Elsevier

Energy

Volume 149, 15 April 2018, Pages 903-913
Energy

Improvement on durability and thermal cycle performance for solid oxide fuel cell stack with external manifold structure

https://doi.org/10.1016/j.energy.2018.02.072Get rights and content

Highlights

  • Two 5-cell SOFC stacks with external manifold structure was designed and constructed.

  • A comparative study of the two stacks was operated about degradation behavior.

  • Improved stack showed excellent long-term stability and thermal cycle performance.

Abstract

Two 5-cell solid oxide fuel cell (SOFC) stacks with an external manifold structure are constructed and their degradation and thermal cycle performance are investigated at 750 °C. The cell consists of anode-supported cells with a size of 11 × 11 cm. In Stack A, the voltage degradation rate during 140 h tests at a current density of 400 mA/cm2 is about 50%/1000 h. The factors influencing the performance of stack are investigated by post-test analysis. We found partial decomposition of the cathode contact materials LaCo0.6Ni0.4O3-δ and higher oxidation rate of metallic interconnect, resulting in an increase of the electrical resistance of the stack. Owning to the improvement of suitable sealing materials and interconnect, the resulting Stack B exhibited reasonable degradation rate of about 5%/1000 h during 140 h at a current density of 500 mA/cm2 together with a good thermal cycle stability. The applicability of stacks with an external manifold structure can be demonstrated in planar intermediate temperature SOFC.

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)

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