Electrocatalytic activity of a Gd2Ti0.6Mo1.2Sc0.2O7-δ anode towards hydrogen and methane electro-oxidation in a solid oxide fuel cell

Mixed conducting oxide anodes are being considered for the direct utilisation of natural gas in high temperature fuel cells. This work refers to the electrochemical characterization of the pyrochlore Gd₂Ti_0.6Mo_1.2Sc_0.2O_7-δ (GTMS) as anode in a solid oxide fuel cell running in low humidity hydrogen or methane. The electro-oxidation reaction was investigated using impedance spectroscopy, potentiostatic measurements and cyclic voltammetry. Kinetic data were obtained for different fuels in the temperature range 845–932 °C. In a methane-fuelled cell, steam reforming appears to be the rate-limiting step. The overall polarisation resistance of the anode under open circuit conditions at 932 °C was 6.86 Ω·cm² in 97% H₂/3% H₂O, and 43 Ω·cm² in 97% CH₄/3% H₂O. For a 97% fuel-3% H₂O/GTMS//YSZ-Al₂O₃//Pt/air cell, the maximum power output at 932 °C was 9.5 mW/cm² and 1.8 mW/cm² in hydrogen and methane, respectively. First investigations on this type of electrode material show unidentified peaks on XRD spectra after electrochemical test, which indicate GTMS instability under experimental conditions. Paper presented at the 7th Euroconference on Ionics, Calcatoggio, Corsica, France, Oct. 1–7, 2000.     

Fuel quality and operational issues for polymer electrolyte membrane (PEM) fuel cells

Polymer electrolyte membrane (PEM) fuel cells are susceptible to degradation due to the catalyst poisoning caused by CO present in the fuel above certain limits. Although the amount of CO in the fuel may be within the permissible limit, the fuel composition (% CO₂, CH₄, CO and H₂O) and the operating conditions of the cell (level of gas humidification, cell temperature and pressure) can be such that the equilibrium CO content inside the cell may exceed the permissible limit leading to a degradation of the fuel cell performance. In this study, 50 cm² active area PEM fuel cells were operated at 55–60 °C for periods up to 250 hours to study the effect of methane, carbon dioxide and water in the hydrogen fuel mix on the cell performance (stability of voltage and power output). Furthermore, the stability of fuel cells was also studied during operation of cells in a cyclic dead end / flow through configuration, both with and without the presence of carbon dioxide in the hydrogen stream. The presence of methane up to 10% in the hydrogen stream showed a negligible degradation in the cell performance. The presence of carbon dioxide in the hydrogen stream even at 1–2% level was found to degrade the cell performance. However, this degradation was found to disappear by bleeding only about 0.2% oxygen into the fuel stream.     

Recent advances in single-chamber fuel-cells: Experiment and modeling

Single-chamber fuel cells (SCFC) are ones in which the fuel and oxidizer are premixed, and selective electrode catalysts are used to generate the oxygen partial pressure gradient that in a conventional dual-chamber design is produced by physical separation of the fuel and oxidizer streams. SCFCs have been shown capable of generating power densities above 700 mW/cm² with appropriate catalysts, making them potentially useful in many applications where the simplicity of a single gas chamber and absence of seals offsets the expected lower efficiency of SCFCs compared to dual-chamber SOFCs.SCFC performance is found to depend sensitively on cell microstructure, geometry, and flow conditions, making experimental optimization tedious. In this paper, we describe recent work focused on developing a quantitative understanding the physical processes responsible for SCFC performance, and the development of an experimentally-validated, physically-based numerical model to allow more rational design and optimization of SCFCs. The use of the model to explore the effects of fuel/oxidizer ratio, anode thickness, and flow configuration is discussed.     

Direct conversion of solid hydrocarbons in a molten carbonate fuel cell

Electrical characteristics of a molten carbonate fuel cell allowing direct electrochemical oxidation of dispersed hydrocarbons have been examined. As the fuel, graphite, anthracite, and cannel coal samples were used. Data illustrating the effect of electrolyte temperature, fuel type and dispersion, and also reactant gas mixture composition on the performance characteristics of the fuel cell, were obtained. Correlation between the specific characteristics of the fuel cell and the hydrogen content of fuel material was established. The maximum current-density values were achieved with hydrogen-rich cannel coal. For dispersed fuel samples, interparticle contact losses were found to have influence on the cell-generated voltage. The maximum cell opencircuit voltage was reached with stoichiometric oxygen-carbon dioxide mixture blown into the cathode. Yet, the largest current-density values were obtained when carbon dioxide lean mixtures were used. Even at zero carbon dioxide concentration the range of cathode polarizations was less than that observed with stoichiometric mixture. The processes proceeding in the cathode and anode packs of the fuel cell are believed to be interrelated processes. In a model fuel cell fueled with dispersed coal, current densities up to 140 mA/cm² and specific powers up to 70 mW/cm² were achieved.     

