Layered MnO2 Research Articles

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.

High-capacity K+-pillared layered manganese dioxide as cathode material for high-rate aqueous zinc-ion battery

Sluggish kinetics and severe structural instability of manganese-based cathode materials for rechargeable aqueous zinc-ion batteries (ZIBs) lead to low-rate capacity and poor cyclability, which hinder their practical applications. Pillaring manganese dioxide (MnO2) by pre-intercalation is an effective strategy to solve the above problems. However, increasing the pre-intercalation content to realize stable cycling of high capacity at large current densities is still challenging. Here, high-rate aqueous Zn2+ storage is realized by a high-capacity K+-pillared multi-nanochannel MnO2 cathode with 1 K per 4 Mn (δ-K0.25MnO2). The high content of the K+ pillar, in conjunction with the three-dimensional confinement effect and size effect, promotes the stability and electron transport of multi-nanochannel layered MnO2 in the ion insertion/removal process during cycling, accelerating and accommodating more Zn2+ diffusion. Multi-perspective in/ex-situ characterizations conclude that the energy storage mechanism is the Zn2+/H+ ions co-intercalating and phase transformation process. More specifically, the δ-K0.25MnO2 nanospheres cathode delivers an ultrahigh reversible capacity of 297 mAh g−1 at 1 A g−1 for 500 cycles, showing over 96 % utilization of the theoretical capacity of δ-MnO2. Even at 3 A g−1, it also delivered a 63 % utilization and 64 % capacity retention after 1000 cycles. This study introduces a highly efficient cathode material based on manganese oxide and a comprehensive analysis of its structural dynamics. These findings have the potential to improve energy storage capabilities in ZIBs significantly.

Deactivation of layered MnO2 catalyst during room temperature formaldehyde degradation and its thermal regeneration mechanism

Layered δ-MnO2 prepared by a one-step redox method was shown to be deactivated during oxidation of formaldehyde at room temperature. Recovery of the formaldehyde degradation activity was investigated after thermal regeneration at different temperatures. XRD, SEM, TEM, H2-TPR, XPS, TGA and DRIFTS characterization were used to analyze the physical properties of fresh, deactivated and thermally regenerated catalysts. The results showed that deactivation of catalyst was caused by less oxygen vacancies due to increased Mn4+ content during formaldehyde degradation and formation of formate blocking active sites. Thermal regeneration helped to decompose formate at the catalyst surface, restoring some of the active sites. Interplanar spacing of MnO2 became wider and the number of Mn3+ on exposed crystal faces increased. More oxygen vacancies were formed. The activity of deactivated catalyst was restored. Formaldehyde degradation rate of catalyst regenerated at 200 °C remained above 80 % after 6 h, demonstrating the possibility of waste layered catalyst recycling.

Enhancing zinc-ion batteries: PEDOT-MnO2 cathodes for superior stability and capacity

Of late, zinc-ion batteries (ZIBs) are emerging as a vital component in sustainable energy storage, ensuring an environmentally friendly alternative to traditional battery systems. Herein, this study provides a novel approach to ZIB research, undertaking to encapsulate PEDOT in a layered MnO2 structure. Through the use of comprehensive analytical techniques, including X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and X-ray absorption spectroscopy (XAS), a detailed analysis of the enhanced PEDOT-MnO2 heterostructure is presented. Our research demonstrates that the encapsulation of PEDOT results in a substantial increase in initial discharge capacity (∼40%) compared to traditional MnO2 alongside improvements in structural stability. These findings, facilitated by in-situ X-ray absorption near-edge structure (XANES) and ex-situ extended X-ray absorption fine structure (EXAFS) techniques, underscore the potential of PEDOT-MnO2 in advancing the development of manganese-based cathode materials for non-aqueous electrolyte ZIBs. The work represents a crucial advancement in the field of sustainable energy storage solutions, highlighting the role of innovative material design in improving the efficiency and stability of rechargeable battery systems.

