Nitrogen fixation in a non-equilibrium spatially distributed electric field

Domain: chemistry
Published: 2026-03-09
Editorial outcome: minor_revision

Abstract
Nitrogen fixation heavily relies on the energy-intensive Haber-Bosch process, necessitating renewable alternatives. Here, we introduce a non-equilibrium spatially distributed electric field (SD-EF) strategy for nitrogen fixation in ambient air plasma. The optimized SD-EF strategy gives a NO x − yield of 9.8 mmol/h, tripling that of a uniform electric field and the N conversion is three times higher than most discharge configurations at similar or lower energy consumptions. This high NO − yield is achieved through simultaneously activating two beneficial kinetic networks via SD-EF by having both high and low electric fields present: O and vibrational excitation of N (N (v)) sub-mechanisms, which are revealed by developing a photonic crystal fiber diagnostic for in-situ quantification of molecules and ions (NO, NO , N O, O , NO −, and NO −) in gas-liquid plasma. The establishment of the SD-EF strategy, coupled with in-situ gas-liquid diagnostics, is broadly applicable to plasma-assisted nitrogen fixation and holds promise for other plasma-assisted chemical conversion processes.

Introduction
Nitrogen fixation is essential for fertilizer production, nitrogenous chemical synthesis, and sustainable fuel production for transportation. The conventional nitrogen fixation route involves the Haber-Bosch (H-B) process by synthesizing ammonia (NH ) from nitrogen (N ) and hydrogen (H ) under high temperatures (>350 °C) and high pressures (>200 atm), N + 3H ⇌ 2NH (R )1, and NH is then used for the synthesis of other nitrogenous chemicals2. The H-B process revolutionized nitrogen fixation and is considered among the most important discoveries of the 20th century. However, this process consumes around 1–2% of the total global energy production because of the required high temperatures and pressures and large quantities of H , which is typically obtained from steam reforming of natural gas that contributes up to 1.2% of global carbon dioxide emissions3. To minimize the environmental impact of carbon emissions, increasing attention has been devoted to biological, photocatalytic, and electrified nitrogen fixation4-6. Among these process, biological nitrogen fixation converts atmospheric N into NH + through symbiotic and free-living nitrogen fixation microorganisms7-9. Photocatalytic nitrogen fixation uses solar energy and water to drive NH synthesis on catalytic surfaces such as transition-metal-based10, plasmonic metal11,12, and carbon-based materials13. Electrified nitrogen fixation utilizes renewable electricity to achieve nitrogen fixation under ambient conditions via an environmentally friendly way, such as through electrocatalytic or non-thermal plasma (NTP) strategies2,14-16. It should be noted that the electrified nitrogen fixation is considered to hold substantial promise for practical deployment because of its compatibility with rapidly expanding distributed renewable power grids4. However, conventional electric nitrogen fixation, such as electrocatalysis or direct N reduction from H O and H by NTP suffers from a high energy consumption and a low nitrogen fixation yield due to a high activation energy of N 17-23.

As an alternative, direct N oxidation by O through the reaction N + xO → 2NO (R ) is another industrial nitrogen fixation method via the Birkeland-Eyde process. But the thermodynamic equilibrium composition for direct N oxidation severely limits NO yield at moderate temperatures. Even if NO products form in a non-equilibrium plasma, decomposition back into much more stable N and O will be favored once the gas equilibrates at low temperature. To address this challenge, a plasma bubble reactor integrating plasma generation and water absorption is designed. The schematic of plasma bubble reactor is shown in Supplementary Fig. 1. In this configuration, gas bubbles are formed within a liquid phase, and plasma discharge occurs either inside the bubbles or at the gas–liquid interface, producing abundant radicals and reactive species that drive chemical reactions under mild conditions. This approach has been explored for nitrogen fixation and ammonia synthesis24-28. However, the nitrogen fixation yield and the energy consumption are still far from the H-B benchmark29. A major challenge hindering progress is the lack of kinetic understanding of the complex plasma-assisted catalysis process at the gas-liquid interface.

For plasma-assisted nitrogen fixation in R , the rate-controlling step(s) for nitrogen fixation is poorly understood because of the complex coupling reaction networks of the N and O excitations in air plasma30. It is known that NTP generates a large number of free radicals and vibrationally excited molecules, which are considered capable of promoting NO synthesis31. But there is a lack of direct research on the effects of NTP types on nitrogen fixation and a need for accurate in situ diagnostics of the gas-liquid interface under high electromagnetic fields in plasma. Fiber optics offers a promising solution for operando quantifying key characteristics, such as temperature, pressure, and species in gas and liquid in plasma, due to their compact size (diameter ~125 μm), high temporal sensitivity, time–resolved response, and immunity to electromagnetic interference32-35.

In this work, we employ a plasma bubble reactor to fix nitrogen from air by using spark and dielectric barrier discharge (DBD). Novel in situ fiber diagnostics with hollow-core photonic crystal fibers (HC-PCF) based photothermal spectroscopy (PTS) are used for the first time to simultaneously quantify key molecules (NO, NO , and O ) and ions (N O − and NO −) in plasma in both the gas phase and aqueous solution. HC-PCF achieves a temporal resolution of 1 Hz and a detection limit of 10 ppm for the species above, and exhibits an excellent resistance to high electromagnetic fields. Furthermore, we identify two different beneficial reaction networks for nitrogen fixation in different electric field strengths, that is, the N (v) sub-mechanism in a low electric field and the O sub-mechanism in a high electric field. Guided by the kinetic results, we establish a non-equilibrium spatially distributed electric field (SD-EF) strategy for nitrogen fixation in air plasma. SD-EF enables the coexistence of a lower electric field for the vibrational excitation of N and a higher electric field for the dissociative excitation of O by employing a dielectric barrier with varied thickness. This new strategy provides an energy efficiency of 1.6 kWh/mol and a NO − yield of 9.8 mmol/h, which is three times higher NO − yield than the uniform electric field at a similar energy consumption. We believe the SD-EF strategy to simultaneously activate multiple beneficial kinetic networks, coupled with in situ gas-liquid diagnostics, will be broadly useful for plasma-assisted nitrogen fixation, as well as other plasma catalysis processes.

Results
Optic fiber diagnostics in plasma
A plasma reactor (Fig. 1a) is used for the nitrogen fixation experiments, featuring a central high voltage electrode and a quartz cylinder that has 12 micro holes with a diameter of 0.4 mm near the sealed base. The entire reactor is immersed in a water-filled cylindrical vessel, which simultaneously serves as an absorber vessel and ground electrode. Air is introduced into the reactor through a three-way plastic joint, where discharge occurs in the region between the high-voltage electrode and the water-immersed section of the reactor wall (purple area in Fig. 1a). The micro holes allow reactive species generated by the plasma to flow into the water, forming bubbles and enabling the dissolution of NO species into the liquid. The detailed description is given in the Methods. Moreover, two types of discharge configurations, DBD and spark modes, are used for nitrogen fixation. The discharge images are included in Supplementary Fig. 2a and b, respectively. The typical voltage and current waveforms of the DBD and spark modes are shown in Supplementary Fig. 3. Spark is characterized by a low electric field, high gas temperature, and high electron density, whereas DBD is characterized by a high electric field, low gas temperature, and low electron density.

We use photothermal spectroscopy (PTS) through hollow-core photonic crystal fibers (HC-PCF) to quantify in situ stable species in plasma. After drilling holes in the HC-PCF, the processed HC-PCF is not only suitable for the gas phase but also for the liquid phase. This has been verified by our optic mode analysis (Supplementary Fig. 4). Our results show that the light can transmit well in our HC-PCF. Fiber optic probes are placed at different locations within the reactor, each labeled to indicate its measurement position. Position I refers to the bottom of the quartz tube of the reactor, where the probe measures the NO, NO , N O, and O concentrations in the gas phase before absorption. Position II is located within the absorption solution, enabling measurement of dissolved NO − and NO − species. Position III is situated at the reactor outlet above the liquid surface, where the NO, NO , N O, and O concentrations are monitored after gas-liquid interaction. The specific arrangements of Position I, II, and III are shown in Fig. 1a. The measurement principle of the HC-PCF based PTS is shown in Fig. 1b. The molecules diffuse into the interior of the PCF through the micro holes made by focused ion beam on the PCF, and then absorb photons at specific wavelengths from a beam of pump light within the PCF. Due to the photothermal effect of different excited molecules, a periodic heat release appears in the central core of the fiber and changes the local temperature and the refractive index of the filling medium. Meanwhile, a beam of probe light is introduced into the PCF along with the pump light. Then the probe light reflects back at the other end of the fiber. The optical phase change between the reflected and incoming probe lights linearly corresponds to the change in the refractive index or local temperature. Hence, the species concentrations of interest can be in situ quantified by measuring the optical phase change of probe light according to their different photothermal effects. The detailed measurement method is described in the Methods.

The calibrations of gaseous species and ions, such as NO, NO , N O, O , NO −, and NO − are detailed in the Methods. Here, we show the calibration curves of NO and NO − in Fig. 1c and d as an example to indicate the excellent linear response of the HC-PCF based PTS to the NO concentration. Subsequently, the transient evolutions of NO, NO , and O concentrations before absorption in the solution (Position I) are measured using the HC-PCF based PTS with a temporal resolution of 1 Hz (Fig. 1e). After the power is on, the gas concentrations of these species before absorption continuously increase after a delay of around 600 s before reaching equilibrium due to the gas diffusion into PCF and the gas temperature fluctuation in plasma. Therefore, the steady-state species quantification by using the HC-PCF based PTS is employed after 600 s in each experiment.

Kinetic mechanisms of N (v) and O in plasma-assisted nitrogen fixation
The NO, NO , N O, and O concentrations in Position I are separately measured in DBD and spark by using the HC-PCF based PTS (Fig. 2a). The spark mode produces a higher NO concentration than that in the DBD mode. However, the spark mode forms negligible O , which is around 1800 ppm in the DBD mode. Both modes produce little N O, indicating that N O in air plasma is not a major nitrogen-containing product. The NO − concentration in Position II is measured by using the HC-PCF based PTS after 600 s (Fig. 2b). The NO − concentration of DBD is 2.0 mmol/L, which is similar to the situation in spark (2.2 mmol/L). But the selectivity of NO − to NO − is 97.5 % using DBD, while it is 39.9 % using spark. Furthermore, the NO, NO , N O, and O concentrations in Position III are separately quantitatively detected in DBD and spark by using the HC-PCF based PTS (Fig. 2c). After absorption, the NO concentrations in DBD and spark are 589 ppm and 563 ppm, respectively. As for the absorption performance of NO, the NO concentration clearly decreases to 1083 ppm in DBD, but the absorption of NO is limited in spark. To evaluate the performance of the NO x absorption, the NO x absorption rate is defined as the difference between 1 and the ratio of the NO concentration after and before absorption. The NO absorption ratio is similar between the different discharge modes, while the NO absorption ratio in DBD with O is higher than that in spark without O (Fig. 2d). In general, considering the requirement of O radical for the synthesis of NO and O , we postulate that sufficient O radical concentration is crucial for enhancing N fixation performance through oxidizing NO.

To understand this discrepancy between the two discharge modes, the electron energy transfer to vibrational (v), ionizing (ion), dissociative (dis), and electronic (ele) excitations of N and O is calculated under varied reduced electric fields (E/N) in Fig. 2e, which is consistent with previous work36. The energy loss fractions are calculated by BOLSIG+37 and computational details are in the Methods. This analysis reveals that electron energy predominantly transfers to the vibrational excitation of N under a low E/N, which is in the range of spark mode. In the DBD mode, however, the energy of transferring to the N (v) channel diminishes, while the N (dis) and O (dis) channels are dominant to form N, O, and O , which agrees with our experimental observation of higher O but lower NO − concentrations in the DBD mode in Fig. 2b. Therefore, the kinetic discrepancy in nitrogen fixation under different configurations is attributed to the shift in the electron energy deposition.

To explore the reaction pathway for N fixation, we perform zero-dimensional (0D) numerical simulations of plasma discharge and gas absorption in solutions in DBD and spark discharge, respectively. The model neglects gas flow and bubble dynamics, focusing instead on the temporal evolution of plasma chemistry. The physical simulation scheme, illustrated in Supplementary Fig. 5, simplifies the process into two stages: a zero-dimensional plasma discharge stage and a zero-dimensional absorption stage in solution. The reaction times for both stages are determined by the gas residence time in the plasma region and the bubble residence time in the solution, both estimated as 0.1 s in this study. The two stages are coupled through Henry’s law to account for gas-liquid mass transfer. This calculation is conducted using ZDPlasKin38 with a validated N /O plasma chemistry set, tracking the temporal evolution of gas temperature and species densities, including vibrationally and electronically excited states, radicals, molecules, ions, and electrons. The nitrogen fixation kinetics explicitly incorporates the vibrational dynamics, accounting for impact excitation, vibrational-translational relaxation, and vibrational-vibrational (V-V/V-V’) exchanges based on the Schwartz-Slawsky-Herzfeld (SSH) theory39. Chemical abstraction reactions are described by the Fridman-Macheret α-model40. The discharge power density is used to self-consistently determine the reduced electric field within the plasma. The details of the simulations are in the Methods, and the simulation results are included in Supplementary Figs. 6-10.