Effect of anode microstructure on the performance of micro tubular SOFCs

Here we report the effect of anode microstructure on the SOFC performance using two types of micro tubular SOFCs, 0.8 and 1.6mm in diameter with different anode microstructure (Cells A and B). The cells consisted of NiO-Gd doped ceria (GDC) as an anode (support tube), GDC as an electrolyte and (La, Sr)(Fe, Co)O₃ (LSCF)-GDC as a cathode. The anode tube for Cell A was prepared using NiO (d_g~ 5µm) and GDC (d_g~ 0.2µm) powders without using a pore former, while the anode tube for Cell B was prepared using NiO (d_g~ 0.5µm)and GDC(d_g~ 0.2µm) powders using a pore former. The peak power density of these cells were shown to be 203, 400 and 857mW cm^{? 2} for Cell A and 273, 628 and 1017mW cm^{? 2} for Cell B, respectively at 450, 500, and 550°C operating temperature. Both cells showed outstanding performance, and furthermore the anode microstructure of Cell B was shown to be more optimized for better cell performance.     

Chemical and redox stabilities of a solid oxide fuel cell with BaCe0.8Y0.2O3−α functioning as an electrolyte and as an anode

The operation of a solid oxide fuel cell (SOFC) based on BaCe_0.8Y_0.2O_3−α (BCY20) at 800 °C was studied without using an anode material. A porous, Ce-rich phase with a fluorite structure was formed at a depth of approximately 10 μm from the BCY20 surface by heat treatment at 1700 °C. This was due to the vaporization of BaO from the BCY20 surface. This treatment improved the cell performance and chemical stability to CO₂ because the Ce-rich phase functioned as an electrically conducting and protective layer. The heat-treated BCY20 also had better chemical and redox stabilities over a Ni–Ce_0.8Sm_0.2O_1.9 (SDC) cermet anode attached to the SDC electrolyte. The cell with the heat-treated BCY20 operated well on unhumidified methane, ethane, propane, and butane without carbon deposition, while the cell with the Ni–SDC cermet anode degraded within a few hours. Moreover, the BCY20 showed higher tolerance to 10 ppm H₂S and stability over 20 times redox cycling in comparison to the Ni–SDC cermet anode.     

Processing and characterization of ultra-thin yttria-stabilized zirconia (YSZ) electrolytic films for SOFC

Sub-micron yttria-stabilized zirconia (YSZ) electrolyte layer was prepared by a liquid state deposition method and with an average thickness of 0.5 μm to improve the performance of the anode-supported solid oxide fuel cell (SOFC). The YSZ precursors, containing yttrium and zirconium species and an additive, poly-vinyl-pyrrolidone (PVP), were spin-coated on a Ni/YSZ anode substrate. Several properties, including crystalline phases, microstructures, and current–voltage (I–V) characteristics, were investigated. The thin film of 4 mol% Y₂O₃-doped ZrO₂ (4YSZ) consisted of cubic, tetragonal, and a trace of monoclinic phases, and showed a crack-free layer after sintering at 1300 °C. The anode supported SOFC, which consists of the Ni–YSZ anode, 4YSZ electrolyte, and Pt/Pd cathode, showed power densities of 477 mW/cm² at 600 °C, and 684 mW/cm² at 800 °C. Otherwise, the surface cracks of the other YSZ-coated samples (e.g. 8YSZ) can be repaired by a multi-coating method.     

Preparation of YSZ films by magnetron sputtering for anode-supported SOFC

YSZ films for anode-supported SOFCs were prepared by reactive sputtering method. It was found that the surface morphology of anode substrate has a very important effect on the quality of sputtered films. By applying an anode functional layer and making the anode surface smooth, dense and uniform YSZ films of 10 µm in thickness were successfully fabricated. The sintering behaviors of the sputtered YSZ films were also discussed. It is suggested that the optimized densification condition for the deposited YSZ films is sintering at 1250 °C for 4 h. Single cells with sputtered YSZ film as electrolyte and LSM-YSZ as active cathode materials were tested. 1.08 V open circuit voltage and a 700 mW/cm² maximum power density were achieved at 750 °C by using humidified H₂ as fuel and air as oxidant.     

