Studies on elements diffusion of Mn/Co coated ferritic stainless steel for solid oxide fuel cell interconnects application

The spinel structure of manganese cobalt oxide (Mn,Co)₃O₄ is one of the most promising coatings for solid oxide fuel cell (SOFC) stainless steel interconnects. The stoichiometric Mn_1.5Co_1.5O₄ composition has properties that are preferable to other Mn/Co ratios, for example a higher conductivity and a thermal expansion coefficient that matches the typical steel substrate. However, previous work showed the Mn/Co ratio changes during operation due to the diffusion of Mn from the substrate. The results presented here are on three coatings with different compositions (namely; pure Co, Mn₂₀Co₈₀, and Mn₄₀Co₆₀) with each coating composition deposited to a thickness of 800 nm, 1500 nm, and 3000 nm. The coatings were applied by DC magnetron sputtering and then machine cut into coupons for isothermal annealing at 800 °C in air using a batch-type furnace for 2, 10, 50, 250, and 1000 h. The morphology, chemical composition (including surface and cross sections of the layers) and structures of the oxides formed were analyzed by SEM, EDS and XRD. Analysis of the element diffusion (Mn, Co, Cr, Fe) shown here points to an optimized coating recipe of Mn₄₀Co₆₀.     

Electrochemical performance of spinel LiMn2O4 cathode materials made by flame-assisted spray technology

Spinel lithium manganese oxide LiMn₂O₄ powders were synthesized by a flame-assisted spray technology (FAST) with a precursor solution consisting of stoichiometric amounts of LiNO₃ and Mn(NO₃)₂·4H₂O dissolved in methanol. The as-synthesized LiMn₂O₄ particles were non-agglomerated, and nanocrystalline. A small amount of Mn₃O₄was detected in the as-synthesized powder due to the decomposition of spinel LiMn₂O₄ at the high flame temperature. The impurity phase was removed with a post-annealing heat-treatment wherein the grain size of the annealed powder was 33 nm. The charge/discharge curves of both powders matched the characteristic plateaus of spinel LiMn₂O₄ at 3 V and 4 V vs. Li. However, the annealed powder showed a higher initial discharge capacity of 115 mAh g⁻¹ at 4 V. The test cell with annealed powder showed good rate capability between a voltage of 3.0 and 4.3 V and a first cycle coulombic efficiency of 96%. The low coulombic efficiency from capacity fading may be due to oxygen defects in the annealed powder. The results suggest that FAST holds potential for rapid production of uniform cathode materials with low-cost nitrate precursors and minimal energy input.     

Post-mortem evaluation of oxidized atmospheric plasma sprayed Mn–Co–Fe oxide spinel coatings on SOFC interconnectors

Interconnects employed in solid oxide fuel cells require electrically conductive protective coatings such as those based on manganese cobalt oxide spinels in order to prevent evaporation of volatile Cr(VI)-compounds and to minimize high temperature corrosion. MnCo_2−xFe_xO₄ based (where x = 0.1 and 0.3) oxide spinel protective coatings were manufactured by the atmospheric plasma spraying process on Crofer 22 APU substrates. The coated substrates were oxidized at 700 °C in air for 1000 h and post-mortem analyses were conducted to study the performance of the thermal sprayed coatings. During the high temperature oxidation, a four-point on-line measurement technique was used for area specific resistance studies. The MnCo_1.7Fe_0.3O₄ coating was tested together with the La_0.85Sr_0.15Mn_1.1O₃-spacer.     

Thermal decomposition of (NH4)2SO4 in presence of Mn3O4

The main objective of this work is to develop a hybrid water-splitting cycle that employs the photon component of sunlight for production of H₂ and its thermal (i.e. IR) component for generating oxygen. In this paper, (NH₄)₂SO₄ thermal decomposition in the presence of Mn₃O₄, as an oxygen evolving step, was systematically investigated using thermogravimetric/differential thermal analyses (TG/DTA), temperature programmed desorption (TPD) coupled with a mass spectrometer (MS), X-ray Diffraction (XRD), and X-ray Photoelectron Spectroscopy (XPS) techniques. Furthermore, thermolysis of ammonium sulfate, (NH₄)₂SO₄, in the presence of Mn₃O₄ was also investigated by conducting flow reactor experiments. The experimental results obtained indicate that at 200-450 °C, (NH₄)₂SO₄ decomposes forming NH₃ and H₂O and sulfur trioxide that in the presence of manganese oxide react to form manganese sulfate, MnSO₄. At still higher temperatures (800∼900 °C), MnSO₄ further decomposed forming SO₂ and O₂.     

