Proton Intercalation Research Articles

EQCM study of intercalation processes into electrodeposited MnO2 electrode in aqueous zinc-ion battery electrolyte

In this work manganese oxide films were obtained by electrodeposition and their electrochemical and mass transfer processes in aqueous zinc-ion battery electrolyte were studied by cyclic voltammetry and electrochemical quartz crystal microbalance (EQCM). Cyclic voltammograms and corresponding mass variation curves of manganese oxide during charge-discharge processes were examined simultaneously on Au-coated quartz crystal electrodes. The investigations were conducted in aqueous electrolytes of different composition (2 M ZnSO4 and 2 M ZnSO4 + 0.1 M MnSO4). Monitoring of electrode mass variation during potential cycling provides direct evidence that redox processes in MnO2 electrodes co-occur with intercalation of protons and zinc ions. Combined CV and EQCM studies reveal that electrodeposited films of MnO2 are unstable in 2 M ZnSO4 electrolyte. The repeated potential cycling in Zn-containing electrolytes leads to rapid deterioration of electrode capacity in the few initial cycles due to the Zn2+ insertion into subsurface structures of MnO2 and blocking of electroactivity of MnO2 film on Au substrate. On the other hand, reversible processes of intercalation of protons and zinc ions occur in 2 M ZnSO4 + 0.1 M MnSO4 electrolyte. Two main steps of mass increase during the discharging process, taking place at 1.4 V (vs. Zn/Zn2+) and in the potential range (1.3–1.0) V were demonstrated by EQCM. The first step of mass increase is mainly related to the intercalation of H+ (as H3O+), whereas the second step of mass increase is mainly associated with formation of surface compounds like zinc sulfate hydroxide salts.

A Review on the Synthesis and Modification of Functional Inorganic‐Organic‐Hybrid Materials via Microwave‐Assisted Method

AbstractThe microwave‐assisted method is an appropriate route; to overcome the energy barrier for stabilizing perovskite structured based inorganic‐organic‐hybrid materials. The motivation for using the microwave‐assisted method in material science as an effective heating technique results from improvement in the topochemical modification of layered perovskite oxides. The microwave‐assisted method causes a fall in time duration from days to hours/minutes and produces good quality products with high crystallinity. Inorganic‐organic‐hybrids can be derived easily by the stepwise modification of layered perovskites. These modification techniques involve proton exchange, grafting, intercalation, and exfoliation steps. The microwave synthesized inorganic‐organic‐hybrid materials are in high demand due to their various interesting properties, potential catalytic, optical, composite, magnetic, and electronic, making them useful for technological applications.

"Water-in-Sugar" Electrolytes Enable Ultrafast and Stable Electrochemical Naked Proton Storage.

Proton is an ideal charge carrier for rechargeable batteries due to its small ionic radius, ultrafast diffusion kinetics and wide availability. However, in commonly used acid electrolytes, the co-interaction of polarized water and proton (namely hydronium) with electrode materials often causes electrode structural distortions. The hydronium adsorption on electrode surfaces also facilitates hydrogen evolution as an unwanted side reaction. Here, a "water-in-sugar" electrolyte with high concentration of glucose dissolved in acid to enable the naked proton intercalation, as well as an extended 3.9V working potential window, is shown. A glucose-derived organic thin film is formed on electrode surface upon cycling. Molecular dynamics simulations reveal the significant decrease of free water in bulk electrolytes, while density functional theory calculations indicate that glucose preferentially binds to the electrode surface which can inhibit water adsorption. The scarcity of free water and the protective organic film work in synergy to suppress water interactions with the electrode surface, which enables the naked proton (de)intercalation. The "water-in-sugar" electrolyte significantly enhances a MoO3 electrode for stable cycling over 100000 times. This facile electrolyte approach opens new avenues to aqueous electrochemistry and energy storage devices.

