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From hydrogen peroxide to fixed nitrogen: air injection through a hollow electrode in a hydrodynamic cavitation discharge

This study demonstrates that injecting ambient air into a hydrodynamic cavitation discharge via a hollow electrode enables the direct aqueous-phase production of fixed nitrogen (nitrite and nitrate) at the expense of hydrogen peroxide yield, achieving a trade-off where approximately one-third of the peroxide output is sacrificed to generate nitrogen species without requiring external gas compression.

Original authors: Radek Horňák, Pavel Sťahel, David Trunec, Blahoslav Maršálek, Petr Lukeš

Published 2026-10-06

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CC BY 4.0

Original authors: Radek Horňák, Pavel Sťahel, David Trunec, Blahoslav Maršálek, Petr Lukeš

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Nitrogen is the silent engine of life on Earth, a fundamental building block for the proteins that make up every plant and animal. Yet, despite being the most abundant gas in our atmosphere, it is locked away in a form that living things cannot use. For over a century, humanity has relied on a massive, energy-hungry industrial process to break these bonds and create fertilizer, a method that consumes a significant slice of the world's energy supply. Scientists have long searched for a way to unlock this nitrogen using electricity, hoping to create a smaller, greener alternative that could be powered by renewable energy right where the crops are grown. The challenge lies in creating a reaction that is efficient enough to be practical, turning invisible air into a liquid nutrient without burning through too much power.

A team of researchers in the Czech Republic has taken a different approach to this problem by combining two distinct physical phenomena: the violent collapse of bubbles in a liquid and the spark of an electrical discharge. They built a device that forces water through a narrow nozzle, creating a region of low pressure where the water boils instantly to form a cloud of vapor bubbles. Inside this cloud, they sustain an electrical glow. In previous versions of this setup, the device acted like a powerful sterilizer, generating hydrogen peroxide by breaking apart water molecules, but it produced almost no nitrogen-based nutrients. The researchers suspected that the key to unlocking nitrogen lay in the gas itself; the bubbles were filled mostly with water vapor, leaving little room for the nitrogen in the air to participate in the reaction.

To solve this, the team modified the reactor by hollowing out the central metal electrode. Instead of a solid rod, they used a tube that allowed them to draw ambient air directly into the heart of the cavitation cloud, right where the electrical spark was happening. Because the pressure inside the bubble cloud is lower than the air outside, the air was sucked in naturally without the need for any external pumps or compressors. This simple change transformed the chemistry of the system. When they ran the experiment with a steady flow of air, the device began to produce significant amounts of nitrite and nitrate, the forms of nitrogen that plants can absorb. The researchers found that by injecting air at a rate of six liters per minute while processing a batch of water, they could generate a combined yield of forty millimoles of fixed nitrogen for every kilowatt-hour of electricity used.

However, this new capability came with a clear trade-off. The same electrical energy that was previously dedicated to making hydrogen peroxide was now being shared with the task of fixing nitrogen. As the air flow increased, the production of hydrogen peroxide dropped by about one-third, falling from two hundred and fifty-four to one hundred and seventy millimoles per kilowatt-hour. The data showed a direct competition between the two chemical pathways; the device could not maximize both outputs simultaneously. The researchers observed that the light emitted by the spark changed dramatically as air was added. Without air, the glow was dominated by the signature of hydroxyl radicals, the fragments of water molecules. As air flowed in, the spectrum shifted to show the distinct bands of excited nitrogen molecules, confirming that the electrical discharge was now interacting primarily with the injected air rather than the water vapor.

The study also revealed that the temperature of the gas inside the bubbles played a crucial role. As more air was added, the gas temperature rose, reaching a range where thermal chemical reactions become significant. This suggests that the nitrogen fixation was not happening solely through high-energy electron collisions, but was also aided by the heat generated within the discharge. Furthermore, the researchers noted that the natural minerals in the tap water they used acted as a buffer, keeping the water's acidity stable. This was vital because, in many similar experiments, the water becomes so acidic that the newly formed nitrite breaks down or reacts away. Here, the water remained neutral, allowing the nitrite to survive and even become the dominant product, making up more than two-thirds of the total nitrogen produced.

While the energy cost of this method is currently much higher than that of traditional industrial fertilizer production, the researchers argue that the value lies in the process itself. The system produces the nutrients directly in the water, ready for use, and operates without the need for complex gas compression equipment. It represents a shift from a device designed purely for water purification to one capable of creating liquid fertilizer on demand. The work demonstrates that by simply changing the gas composition inside a cavitation cloud, it is possible to steer the chemistry from creating oxidizers to creating nutrients, opening a new path for decentralized, electrically driven agriculture.

