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Central to commercializing metal-air batteries is the development of highly efficient and stable catalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). In this study, a composite catalyst with a unique interpenetrating network (denoted as NiCo2O4@MnO2-CNTs-3) was synthesized and exhibited better bifunctional activity (ΔE = 0.87 V) and durability than both Pt/C and Ir/C catalysts. The improved performance arises from three factors: (i) MnO2 promotes the ORR while NiCo2O4 facilitates the OER; (ii) carbon nanotubes improve the electronic conductivity; and (iii) the highly porous structure enables the adsorption-desorption of O2 and enhances the structural stability. As a result, the primary and rechargeable Zn-air battery affords a high power density and specific capacity (722 mA h g-1), an outstanding discharge stability (255 mW cm-2 after 1000 cycles) and a high cycling stability (over 2280 cycles). Electron microscopy and electrochemical analysis revealed that the degradation of the rechargeable Zn-air battery performance resulted from the damage of the air electrode and the hydrogen evolution reaction on the zinc electrode. A flexible Zn-air battery employing a solid-state electrolyte showed an exciting stability (540 cycles) and high power density (85.9 mW cm-2), suggesting that the anion exchange membrane effectively prevents the migration of Zn2+ ions and the deposition of carbonates.

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The formation and propagation of cracks in SOECs originates from an anomalous change in the oxygen chemical potential, μ_(O_2 ), at the oxygen electrode – electrolyte interface. This work offers a theoretical analysis that considers both electrochemistry and elasticity to study the effect of pertinent parameters on the electrochemically driven crack formation, crack growth kinetics, and their dependence on μ_(O_2 ) distributions. Our analysis shows that an electronic insulating and ionic conductive interface between oxygen electrode and electrolyte is in favor of suppressing crack formation and propagation in an SOEC. There exists a critical pressure inside the crack for a YSZ electrolyte, which is 5,878 atm when the diameter of a crack is 1 μm. If the cell voltage is lower than 1.44 V, our analysis shows that no crack propagation is expected under the conditions selected. Conversely, operating the cell at a higher voltage results in a higher pressurization rate, leading to the formation of cracks. The optimal operation between constant current density or constant voltage is discussed by studying the dependence of μ_(O_2 ) on materials transport properties and steam concentration. Results from this work can be used to remedy crack problems and improve performance and durability.

Structure evaluation was comparably studied with the traditional thin anode-supported solid oxide fuel cells based on single-sided electrolyte (SSE-SOFCs) and the thick flat-tube solid oxide fuel cells based on double-sided electrolytes (DSE-SOFCs) under redox conditions. Results showed that the structure of DSE-SOFCs remained unchanged, especially the electrolyte kept un-cracked during the redox process, whereas SSE-SOFC was warped and, in some cases, cracked as the thickness of supported anode was less than 1 mm, which indicated that the DSE-SOFCs exhibited better redox stability than the SSE-SOFCs. K E Y W O R D Sanode-supported solid oxide fuel cells, redox, structure evaluation