A Na+ ion-selective desalination system utilizing a NASICON ceramic membrane.

Seawater is a virtually unlimited source of minerals and water. Hence, electrodialysis (ED) is an attractive route for selective seawater desalination due to the selectivity of its ion exchange membrane (IEM) toward the target ion. However, a solution-like IEM, which is permeable to water and ions other than the target ion, results in the leakage of water as well as extraction of unwanted ions. This degrades the productivity and purity of the system. In this study, A novel desalination system was developed by replacing the cation exchange membrane (CEM) with a Na super ionic conductor (NASICON) in ED. NASICON exceptionally permits Na+ ion migration, and this enhanced the productivity of desalted water by removing 98% of Na+ while retaining water and other cationic minerals. Therefore, the final volume of desalted water in N-ED was 1.36 times larger compared to that of ED. In addition, the specific energy consumption for salt (NaCl) extraction was reduced by ∼13%. Furthermore, the NASICON in N-ED was replaced into a two-sided NASICON-structured rechargeable seawater battery, thereby further conserving ∼20% energy by simultaneously coupling selective desalination with energy storage. Our findings have positive implications and further optimizations of the NASICON will enable practical and energy-effective applications for seawater utilization.

Investigation on the Structure and Properties of Na3.1Zr1.55Si2.3P0.7O11 as a Solid Electrolyte and Its Application in a Seawater Battery.

The ionic conductivity, bend strength, and electrochemical performance in a seawater battery (SWB) of an Na3.1Zr1.55Si2.3P0.7O11 (vA-NASICON) solid electrolyte were compared to those of Na3Zr2Si2PO12 (H-NASICON). vA-NASICON exhibited three times higher total ionic conductivity (8.6 × 10-4 S/cm) than H-NASICON (2.9 × 10-4 S/cm). This is due to the higher bulk ionic conductivity and lower grain boundary resistance of vA-NASICON. The higher bulk conductivity of vA-NASICON is a result of its higher Na content, leading to a larger concentration of charge carriers and/or the formation of a higher conductive rhombohedral phase. The lower grain boundary resistance of vA-NASICON is a result of its larger grain size and reduced ZrO2 content. The bend strength of vA-NASICON (95 MPa) was 30% higher than that of the H-NASICON ceramic. The higher bend strength of vA-NASICON was attributed to its reduced ZrO2 secondary phase (1.1 vol %) compared to that of H-NASICON (2.6 vol %). When the vA-NASICON ceramic was tested in the SWB as a solid electrolyte, an 8.27% improved voltage efficiency and 81% higher power output were demonstrated, compared to those of H-NASICON, as a result of its higher total ionic conductivity and mechanical strength. At the same time, the vA-NASICON membrane revealed comparable cycle life (1000 h) to that of H-NASICON. These results suggest that vA-NASICON can be a better alternative than H-NASICON for use in the SWB.

Nanocrevasse-Rich Carbon Fibers for Stable Lithium and Sodium Metal Anodes.

Metallic lithium (Li) and sodium (Na) anodes have received great attention as ideal anodes to meet the needs for high energy density batteries due to their highest theoretical capacities. Although many approaches have successfully improved the performances of Li or Na metal anodes, many of these methods are difficult to scale up and thus cannot be applied in the production of batteries in practice. In this work, we introduce nanocrevasses in a carbon fiber scaffold which can facilitate the penetration of molten alkali metal into a carbon scaffold by enhancing its wettability for Li/Na metal. The resulting alkali metal/carbon composites exhibit stable long-term cycling over hundreds of cycles. The facile synthetic method is enabled for scalable production using recycled metal waste. Thus, the addition of nanocrevasses to carbon fiber as a scaffold for alkali metals can generate environmentally friendly and cost-effective composites for practical electrode applications.

Rechargeable Seawater Batteries-From Concept to Applications.

Harvesting energy from natural resources is of significant interest because of their abundance and sustainability. Seawater is the most abundant natural resource on earth, covering two-thirds of the surface. The rechargeable seawater battery is a new energy storage platform that enables interconversion of electrical energy and chemical energy by tapping into seawater as an infinite medium. Here, an overview of the research and development activities of seawater batteries toward practical applications is presented. Seawater batteries consist of anode and cathode compartments that are separated by a Na-ion conducting membrane, which allows only Na+ ion transport between the two electrodes. The roles and drawbacks of the three key components, as well as the development concept and operation principles of the batteries on the basis of previous reports are covered. Moreover, the prototype manufacturing lines for mass production and automation, and potential applications, particularly in marine environments are introduced. Highlighting the importance of engineering the cell components, as well as optimizing the system level for a particular application and thereby successful market entry, the key issues to be resolved are discussed, so that the seawater battery can emerge as a promising alternative to existing rechargeable batteries.

A Metal-Organic Framework Derived Porous Cobalt Manganese Oxide Bifunctional Electrocatalyst for Hybrid Na-Air/Seawater Batteries.

Spinel-structured transition metal oxides are promising non-precious-metal electrocatalysts for oxygen electrocatalysis in rechargeable metal-air batteries. We applied porous cobalt manganese oxide (CMO) nanocubes as the cathode electrocatalyst in rechargeable seawater batteries, which are a hybrid-type Na-air battery with an open-structured cathode and a seawater catholyte. The porous CMO nanocubes were synthesized by the pyrolysis of a Prussian blue analogue, Mn3[Co(CN)6]2·nH2O, during air-annealing, which generated numerous pores between the final spinel-type CMO nanoparticles. The porous CMO electrocatalyst improved the redox reactions, such as the oxygen evolution/reduction reactions, at the cathode in the seawater batteries. The battery that used CMO displayed a voltage gap of ∼0.53 V, relatively small compared to that of the batteries employing commercial Pt/C (∼0.64 V) and Ir/C (∼0.73 V) nanoparticles and without any catalyst (∼1.05 V) at the initial cycle. This improved performance was due to the large surface area (catalytically active sites) and the high oxidation states of the randomly distributed Co and Mn cations in the CMO. Using a hard carbon anode, the Na-metal-free seawater battery exhibited a good cycle performance with an average discharge voltage of ∼2.7 V and a discharge capacity of ∼190 mAh g-1hard carbon during 100 cycles (energy efficiencies of 74-79%).