Nitrate is a ubiquitous aqueous pollutant from agricultural and industrial activities. At the same time, conversion of nitrate to ammonia provides an attractive solution for the coupled environmental and energy challenge underlying the nitrogen cycle, by valorizing a pollutant to a carbon-free energy carrier and essential chemical feedstock. Mass transport limitations are a key obstacle to the efficient conversion of nitrate to ammonia from water streams, due to the dilute concentration of nitrate. Here, we develop bifunctional electrodes that couple a nitrate-selective redox-electrosorbent (polyaniline) with an electrocatalyst (cobalt oxide) for nitrate to ammonium conversion. We demonstrate the synergistic reactive separation of nitrate through solely electrochemical control. Electrochemically-reversible nitrate uptake greater than 70 mg/g can be achieved, with electronic structure calculations and spectroscopic measurements providing insight into the underlying role of hydrogen bonding for nitrate selectivity. Using agricultural tile drainage water containing dilute nitrate (0.27 mM), we demonstrate that the bifunctional electrode can achieve a 8-fold up-concentration of nitrate, a 24-fold enhancement of ammonium production rate (108.1 ug h⁻¹ cm⁻²), and a >10-fold enhancement in energy efficiency when compared to direct electrocatalysis in the dilute stream. Our study provides a generalized strategy for a fully electrified reaction-separation pathway for modular nitrate remediation and ammonia production.

Critical minerals are essential for the ever-increasing urban and industrial activities in modern society. The shift to cost-efficient and ecofriendly urban mining can be an avenue to replace the traditional linear flow of virgin-mined materials. Electrochemical separation technologies provide a sustainable approach to metal recovery, through possible integration with renewable energy, the minimization of external chemical input, as well as reducing secondary pollution. In this review, recent advances in electrochemically mediated technologies for metal recovery are discussed, with a focus on rare earth elements and other key critical materials for the modern circular economy. Given the extreme heterogeneity of hydrometallurgically-derived media of complex feedstocks, we focus on the nature of molecular selectivity in various electrochemically assisted recovery techniques. Finally, we provide a perspective on the challenges and opportunities for process intensification in critical materials recycling, especially through combining electrochemical and hydrometallurgical separation steps.

Selection of an appropriate electrolyte medium is essential for successful NH₃ electro-synthesis at low temperature and pressure. In this study, 2-propanol was employed as an electrolyte medium and its effectiveness in the electro-reduction of N₂ to NH₃ under ambient conditions was evaluated. NH₃ synthesis and faradaic efficiency using a mixture of 2-propanol/water (9:1, v/v) surpassed those when electrosynthesis was carried out using solely water. The concentration of H₂SO₄ and the applied current density influenced NH₃ synthesis in this 2-propanol-based system, and the optimal conditions led to maximized N₂ reduction, indicating that the competing and electron-losing reaction of H₂ evolution was relatively well suppressed.

In this study, a novel electrolysis cell based on ethylenediamine (EDA) as a cathodic solvent was developed for NH₃ electro-synthesis. The NH₃-generating cathode chamber was filled with 0.1 M LiCl/EDA and separated by a cation exchange membrane from the anodic compartment, which was filled with 0.05 M H₂SO₄ aqueous solution. It appeared that EDA was cathodically stable, and thus electron-stealing medium destruction was substantially avoided. The faradaic efficiency for NH₃ synthesis was 17.2%, producing 7.73 × 10⁻⁷ mol NH₃ for 1 h electrolysis at a cell voltage of 1.8 V with the charge consumption of 1.3 C.