## 4 Conclusions  
This work is dedicated to a fundamental study of plasma-assisted nitrogen fixation towards nitrogen oxides formed in the absence and presence of a plasma/liquid interface, evaluating the effect of a multi-pin plasma reactor driven by a pulsed DC voltage. Metal-metal (Me-Me) and metal-liquid (Me-L) electrode arrangements were compared regarding their energy costs to fix nitrogen molecules within a broad specific energy input range. To obtain insights into NF, we first studied the physical properties of the investigated plasma setups via several plasma diagnostic techniques. Through the investigation, the sustainability of plasmas under various air flow rates was observed. It was evident that plasmas were sustained in different air flow rates of 4, 6, 8, 10, and 12 L min\({}^{-1}\). They all operated within an identical power range (from 20 to 90 W), indicating a consistent behavior of the plasmas regardless of the air flow rate applied. The plasmas exhibited a transient spark regime with an electron density of 10\({}^{16}\) cm\({}^{-3}\). Furthermore, it was observed that the plasma frequencies varied within a wide range from 1 to 100's of Hz. In addition, the Me-L system showed a higher gas temperature (increase by 400 K) in the core of the plasma streamer. In both the Me-Me and Me-L configurations, the nitrogen-fixed products were evaluated in the gas and liquid phases and the configurations were compared in terms of energy cost per fixed N\({}_{2}\) molecule. Analysis revealed that the nitrogen fixation process is strongly dependent on the plasma operational conditions, particularly the plasma operation frequency. An increase in this frequency can trigger reverse chemical processes (formation of N\({}_{2}\) and O\({}_{2}\) from NOx) and a significant thermal effect. The obtained results also showed that plasma-assisted NF is more energy-demanding in the presence of water, showing an increase in the energy cost (based on plasma-deposited power) by approximately 14 \(\pm\) 8%. This was associated with energy losses during water evaporation and dissociation. Furthermore, we demonstrated that although the multi-pin plasma system with a liquid electrode suffers from the same drawbacks as a single-pin plasma [29], it allows a drastic increase in the production rate, showing the possible up-scalability and, hence, industrial potential of these plasma systems. **Supplementary Materials:** The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app13137619/s1](https://www.mdpi.com/article/10.3390/app13137619/s1)]([https://www.mdpi.com/article/10.3390/app13137619/s1](https://www.mdpi.com/article/10.3390/app13137619/s1)), Figure S1. Electrical circuit of the pulsed plasma generator. Figure S2. Voltage curve as a function of time for a plasma power of 20 W and a gas flow rate of 4 L-min\({}^{-1}\). T represents the plasma period. Figure S3. Example of MassiveOES simulation of the OES spectrum of the Me-Me electrode system for a gas flow rate of 4 L min-1 and a discharge power of 90 \(\pm\) 2 W. Figure S4. Discharge frequency variation in the chopping gap as a function of power for the different flow rates under study in the Me-Me and Me-L systems. Figure S5. Gas temperature as a function of plasma power for the different flow rates under study in the Me-Me and Me-L systems. Figure S6. Estimated electron density as a function of plasma power for the different flow rates under study in the Me-Me and Me-L systems. Figure S7. (a,c) No and (b,d) NO\({}_{2}\) concentrations as a function of SEI for different gas flow rates for the Me-Me and Me-L systems. Figure S8. Evolution of (a) nitrite and (b) nitrate concentrations generated in the liquid phase as a function of SEI in different gas flow rates for the Me-L system. Figure S9. Energy cost of the pulsed discharge as a function of SEI for different gas flow rates in both electrode configurations. Table S1. Possible chemical reactions occurring in the gas and liquid phases.