Improvement on durability and thermal cycle performance for solid oxide fuel cell stack with external manifold structure
Introduction
SOFC is a promising energy conversion device for producing electric energy from hydrocarbon fuels. It has many advantages such as high conventional efficiency, reliability, modularity, fuel flexibility, and environment-friendly [1]. It can be used as a power supply source for various applications including residential houses, mobile facilities and automotive. Due to the use of sufficiently thin electrolyte and optimized catalyst materials, the operation temperature of SOFC has been reduced from 1000 °C to 650–800 °C. Hence, using metal as interconnects becomes possible, which can speed up the commercialization of SOFC.
Much research effort on SOFC stack has already been focused on improving its durability, thermal cycle stability and cost reduction, which are all related to material, component and stack structure designs and management strategy [[2], [3], [4], [5]]. Several stack designs have been developed including internal manifold [6,7], external manifold [8,9], disk type [10], envelope type [11] and tubular type [12,13]. Among these designs, the internal manifold stack has been commonly adopted. However, the complicated interconnect construction and assembling process increase its manufacturing cost [14]. Accordingly, we have developed a novel external-manifold SOFC stack with a simple metallic interconnect design, as shown in Fig. 1 [9,15]. Compared with the internal manifold stack design, the external manifold stack design offers advantages of fewer numbers of components, easier assembly process and lower cost. Another important improvement in the new design is that the stack core is less subjected to thermal effect from the outer environment, because the manifold is decoupled from the active reaction area of the stack, resulting in a more even temperature distribution in the stack. With the internal manifold stacks design, heat generation and distribution usually limit the number of cells to 30 in one stack. However, the external manifold stack design allows more cells to be assembled into one stack to produce higher power output due to its excellent thermal balance ability as demonstrated by molten carbon fuel cell stack [16,17].
Though the external manifold SOFC stack is a promising design for large scale stationary power, over the last decade, only some mathematical models were established to describe the flow distribution in stack [18,19]. For getting a better operation characteristic of external manifold stack, it is necessary to performance experimental testing to find out the causes relating to degradation rate with the help of post-test analysis. The feasibility and durability of external manifold stack have been shown in our previous study in which a stack with 3 cells was operated for 70 h [9]. The composite cathode was later screen printed onto the electrolyte and sintered. A Cell with a dimension of 11 × 11 cm (active reaction area of 9 × 9 cm) showed an open circuit voltage (OCV) of 1.15 V and a peak power density of 770 mW/cm2 at 750 °C [20]. A 1-cell stack was tested for nearly 4000 h and showed a degradation rate of 1.7%/1000 h [21]. Also, a 10-cell stack was assembled and tested for 700 h. The average degradation rate was 17.6%/1000 h at 750 °C and 370 mA/cm2 [15]. However, the effect of thermal cycles on performance degradation has not been considered. Therefore, durability and thermal cycle performance of two external manifold stacks with five anode-supported cells are analyzed and the stacks are also post-test examined in this work. Based on the results obtained, improvements to the structure and components of the stack can be made to reduce the degradation rate of the stack with an external manifold structure.
Section snippets
Material and operation of stack
Cells used in stacks were anode-supported SOFC with porous Ni-YSZ anode and dense YSZ electrolyte [22]. The interconnect used was made of SUS430 ferrite stainless steel. Corrugated plates and Ni foam were used as cathode and anode current collectors as well as the gas distributors. To reduce the contact electrical resistance, LaCo0.6Ni0.4O3-δ (LCN) and Ni paste were used as cathode and anode interface contact materials respectively between current collectors and cells [23]. The compressive seal
Performance and durability test of stack A
Fig. 5 (a) shows voltage and power output measured at 750 °C as a function of current density for 5-cell Stack A with an active reaction cell area of 9 × 9 cm. In order to compare performance between cell and Stack A, voltage and power density are plotted as a function of current density in Fig. 5 (b). In this case, pure hydrogen was used as fuel on the anode side. It can be seen from Fig.5 that the OCV and power density of Stack A were 5.5 V and 475 mW/cm2 (at 1000 mA/cm2), respectively,
Improved test of stack
Post-test analysis from 5-cell stack demonstrated that the decomposition of cathode contact material and higher oxidation kinetic of metallic interconnect contributed to the main performance degradation in Stack A. Both phenomena were closely related to gas leakage and uneven stress distribution. Thus, following improvements were made in the case of Stack B to enhance the reliability of seals and optimize the cathode interface contact stress distribution:
- (1)
Spot and laser welding in the
Conclusion
Two 5-cell stacks with an external manifold structure were tested for 250 h, including being subjected to five thermal cycles. The following conclusions can be made:
- (1)
By improving the metallic interconnect and seal, the stack with external manifold exhibits excellent thermal cycling stability both at OCV and constant current discharging. The degradation rate of the stack during 140 h test is reduced from 50%/1000 h to 5%/1000 h, even though the stack is operated under a higher current density.
