Unconventional Phase Metal Nanomaterials for High-Performance Batteries

Abstract

Developing next-generation batteries with high energy density and environmentally-friendly properties is a challenging but meaningful task in the big picture of sustainable development towards carbon-neutral world. Constructing metal-electrocatalysis coupled novel batteries is a promising technical route approaching this goal. As an important component, cathode metal catalysts heavily determine the electrochemical performance of the designed hybrid batteries. Current optimization strategies on metal catalysts focus on their morphology, size, constituent and distribution, while another critical structural parameter, crystal phase, has been rarely investigated in these fields. In fact, even for the same metal element, different crystal phases have obvious discrepancies on atomic arrangement so that their intrinsic catalytic properties should differ given that electronic structure is highly sensitive to atomic configuration. Therefore, it is believed phase engineering of nanomaterials (PEN) is a feasible and effective strategy to greatly enhance the electroactivity of metal nanocatalysts towards various electrocatalytic reactions, paving the industrialization way for future high-performance and multifunctional batteries.

This thesis aims to design and synthesize a series of metal nanostructures with common and unconventional phases, reveal the general crystal phase-performance relationships, and employ the optimal metal catalysts to construct high-performance hybrid batteries. Particularly, three research projects will be discussed in detail.

Firstly, unconventional phase 4H/face-centered cubic (fcc) iridium (Ir) nanostructures are synthesized on 4H/fcc Au substrate through a facile epitaxial growth method. When used as the bifunctional catalysts for aprotic Li-CO₂ electrochemistry, we observe a phase-dependent electrochemical behaviour. 4H/fcc Ir exhibits the superior electrochemical performance over common fcc Ir in facilitating the round-trip reaction kinetics of Li+-mediated CO₂ reduction and evolution. A low charge plateau of below 3.61 V and a high energy efficiency of 83.8% are achieved by 4H/fcc Ir. Experimental and simulation results unveil that the more reversible generation of amorphous/low-crystalline discharge products induced by strong coordinative electron transfer should be responsible for the higher catalytic activity of 4H-Ir.

Secondly, a two-step synthetic route comprised of capping agent-assisted one-pot wet-chemical growth and facile ligand exchange is developed to prepare tetraphenylporphyrin (tpp)-modified amorphous/crystalline rhodium-copper alloy metallenes (RhCu M-tpp). The as-synthsized RhCu M-tpp can work as a bifunctional catalyst to enable effective electrocatalytic nitrate reduction reactions (NO₃RR) and ethanol oxidation reactions (EOR) in neutral solution, thereby constructing a rechargeable, low-overpotential zinc-nitrate/ethanol battery. It delivers a high energy density of 117364.6 Wh kg^-1_cat, superior rate capability, excellent cycling stability of ~400 cycles and ammonium acetate production. Experimental and theoretical results suggest that there is a molecule-metal relay catalysis in neutral NO₃RR over RhCu M-tpp that significantly facilitates the NH3 selectivity and reaction kinetics via a low energy barrier pathway.

Thirdly, by multi-step epitaxial growth and selective etching, a specially-designed unconventional 4H/fcc ruthenium (Ru) - nickel (Ni) Janus nanostructures have been synthesized as efficient cathode catalysts for aprotic Li-CO₂ and Li-air batteries. After matching with an adaptable dimethylsulfoxide-based organic electrolytes, this heterophase 4H/fcc Ru-Ni Janus nanostructure is able to deliver an optimal discharge-charge difference of 0.65 V, excellent rate capability and long-term cycling stability of 220 cycles at 250 mA g^-1. Benefit from the efficient Li-CO₂ electrochemistry, the assembled Li-air cells can also steadily last for above 150 cycles in ambient air. Experimental and simulation studies reveal that 4H/fcc Ru-Ni with higher-electroactivity facets can not only boost the redox reaction kinetics but tune several discharge reactions towards Li₂C₂O₄ path, thereby alleviating the electrolyte and catalyst failures caused by aggressive singlet oxygen from rapid solo electrochemical degradation of Li₂CO₃.

Date of Award	11 Jul 2024
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Zhanxi FAN (Supervisor)