Kinetics versus Charge Separation: Improving the Activity of Stoichiometric and Non-Stoichiometric Hematite Photoanodes Using a Molecular Iridium Water Oxidation Catalyst

Abstract

Oxygen-deficient iron oxide thin films, which have recently been shown to be highly active for photoelectrochemical water oxidation, were surface-functionalized with a monolayer of a molecular iridium water oxidation cocatalyst. The iridium catalyst was found to dramatically improve the kinetics of the water oxidation reaction at both stoichiometric and nonstoichiometric α-Fe 2 O 3-x surfaces. This was found to be the case in both the dark and in the light as evidenced by cyclic voltammetry, Tafel analysis, and electrochemical impedance spectroscopy (EIS). Oxygen evolution measurements under working conditions confirmed high Faradaic efficiencies of 69−100% and good stability over 22 h of operation for the functionalized electrodes. The resulting ∼200−300 mV shift in onset potential for the iridiumfunctionalized sample was attributed to improved interfacial charge transfer and oxygen evolution kinetics. Mott−Schottky plots revealed that there was no shift in flat-band potential or change in donor density following functionalization with the catalyst. The effect of the catalyst on thermodynamics and Fermi level pinning was also found to be negligible, as evidenced by opencircuit potential measurements. Finally, transient photocurrent measurements revealed that the tethered molecular catalyst did improve charge separation and increase charge density at the surface of the photoanodes, but only at high applied biases and only for the nonstoichiometric oxygen-deficient iron oxide films. These results demonstrate how molecular catalysts can be integrated with semiconductors to yield cooperative effects for photoelectrochemical water oxidation.

Key Strategies to Advance the Photoelectrochemical Water Splitting Performance of α‐Fe2O3 Photoanode

“…The deeper insight of the charge transfer mechanism and water oxidation kinetics for Ir‐OEMC loaded on stoichiometric as well as nonstoichiometric α‐Fe 2 O 3 photoanode surfaces has been provided by Moir et al . As schematically illustrated in Figure , the surface functionalization of oxygen deficient α‐Fe 2 O 3‐x with a monolayer of Ir‐OEMC substantially enhances the charge mobility while having a negligible effect on the thermodynamics and Fermi level of photoanode.…”

“… The Schematic representation of improved oxygen evolution kinetics and restrained surface charge recombination on surface functionalization of hydrogen treated α‐Fe 2 O 3‐x photoanode. Reproduced with permission . Copyright 2016, ACS.…”

Graphite-protected CsPbBr3 perovskite photoanodes functionalised with water oxidation catalyst for oxygen evolution in water

“…To test the versatility of our encapsulated perovskite-based photoanodes and to further improve their efficiency at lower potentials, the addition of an iridium-based water oxidation catalyst (WOC) [48][49][50] was explored. The [Ir(μO)(pyalk)(H 2 O) 2 ] 2 2+ catalyst chosen has previously been shown to rapidly and irreversibly adsorb to the surface of indium tin oxide (ITO) and α-Fe 2 O 3 as a minimally thin molecular monolayer [50][51][52][53] . Grafitingly, we also found it to robustly bind to graphitic surfaces after simply floating the GS on an activated WOC solution at room temperature for 16 h (Fig.…”

Evidence for tetranuclear bis-μ-oxo cubane species in molecular iridium-based water oxidation catalysts from XAS analysis

“…19 After activation 20 these can even be grafted onto conducting metal oxides as molecular monolayers to yield exceptionally efficient and durable anodes for electrochemical water oxidation. [21][22][23] Although a number of spectroscopic features have been analyzed, 24 the exact molecular structure of these highly active catalysts has not yet been elucidated. A previous study of a pyridine-alkoxide ligated Ir WOC using high-energy X-ray scattering (HEXS) analysis and density functional theory (DFT) modelling suggested a mono-m-oxo Ir dimer as the resting state of the activated catalyst, 25 and in the following a series of coordinately saturated mono-m-oxo dimers with two pyalk ligands per iridium centre have been synthesised.…”