Mechanism of oxygen electrode delamination in solid oxide electrolyzer cells

An electrochemical model for degradation of solid oxide electrolyzer cells is presented. The model is based on concepts in local thermodynamic equilibrium in systems otherwise in global thermodynamic non-equilibrium. It is shown that electronic conduction   through the electrolyte, however small, must be taken into account for determining local oxygen chemical potential, μ_O2${\mu }_{{\mathrm{O}}_2}$, within the electrolyte. The μ_O2${\mu }_{{\mathrm{O}}_2}$ within the electrolyte may lie out of bounds in relation to values at the electrodes in the electrolyzer mode. Under certain conditions, high pressures can develop in the electrolyte just near the oxygen electrode/electrolyte interface, leading to oxygen electrode delamination. These predictions are in accord with the reported literature on the subject. Development of high pressures may be avoided by introducing some electronic conduction in the electrolyte.     

High temperature electrolyzer based on solid oxide co-ionic electrolyte: A theoretical model

In the present work a theoretical model of a solid oxide electrolyzer based on an electrolyte having both oxygen ion and proton conductivity is considered. The main parameters of the electrolytic process and an electrolyzer (distribution of gas components, electromotive forces and current densities along the electrolyzer channel, average values of electromotive forces and current densities) were calculated depending on a proton transport number and mode of the reactants’ feeding (co- and counter-flow). The performed analysis demonstrates considerable influence of the mode of feeding on all parameters of the electrolyzer: operation under the counter-flow mode is preferable as regards the specific characteristics and uniformity of their distribution along the electrolyzer. It is shown that the electrolyser's specific characteristics increase with the increase of the proton transport number.     

Parametric study of solid oxide steam electrolyzer for hydrogen production

A theoretical model was developed to study the electrical characteristics of a solid oxide steam electrolyzer (SOSE) for hydrogen production. The activation and concentration overpotentials at the electrodes as well as the ohmic overpotential at the electrolyte were considered as the main sources of voltage loss. The Butler–Volmer equation, Fick's model, and Ohm's law were applied to characterize the overpotentials. The theoretical model was validated as the simulation results agreed well with the experimental data from the literature. In the study of the component thickness effect, anode-support SOSE configuration was identified as the most favorable design. Further parametric analyses were performed to study the effects of material properties and operating conditions on the anode-supported SOSE cell performance. The results have shown that increasing electrode porosity and pore size can reduce the voltage loss. In the operation, both temperature and steam molar fraction can be increased to enhance the SOSE electrical efficiency. The pressure should be regulated depending on the current density. The electrochemistry model can be used to perform more analyses to gain insightful understanding of the SOSE hydrogen production principles and to optimize the SOSE cell and system designs.     

Atomic-scale mechanisms of oxygen electrode delamination in solid oxide electrolyzer cells

Materials used for different components (electrodes, electrolyte, steel interconnects, etc.) of solid oxide electrolyzer cell (SOEC) devices for hydrogen production have to function in aggressive, corrosive environments and in the presence of electric fields. This results in a number of degradation processes at interfaces between components. In this study, we used a combination of first-principles, density-functional-theory (DFT) calculations and thermodynamic modeling to elucidate the main processes that contribute into the oxygen delamination in typical SOEC device consisting of yttria-stabilized zirconia (YSZ) electrolyte and Sr-doped LaMnO₃ (LSM) oxygen electrode. We found that high temperature inter-diffusion of different atoms across the LSM/YSZ interface significantly affects structural stability of the materials and their interface. In particular, we found that La and Sr substitutional defects positioned in ZrO₂ oxide and near LSM/YSZ interface significantly change oxygen transport which may develop pressure buildup in the interfacial region and eventually develop delamination process. Simple models for estimating these effects are proposed, and different possibilities for inhibiting and/or mitigating undesirable delamination processes are discussed.     

Thermodynamic and electrochemical analyses of a solid oxide electrolyzer for hydrogen production

In this paper, a modeling of the Solid Oxide Electrolysis Cell (SOEC), through energetic, exergetic and electrochemical modeling approaches, is conducted, and its performance, particularly through exergy efficiency, is analyzed under various operating conditions and state properties for optimum hydrogen production. In a comprehensively performed parametric study, at a single electrolysis cell scale, the effects of varying some operating conditions, such as temperature, pressure, steam molar fraction and the current density on the cell potential and hence the performance are investigated. In addition, at the electrolyzer system scale, the overall electrolyzer performance is investigated through energy and exergy efficiencies, in addition to the system's power density consumption, hydrogen production rate, heat exchange rates and exergy destruction parameters. The present results show that the overall solid oxide electrolyzer energy efficiency is 53%, while the exergy efficiency is 60%. The exergy destruction at a reduced operating temperature increases significantly. This may be overcome by the integration of this system with a source of steam production.     