Interaction of samaria-doped ceria anode with highly dispersed Ni catalysts in a medium-temperature solid oxide fuel cell during long-term operation

Mixed conducting samaria-doped ceria (SDC) anode with highly dispersed Ni catalysts exhibited a stable high performance over 1100 h in solid oxide fuel cell (SOFC) operated at constant current density of 0.6 A cm^− 2 at 800 °C. While the apparent average size of Ni particles was found to be increased, both the IR-free polarization performance (reflecting an effective reaction area) and the ohmic resistance (reflecting an electronic network) were not changed noticeably during the long-term operation. It was found by scanning electron microscope (SEM) and scanning transmission electron microscope (STEM) that Ni particles were rather stabilized by changing the morphology at the portion contacting with SDC surface presumably due to a strong interaction.     

Fe alloying effect on the performance of the Ni anode in hydrogen fuel

Composite materials of YSZ (yttria-stabilized zirconia) with various Ni–Fe alloys were synthesized and evaluated as the solid oxide fuel cell (SOFC) anode using a 200-µm thick YSZ electrolyte as support and YSZ +La_0.8Sr_0.2MnO₃ (LSM) as cathode. The single cell with the YSZ + Ni_0.75Fe_0.25 anode exhibited the highest performance among all the investigated cells, e.g. a peak power density of 403, 337, 218 and 112 mW/cm² was achieved with H₂ fuel at 900, 850, 800 and 750 °C, respectively. The composite anode with the Ni_0.75Fe_0.25 alloy also had the lowest polarization resistance of 0.55 Ω·cm² at 800 °C among all the alloy compositions, indicating that this specific alloy offered a better anode composition than pure Ni. The possible mechanism for the improved performance of Ni with the Fe alloying addition towards H₂ oxidation was discussed.     

Direct ammonia proton-conducting solid oxide fuel cells prepared by a modified suspension spray

A dense (BCSO) membrane was fabricated by a modified suspension spraying on porous NiO–BCSO anode support. In the process, the suspension was directly prepared by ball-milling the BaCO₃, CeO₂, and Sm₂O₃ powders in ethanol. A dense and uniform electrolyte layer in the thickness of 10 μm was successfully prepared on porous anode support by suspension spray process after co-sintering at 1,400 °C for 5 h. With (NSMO) cathode, a single cell was assembled and tested with hydrogen and ammonia as fuels, respectively. The hydrogen-fueled cell exhibits 1.01 V for open circuit voltage (OCV) and 560 mW/cm² for peak power density at 700 °C. In comparison, the cell in ammonia displays a similar performance (1.02 V for OCV and 530 mW/cm² for output), which indicates the liquid ammonia is a promising substitute for hydrogen. Moreover, the fuel cell displays good interface contacts. To sum up, ammonia-fueled solid oxide fuel cells prepared by this simple suspension spray is an alternative way to promote the commercialization.     

Improvement of Cu–CeO2 anodes for SOFCs running on ethanol fuels

Ethanol is considered to be an attractive green fuel for solid oxide fuel cells (SOFCs) due to several advantages. In this paper, we presented recent progress of our group in Cu–CeO₂ anodes for SOFCs with ethanol steam as a fuel. Cu–CeO₂–ScSZ (scandia stabilized zirconia)anodes with different ratios of copper versus ceria were fabricated and the impedance spectra of symmetric cells were measured to optimize the anode composition. Area specific resistance (ASR) of these anodes was examined to prove the thermal stability of them, and possible reasons for degradation were analyzed. Furthermore, a Ni–ScSZ interlayer was added between Cu–CeO₂–YSZ (yttria stabilized zirconia) anode and ScSZ electrolyte to improve the anode performance, and the three-layer structure was fabricated by acid leaching of nickel and wet impregnation method. The maximum power density of the single cell reached 604 mW cm^? 2 and 408 mW cm^? 2 at 800 °C in hydrogen and ethanol steam respectively, and the cell obtained stable output in ethanol steam over an operation period of 50 h.     

