Layered MnO2 Research Articles

Structural transformation and electrochemical study of layered MnO2 in rechargeable aqueous zinc-ion battery

Layered MnO2 is very attractive cathode material for zinc-ion battery (ZIB) due to its large interlayer distance, high discharge capacity, low cost, and environmental benignity. However, layered MnO2 exhibits capacity fading. Therefore, detailed studies of the structural transformation and electrochemical mechanism of layered MnO2 during cycling are urgently required for performance improvement. In this contribution, we have utilized in situ synchrotron, ex situ X-ray diffraction, and ex situ synchrotron X-ray absorption spectroscopy analyses in order to evaluate the structural transformation of a layered MnO2 during Zn-ion insertion. We found that during initial cycles, the electrode was able to maintain its layered structure; however, after prolonged cycles, it completely transformed into an irreversible spinel structure. We also observed the manganese dissolution from the electrode into the electrolyte during continuous cycling. The formation of irreversible spinel phase and manganese dissolution are responsible for capacity fading. Our findings provide the understanding for further improvement of layered MnO2 as cathode material for next generation ZIB systems.

Promising electrode material using Ni-doped layered manganese dioxide for sodium-ion batteries

Birnessite MnO2, a hydrated layered manganese dioxide with a layered structure, is a promising candidate for sodium-ion batteries because of a significant interlayer distance for the reversible insertion of sodium ions. This work proposes to improve the electrochemical performance of layered manganese dioxide by metal cation doping. Nickel-doped layered MnO2 (0.05–0.15 wt%) prepared by a sol–gel method using a chelate agent of fumaric acid showed a gradual increase of interlayer distance with an increase of Ni-doping amount. Moreover, ex situ XRD results during the first cycle confirmed a stabilization of the layered structure during the sodium insertion. During the charge–discharge test, the initial capacity of 15% Ni layered MnO2 was 140 mAh/g, with small capacity fade over 20 cycles. Layered structure of MnO2 and sodium diffusion along [001] and [010] direction.

Electrostatically Sprayed Manganese Oxide Films – from Pseudocapacitive Charge Storage Materials to 3D Microelectrode Integrands

Electrochemical capacitors or “supercapacitors” essentially bridge the gap between traditional capacitors and rechargeable batteries with their high specific power and moderate energy densities. Commercial supercapacitors typically comprise activated carbon (AC) electrodes, which rely on electrostatic double layer charge storage mechanisms. Pseudocapacitors, on the other hand, benefit from surface redox reactions which lend them superior capacitance than their double layer counterparts. Conducting polymers and transitional metal oxides (TMOs) account for pseudocapacitors in general and among TMOs, manganese oxides have widely been investigated owing to their high specific capacitances, low cost, and environmental benignity. In this work, porous manganese oxide (MnOx) thin films were synthesized via electrostatic spray deposition (ESD) and evaluated as pseudocapacitive electrode materials in aqueous media. Very interestingly, the gravimetric specific capacitance of the ESD based MnOx electrodes underwent a marked enhancement upon electrochemical cycling, from 72Fg-1 to 225Fg-1, with a concomitant improvement in kinetics and conductivity. The change in capacitance and resistivity is attributed to a partial electrochemical phase transformation from the spinel type hausmannite Mn3O4 to the conducting layered birnessite MnO2. Furthermore, the films were able to retain 88.4% of the maximal capacitance after 1000 cycles. Upon verifying the viability of the manganese oxide films for pseudocapacitive applications, the thin films were integrated onto carbon micro-pillars created via carbon microelectromechanical systems (C-MEMS) for examining their application as potential microelectrode candidates. In a symmetric two-electrode cell setup, the MnOx/C-MEMS microelectrodes were able to deliver specific capacitances as high as 0.055 Fcm-2 and stack capacitances as high as 7.4Fcm-3, with maximal stack energy and power densities of 0.51 mWhcm-3 and 28.3 mWcm-3, respectively. The excellent areal capacitance of the MnOx-MEs is attributed to the pseudocapacitive MnOx as well as the three-dimensional architectural framework provided by the carbon micropillars.

