Understanding of CO2 Electrochemical Reduction Reaction Process via High Temperature Solid Oxide Electrolysers

Composite electrodes comprising a rare-earth nickelate phase and a doped rare-earth ceria phase with a high level of rare-earth doping have been studied as oxygen electrodes. While rare-earth nickelates have shown great promise as oxygen electrodes in both solid oxide fuel cells (SOFC) and solid oxide electrolysis cells (SOECs), they become unstable in contact with rare-earth doped ceria. In this work we describe a strategy to eliminate this instability and enable the use of these electrodes as stable high performance oxygen electrodes in both SOFC and SOEC applications. The performance of such composite electrodes is explored using a symmetrical cell approach, and through the electrical conductivity relaxation (ECR) method and compare it to state-of-the-art oxygen electrodes comprising strontium doped lanthanum manganite (LSM) – yttria stabilized zirconia (YSZ) composite electrodes. The symmetrical cell tests which reveal the stabilized nickelate-ceria electrodes to be superior, are also well-correlated with the results of testing complete cells in both SOFC and SOEC modes. Based on these results we make the case for deploying these stabilized nickelate-ceria composite electrodes as oxygen electrodes in reversible SOFC-SOEC systems.

Introduction With the recent enhancement of renewable energy utilization, large scale energy conversion and storage become more important to control the surplus electric power due to the unsteady power generation. Reversible hydrogen energy storage system via solid oxide electrolysis cell (SOEC) and solid oxide fuel cell (SOFC) are expected as a high efficient device because of its high operating temperature (600℃~1000℃). It is possible to unify them to one device as reversible system (RSOFC/EC), but it is necessary to understand the mechanism of FC/EC reactions and design the device with a good balance between FC and EC in order to apply them for an actual energy system. While SOFC reaction models and many electrochemical data of electrodes have been developed and reported by many researchers, there are few reports to investigate the reaction model of SOEC. Thus, it is possible to predict the performance of SOEC based on a comprehensive SOFC/EC models and experimental data of SOFC, because the reactions of SOFC and SOEC are a reversible relationship and the equal value of IR loss can be applied. However, the comprehensive SOFC/EC electrochemical model has not been established. A series of experimental data of anodic and cathodic current density vs. the potential should be investigated at various conditions to develop the SOFC/EC model. In this work, a series of anodic and cathodic electrode performance was taken using a SOFC in various conditions. Furthermore, attempting to predict SOEC performance from SOFC performances, we proposed and discussed a new model of electrolysis based on our developed SOFC model. Experimental The Disks of 8 mol% Y2O3-ZrO2 (YSZ; 0.25 mm thickness, 20 mm diameter) were used as the electrolyte and the anode was a Ni/YSZ or Ni/GDC composite, and the cathode was a La0.8Sr0.2MnO3/ScSZ composite. We measured the dependence of the current density of anodes on oxygen activity with wide range to get both power generation and electrolysis data for various gas composition (PH2=0.3~0.99 atm, PH2O=0.01, 0.03, 0.05 atm). Results and Discussion 1. Method to fit parameters on the SOFC/EC model We have developed the electrochemical reaction model on the fuel electrode (=anode) based on the equilibrium of competitive adsorption and oxygen activity, ao , specified by anode potential (Eq.1, Table.1) [1]. The SOFC model was constructed by assuming competitive adsorption equilibrium of H2, H2O and O on Ni surfaces at three-phase boundary and the rate-determining step to be Langmuir-type reactions of H with O (Table.1). We obtained an equation of the electrical current density, i, as a function of the oxygen activity (ao ) calculated from the anode potential which is simply measured from experiments. To confirm effectiveness of the model, we fitted the model to actual experimental data described by i vs. ao . From the equations of model on Table.1, each current density of SOFC (ia ) and SOEC(ic ) are described as Eq.2. To draw the plot of i vs. ao on the model, the rate constants (ka, kc ) and the equilibrium constants (KO, KH, KH2O ) need to be defined as reliable value. Thus, from the equilibrium potential vs. 1 atm O2 (Eq.3), we defined the rate constant of kc as Eq.4. When the kc became a dependent variable of ka , then we fitted ia to power generation experimental data described by i vs. ao so that the values of KHKO/KH2O were fitted approximately. Finally, i c model could be drawn on the plot of i vs. ao and the maximum value of i c was obtained by Eq.5 which is independent of ao and Eq.6 was obtained using Eq.4. 2. Experimental results and parameter fitting on the SOFC/EC model As the expreimental results, the maximum power density of SOFC with Ni/YSZ was 588.5 mW/cm2 for FC mode at PH2=0.99 atm, PH2O=0.01 atm, and ic(max) was 0.285 A/cm2 for EC mode at PH2O=0.03 atm. However, ic(max) calculated by the model (Eq.6) was 0.050 A/cm2. Also, in case of SOFC with Ni/GDC, ic(max) was 0.400 A/cm2 for EC mode at PH2O=0.03 atm, while ic(max) calculated by the model (Eq.6) was 0.055 A/cm2. The electrolysis current density measured by our RSOFC/EC showed 6 times higher than model values approximately. This result suggested the possibility that additional parallel reaction path was existed. In this presentation, we will discuss the possible electrochemical paths to explain the disagreement between the model and the experimental data. Finally, the comprehensive SOFC/EC model will be proposed to predict EC performance using SOFC data.

