The influence of polymer electrolyte fuel cell cathode degradation on the electrode polarization

The electrode of polymer electrolyte fuel cell (PEFC) consists of the porous catalyst layer and gas diffusion layer (GDL). Quantitative evaluation of the influence of these porous layers’ degradation on the cell performance was attempted. The cell was assembled by using the catalyst layer or GDL, which had been corroded ex situ, as the cathode and the cell performance was characterized. The oxygen diffusion polarizations of the catalyst layer and that of the GDL were evaluated from the polarization curves. The polarization curves before and after a long-term operation were also analyzed by the same way, and the influences of the degradation of catalyst layer and GDL were evaluated. The increase of the gaseous diffusion loss in the catalyst layer was found to cause the cell performance loss mainly from the analysis of the simulated corrosion test and the long-term operation cell.     

Effect of cathode sheet resistance on segmented-in-series SOFC power density

Segmented-in-series solid oxide fuel cells with relatively short cell lengths of 1.4 mm were fabricated with varying LSM cathode current collector thicknesses. Increasing the LSM thickness from 11 to 91 μm yielded a factor of 2–3 area-specific resistance decrease and a similar power density increase. The maximum power density measured at 800 °C was 0.53 W cm⁻² calculated based on total array area (including interconnect), and 0.9 W cm⁻² calculated based on active cell area. A segmented-in-series electrical model was used to quantitatively explain the results based on the decreased cathode sheet resistance. The model also showed that the cell lengths were near optimal for maximizing the power density of these cells.     

Electrochemical performance of Ba0.5Sr0.5CoxFe1−xO3−δ (x = 0.2–0.8) cathode on a ScSZ electrolyte for intermediate temperature SOFCs

Intermediate temperature solid oxide fuel cell cathode materials (Ba, Sr)Co_xFe_1−xO_3−δ [x = 0.2–0.8] (BSCF), were synthesized by a glycine-nitrate process (GNP) using Ba(NO₃)₂, Sr(NO₃)₂, Co(NO₃)₂·6H₂O, and Fe(NO₃)₃·9H₂O as starting materials and glycine as an oxidizer and fuel. Electrolyte-supported symmetric BSCF/GDC/ScSZ/GDC/BSCF cells consisting of porous BSCF electrodes, a GDC buffer layer, and a ScSZ electrolyte were fabricated by a screen printing technique, and the electrochemical performance of the BSCF cathode was investigated at intermediate temperatures (500–700 °C) using AC impedance spectroscopy. Crystallization behavior was found to depend on the pH value of the precursor solution. A highly acidic precursor solution increased the single phase perovskite formation temperature. In the case of using a precursor solution with pH 2, a single perovskite phase was obtained at 1000 °C. The thermal expansion coefficient of BSCF was gradually increased from 24 × 10⁻⁶ K⁻¹ for BSCF (x = 0.2) to 31 × 10⁻⁶ K⁻¹ (400–1000 °C) for BSCF (x = 0.8), which resulted in peeling-off of the cathode from the GDC/ScSZ electrolyte. Only the BSCF (x = 0.2) cathode showed good adhesion to the GDC/ScSZ electrolyte and low polarization resistance. The area specific resistance (ASR) of the BSCF (x = 0.2) cathode was 0.183 Ω cm² at 600 °C. The ASR of other BSCF (x = 0.4, 0.6, and 0.8) cathodes, however, was much higher than that of BSCF (x = 0.2).     

An oxygen reduction reaction active and durable SOFC cathode/electrolyte interface achieved via a cost-effective spray-coating

One of the critical obstacles for commercialization of solid oxide fuel cells (SOFCs) technology is to develop efficient interfaces between cathode and electrolyte that enable high activity toward oxygen reduction reaction (ORR) while maintain long-term durability. Here, we report a cost-effective spray-coating process that applied in the building of an ORR active and durable cathode/electrolyte interface. When tested at 750 °C, such spray-coated cathodes show a typical interfacial polarization resistance of ~0.059 Ωcm², much lower than that of ~0.10 Ωcm² for screen-printed cathodes. Detailed distribution of relaxation time analyses of the impedance spectra over time indicates that the capability of mass transfer and surface exchange process in the spray-coated cathode/electrolyte interface has been enhanced and maintained in the testing periods of ~100 h. As a result, a Ni-based anode supported cell with thin electrolyte and spray-coated cathodes shows an excellent peak power density of 1.012 Wcm⁻², much higher than that of 0.712 Wcm⁻² for cells with screen-printed cathodes, when tested at 750 °C using wet H₂ as fuel and ambient air as oxidant. It is demonstrated that ORR activity and durability of the SOFC cathodes can be dramatically enhanced via a cost-effective spray-coating process.     

