Study of the Electrocatalytic Performance of Two-dimensional Transition Metal Phosphides by On-chip Electrocatalytic Microdevices

Abstract

The rapid development of industrial society demands inexhaustible energy sources, which cannot be entirely fulfilled by fossil fuels. The global energy crisis is urging the exploration of clean, renewable and sustainable energy sources as substitutes for fossil fuels. Electrocatalysis, an advanced technology that converts earth-abundant water, carbon dioxide and nitrogen to valuable chemicals and fuels like hydrogen, hydrocarbons and ammonia, holds great potential in the coming energy revolution. Sitting in the core of electrocatalysis is developing high-performance electrocatalysts. Hydrogen is considered the most promising among the emerging renewable energy sources owing to its high energy density and the water-only byproduct. Therefore, electrocatalytic hydrogen production from water splitting has attracted extensive attention. Current commercial electrocatalysts are based on noble metals, whose high cost and crustal scarcity greatly hinder their scalability.

Transition metal phosphides (TMPs) have drawn extensive attention in electrocatalytic hydrogen evolution reaction (HER) owing to their distinct physiochemical properties, efficient charge transport and abundant earth reserves. On the other hand, two-dimensional (2D) materials possess significant potential for electrocatalysis because of the large surface area and efficient charge transfer. However, most TMPs intrinsically possess three-dimensional (3D) nonlayered crystal lattices, making it extremely challenging to synthesize TMPs with two-dimensional (2D) morphology. Hence, this thesis has committed to synthesizing 2D molybdenum phosphides (a subset of TMPs) and exploring their electrocatalytic HER performance. By using 2D transition metal dichalcogenides (TMDs) precursors (MoS₂) via gas-phase transformation, 2D molybdenum phosphides were successfully synthesized. The chemical composition and properties of the converted MoS₂ significantly depend on the reaction conditions, the reaction temperature and the thickness of precursor MoS₂ nanosheets. Phosphorus-doped MoS₂ (P-MoS₂), single-crystalline MoP, amorphous MoP_x and polycrystalline MoP₂ nanosheets are successfully prepared by tuning the above parameters. Then the obtained 2D TMPs were comprehensively characterized, including their chemical compositions, crystal structures and charge transport properties.

In the past decade, on-chip electrocatalytic microdevices (OCEMs) have emerged as an electrochemical platform specialized for investigating nanocatalysts at the microscopic level, which allows high-precision electrochemical measurements at the individual nanomaterial level and, more importantly, offers unique perspectives inaccessible with conventional electrochemical methods. OCEMs are exceptionally effective in characterizing the intrinsic electrocatalytic properties of 2D materials, including TMDs and TMPs. However, the use of OCEMs often lacks caution, which produces results with poor accuracy and reliability. Therefore, this thesis also establishes the protocol describing the critical concepts, experimental standardization, operational principles and data analysis of OCEMs. Specifically, MoS₂ grown by chemical vapor deposition is utilized as the model material to conduct the electrocatalytic HER measurement in OCEMs with data validation, interpretation and benchmarking. A series of factors, such as the exposed area of material, that could influence the accuracy and reliability of measurement are illustrated. In addition, the protocol for in situ electrical transport measurement is detailed as an example of the adaptations of OCEMs.

Finally, the electrocatalytic HER performance of the synthesized 2D materials is investigated by OCEMs. By virtue of the spatially resolved measurement of OCEMs, P-MoS₂ exhibits edge-dominated electrocatalytic HER activity, clarifying the effect of edge sites on heteroatom-doped 2D layered materials. In situ transport measurements identify the improved intrinsic activity of P-MoS₂ and clarify the effect of phosphorus doping on the electronic structure of MoS₂. This insight is later applied to develop a high-performance catalyst based on transition metal dichalcogenides. Moreover, MoP nanosheets are proven excellent for electrocatalytic HER in both the basal plane and edge sites, which differs from the established edge-oriented electrocatalytic activity of MoS₂. The catalytic active site distribution in layered 2D materials and nonlayered 2D materials provides invaluable guidance for developing 2D catalysts. Furthermore, 2D MoP₂ nanosheets are found to be highly efficient and stable for electrocatalytic HER in both neutral and acidic electrolytes. 2D MoP₂ nanosheets also demonstrate better durability than amorphous Pt films, demonstrating the possibility of MoP₂ being a hopeful alternative HER electrocatalyst for Pt.

In the end, this thesis shares personal understandings regarding the existing challenges and perspectives in the fields of 2D nanocatalysts, OCEMs and their adaptations.

Date of Award	19 Aug 2024
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Qiyuan HE (Supervisor)