Intercalation Of Cations Research Articles

Graphene delamination using 'electrochemical methods': an ion intercalation effect.

The mechanism of graphene delamination from a Pt catalyst growth surface with electrochemical methods is studied. After a water intercalation step, an electrochemical graphene delamination process is done with a variety of different electrolytes. It is shown that (hydrogen or oxygen) bubble formation is not the main driving force to decouple graphene from its catalyst growth substrate. Ion intercalation is identified as the primary component for a fast graphene delamination process from its catalytic growth substrate. When the Pt/graphene sample is negatively charged, cations will intercalate, assuming they do not reduce within the electrochemical window of the solvent. This cation intercalation does result in graphene delamination. In the same way, anions intercalate in positively charged Pt/graphene samples when they do not react within the electrochemical window of the solvent. Furthermore, it is shown that applying a potential is sufficient (current is not needed) to induce ion intercalation and, as a result, graphene delamination. These findings open the door to avoid Na+ or K+ contamination introduced during currently described electrochemical graphene delamination. Alternative electrolytes (i.e. ammonium hydroxide and tetraethylammonium hydroxide) are proposed, due to the absence of alkali contaminants and rapid cation intercalation to delaminate graphene.

Benign Solvation Effect on Electrochemical Intercalation of Triethylmethyl Ammonium into Graphite from Propylene Carbonate

Tetraethyl ammonium/triethylmethyl ammonium tetrafluoroborates dissolved in propylene carbonate (PC) are the most common non-aqueous electrolyte solutions for supercapacitors using both carbonaceous electrodes. In this work, they are applied in the graphite/activated carbon asymmetric capacitors, in which the charge storage at graphite negative electrode is based on the intercalation of quaternary alkyl ammonium cations. The capacitor containing triethylmethyl ammonium (TEMA+) shows a much higher capacity and superior cycle-ability than that containing tetraethyl ammonium (TEA+). In situ X-ray diffraction and in situ dilatometry are combined to determine the larger recoverable expansion in graphite negative electrodes due to a reversible intercalation of TEMA+. Electrochemical quartz crystal microbalance verifies the dramatic structural changes stem from the considerable co-intercalation PC molecules together with TEMA+. A benign solvation effect is proposed for facilitating the transportation and storage of TEMA+ inside graphite.

Prediction of optimal structural water concentration for maximized performance in tunnel manganese oxide electrodes.

Crystal water has been shown to stabilize next-generation energy storage electrodes with structural tunnels to accommodate cation intercalation, but the stabilization mechanism is poorly understood. In this study, we present a simple physical model to explain the energetics of interactions between the electrode crystal lattice, structural water, and electrochemically cycled ions. Our model is applied to understand the effects of crystal water on sodium ion intercalation in a tunnel manganese oxide structure, and we predict that precisely controlling the crystal water concentration can optimize the ion intercalation voltage and capacity and promote stable cycling. The analysis yields a critical structural water concentration by accounting for the interplay between elastic and electrostatic contributions to the free energy. Our predictions are validated with first-principles calculations and electrochemical measurements. The theoretical framework used here can be extended to predict critical concentrations of stabilizing molecules to optimize performance in next-generation battery materials.

Electron Conduction in Nanoparticle Agglomerates Limits Apparent Na+ Diffusion in Prussian Blue Analogue Porous Electrodes

Prussian Blue and its analogues (PBAs) are promising cation intercalation materials for energy storage and environmental applications. Here, we investigate Na+ diffusion in porous electrodes comprised of nickel hexacyanoferrate (NiHCF) PBA nanoparticles (NPs), conductive carbon additive, and polymer binder. We combine experimental characterization, an electronically limited version of porous electrode theory, and simulation to link rate limitations in galvanostatic cycling to electron conduction through NP agglomerates. Using potentiostatic intermittent titration (PITT), we find that the apparent diffusion coefficient of Na+ within NiHCF electrodes varies non-monotonically between 10−11 cm2/sec (at 50% degree of intercalation, DOI) and 10−10 cm2/sec (at DOIs of 0% and 100%). Galvanostatic cycling of electrodes with different average NP-agglomerate sizes reveals that two-fold higher rate capability is achievable when agglomerate radius reduces two-fold, despite having the same NP size distribution. We subsequently introduce and validate theory that explains the variation of diffusion coefficient with DOI, yielding a simple expression for the apparent diffusion coefficient that is proportional to the effective electronic conductivity through NP agglomerates. Finally, using DOI-dependent PITT data we model galvanostatic (dis)charge through electroactive spheres and show agreement with experimental results, confirming that electron conduction through NP agglomerates limits the rate capability of NiHCF electrodes.

