Elsevier

Ceramics International

Volume 48, Issue 7, 1 April 2022, Pages 9673-9680
Ceramics International

Low-temperature constrained sintering of YSZ electrolyte with Bi2O3 sintering sacrificial layer for anode-supported solid oxide fuel cells

https://doi.org/10.1016/j.ceramint.2021.12.168Get rights and content

Abstract

Solid oxide fuel cells (SOFCs) have strong potential for next-generation energy conversion systems. However, their high processing temperature due to multi-layer ceramic components has been a major challenge for commercialization. In particular, the constrained sintering effect due to the rigid substrate in the fabrication process is a main reason to increase the sintering temperature of ceramic electrolyte. Herein, we develop a bi-layer sintering method composed of a Bi2O3 sintering sacrificial layer and YSZ main electrolyte layer to effectively lower the sintering temperature of the YSZ electrolyte even under the constrained sintering conditions. The Bi2O3 sintering functional layer applied on the YSZ electrolyte is designed to facilitate the densification of YSZ electrolyte at the significantly lowered sintering temperature and is removed after the sintering process to prevent the detrimental effects of residual sintering aids. Subsequent sublimation of Bi2O3 was confirmed after the sintering process and a dense YSZ monolayer was formed as a result of the sintering functional layer-assisted sintering process. The sintering behavior of the Bi2O3/YSZ bi-layer system was systematically analyzed, and material properties including the microstructure, crystallinity, and ionic conductivity were analyzed. The developed bi-layer sintered YSZ electrolyte was employed to fabricate anode-supported SOFCs, and a cell performance comparable to a conventional high temperature sintered (1400 °C) YSZ electrolyte was successfully demonstrated with significantly reduced sintering temperature (<1200 °C).

Introduction

Currently, renewable energy has attracted worldwide attention due to the depletion of fossil fuels, growing global energy consumption, and environmental threats of conventional energy sources including nuclear energy [[1], [2], [3]]. Among the various types of eco-friendly energy sources, fuel cells have been considered as a next-generation energy conversion system because of their high energy conversion efficiency, quiet operation, and independence under certain temperature and climate conditions. Among them, the solid oxide fuel cell (SOFC) is one of the most promising fuel cell systems due to its high energy conversion efficiency and the flexibility to use various types of fuels such as hydrocarbons including methanol and ethanol as well as hydrogen for the energy conversion process [[4], [5], [6], [7]]. The fuel flexibility of SOFCs makes it attractive for use in a variety of applications ranging from transportation to stationary systems.

SOFCs are composed of a ceramic multilayer structure that requires compatible interfacial characteristics and microstructural uniformity of each component layer for high cell performance and durability. To obtain the material properties, high-temperature heat treatment processes are required. In the presence of mutual interactions between the component layers, the electrolyte and electrode layers should be carefully sintered at the desirable temperatures of each to obtain excellent material properties while avoiding unwanted chemical reactions [[8], [9], [10]]. The sintering of a ceramic electrolyte layer usually requires a relatively high sintering temperature (>1400 °C) to obtain a fully densified microstructure to ensure gas-tightness and prevent electrical short circuits, while the electrode layer is usually sintered at a relatively low temperature to obtain a sufficient porous microstructure and connectivity for efficient gas and electron phase transport [[11], [12], [13], [14]]. For this reason, the electrolyte layer and electrode layer are usually sintered separately. As a result, the sintering of a constrained film on a rigid substrate is frequently encountered in the fabrication process of SOFCs [[15], [16], [17]].

Constrained sintering is a feature of many ceramic processing technologies, including low-temperature co-fired ceramic (LTCC) [18,19] and multi-layer ceramic capacitors (MLCC) [20,21] as well as SOFCs accompanied by considerable fundamental interests. In the constrained sintering condition, the tensile stress field caused by a rigid substrate during the sintering process retards the densification of the deposited film because the film can shrink only perpendicular to the substrate rather than in the lateral directions to the substrate due to the absence of shrinkage in the rigid substrate [8,17,22]. Therefore, it is difficult to achieve a highly dense electrolyte layer, which is essential for SOFC application under the constrained sintering conditions. To overcome this issue, a high sintering temperature above 1400 °C is employed in the electrolyte layer fabrication process [23,24]. However, this high sintering temperature has drawbacks including unwanted coarsening, migration of metal atoms in the anode support, and excessive thermal stress, which hinder improvement of the cell performance. In addition, long heating and cooling times increase the overall processing time, which increases the manufacturing cost. Therefore, one of the key challenges associated with the fabrication of anode-supported SOFCs is to reduce the sintering temperature of the electrolyte layer.

