(620g) An Asymmetric Iron-Based Redox-Active System for Electrochemical Ion Separation in Aqueous Media

Authors: 
Tan, K. J., Massachusetts Institute of Technology
Hatton, T. A., Massachusetts Institute of Technology
Su, X., Massachusetts Institute of Technology
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 The flip side to the same coin is the critical relevance of separations in aqueous media for environmental remediation and drinking water pollution, which often require the removal of trace molecules of interest among natural constituents.

Electrochemical processes are an attractive modular platform for water purification, 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 Electrochemical separations can be further designed to selectively target certain species where methods such as distillation or chromatography may be inefficient.1,2 Redox-active materials such as metallopolymers have previously been shown to be able to electrochemically separate targeted inorganic oxyanions3 and organic anions in aqueous4,5 and organic6 solution with high selectivity over competing species via the activation of specific chemical binding. The adsorption and desorption processes are reversible and switched via an electrochemical stimulus.

We report a new asymmetric dual-functionalized Faradaic system that leverages the redox-active iron centers of a metallopolymer at the anode and tunable inorganic mixed-valence crystalline intercalating species at the cathode to carry out enhanced electrochemical separation of transition metal oxyanions in aqueous media. The process uses heterogeneous conductive composite electrodes containing these species to suppress solution pH changes from the parasitic water splitting side reactions, provide narrower potential windows to perform the separation, facilitate increased anionic adsorbate removal via cathodic cation intercalation, and also prevent oxidation state changes of homogeneous redox-active species during electrosorption.

References:

(1) Anastas, P. T.; Zimmerman, J. B. Environ. Sci. Technol. 2003, 37, 94A.

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

(3) Su, X.; Kushima, A.; Halliday, C.; Zhou, J.; Li, J.; Hatton, T.A. Nat. Commun. 2018, 9, 4701.

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

(5) Su, X.; Tan, K. J.; Elbert, J.; Rüttiger, C.; Gallei, M.; Jamison, T. F.; Hatton, T. A. Energy Environ Sci. 2017, 10, 1272-1283.

(6) Achilleos, D.; Hatton, T. A. ACS Appl. Mater. Interfaces 2016, 8, 32743-32753.