Silver nanoparticle−decorated MnO2 for promoted Mn4+/Mn2+ redox reaction in aqueous Zn−MnO2 batteries

Rechargeable aqueous Zn−MnO2 batteries have garnered increasing attention recently due to intrinsic safety, high theoretical capacity, appealing output voltage, and low material cost. However, challenges still exist in exploring for high−performance manganese oxides capable of delivering comparable capacity to the Zn anode. Herein, a composite material made of layered MnO2 decorated with Ag nanoparticles (Ag NPs) can be prepared by a one−step hydrothermal reaction. The silver decoration strategy can provide facile electron transport of the composite cathode and induce structural vacancies via forming Ag−O−Mn bond. These features enable the AgMO cathode to exhibit smaller charge−transfer resistance (23.0 vs 41.3 Ω) and accelerated ion transport kinetics (1.25×10−10 vs 3.90×10−11 cm2 s−1). More importantly, the presence of Ag NPs is found to promote the reaction kinetics of Mn4+/Mn2+ redox. By taking these advantages, the AgMO sample exhibits a higher attainable capacity of 333 mAh g–1 at 0.3 A g–1 compared with the Ag−free counterpart (MO, 268 mAh g–1). Moreover, AgMO demonstrates better rate performance with the discharge capacity of 109 mAh g–1 at 8 A g–1 (90 mAh g–1 for MO), and a specific capacity of 93 mAh g–1 can still be achieved for AgMO after 2000 cycles at 4 A g–1 (55 mAh g–1 for MO). This work presents a holistic approach to enhance the energy storage capability of MnO2 cathode in rechargeable aqueous Zn−ion batteries.

Benzoquinone-Lubricated Intercalation in Manganese Oxide for High-Capacity and High-Rate Aqueous Aluminum-Ion Battery.

Aqueous aluminum-ion batteries are attractive post-lithium battery technologies for large-scale energy storage in virtue of abundant and low-cost Al metal anode offering ultrahigh capacity via a three-electron redox reaction. However, state-of-the-art cathode materials are of low practical capacity, poor rate capability, and inadequate cycle life, substantially impeding their practical use. Here layered manganese oxide that is pre-intercalated with benzoquinone-coordinated aluminum ions (BQ-AlxMnO2) as a high-performance cathode material of rechargeable aqueous aluminum-ion batteries is reported. The coordination of benzoquinone with aluminum ions not only extends interlayer spacing of layered MnO2 framework but reduces the effective charge of trivalent aluminum ions to diminish their electrostatic interactions, substantially boosting intercalation/deintercalation kinetics of guest aluminum ions and improving structural reversibility and stability. When coupled with Zn50Al50 alloy anode in 2m Al(OTf)3 aqueous electrolyte, the BQ-AlxMnO2 exhibits superior rate capability and cycling stability. At 1Ag-1, the specific capacity of BQ-AlxMnO2 reaches ≈300mAhg-1 and retains ≈90% of the initial value for more than 800 cycles, along with the Coulombic efficiency of as high as ≈99%, outperforming the AlxMnO2 without BQ co-incorporation.

Regulation of surface acidic sites on birnessite-type MnO2 and its function in the adsorption of gaseous ammonia

Ammonia (NH3) is not only a characteristic malodorous contaminant, but also one of the important precursors in the formation of haze. In engineering practice, the most widely used technology for NH3 purification is adsorption, and the development of adsorbent with excellent NH3 adsorption capacity is the key to adsorption method. Herein, the surface acidic sites of layered birnessite-type MnO2 were regulated for the adsorption of NH3. Results presented that the adsorption capacity of birnessite-type MnO2 reinforced by surface acidic sites could reach 36.1 mg/g, which significantly superior to pristine MnO2 and other commercial carbon materials. In addition, the effects of space velocity, NH3 concentration, adsorption temperature and relative humidity on its adsorption performance were also studied. It was found that the effect of space velocity and relative humidity had no significant effect on the equilibrium NH3 adsorption capacity; while the equilibrium adsorption capacity showed an increasing trend with the rise of NH3 concentration and the decrease of adsorption temperature. Furthermore, the function of surface acidic sites, especially Lewis and Brønsted acidic sites, in the adsorption of birnessite-type MnO2 for NH3 was further revealed. On the one hand, the acidic site strengthening method of acid impregnation can significantly increase the specific surface area of birnessite-type MnO2, which is beneficial for the physical adsorption of NH3. Importantly, this acidic site strengthening method significantly enhances the number and intensity of surface acidic sites of birnessite-type MnO2, especially the Brønsted acidic sites, which greatly promoted the adsorption of alkaline gas NH3. Finally, the cyclic thermal regeneration performance at low temperatures was further studied. This work reports a strategy to improve NH3 adsorption performance by enhancing acidic sites, and further reveals the functional role of surface acidic sites in NH3 adsorption.