A Permselective Graphene Oxide/Polyvinyl Alcohol Composite Membrane for Rechargeable Alkaline Zinc Manganese Dioxide Batteries

Rechargeable alkaline Zn/MnO2 batteries are currently attractive candidates for grid-scale energy storage due to its low cost, safety characteristics, and high gravimetric capacity of MnO2 (308 mAh/g per electron). However, commercialization of this battery has not achieved big success due to its limited cycle life and restricted depth of discharge (DOD). Accessing higher DOD results in higher energy densities, but leads to detrimental characteristics like phase transformation of MnO2 and the zinc redistribution. Recently, a class of Bi-birnessite (a layered MnO2 structure with bismuth oxide) material intercalated with Cu2+ has been reported as being able to stabilize the MnO2 structure and deliver near-full two-electron capacity for more than 6,000 cycles in the absence of zinc [1]. However, when a zinc anode is used, the most detrimental effect arises from zincate ions ([Zn(OH)4]2-) that traverse to the cathode side and react to form a spinel phase ZnMn2O4, a highly resistive and electrochemically inactive compound, leading to rapid energy density loss and battery failure [2]. Therefore, inhibiting ZnMn2O4 formation remains a major challenge for the long-term cycling of an energy dense secondary Zn/MnO2 battery. Previously, we reported the use of a zincate-absorbing interlayer fabricated of calcium hydroxide, which successfully prevents ZnMn2O4 formation [3]. In this presentation, we report the strategy of developing a separator featuring selective transport of OH- and [Zn(OH)4]2- ions. We fabricate a graphene oxide-poly(vinyl alcohol) (GO-PVA) composite membrane with a layered structure. The GO membrane, which is composed of stacked and overlapped GO monolayers with a narrow interlayer spacing (~13.5Å for GO laminates swelled in water), has been demonstrated to be excellent filters for gases and liquids. The nanocapillaries formed within the membranes as well as the oxygen functional groups which repel the bulky [Zn(OH)4]2- anions are responsible for its ion sieving properties. PVA is impregnated to provide a conductive pathway for OH- ions and to improve the mechanical stability. We successfully demonstrate the superiority of the GO/PVA composite membrane in suppressing zincate ion crossover (Fig.1(a)), while minimally impairing OH- conduction. Its application in primary batteries significantly increases the energy density by obtaining capacity of more than 1.5 e- per Mn atom above 0.9V (Fig.1(b)). In rechargeable batteries, it slows down the capacity fade of a conventional zinc/electrolytic γ-MnO2 (Zn/EMD) battery during deep cycling within the 1st electron capacity (Fig.1(c)), while it helps to achieve more than 200 cycles with full 2-electron capacity retained in a zinc/birnessite battery (Fig.1(d)).

Synthesis of Few Layered MnO2 Nanosheets Encapsulated TiO2 Nanorods Electrode for Supercapacitor Application