Asymmetric behavior of solid oxide cells between fuel cell and electrolyzer operations

The electrochemical performance of solid oxide cells (SOCs) is investigated under both fuel cell and electrolyzer operations to understand their asymmetric behavior between the two operation modes. The current–voltage and electrochemical impedance characteristics of a hydrogen-electrode-supported cell are experimentally analyzed. Also, a numerical model is developed to reproduce the cell performance and to understand the internal resistances of the cell. Partial pressures of supplied gas and load current are varied to evaluate their effects on the cell performance. The gas partial pressures of hydrogen and steam supplied to the hydrogen electrode are kept equivalent so that the cell performance can be fairly compared between the two operation modes when the same current is applied. It is found that the origin of the asymmetry is mostly from the hydrogen electrode; both activation and concentration overpotentials show asymmetric behavior particularly at high current densities. A numerical experiment is also conducted by deliberately changing parameters in the model. Asymmetry in the activation overpotential is found to be originated from the non-identical charge-transfer coefficients in the Butler–Volmer equation and also from the non-uniform gas concentration formed in the hydrogen electrode under current-biased conditions. On the other hand, asymmetry in the concentration overpotential is associated with the non-equimolar counter diffusion of hydrogen and steam caused by the effect of Knudsen diffusion. Therefore, enhancing gas transport in the hydrogen electrode and reducing the contribution of Knudsen diffusion are effective approaches to reduce asymmetry not only in the concentration overpotential but also in the activation overpotential.     

Compatibility between AISI441 alloy interconnect and representative seal glasses in solid oxide fuel/electrolyzer cells

AISI441 alloy, an advanced ferritic stainless steel, is considered as a promising metallic interconnect material for solid oxide fuel/electrolyzer cells. In this work, the compatibility between the AISI441 alloy and four representative seal glasses (YSO-4, G18, SABS-0, and SCAN2) has been studied. The AISI441/glass couples are thermally treated in both air and H₂/H₂O atmospheres at 800 °C for up to 500 h. Interfacial morphology shows that the YSO-4 glass and the SCAN2 glass have relatively high reaction tendency with the AISI441 alloy; the G18 glass devitrifies extensively during the thermal treatment; and the SABS-0 glass shows very little interaction or devitrification. In the H₂/H₂O atmosphere, the AISI441 alloy has less interaction with the sealing glasses compared with the air condition. Different phases from interfacial reaction and devitrification have been identified for each of the sealing glasses. Diffusion, devitrification, and reaction are the events occurring at the interface. Overall, the AISI441/SABS-0 glass couple shows the least interfacial reaction and devitrification and the best thermal stability.     

Fabrication of hydrogen electrode supported cell for utilized regenerative solid oxide fuel cell application

A utilized regenerative solid oxide fuel cell (URSOFC) provides the dual function of performing energy storage and power generation, all in one unit. When functioning as an energy storage device, the URSOFC acts like a solid oxide electrolyzer cell (SOEC) in water electrolysis mode; whereby the electric energy is stored as (electrolyzied) hydrogen and oxygen gases. While hydrogen is useful as a transportation fuel and in other industrial applications, the URSOFC also acts as a solid oxide fuel cell (SOFC) in power generation mode to produce electricity when needed. The URSOFC would be a competitive technology in the upcoming hydrogen economy on the basis of its low cost, simple structure, and high efficiency. This paper reports on the design and manufacturing of its anode support cell using commercially available materials. Also reported are the resulting performance, both in electrolysis and fuel cell modes, as a function of its operating parameters such as temperature and current density. We found that the URSOFC performance improved with increasing temperature and its fuel cell mode had a better performance than its electrolysis mode due to a limited humidity inlet causing concentration polarization. In addition, there were great improvements in performance for both the SOFC and SOEC modes after the first test and could be attributed to an increase in porosity within the oxygen electrode, which was beneficial for the oxygen reaction.     