Oxygen vacancy-rich WO3 heterophase structure: A trade-off between surface-limited pseudocapacitance and intercalation-limited behaviour

Intercalation-pseudocapacitance materials are attracting increasing interest as promising electrodes for use in high-capacitance supercapacitors. However, these materials typically exhibit unsatisfactory rate performances due to their relatively slow cation-insertion process. Under high mass loading, their rate performances are even further degraded. Herein is presented our fabrication of an oxygen vacancy-rich h-WO3/ort-WO3·0.33H2O heterophase structure (HOHS) by a facile hydrothermal synthesis. The HOHS has a split-level nanotubes-on-nanoplates morphology and its formation and energy-storage mechanisms are discussed in detail. The HOHS exhibits a collaborative charge-storage mechanism involving surface redox and proton intercalation, and the capacitance contribution associated with the proton intercalation can be regulated over a wide range. By achieving a trade-off between the surface-limited pseudocapacitance and intercalation-limited behaviours and regulating its morphology, the HOHS electrode with an ultra-high mass loading of 10.8 mg cm−2 delivers a high areal capacitance of 2552 mF cm−2 at 1 mA cm−2 and excellent long-term stability. More importantly, the rate performance of the HOHS (78% capacitance retention at 20 mA cm−2 in comparison to 1 mA cm−2) is better than those reported for WO3-based materials. This strategy opens avenues for the fundamental study of the regulation of the energy storage mechanism and the achievement of a trade-off between the capacitance and rate capability in high-mass-loading electrodes.

The Importance of Li-Ion Nature and Stacking in Layered Cathode Materials for Aqueous Ion Batteries: From Bulk to Surface Models

The performance of aqueous ion batteries has been demonstrated since the foundational work from Dahn et al. who combined an anode of VO2 and a cathode of LiMn2O4 in an aqueous electrolyte. The use of aqueous electrolytes opens the possibility of proton co-insertion, one of the common assumptions to explain performance issues in aqueous electrolytes. Experimental and computational works compared the feasibility of several cathode materials to be intercalated by H+ concluding likewise that non-layered cathode materials are less prone to proton intercalation than layered materials.Therefore, layered compounds should not be good candidates as cathodes of aqueous batteries, although Li+ insertion/extraction using high concentrated LiNO3 aqueous solution as electrolyte, show results consistent with the Li (de)intercalation in organic electrolytes between the ranges from x equals 0.5-1.0 in layered LixCoO2. Thus, could layered materials have a good performance in aqueous environment despite of their tendency to accommodate protons?In this work, we use ab-initio computations to investigate the behavior of LixCoO2 in aqueous electrolytes analyzing the proton insertion to determine which specific chemical and structural features lead to electrochemically stable materials for aqueous batteries by means of bulk and surface models. The investigations have concerned layered NaxCoO2 in order to investigate the role of stacking and ion material.Our simulations reveal the excellent performance of LixCoO2 in contrast to NaxCoO2. By means of DFT calculations, the H+/ion exchange have been studied and the grand potential phase diagram and voltage-composition curves have been built considering different phases of LixCoO2 and NaxCoO2. Our simulations predicts the CoO(OH) as the most stable insertion/extraction product at battery operation conditions, due to the formation of O-H-O bonds. However, LixCoO2 is not affected by the presence of protons since they co-existence is not stable in comparison to full lithiated and full protonated systems. Our computations reveal that H+ could have a secondary role as surface detrimental agent, without compromising the battery performance. Nevertheless, the nature of Na and the stacking of NaxCoO2 favors the accommodation of protons, showing different performance than LixCoO2, due to the different stacking and different behavior of Na.Our work suggests that to be layered is not enough justification to be discarded as cathode material for aqueous electrolytes. We suggest that the stacking, the alkali-proton repulsion, and specially the operation conditions (voltage window, pH...) are key factors to investigate the suitability or not of a layered material as positive electrode for ion aqueous batteries.

Optimal Linear Water Density for Proton Transport in Tunnel Oxides

Proton intercalation and transport is key to pseudocapacitive energy storage of oxide electrodes in acidic electrolytes. Although proton transport in water confined in layered or 2D materials is attracting great interest, much less is known about how the proton is transported in the 1D channels of tunnel oxides such as hexagonal WO3 (h-WO3). Here, we use first-principles molecular dynamics to reveal an optimal linear density of four water molecules per nanometer that yields the highest proton diffusivity. The volcano shape of proton diffusivity versus linear water density is a result of balancing the linear hydrogen-bond chain and the rotation of the water molecules to enable the Grotthuss mechanism. This insight provides a unifying view of proton transport along a single file of water molecules confined in hydrophilic 1D channels.

High-Capacity Aqueous Storage in Vanadate Cathodes Promoted by the Zn-Ion and Proton Intercalation and Conversion-Intercalation of Vanadyl Ions.