Supercapacitors have attracted in the field of portable electronic devices and electronic vehicles due to its long cycle stability and high power density. To achieve high energy density, researchers concentrated on the development of transition metal oxide and its composites based electrode material for supercapacitor device fabrication. Among them, MnO2 and TiO2 got great importance due to its low cost and environment friendly nature. Further, use of binary metal oxide nanocomposite electrode materials has been received much attention due to its exceptional physical properties and potential applications. In this work, MnO2 nanosheets encapsulated TiO2 nanorods have been synthesized by two step hydrothermal process for supercapacitors. Initially, TiO2 nanorods were synthesized by ion intercalation assisted hydrothermal process and the MnO2 nanosheets were encapsulated over the TiO2 nanorods by the simple hydrothermal technique with controlled pH condition. The physio chemical properties of the synthesized MnO2 nanosheets encapsulated TiO2 nanorods were studied by various characterization techniques. The crystalline nature of the synthesized material was studied by powder X-ray diffraction technique. Specific surface area and porous nature of the material was studied by nitrogen adsorption techniques and the measured specific surface area of the synthesized MnO2 nanosheets encapsulated TiO2 nanorods is 118 m2 g-1. Few layered MnO2 nanosheets and its encapsulation over TiO2 nanorods was confirmed by field emission electron microscope (FESEM) technique and the elemental analysis was carried out using EDS analysis. The electrochemical behavior of the synthesized MnO2 nanosheets encapsulated TiO2 nanorods was studied by cyclic voltammetry (CV), charge – discharge and electrochemical impedance techniques in 1 M Li2SO4 aqueous electrolyte. The quasi – rectangular shaped cyclic voltammetry curve was observed and confirmed the capacitive behavior of the synthesized material. The specific capacitance values of the synthesized MnO2 nanosheets encapsulated TiO2 nanorods electrode material was calculated from Galvanostatic charge-discharge technique at a various applied current density of 1 to 5 A g-1. The synthesized MnO2 nanosheets encapsulated TiO2 nanorods electrode shows high specific capacitance and energy density value of 1388.28 F g-1 and 123.41 Wh. kg-1, respectively at the applied current density of 1 A g-1. The electrochemical impedance analysis was performed within the frequency range of 10 mHz to 100 kHz to analyze the resistive and capacitive nature of the synthesized electrode material. The plotted Nyquist, Bode angle and Bode absolute plots confirmed the existence of pseudo capacitance and EDLC nature of the electrode material. The above results confirmed that the synthesized MnO2 nanosheets encapsulated TiO2 nanorods is a suitable material for supercapacitor application.

Charge storage mechanisms of birnessite-type MnO2 nanosheets in Na2SO4 electrolytes with different pH values: In situ electrochemical X-ray absorption spectroscopy investigation

The charge storage mechanism of birnessite-type MnO2 nanosheet electrode with ca. 100 μm in thickness in 0.5 M Na2SO4 electrolytes with different pH values was investigated by an in situ electrochemical X-ray absorption spectroscopy (XAS) technique. It is found that the charge storage capacity of MnO2 is controlled by both surface adsorption and redox reaction. At pH 1 and 2, the rate of surface adsorption is faster than that of the surface redox reaction. Whilst, at pH 3, 4, and 5.9, the major capacitance originates from the redox reaction or the insertion/de-insertion of solvated cations into the layered MnO2 nanostructures. As a result, the MnO2-based supercapacitor electrode in 0.5 M Na2SO4 with diluted H2SO4 (pH 4) exhibits higher specific capacitance than that in electrolytes with pH 1–2. The decrease of capacitance at low pH is due to the partial dissolution of MnO2. This finding may lead to better understanding on the charge storage mechanism of the MnO2 materials.

Insights into the surface-defect dependence of molecular oxygen activation over birnessite-type MnO2

In establishing the kinetics, energetics and mechanisms of phenolic degradation reactivity, active reactive oxygen species (ROS) on catalysts surface could exert a vital part. This paper attempts to account for different ROS at the atomic level using octahedral layered birnessite-type MnO2 as a platform with different crystal planes which could induce the Jahn-Teller effect and further realize deep mineralization of phenolic pollutants at low temperature. The catalytic degradation phenol rate of (100) MnO2 is 3 times as much as that of (001) MnO2, and the activation energy of the catalytic reaction is reduced by 11 KJ/mol. The degradation content of (100) MnO2 surpasses 30% than that of (001) MnO2. Both spin-trapping EPR and DFT results show superoxide (O2−) species could exist on (001) MnO2 through one electron transfer, while the peroxide (O22−) species exist on (100) MnO2via two electrons transfer. All the results illustrate that birnessite MnO2 possesses surface-dependent molecular oxygen activation properties.

Origin of unusual spinel-to-layered phase transformation by crystal water.

