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MnO2-based aqueous Zn-ion batteries (ZIBs) hold great promising for large-scale energy storage applications owing to their safe and sustainable nature. However, rapid capacity decay under high depth of discharge limits the applications of MnO2 cathodes. In the meantime, the reaction chemistry and degradation process of MnO2 cathodes cannot be fully understood, leading to improvement of their cycling stability lacks of robust methods. Herein, ZIB performances of MnO2 polymorphs are investigated to disclose their detailed reaction chemistry and degradation mechanisms. Ex situ characterizations at different cycles exhibit evolution of active materials (original MnO2→Mn2+→birnessite→ZnMn2O4/Mn3O4) and coexisted reactions from co-insertion, dissolution/deposition and chemical conversion mechanisms. Variational contributions from these intermediate products and different reaction mechanisms cause fluctuated performance during cycling. Initial performance activation is from enhanced activity of birnessite, while the degradation is caused by its conversion to electrochemically inactive ZnMn2O4 and Mn3O4. By optimizing tunnel structures, it is found that R-MnO2 shows low manganese dissolution with its reaction mainly achieved by intercalation/extraction of Zn2+/H+, and theoretical calculations verify its low Jahn-Teller distortion at discharged state. This specific property circumvents conversion of R-MnO2 to metastable birnessite, giving rise to a stable capacity under high depth of discharge.

High electrochemically active birnessite is always desirable pseudocapacitive material for supercapacitor. Here, two-dimensional (2D) compulsive malposition parallel birnessite standing on β-MnO2 interconnected networks have been designed. Due to the restriction of β-MnO2, compulsive malposition, slippage of MnO6 slab, occurred in birnessite resulting in weaken binding force between birnessite slab and interlayer cations, which enhanced their electrochemical performances. Additionally, the electrical conductivity of the structure was largely promoted by the 2D charge transfer route and double-exchange mechanism in birnessite, also leading to desirable electrochemical properties. Based on the fraction of as-prepared nanostructure, the parallel birnessite exhibited good pseudocapacitance performance (660 ​F ​g−1) with high rate capability. In addition, the asymmetric supercapacitor assembled by reduced graphene oxide (RGO) and as-prepared nanostructure, which respectively served as the negative and positive electrode, delivered an energy density of 33.1 ​Wh kg−1 and a maximum power density of 64.0 ​kW ​kg−1 with excellent cycling stability (95.8% after 10000 cycles). Finally, the study opens new avenues for synthesizing high electrochemically active birnessite structure for high-performance energy storage devices.

The necessity of arsenic (As) removal from metallurgical wastewaters is increasing. Despite its wide recognition as a natural oxidant, the utility of Mn oxide for scorodite production is mostly unknown. In acidic solutions containing both As(III) and ^^^, simultaneous oxidation of the two progressed by MnO_2 and the resultant As(V) and ^^^ triggered the formation of crystalline scorodite (FeAsO_4·2H_2O). At 0.5% or 0.25% MnO_2, 98% or 91% As was immobilized by day 8. The resultant scorodite was sufficiently stable according to the TCLP test, compared to the regulatory level in US and Chile (5 mg/L): 0.11 « 0.01mg/L at 0.5% MnO_2, 0.78 « 0.05mg/L at 0.25% MnO_2. For the oxidation of As(III) and ^^^, 54% (at 0.5% MnO_2) or 14% (at 0.25% MnO_2) of initially added MnO_2 remained undissolved and the rest dissolved in the post As(III) treatment solution. For the Mn recycling purpose, the combination of ^^^-oxidizing bacteria and biogenic birnessite (as homogeneous seed crystal) was used to recover up to 99% of dissolved ^^^ as biogenic birnessite ((Na, Ca)0.5(^^^,^^^III)_2O_4·1.5H_2O), which can be utilized for the oxidation treatment of more dilute As(III) solutions at neutral pH. Although further optimization is necessary, the overall finding in this study indicated that Mn oxide could be utilized as a recyclable oxidant source for different As(III) treatment systems.

Structural unsteadiness and limited electrochemical kinetics upon cycling seriously impede further applications of birnessite cathodes for rechargeable aqueous zinc‑ion batteries (ZIBs), even though they have high voltage platforms and distinctively layered structures for preferable (de)intercalation of zinc ions. Herein, yolk-shell structured K-birnessite (K0.48Mn2O4·0.49H2O)@mesoporous carbon nanospheres (KMOH@C) with rich oxygen vacancies are synthesized for the first time with a two-step diffusion-driven strategy of hydrothermal synthesis followed by etching with KOH. The transport of reaction ions is regulated by surface charge and pore structure of the carbon shells, thus K-birnessite is preciously transferred into the hollow mesoporous carbon (HMC) nanospheres. Furthermore, the etching effect of KOH and the confinement effect of HMC nanospheres generate intercalated K+ and abundant oxygen vacancies into KMOH, leading to an excellent electrochemical kinetics. Meanwhile, HMC nanospheres also endow rapid electron/ion transport and stabilize the crystal structure of K-birnessite. Therefore, KMOH@C exhibits superior electrochemical performances with high reversible capacities of 412.7 and 122.2 mA h g‒1 at 0.5 and 10.0 A g‒1 than reported cathodes, respectively. Moreover, an exceptional cyclability of 129.6 mA h g‒1 even after 6000 cycles at 3.0 A g‒1 is achieved, making the KMOH@C cathode highly competitive for eco-friendly aqueous ZIBs.

Birnessite-MnO2 nanoflakes were synthesized via an aqueous oxidation method at 90 °C using Mn(CH3COO)2, NaOH, and KMnO4. The samples’ morphology, crystalline structure, and optical property were determined by field emission scanning electron microscopy, X-ray powder diffraction and UV-Vis spectrophotometry. The birnessite-MnO2 nanoflakes were converted to KxMn8O16 and Mn suboxides following a decrease in the concentration of KMnO4 in the reaction. The amount of NaOH in the reaction determined the type of precursor. Without NaOH, the precursor was converted from Mn(OH)2 to Mn2+ (from Mn(CH3COO)2), thereby enabling the synthesis of birnessite-MnO2 nanoflowers. The formation mechanism of birnessite-MnO2 nanoflowers and nanoflakes was clarified via the corresponding simulated crystal structures. Evaluation of the synthesized samples confirmed that the birnessite-MnO2 nanoflakes and nanoflowers exhibited excellent degradation properties.

