Tunnel-structured Manganese Oxide Research Articles

Theoretical investigations of the potential application of PVA-mixed-valent tunnel structured manganese oxide nano-composite in continuous simultaneous removal of multi-contaminants from aqueous waste stream

The potential use of PVA-mixed-valent tunnel structured manganese oxide nano-composite in the removal of multi-contaminants form aqueous solutions was assessed by studying the continuous simultaneous removal of lead, caesium, and cobalt. Within this context, the morphology and the nature of nanoparticle inclusion into the PVA matrix was assessed using SEM–EDX analysis. The nanoparticles are homogenously distributed in the polymeric matrix with some agglomerated inclusions of these particles. The thermal and chemical stability analyses prove the stability of the material up to 180 °C and in slightly acidic to slightly alkaline solutions. The analysis of the gravimetric thermal data shows that the thermal treatment is a feasible end of life management route for this material. The values of percentage uptake and bed capacity indicate the feasibility of the use of this material in the simultaneous removal of lead, caesium and cobalt. The breakthrough curves analyses provide insights into the breakthrough characteristics and underlying removal mechanisms. It was found that the removal reaction follows Langmuir kinetics of adsorption–desorption and that the rate driving forces follow second order reversible reaction kinetics, where the sorption occur at energetically equal sites.

Low temperature oxidation of diesel soot particles over one-dimensional 2 × 3 tunnel-structured Na2Mn5O10 catalysts

Tunnel-structured manganese oxides have drawn widespread research attention due to their excellent oxidation properties for the soot oxidation reaction. In this study, a series of romanechite Na2Mn5O10 catalysts with a 2 × 3 tunnel structure were synthesized via a facile hydrothermal method. The as-synthesized catalysts were investigated using X-ray diffraction, scanning and transmission electron microscopy, H2 and soot temperature-programmed reduction, O2 temperature-programmed desorption, NO temperature-programmed oxidation, X-ray photoelectron spectroscopy, and other measurements. Among all the catalysts, the Na1MnyOδ-120 catalyst exhibited the best catalytic performance for soot combustion, with T10, T50, and T90 values of 281 ℃, 317 ℃, and 343 ℃, respectively, which were considerably reduced to 271 ℃, 310 ℃, and 335 ℃ in the existence of 10% H2O. The results of multiple characterization methods indicated that the Na2Mn5O10 catalysts exhibit good reducibility, strong oxygen adsorption and activation capacity, and NO to NO2 oxidation ability. Moreover, the active sites (Mn and oxygen vacancies) and reaction mechanism (Langmuir–Hinshelwood) were also revealed by in-situ DRIFT and density functional theory calculations results. This study details a new strategy for the design and synthesis of efficient soot oxidation catalysts with practical application prospects.

Synthesis of Ag-OMS Catalyst for Sunlight-Assisted Photodegradation of Crystal Violet Dye

The contamination of water with organic pollutants, such as dyes, has become a serious threat to the environment. Therefore, the development of a cost-effective, eco-friendly, proficient, and visible-light-driven catalyst for the treatment of organic dye-contaminated wastewater has been a burning issue recently. Photocatalysis is suggested as a potential treatment technique for the eradication of organic pollutants. The 1D tunnel-structured manganese oxide octahedral molecular sieve (OMS) is a suitable substance to be tested as a visible-light-driven photocatalyst for the degradation of organic contaminants. However, the fast recombination of photoinduced charges (h+/e−) limits its photocatalytic application. The development of heterojunctions between OMS and other metals, such as Ag, is a suitable technique for improving the photocatalytic performance of OMS. In this study, Ag-OMS with plasmon-enhanced photocatalytic activity is reported for the photodegradation of crystal violet dye. Manganese oxide OMS was prepared by an acidic precipitation method using potassium permanganate, manganese acetate, and nitric acid as precursor materials. Ag nanoparticles were deposited on OMS using leaf extracts of Calotropis gigantea. The deposition of Ag enhanced the photocatalytic performance of OMS from 68 to 95%. The effects of Ag contents, catalyst dosage, and concentration of crystal violet dye on catalytic performance were explored as well. Approximately 100, 95, and 75% photodegradation of 50, 100, and 150 mg/L crystal violet dye was observed in 90, 120, and 120 min in the presence of 10% Ag-OMS, respectively. Excellent photocatalytic performance, low dose utilization, and reusability proved that Ag-OMS might have practical environmental applications.