It is well known that many layered transition metal oxides can transform into a spinel structure upon repeated battery cycling, but a phase transition in the opposite direction is rare. Recently, the transformation from spinel Mn3O4 to layered MnO2 was observed during the operation of a Mg battery in aqueous conditions, resulting in high performance Mg batteries. We hereby use ab initio calculations to unveil the mechanism by which crystal water plays a critical role in this unique transformation. Once inserted into the spinel form, a water molecule donates an electron, offering a key structural and thermodynamic driving force to initiate the transformation process. These crystal water molecules then get favorably clustered into a planar form in the layered structure and act as a stabilizing agent for birnessite. Kinetically, the inserted crystal water dramatically promotes the necessary rearrangement of Mn during the transition by lowering the activation barrier by >2 eV. The present structural, thermodynamic and kinetic understanding of the crystal water-driven phase transition provides novel insights to further the design of related low dimensional hydrated materials for multi-valent cathodes.

Degradation of Crystal Violet Dye from Waters by Layered MnO2 and Nanocomposite-MnO2@MnFe2O4 Catalysts

Degradation of Crystal Violet Dye from Waters by Layered MnO2 and Nanocomposite-MnO2@MnFe2O4 Catalysts

Constructing Sheet-On-Sheet Structured Graphitic Carbon Nitride/Reduced Graphene Oxide/Layered MnO₂ Ternary Nanocomposite with Outstanding Catalytic Properties on Thermal Decomposition of Ammonium Perchlorate.

We unprecedentedly report that layered MnO2 nanosheets were in situ formed onto the surface of covalently bonded graphitic carbon nitride/reduced graphene oxide nanocomposite (g-C3N4/rGO), forming sheet-on-sheet structured two dimension (2D) graphitic carbon nitride/reduced graphene oxide/layered MnO2 ternary nanocomposite (g-C3N4/rGO/MnO2) with outstanding catalytic properties on thermal decomposition of ammonium perchlorate (AP). The covalently bonded g-C3N4/rGO was firstly prepared by the calcination of graphene oxide-guanidine hydrochloride precursor (GO-GndCl), following by its dispersion into the KMnO4 aqueous solution to construct the g-C3N4/rGO/MnO2 ternary nanocomposite. FT-IR, XRD, Raman as well as the XPS results clearly demonstrated the chemical interaction between g-C3N4, rGO and MnO2. TEM and element mapping indicated that layered g-C3N4/rGO was covered with thin MnO2 nanosheets. Furthermore, the obtained g-C3N4/rGO/MnO2 nanocomposite exhibited promising catalytic capacity on thermal decomposition of AP. Upon addition of 2 wt % g-C3N4/rGO/MnO2 ternary nanocomposite as catalyst, the thermal decomposition temperature of AP was largely decreased up by 142.5 °C, which was higher than that of pure g-C3N4, g-C3N4/rGO and MnO2, respectively, demonstrating the synergistic catalysis of the as-prepared nanocomposite.

Biphase Cobalt–Manganese Oxide with High Capacity and Rate Performance for Aqueous Sodium‐Ion Electrochemical Energy Storage

AbstractManganese‐based metal oxide electrode materials are of great importance in electrochemical energy storage for their favorable redox behavior, low cost, and environmental friendliness. However, their storage capacity and cycle life in aqueous Na‐ion electrolytes is not satisfactory. Herein, the development of a biphase cobalt–manganese oxide (CoMnO) nanostructured electrode material is reported, comprised of a layered MnO2⋅H2O birnessite phase and a (Co0.83Mn0.13Va0.04)tetra(Co0.38Mn1.62)octaO3.72 (Va: vacancy; tetra: tetrahedral sites; octa: octahedral sites) spinel phase, verified by neutron total scattering and pair distribution function analyses. The biphase CoMnO material demonstrates an excellent storage capacity toward Na‐ions in an aqueous electrolyte (121 mA h g−1 at a scan rate of 1 mV s−1 in the half‐cell and 81 mA h g−1 at a current density of 2 A g−1 after 5000 cycles in full‐cells), as well as high rate performance (57 mA h g−1 a rate of 360 C). Electrokinetic analysis and in situ X‐ray diffraction measurements further confirm that the synergistic interaction between the spinel and layered phases, as well as the vacancy of the tetrahedral sites of spinel phase, contribute to the improved capacity and rate performance of the CoMnO material by facilitating both diffusion‐limited redox and capacitive charge storage processes.

