Manganese Oxide Birnessite Research Articles

Understanding the Effect of Pre-Intercalated Cations on Zn-Ion Storage Mechanism of Layered Birnessite Manganese Oxide for Aqueous Zn-ion Batteries

With the rapid growth of energy consumption, tremendous research efforts have been dedicated to achieving sustainable and green energy storage systems, owing to environmental concerns. Over the past few years, rechargeable aqueous zinc-ion batteries (ZIBs) have become a compelling alternative to lithium-ion batteries (LIBs). Although LIBs have been successfully commercialized due to their high energy density, their organic-based electrolytes are highly volatile and flammable. Therefore, aqueous Zn-ion batteries have emerged as promising energy storage devices, owing to the benefits of water-based electrolytes such as low-cost, high safety, and high-power density as well as the advantages of zinc metal such as its natural abundance, low redox potential (-0.76 V vs standard hydrogen electrode), and high theoretical capacity (840 mAh g-1). Among cathode materials, birnessite manganese oxide (δ-MnO2) is one of the promising intercalation cathode materials for ZIBs due to its layered structure, which can facilitate H+/Zn2+ diffusion, leading to enhanced rate performance and improved capacity. However, the δ-MnO2 also suffers from some critical drawbacks such as structural phase transformations and collapse of the layered structure, resulting in poor cycling stability. To address these issues, a pre-intercalation strategy has been proved as an effective way to stabilize the layered MnO2 by using various organic polymers and metal cations such as polyaniline, Li+, K+, and Ca2+. In this work, we studied the effect of charge density of pre-intercalated cation on the Zn2+ storage mechanism of the δ-MnO2 in a mild aqueous electrolyte of 1 M ZnSO4. Overall, the results showed that a small amount of highly charged pre-intercalant (Al3+) can efficiently stabilize the layered structure of the δ-MnO2, resulting in more accommodation space for Zn2+ insertion, which can lead to improved capacity for the Al-MnO2 of 210 mAh g-1 at 0.1 A g-1. Apart from high capacity, the Al-MnO2 also exhibits excellent cycling stability with a capacity retention of 84% after 2000cycles at 2 A g-1. This is because highly charged intercalants can induce a reduction of binding energy between Zn2+ and host structure, leading to a highly reversible (de)intercalation of Zn2+ and stable structure of the δ-MnO2 during charge/discharge, which was confirmed by ex-situ XRD and DFT calculations. KEYWORDS: rechargeable aqueous Zn-ion batteries, birnessite manganese oxide, intercalation

Enhancement in the Charge-Transfer Kinetics of Pseudocapacitive Iridium-Doped Layered Manganese Oxide.

Birnessite manganese oxide is a promising candidate as an electrode material for aqueous supercapacitors owing to its pseudocapacitance associated with fast redox processes. While manganese oxides are semiconductive, the conductivity is much lower than that of typical materials used for capacitive electrodes such as activated carbon or ruthenium oxide. In an attempt to increase the electronic conductivity of birnessite, a new solid solution phase, Ky(Mn1-xIrx)O2, was synthesized, and the electrochemical charge storage capability of Ir-doped birnessite was studied in aqueous Li2SO4. Structural characterization revealed that the single-phase Ky(Mn1-xIrx)O2 could be synthesized up to x = 0.1. An increase in the pseudocapacitive charge was observed with the increase in Ir content. In addition to the increase in the pseudocapacitive charge, an unusual change in the peak potential was observed. The peak-to-peak difference for the Mn4+/Mn3+ redox decreased with increasing Ir content, indicating an increase in the reversibility of the pseudocapacitive process. The decrease in peak-to-peak difference was observed only by Ir substitution and was not observed for physical mixtures of K0.28MnO2 and IrO2, suggesting a strong electronic interaction between the host Mn ion and the substituting Ir ion.

The charge density of intercalants inside layered birnessite manganese oxide nanosheets determining Zn-ion storage capability towards rechargeable Zn-ion batteries

Rechargeable aqueous Zn–MnO2 batteries have been considered as one of the promising alternative energy technologies due to their high abundance, environmental friendliness, and safety of both Zn–metal anodes and manganese oxide cathodes.

