## 1 Introduction  
Ammonia is a vital chemical used primarily in agriculture and pharmaceuticals and has significant potential as a green energy carrier [1, 2]. However, natural nitrogen fixation is insufficient to meet global demand, necessitating reliance on the Haber-Bosch process [3]. This process operates at elevated temperatures and pressures, consumes significant energy, and contributes 2-3% of global CO\({}_{2}\) emissions [4, 5, 6, 7]. Although the process has seen improvements, it remains energy-intensive and unsuitable for small-scale or renewable energy production [8, 9, 10], driving research into more sustainable methods for ammonia synthesis. The unique properties of non-thermal plasma allow it to be integrated with distributed energy sources to produce ammonia via nitrogen reduction reactions, making it a promising method for ammonia synthesis [11]. Currently, the most commonly used non-thermal plasma-assisted ammonia synthesis is typically carried out in the gas phase, where the low product concentration complicates separation processes and requires hydrogen as a feedback [5, 12]. Increased ammonia synthesis rates and reduced energy consumption are generally achieved by plasma co-catalysis [13]. The emergence of plasma gas-liquid interaction systems addresses some of the limitations of gas-phase plasma by allowing products to dissolve directly in water, thereby avoiding decomposition and loss in the plasma discharge region [14]. In a typical plasma gas-liquid interaction, liquid water serves as the sole hydrogen source for ammonia and provides oxygen atoms, leading to complex reactions that produce nitrogen fixation products, including NH\({}_{4}\)\({}^{+}\) and NO\({}_{\mathrm{x}}\) mixed solutions [15]. At the beginning of the 21 st practical year, [PERSON] et al. [16] proposed the technology of plasma jet treatment of water for ammonia synthesis. In recent years, several approaches have been reported to improve the energy efficiency of nitrogen fixation and NH\({}_{4}\)\({}^{+}\) selectivity of plasma gas-liquid reactions. These approaches include the addition of auxiliary devices [7, 17], modification of the solution properties [18], and the use of catalysts [19]. [PERSON] et al. [20] used a spray plasma jet and UV irradiation to enhance the ammonia synthesis rate and selectivity, achieving an ammonia yield of 2.5 \(\upmu\)mol\(\cdot\)min\({}^{-1}\) and a selectivity of 4.32%. In the study by [PERSON] et al. [4], the ammonia selectivity could be effectively enhanced by adding water vapor to the N\({}_{2}\) plasma jet and showed that excellent ammonia selectivity (95%) could be obtained by adding small amounts of water vapor. [PERSON] et al. [7] synthesized ammonia using a hybrid electrolysis system combining plasma jet and electrolysis at atmospheric pressure and temperature, with a yield of 0.43 \(\upmu\)mol\(\cdot\)min\({}^{-1}\). It was found that the formation of solvated electrons at the gas-liquid interface also has the potential to achieve high selectivity for ammonia. Recently, [PERSON] et al. [21] synthesized ammonia using a DBD plasma in synergy with a catalyst, with water vapor acting as a hydrogen source, which greatly enhanced the ammonia synthesis rate (0.201 \(\upmu\)mol\(\cdot\)min\({}^{-1}\)). In this study, we investigated a water film-dielectric barrier discharge (WF-DBD) reactor for the selective synthesis of NH\({}_{4}\)\({}^{+}\) by direct contact between N\({}_{2}\) plasma and H\({}_{2}\)O. A highly selective (80.7%) and high-rate (12.69 \(\upmu\)mol\(\cdot\)min\({}^{-1}\)) synthesis of NH\({}_{4}\)\({}^{+}\) was achieved by regulating the gas flow and adjusting the voltage. Various spectroscopic measurements and characterizations were used to analyze correlations between product selectivity and reactive intermediates.