Understanding the “seesaw effect” of interlayered K+ with different structure in manganese oxides for the enhanced formaldehyde oxidation

Although the potassium-promoted catalysts exhibit improved catalytic activities, the role of potassium with different structural sites still needs to be studied by more experiments and theory calculation. Herein, potassium (K+) formed via different binding sites (isolated and localized) over layered MnO2 was investigated. The isolated K+ dissociated between layers via weak chemical bond while localized K+ coordinated with oxygen atoms at vacancy site. K+ with localized type (L-MnO2) exhibited the higher activity and the lower Ea (45.2 kJ/mol), compared with isolated one (58.1kJ/mol) under 200ppm HCHO, 120,000mL/gh GHSV and ∼45% relative humidity. Evidenced by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), hydrogen temperature programmed reduction (H2-TPR) and density functional theory (DFT) simulation, the isolated K+ in interlayer are energetically favourable for dissociation of O2 as well as H2O by charge transfer from potassium to oxygen. As a result, the desorption of H2O was inhibited, thus, became the kinetic barriers during the whole reaction process. K+ with localized form possessed suitable ability for O2 activation and weak adsorption of H2O, exhibiting enhanced catalytic activity. Inspired by the seesaw effect, a clear understanding of K+ effect on layered MnO2 towards O2 adsorption and H2O desorption was illustrated.

Dual Core-Shell-Structured S@C@MnO2 Nanocomposite for Highly Stable Lithium-Sulfur Batteries.

Lithium-sulfur (Li-S) batteries have currently excited worldwide academic and industrial interest as a next-generation high-power energy storage system (EES) because of their high energy density and low cost of sulfur. However, the commercialization application is being hindered by capacity decay, mainly attributed to the polysulfide shuttle and poor conductivity of sulfur. Here, we have designed a novel dual core-shell nanostructure of S@C@MnO2 nanosphere hybrid as the sulfur host. The S@C@MnO2 nanosphere is successfully prepared using mesoporous carbon hollow spheres (MCHS) as the template and then in situ MnO2 growth on the surface of MCHS. In comparison with polar bare sulfur hosts materials, the as-prepared robust S@C@MnO2 composite cathode delivers significantly improved electrochemical performances in terms of high specific capacity (1345 mAh g-1 at 0.1 C), remarkable rate capability (465 mA h g-1 at 5.0 C) and excellent cycling stability (capacity decay rate of 0.052% per cycle after 1000 cycles at 3.0 C). Such a structure as cathode in Li-S batteries can not only store sulfur via inner mesoporous carbon layer and outer MnO2 shell, which physically/chemically confine the polysulfides shuttle effect, but also ensure overall good electrical conductivity. Therefore, these synergistic effects are achieved by unique structural characteristics of S@C@MnO2 nanospheres.

A binder-free thin film anode composed of Co2 +-intercalated buserite grown on carbon cloth for oxygen evolution reaction

We present a binder-free catalytic anode for highly efficient and stable oxygen evolution reaction in alkaline media. The catalyst consists of a thin film of buserite-type layered manganese dioxide (MnO2) intercalated with Co2+ ions, resulting from electrodeposition of the layered MnO2 film with tetrabutylammonium (Bu4N+) ions on a carbon cloth, followed by ion-exchange of the initially incorporated Bu4N+ with Co2+ in solution. The electrode is capable to produce a current density of 10mAcm−2 at an overpotential (η) of 377mV with a Tafel slope of 48mVdec−1, much superior to the layered MnO2 without Co2+.

