The Developments of Highly Active and Durable Nanomaterials for Electrocatalysts

Abstract

This thesis represents research on the fabrication of nanoparticles for electrocatalysis application. It explores their synthesis process, physical, electronic and electrochemical properties. Chapter one provides a brief introduction to the exploration of nanomaterials and their applications for electrocatalysts. Chapter two describes the study about metal nanocrystals (NCs) with controlled compositional and distributional structures that have gained increasing attention due to their unique properties and broad applications, particularly in fuel cell systems. However, despite the significant importance of composition in metal NCs and their electrocatalytic behavior, comprehensive investigations into the relationship between atomic distribution and electrocatalytic activity remain scarce. In this study, we present the development of four types of nanocubes with similar sizes and controlled compositions (Pd– Pt alloy, Pd@Pt core–shell, Pd, and Pt) to investigate their influence on electrocatalytic performance for methanol oxidation reaction (MOR). The electrocatalytic activity and stability of these nanocubes exhibited variations based on their compositional structures, potentially affecting the interaction between the surface-active sites of the nanocrystals and reactive molecules. As a result, leveraging the synergistic effect of their alloy nanostructure, the Pd–Pt alloy nanocubes exhibited exceptional performance in MOR, surpassing the catalytic activity of other nanocubes, including Pd@Pt core–shell nanocubes, monometallic Pd and Pt nanocubes, as well as commercial Pd/C and Pt/C catalysts. Chapter three discusses the exploration of efficient nanocatalysts with high activity and stability towards water electrolysis and fuel cell applications is extremely important for the advancement of electrochemical reactions. However, it remains challenging. Controlling the morphology of bimetallic Pd–Pt nanostructures can be a great way to improve their electrocatalytic properties compared with previously developed catalysts. Herein, we synthesize bimetallic Pd–Pt nanodendrites, which consist of a dense matrix of unsaturated coordination atoms and high porosity. The concentration of cetyltrimethylammonium chloride was significant for the morphology and size of the Pd–Pt nanodendrites. Pd–Pt nanodendrites prepared by cetyltrimethylammonium chloride (200 mM) showed higher activities towards both the hydrogen evolution reaction and methanol oxidation reaction compared to their different Pd–Pt nanodendrites counterparts, commercial Pd, and Pt catalysts, which was attributed to numerous unsaturated surface atoms in well-developed single branches. Chapter four deeply study about ultrathin two-dimensional (2D) metal nanosheets have attracted significant attention in the field of electrocatalysis. However, only a few of ultrathin nanosheets with limited elements were developed due to difficulties in restraining three-dimensional (3D) growth of metals and stabilizing unstable edge sites. Herein, for the first time, we present a universal synthetic approach for edge-rich holey ultrathin Pt3M alloy nanosheets (Pt3M HU-NSs, where M = Ni, Co, Cu, Ir, Pd, Ru, Rh, Fe, and Mn) with approximately 3 nm thickness and abundant edge sites through a two- step template-based method involving precisely controlled Pt reduction rates and thermal treatment. The Pt3Ni HU-NSs displayed significantly enhanced oxygen reduction reaction activity and stability compared to other Pt3M HU-NSs, pure Pt HU-NSs, and the state-of-the-art Pt/C catalysts, attributing to the retention of distinctive morphology and composition of Pt3Ni HU-NSs. We believe that highly active ultrathin 2D alloy structures with abundant edge sites and desirable catalytic functions offer a promising avenue for the development of advanced electrocatalysts. Chapter five study about the exploration of tuning the crystal structure of bimetallic nanocrystals, especially Pd-based nanocatalysts with high activity and stability toward water electrolysis application, is essential for improving electrochemical reactions. Introducing the foreign element into the lattice of Pd-Pt nanocrystals (NCs) can be a great way to modify their neighbor lattice environment and improve their electrocatalytic properties compared with their traditional structure (fcc) of Pd-Pt NCs. Herein, we introduce the B insertion into PdPt NCs, which produces the change of crystal structure from fcc-to- hcp for the hydrogen evolution reaction (HER). We experimentally achieved this with the highly crystalline of PdPt–B NCs. Due to the presence of B atoms, the PdPt–B NCs exhibit higher activities and are incredibly stable towards HER compared to their pristine PdPt NCs, Pt/C, and Pd/C catalysts.