When producing a multi-layer photoelectrode for solar fuel production, selecting appropriate bulk materials to use as a semiconductor, a catalyst, and a protection layer is important. However, optimizing the surface of each component and the interfaces between the components is just as critical to maximize the overall performance of the photoelectrode. Our research team has been at the forefront of demonstrating and elucidating the impact of the photoelectrode surfaces and interfaces on the overall performance of the photoelectrodes. For example, our team has shown that when a ternary oxide containing two different metal ions, such as BiVO₄, is used as a photoanode, the surface metal composition (i.e., the surface Bi:V ratio) may not necessarily be the same as the bulk metal composition (Bi:V = 1:1) and it can also be intentionally modified. We showed that changes in the surface composition while using the same underlying bulk photoelectrode can have an immense impact on the band edge positions and work function, which have a direct impact on electron-hole separation and photocurrent generation, even for the same facet exposed on the surface.[1] This observation made us wonder how varying the surface composition of the same photoelectrode can impact the photoelectrode/catalyst junction when the same catalyst layer is deposited on the photoelectrode. In order to explicitly demonstrate and investigate how the detailed features of the photoanode/OEC interface affect interfacial charge transfer and photocurrent generation for water oxidation, we prepared two BiVO₄(010)/FeOOH photoanodes with different Bi:V ratios at the outermost layer of the BiVO₄ interface (close to stoichiometric vs Bi-rich) while keeping all other factors in the bulk BiVO₄ and FeOOH layers identical. The resulting two photoanodes show striking differences in the photocurrent onset potential and the photocurrent density for water oxidation.[2] In this presentation, we explain the atomic origin of the experimentally observed difference by revealing the impact of the surface Bi:V ratio on the hydration of the BiVO₄surface and bonding with the FeOOH layer, which in turn affect the band alignments between BiVO₄ and FeOOH.