In-Liquid Plasma Catalytic Cells

• In-liquid plasma catalytic cells are hybrid plasma–electrochemical systems that utilize nanometer-scale plasma–liquid interfaces to generate high-energy radicals and solvated electrons.
• They facilitate the activation of stable molecules like N₂ and CO₂ through controlled redox processes and enable catalyst regeneration via pulsed plasma discharges.
• Optimized reactor configurations, pulse regimes, and thermal management—combined with reaction–diffusion analysis—enhance efficiency and selectivity in these systems.

In-liquid plasma catalytic cells are a class of @@@@2@@@@ plasma–electrochemical reactors in which plasma discharges, generated within or immediately adjacent to a liquid phase (often aqueous electrolyte), drive interfacial redox chemistry and couple directly to catalytic surfaces. These systems harness unique, highly non-equilibrium conditions at the plasma–liquid interface to yield radicals, solvated electrons, excited molecules, ions, and other transitory species at fluxes and energies far exceeding those accessible by thermal or purely electrochemical means. As a result, in-liquid plasma catalytic cells can enable activation and conversion of stable molecules—such as N₂ or CO₂—or persistent regeneration of catalyst surfaces under conditions that decouple redox rates from bulk thermodynamic constraints.

1. Plasma–Liquid Interface: Reaction Zone Structure and Redox Mechanisms

The plasma–liquid interface establishes a nanometer-scale region of intense electrochemical activity dominated by the interplay of highly reducing solvated electrons (eₐq⁻) and strongly oxidizing hydroxyl radicals (•OH). Lock-in total internal reflection absorption spectroscopy, as developed in atmospheric-pressure argon plasma studies (Lee et al., 2024), quantifies this nanoscopic confinement: for applied bias voltages $V_\mathrm{b}$ between 1000 V and 2500 V at the electrolyte anode, the solvated electron penetration depth remains $\ell_e \approx 10$ nm, with a peak interfacial concentration $[\mathrm{e}_{\mathrm{aq}^-}]_0 \approx 1$ mM.

Despite large changes in applied voltage, both $\ell_e$ and peak $[\mathrm{e}_{\mathrm{aq}^-}]_0$ are invariant, attributed to the saturation of plasma current density ($j \approx 5$–$6\ \mathrm{A}\,\mathrm{m}^{-2}$) and lateral expansion of the plasma footprint rather than increased local flux. The distribution and fate of these species are governed by coupled reaction–diffusion equations:

$$\frac{\partial n_e}{\partial t} = D_e\,\frac{\partial^2 n_e}{\partial x^2} - k_{\mathrm{OH},e}\, n_e\, n_{\mathrm{OH}}$$

$$\frac{\partial n_{\mathrm{OH}}}{\partial t} = D_{\mathrm{OH}}\,\frac{\partial^2 n_{\mathrm{OH}}}{\partial x^2} + J_{\mathrm{OH}}\delta(x) - k_{\mathrm{OH},e}\, n_e\, n_{\mathrm{OH}} - 2 k_{\mathrm{OH},\mathrm{OH}}\, n_{\mathrm{OH}}^2$$

Here, $$D_e = 4.8\times10^{-9}\ \mathrm{m}^2\,\mathrm{s}^{-1}$$ and $$D_{\mathrm{OH}} \sim 10^{-9}\ \mathrm{m}^2\,\mathrm{s}^{-1}$$ are diffusivities, $$k_{\mathrm{OH},e} = 3.0\times10^{10}\ \mathrm{M}^{-1}\mathrm{s}^{-1}$$ is the eaq–OH radical scavenging rate, and $$k_{\mathrm{OH},\mathrm{OH}} \approx 5\times10^{9}\ \mathrm{M}^{-1}\mathrm{s}^{-1}$$ is the OH radical self-recombination rate. Surface production of •OH is described by a flux $$J_{\mathrm{OH}} \lesssim 10^{22}\,\mathrm{m}^{-2}\,\mathrm{s}^{-1}$$. The steady-state solution yields exponentially decaying concentration profiles, with $\ell_e\approx10$ nm and $\ell_{\mathrm{OH}}\approx5$–$10$ nm.