Bulk defect-induced phase transition assisting high-flux magnesium ions intercalation in manganese tetroxide

The industry consensus is that finding appropriate electrode materials is a major obstacle to the widespread use of magnesium ion (Mg2+) energy storage devices due to challenges like high charge density, intense polarization, and strong interaction with the surrounding matrix of Mg2+. Manganese tetroxide electrodes for magnesium ion energy storage suffer from the limited capacity resulting by pseudocapacitance reaction on the subsurface. Herein, manganese tetroxide with bulk cation-anion dual defects (CADDs-Mn3O4) is constructed for the first time by a facile hydrothermal reduction method. The as-prepared bulk CADDs-Mn3O4 electrode in the three-electrode system exhibits a high discharge capacity of 320.93 mAh g−1 contributing by the intercalation of high flux Mg2+, which achieves capacity breakthrough that ever reported. Coupled with the activated carbon anodes in a full-cell configuration, aqueous magnesium ion capacitors display an energy density of 128.39 Wh/kg with an ultra-long cycling capability, giving 85 % capacitance retention after 6000 cycles. This outstanding performance can be attributed to the metastable region created by unique bulk cation-anion dual defects. The transition of Mn3O4 to layered MnO2 is promoted, achieving the quick migration of magnesium ions in the bulk phase in a large flux, thus breaking the constraint of the spinel structure on the insertion of divalent magnesium ions. Moreover, the manganese cation defects reduce the band gap and increase the electrical conductivity of the electrodes, and the oxygen anion defects change the spin state of the electron and weaken the bonding energy of Mg-O. This work provides a detailed understanding of the structure-function relationship based on the introduction of cation-anion dual defects, and thus opens up new pathways for modifying energy storage materials.

Li-Ion and Na-Ion Intercalation in Layered MnO2 Cathodes Enabled by Using Bismuth as a Cation Pillar

