Rectified Ion Transport with Low-Cost Ionic Diodes Based on Sulfonated Poly-(Phenylene-Oxide)

Ionic rectifiers/diodes are devices with potential applications in water treatment (desalination) and sensors. Diode preparation sometimes involves expensive processes such as laser drilling and thus the need for simpler and low-cost methods. We developed a low-cost cation rectifying hybrid membrane ionic rectifier where a microporous sulfonated poly-(2,6-dimethyl phenylene oxide) (SPPO) was asymmetrically attached to a microhole region in a polypropylene adhesive tape substrate. Contact angle measurements showed that SPPO is hydrophilic with a 62° angle. In comparison, the polypropylene adhesive tape and SPPO within it are hydrophobic with angles of 101° and 87°, respectively, contributing to the diode's stability in water. Zeta potential measurements confirmed the negative charge of the SPPO surface in all electrolyte solutions. Scanning electron microscopy revealed that the punctured tape substrate has a 3–4 µm microhole and a thickness of about 25 µm. The SPPO membrane is ≈29 µm thick with a smooth cross-sectional surface. The effects of electrolyte, ionic strength and microhole diameter on ionic diode performance were investigated using cyclic voltammetry and chronoamperometry in a 4-electrode measurement cell. The reported ionic rectifier is simple to fabricate, and it shows good rectification even in high ionic strength media.