Study of the effect of ceria on the activity and selectivity of Co and Ce co-doped birnessite manganese oxide for formaldehyde oxidation

Catalytic oxidation is a promising approach to eliminating formaldehyde (HCHO) to improve indoor air quality. Herein, CeO2 was explored due to its remarkable properties for oxygen storage and oxygen transfer capability for co-doping δ-MnO2 alongside cobalt for enhanced low-temperature oxidation of HCHO. Various characterization techniques were deployed to understand the morphology and physicochemical properties of the synthesized catalysts. The Co-Ce co-doped catalysts with low CeO2 loading (0.05 and 0.1) showed higher catalytic activity for HCHO oxidation due to their higher concentration of surface-active oxygen species. Catalytic oxidation results showed that the presence of CeO2 leads to the generation of methanol as a secondary hazardous pollutant. Methanol selectivity increases with increasing CeO2 loading in the catalysts. The results from in-situ DRIFTS confirmed the formation of methoxy species in the presence of CeO2, which are intermediates for methanol generation. Considering the recent interest in CeO2 as a potential catalyst for practical abatement of HCHO from the indoor environment, this work has thus raised questions on the safety of using CeO2 as a catalytic material for HCHO oxidation. It also provides insights into the surface reaction mechanism leading to the generation of methanol in the presence of CeO2.

Effect of Growth-Time on Electrochemical Performance of Birnessite Manganese Oxide (δ-MnO2) As Electrodes for Supercapacitors: An Insight into Neutral Aqueous Electrolytes

This study demonstrates successful synthesis of nanoflower-like birnessite δ-MnO2 materials through a simple and effective hydrothermal technique. Characterization of the flower-like materials grown at various dwell times were carried out by employing scanning electron microscopy (SEM), X-ray diffractometry (XRD), Brunauer-Emmett-Teller (BET) and Barrett–Joyner–Halenda (BJH) models, and electrochemical analysis to understand the effect of different growth time on their morphological, structural, surface area along with pore size distribution and electrochemical characteristics as electrodes for electrochemical capacitors. Attention is mainly based on electrochemical assessment of the as-synthesized materials with an insight into some neutral aqueous electrolytes, mainly lithium sulphate (1 M Li2SO4) and sodium sulphate (1 M Na2SO4) solutions as potential medium owing to their non-corrosive nature, cost-effectiveness, electrochemical stability and environmentally friendliness. Significant charge propagation, with a high specific capacitance of 387.1 F g-1 was achieved for the half-cell electrode in 1 M Li2SO4 alongside excellent cycling stability for up to 5 000 cycles performed at 3 A g-1 specific current. A comprehensive electrochemical assessment is performed to understanding the relationship between solvated and diffused ions of the neutral electrolytes that could result in fast charge storage kinetics as well as high specific capacitance.

Stabilization of chromium(VI) by hydroxysulfate green rust in chromium(VI)-contaminated soils

Chromium (Cr)-contaminated soils pose a great environmental risk, with high solubility and persistent leaching of Cr(VI). In this study, hydroxysulfate green rust (GRSO4), with the general formula Fe(II)4Fe(III)2(OH)12SO4·8H2O, was evaluated for its efficiency in Cr(VI) stabilization via Cr(VI) reduction to Cr(III) in four representative Cr(VI)-spiked soils. The initial concentrations of phosphate buffer-extractable Cr(VI) (Cr(VI)b) in soils 1, 2, 3, and 4 were 382.4, 575.9, 551.3, and 483.7 mg kg–1, respectively. Reduction of Cr(VI) to Cr(III) by structural Fe(II) (Fe(II)s) in GRSO4 in all studied soils was fast, wherein the application of GRSO4 markedly decreased the amount of Cr(VI)b at the Cr(VI)b/Fe(II)s stoichiometric mole ratio of 0.33. The kinetics of Cr(VI) reduction by GRSO4 could not be determined as this reaction coincided with the release of Cr(VI) from soil during the experiment. The concentration of Cr(VI)b decreased, as the Cr(VI)b/Fe(II)s ratio decreased from 0.46 to 0.20, generally to below 10 mg kg–1. Back-transformation of the generated Cr(III) was examined in the presence of manganese oxide birnessite at the birnessite/initial Cr(III) mole ratio of 4.5. The results of batch tests showed that only 5.2% of the initial Cr(III) was converted to Cr(VI) after two months, while under field capacity moisture conditions, less than 0.05% of the initial Cr(III) was oxidized to Cr(VI) after six months. The results illustrated that remediation of Cr(VI)-contaminated soils would be fast, successful, and irreversible with an appropriate quantity of fresh GRSO4.