Host-Guest Interaction Creates Hydrogen-Evolution Electrocatalytic Active Sites in 3d Transition Metal-Intercalated Titanates.

The hydrogen evolution reaction (HER) is involved in energy-intensive water- and chlor-alkali electrolyzers, and thus, highly active and stable HER electrocatalysts in alkaline media are needed. Titanates, a family of representative two-dimensional materials with negatively charged main layers, are chemically and structurally stable under strongly basic conditions, but they have never been shown to have electrocatalytic activity for HER. Herein, we report that intercalating 3d metal cations, including Fe3+, Co2+, Ni2+, and Cu2+ ions, into the interlayer regions of titanates yields efficient and robust electrocatalysts for the alkaline HER. The intercalation of 3d metal cations in titanates is achieved by rapid cation-exchange reaction between Na+-containing titanates and 3d metal cations at room temperature. Among the 3d metal-intercalated titanates we synthesize, the Co2+-containing material is found to show the best electrocatalytic activity. Experimental and theoretical results reveal that the strong electronic interaction between 3d metal cations and negatively charged main [TiO6]∞ layers renders good catalytic activity to the outermost oxygen atoms in the [TiO6]∞ layer, further making 3d metal-intercalated titanate an efficient electrocatalyst for the HER.

Mitigating Cation Diffusion Limitations and Intercalation-Induced Framework Transitions in a 1D Tunnel-Structured Polymorph of V2O5

The design of cathodes for intercalation batteries requires consideration of both atomistic and electronic structure to facilitate redox at specific transition metal sites along with the concomitant diffusion of cations and electrons. Cation intercalation often brings about energy dissipative phase transformations that give rise to substantial intercalation gradients as well as multiscale phase and strain inhomogeneities. The layered α-V2O5 phase is considered to be a classical intercalation host but is plagued by sluggish diffusion kinetics and a series of intercalation-induced phase transitions that require considerable lattice distortion. Here, we demonstrate that a 1D tunnel-structured ζ-phase polymorph of V2O5 provides a stark study in contrast and can reversibly accommodate Li-ions without a large distortion of the structural framework and with substantial mitigation of polaronic confinement. Entirely homogeneous lithiation is evidenced across multiple cathode particles (in contrast to α-V2O5 partic...

Efficient cationic agents for exfoliating two-dimensional nickel oxide sheets

Two-dimensional catalysis is a very active research area in recent years. Water splitting catalysis may be achieved with the outstanding efficiency of the nickel oxyhydroxide (NiOOH) catalyst. NiOOH is a layered material, which constitutes of partially hydrogenated nickel oxide (NiO2) layers that are stacked together with the possible intercalation of water and electrolyte species. Recently, various studies have reported that NiO single sheets could be prepared and exhibit interesting photocatalytic properties. However, there is a lack of knowledge in developing efficient exfoliation processes adapted to such materials. We investigate with density functional theory the effect of intercalating cations, which is a strategy demonstrated recently to be efficient for exfoliation of various metal oxides. We observed that earth–alkali cations significantly increase the strength and amount of H bonds and inhibit exfoliation. Hence, we conclude that small alkali species are preferable for an exfoliation process of NiOOH.

Optimisation of electrolytic solvents for simultaneous electrochemical exfoliation and functionalisation of graphene with metal nanostructures

