Mn-rich Surface Research Articles

Kinetics of methane combustion on model supported Pd/LaxMnO3 natural gas vehicle catalysts: Sensitivity of La-stoichiometry on the catalytic properties of Pd

The kinetics of the catalytic CH4/O2 reaction has been studied on 1 wt% Pd/LaxMnO3 with x = 0.7, 1.0 and 1.3. Steady-state kinetic measurements have been performed in lean conditions at 400 °C on pre-reduced and on aged catalysts after exposure to wet atmosphere at 750 °C. A single site reaction mechanism occurs preferentially on Pd/La1.3MnO3 exhibiting a La-rich surface. In contrast a dual-site reaction mechanism, combining Pd and surface reactive oxygen species from the support, is preferentially involved on Pd-La0.7MnO3 characterized by a Mn-rich surface. Pd/LaMnO3 exhibits a mixed regime but subsequent deterioration of the Pd-LaxMnO3 interface during thermal aging leads to increasing contribution of the single site reaction mechanism. High surface Pd/Mn ratio and high concentration of Pdn+ with n > 2 can be considered as good descriptors to probe the efficacy of Pd/LaxMnO3 catalysts. Remarkably, the high thermal stability of the Pd-La0.7MnO3 interface, preserving the Pd dispersion on Pd/La0.7MnO3, leads to lower sensitivity to deactivation.

Preparation of long-term cycling stable ni-rich concentration–gradient NCMA cathode materials for li-ion batteries

Nickel-rich (Ni > 90 %) cathodes are regarded as one of the most attractive because of their high energy density, despite their poor stability and cycle life. To improve their performance, in this study we synthesized a double concentration-gradient layered Li[Ni0.90Co0.04Mn0.03Al0.03]O2 oxide (CG-NCMA) using a continuous co-precipitation Taylor–Couette cylindrical reactor (TCCR) with a Ni-rich-core, an Mn-rich surface, and Al on top. The concentration-gradient morphology was confirmed through cross-sectional EDX line scanning. The as-synthesized sample exhibited excellent electrochemical performance at high rates (5C/10C), as well as cyclability (91.5 % after 100 cycles and 70.3 % after 500 cycles at 1C), superior to that (83.4 % and 47.6 %) of its non-concentration-gradient counterpart (UC-NCMA). The Mn-rich surface and presence of Al helped the material stay structurally robust, even after 500 cycles, while also suppressing side reactions between the electrode and electrolyte, resulting in better overall electrochemical performance. These enhancements in performance were studied using TEM, SEM, in-situ-XRD, XPS, CV, EIS and post-mortem analyses. This synthetic method enables the highly scalable production of CG-NCMA samples with two concentration-gradient structures for practical applications in Li-ion batteries.

Constructing a NiMnS electrode with a Mn-rich surface for hydrogen production in anion exchange membrane water electrolyzers.

Efficient alkaline hydrogen evolution electrodes must be used for hydrogen production in anion exchange membrane water electrolyzers (AEMWEs). Therefore, we fabricated a NiMnS catalyst with a Mn-rich surface, which was self-supported on Ti paper through one-step electrodeposition. Mn doping endowed the catalyst with a unique hollow morphology and lattice-distorted structure. Consequently, the NiMnS/Ti electrode exhibited a large number of exposed electrochemical surfaces and effective active sites and a high hydrogen evolution reaction (HER) activity. Specifically, in half-cell measurements, the NiMnS/Ti electrode exhibited an overpotential of 65 mV at 10 mA cm-2, which was lower than that of NiS/Ti (102 mV) and indicated its superior HER activity. When the proposed cathode was applied in an AEMWE single cell, the device achieved a high current density of up to 0.9 A cm-2 at a cell potential of 2.0 V.

Hierarchical Regulation of LaMnO3 Dual-Pathway Strategy for Excellent Room-Temperature Organocatalytic Oxidation Performance.

