Preliminary Study on Direct Operation of LT‐SOFCs Based on GDC Electrolyte Film With DME Fuel

Direct operation of anode‐supported cone‐shaped tubular low temperature solid oxide fuel cells (LT‐SOFCs) based on gadolinia‐doped ceria (GDC) electrolyte film with dimethyl ether (DME) fuel was preliminarily investigated in this study. The single cell exhibited maximum power densities of 500 and 350 mW cm^–2 at 600 °C using moist hydrogen and DME as fuel, respectively. A durability test of the single NiO‐GDC/GDC/LSCF‐GDC cell was performed at a constant current of 0.1 A directly fuelled with DME for about 200 min at 600 °C. The results indicate that the single cell coking easily directly operated in DME fuel. EDX result shows a clear evidence of carbon deposition in the anode. Further studies are needed to develop the novel anti‐carbon anode materials, relate the carbon deposition with anode microstructure and cell‐operating condition.     

Interaction of a ceria‐based anode functional layer with a stabilized zirconia electrolyte: Considerations from a materials perspective

We report on the materials interaction of gadolinium‐doped ceria (GDC) and yttria‐stabilized zirconia (YSZ) in the context of high‐temperature sintering during manufacturing of anode supported solid oxide fuel cells (AS–SOFC). While ceria‐based anodes are expected to show superior electrochemical performance and enhanced sulfur and coking tolerance in comparison to zirconia‐based anodes, we demonstrate that the incorporation of a Ni–GDC anode into an ASC with YSZ electrolyte decreases the performance of the ASC by approximately 50% compared to the standard Ni–YSZ cell. The performance loss is attributed to interdiffusion of ceria and zirconia during cell fabrication, which is investigated using powder mixtures and demonstrated to be more severe in the presence of NiO. We examine the physical properties of a GDC–YSZ mixed phase under reducing conditions in detail regarding ionic and electronic conductivity as well as reducibility, and discuss the expected impact of cation intermixing between anode and electrolyte.     

Characterization of core-shell structured Ni@GDC anode materials synthesized by ultrasonic spray pyrolysis for solid oxide fuel cells

Core-shell structured NiO@GDC powders with NiO cores and GDC shells were synthesized by ultrasonic spray pyrolysis (USP) with a four-zone furnace. The morphology of the as-synthesized powders can be modified by controlling parameters such as the precursor pH, carrier gas flow rate, and zone temperature. At high carrier gas flow rates, the as-synthesized core-shell structured NiO@GDC powders have raisin-like morphology with a rough surface; this is due to fast gas exhaustion and insufficient particle ordering. The core-shell structured Ni@GDC anode showed considerable electrochemical performance enhancement compared to the conventionally-mixed Ni-GDC anode. The polarization resistance (R_p) of conventionally-mixed Ni-GDC anodes increases gradually as a function of the operation time. Alternatively, the core-shell structured Ni@GDC anode synthesized by USP does not exhibit any significant performance degradation, even after 500 h of operation. This is the case because the rigid GDC ceramic shell in the core-shell structured Ni@GDC may restrain Ni aggregation.     

Effects of mass fraction of La0.9Sr0.1Cr0.5Mn0.5O3-δ and Gd0.1Ce0.9O2-δ composite anodes for nickel free solid oxide fuel cells

The optimal anode mass fraction of La_0.9Sr_0.1Cr_0.5Mn_0.5O_3-δ (LSCM) and Gd_0.1Ce_0.9O_2-δ (GDC) is evaluated in this study. The anodes with GDC share of 30–100 wt.% are investigated. Initial polarization resistance decreased as the GDC share increased. However, anodes with GDC share over 80 wt.% significantly deteriorated in the degradation tests. Nano-scale cracks were observed in the GDC phase at the grain boundaries after the test. These nano-cracks were not observed in composite anodes, from which it is implied that LSCM has stabilization effect on GDC structure. The mass fraction of LSCM : GDC = 30 : 70 wt.% is found to be optimal in terms of initial electrochemical performance and stability. The optimal LSCM-GDC shows lower polarization resistance than conventional Ni-YSZ at low temperatures, which is comparable to Ni-GDC anode.     

