The synthesis of NH3 from N2 and water was carried out in a quartz reaction cell sealed with a polytetrafluoroethylene (PTFE) lid. The PTFE lid contained feedthroughs for the electrodes that consisted of a stainless steel capillary tube (outer diameter, 1.59 mm; inner diameter, 0.508 mm; and length, 10 cm; Restek Inc.) held 1 mm above the electrolyte solution surface and a Pt foil (purity, 99.9%; thickness, 0.0254 mm; Alfa Aesar) immersed in the electrolyte solution. A plasma was ignited in the gas gap between the tube and solution surface in a flow of either Ar (airgas, 99.5%) or N2 (airgas, 99.99%+), using a negative high-voltage power supply (RR15-10R, Gamma High Voltage Research) that was ballasted (0.25 to 1 megohm). The plasma served as the cathode and the Pt foil as the anode. Two additional feedthroughs were used to bubble the solutions before each experiment and exhaust the gas. The H-shaped split cell was similar except that the plasma cathode and Pt anode were in separate compartments, each with its own PTFE lid and feedthroughs for bubbling and exhaust, connected by a glass frit. The electrolyte solution was a mixture of 18.2 megohm water and sulfuric acid (purity, 99.999%; Sigma-Aldrich), typically at pH 3.5. The solution was bubbled with either Ar or N2 for 30 min before igniting the plasma to ensure the removal of any dissolved gases and to purge the headspace of the vessel.

The voltage and current were measured by a previously reported electrical circuit (19). Briefly, the voltage was measured between the capillary tube and the Pt anode by using a digital multimeter, and the current was obtained from the voltage drop across a resistor in series between the Pt anode and the power supply. The current was measured in real time using LabVIEW. Temperature measurements of the solution surface as a function of position and time during a process run were performed using an FLIR i3 infrared camera with the PTFE lid removed to allow optical access. An emissivity, ε, of 0.6 was assumed, and the accuracy of the temperatures was verified by a thermometer to be ±2°C.

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