Development of Transition Metal Dichalcogenides-Based Heterostructures for Electrochemical Applications in Hydrogen Economy

School of Energy and Chemical Engineering (Energy Engineering)Energy demand has continuously increased with the development of global industries and rapid population growth. Among them, fossil fuels including coal, petroleum oil, and natural gas have been powering economies for over 150 years resulting in the inevitable emission of greenhouse gases. The heavy reliance on fossil fuels gives rise to serious trouble in the environment called global warming. Therefore, technologies for clean and sustainable energy have been developed such as solar, wind, and marine energy. However, they have some difficulties in substituting the current energy system due to regional and seasonal factors. In this respect, an efficient energy conversion and storage system are needed to resolve intermittent availability. Hydrogen as an energy carrier would be one of the leading options for future carbon-neutral energy systems in the sense that it has the highest gravimetric energy density. Hydrogen is not only being used in many existing industries such as refining petroleum, producing fertilizer, but also can be used as the primary energy carrier for clean energy technologies like fuel cells in the hydrogen economy. As the worldwide energy trend gradually moves toward a carbon-neutral society, the demand for hydrogen is increasing. However, most hydrogen is still being produced from fossil fuels emitting harmful CO2 with a low production rate as well as containing CO2 as an impurity leading to a decline in the durability of the fuel cell due to poisoning the noble metal-based electrocatalysts. Considering both the environmental and the efficiency aspects, it is necessary to advance the water electrolysis technology combined with an eco-friendly energy grid such as solar energy. The main half-reactions of water electrolysis, HER and OER, require efficient and durable electrocatalyst with low cost to replace commercial Pt or Ir-based expensive catalyst. Two-dimensional transition metal dichalcogenides (TMD) are one of the most promising candidates due to their proper hydrogen adsorption/desorption properties. However, the limited number of active edge sites and poor conductivity of the basal plane make it difficult to use TMD as a practical electrocatalyst in water electrolysis. In this dissertation, MoS2, a typical TMD, is used as a bifunctional electrocatalyst by heterostructuring with multiple active components. In Chapter 1, the broad introduction of the hydrogen economy would be addressed. In addition, the water electrolysis, the mechanism of each half-reaction, and several evaluation parameters would be presented. The structure of TMD and some strategies to improve the electrochemical application are also explained. Constructing TMD-based heterostructure, the transition metal phosphides, other sulfides are introduced and grafted on conductive substrates in this work. In Chapter 2, nickel phosphide (Ni2P) and nitrogen-doped carbon (N:C) support are used to adjust the electronic structure of MoS2 as well as increase the number of active sites. As a result, the electronic conductivity of Ni2P/MoS2/N:C catalyst was highly enhanced making phosphosulfide bonding. Besides, the number of active sites is greatly increased. Due to the synergistic effect between Ni2P and MoS2, Ni2P/MoS2/N:C shows Pt-like HER performance in the acidic electrolyte. Chapter 3 is focused on the electrocatalyst working in wide pH electrolytes. The other sulfide, cobalt sulfide (Co9S8), was chosen for the formation of heterostructure with MoS2 because Co9S8 has been studied for hydrodesulfurization (HDS) which has similar reaction properties to HER in the sense that the key factor is adsorption of hydrogen species. Besides, Co9S8 can be applied to OER as well as HER. The electronic structure was reconstructed, and the number of active sites is increased by heterostructuring. Therefore, the Co9S8-MoS2 shows remarkable HER performance in acidic, neutral, and alkaline electrolytes. The actual active site in Co9S8-MoS2 heterostructure was also discussed in detail. In line with research in Chapter 3, the overall water splitting system is realized in Chapter 4. As mentioned, Co9S8 has OER activity as well as HER activity. Thus, Co9S8-MoS2 was also tested as a bifunctional electrocatalyst for overall water electrolysis. The Co9S8-MoS2 as a pre-catalyst for OER shows exceptional activity and stability in alkaline media. The Co9S8 in the heterostructure transforms to cobalt oxide species as an OER active site during the oxidative OER while MoS2 in the heterostructure forms porosity leading to an increase in the electrochemically active surface area. Combining HER system, the Co9S8-MoS2 exhibits excellent overall water splitting performance in a two-electrode system. Finally, the electrochemical system was connected with a photovoltaic cell called PV-EC system using Co9S8-MoS2/NF for both cathode and anode. In Chapter 5, the development of transition metal dichalcogenides by heterostructuring for water electrolysis is summarized. After that, a mixed phase of MoS2 would be introduced for future work which can be used in spray or printing methods. Thus, the electrode can be used for green ammonia production as a hydrogen carrier.clos