Toward a mechanistic understanding of cesium adsorption to todorokite: A molecular dynamics simulation study

A mechanistic understanding of cesium (Cs) adsorption to soil mineral phases is essential for effective mitigation of Cs mobility in the subsurface environment. Todorokite, a common tunnel-structured manganese oxide in soil, exhibits sorption capacity for Cs comparable to the capacities of clay minerals. However, the adsorption sites and molecular species of Cs+ adsorbed to todorokite remain uncertain in comparison with those of clay minerals. In this study, we explored adsorption of Cs+ to hydrated todorokite surfaces via atomistic molecular dynamics (MD) simulations. We performed the first MD simulations based on atomic pair potentials for Mn-oxide edge surfaces interfaced with an aqueous solution. MD simulations predicted that Cs+ forms only inner-sphere (IS) complexes within todorokite tunnels; however, Cs+ forms both IS and outer-sphere (OS) complexes at the external (010) and (100)/(001) external surfaces. On the (010) surface, the positions between IS and OS complexes of Cs+ were interchangeable during MD simulations. Detailed molecular structures of IS and OS Cs+ surface complexes are compared to those of Cs+ in an aqueous solution. The current MD simulation results can be used as an atomistic structural proxy for spectroscopic analysis of adsorbed metal speciation and surface complexation modeling of metal adsorption to Mn oxides.

Flower-like manganese oxide with intercalated nickel ions (Ni3+) as a catalytic electrode material for urea oxidation

The Na+-intercalated manganese oxide nanoflowers and Na+-free manganese oxide nanoparticles were anodically electrodeposited on carbon fibers from manganese (II) acetate solutions with and without sodium sulfate, respectively. After intercalation of nickel ions, the Na+-intercalated manganese oxide remained its original form of nanoflowers (denoted as MnO2/Na/Ni), while the Na+-free manganese oxide underwent severe deformation to form a dense film with aggregated nanoparticles (denoted as MnO2/Ni). It was evident that the preintercalated Na+ ions were replaced with Ni3+ ions which were inserted in the interstitial space of the tunnel-structured manganese oxide. Cyclic voltammetry revealed that the well-dispersed Ni3+ species in MnO2/Na/Ni could behave as a catalytic center for facilitating the urea oxidation reaction (UOR) in an alkaline electrolyte, resulting in superior catalytic capability than the MnO2/Ni and bulk Ni(OH)2. Electrochemical impedance spectroscopy demonstrated that an impressive electrocatalytic activity of MnO2/Na/Ni electrode was largely ascribed to its lower charge-transfer resistance of the Ni3+/Ni2+ redox reaction in indirect UOR process and smaller direct UOR resistance on Ni3+ catalyst than the MnO2/Ni electrode with aggregated catalyst layer.

Photocatalytic Oxidation of Dissolved Mn2+ by TiO2 and the Formation of Tunnel Structured Manganese Oxides

Photocatalytic Oxidation of Dissolved Mn²⁺ by TiO₂ and the Formation of Tunnel Structured Manganese Oxides

Antimonite oxidation and adsorption onto two tunnel-structured manganese oxides: Implications for antimony mobility

Manganese (Mn) oxides have high oxidation and sorption capacities, which enables them to play important roles in antimony (Sb) migration and transformation. In this study, the oxidation and adsorption behaviors of antimonite (Sb(III)) on two tunnel-structured Mn oxides, pyrolusite (β-MnO2) and manganite (γ-MnOOH), were investigated and compared using batch kinetic techniques; the adsorption isotherms, edges and spectroscopic characteristics of Sb(III) and Sb(V) on the two minerals were also compared and modeled. The adsorption isotherms and edges indicated that the affinity of Sb(III) and Sb(V) toward the two Mn oxides depended on the Sb species, the pH of the solution and the characteristics of the two minerals. Acidic pH favored the sorption of both Sb(III) and Sb(V) onto the two minerals, with the rate of sorption decreasing subsequently with the increase in pH. The sorption capacity of β-MnO2 for both Sb(III) and Sb(V) was much lower than that of the γ-MnOOH. Sb(V) was the main species after Sb(III) and Sb(V), which adsorbed onto the surface of both β-MnO2 and γ-MnOOH. Furthermore, Sb(V) combined with γ-MnOOH through inner-sphere complexation and with β-MnO2 through outer-sphere complexation. These results therefore provide new insights into the mechanism of Sb(III) oxidation and adsorption on the surfaces of two tunnel-structured Mn oxides and indicates that γ-MnOOH would exhibit a huge capacity in Sb oxidation and sorption.