The development of a simple, simultaneous electrochemical exfoliation and functionalisation of graphene with metal nanostructures in a one-pot, single step process is reported. This approach is useful in terms of the reduction in processing time and cost, as well as aiding the control of the aggregation of graphene sheets. This first part of this work compares the efficiency of electrochemical graphite exfoliation in dimethyl sulfoxide (DMSO), N-methylpyrrolidone (NMP) and in a mixture of dimethyl carbonate (DMC) and ethylene carbonate (EC) in an electrolyte consisting of LiClO4 and tetraethylammonium tetrafluoroborate. In the second part, the best performing electrolytic solvent was used for in-situ functionalisation of graphene sheets with gold or cobalt nanostructures. The formation of solid layer electrolyte interface in the DMC/EC system is believed to stabilise the graphite from premature exfoliation and allowed the ions to intercalate efficiently to produce a relatively high yield of monolayer graphene sheets. By contrast, the electrochemical exfoliation of graphite in the other two solvents (DMSO and NMP) produced lower yields of few layer graphene. In particular, the co-intercalation of DMSO fragments the electrode by its decomposition by-products (sulfur/carbon oxides) before sufficient cation intercalation occurs. The simulations electrochemical exfoliation and functionalisation of graphene at a single applied potential in the presence of Au salt in DMC/EC solution resulted in the functionalisation of graphene sheets with a variety of high surface area Au nanowhiskers, nanodendrites, nanowires and lamellar nanoparticles. Alternatively, the use of Co(II) salt in the exfoliation solution resulted in the co-deposition of uniformly grown Co nanoparticles on graphene sheets. The metal-functionalised graphene sheets showed high catalytic activity and stability when used as an electrocatalyst for hydrogen evolution reactions. This process could be extended to other metal salts, or mixtures of metal salts, to form graphene-metal alloy composites for use in various applications.

Na-Cation-Assisted Exfoliation of MX2 (M = Mo, W; X = S, Se) Nanosheets in an Aqueous Medium with the Aid of a Polymeric Surfactant for Flexible Polymer-Nanocomposite Memory Applications.

2D nanosheets of transition metal dichalcogenides (TMDCs) have been attracting attention due to their sizable band gap. Facile and effective Na-cation-assisted exfoliation of TMDC (MX2 , M = Mo, W; X = S, Se) nanosheets in an aqueous medium and their application as a composite filler in a polyvinyl alcohol (PVA) matrix are explored in this work. The presence of Na cations is highly beneficial for exfoliating defect-free and few-layer MX2 nanosheets in water in the presence of small-sized micelles of polymeric surfactant, and significantly elevates the exfoliation yield by more than one order of magnitude compared to a conventional surfactant-assisted exfoliation. The strategy suggested in this work is very advantageous compared to both Li cation intercalation in organic solvents and conventional low-yield surfactant-assisted exfoliations. As an application of the exfoliated nanosheets, the fabrication of memory devices with the configuration of Ga-doped ZnO/MX2 -PVA/Ag is demonstrated, and they exhibit bistable and write-once-read-many-times resistive switching behavior with a high ON/OFF current ratio of 3 × 103 at -1.0 V (for WS2 ) and 2.0 V (for MoS2 ). Furthermore, MX2 -PVA nanocomposite fibrous films and mats are successfully fabricated using an electrospinning technique, which can expand the use of TMDC nanofillers in applications involving highly flexible polymer-based MX2 composites.

Effect of Intercalated Metals on the Electrocatalytic Activity of 1T-MoS2 for the Hydrogen Evolution Reaction

We show that intercalation of cations (Na+, Ca2+, Ni2+, and Co2+) into the interlayer region of 1T-MoS2 is an effective strategy to lower the overpotential for the hydrogen evolution reaction (HER). In acidic media the onset potential for 1T-MoS2 with intercalated ions is lowered by ∼60 mV relative to that for pristine 1T-MoS2 (onset of ∼180 mV). Density functional theory (DFT) calculations show a lowering in the Gibbs free energy for H-adsorption (ΔGH) on these intercalated structures relative to intercalant-free 1T-MoS2. The DFT calculations suggest that Na+ intercalation results in a ΔGH close to zero. Consistent with calculation, experiments show that the intercalation of Na+ ions into the interlayer region of 1T-MoS2 results in the lowest overpotential for the HER.

Surfactant-intercalated smectite modified electrode: sensitive electrochemical detection of methyl orange dye

ABSTRACTThis work describes the use of organosmectite modified electrode to evaluate the electrochemical behaviour and to develop an electroanalytical procedure for the determination of methyl orange (MO) dye in natural water. Organosmectites were prepared by intercalation of hexadecyltrimethylammonium cations at various ratios into the interlayer of smectite. The synthesised organosmectites were characterised by various physicochemical techniques such as Fourier transform infrared spectroscopy, X-ray diffraction, field emission scanning electron microscopy and transmission electron microscopy. An amperometric sensor based on organosmectite as electrode modifier for MO sensing purposes was then evaluated by means of clay-film modified electrode using square wave voltammetry (SWV). The electrochemical procedure for MO analysis by stripping voltammetry involves two successive steps: accumulation of MO at open circuit conditions followed by a voltammetric detection in a same medium by the SWV technique. The peak current obtained (after 5 min preconcentration of 15 µmol L−1 MO solution) on a glassy carbon electrode coated by a thin film of the modified clay was more than 2.5 times higher than that exhibited by the same substrate covered by a film of the pristine clay. Under optimised conditions, a linear calibration curve for MO was obtained in the concentration range from 0.1 to 1.6 µmol L−1, leading to a detection limit of 4 × 10−8 mol L−1 (signal-to-noise ratio equal to 3). The interfering effect of various inorganic and organic ions likely to influence the stripping determination of the MO was also examined. To further validate application of this sensor, the proposed method was successfully used to the determination of MO in natural water with satisfactory results.

