Functionalized p-silicon photocathodes for solar fuels applications: Insights from electrochemical impedance spectroscopy

In this contribution we report on an in-depth electrochemical impedance spectroscopy (EIS) study of the interfaces and processes involved in the photoelectrochemical hydrogen production by efficient p-type silicon (pSi) photocathodes, designedly functionalized with protective and catalytic layers. In particular, we have optimized the thermal evaporation of compact and conformal ultra-thin (5 nm) Ti films on pSi, with the double aim of (i) protecting the pSi surface from passivation and photocorrosion, as well as of (ii) introducing platforms with improved adhesion properties for the further functionalization with co-catalysts (in the specific, Pt nanoparticles). The best performing electrodes (labelled as pSi-5Ti-Pt) display an improved photocurrent onset (ca. 200mV positive shift in 1MH2SO4), as well as a more than doubled saturation photocurrent (up to 27 mA/cm2) when compared to the unmodified pSi photocathodes. As evidenced by the EIS analysis, the proposed modifications of pSi surface led to an enhancement of the charge extraction from the semiconductor, most likely due to surface dipole effects able to reduce the resistance associated to the transport through the space charge layer. With respect to unmodified silicon, the presence of the Ti overlayer, partially oxidized to TiO2, also allows for a denser surface coverage of the electrodeposited Pt nanoclusters, resulting in decreased interfacial charge transfer resistance for hydrogen evolution. The proposed functionalization strategy relies on common fabrication methods (already applied at industrial level) and can be easily extended to other photoelectrode materials prone to passivation/ photocorrosion and/or difficult to functionalize, thus introducing more flexibility in the choice of materials for photoelectrochemical cells.