We fabricated a thin film of layered MnO2 whose interlayer space was occupied by hydrated Ni2+ and Cu2+ ions. The process consisted of electrodeposition of layered MnO2 intercalated with tetrabutylammonium cations (TBA+) by anodic oxidation of aqueous Mn2+ ions in the presence of TBA+, followed by ion exchange of the initially incorporated bulkier TBA+ with the denser transition metals in solution. The resulting layered MnO2 co-intercalated with Ni2+ and Cu2+ ions (NiCu/MnO2) catalyzed the ammonia oxidation reaction (AOR) in an alkaline electrolyte with a much lower overpotential than its Ni2+- and Cu2+-intercalated single-cation counterparts. Surprisingly, the NiCu/MnO2 electrode achieved a faradic efficiency as high as nearly 100% (97.4%) for nitrogen evolution at a constant potential of +0.6 V vs Hg/HgO. This can be ascribed to the occurrence of the AOR in the potential region where water is stable and dimerization of the partially dehydrogenated ammonia species is preferred, thereby forming an N-N bond, rather than to be further oxidized into NOx species.

Room-temperature catalytic decomposition of low-concentration volatile organic compounds (VOCs) in indoor air is an exciting dream to solve their pollution. Herein, two kinds of rare-earth elements (Y and La) were separately doped into birnessite-type MnO2 nanosheets in the form of single atoms by the hydrothermal method. As-synthesized La/MnO2 achieved 100% removal of 10 ppm toluene at 40 °C under the gas hourly space velocity of 60 L g-1 h-1, which was even somewhat better than the single Pt atom-doped MnO2. In addition, La/MnO2 showed the good durability at room temperature for 0.5 ppm toluene removal under the GHSV of 300 L g-1 h-1 and could be effectively regenerated at 105 °C. GC/FID, online-MS and TD-GC/MS analysis demonstrated that only ignorable trace benzene (∼3.4 ppb, < one thousandth of inlet toluene) was generated in the gas phase during catalytic decomposition of 10 ppm toluene at room temperature. This research sheds light on the development of low cost and high activity catalysts for low-concentration VOC oxidation at room temperature.

Li-CO2 batteries are one type of promising energy storage and conversion devices to capture and utilize the greenhouse gas CO2, mitigating global temperature rise and climate change. Catalysts that could effectively decompose the discharge product, Li2CO3, are essential for high-performance Li-CO2 batteries. Benefiting from the interconnected porous structure, favorable oxygen vacancy, and the synergistic effects between the carbon nanotube (CNT) and layered birnessite δ-MnO2, our Li-CO2 cathodes with the as-prepared CNT@δ-MnO2 catalyst can efficiently afford a large reaction surface area and abundant active sites, provide sufficient electron/Li+ transport pathways, and facilitate electrolyte infiltration and CO2 diffusion, demonstrating low overpotential and superior cycling stability, which have been proven by both experimental characterization and theoretical computation. It is expected that this work can provide guidance for the design and synthesis of high-performance electrochemical catalysts for Li-CO2 batteries.

Rechargeable aqueous zinc-ion batteries (ZIBs) are regarded as an ideal choice for next-generation energy-storage devices. Nevertheless, the ZIB performance is still far from satisfactory due to th...

Birnessite is a layered MnO2 mineral capable of intercalating nanometric water films in its bulk. With its variable distributions of Mn oxidation states (MnIV, MnIII, and MnII), cationic vacancies, and interlayer cationic populations, birnessite plays key roles in catalysis, energy storage solutions, and environmental (geo)chemistry. We here report the molecular controls driving the nanoscale intercalation of water in potassium-exchanged birnessite nanoparticles. From microgravimetry, vibrational spectroscopy, and X-ray diffraction, we find that birnessite intercalates no more than one monolayer of water per interlayer when exposed to water vapor at 25 °C, even near the dew point. Molecular dynamics showed that a single monolayer is an energetically favorable hydration state that consists of 1.33 water molecules per unit cell. This monolayer is stabilized by concerted potassium–water and direct water–birnessite interactions, and involves negligible water–water interactions. Using our composite adsorption–condensation–intercalation model, we predicted humidity-dependent water loadings in terms of water intercalated in the internal and adsorbed at external basal faces, the proportions of which vary with particle size. The model also accounts for additional populations condensed on and between particles. By describing the nanoscale hydration of birnessite, our work secures a path for understanding the water-driven catalytic chemistry that this important layered manganese oxide mineral can host in natural and technological settings. Originally included in thesis in manuscript form.

Lattice collapse and associated mechanical fracture frequently occur in Li-intercalated metal oxide cathodes at the deep charge state upon Li-ion removal, governed by chemical compositions and the resulting electron density of the oxygen atoms. However, similar lattice collapse for metal oxide electrodes in aqueous storage and its mitigation have not been well studied. Herein, we reported the lattice collapse of MnO2 layered birnessite during the aqueous de-sodiation process at high voltage due to the structural water motion, as evidenced by in situ XRD. Unlike non-aqueous Li-intercalated electrodes, Ni-dopants mitigated the lattice collapse of birnessite at deep charge states. Moreover, density functional theory (DFT) calculations showed that Ni doping induces charge depletion of lattice oxygen due to its higher electronegativity than Mn. This charge reduction, in turn, yields a significant decrease in electrostatic repulsion between the oxygens belonging to lattice and structural water. Classical molecular dynamics simulations based on atomic charges obtained from DFT elucidate that Ni-doping facilitates immobilization of structural water in (Ni)MnO2 and prevents lattice collapse upon Na-ion removal. (Ni)MnO2 exempted from lattice collapse shows an improved storage capacity relative to MnO2 while maintaining similar cycling stability.

