Oxygen Diffusion Coefficient Research Articles

Properties and degradation of manganese(III) porphyrin thin films formed by high vacuum sublimation

Manganese porphyrins are of interest due to the optical, electronic and magnetic properties of the central metal ion, coupled to the low bandgap of the polyaromatic ring. These attractive characteristics are harnessed in solutions or in ultra-thin films, such as, for example, self-assembled monolayers. However, for devices, thicker films deposited using a controlled and reproducible method are required. Here we present the morphological, structural, chemical and optical properties of manganese(III) tetraphenylporphyrin chloride (MnTPPCl) thin films deposited using organic molecular beam deposition, typically employed to process analogue molecules for applications such as organic photovoltaics. We find, using a combination of UV-vis and X-ray photoelectron spectroscopies, that the sublimation process leads to the scission of the Mn–Cl bond. The resultant film is a Mn(II)TPP:Mn(III)TPPCl blend where approximately half the molecules have been reduced. Following growth, exposure to air oxidizes the Mn(II)TPP molecule. Through quantitative analysis of the time-dependent optical properties, the oxygen diffusion coefficient (D) [Formula: see text] is obtained, corresponding to a slow bulk oxidation following fast oxidation of a 8-nm-thick surface layer. The bulk diffusion D is lower than for analogous polycrystalline films, suggestion that grain boundaries, rather than molecular packing, are the rate-limiting steps in oxidation of molecular films. Our results highlight that the stability of the axial ligands should be considered when depositing metal porphyrins from the vapor phase, and offer a solvent-free route to obtain reproducible and smooth thin films of complex materials for engineering film functionalities.

Incorporation of silver graphene oxide and graphene oxide nanoparticles in sulfonated polyether ether ketone membrane for power generation in microbial fuel cell

This study aims to design a new membrane for Microbial Fuel Cell (MFC) applications, where the membrane should have good proton conductivity, low oxygen permeability and anti-biofouling properties. The self-fabricated composite membranes are Graphene Oxide/Sulfonated Polyether Ether Ketone (GO-SPEEK) and Silver Graphene Oxide/Graphene Oxide/Sulfonated Polyether Ether Ketone (AgGO-GO-SPEEK). The proton conductivity of AgGO-GO-SPEEK membrane is 54.2% higher and the oxygen diffusion coefficient is 76.7% lower than Nafion® 117 membrane, yielding higher selectivity as separator in MFC applications. At the earlier optimum stage, MFC with GO-SPEEK generates the highest maximum power density (1134 mW m−2). Voltage achieved for first three feed cycles after enrichment are 620 mV–650 mV for all MFC system. However, the highest maximum power density (896 mW m−3) is attained by MFC with AgGO-GO-SPEEK membrane after 100 days of operation. The Electrochemical Impedance Spectroscopy (EIS) results show that AgGO-GO-SPEEK membrane has the lowest increment in terms of the membrane's resistance. AgGO-GO-SPEEK,a better anti-biofouling property membrane, leads to a longer and stable power production, caused by the better ion transport through membrane. The results indicated that AgGO-GO-SPEEK membrane is a promising alternative membrane to replace Nafion® 117 as a separator in the MFC.

Diffusion of oxygen and nitrogen into titanium under laser irradiation in air

This paper examines the dynamic interactions between pure titanium and ambient air under transient laser processing conditions. The microstructures and composition distributions of the reaction products on and within the titanium samples are reported, which lead to the estimation of high-temperature diffusion coefficients for nitrogen and oxygen into the titanium substrate. In-situ synchrotron X-ray diffraction testing is conducted to reveal the high temperature reaction steps between titanium and air. The formation mechanisms of both titanium nitrides and titanium oxides are discussed.

Comparative Analysis of the Methods for Obtaining Superconducting Fe1 + ySexTe1 – x Single Crystals

The Fe1 + ySexTe1 – x single crystals grown by the KCl flux and Bridgman methods have been compared using the same investigation techniques. It is shown that the KCl flux growth yields high-quality Fe1 + ySexTe1 – x superconducting single crystals. The oxygen diffusion coefficient in Fe1 + ySexTe1 – x has been estimated. During the Bridgman crystal growth, a stable nonsuperconducting phase can be formed, which suggests a need for refining the phase diagram of this compound.

