Designing Two-Dimensional Nanomaterials for Electrocatalytic Clean Energy Conversion

Abstract

The development of efficient and clean energy conversion technologies is a key issue for the sustainability of energy technologies. Hydrogen is one of the best fuels for clean energy systems as its combustion product is only water. Therefore, the development of cost-effective energy conversion technologies for hydrogen generation is significant. Electrocatalytic water splitting using renewable energy is one of the best ways to obtain high-purity hydrogen and the process emits no carbon dioxide. Electrocatalytic reactions occur on the surface of electrode materials. Consequently, understanding and tuning the surface properties of electrode materials is a key aspect in the design and preparation of efficient electrocatalysts. Compared to other nanomaterials, such as nanowires or nanoparticles, the most important two features of two-dimensional (2D) nanomaterials for electrocatalysis are their tunable and uniformly exposed lattice plane and unique electronic state. This Thesis aims to synthesize and optimize novel 2D nanomaterials for the study of hydrogen-related electrocatalytic reactions. Our target reactions include the hydrogen evolution reaction (HER) and the nitrogen reduction reaction (NRR), which have great potential in hydrogen-related clean energy conversion systems. The first two chapters provide a systematic review of the development of 2D nanomaterials for electrocatalysis. The unique advances of 2D electrocatalysts are discussed based on different compositions and functions followed by specific design principles. Following this, various 2D electrocatalysts for a series of electrocatalytic processes involved in the water cycle, carbon cycle, and nitrogen cycle are discussed from their fundamental conception to their functional application. A significant emphasis is placed on various engineering strategies for 2D nanomaterials and their influence on intrinsic material performance, such as electronic properties and adsorption energetics. The first part of this Thesis focuses on the understanding of alkaline HER mechanism using 2D electrocatalyst as the platform. So far, the mechanistic understandings of alkaline HER are inapplicable to highly active nanostructured catalysts in practice. This is because most of nanostructured catalysts have complicated active sites, which cannot be identified carefully using theoretical calculations. Compared to other nanomaterials, 2D nanomaterials have uniformly exposed lattice plane which is considered as the ideal platform for the investigation of electrocatalytic reactions. Consequently, various 2D electrocatalysts with tunable active sites were synthesized and studied via advanced experimental measurements and theoretical calculations. First, a hybrid material of 2D C3N4@MoN was prepared using an interface engineering strategy. The intimate interaction of both inert C3N4 and MoN surfaces induced a highly active interface with tunable dual-active sites for alkaline HER. The enhanced activity originates from the synergy between the optimized hydrogen adsorption energy on the g-C3N4 sites and enhanced hydroxyl adsorption energy on the MoN sites. Second, atomically thin nitrogen-rich nanosheets, Mo5N6, were synthesized using a Ni-induced growth method. The 2D Mo5N6 nanosheets exhibit high HER activity and stability in natural seawater, which were superior to other TMNs and even the Pt benchmark. The superior performance of the nitrogen-rich Mo5N6 nanosheets originates from their Pt-like electronic structure and the high valence state of Mo atoms. Thirdly, a multi-faceted heteroatom-doping method (nitrogen, sulfur, and phosphorus) was applied to tune the electronic structure and HER activity of non-noble metals (Ni and Co) 2D layer directly and continuously without changing their chemical composition. The dopant-induced charge redistribution in the Ni metal 2D layer significantly influences its catalytic performance for the HER in alkaline media. The principle that bridges the dopant effect with the resultant HER activity is visualized with a volcano relationship. The second part of the thesis focuses on the exploration and synthesis of new 2D layered transition metal nitrides (TMNs) for hydrogen-related energy conversion. Firstly, the 2D layered W2N3 nanosheets with nitrogen vacancies was successfully obtained for the NRR. In this work, a new 2D layered W2N3 nanosheet was syntheiszed and the nitrogen vacancies demonstrate activity for electrochemical NRR A series of ex-situ synchrotron based characterizations show that the nitrogen vacancies in the 2D W2N3 nanosheets are stable due to the high valence state of the tungsten atoms and 2D confinement effect. Density function theory calculations suggest that the nitrogen vacancies provide an electron-deficient environment which facilitate nitrogen adsorption and lower the thermodynamic limiting potential of the NRR. Secondly, alkali molten salts were employed as the catalyst to explore and synthesize a new family of 2D layered TMNs under atmospheric pressure. The resultant 2D layered TMNs show superior performance for the HER with small overpotentials of 129 mV and 122 mV at a current density of 10 mA cm-2 in 0.5 M H2SO4 and 1 M KOH, respectively. This level of performance surpasses most of the 2D layered electrocatalysts reported in the literature. They also exhibit excellent oxidation resistance and film-forming properties for practical applications. At last, the challenges and perspectives of 2D nanomaterials for electrocatalysis were discussed. The novel 2D nanomaterials demonstrate great potential for energy-relevant electrocatalytic processes such as HER and NRR. By rationally modifying the surface property and electronic structure at atomic level, the 2D nanomaterials can be extended to more research areas. Moreover, insightfully unveiling the reaction mechanisms of electrocatalysis can lay a solid foundation for designing more efficient 2D electrocatalysts.