Demonstration of artificial photosynthesis with peeled silicon microrod arrays

Abstract

The sustained use of solar energy is plagued by the intermittency of sunlight. If the energy from fleeting solar photons can be stored in chem. bonds through electrochem. reactions, this intermittency can be levelized to power demands. Although water is often proposed as the electron source for these reactions, myriad electron sources exist that may be better matched to the voltage output of existing photovoltaics. In light of this, peeled silicon microrod arrays, partially embedded in a Nafion proton-exchange membrane, were used to drive solar hydroiodic-acid splitting (i.e. HI --> H_2 + I_3-). Open-circuit photovoltages measured for p-type and n-type microrod arrays driving aq. H_2 evolution and iodide oxidn., resp., were 450 - 500 mV. Corrosion of Si was attenuated through methylation of Si atop sites via a two-step chlorination-alkylation surface chem. procedure. This resulted in stable iodide oxidn. under continuous simulated sunlight illumination for weeks. Amorphous MoSx, Pt, or PEDOT:PSS electrocatalysts were deposited on the microrods to catalyze H2 evolution and iodide oxidn. Si microrod arrays were partially embedded in Nafion and mech. removed from the Si substrate to yield free-standing, flexible devices. Solar-to-hydrogen efficiencies and faradaic yields for product formation were quantified. These systems are sustainable because the HI fuel precursor is inorg., thus not capable of liberating CO2, and HI can be regenerated in a flow battery or fuel cell as H_2 + I_3-/I_2.