Cell/stack electrochemical performance and stability of cone-shaped tubular SOFCs for direct operation with methane

Cone-shaped tubular anode-supported solid oxide fuel cells (SOFCs) and two-cell-stack based on NiO-YSZ traditional anodes direct utilization methane as fuel were successfully developed in this study. The single cell exhibited maximum power densities of 1.255 W cm⁻² for hydrogen and 1.099 W cm⁻² for methane at 800 °C, respectively. A stability test of the single cell was performed with different constant current densities at 700 °C in methane. The results indicated that the single cell can be operated stable at high current density in methane. And EDX results showed that there is no measurable coking effect of operation in methane at relatively high current density.A two-cell-stack based on the above-mentioned SOFCs was fabricated and tested by direct utilization of methane. Its typical electrochemical performance was investigated. The two-cell-stack provided a maximum power output of about 3.5 W (350 mW cm⁻² calculated using effective cathode area) by directly using methane at 800 °C. The stack experienced 20 h durability testing. The results demonstrated that the stack was kept at around 1 V (J = 0.05 A cm⁻²) at 700 °C. The stack presented basically stably during the whole test, and the performance of the stack is acceptable for application.     

Tailoring cathode composite boosts the performance of proton-conducting SOFCs fabricated by a one-step co-firing method

A strategy of tailoring the ceramic cathode composite is presented to improve the performance of proton-conducting solid oxide fuel cells (SOFCs) prepared by a one-step co-firing process. Comparing to the conventional way of using BaCe_0.7Zr_0.1Y_0.2O_3-δ (BCZY) in the composite cathode for BCZY-electrolyte based cells, the replacement of BCZY by BaZr_0.8Y_0.2O_3-δ (BZY) mitigates the reaction between the two ceramic phases in the composite cathode during the co-firing process and also keeps the cathode with sufficient porosity for ample gas diffusion which could assist in adequate cathode reactions. As a result, the BCZY-electrolyte based cell with Sm_0.5Sr_0.5CoO_3-δ (SSC)-BZY composite cathode shows a power output of 300?mW?cm^?2 at 600?°C, which is the largest ever reported for proton-conducting SOFCs prepared by a one-step co-firing process. The strategy of tailoring the composite cathode offers both small ohmic resistance and polarization resistance, providing a promising way to develop single-step co-fired proton-conducting SOFCs.     

Effective buffer layer thickness of La-doped CeO2 for high durability and performance on La0.9Sr0.1Ga0.8Mg0.2O3-δ electrolyte supported type solid oxide fuel cells

Solid oxide fuel cells (SOFCs) have been gaining increased attention in the energy sector. Commonly, yttria-stabilized zirconia is widely employed as commercial electrolyte, however, resulted in drawbacks such as high-temperature operating and low conductivity which negatively affect the durability and efficiency. Thus there are many efforts to find high-ionic conductors. From the point of manufacturing, the major difficulty of LaGaO₃-based electrolyte as a high-ionic conductor is its incompatibility with commercial Ni-based anodes during high-temperature processes as well as operating. Several interlayers have been introduced to prevent the reaction between LaGaO₃-based electrolyte and Ni-based anode. In this study, we investigate the optimal thickness of the La-doped CeO₂ (LDC) interlayer by the screen-printing method using La_0.9Sr_0.1Ga_0.8Mg_0.2O_3-δ for the commercial electrolyte supported SOFCs. As a result, the superior power performance of 2.2 W·cm^?2 at 1123 K is achieved through the optimized LDC thickness of 20 μm through not lab-scaled but commercial ceramic manufacturing processing.     

3D imaging and quantification of interfaces in SOFC anodes

Solid oxide fuel cells (SOFCs) are functional electrochemical conversion devices whose performance is strongly dependent on electrode microstructure. Both the performance and lifetime of these electrochemical devices can be considerably enhanced by the ability to design better electrodes. Data acquired from high resolution 3D imaging techniques were used in the quantification of two electrode structures of different compositions. The quantified nickel-based anode data through the analysis of particle sizes with their metal–metal and ceramic–ceramic neck sizes, metal–ceramic interface sizes, volume fractions, and triple-phase boundary densities, demonstrate it is possible to understand how microstructure contributes to differences observed in electrochemical and mechanical performance; facilitating optimisation of electrode micro/nano structure towards improved performance. In doing so, new insights are gained that could be used to develop better electrodes.     

