Nanostructuring Copper Tungstate Photoanodes: Performance Enhancement and Fundamental Limitations

Copper(II) tungstate (CuWO4) 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 (PEC) performance is severely limited by the very short hole diffusion length. To mitigate this limitation, we prepared nanostructured mesoporous CuWO4 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 vs. RHE, nearly an order of magnitude higher than benchmark compact films and among the highest values reported for CuWO4. 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 vs. 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 CuWO4. These findings indicate that, despite nanostructuring, CuWO4 is fundamentally unsuitable for constructing high-efficiency photoanodes for solar water splitting.