Study on optical emission analysis of AC air–water discharges under He, Ar and N2 environments

Abstract

In this paper, hybrid air–water discharges were used to develop an optimal condition for providing a high level of water decomposition for hydrogen evolution. Electrical and optical phenomena accompanying the discharges were investigated along with feeding gases, flow rates and point-to-plane electrode gap distance. The experiments were primarily focused on the optical emission of the near UV range, providing a sufficient energy threshold for water dissociation and excitation. The OH(A 2Σ+ → X 2Π, Δν = 0) band optical emission intensity indicated the presence of plasma chemical reactions involving hydrogen formation. Despite the fact that energy input was high, the OH(A–X) optical emission was found to be negligible at the zero gap distance between the tip of the metal rod and water surface. In the gas atmosphere saturated with water vapour the OH(A–X) intensity was relatively high compared with the liquid and transient phases although the optical emission strongly depended on the flow rate and type of feeding gas. The gas phase was found to be more favourable because of less energy consumption in the cases of He and Ar carrier gases, and quenching mechanisms of oxygen in the N2 carrier gas atmosphere, preventing hydrogen from recombining with oxygen. In the gas phase the discharge was at a steady state, in contrast to the other phases, in which bubbles interrupted propagation of the plasma channel. Optical emission intensity of OH(A–X) band increased according to the flow rate or residence time of the He feeding gas. Nevertheless, a reciprocal tendency was acquired for N2 and Ar carrier gases. The peak value of OH(A–X) band optical emission intensity was observed near the water surface; however in the cases of Ar and N2 with a 0.5 SLM flow rate, it was shifted below the water surface. Rotational temperature was estimated to be in the range of 900–3600 K, according to the carrier gas and flow rate, which is sufficient for hydrogen production.