## 3 Results and Discussion  
### Plasma Diagnostics  
To compare the energy efficiency of NF in the Me-Me and Me-L electrode configurations, it is crucial to ensure that both systems operate at identical conditions. Therefore, plasma diagnostics were first carried out to determine the plasma powers, discharge regimes, corresponding gas temperatures, and to identify the reactive plasma species in both reactor configurations. It is worth noting that the experimental results obtained for different gas flow rates were compared and consistent trends were noted within the studied range of gas flow rates. Therefore, the article presents only the data for the gas flow rates of 4, 8, and 12 L min\({}^{-1}\) for clearer data visualization. The rest of the data can be found in the SI. A typical current-voltage characteristic recorded after the spark gap is shown in Figure 2a, while this figure's inset shows a zoom-in of the time period when a main plasma pulse occurs. Under this condition, the discharge power was calculated to be 20 \(\pm\) 2 W, and the peak current reached 11 A. It is important to note that such current values normally correspond to streamer or spark plasmas [42]. Meanwhile, an increase in the applied DC voltage (and thus an increase in plasma power) and the gas flow rate results in a more frequent formation of voltage pulses from 40 up to 250 Hz, as shown in Figure 2b, independently from the electrode configurations. This leads to a higher number of discharges (current peaks) observed within one period (from 1 at 20 W to 100's at 90 W). The same trend can also be observed from the time-averaged plasma images shown in Figure 2c, where at 20 W, the plasma represents a single filament; while reaching 90 W, the number of filaments significantly increases, concentrating in the middle of the plasma gap. In addition, based on the images shown in Figure 2c, the plasma volume shows a 15 times increase when increasing the plasma power from 20 to 90 W, indicating a nonlinear increase in the plasma-treated gas volume. The plasma volume and number of discharges should strongly impact the overall plasma-triggered chemistry via a higher collision possibility between reactant molecules and electrons. Finally, Figure 2d shows the plasma power as a function of the parameter VA (applied power) averaged over all flow rates under investigation. This result illustrates that both plasma systems with and without water presence operate within the same power range, showing the similarity in electrical properties, thereby allowing their direct comparison. OES was employed to determine the difference in the gas phase compositions as well as the gas temperatures in the Me-Me and Me-L setup configurations. First, the emission spectra in a wide wavelength range from 200 to 900 nm were recorded, and an example of the observed spectra is shown in Figure 3. The spectra for the Me-Me and Me-L electrode arrangements repeat each other well and illustrate emissions from the first positive system of N\({}_{2}\) (N\({}_{2}\)\(\left(\text{B}^{3}\Pi_{\text{u}}\rightarrow\text{A}^{3}\Sigma_{\text{u}}\right)\)), the second positive system of N\({}_{2}\) (N\({}_{2}\)\(\left(\text{C}^{3}\Pi_{\text{u}}\rightarrow\text{B}^{3}\Pi_{\text{g}}\right)\)), and the first negative system of N\({}_{2}^{+}\) (N\({}_{2}^{+}\)\(\left(\text{B}^{2}\Sigma\rightarrow\text{X}^{2}\Sigma\right)\)) [40]. However, a clear difference between the two plasma systems is observed in the wavelength range from 300 to 315 nm, indicating the presence of the OH\(\left(\text{A}^{2}\Sigma^{+}\rightarrow\text{X}^{2}\Pi\right)\) rotational band in the case of the liquid electrode, which can be more clearly seen from the high-resolution spectrum in the inset of Figure 3. These OH radicals originate from the dissociation of water available in the Me-L system. The presence of water in the gas phase is a result of plasma/liquid interactions, leading to water evaporation and decomposition into OH and H, which contributes to the OH radical emission [43; 44].