Study on degradation of solid oxide fuel cell anode by using pure nickel electrode

In this study, the interactions between Ni and YSZ in solid oxide fuel cell anode and the influence of glass seal to anode performances have been investigated by pure Ni anode sintered on YSZ pellet. The evolution of Ni-YSZ interface in 100 h galvanostatic polarization in hydrogen is studied with different humidities in hydrogen. Electrochemical impedance spectroscopy was applied to analyze the time variation of the anode electrochemical characteristics. The interface microstructural changes were characterized by scanning electron microscopy. The influence of bulk gas humidity, gas-sealing material and Ni coarsening on anode durability was studied. The degradation of pure Ni anode is considered to be determined by the competition among the mechanisms of silicon deposition, YSZ interface morphological change and Ni coarsening.     

Development of redox stable fuel electrode supported solid oxide cells

A novel solid oxide cell concept, named as redox solid oxide cell, is proposed in this work. To demonstrate the concept, solid oxide cells with doped-SrTiO₃ fuel electrodes and modified NiO-3YSZ fuel electrode support were developed to realize the redox-stable solid oxide cells. By modifying the particle characteristics of NiO, 3YSZ, slurry composition and sintering profile, a redox stable and multifunctional NiO-3YSZ fuel electrode support was successfully developed. Furthermore, two different types of doped-SrTiO₃ (Sr_0.94Ti_0.9Nb_0.1O₃ and La_0.49Sr_0.31Fe_0.03Ni_0.03Ti_0.94O₃) fuel electrode materials were successfully integrated in to the half-cells with redox stable NiO-3YSZ support. Defect free solid oxide cells of 12 cm × 12 cm size were fabricated. The redox stability of these cells was evaluated and compared with the state-of-the-art NiO-3YSZ solid oxide cells at 850 °C. It was clearly demonstrated that the newly developed redox solid oxide cells have superior stability compared to the state-of-the-art cells. In order to establish the potential of the newly developed redox solid oxide cells, the evaluation of the electrochemical performance is required.     

Oxygen electrode degradation in solid oxide cells operating in electrolysis and fuel cell modes: LSCF destabilization and interdiffusion at the electrode/electrolyte interface

Three long-term experiments have been performed in SOEC and SOFC modes at different operating temperatures. The durability tests confirm a higher degradation in electrolysis mode with respect to fuel cell operation. In addition, a larger increase of the ohmic resistance is observed for the cell operated at higher temperature in electrolysis mode. The oxygen electrodes of the pristine and tested cells have been characterized by synchrotron X-ray micro-diffraction and micro-fluorescence to assess the relation between the material destabilization and the formation of insulating phases due to interlayer diffusion. The analyses of the pristine cell confirm the presence after the electrode sintering of strontium zirconate and a Gd-rich interdiffusional layer in the electrolyte just below the zirconates. Moreover, evolutions in the LSCF unit cell volume reveal strontium segregation after aging. The associated material destabilization is linked to the accumulation of SrZrO₃ at the barrier layer/interdiffusional layer interface in operation and both phenomena are found to be thermally-activated and promoted in electrolysis mode. Finally, the crystallographic evolution of the interdiffusional layer in electrolysis mode has been investigated by X-ray diffraction. A slight increase of the phase peaks intensity detected at the highest temperature is correlated to the largest formation of SrZrO₃ observed in this condition. Based on these preliminary results, it is proposed that the loss of Zr⁴⁺ from the electrolyte due to the zirconates formation could facilitate the interdiffusion of Gd, reducing the local ionic conductivity and thus significantly contributing to the largest increase in the ohmic resistance observed in this case.     

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.     

3D Microstructural characterization of a solid oxide fuel cell anode reconstructed by focused ion beam tomography

The three-dimensional microstructure of an SOFC anode has been characterized using a focused ion beam-scanning electron microscope. The sample preparation and the experimental milling and imaging parameters have been optimized in order to obtain a high-quality 3D reconstruction. Volume fractions and the volumetric connectivity of the individual phases, specific surface and interface areas and the three-phase boundary length have been estimated. Effective thermal, electronic and ionic conductivities of the sample as well as the tortuosity of the solid phases have been evaluated by solving the diffusive transport equation with an implicit 3D finite difference method.     