SOFC中不同浓度干甲烷在Ni-YSZ阳极上的反应

引言 天然气是适于固体氧化物燃料电池(SOFC)应用的燃料之一,天然气中主要成分是甲烷.甲烷通过全氧化或部分氧化[1-4]反应,在发电的同时,生成适于发电或其他用途的富含H2、CO的气体.     

Carbon-tolerance effects of Sm0.2Ce0.8O2−δ modified Ni/YSZ anode for solid oxide fuel cells under methane fuel conditions

Samarium-doped ceria (SDC) is coated onto a Ni/yttria-stabilized zirconia (Ni/YSZ) anode for the direct use of methane in solid-oxide fuel cells. Porous SDC thin layer is applied to the anode using the sol–gel coating method. The experiment was performed in H₂ and CH₄ conditions at 800 °C. The cell performance was improved by approximately 20 % in H₂ conditions by the SDC coating, due to the high ionic conductivity, the mixed ionic and electronic conductive property of the SDC, and the increased triple phase boundary area by the SDC coating in the anode. Carbon was hardly deposited in the SDC-coated Ni/YSZ anode. The cell performance of the SDC-coated Ni/YSZ anode did not show any significant degradation for up to 90 h under 0.1 A cm^?2 at 800 °C. The porous thin SDC coating on the Ni/YSZ anode provided the electrochemical oxidation of CH₄ over the whole anode, and minimized the carbon deposition by electrochemical carbon oxidation.     

Direct-methane anode-supported solid oxide fuel cells fabricated by aqueous gel-casting

Direct methane Solid Oxide Fuel Cells (SOFCs) operated under catalytic partial oxidation (CPOX) conditions are investigated, focusing on the processing of the anode support and the anode deactivation caused by carbon deposition. Anode-supported SOFCs based on gadolinium-doped ceria (GDC) electrolyte, and NiO-GDC anode support were fabricated by the gel-casting method. Suitable aqueous slurries formulations of NiO–GDC were prepared, starting NiO-GDC nanocomposite powders, agarose as gelling agent and rice starch as pore former. Electrochemical and mechanical tests evidenced that the support of 550 ± 50 µm thickness and 10 wt% pore former is a good candidate for direct-methane SOFCs. The cells operating under stoichiometric conditions of CPOX reached a performance of 0.64 W·cm^?2 at 650 ºC, a very close value to that measured under humidified hydrogen (0.71 W·cm^?2). The best electrochemical stability of the cell is achieved at a CH₄/O₂ ratio of 2.5, showing no evidence of carbon deposition and reducing nickel re-oxidation significantly.     

Cu-Ce-Zr-O／YSZ阳极材料以甲烷为燃料时的抗积炭性能

为研究甲烷在固体氧化物燃料电池中操作稳定性,分别采用共沉淀法和柠檬酸溶胶.凝胶法制备了10%CuO-Ce0.15Zr0.85O2催化剂,并以此为阳极催化剂、LSM为阴极制成了YSZ电解质支撑的SOFC单电池.用XRD对材料进行表征;用SEM对阳极,阴极进行表征.以甲烷为燃料对单电池发电性能进行测试,研究了两种不同方法制备的Cu-Ce-Zr-O阳极催化剂的抗积炭性能.相对于共沉淀法,溶胶-凝胶法制备的阳极结构和发电性能都要优于前者.长期稳定性方面,共沉淀法和溶胶.凝胶法制备的Cu-Ce-Zr-O/YSZ阳极都较传统的Ni-YSZ阳极更能够长期稳定运行.     

