H2 H2O Mixtures Research Articles

Distribution of Relaxation Times of Fuel Electrodes for Reversible Solid Oxide Cells Fabricated Under Various Conditions

Introduction Reversible solid oxide cells (r-SOCs) are electrochemical energy devices that can switch reversibly between power generation as a solid oxide fuel cell (SOFC) and hydrogen production as a solid oxide electrolysis cell (SOEC). It is important to systematically understand the electrode reaction processes and degradation factors in these two modes to tailor high-performance and durable r-SOC electrodes. The dependence of polarization resistance on cell fabrication conditions and operating conditions has been individually investigated for SOFC and SOEC by impedance measurement and distribution of relaxation times (DRT) analysis [1-4], but it is necessary to evaluate r-SOC as a whole [5]. Here in this study, we aim to systematically clarify the similarities and differences in the electrode reaction processes at the fuel electrodes of both SOFC and SOEC modes of r-SOCs, with respect to fuel electrode fabrication conditions. Experimental Several types of cells were fabricated with different fuel electrode constituent materials, thicknesses, and sintering temperatures. Three types of fuel electrode materials were used: Ni-GDC cermet, a composite of Ni and mixed ionic-electronic conductor Gd0.9Ce0.1O3 (GDC); Ni-YSZ cermet, a composite of Ni and pure ionic conductor YSZ (8 mol% Y2O3 - stabilized ZrO2); and Ni-GDC co-impregnated electrode with LST-GDC as a backbone support (LST: La0.1Sr0.9TiO3). Scandia-stabilized zirconia (ScSZ) was used as the electrolyte plate, and (La0.6Sr0.4)(Co0.2Fe0.8)O3 (LSCF) was used as the air electrode. GDC buffer layer was prepared between the electrolyte and the air electrode to suppress the formation of interfacial insulting layers.Electrochemical impedance measurements were performed for the fuel electrode of the fabricated cell under SOFC and SOEC modes by supplying H2-H2O mixture and varying operating temperature, fuel humidification, and current density. The DRT analysis was conducted to separate various polarization resistance components of the fuel electrode with different relaxation times. The area of each DRT peak corresponds to the resistance of each electrode reaction process. In this study, activation energy was derived based on the assumption that the electrical conductance is of an Arrhenius-type thermally activated character. Its dependence on the fuel electrode fabrication conditions was investigated. Furthermore, electrode microstructure was observed by using focused-ion beam scanning electron microscopy (FIB-SEM). Results and Discussion Figure 1 shows typical DRT peaks of the Ni-GDC cermet fuel electrode in (a) SOFC and (b) SOEC modes, measured at different temperatures. In the range of 10-1 to 106 Hz where the frequency response was measured, three DRT peaks appeared, P1: 10-1 to 100 Hz, P2: 100 to 102 Hz, and P3: 102 to 103 Hz, all with a DRT peak area decreasing with increasing operating temperature. This indicates that there are at least three electrode reaction processes with different relaxation times, which are thermally active processes. Activation energies of 0.1-0.3 eV (SOFC mode) and 0.2-0.5 eV (SOEC mode) were obtained for P1, and 0.9-1.0 eV (SOFC mode) and 1.1-1.3 eV (SOEC mode) for P2. The activation energies in SOEC mode were higher than those in SOFC mode for both P1 and P2. In the presentation, the dependence of activation energies on the fuel electrode constituent materials and related electrode reaction processes will be discussed.

Thermodynamic modeling of iron and zinc reduction from B<sub>2</sub>O<sub>3</sub> ‒ CaO ‒ Fe<sub>2</sub>O<sub>3</sub> ‒ ZnО melt by СО ‒ CO<sub>2</sub> and H<sub>2</sub> ‒ Н<sub>2</sub>О mixtures

