Recent progress of bismuth vanadate-based photoelectrocatalytic water splitting

With increasing energy demands and ever-growing environmental concerns, solar energy and hydrogen energy have attracted worldwide attention. In particular, hydrogen energy not only has a high energy density, but also is clean, renewable, and carbon-free, when compared with primary energy sources such as coal, oil, and natural gas and secondary energy sources such as coal gas, petrol, and diesel. Photoelectrocatalytic (PEC) water splitting for hydrogen generation is a process in which a PEC cell containing photoelectrodes and electrolyte is used to split water into hydrogen and oxygen by solar energy. Therefore, PEC water splitting is one of the ideal ways to covert and store solar energy to hydrogen energy in terms of chemical bond energy. In a PEC cell, the photoanode is commonly based on n-type semiconductors and the photocathode based on p-type semiconductors. The efficiency of a PEC cell is determined by performance of these photoelectrodes interfaced with the electrolyte. However, because the oxygen evolution reaction on the photoanode is kinetically sluggish involving four electrons and the valance band maximum of photoanodes must be more positive than 1.23 V versus reversible hydrogen electrode, suitable n-type semiconductors are quite few for this purpose, limiting the common photoanodes to low efficiencies for PEC water splitting. In recent years, the bismuth vanadate photoanode has attracted great attention due to its relatively high theoretical maximum photocurrent density (~7 mA cm^–2) and suitable band structure for water splitting, compared with other traditional photoanodes such as titanium dioxide, tungsten oxide, and zinc oxide. Extensive efforts have been made to unleash the full potential of the bismuth vanadate photoanode for PEC water splitting. In this mini review, we survey and analyze the design ideas and synthesis methods of high-performance bismuth vanadate photoanodes by looking back at the research progress made over the past few years on improving the light harvesting efficiency, photo-generated carrier separation efficiency and surface oxygen evolution efficiency of bismuth vanadate photoanodes. The strategies for improving the efficiencies of the bismuth vanadate photoanodes include defect state introduction, crystal facet and morphology control, and heterojunction engineering. Among the strategies, a single one, such as the defect state introduction, may enhance efficiencies of several processes (e.g., photo-generated carrier separation efficiency and surface oxygen evolution efficiency) of bismuth vanadate at the same time, but sometimes, it may enhance the efficiency of one process but degrade the efficiencies of others for the bismuth vanadate photoanode. Thus, how to comprehensively consider the cooperative mechanism to enhance the efficiencies of all the processes involved in PEC water splitting is the key to obtaining high performance bismuth vanadate photoanodes. At present, bismuth vanadate-based photoanodes have exhibited an extremely high photocurrent density and photo-generated carrier separation efficiency at higher bias voltage (over 5 mA cm^–2 at 1.23 V versus reversible hydrogen electrode with over 90% photo-generated carrier separation efficiency), but the light reflection of bismuth vanadate-based photoanodes makes it unable to reach the theoretical maximum photocurrent density (~7 mA cm^–2). Moreover, the efficiencies of bismuth vanadate-based photoanodes at low bias voltages are still too low. Therefore, the future development direction should be to obtain higher photocurrent density at a lower voltage, and increase the absorption efficiency and wavelength range of bismuth vanadate to reach the theoretical photocurrent density and beyond. Although bismuth vanadate photoanodes are not necessarily the final large-scale application scheme of PEC water splitting in the future, their studies will help to provide guidelines for searching new high-performance photoanode materials.