Electrochemical characteristics of electrospun La0.6Sr0.4Co0.2Fe0.8O3−δ-Gd0.1Ce0.9O1.95 cathode

The poor activity of cathode materials for electrochemical reduction of oxygen in intermediate and low temperature regime (<700 °C) is a key obstacle to reduced-temperature operation of solid oxide fuel cells (SOFCs). In our previous work, the direct methane fuel cell exhibits approximately 1 W cm⁻² at 650 °C in hydrogen atmosphere without any functional layers when the electrospun LSCF–GDC cathode was applied into the La₂Sn₂O₇–Ni–GDC anode-supported cell, which is approximately two times higher performance than 0.45 W cm⁻² of the cell with the conventional LSCF–GDC cathode. For detailed analysis of the fibrous cathode, the symmetrical cells with the electrospun and conventional LSCF–GDC cathode are fabricated, and then their electrochemical characteristics are measured by using electrochemical impedance spectroscopy (EIS). Each resistance contribution is determined by equivalent circuit consisting of a series resistance (R_s) and three arcs to describe the polarization resistance of the cathode. Total polarization resistance of the electrospun LSCF–GDC cathode is approximately two times lower than that of the conventional LSCF–GDC cathode at 650 °C, which is attributed to fibrous microstructures and large amount of pores in 100–200 nm. The results correspond to the difference in the cell performances obtained from our previous work.     

SOFC fuelled by methane without coking: optimization of electrochemical performance

In recent years, fuel cell technology has attracted considerable attention from several fields of scientific research as fuel cells produce electric energy with high efficiency, emit little noise, and are non-polluting. Solid oxide fuel cells (SOFCs) are particularly important for stationary applications due to their high operating temperature (1,073–1,273 K). Methane appears to be a fuel of great interest for SOFC systems because it can be directly converted into hydrogen by direct internal reforming (DIR) within the SOFC anode. Unfortunately, internal steam reforming in SOFC leads to inhomogeneous temperature distributions which can result in mechanical failure of the cermet anode. Moreover this concept requires a large amount of steam in the fed gas. To avoid these problems, gradual internal reforming (GIR) can be used. GIR is based on local coupling between steam reforming and hydrogen oxidation. The steam required for the reforming reaction is obtained by the hydrogen oxidation. However, with GIR, Boudouard and cracking reactions can involve a risk of carbon formation. To cope with carbon formation a new cell configuration of SOFC electrolyte support was studied. This configuration combined a catalyst layer (0.1%Ir–CeO₂) with a classical anode, allowing GIR without coking. In order to optimise the process a SOFC model has been developed, using the CFD-Ace+ software package, and including a thin electrolyte. The impact of a thin electrolyte on previous conclusions has been assessed. As predicted, electrochemical performances are higher and carbon formation is always avoided. However a sharp decrease in the electrochemical performances appears at high current densities due to steam clogging.     

Fabrication of thin-film gadolinia-doped ceria (GDC) interdiffusion barrier layers for intermediate-temperature solid oxide fuel cells (IT-SOFCs) by chemical solution deposition (CSD)

A dense gadolinia-doped ceria (GDC) interdiffusion barrier layer as thin as 300 nm was successfully fabricated on a rigid anode/electrolyte bilayer substrate using the chemical solution deposition (CSD) process for intermediate temperature solid oxide fuel cells (SOFCs). Drying-related macro-defects were removed by employing drying control chemical additives (DCCA), which effectively relieved drying stresses. The major process flaws caused by the constraining effects of the rigid substrate were completely eliminated by the addition of GDC nanoparticles into the chemical solution, which suppressed the generation of microstructural anisotropy by mitigating the predominant bi-axial substrate constraints. As a consequence, a thin film GDC interlayer was successfully deposited with a high volumetric density, effectively preventing the chemical interaction between the electrolyte and cathode during the fabrication process and subsequent operation. The cell test and microstructural analysis confirmed excellent electrochemical performance and structural and chemical stability. The CSD process presented in this paper is considered to be a promising technology for the practical preparation of GDC thin film barrier layers for intermediate temperature SOFCs based on the film quality, processing costs and potential for large-scale production.     