Modeling time-dependent circumferential non-uniform corrosion of steel bars in concrete considering corrosion-induced cracking effects

Circumferential non-uniform corrosion of steel bars usually occurs in chloride contaminated RC structures and often lead to premature concrete cover cracking. This paper presents a numerical model developed to investigate the time-dependent process of non-uniform corrosion for steel bars at different locations in concrete. The model also considers the effects of longitudinal cracking on the chloride and oxygen diffusion. Corrosion is triggered around a steel bar once the chloride content reaches the chloride threshold at that point, which is determined based on chloride penetration. Thereafter, time-dependent corrosion rate around the circumferential surface of a steel bar is obtained through electrochemical theory, which make it possible to describe a cross-sectional corrosion profile showing the timeline of damage. A case study is developed to demonstrate the analysis procedure of the proposed time-dependent non-uniform corrosion model and to present the differences in the corrosion process between the side-located rebar and corner-located rebar. Then, the derived model can be partially verified with experimental results. In this research, the simulation results with the verified model indicates that the corrosion of side-located rebar at the top and bottom areas are more severe based on the effects of corrosion-induced cracking which allows more chloride penetration and oxygen diffusion. Also, the conducted parametric study shows that the time it takes to obtain the same critical corrosion angle increases with the increase of concrete cover thickness and steel bar diameter, but the safe time decreases with the increase of chloride penetration and oxygen diffusion coefficient.

A model for concentration impedance of a PEM fuel cell

Abstract We report an analytical physics–based model for oxygen concentration perturbation spectra of a PEM fuel cell cathode side. An expression for the concentration impedance (CI) taking into account oxygen transport in the cathode catalyst layer (CCL) and the gas diffusion layer (GDL) is derived. In the static limit the CI gives the limiting current density due to oxygen transport in the GDL. The Bode plots of real and imaginary part of the CI have minima at the characteristic frequencies corresponding to oxygen transport in the GDL and the CCL, respectively. These features enable direct estimation of the respective oxygen diffusion coefficients without curve fitting.

Modeling oxygen redistribution in UO2+x fuel pellet

Oxygen redistribution in UO2 fuel pellet occurring during reactor operation, where temperature gradient across pellet prevails, is modeled for steady state and transient situations. This phenomenon, called the Soret effect, is due to a positive shift of hyperstoichiometry in the fuel, i.e. from UO2 to UO2+x, and the prevailing temperature gradient during normal reactor operation. Unique solutions to the thermodiffusion equation for oxygen describing the phenomenon are found for various forms of the heat of transport during the steady state and under transient conditions. The solutions are used to compute radial oxygen profile across fuel pellet as per burnup. The relaxation time to thermal equilibrium during a transient is evaluated. This quantity depends on the oxygen diffusion coefficient and the eigenvalues of the putative equation, which are both a strong function of temperature.

Dynamical Insights into Oxygen Diffusion in BaTiO3 and SrTiO3

Oxygen diffusion is a key process in BaTiO3 and SrTiO3 that determines many of their important electronic properties. Despite the importance of oxygen diffusion in these systems, there is still currently significant variation in the oxygen diffusion coefficients for BaTiO3 and SrTiO3 reported in the literature from both experimental and computational methods. Using molecular dynamics simulations, the effect of oxygen vacancy concentration on the oxygen diffusion properties is investigated for these perovskite materials. Oxygen diffusion coefficients and activation energies for both materials are calculated over a temperature range of 900–1500 K. Oxygen vacancy charge compensation is achieved by the reduction of Ti4+ to Ti3+ ions, in agreement with experimental results for undoped BaTiO3 and SrTiO3. The findings from this study yield an accurate reference point for the calculation of oxygen diffusion in these materials and also the application of molecular dynamics in studying such phenomena.

