Effect of air addition to the air electrode on the stability and efficiency of carbon dioxide electrolysis by solid oxide cells

In this study, the effect of air addition to the air electrode on the long-term stability and efficiency of solid oxide cells for CO₂ electrolysis, with 23.8% CO as protective gas in the fuel electrode, has been investigated. The results show that without continuous purging of the air in the air electrode (Cell-1), the degradation rate was 8.37%/kh in the 1070 h electrolysis process, while with 5 L/min air supplied to the air electrode (Cell-2), the degradation rate was 24.41%/kh. Impedance analysis indicates that the degradation of Cell-1 was mainly because of the increase in O^2? exchange polarization impedance, while the degradation of Cell-2 was caused mainly by the variation of ohmic impedance. The microstructural characterization indicated a decrease in active Ni in the fuel electrode in both Cell-1 and Cell-2, but the degree of nickel loss depended on the test time. At the outlet of the Cell-2, the appearance of carbon further explains the faster degradation rate, although the carbon deposition was not directly caused by the introduction of air into the air electrode. Energy spectral analysis shows that the air electrode in Cell-1 generated Sr rich phases, which indicates that the absence of air in the air electrode in the electrolysis process indeed causes more serious microstructure damage. The energy conversion efficiency could exceed 86% if the energy consumed for heating the air is ignored. This work provides a scenario for the application of solid oxide cells for CO₂ electrolysis without air purging in the air electrode.     

The effect of gas compositions on the performance and durability of solid oxide electrolysis cells

High-temperature electrolysis with various gas compositions has been performed to investigate the effects of the hydrogen partial pressure and the humidity generated by the steam electrode on the performance and durability of solid oxide electrolysis cells. The power density of the button cell used in this research is 0.48 W cm⁻² at 750 °C, and the flow rates of the air and humidified hydrogen are 100 cc min⁻¹. By changing the flow ratio of H₂:Ar:H₂O(g) from 10:0:4 to 1:9:4, the cell's OCV decreases from 0.973 V to 0.877 V, and the charge transfer resistance increases from 1.126 Ω cm² to 1.645 Ω cm². The close relationship between the conversion efficiency of high-temperature electrolysis and steam composition is evident in the increase in the cell's charge transfer resistance from 0.381 Ω cm² to 1.056 Ω cm² as the steam content changed from 40 vol% to 3 vol%. Although the electrochemical splitting of water is stimulated in the short term by excessive steam flow, the Ni-YSZ electrodes have been damaged by the steam electrode's low H₂ partial pressure. Consequently, the steam electrode's gas composition must be optimized in the long-term because of the trade-off between performance and durability, which depends on the water concentration of the steam electrodes.     

Performance evaluation of ammonia-fueled flat-tube solid oxide fuel cells with different build-in catalysts

The corrosion of ammonia atmosphere to Ni-cermet anode significantly affects the performance and durability of direct ammonia fuel solid oxide fuel cells. In this study, we investigate the impedance behavior of the flat-tube solid oxide fuel cells (SOFCs) fed with pure ammonia in details, assisted with distributions of relaxation time analysis under different fuels, operating temperatures, flow rates, and cell voltages. In addition, we use Ni/Al₂O₃ particles and nickel foam strips as the built-in catalysts to enhance ammonia decomposition within the anode support of the flat-tube cells. It is confirmed that the increased catalytic activity of ammonia decomposition brought by the built-in catalyst and the change of internal fuel gas resistance would affect the initial performance of the cell. In addition, enhancing the catalytic activity of ammonia decomposition through the built-in catalyst could effectively inhibit the corrosion effect of ammonia on the Ni surface in the anode support, thus enhancing the long-term durability of ammonia-fueled SOFC.     

