Improving charge collection with delafossite photocathodes: a host-guest CuAlO2/CuFeO2 approach

p-Type delafossite CuFeO2 has recently been reported as a promising candidate for direct photoelectrochemical solar water reduction in alkaline conditions. However, despite its favorable optical band gap energy and light absorption, this material suffers from poor electron-hole separation that limits its optimum thickness to a few hundred nanometers. This limitation can be addressed by a hostguest strategy, where a mesoporous p-type scaffold is used to support a thin-film of the light absorber. Here we demonstrate this host-guest approach for the first time with CuFeO2 using p-type transparent CuAlO2 as a scaffold. Optimizing the scaffold layer thickness at 2 mm resulted in a 2.4-fold increase in photocurrent in the presence of O-2-a sacrificial electron scavenger-reaching 2.4 mA cm(-2) at +0.4 V vs. RHE. Moreover, comparing the performance to host-guest electrodes prepared with an insulating SiO2 scaffold as a control suggested that the observed improvement with CuAlO2 was due to a decreased recombination stemming from improved charge separation in addition to improved charge transport through the scaffold compared to the CuFeO2 film alone.