Higher oxygen vacancy content and hydration capability enabled by magnesium doping for high-activity protonic ceramic fuel cell cathode

Protonic ceramic fuel cells (PCFCs) are promising power generation equipment because of their high efficiency and low operating temperature. However, the sluggish oxidation-reduction reaction (ORR) kinetics of the cathode seriously limits their further development. Here, a low-valent alkaline-earth metal Mg-doped BaCo_0.4Fe_0.4Zr_0.2O_3-δ (BaCo_0.4Fe_0.4Zr_0.1Mg_0.1O_3-δ = BCFZMg0.1) cathode is developed to increase oxygen vacancy concentration and hydration capability of mixed oxygen ion-electron conducting (MIEC) electrode materials. The phase composition, microstructure, and stability in wet air were examined, while the oxygen vacancy, hydration capability, conductivity, and surface species of the materials were investigated. Experimental results demonstrate that the oxygen vacancy concentration, hydration capability, and conductivity are all increased by Mg doping. Consequently, the ORR active sites were expanded to the whole electrode surface, and the electrochemical performance of PCFCs with BCFZMg0.1 cathode was greatly improved (peak power density: >40% increase; polarization resistance: 60–70% decrease). This work provides a new strategy to develop cathodes with high ORR activity for PCFCs by doping Mg into the MIEC perovskite B-site.     

Mixed ionic electronic conducting perovskite anode for direct carbon fuel cells

The conversion of carbonaceous materials to electricity in a Direct Carbon Fuel Cell (DCFC) offers the most efficient process with theoretical electric efficiency close to 100%. One of the key issues for fuel cells is the continuous availability of the fuel at the triple phase boundaries between fuel, electrode and electrolyte. While this can be easily achieved with the use of a porous fuel electrode (anode) in the case of gaseous fuels, there are serious challenges for the delivery of solid fuels to the triple junctions. In this paper, a novel concept of using mixed ionic electronic conductors (MIEC) as anode materials for DCFCs has been discussed. The lanthanum strontium cobalt ferrite, La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF) was chosen as the first generation anode material due to its well known high mixed ionic and electronic conductivities in air. This material has been investigated in detail with respect to its conductivity, phase and microstructural stability in DCFC operating environments. When used both as the anode and cathode in a DCFC, power densities in excess of 50 mW/cm² were obtained at 804 °C in electrolyte supported small button cells with solid carbon as the fuel. The concept of using the same anode and cathode material has also been evaluated in electrolyte supported thick wall tubular cells where power densities around 25 mW/cm² were obtained with carbon fuel at 820 °C in the presence of helium as the purging gas. The concept of using a mixed ionic electronic conducting anode for a solid fuel, to extend the reaction zone for carbon oxidation from anode/electrolyte interface to anode/solid fuel interface, has been demonstrated.     

Atomistic simulations of anisotropic mechanical behavior in non-stoichiometric gadolinia-doped ceria solid electrolytes

Gadolinia-doped ceria (GDC), as an electrolyte of solid oxide fuel cells, reduces its mechanical property when exposed to reducing conditions. In this paper, the anisotropic mechanical behavior of non-stoichiometric GDC solid electrolytes is investigated by using the molecular dynamics (MD) method. It is found that the oxygen vacancies whether from doping Gd³⁺ ions or generating Ce³⁺ ions by reduction have a serious impact on the mechanical properties such as phase transformation and fracture strength. The defect-dependent tensile strength is observed to be consistent with reported experimental measurements. Moreover, increasing temperature reduces the fracture strength. The influence of temperature on the critical nucleation stress and the fluorite phase strength is further analyzed. Beyond that, GDC undergoes volumetric expansion due to non-stoichiometric effects. The linear chemical strain and coefficient of compositional expansion (CCE) are calculated and compared with the experimental results.     

Potassium permanganate as an electron receiver in a microbial fuel cell

Microbial fuel cells with air as a cathode electron receiver are simple systems but they need expensive catalysts. In comparison to microbial fuel cells with oxygen as an electron receiver, microbial fuel cells with potassium permanganate produce higher voltage. In this study, electrical performance of a microbial fuel cells containing anaerobic sludge and potassium permanganate as an oxidizing agent was investigated. Glucose (1 g/l) was used as a carbon and energy source. The maximum power density and current density at the maximum power density were 93.13 mW/m² and 0.030 mA/cm² with respect to a potassium permanganate concentration of 400 µM. It is observed that the maximum power density increased to 114.00 mW/m² using an acid-heat treated carbon brush anode. Also, chemical oxygen demand removal was 51% when the microbial fuel cells was operated using 400 µM of potassium permanganate.     