The simulations show that DBD produces a significant amount of O , which assists NO absorption in water by oxidizing NO, whereas spark produces more NO through the N (v) channel with less NO absorption in water. Both of these two plasma configurations are beneficial to nitrogen fixation in water, which validates our main experimental results above. In addition, we explore the N O generation pathway (Supplementary Fig. 10), N O is mainly formed through reaction N (A3Σ +) + O → N O + O (R ). But the electronic excitation of N requires a high electric field strength (> 130 Td), so low electric field is not suitable for the generation of N O in spark discharge. Meanwhile, the abundance of O radical promotes the conversion of N O into NO through reaction N O + O → 2NO (R ), leading to a low N O concentration in the DBD mode. Overall, N O plays a minor role in plasma-assisted nitrogen fixation.

Based on the results of the reaction pathways in Supplementary Figs. 7-10, we summary the reaction pathway of plasma-assisted nitrogen fixation to NO − with two major sub-mechanisms: the O sub-mechanism in DBD and the N (v) sub-mechanism in spark, as shown in Fig. 2f. For the O sub-mechanism at a high reduced electric field, O is mainly produced from the dissociative excitation of O in DBD. It is noted that the water layer primarily enables the transformation of NO and NO into NO − and NO − rather than merely preventing their reduction. The high solubility of O in the aqueous phase plays a crucial role in driving this overall conversion process, which contributes to the nitrogen fixation both through NO + O → NO + O (R ) in the gas phase and NO − + O → NO − + O (R ) in the water solution. The present pathway explains the higher O but lower NO concentrations in Fig. 2a and the much lower NO − concentration in Fig. 2b in the DBD plasma. For the N (v) mechanism at a low reduced electric field, the vibrational excitation of N is dominant for the N excitation in spark and contributes to the nitrogen fixation through N (v) + O → NO + N (R ). The reaction rate increases significantly when the vibrational level of N (v) is over 12 (Supplementary Fig. 11). However, without the assistance of O , less NO is converted to NO and NO −. The present pathway explains the higher NO concentration in Fig. 2a and the lower NO absorption ratio in Fig. 2d in the spark plasma. As a result, both the N (v) sub-mechanism at low E/N and the O sub-mechanism at high E/N are important for nitrogen fixation in the solution. The former provides reactive vibrationally excited N molecules for overcoming the large energy barrier of N≡N and producing NO, while the latter provides active oxidizers, O and O , to convert NO to NO and NO − to NO − for nitrogen fixation in the solution.

Spatially distributed electric field strategy
Based on the reaction pathway analysis above, nitrogen fixation can be accelerated from the N (v) and the O sub-mechanisms at low and high reduced electric fields, respectively. Therefore, we propose a novel spatially distributed electric field (SD-EF) DBD strategy, where a non-equilibrium electric field is formed for the nitrogen fixation in DBD (Fig. 3a).

Here, we design a dielectric with non-uniform thickness to achieve SD-EF (Supplementary Fig. 12). This coaxial dielectric is made of polytetrafluoroethylene (PTFE) with periodic diameters of 6 and 8 mm (5 mm length for each). The formed plasma in the narrower gaps diffuses to the thicker gaps and reduces their breakdown voltages, achieving a spatially distributed stable discharge in the entire reactor in the plasma image of Fig. 3a. The thickness of the dielectric is specifically designed to achieve a low reduced electric field around 100 Td for the N (v) mechanism and a high reduced electric field around 150 Td for the O mechanism in DBD in the present experiments. We further simulate the distribution of the electric field strength within the reactor for SD-EF in Supplementary Fig. 13. It exhibits a higher reduced electric field at the narrower gap and a lower reduced electric field at the thicker gap. The detailed description is given in the Methods.

We directly apply the SD-EF strategy to nitrogen fixation at the same flow and discharge conditions of the uniform DBD and spark experiments. Its main product concentrations are compared with those in DBD and spark before absorption (Fig. 3b) and in the water solution (Fig. 3c). Before the optimization of work conditions, it is observed in Fig. 3b that SD-EF enhances the NO yield by 34% and 49 % than spark and uniform DBD, respectively. But there is no significant difference in NO, NO , and N O selectivities between SD-EF, DBD, and spark discharge (Supplementary Fig. 14). Meanwhile, SD-EF keeps a similar level of O concentration around 1000 ppm with uniform DBD for NO oxidation and absorption in water, unlike the absence of O in the spark discharge. As shown in Fig. 3c, the SD-EF strategy produces 0.10 mmol/L NO − and 2.81 mmol/L NO −, which is an increase of 38% over uniform DBD and spark in nitrogen fixation in water, confirming the advantage of SD-EF over uniform DBD and spark. To assess the long-term stability of the SD-EF system, we conduct a 3-hour SD-EF experiment, during which the NO concentration at Position I is monitored (Supplementary Fig. 15). The NO concentration stabilizes at approximately 3000 ppm, indicating that the SD-EF system maintains stable performance over short to moderate timescales.

To validate the discussion of kinetics on uniform DBD and non-uniform DBD, such as SD-EF, we simulate nitrogen fixation under different reduced electric fields in DBD at low uniform-EF (~100 Td), at high uniform-EF (~150 Td), and SD-EF in Fig. 3d-f. The low uniform-EF and the high uniform-EF refer to the reduced electric field at a thicker gap and a narrower gap of our SD-EF configuration at 15 kV. Due to the presence of both the N (v) and O mechanisms, SD-EF exhibits higher productions of NO around 1238 ppm and NO around 655 ppm at 0.1 s than that in uniform DBDs at 100 and 150 Td. Based on the preliminary validation of SD-EF against experiments and simulations above, we further optimize the discharge and flow conditions of SD-EF, respectively, for the NO – yield in water and energy consumption (EC) in Fig. 3g-h. A minimum EC around 1.6 kWh mol-1 NO – and a maximum NO – yield around 9.8 mmol/h are achieved at 18 kV and 2 L/min flow rate in SD-EF, which is approximately three times the NO − yield in a traditional DBD with a uniform electric field. In addition, it is seen from Fig. 3g-h that a higher voltage generally benefits NO – yield in SD-EF, while it needs to be restricted with both low and high reduced electric field modes. The NO – yield against time generally increases with the flow rate of reactants, while it decreases at a high flow rate level as the nitrogen fixation reactions are incomplete at a low residence time. We also investigate the effect of the N /O ratio; the results indicate that it has little influence on nitrogen fixation (Supplementary Fig. 16). Moreover, using air directly (N /O = 4:1) eliminates the need for gas separation, making it more practical for scalable applications.

Finally, we compare the performance of our proposed nitrogen fixation strategy with state-of-the-art results in the literature, including electrocatalytic nitrogen reduction reaction (eNRR) and various discharge configurations, such as DBD, spark, gliding arc, etc., as summarized in Supplementary Table 1. Since electrocatalysis typically produces NH rather than NO −, the N conversion is used as a unified metric to quantify the performance of N fixation. After optimization, our proposed SD-EF strategy exhibits three times higher N conversion than most discharge configurations at similar or lower energy consumptions. Although the spark configuration achieves a higher N conversion than SD-EF, its energy consumption is approximately one order of magnitude higher. Moreover, SD-EF demonstrates a high NO − selectivity of approximately 96.5% among NO − in water solution (Fig. 3b). In contrast to previous studies conducted in alkaline solutions 15, 27, 41, where NO − is the main product. In addition to the plasma system, eNRR is also included in the comparison. Although the energy consumption of our proposed strategy is about 10 times higher than that of electrocatalysis, the N conversion is approximately 100 times higher. This indicates that our approach is more suitable for small-scale distributed nitrogen fixation.

Based on the results above, SD-EF exhibits a higher NO – yield, a higher NO – selectivity, and a lower energy consumption than existing plasma configurations. The SD-EF strategy is fundamentally based on the modulation of kinetic networks and thus represents a general concept, independent of reactor type or power source. It can be adapted to other plasma-assisted nitrogen fixation systems, such as atmospheric pressure glow discharge plasma jet. Moreover, the strategy of the spatially non-equilibrium electricity field under kinetic networks provides an opportunity for plasma-assisted catalysis not only for nitrogen fixation, but also for other processes, such as CO conversion to methanol, H production from methane pyrolysis, and plastic recycling and upgrading. In addition, the time-resolved in situ optics by using HC-PCF also provides an important diagnostic approach of molecules and ions in gas and liquid phases under high electromagnetic fields for understanding their kinetic mechanisms.

Methods
Plasma discharge system and gas distributions system
Experiments of plasma-assisted nitrogen fixation is conducted using a plasma reactor, as depicted in Fig. 1a. The plasma reactor is fabricated using a quartz tube (10.5 mm I.D., 12.7 mm O.D.) with one end sealed and a stainless-steel rod (4.0 mm dia.) at the reactor centre line as a high voltage electrode. 12 micro holes with a diameter of 400 μm are positioned 1 mm above the sealed base, allowing for ventilation into water and facilitating gas-liquid interface for plasma generation. This reactor is submerged within a long gas cylinder (48.0 mm I.D., 52.0 mm O.D.) containing the deionized water. The water is regarded as an absorber and a ground electrode. The plasma is generated by an AC power supply (CTP 2000K), which generally set a discharge frequency of 10 kHz and a voltage of 10 kV in our experiments. High purity N and O are supplied from a pressurized gas cylinder, and the gas flow rates are controlled by mass flow controllers (Seven star CS200A), which are set to the flow rates of 2000 ml/min.

Three discharge modes, involving DBD, spark, and spatial-distributed electric field (SD-EF), are employed to investigate the effect of the electric field intensity. In the DBD mode, the high voltage electrode is enclosed within a quartz cylinder (6.0 mm O.D.) as a dielectric barrier. In the spark discharge mode, the stainless-steel rod is replaced by a stainless-steel needle. In the SD-EF configuration, the high voltage electrode is enclosed by a coaxial dielectric cylinder made of polytetrafluoroethylene (PTFE) featuring periodic diameters of 6 and 8 mm (each with a length of 5 m). The geometric parameters of the SD-EF configuration have been optimized to balance electric field modulation and gas residence time. The periodic alternation between the high and low electric fields influences the residence time of the gas within each region. It is noted that the characteristic lifetimes of intermediates such as N (v), O, and N are on the microsecond scale41, far shorter than the millisecond-level gas residence time in the SD-EF configuration. However, the characteristic lifetime of O is on a minute scale, which can transit through these electric field regions. The SD-EF strategy enables O generation in the regions of high electric field and its subsequent utilization in the regions of low electric field to oxide NO to NO in the regions of low electric field. Moreover, the spatial electric field distribution is highly sensitive to the thickness and dielectric constant of dielectric barrier. Variations in dielectric thickness or permittivity primarily affect the local electric field gradient and the spatial distribution of electric field strength, thereby influencing nitrogen fixation performance in SD-EF strategy. SD-EF simulations for different high-to-low electric field strength ratios (Supplementary Fig. 17) indicate that the performance of nitrogen fixation is optimized at a ratio of 1.5. Accordingly, we optimize the SD-EF strategy by selecting appropriate dielectric materials and adjusting the discharge gap to achieve a high-to-low electric field strength ratio of approximately 1.5 in this work.

Electrical measurements of plasma discharge
The signals for the applied voltage and the applied current measured by a voltage probe (RIGOL RP1018H) and a current probe (Cybertek cp0030A), respectively, are recorded by an oscilloscope (SDS814X HD). The time-averaged discharge power is calculated from the measured discharge voltage and current with equation (1)

(1)

where P is the discharge power; f is the discharge Frequency; T is the discharge period time; u is the voltage; and i is the current. To ensure the accuracy of the power, all measurements are performed by taking the average over 20 cycles.

Photothermal spectroscopy diagnostic system
The major species NO, NO , and O in gas phase, as well as NO – and NO – in aqueous phase are measured by hollow core photonic crystal fiber (HC-PCF) based photothermal spectroscopy (PTS). Three PCFs serve as sensing units and are placed in the plasma reactor, in the water, and above the surface of water. The PCFs are processed with focused ion beam (FIB) to create surface micro-channels with the diameter of 5 μm for molecule diffusion. The PCF in the plasma reactor is through the gas fitting that connects gas tube with quartz tube into the plasma reactor. For the PTS sensing system (Fig. 1a), an overview diagram of photothermal spectroscopy in a PCF is shown in Supplementary Fig. 18. The modulated pump light with the modulated frequency of 20 kHz emitted by the distributed feedback laser (Nanoplus DFB) is amplified by an optical amplifier and then directed into the PCF. After the pump light reaches the PCF, the molecules of the target substance absorb the pump light to induce a modulated photothermal effect. Additionally, another probe light is emitted by a RIO Orion™ laser and is split into two beams, which propagate through the sensing arm and the reference arm, respectively. The probe light transmitted in the sensing arm will be reflected by the fiber Bragg gratings (FBGs) distributed at the end of PCFs, whose central wavelengths are same with the wavelength of probe light. The probe lights transmitted in the sensing arm and reference arm are heterodyne interfered at the optical fiber coupler. The signal of the interference light is converted to electric signal using a photodetector (THORLABS PDB435C). Moreover, the optical phase information contains in electric signal is demodulated by a self-made demodulate circuit, and then gets recorded by a date acquisition card (PSicoScope 6000E).

