(536b) Copper Hexacyanoferrate Hydrogel Electrodes for Electrochemically-Mediated Cation Separations

Kai-Jher Tan, Xiao Su, Johannes Elbert,and T. Alan Hatton^*

Department of Chemical Engineering, Massachusetts Institute of Technology

25 Ames Street, Cambridge, MA, U.S.A. 02139

*E-mail: tahatton@mit.edu

Chemical separations are of crucial importance and are used ubiquitously in the large-scale production of commodity chemicals and valuable species. However, as the purification steps in industrial manufacturing are often highly material and energy-intensive, there is a clear urgency for pursuing green engineering to develop and implement clean energy-efficient separation processes to reduce the economic and environmental impacts of high global energy consumption.^1,2 In particular, separations from aqueous media are of great interest. The removal of trace contaminants from water has been highlighted recently as a critical area for improvement, with applications to heavy metal recovery and seawater desalination.²

Electrochemical processes are an attractive platform for conducting separations, as they do not require solvents that change solution chemistry or impart environmental effects downstream, as well as heat and pressure inputs like conventional techniques.¹ Furthermore, they can be designed to target certain chemical species, whereas at the molecular scale, processes like distillation or chromatography can be inefficient.^1,2 Previously, redox-active materials such as metallopolymers have been shown to perform highly selective faradaic separations³, as well as intercalating-type materials. One such crystalline candidate is the mixed-valence transition metal hexacyanoferrate class of compounds, whose analogues have been used in ion exchangers, ion-sensing, electrochromic applications, and lately, as an energy storage material for batteries^4,5,6.

We report a new surfmer-based method to produce functionalized redox-active copper hexacyanoferrate (CuHCF) composite hydrogels for electrochemically separating ions in aqueous media. The developed technique is a straightforward, inexpensive, one-pot preparation process that can consistently produce self-contained redox-active electrodes, without the need for manual filtration or grinding steps. The resulting electrodes are uniform, stable in water, and capable of withstanding up to ~100 separation cycles without gel leaching.

CuHCF exhibits strong redox response as well as affinity for cesium cations, whose radioactive isotope, ¹³⁷Cs, is present in nuclear waste and is both an environmental and health concern.⁷ CuHCF nanoparticles can be further applied to remove other metal cations with similar hydrated ionic radii.^4,6 We also investigate methods to further improve the operation of the hydrogel in an electrochemical system by using specific dual-functionalized design schemes to tune the counter electrode⁸ for increasing adsorption and energy efficiency.

In conclusion, we present a novel technique to conveniently generate stable CuHCF composite hydrogels, which demonstrate robust redox activity and applicability in separating heavy metal ions from aqueous media for both water remediation and valuable cation recovery.

References:

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(2) Sholl, D. S.; Lively, R. P. Nature, 2016, 532, 435.

(3) Su, X.; Kulik, H. J.; Jamison, T. F.; Hatton, T. A. Adv. Funct. Mater. 2016, 26, 3394.

(4) de Tacconi, N. R.; Rajeshwar, K.; Lezna, R. O. Chem. Mater. 2003, 15, 3046.

(5) Wessells, C. D.; Huggins, R. A.; Cui, Y. Nat. Commun. 2011, 2, 550.

(6) Wang, R. Y.; Shyam, B.; Stone, K. H.; Weker, J. N.; Pasta, M.; Lee, H.; Toney, M. F.; Cui, Y. Adv. Energy Mater. 2015, 5, 1401869.

(7) Chen, R.; Tanaka, H.; Kawamoto, T.; Asai, M.; Fukushima, C.; Kurihara, M.; Ishizaki, M.; Watanabe, M.; Arisaka, M.; Nankawa, T. ACS Appl. Mater. Interfaces. 2013, 5, 12984.

(8) Su, X.; Tan, K. J.; Elbert, J.; Rüttiger, C.; Gallei, M.; Jamison, T. F.; Hatton, T. A. Energy Environ Sci. 2017, Accepted.