Can crystalline phases be self-healing sealants for solid oxide fuel cells?

The poor thermodynamic and thermal stability of self-healing glass sealants restrict their applications in planar solid oxide fuel cells (SOFCs). In this paper, a calcium borate crystalline is prepared by melting-quench and a subsequent crystallization. The crystalline phases include CaB₄O₇ and CaB₂O₄. The in situ observation reveals that the micro-indentation on the surface of such crystalline can be healed when heating from room temperature to 840 °C at a heating rate of 40 °C min⁻¹. Combining with the improved thermal stability, the crystalline sealant with desired self-healing property, at the operational temperature of SOFCs (e.g., 700-900 °C), provides additional solution for the sealing challenge of SOFCs.     

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

In this paper, a novel process for the production of pure hydrogen from natural gas based on the integration of solid oxide fuel cells (SOFCs) and solid oxide electrolyzer cells (SOECs) is presented. In this configuration, the SOFC is fed by natural gas and provides electricity and heat to the SOEC, which carries out the separation of steam into hydrogen and oxygen. Depending on the system layout considered, the oxygen available at the SOEC anode outlet can be either mixed with the SOFC cathode stream in order to improve the SOFC performance or regarded as a co-product. Two configurations of the cell stack are studied. The first consists of a stack with the same number of SOFCs and SOECs working at the same current density. In this case, since in typical operating conditions the voltage delivered by the SOFC is lower than the one required by the SOEC, the required additional power is supplied by means of an electric grid connection. In the second case, the electricity balance is compensated by providing additional SOFCs to the stack, which are fed by a supplementary natural gas feed. Simulations carried out with Aspen Plus show that pure hydrogen can be produced with a natural gas to hydrogen LHV-efficiency that is about twice the value of a typical water electrolyzer and comparable to that of medium-scale reformers.     

Synthesis and characterization of NiO/GDC–GDC dual nano-composite powders for high-performance methane fueled solid oxide fuel cells

GDC (gadolinium-doped ceria) is well known as a high oxygen ionic conductor and is a catalyst for the electrochemical reaction with methane fuel leading to the oxidation of deposited carbon that can clog the pores of the anode and break the microstructure of the anode. NiO/GDC–GDC dual nano-composite powders were synthesized by the Pechini process, which were used as an AFL (anode functional layer) or anode substrates along with a GDC electrolyte and LSCF–GDC cathode. The anodes, AFL, and electrolyte were fabricated by a tape-casting/lamination/co-firing. NiO–GDC anode and NiO/GDC–GDC anode-supported unit cells were evaluated in terms of their power density and durability. As a result, the NiO/GDC–GDC dual nano-composite demonstrated an improved power density from 0.4 W/cm² to 0.56 W/cm² with H₂ fuel/air and from 0.3 W/cm² to 0.56 W/cm² with CH₄ fuel/air at 650 °C. In addition, it could be operated for over 500 h without any degradation with CH₄ fuel.     

A new nickel–ceria composite for direct-methane solid oxide fuel cells

Various Ni–La_xCe_1−xO_y composites were synthesized and their catalytic activity, catalytic stability and carbon deposition properties for steam reforming of methane were investigated. Among the catalysts, Ni–La_0.1Ce_0.9O_y showed the highest catalytic performance and also the best coking resistance. The Ni–La_xCe_1−xO_y catalysts with a higher Ni content were further sintered at 1400 °C and investigated as anodes of solid oxide fuel cells for operating on methane fuel. The Ni–La_0.1Ce_0.9O_y anode presented the best catalytic activity and coking resistance in the various Ni–La_xCe_1−xO_y catalysts with different ceria contents. In addition, the Ni–La_0.1Ce_0.9O_y also showed improved coking resistance over a Ni–SDC cermet anode due to its improved surface acidity. A fuel cell with a Ni–La_0.1Ce_0.9O_y anode and a catalyst yielded a peak power density of 850 mW cm⁻² at 650 °C while operating on a CH₄–H₂O gas mixture, which was only slightly lower than that obtained while operating on hydrogen fuel. No obvious carbon deposition or nickel aggregation was observed on the Ni–La_0.1Ce_0.9O_y anode after the operation on methane. Such remarkable performances suggest that nickel and La-doped CeO₂ composites are attractive anodes for direct hydrocarbon SOFCs and might also be used as catalysts for the steam reforming of hydrocarbons.     