Characterizing transport mechanism of cathodic oxygen ions in SOFC by molecular dynamics

This paper investigates the oxygen ion conductivity (OIC) of cathodic materials in Solid Oxide Fuel Cells (SOFCs) by molecular dynamics simulation. Although the oxygen diffusion coefficient (ODC) is known to be one of the major factors associated with the ion conductivity of cathodic materials in the SOFC, other major process factors are remained to be further investigated so that the related oxygen ion conductivity can be fully characterized and enhanced. In order to search for significant factors that affect the OIC in cathode, the characteristics of doping effect and temperature change are simulated via Molecular Dynamics approach and the results are presented in details in this study. Based upon the modeling results, it is realized that the OIC at 773 K can be greatly improved by using the catalyst of Gd1-xBaxCoyFe1-y (GBCF). And also, the highest OIC at 973 K can be greatly enhanced by using the catalyst of La1-xSrxCoyFe1-y (LSCF).In addition, the modeling results also indicate that both the doping ratio and operating temperature are of significant factors for both ODC and OIC. Although ODC is a metrics that can be used to evaluate the mobility of oxygen ions inside the catalyst for cathode materials, it does not comprehend the characters of materials. In order to be able to fully characterize the conductivity of cathodic materials in SOFC, significant factors of OIC including the lattice structure of materials, the concentration of oxygen vacancy, and the operating temperature are fully explored in this paper for detailed system characterization.The modeling results clearly indicate that as the working temperature increasing from 773K to 973K, the largest OIC change is given by LSCF, and the smallest OIC change is provided by GBCF. Also, the ODC curve of the GBCF possesses highest ODC values in medium working temperature range due to large numbers of oxygen vacancy generated by the cathode.

Upgrading the electrochemical performance of graphene oxide-blended sulfonated polyetheretherketone composite polymer electrolyte membrane for microbial fuel cell application

Abstract In the present study, composite polymer electrolyte membranes were synthesized by the incorporation of graphene oxide (GO) in sulfonated polyetheretherketone (SPEEK) for simultaneous wastewater treatment and electricity generation. A variety of techniques were applied to characterize the chemical, crystal and morphological structure of the as-synthesized membranes. In addition, the water uptake, mass transfer, and oxygen diffusion coefficients, proton conductivity, ion exchange capacity, internal resistance, power density, coulombic efficiency and COD removal of the MFC system applied with SPEEK, SPEEK/GO composite membranes were studied and compared with the commercial Nafion117. The incorporation of various GO contents caused a decrease in oxygen diffusion and mass transfer coefficients of 7.45 × 10-6 cm2 s-1 and 0.00048 cm s-1, respectively. This was also shown that the SPEEK/GO3 membrane with proton conductivity 2.88 mS cm-1, power density 53.12 mW m−2, voltage 883 mV, and CE 3.74% showed higher performance compared to Nafion 117 membrane as a separator in MFC. Furthermore, the MFC with SPEEK/GO3 membrane gave slightly higher COD removal (88.7%) than that of MFC incorporating the Nafion 117 membrane (81.9%). The ion exchange capacity (IEC) and WU of as-prepared membranes were found to vary from 1 to 2.21 meq g-1 and from 44.31 to 67.85%, respectively. The results obtained were revealed that the MFC assembled SPEEK/GO3 composite membrane is demonstrated significant improvement in cell performance compared with commercial Nafion 117 membrane that qualifies it to be a promising candidate as a proton exchange membrane in the lab-scale microbial fuel cells.

Oxygen isotope exchange in proton-conducting oxides based on lanthanum scandates

A method of the oxygen isotope exchange with equilibration of the gas phase was used in order to obtain the temperature dependences of the oxygen surface exchange and diffusion coefficients of the proton-conducting oxides La1–xSrxScO3–δ (x = 0; 0.04; 0.09) in the temperature range of 600–900 °C and oxygen pressure of 1.01 kPa. The diffusion and oxygen surface exchange coefficients increase with the strontium content in oxides. The rates of the individual stages of the oxygen exchange process on the surface of the oxides were determined. The oxygen incorporation was revealed to be rate-determining for the undoped LaScO3 oxide. However, for the strontium-doped oxides La1–xSrxScO3–δ (x = 0.04; 0.09), the difference between the rates of oxygen dissociative adsorption and oxygen incorporation decreases with increasing the strontium concentration, in a way that for the La0.91Sr0.09ScO3−δ oxide these stages become competing. The possible reasons for these differences in oxygen surface exchange kinetics were analyzed in this paper. Using the obtained oxygen diffusion coefficients, the oxygen-ionic conductivities in accordance with the Nernst-Einstein equation, and the contributions of the oxygen-ion and proton components of the total conductivity of the La1–xSrxScO3–δ (x = 0.04; 0.09) oxides in the wet reducing atmosphere (pH2O = 2.35 kPa, pO2 = 10−15 Pa) were calculated. The proton transference numbers were found to be close to unit at the wet reducing atmospheres in the temperature range of 500–600°С.