A novel system for the production of pure hydrogen from natural gas based on solid oxide fuel cell–solid oxide electrolyzer

Mathematical approaches to modelling the mass transfer process in solid oxide fuel cell anode

To improve solid oxide electrolysis cell (SOEC) system designs and fabrication quality, endurance testing of SOEC systems provides a proving ground on which real-world usage can be simulated, and quality assurance issues can be identified early on. A test facility at Idaho National Laboratory (INL) was designed, built, and operated to facilitate endurance testing of a reversible system by Oxeon Energy. Designated Emerald, the test facility is currently the only test facility at INL designed to allow both SOEC and solid oxide fuel cell (SOFC) testing of the same module. To safely operate a reversible SOEC test facility, several safety features are required, and a hazard and operability analysis is of Emerald is discussed here. Design challenges due to differences in the endothermic SOEC and exothermic SOFC reactions were addressed in the process recuperators. Controls for the Emerald test facility were built in an open-source JavaScript runtime environment, Node.js and executed on a Linux-based industrial controller. The Oxeon Energy module has been successfully brought up to SOEC ready conditions. Initial operations of the Emerald test facility are being improved to eventually achieve non-stop operations and thousands of hours of testing of both SOEC and SOFC.

High temperature electrolysis is a promising option for carbon-free hydrogen production and huge energy storage with high energy conversion efficiencies from renewable and nuclear resources. Over the past few decades, yttria-stabilized zirconia (YSZ) based ion conductor has been widely used as a solid electrolyte in solid oxide electrolysis cells (SOECs). However, its high operation temperature and lower conductivity in the appropriate temperature range for solid electrochemical devices were major drawbacks. Regarding improving ionic-conducting electrolytes, several groups have contributed significantly to developing and applying LaGaO3 based perovskite as a superior ionic conductor. La(Sr)Ga(Mg)O3 (LSGM) electrolyte was successfully validated for intermediate-temperature solid oxide fuel cells (SOFCs) but was rarely conducted on SOECs for its high efficient electrolysis performance. Their lower mechanical strengths or higher reactivity with electrode compared with the YSZ electrolysis cells, which make it difficult to choose compatible materials, remain major challenges. In this field, SOECs have attracted a great attention in the last few years, as they offer significant power and higher efficiencies compared to conventional YSZ based electrolysers. Herein, SOECs using LSGM based electrolyte, their applications, high performance, and their issues will be reviewed.