Cu- and Ni-doped Mn1.5Co1.5O4 spinel coatings on metallic interconnects for solid oxide fuel cells

An interconnect in solid oxide fuel cells electrically connects unit cells and separates fuel from oxidant in the adjoining cells. Metallic interconnects are usually coated with conductive oxides to improve their surface stability and to mitigate chromium poisoning of a cathode. In this study, Mn_1.5Co_1.5O₄ (MCO) spinel oxides doped with Cu and Ni are synthesized and applied as protective coatings on a metallic interconnect (Crofer 22 APU). Doping of Cu and Ni into MCO improves sintering characteristics as well as electrical conductivity and thermal expansion match with the Crofer interconnect. The dense layers of Cu- and Ni-doped MCOs are fabricated on the interconnects by a slurry coating process and subsequent heat-treatment. The coated interconnects exhibit area-specific resistances as low as 13.9–17.6 mΩ cm² at 800 °C. The Cu-doped MCO coating acts as an effective barrier to evaporation and migration of Cr-containing species from the interconnect, thereby reducing Cr poisoning of a cathode.     

Fine-sized LiNi0.8Co0.15Mn0.05O2 cathode powders prepared by combined process of gas-phase reaction and solid-state reaction methods

The Ni-rich precursor powders with spherical shape and filled morphologies were prepared by spray pyrolysis from the spray solution with citric acid, ethylene glycol and a drying control chemical additive. The precursor powders with controlled morphologies formed the LiNi_0.8Co_0.15Mn_0.05O₂ cathode powders with spherical shape and fine size by solid-state reaction with lithium hydroxide. However, the cathode powders prepared from the spray solution without additives had irregular morphologies and were large in size. The precursor powders with hollow and porous morphologies formed cathode powders with irregular and aggregated morphologies. The composition ratios of the nickel, cobalt and manganese components were maintained in the as-prepared, precursor and cathode powders. The initial discharge capacity of the LiNi_0.8Co_0.15Mn_0.05O₂ cathode powders with spherical shape and fine size tested at a temperature of 55 °C under a constant current density of 0.5 C was 215 mAh g⁻¹. The discharge capacity of the LiNi_0.8Co_0.15Mn_0.05O₂ cathode powders decreased to 81% of the initial value after 30 cycles.     

Preparation of La0.75Sr0.25Cr0.5Mn0.5O3−δ fine powders by carbonate coprecipitation for solid oxide fuel cells

A range of La_0.75Sr_0.25Cr_0.5Mn_0.5O_3−δ (LSCM) powders is prepared by the carbonate coprecipitation method for use as anodes in solid oxide fuel cells. The supersaturation ratio (R = [(NH₄)₂CO₃]/([La³⁺] + [Sr²⁺] + [Cr³⁺] + [Mn²⁺])) during the coprecipitation determines the relative compositions of La, Sr, Cr, and Mn. The composition of the precursor approaches the stoichiometric one at the supersaturation range of 4 ≤ R ≤ 12.5, whereas Sr and Mn components are deficient at R < 4 and excessive at R = 25. The fine and phase-pure LSCM powders are prepared by heat treatment at very low temperature (1000 °C) at R = 7.5 and 12.5. By contrast, the solid-state reaction requires a higher heat-treatment temperature (1400 °C). The catalytic activity of the LSCM electrodes is enhanced by using carbonate-derived powders to manipulate the electrode microstructures.     

