Quantification of plasma produced OH and electron fluxes at the liquid anode and their role in plasma driven solution electrochemistry

Plasma-liquid interactions enable various applications through the generation of a large range of reactive species in solution. In this work, we report on the interaction of a pulsed atmospheric pressure glow-like discharge with a liquid anode. Particularly, the flux of hydroxyl (OH) radicals and electrons in the plasma at the liquid anode are measured by laser induced fluorescence (LIF) spectroscopy and current measurements to investigate the role of OH and electrons in plasma-enabled redox chemistry in solution. The impact of the voltage pulse width, voltage amplitude, liquid temperature and conductivity on the OH density distribution was also investigated. We observed a significant OH density near the liquid surface, which showed a transition from a ring-shaped structure to a more uniform structure with increasing plasma power. This transition coincided with a similar transition in the plasma emission intensity and electron density profile. A Raman laser scattering study indicated that this transition can be attributed to an enhanced N₂ mixing at larger plasma-dissipated powers. Besides, a time resolved measurement showed that the OH density segregates radially in the afterglow at velocities exceeding the gas velocity at room temperature due to enhanced gas convection resulting from the plasma-induced gas heating. While the OH flux was of the order of ∼10²¹ m⁻² s⁻¹, approximately two orders of magnitude lower than the electron flux, significant reduction in the solution occurs during the voltage pulse. Nonetheless, a slow oxidation was observed in the afterglow due to the much longer lifetime of OH radicals compared to electrons. The Faradaic efficiency of the liquid redox chemistry was evaluated with H cell measurements and showed a good agreement with a 1D liquid phase model with the measured electron and OH fluxes as the input. This result shows the capability to quantitatively describe the plasma-driven solution electrochemistry for a model redox couple based on OH and electron driven reactions.