Organometal Halide Perovskite-Based Photoelectrodes for Efficient and Stable Photoelectrochemical Water Splitting

Abstract

Hydrogen (H2) is an attractive renewable energy source, which can be used in various applications including electricity generation and fuel for automobiles. However, the most of H2 production process occurs carbon dioxide (CO2) emission from the reforming of fossil fuels at high temperature. Therefore, it is necessary to develop an eco-friendly and sustainable H2 production using renewable energy sources. Photoelectrochemical (PEC) water splitting is considered one of the most attractive technologies for eco-friendly and sustainable production of H2 by utilizing solar energy without CO2 emission. In particular, high efficiency unassisted solar water splitting is quite valuable, but challenging because it requires high PEC performance of each photoelectrodes composing the PEC systems. To achieve cost-effective, efficient, and stable unassisted solar water splitting, development of organometal halide perovskite (OHP)-based photoelectrodes is highly desired due to the excellent optoelectronic characteristics and low temperature solution processibility of OHPs. Although development of OHPs as photoelectrodes for PEC water splitting is quite valuable due to these advantages, it is also challenging because they are vulnerable to water molecules, heat, and light. The focus of this thesis is fabrication and characterization of the OHP-based photoelectrodes in order to ultimately realize the cost-effective, efficient, and stable unassisted solar water splitting. In the first part of this thesis, the OHP-based photocathodes integrated with MoS2 catalysts were investigated. Although OHP-based photocathodes have been reported so far, most of them used scarce and high-cost Pt as catalysts. For future practical applications of OHP-based photocathodes, however, high PEC performance should be achieved while combining earth-abundant and cost-effective hydrogen evolution reaction (HER) catalysts with OHP photocathodes, instead of Pt. To this end, we successfully deposited edge-exposed MoS2 catalysts on metal foils by pulsed laser deposition and integrated them with OHP photovoltaic (PV) cells for fabrication of OHP photocathodes. As a result, fabricated MoS2/Ti foil/OHP photocathodes exhibited the highest half-cell solar-to-hydrogen conversion efficiency (HC-STH) among the previously reported OHP-based photocathodes with a high photocurrent density and positive onset potential. More interestingly, the MoS2/Ti foil/OHP photocathodes also achieved a record long-term PEC stability among the OHP-based photocathodes. In the second part of this thesis, the OHP-based photoanodes integrated with Fe-doped Ni3S2 catalysts were investigated. Although OHP-based photoanodes have been reported so far, consideration of integrating the oxygen evolution reaction (OER) catalysts, which promote the PEC performance of OHP photoanode, is still insufficient. We successfully integrated the topotactically transformed OER catalysts showing high OER catalytic activity, Fe-doped Ni3S2 with OHP photoanodes. As a result, fabricated Fe-doped Ni3S2/Ni foil/OHP photoanodes exhibited higher PEC performance than Ni-Fe LDH/Ni foil/OHP and bare Ni foil/OHP photoanodes. In addition, the highest applied bias photon-to-current efficiency (ABPE) among the previously reported photoanodes for PEC water splitting was achieved. In the third part of this thesis, the OHP-based dual photoelectrodes for unassisted solar water splitting were investigated. Although OHP-based dual photoelectrodes have been reported so far, their PEC performance is still insufficient. We successfully fabricated OHP-based PEC systems composed of highly efficient and stable OHP photocathode and photoanodes with rational design. As a result, fabricated OHP dual photoelectrodes with Ni-Fe LDH/Ni foil/OHP photoanodes and NiMo/Ni foil/OHP photocathodes exhibited the highest STH without external bias among the previously reported OHP-based PEC systems. More interestingly, fabricated OHP dual photoelectrodes exhibited long-term PEC stability without external bias. We believe that the OHP-based PEC systems reported in this thesis afford us opportunities to achieve the future practical application of OHP-based PEC systems for efficient, stable, and cost-effective solar H2 production.