Cobalt-Free Perovskite-Type Oxygen Reduction Reaction Catalysts for Energy Conversion

Alkaline fuel cells and metal–air batteries are emerging as an alternative energy conversion devices which can use non-precious and earth-abundant materials as electrocatalysts for oxygen reduction reaction (ORR) [1-3]. Perovskite-type oxides have been widely investigated due to their high activity, long durability and low cost [4]. Mn and Co-based perovskites have shown stable and high ORR performance and their electrochemical properties was explained using the molecular orbital theory. It was shown that the transition-metal-oxide electrocatalysts exhibit maximum catalytic activity when the occupancy of the eg orbital is ~1 which is the primary activity descriptor, whereas spin-state and electronegativity are secondary activity descriptors [5]. However, Co-based oxides have several disadvantages such as high volatility, thermal expansion coefficient, and cost [6-8]. Thus, there is need for alternative Co-free materials for next generation fuel cells. Here, we report Co-free perovskite-type Ba0.5Sr0.5Fe0.8Cu0.2O3-δ as ORR electrode for fuel cells. Effect of Cu-doping in Ba0.5Sr0.5Fe1-xCuxO3-δ (x = 0-0.2) towards the ORR activity in alkaline media has been studied. Partial substitution of Cu in the Fe-site in (Sr,Ba)FeO3 could influence the transition-metal oxygen covalency which replaces and regenerates OH— ions in alkaline ORR process. Thin film rotating-disk electrode (RDE) method with well-defined oxygen transport was previously developed to study the ORR activity of perovskite catalysts [4, 9]. Evaluating the effects of such molecular level engineering on the desired outcome is challenging, since the ORR process consists of multiple adsorption/desorption step involving oxygen containing species. There are fewer literature on understanding the ORR process on transition metal oxides involving the transfer of four electrons and four OH— ions to O2 with subsequent cleavage of double bond O2. Ba0.5Sr0.5Fe1-xCuxO3-δ (x = 0-0.2) powders were prepared through sol gel synthesis method. Powder X-ray diffraction was employed for structural characterization and the activity towards ORR was studied using RDE method. RDE experiments were performed in oxygen saturated electrolyte at 100-1600 rpm. The voltage scan was performed from 0.4-0.9 V vs. RHE. The relation between current and rotational speed was evaluated through a Koutechy-Levich (K-L) plot to deduce the number of electrons (n) involved in the ORR process, i.e. n = (0.62∙F∙D0 2/3∙ v-1/6∙Co∙slope)-1 Where, F is Faraday’s constant, F = 96485 C.mol-1, (Do) diffusion coefficient of oxygen in 0.1M KOH = 1.93x10-5 cm2.s-1, (ν) Kinematic viscosity of 0.1 M KOH = 1.09x10-2 cm2.s-1, (Co) saturation concentration of O2 in 0.1 M KOH at 1 atm O2 pressure = 1.26x10-6 mol.cm-3, and slope – slope from K-L plot. Preliminary results showed that Cu doping in the lattice is affecting the ORR properties of perovskite in alkaline media. The slope of K-L plot gradually increases with increase in Cu content and decreases beyond 0.05 Cu until 0.2 Cu. The number of electron involved in the ORR process is mostly restricted to two. Acknowledgements This research was supported by the Canada First Research Excellence Fund (CFREF) at the University of Calgary. References Lu, J. Pan, A. Huang, L. Zhuang, J. Lu, Proc. Natl. Acad. Sci. U. S. A. 105, 20611 (2008).Pan, C. Chen, L. Zhuang, J. Lu, Acc. Chem. Res. 45, 473 (2012).Oz, K. Singh, D. Gelman, V. Thangadurai, Y. Tsur, J. Phys. Chem. C. 122 (27) 15097 (2018).Ge, A. Sumboja, D. Wuu, T. An, B. Li, F. W.T. Goh, T.S.A. Hor, Y. Zong, Z. Liu, ACS Catal. 5 , 4643 (2015).Suntivich, H. A. Gasteiger, N.Yabuuchi, H. Nakanishi, J.B. Goodenough, Y. Shao-Horn, Nat. Chem. 3, 546 (2011).McIntosh, J.F.Vente, W.G. Haije, D.H.A. Blank, H.J.M. Bouwmeester, Chem. Mater. 18, 2187 (2006).Teraoka, H. Shimokawa, C. Y. Kang, H. Kusaba, K. Sasaki, Solid State Ionics 177 , 2245 (2006).Wang, C. Tablet, A. Feldhoff, J. Caro, Adv. Mater. 17, 1785 (2005).Ge, A. Sumboja, D. Wuu, T. An, B. Li, F. W. T. Goh, et al., ACS Catal. 5, 4643 (2015). Figure 1