Stability study of cermet-supported solid oxide fuel cells with bi-layered electrolyte

Performance and stability of five cermet-supported button-type solid oxide fuel cells featuring a bi-layered electrolyte (SSZ/SDC), an SSC cathode, and a Ni-SSZ anode, were analyzed using polarization curves, impedance spectroscopy, and post-mortem SEM observation. The cell performance degradation at 650 °C in H₂/air both with and without DC bias conditions was manifested primarily as an increase in polarization resistance, approximately at a rate of 2.3 mΩ cm² h⁻¹ at OCV, suggesting a decrease in electrochemical kinetics as the main phenomenon responsible for the performance decay. In addition, the initial series resistance was about ten times higher than the calculated resistance corresponding to the electrolyte, reflecting a possible inter-reaction between the electrolyte layers that occurred during the sintering stage. In situ and ex situ sintered cathodes showed no obvious difference in cell performance or decay rate. The stability of the cells with and without electrical load was also investigated and no significant influence of DC bias was recorded. Based on the experimental results presented, we preliminarily attribute the performance degradation to electrochemical and microstructural degradation of the cathode.     

High-conductivity electrolyte with a low sintering temperature for solid oxide fuel cells

Bismuth oxide and scandia co-doped zirconia (Sc₂O₃)_0.06(Bi₂O₃)_x(ZrO₂)_0.94–x (ScSZB, x = 0, 0.01, 0.03, 0.05, 0.07, 0.1) powders are prepared via a citrate sol-gel method. Bi₂O₃ promotes the sintering process of scandia stabilized zirconia (ScSZ) and increases electrical conductivity of system. A high conductivity of ～0.094 S/cm at 800 °C is achieved on 5 mol% Bi₂O₃ doped ScSZ (ScSZB05). X-ray Rietveld refinement and transmission electron microscope (TEM) analysis of the ScSZB05 reveal the formation of cubic phase and rhombohedral phase at room temperature. The electrolyte-supported cell constructed by the ScSZ electrolyte gives the maximum power density of 258.3 mW/cm² at 800 °C, while the cell with ScSZB05 electrolyte shows a higher value of 387.6 mW/cm². The performance obtained by theoretical simulation of the two electrolyte-supported cells is in good agreement with the experimental results.     

Bismuth oxide doped scandia-stabilized zirconia electrolyte for the intermediate temperature solid oxide fuel cells

Electrical and structural properties of bismuth oxide doped scandia-stabilized zirconia (ScSZ) electrolyte for solid oxide fuel cells (SOFCs) have been evaluated by means of XRD, TGA, DTA, and impedance spectroscopy. The amount of Bi₂O₃ in the ScSZ was varied in the range of 0.25–2.0 mol%. The original ScSZ samples indicated a rhombohedral crystalline structure that in general has lower conductivity than the cubic phase. However, the addition of Bi₂O₃ to ScSZ electrolyte was found to stabilize the cubic crystalline phase as detected by XRD. Impedance spectroscopy measurements in the temperature range between 350 and 900 °C indicated a sharp increase in conductivity for the system containing 2 mol% of Bi₂O₃ that is attributed to the presence of the cubic phase. In addition, impedance spectroscopy measurements revealed significant decrease of both the grain bulk and grain boundary resistances with respect to the temperature change from 600 to 900 °C and concentration of Bi₂O₃ from 0.5 to 2 mol%. The electrical conductivity at 600 °C obtained for 2 mol% Bi₂O₃ doped ScSZ was 0.18 S cm⁻¹.     

Integrated thermal management strategy and materials for solid oxide fuel cells

The application of positive temperature coefficient (PTC) thermistors to thermal management of solid oxide fuel cells (SOFCs) is considered. A strategy is proposed for eliminating hotspots and reducing temperature gradients in SOFCs via inclusion of a material that exhibits a dramatic change in resistance near the desired maximum cell temperature. Three PTC thermistor materials with transition temperatures near the maximum desirable SOFC operating temperature are identified and screened for compatibility with common SOFC materials. All are found to be unsuitable because of reaction with the SOFC materials or unacceptably small PTC effect.     

