Applicable tolerance evaluations of ion-doped carbon nanotube/polypyrrole electrode under adverse solution conditions for capacitive deionization process

In the capacitive deionization (CDI) process, the composites of carbon nanotube/polypyrrole (CNT/PPy) doped with chloride (Cl−) and dodecyl benzene sulfonate (DBS−) were respectively made into two types of coating electrode (CNT/PPy-Cl and CNT/PPy-DBS) to evaluate the applicable tolerance of the electrodes under different solution conditions. The effects of the solution conditions, including pH and temperature, on the ion-doped CNT/PPy electrodes were typically investigated in terms of the electrochemical characteristic, the ion adsorption performance and the stability. The results indicated that the appropriate solution conditions of ion-doped CNT/PPy electrodes were with the solution pH 6-9 and the solution temperature 5-25 °C, under which the ion adsorption performance could be at least 75.0% that in the optimum solution conditions (pH = 7, T = 15 °C). For the two electrodes, the poor solution tolerance in the strong acidic and alkaline solution can be ascribed to the interaction of hydrogen ions or hydroxide ions with the electrode structure which affects the conjugate effect between the doped ions and the PPy chains or accelerates the oxidation reactions on the electrode surface. The decrement of the electrode performance with increasing solution temperature from 15 °C to 55 °C mainly arises from the hydrophilic-hydrophobic transition on the surface of electrodes. The research results lay a good foundation for the practical application of CNT/PPy electrodes under different solution conditions.