TRANSITION METAL-BASED CATALYSTS FOR ENHANCED OXYGEN ELECTROCATALYTIC REACTION

The development of high efficiency and cost-effective bifunctional electrocatalysts for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is critical to a wide commercialization of metal-air batteries and unified regenerative fuel cells. Pt and IrO2 and their alloys have been applied as favorable catalysts due to their prominent performance. However, their high cost, low stability and susceptibility to poisoning greatly limit their widespread commercial adoption. Transition metal has been regarded as a good alternative to noble metal-based catalysts because of their tunable valence state, cost-effectiveness and variety of chemical composition, structure, and morphology. However, the catalytic activity of the transition metal electrocatalysts is largely hindered by their inherent corrosion and oxidation susceptibility. Therefore, my research mainly focusses on the development and modification of transition metal-based catalysts with enhanced catalytic activity towards ORR and OER.In the first project (Chapter 4), I propose to investigate the beneficial effect of atomic layer deposition (ALD)-derived incorporation of transition metal-based materials for electrocatalytic oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Gas-evolving electrochemical reactions primarily occurs at the triple phase boundary regions where electrons, ions and reactant molecules meet altogether. Therefore, ALD-based dispersion of atomic-scale transition metal species can be an ideal approach for maximizing catalytically active sites. Our proposed research is to investigate the effect of ALD treatment of functionalized carbon-based materials with different transition metals on electrocatalytic ORR and OER reactions via both experimental and theoretical studies. In the first work, a mixed metal/metal oxide-integrated N-doped carbon is prepared by performing atomic layer deposition (ALD) of CeO2 nanodots on a three-dimensional Co and N co-doped carbon polyhedron nanostructure derived from zeolitic imidazolate framework (ZIF; a type of metal-organic framework). An optimally prepared hybrid catalyst achieved excellent bifunctional oxygen electrocatalytic performance comparable to or even better than noble metal-based benchmark catalysts (Pt/C for ORR and IrO2 for OER) thanks to the synergistic effect between Co and Ce, high-surface-area backbone structure, rich Co-Nx moieties and oxygen vacancies. This work proves the effectiveness of ALD in uniformly incorporating the second metal/metal oxide onto a monometallic system for enhanced electrocatalytic performance.In the second project (Chapter 5), I demonstrate a unique 3D core-shell nanostructure for efficient OER electrocatalysis. Two-dimensional (2D) layered double hydroxides (LDHs) are promising as an effective electrocatalyst towards OER, but their poor conductivity and tendency to stack together limits their activity and durability as an electrocatalyst. Herein, 3D core-shell structures are synthesized through a facile one-step reaction strategy, in which the terephthalic acid and urea is employed as the organic ligand for the metal organic framework (MOF) precursor and surface coordination buffer between LDH and MOF. Benefiting from the hierarchical 3D microstructure with uniform nanosheets grown on the surface, the as-prepared electrocatalyst exhibits rich carbon edge sites and high electrochemical surface area. The representative sample (CoNi-BDC@LDH) achieves an OER activity with the overpotential of 280 mV at 100 mA cm−2 and robust cyclic and chronoamperometric stability. A series of quasi-operando studies by X-ray absorption spectroscopy, X-ray photoelectron spectroscopy, Raman and Fourier-transform infrared spectroscopy further elucidate that Co-Ni acts as the main active site while the high valence state of Ni facilitates O2 desorption from O-O bond linking metal sites. The high OER performance is additionally attributed to a high valence state of metal ions in γ-NiOOH/CoOOH, lattice edge sites of carbon, and a synergistic effect between neighboring metal atoms.In the third study (Chapter 6), a Co-Ni based LDH structure with thin layer coating of ceria and/or titania by ALD is synthesized. Benefiting from the result of the first two projects, ALD was applied to the LDH structure directly. The effects of tri-metal incorporation on the electrocatalytic properties of the resulting hybrid systems toward OER is further investigated. The primary electrochemical studies and XPS characterization results suggests the improved OER performance is mainly attributed to the synergistic effect between metal cations