An investigation into the manganese oxide nanoparticles for efficient oxygen evolution reaction: Effect of crystal structure and nano size on the catalytic activity

Abstract

Development of outstanding oxygen evolution reaction(OER) catalyst is essential for efficient hydrogen gas production which is attractive sustainable energy in the future. For this reason, various manganese oxides with bulk scale have been developed and investigated. However, OER activity of conventional manganese oxides in neutral condition is still poor. Thus, our group solved the low activity issue of bulk manganese oxides by introducing MnO nanoparticles in previous research. In this research, we tried to investigate the effect of crystal structure on the OER activity of manganese oxide nanoparticles for more activity enhancement. To clearly see the effect of crystal structure, manganese oxide nanoparticles with various crystal structure(Mn3O4, Mn5O8 and Mn2O3) were obtained with little morphology change via the controlled oxidation of MnO nanoparticles. The oxygen evolution reaction (OER) properties of manganese oxide nanoparticles were evaluated using cyclic voltammetry (CV). Mn3O4 and Mn2O3 nanoparticles showed similar activity and Mn5O8 nanoparticles showed less active property than Mn3O4 and Mn2O3. This observation indicates that effect of crystal structure on OER activity of Mn oxides is significantly reduced in nano scale. To understand the origin of different OER activity between Mn5O8 and other nanoparticles, electrokinetics and redox peak analysis were conducted. As a result, Mn5O8 showed different mechanism (1 electron, 2 proton involved in the reaction before rate determining step) from Mn3O4 and Mn2O3 (1 electron, 1 proton involved in the reaction before rate determining step), which may cause less active property of Mn5O8. Next, we found that manganese oxide nanoparticles have much higher OER activity than bulk manganese oxide materials. Thus, Mn5O8 materials, which has never been investigated as OER catalyst, were further studied to understand why nanoparticles are more active catalyst than bulk manganese oxides. BET analysis and mass activity comparison clearly showed that Mn5O8 nanoparticles have much higher catalytic activity than micron sized Mn5O8 although surface area effect is considered. In addition, catalytic stability is also enhanced in nanoparticles. These results imply that intrinsic catalytic property can be improved as particle size becomes nanoscale. To find the clue of this intrinsic difference, we conducted EPR analysis to observe valence behavior of Mn5O8 materials. As a result, we found that Mn5O8 nanoparticles and micron sized particles have different Mn3+ stability during OER catalysis which may cause intrinsic OER property difference between Mn5O8 materials. In this research, we could understand the effect of crystal structure and nano size on OER activity. Understanding those effect is important because they will be a valuable guide to develop outstanding nano sized OER catalyst in neutral condition. Therefore, further research should be conducted to understand completely the effect of crystal structure in nano scale and nano size on the OER activity.