Imaging emulsions: The effect of salinity on North Sea oils

A workflow to quantify emulsification in water-in-oil (W/O) systems by means of optical microscopy has been applied to study the effect of salinity on emulsification with addition of KCl solutions with increasing concentration to a crude oil sample. A second order relation between droplet size and salinity has been observed for KCl. The effect of the addition of a different salt (NaCl) has also been studied, thereby monitoring the relation between droplet size and the size of the cation. For KCl the droplet size increases up until 5 w% after which it decrease; for NaCl the behaviour is opposite. The effect of interfacially active material isolated from the crude sample has been tested, and the droplet size is substantially affected, with the formation of droplets of considerably smaller diameter. We propose that this observation is due to the presence of two regimes: one where the droplet motive is strongly impacted by cation intercalation in the micellar structure (large droplets) and one where the anions determines the outcome by binding to the surfactants (small droplets).

Nickel Hexacyanoferrate Electrodes for Continuous Cation Intercalation Desalination of Brackish Water

Using porous electrodes containing redox-active nickel hexacyanoferrate (NiHCF) nanoparticles, we construct and test a device for capacitive deionization in a two flow-channel device where the intercalation electrodes are in direct contact with an anion-exchange membrane. Upon reduction of NiHCF, cations intercalate into it and the water in its vicinity is desalinated; at the same time water in the opposing electrode becomes more saline upon oxidation of NiHCF in that electrode. In a cyclic process of charge and discharge, fresh water is continuously produced, alternating between the two channels in sync with the direction of applied current. We present proof-of-principle experiments of this technology for single salt solutions, where we analyze various levels of current and cycle durations. We analyze salt removal rate and energy consumption. In desalination experiments with salt mixtures we find a threefold enhancement for K+ over Na+-adsorption, which shows the potential of NiHCF intercalation electrodes for selective ion separation from mixed ionic solutions.

A rechargeable iodine-carbon battery that exploits ion intercalation and iodine redox chemistry

Graphitic carbons have been used as conductive supports for developing rechargeable batteries. However, the classic ion intercalation in graphitic carbon has yet to be coupled with extrinsic redox reactions to develop rechargeable batteries. Herein, we demonstrate the preparation of a free-standing, flexible nitrogen and phosphorus co-doped hierarchically porous graphitic carbon for iodine loading by pyrolysis of polyaniline coated cellulose wiper. We find that heteroatoms could provide additional defect sites for encapsulating iodine while the porous carbon skeleton facilitates redox reactions of iodine and ion intercalation. The combination of ion intercalation with redox reactions of iodine allows for developing rechargeable iodine–carbon batteries free from the unsafe lithium/sodium metals, and hence eliminates the long-standing safety issue. The unique architecture of the hierarchically porous graphitic carbon with heteroatom doping not only provides suitable spaces for both iodine encapsulation and cation intercalation but also generates efficient electronic and ionic transport pathways, thus leading to enhanced performance.

Layer Structured Materials for Advanced Energy Storage and Conversion.

Owing to the strong in-plane chemical bonds and weak van der Waals force between adjacent layers, investigations of layer structured materials have long been the hotspots in energy-related fields. The intrinsic large interlayer space endows them capabilities of guest ion intercalation, fast ion diffusion, and swift charge transfer along the channels. Meanwhile, the well-maintained in-plane integrity contributes to exceptional mechanical properties. This anisotropic structural feature is also conducive to effective chemical combination, exfoliation, or self-assembly into various nanoarchitectures, accompanied by the introduction of defects, lattice strains, and phase transformation. This review starts with a brief introduction of typical layered materials and their crystal structures, then the structural characteristics and structure oriented unique applications in batteries, capacitors, catalysis, flexible devices, etc., are highlighted. It is surprising to observe that layered materials possess: (1) high reactivity, high reversibility, and enhanced performance via forming additional chemical bonds in alkali-metal ion batteries; (2) facile phase modulation, great feasibility for in-plane/sandwich device design, and cation intercalation enabled high capacitance in supercapacitors; (3) promoted structural diversity, effective strain engineering, and capabilities to function as ideal supporting materials/templates in electrocatalysis field. Finally, the future prospects and challenges faced by layered materials are also outlined.

