MnO2 Polymorphs Research Articles

Design Guidelines for Cathode/Electrolyte Interface of Room-Temperature Rechargeable Magnesium Batteries with MnO2 Cathodes

Rechargeable magnesium batteries (RMBs) are strong candidates for large-scale energy storage systems due to their high theoretical energy density, high safety, and low production costs. However, the sluggish solid-state diffusion of Mg2+ in cathode materials and surface passivation of Mg metal anode in organic electrolytes are the main reasons for the stagnant development of RMBs. Since 2015, we have first reported the reversible Mg2+ insertion in a series of oxide cathode materials (MgCo2O4, etc.) at 150 ℃.[1] To explore suitable host structures at room temperature (RT), we have been comprehensively investigated the phase transformation pathways of various MnO2 polymorphs (α, β, γ, δ, and λ) and revealed that the hollandite-type α-MnO2 allows topotactic Mg2+ intercalation up to 200 mAh g-1 as a metastable host structure.[2] To construct full cells, we further investigated the reaction behavior of MnO2 cathodes in two representative electrolytes at room temperature.[3] In 2.26 M Mg[TFSA]2/G3 (saturated concentration at RT), the α-MnO2 cathode had a discharge capacity of about 110 mAh g-1 that was confirmed to be mainly derived from the Mg2+ intercalation. However, the Mg metal anodes were easily passivated in this electrolyte, leading to a significant decrease in discharge voltage. On the other hand, in 0.3 M Mg[Al(hfip)4]2/G3,[4] which allows reversible deposition/dissolution on Mg metal anodes, the MnO2 cathode was involved into the cathodic decomposition of the electrolyte species, where the Mg2+ intercalation was difficult to be maintained.To establish design guidelines for a stable and functional cathode/electrolyte interface, we revealed the reaction mechanism of the α-MnO2 cathode in these two electrolytes with various experimental and computational methods. In 2.26 M Mg[TFSA]2/G3, [TFSA]- has close coordination with Mg2+ as contact-ion pairs (CIPs) and a lower energy level of lowest unoccupied molecular orbital (LUMO) compared to the free state.[5] As a result, [TFSA]- is prone to be involved in the cathodic decomposition and significantly increase the F ratio on the α-MnO2 cathode after discharge. As shown in Fig .1, the decomposition products formed an SEI-like layer which would protect the active particles to enable the genuine Mg2+ intercalation. Conversely, in 0.3 M Mg[Al(hfip)4]2/G3, because [Al(hfip)4]- can only exist in free state and is almost not involved the cathodic decomposition, the protective layer was absence on the α-MnO2 cathode. This results in an unstable cathode/electrolyte interface, from where O2- was released from the host structure to the electrolyte instead of the expected Mg2+ intercalation for charge compensation of Mn reduction.[1] S. Okamoto, T. Ichitsubo et al, Adv. Sci. 2015, 2, 1500072. [2] T. Hatakeyama, H. Li, T. Ichitsubo et al, Chem. Mater. 2021, 33, 6983. [3] X. Ye, H. Li, T. Ichitsubo et al, ACS Appl. Mater. Interfaces. 2022, 14, 56685-56696. [4] J. Herb et al, ACS Energy Lett. 2016, 1, 1227–1232. [5] H. Li, T. Ichitsubo et al, Cell Reports Phys. Sci. 2022, 3, 100907. Figure 1

Challenges on Developing Room-Temperature Rechargeable Magnesium Batteries with Oxide Cathodes