Reduction of chromium vaporization from SOFC interconnectors by highly effective coatings

The vaporization of Cr-rich volatile species from interconnector materials is a major source of degradation that limits the lifetime of planar SOFC systems with metallic interconnects. In this study, the vaporization of Cr species of a variety high chromium alloys was studied at 800 °C in air using the transpiration method. The measured release of Cr species of the different alloys was correlated with the formed outer oxide scales. A quantitative estimation showed that all the investigated alloys failed to meet the requirements concerning the Cr release from interconnector materials for SOFCs or formed oxide scales which possessed too high electrical resistances. Sputtered ceramic coatings of LSM and LSC and metallic coatings of Co, Ni and Cu were tested with regard to their suitability for Cr retention. The sputtered perovskite coatings turned out to be ineffective in reducing the Cr release to the desired levels. With metallic coatings of Co, Ni or Cu the Cr release could be reduced by more than 99%. The metallic coatings and their oxides effectively reduced the growth of the oxide scale on the steel substrate and showed negligible vaporization rates for Co, Cu and Ni, respectively. Therefore, Co, Ni or Cu were identified as promising and cheap coating materials for metallic interconnectors.     

Synthesis, structural analysis and electrochemical performances of BLSITCFx as new cathode materials for solid oxide fuel cells (SOFC) based on BIT07 electrolyte

BaIn_0.3Ti_0.7O_2.85 (BIT07) is a suitable electrolyte for Solid Oxide Fuel Cell (SOFC) but half cells based on La_0.58Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF) as a cathode material show a degradation of the Area Specific Resistance (ASR) at 700 °C with time. This study deals with the characterization of alternative cathode materials showing a better compatibility with BIT07 than LSCF. A new solid solution, Ba_xLa_0.58(1−x)Sr_0.4(1−x)In_0.3xTi_0.7xCo_0.2(1−x)Fe_0.8(1−x)O_3−δ, with 0 ≤ x ≤ 1, also called BLSITCFx, with in this case x expressed in molar %, derived from BIT07 and LSCF, has been synthesized at 1350 °C in air using BIT07 and LSCF powders. Two compositions, BLSITCF12 and BLSITCF25, have been selected due to their thermal expansion and conductivity properties. Symmetrical half cells based on these two new materials deposited on BIT07 electrolyte have been studied by complex impedance spectroscopy in air versus temperature and time. Their behaviour is comparable to LSCF's, with ASR values never exceeding 0.2 Ωcm² at 700 °C, and moreover their less important Thermal Expansion Coefficient (TEC) mismatch with BIT07 lead to a better mechanical compatibility with time. These new compounds are therefore better candidates than LSCF as cathode materials for SOFC based on BIT07 electrolyte.     

A new technique of estimating anodic and cathodic charge transfer coefficients from SOFC polarization curves

A new technique is presented for estimating cathodic and anodic transfer coefficients from SOFC voltage–current curves. The new technique for charge transfer coefficients is based on integral characteristics of fuel cell polarization curves. Experimental data from literature are used to illustrate the parameter estimation from SOFC current–voltage curves.     

Anode-supported SOFC with 1Ce10ScZr modified cathode/electrolyte interface

The cathode/electrolyte interface in anode-supported solid oxide fuel cells (SOFCs) with Ni–8 mol% yttria-stabilized zirconia (YSZ) anode, thin YSZ electrolyte and La_0.8Sr_0.2Mn_1.1O_3−δ (LSM)–YSZ composite cathode was modified by dispersed (CeO₂)_0.01–(Sc₂O₃)_0.10–(ZrO₂)_0.89 (1Ce10ScZr) electrolyte. The two electrolytes were co-fired into a dense and continuous electrolyte film. By this method, the electrode polarization resistance, specifically the charge transfer resistance of oxygen ions, was greatly reduced. The modified cell achieved a higher performance than the unmodified one, especially at the lower operation temperatures. The decrease of electrode polarization resistance can be ascribed to the accelerated surface oxygen exchange rate from 1Ce10ScZr electrolyte at the cathode/electrolyte interface.     