Rechargeable zinc-ion batteries represent an emerging energy-storage technology that offers the advantages of low cost, use of abundant and nontoxic materials, and competitive energy content in lightly packaged forms. Nanoscale manganese oxides are among the most promising positive-electrode materials for zinc-ion cells, and their performance is further enhanced when these oxides are expressed as conformal deposits on porous carbon architectures, such as carbon nanfoam paper (CNF). We describe an “in-place” conversion of nanometric birnessite Na+-MnOx@CNF to crystalline spinel ZnMn2O4@CNF, a manganese oxide polymorph that nominally contains sites for Zn2+ insertion. The ZnMn2O4@CNF cathodes are electrochemically conditioned in two-terminal cells and ex situ characterized using X-ray diffraction, scanning electron microscopy, energy-dispersive spectroscopy, and X-ray photoelectron spectroscopy. Despite specific Zn2+ insertion sites in ZnMn2O4, we demonstrate that the predominant discharge mechanism involves coupled insertion of protons and precipitation of Zn4(OH)6SO4·xH2O; upon recharge, protons deinsert and Zn4(OH)6SO4 dissolves.

Birnessite-type manganese oxides, consisting of stacked MnOx sheets, separated by charge-balancing metal ions and structural water are potential candidates for electrochemical applications. Due to their structural complexity, Raman spectroscopy is one of the most widely used techniques for studying this class of materials. However, the interpretation of the Raman spectra is still debated. In fact, the observed Raman bands are often intuitively assigned to either Mn–O or M–O vibrations, where M corresponds to charge balancing metal ions such as Na, Ca, and K-ions which are present in natural forms of birnessite. Here, we report a combined experimental and computational study that, opposite to the commonly accepted assignments, strongly suggests that many of the characteristic Raman bands can be attributed to vibrations related to the interlayer water. Our computational findings are compared with detailed ex situ and in situ Raman spectroscopy/X-ray diffraction, and cyclovoltammetry results for K–birnessite, allowing for a full explanation of potential-dependent changes in the Raman spectra. Furthermore, the excellent correlation between the intensity of a band positively assigned to water vibrations and the d-spacing, give support for the important influence of interlayer water on the Raman spectra. This study points to the crucial role of arrangement and content of structural water and deepens the current understanding of hydrated birnessites.

Due to its low-cost and environmental friendliness, birnessite-type manganese oxide has attracted wide interest for use as a cathode material in electrochemical energy storage applications. The mechanisms of energy storage and release have been studied in some detail during the last decade, but despite some agreement, some aspects of the storage and release mechanisms are still under debate. The main reason for this, we argue, is the varying interpretations of Raman spectroscopy data in the literature. Therefore, we undertook a detailed correlative Raman spectroscopy/XRD study in combination with cyclic voltammetry. Raman spectroscopy allowed for straightforward differentiation between symmetry changes during the (de)intercalation of Na-ions. More specifically, through the use of highly crystalline birnessite samples it is suggested that Raman spectra are sensitive to the lattice parameters β and d001, which allowed us to derive unprecedented details of the changes in the birnessite structure that occur upon Na+ (de)intercalation. Furthermore, it is shown that the reversible hexagonal/monoclinic symmetry transition during the course of a charge/discharge cycle is a prerequisite for effective charge storage. Based on the results, a detailed mechanism describing the (de)intercalation of Na+ from/into birnessite is presented.

Birnessite-like minerals are among the most common Mn oxides in surficial soils and sediments, and they mediate important environmental processes (e.g., biogeochemical cycles, heavy metal confinement) and have novel technological applications (e.g., water oxidation catalysis). Ca is the dominant interlayer cation in both biotic and abiotic birnessites, especially when they form in association with carbonates. The current study investigated the structures of a series of synthetic Ca-birnessite analogs prepared by cation-exchange with synthetic Na-birnessite at pH values from 2 to 7.5. The resulting Ca-exchanged birnessite phases were characterized using powder X-ray diffraction and Rietveld refinement, Fourier transform infrared spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, and scanning and transmission electron microscopy. All samples synthesized at pH values greater than 3 exhibited a similar triclinic structure with nearly identical unit-cell parameters. The samples exchanged at pH 2 and 3 yielded hexagonal structures, or mixtures of hexagonal and triclinic phases. Rietveld structure refinement and X-ray photoelectron spectroscopy showed that exchange of Na by Ca triggered reduction of some Mn3+, generating interlayer Mn2+ and vacancies in the octahedral layers. The triclinic and hexagonal Ca-birnessite structures described in this study were distinct from Na- and H-birnessite, respectively. Therefore, modeling X-ray absorption spectra of natural Ca-rich birnessites through mixing of Na- and H-birnessite end-members will not yield an accurate representation of the true structure.

The high theoretical capacity of sodium birnessite cathode materials for sodium-ion batteries (SIBs) has been restricted due to drastic volume variation and fast capacity fading. The underlying rea...

Ceria doped with manganese in various amounts was prepared via hydrothermal synthesis. XRD revealed that beside ceria, monoclinic birnessite (Na0.55Mn2O4 × 1.5H2O) appears in the samples. The occurrence of birnessite was achieved using different synthesis conditions and precursors. SEM imaging revealed birnessite layered formations surrounded with ceria nanoparticles, while EDS confirmed the presence of both phases. Weight percentage, lattice constants and crystallite size of ceria, as well as birnessite, were obtained through whole powder pattern decomposition. Pure and Mn doped ceria samples with crystallite sizes between 3.1 and 3.4 nm, and specific surface area values between 183 and 212 m2 g−1 were obtained. It was established that the amount of Mn entering the ceria lattice or forming birnessite varies with nominal composition. Introduction of Mn in ceria is evident from the decrease of ceria lattice constant (from 5.4185 A to 5.4002 A). The entrance of manganese in the ceria crystal lattice was confirmed via EDS spectroscopy. Thermally induced changes were monitored via DTA/TGA, XRD and FTIR. Birnessite decomposes below 200 °C, while thermal treatment at 500 °C for 2 h yields Na2Mn5O10. Weight percentage of birnessite was confirmed through calculations based on the loss of interlayer water during TGA analysis. HRTEM revealed nanorod morphology of Na2Mn5O10, while EELS gave insight into the composition of this phase.

