Influence of anode's microstructure on electrochemical performance of solid oxide direct carbon fuel cells

The microstructure of anode has a significant influence on the whole electrochemical performance of solid oxide direct carbon fuel cells (SO-DCFCs). The tubular SO-DCFCs based on cathode supported solid oxide fuel cells was fabricated by dip-coating and co-sintering techniques. As the anode porosity mainly came from the pore former (graphite) in the dip-coating process, different contents of graphite were added into the anode slurries. When the graphite was 10.1% wt., the SO-DCFCs showed the best performance and stability. The peak power density reached 242 mW cm⁻² at 850 °C, with carbon black (located 5% Fe) as the fuel and air as the oxidant.     

Improved electrochemical performance and durability of butane-operating low-temperature solid oxide fuel cell through palladium infiltration

Low-operating-temperature solid oxide fuel cells (LT-SOFCs) with various kinds of fuel, such as hydrocarbons, biogas, natural gas, and oxygenated fuel has been an active SOFC research topic. However, conventional SOFC anodes comprised of nickel metal and yttria-stabilized zirconia composite (Ni-YSZ) experience rapid degradation when operated for the butane direct internal steam reforming (B-DISR), especially at a low temperature (LT) range. This study reveals that the impregnated Pd into the Ni-YSZ anode support of thin-film SOFCs (TF-SOFCs) is effective for achieving better performance and stability regarding the TF-SOFC in B-DISR at 600°C. Adding Pd as a dopant into Ni-YSZ significantly promotes the catalytic activity due to the Pd-Ni alloy formation, both on the YSZ grain and the Ni grain surface. The electrochemical performance of cells without Pd (Ni-YSZ cell) and a Pd-infiltrated Ni-YSZ anode (Pd-Ni-YSZ cell) are compared at 600°C for the B-DISR mode at a ratio of steam-to-carbon of 3. Finally, long-term durability tests were performed at 600°C and under 0.15 A cm⁻². The Pd infiltration decreases the deterioration rate to 0.63 mV h⁻¹ after the first 80 hours of operation for the Pd-Ni-YSZ cell, which was a significant improvement from that of the Ni-YSZ cell, 3.75 mV h⁻¹ after 40 hours of operation.     

Quantitative assessment of anode contribution to cell degradation under various polarization conditions using industrial size planar solid oxide fuel cells

Quantitative assessment of anode contribution to cell performance has been investigated under various polarization in three stack repeating units made of 10 cm × 10 cm planar anode-supported solid oxide fuel cells. The measurement is performed in-situ by inserting an ultra-thin Pt probe at the anode/electrolyte interface. The results reveal that the polarization resistance accounts for more than 90% of the total cell resistance when the cell is operated under the activation and concentration polarizations, respectively. The anode almost has no effect on cell degradation when the cell is operated under activation polarization. However, the anode has an obvious contribution to the cell degradation when the cell is operated under ohmic and concentration polarization, where the anode voltage increases by 23.36%/100 h and 512.28%/100 h, respectively. The anode operated under concentration polarization has twenty times of contribution to cell degradation than that of the ohmic polarization. However, when the cell is operated under the ohmic polarization, the main degradation comes from the ohmic resistance caused by the anode.     

Effect of carbon deposition by carbon monoxide disproportionation on electrochemical characteristics at low temperature operation for solid oxide fuel cells

The deterioration by carbon deposition was evaluated for electrolyte- and anode-supported solid oxide fuel cells (SOFCs) in comparison with carbon monoxide disproportionation and methane cracking. The polarization resistance of the nickel-yttria stabilized zirconia (Ni-YSZ) anode increased with a rise in CO concentration in H₂-CO-CO₂ mixture for the electrolyte-supported cells at 923 K. The resistance, however, did not change against CO concentration for the anode-supported cells. In a methane fuel with a steam/carbon (S/C) ratio of 0.1, the cell performance decreased for both of the cells at 1073 K. A large amount of agglomerated amorphous carbon was deposited from the anode surface to the interface between the anode and the electrolyte after power generation at S/C = 0.1 in methane fuel. On the other hand, the crystalline graphite was deposited only at the anode surface for the anode-supported cell after power generation in CO-CO₂ mixture. These results suggest that the reaction rate of CO disproportionation is faster than that of methane cracking. The deposited carbon near the anode/electrolyte interface caused the increase in the polarization resistance.     