Calibration of the key species
Concentrations of stable species are calibrated directly by a mixture with a known mole fraction of the molecule, such as NO, NO , NO –, NO –, and N O. For different molecules, we use different pump light wavelengths. NO, NO , NO –, NO –, and N O are measured at wavelengths of 1789.8, 1501.4, 1948.4, 1670.2, and 1521.17 nm, respectively. The calibrated curves can be found in Supplementary Fig. 19a-e. These species are measured by the HC-PCF based PTS, except for N O. Due to the lack of a corresponding tunable laser of the model, it is measured by the HC-PCF based absorption spectroscopy (ABS). O Concentrations is estimated using formula with the collected phase information. By comparing with the absorption cross-section in HITRAN library and the measured phase curves near 1631.3 nm, we verified the feasibility of our method (Supplementary Fig. 19f).

Calculation of the NO absorption ratio
The NO absorption ratio is calculated on the basis of the following equation:

(2)

where C represent the concentrations of NO (such as NO , NO, and N O) before absorption in the Position I; C represent the concentrations of NO (such as NO , NO, and N O) after absorption in the Position III.

Calculation of the NO – yield
The NO – yield is calculated according to equation (4):

(3)

where V is the volume of solution; C – represent the concentration of NO – (such as NO – and NO –) in the solution in the Position II.

Kinetic model
Numerical modelling for the time evolution of species densities in the plasma discharge and gas absorption is done using the open-source code ZDPlasKin38. The electron impact reaction rate constants and transport parameters are solved by the Boltzmann equation solver BOLSIG37 incorporated in ZDPlasKin. The global equations are shown as follows,

(4)

where n is the density of species i, a is the stoichiometric coefficients for the general reaction. The gas temperature (T ) is considered to be the experimentally measured temperature.

In order to simulate the possesses of gas discharge and gas absorption, there are two kinetic modes incorporated into a plasma model and an aqueous solution model. There are 62 species in the plasma model, 971 reactions in the plasma model, 80 species in the aqueous solution model, and 163 reactions in the aqueous solution model. Detailed species information can be found in Supplementary Table 2. The plasma model is developed by Boagert’s group43. However, the original model implements 24 vibrational levels for N and 15 levels for O , which is computationally demanding. Therefore, we reduced the vibrational levels for N to 15 and the vibrational levels for O to 8 based on another work44. To evaluate the deviation for the simplification of vibrational energy levels, we extend the model to include higher N vibrational levels (v = 16–58) using cross-section data from the Laporta database in LXCAT. The calculated ratios of energy loss coefficients for N (v = 14–20) relative to N (v = 13) are shown in Supplementary Fig. 20. The energy loss coefficients of N (v > 15) are less than 3% of that for N (v = 13), indicating that the concentration of N (v > 15) is negligible. Moreover, according to the Fridman–Macheret α-model, the reaction rate of N (v) + O → NO + N becomes nearly constant for vibrational levels above v = 12 (Supplementary Fig. 11). Therefore, the contribution of N (v > 15) molecules to the formation of NO via N (v) + O → NO + N is minimal. Overall, this simplification has negligible influence on the reaction mechanism and the conclusions of the study. Furthermore, we modified related reactions pertaining to vibrationally excited N , that is, electron impact reaction, N – O vibrational-vibrational (VV) relaxations, N – N and O – O vibrational-translational (VT) relaxations, and elementary reactions with vibrational energy.

For the electron impact reactions, the cross sections of vibrational excitation, electronic excitation, dissociation, and ionization of N , O , N, and O are obtained from the Bigai database, Phelps database, and Morgan database of the LXCAT45. The relaxation rates for the N (v=1) and O (v=1) were based on Andrey et al.46. Other VV and VT relaxations are calculated based on Schwartz-Slawsky-Herzfeld (SSH) theory and approximations for the Morse oscillator model39.

Vibrational excitations play a critical role in the simulation of plasma. These rate coefficients are typically calculated from the corresponding rate coefficients of the collisions from the ground-state atoms by multiplying with an exponential factor according to the Fridman-Macheret α-model40. In this work, the vibrational level of N is up to 15 and a more elaborate description of the vibrational kinetics of N and O is added (i.e., vibrational-vibrational exchanges and vibrational-translational relaxation). In addition, the rate coefficients of the vibrationally excited species are determined according to the Fridman-Macheret model in which the activation energy is lowered by αE , where α is the vibrational efficiency to lower the activation barrier and E is the vibrational energy40. For the aqueous solution model, most of reactions are obtained from Leitz and Kushner47, and part of reactions are estimated by the gas phase reactions.

Numerical Model
To predict the effective of N fixation and analyze the reaction pathways, the simulation model includes the plasma discharge zone with electric field and the liquid zone without electric field. It is assumed that the gas diffusion can be ignored in bubbles of the water solution for simplifying the calculation. The initial mole fractions of dissolved post-discharge gas are directly calculated by Henry’s law46.

For DBD, our discharge model is developed by Ju’s group48. The reduced electric field intensity E/N in the discharge gap is estimated by

(5)

where U is the applied voltage, L is the discharge gap distance, d is the thickness of the quartz tube, N is the gas density, and ε is the relative dielectric constant of the quartz tube. The pulse duration t is determined by the periodic power density W, which can be obtained by

(6)

(7)

where N is the electron number density, μ is the electron mobility at a given reduced electric field, P is power in every pulse and T is the discharge periodic time.

For spark, we adapt the model of Boagert’s group32. The power is assumed to be the constant. The reduced electric field is calculated by the following equations

(8)

where E is the electric field; N is the gas density; P is discharge power; e is the electron charge; N is the electron number density; μ is the electron mobility at a given reduced electric field. At the beginning of the calculation, an initial electron density of 1010 cm-3 is used. The initial corresponding E/N is calculated by equation. Then, the corrected E/N and electron density will be adapted from the input power density after a few timesteps.

To validate the O /N plasma chemistry for the predictions of NO, NO , and O mole fraction, the comparison with the experiment results is conducted in ZDPlasKin at a 0.8 N /0.2 O DBD at 10 kV (Supplementary Fig. 21).

To get insight into the relationship between the N fixation with the reduce electric field, we use BOLSIG and the cross sections of N and O to calculate the fraction of electron energy transferred to different channels of excitation, as well as ionization and dissociation of N and O , in a 0.8 N /0.2 O under different reduced electric field (E/N), as shown in Fig. 2e.

To understand the mechanism of the plasma-assisted N fixation, we perform the simulations at a power of 10 W in the DBD and spark. The simulation results and reaction pathways are presented in Supplementary Figs. 6-10.

To study the importance of the energy distribution under the different reduced electric field, as shown in Fig. 3d-f. The high uniform-EF, low uniform-EF, and SD-EF are simulated under 0.5 mJ/cm3 per pulse by fixing the reduced electric field of 100 Td, 150 Td, as well as 100 Td and 150 Td, respectively. In each pulse of the high uniform-EF and the low uniform-EF, the energy is only 0.5 mJ/cm3, and the discharge stops after the energy is applied. In addition, during a pulse of the SD-EF, 100 Td and 150 Td apply the deposition energy of 0.125 and 0.375 mJ/cm³, respectively, with no electric field applied during the rest of the time.

Light field power simulation
To validate the feasibility of the optical transmission in the PCF, the optic mode of the HC-PCF is simulated by the Electromagnetic Waves, Frequency Domain (ewfd) module of COMSOL. We establish the simulation structures of the HC-PCF without water and with water for the gas phase and aqueous phase, respectively, as found in Supplementary Fig. 4a and b. In Supplementary Fig. 4a, air, whose refractive index is 1, is filled with the hollow-core region and the air holes within the cladding. The cladding is made of silicon dioxide with the refractive index of 1.44. In Supplementary Fig. 4b, air in the hollow-core region and the drilled holes is replaced with aqueous, while the rest air hole region is still filled with air. To simplify the simulation, the refractive index of aqueous is set to 1.333, the same as that of water. The simulation results of the HC-PCF without and with water are presented in Supplementary Fig. 4c and d.