The paper presents the thermodynamic modeling results of zinc and iron reduction from B2O3 ‒ CaO ‒ Fe2O3 ‒ ZnО melts by CO ‒ CO2 and H2 ‒ H2O mixtures containing 0 – 60 % CO2 (H2O) at 1273 – 1673 K using a technique describing the reduction of metals from an oxide melt by gas in bubbling processes, under conditions that provide an approximation to real systems. Its originality is equilibrium determination for each individual portion of gas supplied into the working fluid. The reducible metals oxides content in each calculation cycle is taken from the previous data. During the calculations, changes in the content of zinc (СZnO ) and iron (СFe2O3 , СFe3O4 and СFeO ) oxides in the melt and the degree of their reduction were estimated. When using CO or H2 as a reducing agent, this process proceeds in three stages. In the first stage, Fe2O3 is reduced to Fe3O4 and FeO. CFe2O3 values decrease to almost zero, while CFe3O4 and CFeO increase simultaneously. By the end of the stage, СFe3O4 reaches its maximum value. At the second stage, the Fe3O4 → FeO transition occurs, when СFeO values reach its maximum. At these stages, there is a slight increase in the CZnO . At the third stage, the values CFeO and CZnO decrease, and iron and zinc are reduced. An increase in temperature dramatically reduces the gas consumption for zinc reduction by 2 – 3 times, and the replacement of CO with H2 reduces it by less than 20 %. In the presence of oxidizing agents (CO or H2O), only zinc is reduced. The process ends when the final content of zinc oxide in the melt corresponds to the equilibrium with the initial gas composition. The higher the temperature, the less CZnO is. The obtained data are useful for the development of technologies for the selective recovery of metals.

Modelling and simulation of H2-H2O bubbly flow through a stack of three cells in a pre-pilot filter press electrocoagulation reactor

Computational fluid dynamics simulations were carried out to describe the hydrodynamic characteristics of a two-phase bubbly flow in a filter-press flow reactor stack of three cells, which is typically used in electrocoagulation (EC). The hydrogen evolution reaction (HER) took place at the cathode; dissolution of aluminium occurred at the anode. The fundamental transport equations of momentum and electrical potential were simultaneously solved to simulate the H2-H2O flow. Continuous (H2O) and dispersed phase (H2) velocity fields were modelled via the Euler-Eulerian approach, using the biphasic Reynolds Averaged Navier-Stokes (RANS) equations and the standard k − ε turbulence model. The influence of volumetric flow rate (1.7 ≤ Q ≤ 15 cm3 s−1) and applied current density (–28 ≤ j ≤ –5 mA cm−2) was systematically addressed to calculate the fraction of dispersed phase and current distribution along the electrodes. The evolved H2 bubbles were transported away from the electrode by the liquid flow. The dispersion of H2 through the electrode gap showed a modest bubble curtain profile due to the liquid flow rate. A homogeneous current distribution along the electrode length was experienced due to the geometrical design of the electrochemical cell and the low degree of H2 dispersion. The velocity profiles of the H2-H2O mixture were different in each cell due to the change of flow direction. H2 bubbles increased the velocity of the liquid phase but the gas fraction of such bubbles resulted in a higher pressure drop. Good agreement between theoretical and experimental residence time distribution curves was achieved; the experimental aluminium dose released by the anode agreed well with the simulations.

Lattice Boltzmann Modeling of Water Cumulation at the Gas Channel-Gas Diffusion Layer Interface in Polymer Electrolyte Membrane Fuel Cells

Several experiments have proved that water in liquid phase can be present at the anode of a PEM fuel cell due to vapor condensation resulting in mass transport losses. Nevertheless, it is not yet well understood where exactly water tends to cumulate and how the design of the gas channel (GC) and gas diffusion layer (GDL) could be improved to limit water cumulation. In the present work, a three-dimensional lattice Boltzmann based model is implemented in order to simulate the water cumulation at the GC–GDL interface at the anode of a PEM fuel cell. The numerical model incorporates the H2–H2O mixture equation of state and spontaneously simulates phase separation phenomena. Different simulations are carried out varying pressure gradient, pore size, and relative height of the GDL. Results reveal that, once saturation conditions are reached, water tends to cumulate in two main regions: the upper and side walls of the GC and the GC–GDL interface, resulting in a limitation of the reactant diffusion from the GC to the GDL. Interestingly, the cumulation of liquid water at the interface is found to diminish as the relative height of the GDL increases.