Constructing sustainable energy systems requires rechargeable batteries with high safety, long cycle life and low production costs. Rechargeable magnesium batteries (RMBs) have been expected to be a promising candidate owing to low resource constraints compared to the commercial Li-based batteries. However, the development of RMBs have been stagnated after the first report of a prototype system, which consists of a Chevrel phase cathode, Mo6S8, and a Cl-based electrolyte.[1] The development of high potential oxide cathodes and non-corrosion Cl-free electrolyte encounters significant difficulties. Specifically, for oxide cathodes, the strong Coulomb interaction between Mg2+ and host structures usually inhibits the solid-state diffusion, causing a significant low discharge capacity compared to the theoretical value and rapid capacity loss during cycling.[2] Besides, non-corrosion electrolytes usually passivate Mg metal anodes and/or induce side reactions on oxide cathodes,[3] where remarkably decreases the cell voltage and accelerates the decomposition of electrolytes and active materials.In this presentation, we introduce our efforts to construct room-temperature RMBs with oxide cathodes and Cl-free electrolytes combining both the experimental and computational approaches. To explore suitable host structures for the topotactic Mg2+ intercalation, we revealed the phase transformation pathways and evaluated cathode properties of various transition oxide cathodes (e.g. MnO2 polymorphs) at both elevated temperature (150℃) and room temperature.[3-6] Moreover, to assess the electrolyte compatibility, we discovered the connection between the solvation environment and the reaction behavior on both the oxide cathodes and Mg metal anodes.[3,7] These findings pave the way for room-temperature RMBs using oxide cathodes.[1] D. Aurbach et al, Nature 407, 724 (2000)[2] T. Ichitsubo et al, J. Mater. Chem. A 21, 11764 (2011)[3] X. Ye, H. Li, T. Ichitsubo et al, ACS Mater. Interfaces 14, 56685 (2022)[4] S. Okamoto, T. Ichitsubo et al, Adv. Sci. 2, 1500072 (2015)[5] K. Shimokawa, T. Ichitsubo et al, Adv. Mater. 33, 2007539 (2021)[6] T. Hayakeyama, H. Li, T. Ichitsubo et al, Chem. Mater. 33, 6983 (2021)[7] X. Ye, H. Li, T. Ichitsubo et al, submitted.

Unveiling the Role of Strong Metal-Support Interactions in Gold-Catalyzed CO Oxidation on MnO2 Polymorphs.

The effectiveness of gold (Au)-based catalysts in CO oxidation is significantly influenced by strong metal-support interactions with surface oxygen structures, the mechanisms of which remain elusive. To investigate this property, we selected γ-MnO2, featuring Mn(-O-)2Mn and Mn-O-Mn structural motifs, and β-MnO2, characterized by Mn-O-Mn linkages, as support materials. The CO oxidation process was investigated by fabricating Au nanoparticles supported on these two MnO2 polymorphs. Our findings reveal that Au supported on β-MnO2 substantially enhanced CO oxidation, in stark contrast to the inhibitory effect observed with Au on γ-MnO2. Using operando diffuse reflectance infrared Fourier transform spectroscopy coupled with mass spectrometry, we detected an increase in the production of surface-adsorbed oxygen following Au deposition on β-MnO2. Conversely, Au supported on γ-MnO2 resulted in a diminished capacity for surface oxygen adsorption. The presence of Au+ and Mn2+ ions was identified as pivotal for CO oxidation. Moreover, the engagement of the Mn(-O-)2Mn structure in the reaction was impaired after Au loading on γ-MnO2, and the regeneration of the Mn-O-Mn linkage was similarly hindered. We propose a mechanism for the interactions between Au and the oxygen species associated with Mn(-O-)2Mn and Mn-O-Mn structures on MnO2, offering insights into the divergent catalytic behaviors exhibited by different MnO2 polymorphs.

Artificially interconnected ion-diffusion nanochannels in ion-indiffusible phase-conversion cathodes for rechargeable aqueous zinc batteries

