Improved ORR/OER bifunctional catalytic performance of amorphous manganese oxides prepared by photochemical metal–organic deposition

Abstract

Transition metal oxide nanomaterials or nanocomposites containing transition metal oxides have the potential to replace traditional catalysts for electrochemical applications, photocatalysis, and energy storage. Amorphous manganese oxide catalysts were prepared via photochemical metal–organic deposition (PMOD). Through XRD, SEM-EDS, Raman spectroscopy, FTIR spectroscopy, HRTEM-EDS, and XPS, we confirmed that amorphous manganese oxide catalysts were successfully prepared. Amorphous catalysts prepared with different photolysis times were compared in terms of their performance for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), and catalyst MnO_x-PMOD48 showed the best performance because of its high Mn³⁺ proportion and electrochemically active surface area. MnO_x-PMOD48 showed better ORR/OER performance than the crystalline MnO_x and MnO_x/Ti₄O₇ catalysts from our previous work. Following our previous work on crystalline manganese oxide catalysts, we added Ti₄O₇ during the PMOD process with 48 h of treatment and obtained the amorphous catalyst MnO_x/Ti₄O₇-PMOD. MnO_x/Ti₄O₇-PMOD was supported by Ti₄O₇ particles, which led to improved stability. The ORR/OER catalytic activity of MnO_x/Ti₄O₇-PMOD was better than that of crystalline catalyst MnO_x/Ti₄O₇-300, which was the best crystalline catalyst in our previous work. We also compared lithium-oxygen batteries assembled with MnO_x/Ti₄O₇-PMOD and MnO_x/Ti₄O₇-300. The battery performance tests confirmed that the amorphous manganese catalyst had better ORR/OER bifunctional catalytic performance than the crystalline manganese catalyst because of its high defect state with more abundant edge active sites and more surface-exposed catalytic active sites.