Electrochemical Performance of Cobalt‐Free Nb and Ta Co‐Doped Perovskite Cathodes for Intermediate‐Temperature Solid Oxide Fuel Cells

Abstract

This work characterizes a series of Ta and Nb co‐doped cobalt‐free perovskite oxides with the compositions of SrNb0.2‐xTaxFe0.8O3‐δ (x=0, 0.05, 0.1, 0.15, and 0.2) for intermediate‐temperature solid oxide fuel cell (IT‐SOFC) applications. Their electronic conductivities displayed semiconducting behavior (i. e. increase with temperature rise) between 300 and 500 °C and metallic behavior (i. e. decrease with temperature rise) from 500 to 900 °C. SrNb0.1Ta0.1Fe0.8O3–δ provided the highest electrical conductivity that reaches 300 S cm−1 at 500 °C. The thermal expansion coefficients for SrNb0.2‐xTaxFe0.8O3–δ were lower than those reported for cobalt‐containing perovskite cathodes. They varied from 12.83×10−6 to 16.31×10−6 K−1 between 30 and 810 °C. Among the five tested cathodes, SrNb0.1Ta0.1Fe0.8O3–δ exhibited the lowest area‐specific resistance between 550 and 750 °C, with a minimum of 0.046 Ω cm2 at 750 °C, when tested in its symmetric cell configuration of cathode|Sm0.2Ce0.8O1.9 (SDC)|cathode. A peak power density of 1223 mW cm−2 was achieved on a SrNb0.1Ta0.1Fe0.8O3‐δ|SDC|yttria‐stabilized zirconia (YSZ)|Ni‐YSZ anode‐supported fuel cell at 700 °C. This work highlights the opportunity to create new perovskite cathode compositions with improved electrochemical performance through a co‐doping strategy.

Characterization of B‐Site Niobium‐Doped Pr0.4Sr0.6 (Co0.3Fe0.6)1‐xNbxO3‐δ (x=0, 0.05, 0.1, 0.2) Perovskites as Cathode Materials for Solid Oxide Fuel Cells

“…For example, PrSrFeCoO 3 (PSCF) is considered as one of the new‐type Fe−Co‐based cathode materials, in which Fe cations are in a mixed valence state ranging from +4 to +3, corresponding to a wide range of oxygen nonstoichiometry (from 0 to 0.5). Thus Nb 5+ doping should obey the charge neutrality criterion since Fe 4+ can be reduced to Fe 3+ . Furthermore, its structure alters from cubic to orthorhombic brownmillerite type with long range ordering of oxygen vacancies …”

Cobalt‐Free Perovskite Cathodes for Solid Oxide Fuel Cells

“…Ta and Nb co‐doping in SrNb 0.2‐ x Ta x Fe 0.8 O 3‐δ ( x= 0, 0.05, 0.1, 0.15, and 0.2) was investigated by Liang and co‐workers . Figure displays the thermal expansion behavior (Δ L / L 0 ) of SrNb 0.2‐ x Ta x Fe 0.8 O 3−δ ( x =0, 0.05, 0.1, 0.15, and 0.2) . These data were attained from dilatometry experiments that were carried out by heating perovskite bars from 30 to 810 °C.…”

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

“…Generally, the electrical conductivity comprises of both electronic and ionic conductivities, with the electronic conductivity dominating the overall conductivity. [22,[41][42] Figure 6a shows that all the oxides follow a typical semi-conductor behavior at temperatures lower than 525°C (i. e. σ increases with temperature), and a metallic conducting behavior at higher temperatures (i. e. σ decreases with temperature). [31,43] The linear relationship between ln(Tσ) and 1/T at temperatures below 525°C as presented in Figure 6b indicates that all the tested oxides underwent a thermally activated polaron hopping mechanism.…”

“…[5] Cobalt-containing perovskite oxides such as SrCo 0.8 Nb 0.1 Ta 0.1 O 3-δ have attracted immense research interest as potential IT-SOFC cathodes because of their high mixed ionic and electronic conductivities (MIECs). [19][20][21][22][23] However, because of their lower MIECs SF-based cathodes normally exhibit a lower ORR activity than Co-based cathodes in the IT range. However, the high cost of cobalt, mismatched thermal expansion coefficients (TECs) with ceria-based electrolytes, and chemical incompatibility with zirconium-based electrolytes are the major concerns for the practical application of cobalt-containing perovskites in IT-SOFCs.…”

“…at the B-site (Fe-site). [19][20][21][22][23] However, because of their lower MIECs SF-based cathodes normally exhibit a lower ORR activity than Co-based cathodes in the IT range. [24][25] To circumvent these challenges, charge imbalance could be introduced at the A-site through incorporating lowvalent cations such as alkali metals.…”