Novel Dye-Sensitized Photocathodes and Semiconductors for Light-Driven Photoelectrochemical Synthesis.

다국어 초록 (Multilingual Abstract)

To meet the ever-increasing worldwide energy consumption, artificial photosynthesis which converts sustainable solar energy into value-added products has been widely investigated. Compared with well-studied water splitting and CO2 reduction reactions, oxygen reduction reaction for hydrogen peroxide (H2O2) production and photo-electrosynthesis is a lesser-known process. This dissertation details the study in (1) using dye-sensitized photocathodes for oxygen reduction to produce H2O2 efficiently and aprotic redox reactions (2) applying p-type Cu2O semiconductors for the photoelectrochemical synthesis of value-added chemicals.H2O2 is a versatile energy carrier and can be produced by reduction of O2 on a dye-sensitized photocathode. In this dissertation, using a hydrophobic donor-double-acceptor dye (denoted as BH4) sensitized NiO photocathode, H2O2 can be produced efficiently by reducing O2 with current density up to 600 mA cm−2 under 1 sun conditions (Xe lamp as sunlight simulator, wavelength > 400 nm). The dye-sensitized photoelectrochemical cells (DSPECs) maintain currents greater than 200 mA cm−2 at low overpotential (0.42 V vs. RHE) for 18 h with no decrease in the rate of H2O2 production in aqueous electrolyte. Moreover, the BH4 sensitized NiO photocathode was for the first time applied in an aprotic electrolyte for oxygen reduction. The corresponding photocurrent generated by this photoelectrosynthesis is up to 1.8 mA cm−2. Transient absorption spectroscopy also proves that there is an effective electron transfer from reduced BH4 to O2 with a rate constant of 1.8 x 106 s−1. Although using dye-sensitized photocathode shows high current for O2 reduction, the poor faradaic efficiency of H2O2 production and decomposition limit the development of dye-sensitized electrodes. Inspired by the organic synthesis of H2O2 with anthraquinone method and redox mediators used in Li-O2 batteries, anthraquinone (AQ) redox mediators are introduced to metal-free organic DSPECs for the generation of H2O2. Instead of directly reducing O2 to produce H2O2, visible light-driven AQ reduction occurs in the DSPEC and the following autooxidation with O2 allows for H2O2 accumulation and AQ regeneration. This AQ-relay DSPEC exhibits the highest photocurrent so far in non-aqueous electrolyte for H2O2 production and excellent acid stability in aqueous electrolyte, which provides a practical and efficient strategy for visible-light driven H2O2 production. Continuing the study in photoelectrosynthesis, p-type Cu2O photoelectrodes were designed and synthesized by oxidation and thermal annealing approach. Superior performance of the as-synthesized Cu2O as photoelectrodes in both aqueous and non-aqueous electrolyte provides new insight in developing numerous organic synthesis reactions, through a solar- and electricity-driven process. Perovskites with p-i-n structure were applied to photoelectrochemical benzoquinone reduction. Further, anthraquinone molecules with NH2 function groups were proposed and synthesized for surface passivation of perovskites, which provides the opportunity of enhancing catalytic O2 reduction and the fabrication of novel low-dimension halide perovskites.