Aqueous Zn-ion batteries (AZIBs) are promising alternatives to lithium-ion batteries in stationary storage. However, limited storage capacity and cyclic life impede their large-scale implementation. We report reversible electrochemical insertion of multi-ions into sodium vanadate (NaV3O8) cathode materials for AZIBs, achieving a maximum storage capacity of 450 mAh g-1 at 0.05 A g-1 and a capacity retention of 82% after 500 cycles at 0.4 A g-1. In addition to Zn2+ and H+ insertion, in situ X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) collectively provide explicit evidence on vanadyl ions (VO2+) conversion-intercalation at the NaV3O8 cathode, showing the deintercalation of VO2+ from NaV3O8 and the consequent conversion of VO2+ into V2O5 on charging, and vice versa on discharging. Our study is the first to report on the cation conversion-intercalation mechanism in AZIBs. This reversible multi-ion storage mechanism provides a design principle for developing high-capacity aqueous electrode materials by engaging both the intercalation and conversion of charge carriers.

Electrochemical Proton Intercalation in Vanadium Pentoxide Thin Films and its Electrochromic Behavior in the near-IR Region.

This work examines the proton intercalation in vanadium pentoxide (V2O5) thin films and its optical properties in the near‐infrared (near‐IR) region. Samples were prepared via direct current magnetron sputter deposition and cyclic voltammetry was used to characterize the insertion and extraction behavior of protons in V2O5 in a trifluoroacetic acid containing electrolyte. With the same setup chronopotentiometry was done to intercalate a well‐defined number of protons in the HxV2O5 system in the range of x=0 and x=1. These films were characterized with optical reflectometry in the near‐IR region (between 700 and 1700 nm wavelength) and the refractive index n and extinction coefficient k were determined using cauchy’s dispersion model. The results show a clear correlation between proton concentration and n and k.

Electrochemical performance of spherical LiCoO2 with high stability in neutral aqueous electrolyte

Layer-structured lithium cobalt oxide (LCO) is one of promising electrode materials for secondary aqueous lithium-ion batteries, yet the effect of structural proton insertion in LCO in neutral aqueous electrolytes cannot be ignored. Present study investigates the electrochemical performance of polycrystalline spherical LCO in neutral aqueous saturated Li2SO4 solution. Herein, we for the first time demonstrate the dependence of LCO stability on the discharge cutoff potential. The applied LCO electrodes show good cycling stability within the potential window of 0.65–1.1 V vs. SCE, while electrochemical impedance spectrum (EIS) analysis detects no sign of proton intercalation. Moreover, the spherical LCO free from the proton intercalation exhibits a superior rate capability with 78% discharge capacity retention at 80 C. The lithium-ion chemical diffusion coefficient being seven times than that of irregular shaped LCO sample can be responsible for such significant rate capability. The cyclability testing depicts the better performance of spherical LCO in comparison with the counterpart, especially in terms of electrode activation time. Post cycling electrode characterization displays that the discharge capacity fading of LCO mainly results from the crystal grain deformation due to high potential cycling and can be alleviated by reducing the depth of charge.

High-performance multi-dimensional nitrogen-doped N+MnO2@TiC/C electrodes for supercapacitors

Intercalation of alkali cations (Na+, K+, etc.) or protons (H+) on the surface of MnO2 can improve the theoretical pseudocapacitance in the redox reactions. However, the poor electrical conductivity and vulnerability to agglomeration and dissolution during the redox reactions on MnO2 have hampered practical application. In this work, a network composed of TiC/C nanowires is prepared on Ti6Al4V to produce a conductive three-dimensional substrate on which two-dimensional MnO2 nanosheets are fabricated to form a multi-dimensional MnO2@TiC/C composite electrode. The three-dimensional TiC/C nanowire network increases the specific surface area between the active species and electrolyte and electron transport efficiency to yield a specific capacitance of 284.8 F g−1 or 76.0 mAh g−1. To further improve the conductivity and increase the active sites on the pseudocapacitive materials, nitrogen is incorporated into MnO2 to form the N+MnO2@TiC/C electrode with a specific capacity of 371.1 F g−1 or 103 mAh g−1 rendering it very attractive to high-performance supercapacitors. After nitrogen doping, the cyclic stability of the electrode is improved greatly, and 91% retention of the gravimetric capacity is accomplished after 50,000 cycles. The mechanism is investigated by analyzing the pseudocapacitances of the composite electrodes with and without N doping. Nitrogen doping not only changes the elemental composition of the electrode and enhances the conductivity, but also increases the active sites on MnO2. As a result, the pseudocapacitance contribution increases from 66.4% to 76.3% at 1 mV s−1. The supercapacitor constructed with N+MnO2@TiC/C as the positive electrode and nickel foam coated with activated carbon (AC) as the negative electrode shows excellent power density of 5400 W kg−1 at 23.9 Wh kg−1. Owing to the excellent electrochemical characteristics and cycling stability, the materials and associated design concept have large potential in energy storage applications.

