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

Jonathon W Moir,Geoffrey A Ozin,Emma V Sackville,Ulrich Hintermair

doi:10.1021/acs.jpcc.6b00735

Jonathon W Moir, Geoffrey A Ozin + Show 2 more

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https://doi.org/10.1021/acs.jpcc.6b00735

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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-Fe2O3-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 iridium-functionalized 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 fo...

Highlights

A stable molecular iridium water oxidation catalyst was successfully tethered to ultrathin hematite photoanodes, as confirmed using X-ray photoelectron spectroscopy (XPS) and cyclic voltammetry
In the case of the air-treated films, the iridium catalyst increased the electrocatalytic activity of the films in the dark, transfer of photoholes from the hematite photoanode to the iridium catalyst was minimal due to insufficient band bending and high rates of recombination in the iron oxide, resulting in limited photoactivity
In the case of the H2-treated films, the iridium catalyst improved the kinetics of the water oxidation reaction in both the dark and in the light, and increased the density of photoholes at the surface and decreased the rate of recombination of majority charge carriers with these surfacebound charges at moderate to high applied potentials

Summary

Introduction

The renewable production of clean, nonfossil-derived hydrogen is a critical part of a sustainable future economy, due to the importance of hydrogen in a variety of industrial applications including the Haber−Bosch process,[1] the Fischer−Tropsch process,[2,3] hydrogenation reactions, and organic transformations[4−6] and as transportation fuel in hydrogen fuel cells.[1,7] In principle, photoelectrochemical water splitting provides a facile route to clean hydrogen from water and enables simple monitoring of photoelectrode kinetics and activity.[8−14] Hematite (α-Fe2O3) is a primary photoanode candidate due to its many beneficial features including stability in aqueous alkaline media, Earth abundance, low toxicity, low cost, and its ability to theoretically absorb a large portion of the solar spectrum (∼18% of photons for AM1.5).[15]. A recent study by Forster et al confirmed this latter effect for oxygen-deficient hematite photoanodes prepared in an oxygen-deficient atmosphere.[25] Transient absorption spectroscopy (TAS) and transient photocurrent (TPC) measurements performed in situ confirmed the buildup of photogenerated holes at the surface and a decrease in back electron−hole recombination due to a change in band bending close to the semiconductor−liquid junction (SCLJ). The results of these studies, along with other recent reports,[22,26,27] highlight the importance of charge separation in hematite photoanodes for efficient photoelectrochemical water oxidation

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