Electric field simulation
In this study, the electric field distribution in the SD-EF and low uniform-EF is simulated by the Electrostatic (es) module of COMSOL, and the simulation structures of the SD-EF and low uniform-EF are determined by our experimental configurations. To compare the discharge characteristics in the different configurations, we perform the simulations of the electric field distribution of the SD-EF and low uniform-EF. When the inner high voltage electrodes and the outer ground electrode are applied a voltage of 10 kV, the electric field distribution of the inner electrode center in the chamber is shown in Supplementary Fig. 13.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Nitrogen fixation via plasma electrocatalysis has the potential to transform ammonia and fertiliser production by enabling\ndecentralised, low-temperature routes that are compatible with renewable electricity. The ability to perform real-time, in situ\nspecies quantification in high-field, non-equilibrium conditions represents an important step forward in mechanistic\nunderstanding. The authors present a plasma-assisted nitrogen fixation approach that leverages a spatially distributed\nelectric field (SD-EF) in a dielectric-barrier discharge reactor, coupled with operando hollow-core photonic-crystal-fibre\nphotothermal spectroscopy (HC-PCF PTS) to monitor key intermediates (NO, NO₂, N₂O, O₃, NO₃⁻, NO₂⁻) directly at the gas-\nliquid interface. Given the compelling improvement in yield and energy efficiency, as well as the clear elucidation of dual\nkinetic networks, I believe this work is suitable for publication in Nature Communications with minor revisions.\nI offer three questions to strengthen the manuscript:\n1- A study on nanosecond-pulsed plasma (J Sun et al., Applied Catalysis B: Environmental 342, 123426, 2024) found that\nalternating on/off periods can reduce backward recombination by allowing radicals to react during \"off\" times, rather than just\nmodulating field strength. Could a similar kinetic effect happened here? Is the SD-EF waveform timing effective in preventing\nreverse NO ⇌ N₂ or NOₓ recombination, in addition to or instead of spatial field distribution?\n2-Have the authors considered sub-second, time-resolved measurements around discharge switching events to detect any\nsuppression of reverse reactions? Such information would help determine whether the enhancements are based on field\ndistribution or simply temperature effect on the reactions kinetics.\n3-The dual N₂(v) and O₃ sub-mechanisms are well-defined. Could the authors explain how the lifetimes of key intermediates\n(e.g., N₂(v) vibrational de-excitation versus O₃ decay) match the spatial scales and residence times in the SD-EF reactor\ndesign?\n4- When reactive intermediates (e.g. N₂(v), NO, O₃) transit between low- and high-field regions, do they undergo quenching,\nfurther oxidation/reduction with co-generated species, or recombination? Can the HC-PCF PTS approach be adapted to\ntrack these transient changes in concentration or energy state as the species cross field-strength boundaries?\n5- How does prolonged operation under SD-EF affect reactor performance and dielectric integrity? Can any drift in species\nyields or energy efficiency be quantified with HC-PCF PTS over tens of hours?\n6- Since water vapour and trace O₂ can alter plasma chemistry, have the authors examined how varying relative humidity or\ngas impurities impacts intermediate lifetimes and product selectivity? Could their in situ probe detect those effects?\n7- Beyond time resolution, can the HC-PCF PTS approach be spatially mapped across the gas–liquid interface to confirm\nthe two-mechanism model’s localisation of N₂(v) vs. O₃ chemistry?\n8- Does the water layer merely absorb radicals, or does its rapid quenching actually stabilise reactive intermediates (e.g.\nNO, N₂(v)), thereby influencing selectivity and yield?\n9- Could the DBD be operated entirely in the gas phase, with the gaseous products then routed to an external liquid\nabsorber? This design might decouple plasma generation from liquid handling (e.g. bubble dynamics), simplifying reactor\nconstruction and scaling.", "reviewer_2": "This manuscript describes experiments and models of plasma-driven N2 oxidation to NOx and ultimately nitrate, including a\nproposed strategy for improving oxidation yields. The work is potentially of interest for Nat Comm but needs major\nimprovements in presentation first.\n1. The Introduction needs a schematic or illustration to clarify the physical system and relevant processes. The relevant\nparagraph on page 2 is insufficient for understanding the concept the authors are trying to pursue. The Nat Comm\nreadership will not get the key ideas from \"plasma bubble reactor\".\n2. Related, on first readings of Introduction I mistakenly thought the paper addressed the entire N2 -> NH3 system. I suggest\na rewrite to focus reader attention on the content of the work.\n3. Figure 1b is the closest the manuscript comes to representing the system. The analytical component of the experiment is\nclear, but the experiment itself is not. Where is plasma generated? What do purple dots represent? Where/how are reactants\nfed? Where are reactions happening? Where is material flow? Essentially, what is the reactor, and how is the reaction space\nenvisioned?\n4. Unclear what \"Position I,\" \"Position II,\" \"Position III\" mean.\n5. Similarly page 7, models. It is fine to put details in SI, but narrative must provide enough information to understand\nessential of modeled system and the key inputs and assumptions. What element of Figure 1b is being modeled? Entire\nreactor? A bubble? How does a zero-dimensional model describe both discharge and adsorption? How is discharge\ndescribed? What is origin of kinetic parameters? Figure 2f is based on what analysis? (Fig 2f caption describes reaction\nconditions, but narrative lacks information to understand the meaning of those conditions.)\n6. Fig 3d-f, are these from measurements or models?\n7. \"9.8 nmol/h\" needs some sort of volume or mass or other appropriate normalization to transform it into an intensive\nquantity. How does one make sense of Fig 3i? How can unnormalized yields of different experiments be compared?", "reviewer_3": "This work presented a non-equilibrium spatially distributed electric field strategy in a dielectric barrier discharge to optimize\nnitrogen fixation in air plasma under ambient conditions. The optimized SD-EF strategy gives a NOx− yield of 9.8 mmol/h,\nwhich is three times higher than that using a uniform electric field and up to eighty times higher than most discharge\nconfigurations at similar or lower energy consumptions. This high NOx−yield is achieved through simultaneously activating\ntwo beneficial kinetic networks via SD-EF by maintaining both high- and low-electric fields present: O3 and N2(v) sub-\nmechanisms, which are revealed by developing a photonic crystal fiber diagnostic for in-situ quantification of molecules and\nions in gas-liquid plasma. This work is very interesting, and the writing is clear. I recommend the publication of this work after\nminor revisions.\n(1) Can the strategy presented in this work be adapted for broad applications in nitrogen fixation?\n(2) The authors proposed a kinetic mechanism. Based on this, they designed a spatially distributed electric field strategy.\nSD-EF exhibits a higher NOx- yield, a higher NO3- selectivity, and a lower energy consumption than existing plasma\nconfigurations. Can the authors provide direct evidence to prove that both proposed reaction pathways occur within this non-\nuniform electric field?\n(3) To emphasize the novelty of this work, the authors should briefly introduce other nitrogen fixation pathways and related\nresearch progress in the introduction, including photocatalytic nitrogen fixation (J. Am. Chem. Soc. 2019, 141, 5083; J. Am.\nChem. Soc. 2024, 146, 7734).\n(4) In Figure 3e, how long does it take to reach the optimal NO concentration?\n(5) What are the values of the electric field, gas temperature, and electron density for the Spark and DBD discharges?\n(6) What are the selectivities for the conversion of N2 to NO, NO2, and N2O?", "reviewer_4": "The authors introduced a novel spatially distributed electric field (SD-EF) strategy for plasma-assisted nitrogen fixation\nunder ambient conditions. The optimization of the electric field distribution through a dielectric barrier with nonuniform\nthicknesses led to the simultaneous activation of two beneficial reaction networks, N2 vibrational excitation (N2(v)) at low\nelectric fields and O3 formation at high electric fields. The approach gave a NOx- yield of 9.8 mmol h-1 with an energy\nconsumption of 1.6 kWh mol-1, outperforming traditional uniform dielectric barrier discharges (DBD) and spark discharge\nsystems in efficiency. The study also employed a novel hollow-core photonic crystal fiber (HC-PCF)-based photothermal\nspectroscopy (PTS) to achieve real-time, in situ quantification of the key gas and liquid species with a detection limit of 10\nppm.\nPlasma-assisted nitrogen fixation is considered a promising alternative to the conventional Haber-Bosch process, but its\ncomplicated mechanism has not been understood. This work presents a significant advancement in the field of plasma-\nassisted nitrogen fixation by addressing some key challenges in energy efficiency and product selectivity. The combination\nof SD-EF with in situ diagnostics is innovative and demonstrates clear improvements over traditional configurations.\nHowever, the authors could provide deeper discussion on practical challenges, such as the system sensitivity, scalability,\nand potential environmental impacts, to enhance its applicability and implementation. I would recommend the publication of\nthis work on Nature Communications after minor revision.\n(1) Can the gas-liquid diagnostics capture the signals from short-lived free radicals?\n(2) The study focuses on generating NOx-, but how can this intermediate be efficiently converted to ammonia? Could this\ntwo-step process compete with direct plasma-driven ammonia synthesis?\n(3) What are the advantages of the PTS diagnostic system? What are the limitations of the detection method?\n(4) Does the generation of O3 or other reactive species pose any risks for environmental safety or downstream processes?\n(5) The SD-EF strategy relies on non-uniform electric fields. How sensitive is the system to variations in the dielectric\nthickness or material properties?\n(6) While the energy consumption is lower than traditional methods, how is it compared to electrocatalysis or photocatalysis?\n(7) Have the authors optimized the ratio of the reaction gases? Will different N2/O2 ratios affect the reaction pathway?\n(8) The authors used a photothermal spectroscopy-based in situ measurement. What are the advantages of this method\ncompared to other in situ characterization methods, such as in situ DRFITS and in situ Raman spectroscopy?\n(9) There are many clerical errors in this manuscript. The authors need to make careful changes. For example, on page 6,\nline 119, the second “DBD” should be changed to “spark”.\n(10) For the spark discharge mode, since N2 can be activated through vibrational excitation, the activated N2 molecules may\nreact with O radicals to form N2O molecules. But why is the amount of detected N2O molecules negligible?\n(11) Given the competitive nature of O3 and NO formation, how does the system balance these pathways to maximize NOx\nproduction without excessive energy losses?\n(12) Has the system been tested over extended operation periods? How does the dielectric material and electrode\nconfiguration hold up under prolonged discharge conditions?\n(13) The vibrational energy levels of N2 and O2 have been simplified in the kinetic model. Could this potentially affect the\naccuracy of the reaction pathway?"}, "author_response": "We would like to thank the reviewers and editor for their detailed feedback to\nimprove our manuscript. We have considered the comments carefully and revised our\nmanuscript to address the issues raised. All revisions are clearly marked in Red\nthroughout the manuscript for ease of review.\nReviewer #1:\nNitrogen fixation via plasma electrocatalysis has the potential to transform ammonia\nand fertilizer production by enabling decentralized, low-temperature routes that are\ncompatible with renewable electricity. The ability to perform real-time, in situ\nspecies quantification in high-field, non-equilibrium conditions represent an important step\nforward in mechanistic understanding. The authors present a plasma-assisted nitrogen\nfixation approach that leverages a spatially distributed electric field (SD-EF) in a\ndielectric-barrier discharge reactor, coupled with operando hollow-core photonic-\ncrystal-fibre photothermal spectroscopy (HC-PCF PTS) to monitor key intermediates\n(NO, NO₂, N₂O, O₃, NO₃⁻, NO₂⁻) directly at the gas-liquid interface. Given the\ncompelling improvement in yield and energy efficiency, as well as the clear elucidation\nof dual kinetic networks, I believe this work is suitable for publication in Nature\nCommunications with minor revisions.\nMajor comments:\nComment 1: A study on nanosecond-pulsed plasma (J Sun et al., Applied Catalysis B:\nEnvironmental 342, 123426, 2024) found that alternating on/off periods can reduce\nbackward recombination by allowing radicals to react during \"off\" times, rather than\njust modulating field strength. Could a similar kinetic effect happened here? Is the SD-\nEF waveform timing effective in preventing reverse NO ⇌ N or NO recombination,\n2 x\nin addition to or instead of spatial field distribution?\nResponse: The SD-EF strategy provides a fundamentally different electric field\nmodulation compared with the on/off mode in nanosecond-pulsed plasma. The latter\nenhances the formation of NO by promoting its two production pathways from N\n2\nvibrational excitation and O radical generation instead of suppressing the reverse N (v)\n2\n+ O → NO + N (R 1 ).\nOur new experimental measurements confirm it in the response to Comment 2, and\nwe also add a 0-D simulated production flux of NO in DBD and SD-EF in Fig. R1 to\nclarify it here. The results indicate that the reverse reaction of N 2 (v) + O → NO + N\n(R ) still exists in SD-EF, as in DBD. The reverse reaction R accounts for about 1-2%\n1 1\nof the NO consumption pathway flux in both DBD and SD-EF. However, the forward\nreaction of N 2 (v) + O → NO + N (R 1 ) is significantly enhanced in the SD-EF strategy,\nwith its path flux increasing from about 2% in DBD to 10%. This enhancement arises\nbecause the low electric field region promotes the vibrational excitation of N₂, while\nthe high-field region facilitates the generation of O radicals, thereby enhancing the\nforward reaction N 2 (v) + O → NO + N (R 1 ). In summary, the SD-EF strategy enhances\nthe formation of NO by promoting its two production pathways from N vibrational\n2\nexcitation and O radical generation instead of suppressing the reverse N 2 (v) + O → NO\n+ N (R ).\npercentages with black (green) color indicate the contributions accounting for the total\nspecies production (consumption).