Improving Plasma-Catalytic Ammonia Synthesis Using a Coaxial Double-Helix-Electrode Reactor

Developing energy-efficient ammonia synthesis under mild and carbon-neutral conditions remains a major challenge for sustainable nitrogen fixation. Here, we present a coaxial double-helix-electrode-based double-dielectric barrier discharge (DBD) reactor, termed a "double-helix" design, featuring dual quartz barriers and symmetric high-voltage and grounded electrodes to achieve uniform, high-intensity volume discharge for plasma-catalytic ammonia synthesis. Three-dimensional electrostatic simulations demonstrate that this configuration generates a strongly coupled and spatially homogeneous electric field (∼7 × 10⁶ V m^-1), significantly outperforming conventional single-dielectric DBD designs (∼1 × 10⁶ V m^-1). An optimized Ni electrode with a 1 mm winding pitch increases electron density, as evidenced by optical emission spectroscopy (OES, I_N2+(425 nm)/I_N2*(335 nm) = 0.15). Under plasma-only operation, the double-helix DBD reactor produces approximately 2.5-fold higher NH₃ concentration than a conventional DBD at identical power input. When integrated with a Ni/Al₂O₃ catalyst, synergistic plasma-catalyst interactions further enhance ammonia yield and energy efficiency, achieving an energy yield of up to 3.68 g NH₃ kWh^-1 under 5.92 W discharge. Comprehensive analysis combining electric-field simulations, transient discharge imaging, and catalytic performance measurements elucidates the intrinsic coupling between electrode architecture, discharge physics, and catalytic function. This work demonstrates that electric-field engineering is an effective strategy for enabling stable volume discharge and enhancing plasma-catalytic ammonia synthesis, offering a generic design principle for next-generation low-carbon nitrogen-fixation systems.