Electrodeless discharge in water: Reactive species in liquid and gas phase and energy cost for nitrogen fixation

Abstract

AbstractOne of the main barriers stopping plasma activated water (PAW) in many practical applications is its high energy cost. On the other hand, a careful review of published literature finds that all the studies reported on the PAW only focus on the reactive species generated in liquid, and there is no measurement of the possible reactive species in the gas phase during the generation of PAW, which might contain high concentration reactive species especially for the cases when gas bubbling is used during the generation of PAW. In this paper, an electrodeless plasma device is reported, where distilled water, tap water, and saline water are used as working liquid while O2, N2, and air are flowing through the liquid. Not only the reactive species presented in the liquid phase, the reactive species contained in the gas phase were analyzed for the first time in the field of PAW nitrogen fixation. It was found that the total number of moles of reactive species in gas phase is actually several times higher than that in liquid phase. Only when considering the reactive nitrogen species (RNS) in the liquid phase, the lowest energy cost is about 18.97 MJ/mol when air is flowing though the tap water. On the other hand, when both the RNS in liquid phase and in gas phase are included, the lowest energy cost achieved is 5.53 MJ/mol when air is flowing through distilled water, which is only one‐seventh of the lowest energy cost reported on PAW. This study helps us shed light on plasma nitrogen fixation by using PAW, which has been greatly underestimated due to the overlooking of gas phase products of PAW in the past.