Effects of supersonic treatment on the electrochemical properties and crystal structure of LiMn1.5Ni0.5O4 as a cathode material for Li ion batteries

The electrochemical properties and crystal structure of LiMn_1.5Ni_0.5O₄ treated with supersonic waves in an aqueous Ni-containing solution were investigated by performing charge-discharge tests, inductively coupled plasma (ICP) analysis, scanning electron microscopy (SEM), iodometry, X-ray diffraction (XRD), powder neutron diffraction and synchrotron powder XRD. The charge-discharge curve of LiMn_1.5Ni_0.5O₄ versus Li/Li⁺ has plateaus at 4.1 and 4.7 V. The 4.1 V versus Li/Li⁺ plateau due to the oxidation of Mn^3+/4+ was reduced by the supersonic treatment. During the charge-discharge cycling test at 25 °C, the supersonic treatment increased the discharge capacity of the 50th cycle. Rietveld analysis of the neutron diffraction patterns revealed that the Ni occupancy of the 4b site in LiMn_1.5Mn_0.5O₄, which is mainly occupied by Ni, was increased by the supersonic treatment. This result suggests that Ni²⁺ is partially substituted for Mn^3+/4+ during the supersonic treatment.     

Effects of crystallinity and morphology of solution combustion synthesized Co3O4 as a catalyst precursor in hydrolysis of sodium borohydride

Solution combustion synthesized (SCS) cobalt oxide (Co₃O₄) powder has been studied as a catalyst precursor for the hydrolysis of sodium borohydride (NaBH₄). Synthesis is completed in less than two minutes and results indicate SCS is capable of reproducibly synthesizing 98.5–99.5% pure Co₃O₄ nano-foam materials. SCS materials demonstrate an as-synthesized specific surface area of 24 m² g⁻¹, a crystallite size of 15.5 nm, and fine surface structures on the order of 4 nm. Despite having similar initial surface areas and sample purities, SCS-Co₃O₄ outperforms commercially available Co₃O₄ and elemental cobalt (Co) nano powders when used as a catalyst precursor for NaBH₄ hydrolysis. Hydrogen generation rates (HGR) using 0.6 wt% NaBH₄ in aqueous solution at 20 °C were observed to be 1.24 ± 0.2 L min⁻¹ g_cat⁻¹ for SCS nano-foam Co₃O₄ compared to 0.90 ± 0.09 and 0.43 ± 0.04 L min⁻¹ g_cat⁻¹ for commercially available Co₃O₄ and Co, respectively. The high catalytic activity of SCS-Co₃O₄ is attributed to its nano-foam morphology and crystallinity. During the hydrolysis of NaBH₄, the SCS-Co₃O₄ converts in-situ to an amorphous active catalyst with a specific surface area of 92 m² g⁻¹ and exhibits a honeycomb type morphology.     

MnxCo3-xO4 spinel oxides as efficient oxygen evolution reaction catalysts in alkaline media

The design of efficient electrocatalysts for oxygen evolution reaction (OER) is an essential task in developing sustainable water splitting technology for the production of hydrogen. In this work, manganese cobalt spinel oxides with a general formula of Mn_xCo_3-xO₄ (x = 0, 0.5, 1, 1.5, 2) were synthesised via a soft chemistry method. Non-equilibrium mixed powder compositions were produced, resulting in high electrocatalytic activity. The oxygen evolution reaction was evaluated in an alkaline medium (1 M KOH). It was shown that the addition of Mn (up to x ≤ 1) to the cubic Co₃O₄ phase results in an increase of the electrocatalytic performance. The lowest overpotential was obtained for the composition designated as MnCo₂O₄, which exhibited a dual-phase structure (∼30% Co₃O₄ + 70% Mn_1.4Co_1.6O₄): the benchmark current density of 10 mA cm⁻² was achieved at the relatively low overpotential of 327 mV. The corresponding Tafel slope was determined to be ∼79 mV dec⁻¹. Stabilities of the electrodes were tested for 25 h, showing degradation of the MnCo₂O₄ powder, but no degradation, or even a slight activation for other spinels.     