A novel cobalt-free layered GdBaFe2O5+δ cathode for proton conducting solid oxide fuel cells

While cobalt-containing perovskite-type cathode materials facilitate the activation of oxygen reduction, they also suffer from problems like poor chemical stability in CO₂ and high thermal expansion coefficients. In this research, a cobalt-free layered GdBaFe₂O_5+δ (GBF) perovskite was developed as a cathode material for protonic ceramic membrane fuel cells (PCMFCs) based on proton conducting electrolyte of stable BaZr_0.1Ce_0.7Y_0.2O_3−δ (BZCY7). The button cells of Ni-BZCY7|BZCY7|GBF were fabricated and characterized using complex impedance technique from 600 to 700 °C. An open-circuit potential of 1.007 V, maximum power density of 417 mW cm⁻², and a low electrode polarization resistance of 0.18 Ω cm² were achieved at 700 °C. The results indicate that layered GBF perovskite is a good candidate for cobalt-free cathode material, while the developed Ni-BZCY7|BZCY7|GBF cell is a promising functional material system for solid oxide fuel cells.     

Development of lanthanum strontium cobalt ferrite perovskite electrodes of solid oxide fuel cells – A review

There is an enormous driving force in solid oxide fuel cells (SOFCs) to reduce the operating temperatures from high temperatures (800–1000 °C) to intermediate and low temperatures (400–800 °C) in order to increase the durability, improve thermal compatibility and thermal cycle capability, and reduce the fabrication and materials costs. One of the grand challenges is the development of cathode materials for intermediate and low temperature SOFCs with high activity and stability for the O₂ reduction reaction (ORR), high structural stability as well as high tolerance toward contaminants like chromium, sulfur and boron. Lanthanum strontium cobalt ferrite (LSCF) perovskite is the most popular and representative mixed ionic and electronic conducting (MIEC) electrode material for SOFCs. LSCF-based materials are characterized by high MIEC properties, good structural stability and high electrochemical activity for ORR, and have played a unique role in the development of SOFCs technologies. However, there appears no comprehensive review on the development and understanding of this most important MIEC electrode material in SOFCs despite its unique position in SOFCs. The objective of this article is to provide a critical and comprehensive review in the structure and defect chemistry, the electrical and ionic conductivity, and relationship between the performance, intrinsic and extrinsic factors of LSCF-based electrode materials in SOFCs. The challenges, strategies and prospect of LSCF-based electrodes for intermediate and low temperature SOFCs are discussed. Finally, the development of LSCF-based electrodes for metal-supported SOFCs and solid oxide electrolysis cells (SOECs) is also briefly reviewed.     

Thermal stresses in an operating micro-tubular solid oxide fuel cell

A multi-physics model is developed to investigate the thermal stresses in a micro-tubular SOFC, based on a previously developed thermal-fluids model predicting cell operation. Mechanical properties of the anode and cathode are determined theoretically through composite structure approximation. Residual stresses arisen during the fabrication of the cell due to the mismatch in thermal expansion coefficients are calculated by accounting for each fabrication process separately. The interactions between the cell, the sealant and the alumina tube are accounted for a better representation of the actual fuel cell test setup. The effect of sealant and alumina tube on the stress distribution in the cell is investigated and it is found out that near the fuel cell-sealant interface stress distribution changes significantly. The effect of spatial temperature gradient on the stress distribution is also analyzed and found to have a minimal impact for a typical fuel cell operation at mid-range current densities. The effects of oxygen vacancies caused by the reduction of the GDC electrolyte on the overall stress distribution are also shown. Oxygen vacancies of the electrolyte result in relaxation of the stresses due to the alleviation of mismatch in Young's modulus between different layers of the cell.     