The performance-enhancing strategy of a single pathway for perovskite has been widely studied. In this work, the dual-pathway strategy of A-site Ce substitution and nitric acid selective dissolution was proposed. The catalytic oxidation performance of LaMnO3 exhibits the characteristic of hierarchical regulation, that is, a steplike improvement, which avoids the limitation of performance improvement of the single pathway. The B-site Mn with catalytic activity was in situ reconstituted on the surface to build a Mn-rich surface. The obtained sdLa0.7Ce0.3MnO3 has the advantages of good oxygen mobility, high Mn4+/Mn3+ molar ratio, and large specific surface area, and this material showed excellent catalytic oxidation performance for organics, which can realize colorimetric chemical oxygen demand detection at room temperature. Here, Ce substitution improved the oxidation capacity by improving the oxygen mobility and the ratio of Mn4+/Mn3+, and further nitric acid treatment not only accelerated the in situ reconstruction of B-site Mn but also increased the specific surface area.

Alloying of TiC-FeCr cermet in manganese vapor

Conventional vacuum sintering is not suitable for producing manganese containing cermets because of the physical-chemical properties of manganese, high vapor pressure, combined with high sintering temperature of cermets (1400-1600 °C). Sintering in Mn-rich microatmosphere does not only prevent Mn loss, but also enables additional in-situ alloying of the binder phase during sintering. We studied alloying of TiC-based cermet bonded with high chromium steel during sintering in Mn-rich atmosphere. Sintering in manganese vapor was found to increase sinterability of the TiC-FeCr cermet, resulting in the formation of ∼1 mm thick Mn-rich surface layer with homogeneous microstructure while the core region of the material remained unaffected. This core region exhibited highly increased Mn content and competitive mechanical properties – hardness of ∼1200 HV30 and indentation fracture toughness of ∼13 MPa*m1/2. In addition, an unusual ε-type martensitic phase was observed in the surface layer.

Elevated manganese concentrations in shallow groundwater of various aquifers in a rapidly urbanized delta, south China

High concentration of manganese (Mn) in groundwater is a major concern because of its harmful to human health, and the origin of which in urbanized areas is often complicated. The present study aims to delineate spatial distributions of groundwater Mn in various aquifers and in areas with different urbanization levels in the Pearl River Delta (PRD), and to identify the origins of groundwater Mn in this region. Nearly 400 groundwater samples collected, and 14 chemicals were analyzed. The results show that approximately 20% groundwater in granular aquifers showed elevated-Mn (>0.4 mg/L), and was more than two times of that in fissured aquifers, while that in karst aquifers was absent. The proportions of elevated-Mn groundwater in urbanized areas and peri-urban areas were higher than that in non-urbanized areas. The decomposition of organic matter and reduction of Fe (hydr)oxides in sediments with reducing condition was likely to be the main factor controlling elevated-Mn groundwater in granular aquifers at a regional scale. By contrast, elevated-Mn groundwater in fissured aquifers was likely mainly affected by the urbanization accompanied with the leakage of low-oxygen domestic sewage and the industrialization accompanied by the leakage of industrial wastewater. In addition, Mn-rich surface water was also probably an important source for groundwater Mn in river network areas. Therefore, it is necessary to make a long-term monitoring for groundwater Mn in granular aquifers, especially in urbanized areas and river network areas, because of the high proportion of elevated-Mn.

Facile Mn Surface Doping of Ni-Rich Layered Cathode Materials for Lithium Ion Batteries.

A facile Mn surface doping process is proposed to improve the thermal and structural stabilities of Ni-rich layered cathode materials (Ni ≥ 80%) for lithium-ion batteries in electric vehicles. Herein, we demonstrate that the surface structure of the Ni-rich layered cathode materials can be stabilized by the introduction of a thin Mn-rich surface layer. This layer effectively reduces the direct exposure of the highly reactive Ni on the surface of the cathode materials, thus enhancing thermal stability and mitigating side reactions associated with highly reactive Ni that causes the loss of reversible capacity. In practice, the Mn surface-doped Ni-rich layered cathode material exhibits a high specific capacity with an improved cycling stability even at a high temperature (60 °C). We believe that our simple approach offers more opportunities to upscale production without any extra caution.

