Modification of Ion Exchange Membrane Surface by Plasma Process: I . H+ Ion Perm‐Selective Membrane from Nafion for Redox‐Flow Battery

The proton perm‐selectivity of a perfluorosulfonate cation exchange membrane, Nafion 117, was enhanced by depositing on its surface an ultra‐thin anionic exchange layer containing fixed pyridine rings. The thin layer was deposited from 4‐vinylpyridine monomer vapor using a glow‐discharge (plasma) polymerization technique. The resulting plasma polymer layers were found to be pinhole‐free and of uniform thickness (∼0.2 μm). The influence of the plasma parameters (monomer flow rate, applied power) on the plasma polymer deposition rate was investigated. The IR spectra of the plasma polymers showed the preservation of pyridine rings in the polymers; the lower the applied RF power, the greater the retention of pyridine rings. The proton perm‐selectivity of plasma‐modified Nafion membrane was determined by using it as the separator in a typical two‐compartment cell and measuring , the transference number of the Fe++ ion through the membrane. Pretreatment of the Nafion membrane by oxygen sputtering increased its proton perm‐selectivity and enhanced the adhesion of plasma polymer onto its surface. Plasma‐modified Nafion membranes exhibited very high proton perm‐selectivities, but at the cost of high membrane resistances. For example, membranes having values of 0.00034 and 0.012 had corresponding resistances of 40 and 4.1 Ω‐cm2, respectively.