Charge-storage mechanism of highly defective NiO nanostructures on carbon nanofibers in electrochemical supercapacitors.

An electrode composed of highly defective nickel oxide (NiO) nanostructures supported on carbon nanofibers (CNFs) and immersed in an Li+-based aqueous electrolyte is studied using Raman spectroscopy under dynamic polarization conditions to address the charge-storage phenomenon. By this operando technique, the formation of Li2SO4·H2O during the discharge process is verified. At the same time, we observed the phase transformation of NiO to NiOOH. The Ni(OH)2/NiOOH redox couple is responsible for the pseudocapacitive behavior with intercalation of cationic species in the different Ni structures. A 'substitutive solid-state redox reaction' is proposed to represent the amphoteric nature of the oxide, resulting in proton intercalation, while the insertion of Li+ occurs to a less extent. The electrode material exhibits outstanding stability with 98% coulombic efficiency after 10 000 charge-discharge cycles. The excellent electrode properties can be ascribed to a synergism between CNFs and NiO, where the carbon nanostructures ensured rapid electron transport from the hydrated nickel nanoparticles. The NiO@CNF composite material is a promising candidate for future applications in aqueous-based supercapacitors. DFT simulation elucidates that compressive stress and Ni-site displacement lead to a decrease up-to 3.5-fold on the electron density map located onto the Ni-atom, which promotes NiO/Ni(OH)2/NiOOH transition.

Building Analogies between the Thermal and Electrochemical Reactivity of Hydrogen Using Proton-Intercalating Metal Oxides

Motivated by the challenges of intermediate scaling relations, our research group is working to develop catalytic assemblies that integrate electrochemical and thermal activation to accelerate multi-step redox reactions. This presentation focuses on one approach that involves using transition metal hydrogen bronze compounds (HxMOy) as solid-state hydrogen sources where the hydrogen can be readily replenished via proton intercalation from water or acid equivalents. This approach enables the design of "thermo-electrochemical" reactor systems based on spatial or temporal coupling of thermal and electrochemical redox reactions. Initial modeling and experimental results will be discussed, which indicate that it is possible to execute hydrogenation reactions under conditions that would be impossible to achieve using traditional thermal hydrogenation reactors and catalysts. Figure 1

Shedding Light about the Role of Proton Intercalation in Layered Cathode Materials By Means of First Principle Computations

Flammability, toxicity, and cost are only a few drawbacks about the use of organic liquid electrolytes. On the other hand, working in aqueous electrolyte would therefore be extremely beneficial especially to lower production cost and obtain higher safety and lower toxicity.In contrast to organic electrolytes, where only the inserted metal ions can (de)intercalate in the electrode material, the use of aqueous electrolytes opens the possibility of proton co-insertion. But are protons able to intercalate in the electrodes? and if the answer is affirmative, which is the effect of proton intercalation in the battery performance?Several (electro)chemical studies combined with first-principles computations have confirmed the easy protonation of layered electrodes at different ranges of pH values and this fact has been directly related with the detrimental in cathode materials. Nevertheless, several studies reported the excellent performance of layered LiCoO2 and LiCo1/3Ni1/3Mn1/3O2 as positive electrode -without coating- using neutral or basic salt in water electrolytes. Thus, it remains an open question: Why these layered cathode materials so prone to proton intercalation could perform as well in aqueous environment?In this work, we use density functional computations to investigate the lithium/proton exchange in some well-known cathode materials. The analysis of the proton insertion mechanism has been carried out in the bulk and surfaces of these materials in order to demonstrated which specific chemical and structural features lead to electrochemically stable materials for aqueous batteries. To analyze the Li+/H+ exchange, several Li+/H+/vacancy orderings and arrangements were computed to study the phase stability at different pH values. This study provides fundamental investigations about the role of H-intercalation in the performance of aqueous battery operations.