Oxidation resistance and electrical properties of anodically electrodeposited Mn–Co oxide coatings for solid oxide fuel cell interconnect applications

Co-rich and crack-free Mn–Co oxide coatings were deposited on AISI 430 substrates by anodic electrodeposition from aqueous solutions. The as-deposited Mn–Co oxide coatings, with nano-scale fibrous morphology and a metastable rock salt-type structure, evolved into a (Cr,Mn,Co)₃O₄ spinel layer due to the outward diffusion of Cr from the AISI 430 substrates when pretreated in air. The Mn–Co oxide coatings were reduced into metallic Co and Mn₃O₄ phases when annealed in a reducing atmosphere of 5% H₂–95% N₂. In contrast to the degraded oxidation resistance and electrical properties observed for the air-pretreated Mn–Co oxide coated samples, the H₂-pretreated Mn–Co oxide coatings not only acted as a protective barrier to reduce the Cr outward diffusion, but also improved the electrical performance of the steel interconnects. The improvement in electronic conductivity can be ascribed to the higher electronic conductivity of the Co-rich spinel layer and better adhesion of the scale to the steel substrate, thereby eliminating scale spallation.     

Nonflammable electrolyte for 3-V lithium-ion battery with spinel materials LiNi0.5Mn1.5O4 and Li4Ti5O12

The compatibility between dimethyl methylphosphonate (DMMP)-based electrolyte of 1 M LiPF₆/EC + DMC + DMMP (1:1:2 wt.) and spinel materials Li₄Ti₅O₁₂ and LiNi_0.5Mn_1.5O₄ was reviewed, respectively. The cell performance and impedance of 3-V LiNi_0.5Mn_1.5O₄/Li₄Ti₅O₁₂ lithium-ion cell with the DMMP-based nonflammable electrolyte was compared with the baseline electrolyte of 1 M LiPF₆/EC + DMC (1:1 wt.). The nonflammable DMMP-based electrolyte exhibited good compatibility with spinel Li₄Ti₅O₁₂ anode and high-voltage LiNi_0.5Mn_1.5O₄ cathode, and acceptable cycling performance in the LiNi_0.5Mn_1.5O₄/Li₄Ti₅O₁₂ full-cell, except for the higher impedance than that in the baseline electrolyte. All of the results disclosed that the 3 V LiNi_0.5Mn_1.5O₄/Li₄Ti₅O₁₂ lithium-ion battery was a promising choice for the nonflammable DMMP-based electrolyte.     

Characteristics of Mn-Co and Ni-Co metal powders mixed with oxides as cathode contact materials for SOFC

Manganese, cobalt and nickel metal powders are mixed with 10 wt% oxides: La_0·6Sr_0·4CoO₃, La_0·6Sr_0·4Co_0·2Fe_0·8O₃, La_0·8Sr_0·2MnO₃ and MnCo₂O₄, to form ten compositions. Powder mixtures are studied as cathode contact materials in solid oxide fuel cell stacks. Electrical resistance is measured on samples of powder pellets and steel/powder/steel assemblies at 800 °C for over 10000 h. Microstructures are examined after measurements. For the powder pellets, Mn-Co (mole ratio 1:2) and Ni-Co (mole ratio 1:2) with MnCo₂O₄ exhibit minimum resistivity in the Mn-Co and Ni-Co series respectively. For the steel/powder/steel assemblies, Mn-Co (mole ratio 1:1) with La_0·6Sr_0·4CoO₃ and Ni-Co (mole ratio 1:2) with La_0·8Sr_0·2MnO₃ show close low values. Oxide scales grown on steel surfaces which contact powders with lanthanum-based perovskites are irregular and thinner. Other powders result in more regular and thicker oxide layers. The two forms of oxide structures are in accordance with the two trends of electrical resistance change observed in the assemblies. The addition of lanthanum-based perovskites reveals phenomenal effects on reducing the resistivity of powder mixtures and suppressing the growth of steel oxide scales.     

