Solar to chemical conversion system using crystalline silicon solar cells

Abstract

Solar energy is attracting attention as an unlimited and clean energy source. The solar energy reaching the earth in 1 year is 10,000 times greater than the annual amount of energy consumption. Using this abundant sunlight, people have attempted to produce essential chemicals (hydrogen, hydrogen peroxide, hydrocarbons, carbon dioxide, and fixed nitrogen compounds) using photoelectrochemical cells (PEC). Among them, hydrogen is in the spotlight as a clean energy source because it does not emit carbon oxide after combustion. Thermodynamically, the theoretical electrical energy required for the water splitting reaction is 1.23 eV (ΔG = 475 kJ mol-1). However, when considering side reactions such as interfacial resistance, material diffusion, the energy of about 1.5 to 2 eV is required for hydrogen production. Metal oxide materials such as TiO₂, Fe₂O₃, WO₃, BiVO₄, Ta₃N₅, and SrTiO₃, which are used as PEC materials, are generally stable in water and abundant on Earth. Numerous studies have been conducted on these materials. However, due to the relatively high bandgap (Eg = 2.1 – 3.7 eV), these materials mainly absorb only the UV region, which occupies about 4% of the entire solar spectrum, so they cannot absorb more than 96% of the rest of the sunlight. As a results, they show a low half solar-to-hydrogen efficiency (half-STH) of 0.01 – 2.5 %. On the other hand, crystalline silicon (c-Si) has an appropriate bandgap (Eg = 1.1 eV) that can effectively absorb the solar spectrum, and it is an earth-abundant material. Therefore, many studies are being conducted to use c-Si as a PEC absorber. However, using c-Si as a PEC absorber requires the use of a catalyst to promote the electrochemical reaction. Also, there is a problem that c-Si is corroded in an electrolyte environment, mainly under acid or basic conditions. In this research, we design and develop solar to chemical conversion using c-Si solar cells that solves the disadvantages of c-Si photoelectrode. In Chapter 1, we briefly explain the basic principles of electrochemistry and c-Si solar cells. Utilizing this foundation, we elucidate the mechanisms of solar to chemical conversion reactions and outline how efficiency is calculated. In Chapter 2, we developed a photoelectrochemical cell using p-n junction c-Si photoelectrode. Conventional c-Si/electrolyte heterojunction has limited photovoltage and has sluggish reaction kinetics and photo-corrosion in aqueous solutions. For this reason, the efficiency of c-Si photoelectrochemical cell was very low. However, c-Si p-n junctions demonstrate higher applied bias photon-to-current efficiencies (ABPE) due to the energy difference between quasi-Fermi levels generated by Fermi-level splitting within the p–n junction under light illumination. For these reasons, we developed high efficiency c-Si p-n junction photoelectrode. Thermodynamically, the theoretical electrical energy required for the water splitting reaction is 1.23 eV. However, the built-in potential that can be seen in actual c-Si p-n junctions is 0.6V, it is difficult to unassisted water splitting using a single p-n junction. To solve this problem, we considered a module that connects c-Si p-n junctions in series. In chapter 3, we developed c-Si solar module photoelectrode. We use the all back contact (ABC) solar cell that all electrodes are represent at bottom. Using this unique ABC structure, we developed natural leaf inspired solar water splitting system called artificial leaf. In Chapter 4, we developed the large-scale Block-type Photoelectrochemical all-in-one Water splitting System. Research on solar water splitting hydrogen production systems is being conducted in the industrial field, but system optimization has not been done because it remains at the level of water splitting using power obtained from solar cells. Existing solar water splitting hydrogen production systems require a new type of solar water splitting hydrogen production system due to limitations in installation areas and power loss due to non-optimized systems. So, we developed the large-scale Block- type Photoelectrochemical all-in-one Water splitting System. So, we developed the c-Si unit cell photoelectrode, c-Si module photoelectrode and c-Si photoelectrochemical system. Based on these results, in Chapter 5, we tried to apply it to various solar to chemical conversion reaction. If our c-Si photoelectrode is designed according to the chemical reaction, various solar to chemical conversions are possible. Solar to chemical conversion of various chemicals such as hydrogen peroxide, ammonia, and carbon dioxide is possible. Among them, we developed the hydrogen peroxide and nitrate reduction to ammonia.