Effect of growth-time on electrochemical performance of birnessite manganese oxide (δ-MnO2) as electrodes for supercapacitors: An insight into neutral aqueous electrolytes

This study demonstrates successful synthesis of nanoflower-like birnessite δ-MnO2 materials through a simple and effective hydrothermal technique. Characterization of the flower-like materials grown at various dwell times were carried out by employing scanning electron microscopy (SEM), X-ray diffractometry (XRD), Brunauer-Emmett-Teller (BET) and Barrett–Joyner–Halenda (BJH) models, and electrochemical analysis to understand the effect of different growth time on their morphological, structural, surface area along with pore size distribution and electrochemical characteristics as electrodes for electrochemical capacitors. Attention is mainly based on electrochemical assessment of the as-synthesized materials with an insight into some neutral aqueous electrolytes, mainly lithium sulphate (1 M Li2SO4) and sodium sulphate (1 M Na2SO4) solutions as potential medium owing to their non-corrosive nature, cost-effectiveness, electrochemical stability and environmentally friendliness. Significant charge propagation, with a high specific capacitance of 387.1 F g-1 was achieved for the half-cell electrode in 1 M Li2SO4 alongside excellent cycling stability for up to 5 000 cycles performed at 3 A g-1 specific current. A comprehensive electrochemical assessment is performed to understanding the relationship between solvated and diffused ions of the neutral electrolytes that could result in fast charge storage kinetics as well as high specific capacitance.

Effect of Intercalants inside Birnessite-Type Manganese Oxide Nanosheets for Sensor Applications.

Hydrazine is a common reducing agent widely used in many industrial and chemical applications; however, its high toxicity causes severe human diseases even at low concentrations. To detect traces of hydrazine released into the environment, a robust sensor with high sensitivity and accuracy is required. An electrochemical sensor is favored for hydrazine detection owing to its ability to detect a small amount of hydrazine without derivatization. Here, we have investigated the electrocatalytic activity of layered birnessite manganese oxides (MnO2) with different intercalants (Li+, Na+, and K+) as the sensor for hydrazine detection. The birnessite MnO2 with Li+ as an intercalant (Li-Bir) displays a lower oxidation peak potential, indicating a catalytic activity higher than the activities of others. The standard heterogeneous electron transfer rate constant of hydrazine oxidation at the Li-Bir electrode is 1.09- and 1.17-fold faster than those at the Na-Bir and K-Bir electrodes, respectively. In addition, the number of electron transfers increases in the following order: K-Bir (0.11 mol) < Na-Bir (0.17 mol) < Li-Bir (0.55 mol). On the basis of the density functional theory calculation, the Li-Bir sensor can strongly stabilize the hydrazine molecule with a large adsorption energy (-0.92 eV), leading to high electrocatalytic activity. Li-Bir also shows the best hydrazine detection performance with the lowest limit of detection of 129 nM at a signal-to-noise ratio of ∼3 and a linear range of 0.007-10 mM at a finely tuned rotation speed of 2000 rpm. Additionally, the Li-Bir sensor exhibits excellent sensitivity, which can be used to detect traces of hydrazine without any effect of interference at high concentrations and in real aqueous-based samples, demonstrating its practical sensing applications.

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.

Preparation of novel hollow δ-MnO2 composite sphere for supercapacitors and degradation of bisphenol A

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.