Surface modification of LiNi0.5Mn1.5O4 by ZrP2O7 and ZrO2 for lithium-ion batteries

The spinel LiNi_0.5Mn_1.5O₄ has been surface modified separately with 1.0 wt.% ZrO₂ and ZrP₂O₇ for the purpose of improving its cycle performance as a cathode in a 5-V lithium-ion cell. Although the modifications did not change the crystallographic structure of the surface-modified samples, they exhibited better cyclability at elevated temperature (55 °C) compared with pristine LiNi_0.5Mn_1.5O₄. The material that was surface modified with ZrO₂ gave the best cycling performance, only 4% loss of capacity after 150 cycles at 55 °C. Electrochemical impedance spectroscopy demonstrated that the improved performance of the ZrO₂-surface-modified LiNi_0.5Mn_1.5O₄ is due to a small decrease in the charge transfer resistance, indicating limited surface reactivity during cycling. Differential scanning calorimetry showed that the ZrO₂-modified LiNi_0.5Mn_1.5O₄ exhibits lower heat generation and higher onset reaction temperature compared to the pristine material. The excellent cycling and safety performance of the ZrO₂-modified LiNi_0.5Mn_1.5O₄ electrode was found to be due to the protective effect of homogeneous ZrO₂ nano-particles that form on the LiNi_0.5Mn_1.5O₄, as shown by transmission electron microscopy.     

Solution combustion synthesized cobalt oxide catalyst precursor for NaBH4 hydrolysis

In this paper we report the solution combustion synthesis of cobalt oxide nanofoam from solutions of cobalt nitrate and glycine and subsequent use as an effective catalyst precursor for NaBH₄ hydrolysis. The catalytic activity results show that the hydrogen generation rate (HGR) at room temperature was much higher for the solution combustion synthesized material than for commercial Co₃O₄ nanopowder, though their specific surface areas were comparable (∼26–32 m²/g). Using a 0.6 wt.% aqueous solution of NaBH₄ at 20 °C and a 5 wt.% catalyst precursor loading, a HGR of 1.93 L min⁻¹ g_cat⁻¹ was achieved for solution combustion synthesized Co₃O₄. In contrast, at the same conditions, for commercial Co₃O₄ and elemental Co powders HGRs of 0.98 and 0.49 L min⁻¹ g_cat⁻¹ were achieved respectively. This type of synthesis is amenable to many complex metal oxide catalysts as well, such as LiCoO₂, which have also been shown to be good catalyst precursors for hydrolysis of NaBH₄.     

Preparation of NiCo2O4 microspheres employing hydrothermal approach

In this study, nickel cobalt oxide (NiCo₂O₄) microspheres were prepared by using facile hydrothermal route. The structural confirmation of bimetal oxide formation was acquired by X-ray powder diffraction and Raman studies. The formation of microspheres combined via irregular nanosheets was confirmed by scanning electron microscopy. Highly oriented NiCo₂O₄ microspheres yielded a high current density (258 mA/g) at 10 mV/s and low overpotential (224 V). The highly active electrode showed efficient electron transportation towards oxygen evolution reaction. Long-term stability over 16 h was achieved by the fabricated high-performance NiCo₂O₄ electrode. It is recommended that NiCo₂O₄ microspheres obtained from 3:1 stoichiometry ratio of Ni and Co would lead to new electrocatalysts that give best performance than expensive catalysts used currently in the water oxidation process.     

Effect of sulfur and nickel doping on morphology and electrochemical performance of LiNi0.5Mn1.5O4−xSx spinel material in 3-V region

The effect of nickel and sulfur substitution for manganese and oxygen on the structure and electrochemical properties of the LiNi_0.5Mn_1.5O_4−xS_x is examined. The LiNi_0.5Mn_1.5O_4−xS_x (x = 0 and 0.05) compounds are successfully synthesized at 500 and 800 °C by co-precipitation using the metal carbonate (Ni_0.5Mn_1.5)CO₃ as a precursor. The resulting powder with sulfur doping exhibits different morphology from a Ni-only doped spinel in terms of particle size and surface texture. The LiNi_0.5Mn_1.5O_4−xS_x (x = 0 and 0.05) powders are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and galvanostatic charge–discharge cycling. The nickel- and sulfur-doped spinel displays excellent capacity retention and rate capability in the 3-V region, compared with Ni-only doped spinel material.     

