CoPi-Modified Mesoporous Titania Photoelectrodes for Water Splitting: Why Less Is More

Solar technologies have emerged as a clean and sustainable source of energy toward the mitigation of climate change. In the last years, increasing interest has been devoted to obtaining solar fuels such as hydrogen through photoelectrochemistry. In this framework, titanium dioxide is a sound and promising platform for photoelectrochemical water splitting; however, its performance is limited due to the sluggish oxygen evolution reaction (OER). In this work, a photoactive electrode was developed by combining ordered mesoporous TiO₂ thin films with a cobalt oxo-phosphate (CoPi) OER catalyst. We conducted detailed structural and electrochemical characterization of the TiO₂–CoPi nanocomposite. We compared the performance of dense and mesoporous TiO₂ films on different substrates as photoelectrodes for water splitting prepared by evaporation induced self-assembly. All studied photoelectrodes exhibit high stability, reproducibility, and cycling durability, with consistent photocurrent densities. Controlled amounts of CoPi were deposited in this matrix. Low loadings (0.04 mC/cm²) resulted in a 20% increase in photocurrent (32 μA at 1.23 V vs reversible hydrogen electrode (RHE), compared to 23 μA for the bare TiO₂ film), whereas higher loadings suppressed the photocurrent due to recombination with the TiO₂ matrix or the underlying fluorine-doped tin oxide (FTO). In this study, we demonstrate the importance of optimizing the cocatalyst loading based on the interactions between different components in a nanocomposite photoanode, with a focus on understanding the recombination pathways that appear when working with nanostructured semiconductors.