Unassisted Solar Fuel and Food Production using Silicon Photoelectrode

Abstract

The technology that can efficiently utilize a huge amount of solar energy is modern society’s biggest challenge to overcome serious environmental problems. Photo-electrochemistry is one of the most promising strategies that can convert the intermittent nature of sunlight into storable chemical fuel. It has been considered to play important role in the development of energy resources and diverse chemical synthesis in the next-generation society. To realize the viability of photo-electrochemical value-added chemical production, the development of an efficient and low-price photocatalyst, long- term stability, and further application system are highly required. In this dissertation, we introduce several strategies to enhance the performance and durability of crystalline silicon (c-Si) photoelectrode and expand its applicability to the development of an unbiased photoelectrochemical system for fuel and food production. c-Si has been a promising photocatalyst material due to its abundance, low cost, and appreciate bandgap that enables efficient solar light absorption, yet it suffers from limited stability under ambient or aqueous conditions and insufficient photovoltage. In chapter 1, we briefly describe the importance and principle of solar-driven direct chemical synthesis from photo-electrochemical system. After then, the current state and challenges of photoelectrochemical system and developed photocatalysts are explained, and advantages and bottlenecks of silicon as promising photocatalyst material are addressed. Following chapters 2 and 3 address the development of a high-performance crystalline silicon photoelectrode to enable efficient photoelectrochemical value-added chemical production. Next, developments of unassisted photoelectrochemical systems for fuel and food production with the application of the developed c-Si photoelectrode are addressed in chapters 3 and 4. In chapter 2, we report on the development of crystalline silicon photocathode having structure to achieve high current density and stability in photoelectrochemical system. It includes the strategies to decouple light absorption and catalytic sites, to effectively encapsulate the front side and back side of crystalline silicon photoelectrode maintaining catalytic activity and stability in an electrolyte. From the developed structure, light absorption of crystalline silicon is optimized with nanostructure. The developed crystalline silicon photocathode having tapered microwire arrays achieve high photocurrent close to theoretical limit of silicon and long-term stability in photoelectrochemical water splitting reaction. In chapter 3, we address the development of unassisted wireless photoelectrochemical water splitting system to produce hydrogen, called artificial leaf. Wireless photoelectrochemical system is ideal for practical applications of PEC chemical production compared with wired configuration. However, the need for a deposition of both oxygen evolution catalyst or hydrogen evolution cocatalyst layer on sole photoelectrode for wireless configuration makes the efficiency loss of artificial leaf because of interruption of light absorption by catalyst layer or need of expanded area of wireless device. On the other hand, it is difficult to apply crystalline silicon photoelectrode in application to unbiased solar-to-chemical production despite several advantages of crystalline silicon as photocatalyst materials, because of low photovoltage problem of c-Si. In Chapter 3, the development of c-Si module photoelectrode having interdigitated back contact (IBC) structure is addressed to overcome both problem of the artificial leaf structure and the difficulty of c-Si to apply in unbiased PEC reaction. Inspired from the structure of natural leaf, the utilization of c-Si IBC structure to photoelectrode makes the deposition of both OER and HER catalyst on the rear-side of photoelectrode possible, making decoupling light absorption and catalytic reaction. Plus, easiness and seamless modulization of IBC structure make the development of a compact c-Si module photoelectrode enabling unbiased solar-to- chemical reaction. In chapter 4, we address the development of unassisted photoelectrochemical CO2 reduction for protein production using c-Si photoanode. Livestock production for ruminant meat has contributed largely to the greenhouse gas emissions and various environmental pollutions, highlighting the importance of developing alternative protein sources. Alternative proteins offer an environmentally sustainable solution, yet current production methods heavily depend on agricultural or costly feedstocks and external energy. This chapter investigates the development of an integrated three-compartment photoelectrochemical system employing enzymatic and microbial biocatalysts, enabling unassisted solar CO2-to-protein production. The system comprises a crystalline silicon photoanode for water oxidation, a cathodic compartment converting CO₂ to formate using ethyl viologen-mediated electron transfer and a formate dehydrogenase enzyme, and a microbial compartment where Methylobacterium extorquens AM1 ferments the formate to produce protein. In addition, it presents the advancement of key system components such as electrolytes and membranes as strategies to enhance the efficiency and stability of solar-driven protein production. Ion-selective membranes separate each chamber, ensuring selective transport of formate while blocking harmful mediators to the microbe. The system operates under 1 sun irradiation without external energy input, demonstrating stable CO₂-to-formate conversion with near 100% selectivity for CO2 reduction to formate and subsequent microbial protein production and representing carbon-neutral food production as a promising strategy for simultaneously addressing climate and nutritional challenges.