Performance and Stability Optimization of Solar Fuel Devices

Citation

Zhou, Xinghao (2018) Performance and Stability Optimization of Solar Fuel Devices. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/SSNP-XW29. https://resolver.caltech.edu/CaltechTHESIS:05062018-164727269

Abstract

Fossil fuels enabled the Industrial Revolution, and have been the most important power for promoting the world's economic growth ever since. However, burning fossil fuels have also been causing severe air pollution, and global warming is also related to excessive use of fossil fuels. Solar energy is considered to be the largest renewable clean energy resource. The principal problems of solar energy are low energy concentration and intermittency. Storing solar energy in chemical bonds, similar to photosynthesis in nature, is a possible way to overcome these two problems. Carbon-free chemicals, like hydrogen gas produced by solar-driven water splitting, or carbon-neutral chemicals, like methane, ethylene, formic acid, carbon monoxide, etc. produced from solar-driven CO₂ reduction, are all promising clean fuels for solar storage, as they feature high energy/power intensity, are easy and cheap to store and transport, and have direct interface with existing infrastructures.

In this thesis, we focus on improving the efficiency and stability of the solar-driven fuel generation devices, which consist of (photo-)anode and (photo-)cathode. For the anode part, cobalt oxide Co₃O₄ ultrathin (2 nm) films by atomic layer deposition (ALD) were deposited onto silicon photoanode prior to deposition of thick nickel oxide (NiO_x) layers. The photovoltage of the photoanode increased from 200 mV to 580 mV after including the interfacial Co₃O₄ layer, and the anode was stable in 1.0 M KOH(aq) for 1700 hours, which was equivalent to one year of operation in the field at a maximum photocurrent density of 30 mA/cm² assuming a 20% solar capacity factor. Furthermore, the non-uniform sputtered (NiO_x) layer of the n-Si/SiO_x/Co₃O₄/NiO_x photoanode was removed, and the 2 nm Co₃O₄ film was thickened to 50 nm, and the stability of n-Si/SiO_x/50 nm-Co₃O₄ was improved to 2500 hours with lower efficiency decay rate. For the cathode part, an optimized Pd/C nanoparticle coated Ti mesh cathode exhibited < 100 mV overpotential at 8.5 mA/cm² current density, and > 94% Faradaic efficiency for the reduction of 1 atm of CO₂(g) to formate in 2.8 M KHCO₃. A solar-driven CO₂ reduction (CO₂R) cell was constructed with this cathode, showing 10% solar-to-fuels conversion efficiency.

This thesis can be divided into three parts. The first part discusses importance of solar fuels, as well as gives an introduction of solar-fuel generators. The second part includes Chapter II and Chapter III, which deal with performance improvement of silicon photoanode with ALD Co₃O₄ thin films. The third part is Chapter IV, in which we study the cathode for CO₂ reduction to formate, and demonstrate a 10% efficiency solar-driven CO₂ reduction cell with the cathode.

Thesis Files