Novel P2-type concentration-gradient Na0.67Ni0.167Co0.167Mn0.67O2 modified by Mn-rich surface as cathode material for sodium ion batteries

Developing rechargeable sodium ion batteries with high capacity and long cycle life is still a big challenge. Herein, we present P2-type concentration-gradient material with average composition of Na0.67Ni0.167Co0.167Mn0.67O2, which exhibits relatively high discharge capacity and excellent stability. The material delivers higher discharge capacity and better rate performance than that of the concentration-constant Na0.67Ni0.167Co0.167Mn0.67O2 during cycling. Also, the material exhibits excellent capacity retention of ∼87% after 100 cycles. The high discharge capacity is attributed to the Ni-rich core and the improvement in cycling stability is due to a gradual and continuous increase of Mn4+ in the concentration-gradient spherical particles. The primary particles on the Mn-rich surface have smaller size than that of the constant-concentration samples, leading to a preferential crystalline state, therefore they can facilitate excellent transport properties for Na ions insertion/extraction process. The P2-type concentration-gradient material provides a new way for the development of advanced sodium ion batteries with high capacity and long cycle life.

Diagnostic Studies on Ni-Rich NMC Layered Cathode Materials By Using Synchrotron-Based X-Ray Techniques

Layered Ni-rich LiNixMnyCozO2 (NMC, x ≥ 0.6, x+y+z=1) cathode have attracted significant interest as the cathode materials for rechargeable lithium batteries (LIBs) owing to their high capacity, excellent rate capability and low cost. However, some drawbacks such as capacity fade during cycling and low thermal-abuse tolerance at elevated temperature, prohibiting their use in practical batteries. In addition, when in highly charged states, the reduction of Ni ions during thermal heating releases oxygen from the crystal structure, which can lead thermal runaway and violent reactions with the flammable electrolyte. To overcome performance degradation and improve battery safety, several strategies including lattice doping, surface treatment/modification, tuning the material compositions has been proposed and demonstrated. As one of promising approaches, concentration gradient LiNixMnyCozO2 (CG-NMC) cathode material, in which the Ni concentration decreases and the Mn concentration increases linearly toward the particle surface, has received considerable attention due to its high capacity and improved structural stability. [1-3] The concept of CG-NMC opens many new possibilities for developing high Ni content NMC cathode with high capacity and improved safety characteristic for practical LIB applications. It is well-accepted that the material design of Ni-rich core and Mn-rich surface leads to the high capacity and better thermal stability, compared with normal NMC cathode materials. However, more detailed working mechanisms of CG-NMC cathode have not been fully understood yet. In this study, the structural and chemical changes of normal NMC and CG-NMC cathode materials with various testing conditions including cycling numbers, different cut-off voltages, thermal heating, will be investigated by using synchrotron based time-resolved X-ray diffraction (TR-XRD), hard and soft X-ray absorption spectroscopy (XAS) techniques. Due to its structural and chemical inhomogeneity of CG-NMC cathode materials, the combined use of various types of characterization techniques is essential. Combinations of TR-XRD, hard and soft XAS techniques allow us to distinguish the electronic and crystal structure differences between the bulk and surface of cathode materials in various conditions. The result will provide valuable information for the development and optimize Ni-rich NMC cathode materials with increased capacity and better thermal stability. Acknowledgement The work at Brookhaven National Lab. was supported by the U.S. Department of Energy, the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies (BMR and VTO Battery500 projects) under Contract Number DE-AC02-98CH10886. The work was collaborated with UChicago Argonne, LLC, Operator of Argonne National Laboratory (“Argonne”). Argonne, a U.S. Department of Energy Office of Science Laboratory, is operated under Contract No. DE-AC02-06CH11357.

Countering the Segregation of Transition-Metal Ions in LiMn1/3 Co1/3 Ni1/3 O2 Cathode for Ultralong Life and High-Energy Li-Ion Batteries.

