Abstract
Although ammonia is expected as the next generation of energy medium, even platinum, which is a relatively active ammonia oxidation (AO) catalyst, have an insufficient performance for direct ammonia fuel cells (DAmFCs) operating below 100°C. Highly active AO catalysts at low temperatures needs to be developed in order to use ammonia as a fuel for DAmFCs. AO mechanism on platinum is reported as follows: After NH3 is adsorbed on the platinum surface, the AO proceeds via two competitive paths. One is the formation of hydrazine analogues, followed by the oxidation of the analogues to release N2 molecules. The other is the oxidation of an ammonia molecule to form nitrogen atoms adsorbed onto the surface of platinum (Nads) known as poisoning species. However, after the surface of the platinum is covered with Nads, the elimination process of Nads is not known. In this study, the elimination process was studied by a normal pulse voltammetry.
33.6 wt% Pt/MWCNT (average particle size: 5.57 ± 0.11 nm) catalyst was synthesized by a microwave polyol method. A 2 mg of Pt/MWCNT was suspended in 1 mL of Pt/MWCNT-0.1 wt% Nafion-MeOH by ultrasonication for 10 min. 10 μL of the suspension was cast onto a glassy carbon (GC) disk electrode (6 mm φ), followed by drying for 1 h in air. Electrochemical measurements were performed with a conventional three electrode system equipped with the modified GC electrode, an Au wire counter electrode and a reversible hydrogen electrode (RHE) in 0.1 M KOH or 0.1 M NH3-0.1 M KOH. The normal pulse voltammetry was performed as follows: After the Nads was formed by applying 0.6 V for 180 s, Erecover (V) was applied for various times (1 s，5 s，10 s，20 s，60 s, and 180 s) to eliminate Nads. Then, the AO current at 0.6 V after 1 s was used as an indicator for the amount of Nads poisoning.
Fig. 1 shows the dependence of AO current at 0.6 V after 1 s on recovery potential. The AO current gradually recovered by maintaining the potential below 0.4 V vs. RHE, indicating that the adsorbed N atoms were eliminated from the catalyst surface. It took a few minutes to recover the AO current. In addition, the decrease in potential accelerated the recovery of the AO current, especially below 0.3 V vs. RHE. The final percentage of the recovery increased with a decrease in the potential. The recovery rates below 0.25 V called hydrogen adsorption/desorption region increased with decreasing potential, probably due to the reduction of Nads. The time dependence of the recovery rate was analyzed by the sum of two exponentials, suggesting that the fast and slow reactions exists in the recovery process.
Figure 1