Hydrogen and synthetic fuel production using pressurized solid oxide electrolysis cells

Wind and solar power is troubled by large fluctuations in delivery due to changing weather. The surplus electricity can be used in a Solid Oxide Electrolyzer Cell (SOEC) to split CO₂ + H₂O into CO + H₂ (+O₂). The synthesis gas (CO + H₂) can subsequently be catalyzed into various types of synthetic fuels using a suitable catalyst. As the catalyst operates at elevated pressure the fuel production system can be simplified by operating the SOEC at elevated pressure. Here we present the results of a cell test with pressures ranging from 0.4 bar to 10 bar. The cell was tested both as an SOEC and as a Solid Oxide Fuel Cell (SOFC). In agreement with previous reports, the SOFC performance increases with pressure. The SOEC performance, at 750 °C, was found to be weakly affected by the pressure range in this study, however the internal resistance decreased significantly with increasing pressure.     

Thermal stability of a new solid oxide fuel/electrolyzer cell seal glass

Long-term thermal stability of sealing glass is critical for hermetic seals of solid oxide fuel and electrolyzer cell stacks. In this work, a SrO–La₂O₃–Al₂O₃–SiO₂ glass (SABS-0 glass) has been evaluated as a high temperature sealant by thermal treatment. Powder and bulk SABS-0 glasses are studied in both air and H₂/H₂O atmospheres at 800 °C for up to 1000 h. Weight measurements show negligible SABS-0 glass vaporization during the thermal treatment. Both SABS-0 powder and bulk samples show some surface devitrification but the SABS-0 glass bulk remains amorphous at all the thermal treatment conditions. On the polished bulk SABS-0 surface, needle-shaped crystals are observed for both the air and the H₂/H₂O thermal treatment conditions. Polishing is believed to be the initiator for the SABS-0 glass surface devitrification. The crystalline phases, indentified as silicates and aluminates, increase with the thermal treatment time. However, the crystalline phases on the polished SABS-0 glass surface are very limited and only exist on the very surface of both the air and the H₂/H₂O atmosphere treated samples. The SABS-0 glass has excellent thermal stability in solid oxide fuel/electrolyzer operating environments and is a promising sealant material for such applications.     

Gas transport in hydrogen electrode of solid oxide regenerative fuel cells for power generation and hydrogen production

To further develop solid oxide regenerative fuel cell (SORFC) technology, the effect of gas diffusion in the hydrogen electrode on the performance of solid oxide fuel cells (SOFCs) and solid oxide electrolysis cells (SOECs) is investigated. The hydrogen electrode-supported cells are fabricated and tested under various operating conditions in both the power generation and hydrogen production modes. A transport model based on the dusty-gas model is developed to analyze the multi-component diffusion process in the porous media, and the transport parameters are obtained by applying the experimentally measured limiting current data to the model. The structural parameters of the porous electrode, such as porosity and tortuosity, are derived using the Chapman–Enskogg model and microstructural image analysis. The performance of an SOEC is strongly influenced by the gas diffusion limitation at the hydrogen electrode, and the limiting current density of an SOEC is substantially lower than that of an SOFC for the standard cell structure under normal operating conditions. The pore structure of the hydrogen electrode is optimized by using poly(methyl methacrylate) (PMMA), a pore-forming agent, and consequently, the hydrogen production rate of the SOEC is improved by a factor of greater than two under moderate humidity conditions.     

Investigation of iron–chromium–niobium–titanium ferritic stainless steel for solid oxide fuel cell interconnect applications

As part of an effort to develop cost-effective ferritic stainless steel-based interconnects for solid oxide fuel cell (SOFC) stacks, both bare AISI441 and AISI441 coated with (Mn,Co)₃O₄ protection layers were studied in terms of its metallurgical characteristics, oxidation behavior, and electrical performance. The addition of minor alloying elements, in particular Nb, led to formation of Laves phases both inside grains and along grain boundaries. In particular, the Laves phase which precipitated out along grain boundaries during exposure at intermediate SOFC operating temperatures was found to be rich in both Nb and Si. The capture of Si in the Laves phase minimized the Si activity in the alloy matrix and prevented formation of an insulating silica layer at the scale/metal interface, resulting in a reduction in area-specific electrical resistance (ASR). However, the relatively high oxidation rate of the steel, which leads to increasing ASR over time, and the need to prevent volatilization of chromium from the steel necessitates the application of a conductive protection layer on the steel. In particular, the application of a Mn_1.5Co_1.5O₄ spinel protection layer substantially improved the electrical performance of the 441 by reducing the oxidation rate.     