Performance analysis for the part-load operation of a solid oxide fuel cell–micro gas turbine hybrid system

In this paper, the performance evaluation of a solid oxide fuel cell (SOFC)–micro gas turbine (MGT) hybrid power generation system under the part-load operation was studied numerically. The present analysis code includes distributed parameters model of the cell stack module. The conversions of chemical species for electrochemical process and fuel reformation process are considered. Besides the temperature distributions of the working fluids and each solid part of cell module by accounting heat generation and heat transfers, are taken into calculation. Including all of them, comprehensive energy balance in the cell stack module is calculated. The variable MGT rotational speed operation scheme is adopted for the part-load operation. It will be made evident that the power generation efficiency of the hybrid system decreases together with the power output. The major reason for the performance degradation is the operating temperature reduction in the SOFC module, which is caused by decreasing the fuel supply and the heat generation in the cells. This reduction is also connected to the air flow rate supplement. The variable MGT rotational speed control requires flexible air flow regulations to maintain the SOFC operating temperature. It will lead to high efficient operation of the hybrid system.     

A general electrolyte–electrode-assembly model for the performance characteristics of planar anode-supported solid oxide fuel cells

A general electrode–electrolyte-assembly (EEA) model has been developed, which is valid for different designs of solid oxide fuel cells (SOFCs) operating at different temperatures. In this study, it is applied to analyze the performance characteristics of planar anode-supported SOFCs. One of the novel features of the present model is its treatment of electrodes. An electrode in the present model is composed of two distinct layers referred to as the backing layer and the reaction zone layer. The other important feature of the present model is its flexibility in fuel, having taking into account the reforming and water–gas shift reactions in the anode. The coupled governing equations of species, charge and energy along with the constitutive equations in different layers of the cell are solved using finite volume method. The model can predict all forms of overpotentials and the predicted concentration overpotential is validated with measured data available in literature. It is found that in an anode-supported SOFC, the cathode overpotential is still the largest cell potential loss mechanism, followed by the anode overpotential at low current densities; however, the anode overpotential becomes dominant at high current densities. The cathode and electrolyte overpotentials are not negligible even though their thicknesses are negligible relative to the anode thickness. Even at low fuel utilizations, the anode concentration overpotential becomes significant when chemical reactions (reforming and water–gas shift) in the anode are not considered. A parametric study has also been carried out to examine the effect of various key operating and design parameters on the performance of an anode-supported planar SOFCs.     

Ceria-based electrolyte reinforced by sol–gel technique for intermediate-temperature solid oxide fuel cells

High performance solid oxide fuel cells (SOFCs) based on gadolinia-doped ceria (GDC) electrolyte are demonstrated for intermediate temperature operation. The inherent technical limitations of the GDC electrolyte in sinterability and mechanical properties are overcome by applying sol–gel coating technique to the screen-printed film. When the quality of the electrolyte film is enhanced by the additional sol–gel coating, the OCV and maximum power density increase from 0.73 to 0.90 V and from 0.55 to 0.95 W cm⁻², respectively, at 650 °C with humidified hydrogen (3% H₂O) as fuel and air as oxidant. The impedance analysis reveals that the reinforcement of the thin electrolyte with sol–gel coating significantly reduces the polarization resistance. Elementary reaction steps for the anode and cathode are analyzed based on the systematic impedance study, and the relation between the structural integrity of the electrolyte and the electrode polarization is discussed in detail.     