A substantial proportion of multivalent-ion battery cathodes undergo solid-solid phase conversion that embrace adequate energy densities, but considerable gaps remain between theoretical prediction and experimental realization of such features owing to thermodynamically large-energy-barrier or even infeasible ion migration between interstitial sites of close-packed anionic frameworks. We build artificially interconnected ion-diffusion nanochannels in ion-indiffusible cathodes, or bixbyite α-phase Mn2O3 more specifically, on the basis of grain boundary (GB) networks, realized via a glucose-assisted solution thermal shock strategy. Remarkable tensile and shear strains coexist along GB grids and Zn2+-diffusion energy barriers within them are merely ∼56% of that in ion-diffusible layered MnO2 polymorphs, causing two orders of magnitude increase in ion-diffusion coefficients, modulated diffusion-/capacitance-controlled kinetic processes and reduced charge-transfer resistance. Consequently, this phase-transition cathode is characterized by faster kinetics than reported analogues, delivering specific capacities of 365 mAh g−1 at 0.1 A g−1 and 108 mAh g−1 at 10 A g−1, along with impressive cycling stability with 91% capacity retention at 1A g−1 after 1000 cycles. Designing analogously hierarchical architectures provides a promising direction for further optimization of cathodes with close-packed anionic frameworks.

Polymorphic Engineering of MnO2 Structure as a Diffusion Blocking Layer for Selective Seawater Electrolysis

The water splitting via renewable energy is one of the sustainable process to generate the future clean energy in form of hydrogen. One of the key challenge for the large-scale water electrolysis is the availability of pure water. The sweater can be consider as an alternate feedstock for the large-scale water electrolysis. However, the presence of undesirable components like chlorides, bromides etc. are the potential challenge to make the use of seawater as a facile feedstock. As per the electrochemical potentials, chlorine evolution reaction (CER) along with the oxygen evolution reaction (OER) at the anode is one of the primary existing challenge in seawater electrolysis. Where thermodynamics favour OER and kinetic benefits, the CER process with two-electron transfers reaction. As of now, precious noble metal catalysts such as ruthenium and iridium-based oxides have been explored as a catalyst for seawater electrolysis. However, their high cost does not recommend the use of these catalysts for the large-scale production and limits its commercialization. Here, we are exploring the non-precious Mn based catalyst for OER vs. CER and make make seawater electrolysis as a commercial favourable process. We have explored various polymorphs of MnO2, such as α, β, γ, and δ-MnO2, considering the MnO2 as diffusion blocking layer for the selective OER process during the seawater electrolysis. The polymorphic engineering provide the window to tune the catalytic properties to make it selective towards the OER. Further, the results suggest that α- and β-MnO2 are OER selective however, the γ-, and δ- MnO2 are CER selective. We have identified the reaction mechanism using various electrochemical tools and shown the structure-activity-selectivity trends for the polymorphic structure of MnO2.

Spatial geometric effect driven by the different [MnO6] octahedra entity stacking configurations to facilitate the catalytic decomposition of H2O2 in wastewater

Achieving rapid decomposition of H2O2 is crucial for broadening its applications across various fields, as improper disposal or leakage can lead to serious water pollution. In this work, we explore how the octahedral coordination entity [MnO6] stacking configurations in different MnO2 polymorphs (α-, β-, γ- and δ-) affect their catalytic performance towards H2O2 decomposition. Combining experiments and theoretical simulations, we find that the catalytic activity for H2O2 decomposition strongly depends on the [MnO6] stacking configurations, following the order of δ- < α- < γ- < β-MnO2. δ-MnO2 displays edge-sharing octahedral stacking in a layered structure and thus exhibits the highest catalytic activity, efficiently decomposing H2O2 (0.5 M) in just 2 min at 25 °C without excessive energy consumption. Theoretical calculations further show that the superior active sites are located at the edge Mn-site of its 2D layered structure, with the rate-determining step of H2O desorption (0.55 eV). This study provides crucial insights into the impact of coordination entity stacking configurations on the catalytic activity differences.

First-principles evaluation of MnO2 polymorphs as cathode material in lithium-ion batteries

MnO2 polymorphs show distinct advantages and disadvantages when used as cathode material for Li-ion batteries (LIBs).