Oxygen Reduction Reactions in Low-Viscosity Ionic Liquids Based on Phosphonium Cations

There is high expectation for the lithium-oxygen (Li-O2) battery due to its high energy density because the cathodic active material is O2 in the air. The oxygen reduction reaction (ORR) at the cathode is of great importance, and it is strongly influenced by types of electrolytes. In particular, mass transport of oxygen molecules and its parameter, i. e. the solubility and the diffusion coefficient, are important for high performance of Li-O2 battery. Ionic liquids (ILs) have drawn attentions as an electrolyte for Li-O2 battery, because of their unique properties like incombustibility, low vapor pressure, good ionic conductivity and wide electrochemical potential window [1]. Major ILs used for such electrochemical devices include imidazolium, pyridinium, quaternary ammonium, pyrrolidinium and piperidinium derivatives. On the other hand, phosphonium ILs have the remarkable features such as their chemical and thermal stabilities, and lower viscosity, compared to the ammonium counterparts [2]. Therefore, we focused on the phosphonium ILs. Recently, much lower viscosity was reported that based on small phosphonium cation such as triethylalkylphosphonium cation or tri-n-butylalkylphosphonium cation [2,3]. All electrochemical measurements such as cyclic voltammetry (CV) and potential-step chronoamperometry (CA) were performed using a three electrode type electrochemical cell at 25oC. A glassy carbon (GC) disk electrode (d = 1 mm) or a carbon fiber (CF) microdisk electrode (d = 33 μm) were used as the working electrode. Counter electrode and reference electrode were Pt wire and Ag+/Ag in 0.1 M Ag-OTf / EMI-TFSA , respectively. CAs were performed on the CF microdisk electrode to estimate the solubility and the diffusion coefficient of oxygen in the ILs, individually, by the micro-electrode technique [4]. Two typical groups which consisted of triethylalkyl or tri-n-butylalkyl phosphonium cation were observed in the solubility correlated with molar volume. Moreover, two linear relationship between the diffusion coefficient and the viscosity of ILs were observed. In addition, the diffusion coefficients were correlated to occupied molar volumes which were calculated by the oxygen solubility in each IL. These result suggested that diffusion coefficient was not only affected the viscosity but also diffusion path originated from the cation structure. [1] A. Khan, C. Zhao, ACS sustainable Chem. Eng. 4, 506 (2016) [2] K. Tsunashima, M. Sugiya, Electrochem. Commun. 9, 2353 (2007) [3] K. Tsunashima, M. Sugiya, Electrochemistry, (2007), in press. [4] J. Maruyama, M. Inaba, T. Morita, Z. Ogumi, J. Electroanal. Chem. 504, 208 (2001)

Effects of Antioxidants Particle Size on Oxidation Resistance of MgO-C Refractory