An electrical conductivity relaxation study of oxygen transport in samarium doped ceria

The efficacy of the electrical conductivity relaxation (ECR) technique for investigating the oxygen transport properties of mixed conducting oxides has been evaluated. Fifteen mol% samarium doped ceria (SDC15), for which approximate values of the two principal transport properties, bulk oxygen diffusivity, DChem, and surface reaction rate constant, kS, can be found in the literature, was chosen as the benchmark material against which to validate the methodology. Measurements were carried out at temperatures between 750 and 850 °C and over a wide range of oxygen partial pressures. An unexpectedly high p-type electronic transference number enabled ECR measurements under oxidizing conditions. A systematic data analysis procedure was developed to permit reliable extraction of the kinetic parameters even in the general case of simultaneous bulk and surface limitation. The DChem from this study showed excellent qualitative and quantitative agreement with expected values, falling in the range from ∼2 × 10−5 to 2 × 10−4 cm2 s−1. The surface reaction constant under H2–H2O mixtures also showed good agreement with literature results. Remarkably, this value increased by a factor of 40 under mixtures of CO–CO2 or O2–Ar. This observation suggests kinetic advantages for production of CO rather than H2 in a two-step solar-driven thermochemical process based on samarium doped ceria.

The retardation kinetics of magnetite reduction using H2 and H2–H2O mixtures

Cylindrical compacts of magnetite were isothermally reduced at 773–1273 K with pure H2 or H2–H2O mixtures. The initial reduction rates increased with temperature and partial pressures of H2 in the H2–H2O mixtures. However, with progressing reduction, a dense iron layer formed around the wüstite grains and the rate significantly reduced. In this regime, solid state oxygen diffusion through the dense iron layer was rate limiting. This retardation of reduction occurred at degrees of reduction of 51–89%, depending on the temperature and H2 partial pressure, which has a linear relationship with the dimensionless kinetic parameter, k1mixed/k2mixed, (k1mixed, k2mixed: contribution of gaseous mass transport (GMT) and interfacial chemical reaction (ICR) to the reduction rate, respectively) in the reaction-regime controlled by a combination of both mechanisms. However, under certain conditions (H2, H2–10%H2O, 773 K//H2–10, 20%H2O, 873 K//H2–20%H2O, 973 K) the retardation was absent because of the formation of a microporous iron layer product.

Study of Conductivity of K41X Chromia Forming Alloy in High Temperature Electrolysis Environment

Alloy K41X has been proposed as interconnect material for high temperature vapor electrolysis (HTVE) devices. This chromia forming alloy (alloy K41X) was oxidized at 800 °C in a thermobalance in oxidizing (synthetic air) and reducing (Ar–1 %H2–9 %H2O) environments for 250 h. The evolution of the contact resistance was evaluated using a dedicated device under the same conditions. There were higher oxidation kinetics rate in air than in Ar–1 %H2–9 %H2O but surprisingly, the corresponding area specific resistance (ASR) values were 20 times higher in Ar–1 %H2–9 %H2O mixture than in air. Additional tests and analyses (exposure in Ar–D2–H2O environment, GD-OES and SIMS analyses) clearly showed that the higher ASR value can be attributed to the presence of hydrogen in the oxide scale when exposed in Ar–H2–H2O mixture. In situ changes of atmosphere during ASR measurement showed the rapid kinetics for hydrogen desorption.

Improving the Physicochemical Properties of Fe–25Cr Ferritic Steel for SOFC Interconnects via Y-Implantation and Y2O3-Deposition

The present study investigated the role of the reactive-element effect (REE) in improving the corrosion resistance, chromium vaporization rate, and electrical conductivity of the Fe–25Cr ferritic steel modified either by means of yttrium implantation or chemical deposition of yttrium oxide from metaloorganic compound vapors. The corrosion kinetics of the Fe–25Cr steel, both pure and modified, were determined under isothermal conditions in air and an Ar–H2–H2O gas mixture at 1,073 K. A significant improvement in corrosion resistance was observed after surface modification. XRD and SEM–EDS investigations showed that the protective Cr2O3 layer formed the main part of the scale. Measurements of Cr vaporization rate in the air–H2O gas mixture revealed that both surface modifications of the steel significantly suppressed the formation of volatile chromium compounds to a large degree. The yttrium-implanted steels oxidized both in air and the Ar–H2–H2O mixture were characterized by the lowest area specific resistance and thereby did not exceed the acceptable ASR level (0.1 Ω cm2) for interconnect materials in the temperature range of 973–1,073 K, unlike pure steel and the steel coated with Y2O3.