Monitoring of Energy Storage Mechanism of Zn/Meso-MnO2 Battery System Using X-Ray Absorption Fine Structure

As energy storage systems, LIBs, which are most frequently used, have excellent capacity, power, and long lifespan performance [1]. However, as the reserves of the raw minerals used—lithium and cobalt—are exhausted, prices are escalating. A further issue with LIB is that it heats up to a high temperature during charging, which leads to a string of fire or explosion catastrophes. To solve these safety or cost challenges, various next-generation batteries are being developed, and we are exploring about aqueous zinc ion battery (AZIB), which attracting attention as the next-generation battery due to outstanding safety and cost competitive advantage. Generally, zinc metal acting as an anode, has a high theoretical capacity of 820mAh/g as it is gives two electrons (2e-). Also, because of its sparse reactivity with water and ability to employ water-based electrolytes with strong ion conductivity and low ignition risk, it is particularly advantageous in terms of safety. There have been a variety of cathode materials utilized, such as manganese oxide, vanadium oxide, Prussian blue analogues (PBAs), polyanion compounds, and Chevrel phase compounds [2-4]. Especially, manganese dioxide (MnO2) has been addressed by prior studies that Zn2+ can be inserted into crystal structure because it exhibits several crystal structures (α, β, γ, δ, and λ) with tunnel-type or layer-type structure. Mn4+ ions occupying octahedral holes formed by hexagonal close-packed (hcp) oxide ions compose the fundamental unit of the crystal structure of MnO2 polymorphs. To construct a [MnO6] octahedral unit, each Mn4+ ion is precisely surrounded by six oxygen neighbors, and these units are connected by the edges and/or corners [5]. Understanding the energy storage mechanisms is essential for the one-step evolution of cathode materials for AZIB. However, these polymorphs significantly affect the electrochemical reaction, thus involving complex reaction mechanisms [5-7]. Additionally, it is extremely challenging to research the mechanisms of AZIB since protons (H+) are known to participate in the reaction by acting as charge transfer as well as Zn2+ ions. Therefore, while several efforts are being conducted to unveil the exact reaction mechanism, a mesoporous nanostructured material with a large specific surface area is adopted, which can more precisely grasp the electrochemical reaction studies of electrode/electrolyte surface [8,9]. X-ray absorption fine structure (XAFS) is a powerful technique used for the analysis of nanostructured materials, widely used in the field of battery research [10]. It involves the measurement of the absorption of X-ray by the material, which can provide information about oxidation state, chemical bonding of the materials and the local geometry structure during charge/discharge process. This analysis method gives great insight into the complex energy storage pathway of MnO2 materials with various polymorphs. Based on this knowledge, we synthesized ordered mesoporous β-MnO2 through nano-casting method by using KIT-6 as hard-template and used it as the cathode active material. As depicted in Figure 1, we performed ex-situ XAFS analysis at the states of charge during 2nd cycle to monitor energy storage mechanisms. The oxidation state changes could be quantitatively estimated by edge energy analysis by comparing the reference MnO2 and Mn2O3 samples. It was also possible to unveil further about the reaction mechanism at each voltage by examining the structural change in the local geometry of [MnO6] unit through a careful analysis of pre-edge region and EXAFS spectra. During discharge process, insertion mechanism, which causes relatively larger distortion in the local geometry compared to conversion mechanism, results in a increment of pre-edge intensity, which opposes the decline in pre-edge intensity due to the reduction of the oxidation state. This fact demonstrates that although H+ conversion could not have a significant effect on pre-edge intensity in the early discharge stage, Zn2+ insertion was the phenomenon responsible for the rise in pre-edge intensity during late discharge stage. In addition, through EXAFS spectra analysis, the irreversible change in corner sharing Mn-Mn bond distance during charge/discharge process could be seen that an irreversible phase transition of MnO2 occurred due to Zn2+ insertion, indicating clue to identify the cause of capacity fading. These findings contribute to further understanding of the reaction mechanisms and capacity fading phenomenon and suggest practical strategies for next-generation zinc-ion batteries.

