Nanostructuring Copper Tungstate Photoanodes: Performance Enhancement and Fundamental Limitations

Copper(II) tungstate (CuWO 4 ) combines favorable properties for photoanodes, such as stability in neutral electrolytes, a suitable bandgap of ~2.3 eV, and intrinsically efficient water oxidation catalysis. Nevertheless, its photoelectrochemical performance is severely limited by the very short hole diffusion length. To mitigate this limitation, we prepared nanostructured mesoporous CuWO 4 photoanodes using the Pechini method, aiming to shorten the transport distance for photogenerated holes and enhance collection efficiency. The optimized mesoporous electrodes achieved a photocurrent of 0.6 mA cm −2 at 1.23 V versus reversible hydrogen electrode (RHE), nearly an order of magnitude higher than benchmark compact films and among the highest values reported for CuWO 4 . However, the performance remains far below the theoretical limit, both in terms of photocurrent (lower by a factor of ~15) and maximum achievable photovoltage, with a photocurrent onset potential at ca. +0.6 V versus RHE. Mechanistic analysis reveals that the low efficiency arises from severe bulk recombination and inefficient electron transport, as indicated by charge‐separation efficiencies below 10% and electron collection lengths ~5 times shorter than the thickness required for optimal light harvesting. While electron transport might be partially improved by doping, fast recombination via deep ligand‐field–induced mid‐gap states appears intrinsic to CuWO 4 .