| Summary: | 博士 === 國立臺灣科技大學 === 化學工程系 === 107 === In the past decades, photoelectrochemical (PEC) solar water splitting has become a central research theme and considerable efforts have been devoted for the development of efficient catalyst materials for chemical fuel generation. One direction is to design and search new semiconductor materials through a combination of computational and experimental efforts. The other one is to develop efficient strategies to address the existing limitations in the commonly used semiconductor materials. However, the efficient methodologies for charge transfer at electrode/electrolyte interface remain underdeveloped. Motivated by these challenges, this dissertation focuses on improving PEC materials, namely BiVO4, by surface engineering.
The first experimental chapter presents solvent-engineering approach for fabricating well-crystalized monoclinic BiVO4 with robust mechanistic properties as electrode material for solar water splitting. When applied as electrode material for water splitting in borate electrolyte solution, the BiVO4 electrode exhibited a relatively low onset potential of 0.4 V vs. RHE and a high photocurrent density of 1.04 mA/cm2 at 1.23 V vs. RHE which is among top 1% the highest results reported in the literature for bare BiVO4. Long-term stability witnessed a fairly stable behavior in which around 70% photo-induced current was maintained after one hour. In the second experimental chapter of this thesis, a new concept that integrates an appropriate hole transport material into the conventional photoelectrochemical cell is introduced by inspiring the devotion of hole transport material in the solar cell. With the creation of BiVO4/CuSCN heterojunction photoanodes, the photocurrent density increased to 1.78 mA cm-2 compared to 1.22 mA cm-2 of bare BiVO4. More importantly, the onset potential for oxygen evolution reaction exhibits a dramatic cathodic shift (~230 mV). The heterojunction also possesses internal quantum efficiency of approximately 50% in the range from 350-450 nm with relatively high solar energy conversion efficiency (0.5%) and much higher water oxidation efficiency (~90%). The unique electrode architecture design favoring the simple water splitting process over conventionally fabricated electrode by providing more active sites and facilitates transportation and consumption of photoinduced holes. The next section demonstrates a proof-of-concept electrochemical approach to synthesize hierarchical layered double hydroxides (LDH) for as a new co-catalyst for enhancing photoelectrochemical water oxidation of bismuth vanadate photoanode. The modified photoanodes exhibited 1.7 times increase in photocurrent density and a 300 mV cathodic shift in onset potential. The improved PEC performance of the composite photoanode could be attributed to the multifunctional roles of LDH that reduce kinetic barrier, facilitate photogenerated charges separation, thus retarded the recombination of photogenerated charges. In the last experimental chapter, a facile process is developed for preparing a new type of low-cost ferrite phosphate as an efficient co-catalyst to suppress charge recombination and stabilize bismuth vanadate (BiVO4) photoelectrodes. The composite photoanode exhibit a high photocurrent density of 2.28 mAcm-2, which corresponds to a 250% increase compared to that of pristine BiVO4. Deposition of cocatalyst has yielded a large cathodic shift (∼500 mV) in the onset potential, high oxidation efficiency of about 90% and a good stability of over 120 minutes in a mild basic medium. Comprehensive photoelectrochemical studies suggest that ferrite phosphate could boost the photoelectrochemical properties of the BiVO4 underlayer by mediating hole extraction across the photoexcited semiconductor-electrolyte interface. This in turn enhances photoconversion efficiency and prevents the photooxidation of the photoanode, ensuring prolonged stability.
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