High Fuel Utilization Research Articles

Degradation Analysis of SOFC Performance for Long-Term Operation with High Fuel Utilization (2)

Solid oxide fuel cells (SOFCs) are a power generation technology with its high efficiency and low environmental impact, and the most important issue for SOFC for practical use is to improve reliability as well as high efficiency. A national project on SOFC development in Japan, “Development of advanced evaluation and analysis technologies for the durability of Solid Oxide Fuel Cells stacks (2020-),” has been initiated as a NEDO project in order to clarify their long-term durability and find dominant degradation factor for each SOFC stacks. Central Research Institute of Electric Power Industry (CRIEPI) has operated three kinds of different SOFC cell/stacks (flat-tubular and two kinds of planer) for durability under NEDO’s project, which have been developed by KYOCERA, MORIMURA SOFC Technology, and DENSO.Long-term durability tests of 20,000 hours have been conducted under high fuel utilization of 80% and 85%, which is close to the operating limit of each SOFC cell/stack for high efficiency, to evaluate the effects on the performance and degradation behavior of the cell/stack. In previous projects, we have developed an electrode polarization model that can accurately represent SOFC performance, including cell voltage and each overvoltage, under a variety of conditions. The degradation factors for durability of each SOFC cell/stack have been analyzed by using this model. The analysis revealed that the dominant degradation factors were increased cathode overvoltage and IR loss, similar to the SOFC cell/stacks that were modified from the previous project to improve their long-term durability. On the other hand, the degradation rate of cell voltage was higher at 85% than at 80% fuel utilization in one cell/stack, and the performance degradation factor analysis revealed that this difference was due to an increase in anode overvoltage at 85% than at 80% fuel utilization, where anode overvoltage was not the primary degradation factor.This work has been carried out under the projects (JPNP13001 and JPNP20003) of the New Energy and Industrial Technology Development Organization (NEDO), Japan as contract research. We would like to express our gratitude to NEDO, many SOFC developers, as well as to university partners and National Institute of Advanced Industrial Science and Technology (AIST).

Degradation Analysis of SOFC Performance for Long-Term Operation with High Fuel Utilization (2)

Three types of SOFC stacks have been operated in a NEDO’s project from FY2020 to FY2024 in order to clarify the technical issues for high efficiency and toughness of SOFC stacks and establish the methods for advanced degradation diagnosis of SOFC stacks. Long-term durability tests of two types of SOFC stacks as flatten-tubular type and planar type have been conducted toward 20,000 h under the high fuel utilization at 80 and 85 %, respectively close to the operational limits of the stacks. In addition, the degradation factors of SOFCs were analyzed by using the electrode polarization model developed by CRIEPI. As a result, cathode overvoltage and IR loss were the major degradation factors in the durability tests over 10,000 h.

Numerical and Experimental Analysis of the Effects of Tar Components on Single Planar Sofc Under High Fuel Utilization

Numerical and Experimental Analysis of the Effects of Tar Components on Single Planar Sofc Under High Fuel Utilization

Degradation Behavior of Solid Oxide Fuel Cells Operated at High Fuel Utilization

High energy conversion efficiency in solid oxide fuel cell (SOFC) systems can be achieved by improving the fuel utilization. However, an operation at high fuel utilizations negatively affects Ni-based cermet anodes; the downstream part of the anode is exposed to high P(O2) atmospheres due to the depletion of fuel and the high steam concentration. At this stage, no reports dealt with the degradation phenomena of the anode under such severe conditions. In this study, we aimed to clarify degradation phenomena at the downstream part of practical cells operated at high fuel utilizations by using bottom cells. Fuel gases simulated severe conditions were supplied to single cells. The change in electrochemical properties was monitored during discharge operation, and the microstructural evolution of the anode was quantitatively analyzed by the focused ion beam–scanning electron microscope (FIB–SEM) technique.