Insight into charge storage mechanisms of layered MnO2 nanosheets for supercapacitor electrodes: In situ electrochemical X-ray absorption spectroscopy

Manganese dioxide (MnO2) nanostructures have been widely used as the supercapacitor electrode. However, their charge storage mechanism has not yet been fully understood. Herein, the charge storage mechanism of layered MnO2 nanosheets has been investigated. It was found that they store charges via three processes: electrochemical double layer capacitance (EDLC), surface redox reaction, and intercalation/de-intercalation. The EDLC of the layered MnO2 is equal to 71.6Fg−1 at all scan rates used (10–100mVs−1) without a diffusion limit. The surface redox contribution owing to the chemisorption of Na+ on the surface of MnO2 nanosheets is 30.8, 18.0, 20.0, 17.0, 13.5Fg−1 at 10, 25, 50, 75, and 100mVs−1, respectively. The intercalation contribution thanks to the solvated Na+ insertion into the crystalline bulk structure of layered MnO2 is 96.4, 57.1, 38.6, 30.5, and 25.3Fg−1 at 10, 25, 50, 75, 100mVs−1, respectively. The surface redox reaction and intercalation are still under diffusion limit. Understanding the charge storage mechanism/contribution of MnO2 nanosheets may lead to further development of these materials for practical applications.

Electrostatic Spray Deposition-Based Manganese Oxide Films-From Pseudocapacitive Charge Storage Materials to Three-Dimensional Microelectrode Integrands.

In this study, porous manganese oxide (MnOx) thin films were synthesized via electrostatic spray deposition (ESD) and evaluated as pseudocapacitive electrode materials in neutral aqueous media. Very interestingly, the gravimetric specific capacitance of the ESD-based electrodes underwent a marked enhancement upon electrochemical cycling, from 72 F∙g−1 to 225 F∙g−1, with a concomitant improvement in kinetics and conductivity. The change in capacitance and resistivity is attributed to a partial electrochemical phase transformation from the spinel-type hausmannite Mn3O4 to the conducting layered birnessite MnO2. Furthermore, the films were able to retain 88.4% of the maximal capacitance after 1000 cycles. Upon verifying the viability of the manganese oxide films for pseudocapacitive applications, the thin films were integrated onto carbon micro-pillars created via carbon microelectromechanical systems (C-MEMS) for examining their application as potential microelectrode candidates. In a symmetric two-electrode cell setup, the MnOx/C-MEMS microelectrodes were able to deliver specific capacitances as high as 0.055 F∙cm−2 and stack capacitances as high as 7.4 F·cm−3, with maximal stack energy and power densities of 0.51 mWh·cm−3 and 28.3 mW·cm−3, respectively. The excellent areal capacitance of the MnOx-MEs is attributed to the pseudocapacitive MnOx as well as the three-dimensional architectural framework provided by the carbon micro-pillars.

Electrosynthesis of Layered Organo-Manganese Dioxide Framework-Doped with Cobalt for Iodide Sensing.

Aqueous Mn2+ ions were anodized at 70 °C with Co2+ in the presence of cationic surfactant, cetyltrimethylammonium (CTA). X-ray diffraction (XRD) analysis revealed that the deposited film possesses a layered structure of MnO2, the interlayer of which is occupied with the assembled CTA molecules. Inclusion of Co ions in the MnO2 film was evidenced by X-ray photoelectron spectroscopy (XPS). They were located in the MnO2 framework, not in the interlayer. The thus-obtained film, CTA-intercalated Co-framework-doped layered MnO2 (CTA/Co-MnO2), was applied as an electrochemical sensor toward iodide (I-), a hydrophobic anion. The organic phase between MnO2 layers could extract I- ions from solution, providing a better sensitivity than a film consisting of layered MnO2 with hydrated alkali metals. On the other hand, the Co-doped layers of MnO2 achieved faster electron transfer kinetics for the oxidation of I-, which resulted in a drastic reduction in response time compared to the nondoped CTA/MnO2.

