Proton Intercalation Research Articles

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.

Protonation of Lix(Co,Ni,Mn)O2 compounds in acidic conditions

The easy intercalation/deintercalation of Li+ ions is the key factor for the functionality of the layered LixCoO2 compound as a Li-ion battery cathode material. However, the hexagonal CoO2-layer piling in the LixCoO2 structure is prone to incorporate not only monopositively-charged lithium ions but also monopositive protons. Here we employ a hydrothermal acid treatment for a gradual (controlled by the treatment time) de-intercalation of Li+ ions and a subsequent intercalation of protons between the CoO2 layers in LixCoO2. We demonstrate that the lithium-for-proton exchange reaction can be monitored by AAS for the Li-content, by XRD for the proton-containing phase formation, and by FTIR and TG for the appearance of specific features indicative of the incorporated protons. With XPS we verify that upon the lithium-for-proton exchange the valence of cobalt remains the same, i.e. the proton incorporation compensates the positive charge loss due to the Li+-ion extraction. Finally, we show that essentially parallel lithium-for-proton exchange reactions occur also for Ni- and Mn-substituted Li(Co,Ni,Mn)O2 samples, that is, for compounds that are highly relevant in current Li-ion battery technology.

Are Protons Involve in the Fading Capacity in Li Ion Aqueous Batteries? Shedding Light By Means of First Principle Computations

Commercial lithium ion batteries are composed of an anode based on carbon, a transition metal oxide cathode separated by an organic liquid electrolyte containing a lithium salt such as LiPF6. Nevertheless, operating with an organic electrolyte presents many drawbacks. The organic electrolyte is flammable, expensive and need to be kept away from any water contamination. 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. The performance of aqueous based LIBs 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.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. Proton intercalation has been one of the common assumptions to explain performance issues in aqueous electrolytes such as capacity fading and the increase in lithium diffusion barriers. 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 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.Given that the importance of proton insertion for aqueous batteries, this study provides fundamental investigations about the role of H-intercalation in the performance of aqueous battery operations.

Vanadium hexacyanoferrate as high-capacity cathode for fast proton storage.

Proton electrochemistry is promising for developing future energy storage devices with both high capacity and good rate capability. However, the development of this technology is now hindered by the limited choice of accessible electrodes, especially for cathodes. Herein, we report vanadium hexacyanoferrate (VHCF) as a candidate cathode for proton batteries. Exploiting dual redox-centers of vanadium and iron, VHCF delivers a high specific capacity of 108 mA h g-1. Furthermore, an outstanding rate capability (∼60% of initial value at 100C) and stable cycling for tens of thousands of cycles are also demonstrated. These results are anticipated to inspire searches for more accessible materials and accelerate advances in energy storage devices.

Interlayer separation in hydrogen titanates enables electrochemical proton intercalation

Interlayer structural protons in H2Ti3O7 are identified as the key structural feature to enable electrochemical proton intercalation beyond the near-surface because they effectively reduce interconnections of the titanate layers.

The synergistic effect of proton intercalation and electron transfer via electro-activated molybdenum disulfide/graphite felt toward hydrogen evolution reaction

Molybdenum disulfide (MoS2) possesses promising prospects of application in electrocatalytic hydrogen evolution. However, the MoS2 prepared by traditional hydrothermal method is usually 2H (hexagonal) semiconducting state with limited active sites and unsatisfied conductivity, hindering the improvement of hydrogen evolution efficiency. Herein, the MoS2 nanoflowers are in-situ grown successfully on graphite felt (GF) by hydrothermal method, and a simple and effective electrochemical activation is proposed to enhance the hydrogen evolution activity of the composite. The electrical activation in acidic medium guides the intercalation of protons between MoS2 and GF, and the increased interlayer spacing between MoS2 is conducive to the adsorption and release of hydrogen. The synergistic effect between MoS2 and GF is enhanced, resulting in accelerated electron transfer through S sites in MoS2 and oxygen-containing functional group in GF. The experimental results and density functional theory (DFT) computations demonstrate that the electrical activated catalyst possesses excellent electrocatalytic activity and long-term operation stability (Tafel slope is 48 mV dec-1, the overpotential is 82 mV at current density of −10 mA cm−2). Our research confirms the electrochemical activation is a simple technique for enhancing the hydrogen evolution efficiency of MoS2/GF composite as well as other transition metal chalcogenides and carbon materials.

Versatile and Highly Efficient Controls of Reversible Topotactic Metal–Insulator Transitions through Proton Intercalation

AbstractThe ability to tailor a new crystalline structure and associated functionalities with a variety of stimuli is one of the key issues in material design. Developing synthetic routes to functional materials with partially absorbed nonmetallic elements (i.e., hydrogen and nitrogen) can open up more possibilities for preparing novel families of electronically active oxide compounds. Fast and reversible uptake and release of hydrogen in epitaxial ABO3 manganite films through an adapted low‐frequency inductively coupled plasma technology is introduced. Compared with traditional dopants of metallic cations, the plasma‐assisted hydrogen implantations not only produce reversibly structural transformations from pristine perovskite (PV) phase to a newly found protonation‐driven brownmillerite one but also regulate remarkably different electronic properties driving the material from a ferromagnetic metal to a weakly ferromagnetic insulator for a range of manganite (La1−xSrxMnO3) thin films. Moreover, a reversible perovskite‐brownmillerite‐perovskite transition is achieved at a relatively low temperature (T ≤ 350 °C), enabling multifunctional modulations for integrated electronic systems. The fast, low‐temperature control of structural and electronic properties by the facile hydrogenation/dehydrogenation treatment substantially widens the space for exploring new possibilities of novel properties in proton‐based multifunctional materials.