Advanced electrocatalytic activity of praseodymium-deficient copper-based oxygen electrodes for solid oxide fuel cells

Herein praseodymium-deficient Pr_1.90−xCe_0.1CuO₄ oxides are evaluated as potential oxygen electrodes for solid oxide fuel cells (SOFCs). Introducing Pr-deficiency promotes the oxygen vacancy concentration, further improving electrocatalytic activity of the Pr_1.90−xCe_0.1CuO₄ electrodes towards oxygen reduction reaction (ORR). The Pr_1.75Ce_0·.1CuO₄ (P_1·75CC) component exhibits outstanding electrode performance, as supported by a polarization resistance as low as 0.025 Ω cm² and high peak power density of the single cell (1.11 W cm⁻²) at 700 °C. In addition, the rate-limiting steps for ORR kinetics are determined to be the charge transfer reaction and oxygen adsorption/diffusion process on the electrode surface. This work highlights an effective way for developing the cathode candidates with high electrocatalytic activity and superior stability.     

High performance and stability of nanocomposite oxygen electrode for solid oxide cells

Solid oxide electrochemical cell (SOC) is a highly promising alternative for fuel conversion and power-to-gas due to its high efficiency and low emission. However, degradation resulting from the electrolyte-electrode interface is a major challenge in both fuel cell mode and electrolysis mode. Here, a co-sintering tri-layer structure cell with nanocomposite oxygen electrode is developed to mitigate the interface issue. A 10 × 10 cm² NiO/YSZ||YSZ||YSZ-La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF) cell has been conducted under different fuels in SOFC mode. A power density output of 558 mW/cm² @0.7 V-800 °C in wet H₂ and a durability of 300 h in simulated syngas have been obtained. The performance of LSF, LSCF and SSC oxygen electrodes have been studied in both SOFC and SOEC modes. It suggests that three oxygen electrodes have an order of SSC > LSCF > LSF in electrochemical performance, and an opposite order in stability of SOEC. The degradation of the LSCF and SSC can be derived from the solid-state reactions at the interface between Co-containing perovskites and YSZ during operation. It demonstrates that GDC and Ag modification can enhance the oxygen electrode stability by impeding the solid-state reactions and the nanoparticles sintering. Results suggest that GDC has a negative effect on the cell performance and Ag has a positive effect, implying that enhancing the electric conductivity of YSZ-LSCF is the key to improve the cell performance. Moreover, cell with YSZ-SFM/GDC has been applied in CH₄ assisted SOEC process (CH₄-SOEC), in which a significant reduction of electricity consume can be realized.     

Synergistically enhancing CO2-tolerance and oxygen reduction reaction activity of cobalt-free dual-phase cathode for solid oxide fuel cells

High-performance cathodes with adequate CO₂ tolerance are vital for further development of intermediate-temperature solid oxide fuel cells (IT-SOFCs). However, there is always a trade-off between CO₂ tolerance and oxygen reduction reaction (ORR) performance for single-phase cathodes. Here, we report a cobalt-free Ba_0.6La_0.4FeO_3-δ-Ce_0.8Sm_0.2O_2-δ (BLF-SDC) dual-phase cathode with excellent ORR activity and CO₂ tolerance. Introducing ionic conductor Ce_0.8Sm_0.2O_2-δ (SDC) into the Ba_0.6La_0.4FeO_3-δ (BLF) phase can boost ORR activity due to the extended active sites and enhanced oxygen surface exchange process with a polarization resistance of 0.121 Ω cm² for the BLF-30% SDC (weight ratio, BLF-30SDC) cathode at 700 °C. The CO₂ resistance of the BLF-30SDC composite cathode outperforms BLF cathode by three times at 600 °C. This stability enhancement is owing to low CO₂ adsorption of SDC, which is confirmed from thermodynamic calculation. This work indicates that dual-phase mixed conductors can be developed as highly active and stable cathodes for IT-SOFCs.     

