Cross-linked Polyethylene Imine - Preparation And Properties Of The Cobalt Complex Membrane

Polyethylenimine (PEI) is a water-soluble polymer with imine groups. Imine groups in PEI can complex with cobalt ions and form PEI-Co complex. PEI-Co complex displays reversible oxygen-binding properties, which could be used to modify the oxygen electrode to increase the concentration of oxygen and increase the reduction current of oxygen electrode. However, PEI-Co complex is water-soluble and could be easily washed away when the water produced in fuel cell drained away, which impedes the application of PEI-Co complex in the modification of oxygen electrode. In this thesis, the water-insoluble PEI-Co complex was prepared, the swelling behavior of PEI-Co complex in water and the reduction behavior of the oxygen electrode modified by PEI-Co complex were studied.Cross-linked PEI membrane was prepared through the reaction between secondary amino groups in PEI with chloromethyl groups in polyepichlorohydrin (PECH). The solvent used is N,N-dimethylformamide, the cross-linking temperature and time are 100℃ and 3h, respectively. The complexation reaction of cross-linked PEI with cobalt ions was studied in terms of time, temperature and concentration of cobalt ions. It was found that the binding amount of cobalt ions by cross-linked PEI is closely related to the concentration of cobalt ions. The binding amount of cobalt ions at equilibrium increases as the concentration of cobalt ions increases. With the increase of temperature, the binding amount at equilibrium decreases. The binding amount of cobalt ions at equilibrium obeys Langmuir model, and the adsorption constants under various temperatures were calculated. It was found that the adsorption constant decreases with the increase of temperature. The enthalpy and entropy were determined to be -13 kJ·mol^-1 and -107 J·mor^-1·K^-1, respectively.The swelling behavior of crosslinked PEI-Co complex membranes in water was studied. The equilibrium swelling ratio of PEI-Co complex membranes is lower and the swelling rate is slower than that of crosslinked PEI membranes without cobalt ion complexation. Both the equilibrium swelling ratio and swelling rate of PEI-Co complex membranes increase with increasing the molar ratio of PEI to PECH. The equilibrium swelling ratio of PEI-Co complex membranes remains almost constant with the change of the membrane thickness or temperature. The diffusion mechanismof water through PEI-Co complex membranes was also discussed, and it is found that the diffusion of water primarily obeys Fickian's law, and the activation energy of diffusion is calculated to be 40.4 kJ/mol.When the oxygen electrode is modified by cross-linked PEI-Co complex membrane, the electrode current increases significantly, while the current of PEI-modified electrode decreases. It was demonstrated that the cross-linked PEI membrane without complexation with cobalt ions can not enrich oxygen, and the cross-linked PEI-Co complex membrane can bind with oxygen reversibly. Therefore, cross-linked PEI-Co complex membrane can be used to modify the oxygen electrode to increase the oxygen concentration and the reduction current.