Enhancing Multifunctionalities of Transition-Metal Dichalcogenide Monolayers via Cation Intercalation.

We have demonstrated that multiple functionalities of transition-metal dichalcogenide (TMDC) monolayers may be substantially improved by the intercalation of small cations (H+ or Li+) between the monolayers and underlying substrates. The functionalities include photoluminescence (PL) efficiency and catalytic activity. The improvement in PL efficiency may be up to orders of magnitude and can be mainly ascribed to two effects of the intercalated cations: p-doping to the monolayers and reducing the influence of substrates, but more studies are necessary to better understand the mechanism for the improvement in the catalytic functionality. The cation intercalation may be achieved by simply immersing substrate-supported monolayers into the solution of certain acids or salts. It is more difficult to intercalate under the monolayers interacting with substrates stronger, such as as-grown monolayers or the monolayers on 2D material substrates. This result presents a versatile strategy to simultaneously optimize multiple functionalities of TMDC monolayers.

(Invited) Using Intercalation Compounds for Electrochemical Water Desalination

Electrochemical desalination technologies offer benefits in both scalability and energy consumption relative to pressure-driven processes (e.g., reverse osmosis). Electrodialysis using gas-evolution electrodes that sandwich a series of ion-exchange membranes is the most used of such technologies to date, but alternative electrochemical technologies using electrosorptive interactions have gained significant research attention to date. For instance capacitive deionization (CDI) uses ion adsorption within solution-phase electric double layers to remove salt. Such technology has been applied readily in brackish water desalination. Alternatively, solid-state faradaic electrodes have also been used to remove salt from water through both intercalation and chemical conversion reactions. Specifically, the desalination battery used a Na-ion intercalation cathode paired with an Ag/AgCl conversion anode.1 Intercalation materials have also been used in conjunction with capacitive electrodes to produce a hybrid CDI configuration with enhanced salt adsorption capacity per unit mass of electrode material.2 Here we report on a desalination device in which cation intercalation host compounds (IHCs) of identical chemical composition are used in both electrodes, a concept originally referred to as Na-Ion Desalination (NID).3 While the use of the same IHC in both electrodes is impractical for battery use, we show through computational modeling3,4 and later confirm by experiments5 that desalination is possible with electrodes of identical composition. We show that this mode of desalination is most efficient if an anion exchange membrane (AEM) is used to suppress Na-ion transport between the electrodes, rather than an unselective porous separator used in Na-ion batteries.3 This concept, which we refer to presently as cation intercalation desalination (CID), can be employed with generic intercalation host compounds. In our original work we predicted the performance of CID using Na0.44MnO2 (NMO) and NaTi2(PO4)3 (NTP) intercalation electrodes, which exhibit sizable volumetric charge capacities and therefore produce substantial degree of desalination even for seawater level salt concentrations. Specifically, a Na0.44MnO2-based CID-cell with 0.5 mm-thick electrodes desalinated 700 mM NaCl influent by 63% while consuming only 50% more energy (0.74 kWh/m3) than thermodynamic minimum when cycled at C/2 rate. The high volumetric charge-capacity of Na-ion battery IHCs enabled 59–64% drop in influent salinity with water-recovery levels up to 80% and 95% for 700 mM and 70 mM influent, respectively. Prussian Blue Analogues (PBAs) are an alternative class of intercalation host compounds that have shown facile intercalation and long cycle life in a variety of aqueous cation batteries. We have modeled a particular PBA, Na2NiFe(CN)6 (NiHCF), in CID operation and show that, despite its low charge capacity relative to NMO and NTP, efficient desalination of seawater-level concentrations is possible in a range of CID device configurations.4 In addition a variety of electrochemical cell architectures were explored ranging from the use of flow-through electrodes and extending to electrodialysis stacks using intercalation electrodes.4 We observe that the distribution of ionic current within flow channels is biased toward the inlet when concentrate and diluate streams flow in the same direction (i.e., parallel-flow configuration), but these effects can be mitigated by flowing concentrate and diluate streams in opposing directions (i.e., in counterflow configuration). Finally, we report on experiments using porous electrodes containing redox-active NiHCF nanoparticles in a CID configuration.5 On the basis of active-material mass NiHCF-based CID cells are shown to achieve a salt adsorption capacity in excess of typical CDI cells. These results show potential for decreased energy consumption relative to CDI cells. Initial results with electrolyte influent containing mixtures of cations also reveal preferential adsorption of K+ over Na+.