Chemical durability of Solid Oxide Fuel Cells: Influence of impurities on long-term performance

Because of the fuel flexibility of Solid Oxide Fuel Cells (SOFCs), various types of fuels may be applied directly or via a simple reforming process, including hydrocarbons, alcohols, coal gas, biogas, besides hydrogen. However, various types of minor constituents in practical fuels and/or from the system components can cause chemical degradation of SOFCs, such as anode and cathode poisoning phenomena. In this study, we compare the influence of various external impurities, including sulfur, chlorine, phosphorus, boron, and siloxane for anodes, and H₂O and SO₂ for cathodes, on SOFC performance to have a general overview on long-term chemical durability of SOFCs. Chemical compatibility of Ni with foreign species has also been thermochemically considered. Using common model cells, the stability of cell voltage, electrode overpotential, and ohmic loss up to 3000 h has been experimentally examined for H₂-based fuels, for hydrocarbon-based fuels, and for partially pre-reformed CH₄-based fuels. Increase in degradation rate by impurities was verified for various operational parameters. Impurity poisoning mechanisms are discussed for each specific impurity.     

Novel perovskite-spinel composite conductive ceramics for SOFC cathode contact layer

Perovskite-spinel composite conductive ceramics are developed for solid oxide fuel cell (SOFC) cathode contact layer. The precursor of the composite materials includes micron-sized La_0.6Sr_0.4Co_0.2Fe_0.8O₃(LSCF) perovskite particles coated by the reduced nano-sized Mn_0.9Y_0.1Co₂O₄ (MYC) spinel material, and then it is sintered in-situ to obtain perovskite-spinel composites. The sintering activity of the composites is enhanced by using the reduced spinel powders. The conductive performance of the composite materials is effectively improved due to the high conductivity of perovskite LSCF particles utilized. Measured at 750^οC under constant current density of 400 mA/cm², after running 200 h, the area specific resistance (ASR) value of cathode contact layer remains relatively stable at around 5.4 mΩ cm². The developed LSCF-MYC composite as cathode contact layer presents good bonding strength with both cathode and interconnection, and shows obviously low contact resistance and high stability.     

Solid oxide fuel cell: Materials for anode,cathode and electrolyte

Solid oxide fuel cell technology is the technology which can be driving force to change the course of action of the modern era due to its optimal power generation features with maximum electrical efficiency for automobiles and household devices. Fuel cells can be best described as electrochemical devices that make use of fuel oxidation to convert chemical energy into electrical energy and also lower the amount of oxidant simultaneously.A typical SOFC consists of a cathode, anode and an electrolyte constituting a single cell. These single cells are stacked together for a bigger assembly to produce higher degree of power. The solid electrolyte fills the gap between the cathode and anode transporting O²⁻ ions only. This leaves out electrons as transporting medium, which then pass through the cell via external circuit. Out of the two electrodes, oxidation of fuel takes place at the anode and reduction of oxygen takes place at the cathode. The SOFCs operate at higher temperatures of 600–1200°C producing heat as a byproduct of high quality, actively encouraging quick electrocatalysis utilizing non-precious metals and allowing internal restructuration. The SOFC can also work with high purity hydrogen for proton transport other than O²⁻ ion transport. There are many ceramic materials which have been engineered to act as efficient electrolyte materials. Yttria-stabilized zirconia (YSZ) is the most widely used material as solid electrolyte in SOFC.The present review presents a detailed overview of the SOFC related materials and devices and is an effort to present various reported works in a concise manner.     

Fast performance degradation of SOFC caused by cathode delamination in long-term testing

The performance degradation of a solid oxide fuel cell (SOFC) single cell caused by delamination of the cathode is investigated. When the TEC difference between the cathode and the electrolyte is large, detachment of the interface occurs. This is confirmed by SEM images and impedance analysis during long-term operation. As delamination occurs, the active area naturally decreases, and the oxygen reduction reaction concentrates into the intact area. This leads to an increase in polarization and ohmic losses so that the performance of the single cell degrades by approximately 7.7% during 1000 h of operation. Assuming that the drop of power density was due only to delamination, the area change is approximately 10% when the TEC difference is 7 × 10⁻⁶ K⁻¹.     

Enhancement of the cathode/electrolyte interface by a sintering-active barrier layer for solid oxide fuel cells

It's a critical issue for the successful commercialization of solid oxide fuel cells (SOFCs) to achieve long-term operation and thermal cycling stability without significant degradation. Current work reports an almost-dense, sintering-active and Sr-blocking cathode/electrolyte interface, fabricated through a cost-competitive and scalable method of modifying porous Gadolinia doped Ceria (GDC) barrier layer by in-situ grown GDC nanoparticles. Result show that the robust interface enables improved durability and thermal cycling stability. The hydrothermal modified anode supported cell performance only deteriorates by ∼0.5% after 20 times thermal cycles between 200 and 750 °C, which is more prominent enhancement than ∼16% for pristine cell. The modified symmetric cell shows smaller cathode polarization resistance and well-attached cathode/electrolyte interfaces after ∼1200 h of operation, including 4 times thermal cycles. While the pristine cell shows more obvious area specific resistances increases and the peeled-off cathode layer. It is discussed and concluded that the hydrothermal modified sintering-active barrier layer has contributed mainly to the construction of robust cathode/electrolyte interface, yielding the improved performance and prolonged operation.     