Technical Summary: From Hydrogen Peroxide to Fixed Nitrogen via Air Injection in Hydrodynamic Cavitation Discharge

Problem Statement
Plasma-activated water (PAW) offers a decentralized, electrically driven route to fixed nitrogen for agriculture, potentially bypassing the energy-intensive Haber–Bosch process. However, existing PAW reactors often suffer from low throughput (milliliters to liters per hour) and complex scale-up requirements. Furthermore, hydrodynamic cavitation reactors (specifically the CaviPlasma device), which utilize a macroscopic cavitation cloud to sustain a glow-like discharge at high flow rates, have previously operated primarily as advanced oxidation processes. In these water-vapour-dominated regimes, the discharge generates hydrogen peroxide ( $H_2O_2$ ) but fails to produce measurable quantities of fixed nitrogen (nitrite or nitrate), despite the presence of dissolved nitrogen in the water. The challenge is to enable nitrogen fixation in this high-throughput, liquid-phase system without requiring external gas compression or sacrificing the reactor's hydraulic efficiency.

Methodology
The study modifies the CaviPlasma reactor by replacing the solid electrode at the nozzle throat with a hollow stainless steel tube (4 mm inner bore). This design leverages the sub-atmospheric pressure within the cavitation void to aspirate ambient air directly into the discharge zone without external compressors.

Experimental Setup: Experiments were conducted using a 6 L batch of municipal tap water (Prague) recirculated at 30 L/min. The discharge-generator wall-plug power was fixed at 1200 W.
Variables: The air flow rate was varied from 0 to 6 L/min.
Measurements:
- Liquid Phase: Samples were taken at 36, 72, and 108 seconds to quantify hydrogen peroxide (via titanyl sulfate method), nitrite, and nitrate (via ion chromatography).
- Gas Phase/Discharge: Optical emission spectroscopy (OES) was performed using high-resolution spectrometers to analyze the $N_2$ second positive system and the OH(A–X) band. Rotational ( $T_{rot}$ ) and vibrational ( $T_{vib}$ ) temperatures were derived by fitting simulated spectra to the experimental data.
Conditions: The tap water's natural alkalinity (2.8 mM bicarbonate) maintained the bulk pH at 7.35 throughout the treatment, preventing the acidification that typically destabilizes nitrite in PAW systems.

Key Results

Activation of Nitrogen Fixation: In the absence of air injection, no measurable nitrite or nitrate was produced, confirming that dissolved nitrogen in the water is insufficient for fixation under these conditions. Air injection through the hollow electrode successfully opened a nitrogen-fixation pathway.
Trade-off in Product Yields: A distinct trade-off was observed between oxygen- and nitrogen-derived species. As air flow increased to 6 L/min:
- The combined yield of nitrite and nitrate rose from undetectable levels to 40.3 mmol/kWh (27.8 mmol/kWh nitrite + 12.5 mmol/kWh nitrate).
- The hydrogen peroxide yield decreased monotonically from 254 mmol/kWh (0 L/min) to 170 mmol/kWh (6 L/min), a reduction of approximately one-third.
Discharge Regime Transition: OES data revealed a shift in the dominant emission species. Without air, the spectrum was dominated by the OH(A–X) band (309 nm), indicative of water-vapour dissociation. With air injection, the $N_2$ $N_{2}$ second positive system became dominant.
- Temperature Trends: The $N_2(C)$ rotational temperature ( $T_{rot}$ ), used as a proxy for gas temperature, increased from 1700 K to 2150 K as air flow rose from 2 to 4 L/min at 1200 W. This range spans the threshold where thermal Zeldovich chemistry becomes significant. Conversely, the fitted vibrational temperature ( $T_{vib}$ ) did not increase with air addition, remaining between 5360 K and 6480 K, suggesting the rise in nitrogen yield is driven by gas composition and thermal effects rather than increased vibrational excitation.
Product Stability and Distribution: The natural alkalinity of the tap water suppressed the acid-catalyzed reaction between nitrite and hydrogen peroxide, allowing both to coexist. At 6 L/min, nitrite was the dominant nitrogen product, with a molar $NO_2^-/NO_3^-$ ratio of 2.2. This ratio increased with air flow, suggesting shorter residence times in the active zone limit further oxidation.

Significance and Claims
The authors claim that this configuration enables direct aqueous-phase nitrogen fixation in a rapidly recirculating liquid stream without the need for external gas compression. The study quantifies the energetic cost of this process as 89 MJ per mole of fixed nitrogen (based on wall-plug power), which is acknowledged to be significantly higher than dedicated gas-phase plasma reactors (few MJ/mol) or the Haber–Bosch process.

The paper positions the technology's value not in energy efficiency, but in its process integration: the ability to treat large volumes of water (tens of liters per minute) in a single hydraulic unit, coupling fixation directly with the liquid phase. The work demonstrates that nitrogen fixation in this system is a "trade" rather than a simple additive process; optimizing for fixed nitrogen inherently reduces peroxide production. The authors conclude that while the energy cost is currently high, the configuration offers a unique route for decentralized, point-of-use production of reactive nitrogen species in water, provided the application can accommodate the dilute product concentrations (approx. 3.8 mg/L N after 108 s) and the trade-off with peroxide yield.

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