Electrochemical Characteristics of SOFC Anodes Operated at High Fuel Utilization

High energy conversion efficiency in solid oxide fuel cell (SOFC) systems can be achieved by improving the fuel utilization. However, when the SOFCs are operated under high fuel utilization, the downstream part of the anode is exposed to high steam concentration and hence high P(O2) atmospheres, and this has a negative impact on the stability of Ni-based cermet anodes. At this stage, no reports dealt with the degradation phenomena of the anode under such severe conditions. In this study, we aimed to clarify degradation phenomena at the downstream part of practical cells operated at high fuel utilizations by using bottom cells. Fuel gases that simulated severe conditions were supplied to single cells. The change in electrochemical properties was monitored during discharge operation, and the microstructural evolution of the anode was quantitatively analyzed by the FIB–SEM technique.

Degradation Analysis of SOFC Performance for Long-Term Operation with High Fuel Utilization

A national project in Japan for SOFC development has been initiated from FY2020 to FY2024 in NEDO’s project in order to establish the method for degradation diagnosis of SOFC stacks with durability and reliability for 130,000 hours or the method for evaluation of expanding application. Central Research Institute of Electric Power Industry (CRIEPI) has operated three types of SOFC stacks, which are developed by KYOCERA, MORIMUA SOFC TECHNOLOGY, and DENSO and investigated their long-term durability in this project. We have developed the electrode polarization model for SOFC performance which is able to express the SOFC performance such as Nernst loss, cathode overvoltage, anode overvoltage, and internal resistance (IR) loss under various conditions with high prediction accuracy. Each SOFC stack has been operated with high fuel utilization (Uf = 80% and 85%) as a severe condition for high efficiency towards 20,000-hours operation, and degradation factors for SOFC durability has been analyzed by using our electrode polarization model. Analysis results until now showed that dominant degradation factors for both Uf = 80% and 85% were increase of cathode overvoltage or IR loss as same as that of the SOFC stacks in the previous term [1]. Although anode overvoltage was a minor degradation factor, increase rate of anode overvoltage under Uf = 85% was relatively larger than that under Uf = 80%. In addition, thermal property testing will be conducted for each SOFC stack in order to include temperature term into our electrode polarization model. This new model can predict SOFC performance more accurately under various operating conditions. This work has been carried out under the project of the New Energy and Industrial Technology Development Organization (NEDO), Japan as a contract research. We would like to express our gratitude to NEDO, KYOCERA, MORIMUA SOFC TECHNOLOGY, Denso as well as to university partners and National Institute of Advanced Industrial Science and Technology (AIST).Reference[1] A. Ido et al., ECS Transactions, 91(1), 801-808 (2019).

Electrochemical Characteristics of SOFC Anodes Operated at High Fuel Utilization

High energy conversion efficiency in solid oxide fuel cell (SOFC) systems can be achieved by improving the fuel utilization. However, when the SOFCs are operated under high fuel utilization, the downstream part of the anode is exposed to high steam concentration and hence high P(O2) atmosphere. This has a negative impact on the stability of Ni-based cermet anodes. At this stage, no reports dealt with the degradation phenomena of the anode under such severe conditions. In this study, we aimed to clarify degradation phenomena at the downstream part of practical cells operated at high fuel utilizations by using bottom cells. The change in electrochemical properties of single cells was monitored during discharge operation with supplying fuel gases that simulated severe conditions.

Degradation Analysis of SOFC Performance for Long-Term Operation with High Fuel Utilization

In a NEDO’s project from FY2020 to FY2024, Central Research Institute of Electric Power Industry (CRIEPI) has operated three types of SOFC stacks, which are developed by KYOCERA, MORIMURA SOFC TECHNOLOGY (MST), and DENSO in order to clarify the technical issues for high efficiency and toughness of SOFC stacks and establish the method for degradation diagnosis of SOFC stacks. Each SOFC stack has been operated with the high fuel utilization (Uf = 80 and 85%) as a severe condition for high-efficiency and degradation factors for SOFC durability have been analyzed by using the electrode polarization model which we have developed. Analysis results until now showed that dominant degradation factor was increase of cathode overvoltage or IR loss as same as that in the previous term. Moreover, thermal property testing has been conducted to apply the effect of temperature to our polarization model. It was clarified that internal resistance and electrode resistance coefficients can be expressed by Arrhenius equation.