In recent years, solid oxide electrolysis cells (SOEC) have attracted considerable attention due to its high efficiency, stability, and environmental friendliness. The primary motivation of this chapter was to provide an overview of the current status of CO2 electrolysis in SOECs. In this chapter, the basic working principles of SOEC technology were briefly introduced, the development of key materials involved in SOEC were comprehensively reviewed, and the main applications of SOEC to CO2 electrolysis were summarized. In addition, the challenges faced by SOEC technology in CO2 electrolysis were systematically investigated. It is envisioned that the information collected in this chapter will be a valuable information source for researchers and provide direction and guidance for future research in SOEC technology.

In order to address the issue of the anthropogenic greenhouse effect, the supply with renewable energy will play a major role in the future energy systems. A particularly promising approach to store or even use renewable energy in sectors where electricity cannot be used effectively yet (for example in aviation) are synthetic fuels. Within the synthetic fuel generation process, known as Power-to-X, syngas (H2+CO) can be synthesized into liquid hydrocarbons, such as gasoline or jet fuel, via the Fischer-Tropsch process. In comparison with alternative processes, such as a steam electrolyzer in combination with a reverse water gas shift reactor, solid oxide electrolysis cells (SOEC) offer great potential for producing the required syngas in a single, less complex, and more efficient step within Power-to-X utilizing only steam (H2O), carbon dioxide (CO2), and electricity from renewable sources.This so-called co-electrolysis, which involves the simultaneous reduction of H2O and CO2 in SOECs, is of particular importance. It enables direct and efficient syngas production with a tailored H2/CO ratio, thereby reducing the need for additional downstream processing steps. This integrated approach enhances overall system efficiency, minimizes energy losses, and supports carbon utilization efforts, making it a crucial technology for sustainable fuel and chemical production. However, different syngas applications and associated SOE operating conditions can cause performance deterioration of SOECs. Consequently, a comprehensive understanding of losses incurred by diverse inlet mixtures, in conjunction with other operational conditions, such as temperature or the total flow rate, during electrolysis is essential.To the best of the authors’ knowledge, there is currently a lack of literature on operating cells with CO at the inlet, especially in CO2-electrolysis, where H2 is often added to avoid oxidation of Nickel in the fuel electrode. In order to investigate the influence of different gas components (H2O, H2, CO2, CO) of inlet compositions on performance and losses of SOECs, a series of experiments was performed on a commercial fuel electrode-supported cell with an active area of 81 cm2. To maintain a total flow rate of 2.2 slpm nitrogen was added, and the amount of H2O, H2, CO2, CO was varied, depending on the desired mixture. It is noteworthy that the air flow rate applied to the air electrode remained constant at 2.2 slpm throughout the experimental series. Initially, H2O electrolysis was conducted as a reference, followed by various mixtures for co-electrolysis and CO2 electrolysis, each with distinct H2O/CO2-, H2/CO-, and product/reactant-ratios. In the final experiment, pure CO2 was utilized as the feedstock for CO2-electrolysis, aiming to determine the losses in electrolysis in the absence of product species (CO).During the experiments polarization curves and electrochemical impedance spectroscopy (EIS) in 50 mA/cm2 steps were used for characterizing the performance of the cell at current densities up to 350 mA/cm2. The monitoring of the temperature distribution over the active area was realized through thermocouples integrated in the cell-housing near the gas flow channels on both the anode and cathode side. This approach enables further analysis of the reaction mechanism over the cell area, correlation of impedance data with thermal effects, and detection of hotspots. The outlet gas composition was monitored via a gas analyzer to determine the dry composition.This study investigates the impact of gas components at the inlet in co- and CO2-electrolysis on the performance of fuel electrode-supported SOECs, emphasizing losses in specific operating conditions. Furthermore, it offers a distinctive perspective on CO2 electrolysis with pure CO2 in an industrially sized cell, emphasizing substantial losses with no products at the inlet. The results show that the resistance in co-electrolysis is close to H2O-electolysis and way lower than in CO2-electrolysis which indicates that the main electrochemical reaction is the H2O-electolysis and CO2 is mostly converted to CO via the reverse water gas shift reaction. Figure 1

Electrochemical performance and long-term durability of a 200 W-class solid oxide regenerative fuel cell stack