Rechargeability of low-cost MnO2 cathodes in aqueous cells has the potential to be an enabling technology for low-cost (<$50/kWh) grid-scale batteries. This rechargeability is imparted by modifying the MnO2 with Bi. In this talk we detail using Bi doping for structural stabilization of MnO2 cathodes for Li-ion and Na-ion batteries in dry, non-aqueous electrolyte. We do this by preparing a series of Bi-doped layered MnO2 compounds, which also contain K ions and crystal water. These phyllomanganates or birnessite-type compounds have the general formula KxBiy-MnO2 • nH2O. The materials are structurally characterized by synchrotron X-ray powder diffraction (XPD) and confocal Raman microscopy. It is found that incorporation of Bi in the structure increases the crystallinity of the material by ordering the interlayer cations. This ordering results in the emergence of a structural superlattice in the material, which has been reported previously for birnessite inserted with other heavy, multivalent cations such as as Sr2+, Ba2+, and Ca2+.We selected three Bi doping levels to study in Li-ion and Na-ion batteries: undoped (y = 0), low-doping (y = 0.013 or 1.3%), and high doping (y = 0.043 or 4.3%). Both doped materials had capacity over 200 mAh/g and allowed for far more stable cell cycling than the undoped Kx-MnO2. The cells made with 4.3% doping of Bi were by far more stable. These cycled at 175-190 mAh/g specific capacity through 100 cycles with a nominal voltage of 2.9 V. Operando XRD experiments to characterize the cathodes within sealed coin cells showed that high-level doping (4.3%) promoted dehydration of the KxBiy-MnO2 interlayer, which had a distance of 6.4 Å. This showed that loss of the crystal water from the interlayer was key to achieving maximum stability during cycling. Lower Bi doping (1.3%) did not cause loss of the crystal water once the cathode was wetted with electrolyte. Previous reports have suggested that Bi doping of 20% results in stabilized Li-ion cycling in layered MnO2. However, these reports involved largely amorphous active material. Our results showed that Bi doping of only 1.3% still provided a large stability benefit, and the high crystallinity of our material allowed it to be structurally characterized, both in the as-prepared state and operando during battery cycling. Results from Na-ion cells will also be presented.Stabilized MnO2-based cathodes that cycle with good capacity and somewhat lower cell voltage than traditional Li-ion cells are a promising strategy for low-cost batteries for the grid. The possibility of using Na as the working ion is also useful for large-scale deployment of batteries to balance renewable power. Acknowledgments This work was supported by the U.S. Department of Energy (DOE) Office of Electricity Delivery and Energy Reliability, Dr. Imre Gyuk, Energy Storage Program Manager. This research used resources of the Advanced Photon Source beamline 6-BM, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. This research also used resources at beamlines 7-BM (QAS) and 28-ID-2 (XPD) of the National Synchrotron Light Source II, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Brookhaven National Laboratory under Contract No. DE-SC0012704.

Hollow SiC@MnO2 nanospheres with tunable core size and shell thickness for excellent electromagnetic wave absorption

The hollow microstructure can permit more incident waves to enter the absorber and increase the attenuation ability by multiple reflections and diffraction. In addition, different substances can introduce the heterogeneous interface and further attenuate electromagnetic waves through interfacial polarisation relaxation. Here, hollow SiC@MnO2 nanospheres are synthesised with tunable hollow SiC core size and flower-like layered MnO2 shell thickness. By adjusting the concentration of KMnO4, different shell thicknesses of hollow SiC@MnO2 nanospheres can be prepared, ranging from 40 nm to 110 nm. Furthermore, hollow SiC@MnO2 composites with different inner diameters between 300 nm and 470 nm can be obtained by tailoring the amount of tetraethyl orthosilicate. The results indicate that the effective absorbing bandwidth of hollow SiC@MnO2 can reach 5.71 GHz with a core size of 360 nm and a shell thickness of 90 nm at only 1.8 mm. This work provides a valuable core–shell strategy of hollow SiC@MnO2 towards excellent electromagnetic wave-absorbing properties.

Multi-electron reaction and fast Al ion diffusion of δ-MnO2 cathode materials in rechargeable aluminum batteries via first-principle calculations

Rechargeable aluminum batteries with multi-electron reaction have a high theoretical capacity for next generation of energy storage devices. However, the diffusion mechanism and intrinsic property of Al insertion into MnO2 are not clear. Hence, based on the first-principles calculations, key influencing factors of slow Al-ions diffusion are narrow pathways, unstable Al-O bonds and Mn3+ type polaron have been identified by investigating four types of δ-MnO2 (O3, O’3, P2 and T1). Although Al insert into δ-MnO2 leads to a decrease in the spacing of the Mn-Mn layer, P2 type MnO2 keeps the long (spacious pathways) and stable (2.007–2.030 Å) Al-O bonds resulting in the lower energy barrier of Al diffusion of 0.56 eV. By eliminated the influence of Mn3+ (low concentration of Al insertion), the energy barrier of Al migration achieves 0.19 eV in P2 type, confirming the obviously effect of Mn3+ polaron. On the contrary, although the T1 type MnO2 has the sluggish of Al-ions diffusion, the larger interlayer spacing of Mn-Mn layer, causing by H2O could assist Al-ions diffusion. Furthermore, it is worth to notice that the multilayer δ-MnO2 achieves multi-electron reaction of 3|e|. Considering the requirement of high energy density, the average voltage of P2 (1.76 V) is not an obstacle for application as cathode in RABs. These discover suggest that layered MnO2 should keep more P2-type structure in the synthesis of materials and increase the interlayer spacing of Mn-Mn layer for providing technical support of RABs in large-scale energy storage.