Optimization on technical parameters for fabrication of SDC film by screen-printing used as electrolyte in IT-SOFC

Sm-doped Ceria (SDC) electrolyte film was successfully fabricated on anode substrate of NiO-SDC by screen-printing. Some technical parameters for fabrication were investigated and optimized, including printing times, ink composition and sintering temperature. Scanning electron microscope (SEM) measurement was done to check the microstructures of SDC film and single cell. The parameters greatly affected the quality of SDC film and cell performance. The single cell with the optimum parameters exhibited an OCV of 0.82 V and a power density of 0.5 W/cm² at 600 °C.     

Evaluation of a cobalt-free Ba0.5Sr0.5Fe0.9Nb0.1O3?δ cathode material for intermediate-temperature solid oxide fuel cells

A cobalt-free Ba_0.5Sr_0.5Fe_0.9Nb_0.1O_3??? (BSFNb) perovskite-type oxide was investigated as the cathode material for intermediate-temperature solid oxide fuel cells (IT-SOFCs) with Sm_0.2Ce_0.8O_1.9 (SDC) electrolyte. XRD results showed that BSFNb cathode was chemically compatible with the electrolyte SDC up to 1,000?°C. The maximum output of anode-supported thin-film SOFC reached 503?mW?cm^?2 at 650?°C when employing humidified H₂ as fuel and static air as oxidizer. The electrode polarization resistance was low as 0.078????cm² at 650?°C, and the activation energy of the electrode polarization resistance was 129.72?kJ?mol^?1. The experimental results indicated that the cobalt-free BSFNb was a promising cathode candidate for IT-SOFCs.     

Oxy-combustion of coal in liquid-antimony-anode solid oxide fuel cell system

A novel system based on the indirect oxy-combustion of coal in a liquid Sb anode solid oxide fuel cell (SOFC) has been used to produce electricity for over 48?h. Pulverized anthracite was fed to the liquid-antimony-anode of the fuel cell, and a peak power density of 47?mW cm^?2 was reached at 1023?K and 35?mW cm^?2 at 973?K. The fuel cell was prepared using a porous stainless-steel tube as a support for an LSM cathode, antimony oxide (Sb₂O₃)/yittria stabilized zirconia (YSZ, Y_0.08Z_0.92O_1.96) composite electrolyte (membrane), while liquid antimony acted as the anode. Liquid antimony/antimony oxide served as the intermediate medium for coal oxidation producing mainly carbon dioxide, which evolved as a separate gas stream. The fuel cell will facilitate carbon capture process, and simultaneously convert the chemical energy of coal directly to electricity. The experiment showed that while the fabricated electrolyte was porous, it became dense during the actual operation, preventing nitrogen leakage into the Sb/C side and producing reasonable open circuit voltage. Analysis of the experimental EIS data illustrates that the Ohmic resistance was the primary loss mechanism in the system. It further suggests approaches to improve the design. Continuous operation of this coal fueled oxy-combustion/fuel cell system achieved an overall efficiency of 28.2% despite of its tiny scale. Simple technologies can be employed to scale up this system at relatively low cost of fabrication and materials.     

Performance of La-doped strontium titanate (LST) anode on LaGaO3-based SOFC

Ni-containing anode is currently used with many electrolytes of solid oxide fuel cells (SOFCs). However, Ni is easily oxidized and deteriorates the LaGaO₃-based electrolyte. A La-doped SrTiO₃ (LST, La_0.2Sr_0.8TiO₃) is a candidate as an anode material to solve the Ni poisoning problem in LaGaO₃-based SOFC. In this study, a single-phase LST and an LST-Gd_0.2Ce_0.8O_2 ? δ (GDC) composite were tested as the possible anodes on La_0.9Sr_0.1Ga_0.8Mg_0.2O_3 ? δ (LSGM) electrolyte. In order to further improve the anodic performance, Ni was impregnated into the LST-GDC composite anode. The performance was examined from 600 °C to 800 °C by measuring impedance of the electrolyte-supported, symmetric (anode/electrolyte/anode) cells. A polarization resistance (R_p) of LST-GDC anode was much reduced from that of LST anode. When Ni was impregnated into LST-GDC composite, the R_p value was further reduced to ~ 10% of the single-phase LST anode, and it was 1 Ωcm² at 800 °C in 97% H₂ + 3% H₂O atmosphere. A single cell with Ni-impregnated LST-GDC as an anode, Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3 ? δ (BSCF) as a cathode and LSGM as an electrolyte exhibited the maximum power density of 275 mW/cm² at 800 °C, increased from ~ 60 mW/cm² for the cell using the LST-GDC as an anode. Thus, LST-GDC composite is promising as a component of anode.     

