CHAPTER 2. Electrolyte Materials for Solid Oxide Fuel Cells (SOFCs)

Solid oxide fuel cells (SOFCs) are emerging as energy conversion devices for large-scale electrical power generation because of their high energy conversion efficiency, excellent ability to minimize air pollution, and high fuel flexibility. In this context, this critical review has focussed on the recent advancements in developing a suitable electrolyte for SOFCs which has been required for the commercialization of SOFC technology after emphasizing the literature from the prior studies. In particular, the significant developments in the field of solid oxide electrolytes for SOFCs, particularly zirconia- and ceria-based electrolytes, have been highlighted as important advancements toward the production of sustainable and clean energy. It has been reported that among various electrolyte materials, zirconia-based electrolytes have the potential to be utilized as the electrolyte in SOFC because of their high thermal stability, non-reducing nature, and high mechanical strength, along with acceptable oxygen ion conductivity. However, some studies have proved that the zirconia-based electrolytes are not suitable for low and intermediate-temperature working conditions because of their poor ionic conductivity to below 850°C. On the other hand, ceria-based electrolytes are being investigated at a rapid pace as electrolytes for intermediate and low-temperature SOFCs due to their higher oxygen ion conductivity with good electrode compatibility, especially at lower temperatures than stabilized zirconia. In addition, the most emerging advancements in electrolyte materials have demonstrated that the intermediate temperature SOFCs as next-generation energy conversion technology have great potential for innumerable prospective applications.

Solid oxide fuel cell (SOFC) is well known as power and heat generation device which converts chemical energy directly from fuel into electricity. SOFC operate at high temperature becomes obstacle for SOFC which reducing ionic conductivity material of current electrolyte, reduce lifetime of cell components, high fabrication cost, limited durability and performance issues. This introduce to environment pollution and decrease the SOFC lifetime. The fabrication of durability and stability composite cathode are comprised from mixing of perovskite La0.6Sr0.4CO0.2Fe0.8 (LSCF) powders with nanoscale ionically conducting ceramic electrolyte materials, SDC-carbonate (SDCc) was overcome this problems. Powder preparation and composite cathode fabrication must consider which as main factors in the development of durability and stability of LSCF-SDCc composite cathode. Powders must in nanoscale to enhance the conductivity and decrease the interfacial polarization resistance and the composite cathode should in nanoporous morphology for achieve high power density over than 500 h and remarkable durability. Calcination also plays in important role and its operations will effects to the SOFC durability and performance. The necessary to prolong the lifetime and increase the SOFC performance has lead to development of durability and stability of SOFC. This paper reviews the durability and stability of the composite cathode and focus on the challenges in material technology.