Redox coupling is further enforced by the large ($k_{\mathrm{OH},e} [e][•\mathrm{OH}] \sim 10^4$ M s⁻¹) rates, causing nearly all interfacial •OH produced by plasma diffusion to be scavenged by eₐq⁻, such that the net oxidative flux into the bulk anolyte is negligible. Under cathodic bias, ionic bombardment sharply increases •OH production through water dissociation and ionization, leading to hydrogen peroxide accumulation via

$$2\,\cdot\mathrm{OH}\;\xrightarrow{k_{\mathrm{OH},\mathrm{OH}}}\;\mathrm{H}_{2}\mathrm{O}_{2}$$

with measured bulk [H₂O₂] reaching 150 mM within five minutes, whereas no H₂O₂ is detected under anode bias (Lee et al., 2024).

2. Cell Architectures, Discharge Regimes, and Electrolyte Management

In-liquid plasma catalytic reactors exhibit diverse cell configurations, contingent upon the targeted transformation and plasma regime.

• For nitrogen reduction, as demonstrated by Grosse et al. (Grosse et al., 26 Jun 2025), the architecture is a single-compartment, two-electrode cell containing 40–50 mL 0.1 M KOH (pH ≈14). The working electrode is a 1.0 mm-diameter Pt wire threaded through an alumina tube (serving as gas inlet), with wire length L = 0.5–4 mm protruding into the electrolyte. The counter-electrode is a Pt foil placed ~5 mm away. Cooling jackets stabilize bulk temperature at 25–30 °C.
• In catalyst regeneration studies (Pottkämper et al., 12 May 2025), reactors comprise a 25 mL PMMA chamber filled with purified water or very dilute KOH. In nanosecond-pulse operation, a sharp (50 μm) tungsten HV electrode is paired with a steel counter electrode, with separations of 0.5–2 cm.

Plasma formation in these geometries is enabled by gas–vapor sheaths (e.g., N₂/H₂O), generated through gas flow and/or electrolysis. At potentials $\gtrsim$2 V vs. RHE, hydrogen evolution induces bubble nucleation around the electrode, supporting the ignition of contact glow or arc-like plasmas at 80–230 V (Pt in KOH) (Grosse et al., 26 Jun 2025). The thermal and plasma-induced water splitting at Pt electrodes at T > 900 K results in H* and OH* formation.

Discharge mode—pulsed nanosecond, microsecond, DC, or arc—modulates energy deposition, reactive species generation, and thermal management. Nanosecond pulses (e.g., 20 kV, 12 ns, 10–50 Hz) minimize bulk heating and maximize radical yield per joule (H₂O₂ energy efficiency ≈0.5 g kWh⁻¹) (Pottkämper et al., 12 May 2025), whereas microsecond pulses or DC arcs deliver greater absolute energy but incur higher liquid heating and potential for H₂O₂ decomposition.

3. Radical and Electron Generation: Interfacial Kinetics and Reaction–Diffusion Balances

The local chemistry at the plasma–liquid interface is dictated by sequential and parallel electron-transfer, excitation, and recombination events:

• Electron-impact dissociation of water produces H·, OH·, and eₐq⁻:

$$\mathrm{e}_{\mathrm{aq}}^- + \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{H}^\cdot + \mathrm{OH}^-$$

$$\mathrm{H}_2\mathrm{O} + \mathrm{e}_{\mathrm{aq}}^- \rightarrow \mathrm{H}^\cdot + \cdot\mathrm{OH} + \mathrm{e}^-$$

• Recombination yields H₂O₂:

$$2\,\cdot\mathrm{OH} \rightarrow \mathrm{H}_2\mathrm{O}_2$$

The net reaction via plasma activation:

$$2\,\mathrm{H}_2\mathrm{O} \rightarrow \mathrm{H}_2\mathrm{O}_2 + \mathrm{H}_2$$

is highly dependent on pulse duration and energy density.