Investigating the H2–He–H2O–CH4 equation of state in the deep troposphere of Jupiter

In this work, a new equation of state for a H2–He–H2O–CH4 mixture is presented. The equation is optimized for the deep jovian atmosphere (∼100bars) where the NASA Juno Microwave Radiometer (MWR) will probe. The methodology used is based upon that of Lemmon and Jacobsen (Lemmon, E.W., Jacobsen, R.T. [2004]. J. Phys. Chem. Ref. Data 33, 593–+) and Kunz et al. (Kunz, O., Klimeck, R., Wagner, W., Jaeschke, M. [2006]. Technical Monograph, VDI-Verlag). This methodology is used in combination with available published thermodynamic measurements and with new pressure–Volume–Temperature (pVT) measurements of H2–H2O mixtures conducted with the jovian simulator described in Karpowicz and Steffes (Karpowicz, B.M., Steffes, P.G. [2011]. Icarus 212, 210–223). In addition to being necessary to interpret laboratory measurements, the new equation of state is important in developing temperature pressure profiles of the deep jovian atmosphere. This is demonstrated by incorporating the new equation of state into an updated version of the DeBoer (DeBoer, D.R. [1995]. Ph.D. Thesis, Georgia Institute of Technology) Thermo-Chemical Model (TCM), and viewing its effect on the resulting simulated jovian atmospheric profiles.

Microstructure and deuterium permeation resistance of the oxide scale prepared by initial oxidation method on vacuum solar receiver

Deuterium permeation resistant scales on vacuum solar receiver were developed by initial oxidation method. The initial oxidation atmosphere was produced by the H2–H2O mixtures at 827°C, with a low oxygen partial pressure of about 10−19Pa. The phases and morphologies of the oxide scales were characterized by using X-ray diffraction (XRD), scanning electron microscope (SEM), X photoelectron spectroscopy (XPS) and transmission electron microscope (TEM). The formation of an oxide scale was observed with an inner layer essentially composed of chromium oxide and an outer layer mainly consisting of Mn1.5Cr1.5O4 spinels. The deuterium permeation of 316L stainless steel coated with Cr2O3 scale was only 1/80 of that of uncoated 316L stainless steel under loading of 80KPa at 600°C. The chromium oxide developed on the surface of 316L stainless steel revealed a marked inhibiting effect on the permeability of the deuterium and the associated mechanisms were discussed.

Influence of Hafnium Additions and Preoxidation Treatment on the Metal Dusting of Ni–Al Alloys

Alloys of γ-Ni(Al), γ–γ′ Ni(Al), γ′-Ni3Al and β-NiAl, each containing 0.5 wt% hafnium, were exposed in 1 h cycles to a carbon supersaturated CO–H2–H2O atmosphere (aC = 36.7, \( p_{{{\text{O}}_{2} }}\) = 2.83 × 10−26 atm) at 650 °C and an overall pressure of 1 atm. The alloys accumulated surface coke deposits and, to varying degrees, underwent metal dusting. While morphologies of the reacted samples were found to be very similar to those of equivalent hafnium-free alloys, the weight uptake kinetics were faster for alloys containing 0.5 wt% Hf. The hafnium-containing alloys were preoxidised in a H2–H2O mixture at 1,100 °C to form α-Al2O3 prior to metal dusting exposure. This treatment led to external oxide formation on all alloys except the low Al γ-Ni(Al) alloy, which formed internal oxides. Cyclic metal dusting resistance of all alloys was improved significantly by the preoxidation treatment, as alumina forms a barrier to inward carbon diffusion, and hafnium improves the quality of the barrier oxide.

Temperature and gas composition dependence of internal oxidation kinetics of an Fe10%Cr alloy in water vapour containing environments

AbstractRecently it was proposed, that the hampered formation of external protective chromia scales on FeCr‐alloys in water vapour containing, low‐pO2 gases is correlated with enhanced internal oxidation of chromium. In the present study the internal oxidation kinetics of Fe10Cr (in mass%) during isothermal oxidation in ArH2H2O mixtures at temperatures in the range 800–1050 °C has been investigated. It was found that the tendency for Cr to become internally oxidized decreased with decreasing temperature. At the higher test temperatures the internal oxide precipitates consisted of Fe/Cr‐spinel. With decreasing temperature the precipitates near the oxidation front gradually exhibited increasing amounts of chromia. At 900 °C the oxidation morphology in the ArH2 base gas mixture changed from exclusive internal oxidation of Cr at a water vapour content of 2% towards a combined internal Cr oxidation and external Fe‐oxide formation at higher water vapour partial pressures.