- (2)
Acknowledgments
This research was financially supported by National Science Foundation of China (51572099, 51772114), National Key Research & Development Project-International Cooperation Program (2016YFE0126900). The SEM and XRD characterization was assisted by the Analytical and Testing Center of Huazhong University of Science and Technology. The authors would like to thank Dr. Jen-Jung Fan for helpful discussions on external manifold stack.
References (41)
- et al.
Chapter 1-Introduction to SOFCs
- et al.
Life cycle assessment (LCA) of biogas-fed solid oxide fuel cell (SOFC) plant
Energy
(2017) - et al.
Performance characterization and modelling of syngas-fed SOFCs (solid oxide fuel cells) varying fuel composition
Energy
(2015) - et al.
Long-term performance degradation study of solid oxide carbon fuel cells integrated with a steam gasifier
Energy
(2016) - et al.
Promoted electrochemical performance of intermediate temperature solid oxide fuel cells with Pd0.95 Mn0.05 O-infiltrated (La0.8 Sr0.2 )0.95 MnO3−δ –Y0.16 Zr0.84 O 2 composite cathodes
J Power Sources
(2016) - et al.
From rare earth doped zirconia to 1 kW solid oxide fuel cell system
J Alloys Compd
(2006) - et al.
Feasibility study of an external manifold for planar intermediate-temperature solid oxide fuel cells stack
Int J Hydrogen Energy
(2013) - et al.
Development of envelope-type solid oxide fuel cell stacks
J Power Sources
(2006) - et al.
Performance evaluation of a tubular direct carbon fuel cell operating in a packed bed of carbon
Energy
(2014) - et al.
Performance degradation and analysis of 10-cell anode-supported SOFC stack with external manifold structure
Energy
(2017)
Industrial experience on the development of the molten carbonate fuel cell technology
J Power Sources
Fabrication and performance evaluation of planar solid oxide fuel cell with large active reaction area
Int J Hydrogen Energy
Degradation analysis and durability improvement for SOFC 1-cell stack
Appl Energy
Electrochemical performance and thermal cyclicability of industrial-sized anode supported planar solid oxide fuel cells
J Power Sources
LaCo0.6Ni0.4O3−δ as cathode contact material for intermediate temperature solid oxide fuel cells
Int J Hydrogen Energy
Development of novel glass-based composite seals for planar intermediate temperature solid oxide fuel cells
Int J Hydrogen Energy
Development of Al2O3/glass-based multi-layer composite seals for planar intermediate-temperature solid oxide fuel cells
Int J Hydrogen Energy
Investigation of thermal control for different SOFC flow geometries
Appl Energy
Experimental investigation of temperature distribution over a planar solid oxide fuel cell
Appl Energy
Effect of contact between electrode and current collector on the performance of solid oxide fuel cells
Solid State Ionics
Cited by (24)
Structural design and optimization for a 20-cell solid oxide fuel cell stack based on flow uniformity and pressure drop
2023, International Journal of Hydrogen EnergyCharacterization of metallic interconnects extracted from Solid Oxide Fuel Cell stacks operated up to 20,000 h in real life conditions: The fuel side
2021, International Journal of Hydrogen EnergyCFD model for tubular SOFC directly fed by biomass
2021, International Journal of Hydrogen EnergyCitation Excerpt :The advantages of this coupling are numerous: lower emissions and related impacts from Organic Wastes (OW) [13,14], and high-efficiency values and high exploitability of the starting biomass using SOFC systems [14,15]. SOFC systems suffer degradation phenomena mainly at the anode compartment due to the combination of trace contaminants plus carbon-rich fuels [16–18] as well as thermal cycles [19–21]. The impact of catalytic degradation is crucial for such systems [22–24].