Increasing demand for long-term energy storage and reduced carbon emissions promotes solid oxide electrolysis cell (SOEC) technology. SOEC systems can achieve high hydrogen production rates with near 100 percent stack electrical efficiency while co-electrolysis of carbon dioxide and water leads to production of syngas. With over 25 years’ experience building and testing solid oxide fuel cell (SOFC) materials, components and stacks, Nexceris has a proven track-record of commercializing SOFC products to major OEMs. The established manufacturing platforms at Nexceris enable smooth expansion into SOEC applications.In this talk, development of new SOEC electrode materials, stack components and stacks will be discussed. Over 50 down-selected air and fuel electrodes were electrochemically screened on 25 cm2 electrolyte-supported single cells and compared to baseline electrodes. Multiple electrode sets provided current density over -1.5 A/cm2 at 1.3 V and 800 °C with 75 percent steam/25% percent hydrogen fuel and air. The promising electrodes were implemented into short stacks with active cell area of 42 cm2. Stacks were operated continuously in SOEC mode at 1.3-1.4 V for ≥ 500 hours. The resulting short stack demonstrations show promising SOEC baseline performance and durability platform for future cell and stack development. Co-electrolysis and water electrolysis tests were also conducted and yielded comparable performance. Nexceris’ ChromLok coatings for bipolar plates were also integrated to stacks and tested under realistic conditions.

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A series of bifunctional ReBaCo2O5+δ perovskite catalysts towards intermediate-temperature oxygen reduction reaction and oxygen evolution reaction

A novel solid oxide electrolysis cell (SOEC) to separate anodic from cathodic polarization under high electrolysis current

The nickel(Ni)-patterned electrode enables researchers to quantify the triple-phase boundary (TPB) length and Ni surface area, exclude the influence of bulk gas diffusion and clearly separate the active regions of the chemical/electrochemical reactions. This tool has been studied for identifying the reaction mechanism in SOFC but rarely in SOEC. In this study, the Ni-patterned electrodes with the stripe width of 100 μm were tested in both the solid oxide fuel cell (SOFC) and solid oxide electrolysis cell (SOEC) modes at the atmosphere of H2O/H2 and further H2O/CO2/H2/CO. The experimental test shows the stability of the Ni-patterned electrode in the H2O/H2 atmosphere was much poorer than that in the CO/CO2 atmosphere due to the nickel reacting with steam. Thus, the Ni-patterned electrode needed to operate at a temperature of ≤700oC with the inlet H2/H2O molar ratio of >7 and the current-applied time shouldn’t be over 5 h to keep the accuracy of the TPB length and Ni surface area. The effects of the temperature, partial pressure of H2O and H2 are investigated. The activation energy of the Ni-patterned electrodes was 0.386 eV. The electrochemical performance had a positive correlation with both the partial pressure of H2 and H2O, especially showed higher sensitivity to the partial pressure of H2O. Further, a possible mechanism of the H2O/H2 electrochemical conversion was proposed according to the experimental data and existing literature, which contains two-step charge-transfer reaction: H(Ni)+O2-(YSZ)OH-(YSZ)+(Ni)+e- and H(Ni)+OH-(YSZ)H2O(YSZ)+(Ni)+e-. Based on the experiments, an analytical calculation was performed to investigate the rate-determining steps in the Ni-patterned electrodes by bridging the connection between the kinetic parameters and operating conditions. The results indicate the rate-limiting steps may be different for the SOFC and SOEC modes. In SOFC mode, the H2 electrochemical oxidation could depend more on the charge-transfer reaction H(Ni)+O2-(YSZ)→OH-(YSZ)+(Ni)+e-, however in SOEC mode, the rate-determining step of the H2O electrochemical reduction should be H2O(YSZ)+(Ni)+e-→OH-(YSZ)+H(Ni). In this way, a reversible mechanism could be applied for reversible SOFC (RSOFC) to describe the conversion between hydrogen and steam by different charge-transfer-reaction domination in the SOFC and SOEC mode. Figure 1

A novel solid oxide electrolytic cell with reduced endothermic load for CO2 electrolysis using (La0.80Sr0.20)0.95MnO3-δ cathode