Synthesized akhtenskites remove ammonium and manganese from aqueous solution: removal mechanism and the effect of structural cations.

Ammonium and manganese removal by tunnel-structured manganese oxide is still enigmatic. Herein, tunnel-structured akhtenskites with different structural cations (Na–MnOx, Mg–MnOx Ca–MnOx, Fe–MnOx) were synthesized by the KMnO4 and Mn2+ reaction in the presence of different metal cations, and were used to remove ammonium and manganese from aqueous solution. The results of the batch adsorption experiments indicated that akhtenskites effectively removed NH4+ and Mn2+, and the removal process fitted the pseudo-second-order model. By measuring the concentration of nitrate and nitrite, discriminating the adsorbed and oxidized Mn2+, and testing the zeta potential of the oxides, it can be concluded that NH4+ was merely removed by electrostatic adsorption via Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> Mn–O−; Mn2+ could also be adsorbed by ion exchange with Mn–OH, and the adsorbed Mn2+ could be partly oxidized. The structural properties of the akhtenskites were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Brunauer–Emmett–Teller (BET) specific area, and X-ray photoelectron spectroscopy (XPS). The experimental results showed that ions with higher valence can result in a higher percentage of Mn(iii) in akhtenskite. Mg2+ can result in a lower proportion of lattice oxygen in the oxide, and Fe3+ can increase the pH of the point of zero charge. Both of them were unfavored for the oxidation of Mn2+. Moreover, it was found that Ca–MnOx had optimal removal performance in the catalytic oxidation of Mn2+ owing to appropriate percentages of Olatt and Mn(iii) and lower zeta potential. This study provides new insights into the synthesis and application of manganese oxides.

Engineering sodium-rich manganese oxide with robust tunnel structure for high-performance sodium-ion battery cathode application

Layer sodium-based manganese oxides are one of potentially applicable sodium-ion battery (SIB) cathode materials mainly because of their lower cost and relatively reversible Na+ intercalation/extraction attributes. However, unavoidable Jahn-Teller effect arisen from manganese redox reactions causes serious lattice distortion, leading to distinct capacity fade. NaxMnO2 (x < 0.44) with tunnel structure shows good tolerance of deformation, but its intrinsically low sodium compositions determine its limited capacity. Herein, we report a novel sodium-rich tunnel-type Na0.6MnO2 material fancily prepared with the guidance of cetyltrimethylammonium bromide (CTAB) surfactant. Due to the robust structure and high sodium contents, the tailored CTAB-Na0.6MnO2 exhibits outstanding cycling durability with high discharge capacity. The fast Na+ diffusion kinetics and the restorable framework during charge/discharge cycling are further approved by galvanostatic intermittent titration technique (GITT) and ex situ x-ray diffraction (XRD) techniques. The sodium-rich tunnel-structured manganese oxide provides a new perspective of engineering high-performance SIB cathode materials.

PH-Dependent Degradation of Diclofenac by a Tunnel-Structured Manganese Oxide

The mechanism of diclofenac (DIC) degradation by tunnel-structured γ-MnO2, with superior oxidative and catalytic abilities, was determined in terms of solution pH. High-performance liquid chromatography with mass spectroscopy (HPLC–MS) was used to identify intermediates and final products of DIC degradation. DIC can be efficiently oxidized by γ-MnO2 in an acidic medium, and the removal rate decreased significantly under neutral and alkaline conditions. The developed model can successfully fit DIC degradation kinetics and demonstrates electron transfer control under acidic conditions and precursor complex formation control mechanism under neutral to alkaline conditions, in which the pH extent for two mechanisms exactly corresponds to the distribution percentage of ionized species of DIC. We also found surface reactive sites (Srxn), a key parameter in the kinetic model for mechanism determination, to be exactly a function of solution pH and MnO2 dosage. The main products of oxidation with a highly active hydroxylation pathway on the tunnel-structured Mn-oxide are 5-iminoquinone DIC, hydroxyl-DIC, and 2,6-dichloro-N-o-tolylbenzenamine.

