CoPi-modified mesoporous titania photoelectrodes for water splitting: why less is more

Solar technologies have emerged as a clean and sustainable source of energy towards the mitigation of climate change. In the last years, an 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 OER catalyst (CoPi). We conducted a 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. All studied photoelectrodes exhibit high stability, reproducibility and cycling durability, with consistent photocurrent densities. Controlled amounts of CoPi were deposited on this matrix. Low loadings resulted in a 20% increase in photocurrent, whereas higher loadings suppressed the photocurrent due to recombination with the TiO2 matrix or the underlying FTO. In this study, we demonstrate the importance of optimizing the co-catalyst 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.