Performance analysis of mixed ionic-electronic conducting cathodes in anode supported cells

The analysis of mixed ionic-electronic conducting (MIEC) cathodes with respect to operation temperature and time is essential for a target-oriented development of anode-supported solid oxide fuel cells (ASCs). This study tracks both issues by impedance spectroscopy on a high-performance cathode with the composition La_0.58Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF).A wide set of impedance spectra were sampled at 600, 750 and 900 °C over the entire operation time of 1000 h. The identification and quantification of the individual anodic and cathodic contributions to the polarization losses of an ASC were enabled by an appropriate equivalent circuit model. For this purpose, the impedance data sets were evaluated subsequently by (i) a DRT (distribution of relaxation times) analysis followed by (ii) a CNLS fit. The cathodic polarization resistance is attributed to the oxygen surface exchange and the bulk diffusion of oxygen ions and is described by a Gerischer element.The anodic polarization resistance is described by a Warburg element and two RQ elements according to physical origins. The thorough analysis of all data sets leads to the surprising outcome that the cathode degradation is most pronounced and moreover, increases with decreasing temperature. After 1000 h of operation, the cathode polarization resistance raised steeply from 0.012%/h at 900 °C over 0.28%/h at 750 °C to 1.49%/h at 600 °C. These latest findings will have far-reaching implications for the development of MIEC cathodes.     

Cathode reaction mechanism of porous-structured Sm0.5Sr0.5CoO3−δ and Sm0.5Sr0.5CoO3−δ/Sm0.2Ce0.8O1.9 for solid oxide fuel cells

The cathode reaction mechanism of porous Sm_0.5Sr_0.5CoO_3−δ, a mixed ionic and electronic conductor (MIEC), is studied through a comparison with the composite cathode Sm_0.5Sr_0.5CoO_3−δ/Sm_0.2Ce_0.8O_1.9. First, the cathodic behaviour of porous Sm_0.5Sr_0.5CoO_3−δ and Sm_0.5Sr_0.5CoO_3−δ/Sm_0.2Ce_0.8O_1.9 are observed for micro-structure and impedance spectra according to Sm_0.2Ce_0.8O_1.9 addition, thermal cycling and long-term properties. The cathode reaction mechanism is discussed in terms of frequency response, activation energy, reaction order and electrode resistance for different oxygen partial pressures p(O₂) at various temperatures. Three elementary steps are considered to be involved in the cathodic reaction: (i) oxygen ion transfer at the cathode-electrolyte interface; (ii) oxygen ion conduction in the bulk cathode; (iii) gas phase diffusion of oxygen. A reaction model based on the empirical equivalent circuit is introduced and analyzed using the impedance spectra. The electrode resistance at high frequency (R_c,HF) in the impedance spectra represents reaction steps (i), due to its fast reaction rate. The electrode resistance at high frequency is independent of p(O₂) at a constant temperature because the semicircle of R_c,HF in the complex plane of the impedance spectra is held constant for different values of p(O₂). Reaction steps (ii) and (iii) are the dominant processes for a MIEC cathode, according to the analysis results. The proposed cathode reaction model and results for a solid oxide fuel cell (SOFC) well describe a MIEC cathode with high ionic conductivity, and assist the understanding of the MIEC cathode reaction mechanism.     

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.     

Numerical simulation and analysis of thermal stress distributions for a planar solid oxide fuel cell stack with external manifold structure

A three-dimensional numerical model based on the finite element method (FEM) is constructed to calculate the thermal stress distribution in a planar solid oxide fuel cell (SOFC) stack with external manifold structure. The stack is composed of 5 units which include cell, metallic interconnect, seal and anode/cathode current collectors. The temperature profile is described according to measured temperature points in the stack. It can be clearly seen that the maximum stress concentration area appears at the corner of the components when the stack is heated from room temperature (RT) to 780 °C. The effects of stack components on maximum stress concentration have been investigated under the operation temperature, as well as the thermal stress simulation results. It is obvious that the coefficient of thermal expansion (CTE) mismatch between the interconnect and the seal plays an important role in determining the thermal stress distribution in the stack. However, different compressive loads have almost no effect on stress distribution, and the influence of glass-based seal depends on the elastic modulus. The simulation results can be applied for optimizing the structural design of the stack and minimizing the high stress concentration in components.     

Control of solid oxide fuel cells damage using infrared thermography

The high operating temperature of solid oxide fuel cells, can cause serious damage such as cathode delamination or cracking of the SOFC layers, which cause a decrease in the performance of the latter, a rigorous nondestructive inspection is necessary, in particular that their prices are high. In this paper we have treated thermal control by pulsed infrared thermography of the solid oxide fuel cells electrodes, using 3D finite elements method. The thermal images obtained were processed, and a quantitative analysis was performed to obtain information concerning cracks in electrolyte supported and anode supported planar cells, and cathode delamination in electrolyte supported and cathode supported cells. The results obtained as the temperature distribution in the fuel cells, with and without defects, shows the difference between the flawed and the unflawed cells. The thermal response informs us about the presence of the defects, and in addition it informed us about their depths.     

