In Operando Visualization of Charge Transfer Dynamics in Transition Metal Compounds on Water Splitting Photoanodes

Solar-driven photoelectrochemical (PEC) water splitting provides a highly promising solution for converting solar energy to chemical fuels. The semiconductor (SC) based photoanode often exhibits enhanced PEC performance when coated with a transition metal compound (TMC) overlayer that is merely regarded as a cocatalyst for the oxygen evolution reaction (OER). However, the origin of this improvement and the distinct roles of TMCs remain controversial topics. This is mainly due to a lack of advanced characterization techniques that can in operando capture the photogenerated charge transfer dynamics in such multicomponent SC/TMC systems. Herein, how the aforementioned issue can be addressed using in situ visualization characterization is presented, i.e., scanning photoelectrochemical microscopy (SPECM), and ultraviolet/visible-spectroelectrochemistry (UV/vis-SEC). By employing these techniques to BiVO₄ (BV) combined with various TMCs (e.g., CoPi, Ni(OH)_x, and Fe(OH)_x), it is found that in addition to the superior OER activity of TMC overlayers, special attention should be paid to the fast hole transfer dynamics, especially for achieving the desirable PEC performance. As expected, further loading iron-nickel oxyhydroxide (FeNi-H) layer onto the BV/Fe(OH)_x photoanodes (relatively fast hole transfer ability), the BV/Fe(OH)_x/FeNi-H achieves the highest photocurrent density among all counterparts.