In this study, Si and SiC powder with a critical particle size ranged from 0.1 to 0.4mm was added into MgO-C refractory as antioxidants. At 1200°C in air atmosphere, oxidation weight losses of cylindrical specimens with additives were measured and the effective diffusion coefficient of oxygen in refractories was calculated from the results. Thus, the effects of antioxidants particle size on the oxidation resistance were researched. The result shows that the particle size of antioxidant has a considerable influence on oxidation resistance of material. The oxidation resistance of MgO-C refractories increased at first as the critical particle size of Si powder increased from 0.1 to 0.2mm and then decreased as the critical particle size increased up to 0.4mm, while the oxidation resistance of MgO-C refractories decreased as the critical particle size of SiC additives increased from 0.1 to 0.4mm. The minimum effective diffusion coefficients of oxygen in MgO-C refractories added by Si and SiC were 10.90 and 14.09cm2/min, individually.

Influence of strontium content on the oxygen surface exchange kinetics and oxygen diffusion in La1–xSrxCoO3–δ oxides

Oxygen surface exchange kinetics between the gas phase oxygen and La1–xSrxCoO3–δ (x = 0, 0.3, 0.4, 0.6) oxides at 0.67 kPa oxygen pressure in the 600–850°°С range were investigated. The mechanism within the two-step model including two consecutive stages, oxygen dissociative adsorption and oxygen adatom incorporation into the oxide lattice, was analysed. The oxygen diffusion coefficient (D), the rate of heterogeneous oxygen exchange (rH) and the elementary rates of this process — dissociative adsorption (ra) and incorporation (ri) — were determined. The effect of oxygen vacancy concentration, strontium segregation on the surface, temperature and oxygen pressure on rH, ra and ri are discussed.

Novel low cost proton exchange membrane made from sulphonated biochar for application in microbial fuel cells

A novel low cost proton exchange membrane (PEM) was synthesized, using biochar derived from food waste by pyrolysis at 600 °C followed by sulphonation and using a poly vinyl alcohol based matrix, named as SBC-600, for application in microbial fuel cell (MFC). Membrane properties such as proton conductivity, ion transport number and oxygen diffusion coefficient were estimated and found to be 0.07 S cm−1, 0.891 and 6.46 × 10−9 m2 s−1, respectively. Proton conductivity per unit cost of SBC-600 membrane (0.42 S cm−1 $−1) was found to be 32 times higher than the Nafion membrane. The MFCs with SBC-600 membrane (MFC-SBC) and Nafion 117 as PEM (MFC-N) exhibited chemical oxygen demand removal efficiencies of 81 ± 6.6% and 88 ± 4.9%, respectively. Power harvested per unit cost of membrane was 26 times higher for MFC-SBC (0.278 W $−1) than MFC-N (0.011 W $−1) offering a low cost alternative to the costly PEM presently used in MFCs for its field scale applications.

Calculation of Oxygen Diffusion Coefficients in Oxide Films Formed on Low-Temperature Annealed Zr Alloys and Their Related Corrosion Behavior

The growth of oxide film, which results from the inward oxygen diffusion from a corrosive environment, is a critical consideration for the corrosion resistance of zirconium alloys. This work calculates the oxygen diffusion coefficients in the oxide films formed on zirconium alloys annealed at 400~500 °C and investigates the related corrosion behavior. The annealed samples have a close size for the second-phase particles but a distinctive hardness, indicating the difference in substrate conditions. The weight gain of all samples highly follows parabolic laws. The weight gain of the sample annealed at 400 °C has the fastest increase rate at the very beginning of the corrosion test, but its oxide film has the slowest growth rate as the corrosion proceeds. By contrast, the sample annealed at 500 °C shows the lowest weight gain but the highest corrosion rate constant. Such a corrosion behavior is attributed to the amount of defects existing in the oxide film formed on the annealed samples; fewer defects would provide a lower fraction of short-circuit diffusion in total diffusion, resulting in a lower diffusion coefficient of oxygen in the oxide film, thereby producing better corrosion resistance. This is consistent with the calculated diffusion coefficients of oxygen in the oxide films: 3.252 × 10−11 cm2/s, 3.464 × 10−11 cm2/s and 3.740 × 10−11 cm2/s for the samples annealed at 400 °C, 450 °C, and 500 °C, respectively.