High-voltage layered lithium transition-metal oxides are very promising cathodes for high-energy Li-ion batteries. However, these materials often suffer from a fast degradation of cycling stability due to structural evolutions. It seriously impedes the large-scale application of layered lithium transition-metal oxides. In this work, an ultralong life LiMn1/3 Co1/3 Ni1/3 O2 microspherical cathode is prepared by constructing an Mn-rich surface. Its capacity retention ratio at 700 mA g(-1) is as large as 92.9% after 600 cycles. The energy dispersive X-ray maps of electrodes after numerous cycles demonstrate that the ultralong life of the as-prepared cathode is attributed to the mitigation of TM-ions segregation. Additionally, it is discovered that layered lithium transition-metal oxide cathodes with an Mn-rich surface can mitigate the segregation of TM ions and the corrosion of active materials. This study provides a new strategy to counter the segregation of TM ions in layered lithium transition-metal oxides and will help to the design and development of high-energy cathodes with ultralong life.

Recent Advances in Cathode Materials for Li-Ion Batteries

In this presentation, we report on studies of novel positive electrode materials for lithium-ion batteries with the main emphasis on their structural and surface modifications by cation doping and coating. We have chosen three families of materials: Li- and Mn-rich high-energy density and high capacity (HE-NCM)(1), Ni-rich layered materials Li[NixCoMn]O2 (x>0.5),(2) and gradient materials of the general formulae of LiNi0.65Co0.08Mn0.27O2 . Al3+ and other cations as dopants(3) and salts such as AlF3 as coating materials were studied(4). We have demonstrated the impact of a minor level of Al-doping on the electrochemical characteristics of LiNi0.5Co0.2Mn0.3O2 electrodes and on the interfacial reactions.(4) We propose that the lower capacity fading of the Al-doped electrodes upon aging of the cells in a charged state (4.3 V) at 60 0C in comparison with their undoped counterparts, as well as more stable mean voltage behavior, are likely due to the chemical and structural modifications of the electrode/solution interface. The lower electrochemical impedance of Al-doped LiNi0.5Co0.2Mn0.3O2 electrodes can be explained by more stable surface chemistry developed on the doped particles due to the interfacial reactions of the dopant in Al3+ enriched surface layer (“segregated” aluminum) with an EC-EMC/LiPF6 solution. The modified interface on the Al-doped particles is less resistive and comprises the Li+-ion conducting nano-sized centers like AlF3, which promote Li+ ionic transport to the bulk. Furthermore, we present our recent results on the study of Ni-rich, layered-structure LiNi0.65Co0.08Mn0.27O2 cathode materials and compare their electrochemical performance with materials of the same overall composition, but with a concentration gradient throughout the particles. The gradient was organized as follows: the Ni concentration is higher at the center of the particles but lower at surface, while the Mn concentration is higher at the surface and lower at the center. The synthesis parameters of the co-precipitation method were optimized comparing annealing periods, followed by electrochemical testing. Three different sets of gradient and standard non-gradient materials were explored, and all gradient materials provided superior capacity and rate capability than their respective non-gradient materials. The reasons for the improved discharge capacity of the gradient materials at moderate temperatures and cut-off potentials were explored through impedance spectroscopy and post-mortem characterization. The Mn-rich surface of the gradient material limits the growth of too resistive surface films during cycling, even at extreme temperatures and potentials, improving stability of these cathode materials. The evolution of the gradient structure was examined via TEM and EDX of FIB-produced particle cross-sections. We have established that prolonged cycling, even at elevated temperatures, did not change the initial concentration profiles determined by the synthesis. Transition metal ion dissolution from the cathode was confirmed via ICP of dissolved Li metal anodes, showing a greater degree of Mn dissolution from the non-gradient materials, possibly due to nickel-manganese segregation tendencies. This greater degree of Mn dissolution from non-gradient materials was confirmed in EDX of cycled particles. Electron diffraction measurements of these cycled particles show that spinel formation during cycling of the gradient materials is limited or even eliminated likely due to the higher concentration of Ni in the bulk of the gradient materials opposed to the non-gradient materials. Finally, we demonstrate long-term, (>1000 cycles) experiments of the gradient material electrodes vs. graphite electrodes in full cells that were performed in order to explore the practical advantage of these materials. 1. P. K. Nayak, J. Grinblat, M. Levi, B. Markovsky and D. Aurbach, Journal of the Electrochemical Society, 161, A1534 (2014). 2. C. Ghanty, B. Markovsky, E. M. Erickson, M. Talianker, O. Haik, Y. Tal-Yossef, A. Mor, D. Aurbach, J. Lampert, A. Volkov, J.-Y. Shin, A. Garsuch, F. F. Chesneau and C. Erk, Chemelectrochem, 2, 1479 (2015). 3. D. Aurbach, O. Srur-Lavi, C. Ghanty, M. Dixit, O. Haik, M. Talianker, Y. Grinblat, N. Leifer, R. Lavi, D. T. Major, G. Goobes, E. Zinigrad, E. M. Erickson, M. Kosa, B. Markovsky, J. Lampert, A. Volkov, J.-Y. Shin and A. Garsuch, Journal of the Electrochemical Society, 162, A1014 (2015). 4. F. Amalraj, M. Talianker, B. Markovsky, L. Burlaka, N. Leifer, G. Goobes, E. M. Erickson, O. Haik, J. Grinblat, E. Zinigrad, D. Aurbach, J. K. Lampert, J.-Y. Shin, M. Schulz-Dobrick and A. Garsuch, Journal of the Electrochemical Society, 160, A2220 (2013).