In anode-biased operation, the reaction–diffusion system ensures nearly all interfacial •OH is consumed via

$$\mathrm{e}_{\mathrm{aq}}^- + \cdot\mathrm{OH} \xrightarrow{k_{\mathrm{OH},e}} \mathrm{OH}^-$$

Confined within a 10 nm region, this redox balancing suppresses transfer of both oxidants and reductants into the bulk, maximizing selectivity. For cathode bias, eₐq⁻ are depleted and •OH generation dominates, with hydrogen peroxide formation and bulk accumulation (Lee et al., 2024).

4. Plasma-Catalytic Synthesis and Catalyst Regeneration

Nitrogen Reduction Reaction (NRR):

In-liquid plasma catalysis for NRR integrates direct plasma activation of N₂ within the gas–liquid sheath and classical electrocatalytic hydrogenation on a Pt surface (Grosse et al., 26 Jun 2025). The mechanism encompasses:

• Vibrational and electronic excitation/ionization of N₂ by plasma electrons in the sheath, producing $\mathrm{N}_2^*$ and $\mathrm{N}_2^+$, with $E/N \approx 37$ Td and $n_e \approx 10^{16}\,\mathrm{cm}^{-3}$ under hot-plasma (200–230 V) conditions.
• Surface adsorption and stepwise hydrogenation of atomic N intermediates via a Langmuir–Hinshelwood/Eley–Rideal (LH/ER) mechanism, where hydrogen atoms are supplied by both thermally and plasma-induced water splitting at $T_\mathrm{elec}$ up to 1600 K.

Key kinetic parameters include partial NH₃ current densities up to 250 mA cm⁻², NH₃ yields of 3.0 mmol h⁻¹ cm⁻², and Faradaic efficiencies up to 8%. The necessity of both plasma and catalytic hydrogenation is validated by control experiments: in the absence of either, NH₃ production drops two orders of magnitude (Grosse et al., 26 Jun 2025).

Copper Oxide Catalyst Regeneration:

Pulsed in-liquid plasmas can oxidize, dissolve, and regenerate Cu catalyst surfaces via hydrogen peroxide formation, as shown in Pottkämper et al. (Pottkämper et al., 12 May 2025). Nanosecond pulses yield higher H₂O₂ energy efficiency and produce more uniform Cu₂O nanocubes, with SEM revealing {100}-faceted crystals from hundreds of nanometers to ≈1 μm. Cyclic voltammetry confirms crystallinity via distinct reduction peaks. This self-regeneration pathway leverages:

• Alkaline H₂O₂ oxidation of Cu to soluble Cu(II) complexes,
• Reprecipitation as Cu₂O at the electrode interface once local H₂O₂ is consumed.

These findings suggest a strategy for maintaining active catalyst morphologies for prolonged CO₂ electroreduction, though specific CO₂RR metrics are not reported within the referenced study (Pottkämper et al., 12 May 2025).

5. Performance Metrics, Efficiency, and Control Parameters

Design and operation of in-liquid plasma catalytic cells target figures of merit such as current density, product yield, Faradaic efficiency (FE), and energy efficiency ($\eta_E$):

Metric	Typical Value / Range	Context
NH₃ yield (NRR, Pt)	3.0 mmol h⁻¹ cm⁻²	Hot plasma, Pt cathode (Grosse et al., 26 Jun 2025)
Partial current density	250 mA cm⁻²	Hot state, Pt, N₂–H₂O sheath
Faradaic efficiency	up to 8%	NRR under plasma conditions
H₂O₂ energy efficiency	0.5 g kWh⁻¹ (ns pulsed)	Cu oxide regrowth (Pottkämper et al., 12 May 2025)
Interfacial eₐq⁻ conc.	1 mM (depth 10 nm)	Anode-biased, Ar plasma (Lee et al., 2024)

Faradaic efficiency for NH₃ is

$$\mathrm{FE} = \frac{n_e\,F\,\dot{n}_{\mathrm{NH}_3}}{I} \times 100\%$$

where $n_e=6$, $F=96485$ C mol⁻¹, $\dot{n}_{\mathrm{NH}_3}$ is the molar NH₃ rate, and $I$ is the total current. Energy efficiency ($\eta_E$) is computed relative to reaction free energy and input electrical work, yielding $\eta_E \approx 2$–3% for NRR at $V_\mathrm{cell}\sim200$–230 V (Grosse et al., 26 Jun 2025).