SOFC Anodes Impregnated with Noble Metal Catalyst Nanoparticles for High Fuel Utilization

In order to improve the stability under high fuel utilization, alternative anodes are fabricated with ionic (mixed) conducting GDC (Ce0.9Gd0.1O2) and electronic conducting LST (Sr0.9La0.1TiO3), both of which act as stable ion- and electron-conducting frameworks against reduction-oxidation (redox) cycles, respectively. Noble metal catalyst nanoparticles (Rh, Pt, or Pd) are incorporated via impregnation with GDC on the LST-GDC backbones. The electrochemical characteristics, such as the stability against redox cycling and under high fuel utilization, of SOFC single cells using these anodes are characterized in humidified H2 at 800°C. Moreover, the changes of the noble metal catalyst nanoparticles before/after the high fuel utilization durability tests are analyzed and discussed.

Degradation Analysis of SOFC Performance (1) — Severe Operation with High Fuel Utilization

Central Research Institute of Electric Power Industry (CRIEPI) has operated four types of SOFC stacks, which are developed by Kyocera, NGK SPARK PLUG (NTK), NGK INSULATORS (NGK), and Denso in a NEDO’s project since FY2018 to FY2019 in order to clarify the technical issues for high-efficiency and toughness of SOFC stacks or make a concept for the high-efficient stacks. Each SOFC stack has been operated with the high fuel utilization (Uf = 80%) as a severe condition for high-efficiency for more than 3,000 hours, and degradation factors for SOFC durability have been analyzed by using the electrode polarization model which we have developed. Analysis results until now showed that dominant degradation factor was increase of cathode overvoltage or IR loss as same as that of the SOFC stacks which were modified for improving long-term durability in the previous term.

Oxygen-Electrode-Supported Solid Oxide Cells with High Fuel and Steam Utilization

Oxygen-electrode-supported solid oxide cell (SOC) designs are potentially advantageous for electrolysis and reversible energy-storage applications. In a reversible SOC application, the oxygen produced during electrolysis can be stored and then used during fuel cell operation. In this case, the presence of pure oxygen at the oxygen electrode mitigates the significant gas diffusion limitations encountered with thick oxygen electrode supports when using air. Furthermore, supporting the cell mechanically with the oxygen electrode allows the fuel electrode to be significantly thinner, which alleviates gas diffusion limitations that make it difficult to achieve high steam utilization during electrolysis operation, and high fuel utilization in fuel cell operation. Nevertheless, development of oxygen-electrode-supported cells has been hampered by processing limitations, particularly the difficulty in co-firing oxygen electrode materials with higher melting temperature electrolyte materials to achieve a fully densified electrolyte while retaining satisfactory electrode porosity, surface area, and triple-phase boundary density (TPB), and avoiding undesirable interfacial reactions or interdiffusion. Electrolyte sintering aids can help reach desirable co-firing temperatures. In previous work, sintering aids were used for single-step co-firing temperature of fuel-electrode supported SOCs and resulted in cells with desirable microstructure, free of significant interdiffusion, and relatively good performance, but also with low TPB density and therefore high polarization resistance in the oxygen electrode compared to cells fabricated by a two-step firing process. In this work, we demonstrate successful fabrication of oxygen-electrode-supported SOCs consisting of LSM current collector, LSM-YSZ oxygen-electrode, YSZ electrolyte, and NiO-YSZ fuel electrode, using a reduced-temperature single-step firing process. Microstructural and chemical characteristics of the cell components are studied using scanning electron microscopy and energy dispersive spectroscopy. The electrochemical characteristics are studied using current-voltage measurements and impedance spectroscopy, with both air and pure oxygen at the oxygen electrode and with different H2/H2O contents at the fuel electrode. These cells achieve much better fuel or steam utilization than anode-supported cells due to the relatively thin fuel electrode, and the presence of pure oxygen in the oxygen-electrode support mitigates mass transport limitations. Cells operating in fuel cell mode tested with hydrogen and air produce a maximum power density (Pmax) of 325 mW/cm2 at 800 °C, with a limiting current density of 0.9 A/cm2. Pmax increases to 575 mW/cm2 when using pure oxygen as oxidant, due to the improved gas diffusion and a larger exchange current density in the cathode. LSM-YSZ electrodes of some cells are infiltrated with (Sm0.5Sr0.5)CoO3 (SSC), and enhancement of the electrode in this manner is critical for obtaining high cell power density. Incorporation of SSC nanoparticles increases Pmax to 580 mW/cm2 for a cell operating in fuel cell mode in hydrogen and air, with much smaller initial resistance compare to the non-infiltrated cell. Limiting current is similar to the non-infiltrated cells, suggesting large gas diffusion limitation due to the thick oxygen electrode. Changing the oxidant from air to pure oxygen results in significant improvement in cell performance, with Pmax of 1400 mW/cm2 and no evidence of limiting current up to 6 A/cm2, the highest current measured. Life tests were carried out on SSC-infiltrated cells operating at a direct current of 1 A/cm2 and temperature of 800°C in both fuel and electrolysis modes. Significant degradation was observed, primarily due to coarsening of the infiltrated SSC. Recent results using a different infiltrant, Sr(Ti,Fe)O3, which show better stability, will be presented. Finally, initial results on another method for reducing concentration polarization, via the use of patterned electrode supports made using an extrusion-based 3D-printing process, will be presented.