(Invited) Self-Doped TiO2 Nanotube Array for Photoelectrochemical Water Treatment: Effects of Cathodization Condition

Electrochemical self-doping of TiO2 by cathodization in mild conditions has been evinced to enhance the photoelectrochemical (PEC) activities for water treatment and molecular hydrogen generation. In this study, the effects of doping conditions such as annealing, current density, electrolyte (pH, buffering intensity), and duration were systematically interrogated with respect to the physico-chemical properties and water treatment efficacy of the TiO2 nanotube array (TNA). The incident-photon-to-electron conversion efficiency (IPCE) and the rates of model organic compounds degradation were comparatively evaluated. The self-doping of crystalline (anatase) TNA reduced the band gap to 2.4 eV (blue TNA), while the same sequence for amorphous TNA even further narrowed the gap to 1.4 eV (black TNA). The level of passed charge during the cathodization showed marginal influence. Depth profiles of X-ray photoelectron spectroscopy indicated that the dopants (Ti3+) were exclusively located on surface for the blue TNA, whereas they penetrated into the bulk tube structure for the black TNA which was accompanied with quasi-permanent lattice distortion as confirmed by X-ray diffraction patterns. Therefore, the electrical conductivity in terms of donor density was found to be greater for the black TNA. The buffering intensity of the cathodization electrolyte switch the relative level of proton intercalation in comparison with oxygen vacancy formation, which was demonstrated by time-of-flight and dynamic secondary ion mass spectrometry analysis. Even though the black TNA showed superior double layer capacitance and dark electrochemical oxidation, the blue TNA marked greater IPCE and PEC water treatment efficiency. The findingss of this study would broaden the usage of TiO2 nanomaterials by tuning the physico-chemical characteristics depending on variable water treatment purposes (mineralization of organic compounds versus disinfection).

Effect of microstructure change on resistance of spherical LiCoO2 to electrode degradation for proton intercalation

Lithium cobalt oxide (LCO) with layered crystal structure suffers the structural proton intercalation in aqueous electrolytes of low pH values, little information is available about the effect of microstructure change on the cycling stability of LCO in response to the proton intercalation. In this work, electrochemical properties of three kinds of LCO spheres with different microstructures are studied in neutral aqueous 0.5 M Li2SO4 solution. The investigated materials were obtained by calcining the spherical LCO precursors at various temperatures, which were synthesized via a modified solid phase method for lithiation of spherical Co3O4. Structure and morphology of materials were characterized by X-ray diffraction (XRD) and field emission scanning electron microscope (FE-SEM). The spherical LCO prepared at lower temperature shows more superior electrochemical stability. Herein, the resistance of spherical LCO with a particular microstructure to the electrode degradation for the proton intercalation can be measured by the frequency of occurrences of greater-than-100% coulombic efficiencies during cycling. Meanwhile, the capability of retaining the capacity contribution from the order-to-disorder transformation of lithium ions on the hexagonal lattice of host site after the first-order phase transition was proposed to compare and investigate the cyclability of the three kinds of spherical LCO with different microstructures.

Two-Phase Electrochemical Proton Transport and Storage in α-MoO3 for Proton Batteries

Summary Diffusion-controlled charge storage and phase transitions of electrodes are typical indicators of sluggish kinetics in battery chemistries. However, fast rate capabilities are found in an α-MoO3 proton intercalation electrode that presents both features. Here, the unique topochemistry is shown to involve multiple ion-electrode interactions and proceeds via two key steps: hydronium adsorption on surfaces and proton insertion into bulk lattices. This triggers structure transitions from MoO3 to hydrogen molybdenum bronzes (HMBs). Following the first process, subsequent rearrangements proceed only among HMBs phases with high reversibility and kinetics, thus providing structural explanations to the fast rate capability. At electrode-electrolyte interfaces, hydronium is the active charge carrier that initiates charge transfer and surface hydration, which are accompanied by water adsorption/desorption with reduced polarization and enhanced kinetics. Further water activity is shown to induce material dissolution during function. These findings offer fundamental insights in proton chemistries, which may form the basis of future high rate and capacity energy storage.

