High-Performanced Cathode with a Two-Layered R–P Structure for Intermediate Temperature Solid Oxide Fuel Cells

Abstract

Driven by the mounting concerns on global warming and energy crisis, intermediate temperature solid-oxide fuel cells (IT-SOFCs) have attracted special attention for their high fuel efficiency, low toxic gas emission, and great fuel flexibility. A key obstacle to the practical operation of IT-SOFCs is their sluggish oxygen reduction reaction (ORR) kinetics. In this work, we applied a new two-layered Ruddlesden-Popper (R-P) oxide, Sr3Fe2O7-δ (SFO), as the material for oxygen ion conducting IT-SOFCs. Density functional theory calculation suggested that SFO has extremely low oxygen ion formation energy and considerable energy barrier for O(2-) diffusion. Unfortunately, the stable SrO surface of SFO was demonstrated to be inert to O2 adsorption and dissociation reaction, and thus restricts its catalytic activity toward ORR. Based on this observation, Co partially substituted SFO (SFCO) was then synthesized and applied to improve its surface vacancy concentration to accelerate the oxygen adsorptive reduction reaction rate. Electrochemical performance results suggested that the cell using the SFCO single phase cathode has a peak power density of 685 mW cm(-2) at 650 °C, about 15% higher than those when using LSCF cathode. Operating at 200 mA cm(-2), the new cell using SFCO is quite stable within the 100-h' test.

A first-principles study on divergent reactions of using a Sr3Fe2O7 cathode in both oxygen ion conducting and proton conducting solid oxide fuel cells

“…2(a)) in SFO bulk, 12 suggesting that protons are more easily to locate at rock-salt layer while oxygen vacancies at the central of perovskite layers. Moreover, compared with those in Sr 3 Fe 2 O 7 bulk model, 12,16 values of E vac and E hyrd are much lower for surfaces, suggesting that oxygen vacancies and protons are more easily to form at surface than in bulk.…”

“…The relatively lower oxygen vacancies form energy and lower energy barrier for oxygen diffusion makes SFO have a better oxygen ion conduction than LSCF, which is good agreement with experimental investigations. 16 In conclusion, compared with perovskite oxides, SFO cathode has low oxygen vacancies formation energies and low ion diffusion energy barriers, which can bring forth excellent oxygen ion conduction. Nevertheless, its high energy barrier for O 2 dissociation may restrict its application as an excellent cathode in O-SOFCs.…”

“…E vac and E hyrd at O1, O3, and O4 sites in SFO bulk are also given for comparison. 12,16 The suffix -surf and -bulk denotes the values acquired on the terminal surface model and the corresponding bulk model, respectively. E vac and E hyrd at LSCF (001) surface are also calculated as comparison Table 2, which could provide qualitative analysis on their bonding strength to the surface.…”

“…This is in good agreement with experimental observation that SFO is more effective than both LSCF and LSCF-BZCY as a cathode for P-SOFCs; while for O-SOFCs, opposite is the case. 12,16 Moreover, DFT investigations suggest that the high proton formation energy of LSCF makes it unfavorable for proton formation, and therefore, composite cathodes consisting LSCF and proton conductors (for example, BZCY) have to be made up to effectively accelerate cathode reactions.…”

“…15 Recently, we found that Sr 3 Fe 2 O 7 (Ruddlesden-Popper oxides) has low formation energies and diffusion energy barriers for both oxygen vacancies and protons in bulk model, which make it suitable for both O-SOFCs and P-SOFCs. 12,16 In this work, we performed a rst-principles study on the oxygen reduction process over the Sr 3 Fe 2 O 7 surfaces which acts as a single phase cathode in both O-SOFCs and P-SOFCs, as shown in Fig. 1.…”