Open-Circuit Stability and Integration of Silicon Electrodes for Solar Fuels Devices

Citation

Fu, Harold Jin (2022) Open-Circuit Stability and Integration of Silicon Electrodes for Solar Fuels Devices. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/pgx5-fj77. https://resolver.caltech.edu/CaltechTHESIS:10092021-015926914

Abstract

Two significant challenges that impede the realization of inexpensive, solar-driven water electrolysis involve the corrosion and integration of component materials. For instance, Si is a prominent light absorbing material that readily corrodes in alkaline electrolyte unless subjected to an oxidative potential. Although a protective coating can be applied to mitigate corrosion, the underlying semiconductor remains exposed to electrolyte at pinholes on the protective coating. Illumination slows the dissolution of Si photoanodes further by 2-3 orders of magnitude via oxidation to SiO_x. However, Si is still susceptible to corrosion under nighttime conditions and device stability must be maintained regardless of diurnal patterns of sunlight. This thesis explores two approaches to drive Si passivation in the dark at open circuit. First, a protective electrolyte can be introduced to solution that acts as an oxidizing agent to Si. Secondly, a catalytic thin film like NiO_x on Si can drive the electrode potential positive by catalyzing O₂ in electrolyte. Applying either passivation strategy yielded extended stability of Si photoanodes subjected to simulated day/night cycling. In addition to corrosion, device performance is critically dependent on the integration of component materials. Efficient water splitting requires that at least two semiconductors be connected in series to drive the reaction, while lateral resistance losses in electrolyte preclude large (> cm²) planar photoelectrode areas. Si can be vertically arranged as high aspect ratio microwires that can be embedded in an ion exchange membrane. This assembly can be laminated to a tandem partner arranged in a similar configuration using an electrically conductive interlayer. This thesis additionally investigates the bulk and interfacial properties of Nafion-PEDOT:PSS composite films as a candidate material for this interlayer. After solvent treatment, the composite film exhibited percolation of electrically conductive PEDOT domains even at dilute PEDOT concentrations (~ 0.2 wt%). Despite the presence of an insulating Nafion-rich layer on the surface, the composite forms a low resistance contact to CH₃-terminated p-Si, thereby making the composite a viable interlayer for use in a fully integrated, tandem water splitting device.

Thesis Files