Novel layered perovskite GdBaCoFeO5+δ as a potential cathode for proton-conducting solid oxide fuel cells

While cobalt-containing perovskite-type cathode materials facilitate the activation of oxygen reduction, they also suffer from problems like poor chemical stability in CO₂, high thermal expansion coefficients, etc. Partial B site substitution with Fe element is expected to be able to mitigate these problems while keeping high catalyst performance. In this paper, a layered perovskite GdBaCoFeO_5+δ (GBCF) was developed as a cathode material for protonic ceramic membrane fuel cells (PCMFCs) based on proton-conducting electrolyte of stable BaZr_0.1Ce_0.7Y_0.2O_3−δ (BZCY7). The button cells of Ni-BZCY7|BZCY7|GBCF were fabricated and tested from 600 to 700 °C with humidified H₂ (∼3% H₂O) as a fuel and ambient oxygen as oxidant. An open-circuit potential of 1.002 V, maximum power density of 482 mW cm⁻², and a low electrode polarization resistance of 0.11 Ωcm² were achieved at 700 °C. The experimental results indicated that the layered perovskite GBCF is a good candidate for cathode material, while the developed Ni-BZCY7|BZCY7|GBCF cell is a promising functional material system for intermediate temperature solid oxide fuel cells.     

Model-based control of cathode pressure and oxygen excess ratio of a PEM fuel cell system

For PEM fuel cells supplied with air, pressure and flow control is a key requirement for an efficient and dynamic operation because fuel cells are in risk of starvation when the partial pressure of oxygen at the cathode falls below a critical level. To avoid oxygen starvation and, at the same time, to allow for a dynamic operation of the fuel cell system, both excess ratio of oxygen and cathode pressure need to be adjusted rapidly.     

High-performance La0.5(Ba0.75Ca0.25)0.5Co0.8Fe0.2O3-δ cathode for proton-conducting solid oxide fuel cells

La_0.5(Ba_0.75Ca_0.25)_0.5Co_0.8Fe_0.2O_3-δ, a simple perovskite cathode material with high electrical conductivity (940 S cm^?1 at 600 °C) and impressive surface catalytic activity, was prepared and used in proton-conducting solid oxide fuel cells. As its thermal expansion coefficient is higher than that of the electrolyte material BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3-δ, they were combined and used as a composite cathode. The crystal structure, chemical compatibility, electrical conductivity, cell performance, and the oxygen reduction reaction of the cathode material were explored, and we found that the single fuel cell developed with the composite cathode achieved excellent electrochemical performance, with both a low polarization resistance and high peak power density (0.044 Ω cm² and 1102 mW cm^?2 at 750 °C, respectively). Outstanding stability was also achieved, as indicated by a long-term 100-h test. Additionally, the rate-limiting steps of the oxygen reduction reaction were the oxygen adsorption, dissociation, and diffusion processes.     

Simulation of thermal stresses in anode-supported solid oxide fuel cell stacks. Part I: Probability of failure of the cells

Structural stability issues in planar solid oxide fuel cells arise from the mismatch between the coefficients of thermal expansion of the components. The stress state at operating temperature is the superposition of several contributions, which differ depending on the component. First, the cells accumulate residual stresses due to the sintering phase during the manufacturing process. Further, the load applied during assembly of the stack to ensure electric contact and flatten the cells prevents a completely stress-free expansion of each component during the heat-up. Finally, thermal gradients cause additional stresses in operation.The temperature profile generated by a thermo-electrochemical model implemented in an equation-oriented process-modelling tool (gPROMS) was imported into finite-element software (ABAQUS) to calculate the distribution of stress and contact pressure on all components of a standard solid oxide fuel cell repeat unit.The different layers of the cell, i.e. anode, electrolyte, cathode and compensating layer were considered in the analysis by using the sub-modelling capabilities of the finite-element tool. Both steady-state and dynamic simulations were performed, with an emphasis on the cycling of the electrical load. The study includes two different types of cells, operation under both thermal partial oxidation and internal steam-methane reforming and two different initial thicknesses of the air and fuel compressive sealing gaskets.The results generated by the models are presented in two papers: Part I, focuses on the assessment of the risks of failure of the cell, which was performed by Weibull analysis, while the issues related to the other components are discussed in Part II.Only the anode support contributed to the probability of failure, since the other layers underwent compressive stresses independently of the operating conditions. The cell at room temperature after the reduction procedure was revealed as a critical case. Thermal gradients and the shape of the temperature profile generated during transient operation induced high probabilities of failure. The computed reliability is incompatible with commercialisation, but the scatter induced by the experimental data covers several orders of magnitude. Alternatively, the computed required strength of the anode material to fulfil a probability of failure of 10⁻² in a 50-cells stack during steady-state operation appears achievable. Finally, extreme care is required when using the maximum thermal gradient or temperature difference over the SRU as an indicator for cell cracking.     