Tailoring Oxygen Vacancies and Active Surface Oxygen Species in Copper-Doped MnO2 Catalysts for Total Catalytic Oxidation of VOCs

Catalytic oxidation has been considered an essential route to tackle volatile organic compounds (VOCs)-driven pollution. To this point, surface engineering has emerged as a robust pathway to promote the efficiency of MnO2-based materials toward VOC oxidation. However, regulating such surface properties has been a critical challenge. This investigation attempts to tailor surface oxygen vacancies and active surface oxygen for the total catalytic oxidation of various VOCs (e.g., formaldehyde, isopropanol, and toluene) via doping copper into cryptomelane and birnessite-type MnO2 catalysts. The catalysts are synthesized by a novel facile successive redox precipitation method between KMnO4 and an ethanol solution containing copper nitrate precursors, followed by annealing. It turns out that the low amount of Cu dopants, corresponding to the copper-to-manganese ratio of 0.17 (RCu/Mn = 0.17), generates a tunnel-structured cryptomelane-type MnO2 with strong bulk oxygen mobility. Such a crystalline structure exhibits the best catalytic performance for the total catalytic oxidation of toluene. The higher copper loading (RCu/Mn = 0.27–0.34) drives the formation of Cu-doped layered birnessite-type MnO2 materials containing rich oxygen vacancies and high active surface oxygen species. Those catalysts promote the oxidation of isopropanol and formaldehyde, respectively. To this end, oxygen vacancies and surface oxygen species crucially steer the molecular oxygen activation and interactions toward adsorbed oxygen species, significantly promoting the total oxidation of isopropanol and formaldehyde, respectively. The explored finding could disclose an outstanding platform toward highly selective and efficient oxidation of VOCs.

Layer symmetry and interlayer engineering of birnessites towards high-performance rechargeable aqueous Zn-MnO2 batteries

Rechargeable aqueous Zn-MnO2 batteries are promising candidates for large-scale energy storage systems, yet still plagued by the phase transition and structural collapse issues of MnO2 cathodes during cycling. Interlayer intercalation for the layered MnO2 turns out to be a viable alternative and become the mainstream structure design strategy. However, the characteristics of Mn octahedral layers are generally neglected. Herein, for the first time we elucidate apart from interlayer ions, how layer symmetry of birnessites exerts on the electrochemical performance. The Mn(II) ions stabilized hexagonal birnessite exhibits elevated charge storage performance than its monoclinic precursor, attributing to the layer cation vacancies generated after symmetry transformation, interlayer Mn(II) ions and nanosized morphology. A high specific capacity of 279 mAh g−1 at 1 C is achieved, as well as an outstanding long-term cycling stability with 97% retention over 8000 cycles. The reaction mechanism is comprehensively illustrated. This work previews a new gateway for the design of high-performance layered cathode materials by synergistic manipulation of the crystal structure of the layers and the interlayer environment.

In-situ cation-inserted MnO2 with selective accelerated intercalation of individual H+ or Zn2+ ions in aqueous zinc ion batteries

The recognized energy storage mechanism of neutral aqueous zinc–manganese batteries is the co–insertion/extrusion of H+ and Zn2+ ions. However, modulating the kinetics of a single H+ or Zn2+ ion is scarce, which can provide meaningful insights into the energy storage mechanism of Zn ion batteries. Herein, a distinctive doubly electric field in-situ induced cationic anchoring of two-dimensional layered MnO2 is successfully constructed to modulate the insertion/extrusion of a single H+ or Zn2+ ion. As a result, regulating the intercalation of different metal ions can precisely achieve the accelerated induction for the individual H+ or Zn2+ ions intercalation/deintercalation. Moreover, the introduction of metal ions stabilizes the lattice distortion and alleviates the irreparable structural collapse, leading to an increase in the H+/Zn2+ storage sites, efficiently diminishing the stagnation of the ordered structure and creating the more open channels, which is conducive to facilitating the diffusion of ions. This work delivers some innovative insights into pre-embedding strategies, and also serves as a precious reference for the cathode development of advanced aqueous batteries.

