A hydrogen energy society is one of the solutions to overcome the problem of global warming and to build a sustainable society. If the electrochemically synthesis of highly efficient at low temperatures and atmospheric pressure, ammonia can be used as a hydrogen carrier using the electric energy generated by natural energies. However, there is no active nitrogen reduction catalyst at the conditions. In this study, electrochemical nitrogen reduction was performed under low temperature and atmospheric condition using carbon-supported WO_x/RuO_y nanoparticles prepared by a low-voltage low-frequency solution plasma method and a hydrogen reduction method.

2. Experimental

2.1 Synthesis of WO_x/KB

Ketjenblack (KB) supported WO_x (denoted as WO_x/KB) were prepared by two methods: the reduction of Na₂WO₄∙2H₂O by H₂ (denoted as -H₂) and a low-voltage and low-frequency solution plasma method (denoted as -LVLFSP).

RuO_y loading on WO_x/KB was performed as follows: each WO_x/KB was suspended in RuCl₃ aqueous solution, and was stirred at 80℃ for 24 h, followed by drying under vacuum to obtain each precursor. WO_x/RuO_y/KB-H₂ and WO_x/RuO_y/KB-LVLFSP were obtained by the heat-treatment of the precursors at 250℃ for 24 h under 300 mL/min of H₂ flow.

2.2 Electrochemical measurements

5 wt% Nafion^®-2-PrOH solution and Milli-Q water was added to the prepared catalyst, and the planetary ball mill was stirred for 180 rpm and 30 min to obtain a catalyst suspension. The suspension was coated with a spray method on a carbon paper and dried. The carbon paper and Nafion^® film (NRE-212) were thermally crimped at 130℃ and were incorporated into an electrolysis cell shown in Fig. 1(a). The gas flowing out of the cell was vented into two serially connected vials (4 mM methanesulfonic acid solution for NH₃ collection and only gas phase for H₂). The electrochemical reduction of nitrogen was carried out at 0.6 V vs. RHE for 1 h under 5 mLmin^-1 N₂ flow, followed by the electrolysis at –1.0 V vs. RHE. The amount of ammonia production was quantified by cation chromatography using an IC YS-50 (Shodex) column with 4 mM methanesulfonic acid as eluent at 40℃.

3. Results and Discussion

SEM-EDX revealed that each metal is uniformly dispersed on the KB. The bulks of WO_x/RuO_y/KB -H₂ and WO_x/RuO_y/KB-LVLFSP were amorphous and a WO₃ crystal phase, respectively. Diffraction peaks derived from Ru element were not observed in both the samples.

Fig. 1(b) shows the result of electrosynthesis of ammonia using WO_x/RuO_y/KB catalysts. The ammonia production rate of WO_x/RuO_y/KB-H₂ was about 2 times as large as that of WO_x/RuO_y/KB-LVLFSP, suggesting that the amorphous tungsten oxide suppresses a competitive proton reduction reaction．Faraday efficiency and energy conversion efficiency of both the catalysts at –0.6 V vs. RHE were larger than those of –1.0 V vs. RHE, probably owing to the increasing rate of the competitive reaction.