Tetramethyl ammonium cation intercalated layered birnessite manganese dioxide for high-performance intercalation pseudocapacitor

Abstract We develop an effective strategy to prepare intercalation pseudocapacitor by enlarging interlayer spacing of layered birnessite MnO2 through a controllable intercalation of K+, H+, TMA+/H+, TMA+ cations. The interlayer spacing of K-birnessite is swollen from ∼0.52 nm to 0.77 nm of TMA-birnessite. TMA+/H+ = 1000 sample exhibits an impressively high specific capacitance of 580.05 F g−1, much larger than that of K-birnessite, H-birnessite, TMA+/H+ = 800 (352.8, 414.9, 497.25 F g−1) at current density of 2 A g−1, showing excellent cycling performance with 112% capacitance retention after 10000 cycles. Even upon 20 A g−1, the capacity of TMA+/H+ = 1000 still remains a high level of 503 F g−1, while K-birnessite, H-birnessite, TMA+/H+ = 800 undergo a severe degradation to 180, 216, 433 F g−1. The expanded interlayer spacing enlarges the ion diffusion tunnels, weakens the layer interaction and decreases the resistance of charge diffusion, which easily enables the sufficient utilization of the active material through rapid ion intercalation. More importantly, the molecular-level thickness of host MnO2 layers that expose almost all the active sites on both sides of diffusion tunnels, thus greatly enhances the electrochemical storage capacitance. The results prove the feasibility of this method to prepare new intercalation pseudocapacitor and provide hints for other layered materials.

Highly sensitive GQDs-MnO2 based assay with turn-on fluorescence for monitoring cerebrospinal acetylcholinesterase fluctuation: A biomarker for organophosphorus pesticides poisoning and management

In this study, we demonstrated an assay with turn-on fluorescence for monitoring cerebrospinal acetylcholinesterase (AChE) fluctuation as a biomarker for organophosphorus pesticides (OPs) poisoning and management based on single layer MnO2 nanosheets with graphene quantum dots (GQDs) as signal readout. Initially, the fluorescence of GQDs was quenched by MnO2 nanosheets mainly due to the inner filter effect (IFE). However, with the presence of reductive thiocholine (TCh), the enzymatic product, hydrolyzed from acetylthiocholine (ATCh) by AChE, the redox reaction between MnO2 and TCh occurred, leading to the destruction of the MnO2 nanosheets, and thereby IFE was diminished gradually. As a consequence, the turn-on fluorescence of GQDs with the changes in the spectrum of the dispersion constituted a new mechanism for sensing of cerebrospinal AChE. With the method developed here, we could monitor cerebrospinal AChE fluctuation of rats exposed to OPs before and after therapy, and could thereby open up the pathway to a new sensing platform for better understanding the mechanism of brain dysfunctions associate with OPs poisoning.

Redox Active Cation Intercalation/Deintercalation in Two-Dimensional Layered MnO2 Nanostructures for High-Rate Electrochemical Energy Storage.

Two-dimensional (2D) layered materials with a high intercalation pseudocapacitance have long been investigated for Li+-ion-based electrochemical energy storage. By contrast, the exploration of guest ions other than Li+ has been limited, although promising. The present study investigates intercalation/deintercalation behaviors of various metal ions in 2D layered MnO2 with various interlayer distances, K-birnessite nanobelt (K-MnO2), its protonated form (H-MnO2), and a freeze-dried sample of exfoliated nanosheets. Series of metal ions, such as monovalent Li+, Na+, and K+ and divalent Mg2+, exhibit reversible intercalation during charge/discharge cycling, delivering high-rate pseudocapacitances. In particular, the freeze-dried MnO2 of exfoliated nanosheets restacked with the largest interlayer spacing and a less compact 3D network exhibits the best rate capability and a stable cyclability over 5000 cycles. Both theoretical calculation and kinetic analysis reveal that the increased interlayer distance facilitates the fast diffusion of cations in layered MnO2 hosts. The results presented herein provide a basis for the controllable synthesis of layered nanostructures for high-rate electrochemical energy storage using various single- and multivalent ions.