Nano-sized Sm0.5Sr0.5CoO3−δ as the cathode for solid oxide fuel cells with proton-conducting electrolytes of BaCe0.8Sm0.2O2.9

Nano-sized Sm_0.5Sr_0.5CoO_3−δ (SSC) was fabricated onto the inner face of porous BaCe_0.8Sm_0.2O_2.9 (BCS) backbone by ion impregnation technique to form a composite cathode for solid oxide fuel cells (SOFCs) with BCS, a proton conductor, as electrolyte. The electro-performance of the composite cathodes was investigated as function of fabricating conditions, and the lowest polarization resistance, about 0.21 Ω cm² at 600 °C, was achieved with BCS backbone sintered at 1100 °C, SSC layer fired at 800 °C, and SSC loading of 55 wt.%. Impedance spectra of the composite cathodes consisted of two depressed arcs with peak frequency of 1 kHz and 30 Hz, respectively, which might correspond to the migration of proton and the dissociative adsorption and diffusion of oxygen, respectively. There was an additional arc peaking at 1 Hz in the Nyquist plots of a single cell, which should correspond to the anode reactions. With electrolyte about 70 μm in thickness, the simulated anode, cathode and bulk resistances of cells were 0.021, 0.055 and 0.68 Ω cm² at 700 °C, relatively, and the maximum power density was 307 mW cm⁻² at 700 °C.     

Ni–LnOx (Ln = Dy, Ho, Er, Yb and Tb) cermet anodes for intermediate-temperature solid oxide fuel cells

Ni–LnO_x (Ln = Dy, Ho, Er, Yb and Tb) cermets are investigated as the anodes of intermediate-temperature solid oxide fuel cells using ceria-based electrolyte to seek insights into the properties and electrocatalytic activity of these lanthanide oxides, whose oxygen ion conductivity is negligible. They have displayed similar electrochemical activity which is comparable to, if not higher than, those of the commonly Ni-doped ceria cermets. The anode performance has been found to depend strongly on cermet composition and porosity. Temperature programmed reduction study and EIS analysis under different hydrogen partial pressure suggest that the catalytic activity of the Ni–LnO_x cermets might be originated from the hydrogen adsorption ability on the LnO_x surface, promoting hydrogen spillover process, and consequently enhancing the electrochemical oxidation of the fuel.     

Chromium poisoning for prolonged lifetime of electrodes in solid oxide fuel cells - Review

Solid Oxide Fuel Cells (SOFCs) offer low carbon emission and high efficient energy conversion systems. For the wide commercial distribution of this system, one of the technological issues and challenges is prolonged durability: the SOFC systems should have a long lifetime of more than 10 years. The volatile chromium species poisoning is the key degradation factor to overcome at the functional ceramics of air electrode (cathode)/interlayer/electrolyte interfaces in the SOFC system among many degradation factors. This paper reports recent degradation mechanisms, especially on the chromium (Cr) vapors poisoning at the perovskite oxide cathode. The Cr-concentration levels at cathodes were evaluated from the reported data at small cells and practical cell-stacks. The interactions of volatile Cr species and perovskite oxide cathode surface were evaluated by the chemical reaction of cathode materials with Cr-vapors to form SrCrO₄ and the electrochemical induced Cr-vapors reduction (Cr⁶⁺ to Cr³⁺) to form Cr₂O₃ at (La,Sr,Ca)MnO₃-based and (La,Sr)(Co,Fe)O₃-based materials. Recovery mechanism from Cr-poisoning was reanalyzed at the (La,Sr)(Co,Fe)O₃/ceria-based interlayer/YSZ electrolyte interfaces by Cr-cleaning reaction with the evaporation of Cr₂O₃/SrCrO₄ and nano-meter level cation migration/rearrangement effects with phase separation and new phases formation. This paper is covering not only the elucidation of degradation mechanism but also the fundamentals of physical and chemical analyses on perovskite oxide cathode surface and interfaces. An insight for new materials combination for the next-generation SOFCs is also included.     

