Stability of Photo-Electrochemical Interface for Solar Fuels

Citation

Yu, Weilai (2021) Stability of Photo-Electrochemical Interface for Solar Fuels. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/2z16-d005. https://resolver.caltech.edu/CaltechTHESIS:03172021-221106133

Abstract

Photoelectrochemical (PEC) water splitting is a promising approach to convert renewable solar energy to clean hydrogen (H₂) fuels in one simple step. Although Ⅲ-Ⅴ semiconductors are attractive candidates as light-absorbers in tandem solar-fuel devices, their long-term stability for the hydrogen-evolution reaction (HER) in either acidic or alkaline aqueous electrolytes needs to be established. Chapter 2-5 of this thesis first aims at revealing the underlying corrosion chemistry for a variety of Ⅲ-Ⅴ semiconductors specifically under the HER conditions, offering a rational understanding towards the stability of semiconductor photoelectrode.

In Chapter 2, we start from p-InP and reveal its susceptibility to cathodic photocorrosion forming metallic In⁰, which however can be completely mitigated by the presence of Pt catalyst due to kinetic stabilization. We also show that the resulting PEC performance of p-InP/Pt electrodes is sensitive to the changes in surface stoichiometry, whereas an InO_x-rich surface developed in KOH caused a substantial degradation in the current density-potential (J-E) behavior. In Chapter 3, we discovered that a non-stoichiometric and As⁰-rich surface of p-GaAs, resulting from a galvanic corrosion by Pt, led to mid-gap surface states as well as a complete loss in photoactivity. In Chapter 4-5, we demonstrate similar kinetic stabilization applied to both p-InGaP₂/Pt and pn⁺-InGaP₂/Pt photocathodes for the HER at both pH 0 and pH 14. Additionally, we found that the corrosion of underlying GaAs substrates for the pn⁺-InGaP₂/Pt photocathodes at positive potentials caused damage of structural integrity as well as instability in electrode performance. Altogether these works underscore the mutual dependence of the physical and electrochemical stability of semiconductor photoelectrodes during the HER, which also need to be considered separately. Moreover, both catalytic kinetics and surface stoichiometry are crucial factors for defining long-term corrosion chemistry for semiconductor photoelectrode.

In Chapter 6-7, we further explore solar fuels beyond H₂, namely electrochemical N₂-to-NH₃ conversion. We first establish a new analytical method to isotopically quantify the concentrations of ¹⁵NH₃ in aqueous solutions with a high sensitivity and a low limit-of-detection of <1 μM. Further we applied this advanced method to rigorously verify the electrocatalytic activity of a CoMo electrode for reducing N₂(g) to NH₃. We show that the additional ammonia detected in electrolyte was instead attributed to the corrosion of N impurities present in the CoMo electrode under cathodic bias, thus giving false positive results. These works emphasize the importance of both rigorous product analysis and experiment design in further catalyst development.

Thesis Files