Electrolyte degradation in anode supported microtubular yttria stabilized zirconia-based solid oxide steam electrolysis cells at high voltages of operation

Degradation of solid oxide electrolysis cells (SOECs) is probably the biggest concern in the field of high temperature steam electrolysis (HTSE). Anode supported, YSZ-based microtubular solid oxide fuel cells (SOFC) have been tested in fuel cell mode and also at high voltages (up to 2.8 V) under electrolysis mode. At high steam conversion rates the cell voltage tends to saturate. Our hypothesis is that this effect is caused by the electroreduction of the thin YSZ electrolyte which induces electronic conduction losses. YSZ reduction increases the cathode activity and reduces cathode overpotential. Operation of the cell in severe electrolyte reduction conditions induces irreversible damage at the YSZ electrolyte as observed in SEM experiments by the formation of voids at the grain boundaries of the dense YSZ electrolyte. Evidence of this damage was also given by the increase of the ohmic resistance measured by AC impedance. Signs of electrolyte degradation were also found by both EDX analysis and micro-Raman spectroscopy performed along a transverse-cross section of the cell. The observed oxygen electrode delamination is associated to the high oxygen partial pressures gradients that take place at the electrolyte/oxygen electrode interface.     

燃料电池水下应用可行性研究

目的  为了进一步提升水中装备的续航作战能力,高比能量电能源系统是解决问题的关键,通过对比不同燃料种类对系统比能量的影响,探究燃料电池在水下应用的可行性。 方法  通过对目前广泛研究的质子交换膜燃料电池和固体氧化物燃料电池的特点进行对比分析,依据指标要求对比不同储氢和储氧的方式,确定燃料系统阴极侧采用液氧方式供给可以满足设计要求,不同燃料电池类型其阳极侧可采用的供给方式不同,液氢、有机液体、甲醇重整、直接甲醇和直接丙烷具有应用潜力。 结果  结合不同燃料电池的特点,分析尾气处理装置参数,综合比较水下应用燃料电池能源系统的可行方案,以液氧、液化丙烷或有机液体为燃料的固体氧化物燃料电池能源系统和以液氧、有机液体为燃料的质子交换膜燃料电池可以满足设计需求。 结论  燃料电池能源系统可以显著提升能源系统的比能量,燃料的供给形式是影响电能源系统比能量的主要因素。     

Study on a zero CO2 emission SOFC hybrid power system integrated with OTM using CO2 as sweep gas

A new zero CO₂ emission solid oxide fuel cell (SOFC) hybrid power system integrated with the oxygen ion transport membrane using CO₂ as sweep gas is proposed in this paper. The pure oxygen is picked up from the cathode outlet gas by the oxygen ion transport membrane with CO₂ as sweep gas; the oxy‐fuel combustion mode in the afterburner of SOFC is employed. Because the combustion product gas only consists of CO₂ and steam, CO₂ is easily captured with lower energy consumption by the condensation of steam. With the aspen plus soft, this paper builds the simulation model of the overall SOFC hybrids system with CO₂ capture. The exergy loss distributions of the overall system are analyzed, and the effects of the key operation parameters on the overall system performance are also investigated. The research results show that the new system still has a high efficiency after CO₂ recovery. The efficiency of the new system is around 65.03%, only 1.25 percentage points lower than that of the traditional SOFC hybrid power system(66.28%)without CO₂ capture. The research achievements from this paper will provide the valuable reference for further study on zero CO₂ emission SOFC hybrid power system with higher efficiency. Copyright © 2013 John Wiley & Sons, Ltd.