Pseudocapacitive contributions to enhanced electrochemical energy storage in hybrid perovskite-nickel oxide nanoparticles composites electrodes

Composite materials of metal halide hybrid perovskites (MBI and MPI) and nickel oxide (NiO) nanoparticles, synthesized by a wet chemical route, were used to fabricate a supercapacitor by solution process spin coating technique. Films of the nanocomposite materials were characterized with different techniques to study their morphology, crystal nature, surface analysis and elemental composition. Charge storage due to surface faradaic and diffusion-controlled proton intercalation processes were distinguished, both qualitatively and quantitatively, by analyzing the cyclic voltammetric (CV) sweep data. With addition of the perovskites, pseudocapacitive contributions become increasingly important and lead to greater amounts of total stored charge. The electrochemical performance of the pristine NiO, MBI@NiO and MPI@NiO based devices were investigated as well as the two composite electrodes and exhibit specific capacitances of 407 Fg−1 and 368 Fg−1, respectively, at 10 mVs−1, with corresponding energy densities of 56.5 Whkg−1 and 51.1 Whkg−1. The devices also show capacitance retention efficiencies of more than 90% after 3000 CV cycles. The device were further investigated by electrochemical impedance spectroscopy and the results are in agreement with CV analysis. Besides the charge storage, the stability benefits derived from decorating perovskites with metal oxide provide an interesting direction for the design of materials that offer both high energy density and power density.

An organic synaptic transistor with Nafion electrolyte

An organic electrochemical transistor with the organic semiconductor poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) film as the channel and the Nafion film as the solid electrolyte has been introduced. The channel current of this transistor can be modulated by proton intercalation driven by the gate voltage pulses. Moreover, the change ratio of the channel current strongly depends on the relative humidity (RH) of environment. With the RH of 26.1%, synaptic plasticity such as short-term potentiation (STP), paired-pulse facilitation (PPF), and the STP to long-term potentiation (LTP) transition have been emulated based on this device, where the channel conductance acts as the synaptic weight and the gate voltage pulses are regarded as the action potential. This kind of organic synaptic transistor is easy for fabrication and stable at temperatures up to 190 °C, which provides a potential candidate for flexible and biocompatible electronic devices.

Gate-Tuned Interlayer Coupling in van der Waals Ferromagnet Fe_{3}GeTe_{2} Nanoflakes.

The weak interlayer coupling in van der Waals (vdW) magnets has confined their application to two dimensional (2D) spintronic devices. Here, we demonstrate that the interlayer coupling in a vdW magnet Fe_{3}GeTe_{2} (FGT) can be largely modulated by a protonic gate. With the increase of the protons intercalated among vdW layers, interlayer magnetic coupling increases. Because of the existence of antiferromagnetic layers in FGT nanoflakes, the increasing interlayer magnetic coupling induces exchange bias in protonated FGT nanoflakes. Most strikingly, a rarely seen zero-field cooled (ZFC) exchange bias with very large values (maximally up to 1.2kOe) has been observed when higher positive voltages (V_{g}≥4.36 V) are applied to the protonic gate, which clearly demonstrates that a strong interlayer coupling is realized by proton intercalation. Such strong interlayer coupling will enable a wider range of applications for vdW magnets.

Protonic solid-state electrochemical synapse for physical neural networks

Physical neural networks made of analog resistive switching processors are promising platforms for analog computing. State-of-the-art resistive switches rely on either conductive filament formation or phase change. These processes suffer from poor reproducibility or high energy consumption, respectively. Herein, we demonstrate the behavior of an alternative synapse design that relies on a deterministic charge-controlled mechanism, modulated electrochemically in solid-state. The device operates by shuffling the smallest cation, the proton, in a three-terminal configuration. It has a channel of active material, WO3. A solid proton reservoir layer, PdHx, also serves as the gate terminal. A proton conducting solid electrolyte separates the channel and the reservoir. By protonation/deprotonation, we modulate the electronic conductivity of the channel over seven orders of magnitude, obtaining a continuum of resistance states. Proton intercalation increases the electronic conductivity of WO3 by increasing both the carrier density and mobility. This switching mechanism offers low energy dissipation, good reversibility, and high symmetry in programming.