High efficiency at low operating temperatures is essential for present and future solid oxide fuel cell (SOFC) applications. This requires the selection of materials with compatible physical and chemical properties for the different SOFC components. La0.58Sr0.4Co0.2Fe0.8O3-δ (LSCF) has been shown to be a highly performing cathode material [1] while 8 mol% Y2O3-doped ZrO2 (YDZ) is still the state-of-the-art electrolyte material. However, it has been shown that an insulating SrZrO3 (SZO) interlayer forms between LSCF and YDZ during temperature treatment exceeding 900 °C, drastically reducing SOFC performance [2]. One idea of overcoming this problem is the introduction of a chemically compatible electrolyte material, like Gd2O3-doped ceria (GDC), at the interface between LSCF and YDZ. GDC acts as a diffusion barrier and prevents SZO formation. Goal of the present study is a detailed microstructural characterization of the LSCF-GDC-YDZ interfaces by analytical transmission electron microscopy (TEM). We focus particularly on the effect of the GDC sintering temperature on SZO formation. Symmetric-cell samples were fabricated by screen-printing a 5 µm thick Gd0.2Ce0.8O2-δ(GDC) layer on both sides of YDZ substrates. GDC sintering was performed at 1100°C, 1250°C or 1400°C for 3 hours, followed by screen-printing a LSCF layer on top of the GDC. A sintering temperature of 1080°C was used for all LSCF cathode layers. The samples were studied after GDC sintering (i.e. before LSCF deposition), after LSCF sintering and after electrical characterization at application-relevant temperatures (500 – 800 °C) [3]. The element distribution at the LSCF-GDC and GDC-YSZ interfaces, as well as the chemical composition within the layers themselves, were determined by energy dispersive X-ray spectroscopy (EDXS). Imaging was performed by TEM and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), complemented by selected area electron diffraction (SAED) for crystal structure analysis. The study gives new insights into material interactions at the interfaces. Figure 1 shows an overview HAADF-STEM image of the LSCF-GDC-YSZ interface region in the sample with 1100 °C GDC sintering temperature, after LSCF deposition and electrical characteri­zation. It can be clearly seen that the GDC layer has a thickness of approximately 5 µm and partially retains some porosity after fabrication. Moreover, an inter-diffusion region between YDZ and GDZ is revealed by HAADF-STEM (arrows in Figure 1), suggesting chemical composition variation. Hence, detailed elemental distributions of the YDZ-GDC interface were determined using EDXS mapping. Figure 2 shows (a) the HAADF-STEM image of the interface region and (b) the Zr, (c) the Gd and (d) the Sr elemental distributions in the YDZ-GDZ interface region. It is of particular interest that a substantial Sr concentration (Fig. 2(d)) is observed between GDC and YDZ, despite the GDC diffusion barrier layer. This indicates Sr transport from the LSCF cathode through the porous GDC layer. The same region also contains Zr (Fig. 2(b)) which indicates SZO formation. In addition, a second phase containing up to 30 at% Gd (Fig. 2(c)) is observed between SZO and YDZ. Crystal structure determination by SAED is currently in progress. We will also demonstrate that the GDC sintering temperature strongly influences SZO interlayer formation. While the SZO layer is continuous for 1100 °C GDC sintering temperature, sintering at 1400 °C strongly reduces SZO layer formation. In contrast, the Gd-rich layer between GDC and YDZ increases in thickness with increasing GDC sintering temperature. We will discuss the origin of the phase formation between GDC and YDZ and the effects of this microstructural development on electrochemical SOFC performance.

Solid oxide fuel cells (SOFCs) are an attractive power generation due to their high energy efficiency and possibility of waste heat recovery with operating at high temperature. SOFCs were fabricated by commonly three steps with pre-sintering of anode, dip-coating electrolyte and screen printing cathode materials cause of different shrinkages. It is hard to commercialize with these high manufacturing costs of multi-step process. SOFCs have been focused on low-cost materials and process for commercializing with high efficiency and performance. 8 mol % yttria-stabilized zirconia (8 YSZ) has been considered as one of the most conventional material for SOFC electrolyte. The 8 YSZ electrolytes doped with Cu contents which used as a sintering aid leads to densify the electrolyte and increase the shrinkage and performance. So we attempted to fabricate with one step co-firing process with 8 YSZ electrolytes at low temperature as 1250 oC. First of all, we found addition of trace Cu is effective to improve the sinterability of the YSZ electrolytes. The YSZ electrolyte with 100ppm CuO shows better sinterability and ionic conductivity than the pure YSZ electrolyte. The YSZ electrolyte with 100ppm CuO was densified below 1300 oC and the increased oxygen vacancy concentration leads to have higher ionic conductivity. Furthermore, the performances of the single-layer YSZ cells with 100ppm CuO were about 1.5 times higher(0.51 W cm2) than the cell with the pure YSZ electrolyte (0.36 W cm2) at 800 oC. The GDC/YSZ (100 ppm CuO) bi-layer cells also showed higher cell performance at 700 oC which shows similar tendency to the single-layer YSZ cell tests. These show that addition of 100 ppm CuO into the YSZ electrolyte is greatly effective for improvement of SOFC performance with energy saving during sintering process. According to these results of enhancing the sinterability and ionic conductivity, co-firing process was developed by addition of 5000 ppm Cu to the YSZ electrolytes and anodes at 1250 oC which is seriously lower than conventional multi-step fabrications with the sintering temperature above 1500 oC. The sinterability of the YSZ electrolyte was significantly improved 1250 oC by the 5000 ppm Cu. The co-fired cells with one- and two-step exhibited reasonable OCV value of 1.016 V and 1.011 V, respectively. Maximum power density of two-step cell with 0.71 W cm-2 at 850 oC which was similar value compared to conventional cell but the one-step cell has low maximum power density with 0.14 W cm-2. Above 1200 oC as sintering temperatures, the LSM/YSZ cathode produces insulating phases at the interfaces and low porosity makes cathode polarization. As a result, the cathode should be improved to be stable at 1250 oC, or the co-firing temperatures should be reduced below 1200 oC.

