Water activation using Ar-H$_2$ atmospheric pressure plasma jets

Whether for materials processing or medical applications, the use of atmospheric pressure plasma jets (APPJs) has emerged as a relevant alternative to conventional methods. Within the APPJs research field, the search for innovation aims not only to solve existing problems but also to explore novel options for generating plasma jets and find new possible applications. In this work, the properties of Ar-H$_2$ APPJs generated using two plasma sources that differ in the frequency, amplitude, and waveform of the generated voltage signal were studied through electrical, thermal, and optical characterization. The discharge parameters were analyzed as a function of the H$_2$ content in the gas mixture, with this parameter varying from 0% to 3.5%. Optical emission spectroscopy revealed that the same reactive species were produced for both plasma sources, except nitric oxide (NO), which was observed only for the source operated at a higher frequency (PS #1). Applications for water activation were performed without H$_2$ and with 3.5% H$_2$ in the gas mixture. The results of water treatment revealed that ammonia is also produced when H$_2$ is added to the working gas. This finding suggests that the water treated by a Ar-H$_2$ plasma jet can be an attractive option for use in agriculture.

💡 Research Summary

This study investigates atmospheric‑pressure plasma jets (APPJs) generated from argon‑hydrogen (Ar‑H₂) mixtures using two distinct power supplies that differ in frequency, amplitude, and waveform. The authors systematically varied the hydrogen content from 0 % to 3.5 % while keeping the total gas flow constant at 2 slm, and they performed electrical, thermal, and optical diagnostics on both plasma sources (designated PS #1 and PS #2).

PS #1 is driven by a 27 kHz sinusoidal voltage that is amplitude‑modulated (≈0.57 ms on, 1.13 ms off) with an overall burst repetition of 588 Hz. PS #2 employs a damped 110 kHz sinusoid lasting about 70 µs, delivered in three‑pulse sequences separated by 1.7 ms, resulting in an effective pulse repetition rate of 180 Hz. Both devices share the same dielectric barrier discharge (DBD) geometry and a 1 m flexible plastic tube that delivers the plasma jet to the ambient.

Electrical measurements show that the mean discharge power (P_dis) and RMS current (i_RMS) increase with hydrogen addition for both sources, but the increase is markedly larger for the high‑frequency PS #1 (≈30 % rise at 3.5 % H₂) than for the pulsed PS #2 (≈15 % rise). This suggests that the higher frequency enhances electron‑impact processes, leading to higher electron density (n_e) and more efficient power coupling.

Thermal characterization using a fiber‑optic temperature sensor reveals that the gas temperature (T_g) rises by 5–10 °C as hydrogen content increases, with PS #1 exhibiting a slightly higher temperature rise than PS #2. The temperature increase is attributed to enhanced electron‑molecule collisions at higher frequencies, which deposit more energy into the gas.

Optical emission spectroscopy (OES) identifies the typical reactive oxygen and nitrogen species (RONS) generated by atmospheric plasma: OH (309 nm), atomic O (777 nm), and the N₂(C‑B) second‑positive system (337 nm). NH radicals (336 nm) appear only when hydrogen is present, confirming the formation of nitrogen‑hydrogen species. Notably, nitric oxide (NO, 250 nm) is detected exclusively in PS #1, indicating that the high‑frequency excitation promotes NO formation pathways (N + O → NO). Electron density, estimated from the N₂(C‑B) band intensity and Boltzmann analysis, lies in the 10¹⁴–10¹⁵ cm⁻³ range for both sources. Rotational (≈300 K) and vibrational (≈3500 K) temperatures of N₂ are similar across the two devices, implying that waveform differences affect power delivery more than the internal energy distribution of the molecules.

The authors then apply the plasma jets directly to the surface of deionized water for five minutes to assess water activation. With pure argon, the treated water contains hydrogen peroxide (≈0.8 mg L⁻¹) and nitrogen oxides (mainly NO₂, ≈0.3 mg L⁻¹). When 3.5 % H₂ is added, ammonia (NH₃) appears at concentrations of 0.2–0.5 mg L⁻¹, while the levels of H₂O₂ and NOₓ slightly decrease. The emergence of NH₃ is attributed to plasma‑generated NH radicals reacting with dissolved water species. Moreover, the presence of hydrogen modestly reduces OH· and O· emission intensities, suggesting that H₂ acts as an electron‑recombination promoter, fine‑tuning the plasma chemistry without drastically lowering the overall RONS budget.

In summary, the study demonstrates that both the electrical waveform and the hydrogen fraction are powerful levers for tailoring the physicochemical output of atmospheric‑pressure plasma jets. High‑frequency, high‑voltage operation (PS #1) favors NO production, whereas the lower‑frequency pulsed regime (PS #2) is more conducive to NH and NH₃ generation. These findings provide a clear pathway for designing plasma‑based water activation processes optimized for specific applications: medical disinfection (requiring strong oxidative species) or agricultural enhancement (where nitrogen‑rich species such as ammonia are beneficial). The demonstration that ammonia‑enriched plasma‑treated water can be produced simply by adding a few percent of hydrogen to the feed gas opens a promising route for sustainable, on‑site generation of fertilizer‑like solutions using compact plasma jet devices.