Manufacturing and characterization of metal-supported solid oxide fuel cells

A metal-supported solid oxide fuel cell design offers competitive advantages, for example reduced material costs and improved robustness. This paper reports the performance and stability of a recently developed metal-supported cell design, based on a novel cermet anode, on a 25 cm² (1 cm²/16 cm² active area) cell level. An electrochemical performance comparable to state-of-the-art anode-supported cells is demonstrated.Detailed electrochemical analysis allowed assignment of the overall polarization losses quantitatively to gas diffusion in the metal support, electrooxidation in the anode functional layer, oxygen reduction in the mixed ionic-electronic conducting cathode and an additional polarization process with a rather high relaxation frequency, which may be assigned to an insulating corrosion interlayer.The durability of the cells was investigated by means of galvanostatic operation for periods of up to 1000 h as well as the dynamic behavior, such as redox-, load- and thermal cycling tests.The galvanostatic stability tests indicated a fair, but significant degradation rate (∼5% decrease in cell voltage/1000 h at 650 °C and 0.25 A cm⁻²). Furthermore, the metal-supported cells underwent an endurance test of 100 redox cycles at 800 °C without severe degradation nor total failure.     

Ink-jet printing of electrolyte and anode functional layer for solid oxide fuel cells

In this work, solid oxide fuel cells were fabricated by ink-jet printing. The cells were characterized in order to study the resulting microstructure and electrochemical performance. Scanning electron microscopy revealed a highly conformal 6–12 μm thick dense yttria-stabilized zirconia electrolyte layer, and a porous anode-interlayer. Open circuit voltages ranged from 0.95 to 1.06 V, and a maximum power density of 0.175 W cm⁻² was achieved at 750 °C. These results suggest that the ink-jet printing technique may be used to fabricate stable SOFC structures that are comparable to those fabricated by more conventional ceramics processing methods. This study also highlights the significance of overall cell microstructural impact on cell performance and stability.     

On the single chamber solid oxide fuel cells

Single chamber solid oxide fuel cells, SC-SOFCs, design and performance is discussed. It is shown that all of them operate on non-selective anodes. They operate on cathodes that become selective only under short residence times. As a result these cells are not functioning as true mixed reactant solid oxide fuel cells (MR-SOFC). The lack of selectivity is a serious draw back. True MR-SOFC can be constructed in ways that make them cheaper in fabrication, providing high power density, high fuel utilization and reduced explosion risk. The fact that SOFCs operate in a single cell is a necessary but not sufficient condition for the proper operation of MR-SOFCs.     

Low temperature solid oxide fuel cells with pulsed laser deposited bi-layer electrolyte

Solid oxide fuel cells (SOFC) using a pulsed laser deposited bi-layer electrolyte have been successfully fabricated and have shown very good performance at low operating temperatures. The cell reaches power densities of 0.5 W cm⁻² at 550 °C and 0.9 W cm⁻² at 600 °C, with open circuit voltage (OCV) values larger than 1.04 V. The bi-layer electrolyte contains a 6–7 μm thick samarium-doped ceria (SDC) layer deposited over a ∼1 μm thick scandium-stabilized zirconia (ScSZ) layer. The electrical leaking between the anode and cathode through the SDC electrolyte, which due to the reduction of Ce⁴⁺ to Ce³⁺ in reducing environment when using a single layer SDC electrolyte, has been eliminated by adopting the bi-layer electrolyte concept. Both ScSZ and SDC layers in the bi-layer electrolyte prepared by the pulsed laser deposition (PLD) technique are the highly conductive cubic phases. Poor conductive (Zr, Ce)O₂-based solid solutions or β-phase ScSZ were not found in the bi-layer electrolyte prepared by the PLD due to low processing temperatures of the technique. Excellent reliability and flexibility of the PLD technique makes it a very promising technique for the fabrication of thin electrolyte layer for SOFCs operating at reduced temperatures.     

