Surface and Interface Structure Design of Nanomaterials for Efficient Heterogeneous Electrocatalysis in Liquid Solution

Increasing demands on energy resources have largely constrained the future development of modern society. Currently, people rely strongly on the combustion of fossil fuels creating massive CO2 and causing severe greenhouse effects. Hence, the expansion of renewable energy technologies becomes necessary for the next generation of industrialization. In many novel approaches for solving these growing concerns, converting earth-abundant chemicals into chemical fuels and utilizing generated fuels in fuel cells with cost-effective electrocatalysts have been considered as a substitution of fossil fuels. In Chapters 1 and 2, recent progress and research methods of electrocatalysis for the electrochemical energy conversion and storage in liquid solution have been summarized in detail. It has been widely recognized that the electrocatalyst surface and interface structure play a deceive role in their electrocatalytic performance. However, the studies on the surface and interface of electrocatalyst are still at their very early stage, and the advancements in new materials need to be made available in large quantities at low cost while satisfying various harsh industry conditions is a key part of developing electrochemical energy conversion and storage technology. Chapter 3 describes the development of partial oxidation methods to synthesize δ-MnO2/Mn3O4 nanocomposites with a tunable surface electronic structure. The δ-MnO2/Mn3O4 nanocomposites exhibit significantly improved ORR activity with a half-wave potential of 750 mV vs. RHE, which is ~110 mV and ~90 mV lower than those of the Mn3O4 nanocrystal and the δ-MnO2 nanoflakes in their pure forms, respectively. Chapter 4 focuses on improving the selectivity of CO2 reduction by the atomically dispersive Ni active atoms. The Ni single atom electrocatalyst possesses the maximum CO FE of over 95% at −1600 mV vs. Ag/AgCl, which is about 30% higher than the standard Ni nanoparticles on the nitrogen-doped carbon nanofiber. In Chapter 5, we develop an electrocatalyst of metal nitride nanosheets by controlling the composition, atomic and electronic structure, and morphology for efficient ammonia oxidation reaction (AOR). The AOR onset overpotential of NiCo2N nanosheets is 550 mV, which is about 250 mV lower than that of the Pt/C electrocatalyst. Our ultraviolet-visible and mass spectroscopy results reveal that the NiCo2N nanosheets bypass the formation of the soluble metal-amine complex and preferentially oxidize ammonia to environmentally friendly diatomic nitrogen with a Faradic efficiency of over 90%. Moreover, High entropy material offers a surface atomic structure that cannot be obtained previously by virtue of engineering surface through directly controlling element compositions. These materials represent a new direction in materials research because their diverse compositions can resolve some of the long-standing all-in-one bottlenecks to tune the multiple chemical reaction process in the electrocatalyst industry. In Chapter 6, we report that the high entropy (Mn, Fe, Co, Ni, Cu)3O4 oxides can achieve a high electrocatalytic activity for AOR in non-aqueous solutions. In Chapter 7, I summarize the main scientific achievement and contribution of this dissertation is that we have introduced several well-defined surfaces and interface structures of nanomaterials to significantly enhance their electrocatalytic performance, which paves the way for efficient energy conversion technology and beyond.