A cost-effective process for fabrication of metal-supported solid oxide fuel cells

Metal-supported SOFC cells with Y₂O₃ stabilized ZrO₂ as the electrolyte were prepared by a low cost and simple process involving tape casting, screen printing and co-firing. The interfaces were well bonded after the reduction of NiO to Ni in the support and the anode. AC impedance was employed to estimate the cell polarizations under open circuit conditions. It was found that the electrode polarization resistance was high at low temperatures and became equivalent to the ohmic resistance at higher temperatures near 800°°C. The cell performance was evaluated with H₂ as the fuel and air as the oxidant, and maximum power density between 0.23 and 0.80  W/cm² was achieved in the temperature range of 650–800°C, which confirms the applicability of the cost-effective process in fabrication of metal-supported SOFC cells.     

Pd-YSZ composite cathodes for oxygen reduction reaction of intermediate-temperature solid oxide fuel cells

Pd-Y₂O₃ stabilized ZrO₂ (YSZ) composite cathodes are prepared by conventional mechanical mixing and infiltration methods. In the case of infiltration, thermal decomposition and chemical reduction processes are used to form Pd particles on the YSZ scaffold. The phase structure, morphology and electrochemical performance of the Pd-YSZ composite cathodes are characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM) and electrochemical impedance spectroscopy (EIS). The performance of mechanically mixed Pd-YSZ composite cathodes is inadequate due to significant growth and sporadical distribution of Pd particles. The 5 wt.% Pd-loaded cathode prepared by infiltration-thermal decomposition process shows the lowest polarization resistance, i.e. between 0.042 Ω cm² and 1.5 Ω cm² in the temperature range of 850-600 °C, benefited from the formation of nano-sized Pd particles and the presence of well connected Pd network. The effect of Pd loading on the performance of the infiltrated-thermal decomposed Pd-YSZ composite cathodes is also evaluated, 5 wt.% Pd loading results in the lowest polarization resistances.     

PbO effect on the oxygen reduction reaction in intermediate-temperature solid oxide fuel cell

Ag paste is often used in solid oxide fuel cells (SOFCs) as the current collector. This study investigates the effect of lead oxide (PbO), which is usually present in Ag paste, on the oxygen reduction reaction (ORR) at the typical electrocatalysts, lanthanum strontium cobalt ferrite (LSCF) and lanthanum strontium ferrite (LSF), for intermediate-temperature SOFCs. The chemical oxygen surface exchange coefficients are greatly increased by the deposition of PbO particles on the LSCF and LSF bar surfaces as demonstrated by the electrical conductivity relaxation technique. For example, at 800 °C, the deposition of 0.074 mg cm⁻² PbO particles increases the coefficients from 4.68 × 10⁻⁵ to 7.2 × 10⁻⁴ cm s⁻¹ and from 2.62 × 10⁻⁵ to 1.60 × 10⁻⁴ cm s⁻¹ for LSCF and LSF, respectively. Furthermore, the infiltration of PbO into porous LSCF and LSF electrodes significantly improves the ORR as determined with AC impedance spectroscopy using symmetrical cells comprising of samaria-doped ceria as the electrolyte. When 5.88 and 5.67 wt % PbO is infiltrated into the porous backbone of LSCF and LSF electrodes, at 650 °C, the area specific interfacial polarization resistance (ASR) is reduced from 0.26 to 0.14 Ω cm² and from 1.34 to 0.37 Ω cm², respectively. The distribution of relaxation time (DRT) analysis suggests that PbO efficiently accelerate the charge transfer and oxygen incorporation processes.     

Basic properties of a liquid tin anode solid oxide fuel cell

An unconventional high temperature fuel cell system, the liquid tin anode solid oxide fuel cell (LTA-SOFC), is discussed. A thermodynamic analysis of a solid oxide fuel cell with a liquid metal anode is developed. Pertinent thermochemical and thermophysical properties of liquid tin in particular are detailed. An experimental setup for analysis of LTA-SOFC anode kinetics is described, and data for a planar cell under hydrogen indicated an effective oxygen diffusion coefficient of 5.3 × 10⁻⁵ cm² s⁻¹ at 800 °C and 8.9 × 10⁻⁵ cm² s⁻¹ at 900 °C. This value is similar to previously reported literature values for liquid tin. The oxygen conductivity through the tin, calculated from measured diffusion coefficients and theoretical oxygen solubility limits, is found to be on the same order of that of yttria-stabilized zirconia (YSZ), a traditional SOFC electrolyte material. As such, the ohmic loss due to oxygen transport through the tin layer must be considered in practical system cell design since the tin layer will usually be at least as thick as the electrolyte.