Comparison between two optimization strategies for solid oxide fuel cell–gas turbine hybrid cycles

This paper compares the performance characteristics of a combined power system with solid oxide fuel cell (SOFC) and gas turbine (GT) working under two thermodynamic optimization strategies. Expressions of the optimized power output and efficiency for both the subsystems and the SOFC-GT hybrid cycle are derived. Optimal performance characteristics are discussed and compared in detail through a parametric analysis to evaluate the impact of multi-irreversibilities that take into account on the system behaviour. It is found that there exist certain new optimum criteria for some important design and operating parameters. Engineers should find the methodologies developed in this paper useful in the optimal design and practical operation of complex hybrid fuel cell power plants.     

High-performance gas–electricity cogeneration using a direct carbon solid oxide fuel cell fueled by biochar derived from camellia oleifera shells

Direct carbon solid oxide fuel cells (DC-SOFCs) are promising for generating electricity cleanly and efficiently from solid carbon fuel. Biochar from Camellia oleifera shells is used in a tubular electrolyte-supported 2-cell DC-SOFC stack with a yttrium-stabilized zirconia (YSZ) electrolyte and silver–gadolinium-doped ceria (Ag-GDC) as symmetrical electrodes. The DC-SOFC exhibits comparable electrical performance to the same cell operated on hydrogen fuel and can cogenerate CO and electricity when fueled by biochar. The gas–electricity cogeneration performance of the DC-SOFC is tested under constant-current discharge in terms of electrical power output, CO output rate and purity, electrical conversion efficiency, and gas–electrical cogeneration conversion efficiency. The purity of the output CO can reach more than 80%. Considering the chemical energy of CO a part of the output power, the energy conversion efficiency of >70% is attained. Furthermore, the gas–electricity cogeneration performance is relatively stable before the biochar fuel is exhausted.     

Performance and durability of Ni–Co alloy cermet anodes for solid oxide fuel cells

Ni alloys are examined as redox-resistant alternatives to pure Ni for solid oxide fuel cell (SOFC) anodes. Among the various candidate alloys, Ni–Co alloys are selected due to their thermochemical stability in the SOFC anode environment. Ni–Co alloy cermet anodes are prepared by ammonia co-precipitation, and their electrochemical performance and microstructure are evaluated. Ni–Co alloy anodes exhibit high durability against redox cycling, whilst the current-voltage characteristics are comparable to those of pure Ni cermet anodes. Microstructural observation reveals that cobalt-rich oxide layers on the outer surface of the Ni–Co alloy particles protect against further oxidation within the Ni alloy. In long-term durability tests using highly humidified hydrogen gas, the use of a Ni–Co cermet with Gd-doped CeO₂ suppresses degradation of the power generation performance. It is concluded that Ni–Co alloy cermet anodes are highly attractive for the development of robust SOFCs.     

Demonstrating the dual functionalities of CeO2–CuO composites in solid oxide fuel cells

Nowadays, lowering the operating temperature of solid oxide fuel cells (SOFCs) is a major challenge towards their widespread application. This has triggered extensive material studies involving the research for new electrolytes and electrodes. Among these works, it has been shown that CeO₂ is not only a promising basis of solid oxide electrolytes, but also capable of serving as a catalytic assistant in anode. In the present work, to develop new electrolytes and electrodes for SOFCs based on these features of CeO₂, a new type of functional composite is developed by introducing semiconductor CuO into CeO₂. The prepared composites with mole ratios of 7:3 (7CeO₂–3CuO) and 3:7 (3CeO₂–7CuO) are assessed as electrolyte and anode in fuel cells, respectively. The cell based on 7CeO₂–3CuO electrolyte reaches a power outputs of 845 mW cm^?2 at 550 °C, superior to that of pure CeO₂ electrolyte fuel cell, while an Ce_0.8Sm_0.2O_2-δ electrolyte SOFC with 3CeO₂–7CuO anode achieves high power density along with open circuit voltage of 1.05 V at 550 °C. In terms of polarization curve and AC impedance analysis, our investigation manifests the developed 7CeO₂–3CuO composite has good electrolyte capability with a hybrid H⁺/O^2? conductivity of 0.1–0.137 S cm^?1 at 500–550 °C, while the 3CeO₂–7CuO composite plays a competent anode role with considerable catalytic activity, indicative of the dual-functionalities of CeO₂–CuO in fuel cell. Furthermore, a bulk heterojunction effect based on CeO₂/CuO pn junction is proposed to interpret the suppressed electrons in 7CeO₂–3CuO electrolyte. Our study thus reveals the great potential of CeO₂–CuO to develop functional materials for SOFCs to enable low-temperature operation.     