Fabrication and performance of PEN SOFCs with proton-conducting electrolyte

A positive-electrolyte-negative (PEN) assembly solid oxide fuel cell (SOFC) with a thin electrolyte film for intermediate temperature operation was fabricated. Instead of the traditional screen-printing method, both anode and cathode catalysts were pressed simultaneously and formed with the fabrication of nano-composite electrolyte by press method. This design offered some advantageous configurations that diminished ohmic resistance between electrolyte and electrodes. It also increased the proton-conducting rate and improved the performance of SOFCs due to the reduction of membrane thickness and good contact between electrolyte and electrodes. The fabricated PEN cell generated electricity between 600°C and 680°C using H₂S as fuel feed and air as oxidant. Maximum power densities 40 mW·cm⁻² and 130 mW·cm⁻² for the PEN configuration with a Mo-Ni-S-based composite anode, nano-composite electrolyte (Li₂SO₄+Al₂O₃) film and a NiO-based composite cathode were achieved at 600°C and 680°C, respectively.     

A review study on titanium niobium oxide-based composite anodes for Li-ion batteries: Synthesis,structure, and performance

The growing demands for Li-ion batteries (LIBs) in the electrification revolution, require the development of advanced electrode materials. Recently, intercalating titanium niobium oxide (TNO) anode materials with the general formula of TiNb_xO_2+2.5x have received lots of attention as an alternative to graphite and Li₄Ti₅O₁₂ commercial anodes. The desirability of this family of compounds stems from their high theoretical capacities (377–402 mAh/g), high safety, high working voltage, excellent cycling stability, and significant pseudocapacitive behavior. However, the rate performance of TNO-based anodes is poor owing to their low electronic and ionic conductivities. TNO-based composites generally are prepared with two aims of enhancing the conductivity of TNO and achieving a synergic effect between the TNO and the other component of the composite. Compositing with carbon matrices, such as graphene and carbon nanotubes (CNTs) are the most studied strategy for improving the conductivity of TNO and optimizing its high-rate performance. Also, for obtaining anode materials with high capacity and high long-term stability, the composites of TNO with transition metal dichalcogenides (TMDs) materials were proposed in previous literature. In this work, a comprehensive review of the TNO-based composites as the anodes for LIBs is presented which summarizes in detail the main recent literature from their synthesis procedure, optimum synthesis parameters, and the obtained morphology/structure to their electrochemical performance as the LIBs anode. Finally, the research gaps and the future perspective are proposed.     

Improving the sealing performance of glass-ceramics for SOFCs applications by a unique ‘composite’ approach: A study on Na2O-SiO2 glass-ceramic system

The rigid nature of sealing glass-ceramics restricts the thermal cycling stability of Solid Oxide Fuel Cells (SOFCs), which thus evokes an interest in designing a sealing glass without crystallization under the operational condition of SOFCs. In this paper, we report that the sealing performance of 30Na₂O-70SiO₂ (in mole%) glass-ceramic can be significantly improved by Fe₂O₃ dopant through a composite approach. In particular, the crystallization in glass can be suppressed by appropriate Fe₂O₃ dopant amount (8?mol%), which results in the improved sealing property of glass. In addition, the glass modified with Fe₂O₃ shows good chemical compatibility with 8?mol% yttria-stabilized zirconia (8YSZ) electrolyte and metallic interconnect (430 stainless steel) in dual atmospheres. The possible mechanism for the improved sealing performance of 30Na₂O-70SiO₂ glass-ceramic by this unique composite approach is also discussed.     

FexCo0.5−xNi0.5-SDC anodes for low-temperature solid oxide fuel cells

Trimetal alloys, Fe_xCo_0.5−xNi_0.5 (x = 0.1, 0.2, 0.25, 0.3, 0.4), were studied as anodes for low-temperature solid oxide fuel cells (LT-SOFCs) based on GDC (Ce_0.9Gd_0.1O_1.95) electrolytes. The alloys were formed by in situ reduction of Fe_xCo_0.5−xNi_0.5O_y composites, which were synthesized using a glycine-nitrate technique. Symmetrical cells consisted of Fe_xCo_0.5−xNi_0.5-SDC electrodes and GDC electrolytes, and single cells consisted of Fe_xCo_0.5−xNi_0.5-SDC (Ce_0.8Sm_0.2O_1.9) anodes, GDC electrolytes, and SSC (Sm_0.5Sr_0.5CoO₃)-SDC cathodes were prepared using a co-pressing and co-firing process. Interfacial polarization resistances and I-V curves of these cells were measured at temperature from 450 to 600 °C. With Fe_0.25Co_0.25Ni_0.5-SDC as anodes, the cells showed the lowest interfacial resistance and highest power density. For example, at 600 °C, the resistance was about 0.11 Ω cm² and power density was about 750 mW cm⁻² when humidified (3% H₂O) hydrogen was used as fuel and stationary air as oxidant. Further, the cell performance was improved when the molar ratio of Fe:Co:Ni approached 1:1:2, i.e. Fe_0.25Co_0.25Ni_0.5. In addition, higher power density and lower interfacial resistance were obtained for cells with the Fe_0.25Co_0.25Ni_0.5-SDC anodes comparing to that with Ni-SDC anodes, which have been usually used for LT-SOFCs. The promising performance of Fe_xCo_0.5−xNi_0.5 as anodes suggests that trimetallic anodes are worth considering for SOFCs that operate at low-temperature.     

