Boosting Photoelectrochemical Water Oxidation of BiVO 4 Photoanodes With an Iron‐Based Metal–Organic Framework as a Hole Transport Layer

ABSTRACT Sluggish bulk hole transport fundamentally limits the performance of bismuth vanadate (BiVO 4 ) photoanodes in photoelectrochemical (PEC) water splitting. Herein, we report a BiVO 4 /Fe‐MOF/NiFeOOH trilayer photoanode, wherein an iron‐based bipyridine dicarboxylate metal–organic framework (FeBPDCA), grown on BiVO 4 via coordination‐assisted in situ synthesis, serves as a dedicated hole‐transport layer. The Fe‐MOF layer provides a favorable band alignment and a built‐in electric field at the BiVO 4 /Fe‐MOF interface to drive directional bulk hole extraction, while NiFeOOH acts as a surface water‐oxidation cocatalyst. Experimental characterization and density functional theory (DFT) calculations reveal that the Fe‐MOF primarily accelerates bulk hole extraction, whereas NiFeOOH suppresses surface recombination through rapid water oxidation, together boosting both bulk charge‐separation efficiency ( η bulk ) and surface charge‐transfer efficiency ( η trans ) up to 97%. The optimized photoanode achieves a high photocurrent density of 7.15 mA cm −2 at 1.23 V vs. RHE, with an onset potential of 0.24 V and a maximum applied bias photon‐to‐current efficiency (ABPE) of 2.56%. This work demonstrates that deliberate tuning of MOF energy levels through metal node and ligand selection enables the simultaneous engineering of favorable band alignment and efficient charge separation at semiconductor/MOF junctions, providing a practical design strategy for photoanodes where bulk charge extraction is the primary efficiency bottleneck.