© European Physical Society, EDP Sciences, 2025

With the escalating effects of global warming worldwide transition to green energy sources is imminent, with the aim of reaching zero carbon emissions in the near future. For instance, in the EU the share of renewable sources in total energy production has already reached 46% in 2023, with an expected increasing trend internationally [1]. However, green energy sources are infamous for their intermittency and unpredictability, which is especially true for the most promising: wind and solar. To make things worse, intermittent sources can cause various technical problems in the electrical grid, requiring a constant balance between supply and demand. The problem of excess energy at certain times and almost no production at other, poses a challenge that can be solved only through effective energy storage.

Despite the aforementioned drawbacks, wind and solar have become increasingly popular among individual customers, such as farmers, since they offer cost savings, energy independence, and environmental sustainability. However, challenges such as unpredictable generation, storage costs, and land use must be carefully managed. Since energy storage is crucial for managing the intermittent nature of renewable energy sources, technologies are being developed and deployed to store excess energy when production is high and release it when production is low. For individual owners, the best energy storage solutions depend on factors such as cost, scalability, reliability. For the self-consumption, farmers can use energy storage techniques that could be short-term (lithium-ion batteries, lead-acid batteries) or long-term (thermal storage, hydrogen production, nitrogen fixation). Here we will focus on one of the unconventional long-term energy storage methods - nitrogen fixation. This promising method uses the excess energy to produce useful chemicals instead of storing energy which can be directly utilized at a later time. In other words, this method is deemed energy storage even though the energy is stored in chemical bonds of the produced compounds, like ammonia (NH₃) or nitrous oxides (NO_x) and not in a battery-like source. The most appealing advantage of nitrogen fixation is its local use in the mentioned small farms, or remote households, where induvial consumers would be able to use their locally produced excess energy for ammonia based fertilizer production - thereby “storing” the energy. As always, when applying new approaches, practical feasibility will depend on scale, cost, and type of technology. We should note that while technically possible, small-scale ammonia or NO_x production is not yet cost-effective for individual renewable energy owners.

But, the increasing demand for reduced carbon foot-print, smaller and more flexible distributed processes, and environmental protection is driving the need to expand the scope of oxidative and reductive nitrogen transformations based on the use of electricity. The possible alternative methods include electrochemical process and nonthermal plasmas. The use of plasma avoids the generation of contaminated bulk electrolyte waste typical of electrochemical processes and can be designed to recycle byproducts like NOx. Plasma is the ionized gas simultaneously containing free electrons and atomic and molecular ions, a state of matter commonly found at high temperatures in nature or in some special laboratory conditions. However, the low-temperature non-equilibrium plasma can be obtained as an electrical discharge at atmospheric pressure. This type of plasma is characterized by a special feature: atoms and molecules are at just above the room temperature, while electrons are drastically hotter, typically reaching ~20000 K, hence the designation. Also, the fraction of electrons and ions in the total gas particles is very small ~0.01% i.e. the ionization level is low. These characteristics, along with UV radiation, make low-temperature plasma an ideal environment for production and/or destruction of chemical compounds. The maintenance requirements for such plasma reactors are relatively simple, while the operation conditions can be fine-tunned for a specific application. For these reasons low-temperature atmospheric pressure plasma is already applied in bio-medicine, material treatment and purification of air and water. Plasma can be used for creating new pathways for nitrogen transformation but it will require a molecular-level understanding of the underlying reaction mechanisms in plasma. Next step would be to translate these fundamental insights into the development of practical methods in order to optimize energy consumption. Such research should combine experimental and theoretical approaches across diverse fields of molecular and plasma physics, gas and plasma chemistry, including homogeneous and heterogeneous catalysis, photon and electron-driven processes. One of the directions to follow is applying atmospheric pressure plasma for green, small-scale, fertilizer production. This is especially important, since ammonia is presently produced on an industrial-scale using Haber-Bosch process, a system which consumes enormous amounts of water and energy, accounting for ~2% of global carbon emissions [2]. Contrary to Haber-Bosch, the new method would enable on-demand local synthesis of fertilizers using only air and water as feedstocks. The concept of the plasma driven nitrogen fixation into NO_x and NH₃, and ultimately into the fertilizer NH₄NO₃ was tested in the last five years, attracting the highest-level funding from ERC and DOE [3, 4]. Additionally, plasma produced green ammonia has been considered as the hydrogen carrier or directly as a fuel [5]. While the energy consumption for a modern Haber-Bosch plant is 0.5–0.7 MJ/mol NH₂, emerging renewable-driven processes can have a higher energy cost while still being considered acceptable if they drastically cut emissions. For instance, plasma-based nitrogen fixation coupled with catalytic reduction to ammonia has achieved an energy consumption of 2.1 MJ/mol NH₃ [6]. Although this is higher, it is potentially viable because it can reduce associated CO₂ emissions by up to 90% compared to the Haber-Bosch process.