\nComment 2: Have the authors considered sub-second, time-resolved measurements\naround discharge switching events to detect any suppression of reverse reactions? Such\ninformation would help determine whether the enhancements are based on field\ndistribution or simply temperature effect on the reaction kinetics.\nResponse: This is an excellent comment. To address it, we design a nanosecond-pulsed\ndischarge experiment in a closed DBD reactor at 30 kPa and 7 kV to explore the\nsuppression of reverse reactions, as shown in Fig. R2. The time-resolved measurements\nof NO and NO concentrations (Fig. R3) are performed using our HC-PCF-based PTS\n2\nmethod with “plasma on” for 300 s and with “plasma off” for another 300 s after the\ndischarge switch. The transient measurement resolution is within 100 ms. The NO\nx\nconcentrations initially increase in plasma and then reaches a steady state near 250 s.\nWhen discharge ceases at 300 s, the NO concentration decreases while NO slightly\n2\nincreases, indicating further formation of NO from NO. However, the total NO\n2 x\nconcentration decreases, indicating the presence of reverse reactions and the reduction\nof NO to N . Therefore, the on/off mode in plasma does not completely suppress\nx 2\nreverse reactions. Moreover, we have measured the internal temperatures in plasma on\nand plasma off, which are 18.5 ℃ and 21.3 ℃, respectively. The small temperature\ndifference above indicates that the thermal effect is minimal.\nFurthermore, we also performed a new measurement on NO concentration in the\nx\nhigh and low electric fields, respectively, to provide kinetic evidence of nitrogen\nfixation pathways by modulating the reduced electric field, which is added in response\nto Comment 4 of Reviewer 1.\nFig. R2 Digital image of a closed flat DBD discharge reactor.\nFig. R3 Time resolved evolution of NO and NO concentrations in the closed flat DBD\n2\ndischarge reactor.\nComment 3: The dual N₂(v) and O₃ sub-mechanisms are well-defined. Could the\nauthors explain how the lifetimes of key intermediates (e.g., N₂(v) vibrational de-\nexcitation versus O₃ decay) match the spatial scales and residence times in the SD-EF\nreactor design?\nResponse: The nitrogen fixation modulation under the SD-EF strategy is mainly\nthrough local N (v) and O productions in either a high or low electric field region,\n2\ninstead of direct transit of these transient species between different electric field regions.\nIn fact, the characteristic lifetimes of intermediates such as N (v), O, and N are on the\n2\nmicrosecond scale (J. Chem. Phys., 59, 2787–2795, 1973), far shorter than the\nmillisecond-level gas residence time in the SD-EF configuration. However, the\ncharacteristic lifetime of O is on a minute scale, which can transit through these electric\n3\nfield regions. The SD-EF strategy enables O generation in the regions of high electric\n3\nfield and its subsequent utilization to oxide NO to NO in the regions of low electric\n2\nfield. Overall, the SD-EF configuration establishes distinct low and high electric field\nregions to simultaneously promote NO formation under the N (v) and O sub-\n2 3\nmechanisms, respectively. This spatial decoupling allows both reaction networks to\ncoexist and complement each other effectively.\nWe added the following text to the second paragraph of the first section of Methods\nin the revised manuscript: “In the SD-EF configuration, the high voltage electrode is\nenclosed by a coaxial dielectric cylinder made of polytetrafluoroethylene (PTFE)\nfeaturing periodic diameters of 6 and 8 mm (each with a length of 5 m). The geometric\nparameters of the SD-EF configuration have been optimized to balance electric field\nmodulation and gas residence time. The periodic alternation between the high and low\nelectric fields influences the residence time of the gas within each region. It is noted\nthat the characteristic lifetimes of intermediates such as N (v), O, and N are on the\n2\nmicrosecond scale41, far shorter than the millisecond-level gas residence time in the\nSD-EF configuration. However, the characteristic lifetime of O is on a minute scale,\n3\nwhich can transit through these electric field regions. The SD-EF strategy enables O\n3\ngeneration in the regions of high electric field and its subsequent utilization in the\nregions of low electric field to oxide NO to NO in the regions of low electric field.”\nhigh-field regions, do they undergo quenching, further oxidation/reduction with co-\ngenerated species, or recombination? Can the HC-PCF PTS approach be adapted to\ntrack these transient changes in concentration or energy state as the species cross field-\nstrength boundaries?\nResponse: We added measurements of NO, NO , and O concentrations in the low and\n2 3\nhigh electric field regions, respectively, to investigate the reaction changes when\nreactive intermediates transit between the low and high field regions in the SD-EF\nstrategy.\nAs is explained in Comment 3, N (v) is difficult to transit between electric field (EF)\n2\nregions, while NO and O could pass through these regions and undergo secondary\n3\nreactions. O formed in the high-electric field region decomposes into O radical upon\n3\nentering low-electric field regions, where O radical subsequently reacts with N (v)\n2\ngenerated in the low electric field region to form NO. These transport-driven\ntransformations indicate a strong coupling between spatially separated sub-mechanisms.\nThe HC-PCF-based PTS provides species measurements in a millimeter-scale spatial\nresolution, enabling direct quantification of NO, NO , and O in these electric field\n2 3\nregions, respectively. Fiber I and II are placed in the high and low electric field regions\n(Fig. R4), respectively, to quantify the spatial distribution of NO, NO , and O\n2 3\nconcentrations at 10 kV and 500 sccm (Fig. R5). In this measurement, flow goes\nthrough the high electric field region (Fiber I) and low electric field region (Fiber II) in\nturn. It is seen in Fig. R5 that the concentrations of NO and NO in the low electric field\n2\nregion are higher than those in the high electric field region, reflecting the accumulation\nof NO from the high field to the low field over reaction time in this measurement. It is\nx\nwell known that N (v) is mainly produced below 100 Td in a low electric field\n2\n(ChemSusChem, 10, 2145-2157, 2017) and further promotes the NO formation in this\nx\nregion. However, the O concentration is slightly higher in the high electric field region\n3\nthan in the low electric field region, indicating that O is mainly produced in the high\n3\nelectric field region, while is not significantly formed in the low electric field. In\naddition, O in the low field is mainly produced through the transit from the high field,\n3\nand it is also consumed in the low field to oxidize NO. These measurements\nexperimentally verify that the SD-EF strategy can achieve the coexistence of N (v) and\nmaterial. Figure R4 and Figure R5 can be found in Supplementary Fig. 22 and Fig. 23,\nrespectively.\nFig. R4 Schematic diagram of the position for detecting optical fibers.\nFig. R5 Concentrations of NO, NO , and O in the low and high electric field regions.\n2 3\nComment 5: How does prolonged operation under SD-EF affect reactor performance\nand dielectric integrity? Can any drift in species yields or energy efficiency be\nquantified with HC-PCF PTS over tens of hours?\nResponse: The SD-EF reactor maintains stable discharge characteristics for at least 3\nhours at 10 kV and 10 kHz (Fig. R6). The NO concentration stabilizes at approximately\n3000 ppm, indicating consistent plasma activity and reproducible nitrogen fixation.\nHowever, long-term operation is not recommended, as temperature rise and electron-\ninduced erosion in plasma can cause irreversible damage to the dielectric barrier,\nthereby compromising the discharge performance and nitrogen fixation. Additionally,\nthe laser source of the HC-PCF based PTS also experiences thermal accumulation\nduring extended operation.\nThe sentence “To assess the long-term stability of the SD-EF system, we conduct a\n3-hour SD-EF experiment, during which the NO concentration at Position I is\nmonitored (Supplementary Fig. 15). The NO concentration stabilizes at approximately\n3000 ppm, indicating that the SD-EF system maintains stable performance over short\nto moderate timescales.” has been added in the third paragraph of the third section of\nResults in the revised manuscript. Figure R6 can be found in Supplementary Fig. 15.\nFig. R6 Evolution of the NO concentration during a 3-hour operation test in the SD-EF.\nComment 6: Since water vapour and trace O₂ can alter plasma chemistry, have the\nauthors examined how varying relative humidity or gas impurities impacts intermediate\nlifetimes and product selectivity? Could their in situ probe detect those effects?\nResponse: The gases used in this experiment are commercial N and O mixed in a 4:1\n2 2\nratio. Theoretically, there is no water in our feed gas. In order to verify the purity of the\ngas in the reactor, we select the wavelength of 1513.78 nm to measure the water\nconcentration when the plasma is off. The detection limit is 10 ppm. The results show\nthat the signal intensity is below the noise signal, indicating it is below 10 ppm. In\naddition, there is already O in the system, so the mixture of trace O will not have a\n2 2\nsignificant impact.\nFurthermore, we add 10 ppm O and 10 ppm H O to the original SD-EF simulation\n2 2\n(Fig. R7). The H O discharge kinetics follow Green Chem., 24, 7458–7468, 2022. The\n2\nresults show that the addition of these trace gases increases NO concentration by\n5.3×10-6 ppm and increases NO₂ concentration by 9.9×10-5 ppm, indicating a negligible\noverall effect on the system.\nFig. R7 Numerical simulation for NO, NO , and O concentrations before and after\n2 3\nadding 10 ppm O and 10 ppm H O in SD-EF.\n2 2\nComment 7: Beyond time resolution, can the HC-PCF PTS approach be spatially\nmapped across the gas–liquid interface to confirm the two-mechanism model’s\nlocalisation of N₂(v) vs. O₃ chemistry?\nResponse: The HC-PCF based PTS enables the measurements of key species in both\ngas and liquid phases. In Fig. 2 of the manuscript, we quantify NO, NO , and O in the\n2 3\ngas phase in the DBD and spark mode in the plasma reactor before gases enter the\nsolution. Here, we further perform the measurements for these species in both DBD and\nspark modes directly in solution in Fig. R8. In addition, Fig. R8 indicates that DBD\nproduces a significantly higher O concentration than spark, while its NO concentration\n3\nis only half of that in spark. This observation is consistent with our discussion that O\n3\nassists in oxidizing NO to NO in DBD in Fig. 2a-b of the manuscript.\n2 3\nspark.\nComment 8: Does the water layer merely absorb radicals, or does its rapid quenching\nactually stabilise reactive intermediates (e.g. NO, N₂(v)), thereby influencing selectivity\nand yield?\nResponse: The role of water in the nitrogen fixation process extends beyond simply\nquenching reactive radicals such as N and O or preventing the reduction of stable\nintermediates like NO and NO back to N . Although water can react with these radicals\n2 2\nto generate reactive species such as OH and NH (J. Phys. D: Appl. Phys., 49, 425204,\n2 2016), this is not its primary function in our experiment.\nThe main purpose of the water layer in the SD-EF strategy is to serve as a reactant\nthat facilitates the conversion of NO and NO into the final nitrogen fixation products,\n2 3 x 2\nwhereas NO exhibits much lower solubility. A major feature of SD-EF strategy is that\nit ensures the generation of O even in the presence of high concentrations of NO and\n2 3 2\nhigher than O and NO), thereby reducing the NO concentration in the solution and\n2\npromoting its subsequent dissolution.\nThe sentence “It is noted that the water layer primarily enables the transformation of\nNO and NO into NO − and NO − rather than merely preventing their reduction. The\n2 2 3\nhigh solubility of O in the aqueous phase plays a crucial role in driving this overall\n3\nconversion process, which contributes to the nitrogen fixation both through NO + O\n3\n→ NO + O (R ) in the gas phase and NO − + O → NO − + O (R ) in the water\n2 2 5 2 3 3 2 6\nsolution.” has been added in the sixth paragraph of second section of Results in the\nrevised manuscript.\nComment 9: Could the DBD be operated entirely in the gas phase, with the gaseous\nproducts then routed to an external liquid absorber? This design might decouple\nplasma generation from liquid handling (e.g., bubble dynamics), simplifying reactor\nconstruction and scaling.\nResponse: Agreed. A DBD operated solely in the gas phase, followed by subsequent\nliquid absorption, is technically feasible. In such a configuration, the solution phase\nprimarily serves to enable subsequent electrocatalytic reduction of NO - to NH .\nx 3\nTherefore, direct integration of the plasma and liquid phases is not strictly required.\nIndeed, separate gas-phase nitrogen fixation systems have been reported. (Appl. Catal.\nB Environ., 299, 120667, 2021; Nat. Sustain., 7, 652–660, 2024). However, these\nconfigurations result in loss of reactive N and O species prior to absorption in liquid\nand kill plasma–water interactions involving in active radicals for NO formation.\nIn\nx\ncontrast, the integrated SD-EF reactor preserves direct plasma–liquid contact,\nminimizes diffusion distances, and maximizes the utilization of reactive species.\nReviewer #2:\nThis manuscript describes experiments and models of plasma-driven N oxidation to\nx\nyields. The work is potentially of interest for Nat Comm but needs major improvements\nin presentation first.\nMajor comments:\nComment 1: The Introduction needs a schematic or illustration to clarify the physical\nsystem and relevant processes. The relevant paragraph on page 2 is insufficient for\nunderstanding the concept the authors are trying to pursue. The Nat Comm\nreadership will not get the key ideas from \"plasma bubble reactor\".\nResponse: Thank you for the helpful feedback. A schematic of the plasma-bubble\nreactor is provided in Fig. R9. The plasma bubble reactor is designed to integrate\nplasma generation and water absorption. In this configuration, gas bubbles are formed\nwithin a liquid phase, and plasma discharge occurs either inside the bubbles or at the\ngas–liquid interface, producing abundant radicals and reactive species that drive\nchemical reactions under mild conditions. This approach has been explored for nitrogen\nfixation and ammonia synthesis.\nThe following text has been added in the second paragraph of the Introduction in the\nrevised manuscript to facilitate reader understanding. Fig. R9 is added as\nSupplementary Fig. 1 in the revised manuscript. “To address this challenge, a plasma\nbubble reactor integrating plasma generation and water absorption is designed. The\nschematic of plasma bubble reactor is shown in Supplementary Fig. 1. In this\nconfiguration, gas bubbles are formed within a liquid phase, and plasma discharge\noccurs either inside the bubbles or at the gas–liquid interface, producing abundant\nradicals and reactive species that drive chemical reactions under mild conditions. This\napproach has been explored for nitrogen fixation and ammonia synthesis24-28.”