Investigations of Metal Oxide Photocathode Protection Layers and Interfacial Charge Transfer Rates on Graphene Electrodes

Coatings of HfO2, Ta2O5, TiO2, and Nb2O5 were evaluated as protection layers for p-InP photocathodes in aqueous acidic electrolytes. Each candidate protection layer was characterized based on several criteria: interfacial conduction of photogenerated electrons, thermodynamic and kinetic stability of the overlayer throughout the relevant potential and pH range for cathodic fuel-forming half-reactions, and inhibition of the primary anodic and/or chemical dissolution processes that limit electrode stability. Both conduction of photogenerated electrons and inhibition of anodic oxidation were evaluated using V3+/2+ in 5.0 M HCl(aq) as a one-electron redox couple with a potential close to that of hydrogen evolution in acidic media. Failure modes due to cathodic plating of metal, as well as anodic dissolution and chemical dissolution processes, were evaluated for photocathodes made from etched p-InP, platinized p-InP, and p-InP photoelectrodes that had been coated with the various oxide films and then platinized for use in photoelectrochemical hydrogen evolution in acidic aqueous electrolytes. Nb2O5 best met all of the previously specified criteria, in contrast, TiO2 was found to be thermodynamically and kinetically unstable in acid over the potential range relevant for the hydrogen evolution and/or CO2 reduction electrochemical half-reactions. The protocols developed in this work are broadly applicable for determining the effectiveness of other materials and semiconductors as protection layers for photocathodes. The electrochemical charge transfer rate at mechanically exfoliated graphene electrodes have also been investigated to probe the coupling distance into the electrode by which redox species in solution electronically couple. Scanning electrochemical cell microscopy (SECCM) was used to measure the rates of charge transfer and to reduce the effect of graphene defects on the electrochemical measurements. The rates of charge transfer with Co(en)33+/2+, Ru(NH3)63+/2+, and IrCl62-/3- redox couples were determined on graphene with different thicknesses, suggesting a coupling distance of 0.3 nm. However, real coupling distance may differ due to the convoluting effects of surface defects, errors inherent to SECCM, and uncertainty in the thickness of graphene.