A solid oxide fuel cell (SOFC) is expected to be applied to the distributed energy systems because of its high thermal efficiency and exhaust gas utilization. The exhaust heat from the SOFC can be transferred to the electric power by a gas turbine, and the high efficiency power generation can be achieved by constructing the SOFC and gas turbine hybrid system. In this study, we analyze the local processes in the electrodes and electrolyte of unit SOFC taking into account the heat conduction, mass diffusion, electrode reactions and the transport of electron and oxygen ion. The temperature and concentration distributions perpendicular to the electrolyte membrane are shown. The effects of the operating conditions on the cell performance are also shown. Furthermore, the entropy generation and exergy loss of each process in the electrodes and electrolyte are analyzed and the reason for generating the exergy loss in the SOFC is clarified. It is noted that two electrode reactions are responsible for the major exergy loss.

This research presents a thorough evaluation of an integrated system comprising a Solid Oxide Fuel Cell (SOFC), Organic Rankine Cycle (ORC), and Absorption Chiller (AC). The study employs ASPEN PLUS V10 to assess the system's energy, exergy, economic, and environmental performance. The Integrated SOFC-ORC-AC system offers trigeneration capabilities, generating electricity, cooling, and heating from a single fuel source. It demonstrates potential as an efficient and environmentally friendly energy generation method. The system can be fuelled by steam-reformed natural gas or renewable fuels like biogas or syngas. The SOFC, employing a solid electrolyte, facilitates an electrochemical reaction with hydrogen, producing electricity and heat. The exhaust gas further powers the ORC and AC units. The study builds a mathematical model, assuming steady-state, isothermal, and chemically equilibrated conditions. The SOFC and ORC simulations utilize the Peng Robinson model, while the LiBr absorption chiller employs the ELECNRTL property method for fluid thermodynamic properties. The SOFC electrical model was validated against real-world data, ensuring accuracy. The study also tested a single-effect LiBr absorption chiller, comparing results with experimental data. The operating pressure's effect on SOFC performance was evaluated, demonstrating reduced voltage losses and increased cell voltage and power density at higher pressures. Operating temperature elevation enhanced electrochemical reactions, resulting in higher cell voltage and power density, despite increased voltage losses. Augmenting the fuel utilization factor reduced voltage losses, leading to increased cell voltage and power density. The SOFC-ORC system efficiency peaked at 58.9% at the highest operating pressure, influenced by factors like compressor consumption and high fuel flow rate. Redirecting exhaust gas for waste heat recovery produced hot water, influencing the Coefficient of Performance (COP) of the LiBr absorption chiller. Mass flow rate had a smaller impact compared to hot water temperature. Exergy analysis revealed the SOFC's high efficiency (83.92%), while the steam turbine and LiBr absorption chiller demonstrated lower exergy efficiencies (70% and 32.6% respectively). Cost analysis indicated that the SOFC power plant was the most significant investment at 130,715 $, highlighting the long-term benefits of the integrated system in terms of high efficiency, low emissions, and fuel flexibility. This research offers a comprehensive assessment of the integrated SOFC-ORC-AC system, shedding light on its potential as an efficient and environmentally friendly energy generation solution. The study's findings contribute to the advancement of sustainable energy technologies, emphasizing the importance of trigeneration systems for future energy landscapes.

