## 3 Results and Discussion
### Energy Cost of the Nitrogen Fixation Process in the Multi-Pin System with and without Water  
The final evaluation of the Me-Me and Me-L systems is performed in terms of energy cost (EC) of the fixed nitrogen molecule, considering all detected nitrogen-containing species in gas and liquid phases. The EC profiles of plasma-assisted NF in the Me-Me and Me-L configurations are presented in Figure 8a for gas flow rates of 4, 8, and 12 L min\({}^{-1}\), while the results for other investigated gas flow rates can be found in the SI, Figure S9. All EC curves presented in Figure 8a have a similar shape, passing through a well-defined maximum. This behavior can be explained by dividing the EC profiles into three distinct  
Figure 7: Evolution of (**a**) nitrite and (**b**) nitrate concentrations generated in the liquid phase as a function of SEI at different gas flow rates in the Me–L system. regions, as shown in the inset of Figure 8a. Here, region I, which is characterized by an almost linear increase in EC with increasing SEI values, represents the SEI range where the increase in the number of plasma filaments promotes the reverse reactions, leading to the formation of the initial products, namely, N\({}_{2}\) and/or O\({}_{2}\), reducing the energy efficiency of the NF process. The same conclusion was made in [52], investigating the N\({}_{2}\)-oxidation kinetics at similar conditions. Region II shows the SEI range where the formation and loss reactions of NF are equilibrated. Lastly, region III, showing a sharp decrease in EC with increasing SEI, can be attributed to the significant, nonlinear increase in the number of plasma discharges, meaning a higher gas conversion. At the same time, this plasma behavior can escalate the overall gas temperature in the gap (see Section 3.1 for more details), providing an additional boost in reaction rates and an additional chemical pathway through which nitrogen fixation can proceed, particularly thermal-driven mechanisms. This all shows that the plasma frequency strongly impacts plasma-assisted NF and that an optimal balance between plasma frequency and gas residence time must be found to reach a minimal EC. An additional point toward this conclusion is the behavior of the EC curves at different gas flow rates. As shown in Figure 8a, the EC of the process within region I increases with the gas flow rate. This correlates with the high discharge frequency at a higher gas flow rate (see Figure 2b) that triggers reverse processes, as discussed above. However, a pronounced reduction in EC values at the end of region III at the same SEI but different gas flow rates (e.g., the EC at 450 (J/L) in the Me-Me system is 28, 29, 23.5 MJ/mol for 4, 8, and 12 L min\({}^{-1}\), respectively) underlines the best balance between gas residence time and plasma frequency. Literally, this means that a higher gas flow rate brings an additional cooling effect, suppressing the reverse processes. It is worth noting that although the EC of the process is considerably low at both low and high SEI values for all studied gas flow rates, the production rate of NF products increases significantly with an increase in SEI, as shown in Figure 8b. For example, in the Me-L system, with a flow rate of 4 L min\({}^{-1}\), the EC is approximately 27 MJ/mol and 23 MJ/mol at an SEI value of 300 and 1400 J/L, respectively, which are very similar EC values. However, when looking at the production rate at these two SEI values, an increase of nearly 13 times could be observed in the production rate (from 1.0 to 13.4 mmol/h, as marked in Figure 8b). This clearly indicates that a high production rate can be achieved in combination with a low EC, which is possible in region III of the EC profile. Finally, when comparing the EC plots of the NF process with (Me-L system) and without (Me-Me system) water presence (Figure 8a), it becomes evident that the water electrode has a negative impact on nitrogen oxide formation, reducing it by 14 \(\pm\) 8% (on average) at different gas flow rates and SEIs.