In this regard, much effort has been made to lower the sintering temperature of YSZ electrolyte, which is the most frequently used main electrolyte for SOFCs. In the past decade, a co-sintering technique that simultaneously sinters the anode support tape and the electrolyte layer has been frequently employed to fabricate an electrolyte layer at reduced temperature [25,26]. In this method, since the electrolyte layer can shrink with the anode support tape, it is effective to obtain the highly dense electrolyte layer at low temperature. However, the problems of warpage, delamination, and film defects due to the differences in shrinkage rate and thermal expansion coefficients between the anode and electrolyte require strict shrinkage matching and subsequent flat-firing processes [26,27]. On the other hand, another method of lowering the sintering temperature of the electrolyte is to apply a sintering aid. Many previous studies have reported that the addition of transition metal oxides such as Li2O3, Fe2O3, Co3O4, CuO, and Bi2O3 could improve the sinterability of the YSZ electrolyte either by improving the grain boundary mobility via lattice dissolution or inducing the liquid phase sintering process [28,29]. Among the various sintering additives, Bi2O3 is well known as a very effective aid for lowering the sintering temperature of YSZ because Bi2O3 has a significantly lower melting point (∼825 °C) than other additives and can induce liquid phase sintering at a significantly lower temperature. Accordingly, some research adopted and demonstrated the effect of a Bi2O3 sintering aid to reduce the sintering temperature of the YSZ electrolyte. For instance, Yeh et al. reported that the addition of 5 mol% Bi2O3 could reduce the sintering temperature of YSZ from 1500 to 1200 °C [30]. In addition, Spiridigliozzi et al. reported that the addition of 4 mol% Bi2O3 reduced the sintering temperature of YSZ to 1100 °C [31].

Although previous studies have reported the effect of sintering aids on reducing the YSZ electrolyte sintering temperature, most of these studies were conducted in the free-sintering condition using YSZ pellets for electrolyte material characterization and adopted an electrolyte-supported SOFC structure for fuel cell fabrication. The electrolyte-supported structure is favorable to achieve a dense electrolyte layer because the electrolyte can be sintered under free conditions. However, it has a comparably thick electrolyte, as it is responsible for mechanical rigidity. Therefore, the cells have a high ohmic loss and a limitation in achieving high fuel cell performance. On the contrary, anode-supported SOFCs have the advantage of a relatively high power density owing to the thin electrolyte layer. However, it is very difficult to fabricate a dense electrolyte layer due to the constraints effect at the interface of the rigid anode substrate. Because the sintering behavior of the electrolyte is fundamentally different between the two support types, the simple application of sintering aids has a limited effect on reducing the sintering temperature of the electrolyte layer due to the constrained shrinkage of the layers during the sintering process [32,33]. For this reason, the practical application of the sintering additive to the electrolyte layer structure is limited, and a novel method for applying a sintering aid is required to improve the sinterability of the YSZ electrolyte layer for low temperature processing and high performance anode-supported SOFCs.

Furthermore, another important point to consider in the use of sintering aids to lower the sintering temperature is the possible negative effects resulting from residues of the sintering aid after the sintering process. It has been reported that some residual sintering additives have a negative effect on the durability of cells by inducing electronic conduction, reducing the oxygen ionic conductivity of YSZ, and causing chemical instability between cell components [[34], [35], [36]]. For instance, it has been reported that residual lithium additive at the grain boundaries of the electrolyte lowers the oxygen ionic conductivity, suggesting that the residue must be removed after the sintering process [15]. Knibbe et al. also reported that the residue of Al2O3 additive caused delamination of the La1-xSrxMnO3 (LSM) cathode from the YSZ electrolyte, resulting in failure of the system [37]. In addition, even when Bi2O3 is used as a sintering aid, it was reported that the residue of Bi2O3 additive tends to form unwanted compounds with the cathode material due to the high reactivity of Bi2O3 [38]. Therefore, in order to maintain the material property and cell stability, non-residual sintering aids are preferred, and post sintering removal of the residual additives is frequently recommended [15,39].

In this study, we developed an effective method to apply sintering aids that can overcome the limitation of the constrained sintering condition and sinter the YSZ electrolyte layer at significantly lower temperatures. Moreover, the sintering aid was completely removed after the controlled sintering process, eliminating possible factors of degradation of the cell performance. A bi-layer structure composed of a Bi2O3 sintering sacrificial layer and YSZ main electrolyte layer was designed to uniformly infiltrate the Bi2O3 sintering aid into the YSZ layer during the sintering process and induce liquid phase sintering to obtain a highly dense YSZ layer at a significantly reduced sintering temperature. Furthermore, by taking advantage of the sublimation characteristics of the Bi2O3 sintering aid, the negative effects from the residual additives can be avoided by optimizing the processing temperature. The sintering behavior of the Bi2O3/YSZ bi-layer structure was systematically analyzed, and the material properties including the microstructure, crystallinity, and ionic conductivity were analyzed. The developed bi-layer sintered YSZ electrolyte layer was employed to fabricate anode-supported SOFCs, and the cell performance of the low-temperature-sintered cell was compared to conventional high-temperature-sintered cells. We believe that the results presented here will help to overcome the fabrication issues associated with many ceramic technologies based on multi-component ceramic layers and to facilitate their commercialization.

