Insert overmoulding of the dielectric-coated electrodes of a DBD plasma ozone generator: Proof-of-concept for automotive applications

The development of new industrially-relevant technologies based on the use of ozone (O3) requires the design and implementation of on-site O3 generators adapted to the targeted applications in terms of O3 production method, architecture, dimensioning and construction materials. The most effective method for O3 generation relies on the use of atmospheric pressure cold plasmas and, in particular, of dielectric barrier discharges (DBDs). This paper reports on the fabrication and testing of the dielectric-coated electrodes of a DBD-type plasma O3 generator for automotive applications and, specifically, for the control of fuel ignition timing and combustion kinetics in internal combustion engines. A novel overmoulding process based on injection moulding was developed to coat stainless steel tubular electrodes with a 1 mm thick layer of a dielectric material consisting of either pure polyvinylidene fluoride (PVDF) or PVDF compounded with alumina (Al2O3). The use of alumina as inert inorganic filler for incorporation into the fluoropolymer was intended to enhance the chemical durability of the dielectric layer upon long-term plasma exposure during O3 production. The optimization of the injection overmoulding process was carried out using a one-factor-at-a-time (OFAT) approach, which offered a straightforward and effective strategy for identifying key processing parameters such as mould and barrel temperatures, fill pressure and speed, pack and hold conditions as well as screw recovery. Experiments were carried out to compare the performance of the coated electrodes when integrated into a DBD cell for O3 production fed with dry and humid air streams. Both types of electrodes allowed obtaining comparable and stable values of O3 output and production efficiency for 12 h of DBD cell operation (∼0.016 g·min−1 and ∼58 g·kWh−1 for dry air). However, the hybrid PVDF/Al2O3 composite demonstrated to maintain steady O3 production performance over 24 h, while exhibiting enhanced chemical resistance upon exposure to the reactive plasma environment, as confirmed by material characterization using attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) and white-light vertical scanning interferometry (WLVSI). Overall, this proof-of-concept study demonstrates the potential of injection overmoulding technology for the cost-effective scalable fabrication of durable composite-coated electrodes of an onboard plasma O3 generator for automotive applications.