The Kinetics of Reduction of Dense Synthetic Nickel Oxide in H2-N2 and H2-H2O Atmospheres

An investigation of the kinetics of reduction of dense synthetic nickel oxide has been carried out in H2-N2 and H2-H2O mixtures between 500 °C and 1000 °C. The progress of the reduction was followed metallographically by the measurement of the advance of the nickel product layer. The influences of hydrogen partial pressure, hydrogen-steam ratio, and temperature were systematically investigated in both sets of the mixtures. Increasing hydrogen partial pressure under all conditions investigated results in an increase in the reduction rate. In H2-N2 mixtures and H2-H2O mixtures with low steam content, the initial reduction rate was found to be first order with respect to hydrogen partial pressure. In both sets of mixtures, it was found that the progress of Ni thickness was not a monotonic function of temperature. A minimum rate of advancement of Ni product was observed between 600 °C and 800 °C, depending on the hydrogen partial pressures and reduction time. The change in reduction behavior is shown to be directly linked to changes in Ni product microstructure.

Effect of specimen thickness on the growth rate of chromia scales on Ni-base alloys in high- and low-pO2 gases

Two types of experiments were carried out to study the oxidation behaviour of NiCr-base model alloys; thermogravimetric measurements for up to 72 h exposure at 1000 °C in synthetic air and isothermal exposures for up to 100 h at 1000 °C in laboratory air and in Ar–4%H2–2%H2O. It was found that there is a significant effect of specimen thickness on the oxidation rate of all studied alloys. The growth rates of the chromia scales decrease with increasing specimen thickness in both air and the Ar–H2–H2O mixture. Based on the present results in combination with recent literature data for ferritic steels, it is postulated that the decrease in rate is the result of compressive growth stresses in the scales which can relax by plastic deformation of the substrate for thin specimens but are maintained at significant levels for thick specimens.

Numerical modeling of an anode-supported SOFC button cell considering anodic surface diffusion

A two-dimensional isothermal mechanistic model of an anode-supported solid oxide fuel cell was developed based on button-cell geometry. The model coupled the intricate interdependency among the ionic conduction, electronic conduction, gas transport, and the electrochemical reaction processes. All forms of polarizations were included. The molecular diffusion, Knudsen diffusion, as well as the simplified competitive adsorption and surface diffusion were also considered. An electric analogue circuit was used to determine the effective hydrogen diffusivity. The model results showed good agreement with the published experimental data in different H2–H2O mixtures without any other calibrations after the parameter estimation according to the experimental data in baseline operating condition. The distributions of species concentration and current density were predicted and the effects of cathode area, gas components, and anode thickness on the cell performance were studied.

Effect of water vapour on growth and adherence of chromia scales formed on Cr in high and low pO2-environments at 1000 and 1050°C

The oxidation behaviour of pure Cr at 1000 and 1050°C was studied in Ar–O2 and Ar–H2–H2O mixtures. It was found that in the low-pO2 gases the oxide scales exhibited higher growth rates than in the high-pO2 gases. The scales formed in the low-pO2 gases showed substantially better adherence during cooling, than scales formed in the high-pO2 gases. These differences in growth rate and adherence can be correlated with differences in size and location of the in-scale voids formed during the isothermal exposure. Exposures in Ar-O2-H2O mixtures revealed that the differences in scale growth rates as well as in scale void formation and growth are not primarily related to differences in the oxygen partial pressure of the atmosphere but to the presence of water vapour in the test gas. At sufficiently high H2O/O2-ratios, water vapour promotes oxide formation at the scale/metal interface thereby suppressing excessive growth of existing voids, and also as a consequence improved scale adherence. Whether the enhancement of inward scale growth is related to transport of H2O- or H2-molecules or due to OH− ions, cannot be derived with certainty from the present results.