Electrochemical Partial Oxidation of Methane in Solid Oxide Fuel Cells: Effect of Anode Reforming Activity

Direct-methane solid oxide fuel cells were used to produce electricity and syngas. During initial operation at 750 °C, the cells produced 0.9 W/cm² and ≈90% methane conversion to syngas at a rate of 30 sccm/cm². However, the methane conversion decreased continuously over the first 30–40 h of operation, even though the solid oxide fuel cells (SOFC) electrical performance was stable. An additional catalyst layer on the anode yielded more stable methane conversion to syngas.     

Durability and performance of CGO barriers and LSCF cathode deposited by spray-pyrolysis

Ce_0.9Gd_0.1O_1.95 (CGO) protective layers are prepared by two different methods to prevent the reaction between the Zr_0.84Y_0.16O_1.92 (YSZ) electrolyte and the La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF) cathode. In the first method, the CGO layers are deposited by an airbrushing technique from an ink containing CGO particles without and with cobalt as sintering aids. The second strategy consists in preparing both a dense CGO barrier layer and a porous LSCF cathode by spray-pyrolysis deposition, in order to further reduce the fabrication temperature and minimize the reaction between the cell components. The samples prepared by spray-pyrolysis exhibit better performance and durability than those obtained by conventional sintering methods. The results suggest that the interfacial reactivity between YSZ and LSCF as well as the Sr-enrichment at the cathode surface can be avoided by using low-temperature fabrication methods and by operating at temperatures lower than 650?°C.     

Ni1−xCox/YSZ cermet anodes for solid oxide fuel cells

Nanocrystalline and homogeneous powder mixtures of (Ni_1−xCo_xO_y)+YSZ were obtained by combustion synthesis, and reduced in H₂ at 800 °C to obtain Ni_1−xCo_x/YSZ cermets, and three layer symmetrical cells cermet/YSZ/cermet. These three layer cells were co-firing at 1450 °C, and then reduced to obtain porous Ni_1−xCo_x/YSZ cermet layers with good adhesion to the electrolyte. Results obtained under OCV show that partial substitution of Ni with Co lowers the polarisation resistance, especially the main contribution which is usually most dependent on the cermets microstructure. This trend is reverted for high fractions of Co, and the polarisation resistance obtained for Co/YSZ cermets is much higher than for Ni/YSZ. The low frequency contribution of the polarisation resistance was mainly dependent on the partial pressures of H₂ and H₂O, and is less dependent on the substitution of Ni with Co.     

Low-temperature SOFCs using biomass-produced gases as fuels

The electromotive force (e.m.f) is calculated for solid oxide fuel cells (SOFCs) based on doped ceria electrolytes using biomass-produced gases (BPG, 14.7% CO, 14.2% CO₂, 15.3% H₂, 4.2% CH₄, and 51% N₂) as fuels and air as oxidant. It reveals that the BPG derived e.m.f. is very close to hydrogen when doped ceria is used as the electrolyte. A 35-m-thick samaria-doped ceria based single cell was tested between 450 and 650°C using BPG as fuel. Maximum power density of about 700 mW cm^–2 was achieved at 650 °C. The open-circuit voltage at 450 °C was 0.96 V, close to the calculated value. However, the cell power density using BPG as fuel was relatively lower than that using humidified hydrogen (3% H₂O), and close to that using humidified methane (3% H₂O). Impedance measurements indicate that the relatively lower power output may be attributed to the high anode--electrolyte interfacial polarization resistance when BPG is used as fuel.     

