Enhancing Oxygen Reduction Reaction Activity and CO₂ Tolerance of Cathode for Low-Temperature Solid Oxide Fuel Cells by in Situ Formation of Carbonates

Abstract

Development of low-cost and cobalt-free efficient cathode materials for oxygen reduction reaction (ORR) remains one of the paramount motivations for material researchers at a low temperature (<650 °C). In particular, iron-based perovskite oxides show promise as electrocatalysts for ORR because Fe metal is cheaper and naturally abundant, exhibit matched thermal expansion with contacting components such as electrolytes, and show high tolerance in a CO 2 -containing atmosphere. Herein, we demonstrated a new mechanism, the in situ formation of alkali metal carbonates at the cathode surface. This new mechanism leads to an efficient and robust cobalt-free electrocatalyst (Sr 0.95 A 0.05 Fe 0.8 Nb 0.1 Ta 0.1 O 3−δ , SAFNT5, A = Li, Na, and K) for the application of low-temperature solid oxide fuel cells (LT-SOFCs). Our results revealed that the formation of Li\K carbonates boosts the ORR activity with an area-specific resistance as low as 0.12 and 0.18 Ω cm 2 at 600 °C, which show the highest performance of the cobalt-free single-phase cathode that has been ever reported so far. We also find that the chemical stability and tolerance of tested cathodes toward CO 2 poisoning significantly improved with alkali carbonates, as compared to the pristine SrFe 0.8 Nb 0.1 Ta 0.1 O 3−δ (SFNT) at 600 °C. This work demonstrates the conclusive role of alkali carbonates in developing highly efficient and stable cobalt-free cathodes for LT-SOFCs and CO 2 neutralization.

Unveiling Lithium Roles in Cobalt‐Free Cathodes for Efficient Oxygen Reduction Reaction below 600 °C

“…The lattice parameter reduction originates from the incorporation of the smaller Li + into the Sr 2 + host. [30] This is confirmed by iodometric titration ( Table 1) which shows that the averaged iron valence in the oxides decreases with increasing Li concentration.…”

“…[13][14][15] Ferrite-based perovskites, especially the cubic-structured SrFeO 3-δ (SF)-based perovskites, are promising cobalt-free cathode materials. [30] In particular, we found that the size mismatch of Li + (0.92 Å) dopant with the Sr 2 + (1.64 Å) host could also render a significant amount of Li + migrating to the surface and creating A-site deficiency in the bulk at high temperature. [16][17][18][19] The cubic SF structure can be stabilized with rare earth metals such as Bi 3 + , Sm 3 + , La 3 + at the A-site (Sr-site) and transition metals such as Ti 3 + , Mn 3 + , Ta 5 + , Nb 5 + , Hf 4 + , W 5 + , etc.…”

“…[26][27][28][29] In our recent work, a synergistic effect was observed from both the doped monovalent Li or K ions and the A-site deficiency, which bestows the pristine SrFe 0.8 Nb 0.1 Ta 0.1 O 3-δ perovskite with a remarkably high ORR activity even comparable to the benchmark cobalt-containing Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3-δ perovskite at IT. [30] In particular, we found that the size mismatch of Li + (0.92 Å) dopant with the Sr 2 + (1.64 Å) host could also render a significant amount of Li + migrating to the surface and creating A-site deficiency in the bulk at high temperature. Although such migration induces further charge imbalance in the oxides, it could also have a significant impact on the cathode surface and the crystal structure integrity, with both being crucial for an optimal ORR activity.…”

“…From the scale factors of the main cubic perovskite and impurity (brownmillerite phase), the relative amount of Sr 2 Fe 2 O 5 was determined to be 4.3(5)%. [30] However, a high concentration (> 0.05) of Li + at the A-site destabilizes the beneficial cubic structure. [33][34][35] The main primitive cubic phase structure of the x = 0.05 sample was further confirmed by the high-resolution transmission electron microscopy (HR-TEM).…”

“…[30] The finally prepared powder samples were referenced as Sr 0. [30] The finally prepared powder samples were referenced as Sr 0.…”