Classical mechanical simulations of layer- and tunnel-structured manganese oxide minerals

Tecto- and phyllo-manganate minerals serve as important controls in the geochemical cycle of many elements in soils and sediments due to their high surface areas and chemical reactivity. These nanoscale Mn oxides are mainly composed of Mn octahedra in edge- and corner-sharing configurations that produce a range of tunnel and layer structures. Some fundamental aspects of their atomic structures, however, are difficult to characterize unambiguously because of dynamic structural transformations and a wide range of the structural and chemical compositions. Molecular dynamics (MD) simulations based on interatomic pair potentials provide a perspective on nanoscale Mn-oxides that can be inaccessible to experiment or density functional theory (DFT) calculations. We recently introduced a set of interatomic potentials for the atomistic simulation of Mn-oxides (MnFn) that reproduced many of the subtle phyllomanganate structural variations associated with pH, vacancy content, and Mn oxidation state. In this manuscript, we expand the roster of simulated Mn-oxide minerals to include hydrous and anhydrous tunnel structures and δ-MnO2 with octahedral vacancy contents up to 20%. The MnFn potentials reproduced the experimental tectomanganate lattice parameters and interatomic distances to within 5% and the relative energetics of tectomanganates as predicted by a recent DFT study. We also provide new insights into the Mg site occupancy in todorokite and the Zn coordination in vernadite through large-scale atomistic simulations with MnFn that are challenging to simulate with DFT. In the 3 × 3 tunnel structures of todorokite, the central tunnel site is the predominant site for fully-hydrated Mg2+·6H2O with a minority of partially-hydrated Mg2+ cations binding to corner sites at decreased water/cation ratios. The occurrence of tetrahedral Zn complexes is dependent upon not only the Mn(IV) vacancy content but also the layer spacing and sheet registry. These atomistic simulation results demonstrate the ability of the MnFn potentials to reproduce the basic structures of nanophase Mn-oxides and the capability to perform large-scale modeling that is required to address the subtle structural variations associated with changes in environmental conditions.

Synthesis and Catalytic Activities of Manganese Oxides Prepared by Precipitation Method: Effects of Mixing Modes of Reactants and Calcination Process

The two phases of manganese oxides are prepared by two different mixing modes using the same precipitation method from the solutions of KMnO4 dan maltose, resulting in marked different phase structures and catalytic activities. The oxides synthesized by adding dropwise of the maltose solution into KMnO4 solution (method A) resulted in the formation of layer manganese oxide birnessite, which turns into tunnel structured manganese oxide cryptomelane following the calcination at 600°C for 4 hours. The simultaneous addition of KMnO4 solution and maltose solution (method B) also produced birnessite before calcination, but remain unchanged as birnessite phase after calcination with cryptomelane as minor product. The samples without calcination obtained from method A posseses higher surface area and poor crystallinity compared to that with calcination. The catalytic test using Fenton-like Reaction for methylene blue (MB) degradation indicated the tremendeous difference in their catalytic activities for both samples without and with calcination. The birnessite catalysts (without calcination) prepared using method A show the highest activity and are able to degrade 93% methylene blue within 10 minute, much higher than other samples.

Cd2+ adsorption performance of tunnel-structured manganese oxides driven by electrochemically controlled redox

The heavy metal ion adsorption performance of birnessite (a layer-structured manganese oxide) can be enhanced by decreasing the Mn average oxidation state (Mn AOS) and dissolution−recrystallization during electrochemical redox reactions. However, the electrochemical adsorption processes of heavy metal ions by tunnel-structured manganese oxides are still enigmatic. Here, tunnel-structured manganese oxides including pyrolusite (2.3 Å × 2.3 Å tunnel), cryptomelane (4.6 Å × 4.6 Å tunnel) and todorokite (6.9 Å × 6.9 Å tunnel) were synthesized, and their electrochemical adsorptions for Cd2+ were performed through galvanostatic charge−discharge. The influence of both supporting ion species in the tunnel and tunnel size on the electrochemical adsorption performance was also studied. The adsorption capacity of tunnel-structured manganese oxides for Cd2+ was remarkably enhanced by electrochemical redox reactions. Relative to K+ in the tunnel of cryptomelane, the supporting ion H+ was more favorable to the electrochemical adsorption of Cd2+. With increasing initial pH and specific surface area, the electrochemical adsorption capacity of cryptomelane increased. The cryptomelane electrode could be regenerated by galvanostatic charge−discharge in Na2SO4 solution. Due to the differences in their tunnel size and supporting ion species, the tunnel-structured manganese oxides follow the order of cryptomelane (192.0 mg g−1) > todorokite (44.8 mg g−1) > pyrolusite (13.5 mg g−1) in their electrochemical adsorption capacities for Cd2+.

