Aqueous and Nonaqueous Sodium-Air Cells with Nanoporous Gold Cathode

Two types of sodium-air cells, i.e., nonaqueous and mixed aqueous-nonaqueous (abbreviated as 'aqueous') cells, have been compared to elucidate factors limiting performances of nonaqueous air cells and how the aqueous electrolyte is effective to reliving these limitation. The two cells have the same configuration consisting of a nanoporous gold (NPG) air electrode and a ceramic separator of fast sodium ion conductor, NASICON. Only the selection of catholyte, either alkyl carbonate-based nonaqueous solution of NaClO₄ or aqueous solution of NaOH, is different. All performances are demonstrated to be better for the aqueous one. Lower overpotential of aqueous cell leads to better round-trip efficiency. A large resistance relevant to oxygen reduction reaction in the nonaqueous cell is relieved by changing the catholyte to the aqueous electrolyte, affording higher rate capability and power density. The aqueous electrolyte is also effective to remove the limitation of the discharge capacity defined by the volumetric amount of air electrode, as has been claimed in previous studies on aqueous Na-air cells. Furthermore, the aqueous cell is demonstrated to be robust and less sensitive and to atmosphere. The NPG electrode works reversibly in the half-cell reaction of the aqueous cell, while an electrodeposition of metallic Na on the anode during charging requires an improvement.