High Utilization of Humidified Ammonia and Methane in Solid Oxide Fuel Cells: An Experimental Study of Performance and Stability

Wastewater contains high amounts of unused energy in the form of dissolved ammonia, which can easily be converted into gaseous humidified ammonia via membrane distillation, thus providing a potential fuel for solid oxide fuel cells. This study presents comprehensive investigations of the use of humidified ammonia as the primary fuel component in high-fuel utilization conditions. For these investigations, large planar anode- and electrolyte-supported solid oxide single cells were operated at the respective appropriate temperatures, 800°C and 850°C. Fueled with ammonia, both cells exhibited excellent ammonia conversion ( > 99.5%) in addition to excellent performance output and fuel utilization. In 100 h stability tests performed at 80% fuel utilization, the cells exhibited stable performance, despite scanning electron microscopy analyzes revealing partial impairments to the nickel parts of both cells due to the formation and subsequent decomposition of nickel nitride. This study also demonstrates that methane is a perfect additional fuel component for humidified ammonia streams, as steam supports the internal reforming of methane. Alternating and direct current as well as electrochemical impedance measurements with a variety of ammonia/steam/methane/nitrogen fuel mixtures were used to evaluate the performance potential of the cells, and proved their stability over 48 h in highly polarized conditions.

Enhancing Anodic Catalytic Activity at High Fuel Utilization By Infiltration of Ni Nanoparticles

The performance of SOFCs degrades with increasing fuel utilization, since the fuel oxidation reaction faces increased competition for the three-phase-boundary (TPB) sites in the anode from adsorbed water vapor species. Introduction of addition TPB sites by infiltration of Ni nanoparticles into the anode is an effective way to alleviate this problem. Both liquid and vapor phase infiltration of Ni nanoparticles into commercially available Ni/YSZ cermet anodes have been explored. The microstructure of the anode has been examined by µ-CT and FIB/SEM 3-D reconstructions. The cells have been electrochemically characterized by I-V and EIS techniques in a temperature range of 600°C-800°C. The infiltrated Ni nanoparticles have been characterized by image analysis of SEM micrographs from fracture cross-sections, before and after electrochemical testing. The effect of the nanoparticles on cell electrochemical performance as a function of temperature and anode input water vapor content, the stability of the nanoparticles, as well as the thermodynamics of vapor phase infiltration of Ni will be presented.