The relative contributions of Mn(III) and Mn(IV) in manganese dioxide polymorphs to bisphenol A degradation

Polymorphs of MnO2 comprise Mn(III) and Mn(IV), which are both strong oxidants capable of BPA degradation, but their relative contributions are unclear. To advance process understanding, the reactivities of biogenic MnO2 prepared using Roseobacter sp. AzwK-3b and synthetic MnO2 (i.e., hexagonal and triclinic birnessite) toward BPA were compared. Both colloidal and particulate biogenic MnO2, as well as triclinic birnessite, showed insignificant reactivity towards BPA, but degradation did occur when pyrophosphate (PP), a ligand for Mn(III), was present. Despite higher Mn(III) content of triclinic birnessite (38.6 %), only hexagonal birnessite with an Mn(III) content of 30.4 % degraded BPA without PP, and no rate increases were observed following the addition of PP. Similarly, colloidal MnO2 degraded BPA with nearly double the rate measured with particulate MnO2 (i.e., 1.24 ± 0.10 versus 0.73 ± 0.08 h−1), even though the Mn(III) contents were only 10 % different. The Mn(III) release rates from each MnO2 polymorph in the presence of PP correlated more strongly with the observed BPA degradation rates than with Mn(III) content, suggesting that both Mn(III) release rate and Mn(III) content govern MnO2-mediated BPA degradation. In natural settings, Mn(III) generally occurs in complexed form suggesting that laboratory testing should include ligands to derive environmentally relevant information about MnO2-mediated degradation of BPA and other compounds of concern.

Effect of crystalline phase of MnO2 on the degradation of Bisphenol A by catalytic ozonation

In recent decades, manganese oxide (MnO2) has been an attractive catalyst for organic pollutants removal from wastewater by ozonation. However, the effect of the MnO2 crystalline phase on Bisphenol A ozonation in water has never been investigated. In the present study, three crystalline phases of MnO2 were synthesized by hydrothermal method, including α-, γ-, and δ-MnO2, the last generating three different kinds. The as-prepared MnO2 were evaluated in the ozonation of Bisphenol A (BPA) and correlated with their physicochemical properties obtained by different characterization techniques. The activity of MnO2 polymorphs for TOC removal followed the order of γ-MnO2 > δ3-MnO2 > δ1-MnO2 > α-MnO2 > δ2-MnO2. The catalytic activity of MnO2 strongly depended on the presence of active sites such as density of the surface oxygen vacancy, Mn+3/Mn+4 ratio, and OH-groups on the catalyst surface. The catalysts with the highest amount of oxygen vacancies (but the minor density of the surface oxygen vacancy) and OH-groups were δ3-MnO2 and γ-MnO2, respectively, demonstrating excellent catalytic activity on BPA removal. The effect of stabilities of δ3-MnO2 and γ-MnO2 was also investigated in this research. Furthermore, the toxicity bioassay based on Lactuca sativa seeds demonstrated that by-products generated during conventional and catalytic ozonation were non-toxic. The elimination of the model mixture of emergent compounds in four types water were also tested with δ3-MnO2 and γ-MnO2, obtaining mineralization values around 47–59 % and 40–55 %, respectively, at 120 min compared with 18–23 % of conventional ozonation.

Ultrahigh-Loading Manganese-Based Electrodes for Aqueous Batteries via Polymorph Tuning.

Manganese-based aqueous batteries utilizing Mn2+ /MnO2 redox reactions are promising choices for grid-scale energy storage due to their high theoretical specific capacity, high power capability, low-cost, and intrinsic safety with water-based electrolytes. However, the application of such systems is hindered by the insulating nature of deposited MnO2 , resulting in low normalized areal loading (0.005-0.05mAhcm-2 ) during the charge/discharge cycle. In this work, the electrochemical performance of various MnO2 polymorphs in Mn2+ /MnO2 redox reactions is investigated, and ɛ-MnO2 with low conductivity is determined to be the primary electrochemically deposited phase in normal acidic aqueous electrolyte. It is found that increasing the temperature can change the deposited phase from ɛ-MnO2 with low conductivity to γ-MnO2 with two order of magnitude increase in conductivity. It is demonstrated that the highly conductive γ-MnO2 can be effectively exploited for ultrahigh areal loading electrode, and a normalized areal loading of 33mAhcm-2 is achieved. At a mild temperature of 50°C, cells are cycled with an ultrahigh areal loading of 20mAhcm-2 (1-2 orders of magnitude higher than previous studies) for over 200 cycles with only 13% capacity loss.