Tunable dielectric response and electronic conductivity of potassium-ion-doped tunnel-structured manganese oxides

Potassium-ion-doped MnO2 has been successfully synthesized using the hydrothermal method, and the influence of the doped potassium ions on the electrical conductivity and permittivity is studied. X-ray powder diffraction, scanning electron microscopy, electron-probe micro-analysis, and a vector network analyzer are used to perform characterization. The densities of states of doped and undoped MnO2 tunnel structures are also discussed based on first-principles calculations. Results show that the conductivity and dielectric resonance of MnO2 can be elevated by means of K+ doping. The conductivity of K+-doped MnO2 prepared at different reaction times shows a decreasing trend and is generally 1 order of magnitude higher than that of pure MnO2. The electrical conductivity of K+-doped MnO2 (R3) shows the highest value of 3.33 × 10−2 S/cm at the reaction time of 24 h, while that of pure MnO2 is 8.50 × 10−4 S/cm. When treated with acid, the conductivity of samples remains basically stable along with the increase of treatment time. In addition, acid treatment plays a very significant role in controlling the amount of K+ ions in crystals. The K+ contents of acid-treated samples are 5 times lower than that of the untreated R1. The dielectric losses of the samples with different reaction times are enhanced markedly with frequency increment. The complex permittivity of pure MnO2 only exhibits a resonance at ∼12 GHz, while K+-doped MnO2 exhibits another resonance behavior at ∼9 GHz. The capacity of the dielectric property in the net structure is enhanced by the interfacial polarization, dielectric relaxation, multiple internal reflections, and multiple scattering benefiting.

Stabilization of Tunnel Manganese Oxide Electrodes in Li-Ion and Na-Ion Batteries

Tunnel structured manganese oxides, built from corner and edge sharing MnO6 octahedra arranged around stabilizing cations and water molecules, are of significant interest for energy storage applications such as Li-ion and Na-ion batteries (LIBs and SIBs, respectively) due to their low cost, environmental friendliness, and high electrochemical activity. Moreover, the structural tunnels in these materials act as 1D diffusion channels and provide crystallographic sites for ion insertion. However, tunnel manganese oxide phases face challenges in regards to long term cycling stability in intercalation-based batteries, which has been attributed to structural instabilities resulting from Jahn-Teller distortions induced by manganese reduction [1, 2]. Therefore, it is imperative to stabilize the structure of tunnel manganese oxides for improved electrochemical stability as cathode materials in intercalation-based batteries. Here, we demonstrate stabilization of a newly reported tunnel manganese oxide phase, 2xn-MnO2, with the crystal structure consisting of 2x2, 2x3, and 2x4 octahedra tunnels containing Na+ stabilizing cations and water molecules. Building off of previous work by C.S. Johnson et al. [3], where the smaller tunnel structured α-MnO2 was stabilized with Li2O, we modified the structure and chemical composition of 2xn-MnO2 via mixing in methanol and lithium hydroxide, resulting in insertion of Li2O into the structural tunnels of 2xn-MnO2. In LIBs, the Li2O-stabilized 2xn-MnO2 demonstrates both higher initial capacity and superior capacity retention after 100 cycles. The initial capacity is improved from 93 mAh g-1 for the pristine material to 168 mAh g-1 for the stabilized 2xn-MnO2. Moreover, the galvanostatic discharge-charge profile for the stabilized 2xn-MnO2 exhibits a better pronounced plateau-shaped region compared to the original 2xn-MnO2 phase, indicating that the stabilized material contains more well-defined insertion sites. Similarly, in SIBs, the capacity of the stabilized 2xn-MnO2 was improved by 35% to 81 mAh g-1, and capacity retention increased from 43% to 75% after 100 cycles, demonstrating the effectiveness of this stabilization approach in both Li-ion and Na-ion battery systems. Thus, we show that stabilization with Li2O can be applied to various tunnel manganese oxide phase to improve their electrochemical performance as intercalation-based battery electrodes. References Y. Yuan et al., Nano Energy (2016) 19, 382A. Tompsett and M.S. Islam, Chem. Mater. (2013) 25, 2515S. Johnson et al., J. of Power Sources (1997) 68, 570