Enhancement of acidic sites in layered MnO2 for the highly efficient selective catalytic oxidation of gaseous ammonia

Ammonia (NH3), as a typical harmful gaseous pollutant in atmosphere, poses a serious threat to human health and ecological environment. It is of great significance to develop the cost-effective catalysts with both excellent selective catalytic oxidation (SCO) activity and N2 selectivity. In this study, the acidic sites of layered MnO2 were regulated and its NH3-SCO performance was investigated. The enhancement of acidic sites in layered MnO2 significantly improved the SCO of NH3, and the resulting MnO2 could achieve complete conversion of NH3 at as lower as 100 °C, even better than the reported noble-metal catalysts. Furthermore, the improvement in NH3-SCO activity can be attributed to the abundant active oxygen species and enhancement of acidic sites (especially stronger Brønsted acidic sites). On the one hand, more active oxygen species can enhance the catalytic oxidation of NH3. On the other hand, Brønsted acidic sites acted not only as adsorption sites but also reaction sites, the increase in the content and strength of acidic sites facilitated the adsorption and activation of NH3. This research reveals the functional role of acidic sites in NH3-SCO, and provides a suggestive for the rational design of catalysts with excellent N2 selectivity and low-temperature NH3-SCO activity.

Yttrium-preintercalated layered manganese oxide as a durable cathode for aqueous zinc-ion batteries.

Rechargeable aqueous zinc-ion batteries (RAZIBs) are regarded as competitive alternatives for large-scale energy storage on account of cost-effectiveness and inherent safety. In particular, rechargeable Zn-MnO2 batteries have drawn increasing attention due to high manufacturing readiness level. However, obtaining MnO2 with high electrochemical activity and high cyclic stability toward Zn2+/H+ storage still remains challenging. Herein, we reveal that incorporating yttrium ions (Y3+) into layered MnO2 can regulate the electronic structure of the MnO2 cathode by narrowing its band gap (from 3.25 to 2.50 eV), thus boosting the electrochemical performance in RAZIBs. Taking advantage of this feature, the optimized Y-MnO2 (YMO) sample exhibits greater capacity (212 vs. 152 mA h g-1 at 0.5 A g-1), better rate capability (94 vs. 61 mA h g-1 at 8 A g-1), reduced charge-transfer resistance (79 vs. 148 Ω), and promoted mass transfer kinetics (3.13 × 10-11vs. 2.37 × 10-11 cm2 s-1) in comparison with Y-free MnO2 (MO). More importantly, compared to MO, YMO-0.1 exhibits enhanced energy storage capability by nearly 40% (309 vs. 222 W h kg-1) and stable cycle performance (94 vs. 52 mA h g-1 after 3000 cycles). In situ Raman microscopy further reveals that the presence of Y3+ endows MnO2 with remarkable electrochemical reversibility during charge/discharge processes. This work highlights the importance of the Y3+ preintercalation strategy, which can be further developed to obtain better cathode materials for aqueous batteries.

Li-ion and Na-ion intercalation in layered MnO2 cathodes enabled by using bismuth as a cation pillar

Layered MnO2 with Bi inserted into the interlayer stabilizes Li-ion and Na-ion cycling. The amount of Bi affects the crystal water in the structure.