The current density and temperature distributions of anode-supported flat-tube solid oxide fuel cells affected by various channel designs

The advantages of a flat-tube solid oxide fuel cell (FT-SOFC) include easy sealing, low stack volume and low resistance to current collection. Because the performance of the FT-SOFC is closely linked to the interior hydrogen channel design, this paper studied various channel designs using a numerical approach. Ordinary FT-SOFCs have many channels with small cross-sectional areas. Unfortunately, this design makes it possible for the sealing material to block the channel entrance. Furthermore, it is difficult for the hydrogen to be evenly distributed under this type of design. To overcome these problems, a new design was developed to reduce the number of channels and increase the cross-sectional area. Contrary to expectations, the numerical approach applied here revealed that new design resulted in poor hydrogen transport into the support region and an average current density of 2846.7 A/m², lower than that of the traditional design. Another design with a gradually increasing width from inlet to outlet was applied to increase the performance, maintain the mechanical strength and reduce the pressure drop of SOFCs. This design improved the average current density to 3135.4 A/m². However, the cell performance with this channel design decreased significantly when the channel inlet became too narrow or when the outlet became too wide.     

Mechanisms of performance degradation induced by thermal cycling in solid oxide fuel cell stacks with flat-tube anode-supported cells based on double-sided cathodes

In this work, the degradation in output power of a stack with flat-tube anode-supported cells based on double-sided cathodes and its mechanism are studied. After 102 thermal cycles, the OCV keeps about 1.1 V and remains stable, showing that the one-cell stack exhibits a good sealing performance. During the first 100 thermal cycles, when the temperature ranges from 750 to 200 °C with a heating/cooling rate of 3 °Cmin⁻¹, the stack degradation mainly occurs during the first 34 thermal cycles, and the degradation rate is ~0.89%/cycle. During the 101th and 102nd thermal cycles, an additional loading force is applied on the cathode side of the stack at room temperature, and the results shows that the output power at 750 °C increases and finally exceeds the initial output. As a result, the primary cause for degradation induced by thermal cycling is believed to originate from the weak interface between the cathode and the interconnect, resulting in an increase in ohmic resistance. The stack degradation can therefore be recovered by a secondary loading force on the cathode side.     

Numerical study of a flat-tube high power density solid oxide fuel cell: Part I. Heat/mass transfer and fluid flow

The flat-tube high power density (HPD) solid oxide fuel cell (SOFC) is a new design developed by Siemens Westinghouse, based on their formerly developed tubular type SOFC. It has increased power density, but still maintains the beneficial feature of secure sealing of a tubular SOFC. In this paper, a three-dimensional numerical model to simulate the steady state heat/mass transfer and fluid flow of a flat-tube HPD-SOFC is developed. In the numerical computation, governing equations for continuity, momentum, mass, and energy conservation are solved simultaneously. The highly coupled temperature, concentration and flow fields of the air stream and the fuel stream inside and outside the different chambers of a flat-tube HPD-SOFC are investigated. The variation of the temperature, concentration and flow fields with the current output is studied. The heat/mass transfer and fluid flow modeling and results will be used to simulate the overall performance of a flat-tube HPD-SOFC, and to help optimize the design and operation of a SOFC stack in practical applications.     

Understanding thermal and redox cycling behaviors of flat-tube solid oxide fuel cells

The performance stability of solid oxide fuel cells (SOFCs) under thermal and redox cycles is vital for large-scale applications. In this work, we investigated the effects of thermal and redox cycles on cell performances of flat-tube Ni/yttria-stabilized zirconia (Ni/YSZ) anode-supported SOFCs. Cell performance was considerably affected by the duration of oxidation during redox cycles and the heating rate during the thermal cycles. The cell tolerated 20 short-term redox cycles (5 min oxidation) without significant performance degradation. Besides, the cell exhibited superior stability during 8 thermal cycles with a slow heating rate (4 °C min⁻¹) to that with a fast heating rate (8 °C min⁻¹). These results reflected that the thick anode support (2.7 mm) offered strong resistance to the shocks caused by redox and thermal cycling. Moreover, the morphological changes of the Ni phase during the redox and thermal cycling were investigated using Ni-film anode cells. Agglomeration of Ni particles and dissociation between the Ni film and the YSZ substrate were confirmed after 5 redox cycles, whereas no significant changes in Ni film emerged after 8 thermal cycles.