\nFig. R9 Schematic of the nitrogen fixation process by plasma-bubble reactor.\nComment 2: Related, on first readings of Introduction I mistakenly thought the paper\naddressed the entire N → NH system. I suggest a rewrite to focus reader attention on\n2 3\nthe content of the work.\nResponse: To focus on the nitrogen fixation instead of the ammonia synthesis, the first\ntwo paragraphs of the Introduction have been revised to clearly distinguish between\ndifferent nitrogen fixation pathways, including biological, photocatalytic, and\nelectrified nitrogen fixation. The revised text emphasizes that electrified nitrogen\nfixation is particularly compatible with distributed renewable energy systems and\nclarifies that this work focuses on plasma-driven N oxidation to NO , rather than NH\n2 x 3\nsynthesis.\nThe first and second paragraphs of the Introduction have been modified as “To\nminimize the environmental impact of carbon emissions, increasing attention has been\ndevoted to biological, photocatalytic, and electrified nitrogen fixation4-6. Among these\nprocess, biological nitrogen fixation converts atmospheric N into NH + through\n2 4\nsymbiotic and free-living nitrogen fixation microorganisms7-9. Photocatalytic nitrogen\nfixation uses solar energy and water to drive NH synthesis on catalytic surfaces such\n3\nas transition-metal-based10, plasmonic metal11,12, and carbon-based materials13.\nElectrified nitrogen fixation utilizes renewable electricity to achieve nitrogen fixation\nunder ambient conditions via an environmentally friendly way, such as through\nelectrocatalytic or non-thermal plasma (NTP) strategies2,14-16. It should be noted that\nthe electrified nitrogen fixation is considered to hold substantial promise for practical\ndeployment because of its compatibility with rapidly expanding distributed renewable\npower grids4. However, conventional electric nitrogen fixation, such as electrocatalysis\nor direct N reduction from H O and H by NTP suffers from a high energy\n2 2 2\nconsumption and a low nitrogen fixation yield due to a high activation energy of N 17-\n2\n23.\nAs an alternative, direct N oxidation by O through the reaction N + xO → 2NO\n2 2 2 2 x\n(R ) is another industrial nitrogen fixation method via the Birkeland-Eyde process. But\n2\nthe thermodynamic equilibrium composition for direct N oxidation severely limits\nx x\nplasma, decomposition back into much more stable N and O will be favored once the\n2 2\ngas equilibrates at low temperature. To address this challenge, a plasma bubble reactor\nintegrating plasma generation and water absorption is designed. he schematic of plasma\nbubble reactor is shown in Supplementary Fig. 1. In this configuration, gas bubbles are\nformed within a liquid phase, and plasma discharge occurs either inside the bubbles or\nat the gas–liquid interface, producing abundant radicals and reactive species that drive\nchemical reactions under mild conditions. This approach has been explored for nitrogen\nfixation and ammonia synthesis24-28. However, the nitrogen fixation yield and the\nenergy consumption are still far from the H-B benchmark29. A major challenge\nhindering progress is the lack of kinetic understanding of the complex plasma-assisted\ncatalysis process at the gas-liquid interface.” in the revised manuscript.\nComment 3: Figure 1b is the closest the manuscript comes to representing the system.\nThe analytical component of the experiment is clear, but the experiment itself is not.\nWhere is plasma generated? What do purple dots represent? Where/how are reactants\nfed? Where are reactions happening? Where is material flow? Essentially, what is the\nreactor, and how is the reaction space envisioned?\nResponse: Figure 1b illustrates the principle of the HC-PCF-based photothermal\nspectroscopy (PTS) diagnostic, rather than the reactor itself. Figure 1a depicts the\noverall experimental setup, with the reactor design shown on the right and the fiber-\noptic topology on the left. In the reactor, plasma is generated between the central high-\nvoltage electrode and the grounded reactor wall beneath the liquid surface, represented\nby the purple region in Fig. 1a. N and O gases are introduced into the reactor through\n2 2\nan external three-way connector, pass through the plasma region where NO and O are\nx 3\nformed, and then enter the liquid as bubbles through small holes at the bottom, enabling\nthe dissolution of NO species into the liquid.\nx\nWe added the following text in the first paragraph of the Results. “The entire reactor\nis immersed in a water-filled cylindrical vessel, which simultaneously serves as an\nabsorber vessel and ground electrode. Air is introduced into the reactor through a three-\nway plastic joint, where discharge occurs in the region between the high-voltage\nelectrode and the water-immersed section of the reactor wall (purple area in Fig. 1a).\nThe micro holes allow reactive species generated by the plasma to flow into the water,\nforming bubbles and enabling the dissolution of NO species into the liquid.”\nx\nComment 4: Unclear what \"Position I,\" \"Position II,\" \"Position III\" mean.\nResponse: To address your comment, we added the following text to the second\nparagraph of first section of the Results in the revised manuscript. “Fiber optic probes\nare placed at different locations within the reactor, each labeled to indicate its\nmeasurement position. Position I refers to the bottom of the quartz tube of the reactor,\nwhere the probe measures the NO, NO , N O, and O concentrations in the gas phase\n2 2 3\nbefore absorption. Position II is located within the absorption solution, enabling\nmeasurement of dissolved NO − and NO − species. Position III is situated at the reactor\n2 3\noutlet above the liquid surface, where the NO, NO , N O, and O concentrations are\n2 2 3\nmonitored after gas-liquid interaction. The specific arrangements of Position I, II, and\nIII are shown in Fig. 1a”\nComment 5: Similarly page 7, models. It is fine to put details in SI, but narrative must\nprovide enough information to understand essential of modeled system and the key\ninputs and assumptions. What element of Figure 1b is being modeled? Entire reactor?\nA bubble? How does a zero-dimensional model describe both discharge and adsorption?\nHow is discharge described? What is origin of kinetic parameters? Figure 2f is based\non what analysis? (Fig 2f caption describes reaction conditions, but narrative lacks\ninformation to understand the meaning of those conditions.\nResponse: The modeling in this study is based on a zero-dimensional (0D) model,\nwhich neglects gas flow and bubble dynamics. The physical simulation scheme,\nillustrated in Fig. R11, simplifies the process into two stages: zero-dimensional plasma\ndischarge and zero-dimensional solution absorption. The reaction times for these two\nstages are determined by the gas residence time in the plasma region and the bubble\nresidence time in the solution, both estimated as 0.1 s in this study. The two stages are\nlinked through Henry’s law. The plasma discharge process occurs under the influence\nof an electric field, whereas the solution region is not affected by the electric field. zero-\ndimensional (0D) simulations are conducted using the plasma kinetics solver\nZDPlasKin, which numerically integrates the time evolution of plasma species densities\nbased on a system of coupled rate equations. The fundamental principle of ZDPlasKin\nis to describe the interactions between species, such as N₂ and O₂, through chemical\nreactions (e.g. Eq. (R1)), and transform these reactions into a system of ordinary\ndifferential equations using chemical reaction rate equations (Eq. (R2)).\naAbB  a'AcC (R1)\nd[n]\ni  a k nl (R2)\ndt j ij j j\nl\nwhere n is the density of species i, a is the stoichiometric coefficients for the general\ni ij\nreaction. The gas temperature (T ) is considered to be the experimentally measured\ng\ntemperature.\nEquation (R2), which describes the temporal evolution of chemical species during\nthe reaction process, is solved by importing the reaction mechanism file into ZDPlaskin\nfor numerical computation. The plasma model used in this study is based on that\ndeveloped by ACS Sustainable. Chem. Eng., 8, 9711–9720, 2020. However, the\nBoagert’s model includes 24 vibrational levels for N and 15 vibrational levels for O ,\n2 2\nresulting in high computation demand. Therefore, the vibrational levels are reduced to\n15 for N and 8 for O to improve computational efficiency.\n2 2\nFor the aqueous absorption stage, the reaction mechanism is adopted from J. Phys.\nD: Appl. Phys., 49, 425204, 2016. To gain a systematic understanding of the nitrogen\nfixation mechanism, simulations are conducted for two types of discharge modes: DBD\nand spark discharge. The key difference between these modes lies in the setting of the\nreduced electric field: for DBD, it follows Proc. Combust. Inst., 37, 5545–5552, 2019,\nwhile for spark discharge, it follows ACS Sustainable Chem. Eng., 8, 9711–9720, 2020.\nThe simulation results are provided in Supplementary Figs. 6–10. Figure 2f summarizes\nthese simulation results, highlighting the two most critical sub-mechanisms in plasma-\nassisted nitrogen fixation: the N (v) sub-mechanism and the O sub-mechanism.\n2 3\nThe second and third paragraphs of the Introduction have been modified as “To\nexplore the reaction pathway for N fixation, we perform zero-dimensional (0D)\n2\nnumerical simulations of plasma discharge and gas absorption in solutions in DBD and\nspark discharge, respectively. The model neglects gas flow and bubble dynamics,\nfocusing instead on the temporal evolution of plasma chemistry. The physical\nsimulation scheme, illustrated in Supplementary Fig. 5, simplifies the process into two\nstages: a zero-dimensional plasma discharge stage and a zero-dimensional absorption\nstage in solution. The reaction times for both stages are determined by the gas residence\ntime in the plasma region and the bubble residence time in the solution, both estimated\nas 0.1 s in this study. The two stages are coupled through Henry’s law to account for\ngas-liquid mass transfer. This calculation is conducted using ZDPlasKin37 with a\nvalidated N /O plasma chemistry set, tracking the temporal evolution of gas\n2 2\ntemperature and species densities, including vibrationally and electronically excited\nstates, radicals, molecules, ions, and electrons. The nitrogen fixation kinetics explicitly\nincorporates the vibrational dynamics, accounting for impact excitation, vibrational-\ntranslational relaxation, and vibrational-vibrational (V-V/V-V’) exchanges based on\nthe Schwartz-Slawsky-Herzfeld (SSH) theory38. Chemical abstraction reactions are\ndescribed by the Fridman-Macheret α-model39. The discharge power density is used to\nself-consistently determine the reduced electric field within the plasma. The details of\nthe simulations are in the Methods and the simulation results are included in\nSupplementary Figs. 6-10.” The Fig. R10 is added in Supplementary Fig. 5.\nFig. R10 Schematic of the simulation model for plasma discharge and solution\nabsorption.\nComment 6: Fig 3d-f, are these from measurements or models?\nResponse: Figures 3d–f are obtained from numerical simulations and show the\ntemporal evolutions of NO, NO , and O concentrations in low uniform-EF, high\n2 3\nuniform-EF, and SD-EF.\nComment 7: “9.8 nmol/h” needs some sort of volume or mass or other appropriate\nnormalization to transform it into an intensive quantity. How does one make sense of\nFig 3i? How can unnormalized yields of different experiments be compared?\nResponse: Figure 3i compares the performance of different nitrogen fixation systems\nin terms of energy consumption and N atomic fixation yield. The yields are typically\nnormalized to the number of moles of target product produced per unit mass of catalyst\nper unit time, but there is no catalyst in our work. Therefore, we multiply the normalized\nyield by the mass of catalyst to obtain the total amount of target product generated per\nunit time by the reaction system for literature using catalysts. For our work and other\nsimilar catalyst-free studies, the yield is calculated directly as the number of moles of\ntarget product produced per unit time in the reactor. Accordingly, the yield comparison\npresented in Figure 3i provides a direct measure of the nitrogen fixation rate across\ndifferent processes.\nReviewer #3:\nThis work presented a non-equilibrium spatially distributed electric field strategy in a\ndielectric barrier discharge to optimize nitrogen fixation in air plasma under ambient\nconditions. The optimized SD-EF strategy gives a NOx− yield of 9.8 mmol/h, which is\nthree times higher than that using a uniform electric field and up to eighty times higher\nthan most discharge configurations at similar or lower energy consumptions. This high\nNO −yield is achieved through simultaneously activating two beneficial kinetic\nx\nnetworks via SD-EF by maintaining both high- and low-electric fields present: O3 and\nN2(v) sub-mechanisms, which are revealed by developing a photonic crystal fiber\ndiagnostic for in-situ quantification of molecules and ions in gas-liquid plasma. This\nwork is very interesting, and the writing is clear. I recommend the publication of this\nwork after minor revisions.\nMajor comments:\nComment 1: Can the strategy presented in this work be adapted for broad applications\nin nitrogen fixation?\nResponse: The SD-EF strategy is fundamentally based on the modulation of kinetic\nnetworks and thus represents a general concept, independent of reactor type or power\nsource. It can be adapted to other plasma-assisted nitrogen fixation systems. For\nexample, we can use the SD-EF strategy to modify the uniform discharge gap of the\nconventional atmospheric pressure glow discharge (APGD) plasma jet system into a\nnon-uniform discharge gap, enhancing the nitrogen fixation.\nBeyond nitrogen fixation, this kinetic network-guided approach by understanding of\nkinetic networks is also applicable for plasma-assisted processes, including H\n2\nproduction from methane pyrolysis, CO conversion to fuels, and plastic recycling and\n2\nupgrading, etc. Furthermore, owing to the modular and distributed nature of plasma\nnitrogen fixation devices, the SD-EF strategy can be effectively integrated with\nrenewable electricity sources such as solar and wind power, highlighting its strong\npotential for broad and decentralized applications.\nWe added the following text in the sixth paragraph of the third section of Results in\nthe revised manuscript. “The SD-EF strategy is fundamentally based on the modulation\nof kinetic networks and thus represents a general concept, independent of reactor type\nor power source. It can be adapted to other plasma-assisted nitrogen fixation systems,\nsuch as atmospheric pressure glow discharge (APGD) plasma jet. Moreover, the\nstrategy of the spatially non-equilibrium electricity field under kinetic networks\nprovides an opportunity for plasma-assisted catalysis not only for nitrogen fixation, but\nalso for other processes, such as CO conversion to methanol, H production from\n2 2\nmethane pyrolysis, and plastic recycling and upgrading.”