Boosting the Zn-ion storage capability of birnessite manganese oxide nanoflorets by La3+ intercalation

A kind of La3+ inserted δ-MnO2 was synthesized by a facile process. Benefiting from the larger interlamellar spacing and reduced Zn2+ (de)insertion resistance after intercalation, the capability and rate performance is significantly boosted.

Oxidative Deposition of Manganese Oxide Nanosheets on Nitrogen-Functionalized Carbon Nanotubes Applied in the Alkaline Oxygen Evolution Reaction.

The development of nonprecious catalysts for water splitting into hydrogen and oxygen is one of the major challenges to meet future sustainable fuel demand. Herein, thin layers of manganese oxide nanosheets supported on nitrogen-functionalized carbon nanotubes (NCNTs) were formed by the treatment of NCNTs dispersed in aqueous solutions of KMnO4 or CsMnO4 under reflux or under hydrothermal (HT) conditions and used as electrocatalysts for the oxygen evolution reaction (OER) in alkaline media. The samples were characterized by X-ray photoelectron spectroscopy, X-ray diffraction, transmission electron microscopy, and Raman spectroscopy. Our results show that the NCNTs treated under reflux were covered by partly amorphous and birnessite-type manganese oxides, while predominantly crystalline birnessite manganese oxide was observed for the hydrothermally treated samples. The latter showed, depending on the temperature during synthesis, an electrocatalytically favorable reduction from birnessite-type MnO2 to γ-MnOOH. OER activity measurements revealed a decrease of the overpotential for the OER at a current density of 10 mA cm–2 from 1.70 VRHE for the bare NCNTs to 1.64 VRHE for the samples treated under reflux in the presence of KMnO4. The hydrothermally treated samples afforded the same current density at a lower potential of 1.60 VRHE and a Tafel slope of 75 mV dec–1, suggesting that the higher OER activity is due to γ-MnOOH formation. Oxidative deposition under reflux conditions using CsMnO4 along with mild HT treatment using KMnO4, and low manganese loadings in both cases, were identified as the most suitable synthetic routes to obtain highly active MnOx/NCNT catalysts for electrochemical water oxidation.

Birnessite manganese oxide nanosheets assembled on Ni foam as high-performance pseudocapacitor electrodes: Electrochemical oxidation driven porous honeycomb architecture formation

It has not been considered in the reported literatures that the current or the cyclic number of galvanostatic charge-discharge process controls the morphology growth of manganese oxide materials. Here, standing birnessite nanosheets assembled porous honeycomb architecture is in situ formed on Ni foam by electrochemical oxidation of hydrothermally synthesized Mn3O4 nanospheres electrodes via a electrochemical cycling technique. The birnessite morphology much depends on the conditions of the electrochemical cycling. By tuning the current and cycle number, Mn3O4 nanospheres are converted to birnessite with optimal morphology, while a high pseudocapacitive performance is achieved. The optimized birnessite electrodes exhibit a high level of specific capacitance (377 F g−1 at 1 A g−1), a superior rate capability (227 F g−1 at 30 A g−1) and a good cycling stability (75% retention after 4000 cycles). This work provides useful strategies for developing high-performance manganese oxide electrodes.

Adsorption and transformation of the anthelmintic drug niclosamide by manganese oxide

The manganese oxide birnessite adsorbed and catalyzed the transformation of the anthelminthic drug niclosamide (NIS) into 2-chloro-4-nitroaniline (CNA) and 5-chlorosalicylic acid (CSA) at acidic pH. The adsorption of NIS was fitted using a linear isotherm for all conditions and reaction times. Linear adsorption constant Kd was 103 000 L kg−1 at pH 5.0. The rate of transformation was first order with respect to both MnO2 and NIS. At pH 5.0, the second order rate constant was 3.3 (±0.3) × 10−1 M−1 s−1. The adsorption constants and the rates of transformation decreased when pH increased from 4.0 to 5.5 because of increasing electrostatic repulsions between both negatively charged manganese oxide surface (pHzpc = 2.5) and NIS (pKa = 6.38). The presence of natural organic matter (NOM) extracted from surface water also significantly decreased the adsorption and the rates of transformation of NIS. The rate of transformation decreased by a factor of 20 in presence of 1.6 mgC L−1 even though significant amounts of NIS were adsorbed onto MnO2. The interactions between NOM and NIS were investigated by using the fluorescence quenching method and would explain that NIS adsorbed on the surface of manganese oxide was stable in presence of NOM. Thus, hydrolysis catalyzed by manganese oxide is probably not an important process compared to biodegradation and adsorption because of the presence of organic matter and pH values usually >5.5 in aquatic environment.

