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.

In this work, carbon electrode modified polymer membrane and free-standing polymer membranes are applied for desalination via capacitive deionization with microporous activated carbon. The desalination performance under different concentrations (20 mM and 600 mM) of feed water and different voltage operation mode (zero-volt and voltage-reversal operation). For the ion exchange membrane coating under 20 mM NaCl solution at a cell voltage of 0/+1 V, the desalination capacity reached 22.7 mg/g at the charge efficiency of 142.1%. In the case of the free-standing membrane, under 600 mM NaCl solution at a voltage range of −1/+1 V, the desalination capacity reached 26.0 mg/g with a charge efficiency of 61.5%. These results not only show the possibility of membrane capacitive deionization (MCDI) application in seawater desalination but also reveals different ion transport mechanisms of the coated membrane and free-standing membranes under different molar strength of feed water.

Abstract Transition metal dichalcogenides are attractive two-dimensional electrode materials for electrochemical energy storage devices due to their high reversible charge storage capacity. Hybridization of these materials with carbon promises enhanced performance by facilitating the access to reactive sites and extended mechanical stabilization. Herein, we introduce a NbS2/C hybrid material exhibiting a gyroidal microstructure synthesized through macromolecular co-assembly of a tailored block copolymer and an organometallic niobium precursor and subsequent sulfidation. Our synthesis allows the preparation of mechanically stable monoliths with NbS2 nanocrystals engulfed in a highly porous carbon shell. Due to the curvature of the gyroidal structure, abundant reactive sites are exposed that lead to an attractive performance in a lithium-containing electrolyte with a capacity of up to 400 mAh/g.

The Faradaic processes related to electrochemical water reduction at the nanoporous carbon electrode under negative polarization are reduced when the concentration of aqueous sodium nitrate (NaNO3) is increased or the temperature is decreased. This effect enhances the relative contribution of ion electrosorption to the total charge storage process. Hydrogen chemisorption is reduced in aqueous 8.0 m NaNO3 due to the low degree of hydration of the Na+ cation; consequently, less free water is available for redox contributions, driving the system to exhibit electrical double-layer capacitive characteristics. Hydrogen adsorption/desorption is facilitated in 1.0 m NaNO3 due to the high molar ratio. The excess of water shifts the local pH in carbon nanopores to neutral values, giving rise to a high overpotential for dihydrogen evolution in the latter. The dilution effect on local pH shift in 1.0 m NaNO3 can be reduced by decreasing the temperature. A symmetric activated carbon cell assembled with 8.0 m NaNO3 exhibits a high capacitance and coulombic efficiency, a larger contribution of ion electrosorption to the overall charge storage process, and a stable capacitance performance at 1.6 V.

Abstract We present a facile two-step synthesis of vanadium (III) oxide/carbon core/shell hybrid material for application as lithium-ion battery electrode. The first step is a thermal treatment of a mixture of vanadium carbide (VC) and NiCl2 ⋅ 6H2O at 700 °C in an inert gas atmosphere. Elemental nickel obtained from decomposing NiCl2 ⋅ 6H2O served as a catalyst to trigger the local formation of graphitic carbon. In a second step, residual nickel was removed by washing the material in aqueous HCl. By replacing NiCl2 ⋅ 6H2O with anhydrous NiCl2, we obtained a hybrid material of vanadium carbide-derived carbon and a vanadium carbide core. Material characterization revealed a needle-like morphology of the rhombohedral V2O3 along with two carbon species with a different degree of graphitic ordering. We varied the NiCl2 ⋅ 6H2O-to-VC ratio, and the optimized material yielded a capacity of 110 mAh ⋅ g−1 at 2.5 A ⋅ g−1 which increased to 225 mAh ⋅ g−1 at 0.1 A ⋅ g−1 after 500 cycles in the potential range of 0.01-3.00 V vs. Li/Li+. This enhanced performance is in stark contrast to the loss of lithium uptake capacity when using commercially available V2O3 mixed with carbon black, where 93 % of the initial capacity was lost after 50 cycles.

