Oxygen Evolution Catalysts at Transition Metal Oxide Photoanodes: Their Differing Roles for Solar Water Splitting

Abstract

In the field of photoelectrochemical water splitting for hydrogen production, dedicated efforts have recently been made to improve water oxidation at photoanodes, and in particular, to accelerate the poor kinetics of the oxygen evolution reaction which is a key step in achieving a viable photocurrent density for industrialization. To this end, coating the photoanode semiconductors with oxygen evolution catalysts (OECs) has been one of the most popular options. The roles of OECs have been found to be multifold, as opposed to exclusively catalytic. This review aims to unravel the complexity of the interfacial processes arising from the material properties of both semiconductors and OECs, and to rationalize the variation in findings in the literature regarding the roles of OECs. Light is also shed on some of the most useful characterization techniques that probe the dynamics of photogenerated holes, to answer some of the field's most challenging mechanistic questions. Finally, some ideas and suggestions on the design principles of OECs are proposed.

Enhanced Surface Kinetics and Charge Transfer of BiVO4 Photoanodes by Rh2O3 Cocatalyst Loading for Improved Solar Water Oxidation

“…Clearly, Rh 2 O 3 /BiVO 4 immersed in 50 mL H 2 O containing 50 μL RhCl 3 solution for 12 h outperforms all other Rh 2 O 3 /BiVO 4 photoanodes, that is 0.88% mass ratio of Rh 2 O 3 :BiVO 4 is the most satisfactory one, while whether insufficient or redundant Rh 2 O 3 works greatly to the disadvantage of performance. Owing to enhanced surface kinetics and carrier transfer efficiency, holes are quicker to participate in evolving O 2 under illumination after the decoration of Rh 2 O 3 ; however, holes and electrons meanwhile prefer to reach and react at the same sites of the cocatalyst, meaning that these active sites actually incur more recombination [21] . As a result, performance critically depends on the rate of kinetics and efficiency enhancement compared with the rate of recombination at the active sites.…”

“…Owing to enhanced surface kinetics and carrier transfer efficiency, holes are quicker to participate in evolving O 2 under illumination after the decoration of Rh 2 O 3 ; however, holes and electrons meanwhile prefer to reach and react at the same sites of the cocatalyst, meaning that these active sites actually incur more recombination. [21] As a result, performance critically depends on the rate of kinetics and efficiency enhancement compared with the rate of recombination at the active sites. This is why both impregnation times and impregnation concentrations show volcano relationships with photocurrent densities.…”

Opportunities from Doping of Non‐Critical Metal Oxides in Last Generation Light‐Conversion Devices

“…Among the commonly employed photocatalytic approaches, [203] i.e., the colloidal-phase photocatalytic (PC) approach and the photoelectrochemical (PEC) approach, the latter allows for a modular development of materials specifically suitable for photocathodes or photoanodes in separate environments, before combining them into a single water splitting device. This is particularly important for MOSs, [204] since their valence band position is often driven by highly positive O 2p orbital (+3.0 V vs normal hydrogen electrode, NHE), thus limiting the maximum reduction potential obtained with a photoexcited narrow bandgap oxide. As a consequence, overall water splitting is rarely achieved with a single MOS, but rather by fashioning complex Z-scheme photocatalytic heterostructures or bias-free tandem photoelectrochemical systems (Figure 7a).…”

Decorating the Cocatalyst Membrane with Coordinated Tannic Acid and Ternary Metal for Advancing Photoelectrochemical Performance of F-Doped Hematite Photoanodes

“…However, these types of semiconductor photoanodes are still plagued by the severe photoexcited electron−hole pair recombination damage both in the bulk and at the electrode/electrolyte interfaces, which has not been obtained through major breakthrough. 8,9 Even so, hematite (α-Fe 2 O 3 ) still stands out from the crowd due to its suitable band gap width (2.0−2.2 eV) that allows a theoretical maximum of 16.8% STH efficiency and excellent stability. 10,11 At present, the improvement of the α-Fe 2 O 3 is considered to be the most promising development direction for promoting the PEC water splitting efficiency.…”