Effect of GDC interlayer thickness on durability of solid oxide fuel cell cathode

Long-term performance degradation of solid oxide fuel cell (SOFC) cathode as a function of gadolinium doped ceria (GDC) interlayer thickness has been studied under accelerated operating conditions. For this purpose, SOFC half-cells with GDC interlayer thicknesses of 2.4, 3.4 and 6.0 µm were fabricated and tested for 1000 h at 900 °C under constant current density of 1 A/cm². The half-cells consisted of lanthanum strontium cobalt ferrite (LSCF)/GDC composite cathode, GDC interlayer, scandia-ceria stabilized zirconia electrolyte and platinum anode as a counter electrode. Area specific resistance (ASR) of the half-cells was continuously measured over time. Higher increase in ASR was observed for the half-cells with GDC interlayer thickness of 2.4 and 6.0 µm, which is attributed to higher strontium (Sr) diffusion towards electrolyte and to cathode/GDC interface delamination coupled with small Sr diffusion, respectively. However, half-cell with GDC interlayer thickness of 3.4 µm showed smaller degradation rate due to highly dense GDC interlayer which had less interfacial resistance and suppressed Sr diffusion towards electrolyte.     

Lowering the sintering temperature of solid oxide fuel cell electrolytes by infiltration

A dense electrolyte with a relative density of over 95% is vital to prevent gas leakage and thus the achievement of high open circuit voltage in solid oxide fuel cells (SOFCs). The densification process of ceria based electrolyte requires high temperatures heat treatment (i.e. 1400–1500 °C). Thus, the minimum co-sintering temperatures of the anode-electrode bilayers are fixed at these values, resulting in coarse anode microstructures and consequently poor performance. The main purpose of this study is to densify gadolinia doped ceria (GDC), a common SOFC electrolyte, at temperatures lower than 1400 °C. By this aim, an approach involving the infiltration of polymeric precursors into porous electrolyte scaffolds, a method commonly used for composite SOFC electrodes, is proposed. By infiltrating polymeric precursors of GDC into porous GDC scaffolds, a reduction in the sintering temperature by at least 200 °C is achieved with no additives that might affect the electrical properties. Energy dispersive x-ray spectroscopy line scan analyses performed on porous GDC scaffolds infiltrated by a marker solution (polymeric FeOx precursor in this case) reveals a homogeneous infiltrated phase distribution, demonstrating the effectiveness of polymeric precursors.     

Improving the performance of silicon anode in lithium-ion batteries by Cu2O coating layer

The influence of a 200 nm Cu₂O coating layer on the electrochemical performance of an 800 nm Si thin-film anode was investigated by cyclic voltammetry, electrochemical impedance spectroscopy, and galvanostatic charge/discharge measurements. The electrochemical performance of the Si thin-film anode was improved by the coating layer. The coated Si anode exhibited higher values of conductivity in comparison with the pristine Si anode. Scanning electron microscopy images of the anodes after cycling test showed that the coated Si anode after cycling test had less cracks than the pristine Si anode. The galvanostatic charge/discharge measurements reveal that the cyclability and rate capability of the coated Si thin-film anode were better than the pristine Si thin-film anode.     

Effect of Ethane and Propane in Simulated Natural Gas on the Operation of Ni–YSZ Anode Supported Solid Oxide Fuel Cells

Solid oxide fuel cells with Ni–yittria‐stabilised zirconia (YSZ) anode supports were tested on surrogate natural gas fuels (methane containing 2.5–10% ethane and 1.25–5% propane) and compared with results for pure methane. Inert anode‐side diffusion barriers were found to help suppress coking on the Ni–YSZ anodes. However, carbonaceous deposits were observed on anode compartment surfaces and the barrier layers for all of the natural gas compositions tested. The addition of air to the natural gas was shown to suppress this coking. For natural gas with 5% ethane and 2.5% propane, the addition of 33% air yielded stable, coke‐free operation at 750 °C and 800 mA cm^–2. Cell performance on this fuel was only slightly worse than for the same cell operated with dry hydrogen.     

