Quasi‐Steady‐State Polarization Reveals the Interplay of Capacitive and Faradaic Processes in Capacitive Deionization

Abstract

We employ slow cyclic voltammetry to study the quasi‐steady‐state capacitive deionization (CDI) of aerated 10 mM NaCl from Vcell=−0.2 to 2 V. The method allows for the deconvolution of capacitive and faradaic processes, which are significant even at low voltage, in conjunction with pH, conductivity, dissolved oxygen (DO), and electrode potential measurements. The pH at three locations and effluent DO data identify asymmetric electrosorption in the cell configured with pristine activated carbon electrodes, where charge is irreversibly consumed to produce H+/OH− instead of reversibly adsorbing Cl−/Na+. Implementing a reference electrode resolves ion adsorption as a function of the individual potential at each electrode. Near‐electrode pH probes reveal pH environments that diverge by up to 8 units during cell polarization and allow more accurate calculation of faradaic redox potentials in a flow‐by CDI cell. In the aerated NaCl solution that will be relevant to industrial‐scale water treatment, we find that DO reduction occurs to a greater degree than Na+ adsorption at the cathode until DO removal becomes mass transfer limited at <−0.2 V vs. SHE. Two cell architectures – flow‐by and flow‐through – corroborate our findings. Finally, we reconcile all measurements to generate a map displaying how the potential is partitioned amongst the capacitive and faradaic processes occurring during CDI operation over a 2.2 V window.

Interconnected metal oxide CNT fibre hybrid networks for current collector-free asymmetric capacitive deionization

“…The analysis of the pH evolution (see Fig. 6a) indicates a slight acidication of the electrolyte solution during the rst cycle, most likely attributed to faradaic reactions associated with the reduction of dissolved oxygen 72,73 and thus not present in subsequent cycles. Accordingly, due to these pH uctuations conductivity measurements could be inaccurate and masking capacitive performance.…”

Water Desalination with Energy Storage Electrode Materials

“…4 Another challenge is a limited cycle life due to corrosion of the positively charged carbon electrode during cell charging, especially when cells are operated at voltages well above 1 V. CDI cells typically achieve less than 1,000 charge/ discharge cycles, 22 with cycle life depending on the type and composition of the feedwater stream, the electrode material, cell design, and other operational parameters. 23 To develop next-generation electrochemical systems for water desalination, it is convenient to take inspiration from the highly developed energy storage field. As demonstrated by CDI cells, energy storage electrodes can be successfully applied as efficient water desalination electrodes (while maintaining their energy storage functionality).…”

Desalination with porous electrodes : Mechanisms of ion transport and adsorption

“…• the oxidation of water at the anode (2 H Although CDI is based on a capacitive process, faradaic reactions may also occur [58][59][60][61][62][63][64][65]. These faradaic reactions can be unfavorable in CDI, as they…”

“…• reduce charge and energy efficiency of the process (electrons involved in faradaic reactions do not contribute to ion adsorption in EDLs); • can result in unwanted pH changes of the effluent solution, as some reactions involve the production or consumption of protons or hydroxyl ions, which can result in precipitation of salt on membranes or into electrodes, and thereby affect the long-term stability of the cell [64,[66][67][68]; • can result in oxidation of carbon -often the main electrode material in CDI -and thereby also affect the stability of the CDI desalination process [63][64][65].…”

“…Whereas MCDI has shown stable long-term performance [102], CDI shows a declining desalination performance over time, especially if dissolved oxygen is present in the feed water [3,64,100,120,170,171]. This decrease in desalination performance has been mainly attributed to faradaic reactions involving carbon oxidation [63][64][65].…”