浸渗法制备固体氧化物燃料电池复合阴极研究进展

中低温化是目前固体氧化燃料电池研究的主要方向,影响其发展的主要问题是电解质及阴极材料的研制.浸渗法制备复合阴极能够显著提高电池在中低温下的电化学性能和效率,是目前研究的热点之一.本文介绍了近年来采用具有催化活性的电极材料、贵金属、氧离子传导材料等作为浸渗剂制备复合阴极的研究现状,并对其发展方向进行了展望.     

Effects of manganese substitution at the B-site of lanthanum-rich strontium titanate anodes on fuel cell performance and catalytic activity

Sr_0.4La_0.6Ti_1−xMn_xO_3−δ with rhombohedral structure has been investigated in terms of their electrochemical performance, redox stability, and electro-catalytic properties for solid oxide fuel cell anodes. The performance of Sr_0.4La_0.6Ti_1−xMn_xO_3−δ anodes for solid oxide fuel cells strongly depends on the Mn substitution at the B-site of the perovskites. Electrical conductivity of Sr_0.4La_0.6Ti_1−xMn_xO_3−δ increases with increasing Mn content. X-ray photoelectron spectroscopy analysis reveals that the amount of Mn³⁺ and Ti³⁺, which is an electronic charge carrier, increases with Mn doping. The reduced anode powders with high Mn/Ti ratio show oxygen storage capability and a low carbon deposition rate. Linear thermal expansion coefficients of Sr_0.4La_0.6Ti_1−xMn_xO_3−δ anodes range from 9.46×10⁻⁶ K⁻¹ to 11.3×10⁻⁶ K⁻¹. The maximum power densities of the single cell with the Sr_0.4La_0.6Ti_0.2Mn_0.8O_3−δ anode in humidified H₂ and CH₄ at 800 °C are 0.29 W cm⁻² and 0.24 W cm⁻², respectively.     

Performance of an anode-supported tubular solid oxide fuel cell (SOFC) under pressurized conditions

An anode-supported tubular solid oxide fuel cell (SOFC) with a 15-μm thick YSZ electrolyte and an active area of 100 cm² was successfully fabricated by co-firing process, and the cell performance was measured under both atmospheric and pressurized conditions. The experimental results showed that the cell performance was significantly improved under the pressurized condition. When the pressure was increased from 1 to 6 atm, the maximum power density increased from 135.0 to 159.0 mW cm⁻² at 650 °C, and from 266.7 to 306.0 mW cm⁻² at 800 °C. The maximum power density at 800 °C and 4 atm was decreased from 334.8 to 273.9 mW cm⁻² when increasing the fuel utilization from 10% to 90%. Under the test condition of 70% fuel utilization, 800 °C and 4 atm, the cell could run stably at 0.7 V and 350 mA cm⁻² for 50 h, almost without any performance loss.     

Correlation between yttria stabilized zirconia particle size and morphological properties of NiO–YSZ films prepared by spray coating process

A systematic approach was taken to investigate the morphology of NiO–yttria stabilized zirconia (YSZ) films deposited by a spray coating process. The final morphological aspects of anode films were influenced by the particle size of YSZ powders and the milling time of the slurries used for film deposition. YSZ powders with average particle size of 17 and 52 nm were obtained from powders calcined at 800 and 1000 °C, respectively. The results obtained by rheological studies pointed out that slurries prepared from YSZ powders calcinated at 1000 °C and milling time of 20 h had more stability. All slurries presented thixotropic and pseudoplastic behaviors.     

A novel BaCe0.5Fe0.3Bi0.2O3–δ perovskite-type cathode for proton-conducting solid oxide fuel cells

A perovskite-type BaCe_0.5Fe_0.3Bi_0.2O_3-δ (BCFB) was employed as a novel cathode material for proton-conducting solid oxide fuel cells (SOFCs). The single cells with the structure of NiO-BaZr_0.1Ce_0.7Y_0.2O_3-δ (BZCY7) anode substrate|NiO-BZCY7 anode functional layer|BZCY7 electrolyte membrane|BCFB cathode layer were fabricated by a dry-pressing method and investigated from 550 to 700 °C with humidified hydrogen (~3% H₂O) as the fuel and the static air as the oxidant. The low interfacial polarization resistance of 0.098 Ω cm² and the maximum power density of 736 mW cm⁻² are achieved at 700 °C. The excellent electrochemical performance indicates that BCFB may be a promising cathode material for proton-conducting SOFCs.