\nComment 2: The authors proposed a kinetic mechanism. Based on this, they designed\na spatially distributed electric field strategy. SD-EF exhibits a higher NO - yield, a\nx\nhigher NO - selectivity, and a lower energy consumption than existing plasma\n3\nconfigurations. Can the authors provide direct evidence to prove that both proposed\nreaction pathways occur within this non-uniform electric field?\nResponse: We performed in situ measurements using the HC-PCF-based PTS\ndiagnostic system to directly verify the reaction pathways under different electric field\nstrength. Please refer to our response to Comment 4 in Reviewer 1 for detailed\ninformation.\nComment 3: To emphasize the novelty of this work, the authors should briefly introduce\nother nitrogen fixation pathways and related research progress in the introduction,\nincluding photocatalytic nitrogen fixation (J. Am. Chem. Soc. 2019, 141, 5083; J. Am.\nChem. Soc. 2024, 146, 7734).\nResponse: To address your comment, we added the following text to the Introduction:\n“To minimize the environmental impact of carbon emissions, increasing attention has\nbeen devoted to biological, photocatalytic, and electrified nitrogen fixation4-6. Among\nthese, biological nitrogen fixation converts atmospheric N into NH + through\n2 4\nsymbiotic and free-living nitrogen fixation microorganisms7-9. Photocatalytic nitrogen\nfixation uses solar energy and water to drive NH synthesis on catalytic surfaces such\n3\nas transition-metal-based10, plasmonic metal11,12, and carbon-based materials13.\nElectrified nitrogen fixation utilizes renewable electricity to achieve nitrogen fixation\nunder ambient conditions via an environmentally friendly way, such as through\nelectrocatalytic or non-thermal plasma (NTP) strategies2,14-16. It should be noted that\nthe electrified nitrogen fixation is considered to hold substantial promise for practical\ndeployment because of its compatibility with rapidly expanding distributed renewable\npower grids4.”\nComment 4: In Figure 3e, how long does it take to reach the optimal NO concentration?\nResponse: We further performed an additional long-term zero-dimensional simulation\nlasting 30 s under the same conditions as in Fig. 3e. The results indicates that NO\nconcentration reaches its maximum after approximately 20–30 s.\nComment 5: What are the values of the electric field, gas temperature, and electron\ndensity for the Spark and DBD discharges?\nResponse: In this paper, the typical parameters are as follows: (i) Spark discharge:\nreduced electric field 50 Td, gas temperature around 900 K, electron density 1011–1013\ncm⁻3. (ii) DBD discharge: reduced electric field above 130 Td, gas temperature around\nreported ranges for similar plasma configurations (Applied Catalysis B: Environmental\n342, 123426, 2024).\nComment 6: What are the selectivities for the conversion of N to NO, NO , and N O?\n2 2 2\nResponse: The selectivities for the conversion of N to NO, NO and N O can be found\n2 2, 2\nin Fig. R13. In DBD, the selectivities are 56.5% for NO, 38.9% for NO , and 4.6% for\n2\n38.8% for NO , and 3.0% for N O, respectively. In SD-EF, the selectivities are 55.6%\n2 2\nfor NO, 40.9% for NO , and 3.5% for N O.\n2 2\nThe sentence “But there is no significant difference in NO, NO , and N O\n2 2\nselectivities between SD-EF, DBD, and spark discharge (Supplementary Fig. 14).” has\nbeen added in the third paragraph of the third section of Results in the revised\nmanuscript. The sentence “In DBD, the selectivities are 56.5% for NO, 38.9% for NO ,\n2\nand 4.6% for N O, respectively. In spark discharge, the selectivities are approximately\n2\n58.1% for NO, 38.8% for NO , and 3.0% for N O, respectively. In SD-EF, the\n2 2\nselectivities are 55.6% for NO, 40.9% for NO , and 3.5% for N O.” has been added in\n2 2\nthe caption of Supplementary Fig. 14. Figure R11 can be found in Supplementary Fig.\n14.\nFig. R11 Comparison of selectivities for the conversion of N to NO, NO , and N O\n2 2 2\nbetween different discharge modes.\nReviewer #4:\nThe authors introduced a novel spatially distributed electric field (SD-EF) strategy\nfor plasma-assisted nitrogen fixation under ambient conditions. The optimization of the\nelectric field distribution through a dielectric barrier with nonuniform thicknesses led\nto the simultaneous activation of two beneficial reaction networks, N vibrational\n2\nexcitation (N (v)) at low electric fields and O3 formation at high electric fields. The\n2\napproach gave a NO - yield of 9.8 mmol h-1 with an energy consumption of 1.6 kWh\nx\nmol-1, outperforming traditional uniform dielectric barrier discharges (DBD) and\nspark discharge systems in efficiency. The study also employed a novel hollow-core\nphotonic crystal fiber (HC-PCF)-based photothermal spectroscopy (PTS) to achieve\nreal-time, in situ quantification of the key gas and liquid species with a detection limit\nof 10 ppm.\nPlasma-assisted nitrogen fixation is considered a promising alternative to the\nconventional Haber-Bosch process, but its complicated mechanism has not been\nunderstood. This work presents a significant advancement in the field of plasma-\nassisted nitrogen fixation by addressing some key challenges in energy efficiency and\nproduct selectivity. The combination of SD-EF with in situ diagnostics is innovative\nand demonstrates clear improvements over traditional configurations. However, the\nauthors could provide deeper discussion on practical challenges, such as the system\nsensitivity, scalability, and potential environmental impacts, to enhance its\napplicability and implementation. I would recommend the publication of this work on\nNature Communications after minor revision.\nMajor comments:\nComment 1: Can the gas-liquid diagnostics capture the signals from short-lived free\nradicals?\nResponse: The short-lived free radicals can be detected by Mass spectrometry (MS) or\nspatial optical diagnostic methods in principle. However, these techniques are not\ncompatible with our work. Specifically, MS requires an additional quartz capillary to\ndraw gases from the reactor into the MS, but reactive species gradually deactivate\nduring their transport through this capillary, making it impossible for the MS to detect\nshort-lived radicals. On the other hand, optical diagnostic methods need optical\nobservation windows, which impose additional constraints on reactor design and cannot\nbe directly applied to our system. Thus, we chose the HC-PCF-based PTS as the\ndetection method in this work due to its compact size (diameter ~125 μm), high\ntemporal sensitivity, time–resolved response, and immunity to electromagnetic\ninterference. It cannot be used to quantify active radicals due to their prompt quenching\nonce they enter HC-PCF. The current PTS system is therefore designed for quantifying\nstable or quasi-stable species in both gas and liquid phases to explore their kinetic\npathways for nitrogen fixation. Future studies on enabling rapid exchange of species\ninside and outside the optical fiber are expected to address this issue.\nComment 2: The study focuses on generating NOx-, but how can this intermediate be\nefficiently converted to ammonia? Could this two-step process compete with direct\nplasma-driven ammonia synthesis?\nResponse: After forming NO −, it can be further reduced to NH through\nx 3\nelectrocatalytic NO reduction reactions (eNO RR). This two-step approach, known as\nx x\nplasma-electrocatalysis, has been demonstrated to achieve higher yields and lower\nenergy consumption than direct plasma-assisted N reduction using H or H O as\n2 2 2\nhydrogen sources. This concept has been experimentally validated in recent reports\n(Energy Environ. Sci., 2021, 14, 865; Curr. Opin. Green Sustainable Chem., 2025, 51,\n100986). Therefore, coupling SD-EF based NO formation with eNO RR represents a\nx x\nhighly efficient route for sustainable ammonia synthesis.\nComment 3: What are the advantages of the PTS diagnostic system? What are the\nlimitations of the detection method?\nResponse: The HC-PCF-based PTS system offers high detection sensitivity and\ncompact size, relatively high temporal resolution, strong resistance to electromagnetic\ninterference, and the capability for in situ monitoring in both gas and liquid phases.\nHowever, the main limitation arises from molecule diffusion, which restricts the\nresponse speed. As a result, the system is unsuitable for extremely fast transient\nphenomena or the detection of short-lived radical species, because its response time is\non the order of seconds to minutes. Future studies on enabling rapid exchange of species\ninside and outside the optical fiber are expected to address this issue.\nThe above comment has been added in the Supplementary Note 2 in the\nSupplementary materials.\nComment 4: Does the generation of O or other reactive species pose any risks for\n3\nenvironmental safety or downstream processes?\nResponse: Although reactive species such as O could be harmful if directly released\n3\ninto the atmosphere, the reactor design ensures that the exhaust gases are collected and\nabsorbed into the liquid phase. For the application, O can easily be decomposed into\n2\nsignificant environmental or safety risks.\nComment 5: The SD-EF strategy relies on non-uniform electric fields. How sensitive\nis the system to variations in the dielectric thickness or material properties?\nResponse: We perform the SD-EF simulation under different high-to-low electric field\nstrength ratios to optimize the SD-EF configuration. Because the spatial electric field\ndistribution is highly sensitive to the thickness and dielectric constant of the dielectric\nbarrier. Variations in dielectric thickness or permittivity primarily affect the local\nelectric field gradient and the spatial distribution of electric field strength, thereby\ninfluencing nitrogen fixation performance in the SD-EF strategy. The SD-EF\nsimulations for different high-to-low electric field strength ratios (Fig. R14) indicate\nthat the performance of nitrogen fixation is optimized at a ratio of 1.5. Accordingly, we\noptimize the SD-EF strategy by selecting appropriate dielectric materials and adjusting\nthe discharge gap to achieve a high-to-low electric field strength ratio of approximately\n1.5 in this work. Moreover, the secondary electron emission coefficient of the dielectric\nmaterial can impact discharge uniformity. Thus, proper optimization of dielectric\ngeometry and material properties is crucial for the stable operation of the SD-EF\nstrategy.\nWe added the following text in the Methods in the revised manuscript. Figure R12\ncan be found in Supplementary Fig. 17. “Moreover, the spatial electric field distribution\nis highly sensitive to the thickness and dielectric constant of dielectric barrier.\nVariations in dielectric thickness or permittivity primarily affect the local electric field\ngradient and the spatial distribution of electric field strength, thereby influencing\nnitrogen fixation performance in SD-EF strategy. SD-EF simulations for different high-\nto-low electric field strength ratios (Supplementary Fig. 17) indicate that the\nperformance of nitrogen fixation is optimized at a ratio of 1.5. Accordingly, we\noptimize the SD-EF strategy by selecting appropriate dielectric materials and adjusting\nthe discharge gap to achieve a high-to-low electric field strength ratio of approximately\n1.5 in this work.”\nFig. R12 Numerical simulation of SD-EF under different high-to-low electric field\nstrength ratios.\nComment 6: While the energy consumption is lower than traditional methods, how is\nit compared to electrocatalysis or photocatalysis?\nResponse: We have added a direct comparison with electrocatalysis approaches in Fig.\nR12. Since electrocatalysis typically produces NH rather than NO −, the vertical axis\n3 x\nis converted from NO − yield to N atomic fixation yield. Although the energy\n3 x\nconsumption of our method is about 10 times higher than that of electrocatalysis, the\nnitrogen fixation yield is approximately 1000 times higher. This indicates that our\napproach is more suitable for small-scale distributed nitrogen fixation. In addition, the\nphotocatalytic systems are not included in this comparison because they operate under\nfundamentally different energy input conditions (i.e., solar illumination rather than\nelectrical energy).\nWe add the following text to the fifth paragraph of the third section of the Results in\nthe revised manuscript. Fig. 3i in the revised manuscript has been replaced by Fig. R13.\n“Beyond the plasma system, the electrocatalytic nitrogen reduction reaction (eNRR) is\nalso compared with our proposed method. Although the energy consumption of our\nmethod is about 10 times higher than that of electrocatalysis, the nitrogen fixation yield\nis approximately 1000 times higher. This indicates that our approach is more suitable\nfor small-scale distributed nitrogen fixation.”\nFig. R13 Comparison of nitrogen atomic fixation yield and energy consumption across\ndifferent configurations.\nComment 7: Have the authors optimized the ratio of the reaction gases? Will different\nN /O ratios affect the reaction pathway?\n2 2\nResponse: We have added experiments investigating the effect of the N /O ratio, as\n2 2\nshown in Fig. R16. As the N /O ratio decreases from 4:1 to 1:4, the NO concentration\n2 2\ndecreases continuously, while the NO concentration initially decreases and then\n2\nincreases. The optimal N /O ratio for NO generation is 4:1, reaching 2813 ppm,\n2 2 2\ncompared to 2280 ppm at a 0.5 N /0.5 O ratio. However, the overall NO concentration\n2 2 x\n(the sum of NO and NO ) shows little difference between a 0.5 N /0.5 O ratio (5334\n2 2 2\nppm) and a 0.8 N /0.2 O ratio (5455.1 ppm). Moreover, using air directly (N /O = 4:1)\n2 2 2 2\navoids the need for air separation, making it more practical for scalable applications.\nFor this reason, we think N /O ratios have less impact on the reaction pathway, only\n2 2\naltering the relative proportions of path fluxes of nitrogen- and oxygen-containing\nreactions. The experiments presented in the manuscript are based on the air composition\nratio.\nThe following text has been added in the fourth paragraph of the third section of\nResults in the revised manuscript. Figure. R14 can be found in the Supplementary Fig.\n16. “We also investigate the effect of the N /O ratio; the results indicate that it has little\n2 2\ninfluence on nitrogen fixation (Supplementary Fig. 16). Moreover, using air directly\n(N /O = 4:1) eliminates the need for gas separation, making it more practical for\n2 2\nscalable applications.”\nFig. R14 NO and NO concentrations at different N /O ratios in SD-EF\n2 2 2\nComment 8: The authors used a photothermal spectroscopy-based in situ measurement.\nWhat are the advantages of this method compared to other in situ characterization\nmethods, such as in situ DRFITS and in situ Raman spectroscopy?