| Solar energy is a tremendous natural energy source for human, and fulfilling the conversion and storage of solar energy would be a feasible approach for human society to alleviate the excessive dependency on the fossil fuels. Hydrogen energy, which is a clean and renewable energy source, has been a key medium in an attractive sustainable energy utilization strategy called hydrogen economy. How to produce hydrogen with low energy cost is a fundamental problem to solve for this energy loop, while natural photosynthesis gives the answer. Artificial photosynthesis stores solar energy into chemical bonds, representing a high efficient approach for conversion and storage of solar energy. This thesis focuses on the highly efficient photoanodes in the artificial photosynthesis, and the research progress includes following parts:1. The oxidation half reaction of the PEC water splitting is a sluggish 4-electron transfer process, which limits the conversion efficiency greatly. In this thesis, the attractive hematite (?-Fe2O3) photoanode was adopted as the study object. Aiming at the material drawbacks of hematite, an investigation on the approaches to increase the PEC reaction activity was conducted with purpose, and these approaches could be divided into nanostructure construction and surface modification. We adapted the atmosphere pressure chemical vapor deposition (APCVD) and set up a batch-preparation system for deposing hematite. After the conditions were optimized, the prepared hematite showed nanorod structure with dendrites, which had a much larger specific surface area and a high [110] preferential orentation compared to the plain film via the normal solution-phase synthesis. Moreover, the plateau photocurrent of the nanostructured photoanode raised to 1.2 mAcm-2, which was relatively high compared to the results of the literature.2. Until now, in-depth studies on the morphology effect of the hematite have been rare, while in this thesis, a combining method of two-target cosputtering and dealloying has been developed to construct the nanoporous gold (NPG) on the conductive glass, followed by the APCVD method to obtain the ?-Fe2O3/NPG photoanode. As the space between the nanorods were remarkably enlarged in ?-Fe2O3/NPG, the PEC performance increased, proving the morphology effect of the NPG substrate. However, ?-Fe2O3/NPG showed no more preferential orientation, thus a "gold islands" (GIs) substrate was designed. The gold film would turn into many GIs with different sizes and intervals according to the thermal conditions, and then Fe2O3/GIs was obtained. The preferential orientation was regained to a high level surprisingly, with a further higher PEC performance with a plateau photocurrent of 2 mAcm-2. This research revealed that the PEC performance was a comprehensive result of the morphology effect and other factors.3. Surface modification with cocatalysts has been recognized as an important approach to raise the PEC performance in a view of kinetics. These years, the non-noble OER catalysts have been paid much attention to. We compared the effect of Co-, Fe-, Ni-based OER cocatalysts, only to find that a good OER catalyst didn't mean a good cocatalyst necessarily. In order to control the surface state density of the photoanode, which was detrimental to the PEC performance, a reactive sputtering technique was adopted to modify CoOx cocatalyst on ?-Fe2O3 surface, and the onset potential move cathodically with 0.184 V, similar to the highest result of IrO2 cocatalyst in the literature.4. Considering the solar energy conversion and storage process, replacing the OER with the chlorine evolution reaction (CER) in the anodic half cell would be a quite advantageous approach in a view of thermodynamics and kinetics. In this thesis, a single-crystalline n-Si photoanode was used to investigate the chemical approach to raise the solar-to-chemical (STC) efficiency. A composite photoanode of RuO2/TiO2/n-Si was prepared via the reactive sputtering, which has a highly catalysis preference towards CER. When applied to the splitting of the acidic saturated NaCl solution, the photoanode functioned stably in a relatively long time range. Meanwhile, the in-situ differential electrochemical mass spectrum (DEMS) proved the product was only chlorine with no detectable oxygen. Calculation results gave an 8% STC efficiency of the chlorine electrode, and the short-circuit current (jsc) and the open-circuit voltage (Voc) were 35 mAcm-2 and 0.55 V, respectively, much higher than the oxygen electrode, and in a leading level compared to those results in the literature.5. It is necessary to raise the overall energy conversion efficiency of the artificial photosynthesis before it turns into practical use. For this purpose, we designed and made a 2-photoelectrode (PE) PEC flow cell, to investigate the process of energy conversion during the production of (H2+Cl2) fuels in the PEC system containing the chloride. After optimization and modified with Pt cocatalyst, the p-Si nanowire (p-Si NW) photocathode had a similar PEC HER performance to those of the literature. When the p-Si NW photocathode was combined with the stable TiO2 photoanode, the spectrum absorption of either one was complementary with the other, so as to convert and store more solar enenrgy. Finally, we fulfilled the goal of unbiased PEC splitting of brine. |
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