Evidence for the electrolysis of water by atmospheric-pressure plasmas formed at the surface of aqueous solutions

The formation of atmospheric-pressure plasmas with liquid electrodes holds great importance for a number of emerging technologies and applications, yet fundamental questions remain about the nature of the interactions at the plasma/liquid interface. In particular, when the liquid serves as the anode, the plasma supplies gas-phase electrons to the liquid surface, and how these electrons interact with the liquid has not been fully explained. In this work, we present experimental evidence that in the case of water, plasma electrons are involved in electrolytic reactions leading to the conversion of protons (H⁺) to hydrogen gas. Reactions associated with water electrolysis are indirectly characterized by pH measurements that show qualitatively and quantitatively that the liquid at the plasma interface increases in basicity, consistent with the reduction of protons by plasma electrons. Mass spectrometry measurements confirm the evolution of hydrogen gas, directly providing evidence of water electrolysis. This work highlights the critical role that plasma electrons can play in plasma/liquid interactions with broad implications for any plasma system involving an aqueous electrode, including emerging applications in plasma medicine and plasma-liquid materials synthesis.