The catalysts play an important role in energy devices including fuel cell and Li-air batteries. Usually, platinum (Pt) and platinum alloy (Pt alloy) are used in these devices, due to their good catalytic performance such as high oxygen reduction reaction (ORR) activity and high onset potential. However, Pt is very expensive in comparison with the other metal and non-metal catalysts, which limits commercialization of energy technologies. Cost-effective perovskite structure transition-metal oxide catalysts have shown reasonably high activity for the ORR as well as high electronic and ionic conductivity.[1] Especially, LaMnO_3+δ exhibits the highest ORR activity value among perovskite catalysts because it satisfies the Sabatier’s principle as possessing approximately 1 electron in e_g band of d orbital.[2]

In the catalytic reaction, nanometer-sized (<10 nm) particles possess a large surface area which leads to a high catalytic efficiency; however, it is hard to achieve nanometer-sized particles from typical chemical reaction synthesis. Laser ablation synthesis in solution (LASiS), which is a simple, fast, and clean process, is suitable to achieve nanometer-sized pure LaMnO_3+δ particles by controlling laser parameters. In this study, we fabricated the LaMnO_3+δ catalyst powder which has large surface area for improving catalytic efficiency using LASiS. Also, we compared the ORR efficiency among synthesized LaMnO_3+δ/GCB powder, LaMnO_3+δ bulk, and Pt/C through a rotation disk electrode (RDE) method.

The LaMnO_3+δ target for LASiS was prepared using solid state reaction with La₂O₃ and MnO₂ powder. The powder was put in zirconia jar with isopropyl alcohol (IPA) and zirconia ball, and then mixed using planetary ball mill for 120 minutes. Mixed powder was dried at 100°C in oven and then calcined at 900°C for 12 hours under air atmosphere in furnace. A disk- pellet was made using cold isostatic pressure (CIP) method. The pellets were sintered at 1200°C for 24 hours under the air atmosphere. The target soaked in ethanol was ablated to a Q-switched Nd:YAG pulsed laser source providing a 8-ns pulse width with wavelength of 266 nm with repetition rate of 10 Hz. The laser energy and spot size were around 65 mJ/pulse and 1.5 mm respectively, which corresponded to a laser energy density of 3.67 J/cm². Colloidal solution from LASiS was mixed with 6M HCl treated GCB before drying at 80°C in oven and then obtained powder were annealed at 500°C to 800°C for 30 minutes under the air atmosphere in furnace. We found that the synthesized LaMnO_3+δ/GCB powder annealed at 800°C for 30 minutes shows the highest ORR activity. Its half wave potential value was around 0.83 V versus reversible hydrogen electrode (vs. RHE), corresponding to around 150 mV higher than bulk LaMnO_3+δ. Also, the conventional Tafel plot results indicate that the ORR efficiency of LaMnO_3+δ/GCB catalyst is similar to that of Pt/C 20 wt%. Specific activity at 0.8 V vs. RHE was improved by more than 15 times in comparison to bulk LaMnO_3+δ.

 

[1] D. Chen et al., Chem. Rev. 115, 9869 (2015).

[2] J. Suntivich et al., Nature Chemistry. 3, 546 (2011).