Defect engineering is an effective strategy to enhance the activity of catalysts for various applications. Herein, it was demonstrated that in addition to enhancing surface properties via doping, the influence of dopants on the surface-intermediate interaction is a critical parameter that impacts the catalytic activity of doped catalysts for low-temperature formaldehyde (HCHO) oxidation. The incorporation of Co into the lattice structure of δ-MnO2 led to the generation of oxygen vacancies, which promoted the formation of surface active oxygen species, reduced activation energy, and enhanced catalytic activity for low-temperature oxidation of HCHO. On the contrary, Cu doping led to a drastic suppression of the catalytic activity of δ-MnO2, despite its enhanced redox properties and slight increase in the surface concentration of active oxygen species, compared to pristine δ-MnO2. Diffuse reflectance infrared Fourier transform analysis revealed that in the presence of Cu, carbonate intermediate species accumulate on the surface of the catalysts, leading to partial blockage of active sites and suppression of catalytic activity.

Various manganese oxides are considered as rather active ORR catalysts and/or supercapacitor materials for operation in alkaline medium. However all these oxides are thermodynamically unstable at high pH and undergo more or less slow transformations to non-stoichiometric oxides containing alkali metal cations M (MxMnO2*nH2O, known also as birnessite mineral). We report a possibility to deposit birnessite thin layers by means of permanganate cathodic reduction. We consider ORR activities and reversible recharging data for birnessite deposited at various potentials with account for XRD information on birnessite cell disordering and microscopic data on the morphology of the deposits. Deposition is complicated by formation of soluble Mn(V) and Mn(VI) species and their disproportionation. However fortunately these by-side reactions appear to be strongly inhibited by forming birnessite solid, and the resulting current efficiencies are high enough. We compare deposition efficiency and adhesion for Pt, Ni, and glassy carbon supports. The best adhesion is found for Ni support. Deposits morphology strongly depends on deposition potential. Thin deposits formed at low overvoltage demonstrate homogeneously distributed pores, and only starting from sub-mkm thickness branched flakes appear, which are typical for natural birnessite morphology. Contrary, at higher overvoltage the deposits start to branch from the very beginning, and consist from thin flakes exclusively. This morphology is unfavorable for electrode behavior because of too high material resistance. Deposition transients allow to monitor the morphology. For flakes formation, oscillatons at deposition transients are observed, which can be associated with layer-by-layer growth. The recharging of all birnessite materials is reversible, but the specific capacity strongly depends on deposition potential and deposit thickness. The best results (ca. 500 mkF/cm2 at scan rates up to 0.1 V/s) are found for thin deposits fabricated at low overvoltage. ORR mass activity is comparable with observed earlier for MnO2, which is believed to undergo slow transformation to birnessite in alkaline solutions. The important advantage of electrodeposited birnessite as compared to the majority of chemically synthesized manganese oxides is a possibility to operate in the absence of conducting binder.

Octahedral layered birnessite (OL) was synthesized by redox method, and OL supported Ag catalysts (xAg/OL, x = 0.1wt%, 0.2wt%, 0.3wt%, 0.5wt%) were prepared by ion exchange method. Then catalysts were characterized by XRD, SEM, BET, H2-TPR, TG, O2-TPD and in-situ DRIFTS, while the catalytic activity of CO was evaluated. Among xAg/OL samples, the 0.3Ag/OL exhibited the best catalytic activity for CO oxidation (T50 = 105 oC and T90 = 135 oC). The results show that the chemical adsorption of oxygen, the low-temperature reducibility and the strong interaction between the Ag species and OL are related to the excellent catalytic activity of xAg/OL. The reaction mechanism was studied by in-situ DRIFTS. First, O2 was adsorbed and activated on the oxygen vacancies of xAg/OL, then formed oxygen free radical attacked the adsorbed CO and produced CO2, subsequently CO2 desorbed from the catalyst surface. Oxygen vacancies was supplemented by gas O2, thus circulating.

The layered manganese dioxide (δ-type MnO2) film electrodeposited on carbon cloth (CC) functioned as a cathode in aqueous zinc-ion batteries. Battery performance was notably improved by doping MnO2...

The development of efficient and noble-metal-free electrocatalysts for the challenging oxygen evolution reaction (OER) is crucial for sustainable energy solutions. In this work, a facile co-precipitation method, followed by thermal postsynthetic treatment in N2/air, was developed to synthesize molybdenum-doped α-Mn2O3 materials (Mn2O3:1.72%Mo, Mn2O3:2.64%Mo, Mn2O3:32.23%Mo, and Mn2O3:49.67%Mo) as low-cost water-oxidizing electrocatalysts. Powder X-ray diffraction (PXRD), extended X-ray absorption fine structure (EXAFS), X-ray photoelectron spectroscopy (XPS), and high-resolution transmission electron microscopy (HRTEM) investigations showed the presence of strong distortions in the molybdenum-doped α-Mn2O3 host lattice (Mn2O3:2.64%Mo) and an average oxidation state of Mn2.8+. Several test assays demonstrated that these structural features significantly promote the OER activity. Mn2O3:2.64%Mo was found to exhibit very good activity among the series in cerium ammonium nitrate (CAN)-assisted water oxidation with a maximum turnover frequency (TOF) of 585 μmol O2 m–2 h–1, which is a 15-fold improvement of the pure α-Mn2O3 activity and higher than the value of the previously reported benchmark Mn-based catalyst, birnessite. The optimized catalyst (Mn2O3:2.64%Mo) excelled through a low onset potential (300 mV) and a promising overpotential of 570 mV for OER at a current density of 10 mA cm–2, which is only 20 mV above that of the noble metal benchmark RuO2 electrode and competitive with that of the most active Mn-based OER catalysts reported to date. Electrochemical impedance spectroscopy (EIS) studies demonstrated that the catalytically active surface area of Mn2O3:2.64%Mo is much higher than that of α-Mn2O3 for the OER at the applied potential. In addition, stability during 30 h without degradation was achieved, which exceeds that of a wide range of current noble-metal-free electrocatalysts. Our study provides a facile and effective approach for the preparation of economical and high-performance manganese-based electrocatalysts for water oxidation.

