Carbon fibre mat (Toray) consisting of 10 μm fibres compressed into a thin three dimensional mesh was used as the working electrode with the circuit connection accomplished through the use of an adhesive copper tape. A thin layer of cellulose acetate phthalate (Sigma) was solvent cast from a 10 mg/mL acetonitrile solution on to a layer of carbon-polyethylene (Goodfellow). The CAP film was carefully removed and placed on top of the carbon fibre and both sandwiched between polyester laminate sheets that had been pre-patterned to reveal a square window on either side. The basic approach has been described previously [17] but in this instance, rather than have carbon exposed on both sides of the window, the CAP film completely blocks and insulates one side.