Composite air electrode for low-temperature protonic ceramic fuel cells

What is it about?

Protonic ceramic fuel cells (PCFCs) are more suitable for operation at low temperatures due to their smaller activation energy (Ea). Unfortunately, the utilization of PCFC technology at reduced temperatures is limited by the lack of durable and high-activity air electrodes. A lot number of cobalt-based oxides have been developed as air electrodes for PCFCs, due to their high oxygen reduction reaction (ORR) activity. However, cobalt-based oxides usually have more significant thermal expansion coefficients (TECs) and poor thermomechanical compatibility with electrolytes. These characteristics can lead to cell delamination and degradation. Herein, we rationally design a novel cobalt-containing composite cathode material with the nominal composition of Sr4Fe4Co2O13+δ (SFC). SFC composed of tetragonal perovskite phase (Sr8Fe8O24, I4/mmm, 81wt.%) and spinel phase (Co3O4, Fd-3m, 19 wt.%). The SFC composite cathode displays an ultra-high oxygen ionic conductivity (0.053 S cm-1 at 550 oC), superior CO2 tolerance, and suitable TEC value (17.01×10−6 K−1). SFC has both the O2-/e- conduction function, and the triple conducting (H+/O2-/e-) capability was achieved by introducing the protonic conduction phase (BaZr0.2Ce0.7Y0.1O3-δ, BZCY) to form SFC+BZCY (70wt.%: 30 wt.%). The SFC+BZCY composite electrode exhibits superior ORR activity at a reduced temperature with extremely low area-specific resistance (ASR, 0.677 Ω cm2 at 550 oC), profound peak power density (PPD, 535 mW cm-2 and 1.065 V at 550 oC), extraordinarily long-term durability (>500 h for symmetrical cell, 350 h for single cell). Moreover, the composite has an ultra-low TEC value (15.96×10−6 K−1). This study provides that SFC+BZCY with triple conducting capacity is an excellent cathode for low-temperature PCFCs.

Featured Image

Photo by National Cancer Institute on Unsplash

Photo by National Cancer Institute on Unsplash

Why is it important?

SFC cathode has been synthesized via self-assembled. SFC is composed of a tetragonal perovskite phase (mass ratio 81 wt.%) and a spinel phase (19 wt.%). In addition, the SFC cathode has outstanding CO2 tolerance, superior oxygen ionic conductivity, and ultra-low TEC. And the PPD value of SFC-based PCFC was 356 mW cm-2 at 550 oC. The triple-conducting composite cathode was prepared by introducing protonic conducting capability into the SFC cathode via physically mixing the SFC cathode and the BZCY electrolyte at a 7:3 mass ratio. And expanding ORR active sites from TPB to the entire cathode region, thus effectively promoting the oxygen ion migration rate and improving the oxygen surface process. As for PCFCs performance, the SFC+BZCY cathode takes an excellent PPD of 535mW cm-2 at 550 oC, 50% higher than the SFC cathode. At the same time, the SFC+BZCY cathode showed an exceeding long-term operational durability (symmetric cell for over 500 h at 550 oC, and the single cell for 350 h at 550 oC). These studies reveal that SFC+BZCY is a highly active and durable triple-conducting composite air electrode material for low-temperature PCFCs.

Perspectives