Although δ-MnO2 with a layered structure is a promising electrode material for supercapacitors, its poor electronic conductivity and structural instability limit its electrochemical performance. In this work, structure reinforced birnessite with improved electron transfer is developed via Cr doping in δ-MnO2 using hydrothermal synthesis. The Cr-doped δ-MnO2 exhibits significantly improved specific capacitance and cycling stability in an extended potential window (0–1.2 V vs. Ag/AgCl) compared to pristine δ-MnO2. Specifically, the 2 mol% Cr-doped δ-MnO2 (0.02CrMO) electrode exhibits large specific capacitances of 250 F g−1 at 0.2 A g−1 and 150 F g−1 at 10 A g−1 (∼5 mg cm−2 mass loading) as well as an outstanding capacitance retention of 82.6% after 30 000 cycles. Both calculations and experimental results demonstrate that Cr doping in δ-MnO2 can narrow the band gap, enhance electron transfer, and strengthen the layered structure with suppressed Jahn–Teller distortion, resulting in reduced Mn dissolution and phase transition.

Birnessite-type manganese oxides and their magnetic composites with iron oxides are known due to their effective properties in water oxidation catalysis (WOC). The present work demonstrates how transition metal ion doping influences the WOC performances of magnetic and non-magnetic MnOx catalysts in a comparative manner. Transition metal cation (TM = Cr3+, Co2+, Ni2+, Cu2+, Zn2+) doped layered birnessite and birnessite-manganese ferrite composite nanocatalysts were synthesized and characterized. Core-shell type, magnetic composite catalyst particles of similar to 150 nm diameter were obtained by in situ deposition of birnessite shells onto 50-60 nm sized manganese ferrite spheres by reducing MnO4- ions with propionic acid in the presence of the TM ions. The resulting TM-doped catalyst materials were analyzed by XRD, SEM, TEM, IR, TGA and AAS to verify their compositions, structures and morphologies. WOC activities and stabilities were comparatively examined in both chemical (using Ce4+ as sacrificial oxidant at pH similar to 1-2) and electrochemical (pH 7, screen-printed particles on FTO) test setups. In both types of experiments, the highest catalytic activity was observed for catalysts doped with Co2+ ions, followed by Ni2+-doped samples. In comparison to the K-birnessite reference material, the Co2+- and Ni2+-doped analogs were additionally found to be more stable and more active in the chemical and long-term electrocatalysis measurements, demonstrating the potential of TM-doping to improve the performance of magnetic-MnOx-based WOC materials. In addition, the formation of a magnetic core-shell structure converts the micron-sized amorphous MnO2 particles to spherical at the nanoscale and thus facilitates the mass-energy transfer throughout the entire catalyst surface.

In the past few years, intensive attention has been focused on birnessite based electrodes for supercapacitors. Much progress has been achieved in developing birnessite based nanostructures with high electrochemical performance. However, challenges still remain in taking full advantage of birnessite and building smart structures to overcome the gap between the obtained capacitance and its theoretical capacitance. In this review, the basic information on birnessite and its preparation strategies are summarized and the current challenges are put forward. Finally, some new strategies for preparing high electrochemical performance birnessite based nanostructures are highlighted.

Manganese oxides (MnO2) are a class of promising electrode materials for high-performance supercapacitors. Among various possible crystal phases of MnO2, the two-dimensional layered birnessite (δ-MnO2) is considered to facilitate cation intercalation/deintercalation with little structural rearrangement during the charging and discharging process. Here, we report a facile strategy for synthesis of δ-MnO2 with significantly enhanced electrochemical capacitive energy storage performance via phase transformation of orthorhombic MnO2. The resultant δ-MnO2 delivers a specific capacitance of 251.4 F g−1 at a current density of 1 A g−1, which is almost three times higher than that of the original orthorhombic phase. The solid-state flexible asymmetric supercapacitor based on birnessite MnO2 as the positive electrode possesses an operating potential window as high as 2.0 V as well as high energy density and power density. This work demonstrates the post crystal engineering of MnO2 is an effective way to optimize their electrochemical properties.

The layered birnessite derivatives as intercalation cathode materials for rechargeable batteries and supercapacitors receive great attention due to the abundant, low-cost, highly safe, and environmentally friendly manganese element. However, the practical application of chemically synthesized birnessite in energy storage has been restricted by low specific capacity and poor cyclability because of the limited interlayer metal ions intercalation and inferior structural stability during cycling. In this focused review, we discuss the origin of unsatisfying charge storage performance of the chemically synthesized birnessite and disclose the reinforced birnessite structures derived from spinel Mn3O4 by in-situ electrochemical conversion. With enhanced structural stability and large interlayer distance, the electrochemically converted birnessite shows promising electrochemical performance in various batteries and supercapacitors. Finally, critical perspectives on the future development of layered birnessite from spinel Mn3O4 are provided, which may guide advanced electrode design for high-performance sustainable batteries.

Novel nanocomposite adsorbent based on manganese dioxide-loaded polyacylonitrile propylene fibers was synthesized for selective removal of strontium ions from contaminated waters by in situ formation of manganese dioxide with layered structure (birnessite) nanoparticles on the fibers surface. Data of scanning electron microscopy and X-ray diffraction confirmed the formation of birnessite homogeneous layer on the fibers surface, which consisted of rounded nanoparticles (50 to 70 nm). The efficiency of the synthesized adsorbent for removal of strontium ions was studied under various experimental conditions. It has demonstrated a rapid adsorption process, high adsorption efficiency over a wide pH range, and selectivity in Sr ions removal from model solutions with high salt content.