Ir Nanoconfined and Doped MnO2 Nanosheets for an Enhanced Electrocatalytic Oxygen Evolution Reaction in Acidic and Basic Medium

One of the promising strategies to generate hydrogen for a future hydrogen economy is via electrocatalytic water splitting, where renewable electricity would be used to convert water into hydrogen and oxygen on an electrocatalyst within an electrolyzer. Oxygen evolution reaction (OER), the anodic half-reaction of water splitting is, however, kinetically sluggish and requires four electron-proton transfers making the overall reaction energy-intensive. For the economical production of hydrogen, improving the reaction kinetics in both low and high pH is of utmost necessity by designing an active and stable OER catalyst. Precious metals like Ir, Ru, and their oxides are known to be excellent OER catalysts but their large-scale application is hindered due to their low earth-abundancy. To address this issue, we have synthesized two types of OER catalysts with varying Ir wt.% to achieve a higher OER-activity per mass of Ir in the catalyst: Ir/MnO2, Ir nano-confined in the interlayer of layered MnO2 nanosheets and Ir-doped MnO2 nanosheets. In both acidic and basic environments, Ir/MnO2 exhibited better OER activity than Ir-doped MnO2. In 0.5 M H2SO4, 16.7 wt.% Ir/MnO2 and 17.5 wt.% Ir-doped MnO2 exhibited OER overpotentials (η) of 298 mV and 320 mV, respectively, at a current density of 10 mA cm-2. In 1M KOH electrolyte, 15.4 wt.% Ir/MnO2 and 17.5 wt.% of Ir-doped MnO2 exhibited OER η values of 213 mV and 245mV, respectively at 10 mA cm-2. These overpotentials are ~500 mV lower than the pristine MnO2 nanosheets and ~60-120 mV lower than commercial reference catalysts (20 wt.% Ir/C and IrO2) at both low and high pH conditions, respectively. These synthesized catalysts were able to outperform the commercial materials by exhibiting four times higher OER-activity per mass of Ir, in both acidic and basic media. Hence, this synthetic approach may be a method to achieve enhanced OER activity using a lower mass of Ir. The physical characterization of the materials was carried out with X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray absorption spectroscopy (XAS).

Construction of oxygen vacancies in δ-MnO2 for promoting low-temperature toluene oxidation

Oxygen vacancy engineering is a proven method to promote catalytic performance. In this paper, a series of alkaline earth metal-doped layered MnO2 catalysts were prepared to modulate the oxygen vacancies in MnO2 and applied to catalytic oxidation of toluene at low-temperature. Various characterizations and DFT calculations show that the alkaline earth metals enter the MnO2 lattice, leading to the distortion of the lattice and the formation of oxygen vacancies, which eventually lead to the activation of the lattice oxygen. Ba-doped MnO2 shows the highest content of oxygen vacancies and the highest lattice oxygen activity, thus exhibiting the best low-temperature catalytic performance for toluene (T90 = 182 °C). This work provides an innovative approach for the construction of oxygen vacancies on high-performance catalysts for toluene oxidation.

Potassium-doped hydrated manganese dioxide nanowires-carbon nanotubes on graphene for high-performance rechargeable zinc-ion batteries

Aqueous rechargeable Zn-ion batteries (ARZIB) are promising candidates for next-generation batteries because of their high safety, low cost, and relatively high capacity. In this study, we developed hydrated and potassium-doped manganese dioxide (MO) nanowires mixed with carbon nanotubes (CNT) on a graphene substrate (hydrated KMO-CNT/graphene) for ARZIB. A simple polyol process using poly(ethyl glycol), KMnO4, CNT, and graphene was utilized to fabricate hydrated KMO-CNT/graphene. MnO2 nanowires with diameters of 15–25 nm possess a high specific capacity with a short diffusion path. The intercalated K ions and hydrates in the layered MnO2 nanowires maintained the MO structure during the charge and discharge processes, whereas carbon nanomaterials (CNT and graphene) enhanced the conductivity of the material. Consequently, hydrated KMO-CNT/graphene demonstrated good ARZIB performance. A high capacity of 359.8 mAh g–1 at 0.1 A g–1, and at a high current density of 3.0 A g–1, a capacity of 129 mAh g–1 with 77% retention after 1000 cycles, were achieved.