Enhancing Anodic Catalytic Activity at High Fuel Utilization By Infiltration of Ni Nanoparticles

The performance of SOFCs degrades with increasing fuel utilization, since the fuel oxidation reaction faces increased competition for the triple-phase-boundary (TPB) sites in the anode from adsorbed water vapor species. Introduction of addition TPB sites by infiltration of Ni nanoparticles into the anode is an effective way to alleviate this problem. Both liquid and vapor phase infiltration of Ni nanoparticles into commercially available Ni/YSZ cermet anodes have been explored. The microstructure of the anode before infiltration has been examined by FIB/SEM 3-D reconstruction to quantify the TPB length. Infiltrated and un-infiltrated cells have been electrochemically characterized by I-V and electrochemical impedance spectroscopy (EIS) techniques in a temperature range of 600°C-800°C at varying water vapor partials pressures. The infiltrated Ni nanoparticles have been characterized by image analysis of SEM micrographs from fracture cross-sections, before and after electrochemical testing. The effects of the nanoparticles on cell electrochemical performance as a function of temperature and anode input water vapor content, the stability of the nanoparticles, as well as the thermodynamics of vapor phase infiltration of Ni are presented.

Alternative Ni-Impregnated Mixed Ionic-Electronic Conducting Anode for SOFC Operation at High Fuel Utilization

Redox-stable anodes are developed for zirconia-based electrolyte-supported SOFCs in order to improve the durability against fuel supply interruption and for higher fuel utilization, as an alternative to the conventional Ni-YSZ cermet. GDC (Ce0.9Gd0.1O2) is utilized as a mixed ionic-electronic conductor (MIEC), and combined with LST (Sr0.9La0.1TiO3) as an electronic conductor. Ni catalyst nanoparticles are incorporated via impregnation. The electrochemical characteristics of SOFC single cells using these anode materials are investigated in humidified H2 at 800°C. The stability against redox cycling and under high fuel utilization is analyzed and discussed. Ni-impregnated anodes with dispersed Ni catalyst nanoparticles on conducting oxide LST-GDC backbones exhibit lower anode non-ohmic overvoltage, and improve I-V performance. These anodes also show better redox stability compared to conventional anodes because of the isolation of Ni catalysts, preventing their agglomeration. Moreover, the co-impregnation of Ni catalysts and GDC nanoparticles further improves electrochemical characteristics due to a decrease in anode ohmic (IR) loss and non-ohmic overvoltage. This anode shows comparable I-V performance to conventional anodes for typical humidified hydrogen fuels, and is a promising redox-stable alternative for application at high fuel utilization.

Processing of Solid Oxide Fuel Cell Anodes for Enhanced Intermediate Temperature Catalytic Activity at High Fuel Utilization

Despite many years of research on solid oxide fuel cell (SOFC) electrolyte and electrode materials, the conventional anode formulation of nickel and yttria-stabilized zirconia (Ni-YSZ) cermet has not changed much, which highlights the effectiveness of nickel as a catalyst for the fuel oxidation reaction occurring at the triple phase boundaries (TPBs). However, most studies on single cells report results only under fuel rich atmospheric conditions in the anode, which is not representative of how fuel cell stacks are operated. SOFC stacks can easily reach fuel utilizations of 85%. High fuel utilization conditions result in lower hydrogen and higher water vapor partial pressures and an increase in anodic polarization, reducing the electrochemical performance of SOFC anodes. It is postulated that fuel oxidation reaction kinetics can be improved by increasing the TPB length via the deposition of sub-micron sized nickel particles in the cermet anode. This research analyzes the effects of various such nickel infiltration and deposition processes on the performance of anode-supported SOFC over a wide range of hydrogen and water vapor partial pressures at intermediate temperatures (600-800 Celsius). Infiltration and stability or coarsening of sub-micron nickel particles in Ni-YSZ cermets is a well-documented problem. So the impacts of various processing techniques derived from literature and developed internally were examined through long-term or accelerated testing. The cell performance was characterized using I-V and EIS techniques. The cell microstructure was characterized using SEM and image analysis techniques. Out-of-cell measurements on electrodes were also conducted to model its performance. The results were then all combined to identify the role of the sub-micron size nickel particles and the deposition process in enhancing the anode performance at high fuel utilization.