Thermodynamic analysis of solid oxide fuel cells operated with methanol and ethanol under direct utilization,steam reforming,dry reforming or partial oxidation conditions

There is increasing interest in developing solid oxide fuel cells (SOFC) for portable applications. For these devices it would be convenient to directly use a liquid fuel such as methanol and ethanol rather than hydrogen. The direct utilization of alcohol fuels in SOFC involves several processes, including the deposition of carbon, which can lead to irreversible deactivation of the fuel cell. Several publications have addressed the thermodynamic analysis of the reforming of methanol (MeOH) and ethanol (EtOH) in SOFC, but none have considered the direct utilization of these fuels. The equilibrium compositions, the carbon deposition boundaries, and the electromotive forces for the direct utilization and partial oxidation of methanol and ethanol in SOFC as a function of the fuel utilization are obtained in this study. In addition, the minimum amounts of H₂O, and CO₂ for direct and indirect reforming with MeOH and EtOH to avoid carbon formation are calculated.     

Methane-free biogas for direct feeding of solid oxide fuel cells

This paper deals with the experimental analysis of the performance and degradation issues of a Ni-based anode-supported solid oxide fuel cell fed by a methane-free biogas from dark-anaerobic digestion of wastes by pastry and fruit shops. The biogas is produced by means of an innovative process where the biomass is fermented with a pre-treated bacteria inoculum (Clostridia) able to completely inhibit the methanization step during the fermentation process and to produce a H₂/CO₂ mixture instead of conventional CH₄/CO₂ anaerobic digested gas (bio-methane). The proposed biogas production route leads to a biogas composition which avoids the need of introducing a reformer agent into or before the SOFC anode in order to reformate it.In order to analyse the complete behaviour of a SOFC with the bio-hydrogen fuel, an experimental session with several H₂/CO₂ synthetic mixtures was performed on an anode-supported solid oxide fuel cell with a Ni-based anode. It was found that side reactions occur with such mixtures in the typical thermodynamic conditions of SOFCs (650–800 °C), which have an effect especially at high currents, due to the shift to a mixture consisting of hydrogen, carbon monoxide, carbon dioxide and water. However, cells operated with acceptable performance and carbon deposits (typical of a traditional hydrocarbon-containing biogas) were avoided after 50 h of cell operation even at 650 °C. Experiments were also performed with traditional bio-methane from anaerobic digestion with 60/40 vol% of composition. It was found that the cell performance dropped after few hours of operation due to the formation of carbon deposits.A short-term test with the real as-produced biogas was also successfully performed. The cell showed an acceptable power output (at 800 °C, 0.35 W cm⁻² with biogas, versus 0.55 W cm⁻² with H₂) although a huge quantity of sulphur was present in the feeding fuel (hydrogen sulphide at 103 ppm and mercaptans up to 10 ppm). Therefore, it was demonstrated the interest relying on a sustainable biomass processing which produces a biogas which can be directly fed to SOFC using traditional anode materials and avoiding the reformer component since the methane-free mixture is already safe for carbon deposition.     

Research of carbon deposition formation and judgment in Cu-CeO2-ScSZ anodes for direct ethanol solid oxide fuel cells

We have been researching Solid oxide fuel cells (SOFCs) with Cu-CeO₂-ScSZ (scandia stabilized zirconia) anodes operating directly on ethanol fuels in recent years. In this paper, Cu-CeO₂-ScSZ anodes with different Cu/CeO₂ composition are fabricated by dry pressing, sintering and wet impregnation technologies. The photographs and SEM images of these samples after exposure to ethanol fuels for 300 h are observed to characterize their carbon deposition behaviors. The different deposited carbon morphologies in the anodes with different compositions are recorded, and possible reaction mechanisms and prevention methods are discussed. Based on these results; we demonstrate the carbon deposition behaviors and degradation reasons for the single cell running in ethanol fuels.     

Factors affecting limiting current in solid oxide fuel cells or debunking the myth of anode diffusion polarization

Limiting current densities for solid oxide fuel cells were measured using both button cells and a flow-through cell. The cell anodes were supplied with mixtures of humidified hydrogen and various inert gasses. It was demonstrated that the true limiting current in flow-through cells is reached when either: the hydrogen is nearly or completely depleted at the anode-electrolyte interface near the outlet; or when the concentration of steam at that interface becomes high enough to interfere with adsorption or transport of the remaining hydrogen near the triple-phase boundaries. Choice of inert gas had no effect on limiting currents in the flow-through tests, indicating that diffusion within the porous anode had no significant effect on cell performance at high currents. In the button cells, the apparent limiting currents were significantly changed by the choice of inert gas, indicating that they were determined by diffusion through the bulk gas within the support tube. It was concluded that the apparent limiting currents measured in button cells are influenced more by parameters of the experimental setup, such as the proximity of the fuel tube outlet, than by the physical properties of the anode.     