\nResponse: Photothermal spectroscopy (PTS) detects signals through the photothermal\neffects generated by absorbed light energy. Compared to DRIFTS and Raman\nspectroscopy, PTS offers three main advantages (Rev. Sci. Instrum., 2001, 72, 1937–\n1955; Appl. Phys. B, 2011, 110, 163–175):\ni) Higher sensitivity and lower detection limits: PTS does not rely on the radiative\nscattering or absorption strength of molecules. Instead, it detects the local temperature\nrise induced by non-radiative relaxation processes, producing strong signals even for\nmolecules with weak adsorption. In contrast, direct infrared absorption spectroscopy is\nlimited by strong laser output and detector saturation, while Raman spectroscopy\nsuffers from inherently weak scattering intensity, making it difficult to achieve\ncomparable sensitivity even with enhancement techniques.\nii) Broader dynamic range and stronger resistance to interference: The PTS\nsignal depends on variations in medium properties, making it less susceptible to\nbackground noise. Conversely, direct infrared absorption spectroscopy is often affected\nby laser output fluctuation, and Raman signals are easily overwhelmed by fluorescence\nbackgrounds.\niii) Wider applicability: PTS is particularly suitable for non–infrared-active or\nweakly scattering molecules because detection does not rely on a molecular dipole\nmoment. Any absorption or scattering process that generates a thermal effect can be\nmeasured, allowing PTS to identify species with weak infrared absorption, providing\nmeasurable signals in systems that are otherwise spectroscopically challenging.\nThese characteristics make PTS particularly suitable for highly sensitive in situ\ndetection of trace species in plasma environments under strong electromagnetic fields.\nThe above comment has been added in the Supplementary Note 3 in Supplementary\nmaterials.\nComment 9: There are many clerical errors in this manuscript. The authors need to\nmake careful changes. For example, on page 6, line 119, the second “DBD” should be\nchanged to “spark”.\nResponse: We have carefully proofread the entire manuscript and SI. The errors have\nbeen carefully corrected throughout the revised manuscript.\nComment 10: For the spark discharge mode, since N can be activated through\n2\nvibrational excitation, the activated N molecules may react with O radicals to form\n2 2\nResponse: We summarize the reaction flux of N O in spark discharge in simulation\n2\n(Fig. R17). The results indicate that N O is mainly product through reaction N (A3Σ +)\n2 2 u\n+ O 2 → N 2 O + O (R 3 ). But the electronic excitation of N 2 requires a high electric field\nstrength (> 130 Td), so a low electric field is not suitable for the generation of N O in\n2\nspark discharge. Meanwhile, the abundance of O radical promotes the conversion of\nN 2 O into NO through reaction N 2 O + O/O(1D) → 2NO (R 6 ), leading to a low N 2 O\nconcentration in the DBD mode with a high electric field. Therefore, N O plays a minor\n2\nrole in plasma-assisted nitrogen fixation.\nThe following text has been added in the fifth paragraph of the second section of\nResults in the revised manuscript. “In addition, we explore the N O generation pathway\n2\n(Supplementary Fig. 10), N 2 O is mainly formed through reaction N 2 (A3Σ u +) + O 2 →\nN O + O (R ). But the electronic excitation of N requires a high electric field strength\n2 3 2\n(> 130 Td), so low electric field is not suitable for the generation of N O in spark\n2\ndischarge. Meanwhile, the abundance of O radical promotes the conversion of N O into\nin the DBD mode. Overall, N O plays a minor role in plasma-assisted nitrogen fixation.”\n3\nsystem balance these pathways to maximize NO production without excessive energy\nx\nlosses?\nResponse: The SD-EF strategy is specifically designed to balance these two pathways\nby establishing the high and low electric field regions within a single reactor. In the low\nelectric field region (~100 Td), electrons mainly excite N vibrationally, forming N (v),\n2 2\nwhich reacts via O + N (v) → NO + N (R ) to produce NO. In the high electric field\n2 1\nregion (~150 Td), O dissociation and O formation dominate, promoting oxidation\n2 3\nreactions such as NO + O → NO + O (R ) and NO − + O → NO − + O (R ).\n3 2 2 5 2 3 3 2 6\nBy spatially separating these sub-mechanisms, the SD-EF strategy ensures that each\nreaction occurs under its optimal electric field. The balance between O and NO\n3\npathways can be tuned by adjusting the relative volume ratio of high and low electric\nfield regions, enabling efficient and stable nitrogen fixation.\nComment 12: Has the system been tested over extended operation periods? How does\nthe dielectric material and electrode configuration hold up under prolonged discharge\nconditions?\nResponse: We perform a 3-hour SD-EF experiment to assess the long-term stability of\nthe SD-EF system. Please refer to our response to Comment 5 of Reviewer 1.\nComment 13: The vibrational energy levels of N and O have been simplified in the\n2 2\nkinetic model. Could this potentially affect the accuracy of the reaction pathway?\nResponse: The simplification of vibrational energy levels may slightly affect accuracy.\nTo assess the influence, we extend the model to include higher N₂ vibrational levels (v\n= 16–58) using cross-section data from the Laporta database in LXCAT. The calculated\nratios of energy loss coefficients for N (v = 14–20) relative to N (v = 13) are shown in\n2 2\nFig. R18. The energy loss coefficients of N (v > 15) are less than 3% of that of N (v =\n2 2\n13), indicating that the concentration of N (v > 15) is negligible. Moreover, according\n2\nto the Fridman–Macheret α-model, the reaction rate of N (v) + O → NO + N (R )\n2 1\nbecomes nearly constant for vibrational levels above v = 12 (Fig. R19). Therefore, the\ncontribution of N (v > 15) molecules to the formation of NO via N (v) + O → NO + N\n2 2\n(R ) is minimal. Overall, this simplification has a negligible influence on the reaction\n1\nmechanism and the conclusions of the study.\nThe following text has been added to Methods in the revised manuscript. Fig. R15\nand Fig. R16 can be found in the Supplementary Fig. 20 and 11, respectively. “To\nevaluate the deviation for the simplification of vibrational energy levels, we extend the\nmodel to include higher N vibrational levels (v = 16–58) using cross-section data from\n2\nthe Laporta database in LXCAT. The calculated ratios of energy loss coefficients for\nN (v = 14–20) relative to N (v = 13) are shown in Supplementary Fig. 20. The energy\n2 2\nloss coefficients of N (v > 15) are less than 3% of that for N (v = 13), indicating that\n2 2\nthe concentration of N (v > 15) is negligible. Moreover, according to the Fridman–\n2\nfor vibrational levels above v = 12 (Supplementary Fig. 11). Therefore, the contribution\nof N (v > 15) molecules to the formation of NO via N (v) + O → NO + N is minimal.\n2 2\nOverall, this simplification has negligible influence on the reaction mechanism and the\nconclusions of the study.”\nFig. R15 Ratio of the energy loss coefficients of N (v = 14-20) to that of N (v = 13)\n2 2\nfor a 0.8 N /0.2 O mixture as a function of E/N.\n2 2\nFig. R16 Reaction rate O + N (v) → NO + N (RS6) against temperature under different\n2\nvibrational levels."}, {"round": 2, "reviewer_comments": {"reviewer_2": "The authors have done an admirable job of addressing most of my prior comments, and the presentation is much the better\nfor it. Two points need further clarification in the manuscript:\n1. Figure 3i remains unclear. The horizontal axis in intensive: energy per N converted. Vertical axis is extensive,\namount/hour, which simply scales with the size of the system (ie, doubling the number of reactors doubles the vertical axis\nwhile keeping horizontal constant). At a minimum, authors need to explain this point and the rationalization of the\ncomparison. Much better would be to make the vertical axis intensive.\n2. That Figure 3d-f are from observations remains unclear. I recommend modification to the caption.\n3. Are the authors aware of NATURE COMMUNICATIONS | (2022)13:402? This paper also addresses N2 oxidation. It\nshould be consulted and cited, potentially incorporated into figure 3i.", "reviewer_3": "The revised manuscript may be accepted for pubication.", "reviewer_4": "The authors reported on plasma-driven N2 oxidation using a spatially distributed electric field (SD-EF) strategy, coupled with\noperando HC-PCF photothermal spectroscopy to detect gas–liquid species. The SD-EF strategy can co-activate N2(v) at\nlow field and O3 at high field within one device, delivering an excellent NOx generation rate of ~9.8 mmol h−1 1.6 kWh\nmol−1.\nIn their response, the authors provided point-by-point replies to all of the reviewers’ comments, addressing both technical\nand conceptual concerns. In the revised manuscript, the introduction was rewritten to focus on plasma-driven N2 oxidation\nand supplemented with a clear plasma-bubble-reactor schematic. More details of kinetic model simulation were\nsupplemented. A control “plasma on/off” experiment was conducted to enhance the reliability of the results. The liquid-phase\nmeasurements and a 3-hour stability test were also added, further improving the data comprehensiveness.\nOverall, although the previous version had certain shortcomings, they have been addressed comprehensively in the revised\nmanuscript. Therefore, I think this manuscript has met the standard of Nat. Commun. and I recommend its publication."}, "author_response": "Response to reviewers\nWe would like to thank the reviewers and editor for their detailed feedback to\nimprove our manuscript. We have considered the comments carefully and revised our\nmanuscript to address the issues raised. All revisions are clearly marked in Red\nthroughout the manuscript for ease of review.\nReviewer #2:\nThe authors have done an admirable job of addressing most of my prior comments, and\nthe presentation is much the better for it. Two points need further clarification in the\nmanuscript:\nMajor comments:\nComment 1: Figure 3i remains unclear. The horizontal axis in intensive: energy per N\nconverted. Vertical axis is extensive, amount/hour, which simply scales with the size of\nthe system (ie, doubling the number of reactors doubles the vertical axis while keeping\nhorizontal constant). At a minimum, authors need to explain this point and the\nrationalization of the comparison. Much better would be to make the vertical axis\nintensive.\nResponse: We thank the reviewer for this important and insightful comment. We agree\nwith your suggestion to replace the vertical axis in the revised Fig. 3i with N\n2\nconversion, an intensive metric that quantifies the fraction of N converted. The revised\n2\nconsumption (EC) between our work and previous studies, including eNRR, spark\ndischarge, DBD, gliding arc discharge, and combined spark + DBD systems.\nThe fifth paragraph of the third section of results has been modified as “Finally, we\ncompare the performance of our proposed nitrogen fixation strategy with state-of-the-\nart results reported in the literature, including electrocatalytic nitrogen reduction\nreaction (eNRR) and various discharge configurations such as DBD, spark, gliding arc,\netc., as summarized in Supplementary Table 1. Since electrocatalysis typically\nproduces NH rather than NO −, the N conversion is used as a unified metric to quantify\n3 x 2\nthe performance of N fixation. The comparison is visualized in Fig. 3i. After\n2\noptimization, our proposed SD-EF strategy exhibits three times higher N conversion\n2\nthan most discharge configurations at similar or lower energy consumptions. Although\nthe spark configuration achieves a higher N conversion than SD-EF, its energy\n2\nconsumption is approximately one order of magnitude higher. Moreover, SD-EF\ndemonstrates a high NO − selectivity of approximately 96.5% among NO − species in\n3 x\nwater solution (Fig. 3b), in contrast to previous studies conducted in alkaline solutions\n15, 27, 41, where NO − is the main product. In addition to the plasma systems, eNRR is\n2\nalso included in the comparison. Although the energy consumption of our proposed\nstrategy is about 10 times higher than that of electrocatalysis, the N conversion is\n2\napproximately 100 times higher. This indicates that our approach is more suitable for\nsmall-scale distributed nitrogen fixation.”\nFig. R17 Comparison of NO, NO , and O concentrations between the three different\n2 3\nconfigurations.\nComment 2: That Figure 3d-f are from observations remains unclear. I recommend\nmodification to the caption.\nResponse: The caption has been modified to “d, e, f, Simulation results of the time\nevolution of O (d), NO (e) and NO (f) concentrations at a pulse energy density of 0.5\n3 2\nmJ/cm3 in high uniformity EF, low uniformity EF and SD-EF” in the revised\nmanuscript.\nComment 3: Are the authors aware of NATURE COMMUNICATIONS | (2022)13:402?\nThis paper also addresses N2 oxidation. It should be consulted and cited, potentially\nincorporated into figure 3i.\nResponse: The study published in Nature Communications13, 402 (2022) explored the\ninteraction mechanism between plasma and Pt catalyst during nitrogen fixation,\nhighlighting the role of the vibrationally excited N in promoting nitrogen fixation.\ndiscussed in our manuscript. Therefore, this reference has now been cited and discussed\nin the revised version. The sentence “It is known that NTP generates a large number of\nfree radicals and vibrationally excited molecules, which are considered capable of\npromoting NO synthesis31” has been added in the third paragraph of the Introduction\nx\nin the revised manuscript.\nHowever, this work does not report quantitative energy consumption metrics under\ncomparable conditions, which are required for the analysis presented in Fig. 3i.\nTherefore, it is not feasible to directly incorporate its data into Fig. 3i, which focuses\non a comparative evaluation of energy efficiency.\nReviewer #3:\nThe revised manuscript may be accepted for publication.\nResponse: We thank the reviewer for the positive evaluation and support for\npublication.\nReviewer #4:\nThe authors reported on plasma-driven N oxidation using a spatially distributed\n2\nelectric field (SD-EF) strategy, coupled with operando HC-PCF photothermal\nspectroscopy to detect gas–liquid species. The SD-EF strategy can co-activate N (v) at\n2\nlow field and O at high field within one device, delivering an excellent NO generation\n3 x\nrate of ~9.8 mmol h−1 1.6 kWh mol−1.\nIn their response, the authors provided point-by-point replies to all of the reviewers’\ncomments, addressing both technical and conceptual concerns. In the revised\nmanuscript, the introduction was rewritten to focus on plasma-driven N oxidation and\n2\nsupplemented with a clear plasma-bubble-reactor schematic. More details of kinetic\nmodel simulation were supplemented. A control “plasma on/off” experiment was\nconducted to enhance the reliability of the results. The liquid-phase measurements and\na 3-hour stability test were also added, further improving the data comprehensiveness.\nOverall, although the previous version had certain shortcomings, they have been\naddressed comprehensively in the revised manuscript. Therefore, I think this\nmanuscript has met the standard of Nat. Commun. and I recommend its publication.\nResponse: We thank the reviewer for the positive evaluation and support for\npublication."}]

Decision letter