Electrochemical Conditioning, Energy Consumption, and Long-Term Performance of Inverted Capacitive Deionization Cells

Abstract

Capacitive deionization (CDI) can desalinate brackish or lower salt content water at energy costs competitive with reverse osmosis [1]. The brief lifetime of CDI anodes has recently been overcome by operating in an inverted CDI mode (i-CDI), whereby chemical surface charges adsorb ions and an applied potential is subsequently used to repel them to form a concentrated stream [2]. The work here presents new insights on these next generation CDI cells with a focus on in situ anode pre-treatments, energy use, and system longevity for practical considerations. Electrochemically conditioning pristine anodes for 12 hours at +2.5 V was performed in a water stream, i.e. in situ, and immediately deliver peak salt capacities of approximately 6 mg g-1 for SpectraCarb activated carbon cloth electrodes in i-CDI mode (see conditions in Fig. 1). This is in contrast to common ex situ anode pretreatment methods using chemical oxidation in HNO3, which can require lengthy immersion durations, extensive neutralization, and up to 50 cycles (100 h) to reach a similar salt capacity (Fig. 1). This new electrochemical conditioning method can thus save days of processing time to attain steady desalination performance compared to the chemical pretreatment of electrodes and limit the volume of waste stream produced from the treatment process. Moreover, the charge efficiency of electrochemically prepared i-CDI cells is superior to chemical treatment until convergence after 80 cycles. Energy consumption of the cells reaches a minimum of 0.21 kWh m-3, which is approximately 80% lower than reverse osmosis at the feed concentration used here, and approaches the expected energy consumption by CDI desalination [1]. The long-term experiments shown in Figure 1 are run from feed tanks that are not purged of dissolved oxygen in order to more accurately simulate real-world operation. This gives rise to an observed accelerated performance loss with time, independent of the electrode pre-treatment method used. Surface area and sheet resistance measurements suggest that anode degradation (or further oxidation) may be responsible for the decline in performance: anode resistance (HNO3-treated) increased from 60 Ω □-1 before cycling to 600 Ω □-1 after cycling while specific surface area declined from 1390 to 1050 m2 g-1. Frequently, during capacitive deionization experiments, anode degradation is obscured or delayed by the use of membranes, by desalinating for only several cycles, and/or by purging the influent water stream of dissolved O2 with an inert gas such as nitrogen. While these controlled conditions have unveiled critical insights and allowed CDI to flourish in the recent literature, steady performance decline due to anode chemistry may continue to stall commercial adoption, and strategies to mitigate anode oxidation will be reviewed. Figure 1. Long-term salt adsorption capacity and charge efficiency for i-CDI cells operated at +0.8/0 V after electrochemical and nitric acid anode pre-treatments; 4.5 mM NaCl solution was circulated at 20 ml min-1 in single-pass mode through 3.5 g of SpectraCarb electrodes. [1] Oren Y. Capacitive delonization (CDI) for desalination and water treatment - past, present and future (a review). Desalination 2008;228:10-29. [2] Gao X, Omosebi A, Landon J, Liu KL. Surface charge enhanced carbon electrodes for stable and efficient capacitive deionization using inverted adsorption-desorption behavior. Energ Environ Sci 2015;8:897-909. Acknowledgements These authors are grateful to the U.S.-China Clean Energy Research Center, U.S. Department of Energy (DE-PI0000017) for project funding. Figure 1