Tunnel structured manganese oxide nanowires as redox active electrodes for hybrid capacitive deionization

Hybrid capacitive deionization (HCDI), which combines a capacitive carbon electrode and a redox active electrode in a single device, has emerged as a promising method for water desalination, enabling higher ion removal capacity than devices containing two carbon electrodes. However, to date, the desalination performance of few redox active materials has been reported. For the first time, we present the electrochemical behavior of manganese oxide nanowires with four different tunnel crystal structures as faradaic electrodes in HCDI cells. Two of these phases are square tunnel structured manganese oxides, α-MnO2 and todorokite-MnO2. The other two phases have novel structures that cross-sectional scanning transmission electron microscopy analysis revealed to have ordered and disordered combinations of structural tunnels with different dimensions. The ion removal performance of the nanowires was evaluated not only in NaCl solution, which is traditionally used in laboratory experiments, but also in KCl and MgCl2 solutions, providing better understanding of the behavior of these materials for desalination of brackish water that contains multiple cation species. High ion removal capacities (as large as 27.8mgg−1, 44.4mgg−1, and 43.1mgg−1 in NaCl, KCl, and MgCl2 solutions, respectively) and high ion removal rates (as large as 0.112mgg−1 s−1, 0.165mgg−1 s−1, and 0.164mgg−1 s−1 in NaCl, KCl, and MgCl2 solutions, respectively) were achieved. By comparing ion removal capacity to structural tunnel size, it was found that smaller tunnels do not favor the removal of cations with larger hydrated radii, and more efficient removal of larger hydrated cations can be achieved by utilizing manganese oxides with larger structural tunnels. Extended HCDI cycling and ex situ X-ray diffraction analysis revealed the excellent stability of the manganese oxide electrodes in repeated ion removal/ion release cycles, and compositional analysis of the electrodes indicated that ion removal is achieved through both surface redox reactions and intercalation of ions into the structural tunnels. This work contributes to the understanding of the behavior of faradaic materials in electrochemical water desalination and elucidates the relationship between the electrode material crystal structure and the ion removal capacity/ion removal rate in various salt solutions.

Chemical, Morphological, and Structural Tuning of Tunnel Manganese Oxides for Intercalation Batteries

In this work, we investigate the effect of tunnel dimension, material morphology, and structural water content on the performance of two tunnel structured manganese oxides in intercalation batteries. Tunnel manganese oxides represent an attractive family of materials for intercalation-based energy storage due to their open crystal structures consisting of well-defined structural tunnels that allow for the reversible insertion/extraction of charge-carrying ions, as well as their low cost, low environmental impact, and high electrochemical activity. Facile hydrothermal methods can be used to synthesize tunnel manganese oxides, which have the general formula AxMnO2, where “A” is a stabilizing cation present within the structural tunnels built from MnO6 octahedra. We focus on comparing the performance of two tunnel manganese oxides with square tunnel structures, α-MnO2 (K0.11MnO2), which contains tunnels of 2x2 octahedra dimension, and todorokite MnO2 (Mg0.20MnO2), which contains tunnels of 3x3 octahedra dimensions. When tested as intercalation electrodes in Na-ion batteries, we find that the material with smaller structural tunnels, α-MnO2, achieved a higher initial capacity of 330 mAh g-1­, while todorokite MnO2, which contains larger 3x3 octahedra tunnels, achieved a capacity of 157 mAh g-1. However, the larger tunnel structure proved to be more stable in a Na-ion battery, demonstrating superior capacity retention, as well as superior rate performance. These results suggest that although smaller tunnels may initially show higher capacity, larger tunnels are preferable for superior electrochemical stability. Further, the effect of material morphology is also shown by synthesizing todorokite MnO2 with both a platelet and a nanowire morphology. In Li-ion batteries, the nanowire morphology exhibited a 30% higher capacity retention after 100 cycles, demonstrating the advantage of a nanowire morphology in terms of enhanced cycling stability. Moreover, the effect of structural water in todorokite MnO2 (0.42 H2O molecules per Mn) is discussed, elucidating the role of structural water on ion insertion of into tunnel structured materials. Lastly, we discuss the effect of controlling the electrochemical potential window used during cycling, the trade off between capacity and capacity retention, and how the stability of these materials can be improved. This work describes the role of tunnel size, ionic content, and material morphology on the behavior of tunnel manganese oxides and thus provides insight into tailoring of tunnel structured materials for intercalation batteries.

