Unveiling the Hydration Structure of Ferrihydrite for Hole Storage in Photoelectrochemical Water Oxidation

Abstract

Ferrihydrite (Fh) has been demonstrated acting as a hole‐storage layer (HSL) in photoelectrocatalysis system. However, the intrinsic structure responsible for the hole storage function for Fh remains unclear. Herein, by dehydrating the Fh via a careful calcination, the essential relation between the HSL function and the structure evolution of Fh material is unraveled. The irreversible and gradual loss of crystal water molecules in Fh leads to the weakening of the HSL function, accompanied with the arrangement of inner structure units. A structure evolution of the dehydration process is proposed and the primary active structure of Fh for HSL is identified as the [FeO6] polyhedral units bonding with two or three molecules of crystal water. With the successive loss of chemical crystal water, the coordination symmetry of [FeO6] hydration units undergoes mutation and a more ordered structure is formed, causing the difficulty for accepting photogenerated holes as a consequence.

Charge Relays via Dual Carbon‐Actions on Nanostructured BiVO4 for High Performance Photoelectrochemical Water Splitting

“…[ 1 ] The solar‐to‐hydrogen conversion efficiency of a PEC water splitting system is ultimately determined by physical properties such as charge separation and charge transfer capability. [ 2 ] Among the materials of recent interest, bismuth vanadate (BiVO 4 ) deserves particular attention because of its suitable bandgap, abundance, nontoxicity, and excellent catalytic performance. [ 3 ] However, its PEC performance is still limited due to its poor electrical conductivity, severe charge recombination, and sluggish surface kinetics.…”

Ultrathin Cobalt Oxide Interlayer Facilitated Hole Storage for Sustained Water Oxidation over Composited Tantalum Nitride Photoanodes

“…The evaluation for hole-storage behaviors of the above-studied photoanodes could be further carried out by transient photocurrent responses under applied stepped potentials, as shown in Figure S9. The evident transient cathodic peaks appear upon light on/off, indicating that the photogenerated holes accumulated on the interface of Ta 3 N 5 and the electrolyte would in turn participate in the recombination process with conduction band electrons. − The quantity of stored holes, integrating from cathodic current peaks, represents the accumulated holes prepared for taking part in PEC water oxidation reaction using our previous calculation methods. , Figure d demonstrates that the charge-storage capacity of Ta 3 N 5 is increased by at least 2 orders of magnitude after the addition of the CoO x /Ni­(OH) x bilayer. Meanwhile, charge-storage quantities for Ta 3 N 5 /CoO x and Ta 3 N 5 /Ni­(OH) x are also presented for comparison.…”

“…42−44 The quantity of stored holes, integrating from cathodic current peaks, represents the accumulated holes prepared for taking part in PEC water oxidation reaction using our previous calculation methods. 18,44 Figure 2d demonstrates that the charge-storage capacity of Ta 3 N 5 is increased by at least 2 orders of magnitude after the addition of the CoO x /Ni(OH) x bilayer. Meanwhile, charge-storage quantities for Ta 3 N 5 /CoO x and Ta 3 N 5 /Ni-(OH) x are also presented for comparison.…”

Coupling effect between hole storage and interfacial charge transfer over ultrathin CoPi-modified hematite photoanodes

“…19,20 Hole-storage-layer (HSL) materials including ferrihydrite (denoted as Fh), Ni(OH) x /MoO 3 , Ni(OH) x /CoO x , and Ni(OH) x / Fh have been demonstrated to be effective in extracting and storing photogenerated holes from the semiconductor, resulting in a remarkable improvement in the PEC durability and activity. [21][22][23][24][25][26] Moreover, some typical water oxidation cocata-lysts, such as cobalt phosphate (CoPi) and the NiOOH/black phosphorene bilayer, have also been identified to possess superior hole-extraction ability. 27,28 In this regard, it is important to reveal the intrinsic roles of the modification layers in terms of the charge carrier kinetics and interfacial energetics at the SCLI.…”