Interfacial engineering of 3D metal-based electrocatalysts for energy conversion

• Main Author: Lan, Yang
• Other Authors: Alex Yan Qingyu
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2022
• Subjects: Engineering::Materials::Energy materials
• Online Access: https://hdl.handle.net/10356/163605
• Institution: Nanyang Technological University

Considering the increasing environment problems, the development of efficient clean energy to replace fossil fuels is the trend of today’s world. Hydrogen (H2) and ammonium (NH3) are promising candidates with high energy density and environment friendliness. To achieve large-scale production of H2 and NH3, the electrocatalytic water splitting and nitrogen fixation under ambient condition paves encouraging avenue in the field. However, the electrocatalysts for accelerating the reaction speed and overcoming the reaction barrier are mainly focused on noble metals, such as Ir, Ru, Pt, Au, etc. The high-cost of these noble metals significantly hinders their large-scale application, which triggers the research into highly efficient non-noble-metal-based catalysts. In the past few years, 3d transition metal-based materials have attracted tremendous attention due to their outstanding catalytic performance and simple synthesis process. However, the performance of these materials is still far from satisfaction for industrial application, and the mechanisms are not deeply understood. Therefore, this dissertation focuses on the interfacial engineering including heterostructure engineering, surface reconstruction and surface morphology modification of the 3d metal-based electrocatalysts to optimize their catalytic activity towards water splitting and nitrogen fixation, and exploration of the reaction mechanisms. In the first research work, the amorphous Ni-doped cobalt phosphates (Ni-CoPi) with highly ordered mesoporous structure have been synthesized. The design of amorphous structure improves the density of active sites beneficial from its short-range ordering. The highly ordered mesopores enlarges the surface area of the materials and help expose more active sites. After the electrochemical oxidation, an amorphous layer of Co (oxy)hydroxides in-situ formed on the surface of the cobalt phosphates and serves as true active species. This layer of Co (oxy)hydroxides exhibits a decent overpotential (η10) of only 320 mV at the current density of 10 mA cm-2 and a small Tafel slope of 44.5 mV dec-1 in 1M KOH. The surface reconstructed material also shows remarkable durability for 20h in the alkaline media. A cyclic voltammetry (CV) method was carried out to explore the significance of Ni doping and it turns out that Ni improves the synergistic work and facilitates the surface reconstruction of cobalt phosphates. In the second part, Co2P|Co4N heterojunction nanosheets have been prepared. The heterostructure engineering significantly exposes more active sites, facilitates the charge transfer and improves the synergistic work. Surface reconstruction results in-situ generated amorphous CoOOH (SRA-CoOOH) layer on the Co2P|Co4N surface, which exhibits a decent η10 of 290 mV and a small Tafel slope of 55.6 mV dec-1 toward OER in alkaline media. Chemical probe is used to explore the reaction pathway of OER and proves that the surface reconstructed catalysts take the direct O-O coupling lattice oxygen mechanisms (LOM). A kinetics modeling reveals that the Gibbs free energy of the rate determining step (RDS) is significantly lowered through the LOM pathway, which facilitates the OER performance. Moreover, Cr doping boosts the HER performance of the Co2P|Co4N catalysts with a η10 of 120 mV in the alkaline media. When assembled with the SRA-CoOOH, the two-electrode system exhibits remarkable overall water splitting activity, requiring a small cell voltage of 1.58 V to deliver the current density of 50 mA cm-2 in 1M KOH. In the third work, the V4C3Tx MXene decorated with Ni nanoparticles has been explored for ammonium generation through nitrogen reduction reaction (NRR). MXene is well-known for its large surface area and flexibility to tune its surface properties. DFT calculation reveals that the synergistic work effect between Ni nanoparticles and V4C3Tx MXene changes the reaction mechanism from sluggish distal pathway to the more efficient alternating pathway, which significantly boosts the NRR performance. The prepared catalysts achieve the highest NH3 yield rate of 21.29 µg h-1mgcat-1 at the current density of 0.2 mA cm-2 and the largest Faradic efficiency of 14.86% at the current density of 0.1 mA cm-2. The performance is even superior to most of the reported MXene derivatives for nitrogen reduction reaction.