Study on the Manganese based Water Oxidation Catalyst

Abstract

Hydrogen energy is considered as the most promising alternative energy resources, due to its high energy density and environmentally friendly nature. Historically, hydrogen production has been mainly progressed by gas reforming process, which requires high pressure and high temperature and produces CO2 and other pollutants. Recently, electrochemical hydrogen production has drawn great attention because byproduct of the reaction is only oxygen gas. To operate overall electrochemical water splitting reaction, anodic oxygen evolution reaction (OER) is considered as the rate determining step. Four electrons and four proton involved reaction kinetics results in relatively large overpotential values, compared to cathodic hydrogen evolution reaction. For the decades a lot of research efforts have been dedicated to develop robust and efficient water oxidation catalyst. On the other hand, in nature, there exist water oxidizing complex of which active site is consist of manganese and calcium elements. On the contrary to the fact that noble metal based catalysts, IrO2, RuO2 materials are used in industry, nature chose to use earth abundant manganese and calcium to oxidize water. Moreover, surprisingly, catalytic efficiency of Mn4Ca cluster in photosystem II, is superior to previously reported synthetic catalysts. Such an exceptional performance of the biological system has inspired to study manganese based catalysts. However, unfortunately, manganese based water oxidation catalysts have suffered from the serious activity degradation problem under neutral condition. Although many researchers have tried to address this issue, fundamental resolution is not suggested yet. In this thesis, we report efficient and robust manganese based water oxidation catalysts. From the our designed manganese based catalysts, we believe that long lasting issue in Mn catalysts is completely resolved. In chapter 2, we discovered new crystal structure, manganese phosphate compound(Mn3(PO4)2-3H2O) as water oxidation catalyst. Due to the bulky phosphate groups, highly distorted crystal structure are generated. Computational analysis clearly revealed that phosphate ligations in structure could make relatively longer Mn-O bonding and more distorted geometry, compared to previously reported Mn-based oxides. Unique structural flexibility can stabilize Jahn-Teller distorted Mn(III) and thus facilitate Mn(II) oxidation, as verified by electron paramagnetic resonance spectroscopy. In chapter 3, as an another strategy to stabilize Mn(III) species on the catalyst surface, we developed nanosized manganese based water oxidation catalysts. Sub 10 nm sized manganese oxide nanoparticles were synthesized via hot injection method. The surface treatment and ambient heat treatment process activate manganese oxide nanoparticles which results in high catalytic efficiency under neutral pH. Catalytic performance of manganese oxide nanoparticles are the one of the best record among the that of first-row transition metal based catalysts. In chapter 4, From the various in-situ analysis and electrokinetic study, we demonstrated the detailed oxygen evolving catalytic cycle of manganese oxide nanoparticles, which is totally different from that of bulk Mn materials. Proton coupled electron transfer occurs before and during water oxidation catalysis and thus high valent Mn(IV)=O species can participate in the rater determining step for oxygen evolution. In chapter 5, We developed Ni-decorated Mn¬3O4 nanoparticles as new water oxidation catalysts. In this work, we newly discovered that under ambient annealing process, atomic nickel diffusion phenomena occurs at the metal oxide interface. With our methodology surface specific heteroatom doping can be possible. Ni-decorated Mn¬3O4 nanoparticles exhibit outstanding performance under neutral and basic condition. Interestingly, we discovered that during water oxidation catalysis, unique Mn status is generated on the catalyst surface. From the EPR analysis, we verified low-spin Mn(IV) species are formed, which has never been reported before. Computation study also supported that atomically doped Ni atom induced distorted Mn geometry in Mn¬3O4 nanoparticles structure and thus stabilize low Mn(IV) species. In conclusion, we discovered various manganese based water oxidation catalysts. Our designed catalysts could overcome the issues in manganese catalysts and finally achieved excellent performance for oxygen evolution catalysis.