Ag current collector for honeycomb solid oxide fuel cells using LaGaO3-based oxide electrolyte

Honeycomb type solid oxide fuel cell (SOFC) using a Ag mesh as a current collector and La_0.9Sr_0.1Ga_0.8Mg_0.2O₃ (LSGM) as an electrolyte was studied for reducing production cost. When an Ag mesh was used as a current collector, the power density of the cell became lower than that of a cell using a Pt mesh due to the relatively worse contact caused by the lower calcination temperature, particularly in the case of the anode. The preparation method and the electrode structure were investigated for the purpose of increasing the power density of the cell using the Ag current collector. It was found that an interlayer of Ni–Sm_0.2Ce_0.8O_1.9 (1:9) between the NiFe–LSGM cermet anode and the LSGM electrolyte was effective for decreasing the pre-calcination temperature for anode fabrication. Much higher power densities of 300 mW cm⁻² and 140 mW cm⁻² at 1073 K and 973 K, respectively, were achieved by inserting an interlayer. These results for the power density of the cell using the Ag mesh current collector after the optimization of the electrode structure and the preparation procedure are close to those of the cell using the Pt mesh current collector cell presented in our previous work.     

Direct ceramic inkjet printing of yttria-stabilized zirconia electrolyte layers for anode-supported solid oxide fuel cells

Electromagnetic drop-on-demand direct ceramic inkjet printing (EM/DCIJP) was employed to fabricate dense yttria-stabilized zirconia (YSZ) electrolyte layers on a porous NiO-YSZ anode support from ceramic suspensions. Printing parameters including pressure, nozzle opening time and droplet overlapping were studied in order to optimize the surface quality of the YSZ coating. It was found that moderate overlapping and multiple coatings produce the desired membrane quality. A single fuel cell with a NiO-YSZ/YSZ (∼6 μm)/LSM + YSZ/LSM architecture was successfully prepared. The cell was tested using humidified hydrogen as the fuel and ambient air as the oxidant. The cell provided a power density of 170 mW cm⁻² at 800 °C. Scanning electron microscopy (SEM) revealed a highly coherent dense YSZ electrolyte layer with no open porosity. These results suggest that the EM/DCIJP inkjet printing technique can be successfully implemented to fabricate electrolyte coatings for SOFC thinner than 10 μm and comparable in quality to those fabricated by more conventional ceramic processing methods.     

Synthesize and characterization of ceria based nano-composite materials for low temperature solid oxide fuel cell

The present study is focused on ceria based mixed (ionic and electronic conductor) composite Al_0.05Ni_0.1Ti_0.05Zn_0.80-SDC (ATZN-SDC) oxide material was prepared by solid state reaction, which can be used as anode materials for solid oxide fuel cell. The effect of Ti and Al oxides were analyzed on the NiZn-SDC composite with respect to its conductivity and catalytic activity in hydrogen atmosphere. The average crystallite size of the composite was found to be 40–100 nm by XRD and SEM. The DC conductivity was determined by 4-probe technique. The electrochemical impedance spectrum (EIS) was also examined in hydrogen atmosphere within a temperature range of 350–550 °C. The maximum power density 370 mW/cm² was achieved at 650 °C.     

Characterization of electrolyte–electrode interlayers in thin film solid oxide fuel cells

In order to reduce the operating temperature of solid oxide fuel cells (SOFCs), anode-supported cells incorporating thin film (∼10 μm) electrolytes in conjunction with anode/electrolyte and cathode/electrolyte interlayers were studied. SOFC button cells were prepared through deposition of colloidal slurries onto anode supported substrates and were analyzed as a function of temperature and polarization via voltammetry and electrochemical impedance spectroscopy (EIS). It was found that the electro-catalytic activity or electrode/electrolyte interfacial areas were enhanced through the addition of these interlayers. This performance improvement was attributed to the introduction of a diffuse mixed conduction region associated with these interlayers. The cathode is thought to benefit disproportionately from this enhancement. Single SOFC button cells with electrode interlayers were then characterized as a function of temperature and polarization to assess the involvement of these interfacial layers. EIS was applied and the data were used to deconvolute component impedances. Finally electrochemical models were developed to provide a more complete understanding of these assemblies under operation.     

