Facet Engineering of Low-Cost Transitional Metal Oxides for Tunable ORR/OER Kinetics

Abstract

To meet future needs for industries from personal devices to automobiles, energy storage devices are transitioning into the next-generation advanced battery systems beyond lithium ions with higher energy density. Among them, lithium-oxygen (Li-O2) battery stands out due to its highest theoretical energy density among all rechargeable ion battery systems. Yet, its application is severely disabled by the sluggish reaction kinetics during oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). To address this problem, various catalysts have been developed with the most promising materials being the noble metals (Pt, etc.) and transitional metal oxides (TMOs such as CoxOy, MnxOy, etc.). While the noble metal catalysts offer superior performance in terms of improving the reaction kinetics, the high cost and environmental sensitivity prevent the large-scale application. As an alternative, TMOs are attracting more and more interest because they exhibit satisfactory catalytic performance while their cost and cycling durability are much better than that of noble metals. While most studies have focused on the development of different TMO structures, phases and compositions, few studies have reported the advancement in the crystalline facet engineering, particularly for the low-cost TMOs such as manganese oxides, and its effect on ORR/OER kinetics. In this work, beta-(β-)MnO2 crystals with either octahedron or rod-like shapes were successfully synthesized separately using a simple hydrothermal method. Combining various electron microscopic methods, the octahedron’s surface is seen to mostly occupied by MnO2-{111} facets, while rod’s surface is occupied by MnO2-{100}. As the cathode for Li-air batteries, the β-MnO2 with {111} facets and {100} facets catalyzed the formation of Li2O2 into large toroids and thin film, respectively. For the first time, we revealed that the control of crystal facets can switch the Li2O2 formation mechanism from the surface mode to the solution mode even in the low donor number TEGDME electrolyte. To further understand the formation kinetics of Li2O2, we referred to operando liquid cell transmission electron microscopy, by which a LiO2 phase was identified to function as the intermediate discharge products which significantly affect the formation of Li2O2 as the final product. Following this finding, DFT results further showed that the different catalytic properties were due to the different adsorption free energies for the LiO2 intermediate on the {111} and {100} facets of MnO2. Our findings revealed that facet-engineering of the low-cost TMO catalysts would open a new way for the design of lithium-air batteries with high stabilities and long cycle life. Figure 1