Study of the first step of the Mn2O3/MnO thermochemical cycle for solar hydrogen production

In this work, a complete thermodynamic study of the first step of the Mn₂O₃/MnO thermochemical cycle for solar hydrogen production has been performed. The thermal reduction of Mn₂O₃ takes place through a sequential mechanism of two reaction steps. The first step (reduction of Mn₂O₃ to Mn₃O₄) takes place at teomperatures above 700 °C, whereas the second reaction step (reduction of Mn₃O₄ to MnO) requires temperatures above 1350 °C to achieve satisfactory reaction rates and conversions. Equilibrium can be displaced to lower temperatures by increasing the inert gas/Mn₂O₃ ratio or decreasing the pressure. The thermodynamic calculations have been validated by thermogravimetric experiments carried out in a high temperature tubular furnace. Experimental results corroborate the theoretical predictions although a dramatically influence of chemical kinetics and diffusion process has been also demonstrated, displacing the reactions to higher temperatures than those predicted by thermodynamics. Finally, this work demonstrates that the first step of the manganese oxide thermochemical cycle for hydrogen production can be carried out with total conversion at temperatures compatible with solar energy concentration devices. The range of required temperatures is lower than those commonly reported in literature for the manganese oxide cycle obtained from theoretical and thermodynamic studies.     

A hierarchical nanostructure consisting of amorphous MnO2, Mn3O4 nanocrystallites, and single-crystalline MnOOH nanowires for supercapacitors

In this communication, a porous hierarchical nanostructure consisting of amorphous MnO₂ (a-MnO₂), Mn₃O₄ nanocrystals, and single-crystalline MnOOH nanowires is designed for the supercapacitor application, which is prepared by a simple two-step electrochemical deposition process. Because of the gradual co-transformation of Mn₃O₄ nanocrystals and a-MnO₂ nanorods into an amorphous manganese oxide, the cycle stability of a-MnO₂ is obviously enhanced by adding Mn₃O₄. This unique ternary oxide nanocomposite with 100-cycle CV activation exhibits excellent capacitive performances, i.e., excellent reversibility, high specific capacitances (470 F g⁻¹ in CaCl₂), high power property, and outstanding cycle stability. The highly porous microstructures of this composite before and after the 10,000-cycle CV test are examined by means of scanning electron microscopy (SEM) and transmission electron microscopy (TEM).     

Preparation and characterization of sub-micro LiNi0.5−xMn1.5+xO4 for 5 V cathode materials synthesized by an ultrasonic-assisted co-precipitation method

Sub-micro spinel LiNi_0.5−xMn_1.5+xO₄ (x < 0.1) cathode materials powder was successfully synthesized by the ultrasonic-assisted co-precipitation (UACP) method. The structure and electrochemical performance of this as-prepared powder were characterized by powder XRD, SEM, XPS, CV and the galvanostatic charge–discharge test in detail. XRD shows that there is a small Li_yNi_1−yO impurity peak placed close to the (4 0 0) line of the spinel LiNi_0.5−xMn_1.5+xO₄, and the powders are well crystallized. XPS exhibits that the Mn oxidation state is between +3 and +4, and Ni oxidation state is +2 in LiNi_0.5−xMn_1.5+xO₄. SEM shows that the prepared powders (UACP) have the uniform and narrow size distribution which is less than 200 nm. Galvanostatic charge–discharge test indicates that the initial discharge capacities for the LiNi_0.5−xMn_1.5+xO₄ (UACP) at C/3, 1C and 2C, are 130.2, 119.0 and 110.0 mAh g⁻¹, respectively. After 100 cycles, their capacity retentions are 99.8%, 88.2%, and 73.5%, respectively. LiNi_0.5−xMn_1.5+xO₄ (UACP) at C/3 discharge rate exhibits superior capacity retention upon cycling, and it also shows well high current discharge performance. CV curve implies that LiNi_0.5−xMn_1.5+xO₄ (x < 0.1) spinel synthesized by ultrasonic-assisted co-precipitation method has both reversibility and cycle capability because of the ultrasonic-catalysis.