Investigating the electrochemical performance of MnO2 polymorphs as cathode materials for aqueous proton batteries

Aqueous proton batteries (APBs) have attracted considerable attention due to the small size and fast kinetics of protons. However, the study of the cathode materials for APBs is unsatisfactory due to either low specific capacity or poor stability in acidic electrolytes, which greatly hinders the development of APBs. In this work, for the first time, six kinds of MnO2 polymorphs (α-, β-, δ-, γ-, λ-, and R-MnO2) are synthesized and systematically investigated to evaluate their potential as cathodes for APBs, among which the λ-MnO2 stands out by virtue of the 235 mAh g−1 specific capacity and 85% capacity retention rate over 500 cycles. To explain the reason for the stability of the λ-MnO2, DFT calculations are conducted, and the results indicate that the λ-MnO2 sustains minute volume expansion after proton insertion due to the slight Jahn-Teller distortion and robust all edge-sharing structure. The full battery performance shows the potential of the λ-MnO2 as a promising cathode for APBs with an excellent capacity of 202 mAh g−1 at 0.1 A g−1 and an outstanding retention rate of 80% after 1000 cycles. These results broaden the range of cathode materials and shed new light on the performance improvement of APBs.

Electrochemical performance of K+ -intercalated MnO2 nano-cauliflowers and their Na-ion-based pseudocapacitors

The polymorphs of MnO2 attract great deal of attention as electrode materials due to their layer/ tunnel structures which allow for rapid intercalation/deintercalation and facilitate fast charge–discharge. Pre-intercalating MnO2 with alkali cations is reported to enhance ion diffusion yielding better electrochemical response. Taking advantage of crystal structure of MnO2 and intercalated K+-ions, a facile synthesis of cauliflower-shaped MnO2 nanoparticles that yields self-intercalation of K+ ions is reported. Controlled sample is obtained by thoroughly washing as-prepared sample and their electrochemical response for Na-ion pseudocapacitors is compared. The sample with K+ exhibits ∼20 % enhancement in specific capacitance value. Both the samples yield better electrochemical properties in basic electrolyte (NaOH) as compared to neutral electrolyte (NaNO3). The asymmetric device fabricated using MnO2-K as the positive electrode and activated carbon (AC) as the negative electrode yields an impressive energy density of ̴ 23 Wh kg−1 with the cycling stability of 98 % after 1000 cycles.

Significance of MnO2 Type and Solution Parameters in Manganese Removal from Water Solution.

A very low concentration of manganese (Mn) in water is a critical issue for municipal and industrial water supply systems. Mn removal technology is based on the use of manganese oxides (MnOx), especially manganese dioxide (MnO2) polymorphs, under different conditions of pH and ionic strength (water salinity). The statistical significance of the impact of polymorph type (akhtenskite ε-MnO2, birnessite δ-MnO2, cryptomelane α-MnO2 and pyrolusite β-MnO2), pH (2-9) and ionic strength (1-50 mmol/L) of solution on the adsorption level of Mn was investigated. The analysis of variance and the non-parametric Kruskal-Wallis H test were applied. Before and after Mn adsorption, the tested polymorphs were characterized using X-ray diffraction, scanning electron microscope techniques and gas porosimetry analysis. Here we demonstrated the significant differences in adsorption level between MnO2 polymorphs' type and pH; however, the statistical analysis proves that the type of MnO2 has a four times stronger influence. There was no statistical significance for the ionic strength parameter. We showed that the high adsorption of Mn on the poorly crystalline polymorphs leads to the blockage of micropores in akhtenskite and, contrary, causes the development of the surface structure of birnessite. At the same time, no changes in the surfaces of cryptomelane and pyrolusite, the highly crystalline polymorphs, were found due to the very small loading by the adsorbate.