Novel doped barium cerate–carbonate composite electrolyte material for low temperature solid oxide fuel cells

A composite of a perovskite oxide proton conductor (BaCe_0.7Zr_0.1Y_0.2O_3−δ, BCZ10Y20) and alkali carbonates (2Li₂CO₃:1Na₂CO₃, LNC) is investigated with respect to its morphology, conductivity and fuel cell performance. The morphology shows that the presence of carbonate phase improves the densification of oxide matrix. The conductivity is measured by AC impedance in air, nitrogen, wet nitrogen, hydrogen, and wet hydrogen, respectively. A sharp increase of the conductivity at certain temperature is seen, which relates to the superionic phase transition at the interface phases between oxide and carbonates. Single cell with the composite electrolyte is fabricated by dry-pressing technique, using nickel oxide as anode and lithiated nickel oxide as cathode, respectively. The cell shows a maximum power density of 957 mW cm⁻² at 600 °C with hydrogen as the fuel and oxygen as the oxidant. The remarkable proton conductivity and excellent cell performance make this kind of composite material a good candidate electrolyte for low temperature solid oxide fuel cells (SOFCs).     

The design of stationary and mobile solid oxide fuel cell–gas turbine systems

A general thermodynamic model has shown that combined fuel cell cycles may reach an electric-efficiency of more than 80%. This value is one of the targets of the Department of Energy (DOE) solid oxide fuel cell–gas turbine (SOFC–GT) program. The combination of a SOFC and GT connects the air flow of the heat engine and the cell cooling. The principle strategy in order to reach high electrical-efficiencies is to avoid a high excess air for the cell cooling and heat losses. Simple combined SOFC–GT cycles show an efficiency between 60 and 72%. The combination of the SOFC and the GT can be done by using an external cooling or by dividing the stack into multiple sub-stacks with a GT behind each sub-stack as the necessary heat sink. The heat exchangers (HEXs) of a system with an external cooling have the benefit of a pressurization on both sides and therefore, have a high heat exchange coefficient. The pressurization on both sides delivers a low stress to the HEX material. The combination of both principles leads to a reheat (RH)-SOFC–GT cycle that can be improved by a steam turbine (ST) cycle. The first results of a study of such a RH-SOFC–GT–ST cycle indicate that a cycle design with an efficiency of more than 80% is possible and confirm the predictions by the theoretical thermodynamic model mentioned above. The extremely short heat-up time of a thin tubular SOFC and the market entrance of the micro-turbines give the option of using these SOFC–GT designs for mobile applications. The possible use of hydrocarbons such as diesel oil is an important benefit of the SOFC. The micro-turbine and the SOFC stack will be matched depending on the start-up requirements of the mobile system. The minimization of the volume needed is a key issue. The efficiency of small GTs is lower than the efficiency of large GTs due to the influence of the leakage within the stages of GTs increasing with a decreasing size of the GT. Thus, the SOFC module pressure must be lower than in larger stationary SOFC–GT systems. This leads to an electrical-efficiency of 45% of a cycle used as a basis for a design study. The result of the design study is that the space available in a mid-class car allows the placement of such a system, including space reserves. A further improvement of the system might allow an electrical-efficiency of about 55%.     

Anode performance of LST–xCeO2 for solid oxide fuel cells

La-doped SrTiO₃ (LST)–xCeO₂ (x = 0, 30, 40, 50) composites were evaluated as anode materials for solid oxide fuel cells in terms of chemical compatibility, electrical conductivity and fuel cell performance in H₂ and CH₄. Although the conductivity of LST–xCeO₂ composite slightly decreased from 4.6 to 3.9 S cm⁻¹ in H₂ at 900 °C as the content of CeO₂ increased, the fuel cell performance improved from 75.8 to 172.3 mW cm⁻² in H₂ and 54.5 to 139.6 mW cm⁻² in CH₄ at 900 °C. Electrochemical impedance spectra (EIS) indicated that the addition of CeO₂ into LST can significantly reduce the fuel cells polarization thus leading to a higher performance. The result demonstrated the potential ability of LST–xCeO₂ to be used as SOFCs anode.     

