Solar-driven photoelectrochemical (PEC) water splitting offers a promising route for the sustainable production of green hydrogen by directly converting sunlight into chemical fuels. However, conventional PEC devices remain constrained by planar electrode architectures that limit light harvesting, charge collection, and mass transport while hindering large-area scalability and device integration. Overcoming these limitations requires new electrode designs that simultaneously optimize optical, electrical, and electrochemical performance.

Chapter 2 introduces a transparent porous conductive substrate (TPCS) as a multifunctional platform for next-generation PEC devices. The TPCS consists of a highly porous quartz fiber scaffold conformally coated with fluorine-doped tin oxide (FTO), combining optical transparency, inherent electrical conductivity, mechanical robustness, and interconnected porosity within a single architecture. Through optimization of substrate fabrication, mechanical reinforcement, and conductive coating processes, an easy-to-scale platform was developed for integrating semiconductor photoelectrodes.

Building on this platform, Chapter 3 presents a monolithic Janus PEC architecture in which complementary photoelectrodes are selectively integrated onto the opposite faces of a single porous conductive substrate. A BiVO4/NiFe-layered double hydroxide photoanode and a PBDB-T:ITIC organic bulk-heterojunction photocathode modified with a 2PACz self-assembled monolayer and a RuOX catalyst are combined into a bias-free tandem device capable of overall solar water splitting. The porous architecture provides efficient electronic coupling between both photoelectrodes while substantially shortening ionic transport pathways compared with conventional planar artificial leaf designs. Electrical resistance measurements, IR-drop analysis, and electrochemical impedance spectroscopy demonstrate that this architecture effectively suppresses ionic transport losses, particularly as device dimensions increase. As a result, the monolithic artificial leaf achieves stable, unassisted evolution of hydrogen and oxygen under continuous solar illumination.

In Chapter 4, the versatility of the TPCS is further demonstrated by its integration into an anion-exchange membrane photoelectrochemical (AEM-PEC) system that employs Hematite-based photoelectrodes for Gas-phase water splitting. This application highlights that the substrate is not limited to any specific semiconductor or device configuration; rather, it serves as a versatile platform for multiple PEC technologies.

Overall, this work establishes TPCS as a new architectural strategy for photoelectrochemical energy conversion. By integrating optical transparency, electrical conductivity, and three-dimensional porosity within a scalable monolithic platform, this thesis addresses fundamental transport limitations in PEC systems. It provides a versatile foundation for developing efficient, scalable solar fuel generators.