Section snippets

Sample preparation

To prepare the Bi2O3 sintering functional layer paste, a solvent (α-terpineol, Tokyo Chemical Industry (TCI), Japan) and a polymeric dispersant (Hypermet KD-6) were mixed and homogenized for 24 h using a planetary mill (Fritsch, Germany). Then, bismuth oxide powder (specific surface area (SSA) = 3.8–5.5 m2/g, Alfa Aesar, USA) was added to the prepared organic mixture and mixed for 24 h. Finally, a binder (BH-3, Sekisui Chemical, Japan) and plasticizer (dibutyl phthalate (DBP), Sigma-Aldrich,

Results and discussion

A high sintering temperature over 1400 °C is generally required to fabricate a YSZ electrolyte layer to obtain a highly dense microstructure with gas-tightness. Although many previous studies have demonstrated the effect of Bi2O3 sintering additive in lowering the sintering temperature of the YSZ electrolyte, the sintering aid only has a limited effect in the sintering of the YSZ film structure due to the effect of substrate constraints. Fig. 1 shows the microstructure of the YSZ layers

Conclusion

A low-temperature fabrication process for a YSZ electrolyte was developed by adopting a bi-layer structure of YSZ and a Bi2O3 sintering functional layer. The Bi2O3 sintering functional layer applied on the YSZ electrolyte melted at the melting point of Bi2O3 and infiltrated into the entire YSZ layer at this temperature. The melted Bi2O3 phase effectively induced liquid phase sintering and significantly reduced the sintering temperature of the YSZ electrolyte to 1100–1,200 °C. Subsequent

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

This work was supported by a National Research Foundation of Korea grant funded by the Korean government (MSIT) (No. NRF-2021R1A2C2013203). This work was also partly supported by an Institute of Information & Communications Technology Planning & Evaluation (IITP) grant funded by the Korean government (MSIT) (No.2020-0-01373, Artificial Intelligence Graduate School Program (Hanyang University)) and by the research fund of Hanyang University (HY-202000000790009).

References (44)

  • S. Le et al.

    Constrained sintering of Y2O3-stabilized ZrO2 electrolyte on anode substrate

    Int. J. Hydrogen Energy

    (2012)
  • G. Okuma et al.

    Microstructural evolution of electrodes in sintering of multi-layer ceramic capacitors (MLCC) observed by synchrotron X-ray nano-CT

    Acta Mater.

    (2021)
  • X. Wang et al.

    Constrained sintering of 8 mol% Y2O3 stabilised zirconia films

    J. Eur. Ceram. Soc.

    (2012)
  • X. Ge et al.

    Screen-printed thin YSZ films used as electrolytes for solid oxide fuel cells

    J. Power Sources

    (2006)
  • P. Ried et al.

    Processing of YSZ screen printing pastes and the characterization of the electrolyte layers for anode supported SOFC

    J. Eur. Ceram. Soc.

    (2008)
  • A.S. Mehranjani et al.

    Low-temperature co-sintering for fabrication of zirconia/ceria bi-layer electrolyte via tape casting using a Fe2O3 sintering aid

    J. Eur. Ceram. Soc.

    (2017)
  • K. Azari et al.

    The effect of cell shape on the warpage in solid oxide fuel cells

    J. Power Sources

    (2015)
  • A.J. Flegler et al.

    Cubic yttria stabilized zirconia sintering additive impacts: a comparative study

    Ceram. Int.

    (2014)
  • Y.-S. Jung et al.

    Liquid-phase redistribution during sintering of 8 mol% yttria-stabilized zirconia

    J. Eur. Ceram. Soc.

    (2003)
  • T.H. Yeh et al.

    Effect of sintering process on the microstructures of Bi2O3-doped yttria stabilized zirconia

    Mater. Res. Bull.

    (2010)
  • S.-I. Lee et al.

    Fabrication of dense and defect-free diffusion barrier layer via constrained sintering for solid oxide fuel cells

    J. Eur. Ceram. Soc.

    (2017)
  • C. Nicollet et al.

    Gadolinium doped ceria interlayers for Solid Oxide Fuel Cells cathodes: enhanced reactivity with sintering aids (Li, Cu, Zn), and improved densification by infiltration

    J. Power Sources

    (2017)
  • Cited by (13)

    View all citing articles on Scopus
    View full text