Highly Durable Cathode Materials with High Performance for Protonic Ceramic Fuel Cells

Solid oxide fuel cells (SOFCs) have attracted clean alternative power sources because of their have high energy conversion efficiency, multi-fuel flexibility and no use of precious metal catalysts. However, high operating temperatures (800 – 1000 ^oC) deteriorate the long term durability and fabrication cost of SOFCs [1]. Recently protonic ceramic fuel cells (PCFCs) based on proton-conducting electrolytes may be one of solutions to decrease operating temperatures due to high proton conductivity at intermediate temperature (IT; 400 – 600 ^oC) [2].

Doped barium zirconate (BZO) and barium cerate (BCO) based electrolytes have been studied extensively for use in PCFCs [3]. Even though BCO exhibits high proton conductivity, it is not suitable owing to its poor CO₂ tolerance. On the other hand, BZO shows high chemical stability with good mechanical strength under CO₂atmospheres. However, the proton conductivity of BZO-based materials is lower than that of BCO at the IT.

Hence, the compound materials (BZCO) of BZO and BCO have attracted as a candidate electrolyte for PCFCs, BZCO exhibits high proton conductivity as well as maintains good chemical stability. In particular, co-doped BZCO materials with Y and Yb (BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3–δ; BZCYYb) have been reported to exhibit high proton conductivity and the excellent catalytic activity for reforming of hydrocarbons with the conversion of H₂S to SO₂[3]

For PCFCs, the reactions at the cathode can be expressed as Eq. (1) and water generates at three phase boundaries (TPBs) of cathode side [4]:

              (1)         4OH^._o + O₂ + 4e^- --> 2H₂O + 4O^x_o

This phenomenon makes their cathode reactions more complex and results in high cathode polarization resistances. Until now, however, mixed ionic and electronic conductors for oxygen conducting SOFCs have used as the cathode materials for PCFCs, such as Ba_0.6Sr_0.4Co_0.2Fe_0.8O_3–δ (BSCF), La_0.6Sr_0.4Co_0.2Fe_0.8O_3–δ(LSCF).

Therefore, in this work suitable cathode materials are proposed for highly durable PCFCs with the improved performance. The cathode materials for PCFCs are required to the properties of high electric and ionic conductivity, high catalytic activity, good thermal compatibility with the electrolyte, and chemical stability in a humid oxidizing atmosphere [5-7]. For achieving these goals, several double perovskite-structure cathode materials are investigated under various operating conditions, such as flow rate of gas, amount of humidification and operating temperature in terms of the performance and long-term durability through the use of electrochemical and physicochemical analysis.

 

1. Materials for fuel-cell technologies, Brain C. H. Steele, Angelika Heinzel, Nature, 414, 345 (2001).
2. Lowering the temperature of solid oxide fuel cells, Kang Taek Lee, Eric D. Wachsman, Science, 334, 935 (2011).
3. Enhanced sulfur and coking tolerance of a mixed ion conductor for SOFCs: BaZr_0.1Ce_0.7Y_0.2–xYb_xO_3-δ, Lei Yang, Shizhong Wang, Kevin Blinn, Mingfei Liu, Ze Liu, Zhe Cheng, Meilin Liu, Science, 326, 9127 (2009).
4. Reaction model for cathodes cooperated with oxygen-ion conductors for solid oxide fuel cells using proton-conducting electrolytes, Ling Zhao, Beibei He, Jiaqiang Gu, Feng Liu, Xiangfeng Chu, Changrong Xi, Int. J. Hydrogen energy, 37, 548 (2012).
5. Anode-supported tubular SOFCs based on BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3−δ electrolyte fabricated by dip coating, Changcheng Chen, Mingfei Liu, Yaohui Bai, Lei Yang, Erqing Xie, Meilin Liu, Electrochemistry Communications, 13, 615 (2011).
6. BaZr_0.9Yb_0.1O_3-δ-modified bi-electrode supported solid oxide fuel cells with enhanced coking and sulfur tolerance, Kevin S. Blinn, Meilin Liu, J. Power Sources, 243, 24 (2013).
7. Comparative study of electrochemical properties of different composite cathode materials associated to stable proton conducting BaZr_0.7Pr_0.1Y_0.2O_3-δ electrolyte, Electrochimica Acta, 146, 1 (2014).

* Corresponding author: jyoung@sejong.ac.kr (J.-Y. Park).