenges were overcome, including the integration of a custom imaging system with an optical microscope that captures the electrosorption process within the narrow spacer region. A pH-sensitive dye was formulated with a rainbow hue range spanning pH 4 (red) through pH 7 (green) to pH 11 (violet) for the purpose of creating a visual pH map of the electrolyte phase. The initial solution prepared at 5000 mg/L NaCl was the source of dissolved ions for the batch. Dye was then added and adjusted to pH 7. Constant-voltage operation for 180 s consisted of observations of pH at the anode and cathode. The experiment was repeated at voltages between 0.9 and 1.8 V. Rapid lowering of the pH adjacent to the anode was observed from 20 s onward (indicated by a vivid orange color). Comparing the pH at the anode at 1.3 V (pH 5.8), 1.5 V (pH 4.7), and 1.8 V (pH 4.2) indicates that Faradaic reaction is the dominant mechanism, driven by increasing overpotential. This technique opens the opportunity to explore means of mitigating the adverse effects of pH by direct observation of the phenomena.