Introduction For many potential mobile robot applications, current power supply technology (such as the lithium ion battery technology) is in fact a key limiting factor due to its low energy density. Solid oxide fuel cells (SOFCs) are a promising alternative and can overcome limitations of current battery technology. This is because SOFCs can, in principle, use not only hydrogen but also hydrocarbon fuels which have high energy density such as gasoline and so on.Ni-YSZ cermet is the most promising anode material for SOFCs because of its low cost, chemical stability, and closely matched thermalexpansion coefficients with the dense YSZ electrolyte. Ni is an excellent catalyst for hydrocarbon fuel reforming and cracking, which leads to rapid carbon deposition inside the SOFC anode and causes coking and deactivation1). In principle, the carbon deposition on Ni can be depressed by adding steam in the hydrocarbon fuel2,3), whereas few detailed investigations of SOFCs that operate on gasoline fuel have been reported.In this work, we studied the effect of steam content and demonstrated the direct operation of SOFCs by octane which is a main ingredient of gasoline. In the case of the Ni-YSZ anode, the operation temperature must be high in order to avoid sulfur poisoning4 -6 ), because most commercially available fuels contain sulfur. Therefore, in this study, the SOFCs operation was conducted at high temperature of 950 oC. Experimental Surface-polished YSZ pellets of 2 mm in thickness were used for the electrolyte. Pt cathode electrode and Pt reference electrode were prepared on the surface and side of the pellets, respectively, by firing Pt paste at 1300 oC. Then, NiO powder and GDC powder (NiO/YSZ=6/4 (wt)) were thoroughly mixed in a vacuum using the planetary ball mill (200 rpm, 12 h), and made into NiO-YSZ pastes using ethyl-cellulose and organic solvent. The NiO-YSZ paste was screen-printed in diameter of 8 mm on the YSZ pellets and sintered at 1200 oC for 1 h.Octane and steam were supplied to the SOFC cell by bubbling carrier gas (He) at a constant rate. Octane bubbler temperature was 30 oC, and Steam/Carbon ratio (S/C) were controlled by changing the temperature of the water bubbler from 40 to 70 oC. Electrochemical properties (open circuit voltage, anode electrode resistance, I - V and I - P characteristics, stable power generation property) were measured over a temperature range of 950-700 oC. During the constant current generation, exhaust gas was analyzed by gas chromatography. Results and Discussion Figure 1 shows I - V and I - P characteristics for the cells at 950 oC in wet octane. Maximum power density of 35 mW/cm2 was obtained by S/C of 8.9. The power density decreased with S/C quickly.Figure 2 shows the time course of cell voltage at a drawn current density of 5.2 mA/cm2 for S/C of 3.5. The cell voltage was stable. That is, carbon deposit does not arise even in the cell of the Ni-YSZ anode in low S/C of 3.5 when the operation temperature is high of 950 oC. References 1) S. Mclntosh and R.J. Gorte, “Direct Hydrocarbon Solid Oxide Fuel Cells,” Chemical Reviews, 104 (2004) 4845- 4865.2) K. Sasaki and Y. Teraoka, “Equilibria in Fuel Cell Gases: I. Equilibrium Compositions and Reforming Conditions,” Journal of Electrochemical Society, 850 (7) (2003) A878-A884.3) K. Sasaki and Y. Teraoka, “Equilibria in Fuel Cell Gases: II. The C-H-O Ternary Diagrams,” Journal of Electrochemical Society, 850 (7) (2003) A885-A888.4) K. Sasaki, K. Susuki, A. Iyoshi, M. Uchimura, N. Imamura, H. Kusaba, Y. Teraoka, H. Fuchino, K. Tsujimoto, Y. Uchida, N. Jingo, “H2S Poisoning of Solid Oxide Fuel Cells,” J. Electrochem. Soc., 153 (2006) A2023–A2029.5) P. Lohsoontorn, D.J.L. Brett, N.P. Brandon, “Thermodynamic predictions of the impact of fuel composition on the propensity of sulphur to interact with Ni and ceria-based anodes for solid oxide fuel cells,” Journal of Power Sources, 175 (2008) 60–67.6) H. Kishimoto, Y-P Xiong, K. Yamaji, T. Horita, N. Sakai, M.E. Brito, H. Yokokawa, “Stability of Ni base anode for direct hydrocarbon SOFCs, ” Journal Chemical Engineering of Japan, 40 (13) (2007) 1178-1182.