Surface-modified concentration-gradient Ni-rich layered oxide cathodes for high-energy lithium-ion batteries

Concentration-gradient layered Li[Ni0.8Co0.2]0.7[Ni0.2Mn0.8]0.3O2 oxide with Ni-rich in the core and Mn-rich in the surface region has been synthesized through a condition-controlled tank reactor reaction. While the Ni-rich core facilitates high capacity, the Mn-rich surface enables good cyclability and thermal stability. The concentration-gradient sample exhibits a higher capacity of 204mAhg−1 at C/5 rate with superior cyclability and thermal stability compared to the constant-concentration LiNi0.62Co0.14Mn0.24O2 sample, which has the same net Ni, Co, and Mn contents as those present in the concentration-gradient sample. The concentration-gradient sample was also coated with a thin layer of Al2O3 on the surface to stabilize the electrode/electrolyte interface and thereby further improve the electrochemical performance. Both the structural (gradient structure) and surface (Al2O3 coating) modifications help suppress side reactions between electrode and electrolyte and reduce the decline in voltage during cycling. The Al2O3-coated concentration-gradient sample exhibits improved long-term cyclability, rate capability, and thermal stability compared to the pristine uncoated sample.

Size matters: Mechanism of metal release from 316L stainless steel particles is governed by size-dependent properties of the surface oxide

Size-dependent health aspects due to exposure to micro- or nano-sized particles can only be fully understood if their physicochemical properties are well characterized. The aim of this study was to explain the process of metal release from well-characterized inert gas atomized stainless steel 316L particles, sized<4µm (fine) and<45µm (coarse), in aggressive environments of relevance for inhalation and cellular uptake. This was accomplished by correlating new results from real-time metal release measurements with particle- and surface oxide characteristics. In simulated biological media with complexing properties, a complexation (ligand)-induced dissolution mechanism is dominating the metal release from fine 316L particles (having a homogeneous and amorphous Mn-rich surface oxide due to rapid cooling). At similar conditions, the coarse 316L particles show a metal release mechanism dominated by fast dissolution of surface oxide nanoparticles (rich in Mn, Fe, and some S), acting as initiation sites for metastable pitting corrosion.

Diffusion of Mn interstitials in (Ga,Mn)As epitaxial layers

Magnetic properties of thin (Ga,Mn)As layers improve during annealing by out-diffusion of interstitial Mn ions to a free surface. Out-diffused Mn atoms participate in the growth of a Mn-rich surface layer and a saturation of this layer causes an inhibition of the out-diffusion. We combine high-resolution x-ray diffraction with x-ray absorption spectroscopy and a numerical solution of the diffusion problem for the study of the out-diffusion of Mn interstitials during a sequence of annealing steps. Our data demonstrate that the out-diffusion of the interstitials is substantially affected by the internal electric field caused by an inhomogeneous distribution of charges in the (Ga,Mn)As layer.