The effect of ceria content in nickel–ceria composite anode catalysts on the discharge performance for solid oxide fuel cells

Optimum ceria content in nickel–ceria composite anode catalyst from the point of discharge performance is discussed. The ohmic loss increased when the ceria content was higher than 30 mol%. Even though the electrical conductivity of the anode decreased with increasing ceria content in the anode catalyst in association with decreasing nickel content, the ohmic loss was kept low until the ceria content was ≤30 mol% because the semiconducting ceria compensated for the decreased current path owing to the decreasing nickel content. The lowest activation loss was observed when the ceria content in the nickel anode catalyst was 30 mol% and the maximum activation loss was obtained for ceria content of 2 mol%. Ceria content in nickel anode influenced microstructure of the anode matrix. When the CeO₂ content was 2 mol%, sintering of anode catalyst was evident and the porosity of anode matrix was almost 57% - highest in this study. Whereas sintering of anode catalyst was not evident and the porosity of anode matrix was 46% when the ceria content in the nickel anode catalyst was 30 mol%. Activation loss was strongly influenced by microstructure of anode matrix, and highest activation loss when the CeO₂ content was 2 mol% was owing to the inappropriate microstructure for electrochemical reaction: sintering of the anode catalyst and excessive porosity of the anode.     

Analysis of a pressurized solid oxide fuel cell–gas turbine hybrid power system with cathode gas recirculation

A pressurized solid oxide fuel cell–gas turbine hybrid system (SOFC–GT system) has been received much attention for a distributed power generation due to its high efficiency. When considering an energy management of the system, it is found that a heat input is highly required to preheat air before being fed to the SOFC stack. The recirculation of a high-temperature cathode exhaust gas is probably an interesting option to reduce the requirement of an external heat for the SOFC–GT system. This study aims to analyze the pressurized SOFC–GT hybrid system fed by ethanol with the recycle of a cathode exhaust gas via a simulation study. Effect of important operating parameters on the electrical efficiency and heat management of the system is investigated. The results indicate that an increase in the operating pressure dramatically improves the system electrical efficiency. The suitable pressure is in a range of 4–6 bar, achieving the highest system electrical efficiency and the lowest recuperation energy from the waste heat of the GT exhaust gas. In addition, it is found that the waste heat obtained from the GT is higher than the heat required for the system, leading to a possibility of the SOFC–GT system to be operated at a self-sustainable condition. Under a high pressure operation, the SOFC–GT system requires a high recirculation of the cathode exhaust gas to maintain the system without supplying the external heat; however, the increased recirculation ratio of the cathode exhaust gas reduces the system electrical efficiency.     

Catalytic activity of Ni–YSZ–CeO2 anode for the steam reforming of methane in a direct internal-reforming solid oxide fuel cell

As one of the key technologies in the development of a direct internal-reforming solid oxide fuel cell, catalytic activity and stability of a Ni–YSZ–CeO₂ anode on a zirconia electrolyte for the steam reforming of methane was investigated by experiments using a differential fuel cell reactor. The effects of the partial pressure of CH₄, H₂O and H₂, and temperature as well as the electrochemical oxidation on the catalytic activity were analyzed. It was found that the catalytic activity of the Ni–YSZ–CeO₂ anode was higher than that of the Ni–YSZ reported especially at low temperature. A deterioration of the catalytic activity of the anode was observed at low P_H2 and high P_H2O atmosphere, and also at high current densities. This might be caused by the oxidation of the Ni surface by H₂O in the reaction gas and that produced by the anodic reaction. A rate equation for a fractional function for the steam reforming on open circuit was also proposed.     