Effect of Fe addition on the microstructure and oxidation behavior of MgO–C refractory

With the aim to solve the problem of structure degradation of MgO–C refractory during high-temperature oxidation, iron powder (Fe) was introduced as both antioxidant and ceramic bond former, and its effect on the microstructure evolution, physical and mechanical properties was investigated. Oxidation kinetics analysis, based on the shrinking core model, was also performed to reveal the quantitative effect of Fe on the oxidation resistance. Compared to the traditional MgO–C refractory, both densification and bonding degree of Fe-containing refractory after high-temperature oxidation were greatly improved by the formation of Mg(Fe1·2Al0.8)O4 bridges and magnesio-wustite [(Mg,Fe)Oss]. This change of microstructure caused a dramatic decrease in the effective diffusion coefficient of oxygen (Deff, decreased from 18.3 to 5.0 cm2/min), which contributed the most to the improvement of the oxidation resistance. The maintenance of strength against oxidation time was another important finding for the newly designed refractory, providing a basis for developing new carbon-containing refractories with self-repairing function.

Tuning anion solvation energetics enhances potassium–oxygen battery performance

The oxygen reduction reaction (ORR) is a critical reaction in secondary batteries based on alkali metal chemistries. The nonaqueous electrolyte mediates ion and oxygen transport and determines the heterogeneous charge transfer rates by controlling the nature and degree of solvation. This study shows that the solvent reorganization energy (λ) correlates well with the oxygen diffusion coefficient [Formula: see text] and with the ORR rate constant [Formula: see text] in nonaqueous Li-, Na-, and K-O2 cells, thereby elucidating the impact of variations in the solvation shell on the ORR. Increasing cation size (from Li+ to K+) doubled [Formula: see text], indicating an increased sensitivity of k to the choice of anion, while variations in [Formula: see text]were minimal over this cation size range. At the level of a symmetric K-O2 cell, both the formation of solvent-separated ion pairs [K+-(DMSO)n-ClO4- + (DMSO)m-ClO4-] and the anions being unsolvated (in case of PF6-) lowered ORR activation barriers with a 200-mV lower overpotential for the PF6- and ClO4- electrolytes compared with OTf- and TFSI- electrolytes with partial anion solvation [predominantly K+-(DMSO)n-OTf-]. Balancing transport and kinetic requirements, KPF6 in DMSO is identified as a promising electrolyte for K-O2 batteries.

Investigation of the initial stage of iron oxidation in the air. Part 2. Temperature range 400-625 ºС

Iron oxidation in the case of formation of thin oxide films on the surface of iron plates as a result of their oxidation in air by a parabolic law at temperatures above(400-550 ºС) and below (575 – 600 ºС) the Shadronpoint (575 ºС)has been studied.Studies are performed on the experimental plant described in the first part of the article, published in this issue of the journal.The experimental determination of the thickness of the oxide film depending on time is based on the method of occurrence of "variability" colors on the surface of the oxidized iron plate. With an increase in the thickness of the oxide film its color as a result of light's interference will consistently vary according to the series: yellow, orange, red, purple, violet and blue. This corresponds to the range of the oxide film's thickness from 46 to 72 nm.In the temperature range of 400-550 °C, a stable oxide Fe3O4 (magnetite) appears on the iron’s surface. It was shown that the dependences lnh of lnτ for all investigated temperatures are linear and have tangents of inclination angles equal to 0.5. The obtainedvalue of the activation energyis constant and equal to E = 43.0 kJ/mol.The oxygen diffusion coefficient D = 1.73 · 10–13 m2/s was obtained according to the oxidation model of a metal plate, which is based on the determinant role of oxygen diffusion through an oxide film.In the temperature range of 550 - 625 ºC, the change in the rate of the constant of the interaction between iron and oxygen from the air is due to the stable formation ofawustiteFeO. All of the oxidation isotherms can be described by the linear dependence in the coordinates lnh and lnτ. It was shown that the index of degree in the oxidation law at the temperatures below the Shadronpoint is equal to 2 (the parabolic law), and above the Shadron point it increases with increasing temperature of the process in accordance with the obtained dependence n = 2 + 0.008 (T-823).