Ordered and Disordered Hybrid Tunnel Structured Manganese Oxides for Na-Ion Batteries

Through modification of previously developed hydrothermal techniques, we synthesize two tunnel structured manganese oxides with novel hybrid tunnel configurations and investigate their behavior in Na-ion batteries. Tunnel manganese oxides represent a promising class of materials for Na-ion batteries due to their low cost, low toxicity, and high electrochemical activity. The two materials studied in this work contain unique combinations of structural tunnels composed of varying amounts of MnO6 octahedra surrounding stabilizing Na+ ions, resulting in numerous different tunnel dimensions within individual nanostructures. Through cross-sectional TEM analysis, the ordered phase was found to consist of a systematic combination of 2x2, 2x3, and 2x4 octahedra tunnels. The disordered phase, on the other hand, was found to consist of a more random and nonuniform distribution of not only 2xn (n=2, 3, 4, 5, 6) tunnels, but also larger 3x3 octahedra tunnels. Moreover, although both materials were synthesized with a nanowire morphology, the disordered phase is found to be composed of nanowires with smaller diameter. We investigate these two materials as cathodes for Na-ion batteries and find that upon first discharge, the ordered phase exhibits a capacity nearly 300 mAh g-1, while the disordered phase exhibits a capacity of just below 200 mAh g-1. Upon further cycling, however, the disordered phase maintains a greater amount of its initial capacity, indicating that although the better defined tunnel structure may be advantageous for greater initial ion intercalation capacity, the disordered phase shows a greater degree of electrochemical stability. The rate performance of these two materials is discussed in relationship to the different diffusion pathways present in the materials, as well as the structural stability of the two phases upon repeated ions insertion and deinsertion. The performance of these materials is also compared to tunnel manganese oxides with pure tunnel configurations, and the advantage of having such a hybrid tunnel configuration is also emphasized. This work highlights the performance of two similar novel cathode materials for Na-ion batteries, as well as the relationship between the degree of structural order in manganese oxides and their electrochemical behavior in Na-ion batteries.

Electrode Properties of Todorokite-type Tunnel-structured Manganese Oxide for Calcium Secondary Batteries

The electrode properties of todorokite-type tunnel-structured manganese oxides (TodMO) were examined for their potential use as cathode materials in calcium batteries. TodMO with a chemical composition of Mg0.19Na0.07MnO2·0.37H2O was prepared through hydrothermal treatment of layer-structured manganese oxide using magnesium ions as interlayer guest ions. The TodMO exhibited a dis-charge and charge capacity of 100 and 80 mAh g−1, respectively, at a relatively large current density of 100 mA g−1. The reaction mecha-nism was studied in detail using X-ray absorption spectroscopy measurements. Consequently, it became clear that the newly formed Mn3+-compound converted from TodMO is responsible for the reversible capacity.

The role of electronic and ionic conductivities in the rate performance of tunnel structured manganese oxides in Li-ion batteries

Single nanowires of two manganese oxide polymorphs (α-MnO2 and todorokite manganese oxide), which display a controlled size variation in terms of their square structural tunnels, were isolated onto nanofabricated platforms using dielectrophoresis. This platform allowed for the measurement of the electronic conductivity of these manganese oxides, which was found to be higher in α-MnO2 as compared to that of the todorokite phase by a factor of ∼46. Despite this observation of substantially higher electronic conductivity in α-MnO2, the todorokite manganese oxide exhibited better electrochemical rate performance as a Li-ion battery cathode. The relationship between this electrochemical performance, the electronic conductivities of the manganese oxides, and their reported ionic conductivities is discussed for the first time, clearly revealing that the rate performance of these materials is limited by their Li+ diffusivity, and not by their electronic conductivity. This result reveals important new insights relevant for improving the power density of manganese oxides, which have shown promise as a low-cost, abundant, and safe alternative for next-generation cathode materials. Furthermore, the presented experimental approach is suitable for assessing a broader family of one-dimensional electrode active materials (in terms of their electronic and ionic conductivities) for both Li-ion batteries and for electrochemical systems utilizing charge-carrying ions beyond Li+.