Electrochemical Deoxygenation of Aqueous Solutions Using Symmetric Activated Carbon Electrodes in Flow-through Cells

We present a simple electrochemical flow cell using symmetric, unmodified commercially available carbon cloth electrodes that removes 97+% of incoming aqueous dissolved oxygen (DO). The electro-deoxygenation (EDO) cell achieves net O2 removal by leveraging the high overpotential of the oxygen evolution reaction on activated carbon and its propensity to oxidize under anodic polarization in aqueous solution: oxygen is reduced at the cathode via the oxygen reduction reaction (ORR) while water is oxidized and incorporated into surface oxide functional groups at the anode, effectively sequestering dissolved oxygen. The polarized electrodes promote the two-step reduction of DO resulting in some residual hydrogen peroxide in the effluent, which may be beneficial for certain water treatment applications. We modified a cell with Ni cathodes downstream to reduce all H2O2 to water for particularly sensitive applications; in this cell >99% of incoming DO could be removed to sub-10 ppb levels. EDO cells, which currently employ sacrificial anodes, can deaerate 30 L g-1 anode of water at an energy cost of 1 kWh (~$0.10) per 104 L. The technique is versatile, successfully deoxygenating solutions from dilute to seawater concentrations and at flow rates beyond 50 ml min-1 (O2 flux = 10-4 mol s-1 m-2), more than 50x higher throughput DO removal than similar technologies. We demonstrate the utility of an EDO cell as a component in an electrochemical water desalination process; dissolved oxygen removed upstream of a capacitive deionization cell enhances salt adsorption capacity and cell lifetime. Figure 1. (a) Dissolved oxygen removal by an electro-deoxygenation cell stack with six pairs of electrodes (up to t = 1.1 h) and subsequently with an additional H2O2 removal stack (six Ni cathodes and two additional anodes electrically connected in parallel downstream, t > 1.1 h). The device is operated by holding the cathodes, connected in parallel, at an oxygen-reducing potential versus an in situ reference electrode to achieve E c = -0.2 V vs. SHE. The electrolyte is 0.1 M NaCl with an initial flow rate of 50 ml min-1, and the effect of increasing flow to 75 and 100 ml min-1 is shown at t > 4.2 h. (b) Salt removal performance of a capacitive deionization (CDI) cell downstream of our EDO cell; the electrolyte is 10 mM NaCl flowing at 20 ml min-1 and the CDI cell consists of four pairs of carbon cloth electrodes totaling 3.2 g with charging/discharging at V cell = 1.2/0.0 V in a single-pass mode. At t = 24 h, the EDO cell is turned off to highlight its effect on deionization performance. Figure 1