Experimental investigation of direct internal reforming of biogas in solid oxide fuel cells

This work investigates the behaviour of planar solid oxide fuel cells (SOFCs) fed by two different fuel mixtures that simulate biogases coming from anaerobic digestion. The fuel mixtures are namely bio-methane and bio-hydrogen. The first composition is the conventional one, where a biological process of fermentation is carried out to produce a gas that contains a mixture of methane and carbon dioxide with traces of H₂S and other organic sulphur compounds. The second mixture is representative of a biogas produced through a novel routine: a particular pre-treatment of the bacteria inoculum (generally clostridia bacteria) is performed in order to inhibit the methanogenic step in the fermentation process, such that bio-hydrogen is produced as the only effluent of the digester (a mixture of H₂/CO₂, with no traces of methane).     

Effect of nickel-phosphorus interactions on structural integrity of anode-supported solid oxide fuel cells

An integrated experimental/modeling approach was utilized to assess the structural integrity of Ni-yttria-stabilized zirconia (YSZ) porous anode supports during the solid oxide fuel cell (SOFC) operation on coal gas containing trace amounts of phosphorus impurities. Phosphorus was chosen as a typical impurity exhibiting strong interactions with the nickel followed by second phase formation. Tests were performed using Ni-YSZ anode-supported button cells exposed to 0.5-10 ppm of phosphine in synthetic coal gas at 700-800 °C. The extent of Ni-P interactions was determined by a post-test scanning electron microscopy (SEM) analysis. Severe damage to the anode support due to nickel phosphide phase formation and extensive crystal coalescence was revealed, resulting in electric percolation loss. The subsequent finite element stress analyses were conducted using the actual anode support microstructures to assist in degradation mechanism explanation. Volume expansion induced by the Ni phase alteration was found to produce high stress levels such that local failure of the Ni-YSZ anode became possible under the operating conditions.     

The effect of HCl in syngas on Ni-YSZ anode-supported solid oxide fuel cells

The Ni-YSZ cermet anode of the solid oxide fuel cell (SOFC) has excellent electrochemical performance in a clean blended synthetic coal syngas mixture. However, chloride, one of the major contaminants existing in coal-derived syngas, may poison the Ni-YSZ cermet and cause degradation in cell performance. Both hydrogen chloride (HCl) and chlorine (Cl₂) have been reported to attack the Ni in the anode when using electrolyte-supported SOFCs. In this paper, a commercial anode-supported SOFC was exposed to syngas with a concentration of 100 ppm HCl under a constant current load at 800 °C for 300 h and 850 °C for 100 h. The cell performance was evaluated periodically using electrochemical methods. A unique feature of this experiment is that the active central part of the anode was exposed directly to the fuel without an intervening current collector. Post-mortem analyses of the SOFC anode were performed using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). The results show that the 100 ppm concentration of HCl causes about 3% loss of performance for the Ni-YSZ anode-supported cell during the 400 h test. Permanent changes were noted in the surface microstructure of the nickel particles in the cell anode.     

The effect of phosphine in syngas on Ni-YSZ anode-supported solid oxide fuel cells

Ni-YSZ cermet is commonly used as the anode of a solid oxide fuel cell (SOFC) because it has excellent electrochemical performance, not only in hydrogen fuel, but also in a clean blended synthetic coal syngas mixture (30% H₂, 26% H₂O, 23% CO, and 21% CO₂). However, trace impurities, such as phosphine (PH₃), in coal-derived syngas can cause degradation in cell performance [J.P. Trembly, R.S. Gemmen, D.J. Bayless, J. Power Sources 163 (2007) 986-996]. A commercial solid oxide fuel cell was exposed to a syngas with 10 ppm PH₃ under a constant current load at 800 °C and its performance was evaluated periodically using electrochemical methods. The central part of the anode was exposed directly to the syngas without an intervening current collector. Post-mortem analyses of the SOFC anode were performed using Raman spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). The results show that the impurity PH₃ caused a significant loss of the Ni-YSZ anode electrochemical performance and an irreversible Ni-YSZ structural modification. Ni₅P₂ was confirmed to be produced on the cell surface as the dominant nickel phosphorus phase.     

