Design and characterizations of carbon confined noble metal catalysts and their application in electrocatalysis

Abstract

Fuel cells, such as proton exchange membrane fuels cells and direct formic acid fuel cells etc., have been regarded as promising energy conversion devices, as they can effectively convert chemical energy into electrical energy without causing any pollutants. Despite intensive efforts that have been made to explore noble metal-catalysts for cathodic reaction e.g. oxygen reduction reaction (ORR) and anodic reaction e.g. formic acid oxidation reaction (FAOR), the activity and stability remain the key issues hindering their widespread applications. On the other hand, the CO poisoning of catalysts can easily happen on continuous metal sites, which greatly decreases the FAOR performance. Recently, constructing noble metal based catalysts with elaborate surface nanostructure is developed as an effective way to improve both catalytic activity and stability. Specifically, carbon encapsulated noble metal catalysts have been widely studied. However, the role of carbon encapsulation on catalytic performance and materials synthesis still needs further investigation. Therefore, in this thesis, a series of carbon confined noble metal catalysts have been developed, and superior ORR and FAOR performances are achieved. Specifically, the role of surface nanostructure is investigated. The main achievements are concluded as follows: (1) To avoid serious corrosion of non-noble metals in efficient low-Pt alloys, a hollow carbon confined PtCo3 intermetallic (O-PtCo3@HNCS) is achieved using Co pre-embedding and subsequent impregnation-reduction method. During the synthesis, Co pre-embedding ensures the formation of rich mesoporous structure, and subsequent impregnation-reduction at high temperature is responsible for the formation of Pt-Co intermetallics and carbon confinement. As expected, the obtained O-PtCo3@HNCS exhibits superior ORR performance with negligible degradation after stability test. The superior stability can be ascribed to the combination of ordered structure and carbon encapsulation that particle aggregation, sintering and dissolution are greatly avoided. (2) To expand the scope of high entropy alloy (HEA) nanoparticles (NPs) with uncharted composition and performance, a general synthetic approach using hollow carbon confinement assisted furnace annealing method is developed to achieve nonequilibrium HEA-NPs. The obtained HEA-NPs exhibit a uniform particle size of 5.9 nm. Owing to the facile temperature control, various ex situ characterizations could be applied to reveal the detailed formation pathway. Besides, by changing the annealing temperature, HEA-NPs from crystalline solid solution to intermetallic can be tailored. During the ordinary furnace annealing synthesis, the pre-confinement of metal precursor is believed to prevent the thermodynamical favored phase separation by ensuring uniform distribution of metal particles in such confined volume. In addition, the synthesized HEA-NPs exhibit remarkable ORR performance, and also superior structural stability as revealed by identical location transmission electron microscopy. (3) To make the single atoms trapped inside the carbon matrix accessible to the reactant, a hollow N-doped carbon nanorod confined Rh single atoms catalyst (Rh-SACs/HCNR) is developed via a coordination-template method. The uniform distribution of Rh single atom on the hollow carbon nanorod is revealed using aberration-corrected scanning transmission electron microscopy and energy dispersive X-ray spectroscopy mapping. Using X-ray photoelectron spectroscopy, the role of carbon confinement of hollow structure in preventing the conversion from favorable pyridinic/pyrrolic N to unfavorable graphic N during high temperature treatment is identified. As proof of concept, the obtained Rh-SACs/HNCR shows extraordinary FAOR activity and selectivity.