Activating reversible carbonate reactions in Nasicon solid electrolyte-based Na-air battery via in-situ formed catholyte.

Out of practicality, ambient air rather than oxygen is preferred as a fuel in electrochemical systems, but CO2 and H2O present in air cause severe irreversible reactions, such as the formation of carbonates and hydroxides, which typically degrades performance. Herein, we report on a Na-air battery enabled by a reversible carbonate reaction (Na2CO3·xH2O, x = 0 or 1) in Nasicon solid electrolyte (Na3Zr2Si2PO12) that delivers a much higher discharge potential of 3.4 V than other metal-air batteries resulting in high energy density and achieves > 86 % energy efficiency at 0.1 mA cm-2 over 100 cycles. This cell design takes advantage of moisture in ambient air to form an in-situ catholyte via the deliquescent property of NaOH. As a result, not only reversible electrochemical reaction of Na2CO3·xH2O is activated but also its kinetics is facilitated. Our results demonstrate the reversible use of free ambient air as a fuel, enabled by the reversible electrochemical reaction of carbonates with a solid electrolyte.

Ultrafast and Reversible Superwettability Switching of 3D Graphene Foams via Solvent-Exclusive Plasma Treatments.

For highly active electron transfer and ion diffusion, controlling the surface wettability of electrically and thermally conductive 3D graphene foams (3D GFs) is required. Here, we present ultrasimple and rapid superwettability switching of 3D GFs in a reversible and reproducible manner, mediated by solvent-exclusive microwave arcs. As the 3D GFs are prepared with vapors of nonpolar acetone or polar water exclusively, short microwave radiation (≤10 s) leads to plasma hotspot-mediated production of methyl and hydroxyl radicals, respectively. Upon immediate radical chemisorption, the 3D surfaces become either superhydrophobic (water contact angle = ∼170°) or superhydrophilic (∼0°), and interestingly, the wettability transition can be repeated many times due to the facile exchange between previously chemisorbed and newly introduced radicals via the formation of methanol-like intermediates. When 3D GFs of different surficial polarities are incorporated into electric double-layer capacitors with nonpolar ionic liquids or polar aqueous electrolytes, the polarity matching between graphene surfaces and electrolytes results in ≥548.0 times higher capacitance compared to its mismatching at ≥0.5 A g-1, demonstrating the significance of wettability-controlled 3D GFs.