Atomically Dispersed Ir Catalysts on Si Photoanode for Efficient Photoelectrochemical Water Splitting

Hydrogen is expected to become sustainable future energy due to zero-emission, non-toxicity, and large energy density. However, it usually exists in covalent compounds tightly bonded with other elements such as oxygen, nitrogen, and organic materials. One of the promising electrochemical methods to obtain pure hydrogen from these compounds is a solar water splitting. As a photoelectrode, Si has been widely investigated owing to its excellent charge carrier mobility, long carrier diffusion length, earth abundance, and wide-range solar spectrum absorption. Nevertheless, the poor catalytic activity, chemical corrosion in aqueous electrolyte, and negative valence band position limit its practical application. Hence, it is necessary to apply the photoelectrochemical catalysts having large amounts of active sites with complete coverage of photoelectrodes.<br/> Single atom catalysts (SACs) have been recently investigated in electrochemical (EC) reaction to not only maximize the atomic efficiency of platinum group element (PGE) catalysts but also introduce unconventional geometric and electronic structures. The modification of the chemical state of single atoms induced by electronic interactions with the support matrix plays an essential role in improving catalytic activity and stability. However, stabilizing atomically dispersed PGE single atoms on Si photoanodes for photoelectrochemical-oxygen evolution reaction is still challenging due to the scarcity of anchoring sites.<br/> Here, we demonstrate the decoration of Ir SAs on Si photoanodes and reveal the role of SAs on the separation and transfer of photogenerated charge carriers. NiO/Ni thin film, an active and highly stable catalyst, enables to embed the Ir SAs in its lattices by locally modifying the electronic structure. The existence of atomically dispersed atoms, electronic structure, and environmental coordination were identified by high-resolution transmission electron microscope (TEM), X-ray absorption near edge structure (XANES), and X-ray absorption fine structure (EXAFS) analysis. In addition, the roles of the isolated Ir SAs, enabling effective photogenerated charge transport by suppressing the charge recombination and lowering the thermodynamic energy barrier in the rate-determining step, were revealed by intensity-modulated photocurrent spectroscopy (IMPS) and DFT calculations. As a result, the Ir SAs/NiO/Ni/ZrO2/n-Si photoanode shows a benchmarking photoelectrochemical performance with a high photocurrent density of 27.7 mA cm-2 at 1.23 VRHE and 130 h stability. This study proposes the rational design of SAs on Si photoelectrodes and reveals the potential of the PGE SAs to boost photogenerated charge carrier kinetics.