Electrochemical production of hydroxylamine from nitrate on metal electrodes: A comparative study of selectivity and efficiency.

Despite the great potential of electrochemical nitrate reduction as a hydroxylamine production method, this strategy has not been sufficiently examined, and the effects of electrode material type on the selectivity and efficiency of this reduction remain underexplored. To bridge this gap, the present study evaluated six metals (Ag, Cu, Ni, Sn, Ti, and Zn) as cathode materials for the electrochemical reduction of nitrate to hydroxylamine, showing that the selectivity of hydroxylamine production was maximal for Sn, while the corresponding faradaic and energy utilization efficiencies were maximal for Ti. Although all tested materials favored nitrate reduction over hydrogen evolution, the disparity in the onset potentials of these reactions did not adequately explain the variations in nitrate removal efficiency, which was found to be influenced by material resistance and charge-transfer properties. The rate constants of elementary nitrate reduction steps determined from the time-dependent concentrations of nitrate and its reduction products (nitrous acid, hydroxylamine, and ammonium) were used to calculate the selectivity and efficiency of hydroxylamine production for each electrode. In turn, these selectivities and efficiencies were correlated with the density functional theory-computed adsorption energies of a key hydroxylamine precursor on different electrodes to afford a volcano-type plot with Ti and Sn at its pinnacle. Thus, this study introduces valuable descriptors and methods for the further screening of electrocatalysts for hydroxylamine generation and the establishment of more environmentally friendly hydroxylamine production techniques utilizing sustainable electricity.

Surface Electrochemistry of Carbon Electrodes and Faradaic Reactions in Capacitive Deionization.

Recent advances in electrochemical desalination techniques have paved way for utilization of saline water. In particular, capacitive deionization (CDI) enables removal of salts with high energy efficiency and economic feasibility, while its applicability has been challenged by degradation of carbon electrodes in long-term operations. Herein, we report a thorough investigation on the surface electrochemistry of carbon electrodes and Faradaic reactions that are responsible for stability issues of CDI systems. By using bare and membrane CDI (MCDI) as model systems, we identified various electrochemical reactions of carbon electrodes with water or oxygen, with thermodynamics and kinetics governed by the electrode potential and pH. As a result, a complete overview of the Faradaic reactions taking place in CDI was constructed by tracing the physicochemical changes occurring in CDI and MCDI systems.

Understanding the Behaviors of λ-MnO2 in Electrochemical Lithium Recovery: Key Limiting Factors and a Route to the Enhanced Performance.

Recently developed electrochemical lithium recovery systems, whose operation principle mimics that of lithium-ion battery, enable selective recovery of lithium from source waters with a wide range of lithium ions (Li+) concentrations; however, physicochemical behaviors of the key component-Li+-selective electrode-in realistic operation conditions have been poorly understood. Herein, we report an investigation on a λ-MnO2 electrode during the electrochemical lithium recovery process with regards to the Li+ concentration in source water and operation rate of the system. Three distinctive stages of λ-MnO2 originating from different limiting factors for lithium recovery are defined with regard to the rate of Li+ supply from the electrolyte: depleted, transition, and saturated regions. By characterization of λ-MnO2 at different stages using diverse X-ray techniques, the importance of Li+ concentration in the vicinity of the electrode surface is revealed. On the basis of this understanding, increasing the density of the electrode/electrolyte interface is suggested as a realistic and general route to enhance the overall lithium recovery performance and is experimentally corroborated at a wide range of operation environments.

Rapid and selective lithium recovery from desalination brine using an electrochemical system.

Due to the steep increase in the use of mobile electronics and electric vehicles, there has been a dramatic rise in the global lithium consumption. Although seawater is considered as an ideal future source of lithium, technological advances are necessary to ensure the economic feasibility of lithium recovery from seawater because the concentration and portion of Li+ are extremely low in seawater. Especially, battery-based electrochemical systems for lithium recovery have been considered as promising lithium recovery methods, though they have not been considered for seawater applications due to the extremely low concentration of Li+. In this study, we demonstrate that an electrochemical system based on a battery electrode material (λ-MnO2) can be used for efficient lithium recovery from desalination brine (2-3 times concentrated seawater). Our approach was able to capture Li+ within a substantially short period of time compared to conventional processes at a rate that was at least 3 times faster than that of adsorption processes, and our approach did not require acid or toxic chemicals unlike the other recovery technologies. Moreover, by consecutive operation of the system, a lithium recovery solution containing 190 mM of Li+ was obtained with only a small consumption of energy (3.07 Wh gLi-1), and the purity of Li+ was increased to 99.0%.