Differential capacity of kaolinite and birnessite to protect surface associated proteins against thermal degradation

It is widely accepted that soil organic carbon cycling depends on the presence and catalytic functionality of extracellular enzymes. Recent reports suggest that combusted and autoclaved soils may have the capacity to degrade organic test substrates to a larger extent than the living, enzyme-bearing soils. In search of the underlying mechanisms, we adsorbed Beta-Glucosidase (BG) and Bovine Serum Albumin (BSA) on the phyllosilicate kaolinite and the manganese oxide birnessite at pH 5 and pH 7. The protein-mineral samples were then subjected to gradual energy inputs of a magnitude equivalent to naturally occurring wildfire events. The abundance and molecular masses of desorbed organic compounds were recorded after ionization with tunable synchrotron vacuum ultraviolet radiation (VUV). The mechanisms controlling the fate of proteins varied with mineralogy. Kaolinite adsorbed protein largely through hydrophobic interactions and, even at large energy inputs, produced negligible amounts of desorption fragments compared to birnessite. Acid birnessite adsorbed protein through coulombic forces at low energy levels, became a hydrolyzing catalyst at low energies and low pH, and eventually turned into a reactant involving disintegration of both mineral and protein at higher energy inputs. Fragmentation of proteins was energy dependent and did not occur below an energy threshold of 0.20 MW cm−2. Neither signal abundance nor signal intensity were a function of protein size. Above the energy threshold value, BG that had been adsorbed to birnessite at pH 7 showed an increase in signal abundance with increasing energy applications. Signal intensities differed with adsorption pH for BSA but only at the highest energy level applied. Our results indicate that proteins adsorbed to kaolinite may remain intact after exposure to such energy inputs as can be expected to occur in natural ecosystems. Protein fragmentation and concomitant loss of functionality must be expected in surface soils replete with pedogenic manganese oxides. We conclude that minerals can do both: protect enzymes at high energy intensities in the case of kaolinite and, in the case of birnessite, substitute for and even exceed the oxidative functionality that may have been lost when unprotected oxidative enzymes were denatured at high energy inputs.

Novel CaxMnOy/TiO2 composites for efficient photocatalytic degradation of methylene blue and the herbicide imazapyr in aqueous solution under visible light irradiation

CaxMnOy-TiO2 composites with different manganese oxide birnessite fractions (1–50 wt.%) have been synthesized based on the precipitation of birnessite and mixing with commercial titanium dioxide TiO2 by mechanical grinding. The CaxMnOy-TiO2 composites exhibited high photoactivity for the decomposition of imazapyr herbicide and methylene blue under UV–vis illumination. The results suggested that a correlation exists between the calcium manganese oxide content, oxidation state and the changes in the UV–vis absorption properties. The composite with 5% Ca0.1MnOy/TiO2 possessed the highest photocatalytic performance among all the as-obtained catalysts, the enhanced photocatalytic activity was attributed to the reduction of the band gap from 3.18 eV to 2.89 eV for pure TiO2 and 5% Ca0.1MnOy/TiO2 composite respectively. In addition, an efficient inhibition of electron-holes recombination rates due to the deposition of manganese oxide mainly on the surface of titanium dioxide producing an efficient charge separation, and transfer between the materials. Furthermore, the photocatalytic rate constant was found to increase from 0.124 min−1 to 0.542 min−1 and from 0.155 min−1 to 0.425 min−1 for methylene blue dye and imazapyr, in the presence of commercial titanium dioxide TiO2 and 5% Ca0.1MnOy/TiO2 composite, respectively.

