Development of Organic Photoelectrodes and Inorganic Layered Material Electrocatalysts for Efficient and Stable Solar Water Splitting

Abstract

Hydrogen (H2) is a promising chemical energy carrier due to its zero-carbon emission characteristic and higher specific mass energy density than that of natural gas, gasoline, and diesel. Among the various H2 production methods, solar-driven water electrolysis is considered one of the good options for generating green hydrogen. Photoelectrochemical (PEC) water splitting systems have attracted attention for green hydrogen production since they generate hydrogen using only solar energy and water. Especially, designing the high-performance and stable photoelectrode is a top priority to commercializing PEC water splitting systems. It is necessary to consider suitable photo-absorbing semiconductor materials and electrocatalysts used as a surface cocatalyst. Since the first demonstration of PEC water splitting with TiO2 photoanode in 1972 by Honda and Fujishma, various types of inorganic metal oxides-based semiconductor materials such as BiVO4, Fe2O3, WO3, and Cu2O have been actively studied. Despite extensive studies, no inorganic semiconductors have been developed that fully satisfy all the criteria for an effective PEC water splitting system, including suitable band energy levels, and charge transfer features. Furthermore, research on highly active electrocatalysts for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is needed for the efficient utilization of photo-generated charge to electrochemical reaction. Thus, new strategies for developing PEC cells must be established to overcome the performance limitations of inorganic semiconductor materials-based PEC systems. In this dissertation, the strategy of utilizing organic semiconductors as the photoactive materials is introduced for developing high-performance and stable PEC water splitting cells, considering their superior charge-transfer characteristics, high tunability of band energy levels, and cost-effectiveness. Moreover, inorganic layered double hydroxide (LDH) materials are studied as OER electrocatalyst, applied to the surface of organic semiconductor-based photoanode for preventing water intrusion and maximizing the charge-separation efficiency, which affects enhanced performance and stability. Thus, various directions of research were investigated on the photo-absorbers (organic semiconductor) and electrocatalysts (inorganic layered materials) part of solar water splitting systems. In Chapter 2, the design of high-performance and stable organic semiconductors-based photoanodes consisting of bulk heterojunction (BHJ) system with p-type polymers and n-type non-fullerene materials was introduced. BHJ-based organic photoactive layers were passivated using nickel foils, GaIn eutectic, and LDHs as model materials for preventing the electrolyte permeation to the organic layer and enhancing the OER activity on surface of the electrode. Based on this configuration of photoanode, the designed organic semiconductor-based photoanode showed the possibility of overcoming the performance of inorganic semiconductor-based PEC water splitting cells. Further, 90% of the initial photocurrent density was retained after conducting the stability test for 10 h, whereas the photoactive layer without passivation lost its activity within a few minutes. According to previous research, the destabilization of LDH-loaded metallic foil encapsulated organic semiconductor-based photoanode was due to the low photostability of the organic photoactive materials under irradiation comprising the UV region. In Chapter 3, the newly designed photon downshifting Pt(II) complex (denoted as PtCP) was applied as an additive to highly active BHJ organic photoanode to effectively improve stability. PtCP clearly showed the UV-to-visible light conversion and strong intermolecular interactions with the organic semiconductors. Thus, the PtCP-doped BHJ organic photoanode exhibited a high performance and stability, particularly 82% of initial performance was retained after 120 h durability test. To our best knowledge, the performance and stability of PtCP-doped BHJ organic photoanode were the highest among all reported organic semiconductor-based photoanodes. Due to the beneficial characteristics of LDH such as flexible chemical composition, intrinsic layered structure, and high crystallinity, LDH-based electrocatalysts can be designed for applying to various operating conditions and electrochemical reactions. In Chapter 4, Ir species nanocluster-anchored CoFe-LDH nanostructures were introduced, wherein the crystalline nature of LDH restrained corrosion associated with H+ and the Ir species dramatically enhanced the OEC kinetics at neutral pH. The optimized OER electrocatalyst demonstrated high OER activity in neutral electrolyte. When it was integrated with an organic semiconductor-based photoanode, we obtained a high photocurrent density in neutral electrolyte, which is the highest among all reported photoanodes for neutral OER to our knowledge. Chapter 5 was focused on the huge potential of LDH structure as OER electrocatalysts. Multi-metal-based (hydr)oxides show promise as OER electrocatalysts due to potential electronic interactions among constituent metal cations, but complex compositions may not always enhance performance. Thus, an approach reported to control the interlayer metal cation distribution in LDHs to improve their OER performances based on the unique characteristics of LDHs in Chapter 5. Restacking of exfoliated NiFe and CoAl LDH nanosheets led to electrochemical synergistic effects between different nanosheets. The restacked LDH showed high activity for OER compared to randomly distributed NiFeCoAl quaternary LDHs, and it was well exhibited in a photovoltaic-electrochemical (PV-EC) water splitting system. Thus, a new design approach is suggested to enhance the electrochemical performances of LDHs by relaxation the degree of cation mixing.