We report on the growth of single-crystal potassium birnessite (K0.31MnO2*0.41H2O) and present both the average and local structural characterization of this frustrated magnetic system. Single crystals were obtained employing a flux growth method with a KNO3/B2O3 flux at 700 {\deg}C. Single-crystal X-ray diffraction revealed an average orthorhombic symmetry, with space group Cmcm. A combination of high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) with atomic resolution energy dispersive X-ray spectroscopy (EDS) demonstrated the layered structure of potassium birnessite with manganese-containing planes well separated by layers of potassium atoms. MnO6 octahedra and the K/H2O planes were clearly imaged via integrated differential phase contrast (iDPC) STEM. Furthermore, iDPC-STEM also revealed the existence of local domains with alternating contrast of the manganese oxide planes, most likely originating from charge ordering of Mn3+ and Mn4+ along the c-axis. These charge-ordered domains are clearly correlated with a reduction in the c-lattice parameter compared to the rest of the matrix. The insight gained from this work allows for a better understanding of the correlation between structure and magnetic properties., Comment: A high-resolution version with supplementary information can be found at https://pubs.rsc.org/en/content/articlelanding/2021/TC/D0TC05396A#!divAbstract

Catalytic oxidation is an efficient and cost-effective technology to eliminate HCHO, and MnO2 shows excellent performance towards this kind of reactions. In this work, a birnessite type manganese dioxide nanoparticles modified silica-doped titanium dioxide (MnO2@SiO2–TiO2 ) nanofibrous membranes were successfully synthesized by using an electrospinning technique and liquid phase synthesis . It is found that MnO 2@SiO2–TiO2-4 nanofibrous membrane showed the best catalytic activity for HCHO removal. The as-fabricated nanofibrous membranes exhibited good and reversible catalytic oxidation activity towards formaldehyde, and more than 90% of the catalytic performance could remain after 5 testing cycles. The successfully constructed MnO2 nanoparticles decorated soft SiO2–TiO2 nanofibrous membranes, which can be a great promise for effectively solving the problem of environmental remediation.

The aim of this work was to study the effect of the microwave radiation on the thermal and spectroscopic features, as well as about arrangement (order-disorder) and morphological properties, of the layered manganese oxide with birnessite-type structure. The route employed to obtain Na-birnessite matrix was redox precipitation. The products were characterized by X-ray diffraction, thermal analysis (TG-DTG-DSC), infrared (FTIR) and Raman spectroscopy, scanning electron microscopy (SEM) and nitrogen adsorption-desorption technique. The results showed that microwave radiation influenced in a short time (5 min) the octahedral ordering of birnessite, as well as in increasing the crystallite size. Thermal analysis showed that the thermal behavior of the lamellar matrix was different from that of birnessite under microwave radiation. After microwave-assisted hydrothermal treatment, FTIR and Raman spectroscopy investigations were used to differentiate ordered and disordered birnessites. Otherwise, there were no changes in SEM morphology of the lamellar-type phases, but the particle size changed. Resumo No presente trabalho, apresenta-se um estudo da influência da radiação micro-ondas nas propriedades térmica e espectroscópica, assim como estrutural (ordem-desordem) e morfológica, de óxido de Mn estruturado em camada tipo Na-birnessita. A rota empregada para obtenção da matriz lamelar foi a precipitação redox. Os produtos foram caracterizados por difração de raios X, análise térmica (TG-DTG-DSC), espectroscopia de infravermelho (FTIR) e Raman, microscopia eletrônica de varredura e técnica de adsorção-dessorção de N2. Os resultados mostraram que a radiação por micro-ondas influenciou em um curto tempo (5 min) no ordenamento das folhas octaédricas de birnessita, assim como no aumento de tamanho de cristalito. Medidas de análise térmica revelaram que o comportamento termal da matriz lamelar birnessita foi diferente em relação à birnessita irradiada por micro-ondas. Após tratamento por radiação de micro-ondas, estudos por espectroscopias FTIR e Raman foram usadas para diferenciar birnessita ordenada de desordenada. A morfologia em placas dos produtos envolvidos não sofreu mudança após radiação por micro-ondas, mas o tamanho das partículas alterou.

Insight into the influence of hydration energy of structural cations within birnessite-type layered MnO2 on charge storage mechanisms via redox reaction and intercalation/deintercalation processes ...

Na4Mn9O18 was recognized as the most interesting material for sodium ion batteries due to its low cost, high specific capacity and good cycle performance. The excellent electrochemical performance of Na4Mn9O18 nanostructures was shown in literature. In this work, Na4Mn9O18 nanowires were synthesized by hydrothermal reactions of Mn2O3 powder and NaOH solution at the temperatures of 185–205 °C for 48–96 h. The investigation of composition and structure of the synthesized products via scanning electron microscopy and X-ray diffraction analyses showed that major intermediate products at the low and high temperatures were Mn3O4 and birnessite Na0.55Mn2O41.5H2O, respectively. The synthesized Na4Mn9O18 nanowires showed a good electrochemical performance with discharge capacities of over 90 mAhg−1, and Coulombic efficiencies of more than 91% at a rate of 0.2C during 30 cycles of charge/discharge.

By adjusting the amount of Cu(NO 3 ) 2 ·3H 2 O, Cu 2+ -doped birnessite δ-MnO 2 spherical substances were synthesized by a simple hydrothermal synthesis process without any templates and surfactants. The structure, morphology, and specific surface area were characterized by XRD, SEM, TEM and BET. Further study shows that the 0.25 mmol Cu-doped MnO 2 sample is expected to provide higher specific capacitance (636.3 F g -1 at 1 A g -1 current density) compared with pure δ-MnO 2 (335.6 F g -1 at the current density of 1 A g -1 ) and long-term cyclic stability (105.01 % capacitance retention after 1500 cycles at current density of 5 A g -1 ). Electrochemical impedance spectroscopy proved the low resistance characteristics of the prepared samples. All the results show that the copper-doped MnO 2 material is not only low cost, but also of excellent electrochemical performance, thus possesses great potential in future energy development.