The ideal conceptual solution for fertilizer production near agricultural fields, utilizing plasma technology, is illustrated in Figure 1. Historically, this approach can be viewed as an evolution of the two-century-old renewable energy-based windmill water pump. By leveraging plasma technology, the innovative use of intermittent energy sources can be elevated to enable self-sufficient agricultural systems. Such systems would autonomously irrigate crops with fertilized water, reducing dependence on external markets and geopolitical influences.

Fig. 1 Scheme of the plasma technology implementation for the on-sight fertilizer production from water and air using renewable, solar and wind, energy.

Intesertingly, the first industrial implementation of plasma for nitrogen fixation was performed in 1900 by Birkeland and Eyde using the same concept of nitrogen fixation by oxidation. They mimicked nature’s lightning by developing thermal electric arc furnaces at ~3000°C to split atmospheric nitrogen (N₂) and oxygen (O₂) molecules, which would then react to form nitric oxide (NO). This was then further oxidized and absorbed in water to produce nitric acid (HNO₃). Based on the current state of the art, the most attractive pathway for the nitrogen fixation is likewise: initial plasma oxidation directly from air and accumulation of the products in the liquid water solution in the form of HNO_x. Using subsequent electrochemical or catalytic reduction, nitrogen can be fixed in the form of NH₃. Water can be used either as trapping medium for species produced in a distant plasma or as part of an electrical discharge where the plasma is in contact with it, see Fig. 2.

Fig. 2 Three discharge types: a) Dielectric barrier discharge with liquid electrode; b) Gliding arc with a liquid electrode; and c) Pulsed discharge over the liquid surface.

One has to have in mind that plasma-liquid interaction is extremely complex as it involves gas phase chemistry, multiphase species transport, mass and heat transfer, interfacial reactions and liquid phase chemistry. Direct formation of NO_x and NH₃ in plasma with or without catalyst, that implies N≡N bond dissociation (9.7 eV), is still energy inefficient with low throughput, but it is attractive due to straightforward implementation and operation at ambient temperature that enables one pot solution. To increase efficiency, it is necessary to conduct in-depth studies of nitrogen oxidation and reduction by plasma. This study should aim to investigate the time evolution of fundamental processes occurring within a single discharge pulse or voltage cycle, with the aim of optimizing plasma parameters to enhance desired processes while minimizing the decomposition of target species. This can be done by changing the plasma discharge via alteration of the voltage amplitude, pulse duration and frequency. Also, electrode geometry and other nonelectrical variables can be modified, that may influence discharge parameters, such as electric field strength. As different types of the atmospheric pressure discharge produce plasmas with different parameters, the research should be focused on the characterization and comparison of several discharges e.g. dielectric barrier discharge (DBD), gliding arc discharge (GAD), and pulsed discharge over a liquid surface, see Fig. 2. All of the mentioned discharges consist of many microdischarges that represent the fundamental building blocks of the plasma reactor to be utilized for nitrogen fixation experiments.

From a physicist point of view, a comprehensive diagnostic approach must be employed to analyze the properties of these microdischarges, including: electron number density, temperatures, electric field strength, followed by the measurements of the excited species concentration. The experimental data obtained from the diagnostics will serve as essential input parameters for a numerical model of plasma-chemical kinetics. This model has to simulate the generation and decay of key species, providing insights into the underlying mechanisms of nitrogen fixation and enabling the optimization of plasma reactor performance. The integration of experimental diagnostics and numerical modeling is necessary to facilitate a deeper understanding of the microdischarge behavior and its role in the plasma-assisted nitrogen fixation processes. In addition, plasma-surface interactions have to be investigated by examining the interaction of a single microdischarge with liquid surfaces and heterogeneous catalysts, both of which are critical for the nitrogen fixation process. In plasma catalysis, the catalyst acts as a physical mediator that locally enhances electric fields to modify ionization and discharge characteristics, while simultaneously lowering activation barriers for key surface reactions. The interaction with liquids should involve quantifying the concentration of solvated electrons in liquid and correlating this with plasma density above the liquid. All this careful research, within present and future projects will achieve an optimized and cost-effective nitrogen-fixation technology.

Though it seems like a very challenging task, in the future, plasma-assisted nitrogen fixation may prove to be one of the key elements for expansion of wind and solar energy, due to its benefits for remote and isolated households. If the world aims at completely replacing fossil fuels, large number of consumers in the developing countries need to convert to green energy sources, and plasma-assisted nitrogen fixation may be one of the technologies to aid the process.

About the Authors

Bratislav Obradović is a professor at the University of Belgrade, Faculty of Physics. He is President of the Serbian Physical Society.

Goran Sretenović is a senior research associate at the University of Belgrade, Faculty of Physics.

Nikola Cvetanović is an associate professor at the University of Belgrade, Faculty of Transport and Traffic Engineering.

Vesna Kovačević is an assistant professor at the University of Belgrade, Faculty of Physics.

References

All Figures

	Fig. 1 Scheme of the plasma technology implementation for the on-sight fertilizer production from water and air using renewable, solar and wind, energy.
In the text

	Fig. 2 Three discharge types: a) Dielectric barrier discharge with liquid electrode; b) Gliding arc with a liquid electrode; and c) Pulsed discharge over the liquid surface.
In the text