An aqueous solution of a [poly(ethyleneiminato)]cobalt(II) complex that reversibly binds and releases O2 functions as an O2-enriching medium for an O2 electrode to enhance the current for the uncatalyzed reduction of O2. The effect of the electrochemically inactive cobalt complex is to supply reducible O2 to the solution within the diffusion layer near the electrode by releasing the bound O2 from the O2 adduct, which contributes to a decrease in the diffusion layer thickness of O2. Based on the rapid release of O2 from the O2 adduct, a diffusion-limited current is obtained for the reduction of O2. The hypothetical concentration of the enriched O2 supplied under pure diffusion control (i.e. unperturbed by the kinetics of the O2 binding) is several times larger than that of the actual concentration of O2 due to the higher solubility of the O2 adduct than the physical solubility of O2 in H2O at room temperature under an atmosphere of air. An insoluble membrane of a cobalt(II) complex with a cross-linked poly(ethyleneimine) ligand that has the ability to swell serves as an O2-enriching material to concentrate O2 from aqueous electrolyte solutions. A new type of O2-diffusion electrodes for metal/air batteries and fuel cells is proposed using the O2-enriching material immobilized at the electrode surface.
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