Development of chromium barrier coatings for solid oxide fuel cells

The performance of electrolyte supported solid oxide fuel cell is impaired primarily due to poisoning of electrodes due to contaminants generated from the metallic components of the stack.Ferritic stainless steels are commonly used as stack material under severe operating conditions of SOFC environment. However, the high chromium content in this type of steels tends to form gaseous oxides and/or hydroxides which volatilize and condense on various components of stack assembly, particularly cathodes, resulting in performance degradation of the system.Two types of barrier coatings have been developed to minimize the chromium volatilization. In one case, coatings of oxide species were deposited by processes such as thermal and plasma spraying, and the other is by diffusion coating process such as aluminizing.This presentation will describe various barrier coatings, barrier properties provided by the coatings, and transpiration measurements adopted to evaluate the efficiency of those coatings.     

Characterization of La2−xSrxCoTiO6 (0.6 ≤ x ≤ 1.0) series as new cathodes of solid oxide fuel cells

La_2−xSr_xCoTiO₆ (0.6 ≤ x ≤ 1.0) compound series is prepared by Sr-substitution in the A-site of the perovskite by a modified Pechini procedure under air. Charge compensation as Sr²⁺ content increase occurs by Co²⁺ oxidation to Co³⁺. Reduced samples are obtained by further treatment under 5%H₂/Ar and characterized by Neutron Powder Diffraction. Upon redution, Co³⁺ to Co²⁺ reduction and oxygen vacancies creation are detected. Dependence of total conductivity with temperature and pO₂ exhibits a typical p-type semiconducting behaviour. Results show that the higher the Sr content, the higher holes (Co³⁺) concentration and consequently, La_2−xSr_xCoTiO₆ (x = 1.0) shows the highest conductivity (13.23 S/cm at 1073 K in air). The negligible reactivity with YSZ, used as the electrolyte, of symmetrical cells under oxidant conditions and the moderate thermal expansion found by XRD point to their possible use as SOFC cathodes. Thus, La_1.2Sr_0.8CoTiO₆-based cathodes display polarization resistance of 0.9 Ω cm² at 1073 K in oxygen, only slightly above than the current state-of-the-art.     

Electrochemical performance of novel NGCO-LSCF composite cathode for intermediate temperature solid oxide fuel cells

In this study, a co-dopant CGO was synthesized to produce more efficient cathode materials for intermediate temperature solid oxide fuel cell (IT-SOFC) applications. Neodymium (Nd) was doped into CGO in four different weight ratios in the formula Nd_xGd_0.15Ce_0.85-xO_2-δ (NGCO); the selected percentages for x were 1%, 3%, 5% and 7%. XRD patterns showed pure phase for all synthesized compositions and good compatibility at high temperature under static air with the most common ceramic cathode material in IT-SOFC (La_0·60Sr_0·40Co_0·20Fe_0·80O_2-ä, LSCF). Impedance spectroscopic characterization of symmetrical cells of the composite NGCO-LSCF at different temperatures (650–800 °C in steps of 50 °C) and a frequency range of 0.1–1 MHz in synthetic air revealed interesting results. The lowest polarization resistance (R_p) was achieved for Nd_0.05Gd_0.15Ce_0·80O_2-δ (0.06 Ω cm² at 800 °C, 0.17 Ω cm² at 750 °C, 0.31 Ω cm² at 700 °C, and 0.59 Ω cm² at 650 °C). The expected decrease in R_p was not observed for the sample with higher Nd content (7% Nd). Thus, it can be said that there is a distinction between the compositions Nd_0.05Gd_0.15Ce_0·80O_2-δ and Nd_0.07Gd_0.15Ce_0·78O_2-δ; the co-doping of Nd in NGCO incremented the oxygen ion diffusion path, thereby optimization in the triple phase boundary (TPB) sites was obtained. Furthermore, SEM and TGA measurements were conducted to clarify the reasons of such improvements. This work showed that an NGCO-LSCF composite can be considered as a potential candidate for cathode material for future IT-SOFC applications.     