Examining Electrolyte Compatibility on Polymorphic MnO2 Cathodes for Room-Temperature Rechargeable Magnesium Batteries

Rechargeable magnesium batteries are promising candidates for post-lithium-ion batteries, owing to the large source abundance and high theoretical energy density. However, there remain few reports on constructing practical cells with oxide cathodes and Mg anodes at room temperature. In this work, we compare the reaction behavior of various MnO2 polymorph cathodes in two representative electrolytes: Mg[TFSA]2/G3 and Mg[Al(hfip)4]2/G3. In Mg[TFSA]2/G3, discharge capacities of the MnO2 cathodes are well consistent with the changes in Mg composition, where nanorod-like α-MnO2 and λ-MnO2 show the capacities of about 100 mA h g-1 at room temperature. However, this electrolyte has the disadvantage that the Mg anodes are easily passivated. In contrast, Mg[Al(hfip)4]2/G3 allows highly reversible deposition/dissolution of Mg anodes, whereas the discharge process of the MnO2 cathodes involves a large part of side reactions, in which the MnO2 active material takes part in some reductive reaction together with electrolyte species instead of the expected Mg2+ intercalation. Such an unstable electrode/electrolyte interface would lead to continuous degradation on/near the cathode surface. Thus, the interfacial stability between the oxide cathodes and the electrolytes must be improved for practical applications.

On the Mechanism of Heterogeneous Water Oxidation Catalysis: A Theoretical Perspective

Earth abundant transition metal oxides are low-cost promising catalysts for the oxygen evolution reaction (OER). Many transition metal oxides have shown higher OER activity than the noble metal oxides (RuO2 and IrO2). Many experimental and theoretical studies have been performed to understand the mechanism of OER. In this review article we have considered four earth abundant transition metal oxides, namely, titanium oxide (TiO2), manganese oxide/hydroxide (MnOx/MnOOH), cobalt oxide/hydroxide (CoOx/CoOOH), and nickel oxide/hydroxide (NiOx/NiOOH). The OER mechanism on three polymorphs of TiO2: TiO2 rutile (110), anatase (101), and brookite (210) are summarized. It is discussed that the surface peroxo O* intermediates formation required a smaller activation barrier compared to the dangling O* intermediates. Manganese-based oxide material CaMn4O5 is the active site of photosystem II where OER takes place in nature. The commonly known polymorphs of MnO2; α-(tetragonal), β-(tetragonal), and δ-(triclinic) are discussed for their OER activity. The electrochemical activity of electrochemically synthesized induced layer δ-MnO2 (EI-δ-MnO2) materials is discussed in comparison to precious metal oxides (Ir/RuOx). Hydrothermally synthesized α-MnO2 shows higher activity than δ-MnO2. The OER activity of different bulk oxide phases: (a) Mn3O4(001), (b) Mn2O3(110), and (c) MnO2(110) are comparatively discussed. Different crystalline phases of CoOOH and NiOOH are discussed considering different surfaces for the catalytic activity. In some cases, the effects of doping with other metals (e.g., doping of Fe to NiOOH) are discussed.

Enhancing arsenic (III) removal by integrated electrocatalytic oxidation and electrosorption reactions on nano-textured bimetal composite of iron oxyhydroxide and manganese dioxide polymorphs (α-, γ-, β-, and ε-MnxFe1−xO)

A composite electrode of manganese oxide (MnO2) incorporated with iron oxide (α-FeOOH) was synthesized for arsenite (As(III)) oxidation and subsequent arsenate (As(V)) electrosorption. The crystal structure and chemical state of MnO2 polymorphs were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS), and BET surface area. The redox couple of Mn(III)/Mn(IV) mediated the catalytic electron transfer with respect to As(III)/As(V) redox equilibrium. The Mn site contributed a high diffusive current to the redox capacitance, meanwhile the Fe site better provided the double-layer capacitive deionization for arsenic species. Electrolysis of arsenite under constant anodic potential mode (+1.0 V vs. Ag/AgCl) enabled assess the performance of the electrodes. Among the polymorphs, γ-Mn0.2Fe0.8O exhibited the best arsenic adsorption capacity of 48 mg-As g−1, compared to that of α-FeOOH NPs (15 mg-As g−1) and γ-MnO2 (7 mg-As g−1), based on multilayer Langmuir adsorption model.