Fabrication of anode supported thick film ceria electrolytes for IT-SOFCs

Anode supported thick film ceria electrolyte unit cells were fabricated using a colloidal dip coating method for IT-SOFCs. Pre-sintering temperature of the anode substrate and the final sintering temperature were found to be the primary parameters determining the density of the film. With Ni-Ce_0.89Gd_0.11 O_2–δ cermet anode, La_0.6Sr_0.4Co_0.2Fe_0.8O₃ cathode and 15 μm Ce_0.89Gd_0.11 O_2–δ electrolyte, the cells were tested in a fuel cell configuration with air at the cathode and moist H₂ at the anode. At 650 °C, the cell indicated a maximum power density of ∼0.27 W/cm² at a current density of 0.62 A/cm². Cell performance was compared with oxygen at the cathode and the cell indicated a maximum power density of ∼0.50 W/cm² at 1.14 A/cm², 650 °C. Activation energy for the area specific resistance (ASR) of the cell suggests that with air at cathode, the cell performance was limited by gaseous diffusion at cathode and with oxygen at cathode, by oxygen ion transport across the electrolyte.     

Fabrication of LSM-SDC composite cathodes for intermediate-temperature solid oxide fuel cells

Microstructure, interfacial resistance, and activation energy for composite cathodes consisting of 50 wt% (La_0.85Sr_0.15)_0.9MnO_3-δ (LSM) and 50 wt% Sm_0.2Ce_0.8O_1.90 (SDC) were studied for intermediate-temperature solid oxide fuel cells based on SDC electrolytes. Microstructure and interfacial resistance were greatly influenced by the characteristics of starting powder and temperatures sintering the electrodes. Optimum sintering temperatures were 1100 and 950 °C, respectively, for electrodes with SDC prepared using oxalate coprecipitation technique (OCP) and glycine-nitrate process (GNP). Area-specific resistances determined using impedance spectroscopy were 0.47 and 0.92 Ω cm² at 800 °C for LSM-SDC/OCP and LSM-SDC/GNP, respectively. The high electrochemical performance is attributed to small grain size, high porosity, and high in-plane electrical conductivity of composite cathode, demonstrating the dramatic effects of microstructure on electrode performance. To increase the electrode performance, it is critical to enhance the diffusion rate of oxygen species.     

Influence of YSZ electrolyte thickness on the characteristics of plasma-sprayed cermet supported tubular SOFC

Novel Ni–Al₂O₃ cermet-supported tubular SOFC cell was fabricated by thermal spraying. Flame-sprayed Al₂O₃–Ni cermet coating played dual roles of a support tube and an anode current collector. Y₂O₃-stabilized ZrO₂ (YSZ) electrolyte was deposited by atmospheric plasma spraying (APS) to aim at reducing manufacturing cost. The gas tightness of APS YSZ coating was achieved by post-densification process. The influence of YSZ coating thickness on the performance of SOFC test cell was investigated in order to optimize YSZ thickness in terms of open circuit voltage of the cell and YSZ ohmic loss. It was found that the reduction of YSZ thickness from 100 μm to 40 μm led to the increase of the maximum output power density from 0.47 W/cm² to 0.76 W/cm² at 1000 °C. Using an APS 4.5YSZ coating of about 40 μm as the electrolyte, the test cell presented a maximum power output density of over 0.88 W/cm² at 1030 °C. The results indicate that SOFCs with thin YSZ electrolyte require more effective cathode and anode to improve performance.     

Effect of starting powder on screen-printed YSZ films used as electrolyte in SOFCs

Screen-printing technology was developed to fabricate dense YSZ electrolyte films onto NiO–YSZ porous anode substrates. A single fuel cell of Ni-YSZ/YSZ (31 μm)/LSM-YSZ was successfully prepared by screen-printing technology. Using humidified hydrogen as fuel and ambient air as oxidant, the fuel cell provided the maximum power densities of 0.18, 0.33, 0.58, 0.97 and 1.3 W/cm² at 650, 700, 750, 800 and 850 °C, respectively. The properties of the starting YSZ powder exerted a significant effect on the characteristics of the screen-printed YSZ electrolyte films. The aggregates of the powder could be partially broken by ball milling. The YSZ powder with a small particle size and a narrow particle size distribution helped to obtain dense YSZ films. The films prepared from the YSZ powder with high aggregates were very porous, which resulted in a low open circuit voltage, a high ohmic resistance, a high polarization resistance and thus a poor cell performance.