Systematic Doping of Cobalt into Layered Manganese Oxide Sheets Substantially Enhances Water Oxidation Catalysis.

The effect on the electrocatalytic oxygen evolution reaction (OER) of cobalt incorporation into the metal oxide sheets of the layered manganese oxide birnessite was investigated. Birnessite and cobalt-doped birnessite were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and conductivity measurements. A cobalt:manganese ratio of 1:2 resulted in the most active catalyst for the OER. In particular, the overpotential (η) for the OER was 420 mV, significantly lower than the η = 780 mV associated with birnessite in the absence of Co. Furthermore, the Tafel slope for Co/birnessite was 81 mV/dec, in comparison to a Tafel slope of greater than 200 mV/dec for birnessite. For chemical water oxidation catalysis, an 8-fold turnover number (TON) was achieved (h = 70 mmol of O2/mol of metal). Density functional theory (DFT) calculations predict that cobalt modification of birnessite resulted in a raising of the valence band edge and occupation of that edge by holes with enhanced mobility during catalysis. Inclusion of extra cobalt beyond the ideal 1:2 ratio was detrimental to catalysis due to disruption of the layered structure of the birnessite phase.

Decoration of the layered manganese oxide birnessite with Mn(II/III) gives a new water oxidation catalyst with fifty-fold turnover number enhancement.

The role of the manganese average oxidation state (AOS) in water oxidation catalysis by birnessite was investigated. Low AOS samples were most active, generating O2 immediately. Samples with a relatively high AOS showed an initial induction period and decreased turnover. Mn(ii- and iii)-enriched samples gave a 10-50 fold enhancement in turnover number.

Three-dimensional honeycomb-like networks of birnessite manganese oxide assembled by ultrathin two-dimensional nanosheets with enhanced Li-ion battery performances.

Three-dimensional (3D) honeycomb-like birnessite networks composed of ultrathin two-dimensional (2D) nanosheets were firstly synthesized through a facile and low-cost synthetic route. By using carbon microspheres as a template instead of graphene, hierarchical birnessite structures assembled by ultrathin nanosheets including york-shell and hollow structures were obtained besides the ultrathin birnessite nanosheets with a thickness of about 0.7 nm. By assembling carbon spheres into an ordered 3D array, novel 3D honeycomb-like birnessite structures assembled by ultrathin nanosheets were firstly prepared. When evaluated as an anode material for Li-ion batteries, the 3D honeycomb-like networks show enhanced electrochemical performances with high capacities, excellent cycling stability and good rate capability, which can be ascribed to the novel 3D honeycomb-like macroporous structure with a 3D inverse opal structure, well-ordered macropores, interconnected walls and a regular periodicity.

Achieving electrochemical capacitor functionality from nanoscale LiMn2O4 coatings on 3-D carbon nanoarchitectures

Conformal nanoscale coatings of Na+-birnessite manganese oxide (MnOx) produced via redox reaction between aqueous permanganate (NaMnO4·H2O) and the carbon surfaces of fiber-paper-supported carbon nanofoams are converted to LiMn2O4 spinel through topotactic exchange of Na+ for Li+ in the as-deposited lamellar birnessite, followed by mild thermal treatments to complete the transformation to LiMn2O4. The evolution of the birnessite-to-spinel conversion is verified with X-ray diffraction, solid-state nuclear magnetic resonance, X-ray absorption spectroscopy, electron microscopy, cyclic voltammetry, and electrochemical impedance spectroscopy. The mild conditions used to convert birnessite to spinel ensure that the conformal nanoscale nature of the oxide coating is retained throughout the macroscopically thick (170 μm) carbon nanofoam substrate during the conversion process. The architecture of the LiMn2O4–carbon nanofoam facilitates rapid ion/electron transport, enabling the LiMn2O4 to insert and extract Li+ from aqueous electrolytes at scan rates as high as 25 mV s−1, and with a relaxation time of 37 s as derived from electrochemical impedance. This architectural expression of nanoscale LiMn2O4 delivers full theoretical capacity (148 mA h g−1) at 2 mV s−1.