Use of La0.75Sr0.25Cr0.5Mn0.5O3 materials in composite anodes for direct ethanol solid oxide fuel cells

The perovskite system La_1−xSr_xCr_1−yM_yO_3−δ (M, Mn, Fe and V) has recently attracted much attention as a candidate material for the fabrication of solid oxide fuel cells (SOFCs) due to its stability in both H₂ and CH₄ atmospheres at temperatures up to 1000 °C. In this paper, we report the synthesis of La_0.75Sr_0.25Cr_0.5Mn_0.5O₃ (LSCM) by solid-state reaction and its employment as an alternative anode material for anode-supported SOFCs. Because LSCM shows a greatly decreased electronic conductivity in a reducing atmosphere compared to that in air, we have fabricated Cu-LSCM-ScSZ (scandia-stabilized zirconia) composite anodes by tape-casting and a wet-impregnation method. Additionally, a composite structure (support anode, functional anode and electrolyte) structure with a layer of Cu-LSCM-YSZ (yttria-stabilized zirconia) on the supported anode surface has been manufactured by tape-casting and screen-printing. Single cells with these two kinds of anodes have been fabricated, and their performance characteristics using hydrogen and ethanol have been measured. In the operation period, no obvious carbon deposition was observed when these cells were operated on ethanol. These results demonstrate the stability of LSCM in an ethanol atmosphere and its potential utilization in anode-supported SOFCs.     

Experimental analysis of performance degradation of micro-tubular solid oxide fuel cells fed by different fuel mixtures

This paper analyzes the thermodynamic and electrochemical dynamic performance of an anode supported micro-tubular solid oxide fuel cell (SOFC) fed by different types of fuel. The micro-tubular SOFC used is anode supported, consisting of a NiO and Gd_0.2Ce_0.8O_2−x (GDC) cermet anode, thin GDC electrolyte, and a La_0.6Sr_0.4Co_0.2Fe_0.8O_3−y (LSCF) and GDC cermet cathode. The fabrication of the cells under investigation is briefly summarized, with emphasis on the innovations with respect to traditional techniques. Such micro-tubular cells were tested using a Test Stand consisting of: a vertical tubular furnace, an electrical load, a galvanostast, a bubbler, gas pipelines, temperature, pressure and flow meters. The tests on the micro-SOFC were performed using H₂, CO, CH₄ and H₂O in different combinations at 550 °C, to determine the cell polarization curves under several load cycles. Long-term experimental tests were also performed in order to assess degradation of the electrochemical performance of the cell. Results of the tests were analyzed aiming at determining the sources of the cell performance degradation. Authors concluded that the cell under investigation is particularly sensitive to the carbon deposition which significantly reduces cell performance, after few cycles, when fed by light hydrocarbons. A significant performance degradation is also detected when hydrogen is used as fuel. In this case, the authors ascribe the degradation to the micro-cracks, the change in materials crystalline structure and problems with electrical connections.     

Effect of anode and Boudouard reaction catalysts on the performance of direct carbon solid oxide fuel cells

Tubular electrolyte-supporting solid oxide fuel cells directly operated on carbon fuel were fabricated and tested. Gadolinia doped ceria (GDC) mixed with silver was used as the anode to catalyze the electrochemical oxidation of CO while Fe-based catalyst was loaded on the carbon fuel to enhance the Boudouard reaction. The performance was significantly improved, with a maximum power density of 45 mW cm⁻², 10 times higher than that of the cell without any catalyst. Impedance measurements showed that the polarization resistance was decreased by tens of times through applying catalysts in the cell. An operation life of 10 h was observed at a constant current of 70 mA. The mechanism of the cell reaction was analyzed.     

Doubling the diffusivity measurement efficiency in solid oxide fuel cells (SOFCs) via a bi-sensor electrochemical cell

To improve the efficiency of the diffusivity measurement in solid oxide fuel cells (SOFCs), a bi-sensor electrochemical cell is proposed and analyzed. The cell consists of an oxygen pump and two oxygen sensors. This bi-sensor cell doubles the efficiency of binary diffusivity measurements in both porous anodes and cathodes in SOFCs. The design provides an efficient platform to estimate the electrode limiting current density (LCD) and concentration polarization (CP) in SOFCs. Mechanism of measuring the CP with different electrode configurations was discussed in depth. The design was verified by studying the effect of electrode thickness on the CPs at different operating temperatures.     

