THE EFFECT OF ALTERNATING DISTRIBUTION OF TRANSITION METALS IN LAYERED MATERIALS ON OXYGEN EVOLUTION CATALYSIS

Abstract

The goal of this project is the design of heterogeneous catalysts to facilitate the oxygen evolution reaction (OER). Considering the industrial feasibility for this reaction, first-row transition-metal-based materials are good candidates since they are cheap, abundant and possess variable oxidation states. However, most of them give only moderate catalytic activities, compared with noble-metal-based materials. To achieve efficient catalysts while maintaining low cost, it is important to discover and modify new systems based on the study of existing materials.In chapter 3 we present a study of the effect of surface reduction of birnessite on catalytic activity. A sample of birnessite was reduced by stirring with sodium dithionite, in which case the oxidation states of surface Mn decreased faster than those of inside Mn. We characterized the difference between the oxidation states of Mn of surface and inside (ΔAOS) and further investigate the effect of ΔAOS on catalysis. The catalytic activity was examined by reaction of birnessite samples with ceric ammonium nitrate, and O2 evolution was monitored using a dissolved oxygen probe with respect to time. The most reduced samples with ΔAOS of 0.15 was found to possess a turnover number (TON) of 36 mmol O2 per mol Mn, a value 10-fold higher than the unmodified sample. This result suggests oxidation state differential across layers aids the catalysis. In chapter 4, a more rigorous study is conducted by the examination of few-layer catalysts constructed by manganese oxide sheets with different oxidation states. We stacked low-AOS manganese oxide sheets with high-AOS manganese oxide sheets in various ordered combinations to obtain few-layer birnessite samples with non-uniform distribution of Mn(III). We found samples with more variation in AOS had a lower overpotential (~510 mV) in electrochemical OER catalysis than uniform stacks of the parent manganese oxide sheets (~750 mV for low-AOS sheets, >1000 mV for high-AOS sheets. The result indicates that the distribution of Mn(III) in stacking direction was the dominant factor for OER catalysis in birnessite and is more important than the overall Mn(III) content. We also found the band structures via scanning tunneling microscopy (STM) and provide an electronic-structure-based explanation of the observed activity. In chapter 5 an analogous strategy to that used in chapter 4 is applied to optimize lithium cobalt oxide (LCO) and lithium nickel oxide (LNO) layered catalysts. LCO and LNO contains various oxidation states (or spin states) of cobalt and nickel atoms. With alternatively stacking a high-AOS and a low-AOS cobalt (or nickel) oxide sheets one by one, the electrochemical OER catalytic activity of the obtained few layer LCO (or LNO) sample was enhanced. The results indicated that the structural feature of the alternating distribution of oxidation states affected not only the birnessite catalysts but also both cobalt and nickel oxide materials. In chapter 6 we incorporated both cobalt and nickel oxide sheets into layered heterostructured catalysts. We present findings that mixed transition metal oxide material K-CoxNiyO2 with alternating distribution of cobalt and nickel oxide layers showed enhanced activity mixed Ni-Co metal oxides with homogeneously distributed transition metals. The overpotential of the sample K-Co0.5Ni0.5O2 with alternating distribution of Co and Ni is 460 mV, 190 mV smaller than that of the sample with homogeneously distributed Co and Ni, even though they had a similar elemental composition.