Molecular level design is crucial for the development of new, more efficient electrochemical interfaces for a wide range of industrial and environmental applications. In particular, selective separations remain one of the most important processes in the chemical and biochemical industries, and are crucial for water purification and environmental remediation. Thermal and pressure-based systems can incur high energetic costs, while adsorption technologies often require harmful solvents and regenerants. Despite the great interest in electrochemical systems for energy storage and electrocatalysis, a lack of molecular selectivity and the relatively narrow range of materials chemistry have limited their applications in separations science.

Redox-functionalized electrode materials offer an attractive platform for performing selective electrochemical separations. Organometallics, metallopolymers and associated metal-organic complexes offer a wealth in flexibility in terms of metal/ligand design, and control of electronic properties. First, the development of a range of redox-active metallopolymer electrodes is presented, with specific interactions towards micropollutant anions of concern [1-2]. The underlying intermolecular mechanisms are then unraveled by a combination of electronic structure calculations and spectroscopy, and leveraged for fine chemical separations. Second, the capabilities of redox-electrodes are leveraged towards not only selective capture, but tandem environmental transformation of emerging contaminants and heavy metal pollutants, as a pathway towards process intensification through electrification. Chromium and arsenic oxyanions are separated selectively in the presence of excess competing ions, and down to 10-100 ppb concentrations, based on differential charge-transfer interactions [3]. The integrated capture and electro-reduction of chromium was enabled based on a judicious choice of operating voltage windows, and consideration of thermodynamic partitioning of the transition metal elements.

From a fundamental perspective, these concepts point towards an emerging direction in electrochemical interface design – by superimposing properly tuned chemical interactions, we can reach beyond double-layer effects and achieve unprecedented molecular selectivity. From a practical perspective, electrochemically-responsive materials are expected to provide a sustainable and energy-efficient platform for sustainable separations and environmental remediation.

References.

[1] X. Su, et al, “Anion-selective redox electrodes: electrochemically-mediated separation with organometallic interfaces,” Advanced Functional Materials. 2016, 26(20), 3394-3404.

[2] X. Su, et al, “Asymmetric Faradaic systems for selective electrochemical separations,” Energy & Environmental Science, 2017, 10, 1272-1283.

[3] X. Su, et al. “Electrochemically-mediated selective capture of heavy metal oxyanions chromium and arsenic from water,” Nature Communications, 2018, 9, 4701.