In this study, birnessite manganese oxide nanosheets (δ-MnO2) are growth on hollow carbon sphere (HCS) by a facile hydrothermal reaction to prepare a novel multifunctional nanocomposite (δ-MnO2/HCS). The obtained δ-MnO2/HCS nanocomposite possesses a unique hollow structure, which exhibited excellent adsorption of electrolyte ions and macromolecules. When used as electrode for supercapacitors (SCs), it exhibits excellent electrochemical ability. The specific capacitance is 370 F/g at 5 mV/s, and 93.3% of the initial capacitance is retained after 5000 cycles. Additionally, when δ-MnO2/HCS acted as a catalyst for the degradation of bisphenol A (BPA), the removal rate is 95% within 10 min and BPA is almost entirely decomposed in 30 min. This multifunctional δ-MnO2/HCS composite may find novel applications in future environmentally friendly energy technologies.

Birnessite, primarily formed by biooxidation, is a common semiconducting Mn oxide in nature and a major controller of heavy metals cycling processes. In turn, the heavy metal sorption pathway alters its structural chemistry and thus the electronic structure. We synthesize three kinds of birnessite, namely biogenic (bio-birnessite), Cu coprecipitated (co-birnessite) and Cu adsorbed birnessite (ad-birnessite), to investigate the influence of Cu(II) occupancy on the structure, semiconducting property and photostability of birnessite. XRD show co-birnessite holds the same hexagonal symmetry as bio-birnessite, whereas ad-birnessite transforms to triclinic symmetry as reflected by the splitting reflections at 2.44 and 1.42 A. The adsorption process is accompanied by a more intense releasing of Mn(II) (156.88 μM/L) from a bio-birnessite analog δ-MnO2 in light than in dark (19.55 μM/L), suggesting the photoreductive releasing of Mn(II) promotes structure transformation. Conformably, both the Mn average oxidation state (AOS) (3.32) and mole ratio of Cu/Mn (0.08) in ad-birnessite is lower than those in co-birnessite (AOS: 3.47, Cu/Mn ratio: 0.17). EXAFS demonstrate different Cu complexing features in ad- and co-birnessite that the ratio of Cu incorporated (INC) into vacancies is 1.16 and 0.45 for ad- and co-birnessite, respectively. UV–vis diffuse reflection spectra (DRS) and photoelectron spectrometer (PS) exhibit the band gap (Eg) and valence band (VB) of bio-birnessite are 2.05 and −5.58 eV, while there is 0.1 and 0.05 eV decrease of Eg, and 0.08 and 0.16 eV lowering of VB in co- and ad-birnessite, respectively. The reduced Eg guarantee them to generate photoelectron-hole pairs under mild indoor visible light, and the lower energy level of VB in ad-birnessite makes its VB holes more reactive to accept electrons from electron donors. Further density functional theory (DFT) calculations focus on interpreting the fine structures brought by different Cu sorption pathways on the electronic structure of birnessite. A Mn vacancy in a 2 × 2 × 1 hexagonal birnessite cell significantly reduces Eg from 1.60 to 0.28 eV by hybridizing Mn 3d and O 2p states in VB. The doped Cu reduces Eg mainly through incorporating Cu 3d orbital in VB. The INC Cu in hexagonal birnessite contributes more to reduce Eg (1.60–0.34 eV) than TCS Cu (1.60–1.20 eV), while in triclinic birnessite cell, TCS Cu causes more obvious reduction of Eg (1.68–0.28 eV) than INC Cu (1.68–1.55 eV). Thus, we conclude the vacancy imposes more effect on the electronic structure of hexagonal co-birnessite than doped Cu in TCS or INC sites; and the band structure of co-birnessite is more sensitive to INC Cu, in contrast to TCS Cu affecting more in ad-birnessite. Overall, the crystal structure differs according to Cu fixation pathway, which imposes a comprehensive effect on band structure and photostability contributed by vacancies, Cu occupancy sites and structural symmetry. Cu coprecipitating with birnessite is suggested as a preferred method in practical clean-up of metals such as Cu, due to the higher adsorption capacity for Cu, better stability of TCS Cu and better photostability influenced by Cu.

Iron and manganese have been studied as a proposal for new materials that could be used for the adsorption of arsenic (V) (As (V)), in order to remove this contaminant. The objective of this work was to study the effect of the molar ratio of three Mn/Fe + Mn composites (X = 0.17, 0.32, 0.47) on the properties of adsorbent media, and to determine their influence on arsenic removal by comparing them with two metallic oxyhydroxides, that are commonly used as adsorbents of As(V) in aqueous solution (goethite and birnessite). These media were synthesized by chemical precipitation while controlling particle size. They were characterized using X-ray diffraction (XDR), scanning electron microscopy with X-ray dispersive energy spectrometry (SEM-EDS), surface area analysis by the BET method, Fourier transform infrared spectroscopy (FTIR), and isoelectric point analysis (IEP). The surface area (286 m2/g) of the composite with a molar ratio of X = 0.17 was larger than that of the other media. The adsorption kinetics and isotherms were fitted to the mathematical models, specifically, the pseudo-second order and Langmuir, respectively. The X = 017 composite had an adsorption capacity of 3.28 mg/g and removed 99% of As(V) with an initial concentration of 0.5 and 97% with an initial concentration of 10 mg/L, at 180 min, 25 °C, and pH 7. The five adsorbent media were tested with well water with an initial As(V) concentration of 0.075 mg/L, and the best behavior was exhibited with a molar ratio of X = 0.17 at 90 min, resulting in 100% removal of As(V). The results suggest that this material is an effective and viable alternative to remove this contaminant from water.

