Direct ammonia solid oxide fuel cells based on spinel ACo₂O₄ (A=Zn, Fe, Ni) composite cathodes at intermediate temperature

ABSTRACT

Direct ammonia solid oxide fuel cell (DA-SOFC) is a promising device to realize high-efficiency NH₃-to-power. In this study, we fabricate a series of cobalt-based spinel ACo₂O₄ (ACO, A = Zn, Fe, Ni) oxide as DA-SOFC cathodes using the citric acid complexing method. We prepare three cobalt-based spinel oxides with different A-site elements, i.e., Zn, Fe, and Ni, to establish the relationships between the valence and active site of ACO spinel oxide. The structural analysis based on XRD, XPS, TEC, and UV-Vis diffuse reflectance confirms that NCO exhibits smaller band gap, higher electronic conductivity, and better compatibility with electrolyte. The electrochemical results indicate that the cathodic surface of NCO containing rich Co³⁺ and O_ads leads to better oxygen reduction reaction (ORR) activity. The maximum power density of the DA-SOFC using NCO cathode reaches 1.06 W·cm⁻² at 800°C, superior to those of using ZCO (0.83 W·cm⁻²) or FCO (0.92 W·cm⁻²) cathode. Kinetic analysis based on the distribution of relaxation time (DRT) and electrochemical impedance spectroscopy (EIS) indicates ${\mathit{O}}_{\mathit{T}\mathit{P}\mathit{B}}+{\mathit{e}}^{-}\leftrightarrow {\mathit{O}}_{\mathit{T}\mathit{P}\mathit{B}}^{-}$ is rate-limiting.

KEYWORDS:

• Direct ammonia solid oxide fuel cell
• spinel oxide
• oxygen reduction reaction
• distribution of relaxation time
• kinetics analysis

Highlights

• Elaborate the effect of the valence on active site of ACo₂O₄ spinel oxide

• Confirm NiCo₂O₄ higher ORR activity due to smaller band gap and higher conductivity

• Optimal cathode is obtained over NiCo₂O₄ owing to richer Co³⁺ active site

• Kinetic analysis based on DRT+EIS indicates ${\mathit{t}\mathit{O}}_{\mathit{T}\mathit{P}\mathit{B}}+{\mathit{e}}^{-}\leftrightarrow {\mathit{O}}_{\mathit{T}\mathit{P}\mathit{B}}^{-}$ is rate-limiting

Acknowledgments

This work was supported by the National Key R&D Program of China (Grant No. 2020YFB1505604), National Natural Science Foundation of China (Grant No. 21908028, U2005215), Fujian Science and Technology Major Project (2020HZ07009) and Natural Science Foundation of Fujian Province (Grant No. 2019J01257).

Disclosure statement

No potential conflict of interest was reported by the author(s).

Supplementary material

Supplemental data for this article can be accessed on the publisher’s website.