Electrochemical performances of LiNiCoMn bifunctional electrode for low temperature solid oxide fuel cells

Lithium transition metal oxides LiNi_0.83Co_0.11Mn_0.06O₂ (NCM-83) and LiNi_0.8Co_0.1Mn_0.1O₂ (NCM-811) are prepared and acted as cathodes and bifunctional electrodes for low temperature solid oxide fuel cells with H₂ and CH₄ fuels. The Ni anode-supported cell with NCM-83 cathode exhibits maximum power density (P_max) of 0.72 W cm⁻² with H₂ fuel at 600 °C. The symmetric cell with NCM-83 electrodes shows high P_max of 0.465 W cm⁻² with H₂ fuel and 0.354 W cm⁻² with CH₄ fuel at 600 °C. And the P_max of the cell with NCM-811 as anode and NCM-83 as cathode is 0.204W cm⁻² with H₂ fuel at 600 °C. The oxygen vacancies in NCM materials are conducive to the rapid oxygen ion conduction of the cathode, and in the anodic reduction atmosphere, the NCM materials will generate Ni/Co active particles in situ, proving the NCM materials can be advanced bifunctional electrode materials for hydrogen oxidation reaction and oxygen reduction reaction at low temperature.     

Crack severity in relation to non-homogeneous Ni oxidation in anode-supported solid oxide fuel cells

The full oxidation of Ni-YSZ anode-supported cells at high temperatures (>700 °C) is shown here to lead to much more severe degradation (larger quantity and wider cracks in the electrolyte) than at lower temperatures. This correlates with the linear mass gain/time profile observed in TGA experiments at high temperatures, indicative of diffusion controlled Ni oxidation and thus the presence of O₂ (and Ni/NiO) concentration gradients into the depth of the anode layer. At low partial pressures of O₂, the severity of cracking also increases. SEM studies of partially oxidized anode layers confirmed that Ni oxidation is non-homogeneous when carried out at either high temperatures or low pO₂, in which case the outer regions of the anode (near the anode/air interface) become almost fully oxidized, while the inner regions (near the electrolyte) remain metallic. Under these conditions, the continued volume expansion associated with NiO formation can then only occur towards the electrolyte, increasing the compressive stress inside the anode as the Ni continues to be oxidized, leading to electrolyte cracking and warping (convex to the electrolyte). To prevent severe degradation to the cell, efforts should therefore be made to avoid gradients in NiO/Ni content during oxygen exposure of Ni-YSZ anode-supported cells at high temperatures.     

Performance analysis of hollow fibre-based micro-tubular solid oxide fuel cell utilising methane fuel

Solid oxide fuel cell (SOFC) has been studied as one of the most amazing development in energy production that could work directly with hydrocarbon fuel without reforming procedure. This study was conducted to analyse the micro-tubular solid oxide fuel cell (MT-SOFC) in terms of its performance by utilising methane as the fuel, subsequently compared with hydrogen. MT-SOFC that was investigated in this work consisted of thin cathode layer, coated onto co-extruded anode/electrolyte dual-layer hollow fibre (HF); in which its anode was made of nickel (Ni), coupled with cerium-gadolinium oxide (CGO) as an electrolyte, whereas the cathode was lanthanum strontium cobalt ferrite (LSCF) and CGO. The physical analyses carried out were three-point bending test and scanning electron microscopy (SEM). X-ray diffraction (XRD) analysis was further conducted to examine the carbon deposition in HFs. In evaluating the performance of HFs, current-voltage (IV) measurement, as well as impedance analysis of various temperatures range from 500 °C to 700 °C were performed. Based on the results, the OCV, maximum power density and ohmic ASR of MT-SOFC exposed to methane fuel, were at 0.79 V, 0.22 W cm⁻² and 0.31 Ω cm²; compared to the other that was exposed to hydrogen fuel, recorded at 0.89 V, 0.67 W cm⁻² and 0.19 Ω cm² respectively. This indicates that there was a significant reduction in cell performance when methane was used as the fuel, due to the carbon deposition as proven by SEM, three-point bending and XRD.