Comparative study of perovskites as cathode contact materials between an La0.8Sr0.2FeO3 cathode and a Crofer22APU interconnect in solid oxide fuel cells

Perovskites of different compositions were tested as cathode contact material between an La_0.8Sr_0.2FeO₃ cathode and a Crofer22APU interconnect by resistance measurements at 800 °C. The materials tested were LaNi_0.6Fe_0.4O₃ and La_0.8Sr_0.2FeO₃ which are also used as cathodes; La_0.8Sr_0.2Mn_0.5Co_0.5O₃ and La_0.8Sr_0.2Mn_0.1Co_0.3Fe_0.6O₃, selected for comparing perovskites with different Mn contents; and La_0.8Sr_0.2Co_0.75Fe_0.25O₃ and La_0.8Sr_0.2Co_0.75Cu_0.25O₃ for comparing perovskites with high Co content and two possible partial substitutions of the Co. The initial area-specific contact resistance (ASR) was found to depend on the electrical conductivity of the measured perovskites. Time evolution of the ASR depended on the interactions between the contact material and the interconnect, showing the highest degradation rates for LaNi_0.6Fe_0.4O₃ and La_0.8Sr_0.2FeO₃. Chromium from the interconnect reacted with the Sr-containing perovskites forming SrCrO₄. With the contact material without strontium chromium-containing perovskites were formed. A reduced interfacial reaction was achieved by application of a MnCo_1.9Fe_0.1O₄ spinel protection layer on Crofer22APU in terms resulting in low and stable ASR.     

Selection of cathode contact materials for solid oxide fuel cells

The goal of this work is to identify suitable cathode contact materials (CCM) to bond and electrically connect LSCF cathode to Mn_1.5Co_1.5O₄-coated 441 stainless steel after sintering at the relatively low temperature of 900-1000 °C. A wide variety of CCM candidates are synthesized and characterized. For each, the conductivity, coefficient of thermal expansion, sintering behavior, and tendency to react with LSCF or Mn_1.5Co_1.5O₄ are determined. From this screening, LSCF, LSCuF, LSC, and SSC are selected as the most promising candidates. These compositions are applied to LSCF and Mn_1.5Co_1.5O₄-coated 441 stainless steel coupons and subjected to 200 h ASR testing at 800 °C. After area-specific resistance testing, the specimens are cross-sectioned and analyzed for interdiffusion across the CCM/LSCF or CCM/Mn_1.5Co_1.5O₄ interfaces. A relatively narrow band of interdiffusion is observed.     

Effects of chromium poisoning on the long-term oxygen exchange kinetics of the solid oxide fuel cell cathode materials La0.6Sr0.4CoO3 and Nd2NiO4

The influence of chromium poisoning on the long-term stability of the oxygen exchange kinetics of the promising IT-SOFC cathode materials La_0.6Sr_0.4CoO_3−δ (LSC) and Nd₂NiO_4+δ (NDN) is investigated in-situ by dc-conductivity relaxation experiments. The as-prepared LSC and NDN samples show high chemical oxygen surface exchange coefficients k_chem. After the deposition of a 10 nm thick Cr-layer onto the surface, k_chem of LSC decreases to 50% of the initial value. Additional chromium deposition of 20 nm on LSC leads to a further decrease of k_chem to 27% of the initial value. In contrast, the effect of a 10 nm thick Cr-layer on k_chem of NDN is negligible. Even with additional 20 nm of chromium and a total testing time of 1750 h, the nickelate retains a k_chem of 60% of the initial value. X-ray photoelectron spectroscopy (XPS) of the degraded. LSC shows a significantly altered surface cation composition with Sr-enrichment down to 30 nm depth while XPS analysis of the degraded NDN reveals a thin surface zone of approximately 30 nm containing nickel and chromium. In contrast to LSC, the changes in the surface composition of NDN due to Cr-poisoning ultimately had only a minor influence on the surface exchange properties.     

Total polarization effect on the location of maximum temperature value in planar SOFC

The aim of the present study is the evaluation and the location of the maximum temperature values within the solid and porous components of a planar SOFC under the effect of total polarization: Ohmic, activation, concentration and the chemical reaction.The temperature field in SOFC components (interconnection, cathode, anode and electrolyte) is obtained by developing a mathematical model in FORTRAN language.The mathematical model predictions show the effect of the overpotentials on the thermal gradient and its locations in an SOFC with two geometries: i) anode or ii) electrolyte supported. The results are also discussed, following the SOFC low or high operating temperatures.