Electrochemical water-splitting is a promising pathway for producing sustainable and environmentally friendly hydrogen energy. The oxygen evolution reaction (OER) generally requires high overpotential due to the slow reaction kinetics to overcome. In photosystem II, water oxidation complex (WOC) composed of non-precious element Mn and Ca, efficiently catalyzes OER. Inspired by Mn4CaO5 cluster in nature, we developed Mn3(PO4)2-3H2O catalysts and revealed that its asymmetric structure can stabilize Jahn-Teller distorted Mn(III) leading to the facile Mn redox during OER. Also through delithiation process in LiMnP2O7 catalysts, we verified the importance of Mn(III) species on water oxidation catalysis. On the basis of these findings, our group synthesized partially oxidized sub-10 nm MnO NPs which show superior catalytic performance compared to the bulk manganese oxides. Also clear size dependency on OER performance enhancement was observed in size range from 60~80 nm to 10 nm. In addition, stabilization of Mn(III) species on the surface of MnO NPs was confirmed through EPR analysis. Furthermore, we verified completely different OER mechanism of our MnO NPs compared to the conventional Mn-based catalyst via various electrochemical analysis and in-situ spectroscopic tools such as XAS, UV-VIS and Raman. Herein, we extend our understanding on nano-effects to nano-scale whether the OER behavior will be similar or different among 4 nm and 10 nm Mn-oxide NPs. 4 nm Mn3O4 NPs were successfully synthesized via simple, one-pot thermal decomposition method through modification on previously reported 6 nm Mn3O4 NPs. OER activity of 4 nm Mn3O4 NPs increased up to 45% compared to 10 nm Mn3O4 NPs at 1.3V (vs. NHE). However, there was no change in OER mechanism between 4 nm and 8 nm Mn3O4 NPs. Finally, to verify the origin of the enhanced activity in 4 nm Mn3O4 NPs, electrochemically active surface area (ECSA), electrochemical impedance spectroscopy (EIS) and redox analysis were implemented. Consequently, increased active surface area was confirmed in 4 nm Mn3O4 NPs and we believe 4 nm NPs can serve as a proper platform for designing efficient electrocatalysts.

Uniform, assembled sub-5 nm Mn3O4 nanoparticles as water oxidation electrocatalysts at neutral pH

2019 Fall National Meeting

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