Negative electrode materials that possess fast lithium insertion kinetics are in high demand for high power lithium-ion batteries and hybrid supercapacitor applications. In this work, hydrogen titanium oxides are synthesized by a proton exchange reaction with sodium titanium oxide, resulting in the H2Ti3O7 phase. We show that a gradual water release in four steps yields intermediate phases of hydrogen titanate with different degrees of interlayer protonation. In addition, a synthesis route using zinc nitrate is explored yielding H2Ti3O7 with a high rutile content. This material dehydrates already at a lower temperature, resulting in a lamellar rutile titania phase. The hydrogen titanate materials with partially protonated interlayers are tested as negative electrodes in a lithium-ion battery and hybrid supercapacitor setup, showing an improved performance compared to the fully protonated phases. The performance in half-cells reaches around 168 mAh/g, with high retention of 42 mAh/g at 10 A/g. This translates to an energy of 88 Wh/kg for a full-cell with a maximum power of 9.2 kW/kg and high cycling stability over 1000 cycles.

Asymmetric hybrid supercapacitors (AHSCs) combine high specific energy and power by merging two electrodes with capacitive and Faradaic charge storage mechanisms. In this study, we introduce AHSC cells that use lithium titanate and activated carbon electrodes in an alkali-ion containing ionic liquid electrolyte. With this cell concept, it is possible to operate the activated carbon electrode in a higher potential window. Consequently, higher cell voltages and a reduced carbon electrode mass can be used, resulting in significantly increased energy compared to aqueous or organic electrolytes. We demonstrate the feasibility of this cell concept for both lithium- and sodium-ion intercalation, underlining the general validity of our approach. Our prototype cells already reach high specific energies of 100 W h/kg, while maintaining a specific power of up to 2 kW/kg and cycling stability of over 1500 cycles. Owing to the IL electrolyte, stable cycling of an AHSC at 80 °C is demonstrated for the first time.

In recent years, a wealth of new desalination technologies based on reversible electrochemical redox reactions has emerged. Among them, the use of redox-active electrolytes is highly attractive due to the high production rate and energy efficiency. Yet, these technologies suffer from the imperfect permselectivity of polymer membranes. Our present work demonstrates the promising desalination performance of a sodium superionic conductor (NASICON) for selective removal of sodium against iodide in a half-cell configuration consisting of an activated carbon electrode in aqueous 600 mM NaI solution. For feedwater with aqueous 600 mM NaCl, the desalination cell exhibited a stable performance over a month with more than 400 operation cycles with the aid of high sodium permselectivity of the NASICON membrane against iodide (99.9–100%). The cell exhibited a maximum sodium removal capacity of 69 ± 4 mg/g (equivalent to the NaCl salt uptake capacity of 87 ± 4 mg/g) with a charge efficiency of 81 ± 3%.

Over recent decades, a new type of electric energy storage system has emerged with the principle that the electric charge can be stored not only at the interface between the electrode and the electrolyte but also in the bulk electrolyte by redox activities of the electrolyte itself. Those redox electrolytes are promising for non-flow hybrid energy storage systems, or redox electrolyte-aided hybrid energy storage (REHES) systems; particularly, when they are combined with highly porous carbon electrodes. In this review paper, critical design considerations for the REHES systems are discussed as well as the effective electrochemical characterization techniques. Appropriate evaluation of the electrochemical performance is discussed thoroughly, including advanced analytical techniques for the determination of the electrochemical stability of the redox electrolytes and self-discharge rate. Additionally, critical summary tables for the recent progress on REHES systems are provided. Furthermore, the unique synergistic combination of porous carbon materials and redox electrolytes is introduced in terms of the diffusion, adsorption, and electrochemical kinetics modulating energy storage in REHES systems.

In recent years, electrochemical water desalination with battery electrode materials has emerged as a promising solution for energy-efficient salt-water desalination. Here, we report the promising desalination performance of a hydrothermally synthesized vanadyl phosphate material (mixed phases of sodium vanadyl phosphate dehydrate and vanadyl hydrogen phosphate hemihydrate) as a new electrode material. We observed robust stability of the synthesized electrode material over 280 cycles during desalination operation for 100 mM NaCl feedwater which was continuously flowing along the electrode material. During the first 100 cycles, the charge storage capacity was enhanced by 47%. This enhancement seems to be caused by a continuous conversion to vanadyl phosphate monohydrate from initial phases according to the post-mortem analysis by X-ray diffraction and infrared spectroscopy. The maximum sodium uptake capacity of the vanadyl phosphate electrode was 24.3 mg g−1 with charge efficiency of around 85%. We found no detectable level of contamination by phosphor nor vanadium from the treated water stream indicating that our synthesized electrode is also environmentally safe for water desalination applications.