Energy Materials

Reversible electrochemical processes are a promising technology for energy-efficient water treatment. Electrochemical desalination is based on the compensation of electric charge by ionic species, through which the ions are immobilized and, thereby, removed from a feed-water stream flowing through a desalination cell. For decades, electrochemical desalination has focused on the use of carbon electrodes, but their salt-removal ability is limited by the mechanism of ion electrosorption at low molar concentrations and low charge-storage capacity. Recently, charge-transfer materials, often found in batteries, have demonstrated much larger charge-storage capacities and energy-efficient desalination at both low and high molar strengths. In this Review, we assess electrochemical-desalination mechanisms and materials, including ion electrosorption and charge-transfer processes, namely, ion binding with redox-active polymers, ion insertion, conversion reactions and redox-active electrolytes. Furthermore, we discuss performance metrics and cell architectures, which we decouple from the nature of the electrode material and the underlying mechanism to show the versatility of cell-design concepts. These charge-transfer processes enable a wealth of environmental applications, ranging from potable-water generation and industrial-water remediation to lithium recovery and heavy-metal-ion removal.

Lithium-ion batteries are the primary power source for electric vehicles and portable electronic devices, creating a massive demand to mine and extract lithium. So far, lithium extraction has focused on brine and geological deposits. Yet, access to the enormous amount of lithium (at low concentration) in the earth’s oceans and other aqueous media remains challenging. Electrodialysis with Li-selective ceramic membranes could effectively separate lithium from seawater but at a high energy cost. Reversible electrochemical processes, like redox flow batteries, can overcome the limitation of electrodialysis-based systems. Herein we propose a system combining Li-selective ceramic membranes and a simple redox flow electrolyte to accomplish continuous lithium recovery from seawater. The lithium-extraction redox flow battery (LE-RFB) extracts dissolved lithium with a purity of 93.5% from simulated seawater, corresponding to a high Li/Mg selectivity factor of about 500.000:1. Benefiting from a low operating voltage, 1 g of lithium is extracted with only 2.5 Wh of energy consumption.

The capacitance of the electrochemical interface has traditionally been separated into two distinct types: non-Faradaic electric double-layer capacitance, which involves charge induction, and Faradaic pseudocapacitance, which involves charge transfer. However, the electrochemical interface in most energy technologies is not planar but involves porous and layered materials that offer varying degrees of electrolyte confinement. We suggest that understanding electrosorption under confinement in porous and layered materials requires a more nuanced view of the capacitive mechanism than that at a planar interface. In particular, we consider the crucial role of the electrolyte confinement in these systems to reconcile different viewpoints on electrochemical capacitance. We propose that there is a continuum between double-layer capacitance and Faradaic intercalation that is dependent on the specific confinement microenvironment. We also discuss open questions regarding electrochemical capacitance in porous and layered materials and how these lead to opportunities for future energy technologies.

Due to their versatile properties and excellent electrical conductivity, MXenes have become attractive materials for alkali metal-ion batteries. However, as the capacity is limited to lower values due to the intercalation mechanism, these materials can hardly keep up in the ever-fast-growing community of battery research. Antimony has a promisingly high theoretical sodiation capacity characterized by an alloying reaction. The main drawback of this type of battery material is related to the high volume changes during cycling, often leading to electrode cracking and pulverization, resulting in poor electrochemical performance. A synergistic effect of combing antimony and MXene can be expected to obtain an optimized electrochemical system to overcome capacity fading of antimony while taking advantage of MXene charge storage ability. In this work, variation of the synthesis parameters and material design strategy have been dedicated to achieving the optimized antimony/MXene hybrid electrodes for high-performance sodium-ion batteries. The optimized performance does not align with the highest amount of antimony, the smallest nanoparticles, or the largest interlayer distance of MXene but with the most homogeneous distribution of antimony and MXene while both components remain electrochemically addressable. As a result, the electrode with 40 mass% MXene, not previously expanded, etched with 5 mass% HF and 60% antimony synthesized on the surfaces of MXene emerged as the best electrode. We obtained a high reversible capacity of 450 mA h g−1 at 0.1 A g−1 with a capacity retention of around 96% after 100 cycles with this hybrid material. Besides the successful cycling stability, this material also exhibits high rate capability with a capacity of 365 mA h g−1 at 4 A g−1. In situ XRD measurements and post mortem analysis were used to investigate the reaction mechanism.