Thermodynamic analysis of ammonia fed solid oxide fuel cells: Comparison between proton-conducting electrolyte and oxygen ion-conducting electrolyte

A thermodynamic analysis has been performed to compare the theoretical performance of ammonia fed solid oxide fuel cells (SOFCs) based on proton-conducting electrolyte (SOFC-H) and oxygen ion-conducting electrolyte (SOFC-O). It is found that the ammonia fed SOFC-H is superior to SOFC-O in terms of theoretical maximum efficiency. For example, at a fuel utilization of 80% and an oxygen utilization of 20%, the efficiency of ammonia fed SOFC-H is 11% higher than that of SOFC-O. The difference between SOFC-H and SOFC-O becomes more significant at higher fuel utilizations and higher temperatures. This is because an SOFC-H has a higher hydrogen partial pressure and a lower steam partial pressure than an SOFC-O. In addition, an increase in oxygen utilization is found to increase the efficiency of ammonia fed SOFCs due to an increase in oxygen molar fraction and a reduction in steam molar fraction. With further development of new ceramics with high proton conductivity and effective fabrication of thin film electrolyte, the SOFC based on proton-conducting electrolyte is expected to be a promising approach to convert ammonia fuel into electricity.     

Mesoscale-structure control at anode/electrolyte interface in solid oxide fuel cell

To enhance the power density of a solid oxide fuel cell, a mesoscale-structure control of an electrode/electrolyte interface was proposed; here, the mesoscale means a size range of 10-100 μm, which is larger than the microscale of the electrode particles but smaller than the macroscale of the cell geometries. Therefore, the mesoscale structure does not only change the local thickness of the electrolyte and electrode but also enlarge the electrode/electrolyte interface area, and thus influence the cell performance. First, to find effective conditions for the mesoscale-structure control, a preliminary theoretical analysis in a conventional flat cell was performed focusing on the ratio of the ion-conducting resistance to the reaction resistance. In the light of this basic knowledge, as a second step, the effects of the mesoscale structure on an anode side of an electrolyte-supported cell were studied numerically and experimentally. A 2D numerical simulation based on an equivalent electrical circuit model and the dusty-gas model was carried out. As a result, the mesoscale-grooved structure was found to be effective for enhancement of the power generation, if the groove scale is sufficiently larger than that of the active reaction region of the electrode. Qualitatively similar results were obtained from the experiments using a segmented electrode with both flat and mesoscale-grooved surface in a button-type cell.     

High performance metal-supported solid oxide fuel cells with Gd-doped ceria barrier layers

Metal-supported solid oxide fuel cells are believed to have commercial advantages compared to conventional anode (Ni-YSZ) supported cells, with the metal-supported cells having lower material costs, increased tolerance to mechanical and thermal stresses, and lower operational temperatures. The implementation of a metallic support has been challenged by the need to revise the cell fabrication route, as well as electrode microstructures and material choices, to compete with the energy output and stability of full ceramic cells.The metal-supported SOFC design developed at Risø DTU has been improved, and an electrochemical performance beyond the state-of-the-art anode-supported SOFC is demonstrated possible, by introducing a CGO barrier layer in combination with Sr-doped lanthanum cobalt oxide (LSC) cathode. Area specific resistances (ASR) down to 0.27 Ω cm², corresponding to a maximum power density of 1.14 W cm⁻² at 650 °C and 0.6 V, were obtained on cells with barrier layers fabricated by magnetron sputtering. The performance is dependent on the density of the barrier layer, indicating Sr²⁺ diffusion is occurring at the intermediate SOFC temperatures. The optimized design further demonstrate improved durability with steady degradation rates of 0.9% kh⁻¹ in cell voltage for up to 3000 h galvanostatic testing at 650 °C and 0.25 A cm⁻².