Oxidation-Induced Degradation of SOFC Ni Anodes at High Fuel Utilizations

In the downstream of SOFC systems, higher oxygen partial pressure may cause oxidation-induced Ni anode degradation, associated with the formation of NiO and/or Ni(OH)2. Under experimental conditions for Ni anodes exposing to high oxygen partial pressures, conduction pathways could be destroyed resulting in cell performance degradation. This study is therefore focusing on the changes in microstructure and cell performance at high fuel utilizations. The objective of this study is to derive guidelines and limit for high fuel utilization operation towards higher power generation efficiency. The cells used consist of Ni-based anode with scandia-stabilized zirconia (ScSZ), ScSZ electrolyte, and lanthanum-strontium-manganite(LSM)-ScSZ cathode. Reference electrode made from Pt paste was attached to the cathode side to separate anode and cathode overvoltages. The electrochemical properties of the cells were measured at 800oC by feeding humidified fuel, H2 (20 cc/min)-H2O (80 cc/min)-N2 (100 cc/min), to the anode, and air (150 cc/min) to the cathode. As carrier gas, N2was fed to lower the humidifier temperature for simulating higher humidity conditions. The condition for Ni oxidation was estimated by thermochemical equilibrium calculation software, HSC Chemistry. Performance stability was examined at a constant cell voltage for a constant partial oxygen pressure at the anode for 100h. I-V characteristics before and after the 100h test were measured for confirming performance changes. Focused-ion-beam-scanning electron microscopy (FIB-SEM) observation was conducted for the anode, reconstructed by the brightness differences of BSE images, and microstructures were observed in detail by STEM. Anode voltage threshold was derived from the oxygen partial pressure at the boundary where both Ni and NiO coexist in the phase stability diagram as shown in Fig. 1. The boundary shifted to higher oxygen partial pressure as temperature became higher. The threshold voltage was 0.701 V at 800oC. The performance stability was examined for 100h at the constant cell voltage of 0.6 V, showing stable performance. The resistances did not change before and after the test, and no microstructural change of the anode was observed. The performance stability was also examined at the constant cell voltage of 0.5 V in order to set the experimental condition near the threshold. Anode voltage started to vibrate during the test when anode voltage approached about 0.7 V. The anode-side ohmic and nonohmic resistances increased during the vibration, suggesting Ni oxidation. But the electrochemical measurements revealed no change in anode resistances, indicating that the oxidation of Ni was limited to their surfaces during power generation. The number of isolated Ni particles was still small judging from FIB-SEM reconstruction images, maintaining their conduction pathways. The presence of NiO was also not confirmed by STEM mapping images. The performance stability was also examined at the constant cell voltage of 0.65 V when further humidified fuel, H2 (5 cc/min)-H2O (95 cc/min)-N2 (100 cc/min), was supplied. The performance was stable as shown in Fig. 2 and it was verified that power generation was possible even under such a condition if the anode voltage was kept sufficiently higher than the threshold. Figure 1

High Fuel Utilization in Solid Oxide Fuel Cells: Experimental Characterization and Data Analysis with Continuous Wavelet Transform

The on-line diagnostics of Solid Oxide Fuel Cells (SOFCs) is a critical tool to achieve optimal performance and extend the lifetime. The Continuous Wavelet Transform (CWT) methodology was applied to the SOFC voltage signal to detect signatures that reveal the presence of a fault in the cell/stack. The selected fault was anode re-oxidation caused by high Fuel Utilization (FU) (higher then nominal). To experimentally emulate the high FU faults, a standard test procedure was developed, which was used to characterize a μ-CHP system at high FU operation. To complete the analysis, data collected on Single Cells were exploited too. The CWT was applied to the voltage signal for each FU level to verify the qualitative difference (signature) between the signals at different FU's within the same tests as well as the correspondence between the same conditions over different tests. A statistical study was performed to quantify the observed differences and to determine the correspondence between CWT coefficients and operating conditions. The approach proves to be suitable to diagnose high FU in SOFC, showing a successful detection rate above 76%. The results show the good potential of using the CWT methodology as diagnostic tools for SOFCs from cell to stack level.

