Chemistry in nanosecond plasmas in water

Discharges in liquids are the basis of a range of applications in electrochemistry, wastewater treatment, or plasma medicine. One advantage of discharges in water is their ability to produce radicals and molecules directly inside liquid with a high conversion efficiency. In this study, H2O2 production in a 10 ns pulsed discharge in water is investigated. The dynamic of these discharges is based on plasma ignition directly inside liquid followed by the formation of a bubble that expands in time before it eventually collapses. This sequence can be well described by cavitation theory. H2O2 is produced using different plasma conditions varying the treatment time, the pulse frequency between 1 and 100 Hz, and the applied voltage in a range from 15-30 kV. The resulting H2O2 concentration is measured using absorption spectroscopy ex situ based on a colorimetry method. The results indicate that the main parameter controlling the H2O2 production constitutes the applied voltage. The measured concentrations are compared with a global chemistry model simulating the chemistry involved during a single pulse using pressures and temperatures from the cavitation model. In addition, a global chemical equilibrium model for H2O2 creation is evaluated as well. The models show a good agreement with the data. The energy efficiency for the production of H2O2 reaches values up to 2 g/kWh.