Rechargeable aqueous zinc-manganese dioxide batteries with high energy and power densities

Although alkaline zinc-manganese dioxide batteries have dominated the primary battery applications, it is challenging to make them rechargeable. Here we report a high-performance rechargeable zinc-manganese dioxide system with an aqueous mild-acidic zinc triflate electrolyte. We demonstrate that the tunnel structured manganese dioxide polymorphs undergo a phase transition to layered zinc-buserite on first discharging, thus allowing subsequent intercalation of zinc cations in the latter structure. Based on this electrode mechanism, we formulate an aqueous zinc/manganese triflate electrolyte that enables the formation of a protective porous manganese oxide layer. The cathode exhibits a high reversible capacity of 225 mAh g−1 and long-term cyclability with 94% capacity retention over 2000 cycles. Remarkably, the pouch zinc-manganese dioxide battery delivers a total energy density of 75.2 Wh kg−1. As a result of the superior battery performance, the high safety of aqueous electrolyte, the facile cell assembly and the cost benefit of the source materials, this zinc-manganese dioxide system is believed to be promising for large-scale energy storage applications.

Electrochromic properties of TiO2 thin films grown by thermionic vacuum arc method

Titanium dioxide (TiO2) thin films were deposited on fluorine tin oxide/glass and indium tin oxide/glass substrates by thermionic vacuum arc in this work. The deposited films phases were identified as anatase and rutile from X-ray Diffractions. The bandgap of the samples has been found ~3.0eV. The electrochromic properties of TiO2 thin films were investigated using cyclic voltammograms, chronoamperometry and chronocoulometric studies. Electrochemical cation intercalation was promoted using 0.1M LiClO4 in propylene carbonate. Inserted/extracted charge, diffusion coefficient, coloration efficiency, optical modulation, and reversibility was calculated. Best results were obtained for TiO2/FTO sample with 18.6cm2/C coloration efficiency. Deposited TiO2 thin films optical and morphological studies also covered.

Electrochemical Intercalation of Mg2+ into Anhydrous and Hydrated Crystalline Tungsten Oxides

The reversible intercalation of multivalent cations, especially Mg2+, into a solid-state electrode is an attractive mechanism for next-generation energy storage devices. These reactions typically exhibit poor kinetics due to a high activation energy for interfacial charge-transfer and slow solid-state diffusion. Interlayer water in V2O5 and MnO2 has been shown to improve Mg2+ intercalation kinetics in nonaqueous electrolytes. Here, the effect of structural water on Mg2+ intercalation in nonaqueous electrolytes is examined in crystalline WO3 and the related hydrated and layered WO3·nH2O (n = 1, 2). Using thin film electrodes, cyclic voltammetry, Raman spectroscopy, X-ray diffraction, and electron microscopy, the energy storage in these materials is determined to involve reversible Mg2+ intercalation. It is found that the anhydrous WO3 can intercalate up to ∼0.3 Mg2+ (75 mAh g-1) and can maintain the monoclinic structure for at least 50 cycles at a cyclic voltammetry sweep rate of 0.1 mV s-1. The kinetics of Mg2+ storage in WO3 are limited by solid-state diffusion, which is similar to its behavior in a Li+ electrolyte. On the other hand, the maximum capacity for Mg2+ storage in WO3·nH2O is approximately half that of WO3 (35 mAh g-1). However, the kinetics of both Mg2+ and Li+ storage in WO3·nH2O are primarily limited by the interface and are thus pseudocapacitive. The stability of the structural water in WO3·nH2O varies: the interlayer water of WO3·2H2O is removed upon exposure to a nonaqueous electrolyte, while the water directly coordinated to W is stable during electrochemical cycling. These results demonstrate that tungsten oxides are potential candidates for Mg2+ cathodes, that in these materials structural water can lead to improved Mg2+ kinetics at the expense of capacity, and that the type of structural water affects stability.