MnO2 Polymorphs as Cathode Materials for Rechargeable Ca‐Ion Batteries

AbstractCalcium‐ion batteries (CIBs) have attracted extensive interest for energy storage due to the high operating potentials and the abundant resources of calcium. However, the application of CIBs has been limited by the absence of high‐voltage and high‐performance cathode materials. Herein, for the first time, four kinds of MnO2 polymorphs (α, β, γ, and δ‐phase) as cathode materials for CIBs are reported, and the δ‐MnO2 exhibits the highest electrochemical performance, which is mainly due to the higher Ca‐ion diffusivity of δ‐MnO2. The excellent rate capability obtained herein outperforms those of previously reported for all oxides as cathode materials for CIBs. Furthermore, the Ca‐ion storage mechanism of δ‐MnO2 based on the reversible order–disorder transformation is demonstrated by the in situ/ex situ characterizations. In addition, the capacity decay mechanism of δ‐MnO2 during the cycling process is confirmed as the dissolution of Mn and the optimization strategy is proposed. These results demonstrate that the δ‐MnO2 is a promising cathode material for CIBs, providing new opportunity for the development of high‐performance CIBs.

Effect of MnO2 Polymorphs’ Structure on Low-Temperature Catalytic Oxidation: Crystalline Controlled Oxygen Vacancy Formation

MnO2 polymorphs (α-, β-, and ε-MnO2) were synthesized, and their chemical/physical properties for CO oxidation were systematically studied using multiple techniques. Density functional theory (DFT) calculations and temperature-programmed experiments reveal that β-MnO2 shows low energies for oxygen vacancy generation and excellent redox properties, exhibiting significant CO oxidation activity (T90 = 75 °C) and stability even under a humid atmosphere. For the first time, we report that the specific reaction rate for β-MnO2 (0.135 moleculeCO·nm-2·s-1 at 90 °C) is roughly approximately 4 and 17 times higher than that of ε-MnO2 and α-MnO2, respectively. The specific reaction rate order (β-MnO2 > ε-MnO2 > α-MnO2) is not only in good agreement with reduction rates (CO-TPSR measurements) but also agrees with the DFT calculation. In combination with in situ spectra and intrinsic kinetic studies, the mechanisms of CO oxidation over various crystal structures of MnO2 were proposed as well. We believe the new insights from this study will largely inspire the design of such a kind of catalyst.

Understanding the As(III) oxidative performance of MnO2 polymorphs (α, β, and γ) and synthesis of an efficient nanocomposite of iron ore slime derived 2-line ferrihydrite and γ-MnO2 for sequestration of total arsenic from aqueous solution

The present research was performed with a primary objective to study the performance of different MnO2 polymorphs for oxidative transformation of As(III). Five different MnO2 polymorphs with distinct morphologies were hydrothermally synthesized by altering the precursor and synthesis condition. The As(III) oxidative reactivity of MnO2 polymorphs, as confirmed by the kinetic experiment, were found to be in the order: γ-M2 > α-M2 > α-M1 > γ-M1 > β-M1. The study confirms that the surface Mn(III) content, crystallinity, reducibility, and synthesis temperature are the primary deciding factors that regulate the As(III) oxidative performance of MnO2. In addition, 2-line ferrihydrite nanoparticles were synthesized from iron ore slime for removal of As(V). A new nanocomposite of 2-line ferrihydrite and γ-MnO2 was prepared for the removal of both As(III) and As(V). The nanocomposite with high As(III) and As(V) removal affinity, excellent reusability and outstanding performance in groundwater environment demonstrate its potential as a front-line arsenic removal material.