Spectroscopic characterization of Prussian blue analogues as photoanodes in water oxidation

Abstract

For the past fifty years, utilizing solar energy for water splitting has garnered significant attention as a potential method to convert solar energy into chemical energy, specifically clean and renewable hydrogen fuel. Various methods for water splitting, including photocatalytic (PC) and photoelectrocatalytic (PEC) methods, utilize either homogeneous or heterogeneous catalysts. Although a wide range of semiconductor metal oxides and co-catalyst–modified oxides can generate hydrogen and/or oxygen, their overall energy-conversion efficiencies remain low and insufficient for real-world applications. This inefficiency largely arises because among the three essential stages of the water splitting reaction—solar light absorption, charge separation and transport, and catalytic reduction and oxidation reactions—the rapid charge recombination makes the efficiency insufficient, or the charge separation fails to complement the other two stages of the reaction. In this respect, Prussian blue analogues (PBA) have been identified as one of the promising and efficient cocatalysts in the PC and PEC water oxidation processes. The incorporation of PBAs as cocatalysts in semiconductors can improve oxidation and reduction reactions by offering active sites for reactions, while minimizing charge recombination and reversal. However, the application is still limited, and the analysis of the charge carrier dynamics is a key approach for improving the performance and quantifying the loss mechanisms of PBA systems. With the aim of providing researchers with the necessary knowledge to choose the appropriate measurement methods, this mini review provides a comprehensive overview of the different spectroscopy techniques, namely electrochemical impedance spectroscopy (EIS), transient absorption spectroscopy (TAS), and time-resolved photoluminescence (TRPL) to assess the carrier dynamics from activation energy and time resolution from picoseconds to milliseconds/second. The selected spectroscopic techniques will shed light on the electrocatalytic, photocatalytic, and photoelectrocatalytic behaviours of PBA systems incorporated into various metal oxides.