Integrated gasification fuel cell (IGFC) systems combining coal gasification and solid oxide fuel cells (SOFC) are promising for highly efficient and environmentally friendly utilization of coal for power production. Most IGFC system analyses performed to-date have used nondimensional thermodynamic SOFC models that do not resolve the intrinsic constraints of SOFC operation. In this work a quasi-two-dimensional (2D) finite volume model for planar SOFC is developed and verified using literature data. Special attention is paid to making the model capable of supporting recent SOFC technology improvements, including the use of anode-supported configurations, metallic interconnects, and reduced polarization losses. Activation polarization parameters previously used for high temperature electrolyte-supported SOFC result in cell performance that is much poorer than that observed for modern intermediate temperature anode-supported configurations; thus, a sensitivity analysis was conducted to identify appropriate parameters for modern SOFC modeling. Model results are shown for SOFC operation on humidified H2 and CH4 containing syngas, under coflow and counterflow configurations; detailed internal profiles of species mole fractions, temperature, current density, and electrochemical performance are obtained. The effects of performance, fuel composition, and flow configuration of SOFC performance and thermal profiles are evaluated, and the implications of these results for system design and analysis are discussed. The model can be implemented not only as a stand-alone SOFC analysis tool, but also a subroutine that can communicate and cooperate with chemical flow sheet software seamlessly for convenient IGFC system analysis.

Integrated gasification fuel cell (IGFC) systems combining coal gasification and solid oxide fuel cells (SOFC) are promising for highly efficient and environmentally friendly utilization of coal for energy production. Most IGFC system analyses performed to date have used non-dimensional thermodynamic SOFC models that do not resolve the intrinsic constraints of SOFC operation. In this work, a one-dimensional finite volume model for planar SOFC is developed and verified using literature data. Special attention is paid to making the model capable of supporting recent SOFC technology improvements, including the use of anode-supported configurations, metallic interconnects, and reduced polarization losses. Results are presented for SOFC operation on humidified hydrogen and methane-containing syngas, under co-flow and counter-flow configurations; detailed internal profiles of species mole fractions, temperature, current density and electrochemical performance are obtained. The effects of performance, fuel composition and flow configuration on SOFC performance and thermal profiles are evaluated, and the implications of these results for system design and analysis are discussed.

Influence of carbon deposition on the anode performance of solid oxide fuel cells

Solid oxide fuel cells (SOFCs) are being extensively researched for clean power generation from coal-derived syngas. Some of the contaminants in syngas such as phosphine (PH3) may interact with the SOFC anode material, and degrade its electrochemical performance and material properties. In this paper, a modified Sagnac interferometry method is utilized to monitor the anode surface transient temperature as a function of applied current densities under hydrogen and simulated coal syngas. Moreover, the poisoning effects of PH3 contaminant on the SOFCs performance are investigated in dry and moist conditions. The experimental results indicate that the Ni-cermet-based SOFC anode is more susceptible to degradation due to PH3 in the presence of steam than under dry conditions. These experiments are valuable for the validation and the development of SOFC electrochemical models, and understanding the anode-contaminant interaction.

The use of Solid Oxide Fuel Cells (SOFCs) continues to spread in Japan. This paper clarifies the simple linear relation between their gas consumption and their generated electric power. This means that the total amount of gas consumption only depends on the total electric power, even if plural SOFCs operate in parallel. The total gas amount doesn't depend on SOFC power distribution. To realize parallel SOFC operation, this paper presents a new current transformer (CT) circuit that realizes equal power distribution. This paper also proposes a small-scale Community Energy Management System (CEMS), where three houses share two SOFCs and one battery cell. The electric consumption of each house is hidden from the other houses to protect privacy in the proposed system. To evaluate our proposed system, a prototype CEMS was implemented, where the SOFCs and a battery cell were simulated by software. Finally, we introduced a prototype CEMS into three actual houses and conducted a social experiment that shows that providing low-priced electricity generated by a SOFC changes user awareness of energy usage and promotes energy reduction.

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.

Proton-conducting barium stannate for high-temperature purposes: A brief review

The formation of insulating phases at the interface between cathode and electrolyte materials during solid oxide fuel cell (SOFC) operation leads to deleterious effects on SOFC properties. We report in situ impedance spectroscopy measurements of the degradation process of porous La0.65Sr0.35MnO3/Zr0.84Y0.16O1.92 electrodes submitted to annealing temperatures above normal SOFC operating temperature, which is around 1173 K, simulating long-term SOFC operation. To measure the remaining effects after high-temperature annealing, impedance diagrams were plotted during the entire heating process up to the annealing temperatures, and during the complete cooling process. These diagrams were also plotted during the aging periods to follow the overall degradation. It was possible to detect and quantify electrical and electrochemical degradations, which are related to the formation of insulating phases at the electrode/electrolyte interface. Samples were annealed for about 200 h in air at temperatures between 1373 an...