A novel Nb and Cu co-doped SrCoO3-δ cathode for intermediate temperature solid oxide fuel cells

The extensive explorations of potential cathode materials are prominently critical for the rapid development of high performance solid oxide fuel cells (SOFCs). Herein, we develop a novel Nb and Cu co-doped SrCoO_3-δ (SCNC) cathode base on solid state reaction, which exhibits decent compatibility with gadolinium doped cerium oxide (GDC) electrolyte. The SCNC is successfully stabilized with cubic structure at room temperature when incorporating of small amount of high valence Nb⁵⁺. Meanwhile, the oxygen vacancy concentration of SCNC is efficiently improved with the addition of Cu. The Nb and Cu co-doping also substantially promotes the electronic conductivity, achieving 550 S cm⁻¹ for the optical doped SrCo_0.85Nb_0.05Cu_0.10O_3-δ (SCNC10) at 400 °C. In addition, the polarization of SCNC is remarkably reduced, reaching as low as 0.021 Ω cm² for SCNC10 at 700 °C. The activation energy for reaction is also significantly lowered to 0.78 eV. The reaction order m is deduced to be about 0.30, implying that the rate determination step for SCNC10 is the charge transfer reaction. The peak power density of the single cell reaches 780 mW cm⁻² at 800 °C. All these outstanding performances demonstrate that SCNC is a promising cathode for SOFCs when operating at intermediate temperature (IT).     

Chromium deposition and poisoning of cathodes of solid oxide fuel cells – A review

Intermediate temperature solid oxide fuel cells (IT-SOFCs) using chromia-forming alloy interconnect requires the development of cathode not only with high electrochemical activity but also with the high resistance or tolerance towards Cr deposition and poisoning. This is due to the fact that, at SOFC operating temperatures, volatile Cr species are generated over the chromia scale, poisoning the cathodes such as (La,Sr)MnO₃ (LSM) and (La,Sr)(Co,Fe)O₃ (LSCF) and causing a rapid degradation of the cell performance. Thus, a fundamental understanding of the interaction between the Fe–Cr alloys and SOFC cathode is essential for the development of high performance and stable SOFCs. The objective of this paper is to critically review the progress and particularly the work done in the last 10 years in this important area. The mechanism and kinetics of the Cr deposition and Cr poisoning process on the cathodes of SOFCs are discussed. Chromium deposition at SOFC cathodes is most likely dominated by the chemical reduction of high valence Cr species, facilitated by the nucleation agents on the electrode and electrolyte surface and/or at the electrode/electrolyte interface, i.e., the nucleation theory. The driving force behind the nucleation theory is the surface segregation and migration of cationic species on the surface of perovskite oxide cathodes. Overwhelming evidences indicate that the surface segregation plays a critical role in the Cr deposition. The prospect of the development in the Cr-tolerant cathodes for SOFCs is presented.     

Direct reforming of Methane–Ammonia mixed fuel on Ni–YSZ anode of solid oxide fuel cells

To control the temperature distribution in the Ni–YSZ (yttria-stabilized zirconia) anode of solid oxide fuel cells (SOFCs) by efficiently utilizing the heat generated by electrochemical reactions, the supply of methane–ammonia mixed fuel is proposed. The reaction characteristics of reforming/decomposition of the mixed fuel on a Ni–YSZ catalyst are experimentally investigated. A mixture gas of methane, steam, ammonia, and balance argon is supplied to a packed bed catalyst placed in a quartz tube in an electric furnace. The crushed Ni–YSZ anode of SOFCs is used as the catalyst. The exhaust gas composition is analyzed by gas chromatography and the streamwise temperature distribution of the catalyst bed is measured by an infrared camera. It is found that ammonia decomposition preferentially proceeds and steam methane reforming becomes active after sufficient ammonia has been consumed. A low-temperature region is formed by steam methane reforming owing to its strongly endothermic nature. Its position moves downstream while its magnitude decreases as the ammonia concentration in the fuel increases. This shows that the local temperature distribution can be controlled by tuning the ratio of methane to ammonia in the mixed fuel. It is also found that, at a certain mixture ratio, the mixed fuel realizes a hydrogen production rate higher than that for only methane or ammonia.