Effect of the polyethyleneimine-cobalt complex on reduction current of oxygen

The reversible oxygen-binding properties of the polyethyleneimine-cobalt (PEI-Co) complex and the effect of the PEI-Co complex on the reduction current of oxygen were studied. The color of the PEI-Co complex in the electrolyte solution changed on exposure to oxygen atmosphere, which was monitored by UV-visible absorption spectrometry. The UV-visible absorption spectra of the PEI-Co complex displayed a new adsorption band at 310 nm, with an isosbestic point at 280 nm. The spectral change was attributed to the reversible oxygen-adduct formation of the PEI-Co complex even in the electrolyte solution. The oxygen-binding equilibrium curve of the PEI-Co complex obeyed a Langmuir isotherm, to give the high oxygen-binding affinity (p₅₀ = 0.667 kPa). And the apparent dissociation rate constant of oxygen from the PEI-Co complex (k_d = 1.1 × 10⁵ s^-1) was also high. The aqueous solution of the PEI-Co complex that had rapid and reversible oxygen-binding properties functioned as an oxygen-enriching medium for oxygen electrode to enhance the diffusion-controlled current for the oxygen reduction. Based on the rapid release of oxygen from the PEI-Co complex, a high current was obtained for the reduction of oxygen in the presence of the PEI-Co complex, and the current increased with the increase of the concentration of the PEI-Co complex and the oxygen concentration in the atmosphere. The reduction current reached a saturated value near [ethyleneimine unit]/[Co] = 5, which suggested the structure of a six-coordinate μ-dioxo dinuclear complex [N ₅Co^III-O₂-Co^IIIN₅]. A new type of oxygen-diffusion electrode for metal/air batteries and fuel cells is proposed using the oxygen-enriching material immobilized at the electrode surface.