Extraction of Hydrogen from Water Vapor by Hydrogen Pump Using Ceramic Protonic Conductor

A blanket tritium recovery system that uses an electrochemical hydrogen pump with a protonic conductor membrane is proposed. One of the advantages of this system is the potential for processing the blanket sweep gas without fractionation of hydrogen isotopes and water vapor. In this work, hydrogen in a water molecule is extracted by a hydrogen pump using a Perovskite-type ceramic such as SrCe0.95Yb0.05O3-α. The threshold, which corresponds to the energy of H2O decomposition, for hydrogen extraction from the water molecule is 500 to 600 mV at 873 K. The threshold becomes smaller with increases of the partial pressure of the water vapor. In the case of pumping of the H2-H2O mixture gas, transportation of H2 precedes H2O decomposition below the threshold (H2O decomposition voltage), and the threshold becomes larger. In order to process the blanket sweep gas without fractionation of hydrogen isotope and water vapor, comparatively high applied voltage is required.

Role of sulphur in carburization, carbide formation and metal dusting of iron

AbstractThe kinetics and mechanisms of reactions on iron in CH4–H2 and CO–H2–H2O mixtures have been studied, also in the presence of H2S, to understand carburization, iron carbide formation, metal dusting and carbon deposition. Rate equations and mechanisms of the carburization in CH4–H2 and CO–H2–H2O have been elucidated. Both reactions are retarded by adsorbed sulphur, the rates becoming inversely proportional to the sulphur activity aS ∼ p(H2S)/p(H2) with increasing aS. At carbon activities aC > 1, cementite growth can be started in both gas mixtures on iron but the decomposition of this unstable carbide gives rise to a corrosion process called ‘metal dusting’, i.e. a disintegration of iron and steels to a dust of metal particles and graphitic carbon. This disintegration can be prevented by adsorbed sulphur, which hinders the nucleation of graphite. The stabilizing effect of sulphur on cementite and higher carbides such as Hägg carbide allows fundamental studies to be carried out on their thermodynamics, non‐stoichiometry and diffusional growth mechanisms, and the presence of sulphur will allow iron carbide production in a process of great present interest—the direct reduction of iron ores in carburizing gas mixtures. Kinetic studies in the system Fe–C–S are supplemented by AES and low‐energy electron diffraction investigations on the adsorption and segregation of sulphur on iron and cementite. Copyright © 2002 John Wiley & Sons, Ltd.

On the kinetics of chromite reduction

We reduced a natural chromite under controlled geometrical and thermodynamic conditions. The reaction between graphite and chromite in argon at 1200 and 1400°C is apparently rate-limited by a step that either involves the reductant or the gas phase. The reduction of chromite in an H2-H2O mixture at 1200°C follows the parabolic rate law; i.e., it is rate-limited by diffusion in the oxide.

Formation of chromium rich oxide scales for protection against metal dusting

Metal dusting is a disintegration of metals and alloys into graphite and metal particles, caused by strongly carburizing gas mixtures mainly in the temperature range 400–700°C. Protection of steels against metal dusting is possible through the formation of dense chromium rich oxide scales but it is not guaranteed that such scales are formed at low temperatures, even on high Cr-steels.Surface analytical studies have been conducted on the formation and composition of the oxide scales on 9–20%Cr steels. The growth of oxide films was followed by AES for 3 hours at 10–7 mbar O2 great differences were observed in dependence on surface finish. On ground samples, Mn and Si appeared early and Cr-rich oxide was formed, whereas on chemically etched samples Fe-rich oxides grew.After long term exposures (240 h) under metal dusting conditions, i.e. in CO–H2–H2O mixtures at 600°C, thin Cr-rich scales were observed on ground steels which were impermeable to carbon whereas on chemically etched steels thick Fe-rich scales had grown and carbon penetration was detectable. Accordingly, the oxide formation on Cr-steels at relatively low temperatures strongly depends on the surface treatment. Any surface working such as grinding and sand-blasting etc. introduces dislocations and causes a fine-grained microstructure near the surface, and the dislocations and grain boundaries act as rapid diffusion paths for supply of Cr to the surface in the first minutes of exposure, which leads to the formation of a protective oxide scale.