Sunlight drives photosynthesis and associated biological processes, and also influences inorganic processes that shape Earth’s climate and geochemistry. Bacterial solar-to-chemical energy conversion on this planet evolved to use an intricate intracellular process of phototrophy. However, a natural nonbiological counterpart to phototrophy has yet to be recognized. In this work, we reveal the inherent “phototrophic-like” behavior of vast expanses of natural rock/soil surfaces from deserts, red soils, and karst environments, all of which can drive photon-to-electron conversions. Using scanning electron microscopy, transmission electron microscopy, micro-Raman spectroscopy, and X-ray absorption spectroscopy, Fe and Mn (oxyhydr)oxide-rich coatings were found in rock varnishes, as were Fe (oxyhydr)oxides on red soil surfaces and minute amounts of Mn oxides on karst rock surfaces. By directly fabricating a photoelectric detection device on the thin section of a rock varnish sample, we have recorded an in situ photocurrent micromapping of the coatings, which behave as highly sensitive and stable photoelectric systems. Additional measurements of red soil and powder separated from the outermost surface of karst rocks yielded photocurrents that are also sensitive to irradiation. The prominent solar-responsive capability of the phototrophic-like rocks/soils is ascribed to the semiconducting Fe- and Mn (oxyhydr)oxide-mineral coatings. The native semiconducting Fe/Mn-rich coatings may play a role similar, in part, to photosynthetic systems and thus provide a distinctive driving force for redox (bio)geochemistry on Earth’s surfaces.

Long-term cycling stability is a main issue for the application of manganese-based electrodes in pseudocapacitors. Herein, we demonstrate a facile approach to enhance the cycling stability of birnessite electrodes by pre-adding slight Mn2+ into the aqueous electrolyte. The cycling performance of the electrode and the morphology of birnessite change with Mn2+ concentration, which are ascribed to the electrochemical oxidation of Mn2+ or the reduction of birnessite during the charge/discharge cycling process. By controlling a low concentration of Mn2+, the birnessite electrode exhibits enhanced capacitance and cycling stability, while a hierarchical nanosheets assembled nanowall array architecture is achieved after the prolonged cycling. The resulting electrode delivers a superior rate capability (60% retention at 20 A g−1) and a high cycling stability (93% retention after 10000 cycles). This work opens up a new strategy for the development of manganese-based electrodes with high rate performance and long-term cycling stability.

Recent research on materials for capacitive deionization (CDI) has shown that intercalation materials have salt removal capacities (>40 mg g–1) much higher than those of carbon materials (∼15 mg g–...

Graphene oxide (GO)/MnO2 nanocomposites were synthesized by adding KMnO4 in a solution of water and ethanol (3:1), containing 10 mg of GO. Brown precipitates were obtained after a continuous stirring for 1 hr. The precipitates were then washed with deionized water (DI) water and dried to obtain the MnO2-GO nanocomposites. Pure MnO2 was also synthesized using the same method without GO for the comparison. X-ray diffraction pattern confirm δ-MnO2 type of MnO2 with birnessite type MnO2 structure. The TEM images show the average diameter of MnO2 nanorods as 15 nm. Electrochemical characterizations were carried out in an aqueous solution of 3M KOH. Charge-discharge studies were carried out between 1A/g to 20 A/g current range. The MnO2-GO nanocomposites showed improved electrochemical performances. The capacitance of MnO2 and MnO2-GO electrodes was found to be as 300 F/g, and 350 F/g, respectively at a current of 0.5 A/g.

Pseudocapacitance is generally associated with either surface redox reactions or ion intercalation processes without a phase transition. Typically, these two mechanisms have been independently studied, and most works have focused on optimizing one or the other in different material systems. Here we have developed a strategy based on solubility contrast, in which the contribution from the two capacitive mechanisms is simultaneously optimized. Taking layered birnessite MnO2 as a model, controllable nanostructures and oxygen vacancies are achieved through a simple coprecipitation process. Simultaneously controlling crystallite size and defect concentration is shown to enhance the charging-discharging kinetics together with both redox and intercalation capacitances. This synergistic effect results from enhanced ionic diffusion, electronic conductivity, and large surface-to-volume ratio. In addition, considerable cycling durability is achieved, resulting from improved framework strength by defect creation and the absence of proton (de)intercalation during discharge/charge. This work underscores the importance of synergistically regulating nanostructure and defects in redox-active materials to improve pseudocapacitive charge storage.

Supercapacitors as an important component of sustainable energy system attract tremendous attention, but improvement of their performance in energy density and power capability is still considered as a challenging task. Herein, a versatile electrochemical activation strategy is introduced for a prototypical MnO2 film-coated carbon nanotubes (MnO2@CNTs) based electrode to boost its electrochemical energy storage performance. The electrochemical activation process was completed by an initial charge–discharge cycling operation, which led to the dense MnO2 film transforming into ultrathin three-dimensional (3D) nanosheets birnessite with an unexpected Na+ ions diffusion path. By adjusting electrochemical activation parameters, the nanostructure and intercalated Na+ content of birnessite was optimized. The electrochemical activated MnO2@CNTs-based supercapacitor showed a specific capacitance as high as ∼404 F g−1 at a current density of 0.5 A g−1, and about 78.7% of the initial capacitance was retained at a high current density of 100 A g−1. This should be ascribed to the optimized MnO2 3D nanosheets structure and uniformly embedded Na+ ions after electrochemical activation operations, which greatly enhanced the surface- and diffusion-controlled capacitance during the electrochemical cycling processes. It is believed that this work will provide a significant strategy for the development of high-performance supercapacitor electrodes.

High purity birnessite was synthesized and exfoliated into a negatively charged monolayer structure. A positively charged 5, 10, 15, 20-tetrakis (4-aminophenyl) manganese porphyrin (MnTAPP) was synthesized. Driven by the electrostatic force and the coordination effect of the amino nitrogen on the manganese ion in birnessite, the single-layer birnessite was reassembled with MnTAPP, forming a new sandwich-type catalytic composite MnTAPP@bir. The structure and chemical properties of the composite were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermogravimetric analysis (TGA) and Brunauer–Emmett analysis (BET). Electrocatalytic studies showed that the MnTAPP@bir exhibited an overpotential for water oxidation of 450 mV (at 10 mA cm−2) and a Tafel slope of 121.5 mV dec−1 compared to birnessite with 700 mV (at 10 mA cm−2) and 230 mV dec−1. Impedance spectroscopy results suggested that the charge transfer resistivity of MnTAPP@bir was significantly lower than that of birnessite, suggesting that MnTAPP in the interlayer increased the conductivity of birnessite. Through a chronoamperometry test, the new material also showed excellent stability within 4000 s.