Development of a novel test system for in situ catalytic, electrocatalytic and electrochemical studies of internal fuel reforming in solid oxide fuel cells

A test system based around a thin‐walled extruded solid electrolyte tubular reactor has been developed, which enables the fuel reforming catalysis and surface chemistry occurring within solid oxide fuel cells and the electrochemical performance of the fuel cell to be studied under genuine operating conditions. It permits simultaneous monitoring of the catalytic chemistry and the cell performance, allowing direct correlation between the fuel cell performance and the reforming characteristics of the anode, as well as enabling the influence of drawing current on the catalysis and surface reaction pathways to be studied. Temperature‐programmed reaction measurements can be carried out on anodes in an actual SOFC, and have been used to investigate the reduction characteristics of different anode formulations, methane activation and methane steam reforming, and to evaluate the nature and level of carbon deposition on the anode during reforming. This revised version was published online in July 2006 with corrections to the Cover Date.     

操作条件对固体氧化物燃料电池阳极反应转变的影响

研究了不同电流密度下,甲烷浓度、反应温度对甲烷在SOFC中反应由部分氧化到完全氧化转变的规律;测量了不同电流密度下,阳极出口气体产生速率;确定了甲烷浓度和电池反应温度变化时甲烷电化学反应由部分氧化转变为完全氧化的电流密度门槛值,及该门槛值与甲烷浓度、电池操作温度的变化关系.结果说明甲烷开始发生完全氧化的电流密度门槛值与甲烷浓度成正比;甲烷浓度一定,温度升高,甲烷开始发生完全氧化的电流密度的门槛值也随之提高.     

复合阳极Ni-Fe/Ce0.9Gd0.1O1.95在阴极支撑单电池的性能及其表征

为了提高固体氧化物燃料电池在中温条件下的电性能,探索了一种双金属阳极的阴极支撑单电池。单电池以La_0.6Sr_0.4CoO₃（LSC）-Ce_0.9Gd_0.1O_1.95（GDC）为阴极支撑体,旋涂了甘氨酸-硝酸盐法制备的La_0.9Sr_0.1Ga_0.8Mg_0.2O_3-δ（LSGM）电解质及Sm_0.2Ce_0.8O_1.9（SDC）缓冲层,涂覆了由硬模板法和浸渍法结合制备的Ni-Fe/GDC双金属阳极。对制备材料进行了XRD和微观形貌分析,单电池电化学测试在800 ℃和750 ℃下,以氢气为燃料的最大功率密度达0.73 W/cm²和0.64 W/cm²,以甲烷为燃料时达0.41 W/cm²和0.40 W/cm²。测试后的SEM表明,阳极具有多孔的微观结构,金属颗粒均匀包覆蠕虫状GDC,保证了单电池具有较高的发电性能。     

Non-stationary catalytic cracking of methane over ceria-based catalysts: Mechanistic approach and catalyst optimization

The non-stationary cracking of methane over various noble metal/CeO₂-doped catalysts at 400 and 600 °C was followed by DRIFT spectroscopy and on the basis of the identified elementary steps a simplified kinetic modeling is proposed. The production of H₂ by direct decomposition of CH₄ on the noble metal is improved by the capacity of ceria to store carbonaceous surface species thanks to: (i) the spillover of carbonyls from noble metal particles towards basic hydroxyls formed on partially reduced Ce sites and (ii) the reverse spillover of ceria oxygen towards metal to oxidize the carbon issued from methane cracking. The resulting formate adspecies are in turn oxidized into carbon dioxide during the regeneration step. Doping the ceria with basic lanthanide oxides and replacing Pt by more efficient and eventually better dispersed metals for methane decomposition like Rh and Ir lead to significant improvements in the hydrogen productivity.     

Kinetics of internal steam reforming of methane on Ni/YSZ-based anodes for solid oxide fuel cells

The kinetics of steam reforming of methane was determined on a Ni-YSZ anode, which we refer to as anode ‘A’ and on a Ni-YSZ anode modified by the addition of a basic compound, which we refer to as anode ‘B’. A salient feature of our work is that the data were collected on 50 μm thick anodes screen-printed on 110 μm thick YSZ electrolytes and the experiments were carried out in a fuel cell configuration. Orders in methane and steam were both higher on the modified Ni-YSZ anode. Activation energy was also higher on this anode suggesting different nature of sites in the two anodes. In the present study we have attempted to generate kinetic data at steam/carbon ratios which are economically attractive for fuel cell operation.