High-performance Solar Hydrogen peroxide production by Photovoltaic material based Photoelectrochemical cell

School of Energy and Chemical Engineering (Chemical Engineering)At present, approximately, 80% of world???s energy consumption is fulfilled from non-renewable resources such as fossil fuels. When burned, these fossil fuels release a huge amount of carbon dioxide (Co2), which can seriously damage the environment. Reduced reserves of fossil fuels and environmental concerns has prompted substantial research efforts toward renewable energy. Countries around the world expect a low-carbon future by replacing renewable energy such as solar, biomass, wind, geothermal and other renewable sources. Considering to its high energy density and the clean energy conversion process, hydrogen has been regarded as the most promising energy source. However, in order to store hydrogen as an energy source, there is a problem that a high pressure of almost 700 bar is required. Hydrogen peroxide (H2O2) is a green oxidizing agent that has a high energy density comparable to hydrogen and is convenient to transport and store in a liquid state, making it promising as a new future energy carrier. Currently, the anthraquinone oxidation (AO) process is the best-known process for producing H2O2. This method requires a noble metal-based catalyst and high energy input, toxicity and has a risk of explosion. For this reason, other eco-friendly methods for H2O2 production such as electrochemical processes and photocatalyst methods have been developed. Although this process has been studied widely, it still faces low efficiency and selectivity to generate H2O2 due to the use of low performance of inorganic semiconductor-based electrodes. Accordingly, there is a need for research to improve the high concentration and efficiency of H2O2 replacing conventional inorganic semiconductor-based material. In this thesis, high-performance unassisted solar H2O2 production have been conducted using photovoltaic (PV) material-based photoelectrochemical (PEC) cells. PV-based materials such as perovskite and dye-sensitized solar cell (DSSC) were used to overcome the limitations of the low current and efficiency of the typical metal-oxide materials. In chapter 2, an inorganic perovskite photoanode was fabricated to improve charge transfer, and high selectivity toward H2O2 from oxygen reduction reaction (ORR) was generated using a reduced graphene oxide (rGO) electrocatalyst. The onset potential and stability were enhanced by using CsPbI2Br-based inorganic perovskite with a large band gap and more stable in heat and moisture than organic perovskite. An additive layer using PEAI was added to further ensure stability, and stability of 25hs could be achieved. In cathode part, carbon-based electrocatalysts such as rGO, which are inexpensive and have high performance, was applied instead of a commonly used noble metal catalyst to control the aspect of cost. By reducing GO, rGO was synthesized, and the defects of rGO showed high current density and selectivity toward H2O2 generation. As a result, H2O2 can be produced with high selectivity over 83% by using a two-electrode system with perovskite photoanode and rGO electrocatalyst. In this way, H2O2 is generated only by solar energy without additional bias, and is considered to be a more eco-friendly and renewable method. In chapter 3, other PV-based materials, dye-sensitized photoelectrochemical cell (DSPEC), were fabricated to improve H2O2 efficiency. DSPECs can be made by earth-abundant materials, enabling low-cost and simple fabrication. The existing published DSPECs have poor charge transfer and fast recombination, resulting the low current density about 2 mA cm-2. To overcome this problem, a simple passivation structure using nickel foil was used to improve efficiency and stability in alkaline condition. In this structure, the nickel foil not only protects the dye from harsh water condition but also allows effective charge transfer. The charge transfer efficiency was further enhanced by using an NiFe-LDH oxygen evolution reaction (OER) catalyst grown on a nickel foil. With this catalyst, the device can be achieved high performance and maintain stability for 30 h. As a well-known efficient H2O2 catalyst, the oxidized-bucky paper (O-BP) made of cross-linked O-CNTs was applied to a cathode. It produces a high concentration and selectivity toward H2O2 almost 85% with 2-electrode system using DSPEC in 1M NaOH at pH 13.7 without additional bias or sacrificial agent. In consequence, it was able to generate high efficiency of H2O2 using DSPEC with high performance and stability in aqueous condition. A system using PV can be a superior application for other reactions as further research. .ope