High Fuel Utilization Operation of Solid Oxide Fuel Cells - a Modeling Study

High fuel utilization (Uf ) is desirable in fuel cells as it has a direct impact on efficiency – the higher the Uf the higher the fuel cell efficiency. However, the Ni phase in Ni based SOFC anodes is susceptible to oxidation to NiO when the fuel cell is operated at high fuel utilizations. This phase transition is problematic because a) NiO is inactive to the fuel oxidation reactions, and b) the larger specific volume of NiO results in damage to the anode microstructure if this Ni to NiO transition is reversed and repeated a few times. The second problem listed above is known as Ni redox cycling. Thus, there are conflicting objectives where the designer/operator of a SOFC system desires a high fuel utilization Uf while avoiding Ni oxidation. Ideally, one would like to identify a safe operating zone where the fuel utilization Uf can be maintained in a high enough range without risking Ni oxidation. In this presentation we will use simulation results from a detailed multiphysics model to outline some of the challenges of operating a SOFC at high fuel utilization and attempt to identify the above mentioned “safe operating zone”. The multiphysics model used in this work is for a planar anode supported geometry and includes a fully coupled description of the flow and mass transfer in the air/fuel channels and the electrodes, and the current/potential distribution in the electrodes. One of the mechanisms for Ni oxidation is through reaction (1). Ni + H2O ↔ NiO + H2 (1) The favourability of reaction (1) at a particular location in the anode is dictated by the thermodynamics of the reaction and the local fuel composition. As the model is able to predict the fuel side composition as a function of position, the output of the model can be used to predict whether there is a risk of Ni oxidation in the anode at any given set of operating conditions. We will present results from a parametric study over a wide range of different temperatures, cell voltage, as well as inlet fuel flow-rates and composition. An example set of results is given in Fig 1 where the inlet flow-rate is varied for a given cell voltage and temperature. The Ni oxidation reaction is thermodynamically favourable in the latter section of the anode where the partial pressure ratio PH2O/PH2 exceeds the threshold value given by the thermodynamics of the reaction and shown as a dot-dash line in the bottom plot. According to Fig 1, the threshold Uf for these conditions is ~99%. This is a much higher value than the typical 80-85% used in most SOFC designs. Another noteworthy point is that at this high fuel utlization, the last 1/3 of the cell is producing a very small fraction of the total current. Figure 1

Oxidation-Induced Degradation of SOFC Ni Anodes at High Fuel Utilizations

In the downstream of SOFC systems, higher oxygen partial pressure may cause oxidation-induced Ni anode degradation, associated with the formation of NiO and/or Ni(OH)2. Under experimental conditions for Ni anodes exposing to high oxygen partial pressures, conduction pathways could be destroyed resulting in cell performance degradation. This study is therefore focusing on the changes in microstructure and cell performance at high fuel utilizations. The objective of this study is to derive guidelines for high fuel utilization operation towards higher power generation efficiency. The cells used consist of Ni-based anode with scandia-stabilized zirconia (ScSZ), ScSZ electrolyte, and lanthanum-strontium-manganite(LSM)-ScSZ cathode. The electrochemical properties of the cells were measured at 800oC by feeding humidified fuel, H2 (20cc/min)-H2O (80cc/min), to the anode, and air (150cc/min) to the cathode. The condition for Ni oxidation was estimated by thermochemical equilibrium calculation software, HSC Chemistry. Performance stability was examined at a constant cell voltage for a constant partial oxygen pressure at the anode for 100h. I-V characteristics before and after the 100h test were measured and focused-ion-beam-scanning electron microscopy (FIB-SEM), and SEM-energy dispersive X-ray spectroscopy (SEM-EDX) analysis were conducted for microstructural characterizations. Anode voltage threshold was derived from the oxygen partial pressure at the boundary where both Ni and NiO coexist in the phase stability diagram. The threshold voltage was 0.701 V at 800oC. The performance stability was examined for 100h at the constant cell voltage of 0.6 V, showing stable performance as shown in Image1. The resistances did not change before and after the test, and no microstructural change of the anode was observed. The performance stability was also examined at the constant cell voltage of 0.5 V in order to set the experimental condition near the threshold. As shown in Image 2, anode voltage started to vibrate during the test. The anode-side ohmic resistance increased during the vibration, suggesting Ni oxidation. But the electrochemical measurements revealed no change in anode resistances, indicating that the oxidation of Ni is limited to their surfaces. While slight agglomeration of Ni particles was observed, the number of isolated Ni particles was still small judging from FIB-SEM reconstruction images, maintaining their conduction pathways. Figure 1