For peroxide generation in Cu systems, nanosecond pulsing delivers 2–3× higher energy efficiency than microsecond pulses, as shorter durations avoid thermal decomposition. Bulk temperature remains near 20 °C in ns mode, compared to up to 45 °C in μs mode (Pottkämper et al., 12 May 2025).

6. Design Considerations and Reactor Optimization

Optimization of in-liquid plasma catalytic systems involves several interdependent variables:

• Electrode polarity: Anode bias localizes reducing eₐq⁻ within a thin layer for selective reduction chemistry; cathode bias maximizes oxidative flux and H₂O₂ generation (Lee et al., 2024).
• Pulse regime: Nanosecond pulses are preferred for peroxide-mediated catalyst regeneration and for suppressing bulk heating; microsecond and DC arcs serve when higher radical densities or thermal activation are needed (Pottkämper et al., 12 May 2025).
• Catalyst support: In NRR, use of Pt wire maximizes both plasma resilience and electrocatalytic activity (Grosse et al., 26 Jun 2025); for CO₂RR, plasma-recrystallized Cu₂O nanocubes offer sustained active surfaces (Pottkämper et al., 12 May 2025).
• Gas/sheath dynamics: Management of gas–vapor sheath thickness and composition is critical; optimal N₂ flow (0.07 L min⁻¹) maintains sheath stability and high NH₃ yields (Grosse et al., 26 Jun 2025).
• Thermal control: External cooling and limiting discharge duration prevent electrolyte heating, suppressing competing decay reactions and outgassing.
• Catalyst regeneration: Intermittent plasma exposure between electrolysis cycles can mitigate surface deactivation and morphological collapse (Pottkämper et al., 12 May 2025).

A plausible implication is that such fine-tuning of pulsing, electrode geometry, and catalyst proximity enables precise control over the local radical population, redox lifetimes, and catalytic turnover, supporting adaptive operation for diverse chemical transformations.

7. Challenges, Limitations, and Outlook

Current limitations in in-liquid plasma catalytic cells include:

• Electrode lifetime: Only Pt demonstrates long-term stability under hot-plasma NRR; non-noble metals corrode rapidly (Grosse et al., 26 Jun 2025).
• Energy efficiency: Absolute yields and Faradaic efficiencies remain an order of magnitude below those required for large-scale deployment; improvements in plasma coupling, sheath uniformity, and heat management are required (Grosse et al., 26 Jun 2025).
• Catalyst–radical interface engineering: For CO₂ reduction, in situ performance metrics (current density, rate constants, selectivity) for plasma-regenerated catalysts are not yet established (Pottkämper et al., 12 May 2025).
• Scalability: Extension to parallelized electrodes and optimized heat removal is needed to achieve current densities and productivities relevant for distributed manufacturing (Grosse et al., 26 Jun 2025).

Future work must address quantitative accounting of all plasma-generated intermediates, direct measurement of nanoscale interfacial profiles under relevant operation, and integration of real-time diagnostics to inform closed-loop reactor control.

• (Lee et al., 2024) Solvated Electrons and Hydroxyl Radicals at the Plasma-Liquid Interface
• (Grosse et al., 26 Jun 2025) In-liquid Plasma Catalysis for Nitrogen Reduction
• (Pottkämper et al., 12 May 2025) Nanosecond and microsecond-pulsed plasma-in-liquid treated copper oxide surfaces