Structural and magnetic properties of the molecular beam epitaxy grown MnSb layers on GaAs substrates

The structural and magnetic properties of MnSb layers grown on two differently oriented GaAs substrates are reported. The MnSb compounds grow nonhomogenously both on GaAs(111)B and on GaAs(100) substrates. In x-ray diffraction studies the formation of two epitaxial domains is observed depending on the crystallographic orientation of the substrate. The observed diffusion of Ga atoms from the substrate to the layers results in the formation of an additional Mn-rich cubic phase of GaMnSb. In the case of the (100) oriented substrate, the diffusion of Mn into the substrate was additionally found. Traces of other phases were also noticed. The complex morphology of the layers is found to influence their magnetic properties. Magnetic force microscopy images revealed an inhomogenous distribution of the magnetic force gradient on the surface and the formation of magnetic domains in the samples. X-ray absorption studies of the chemical bonding and local atomic structure around Mn atoms confirmed high structural and chemical disorder in the samples. The chemical bonding of the dominating fraction of Mn atoms is found, however, similar to that in the reference MnSb powder. The x-ray magnetic circular dichroism measurements reveal an enhanced orbital moment and a reduced spin moment, which is most likely caused by the presence of different phases and a Mn-rich surface in the investigated samples.

Photoelectron spectroscopy study of Ga 1− xMn xAs(0 0 1) surface oxide and low temperature cleaning

X-ray photoemission spectroscopy has been used to study Ga 0.933Mn 0.067As surface cleaning using atomic hydrogen and ex situ HCl etching. Both cleaning techniques are found to reduce the Ga and As surface oxides. Quantitative compositional analysis indicates that a Mn rich surface contamination layer of average thickness 3–4 Å remains in both cases. Beneath the contamination layer the bulk crystal is terminated with a 2 Å thick As layer in the case of etching and a 0.9 Å coverage of Ga for the atomic hydrogen cleaned case.

Burrow stratigraphy in relation to manganese diagenesis in modern deep-sea carbonates

Box cores of modern deep-sea carbonates collected from various parts of the Pacific Ocean (Ontong Java Plateau, Shatsky Rise, and the landward margin of the Ogasawara Trench) have a distinct three-layer burrow stratigraphy: a surface monotonic layer in which trace fossils are not visible overlies an intermediate mottled layer in which trace fossils are enhanced by colour contrasts and this in turn overlies a basal faded layer in which trace fossils are faint or not visible. Examination of the vertical distributions of benthos (in particular meiobenthos), solid phase Mn and Fe, and CaCO 3 indicates that the layering is caused primarily by variations in the content of solid phase Mn. In particular, the layering depends on the depth of occurrence of an Mn-rich layer interpreted to be of diagenetic origin. In cores with a subsurface (19- to 27-cm depth) peak in the vertical distribution of Mn, the dark Mn-rich layer corresponds to the intermediate mottled layer, and examination of the micro-distribution of solid phase Mn and Fe in and around trace fossils suggests that mottling results from the recent infilling of deep burrows with light coloured Mn-poor surface sediments and from the localized reduction of Mn within old infilled burrows. In cores with a near-surface Mn peak (2 to 4 cm), the dark Mn-rich layer corresponds to the surface monotonic layer and mottling in the underlying light coloured Mn-poor layer results from the infilling of recent deep burrows (maximum depth 27 cm) with dark Mn-rich surface sediments. The deepest infilled burrows in the latter cores are tiny Chondrites only a few tenths of a millimetre in diameter, thought to be produced by some meiobenthic organism, perhaps a nematode. Multiple linear regression analysis suggests that the major factor causing variability in the depth of occurrence of the Mn peak is the rate of burial of organic matter.